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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. [ABSTRACT FROM AUTHOR]