Improving the efficiency of OBBP allocation algorithm

ABSTRACT: Mobile WiMAX based on IEEE 802.16e is a broad band wireless access technology which has been widely accepted as the best solution for wireless broad band services. This technology is implemented by Orthogonal Frequency Division Multiple Access (OFDMA) that breaks down the spectrum into narrower bands with smaller number of subcarriers. In this thesis a comprehensive study of WiMAX system was carried out. We mainly focus on downlink transmission scenario where users assigned by the scheduler should be placed in DL subframe. Different frame packing algorithms are implemented and the work also introduces a new strategy to improve the packing efficiency of the standard Orientation Based Burst Packing (OBBP) algorithm. The aim while packing is to maximize the utilization of the DL subframe space and also at the same time to minimize the wasted slots. Enhanced One Column Striping with non-increasing Area first mapping (eOCSA) and the OBBP algorithms were first implemented using the MATLAB platform and by introducing a new strategy in the 3rd stage of the OBBP frame packing algorithm a substantial improvement in frame utilization and hence efficiency has been obtained. To start with the efficiency of the modified OBBP (MOBBP) and standard OBBP algorithms were obtained through simulations where a subframe with a capacity of 840 slots (60 subchannels × 28 symbols) was assumed and the instantaneous offered load was varied in the range 0.8 to 3.2. For this set up it was observed that for all offered loads MOBBP had higher efficiencies in comparison to OBBP. The efficiency gain varied between 1-3 % and was most distinct when the load was around 1 %. In a second experiment, the (eOCSA), OBBP and MOBBP algorithms were compared assuming a subframe with 360 slot capacity (30 subchannels × 12 symbols). For this experiment till the instantaneous load reached 1.5 % the MOBBP would have a 4-5 % improvement in efficiency over OBBP. When the offered load exceeded 1.5 % the gain in efficiency would gradually drop. Comparing eOCSA with OBBP and MOBBP clearly shows that efficiency for eOCSA is consistently better than both over all offered loads. The difference between MOBBP and eOCSA is around 2-2.5 % after the offered load exceeds 2 %. The thesis also provides the mean over allocated slots per frame for the three algorithms compared. By far the eOCSA has the highest over allocated slots among the three compared algorithms. A third experiment was conducted to compare the OBBP and MOBBP under real traffic using the COST-231 Hata Extended channel model. The distance of each user from the base station and speed of user’s have been selected from a uniform distribution. We have varied the number of users between 20 and 40. Packing efficiency and number of padded slots in the two algorithms have been compared. For the (30 × 24) DL subframe the results show that MOBBP is again consistently better than the standard OBBP. It was observed that the gain in the frame packing efficiency would change up to 1.2 %. Keywords: eOCSA; OBBP; MOBBP; OFDMA; DL-PUSC; WiMAX. …………………………………………………………………………………………………………………………