This paper presents a comparison between coherent and non-coherent multi-antenna systems for frequency and time selective channel. Performance of a coherent system mainly depends on channel estimation at the receiver. This estimation generally involves insertion of pilot symbols in a frame and thus a loss of spectral efficiency. Differential space time modulation (DSTM), which is an extension of SISO differential techniques to MIMO case, seems to be an alternative solution to attain high spectral efficiency and good performance in fast time-varying channels, where estimation of channels may be difficult. When comparisons between DSTM and coherent system are provided, perfect channel knowledge at the receiver is generally assumed. In this paper, we propose to include realistic channel estimation for coherent system as well as degradation of spectral efficiency due to necessary pilot insertion. Simulation results show that in very fast time-varying channel non-coherent systems outperform systems with channel estimation. Besides, performance of several non-coherent receivers are provided for different channels and with channel coding.
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