A comparison of differential and coherent reception for a coded OFDM system in a low C/I environment

This paper compares the performance of differential and coherent QPSK for a high data rate slow frequency hopping (SFH) OFDM system in a channel with strong co-channel interference. Two-branch diversity reception and channel coding are included in the comparison. The coherent system uses a rate 1/2, 64-state convolutional code and includes a relatively high amount of pilot symbol overhead to facilitate the measurement of channel state information (CSI) including the instantaneous co-channel interference level. The CSI is incorporated in the combining and decoding process to improve the decoded bit error rate performance. Although the differential system does not support the measurement of the co-channel interference level, the savings in pilot symbol overhead enables the use of a more powerful rate 1/3, 64-state convolutional code. By using a rate 1/2 code in the CSI-coherent system and a rate 1/3 code in the differential system, the information data rates of the two systems are similar. Results indicate that CSI-coherent system performs better than the differential system due to the inclusion of accurate CSI in the combining and decoding process.