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Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)

Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: FPP-SUN Implementation Considerations Date Submitted: July 5, 2009 Source: Rishi Mohindra, MAXIM Integrated Products Contact: Rishi Mohindra, MAXIM Integrated Products

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Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)

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  1. Rishi Mohindra, MAXIM Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: FPP-SUN Implementation Considerations Date Submitted: July 5, 2009 Source: Rishi Mohindra, MAXIM Integrated Products Contact: Rishi Mohindra, MAXIM Integrated Products Voice: +1 408 331 4123 , E-Mail: Rishi.Mohindra@maxim-ic.com Re: TG4g Call for proposals Abstract: PHY proposal towards TG4g 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

  2. Future Proof Platform for SUN: Implementation Considerations Rishi Mohindra IEEE 802 Plenary Session San Francisco July 2009 Supporters: Emmanuel Monnerie [Landis & Gyr], Partha Murali [Redpine], Steve Shearer [Independent], Shusaku Shimada [Yokogawa Electric Co.], Bob Fishette [Trilliant], Sangsung Choi, [ETRI], Roberto Aiello [Independent], Kendall Smith [Aclara], David Howard [On-Ramp]

  3. Contents • OFDM as Future Proof Technology • Path loss • OFDM implementation considerations • Zero-IF Transceiver Architecture advantages • Low-IF receivers for GFSK • Phase Noise • Receiver dynamic range • Crystal Tolerance and Coarse + Fine Frequency Error Correction • Spectral mask & PA backoff • Battery Life Calculations • Legacy Meter Reading example use cases • Calculations • Gate count and relative cost • Conclusions

  4. Legacy Meter Reading technology issues: Different frequency bands, modulations, network topologies, interference conditions, MACs, Network Layers etc. No single existing flavor can work in all situations Data rates & modulation techniques are “old” and there is lack of advantages that are offered by today’s modern VLSI digital technologies. Digital gate count and signal processing capabilities reflect 15 year old 0.35 um CMOS technology. No existing deployment technology offers the possibility of “future proofing” through improved data rates and range Poor spectral efficiency (bits/s/Hz) Possible misconception that “simple is cheap” without a broader vision of modern technology advantages relating to cost and performance The 802.15.4g FSK proposals do not fully resolve the above issues, and will largely provide a new face to old technology with “commonality” and legacy compatibility as the driver instead of “Future Proofing.” Slide 4

  5. OFDM as Future Proof Technology Physical Layer feature requirements 40kb/s to 1Mb/s through put data rates at nodes 20 year battery life for all non-AC powered nodes Cost Requirements Solution cost to be comparable to legacy solutions while supporting significantly higher data rates 1Mb/s solution cost should only be a small increment to the 40kb/s BOM Special Network Requirements for Gas & Water meters Not depend on AC powered nodes for majority of repeaters or Hops. Support max # nodes (e.g. 100000 nodes) per BS in a MESH or STAR network 140 dB desirable link budget for up to a few miles range Should not be limited in range due to multipath conditions i.e. work well in rms delay spreads in excess of 10 usec. Special Requirements for MESH networks with AC powered hops Support 10x increase in network throughput with 100 kb/s link data rates at battery operated nodes. Allow upgrade paths for up to 1 Mb/s link data rates between AC powered nodes or hops for improved network capacity in the future Only OFDM offers a common future-proof technology that is scalable according the above needs Slide 5

  6. A look at Path Loss Path Loss analysis set up: Freq = 900 MHz MESH: Transmitter and Receiver antenna heights = 0.5m STAR: Base Station antenna height = 8.9m, nodes antenna height = 0.5m Path Loss models: Free Space 2-ray LOS with loss-less ground reflection for STAR and MESH Hata urban for STAR and MESH Hata suburban for STAR Hata rural for STAR Slide 6

  7. Path Loss Models Slide 7

  8. Hata Path Loss data Freq = 1.8GHz, MS height = 2m, BS height = 1m Typical urban in Southern England X-axis: 3 = 1000m Slide 8

