US20030060224A1 - Reverse link initial power setting using effective radiated power message to compute path loss - Google Patents

Abstract

In an illustrative embodiment of the present invention, a reference signal including pilot information is transmitted from a base station to one or multiple field units over a pilot channel. A message is also sent to the field units over a paging channel to indicate an effective radiated power level at which the reference signal is transmitted on the pilot channel. Based on a received power level of the reference signal at a field unit and the effective radiated power level of the reference signal, a forward path loss is estimated at the field unit for the forward link between the base station and field unit. Assuming the path loss in the reverse link is approximately the same as the estimated forward link path loss, the field unit can transmit a reply message in the reverse link so that the base station generally receives a message at a desired power level.

Description

RELATED APPLICATION(S)
• This application claims the benefit of U.S. Provisional Application No. 60/184,223 filed on Feb. 23, 2000, the entire teachings of which is incorporated herein by this reference.[0001]
BACKGROUND OF THE INVENTION
• A specific protocol has been developed for allowing multiple users to transmit over a shared radio channel. For example, the IEEE (Institute of Electrical and Electronics Engineers) 802.11 Standard generally supports access to radio channels based on a method known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA).[0002]
• In simple terms, this method is based on a “listen before talk” scheme. A transmitter device monitors traffic on a shared radio channel to determine if another transmitter device is presently transmitting on the same channel. If the radio channel is in use, the transmitter device will continue to monitor the channel until it is clear. When the radio channel is finally clear, the transmitter will then transmit over the radio channel.[0003]
• Ideally, another transmitter device will not simultaneously transmit during the same time. However, a collision can occur on the radio channel when two or more transmitter devices transmit on the radio channel simultaneously. Consequently, neither message transmission would be intelligible and both transmitter devices must re-transmit their messages again to a corresponding target device.[0004]
• Based on this CSMA/CA scheme, re-transmission of data due to a collision cannot occur before a minimum time gap. After the minimum time gap has passed, the transmitter device selects a random “backoff interval,” which is the wait time before the radio channel is again monitored to determine whether the radio channel is clear to transmit. If the channel is still busy, another shorter “backoff interval,” is selected for a subsequent message transmission. This process is repeated until the transmitter device is able to transmit data.[0005]
• Another standard for transmitting data on a shared radio channel is based on IS-95, in which multiple field units can transmit at the same time.[0006]
• The IS-95 standard suggests a method of ramping RF power of a field unit until a message from the field unit is transmitted at a power level that is detectable at a base station. According to this method, a field unit transmits an access request message to a base station for the assignment of wireless resources on a reverse link.[0007]
• After transmitting an access request message on an access channel, the field unit monitors a paging channel for an acknowledgment message from the base station indicating that the access request message was properly received. If no acknowledgment message is sent to the requesting field unit, it is presumed that the message from the field unit was not transmitted at an appropriate power level. That is, the power output level of the field unit is so low that the base station did not detect a previously transmitted access request message. The access request message is then re-transmitted over the access channel at a higher power level.[0008]
• This process is subsequently repeated until the field unit transmits a message at a power level that is high enough for the base station to properly receive the message. Similar to the IEEE 802.11 standard, a collision can occur on the shared radio channel when two or more field units simultaneously transmit a message.[0009]
SUMMARY OF THE INVENTION
• The present invention is generally directed towards an apparatus and method for enhancing the utilization of resources in a wireless communication system. In an illustrative embodiment, a reference signal is transmitted from a first transceiver to a second transceiver or group of target transceivers. The first transceiver also transmits a first message, which includes information indicating an effective radiated power level at which the first transceiver transmits the reference signal. The received power level of the reference signal is then measured at the second transceiver to estimate a path loss between the first transceiver and second transceiver. More specifically, a path loss can be calculated by comparing the received power level of the reference signal with the effective radiated power level information as indicated by the first message.[0010]
• A second message is optionally sent from the first transceiver to the second transceiver. This second message can include information indicating a desired power level at which subsequent messages in a reverse direction should be received at the first transceiver.[0011]
• In certain applications, the forward path loss between the first transceiver and second transceiver is approximately the same as a reverse path loss for message transmissions from the second transceiver back to the first transceiver. Consequently, the second transceiver can adjust its power output level so that the first transceiver receives a message at the desired power level taking into account the estimated path loss as previously discussed. Of course, the path loss can be different in the reverse link direction than that of the forward link and an estimated power setting at which the second transceiver transmits a message can be a starting point for transmitting subsequent messages. For example, the power output level of the second transceiver can be increased for subsequent message transmissions until the first transceiver detects the message.[0012]
• In a specific application of the present invention, the reference signal is transmitted over a pilot channel of a CDMA (Code Division Multiple Access) communication system. The reference signal itself optionally includes a marker such as pilot symbols that are monitored at a second transceiver. The first message as transmitted by the first transceiver can be transmitted over a paging channel, while a reply message from the second transceiver to the first transceiver can be transmitted over an access channel in which multiple transceivers compete to transmit messages to the first transceiver. The access channel is optionally divided into time slots in which a transceiver sends a message to the first transceiver.[0013]
• One message type that can be transmitted on the access channel is an access request message. Such a message is an indication to the first transceiver that a more formal communication link should be established between the first transceiver and second transceiver. Consequently, the more formal communication link can be allocated to support more efficient, on-demand data transfers.[0014]
• An access request message is optionally encoded so that it includes timing alignment information. For example, the first transceiver can analyze a reply message such as an access request message including a timing marker and provide feedback indicating whether the reply is appropriately transmitted within a time slot. Other types of reply messages can also include a reference marker for timing alignment. In one application, a reference marker is a string including pilot information such as one or multiple pilot symbols.[0015]
• In addition to monitoring the timing of a reply message, the received power level of a reply message can be monitored to determine whether a transceiver is transmitting a message so that it is received at a desired power level. This can be achieved by comparing a power level at which a reply message is received to a desired power level at which the reply message should be received. Based on this comparison, a power adjustment message is optionally transmitted to a corresponding target transceiver. Thus, subsequent message transmissions from a second transceiver to the first transceiver can be optimally adjusted based on operating conditions of the wireless communication system at an instant in time.[0016]
