List of Authors France Telecom – Jean Schwoerer, Benoit Miscopein, Stephane Mebaley-Ekome (1)



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List of Authors

  • France Telecom – Jean Schwoerer, Benoit Miscopein, Stephane Mebaley-Ekome (1)

  • CEA-LETI – Laurent Ouvry, Raffaele D’Errico, François Dehmas, Mickael Maman, Benoit Denis, Manuel Pezzin (2)

  • THALES – Arnaud Tonnerre (3)

  • University of Surrey, UK – Ehsan Z. Hamadani

  • INSA-Lyon, FR – Jean-Marie Gorce



Outline

  • Introduction

  • PHY proposal

    • Advantages of UWB
    • Band Plan and PLL reference diagram
    • Pulse Repetition Frequency, Preamble, Modulation
    • Variable bit rate & throughput
    • Link Budget, Performances
    • Feasibility examples
  • MAC proposal

    • Beacon-based mode: Evolution of IEEE 802.15.4 MAC
    • Beacon-disabled mode: Preamble sampling approach
    • Upper layers responsibility
  • Conclusions & References



PHY Proposal



Introduction

  • Proposal main features:

    • Based on IEEE802.15.4a-2007 where a very reduced set of mode is selected for the BAN context
    • UWB Impulse-radio based
    • Support for different receiver architectures (coherent/non-coherent)
    • Flexible modulation format
    • Support for multiple rates
    • Support for multiple SOP


Low radiated power

  • Low radiated power

    • Low PSD, low interference, low SAR
    • High co-existence with existing 802.x standards
    • Real potential for low power consumption
  • Large bandwidth worldwide

    • Spectrum is worldwide available
    • Robust to multipath and fast varying channels
  • Flexible, scalable (e.g. data rates, users)

  • Low complexity HW/SW solutions in advanced development (eg 802.15.4a)





Band plan (selection from 15.4a)



Band plan versus regulation for UWB



PLL Reference Diagram



Pulse Repetition Frequency

  • Selected value

    • 15.6 MHz PRF (64.10 ns of pulse repetition period PRP)
    • Use of binary codes, No bursts => mean PRF = peak PRF
  • Rationale

    • Typical channel delay spread for indoor applications below 25 ns in 90% of channel realizations => very limited ISI (or IPI)
    • Integer relationship with base frequency of the bandplan (499.2 MHz / 32) => no fractional PLL
    • Maximized Pulse amplitude
      • => better for transmitter power consumption (between-pulses duty cycling)
      • => better for pulse detectability in the receiver
      • => better for threshold crossing receivers
      • => compatible with low voltage CMOS technologies without I/O « tricks » (around 780 mVp-p)
    • A single value => simplicity
    • Compatible with bit rate scalability with short spreading factors
    • No high speed clock / long FIRs filters to generate or correlate with bursts of pulses


Preamble

  • PRF is the same as one of the mean PRF in 15.4a

  • Sp code duration:

    • Example with N=31 : Tpsym = 31 * PRP = 1.9871 us
    • To be checked if enough for a correct (Pd,Pfa) versus complexity
  • Number of Sp codes in the preamble

    • Example with Nsp = 16 : Tsync = 16 * Tpsym = 31.7936 us
    • This is probably a maximum value
    • To be checked if enough for a correct (Pd,Pfa) versus overhead


Modulation

  • PRF is ~ the same as one of the mean PRF in 15.4a

  • Spreading code length Sd

    • Example with N=7 (BARKER CODE) : Symbol duration Ts = 7 * PRP = 0.4488 us
  • Modulation : 1 bit per symbol + second bit used for redundancy OR non coherent demod

    • DBPSK : BPSK with differential encoding (at symbol level)
      • Sub-optimal but easier to implement and less sensitive to clock drift
    • Same + PPM : the whole S code can be shifted within the PRP with ½ the PRP value
      • Does not affect the mean PRF value and the spectrum shape
      • Is simple to implement (though a little more complex than pure DPBSK)
      • Is compatible with non coherent / threshold crossing detectors
      • Is ISI compatible in the BAN context


Modulation : Bit-DBPSK + 2-PPM (an orthogonal keying modulation)



