Integer QP Relaxation Based Algorithms for ICI Reduction in OFDM Systems

In a fast fading channel, Doppler spread caused by user mobility destroys the orthogonality among orthogonal frequency division multiplexing (OFDM) subcarriers, resulting in intercarrier interference (ICI). In this paper, the OFDM ICI reduction problem is first formulated as a combinatorial optimization problem with integer constraints. Two relaxation methods are then utilized to relax the maximum likelihood (ML) detection problem into convex quadratic programming (QP) problems. To further reduce computational complexity, the QP problems are solved by limiting the search to a 2-dimensional subspace. A low-bit descent search (LBDS) can also be employed to improve the system performance. Performance results are given which demonstrate that the integer QP relaxation based algorithms provide excellent performance with reasonable computational complexity.