Chirp Spread Spectrum (CSS) PHY Submission

1
Project IEEE P802.15 Working Group for Wireless
Personal Area Networks (WPANs) Submission Title
Chirp Spread Spectrum (CSS) PHY Presentation for
802.15.4a Date Submitted January 04,
2005 Source John Lampe Company Nanotron
Technologies Address Alt-Moabit 61, 10555
Berlin, Germany Voice 49 30 399 954 135, FAX
49 30 399 954 188, E-Mail j.lampe_at_nanotron.com
Re This is in response to the TG4a Call for
Proposals, 04/0380r2 Abstract The Nanotron
Technologies Chirp Spread Spectrum is described
and the detailed response to the Selection
Criteria document is provided Purpose Submitted
as the candidate proposal for TG4a
Alt-PHY 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
Chirp Spread Spectrum (CSS)PHY Presentationfor
802.15.4a

• by
• John Lampe, Rainer Hach, and Lars Menzer
  Nanotron Technologies GmbHBerlin, Germany
• www.nanotron.com

3
CSS Proposal Presentation Contents

• CSS System Overview
• Channel Models Simulation Results
• Ranging
• SCD Topics
• Backward Compatibility
• Clear Channel Assessment
• Other Topics
• PAR and 5C Requirement Checklist
• Summary
• For more detail, please see our proposal, doc
  15-04-0689

4
Logical Diagram of the Proposed System

• Simplicity
• Basically a 2 ary transmission system
• The windowed chirp is a linear frequency sweep
  with a total duration of 1 µs

5
Block-diagram Tx
Digital Block
Low-Pass Filter
I/Q Modulator
LP
I
fT f 10 MHz
LP
Q
LO
f 2.412, 2.437 or 2.462 GHz
6
Block-diagram Rx (Direct Conversion)
2
Low-Pass Filter
I/Q Demodulator
3
DDDL
Up
1
LP
ADC
I
Down
Digital Block
LP
ADC
Q
LO
fR fLO 10 MHz
RSSI
Chirp pulse
1
Correlation pulse
2
fLO 2.412, 2.437 or 2.462 GHz
Trigger signal with adaptive threshold
3
7
Chirp Properties

• Complex values of a windowed baseband up-chirp
  and down-chirp signals each with a total duration
  of 1µs
• Flat magnitude
• Plenty of roll-off time (easy to implement meet
  regulatory requirements)
• Significant simplification of correlator due to
  up-chirp and down-chirp similarities

8
Chirp Properties (cont.)

• Up-converted up-chirp and down-chirp signals each
  with a total duration of 1µs

9
Chirp Properties (cont.)

• Figure shows the autocorrelation function (ACF)
  of a chirp and cross-correlation function (CCF)
  of an up- and down-chirp
• Note that the CCF has a constant low value
  (compared to DSSS sequences).

10
Chirp Properties (cont.)

• This figure shows the PSD of the signal with a
  power of 10 dBm with 100 kHz resolution
• At 12 MHz offset from the center is below -30dBm
  (which is the ETSI requirement for out of band
  emissions)

11
System Properties

• Further processing of the signals Sig A and Sig B
  for symbol detection can be done either in a
  coherent (real part processing) or non-coherent
  manner (envelope filtering).
• Since the analytical results are well known for
  AWGN channels we will mention these
• Simulations over other channels will all refer to
  the non coherent system as drawn below.

12
System Properties (cont.)

• This figure shows the analytical BER values for
  2-ary orthogonal coherent and non coherent
  detection and the corresponding simulation
  results (1E7 symbols) for up down chirp (using
  the chirp signals defined above)
• The performance loss due to the non-orthogonality
  of up and down chirps is very small.

13
Channel Models

• Since this proposal refers to the 2.4GHz ISM
  band, only channel models with complete parameter
  sets covering this frequency range can be
  considered
• These are LOS Residential (CM1) and NLOS
  Residential (CM2).
• The SCD requirements on the payload size to be
  simulated seem to be somewhat inconsistent. At
  some point 10 packets with 32 bytes are mentioned
  which would be a total of 2560 bits. On the other
  hand a PER of 1 is required which mean
  simulating much more than 100 packets or 25600
  bits.
• Accurate results are obtained when large number
  of independent transmissions of symbols are
  simulated.
• BER is , with N number bits.
• For example, with PER1 and N256 (32 octets) we
  get BER3.9258E-5

