Method and apparatus for determining content of burst to be transmitted from base stationTechnical Field
The present invention relates generally to a method and apparatus for determining the content of bursts to be transmitted from a base station on a broadcast frequency. In particular, the present invention is used to enable accurate identification of base stations when measuring received signals and to avoid burst confusion.
Background of the invention and Prior Art
A mobile communication network comprises a plurality of base stations connected together by switching nodes, such as Base Station Controllers (BSCs), Mobile Switching Centers (MSCs) and serving GPRS (general packet radio service) support nodes (SGSNs). Each base station provides radio coverage over an area known as a cell for radio communication with mobile terminals located therein. The mobile network is allowed to use some limited radio spectrum for transmissions between the base station and the mobile terminal. Thus, data is transmitted over various logical channels on physical frequency channels within the allocated spectrum.
In a typical cellular network configuration, each cell is allocated a number of specified physical frequency information for call connections and for broadcasting information to mobile terminals. Since the total number of available physical frequency channels is limited, they must be reused to some extent in a plurality of cells throughout the network. However, the frequency channels can only be reused between cells that are sufficiently far away from each other to avoid excessive interference with each other, but the mobile terminal and the base station are adapted to handle some degree of interference.
Cell planners are concerned with assigning frequencies to cells in a network that employ certain reuse patterns, as is generally well known in the art. Much work has been done to be able to adopt a compact reuse pattern, i.e. a reuse distance as short as possible in order to maximize the traffic capacity of the network. An important factor to consider is to keep the call and broadcast transmit power levels as low as possible without compromising radio coverage, thereby reducing the amount of interference, which in turn will allow short reuse distances. High precision power control mechanisms for ongoing calls have therefore been developed.
Another area of concern is the selection of the most suitable base station to connect with the mobile terminal. For example, if a mobile terminal connected to a serving base station moves from that base station to a neighboring base station, the received signal strength or link quality of the old base station will decrease, while the received signal strength or link quality of the new base station will increase. Thus, the new base station may become more suitable for the connection, requiring less transmit power to achieve acceptable link quality.
There are various known mechanisms for switching a connection from a serving base station to a new base station, which are called "handover" or "handover" when the terminal is in busy mode, i.e. participating in a call, and which are called "cell selection" or "cell reselection" when the terminal is in idle mode, i.e. not participating in a call but powered up. The handover at the serving base station may also be performed for mobile terminals in packet switched transmission mode (as opposed to circuit switched communication). For either mode, this will be referred to as "base station selection" hereinafter for brevity. Thus, correct base station selection will result in low interference and enable compact reuse patterns, as well as saving battery power consumption in the terminal.
In most cellular networks today, the mobile terminal is required to make measurements on signals from neighboring base stations as well as from the serving base station and to report the measurement results to the serving base station. The reported measurements can then be used by the network for different purposes, such as:
1) the process of selecting the most suitable base station for a particular connection is supported, as described above. The reported measurements of signals from neighboring base stations and from the serving base station are compared and the "best" base station is selected for the connection, preferably after some predetermined threshold condition has also been reached. In the case of busy mode handovers, these measurements are often referred to as MAHO (mobile assisted handover) measurements.
2) Mechanisms that support determining the location of a terminal (commonly referred to as "positioning") require timing and/or signal strength information of the signals received from each target base station. The terminal may measure and report the reception delay and/or signal strength of signals from preferably at least three base stations. From this information and the known location of each base station, the current location of the terminal can be calculated.
3) Network attributes are characterized, such as calculating cell relationships and evaluating different cell patterns or plans and various algorithms and parameters used to operate the network. Various measurements of link quality from different base stations may form the basis for network planning and network configuration efforts. The cell relationships include, for example, interference levels estimated when multiple cells are assigned the same or adjacent frequency channels for transmission. Typical network rule tasks include: setting a cell mode and a transmission power level, carrying out antenna adjustment and setting a frequency distribution parameter and a switching threshold.
From the foregoing, it is apparent that it is desirable for a mobile terminal to make accurate and reliable measurements of signals transmitted from different base stations. Thus, the effects of reported inaccurate or misleading measurements may adversely affect the above-described activities.
To enable such measurements, each base station constantly transmits signals at a fixed power level on at least one broadcast frequency or pilot channel, which in GSM is called BCCH (broadcast control channel) frequency. Mobile terminals present in the network may then make measurements on the broadcasted signal, e.g. in terms of link quality or timing estimation.
