Massive MIMO in LTE with MRT Precoder : Channel Ageing and Throughput Analysis in a Single-Cell Deployment
Mobile data traffic is growing exponentially due to the popularization of smart phones, tablets and other data traffic appliances. One way of handling the increased data traffic is to deploy large antenna arrays at the base station, also known as Massive MIMO. In Massive MIMO, the base station having excessive number of transmit antennas, can achieve increased data rate by spatial-multiplexing terminals into the same time-frequency resource.
This thesis investigates Massive MIMO in LTE in a single-cell deployment with up to 100 base station antennas. The benefits of more antennas are investigated with single-antenna terminals in a typical urban environment. The terminal transmitted sounding reference signals (SRS) are used at the base station to calculate channel state information (CSI) in order to generate an MRT precoder. With perfect CSI, the results showed that the expected terminal SINR depends on the antenna-terminal ratio. It was also showed that with spatial-multiplexed terminals and 100 base station antennas, the maximum cell throughput increased 13 times compared with no spatial-multiplexed terminals.
Channel ageing causes inaccuracy in the CSI, the thesis showed that the variation in terminal SINR increased rapidly with less frequent SRS transmissions. When having moving terminals at 3 km/h, the difference between the 10th and 90th SINR percentile is 1 dB with an SRS transmission periodicity of 20 ms, and 17 dB with an SRS transmission periodicity of 80 ms. With 100 base station antennas and moving terminals at 3 km/h with an SRS periodicity of 20 ms, the maximum cell throughput decreased with 13% compared to when the base station has perfect CSI.
The result showed that the maximum cell throughput scaled linearly with the number of base station antennas. It also showed that having the number of spatial-multiplexed terminals equal to the number of antennas is a reasonable assumption when maximizing the cell throughput.
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