CHARACTERIZATION AND MITIGATION OF LINEAR AND NON-LINEAR DISTORTIONS IN SIX-PORT MODULATORS

Abstract

There is an increasing demand of lowering the complexity and power consumption in the design of the wireless terminals. Further, wideband, compact size, and low-cost digital transceivers are required to meet the demands of upcoming mobile and wireless communications. With the advent of 5G wireless communication, the expected data rate is going to exceed to an order of Gbps. This is possible with high spectral efficient modulation scheme or multiplexing techniques, where, maximum data can be transmitted within limited bandwidth. These schemes such as higher order quadrature amplitude modulation and orthogonal frequency division multiplexing result into envelope varying signals with high crest factor; which generates several linear and non-linear distortions in radio frequency modulators. The crest factor of any envelope varying signal is defined as the ratio of peak amplitude divided by r.m.s value of the signal. Moreover, the bandwidth requirement is still high to handle high data rate expected for 5G communication. Therefore, based on the availability of spectrum and the spectral efficiency of modulation scheme, both sub-6 GHz band and millimetre-wave band are recently opened for 5G communication. The power amplifiers an inevitable component of radio frequency chain which produces most of the distortion in transmitter. Therefore, one must restrict preceding stages such as RF modulator to add any distortion which may be further amplified with the PA. The conventional quadrature modulator uses mixers which are inherently non-linear components and can generate non-linear distortion. This will further reduce spectral efficiency. However, six-port reflectometer based modulator which does not use mixer is inherently linear and can be a good choice for upcoming wireless vi transmitters, where, spectral efficiency requirement is quite high. Moreover, the architecture of Six-Port Modulator is simple which uses conventional passive components and schottky diodes and therefore, can easily be scaled to millimetre wave frequency. Although, these modulators are inherently linear in practice, there are several linear and non-linear distortions which appear in case of envelope varying signals with high crest factor. This thesis presents an experimental methodology to characterize these distortions using an instrument based test-bed. A hardware prototype of six-port modulator was developed to modulate in-phase (I) and quadrature-phase (Q) components of baseband signal to RF carrier of 2 GHz. Later, appropriate digital pre-compensation schemes are implemented to mitigate the distortions generated in SPM. With the implementation of proposed scheme, the SPM hardware prototype presents an excellent performance in terms of reduced error vector magnitude and adjacent channel leakage ratio.