Training symbol placement for packet transmissions under asynchronous interference

In a multiple access communication system that uses packet transmissions, the packets of one user might be subject to asynchronous interference from the packets of other users in the system. This paper analyses the influence of the placement of training symbols on the performance of the channel estimator in this scenario. The analysis of the mean square error (MSE) of the minimum mean square error estimator (MMSE) is shown to be equivalent to the analysis of the Fisher information matrix (FIM) for mixtures of Gaussian distributions. A complete solution to this problem is hard to find, but the bounds, asymptotics and simulations suggest that the best placement of training symbols is in two clusters of equal or quasi-equal size at the two ends of the data packet.