OFDM COMMUNICATION SYSTEM, COMMUNICATION UNIT AND METHOD OF COMMUNICATING OFDM SIGNALS

Field of the Invention

The present invention relates to communication of signalling information in an orthogonal frequency division multiplex (OFDM) communication system. In particular, but not exclusively, the present invention relates to communication of signalling information related to physical layer characteristics in an IEEE 802.11 communication system.

Background of the Invention

In recent years, wireless data communication, in both domestic and industrial environments, has become increasingly commonplace, leading to an increasing number of wireless communication systems having been designed and deployed. In particular, the use of wireless networking has become prevalent. As a consequence, wireless network standards, such as IEEE 801.11a and IEEE 801. Hg, have become industry standards to ensure compatibility between numerous products.

Furthermore, the requirement for increased data rates, communication capacity and improved quality of service has led to an increased level of research and new techniques and standards being developed for wireless networking. One such standard is the IEEE 801. Hn standard which is currently under development. IEEE 801. Hn is expected to operate in the 2.4GHz or 5GHz

frequency spectrum and promises data rates of around lOOMbps and above, on top of the medium access layer (MAC) layer. In the context of the present invention, the expression ^λ on top of the MAC may be understood to encompass the actual data rate that is available to the user, i.e. the theoretical throughput of the physical layer (inside the part of the frame attributed to the user) minus any overhead such as preambles, signalling, etc .

It is proposed that IEEE 801. Hn will use many techniques that are similar to the earlier developed IEEE 801.11a and IEEE 801. Hg standards. Thus, in this manner, the standard is to a large extent compatible with many of the characteristics of the earlier standards, thereby allowing reuse of techniques and circuitry developed for such standards. For example, as in the previous standards IEEE 801.11a and IEEE 801.Hg, IEEE 801.Hn will use Orthogonal Frequency Division Multiplex (OFDM) modulation for transmission over the air-interface.

Furthermore, in order to improve efficiency and to achieve the high data rates, IEEE 801.Hn is planned to introduce a number of advanced, enhanced techniques, over and above the techniques employed in IEEE 801.Ha and

IEEE 801.Hg. For example, IEEE 801.Hn communication is expected to be typically based on a wireless network that uses a plurality of transmit and receive antennas. Furthermore, rather than merely providing diversity from spatially separated transmit antennas, the IEEE 801.Hn standard will utilise transmitters having at least partially separated transmit circuitry for each antenna,

thereby allowing different, respective sub-signals to be transmitted from each of the antennas.

The corresponding receivers may receive signals from a plurality of receive antennas, and may therefore perform a joint detection of the respective received signals, taking into account the number of, and individual characteristics, associated with each of the plurality of receive antennas. Specifically, it appears that the IEEE 801. Hn standard will specify the use of a Multiple-

Transmit-Multiple-Receive (MTMR) antenna concept, which exploits Multiple-Input-Multiple-Output (MIMO) channel properties to improve performance and throughput.

In order to enable or facilitate reception, the standards of IEEE 802.11a/g, as well as all 802. Hn, prescribe that all data packets are preceded by a physical layer preamble. The physical layer preamble comprises known data that facilitates receiver gain setting, synchronization and channel estimation. In addition, a dedicated OFDM symbol is included, which conveys physical layer signaling required for the decoding of the data packet. This information includes, amongst others, information of the modulation scheme, coding rate and packet length for the subsequent data packet. This signaling is known as the SIG field.

Since IEEE 802. Hn receivers require information relating to multiple antennas, the signaling field has been enhanced for IEEE 802. Hn, and is generally referred to as SIG-N. The SIG-N fields are communicated as

quadrature phase shift keyed (QPSK) symbols in the sub- carriers of the dedicated SIG-N field OFDM symbol.

Specifically, SIG-N mapping (QPSK) has been defined in the context of a proposal to IEEE802.11n (authored by Cenk Kose and Bruce Edwards: "WWiSE Proposal: High throughput extension to the 802.11 Standard", IEEE document number 11-05-0149-02-00On) .

In such proposed systems, no signaling information is available prior to the transmission of the SIG-N field. The receiver must therefore be able to decode this field without any prior information about its nature (this is necessary for compatibility reasons with earlier standards) .

