Spectrum Sharing with a Primary ARQ System
Krishnan Eswaran, Kannan Ramchandran and Michael Gastpar
While the majority of spectrum currently gets allocated for specific systems with a specific use, one hope is that certain parts of the spectrum can be reused by different systems. The challenge is to reuse the spectrum in such a way that a primary system for which the spectrum was originally allocated continues to function. Given this restriction, a secondary system wants to achieve the maximum allowable throughput.
We study a case in which the primary system employs ARQ feedback in its design. In order for the primary to achieve a specified target rate, the secondary must transmit within an interference budget. Unfortunately, the interference budget is unknown to the secondary. We propose a scheme in which the secondary can stay within its interference budget through the primary's ARQ feedback without explicitly knowing it a priori. Such schemes satisfy a certain robustness property, and under said robustness assumption, there exists an information-theoretically optimal rate-interference budget (RIB) tradeoff. We compare how far fixed strategies are from this RIB function as we vary the interference budget. Further, we exhibit a strategy that is optimal beyond a threshold interference budget and within 1 bit/channel use away from the RIB function for all others.
Figure 1: RIB function for a specific channel