FIELD OF THE INVENTION

[0001] The present invention relates generally to communication systems, and particularly to methods and systems for transmission using multiple antennas. BACKGROUND OF THE INVENTION

[0002] Some communication systems transmit data from a transmitter to a receiver over multiple communication channels, using multiple transmit antennas and multiple receive antennas. Multiple-channel transmission is used, for example, in spatial multiplexing schemes that achieve high throughput, in beam-forming schemes that achieve high antenna directivity and in spatial diversity schemes that achieve high resilience against channel fading and multipath. These schemes are often referred to collectively as Multiple-Input Multiple-Output (MIMO) schemes.

[0003] MIMO schemes are contemplated, for example, for use in Evolved Universal Terrestrial Radio Access (E-UTRA) systems, also referred to as Long Term Evolution (LTE) systems. The Third Generation Partnership Project (3GPP) E-UTRA standards specify MIMO schemes for use by E-UTRA User Equipment (UE) and base stations (eNodeB). These schemes are described, for example, in 3GPP Technical Specification 36.211, entitled "Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)," (3GPP TS 36.21 1), version 8.6.0, March, 2009, in 3GPP Technical Specification 36.213, entitled "Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E- UTRA); Physical Layer Procedures (Release 8)," (3GPP TS 36.213), version 8.6.0, March, 2009, and in 3GPP Technical Report 36.814, entitled "Technical Specification Group Radio Access Network; Further Advancements for E-UTRA Physical Layer Aspects (Release 9)," (3GPP TR 36.814), version 0.4.1 , February, 2009, which are incorporated herein by reference.

SUMMARY OF THE INVENTION

[0004] An embodiment of the present invention provides a method for communication in a transmitter having a first number of transmit antenna ports. In accordance with the disclosed method, an upper limit is set on a second number of spatial layers to be used by the transmitter to be less than the first number. An actual number of the spatial layers, which does not exceed the upper limit, is allocated for transmission to a given receiver. One or more streams of modulated symbols are mapped onto the allocated actual number of the spatial layers. The actual number of the spatial layers is transmitted from the transmitter to the given receiver.

[0005] In some embodiments, transmitting the spatial layers includes applying a preceding operation that maps the spatial layers onto the transmit antenna ports, and setting the upper limit includes setting a first upper limit when the preceding operation depends on feedback from the given receiver, and setting a second upper limit, which does not exceed the first upper limit, when the precoding operation is not dependent on the feedback.

[0006] In an embodiment, input data is encoded with an Error Correction Code (ECC) to produce a given number of code words, and the code words are modulated to produce the respective given number of the streams of the modulated symbols, wherein the given number of the code words is restricted to be at most two.

[0007] In a disclosed embodiment, when the actual number of spatial layers is between one and four, the modulated symbols are mapped onto the layers in accordance with the table shown in Fig. 4A in which d^ (jϊ) denotes an n modulated symbol originating from a code word q, and X^ (rϊ) denotes an n value of a spatial layer/?.

[0008] In another embodiment, when the actual number of spatial layers is between five and eight, the modulated symbols are mapped onto the layers in accordance with the table shown in Figs. 4A and 4B.

[0009] In yet another embodiment, when the actual number of spatial layers is between five and eight, the modulated symbols are mapped onto the layers in accordance with the table shown in Figs. 5 A and 5B.

[0010] In still another embodiment, when the actual number of spatial layers is between five and eight, the modulated symbols are mapped onto the layers in accordance with the table shown in Figs. 6A and 6B. [0011] In another embodiment, when the actual number of spatial layers is between five and eight, the modulated symbols are mapped onto the layers in accordance with the table shown in Figs. 7A and 7B.

[0012] In some embodiments, the first number is greater than four. In an embodiment, the first number is equal to eight, and the upper limit is between four and seven. [0013] In an embodiment, allocating the actual number of the spatial layers includes signaling the actual number to the given receiver using a signaling protocol in which a data structure allocated to signaling the actual number is insufficient for signaling values higher than the upper limit. Additionally or alternatively, allocating the actual number of the spatial layers may include signaling from the given receiver to the transmitter a preferred number of the spatial layers using a signaling protocol in which a data structure allocated to signaling the preferred number is insufficient for signaling values higher than the upper limit. In a disclosed embodiment, the spatial layers include respective parallel streams transmitted concurrently from the transmitter to the given receiver.