In addition to providing high data rate services, IEEE 802. Hn is also expected to be used for a variety of applications having different requirements and characteristics. For example, IEEE 802. Hn may be used for lower data rate applications, such as mobile Voice over Internet Protocol (VoIP) , and mobile multimedia streaming, for handheld devices. Although these applications have relatively low data rate, it is desired that they can be accessed over a large area and therefore it is desirable for IEEE 802. Hn cells to have as large a coverage area as possible.

However, it is noteworthy that the IEEE802.11n standard is also targeted to operate in both a ^λ Green-Field' mode (i.e. where no legacy IEEE802.Ha/g devices exist, but only IEEE802.11n devices are present) and a ^λ Mixed' mode

(i.e. where there is a coexistence between IEEE802.11n and legacy IEEE802. lla/g devices).

A mechanism for detecting the presence of an IEEE802.11n signal in a mixed-mode only scenario has been proposed within the following IEEE802.11n draft proposals:

(i) TGnsync proposal technical specification, authored by Syed Aon Mujtaba, 2005, IEEE document number 11-04-0889-04-00On; and (ii) WWiSE proposal technical specification, authored by Sean Coffey, 2005 IEEE document number 11-04-

0886-06-00On.

The above documents may be obtained from: htt:p : //www.802wirelessworld. com/ index . j sp .

Notably, these proposals provide means to detect the presence of the IEEE802.11n mixed mode only, but notably late in the frame.

Notably, in (i) , the detected information is communicated in the ^λ Mixed' -mode only, and is available only after the legacy preamble part (for example, by means of an indication bit in the IEEE802.11n signaling field. In (ii) , the detected information is communicated by using a specific constellation in the signaling field that can be easily detected: for example a binary phase-shift keyed (BPSK) constellation that is rotated by 90 degrees) . Again, this detection must be performed well after the reception of the legacy part of the preamble, as illustrated in FIG. 1, which leads to corresponding latency requirements in the receiver design.

Thus, FIG. 1 illustrates a proposed frame structure 100, comprising first antenna 115 and second antenna 110 transmitting a signal comprising a legacy part of a preamble 105 and a new IEEE802.11n preamble part 140. The legacy part of a preamble 105 comprises an δμsec short sequence 120, followed by an δμsec long sequence 125 and 4μsec signalling 130 comprising a SIG-MM field. The new IEEE802.11n preamble part 140 then comprises an δμsec short sequence 145, followed by an δμsec long sequence 150 and two portions of 4μsec signalling SIG-N fields 155 and 160. Thus, as shown, the receiver needs to decode the legacy part of the preamble 105 and a significant portion of the new IEEEδ02.11n preamble part 140 before it is able to identify the nature of the transmission by decoding the SIG-N field 155, 160.

However, the inventors of the present invention have recognised that in a ^λ Mixed' -mode scenario, the receiver should identify, as soon as possible in the frame, whether a pure legacy device is transmitting or whether a new IEEEδ02.11n device is operating in a "Mixed-Mode".

Thus, a need exists for an improved mechanism for detecting a ^λ Mixed' -mode scenario and method of operation therefor. In particular, an improved mechanism for detecting much earlier whether the received mixed-mode signal frame relates to a pure legacy device or a new IEEEδ02.11n device, is needed.

Summary of the Invention

In accordance with aspects of the present invention, there is provided a wireless communication system, a wireless communication unit, and method of operation therefor, as defined in the appended Claims.

Brief Description of the Drawings

FIG. 1 illustrates a known frame structure, as proposed for the IEEE802.11n wireless communication network standard.

Exemplary embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 2 illustrates a wireless communication unit capable of Orthogonal Frequency Division Multiplexing (OFDM) communication in accordance with embodiments of the present invention;

FIG. 3 illustrates an Orthogonal Frequency Division Multiplexing (OFDM) transmitter in accordance with embodiments of the present invention;

FIG. 4 illustrates a frame structure for the IEEE802.11n wireless communication network standard, adapted in accordance with embodiments of the present invention;

FIG. 5 illustrates a spectral plot of the legacy IEEE802.il spectrum and the increased spectral capacity

of IEEE802.11n, as utilised in embodiments of the present invention;

FIG. 6 illustrates an OFDM receiver in accordance with embodiments of the present invention; and

FIG. 7 illustrates a method of detecting a mode of operation of a mixed mode IEEE802.il and IEEE802.11n communication system in accordance with embodiments of the present invention.

