Protocols and Performance Limits for Half-Duplex Relay Networks

The density of radio access points in currently deployed cellular networks increases in order to improve the reuse of available resources and to serve more user terminals at high data rates. This implies higher deployment costs, which demands low-cost and flexible alternatives. However, this ambitious goal cannot be accomplished with conventional cellular systems but requires new technologies such as multi-cell cooperation, movable radio access points without backhaul connection, and multiple-antenna transmission. This paper concentrates on relaying as one possibility to improve data rates in next-generation mobile networks. More specifically, it introduces protocols and analyzes performance limits in the half-duplex relay channel where multiple relay nodes support a single communication pair. In this channel, nodes underly the 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 analyzed scenarios compress-and-forward based protocols provide for a wide range of parameters performance gains over decode-and-forward protocols; this is in contrast to full-duplex networks, where decode-and-forward dominates compress-and-forward in most scenarios. In this paper, a protocol with one compress-and-forward and one decode-and-forward based relay node is introduced. Just as the cut-set upper bound, which operates in a mode where relay nodes transmit alternately, both relays support each other. Furthermore, the performance gains through a random channel access in wireless networks are analyzed and it is shown that in practical systems this strategy provides only marginal performance gains if any.