| The characteristics of Wireless Local Area Networks(WLANs), such as high data rate, low cost and convenient use, have made it a main way to access to the Internet. The scale and density of the system increase with the number of wireless broadband users, which would heat up the competition. Meanwhile, inter-cell interference reduces the access opportunity for each node and infects resource utilization.To address this issue, researchs have been done to improve access efficiency on physical rate, power, carrier sensing and MAC mechanism. Currently, high efficiency access technology still includes these problems, 1). The spatial reuse is low and the access opportunity for each node is limited. 2). Uneven transmission caused by the edge stations would reduce the spectrum efficiency. 3). In dynamic bandwidth mode, one user would not occupies the whole channel bands and the spectrum efficiency is low. In order to solve these problems, this paper study the high efficiency access technology based on National Basic Research Program of China(973 program). This paper mainly talk about how to improve spatial reuse and spectrul efficiency of the high density WLANs.The paper first establishs a throughput model for WLAN based on stochastic geometry and proposes a method named as Dynamic Sensitive Control(DSC) to improve spatial reuse. Then we proposed an OFDMA access technology with relays and an opportunistic dynamic bandwidth channel selection algorithms in order to improve spectrum efficiency. The main content includes:1. A throughput model for WLAN based on stochastic geometry has been established. The WLAN equipment are distributed irregularly, then both the uplink and downlink interference models are established in which APs(Access Points) and STAs(Stations) are distributed according to random Poisson distributions in this network. Data transmitting rate is selected based on the Signal to Interference plus Noise Ratio(SINR) of each link and the mean throughput in uplink and downlink is calculated as well. This model can be used to measure the proposed algorithm.2. DSC algorithm which can improve the network spatial reuse has been proposed. Firstly, STAs are divided into Cell Center Stations(CCS) and Cell Edge Stations(CES) according to the SINR in each cell. CES adjust its sensitivity for channel sensing, i.e. carrier sensing carrier threshold according to the average RSS(Receive Signal Strength) of AP Beacon signal. AP confirms its location and the density of APs around according to the RSS of Beacon signal, and adjust its carrier sensing carrier threshold according to mean value of the weakest RSS and other parameters which can reflect the AP density. Simulation results show that by utilizing DSC algorithm in this study, the uplink average throughput in WLAN can be improved by 3 times and the downlink throughput is improved by nearly 2.5 times.3. An OFDMA access mechanism based on rely stations(RSs) has been proposed. Some of the STAs are changed to perform as RSs, and the ODFMA access mechanisim is utilized in downlink in order to provide orthogonal carrier group without interference for STAs and RSs in the same cell, then the spectral efficiency is improved. Based on the utilization of network spectral efficiency, the transmitting power and density of both the APs and the RSs are analyzed and simulated.4. Dynamic bandwidth cooperative access method based on the collision probability has been proposed as well. Firstly, the factors of the spectrum utilization model is calculated, the idle period expectation are present as well. Secondary user choosees the channels with the maximum transmit period and decides proper channel bandwidth and channel number. Simulation results show that the performances of the proposed algorithms improves the throughput of the secondary user and dose not improve collision probability. |
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