WO2003003617A2 - Systemes de communication - Google Patents

Systemes de communication Download PDF

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Publication number
WO2003003617A2
WO2003003617A2 PCT/GB2002/003009 GB0203009W WO03003617A2 WO 2003003617 A2 WO2003003617 A2 WO 2003003617A2 GB 0203009 W GB0203009 W GB 0203009W WO 03003617 A2 WO03003617 A2 WO 03003617A2
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WIPO (PCT)
Prior art keywords
channel
timeslots
communications
carrier
allocating
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PCT/GB2002/003009
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English (en)
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WO2003003617A3 (fr
Inventor
Diana Margaret Ball
Mark Wentworth Rayne
John Francis Wilson
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Sepura Limited
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Publication date
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Publication of WO2003003617A2 publication Critical patent/WO2003003617A2/fr
Publication of WO2003003617A3 publication Critical patent/WO2003003617A3/fr

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/24—Radio transmission systems, i.e. using radiation field for communication between two or more posts
    • H04B7/26—Radio transmission systems, i.e. using radiation field for communication between two or more posts at least one of which is mobile
    • H04B7/2615—Radio transmission systems, i.e. using radiation field for communication between two or more posts at least one of which is mobile using hybrid frequency-time division multiple access [FDMA-TDMA]
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/24—Radio transmission systems, i.e. using radiation field for communication between two or more posts
    • H04B7/26—Radio transmission systems, i.e. using radiation field for communication between two or more posts at least one of which is mobile
    • H04B7/2643—Radio transmission systems, i.e. using radiation field for communication between two or more posts at least one of which is mobile using time-division multiple access [TDMA]
    • H04B7/2656—Radio transmission systems, i.e. using radiation field for communication between two or more posts at least one of which is mobile using time-division multiple access [TDMA] for structure of frame, burst
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J4/00—Combined time-division and frequency-division multiplex systems

Definitions

  • the present invention relates to communications systems, and in particular to mobile communications systems such as private and public mobile telephony (radio) systems.
  • TDMA time division multiple access
  • radio frequency carriers are each composed of a number of discrete timeslots, with each timeslot consisting of a (and usually the same) fixed period of time.
  • TDMA time division multiple access
  • a fixed number of timeslots are grouped together to make a "frame", and the frames repeat regularly in time.
  • communications channels each of which may serve a different user or group of users, are created and destroyed as communications requirements change, with each such channel usually being made up of the same particular timeslots in each frame.
  • a single radio frequency carrier will typically be divided up into one or more channels (depending on the number of timeslots in a frame and how those timeslots are allocated to the communication channels) .
  • Examples of such communications systems arrangements include GSM (particularly in its GPRS form where multiple timeslot allocations are possible) , TETRA and DECT.
  • a single timeslot lasts 0.577 ms .
  • the timeslots are arranged in repeating groups of eight timeslots, each group making up a regularly repeating "frame" of length 4.615 ms .
  • frame of length 4.615 ms .
  • each timeslot lasts 14.167 ms, and there are 4 timeslots in a repeating frame of length 56.67 ms . 18 such frames constitute a multi-frame, and 60 multi-frames constitute a hyper-frame.
  • a transmitting mobile station may transmit on one to four timeslots per frame, and a base station usually transmits on all four timeslots per frame (to the same or differing mobile stations) . On some occasions (e.g. for random access signalling) , a mobile station may transmit short bursts within the first or second half of a timeslot. It is possible to allocate from 1 to 4 timeslots per frame to a single TETRA communications channel to provide different data transmission rate channels.
  • New developments in signal processing now make it possible to increase the number of bits of information that can be transmitted in a given time period, for example by increasing the modulation rate or level of the signalling, or by increasing the band width of the transmission (e.g. by transmitting on more carrier frequencies simultaneously) , or both. It is becoming increasingly desirable to enhance existing communications systems in this manner. However, the Applicants have recognised that when this is done, it would be desirable to maintain backward compatibility with the original transmission format and protocol, and to, for example, use the existing transmission burst structure and protocol methods with the enhanced data rate.
  • TDMA time division multiple access
  • a method of operating a time division multiple access communications system in which plural individual carriers are divided into regularly repeating frames each comprising a predetermined number of timeslots, the method comprising: allocating for use as a communications channel, timeslots on more than one carrier frequency in each transmission frame, and wherein the allocated timeslot pattern repeats for selected succeeding transmission frames.
  • an apparatus for use in a time division multiple access communications system in which plural individual carriers are divided into regularly repeating frames each containing the same number of timeslots, the apparatus comprising: means for allocating for use as a communications channel a pattern of timeslots on more than one carrier frequency in a single transmission frame which repeats for selected succeeding transmission frames .
  • timeslots on different radio frequency carriers are allocated into a single communications channel, thereby providing a higher data rate channel. Furthermore, the same, fixed pattern of timeslots is used for selected successive frames. In other words, communications channels are created by assigning frame-sized patterns of timeslots across multiple carriers. This provides a convenient timeslot structure for the enhanced data rate capacity channel that is in particular compatible with the existing transmission structure, format and protocol, etc., of the communications system.
  • the timeslot pattern preferably repeats for each successive transmission frame until the channel is no longer needed.
  • the allocation of timeslots to a channel can be carried out as desired, and is preferably fully flexible as far as possible within a single frame, e.g. such that timeslots can effectively be allocated to a channel at random within a single frame. It is in particular preferred to be able to use single, isolated timeslots on a carrier in any communications channel. Thus, preferably, a channel can be allocated any unused timeslots in a frame in any pattern or combination whatsoever. Having a more flexible timeslot allocation process helps to facilitate the maximum possible utilization of bandwidth when providing multiple separate channels (unlike, for example, where timeslot allocations have to take place in blocks of fixed dimensions) , since it permits the available radio carrier frequencies to be fully utilised.
