Influence of Impairments in Coordinated Cellular Networks and Compensation Strategies

The fifth generation (5G) of communication systems will need to support new applications with a variety of requirements, such as high data rates, low latency, enhanced indoor coverage and a large number of devices. It is strongly believed that the expected demands can be satisfied by combining the usage of additional spectrum at higher frequency bands, small cells, flexible air interfaces and spectrally efficient transmission by using multiple antennas. In terms of spectral efficiency enhancement, base station cooperation is a powerful technique, as it has the potential to eliminate inter-cell interference and is therefore considered as a promising candidate for 5G. The present article provides an overview on the influences of real-world channel and synchronization impairments on cooperating base stations and further focuses on new strategies for compensating the performance losses. Recent analytical results and numerical evaluations reveal the importance of accurate channel knowledge used for data precoding and synchronization among the base stations. The transition from theoretical to practical coordination concepts can be realized by impairments' mitigation through multi-cell channel estimation, flexible clustering and user selection as well as adaptive feedback compression. It is further shown that advanced channel prediction can compensate the performance degradation, whereas precise synchronization among distributed base stations must be guaranteed as well. In this way, limitations in mobility, feedback delay and number of supported users can be significantly relaxed. Complementary to the impairments' mitigation, robust coordination schemes relying on partial channel knowledge are also proposed for as an alternative solution to the classical schemes which require full channel knowledge.