IEEE 802.15 PHY Proposal

1
Project IEEE P802.15 Working Group for Wireless
Personal Area Networks (WPANs) Submission Title
Proposed Code Sequences for IEEE 802.15.4a
Alt-PHY Date Submitted 16 Jan 2005 Source
Francois Chin, Sam Kwok, Xiaoming Peng, Kannan,
Yong- Huat Chew, Chin-Choy Chai, Hongyi Fu,
Manjeet, Tung-Chong Wong, T.T. Tjhung, Zhongding
Lei, Rahim Company Institute for Infocomm
Research, Singapore Address 21 Heng Mui Keng
Terrace, Singapore 119613 Voice 65-68745687
FAX [phone] E-Mail chinfrancois_at_i2r.a-
star.edu.sg Re Response to the call for
proposal of IEEE 802.15.4a, Doc Number
15-04-0380-02-004a Abstract I2Rs Proposal to
IEEE 802.15.4a Task Group Purpose For
presentation and consideration by the
IEEE802.15.4a committee 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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Proposed Code Sequences, Modulation Coding
for IEEE 802.15.4a Alt-PHY

• Francois Chin
• Institute for Infocomm Research
• Singapore

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Proposal Motivation

• To satisfy IEEE 802.15.4a technical requirements,
  low power consumption is crucial
• Conventional coherent UWB system based on
  correlator in the receiver can provide fairly
  good performance, but at the expense of
  implementation complexity, and consequently power
  consumption and system cost
• To meet low power and low cost requirement, UWB
  system with OOK (On-Off Keying) modulation and
  noncoherent detection is proposed
• In the proposed UWB OOK system, the signal
  demodulation is performed by simply integrating
  signal energy, thus omitting signal / pulse
  generator, significantly relieve the strict
  synchronization requirement and greatly simplify
  transceiver structure with the minimal power and
  cost demand
• However, some challenges of such OOK system are
  threshold setting, simultaneous operating
  piconets (SOP) receiver timing sampling
  boundaries
• This proposal contains techniques that will
  overcome such limitation, and improve the overall
  system performance of the UWB OOK system

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Challenges for OOK systems

• Conventional OOK systems face challenges in
• Receiver On-Off threshold setting
• Chip period boundary determination
• Other piconet interference
• This proposal intend to overcome these issues
  with
• Despreading using soft decision chip values
  instead of hard thresholding to better suppress
  other piconet interference
• Oversampling, together with properly chosen
  orthogonal code sequences, to recover chip timing
• The success of this hinges on the choice of code
  sequence !!

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Features of Proposal

• Use soft chip values after energy integrator at
  receiver with oversampling will eliminate the
  need for
• On-Off threshold setting
• Chip period boundary determination
• Use chip sequences for symbol mapping to carry
  more energy per chip (which is essential for OOK
  systems) and to suppress Other piconet
  interference
• Chip repetition to provide data rate / baseband
  operation frequency / power scalability

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Proposed System Parameters
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Modulation Coding
Binary data From PPDU
On-Off control
Chip Repetition
Bit-to- Symbol
Symbol- to-Chip
0,1 Sequence
Pulse Generator

• Bit to symbol mapping
• group every 4 bits into a symbol (for Mandatory
  bit rate of 2.75MHz)
• Symbol-to-chip mapping
• Each symbol is mapped to a 32-chip 0,1
  sequence, according to Gray Coded Code Position
  Modulation (CPM)
• Chip Repetition On Off Control (Output _at_ 22Mcps
  / K)
• Depending on the type of devices
• E.g. Factor of K11 corresponds an On-off
  control output switching _at_ 2 MHz, giving 25 kbps
• During a wireless transmission from a FFD to
  RFD, FFD can run at 22 MHz with Chip repetition
  RFD runs at 22MHz / K

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Modulation Coding
Binary data From PPDU
On-Off control
Chip Repetition
Bit-to- Symbol
Symbol- to-Chip
0,1 Sequence
Pulse Generator

• Pulse Generator
• can be one of the following
• A. 1,-1 bipolar RNS pulse generator _at_ 132 M
  pulse / sec (Mpps)
• B. 1,0 unipolar pulse sequence generator _at_
  132 M pulse / sec (Mpps) (with random pulse
  timing jittering)
• C. 1,0 unipolar chaotic signal generator with
  periodic on-off frequency _at_ 22 MHz

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Band Plan

• Proposed operating band 3.1 5.1 GHz
• To meet the FCC spectrum requirement for UWB
  systems
• To avoid Interferences from 802.11a,n and other
  sources
• Bands for the future Approximately 6 10 GHz

