The MIMO sounding sequences may also be used by the receiving station to
determine the number of active spatial streams and data rates that may reliably be
supported. The rate recommendations can then be fed back to the transmitting station.
9.5 Receiver Structures
The theoretically optimal receiver for the transmission schemes described above is a
maximum likelihood (ML) sequence receiver that is capable of making joint decisions on
all the information bits using knowledge of the correlation introduced by the channel code
across blocks, data subcarriers and all the OFDM symbols in a packet. Such a receiver
would be prohibitively complex, as it would need to perform an exhaustive search over all
combinations of bits for the whole sequence of information bits transmitted in a packet. In
this section, we shall instead investigate some more practical receiver structures.
9.5.1 Near-Optimal Iterative Receiver
Near-optimal performance can be achieved with a receiver that performs iterative joint
detection and decoding, as shown in Figure 9.4. This receiver consists of a MIMO detector
and a channel decoder, both of which compute soft decisions on the coded bits.
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