Abstract:
The disclosure relates to transmission of user data over multiple transmission layers in a wireless communication system with single-carrier orthogonal frequency division multiple access. A wireless terminal performs transform precoding on a vector of digital modulation symbols and the resulting complex-valued symbols are mapped to frequency/time/space resources. The digital modulation symbols are reordered, modified by a setting of complex-valued functions, and transform precoded. The resulting second set of complex-valued symbols are transform precoded and mapped to frequency/time/space resources.

Description:
FIELD OF THE DISCLOSURE 
       [0001]    The present disclosure relates generally to wireless communications and more particularly to spatial diversity transmission in single carrier FDMA systems. 
       BACKGROUND 
       [0002]    Single-carrier Frequency Division Multiple Access (SC-FDMA) is a multiplexing scheme used in wireless communication systems that first transform precodes, typically with the discrete Fourier transform (DFT), a set of modulation symbols, to generate a set of frequency domain samples which are then mapped to a set of subcarriers in a SC-FDMA symbol period. An inverse discrete Fourier transform (IDFT) will then be applied onto the set of frequency domain samples to generate a set of time samples which have the desirable property of a low peak-to-average power ratio. Transmit waveforms with low peak-to-average power ratios can be amplified with highly efficient power amplifiers that require minimum bias current and therefore SC-FDMA is a popular choice for mobile wireless systems such 3GPP LTE Releases 8,9, and 10. 
         [0003]    Modern wireless communication systems often employ multiple antennas at both the transmitter and receiver for either transmission/reception diversity or spatial multiplexing of multiple user data streams. Spatial multiplexing is advantageous when propagation conditions include scattering and reflections of a transmitted signal thereby causing multiple paths between transmitter and receiver. With appropriate transmission techniques, different user data streams can be directed along each path, the maximum number of streams being equal to the minimum of the number of transmit and receive antennas. A common transmission technique used to enable spatial multiplexing is spatial precoding. In spatial precoding two or more transmission layers are formed from the coded modulation symbols of two or more user data streams. A typical example is when the coded symbols of the first user data stream are sent on the first transmission layer and the coded symbols of the second user data stream are sent on the second transmission layer. At each SC-FDMA symbol, the frequency domain samples corresponding to the two data streams are multiplied by a precoding matrix. The components of the resulting signals are transmitted on a set of antennas, each component corresponding to a different transmit antenna. Note that if the precoding matrix is not square, the number of transmission layers and number of transmission antennas will not be equal. The performance of spatial multiplexing is heavily dependent on how the precoding matrix is chosen. In open loop schemes, it may be a constant, while in closed-loop schemes the user may feed back to the transmitter either a recommended precoding matrix as in the downlink of LTE or may give an explicit instruction to the transmitter on which precoding matrix to use, as in the uplink of 3GPP LTE Release 10. Regardless of how the precoding matrix is determined, spatial precoding of, for example two transmission layers, allows two user data streams to be transmitted over the channel instead of one. 
         [0004]    However spatial multiplexing is not suited to all transmission scenarios. One such scenario is the transmission of fixed-payload, low-rate data, possibly in parallel with multiple spatially multiplexed data streams. This scenario occurs in the uplink of 3GPP LTE Release 10 when the so-called UCI (user control information) symbols are multiplexed with user data onto the physical uplink shared channel (PUSCH) and the mobile station, or UE, is transmitting in spatial multiplexing mode. Because the low-rate payload in this scenario is fixed, spatial multiplexing mode of transmission, which targets high spectral efficiency, may not be that helpful for the UCI payload which prefers robustness. The other option is to generate twice the number of coded symbols and transmit half the symbols on each layer. This offers some spatial diversity since the fading on the layers will be at east partially uncorrelated. However the diversity gain of this scheme is limited by the interference between symbol streams, termed inter-layer interference, observed at the receiver. 
         [0005]    Therefore a need exists for a single-carrier FDMA transmission scheme that provides transmit spatial diversity across multiple transmission layers for one type of information symbol while still providing spatial multiplexing across the multiple transmission layers for a different type of information symbol. 
         [0006]    The various aspects, features and advantages of the invention will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description thereof with the accompanying drawings described below. The drawings may have been simplified for clarity and are not necessarily drawn to scale. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  illustrates a wireless communication system according to a possible embodiment; 
           [0008]      FIG. 2  illustrates space-time transform precoding according to a possible embodiment. 
           [0009]      FIG. 3  illustrates reordering and symbol modification according to a possible embodiment; 
           [0010]      FIG. 4  illustrates the mapping of digital modulation symbols to time and layers according to a possible embodiment 
           [0011]      FIG. 5  illustrates the mapping of digital modulation symbols to time and layers according to a second possible embodiment 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Additional features and advantages of the disclosure will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the disclosure. The features and advantages of the disclosure may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present disclosure will become more fully apparent from the following description and appended claims, or may be learned by the practice of the disclosure as set forth herein. 
