WO2010149207A1

WO2010149207A1 - Method and apparatus for precoding codebook selection

Info

Publication number: WO2010149207A1
Application number: PCT/EP2009/057837
Authority: WO
Inventors: Kari Hooli; Kari Pajukoski; Esa Tiirola
Original assignee: Nokia Siemens Networks Oy
Priority date: 2009-06-23
Filing date: 2009-06-23
Publication date: 2010-12-29

Abstract

Apparatus comprising: a spatial layer precoder using a precoding code book for controlling a mapping for at least three spatial layers to four antenna ports, said code book comprising a plurality of entries, wherein said entries comprise at least one of: a first set of codebooks entries where the amplitude of each element of the entry is such that the power per spatial layer symbol is substantially equal for all code words; and a second set of codebook entries where the number of antenna ports receiving a spatial layer of the entry is such that the power per spatial layer symbol is as equal as possible over all codewords.

Description

METHOD AND APPARATUS FOR PRECODING CODEBOOK SELECTION

Embodiments of the present invention relate to a method and apparatus and, in particular but not exclusively, to apparatus and a method for use in a multiple input multiple output wireless telecommunications network.

It has been proposed to improve the coverage and capacity of communication by use of spatial diversity or spatial multiplexing. By using spatial multiplexing, the data rate can be increased by transmitting independent information streams from different antennas but using the same channel as defined by frequency and time resource and possibly spreading code.

These systems may be referred to as multiple input multiple output (MIMO) systems. These systems require complex controllers to control both the transmission and receiving elements of the mobile station and the base station.

Multi-stream single user MIMO transmission has been proposed and forms part of WCDMA (wideband code division multiple access), 3GPP LTE (Third generation partnership project -long term evolution) and WiMax system standards. In single user multiple input multiple output (SU-MIMO), a MIMO receiver with multiple antennas and receiving circuitry receives the multiple streams, separates the multiple streams and determines the transmission symbols sent over each stream of the spatially multiplexed data streams originating from single user equipment.

In the 3GPP forum, LTE-Advanced has been proposed to be an evolution of LTE Rei'8 system to address the ITU-R (International Telecommunications Union Radio communication Sector) requirements for IMT (International Mobile Telecommunications)-Advanced and most likely to be part of 3GPP LTE ReI. 10. 3GPP approved a new Study Item on LTE-Advanced in RAN#39 (March 2008). It has been proposed that SU-MIMO with 2-4 transmission antennas at the UE (user equipment) will be part of LTE-Advanced [TR 36.913 vδ.0.0].

It has been proposed that SU-MIMO UL (uplink) transmissions will involve transmission precodiπg techniques and that this precoding utilizes fixed codebooks.

Furthermore the LTE-Advanced Study Item has decided that in single user multiple-input multiple-output (SU-MIMO) uplink (UL) transmission precoding will be based on codebooks, that there are a maximum of two codewords input to the codeword to layer mapper and precoder, and that the codeword to layer mapping will be similar to the LTE Release 8 Downlink codeword - to-layer mapping. In particular, there is a trend to favour codebooks which preserve cubic metric (CM) or are at least CM friendly. For example, the agreed 2-Tx (transmitter) antenna codebook for rank 2 and 4-Tx antenna codebooks for transmission ranks 2 and 4 preserve CM (additionally rank-1 codebooks naturally preserve CM properties).

A cubic metric (CM) friendly codebook may be understood to cover two types of codebooks. Firstly, where the codebook contains some precoding matrices that preserve the cubic metric and the cubic metric is not taken into account in the rest of precoding matrices. Secondly, where there is a cubic metric value increase due to the precoding which is alleviated by limiting the number of spatial layers mapped onto a single antenna to be less that the transmission rank. In other words, in the case of a rank 3 transmission, only two spatial layers (out of a maximum of three) are mapped onto a single antenna.

A cubic metric (CM) preserving codebook is one where the cubic metric value of a single antenna single carrier frequency division multiple access (SC-FDMA) transmission is not increased. This, for example, may be achieved by restricting precoding so that only one spatial layer is mapped on each individual antenna. This desire to have cubic metric preserving or cubic metric friendly precoding causes significant restrictions to the codebook design and furthermore may limit the achievable precoding gains available.

For example, in an agreed 3GPP codeword to layer mapping for rank 3 systems there may be two codewords and three layers as defined by the rank. These CW-to-layer mapping standards indicate that the first codeword CW#1 is mapped as such on to the first spatial layer SL#1 , and the second codeword CW#2 is mapped with a serial-to parallel conversion to the second and third spatial layers SL#2 and SL#3. In this situation the second codeword CW#2 will contain twice the number of symbols as the first codeword CW#1. This non-symmetrical codeword design is problematic in defining precoding codebooks with of cubic metric preserving or cubic metric friendly properties.

The Householder matrix based codebooks used in LTE Release 8 Downlink (DL) as described in the standard 3GPP TS36.211 Sec. 6.3.4.2.3 furthermore can not be regarded as cubic metric preserving or cubic metric friendly codebooks. For example, in Release 8 Download codebooks, all three layers are mapped to each transmit antenna which increases both the cubic metric (CM) and also the peak to average power ratio (PAPR).

In other precoding codebook designs, a 4-Tx precoding codebook may preserve the favourable PAPR properties where only a single layer is mapped to each transmit antenna - in other words that the first layer, layer 1, is mapped to two transmit antennas and the second and third spatial layers, layers 2 and 3, are mapped to one transmit antenna each.

Furthermore, other proposals include mapping two spatial layers for each antenna. In other words, the first spatial layer, layer 1 , is mapped onto four transmit antennas and the second and third spatial layers, layers 2 and 3, are mapped onto two transmit antennas each. However in the above cases, the transmit power per modulated symbol in the first codeword CW#1 would be twice the transmit power per modulated symbol in the second codeword CW#2. In other words the first codeword CW#1 and the second codeword CW#2 are transmitted with equal power, even though the second codeword CW#2 contains twice the number of symbols of the first codeword CW#1

Based on numerical analysis of the above methods it appears that the above methods do not reach the performance results of 3GPP LTE Rel'8 DL Codebook, The same analysis points to the imbalance between the modulated symbols as the main reason for the degraded performance results.