• Certain aspects of the present invention reduce co-channel interference and generally increase the throughput capability of a wireless communication system. As previously discussed, an initial power output level of a field unit can be adjusted so that it minimally interferes with others when it initially transmits messages or transmits subsequent messages. When message transmissions on an allocated channel such as an access channel are minimized, more wireless resources can otherwise be allocated for supporting higher speed data transfers in a wireless communication system.[0017]
• Of course, the initial power output level of a field unit can be so low that a message transmission is not detected at a target device such as a base station. In this case, a power output level can be increased accordingly for subsequent message transmissions until the message is detected at a target transceiver. This process of incrementally increasing the power level can be a time consuming process, especially if the power output of the transceiver were to start transmitting at a lowest possible power level. Thus, it can take a considerable amount of time to successfully transmit a message to a target receiver such as a base station.[0018]
• The principles of the present invention can be used to simultaneously reduce the effective time it takes to transmit a message to a base station while minimally interfering with other channels of the wireless communication system. This is achieved, at least in part, by approximating a path loss between a field unit and base station and transmitting a message from the field unit so that a message is received at a desired power level at the base station. Since the initial power setting of the field unit is approximately set to a detectable power level for data transmissions to a target receiver, it typically requires less time to transmit an initial message to a target receiver when power is incrementally increased so that a message is eventually detected.[0019]
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a bock diagram of an illustrative wireless communication system according to certain principles of the present invention.[0020]
• FIG. 2 is a timing diagram illustrating multiple channels on which messages are transmitted according to certain principles of the present invention.[0021]
• FIG. 3 is a timing diagram illustrating a use of data fields within a time-slotted channel according to certain principles of the present invention.[0022]
• FIG. 4 is a pictorial diagram illustrating details of a message according to certain principles of the present invention.[0023]
• FIG. 5 is a flow chart for processing messages at a target receiver according to certain principles of the present invention.[0024]
• FIG. 6 is a flow chart for transmitting messages to a target receiver according to certain principles of the present invention.[0025]
• FIG. 7 is a timing diagram illustrating multiple channels on which messages are transmitted according to certain principles of the present invention.[0026]
• FIG. 8 is a diagram of a monitored reference signal according to certain principles of the present invention.[0027]
• FIG. 9 is a flow chart for setting an initial power output level of a transmitter device according to certain principles of the present invention.[0028]
• 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.[0029]
DETAILED DESCRIPTION OF THE INVENTION
• A description of preferred embodiments of the invention follows.[0030]
• FIG. 1 is a block diagram illustrating a wireless communication system supporting the transmission of data information over multiple allocated communication channels according to certain aspects of the present invention. As in many wireless communication systems, users compete for wireless bandwidth allocation. Hence, it is desirable that the[0031]wireless communication10 is optimized for data throughput and, in certain applications, hi-speed bursts of data throughput.
• Certain aspects of the present invention are based on the recognition that the power output of a field unit transmitting over a wireless channel can be controlled so that it minimally interferes with other field units using the same general wireless airspace. In particular, a power output level of a newly transmitting field unit is initially set so low that a base station may not detect messages transmitted by the field unit. This initially low power setting of a field unit reduces co-channel interference because the transmitter device is not transmitting at excessive power levels. During subsequent communication attempts with the base station, the power output of a field unit is then ramped up until messages are acknowledged at the base station.[0032]
• In one application, one or multiple field units randomly transmit messages on a first allocated channel. When two field units transmit a message simultaneously on this first allocated channel, there can be a message collision at the base station. The base station may be able to detect that a message was transmitted by a field unit and there was a message collision. However, the base station may not be able to decipher the contents of the message and determine from which field unit transmitted a message. Thus, in certain situations, the base station can not transmit a message directed specifically to a particular field unit indicating that a collision occurred for a previous message transmission.[0033]
• One aspect of the present invention involves providing general feedback information to the field units indicating that a collision was detected. Consequently, a previous undetected message can be re-transmitted by a field unit. If there was no collision detected and no acknowledgment received by the filed unit, the field unit can successively ramp up its power output setting for subsequent message transmission attempts to ensure that a message will eventually be acknowledged by the base station.[0034]
• According to the following description of a preferred embodiment,[0035]communication system10 is described as a wireless communication link such as a CDMA radio channel utilizing shared channel resources. However, it should be noted that the techniques described herein can be applied in other applications supporting shared access. For example, the principles of the present invention can be applied to other general applications such as telephone connections, computer network connections, cable connections, or other physical media to which allocation of resources such as data channels are granted on an as-needed basis.
• As shown,[0036]communication system10 includes a number of Personal Computer (PC) devices12-1,12-2, . . .12-h, . . .12-m, corresponding field units or terminals14-1,14-2, . . .14-h, . . .14-m, and associated directional antenna devices16-1,16-2, . . .16-h, . . .16-m. Centrally located equipment includes abase station antenna18, and acorresponding base station20 that includes high speed processing capability.
• [0037]Base station20 and related infrastructure provides connections to and from anetwork gateway22,network24 such as the Internet, andnetwork file server30.Communication system10 is preferably a demand access, point to multi-point wireless communication system such that thePC devices12 can transmit data to and receive data fromnetwork server30 based on a logical connection including bi-directional wireless connections implemented overforward links40 andreverse links50. That is, in the point to multi-point multiple accesswireless communication system10 as shown, a givenbase station20 typically supports communication with a number ofdifferent field units14 in a manner which is similar to a cellular telephone communication network. Accordingly,system10 can provide a framework for a CDMA wireless communication system where digital information is relayed on-demand between multiple mobile cellular users and ahardwired network24 such as the Internet.
• [0038]PC devices12 are typically laptop computers, handheld units, Internet-enabled cellular telephones, Personal Digital Assistant (PDA)-type computers, digital processors or other end user devices, although almost any type of processing device can be used in place ofPC devices12. One ormultiple PC devices12 are each connected to arespective subscriber unit14 through a suitable hard wired connection such as an Ethernet-type connection viacable13.
• Each[0039]field unit14 permits its associatedPC device12 access tonetwork file server30. In thereverse link50 direction, that is, for data traffic transmitted from thePC12 towards theserver30, thePC device12 transmits information tofield unit14 based on, for example, an Internet Protocol (IP) level network packets. Thefield unit14 then encapsulates the wired framing, i.e., Ethernet framing, with appropriate wireless connection framing so that data packets can be transmitted over the wireless link ofcommunication system10. Based on a selected wireless protocol, the appropriately formatted wireless data packet then travels over one of the radio channels that comprise thereverse link50 throughfield unit antenna16 tobase station antenna18. At the central base station location, thebase station20 then extracts the radio link framed data packets and reformats the packets into an IP format. The packets are then routed throughgateway22 and any number or type ofnetworks24 to an ultimate destination such as anetwork file server30.
