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1 . Wireless Technologies – A case study for IEEE 802.11g OFDM system Arpan Pal Center of Excellence for Embedded Systems TCS, Kolkata [email protected]

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  1. 1. 1. Wireless Technologies A case study for IEEE 802.11g OFDM system Arpan Pal Center of Excellence for Embedded Systems TCS, Kolkata [email protected]
  2. 2. 2. Introduction Cellular Wireless Systems & Wireless Networks 802.11g OFDM PHY development System Description Development Framework Simulation Issues Receiver Algorithms Implementation Issues Simulation Results Security Algorithms Areas of future - Convergence to 4G Agenda
  3. 3. 3. Wireless is the next giant leap in information services. The new paradigm for connectivity enables business to operate faster better more cost effectively and more profitably through the use of always on, always connected, and always available content and applications. With the tremendous increase in wireless LANs, Mobile phones, PDAs, and other mobile devices, the merging of computation and telecommunication technologies is a fundamental part of modern society. Can broadly be classified into two types Cellular Wireless Systems and Wireless Networks Introduction
  4. 4. 4. CellularWireless Systems Roadmap 9.6 k 64 k 384 k 1000 k 2000 k 5000 k 1995 2000 2005 AMPS TACS NMT IS136 GSM IS95A IS95B GSM-GPRS 1980 CDMA2000 W-CDMA 4G3G2.5G2G1G ???
  5. 5. 5. Wireless Networks PAN IEEE 802.15 LAN IEEE 802.11 MAN IEEE 802.16
  6. 6. 6. OFDM Overview Multi-Carrier Modulation Technique Carrier spacing kept minimum maintaining orthogonality Multi-path robustness due to multi-carrier Reduction of ISI through Guard Bands Robust against narrow-band interference Efficient FFT based receiver structures Simpler Frequency Domain Channel Equalization Simpler Receiver Synchronization Techniques High peak-to-average power levels Susceptible to RF Front-end non-linearity Susceptible to LO frequency offset / drift
  7. 7. 7. 802.11g OFDMPHY Assemble frame Scrambler Convolution Block Interleaver Bit Mapper IFFT Add Guard Interval Window MAC Layer DAC RF Transmitter Transmit Remove Guard Interval FFT Channel / Phase Correction De-mapper De-interleaver Viterbi Decoder Descrambler Disassemble Frame Channel Estimator Frequency/Phase Correction MAC Layer ADC RF Receiver AGC AFC Receive Management Entity PLCP PMD PMD PLCP Frame Sync & Coarse Frequ. Correct Receiver Sync PLME Preamble & Pilot Insertion
  8. 8. 8. Development Framework Simulation Environment Fixed point Model Floating point Model MATLAB model Analysi s Result C System level model Analysi s Result VHDL / Verilog Simulation Synthesis Rest of the process
  9. 9. 9. Simulation Issues Proper selection of channel models with various delay-spreads and Doppler shifts. Proper simulation of Sampling Clock error. Proper simulations of Phase and Frequency Error. Proper modeling of Phase Noise. Proper modeling of I-Q imbalance. AGC Proper modeling of the LNA and anti-aliasing filter. AWGN noise.
  10. 10. 10. ReceiverAlgorithms Time Synchronization Packet Detection Energy Based Frame Synchronization Short Training Sequence Cross-Correlation Based False Alarm Reduction Short Training Sequence Auto-Correlation Based
  11. 11. 11. ReceiverAlgorithms Frequency Synchronization Frequency Offset Estimation Coarse estimate based on Short Training Sequence Cross-Correlation Fine estimate based on Long Training Sequence Cross-Correlation Frequency Offset Correction Time domain (pre-FFT) rotation based on estimated offset Carrier Phase Tracking Phase offset due to residual frequency offset and sampling clock error Pilot based estimation for phase Each OFDM symbol contains 4 pilots Frequency domain (post FFT) rotation for phase correction
  12. 12. 12. ReceiverAlgorithms Channel Estimation Assumes quasi-stationary channels (does not change within a packet) Channel Transfer function estimated from long training sequence (LTS) Estimated Channel Transfer Function = FFT(Received LTS) / FFT(ideal LTS) Takes care of indoor channels along with gain variation Channel compensation done post FFT by dividing with estimated Channel Transfer Function
  13. 13. 13. Implementation Issues Use of Radix-22 FFT /IFFT algorithm instead of a Radix-2 or a Radix-4 implementation. Use of CORDIC (COordinate Rotation DIgital Calculation) for performing complex multiplication and division. Determining the scaling factor to be used after every stage of the FFT block. Deciding on the number of iterations to be used for implementing CORDIC. Approximating all sqrt(x2 + y2 ) with (|x| + |y|) for hardware simplification and altering the various threshold values accordingly. LUT implementation of various mathematical calculations. Deciding on the number of bits to be used for ADC Fixing the number of bits to be used for implementing FFT/IFFT.
  14. 14. 14. Simulation Results 2 3 4 5
  15. 15. 15. Simulation Results Channel Model 3 used
  16. 16. 16. Security Algorithms An Alternative Approach for Enhancing Security of Wireless Networks using Physical Layer Encryption patent filed Provides enhanced security against - Data Privacy - Data Forgery - Denial of Service Two KEY Security method (secure KEY delivery assumed) KEYS used to encrypt/modify physical layer parameters like - Error Control Coding Rate - Type of Modulation / Constellation Mapping - Length of Packet - Interleaving Pattern - Phase offset Contribution presented in IEEE 802.20 Standard body meeting in September, 2003
  17. 17. 17. Areas of future Convergence to 4G 4G is whatever that is beyond 3G To be used for Real-Time Video Delivery and similar applications QOS is important Calls for more efficient Modulation, ECC and Equalization Incorporation of Mobility and stringent Multipath including non- LOS scenario into Wireless Networks can lead to 4G OFDM and UWB likely candidates for 4G PHY Space-Time Diversity, Smart Antenna Processing and Multiple-Input- Multiple-Output (MIMO) systems are also likely to be used CDMA with its multi-user capabilities can provide the access mechanism Calls for implementation of a MAC layer that seamlessly integrates all the above features of PHY