There is provided according to a first aspect of the invention a method comprising: using a precoding code book for controlling a mapping for at least three spatial layers to four antenna ports, said code book comprising a plurality of entries, wherein said entries comprise at least one of: a first set of codebooks entries where the amplitude of each element of the entry is such that the power per spatial layer symbol is substantially equal for all code words; and a second set of codebook entries where the number of antenna ports receiving a spatial layer of the entry is such that the power per spatial layer symbol is as equal as possible over all codewords.

The method may comprise providing a plurality of precoding code books, each of said precoding code books being associated with a rank 3.

The method may comprise providing precoding code books for two codewords mapped into three spatial layers.

The method may comprise providing for the first set of codebooks codebook entries with precoding weights which map the first spatial layer to all four antenna ports using a first amplitude weighting, the second spatial layer to a selection of two antenna ports from the four antenna ports using a second amplitude weighting and the third spatial layer to the two antenna ports not selected using the second amplitude weighting.

The ratio between the first amplitude weighting and the second amplitude weighting is preferably 1/V2 .

The first codeword may have fewer symbols than the second codeword.

The method may comprise providing for the second set of codebook entries, codebook entries with precoding weights which map the first codeword as represented by the first spatial layer to one antenna port using a first weighting value, and the second codeword represented by the second and third spatial layers to the other three antenna ports.

The method may comprise providing for the second set of codebook entries, codebook entries with precoding weights which map the first codeword as represented by the first spatial layer to a first antenna port using a first weighting vaiue, and the second spatial layer to a second antenna port using a second weighting value and the third spatial layer to the third antenna port using a third weighting value and to the fourth antenna port using a fourth weighting value.

The first weighting vaiue, the second weighing value and the third weighting value are preferably equal.

The fourth weighting value is preferably one of 1/2,-1/2 J/2, and -j/2.

The method may further comprise providing for the second set of codebook entries, codebook entries wherein a spatial correlation value between the third and fourth antenna port is greater than or equal to a spatial correlation value between any other combination of the antenna ports. At least one antenna port is preferably arranged in accordance with spatial correlation properties of an antenna

The different spatial correlation properties may comprise at least one of antenna polarization and antenna position

The method may comprise transmitting at least one antenna port from a plurality of antenna

The method may comprise controlling the codeword power value

A computer program may comprise program code means adapted to perform any of the steps of described above

According to a second aspect of the invention there is provided a method comprising selecting one of a plurality of entries in a precoding code book for controlling transmissions from four antennas of a device, said code book comprising a plurality of entries, wherein said entries comprise at least one of a first set of codebooks entries where the amplitude of each element of the entry is such that the power per spatial layer symbol is substantially equal for ail code words, and a second set of codebook entries where the number of antenna ports receiving a spatial layer of the entry is such that the power per spatial layer symbol is as equal as possible over ail codewords

Each of said precoding code books may be associated with a rank 3

The precoding code books may map two codewords into three spatial layers

The method may further comprise communicating the selection to an apparatus for controlling a mapping for at least three spatial layers to four antenna ports According to a third aspect of the invention there is provided apparatus comprising: a spatial layer precoder configured to control a mapping for at least three spatial layers to four antenna ports using a precoding code book, said code book comprising a plurality of entries, wherein said entries comprise at least one of: a first set of codebooks entries where the amplitude of each element of the entry is such that the power per spatial layer symbol is substantially equal for all code words; and a second set of codebook entries where the number of antenna ports receiving a spatial layer of the entry is such that the power per spatial layer symbol is as equal as possible over all codewords.

The apparatus may further comprise a memory for storing a plurality of precoding code book entries, each of said precoding code book entries being associated with a rank 3.

The precoding code book entries are preferably configured to control a mapping for two codewords mapped into three spatial layers.

The first set of codebook entries are preferably configured with precoding weights which: map the first spatial layer to al! four antenna ports using a first amplitude weighting; map the second spatial layer to a selection of two antenna ports from the four antenna ports using a second amplitude weighting; and map the third spatial layer to the two antenna ports not selected using the second amplitude weighting.

The first codeword has preferably fewer symbols than the second codeword.

The second set of codebook entries are preferably configured with precoding weights which: map the first codeword as represented by the first spatial layer to one antenna port using a first weighting value; and map the second codeword represented by the second and third spatial layers to the other three antenna ports. The second set of codebook entries are preferably configured with precoding weights which: map the first codeword as represented by the first spatial layer to a first antenna port using a first weighting value; map the second spatial layer to a second antenna port using a second weighting value; map the third spatial layer to the third antenna port using a third weighting value; and map the third spatial layer to the fourth antenna port using a fourth weighting value.

The first weighting value, the second weighing value and the third weighting vaiue are preferably equal.

The fourth weighting value is preferably one of 1/2,-1/2, j/2, and -j/2.

The spatial layer precoder may be configured to control a mapping using the second set of codebook entries wherein a spatial correlation value between the third and fourth antenna port is greater than or equal to a spatial correlation value between any other combination of the antenna ports.

The apparatus may further comprise a plurality of antenna wherein at least one antenna port is arranged in accordance with spatial correlation properties of each antenna.

The different spatial correlation properties may comprise at least one of antenna polarization and antenna position.

The apparatus may further comprise a power controller configured to control the codeword power value.

The apparatus may further comprise a processor configured to control at least one of the spatial layer encoder, the memory, the power controller and the plurality of antenna.

According to a fourth aspect of the invention there is provided an apparatus comprising: a codebook entry selector configured to select one of a plurality of entries in a precoding code book for controlling transmissions from four antennas of a device, said code book comprising a plurality of entries, wherein said entries comprise at least one of: a first set of codebooks entries where the amplitude of each element of the entry is such that the power per spatial layer symbol is substantially equal for all code words; and a second set of codebook entries where the number of antenna ports receiving a spatial layer of the entry is such that the power per spatial layer symbol is as equal as possible over all codewords.

