Abstract:
A method of multiple input multiple output downlink communications including mapping complex-valued modulation symbols onto one or more transmission layers by mapping complex-valued modulation symbols onto the layers and preceding the complex-valued modulation block of vectors and generating a block of vectors mapped onto at least four antenna ports. A first embodiment employs a maximum of two layers per codeword. A second embodiment employs no more than two codewords.

Description:
CLAIM OF PRIORITY 
     This application claims priority under 35 U.S.C. 119(e)(1) to U.S. Provisional Application No. 61/078,231 filed Jul. 3, 2008. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The technical field of this invention is wireless communication. 
     BACKGROUND OF THE INVENTION 
     The current Evolved Universal Terrestrial Radio Access (E-UTRA) Long Term Evolution (LTE) Rel. 8 specification supports up to 4-layer spatial multiplexing. As the enhancement for LTE is coming due to the IMT-Advanced call-of-proposal for yet another generation of upgrade in cellular technology. Thus the various aspects of LTE need to be reevaluated and improved. Of a particular interest is to increase the downlink (DL) peak data rate by a factor of 2 and an increase in the DL spectral efficiency to meet the IMT-Advanced requirements. Since LTE Rel. 8 already supports 64 Quadrature Amplitude Modulation (QAM) and higher-order modulation is infeasible in terms of the error vector magnitude (EVM) requirements, the support of higher-order spatial multiplexing (up to 8 layers) is inevitable. 
       FIG. 1  shows an exemplary wireless telecommunications network  100 . The illustrative telecommunications network includes base stations  101 ,  102  and  103 , though in operation, a telecommunications network necessarily includes many more base stations. Each of base stations  101 ,  102  and  103  are operable over corresponding coverage areas  104 ,  105  and  106 . Each base station&#39;s coverage area is further divided into cells. In the illustrated network, each base station&#39;s coverage area is divided into three cells. Handset or other user equipment (UE)  109  is shown in Cell A  108 . Cell A  108  is within coverage area  104  of base station  101 . Base station  101  transmits to and receives transmissions from UE  109 . As UE  109  moves out of Cell A  108  and into Cell B  107 , UE  109  may be handed over to base station  102 . Because UE  109  is synchronized with base station  101 , UE  109  can employ non-synchronized random access to initiate handover to base station  102 . 
     Non-synchronized UE  109  also employs non-synchronous random access to request allocation of up-link  111  time or frequency or code resources. If UE  109  has data ready for transmission, which may be traffic data, measurements report, tracking area update, UE  109  can transmit a random access signal on up-link  111 . The random access signal notifies base station  101  that UE  109  requires up-link resources to transmit the UE&#39;s data. Base station  101  responds by transmitting to UE  109  via down-link  110 , a message containing the parameters of the resources allocated for UE  109  up-link transmission along with a possible timing error correction. After receiving the resource allocation and a possible timing advance message transmitted on down-link  110  by base station  101 , UE  109  optionally adjusts its transmit timing and transmits the data on up-link  111  employing the allotted resources during the prescribed time interval. 
       FIG. 2  illustrates the overall process of downlink communication of the prior art.  FIG. 2  illustrates only two input codewords according to the prior art. The baseband signal representing a downlink physical channel is produced by the following steps. Each input codeword to be transmitted on a physical channel has its bits scrambled by respective scrambling circuits  201  and  202 . Corresponding modulators  202  and  212  modulate the scrambled bits generating complex-valued modulation symbols. A single layer mapper  203  maps the complex-valued modulation symbols onto one or more transmission layers. Complex-valued modulation symbols d (q) (0), . . . ,d (q) (M symb   (q) −1) for code word q are mapped onto the layers x(i)=[x (0) (i) . . . x (v−1) (i)] T , i=0, 1, . . . , M symb   layer −1 where v is the number of layers and M symb   layer  is the number of modulation symbols per layer. Single preceding circuit  204  precodes the complex-valued modulation symbols on each layer for transmission on the antenna ports. Precoder circuit  204  input a block of vectors x(i)=[x (0) (i) . . . x (v−1) (i)] T , i=0, 1, . . . , M symb   layer −1 from layer mapper  203  and generates a block of vectors y(i)=[ . . . y (p) (i) . . . ] T , i=0, 1, . . . , M symb   ap −1 to be mapped onto resources on each of the antenna ports, where y (p) (i) represents the signal for antenna port p. Corresponding resource channel mappers  205  and  215  map the complex-valued modulation symbols for each antenna port to resource elements. Corresponding OFDM signal generation circuits  206  and  216  generate complex-valued time-domain the Orthogonal Frequency Division Multiplexing (OFDM) signal for each antenna port. 
