GB2478735A

GB2478735A - Antenna selection procedure which involves receiving data on a first antenna at the same time as receiving sounding signals on a range of other antennae

Info

Publication number: GB2478735A
Application number: GB201004310A
Authority: GB
Inventors: Magnus Stig Torsten Sandell; Justin Coon
Original assignee: Toshiba Research Europe Ltd
Current assignee: Toshiba Europe Ltd
Priority date: 2010-03-15
Filing date: 2010-03-15
Publication date: 2011-09-21
Anticipated expiration: 2030-03-15
Also published as: GB2478735B; GB201004310D0

Abstract

This invention is applicable to a multi-antenna receiver which has more antennae than RF chains. While maintaining at least one antenna dedicated to data reception, at least one other antenna is selected for receiving sounding signals and performing channel information gathering. The antenna used for channel information gathering is rotated so that channel information can be obtained for all channels. Preferably, in a first mode, only data are received in a first subset of antennas and then in a second mode sounding signals are received in antennae outside the first subset.

Description

CHANNEL ESTIMATION IN SUPPORT OF ANTENNA SWITCHING

The present invention concerns channel estimation in a multiantenna channel1 particularly for implementations using antenna switching.

In a reciprocal transmission, the channel between two devices is the same in both directions. This can be exploited to reduce the amount of training and hence reduce overhead. If the condition of the channel, such as described by channel state information (CSI), is known at the transmitter, appropriate modifications can be made to the format of the next transmission to ensure that transmission is as effective as possible. One approach is to use precoding, which modifies the transmitted data to account for the channel state. One form of precoding is beamforming, which generally requires access to complete CSI. Another is antenna selection which can be employed when only the power at the various receive antennas are known. Full or partial CSI can be obtained during reception ("the uplink") and used for transmission ("the downlink"), as illustrated in Figure 1.

In Figure 2, the transmission and reception in the uplink and downlink are shown. First, the user device transmits its signal ("uplink") to the base station and the base station receives the signal and detects the data in the signal. The channel may be estimated by means of pilots (known symbols) in the uplink packet. Based on the data received in the uplink, the base station decides which antennas to use for the downlink transmission. In addition, the base station precodes the signal in the downlink transmission. It will be appreciated that the user device can also be configured to perform a similar operation, although no antenna selection will be performed in an example where the user device has only one antenna.

Figure 2 describes transmission in the uplink and subsequent transmission in the downlink, for a wireless communication network comprising a user device and a base station. In the given example, the base station possesses four antennas, available for selection. The graph illustrates activity at the antennas of the base station. It will be appreciated that antenna selection, and similar graphs of activity, could be also be replicated at the user station.

Subsequent transmissions are made on the same basis. Antenna selection is used, from time to time, to switch the antennas in use (as illustrated in blocks C and D), for operational reasons.

In a multiantenna system, the cost of antennas is usually much lower (or even negligible) than the cost of the RF chain (filtering, down-converting, sampling etc) driving the antennas. Hence antenna selection (such as described in A. Molisch and M. Win, "MIMO systems with antenna selection", IEEE Microwave Magazine, vol 5, no 1, pp 46-56, March 2004) provides a cost effective way of obtaining diversity and/or coding gains in a multiantenna system, by having more antennas than RF chains and switching dynamically between the antennas when necessary to make sure the best (or at least an acceptably good) antenna is used. A problem with this approach is that it is very difficult to be certain that one antenna is better than another when only one of them is in use.

One possible way of finding the best antenna in an antenna switching system is by regularly transmitting training sequences known to the receiver. If M antennas in total are available with N RF chains, then MIN training sequences would be sent at the head of a transmission packet to allow the estimation of all M antennas, since the receiver can only estimate N antennas at the time, The receiver would then switch antennas between each sequence to estimate the channels and determine which one is best.

The data portion of the packet would then follow, using the previously determined best" antenna. The drawback with this approach is that there is inevitably a large overhead in sending multiple training sequences at regular intervals; this is especially a problem on a fast fading channel since these intervals must inevitably be very short to respond to frequent changes in channel state. There is also the problem of switching time; the antenna selector needs some time to switch between antennas and hence the multiple training sequences would need to accommodate this, thereby increasing the overhead.

