  Two-way relaying promises considerable improvements on spectral efficiency in
wireless relay networks. While most existing works focus on physical layer
approaches to exploit its capacity gain, the benefits of two-way relaying on
upper layers are much less investigated. In this paper, we study the
cross-layer design and optimization for delay quality-of-service (QoS)
provisioning in two-way relay systems. Our goal is to find the optimal
transmission policy to maximize the weighted sum throughput of the two users in
the physical layer while guaranteeing the individual statistical delay-QoS
requirement for each user in the datalink layer. This statistical delay-QoS
requirement is characterized by the QoS exponent. By integrating the concept of
effective capacity, the cross-layer optimization problem is equivalent to a
weighted sum effective capacity maximization problem. We derive the jointly
optimal power and rate adaptation policies for both three-phase and two-phase
two-way relay protocols. Numerical results show that the proposed adaptive
transmission policies can efficiently provide QoS guarantees and improve the
performance. In addition, the throughput gain obtained by the considered
three-phase and two-phase protocols over direct transmission is significant
when the delay-QoS requirements are loose, but the gain diminishes at tight
delay requirements. It is also found that, in the two-phase protocol, the relay
node should be placed closer to the source with more stringent delay
requirement.