[0014] There is additionally provided, in accordance with an embodiment of the present invention, a method for communication in a transmitter having a plurality of transmit antenna ports and is operative to map streams of modulated symbols onto spatial layers. In accordance with the disclosed method, a precoding operation is selected for use in mapping the spatial layers onto the transmit antenna ports. An upper limit is set on a number of the spatial layers depending on the selected precoding operation. One or more of the streams of the modulated symbols are mapped onto the number of spatial layers that does not exceed the upper limit. The selected precoding operation is applied to the spatial layers so as to map the spatial layers onto the transmit antenna ports. The precoded spatial layers are transmitted over the transmit antenna ports to a receiver.

[0015] In some embodiments, setting the upper limit includes setting a first upper limit when the selected precoding operation depends on feedback from the receiver, and setting a second upper limit, which is less than the first upper limit, when the selected precoding operation is not dependent on the feedback. In an embodiment, input data is encoded with an Error Correction Code (ECC) to produce a given number of code words, and the code words are modulated to produce the streams of the modulated symbols, wherein the given number of the code words is restricted to be at most two. In a disclosed embodiment, mapping the streams onto the spatial layers includes signaling the number of the spatial layers to the receiver using a signaling protocol in which a data structure allocated to signaling the number of the spatial layers is insufficient for signaling values higher than the upper limit. Additionally or alternatively, mapping the streams onto the spatial layers includes signaling from the receiver to the transmitter a preferred number of the spatial layers using a signaling protocol in which a data structure allocated to signaling the preferred number is insufficient for signaling values higher than the upper limit. In some embodiments, the spatial layers include respective parallel streams transmitted concurrently from the transmitter to the receiver.

[0016] There is also provided, in accordance with an embodiment of the present invention, a communication apparatus, which includes a transmitter and a first number of transmit antenna ports. The transmitter is configured to set an upper limit on a second number of spatial layers to be used by the transmitter to be less than the first number, to allocate an actual number of the spatial layers, which does not exceed the upper limit, for transmission to a given receiver, to map one or more streams of modulated symbols onto the allocated actual number of the spatial layers, and to transmit the actual number of the spatial layers simultaneously to the given receiver. The transmitter may be included in a mobile communication terminal or in a base station.

[0017] There is further provided, in accordance with an embodiment of the present invention, a communication apparatus, which includes a transmitter and a first number of transmit antenna ports. The transmitter is configured to map streams of modulated symbols onto spatial layers, to select a precoding operation for use in mapping the spatial layers onto the transmit antenna ports, to set an upper limit on a number of the spatial layers depending on the selected precoding operation, to map one or more of the streams of the modulated symbols onto the number of spatial layers that does not exceed the upper limit, to apply the selected precoding operation to the spatial layers so as to map the spatial layers onto the transmit antenna ports, and to transmit the precoded spatial layers over the transmit antenna ports to a receiver. The transmitter may be included in a mobile communication terminal or in a base station.

[0018] The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Fig. 1 is a block diagram that schematically illustrates a transmitter having multiple antennas, in accordance with an embodiment of the present invention;

[0020] Figs. 2 and 3 are flow charts that schematically illustrate methods for transmission via multiple antennas, in accordance with embodiments of the present invention; and

[0021] Figs. 4A-7B are tables showing mapping of code words to spatial layers, in accordance with embodiments of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0022] In some known MIMO schemes, a transmitter maps streams of modulated symbols onto spatial layers, i.e., signals that are to be transmitted over different MIMO transmission channels. The spatial layers are also referred to as transmission layers or spatial streams, or simply layers for brevity. The transmitter then applies a precoding operation to map each spatial layer onto a respective set of antenna ports. A transmission process of this sort, as performed in the downlink of a E-UTRA eNodeB, is described in detail in section 6.3 of the 3GPP TS 36.211 specification, cited above.

[0023] Embodiments of the present invention that are described hereinbelow provide improved transmitter configurations and transmission methods, which reduce the complexity of MIMO transmitters and simplify the above-mentioned transmission process and associated signaling.

