US20060274711A1 - Maintenance link using active/standby request channels - Google Patents

Abstract

Multiple field units in a CDMA system are synchronized for communication with a base station using shared forward and reverse link channels. In an illustrative embodiment, each field unit is assigned a time slot in a forward link channel to receive messages from the base station. Likewise, each field unit is assigned a time slot on a common reverse link channel for transmitting messages to the base station. Timing alignment and power level control among each of many field units and the base station is achieved by analyzing messages received at the base station in a corresponding time slot as transmitted by each field unit. Thereafter, a message is transmitted from the base station in a corresponding time slot to a particular field unit for adjusting its timing or power level so that future messages transmitted from the field unit are received in the appropriate time slot at the base station at a desired power level. In this way, minimal resources are deployed to maintain communication and precise synchronization between a base station and each of multiple users, minimizing collisions between field units transmitting in adjacent time slots on the reverse link. This method reduces the frequency a field unit must rely on the use of a slotted aloha random access channel according to IS-95.

Description

RELATED APPLICATION
• This application is a continuation of application Ser. No. 09/775,305, filed Feb. 1, 2001, which claims the benefit of U.S. Provisional Application Ser. No. 60/180,598 filed on Feb. 7, 2000. The entire teachings of the above applications are incorporated herein by reference.
BACKGROUND OF THE INVENTION
• Increasing use of wireless telephones and personal computers has led to a corresponding increase in demand for advanced telecommunication services that were once thought practical only for specialized applications. In the 1980s, wireless voice communication became widely available through cellular telephone networks. Such services were thought at first to be for the exclusive province of businessmen because of expected high subscriber costs. The same was also true for access to remotely distributed computer networks, whereby until very recently, only business people and large institutions could afford the necessary computers and wireline access equipment.
• As a result of the widespread availability of affordable new technologies, the general population now increasingly desires to have not only wireline access to networks such as the internet and private intranets, but also wireless access as well. Wireless technology is particularly useful to users of portable computers, laptop computers, hand-held personal digital assistants and the like who prefer access to such networks without being tethered to a telephone line.
• There still is no widely available satisfactory solution for providing low cost, high speed access to the Internet, private intranets, and other networks using the existing wireless infrastructure. This is most likely an artifact of several unfortunate circumstances. First, the typical manner of providing high speed data service in the business environment over a wireline network is not readily adaptable to the voice grade service available in most homes or offices. For example, such standard high speed data services do not necessarily lend themselves to efficient transmission over standard cellular wireless handsets because wireless networks were originally designed only to provide voice services. As a result, present day digital wireless communication systems are optimized for voice transmissions, although certain schemes such as CDMA do provide some measure of asymmetrical behavior for the accommodation of data transmissions. For example, the data rate specified by the Telecommunication Industry Association (TIA) for IS-95 on the forward traffic channel is adjustable in increments from 1.2 kbps up to 9.6 kbps for so-calledRate Set 1, and increments from 1.8 kbps up to 14.4 kbps for Rate Set 2. On the reverse link traffic channel, however, the data rate is fixed at 4.8 kbps.
• At best, existing wireless systems therefore typically provide a radio channel that can accommodate maximum data rate transfers of 14.4 kilobits per second (kbps) over a forward link direction. Such a low data rate channel does not lend itself directly to transmitting data at rates of 28.8 or even 56.6 kbps that are now commonly available using inexpensive wireline modems, not to mention even higher rates such as the 128 kbps that are available with Integrated Services Digital Network (ISDN) type equipment. Data rates at these levels are rapidly becoming the minimum acceptable rates for activities such as browsing web pages.
• Although wireline networks were known at the time when cellular systems were initially developed, for the most part, there was no provision made for such wireless systems to provide higher speed ISDN- or xDSL-grade data services over cellular network topologies.
• In most wireless systems, there are many more potential users than radio channel resources. Some type of demand-based multiple access system is therefore required.
• Whether the multiple access is provided by the traditional Frequency Division Multiple Access (FDMA) using analog modulation on a group of radio frequency carrier signals, or by schemes that permit sharing of a radio carrier frequency using Time Division Multiple Access (TDMA), or Code Division Multiple Access (CDMA), the nature of the radio spectrum is such that it is expected to be shared. This is quite dissimilar to the traditional environment supporting data transmissions in which the wireline medium is relatively inexpensive and is not typically intended to be shared.
• Other factors to consider in the design of a wireless system are the characteristics of the data itself. For example, consider that access to web pages generally is burst-oriented, with asymmetrical data rate transmission requirements in a reverse and forward direction. In a common application, a user of a remote client computer first specifies the address of a web page to a browser program. The browser program then sends the web page address data, which is usually 100 bytes or less in length, over the network to a server computer. The server computer then responds with the content of the requested web page, which may include anywhere from 10 kilobytes to several megabytes of text, image, audio, or even video data. The user thereafter may spend several seconds or even several minutes reading the content of the page before downloading another web page.
• In an office environment, the nature of most employees' computer work habits is typically to check a few web pages and then to do something else for an extended period of time, such as accessing locally stored data or even terminating use of the computer altogether. Therefore, even though such users may remain connected to the Internet or private intranet continuously during an entire day, actual use of the high speed data link is usually quite sporadic.
• If wireless data transfer services supporting Internet connectivity are to coexist with wireless voice communication, it is becoming increasingly important to optimize the use of available resources in wireless CDMA systems. Frequency re-use and dynamic traffic channel allocation address some aspects of increasing the efficiency of high performance wireless CDMA communication systems, but there is still a need for more efficient utilization of available resources.
SUMMARY OF THE INVENTION
• The present invention provides several novel features for optimizing wireless links in a CDMA communication system. In an illustrative embodiment, a pair of shared channels is allocated for transmitting a message at a specified time from a field unit to a base station for synchronizing the base station to the field unit. More specifically, a message received at the base station from the field unit on either one of the pair of shared channels is analyzed to determine whether the field unit is properly synchronized with the base station to transmit properly within an assigned time segment. Minimal timing feedback messages to advance or retard timing are then transmitted in a forward link to the field unit so that subsequent messages by the field unit are transmitted within the appropriate time segment. Preferably, synchronization of links is maintained between a base station and each of multiple field units simultaneously. Thus according to one aspect of the shared communication system of the present invention, minimal resources are allocated for maintaining synchronization links with multiple field units so that wireless resources can be reserved for high bandwidth data transfers.
• In a preferred embodiment, a forward link channel is allocated to support time-slotted message transmissions from a base station to each of multiple field units. In an opposite direction, reverse link channels are allocated to support time-slotted message transmissions from the field units back to the base station. Thus, each channel is partitioned into time slots or time segments, where a time slot in the forward channel is assigned to a field unit for directing communications from the base station to the corresponding field unit. Likewise, a time slot in a reverse channel is assigned to a field unit for transmitting messages to the base station. For both forward and reverse link communications, the relative position of a time slot itself indicates to which field unit a message is directed from the base station or from which field unit a message is transmitted to the base station.
• In one application, a forward link channel is partitioned to include both “active” and “standby” time slots, where an active time slot is assigned to a field unit to indicate that reverse link traffic channels are presently being used for transmitting a data payload to the base station in the reverse link. A standby time slot is assigned to a field unit for maintaining synchronization with the base station even though the corresponding field unit is not presently transmitting a data payload over the reverse link. Consequently, a field unit in the standby mode can immediately transmit on newly assigned reverse link channel without a prolonged delay of re-synchronizing since the field unit is already synchronized with the base station.
• The base station monitors the field units, acknowledging requests by corresponding field units to be placed in the active mode. If available, an active time slot is assigned to the link requesting field unit and traffic channels are allocated in the reverse link to support a data payload transfer between the requesting field unit and the base station. Following a data transfer on a particular link, the field unit is reassigned to a standby time slot for maintaining a minimal but synchronized link with the base station.
• In a preferred embodiment, there are a predetermined number of periodically repeating time slots allocated within each of the forward link and a pair of reverse link coded channels. Hence, messages from the base station or field units can be transmitted on a periodic basis in the appropriate time slots of the shared channels. Preferably, the time slot assigned to a field unit in the first channel for receiving data from the base station is offset in time with respect to the corresponding time slot assigned in the pair of channels for transmitting a timing indication marker from the field unit to the base station. This allows the base station time to analyze a timing of a received message in a time slot, generate a timing adjustment feedback message and transmit it to the corresponding field unit in the appropriate time slot of the forward channel.
