GB2434949A

GB2434949A - A multiple access technique for multi-band OFDM systems

Info

Publication number: GB2434949A
Application number: GB0602437A
Authority: GB
Inventors: Justin Coon; Jiun Siew
Original assignee: Toshiba Research Europe Ltd
Current assignee: Toshiba Europe Ltd
Priority date: 2006-02-07
Filing date: 2006-02-07
Publication date: 2007-08-08
Anticipated expiration: 2026-02-07
Also published as: GB2434949B; GB0602437D0

Abstract

An Orthogonal Frequency Division Multiplexing (OFDM) system employs block spreading codes common to Multi-Carrier Code Division Multiple Access (MC-CDMA) systems in order to increase the number of interference-free logical channels (piconets), and hence the network capacity, of Multi-Band OFDM Alliance (MBOA) schemes. MB-OFDM systems use different frequency band groups (Fig. 1) in sequences specified by Time Frequency Codes (TFCs - Fig. 2) to allow multiple simultaneous users of a given frequency band which can be separated at the receiver. The spread code symbols include pseudo-random sequences, and conjugate symmetry where no reversal of the subcarriers is required. If original and time spread blocks (Fig. 3) are transmitted on different channels as in TFC1 and TFC2, a symbol interleaver is required (Fig. 4), and the OFDM signal may be transmitted over multiple antennas in a MIMO system.

Description

A Multiple Access Technique for Multi-band OFDM Systems This invention

relates to signals, apparatus and methods for use in OFDM (Orthogonal Frequency Division Multiplexed) communication systems. More particularly it relates to a multiple access scheme for use in multi-band orthogonal frequency division multiplexing (OFDM) systems.

A personal area network (PAN) is a computer network used for communication among devices (including, among others, e.g. telephones and personal digital assistants) close to one person. The reach of a PAN is typically a few meters. PANs can be used for communication among the devices themselves (intra-device communication), or for connecting to a higher level network or the Internet (an uplink). A PAN may be wired with computer buses such as USB and Firewire. Increasingly popular are wireless personal area network (WPAN) made possible with network technologies such as IrDA or Bluetooth. A network of computing devices using e.g. Bluetooth technology protocols allows one master device to interconnect with a number of active slave devices forming a so-called piconet.

The current generation of high data rate wireless local area network (WLAN) standards, such as Hiperlanl2 and IEEE8O2.lla, provide data rates of up to 54 Mbit/s. However, the ever-increasing demand for even higher data rate services, such as Internet, video and multi-media, have created a need for improved bandwidth efficiency from next generation wireless LANs. The current IEEE8O2. 11 a standard employs the bandwidth efficient scheme of Orthogonal Frequency Division Multiplex (OFDM) and adaptive modulation and demodulation. The systems were designed as single-input single-output (SISO) systems, essentially employing a single transmit and receive antenna at each end of the link. However within ETSI BRAN some provision for multiple antennas or sectorised antennas has been investigated for improved diversity gain and thus link robustness.

Orthogonal frequency division multiplexing is a well-known technique for transmitting high bit rate digital data signals. Rather than modulate a single carrier with the high speed data, the data is divided into a number of lower data rate channels each of which is transmitted on a separate subcarrier. In this way the effect of multipath fading is mitigated. In an OFDM signal the separate subcarriers are spaced so that they overlap and the subcarrier frequencies are chosen that so that the subcarriers are mutually orthogonal, enabling the separate signals modulated onto the subcarriers to be recovered at the receiver. One OFDM symbol is defined by a set of symbols, one modulated onto each subcarrier (and therefore corresponds to a plurality of data bits). The subcarriers are orthogonal if they are spaced apart in frequency by an interval of lIT, where T is the OFDM symbol period.

An OFDM symbol can be obtained by performing an inverse Fourier transform, preferably an Inverse Fast Fourier Transform (IFFT), on a set of input symbols. The input symbols can be recovered by performing a Fourier transform, preferably a fast Fourier transform (FFT), on the OFDM symbol. The FF1 effectively multiplies the OFDM symbol by each subcarrier and integrates over the symbol period T. It can be seen that for a given subcarrier only one subcarrier from the OFDM symbol is extracted by this procedure, as the overlap with the other subcarriers of the OFDM symbol will average to zero over the integration period T. Often the subcarriers are modulated by QAM (Quadrature Amplitude Modulation) symbols, but other forms of modulation such as Phase Shift Keying (PSK) or Pulse Amplitude Modulation (PAM) can also be used.