[0024] Generally, the number of spatial layers that are actually used for transmission from a given transmitter to a given receiver may be varied according to channel conditions. In conventional E-UTRA systems, for example, the number of spatial layers may reach InIn(N ₇ ;;^, Nβ _Λ ⁷ ), wherein Nγx and NRX denote the number of transmit and receive antenna ports, respectively. In many cases, however, it may be advantageous to use an even lower number of spatial layers.

[0025] Typically, the actual number of layers that can be used depends on the level of correlation among the different communication channels between the transmitter and the receiver (i.e., between different transmit and receive antenna pairs). Low correlation usually implies that a large number of parallel transmission streams can be transmitted and reconstructed successfully, meaning that a large number of layers can be used. High correlation usually means that the number of layers should be small.

[0026] In most cases, at least some correlation exists among the multiple communication channels, and therefore the likelihood of exploiting the maximum theoretical number of layers is small. Consequently, in most practical scenarios, it is sufficient to limit the number of actual layers to a value that is less than the maximum theoretical limit of mm{Nγχ,

NRX)- Thus, in some embodiments of the present invention, the transmitter limits the number of layers to be less than the number of transmit antennas. This limit is typically set a-priori, irrespective of channel conditions or the number of receive antennas in any given receiver. Limiting the maximum number of layers reduces the complexity of the transmitter considerably. The benefit of this technique is particularly significant in evolving LTE- Advanced (LTE-A) systems, which may use up to eight transmit antennas and eight receive antennas. [0027] In some embodiments, the transmitter sets different upper limits on the number of layers depending on feedback from the receiver and/or the type of precoding operation used. For example, when precoding is adaptive based on feedback from the receiver, in other words when precoding is performed in closed loop, the ability to exploit the spatial multiplexing gain of the multiple channels is relatively high, and therefore the transmitter may allow a higher maximum number of layers. On the other hand, when precoding is performed in open loop, i.e., without feedback from the receiver, the transmitter may set a lower limit on the maximum number of layers.

[0028] Fig. 1 is a block diagram that schematically illustrates a transmitter 20 having multiple antennas, in accordance with an embodiment of the present invention. The description that follows refers to a transmitter of an LTE-A eNodeB, although other transmitters are contemplated. In alternative embodiments, for example, the methods and systems described herein can be used in transmitters operating in accordance with any other suitable communication standard or protocol, such as IEEE 802.16 (also referred to as WiMAX), for example. Although the description that follows refers mainly to downlink transmission from the eNodeB to the UE, the disclosed methods and systems may be applicable to uplink transmission, as well. [0029] Transmitter 20 comprises one or more modulation chains, each comprising an Error Correction Code (ECC) encoder 24, a scrambler 28 and a modulation mapper 32. Data for transmission is encoded by ECC encoders 24, to produce respective ECC code words. (The example of Fig. 1 shows two separate ECC encoders for clarity. In practice, however, the transmitter may comprise a single ECC encoder that produces code words for the different modulation chains.) The number of code words that are used for encoding a given transmission is referred to as New- Certain aspects regarding the choice of this value are addressed further below.

[0030] The bits of each code word are scrambled by a respective scrambler 28, and then modulated by a respective modulation mapper 32. Each modulation mapper produces a stream of complex-valued modulated symbols. Any suitable modulation scheme, such as

Quadrature Phase Shift Keying (QPSK) or Quadrature Amplitude Modulation (QAM), can be used. A given modulation mapper 32 operates on the scrambled bits of a given code word denoted q (q-0,\,...,Ncw^) to produce a block of M _syrnb complex-valued modulated symbols denoted G^(O), d ⁽ ^ (l), ... , d™ (M^ - l). [0031] A layer mapper 36 maps the modulated symbol streams produced by modulation mappers 32 onto one or more spatial layers. (For a given set of time and frequency resources allocated to a certain communication channel, the multiple transmit and receive antennas add another "spatial" dimension to those resources. One of the possibilities to exploit the additional spatial dimension is by increasing the number of independent modulated symbols transmitted per time-frequency resource. The factor of increase, relative to the case of a single transmit antenna and a single receive antenna, is defined as the number of spatial layers.)

