Rake Span Requirements for Multi-band UWB Systems

1
Project IEEE P802.15 Working Group for Wireless
Personal Area Networks (WPANs) Submission Title
Rake Span Requirements for Multi-band UWB
Systems Date Submitted 14 May, 2003 Source
Jaiganesh Balakrishnan et al. Company Texas
Instruments Address 12500 TI Blvd, MS 8649,
Dallas, TX 75243 Voice214-480-3756, FAX
972-761-6966, E-Mailjai_at_ti.com Re
Abstract This document describes the rake
span requirements for multi-band UWB
systems. Purpose For discussion by IEEE 802.15
TG3a. 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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Rake Span Requirements for Multi-band UWB
Systems

• Jaiganesh Balakrishnan, Anuj Batra Anand Dabak,
  Jerry Lin, Ranjit Gharpurey, and Simon Lee
• Texas Instruments12500 TI Blvd, MS 8649Dallas,
  TX
• May 14, 2003

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Outline

• Overview of multi-path energy capture.
• RAKE design parameters.
• RAKE design no group delay variations.
• RAKE design group delay variations.
• Buildability issues with multiple RX chains.
• Conclusions.

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Multi-path Energy Capture

• In multi-path environments, the RMS delay spreads
  for a UWB channel can be large (14 ns for CM3, 25
  ns for CM4).
• Uncaptured multi-path energy results in loss in
  performance of the UWB device.
• One method for energy collection is to use a RAKE
  receiver.

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RAKE Design Parameters

• There are two main parameters that need to be
  considered when designing a RAKE receiver for a
  multi-band system
• The total number of RAKE fingers (see also
  03/210r0)
• Span of the RAKE receiver.
• The total number of RAKE fingers determines the
  RX digital complexity.
• Span of the RAKE receiver determines the number
  of analog RX chains needed.
• Optimal RAKE finger placement does not need to be
  contiguous.
• Ex 2 RAKE fingers can be separated by a
  considerable number of samples.

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Design of a RAKE

• Assumption
• System 1 A 7-band multi-band system that
  transmits a 3.9 ns pulse once every 7.8 ns in
  each sub-band (132 Msps).
• System 2 A 7-band multi-band system that
  transmits a 3.9 ns pulse once every 3.9 ns in
  each sub-band (264 Msps).
• Impact of ISI is expected to be negligible and
  hence not considered.
• A smaller RAKE span results in loss of collected
  multi-path energy.
• Inherent trade-off
• Number of RX chains vs. multi-path energy
  collection.

Symbol Rate Rake Span for 1 RX Chain Rake Span for 2 RX Chains Rake Span for 3 RX Chains
132 Msps 7.8 ns 15.6 ns 23.4 ns
264 Msps 3.9 ns 7.8 ns 11.7 ns
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RAKE Design No Group Delay (1)

• Assumption
• Synchronous hopping across the sub-bands.
• No group delay due to front-end filtering.
• Optimal timing is typically not feasible with a
  single receive chain
• Reason optimal sampling time for RAKE in
  sub-bands 2 and 3 overlap. Impossible to do
  with a single receive chain.

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RAKE Design No Group Delay (2)

• To ensure that multi-path energy is collected
  across all sub-bands, we need to constrain the
  RAKE fingers to be in the same location
  regardless of the sub-band.
• Location is chosen to maximize the overall
  received energy.

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RAKE Design No Group Delay (3)

• Assumptions
• Ideal channel estimation.
• No front-end group delay variations.
• Zero switching time.
• Sample timing chosen to maximize collected energy
  for symbol spaced sampling (264 MHz).
• Normalized the channel impulse responses to unity
  to remove effects of shadowing/fading.
• Captured energy averaged over 100 channel
  realizations.

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RAKE Design No Group Delay (4)

• Captured energy versus RAKE span for CM2 channel
  environment
• A 3 dB performance loss ? 30 loss in range.
• Conclusion to achieve less than 3 dB performance
  loss in CM2
• Need 2 receive chains for a 132 Msps systems
• Need 3 receive chains for a 264 Msps systems.

Loss In Captured Multi-path Energy Number of RX Chains for 132 Msps System Number of RX Chains for 264 Msps System
5.3 dB 1 1
3.9 dB 1 2
3.0 dB 2 3
2.4 dB 2 4
1.9 dB 3 5
1.5 dB 3 6
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RAKE Design No Group Delay (5)

• Captured energy versus RAKE span for CM3 channel
  environment
• A 3 dB performance loss ? 30 loss in range.
• Conclusion to achieve less than 3 dB performance
  loss in CM3
• Need 3 receive chains for a 132 Msps systems
• Need 5 receive chains for a 264 Msps systems.

Loss In Captured Multi-path Energy Number of RX Chains for 132 Msps System Number of RX Chains for 264 Msps System
7.4 dB 1 1
5.1 dB 1 2
4 dB 2 3
3.3 dB 2 4
2.8 dB 3 5
2.4 dB 3 6
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RAKE Design Group Delay (1)

• Consider a multi-band system whose operating
  bandwidth includes the U-NII band.
• If a notch filter is used to suppress the
  interference from the U-NII band, then there
  could be significant group delay variations on
  sub-bands on either side of the notch.
• Notch filter is one of the components that will
  result in group delay variations. Other
  components include antenna, LNA, etc.

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RAKE Design Group Delay (2)

• Assumption
• CM1 channel environment.
• Synchronous hopping across the sub-bands.
• Due to group delay variations, impulse response
  for sub-bands 4 and 6 are delayed by 3.9 ns
  relative to the other sub-bands.
• Conclusions
• With one RAKE finger, the loss in performance is
  nearly 1 dB.
• However, as the number of RAKE fingers increases
  the additional degradation due to group delay
  variations is small (also true for CM2, CM3, and
  CM4).

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Buildability Issues with Multiple RX Chains

• Hardware penalty for multiple receive chains.
• Need to duplicate entire receive chain after LNA,
  including mixer, VGA, channel select filter, and
  ADC.
• Increased die size, power consumption, and cost.
• Analog section does not scale with improvements
  in technology node.
• Potential issues
• LNA loading results in a trade-off between
  bandwidth, power, and noise figure.
• Cross-talk between receiver chains.
• Increased design time.

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Conclusions

• Studied RAKE span requirements for multi-band UWB
  systems.
• Multi-band UWB systems need multiple RX chains
  for CM2, CM3 and CM4 environments.
• If more than 1 Rx chain is used, the impact due
  to group delay variations is negligible.
• Multiple receive chains results in a hardware
  penalty and has potential implementation issues.