Description of Embodiments of the Invention

In one embodiment of the present invention, an orthogonal frequency division multiplex (OFDM) wireless communication system supporting two OFDM communication modes of operation is described. A first OFDM communication mode of operation utilises a first communication bandwidth. A second OFDM communication mode of operation utilises a second communication bandwidth that incorporates the first communication bandwidth and at least one side carrier of the first communication bandwidth. The at least one side carrier is arranged to carry no power in the first mode of operation. Signalling information is communicated in the at least one side-carrier in the second mode of operation .

In one embodiment of the present invention a transmitter may transmit the signalling information in a preamble of the frame format of the at least one side-carrier to

identify to a receiving unit that it is to communicate in at least the second OFDM communication mode.

In one embodiment of the present invention the transmitter may modulate a non-zero power symbol on the at least one side-carrier to identify to the receiving unit that it is to communicate in at least the second OFDM communication mode.

In one embodiment of the present invention, a receiving OFDM communication unit comprises a receiver arranged to receive the signalling information in a preamble of the frame format of the at least one side-carrier, thereby identifying that it is to communicate in the second OFDM communication mode if the at least one side-carrier has been modulated with a non-zero power symbol or identifying that it is to communicate in the first OFDM communication mode if the at least one side-carrier has been modulated with a zero power symbol.

In one embodiment of the present invention, the first and second OFDM communication modes may comprise IEEE802.il communication modes. The mode of operation may be selected by encoding or decoding at least one side- carrier of a SIG-MM field of the IEEE802.il preamble, which may indicate a transmission of an IEEE802.11n signal in a mixed mode of operation.

In one embodiment of the present invention, a transmitting OFDM communication unit may modulate

information bits on at least one side-carrier in an IEEE802.11n frame format, which is unused in a legacy IEEE802.il standard.

In this manner, the legacy part of the IEEE802.11n preamble may be used for communicating an indication of frame format, whilst enabling legacy devices to still synchronize efficiently and decode the SIG-MM field. Furthermore, in this manner, the receiver latency requirements are kept to a minimum, since the communication occurs very early in the frame.

In one embodiment of the present invention, a receiving OFDM communication unit may comprise a detector arranged to detect a presence of the at least one side-carrier using a spectral power detection mechanism or using a probabilistic detection of a presence of a side-carrier, which may interpolate channel coefficients of the side- carriers or interpolate between channel phase or channel amplitude coefficients of carriers.

In one embodiment of the present invention, the transmitting OFDM communication unit may comprise data generating logic for generating a set of data symbols indicative of a presence of a mixed mode of operation frequency selection logic arranged to locate the set of data symbols in the at least one side-carrier and a radio transmitter for transmitting the set of data symbols in the at least one side-carrier.

Embodiments of the present invention provide a mechanism to detect, in a ^λ mixed' -mode scenario, whether a received frame is a new IEEE802.11n or a legacy device.

Advantageously, from a decoding latency point of view, the inventive concept is able to communicate a presence of a ^λ mixed' mode device, i.e. whether it is a legacy device or a new IEEE802.11n device, much earlier in the frame than prior art mechanisms, and optimally communicate its presence in the legacy part of the IEEE802.il frame.

Notably, embodiments of the present invention provide a mechanism that is able to recognise a transmission of a side-carrier as an IEEE802.11n transmission, particularly in both the presence of noise and a random gain in the propagation channel, in contrast to receiving an IEEE802. lla/g transmission that would not transmit in the side-carrier.

In the context of the present invention, the expression ^λ side-carrier' encompasses any OFDM carrier that is typically arranged to carry an amplitude of value zero (i.e. it carries no power during a normal mode of operation) .

Embodiments of the present invention are described in the context of the Technical Specifications and proposals for an IEEE 802.11 communication system, and in particular a mixed mode IEEE 802. Hn and IEEE802. lla/g communication system. However, it will be appreciated that the invention is not limited to this application but may be applied to many other OFDM communication systems including, for example, other IEEE 802.11 communication systems .