[0032] The actual number of spatial layers used by mapper 36 is denoted N _/ ^yE _& s ¹ , and is a selectable parameter. The choice of this value may depend, for example, on the channel conditions between transmitter 20 and a given receiver to which the transmission is intended. Each spatial layer comprises a stream of complex values, which are to be subsequently transmitted over the MIMO communication channel. In some embodiments, transmitter 20 sets an upper limit on the value of NI^YERS, as will be discussed in detail further below. Several suitable mapping schemes that can be used by layer mapper 36 are shown in Fig. 4A-7B below.

[0033] The mapped spatial layers are provided to a precoder 40. Precoder 40 maps the Ni _A YER _S spatial layers onto NJX transmission channels, corresponding to NJX antenna ports 52 of the transmitter. (Note that a given antenna port may not necessarily correspond to a single physical antenna, but may correspond to a "virtual antenna" whose transmitted signal is generated - in a manner that the receiver need not necessarily be aware of - as a superposition (a weighted sum) of the signals stemming from a number of physical antennas. Note also that the number of antenna ports may be larger than the number of layers.) Resource mappers 44 allocate resource elements (time-frequency allocations) to the respective transmission channels. The outputs of mappers 44 are processed by respective Orthogonal Frequency Division Multiplexing (OFDM) generators 48, which produce OFDM signals that are transmitted via antenna ports 52 toward the receiver. [0034] Transmitter 20 comprises a controller 56, which configures and controls the different transmitter elements. In particular, controller 56 comprises a layer and code word setting module 60, which sets the number of spatial layers and the number of code words to be used by the transmitter. The functions of module 60 are explained in detail below.

[0035] The transmitter configuration shown in Fig. 1 is a simplified example configuration, which is depicted for the sake of conceptual clarity. In alternative embodiments, any other suitable transmitter configuration can also be used. For example, although the embodiments described herein refer mainly to transmitters having eight transmit antenna ports, the methods and systems described herein can be used with any other suitable number of antenna ports. Transmitter elements that are not mandatory for explanation of the disclosed techniques, such as various Radio Frequency (RF) elements, have been omitted from Fig. 1 for the sake of clarity.

[0036] The different components of transmitter 20 may be implemented using dedicated hardware, such as using one or more Application-Specific Integrated Circuits

(ASICs) and/or Field-Programmable Gate Arrays (FPGAs). Alternatively, some transmitter components may be implemented using software running on general-purpose hardware, or using a combination of hardware and software elements. Typically, controller 56 comprises a general-purpose processor, which is programmed in software to carry out the functions described herein, although it too may be implemented on dedicated hardware. The software may be downloaded to the processor in electronic form, over a network, for example, or it may, alternatively or additionally, be provided and/or stored on tangible media, such as magnetic, optical, or electronic memory.

[0037] When transmitter 20 transmits to a given receiver (not shown in the figures), module 60 typically sets the actual number of spatial layers (NL _Λ røΛs) according to the channel conditions between the transmitter and the receiver. In accordance with an embodiment, the maximum possible number of layers is given by min(Nfχ, N^γ), wherein

Nfx and N _# χ denote the number of transmit and receive antenna ports, respectively. Module

60 typically selects the actual number of layers adaptively, in an attempt to maximize the data throughput that can be transferred reliably to the receiver over the present channel (and noise- plus-interference).

[0038] The number of layers that is predicted to yield the maximal throughput is sometimes called the "MIMO channel rank." In some cases (e.g., when the transmitter lacks reliable information as to the channel conditions) the receiver notifies the transmitter as to the "preferred rank" (e.g., a rank indicator, in the terminology of section 7 of the 3GPP TS 36.213 specification, cited above). In some embodiments, module 60 sets the actual number of layers based on the preferred rank that is fed-back from the receiver. The statistics of which ranks are preferable in scenarios of varying (e.g. fading) channels typically depends on the amount of correlation among the different communication channels between the transmitter and the receiver (i.e., between the different transmit and receive antenna pairs) and on the Signal-to- Νoise Ratio (SΝR) at the receiver. Lower ranks are more likely to be preferable at low SΝR and/or in highly-correlated channels, and vice versa. [0039] When the communication channels exhibit little correlation and/or provide a relatively high SNR, the receiver is more likely to succeed decoding a large number of spatial layers. In such cases, in accordance with an embodiment of the invention, module 60 typically sets a relatively large number of layers, so as to provide a relatively high data throughput. However, when the communication channels are highly correlated and/or lead to a relatively low SNR, the receiver is only likely to succeed decoding a smaller number of spatial layers. In such cases, module 60 may reduce the number of actual layers accordingly.

