  In this paper, we consider a simple network consisting of a source, a
half-duplex decode-and-forward relay, and a destination. We propose a new
relaying protocol employing adaptive link selection, i.e., in any given time
slot, based on the channel state information of the source-relay and the
relay-destination link a decision is made whether the source or the relay
transmits. In order to avoid data loss at the relay, adaptive link selection
requires the relay to be equipped with a buffer such that data can be queued
until the relay-destination link is selected for transmission. We study both
delay constrained and delay unconstrained transmission. For the delay
unconstrained case, we characterize the optimal link selection policy, derive
the corresponding throughput, and develop an optimal power allocation scheme.
For the delay constrained case, we propose to starve the buffer of the relay by
choosing the decision threshold of the link selection policy smaller than the
optimal one and derive a corresponding upper bound on the average delay.
Furthermore, we propose a modified link selection protocol which avoids buffer
overflow by limiting the queue size. Our analytical and numerical results show
that buffer-aided relaying with adaptive link selection achieves significant
throughput gains compared to conventional relaying protocols with and without
buffers where the relay employs a fixed schedule for reception and
transmission.
