What is Really Fundamental?

1
Project IEEE P802.15 Working Group for Wireless
Personal Area Networks (WPANs) Submission Title
What is really fundamental? Date Submitted
12 September, 2004 Source A. Batra, J.
Balakrishnan, A. Dabak, S. Lingam Company Texas
Instruments Address 12500 TI Blvd, MS 8649,
Dallas, TX 75243 Voice214-480-4220, FAX
972-761-6966, E-Mailbatra_at_ti.com Re
FYI Abstract This document examines what is
fundamental. 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.
2
What is Really Fundamental?

• Anuj Batra, Jaiganesh Balakrishnan, Anand Dabak,
  Srinivas Lingam
• Texas Instruments12500 TI Blvd, M/S 8649Dallas,
  TX 75204September 12, 2004

3
Motivation

• This presentation is going to look at the some of
  the fundamental issues concerning both
  proposals
• Is the 6-dB gap for 480 Mbps MB-OFDM a
  fundamental gap?
• Implementation losses associated with the DS-UWB.

4
Is the 6 dB Gap Fundamental?
5
Fundamental Concepts in 802.15.3a?

• According to Document 15-04/022r0
• Rayleigh fading for MB-OFDM cannot be mitigated
  by any amount of added signal processing
• High rate modes degraded by 6 dB or more relative
  to AWGN
• Rayleigh fading performance does not improve
  with process technology or added digital
  processing
• The DS-UWB authors have REPEATEDLY stated that
  this is a fundamental problem with Multi-band
  OFDM.
• Question
• Is this a fundamental concept that we cannot
  violate?

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Latest MB-OFDM Proposal (1)

• Guard tone mapping was added to the MB-OFDM
  proposal in order to address concerns raised by
  the Task Group.
• Exact mapping of tones is shown below
• Equivalent to
• Frequency-domain spreading for the lower rates of
  MB-OFDM system.
• Excess BW used by single-carrier systems.

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Latest MB-OFDM Proposal (2)

• Mathematically, the mapping of the data on to the
  Guard Tones can be written as follows
• where Pn are the Guard Tones and Cn are the Data
  Carrier Tones.
• Let us now consider the effect that the Guard
  Tones has on the system performance of the
  MB-OFDM solution for the 110, 200, and 480 Mbps
  modes.

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Simulation Parameters

• Assumptions
• System as defined in 03/268.
• Clipping at the DAC (PAR 9 dB).
• Finite precision ADC (4 bits for 110, 200 Mbps
  and 5 bits for 480 Mbps).
• No attenuation on the Guard Tones.
• Degradations incorporated
• Front-end filtering.
• Multi-path degradation.
• Shadowing.
• Clipping at the DAC.
• Finite precision ADC.
• Crystal frequency mismatch (?20 ppm _at_ TX, ?20 ppm
  _at_ RX).
• Channel estimation.
• Carrier/timing offset recovery.
• Carrier tracking.
• Packet acquisition.

9
System Performance with Guard Tones

• The distance at which the Multi-band OFDM system
  can achieve a PER of 8 for a 90 link success
  probability is tabulated below
• Includes losses due to front-end filtering,
  clipping at the DAC, ADC degradation, multi-path
  degradation, channel estimation, carrier
  tracking, packet acquisition, etc.

Range AWGN CM1 CM2 CM3 CM4
110 Mbps 21.5 m New 12.0 m Original 11.4 m New 11.4 m Original 10.7 m New 12.3 m Original 11.5 m New 11.3 m Original 10.9 m
200 Mbps 14.8 m New 7.4 m Original 6.9 m New 7.1 m Original 6.3 m New 7.5 m Original 6.8 m New 6.6 m Original 4.7 m
480 Mbps 9.1 m New 3.2 m Original 2.9 m New 3.0 mOriginal 2.6 m N/A N/A
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Improvement with Guard Tones

• System performance improves for both channel
  models
• CM1 2.9 m ? 3.2 m (0.9 dB improvement).
• CM2 2.6 m ? 3.0 m (1.2 dB improvement).
• Using the fact that shadowing contribution is
  3.9 dB to the overall degradation, the gap from
  AWGN to the 480 Mbps mode using Guard Tones has
  already been reduced by 0.8 dB!
• This analysis shows that the Rayleigh fading for
  MB-OFDM can be mitigated by additional signal
  processing.
• Gap of 6 dB in fading is NOT a fundamental issue.

11
Examine Implementation Losses Associated with the
DS-UWB Proposal
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Implementation Losses (1)

• Lets review the performance results given by the
  DS-UWB authors
• Simulation results used to show an implementation
  loss of 0.8 dB, but now show an implementation
  loss of 0.4 dB.
• Some examples of implementation loss
• Degradations due to finite-precision ADC.
• Degradations due to timing synchronization
  errors.
• Degradations due to carrier frequency
  synchronization errors.
• Degradations due to channel estimation errors.
• Degradations due to finite-precision effects in
  the digital domain.
• In contrast, the MB-OFDM proposal has shown
  simulation results with a 2.5 dB of
  implementation loss for 110 Mbps.

