IQ Offset and Imbalance Estimation for LTE Basestation Receivers and Measurement Devices

Low-cost mobiles for the LTE communication system usually generate transmit signals which are strongly influenced by effects of the IQ offset. The specified limits concerning the maximum value for this distortion that has to be accepted requires its compensation at basestation receivers for a reliable data detection. Another need exists in the field of communication testers for mobile devices. To assess the quality of a transmit signal, an IQ offset compensation is mandatory. Moreover, it is required for a reliable computation of the corresponding quality measure especially for highly corrupted output signals of mobile devices. Due to deviations of the LTE transmission scheme from classical OFDM, the IQ offset has to be estimated jointly with the IQ imbalance and the channel. For the general case, a pilot-aided approach leads to an underdetermined estimation problem. In this context, a solution is presented which approximately solves the problem according to the Maximum-Likelihood criterion. Simulation results show that the introduced approximation is valid and significantly improves the receiver performance.