Abstract:
The invention relates to an apparatus of transmitter and receiver for Multiple Input/Multiple Output MultiCarrier-Code Division Multiple Access (MIMO MC-CDMA) systems. At the transmitter, modified orthogonal transmit diversity (MOTD) encoders are used for increasing space and time transmission diversity. A P-way combiner is used to connect the MOTD encoders and P multi-carrier modulators. Each modulator is connected to a set of antennas. At the receiver, each multi-carrier demodulator has an amplitude/phase compensator to compensate distortion at every sub-carrier. Similarly, a combiner is used to connect the demodulators and the MOTD decoders. Upon the invention, the space, time, and frequency diversity can be explored.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to the technical field of wireless communications and, more particularly, to a transmitter and receiver apparatus for Multiple Input/Multiple Output MultiCarrier-Code Division Multiple Access (MIMO MC-CDMA) systems. 
   2. Description of Related Art 
   A new generation cellular mobile communication system has to provide wireless network applications with high-speed information transmission. Currently, it is known that MultiCarrier-Code Division Multiple Access (MC-CDMA) technique is one of the best solutions. CDMA is a core technique for the Third Generation Mobile Communication System, which applies Gold codes and OVSF (Orthogonal Variable Spreading Factor) codes to the system for allowing multiple users to concurrently transmit data on a same band. 
   Multi-carrier modulation is the principle of transmitting data by dividing a high-rate data stream into several parallel low-rate data streams onto individual carriers or subcarriers. By transmitting several symbols in parallel, the symbol duration is increased proportionately, which reduces the effects of ISI (Inter-Symbol Interference) caused by the dispersive Rayleigh-fading environment. By transmitting symbols onto subcarriers, frequency diversity is gained and thereby mitigating the effects of narrow band interference and frequency selective fading. Due to the advance of digital signal processing (DSP) and very large-scale integrated-circuit (VLSI), multi-carrier modulation is widely used in high-rate digital communications, such as digital broadcast, digital television and wireless local area network (WLAN). It is important in the present and future wireless multimedia communications. 
   In communication theory, multiple input/multiple output (MIMO) refers to radio links with multiple antennas at the transmitter and the receiver side. Given multiple antennas, the spatial dimension can be exploited to improve the performance of the wireless link. 
   In a typical MIMO MC-CDMA system, one multi-carrier modulator is connected to one transmitting antenna. That is, the transmitter has the same amount of multi-carrier modulators and transmitting antennas. The Inverse fast Fourier transform (IFFT) unit, which often occupies a large area in an Integrated Circuit (IC), is one of the most important, complicated and expensive units in a multi-carrier modulator. It means that multiple expensive IFFT units are necessary for a typical MIMO MC-CDMA system. 
   SUMMARY OF THE INVENTION 
   The object of the present invention is to provide an apparatus of transmitter and receiver for multiple input/multiple output multicarrier-code division multiple access (MIMO MC-CDMA) systems, thereby effectively improving the system performance. 
   According to the present invention, an apparatus of transmitter for MIMO MC-CDMA systems is provided. The apparatus of transmitter includes a modified orthogonal transmit diversity (MOTD) encoder, a combiner, at least one code spreader, at least one multi-carrier modulator and at least one set of transmitting antennas. The information used for communication is divided into at least one data stream, which is then fed into the MOTD encoder. The encoder consists of a symbol-mapping unit, a serial to parallel (S/P) converter, a splitter, and at least one code spreader in which orthogonal codes are generally applied for. Thereby the at least one spread symbol stream is obtained and connected to the combiner. At least one combined symbol stream is output from the combiner. Each of it is spread by a code spreader in which PN codes, Gold codes, or orthogonal codes are generally applied for. The at least one code spreader works as similar as the aforementioned code spreader in the MOTD encoder. The code spreader in the MOTD encoder is named as the first code spreader and the code spreader here is named as the second code spreader. Each output of the at least one second code spreader is connected to one multi-carrier modulator. In general, the modulator consists of an S/P converter, an IFFT unit, a parallel to serial (P/S) converter, and a cyclic prefix (CP) inserter. At least one set of transmitting antennas is used for radiating the RF (Radio Frequency) signal stream, which is up converted from the output of the modulator. Each radiated signal stream by an antenna in one set is delayed by small time duration. 
