  We consider slow fading relay channels with a single multi-antenna
source-destination terminal pair. The source signal arrives at the destination
via N hops through N-1 layers of relays. We analyze the diversity of such
channels with fixed network size at high SNR. In the clustered case where the
relays within the same layer can have full cooperation, the cooperative
decode-and-forward (DF) scheme is shown to be optimal in terms of the
diversity-multiplexing tradeoff (DMT). The upper bound on the DMT, the cut-set
bound, is attained. In the non-clustered case, we show that the naive
amplify-and-forward (AF) scheme has the maximum multiplexing gain of the
channel but is suboptimal in diversity, as compared to the cut-set bound. To
improve the diversity, space-time relay processing is introduced through the
parallel partition of the multihop channel. The idea is to let the source
signal go through K different "AF paths" in the multihop channel. This parallel
AF scheme creates a parallel channel in the time domain and has the maximum
diversity if the partition is properly designed. Since this scheme does not
achieve the maximum multiplexing gain in general, we propose a flip-and-forward
(FF) scheme that is built from the parallel AF scheme. It is shown that the FF
scheme achieves both the maximum diversity and multiplexing gains in a
distributed multihop channel of arbitrary size. In order to realize the DMT
promised by the relaying strategies, approximately universal coding schemes are
also proposed.
