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Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: Multi-Rate PHY Proposal for the TG4g PHY Amendment Date Submitted: March 2009 Source: Michael Schmidt, Dietmar Eggert, Frank Poegel, Torsten Bacher, Sascha Beyer, Atmel

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  1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: Multi-Rate PHY Proposal for the TG4g PHY Amendment Date Submitted: March 2009 Source: Michael Schmidt, Dietmar Eggert, Frank Poegel, Torsten Bacher, Sascha Beyer, Atmel Contact: Michael Schmidt, Atmel Voice: +49 351 6523-436 , E-Mail: michael.schmidt@atmel.com Re: TG4g Call for proposals Abstract: PHY enhancements towards TG4g supporting multiple data rates Purpose: PHY proposal for the TG4g PHY amendment 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. Slide 1 Michael Schmidt

  2. Overview DSSS versus Narrow-Band Coding and Modulation Preamble Design Europe Slide 2 Michael Schmidt

  3. DSSS versus Narrow Band Myth: DSSS obtains more range due to processing gain. • In principal, there is no clear advantage of either one. The range is mainly determined by the data rate itself. • DSSS with a spreading factor k introduces k-times more noise due to bandwidth extension (-174 dBm/Hz) which is again compensated by the processing gain. • DSSS may yield an additional coding gain but it is usually far away from the Shannon limit. Current powerful capacitiy achieving codes (LDPC, Turbo Codes) are usually ½ rate codes. Slide 3 Michael Schmidt

  4. DSSS versus Narrow Band Myth: DSSS outperforms NB in case of Multipath • Current IEEE 802.15.4 O-QPSK PHYs apply C(16,4) coding and C(32,4) coding, leading to a fairly moderate spreading factor k of 4 and 8, respectively. • A Rake receiver is not applicable for small k. (It is well known that DSSS with a Rake receiver approaches the Matched Filter Bound, assuming long and good PN DSSS sequences.) • For DSSS-(G)MSK, detection suffers from additional distortions due to inter-chip interference, since it operates at a k-fold chip rate according to the spreading factor k. This may not be completely compensated by the processing gain if k is small or if M-ary code sets are applied. Slide 4 Michael Schmidt

  5. DSSS versus Narrow Band Cont. ~ Myth: DSSS outperforms NB in case of Multipath • For DSSS with a spreading factor k >= 4, chip based equalization is not really applicable, since the SNR at the chip level is decreased by k, leading to unreliable estimates. • chip based equalization is only useful for very mild spreading (1/2 rate coding) or no spreading at all. • Example: PHY according to the GSM standard • applies GMSK-BT=0.3 with ½ rate convolutional coding • trellis based SISO equalization is supported: • trainings sequences for channel estimation • trailing zeros Slide 5 Michael Schmidt

  6. DSSS versus Narrow Band Cont. ~ Myth: DSSS outperforms NB in case of Multipath • When applying (G)MSK-DSSS: • seek for improved code sets compared to C(32,4) and C(16,4) of IEEE-802.15.4-2006 • consider traditional large spreading gains (64 … 128) based on PN sequences. This, however, may lead to low data rates, conflicting with a target PSDU length of 1500 octets. Slide 6 Michael Schmidt

  7. DSSS versus Narrow Band Targeting low data rates NB: • Low data rates are obtained by a low chip rate equal to the bit rate. • This requires appropriate narrow band filtering, which is challenging for both low-IF and ZIF receivers. • Requires low IF in order to avoid a high pole-Q • Reduced IF increases 1/f noise • More complicated DC removal • Requires advanced AD conversion and base band filtering when targeting a high IF/BW ratio Slide 7 Michael Schmidt

  8. DSSS versus Narrow Band Cont. ~ Targeting low data rates DSSS: • Low data rates can be obtained by appropriate spreading at a certain fixed chip rate. • Different data rates can be obtained by simply changing the spreading gain. • Bandwidth of the receive filter can be kept constant with a fixed IF • Simplifies design of the analog receiver front end Slide 8 Michael Schmidt

  9. DSSS versus Narrow Band Cont.~ Targeting low data rates Slide 9 Michael Schmidt

  10. DSSS versus Narrow Band Impairments • Narrow band interferer (e.g. distortions due to CW signal ) • Clock offset Interference Avoidance • NB is more flexible when FH or AFA is applied Slide 10 Michael Schmidt

  11. DSSS versus Narrow Band The choice of DSSS or NB is influenced by Regularity Requirements FCC CFR 47: • For a BW < 500 kHz, FH is mandatory • BW >= 500 kHz avoids the need for hopping but reduces the number of hopping channels for AFA ETSI, ERC • Output power: • 6.2 dBm/100 kHz for restricted wideband operation within 865-868 MHz • DSSS appears to be useful here • Operation within 868.000-869.650 MHz is too limited for DSSS • Pure GFSK modulation seems to be useful as suggested in [3] • Out of Band Power: -36 dBm/100 kHz Slide 11 Michael Schmidt

  12. Coding and Modulation Slide 12 Michael Schmidt

  13. Coding • Current IEEE 802.15.4 DSSS codes C(32,4) and C(16,4) obtain certain constrains which are obsolete when applying a chip scrambler • The preamble should be independently designed (must not be necessarily assembled with code words) Slide 13 Michael Schmidt