14
Channel Model 1 LOS Residential
15
Channel Model 2 NLOS Residential
16
System Performance (CM2) BER
17
System Performance (CM2) PER

• Transmit power of 10 dBm
• 1 MBit / sec
• No FEC

18
System Performance

• Simulation over 100 channel impulse responses (as
  required in the SCD) were performed for channel
  model 1 and channel model 2.
• No bit errors could be observed on channel model
  1 (simulated range was 10 to 2000m). This is not
  really surprising because this model has a very
  moderate increase of attenuation over range
  (n1.79)
• The results for channel model 2 are presented.
  The parameter n4.48 indicates a very high
  attenuation for higher ranges. The results were
  interpreted as PER respectively and for
  convenience were plotted twice (linear and log y
  scale).

19
Ranging Topics

• TOA estimation
• Dithering and Averaging
• TOA processing
• SDS TWR Technique
• Error Sensitivity Analysis
• Simulation Results

20
TOA Estimation for Ranging
Noise and Jitter of Band-Limited Pulse Given a
band-limited pulse with noise su we want to
estimate how the jitter (timing error) st, is
affected by the bandwidth B. Jitter can be
represented as a variation in the rising edge of
a pulse through a given threshold,
21
TOA Estimation for Ranging

• The SN at the matched filter output is 2Es/N0
• If we assume a pulse with a rise time trise which
  is the inverse of the pulse bandwidth B (trise
  1/B) we can derive
• Bandwidth and signal to noise ratio can be traded
  against each other.

22
Principle of Dithering and Averaging
pulse to transmit
PHY packet to transmit
Ti
Ti

• Dithering

transmitted pulse
PHY packet transmitted
t
?Ti2
?Ti3
?Tin
?Ti1
PHY packet received

• Averaging

t
?TiRX3
?TiRXn
?TiRX2
?TiRX1
Ti
Ti
Ti
Ti
tToA
23
TOA ProcessingSymmetrical Double-Sided Two-Way
Ranging (SDS TWR)
Tround ... round trip time Treply ... reply
time Tprop ... propagation of pulse
Double-Sided Each node executes a round trip
measurement. Symmetrical Reply Times of both
nodes are identical (TreplyA TreplyB). Results
of both round trip measurements are used to
calculate the distance.
24
TOA ProcessingEffect of Time Base Offset Errors
Assuming offset errors eA, eB of the timebases
of node A and B we get
On the condition that the two nodes have almost
equal behavior, we can assume
This has the effect that timebase offsets are
canceled out
Calculations show that for 40 ppm crystals and 20
µs max difference between TroundA and TroundB
and between TreplyA and TreplyB an accuracy
below 1 ns can easily be reached!
25
Influence of Symmetry ErrorCalculation of an
SDS TWR Example System

• Example system EtA 40 ppm, EtB 40 ppm
  (worst case combination)

d ?d (?Treply 20 ns) ?d (?Treply 200 ns) ?d (?Treply 2 µs) ?d (?Treply 20 µs)
10 cm 0.012 cm 0.12 cm 1.2 cm 12 cm
1 m 0.012 cm 0.12 cm 1.2 cm 12 cm
10 m 0.05 cm 0.12 cm 1.2 cm 12 cm
100 m 0.4 cm 0.4 cm 1.2 cm 12 cm
1 km 4 cm 4 cm 4 cm 12 cm
10 km 40 cm 40 cm 40 cm 40 cm

• A ?Treply of between 2 µs and 20 µs is typical
  for a low-cost implementation.
• Implementations with symmetry error below 2 µs
  are feasible.
• Conclusion Even a 20 µs symmetry error allows
  excellent single-pulse accuracy of distance.

26
Simulation of a SDS TWR System
Example system Simulates SDS TWR Dithering
Averaging Crystal Errors 40 ppm Single shot
measurements _at_ 1 MBit/s data rate
(DATA-ACK) Transmit Jitter 4 ns
(systematic/pseudo RN-Sequence) Pulse detection
resolution 4 ns Pulses averaged per packet
32 Symmetry error 4 µs (average) Distance 100
m Results of Distance Error ?d ?dWC lt 50
cm ?dRMS lt 20 cm
27
Selection Criteria Document Topics
28
Backward Compatibility Option

• Due to the similarities with DSSS it is possible
  to implement this proposal in a manner that will
  allow backward-compatibility with the 802.15.4
  2.4 GHz standard.
• The transmitter changes are relatively
  straightforward.
• Changes to the receiver would include either dual
  correlators or a superset of CSS and DSSS
  correlators.
• Optional methods for backward-compatibility could
  be left up to the implementer
• mode switching
• dynamic change (on-the-fly) technique
• This backward-compatibility would be a
  significant advantage in the marketplace by
  allowing these devices to communicate with
  existing deployed 802.15.4 infrastructure and
  eliminating customer confusion.