Assumptions / Value DS-UWB AWGNLink Budget Table DS-UWB AWGNSimulation Results (1) DS-UWB AWGNSimulation Results (2)
Range 18.3 meters 22.2 meters 23.4 meters
Noise Figure 6.6 dB 6.6 dB 6.6 dB
Implementation Loss 2.5 dB Not Given,Inferred Value 0.8 dB Not Given,Inferred Value 0.4 dB
Results extracted from IEEE 802.15-04/099r2.
Results extracted from IEEE 802.15-04/483r2.
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Implementation Losses (2)

• Question
• Is it possible to design a system with an
  implementation loss of 0.4 dB?
• In this presentation, we start by looking the
  degradations that are finite-precision ADC
  degradations and caused by timing synchronization
  errors .
• In follow-up presentations, we hope to look at
  the some of the remaining categories
• Carrier-frequency synchronization errors.
• Channel estimations errors.
• Etc.

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Degradations Due to a 3-bit ADC (1)

• System model
• Simulation assumptions
• Considered 3-bit and 20-bit ADC.
• Channel AWGN.
• K 6, R 1/2 convolutional code (as specified
  by DS-UWB authors).
• Rates 110 and 200 Mbps.
• Perfect channel estimation.
• No frequency offset / 0 ppm crystal error.
• No fixed-point effects other than ADC.

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Degradations Due to a 3-bit ADC (2)

• Simulation results
• At a BER 104, loss due to a 3-bit ADC is 0.4
  dB.
• Still need to examine the performance
  degradations due to finite-precision ADCs in
  multi-path channel environments.

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Nyquist Sampling

• Nyquist theorem states that sufficient statistics
  can be obtained if and only if a signal is
  sampled at twice its largest bandwidth.
• Is there a fundamental limitation in sampling a
  system with sub-Nyquist sampling?
• Yes. The answer is Aliasing.
• Aliasing may result in destructive interference
• Results in loss in base-band sampled signal
  energy.
• Destroys the flatness of the signal spectrum and
  requires signal processing to invert the channel.

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Sub-Nyquist Sampling

• Assumptions for analyzing the impact of 1X
  sampling
• DS-UWB system with a chip rate 1326 MHz, excess
  BW of 50 ? a max BW of 2 GHz.
• AWGN Channel
• 0 ppm crystal mismatch between transmitter and
  receiver
• UNKNOWN PROPAGATION DELAY
• Ideal Rake to collect energy in all the paths.
• Perfect equalization (which does not exist in
  practice) DOES NOT INCLUDE IMPACT DUE TO ISI.

18
Additional Items for Analysis

• We hope to have additional simulations /
  analysis in the future that examine the effects
  of the following items on implementation loss
• Carrier-frequency synchronization errors.
• Channel estimations errors.
• Etc.

19
Other Implementation Issues for DS-UWB

• Are there any problems that may arise when
  multiple piconet overlaps?
• Recall all DS-UWB piconets use the same
  spreading code.
• Only difference between piconets is a
  carrier-frequency offset of just 13 MHz.
• Q Is the 13 MHz carrier frequency spacing enough
  to separate multiple piconets?
• A No. As well will show, the resulting SINR
  (signal-to-interference and noise-ratio) appears
  to FADE every 6 chips or so.
• Thus, the desired piconet sees the equivalent of
  a highly-time selective fading channel.
• Thus, the advantage of using a large BW channel
  is LOST for a DS-UWB system.
• More details are shown on the next slide.

20
Multiple Piconets

• DS-UWB System Assumptions
• Chip Rate of desired Piconet 1326 MHz.
• Chip Rate of Interfering Piconet 1339 MHz.
• Spreading factor of Interfering Piconet 6
  (i.e., 114 Mbps)
• SNR of desired signal 15 dB.
• dint/dref 1
• AWGN channel for both desired and interfering
  piconet.
• Zero sampling offset (Best case assumption, but
  not practical)
• 0 ppm crystal mismatch (Best case assumption, but
  not practical)
• Instantaneous SINR shows deep fades.
• This is a true FUNDAMENTAL limitation of using
  the same spreading code in the impulse radio like
  DS-UWB system.

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Conclusions (1)

• Rayleigh fading for the Multi-band OFDM system
  can be mitigated by additional signal processing.
• So-called Fundamental-Gap of 6 dB in fading is
  NOT a fundamental issue after all.
• DS-UWB authors show an implementation loss of 0.4
  dB in their simulation/performance results
• Shown that a 3-bit ADC results in a 0.4 dB loss
  when compared to 20-bit ADC. Shown that a timing
  synchronization error can be high as 1.25 dB.
• Finite-precision ADC degradations and timing
  synchronization errors are ONLY TWO OF THE
  COMPONENTS that make up implementation loss.

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Conclusions (2)

• In addition, we have shown that the DS-UWB system
  experiences an equivalent highly-selective fading
  channel when multiple piconets overlap.
• The so-called ultra-wideband BW advantage
  (using a very large BW) disappears for the DS-UWB
  system.