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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: [Elster & France Telecom proposal] Date Submitted: [15 September, 2011] Source: [Jean Schwoerer, Nicolas Dejean] Company [France Telecom R&D, Elster]

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

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  1. J. Schwoerer (France Telecom) – N. Dejean (Elster)) Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [Elster & France Telecom proposal] Date Submitted: [15 September, 2011] Source: [Jean Schwoerer, Nicolas Dejean] Company [France Telecom R&D, Elster] Address [28 chemin du vieux chênes 38240 FRANCE ] Voice:[+33 4 76 76 44 83], FAX: [+33 4 76 76 44 50], E-Mail:[jean.schwoerer@orange-ftgroup.com] Re: [.] Abstract: [This document give preliminary information on the proposal that we submit] Purpose: [Description of what the author wants P802.15 to do with the information in the document] 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.

  2. Orange - France Telecom / ELSTERTechnical Proposal

  3. Agenda • LECIM devices needs : • Long range • Low power • Proposed network capabilities • Proposed PHY features • RF characteristic • FEC and interleaving • Frequency Hopping • MAC adaptation layer • Coexistence • Conclusion

  4. Sub-GHz wireless connectivity platform • Large scale wireless sensor network needs : • Typical network structure (star and tree)at reasonable cost • 1-hop mesh / relaying for hard the few reach endpoints or to recover wireless connectivity after major events • Efficient power management to maximize endpoint battery life • “almost” always on and limited latency for applications involving IP or human interaction (it’s not just remote meter reading) • Battery powered LECIM devices will provides : • Long range (high link budget) for cost effective network infrastructure and long operating range • Ultra-low power management to reach multi-year operation • Ability to peacefully coexist with other devices • permanent reachability with human acceptable turn-around time (a few 10’s of sec)

  5. Sub-GHz wireless connectivity platform • How to get long range ? there is two way • Go wide band thanks to spread spectrum, and get benefit from diversity and de-spreading gain • Go narrow band and take benefit from • Increased sensitivity (less noise) • Limited cost and complexity • Reduced spectrum use (better coexistence) • In addition, Frequency Hopping and efficient FEC and interleaving can bring diversity and reduce system margin, even for narrow band system

  6. Sub-GHz wireless connectivity platform • Ultra-low power design to reach multi-year operation : • The best way to save power : just do nothing ! • But we also need to save desired features : bidirectionnality and limited latency • Network synchronization : • Allow endpoint s to sleep as soon as no activity is planned for them • Minimize unwanted wake up and coordinate RX windows : each endpoint stay reachable in acceptable time (always-on illusion) • If network is asynchronous : very short media probing at regular interval • Keep network probing duration as short as possible (direct impact on the duty cycle!) and as low level as possible : Full wake up occurs only if some activity is detected on the channel (CSL mode introduced by 4e) • Probing period can be in the order of one to a few second (low latency) • Efficient when network activity is really low (few messages / day / endpoint)

  7. Fundamentals – RF & Modulation • Sub-GHz ISM license free bands 915MHz, 868MHz, 316-433MHz • better propagation properties and less interferences than 2.4 Ghz • Simple to implement modulation : GFSK / FSK • Endpoint will benefit of simplicity : several very power efficient transceiver are already available for endpoint • Concentrator can offer more complex receiver (coherent, soft decision..) • Low data rate and narrow bandwidth : 15 Kbps GFSK modulation • BT = 0.5 / Modulation Index = 1 • 40 KHz bandwidth and 50 kHz channelization • Limited noise bandwidth : - 127 dBm thermal noise • RX sensitivity up to – 115 dBm (endpoint) • Low spectrum occupancy : better coexistence properties and easy use on every ISM band

  8. Fundamentals – Frequency Hopping • Intra Frame Frequency Hopping : Provide channel diversity • Frame is sliced into several data blocks • Hopping occurs between each data block over a N-hopping sequence • Hopping rate depend on data block duration • up to 63 50 kHz channels in EU (863-870 MHz) and at least 50 channels or less to comply with FCC part.15-247 (915 MHz) • Real channel diversity and real co-existence tool • Adaptative FH to avoid busy channel is do-able Frame SHR + PHR PSDU Data block #N Data block #1 Data block #2 Chan. #1 SHR + PHR PSDU PSDU – subpart #1 Chan. #2 PSDU – subpart #N Chan. #N