• To maintain synchronization, messages received at the base station in assigned time slots from each of many field units are analyzed to determine how to adjust the timing of future message transmissions at the corresponding field units. In particular, message transmissions from a field unit are adjusted so that a message is received at the base station in an assigned time slot or time segment. This is achieved by transmitting a timing adjustment message from the base station over the forward link to the field unit, where the timing adjustment message indicates whether the field unit should advance or retard timing of future message transmissions so that they fall within a desired time slot. In this way, field units transmitting messages in adjacent time slots of the same reverse link channel typically will not interfere with each other. This reduces co-channel interference.
• Messages transmitted over a reverse link channel from a field unit to the base station preferably include markers for synchronizing the corresponding field unit with the base station. The markers transmitted within a message provide a precise reference point within a time slot itself for generating timing correction information as described above. For example, the base station analyzes a marker within a time slot and determines a degree to which the field unit must adjust its timing in the reverse link during that subsequent messages are transmitted by the field in the appropriate time segment. Preferably, a well-placed string of pilot symbols is used as a time reference marker in a time slot. Alternatively, a short pseudo random noise (PN) code may be used as a marker in a time slot. Of course, any type of marker can be used as long as it is detectable by the base station.
• While in the active mode, a marker in a corresponding time slot of the traffic channels is analyzed to support timing alignment between the field unit and base station. While in the inactive or standby mode, a marker in a time slot of either one of the pair allocated reverse link channels is analyzed to support timing alignment between the field unit and base station.
• As previously discussed, the shared forward link channel is used to transmit link quality messages or timing adjustment messages from a base station to multiple field units. In an opposite direction, the reverse link includes a pair of uniquely coded first and second channels. Each channel of the pair serves a particular purpose. For example, a time segment or time slot of the reverse link channels is assigned to a field unit for transmitting an indication message to the base station. A data transmission from a field unit over the first channel indicates a first type of message to the base station while a data transmission from the field unit over the second channel indicates a second type of message to the base station. Accordingly, the mere transmission on a particular channel itself provides an indication or message to the base station. Consequently, an encoded data payload need not be transmitted to communicate with the base station.
• In one application, a transmission of a marker in a time slot over one channel indicates a request by the corresponding field unit to go active. That is, transmission of a marker in an assigned time slot indicates that the field unit is requesting that reverse link traffic channels be assigned to the user for transmitting a data payload from the field unit to the base station. This presumes that the field unit is presently in the standby mode. Alternatively, a field unit transmits a marker over a second channel of the pair of reverse link channels to indicate that the field unit is not requesting to be placed in the active mode. For example, the field unit does not want to transmit data on a reverse link channel. Rather, the field unit requests to remain inactive but synchronized with the base station so that the field unit can immediately go active again at any moment.
• As previously discussed, synchronization is achieved by monitoring markers received at the base station for each of multiple field units and transmitting an adjustment message in reverse link so that future transmissions are properly aligned within a time slot. In a preferred application, a field unit transmits a marker in at least one of three reverse link channels so that the base station and field units can be synchronized. For example, precise timing alignment is achieved while a field unit is in the standby mode via a marker transmitted in a time slot of the first shared reverse link channel. When a field unit desires to go active, the field unit then discontinues transmitting the marker in first shared channel and transmits a marker in the same time segment over the second channel. This transmission over the second channel serves two purposes. The first purpose is to indicate that the field unit desires to be placed in the active mode. The second purpose is to transmit a marker from the field unit so that the base station can continue to analyze whether a field unit is properly synchronized. In an instance where reverse link traffic channels are assigned for use by a particular field unit, the field unit discontinues transmitting a marker in either the first or second channel to maintain synchronization. Rather, the field unit then transmits a marker in the assigned reverse link traffic channels. Similar to the method as previously discussed, the base station will now monitor a portion of a marker transmitted in the reverse link traffic channel to determine whether the field unit is properly synchronized to transmit within a time slot. In this way, feedback to each of the field units ensures that they are precisely aligned for transmitting data within an assigned time slot no matter the mode of the field unit. In other words, precise timing alignment for a particular link to a field unit is optimally maintained whether the field unit is in the standby mode, request-to-go active mode, or presently transmitting a data payload on one or multiple assigned reverse link traffic channels.
• In a preferred embodiment, the message sent from the base station to a field unit for adjusting its timing with respect to the base station includes a single bit indicating whether to advance or retard the slot timing reference at the field unit. Based on the state of the bit as received in a time slot at a corresponding field unit, the field unit then advances or retards its slot timing a predefined amount of time such as ⅛ of a chip. If a state of the single bit as received in successive feedback messages at the field unit is in a same state n times in a row, i.e., n logic ones or zeros are received in a row, a subsequent bit in the same state will cause the field unit to advance or retard its timing based on a larger predefined time such as ½ of a chip rather than ⅛ of a chip. Consequently, a field unit that is grossly out of synchronization with the base station will be synchronized more quickly than if a smaller fixed adjustment time such as ⅛ of a chip were used to continuously adjust timing.
• To synchronize newly powered field units, the base station monitors an access channel for requests by such field units to establish new links with the base station. The time a random or pseudo-random link request message is received over the access channel at the base station can be used to generate an initial timing adjustment to be made at the requesting field unit for achieving timing synchronization with the base station.
• More particularly, to achieve initial synchronization of time slots between the base station and field unit, a coarse timing adjustment message is transmitted to a respective field unit indicating how to initially achieve a gross timing alignment with the base station. The coarse timing adjustment message is preferably a multi-bit value notifying the field unit of an amount to advance or retard its initial timing to achieve synchronization with the base station. Accordingly, a field unit can be coarsely or precisely synchronized for receiving and transmitting messages in an assigned time slot.
• Preferably, the coarse timing adjustment message is sent to the link requesting field unit over a paging channel. Likewise, dynamic time slot assignment information is transmitted to a field unit over the paging channel.
• In another illustrative embodiment of the present invention, a coded channel of the wireless communication system is allocated to support synchronized communications from each of multiple transmitters and a receiver. A first portion of the coded channel is assigned for use by a first transmitter to transmit a reference signal to the receiver, while a second portion of the channel is assigned for use by a second transmitter to transmit a message to the receiver. Preferably, the coded channel is divided into time slots and each transmitter transmits in the appropriate time slot to the receiver.
• According to this method, multiple transmitters can transmit different types of information to the receiver over the coded channel. For example, the reference signal transmitted by the first transmitter is optionally a timing signal analyzed by the receiver. Timing adjustment messages in a feedback link can then be communicated to the first transmitter to adjust its timing. At the same time, a message is transmitted by the second transmitter to the receiver over the coded channel.
• Preferably, the reference signal generated by the first transmitter does not include forward error correction information, while the message transmitted by the second transmitter does include forward error correction information. In a specific application, the message transmitted from the second transmitter to the receiver is a feedback message including a link quality message indicating a reverse link quality of transmissions between the receiver and corresponding transmitter.
• In another illustrative embodiment of the present invention, a first and second coded channels are allocated to support synchronized communications from a transmitter to a receiver. Preferably, the first and second coded channels are divided into time slots in which a transmitter communicates an indication to a target receiver by generating a signal at an adjusted power level from the transmitter to the receiver. As previously discussed, the transmission over the first or second coded channel itself indicates a message to the receiver. For example, transmission within a time slot of the first coded channel indicates to the receiver that the field unit desires to remain in a standby mode, while a transmission over the second coded channel indicates to the receiver that the transmitter would like to transmit a data payload to the receiver over additionally assigned traffic channels.
• The indication transmitted over the first or second channel in a time slot is analyzed at the receiver to determine the strength of the received signal. If the signal strength is lower than a desired level, a feedback message generated by the receiver is communicated to the transmitter for adjusting its power level for subsequent transmissions on the first or second coded channel. In this way, the power level of a transmitter can be adjusted to reduce co-channel interference in a wireless system. More specifically, the power level of a transmitting device can be gradually increased or decreased so that it has minimal impact on other channels.
• A power adjustment message communicated from the receiver to the transmitter can include a single bit indicating whether the transmitter should increase or decrease its power output for subsequent transmissions. Thus, power can be optimally adjusted even in a dynamic environment where the signal to noise ratio can change almost instantaneously.
• This method of adjusting power is similar to the method as previously described for synchronizing a transmitter to a receiver via feedback messages. However, in this illustrative embodiment, the power level output of the transmitter is adjusted via feedback messages instead of timing. Thus, the power level of a transmitter can be adjusted while in a standby mode so that, in the event that the transmitter goes active transmitting a data payload to the receiver, the power level of the transmitter is optimized to reduce co-channel interference. Both power and timing feedback loops can be implemented simultaneously so that the power output level and timing of a transmitter is optimized for potentially sporadic data transmissions.
