WiMAX Capacity Enhancement: Capacity Improvement of WiMAX Networks by Dynamic Allocation of Subframes

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1 WiMAX Capacity Enhancement: Capacity Improvement of WiMAX Networks by Dynamic Allocation of Subframes Syed R. Zaidi, Shahab Hussain, M. A. Ali Department of Electrical Engineering The City College of The City University of New York, New York, NY, USA Abstract A WiMAX TDD frame contains one downlink subframe and one uplink subframe, the capacity allocated to each subframe is a system parameter that should be determined based on the expected traffic conditions. Normally, the load in downlink is higher compared to uplink, hence the downlink subframe is expected to have a longer duration. The split between downlink and uplink subframes is usually decided once, based on average statistical traffic behavior, setting the system accordingly. Nevertheless, it is also possible that through real-time monitoring, the system itself may adapt the uplink and downlink boundary dependent on the current traffic conditions. We devise a custom algorithm which assigns the network resources dynamically via adaptive allocation of uplink and downlink subframes. and components among variety of vendors, WiMAX forum also conducts certification program for base stations, network equipments and end user devices. Fig. 1 shows one of the practical WiMAX network architecture [5, 6]. Keywords- WiMAX; 4G; Capacity, , TDM PON, Dynamic Allocation, Subframes I. INTRODUCTION There is on-going research studies on the assignment of WiMAX TDD subframes. In this regard Filin et al.[1] proposed adaptive subframe algorithm which is fast and efficient. Adhicandra [2] proposed simple subframe assignment algorithm. Pries et al. [3] studied different case studies of TDD frame split scenarios. We propose a simple yet efficient subframe assignment scheme which greatly improves the capacity of WiMAX networks. When WiMAX network started to deploy, it targeted only stationary devices with roof mounted antennas or indoor WiMAX routers. That version of WiMAX is known as IEEE or d. Since these networks designed to handle stationary devices, so no handovers of connections between base stations are required. Second version of WiMAX known as IEEE or e or mobile WiMAX is the advanced air interface which not only supports mobility of subscribers but also handovers between base stations. Since mobile subscribers and handovers require more administration so it became necessary to define the network beyond base stations. As IEEE is only responsible for the air interface so that task was carried over by WiMAX Forum. Moreover in order to ensure interoperability of devices Figure 1. WiMAX Network Architecture As evident from the figure R1 reference point is the air interface between mobile devices and base station. This is fully based on IP protocol. The protocol used over this interface is either IEEE d for only fixed clients or IEEE e for both mobile and fixed clients. Base stations can carry out smooth handovers among them using R8 interface. R6 interface is between base stations and Access Service Network Gateway (ASN-GW). ASN-GW is the gateway between radio network and core network. In LTE, this is comparable to 48

