1. March 2003
Project: IEEE P Working Group for Wireless Personal Area Networks (WPANs)
Submission Title: Ultra Wide-Band Modulation Schemes: A Communications Theory Perspective
Date Submitted: March 3, 2003
Source: Eric Ojard and Jeyhan Karaoguz Company: Broadcom Corporation
Address: 190 Mathilda Place, Sunnyvale, CA 94086
Voice:
Re: [ a Call for proposal]
Abstract: Ultra Wide-Band Modulation Schemes: A Communications Theory Perspective
Purpose: [TG3a-Broadcom-CFP-Presentation .]
Notice: This document has been prepared to assist the IEEE P 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 P
Eric Ojard, Broadcom Corp.

2. Ultra-Wide-Band Modulation Schemes A Communications Theory Perspective
Eric Ojard
Broadcom Corporation

3. doc.: IEEE 802.15-<doc#>
<month year>
doc.: IEEE <doc#>
March 2003
Introduction
This presentation is a tutorial on some of the options available in designing a PHY protocol for UWB
Key questions:
Channelization for uncoordinated piconets: Frequency Division or Code Division?
How much bandwidth should we use?
Coding Scheme
Modulation & Spreading Options
Various options are considered and the trade-offs are analyzed
Eric Ojard, Broadcom Corp.
<author>, <company>

4. Constraints and Requirements
March 2003
Constraints and Requirements
FCC allows use of GHz at –41 dBm/MHz
SG3a Target Rates
m required
200 4 m required
480 1m desired
Should operate in the presence of 3 other uncoordinated piconets (requires some type of channelization)
Eric Ojard, Broadcom Corp.

5. doc.: IEEE 802.15-<doc#>
<month year>
doc.: IEEE <doc#>
March 2003
Outline
Theoretical Capacity in Low-SNR regime
Coding & Spreading Examples
Uncoordinated Piconets: Frequency Division vs Code Division
Bandwidth Usage
Code Division Spreading Options
Eric Ojard, Broadcom Corp.
<author>, <company>

6. Theoretical Capacity March 2003 Eric Ojard, Broadcom Corp.
slope: 1 bit/s/Hz per 3 dB
log2(1+SNR)
SNR log2(e)
slope: 2X per 3 dB
2(1-H(Q(sqrt(SNR))))
Eric Ojard, Broadcom Corp.

7. Theoretical Capacity (cont’d)
March 2003
Theoretical Capacity (cont’d)
The graph can be divided into two distinct regions:
High SNR regime (SNR > 0 dB or C > 1 bit/s/Hz) e.g a/b/g,
Low SNR regime (SNR < 0 dB or C < 1 bit/s/Hz) e.g. UWB
The curves behave differently in the low-SNR and high-SNR regimes:
At high SNR, capacity increases by 1 bit/s/Hz for every 3 dB
At low SNR, capacity increases by a factor of 2 for every 3 dB
Coding has a much bigger payoff in the low-SNR regime than in the high-SNR regime.
At low SNR, binary modulation is optimal – nothing to be gained from higher-order constellations.
In theory, very high rates are achievable at very low SNR:
@ -10 dB, 7 GHz * 0.15 bits/s/Hz = 1 Gbit/s
@ -13 dB, 7 GHz * bits/s/Hz = 500 Mbps
@ -16 dB, 7 GHz * bits/s/Hz = 250 Mbps
Eric Ojard, Broadcom Corp.

8. Theoretical Capacity (cont’d)
*plots generated by function ~/research/uwb/low_snr_cap_plots.m
March 2003
Theoretical Capacity (cont’d)
½ log2(1+SNR)
1-H(Q(sqrt(SNR)))
10log10(p/2)=1.96 dB
10*log10(ln(2))=-1.59 dB
Another way of viewing the same curves: Eb/N0=SNR/2R
Eric Ojard, Broadcom Corp.

9. March 2003
Coding & Spreading
The previous slides showed only the Theoretical Capacity.
At low SNR, reduce rate by 2X for every 3 dB from the Shannon limit.
It is straightforward to combine well-known binary codes with spreading sequences, as shown in the following slides.
easy to get within 6 dB of Shannon limit using convolutional codes
possible to get much closer to Shannon limit using concatenated codes and/or iterative decoding.
Eric Ojard, Broadcom Corp.