  • a given channel can preferably include a different number of timeslots on each different carrier frequency making up the channel .
  • the channel may include simultaneous timeslots on more than one different carrier, this is not mandatory, and the different carriers of the channel could each use different timeslots in the frame.
  • the timeslot allocation is, as far as possible, symmetrical and/or is such that (adjacent) timeslots on a given carrier frequency are at regular (preferably equal) intervals (i.e. the spacings between (adjacent) timeslots are regular (and preferably equal) .
  • the channel would be allocated slots 1 and 3 or slots 2 and 4 on a carrier frequency of the channel (and preferably on all carrier frequencies for the channel) in each TETRA frame of the channel.
  • the Applicants have found that using such a more "balanced" timeslot allocation can help to avoid unwanted frequency peaks in the transmitted signal .
  • a method of operating a time division multiple access communications system in which plural individual carriers are divided into regularly repeating frames each comprising a predetermined number of timeslots, the method comprising: allocating for use as a communications channel a pattern of timeslots on more than one carrier frequency in a single transmission frame which repeats for selected succeeding transmission frames, and in which pattern adjacent timeslots on a carrier frequency of the channel are at regular intervals.
  • an apparatus for use in a time division multiple access communications system in which plural individual carriers are divided into regularly repeating frames each containing the same number of timeslots, the apparatus comprising: means for allocating for use as a communications channel a pattern of timeslots on more than one carrier frequency in a single transmission frame which repeats for selected succeeding transmission frames and in which adjacent timeslots on a carrier frequency of the channel are at regular intervals.
  • the timeslot allocation is such that adjacent timeslots on each carrier frequency used for the channel are at regular (and preferably identical) intervals (i.e. have regular, preferably identical spacings) .
  • timeslot allocation there will be, as will be appreciated by those skilled in the art, some inevitable restrictions on timeslot allocation, such as the constraints of the frame size and number of available carriers, and any pre-existing timeslot allocations that may be in place at the time (i.e. such that some timeslots are not available for the channel as they are already in use) .
  • the timeslot allocation is based (as far as possible) on certain defined criteria.
  • the timeslot assignment for channels to be used by such users is preferably appropriately restricted to carrier frequencies that the user can use.
  • the communications system supports plural different modulation schemes (rates and/or levels) , it is preferably possible for these modulation schemes to be available for the multi-carrier channel.
  • the ability to use different modulation schemes further enhances the flexibility of the channel allocation arrangement and the efficient use of frequency spectrum bandwidth.
  • the system preferably selects which modulation scheme to use for a given channel in accordance with one or more selected, preferably predefined, criteria.
  • the modulation scheme used could be selected based on the desired data rate for the channel and/or the number of available timeslots. It is also the case that at higher modulation levels (for a given transmitter power) , the energy per bit and hence transmission range is reduced. Thus the signal level and/or bit error rate could also be taken into account when selecting which modulation scheme to use.
  • the interleaving scheme used for the multi-carrier channel can be selected as desired, and preferably plural interleaving depths are available for each multi-carrier channel.
  • interleaving does not take place across the carrier frequencies (i.e. the interleaving is carried out independently on each carrier frequency) .
  • the interleaving scheme to vary according to the number of frequencies in use, which would make it difficult to vary the number of frequencies used in a channel (since the interleaving scheme would have to vary as the number of frequencies is varied, thereby leading to variable performance and protocol complexities) .
  • a method of interleaving data to be transmitted on a communications channel comprising plural different carrier frequencies, the method comprising: interleaving the data on each carrier frequency of the channel separately to the data on the other carrier frequencies of the channel .
  • an apparatus for interleaving data to be transmitted on a communications channel comprising plural different carrier frequencies, the apparatus comprising: means for interleaving the data on each carrier frequency of the channel separately to the data on the other carrier frequencies of the channel.
  • carrier frequencies can be added to or removed from a channel as desired, and there is no need for there to be a fixed number of frequencies in use. This again enhances the flexibility (and thus potential efficiency) of the timeslot allocation process.
  • the timeslot pattern, modulation scheme, or both, etc., allocated to a communications channel may be modified while the channel is in use. This could be done, for example, if the channel quality changes, or if higher priority users request channels, or if the users of the channel themselves request a different number of timeslots or data rate per frame.
  • plural simultaneous multi-carrier channels in accordance with the present invention can be assigned (e.g. to different users) . This could be done, for example, using different interlocking timeslot patterns.
  • the timeslots allocated to each channel can be picked from any unused timeslots, i.e. the different channels can be allocated timeslots from all available timeslots.
  • a channel becomes disused its timeslots can be assigned to one or more new channels, as required.
  • each timeslot allocated to a particular channel preferably uses the same modulation scheme (as that will simplify higher-layer protocol considerations, which could otherwise be very complex) .
  • This type of arrangement would require the appropriate communications station's, e.g. base station's transmitter and receiver to be capable of switching their modulation level and/or type in between timeslots. This could be assisted by, for example, the transmitter introducing special phasing bits to avoid large phase discontinuities and consequent spurious emissions at the change-over point, or by the transmitter in a controlled fashion switching off the RF carrier between slots, changing the modulation type and switching the carrier on again.
  • individual channels can use different modulation rates, channel coding and/or interleaving methods (where appropriate) .
  • the radio frequency carriers can (selectively) be used for different purposes, such as for a multiple carrier channel in accordance with the present invention, a standard voice call, etc., with the purpose of an individual carrier being assigned appropriately.
