IEEE Transactions on Vehicular Technology, vol. 58, no. 6, pp. 2803-2814, July 2009.
Pilot-aided OFDM channel estimation has thus far only been optimized for open-loop systems. Such studies have concluded in the optimality of uniformly-spaced pilot symbols of equal power. In this paper, examine closed-loop OFDM systems and show how uniform pilots may no longer be optimal. In doing so, we propose a feedback technique in which the pilot allocation mechanism is adapted to the mobile channel conditions. After deriving a general expression for average symbol error rate as a function of pilot allocation, we optimize the pilot locations and also propose two practical solutions using upper-bounds on the objective function.We proceed to formulate pilot allocation based on maximal average OFDM channel capacity. We examine the feedback overhead associated with our method and make comparisons with the conventional uniform framework. Finally, we suggest the use of vector quantization in the context of the generalized Lloyd algorithm to reduce feedback, and also extend our method to a MIMO-OFDM system based on the Alamouti space-time block code. We conduct coded and uncoded simulations to illustrate the error-rate/capacity performance gains via non-uniform pilots.