Mobile WiMAX is one of the candidate technologies for 4G wireless systems, promising high data rates and affluent multimedia services. The mobile WiMAX standard (IEEE802.16e-2005)is based on Orthogonal Frequency Division Multiple Access (OFDMA), which allows a very efficient use of bandwidth in a wide frequency range, this is due to the fact that OFDMA uses multi-channel OFDM approach and provides subcarrier access in the time and the frequency domains. In this paper the impact of downlink distributed (FUSC, PUSC) and adjacent (AMC) subcarrier permutation modes used in mobile WiMAX system are investigated using simulation under various modulation and coding schemes (link speeds)with the support of optimum value of the Cyclic Prefix (CP) for the OFDMA symbol duration. The results are expressed in terms of the maximum achievable throughput and operating distance from base station in pedestrian multipath fading channel (ITU-R Ped. B at 3Km/h speed) under a UDP-based application (video streaming), which is the proper application for multimedia services. The results show the optimum cyclic prefix valve for the FUSC, PUSC and AMC subcarrier permutation modes to achieve the best system performance, also the results show differences in PER performance atvarious modulation and coding schemes for the three subcarrier permutation modes (FUSC, PUSC and AMC). These differences in performance are exploited in the present work to show the best threshold SNR to switch from link speed to another link speed in terms of throughput and operating distance.

In this paper, the effect of physical layer limitations on the MAC layer capacity of mobile WiMAX system is analyzed mathematically. The downlink physical layer simulation results show the best burst profiles for a mobile WiMAX system working in a multipath fading channel under the Quality of Service (QoS) requirements for video streaming applications. The capacity mathematical results are obtained based on the physical layer constraints after discarding the MAC layer overheads. Then, a complete simulation model for physical and MAC layers are built to validate the capacity analytical results. The simulation results show a good degree of convergence between the analytical and simulation capacity results. Also, the results show that the maximum number of users that can be served instantaneously by the BS are different depends on the used distributed subcarrier permutation mode.

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In this paper, capacity analysis for mobile WiMAX working in distributed subcarrier permutation modes is investigated mathematically depending on the physical layer simulation results. These simulation results of physical layer dispose exact values for the number of best burst profiles that candidate to carry Medium Access Control (MAC) payload in a proposed ITU-R mixed user channel. The complete simulation model for physical and MAC layers has been made using OPNET 14.5 to validate the analytical results of the obtained capacity. This approach for capacity analysis based on simulation results is proposed to overcome the lack of interests by other researches in the significant influence of the physical layer constraints on the MAC layer capacity calculations. As well as in this paper, a proposed ITU-R mixed user channel is performed to suit the high mobility users involved under distributed subcarrier permutation modes. Moreover, this paper focused on the exact overhead calculations by taking into account all the significant overhead parameters in the MAC layer frame structure that affects the overall system capacity. KeywordsIEEE802.16e-2005, OFDMA, PUSC, FUSC, QoS, proposed ITU-R mixed user channel model, PER.