Batra, A. et al., in "MultiBand OFDM Physical Layer Proposal for IEEE 802.15 Task Group 3a", MBOA-SIG, 14 September 2004, incorporated herein by reference, specifies the physical layer entity for an ultra wide band system that utilizes the unlicensed 3.1 - 10.6 GHz UWB band, as regulated in the United States by the Code of Federal Regulations, Title 47, Section 15. The UWB system provides a wireless PAN with data payload communication capabilities of 53.3, 80, 110, 160, 200, 320, 400, and 480 Mb/s.

The support of transmitting and receiving at data rates of 53.3, 110, and 200 Mb/s is mandatory. The proposed UWB system employs orthogonal frequency division multiplexing using a total of 122 modulated and pilot subcarriers out of a total of 128 subcarriers. Forward error correction coding (convolutional coding) is used with a coding rate of 1/3, 11/32, 1/2, 5/8, and 3/4. The proposed UWB system also utilizes a time-frequency code (TFC) to interleave coded data over up to three frequency bands (called a band group). Four such band groups with three bands each and one band group with two bands ase defined, along with four 3-band TFCs and two 2-band TFCs.

Together, these band groups and the TFCs provide the capability to define eighteen separate logical chaimels or independent piconets. Devices operating in band group #1 (the three lowest frequency bands) are denoted Mode 1 devices, and it shall be mandatory for all devices to support Mode 1 operation, with support for the other band groups being optional and added in the future.

Designing a system that will efficiently and effectively support multiple users is a key challenge in wireless communications today. Many multiple-access (MA) techniques have been devised, and some have found their way into the standards.

For example, many simple specifications allow for user separation by employing time division multiple access (TDMA) or frequency division multiple access (FDMA), whereas second and third generation cellular devices support multiple users through code division multiple access (CDMA).

CDMA, in particular, has received much attention in the research community due to its elegant structure and versatility. Many variants of the conventional direct sequence (DS) approach to CDMA have been studied. These variants include chip interleaved (CI) CDMA, block spread (BS) CDMA and cyclic prefix (CP) CDMA with frequency-domain equalisation (FDE).

Furthermore, the growing popularity of orthogonal frequency division multiplexing (OFDM) has led to research in the area of a hybrid form of CDMA known as multi-carrier CDMA (MC-CDMA), in which conventional CDMA spreading techniques are applied to an OFDM system to improve performance whilst maintaining orthogonality amongst users. It is worth noting that MC-CDMA is very different from orthogonal frequency division multiple access (OFDMA), which does not employ spreading codes to separate users, but instead simply assigns mutually exclusive sets of tones to different users to maintain user orthogonality.

Ultra-wideband (UWB) is a concept that promises extremely high data rates over a short range. UWB systems utilise a large section of bandwidth -at least 500 MHz -to transmit data at low power. As a result of the low transmit power, a robust physical layer (PHY) technique must be used in UWB systems.

Currently, the IEEE 802.15.3a High Rate Alternative PHY Task Group (TG3a) is considering direct sequence CDMA and multi-band (MB) OFDM technologies as possible high-data-rate physical layer enhancements to wireless personal area networks (WPANs). One important issue that is considered in both physical layer techniques is that of exploiting frequency diversity in the channel whilst minimising the interference caused by neighbouring piconets.

Direct sequence CDMA systems address this issue with variable-rate spreading codes.

However, at high data rates, the known DS-CDMA systems can only support one or two piconets in an area. As more piconets enter an area, all systems must decrease their data rates or undergo significant performance losses due to interference.

The multi-band OFDM system proposed by the Multi-Band OFDM Alliance (MBOA) in Batra, A. et a!. (supra) utilises different frequency band groups along with band hopping within a band group via pre-defined time-frequency codes to exploit frequency diversity in the channel whilst minimising interference. According to this proposal an MB-OFDM piconet operates in a specified band group, and each band group comprises up to three sub-bands as shown in Table 1. The width of each sub-band is 528 MHz.