As is known, a broadcast frequency comprises successive bursts of signals transmitted in time slots. A plurality of logical channels are multiplexed onto the physical broadcast frequency according to a specific TDMA (time division multiple access) frame structure. Logical channels may include point-to-multipoint channels such as a paging channel PCH, a Frequency Correction Channel (FCCH), a Synchronization Channel (SCH), and other specific control channels. Logical channels may also include dedicated point-to-point Traffic Channels (TCH) and point-to-point signaling channels, such as stand-alone dedicated control channels (SDCCH), which may be used for mobile terminal connections.
When a mobile terminal connects to a certain base station, that base station will transmit measurement instructions including a list of broadcast channels transmitted by neighbouring base stations to be measured by the terminal. Such a neighbor base station list may be predefined for each cell, at least for the purpose of MAHO measurements, and indicates the broadcast frequency of each target base station and possibly also the identity of the base station. The number of base stations included in the neighbour list is typically in the range of 10 to 32, depending on the cell configuration in the network.
To illustrate this measurement step, FIG. 1 shows a schematic diagram of a mobile terminal 10 operating in a cellular communication network 12, including a plurality of base stations 14, 16a-c interconnected by a switching network 18. In this case, the terminal 10 is currently connected to the serving base station 14. Initially, when the terminal 10 starts a connection with the serving base station 14, the terminal 10 receives a measurement instruction from the network via the base station 14, comprising a list of neighbouring base stations with the broadcast frequencies (and optionally also identities) of a number of predetermined neighbouring base stations, three of which are denoted 16 a-c. The terminal 10 is thus instructed by the network to perform measurements, e.g., for link quality or timing, on the specified broadcast frequency during idle periods and to send one or more reports to the serving base station 14 and the network. Such reports may be sent in response to polling from the network or according to a predetermined schedule specified by usage criteria or measurement instructions sent from the network.
Alternatively, the terminal 10 may not be connected to any serving base station, such as the terminal 10 when it is just powered on. The terminal may then scan the broadcast frequencies and measure them for registration with the base station with the strongest/best signal.
As described above, the mobile terminal may measure the broadcasted signal for link quality or timing. The link quality is measured as at least one of: received Signal Strength (RSS), carrier-to-interference power ratio (C/I), carrier power, Bit Error Rate (BER), or any other link quality related parameter. Timing may be measured by detecting burst offsets relative to a given clock reference provided from the serving base station. However, the present invention is not limited to any particular measurement method or arrangement.
However, due to the co-channel interference problem described above, measurement errors may occur if the frequency reuse distance is relatively short. For example, a terminal may erroneously measure a strong signal from a base station that is not the intended target base station but that reuses the same broadcast frequency. Furthermore, the measured signal is typically the sum of multiple signals transmitted from several sources reusing the same frequency, including reflections thereof, and thus the measured total signal strength and/or quality may be misleading.
In some networks, therefore, the terminal is required to qualify the measurement by checking the target base station being measured before reporting the measurement to the serving base station. By attributing a certain measurement to a certain base station, that measurement will be more reliable, whereas if the terminal cannot identify the base station, the measurement should be discarded.
In WO02/096149 a solution is disclosed for a mobile terminal to accurately attribute measurements to a particular base station. A measurement is qualified only if the identity of the measurement can be determined from the same received signal as measured.
In order to be able to identify the base stations, it has been proposed to add to the signal broadcast from each base station an indication of the identity of the base station which is readable or at least detectable by the terminal. Base station identification is typically further aided by the terminal knowing from the received list of neighbouring base stations which target base station is expected for each measurement frequency. In GSM, a base station identity called BSIC is used, which is contained in the SCH. BSIC includes Network Color Codes (NCC) and base station color codes (BCC). Furthermore, the normal burst transmitted on the BCCH frequency may contain information related to BSIC so that the receiving terminal can obtain BSIC therefrom.
Generally, the transmitted signal, when received by the terminal, has been corrupted to varying degrees in its propagation. Thus, a process called channel estimation is used by the terminal receiver to recover the transmitted signal. The channel estimation utilizes a training bit sequence known to the terminal, which in GSM is 26 bits in length, and is typically embedded in all normal bursts transmitted from the base station, including the bursts in the broadcast channel. Exceptions to the "normal" burst may be the FCCH burst containing the sine wave of the frequency synchronization and the SCH burst containing the longer special training sequence for the initial TDMA burst synchronization. There are many known techniques for channel estimation, which will not be described further herein. In the time slots where no useful information is to be transmitted, so-called "dummy bursts" are transmitted which do not contain data in order to maintain the required continuous transmission on the BCCH frequency.
