Project:%20IEEE%20P802.15%20Working%20Group%20for%20Wireless%20Personal%20Area%20Networks%20(WPANs)

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Project IEEE P802.15 Working Group for Wireless
Personal Area Networks (WPANs) Submission Title
Mitubishi Electric Proposal Time-Hopping Impulse
Radio Date Submitted May 5th, 2003 Source
Andreas F. Molisch et al., Mitsubishi Electric
Research Laboratories Address MERL, 201 Broadway
Cambridge, MA, 02139, USA Voice 1 617 621
7558, FAX [phone] , E-Mail
Andreas.Molisch_at_ieee.org Re Response to Call
for Proposals Abstract We present a standards
proposal for a high-data-rate physical layer of a
Personal Area Network, using ultrawideband
transmission. The air interface is based on
time-hopping impulse radio, using BPSK for the
modulation, and in addition polarity
randomization of the pulses within the symbol.
Combinations of delayed and weighted pulses allow
an efficient shaping of the spectrum. This
provides good suppression of interference, and
guarantees fulfillment of coexistence
requirements. The system is designed to have A/D
conversion and digital processing only at the
symbol rate, not the chip rate. Costs are
comparable to Bluetooth. Purpose Proposing a
PHY-layer interface for standardization by
802.15.3a 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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Ultra WideBand

• Mitsubishi Electric Proposal
• Time-Hopping Impulse Radio
• A. F. Molisch, Y.-P. Nakache, P. Orlik, J. Zhang
• Mitsubishi Electric Research Lab
• S. Y. Kung, Y. Wu, H. Kobayashi, S. Gezici, E.
  Fishler, V. Poor
• Princeton University
• Y. G. Li
• Georgia Institute of Technology
• H. Sheng, A. Haimovich
• New Jersey Institute of Technology

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Contents

• System overview
• Physical-layer details
• Performance evaluation
• Signal robustness
• Coexistence
• Cost analysis
• Summary and conclusions

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Goals and Solutions

• Commonly used technology
• ? Time hopping impulse radio
• Fulfillment of spectral mask, but full
  exploitation of allowed power. Interference
  suppression
• ? Linear combination of basis pulses
• Cheap implementation, robustness to multipath
• ? Few Rake fingers, all A/D conversion and
  computation done at 200MHz
• Scalability
• ? Multi-code transmission

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Creation of Proposal

• Proposal based on
• Scientific experience of leading research groups
  (Princeton, Georgia Tech, MERL, MELCO)
• Practical experience of high-quality product
  development team of Mitsubishi in USA and Japan
• Experience in hardware (RF components, antennas,
  semiconductor, applications,..) and applications
  design

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Transmitter Structure
Sync. Training Sequence
Central Timing Control
Convolutional Code
Multiplexer
Timing Logic
Pulse Gen. TH Seq.-1
Polarity Scrambler
Power Control
Demultiplexer
Data Source
Convolutional Code
Multiplexer
Polarity Scrambler
Timing Logic
Pulse Gen. TH Seq.-N
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Receiver Structure
Synchronization
Timing Control
Channel Estimation
Rake Receiver Finger 1
AGC
Rake Receiver Finger 2
Demultiplexer
MMSE Equalizer
Convolutional Decoder
Summer
Data Sink
Rake Receiver Finger Np
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Contents

• System overview
• Physical-layer details
• acquisition
• channel estimation
• polarity hopping
• spectral shaping
• Rake structure
• Performance evaluation
• Summary and conclusions

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Fast acquisition

• template signal and received signal need to be
  aligned
• standard method serial search (chip by chip)
• but chip duration very short in UWB, takes long
  time
• our solution
• Beacon provides rough timing estimation (within
  runtime of the piconet diameter)
• new block search methods for actual acquisition

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Block Search Algorithms

• Steps in acquisition
• Find delay region where signal is likely to exist
• After finding it, search in more detail for first
  significant path
• Block search algorithm
• Sequantial block search (SBS) integrate output
  of detector over delay region (block), search for
  block with significant energy. Best for LOS
• Average block search (ABS) average over absolute
  values of detector output. Best for NLOS

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Sequential Block Search

• 1) Check the bth block using the first template
  signal (t).
• 2) If the output of the bth block is not higher
  than a block threshold, tb, then, go to step 6.
• 3) If the output of the bth block is higher than
  the block threshold, tb, then search the block in
  more detail, i.e., cell-by-cell serial search
  with a signal threshold ts, using the second
  template signal (t).
• 4) If no signal cell is detected in the block, go
  to step 6.
• 5) If the signal cell is detected in the block,
  DONE.
• 6) Set b (b mod B) 1 and go to step 1.

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Average Block Search

• 1) Check difference between successive averages
  wi mod B - w(i-1) mod B.
• 2) If the difference is not higher than a first
  threshold go to step 6.
• 3) If the difference is higher than, check
• z(i mod B)K1, , z(i mod B)1)K serially,
  comparing to a second threshold, .
• 4) If no signal cells detected, go to step 6.
• 5) If signal cell(s) are detected, DONE.
• 6) Set i (i 1) mod B, and go to step 1.