[0040] In practice, however, the likelihood of reaching and exploiting the theoretical maximum number of layers min(Njχ, NRX) is very low. In most practical scenarios, at least some correlation exists among the multiple communication channels, and the situation where the maximum number of layers is beneficial - namely, a very high SΝR - is seldom reached.

[0041] Therefore, in some embodiments, module 60 in transmitter 20 sets an upper limit, which is less than the above-mentioned theoretical limit, on the number of spatial layers. Typically, this upper limit is applied to the transmitter operation as a whole, irrespective of any given receiver. The upper limit is therefore expressed in terms of the number of transmit antenna ports. In other words, transmitter 20 may limit NIAYERS to a value that is less than (and not equal to) Nj*χ. The upper limit is denoted Λ ^T MAX herein. For example, in an LTE-A eΝodeB having eight transmit antenna ports (M ^r jχ =8), module 60 may set the upper limit to NMAX=^ _I NMAX=S ¹ , NMAX=^ ^or NMAX=I- Alternatively, any other suitable values of N^ and N MAX ^{can a} l ^so be used.

[0042] For a given value of Njx, the choice of NMAX trades-off transmitter complexity and performance. Higher NMAX corresponds to a potentially-higher maximum throughput, but on the other hand means that the transmitter is required to store larger mapping tables and support simultaneous processing (e.g., mapping and precoding) of a higher number of symbol streams. Lower NMAX simplifies the transmitter at the expense of potentially-lower maximum throughput. Since the likelihood of approaching the maximum throughput in real-life scenarios is low, as explained above, limiting N MAX ^t0 values less than the number of transmit antenna ports is often a preferable trade-off.

[0043] Moreover, limiting the number of layers enables reduction of signaling resources, which are used for signaling the actual number of layers between the transmitter and the receiver (and/or signaling the preferred number of layers as feedback from the receiver to the transmitter, as noted above). For example, if the maximum number of layers is reduced from eight to four (i.e., Nz, _v 4 ₇ £ _Λ 5≤Λ0iM _A ⁼ 4), the transmitter can report the value of NI^YERS to the receiver using only two bits instead of three. If N MAX ^ ^set to ^s i ^χ iN LAYERS^ M _A X"^\ three signaling bits are still needed, but only six out of the eight possible bit value combinations are used for signaling NL^YERS- The remaining two bit value combinations are free, and can be reserved for any other suitable purpose. Thus, in some embodiments, transmitter 20 signals N^YgRs to the receiver using a signaling protocol in which the signaling resources (e.g., a field or other data structure), which are allocated to signaling the actual number of layers, are insufficient for signaling values higher than N^ _A χ.

[0044] As noted above, in some embodiments the receiver notifies the transmitter of the preferred number of layers to be used (e.g., using a rank indicator field). In these embodiments, the protocol used for signaling the preferred number of layers to the transmitter may be defined so that signaling resources (e.g., a field or other data structure), which are allocated to signaling the preferred number of layers, are insufficient for signaling values higher than NM ₄ X-

[0045] Fig. 2 is a flow chart that schematically illustrates a method for transmission via multiple antennas, in accordance with an embodiment of the present invention. In the present example, transmitter 20 is disposed in an LTE-A base station (eΝodeB), which may communicate with multiple UEs. The method begins with transmitter 20 setting an upper limit

(NMAX) ^on the number of spatial layers to be used for downlink transmission, at a limiting step 70. The upper limit is less than the number of transmit antenna ports of transmitter 20, i.e.,

NMAX<NTX-

[0046] The eΝodeB, and in particular transmitter 20, establishes communication with a given UE, at a communication set-up step 74. Based on the channel conditions between the eΝodeB and this UE, module 60 in transmitter 20 selects an actual number of spatial layers

(N LAYERS) f° ^r downlink communication with the given UE, at an actual layer selection step

78. Module 60 selects an actual number that does not exceed the upper limit set at step 70 above, i.e., NLΛTE _/f S≤NM _Λ X- (Additionally or alternatively, the actual number of layers may be set based on closed-loop channel condition information that is fed-back from the receiver and/or on a-priori determination of channel condition information. The actual number of layers may be different for closed-loop and open-loop operation. These features are described in detail further below.)

