外文资料（一） On Orthogonal Space-Time Block Codes for MIMO-OFDM Systems Space-time code in mobile communication system, and orthogonal desing in multiple- antennas scneme are dicsussed. By the methods, data is encoded using a space- time block code and is split into several streams which are simultaneously transmitted by antennas. So a maximum- likelihood decoding algorithm can be used at the receiver to achieve the maximum diversity orderIntroduction Most work on wireless communications had focused on having an antenna array at only one end of the wireless link usually at the receiver. Seminal papers by Gerard J. Foschini and Michael J. Gans1, Foschini2 and Emre Telatar3 enlarged the scope of wireless communication possibilities by showing that for the highly-scattering environment substantial capacity gains are enabled when antenna arrays are used at both ends of a link. An alternative approach to utilizing multiple antennas relies on having multiple transmit antennas and only optionally multiple receive antennas. Proposed by Vahid Tarokh, Nambi Seshadri and Robert Calderbank, these spacetime codes (STCs) achieve significant error rate improvements over single-antenna systems. Their original scheme was based on trellis codes but the simpler block codes were utilized by Siavash Alamouti,and later Vahid Tarokh, Hamid Jafarkhani and Robert Calderbank to develop spacetime block-codes (STBCs) 4. STC involves the transmission of multiple redundant copies of data to compensate for fading and thermal noise in the hope that some of them may arrive at the receiver in a better state than others. In the case of STBC in particular, the data stream to be transmitted is encoded in blocks, which are distributed among spaced antennas and across time. While it is necessary to have multiple transmit antennas, it is not necessary to have multiple receive antennas, although to do so improves performance. This process of receiving diverse copies of the data is known as diversity reception and is what was largely studied until Foschinis 1998 paper. An STBC is usually represented by a matrix. Each row represents a time slot and each column represents one antennas transmissions over time. Here, sij is the modulated symbol to be transmitted in time slot i from antenna j. There are to be T time slots and nT transmit antennas as well as nR receive antennas. This block is usually considered to be of length TThe code rate of an STBC measures how many symbols per time slot it transmits on average over the course of one block. If a block encodes k symbols, the code-rate is Only one standard STBC can achieve full-rate (rate 1) Alamoutis codeOrthogonality STBCs as originally introduced, and as usually studied, are orthogonal. This means that the STBC is designed such that the vectors representing any pair of columns taken from the coding matrix is orthogonal. The result of this is simple, linear, optimal decoding at the receiver. Its most serious disadvantage is that all but one of the codes that satisfy this criterion must sacrifice some proportion of their data rate (see Alamoutis code).Moreover, there exist quasi-orthogonal STBCs that achieve higher data rates at the cost of inter-symbol interference (ISI). Thus, their error-rate performance is lower bounded by the one of orthogonal rate 1 STBCs, that provide ISI free transmissions due to orthogonality.Higher order STBCs Tarokhet al. discovered a set of STBCs that are particularly straightforward, and coined the schemes name. They also proved that no code for more than 2 transmit antennas could achieve full-rate. Their codes have since been improved upon (both by the original authors and by many others). Nevertheless, they serve as clear examples of why the rate cannot reach 1, and what other problems must be solved to produce good STBCs. They also demonstrated the simple, linear decoding scheme that goes with their codes under perfect channel state information assumption.4 transmit antennas Two straightforward codes for 4 transmit antennas are:and These codes achieve rate-1/2 and rate-3/4 respectively, as for their 3-antenna counterparts.C4,3 / 4 exhibits the same uneven power problems as C3,3 / 4. An improved version of C4,3 / 4 iswhich has equal power from all antennas in all time-slots.Decoding One particularly attractive feature of orthogonal STBCs is that maximum likelihood decoding can be achieved at the receiver with only linear processing. In order to consider a decoding method, a model of the wireless communications system is needed. At time t, the signal received at antenna j is:,where ij is the path gain from transmit antenna i to receive antenna j, is the signal transmitted by transmit antenna i and is a sample of additive white Gaussian noise (AWGN). The maximum-likelihood detection rule is to form the decision variables where k(i) is the sign of