Referring first to FIG. 2, a block diagram of a wireless communication unit (often referred to as a mobile subscriber unit (MS) or user equipment (UE) in the context of cellular communications) is shown, in accordance with one embodiment of the invention. The UE 200 contains an antenna 202 that may be coupled to a duplex filter or antenna switch 204 that provides isolation between receive and transmit chains within the UE 200. The receiver chain, as known in the art, includes receiver front-end circuitry 206 (effectively providing reception, filtering and intermediate or baseband frequency conversion) . The front-end circuitry 206 is serially coupled to a signal processing function 208. An output from the signal processing function 208 is provided to a suitable output device 210, such as a screen or flat panel display. The receiver chain also includes received signal strength indicator (RSSI) circuitry 212, which in turn is coupled to a controller 214 that maintains overall subscriber unit control. The controller 214 may therefore receive bit error rate (BER) or frame error rate (FER) data from recovered information. The controller 214 is also coupled to the receiver front-end circuitry 206 and the signal processing function 208 (generally realised by a digital signal processor (DSP) ) . The controller is also coupled to a memory device 216, which selectively stores operating regimes, such as decoding/encoding functions, synchronisation patterns, code sequences, RSSI data, direction of arrival of a received signal and the like.

In accordance with embodiments of the present invention, a timer 218 is operably coupled to the controller 214 to control the timing of operations (transmission or reception of time-dependent signals) within the UE 200. As regards the transmit chain, this essentially includes an input device 220, such as a keypad, coupled in series through transmitter/modulation circuitry 222 and a power amplifier 224 to the antenna 202. The transmitter/ modulation circuitry 222 and the power amplifier 224 are operationally responsive to the controller 214. The signal processor function 228 in the transmit chain may be implemented as distinct from the processor in the receive chain. Alternatively, a single processor may be used to implement processing of both transmit and receive signals, as shown in FIG. 2. Clearly, the various components within the UE 200 can be realised in discrete or integrated component form, with an ultimate structure therefore being merely an application-specific or design selection .

In accordance with embodiments of the present invention, the receiver is adapted according to the circuit of FIG. 6 and the transmitter is adapted as illustrated in FIG. 3. FIG. 3 illustrates an Orthogonal Frequency Division Multiplexing, OFDM, transmitter 300 in accordance with some embodiments of the invention.

In one embodiment of the present invention, an OFDM transmitter 300 comprises a transmitter 301, which is arranged to transmit OFDM signals in accordance with the Technical Specifications for the communication system. In the specific example, a plurality of OFDM transmitters

300, in the form of IEEE 802. Hn transmitters, are respectively coupled to a plurality of antennas 303, with one transmitter shown coupled to one antenna for clarity purposes only.

The transmitter 301 is further coupled to a data packet generator 305 which generates data packets that are fed to the transmitter 301 for transmission. Specifically, the data packet generator 305 generates a data packet comprising a number of signalling OFDM symbols and a number of user data symbols.

The data packet generator 305 is coupled to a user data symbol generator 307, which is further coupled to a user data source 309. In the specific example, the user data source 309 is an internal source. However, it will be appreciated that data to be transmitted may be received from any physical, logical or functional external or internal entity.

The user data symbol generator 307 is further coupled to a transmit controller 311 that controls the transmission characteristics of the transmitted signals. In particular, in accordance with the IEEE 802.11 specifications, different characteristics and modes may be used for transmission. Specifically, different forward error correcting (FEC) schemes, modulation schemes and transmission modes may be used for transmission of data packets. The transmit controller 311 is arranged to select the desired parameters and feed these to the user data symbol generator 307.

In response, the user data symbol generator 307 applies the selected error coding and modulation scheme to the user data received from the user data source 309 to generate an OFDM symbol during a MM-SIG field of the IEEE802.il frame. Specifically, the user data symbol generator 307 encodes the data from the user data source 309 in the MM-SIG field to identify the data as being of the IEEE802.11n communication. In accordance with embodiments of the present invention, the data symbol is applied to at least one side-carrier of the OFDM symbol. For example, in some modes the user data symbol generator 307 may apply a 1/2 rate convolutional encoding and generate a QPSK symbol for each side-carrier. Whereas, in other more robust modes, the user data symbol generator 307 may apply a 1/2 rate convolutional encoding and generate a BPSK symbol for each side-carrier.