Each of said precoding code books may be associated with a rank 3.

The precoding code books may map two codewords into three spatial layers.

The apparatus may further comprise a transmitter for communicating the selection to a further apparatus for controlling a mapping for at least three spatial layers to four antenna ports.

An integrated circuit or chip set may comprise an apparatus as described above.

A user equipment may comprising an apparatus as described above.

A user equipment may comprise four antennas.

A base station may comprise an apparatus as described above,

According to a fifth aspect of the invention there is provided apparatus comprising: means for using a precoding code book for controlling a mapping for at least three spatial layers to four antenna ports, said code book comprising a plurality of entries, wherein said entries comprise at least one of: a first set of codebooks entries where the amplitude of each element of the entry is such that the power per spatial layer symbol is substantially equal for all code words; and a second set of codebook entries where the number of antenna ports receiving a spatial layer of the entry is such that the power per spatial layer symbol is as equal as possible over ali codewords.

According to a sixth aspect of the invention there is provided apparatus comprising: means for selecting one of a plurality of entries in a precoding code book for controlling transmissions from four antennas of a device, said code book comprising a plurality of entries, wherein said entries comprise at least one of: a first set of codebooks entries where the amplitude of each element of the entry is such that the power per spatial layer symbol is substantially equal for all code words; and a second set of codebook entries where the number of antenna ports receiving a spatial layer of the entry is such that the power per spatial layer symbol is as equal as possible over all codewords.

Various embodiments of the present invention will now described by way of example only with reference to the accompanying Figures, in which:-

Figure 1 shows a schematic view of a system including an schematic base station and user equipment configuration within which embodiments of the invention may be implemented;

Figure 2 shows a schematic view of some parts embodying the present application;

Figure 3 shows a flowchart of steps taken at the mobile station in some embodiments;

Figure 4 shows a flowchart of steps taken at the base station in some embodiments; and Figure 5 shows a graph showing the data throughput against signal to noise ratio achievable according to some embodiments.

Embodiments of the present invention are described herein by way of particular examples and specifically with reference to preferred embodiments. It will be understood by one skilled in the art that the invention may not be limited to the details of the specific embodiments given herein.

Figure 1 shows a communication network 30 in which some embodiments of the present invention may be implemented. In particular, some embodiments of the present invention may relate to the implementation of radio modulators/demodulators (modems) for a range of devices that may include: user equipment 201 , relays, access points or base stations 101 which communicate over a wireless environment 151.

Embodiments of the present invention may be applicable to communication networks implemented according to a range of standards and their evolution including: WCDMA (Wideband Code Division Multiple Access), 3GPP LTE (Long Term Evolution), WiMax (Worldwide Interoperability for Microwave Access), UMB (Ultra Mobile Broadband), CDMA (Code Division Multiple Access), IxEV-DO (Evolution-Data Optimized), WLAN (Wireless Local Area Network), and UWB (Ultra-Wide Band) receivers.

With respect to figure 1, a schematic view of a system within which embodiments of the invention may be implemented is shown. The communication system 30 is shown with a base station 101 which may be a node B (NB), an enhanced node B (eNB) or any access server suitable for enabfing user equipment 201 to access wirelessly a communication system.

Figure 1 shows a system whereby the base station (BS) 101 may transmit to the user equipment (UE) 201 via the wireless environment communications channel 151 , which may be known as the downlink (DL), and the user equipment (UE) 201 may transmit to the base station (BS) 101 via the wireless environment communications channel 151 , which may be known as the uplink (UL).

The base station 101 can comprise a processor 105 which may be configured to control the operation of the receiver/transmitter circuitry 103. The processor may be configured to run software stored in memory 106. The memory 106 may be further configured to store data and/or information to be transmitted and/or received. The memory 106 may further be used to store configuration parameters used by the processor 105 in operating the base station 101.

The transmitter/receiver circuitry 103 may be configured to operate as a configurable transmitter and/or receiver converting between radio frequency signals of a specific protocol for transmission over (or reception via) the wireless environment and baseband digital signals. The transmitter/receiver circuitry 103 may be configured to use the memory 106 as a buffer for data and/or information to be transmitted over or received from the wireless environment 151.

The transmitter/receiver circuitry 103 may further be configured to be connected to at least one antenna for receiving and transmitting the radio frequency signals over the wireless environment to the user equipment 201. In figure 1 the base station is shown comprising 2 antennas, the first antenna 107i and the second antenna 107₂ both configured to transmit and receive signals. In other embodiments of the invention the base station may have more antennas represented by the dotted antenna 107_m in figure 1. In one preferred embodiment, m may be 4. Four receiving antennas is needed in order to support rank-4 transmission.

The base station 101 may be connected to other network elements via a communications link 111. The communications link 111 may receive data to be transmitted to the user equipment 201 via the downlink and transmits data received from the user equipment 201 via the uplink. This data may comprise data for all of the user equipment within the cell or wireless communications range operated by the base station 101. The communications link 111 is shown in figure 1 as a wired Sink. However it would be understood that the communications link may further be a wireless communications link. In figure 1 , there is shown two user equipment 201 within the range of the base station 101. However it would be understood that there may be more or fewer user equipment 201 within range of the base station 101. The user equipment may be a mobile station, or any other apparatus or electronic device suitable for communication with the base station. For example in further embodiments of the invention the user equipment may be personal data organisers or laptop computers suitable for wireless communication in the environment as described hereafter. It should be appreciated that embodiments of the invention may also be applied to a relay station.

Figure 1 in particular shows a first user equipment UEi 20I ₁ and a second user equipment UE₂ 2OI₂. Furthermore figure 1 shows in more detail the first user equipment UEi 20I ₁. The first user equipment 20I ₁ may comprise a processor 205 configured to contro! the operation of a receiver/transmitter circuitry 203. The processor may be configured to run software stored in memory 207, The processor may further control and operate any operation required to be carried out by the user equipment such as operation of the user equipment display, audio and/or video encoding and decoding in order to reduce spectrum usage, etc.

