GB2415870A

GB2415870A - Adaptive OFDM multi-carrier system using subsets of best quality channels.

Info

Publication number: GB2415870A
Application number: GB0414659A
Authority: GB
Inventors: Hokyu Choi; Thierry Lestable
Original assignee: Samsung Electronics Co Ltd
Current assignee: Samsung Electronics Co Ltd
Priority date: 2004-06-30
Filing date: 2004-06-30
Publication date: 2006-01-04
Anticipated expiration: 2024-06-30
Also published as: GB2415870B; GB0414659D0

Abstract

In an adaptive multicarrier Orthogonal Frequency Divisional Multi-Access (OFDM) system, the receiver includes index data in the control signals (fig. 8) fed back to the transmitter uplink which identifies a predetermined number Lc of subchannels of the best quality and indicates their Signal to Noise Ratios (SNR), as measured from a pilot signal. The number of subchannels Lc is varied in inverse relation to the number of terminals such that the capacity remains monotonic. The index portion is coded as a multi-bit tuple comprising a number of bits B capable of indicating all possible combinations of the Lc subchannels, and the channel quality metric is indicated by m data bits, thus reducing the capacity required on the uplink channel compared to sending a separate indication of the subcarrier number for each channel.

Description

MULTI-CARRIER COMMUNICATIONS

This invention relates to multi-carrier (MC) communications such as those using frequency division multiple access (FDMA), preferably orthogonal frequency division multiple access (OFDMA). It relates to systems, transmitters, receivers and transceivers using FDMA, and for methods of transmitting and receiving a signal, and a signal thus produced.

Orthogonal frequency division multiplexing (OFDM) is a multi-carrier modulation technique, which has been used or proposed for use in wireless tans (WLAN) such as IEEE 802.1 l a/g, in terrestrial digital video broadcasting (DVB-T), digital audio broadcasting (DAB) and next generation (4G) and mobile communications. The present invention is concerned with two way communications having control signalling uplink (from-terminal) and data downlink (to-terminal) channels.

In OFDMA, a number of different terminal equipments (such as mobile terminals) share a common set of frequency sub-carriers. The terminals may, for example, all be located within a given cell of a mobile communications network. Different terminals are allocated different carriers, so that there is no interference between those of different terminals (orthogonal multiple access). A data stream is channel coded to provide error protection, and then distributed across the subset of downlink subcarrier channels allocated to each terminal, and transmitted.

In a radio transmission system, interference sources will sporadically generate noise which will selectively affect different subcarriers. Also, transmission conditions will affect the response on each channel: for example, multi-path effects due to multiple reflections will affect both the gain and phase on each subcarrier differently. These effects are experienced differently by different terminals, depending on their positions relative to the interference sources and reflectors. They will also vary over time, particularly where the terminals are themselves mobile.

In adaptive multi-carrier OFDMA, the coding rate and modulation level are controlled and adapted in dependence on the channel condition - i.e channel quality - and quality of service requirements. For example the signal to noise ration (SNR) per subcarrier can be used as a reference measure for such quality evaluation.

Thus, it is in general possible to select a subset of subcarriers for the different terminals so that each terminal experiences sufficiently good quality transmission. This in turn means that lower power or higher order modulation or a higher coding rate can be used to communicate with the mobiles, which improves the capacity.

Accordingly, each mobile terminal monitors the channels (sub carriers) it can receive, and supplies uplink feedback information on the quality of each channel, from the point of view of that terminal, to the base station. The quality on the different channels is monitored by transmitting a downlink pilot signal at periodic intervals. Use of a pilot signal is generally important with OFDM transmission in any event, since the channels are closely spaced in frequency domain so that phase and frequency errors must be tightly controlled. Typically, the pilot signal comprises a known pilot symbol; the pilot symbols may be on every carrier or every Nth carrier, or in regular patterns distributed over time and frequency (depending on channel coherence bandwidth and coherence time). At the receiver, since the position in time and frequency of the pilot signal is known, as is the value of the pilot symbol, the receiver can calculate the channel properties (i.e. distortion and noise).

It will be appreciated, however, that if there are multiple terminals, each monitoring multiple frequencies, the volume of data signals on the uplink to the base station can become very large. This information is referred to as the uplink feedback overhead. For example, in proposed next generation multi-carrier system proposals, thousands of sub-carriers may be used with hundreds of mobile terminals. The problem is increased where higher numbers of modulation levels are used. Figure 10 shows the increase in the number of signalling bits required at relatively small numbers of subcarriers for various modulation levels.

