Mobile transmitters I/Q imbalances in LTE uplink: Analysis and digital mitigation

The single-carrier frequency-division multiple access (SC-FDMA), also known as discrete Fourier transform (DFT)- spread OFDMA, has been adopted as uplink transmission scheme for the 3GPP Long Term Evolution (LTE). Such OFDM-based systems are known to be particularly sensitive to radio frequency (RF) impairments. The major sources of these RF impairments are in-phase and quadrature-phase imbalance (IQI) and carrier frequency offset (CFO). In this paper, we investigate the impact of radio impairments on the LTE uplink by mathematically formulating the received signal as a function of transmitted signals, IQI, CFO, and channel distortions. Then, two alternative joint equalization schemes are proposed for coping with such impairments, stemming from the minimum mean square error (MMSE) and the zero forcing (ZF) principles, respectively. LTE uplink computer simulations are then carried out to assess the validity and performance of the proposed equalizers. The results demonstrate that the proposed schemes significantly improve the uplink performance, the performance of the MMSE solution being very close to the zero RF impairments scenario.