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An Introduction to Multi-band OFDM Physical Layer Proposal for IEEE 802.15 Task Group 3a

An Introduction to Multi-band OFDM Physical Layer Proposal for IEEE 802.15 Task Group 3a. Presenter: Keng-Hsien Lin Advisor: Prof. Tzi-Dar Chiueh Date: Feb 16, 2004. Outline. Motivation MBOA History Overview Review of Some OFDM Principles PHY Specification of Multi-band OFDM Proposal

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An Introduction to Multi-band OFDM Physical Layer Proposal for IEEE 802.15 Task Group 3a

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  1. An Introduction to Multi-band OFDM Physical Layer Proposal for IEEE 802.15 Task Group 3a Presenter: Keng-Hsien Lin Advisor: Prof. Tzi-Dar Chiueh Date: Feb 16, 2004

  2. Outline • Motivation • MBOA History Overview • Review of Some OFDM Principles • PHY Specification of Multi-band OFDM Proposal • Responses to Issues • Conclusions & Future Work & References

  3. Outline Overview • Motivation • Why I choose this topic? • MBOA History Overview • Review of Some OFDM Principles • PHY Specification of Multi-band OFDM Proposal • Responses to Issues • Conclusions & Future Work & References

  4. Why I Choose This Topic? • First, according to Professor’s email saying: “When I say MS1 student, I actually mean all MS1 students who are free, that is except Mr. Wu and Mr. Jhuang.” I think that’s a hint of the topic of today’s presentation that I should choose. • Second, after I read some background materials of this topic, I really found many attractive things….

  5. PAN/LAN Standards Application Range Can exceed 100 Mbps! [1]

  6. UWB Application Range Local high throughput delivery wired & wireless wired & wireless Broadband wired & wireless Long range delivery wired & wireless wired & wireless wired & wireless [2] All this existing technologies can be replace with UWB tech

  7. IEEE 802.15.3 and 802.15.3afor Wireless Personal Area Network • 802.15.3 • High data rate (up to 55 Mbps) • Low power • Low cost • 802.15.3a • Provide a higher speed (up to 480 Mbps) PHY enhancement amendment to 802.15.3 for applications which involve imaging and multimedia. • Now there are two proposals for this standard • MB-OFDM  Today’s Topic • DS-UWB Use 3.1~10.6 GHz band (Ultra-Wide-Band) to pursue higher data rate [3]

  8. Outline Overview • Motivation • MOBA History Overview • Let us see MBOA’s brief history until last year • Review of Some OFDM Principles • PHY Specification of Multi-band OFDM Proposal • Responses to Issues • Conclusions & Future Work & References

  9. Multi-Band OFDM Alliance’s (MBOA) Brief History Nov 10 • 03/268r2 draft spec proposed [4]

  10. [4]

  11. Outline Overview • Motivation • MBOA History Overview • Review of Some OFDM Principles • Why and how we develop OFDM? • Typical baseband model of an OFDM transceiver • OFDM drawbacks • PHY Specification of Multi-band OFDM Proposal • Responses to Issues • Conclusions & Future Work & References

  12. Why We Develop OFDM? • In limited bandwidth and technology, we must use bandwidth more efficiently to pursue higher data rate • OFDM is an elaborate solution • But overlapped bandwidth may cause severe inter-channel (carrier) interference (ICI) (a) Traditional FDM (b) OFDM [5]

  13. How We Develop OFDM? – Guard Interval and Cyclic Prefix [5]

  14. How Cyclic Prefix Works? F T T’ F’ IFFT Channel FFT WH C W F: Freq Domain Tx Data (Nx1) T: Time Domain Tx Data (Nx1) WH: IDFT matrix (NxN) F’: Freq Domain Rx Data (Nx1) T’: Time Domain Rx Data (Nx1) C: Discrete channel impulse response matrix (NxN) W: DFT matrix (NxN) ∵Cyclic prefix C is equivalent to a circulant matrix  C can be decomposed to WH*D*W, where WH*W = I (orthogonal), D is diagonal matrix with diagonal value = DFT{1st row of C} (proof in [6]) ∴ F‘ = W * T’= W * (C * T) = W * (C * (WH * F)) = W * C * WH * F = W * (WH * D * W) * WH * F= D * F Fk’ = Di * Fk Easier FDE is possible!

