Over-the-Air Tests of High-Speed Moving LTE Users in a Reverberation Chamber

High-speed trains are nowadays a reality in most countries. Internet services and connectivity are required by the passengers on board using smartphones and laptops. Nevertheless, the higher the speed, the bigger the issues that operators have to face to implement mobility and to reduce the effect of fast fading signal and Doppler shift, which may cause throughput reduction and problems to access the network. In the past, operators tried to optimize the cellular access to UMTS cells covering railways. Now that 4G networks have become mature and deployed, a similar approach needs to be optimized and implemented to LTE cells covering the same targets. We present the results of a testing campaign replicating real “high speed train” propagation scenarios, like a train running in open space or in tunnels with the help of a reverberation chamber. The chamber was partially filled by absorbing panels and equipped with rotating stirrers in order to approach the multipath propagation and to replicate the fading conditions that is typical of smartphones in a train coach. The effects of Doppler and its fast variations are presented, together with an analysis of the efficiency of different transmission solutions for tunnels (single input single output – SISO – cells propagating inside them). The testing sessions were performed under a collaboration program between Telecom Italia (in the paper referred as “operator”), Nokia Networks and Università Politecnica delle Marche. After the experiments in our laboratory, some features were introduced in the operator’s live network, covering the high-speed railway between the two Italian cities of Piacenza and Bologna.