Another solution is to use a switch-and-stay algorithm (see, for example, Ko, Alouini, Simon, "Analysis and optimization of switched diversity systems", IEEE Transactions on Vehicular Technology, vol 49, no 5, pp 1813-1831, sep 2000). In this case, one antenna is chosen and remains active as long as it performs above a predetermined threshold, for instance a threshold in SNR. When SNR falls under this threshold, the receiver will switch to another antenna. If, in such an approach, there is no available information about other antennas, another antenna will typically be chosen at random.

US2009/0016312A1 describes a method of selecting antennas in uplink and downlink based on signalling of CSI.

US2008/0051149A1 describes antenna selection based on the sending of pilots for different sets of antennas.

US2009/0054093A1 describes antenna switching based on sounding packets.

If antenna selection (or some other form of precoding) is used at the transmitter, there is a requirement for CSI as noted above. This can be obtained from the received signal if the channel is reciprocal; however this information is only available on the active antennas. The CSI from other antennas may only be available in the form of measurements that occurred in the past when those antennas were active, and thus the CSI may be outdated. Hence, antenna selection in such circumstances may not take advantage of use of the best set of antennas for transmission.

A conventional antenna switching system would estimate the channel in the uplink on the active antennas and use these for downlink transmission. It is possible to use CSI for unused antennas obtained during previous receptions but this is highly likely to be outdated information and would in most practical cases not be useful.

An aspect of the invention provides a method for obtaining CSI for all channels even if only a subset is chosen for reception of data. By having this information, a better selection can be made for transmission since all CSI is up to date. This could mean an improved performance for the transmitted data, when compared with techniques as set out above.

In general terms, an aspect of the invention concerns channel estimation in a reciprocal channel, defined by an uplink and a downlink. It is envisaged that the channel is so defined between two devices, each having a plurality of antennas and capable of transmitting or receiving on those antennas. RF chains are provided to drive the antennas. In such a channel, the uplink can be used for estimation of the channel in the downl ink direction and for determination of a precoding to be applied.

In such an arrangement, antenna switching can be used. In such a technique, the RE chains driving the antennas are caused to switch between antennas in an attempt to find the best antenna or antennas to use in prevailing conditions. In practice it is difficult to establish which antennas are the most appropriate, since it is not possible to obtain any information about inactive antennas. As discussed in the prior art, this has necessitated specially designed training sequences which allow a device to estimate the channel on all antennas. This approach would, however, lead to an increased overhead on transmission.

To reduce this overhead, an aspect of the present invention provides an approach which involves modifying the receiver structure such that part of a received signal is dedicated to sounding inactive channels while simultaneously receiving the data. This allows information to be obtained describing all antennas of the receiver, and not just the ones used for data reception. Possible advantages of using this approach in specific embodiments of the invention can include enhancement of the downlink performance while maintaining uplink performance.

Another aspect of the invention provides a method for obtaining channel state information for all antennas of a multiantenna wireless communication apparatus from an uplink OFDM signal when using antenna switching.

The channel state information can comprise the power of the channel.

The channel state information may comprise all information necessary to define the complete state of the channel. The method may comprise employing decision feedback to obtain said channel state information.

The method may comprise multilevel modulation.

The method may comprise estimating only a subset of the available antennas.

The method may comprise using an interleaver designed to take advantage of variable bit detection reliabilities.

The method may be applied to a single carrier signal.

Another aspect of the invention comprises a wireless communication apparatus comprising a plurality of antennas, and an antenna selector for selecting one or more of said antennas for use in transmitting and or receiving signals, the antenna selector being operable to select antennas for use on the basis of received information describing the nature of transmission from another apparatus, the antenna selector being operable, in a first mode, to select a subset of said antennas for reception of signals and, in a second mode, to switch operation from one or more of said subset of selected antennas for reception of channel information on channels defined by one or more antennas not included in said subset.

Another aspect of the invention provides a method of collecting channel information for use in antenna selection, the method being operable in a wireless communications apparatus comprising a plurality of antennas for use in transmission and reception of signals, the method comprising selecting a subset of said antennas for reception of a signal transmitted by another device, receiving said signal on said subset of said antennas for a data phase and, in a data and sounding phase, maintaining said receiving on at least one but not all of said subset of said antennas while switching to receiving on one or more other antennas not in said subset to collect channel information for said one or more other antennas.

It is noted that it is also possible to perform restricted antenna selection. If, for example, selector A can choose between antennas 1 and 2, and selector B can choose between antennas 3 and 4, there will be four possible selections of two antennas: 1-3, 1-4, 2-3 and 2-4. The two other combinations (1-2 and 3-4) would not be possible. The advantage of this scheme is that the selection may be simpler to implement as it consists of two choose-one-from-two selectors, rather than a choose-two-from-four which may require cross-connections.