• In one application, information generated by[0040]PC device12 is based on a TCP/IP protocol. Consequently, aPC device12 has access to digital information such as web pages available on the Internet. It should be noted that other types of digital information can be transmitted over channels ofcommunication system10 based on the principles of the present invention.
• Data information can also be transferred from the[0041]network file server30 toPCs12 onforward link40. In this instance, network data such as IP (Internet Protocol) packets originating atfile server30 travel onnetwork24 throughgateway22 to eventually arrive atbase station20. As previously discussed for reverse link data transmissions, appropriate wireless protocol framing is then added to raw data such as IP packets for communication of the packets over wirelessforward link40. The newly framed packets then travel via an RF signal throughbase station antenna18 andfield unit antenna16 to the intendedtarget field unit14. An appropriatetarget field unit14 decodes the wireless packet protocol layer, and forwards the packet or data packets to the intendedPC device12 that performs further processing such as IP layer processing.
• A given[0042]PC device12 andfile server30 can therefore be viewed as the end points of a logical connection at the IP level. Once a connection is established between thebase station processor20 andcorresponding field unit14, a user at thePC device12 can then transmit data to and receive data fromfile server30 on an as-needed basis.
• [0043]Reverse link50 optionally includes different types of logical and/or physical radio channels such as anaccess channel51, multiple traffic channels52-1, . . .52-m, and amaintenance channel53. The reverselink access channel51 is typically used by thesubscriber units14 to request an allocation of traffic channels by thebase station20. For example,traffic channels52 can be assigned to users on an as-needed basis. The assignedtraffic channels52 in thereverse link50 then carry payload data fromfield unit14 tobase station20.
• Notably, a given link between[0044]base station20 andfield unit14 can have more than onetraffic channel52 assigned to it at a given instant in time. This enables the transfer of information at higher rates.
• [0045]Maintenance channel53 can be used to carry maintenance information such as synchronization and power control messages to further support transmission of digital information over bothreverse link50 andforward link40.
• Forward link[0046]40 can include apaging channel41, which is used bybase station20 to inform afield unit14 of general information such as that one or multiple forward link traffic channels42 have been allocated to it for forward link data transmissions. Traffic channels42-1 . . .42-n on theforward link40 are used to carry payload information frombase station20 to a correspondingtarget subscriber unit14.
• [0047]Maintenance channel43 can be used to transmit synchronization and power control information onforward link40 frombase station processor20 tofield units14. Additionally,paging channel41 can be used to inform afield unit14 of allocatedtraffic channels52 in thereverse link50 direction.
• Traffic channels[0048]42 of theforward link40 can be shared amongmultiple subscriber units14 based on a Time Division Multiplexing scheme. Specifically, a forward link traffic channel42 is optionally partitioned into a predetermined number of periodically repeating time slots for transmission of data packets from thebase station20 tomultiple subscriber units14. It should be understood that a givensubscriber unit14 can, at any instant in time, have multiple time slots or no time slots assigned to it for use. In certain applications, an entire time-slotted forward or reverse link traffic channel can also be assigned for use by aparticular field unit14 on a continuous basis.
• FIG. 2 is a timing diagram illustrating multiple channels on which messages are transmitted between a[0049]base station20 andfield units14 according to certain principles of the present invention. As shown,field units14 can transmit messages tobase station20 over a dedicated channel such asaccess channel51.Access channel51 supports a transmission of access request messages from afield unit14 tobase station20. An access request message can indicate a request byfield unit14 for a high speed bi-directional communication link.
• Message transmissions over[0050]access channel51 need not be limited to access request type messages. For example,access channel51 can be structured to support other types of messages.
• In the illustrative timing diagram as shown in FIG. 2,[0051]access channel51 is optionally partitioned into periodically repeatingtime slots210 in which messages are transmitted from afield unit14 tobase station20. More specifically, an Epoch on the order of 26.6 mS in duration can be partitioned to includetime slot #0 andtime slot #1 as shown. In this application, a field unit ormultiple field units14 can randomly send messages tobase station20 in either time slot of an Epoch. Providingmultiple time slots210 in which afield unit20 can transmit a message renders it less likely that two randomly transmittingfield units14 will transmit a message in asame time slot210. Depending on an application, afield unit14 can be synchronized withbase station20 usingpilot channel44 so that messages can be transmitted in aparticular time slot210.
• When a collision occurs, i.e., two[0052]field units14 transmit a message in thesame time slot210, a devicemonitoring access channel51 for such messages may not be able to properly decode or decipher the content of either transmitted message. An indication of this condition is transmitted tofield units14.
• [0053]Feedback channel45 is provided so thatbase station20 can sendfeedback messages230 tofield units14. At least a portion of thefeedback channel45 as shown is reserved for transmitting general messages to thecollective field units14 whether or not a message collision occurred onaccess channel51 in a previous Epoch and, more particularly, that a message occurred in aparticular time slot210.
• A[0054]feedback message230 is optionally a sequence of encoded information or single bit indicating whether a collision occurred for a message transmitted tobase station20 in atime slot210. As shown,multiple feedback messages230 can be generated during a given time duration such as an Epoch or half Epoch. For example, feedback information such asfeedback messages230 can be transmitted in duration A of Epoch N+1 to indicate that a collision occurred for the reception of amessage210 intime slot #0 of Epoch N atbase station20. More specifically, alogic 1 transmitted in each of threefeedback messages230 of duration A can indicate that a collision was detected while alogic 0 setting can indicate that no collision was detected.
• Transmitting multiple, spaced[0055]feedback messages230 as shown for duration A provides redundancy to some extent. For example,multiple messages230 transmitted in a given duration, such as duration A, can be part of a redundantly transmitted message to increase the chance that afield unit14 will be properly notified whether a collision occurs. Hopefully, at least one of themultiple feedback messages230 can be identified at afield unit14 even if a message transmission failure occurs for some of themessages230 in, for example, duration A.
• In a similar manner as previously discussed,[0056]feedback messages230 offeedback channel45 transmitted in duration B of Epoch N+1 can indicate whether a message collision is detected intime slot #1 of Epoch N as monitored bybase station20.
• In a reverse link direction, a message transmitted by a[0057]field unit14 tobase station20 onaccess channel51 includes unique information such as the identification number of thefield unit14 transmitting the message. In forward link direction,paging channel41 supports message transmissions frombase station20 tofield units14, where the message transmissions are typically directed to aspecific field unit14. Thus,base station20 can respond to afield unit14 that sent a message onaccess channel51 by transmitting a reply message to thefield unit14 overpaging channel41. Other link information forwarded on thepaging channel41 can be forwarded to afield unit14 to establish a formal bi-directional link betweenbase station20 andfield unit14.