   According to the present invention, an apparatus of receiver for MIMO MC-CDMA systems is provided. The apparatus includes at least one receiving antenna, at least one multi-carrier demodulator, at least one code de-spreader, a combiner, and a MOTD decoder. Each received signal stream, which is down converted from the signal received by a receiving antenna, is connected to a multi-carrier demodulator. In general, the demodulator consists of a CP remover, an S/P converter, an IFFT unit, a phase/amplitude compensator, and a P/S converter. The at least one demodulated symbol stream is combined by a combiner. The output of the combiner is connected to a code de-spreader and the output of the de-spreader is connected to a MOTD decoder. The decoder consists of at least one code de-spreader in which orthogonal codes are generally applied for, a summer, a P/S converter and a symbol-demapping unit. Finally, the information is collected from the at least one data stream which is decoded from the decoder. 
   Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic block diagram of an apparatus of transmitter for MIMO MC-CDMA systems according to an embodiment of the invention; 
       FIG. 2  is a schematic block diagram of the interior of an K-input/K*M-output MOTD encoder according to an embodiment of the invention; 
       FIG. 3  is a schematic block diagram of the interior of a single-input/M-output MOTD encoder according to an embodiment of the invention; 
       FIG. 4  is a schematic diagram of the interior of a splitter according to an embodiment of the invention; 
       FIG. 5  is a schematic diagram of a K*M-input/P-output combiner according to an embodiment of the invention; 
       FIG. 6  a schematic block diagram of an apparatus of receiver for MIMO MC-CDMA system according to an embodiment of the invention; 
       FIG. 7  is a schematic diagram of a Q-input/single-output combiner according to an embodiment of the invention; 
       FIG. 8  is a schematic block diagram of a single-input/K-output MOTD decoder according to an embodiment of the invention; 
       FIG. 9  is a schematic block diagram of the interior of a single-input/single-output MOTD decoder according to an embodiment of the invention; and 
       FIG. 10  is a schematic diagram of the interior of a summer according to an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  is a schematic block diagram of an apparatus of transmitter for MIMO MC-CDMA systems according to an embodiment of the invention. In  FIG. 1 , the apparatus of transmitter includes a K-input/K*M-output modified orthogonal transmit diversity (MOTD) encoder (labeled by K-MOTD-K*M encoder, which means the encoder is with K inputs and K*M outputs)  11 , a K*M-input/P-output combiner  12 , P code spreaders  13 , P multi-carrier modulators  14 , P amplitude adjusters  15 , a plurality of time delays  16  and a plurality of antenna sets  17 , where K≧1, M≧1, and P≧1. Each multi-carrier modulator  14  consists of a serial/parallel (S/P) converter  141 , an Inverse Fast Fourier Transform (IFFT)  142 , a parallel/serial (P/S) converter  143  and a cyclic prefix (CP) unit  144 . 
     FIG. 2  is a schematic block diagram of the interior of a K-MOTD-K*M encoder  11  according to an embodiment of the invention. In  FIG. 2 , the encoder  11  consists of K single-input/M-output MOTD encoders (MOTD-M encoder). As shown in  FIG. 2 , D 1 (t), . . . ,D k (t) can be regarded as data streams for different users or for different applications. Each data stream (such as D 1 (t)) passes through a MOTD-M encoder  211  to increase its transmit diversity and thus M orthogonal symbol streams (such as S 1,1 (t), . . . S 1,M (t)) are obtained in parallel. 
     FIG. 3  is a schematic block diagram of the MOTD-M encoder according to an embodiment of the invention. As shown in  FIG. 3 , the MOTD-M encoder consists of a symbol-mapping unit  31 , an S/P converter  32 , an M×M splitter  33  and M first code spreaders  34  in which (quasi-) orthogonal codes are generally applied for. In this embodiment, the constellation used in the symbol-mapping unit  31  can be any constellation, for example, MPSK or MQAM. The S/P converter  32  converts an M serial input symbols into M parallel output symbols. The M×M splitter  33  makes M copies of each of the M output symbols and then dispatches them in a random or a specific order (as described hereinafter). The output of the M first code spreader  34  are denoted by S 1 (t), . . . S M (t), where O 1 (t), . . . , O M (t) are generally (quasi-) orthogonal. 