Variable bit rates

  • Bit rates

    • Bit rate is adjusted with the number of pulses per symbol keeping a constant mean PRF
    • 2.22 Mbit/s is the default uncoded data rate
    • 5.2 and 15.6 Mbit/s are mandatory => No additional complexity & allows to reduce channel use
    • 31.2 Mbit/s is proposed optionally for coherent receiver (uses both PPM & DPBSK)
    • Proposed FEC : systematic RS (63,55) as in 15.4a (maximum efficiency ~0.87)


Link budget



Performances analysis methodology and channel models

  • Goal:

    • Get/discuss performances at a link budget / outage probability level with the different channel models at the default 2.2 Mbps rate before digging into the design level performances
  • Methodology:

    • Perform extra measurements at CEA-Leti (for UWB 3-5GHz but also for 2.4 GHz)
    • Complement the IEEE802.15.6 CM3 UWB channel model with extra measurements and models and compare channel models with each other
    • Move towards a scenario based approach
      • Scenario = [at least] given (Tx,Rx) couple + given generic environment + given generic movement
      • Justified by the huge dispersion of the BAN radio channel
    • Calculate outage probabilities given the path loss and shadowing statistics


Performances: channel path loss models

  • Available CM3 UWB channel models as in TG6 document :

    • A (source NICT)
      • Indoor and anechoic
    • B (source IMEC)
      • Anechoic
    • C (source Samsung)
      • Indoor and anechoic
  • Conclusion

    • Huge dispersion (tens of dBs) between models
    • Distance is not a relevant parameter to get a path loss model
    • Coming back to the scenario based channel characterization is proposed
  • Reference

    • Roblin C.; D'Errico R.; Gorce J.M.; Laheurte J.M ; Ouvry L., « Propagation channel models for BANs: an overview », COST 2100, 16/02/2009 - 18/02/2009, Braunschweig , Germany


Performances: extra channel measurements

  • 2-5 GHz, indoor and anechoic, 7 subjects, standing/walking/running, scenarios as depicted below

  • Log normal path loss model. Shadowing and small scale fading modeled separately.

  • Reference : D'Errico R.; Ouvry L.,“Time-variant BAN channel characterization” TD(09)879, COST2100, 18-19/05/2009, Valencia, Spain (Measurement set up details available on request)



Performances: extra channel measurements

  • On previous page:

    • Error bar @ 1 std of mean channel gain over subjects
    • Without the slow shadowing std
    • 2.4 GHz and 3-5 GHz for comparison
  • On this page

    • UWB 3-5 GHz only
    • Error bar @ 2 stds of mean channel gain and slow shadowing (95% confidence interval)
  • Conclusions

    • 2.5% outage probability @ ~-70dB channel loss for the 8 scenarios in indoor conditions
    • Higher outage in anechoic chamber depending on the scenario (not shown here)


Performances : outage probability

  • Starting from:

    • The link budget
      • PTx + antenna = -20.5 dBm
      • Sensitivity = -90.5 dBm
        • Includes 6dB NF, 5dB I.L. and 9dB min required EbN0
      • 70dB total link margin
    • The different scenario based path loss models
      • CM3 UWB A back to the scenarios
      • CM3 UWB C back to the scenarios
      • CEA-Leti’s measurements
  • Get the outage probability performance for an EbN0

    • Probability that the received power is higher than the receiver sensitivity
    • from the proposed -90.5dBm sensibility (2.2 Mbps)
    • through to -87.8dBm (5.2 Mbps)
    • and to -82dBm (15.6 Mbps)


Performances : outage probability



Performances : outage probability



Performances : scenario based outage probability

  • Tentative consolidation of

    • CM3 A (NICT)
    • CM3 C (Samsung)
    • CEA-Leti’s measurements
    • for four indoor scenarios :
    • Hip-chest
    • Hip-right wrist
    • Hip-thigh
    • Hip-chest
    • (others available as well, including anechoic chamber cases)
  • Needs further update to refine comparisons, but aims at opening discussions



Performance : EbN0 requirements (DBPSK)

  • Conditions: DBPSK, 20 bytes PSDU, CM3 A channel



Conclusions on link performances on the different channel

  • The proposed link budget and system specification makes the UWB proposal feasible for most of the scenarios