The transmit controller 311 is furthermore coupled to a signalling data generator 313. The signalling data generator 313 generates a set of data symbols that are indicative of physical layer characteristics of data transmissions from the OFDM transmitter 300. In particular, the signalling data generator 313 generates a set of data symbols which define the transmit parameters and characteristics that have been applied to the user data by the user data symbol generator 307. Thus, the set of data symbols may identify the forward error correcting coding and symbol constellations used in the IEEE802.11n transmission and may further include information of other transmission parameters affecting physical layer communication such as e.g. the duration of the data packets.

In the specific example, the signalling data generator 313 generates a SIG-MM field in accordance with the specifications for IEEE 802. Hn communication system.

It will be appreciated that the Physical Layer is the lowest layer of the seven layer Open Systems Interconnection (OSI) or similar network models. The physical layer provides the means to activate and use a physical connection, in this case the radio communication link, for bit transmission. In other words, the physical layer provides the procedures for transferring individual bits across a physical media.

The signalling data generator 313 is coupled to a first symbol generator 315 and a second symbol generator 317, which are both fed the set of data symbols from the signalling data generator 313. In the example of FIG. 1, the signalling data generator 313 generates the set of data symbols as QPSK symbols ready for transmission in at least one OFDM side-carrier.

The first symbol generator 315 generates a first OFDM signalling symbol by allocating the received data symbols to at least one side-carrier of the MM-SIG field of a first OFDM signalling symbol.

Similarly, the second symbol generator 317 generates a second OFDM signalling symbol by allocating the received data symbols to at least one further side-carrier of the second OFDM signalling symbol. Likewise, the second symbol generator 317 may simply select a side-carrier for

each data symbol of the set of data symbols in accordance with a predetermined rule or table. However, the allocation of at least one data symbol is different for the first symbol generator 315 and the second symbol generator 317, thereby resulting in at least some of the data symbols of the set of data symbols being allocated to different side-carriers in the first and the second OFDM signaling symbol.

The first symbol generator 315 and the second symbol generator 317 are further coupled to the data packet generator 305, which is fed the first and second OFDM symbols. The data packet generator 305 then proceeds to generate a data packet comprising the first and second OFDM symbols, as well as the user data OFDM symbols received from the user data symbol generator 307. In the specific example of an IEEE 802.11 transmitter, the data packet generator 305 furthermore inserts a preamble of known data to facilitate reception and in particular to facilitate initial gain setting, synchronisation and channel estimation by the receiver.

The generated data packet is fed to the transmitter 301, which proceeds to transmit the OFDM symbols. In particular the transmitter 301 performs an inverse

Discrete Fourier Transform (iDFT) on the OFDM symbols, upconverts and amplifies the resulting signal, etc. as is known to the person skilled in the art.

The embodiment shown in FIG. 3 illustrates two respective symbol generators 315, 317 providing symbols to be input to a data packet generator 305 and a single transmitter

301. However, as would be appreciated by a skilled person, a plurality of logic elements will be used in parallel for an IEEE802.11n device.

Thus, in the transmitter of FIG. 3, the physical layer signalling data symbols are transmitted twice using different OFDM signalling symbols. Furthermore, the allocation of data to the different sub-carriers is varied between the two (or more) different signalling symbols, thereby obtaining a reliability improvement that exceeds the 3dB gain which is achievable by conventional retransmission .

FIG. 4 illustrates a proposed frame structure 400, comprising first antenna 410 and second antenna 415 transmitting a signal comprising a legacy part of a preamble 405 and a new IEEE802.11n preamble part 440. The legacy part of a preamble 405 comprises an δμsec short sequence 420, followed by an δμsec long sequence 425 and 4μsec signalling 430 comprising a SIG-MM field. The new IEEE802.11n preamble part 440 then comprises an δμsec short sequence 445, followed by an δμsec long sequence 450 and two portions of 4μsec signalling SIG-N fields 455 and 460.

Thus, in order to ensure that legacy devices are still able to decode the legacy carriers in the SIG-MM field 430 correctly, the inventive concept of the present invention proposes to use SIG-MM OFDM side-carriers, which in one embodiment of the present invention are defined to be a logical ^λ 0' to indicate a legacy device.

Notably, these side-carriers are unused in the legacy standard.