[0047] The transmitter communicates with the given UE using the selected actual number of layers. ECC encoders 24 encode the data for transmission so as to produce NQW

ECC code words, at an ECC encoding step 82. Scramblers 28 scramble the bits of each code word, and modulation mappers 32 modulate the scrambled bits to produce streams of encoded symbols, at a modulation step 86. The output of step 86 is a set of Nc^≥l streams of modulated symbols. Layer mapper 36 maps the NQW streams of modulated symbols onto the NITERS spatial layers, at a layer mapping step 90. Any suitable mapping scheme can be used, such as the illustrative mapping schemes described in Figs. 4A-7B below.

[0048] Precoder 40 maps the N _/ ^ YERS spatial layers onto the Nj-χ transmit antenna ports 52 of transmitter 20, at a precoding step 94. The transmitter transmits the precoded spatial layers via the transmit antenna ports to the given UE, at a transmission step 98. [0049] In some embodiments, module 60 selectively limits the maximum number of code words per transmission (New) to be less than the actual number of layers. For example, module 60 may limit the value of Nc^ to no more than 2 (i.e., Nc^e {1,2}). Reducing the number of code words per transmission simplifies the transmitter and reduces the signaling resources needed for signaling the selected New value (and/or other information that is signaled per code word) to the receiver. On the other hand, a lower New value may somewhat degrade the receiver performance, for example in receivers that use Sequential Interference Cancellation (SIC) techniques. Nevertheless, in most cases a maximum of Ncw ⁼ ^ provides good receiver performance, even for NiAγβRg=8. Increasing New beyond this value may not provide additional performance that justifies the associated complexity. The layer mapping examples given in Figs. 4A-7B below demonstrate mapping of one or two code words onto up to eight spatial layers.

[0050] In some embodiments, transmitter 20 sets different upper limits on the number of layers, depending on the type of precoding operation applied by precoder 40.

Precoder 40 may apply closed-loop or open-loop precoding. For example, closed- and open- loop precoding in E-UTRA systems are described in sections 6.3.4.2.1 and 6.3.4.2.2 of 3GPP TS 36.211, cited above, and in section 7 of 3GPP TS 36.213, cited above. In closed-loop precoding, the mapping of spatial layers to antenna ports is adaptive based on feedback provided by the receiver. For example, in some embodiments the transmitter and receiver support a predefined set (a "codebook") of precoding schemes, usually expressed as precoding matrices. The receiver notifies the transmitter which precoding scheme is preferable at a given point in time, and the transmitter selects and applies the mapping scheme requested by the receiver. In open-loop precoding, the transmitter applies a certain precoding scheme irrespective of feedback from the receiver.

[0051] When the transmitter uses closed-loop precoding, link adaptation may track the channel conditions relatively accurately, and the receiver may better exploit the potential spatial multiplexing gain of the multiple channels. When using open-loop precoding, on the other hand, the actual spatial multiplexing gain is likely to be lower. Therefore, using a large number of spatial layers is more likely to produce high performance under closed-loop precoding than under open-loop precoding. Thus, in some embodiments, transmitter 20 sets a certain upper limit (denoted N MAX OU ^on the number of layers when using open-loop precoding, and another upper limit (denoted Λ ^T MAX CL) ^on the number of layers when using closed-loop precoding, wherein N M _A X OL^M _A X CL-