     FIG. 4  is a schematic diagram of the interior of an M×M splitter. As shown in  FIG. 4 , the input symbols are denoted by A(1), . . . , A(M). M copies of them will be dispatched in M time slots with a random or a specific order.  FIG. 4  shows an example of a specific order. At T=0, the order from top to bottom is A(1), . . . , A(M). At T=D, where D is a unit of time delay, the order from top to bottom is A(M), A(1), . . . , A(M−1). Accordingly, at T=(M−1)*D, the order from top to bottom is A(2), . . . , A(M), A(1). 
     FIG. 5  illustrates the K*M-input/P-output combiner  12  with P=1 (i.e., a single output). The output is obtained by simply summing the K*M input symbols. With P&gt;1 (i.e., multiple outputs), the K*M input symbols are mathematically operated and then P output symbols are generated. For example, the K*M input symbols are first divided into P groups. In each group, all the symbols then sum to its output. Note that in  FIG. 1 , the P second code spreaders (C 1 (t), . . . , C p (t))  13  can be the same or not. The P amplitude adjusters  15  are used for adjusting the transmission power of the antennas.  FIG. 1  illustrates a case of equal transmission power to every antenna  17 . The time delays  16 , which values are all smaller than the CP duration, lead to an frequency selective effect in transmission. An interleaver may be inserted in front of the IFFT  142 , thereby fairly treating all the symbol streams in the frequency domain. 
     FIG. 6  is a schematic block diagram of an apparatus of receiver for MIMO MC-CDMA systems according to an embodiment of the invention. In  FIG. 6 , the apparatus of receiver includes Q antennas  61 , Q multi-carrier demodulators  62 , a Q-input/single-output combiner  63 , a code de-spreader  64 , a single-input/K-output MOTD decoder (MOTD-K decoder)  65 , where Q≧1. Each multi-carrier demodulator  62  consists of a cyclic prefix (CP) remover  621 , an S/P converter  622 , a Fast Fourier Transform (FFT)  623 , a phase/amplitude compensator  624  and a P/S converter  625 . 
     FIG. 7  is a schematic diagram of an embodiment of a Q-input/single-output combiner  63 . As shown in  FIG. 7 , the combiner  63  sums the Q input symbols with equal gain to output, can be thereby named as an Equal Gain Combiner (EGC). However, other combining mechanisms can also be used. For example, the Q input symbols are squared before being summed to output. In the case, the combiner can thereby named as a Maximum Ratio Combiner (MRC). 
   The second code de-spreader (C(t))  64  corresponding to one of the P second code spreader  13  in  FIG. 1 , for example, C(t)=C p (t). 
     FIG. 8  is a schematic block diagram of a MOTD-K decoder, which consists of K single-input/single-output MOTD decoders  81 . As shown in  FIG. 8 , D 1 (t), . . . , D k (t) can be regarded as different data streams for different users or for different applications. Each stream (such as D 1 (t)) is obtained by passing through a single-input/single-output MOTD decoder (shown in the following). 
     FIG. 9  is a schematic block diagram of the interior of a single-input/single-output MOTD decoder. As shown in  FIG. 9 , the decoder consists of M first code de-spreader  91  in which (quasi-) orthogonal codes are generally applied for, an M×M summer  92 , a P/S converter  93 , and a symbol-demapping unit  94 . In this embodiment, all the decoding operations are corresponding to the encoding operations in  FIG. 3 . 
     FIG. 10  is a schematic diagram of the interior of an M×M summer. Likewise, corresponding to the operations of the M×M splitter in  FIG. 4 , at T=0, the order from top to bottom is A(1), . . . , A(M). At T=D, where D is a unit of time delay, the order from top to bottom is A(M), A(1), . . . , A(M−1). Accordingly, at T=(M−1)*D, the order from top to bottom is A(2), . . . , A(M), A(1). If an interleaver is applied in a multi-carrier modulator, a de-interleaver (not shown) must be inserted after the FFT  623 . 
   The invention can improve the system performance, which has proved by the simulation in a Rayleigh multi-path attenuation channel with Doppler effect. 
   As cited, the invention adopts the structure of orthogonal transmit branching (for example, multi- (single-) input/multi- (single-) output modified orthogonal transmit diversity (MOTD) decoders (encoders)). The invention also adopts the transceiver structure with multiple input/multiple output (MIMO), thereby effectively improving the system performance. 
   Although the present invention has been explained in relation to its preferred embodiment, it is to be understood that many other possible modifications and variations can be made without departing from the spirit and scope of the invention as hereinafter claimed.