  9. Link budget and receiver sensitivity • Typically 2 km at 900MHz for shown Hata suburban model for STAR

  10. Path Loss Conclusions Path Loss at 1km range based on Hata models: Urban MESH = 154 dB, while Urban STAR = 137 dB due to antenna height difference Suburban STAR = 127 dB Rural STAR = 108 dB Rural STAR (Hata) matches the 2-ray LOS model for larger distances. Allow extra loss for gas/water meter at below ground level Hata model uses narrow band measurements & is prone to more spatial fading fluctuations Using wider modulation bandwidths (up to 1MHz for OFDM) will drastically reduce the deep spatial fades and reduce link budget margin requirements Implementation considerations to combat path loss: use a modulation that requires lowest S/N, e.g. OFDM BPSK requires 1dB S/N versus 11 dB for GFSK. Minimize receiver noise figure & Modem Implementation loss. OFDM has only 0.5dB modem implementation loss (in addition to receiver noise figure). GFSK has 2-3 dB implementation loss (strongly phase noise and filter ISI dependent) without time-domain equalizer. Total Receiver Loss = Noise Figure + Implementation Loss. Total OFDM Receiver Loss can easily be kept below 4dB in 0.13 um RF CMOS technology at very low battery current Slide 10

  11. OFDM Data Rates SNR shown for 1000 bytes 10% PER in AWGN Slide 11

  12. Receiver Sensitivity for 4 dB total loss Slide 12

  13. Receiver Sensitivity for 4 dB total loss Slide 13

  14. Link Budget for 30 dBm transmitter & 4 dB receiver loss,0 dBi antennas Slide 14

  15. Zero-IF Architecture for OFDM • Key Features • Direct conversion between RF and IQ baseband • Same RF Synthesizer frequency between Transmit & Receive: allows fast turn-around for ACK etc • Sharing of IQ Low-pass between transmitter & receiver (due to Time Division Duplexing) means smaller die area Slide 15

  16. Zero-IF Receiver Architecture for OFDM Advantage of Zero-IF No Image rejection issues as far as Blockers are concerned. Low-pass filter at 0 Hz IF frequency consumes significantly lower current than Low-IF Polyphase filters, and also has fewer active stages in filter DC notch has no impact on OFDM since there is no “DC-subcarrier.” Notch is accomplished by digital DC cancellation every packet (OFDM STF field is DC balanced in IQ, unlike GFSK that requires a blank period for DC calibration) Architecture proven to consume lowest current Uncoupled I & Q signal paths allows unlimited Digital Image Rejection calibration Possible disadvantages 1/f flicker noise in IQ base band signal paths can slightly increase noise figure of subcarriers close to DC Spectrum at Base Band dB Blockers DC 0 f Low-pass Channel Filter response Slide 16

  17. Analog Zero-IF + Digital Low-IF Receiver Architecture for OFDM Spectrum at Base Band and Analog+Digital Channel Filtering dB Blockers DC 0 f 0 Analog Low-pass Anti-aliasing Filter response (before A/D) >80 dB Digital Image Rejection in “real-time” over Temperature Analog DC notch Not required Digital Polyphase Channel Filter response (after A/D) Slide 17

  18. Detailed Zero-IF OFDM Receiver Core example Slide 18

  19. Low-IF Architecture for FSK & ASK Disadvantage of Low-IF Poor Image rejection over temperature even after calibration (may not be an issue in ISM bands) Complex Polyphase filter consumes significantly higher current than low-pass filters due to higher operating IF frequency, and more active stages in filter DC notch can have an impact on Group Delay flatness that can degrade ISI for GFSK Significantly larger die area for a given data rate compared to OFDM zero-IF receiver Slow Transmit-Receive switching due to RF Synthizer frequency programming & settling Why used because FSK receivers can’t work well with Zero-IF IQ DC offsets Mitigate close-to-DC 1/f flicker noise in IQ base band signal paths Slide 19

  20. Low-IF Architecture for FSK, ASK & DSSS dB Spectrum at Base Band Blocker atImage DC 0 frequency Image Rejection DC notch Polyphase Channel Filter response Slide 20

