Low-Rate UWB Alternate Physical Layer for TG 802.15.4a

1
Project IEEE P802.15 Working Group for Wireless
Personal Area Networks (WPANs) Submission Title
TG4a-SandLinks-CFP-Presentation Date
Submitted 4 Jan, 2005 Source Dani Raphaeli,
Gidi Kaplan Company SandLinks Address
Hanehoshet 6, Tel Aviv, Israel E-Mail
danr_at_eng.tau.ac.il Re 802.15.4a Call for
proposal Abstract A proposal for the
P802.15.4a alt-PHY standard Purpose Response
to WPAN-802.15.4a Call for Proposals 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.
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Low-Rate UWB Alternate Physical Layer Proposal
Submissionfor TG 802.15.4a

• Jan 05 Meeting
• Dr. Dani Raphaeli Dr. Gideon Kaplan
• SandLinks

3
Outline

• General Overview
• Signal and Packet design
• Communication Performance
• Sensitivity, Acquisition
• Interference Coexistence
• Aggregate Rate
• Ranging
• MAC Protocol Considerations
• Block Diagrams and Technical Feasibility
• Cost/Complexity
• Scalability
• Power Consumption
• Summary

4
Technical Requirements of TG-4a

• Low complexity and cost
• Low power consumption
• Precision location (highly desired relative
  ranging)
• Extended range
• Robustness (against MP, against interference)
• Mobility
• Low bit rate for each individual link
• High Aggregated rate at a collector node
• Random, ad-hoc, topology
• Work under current 15.4 MAC

5
General Overview of Proposal

• Symbol Interleaved Impulse Radio
• 500Mhz bandwidth in UWB band
• Optional 80Mhz in 2.4GHz, 200Mhz in 5.2 Ghz
• May choose (program) one of several Center
  Frequencies
• Use of Round Trip Delay for ranging
• Low data rate per device allows to obtain PER and
  Ranging within substantial distances, for various
  channel models
• High total (aggregate) rate
• Suitable for very low-cost (small die size)
  implementation in a standard process
• Robust, Flexible and Scalable solution.

6
Symbol Interleaved Impulse Radio

• Basic principle Use pulse trains with constant
  large separation between them. Each pulse train
  represents one symbol.
• Pulse train (or sequence) is used instead of
  single pulse to decrease peak to average, which
  serves to
• Simplify implementation
• Meet FCC peak power constraint in the UWB band
• Pulse sequence polarity corresponding to the 11
  bit barker sequence 10110111000

100ns
20?s
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Symbol Interleaved Impulse Radio (cont.)

• Many users can transmit concurrently without
  interference
• (each color represents a different packet from a
  different user).

20?s
Substantial aggregate rate can be achieved (see
in the sequel) the transmission management
mechanism of 15.4 is appropriate.
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Benefits

• There is no need for a difficult and slow
  synchronization process (incurred if several /
  long sequences are used)
• Easy implementation
• Passes FCC rules
• Reduced sensitivity to Multipath (see figure
  below)
• Near-Far Problem is minimized.

9
Signal (Pulse) Design

• A look on an actual pulse train symbol (fc4GHz)
• Zoom on a single pulse
• For average and peak powers- see Appendix A

10
Signal (Pulse) Design

• A look on an actual pulse train symbol (fc4GHz)
  in the frequency domain, Pt-15dbm

11
Packet Structure Design

• Preamble (un-modulated) part enables to
  synchronize on received signal and for receiver
  acquisition and training.
• Data part uses PPM (binary, possibly M-ary) to
  convey message SPDU. Message lengths between
  7 to 128 Octets (MAC limit). Nominal symbol rate
  is 50Ksym/sec.
• Response (un-modulated) part allows for
  synchronous Ack (see in the following) plus data
  response.
• Total packet length typically 10 to 20 msec.

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Packet Structure

Response Period (optional)
Preamble
DATA (MAC fields)
Unmodulated
PPM
Unmodulated
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The Response Period

Response Period
DATA
ACK DATA
ACK Preamble
The ACK is transmitted during the response period
of the original Packet.
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The Synchronous ACK

• The ACK is transmitted during the response period
  of the original packet thereby allowing
  synchronization of the response to measure the
  channel round trip delay.
• The Response Period duration is minimally equal
  to the ACK preamble duration, and at maximum
  lasts for the entire ACK
• The response (the ACK) is transmitted at a fixed
  (known) delay relative to the RP pulses. The Node
  receiving the ACK can measure the RTD and
  calculate the distance accordingly.
• The symbols of the RP are used for synchronizing
  the response
• This allows the use of low accuracy clocks, which
  serves to
• REDUCE THE COST
• MINIMIZE SYSTEM COMPLEXITY (MAC/higher layer not
  involved in generating accurate time base)
• Since the ACKs are transmitted at a fixed delay,
  ACK collisions are avoided as long as the
  original packets were not colliding