• One type of message transmitted on[0058]paging channel41 is an ACK (acknowledgment)message240.ACK messages240 are sent bybase station20 to indicate that a message received in atime slot210 ofaccess channel51 was properly received. Similar to thefeedback messages230 onfeedback channel45, ACK messages transmitted to afield unit14 are also feedback messages. However, anACK message240 indicates that a previous access request message transmitted by afield unit14 was properly received.ACK message240 can also indicate that a formal communication link will be established with the access requestingfield unit14. For instance, traffic channels can be assigned tofield unit14 for transmitting or receiving a data payload.
• It should be noted that[0059]field units14 optionally transmit at such a low power output level that messages transmitted in aparticular time slot210 are not detected atbase station20. In this instance, afield unit14 can adjust its power output level for a subsequent message transmission based on feedback information received frombase station20. More specifically, afield unit14 can adjust its power output level depending on afeedback message230 orACK message240 received onfeedback channel45 orpaging channel41 respectively. Accordingly, the power output level of afield unit14 can be optimized so that it minimally interferes withother field units14 transmitting information over a common radio frequency.
• Although other message types can be supported, the messages transmitted in a[0060]time slot210 ofaccess channel51 are typically access request messages indicating that afield unit14 would like to receive or transmit data payload information on reverselink traffic channels52 or forward traffic channels42.
• One aspect of adjusting the power output of a[0061]field unit14 is to determine whether or not a collision was detected for a previous message transmission from thefield unit14 tobase station20. For example, if a collision is not detected for a previously transmitted message from aparticular field unit14, the power output level for a subsequent message transmission from thefield unit14, if any, can be increased so thatbase station20 is more likely to detect the message transmission. More specifically, the power output level of afield unit14 can be increased a predetermined amount such as +0.5 dB for a subsequent message transmission if no collision was detected for a previous message transmission.
• In the event that a message transmission by a[0062]field unit14 does result in a collision due tomultiple field units14 transmitting in asame time slot210, the power output level of thefield unit14 can be unchanged or potentially reduced for a subsequent message transmission since it is not known whether the message transmission by afield unit14 would have otherwise been detected atbase station20 if only onefield unit14 transmitted a message within aparticular time slot210. Hence, one aspect of the present invention involves adjusting the power output level of afield unit14 so that it minimally interferes with others using the same radio channel.
• This method of transmitting messages can be particularly useful when a user first powers a[0063]field unit14 and must communicate withbase station20. For example, it is undesirable in certain situations to transmit a message at such a high power output level that such message transmissions cause other data transmissions on the radio channel to become corrupted due to excessive noise.
• In a similar manner as previously discussed, the power output level of a[0064]field unit14 can be adjusted depending on whetherbase station20 acknowledges receipt of a message onaccess channel51. Thus, iffield unit14 does not detect areply ACK message240, the power output level offield unit14 can be increased for subsequent message transmissions.
• A maximum power adjustment level such as 60 dBm can be selected in which the[0065]field unit14 will discontinue transmitting if no ACK message is received even at this level.
• A more sophisticated application of the present invention can involve adjusting a power output level of a[0066]field unit14 depending on both afeedback message230 indicating whether a collision was detected and anACK message240 indicating that an access message was acknowledged bybase station20. More specifically, afield unit14 can adjust its power output level for subsequent message transmissions if no collision is detected and no ACK message is received for a previously transmitted message from afield unit14. Otherwise, thefield unit14 can re-transmit at a previous power output level.
• Regardless of which method is used to adjust the power output level, the power level setting at which[0067]base station20 acknowledges receipt of a message from afield unit14 can be used to determine power level settings at which thefield unit14 must transmit other information tobase station20. For example, a message from afield unit14 can be transmitted using a particular modulation rate during the initial message transmission. The power output level of subsequent transmissions from thefield unit14 can be adjusted to accommodate transmitting messages at different modulation rates. For example, it can be determined at what power level a field unit should transmit on allocated traffic channels using a different modulation rate. A history of the power level output adjustments are optionally maintained to track power adjustments and determine at what power level a monitoring device such asbase station20 detects the transmission of a message.
• As previously discussed, one aspect of the present invention involves re-transmitting a message from a[0068]field unit14 so that it can be detected atbase station20. A re-transmission is optionally based on a random back off time so that a collision is less likely to occur on a subsequent attempt to transmit a message.
• Consider a situation where two or[0069]more field units14 transmit a message overaccess channel51 and a collision is detected atbase station20. As previously discussed, a feedback message will be transmitted to thefield units14 indicating that a collision occurred. Bothfield units14 must then re-transmit their corresponding messages tobase station20.
• To avoid another collision, the[0070]field units14 randomly choose a back off time relative to the previous message transmission in which the collision occurred and transmit in anothertime slot210. For example, if field unit A and field unit B transmit a message intime slot #0 of Epoch N, field unit A will choose a back off time such as 3 Epochs and re-transmit a message tobase station20 intime slot #1 of Epoch N+3 while field unit B re-transmits a message based on a random back off time intime slot #0 of Epoch N+2. Accordingly, field units A and B are less likely to cause another collision for a message re-transmission.
• FIG. 3 is a timing diagram illustrating another embodiment of the present invention for transmitting feedback messages to[0071]field units14. Anaccess channel51 is partitioned so that afield unit14 can transmit an access probe or other message in atime slot345. As shown,feedback channel355 is partitioned to include 64 timeslots TS#0,TS#1,TS#2 . . .TS#63 that repeat every Epoch.
• Each time slot[0072]315 offeedback channel355 preferably includes a data field supporting 16 bits of information. In the specific application as shown, ten bits of information are reserved for a general message, one bit is reserved as a collision detectbit325 and five bits are reserved for CRC (Cyclical Redundancy Check)data328.
• [0073]General message320 is optionally a message directed to aparticular field unit14. For example, each ofmultiple field units14 can be assigned use of a particular time slot315 for receiving feedback information frombase station20 tofield units14. When assigned, acorresponding field unit14 monitors an appropriate time slot315 to receive messages frombase station20. One type of specific message in a time slot315 is feedback information to afield unit14 indicating how its timing or power should be adjusted so that messages transmitted from afield unit14 are properly received atbase station20.
• Time slots[0074]315 are optionally unassigned and the message itself can include an address to which field unit14 a message is directed. Thus, in a modified embodiment, feedback messages can be transmitted asynchronously to afield unit14.
• Collision detect[0075]bit325 in a time slot315 is a single bit indicating whether a collision occurred in a monitoredtime slot345. More specifically, collision detectbits325 of timeslot TS#0,TS#1 . . . TS#31 of Epoch M can be used to indicate that a collision occurred in accessprobe slot #0 of Epoch M-1. Accordingly, this string of individual collision detectbits328 over multiple time slots can be set to a same logic state indicating that a collision was detected.