In H. Cheon, B. Park, D. Hong, Adaptive Multicarrier System with Reduced Feedback Information in Wideband Radio Channels, IEEE 1999, adjacent subcarriers are grouped together, and the modulation is adapted on the basis of groups rather than on the basis of individual subcarriers. This obviously reduces the complexity required together with the volume of signalling, but it entails some performance degradation; the more subcarriers are grouped together, the greater the degradation.

Alternatively in some other references, source coding depending on the time and frequency characteristics of the channel is applied to reduce the signalling information.

US published application 2002/0119781 (Li et al) shows an OFDMA system in which the user terminals select only certain channels and then send feedback information on those channels: for example, each user terminal selects the "clusters" (of subcarriers) that are good candidates for communication, and sends back information on those to the base station.

Since the terminals in some embodiments do not transmit data on all subcarriers back to the base station, it is necessary for the signal on the uplink to contain not only quality data on the subcarrier concerned (or cluster of subcarriers concerned) but also an identifier which indicates which subcarrier 1 5 it is.

Although the volume of data may be reduced by not transmitting data for all subcarrier clusters, part of this overhead saving is taken up with the need to transmit extra index data to indicate which channels are concerned. In the above-mentioned US published application, it is proposed firstly to reduce the volume of index information by grouping clusters into groups and then identifying only the groups; or secondly to apply source coding to reduce the volume of information in the index.

The present invention, in one aspect, aims to reduce yet further the volume of overhead information which must be transmitted. In one aspect, it therefore provides an adaptive frequency division multiple access transmission system, in which the uplink control signals comprise index data identifying the number of channels for which data is fed back, and the index data comprises a multi-bit tuple, of length sufficient to code all such possible combinations of channels.

In whatever form the uplink signalling channel is provided, it will have a finite capacity. For each mobile terminal, the uplink feedback signal l O will include data for a plurality of channels, and so the size of the signal will be proportional to the number of channels on which feedback is provided.

However, the total volume of feedback information received by the base station will additionally depend upon the number of mobile terminals sending uplink signals.

We have realised that there is a relationship between the quality of transmission on the downlink, and the number of channels to which uplink information is sent from each mobile terminal: the fewer the channels on which uplink data is transmitted from each mobile terminal, the greater the degradation.

It might therefore be thought desirable to transmit uplink information on the maximum number of channels on each mobile terminal. However, we have also realised that the effect of doing so may be to swamp the capacity on the uplink channel. Accordingly, another aspect of the invention provides an adaptive frequency division multiple access transmission system, in which the number of channels for which data is fed back in uplink control signals is periodically varied in inverse monotonic relation to the number of terminals.

Preferably it is varied in dependence also upon a measure of the downlink degradation which would result from the limiting of the number of channels for which data is fed back.

The channels concerned may be subcarriers, or predetermined groups of subcarriers. Preferably, the number of channels is selected taking into account jointly the number of terminals and the effect on the downlink on the size of the subset of channels selected.

In preferred embodiments, the number of channels thus calculated is periodically transmitted to the terminals, to be used thereafter. The terminals therefore "know" how many channels they are to signal on, and the coding of the index data (indicating which channels data is to be transmitted) can be made more compact.

In another aspect, the present invention provides a method for allocating the number of subcarriers or clusters selected so as to reduce the feedback overhead in the uplink.

In yet another aspect, the present invention provides a method of signalling the number of carriers or channels to be used in this way. Other aspects and preferred embodiments are disclosed in the claims and described hereinafter.

