WO2021224127A1

WO2021224127A1 - Digital radio communications

Info

Publication number: WO2021224127A1
Application number: PCT/EP2021/061398
Authority: WO
Inventors: Juha Heiskala; Heikki Berg; Mikko Kokkonen
Original assignee: Nordic Semiconductor Asa
Priority date: 2020-05-04
Filing date: 2021-04-30
Publication date: 2021-11-11
Also published as: GB202006580D0

Abstract

A digital radio transmitter (110) is arranged to operate according to a predetermined communication protocol and comprises a plurality of transmit antennas (114). The transmitter (110) is arranged to transmit a data packet (200) comprising: a reference signal (408) to allow a receiver (112) to perform channel estimation, a data channel (410) comprising a payload of the data packet (200), and a control channel (406) comprising information required to decode the data channel (410). The transmitter (110) is arranged to transmit each of the reference signal (408), data channel (410) and control channel (406) using beamforming employing a common set of beamforming parameters.

Description

Digital Radio Communications

FIELD

This invention relates to low power digital radio communications. It relates more specifically, although not exclusively, to devices in an Orthogonal Frequency Division Multiplexing (OFDM) radio system supporting Multiple-Input-Multiple- Output (MIMO) or Multiple-Input-Single-Output (MISO) transmission modes and beamforming (precoding).

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 some radio communication applications, the MIMO principle is employed whereby the transmitter and receiver are provided with multiple antennas which can be exploited in a number of ways. MIMO communications are dependent on the fact that transmission antennas are ‘separated’ between each other and receiver antennas are also separated between each other. This separation enables frequency selectivity between transmission paths to be independent. Separation can be either physical (distance) or polarisation diversity (two antennas are orthogonally polarized) or combination thereof.

In the MISO principle, the receiver only has (or uses) as single antenna but the transmitter has multiple antennas.

In some MIMO/MISO radio communication applications, the principle of beamforming is employed whereby a transmitter varies the phase and/or magnitude of signals transmitted from different antennas to generate regions of constructive and destructive interference, thereby allowing the transmitter to ‘direct’ the transmitted signal towards a receiver. Beamforming may be achieved in both the analogue domain and the digital domain (otherwise known as precoding), and can be performed for both open-loop and closed-loop transmission modes. In closed- loop transmission modes, the parameters used for beamforming are dependent on feedback received by the transmitter from a receiver on instantaneous channel conditions; in open-loop transmission modes the parameters used for beamforming are fixed and do not depend on instantaneous channel conditions.

In order to decode the information in channel packet, a receiver is required to estimate channel characteristics (i.e. the effect of multiple paths in wireless radio transmission).. To estimate channel characteristics some radio protocols use reference signals, defined in the frequency domain, to allow a receiver to estimate the channel characteristics. Since these channel characteristics are highly dependent on any beamforming performed by the transmitter, it is necessary for the transmitter to know what beamforming has been applied. The present invention seeks to provide a way of doing this.

SUMMARY

When viewed from a first aspect the invention provides a digital radio transmitter arranged to operate according to a predetermined communication protocol and comprising a plurality of transmit antennas, wherein said transmitter is arranged to transmit a data packet comprising: a reference signal to allow a receiver to perform channel estimation; a data channel comprising a payload of the data packet; and a control channel comprising information required to decode the data channel; wherein the transmitter is arranged to transmit each of the reference signal, data channel and control channel using beamforming employing a common set of beamforming parameters. The invention 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 reference signal; a data channel comprising a payload of the data packet; and a control channel comprising information required to decode the data channel; wherein the receiver is arranged to: use the reference signal for performing channel estimation; and decode said packet by assuming that each of the reference signal, data channel and control channel was transmitted using beamforming employing a common set of beamforming parameters.

The invention also extends to 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 reference signal; a data channel comprising a payload of the data packet; and a control channel comprising information required to decode the data channel; wherein the transmitter is arranged to transmit each of the reference signal, data channel and control channel using beamforming employing a common set of beamforming parameters; the receiver is arranged to use the reference signal for performing channel estimation; and the receiver is arranged to decode said packet by assuming that each of the reference signal, data channel and control channel were transmitted using beamforming employing a common set of beamforming parameters.

The invention also extends to a method of operating a digital radio transmitter according to a predetermined communication protocol, the method comprising transmitting a data packet comprising: a reference signal to allow a receiver to perform channel estimation; a data channel comprising a payload of the data packet; and a control channel comprising information required to decode the data channel; the method further comprising transmitting each of the reference signal, data channel and control channel using beamforming employing a common set of beamforming parameters.

The invention also extends to a method of operating a digital radio receiver according to a predetermined communication protocol, the method comprising receiving a data packet comprising: a reference signal; a data channel comprising a payload of the data packet; and a control channel comprising information required to decode the data channel; the method further comprising: using the reference signal for performing channel estimation; and decoding said packet by assuming that each of the reference signal, data channel and control channel were transmitted using beamforming employing a common set of beamforming parameters.