  • the use of the carriers can be altered dynamically in use, so that, for example, at some times they may be dedicated to voice calls and at others to multi-carrier channel use.
  • some or all (or as many as required) of the carriers used for multi-carrier channels could be reassigned to voice call usage . It would also be possible for a carrier to be used simultaneously for both a multi-carrier channel and some other purpose, such as a voice call. In such a case, one or more timeslots on the carrier could be used for a multi-carrier channel, and other, previously free, timeslots, used for a voice call.
  • a given carrier carries both a multi-carrier channel and a voice call
  • the multi-carrier channel and the voice call it is not necessary for the multi-carrier channel and the voice call to use the same modulation type and/or level, although, as discussed above, to be able to do this, the appropriate communications stations, such as base stations, will need to be able to switch between modulation types at timeslot boundaries.
  • the carrier frequencies allocated to a given multi-carrier channel can be selected as desired (subject to the frequencies available to the communications system) .
  • a given channel is made up of carrier frequencies that are close to each other (e.g. within a certain, preferably predetermined, frequency bandwidth) .
  • the channel is made up of adjacent carriers as far as possible, and preferably of adjacent carriers only (whenever that is possible).
  • communications systems typically arrange their carrier frequencies at fixed frequency spacings (usually 25 kHz in TETRA) across the available frequency spectrum) .
  • the group or pool of carriers available for use for the multi-carrier channels extend over a narrow bandwidth and most preferably comprise a, preferably contiguous, block of adjacent carriers.
  • Using closely spaced and adjacent carriers for a multi-carrier channel can help to simplify receiver design. This is because a receiver must be capable of receiving data on all the carriers on which it is being sent. However, where carriers are spaced further apart, it can be difficult and costly (particularly in the mobile station) to design a single receiver that can receive and demodulate all the carriers simultaneously. This could be achieved by using more than one receiver, but that may be undesirable on grounds of equipment size, cost and power consumption. However, it is possible to design a cost effective single receiver that can receive multiple carriers simultaneously, if the carrier frequencies are adjacent (or at least very close) . The degree of difficulty of implementing such a receiver depends strongly on the bandwidth of the group of carriers forming the channel, and hence the use of adjacent carrier frequencies is preferred.
  • adjacent carriers can also have a beneficial effect on the spectral efficiency of a communications system.
  • the adjacent carrier spurious power levels of a number of communications systems are quite high, which can mean that two neighbouring cells cannot use adjacent carriers due to the unacceptably high levels of interference that might result .
  • GSM communications systems
  • a seventh aspect of the present invention there is provided a method of allocating radio frequency carriers in a communications system to a communications channel that uses plural radio frequency carriers, the method comprising: allocating as far as possible adjacent frequency carriers to the channel .
  • an apparatus for allocating radio frequency carriers in a communications system to a communications channel that uses plural radio frequency carriers comprising: means for allocating as far as possible adjacent frequency carriers to the channel .
  • a communications system in which multiple-carrier frequency channels can be provided, wherein the carrier frequencies available for use in multi-carrier channels comprise a contiguous block of adjacent carrier frequencies.
  • One potential problem with using a limited pool of adjacent carriers for the multi-carrier channels is that that could lead to interference problems, in particular so-called “near-far effect" interference problems when a receiver has to receive two such channels simultaneously (e.g. such as might be the case for base station receivers on the uplink (the communications link between the mobile transmitters and the base station receivers)) .
  • This interference arises where a (e.g. mobile) station which is close to a receiving (e.g. base) station (thereby presenting it with a strong signal) , causes interference to the receiver on one of the other carriers used in the system on which the receiver is receiving a weak signal from another more distant station.
  • This type of interference is usually caused either by spurious power transmitted by the closer station on either side of its wanted transmission frequency, and/or by inadequacies in the receiver (usually termed blocking) .
  • This problem increases as the frequency spacing between the carriers decreases (and can therefore be avoided by allocating carriers with considerable frequency spacing between them (but as noted above this may not be desirable) ) .
  • the timeslot and carrier channel allocation process preferably takes account of possible near-far effect interference problems.
  • the channel allocation (both in terms of timeslots and carriers) is based on a signal strength indication (e.g. the actual received signal strength, or preferably an average thereof, e.g. over a few seconds (to increase the accuracy of the measurement) ) for the received signal (e.g. uplink signal) and/or for the other signals currently being received.
  • a signal strength indication e.g. the actual received signal strength, or preferably an average thereof, e.g. over a few seconds (to increase the accuracy of the measurement)
  • the received signal e.g. uplink signal
  • a method of allocating timeslots and carrier frequencies to a communications channel in a communications system in which channels comprising multiple carriers are possible comprising: allocating timeslots and/or carriers to the channel on the basis of an indication of the signal strength of a signal that is to use the channel and/or on the basis of an indication of the signal strength of a signal that is using an existing channel of the communications system.
  • an apparatus for allocating timeslots and carrier frequencies to a communications channel in a communications system in which channels comprising multiple carriers are possible comprising: means for allocating timeslots and/or carriers to the channel on the basis of an indication of the signal strength of a signal that is to use the channel and/or on the basis of an indication of the signal strength of a signal that is using an existing channel of the communications system.
  • a strong signal e.g. above a particular, predetermined indicated signal strength
  • any new channels, particularly having weak signals are allocated in timeslots that the strong signal does not exist in (does not overlap with in time), i.e.
  • a call or requested channel will have a strong signal (high signal strength, e.g. above some predetermined value)
  • This can be achieved, for example, by using the highest possible data rate for the call (e.g. by using the highest available modulation level) , which can, for example, either reduce the time for the call (e.g. data transfer), or the number of timeslots used for the call per frame.
