Research On Medium Access Control Technology For WiFi In Dense User Scenarios

Due to the low cost, high speed, mobility, easy installation, easy maintenance and other aspects of advantages, WiFi networks have been widely used as the main access to mobile Internet, resulting in network coverage continuing to expand, the number of users increasing rapidly. WiFi adopts CSMA/CA mechanism and SU-MIMO technology, in which only one node is allowed to access the channel for communication at a time. Therefore, in dense user scenarios, system performance will deteriorate severely due to severe channenl competition among the STAs.This thesis focuses on the dense user scenarios which consist of one AP and multiple STAs. First, the reasons that why it is difficult for users to access the channel in dense sce-narios, frequent collisions, throughput degradation and other issues are analyzed. It points out that the existing DCF binary exponential backoff algorithm is not suitable for dense WiFi networks. Then, two solutions to solve this problem are proposed.1. Based on the existing 802.11 CSMA/CA access mechanism, the DCF (Distributed Coordination Function) binary exponential backoff algorithm is improved. The improved algorithm uses a parameter (ration of channnel busy time and channel idle time) to estimate the channel congestion, and then update the contention window adaptively. In addition, a two-stage backoff improvement algorithm is proposed. By controlling the number of the competing STAs, the proposal reduces the probability of conflict to improve system perfor-mance.2. A down/up-link Medium Access Control (MAC) protocol called UDL-MU-MIMO is proposed to enhance the performance of IEEE 802.11 WLANs by exploring the multiuser spatial multiplexing technique. In the downlink, we implement an IEEE 802.11ac compliant MU-MIMO transmission scheme to allow the AP to simultaneously send frames to mutiple STAs. In the uplink, multiple STAs are able to transmit frames to the AP simultaneously. The process of multi-user access, and improved MAC layer control frames are describled elabo-rately in the thesis. We also propose an analytic saturation model to validate the protocol. The result shows that the scheme can effectively improve the transmission efficiency of the system, and it is suitable for ultra-dense user scenarios.