    • outage of 5% as in TRD
    • 1e-2 PER for 20 bytes PSDU, or
    • 1e-1 PER for 256 bytes PSDU
    • Actual EbN0 requirement still to refine (current simulation with realistic receiver on CM3 gives 13dB after RS decoding, within the proposed IL values, but the CM3 multipath model is questionable)
  • However, large variations between the different models (CM3 A is optimistic, CM3 C is pessimistic, CM3 B and CEA-Leti’s measurement are median)

  • Further analysis in the 7.25-8.5 GHz band is necessary



Feasibility example (see references)



Conclusions

  • Proposal based upon UWB impulse radio alt-PHY of 15.4a

    • Advantage
      • Early implementations exist: experienced proposal
      • Selection of the most relevant modes and their adaptation to the BAN context (note: 15.4a mandatory mode is NOT the selected option for 15.6)
      • A standard exists which will speed up the 15.6 standard drafting steps
    • Modulation:
      • DBPSK provide robustness for a limited complexity & 2PPM allow several receiver implementations
      • RS FEC help to improve link budgets and parity bit will improve robustness of the DBPSK receiver
  • System tradeoffs

    • Variable bit rates allow to accommodate all applications envisaged in TG6
    • Minimizing talk time improve energy consumption, SOP performances, and regulatory compliance
  • Flexible implementation of the receiver

    • Compatible with a lot of UWB detectors (coherent, differential, energy, threshold crossing)
    • FEC decoder is optional
  • Fits with multiple technologies

    • Compatible with implementation in low voltage CMOS
    • Very low power integrated solutions already proven (thus to be adapted)
  • The very low transmit power is a very attractive feature for the UWB PHY adoption



MAC Proposal



MAC layer

  • BANs may be coordinated most of the time

    • The coordinator can allocate a negotiated bandwidth to QoS demanding nodes (data rate, BER, latency)
    • A beacon based approach is adapted
  • Some applications imply a lower channel load

    • A beacon-free mode is more efficient
  • An IEEE 802.15.4-like MAC layer is a good base for BANs

    • With a beacon-enabled mode including TDMA and Slotted-ALOHA (for UWB) with relaying
    • With an enhanced beacon-free mode by using Preamble Sampling


Beacon-based MAC mode

  • BAN requires above all:

    • Relaying capability: to cope with low-power emission and severe NLOS conditions
      • Could be limited to 2-hops
    • Reliability: high level of QoS for critical / vital traffic flows
  • Proposed architecture

    • Mesh network centralized on the gateway
      • Full mesh topology based on a scheduling tree
      • Guaranteed access for management and data messages (real TDMA)


Beacon-based MAC mode

  • Superframe structure

    • Based on 802.15.4
    • Inclusion of a control portion for management messages
      • The control portion shall be large enough to allow dynamic changes of topology
    • The CAP is minimized, mostly used for association using slotted ALOHA


Beacon-based MAC mode

  • Fine structure

    • Superframe is divided equally into slots
    • Use of Minislots in the Control portion
      • Provides flexibility: adaptation to different frame durations
    • Guaranteed Time MiniSlot (GTMS) shall be introduced in CFP


Beacon-based MAC mode

  • Control portion structure

    • Beacon period
      • The beacons are relayed along the Scheduling Tree
      • The beacon-frame length shall be minimized
      • Beacon alignment procedure shall be used
    • Request period
      • Request period is a set of GTS dedicated to allocation demands
      • Transmission from the leaves to the coordinator
    • Topology management period
      • Hello frames, for advanced link state procedure
      • Scheduling tree based update


Beacon-free MAC mode

  • However, some situations might not require a beacon-enabled MAC protocol (see references.)

    • Symmetric or asymmetric network, low communication rate and small packets
    • Network set-up, coordinator disappearance, etc
  • In such conditions, the downlink is an issue : nodes must be powered-up to receive data from the coordinator (e.g. polling/by-invitation MAC scheme)

  • We propose a Preamble Sampling MAC protocol for UWB



Beacon-free MAC mode

  • Preamble Sampling principles:

    • Nodes periodically listen to the channel. If it is clear, they go back to sleep; conversly, they keep on listening until data
    • Nodes are not synchronized but share the wake up period (TCI)
    • A packet is transmitted with a preamble as long as TCI


