This invention relates to low power digital radio communications. It relates more specifically, although not exclusively, to establishing connections between devices in an Orthogonal Frequency Division Multiplexing (OFDM) radio system. BACKGROUND

OFDM is a form of radio transmission that is used in various radio protocols such as Long Term Evolution (LTE™), various IEEE™ 802.11 standards, DAB™ radio, DVB-T, and WiMAX™. Rather than encoding data on a single carrier frequency, a data stream is spread over some or all of a radio channel containing multiple OFDM subcarriers. The OFDM subcarriers are typically closely spaced, at regular intervals, across the frequency spectrum, although this is not essential. The subcarriers are orthogonal to avoid mutual interference. OFDM can thereby provide good resilience to multipath fading and to external interference. In order for a data packet to be successfully received by an intended recipient, it is necessary for the recipient to be synchronized to the transmitter (i.e. to determine and correct for frequency and timing offsets), which is achieved by means of a synchronization training field (STF) of the data packet, and to learn details of the specific characteristics of the transmission channel between the transmitter and receiver through means of a channel training field (CTF) where, for example, refinement of frequency and timing can be achieved. This allows the receiver to decode a further part of the packet, such as a physical header field (PHF) which gives information about the transmission, such as the modulation and coding scheme, that will allow the transmitter to decode the subsequent data field (DF) of the packet.

ETSI TR 103514 V1.1.1 (2018-07) ‘Digital Enhanced Cordless Telecommunications (DECT) DECT-2020 New Radio (NR) interface Study on Physical (PHY) layer’ (the “DECT-2020 proposal”) proposes a packet structure in which includes an ‘A0’ field which is equivalent to a PHF and so contains information for decoding the subsequent DF. The DECT-2020 proposal also provides for an option extension to the A0 field - the ‘AT field - which allows extra signalling if required. The presence of the A1 field is signalled in the A0 field. To date it has not been determined whether the A1 field would be of fixed length or whether the receiver would need to ‘blind decode’ the A1 field without knowing its length. Similarly it has not been decided whether the A1 field will have the same modulation and coding scheme as at A0 field and/or the data field or whether blind decoding will need to be employed for this too.

SUMMARY

The Applicant has made certain developments over previous proposals and when viewed from a first aspect the invention provides a digital radio communication system comprising a transmitter and a receiver arranged to operate according to a predetermined communication protocol, wherein said transmitter is arranged to transmit a data packet comprising: a data field; a first physical header field indicating at least a transmission type, information regarding modulation and/or coding of said data field and an indicator that a further physical header field is being transmitted; and a second physical header field indicating at least a transmission type, information regarding modulation and/or coding of said data field and an indicator as to whether a further physical header field is being transmitted.

The invention extends to a digital radio transmitter arranged to operate according to a predetermined communication protocol, wherein said transmitter is arranged to transmit a data packet comprising: a data field; a first physical header field indicating at least a transmission type, information regarding modulation and/or coding of said data field and an indicator that a further physical header field is being transmitted; and a second physical header field indicating at least a transmission type, information regarding modulation and/or coding of said data field and an indicator as to whether a further physical header field is being transmitted. The invention also extends to a digital radio receiver arranged to operate according to a predetermined communication protocol, wherein said receiver is arranged to receive a data packet comprising: a data field; a first physical header field indicating at least a transmission type, information regarding modulation and/or coding of said data field and an indicator that a further physical header field is being transmitted; and a second physical header field indicating at least a transmission type, information regarding modulation and/or coding of said data field and an indicator as to whether a further physical header field is being transmitted.

The invention also extends to a method of operating a digital radio communication system comprising a transmitter and a receiver operating according to a predetermined communication protocol, the method comprising said transmitter transmitting a data packet comprising: a data field; a first physical header field indicating at least a transmission type, information regarding modulation and/or coding of said data field and an indicator that a further physical header field is being transmitted; and a second physical header field indicating at least a transmission type, information regarding modulation and/or coding of said data field and an indicator as to whether a further physical header field is being transmitted.

