Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

MIMO Overview 303From the above general model, two cases exist: (a) case M = 1, resulting in a singleinput/multiple output (SIMO) channel, and (b) case L = 1, resulting in a multipleinput/single output (MISO) channel. In the case of SIMO, conventional receiver diversitytechniques such as MRC can be realized, which can improve power efficiency, especiallyif the channels between the Tx and the Rx antennas are independently faded paths (e.g.Rayleigh distributed), where the multi-path diversity order is identical to the number ofreceiver antennas [21].With diversity techniques, a frequency- or time-selective channel tends to become anAWGN channel. This improves the power efficiency. However, there are two ways toincrease the spectral efficiency. The first one, which is the trivial way, is to increase thesymbol alphabet size and the second one is to transmit different symbols in parallel inspace by using the MIMO properties.The capacity of MIMO channels for an uncoded system in flat fading channels withperfect channel knowledge at the receiver is calculated by Foschini [12] as(C = log 2[det I L + E )]s/N oMh(t)h∗T (t) , (6.2)where ‘det’ means determinant, I L is an L × L identity matrix, and (·) ∗T is the conjugatecomplex of the transpose matrix. Note that this formula is based on the Shannon capacitycalculation for a simple AWGN channel.Two approaches exist to exploit the capacity in MIMO channels. The informationtheory shows that with M transmit antennas and L = M receive antennas, M independentdata streams can be simultaneously transmitted, hence reaching the channel capacity.As an example, the BLAST (Bell-Labs layered space time) architecture can be referredto [12, 28]. Another approach is to use an MISO scheme to obtain diversity, where inthis case sophisticated techniques such as space–time coding (STC) can be realized.All transmit signals occupy the same bandwidth, but they are constructed such that thereceiver can exploit spatial diversity, as in the Alamouti scheme [2]. The main advantageof STC, especially for mobile communications, is that they do not require multiple receiveantennas.6.2.1 BLAST ArchitectureThe basic concept of the BLAST architecture is to increase channel capacity by increasingthe data rate through simultaneous transmission of independent data streams over Mtransmit antennas. In this architecture, the number of receive antennas should at least beequal to the number of transmit antennas L ≥ M (see Figure 6-1).For m-array modulation, the receiver has to choose the most likely out of m M possiblesignals in each symbol time interval. Therefore, the receiver complexity grows exponentiallywith the number of modulation constellation points and the number of transmitantennas. Consequently, sub-optimum detection techniques such as those proposed inBLAST can be applied. Here, in each step only the signal transmitted from a singleantenna is detected, whereas the transmitted signals from the other antennas are canceledusing the previously detected signals or suppressed by means of zero-forcing or MMSEequalization.