Band Group Band_ID Centre Frequency (MHz) 1 1 3432 2 3960 __________ 3 4488 2 4 5016 5544 ___________ 6 6072 3 7 6600 8 7128 ___________ 9 7656 4 10 8184 11 8712 __________ 12 9240 13 9768 __________ 14 10296 Table 1: List of band groups and sub-band centre frequencies for proposed MBOA system.

The ultra-wideband system also utilises a time-frequency code to interleave coded data over up to three frequency bands called a band group. Four such band groups with three bands each and one band group with two bands are defined, along with four 3-band TFCs and two 2-band TFCs. Together, these band groups and the TFCs provide the capability to define eighteen separate logical channels, or independent piconets.

Table 1 outlines the band groups and the centre frequencies of their respective sub-bands. Each multi-band OFDM piconet operating in a band group is assigned a different time-frequency code. Four TFCs are available for use in band groups one through four; two TFCs are available for use in band group five. The four main TFCs are listed in Table 2 and an illustration of these TFCs is shown in Figure 2.

Time Frequency Code Length 6 Time Frequency Code (TFC_Number ___________ ___________ 1 1 2 3 1 2 3 2 1 3 2 1 3 1 3 1 1 2 2 3 3 4 1 1 3 3 2 2 Table 2: Four main time-frequency codes (for use in band groups one through four).

In US 2002/1 26740 Al (Giannakis et a!.), incorporated herein by reference, techniques are described for maintaining the orthogonality of user waveforms in multi-user wireless communication systems, such as systems using the code division multiple access (CDMA) modulation scheme in the presence of frequency-selective fading channels. Unlike conventional systems in which spreading is performed on individual information-bearing symbols, the "chip-interleaved block-spreading" (CIBS) techniques described herein spread blocks of symbols. A transmitter includes a block-spreading unit to form a set of chips for each symbol of a block of information-bearing symbols and to produce a stream of chips in which the chips from different sets are interleaved.

A pulse shaping unit within the transmitter generates a transmission signal from the stream of interleaved chips and transmits the signal through a communication channel.

A receiver includes a block separator to de-interleave the chips, followed by a match filter to separate signals from different users, and followed by any single-user equalizer.

Also, in US 2005/1353 14 Al (Giannakis et a!.), incorporated herein by reference, techniques are described for performing block equalization on a received wireless communication signal formed according to interleaved chips generated from sub-blocks of symbols. For example, a one-step block equalization process is described which produces estimates of the information-bearing symbols from a wireless communication signal received from two or more transmitters in a soft handoff environment. The techniques provide improved performance in high load, soft handoff environments with low complexity, highly flexible equalization. The wireless communication signal may be a CIBS-CDMA signal in which a symbol block is divided into sub-blocks and spread by a user-specific block-spreading matrix. The CIBS signal is received through M subchannels and a de-spreading matrix is applied to produce a multi-user interference (MUI) free sub-block output for the mth channel. One-step block equalization comprises forming a single block from the m de-spread sub-blocks and performing block equalization on the single block.

US 2004/008617 Al (Dabak et al.), incorporated herein by reference, discloses a system and method for a multi-carrier ultra-wideband transmitter. An UWB transmitter is described taking advantage of both code division multiple access and orthogonal frequency division multiplexing techniques to create a multi-carrier UWB transmitter.

The multi-carrier UWB is capable of avoiding interferers by eliminating signal transmissions in the frequency bands occupied by the interferers.

In Zhou, Shengli; Giannakis, G.B.; Le Martret, C.: "Chip-interleaved block-spread code division multiple access", IEEE Transactions on Communications, February 2002, Volume 50, Issue 2, pages 235 -248, incorporated herein by reference, a multiuser-interference-free code division multiple access transceiver for frequency-selective multipath channels is developed. Relying on chip-interleaving and zero padded transmissions, orthogonality among different users' spreading codes is maintained at the receiver even after frequency-selective propagation.