15
Topology Types of Devices

PAN coordinator

• The 802.15.4 defines two types of devices
• The low complexity RFD (Reduced Function Device)
  which can be only a leaf on the network.
• The full complexity FFD (Full Function Device).
• A typical topology composed of many RFDs as the
  sensors or tags and few FFDs as coordinators and
  data concentrators.
• The topology may change in the network.

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Types of Devices (cont)

• We propose asymmetric PHY FFD with higher
  functionality and higher cost and RFD with lower
  functionality and cost.
• The ultra low cost RFD (Reduced Function Device)
  is not required to be able to receive multiple
  packets. It will be capable of
• Responding to FFD requests.
• Sending packets to a FFD
• Requesting for a pending packet
• The FFD (Full Function Device) is expected to be
  able to receive simultaneous multiple packets
  concurrently. It will be capable of
• Receiving many packets at the same time and
  responding each of them with ACK.
• Calculating the distance to each node it received
  ACK from
• Responding to RFD data requests.

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Communication Performance PER vs. Eb/No

• The chosen modulation is PPM
• Coding scheme is still TBD. We use simple (63,57)
  Hamming code (and hard decision decoding) for the
  current presentation however obviously other
  codes, still simple to implement, exist with a
  higher coding gain.
• For 32 octets, to get PER of 1, the BER should
  be
• BER lt 0.01/(328)4e-5
• In the next slide, the theoretical results show
  that Es/N011.5dB is required
• In AWGN channel, for 50Ksym/sec, d100m is
  achieved with 6dB of margin.

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Communication Performance Theoretical BER vs.
Eb/No
19
Link Budget (AWGN channel)
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Performance under Multipath

• From the link budget Receiver Sensitivity is
  -107.5 dBm or, total path loss lt90dB.
• Achievable distances for the 9 channel models
  defined by the TG4a channel modeling subgroup,
  are shown in the next slide.
• PER performance on these channels was checked by
  system simulation. The simulation includes
• Acquisition
• Tracking
• Adaptation
• Demodulation
• Decoding
• Packet processing
• The PER results for several channel models
  (presented next) show good match with the
  theoretical predictions.

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Distance vs. Channel Models50Ksym/sec
Distance m Type of Channel CM
394 Resident. LOS 1
8.2 Resid. hard NLOS (concrete walls) 2
1610 Office LOS 3
20.6 Office NLOS 4
421 Outdoor LOS 5
75 Outdoor NLOS 6
421 Industrial LOS 7
27 Industrial NLOS 8
393.5 Farm 9
The high atten. In 15-04-0290-02-004a taken from
802.15-02/444
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PER curves

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Acquisition

• We assume the super-frame structure includes a
  Beacon transmission
• In a steady-state, all devices synchronize to the
  Beacon transmissions of the PAN coordinator
• A quick re-acquisition (in a short length
  window), to re-align the timing, is performed per
  each received Beacon.
• The device then listens in the address message
  space to check if data is waiting otherwise (if
  the device does not need to transmit) it goes
  back to sleep
• The quick acquisition is performed over the
  standard 4 octets preamble of the Beacon packet
• All normal transmission packets will also include
  a 4 octets preamble, used for fine timing
  acquisition channel model learning.

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Acquisition (cont.)

• In case a new RFD/FFD device joins an existing
  network, it has first to synchronize to the
  super-frame structure (namely to the Beacons
  transmissions)
• One possible mechanism is passive association
• Assuming that the power consumption dictates no
  more than about 1 duty cycle over long periods,
  this passive process will be relatively slow in
  time.
• If active association is used, faster
  synchronization can be achieved.

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Interference Coexistence

• Protection against WLAN and other out of band
  signals (in 2.4Ghz, 5.3Ghz) aided by a 3rd order
  Band-Pass filter in the receiver (or an
  equivalent LPF after down conversion)
• For narrow-band interference (in-band),
• High processing gain inherent in the technique
  (500MHz/50KHz40dB)
• Adaptive or programmable interference rejection
  mechanism (with mild requirements) may be
  employed
• A real life effect which should be considered, is
  the transmission of wide band noise (OOB) by
  other devices, which covers the same freq band as
  the UWB device
• The result show that at most 1m separation
  insures meting the criteria of PERlt1, for UWB
  signal level 6dB above sensitivity level
• For detailed analysis see spreadsheets in
  submitted material.