The invention also extends to a method of operating a digital radio transmitter according to a predetermined communication protocol, the method comprising said transmitter transmitting a data packet comprising: a data field; a first physical header field indicating at least a transmission type, information regarding modulation and/or coding of said data field and an indicator that a further physical header field is being transmitted; and a second physical header field indicating at least a transmission type, information regarding modulation and/or coding of said data field and an indicator as to whether a further physical header field is being transmitted. The invention also extends to a method of operating a digital radio receiver according to a predetermined communication protocol, the method comprising said receiver receiving a data packet comprising: a data field; a first physical header field indicating at least a transmission type, information regarding modulation and/or coding of said data field and an indicator that a further physical header field is being transmitted; and a second physical header field indicating at least a transmission type, information regarding modulation and/or coding of said data field and an indicator as to whether a further physical header field is being transmitted.

The invention also extends to a non-transitory computer readable medium comprising instructions configured to cause a digital radio communication system to operate in accordance with the method set out above.

The invention also extends to a non-transitory computer readable medium comprising instructions configured to cause a digital transmitter to operate in accordance with the method set out above.

The invention also extends to a non-transitory computer readable medium comprising instructions configured to cause a digital radio receiver to operate in accordance with the method set out above.

Thus it will be seen by those skilled in the art that in accordance with the invention, a flexible number of additional PHFs can be included in the packet without the receiver having to ‘blind’ decode them since it can determine from the preceding one whether a further PHF is to be expected. This provides extra signalling capacity when needed - i.e. without permanently lengthening the packet - whilst allowing for efficient decoding by the receiver.

The additional signalling capacity which may be provided in accordance with the invention may be used for a number of different things. Advantageously however it may be used to enable different modulation/coding schemes to be applied to different blocks of data in the packet and thus in a set of embodiments the first physical header field indicates information regarding modulation and/or coding of a first block of data and the second physical header field indicates information regarding modulation and/or coding of a second block of data. The data field would thus comprise the first and second blocks of data.

As will be appreciated, this allows blocks of data with different modulation/coding schemes to be multiplexed in a single packet. This may satisfy, in an efficient manner, requirements at higher levels of the protocol to provide higher quality of service levels to some types of data than to others.

Additionally or alternatively the additional signalling capacity which may be provided in accordance with the invention may be used to enable different recipient addresses to be set for different blocks of data in the packet and thus in a set of embodiments the first physical header field indicates one or more recipients for a first block of data and the second physical header field indicates one or more recipients for a second block of data. As will be appreciated, this allows blocks of data intended for different recipients or groups of recipients to be multiplexed in a single packet.

The second physical header field could be located after the first physical header field, prior to the data field. However in a set of embodiments a first block of data follows the first physical header field, the second physical header field follows the first block of data and a second block of data follows the second physical header field. Having the data blocks alternate with a corresponding PHF is particularly advantageous where, as set out above, the data blocks have different modulation/coding schemes and/or different recipients as it allows the information necessary to decode the respective data blocks to be more closely associated with them. In such embodiments the data field may be distributed across the packet.

The first and second physical header fields may have the same format as each other. However this is not essential. For example as certain details such as the identity of the transmission and, typically, the transmission power will be common to the whole packet, this need not be repeated. This may mean for example that such information is contained only in the first PHF and not in the second, so that the second PHF can contain further information or be shorter than the first. Although not essential (e.g. it could be pre-defined) the first and second physical header fields specify a length of the respective data blocks - e.g. as a number of sub-slots.

In a set of preferred embodiments the first and second data blocks are independent of each other. As will be appreciated by those skilled in the art this may mean that they relate to independent automatic repeat request (ARQ) processes, that the receiver indicates separately for each data block that it had been received correctly and/or they comprise different media access control (MAC) protocol data unit (PDU) types.

In an exemplary set of embodiments the first data block comprises a broadcast or multicast message (i.e. one intended for all or a plurality of receivers) as indicated in the first PHF and the second data block comprises a message for a smaller number of receivers (e.g. a unicast message) as indicated in the second PHF.

Such an arrangement exploits an advantage which may be achieved in accordance with the invention of using a single synchronisation training field in the packet to train all the receivers which enables the two transmission types outlined above. By contrast if it were necessary to transmit separate packets for the two types of message, it would be necessary to include synchronisation training fields in each one - leading to a greater transmission overhead (which may translate into greater power use by the transmitter).

Typically the data packet will comprise a channel training field (CTF). A single CTF could be provided for the packet - e.g. before the first physical header field but in a set of embodiments, at least one further channel training field is provided - e.g. after the first data block and before the second PHF. In other words separate channel training fields may be provided for the respective data blocks. This may enable improved channel estimation quality for the extended parts of the packet.