10. Examples of well-known codes combined with spreading
March 2003
Coding & Spreading
Examples of well-known codes combined with spreading
no spreading
spreading
*code plots assume optimal soft-input decoding
Eric Ojard, Broadcom Corp.

11. March 2003
Coding & Spreading
Examples of well-known codes combined with spreading
(another way of viewing the same data)
no spreading
spreading
*code plots assume optimal soft-input decoding
Eric Ojard, Broadcom Corp.

12. Channelization Options
March 2003
Channelization Options
The solution should support 4 uncoordinated piconets (02/104r15)
Channelization Options:
Code Division Multiplexing (CDM)
Frequency Division Multiplexing (FDM)
FDM: 4 non-overlapping frequency bands:
6 dB penalty in transmitted power
Additional path loss penalty for high-frequency channels: 20 log10(4pfc/c) dB
Potential for better performance compared to CDM in cases where uncoordinated piconets are very close
Lost immunity to frequency-selective fading is minor (see slides on fading vs bandwidth)
CDM: Each piconet has a different spreading code
allows use of maximum transmitted power
maximum immunity to frequency-selective fading
Eric Ojard, Broadcom Corp.

13. March 2003
Link Margin: FDM vs. CDM
Reference: IEEE P /490r0-SG3a
In the following slides, we consider two cases:
(1) FDM: fmin=8.725 MHz, fmax=10.6 MHz (highest-freq channel in a 4-channel system)
PT = dBm
L1 = dB
(2) CDM: fmin=3.1 MHz, fmax=10.6 MHz
PT = dBm
L1 = dB
Eric Ojard, Broadcom Corp.

14. Link Margin From a coding perspective...
*plots generated by function ~/research/uwb/link_margin_plots.m
March 2003
Link Margin
From a coding perspective...
480 1m is very easy
200 4m is harder
110 10m is the hardest
The FDM system* requires ~10.4 dB more coding gain than CDM.
*highest frequency band: GHz
Eric Ojard, Broadcom Corp.

15. March 2003
Link Margin: FDM vs CDM
(1) FDM: fmin=8.725 GHz, fmax =10.6 GHz (highest frequency channel)
requires S+I <= 6.2 dB for m
requires strong code & near-optimal receiver
very little margin
not impossible, but very demanding.
(2) CDM: fmin=3.1 GHz, fmax =10.6 GHz
requires S+I <= 16.7 dB for m
Very easy, even with weak code & high implementation loss.
Eric Ojard, Broadcom Corp.

16. Interfering Transmitter
*plots generated by function ~/research/uwb/piconet_interference_plots.m
March 2003
Uncoordinated Piconets w/ CDM
dref
dint
Desired Transmitter
Receiver
Interfering Transmitter
Assume uncoordinated piconets use the same frequency band with different spreading codes.
Assume true orthogonality isn’t practical due to random multipath & lack of synchronization.
Treat interference as uncorrelated noise with same PSD as desired signal.
Eric Ojard, Broadcom Corp.

17. Uncoordinated Piconets w/ CDM
March 2003
Uncoordinated Piconets w/ CDM
For a CDM system operating within 9 dB of the Shannon Limit, the target rate of 110 Mbps can be dref/dint=3.3
Although strong codes aren’t needed to meet the basic requirements in an interference-free environment, coding gain & receiver performance (S+I) will have a large impact on performance in self-interference environments.
dref/dint increases by 2X per 6 dB of coding gain
Regardless of coding gain, CDM systems will never allow uncoordinated piconets “on top” of each other (“near-far problem”).
In theory, FDM could perform much better when uncoordinated piconets are very close.*
*FDM isn’t the only way to achieve this. This can be achieved by any method that results in truly orthogonal signals (including multipath effects) at the receiver.
Eric Ojard, Broadcom Corp.

18. How Much Bandwidth Should We Use?
March 2003
How Much Bandwidth Should We Use?
Pros of wider bandwidth:
Under the FCC’s UWB regulations, transmit power is proportional to the bandwidth.
But note that the net benefit on link margin is reduced at high frequencies due to L1=20*log10(sqrt(4pfminfmax))
In a self-interference environment (e.g. uncoordinated piconets using same frequency band)...
the achievable rate is proportional to the bandwidth.
for a given target rate, dref/dint ~ sqrt(BW)
Better Immunity to Frequency-Selective Fading
Cons of wider bandwidth:
typically results in higher cost, power.
Eric Ojard, Broadcom Corp.

