Impact of frequency offsets and IQ imbalance on MC-CDMA reception based on channel tracking

Abstract

New air interfaces are currently being developed to meet the high spectral efficiency requirements of the emerging wireless communication systems. Multi-carrier code-division multiple access (MC-CDMA) is seen as a promising candidate for the fourth generation (4G) cellular communication systems because it can interestingly deal with the multipath propagation at a low processing complexity. Besides spectral efficiency and power consumption, the production cost of the transceiver should also be optimized. Direct conversion radio frequency (RF) receivers are appealing because they avoid costly intermediate frequency (IF) filters. However, they imply RF IQ separation, introducing a phase and amplitude mismatch between the I and Q branches. A communication system based on MC-CDMA is sensitive to synchronization errors and front-end non-idealities because it uses a long symbol duration. The goal of this paper is to evaluate the impact of the carrier frequency offset (CFO), the sampling clock offset (SCO) and the IQ imbalance on the MC-CDMA downlink system performance, considering a receiver based on channel tracking designed to cope with high mobility conditions. It is demonstrated that part of the effects is compensated by the channel estimation and an expression of the variance of the remaining symbol estimation error is provided. For the cellular system and the target performance considered in this paper, specifications are defined on the non-idealities. The results are validated with bit error rate (BER) simulations.

Cross-Validation Estimation for Frequency-Dependent I/Q Imbalance in MIMO-OFDM Receivers

“…According to Eqs. (8), (9) and (12), Fig. 6 is the architecture of the crossvalidation estimator, and Fig.…”

“…Therefore, introducing the filter mismatch phenomenon into the system increases the estimation error. The I/Q imbalance with filter mismatch, namely frequency-dependent I/Q imbalance, is much critical for OFDM direct-conversion receivers to make the corresponding estimation and compensation be extremely difficult [3,9].…”

Low-Complexity EM-based Joint CFO and IQ imbalance Acquisition

“…Especially, the carrier frequency offset (CFO), caused by the local oscillators at the transmitter and receiver, and the IQ imbalance, caused by the use of direct-conversion analog front-ends, are destructive [4]. It has been recently shown that those effects can be advantageously compensated digitally [5].…”

Optimal Training Sequences for Joint Channel and Frequency-Dependent IQ Imbalance Estimation in OFDM-based Receivers

“…The time impulse response mismatch, mostly caused by a mismatch between the two LPFs, causes a frequency dependent I/Q imbalance which must be estimated and compensated per carrier. The impact of I/Q imbalance on OFDM and CDMA is shown to be critical in [1] and [2]. Even though the degradation created by the LO imbalance is a dominant effect, the frequency dependent part gives enough degradation to make the use of high modulation schemes impossible.…”