The memory 207 may be further configured to store data and/or information to be transmitted and/or received. The memory 207 may further be used to store configuration parameters used by the processor 205 in operating the user equipment 20I ₁. The memory may be solid state memory, optical memory (such as, for example, CD or DVD format data discs), magnetic memory (such as floppy or hard drives), or any media suitable for storing the programs for operating the processors, configuration data or transmission/reception data.

The transmitter/receiver circuitry 203 may be configured to operate as a configurable transmitter and/or receiver converting between radio frequency signals of a specific protocol for transmission over (or reception via) the wireless environment and baseband digital signals. The transmitter/receiver circuitry 203 may be configured to use the memory 207 as a buffer for data to be transmitted over or received from the wireless environment 151.

The transmitter/receiver circuitry 203 is configured to be connected to at least one antenna for receiving and transmitting the radio frequency signals over the wireless environment to the base station 101. In figure 1 the user equipment is shown comprising 4 antennas, the first antenna 251 n to the fourth antenna 25114.

Although figure 1 and the examples described hereafter describe the user equipment and the bases station as having a processor arranged to carry out the operations described below, it would be understood that in embodiments of the invention the respective processors may comprise a single processor or a plurality of processors. The processors may be implemented by one or more integrated circuits.

Some embodiments of the present invention may be used in the LTE-Advanced system which may be part of 3GPP LTE ReI. 10. However, it should be appreciated that this is by way of example only and embodiments of the invention may be used in alternative systems.

A PUSCH (physical uplink shared channel) precoding scheme for single user MIMO (SU-MIMO), with a precoding codebook design for 4 Tx (Transmission) antennas is discussed. In another embodiment, these techniques could be appiied also to PUCCH Format 2 (for example with single stream precoding). The same techniques may be applied to sounding reference signals.

With respect to Figure 2, a schematic arrangement which may be found within the user equipment 201 or any suitable apparatus employing embodiments of the invention shows the physical uplink shared channel (PUSCH) precoding scheme for a single user multiple-input multiple-output (SU-MIMO) system with a precoding codebook design for 4 Tx (transmission) antennas. The encoder 261 is configured to generate two code words. In some embodiments the encoder 261 may be considered to be a combination of a transport block, cyclic redundancy check coder, radio channel encoder, hybrid automatic retransmission request encoder, scrambler, and modulator, in these embodiments data in from the transport block or transport blocks may be processed by a cyclic redundancy check (CRC) coder which inserts check bits into the data to be used by a receiver to detect errors. Furthermore in these embodiments the output from the cyclic redundancy check coder may be passed to a radio channel encoder which applies a channel coding to the processed transport blocks to provide protection to the payload data from the transport blocks against impairments presented by the radio channel. Furthermore outputs from the modulator may be input to the hybrid automatic retransmission request encoder configured to apply a hybrid automatic retransmission request (HARQ) operation to extract or repeat code bits from the blocks of code bits provided by the channel encoder to generate a precise set of bits to be transmitted within a transmit time interval (TTI) based upon criteria such as the number of assigned resource blocks, the selected modulation scheme and the spatial multiplexing order. The output from the HARQ encoder may then be scrambled using a bit level scrambling sequence or mask in the scrambler. The operation of the scrambler may aid in the reception of the data by suppressing interference to the radio signal. The data output from the scrambler may then be passed to the modulator which may apply a data modulation such as quadrature phase shift-keying (QPSK) 16 quadrature amplitude modulation (16-QAM), or 64 quadrature amplitude modulation (64-QAM) to the transform blocks of scrambled bits. The output of the modulator may then form the output of the encoder. In other words the encoder may in some embodiments output code words in the form of modulated symbols where each transport stream is the origin of a codeword output stream.

in the embodiment shown in Figure 2, the encoder 261 outputs a first codeword CW#1 with symbols X-i, X2 271 and a second codeword with symbols Y-i, Y2, Y₃, Y4273. These codewords are passed to a layer mapper 263. The layer mapper 263 in some embodiments receives the codewords and maps the codewords from the encoder into layers. The iayer mapper in the embodiment shown in figure 2 maps the first codeword CW#1 with the symbols X₁, X₂ 271 and the second codeword CW#2 with the symbols Y₁, Y₂, Y₃, Y₄ 273 to three spatial layers, a first layer (SL#1 or layer 1 ) 281 , a second layer (SL#2 or layer 2) 283, and a third layer (SL#3 or layer 3) 285. In the embodiment shown in Figure 2, the layer mapper maps the first codeword CW#1 271 directly to the first layer, layer 1 281 so that layer 1 contains the symbols X₁, X₂. The layer mapper 263 furthermore comprises a serial to parallel converter 281 which receives the second codeword CW#2 273 and splits the codeword into a first part and a second part. The layer mapper then maps the first part of the second codeword with the symbols Y₁, Y₃ to the second layer, layer 2 283 and maps the second part of the second codeword with the symbols Y₂ and Y₄ to the third layer, layer 3 285. The layer symbols are then input to the precoder 265. The precoder 265 applies a codebook weighting matrix to the layer symbols to generate the antenna (or antenna port) mapped symbols.

In some embodiments of the invention, the SU-MiMO precoding codebooks are arranged to take into account the properties specific for the uplink of LTE-Advanced system.

In a MiMO system, the performance of a radio system is improved by using spatial precoding at a transmitter and spatial postcoding at the receiver. Spatial precoding may comprise spatial beamforming and spatial coding. The spatial precoding is done to enhance the signal power at the destination and to diminish the interfering power.

In single-layer beamforming, the same signal is emitted from each of the transmit antennas with appropriate phase (and optionally gain) weighting such that the signal power is maximized at the receiver input. The benefits of beamforming are to increase the signal gain from constructive combining and to reduce the multipath fading effect. When the receiver has multiple antennas, the transmit beamforming cannot simultaneously maximize the signal level at all of the receive antenna and precoding is used. Precoding requires knowledge of the channel state information (CSl) at the transmitter.

Some embodiments of the invention use a precoding codebook design for simultaneous transmission of up to 2 codewords.

