Compromise for UWB Interoperability PHY Overview

1
Project IEEE P802.15 Working Group for Wireless
Personal Area Networks (WPANs) Submission Title
Compromise for UWB Interoperability PHY
Overview Date Submitted 20 February,
2004 Source John McCorkle Company Motorola,
Inc Address 8133 Leesburg Pike Voice703-269-3
000, FAX [phone], E-Mailjohn_at_xtremespec
trum.com Re IEEE 802.15.3a Call For Intent to
Present for Ad-Hoc Meeting Abstract This
document provides an overview of a proposed
Common Signaling Mode that would allow the
inter-operation or MB-OFDM and DS-UWB
devices. Purpose Promote further discussion
and compromise activities to advance the
development of the TG3a Higher rate PHY
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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Talking with each other Basic Requirements

• Each class of UWB devices (MB-OFDM or DS-UWB)
  needs a way to send messages to the other type
• MB-OFDM ? DS-UWB
• DS-UWB ? MB-OFDM
• Even better, design a common signaling mode that
  can be understood by either class of devices
• Goal Minimize additional complexity for each
  type of device while enabling this extra form of
  communications
• Use existing RF components DSP blocks to
  transmit message to other-class devices
• Also need to support a low-complexity receiver
• Lower rate mode could be acceptable if it can be
  used to provide robust control functions

3
The CSM Waveform

• One waveform that would be straightforward for
  either class of device is a BPSK signal centered
  in the middle of the low band at 4GHz
• Such a signal could be generated by both MB-OFDM
  and DS-UWB devices using existing RF and digital
  blocks
• MB-OFDM device contains a DAC nominally operating
  at 528 MHz
• A 528 MHz BSPK (3 dB BW) signal is likely too
  wide for MB-OFDM band filters
• Instead, DAC can be driven at slightly lower
  clock rate to produce a BPSK signal that will fit
  the MB-OFDM Tx filter
• Result is a 500 MHz wide BPSK signal that a
  DS-UWB device could receive demodulate, as
  would an MB-OFDM receiver
• DS-UWB device contains a pulse generator
• Use this to generate a 500 MHz BPSK signal at
  lower chip rate
• This signal would fit MB-OFDM baseband Rx filter
  and could be demodulated by both the MB-OFDM
  receiver and the DS-UWB receiver

4
CSM Waveform Makes All Connections
XMIT DS
REC DS
XMIT MB-OFDM
REC MB-OFDM
5
MB-OFDM DS-UWB Signal Spectrum with CSM
Compromise Solution
Proposed Common Signaling Mode Band (500 MHz
bandwidth)
Relative PSD (dB)
0
-3
DS-UWB Low Band Pulse Shape (RRC)
-20
3960
3432
4488
Frequency (MHz)
3100
5100
FCC Mask
MB-OFDM (3-band) Theoretical Spectrum
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CSM Interoperability Signal Overview

• 500 MHz BPSK signal has similar characteristics
  to original pulsed-multiband signals
• Proposed by several companies in TG3a CFP
• Adopt MB-OFDM band 2 center frequency for common
  signaling band
• Centered at 3960 MHz with approximately 500 MHz
  bandwidth
• BPSK chip rate easily derived from carrier chip
  rate carrier frequency / 9
• Frequency synthesis circuitry already present in
  MB-OFDM radio
• Does not suffer from Rayleigh fading (gt500 MHz
  BW)
• Uses different CSM piconet code for each piconet
• Each DEV can differentiate beacons of different
  piconets
• Provides processing gain for robust performance
  signal BW is much greater than data rate
• Relatively long symbol intervals (55 ns) used to
  avoid/minimize ISI
• Equalization still very simple in worse multipath
  channels

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MB-OFDM Transceiver Recovery of the CSM Signal

• Proposed MB-OFDM transmitter architecture
  contains almost all required blocks for CSM
  signal generation
• Use real-valued (single) DAC clocked at 440 MHz
  (less than design speed)
• Use length-24 ternary (-1/0/1) per-piconet
  spreading code
• This would be matched in DS-transmitter with a
  324 72 length code
• Result is BPSK signal with 520 MHz bandwidth (at
  -10 dB points)
• BPSK chip is a pulse of nine cycles of a
  sinusoid at 3960 MHz

