Adaptive Modulation With CAZAC Preamble-Based Signal-to-Noise-Ratio Estimator in OFDM Cooperative Communication System.

This paper presents an adaptive modulation in a single-input-single-output (SISO)-OFDM-based cooperative system that employs a Constant Amplitude Zero Autocorrelation (CAZAC) preamble-based SNR estimator. A novel CAZAC preamble-based SNR estimator that does not incur a throughput penalty is derived by exploiting a modified CAZAC preamble structure for the OFDM system. The proposed CAZAC SNR estimator is used to estimate the channel quality by finding the instantaneous SNR of the received cooperative data, which is sent to the transmitter via feedback channel. The modulation scheme is then changed according to the estimated SNR. The performance of the CAZAC preamble-based SNR estimator exhibits less than 0.02 dB and less than 0.2 dB bias when communicating over AWGN and SUI-5 channels, respectively. The proposed adaptive modulation scheme with the CAZAC SNR estimator significantly improved the channel throughput in comparison with the corresponding benchmarker scheme dispensing with adaptive modulation that invokes $M$ -PSK and $M$ -QAM. More explicitly, the proposed adaptive modulation system with the CAZAC SNR estimator that invokes $M$ -PSK exhibits significant improvement by about 5 dB at BER $= 10<^>{-3}$ in comparison to its corresponding benchmarker scheme dispensing with adaptive modulation when communicating over the SUI-5 channel. On the other hand, the relay-aided cooperative transmission exhibited an approximately 6 dB SNR gain in terms of channel throughput in comparison to the corresponding non-cooperative benchmarker scheme, and benefited from spatial diversity.