Edouardo Lopez-Estraviz

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

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

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

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.

Flexible transmission scheme for 4G wireless systems with multiple antennas

New air interfaces are currently being developed to meet the high requirements of the emerging wireless communication systems. In this context, the combinations of the multicarrier (MC) and spread-spectrum (SS) technologies are promising candidates. In this paper, we propose a generic transmission scheme that allows to instantiate all the combinations of orthogonal frequency-division multiplexing (OFDM) and cyclic-prefixed single-carrier (SC) modulations with direct-sequence code-division multiple access (DS-CDMA). The generic transmission scheme is extended to integrate the space-division multiplexing (SDM) and the orthogonal space-time block coding (STBC). Based on a generalized matrix model, the linear frequency-domain minimum mean square error (MMSE) joint detector is derived. A mode selection strategy for up- and downlink is advised that efficiently trades off the cost of the mobile terminal and the achieved performance of a high-mobility cellular system. It is demonstrated that an adaptive transceiver that supports the proposed communication modes is necessary to track the changing communication conditions.

MC-CDMA performance in the presence of carrier frequency offset, sample clock offset and IQ imbalance

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