• In a similar manner, TS#32, TS#33 . . .[0076]TS#63 of Epoch M can be set appropriately to indicate whether a collision occurred on accessprobe slot #1 of Epoch M-1. Thus, amonitoring field unit14 can determine whether a collision occurred atbase station20 based on a single bit, a sequence of multiple bits, or a sequence of spaced bits.
• [0077]CRC data328 is also provided in afeedback message360. TheCRC data328 is optionally decoded at thefield unit14 to ensure that amessage360 is properly received at afield unit14 and, more specifically, that a particular collision detectbit325 is properly received. Other methods can also be used to ensure and verify that a message and data is properly received atfield unit14. For example, a message can be transmitted based on an FEC (Forward Error Correction) code.
• FIG. 4 is a diagram illustrating a format for transmitting messages over the access channel from a field unit to a target receiver according to the principles of the present invention.[0078]
• In one application,[0079]message410 is transmitted by afield unit14 overaccess channel51 and includes two parts. As shown, a first part orpreamble415 ofmessage410 is a coded message indicating a request by thefield unit14 for a communication link. Eachfield unit14 can transmit amessage410 having a commonly codedpreamble415. Thus, if twofield units14 transmit a message including thesame preamble message415,base station20 can determine that at least apreamble message415 was sent by at least onefield unit14. That is, thepreamble message415 as transmitted by one field unit can overlap with thepreamble message415 as transmitted by anotherfield unit14 whenmultiple messages410 are transmitted in the same time slot.
• [0080]Message410 optionally includes adata payload420 that is transmitted tobase station20. In one application,data payload420 includes the serial number of thefield unit14 transmittingmessage410. Typically, some form of redundancy check information such as CRC data is included withmessage410 so thatbase station20 can determine whethermessage410 is properly received without errors.
• If[0081]message410 is received without errors,base station20 can respond accordingly to establish a link with afield unit14 and transmit a “non-collision” message onfeedback channel45 to thefield units14. Alternatively, ifmessage410 includes an errorfree preamble415 but improperly receiveddata payload information420,base station20 can deduce that two or more transmitters sent a message at the same time. A collision detection message is then transmitted overfeedback channel45 indicating that a collision occurred. Thus, a target receiver such asbase station20monitoring messages410 can provide valuable feedback to multiple transmittingfield units14 whether a message collision occurs.
• Another aspect of the present invention involves coding a[0082]preamble415 usingpilot block53 and Barker code block54. Based on this coding or use of a sequence of symbols, afield unit14 can transmit amessage410 tobase station20.
• As shown, a[0083]preamble message415 can include fourpilot blocks53 and four Barker code blocks54. The Barker code blocks54assist base station20 identify a point wherepreamble415 of amessage410 starts. In other words, the information in thepreamble415 can be used for timing purposes at thebase station20 to asynchronously receive a message. Thus, it is not necessary that afield unit14 transmit amessage410 in atime slot210 becausebase station20 can be modified to receive asynchronous messages.
• However, in an application where[0084]messages410 include Barker code blocks54 that are transmitted in atime slot210,base station20 can identify a receivedmessage410 even if a collision occurs because thepreamble415 of amessage410 simultaneously transmitted bymultiple field units14 in a time slot will overlap and, thus, will be detectable atbase station20.
• Each[0085]pilot block53 includes a number of repeating pilot symbols. Preferably, a pilot block includes48 symbols that are used by a target receiver to decodemessage410.
• The second portion of a[0086]message410 can include adata payload420 that is sent tobase station20. Preferably, pilot symbols are also inserted in thedata payload420 portion ofmessage410 for assisting in coherent demodulation of data at a target receiver. Pilot symbols typically include a series of positive data bits and therefore do not themselves inherently include timing information.
• A Barker code block[0087]54 as shown includes a predetermined pattern of bit information. Use of BPSK (Binary Phase Shift Keying) can be used to generate apositive barker sequence450, +B, such as three positive bits, followed by three negative bits, a positive bit, a pair of negative bits, a positive bit and then a negative bit respectively. A Barker code sequence can alternatively be negative such as a negative Barker sequence, −B, further assisting in message processing at a monitoring device.
• Further details for processing[0088]message410 including pilot blocks53 and Barker code blocks54 can be found in co-pending U.S. patent application Ser. No. 09/766,875 filed on Jan. 19, 2001 entitled “Access Channel Structure for Wireless Communication System,” the entire teachings of which are incorporated herein by reference.
• FIG. 5 is a flow chart illustrating a process for monitoring a channel for messages according to the principles of the present invention.[0089]
• [0090]Step500 generally indicates an entry point into the flow chart. Instep510, anaccess channel51 is monitored for message transmissions such as access request messages transmitted by afield unit14. It is then determined instep520 whether the message includes a Barker code or an appropriately receivedpreamble415 of amessage410. If no Barker code orpreamble415 is detected instep520, process flow resumes atstep510 again. Alternatively, if a Barker code is detected instep520, themessage410 is further analyzed to determine if a collision occurs in a time slot. That is, it is determined whether at least a portion of data in a receivedmessage410 is corrupted.
• One way to determine if a message collision occurs is to verify that data in a[0091]message410 was properly received. This can be achieved by analyzing the receivedmessage410 according to redundancy check information. If the data in amessage410 is not properly received atbase station20, a feedback message is transmitted bybase station20 overfeedback channel45 indicating that a collision was detected for a previous access request message instep540. Followingstep540, process flow resumes again atstep510.
• If a collision is not detected for a particular message in[0092]step530, themessage410 is analyzed to determine which ofmultiple field units14 sent the message. Following instep550, anACK message240 is sent to the requestingfield unit14 over thepaging channel41. Also, a message is sent over thefeedback channel45 indicating that no message collision occurred for the correspondingprevious time slot210 ofaccess channel51. Finally, a more formal link is established with the access requesting field unit instep560.
• FIG. 6 is a flow chart illustrating a process flow at a field unit for transmitting a message to a target receiver according to the principles of the present invention. Step[0093]600 generally indicates an entry point into the flow chart.
• In[0094]step620, the status offield unit14 is monitored for an input by a user indicating that thefield unit14 desires to establish a communication link with a target receiver such asbase station20. It is then determined instep620 whether the input indicates that afield unit14 desires to establish a communication link. If not, process flow again resumes atstep610. If so,field unit14 transmits an access request message on theaccess channel51 instep630. Thereafter, thefeedback channel45 is monitored by afield unit14 instep640 for feedback information such as collision detection messages.
• If a collision is detected for a previous transmission by a[0095]field unit14 instep650, process flow continues atstep660 where the power output level is not adjusted for thefield unit14 and amessage410 is subsequently re-transmitted instep630.
• If a collision is not detected in[0096]step650 as indicated by a collision feedback message, it is determined instep670 whether anACK message240 is received at thefield unit14 overpaging channel41. If so, a link is established between thefield unit14 andbase station20 instep690. If not, the power output level of thefield unit14 is increased instep680 and process flow continues atstep630 to re-transmit a message from thefield unit14 tobase station20.
• FIG. 7 is a timing diagram illustrating multiple channels on which messages are transmitted among transceivers according to the principles of the present invention.[0097]
