As more small cells are added to the HetNets, there will be cells not serving any active user at a given time. Even though no traffic is carried in these cells, the downlink common channels and signals will still be transmitted, generating unnecessary interference to neighboring cells. Moreover, the PAs of these cells will be kept on all the time consuming excessive energy. Opportunistic small cells will help address these issues and facilitate HetNets densification.
When there is no user in the serving area, opportunistic small cells will enter the dormant state and stop transmission on the downlink. The PA will be shutdown to improve the energy efficiency and downlink interference to adjacent cells will be removed. In the mean time, the small cell will continue to monitor uplink signals in the dormant state. Upon detection of uplink signals of approaching terminals, the small cell will become active and restore downlink transmission. The terminal detection and activation are performed in advance to ensure IC-capable terminals detecting the weak signals from small cells with range expansion. Opportunistic small cells can be implemented today without standard changes.
One key consideration for small cell deployment is the backhaul. Wireline backhauls such as fiber and Ethernet are not always available at the desired locations. Wireless backhauls using microwave and millimeter-wave require line-of-sight (LOS) to the aggregation point which may be a challenge in urban areas with high-rise buildings. Relays using NLOS wireless backhaul enable small cell deployment in backhaul constraint areas.
Qualcomm Research has implemented and demonstrated prototype opportunistic small cells and relays with LTE-based NLOS wireless backhaul in our OTA networks in San Diego.