Optimal Spectrum Sharing With ARQ-Based Legacy Users Via Chain Decoding

Abstract

This paper investigates the design of access policies in spectrum sharing networks by exploiting the retransmission protocol of legacy primary users (PUs) to improve the spectral efficiency via opportunistic retransmissions at secondary users (SUs) and chain decoding. The optimal policy maximizing the SU throughput under an interference constraint to the PU and its performance are found in closed form.It is shown that the optimal policy randomizes among three modes: Idle, the SU remains idle over the retransmission window of the PU, to avoid causing interference; Interference cancellation, the SU transmits only after decoding the PU packet, to improve its own throughput via interference cancellation;Always transmit, the SU transmits over the retransmission window of the PU to maximize the future potential of interference cancellation via chain decoding. This structure is exploited to design a stochastic optimization algorithm to facilitate learning and adaptation when the model parameters are unknown or vary over time, based on ARQ feedback from the PU and CSI measurements at the SU receiver.It is shown numerically that, for a 10% interference constraint, the optimal access policy yields 15% improvement over a state-of-the-art scheme without SU retransmissions, and up to 2× gain over a scheme using a non-adaptive access policy instead of the optimal one. July 2, 2018 DRAFT arXiv:1801.07877v2 [cs.IT] 29 Jun 2018 2 legacy users (primary users, PUs) and opportunistic users (secondary users, SUs) capable of autonomous reconfiguration by learning and adapting to the communication environment [4]. A central question is: how can opportunistic users leverage side information about nearby legacy users (e.g., activity, channel conditions, protocols employed, packets exchanged [5]) to opportunistically access the spectrum and improve their own performance, with minimal or no degradation to existing legacy users [6]? In this paper, we address this question in the context of the retransmission protocol employed by PUs. We consider a wireless network composed of a pair of PUs and a pair of SUs. The PU employs Type-I HARQ [7] to improve reliability, which results in replicas of the PU packet (re)transmitted over subsequent slots, henceforth referred to as ARQ window. With the scheme developed in [8], the SU receiver attempts to decode the PU packet independently in each slot, and replicas of the PU packet are not exploited; thus, in the example of Fig. 1, no SU packets can be decoded with the scheme [8]. However, the SU may leverage these replicas to improve its own throughput via interference cancellation. In [9], we have investigated a scheme, termed backward interference cancellation (BIC), where the SU receiver decodes the PU packet and removes its interference to achieve interferencefree transmissions over the entire ARQ window of that PU packet. In the example of Fig. 1, this scheme allows the SU receiver to decode packet S4, after removing the interference of P2, decoded in slot 5, thus outperforming [8].In [10], we have...