29
Signal Robustness

• The proposed CSS PHY is designed to operate in a
  hostile environment
• Multipath
• Narrow and broadband intentional and
  unintentional interferers
• Since a chirp transverses a relatively wide
  bandwidth it has an inherent immunity to narrow
  band interferers
• Multipath is mitigated with the natural frequency
  diversity of the waveform
• Broadband interferer effects are reduced by the
  receivers correlator
• Forward Error Correction (FEC) can further reduce
  interference and multipath effects.
• Three non-overlapping frequency channels in the
  2.4 GHz ISM band
• This channelization allows this proposal to
  coexist with other wireless systems such as
  802.11 b, g and even Bluetooth (v1.2 has adaptive
  hopping) via DFS
• CSS proposal utilizes CCA mechanisms of Energy
  Detection (ED) and Carrier Detection
• These CCA mechanisms are similar to those used in
  IEEE 802.15.4-2003
• In addition to the low duty cycle for the
  applications served by this standard sufficient
  arguments were made to convince the IEEE 802
  sponsor ballot community that coexistence was not
  an issue.

30
Support for Interference Ingress

• Example (without FEC)
• Bandwidth B of the chirp 20 MHz
• Duration time T of the chirp 1 µs
• Center frequency of the chirp (ISM band) 2.437
  GHz
• Processing gain, BT product of the chirp 13 dB
• Eb/N0 at detector input (BER10E-4) 12 dB
• In-band carrier to interferer ratio (C/I _at_
  BER10-4) 12 - 13 -1 dB
• Implementation Loss 2 dB

31
Support for Interference Egress

• Low interference egress
• IEEE 802.11b receiver
• More than 30 dB of protection in an adjacent
  channel
• Almost 60 dB in the alternate channel
• These numbers are similar for the 802.11g
  receiver

32
Regulatory

• Devices manufactured in compliance with the CSS
  proposal can be operated under existing
  regulations in all significant regions of the
  world
• Including but not limited to North and South
  America, Europe, Japan, China, Korea, and most
  other areas
• There are no known limitation to this proposal as
  to indoors or outdoors
• The CSS proposal would adhere to the following
  worldwide regulations
• United States Part 15.247 or 15.249
• Canada DOC RSS-210
• Europe ETS 300-328
• Japan ARIB STD T-66

33
Scalability Data Rate

• Mandatory rate 1 Mb/s
• Optional rate 267 Kb/s
• Other possible data rates include 2 Mb/s to allow
  better performance in a burst type, interference
  limited environment or a very low energy
  consumption application
• Lower data rates achieved by using interleaved
  FEC
• Lower chirp rates would yield better performance
• longer range, less retries, etc. in an AWGN
  environment or a multipath limited environment
• It should be noted that these data rates are only
  discussed here to show scalability, if these
  rates are to be included in the draft standard
  the group must revisit the PHY header such as the
  SFD.

34
Scalability Frequency Bands

• The proposer is confident that the CSS proposal
  would also work well in other frequency bands
• Including the 5 GHz UNII / ISM bands

35
Scalability Data Whitener

• Additionally, the group may consider the use of a
  data whitener, similar to those used by Bluetooth
  and IEEE 802.11 to produce a more noise-like
  spectrum and allow better performance in
  synchronization and ranging.

36
Scalability Power Levels

• For extremely long ranges the transmit power may
  be raised to each countrys regulatory limit, for
  example
• The US would allow 30 dBm of output power with up
  to a 6 dB gain antenna
• The European ETS limits would specify 20 dBm of
  output power with a 0 dB gain antenna
• Note that even though higher transmit power
  requires significantly higher current it doesnt
  significantly degrade battery life since the
  transmitter has a much lower duty cycle than the
  receiver, typically 10 or less of the receive
  duty cycle.

37
Simultaneous Operating Piconets

• Separating Piconets by frequency division
• This CSS proposal includes a mechanism for FDMA
  by including the three frequency bands used by
  802.11 b, g and also 802.15.3
• It is believed that the use of these bands will
  provide sufficient orthogonality
• The proposed chirp signal has a rolloff factor of
  0.25 which in conjunction with the space between
  the adjacent frequency bands allows filtering out
  of band emissions easily and inexpensively.

