6.10 SUMMARY

In this chapter we have shown alternative techniques to synthesize the transmission strategy, when the transmitter knows the channel. In the first part of the chapter, we have considered the single-user context and we have provided the general solution according to alternative criteria. We have then specialized the results to SISO, MISO, and, MIMO systems to provide some physical insight into the optimal solutions. Among other alternatives, the minimum BER approach has been shown to be a good candidate as it has limited losses in terms of information rate and it exhibits a good behavior in terms of BER in many different situations. We have initially assumed perfect channel knowledge at both the transmit and receive sides. Then we have tested the resilience of the optimal systems to imperfect channel knowledge. We have shown how to optimize the transmission strategy in the more practical case where the transmitter has only partial knowledge about the channel. An excellent recent reference on the capacity limits of MIMO channels, under different degrees of channel knowledge at the transmit and receive sides is [40].

In the second part, we have considered the multi-user case. We have analyzed, in particular, the multiple access and the broadcast channels. A very interesting approach to the optimal design of multiantenna multiple access and broadcasting systems is based on the duality between the multiple access and the broadcast channel established in [41, 42, 43]. Finally, we have considered the so called cocktail party problem, where a series...