RF impairments: Research challenges and applications in next generation mobile communications

Wireless connectivity has truly become ubiquitous over the last few years, as can be concluded from the increasing number of electronic devices that are being equipped with wireless communication capabilities. Nowadays, the wireless application range has been extended to a wide variety of consumer lifestyle areas. This causes wireless systems to move towards the commodity markets, where price pressure is high. This creates a drive for low-cost solutions. On the other hand, there is a persistent demand for increasing performance of wireless solutions, since the same throughput and quality of service are demanded as that of the wired solutions they are replacing. To meet these requirements, wireless communication systems are continuously applying wider bandwidths, larger signal dynamics, and higher carrier frequencies. This results in an ever-increasing demand on the performance of radio frequency (RF) front-ends, which at the same time have to be low-cost and power-efficient. Since the RF technology is, consequently, pushed to its operation boundaries, the intrinsic imperfections of the RF IC technology are more and more governing the system performance of wireless modems. Direct-conversion transceivers can offer highly integrated low-cost hardware solutions for wireless communication networks. Sch transceivers, however, are also very sensitive to various radio frequency (RF) impairments, which can considerably limit the outage performance. The most prominent imperfections are the nonlinear distortion due to the mixer and power amplifier (PA) nonlinearities, image-frequency interference due to I/Q mismatch, non-constant group delay distortion in bandpass filters, and phase noise (PHN) and carrier frequency offset (CFO) in the oscillators. These impairments, if not properly understood and taken into account, will cause spectrum regrowth, which may violate the spectrum masks imposed by the regulatory bodies, as well as interference and noise, which degrade the transmitted or received signal qualities and thereby limit the performance of the whole communications system. Most of the existing studies in the open literature assume an ideal RF front-end and hence neglect the impact of such practical RF hardware limitations