Downlink multiuser network coding (NC) automatic repeat request (ARQ) systems can provide significant throughput improvements over basic ARQ systems. In this paper, we consider the case of two-unicast flows and analyze their performance in terms of the effective capacity (EC) under retransmission schemes. We extend the derived EC of NC-ARQ for asymmetric channels where users have different data rates. For NC-ARQ, two scenarios at the source are considered to handle the transmission. The EC expression for two-unicast flows using Alamouti space time block coding (STBC) is also derived. We also present a new scheme that combines both NC and STBC and, denoted by NC-STBC-ARQ. Our results show that the new NC-STBC-ARQ improves the EC over basic STBC in the low-to-moderate signal-to-noise ratio (SNR) regime. Besides, the new NC-STBC-ARQ improves the EC compared to basic NC-ARQ for a moderate-to-high SNR regime.

下行多用户网络编码（NC）自动重复请求（ARQ）系统可以提供比基本ARQ系统显着提高的吞吐量。在本文中，我们考虑了双单播流的情况，并根据重传方案下的有效容量（EC）分析了它们的性能。我们将导出的NC-ARQ EC扩展到用户具有不同数据速率的非对称信道。对于NC-ARQ，在源头考虑了两种情况来处理传输。还推导了使用Alamouti时空块编码（STBC）的两个单播流的EC表达式。我们还提出了一种将NC和STBC结合在一起的新方案，并以NC-STBC-ARQ表示。我们的结果表明，新的NC-STBC-ARQ在低到中等的信噪比（SNR）方面比基本STBC改善了EC。除了，