A Low Latency Transmission Method for Massive MIMO Systems with Low Control Signaling Overhead

In this paper we consider the uplink of Massive MIMO systems where the channel capacity is almost deterministic and varies slowly over time. We exploit this fact, by appropriately choosing the length of the codeword only once based on an initial estimate of the uplink channel capacity fed back by the base station (BS). The user terminal (UT) then transmits the codeword over consecutive coherence time intervals without waiting for any ACK/NACK from the BS. The BS sends only a single ACK/NACK after it completes decoding. The proposed method therefore has significantly less latency and signaling overhead when compared to stop-and-wait type HARQ methods (as in LTE uplink) where a lot of delay is introduced as the UT needs to wait for the BS to signal the acknowlegement (ACK/NACK) for every retransmission. Analysis reveals that for sufficiently large number of antennas at the BS, the average codeword length of the proposed method (in number of coherence intervals) is the same as the average number of retransmissions (in number of coherence intervals) required by incremental redundancy (IR) based HARQ and also that the energy efficiency of the proposed method and IR-HARQ is the same.