These and other aspects, preferred features and embodiments will be described further, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a block diagram showing schematically the elements of a communications system according to an embodiment of the invention; Figure 2a shows illustratively the structure along the frequency axis of an orthogonal frequency division multiple access waveform comprising a plurality of subcarriers, each with an associates signal to noise ratio; and Figure 2b shows a selected subset of the subcarriers of Figure 2a; Figure 3 is a block diagram showing schematically the elements of a base station or node B of the network of Figure 1; and Figure 4 is a corresponding block diagram showing schematically the structure of one of a plurality of terminals of that network; Figure 5 is a flow diagram showing schematically the process performed by the base station of Figure I in controlling channel allocation; and Figure 6 is a corresponding flow diagram showing the operation of the terminals; Figure 7 is a timing diagram (with time elapsing downwards on the vertical axes) showing the signalling between the terminal and base station; Figure 8 illustrates the structure of an uplink signal from the mobile terminal; Figure 9 is a graph of required number of bits for feedback against number of selected subcarriers, showing the results for 10 subcarriers and the requests for 20 subcarriers in total; Figure 10 is a corresponding graph relating to the prior art showing the number of signalling bits required for feedback against the total number of subcarriers, for modulation levels of 2, 3 and 4; Figure ll(a) is a graph showing the transmission gain against the number of selected subcarriers for uplink signalling; Figure ll(b) shows the same gain data re-drawn as a percentage efficiency, and a percentage penalty (= 100 - efficiency); Figure 1 l(c) is a graph re-drawing the efficient and penalty figures on an axis of compression ratio (indicating the compression achieved in the upper link signal); and Figure 1 l(d) is a graph showing this compression ratio as a function of the number of selected subcarriers; Figure 12 is a graph showing the gain and penalty data re-drawn against the compression ratio in the same manner as Figure 1 l(c), with step wires representation to take account of integer numbers of subcarriers.

Referring to Figure 1, a communication system such as a mobile telephony system comprises a plurality of user equipment (UK) such as mobile terminals 300a, 300b, in radio communication with a base station lOO, provided within a cell, and having a fixed link connection to a backbone network (such as an IP network) via a switch computer 200. The IP backbone network will not further be discussed since it is of conventional type.

Referring to Figure 2a, the orthogonal frequency division multiple (OFDM) waveform used in the downlink communications comprises a plurality of subcarriers spaced apart along the frequency axis such that the peak of each subcarrier falls within the first null of its neighbour.

Referring to Figure 3, the base station 100 comprises a receiver antenna 102 coupled to an uplink receiver 104 which receives and demodulates uplink signals from the terminals 300. The received signal from the receiver 104 is demultiplexed by a demultiplexer 110 into data signals from each user terminal (which are routed to the IP network computer 200) and uplink control signals, which are supplied to an uplink control unit 120.

On the transmitter side, data signals intended for terminals 300 are received from the network computer 200 and passed to a multiplexer 116 where they are multiplexed, together with control signals from the uplink control unit 120, onto selected OFDM subcarriers generated by an OFDM generator 115. The OFDM signal is then amplified and transmitted by a downlink transmitter 114 coupled to a downlink antenna 112, for reception by the mobile terminals 300.

A carrier allocator 115 determines which subcarriers will be used to communicate with each mobile terminal 300, on the basis of the uplink feedback signals from the mobile terminals, and controls the multiplexer accordingly.

Conveniently, the uplink transmissions are also OFDMA, but it would be possible to use any other multiple access system such as CDMA, TDMA, or random access protocols such as ALOHA.

Referring to Figure 4, the structure of a terminal transceiver 300 of Figure I is shown. A downlink receiver antenna 312 receives the OFDM signal transmitted by the base station 100 and passes it to a downlink receiver 314 at which it is amplified and down converted and supplied to an OFDM demodulator 315. Selected channels from the OFDM demodulator 315, under control of a channel selector 310, pass to a demultiplexer 316 which separates out the control signals intended to control the uplink from data signals. The downlink data signals are then supplied for use, at a data port (for example connected to a display unit, or an analog to digital converter for audio reproduction). Pilot signals are routed to an uplink feedback control unit 320.

Output signals from the uplink feedback control unit are supplied to a multiplexer 306, together with uplink data (for example, user commands or voice data), and the multiplexed uplink signal is supplied to an uplink transmitter 304 for transmission via an uplink antenna 302 to the base station (it will be appreciated that a separate uplink feedback channel could be provided, in which case the demultiplexer would be redundant).

The operation of the system will now be described in greater detail.

Referring to Figure 5, periodically (for example, every frame of a framed communication system) in step 1002 the base station 100 receives uplink feedback signals (described in greater detail below) from the mobile terminals 300. Each uplink feedback signal comprises a data portion comprising m = 4 bits per selected subcarrier indicating the signal to noise ratio (SNR) experienced on that subcarrier. It also comprises an index data portion which indicates the identities of the subcarriers for which the data portion contains data. Of the total number of subcarriers L (which may be in the thousands) only the LC best are signalled on the uplink feedback.

In step 1004, the allocator llO allocates downlink subcarriers to mobile terminals 300 on the basis of the uplink channel quality data received in step 1002. Ideally, each mobile terminal would be allocated the channel for which it has supplied uplink data indicating the highest signal to noise ratio.