         [0013]    Various embodiments of the disclosure are discussed in detail below. While specific implementations are discussed, it should be understood that this is done for illustration purposes only. A person skilled in the relevant art will recognize that other components and configurations may be used without parting from the spirit and scope of the disclosure. 
         [0014]    In the following description, the terms base-station, base unit, eNB, transmitter are used interchangeably to represent a point in a wireless that is transmitting data to another receiving device. The terms receiver, wireless device, UE, are used interchangeably to represent a device receiving data and communicating with a transmitting device. 
         [0015]    The present disclosure comprises a variety of embodiments, such as a method, an apparatus, and an electronic device, and other embodiments that relate to the basic concepts of the disclosure. The electronic device may be any manner of computer, mobile device, or wireless communication device. 
         [0016]      FIG. 1  block diagram of a system  100  for communicating data over multiple transmission layers with SC-FDMA in accordance with an exemplary embodiment of the present invention. The wireless terminal  146  can consist of a plurality of antennas  136  coupled to transceivers  130 . The wireless terminal  146  can also include a receiver  150  coupled to the transceivers  130  and  131  and also coupled to a controller  152 . The receiver  150  may receive control messages from the base unit  142  which are then passed to the controller  152 . The controller  152  can be configured to control operations of the wireless terminal  146 . The information source  108  generates data to be transmitted from the wireless terminal to the base unit. The information source  108  delivers vectors of information bits  114 . The controller  152  also outputs control information signal  102  to the space-time transform coder  118 . The control information signal can be information relating to which transmission layers will be used to carry coded information symbols as mapped by the space-time transform coder  116 . Control information  104  related to the characteristics of the transmission link between the base unit  142  and the wireless unit  146  may be a precoding matrix to be applied by the spatial precoder  124 . 
         [0017]    The spatial channel coder  112  operates on the information bits  114  to generate vectors of digital modulation symbols  116 . The channel coder  112  adds redundancy to the information bits of information bits vectors  114  in order to aid in the correction of errors which occur in the transmission link between wireless terminal  146  and base unit  148  to be corrected by the receiver in the base unit. The vectors of digital modulation symbols  116  may be fed to space-time transform coder  118  which generates blocks of complex-valued symbols  117  and  119  with each block corresponding to a transmission layer. 
         [0018]    The blocks of complex-valued symbols  117  and  119  are then fed to the spatial precoder  124  which can generate the inputs to the wireless terminal antennas  136  through the transceivers  130 . Spatial precoding  124  can be performed with a precoding matrix which is used to form multiple weighted-combinations of the transmitter outputs. The weighted combinations are then applied to multicarrier modulators  160  which modulates each input symbol to equally-spaced subcarriers. 
         [0019]    One skilled in the art can recognize that while the embodiment of the wireless terminal  146  described has two transmission layers of complex-valued symbols  117  and  119  and four transmit antennas on the wireless terminal  136 , other embodiments may have any number of transmit antennas and any number of layers as long as the minimum of the number of transmit antennas  136  and the number of receive antennas is greater than or equal to the number of transmission layers. 
         [0020]      FIG. 2  illustrates one embodiment of the space time transform coder  118 . According to this aspect of the disclosure, a vector of digital modulation symbols  218  is transformed by Discrete Fourier Transform (DFT)  208  to generate a set of complex-valued modulation symbols  226 . The DFT is defined by the equation 
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         [0021]    where s=           0  s 1  L s N−1            is the vector of digital modulation symbols and x=           0 x 1  L x N−1            is the vector of complex-value symbols and N is length of each of these vectors. The notation           indicates matrix transposition. The DFT modulates a set of transform codes, represented as column vectors of length-N with the symbols of its input vector s . The transform code corresponding to the n th symbol is the vector           e j2pn/N  L e j2pn(N−1)/N            . Note that if a second source of user data is to be transmitted, the vector s may contain both the low-rate control information and the user data multiplexed. The complex-modulation symbols  226  are mapped to a set of uplink resources consisting of a set of subcarriers, an SC-FDMA time period, and a spatial layer by the time/frequency/space resource mapper  210 . The time/frequency mapper  210  maps N input symbols x o ,x 1 ,L ,x N-1  onto a set of subcarriers, an SC-FDMA symbol interval, and the second transmission layer. Specifically, the complex-valued symbols x o ,x 1 ,L ,x N−1  are mapped to subcarriersS 0 ,S 1 ,L, S N−1 , SC-FDMA time periods            0 , t 0 +T sc-ofdm           , and spatial transmission layers L 1 . Here is the time duration of an SC-OFDM symbol. An example of subcarrier mapping is localized mapping which maps modulation symbol n to the n 0 +n subcarrier. A second example, based on distributed mapping, maps modulation symbol n to subcarrier n 0 +N Dist n where N Dist  is a positive integer greater than 1 and n 0  is a fixed subcarrier offset. 