A random phase updating algorithm for reducing the peak-to-average power ratio (PAPR) of OFDM signals is addressed in: "Random phase updating algorithm for OFDM transmission with low PAPR", Nikookar, H., Lidsheim, K.S., IEEE Transactions on Broadcasting, June 2002, Volume 48, Issue 2, pages 123 -128, incorporated herein by reference. The phase of each subcarrier is updated by a random increment until the PAPR goes below a certain threshold level. The influence of different distributions for the phase increments and the variance of distributions on the mean and variance of PAPR as well as the number of iterations to reach the threshold is investigated. Further, the random phase updating algorithm has been extended by dynamically reducing the threshold level. After successful updating of the phase shifts, the threshold level is reduced and the variance of the phase increments is changed. It is shown that the random phase updating algorithm with dynamic threshold gives the best results and can reduce the mean power variance of an 8-carrier OFDM signal with BPSK modulation by a factor of 7 dB. In order to reduce the complexity, the random phase updating algorithm is investigated with quantization and grouping of the phase shifts. Results show that for a 16-carrier OFDM system, 2-level quantization of phase shifts in 8 groups of 2 carriers give no significant increase in the power variance while reducing complexity.

In another configuration of the above aspect the frequency band group may advantageously comprise up to three frequency bands.

In another configuration of the above aspect the method relates to a multiple input multiple output (MIMO) system comprising a plurality of antennas for transmitting the OFDM signal, wherein each of the users is allocated a unique block spreading code that is equally applied to the transmissions from all antennas.

In yet another configuration of the above aspect the method relates to a multiple input multiple output (MIMO) system comprising a plurality of antennas for transmitting the OFDM signal, wherein a single user is allocated all unique block spreading codes which are assigned to a corresponding number of subsets of the total number of antennas.

In a further configuration of the above aspect the method comprises the steps of listening to one of the plurality of frequency bands comprised in the frequency band group for a predetermined duration; detecting the block spreading code used for a transmitted symbol and its time-spread counterpart; determining which out of the plurality of the predetermined orthogonal block spreading codes is available.

In a second aspect of the invention a method of receiving an orthogonal frequency division multiplexing (OFDM) signal for transmission of a sequence of symbols of one of a number of simultaneous users over a frequency band group comprising a plurality of frequency bands in which transmission occurs by transmitting each symbol and one or more time-spread counterparts thereof over a predetermined sequence of different frequency bands in comprises the step of: applying one of a plurality of predetermined unique block spreading codes to the sequence of symbols and one or more counterparts thereof in the frequency domain after reception.

In one configuration of the above aspect the block spreading code has a length equal to the maximum number of simultaneous users of the frequency band group.

In another configuration of the above aspect the block spreading code is of length two.

In another configuration of the above aspect the method of reception comprises de-interleaving the symbols and the time-spread counterparts thereof whose reception occurs on the same frequency band within the frequency band group.

In another configuration of the above aspect the frequency band group comprises up to three frequency bands.

In yet another configuration of the above aspect the method relates to a multiple input multiple output (MIMO) system comprising a plurality of antennas for receiving the OFDM signal, wherein each of the users is allocated a unique block spreading code that is equally applied to the reception from all antennas.

In a further configuration of the above aspect the method relates to a multiple input multiple output (MTMO) system comprising a plurality of antennas for receiving the OFDM signal, wherein a single user is allocated all unique block spreading codes which are assigned to a corresponding number of subsets of the total number of antennas.

In another configuration of the above aspect the method comprise: listening to one of the plurality of frequency bands comprised in the frequency band group for a predetermined duration; detecting the block spreading code used for a transmitted symbol and its time-spread counterpart; and determining which out of the plurality of the predetermined orthogonal block spreading codes is used for transmission.

In a third aspect of the invention a transmitter comprises means for generating an orthogonal frequency division multiplexing (OFDM) signal for transmission of a sequence of symbols of one of a number of simultaneous users over a frequency band group comprising a plurality of frequency bands in which transmission occurs by transmitting over different frequency bands in a predetermined sequence, the transmitter further comprising means for applying one of a plurality of predetermined unique block spreading codes to the sequence of symbols in the frequency domain prior to transmission.

