(Chapter 2) ENE 490 MON 13:30-16:30 Asst. Prof. Suwat Pattaramalai
Background on IEEE 802.16 and WiMAX (Table 2.1 and Table 2.2) Salient Features of WiMAX OFDM-based physical layer: good resistance to multipath and operate in NLOS coniditions Very high peak data rates: as high as 74Mbps with 20MHz wide spectrum, 25Mbps-downlink and 6.7Mbps-uplink with 10MHz spectrum: using 64 QAM modulation Scalable bandwidth and data rate: scalable physical-layer architecture Adaptive modulation and coding (AMC): based on channel conditions (SNR) Link-layer retransmissions: Automatic retransmission requests (ARQ) Support for TDD and FDD (Time and Frequency division duplexing) Orthogonal frequency division multiple access (OFDMA): multiuser Flexible and dynamic per user resource allocation: by base station Support for advanced antenna techniques: multiple-antenna, etc. Quality-of-service: constant or variable bit rate, real or non-real time Robust security: Advance Encryption Standard, AES Support for mobility: handovers, power-saving, power control IP-based architecture: end-to-end services, support IP application
WiMAX Physical Layer OFDM Basics: multicarrier modulation with orthogonal subcarriers (in frequency tone), guard interval between symbols larger than delay spread (waste BW and power) OFDM Pros and Cons Reduced computational complexity Graceful degradation of performance under excess delay, good for adaptive modulation and coding Exploitation of frequency diversity: robustness against burst errors Use as a multiaccess scheme: OFDMA in mobile WiMAX Robust against narrowband interference Suitable for coherent demodulation: pilot-based channel Problems: nonlinearities and clipping distortion, power inefficiencies, phase noise, frequency dispersion, need accurate frequency synchronization
OFDM Parameters in WiMAX: Table 2.3 shows parameters used in Fixed and mobile WiMAX Subchannelization: OFDMA Slot and Frame Structure: Slot, data region (Figure 2.1) Adaptive Modulation and Coding (AMC) in WiMAX: variety of modulation and coding based on channel condition, increase system capacity, real-time trade-off between throughput and robustness, Table 2.4 PHY-Layer Data Rates: varies based on operating parameters; channel bandwidth, modulation and coding scheme used, number of subchannels, etc., Table 2.5
Mac-Layer Overview: support IP and Ethernet, fig. 2.2 Channel-Access Mechanisms: based on incoming traffic and requests Quality of Service: using connection-oriented MAC architecture, five services; Unsolicited grant services (UGS), Real-time polling services (rtps), Non-real-time polling services (nrtps), Besteffort (BE) service, Extended real-time variable rate (ERT-VR) service, Table 2.6 Power-Saving Features: sleep mode or idle mode Mobility Support: Normadic, Portable, Simple mobility (up to 60 kmph), Full mobility (Up to 120 kmph), Handover Security Functions: support for privacy (encryption), device/user authentication, Flexible key-management protocol, protection of control messages, support for fast handover Multicast and Broadcast Services: signaling mechanisms Advanced Features for Performance Enhancements
Advanced Features for Performance Enhancements Advanced Antenna Systems: transmit diversity, beamforming, spatial multiplexing (MIMO) Hybrid-ARQ: forward error correction and automatic repeat request Improved Frequency Reuse: dynamic frequency allocation based on loading and interference conditions Reference Network Architecture Fig. 2.3 IP-Based WiMAX Network Architecture: mobile stations, accessservice network (ASN), connectivity service network (CSN) Fig. 2.4 Functions performed across reference points Performance Characterization Throughput and Spectral Efficiency: table 2.7 Sample Link Budgets and Coverage Range : table 2.8
Summary and Conclusions Flexible and robust air interface Physical layer is based on OFDM Support several advanced techniques for increasing the reliability of link layer: error correction coding, ARQ, antenna arrays Advanced signal processing to inprove overall capacity: adaptive modulation, coding, spatial multiplexing, multiuser diversity Flexible MAC layer for variety of traffic types: voice cideo, multimedia, strong QOS Robust security functions: encryption and mutual authentication Seamless handover, low power consumption for portable Flexible all-ip-based network architecture, end-to-end functions High spectral efficiency, MIMO