Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [Narrow Band PHY Prop

1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [Narrow Band PHY Proposal for 802.15.4g] Date Submitted: [10 March 2009] Source: [Cristina Seibert] Company [Silver Spring Networks] [Benjamin A. Rolfe] Company [Blind Creek Associates] [George Flammer] Company [Silver Spring Networks] Address [] Voice:[] E-Mail: [cseibert @ silverspringnet.com] [ben @ blindcreek.com] [gflammer @ silverspringnet.com] Re: [] Abstract: Preliminary Proposal for a Narrow Band PHY for 802.15.4g Purpose: Technical Proposal Notice: This document has been prepared to assist the IEEE P802.15. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.15. C. Seibert, B. Rolfe, G Flammer

2. Narrow-band PHY Proposal for 802.15.4g Smart Utility Networks (SUN) Preliminary Technical Features Summary C. Seibert, B. Rolfe, G Flammer

3. PHY Overview Contents • Key Characteristics of SUN • General PHY Features • Key PHY Techniques • Hopping and Modulation • Data Whitening • PHY Processing Aspects • Data Transfer • PHY Frame (PPDU) • PHY Channel Plan • MAC Support • Conclusions C. Seibert, B. Rolfe, G Flammer

4. Key Characteristics of SUN PHY Some of the characteristics of the W-SUN include: • Low data rate: over the air data rates of 40kb/s up to 1 Mb/s • High resilience and adaptability in the presence of interference and good coexistence properties with both like systems and non SUN systems. • Enhanced IP support (> 1500 octet payload) • Ubiquity • Very high reliability and availability • Dynamic scaling to very large aggregate networks • Peer to Peer, minimal infrastructure-dependent operation MAC C. Seibert, B. Rolfe, G Flammer

5. General PHY Features • Frequency Hopping Spread Spectrum: • Individual channels at nominal < 250 KHz, 20 dB down from peak • 300 KHz channel spacing • Channel plan across multiple bands • Simple PHY frame • Support for 2047 octet payload (802.1 MTU) • w/32-bit CRC on PHY payload (802 standard generator) • Nominal (base)100kbps data rate, alternate rates possible • Simple FSK modulation • Data whitening of payload bits C. Seibert, B. Rolfe, G Flammer

6. Hopping and Modulation • FHSS: • A method of transmitting radio signals • Carrier tunes to various channels • Channel pseudorandom sequence known to both transmitter and receiver. • FSK: • A frequency modulation scheme • Digital information is transmitted through discrete frequency changes of a carrier wave. • MFSK is a spectrally efficient form of FSK. C. Seibert, B. Rolfe, G Flammer

7. Data whitening • Whitens payload data (PSDU/MPDU) to avoid long series of 1’s and 0’s. • 8-bit scrambler (255 bit sequence) • taps at bits [8,4,3,2] • Scrambler re-seeded periodically C. Seibert, B. Rolfe, G Flammer

8. PHY Processing Aspects • The physical layer (PHY) provides the on-air interface between communicating nodes. Its header is sent “in the clear” and its payload (the MAC/DLL data) is scrambled by the PHY Layer. • The major functions of the PHY layer is the recovery of bit timing, determination of start-of-frame, recovery of the scrambling seed, and length of the PHY frame, and performing a 32-bit cyclic-redundancy-check against the unscrambled MAC/DLL data field. • The PHY layer then processes the incoming stream for length bytes, calculating the CRC-32. If the CRC-32 matches that received, the PHY layer passes the frame to the MAC/DLL for subsequent processing. C. Seibert, B. Rolfe, G Flammer

9. Data Transfer C. Seibert, B. Rolfe, G Flammer

10. PHY Frame (PPDU) • Support for 2047 octet payload (802.1 MTU) • IEEE CRC-32 on PHY frame Structure of PPDU C. Seibert, B. Rolfe, G Flammer

11. PHY Channel Plan • Large number of narrow band channels across multiple bands • Hop-able across all bands implemented • At least one band from: • 779-787 MHz (China) • 840 to 956 MHz • 868–868.6 MHz (e.g. Europe, China, others) • 902–928 MHz (e.g., Americas, China, others) • 950-956 MHz (Japan) • 865.6-867.6, 840.5-844.5 • 2400–2483.5 MHz (worldwide) • Other bands available? C. Seibert, B. Rolfe, G Flammer

12. PHY Channel Plan Increase channels per page to support more channels C. Seibert, B. Rolfe, G Flammer

13. PHY Channel Plan Example: 902–928 MHz Chan n: 902.3+(n*0.3) where n=0 to 84 C. Seibert, B. Rolfe, G Flammer

14. MAC Support • Meet regulatory requirements for channel occupancy • E.g. max on channel 0.4 seconds • Visit each channel in sequence before re-visiting • OK to skip channel (black-listing) • Reliability support • Packet acknowledgment • Packet retransmission C. Seibert, B. Rolfe, G Flammer

