Researches On Adaptive Link Control For 802.11 WLAN

In recent years, IEEE 802.11 Wireless Local Network(WLAN) has become one of the main solutions for massive deployment scenarios like large office, high density hotpots and etc. However, as WLAN shares unlicensed frequency bands with other wireless equipments and communication systems, it suffers inevitable inter-systems interferences. Meanwhile, as WLAN is a typical time division communication system without unified link control standard, the interferences and collisions caused by concurrent users are also very serious. Therefore, how to reduce these interferences and improve the network performance under massive deployment scenarios has become the critical challenge for the development of WLAN technology.Starting from the common framework for adaptive link control and the mathematic model for link performance analysis, this dissertation firstly summarizes three key elements that are fundamental for WLAN performance: physical layer rate(PHY_rate), transmission error rate and valid transmission duration. According to the three elements, the researches on the adaptive link control have been launched: using transmission channel control to avoid inter-systems interferences in unlicensed frequency bands and decrease transmission error rates; using transmission scheme control to match the time varying changes of wireless link, reduce transmission error rate and promote the PHY_rate; using transmission timing control to solve the problem of multi-users collision and interference, thus extend valid transmission duration and improve the channel utilization ratio. The main contributions are elaborated as follows:1. An optimization model for WLAN’s transmission channel selection basing on spectrum sense network is proposed. The model takes advantage of sensing network’s features of distributed computing and system synthesis to improve the ability of cooperative spectrum sensing. In the model, sensing node and sink node are designed, and the communication process between nodes is defined. When the sensing reports of sink nodes are assembled to the central control node, the coloring theory and simulated annealing algorithm are applied to deduce the optimal channel combination for the WLAN. Then through the network optimization mechanism, the entire transmission channel can be adjusted adaptively, which greatly degrades the interference level of the whole network. Numerical results and field tests show that comparing with the method of no spectrum sensing and spectrum sensing using only AP, this model can get the more accurate sensing ability on wireless environment and the more flexible ability on transmission channel adjustment, which would benefit to reduce the interferences between inter-systems, decrease the transmission error rate, and improve the performance of the network finally.2. An adaptive Modulation and Coding Scheme(MCS) selection algorithm based on the Minstrel is put forward. It comprises of the following three main parts:①SNR Calibration is a mechanism to estimate the downlink SNR from transmitter side via probing. The more accurate of the SNR estimation brings the more rational of the MCS selection, thus the transmission error rate decreases.②MCS Protection builds a MCS selection experience table according to the statistics of historical transmission and the SNR estimation, which limits the choice of MCS of transmitter in the MCS range and prevents MCS from misadjusting, then guaranteed PHY_rate of transmission.③ Multi-stream Adaption presents a probing algorithm based on the Multi-Armed Bandit, which can select the MIMO scheme adaptively according to the wireless environment, and solve the problem of single stream or multi-stream mis-selecting.The experiments demonstrate that compare with the Minstrel, this algorithm can improve the rationality of MCS selection and the performance of the WLAN in the massive deploy scenario with fierce interference and collision.3. A series of adaptive technologies related to the transmission timing parameters of the MAC layer are proposed.①Transmission Opportunity(TXOP) Adaption. As there is always an optimal TXOP value for specific traffic models, TXOP Adaption adjusts the TXOP Limit parameter adaptively according to the transmission queue length of the AP. Through TXOP Adaption, the users or services with high priority can be scheduled timely, which enhance the Qo S for WLAN and the utilization of the wireless channel.②Contention Window(CW) Adaption. According to the differentiation of using the different CW parameter under the different number of concurrent users, CW Adaption is proposed. And by consideration of both virtual collision rate and channel occupancy rate, the length of CW can be chosen adaptively to realize the optimal prediction throughput, thus improve the performance of the network in high density scenario.③Guard Interval(GI) Adaption. By choosing the GI mode dynamically, the transmitter can adapt to the channel state and extend valid transmission duration, which is also benefit to the performance enhancement.The key technologies proposed in this dissertation have been utilized in commercial infrastructure successfully.