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MIMO-OFDM systems are gaining prominence in high data rate applications due to their increased spectral efficiency. In MIMO-OFDM systems, I/Q mismatch causes interference between the frequency mirror subcarriers. This can significantly degrade and limit the performance of the system. In this paper, we present a mathematical analysis of the effects of I/Q gain, delay and phase mismatches in MIMO-OFDM systems. We show the impact of I/Q mismatch when more antennas are added. We also derive a linear optimal solution to cancel all three I/Q mismatches and an RLS based adaptive filter for the same. We show simulation results and results on an experimental testbed.

MIMO-OFDM is being considered for communication systems where high throughput and spectral efficiency are important factors. Analog impairments like I/Q mismatch and phase noise significantly degrade and limit the performance of communication systems. In this paper, we analyze the impact of I/Q mismatch and phase noise in MIMO-OFDM systems as a function of the number of antennas. We show the improvement in performance that is possible when these impairments are cancelled. We also discuss the impact of correlated and uncorrelated phase noise in MIMO-OFDM systems. Finally, we show the results of I/Q mismatch and phase noise cancellation in wireless measurements performed using a 2x2 MIMO-OFDM testbed.Index Terms-Analog impairments, I/Q imbalance, I/Q mismatch, MIMO-OFDM, phase noise.

Theoretical capacity calculations and corresponding simulations show significant capacity/throughput gains from MIMO systems. Whether these gains are achievable in a real system, deployed in a practical environment, depends on a variety of factors, such as the choice of the communication algorithms, analog impairments and the "quality" of the wireless channel to sustain MIMO communications. In this paper, a 5.25GHz broadband MIMO-OFDM testbed is described along with field measurements conducted with it. The MIMO-OFDM communication algorithms and also the impact of analog impairments on the performance of the system are described. Detailed system calibration results are described which serve as a baseline for results of field measurements. The results of wireless measurements are compared with the theoretical capacity, computed with the channel estimates obtained during the demodulation process. The average achievable capacity in the indoor wireless environment is shown to be 9.97 bps/Hz (bits per sec per Hz) while the capacity loss due to analog impairments and the choice of algorithms is about 2.33 bps/Hz. Also, field measurements conducted with the system in various environments are presented comparing the average throughput/capacity achieved in each of these environments.Index Terms-MIMO-OFDM, testbed, field measurements, channel estimation, MIMO decoder, analog impairments, I/Q mismatch, phase noise.

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