  21. Local Oscillator Phase noise impact on OFDM EVM,with Symbol Time Scaling Option 2 802.11a Option 3 Such good EVM is Only needed for 256-QAM • Conclusion: • 16-QAM OFDM tolerates Phase Noise that is 15 dB relaxed compared to GFSK transceivers • Results in significant Battery current reduction • Out-of-band blocking will largely influence phase noise • BPSK OFDM requires10dB smaller C/I compared to GFSK • BPSK OFDM tolerates 10dB higher phase noise for same blocker and Rx sensitivity Slide 21

  22. Receiver Dynamic range for Active Channel Filter Output Non-coherent GFSK receiver channel filters require 8 – 13 dB higher filter dynamic range compared to OFDM receiver Extra current savings possible for OFDM receiver filter Receiver Saturation Level Receiver Saturation Level Receiver Saturation Level >2 dB GFSK BT=0.5 Df=0.25Rb Non-coherent DR > 15 dB dB 13 dB >3 dB OFDM QPSKr=1/2 DR > 7 dB 4 dB >2 dB DR > 3 dB 1 dB OFDM BPSK r=1/2 Noise Floor Slide 22

  23. Complete OFDM Receiver Dynamic range & Noise Figure based on FSK receiver specifications (16-FFT case) Total effective S/(N+D) curve • Noise Figure (3.2 dB), IP3 (-23 dBm), Phase Noise and blocking specs kept same as FSK receivers • Equivalent receiver performance is shown above • It is able to support even 64-QAM OFDM ! • Over-kill for SUN applications with BPSK to 16QAM Performance looks good for 64-QAM OFDM ! Slide 23

  24. Crystal Tolerance and Short Training Field Local Oscillator Receiver digital channel filterbefore frequency correction Relative Frequency error due to crystal Receiver digital channel filterafter frequency correction Carrier Leakage • Only Null STF Subcarriers lost due to frequency error prior to frequency error correction • These are “used” in the LTF and Symbols, and are shifted back inside channel filter after digital frequency correction Crystal tolerance is not an issue since many STF null subcarriers can be lost due to frequency error before the Freq Error Estimation algorithm is affected. 40 ppm tolerance is OK for 500kHz OFDM bandwidth. Slide 24

  25. Integer and Fractional Frequency Errors Local Oscillator Dfreq2 Short Training Field Subcarriers Dfreq1 Dfreq Center Null subcarrier Null subcarriers Carrier Leakage Total frequency error Dfreq = Dfreq1 (integer error) + Dfreq2 (fractional error) Slide 25

  26. Frequency Error Estimation sequence Short Training Field (10x repetition) Long Training Field (2x repetition) Symbols time Correlation + AGC Coarse Freq error correction + DC cancellation Fine Freq error correction + Channel Estimation Delay Dt X Fractional freq error Subcarrier Correlation Integer freq error Mean FFT STF A* conjugate Slide 26

  27. 500kHz OFDM Modulation bandwidth meets FCC & ETSI Mask Compliance for non-FHSS ETSI allows 25mW transmit power in a 600kHz bandwidth at 868.3MHz Zigbee operates in this band due to the allowed power and bandwidth Spectral plot overlaid with the ETSI emission mask is shown next slide About +-43ppm frequency error can be tolerated at 868MHz before violating the ETSI mask that is fixed at a channel center frequency of 868.3MHz Allowed ETSI transmitted power level in 515kHz bandwidth is +13.3dBm. FCC requires 500kHz min modulation bandwidth for non-FHSS. Up to +30dBm transmit power allowed. Should meet -41dBm/MHz emission limit in Restricted Bands at 2390MHz & 2483MHz. Plot shows full compliance Slide 27

  28. ETSI Spectral Mask and PA backoff • +13 dBm output power • PA = Rapps model with Rho = 3 Slide 28

  29. Rapps PA model. 6 dB back off from saturation power for PA, rho = 1. At 2400.5 MHz channel, meets requirement for 2390MHz restricted band (10 MHz from carrier). At 2477 MHz channel, meets requirement for 2483 MHz restricted band (6 MHz offset from carrier). dBm/MHz dBm/10kHz Slide 29

  30. EVM versus PA Backoff 16-QAM, r=0.5 64-QAM, r=0.5 PA = Rapps model with Rho = 3 PA efficiency can be 25% or higher for BPSK & 16-QAM OFDM due to small back off from saturation to meet EVM requirement Slide 30