• One aspect of the present invention is to provide a link between a base station and each of multiple field units using minimal resources. Providing shared channels using the methods as described above for communicating with a base station reduces the number of CDMA channels necessary to maintain synchronized links with multiple field units. Accordingly, more channels or resources are then available to support data transfers between the base station and field units. This increases the potential throughput on an allocated carrier frequency.
• Another aspect of the present invention involves a novel method of providing a minimal feedback loop for supporting timing alignment between the base station and each of multiple field units. The base station analyzes a marker incorporated in each message received in an assigned time slot to detect whether a field unit message directed to the base station is early or late with respect to a preferred timing with respect to the base station. A message such as a single advance/retard control bit is then transmitted back to the corresponding field unit to advance or retard its timing so that future message transmissions from the field unit are received at the base station in the appropriate time slot. Thus, multiple field units transmitting messages on the shared reverse link channel generally do not interfere with each other when transmitting in adjacent time slots. Additionally, the timing information sent on the forward link enables the field unit to synchronize itself to the base station for receiving messages in its assigned time slot on the forward link. As a consequence of maintaining each of multiple synchronized links, a corresponding field unit can immediately transmit a data payload in a reverse link direction without having to go through a lengthy process of re-synchronizing with the base station prior to a data transmission.
BRIEF DESCRIPTION OF THE DRAWINGS
• The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.
• FIG. 1 is a general diagram illustrating a wireless communication system according to the principles of the present invention.
• FIG. 2 is a timing diagram illustrating heartbeat and Link Quality Management (LQM) slot timing according to the principles of the present invention.
• FIG. 3 is a diagram illustrating an exemplary bit definition of an LQM slot according to the principles of the present invention.
• FIG. 4 is a graph illustrating a field unit requesting to go active and the allocation of traffic channels to transmit a data payload in a reverse link direction according to the principles of the present invention.
• FIG. 5 is a block diagram supporting channel synchronization according to the principles of the present invention.
• FIGS. 6aand6bare flow charts illustrating how forward and reverse channels are synchronized according to the principles of the present invention.
• FIG. 7 is a graph illustrating pulse sampling techniques for identifying a timing mark for synchronizing forward and reverse channels according to the principles of the present invention.
• FIG. 8 is a table showing attributes of an active, standby and idle mode for synchronizing a field unit to a base station according to the principles of the present invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
• FIG. 1 is a diagram of awireless communication system100 according to the principles of the present invention. Abase station25 maintains wireless communication links with each of a plurality offield units42A,42B,42C (collectively, field units42) as shown. Such wireless links are established based upon assignment of resources on aforward link70 and areverse link65 between thebase station25 and field units42. Eachlink65 or70 is typically made up of several logicalreverse link channels55 and several logicalforward link channels60.
• As shown,communication system100 supports wireless communication between aninterface50 andnetwork20. Typically,network20 is a Public Switched Telephone Network (PSTN) or computer network such as the Internet, internet or intranet.Interface50 is preferably coupled to a digital processing device such as aportable computer12, to provide wireless access to thenetwork20. Consequently,portable computer device12 has access tonetwork20 based on communications over a combination of both hard-wired and wireless data links.
• In a preferred embodiment, theforward link channels60 andreverse link channels55 are defined inwireless communication system100 as Code Division Multiple Access (CDMA) channels. That is, each CDMA channel is preferably defined by encoding and transmitting data over the channel with an augmented pseudo random noise (PN) code sequence. The PN coded data is then modulated onto a radio frequency carrier. This enables a receiver to decipher one CDMA channel from another knowing only the particular augmented PN code assigned for a given channel. In accordance with the preferred embodiment, each channel preferably occupies a 1.25 MHz band consistent with the IS-95 CDMA standard and is capable of transmitting at 38.4 kbps.
• Forward link channels70 include at least three logical channels. As shown, this includes a Link Quality Management (LQM)channel60L, apaging channel60P, andmultiple traffic channels60T.
• Reverse link65 includes a heartbeat standby channel55HS, heartbeat request active channel55HRA,access channel55A andmultiple traffic channels55T. Generally, thereverse link channels55 are similar to theforward link channels60 except that each reverselink traffic channel60T can support variable data rates from 2.4 kbps to a maximum of 160 kbps.
• Data transmitted betweenbase station25 andfield unit42A typically consists of encoded digital information, such as web page data. Based on the allocation of multiple traffic channels in thereverse link65 orforward link70, higher data transfer rates can be achieved in a particular link between thebase station25 andfield unit42A. However, since multiple field units42 compete for bandwidth allocation, afield unit42A may have to wait until resources are free to be assigned traffic channels to transmit a data payload.
• As shown inFIG. 2, the forwardlink LQM channel60L is partitioned into a predetermined number of periodically repeatingtime slots310 for the transmission of messages to each of multiple field units42. Each field unit42 identifies messages directed to itself based upon messages received in its assignedtime slot310. In other words, field units42 monitor messages received in their respectively assignedtime slots310 to receive information frombase station25.
• The reverse link heartbeat standby channel55HS and heartbeat request active channel55HRA are also shared among multiple users. These channels are both partitioned into periodically repeatingtime slots310 so that thetime slots310 in each heartbeat channel align with each other and also the time slots of theLQM channel60L. Atime slot310 of the reverse link heartbeat channels55HS or55HRA is assigned to one of many field units42 for transmitting heartbeat-type messages to thebase station25 over either the heartbeat standby channel55HS or heartbeat request active channel55HRA. Accordingly, thebase station25 identifies from whichfield unit42A a message is transmitted based upon the receipt of a message in a particular time slot.
• The pair of shared channels in the reverse link are called heartbeat channels because one aspect of the present invention involves transmitting a minimal indication from afield unit42A to keep it synchronized with thebase station25. The heartbeat channels55HS,55HRA andLQM channel60L are described in more detail below.
• As previously mentioned, another aspect of the present invention involves maintaining a minimal maintenance link between each of multiple field units and abase station25 even when they are not presently transmitting a data payload in a reverse link direction. This scheme maintaining synchronization is particularly advantageous in applications where each of the multiple field units sporadically request to go active and transmit data in a reverse link. Since each of the field units42 is already synchronized with thebase station25 via the minimal link, afield unit42A can be assigned reverselink traffic channels55 and, when assigned, almost immediately transmit a data payload in a reverse link direction without interfering with other channels. That is, afield unit42A does not need to go through a lengthy process of re-synchronizing itself with thebase station25 when traffic channels are eventually assigned for its use.
• In the following description, reference is again generally made toFIG. 1, but more specific details ofLQM channel60 and heartbeat channel55H are referenced toFIG. 2.
• To establish a synchronized link withbase station25, field units42 transmit messages onaccess channel55A tobase station receiver35 viafield unit transmitter40. These messages are then acknowledged and processed atbase station25. If available, resources are allocated bybase station25 to establish a bi-directional communication link with the requestingfield unit42A.
• Within theforward link70, thepaging channel60P is used by thebase station transmitter30 to send overhead and paging messages or commands to thefield unit receiver45. Overhead information includes data such as system configuration parameters for establishing wireless links with field units42.
• As mentioned previously,wireless communication system100 includes a heartbeat channel55HS and heartbeat request active channel55HRA in thereverse link65 and link quality management channel (LQM)60L in theforward link70. These channels are shared between thebase station25 and multiple field units42. That is, thebase station25 transmits messages to multiple field units42 using the same forwardlink LQM channel60L, where a message to aparticular field unit42A is transmitted in an assignedtime slot310. In this way, time slot assignments serve as a way of addressing messages to a particular field unit and corresponding communication link.
• The principles of the present invention are advantageously deployed to support users that require on-demand and sporadic high speed throughput of data on a wireless communication link. For example, remote users atPC device12 can be connected to the Internet over a wireless link that supports on demand high speed throughput capability for downloading an object file such as a web page. Users can remain in a standby mode when no data payloads are transmitted in a reverse link direction. For example, use of a link can be minimal for a period of time so that the user can review a web page. To support such users, it is advantageous to maintain synchronization with thebase station25 even while the link is not actively being used to transmit or receive data. This is achieved inwireless communication system100 by maintaining a minimal connection with thebase station25 even when no data is being actively transferred between thebase station25 and aspecific field unit42A.
• One aspect of the minimal connection between afield unit42A andbase station25 involves adjusting timing of thefield unit42A so that its timing is properly aligned withbase station25. Another aspect of the minimal connection involves adjusting the power level output of thefield unit42A so that it transmits at a low but detectable power level.
• As mentioned, repeatedly creating or reviving connections for users who sporadically need a link can be time consuming and result in the inefficient use of resources. It is also inefficient to reserve resources such astraffic channels55T on a continuous basis for subscribers who are not presently transmitting data. Accordingly,traffic channels55T are allocated on an as-needed basis to support data transfers, optimizing the use of available resources inwireless communication system100.