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Channel Estimation

• Swept delay correlator
• Principle estimating only one channel sample per
  symbol.
• Similar concept as STDCC channel sounder of Cox
  (1973).
• Sampler, AD converter operating at SYMBOL
  frequency
• Requires longer training sequence
• Three-step procedure for estimating coefficients
• With lower accuracy estimate at which taps
  energy is significant
• With higher accuracy determine tap weights
• Determine effective channel seen by equalizer
• Silence periods for estimation of interference

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Channel Estimator Block Diagram
Adj.Weight
Multiplier Low-Pass Filter
S
Rake receiver Output
Programmable Training Waveform Gen.
Rake Finger 1
EQ Output
ReceiverFront End
MMSE Equalizer
Multiplier Low-Pass Filter
Adj.Weight
Programmable Training Waveform Gen.
Rake Finger 2
Coefficients
Multiplier Low-Pass Filter
Adj.Weight
Equalizer Estimator
Programmable Training Waveform GEN.
Rake Finger N
Channel Estimator
EQ Training Sequence
Timing Controller
Channel Estimation Output
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Estimator algorithm
evaluation of one sample per 5ns interval, offset
by Tc
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Estimator

• Multi-step procedure
• estimate which taps have significant weights
• estimate tap weights for L significant taps
• determine Rake receiver weights via minimum mean
  square error criterion
• determine equivalent (symbol-spaced) channel from
  transmitter to output Rake receiver
• find equalizer for this equivalent channel (MMSE)

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Modulation and Multiple Access

• Multiple access
• Combination of pulse-position-hopping and
  polarity hopping for multiple access
• More degrees of freedom for design of good
  hopping sequence than pure pulse-position-hopping
• Short hopping sequences, to make equalizer
  implementation easier
• Modulation BPSK
• Channel coding
• rate ½ convolutional code
• requires 4dB SNR for 10-5 BER
• Improvement by 3dB possible by turbo codes

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Spectral Shaping Interference Suppression

• Basis pulse fifth derivative of Gaussian pulse
• Drawbacks
• Loses 3dB compared to FCC-allowed power
• Strong radiation at 2.45 and 5.2 GHz

Monocycle, 5th derivative of gaussian pulse
Power spectral density of the monocycle
10log10P(f)2 dB
Magnitude of p(t)
Time (s)
frequency (Hz)
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Linear Pulse Combination

• Solution linear combination of delayed, weighted
  pulses
• Adaptive determination of weight and delay
• Number of pulses and delay range restricted
• Can adjust to interferers at different distances
• (required nulldepth) and frequencies
• Weight/delay adaptation in two-step procedure
• Initialization as solution to quadratic
  optimization problem (closed-form)
• Refinement by back-propagating neural network
• Matched filter at receiver ?good spectrum helps
  coexistence and interference suppression

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Initialization

• find modified mask that follows FCC and
  required interference suppression (e.g., 20dB for
  802.11a
• approximate optimum filling of mask as
• solution of this in closed form (eigenvector
  belonging to largest eigenvalue)

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Iterative Refinement

• backpropagating neural network

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Rake Receiver

• Main component of Rake finger pulse generator
• A/D converter 3-bit, operating at 220Msamples/s
• No adjustable delay elements required

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Contents

• System overview
• Physical-layer details
• Performance evaluation
• Signal robustness
• Coexistence
• Cost analysis
• Summary and conclusions

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Link Budget
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PER as Function of Distance
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Probability of Link Success
Sensitivities AWGN 13m cm1 6.8 cm2 6.2 cm3
5.3 cm4 5.0
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Outage vs. SNR
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Single co-channel interferer separation distance
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Single co-channel interferer separation distance
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Single co-channel interferer separation distance
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Single co-channel interferer separation distance
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Susceptibility to Interference

• Piconets
• 20 first realizations of the 4 channel model and
  AWGN
• Desired user 6dB above sensitivity
• admissible distance of interferer between 0.5
  and 1.5m
• 802.11a influence only when interferer less than
  0.4m distance, in CM2
• 802.11b no noticeable influence (even at 0.3m
  distance of interferers) in all cases

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Coexistence (at 1m)
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Cost Estimates (for 110Mbit/s mode)

• TX
• Digital
• Coders 100k gates
• timing logic lt100k gates
• RF
• Pulse generators (4) 0.6mm2
• Polarity scramblers 0.04mm2
• Summers 0.04mm2

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Cost Estimates (for 110Mbit/s mode)

• RX
• Digital
• Viterbi Decoder 100k gates
• timing logic lt100k gates
• MMSE equalizer 50k gates
• Rake finger weighting and summing lt50k gates
• RF
• LNA (11dB SNR) 0.05mm2
• Pulse generators (210) 3.2mm2
• Polarity descramblers 0.04mm2
• Low-pass filters 0.48mm2
• Summers 0.04mm2

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Cost Estimates - Summary

• RF part
• total die size lt10mm2 less than Bluetooth
• 0.18mu CMOS technology sufficient
• Digital part
• Less than 500k gates
• Operation at 220Mbit/s
• Antenna cavity-backed spiral antenna
• Total costs comparable to Bluetooth

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Self-Evaluation (I)
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Self-Evaluation (II)
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Summary and Conclusions

• TH-IR based standards proposal
• Meets targets of 802.15.3a for LOS
• Innovative way to manage spectrum
• Meet FCC requirements
• Improve performance in interference environment
• Decrease interference to other systems
• Allows cheap implementation
• All digital operations at symbol rate, not chip
  rate
• Scaleable
• Multicode / multirate system.