In a fourth aspect of the invention a receiver comprises means for receiving an orthogonal frequency division multiplexing (OFDM) signal for transmission of a sequence of symbols of one of a number of simultaneous users over a frequency band group comprising a plurality of frequency bands in which transmission occurs by transmitting over different frequency bands in a predetermined sequence, the receiver further comprising means for applying one of a plurality of predetermined unique block spreading codes to the sequence of symbols in the frequency domain after reception.

In a fifth aspect of the invention a computer program product is directly loadable into the internal memory of an electronic data processing means, comprising software portions for performing the steps of any of claims 1 to 12 when said product is run on said data processing means.

In yet another aspect of the above invention a data carrier carries the computer program product.

Figure 1 shows the three sub-bands in band group one for an IVIB-OFDM system as proposed by the MBOA.

Figure 2 shows the four main time frequency codes as proposed by MBOA.

Figure 3shows a time-spread MB-OFDM signal employing TFC number three.

Figure 4 shows a symbol-level interleaving operation for TFC number one.

Figure 5 shows packet error rates for systems using all four TFCs (data rate = 200 Mbps).

Figure 6 shows a BS-CDMA spread block.

Referring now to Figure 1, the frequency of operation for Mode 1 devices according to the MBOA proposal is shown. Band group 1 comprises three bands 1, 2, and 3 located around centre frequencies 3432 MHz, 3960 MHz, and 4488 MHz, respectively.

Several multi-band OFDM transmission modes were proposed by the MBOA for use in UWB systems. A list of these modes is given in Table 3.

Data Modulation Coding Conjugate Time Overall Coded bits Rate rate Symmetric Spreading Spreading per OFDM (Mb/s) (R) Input to Factor Gain symbol ______ _________ ______ IFFT (TSF) ________ (NCBPS) 53.3 QPSK 1/3 Yes 2 4 100 QPSK 1/2 Yes 2 4 100 QPSK 11/32 No 2 2 200 QPSK 1/2 No 2 2 200 QPSK 5/8 No 2 2 200 320 QPSK 1/2 No 1 (no 1 200 _______ ___________ ________ ___________ spreading) __________ ___________ 400 QPSK 5/8 No 1 (no 1 200 _______ ___________ ________ ___________ spreading) __________ ___________ 480 QPSK 3/4 No 1 (no 1 200 ______ ___________ I spreading) _________ __________ Table 3: List of transmission modes proposed by the MBOA for use in a UWB MB-OFDM system.

Some of these transmission modes (53.3, 80, 110, 160, and 200 Mb/s) utilise a form of spreading called "time spreading" to create a more robust link. In this operation, the same information is transmitted over two adjacent OFDM symbols.

Time spreading can be explained mathematically as follows. Consider the original OFDM symbol in the time domain at time 2k, which is denoted by t2k. The spread symbol is thus given by _JPk (z(t2k)+J(t2k)) , no conjugate symmetry 2k+11 ( ) 1 Pkt2k, conjugate symmetry where pk is an element of a pseudo-random sequence as defined in the MBOA proposal.

This time spreading operation serves to reverse the order of the subcarriers in the OFDM symbol when no conjugate symmetry is employed, which maximises the frequency diversity gained from the frequency-selective channel. For the case where conjugate symmetry is employed, no subcarrier reversal is needed, and the time-spread signal is simply a repetition of the original signal multiplied by Pk {1,-1}.

The proposed BS-CDMA approach to MB-OFDM with TFCs can be achieved with a minimal amount of modification to the system proposed by the MBOA. With this modification, however, the throughput of the network can be increased. The inventive method exploits the time spreading property of many of the MB-OFDM transmission modes along with the fact that each TFC is defined for six OFDM symbols and only three sub-bands, thus requiring multiple transmissions on the same sub-band within a TFC frame. By applying a unique, i.e. orthogonal, block spreading code of predetermined length to each user, a plurality of synchronous simultaneously operating users that employ the same TFC can be separated at the receiver.

In the preferred embodiment a length-two block-spreading code is used, allowing two users to operate simultaneously using the same TFC, which may result in a doubling of the network throughput. Further embodiments may be contemplated using longer block-spreading codes and enabling a larger number of simultaneously operating users to be separated.