If several terminals indicate the same subcarrier as having the highest signal to noise ratio, then it is allocated to one of them and each of the others is allocated another channel for which returns data indicating a reasonably high signal to noise ratio. The allocation proceeds iteratively until a channel is allocated to each mobile terminal. In some embodiments, multiple channels may be allocated to each mobile terminal. In step 1006, the channel allocation is signalled to the mobile terminals for use in the next frame.

In step 1008, a value is selected for Lc, the number of channels for which mobile terminals should send uplink feedback quality data. If, in step 1010, it is determined that this value LC is different from the existing value, then in step 1012 the new value is signalled on a broadcast control channel to all mobile terminals. The process is then repeated for the next frame, and so on. At the same time, downlink user data is transmitted on the selected downlink channels and uplink user data is received on uplink channels, in conventional OFDMA fashion, as shown in Figure 7.

This signalling process is shown in Figure 7. It comprises the following steps: a. Base station determines/updates the number of subcarriers/clusters selected b. Base station signals the selected number to subscribers through common or dedicated downlink channel periodically or sporadically by necessity with specific action time c. Subscribers receive the signalled number and apply at the action time specified by base station.

Referring to Figure 6, in step 3002, each mobile terminal receives its channel allocation on a dedicated downlink control channel (together with data on the coding rate and modulation level which will be used) and in step 3004 the channel selector 310 selects the specified channels for demultiplexing by the demultiplexer 316 to extract the downlink user data.

In step 3006, the terminal 300 detects whether a new value of LC has been transmitted on a broadcast control channel by the base station. If so, then in step 3008 the new value is stored for future use in place of the existing value.

In step 3010, the next periodic occurrence of a pilot signal is detected and in step 3012 the signal to noise ratio values, on all subcarriers which the receiver can demultiplex, are calculated. In step 3014, the LC highest values of signal to noise ratio are selected and in step 3016 are selected and represented in 4 bit form, to form the data portion of the uplink feedback control signal. In step 3016, the identities of the subcarriers are used to generate an index code portion and in step 3018, the uplink control signal comprising the index portion and the data portion (as shown in Figure 8)is transmitted on an uplink channel to the base station. The process is then repeated on each subsequent frame.

Index Portion Structure Table 1 illustrates the method of deriving the index portion at the terminal 300 by the uplink control circuit 120, and decoding it at the base station 100 by the uplink control circuit 120. It shows a simple case with four subcarriers Sl-S4, where L2 (in other words, each terminal must supply feedback data for two of the subcarriers). There are, therefore, six different cases as shown in Table 1 below: x Ix 1 1 X O X O 001 O X X O 010 X X O X 011 O X O X 100 O O X X 011

Table 1

The required number of bits B in a code capable of indicating all the combinations is B = |log2(C)|, where L denotes the total number of subcarriers, Lo denotes the number of selected subcarriers, and the log term is the number of possible combinations of Lc subcarriers from a total of L subcarriers (rounded up where necessary): N! N n (N - n) n! Each possible combination is therefore represented by a code of B bits.

The receivers are instructed by the base station as to the number of channels for which feedback is to be sent, as indicated above, and the base station and terminals store a codebook corresponding to that shown in Table 1 for each value of the number of channels Lc and select the codebook to use in dependence on the value of Lc signalled by the base station.

The total required number of bits in the uplink per each mobile station in uplink is therefore as follows (where m is the number of bits used to indicate channel quality- for example 4): log2(LCl)|+mx Lc, =B+mxLc By way of comparison, for full feedback of all L channels, the total required number of bits in the uplink per each mobile station would be: mxL and the total number of bits required to signal each of Lc channels per each mobile station by sending a separate indication of the subcarrier number for each channel (which indicator would have to be log2 L bits long) followed by the value would be Lc log2 L + m x Lc Where Lc is much less than L, it can be seen that transmitting feedback on only a subset of the channels substantially reduces the capacity required on the uplink channel. Over other methods, this method (control of number of feedback channels, coupled with codebook for each number to represent the different possible combinations) gives the best performance in reducing the volume of feedback signalling on the uplink.

Figure 9 shows the required number of bits for feeding back only some channels (with 4 bits to represent channel quality SINR data). For example, if each mobile station feeds back just the 3 best results SINRs among total 10 sub-channel groups, then it can reduce uplink overhead by more than 50% compared to full feedback for all channels (L=10, LC=1O) Even though the downlink performance would be degraded to some extent, it is helpful in uplink performance. Based on our heuristic observations, taking only 34 best SINRs from each mobile station would not impact much on the downlink performance.

