Small Cell Wireless Communications Over Licensed and Unlicensed Bands.

摘要

Many cellular user equipments (UEs) today are able to access both the unlicensed band (e.g., via WiFi) and the cellular licensed band. However, in most regions of the world, they are not allowed to access both bands simultaneously for data communications---users can only choose either WiFi or cellular. In addition, currently UEs do not consider channel or interference conditions in their automatic band selections. For instance, many smart phones today simply choose WiFi over cellular when both are available, even though the WiFi connection may be weak. The Small Cell Forum recently proposed a type of small cells that simultaneously access both licensed and unlicensed bands. This study investigates the strategies for small cells to balance their traffic in licensed and unlicensed bands, so that small cells can optimally use the two bands based on their respective channel and interference conditions. The goal is to maximize the sum utility (i.e., user satisfaction) of small cell and non-cellular WiFi WLAN users while keeping the interference from small cell to macrocell below predefined thresholds. The optimal small cell traffic balancing scheme is obtained and implemented in a network simulator which considers the activities and interactions of macrocell, small cell and WiFi WLANs. Simulation results demonstrate that the proposed scheme significantly increases sum utility of all macrocell, small cell and WLAN users, compared with the current practices where users can choose only one band (licensed or unlicensed).;One of the most widely-deployed small cells is the WiFi hotspot. Due to the distributed nature of the WiFi channel access scheme, hidden terminals have significant impact on the WiFi performance. Current WiFi performance analysis are based on simplified assumptions on topology, such as networks without hidden terminals or networks where every station observes the same number of hidden terminals. In practice, the number of hidden terminals observed at a station depends on the locations of all stations in the WiFi hotspot. In this study, the impact of hidden terminals on WiFi hotspots with arbitrary topologies are modeled to obtain the throughput for each individual station. The model is validated by simulations and measurements. Results show that WiFi network topology impacts the number of hidden terminals observed at each station, resulting in throughput disparities among WiFi stations. It is also shown that the RTS/CTS mechanism does not eliminate the hidden terminal problem.