Research On Interference Alignment Technique In High Speed Railway Scenario

With the development of high speed railway communication technique, more diversified information service provided by the train and the requirement for data rate is increasing at an unprecedented speed. To meet the requirement of increasing capacity, D2D (Device to Device) and mobile relay techniques will be applied in railway communication system, which inevitably makes the interference environment more complicated. Interference alignment (IA), as a novel technique of interference management, not only can effectively eliminate the interference, but also can improve the capacity of interference network.This paper researches on the interference problem of various scenarios in high speed railway communication system, i.e., interference channel in D2D scenario for train, passenger access in network by the APs (Access Points) installed in train, and mobile relays communicate with the base station. Moreover, this paper proposes some schemes of interference cancellation.1. For the problem of interference among D2D communication link and the cellular link, this paper studies IA in multi-user interference channel and proposes a novel IA scheme based on gradient decent algorithm. The scheme of distribute IA iteratively optimizes the precoding matrix and interference suppression matrix to achieve interference alignment. However, this method requires strictly channel reciprocity and it converges slowly. So, this paper proposes IA scheme based on gradient decent algorithm, which relaxes the assumption of channel reciprocity by limiting the optimization only at the transmitters’side. More importantly, the simulation results suggest that the gradient decent algorithm converges faster and has smaller remnant interference than other methods.2. For the problem of interference among APs in train, this paper studies IA in random access networks (RANs) and proposes an opportunity interference alignment (OIA) scheme based on cross layer design. In physical layer, the IA scheme based on minimum interference leakage (IL) is employed. In MAC layer, each user applies the opportunism scheme based on its effective generating interference, i.e., the LI metric, to other RANs. In each time slot, each user transmits a packet if the instant IL metric is smaller than a certain threshold. The simulation results demonstrate that the proposed OIA achieves much better throughput than the conventional techniques.3. For the problem of inter-carrier interference (ICI) in the scenario of mobile relays communicating with the base station, this paper studies the method of eliminating ICI and proposes a joint ICI cancellation scheme. In this scheme, the cosine windowing is exploited in transmitter. As a result, the frequency-domain channel matrix reshaped by the cosine windowing can be closely approximated to a strictly banded matrix, which cancels most of ICI. Furthermore, the complex exponential basis expansion model (CE-BEM) is employed for the channel estimation to cancel the residual ICI and enhance BER performance.