     The most crucial matter in extending the maximum number of layers from 4 to 8 is the extension of the codeword-to-layer mapping or simply termed layer mapping. The extension needs to be backward compatible with LTE Rel. 8 and introduce minimum impact on the current LTE specification especially in terms of control signaling.  FIG. 3  illustrates the current layer mapping for two transmitting antennas (2-TX) and for four transmitting antennas (4-TX). For case  311  with one layer and two transmitting antennas, a single codeword CW 1  is supplied to preceding for transmission via two antennas. For case  312  with one layer and four transmitting antennas, a single codeword CW 1  is supplied to preceding for transmission via four antennas. For case  321  with two layers and two transmitting antennas, two codewords CW 1  and CW 2  are supplied to preceding for transmission via two antennas. For case  322  with two layers and four transmitting antennas, two codewords CW 1  and CW 2  are supplied to preceding for transmission via four antennas. For case  323  with two layers and four transmitting antennas, one codeword CW 1  is supplied to a serial to parallel (S/P) converter which splits it into two signals and further supplies preceding for transmission via four antennas. As noted in  FIG. 3  this case occurs only for retransmission using one codeword when the initial transmission used more that one codeword. For case  332  with three layers and four transmitting antennas, one codeword CW 1  is supplied to preceding directly. The second codeword CW 2  is supplied to preceding via an S/P converter which splits it into two signals for supply to preceding. In case  332  preceding drives four antennas. For case  342  with four layers and four transmitting antennas, codewords CW 1  and CW 2  supplies respective S/P converters which each split into two signals and further supply preceding for transmission via four antennas. 
     SUMMARY OF THE INVENTION 
     A method of multiple input multiple output downlink communications including mapping complex-valued modulation symbols onto one or more transmission layers by mapping complex-valued modulation symbols onto the layers and preceding the complex-valued modulation block of vectors and generating a block of vectors mapped onto at least four antenna ports. A first embodiment employs a maximum of two layers per codeword. A second embodiment employs no more than two codewords. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of this invention are illustrated in the drawings, in which: 
         FIG. 1  is a diagram of a communication system of the prior art related to this invention having three cells; 
         FIG. 2  illustrates the overall process of downlink communication of the current art; 
         FIG. 3  illustrates the layer mapping of the current art; 
         FIG. 4  illustrates the layer mapping of a first alternative embodiment of this invention; and 
         FIG. 5  illustrates the layer mapping of a second alternative embodiment of this invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     This invention includes two alternatives embodiments. In the first alternative the maximum number of layers per codeword is 2. This results in 1 to 4 additional codeword(s) when the number of transmission layers is greater than 4. In the second alternative the maximum number of codewords is 2. Thus a maximum number of layers per codeword of 3 and 4 need to be supported when the number of transmission layers is greater than 4. 