The invention also extends to a method of operating a digital radio communication system comprising a transmitter and a receiver according to a predetermined communication protocol, the method comprising the transmitter transmitting a data packet comprising: a reference signal to allow the receiver to perform channel estimation; a data channel comprising a payload of the data packet; and a control channel comprising information required to decode the data channel; wherein the method comprises: the transmitter transmitting each of the reference signal, data channel and control channel using beamforming employing a common set of beamforming parameters; the receiver using the reference signal for performing channel estimation; and the receiver decoding said packet by assuming that each of the reference signal, data channel and control channel were transmitted using beamforming employing a common set of beamforming parameters. The invention also extends to a non-transitory computer readable medium comprising instructions configured to cause a digital radio 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, the same set of beamforming parameters is used for beamforming the reference signal, control channel and data channel. This may mean that it is not necessary to provide a separate reference signal for each of the control channel and data channel, even if the control channel and data channel are transmitted using different transmission modes as may be typical. Instead common reference signals can be used for learning the channel in respect of both the control channel and data channel. This may reduce the radio resource usage required to transmit a data packet and in some embodiments it may be possible to obtain beamforming gain for the control channel, increasing the signal-to-noise ratio of the control channel at reception.

The beamforming parameters could be identical for each of the above-mentioned three parts of the packet: reference signal, control channel and data channel. However in a set of embodiments one or more of the parts employs additional parameters. Additional beamforming parameters may be necessary if more transmit antennas are used to transmit one part of the packet compared to another. Where both of the parts employ additional parameters, these are preferably common to both of them. In other words, all three parts may employ beamforming parameters which are at least a subset of a (larger) common set of parameters.

In a set of such embodiments the reference signal employs all of the parameters employed in the data channel. The control channel may employ all or, more typically a subset, of the parameters used in the data channel. For example in a set of embodiments the data channel is transmitted using a different transmission mode to the control channel. In a set of such embodiments the control channel is transmitted using two effective transmitters. In a set of such embodiments the two effective transmitters are used to employ space-time transmit diversity e.g. using Alamouti encoding. The transmitted signal for the effective transmitters will be formed according to the beamforming parameters.

In a set of embodiments the control channel comprises information indicative of the transmission mode used in the data channel. The receiver may then use the information in the control channel in order to decode the data channel.

In a set of embodiments, a receiver is arranged to receive the data packet using a single antenna. In other embodiments, a receiver is arranged to receive the data packet using a plurality of antennas.

In a set of embodiments the data channel may be transmitted in a closed-loop multi-antenna transmission mode. The set of beamforming parameters may therefore be dependent on feedback from the receiver to the transmitter on instantaneous channel conditions. Alternatively the reciprocity of radio channel transmission between the transmitter and receiver when they act in reverse roles may enable the transmitter to determine instantaneous channel conditions from the transmissions it receives from the receiver.

Additionally or alternatively the data channel may be transmitted in an open-loop multi-antenna transmission mode. In this case, the set of beamforming parameters may be predetermined such that they do not depend on instantaneous channel conditions. The beamforming parameters may be orthogonal.

The data channel could instead be encoded according to the Alamouti procedure before beamforming is applied. In embodiments the same transmission mode would then be used for the data and control channels.

The data channel typically includes a data payload.

In a set of embodiments the data packet comprises a Data Field (DF) comprising the reference signal, control channel and data channel, wherein the reference signal, control channel and data channel are each defined in the frequency domain. In a set of embodiments therefore the reference signal is transmitted alongside the control channel in dedicated sub-carrier frequency allocations. In a set of embodiments the reference signal comprises a plurality of demodulation reference signals (DRS); the control channel comprises a physical control channel (PCC) and the data channel comprises a physical data channel (PDC).

In a set of embodiments the data packet further comprises a Synchronisation Training Field (STF). The STF may be transmitted immediately before the DF. In a set of embodiments the data packet further comprises a Guard Interval (Gl) transmitted immediately before the STF.

In a set of embodiments the STF is transmitted without beamforming. In another set of embodiments, the STF is beamformed, preferably using the common set of the beamforming parameters or at least a subset thereof.

In a set of embodiments, the set of beamforming parameters comprise elements of a beamforming matrix e.g. one defining a set of beamforming weights. In some circumstances, the beamforming matrix may be an identity matrix and therefore not introduce any physical beamforming affect.

In a set of embodiments, the transmitter is arranged to employ Orthogonal Frequency-Division Multiplexing (OFDM) during packet transmission. The packet may therefore comprise a pre-defined number of OFDM symbols.

