Real-time sub-carrier Adaptive Modulation and Coding in wideband Orthogonal Frequency Division Multiplexing wireless systems

Abstract

The increasing demand for high speed wireless connectivity at low cost proposes new challenges for communication systems designers to implement solutions that increase the data rate by utilizing the limited radio resources more efficiently at a low additional complexity. Sub-carrier Adaptive Modulation and Coding (AMC) exploits the high frequency diversity in wideband Orthogonal Frequency Division Multiplexing (OFDM) channels to obtain higher data rates. While prior work has discussed the value of sub-carrier AMC from a theoretical perspective, this work presents the design and performance of a real-time sub-carrier AMC system. We describe our OFDM transceiver prototype, which implements real-time subcarrier AMC for a wideband wireless channel. We discuss how our design achieves accurate and consistent Signal-to-Noise Ratio (SNR) estimates, which are critical for the success of AMC. We compare the performance of sub-carrier AMC with a non-adaptive scheme that assigns the same modulation and channel coding to all sub-carriers that can support that modulation and coding for the target Bit Error Rate (BER). For a conservative comparison, we compare against the uniform modulation/coding assignment that achieves the highest data rate. Our experiments over the wireless channel show that for a target coded BER of 10-5, our system achieves average data rates of 308.3 and 237.1 Mbps across a variety of Line-of-Sight (LOS) and Non Line-of-Sight (NLOS) locations respectively, which result in 34% and 40% gain over the best non-adaptive scheme. Equivalently, such data rate gain from AMC translates to an SNR improvement of 3 dB. Finally, our implementation of AMC incurs a low overhead of 1.1% of the data rate, and a reasonable complexity, occupying 9.95% of the total transceiver gates on the Field Programmable Gate Array (FPGA).