Reduction of Nonlinear Distortion in Multi-Antenna WiMAX Systems

Multiple-input multiple-output (MIMO) techniques in combination with orthogonal frequency-division multiplexing (OFDM) have already found its deployment in several standards for the broadband communications including WiMAX or 3GPP proposal termed as Long Term Evolution (LTE). The MIMO-OFDM allows to substantially increase the spectral efficiency, link reliability and coverage of the signal transmission. With recent advent of the hardware processing enhacements, the processing requirements of MIMO-OFDM might be accomodated in the portable units and thus, it is widely expected that this technology will dominate over the next years in the wireless communications. Despite of its undoubted benefits, MIMO-OFDM transmission systems are also characterized by the large envelope fluctuation of the transmitted signal Drotar et al. (2010a). This requires the application of the high Input Back-off (IBO) at the nonlinear High Power Amplifier (HPA) stage that subsequently results in an inefficient use of HPA and limitation of the battery life in the user mobile stations. In is important to note that nonlinear amplification manifests itself in the form of Bit-Error-Rate (BER) degradation at the receiver side and simultaneously, in the form of the out-of-band radiation Deumal et al. (2008). An intuitive solution to supress the out-of-band radiation and thus, occupy the area within the spectral mask of the transmission is is to deactivate subcarriers at the borders of the used MIMO-OFDM spectrum. However, this approach impairs the spectral efficiency of the transmission and may not be convenient for the high data rate applications. Therefore it is feasible to look for the additional technique that aim to reduce the out-of-band emissions and to maintain the specific spectral mask of the transmission Khan (2009). The possible solution is to design MIMO-OFDM systems such that the signal is less sensitive to the nonlinearity impairments. Lower fluctuation of the signal envelope can be achieved by modifying the transmitted signal prior to the transmission. However, this approach requires additional hardware and signal processing at the transmitter, which is not feasible in some applications. For these applications, the receiver based compensation is of more interest. In the following sections, we will review the details of the most favourite methods reducing envelope fluctuation, which are intended to be used in Single-Input Single-Output (SISO) OFDM and MIMO-OFDM systems. Moreover, we will introduce two novel techniques that 4