Abstract
This paper introduces a novel technique for access by a cognitive Secondary User (SU) using best-effort transmission to a spectrum with an incumbent Primary User (PU), which uses Type-I Hybrid ARQ. The technique leverages the primary ARQ protocol to perform Interference Cancellation (IC) at the SU receiver (SUrx). Two IC mechanisms that work in concert are introduced: Forward IC, where SUrx, after decoding the PU message, cancels its interference in the (possible) following PU retransmissions of the same message, to improve the SU throughput; Backward IC, where SUrx performs IC on previous SU transmissions, whose decoding failed due to severe PU interference. Secondary access policies are designed that determine the secondary access probability in each state of the network so as to maximize the average long-term SU throughput by opportunistically leveraging IC, while causing bounded average long-term PU throughput degradation and SU power expenditure. It is proved that the optimal policy prescribes that the SU prioritizes its access in the states where SUrx knows the PU message, thus enabling IC. An algorithm is provided to optimally allocate additional secondary access opportunities in the states where the PU message is unknown. Numerical results are shown to assess the throughput gain provided by the proposed techniques.
Highlights
Cognitive Radios (CRs) [3] offer a novel paradigm for improving the efficiency of spectrum usage in wireless networks
The former maximizes the instantaneous throughput under interference from the Primary User (PU), neglecting the buffering capability at SUrx; the choice RsU = Rs∗U reflects a pessimistic expectation of the ability of SUrx to decode the PU message and to enable Backward IC (BIC)
From (9) we have RsU = RsK = arg maxRs TsU(Rs, Rp) + ps,buf (Rs, Rp)RsK, RsU = RsK maximizes the sum of the instantaneous throughput and the future throughput possibly recovered via BIC, reflecting an optimistic expectation of the ability of SUrx to decode the PU message, which enables BIC
Summary
Cognitive Radios (CRs) [3] offer a novel paradigm for improving the efficiency of spectrum usage in wireless networks. The PU (underlay cognitive radio paradigm [8]) Within this framework, we propose to exploit the intrinsic redundancy, in the form of copies of PU packets, introduced by the Type-I Hybrid Automatic Retransmission reQuest (Type-I HARQ [10]) protocol implemented by the PU by enabling Interference Cancellation (IC) at the SU receiver (SUrx). BIC relies on buffering of the received signals Based on these IC schemes, we model the state evolution of the PU-SU network as a Markov Decision Process [11], [12], induced by the specific access policy used by the SU, which determines its access probability in each state of the network. In [19], the data transmitted by the PU is obtained causally at the SU receiver This model requires a joint design of the PU and SU signaling and channel state information at the transmitters. The proofs of the lemmas and theorems are provided in the appendix
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