Aspects of the invention may be implemented by way of dedicated hardware, software executed on general purpose hardware, or a mixture of the two. To that end, the implementation may be by way of a digital signal processor, a system on a chip, or other signal processing device with flexibility as to operation, by way of a bespoke software product designed to fulfil all elements of the invention, or by software designed to harness pre-provided facilities (such as libraries or other functional elements) by way of a plug in or other configuration based file. Such a software product can be provided as a computer readable medium, such as a mass storage device (e.g. a flash memory device) or an optically readable device, or alternatively by way of download. For smaller software products, such as might be implemented in the event that a plug in implementation were devised, a short messaging service (SMS) message format might be possible.

Further aspects, features, elements and advantages of the invention will become apparent from the following description of specific embodiments of the invention, in which: Figure 1 illustrates the timing of uplink and downlink transmissions.

Figure 2 illustrates a graph of antenna activity over time for a base station of a wireless communication network in accordance with a prior art example described above; Figure 3 illustrates a wireless communication network in accordance with a specific embodiment of the invention; Figure 4 illustrates a base station of the network illustrated in Figure 3; Figure 5 illustrates a graph of antenna activity for the base station illustrated in Figure 4; and Figure 6 illustrates graphs for simulations of the specific embodiment of the invention alongside simulations of conventional approaches in accordance with the known state of the art.

As illustrated in Figure 3, the specific embodiment of the invention now described comprises a wireless communication network 10 defining an uplink and a downlink between a user station 30 and a base station 20. For completeness, the base station 20 is illustratively connected to an "infrastructure" block 40, which may be representative of the internet, or a backhaul network in general. It is noted that that the communication may also be established between two user devices (for example, "peer-to-peer" communication) without changing the principle.

In comparison with previous arrangements, the embodiment provides a modified reception in the uplink to allow the user device to obtain information about antennas not chosen for reception.

The base station 20, as shown in Figure 4, provides a data source and sink 22 which is capable of generating data for transmission, processing data received by the base station 20, or retrieving data from the infrastructure or any other possible source of data, and delivering the same thereto. In order to effect transmission or reception on the wireless channel, the base station, in this example, comprises two RF chains 24_i and 242, which are capable of generating RF signals for transmission on antennas 281 to 28_4. In practice, therefore, with the number of antennas exceeding the number of RF chains supplied, antenna selection is required, provided in the form of antenna selector 26 as illustrated. The reader will understand that the return path, for reception of signals on the antennas 28_i to 28_4 and processing by the RF chains 24_I and 24_2 is implicit in the above.

The embodiment employs orthogonal frequency division multiplexing (OFDM), and so a data packet consists of a number of OFDM symbols. For some of these OFDM symbols, the receiving antennas can be switched in order to listen" to other antennas.

This approach is illustrated in Figure 5, in which the operation of the four antennas is illustrated. Of these four antennas 28, two are active, as driven by the two RF chains 22_i and 22_2. The operation of the antennas 28 is divided, in time, into two phases.

During a "data phase", data is received using antennas 28_i and 28_2. In one alternative approach, the operation of these two antennas can be combined using, for example, maximal ratio combining (MRC). In the second part of the packet, "data + sounding phase", the antenna selector 26 switches the activity otherwise intended for antenna 28_2 first to antenna 28_3 and then to antenna 284.

It should be appreciated that the switching time from one antenna to the next depends on the type of switch and its speed relative to the OFDM symbol duration. Some switches may be fast but incurs an insertion loss; others may be slower with lower losses. In general, a "switching phase" is provided to allow the RF chains to switch from one antenna to another. The duration of this switching phase may be negligible where fast switches are implemented.

The advantage of this approach is that the base station 20 can estimate the power of the received signal on antennas 28_3 and 28_4, and hence obtain information about their strength. If multilevel modulation is used (and thus the power per data symbol is not constant), the power of the data symbol as transmitted by the user station 30 will affect the received power. This can be mitigated by first estimating the data symbols (using antenna 28_i), then using decision feedback. This would also allow for full CSI estimation and not just estimation of the power of the channel. Alternatively, the multilevel property can be ignored, especially if only the total powers of antenna 28_3 or 28_4 are needed (summed over all subcarriers), since the effects will be averaged out.