In a set of embodiments the indicator in the respective physical header fields comprises a bit which has a first value - e.g. Ί ’ - if a further physical header field is being transmitted and a second value - e.g. Ό’ - if a further physical header field is not being transmitted. As will be appreciated, the invention has been described thus far in terms of a second PHF (and in embodiments a second data block corresponding) but this is of course not limiting and more than two PHFs can be transmitted. Each simply needs to indicate whether it is the last or whether at least one further one is to come and the receiver can decode accordingly. Moreover this can in principle be changed from one packet to the next - providing significant flexibility and efficiency in how the communication operates.

In a set of embodiments the transmitter and receiver are arranged to operate Orthogonal Frequency Division Multiplexing (OFDM). The data field, or the respective data blocks, preferably comprises or each comprise a plurality of OFDM symbols.

In a set of embodiments the protocol is in accordance with DECT-2020 proposal to the extent that it does not conflict with the operation set out herein - for example having a frame of 24 slots, each slot comprising ten OFDM symbols.

In a set of embodiments the transmitter and receiver are each able to function in the other role as well - e.g. they may operate in time division duplexing so that the two communicating devices alternate the transmitter and receiver roles

The transmitter and receiver could be used in any of a wide range of applications such as cordless telephones or wireless microphones but the applications are not limited to transmission of voice and can include e.g. sensors and actuators for smart buildings, factories or cities.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Fig. 1 is a schematic diagram illustrating a radio communication system in accordance with the invention;

Fig. 2 is a schematic diagram of a known data packet structure; Fig. 3 is a schematic diagram of a possible data packet structure in accordance with a first embodiment of the invention;

Fig. 4 is a schematic diagram of an extended data packet structure in accordance with the first embodiment of the invention;

Fig. 5 is a schematic diagram of an alternative extended data packet structure in accordance with a second embodiment of the invention;

Fig. 6 is a flowchart indicating the operation of a receiver device in receiving an extended data packet shown in accordance with the first embodiment of the invention;

Fig. 7 is a schematic diagram of a possible data packet structure in accordance with a third embodiment of the invention;

Fig. 8 is a more detailed schematic diagram of subcarrier allocations of the possible data packet structure shown in Fig. 7;

Fig. 9 is a schematic diagram of an extended data packet structure in accordance with the third embodiment of the invention;

Fig. 10 is a schematic diagram of an alternative extended data packet structure in accordance with a fourth embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a radio system comprising a first radio transceiver device 100 and a second radio transceiver device 102. The first radio transceiver device comprises an antenna 104; and the second transceiver comprises an antenna 106. As will be well understood by those skilled in the art, a number of standard modules such as processors, oscillators, filters, amplifiers, digital to analogue converters (DACs) and analogue to digital converters (ADCs) are provided in the radio transceivers 100 and 102 but the description of these is omitted for the sake of brevity. As will also be understood by one skilled in the art, the radio transceivers 100 and 102 may comprise any number of antennas, e.g. to support operating using MIMO (multiple input, multiple output) transmission modes.

Fig. 1 also shows the signal path 108 from the first transceiver when it is acting as a transmitter through its antenna 104 to the second transceiver 102 acting as a receiver through its antenna 106. The transceivers are configured to operate using OFDM (Orthogonal Frequency Division Multiplexing) as is known perse in the art.

Fig. 2 shows an example structure of a prior art data packet 200, as proposed in DECT(19)000199r2 - Revised Physical layer formats for packet-mode services, Wireless Partners S.L.L. The data packet comprises: a Guard Field (GF) 214, a Synchronisation Training Field (Short) (STFS) 202, a Channel Training Field (CTF) 204, an A0 Field 206, a Channel Training Field for MIMO (CTFM) 207, an A1 Field 211 and a B (Data) Field 208. The lower part of the diagram shows various of these fields in more detail.

In this prior art example, the purpose of the A0 field 206 is to signal the parameters required to decode the B field 208. However, sometimes the amount of data required to do this is larger than can be accommodated by the A0 field 206. This is addressed by the addition of the A1 field 211 which acts as an extension to the A0 field to provide more signalling capacity. The presence of the A1 field signalled in the A0 field but its length may not be which requires the receiver to ‘blind decode’ the A1 field in order to determine its length. There is only a single B field 208 which therefore has the same modulation and coding and can only be addressed to the recipient.