         [0022]    The vector of digital modulation symbols  218  is also fed to a reordering block that permutes the order of the digital modulation symbols within the vector. The permuted vector  220  is then fed to a symbol modification stage  204  which operates on each of the digital modulation symbols to generate a second vector of digital modulation symbols  222 . The symbols modification stage is defined by the N functions f 0 ,f 1 L,f N−1  each of which map complex numbers to complex numbers such that the nth element of  222  has the value f n (s n ). An example of a symbol modification stage is one which outputs the complex conjugate of its input and is therefore defined by the functions f n (s)=s*, n=0,1,L,N−1. Another example of a symbol modification stage is one which rotates its n th input symbol by a phase of α n  and is therefore defined by f n (s)=e jα     n   s, n=0,1,L,N−1. The symbol modification stage can also not modify the symbols at all: f n (s)=s, n=0,1,L,N−1 
         [0023]    The second vector of complex-valued modulation symbols  222  is then transformed by DFT  206  whose operation is the same as described above for DFT  208 . The transformed set of complex-valued modulation symbols is then mapped to a second set of time/frequency resources  234  to a second transmission layer in a manner analogous to the mapping of the complex modulation symbols  226  described above. The SC-FDMA time interval            1 , t 1 +T sc-ofdm )to which the second vector of complex-valued modulation symbols are mapped can be the same or different than the SC-FDMA time interval,            0 , t 0 +T sc-ofdm           ), to which the first vector of complex-valued modulation symbols are mapped. 
         [0024]      FIG. 3  illustrates an embodiment of the symbol modification stage  204 . The reordered digital modulation symbols  302  are fed to complex conjugation blocks  306  which, along with the original modulation symbols  302 , are fed to selectors  312 . The selectors output either the modulation symbol or its complex conjugate depending on the selector inputs  314 ,  320 , and  310 . For example the selector may output the modulation symbols  302  if its selection input is ‘0’ and the complex conjugate of the modulation symbol if the its selection input is ‘1’. The selection inputs can be set at the time of manufacturer or can be programmable. They can also be determined from control messages sent from the base station. The selector outputs are rotated by phase factors  310 ,  316 , and  322  in the multipliers  304  to yield modified complex modulation symbols  340 . The phase factors applied to the set of selector outputs are complex numbers of the form e jα     n   , n=0,1,L,N−1 where 0£ α n &lt;2 p. 
         [0025]      FIG. 4  illustrates the mapping of digital modulation symbols  218  to complex-valued symbols, SC-FDMA symbol intervals, and spatial layers for one embodiment of the disclosure. In this embodiment the reordering performed in  202  consists of taking pairs of digital modulation symbols with indices k and l, k&lt;l respectively and reordering such that symbol s k  is reordered to index l and symbol s, is reordered to index k. In  FIG. 4  the pairs of symbols are consecutive so that symbols within indices 1,2,3,4,5,6 . . . are reordered to have order 2,1,4,3,6,5 . . . . The symbol modification is of the form in  FIG. 3  with selection of the complex conjugate for all symbols and phase rotations which alternate between 0 and 180 degrees. The digital modulation symbols  410  are mapped to the first spatial transmission layer  402  while the reordered digital modulation symbols  408  are mapped to the second transmission layer  404 . Both the original set of modulation symbols and its reordered and phase rotated version is transmitted in the same SC-FDMA symbol interval. 
         [0026]      FIG. 5  illustrates another embodiment of the disclosure where the same reordering and phase factors used in the embodiment of  FIG. 4  are used. In this embodiment however the original vector  218  of complex-valued modulation symbols are first split into two vectors            0  S 2  L S N-2            and            1  s 3  L S N−1            where N assumed to be even . Space transform coding  118  described above is applied to each of these vectors separately. The processing of            0  s 2  L S N−2            will be described first. The vector  506 ,            0  s 2  L S N−2            is transformed to give the vector            0  x 2  L x N−2            which is mapped in frequency by the identity mapping to a first SC-OFDM symbol interval  520 , [t 0 , t 0 +T). The term identity mapping refers to mapping x 0  to the first subcarrier, x 1  to a second subcarrier, and so on. The vector  506 ,            0  s 2  L S N−2            is reordered as described above for the embodiment of  FIG. 4 : indices 0,1,2, . . . N−1 are reordered to 1,0,3,2, . . . ,N−1,N−2. The symbol modification step used is that described in  FIG. 3  where the selector selects the complex conjugate for all symbols and no rotation in phase is performed. The results of the symbol modification step is the set of complex-valued modulation symbols  512  which are mapped to the second layer at a second SC-OFDM symbol interval  522 , [t 1 , t 1 +T). As with the first set of complex-valued modulation symbols, the frequency mapping is the identity mapping. 
         [0027]    A similar sequence of steps as described above is used to perform space transform coding on the vector,            1  s 3  L s N−1             
         [0028]    While the present disclosure and the best modes thereof have been described in a manner establishing possession and enabling those of ordinary skill to make and use the same, it will be understood and appreciated that there are equivalents to the exemplary embodiments disclosed herein and that modifications and variations may be made thereto without departing from the scope and spirit of the inventions, which are to be limited not by the exemplary embodiments but by the appended claims.