  • system can additionally or alternatively alter existing channel allocations to enable a new channel to be assigned timeslots where the possibility of interference is reduced .
  • power control techniques are used to try to ensure that the signals from all transmitters (e.g. mobile stations) arrive at the receiver, e.g. base station, at the same power level (as that would avoid the problem of strong and weak signals interfering (and therefore reduce the need to allocate timeslots as discussed above to try to avoid interference) ) .
  • the first such way often referred to as "open loop” power control, involves, as is known in the art, a mobile station measuring the signal strength received from the base station and using that measurement together with, e.g., offsets and limits based on the required signal level at the base station and the difference in the output of its transmitter compared to the base station, to determine the power output to use for its transmitter.
  • closed loop power control usually known as "closed loop” power control (e.g.
  • the timeslot and carrier allocation may be base the timeslot and carrier allocation on the basis of some form of signal strength indication other than the actual received signal strength. For example, it may be preferable to do it on the basis of what the signal strength would be if power control was not being applied, and/or, for example where this information may not be fully available, on the basis of the (preferably averaged) signal level at call setup or on a random access channel (particularly if power control is not used for random access) , or the level of power control being used (e.g. how much power control the system is asking the mobile station to use) , as well as or instead of on the (preferably averaged) actual received signal level .
  • some form of signal strength indication other than the actual received signal strength. For example, it may be preferable to do it on the basis of what the signal strength would be if power control was not being applied, and/or, for example where this information may not be fully available, on the basis of the (preferably averaged) signal level at call setup or on a random access channel (particularly if power control is not
  • a further consideration with the ability of power control to reduce interference effects relates to its use or otherwise in call setup procedure and random access attempts. If power control is not used for random access attempts, then a wide range of signal levels could be received for such attempts. Thus, where random access and call setup attempts are made on a selected control channel frequency (carrier) , as is usually the case, it is preferred for the control channel (or channels) carrier (s) to be spaced in frequency from the main contiguous group or pool of carriers used for main channel allocation, or at least to be placed on a carrier at the edge of the adjacent contiguous frequency (carrier) group, so that the control channel is less likely to be a source of interference .
  • the above channel allocation criteria can also be used for downlink channel allocations, and this is in particular preferably done where downlink power control is being used (as downlink power control can cause interference problems at the mobile receiver) .
  • a user e.g. mobile station or other party, requesting a channel or call or communication includes in their request information about the type of communication (channel) desired.
  • the system should then respond appropriately and as far as possible, and may grant, for example, a multi-carrier channel in accordance with the present invention, or a normal single carrier channel, etc., depending on, for example, system resources and the channel requested.
  • the information in the request could be, for example, an indication of the number of timeslots required for the channel .
  • the system is not obliged to offer the channel requested (e.g. in terms of its data capacity) and could, for example, if resources are limited, offer a different channel or no channel at all.
  • the channel or communication request includes an indication of the data rate or capacity desired for the communication (channel) .
  • the standard data capacity timeslot used for this purpose is preferably a timeslot using one of the modulation schemes available to the system. It is preferably the lowest data capacity modulation scheme, as then timeslots can be requested in integer values.
  • a method of operating a communications system in which communications channels comprised of plural different carrier frequencies can be used comprising: a station of the communications system transmitting a request for a communications channel, and including in its request an indication of the data transmission rate or capacity required for the communications channel ; and the system in response to such a request allocating a communications channel comprised of timeslots on plural different carriers, wherein the timeslots allocated to the channel are based on the indicated data transmission rate or capacity.
  • a communications system in which communications channels comprised of plural different carrier frequencies can be used, the system comprising : a communications station comprising means for transmitting a request for a communications channel, and including in its request an indication of the data transmission rate or capacity required for the communications channel; and the system further comprising means for, in response to such a request, allocating a communications channel comprised of timeslots on plural different carriers, wherein the timeslots allocated to the channel are based on the indicated data transmission rate or capacity.
  • a communications station of a communications system in which communications channels comprised of plural different carrier frequencies can be used, the communications station comprising: means for transmitting a request for a communications channel, and including in its request an indication of the data transmission rate required for the communications channel .
  • a communications station of a communications system in which communications channels comprised of plural different carrier frequencies can be used, the communications station comprising: means for receiving a request for a communications channel which includes an indication of the data transmission rate required for the communications channel; and means for, in response to such a request, allocating a communications channel comprised of timeslots on plural different carriers, wherein the timeslots allocated to the channel are based on the indicated data transmission rate or capacity.
  • the data to be transmitted can be loaded into the timeslots allocated to the communications channel as desired.
  • the data to be transmitted is placed in the timeslots of a given multi-carrier channel according to a predetermined ordering scheme. This helps to allow a receiver to reassemble the data in the same order as it was transmitted. While any suitable ordering scheme would be possible, so long as it is known to both the transmitter and the receiver, in a preferred such scheme, the loading of the timeslots is ordered firstly by timeslot (i.e. filling up the earliest timeslots in the frame first and so on, with the last timeslots being filled last) , and preferably then by carrier number (preferably counting from the lowest frequency carrier to the highest, or vice-versa) .
  • Ordering by timeslot first ensures that data which is closely related is transmitted more closely in time, thereby, for example, reducing the delay time to request a retransmission of faulty timeslots.
  • a method of transmitting data on a communications channel which uses timeslots on plural carrier frequencies, comprising: allocating the data to the timeslots of the channel for transmission in accordance with a predetermined ordering scheme .