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 first radio communication system in accordance with an embodiment of the invention;

Fig. 2 is a schematic diagram illustrating a second radio communication system in accordance with another embodiment of the invention;

Fig. 3 is a schematic diagram of a data packet structure in accordance with the invention; Fig. 4 is a key showing the different subcarrier allocation types shown in Figs. 5, 6,

7 and 8; Fig. 5 is a schematic diagram illustrating subcarrier allocation for transmission of a data packet using a single space-time stream and two subslots;

Fig. 6 is a schematic diagram illustrating subcarrier allocation for transmission of a data packet using two space-time streams and two subslots;

Fig. 7 is a schematic diagram illustrating subcarrier allocation for transmission of a data packet using four space-time streams and four subslots;

Fig. 8 is a schematic diagram illustrating subcarrier allocation for transmission of a data packet using eight space-time streams and four subslots;

Fig. 9 is a flowchart illustrating the packet transmission process according to an embodiment of the present invention; Fig. 10 is a flowchart illustrating the packet reception process according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a MIMO (multiple input, multiple output) transmission mode equipped digital radio transmitter 100 and a MIMO transmission mode equipped digital radio receiver 102. The transmitter 100 comprises three antennas 104a, 104b and 104c; and the receiver 102 comprises three antennas 106a, 106b and 106c. 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 and analogue to digital converters are provided in the radio transceivers 100, 102 but description of these is omitted for the sake of brevity. It will also be understood that the transmitter is not required to comprise exactly three antennas, but may comprise any integer number of antennas greater than or equal to two. The receiver is also not required to comprise exactly three antennas, but may comprise any integer number of antennas greater than or equal to one. In the case that the receiver comprises only a single antenna, MISO (multiple input, single output) transmission modes may be used instead of MIMO transmission modes.

Fig. 1 also shows the possible signal paths 108 from the transmitter 100 from each of its antennas 104a, 104b and 104c to each of the antennas 106a, 106b and 106c of the receiver 102. As will be appreciated by those skilled in the art, many different MIMO transmission modes are known in the art and could be employed depending on the application from simply transmitting identical signals from each antenna 104a, 104b, 104c (transmit diversity) to open loop spatial multiplexing, closed loop spatial multiplexing, cyclic delay diversity, etc. The transmitter and receiver are configured to operate using OFDM (orthogonal frequency-division multiplexing) modulation as is known perse in the art.

More pertinently, the signals from multiple transmit antennas can be transmitted to a single receive antenna using beamforming. Fig. 2 shows another arrangement of a first beamforming equipped digital radio transceiver device 110 and a second beamforming equipped digital radio transceiver device 112. The first radio transceiver 110 comprises four antennas 114a, 114b, 114c and 114d; and the second radio transceiver 112 comprises a single antenna 116. As before processors, oscillators, filters, amplifiers, digital to analogue converters and analogue to digital converters are omitted for the sake of brevity.

Fig. 2 also shows the possible signal paths 118 from the first transceiver 110 when it is acting as a transmitter through each of its antennas 114a, 114b, 114c and 114d to the second transceiver 112 acting as a receiver through each its antenna 116. In this example, each of the transmit antennas 114a, 114b, 114c and 114d transmit the same signal, but with various phase and magnitude adjustments to the output of each individual antenna in order to beamform the output signal in such a way that some regions have increased signal intensity and other regions have reduced signal intensity as result of constructive and destructive interference between the signals transmitted by different antennas. It will therefore be understood how manipulating the phase and magnitudes of the signals output by each antenna provides the ability to ‘steer’ the transmitted signal towards the receiver 112 to increase the chance of successful reception and decrease signal strength in other regions to reduce interference with other signals. The receiver 112 cannot directly determine that the signal it receives is the result of four separate signals subjected to beamforming. Rather the receiver antenna 116 receives a single signal which is a product of the radio channel conditions along the respective paths 118 between the transmit antennas 114a-d and the ‘weights’ applied to the respective signals from each antenna in order to achieve the beamforming mentioned above. The transmit antennas 114a-d therefore act as single ‘effective’ transmit antenna. In order to decode the signal it receives the receiver 112 must estimate the combined channel between the effective antenna and the receive antenna 116.

The principles of the arrangements show in Figs. 1 and 2 may be combined so that multiple space-time streams are transmitted. For example, although Fig. 2 shows four transmit antennas these could generate two space-time streams, with the additional transmit antennas used to beamform the transmission. Such an arrangement would have two effective transmit antennas.

When such multiple space-time streams are employed they may be used in a number of ways. In the example above of two space-time streams each could be used to provide two separate spatial streams - that is independent data streams carrying different data. This spatial multiplexing allows higher data rates to be achieved. Alternatively the transmitter may perform space-time encoding to increase the robustness of the signal. For example in Alamouti space-time encoding, as will be described in more detail below, the number of space-time streams, N_STS = 2 but the number of spatial streams, N_ss = 1.

Fig. 3 shows an example structure of a data packet 200 in accordance with the invention. The data packet comprises: a Guard Interval (Gl) 202, a Synchronisation Training Field (STF) 204 and a Data Field (DF) 206. The lower part of the diagram shows various of these fields in more detail.