19. March 2003
Bandwidth & Fading
One of the key advantages to Ultra-Wide-Band technology is its inherent immunity to frequency-selective fading.
Narrowband signals cannot resolve multipath components; the entire frequency band could fall in a deep spectral null.
The immunity to fading is a function of the ratio of bandwidth to center frequency.
~18 dB
~20 MHz
Example Channel: 20 MHz channel can have dB fade.
Eric Ojard, Broadcom Corp.

20. Fading Probability vs. Bandwidth
*plots generated by function ~/research/uwb/bw_fade_test.m
March 2003
Fading Probability vs. Bandwidth
CM1
CM2
Eric Ojard, Broadcom Corp.

21. Fading Probability vs. Bandwidth (cont’d)
*plots generated by function ~/research/uwb/bw_fade_test.m
March 2003
Fading Probability vs. Bandwidth (cont’d)
CM3
CM4
Eric Ojard, Broadcom Corp.

22. Bandwidth & Fading (cont’d)
March 2003
Bandwidth & Fading (cont’d)
15 MHz -> 5 GHz: reduction in 1% worst-case fade:
CM1: 15 dB
CM2: 12 dB
CM3: 12 dB
CM4: 9 dB
Not much difference between 1.5 GHz curves and 5 GHz curves: anything with BW > ~1 GHz has good fading immunity.
The accuracy of these results is highly dependent on the accuracy of these channel models.
Eric Ojard, Broadcom Corp.

23. Spreading & Modulation Options
March 2003
Spreading & Modulation Options
Here we consider modulation schemes where uncoordinated piconets share the same frequency band (CDM)
Several possible variations on DSSS* (not an exhaustive list)
Long PN Spreading Sequence
Symbol-Length Spreading Sequence
Multi-Symbol-Length Spreading Sequence
Symbol-Length Spreading with Short Time Hopping
*DSSS = Direct Sequence Spread Spectrum
Eric Ojard, Broadcom Corp.

24. Long Sequence Spreading
March 2003
Long Sequence Spreading
Chip sequence is a long (effectively infinite length) pseudo-noise sequence.
Every symbol has a different spreading sequence.
Every uncoordinated piconet has a different spreading sequence.
Advantages
Perfect autocorrelation properties (flat PSD)
Perfect cross-correlation properties with uncoordinated piconets.
Disadvantages
Near-optimal detection requires a high complexity receiver for a large number of multipath components.
Any additional ISI mitigation requires a more sophisticated receiver design
Eric Ojard, Broadcom Corp.

25. Symbol-Length Spreading (SLS) Sequence
March 2003
Symbol-Length Spreading (SLS) Sequence
The chip sequence is the same for every symbol.
Linear Time Invariant (LTI) modulation
Advantages
Lower-Complexity receiver
Disadvantages
Imperfect Autocorrelation: PSD has ripple (assuming binary spreading sequences)
Imperfect Cross-correlation with uncoordinated piconets.
Random multipath tends to provide low correlation, but this breaks down in free-space.
Trade-off between autocorrelation and cross-correlation becomes harder to manage at higher symbol rates.
Eric Ojard, Broadcom Corp.

26. Symbol-Length Spreading w/ Time-Hopping
March 2003
Symbol-Length Spreading w/ Time-Hopping
The chip sequence is the same for every symbol.
To reduce the correlation with uncoordinated piconets, symbol positions are dithered by a pseudo-random hopping pattern.
Better Cross-correlation properties compared to plain Symbol-Length Spreading.
Time-varying ISI makes optimal detection more complex.
Eric Ojard, Broadcom Corp.

27. Multi-Symbol-Length Spreading Sequence
March 2003
Multi-Symbol-Length Spreading Sequence
The chip sequence repeats every N symbols, where N is a small integer.
Better auto-correlation and cross-correlation properties compared to symbol-length spread-spectrum.
Higher Complexity Detection than Symbol-Length Spreading
Time-varying ISI makes optimal detection more complex.
Eric Ojard, Broadcom Corp.