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Interference Coexistence (cont.)

• Under extreme interference cases, a change of the
  active band may be undertaken (under higher layer
  command).
• Coexistence with other devices (802 type,
  Vsats,..) is achieved with a small distance
  separation, due to the low average power density
  level of UWB transmission (detailed analysis in
  submitted material)
• Co-existence with other Piconets possibly
  co-located may be simply achieved by selection
  of different active frequency bands for the
  Piconets (up to 3).
• The band select filter provides more than 20dB
  attenuation, even for the adjacent bands of 4Ghz
  (centered at 3.5Ghz, 4.5Ghz).
• Further simulation results will be provided later
  on.

27
Band Plan

• The analysis (e.g. Link Budget) was made with a
  Fc4Ghz (Fl3.6Gh, Fh4.4Ghz for -10dB points)
• The UWB freq range can be divided to multi-bands,
  coordinated with other uses defined by the ITU
  and IEEE bodies
• Typically a device may be programmable to one of
  3 bands in the range 3-5GHz (and additional bands
  in 6-10GHz when higher speed processes will be
  cost effective)
• This enhances the robustness of the design and
  may serve to improve acceptance by regulation
  bodies worldwide
• Outside the USA, device will operate in 2.4GHz or
  5.2GHz until UWB will be approved worldwide.
• Nevertheless, since the high aggregate rate
  (10Mbps) enables virtually all multiple uses in
  the same area, the standard should allow for
  lower cost devices to be fabricated for one fixed
  band.

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Aggregate Rate Considerations

• Recall the Interleaved pulsed transmission
  proposed
• There are N200 virtual time slots (of Ts
  100nsec), totaling 20usec, between each
  transmitted symbols of a single packet
• The transmitting / answering devices can chose
  one of the N virtual time slots, to transmit
  their packet
• This choice is kept throughout the packet
• Due to the possible spatial layout of the
  answering devices, round trip delay differences
  can be larger than Ts.
• Thus the basic model is multi-channel (N)
  un-slotted Aloha
• The throughput vs. offered load of such a channel
  is known, and its peak is 1/2e (per slot).

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Aggregate Rate (cont.)

• The ALOHA model assumes that if more than one
  transmission uses the same slot, than there is a
  collision and none gets through
• Recall the Barker sequence (of length 11)
  Processing Gain, allowing for more than one
  reception in a time slot, if their sequences are
  in shift
• However some issues like Near-Far (power ratio)
  and also channel multipath come into play
• First analysis estimates that the effective PG is
  about 3 further simulations are needed to
  justify this estimate.
• Thus the scheme has 3N effective slots, so the
  maximum aggregate rate is
• 3200(1/2e)1/50usec 5.5Msym/sec.

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Aloha Curve(s)
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Aggregate Rate (cont.)

• For a ALOHA channel, insuring stability is of
  importance, by employing simple anti-congestion
  (back-off) mechanisms
• Usage of Guaranteed Time Slots (GTS) can further
  improve the capacity, as these will operate at
  close to 100 efficiency
• However this mode is applicable especially to
  relatively long transmissions.
• Employing a collision avoidance (or CCA)
  mechanism, performance is improved in the
  (contention-based) Aloha slots as well as the
  stability
• With CCA employed, for a propagation delay of
  30nsec, and transmission of 100nsec, theoretical
  capacity grows up to to Capacity 9.6Mbs
• The transmitting / answering devices hear only a
  partial population of all devices, thus the
  actual performance improvement of CCA will be
  assessed via a simulation (per specific channel
  and node locations).

32
Ranging

• Basic method proposed is Round Trip Delay
  measurement (by a FFD).
• Why should we choose RTD for 15.4a?
• No need for fixed expensive infrastructure.
• No need to generate a very accurate time base.
• The only one that can be used in Relative
  systems.
• Each node makes its own measurement autonomously.
• Easy to handle Multipath (take the earliest
  component).
• Straightforward to implement.
• Can handle distance measurement with a single
  node in case x,y,z coordinate is not necessary.

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Ranging (cont.)

• Ranging is performed at same distance coverage as
  is for communications
• The ranging algorithm uses between 30 to 50
  symbols for averaging of the signal
• Simulation results for LOS channel models
  (residential, office, outdoor), the ranging
  accuracy is on the order of 0.3 to 0.5 meter.
• Assuming uncorrelated errors at both
  measurements of the round trip delay, 1.4nsec is
  equivalent to (1-way) distance error of 30cm
• For NLOS channel models that were presented, the
  first path delay varies randomly in a certain
  range, in the model realizations thus, ranging
  has a large error in some of the models.
• For CM4 (office NLOS probably a soft NLOS
  model), the std deviation is about 3nsec (0.66m).
  
