Abstract—In this work, a new method in time-diversity is used with an Orthogonal-Frequency-Division-Multiplexing (OFDM) system to enhance the bit-error-rate (BER) performance without increasing the signal bandwidth or decreasing the transmission rate. The diversity encoder is used to map the modulated symbols to diversity symbols for each OFDM subcarrier. The modulated symbol appears in N diversity symbols and it is transmitted on the same subcarrier through N different OFDM symbols. A Multiple- Input-Multiple-Output (MIMO) transmitter that works in spatial multiplexing mode, is used in this system to increase the spectral efficiency of the transmitted signal. The diversity gain of the proposed time-diversity method is equal to N that the order of the Walsh matrix. Symbols interleaving with programmable widths are used to get independent fading gain at arbitrary Doppler frequency shifts. The proposed system works in a flat fading channel with Doppler frequency shifts from 50 Hz to 7 kHz. In the receiver, a diversity decoder maps the vectors of diversity symbols to vectors of modulated symbols. The performance of the diversity decoder is the same as the performance of the Maximal-Ratio-Combiner (MRC) receiver. The proposed system increases the spectral efficiency of the transmitted signal and increases the Signal-to-Noise Ratio (SNR) of the decision variable by using time diversity without increasing the transmission bandwidth. The proposed system is simulated and implemented in FBGA. The performance of the implemented system is the same as the performance of the N channels diversity system with the Maximal-Ratio- Combiner (MRC) receiver.
 

Keywords—time-diversity, diversity encoder, orthogonalfrequency- division-multiplexing system, maximal ratio combiner, multiple-input-multiple-output system, QR decomposition detector


Cite: Eman Zakaria, Ashraf Y. Hassan, H. EL Hennawy, and Abdelhady M. Abdelhady, “Spectral-Efficient and Power-Efficient MIMOOFDM System with Time Diversity for Flat Fading Channel with Arbitrary Doppler Frequency Shift," Journal of Communications, vol. 19, no. 2, pp. 53-64, 2024.

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