Fig. 3 shows an example structure of a data packet 300 in accordance with a first embodiment of the present invention. The data packet 300 comprises: a Synchronisation Training Field (STF) 302, a Channel Training Field (CTF) 304, a Physical Header Field (PHF) 306, a Data Field (DF) 308, and a Guard Field (GF) 314. The lower part of the diagram shows various of these fields in more detail.

The STF 302 comprises an STF training symbol defined in the frequency domain using every fourth subcarrier. The STF training symbol is converted to the time domain using a Fast Fourier Transform in which it has a size of NFFT made up of four repetitions of a signal pattern each of length NFFT/4. TO this time domain symbol is added a cyclic prefix of size 3*NFFT/4, thus producing complete synchronization training field of length 7*NFFT/4 samples with seven repetitions 310 of a signal pattern. The synchronisation signal is designed in a manner to allow the receiver to correct for receiver gain, frequency and timing errors relative to the transmitter. In order to accomplish this, the signal should have good correlation properties.

Following the STF 302 is the Channel Training Field (CTF) 304. The CTF 304 is used by the receiver to learn further properties of the transmission channel(s) for each transmitted stream and to refine the receiver’s estimate of the timing and frequency offset. The CTF 304 comprises two CTF symbols 316, each preceded by a Cyclic Prefix (CP) 312. In certain embodiments, wherein a MIMO transmission mode is used, each CTF symbol 316 may contain channel training information for up to different four spatial streams. In other embodiments, wherein a MIMO transmission mode is used, the CTF 302 may reserve a symbol for each spatial stream in use.

Following the CTF 304 is the Physical Header Field (PHF) 306, which is subsequently followed by the Data Field (DF) 308. The PHF 306 comprises up to two PHF symbols 320, each preceded by a cyclic prefix 318. In this embodiment, the PHF signals the parameters required by the receiver to decode the DF 308 - specifically the modulation and coding scheme (MCS) - as well as carrying all the other information required by the receiver. These include for example the header type (Broadcast, Multicast, Unicast, or Unicast-no-HARQ), transmit power, length of data filed (in sub-slots) transmission mode, transmitter ID and cyclic redundancy. For unicast transmissions it will also include the DF HARQ process number. Typically the payload bits are masked with a random signal generated for instance from the network ID, frame and slot numbers and the CRC bits are typically masked with the ID of the intended receiver.

In addition to this, the first PHF 306 also comprises a single extension bit that indicates whether a packet will be extended by a further PHF being transmitted. In this example, the extension bit in the PHF 306 is set to 0, indicating that the packet is not extended and thus contains only a single PHF 306 and DF 308. An extended packet will be described in greater detail with reference to Fig. 4.

The DF 308 comprises a number of OFDM data symbols 324, each preceded by a respective cyclic prefix 322. Immediately following the DF 308 is the Guard Field (GF) 314, marking the end of the packet.

Fig. 4 shows an example structure of an extended data packet 400 in accordance with the first embodiment of the invention. In common with the data packet 300 of Fig. 3, the data packet 400 comprises: a Synchronisation Training Field (STF) 402, a Channel Training Field (CTF) 404, a first Physical Header Field (PHF) 406, a first Data Field block (DF block) 408, a second PHF 410, a second DF block 412 and a Guard Field (GF) 414. Each of these fields have the same structure as described previously with reference to Fig. 3.

In this example, the extension bit in the first PHF 406 is set to 1, indicating that the packet is extended: i.e. that a second PHF 410 and DF block 412 will immediately follow the first DF block 408.

The second PHF 410 is of the same structure as the first PHF 406 and thus indicates, inter alia, the length of the second DF block 412, the type of transmission (e.g. unicast), the modulation and coding scheme and the intended recipient. As will be appreciated this allows the second DF block 412 to have a different level of modulation/coding - and so a different level of certainty of being received correctly - than the first DF block 408. This may support a requirement dictated by a higher layer of the protocol to transmit some data with better quality of service parameters than others without requiring a completely separate packet. Additionally or alternatively it allows the second DF block 412 to be sent to a different recipient to the first DF block 408.

