Is the Cyclic Prefix Needed in Massive MIMO?

University essay from Linköpings universitet/Kommunikationssystem


Massive multiple-input-multiple-output (MIMO) is a wireless communication technologythat usesa large number of antennas at the base station and servesmultiple terminals over the same time-frequency resource.This technique can achievehigher data rates than existing communication technology, which only serves one terminal per resource. That is why Massive MIMO is considered a promising candidate for 5G. Orthogonal frequency division multiplexing (OFDM) can be used for transmitting information at different sub-channels. The cyclic prefix (CP) is a repetition of the last samples in a symbol, which is appended at the beginning of the symbol.It serves as a guard interval between consecutive symbols to avoid inter-symbol interference (ISI) and to make sub-channels orthogonal. In this thesis it is proposed to shorten the CP length in Massive MIMO. The shortening of the CP length will increase the effective spectral efficiency but also create additional interference. This trade-off is investigated. A simulation based study is performed to analyse the effective achievable rate of anuplink Massive MIMO system in a single-cell scenario when 10000 Gaussian symbols are transmitted.In the simulation, the number of sub-channels is 128. They are transmitted through a channel with 10 taps. With the classical CP length9samples and a massive MIMO base stationwith 50 antennas serving 3 terminals,the effective achievable rate was 3.863 bits/s/Hz. It was found thatthe effective achievable rate is maximized when the CP length is shortened to 6 samples; the effective achievable rate then became 4.112 bits/s/Hz.In the same system when 100 antennas are used, the corresponding effective achievable rates are 4.791 bits/s/Hz and 4.895 bits/s/Hz with an optimum CP lengthof5 samples. It is shown that the optimum CP length in Massive MIMO is not equal to the number of taps minus one which is the conventional choice. Yes, the CP is needed in Massive MIMO, but it can be shorter than conventionally.

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