Bit error rate minimising pilot symbol arrangement in closed-loop orthogonal frequency division multiplexing systems

The use of a feedback link in wireless orthogonal frequency division multiplexing (OFDM) systems is analysed. Although open-loop scenarios have been rigorously investigated in the literature, closed-loop scenarios, in which there is a feedback link from a strong to a weak user, have not been thoroughly studied. The study mainly focus on the recently proposed dynamic pilot symbol arrangement strategy. It was shown in a recent paper that for coherent OFDM transmission, this strategy is capable of delivering substantial gains in the performance. Pilot symbols are used to sample channel behaviour so that data symbols can be decoded correctly. However, these pilots do not need to be arranged so as to estimate the channel in the best possible way. In this work, the authors propose a pilot symbol arrangement strategy that minimises the bit error rate, we analyse the solution in detail and propose a low complexity search that maintains most of the quality of an exhaustive search. This low complexity search makes the pilot symbol method viable for medium to large-scale OFDM systems. The authors also analyse the use of a Wiener filter for channel estimation combined with the dynamic pilot symbol strategy and show that it delivers good gains in comparison to a simple maximum likelihood channel estimate.