Analysis of LTE in Two-Path Vehicular Repeater Channels

In recent years, many public high-speed trains have been upgraded with active in-train repeaters in order to improve mobile service coverage for train passengers. With a repeater in place, the user equipment perceives a wireless channel with a direct path and an indirect path through the repeater delayed by several micro seconds. In vehicular scenarios, the pick-up antenna experiences a wide range of signal levels. Due to the limited output power of the repeater, the paths may arrive with similar power, which can be a problem for OFDM based systems like LTE. Drive tests in LTE have shown that uplink performance degrades, even if the delay stays within the cyclic prefix. In order to gain a better understanding, we conducted a laboratory experiment to measure, quantify, and analyze the impact of two-path channels, with different delays and power levels, onto a live LTE network. Results show uplink performance degradation and increased transmit power even for delays smaller than the cyclic prefix due to non-optimal time-synchronization. In the system under test this degradation already starts at a path power difference of 15 dB and reaches up to 50% at equal path power levels.