Performance improvement of dense WLAN based on adaptive channel access

Abstract

These days, increasing demands of mobile devices cause the surge of wireless traffic. Data transmission requests are frequently generated in some areas such as airport, shopping center, apartment complex and office building. At these areas, numerous access points ( AP ) are deployed with no planning in the limited space. Dense wireless local area network ( WLAN ) refers the situation where lots of uncoordinated APs are deployed. Also, more stations ( STA ), which generate huge traffic, are connected to each APs in the restricted space. Because frequency resource approved for WLAN is limited, dense WLAN system cannot accommodate all of transmission requests. In such a case, collisions between nodes, which contend for channel assignment simultaneously, are frequently generated. Excessive competitions occupy frequency resource and disturb neighboring transmissions. It results in the degradation of network throughput. Improving network performance is a significant problem of next generation WLAN protocol. To resolve this problem, various researches are under progress. Orthogonal frequency division multiple access ( OFDMA ) based carrier sense multiple access with collision avoidance ( CSMA/CA ) protocol is one of promising techniques for dense WLAN. In OFDMA system, the channel is divided as some narrow sub-channels. Because nodes can know sub-channel states concurrently, nodes can determine whether each sub-channel is available or not. Collisions among nodes will be reduced because nodes have more alternatives of resource. Although there exist some mechanisms that adopt OFDMA based CSMA/CA, most of them are not useful in uncoordinated scenario. They requires the centralized control to improve WLAN throughput in densely deployed situation. In this paper, a distributed OFDMA based CSMA/CA scheme is suggested. To extend channel access timings among nodes, the idea of p-persistent is applied. Channel information is reflected in $\gamma$ values which assist making some decision. Two permissions are assigned to transmission intended nodes: at the procedure of channel selection and the beginning of data transmission. With proposed algorithm, the opportunity of channel access is properly distributed and excessive contention between nodes are prevented. These lead enhancement of throughput degradation problem at dense WLAN. By using the penalty factor, proposed method adjusts transmission probability considering interferences which are caused from other transmissions. To find optimal penalty factor that can maximize network throughput, nonlinear programming is employed. Analysis to find the probability that a STA can transmit successfully in the time slot and the average throughput of a STA are studied. Detailed optimization process and effects of programming are also discussed. Simulation results show proposed method can improve performance of dense WLAN. One of conventional research that uses two dimensional backoff scheme for OFDMA related CSMA/CA protocol is compared with the proposed method. As a result, we can confirm that our proposition represents more ability of improving performance in uncoordinated dense WLAN environment.