38
Clear Channel Assessment

• A combination of symbol detection (SD) and energy
  detection (ED) has proven to be useful in
  practice (e.g. 802.11x, 802.15.4).
• CCA is used by the 15.4 MAC to significantly
  increase the number of active nodes possible by
  reducing the probability of collisions.

39
MAC Enhancements and Modifications

• There are very minimal anticipated changes to the
  15.4 MAC to support the proposed Alt-PHY.
• Three channels are called for with this proposal
  and it is recommended that the mechanism of
  channel bands from the proposed methods of TG4b
  be used to support the new channels.
• There will be an addition to the PHY-SAP
  primitive to include the choice of data rate to
  be used for the next packet. This is a new field.
• Ranging calls for new PHY-PIB primitives are
  expected to be developed by the Ranging
  subcommittee.

40
Frame Structure
Octets 4 1 or 2 1 Variable (up to 256)
Preamble SFD Frame length (8 bits) PHY payload
SHR SHR PHR PSDU

• The SFD structure has different values for, and
  determines, the effective data rate for PHR and
  PSDU
• The Preamble is 32 bits in duration (a bit time
  is 1 µs)
• In this proposal, the PHR field is used to
  describe the length of the PSDU that may be up to
  256 octets in length
• In addition to the structure of each frame, the
  following shows the structure and values for
  frames including overhead not in the information
  carrying frame

41
Throughput
797 Kb/s
330 Kb/s
245 Kb/s
155 Kb/s
267 Kb/s plot
1 Mb/s plot
Tack 192 µs SIFS 192 µs
42
Signal Acquisition

• Although CSS could use a shorter preamble, for
  consistency with IEEE 802.15.4-2003 this CSS
  proposal is based upon a preamble of 32 symbols
  which at 1MS/s is 32 µs
• Existing implementations demonstrate that
  modules, which might be required to be adjusted
  for reception (gain control, frequency control,
  peak value estimation, etc.), can settle in this
  time

43
Link Budget

• footnotes
• 1 Rx noise figure in addition the proposer can
  select other values for special purpose (e.g. 15
  dB for lower cost lower performance system)
• 2 The minimum Rx sensitivity level is defined
  as the minimum required average Rx power for a
  received symbol in AWGN, and should include
  effects of code rate and modulation.

44
Sensitivity

• The sensitivity to which this CSS proposal refers
  is based upon non-coherent detection
• It is understood that coherent detection will
  allow 2 - 3 dB better sensitivity but at the cost
  of higher complexity (higher cost?) and poorer
  performance in some multipath limited
  environments
• The sensitivity for the 1 Mb/s mandatory data
  rate is -92 dBm for a 1 PER in an AWGN
  environment with a front end NF of 7 dB
• The sensitivity for the optional 267 kb/s data
  rate is -97 dBm for a 1 PER in an AWGN
  environment with a front end NF of 7 dB

45
Mobility Values

• Communication
• No system inherent restrictions are seen for this
  proposal
• The processing gain of chirp signals is extremely
  robust against frequency offsets such as those
  caused by the Doppler effect when there is a high
  relative speed vrel between two devices.
• The Doppler effect must also be considered when
  one device is mounted on a rotating machine,
  wheel, etc.
• The limits will be determined by other, general
  (implementation-dependent) processing modules
  (AGC, symbol synchronization, etc.).
• Ranging
• The ranging scheme proposed in this document
  relies on the exchange of two hardware
  acknowledged data packets
• One for each direction between two nodes
• The total time for single-shot (2 data, 2 Ack)
  ranging procedure between the two nodes is the
  time tranging which, depending on the
  implementation, might be impacted by the uC
  performance. During this time the change of
  distance should stay below the accuracy da
  required by the application. The worst case is

• For da 1m
• tranging 2 ms this yields
• vrel ltlt 1000 m/s

46
Power Management Modes

• Power management aspects of this proposal are
  consistent with the modes identified in the IEEE
  802.15.4 2003 standard
• There are no modes lacking nor added
• Once again, attention is called to the 1 Mbit/s
  basic rate of this proposal and resulting shorter
  on times for operation

