How to select the pilot carrier positions in CP-OFDM?

In this paper, we consider the problem of pilot aided channel estimation (PACE) for CP-OFDM based systems. The positions of the pilot carriers have a strong influence on the mean squared error (MSE) of the channel estimate, and thus indirectly on the overall system performance. Hence, we are interested in the pilot carrier placement that minimizes this MSE. However, the optimization problem at hand is an integer combinatorial optimization problem and thus NP hard. In this paper, we propose two heuristic algorithms searching for a good pilot carrier placement with a low MSE. Both algorithms have comparable performance: the resulting MSE is for both algorithms similar, and in the majority of the cases the optimal pilot carrier placement is found. We compare the results from the proposed heuristic algorithms with the results for the maximum distance distribution from [1]. We are able to achieve lower channel estimate MSE than with the maximum distance distribution, although the difference becomes small for large FFT size.