Frequency-dependent modulation and coding rates for LTE link adaptation in static conditions

Abstract

OFDM wireless cellular systems rely on link adaptation in order to match the Modulation and Coding Scheme (MCS) of the transmission blocks with the dynamic channel conditions experienced at the receive side. For simplicity, Long-Term Evolution (LTE) systems consider a single MCS format per codeword, which is optimized according to the average frequency response over the user's allocated bandwidth. When the channel coherence bandwidth is smaller than the user bandwidth, the constituent codeblocks experience significantly different channel responses, thus rendering link adaptation less effective. This paper proposes a frequency-dependent link adaptation mechanism whereby codeblocks are independently assigned different MCS values that can be fine-tuned to the frequency characteristics of the channel. Post-detection SINR values at the receive side are exploited (instead of CQI values), along with horizontal mapping of resources with two variants: direct mapping and diversity mapping. A suitable in-band signaling scheme for the codeblock sizes and modulations is also described. Link-level simulations show moderate increases in median throughput, but significant improvements in instantaneous throughput, for a broad range of SINR conditions. The proposed scheme can be particularly beneficial when very large signal bandwidths are used as foreseen in future 5G cellular systems.