47
Power Consumption

• The typical DSSS receivers, used by 802.15.4, are
  very similar to the envisioned CSS receiver
• The two major differences are the modulator and
  demodulator
• The power consumption for a 10 dBm transmitter
  should be 198 mW or less
• The receiver for the CSS is remarkably similar to
  that of the DSSS with the major difference being
  the correlator
• The difference in power consumptions between
  these correlators is negligible so the power
  consumption for a 6 dB NF receiver should be 40
  mW or less
• Power save mode is used most of the time for this
  device and has the lowest power consumption
• Typical power consumptions for 802.15.4 devices
  are 3 µW or less
• Energy per bit is the power consumption divided
  by the bit rate
• The energy per bit for the 10 dBm transmitter is
  less than 0.2 µJ
• The energy per bit for the receiver is 60 nJ
• As an example, the energy consumed during an
  exchange of a 32 octet PDU between two devices
  would be 70.6 µJ for the sender and 33.2 µJ for
  the receiver

48
Antenna Practicality

• The antenna for this CSS proposal is a standard
  2.4 GHz antenna such as widely used for 802.11b,g
  devices and Bluetooth devices.
• These antennas are very well characterized,
  widely available, and extremely low cost.
• Additionally there are a multitude of antennas
  appropriate for widely different applications.
• The size for these antennae is consistent with
  the SCD requirement.

49
Size and Form Factor

• The implementation of the CSS proposal will be
  much less than SD Memory at the onset
• Following the form factors of Bluetooth and IEEE
  802.15.4 / ZigBee
• The implementation of this device into a single
  chip is relatively straightforward
• As evidenced in the Unit Manufacturing
  Complexity slides

50
Unit Manufacturing Complexity

• Target processRF-CMOS, 0.18 µm feature size

Pos. Block description Estimated Area Unit
1 Receiver with high-end LNA 2.00 mm²
2 Transmitter, Pout 10 dBm 1.85 mm²
3 Digitally Controlled Oscillator miscellaneous blocks 0.62 mm²
4 Digital and MAC support 0.30 mm²
5 Digital Dispersive Delay Line (DDDL) for the proposed chirp duration 0.32 mm²
6 Chirp generator for the proposed chirp duration 0.08 mm²
Occupied chip area for all major blocks required to build complete transceiver chip utilizing CSS technology 5.17 mm²
51
Unit Manufacturing Complexity

• Target processRF-CMOS, 0.13 µm feature size

Pos. Block description Estimated Area Unit
1 Receiver with high-end LNA 1.90 mm²
2 Transmitter, Pout 10 dBm 1.71 mm²
3 Digitally Controlled Oscillator miscellaneous blocks 0.59 mm²
4 Digital and MAC support 0.19 mm²
5 Digital Dispersive Delay Line (DDDL) for the proposed chirp duration 0.21 mm²
6 Chirp generator for the proposed chirp duration 0.06 mm²
Occupied chip area for all major blocks required to build complete transceiver chip utilizing CSS technology 4.66 mm²
52
Time To Market

• No regulatory hurdles
• CSS-based chips are available on the market
• No research barriers no unknown blocks
• Normal design and product cycles will apply
• Can be manufactured in all CMOS

53
Requirements Checklist

• Chirp Spread Spectrum (CSS) Proposal Meets the
  PAR and 5C
• Precision ranging capability accurate to one
  meter or better
• Extended range over 802.15.4-2003
• Enhanced robustness over 802.15.4-2003
• Enhanced mobility over 802.15.4-2003
• International standard
• Ultra low complexity (comparable to the goals for
  802.15.4-2003)
• Ultra low cost (comparable to the goals for
  802.15.4-2003)
• Ultra low power consumption (comparable to the
  goals for 802.15.4-2003)
• Support coexisting networks of sensors,
  controllers, logistic and peripheral devices in
  multiple compliant co-located systems.

54
Summary

• Chirp Spread Spectrum (CSS) is simple, elegant,
  efficient
• Combines DSSS and UWB strengths
• Precise location-awareness
• Robustness multipath, interferers, correlation,
  FEC, 3 channels, CCA
• Mobility enhanced
• Optional backward compatibility with
  802.15.4-2003
• Excellent throughput
• SOPs FD channels
• Signal Acquisition excellent
• Link Budget and Sensitivity excellent
• Very minimal MAC changes, CCA supported
• Power Management and Consumption - meets or
  exceeds requirements
• Antenna many good choices
• Can be implemented with todays technologies
• Low-complexity, low-cost
• Size and Form Factor meets or exceeds
  requirements
• Low power consumption
• Globally certifiable
• Scalability with many options for the future