• As previously discussed, one aspect of the present invention involves setting a[0098]field unit14 to an initial power level so that it minimally interferes with other users during a message transmission. Since power is ramped up based on whether an access message is detected atbase station20, it is preferable that the initial power level of the field unit is reasonably near a power level at whichbase station20 will receive a message at a desired power level. Consequently, afield unit14 will be able to transmit a message tobase station20 and establish a more formal communication link in less time since a power level output offield unit14 will need only minimal adjustments so that a message is received atbase station20.
• One method for initially setting a power output level of a[0099]field unit14 involves transmitting areference signal710 onpilot channel44 frombase station20. Preferably, thereference signal710 is transmitted at an appropriate power level so thatmultiple field units14 in a wireless airspace monitoring thepilot channel44 can identify thereference signal710 and measure a power level at which it is received. In one application, reference signal includes pilot information such as a sequence of pilot symbols, where the pilot symbols are defined by PN (Pseudo Noise) codes. One or multiple pilot correlation filters infield unit14 is used to detect the pilot symbols.
• Each[0100]field unit14 monitoring thepilot channel44 typically includes a power detector circuit to measure a power level of the receivedreference signal710. For example, the power detector is used to measure the strongest pilot path of the receivedreference signal710. This measurement is used to estimate a forward path loss betweenbase station20 andfield unit14.
• The total received signal power level of the[0101]reference signal710 can be computed based on the sum of the magnitude squared of the I and Q channel. Power measurements are optionally filtered for providing a better estimate of a received power level under fading conditions.
• As shown in FIG. 7, messages are transmitted on[0102]paging channel41 frombase station20 tofield units14. One such message is message A that includes information indicating a power level at whichreference signal710 is transmitted frombase station20. This value can be expressed in dBm that already takes into account the gain of the base station antenna. Thus, message A can include effective radiated power level information at whichbase station20 transmitsreference signal710. In harmony with the principles of the present invention, additional messages such as antenna gain information, offset information, correction information and general information can be transmitted to afield unit14.
• [0103]Field unit14 decodes message A to determine a power level at whichreference signal710 is transmitted. The forward path loss betweenbase station20 andfield unit14 is then determined by comparing the received power level of thereference signal710 atfield unit14 with the effective radiated power level as indicated by message A.
• The calculated forward path loss can then be used to estimate a reverse path loss between[0104]field unit14 andbase station20. For example, the reverse path loss is estimated to be about the same as the forward path loss, although it is probably at least slightly different. This estimated path loss is used to determine an initial setpoint at which messages can be transmitted fromfield unit14 tobase station20.
• Consider a case where[0105]base station20 transmits areference signal710 at an effective radiated power level of 55 dBm. As discussed, this information is sent to fieldunits14 via message A generally broadcasted on thepaging channel41. If the received power level of thereference signal710 is 22 dBm, the forward path loss is calculated as 55−22 dBm, or forward path loss=33 dBm. Based on this path loss, afield unit14 can estimate a reasonable power output level for a subsequent attempt to transmit a message tobase station20.
• Additional messages can be sent on[0106]paging channel41 frombase station20 tofield units14. For example, message B is also generally transmitted to field units overpaging channel41. Message B preferably includes encoded information indicating a desired power level at whichbase station20 will receive subsequent messages from afield unit14. This information can also be a specific message directed transmitted to aparticular field unit14. Thus, afield unit14 can use the information to estimate at what level a message should be transmitted so that a message is received at the desired power level. In a case where message B indicates a desired power level of 12 dBm and the forward path loss is approximately 33 dB as discussed,field unit14 can attempt to transmit a message at 33+12 dBm, or 45 dBm, tobase station20.
• Notably, the reverse path loss may be much more than 33 dBm as estimated. In such a situation, base station would not necessarily detect a message transmitted by[0107]field unit14. As previously discussed, however, the power output setpoint of 45 dBm can be a starting point at which messages such as access request messages750 are transmitted overaccess channel51. If a collision is not detected atbase station20 and noACK message240 is received overpaging channel41, the power output offield unit14 can be increased by 1 dBm to 46 dBm for a subsequent attempt to transmit a message. This procedure of adjusting the power output level of afield unit14 can be repeated until a message is detected atbase station20.
• Messages transmitted to[0108]base station20 can also be monitored to determine a power level at which a message is received from afield unit14. To achieve this end, a message such as message C can include pilot information such as a pilot symbol or sequence of pilot symbols. Pilot correlation filters are then used to identify the strongest diversity path and side paths as shown in FIG. 8. One or multiple paths are then used to determine a power level at which the message is received atbase station20 onaccess channel51. To ensure that a message C is properly received, the message is analyzed for errors using error detection information such as CRC check bits. These and other aspects of invention were previously discussed in this specification.
• After message C is properly received at[0109]base station20, a power adjustment message is generated atbase station20 to indicate how thefield unit14 should be adjusted so that subsequent messages tobase station20 are received at a desired power level. For example, ifbase station20 determines that a message is received at 23 dBm,base station20 can send a message overpaging channel41 indicating that the field unit should reduce its power output level for subsequent message transmissions so that a message from a field unit is received at a lower power level such as 12 dBm.
• FIG. 8 is a graph illustrating a received diversity string for a pilot symbol according to the principles of the present invention. A received message such as[0110]reference signal710, message A, message B or message C can include a marker such as one or multiple pilot symbols that are monitored at a receiver to determine a power level of a received message.
• Both[0111]base station20 andfield units14 include pilot correlation filters for identifying a marker such as one or multiple pilot symbols in a transmitted message. This marker aids in analyzing both timing alignment and a received power level of a message. Incidentally, the diversity string illustrates the receipt of a marker in a message as a result of multipath fading. That is, a signal from a transmitter is received at a target at different times due to varying times it takes for the signal to reach the target over different paths between a transmitter and receiver.
• Preferably, the strongest received diversity path will be designated as the time alignment string at[0112]base station20field unit14 for analyzing the timing of a received message. Likewise, the single strongest path is preferably chosen to calculate a power level at which a message is received. However, additional paths are optionally used to determine a received power level of a message.
• Timing alignment and a received power level of a message is determined using the correlation profile of the strongest pilot in a particular string, which is analyzed as mentioned using correlation filters. The output of the correlation filters typically consist of 256 samples, which represent 64 lags at 4 samples per lag. The 256 sample output-window represents the total correlation time span of a receiver device. This can vary depending on the application. Preferably, the time alignment point is[0113]sample number80 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, each[0114]field unit14 transmitting in a time slot 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. Location of the centroid, L, of the samples in FIG. 6 is calculated based on the following equation:$L=\frac{\sum \left[t\times Q\left(t\right)\right]}{\sum Q\left(t\right)}$