In one embodiment of the present invention it is envisaged that, in order to increase robustness, the wireless communication unit may copy (and potentially encode) information over several side-carriers. In one embodiment of the present invention, it is envisaged that a phase of the extra tones located in the side-carriers and arranged to carry the information bits, may be fixed. In addition, it is envisaged that the power may be selected from a given range, for example, the transmitter may decide on particular data tones and the amplitude thereof, as a function of the signal-to-noise ratio (SNR) requirements prevalent at the time. Hence, it may be possible to use high amplitudes for a frame that uses very robust constellations and must work at a low SNR. Similarly, it may be possible to use low amplitudes (and thus, advantageously, more power efficient amplitudes) if the frame can only be decoded if the selected carrier constellations of the data symbols require a high SNR.

In this manner, setting the carriers of the extended part of the SIG-MM spectrum to zero may be used to indicate that no ^λ Mixed' mode signal is present, thereby corresponding to the behaviour of legacy devices.

In embodiments of the present invention, the inventive concept of the present invention exploits knowledge that the IEEE802.11n standard is targeted at supporting an increased number of OFDM carriers than the legacy standard (the IEEE802.11n standard proposes the use of

^λ 56' carriers instead of the ^λ 52' carriers of the legacy mode) . Thus, and advantageously, the additional (side-) carriers will not be filtered by IEEE802.11n filters in the RF front-end (say RF front-end 206 of FIG. 2), so long as the extended part of the used spectrum does not exceed the used part of IEEE802.11n data symbols.

Thus, the inventive concept provides an efficient mechanism for communicating the presence of the ^λ Mixed' mode (or indeed any other indication of a particular service or application) to the receiver. In this manner, the receiver latency requirements are kept to a minimum, since the communication occurs very early in the frame.

In one embodiment of the present invention, the proposed IEEE802.11n frame in FIG. 4 may be preceded by IEEE802.11a preambles using the Signaling field (SIG-A), often used for ^λ spoofing' purposes (i.e. it tells the legacy devices to remain silent during the IEEE802.11n phase) . The inventive concept of the present invention presents a mechanism to communicate the presence of the ^λ Mixed' mode in the legacy part of the preamble, thereby ensuring that legacy devices are still able to use and decode the legacy part correctly. The proposed technique can also be used to transmit additional or alternative information in such an early part in the frame.

The corresponding extension of the SIG-MM carriers, to accommodate IEEE802.11n communication, and concurrently support embodiments of the present invention with respect to side-carriers, is illustrated in the simplified spectral plot 500 of FIG. 5. Here, the simplified

spectral plot 500 comprises frequency 505 versus spectral amplitude 510, with the boundary (frequency range) of the legacy spectrum 415 being identified as less than the boundary (frequency range) of the spectrum usage of IEEE802.11n including spectrum allocated for transmission of the information tones/bits on the side carriers 420.

Hence, as shown in one embodiment of the present invention, the receiver is able to identify the nature of the transmission by decoding the SIG-MM portion of the legacy part of the preamble 405, as transmitted in these side-carriers. It is proposed to encode one or several bits onto these side carriers.

In one embodiment of the present invention, all bits may be modulated onto a carrier, for example by using standard IEEE802.11n constellations (i.e. BPSK, QPSK, 16- QAM or 64-QAM) . Further, in one embodiment, only one bit may be modulated onto side-carriers based on BPSK constellation. Such an embodiment may be optionally combined with a rotation of any kind, such as defined in IEEE 802. Hn, where some signalling fields use a BPSK constellation rotated by 90 degrees, thereby supporting a simple detection algorithm distinguishing between BPSK and rotated-BPSK.

In addition, in one embodiment of the present invention, the amplitudes of all (or substantially all) "K" side- carriers may be used for communicating this additional information to the receiver. Examples of how these side- carriers may be used are illustrated below:

Option 1 :

A single-transmit-antenna case, whereby an initial constellation amplitude may be multiplied by weighting values of say, ^λ +/-l', corresponding to the entries of a predefined row of a NxN Walsh-Hadamard matrix (or indeed any other orthonormal matrix, such as a NxN Fourier matrix) . A definition of the Walsh-Hadamard matrix can be found at : htcp : //mathwqrld. wolfram. com/HadamardMa triz . html

Option 2 :

A single-transmit-antenna case, whereby an initial constellation amplitude may be multiplied by pre-defined, selectable, weighting values of say, ^λ +/-l', such that the time domain Peak-to-Average Power Ratio (PAPR) values are minimised.