To illustrate the proposed BS-CDMA approach according to the preferred embodiment, consider an example with two synchronous MB-OFDM systems that employ conjugate symmetry, time spreading and TFC number three as depicted in Figure 3. Let the /cth original OFDM symbol and its time spread counterpart for the mth user be denoted by t and t, respectively. In the frequency domain -prior to processing with an inverse discrete Fourier transform (IDFT) -these OFDM symbols are denoted by s and s1, respectively. Now, applying the length-two block spreading code c:= (c [O],c' [1])T to the kth symbol group for the mth user gives the modified transmitted frequency-domain signal X2! q = c [q]sk+q.

Note that c is defined such that m=n otherwise for some a > 1. Since both users employ the same TFC, the (2k+q)th received symbol -after removing the cyclic prefix (CP), or executing the overlap-add technique for zero padded (ZP) transmissions, and applying the discrete Fourier transform (DFT) operation -can be written as Y2k+q = Hk+qXk+q + Hk+qXk+q + 2k+q = c [q} Hk+qSk+q + c [q] Hk+qSk+q + 2k+q' q E {O, i} where 2k+q is a vector of zero-mean Gaussian noise samples with covariance matrix a2! and H+q is the diagonal channel frequency response matrix for user m at time 2k+q.

The symbols from each group of original and time spread blocks can be stacked to form a single vector Zk:= (Yk, PkYk+I)T at the receiver. Note that the received signal corresponding to the time-spread symbol is multiplied by pk prior to stacking.

Assuming each user's channel is static over one TFC frame -i.e. H = H1 = Htm - the two users' signals can be separated by multiplying the respective users' block-spreading codes by the stacked received vector. The resulting interference-free received signal from user m is given by U' =--((y' I)zk a 1 (2) = HmS'k a where = (c' [o])* 2k + (c [1])* 2k+1 Note that n is a vector of zero-mean Gaussian random variables with covariance matrix ac2!. Any frequency-domain equaliser (FDE) can, at this point, be applied to recover the transmitted message for the mth user.

A significant advantage of the proposed technique according to the invention is its capability to provide multi-user transmission without requiring each user to know the other user's channel state information. Indeed, only the channel state information for the desired user is required to equalise the received, de-spread signal given in equation (2). As a result, this multiple access technique can easily be used in point-to-point networks.

In the embodiment detailed above, the receiver was able to separate the two users since each user transmitted its original and time-spread OFDM symbol over the same channel, which remained static for those two symbol periods. Note that TFC number four follows this regime; and thus the previous embodiment directly extends to the case where TFC number four is employed by two synchronous users.

Referring to Figure 2, however, it can be seen that systems employing TFC numbers one and two do not transmit the original and corresponding time-spread blocks over the same channel.

Consequently, in order to apply the proposed multiple access scheme with TFC numbers one and two, according to the invention, a symbol-level interleaver is implemented to ensure proper allocation of the symbols to the channels. An embodiment of how this may be achieved for a system using TFC number one is illustrated in Figure 4. A similar interleaving step may be implemented when TFC number two is used.

This interleaving operation reduces the frequency diversity in the MB-OFDM system somewhat since information is transmitted on the same channel twice for all TFCs.

However, all MB-OFDM transmission modes proposed by the MBOA employ a channel code with bit interleaving on the header and the payload of each packet. Since frequency diversity is inherently exploited in each packet by the channel code, the performance loss due to the symbol-level interleaver is minimal. In general, this performance loss is less than a decibel. This fact is illustrated in Figure 5 where the packet error rates (PERs) for conventional MBOA MB-OFDM systems using the four main TFCs and no symbol-level interleaver are shown.

The multiple access technique according to the invention requires a system to implement time spreading as defined in equation (1) for a system with conjugate symmetry. For transmission modes that do not employ conjugate symmetry, the time spreading operation defined in equation (1) replicates the transmitted data signal and reverses the order of the data subcarriers. The subcarrier reversal operation is designed to maximally exploit frequency diversity in the channel. Again, since the bits in eachpacket are encoded and interleaved across the TFC frame, subcarrier reversal caused by time spreading does not provide a significant performance advantage. Consequently, according to a further aspect of the invention it is proposed that time spreading be performed for all systems as if conjugate symmetry were used. It should be noted that the multiple access scheme according to the invention requires the system to employ time spreading in order to work.