  • an apparatus for transmitting data on a communications channel which uses timeslots on plural carrier frequencies comprising: means for allocating the data to the timeslots of the channel for transmission in accordance with a predetermined ordering scheme .
  • the channel allocation, etc., techniques of the present invention can, and preferably are, used for both uplink and downlink channels. It is also preferred that such arrangements can be asymmetric as between uplink and downlink pairs, e.g. as regards carrier allocations and modulation schemes. This could result in different (independent) uplink and downlink slot assignment patterns.
  • a method of operating a communications system comprising allocating an associated uplink and downlink communications channel pair to users of the system, wherein the timeslot pattern allocated to the uplink channel is different to the timeslot pattern allocated to the downlink channel.
  • the present invention is applicable to communications systems and particularly mobile communications systems, generally, it is particularly applicable to the TETRA mobile communications system.
  • the present invention also extends to a TETRA system incorporating any or all aspects and features of the present invention.
  • the methods in accordance with the present invention may be implemented at least partially using software e.g. computer programs. It will thus be seen that when viewed from further aspects the present invention provides computer software specifically adapted to carry out the methods hereinabove described when installed on data processing means, and a computer program element comprising computer software code portions for performing the methods hereinabove described when the program element is run on data processing means.
  • the invention also extends to a computer software carrier comprising such software which when used to operate a communications system or station comprising data processing means causes in conjunction with said data processing means said system or station to carry out the steps of the method of the present invention.
  • Such a computer software carrier could be a physical storage medium such as a ROM chip, CD ROM or disk, or could be a signal such as an electronic signal over wires, an optical signal or a radio signal such as to a satellite or the like. It will further be appreciated that not all steps of the method of the invention need be carried out by computer software and thus from a further broad aspect the present invention provides computer software and such software installed on a computer software carrier for carrying out at least one of the steps of the methods set out hereinabove .
  • Figure 1 shows an example of timeslots on a single radio frequency carrier
  • Figure 2 shows an example of six radio frequency carriers
  • Figure 3 shows examples of multi-carrier channel timeslot allocations in accordance with the present invention for the radio frequency carriers of Figure 2;
  • Figures 4 to 10 show examples of signalling messages that can be used in a TETRA communications system that can operate in accordance with the present invention; and
  • Figures 11 and 12 illustrate further timeslot allocation arrangements.
  • Figure 1 shows an example of timeslots in a TDMA communications system on a single radio frequency carrier 10.
  • the carrier is arranged in regularly repeating frames 11 (frame 1) , 12 (frame 2) , 13 (frame
  • each frame comprising four timeslots.
  • the single radio carrier is divided into different channels, each of which may serve a different user or group of users.
  • a first channel 14 uses timeslot 1 of each frame
  • a second channel 16 uses timeslot 4 of each frame
  • a third channel 15 uses timeslot 2 and 3 of each frame. Since the third channel has twice the number of timeslots per frame as the other two channels, it offers users twice the data rate.
  • the channels shown in Figure 1 can be destroyed and new channel arrangements created.
  • radio transceiver As discussed above, as a result of new processing capabilities it is now becoming possible for a radio transceiver to transmit on multiple radio carriers simultaneously and to receive on multiple radio carriers simultaneously. It is also possible to use higher levels of modulation (which increase the number of information bits that may be transmitted in a single modulation symbol) . Both of these techniques can be combined to increase the data capacity of radio channels .
  • Figure 2 shows a first preferred embodiment of a method of providing higher data rate (capacity) channels in which timeslots on multiple individual radio frequency carriers are allocated into a single communications channel .
  • Figure 2 shows six radio frequency carriers 21 to 26, each being transmitted in the same TDMA frame structure 27, 28, 29 as shown in Figure 1.
  • the frames and timeslots of each carrier frequency shown in Figure 2 are aligned in time and synchronised. This is a typical arrangement for a TETRA or GSM system.
  • each transmitter or receiver would be able to transmit or receive a single carrier and thus a base station would require six transmitters and receivers for the six radio frequency carriers shown in Figure 2 , and a mobile station would transmit or receive one or more timeslots per frame on one of the carriers, or, if in a duplex call, transmit timeslots on one carrier and receive timeslots on a different carrier.
  • a mobile station it is now possible for a mobile station to be given the ability to transmit and/or receive more than one (perhaps up to six) carrier frequencies simultaneously. In such a case, it would be possible to allocate a communications channel consisting of simultaneous timeslots on different carrier frequencies.
  • TDMA carriers are allocated to logical channels is facilitated.
  • Figure 3 gives some examples of suitable channel timeslot allocations. Assuming, for example, that a data rate corresponding to six timeslots per frame
  • channel a which consists of one slot on each of carriers one to six, providing a data rate of six slots per frame, could be used.
  • channel b in Figure 3 provides a rate of three slots per frame, consisting of slot 2 on carriers 1, 2 and 3.
  • Channel c is a more dispersed allocation providing a data rate of six slots per frame, these being slot 3 on carriers 1 to 4 and slot 4 on carriers 3 and 4.
  • Channel d in Figure 3 consists of slot 2 on carriers 5 and 6, slot 3 on carrier 6 and slot 4 on carriers 1 and 2.
  • the slot allocations can be very dispersed. This could be for many reasons, such as historical slot availability. For example, if in the example shown in Figure 3 a new channel e was required, a four-slot allocation could be provided from carrier 4 slot 2, carrier 3 slots 3 and 4 and carrier 6 slot 4.
  • the embodiments shown in Figures 2 and 3 use only six carriers and have four timeslots per frame, the embodiment (and the present invention) can, as will be appreciated by those skilled in the art, be extended to any number of timeslots and carriers. In particular, even though individual communication stations (e.g.