  
• where t=sample time and Q(t) is the magnitude of a sample at a given time.[0115]
• For example, L is calculated based on the results as shown in FIG. 6:[0116]$L=\frac{\left(\mathrm{.25}*76\right)+\left(\mathrm{.5}*77\right)+\left(1.0*78\right)+\left(\mathrm{.8}*79\right)+\left(\mathrm{.6}*80\right)}{\mathrm{.25}+\mathrm{.5}+1.0+\mathrm{.8}+\mathrm{.6}}$

  
• L=78.317[0117]
• Again, the timing alignment error is determined by comparing the timing of the computed centroid to the desired time set point of[0118]80, which is chosen as the reference point for timing alignment within a time slot. Since the centroid in the example above is estimated to be 78.317, timing is early relative to the set point of80. An appropriate message can be sent tofield unit14 indicating how its timing should be finely tuned so that messages fromfield unit14 are received at the appropriate time atbase station20.
• In a similar manner, the diversity string of FIG. 8 can be analyzed to determine a power level at which a message is received. Thus, an appropriate message can be sent to[0119]field unit14 indicating how its power output level should be adjusted so that a message is received at a desired power level.
• As mentioned, this technique can be used to detect a received power level of[0120]reference signal710 atbase station20.
• More details regarding timing alignment and power control between a[0121]base station20 and each ofmultiple field units14 can be found in co-pending U.S. Application No. (2479.1067-001) entitled “Minimal Maintenance Link to Support Synchronization” filed on Feb. 7, 2001, and co-pending U.S. application Ser. No. 09/775,305 entitled “Maintenance Link Using Active/Standby Request Channels” filed on Feb. 1, 2001, the entire teachings of both of which are incorporated herein by reference.
• FIG. 9 is a flow chart illustrating a method for setting a power level output of a field unit based on an estimated path loss according to the principles of the present invention.[0122]
• [0123]Step900 indicates an entry point into the flow chart. Following instep910,field unit14 monitorspilot channel44 for reference signals710. As previously discussed, the field unit determines a power level at which reference signals710 are received using a power detector circuit and pilot correlation filters.
• In[0124]step920,paging channel41 is monitored by afield unit14 for messages transmitted frombase station20. As previously discussed, message A is received onpaging channel41 and decoded to determine an effective radiated power level at which reference signals710 are being transmitted frombase station20.
• Based on the detected power level of[0125]reference signal710 as received atfield unit14 and corresponding effective radiated power level at which thereference signal710 is transmitted frombase station20, a path loss is estimated between the base station andfield unit14 instep930. Preferably, the path loss is estimated by computing the difference between the power level at whichreference signal710 is transmitted frombase station20 and a power level at whichreference signal710 is received atfield unit14.
• Message B is subsequently received at[0126]field unit14 instep940. This message preferably includes information indicating a desired power level at which messages are to be received atbase station20.
• Based on the desired power level at which messages are to be received at[0127]base station20 and the estimated path loss instep930,field unit14 determines a power output setting forfield unit14 so that a message is received at the desired power level atbase station20 instep950. More specifically, it is presumed that an actual path loss from thefield unit14 to base station is approximately the same as the calculated path loss between the base station andfield unit14 based on measurements ofreference signal710. Thus, an appropriate power output level offield unit14 can be determined by adding the estimated path loss to the desired power level setting to determine a power output setting forfield unit14. Consequently, this power output setting of afield unit14 should be a reasonable starting point for attempting to transmit an initial message tobase station20.
• Also in[0128]step950,field unit14 transmits message C such as an access request message tobase station20 overaccess channel51. Upon receipt,base station20 measures a received power level of message C atbase station20. This received power level is then compared to the desired power level so that feedback can be provided tofield unit14 indicating how to adjust its power output level so that subsequent messages are received at the desired power level.
• Following in[0129]step960,field unit14monitors paging channel41 for an ACK message indicating thatbase station20 properly received message C. If an ACK is not received instep970, the power output level of afield unit14 is increased instep975 and the message is subsequently re-transmitted instep960. This loop of increasing power generally repeats untilbase station20 acknowledges receipt of the message.
• When an ACK is received in[0130]step970, process flow continues atstep980 in which additional messages are received frombase station20 indicating whether the message transmitted instep950 was received at the desired power level. As discussed, information can be transmitted to afield unit14 indicating how to adjust its power output level so that subsequent messages are received at the desired power level atbase station20. Consequently, the output power level of thefield unit14 is adjusted accordingly for subsequent message transmissions.
• 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.[0131]