15. Conclusions • Proposal consistent with scope of approved SUN PAR • Support millions of users at low data rates • Robust and available • Low cost and ubiquitous • Operation in unlicensed spectrum • Applications supported by this proposal are consistent with those proposed by utilities and manufacturers during PAR approval process and in tutorials • Some references: • 15-08-0199-00-wng0-the-smart-grid.ppt • 15-08-0455-00-0000-utility-view-of-nan-drivers-and-requirements.pdf C. Seibert, B. Rolfe, G Flammer

16. Part 2Background: Narrow Band PHY ProposalFeature Rational C. Seibert, B. Rolfe, G Flammer

17. General Tradeoffs - FHSS • + Many channels => frequency diversity benefit => good coexistence and interference mitigation • + Narrow channels => long symbol time => low ISI, low processing requirements • + Narrow channels => minimal frequency selective fading => no need for expensive equalizers => low cost and simplicity • + Low PAPR => inexpensive power amplifiers => low cost and simplicity • + Does not rely on high precision on clocks and filters => low cost • + Proven actual deployments on the scale of millions of units • SSN, Coronis/France Telecom, Elster, CellNet/Hunt, GE, Eka • - high bandwidth overhead for high bit rates transmission C. Seibert, B. Rolfe, G Flammer

18. Narrow Band Channels • Supports 100kbps with simple modulation • Allows large number of channels • 84 in 900MHz, 277 in 2.4GHz • Can meet “Frequency Hopper” regulations (FCC, others) • Channel diversity advantages for interference mitigation • Lots of channels good for network capacity • Spectrum scavenging • Utilization of channels across multiple bands • Some that may be under-used now • Adjacent channel rejection (simple inexpensive approach) • Established Low-IF receivers with image ~100KHz • 300KHz spacing parks the next channel’s energy away from the image • Lots of flexibility for designer/implementation C. Seibert, B. Rolfe, G Flammer

19. Channel Diversity Advantage Current 802.15.4 PHY Channels per band: • 902MHz 10 Channels • 2.4GHz 16 Channels NB PHY: • 902MHz 84 Channels • 2.4GHz 277 Channels C. Seibert, B. Rolfe, G Flammer

20. Channel Diversity • Random Channel Access (1,10,84 channels) • Independent access probability (no channel correlation • 1% Duty Cycle average (all nodes) C. Seibert, B. Rolfe, G Flammer

21. Channel Diversity • Random Channel Access (1,10,84 channels) • Independent access probability (no channel correlation • 5% Duty Cycle average (all nodes) C. Seibert, B. Rolfe, G Flammer

22. Channel Diversity • Random Channel Access (1,16,277 Channels) • Independent access probability (no channel correlation • 5% Duty Cycle average (all nodes) C. Seibert, B. Rolfe, G Flammer

23. Channel Diversity • Random Channel Access (1,16,277 Channels) • Independent access probability (no channel correlation • 5% Duty Cycle average (all nodes) C. Seibert, B. Rolfe, G Flammer

24. PHY Frame • Scrambler seed in PHY header • So it can be changed • Allows flexibility in implementation at upper layers • Length to support 2047 octets for IP • Necessitates 32-bit CRC Structure of PPDU C. Seibert, B. Rolfe, G Flammer

25. FSK Modulation • FSK is simple, cheap, proven • Proven technology/Low cost solution (low complexity) • Many low cost implementations available • Constant envelope • Allows non-coherent generation / demodulation or coherent implementations • Allows for a limiter discriminator detector (simple, cheap) • Minimal filtering requirements • 25kHz frequency deviation (±3kHz) • modulation index 0.5 (MSK), has spectral advantages • constant phase modulation • ±3kHz tolerance simplifies implementation (more options) • Easy low cost implementation C. Seibert, B. Rolfe, G Flammer

26. Error Detection Depend on detection and retry • Environment typically interference limited • Long burst errors more likely than random bit or short burst errors • FEC trade-off More bits at low bit rate => more time on air => higher probability of interference • Retry + channel diversity + low duty cycle • Diversity increases prob. of success on retry • Diversity reduces interference (< prob of error) • Low duty cycle reduces probability of interference C. Seibert, B. Rolfe, G Flammer

27. Part 3Background: Data Whitening C. Seibert, B. Rolfe, G Flammer

28. Agenda Problem Statement Existing implementations Receiver designs Demodulator designs Problems with demodulators Whitening effect Problems with whitening approaches Various solutions Benefits of preferred solution C. Seibert, B. Rolfe, G Flammer