2 Access Gateway (AGW). R3 interface is defined between ASN-GW and core network. The paper is organized as follows: In the first part we analyze the frame and application performance of a WiMAX network with static allocation of uplink and downlink subframes. In the second part we devise an algorithm that dynamically changes the capacity allocated to UL and DL subframes. That algorithm is implemented on the WiMAX BS and the network is then simulated. Finally, the results are then compared with the static allocation of subframes. We use OPNET [4] for simulation. A. IEEE Air Interface Some of the parameters of IEEE which is also known as mobile WiMAX are given in the Table 1. Complete description of IEEE can be found in reference [6]. As LTE, WiMAX employs OFDMA in the downlink but for the uplink it differs from the LTE as it defines to use OFDMA also in the uplink. LTE uses SC-FDMA in the uplink. The mobile WiMAX standard supports both TDD and FDD modes but initial deployments are only using TDD. TABLE I. WIMAX PARAMETERS. Parameters Values System Bandwidth ( MHz) Number of subcarriers FFT Size Subcarrier spacing (khz) OFDM symbol duration (µs) 91.4 Cyclic Prefix (µs) 11.4 data rate by two. Moreover MIMO is only implemented in the downlink direction by the base stations. In the uplink direction MIMO is not implemented due to limited antenna size and power constraints. II. PART I: STATIC SUBFRAME ASSIGNMENT The basic simulated network using OPNET simulation tool [4] consists of WiMAX base station with five subscriber stations (SS) as shown in fig. 3. All SS have UL load of 200 Kbps for a total UL load of 1 Mbps. At certain times server generates 400 Kbps of application traffic directed to ss_1, ss_2 and ss_3 for a total downlink load of 1.2 Mbps. The WiMAX network uses 5 MHz of bandwidth with 5 ms of frame size and 512 subcarriers. Modulation used is QPSK ½. OFDMA TDD frame structure is shown in the following figure 2. In OFDMA data transmissions to users are multiplexed in both time and frequency because of number of available sub channels. Figure 3. Basic WiMAX Simulation Network Figure 2. OFDMA TDD Frame structure At the beginning of the frame, Downlink-MAP (DL-MAP) informs devices when and where the data is scheduled for them in the frame. Uplink-MAP (UL-MAP) in the frame assigns uplink transmissions opportunities for this and the frames to follow. B. MIMO Transmissions Like LTE, WiMAX standard also supports the use of MIMO transmissions e.g., 2 x 2 MIMO to increase the overall Users are allocated slots for data transfer and these slots represent the smallest possible data unit. A slot is defined by a time and subchannel dimension and it varies depending on the following operating modes: For downlink FUSC, one slot is a single subchannel by one OFDMA symbol For downlink PUSC, one slot is a single subchannel by two OFDMA symbols. For uplink PUSC, one slot is a single subchannel by three OFDMA symbols. A single packet of user data is distributed over multiple OFDMA symbols for the PUSC modes. Moreover In FUSC, 49

3 there is one set of common pilot subcarriers; in PUSC, each subchannel contains its own set of pilot subcarriers [5]. Since we use QPSK ½, data rate comes out to be around 6.3 Mbps. If an FEC (Forward Error Correction Code) of 1/2 is applied, then the total effective data rate is 3.1 Mbps. In this scenario 34 symbols are used for DL subframe and 12 symbols for UL subframe. Hence DL capacity ~ 2.5 Mbps and hence UL capacity ~ 600 Kbps. List of simulation parameters is given below in Table 2. TABLE II. SIMULATION PARAMETERS WiMAX Base Station: Frame Duration 5 ms Symbol Duration µs TTG µs RTG µs No. of subcarriers 512 Base Frequency 5.8 GHz Bandwidth 5 MHz Duplexing Technique TDD( Time Division Duplex) Frequency Division(DL Zone) FFT-512 FUSC Frequency Division( UL Zone) FFT-512 PUSC Maximum Transmission power 0.5 W Number of initial ranging codes 8 Number of initial BW request codes 8 WiMAX Subscriber Station: Maximum Transmission power 0.5 W Pathloss model Free Space BS MAC address Distance based Ranging power step 0.25 mw Modulation and coding QPSK ½ Piggyback BW request Enabled CQICH Period 3 Figure 5. Sumulation Results The simulation was run and following results are obtained. Figure 6. Sumulation Results Figure 4. Sumulation Results 50

4 As it is evident that Downlink has enough capacity to support 1.2 Mbps but for uplink the load is 1 Mbps but the UL capacity is only around 600 Kbps, so throughput is low and delays are high. Figure 7. Sumulation Results III. PART II: DYNAMIC SUBFRAME ASSIGNMENT In part 1 we observed that DL subframe had enough resources to spare for UL subframe which was starving and its subframe utilization was reaching 100%. Hence in part II we propose an algorithm which dynamically assigns symbols to DL and UL subframes. If the uplink subframe utilization exceeds 90% threshold and at that moment downlink subframe utilization is less than 60% then uplink subframe size is increased. On the other hand if the DL subframe utilization starts increasing 70% threshold then the frame is reset to the initial settings. Algorithm is given below Start If DL subframe already reduced & DL subframe utilization > 70% Reset frame partitions end else If UL subframe utilization > 90% If DL subframe utilization < 60% If DL subframe length N.L.E.Q to min. subframe length Increase UL subframe end else end IV. PART III: RESULTS Since WiMAX BS controls the framing and we use OPNET for simulation so this algorithm is implemented on WiMAX_BS_Control which is a child process of WiMAX_BS_MAC. After compiling WiMAX_BS_Control, we run the simulation again and get following results. Figure 8. Sumulation Results 51