Fig. 2 schematically illustrates an exemplary normal burst 20 transmitted in a time slot of a broadcast frequency channel of a target base station contained in a list of neighbouring base stations received by a terminal. The normal burst 20 may belong to any one of many possible logical channels, such as a traffic burst belonging to a traffic channel, and includes a bit field 22 with a training sequence, typically arranged approximately in the middle of the burst. The burst 20 may also include various fields disposed on either side 24, 26 of the training sequence, such as a header field, a field with payload or control data, a tail bit field, etc.
The training sequence in the normal burst from the serving base station is known by the terminal and is used to assist in synchronization and decoding or detection of the burst. A set of known training sequences is typically defined, e.g. eight different sequences in GSM, and one sequence for a particular burst is identified by a Training Sequence Code (TSC) of e.g. 3 bits. In the GSM common control channel, the TSC is identical to the BCC, while for other channels the TSC is transmitted to the terminal in a channel assignment message.
According to WO02/096149, the base station identity is preferably related to the TSC in a manner known in advance to the mobile terminal by applying a "base station specific" training sequence. Thus, by detecting the expected training sequence in a burst and deriving the TSC therefrom, a mobile terminal receiving a normal burst from a target base station can determine the base station identity regardless of which logical channel the received burst belongs to. In the simplest case, the base station identity, BCC as in GSM, is set to be the same as TSC. However, other relationships are possible.
In WO02/096149, it is also proposed to perform channel estimation for measurement signals from neighboring base stations in order to check the target base station. In addition, channel estimation may be performed on several candidate training sequences to separate components from multiple base stations reusing the same frequency and any reflections thereof. This step helps in detecting the target base station, as described in more detail in WO 02/096149.
In some mobile networks, data bits are transmitted in bursts using different modulation methods. In networks using EDGE (enhanced data rates for GSM/TDMA136 evolution) technology, i.e. GERAN (GSM/EDGE radio access network), two different modulations are used, namely GMSK (gaussian minimum shift keying) using two phases in each symbol to represent one bit (1 or 0), and 8PSK (8-phase shift keying) using eight phases in each symbol to represent three bits (1 or 0 each). In GERAN eight different training sequences with corresponding TSCs have been defined for each modulation form, where each TSC is coupled to one GMSK training sequence and one 8PSK training sequence, respectively. These training sequences/TSCs have also been stored in mobile terminals capable of operating in GERAN.
Base stations reusing the same broadcast frequency may have different BCCs and by coupling TSCs to BCCs, the training sequences will be different as long as the BCCs are different. This is the case, for example, in GERAN base stations supporting a positioning technique called enhanced observed time difference (E-OTD). Thus, if the mobile terminal can identify the training sequence contained in the received burst, the transmitting base station can be identified from the TSC/BCC originating from that training sequence. Here, the terminal can make an evaluation attempt of the same received signal for each possible modulation form, as is also further explained in WO 02/096149. However, the synchronization burst, the frequency correction burst and the dummy burst on the broadcast frequency have no base station specific training sequence, and all other logical channel bursts should contain one of the above-mentioned base station specific training sequences.
It is important that each base station continuously transmits on the broadcast frequency, even in currently unused time slots (i.e. no data to transmit), at least for measurements for base station selection purposes, in order to carry out the measurements at any time. Unused time slots may occur in the broadcast frequency frame structure, e.g. when there is an unoccupied traffic channel due to low traffic load, or when so-called Discontinuous Transmission (DTX) is currently applied to an occupied traffic channel, which is a well-known algorithm for minimizing interference. In DTX, no data is transmitted when not necessary, e.g. due to silence at the transmitting end. In cellular networks, dummy bursts are typically transmitted on a broadcast frequency in unused time slots in order to maintain continuous transmission, as described above.