  9. Fundamentals – Frequency Hopping • Intra Frame Frequency Hopping : between slow and fast FH • Foreseen data block duration : 256 symbols = ~17 ms (directly linked to channel coherence time) • Simpler to implement than fast-FH & way less overhead • Short frame will require one to a few data block -> behave like channel surfing system : if interfered then retry on another channel • Longer frame (4 & more) will took benefit from diversity : thanks to FEC, damaged bloc will be recovered from redundancy included into data blocs who survived, avoiding costly retransmission of a long frame • Has similar spectrum usage than channel surfing system at higher data rate : would ease coexistence with them • Between slow and fast FH : Let’s take the best parts of each…

  10. Example • As an example, with a 15kbps data rate and ½ FEC, 16 data bytes can be transmitted in a ~17ms slot • Data block duration is shortest than channel coherence time • 128 slots are required for transmitting 2047 bytes (longest possible frame) • Average 100 bytes frames will requires 7 data blocks

  11. Fundamentals – Frequency Hopping Frame SHR + PHR PSDU • Intra Frame Frequency Hopping : • R-Sync : 3 bits “000” for carrier re-sync after hopping • Proposed data block length is : 255 symbols ( ~17 ms) + R-Sync • R-Sync overhead is ~1% • Data block duration is shorter than channel coherence time • Only complete data block can be send : add padding bit if required Chan. #N PSDU – subpart #N R-Sync Data block #N Data block #1 Data block #2 3 bits 255 bits

  12. Fundamentals – Frequency Hopping Frame SHR + PHR PSDU • First data block include SHR, SFD, PHR and MHR. • Long SHR used for CSL wake-up frame are obtained by TX-ing several time the first data block. • IPdata is used to encode into the preamble itself, the remaining duration of this preamble (if CSL is used) (Manchester encoded) • PHR contains : • FH mode : on / off / adaptative (can exclude up to 2 channels) • Data rate, Coding scheme, hopping sequence and excluded channel • PSDU size and data block size • MHR : Included in first data block to avoid un-necessary hopping SHR SFD PHR MHR Chan. #N Sync IPData FH mode PHY param PSDU Size 101010..1010 E2 40 bytes Data block #N Data block #2 Data block #1

  13. Fundamentals – FEC & Interleaver • 2 FEC are proposed for asymmetric protection : • Generic use : Convolutional codes K=7 (171,133) • Industry standard : already included in 802.15.4 PHY • Endpoint : efficient protection for reasonable complexity (dmin=10) • Gateway : more complex RX implementation allow to increase performance by 4 dB (soft decision, coherent RX) • Uplink : Endpoint to Gateway -> Turbo Code (option) • Simple encoding is done by the endpoint – more complex decoding is handled by Gateway. • Gateway is able to decode both FEC, whatever endpoint selected • Higher coding rate : limits endpoint TX time to save battery and spectrum use. • Still an open question (complexity vs benefits)

  14. Fundamentals – FEC & Interleaver • Data interleaving • Uniformally spread correlated bits among data blocks • Thanks to FH, each data block is transmitted on a independent channel • Random Interleaver : • Optimal performance require 7 data blocks (typical 100 bytes frame) • But some frame will be shorter. • Random interleaver would allow Variable size between 2 to 7 Data blocks. • Uniform distribution.

  15. Fundamentals – Adaptative FH • Adaptative FH : a tool to improve co-existence • ISM bands are full of potential interferer with 100% Activity factor : RFID reader (up to 36 dBm), Wireless audio, RF Mic.. • Goals : avoid jamming them as well as being jammed by them • Benefits: Help coexistence with NB devices, reduces retries and save endpoint battery • Idea : replace an interfered channel in the FH sequence by a previously defined backup channel. • Adaptative FH is triggered by the interfered devices (usually the endpoint) • Adaptative FH signaling is included into normal frame header and address only the current frame and endpoint (no negociation, no command frame..) • Other devices (usually AP) will remember this until adaptative FH is disabled by the endpoint • Requires that at least one backup channel has been defined

