Low-complexity channel estimation for superimposed pilots in distributed OFDMA systems

In multiple-input multiple-output (MIMO) systems, the requirement for orthogonal pilot sets grows linearly with the number of transmit antennas. The channel estimators based on multiplexed pilot subcarriers used in the current systems such as WiMAX and LTE consume bandwidth allocated for data transmission. An alternative approach based on superimposing the pilots onto the data subcarriers offers a good solution for channel estimation in systems where the maximum data rate can not be sacrificed for the cost of channel estimation accuracy. The concept of using superimposed pilots for channel estimation in a distributed multiple antenna transmission system is studied here with simple and low-complexity algorithms suitable for hardware implementation. The complexity and performance of the system are compared with a conventional multiplexed pilot system. The bit error rate (BER) simulation results demonstrate that superimposed pilots can be used to achieve tolerable performance in adaptive high-data rate systems with up to four transmitting antennas.