Low latency random access with TTI bundling in LTE/LTE-A

To reduce the uplink channel access latency in LTE/LTE-A, we propose a Transmission Time Interval (TTI) bundling scheme for the random access procedure. With the proposed method, a UE sends multiple preambles in consecutive subframes in order to increase the success rate of random access and hence to reduce the latency. We introduce a Semi-Markov model to accurately model and analyze the random access mechanism with TTI bundling. With this model, we formulate the access latency as a function of the number of TTI bundles and select the optimal value which minimizes the channel access latency. The proposed Semi-Markov model is validated against simulation and the performance of the TTI bundling method is also evaluated. We find that channel access latency can be significantly reduced when the preamble collision rate is not high.