Abstract
Wireless high-speed digital communications systems are becoming increasingly commonplace for both commercial and military applications. One of the most effective techniques for combating multipath interference is the multicarrier OFDM (Orthogonal Frequency Division Multiplexing) scheme. However, the high PAPR (Peak-to-Average Power Ratio) of OFDM signals constitutes a problem when non-linear amplifiers are used. In this thesis the PAPR problem is introduced and the LINC (Linear Amplification with Nonlinear Components) technique is considered as a possible solution to counteract the PA (Power Amplifier) nonlinearity distortion. The proposed LINC technique can be implemented using DSP (Digital Signal Processing) techniques, has the potential of high IMD (Inter-modulation Distortion) suppression and it is unconditionally stable, which is a key advantage in broadband applications. It's well known that the unwanted imperfections like the I/Q (In-phase/Quadrature) imbalances present in the analogue IQ modulators in a direct conversion OFDM system contribute to a loss of orthogonality and create ICI (Inter-carrier Interference). These impairments can also severely reduce the efficiency of the LINC technique itself. This thesis investigates a digital compensation mechanism to reduce the IQ imbalance errors in the direct conversion OFDM LINC transmitter. The LINC technique is considered in the first part of this thesis as a possible linearisation technique to counteract the non-linear distortion caused by both the mixer and power amplifier in a single-carrier QPSK (Quaternary Phase Shift Keying) transmitter. Prototype systems for single-carrier QPSK transmitter and multi-carrier OFDM transmitter have been constructed to demonstrate the proposed method's capability.