Position based limited feedback scheme for railway MU-MIMO systems

In future railway communications, the system architecture is highly likely to be based on the mobile relay structure. However, during the transition period, onboard passenger equipments can only directly connect to wayside public mobile networks to get wireless services. As an efficient technology to increase capacity widely used in long term evolution (LTE) public mobile networks, multi-user multiple input multiple output (MUMIMO) requires channel state information (CSI) of users at base stations to suppress inter-user interference, which poses a bandwidth burden on feedback links. Thus, how to reduce the MU-MIMO feedback load becomes an urgent problem. In railway scenarios, onboard passenger equipments of a user set almost have the same signal to noise ratio (SNR), which provides a new way to reduce the MU-MIMO feedback overheads. Accordingly, we propose a position based limited feedback scheme for railway MU-MIMO systems, in which dedicated feedback resource positions are pre-assigned to codebook vectors. A resource position, where users feed back channel quality indicators (CQIs), indicates a special codebook vector. Then, overheads due to precoding matrix index (PMIs) are saved. Even if bandwidth resources assigned to feedback positions may be insufficient to supply every user with a feedback chance, it still has little impact on the total capacity whether the user with the largest CQI in an active user set is selected in highly populated railway scenarios. Besides, a novel performance criterion called feedback efficiency is proposed. Simulation results demonstrate that the proposed scheme can achieve higher feedback efficiency compared with the conventional scheme.