Option 3:

A multiple-transmit-antenna case, suitable for OFDM use, whereby the resulting multiple carrier amplitudes, obtained from the ^λ Single-transmit-antenna case of option

1', may be used. In this embodiment, for each transmit antenna, a distinct cyclic delay is applied in accordance with the cyclic delays in the legacy synchronization preambles.

Option 4 :

A multiple-transmit-antenna case, based on option 2, whereby multiple initial constellation amplitudes are multiplied by weights, for example ^λ +/-l' corresponding to the entries of a predefined row of a NxN Walsh- Hadamard matrix (or NxN Fourier matrix or any other

orthonormal matrix) . In this embodiment, each transmit antenna uses a different row of the NxN Walsh-Hadamard matrix (or NxN Fourier matrix or any other orthonormal matrix) . In this manner, it is possible to reduce the effects of constructive/destructive interference. However, in this embodiment the reception of all MIMO LTS sequences is required before the different carrier amplitudes on the extended SIG field can be exploited.

Referring now to FIG. 6, an OFDM receiver 600 is illustrated in accordance with embodiments of the present invention. The OFDM receiver 600 comprises an antenna 601 connected to a receiver 603, which receives the signal from the OFDM transmitter 300 of FIG. 3. The receiver 603 demodulates, amplifies, down-converts and generates the OFDM symbols by applying a Discrete Fourier Transform (DFT) , as will be known to the person skilled in the art.

The receiver 603 is coupled to a symbol extractor 605 which is further coupled to a first symbol processor 607, a second symbol processor 609 and a user data decoder 611. The symbol extractor 605 extracts the OFDM symbols from the data packet transmitted in the SIG-MM field in at least one side carrier. The symbol extractor 605 specifically forwards the OFDM signaling symbol to the first symbol processor 607 and may transmit a second OFDM signaling symbol to a second symbol processor 609. In addition, the user data OFDM symbols are fed to the user data decoder 611.

The first symbol processor 607 processes the first OFDM signaling symbol and/or the second symbol processor 609 processes the second OFDM signaling symbol such that the corresponding data symbols of the first set of data symbols align in the two OFDM symbols. For example, the second symbol processor 609 may simply reorder the data of the second OFDM signaling symbol such that it is the same as the first OFDM signaling symbol. The first symbol processor 607 and second symbol processor 69 are coupled to a combiner 613 which combines the data of the first and second OFDM signaling symbols.

In a simple embodiment, the combiner 613 simply performs a selection combining wherein the combiner 613 for each sub-carrier selects the data symbol value of either the first or the second OFDM signaling symbol. The combiner may for example select the data symbol having the highest received signal value.

The combiner 613 is coupled to a receive controller 615 which is fed the combined data values. Thus, in the specific example, the receive controller 615 receives the SIG-MM field which reflects the transmission parameters used by the OFDM transmitter (say, transmitter 300 in FIG. 3) for transmitting the user data.

The receive controller 615 is coupled to the user data decoder 611 and feeds the information of the applied physical layer transmission parameters to this. In response, the user data decoder 611 decodes the received OFDM user data symbols. For example, the user data decoder 611 proceeds to apply the appropriate forward

error correcting decoding scheme and to decode the resulting data according to the symbol constellations defined by the SIG-MM field.

In one embodiment of the present invention, it is envisaged that a simple power detection can be applied to detect whether tones are located in the extended side- carriers in the SIG-MM field (thereby indicating the presence of the ^λ Mixed' mode signal. Thus, if the corresponding detected power lies above a given threshold, the SIG-MM field is considered to indicate the presence of the ^λ Mixed' mode.

Alternatively, a more powerful detection mechanism comprises a probabilistic detection of the presence of the additional carriers. In this embodiment, the channel coefficients of the new side carriers may be obtained via interpolation, since they cannot be derived from the transmitted learning symbol (LTS) . The use of interpolation may also be used to set the same carriers to ^λ zero', as the current legacy signaling field does. Thereafter, the corresponding decision is performed by determining a phase of the extra carrier amplitudes. A comparison may then be performed, on a per carrier basis, between a Gaussian distribution centred at zero

(corresponding to the legacy case) , and a Gaussian distribution centred at the considered carrier amplitudes/phases .

