At the receiver, an array of antennas is again used to pick up the multiple transmitted sub streams and their scattered images. Each receiver antenna sees the entire transmitted sub streams super imposed, not separately. However, if the multipath scattering is sufficient is sufficient, then the multiple sub streams are located at different points in space .Using sophisticated signal processing, these slight difference in scattering allow the sub streams to be identified and recovered. In effect the unavoidable multipath is exploited to provide a useful spatial parallelism that is used to greatly improve data transmission rates. Thus when using the BLAST technique, the more multipath, the better, just the opposite of the conventional systems.
The blast signal processing algorithms used at the receiver are the heart of the technique. At the bank of receiving antennas, high speed signal processors look at the signals from all the receiver antennas simultaneously, first extracting the strongest signal have been removed as a source of interference. Again the ability to separate the sub streams depends on the slight differences in the way the different sub streams propagate through the environment. Let us assume a signal transmitted vector symbol with symbol-synchronous receiver sampling and ideal timing. If a= (a1, a2, a3,…. am) T is the vector transmitted symbols, then the receiver N vector is r1=Ha+v, where H is the matrix channel transfer function and V is a noise vector. Signal detection can be done using adaptive, antenna array techniques, sometimes called linear combinational nulling. Each sub stream is sequentially understood as the desired signal. This implies that the other sub stream will be understood as interference. One nulls out this interference by weighting the interfering signals they go to zero (known as zero forcing). While these linear nullings work, on linear approaches can be used in conjunction with them for overall result. Symbol cancellation is one such technique. Using interference from already detected components of interfering signals are subtracted to form the received signal vector. The end result is a modified receiver vector with few interferes present in the matrix. Bell labs actually tried both approaches. The result showed that adding the nonlinear to the linear yielded the best performance and dealing with the strongest channel, first (thus removing it as and interference) give the best overall SNR. If all components of ‘a’ are assumed to be the part of the same constellation, it would be expected that the component with the smallest SNR would dominate the overall error performance. The strongest channel then becomes the place to start symbol cancellation. This technique has been called the “best-first” approach and has become the de-facto way to do signal detection from an RF stream. But what the Bell labs guys found is that if you evaluate the SNR function at each stage of the detection process, rather than just at the beginning, you come up with a different ordering that is also (minmax) optimal.
As its core V-BLAST is an iterative cancellation method that depends on computing a matrix inverse to solve the zero forcing function. The algorithm works by detecting the strongest data stream from the received signal and repeating the process for the remaining data streams. While the algorithm complexity is linear with the number of transmitting antennas, it suffers performance degradation through the cancellation process. If cancellation is not perfect, it can inject more noise in to the system and degrade detection.
The essential difference between D-BLAST and V-BLAST lies in the vector encoding process. In D-BLAST, redundancy between the sub streams is introduced through the use of specialized inter-sub stream block coding. In D-BLAST code blocks are organized along diagonals in space-time. It is this coding that leads to D-BLAST’s higher spectral efficiencies for a given number of transmitters and receivers. In V-BLAST, however, the vector encoding process is simply a demultiplex operation followed by independent bit-to-symbol mapping of each sub stream. No inter-sub stream coding, or coding of any kind, is required, though conventional coding of the individual sub streams may certainly be applied
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