A Novel Mechanism for Contention-based Initial Ranging in IEEE 802.16e Networks

The paper proposes a Waiting-time Dependent In- creasing rate Adapted Backoff (WDIA) algorithm to improve the contention efficiency in the IEEE 802.16e. A contention model is first proposed to analyze the frame-based behavior of contention resolution in 802.16e network. We introduce the concept of reduced overlapping between back-off windows of contending stations and we adopt an adaptable increasing rate. An initial ranging scenario is constructed to evaluate the effectiveness of WDIA. The simulation results show that the WDIA algorithm achieves better performance than Truncated Binary Exponential Backoff (TBEB) algorithm in terms of number of retransmissions, access delay and resource utilization. I. INTRODUCTION In 802.16e, the uplink frame consists in three parts: con- tention interval for initial ranging (IR), contention interval for bandwidth request (BR), and data transmission interval for user traffic. These parts are allocated by the base station (BS). At network entry, subscriber stations (SS) contend for resource on the IR interval in order to access the network. As in many cellular networks, this random access is based on a traditional slotted Aloha in conjunction with a back-off algorithm. Changes to the allocation of the contention interval per- formed by BS can reduce the collision probability of user access, but also reduces the amount of resources dedicated to user data traffic. A better solution is improving the contention efficiency within a relative stable contention interval, while maintaining sufficient resources for data traffic. In this paper, a Waiting-time Dependent and Increasing rate Adapted (WDIA) back-off algorithm is proposed to improve the contention performance (collision rate, access delay and resource utilization) of IEEE 802.16e networks. Waiting-time Dependent Back-off (WDB) has been originally proposed for WLAN (1) and is extended in this paper to the IR process in WiMAX. Focusing on contention performance, the Request- Response mechanism based IR is considered rather than the CDMA ranging code mechanism. Although presented here for IR, WDIA is such a novel algorithm for contention resolution that can also be used for other types of contention-based uplink accesses, including periodic ranging, handoff ranging and bandwidth request. Bianchi (2) has given a simple and accurate framework for the analysis of back-off based MAC protocols but focused on IEEE 802.11 DCF networks. He concluded that maximum performance can be achieved by adaptively tuning the value of the back-off window (BW) size based on the network size. In (3), a distributed backoff algorithm is proposed to maximize the throughput by computing the optimal value of CW for each station. However, the optimization is based on estimating the number of active stations that is inaccu- rate under a distributed contention manner. In (4), collision history based BW adjustment is proposed to randomize the transmission timing and minimize the collision probability. Authors focus on periodic ranging. However periodic ranging is performed using periodically granted bandwidth and not using contention-based ranging unless no grant is received when T4 (35s as maximum defined in 802.16e) expires. In (5), authors propose a Markov chain based analytical model to study initial ranging performance. They point out that a fixed size of initial BW, as proposed in the standard, neglects the effect of varying network load and define a new adaptive algorithm based on the number of contending SSs. Again, this number is difficult to estimate in practice. In our work, we propose a contention model of initial ranging in 802.16e and give the direct relation between contention performance and back-off window overlapping. Based on the analysis, we introduce the concept of reduced overlapping between BWs of contending SSs to improve contention performance. The way BWs are adjusted is based on the time duration requests have already waited, the so called waiting time (WT). And the load of contention is used for optimizing the BW instead of the contending number of SSs since it can be estimated under our separated BW mechanism. The rest of the paper is organized as follows. In Section II, we summarize the procedure of initial ranging and point out the limitations of contention resolution in 802.16e. Section III presents the contention model and gives direct relation between contention performance and backoff window overlap- ping. Section IV proposes our algorithm WDIA and Section V evaluates the improvements of WDIA. Finally, conclusions are drawn in Section VI.