In this paper, we investigate the sum rate (SR) maximization problem for downlink cooperative non-orthogonal multiple access (C-NOMA) system under in-phase and quadrature-phase (IQ) imbalance at the base station (BS) and destination. The BS communicates with users by a half-duplex amplified-and-forward (HD-AF) relay under imperfect IQ imbalance. The sum rate maximization problem is formulated as a non-convex optimization with the quality of service (QoS) constraint for each user. We first use the variable substitution method to transform the non-convex SR maximization problem into an equivalent problem. Then, a joint power and rate allocation algorithm is proposed based on successive convex approximation (SCA) to maximize the SR of the systems. Simulation results verify that the algorithm can improve the SR of the C-NOMA compared with the cooperative orthogonal multiple access (C-OMA) scheme.

In this letter, we propose a blind adaptive algorithm for joint compensation of inter-block interference (IBI) and frequency-dependent IQ imbalance using a single time-domain equalizer. We combine the MERRY algorithm for IBI suppression with the differential constant modulus algorithm to compensate for IQ imbalance. The effectiveness of the proposed algorithm is shown through computer simulations.

This letter proposes an automatic IQ imbalance compensation technique for quadrature modulators by means of spectrum measurement of RF signal using a spectrum analyzer. The analyzer feeds back only magnitude information of the frequency spectrum of the signal. To realize IQ imbalance compensation, the conventional method of steepest descent is modified; the descent direction is empirically determined and a variable step-size is introduced for accelerating convergence. The experimental results for a four-channel transmitter operating at 11 GHz are presented for verification.

A receiver architecture and a digital IQ imbalance compensation method for dual-carrier reception are newly proposed. The impact of IQ imbalance on the baseband signal is mathematically analyzed. Based on the analysis, IQ imbalance parameters are estimated and the coupling effect of IQ imbalance is compensated using digital baseband processing alone. Simulation results show that the proposed IQ imbalance compensation successfully removes IQ imbalance. The deviation from the ideal performance is less than 1 dB when it is applied to the 3GPP-LTE carrier aggregation.

In orthogonal frequency division multiple access (OFD-MA) uplink, the distortions introduced by both multiple carrier frequency offsets (CFOs) and in-phase and quadrature-phase (IQ) imbalances will severely degrade the system performance. With both CFOs and IQ imbalances, signal detection at the receiver becomes hard, if not impossible. In this letter, a linear receiver is proposed to cope with the distortions at a slight drop in system transmission rate. The analysis and simulations demonstrate the effectiveness of the proposed approach.

Recently, the marginalized particle filter (MPF) has been applied to blind symbol detection problems over selective fading channels. The MPF can ease the computational burden of the standard particle filter (PF) while offering better estimates compared with the standard PF. In this paper, we investigate the application of the blind MPF detector to more realistic situations where the systems suffer from analog imperfections which are non-linear signal distortion due to the inaccurate analog circuits in wireless devices. By reformulating the system model using the widely linear representation and employing the auxiliary variable resampling (AVR) technique for estimation of the imperfections, the blind MPF detector is successfully modified to cope with the analog imperfections. The effectiveness of the proposed MPF detector is demonstrated via computer simulations.

Direct conversion receivers in orthogonal frequency division multiplexing (OFDM) systems suffer from direct current (DC) offset, frequency offset, and IQ imbalance. We have proposed an IQ imbalance estimation scheme in the presence of DC offset and frequency offset, which uses preamble signals in the time domain. In this scheme, the DC offset is eliminated by a differential filter. However, the accuracy of IQ imbalance estimation is deteriorated when the frequency offset is small. To overcome this problem, a new IQ imbalance estimation scheme in the frequency domain with the differential filter has been proposed in this paper. The IQ imbalance is estimated with pilot subcarriers. Numerical results obtained through computer simulation show that estimation accuracy and bit error rate (BER) performance can be improved even if the frequency offset is small.

In this paper, we analyze the effects of IQ (In-phase/Quadrature-phase) imbalance at both transmitter and receiver of OFDM (Orthogonal Frequency Division Multiplexing) system and show that more diversity gain can be achieved even though there are unwanted IQ imbalance. When mixed sub-carriers within an OFDM symbol due to the IQ imbalance undergo frequency selective channels, additional diversity effects are expected during the demodulation process. Simulation results on the symbol error rate (SER) performance with ML (Maximum Likelihood) and OSIC (Ordered Successive Interference Cancellation) receiver show that significant performance gain can be achieved with the diversity gain caused by the IQ imbalance combined with the frequency selective channels.

Currently, multiband orthogonal frequency division multiplexing (MB-OFDM) is considered to be one of the modulation schemes of UWB and is being actively investigated. It is necessary to provide low-cost receivers for consumers to receive wide support for the MB-OFDM system. Such receivers can be achieved by utilizing direct-conversion architecture. Direct-conversion architecture suffers from IQ imbalance. IQ imbalance causes intercarrier interference (ICI) in the demodulated signals. In this paper, a new scheme of IQ imbalance compensation using transmit diversity is proposed. This scheme enables the system to achieve frequency diversity and simultaneously compensates for the influence of IQ imbalance. It is shown that the performance of the proposed scheme is better than that of the conventional IQ imbalance compensation scheme.

In this letter, we propose a new detection method for an OFDM signal distorted by IQ imbalance, and a pilot pattern to estimate the channel associated with IQ imbalance. It is shown by computer simulation that the proposed method can achieve robust detection even when severe IQ imbalance exists in OFDM systems with an input of higher-order constellation.

Recently, the direct conversion scheme has been actively investigated for the purpose of cost miniaturization and low power consumption of wireless receivers. IQ imbalance is one of the problems for the direct conversion receiver. In the case of OFCDM modulations, this IQ imbalance causes intercarrier interference (ICI) in the demodulated signals. In this paper, the decision directed scheme for IQ imbalance compensation is proposed. In the proposed scheme, the combination of received symbols which satisfies orthogonality conditions is used for compensation of IQ imbalance. Therefore, in addition to the pilot symbols, the received symbols can be used in order to improve the accuracy of the compensation matrix and BER can be reduced.