The Gl 202 forms the initial part of the packet 200, providing separation between consecutive packets 200. Immediately following the Gl 202 is the STF 204, comprising seven repetitions of a signal pattern 208, with the purpose of enabling synchronisation between the transmitter 100, 110 and the receiver 102,121 by allowing the receiver 102 to correct for receiver gain, frequency and timing errors relative to the transmitter 100.

Following the STF 204 is the DF 206, comprising a number of data field symbols 212 each preceded by a cyclic prefix 210, with the purpose of delivering the payload of the packet 200. The DF 206 is transmitted using OFDM, providing the ability to dedicate different subcarrier frequencies for different signals/channels, as will be explained in more detail below with reference to Figs. 4, 5, 6, 7 and 8.

Fig. 4 is a key showing the different subcarrier allocation types shown in Figs. 5, 6,

7 and 8, illustrating the different shadings and which subcarrier allocation types they correspond to. Shown are the Guard/Empty field 202, the Synchronisation Training Field (STF) 204, the Physical Control Channel (PCC) 406, the Demodulation Reference Signal (DRS) 408 and the Physical Data Channel (PDC) 410. In this example the PCC 406, DRS 408 and PDC 410 make up the Data Field 206 shown in Fig. 3. The DRS 408 is a reference signal which allows the receiver 102 to perform channel estimation as explained in more detail below. The PDC 410 is a data channel comprising the payload of the data packet 200; and the PCC 406 is a control channel comprising various information bits about the transmission which are required to decode the data channel (PDC 410).

The DRS 408 contains no data, instead it provides a reference signal that the receiver may use to estimate the transfer function or channel conditions for each space-time stream transmitted by the transmitter 100, 110. In the case of streams which are beamformed as shown in Fig. 2, the transfer function is dependent on the individual signal paths 118 and the beamforming weights as previously described.

The PCC 406 comprises specific parameters that indicate the transmission mode used for transmitting the PDC 410. These could include, for example: whether beamforming is used; whether open-loop or closed-loop transmission is used; how many spatial streams are used; how many space-time streams are used; and the length of the data packet 200 in a number of subslots or a number of slots. As used herein, the term subslot refers to the transmission of five consecutive OFDM symbols. A slot may be made up of two, four, eight or 16 subslots depending on the subcarrier scaling applied. It is not essential that all of the information mentioned above is conveyed in the PCC. For example some parameters may be predetermined, some modes may not be not be supported or some information may be conveyed in a different way.

Figs. 5, 6, 7, and 8 show examples of subcarrier allocation for the data packet 200. The rows correspond to the subcarriers 302 available to the transmitter 100, 110. The number of each subcarrier 302 corresponds to that subcarrier’s index, with index zero corresponding to the central subcarrier frequency; in the examples given in Figs. 5, 6, 7 and 8 there are sixty-four subcarriers 302 available to the transmitter. The columns correspond to the time-evolution of symbols 304 transmitted by the transmitter 100. The number of each symbol 304 corresponds to that symbol’s index, with index zero being the first symbol transmitted by the transmitter 100,110 and index nine being the latest in time.

Fig. 5 shows an example of a possible subcarrier allocation 400 for the data packet 200, wherein the number of space-time streams N_STS = 1 and the number of subslots N_sub = 2 . This is the situation depicted in Fig. 2 and would mean that both the PDC 410 and PCC 406 would be transmitted without the use of space-time encoding nor the use of MIMO or MISO transmission modes.

Guard allocations 306 correspond to the allocations for guard/empty fields 202. STF allocations 308 correspond to subcarrier allocations for transmitting the Synchronisation Training Field 204. DRS allocations 310 correspond to subcarrier allocations for transmitting the Demodulation Reference Signal 408 for the single space-time stream of index zero. PCC allocations 320 correspond to subcarrier allocations for transmitting the Physical Control Channel 406. PDC allocations 322 correspond to subcarrier allocations for transmitting the Physical Data Channel 408.

T ransmission of the STF 204 is completed by the end of symbol index one, and subsequent symbols are used to transmit the Data Field 206. The DF 206 comprises an initial transmission of the DRS 408 and PCC 406, with each being allocated to different subcarriers. Following transmission of the PCC 408, the transmitter 110 transmits the PDC 410. A second set of DRS allocations 320 are transmitted one subslot after transmission of the first set of DRS allocations 320 so as to allow the receiver to perform up-to-date channel estimation. Fig. 6 shows another example of a possible subcarrier allocation 500 for the data packet 200, wherein the number of space-time streams N_STS = 2 and the number of subslots N_sub = 2. In this example, since there are two space-time streams, the PDC 410 can be transmitted either using a MIMO mode with two separate spatial streams, or the PDC 410 can transmitted in MISO mode using space-time encoding, e.g. Alamouti space-time encoding. The PCC 406 can also be transmitted with two space-time streams, using Alamouti space-time encoding. The increased number of space-time streams N_STS over the example shown in Fig. 5 requires that an extra DRS allocation 311 be transmitted for the extra space-time stream, as one DRS 408 is required for each space-time stream used.