The processor 205 may generate and/or recover and/or load the precoding codebook, in order words the possible matrices which are used in the precoder 265 to map the layer symbols to the output antenna symbols to attempt to preserve the cubic metric values or produce cubic metric friendly precoding matrices. The cubic metric is the metric of the actual reduction in power capability, or power derating of a typical power amplifier in a mobile handset. The cubic metric is therefore a more effective predictor than the peak to average power ratio (PAPR) when evaluating uplink physical channels. The cubic metric (CM) may be calculated by the equation:

CM = CEIL{20log 10[(v_^norm³)_RMs] - 20log 1 u[(v_norrn_ref³)_RMs)]/k,0.5}

where CEIL{x,0.5} is the value of the expression rounded upwards to the closest 0.5 dB, k is equal to a constant and may be 1.41 according to 3GPP working group 4 in the document RP-040372, v_norm³ is the normalised voltage waveform of the input signal, v_norm__ref³ is the normalised voltage wave form of the reference signal.

As indicated previously, cubic metric friendly codebooks are codebooks where although there may be a cubic metric value increase due to precoding, this increase is alleviated by limiting the number of spatial layers mapped on a single antenna to be less than the transmission rank. For example where there is a rank 3 transmission, only two spatial layers are mapped on a single antenna. A cubic metric preserving codebook as described previously is one where the cubic metric value of a single antenna SC-FDMA transmission is not increased. This may be achieved by restricting the precoding so that only one spatial layer is mapped onto each individual antenna. Based on this criteria of designing a codebook capable of cubic metric preserving or being cubic metric friendly, the processor 205 may generate and/or recover and/or load a precoding codebook in one of two ways.

Where the processor 205, in some embodiments, generates/recovers/loads a cubic metric friendly codebook the precoding matrices may be configured so that the single antenna transmits multiple spatial layers and the amplitude of a precoding weight may be different for spatial layers used to transmit different codewords. However, the processor 205 in some embodiments may design/download a codebook with the criteria that the amplitude of a precoding weight remains constant over the precoding weights on layers used to transmit the same codewords. These amplitudes may be selected by the processor so that the transmit power per modulated symbol is equai to all codewords, while the total power per antenna is not altered.

Where the processor 205, in some embodiments, generates/recovers/loads a cubic metric preserving codebook the precoding matrices may be configured so that the single antenna transmits only one spatial layer and the number of transmit antennas used to transmit a spatial layer is chosen by the processor so that the transmit power per modulated symbol is as equal as possible over all codewords. In other words, the processor determines a codebook for a 4 transmitters (Tx) or antennas, with rank three and with two input codewords where one codeword, the second codeword, is twice as large as the other codeword, the first codeword (as described above and shown in Figure 2), the processor 205 may be configured to determine a codebook design so that one transmit antenna is used to transmit the smaller codeword, the first codeword CW#1 , and three transmit antennas are used to transmit the larger codeword, the second codeword CW#2.

The matrix values in the following tables represent the amplitude and phase when a layer X is mapped to antenna Y. An example of the precoding matrix values which may be used and/or generated by the processor 105 for a cubic metric friendly precoding codebook CMF- 1 where two layers are mapped for each antenna so that the first layer is mapped on to four transmit antennas and second and third layers are mapped onto two transmit antennas each are shown below. In these embodiments, the matrix values weights the first layer, SL#1 with the value _Λ/2/6 and weights the second and third layers SL#2 and SL#3 by2/Vβ . Thus, the ratio between the precoding amplitudes for the first layer SL#1 and for the second and third layers SL#2 and SL#3 is l/V-ϊ . Thus in these embodiments the amplitudes of transform matrix may be of the form:-

In the above embodiments when we refer to the codebook as determining the amplitude differences or relative amplitude transformation elements, it may be understood that the phase term of the elements as well as antenna permutation (i.e., permutation of matrix rows in the codebook matrices) may be selected based on other design criteria.

In some other embodiments, the processor may generate a cubic metric friendly precoding codebook, CMF-2, using a modified long term evolution Release 8 Download 2 transmitter antenna, rank 2 precoding matrix configuration for the 4 transmitter rank 3 environment. The processors thus use the 2x2 matrices such

separately as submatrices for the 4x3 precoding matrices where

the sub-matrix is used as: a) the first and second transmit antennas and first and second layers and b) the third and fourth transmit antennas and the first and third layers. The processor may then apply different combinations of the submatrices to obtain precoding vectors for the layers. An example of such combinations can be obtained with following steps:

- four different 2x2 submatrices are obtained from the aforementioned two submatrices by changing the order of the columns so that the precoding vectors

for the second and third layer are of form where x e {l, y>l,y} .

- These four submatrices are mapped to the corresponding elements on the 4x3 precoding matrix. 16 different precoding matrices are obtained as different combinations of the four submatrices.

- Precoding for the first layer may be improved by changing the order of 3^rd and 4^th row for some of the precoding matrices with [0 0 1 1] as the third column.

Finally the precoding weights for layer 1 may be normalised via a factor of I/Λ/2 to obtain the equal transmitted energy per modulation symbol for the both codewords.

An example of such a codebook, containing precoding matrices, with proper overall scaling is shown below:

1 0 r 1/V2 1 0 ]/Λ/ 2 1 0 1/ 2 1 0

1 - J 0 1 y/V2 - y 0 1 _ jf 0 1 /2 - 7 0

1/V2 0 1 V6 1/V2 0 1 Λ/6 V ^■h 0 1 ^■>/6 2 0 1

_- l/V2 0 1 -jj4ϊ 0 Λ 1/ -Jl 0 - 1 _ y^"A /2 0

Examples of the precoding matrix values which may be used and/or generated by the processor for cubic metric preserving precoding where one transmitter antenna is used to transmit the smaller codeword, CW#1 , and three transmit antennas used to transmit the larger codeword, CW#2, may use two separate codebooks. The first codebook CMP-1 with 16 precoding matrices may be given by:

where x can be any one of the values of +1 , -1 , +j and -j. This codebook may be considered to be permutation of a rank 3 codebook where the first codeword is transmitted with one transmit antenna and the second codeword is transmitted with three antennas. For the third layer which in the above embodiments is transmitted with two antennas, the antennas selected are the antennas have or are expected to have a significant spatial correlation (in comparison to correlation values between other antennas). In such embodiments the expectation of the spatial correlation of the antennas may in some embodiments be based e.g. on the antenna layout.

