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

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 elements 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
• DBPSK + 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
• DBPSK + chip-PPM : each chip of the code is shifted according to the chip value => selected option

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)
• The lowest rate is a compromise between “Tx_on time” and range (+clock drift compensation requirements).

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-left ear
• 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

• Same results (with better PER floor in highest rate) for 256 bytes

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

Background design know-how (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 elements 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
• Would be limited to 2-hops in the BAN context
• 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

Questions ?

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