[0052] Fig. 3 is a flow chart that schematically illustrates a method for transmission via multiple antennas, in accordance with an embodiment of the present invention. The method of Fig. 3 begins with transmitter 20 establishing communication with a given UE, at a communication establishing step 100. Controller 56 in transmitter 20 checks whether open- or closed-loop precoding is used with this UE, at a precoding mode checking step 104. If open- loop precoding is used, module 60 sets ^ M _A ]C^ M _A X OLi at an open-loop layer limiting step 108. Otherwise, i.e., if closed-loop precoding is used, module 60 sets ^MA^^MAX CL _> ^{at a} closed-loop layer limiting step 112. As noted above, N MAX OL^ MAX CL-

[0053] Having selectably limited the number of spatial layers to either NMAX OL ^OΓ NMAX CL depending on the precoding mode, transmitter 20 sets the actual number of layers

NITERS to values that do not exceed the applicable upper limit, at a layer setting step 116. From this stage, the method continues similarly to steps 82-98 of the method of Fig. 2 above. [0054] In some embodiments, N^ YERS is limited to be less than N^χ only when the number of antenna ports is high, and is allowed below a certain number of antenna ports. This technique can be used to maintain backward compatibility with conventional schemes, e.g., with LTE systems conforming to the TS 36.211 and 36.213 specifications, cited above. For example, the constraints on the number of layers can be set to 1 ≤ N _LAYERS ≤ min(N _TX , N _RX ) if mm(N _TX , N _RX ) ≤ 4, to

1 ≤ ^ _LA Y _ERS ≤ Ν _M AX_CL < rnin(N _TX , N _RX ) if min(N _TX , N _RX ) > 4 and closed-loop preceding is used, and to 1 < N _LAYERS ≤ Ν _MAX _ _0L < ™-in(N _TX , N _RX ) if min(N _TX , N _RX ) > 4 and open- loop precoding is used. [0055] Figs. 4A-7B are tables showing mapping examples of code words to spatial layers, in accordance with embodiments of the present invention. Figs. 4A and 4B show one mapping example, Figs. 5A and 5B show a second example, Figs. 6A and 6B show a third example, and Figs. 7A and 7B show a fourth example. All four examples refer to an LTE-A eNodeB having eight antenna ports. These four examples are in no way limiting. The methods and systems described herein may use any other suitable mapping scheme.

[0056] In a given example, each row defines the mapping of a certain number of code words (Ncpp) to a certain number of spatial layers (Nx^y^^s)- ^m the examples, dS^ (n) denotes the n'th modulated symbol originating from code word q. X^ (n) denotes the n'th complex value of the p'th spatial layer. As can be seen in the examples, in some cases the symbols of a given code word are de-multiplexed over two or more layers. In other cases, the symbols of a given code word are mapped to a single layer. The examples given herein attempt to distribute the symbols among the layers evenly, although this feature is not necessarily mandatory.

[0057] In all four examples, the mapping of code words to layers for NLAYERS£A conforms to the mapping specified in section 6.3.3.2 of the 3GPP TS 36.211 specification, cited above. This feature maintains backward compatibility, i.e., enables the eΝodeB to communicate with LTE-compliant UEs. This feature is, however, by no means mandatory. Other mapping schemes may differ from the 3GPP TS 36.211 specification as desired.

[0058] In all four examples, the maximum number of code words is two. As noted above, increasing the number of code words beyond two usually does not provide significant performance improvement. Nevertheless, in alternative embodiments, the mapping may specify higher numbers of code words, as well. [0059] As explained above, transmitter 20 may selectably set an upper limit on the number of layers, which is less than the number of antenna ports. The transmitter may set this upper limit, for example, by storing and/or using only a subset of the rows of a given mapping table. For example, when setting NM _/ L¥=6, the transmitter may omit the rows corresponding to NIAYERS>(>- This technique may simplify the design of layer mapper 36 and reduce the memory space used for storing the mapping table.

[0060] Additionally or alternatively, the transmitter may omit one or more of the rows of the mapping table in order to reduce memory requirements, computational complexity and signaling resources. The omitted rows do not necessarily correspond to large numbers of layers. For example, the transmitter may omit the odd-order rows or even-order rows of the mapping table.

[0061] Although the embodiments described herein mainly address setting the number of spatial layers in LTE-A transmitters, the methods and systems described herein can also be used in other applications, such as in IEEE 802.16 transceivers. [0062] It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.