The "legacy" dummy bursts generally do not carry intelligible information and typically convey a predetermined and fixed bit pattern in all dummy bursts that is easily recognized by the mobile terminal. It is therefore not possible to determine any base station identity from such dummy bursts, which sometimes occur quite frequently. The synchronization bursts and frequency correction bursts occur at fixed frame bits known to the mobile terminal, but the occurrence of dummy bursts is to varying degrees unpredictable by them. Furthermore, the fixed bit pattern in the entire burst requires special treatment by the mobile terminal, since the normal training sequence is not included as expected. Thus, the occurrence of dummy bursts makes the measurement of neighboring base stations more difficult.
To overcome these problems, it has been proposed to include in the dummy burst transmitted on the broadcast frequency channel an indication of the identity of the base station, for example a training sequence related to the identity of the base station in a known manner. This will therefore significantly increase the likelihood that the mobile terminal will determine the identity of the neighbouring base station that has sent the received dummy burst and enable successful measurements to support, for example, base station selection and positioning activities.
If a known base station specific training sequence is included in the received dummy burst, the terminal can use it to identify the base station and estimate the timing offset to the respective base station, e.g. in E-OTD, or better estimate the link quality of the burst.
Even if the dummy burst with the base station specific training sequence allows to identify the neighbouring base stations, another problem that will arise is that in some cases the dummy burst may be erroneously interpreted by the mobile terminal as its normal traffic burst. For example, if the mobile terminal is active in a call, but the used downlink traffic channel is currently in DTX mode, i.e. the corresponding timeslot is not used, the base station has to transmit dummy bursts to meet the requirements of continuous broadcasting. The mobile terminal can easily distinguish normal traffic bursts from conventional dummy bursts by their specific bit patterns having very low cross-correlation with the training sequence in the normal traffic burst and interpret such dummy bursts correctly as a continuous DTX pattern. On the other hand, if the transmitted dummy burst contains a base station specific training sequence, it may be erroneously interpreted by the terminal as a regular traffic burst with the same training sequence.
If one or more such dummy bursts are interpreted as traffic bursts, the terminal may erroneously assume that the DTX mode is complete and that a speech frame was received. In that case the terminal will try to decode the error correction code of the assumed speech frame. In addition, it will attempt to decode the error detection code (also known as the cyclic redundancy check [ CRC ] code) of the speech frame to check if it is a valid speech frame. While the error detection code in most cases provides a correct indication of whether a speech frame is valid, it is still possible to incorrectly consider an invalid frame as valid. The likelihood of interpreting an invalid speech frame as a valid speech frame depends on the number of bits used for the CRC. This probability generally decreases exponentially as the number of CRC bits increases. If an invalid speech frame is transmitted to a speech decoder in the terminal, it will result in unwanted noise. This problem is particularly relevant for TCH/FS (traffic channel/full rate speech) and TCH/HS (traffic channel/half rate speech) channels, carrying speech frames suitable for early GSM speech codecs, where only three CRC bits are used. A three bit CRC indicates that an invalid frame near 1/8 is interpreted by the CRC as a valid frame.
The potential problem of misinterpreting traffic bursts is likely to negate the benefits gained for identification and measurement activities and therefore must be taken into account when introducing dummy bursts with base station specific training sequences.
In summary, in order to improve the steps of base station selection, location determination and network evaluation, it is of paramount importance to obtain reliable measurements of neighbouring base stations with high accuracy and to minimize the delay time between such measurements and the corresponding reports. At the same time, it is desirable to avoid as much as possible the risk of misinterpreting different bursts received by the mobile terminal. It is also desirable to reduce the impact of interference on measurement accuracy, thereby allowing tighter frequency reuse.
Summary of The Invention
It is an object of the present invention to reduce or eliminate the above problems. It is an object to determine the content of bursts to be transmitted from a base station of a mobile network on a broadcast frequency such that the risk of burst confusion for connected terminals is minimized while maximally facilitating burst detection for measuring terminals. Another object is to determine burst content so that a mobile terminal can identify a transmitting base station when transmitting a dummy burst. Another object is to reduce interference in the network and the impact of interference on measurement accuracy, thereby allowing tighter frequency reuse in the network.
These and other objects are achieved by providing a method and apparatus for determining the content of bursts to be transmitted from a base station in a mobile network on a broadcast frequency in specific time slots of a broadcast frequency frame structure, wherein each time slot is allocated a logical channel. Information about an upcoming burst to be transmitted is first received and then the current state of the logical channel allocated to the upcoming burst is checked. The content of the upcoming burst is then determined according to the current channel state, such that a dummy burst with a base station specific training sequence is transmitted when there is no data to transmit and the dummy burst is not confused with a regular traffic burst according to the current channel state. The dummy burst to be transmitted has a data field that does not contain information understandable to the mobile terminal.