       FIG. 4  illustrates the first alternative embodiment having a maximum of 2 layers per codeword.  FIG. 4  assumes more than four transmitting antennas. For case  411  with one layer, a single codeword CW 1  is supplied to precoding for transmission via the plural antennas. For case  421  with two layers, two codewords CW 1  and CW 2  are supplied to precoding for transmission via the plural antennas. For case  422  with two layers, one codeword CW 1  is supplied to an S/P converter which splits it into two signals and further supplies precoding for transmission via four antennas. As noted in  FIG. 4  this case occurs only for retransmission using one codeword when the initial transmission used more that one codeword. For case  431  with three layers, one codeword CW 1  is supplied to precoding directly. The second codeword CW 2  is supplied to precoding via an S/P converter which splits it into two signals. In case  431  precoding drives the plural antennas. For case  441  with four layers, codewords CW 1  and CW 2  supplies respective S/P converters which split each into two signals and further supplies precoding for transmission via the plural antennas. For case  451  with five layers, one codeword CW 1  supplies precoding directly. Two codewords CW 2  and CW 3  supply precoding via respective S/P converters which split them into two signals. Precoding supplies the plural antennas. For case  461  with six layers, three codewords CW 1 , CW 2  and CW 3  supply respective S/P converters which split each into two signals and further supplies precoding to drive the plural antennas. For case  471  with seven layers, one codeword CW 1  supplies precoding directly. Three codewords CW 2 , CW 3  and CW 4  supply precoding via respective S/P converters which split each into two signals and further supplies precoding. Precoding supplies the plural antennas. For case  481  with eight layers, four codewords CW 1 , CW 2 , CW 3  and CW 4  supply respective S/P converters which split them into two signals and further supplies precoding to drive the plural antennas. Cases  411 ,  421 ,  422 ,  431  and  441  correspond substantially to respective prior art cases  312 ,  322 ,  323 ,  332  and  342  illustrated in  FIG. 3 . Cases  451 ,  461 ,  471  and  481  are entirely new. 
     With multi-codeword transmission, a successive interference cancellation (SIC) receiver can attain a near-capacity performance. The SIC receiver can also attain the Shannon capacity, but the associated channel quality indicator (CQI) definition is also easy to obtain. This is unlike the maximum likelihood (ML) type receiver or maximum aposteriori probability (MAP) type receiver. Hence, we may be motivated to keep the maximum number of layers per codeword to 2 and increase the number of codewords such that 
                 N   CW     =     [       N   layer     2     ]       ,         
where N CW  is the number of codewords, N layer  is the number of layers and [X] is the greatest integer in X.
 
     The first codeword (CW 1 ) is designated as the codeword with the minimum number of codewords. That is: N layer   CW1 =2−mod(N layer , 2), N layer   CW1 =2 for n&gt;1. Alternatively, the last codeword can be designated as the codeword with the minimum number of codewords. The maximum codeword size is thus twice the codeword size as 1-layer transmission. 
     Three control signals may be required to support codeword to layer mapping: channel quality indicator report indicating supportable modulation and coding schemes (MCS) for each codeword; downlink control signaling to signal the actual MCS used; and an acknowledge/not acknowledge (ACK/NAK) signal for each codeword. The following alternatives are possible for each of these control signals. The first alternative provides one signal per codeword. This results in four signals for an 8-layer transmission. While this is a possible embodiment, it is preferred for backward compatibility with prior art UL and DL control signaling since it requires some new UL control formats and higher DL control overhead for the new DCI formats. The prior art system only supports up to 2 signals. The second alternative includes a maximum of two signals irrespective of the number of codewords. The N CW  codewords are bundled into 2 effective codewords for the purpose of signaling. As an example, for 5-layer and 6-layer transmission, CW 1  and CW 2  are bundled into 1 signal such as one ACK/NAK signal or Hybrid Automatic Repeat Request (HARQ) process number, or CQI report, or MCS for both these codewords, while CW 3  is assigned to the second HARQ process. For 7-layer and 8-layer transmission, CW 1  and CW 2  are bundled into 1 signal while CW 3  and CW 4  are bundled into the second signal. 
     It must be noted that different strategies might be adopted for different control signals. For example, the CQI report and DL signaling might be done separately for each codeword while the ACK/NAK signals are bundled together. This invention includes all combinations of signaling schemes for different signals along the lines mentioned above. 