  15. Typical Baseband Model of an OFDM Transceiver Provide Sync or Channel information Prevent burst errors! As previous mentioned function Fit channel and can reduce DAC clock rate [5]

  16. OFDM Drawbacks (1/3) • Capacity and power loss due to guard interval • The guard interval consumes 20% of the bandwidth and transmit power in IEEE802.11a • High sensitivity to synchronization errors • Interference and loss of orthogonality [7]

  17. OFDM Drawbacks (2/3) • A large peak-to-average ratio (PAPR) problem This term may be large, especially when n=0 [7]

  18. OFDM Drawbacks (3/3) • Solution to PAP Problem • Distortion technique as clipping / peak windowing • Forward error correction coding  as convolutional coding • Scrambling  reduce probability of large PAPR occurrence [7]

  19. Outline Overview • Motivation • MBOA History Overview • Review of Some OFDM Principles • PHY Spec. of Multi-band OFDM Proposal • Overview of Multi-band OFDM • Band plan • PLCP frame format • Architecture discussion – Tx and Rx • System Parameters • Responses to Issues • Conclusions & Future Work & References

  20. Overview of Multi-band OFDM • Basic idea: divide spectrum into several 528 MHz bands. • Information is transmitted using OFDM modulation on each band. • OFDM carriers are efficiently generated using an 128-point IFFT/FFT. • Internal precision is reduced by limiting the constellation size to QPSK. • Information bits are “interleaved across all bands” to exploit frequencydiversity and provide robustness against multi-path and interference. • 60.6 ns (32*1/528MHz) prefix provides robustness against multi-path even in the worst channel environments. • 9.5 ns (5*1/528MHz) guard interval provides sufficient time for switching between bands. [8]

  21. Band Plan (1/2) • Group the 528 MHz bands into 4 distinct groups. • Group A: Intended for 1st generation devices (3.1 – 4.9 GHz)  for Mode1, Mode2. • Group B: Reserved for future use (4.9 – 6.0 GHz). • Group C: Intended for devices with improved “SOP” performance (6.0 – 8.1 GHz)  for Mode2. • Group D: Reserved for future use (8.1 – 10.6 GHz). [8]

  22. Band Plan (2/2) • The relationship between the center frequencyfc and the band numbernb is: [8]

  23. Physical Layer Convergence Procedure (PLCP) Frame Format [8] (a) Multi-band OFDM Compare later! [9] (b) 802.11a packet format

  24. More Details on the PHY Header • PHY Header: • Band Extension (3 bit field): • Indicates the mode of transmission for the payload (Mode 1 or Mode 2). • Rate (4 bit field): • Indicates the rate of transmission for the payload. • Rate field also specifies the coding rate, puncturing pattern, and spreading technique. • Length (12 bit field): • Indicates the number of bytes in the payload (excludes the FEC). • Scrambler (2 bit field): • Conveys information about the scrambler state. [8]

  25. Standard PLCP Preamble Format (1/3) 9.375us [10] (a) Multi-band OFDM (30 OFDM symbol time) [9] (b) 802.11a (4 symbol time)

  26. Standard PLCP Preamble Format (2/3) • Multi-band OFDM preamble is composed of 3 sections: • Packet sync sequence: used for packet detection. • Frame sync sequence: used for boundary detection. • Channel estimation sequence: used for channel estimation • Packet and frame sync sequences are constructed from the same hierarchical sequence. • Correlators for hierarchical sequences can be implemented efficiently: • Low gate count. • Extremely low power consumption. • *Preamble sequences are designed to be extremely robust. * in this slides means the sentence was made by the proposer [8]

  27. Standard PLCP Preamble Format (3/3) • Basic idea: define 4 hierarchical preambles, with low cross-correlation values. • Preambles are generated by spreading (multiply) a length 16 sequence by a length 8 sequence (total length 128 in freq. domain). • *In the multiple overlapping piconet case, it is desirable to use different hierarchical preambles for each of the piconets [8]

  28. Proposed Tx Architecture (1/4) • Block diagram of an example TX architecture: • Architecture is similar to that of a conventional and proven OFDM system. Can leverage existing OFDM solutions for the development of the Multi-band OFDM physical layer. In order to change coding rate! Proposer initially called this “Time-Frequency interleaved” Change frequency! [8]

  29. Tx – Scrambler (2/4) Data In X15 X14 X13 X2 X1 Data Out (a) Multi-band OFDM (b) 802.11a [9]

  30. Tx – Convolutional Encoder (3/4) • Assume a mother convolutional code of R = 1/3, K = 7 (compare with 802.11a, R=1/2, K=7.) Having a single mother code simplifies the implementation. • Generator polynomial: g0 = [1338], g1 = [1458], g2 = [1758]. • Higher rate codes are achieved by puncturing the mother code. [8]

  31. Tx – OFDM Modulation (4/4)Use 128-point IFFT/FFT Guard subcarriers Data subcarriers High-freq subcarriers 128 = 100 + 12 + 10 + 6 Total subcarriers Pilot subcarriers Null subcarriers [10]

