  In this paper, protocols for the half-duplex relay channel are introduced and
performance limits are analyzed. Relay nodes underly an orthogonality
constraint, which prohibits simultaneous receiving and transmitting on the same
time-frequency resource. Based upon this practical consideration, different
protocols are discussed and evaluated using a Gaussian system model. For the
considered scenarios compress-and-forward based protocols dominate for a wide
range of parameters decode-and-forward protocols. In this paper, a protocol
with one compress-and-forward and one decode-and-forward based relay is
introduced. Just as the cut-set bound, which operates in a mode where relays
transmit alternately, both relays support each other. Furthermore, it is shown
that in practical systems a random channel access provides only marginal
performance gains if any.
