TG4a Review of proposed UWB-IR Modulation Schemes

1
Project IEEE P802.15 Working Group for Wireless
Personal Area Networks (WPANs) Submission Title
TG4a Review of proposed UWB-IR Modulation
schemes Date Submitted 21 April 2005 Source
Philip Orlik, Andy Molisch (Mitsubishi
Electric), Gian Mario Maggio (STMicroelectronics),
Ian Opperman (University of Oulu) Contact
Philip Orlik Voice 1 617 621 7570, E-Mail
porlik_at_merl.com Abstract Yet another UWB
waveform Purpose To provide information for
further investigation on and selection of the
modulation /waveform for UWB Impulse Radio (low
bit rate plus ranging) 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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IEEE 802.15.4a PHYUWB-IR Modulation for
multiple receiver types

• Many slides stolen from 15-05-0217-00-004a

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Definitions

• Coherent RX The phase of the received carrier
  waveform is known, and utilized for demodulation
• Differentially-coherent RX The carrier phase of
  the previous signaling interval is used as phase
  reference for demodulation
• Non-coherent RX The phase information (e.g.
  pulse polarity) is unknown at the receiver
• -operates as an energy collector
• -or as an amplitude detector

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Waveform Design (1/2)

• Combination of BPPM with BPSK
• Guarantee coexistence of coherent and
  non-coherent receiver architectures
• Non-coherent receivers just look for energy in
  the early or late slots to decode the bit (BPPM)
• Coherent and differentially-coherent receivers,
  in addition, understand the fine structure of the
  signal (BPSK or DBPSK)
• Principle Non-coherent and differentially-coheren
  t modes should not penalize coherent RX
  performance

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Waveform Design (2/2)

• Two possible realizations
• 1) The whole symbol (consisting of Nf frames) is
  BPPM-modulated
• 2) Apply 2-ary time hopping code, so that each
  frame has BPPM according to TH code
• Coexistence coherent/non-coherent RX
• - Special encoding and waveform shaping within
  each frame
• - Use of doublets with memory from previous bit
  (encoding of reference pulse with previous bit)
• - Proposed 20ns separation between pulses
• - Extensible to higher order TR for either
  reducing the penalty in transmitting the
  reference pulse or increasing the bit rate (see
  15-05-0217-00-004a for detail)
• - Also possible the use of multi-doublets (see
  15-05-0217-00-004a)

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Impulse Radio Modulation Scheme
Coherent Receiver
Hybrid Transmitter
Differentially Coherent Receiver
Non-Coherent Receiver
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Pros/Cons of RX Architectures

• Coherent
• Sensitivity
• Use of polarity to carry data
• Optimal processing gain achievable
• - Complexity of channel estimation and RAKE
  receiver
• - Longer acquisition time
• Differentially-Coherent (or using Transmitted
  Reference)
• Gives a reference for faster channel
  estimation (coherent approach)
• No channel estimation (non-coherent approach)
• - Asymptotic loss of 3dB for transmitted
  reference
• Non-coherent
• Low complexity
• Acquisition speed
• - Sensitivity, robustness to SOP and interferers

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Design Parameters

• Pulse Repetition Period (PRP)
• Proposed range between 40ns (first realization)
  and 125ns (second realization)
• Channelization (In addition to FDM)
• Coherent schemes Use of TH codes and polarity
  codes
• Non-coherent schemes Use of TH codes (polarity
  codes for spectrum smoothing only)
• TH code length
• Variable TH code length proposed range 8-16
• TH code Binary position, bi-phase
• Note For first realization, higher-order TH with
  shorter chip duration (multiples of 2ns) may be
  used

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Differential Encoding Basics
b0
b2
b4
b3
b1
b5
b-1
Tx Bits
0 0 1 1
0 0
1
Reference Polarity
-1 -1 1
1 -1
-1
1 -1 1
-1 1 -1
Ts
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Example Signal Waveforms for data modulation (1)
bi-1 1, bi 1
bi-1 0, bi 1
bi-1 1, bi 0
bi-1 0, bi 0
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Example Signal Waveforms for data modulation (2)
 11 
2-PPM TR base M 2 One bit/symbol
 01 
 10 
 00 
2-PPM 16 chips 2-ary TH code or 2-PPM 8 chips
4-ary TH code
(coherent decoding possible)

• Time hopping code is (2,2) code of length 8/16,
  can be exploited by non-coherent RX
• Effectively, 28 or 216 codes to select for
  channelization for non-coherent scheme

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Example Signal Waveforms for data modulation (3)
2-PPM 16 chips 2-ary TH code
(coherent decoding possible)

• Time hopping code is (2,2) code of length 8/16,
  can be exploited by non-coherent RX
• Effectively, 28 or 216 codes to select for
  channelization for non-coherent scheme

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De-Spreading TH Codes
TH Sequence Matched Filter
r(t)
Bit Demodulation
LNA
Case I - Coherent TH de-spreading
TH Sequence Matched Filter
Bit Demodulation
b(t) soft info
r(t)
LNA
Case II Non-coherent / differential TH
despreading
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Coherent Receiver RAKE Receiver
Channel Estimation
Rake Receiver Finger 1
Rake Receiver Finger 2
Sequence Detector
Demultiplexer
Convolutional Decoder
Summer
Data Sink
Rake Receiver Finger Np

• Addition of Sequence Detector Proposed
  modulation may be viewed as having memory of
  length 2

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Uncoded AWGN Performance
1.5 dB
16
Uncoded CM8 Performance
1 dB
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Advantages

• Coherent RX gets additional benefit from coding
  inherent in the modulation
• Waveform permits polarity scrambling to reduce
  required back-off
• A single waveform for all receiver types

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Coherent view of Hybrid Modulation

• Symbols consist of sequences of doublets that
  have 4 possible forms

bi-1 1, bi 1
bi-1 0, bi 1
bi-1 0, bi 0
bi-1 1, bi 0
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Basis functions

• Symbols can be decomposed using several
  equivalent orthogonal basis functions (2-D)

(
)
(
)
_
_
,
,
1, 0
1/2, 1/2
-1, 0
-1/2, -1/2
0, 1
1/2, -1/2
0, -1
-1/2, 1/2
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Matched Filter Outputs (basis 1)
Template signals constructed from these 2 basis
functions
Barker 13 applied to TR doublets
bi-1 1, bi 1
bi-1 0, bi 1
bi-1 1, bi 0
bi-1 0, bi 0
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Matched Filter Outputs (basis 2)
Template signals constructed from these 2 basis
functions
Barker 13 applied to doublets
_
_
bi-1 1, bi 1
bi-1 0, bi 1
bi-1 1, bi 0
bi-1 0, bi 0
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Ranging Implications

• So what does this mean for ranging?
• Really nothing
• If we want to transmit a barker sequence (or
  something else) we can still use our receiver to
  look for the ranging symbol
• Basically we can use as a
  basis and view the compression code as a polarity
  code applied to the reference pulse and data
  pulse independently.
• Barker 13 1 1 1 1 1 -1 -1 1 1 -1 1 -1 1
• Reference code 1 1 1 -1 1 1 1
• Data code 1 1 -1 1 -1 -1

(
)
_
_
,
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Barker 13 transmission
Correlator output
Tx waveform