  32. Proposed Rx Architecture • Block diagram of an example RX architecture: • Architecture is similar to that of a conventional and proven OFDM system. Can leverage existing OFDM solutions for the development of the Multi-band OFDM physical layer. [8]

  33. System Parameters • System parameters for mandatory and optional data rates: • Clock rate: 132 MHz  The proposal used this to estimate power consumption • Sample rate: 1/528MHz  Ex: 110 Mbps = (100)*(2)*(11/32)*(1/2)*(1/312.5n) [8] * Mandatory information data rate, ** Optional information data rate

  34. Zero-padded Prefix (1/2) • Ripple in the transmitted spectrum (Guess that ∵CP duration isn’t a multiple of OFDM duration (excluding CP duration)) can be eliminated by using a zero-padded prefix. • *Using a zero-padded (ZP) prefix instead of a cyclic prefix is a well-known and well-analyzed technique. • *Almost no ripple in PSD. [8]

  35. Zero-padded Prefix (2/2) • A Zero-padded Multi-band OFDM has the same multi-path robustness as a system that uses a cyclic prefix (60.6 ns of protection). • *The receiver architecture for a zero-padded multi-band OFDM system requires ONLY a minor modification (less than < 200 gates). • Added flexibility to implementer: multi-path robustness can be dynamically controlled at the receiver, from 1.9 ns up to 60.6 ns. [8]

  36. Outline Overview • Motivation • MBOA History Overview • Review of Some OFDM Principles • PHY Specification of Multi-band OFDM Proposal • Responses to Issues (Proposed “Self Evaluation Matrix”) • General aspects • PHY protocol aspects • Conclusions & Future Work & References

  37. General Aspects (Proposed “General Solution Criteria”) [10]

  38. Interference and Susceptibility • Minimum tolerable separation when interfered by other system [10]

  39. Scalability • Data rate scaling: • Data rates from 55 Mb/s to 480 Mb/s has been defined in the current proposal. • Frequency scaling: • Mode 1 (3-bands) and optional Mode 2 (7-band) devices. • *Guaranteed interoperability between different mode devices. • Power scaling: • Implementers could always trade-off power consumption for range and information data rate. • Complexity scaling: • Digital section will scale with future CMOS process improvements. • Implementers could always trade-off complexity for performance. [8]

  40. PHY Protocol Aspects(Proposed “PHY Protocol Criteria”) [10]

  41. System Performance • Range vs link success probability and channel environment is plotted below (at 110 Mbps of mode1) CM1~4: Channel model that was estimated [10]

  42. Sensitivity • Receiver performance requirements (Target: PER < 8% with 1024 bytes payload ) [10]

  43. Outline Overview • Motivation • MBOA History Overview • Review of Some OFDM Principles • PHY Specification of Multi-band OFDM Proposal • Responses to Issues • Conclusions & Future Work & References • Conclusions • Future work • References

  44. Conclusions • MB-OFDM is still under revision, and materials of today’s presentation is up to 2004/02/16. • Let us wait and see what will happen in the future • This proposal doesn’t explain many issues clearly, especially in Rx implemetation • Nevertheless, some nice ideas in MB-OFDM such as time-frequency interleaved (time-frequency codes) is worthy of our further discussion or research

  45. Future Work • I will often open IE to browse and see whether there are new things in MBOA and IEEE802.15.3a websites • If needed, I will do further investigation and find out more materials of implementation issues to understand or even simulation • Welcome to discuss with me and thanks for your attention

  46. References (1/2) • [1] Intel Labs, Jeff Foerster, “Ultra-wideband Technology for Short- Range, High-Rate Wireless Communications” • [2] Intel Corporation, Jeff Foerster, “Ultra-wideband Standards and Technology Development,” Mar. 2003 • [3] Meng-Hau Wu, “Introduction to 802.15.3a and its two standard candidates, MB-OFDM and DS-UWB,” Feb. 2004 • [4] MBOA, Evening Public Session, Nov. 2003 • [5] Prof. Tzi-Dar Chiueh, “OFDM and Its Application to High-Speed Wireless LAN” • [6] Robert M. Gray, Stanford University, “Toeplitz and Circulant Matrices: A review” • [7] MES, Darmstadt University of Technology, “OFDM Basics for Wireless Communications” • [8] MBOA, “Multi-band OFDM Physical Layer Proposal (267r6 ppt file),” Sep. 2003

  47. References (2/2) • [9] IEEE802.11a, “Wireless LAN MAC and PHY specifications,” Sep. 1999 • [10] MBOA, “Multi-band OFDM Physical Layer Proposal (268r2 pdf file),” Nov. 2003

  48. Backup Slides

  49. Piconet

  50. This allows a single transmit and receive analog chain at the receiver at all times,

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