Bits through ARQs
Krish Eswaran, Michael Gastpar and Kannan Ramchandran
While the Federal Communications Commission (FCC) currently allocates the majority of spectrum for specific systems with a specific use, much of this spectrum is underutilized. This raises the possibility that allocated bands can be reused by secondary systems. The challenge is to reuse these bands in such a way that a primary system for which the band was originally allocated continues to function. Given this restriction, a secondary system wants to maximize its transmission rate.
Primary systems have already been designed to satisfy certain specifications ignorant of the presence of a secondary system. Thus, in order for the primary to operate successfully, the interference at the primary's receiver should satisfy certain constraints. For this to happen, the secondary must transmit within an interference budget. Unfortunately, the interference budget is unknown to the secondary a priori, and it must sense the amount of interference it is generating at the primary's receiver.
Given these constraints, we define the rate-interference budget function (RIB) as the information-theoretically optimal tradeoff between the secondary's rate and interference budget. If the primary is a packet system and has a rate target known to the secondary, we present a strategy for the secondary if it can sense the primary's ACK/NAK feedback. This strategy is motivated by congestion control algorithms and allows the primary to maintain its rate target and is robust to fluctuations in the interference budget. Furthermore, if the interference budget is fixed, the strategy allows the secondary to approach rate points on the RIB function.
- K. Eswaran, M. Gastpar, and K. Ramchandran, "Bits through ARQs: Spectrum Sharing with a Primary Packet System," ISIT, Nice, France, 2007.