The second cubic metric preserving example codebook CMP-2 may be the 16 precoding matrices given by:

where x is either a +1 or -1 value.

Thus in at least one embodiment there is a method comprising using a precoding code book for controlling a mapping for at least three spatial layers to four antenna ports. The code book comprises a plurality of entries, and the entries comprise at least one of: a first set of codebooks entries where the amplitude of each element of the entry is such that the power per spatial layer symbol is substantially equal for all code words; and a second set of codebook entries where the number of antenna ports receiving a spatial layer of the entry is such that the power per spatial layer symbol is as equal as possible, or substantially equal, over all codewords. In other words the first set with cubic metric friendly matrices, and the second set of codebook entries with cubic metric preserving matrices.

The above codebooks in the case of the cubic metric friendly precoding codebook generates a codebook which enables transmit power per symbol to be equal over both codewords. Thus in the case of cubic metric preserving precoding, the ratio between different transmit powers is reduced to 1.76 dB from previously 3 dB.

In some embodiments, the processor or a power controller may further perform codeword specific power control, in other words balancing, to further balance the transmit power per symbol between the codewords. The outcome of this balancing by the processor or power controller in some embodiments may be that the transmit power per transmit antenna becomes unbalanced. For these reasons, codeword specific power control may be performed in some embodiments, only where the maximum output power of power amplifier is not limiting the performance. Thus in these embodiments due to unbalanced power per antenna, fuli total transmit power is not available, hence performance should not be limited mainly by thermal noise but by interference.

The processor/power controller may generate or apply specific codeword power balancing so that the balancing may be switched on or off using dedicated higher layer signaling or broadcast system information. In other embodiments, the balancing may be specified such that power balancing takes place only within an available transmission power resources and does not increase or decrease the total transmission power compared to the case without power balancing. In other embodiments of the invention, the codeword specific power balancing may cope with existing power control signaling. In other embodiments the codeword specific parameters (either absolute power balancing or relative power balancing) may be applied. Furthermore in other embodiments the codeword specific power balancing may be applied with certain MIMO receiver types, for example with minimum mean squared error (MMSE) or serial interference cancellation (SIC) receivers.

In the above examples it can be seen that the precoding matrix values for both cubic metric friendly and cubic metric preserving cases may produce suitable results for the four transmitter antenna (4-Tx), rank 3 with 2 codeword environment as used for the physical uplink shared channel (PUSCH) for 3GPP LTE-advanced. The same precoding matrix codebooks would also be suitable for any other 4-Tx, rank 3, 2 codeword communication system or channel.

Similarly it would be understood that although the above precoding matrix values are examples of a three layer to four transmitter element configuration the same matrix values may be easily used in any suitable communication system applying single user multiple input multiple output using suitable numbers of spatial layers, codewords and transmitter elements.

With reference to Figure 3, a simuiation of the 3GPP spatial channel model - extention (SCM-E) urban macro near-line-of-sight (NLoS) channel with precoding codebook evaluation setup as agreed in the 3GPP RAN 1 Meeting, is shown where ports were mapped to transmit antennas so that most correlating transmit antennas are adjacent.

With low correlation between polarized antennas, smalf spatial separation between antenna elements transmit antennas 1 and 3 formed one x-antenna element and transmit antennas 2 and 4 formed the other x-antenna element.

From this Figure, it can be shown that the performance comparison for the example precoding matrices including the cubic metric preserving examples CMP-1 305 and CMP-2 307, and CM friendly codebooks CMF-1 301 and CMF-2 303 produce favorable throughput rates than the current Release 8 down link precoding codebook 351 , the single spatial layer mapped to a antenna element codebook (R1 -091465) 353, a PARP preserving precoding codebook (NSN 2009E00471) 355, a 2 spatial layers for each layer mapping codebook (R1 -091842).

Reference is made to Figure 4 which shows a flow chart embodying the present invention. This is carried out by the user equipment. Additionally layer mapping (i.e. how transport blocks (code words) are mapped to spatial layers is performed in the UE

In the first step one of the cubic metric friendly or cubic metric preserving codebooks using the above criteria is received by the apparatus. In other embodiments the apparatus/user equipment receives information identifying the cubic metric friendly or cubic metric preserving codebook previously stored in the apparatus. In some embodiments the apparatus receives information enabling the apparatus to generate the precoding matrix. For example the apparatus may receive the 'x' value as found in CMP- 1 or CMP-2 and the apparatus then generate the specific CMP-1 or CMP-2 coding matrix from this value. In some embodiments the apparatus further receives information identifying one of the entries of the codebook. This receiving of the codebook/codebook identifier and also receiving information identifying the entry in the codebook to be used for this user is shown in figure 4 by step 401.

The processor may then store the received codebook/codebook identifier in the memory 207. The operation of storing the codebook identifier is shown in figure 4 by step 403.

The data stream(s), in other words the codewords, generated by the encoder 261 and mapped by the layer mapper 263 into spatial layers, are precoded in the precoder 265 using the precoding matrix in accordance with the information of the selected codebook and codebook entry. The precoded data streams are then transmitted by the respective antennas. If necessary the selected precoding matrix may be used in any necessary retransmission. The application of the selected precoding matrix is shown in figure 4 by step 405.

Furthermore reference is made to Figure 5 which shows operations which may be carried out by the base station.

in a first operation the base station is arranged to determine the channel conditions. Preferably, the instantaneous channel conditions are determined. The channel is the channel between the user equipment and the base station. The operation of determining channel conditions is shown in figure 5 by step 501.