If the channel is unoccupied, wherein no connected mobile terminal is listening to the channel, a dummy burst with a base station specific training sequence may be transmitted. The dummy burst may be a GMSK modulated dummy burst if at least two different modulation forms, including GMSK modulation, are available for the broadcast frequency. The transmitted dummy bursts may belong to dummy frames in the form of filler frames generated at higher layers of the network, such as layer 2 filler frames.
If only a single modulation form is available for the broadcast frequency, a dummy burst with a normal fixed bit pattern may be transmitted when the channel is occupied but there is no data to transmit. On the other hand, if at least two different modulation forms are available for the broadcast frequency, then when the channel is occupied but there is no data to transmit, a dummy burst with a base station specific training sequence may be transmitted using a modulation form other than the modulation form currently used for the assigned logical channel. If GMSK modulation and 8PSK modulation are available for the broadcast frequency, GMSK modulated dummy bursts may be transmitted when 8PSK modulation is currently used for the logical channel, and vice versa.
When a dummy burst with a base station specific training sequence is to be transmitted, the transmitted dummy burst may contain a fixed bit pattern located in the burst on at least one side of the base station specific training sequence. The fixed bit pattern preferably has, at least in part, a low cross-correlation with all possible training sequences defined in the network. The transmitted dummy burst may also contain a fixed bit pattern located in the burst on at least one side of the base station specific training sequence, where the fixed bit pattern is related to that training sequence in a known manner. If at least two different modulation forms are available for the broadcast frequency, the fixed bit pattern may be different for the different modulation forms.
For systems using DTX, any DTX mode may also be disabled for logical channels not using the broadcast frequency application hopping contained in the hopping scheme, so that regular traffic bursts may be transmitted even during silence.
Brief description of the drawings
The invention will now be described in more detail with reference to the accompanying drawings, in which:
fig. 1 is a schematic diagram of a mobile communication network that performs measurements on neighboring base stations.
Fig. 2 is a diagram of an exemplary normal burst.
Fig. 3 is a schematic diagram of a mobile network including a base station transmitting bursts that may be determined according to the present invention.
Fig. 4 is a flow chart illustrating steps performed in a procedure for deciding whether to use DTX for a traffic channel.
Fig. 5 is a flow chart illustrating the steps performed in a procedure for determining the content of a burst to be transmitted according to the present invention.
Fig. 6 is a schematic diagram of a dummy burst according to one embodiment.
Fig. 7 is a schematic diagram of a dummy burst according to another embodiment.
Description of The Preferred Embodiment
In fig. 3, three base stations 300a, 300b and 300c are shown belonging to a mobile network (not shown), of which the base stations 300a-c are all connected to a base station control node 302, e.g. a BSC in GSM, which is adapted to control transmissions from the base stations 300a-c via a logic unit 304. The base station 300a is schematically illustrated in more detail in the figure, comprising an antenna 306, a multiplexer 308 and a logic unit 310.
According to the TDMA frame structure, only a small part of which is shown, the base station is adapted to constantly transmit bursts 312 in consecutive time slots on a broadcast frequency carrier 314. In GSM, for example, the multiframe structure of the BCCH frequency defines how logical channels are mapped onto physical channels. Logic unit 310 in that base station 300a or logic unit 304 in control node 302, or both, may determine the content of the burst to be transmitted from base station 300 a.
The base station is of course also provided with many other necessary components, such as transceivers, filters, etc., but not shown here for the sake of simplicity. Furthermore, the base station may also transmit more physical channels on other carrier frequencies than the broadcast frequency from its antenna, by using combiners or the like, according to well known techniques which are not described further herein.
The time slots in each TDMA frame are associated with particular logical channels C1, C2, C3, etc., which may be any type of channel that is transmitted on a broadcast frequency. The logic unit 310 receives information (not shown) about the content transmitted in the respective logical channel and accordingly creates a bit pattern to be transmitted in consecutive time slots. The created bit pattern is then fed to a multiplexer 308 for sequential transmission as bursts from the antenna 306, according to a given multi-frame structure of the broadcast channel. If logic unit 310 does not receive data to be transmitted, a dummy burst may be created and transmitted, as described in detail below. Thus, in this concept, the logical unit will also be informed (by the BSC in GSM) whenever there is no data to transmit.