  14. Coding • Options for improved DSSS block codes C(64,4) C(32,4) C(16,4): • sub codes of near extended BCH codes • sub codes of Reed Muller Codes • Improve existing IEEE 802.15.4 codes by considering MSK modulation rather than O-QPSK and remove constrains • C(8,4) coding seems to have little potential for an additional coding gain (code length is too short). • Either skip ½ rate coding or apply ½ convolutional coding • Consider DSSS codes with a large spreading gain, e.g. C(64,1). This, however, will have a severe impact on the preamble design. (Preamble detection must be able to cope with this kind of spreading gain.) Slide 14 Michael Schmidt

  15. GMSK Why GMSK? • Constant envelope signal • Permits direct modulation transmitter design • GMSK considerably reduces side lopes compared to MSK • Reduced interference • MSK hardly applicable for the EU band • GMSK (GFSK with modulation index h = 0.5) is the preferred choice but a modulation index h < 0.5 may be unavoidable for the EU band 869.400-869.650 MHz • GMSK is detectable for IEEE 802.15.4 legacy devices usingO-QPSK with appropriate coding Slide 15 Michael Schmidt

  16. GMSK Known Issues: • Transmitter design is more complicated compared to MSK • Accurate I/Q based GMSK shaping can be implemented within the base band but a mixer is required. • Avoid mixed architecture by direct modulation. This, however, might be subject to patent issues. • Receiver may require Trellis based detectors • depends on the choice of BT • less relevant for DSSS-GMSK • less relevant if channel equalization is considered anyway (see, e.g. GSM PHY) Slide 16 Michael Schmidt

  17. Preamble Design The length of the preamble should be related to • the maximum frame length • the target of the receiver sensitivity • the maximum clock offset Slide 17 Michael Schmidt

  18. Preamble Design • Good AKF properties for timing synchronisation and clock offset estimation • Channel estimation (useful for equalization when applying ½ rate coding or no coding) • Dissolve IQ images during preamble detection if IF is symmetric w. r. t. channel spacing. Slide 18 Michael Schmidt

  19. Preamble Design Cont.~ Dissolving IQ Images Problem: • Avoid false preamble detection on IQ images, since the transceiver is usually occupied with SFD search during this time period. Slide 19 Michael Schmidt

  20. Preamble Design IQ imbalance: Since is approximately left-analytic Slide 20 Michael Schmidt

  21. Preamble Design Cont.~ Dissolving IQ Images • A conjugate signal can be differentiated from its non-conjugate in case of frequency modulation • This, however, is not possible with a trivial {…0 1 0 1 0 1 0 1 …} preamble sequence, since the time reference is unknown during preamble detection. Slide 21 Michael Schmidt

  22. Preamble Design Possible candidates: • multiple (4…8) extended M-sequences of length: • 32 for NB-GFSK • 64 or 128 for DSSS-GFSK Slide 22 Michael Schmidt

  23. European Frequency Band (863 – 870 MHz) • Harmonized standard ETSI EN 300 220 defines frequency band of 863 – 870 MHz with different power limits for digital modulation systems using DSSS • Duty cycle limitations do not apply to systems using Listen Before Talk techniques combined with Adaptive Frequency Agility (AFA) Slide 23 Michael Schmidt

  24. ERC Recommendation 70-03 Regulatory parameters for 863 – 870 MHz [1] Slide 24 Michael Schmidt

  25. ERC Recommendation 70-03 Regulatory parameters for 863 – 870 MHz [1] Slide 25 Michael Schmidt

  26. EU DSSS-Proposal • DSSS-GMSK BT = 0.3…0.5 • Chip rate: 400 kchip/s • Spreading factors: 1, 2, 4, 8,16, 32, 64 • Data rates: 400, 200, 100, 50, 25, 12.5, 6.25 kbps • 6.2 dBm/100 kHz when restricting to channels {4, 5, 6, 7} • 0.8 dBm/100kHz when restricting to channels {4, 5, 6, 7, 8, 9, 10} • -4.5 dBm/100kHz when applying all channels • LBT with AFA Michael Schmidt

  27. EU DSSS-Proposal Spectral properties: • GMSK • Chip rate: 400 kchip/s • BT = 0.3 Michael Schmidt

  28. EU NB-Proposal • Utilize band 868.000 – 869.650 MHz, similar as suggested in [3] • GFSK, modulation index < 0.5 possibly unavoidable • Chip rate: 100 kchip/s • Data rates: 100 kbps or 50 kbps (preferably by ½ rate convolutional coding) • Channel spacing >= 250 kHz • Output power 14 dBm and possibly 27 dBm for a channel with 869.400-869.650 MHz, however: • Complex PA design • out of band limit of -36 dBm /100 kHz is quite challenging • LBT with AFA Michael Schmidt

  29. Bibliography [1] ERC RECOMMENDATION 70-03 (Tromsø 1997 and subsequent amendments); RELATING TO THE USE OF SHORT RANGE DEVICES (SRD); Recommendation adopted by the Frequency Management, Regulatory Affairs and Spectrum Engineering Working Groups,Version of 18 February 2009. [2] Draft ETSI EN 300 220-1 V2.2.1 (2008-04) Electromagnetic compatibility and Radio spectrum Matters (ERM); Short Range Devices (SRD); Radio equipment to be used in the 25 MHz to 1 000 MHz frequency range with power levels ranging up to 500 mW; Part 1: Technical characteristics and test methods. [3] Khanh Tuan Le, “Preliminary Proposal for a Multi-Regional Sub-GHz PHY for 802.15.4g”, 802 Plenary Meeting 12th March, 2009 Vancouver BC. Michael Schmidt

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