In this paper, we examine message prioritization in terms of both rate and reliability in the two-way relay channel using decode-forward relaying. Each source sends both a low priority message and a high priority message. We design a scheme that routes the high priority messages through the relay and direct link but the low priority message is only decoded by the destination. For fixed message priorities, the rate-optimal power allocation is analytically determined based on the link state. The high priority message is guaranteed to have greater reliability than that of the low priority message. We analytically formulate this outage probability from both the perspective of a single user and that of the overall system, and numerically examine the increased reliability for the high priority message.

In this paper, we re-examine the relay channel under the decode-forward (DF) strategy. Contrary to the established belief that block Markov coding is always the rate-optimal DF strategy, under certain channel conditions (a link regime), independent signaling between the source and relay achieves the same transmission rate without requiring coherent channel phase information. Furthermore, this independent signaling regime allows the relay to conserve power. As such, we design a composite DF relaying strategy that achieves the same rate as block Markov DF but with less required relay power. The finding is attractive from the link adaptation perspective to adapt relay coding and relay power according to the link state. We examine this link adaptation in fading under both perfect channel state information (CSI) and practical CSI in which nodes have perfect receive and long-term transmit CSI, and derive the corresponding relay power savings in both cases. We also derive the outage probability of the composite relaying scheme, which adapts the signaling to the link regime. Through simulation, we expose a tradeoff for relay placement showing that the relay conserves the most power when closer to the destination but achieves the most rate gain when closer to the source.

We establish a novel link-state regime result for a composite decode-forward (DF) two-way relaying scheme with a direct link. During transmission, our scheme combines block Markov coding and an independent coding scheme that resembles network coding at the relay. A developed novel approach optimizes the composite technique by analyzing the dual variable space to identify link-state regimes in which a particular combination of transmission techniques is optimal. Our results expose an interesting trend: when the user-to-relay link is marginally stronger than the direct link, independent coding is optimal and the relay can conserve power. However, for larger user-to-relay link gains, the relay must use full power and supplement independent coding with block Markov coding to achieve the largest rate region. For Rayleigh fading links, we demonstrate that relay power savings are achievable in most node configurations. The link-state regimes are further applied to perform link adaptation in fading to illustrate significant data rate gains over direct transmission even under a more practical, long-term link state information. These link-state regime results are useful for the application of two-way DF relaying in practice.

In this paper, we consider a composite independent and coherent decode-forward (DF) relaying scheme and analytically evaluate its outage performance over Rayleigh fading channels. We examine the link-state regimes in which the optimal strategy is either direct transmission, independent DF (IDF), or coherent DF (CDF), depending on the relation among the links. Outage probabilities at both the relay and destination are determined in order to analytically derive the overall outage performance. With full channel state information (CSI), the relay can save power without affecting the overall outage performance. Specifically, by using only a portion of its maximum power for transmission, the relay conserves power in the IDF regime while still achieving the maximum transmission rate. Even with just statistical CSI, the relay can employ these link regime results to reduce transmit power significantly while maintaining the same outage performance as that of using full power.

We present a link-state based composite decode-forward (DF) scheme for the relay channel comprised of independent coding and coherent block Markov coding. Although it is believed that block Markov coding is the optimal DF technique whenever the source- to-relay link is stronger than the direct link, we show that under some link-state conditions, independent coding achieves the same rate but also results in power savings at the relay. We then apply this composite scheme in fading under a practical channel state information (CSI) assumption in which nodes have perfect receive CSI and long-term transmit CSI. Through simulation, we demonstrate that the throughput with practical CSI achieves a rate comparable to that of block Markov coding, but with less required relay power. Further, we expose a novel trade-off between consumed relay power and rate gain for relay placement. Specifically, the relay conserves the most power when closer to the destination but achieves the most rate gain when closer to the source.

In this paper, we analyze a full-duplex composite decode-and-forward scheme for the two-way relay channel in a Rayleigh fading environment. Designed from a link-state perspective, this scheme combines both block Markov and independent coding, and yields relay power savings in some link-state regimes. For a given distance configuration of nodes, we analytically determine the probability of each link-state regime in Rayleigh fading, which always spans the relay power saving regimes. As such, relay power savings are realizable for almost any distance configuration. Further, we evaluate the throughput when only long-term channel state information (CSI) is available instead of perfect CSI, which still shows substantial gain over not using the relay. Numerical results illustrate significant relay power savings for most distance configurations and highlight an inherent tradeoff between achievable rate and power savings at the relay.