• FIG. 2 is a timing diagram more particularly illustrating the heartbeat standby channel55HS, heartbeat request active channel55HRA andLQM channel60L. Preferably, there are twoLQM channels60L combined with a total of four heartbeat-type channels including two heartbeat standby channels55HS and two heartbeat request active channels55HRA since coded channels are typically allocated in pairs. However, only one of each channel type is shown inFIG. 2 for illustrative purposes. Of course, the paired sets of channels can be used to support twice the number of users.
• As shown, 64 time slots (in each direction) are defined per EPOCH period in each of the heartbeat standby55HS, heartbeat request active channel55HRA andLQM60L channels. Up to 48 field units42 in the standby mode can be supported along with up to 16 users in the active mode. The EPOCH period in the illustrated embodiment is 13.3 mS, so that each time slot is 208 mS or 256 PN code chips. Because time slots repeat on a periodic basis, thebase station25 can exchange information with a particular field unit42 every EPOCH or 13.3 mS.
• Data transmissions on theLQM channel60L are maintained bybase station25, which is preferably used as a master timing reference. Field units42, therefore, must synchronize themselves tobase station25, and specifically to theLQM channel60L, in order to communicate with thebase station25 and transmit within an assigned time slot.
• Generally, a link betweenbase station25 and afield unit42A is maintained in one of three modes: active, standby or idle. Precise synchronization between thebase station25 and aparticular field unit42A is maintained only for field units42 in the active and standby mode.FIG. 7 provides more details about mode types maintained for a particular link between thebase station25 and afield unit42A. This aspect of the present invention will be discussed later in the specification.
• Eachfield unit42A in the standby or active mode is assigned one time slot in the forwardlink LQM channel60L and one time segment in the reverse link heartbeat-type channels. Accordingly, information is targeted to afield unit42A based upon the transmission of a message in a particular time slot. For example, afield unit42A assigned totime slot #1 demodulates and decodes information received intime slot #1 on the forwardlink LQM channel60L, while data transmitted back tobase station25 is transmitted byfield unit42A intime slot #1 of the reverse link heartbeat standby channel55HS or heartbeat request active channel55HRA. Bothbase station25 andfield unit42A identify to which link a message is directed based on receipt of a message in aparticular time slot310.
• Preferably, there is a timing offset between time slots in each respective channel, allowing thebase station25 time to process a message received in an assigned time slot and then respond accordingly over theLQM channel60L in a following portion of a cycle. Thus, messages transmitted over theLQM channel60L include feedback messages that are used to adjust transmitting characteristics of afield unit42A.
• It should be noted that although theLQM channel60L is used as a timing reference as described above, the principles of the present invention equally apply where the heartbeat-type channels55HS and55HRA are used in a forward link and LQM-type channel is used in a reverse link. In other words,base station25 is optionally synchronized with respect to afield unit42A.
• In the standby mode, synchronization is maintained between the forwardlink LQM channel60L and reverse link heartbeat standby channel55HS based upon messages sent in the appropriate time slot on theLQM channel60L indicating to a particular field unit42 whether messages transmitted to thebase station25 from that field unit42 are received in the appropriate time slot. For example, message transmissions from thefield unit transmitter40 tobase station25 are analyzed atbase station receiver35 to achieve fine tuning alignment between thebase station25 and each of multiple field units42.
• As shown inFIG. 2, time slots A₁through A₁₆of theLQM channel60L are reserved for field units42 in the active mode, indicating that traffic channels are assigned to afield unit42A in a reverse link direction and data is being transferred from the field unit42 to thebase station25. Contrariwise, time slots1-48 of theLQM channels60L are reserved for field units42 operating in the standby mode that are not presently transmitting a data payload over a reverse link ofcommunication system100.
• At any given time, there are preferably no more than 48 of the 64 time slots of the heartbeat channel55H orLQM channel60L assigned to respective field units42. This ensures that on completion of a data transfer between afield unit42A andbase station25, afield unit42A in the active mode assigned an active time slot can revert back to the standby mode and consequently be assigned an unused standbymode time slot310 again.
• Preferably, field units42 in the standby mode are assigned an unusedactive time slot310 as close to the EPOCH mark M1 as possible when they are placed in the active mode. For example, if 48 field units are assigned standby mode LQM slots S₁, S₂, . . . S₄₈, afield unit42A set to the active mode would be assigned active mode time slot A₁in the LQM channel. The nextactive time slot310 to be assigned to afield unit42A would be the lowest numbered and unused time slot such as A₂, assuming A₁is then in use.
• It should be noted that heartbeat standby channel55HS also includes additional time slots for transmitting messages from anactive field unit42A, i.e., afield unit42A transmitting data in a reverse link over assigned traffic channels, tobase station20. Preferably, reverseLQM time slots250 are allocated for transmitting link quality information from a correspondingactive field unit42A tobase station20. In this way,base station20 can be notified of a corresponding link quality of transmissions on forward channels between thebase station20 and field unit42.
• In a specific application utilizing the reverseLQM time slots250, afield unit42A can monitor the quality of a forward link signal from thebase station20 and transmit a modulated message including forward error correction information to thebase station20 in an assignedLQM time slot250. Based on these feedback messages transmitted in anLQM time slot250, properties of the transmitted signal frombase station20 can be adjusted so that subsequent messages on the forward link channels to thefield unit42A can be properly detected. For instance,field unit42A can monitor whether a signal transmitted by thebase station20 on a forward link traffic channel is transmitted at an appropriate power level so that the power level of the received signal is within a desired range, e.g., a selected signal-to-noise ratio. In this instance, the message sent in the reverseLQM time slot250 can indicate whetherbase station20 should increase or decrease its power level output on the forward channel.
• The heartbeat standby channel55HS therefore supports at least two types of communications between multiple field units42 andbase station20. Afirst field unit42A in the standby mode transmits a timing reference signal that is monitored atbase station20 for adjusting timing alignment of the corresponding filedunit42A. As recently discussed, asecond field unit42B in the active mode is assigned a reverseLQM time slot250 for transmitting a message tobase station20. Preferably, the message transmitted in a reverseLQM time slot250 includes a data message that is demodulated and decoded atbase station25 to determine the contents of the message.
• The mere RF (Radio Frequency) transmission in a time slot on the heartbeat standby channel55HS by afield unit42A in the standby mode itself is an indication to thebase station20 that thefield unit42A desires to remain in the standby mode. As mentioned, the latter transmission by afield unit42A in the standby mode preferably does not include an encoded and modulated message including forward error correction information.
• FIG. 3 is a timing diagram illustrating an exemplary mapping of bits in a forward linkLQM time slot310 according to the principles of the present invention. As shown, there are 16 bits transmitted in eachtime slot310, although this can vary depending on the application. One bit of theLQM time slot310 isLQM timing bit311 that indicates whether a field unit message transmission received at thebase station25 on a last message cycle is accurately received within an assignedtime slot310. This ensures that other field units42 transmitting messages in adjacent time slots of the samereverse link channel65 do not interfere with each other.
• In a preferred embodiment, theLQM timing bit311 indicates whether afield unit42A is to advance or retard its timing on thereverse link65. A logic one indicates that timing should be advanced ⅛ of a chip while a logic zero indicates that timing should be retarded ⅛ of a chip. In this way, thebase station25 individually synchronizes communication links between thebase station25 and each of a plurality of field units42. Said differently, timing of message transmissions from corresponding field units42 are frequently adjusted, so that corresponding messages are received in the assigned time slots at thebase station25. Consequently, a field unit42 can synchronize itself with thebase station25 even though it is moving very fast relative to thebase station25.
• In a preferred embodiment, thebase station25 transmits information on theLQM channel60L based on BCH coding. This enables a receiving field unit42 to detect and correct errors. For example, the use of a 15,7 code allows up to 2 errors to be corrected and up to 3 errors to be detected. As shown inFIG. 3, there are 8parity bits313 for error correction and detection.
• Referring again toFIG. 2, a timing diagram illustrates the heartbeat standby channel55HS and heartbeat request active channel55HRA. As shown, time slot numbering is selected for both channels so that they line up with each other. For example,time slot #1 for each heartbeat channel are aligned with each other in a given time segment, T_SLOT.
• The heartbeat standby channel55HS and heartbeat request active channel55HRA serve different functions. For example, afield unit42A assigned use of aparticular time slot310 transmits over the heartbeat standby channel55HS in order to provide an indication tobase station25 that the field unit42 desires to remain in the standby mode. On the other hand, afield unit42A alternatively transmits over theappropriate time slot310 of the heartbeat request active channel55HRA to provide an indication to thebase station25 that thefield unit12 desires the allocation of reverse link traffic channels for transmitting a data payload from thefield unit42A tobase station25.
• In a preferred application, the heartbeat standby channel55HS, heartbeat request active channel55HRA andLQM channel60L are all defined by unique code such as long PN (Pseudo-Random Noise) codes. Accordingly,base station25 detects a message from afield unit42A in an assigned time slot by detecting whether or not afield unit42A transmits an RF (Radio Frequency) signal over the corresponding uniquely coded channel. A transmission within an assigned time slot of either heartbeat channel need not include a meaningful data payload that must be demodulated because the mere coded RF transmission by afield unit42A within a time of a channel itself indicates tobase station25 whether thecorresponding field unit42A desires to remain in the standby mode or go active.
• In one application,field unit42A transmits unmodulated data including a short PN code, a long PN code, and an orthogonal code such as a Walsh code in an assignedtime slot310 of the heartbeat-type channel, i.e., the heartbeat standby channel55HS of the heartbeat request active channel55HRA. Thus, the message as received in atime slot310 is easily identified without having to decode a corresponding data payload message. Afield unit42A can then transmit at a lower power level than would otherwise be necessary if the field unit transmitted an indication including a coded message or data payload.