29. Problem Statement • Long runs of 1s or 0s will occur in data (payloads) • Whitened data aids in • bit timing recovery and tracking • Minimize DC bias • Common technique (everyone does it?) • 802.11, 802.15.1, 802.15.3, … • FHSS, DSSS, OFDM • FSK/GFSK, QPSK, DBPSK, DQPSK n-QAM … • Interference limited application scenarios need minimal bandwidth consumption (e.g., as opposed to Manchester encoding) • Arbitrary data can ‘unscramble’ in fixed scrambler • Low or no on-air overhead C. Seibert, B. Rolfe, G Flammer

30. What’s Typical 802.15.1 (Bluetooth) – Fixed ‘seed’ 7-bit LFSR {4,7} Fixed initialization 802.15.3 – four seeds Each a 15-bit sequence 4 different seed values Seed ID in PHY header 802.11 – Variable seed sent as part of header FHSS, DSSS and OFDM use scramblers, 7-bits, taps {4,7} FHSS, DSSS use fixed seed values OFDM sends seed value in PHY header SERVICE field: Pseudo-random non-zero seed set by sender C. Seibert, B. Rolfe, G Flammer

31. Many receiver designs C. Seibert, B. Rolfe, G Flammer

32. Many demodulator designs • Differential • Foster-Seeley • Slope • Ratio • Quadrature • Phase Lock Loop • Foster-Seeley C. Seibert, B. Rolfe, G Flammer

33. FM demodulation The received signal is delayed and mixed with the original signal giving difference signal output representing the frequency/phase modulation in the incoming signal. This is differential detection, and the circuit is widely known as a discriminator. To recover the data, the bit-timing must be recovered and a decision taken on whether the signal represents a “1” or a “0.” The data recovery is done by some method of “data slicing,” followed by a timing-recovery circuit. The data recovery mechanism is critical, as the output of the analog discriminator can sit on a varying DC level.These schemes are well-proven but have a drawback - the need to deal with DC drift in the demodulated output – which is exacerbated by zero-IF receiver designs. C. Seibert, B. Rolfe, G Flammer

34. Problems with demodulators DC coupled demodulators AC coupled demodulators can ‘droop’ toward a rail if extended sequences of ones or zeros are received C. Seibert, B. Rolfe, G Flammer

35. Solution = Whiten data Scrambling or whitening makes long sequential runs of ones or zeros statistically unlikely…. … increasing the average frequency of edge transitions…. …making tracking bit synchronization easier… … and keeps AC-coupled demodulators average signal within the hysteresis zone C. Seibert, B. Rolfe, G Flammer

36. Problems with Scramblers Data is not always white and there exist data patterns which can ‘unscramble’ the scrambler resulting in undesired long runs of ones or zeros This is rare. This is fatal. C. Seibert, B. Rolfe, G Flammer

37. Whitening Solution #1(Manchester encoding) • Manchester encoding introduces a level transition for each bit, converting sequential zeros or ones into an alternating waveform running at twice the data rate. • This means that the worst-case bandwidth required for Manchester (or 802.3) encoding will be twice that required for the data rate in the absence of encoding • Doubling the BW will result in a range reduction… • Next option… C. Seibert, B. Rolfe, G Flammer

38. Scrambler Generic scrambler circuitry constructed from programmable shift register. The Taps can be pre-loaded with arbitrary ‘seeds’ which will produce different output streams based given identical input streams. C. Seibert, B. Rolfe, G Flammer

39. Scrambler Solution #1(try before sending) Use multiple seeds: If data breaks one, less likely to break the other Like 15.3 (4 seeds) Many methods: Guess and adjust A-priori knowledge C. Seibert, B. Rolfe, G Flammer

40. Scrambler Solution #2 In a FHSS system, change the ‘seed’ every channel and if the packet is not received on a particular channel… Just take the hit – knowing that the next channel will scramble differently and will statistically be incredibly unlikely to be non-receivable. C. Seibert, B. Rolfe, G Flammer

41. Additional Benefits of #2 Provides orthogonally for geographically overlapping networks. Provides an ‘extra long’ start word - moving from 16 bits to 24 bits dramatically decreases the packet false starts Provides some protection from ‘replay’ attacks Provides protection from ‘spurious reception’ C. Seibert, B. Rolfe, G Flammer

42. No extra byte needed If the benefits of the separate ‘scramble seed’ are not required, transmitting the seed becomes superfluous. Such applications can eliminate the costs by: • Shortening the preamble by one byte • Setting the first start byte identically to the preamble byte • There is no need to ever change the scrambler seed C. Seibert, B. Rolfe, G Flammer

43. Conclusion(s) • Whitening has been shown to be necessary • Scrambling has been shown to be low cost whitening implementation • Static scrambler seeds have been used to some success (802.11, etc.) • Dynamic seeding of has multiple benefits at same cost(s) Proposal: include dynamic seed for 802.15.4g scrambler. C. Seibert, B. Rolfe, G Flammer