In the example shown, the base station 300a transmits data as "normal" bursts, i.e., containing some received data, in channels C1 and C2. However, no data is to be sent in the channel allocated to the next slot, but a dummy burst D is transmitted. Then, channels C4, C5, and C6 require data to be transmitted in their respective slots, but dummy burst D is transmitted in the next slot due to the lack of data to be transmitted, and so on. As described in the background section, the transmission of conventional dummy bursts with a common fixed bit pattern may in some cases create problems, which the present invention aims to circumvent.
The present invention is directed to a solution in which the content of a burst to be transmitted for a particular logical channel is determined based on the current state of that logical channel. In particular, dummy bursts are transmitted in a manner that is not confused with regular traffic bursts. In the context of the present invention, a "dummy burst" may be any type of burst having a data field that does not contain data that is understandable or meaningful to the mobile terminal.
As mentioned above, the use of DTX may cause burst confusion in some cases. When a logical channel is occupied but its time slot is currently unused, indicating that the mobile terminal is connected but currently has no data to transmit, DTX mode is typically applied. The decision to apply DTX mode may be made anywhere in the transmission path depending on the implementation, but is most often made by the network node controlling the base station, e.g. the BSC in the GSM system. During DTX, the connected terminal listens to a particular slot in the frame structure assigned to the traffic channel in order to detect when a regular traffic burst interrupts the DTX mode. In general, DTX does not have any interference-preventing effect if frequency hopping is not applied to the broadcast frequency, i.e. when the traffic channel is transmitted only on the broadcast frequency due to the continuous transmission requirement of that frequency. However, if frequency hopping is applied, the DTX mode may reduce interference to other frequencies used in the frequency hopping scheme. Therefore, the following steps shown in fig. 4 are preferably performed to minimize interference.
In a first step 400, the properties of a particular traffic channel transmitted on a broadcast frequency are checked for frequency hopping. In a next step 402, it is determined whether frequency hopping is to be applied for that channel. If not, then in step 404 any DTX mode is disabled or simply not applied for that channel due to the continuous transmission requirements. Thus, a regular traffic burst will be transmitted in an unused time slot, i.e. when there is no data to send. In a voice call, such a transmitted traffic burst may contain data describing silence, optionally including some "comfort noise", without using DTX. This data will be decoded by the receiver and the perceived effect will be the same anyway for the user of the receiving terminal, since in DTX the comfort noise, which is normally generated locally in the terminal, is often based on received noise parameters, as is well known in the art. As described above, DTX cannot be used on the broadcast frequency to reduce interference due to the continuous transmission requirement, so disabling DTX mode to transmit regular traffic bursts in this case has no adverse effect.
If, on the other hand, frequency hopping is applied to that traffic channel, DTX can advantageously be applied in step 406, since this will reduce interference to other frequencies used in the frequency hopping scheme. But instead a dummy burst is transmitted for that channel on the broadcast frequency in an unused slot, i.e. during DTX, as described below.
Fig. 5 is a flow chart illustrating the steps of determining the content of a burst to be transmitted from a base station, which is valid for a base station configured to use a single modulation format, such as GMSK in a conventional GSM network, for a broadcast channel or to use multiple modulation formats, in this case GMSK and 8PSK in GERAN.
The steps of fig. 5 are performed each time a traffic channel burst is to be transmitted. In this example, other logical channels, such as various point-to-point or point-to-multipoint control channels, are not considered. In a first step 500, information on what data, if any, is to be transmitted in the next traffic channel burst is received, e.g. in logic unit 310 or 304 of fig. 3. This may be a specific payload or control data to be transmitted, or an indication that there is no data to be transmitted. In a next step 502, the status of the traffic channel allocated to the upcoming burst is checked. In the next step, the burst content depends on the current channel state, as described below.
Following step 502, a determination is made in step 504 as to whether the time slot is unused. If a slot is currently in use and there is data to transmit to a connected mobile terminal listening to that particular slot in the frame structure, then a regular traffic burst containing the required data will naturally be transmitted in step 506. Of course, all regular traffic bursts contain base station specific training sequences, as shown in fig. 2, to enable correct identification of the base station.