• Since field units42 transmit during an assigned time segment over only one of the pair of heartbeat channels including heartbeat standby channel55HS and heartbeat request active channel55HRA, the combination of transmitted RF power on these channels is effectively that of a single channel.
• A marker is preferably included within a time slotted message of either heartbeat channel so thatbase station25 can analyze whether a corresponding field unit42 is properly synchronized. More specifically, field unit42 transmits a marker at a predetermined position in atime slot310 andbase station25 then sends a message in theappropriate time slot310 of the forwardlink LQM channel60L to indicate whether the field unit should advance or retreat its timing of future message transmissions.
• Another aspect of the present invention involves maintaining a power feedback loop between each of multiple field units42 andbase station25. The indication transmitted in a time slot of the heartbeat standby channel55HS or heartbeat request active channel55HRA is analyzed atbase station25 to determine the strength of the received RF signal as transmitted by acorresponding field unit42A. For example, the power output of afield unit42A can be adjusted based on a signal-to-noise ratio of the signal received atbase station25. If the signal strength is lower than a desired level or outside a specified range as detected bybase station25, a feedback message generated bybase station25 is communicated to thefield unit42A in the appropriate forward link for adjusting its power level for subsequent transmissions on the heartbeat type channels. In this way, the power level of afield unit42A can be adjusted to reduce co-channel interference based on power adjustment messages transmitted to afield unit42A in successiveLQM time slots310. The power level of a transmitting device can be gradually increased or decreased so that it has minimal impact on other channels.
• The aforementioned method of adjusting the power output level of afield unit42A is similar to the method as previously described for synchronizing afield unit42A tobase station25 via feedback messages. However, in the power feedback control loop, the power level output of thefield unit42A is adjusted via feedback messages instead of timing. Thus, the power level of afield unit42A can be adjusted while in a standby mode so that, in the event that the transmitter goes active transmitting a data payload to the receiver, the power level of the transmitter is optimized to reduce co-channel interference.
• The power feedback loop provides a reference for transmitting an RF signal at a specified power level so that thefield unit42A can determine at what level thefield unit42A should transmit an FEC (Forward Error Correction) coded message over other channels such as reverse link traffic channels. More specifically, afield unit42A recently assigned to the active mode can determine at what level to transmit a data payload tobase station25 depending on the modulation-type and FEC code to be used for transmitting the data payload using the power level transmission on the heartbeat channel as a reference.
• Both power and timing feedback loops can be implemented simultaneously so that the power output level and timing of afield unit42A is optimized for potentially sporadic data transmissions. Thus, power and timing of afield unit42A is optimally adjusted even in a dynamic environment where the signal to noise ratio and signal path transmission delay of thefield unit42A changes almost instantaneously. A link is maintained even during changing environmental conditions.
• In the standby mode, power level optimization is achieved based upon messages sent in theappropriate time slot310 on theLQM channel60L indicating to aparticular field unit42A whether RF transmissions form thefield unit42A to thebase station25 are received at an appropriate power level. For example, signal transmissions from thefield unit transmitter40 tobase station25 are analyzed atbase station receiver35 to achieve fine tuning power level adjustments for each of multiple field units42.
• FIG. 3 is a timing diagram illustrating an exemplary mapping of bits in a forward linkLQM time slot310 according to the principles of the present invention. As shown, there are at least 16 bits transmitted in eachtime slot310, although this can vary depending on the application.
• One bit of theLQM time slot310 is LQM powerlevel control bit312 that indicates whether a field unit transmission on the heartbeat standby channel55HS or heartbeat request active channel55HRA received atbase station25 in a previous Epoch cycle is detected to be within a desired power level range. This feedback message in theLQM time slot310 is monitored at thefield unit42A to adjust the power output level of thefield unit42A so that power output of thefield unit42A is minimal but detectable atbase station25. Of course, the power output level of thefield unit42A is adjusted above the minimal detectable level so that transmissions from thefield unit42A are still detectable even if there is a slight change in environmental conditions.
• Notably, if the transmission by afield unit42A is so low that it is not detectable atbase station25, a feedback message in theLQM time slot310 will be generated indicating that thefield unit42A shall increase its power output level a predetermined amount so that thebase station25 can hopefully detect a transmission by thefield unit42A in a following Epoch. Power feedback messages transmitted over multiple Epochs to thefield unit42A can be used to gradually adjust its power output level. This gradual change in power output by thefield unit42A minimally impacts the quality of other channels. In other words, thefield unit42A preferably does not transmit at such a high power level that it causes undue interference with other field units42 transmitting on other coded channels.
• In a specific application, the LQM powerlevel control bit312 indicates whether afield unit42A is to increase or decrease its power level output for transmissions on thereverse link65. A logic one indicates that timing should be increased by, for example, ½ dB while a logic zero indicates that power level output of thefield unit42A should be decreased by ½ dB so that the received signal atbase station25 falls within a desired signal-to-noise ratio range. In this way,base station25 individually adjusts the power level of communication links between thebase station25 and each of a plurality of field units42. Said differently, power output levels of corresponding field units42 are frequently adjusted, so that corresponding indications are received at a desired power level atbase station25. Consequently, the power output level of a field unit42 can be continuously adjusted so that it is optimally set even though thefield unit42A may be moving very fast relative to thebase station25, i.e., the reverse link path loss may be changing and the power output level of thefield unit42A will be adjusted accordingly for supporting continued communications withbase station25.
• As mentioned, power adjustments are made at thefield unit42A based upon the state of theLQM timing bit312. Initially, timing is adjusted by a first predetermiend amount such as ½ dB in the appropriate direction depending on the state of this bit. However, if thefield unit42A receives 8 “increase” power bits in a row or 8 “decrease” power bits in a row over as many EPOCHs, power adjustments of thefield unit42A are based on 1 db instead of ½ dB for the following LQMpower control bits312 of the same state. In this way, the optimal power output level of thefield unit42A can be achieved more quickly when the power level for a link is grossly out of adjustment.
• Once thefield unit42A determines that the power output level is overcorrected, i.e., the polarity of the LQM timing bit312 changes state from one epoch to the next, power output adjustments at thefield unit42A revert back to ½ db for each subsequently received LQMpower control bit312. When power synchronization is achieved between a field unit42 andbase station25, the LQMpower control bit312 will typically be set to alternating logic ones and zeros for several successive EPOCH cycles. In other words, power control output at the field unit will jitter ½ dB when synchronization is practically achieved between thebase station25 andfield unit42A. This amount of jitter is tolerable for maintaining such synchronization links. Of course, a filter can be implemented at thefield unit42A so that the power output does not jitter from one Epoch to the next of the filedunit42A.
• Rather than transmit a single LQMpower control bit312, theLQM timing slot310 can also include a multi-bit message indicating an amount that thecorresponding field unit42A is to increase or decrease it power output level.
• FIG. 4 is a timing diagram illustrating afield unit42A requesting to be assigned reverse link traffic channels according to the principles of the present invention. As shown, afield unit42A in the standby mode is assigned aparticular time slot310 in Epoch E₁. As previously discussed, thefield unit42A transmits over the assignedtime slot310 of the heartbeat standby channel55HS to remain in the standby mode. In response to this reverse link indication from thefield unit42A,base station25 transmits a feedback message in theappropriate time slot310 of theLQM channel60L in Epoch E₁for maintaining synchronization of the link. As discussed, this feedback message can include both power and timing control adjustment information.
• Epoch E₂illustrates a similar circumstance where thefield unit42A continues requesting to remain in the standby mode. Consequently, the repetitive function of monitoring a timing marker within atime slot310 and providing corresponding feedback in the reverse link over theLQM channel60L ensures that the corresponding link is synchronized in the event that the field unit42 desires to transmit a data payload in a reverse link direction.
• In following Epoch E₃,field unit42A indicates to base station25 a request to go active so that it will be assigned reverse link traffic channels to transmit a data payload. As mentioned, this is achieved by generating an RF signal in theappropriate time slot310 of the heartbeat request active channel55HRA. Depending on a number of available reverse link traffic channels, there can be a delay between the time a field unit requests to go active and the time traffic channels are actually assigned for use by thefield unit42A. Thus, it is desirable to repeat a request to go active by transmitting in an assignedtime slot310 of the heartbeat request active channel55HRA at thebase station25. Since timing adjustment feedback messages are also transmitted to thefield unit42A based on messages received on the heartbeat request active channel55HRA, precise synchronization and power control of the corresponding link betweenbase station25 and field unit42 is maintained for subsequent Epochs E₄and E₅.
• Prior to or during Epoch E₅,field unit42A is notified which traffic channels are allocated for transmitting its data payload in a reverse link direction.
• Epoch E₆and E₇illustrate thatfield unit42A has been assigned use of reverselink traffic channels55T for transmitting a data payload. Notably, thefield unit42A no longer transmits an indication to thebase station25 over either the heartbeat standby channel55HS or the heartbeat request active channel55HRA. However, a link quality message is still transmitted in a forward link direction from thebase station25 to adjust timing of the field unit42. The timing adjustment feedback messages are based on markers transmitted over the reverselink traffic channels55T. As shown, in Epoch E₆and E₇, an LQM message is transmitted to thefield unit42A in a newly assigned active time slot between A₁and A₁₆. Thus, transmissions from thebase station25 tofield unit42A have shifted to anew time slot310. Of course, prior to Epoch E₆,field unit42A must be notified of thetraffic channels55T on which it is to transmit a data payload and the newly assignedactive time slot310 in which thefield unit42A is to receive a time-slotted LQM message.