Obtaining the Channel Quality metric An analysis of this process will now be given. The process can be subdivided into two main steps: firstly, selecting the strongest Lc subcarriers amongst the total L subcarriers, and secondly, calculating an average channel quality metric.

For the first step, performed by the uplink feedback control unit 320, if we let L be the total number of subcarriers available, and L, be the number of subcarriers finally chosen, then this process can be considered as a Generalized Selective Combining (SC) process.

The strength of each subcarrier is related to their Signal to Noise Ratio (SNR), denoted Yk for the kth subcarrier.

Therefore the resulting SNR of the chosen subcarriers are no longer independent, and their joint Probability Density Function (PDF) can be expressed as the following (from M-S. Alouini, and M.K. Simon, An LCGF Based Performance Analysis of Generalized Selection Combining over Rayleigh Fading Channels, IEEE Trans. On. Communications, Vol.48, N.3, March 2000, pp.401-414): P(Y, Y,2, .., ye,, ) = (Lc!) (L) [P(Y' )t hi P(Y, ) where P(.) and p(.) denote the Cumulative Distribution Function (CDF) and PDF, respectively.

The second step (2), performed by the uplink control circuit 120 at the base station, enables us to derive the Channel Quality metric, in order to get information about the resulting penalty due to the compression (by selection) of feedback information. This Channel Quality metric is defined by addition of the previous selected SNRs: F[c = Yin I=' This step can therefore be considered as Maximal Ratio Combining (MRC) process. Therefore the global channel quality metric is modelled as a combination of both SC, followed by MRC. By applying mathematics developed in the study of Hybrid SC/MRC systems (M-S. Alouini, and M.K.

Simon, An LcGF-Based Performance Analysis of Generalized Selection Combining over Rayleigh Fading Channels, IEEE Trans. On.

Communications, Vol.48, N.3, March 2000, pp.401-414), we can carry out the statistical distribution of the channel quality metric: (-1)+'-' (,L Lc) (Lc) .e-r (LC) lie (LC-l' (e-t r 1 1 Y) Jo where r denotes average SNR of all the subcarriers. Average value of the channel quality metric as a function of selected number subcarriers is given by E{rL}=1+ 1: Lc r We will thus define Gain (or transmission capability) as the normalized expression w.r. t. to average SNR of all the subcarriers: Gain= r =Lc-(l+ I)J With the obtained Gain (or transmission capability), we can now obtain the trade- off relation between transmission capability and feedback compression.

The process is illustrated by Figure ll(a) to (d). We first plot the transmission gain in dB (1) depending on the number of selected Lc subcarriers (Figure I I(a)). Then we can obtain the same gain in terms of efficiency (%), together with its counterpart, seen as a penalty factor, again w.r.t. number of selected subcarriers (Figure 1 l(b)). Now, we focus on the compression ratio (Reduced Feedback Signalling) reached w.r. t. the number of selected subcarriers (Figure I I(c)), and whose analytical relation is given by the above Equation.

It now becomes obvious that we can get the final relation between the transmission capability (or penalty) w.r.t. the compression ratio (Figure I 1 (d)).

This final curve corresponds to the trade-off to be tuned, depending on the constraint target, like the maximum penalty, or the minimum compression ratio to be reached, and so on. Figure 12 shows an example of this final relationship, which is stepwise approximated and stored in a look-up table at the base station in this embodiment (or alternately could be fitted with interpolation polynomials). It can be seen that a 30% compression ratio on the Feedback Signalling information still permits use of allows to use up to 80% of the downlink transmission capability (or, inversely, degrades it by 20%). Using the stored relationship, the base station selects a number of subcarriers to feed back so as to achieves both a high enough degree of compression on the uplink and an acceptable quality on the downlink. This is recalculated each time the number of terminals changes, and the load (e.g. downlink usage) changes.

It will be clear to the skilled person that, other than the modulation and RF components, the blocks making up the embodiment at the base station and the mobile terminals comprise suitably programmed digital signal processor devices (DSPs) or ASICs executing signal processing. Separate dedicated hardware devices may be used for the OFDM operations (specifically the Inverse Fast Fourier Transform or IFFT used in the transmitter components to map the subcarriers into a composite time- domain signal for RF modulation) and the Fast Fourier Transform or FFT used in the receiver components to map the time-domain signal back into component subcarriers).

