Study On Pilot Pollution Elimination Of Multi-cell TDD System

In modern cellular system, the inter-cell interference has become an importantfactor affecting transmission rate. Coordinated beamforming(i.e. CB) is a technology ofCoordinated Multi-Point transmission(i.e. COMP), and it is a key technology to reduceinter-cell interference. CB does not require the exchange of user information, but it stillneeds the exchange of channel side information at the transmitter (i.e. CSIT). The use oflarge-scale antenna technology can reduce the requirement of exchange CSIT, thistechnology can eliminate the interference from other cell users through the use of localchannel precoding vector. Hence, by simply increasing the number of antennas at eachtransmitter can alleviate the need for exchanging CSIT between base stations.However, this conclusion is based on the accurate channel state information(i.e.CSI) obtained from the base station side. In TDD system, through the use of channelreciprocity, we can get downlink CSI by estimating uplink channel in the base station.In practice, it is difficult to obtain a sufficient number of orthogonal pilot sequencessince pilot sequence length is limited by fast changing channel. As a result, pilotpollution will occur for non-orthogonal pilot sequence from users in the neighboringcells. For pilot pollution, CSI can not be guaranteed accurate. Therefore, there is anurgent need for a method to weaken or even eliminate inter-cell pilot pollution.In this paper, the completion of work includes:First, we analyze the reason of pilot pollution in TD-LTE system, and assess thepollution: we simulate the estimation error when using orthogonal and non-orthogonalpilot sequences in the same cell, and show the impact of cell radius and the number ofBS antennas on pilot pollution. The result shows that, orthogonal pilot sequences caneliminate pilot pollution. Then, based on the analysis of multipath channel, we introducea coordinated multi-cell pilot allocation scheme. Compared with randomized multi-cellpilot allocation program, the scheme can weaken the inter-cell pilot pollution effectively.Last, in the time domain, we analyze a time-shifted pilot scheme and verify itssuperiority, it is different with existing TDD aligned pilot transmission scheme. Thisscheme is based on partitioning cells into groups of cells, cells belong to different groups use different shifted frame structure. The simulation result shows that,time-shifted pilot scheme can eliminate pilot pollution, Verify the superiority of thisprogram.