If it is determined in step 504 that the time slot is not in use, it is determined in a next step 508 whether the existing channel is currently occupied by an active connection. If not, a dummy burst with a Base Station (BS) -specific Training Sequence (TS) is transmitted in the time slot, step 510. Such a dummy burst with a base station specific training sequence is not mistaken by any terminal as a regular traffic burst, since no connected terminal is currently listening to that particular slot in the frame structure, since the channel is unoccupied.
In practice, the transmission of dummy bursts in unoccupied time slots can also be achieved by generating dummy frames in the form of filler frames at higher layers in the network. These filler frames typically have the same format as frames of a normal speech or control channel and are therefore treated in the same way by the base station. To the base station, the filler frames of the speech channel appear as ordinary speech frames, so the corresponding dummy bursts appear as any regular traffic bursts with base station specific training sequences. However, the data within these frames is not controlled or detected by any particular mobile terminal. One example of such a filler frame is a "layer 2 filler frame," which is transmitted in unoccupied signaling blocks on the BCCH channel and the Common Control Channel (CCCH) in GSM.
If the base station is configured to use GMSK modulation as well as 8PSK modulation, the dummy burst of step 510 is preferably GMSK modulated to further assist in burst detection for any measuring terminal. For measurement purposes of neighboring cells, GMSK modulation is generally preferred over 8PSK modulation for the following reasons. 8PSK modulated signals have higher peak-to-average power than GMSK modulated signals, which means that the maximum power supported by the base station is generally lower for 8PSK modulated signals. Depending on the nominal output power setting of the cell, 8PSK modulated signals may be transmitted with reduced average power, resulting in greater uncertainty in link quality and timing measurements compared to GMSK. Furthermore, if an interference suppression algorithm such as SAIC (Single antenna interference Cancellation, defined in 3GPP TSG GERAN, GP-022891, word Item for Single antenna interference Cancellation, nov.2002) is used by the connected terminal, it generally provides a greater gain for GMSK interference than for 8PSK interference. Furthermore, the constant envelope property of GMSK modulated signals may be used to further improve robustness against interference, which cannot easily be done for 8PSK modulated signals, since they do not have such a property.
If it is determined in step 508 that the existing channel is occupied, the following steps will depend on whether the base station is configured to use a single modulation form for the broadcast channel (right arrow) or multiple modulation forms, here GMSK and 8PSK (down arrow).
When using a single modulation form, dummy bursts with a common fixed bit pattern, i.e. without base station specific training sequences, such as the conventional dummy bursts used before, should be transmitted in a next step 512. This avoids the risk of a connected mobile terminal confusing it with regular traffic bursts. Furthermore, when applying frequency hopping to a traffic channel, DTX can safely be used for this traffic channel to reduce interference, as determined in step 406 of fig. 4, among other non-broadcast frequencies comprised in the frequency hopping scheme, since dummy bursts are transmitted anyway independent of DTX on the broadcast frequency.
It should be noted that in this case (step 512), dummy bursts with base station specific training sequences should not be transmitted, since there is a risk that connected mobile terminals occupying the channel will mistake such bursts as regular traffic bursts.
If the base station is configured to use multiple modulation forms, it is determined which of two different modulation forms is used for the existing traffic channel in a next step 514 after step 508. In this example, the base station is configured to use GMSK as well as 8PSK modulation, such as EDGE-supported base stations in GERAN. 8PSK provides a higher bit rate of three bits per symbol than one bit per symbol in GMSK, but 8PSK modulated bursts are more sensitive to background interference and noise, and thus more demanding on link quality. Thus, the network can optionally select the most appropriate modulation format for the session, depending on the link quality and bandwidth requirements of the session. The modulation form may also be switched dynamically during the active session if the situation changes. It should be noted that the present invention is not limited to any particular type and number of modulation forms or network types.
For base stations using a single modulation format (right arrow), a dummy burst with a normal fixed bit pattern is transmitted in step 512 to avoid burst aliasing, but to make measurement of that burst by neighboring base stations more difficult, as described above. However, if a different modulation form (down arrow) can be used, then in the following way, dummy bursts with base station specific training sequences can be transmitted while avoiding burst confusion.
After step 508 it is checked in step 514 which modulation form is currently applied to the existing channel. If GMSK modulation is currently applied, a dummy burst with a base station specific training sequence is transmitted using 8PSK modulation in step 516. On the other hand, if 8PSK modulation is currently applied, a dummy burst with a base station specific training sequence is transmitted using GMSK modulation in step 518. By transmitting such dummy bursts using another modulation form than the current application, the risk of confusion for connected terminals is minimized, since the cross-correlation between the training sequences of the two modulation forms is very low. Thus, a connected mobile terminal will not interpret e.g. GMSK dummy bursts as 8PSK traffic bursts and vice versa even if the Training Sequence Code (TSC) is the same, since the training sequences are actually different in different modulations.