  16. Fundamentals – Co-existence • Adaptative FH : How it works ? • Interference detection criteria is implementation specific. • Requires PHY header signalization : 2 bits FHmode field + 6 bits / Xch. • XCh contain the number of the channel than need to be replaced • AP is just supposed to store and maintain A-FH context until further notice • A-FH can be used for broadcast only if AP is aware of a single context. NB interferer on channel 53 AP Endpoint Channel 53 is excluded from the FH sequence used to talk to this given endpoint and replaced by backup channel #1 Data block #3 is corrupted. Ch #53 is replaced by backup ch. #1 Data block #1 to#5 sent on chanel 1, 27, 53, 38, 11 Group Ack for data bloc 1,2,4&5 & no_ack for #3 FH mode swichted to « adaptative » and ch. #53 tagged as first blacklisted channel and replaced by backup channel 1 AP will no longuer transmit on ch. #53 toward this endpoint until ch. #53 is released NB interferer is now protected Data block #3 is re-sent on backup chanel #1

  17. Link Budget – 915 MHz Scenario 1 : 2 km range in a Sub-urban area, between pole and indoor meter Indoor endpoint

  18. Link Budget – 868 MHz 25 mW Scenario 2 : 1 km range in a Sub-urban area, between pole and typical outdoor gaz meter Outdoor endpoint Help to save endpoint power

  19. Link Budget – 868 MHz 25 mW Scenario 3 : 100m range in a Sub-urban area, between a rooftop device and underground water meter (typical relaying scenarios) Wallfish Ikegami model as valid range down to 20m. Underground endpoint

  20. Frame Fragmentation • Transmitting long frame at a low date rate can be problematic : • Channel coherence time is estimated to 20 ms (300bits@15kbit/S) • Frame error rate increased when frame length increase • and 15.4k ask for frame length up to 1500 bytes. • But Fast FH provide a «de-facto» PHY fragmentation : • A frame is sliced in several “data blocks” • Data block duration is shorter than channel coherence time • Frame length and data block size give the number of data block • Each data block is identified by his position in the hopping sequence • Only ACK need to be modified to allows signaling of damaged data block

  21. MAC Layer compatibility • 15.4e TSCH resources management • Time Slot Channel Hopping defines the automatic repetition of a slotframe based on a shared notion of time • TSCH Allows the devices hopping over the entire channel space in a slotted way thus minimizing the negative effects of multipath fading and interference while avoiding collisions • Slotframe is configurable through the definition of the channels used, the number of slots and the duration of the slots • TSCH parameters have to be define with regards to data block duration

  22. MAC Layer compatibility • Packet fragmentation in a slotframe • Fast FH provide a PHY level fragmentation : each data block is a fragment • Data block tightly shorter than channel coherence time : Fragmentation occurs only when needed • Adaptation layer between PHY and MAC uses the 15.4e TSCH channel management scheme for spreading PHY data blocks over channels • The first data block carries PHR and MHR, including FrameID, Frame length and thus the total number of data blocks in the frame. • By knowing the Hopping sequence, data block duration & number, the receiver is able to follow the hopping sequence, even if some data blocks get losts • .

  23. MAC Layer compatibility • Packet fragmentation in a slotframe • Other Data blocks are sent on succession of different channels following a known order. • So, each data block implicitly carry is FragmentID by the channel over which it has been sent. RX just need to increase a counter after each hop. • Consequently, signalization needed is provided for just some control fields added on the PHR (first data block only) => Very low overhead. • After the last data block, a group ACK is sent in the other direction. Each bit of this frame’s payload representing the correct reception of the corresponding data block (max : 128 blocks) • A new frame is sent. First data block will include normal header • Then the missing data blocks, sent in their order in the initial transmission. • By knowing the order of missing data blocks, receiver is able to re-construct the whole frame. • .

  24. Coexistence • Several mean to help coexistence • Narrow channel (50 kHz) : limited spectrum usage • Frequency hopping : • Adequate sequences management mitigate interference between independent networks • Short data block minimize interference on a single channel as individual channel occupancy time remain low • Adaptative FH help to protect from interferer and from interfering

  25. Conclusions • FSK/GFSK are proven solutions : • To address very low power wireless devices • To allow flexible implementations • Narrow band, IF-FH, efficient FEC and interleaving allow supporting path loss larger than 140 dB. • Frequency Hopping bring channel diversity and frame fragmentation “built-in” : improved robustness and low overhead • Relaying allows yet improved network coverage and network resilience against major channel / network changes • but handling yet higher path loss requires other technology • Limited latency and always-on behavior can be provided at an acceptable cost • Will be happy to discuss exchange with everybody interested

  26. Thank You

  27. FSK Receiver implementation • Comparison between : • Low cost non coherent FSK receiver using hard decision and viterbi decoder • Coherent FSK receiver using soft decision and viterbi decoder • Performance increase by 4 dB at BER = 1.10e-3

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