In accordance with the invention, the same DRS 408 is used for both the PCC 406 and PDC 410. As will be explained below, this is enabled because the same beamforming parameters are applied to the DRS, the PCC and the PDC.

As in Fig. 5, transmission of the STF 204 is completed by the end of symbol index one, and subsequent symbols are used to transmit the Data Field 206 comprising the DRS 408, the PCC 406 and the PDC 410. DRS allocations 310 and 311 correspond to subcarrier allocations for transmitting the Demodulation Reference Signal 408 for the space-time streams of index zero and one respectively. PCC allocations 320 correspond to subcarrier allocations for transmitting the Physical Control Channel 406. PDC allocations 322 correspond to subcarrier allocations for transmitting the Physical Data Channel 410. As a result of the extra DRS allocation 311 when compared to the example shown in Fig. 5, an equivalent number of PCC allocations 320 are displaced and transmitted at the same time as the PDC 410, such that there are the same number of PCC allocations 320 in both examples.

Fig. 7 shows another example of a possible subcarrier allocation 600 for the data packet 200, wherein the number of space-time streams N_STS = 4 and the number of subslots N_sub = 4. In one example, since the number of space-time streams N_STS = 4, the PDC 410 is transmitted using a combination of spatial multiplexing and space-time encoding that uses the four space-time streams. The PCC 406 is transmitted with just two space-time streams, as it is transmitted using Alamouti space-time encoding. The increased number of space-time streams N_STS over the example shown in Fig. 6 requires that two extra DRS allocations 312 and 313 be transmitted for the extra space-time streams, as one DRS allocation is required for each space-time stream used. As in the example of Fig. 6, the same DRS 408 is used for both the PCC 406 and PDC 410. The increased number of subslots N_sub over the example shown in Fig. 6 increases the length of the data packet, and increases the separation between the first and second transmission of the DRS 408 to two subslots rather than one as shown in Figs. 5 and 6.

DRS allocations 310, 311, 312 and 313 correspond to subcarrier allocations for transmitting the Demodulation Reference Signal 408 for the space-time streams of index zero, one, two and three respectively. As a result of the extra DRS allocations 312 and 313 when compared to the example shown in Fig. 6, an equivalent number of PCC allocations 320 are displaced and transmitted at the same time as the PDC 410, such that there are the same number of PCC allocations 320 in both examples.

Fig. 8 shows another example of subcarrier allocation 700 for the data packet 200, wherein the number of space-time streams N_ss = 8 and the number of subslots N_sub = 4. The PDC 410 may thus transmitted using a combination of spatial multiplexing and space-time encoding that uses eight space-time streams. The increased number of space-time streams N_STS over the example shown in Fig. 7 requires that four extra DRS allocations 314, 315, 316 and 317 be transmitted for the extra space-time streams. Again, the same DRS 408 is used for both the PCC 406 and PDC 410.

DRS allocations 310, 311, 312, 313, 314, 315, 316 and 317 correspond to subcarrier allocations for transmitting the Demodulation Reference Signal 408 for the space-time streams of index zero, one, two, three, four, five, six and seven respectively. As a result of the extra DRS allocations 314, 315, 316 and 317 when compared to the example shown in Fig. 7, an equivalent number of PCC allocations 320 are displaced and transmitted at the same time as the PDC 410, such that the same number of PCC allocations 320 exist in both examples.

Operation of an embodiment of the invention will now be described with reference to Fig. 2 and further reference to Figs. 9 and 10. As previously described, when a plurality of transmit antennas are used, it is possible to direct a transmitted signal to a receiver using beamforming techniques. It is possible to beamform a transmitted signal in both the digital and analogue domains, wherein digital beamforming is also known as precoding.

More specifically, a block k of a packet field comprising n symbols s_{l k}, s_{2 k}, ... ,s_n>k may be transmitted without beamforming such that the transmitted signal X_k is given by:

In accordance with the invention however a block k of a packet field comprising n symbols s_{l k}, s_{2 k}, ... , s_{n fe} is precoded (beamformed) using a precoding (beamforming) matrix W_k such that the transmitted beamformed signal X_k' is given by:

The received signal R_k is then given by:

R_k = H_kX_k' + N_k = H_kW_kX_k + N_k = H_kX_k + N_k, (3) where H_k is the channel transfer matrix for the block k, N_k represents noise and interference present in the received signal, and H_k is the combined channel matrix, given by:

H_k = H_kW_k. (4)