The extension bit in the second PHF 410 is set to 0, indicating that the corresponding second DF 412 will be the final one in the packet and will not be followed by a further PHF and DF block. Of course the packet may be extended further by setting this bit to in which case the receiver will know that a further PHF and DF block will follow. Fig. 5 shows an example alternative structure of an extended data packet 500 in accordance with a second embodiment of the invention. The data packet 500 comprises: a Synchronisation Training Field (STF) 502, a first Channel Training Field (CTF) 504, a first Physical Header Field (PHF) 506, a first Data Field block 508, a second CTF 509, a second PHF 510, a second DF block 512 and a Guard Field (GF) 514. Each of these fields comprise the same symbol structure as described previously with reference to Fig. 3. In this embodiment, as with the first embodiment shown in Fig. 4, the extension bit in the first PHF 506 is set to 1 , indicating that the packet is extended in the a similar way to that described previously with reference to Figure 4. However in this example the packet comprises a second Channel Training Field (CTF) 509 immediately following the first DF block 508 and immediately preceding the second PHF 510. This is decoded in the same way as described above for the first CTF 504 but improves the channel estimation quality for the extended parts of the packet.

Operation of the receiver device 102 in receiving the extended data packet shown in Fig. 4, will now be described with reference to Fig. 6. At step 600 the receiver 102 receives, using its antenna 106, the first part of the data packet 400, which is the STF 402. The receiver 102 then uses a standard auto-correlation technique on the received signal samples using the following formula: where p _n denotes the auto-correlation value, r _n denotes the received signal samples, L _REP is the length of a period and N _REP is the number of periods used for the process of autocorrelation. The synchronisation signal is declared to be found when

\Pn\ ² > T (2), where T is a threshold value chosen to keep the number of false detections acceptably small. As is well known per se in the art, the timing and frequency of the correlation threshold being reached can be used to give an estimate of the frequency and timing offset of the received signal relative to the receiver. The estimate of the frequency offset Af may be calculated from the maximum autocorrelation value ' p _n ^li max where n _max is the time instant at which the synchronisation signal was declared to be found, T _s is the sampling period of the system, L _REP is the length of the period, and all other symbols have their standard mathematical meaning.

The received signal is then corrected to account for the estimated frequency error Af and the timing error by the receiver (step 602). The STF may be further processed for instance with cross-correlation with a pre-defined synchronization sequence in order to provide a refined frequency and timing offset estimate.

The receiver then receives the CTF 404 (step 604) and calculates estimates of the radio channel impulse response for the transmit antenna (or each transmit antenna if Ml MO is being used) to obtain an estimate of the channel for that antenna (step 606). The channel estimate is then used to demodulate and decode the PHF 406.

The receiver then receives the PHF 406 (step 608). Contained within the PHF 406 are the parameters required to decode the subsequent DF 408, such as the modulation and coding scheme which has been employed and the number of sub slots in the data field block. The DF block 408 is received by the receiver at step 610 and the receiver then decodes it using the above-mentioned parameters at step 612. The first DF block 408 may for example be a broadcast signal, intended for all receivers, and transmitted with a low order modulation.

If the extension bit in the most recently received PHF 408 is set to T, the receiver will return to step 608 and repeat this cycle. It will thus receive and decode the second PHF 410 which contains the parameters for decoding the DF block 412 which follows it. The second PHF 410 might indicate that the data in the second DF block 412 is unicast data, intended for this specific receiver and coded using a specific transmission mode, modulation and coding. Once the receiver returns to the comparison of the extension bit at step 614, in the second PHF 410, this is set to O’, which indicates that there are no more PHFs. The receiver thus knows that the packet will end after the second DF 412 and thus the receiver will end the reception and decoding process (step 616) after receiving the DF 412, and prepare to receive more packets.

If the alternative packet structure shown in Fig. 5 is employed, the method set out above will be modified to decode the additional channel training field 509 which precedes the second PHF 510.

Fig. 7 shows an example structure of a data packet 700 in accordance with a third embodiment of the present invention. The data packet 700 comprises: a Synchronisation Training Field (STF) block 702, a Data Field (DF) block 704, and a Guard Field (GF) block 706. The lower part of the diagram shows various of these fields in more detail.

As in the previous embodiments, the STF block 702 comprises an STF training symbol defined in the frequency domain using every fourth subcarrier, giving seven repetitions 708 of a signal pattern, as described in more detail with reference to the STF 302 of Fig. 3.

