September 2004

Anuj Batra et al., Texas InstrumentsSlide 1

doc.: IEEE 802.15-04/0533r0

Submission

Project: IEEE P802.15 Working Group for Wireless Personal Area Networks Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)(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]Voice:[214-480-4220], FAX: [972-761-6966], E-Mail:[batra@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.

September 2004

Anuj Batra et al., Texas InstrumentsSlide 2

doc.: IEEE 802.15-04/0533r0

Submission

What is Really Fundamental?

Anuj Batra, Jaiganesh Balakrishnan, Anand Dabak, Srinivas Lingam

Texas Instruments12500 TI Blvd, M/S 8649

Dallas, TX 75204

September 12, 2004

September 2004

Anuj Batra et al., Texas InstrumentsSlide 3

doc.: IEEE 802.15-04/0533r0

Submission

Motivation

This presentation is going to look at the some of the “fundamental” issues concerning both proposals:

1. Is the 6-dB gap for 480 Mbps MB-OFDM a fundamental gap?

2. Implementation losses associated with the DS-UWB.

September 2004

Anuj Batra et al., Texas InstrumentsSlide 4

doc.: IEEE 802.15-04/0533r0

Submission

Is the 6 dB Gap Fundamental?

September 2004

Anuj Batra et al., Texas InstrumentsSlide 5

doc.: IEEE 802.15-04/0533r0

Submission

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?

September 2004

Anuj Batra et al., Texas InstrumentsSlide 6

doc.: IEEE 802.15-04/0533r0

Submission

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.

0 5 35

c49 c50 c53 P5 c54 c80 P35 c81DC

Subcarrier numbers

P -55c0

-55 -45 -35

c10 c18 P -35 c19 c27P -45c9c1

-25

P -25 c28

-15

P-15 c37c36

-5

P -5 c46c45

25

c71 P25 c72

15

c62 P15 c63

45

c89 P45 c90

55

c98 P55 c99c4c0

-61

c95

61

c99

CopyCopy

September 2004

Anuj Batra et al., Texas InstrumentsSlide 7

doc.: IEEE 802.15-04/0533r0

Submission

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.

4,3,2,1,061

9557

nCP

CP

nn

nn

September 2004

Anuj Batra et al., Texas InstrumentsSlide 8

doc.: IEEE 802.15-04/0533r0

Submission

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 @ TX, 20 ppm @ RX). Channel estimation. Carrier/timing offset recovery. Carrier tracking. Packet acquisition.

September 2004

Anuj Batra et al., Texas InstrumentsSlide 9

doc.: IEEE 802.15-04/0533r0

Submission

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 mNew: 12.0 mOriginal: 11.4

m

New: 11.4 mOriginal: 10.7

m

New: 12.3 mOriginal: 11.5

m

New: 11.3 mOriginal: 10.9

m

200 Mbps

14.8 mNew: 7.4 m Original: 6.9

m

New: 7.1 mOriginal: 6.3

m

New: 7.5 mOriginal: 6.8

m

New: 6.6 mOriginal: 4.7

m

480 Mbps

9.1 mNew: 3.2 mOriginal: 2.9

m

New: 3.0 mOriginal: 2.6

mN/A N/A

September 2004

Anuj Batra et al., Texas InstrumentsSlide 10

doc.: IEEE 802.15-04/0533r0

Submission

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.

There exist several additional ways to reduce the gap even further!

September 2004

Anuj Batra et al., Texas InstrumentsSlide 11

doc.: IEEE 802.15-04/0533r0

Submission

Examine Implementation Losses Associated with the DS-UWB

Proposal

September 2004

Anuj Batra et al., Texas InstrumentsSlide 12

doc.: IEEE 802.15-04/0533r0

Submission

Implementation Losses (1)

Let’s 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 / ValueDS-UWB AWGN

Link 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.

September 2004

Anuj Batra et al., Texas InstrumentsSlide 13

doc.: IEEE 802.15-04/0533r0

Submission

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.

September 2004

Anuj Batra et al., Texas InstrumentsSlide 14

doc.: IEEE 802.15-04/0533r0

Submission

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.

Encoder SpreaderN-bitADC

De-spreader Decoder

AWGN

September 2004

Anuj Batra et al., Texas InstrumentsSlide 15

doc.: IEEE 802.15-04/0533r0

Submission

Degradations Due to a 3-bit ADC (2)

Simulation results:

At a BER = 10–4, 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.

2 2.5 3 3.5 4 4.5 5 5.5 6

10-4

10-3

10-2

10-1

100

Eb/N0 (dB)

Bit

Err

or R

ate

(BE

R)

Impact finite-precision ADC on BER performance

3 bit ADC20 bit ADC

September 2004

Anuj Batra et al., Texas InstrumentsSlide 16

doc.: IEEE 802.15-04/0533r0

Submission

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.

September 2004

Anuj Batra et al., Texas InstrumentsSlide 17

doc.: IEEE 802.15-04/0533r0

Submission

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.

Maximum loss in signal energy can be as high as 1.25 dB. This is a true FUNDAMENTAL limitation of chip-rate sampling for the DS-UWB system.

September 2004

Anuj Batra et al., Texas InstrumentsSlide 18

doc.: IEEE 802.15-04/0533r0

Submission

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.

September 2004

Anuj Batra et al., Texas InstrumentsSlide 19

doc.: IEEE 802.15-04/0533r0

Submission

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.

September 2004

Anuj Batra et al., Texas InstrumentsSlide 20

doc.: IEEE 802.15-04/0533r0

Submission

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.

September 2004

Anuj Batra et al., Texas InstrumentsSlide 21

doc.: IEEE 802.15-04/0533r0

Submission

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.

Total implementation loss (with only 2 components) is bounded between 0.4 dB and 1.65 dB.

September 2004

Anuj Batra et al., Texas InstrumentsSlide 22

doc.: IEEE 802.15-04/0533r0

Submission

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.