It will be observed that the uplink reception will be degraded somewhat since only one receive antenna is used for data detection during the data + sounding phase; antennas 28_3 and 28_4 cannot be used for data detection as no channel estimates are yet available for those antennas and hence no coherent detection is possible. It is assumed, for the purpose of this embodiment, that channel estimation is aided by a preamble (known training sequence) and only antennas 28_I and 28_2 are estimated using that, since these are the only active antennas during the reception of the preamble.

Because some bits are detected by two antennas (during the data phase) and others by only one antenna (during the data + sounding phase), those detected by two antennas will have a higher average reliability than those detected with only one antenna. If the format of the "data" and "data + sounding" phases are known to the transmitter, this format can be used to design an interleaver that takes the variable reliabilities into account.

Someone with knowledge in the field of the invention will recognise that there are many variations on this idea. For instance, not all non-data receiving antennas need to take part in the data + sounding phase. In the above example, both non-data receiving antennas are used, but it would also be possible to configure such a system such that only antenna 28_3 would be operational to listen to the channel, antenna 28_4 not being used at all. This could improve uplink performance; it could be that the existing information for antenna 28_4 is recent enough to be used for the downlink precoding.

Another arrangement could involve using more than one OFDM symbol per antenna for "sounding".

Finally, although the above description assumes an OFDM system, similar schemes could be devised for, for example, single-carrier transmission approaches.

The described embodiment harnesses the idea that, instead of requiring extra training sequences to obtain information about all antennas in a reciprocal channel, such information can be obtained by devising a "sounding" scheme on the uplink.

Information about the downlink can be obtained from the uplink in antenna switching systems without using extra training sequences.

In Figure 6 the bit-error rate (BER) of the described embodiment is compared to the switch-and-stay algorithm. The system has six antennas, chooses two of these for data transmission and applies per-tone antenna selection to them. The packet consists of 10 OFDM symbols, 8 in the "data phase" and 2 in the "data + sounding phase", and a rate 1/2 convolutional code is used. The channel is constant during the up-and downlink packet but changes for the next transmission; the relative Doppler rate (Doppler frequency times nterpacket separation) is 0.1. It is clear, from the data extracted from the simulation, that the two approaches are similar for the uplink but that the specific embodiment of the present invention outperforms the conventional switch-and-stay scheme in the downhink. This is due to the currency of the CSI which can be obtained using the present embodiment.

While the invention has been exemplified by the above examples of a specific embodiment thereof, the reader will appreciate that nothing in the foregoing should be read as restricting the scope or application of the invention. The scope of the invention should be understood from the appended claims, which may be read in conjunction with (but not limited by) the supporting description and accompanying drawings.

