Impact of Backhaul Subframe Misalignment on Uplink System Performance of LTE-Advanced Relay Networks

Most work on relaying uplink assumes a strictly synchronized subframe partitioning between relay and access links among all eNBs. However, according to 3GPP guidelines eNBs can independently decide on different backhaul subframe configurations to adapt to, e.g. traffic variations, which then implies a backhaul subframe misalignment in the system. This configuration results in a new type of interference, referred to as relay-to-relay interference. In general, uplink interference characteristics will be significantly different compared to conventional fully synchronized configuration since the transmissions of relays and user equipments will interfere with each other. Accordingly, uplink power control parameters should be properly adjusted. In this work, we evaluate the impact of backhaul subframe misalignment on the uplink system performance of relay deployments within the LTE-Advanced framework considering 4 and 10-relay urban and suburban deployments. Besides, we investigate different inter eNB coordination and subframe alignment scenarios. Comprehensive results show that a backhaul subframe misalignment can cause severe losses especially in cell coverage depending on the degree of coordination possible, and the availability of directional antennas at relays for backhauling. It is further shown that a proper power control optimization can cope with such performance degradations in urban deployments. As the cell-edge eNB-served user equipments are power limited in suburban deployments, the impact of power control optimization can be limited. Therefore, relay cell range extension is as well utilized in suburban deployments so that these user equipments are then served by relays. However, the performance degradation cannot be fully compensated by power control and cell range extension in suburban deployments, and thus a certain level of inter-eNB coordination is necessary.