This invention may be used together with a method of dynamic subcarrier allocation as described in our co-pending UK application GB04 (agent's reference J00046534GB, filed on the same day as the present application), and/or with a method of pilot signal adaptation described in our copending UK application GB04 (agent's reference J00046533GB, filed on the same day as the present application), both of which are incorporated herein in their entirety.

It will be apparent from the foregoing that many other embodiments or variants of the above are possible. The present invention extends to any and all such variants, and to any novel subject matter or combination thereof disclosed in the foregoing.

Claims

CLAIMS: 1. A method of transmitting uplink data indicating downlink

quality measurements in an adaptive frequency division multiple access transmission system, comprising the steps of: measuring, at a terminal, the downlink quality on a plurality L of candidate frequency division channels; selecting a subset LC of said channels comprising a predetermined number of those channels having the best downlink quality, and transmitting an uplink signal comprising an index portion identifying the channels which comprise the subset and a data portion containing the channel quality measurements; characterised by: representing the selected subset as a multibit word of length loge (L) bits selected from a set comprising each possible combination of Lc channels selected from L candidates, and substituting for the multibit word a shorter multi-bit tuple, of length B bits, sufficient to code all such possible combinations, and using said tuple as said index portion.
2. A method according to claim 1, further comprising periodically varying the number of channels in the subset Lc.
3. A method controlling an adaptive frequency division multiple access transmission system, comprising the steps of measuring, at each of a plurality of terminal, the downlink quality on a plurality L of candidate frequency division channels; selecting, at each terminal, a subset LC of said channels comprising a predetermined number of those channels having the best downlink quality, and transmitting, from each terminal, an uplink signal identifying the channels which comprise the subset and containing the channel quality 1 0 measurements; characterized by: periodically detecting the number of terminals, and varying the number of channels in the subset LC in inverse monotonic relation to the number of terminals.
4. A method according to claim 3, comprising transmitting the new number of channels in the subset LC for future use to the terminals.
5. A method according to claim 3, comprising calculating the number of channels taking into account jointly; the number of terminals; and the degradation on the downlink caused by the size of the selected subset relative to the total available channels.
6. A terminal for use in an adaptive frequency division multiple access transmission system, comprising a channel quality measurement device measuring the downlink quality on a plurality L of candidate frequency division channels, selecting a subset LC of said channels comprising a predetermined number of those channels having the best downlink quality, and generating channel quality data for each channel of the subset; and an uplink data transmission circuit transmitting an uplink signal comprising an index portion identifying the channels which comprise the subset and a data portion containing the channel quality data; characterised by: a coder connected to the channel quality measurement device and the uplink data transmission circuit, which represents the selected subset as a multibit word of length log2(L) bits selected from a set comprising each possible combination of LC channels selected from L candidates, and substitutes for the multibit word a shorter multi-bit tuple, of length B bits sufficient to code all such possible combinations, and supplies the tuple to the uplink data transmission circuit as said index portion.
7. A terminal for use in an adaptive frequency division multiple access transmission system, comprising a channel quality measurement device measuring the downlink quality on a plurality L of candidate frequency division channels, selecting a subset LC of said channels comprising a predetermined number of those channels having the best downlink quality, and generating channel quality data for each channel of the subset; and an uplink data transmission circuit transmitting an uplink signal identifying the channels which comprise the subset and containing the channel quality data; characterised by: a downlink control circuit receiving a downlink control signal and connected to vary the number of channels to be used in said subset in response thereto.
8. A base station for use in an adaptive frequency division multiple access transmission system, comprising: an uplink channel receiver which receives uplink feedback signals from a plurality of terminals, each uplink signal containing downlink channel quality measurements of the downlink quality on a respective subset LC having the best downlink quality of the frequency division channels associated with one of the terminals, and identifying the channels which comprise the subset for that terminal; and a downlink channel allocator which allocates downlink chancels to said terminals in dependence on said uplink feedback signals; characterized by: an uplink feedback control circuit which periodically detects the number of terminals, varies the number of channels in the subset Lc in inverse monotonic relation to the number of terminals; and transmits the new number of channels in the subset LC for future use to the terminals.
9. An adaptive frequency division multiple access transmission system, in which the number of channels for which data is fed back in uplink control signals is periodically varied in inverse monotonic relation to the number of terminals.
10. An adaptive frequency division multiple access transmission system, in which the uplink control signals comprise index data identifying the number of channels for which data is fed back, and the index data comprises a multi-bit tuple, of length sufficient to code all such possible combinations of channels.
11. A signal for use in a method according to claims 1 to 3 or a system according to claims 9 or 10.