Diversity-Multiplexing Tradeoff in the Multiaccess Relay Channel with   Finite Block Length
  The Dynamic Decode-and-Forward (DDF) protocol and the Hybrid DDF and Amplified-and-Forward (HDAF) protocol for the multiple-access relay channel (MARC) with quasi static fading are evaluated using the Zheng-Tse diversity-multiplexing tradeoff (DMT). We assume that there are two users, one half-duplex relay, and a common destination, each equipped with single antenna. For the Rayleigh fading, the DDF protocol is well known and has been analyzed in terms of the DMT with infinite block length. By carefully dealing with properties specific to finite block length, we characterize the finite block length DMT which takes into account the fact that the event of decoding error at the relay causes the degradation in error performance when the block length is finite. Furthermore, we consider the situation where the destination does not have a priori knowledge of the relay decision time at which the relay switches from listening to transmitting. By introducing a decision rejection criterion such that the relay forwards message only when its decision is reliable, and the generalized likelihood ratio test (GLRT) at the destination that jointly decodes the relay decision time and the information message, our analysis show that the optimal DMT is achievable as if there is no decoding error at the relay and the relay decision time is known at the destination. Therefore, infinite block length and additional overhead for communicating the decision time are not needed for the DDF to achieve the optimal DMT. To further improve the DMT, we propose the HDAF protocol which take advantages of both the DDF and the Amplified-and-Forward protocols by judiciously choosing which protocol to use. Our result shows that the HDAF protocol outperforms the original DDF in the DMT perspective. Finally, a variant of the HDAF protocol with lower implementation complexity without sacrificing the DMT performance is devised. 
Chung-Pi Lee and Hsuan-Jung Su
cs.IT math.IT