The channel transfer matrix H_k is of size n_RX x n_TX, where n_RX is the number of receive antennas 116 in use by the receiver 112 and n_TX is the number of transmit antennas 114a-d in use by the transmitter 110. In the example shown in Fig. 2 the channel transfer matrix would thus have size 1 x 4 , if all available antennas are used by the transmitter 110 and receiver 112 respectively. In order to decode a signal X_k, the receiver 112 must be able to estimate the combined channel matrix H_k. To allow this, the transmitter transmits the Demodulation Reference Signal (DRS) 408, which allows the receiver to perform channel estimation. In accordance with the present invention, one DRS 408 is transmitted for each space-time stream used to transmit the PDC 410, as each space-time stream has a different combined channel matrix H_k. The PCC 406 and PDC 410, however, do not require separate DRS 408 transmission, as will be described further below, as the present invention provides the ability for the same DRS 408 to be used for both the PCC 406 and PDC 410.

In accordance with the present invention, the DRS 408, PCC 406 and PDC 410 are beamformed using subsets of the same precoding matrix W_k. The PDC 410 is precoded as described above using the precoding matrix W_{k PDC}, which has a size of n_TX x N_STS wherein N_STS is the number of space-time streams required for transmission of the PDC 410. The PDC 410 itself may be transmitted using any transmission mechanism, including but not limited to: Alamouti transmit diversity, open-loop transmit diversity, closed-loop transmit diversity, open-loop spatial multiplexing and closed-loop spatial multiplexing.

In the embodiment presently being described, two space-time streams are used. This would correspond to a modification of Fig. 2 to provide the transmitter 110 with eight transmitters, with the additional four transmitters forming a second space-time stream received by a second receive antenna.

The elements of precoding matrix W_{k PDC} depend on the transmission mode used for transmitting the PDC 410. During closed-loop multi-antenna transmission the precoding matrix W_{k PDC} is dependent on feedback from the receiver 112 to the transmitter 110 on instantaneous channel conditions or channel state feedback from receiver to transmitter, enabled by the reciprocity of radio channel conditions.

During open-loop multi-antenna transmission modes, the precoding matrix W_{k PDC} is a fixed, preferably orthogonal, implementation specific precoding matrix that is not dependent on instantaneous channel conditions. For example, the precoding matrix W_{k PD}c may be the identity matrix, which corresponds to directly using the physical antenna beams (i.e. no physical beamforming is performed).

In the embodiment being described here, the PCC 406 is transmitted using Alamouti transmission diversity. Symbols in the PCC 406 are first encoded according to the Alamouti procedure - a simple transmit diversity scheme (space- time code) for use with two space-time streams. On a subcarrier level, the Alamouti procedure involves transmitting a signal X_{k PC}c of the form: si,k ^s2,k

X ki .pcc ~^s . (5 2,k ^si,k. ) wherein s_{1 k} and s_{2 k} are the symbols to be transmitted in block k of the PCC 406 and s _k and s_{2 k} are the complex conjugates of the symbols s_{l k} and s_{2 k} respectively.

After encoding according to the Alamouti procedure, the PCC 406 is precoded with the precoding matrix W_{k PCC}, which has a size of n_TX x 2:

If N_STS > 2 - i.e. the number of space-time streams for transmission of the PDC 410 were greater than that used for transmission of the PCC 406 - then the precoding matrix W_{k PCC} comprises a subset of the larger precoding matrix W_{k PDC}. For example, the precoding matrix W_{k PCC} may comprise the first two columns of the precoding matrix W_{k PDC}.

If N_STS = 2 - i.e. the number of space-time streams for transmission of the PDC 410 is the same as that used for transmission of the PCC 406 (which may be because the PDC 410 is also encoded according to the Alamouti procedure) - then the precoding matrices W_{k PCC} and W_{k PDC} are the same.

The beamforming parameters which are common to the precoding matrix W_{k PCC} and the precoding matrix W_{k PDC} form a common set of beamforming parameters. If W_{k PDC} is larger than W_{k PCC} this could alternatively be described as W_{k PCC} being a subset of W_{k PDC} is largest in size. In order to decode the PCC 406 and the PDC 410, the receiver must estimate the combined channel matrix H_k = H_kW_k for each. This is enabled by the transmission of the DRS 408 by the transmitter 110, which is precoded with the same beamforming parameters as used for the PCC and the PDC, i.e. using a precoding matrix W_{k DRS} comprising whichever of W_{k PCC} and W_{k PDC} is largest in size, and therefore is of size n_TX x max( 2, N_STS ).

