Spectrum Assignment for Flexible Rate Cognitive OFDM Systems

In this paper we present an adaptive algorithm for spectrum assignment applicable for unlicensed systems that share the same spectrum range with licensed systems. To improve the rate flexibility of the unlicensed systems using orthogonal frequency-division multiplexing (OFDM) modulation schemes, we design an algorithm that is implemented as a distributed optimization problem in each transmitted bit. Each bit cost function is minimized by dynamically choosing the bit spectrum pattern that are better suited to the available spectrum range without degrading the licensed system performances. The optimization problem is treated as a constraint problem for which the optimal bit spectrum patterns are designed based on a feasible direction approach. At the optimal equilibrium point, the receiver that employs a conventional correlation operation with the replica of the transmitted signal will have the same efficiency as the minimum mean squared error (MMSE) receiver in the presence of noise and licensed systems.