In a further aspect of the invention, the multiple access method can be extended to Multiple-Input Multiple-Output (MIMO) systems. One possible MIMO method according to the invention is to assign the spreading codes as is done in the single-antenna case: each of two users has a unique code that is equally applied to the transmissions from all antennas.

On the other hand, one user with multiple antennas may be given both codes, and that user can assign one code to a subset of the total number of available antennas and the other code to the remaining antennas. This approach could improve the performance of that user since more degrees of freedom would be allowed to decode the received message(s).

Referring now to Figure 6, it can be seen that conventional block-spreading-CDMA systems apply one element of a code sequence to one block of symbols. This results from the fact that BS-CDMA was originally designed for use in single-carrier systems.

However, in OFDM systems according to the invention, BS-CDMA techniques can be applied to each subcarrier. Consequently, a different -and possibly complex -length-two spreading code can be applied to each subcarrier of the original and time spread frequency-domain OFDM symbols. This could allow for peak-to-average power ratio (PAPR) reduction through phasing of the data subcarriers at the transmitter. Since only two orthogonal spreading codes are used for each subcarrier, the PAPR can be reduced for the original and the time spread symbols of the first user by applying conventional phasing techniques without destroying the orthogonality of the two users' signals. As an example, consider the first user's spreading code for subcarrier n, which could be given by c [n] := eJ91 (c [n],c [])T (3) Where c [n] is a complex number and O4' [n} is the phase offset of subcarrier n for user m at time k. In this case, the second user's spreading code for subcarrier n could be given by c [n] := e ' (c [n],_c [})T (4) where the negation of the second element of the spreading code would not affect the PAPR of the signal.

As described above, the current MBOA proposal calls for a single user or piconet to operate on a given TFC. The multiple access scheme according to the invention allows a plurality of users to simultaneously operate on a given TFC while remaining separable at the receiver.

While conventional multiple access schemes work well in the down-link, they require complex interference suppressionlcancellation techniques in the up-link due to the high-dimensionality of the communication problem in this case. High-dimensionality in the up-link results from the fact that users transmit through different channels, and thus more unknowns are introduced into the system. The present invention can be easily implemented in either the up-link or the downlink since all users' signals are perfectly separated during transmission, thus precluding the need for complex interference suppression or cancellation techniques. Furthermore, the block spreading scheme according to the present invention can be used to allow new users to seamlessly join a network. To illustrate this point, consider a network with one piconet -i.e. only one TFC is used, and that piconet is assigned a single spreading code. If another piconet wishes to join the network, but might interfere with the current piconet, it can simply listen on one sub-band for the first piconet's transmission. By listening for one TFC frame, the second piconet will hear two transmissions on that sub-band (see Figure 2).

Assuming the chaimel remains static during the TFC frame and the spreading codes are designed in a maimer similar to that given by equations (3) and (4), it is easy to see that the two detected transmissions will either be the same (except for noise) or they will be reversed in polarity. The second piconet can observe this difference easily by comparing the two transmissions, and determining which spreading code the first piconet is using. Finally, the second piconet can synchronise to the first and begin transmitting using the other remaining spreading code. The first piconet, throughout this process, does not recognise that the second piconet is joining the network, although the first piconet can simply listen for the second at any time if it needs to know how many piconets are close by.

While the invention has been described based on the number of frequency bands and time-frequency codes as suggested in the MBOA proposal, it will be appreciated that the invention is not limited to these specifications. The invention may equally well be applied to other numbers of frequency bands andlor other numbers of time-frequency codes. Likewise, different lengths of block spreading codes may be contemplated.

No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto. (C-I

Claims

CLAIMS: 1. A method of generating an orthogonal frequency division

multiplexing (OFDM) signal for transmission of a sequence of symbols of one of a number of simultaneous users over a frequency band group comprising a plurality of frequency bands in which transmission occurs by transmitting each symbol and one or more time-spread counterparts thereof over a predetermined sequence of frequency bands, the method comprising the step of: generating one or more time-spread counterparts of the symbols by applying one of a plurality of predetermined unique block spreading codes to the sequence of symbols in the frequency domain prior to transmission.

