Performance Analysis and Mitigation Techniques for I/Q-Corrupted OFDM Systems

Orthogonal Frequency Division Multiplexing (OFDM) has become a widely adopted modulation technique in modern communications systems due to its multipath resilience and low implementation complexity. The direct conversion architecture is a popular candidate for low-cost, low-power, fully integrated transceiver designs. One of the inevitable problems associated with analog signal processing in direct conversion involves the mismatches in the gain and phases of In-phase (I) and Quadrature-phase (Q) branches. Ideally, the I and Q branches of the quadrature mixer will have perfectly matched gains and are orthogonal in phase. Due to imperfect implementation of the electronics, so called I/Q imbalance emerges and creates interference between subcarriers which are symmetrically apart from the central subcarrier. With practical imbalance levels, basic transceivers fail to maintain the sufficient image rejection, which in turn can cause interference with the desired transmission. Such an I/Q distortion degrades the systems performance if left uncompensated. Moreover, the coexistence of I/Q imbalance and other analog RF imperfections with digital baseband and higher layer functionalities such as multiantenna transmission and radio resource management, reduce the probability of successful transmission. Therefore, mitigation of I/Q imbalance is an essential substance in designing and implementing modern communications systems, while meeting required performance targets and quality of service. This thesis considers techniques to compensate and mitigate I/Q imbalance, when combined with channel estimation, multiantenna transmission, transmission power control, adaptive modulation and multiuser scheduling. The awareness of the quantitative relationship between transceiver parameters and system parameters is crucial in designing and dimensioning of modern communications systems. For this purpose, analytical models to evaluate the performance of an I/Q distorted system are considered

SER/BER Expression for M-QAM OFDM Systems with Imperfect Channel Estimation and I/Q Imbalance

We investigate the joint effect of channel estimation and frequency flat transmitter and receiver I/Q imbalance on an Orthogonal Frequency Division Multiplex (OFDM) system. We assume independent fading with identically and independently distributed (i.i.d) mirror carrier channel coefficients. Closed form expressions for symbol error rate and bit error rate of M-QAM modulation are derived. We consider join and separate channel estimation, as well as joint and separate equalization of the mirror carriers. Performance is evaluated by treating the I/Q interference as a non-Gaussian random variable. The results show that a Gaussian approximation of I/Q interference is very good, especially for low order modulations.Peer reviewe

Ultra-reliable link adaptation for downlink MISO transmission in 5G cellular networks Ultra-Reliable Link Adaptation for Downlink MISO Transmission in 5G Cellular Networks

Abstract: This paper addresses robust link adaptation for a precoded downlink multiple input single output (MISO) system, for guaranteeing ultra-reliable (99.999%) transmissions to mobile users served by a small cell network (e.g. slowly moving machines in a factory). Effects of inaccurate channel state information (CSI) caused by user mobility and varying precoders in neighboring cells are mitigated. Both of these impairments translate to changes of received signal-to-noise plus interference ratios (SINRs), leading to CSI mispredictions and potentially erroneous transmissions. Knowing the statistics of the propagation channels and the precoder variation, backoff values can be selected to guarantee robust link adaptation. Combining this with information on the current channel state, transmissions can be adapted to have a desired reliability. Theoretical analysis accompanied by simulation results show that the proposed approach is suitable for attaining 5G ultra-reliability targets in realistic settings