Claims

CLAIMS: 1. A wireless communication apparatus comprising a plurality of antennas, arid an antenna selector for selecting one or more of said antennas for use in transmitting and/or receiving signals, the antenna selector being operable to select antennas for use on the basis of received information describing the nature of transmission from another apparatus, the antenna selector being operable, in a first mode, to select a subset of said antennas for reception of signals and, in a second mode, to switch operation from one or more of said subset of selected antennas for reception of channel information on channels defined by one or more antennas not included in said subset.
2. Apparatus in accordance with claim I wherein said antenna selector is operable to switch less than all of said subset of selected antennas.
3. Apparatus in accordance with claim I or claim 2 and operable to receive a symbol of data over time, wherein said antenna selector is operable in said first mode for a first period of reception of said symbol and in said second mode for a second period of said symbol.
4. Apparatus in accordance with claim 3 wherein, in said second period, said antenna selector is operable to switch operation of antennas not in said subset successively so as to collect channel information for each antenna not included in said subset.
5. Apparatus in accordance with claim 3 or claim 4 wherein said antenna selector is operable to maintain operation of one antenna throughout reception of a symbol, whether in the first period or the second period.
6. Apparatus in accordance with any preceding claim and operable to receive an OFDM symbol.
7. Apparatus in accordance with any preceding claim and operable to transmit a signal on the basis of channel information obtained in a previous received signal.
8. Apparatus in accordance with any preceding claim wherein the channel information is estimated from a preamble and/or data of said signal.
9. A method of collecting channel information for use in antenna selection, the method being operable in a wireless communications apparatus comprising a plurality of antennas for use in transmission and/or reception of signals, the method comprising selecting a subset of said antennas for reception of a signal transmitted by another device, receiving said signal on said subset of said antennas for a data phase and, in a data and sounding phase, maintaining said receiving on at least one but not all of said subset of said antennas while switching to receiving on one or more other antennas not in said subset to collect channel information for said one or more other antennas.
10. A method in accordance with claim 9 and comprising receiving a symbol of data over time, wherein said receiving in said data phase coincides with a first portion of said receiving of said symbol and said receiving in said data and sounding phase coincides with a second portion of said receiving of said symbol.
11. A method in accordance with claim 10 wherein said first and said second period cover reception of said symbol in time.
12. A method in accordance with any one of claims 9 to 11 wherein at least one antenna receives in both said data phase and said data and sounding phase.
13. A method in accordance with any one of claims 9 to 12 and comprising receiving an OFDM symbol.
14. A method of controlling reception and transmission in a multi antenna wireless communication device, comprising performing the method of any one of claims 9 to 13 and selecting antennas for reception and/or transmission of data according to collected channel information.
15. A computer program product comprising computer executable instructions which, when executed by a computer, are operable to cause said computer to perform a method in accordance with any one of c!aims 9 to 14.
16. A computer pr?m product in accordance with claim 15 and including a computer readab storage medium.
17. A computer program product in accordance with claim 15 and including a computer receivable signal.Amendments to the claims have been filed as follows CLAIMS: 1. A wireless communication apparatus comprising a plurality of antennas, and an antenna selector for selecting one or more of said antennas for use in receiving signals, the antenna selector being operable to select antennas for receipt of a signal transmitted from another apparatus, the antenna selector being operable, in a first mode, to select a subset of said antennas for reception of signals and, in a second mode, maintaining said reception on at least one but not all of said subset of said antennas while switching to receive on one or more other antennas not in said subset to collect channel information for said one or more other antennas.2. Apparatus in accordance with claim 1 wherein said antenna selector is operable to switch less than all of said subset of selected antennas.3. Apparatus in accordance with claim I or claim 2 and operable to receive a data packet over time, wherein said antenna selector is operable in said first mode for a first period of reception of data packet and in said second mode for a second period of said data packet.4. Apparatus in accordance with claim 3 wherein, in said second period, said antenna selector is operable to switch operation of antennas not in said subset successively so as to collect channel information for each antenna not included in said subset.5. Apparatus in accordance with claim 3 or claim 4 wherein said antenna selector is operable to maintain operation of one antenna throughout reception of a symbol, whether in the first period or the second period.S.....30. 6. Apparatus in accordance with any preceding claim and operable to receive an OFDM symbol.7. Apparatus in accordance with any preceding claim and operable to transmit a signal on the basis of channel information obtained in a previous received signal.8. Apparatus in accordance with any preceding claim wherein the channel information is estimated from a preamble and/or data of said signal.9. Apparatus in accordance with any preceding claim wherein the antenna selector is configured to select one or more of said antennas for use in transmitting signals.10. A method of collecting channel information for use in antenna selection, the method being operable in a wireless communications apparatus comprising a plurality of antennas for use in reception of signals, the method comprising selecting a subset of said antennas for reception of a signal transmitted by another device, receiving said signal on said subset of said antennas for a data phase and, in a data and sounding phase, maintaining said receiving on at least one but not all of said subset of said antennas while switching to receiving on one or more other antennas not in said subset to collect channel information for said one or more other antennas.11. A method in accordance with claim 10 and comprising receiving a symbol of data over time, wherein said receiving in said data phase coincides with a first portion of said receiving of said symbol and said receiving in said data and sounding phase coincides with a second portion of said receiving of said symbol.12. A method in accordance with claim 11 wherein said first and said second period cover reception of said symbol in time.13. A method in accordance with any one of claims 10 to 12 wherein at least one antenna receives in both said data phase and said data and sounding phase.* * 14. A method in accordance with any one of claims 10 to 13 and comprising receiving an OFDM symbol. ** ** * . ** * 15. A method of controlling reception and transmission in a multi antenna wireless communication device, comprising performing the method of any one of claims to 14 and selecting antennas for reception and/or transmission of data according to collected channel information.16. A method in accordance with any preceding claim wherein the antenna selector is configured to select one or more of said antennas for use in transmitting signals.17. A computer program product comprising computer executable instructions which, when executed by a computer, are operable to cause said computer to perform a method in accordance with any one of claims 10 to 16.
18. A computer program product in accordance with claim 17 and including a computer readable storage medium.
19. A computer program product in accordance with claim 17 and including a computer receivable signal.I..... * * * * S ** ** * S * * I *5 55 * I * * I