5 Figure 9. Sumulation Results Figure 11. Sumulation Results Figure 10. Sumulation Results Figure 12. Sumulation Results 52

6 the downlink utilization in real time and returning DL resources as needed. REFERENCES [1] S. A. Filin, S. N. Moiseev, M. S. Kondakov, "Fast and Efficient QoS Guaranteed Adaptive Transmission Algorithm in the Mobile WiMAX System", IEEE Trans. on Vehicular Technology, Vol. 57, No. 6, [2] Iwan Adhicandra, Adaptive Subframe Allocation in WiMAX Networks, 33rd International Conference on Telecommunications and Signal Processing- TSP 2010 [3] R. Pries, D. Staehle, D. Marsico, "IEEE Capacity Enhancement Using an Adaptive TDD Split", VTC [4] OPNET Technologies, OPNET, Internet: [5] WiMAX FORUM, WiMAX FORUM, Internet: [6] IEEE (2005) IEEE standard for local and metropolitan area networks part 16, amendment 2 and corrigendum 1, Figure 13. Sumulation Results From the graphs we have the following observations: Downlink Traffic: It is obvious that the results are same for downlink traffic ~ 1.2 Mbps with around 8 ms delay when dynamic subframe assignment is used as compared to static subframe downlink traffic. Uplink Traffic: When dynamic subframe assignment is used, uplink throughput is around 700 Kbps with 3 ms delay when there is downlink traffic. However as the downlink traffic decreases, the uplink throughput is increased to around 1 Mbps with 50 ms delay. When static subframe was used, uplink traffic throughput was around 560 Kbps with 3.7 ms delay. Hence there is a big improvement in performance when we use dynamic subframe assignment. The above graph shows the utilization of both downlink and uplink subframes. The main advantage is when download utilization reaches zero and uplink can borrow all resources it requires. But since our algorithm evaluates instantaneous subframe utilization values, then even if the downlink subframe utilization exceeds 60%, the uplink subframe would borrow resources from any downlink subframe whose utilization falls below 60%. V. CONCLUSION Subframe partition size is the main system configuration parameters that directly determines the capacity balance of the cell between uplink and downlink sections. Our proposed mechanism successfully improves the performance of the uplink while taking advantage of the underutilized downlink subframe, allowing uplink subframe to take temporarily the unused resources. On the other hand it also kept the performance of the downlink traffic unaffected by monitoring Syed Rashid Zaidi received the M.S., M.Phil., degrees in Electrical Engineering from City University of, New York, NY, USA. He is currently a Ph.D. Candidate in Electrical Engineering in The City Univeristy of New York. His research areas are LTE, WiMAX and PON. Shahab Hussain received the M.S., and M.Phil., degrees in Electrical Engineering from The City University of, New York, NY. He is currently a Ph.D. Candidate in Electrical Engineering in The City University of New York. His research areas are LTE, WiMAX and Passive Optical Networks. He is working in Alcatel-Lucent since 2000 as Sr. Systems Engineer. He is involved in the design of UMTS/LTE based wireless networks for leading US telecom service providers like AT&T,Verizon, Sprint., and Time Warner cable etc. Mohamed A. Ali received the M.S. and Ph.D. degrees in electrical engineering from the City College of the City University of New York in 1985 and 1989, respectively. He joined the Faculty of Electrical Engineering at the City College of New York in 1989, where he is currently a Professor. His research interests are in the general area of telecommunications networking architecture and technology, encompassing broadband wired/wireless access, IP/MPLS-based layer 1/2/3 Enterprise MAN and VPNs, Fourth-Generation (4G) mobile WiMAX and cellular Long-Term Evolution (LTE) systems, passive optical networks (PONs), data and voice (traditional as well as converged voice-over-packet solutions) networks and fiber optic communication systems, traffic engineering, carrier-grade Ethernet networking technology, services, and architecture, long-haul WDM, and TDM/SONET-based optical transport networks. He has published over 100 papers in professional journals and international conferences. Dr. Ali received the National Science Foundation Faculty Career Development Award. 53