  • the radio system should take account of the bandwidth of individual mobile stations and restrict their timeslot assignments appropriately.
  • one of the channels could be restricted to an RF bandwidth or multi-carrier capability of only four carriers if that is all that a mobile station using the channel can handle.
  • the use of carriers for this purpose can be altered dynamically, such that at some times the carriers may be used for other purposes, such as voice calls, and at other times, would be used for multi-carrier data channels.
  • a carrier might at some time be removed from the pool of available multi-carrier data channels (which may mean that all data channel allocations have to be re-assigned to use available timeslots on the remaining carriers (or dropped) ) , and might later be returned. This could be particularly useful in a communications system used for public safety operations.
  • some or all (or as many as required) of the carriers being used for multi-carrier data communications could be re-assigned by the system for use for the emergency voice calls. It should be noted that in such arrangement it may not be necessary to completely remove a carrier from multi-carrier use in the event that additional capacity of voice calls is required, but rather one or more spare timeslots on a given carrier may be used for a voice call while other timeslots are still used for a multi-carrier channel.
  • slot 2 of carrier 4 is unused and so may be allocated to a voice call of single slot capacity, while the remaining slots on carrier 4 are still used for multi-carrier channels.
  • the modulation level or type for the voice call may differ from the modulation being used for the multi-carrier channel.
  • the base station transmitter must be capable of switching its modulation level or type in between timeslots, and for the base station receiver to be able to switch between modulation levels or type in the same way. This could be further facilitated by, for example, the base station transmitter introducing special phasing bits to avoid large phase discontinuities and consequent spurious emissions at the change-over point, or alternatively, the transmitter could (in a controlled fashion) switch off the RF carrier between slots, change the modulation type, and switch the carrier on again.
  • channel a might use standard TETRA ⁇ /4 DQPSK
  • channel b might use 16-QAM
  • channel c might use 32-QAM
  • channel d might use 64-QAM.
  • each timeslot allocated to a particular multi-carrier channel preferably uses the same modulation type, as that will simplify higher-layer protocol considerations, but timeslots allocated to different multi-carrier channels can use different modulation types.
  • the modulation scheme to be used should be chosen appropriately, for example based on the required data transmission rate for the channels. It is also the case that the energy per bit, and therefore the range, is reduced at higher modulation levels. Therefore the radio system (base station) preferably measures the signal level or bit error rate when the data connection is requested, and from time-to-time thereafter, and uses that information to help it choose the most appropriate modulation scheme.
  • each timeslot allocated to a particular channel preferably uses the same interleaving depth (to simplify higher-layer protocol considerations, which would otherwise be very complex, and to provide a constant time delay) , but timeslots allocated to different channels could use different interleaving depths.
  • channel a could use one-slot interleaving
  • channel b could interleave across four frames
  • channels c and d could interleave across eight frames (which would mean, for example, that for channel c, timeslot 3 on carrier 4, for example, would be interleaved with timeslot 3, carrier 4 of the next eight frames) .
  • While the carrier frequencies made available for multi-carrier channels and allocated to a given multi-carrier channel in the present embodiment can be allocated as desired, and in particular do not need to be adjacent or contiguous, as discussed above it is preferred to use adjacent carriers as far as possible, as a receiver of the multi-carrier channel will require an intermediate frequency bandwidth that is wide enough to receive all of the group of carriers and this is more straightforward when the carriers are close together. Thus, using adjacent carriers is therefore much more cost-effective than when the carriers are spaced further apart .
  • the base station receiver system (it may have a separate receiver for each carrier or a single receiver with a wide enough bandwidth and the appropriate demodulation system for receiving all of the carriers) is likely to suffer from adjacent channel interference due to its own limitations or spurious transmitted power from the mobile station using channel A.
  • this problem can be completely eliminated if the call from the more distant mobile station is placed on timeslot 3 or 4 of any carrier, as the strong mobile station (channel A) is not transmitting during those slots (and the interference protection between timeslots is infinite) .
  • the system can reduce the likelihood of interference by carefully choosing which timeslots and carriers it allocates to a particular channel or call on the basis of the signal strength.
  • power control is also used to try to reduce the effects of interference.
  • power control can be used on the uplink to control the power of the mobile stations in the radio system more accurately at all times so that the signals received by the base station receivers are always at the same power level .
  • the type of power control used can be selected as desired, and could, for example, be one or other or both of the known techniques referred to as open loop power control and closed loop power control .
  • Closed loop power control is preferred, because it is the uplink signal of the base station receiver that is being both measured and controlled, whereas in open loop power control it is the downlink that is measured but the uplink that is controlled (which may lead to inaccuracies because the uplink and downlink frequencies are usually some MHz apart and thus too far apart for the amplitude fading to be correlated) . While power control can help to avoid interference fades, as discussed above, it may not allow them to be avoided in their entirety. Thus even where power control is being used, it is still preferred to allocate timeslots and carriers to channels in accordance with the criteria discussed above, such as signal strength. However, in such a case, it may be that the actual received signal strength is not necessarily the best signal strength measurement on which to base the timeslot and carrier allocation.
  • the allocation could be based on the signal level at call setup or on a random access channel (e.g. if power control is not used on random access signals) , or on the level of power control being used or how much power control is being applied (e.g. how much power control the system is asking the mobile station to use) , or on the actual received signal strength (e.g. where it is below the level of onset of power control or above its top limit or where imperfect power control such as open loop power control is being used) .