2. The method of claim 1 wherein the block spreading code has a length equal to the maximum number of simultaneous users of the frequency band group.

3. The method of claim 2 wherein the block spreading code is of length two.

4. The method of any of the preceding claims comprising the further step of interleaving the symbols and the one ore more time-spread counterparts such that transmission of the each symbol and the corresponding time-spread counterparts occurs on the same frequency band within the frequency band group.

5. The method of any preceding claim wherein the frequency band group comprises up to three frequency bands.

6. The method of any of the preceding claims in a multiple input multiple output (MIMO) system comprising a plurality of antennas for transmitting the OFDM signal, wherein each of the users is allocated a unique block spreading code that is equally applied to the transmissions from all antennas.

7. The method of any of claims I to 5 in a multiple input multiple output (MIMO) system comprising a plurality of antennas for transmitting the OFDM signal, wherein a single user is allocated all unique block spreading codes which are assigned to a corresponding number of subsets of the total number of antennas.

8. The method of any of the preceding claims comprising the steps of: listening to one of the plurality of frequency bands comprised in the frequency band group for a predetermined duration; detecting the block spreading code used for a transmitted symbol and its time-spread counterpart; determining which out of the plurality of the predetermined orthogonal block spreading codes is available.

9. A method of receiving an orthogonal frequency division multiplexing (OFDM) signal for transmission of a sequence of symbols of one of a number of simultaneous users over a frequency band group comprising a plurality of frequency bands in which transmission occurs by transmitting each symbol and one or more time-spread counterparts thereof over a predetermined sequence of frequency bands, the method comprising the step of: applying one of a plurality of predetermined unique block spreading codes to the sequence of symbols and one or more time-spread counterparts thereof in the frequency domain after reception 10. The method of claim 9 wherein the block spreading code has a length equal to the maximum number of simultaneous users of the frequency band group.

11. The method of claim 10 wherein the block spreading code is of length two.

12. The method of any of claims 9 to 11 comprising the further step of de-interleaving the symbols and the one or more time-spread counterparts thereof whose reception occurs on the same frequency band within the frequency band group.

13. The method of any of claims 9 to 12 wherein the frequency band group comprises up to three frequency bands.

14. The method of any of claims 9 to 13 in a multiple input multiple output (MIMO) system comprising a plurality of antennas for receiving the OFDM signal, wherein each of the users is allocated a unique block spreading code that is equally applied to the reception from all antennas.

15. The method of any of claims 9 to 13 in a multiple input multiple output (MIMO) system comprising a plurality of antennas for receiving the OFDM signal, wherein a single user is allocated all unique block spreading codes which are assigned to a corresponding number of subsets of the total number of antennas.

16. The method of any of claims 7 to 11 comprising the steps of: listening to one of the plurality of frequency bands comprised in the frequency band group for a predetermined duration; detecting the block spreading code used for a transmitted symbol and its time-spread counterpart; determining which out of the plurality of the predetermined orthogonal block spreading codes is used for transmission.

17. A transmitter comprising means for generating an orthogonal frequency division multiplexing (OFDM) signal for transmission of a sequence of symbols of one of a number of simultaneous users over a frequency band group comprising a plurality of frequency bands in which transmission occurs by transmitting over different frequency bands in a predetermined sequence, the transmitter further comprising means for applying one of a plurality of predetermined unique block spreading codes to the sequence of symbols in the frequency domain prior to transmission.

18. A receiver comprising means for receiving an orthogonal frequency division multiplexing (OFDM) signal for transmission of a sequence of symbols of one of a number of simultaneous users over a frequency band group comprising a plurality of frequency bands in which transmission occurs by transmitting over different frequency bands in a predetermined sequence, the receiver further comprising means for applying one of a plurality of predetermined unique block spreading codes to the sequence of symbols in the frequency domain after reception.

19. A computer program product directly loadable into the internal memory of an electronic data processing means, comprising software portions for performing the steps of any of claims 1 to 12 when said product is run on said data processing means.

20. A data carrier carrying the computer program product according to claim 19.