  31. ETSI Emission Mask at 868.3 MHz(fe = 300 kHz) Slide 31

  32. ETSI allowed transmitted power level(1% duty cycle alowed with LBT) Slide 32

  33. ARIB Emission Mask for 954 MHz Slide 33

  34. GFSK requires 20 dBm output for the same link performance in AWGN as a 10 dBm OFDM transmitter. Slide 34

  35. Legacy Meter Reading example use cases for battery life calculation • STAR topology cases: • 50000 nodes per BS, 12.5 kHz channels in Licensed bands, GFSK, 30 dBm Tx, expensive crystal with TBD ppm life time accuracy, slow real time frequency error calibration at nodes for sub-ppm timing accuracy, slotted time structure with no collision, 6 hour transmission gaps per node. Lower PA efficiency due to ramp-time for spectral compliance. • Light MESH topology cases: • 25000 nodes per network with one concentrator point, up to 3 hops from end-nodes to concentrator point, ISM band FHSS GFSK, hops over AC powered devices only (1 per sq mile). • IEEE802.11n WLAN at 2.4GHz with 20 year battery life, 2 mile network radius per concentration point, hops over AC powered devices. • MESH topology cases: • 50000 nodes per network including up to 20000 AC powered hop-nodes, FHSS GFSK or MSK. • 2000 nodes per concentrator, multiple hops. Multiple nodes per home. 2.4GHz ISM band, IEEE802.15.4 OQPSK DSSS, 250 kb/s, 30dBm Tx at BS, 24 dBm Tx at gas & water nodes. Rake receiver. 40 ppm crystal tolerance. 2 mile network radius.

  36. Battery Life Assumptions for STAR network • 22000 seconds Frame Interval • 48.675 kb/s data rate for OFDM and GFSK • 250 ms Receiver ON time per Frame (in addition to payload receive bursts) • One transmit and one receive payload burst per frame • 50 bytes Transmit and Receive payload data per burst (excluding PHY overheads) • OFDM transmit power +24 dBm, GFSK transmit power +30dBm for similar AWGN link budget • 25 years battery self-discharge time • Background application programs and synchronization timers running • Current consumptions accounted for: • Transmitter (analog + digital) • Receiver (analog + digital) • MAC • Application (running continuously with 10% duty cycle) • Synchronization timer (running continuously) • Battery self discharge

  37. OFDM Battery Life Calculations (based on STAR network) Slide 37

  38. OFDM mAH contributions (based on STAR network) Slide 38

  39. GFSK Battery Life Calculations (based on STAR network) Slide 39

  40. GFSK mAH contributions (based on STAR network) Slide 40

  41. OFDM data rate = 46.875 kb/s using BPSK r=1/2, 24dBm transmit power • GFSK data rate = 46.875 kb/s, BT=h=0.5, 30dBm transmit power • Same AWGN link budget for OFDM and GFSK • X-axis = #Tx = #Rx bursts per 22000 seconds with 50byte payloads

  42. OFDM data rate = 187.5 kb/s using 16-QAM , r=1/2, 24dBm transmit power • GFSK data rate = 46.875 kb/s, BT=h=0.5, 30dBm transmit power • Assuming OFDM has 8 dB multipath advantage over GFSK as an example • X-axis = #Tx = #Rx bursts per 22000 seconds with 50byte payloads

  43. SoC Gate Count, Die Area & cost • Note: OFDM digital gate count provided by Redpine Signals • Cost • GFSK SoC costs 1.2x more than OFDM Soc • RF Power Amplifier + associated electronics cost: GFSK 30 dBm transmitter costs significantly more than OFDM 20 dBm transmitter Slide 43

  44. Conclusions Advantages of OFDM over GFSK implementations for similar data rates Total chip set costs less Tolerates significantly higher phase noise Potentially use much cheaper crystals Requires simpler RF Transceiver architecture SoC has smaller die area Battery life is better and potentially 2x longer 10 dB link budget advantage in AWGN Potentially 20 to 30 dB link budget advantage in frequency selective fading conditions where GFSK may still be operable Slide 44

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