Study on system latency reduction based on Shorten TTI

System latency is one key metric that guides LTE design. Shortening TTI by reducing subframe length along with backward compatibility design can bring significant latency gains for legacy LTE systems. In general, it is believed that the shorter TTI is, the more gain can be achieved. However, due to the hardware limitation as well as the implementation complexity, it is not easy for ultra-short TTI lengths to adapt to legacy LTE. Therefore, to maximize the latency gain with relative longer TTI length (slot level TTI), this paper proposes another method including reducing UL access delay and HARQ ACK/NACK RTT. It can be verified that combining slot TTI, this method can reduce 45.58% and 40.56% latency at low traffic load for cell edge and center users, higher than the 40.29% and 39.51% latency reduction brought by very short TTIs.