Analysis And Compensation Schemes Of IQ Imbalance In 60GHz Communication Systems

Due to increasing demand in high speed wireless communication, 60 GHz communication systems has attracted more attention as its large bandwidth, fast transmission speed, security and good anti-interference. As a major source of impairments, the IQ imbalance arises from the mismatch between the in-phase(I) and quadrature-phase(Q) branches of the transmitter(TX) up-convertor and the receiver(RX) down-convertor. Ideally, the I/Q branches should have the equal amplitude and exactly 090 phase shift, but this is rarely the case in practice, resulting in IQ imbalance. The IQ imbalance destroys the I/Q orthogonality and incurs crosstalk between the I and Q signals, or equivalently the real and imaginary parts of the complex baseband signal. Such crosstalk is self-interfering and its adverse effect cannot be mitigated by increasing signal power. Therefore, if not compensated, the IQ imbalance may significantly degrade the system performance.When the RF front-end of the high-frequency communication system to relax the requirements for IQ imbalance in order to reduce costs, or existing complementary metal oxide semiconductor technology inevitably produce more severe IQ imbalance, compensating IQ imbalance in the baseband is necessary and critical. In the recent years, much effort has been made to develop techniques that estimate and compensate for IQ imbalance. These techniques include blind compensation and training-based compensation. Blind IQ imbalance compensation schemes do not require training symbols, but rely on some statistical property(e.g. circularity) of received signal, which, unfortunately, may be destroyed by mutipath propagation. Moreover, blind schemes often needs long date sequence to enable its convergence. These factors greatly limit the application scope of blind schemes. For non blind estimation algorithm, based on signal detection theory, the IQ imbalance parameters can also be estimated by sending known symbols. Generally through the system level based algorithm and the traditional least squares, maximum likelihood, the expectation maximization and other criteria, accurate and rapid implementation of the IQ imbalance estimation and compensation. Compared with the blind estimation computation, this compensation scheme easy to implement, and therefore widely used. But this algorithm does not fully exploit the characteristics of the training sequence, and based on the assumption that the channel information is obtained ideal premise.In this thesis, we address the joint estimation and compensation of TX and RX IQ imbalance for single carrier frequency domain equalization(SC-FDE) and orthogonal frequency division multiplexing(OFDM) based 60 GHz communication systems. The main research contents include IQ imbalance simulation models, system models interferenced by IQ imbalance, the effects on system performance by IQ imbalance and compensation algorithm of IQ imbalance.System model is a prerequisite for the establishment of research work. Between this, in the study of IQ imbalance, the thesis used for 60 GHz communication SC-FDE and OFDM system model interferenced by the IQ imbalance. The model is based on 60 GHz communications standards, is the basis of this article. Secondly, through simulation, the thesis analysis the effects of system performance by IQ imbalance, and roughly sum up IQ imbalance parameters in accordance with the actual requirements. Finally, under different IQ imbalance conditions, the thesis gives three compensation algorithm and its performance is verified and analyzed. First, a golay complementary sequences based IQ imbalance compensation scheme for receiver is presented. By utilizing property of golay sequences in preamble of 802.11 ad frame, the signal and its conjugate interference in preamble are separated by correlation. The results show that it has effectively eliminated the influence of the imbalance at a low complexity.Secondly, the method calls for RX IQ imbalance and channel cooperation. The channel estimation method can separate channel from IQ imbalance, but it need to exactly know the channel length beforehand. Finally, a scheme joint estimation and compensation of transmitter and receiver IQ imbalances for SC-FDE and OFDM systems is presented. Therefore, the proposed method can separate TX IQ imbalance, channel and RX IQ imbalance, and enables flexible compensation. It is shown through extensive simulations that the proposed method can achieve performance very close to the ideal case without IQ imbalances.Since IEEE 802.11 ad standard adopts golay sequences as preamble, the method in this thesis could be used directly in an IEEE 802.11 ad system without bringing in any modification in frame structure.