440 MHz DAC clock
Not used for CSM
IFFT
Input
Convolutional
Bit
Constellation
Xmt LPF
DAC
Scrambler
Puncture
Insert Pilots
Data (9.2 Mbps w/ FEC, 18.3 Mbps un-coded)
Mapping
Encoder
Interleaver
Add CP GI
p
cos
(
2
f
t
)
Only required if FEC is used for CSM
c
Apply length-24 (-1/0/1)
Already present in MB-OFDM Transceiver
piconet spreading code
Time Frequency Code
(hold fixed at band 2 frequency 3960 MHz)
Add piconet coder
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MB-OFDM Frequency Synthesis for CSM
Select
Already present in MB-OFDM Transceiver
DAC Clock
440 MHz
Added Divider Selector
Carrier Frequency
Band 2 3960 MHz

• Clock for DAC based on existing MB-OFDM PLL
• 440 MHz Band 2 center frequency / 9

9
MB-OFDM Transceiver Recovery of the CSM Signal

• Data processing speed is much lower due to
  reduced data rates (10x slower)
• No Equalization needed (symbol interval is 55ns,
  almost no ISI, hence 60ns CP)
• Proposed MB-OFDM receiver already contains the
  needed blocks
• MB-OFDM receiver contains both time-domain and
  frequency-domain processing
• Time domain processing of BPSK signal is
  straight-forward
• MB-OFDM already contains correlator blocks used
  for synchronization functions
• Frequency domain processing possible using FFT
  engine for fast correlation
• MB-OFDM receiver uses IQ sampling with 4-5 bits
  resolution, could be under-clocked at 440 MHz
• Could implement RAKE / Channel-matched-filter

Low-complexity BPSK demodulator can use MB-OFDM
DSP blocks
BPSK demodulation And FEC decoding
Already present in MB-OFDM Transceiver
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Simplified DS CSM Signal Generator

• Proposed DS-UWB transmit architecture contains
  all required blocks for CSM generation
• Use length-24 ternary (-1/0/1) per-piconet
  spreading code
• Chipping rate of 440 MHz requires dividing
  chipping rate by 3
• Result is same CSM BPSK signal with 520 MHz
  bandwidth

LPF
Input
Convolutional
Bit
Scrambler
Puncture

Data (9.2 Mbps w/ FEC, 18.3 Mbps un-coded)
Encoder
Interleaver
Only required if FEC is used for CSP
Apply length-72 (-1/0/1)
Piconet spreading code
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Would the CSM mode need to use Forward Error
Correction?

• Based on link budget analysis, an un-coded CSM
  mode (18 Mbps) would have less margin at 10 m
  than the 110 Mbps MB-OFDM
• But we want the CSM to be more robust, not less
• FEC could be added to improve robustness, however
  there is no code that is common to both MB-OFDM
  DS-UWB proposals
• MB-OFDM uses punctured codes based on a rate 1/3
  k7 code
• DS-UWB uses punctured codes based on a rate 1/2
  k7 code
• Adding FEC to the CSM could result in as much as
  5 dB coding gain
• Would require a common code that both receivers
  can decode
• Pick one of the codes from the two proposals, or
• Choose a different code with relatively low
  complexity
• Following slides show link budgets for a few
  sample FEC choices

12
Link Budgets for CSM with Several Possible FEC
Modes
13
FEC Conclusions

• Based on complexity versus performance trade-off
  analysis for convolutional and block codes to
  provide 10 Mbps for CSP
• CSP must provide a more robust link than data
  modes (110 Mbps)
• Requiring either MB-OFDM or DS-UWB receiver to
  implement additional decoder for a different
  convolutional code would increase complexity
• Further analysis is underway, no definitive
  recommendation at this time

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Conclusions

• GOAL A CSM that allows interoperability between
  DS-UWB and MB-OFDM devices
• The efficiency is FAR better than allowing the
  devices to collide.
• A Common Signaling Mode is described that meets
  that goal
• Minimum useful data rate for 15.3 MAC-based
  interoperability is 10 Mbps
• Achieves desired data rates and robust
  performance
• Prevents coexistence problems for two different
  UWB PHYs
• Provides interoperability in a shared piconet
  environment
• The creation of a common signaling mode (CSM) is
  simple to add
• Essentially ZERO cost for both DS and MB-OFDM
• MB-OFDM requires addition of a divide-by-9
• Multiple options for receive using either time or
  frequency domain DSP blocks in MB-OFDM radio
• Using existing MB-OFDM band 2 center frequency
  and bandwidth
• DS requires more change, but is feasible
• changing clocks,
• adding mode to support 1/3 rd chipping rate