Thus, in one embodiment of the present invention, as illustrated with reference to FIG. 7, a method 700 for detecting a presence of IEEE802.11n channel information

(over 56 carriers) based on the legacy LTS field (over 52 carriers) , is described. In one embodiment of the present invention, a threshold may be used to indicate whether a particular simulation value contains sufficient confidence that it can be relied upon. That is, below a given threshold value, directly related to the channel model, each point that fails to satisfy a confidence metric will not be considered.

First, and taking into account the slowly varying behaviour of the IEEE802.11a channel and IEEE802.11n channel, the channel information collected over the legacy LTS field is extrapolated. The extrapolation may be performed by first examining the n first terms and n last terms of the sub-carriers, as shown in step 710. Thereafter, the method determines the number (n _eff ) of the first (respectively last) unusable term discriminated against by the above threshold as in step 715. The method then calculates a polynom with a degree equal to (n _ef f-l) that interpolates the n _eff points, as shown in step 720. The values of the new side carriers are then calculated, in step 725, and these values are then compared to the confidence threshold, in step 730. If the calculated values are below the confidence threshold in step 730, they are set to the first (respectively last) value of the IEEE802.11a channel, as shown in step 735. If the calculated values are above the confidence threshold in step 730, they are used in the decoding process and identify that a pure IEEE802.11n transmission has been received, as shown in step 740.

One embodiment of the present invention will be described in terms of an inclusion of signalling information into parts of the OFDM spectrum that is defined to be unused in the corresponding standard. However, it will be appreciated by a skilled artisan that the inventive concept herein described may be embodied in any type of information (and is therefore not limited to signalling information) . In a number of applications, the inclusion of signalling information in used parts of the spectrum, in accordance with embodiments of the present invention, effectively helps to improve the implementation, since the available signalling information typically reduces latency requirements. In this manner, the inventive concept herein described considerably reduces power consumption requirements of devices.

Furthermore, it is envisaged that the inventive concept is not limited to use in wireless communication systems. It is envisaged that the inventive concept herein described may equally be applied to any OFDM based system.

It will be understood that the improved wireless communication system, wireless communication unit, and method of operation therefor, as described above, aims to provide at least one or more of the following advantages:

(i) The legacy part of the IEEE802.11n preamble is used for communicating an indication of frame format, whilst enabling legacy devices to still synchronize efficiently and decode the SIG-MM field; and

(ii) The receiver latency requirements are kept to a minimum, since the communication occurs very early in the frame.

In particular, it is envisaged that the aforementioned inventive concept can be applied by a semiconductor manufacturer to any encoding/decoding integrated circuit. It is further envisaged that, for example, a semiconductor manufacturer may employ the inventive concept in a design of a stand-alone device, or application-specific integrated circuit (ASIC) and/or any other sub-system element.

It will be appreciated that any suitable distribution of functionality between different functional units or elements or logic circuits, may be used without detracting from the inventive concept herein described.

Hence, references to specific functional devices or logic circuits or elements are only to be seen as references to suitable means for providing the described functionality, rather than indicative of a strict logical or physical structure or organization.

Aspects of the invention may be implemented in any suitable form including hardware, software, firmware or any combination of these. The elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed, the functionality may be implemented in a single unit or IC, in a plurality of units or ICs or as part of other functional units.

Although the present invention has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognize that various features of the described embodiments may be combined in accordance with the invention. In the claims, the term Comprising' does not exclude the presence of other elements or steps.

Furthermore, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. Also, the inclusion of a feature in one category of claims does not imply a limitation to this category, but rather indicates that the feature is equally applicable to other claim categories, as appropriate .

Furthermore, the order of features in the claims does not imply any specific order in which the features must be performed and in particular the order of individual steps in a method claim does not imply that the steps must be performed in this order. Rather, the steps may be performed in any suitable order. In addition, singular references do not exclude a plurality. Thus, references to "a", "an", "first", "second" etc. do not preclude a plurality.

Thus, an improved wireless communication system, wireless communication unit and method of operation therefor have been described, wherein the aforementioned disadvantages with prior art arrangements have been substantially alleviated.