This therefore means that the receiver 112 is able to estimate the combined channel matrices H_{k PCC} and H_{k PDC} from the combined channel matrix H_{k DRS} of the DRS 408. Once the combined channel matrices H_{k PCC} and H_{k PDC} are estimated, the receiver 102 is, by design, able to decode the PCC 406 using an ordinary Alamouti decoder, as it knows in advance that the PCC 406 will be transmitted according to the Alamouti procedure. Once this is done, the receiver 112 has access to the information within the PCC 406, which it may then use to determine the specific transmission mode used in transmission of the PDC 410, as the PCC 406 comprises the side information required to decode the PDC 410, e.g. the number of spatial streams used, the number of space-time streams used, whether open-loop or closed-loop transmission is used, etc. The transmitter may optionally precode the STF 204 with the precoding matrix W_{k STF} which is also a subset of

Wfc ,DRS-

Thus it will be understood by those skilled in the art that it is not necessary to provide a separate reference signal for each of the PCC 406 and PDC 410 despite them not (necessarily) utilising the same transmission mechanism, as channel estimation for both may be performed from the same DRS 408. This reduces the amount of radio resources required for transmission of the data packet 200. In addition, link budget is optimised during transmission of the PCC 406 by using Alamouti transmit diversity and link capacity is optimised during transmission of the PDC 410 by using open or closed loop MIMO modes.

Fig. 9 is a flowchart illustrating the packet transmission process an example transmitter according to an embodiment of the present invention. At step 800, the transmitter determines which transmission mode it will use for the PDC 410. As described previously, possible transmission modes for the PDC 410 include but are not limited to: Alamouti transmit diversity, open-loop transmit diversity, closed-loop transmit diversity, open-loop spatial multiplexing and closed-loop spatial multiplexing. At step 802, the transmitter 110 determines the number of space-time streams N_STS that are required for the selected transmission mode of the PDC.

If N_STS =2, then the transmitter proceeds to step 804, where it determines the precoding matrix W_{k DRS} of size n_TX x 2. As used herein, n_TX corresponds to the physical number of transmit antennas used by the transmitter. The transmitter then precodes the DRS 408 using the precoding matrix W_{k DRS}. Next, the transmitter proceeds to step 806, where it encodes the PCC 406 according to the Alamouti procedure. The transmitter then proceeds to step 808, where it precodes the PCC 406 using the precoding matrix W_{k PCC} which is the same as the precoding matrix W_{k DRS} The transmitter then proceeds to step 810, where it encodes the PDC 410 according to the PDC transmission mode determined at step 800. Next, the transmitter 110 proceeds to step 812, where it precodes the PDC 410 using the precoding matrix W_{k PDC} which is also the same as the precoding matrix W_{k DRS},.

The transmitter then proceeds to step 822, where it further precodes the STF 204 with the precoding matrix W_{k STF} which is the same as or a subset of the precoding matrix W_{k DRS} dependent on which transmit antennas are used in transmission of the STF 204. Once all encoding and precoding has been complete, the transmitter 110 transmits the data packet 200.

If N_STS > 2, then the transmitter 110 proceeds to step 816, where it determines the precoding matrix W_{k DRS} of size n_TX x N_STS. The transmitter then precodes the DRS 408 using the precoding matrix W_{k DRS}. Next, the transmitter proceeds to step 806, where it encodes the PCC 406 according to the Alamouti procedure. The transmitter then proceeds to step 818, where it precodes the PCC 406 using the precoding matrix W_{k PCC} which is a subset of size n_TX x 2 of the precoding matrix W_{k DR}s , dependent on which transmit antennas are to be used in transmission of the PCC 406. The transmitter then proceeds to step 810, where it encodes the PDC 410 according to the PDC transmission mechanism determined at step 800. Next, the transmitter 110 proceeds to step 820, where it beamforms the PDC 410 using the precoding matrix W_{k PDC} which equal to the precoding matrix W_{k DRS}. The transmitter 110 then proceeds to step 822, where it precodes the STF 204 with the precoding matrix W_{k STF} which is a subset of the precoding matrix W_{k DRS} dependent on which transmit antennas are used in transmission of the STF 204. Once all encoding and precoding has been complete, the transmitter 110 transmits the data packet 200.

Fig. 10 is a flowchart illustrating the corresponding packet reception process. At step 900, the receiver receives the data packet 200, through its antennas, as transmitted by the transmitter with beamforming. At step 902, the receiver 112 uses the STF 204 to calculate and correct for frequency and timing errors, as is known in the art. The receiver 112 then proceeds to step 904, where it uses the DRS 408 to perform channel estimation for the PCC 406: using the DRS 408 to calculate the combined channel matrix of the PCC 406. The receiver then proceeds to step 906, where it decodes the PCC 406 using an ordinary Alamouti decoder, as is known in the art.

Having decoded the PCC 406, the receiver is able to determine the transmission mode used for the PDC 410 at step 908, including for example whether open-loop or closed-loop transmission has been used, the number of space-time streams used, the number of spatial streams used, the number of space-time streams used etc. At step 910, the receiver uses the DRS 408 to perform channel estimation for the PDC 410: using its knowledge of the number of space-time streams used to transmit the PDC 410 and the DRS 408 to estimate the combined channel matrix of the PDC 410. The receiver then proceeds to step 910 and decodes the PDC 410 according to the transmission mechanism and parameters transmitted in the PCC 406. Once the PDC 410 has been decoded by the receiver, the data packet 200 has successfully been received by the receiver.

