Nowadays a lot of effort is spent on developing OFDM- based inexpensive wireless transceivers. Direct-conversion radio frequency transceivers are appealing because they avoid costly IF analog components. This kind of transceivers imply analog RF I/Q separation. The mismatch between the analog components in the in-phase and quadrature branches introduces an unwanted in-band interference. Unfortunately, OFDM-based systems are very sensitive to I/Q mismatch, mostly when high order modulation schemes are applied. A digital compensation of this unwanted effect is required. In this paper, we developed a method for compensating the transmitter/receiver frequency-dependent I/Q imbalance jointly with the propagation channel in the frequency domain. Based on a low complexity ML channel estimator, a pilot design criterion is derived for channel and IQ imbalance coefficients estimation. An advanced equalizer is proposed which compensates for the frequency-dependent IQ imbalance. Both steps, estimation and compensation, are low cost in terms of implementation complexity. The proposed algorithm enables the system to achieve high SNRs.

Nowadays OFDM is combined with advanced multiple access and multiple antenna techniques in order to improve the link capacity. In this context, a lot of effort is spent on developing inexpensive wireless OFDM-based receivers. Direct-conversion radio frequency receivers are appealing because they avoid costly IF filters. This kind of receivers implies analog RF I/Q separation introducing an unwanted in-band image interfearence encedue to the mismatch between the in-phase and quadrature branches. Unfortunately, systems combining OFDM with multiple antennas and multiple access techniques are very sensitive to I/Q mismatch, mostly when high order modulation schemes are applied. A digital compensation of this un-wanted effect is required. In this paper, we propose a method for estimating jointly the frequency dependent I/Q imbalance and the propagation channel in the frequency domain. First the channel is estimated based on a pilot optimized to suppress the interference caused by I/Q imbalance. Second I/ C imbalance is estimated and compensated relying on the estimated channel. Both steps are low cost in terms of implementation complexity. Finally, it is shown that the performance of the channel estimate is not degraded by I/Q imbalance. The proposed algorithm enables the system to work at high SNRs.

In the context of 4G wireless mobile systems with direct conversion receivers, an new algorithm for carrier frequency offset (CFO) and IQ-imbalance estimation and compensation at the receiver is proposed. All processing for estimation and compensation of both CFO and IQ-imbalance is done in the time domain. Because the CFO is estimated while taking into account the presence of IQ-imbalance, a very precise estimate of the CFO can be obtained, even in the presence of large IQ-imbalance. The algorithm uses a repetitive low peak-to-average preamble sequence. At the transmitter, in order to improve the estimation result, an artificial CFO is digitally super-imposed on a part of the preamble sequence. The new algorithm yields better results than existing CFO estimation algorithms for SNRs where the IQ imbalance is dominant over the Gaussian noise. The structure of the preamble also allows an effective IQ-imbalance estimation.

New air interfaces are currently being developed to meet the high spectral efficiency requirements of the emerging wireless communication systems. Multicarrier 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, the sampling clock offset, 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 simulations

New air interfaces are currently being developed to meet the high requirements of the emerging wireless communication systems. In this context, MC-CDMA is seen as a promising candidate for the 4G cellular communication systems since it can interestingly deal with the multipath propagation. The goal of this paper is to evaluate the impact of the carrier frequency offset, the sampling clock offset and 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 error is provided. For the setup considered in this paper, mostly IQ imbalance degrades the performance. On the other hand, the impact of sample clock offset is negligible with respect to the one of carrier frequency offset.

Single-carrier code division multiple access (SC-CDMA), also named cyclic-prefix CDMA in the literature, is a promising air interface for the up-link of the 4G cellular wireless communication systems. It enables the high capacity intrinsically offered by CDMA by making the equalization of the multi-path channels and the mitigation of the resulting interference possible at a low complexity. This paper studies the impact of multi-user sample clock offset on the performance of up-link SC-CDMA and proposes a compensation strategy at the receiver. It is shown that clock offset leads to an unacceptable degradation of the performance if it is not compensated. In a simultaneous multi-user up-link system, the sample clock offset is usually pre-compensated at the transmitters. This paper demonstrates that the compensation can also be done at the receiver. In a first step the optimal linear multi-user joint detector is designed according to the MMSE criterion, taking the effect of clock offset into account. In a second step the MMSE joint detector is simplified to a low complexity detector incurring a negligible loss of performance