LTE radio access: An overview

This chapter provides an overview of some of the most important components and features of Long-Term Evolution (LTE). These include transmission schemes, scheduling, hybrid ARQ with soft combining, multi-antenna support, and spectrum flexibility. The LTE downlink transmission scheme is based on orthogonal frequency division multiplex (OFDM). Due to the relatively long OFDM symbol time in combination with a cyclic prefix, OFDM provides a high degree of robustness against channel frequency selectivity. Moreover, OFDM with its inherent robustness to frequency-selective fading is attractive for the downlink, especially when combined with spatial multiplexing. At the heart of the LTE transmission scheme is the use of shared-channel transmission. In this transmission, the time-frequency resource is dynamically shared between users. The use of shared-channel transmission is well matched to the rapidly varying resource requirements posed by packet data and also enables several of the other key technologies used by LTE. Finally, one important part of the LTE requirements in terms of spectrum flexibility is the possibility to deploy LTE-based radio access in both paired and unpaired spectrum that is LTE should support both frequency- and time-division-based duplex arrangements.