Opportunistic spectrum allocation for max-min rate in NC-OFDMA

We envision a scenario for opportunistic spectrum access among multiple point-to-point links when the available spectrum is not contiguous due to the presence of external interference sources. Non-contiguous Orthogonal Frequency Division Multiplexing (NC-OFDM) is a promising technique to utilize such disjoint frequency bands in an efficient manner. In this paper we study the problem of fair spectrum allocation across multiple NC-OFDM-enabled, point-to-point cognitive radio links under certain practical considerations that arise from such noncontiguous access. When using NC-OFDM, the channels allocated to a cognitive radio link are spread across several disjoint frequency bands leading to a large spectrum span for that link. Increased spectrum span requires higher sampling rates, leading to increased power consumption in the ADC/DAC of the transmit/receive nodes. In this context, this paper proposes a framework for spectrum allocation that maximizes the minimum rate achieved by the cognitive radio links, under a constraint on the maximum permissible spectrum span. For constant transmit powers and orthogonal spectrum allocation, such an optimization is an integer linear program and can be solved efficiently. There exists a clear trade-off between the max-min rate achieved and the maximum permissible spectrum span. The spectrum allocation obtained from the proposed optimization framework is shown to be close to the trade-off boundary, thus showing the effectiveness of the proposed technique. We find that it is possible to limit the spectrum span without incurring a significant penalty on the max-min rate under different interference environments. We also discuss an experimental evaluation of the techniques developed here using the ORBIT radio-network testbed that consists of multiple Universal Software Radio Peripherals (USRPs).