  We show that the performance of relay selection can be improved by employing
relays with buffers. Under the idealized assumption that no buffer is full or
empty, the best source-relay and the best relay-destination channels can be
simultaneously exploited by selecting the corresponding relays for reception
and transmission, respectively. The resulting relay selection scheme is
referred to as max-max relay selection (MMRS). Since for finite buffer sizes,
empty and full buffers are practically unavoidable if MMRS is employed, we
propose a hybrid relay selection (HRS) scheme, which is a combination of
conventional best relay selection (BRS) and MMRS. We analyze the outage
probabilities of MMRS and HRS and show that both schemes achieve the same
diversity gain as conventional BRS and a superior coding gain. Furthermore, our
results show that for moderate buffer sizes (e.g. 30 packets) HRS closely
approaches the performance of idealized MMRS and the performance gain compared
to BRS approaches 3 dB as the number of relays increases.