Following the STF block 702 is the Data Field (DF) block 704. The DF block 704 comprises eight symbols 708 to 722, each preceded by a Cyclic Prefix (CP) 724.. In the example shown in Fig. 7, each symbol has a FFT size of sixty-four - i.e. there are sixty-four available subcarriers within each symbol. It will be understood that the FFT size of each symbol is not limited to sixty-four, but may be any number - e.g. 128, 256, 512, 768 or 1024. Various subcarriers of the symbols 708 to 722 are allocated to transmission of Demodulation Reference Signals (DRS) 725, a Physical Control Channel (PCC) 730 and a Physical Data Channel (PDC) 732, as will be described in more detail with reference to Fig. 8.

Fig. 8 shows an example of possible subcarrier allocations for the data packet 700. The vertical axis shows the indices of each subcarrier, and the horizontal axis shows the index of each OFDM symbol. The symbol of index zero is equivalent to the STF 702 of Fig. 7, the symbols of index one to eight are equivalent to the first to eighth DF symbols 708 to 722 of Fig. 7, and the symbol of index nine is equivalent to the Gl 706 of Fig. 7. In symbol zero, every fourth subcarrier is allocated the STF 728 in order to create a repeating signal pattern, as described previously. The other subcarriers in symbol zero are unallocated. In symbol one, some subcarriers are allocated to the DRS 725 and the remaining subcarriers are allocated to the PCC 730. In symbol two, all of the available subcarriers are allocated to the PCC 730. The available subcarriers of the remaining symbols three to eight are allocated to the PDC 732, with the exception of some subcarriers of symbol six which are allocated to the DRS 725.

In the example shown in Fig. 8, the FFT size is equal to sixty-four and the number of transmit antennas is equal to one. It will be understood that other FFT sizes and numbers of transmit antennas may be used, with the subcarrier allocation of the packet 700 being changed accordingly. In general, the transmitter 100 first allocates the DRS 725 to the required number of subcarriers in symbol one (and symbol two if necessary) and symbol six (and symbol seven if necessary). The transmitter 100 then allocates ninety-eight subcarriers (in some embodiments) that are not already allocated to the DRS 725 in the first symbols (symbols one and two) to the PCC 730. In examples where symbols one and two do not contain enough available subcarriers to allocate all necessary subcarriers to the PCC 730, some subcarriers in symbols three and four may also be allocated to the PCC 730.

Once the transmitter 100 has allocated the subcarriers required for the DRS 725 and the PCC 730, the remaining subcarriers of the DF 704 are allocated to the PDC 732. If symbol one contains enough available subcarriers to fully allocate the subcarriers required for the DRS 725 and PCC 730 (e.g. if the FFT size is large), then some subcarriers of symbol one may be allocated to the PDC 732.

The DRS 725 are reference signals which allow the receiver 102 to perform channel estimation for each transmit stream used by the transmitter 100; the subcarriers allocated to the PDC 732 carry the payload of the data packet 700; and the subcarriers allocated to the PCC 730 signal the parameters required by the receiver to decode the PDC 732 - specifically the modulation and coding scheme (MCS) and the total length of the DF 704 - as well as carrying all the other information required by the receiver. The data transmitted in (i.e. the payload of) the PCC 732 is the same as that transmitted over the Physical Header Field (PHF) 306 of Fig. 3, but in this example the name of the field in the physical layer specification is different as the data is further encoded, rate-matched and modulated in order to transmit the data over ninety-eight subcarriers which are allocated to the PCC 732 in the DF block 704. It will be understood that the number of subcarriers allocated to the PCC 732 is not limited to ninety-eight, but may be any number.

In addition to this, in accordance with the invention, the payload of the PCC 730 (which is equivalent to the PHF 306 of Fig. 3) also comprises in the Media Access Control (MAC) layer a single extension bit that indicates whether the packet 700 will be extended, with a further DF block being transmitted. In this example, the extension bit in the payload of the PCC 730 is set to O’, indicating that the packet is not extended and thus the DF block 704 contains only eight symbols 708 to 722. Returning to Fig. 7, the data packet 700 therefore contains only a single full slot in this example - i.e. ten OFDM symbols including the STF 702 and Gl 706. In some examples, the subcarrier width can be increased (e.g. two times, four times or eight times): this may permit a proportional decrease in the duration of each OFDM symbol and therefore allow more symbols to be transmitted in each slot accordingly (e.g. twenty, forty or eighty). An extended packet will be described in greater detail with reference to Figs. 9 and 10.