  • a further issue with the ability of power control to remove near-far interference lies with its use in random access attempts and the call setup procedure. If power control is used during random access then that will reduce near-far interference. However, using power control for random access can result in signals from mobiles transmitting in the same slot to gain access or being at a similar strength, thereby ending up with none of them gaining access and thus extending call setup times . It may therefore be preferred not to use power control on random access channels so as to try to ensure a big enough difference in signal strength between simultaneously transmitting mobiles for one of them to gain access on the first attempt .
  • Random access and call setups are usually done on a control channel and if power control is not being used for random access, then there would be a wide range of signal levels used on the control channel. It is therefore preferred in such situations to place the control channel where it is less likely to be a cause of near- far interference.
  • This may be done by having one or more carriers used for control purposes spaced in frequency from the main contiguous group of carriers to be used for in particular multi-carrier channels. These spaced carriers may be used for all normal purposes in the communications system as well as control channel purposes, except that they are preferably not used where it is necessary to provide a multi-carrier channel. Where this is not possible, the control channel is preferably at least placed on a carrier at the edge of the group of closely spaced channels as it is then less likely to be a source of interference.
  • the base station transmitter when downlink power control is used, the base station transmitter would turn its power down when in a call with a nearby mobile station but calls to other mobile stations at the edge of coverage on other carriers will be sent to the base station's full power and may therefore cause interference. (This situation is very similar to that described for uplink power control).) It is preferred therefore, particularly where downlink power control is used, to also use the above criteria and considerations for the allocation of timeslots and carriers on the downlink particularly when using closely spaced or adjacent carriers. It can be seen from the above that the assignment of timeslots and carriers on the basis of signal strength or potential signal strength (e.g. in a system where power control is used) allows the use of adjacent carriers for multi-carrier channels but with reduced near-far interference problems on the uplink.
  • adjacent carriers also allows the use of relatively simple radio receivers instead of wide band receivers or multiple receivers to receive more than one carrier simultaneously.
  • the use of adjacent carriers can also have a significant beneficial effect on the spectral efficiency of the communications system.
  • mobile stations or other parties requesting a data connection from the system give an indication of their transmission requirements in their channel or communication request.
  • Such an indication could, for example, be given in terms of the number of timeslots per frame.
  • the communications system would then attempt to provide the requested number of slots in a pattern of the radio system's choice. (However, it should be noted that the communications system is not obliged to offer the number of timeslots requested, and, for example, if resources are limited it could offer a reduced number of timeslots or none at all.)
  • channels of various modulation types can be used, which means that the actual data capacity of a given timeslot will depend upon the modulation type being used.
  • standard TETRA ⁇ /4 DQPSK modulation encodes two bits per modulation symbol
  • 16-QAM encodes four bits per symbol
  • 64-QAM encodes six bits per symbol.
  • the capacity of a timeslot modulated with 16-QAM is twice that of a ⁇ /4 DQPSK modulated timeslot with the same symbol rate.
  • the capacity of a 32-QAM timeslot would be 2.5 times that of a ⁇ /4 DQPSK timeslot using the same symbol rate, and the capacity of a 64-QAM timeslot would be three times that of a ⁇ /4 DQPSK timeslot using the same symbol rate.
  • a party requesting a data communications channel requests the channel in terms of the data rate or capacity required.
  • a request could be in terms of the absolute data rate required, but preferably is given in units of equivalent timeslots per frame, where an equivalent timeslot is a timeslot of a standard predetermined data capacity and is the size of a timeslot of a specified modulation type (preferably the lowest modulation level, so that timeslots can be requested in integer values) .
  • an equivalent timeslot is a timeslot of a standard predetermined data capacity and is the size of a timeslot of a specified modulation type (preferably the lowest modulation level, so that timeslots can be requested in integer values)
  • the equivalent timeslot could be the data capacity of a standard TETRA ⁇ /4 DQPSK timeslot, and where higher modulation levels such as 16-QAM, 32-QAM, etc. are available, timeslots modulated using those higher level modulation schemes will have a capacity of plural equivalent timeslots (two equivalent timeslots in the case of 16-QAM, 2.5 equivalent timeslots in the case of 32-QAM, etc.) .
  • the user of channel a in Figure 3 could have requested a rate of six equivalent timeslots and been granted six ⁇ /4 DQPSK timeslots.
  • a user of channel b might have requested six equivalent timeslots per frame and been granted three 16-QAM timeslots (since each 16-QAM timeslot provides two equivalent timeslots) .
  • Users of channel c might have requested fifteen equivalent timeslots per frame and been granted six 32-QAM timeslots each of capacity 2.5 equivalent timeslots.
  • Users of channel d might have requested fifteen equivalent timeslots per frame and been granted five 64 -QAM slots each of capacity three equivalent timeslots, for example if the channel to these users was determined to be of excellent quality.
  • channel d can efficiently provide users with many equivalent timeslots while using a minimal number of the timeslots available per frame, thereby leaving more timeslot capacity available for other users .
  • a base station of the system it is possible for a base station of the system to modify the pattern and modulation level (or both) of the timeslots allocated to a given channel, for example if the channel quality changes or if higher priority users request channels, or the users of the channel themselves request a different number of equivalent timeslots per frame .
  • the data to be transmitted on a given communications channel is placed into the timeslots of the channel in accordance with a predetermined ordering (numbering) scheme. This allows a receiver to reassemble the contained data in the same order as it was transmitted.
  • the timeslots are ordered firstly by timeslot, and then by carrier number, counting from the lowest frequency carrier to the highest frequency carrier (although the reverse direction of frequency would work just as well, as would any pre-agreed ordering, as will be appreciated by those skilled in the art) .