• The random arrival of first cluster in the model
  needs further discussion.

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Max Ranging Error Results LOS channel models,
N50 symbols
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Ranging (cont.)

• Considerations for mobile nodes
• Time for ranging is between 600usec to 1msec.
• For mobility values on the order of 1meter/sec
  (on a mobile luggage conveyer, for example), the
  displacement affected while location is measured
  is negligible on the order of 0.1 cm. This is
  also negligible compared to the wave length
  (8cm).
• Assuming coherence time requirement of 5ms the
  maximum doppler rate is 200Hz, which translates
  to about 15m/s max speed.

36
MAC considerations

• Network includes FFD and RFD devices
• Packet structure adheres to 15.4
• Supports the full set of 15.4 MAC functions
• Ranging result just another parameter
  transferred from Phy to Mac layer after a single
  transaction
• Supporting anti-congestion mechanisms at both
  type of devices.

37
Receiver Block Diagram
38
Transmitter Block Diagram
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Technical Feasibility

• The analog (RF) part can be implemented by either
  SiGe or 0.13u CMOS processes.
• The former has a higher bandwidth / more accurate
  models for high frequencies
• The latter is about 30 lower in cost per mm2.
• Both technologies are in use today for similar
  frequencies (e.g. 802.11a)
• The other high speed elements are also based on
  existing technology and modules
• All in all, the die size estimation is 6.3 mm2
  (see next slide).

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Estimated Size and Power (RFD)
Estimated Die Size mm2 Estimated Power (mW)
Analog Blocks 2.0 2.5
Analog To Digital 0.5 3
Digital Blocks, uP, RAM, ROM 3.3 7.5
Pads 0.5
Total 6.3 13.0
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Power Consumption

• The low power consumption is due to
• Activating the components only when a
  transmission is expected (note the advantage of a
  short pulse sequence!)
• Low power consumption design methodologies of all
  the parts
• Each device typically listens only to the Beacons
  and rest of time is in sleep mode, thus the
  effective average power consumption will be
  reduced by a large factor (e.g. 1), enabling
  long battery life
• When in acquisition, a search for a symbol over
  few hypothesis is made.

42
Scalability

• Higher (peer to peer) data rates can be achieved
  by
• interleaving few packets from same source, which
  essentially mean lower separation between
  consecutive symbols.
• Using higher order PPM
• For example Interleaving 10 packets and using
  16-ary PPM results in 50Kbps1042Mbps
• ALL RATES ARE COMPATIBLE AND COEXISTENT!
• Lower (peer to peer) data rates can also be
  achieved (by using lower coding rates, and
  increasing preamble length accordingly to
  accommodate lower SNR), but not recommended
• Hooks for a cognitive radio can be added in the
  future, for example to add programmable notch
  filters in the transmitter.

43
Summary

• The Symbol Interleaved Impulse Radio system is a
  sound, complete system proposal that
  simultaneously answers all the technical
  requirements of TG-4a of 802.15 and all minimum
  SCD criteria
• Offers large advantages (vs. conventional DS
  solutions)
• in terms of Range, Power, Aggregate rate and
  Cost
• It enables both a robust design in various
  channels and scenarios, flexibility to a
  multitude of applications, and a very low-cost
  solution
• Good distance performance on most channel models
• We will be happy to cooperate with every one that
  is interested in this direction, in order to
  further improve its parameters.

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Appendix A Average and Peak Powers

• Regulation
• Average transmission power is limited to -41.3
  dBm/Mhz, or
• -14.3dBm for a 500Mhz bandwidth
• The peak power per 50Mhz is limited to 0dBm.
• Recall the 11-sequence Barker pulsed transmission
  (eleven 2nsec pulses, with 10nsec intervals)
• To achieve the max. Average power, the peak power
  of each 2nsec pulse will be
• -14.310log (20usec/22nsec) 15dBm
• Now check the peak power measured through a 50Mhz
  wide filter it has a time constant of about 20
  -30nsec, thus the resultant power is
• 15 10log (2nsec/10nsec) 10log(50/500)
  15-7-10 -2dBm
• so that the FCC peak power limit is met.

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Appendix B Interference Spreadsheet (1)
46
Appendix B Interference Spreadsheet (2)
47
App. B Co-Existence Example