       FIG. 5  illustrates the second alternative embodiment having a maximum of 2 codewords.  FIG. 5  assumes more than four transmitting antennas. For case  511  with one layer, a single codeword CW 1  is supplied to precoding for transmission via the plural antennas. For case  521  with two layers, two codewords CW 1  and CW 2  are supplied to precoding for transmission via the plural antennas. For case  522  with two layers, one codeword CW 1  is supplied to an S/P converter which splits it into two signals and further supplies precoding for transmission via four antennas. As noted in  FIG. 5  this case occurs only for retransmission using one codeword when the initial transmission used more that one codeword. For case  531  with three layers, one codeword CW 1  is supplied to precoding directly. The second codeword CW 2  is supplied to precoding via an S/P converter which splits it into two signals. In case  531  precoding drives the plural antennas. For case  532  with three layers, one codeword CW 1  is supplied to an S/P converter which splits it into three signals and further supplies pre-decoding to drive the plural antennas. As noted in  FIG. 5  this case occurs only for retransmission using one codeword when the initial transmission used more that one codeword. For case  541  with four layers, codewords CW 1  and CW 2  supplies respective S/P converters which split each into two signals and further supplies precoding for transmission via the plural antennas. For case  542  with four layers, a single codeword CW 1  supplies an S/P converter which splits it into four signals and further supplies precoding for transmission via the plural antennas. As noted in  FIG. 5  this case occurs only for retransmission using one codeword when the initial transmission used more that one codeword. For case  551  with five layers, one codeword CW 1  supplies a first S/P converter which splits it into two signals and further supplies precoding. A second codeword CW 2  supplies a second S/P converter which splits it into three signals and further supplies precoding. Precoding supplies the plural antennas. For case  661  with six layers, a first codeword CW 1  supplies a first S/P converter which splits it into three signals and further supplies precoding. A second codeword CW 2  supplies a second S/P converter which splits it into three signals and further supplies precoding. Precoding supplies the plural antennas. For case  571  with seven layers, a first codeword CW 1  supplies a first S/P converter which splits it into three parts and further supplies precoding. A second codeword CW 2  supplies a second S/P converter which splits it into four parts and further supplies precoding. Precoding supplies the plural antennas. For case  581  with eight layers, two codewords CW 1  and CW 2  supply respective S/P converters which split each into four parts and further supplies precoding to drive the plural antennas. Cases  511 ,  521 ,  522 ,  531  and  541  correspond substantially to respective prior art cases  312 ,  322 ,  323 ,  332  and  342  illustrated in  FIG. 3 . Cases  542 ,  551 ,  561 ,  571  and  581  are entirely new. 
     This alternative preserves maximum backward compatibility of the UL and DL control signaling with the prior art illustrated in  FIG. 3 . With a maximum number of codewords of two, there is no need for additional number of HARQ processes nor additional number of UL ACK/NAK bits per process. While the SIC receiver gain is limited to two iterations, the loss can be compensated by applying ML type detection for each of the two SIC receiver iterations. 
     The first codeword (CW 1 ) is designated as the codeword with the minimum number of codewords. That is: 
                 N   layer     CW   ⁢           ⁢   1       =     ⌊       N   layer     2     ⌋       ,       N   layer     CW   ⁢           ⁢   2       =         N   layer     -     N   layer     CW   ⁢           ⁢   1         =       ⌈       N   layer     2     ⌉     .               
Alternatively, the last codeword can be designated as the codeword with the minimum number of codewords.
 
     Consequently, the maximum codeword size is two, three or four times codeword size for 1-layer transmission, depending on the number of layers. Related to HARQ, two additional layer mappings for retransmission purposes may be needed for 3-layer and 4-layer retransmissions. These are cases  532  and  542 . Cases  532  and  542  are only used for retransmitting one out of the 2 codewords if only one of the two codewords fails to be decoded. This is analogous to the need for defining a 1-CW 2-layer mapping for the 2-layer retransmission of case  323  illustrated in  FIG. 3 . 
     The same considerations mentioned for first alternative hold here regarding control signaling.