28. Comparison of DSSS variations
March 2003
Comparison of DSSS variations
LTI?
Constant Baud?
Flatness of PSD
Decorrelation of Piconets
Receiver Complexity
Equalzation (if desired)
Long Sequence Spreading No
Yes
Perfect
High
Very Difficult
Symbol-Length Spreading Sequence
Fair
Lowest
Easy
SLS w/ Time Hopping
Good to Perfect
Low
Difficult
Multi-Symbol Spreading Sequence
Good
Medium
Eric Ojard, Broadcom Corp.

29. March 2003
Conclusions
In Theory, the UWB environment enables rates far in excess of the target rates.
Supporting 4 uncoordinated piconets is the biggest challenge.
FDM would require very strong coding to meet the target rates, but could perform better when uncoordinated piconets are very close.
CDM could meet target rates with weaker coding, but performance would be limited when uncoordinated piconets are close.
Wider bandwidth always enables higher performance...
especially when uncoordinated piconets share the same frequency band
but at the expense of higher complexity.
Eric Ojard, Broadcom Corp.

30. March 2003
Backup Slides
Eric Ojard, Broadcom Corp.

31. March 2003
Channel Models
The proposed channel model for simulations is described in /368r5.
3 parts:
Path loss Model
Multipath Model
Shadowing Model
Eric Ojard, Broadcom Corp.

32. L = 20*log10(d) dB, where d is in meters
March 2003
Path Loss Model
L = 20*log10(d) dB, where d is in meters
Eric Ojard, Broadcom Corp.

33. Multipath & Shadowing
March 2003
The Multipath Model is a Saleh-Valenzuela model, modified so that multipath gains have a lognormal distribution rather than a Rayleigh distribution.
4 Multipath Parameter Sets:
CM1: 0-4m LOS
CM2: 0-4m NLOS
CM3: 4-10m LOS
CM4: 4-10m NLOS
Shadowing: log-normal shadowing with 3 dB standard deviation.
Eric Ojard, Broadcom Corp.

34. Example Channels (CM1 & CM2)
*plots generated by function ~/research/uwb/channel_plots.m
March 2003
Example Channels (CM1 & CM2)
CM1: 0-4 m LOS
CM2: 0-4 m NLOS
Eric Ojard, Broadcom Corp.

35. Example Channels (CM3 & CM4)
*plots generated by function ~/research/uwb/channel_plots.m
March 2003
Example Channels (CM3 & CM4)
CM3: 4-10 m LOS
CM4: extreme NLOS
Eric Ojard, Broadcom Corp.

36. Multipath Model Power-Delay Profiles
*plots generated by function ~/research/uwb/channel_plots.m
Multipath Model Power-Delay Profiles
March 2003
Eric Ojard, Broadcom Corp.

37. Coding & Spreading @ Pe=1e-8
March 2003
Coding & Pe=1e-8
Examples of well-known codes combined with spreading
no spreading
spreading
*for Pe=1e-5, shift points left by ~2 dB
Eric Ojard, Broadcom Corp.

38. Interfering Transmitter
*plots generated by function ~/research/uwb/piconet_interference_plots.m
March 2003
Uncoordinated Piconets w/ CDM
dref
dint
Desired Transmitter
Receiver
Interfering Transmitter
Assume uncoordinated piconets use the same frequency band with different spreading codes.
Assume true orthogonality isn’t practical due to random multipath.
Treat interference as uncorrelated noise with same PSD as desired signal.
Eric Ojard, Broadcom Corp.

39. Uncoordinated Piconets w/ CDM
March 2003
Uncoordinated Piconets w/ CDM
For a CDM system operating within 9 dB of the Shannon Limit, the target rate of 110 Mbps can be dref/(dref+dint)=0.77
Although strong codes aren’t needed to meet the basic requirements in an interference-free environment, coding gain & receiver performance (S+I) will have a large impact on performance in self-interference environments.
Regardless of coding gain, CDM systems will never allow uncoordinated piconets “on top” of each other (near-far problem).
In theory, FDM could perform much better when uncoordinated piconets are very close.*
*FDM isn’t the only way to achieve this. This can be achieved by any method that results in truly orthogonal signals (including multipath effects) at the receiver.
Eric Ojard, Broadcom Corp.