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Scalable OFDMA Physical Layer in IEEE 802.16 WirelessMAN

Scalable OFDMA Physical Layer in IEEE 802.16 WirelessMAN. Advisor: Dr. Kai-Wei Ke Speaker: Chao-Sung yah Date:12/3/2007. Outline. Introduction Multicarrier design requirement and tradeoffs Basic of OFDMA frame structure Conclusion Reference Q&A. Introduction(1/3).

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Scalable OFDMA Physical Layer in IEEE 802.16 WirelessMAN

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  1. Scalable OFDMA Physical Layer in IEEE 802.16WirelessMAN Advisor: Dr. Kai-Wei Ke Speaker: Chao-Sung yah Date:12/3/2007

  2. Outline • Introduction • Multicarrier design requirement and tradeoffs • Basic of OFDMA frame structure • Conclusion • Reference • Q&A

  3. Introduction(1/3) • Line of Sight(LOS) • Operation in the 10～66GHz、傳輸距離為5公里左右，當頻寬在28MHz時，速度最高可達約134Mbps • Non Line of Sight (NLOS) • 2～11GHz頻段、傳輸距離約10公里，通道頻寬為20MHz時最高速度約75Mbps

  4. Introduction(2/3) • Support variable bandwidth sizes between 1.25 and 20 MHz for NLOS operations • Makes the need for a scalable design of OFDM signaling inevitable • OFDM andOFDMA ,if no scalability enhancements ,then no guarantee fixed subcarrier spacing

  5. Introduction(3/3) • Other features • AMC subchannels • Hybrid Automatic Repeat Request((H-ARQ) • high-efficiency Uplink (UL) subchannel structures • Multiple-Input-Multiple-Output (MIMO) diversity • enhanced Advanced Antenna Systems (AAS) • coverage enhancing safety channels

  6. Multicarrier designrequirement and tradeoffs • Two main elements of the Wide-Sense Stationary Uncorrelated Scattering (WSSUS) model are briefly discussed here • Doppler spread and coherence time of channel • Multipath delay spread and coherence bandwidth.

  7. Outline • Introduction • Multicarrier design requirement and tradeoffs • Basic of OFDMA frame structure • Conclusion • Reference • Q&A

  8. Doppler spread and coherence time of channel • A maximum speed of 125 km/hr is used here The maximum Doppler shift corresponding to the operation at 3.5 GHz is given by Equation

  9. Doppler spread

  10. Doppler spread and coherence time of channel • The coherence time of the channel, a measure of time variation in the channel, corresponding to the Doppler shift specified above, is calculated in Equation

  11. Doppler spread and coherence time of channel • This means an update rate of ~1 KHz is required for channel estimation and equalization.

  12. Multipath delay spread and coherence bandwidth. The International Telecommunications Union (ITU-R) Vehicular Channel Model shows delay spread values of up to 20 μs for mobile environments.

  13. Multipath delay spread and coherence bandwidth. • This means that for delay spread values of up to 20 μs,multipath fading can be considered as flat fading over a 10 KHz subcarrier width.

  14. Formula EX:5MHz • Fs = floor(8/7 *BW/0.008)x0.008 • over-sampling factor used is 8/7 • Sampling frequency Fs = 8/7 * 5 =~5.714MHz • Sample time = 1/Fs =~175ns • Subcarrier frequency spacing delta F = Fs/Nfft = 5.714M/512 = 11.16KHz • Tb = 1/deltaF = 1/11.16K = 89.6μs • Tg = Tb/8 =11.2μs,Ts=Tb+Tg =100.8μs

  15. WirelessMAN OFDMA supports a wide range of frame sizes to flexibly address the need for various applications and usage model requirements

  16. Drivers of scalability • Subcarrier spacing is independent of bandwidth. • The number of used subcarriers (and FFT size) should scale with bandwidth. • The smallest unit of bandwidth allocation, specified based on the concept of subchannels. • The number of subchannels scales with FFT • Tools are provided to trade mobility for capacity.

  17. Outline • Introduction • Multicarrier design requirement and tradeoffs • Basic of OFDMA frame structure • Conclusion • Reference • Q&A

  18. three types of OFDMA subcarriers • There are three types of OFDMA subcarriers: • Data subcarriers for data transmission. • Pilot subcarriers for various estimation and synchronization purposes. • Null subcarriers for no transmission at all, used for guard bands and DC carriers.

  19. three types of OFDMA subcarriers

  20. Basic of OFDMA frame structure • Subchannel :Active subcarriers are divided into subsets of subcarriers • In FUSC, there is one set of common pilot subcarriers, but in PUSC, each subchannel contains its own set of pilot subcarriers

  21. OFDMA slot • For downlink FUSC and downlink optional FUSC using the distributed subcarrier permutation, one slot is one subchannel by one OFDMA symbol. • For downlink PUSC using the distributed subcarrier permutation , one slot is one subchannel by two OFDMA symbols. • For uplink PUSC using either of the distributed • subcarrier permutations ,one slot is one subchannel by three OFDMA symbols • For uplink and downlink using the adjacent subcarrier permutation, one slot is one subchannel by one, two,three, or six OFDMA symbols.

  22. OFDMA slot

  23. OFDMA frame structure

  24. Subcarrier allocation modes • There are two main types of subcarrier permutations: • distributed : subcarrier permutations perform very well in mobile applications • adjacent :subcarrier permutations can be properly used for fixed, portable, or low mobility environments This mechanism is designed to minimize the probability of hits between adjacent sectors/cells by reusing subcarriers

  25. DL Distributed Subcarrier Permutations: (FUSC)

  26. DL and UL Distributed Subcarrier Permutation:(PUSC)

  27. DL and UL Distributed Subcarrier Permutation:(PUSC)

  28. example

  29. DL and UL Distributed Subcarrier Permutation:(PUSC)

  30. example

  31. Optional DL Distributed SubcarrierPermutation:(OFUSC)

  32. Optional DL Distributed SubcarrierPermutation:(OFUSC) • Compared to FUSC mode, the number of used subcarriers in this method is considerably larger (1681 vs. 1729). As a result, compliance with spectral mask requirements, without a change in the over-sampling factor, may be a challenge for this mode.

  33. Optional UL Distributed SubcarrierPermutation: (OPUSC)

  34. Optional UL Distributed SubcarrierPermutation: (OPUSC)

  35. Optional DL and UL Adjacent SubcarrierPermutation: Advanced Modulation and Coding(AMC)

  36. PUSC Sector 1 FUSC FUSC Sector 3 PUSC PUSC PUSC Sector 1 FUSC Sector 2 Sector 3 Sector 1 PUSC PUSC PUSC PUSC Sector 2 FUSC Sector 3 PUSC Sector 2

  37. Conclusion • The IEEE 802.16 WirelessMAN OFDMA supports a comprehensive set of system parameters and advanced optional features for mobile, portable, and fixed usage models. Scalability enables the technology to operate optimally in different usage scenarios.

  38. Reference • Scalable OFDMA Physical Layer in 802.16 WirelessMAN, IntelTechnology Journal • J. Yun and M. Kavehrad,”PHY/MAC Cross-Layer issues in Mobile WiMAX.” January 2006 Bechtel elecommunications Technical Journal • An Introduction to WiMAX. 暨南大學通訊所. 魏學文 www.cm.nctu.edu.tw/~IEEEITComSoc/slide/2007_summer/20070806B.pdf • IEEE Std 802.16-2004, “IEEE Standard for Local and Metropolitan Area Networks – Part 16: Air Interface for Fixed Broadband Wireless Access Systems,” October 2004. subscribers.

  39. Q&A

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