The base station may then further make a determination as to the rank of the communication. In other words the number of data streams of spatial layers which are to be transmitted at the same time is determined. In the embodiments discussed, the number of data streams spatial layers can be up to m where m is the number of antenna which the UE has, however preferably in embodiments the rank determination is set to 3. The operation of rank determination is shown in figure 5 by step 503. The base station may then selects a cubic metric friendly codebook, such as the examples provided by CMF- 1 and CMF-2 or cubic metric preserving codebook such as the examples provided by CMP-1 and CMP-2 based on the rank and in some embodiments the user equipment capabilities. The base station may then also select a codebook entry from the selected codebook based on the channel conditions and/or the nature of the data streams. Preferably the codebook entry is selected based on the instantaneous channel conditions. The selection of codebook/codebook entry is shown in figure 5 by step 505.

the base station may then send the selected codebook entry and codebook to the user equipment. Alternatively, information identifying the codebook may be sent, with the codebook entry. The signaling of the codebook/codebook entry is shown in figure 5 by step 507.

In alternative embodiments at the receiver side, for example, in the case that demodulation reference signals are not precoded, the processor 105 of the base station receiver needs to calculate the effective channel by combining the selected precoding matrix with channel estimates.

Embodiments of the invention may be used with fewer antennas than four or more than four antennas.

It is noted that whilst embodiments may have been described in relation to user equipment or mobile devices such as mobile terminals, embodiments of the present invention may be applicable to any other suitable type of apparatus suitable for communication via access systems. A mobile device may be configured to enable use of different access technologies, for example, based on an appropriate multi-radio implementation.

It is also noted that although certain embodiments may have been described above by way of example with reference to the exemplifying architectures of certain mobile networks and a wireless local area network, embodiments may be applied to any other suitable forms of communication systems than those illustrated and described herein. It is also noted that the term access system may be understood to refer to any access system configured for enabling wireless communication for user accessing applications.

The above described operations may require data processing in the various entities. The data processing may be provided by means of one or more data processors. Similarly various entities described in the above embodiments may be implemented within a single or a plurality of data processing entities and/or data processors. Appropriately adapted computer program code product may be used for implementing the embodiments, when loaded to a computer. The program code product for providing the operation may be stored on and provided by means of a carrier medium such as a carrier disc, card or tape. A possibility may be to download the program code product via a data network. Implementation may be provided with appropriate software in a server.

For example the embodiments of the invention may be implemented as a chipset, in other words a series of integrated circuits communicating among each other. The chipset may comprise microprocessors arranged to run code, application specific integrated circuits (ASICs), or programmable digital signal processors for performing the operations described above.

Embodiments of the inventions may be practiced in various components such as integrated circuit modules. The design of integrated circuits can be by and large a highly automated process. Complex and powerful software tools may be available for converting a logic level design into a semiconductor circuit design ready to be etched and formed on a semiconductor substrate.

Programs, such as those provided by Synopsys, Inc. of Mountain View, California and

Cadence Design, of San Jose, California may automatically route conductors and locate components on a semiconductor chip using well established rules of design as well as libraries of pre-stored design modules. Once the design for a semiconductor circuit may have been completed, the resultant design, in a standardized electronic format (e.g., Opus, GDSII, or the like) may be transmitted to a semiconductor fabrication facility or "fab" for fabrication.

It is noted herein that while the above describes exemplifying embodiments of the invention, there are several variations and modifications which may be made to the disclosed solution without departing from the scope of the present invention as defined in the appended claims.

Claims

1. A method comprising: using a precoding code book for controlling a mapping for at least three spatial layers to four antenna ports, said code book comprising a plurality of entries, wherein said entries comprise at least one of; a first set of codebooks entries where the amplitude of each element of the entry is such that the power per spatial layer symbol is substantially equal for all code words; and a second set of codebook entries where the number of antenna ports receiving a spatial layer of the entry is such that the power per spatial layer symbol is as equa! as possible over all codewords,

2. A method as claimed in claim 1, comprising providing a plurality of precoding code books, each of said precoding code books being associated with a rank 3.

3. A method as claimed in claim 2, comprising providing precoding code books for two codewords mapped into three spatial layers.

4. A method as claimed in claim 3, comprising providing for the first set of codebooks codebook entries with precoding weights which map the first spatial layer to a!l four antenna ports using a first amplitude weighting, the second spatial layer to a selection of two antenna ports from the four antenna ports using a second amplitude weighting and the third spatial layer to the two antenna ports not selected using the second amplitude weighting.

5. A method as claimed in claim 4, wherein the ratio between the first amplitude weighting and the second amplitude weighting is

.

6. A method as claimed in claim 3, wherein the first codeword has fewer symbols than the second codeword.

7. A method as claimed in claim 6, comprising providing for the second set of codebook entries, codebook entries with precoding weights which map the first codeword as represented by the first spatial layer to one antenna port using a first weighting value, and the second codeword represented by the second and third spatial layers to the other three antenna ports..

8. A method as claimed in claim 6 or 7, comprising providing for the second set of codebook entries, codebook entries with precoding weights which map the first codeword as represented by the first spatial layer to a first antenna port using a first weighting value, and the second spatial layer to a second antenna port using a second weighting value and the third spatial layer to the third antenna port using a third weighting value and to the fourth antenna port using a fourth weighting value.

9. A method as claimed in claim 8, wherein the first weighting value, the second weighing value and the third weighting value are equal.

10. A method as claimed in any of claims 8 to 9, wherein the fourth weighting value is one of 1/2,-1 /2,j/2, and -j/2.

11. A method as claimed in any of the claims of 8 to 10, further comprising providing for the second set of codebook entries, codebook entries wherein a spatial correlation value between the third and fourth antenna port is greater than or equal to a spatial correlation value between any other combination of the antenna ports.

12. A method as claimed in any preceding claim, wherein at (east one antenna port is arranged in accordance with spatial correlation properties of an antenna.

13. A method as claimed in claim 12, wherein said different spatial correlation properties comprise at least one of antenna polarization and antenna position.

14. A method as claimed in any preceding claim, comprising transmitting at least one antenna port from a plurality of antenna.