Beacon-free mode

  • Preamble sampling protocols are known to be the most energy efficient uncoordinated MAC protocols

  • TCI depends on the traffic load and the TRX consumption (TX/RX/listen modes)

  • Can be of the order of 100 ms

  • The preamble is a network specific sequence

    • Either a typical wake-up signal
    • Or a preamble with modulated fields (e.g. @, time left to data)


Beacon-free MAC mode

  • To comply with LDC regulation in the lower band (e.g. Europe), the preamble can be relayed by the BAN nodes (up to 5ms long bursts)



Beacon-free MAC mode

  • Mode 0: only one burst per device is emitted

  • Mode 1: burst is re-emitted by a device, under the LDC limit, until a neighbour relays

  • Mode 2: same as mode 1 + former relays, with emission credits left, can relay again



Upper Layers responsibility

  • Enabling/disabling beacons switching procedure

    • From beacon-free to beacon-based mode
      • BAN formation (coordinator election)
      • New coordinator election if former coordinator leaves
      • Can be triggered by a user requesting a link with high QoS
    • From beacon-based to beacon-free mode
      • Fallback mode if the coordinator leaves
      • If the required BW (rate of GTS requests) is below a given threshold and requested QoS is adequate


Conclusions on MAC

  • Propose a combination of

    • A Beacon-based true TDMA mode including fast relaying and mesh support
    • A Beacon-free mode using preamble sampling and extra features adapted to UWB


References

  • 15-08-0644-09-0006-tg6-technical-requirements-document.doc

  • M. Pezzin, D. Lachartre, « A Fully Integrated LDR IR-UWB CMOS Transceiver Based on "1.5-bit" Direct Sampling », ICUWB 2007, Singapore, September 2007

  • D.Lachartre, B. Denis, D. Morche, L. Ouvry, M. Pezzin, B.Piaget, J. Prouvée, P. Vincent, « A 1.1nJ/b 802.15.4a-Compliant Fully Integrated UWB Transceiver in 0.13μm CMOS», ISSCC 2009, San Francisco, February 2009

  • European ICT PULSERS II and ICT EUWB projects deliverables

  • French ANR "BANET" project

  • Roblin C.; D'Errico R.; Gorce J.M.; Laheurte J.M ; Ouvry L., « Propagation channel models for BANs: an overview », COST 2100, 16-18/02/2009, Braunschweig , Germany

  • D'Errico R.; Ouvry L.,“Time-variant BAN channel characterization” TD(09)879, COST2100, 18-19/05/2009, Valencia, Spain

  • European ICT SMART-Net project deliverables

  • Timmons, N.F.   Scanlon, W.G.   "Analysis of the performance of IEEE 802.15.4 for medical sensor body area networking", IEEE SECON 2004, Santa Clara, Ca, October 2004



Back-up slides



Beacon-free MAC mode

  • In this collaborative scheme, the preamble burst can composed of

    • Destination adress (compuls.)
    • Time left to data (compuls.)
    • Packet length
    • Source adress
  • Need for a relaying collision resolution policy

    • Based on the wake-up instant during the burst (priority is given to the node which has detected the burst first)


Beacon-free MAC mode

  • Relationship between wake-up instant in the burst and back-off length can be of any kind (linear, logarithmic, exponential…)



Beacon-free MAC mode

  • Collaborative scheme is very flexible

    • Nodes can relay once or up to the emission maximum duration (50 ms in Europe)
    • If the source listens to the relays, it can get the wake-up schedules of its neighboors
    • The destinator can emit a specific burst to notify the source
      • Possibility to stop the relay process
      • Reduction of the latency because the source can anticipate the payload emission


Beacon-free MAC mode

  • Exemple of latency reduction



Upper layers responsibility

  • Management of cooperative transmissions

    • One way relay channel (OWRC)
    • A single source node transmits data to a single destination node with the help of some relay nodes
    • Two way relay channel (TWRC)
    • Two nodes like to communicate to each other through the help of a relay


Upper layers responsibility

  • Cooperation possibilities

    • Low data rate transmission
      • Complexity of involved nodes and power consumption are the main issues
      • Low complexity cooperative techniques may be used
    • High data rate transmission
      • Increasing data rate and channel availability are more important
      • Decode and forward schemes in OWRC or even network coding with TWRC are more suitable



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