• As mentioned, markers are included with the data payload transmissions over the reverselink traffic channel55T tobase station25 where they are analyzed. In this instance, the minimal feedback timing adjustment messages are generated based on the markers received within the reverselink traffic channels55T. The timing adjustment messages are transmitted in the newly assigned active time slot A₁of the forwardlink LQM channel60L.
• After a data payload is transmitted over the reverselink traffic channel55T, thefield unit42A is placed in the standby mode as shown in Epoch E₈and E₉.
• Accordingly, synchronization is again maintained based on a feedback loop between afield unit42A andbase station25. More specifically, messages transmitted in atime slot310 of the heartbeat standby channel55HS are again analyzed atbase station25 and timing adjustment feedback information is transmitted in the standby time slot S₁of the forwardlink LQM channel60L to precisely synchronize thefield unit42A with thebase station25.
• FIG. 5 more particularly shows hardware components atbase station25 that are used to achieve synchronization and power control of thereverse link65 andforward link70. Information transmitted in atime slot310 as assigned for use by afield unit42A is analyzed by a corresponding heartbeat correlation filter such as heartbeatstandby correlation filter440 or heartbeat requestactive correlation filter445. Generally, the unique codes of each heartbeat channel are monitored indifferent time slots310 to detect a request by acorresponding field unit42A to be placed in the active mode or remain in the standby mode. Thereafter, thebase station25 will setfield unit42A to the active mode by assigning it the appropriate resources if a request to go active is detected. Note that heartbeatstandby correlation filter440 is used to identify a long PN code corresponding to a request by thefield unit42A to remain in the standby mode, while heartbeat requestactive correlation filter445 atbase station25 identifies a long PN code corresponding with a request to be placed in the active mode.
• Regardless on which heartbeat-type channel afield unit42A transmits in an assignedtime slot310, the marker from thefield unit42A is monitored by apulse timing analyzer422. It is then determined whether the message transmission by acorresponding field unit42A is received early or late within atime slot310 atbase station25. Preferably, the strongest received diversity string in atime slot310 will be designated as the time alignment string for analyzing timing of the message received over heartbeat standby channel55HS or heartbeat request active channel55HRA.
• Time slot alignment is preferably based on the correlation profile of the pilot in a particular string, which is analyzed using correlation filters as mentioned. The output of the correlation filters440,445 include 256 samples, which represent 64 lags at 4 samples per lag. The 256 sample output-window represents the total correlation time span of thebase station25. Preferably, the time alignment point or marker in atime slot310 issample number 80, which allows 20 lags for precursor and 44 lags for post cursor channel information.
• Generally, the computation of the time alignment error is based on a determination of where the centroid or peak lies in a given sample string. For example, eachfield unit42A transmitting in its assignedtime slot310 over either the heartbeat standby channel55HS or heartbeat request active channel55HRA includes a marker, i.e., the peak signal, located at a predetermined position within a time slot. The strongest pilot path for the channel and 2 samples on either side of the main path, i.e., 1 and ¼ chips, is statistically analyzed to determine the centroid or peak of a marker within a time slot. The centroid of the samples inFIG. 6 are calculated based on the following equation:$L=\frac{\sum \left[t⨯Q\left(t\right)\right]}{\sum Q\left(t\right)}$
  where L is a position of the centroid in a time slot, t is the sample time along the X-axis, and Q(t) is the magnitude of a sample at a given sample time. For example, L is calculated based on the results as shown inFIG. 6:$L=\frac{\left(\mathrm{.25}*76\right)+\left(\mathrm{.5}*77\right)+\left(1.0*78\right)+\left(\mathrm{.8}*79\right)+\left(\mathrm{.6}*80\right)}{\mathrm{.25}+\mathrm{.5}+1.0+\mathrm{.8}+\mathrm{.6}}$$L=78.317$
• Again, the timing alignment error is determined by comparing the timing of the computed centroid to the desired time set point of 80, which is chosen as the reference point for timing alignment within atime slot310. Since the centroid in the example above is estimated to be 78.317, timing is early and, therefore, the correspondingLQM timing bit311 will be set to a logic “one” indicating that the corresponding field unit should advance its timing reference by ⅛ of a chip so that subsequent messages are transmitted ⅛ of a chip later intime slot310. This overall feedback technique in the present invention ensures continuous fine-tuning the time alignment betweenbase unit25 and each of multiple field units42.
• Preferably, the time error is calculated by taking the integer of twice the difference between the desiredset point sample 80 and L. For example,
  time_error=integer [(L−80)*2]
• If the time_error result is negative, theLQM timing bit311 is set to a logic “one.” Conversely, theLQM timing bit311 is set to a logic “zero” when time_error is positive.
• Referring again toFIG. 5,processor426 analyzes timing data and generates time_error for synchronizing the reverse link heartbeat channels55H and forwardlink LQM channel60L. LQM time slotted messages are then transmitted byLQM signal generator450 onLQM channel #160L to provide timing adjustments for thecorresponding field unit42A as mentioned.
• If afield unit42A in the standby mode transmits a request to go active by transmitting in an assigned time slot of the heartbeat request active channel55HRA, such a request is detected at heartbeat requestactive correlation filter445. As previously discussed, the timing characteristics of an active mode request detected at heartbeat requestactive correlation filter445 is also analyzed to determine timing errors as described above for maintaining alignment on a particular link between thebase station25 and eachfield unit42A.
• If resources are available for allocatingtraffic channels55T, the requestingfield unit42A is placed in the active mode bybase station25, where configuration details for setting up the data transfer are handled byprocessor426. For example, information regarding new LQM time slot assignments, i.e., assignment of an active mode time slot A₁. . . A₁₆, is sent to acorresponding field unit42A over, for example, thepaging channel60P. Reverselink traffic channels55T are then allocated for transferring a data payload fromfield unit42A tobase station25.
• While in the active mode, synchronization of the forward and reverse link is maintained based on messages transmitted over theLQM channel60L andtraffic channels55T since the heartbeat channel time slot is no longer dedicated on thereverse link65 for use by the transmittingfield unit42A. More specifically, a timing marker is included in the reverse link traffic channel transmissions so thatbase station25 can monitor whether datapayload field unit42A is early or late in its timing.
• Messages transmitted by afield unit42A in the active mode are transmitted tobase station25 overtraffic channels55T and the corresponding traffic channel signal is fed into the trafficchannel correlation filter430 atbase station25 for detection of pilot symbol timing markers. Preferably, afield unit42A transmits a sequence of 32 pilot symbols in an assignedtime slot310 as a timing marker. Thetraffic channel55T is then analyzed bypulse timing analyzer420 to determine whether such messages are early or late with respect to a desired synchronization of the field unit42 withbase station25.
• The process of analyzing a pulse or marker for estimating the centroid is similar to that described earlier in the specification for messages and corresponding markers such as long PN codes on either heartbeat channel55HS or55HRA. However, whenfield unit42A is in the active mode, pilot symbols in thetraffic channels55T are used as a timing reference mark rather than long PN codes. Again, seeFIG. 7 and related discussion above for details regarding how a timing marker is analyzed to identify whether afield unit42A should advance or retard its timing.
• FIG. 8 is a table illustrating different operational modes according to the principles of the present invention and how synchronization is maintained between a field unit and base station for each of the modes.
• Preferably, timing alignment of thebase station25 and field units42 is based upon theLQM timing bit311 as transmitted in an assigned active time slot A₁. . . A₁₆on theforward link70. When the receipt of data messages transmitted by the active field unit42 are received early or late with respect to an assigned time slot, theLQM timing bit311 is set accordingly to advance or retard timing of future message transmissions on thetraffic channels55T.
• Although a single trafficchannel correlation filter430 is shown for detecting a marker in asingle traffic channel55T,multiple traffic channels55T are optionally analyzed to coordinate timing alignment between thereverse link65 andforward link70.
• As mentioned,access channel55A is used by the field units42 to transmit requests for establishing a synchronization link with thebase station25. Typically, messages on theaccess channel55A are transmitted on a random basis. Hence, a message collision may occur if two or more link requesting field units42 happen to transmit a link request message on theaccess channel55A at the same time.
• If a collision is detected on theaccess channel55A, the collision is made known to the field units42 based upon a message generated by pagingchannel signal generator455 overpaging channel60P. Each field unit42 will then retransmit their request to establish a synchronization link on theaccess channel55A based on a random back off time, making it less likely that a collision will occur on a second or other subsequent attempt.
• Access channel55A, also shown inFIG. 5, is fed into accesschannel correlation filter435. Preferably, a field unit42 transmits a sequence of 32 pilot symbols including information identifying thefield unit42A requesting a synchronization link. A received sequence of pilot symbols is analyzed bypulse timing analyzer422 to determine initial timing information of thefield unit42A with respect to thebase station25. Since the field units42 randomly transmit requests on theaccess channel55A, it is necessary to determine an initial timing error between the field unit42 andbase station25 for achieving a coarse synchronization of the forward and reverse link channels.
• If it is determined by thebase station25 that a synchronization link will be established between thebase station25 and requestingfield unit42A, an appropriate acknowledgment message is transmitted over theforward paging channel60P to thebase station25 to thecorresponding field unit42A. Among other information transmitted over theforward paging channel60P to the field unit42, a heartbeat time slot assignment, an LQM time slot assignment, and synchronization information such as coarse timing adjustment information is also transmitted to the field unit42. Thus, afield unit42A newly assigned to the standby mode can transmit an indication over one of the heartbeat-type channels for maintaining more precise synchronization withbase station25.