15. A method as claimed in any preceding claim, comprising controlling the codeword power value.

16. A computer program comprising program code means adapted to perform any of the steps of any of the preceding claims.

17. A method comprising: selecting one of a plurality of entries in a precoding code book for controlling transmissions from four antennas of a device, said code book comprising a plurality of entries, wherein said entries comprise at least one of: a first set of codebooks entries where the amplitude of each element of the entry is such that the power per spatial layer symbol is substantially equal for all code words; and a second set of codebook entries where the number of antenna ports receiving a spatial layer of the entry is such that the power per spatial layer symbol is as equal as possible over all codewords.

18. A method as claimed in claim 17, wherein each of said precoding code books being associated with a rank 3.

19. A method as claimed in claim 18, wherein the precoding code books map two codewords into three spatial layers.

20. A method as claimed in claims 17 to 19, further comprising communicating the selection to an apparatus for controlling a mapping for at least three spatial layers to four antenna ports.

21. Apparatus comprising: a spatia! layer precoder configured to control a mapping for at least three spatial layers to four antenna ports using a precoding code book, said code book comprising a plurality of entries, wherein said entries comprise at least one of: a first set of codebooks entries where the amplitude of each element of the entry is such that the power per spatia! layer symbol is substantially equal for ali code words; and a second set of codebook entries where the number of antenna ports receiving a spatial layer of the entry is such that the power per spatial layer symbol is as equal as possible over all codewords.

22. Apparatus as claimed in claim 21 , further comprising a memory for storing a plurality of precoding code book entries, each of said precoding code book entries being associated with a rank 3.

23. Apparatus as claimed in claim 22, wherein the precoding code book entries are configured to control a mapping for two codewords mapped into three spatial layers.

24. Apparatus as claimed in claim 23, wherein for the first set of codebook entries the codebook entries are configured with precoding weights which: map the first spatial layer to all four antenna ports using a first amplitude weighting; map the second spatial layer to a selection of two antenna ports from the four antenna ports using a second amplitude weighting; and map the third spatial layer to the two antenna ports not selected using the second amplitude weighting.

25. Apparatus as claimed in claim 22, wherein the ratio between the first amplitude weighting and the second amplitude weighting is 1/V2 .

26. Apparatus as claimed in claim 23, wherein the first codeword has fewer symbols than the second codeword.

27. Apparatus as claimed in ciaim 26, wherein for the second set of codebook entries, codebook entries are configured with precoding weights which: map the first codeword as represented by the first spatial layer to one antenna port using a first weighting value; and map the second codeword represented by the second and third spatial iayers to the other three antenna ports.

28. Apparatus as claimed in claim 26 and 27, wherein for the second set of codebook entries codebook entries are configured with precoding weights which: map the first codeword as represented by the first spatial layer to a first antenna port using a first weighting value; map the second spatial layer to a second antenna port using a second weighting value; map the third spatial layer to the third antenna port using a third weighting value; and map the third spatial layer to the fourth antenna port using a fourth weighting value.

29. Apparatus as claimed in claim 28, wherein the first weighting value, the second weighing value and the third weighting value are equal.

30. Apparatus as claimed in any of claims 28 to 29, wherein the fourth weighting value is one of 1/2,-1/2, j/2, and -j/2.

31. Apparatus as claimed in any of claims 28 to 30, wherein the spatial layer precoder is configured to control a mapping using the second set of codebook entries wherein a spatial correlation value between the third and fourth antenna port is greater than or equal to a spatial correlation value between any other combination of the antenna ports.

32. Apparatus as claimed in any of claims 21 to 31 further comprising a plurality of antenna wherein at feast one antenna port is arranged in accordance with spatial correlation properties of each antenna.

33. Apparatus as claimed in claim 32, wherein said different spatial correlation properties comprise at least one of antenna polarization and antenna position.

34. Apparatus as claimed in any of claims 21 to 33, further comprising a power controller configured to control the codeword power value.

35. Apparatus as claimed in any claims 21 to 34, further comprising a processor configured to control at least one of the spatial layer encoder, the memory, the power controller and the plurality of antenna.

36. Apparatus comprising: a codebook entry selector configured to select one of a plurality of entries in a precoding code book for controlling transmissions from four antennas of a device, said code book comprising a plurality of entries, wherein said entries comprise at least one of: a first set of codebooks entries where the amplitude of each element of the entry is such that the power per spatial layer symbol is substantially equal for all code words; and a second set of codebook entries where the number of antenna ports receiving a spatial layer of the entry is such that the power per spatial layer symbol is as equal as possible over all codewords.

37. An apparatus as claimed in claim 36, wherein each of said precoding code books being associated with a rank 3.

38. An apparatus as claimed in claim 37, wherein the precoding code books map two codewords into three spatial layers.

39. An apparatus as claimed in claims 36 to 38, further comprising a transmitter for communicating the selection to a further apparatus for controlling a mapping for at least three spatial layers to four antenna ports.

40. An integrated circuit or chip set comprising an apparatus as claimed in any of claims 21 to 39.

41. A user equipment comprising an apparatus as claimed in any of claims 21 to 35 or any of claims 21 to 35 when appended to claim 20.

42. A user equipment as claimed in claim 41 , comprising four antennas.

43. A base station comprising an apparatus as claimed in claims 36 to 39.

44. Apparatus comprising: means for using a precoding code book for controlling a mapping for at least three spatial layers to four antenna ports, said code book comprising a plurality of entries, wherein said entries comprise at least one of: a first set of codebooks entries where the amplitude of each element of the entry is such that the power per spatial layer symbol is substantially equal for all code words; and a second set of codebook entries where the number of antenna ports receiving a spatial layer of the entry is such that the power per spatial layer symbol is as equal as possible over all codewords.

45. Apparatus comprising: means for selecting one of a plurality of entries in a precoding code book for controlling transmissions from four antennas of a device, said code book comprising a plurality of entries, wherein said entries comprise at least one of: a first set of codebooks entries where the amplitude of each element of the entry is such that the power per spatial layer symbol is substantially equal for all code words; and a second set of codebook entries where the number of antenna ports receiving a spatial layer of the entry is such that the power per spatial layer symbol is as equal as possible over all codewords.