Compensation of I/Q imbalance in OFDM communication systems.

Baha'a Jasim Mousa Al-Joubory
Dr. Ra'ad S. Fyath

Abstract: A major source of impairments in Orthogonal Frequency Division Multiplexing (OFDM) wireless communication systems is the imbalance between the real part (Inphase (I)) and the imaginary part (Quadrature (Q)) of the complex signal. This thesis addresses problem theoretically and introduces a blind estimation algorithm to estimate and then compensate the I/Q imbalance. The preformance of a direct-conversion receiver implemented in OFDM systems is analyzed in the presence of I/Q imbalance. Two types of imbalance (mismatch) between the I-and Q-branches are considered here. These two types are lumped in the amplitude mismatch parameter ''g" and the phase mismatch parameter "Q" which denotes, respectively, the imbalance corresponding to unequal amplitudes and exact phase difference between the I-and Q-branches. Different modulation schemes are considered here: QPSK, 16QAM and 64 QAM. The simulation results performed in Matlab-7 environment indicates clearly that the effect of I/Q imbalance is more pronounced when high level modulation scheme is implemented.A blind estimation algorithm is implemented in the receiver to estimate the I/Q imbalance parameters. The estimated results are used to compensate the effect of I/Q imbalance. The results indicate that for a QPSK system operating at Bit-Error Rate (BER) of 10-5 with g =1.1 and ? = 5o , the required Signal-to- Noise Ratio (SNR) is 12.5, 2.9, and 12.6dB, in the absence, presence, and compensation of I/Q imbalance, respectively. These values are to be compared with 19.5, 21.5, and 19.7dB for 16QAM system and 27.3, 60.0 and 29.6dB for 64QAM system respectively.