Channel Estimation and Equalization in Multi-User Uplink OFDMA and SC-FDMA Systems Under Transmitter RF Impairments

Single-carrier frequency division multiple access (SC-FDMA), a modified form of orthogonal frequency division multiple access (OFDMA), has been adopted as the uplink physical layer radio access technique for the 3GPP Long Term Evolution (LTE) and LTE-Advanced. Radio transceiver implementations for such OFDM-based systems with the direct-conversion architecture are desirable to enable small size, low-cost and low power consumption terminals. However, the associated circuit impairments stemming from the processing of analog radio frequency (RF) signals, such as in-phase and quadrature phase (I/Q) imbalance and carrier frequency offset (CFO) errors, can severely degrade the obtainable link performance. In this paper, we analyze the effects of these radio impairments in multi-user SC-FDMA uplink system , and present digital signal processing (DSP)-based methods for the joint estimation and equalization of impairments and channel distortions on the receiver side with arbitrary number of receiver antennas. For the equalization, linear equalizers such as the zero-forcing (ZF) and the minimum mean-square error (MMSE) equalizers that utilize pairs of mirror subcarriers are formulated, and the MMSE equalizer is developed to effectively handle mirror sub-band users with different power levels. Furthermore, for reduced computational complexity, the joint channel and impairments filter responses are efficiently approximated with polynomial-based basis function models. The parameters of the basis functions are then estimated by exploiting the time multiplexed reference symbols in the LTE uplink sub-frame structure. The performance of the proposed estimation and equalization methods is assessed with extensive multi-user link simulations, with both single-antenna and dual-antenna base station receivers, and the results show that the proposed algorithms are able to significantly reduce the impact of channel distortions and radio impairments. The resulting receiver implementation wit- the proposed techniques enables improved uplink link performance even when the mobile terminals fulfill their emission requirements, in terms of I/Q images, without any changes in the LTE standard’s frame and pilot structures.