Admission Control Of Bandwidth Request In Wireless Metropolitan Area Networks

As an up and coming network, Wireless Metropolitan Area Networks (WMAN) which is possessed with outstanding advantages such as long distance transmission, high speed and efficiency, has become the hot topic in wireless communications in recent years. To support QoS, the request/grant mechanism has been used in WMAN. Under this mechanism, when a subscriber station wants to send data, it needs to send a bandwidth (BW) request to base station firstly. So the admission control of BW request has a great impact on the performance of WMAN and is treated as a fundamental problem in WMAN.At present, there are several BW request methods in IEEE 802.16 which include unicast polling, multicast polling, broadcast polling, piggyback and so on. As for broadcast polling, the mandatory contention resolution that shall be supported by IEEE 802.16 is based on the truncated binary exponential backoff algorithm. All these methods and resolutions have defined the framework for subscriber station to send BW request. While the performance of each method and how to choose the right method according to different parameters of system are out of the range of standard. To fully use the wireless resource and guarantee the QoS, this thesis studies the performance of the contention resolution provided by IEEE 802.16 and presents a centralized contention resolution and a binary multicast polling method. The choice between unicast polling and multicast polling is also discussed and an n modulus multiple access protocol is presented. All these studies are useful for building and running WMAN. The works of this thesis in detail are as followings:(1) Based on the analysis of the differences of the MAC layer between IEEE 802.16 and IEEE 802.11, the metrics to evaluate the performance of BEB based contention resolution, which include the utilization of transmission opportunity u, the delay of BW request d and drop probability of BW request p_d are calculated by using the probability method. With theoretical analysis and extensive simulations, the effects of contention parameters such as initial window w, the Maximum contention window W_max, the number of transmission opportunities N_to, the number of total SSs N and the number of active SSs n are discussed. The principle and method of setting system parameters are also presented.(2) With the performance analysis of contention resolution of IEEE 802.16, we found that the BEB based resolution can not get optimal performance in some situations. So a novel centralized scheduling contention resolution (CSCR) is presented. The basic idea of CSCR is: in each time frame, by estimating the number of active SSs, the BS broadcast an optimal window to all SSs, each SS then individually participates with this window in transmission. Theoretical analysis and extensive simulation results show that even there are errors between the really and estimated number of active SSs, CSCR outperforms BEB based resolution on many aspects.(3) Since the maximum utilities of TO under both BEB and CSCR based contention resolution can only be (1-1/n)^n-1, which is not so good when n is large, we turn to the unicast and multicast polling method and present a binary multicast polling method which divides the whole SSs into N/2 groups, each group consists of two SSs, each SS only contest with the SS in its group. The theoretical analysis indicates that when the number of active SSs is less than 0.757V, the binary multicast polling is better than unicast polling. We also provide an algorithm to make a choice between unicast and broadcast polling.(4) The BW request is a multiple access problem. Typically there are contention based and contention free ways for this problem. For the contention based method, it has good performance when there are only little active SSs, while the contention free method has the contrast situations. To full use the benefit of both types of protocol, an n modulus multiple access protocol is presented. In this protocol, the coordinator broadcasts the number of active nodes to all nodes. Each node divides this number with its own ID number and gets a reminder r and then sends its data at rth slot. The theoretical analysis indicates that the n modulus protocol has a good performance in both heavy and light load.