04-0626r3

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Project IEEE P802.15 Working Group for Wireless
Personal Area Networks (WPANs) Submission Title
Coding for TG4a Date Submitted July
2005 Source Matt Welborn (1), Michael
McLaughlin (2), Shariar Emami (1) Company
(1) Freescale Semiconductor, Inc, (2) decaWave,
Ltd. Address 8133 Leesburg Pike, Vienna VA
22182 Voice703-269-3000, FAX ,
E-Mailmatt.welborn _at_ freescale.com, or
michael_at_decawave.com Re Response to Call for
Proposals Abstract This document describes a
proposal for the TG4a baseline draft
standard. Purpose Proposal Presentation for
the IEEE802.15.4a standard. 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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Support for Coherent and Non-Coherent Modulation

• Two different sets of requirements
• Non-coherent receiver only uses energy detector,
  cannot resolve phase relative to reference
  carrier
• Coherent receiver can also resolve phase
• Idea
• Encode data non-coherently for PPM/OOK
• Add redundant encoding of data in phase of pulses
• How would this work?

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Encode data at two levels

• Data bits map into PPM constellation
• Map zero into
• Map one into
• Now map data into pulse phase as well
• How do we do this?
• Create a redundant mapping using a systematic
  convolutional code

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Use a Systematic Code to Compute a Redundant Bit
x1bk
Convolutional Encoder
bk
x2

• Rate-½ convolutional encoder
• Produce multiple coded bits from each data bit
• Encoder itself is very low complexity
• Special case of convolutional code is a
  systematic code
• First coded bit is same as input data bit
• Second coded bit is computed by encoder
• Code can be chosen to have desired constraint
  length (TBD) code gain (not limited to a
  specific constraint length)
• Mapping coded bits to waveform
• Map first coded bit (systematic bit) into
  position for BPPM
• Map second coded bit into phase
• Can be extended to more general (non-systematic)
  codes very easily

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Non-Coherent and Coherent Demodulation
X1 0, X2 0
X1 1, X2 0
X1 1, X2 1
X1 0, X2 1

• Non-coherent receiver only sees position
• Demodulates only x1
• No Viterbi decoding required (easy since x1bk)
• Achieves no coding gain, assumes bk x1 ? Done.
• Coherent receiver demodulates position and phase
• Decodes x1 x2
• Viterbi decoding used to estimate original bit,
  bk
• Achieves coding gain of original rate ½ code

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Another way to look at this Mapping
4-BOK (coherent) constellation
2-PPM constellation
2-PPM constellation
OOK constellation
Non-coherent receiver cannot see these

• Encoding two coded bits requires a 4-point signal
  constellation
• Each axis represents one of two possible
  positions (orthogonal axes)
• Phase of pulse determines sign of constellation
  point on axis ? 4-BOK
• Non-coherent receiver is insensitive to phase
  see only two points in constellation ? 2-PPM
• Support for OOK receiver is possible by
  demodulating only one of the two dimensions (i.e.
  just look at first position pulse or not?)

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Ternary Modulator II (from 15-05-428r0)
If a guard of 2 pulses is used
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Using multiple pulses per symbol
Tsymbol

• Can use multiple pulses per each burst
• Four shown as an example
• Can using scrambling on pulses to whiten
  spectrum
• Can also include guard pulses if desired
• Also need to ensure that average pulse rate is
  kept sufficiently high

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What about a Differential Recevier?

• Differential receiver cannot discern absolute
  phase, just the phase difference between two
  pulses

Same polarity
Opposite Polarity
Same polarity, inverted
Opposite Polarity, inverted
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Extending the Punctured Code Concept

• Now we need three levels of encoding
• Pulse position detectable by non-coherent
  receiver
• Relative phase detectable by differential
  receiver
• Absolute phase detectable by coherent
  receiver
• Extend the previous concept to use a rate-1/3
  code
• Encoder maps one input data bit to three output
  bits
• Could choose to use one systematic bit of the
  three
• Decode only phase
• Can decode the rate 1 punctured code (only
  systematic bit - trivial)
• Decode position relative phase
• Have two of every three coded bits
• Can treat the code as a rate 1/3 code, punctured
  to rate ½
• Perform decoding and get moderate coding gain
• Decode all three (Position, relative absolute
  phase)
• Use full viterbi decoder to get best performance
  coding gain