Uplink PHY Design with Shortened TTI for Latency Reduction

Shortening the transmit time interval (TTI) has been considered in 3GPP LTE as an effective way for latency reduction. In this paper, we discuss the physical layer design aspects for uplink data transmissions with short TTI (sTTI). Here, an sTTI consists of one or a few SC-FDMA symbols, and its time duration is less than 1 ms. Different methods of demodulation reference signals (DMRS) design, DMRS sharing and DMRS multiplexing, are introduced to reduce the reference signal overhead when shortening the TTI length. Our link level simulation results show that both methods can efficiently reduce the DMRS overhead, and thereby improve the system throughput. Moreover, we show that due to better channel estimates, a shorter TTI, e.g., 2-symbol TTI, is more robust towards high Doppler scenarios. In contrast, for a 7-symbol TTI, which is 0.5 ms, more than one reference symbols can be inserted in order to improve the channel estimates in high Doppler scenarios. Finally, a discussion is given on how several short TTIs can be multiplexed into the frame format used for LTE.