Claims

1. A digital radio transmitter arranged to operate according to a predetermined communication protocol and comprising a plurality of transmit antennas, wherein said transmitter is arranged to transmit a data packet comprising: a reference signal to allow a receiver to perform channel estimation; a data channel comprising a payload of the data packet; and a control channel comprising information required to decode the data channel; wherein the transmitter is arranged to transmit each of the reference signal, data channel and control channel using beamforming employing a common set of beamforming parameters.

2. The digital radio transmitter as claimed in claim 1 wherein the beamforming parameters employed for each of the reference signal, data channel and control channel are at least a subset of a larger common set of parameters.

3. The digital radio transmitter as claimed in any preceding claim wherein the beamforming parameters employed for the reference signal comprise all of the parameters employed for the data channel.

4. The digital radio transmitter as claimed in any preceding claim wherein the beamforming parameters employed for the control channel comprise a smaller subset of the parameters employed for the data channel.

5. The digital radio transmitter as claimed in any preceding claim arranged to transmit the data channel using a different transmission mode to the control channel.

6. The digital radio transmitter as claimed in any preceding claim arranged to transmit the control channel using two effective transmitters to employ Alamouti space-time transmit diversity.

7. The digital radio transmitter as claimed in any preceding claim wherein the information required to decode the data channel comprises information indicative of a transmission mode used in the data channel.

8. The digital radio transmitter as claimed in any preceding claim wherein the transmitter is arranged to transmit the control channel such that a receiver is able to receive the control channel using a single antenna.

9. The digital radio transmitter as claimed in any preceding claim wherein the beamforming parameters are orthogonal.

10. The digital radio transmitter as claimed in any preceding claim wherein the reference signal, control channel and data channel are each defined in the frequency domain.

11. The digital radio transmitter as claimed in any preceding claim wherein the data packet further comprises a Synchronisation Training Field (STF) beamformed using the common set of beamforming parameters or a subset thereof.

12. The digital radio transmitter as claimed in any preceding claim wherein the common set of beamforming parameters comprise elements of a beamforming matrix defining a set of beamforming weights.

13. The digital radio transmitter as claimed in any preceding claim wherein the transmitter is arranged to employ Orthogonal Frequency-Division Multiplexing (OFDM) during packet transmission.

14. 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 reference signal; a data channel comprising a payload of the data packet; and a control channel comprising information required to decode the data channel; wherein the receiver is arranged to: use the reference signal to perform channel estimation; and decode said packet by assuming that each of the reference signal, data channel and control channel was transmitted using beamforming employing a common set of beamforming parameters.

15. The digital radio receiver as claimed in claim 14 arranged to use the reference signal to perform channel estimation in respect of both the control channel and the data channel.

16. 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 reference signal; a data channel comprising a payload of the data packet; and a control channel comprising information required to decode the data channel; wherein the transmitter is arranged to transmit each of the reference signal, data channel and control channel using beamforming employing a common set of beamforming parameters; the receiver is arranged to use the reference signal to perform channel estimation; and the receiver is arranged to decode said packet by assuming that each of the reference signal, data channel and control channel were transmitted using beamforming employing a common set of beamforming parameters.

17. A method of operating a digital radio transmitter according to a predetermined communication protocol, the method comprising transmitting a data packet comprising: a reference signal to allow a receiver to perform channel estimation; a data channel comprising a payload of the data packet; and a control channel comprising information required to decode the data channel; the method further comprising transmitting each of the reference signal, data channel and control channel using beamforming employing a common set of beamforming parameters.

18. A method of operating a digital radio receiver according to a predetermined communication protocol, the method comprising receiving a data packet comprising: a reference signal; a data channel comprising a payload of the data packet; and a control channel comprising information required to decode the data channel; the method further comprising: using the reference signal to perform channel estimation; and decoding said packet by assuming that each of the reference signal, data channel and control channel were transmitted using beamforming employing a common set of beamforming parameters.

19. A method of operating a digital radio communication system comprising a transmitter and a receiver according to a predetermined communication protocol, the method comprising the transmitter transmitting a data packet comprising: a reference signal; a data channel comprising a payload of the data packet; and a control channel comprising information required to decode the data channel; wherein the method comprises: the transmitter transmitting each of the reference signal, data channel and control channel using beamforming employing a common set of beamforming parameters; the receiver using the reference signal to perform channel estimation; and the receiver decoding said packet by assuming that each of the reference signal, data channel and control channel were transmitted using beamforming employing a common set of beamforming parameters.

20. A non-transitory computer readable medium comprising instructions configured to cause a digital radio transmitter to operate in accordance with the method as claimed in claim 17.

21. A non-transitory computer readable medium comprising instructions configured to cause a digital radio receiver to operate in accordance with the method as claimed in claim 18.