  We envision a scenario of opportunistic spectrum access among multiple 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 NCOFDM-enabled point-to-point cognitive radio links
under certain practical considerations that arise from such non-contiguous
access. When using NC-OFDMA, the channels allocated to a cognitive link are
spread across several disjoint frequency bands leading to a large spectral span
for that link. Increased spectral 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 spectrum allocation that maximizes the
minimum rate achieved by the cognitive radio links, under a constraint on the
maximum permissible spectral span. Under constant transmit powers and
orthogonal spectrum allocation, such an optimization is a mixed-integer linear
program and can be solved efficiently. There exists a clear trade-off between
the max-min rate achieved and the maximum permissible spectral span. The
spectral allocation obtained from the proposed optimization framework is shown
to be close to the tradeoff 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 Universal Software Radio Peripheral (USRP)
enabled ORBIT radio network testbed.
