Performance Evaluation of Rank Enhancement Pilot Design for Massive MIMO OFDM

The Demand for higher spectrum efficiency is increasing the number of transmit antennas in MIMO-OFDM systems. MIMO-OFDM with a massive number of transmit antennas (Massive MIMO-OFDM) has been gathering attention. In massive MIMO-OFDM systems, the method of estimating the channel state information (CSI) at the transmitter significantly impacts systems performance. The training frame for CSI estimation decreases MAC efficiency by increasing overheads. This paper presents an OFDM pilot symbol design in which different transmission weights are used in each subcarrier. The proposed pilot symbol design enables a user station (STA) to estimate CSI corresponding to more transmission ranks than the number of pilot symbols. The STA calculates CSI at each subcarrier using the signal received in multiple subcarriers. To evaluate the performance benefits of the proposal, CSI is measured in an actual indoor environment by using a 16 x 4 MIMO-OFDM testbed. Simulations using the IEEE802.11n channel and the measured indoor channel show that the achievable bit rate of the proposed method approaches that of the full antenna CSI even with a very short training frame.