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UWB Pulse Spectrum

• 1.5 ns rectified cosine shape
• 1400 MHz 10-dB bandwidth
• Centre frequency 4 GHz

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Multiple access

• Multiple access within piconet
• TDMA, same as 15.4.
• Multiple access across piconets
• CDM
• Different Piconet uses different Base Sequence

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The receiver
LPF / integrator
Soft Despread
BPF
( )2
ADC
Sample Rate 1/Tc

• Energy detection technique rather than coherent
  receiver, for low cost, low complexity
• Soft chip values gives best results
• Oversampling sequence correlation is used to
  recovery chip timing recovery
• LPF / integrator and ADC sampling rate depends on
  types of devices
• 22MHz for high rate device
• 22MHz / K for low rate device (upto K 55, for 5
  kbps)
• Low rate device can truly run only slower clock
  (e.g. with transmit pulsing jitterling)
• Synchronization fully re-acquired for each new
  packet received (gt no very accurate timebase
  needed)
• Scalability

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Criteria of Code Sequences

• To minimise impact of DC noise effect on receiver
• For OOK signaling, the transmitter transmits
  1,0 unipolar sequences
• Conventional receive code sequence follows
  transmit sequence
• After the energy capture in the receiver, the
  noise has positive DC components in each chip
  error occurs in thresholding, especially at lower
  SNR
• This will accumulate noise unevenly in symbol
  decision
• An ideal receive despreading chip sequence should
  then have bipolar chip values, preferrably with
  equal number of 1 and -1 chips
• This, to certain extent, will nullify DC noise in
  symbol decision
• This, will also nullify unipolar signals from
  other interfering piconets
• Cyclic correlation of any antipodal sequence with
  its corresponding
• A good code set should, so that the DC noise
  effect in the receiver can be minimised
• This will also accumulate unipolar signals from
  other piconets

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Criteria of Code Sequences

• 2. The sequence should have orthogonal cross
  correlation properties to minimise symbol
  decision error

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Base Sequence Set

• 31-chip M-Sequence set
• Only one sequence and one fixed band (no hopping)
  will be used by all devices in a piconet
• Logical channels for support of multiple piconets
• 6 sequences 6 logical channels (e.g.
  overlapping piconets)
• The same base sequence will be used to construct
  the symbol-to-chip mapping table

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Symbol-to-Chip Mapping Gray Coded Code Position
Modulation (CPM)
To obtain 32-chip per symbol, cyclic shift the
Base Sequence first, then append a 0-chip
Base Sequence 1
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Why M-Sequences?

• Cyclic auto-correlation of any bipolar sequence
  gives peak value of 31 and sidelobe value of -1
  throughout
• Cyclic correlation of any bipolar sequence with
  its corresponding unipolar sequence give peak
  value of 16 and correlation with other 15
  unipolar sequences with give zero sidelobe
  throughout
• i.e. Each transmit OOK sequence will give a peak
  correlator output at a correlator with its
  corresponding antipodal sequence ZERO at other
  15 correlators

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Zero Padding Chip

• To avoid / reduce inter-symbol interference in
  channels with excess delay spread
• To ensure same number of 1s and -1s in
  corresponding receive correlation sequences, and
  to remove uneven DC noise distribution across
  symbol decision matric in receiver

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Synchronisation Preamble
Correlator output for synchronisation

• Code sequences has excellent autocorrelation
  properties
• Preamble is constructed by repeating 0000
  symbols

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Frame Format
2
1
0/4/8
2
n
Octets
Data Payload
Frame Cont.
MAC Sublayer
Seq.
Address
CRC
MHR
MSDU
MFR
Data 32 (n23)
TDB
1
1
For ACK 5 (n0)
Octets
PHY Layer
Frame Length
Preamble
SFD
MPDU
SHR
PHR
PSDU
PPDU
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AWGN Performance

• Soft value depreading gives 2 3 dB gain over
  thresholding techniques

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Comparison with other Sequences
M-Sequence has better single isolated piconet
performance due to its excellent cross
correlation between mapping sequences
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Inter-Piconet Interference Suppression
Let investigate the false alarm probability in
the presence of one two overlapping piconets
with asynchronous operation, all piconets using
sequences from either M-Sequence Code Set or Gold
Sequence Code Set
M-Sequence Code Set gives lower false alarm
probability and better suppression
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Inter-Piconet Interference Suppression
Max Corr Value
2 interfering piconet
1 interfering piconet
false alarm
M-Sequence Code Set gives lower false alarm
probability and better suppression
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Inter-Piconet Interference Suppression
M-Sequence Code Set gives lower false alarm
probability and better suppression