In general, dummy bursts with base station specific training sequences are transmitted using a form of modulation other than that currently used for the allocated logical channels. But in some specific cases it is preferred to use GMSK modulation to transmit 8PSK modulated dummy bursts for the logical channels, not the other way around.
After preparing the burst for transmission in the manner described above, the process may be repeated by returning to step 500 to process the next upcoming traffic burst to be transmitted, and so on.
In case a dummy burst with a base station specific training sequence is to be transmitted, as in steps 510, 516 and 518 described above, the above result can also be improved by selecting the appropriate bit pattern before and after the actual training sequence in the burst.
Fig. 6 schematically illustrates an embodiment of a dummy burst 600 with a base station specific training sequence 602 approximately in the middle of the burst, but it may be located virtually anywhere in the burst. Fixed bit patterns 604a and 604b are included on either side of the training sequence 602. At least one of the two modes, preferably both, has a low cross correlation, i.e. low similarity, with all possible training sequences (eight per modulation form in GSM/GERAN) used in the network including the existing one 602 that is specific to the transmitting base station. Thus, the risk of these parts of the burst being erroneously interpreted as a training sequence by a connected or measuring mobile terminal is minimized. Alternatively, the burst may contain such a fixed bit pattern located only on one side of the base station specific training sequence.
Fig. 7 schematically illustrates another embodiment of a dummy burst 700, again with a base station specific training sequence 702 approximately in the middle of the burst. In this case, fixed bit patterns 704a and 704b are included on either side of training sequence 702, and are both related to that particular training sequence in a known manner. These bit patterns 704a and 704b are therefore also base station specific and will be known to the mobile terminal, so that the receiving terminal can detect and identify the base station with the whole dummy burst to varying degrees, as well as measure link quality and make timing measurements when necessary. Thus further increasing the chance of correct burst detection.
The base station specific bit patterns 704a and 704b preferably have a low cross-correlation with all possible training sequences defined in the network, or at least with the existing one 702. As in the previous example, the fixed bit pattern may also be located on only one side of the base station specific training sequence 702. Furthermore, synchronization of the entire dummy burst may be simplified if the autocorrelation properties of the entire dummy burst are also taken into account when designing the fixed bit pattern in the data field.
If at least two modulation forms are used for the broadcast frequency, the fixed bit patterns 604a, 604b, 704a and 704b are preferably different for the different modulation forms, respectively.
In the above examples of transmission strategies and burst structures, the risk of burst confusion for connected terminals is minimized while maximally facilitating burst detection for the measuring terminals. More specifically, bursts with base station specific training sequences can be transmitted as far as possible without the risk of confusion. This is advantageous for burst detection by mobile terminals, which enables good reception quality (e.g. speech quality) as well as reliable measurements and base station identification. When a relatively tight reuse pattern is employed in a cellular network where several related base stations use the same broadcast frequency, it is of paramount importance to be able to correctly identify the base stations or at least to distinguish between different base stations. The signal received by the measurement terminal will typically contain contributions from more than one base station.
Thus, using bursts with base station specific training sequences will facilitate the measurements, since the receiving terminal only needs to receive the burst during the training sequence interval (although some additional symbols may be needed) in contrast to the whole burst in order to obtain a correct synchronization. The timing estimation will be successfully performed more frequently for the purpose of positioning measurements. In a conventional dummy burst with a normal fixed bit pattern, due to its poor auto-correlation properties, the entire burst must be detected to obtain correct synchronization.
The present invention may be implemented by a computer program product directly loadable into the internal memory of a computer or stored on a computer usable medium. The apparatus for performing the inventive method by running said computer program may be located in a base station and/or a network node controlling a plurality of base stations. Thus, the "intelligence" to determine the content of a burst to be transmitted from a base station of a mobile network may be located in that base station or another network node, such as a BSC, or distributed in both.
While the present invention has been described with reference to specific exemplary embodiments, the description is intended to be illustrative of the inventive concepts and should not be taken to limit the scope of the invention. Various alternatives, modifications, and equivalents may be used without departing from the spirit of the invention, as defined by the appended claims.