• As mentioned, coarse timing adjustment information is transmitted on theforward paging channel60P to roughly synchronize the link requestingfield unit42A with respect tobase station25. Preferably, a 10-bit signed number is transmitted to thefield unit42A indicating an amount to advance or retard its timing with respect to the link request message of the field unit42 as previously transmitted on theaccess channel55A. Each least significant bit (LSB) in the 10-bit signed number is appropriately weighted. For example, an LSB can represent 16 chips. Based on this timing correction information, thecorresponding field unit42A adjusts its coarse timing relative to thebase station25. Thereafter, messages are then transmitted in the appropriate reverse link time slot of the heartbeat channel55HS,55HRA ortraffic channel55T. Fine-tuning is thereafter achieved by analyzing transmissions byfield unit42A atbase station25 and providing synchronization information over theLQM channel60L feedback path.
• In addition to transmitting in the appropriate time slot, coarse and fine synchronization with thebase station25 renders it possible for a field unit42 to receive information in its assigned time slot in the forward link.
• FIGS. 5aand5bare a flow charts providing details of how a wireless communication link is established betweenfield unit42A andbase station25. There are typically multiple field units42 requesting communication links in a particular service area, where each mobile orfield unit42A is located at a different distance with respect tobase station25. For example, some field units42 can be located very close tobase station25 while others are located very far away. Hence, the time it takes for a signal to travel from aparticular field unit42A tobase station25 is different for each field unit42. Precise timing alignment of a specific field unit42 and thebase station25 is therefore important to avoid or minimize collisions between field units42 transmitting in adjacent time slots.
• If all field units42 transmitted in real time without taking into account the distance tobase station25 and corresponding delay, message transmissions in an assigned time slot from a particular field unit would be skewed, i.e., messages at the base station would be received slightly out of an assigned time slot. Therefore, message transmissions from eachfield unit42A are precisely adjusted as previously discussed to prevent this skewing phenomenon.
• Not only does distance from afield unit42A tobase station25 effect timing alignment, so does the environment in which afield unit42A transmits a message. For example, building structures, atmospheric conditions and other geographical terrain will effect the path of a signal transmitted from afield unit42A tobase station25.
• Therefore a field unit42 changing position merely a few feet in several seconds can have a substantial impact on timing of a signal path, thus, effecting timing alignment between areverse link65 andforward link70. Based on the principles of the present invention, the previously described method of continuously adjusting timing transmissions in the sharedreverse channel65 minimizes collisions among multiple field units42 transmitting tobase station25 in adjacent time slots.
• Step510 inFIG. 6ashows an entry point of the flow chart for establishing a wireless communication link. Instep515,access channel55A is monitored bybase station25 to detect requests by field units42 to establish wireless synchronization links withbase station25. A link request message received atbase station25 includes a sequence of pilot symbols followed by data identifying the link requestingfield unit42A. Based on the data information received overaccess channel55A,base station25 is able to access characteristics of the corresponding field unit42.
• If no standby time slots are available for establishing a new synchronization link, the connection request by afield unit42A is denied as shown instep525. A message is then transmitted to thecorresponding field unit42A on the forwardlink paging channel60P to indicate that no time slots are available and thefield unit42A must try again at a later time to establish a standby synchronization link.
• If resources are available to establish a new link instep520,base station25 analyzes the timing of the request message as received from afield unit42A onaccess channel55A instep530. As mentioned, the sequence of 32 pilot symbols are analyzed to determine the location of the peak pulse or marker in thereverse link65. Based on the time when this random message is received with respect to the base station's master time reference EPOCH mark, M1, and the distance that thefield unit42A is located frombase station25, a coarse time adjustment message is generated by thebase station25 to synchronize timing between the link requestingfield unit42A andbase station25. This coarse timing information, preferably a 10-bit signed number indicating how a field unit42 should adjust its timing to align the field unit with the base station EPOCH mark, is sent to thefield unit42A over the forwardlink paging channel60P instep535. Thefield unit42A then adjusts its timing reference accordingly so that subsequent messages are transmitted in an assigned time slot on thereverse link65. Timing alignment also ensures that thefield unit42A can receive messages frombase station25 in the appropriate time slot of the forwardlink LQM channel60L.
• Following instep540,base station25 assigns two time slots to the link requestingfield unit42A overpaging channel60P. One time-slot assignment indicates the time slot in which thefield unit42A is to receive LQM messages from thebase station25 over theLQM channel60L. Another time-slot assignment indicates in whichtime slot310 of the reverse link field unit42 is to transmit over a heartbeat-type channel tobase station25. Based upon these time slot assignments, thebase station25 and field units42 can determine to which link a message pertains as the time slot itself indicates to which target a message is directed.
• While in the standby mode,base station25 monitors periodic messages in an assigned time slot for a transmission on either the heartbeat standby channel55HS or heartbeat request active channel55HRA by acorresponding field unit42A. For example, a marker received in a time slot of either channel is analyzed atbase station25 to correct timing alignment as mentioned betweenbase station25 andfield unit42A. If the message in a time slot is received early or late atbase station25, timing of future transmissions by the field unit42 in an assignedtime slot310 on a reverse link heartbeat channel is appropriately retarded or advanced based upon theLQM timing bit311 for aparticular field unit42A instep542.
• Timing adjustments are made at thefield unit42A based upon the state of theLQM timing bit311. Initially, timing is adjusted by ⅛ of a chip in the appropriate direction depending on the state of this bit. However, if thefield unit42A receives 8 retard bits in a row or 8 advance bits in a row over as many EPOCHs, timing adjustments of the reference at thefield unit42A at are based on ½ of a chip instead of ⅛ of a chip for the followingLQM bits311 of the same state. In this way, synchronization between thebase station25 and field unit42 is achieved more quickly when timing for a link is grossly out of adjustment.
• Once thefield unit42A determines that timing is overcorrected, i.e., the polarity of the LQM timing bit311 changes state from one epoch to the next, timing adjustments at the field unit42 revert back to ⅛ of a chip for each subsequently receivedLQM timing bit311. When synchronization is achieved between a field unit42 andbase station25, theLQM timing bit311 will typically be set to alternating logic ones and zeros for several successive EPOCH cycles. In other words, timing at the field unit will jitter ⅛ of a chip when synchronization is practically achieved between thebase station25 andfield unit42A. This amount of jitter is tolerable for maintaining such synchronization links.
• Iffield unit42A receives another 8 cycles of timing adjustment corrections in the same direction such that 16successive LQM bits311 are the same state, the time adjust correction is set to 1 chip per receivedLQM timing bit311. As stated earlier, when over-correction is detected, timing adjustments at the field unit are again based on ⅛ of a chip for each receivedLQM timing bit311 again.
• In addition to monitoring timing pulses for aligning message transmissions of each field unit42,base station25 also determines on which heartbeat channel afield unit42A transmits during its assigned time segment T_SLOT. It is then determined instep547 whether afield unit42A requests to be set to the active mode based on whether thefield unit42A transmits over the heartbeat request active channel55HRA. If so, the base station allocates appropriate resources such astraffic channels55T in thereverse link65 to support the data transfer instep550. Additionally,base station25 is assigned an active time slot for use by a field unit42, i.e., one available time slot between A₁-A₁₆, in the forwardlink LQM channel60L to maintain a synchronization loop. While in the active mode, as mentioned, thefield unit42A maintains synchronization withbase station25 based on a sequence of well-placed pilot symbol markers in thetraffic channels55T, upon which thebase station25 issues timing adjustments in theappropriate time slot310 using the forward linkLQM timing bit311. Additionally, thefield unit42A transmits data over the reverselink traffic channels55T instep555 before returning to the main loop again atstep560. At this re-entry point into the main loop again, the field unit42 is then reassigned a standbymode time slot310.
• If afield unit42A has not been in the standby mode too long instep560,base station25 determines whether thefield unit42A has made a request to terminate a wireless link betweenbase station25 andcorresponding field unit42A instep572. Without the request to tear down a link, processing loops back tostep542.
• If field unit42 generates a request to tear down a corresponding link instep572,base station25 acknowledges such a request instep575 by sending a message to the field unit42 and tears down the communication link. This is one way of terminating the flow chart as shown instep580.
• Referring again to step560, if it is determined that thefield unit42A is inactive too long, i.e., in standby mode not transmitting data, the base station revokes the assigned LQM and heartbeat channel slots for use by other users and maintains an idle connection with thefield unit42A instep565.
• When it is determined thatfield unit42A requests to go active again instep582, process flow continues at the beginning of the flow chart to reestablish a synchronized link instep570. In such a case, connectivity is reestablished based in part on the prior connection. For example, it is not necessary to go through the entire configuration process since data maintained with respect to the corresponding recently active link is advantageously used to minimize the overhead associated with reviving the previous connection.
• Flow continues atstep585 ifbase station25 fails to detect a request by the field unit42 to go active again instep582. Ifbase station25 fails to receive a response from anidle field unit42A in a specified time out period instep585,base station25 pings the field unit42 onforward page channel60P to elicit a response by the field unit42 instep587. If thefield unit42A does not respond instep590, it is assumed that thefield unit42A is shut down and an idle connection is therefore no longer maintained for that particular field unit42. If thefield unit42A responds to the ping instep590, process flow continues instep595 at START (step510) of the flowchart to reestablish the link as a standby connection.
• While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.