Claims (15)

What is claimed is:

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

transmitting a first message from a first transceiver, the first message including effective radiated power level information at which the first transceiver transmits a reference signal; and

measuring a received power level of the reference signal received at a second transceiver;

decoding the first message at the second transceiver to determine the effective radiated power level information; and

estimating a path loss between the first and second transceivers by comparing the received power level and the effective radiated power level information.

2. A method inclaim 1 further comprising the step of:

transmitting a second message from the first transceiver to the second transceiver indicating a desired power level at which subsequent messages shall be received at the first transceiver.

3. A method inclaim 2 further comprising the step of:

transmitting a reply message from the second transceiver to the first transceiver at a power level so that the first transceiver receives the reply message at the desired power level.

4. A method as inclaim 2 further comprising the step of:

determining a power output setting at the second transceiver based on an estimated path loss so that a message is received at the first transceiver at the desired power level.

5. A method as inclaim 1, wherein the first message is transmitted over a paging channel of a CDMA (Code Division Multiple Access) communication system.

6. A method as inclaim 3, wherein the reply message is transmitted over an access channel of a CDMA communication system.

7. A method as inclaim 1, wherein the second transceiver transmits an access request message to the first transceiver in a time slot of an allocated reverse channel.

8. A method as inclaim 7, wherein the access request message from the second transceiver to the first transceiver includes encoded information for measuring timing alignment of the second transceiver.

9. A method as inclaim 1, wherein the second transceiver sends a reply message to the first transceiver, the reply message including a reference marker that is monitored for timing alignment.

10. A method as inclaim 1, wherein the first transceiver transmits the first message over a dedicated channel to multiple transceivers.

11. A method as inclaim 1 further comprising the step of:

measuring a power level at which a reply message transmitted from the second transceiver is received at the first transceiver.

12. A method inclaim 11 further comprising the steps of:

comparing a power level at which the reply message is received at the first transceiver with a desired power level at which messages should be received; and

transmitting a power adjustment message to the second transceiver for subsequent message transmissions.

13. A method as inclaim 1, wherein the reference signal includes pilot symbols.

14. A method as inclaim 1, wherein the reference signal is transmitted over a pilot channel.

15. A method as inclaim 1, wherein a path loss is calculated based on the difference between an effective radiated power level of the reference signal and power level of one or multiple received reference signals.

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