As previously explained, a subset of the available OFDM subcarriers are used to transmit Demodulation Reference Signals (DRS) 725 during the first DF symbol 708. Similarly, during the sixth DF symbol 718 a subset of the available OFDM subcarriers are also used to transmit the DRS 725. .This is indicated by the dark stripes shown on these symbols 708, 718.

The DRS 725 are transmitted regularly (every five symbols in this example) in order to allow the receiver 102 to maintain up-to-date channel estimation for each transmit stream. Consequently, the number of subcarriers used to transmit the DRS 725 is dependent on the number of transmit streams in use by the transmitter 100.

It will be understood that the DRS 725 is not limited to being transmitted during the first DF symbol 708 and sixth DF symbol 718, but may be transmitted over a subset of the available OFDM subcarriers during any of the DF symbols 708 to 722. In embodiments where the number of transmission antennas is two or less, the DRS 725 are transmitted on every fifth DF symbol, starting with the first DF symbol 708.

In embodiments where the number of transmission antennas is greater than two the DRS 725 are transmitted on every tenth DF symbol, starting with the first. Fig. 9 shows an example structure of an extended data packet 800 (relative to the non-extended data packet 700 shown in Fig. 7) in accordance with the third embodiment of the invention. In common with the data packet 700 described above, the data packet 800 comprises corresponding: STF 802, first DF block 804, second DF block 805 and Gl 808. The STF 802, Gl 808 and first DF block 804 have the same structure as described previously with reference to Figs. 7 and 8. The second DF 805 comprises five symbols 826 to 834, each preceded by a CP 836.

In this example, the extension bit in MAC layer of the payload of the PCC (which is equivalent to the PHF 306 of Fig. 3) of the first DF block 804 is set to , indicating that the packet is extended: i.e. that a second DF block 805 will immediately follow the first DF block 804. The combined length of the two parts of the data field 804, 805 is thus thirteen symbols - i.e. five more than in Fig. 7.

The subcarriers allocated to the PCC of the second DF block 805 are of the same structure as the subcarriers allocated to the PCC of the first DF block 804 and thus indicate, inter alia, the length of the second DF block 805, the modulation and coding scheme (MCS) and the intended recipient. As will be appreciated this allows the second DF block 805 to have a different level of modulation/coding - and so a different level of certainty of being received correctly - than the first DF block 804. This may support a requirement dictated by a higher layer of the protocol to transmit some data with better quality of service parameters than others without requiring a completely separate packet. Additionally or alternatively it allows the second DF block 805 to be sent to a different recipient to the first DF block 804.

The extension bit in the MAC layer of the payload of the PCC of the second DF block 805 is set to O’, indicating that the corresponding second DF block 805 will be the final one in the packet and will not be followed by a further DF block. Of course the packet may be extended further by setting this bit to in which case the receiver will know that a further DF block will follow.

Within the second DF block 805, two symbols 826 and 828 which have subcarriers allocated only to the PDC precede the symbols 830 and 832 which have subcarriers allocated to the DRS 824 and PCC in a similar manner to symbols one and two of Fig. 8. The receiver 102 is therefore unable to decode the first two symbols 826 and 828 until after it has received the two symbols 830 and 832. In order to do this, the receiver 102 buffers (i.e. temporarily stores in memory) the first two symbols 826 and 828 it receives so that it can decode them after it receives the two symbols 830 and 832.

In the embodiment shown in Fig. 9, the overhead required to transmit the STF 802 and Gl 806 are subtracted from the first DF block 804. However, in other embodiments, the overhead required to transmit the STF 802 and Gl 806 are subtracted from the second DF block 805 instead of the first DF block 804. An example of this is shown in the further embodiment shown in Fig. 10, in which the data packet 900 comprises a lengthened first DF block 904 and a shortened second DF block 905 when compared to the first DF block 804 and second DF block 805 shown in Fig. 9. The first DF block 904 of Fig. 10 is two symbols longer than the first DF block 804 of Fig. 9, and the second DF block 905 of Fig. 10 is two symbols shorter than the second DF block 805 of Fig. 9. Otherwise, the structure of the data packet 900 is much the same as that described with reference to the data packets 700 and 800 shown in Figs. 7, 8 and 9.