Claims (23)

1. A method of supporting wireless communications, the method comprising:

assigning a first coded channel to a plurality of transmitters to support communications from the plurality of transmitters, the first coded channel having a first portion for transmitting a first type of information from the plurality of transmitters and a second portion for transmitting a second type of information from the plurality of transmitters;

receiving the first type of information from a first transmitter in the plurality of transmitters in the first portion of the first coded channel; and

receiving the second type of information from a second transmitter in the plurality of transmitters in the second portion of the first coded channel.

2. A method as defined inclaim 1 wherein the first type of information is a reference signal.

3. A method as defined inclaim 1 wherein the second type of information is link quality information that indicates a quality of a wireless link to the first transmitter.

4. A method as defined inclaim 1 wherein the first coded channel contains a plurality of time slots wherein each time slot is assigned to a particular transmitter in the plurality of transmitters.

5. A method as defined inclaim 4 further comprising:

receiving a message from a transmitter in the plurality of transmitters; and

analyzing the message to determine a timing adjustment that needs to be made by the transmitter to synchronize the transmitter to transmit a successive message in the transmitter's assigned time slot.

6. A method as defined inclaim 5 further comprising:

generating a feedback message containing the timing adjustment; and transmitting to the transmitter.

7. A method as defined inclaim 5 wherein the timing adjustment indicates whether the transmitter should advance or retard timing of message transmissions.

8. A method as defined inclaim 1 further comprising:

assigning a second coded channel to the plurality of transmitters to support indications from the transmitters to allocate one or more traffic channels for the transmission of data from the transmitters.

9. A method as defined inclaim 8 wherein the second coded channel contains a plurality of time slots wherein each time slot is assigned to a particular transmitter in the plurality of transmitters.

10. A method as defined inclaim 9 wherein an indication from a transmitter in the plurality of transmitters is a transmission by the transmitter in a time slot assigned to the transmitter that is contained in the second coded channel.

11. A method as defined inclaim 8 wherein an indication from a transmitter is a pseudo random noise (PN) code.

12. A method as defined inclaim 8 further comprising:

receiving an indication in the second coded channel from a transmitter that indicates the transmitter is to be set into an active mode; and

allocating a traffic channel for use by the transmitter to transmit data.

13. An apparatus comprising:

a processor configured to:

(a) assign a first coded channel to a plurality of transmitters to support communications from the plurality of transmitters, the first coded channel having a first portion for transmitting a first type of information from the plurality of transmitters and a second portion for transmitting a second type of information from the plurality of transmitters; and

a receiver configured to:

(a) receive the first type of information from a first transmitter in the plurality of transmitters in the first portion of the first coded channel, and

(b) receive the second type of information from a second transmitter in the plurality of transmitters in the second portion of the first coded channel.

14. An apparatus as defined inclaim 13 wherein the first coded channel contains a plurality of time slots wherein each time slot is assigned to a particular transmitter in the plurality of transmitters.

15. An apparatus as defined inclaim 14 wherein the receiver is further configured to receive a message from a transmitter in the plurality of transmitters, and the processor is further configured to analyze the message to determine a timing adjustment that needs to be made by the transmitter to synchronize the transmitter to transmit a successive message in the transmitter's assigned time slot.

16. An apparatus as defined inclaim 13 wherein the processor is further configured to assign a second coded channel to the plurality of transmitters to support indications from the transmitters to allocate one or more traffic channels for the transmission of data from the transmitters.

17. A base station as defined inclaim 16 wherein the receiver is configured to receive an indication in the second coded channel from a transmitter that indicates the transmitter is to be set into an active mode and, the processor is further configured to allocate a traffic channel for use by the transmitter to transmit data.

18. An apparatus comprising:

means for assigning a first coded channel to a plurality of transmitters to support communications from the plurality of transmitters, the first coded channel having a first portion for transmitting a first type of information from the plurality of transmitters and a second portion for transmitting a second type of information from the plurality of transmitters;

means for receiving the first type of information from a first transmitter in the plurality of transmitters in the first portion of the first coded channel; and

means for receiving the second type of information from a second transmitter in the plurality of transmitters in the second portion of the first coded channel.

19. An apparatus as defined inclaim 18 wherein the first coded channel contains a plurality of time slots wherein each time slot is assigned to a particular transmitter in the plurality of transmitters.

20. An apparatus as defined inclaim 19 further comprising:

means for receiving a message from a transmitter in the plurality of transmitters; and

means for analyzing the message to determine a timing adjustment that needs to be made by the transmitter to synchronize the transmitter to transmit in its assigned time slot.

21. An apparatus as defined inclaim 20 further comprising:

mean for transmitting a feedback message containing the timing adjustment to the transmitter.

22. An apparatus as defined inclaim 18 further comprising:

means for assigning a second coded channel to the plurality of transmitters to support indications from the transmitters to allocate one or more traffic channels for the transmission of data from the transmitters.

23. An apparatus as defined inclaim 21 further comprising:

means for receiving an indication in the second coded channel from a transmitter that indicates the transmitter is to be set into an active mode; and

means for allocating a traffic channel for use by the transmitter to transmit data.

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