  • carrier number counting from the lowest frequency carrier to the highest frequency carrier
  • the timeslots of channel a are ordered in the sequence : slot 1 carrier 1 slot 1 carrier 2 slot 1 carrier 3 slot 1 carrier 4 slot 1 carrier 5 slot 1 carrier 6.
  • the timeslots of channel b are ordered in the sequence : slot 2 carrier 1 slot 2 carrier 2 slot 2 carrier 3
  • the timeslots of channel c are ordered in the sequence : slot 3 carrier 1 slot 3 carrier 2 slot 3 carrier 3 slot 3 carrier 4 slot 4 carrier 3 slot 4 carrier 4
  • Figures 4 to 10 show examples of signalling messages which may be used to provide the flexible channel assignments of the present embodiment and invention.
  • the example signalling messages shown are based on an adaption of the TETRA protocol, particularly that relating to the transmission of packet data.
  • Figure 4 shows a modified extended services broadcast information element. This element is included in the SYSINFO message broadcast by TETRA base stations. This SYSINFO message is used by mobile stations searching for a new radio base site, and the modified extended services broadcast information element of Figure 4 is used to indicate to a searching mobile station whether the radio system supports the use of higher-level modulation and multi-carrier operation.
  • the HLM sub-element in the extended services broadcast information element of Figure 4 indicates which modulation types the base station supports.
  • TC 32-QAM and TC 64 -QAM referred to in Figure 4 are trellis-coded versions of 32-QAM and 64-QAM respectively (trellis coding is a particularly effective way of improving the channel coding of the data, albeit at the cost of reducing the effective bit rate per symbol) . )
  • FIG. 5 shows the contents of an exemplary modified U-LOCATION UPDATE DEMAND protocol data unit (PDU) .
  • PDU protocol data unit
  • Figure 6 shows the contents of the "extended capabilities" information element of Figure 5. This element allows the mobile station to indicate to the radio system which higher level modulation types it supports, and how many carriers it can receive and transmit simultaneously.
  • FIG. 7 shows an exemplary AL-SETUP protocol data unit, which a mobile station can use to request a TETRA "advanced link" .
  • This protocol data unit could be used, for example, to request a channel for transmission of packet data.
  • This protocol data unit includes fields whereby the mobile station can request a number of equivalent timeslots per frame, up to a maximum possible value of 60 equivalent timeslots (see Notes 9 and 11 in Figure 7) .
  • the TETRA system would determine how it should allocate the requested equivalent timeslots in terms of numbers of carriers and the modulation level to be employed.
  • the radio system uses the capabilities of the mobile station declared during registration in the extended capabilities information element of the U-LOCATION UPDATE DEMAND protocol data unit as discussed above.
  • Figure 8 shows an exemplary MAC-RESOURCE protocol data unit. This message is used by the base station to allocate channel resources to the requesting mobile station, via a "channel allocation element" included in the message.
  • Figure 9 shows the contents of the channel allocation information element of the MAC-RESOURCE protocol data unit of Figure 8. This element indicates which timeslots (1 to 4, or any combination) have been allocated on the lowest (or it could equally well be the highest) allocated uplink and downlink carriers, and which modulation type is to be used on the channel.
  • the channel allocation information also indicates how many, if any, further carriers have been allocated to the uplink and downlink channels in the "allocation for further carrier" information element. That information element is shown in more detail in Figure 10. It is repeated for each additional carrier and gives the uplink and downlink timeslot allocation for each additional carrier. Asymmetric uplink and downlink carrier and modulation, etc., allocations are possible. It can be seen from the above that these various messages and information elements described give a means of providing a completely flexible allocation of timeslots to channels, in accordance with the present invention.
  • the present invention provides a method of allocating timeslots in a multi-carrier communication channel, in which a channel can be made up of any number or combination of timeslots of equal data capacity.
  • the allocated timeslot pattern repeats in frames but otherwise can be arranged as desired. This allows greater flexibility in adding and removing channels of varying sizes. It is particularly useful for high-speed data communication where multiple radio carriers may be combined to provide the required data rate .

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Mobile Radio Communication Systems (AREA)
  • Time-Division Multiplex Systems (AREA)

Abstract

L'invention concerne un système de communication dans lequel les stations de communication peuvent émettre et recevoir plus qu'une fréquence porteuse. Des canaux de communication constitués de tranches de temps simultanées, sur différentes fréquences porteuses, sont affectés à des parties demandant un appel, en fonction du débit de données particulier requis pour l'appel. Les systèmes déterminent à combien de tranches de temps par trame correspond le débit de données requis, puis il affecte le nombre approprié de tranches de temps au canal. Bien qu'un nombre entier de tranches de temps par trame doivent être affectées au canal, les tranches de temps peuvent, dans d'autres cas, être affectées de n'importe quelle façon désirée. Ainsi, par exemple, un débit de données correspondant à six tranches de temps par trame peut être assuré par un canal (canal a) qui est constitué d'une tranche, sur chacune des six porteuses différentes, ou bien par une affectation plus dispersée de la tranche 3 sur les porteuses 1 à 4, et de la tranche 4 sur les porteuses 3 et 4 (canal c). Un débit de trois tranches par trame pourrait être assuré par un canal (canal b) constitué de la tranche 2 sur les porteuses 1, 2 et 3. Ces répartitions des tranches se répètent pour des trames successives.
PCT/GB2002/003009 2001-06-29 2002-06-28 Systemes de communication WO2003003617A2 (fr)

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US9088384B2 (en) 2005-10-27 2015-07-21 Qualcomm Incorporated Pilot symbol transmission in wireless communication systems
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GB0116015D0 (en) 2001-08-22
WO2003003617A3 (fr) 2003-06-12

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