November 2004

Welborn, FreescaleSlide 1

doc.: IEEE 802.15-04-0626-02-004a

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: [System Design Issues for Low Rate UWB ]Date Submitted: [November 2004]Source: [Matt Welborn ] Company [Freescale Semiconductor, Inc]Address [8133 Leesburg Pike, Vienna VA 22182]Voice:[703-269-3000], FAX: [], E-Mail:[matt.welborn @ freescale.com]Re: [Response to Call for Proposals]

Abstract: [This document describes a number of important design considerations for TG4a]

Purpose: [Preliminary Proposal Presentation for the IEEE802.15.4a 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.

November 2004

Welborn, FreescaleSlide 2

doc.: IEEE 802.15-04-0626-02-004a

Submission

UWB for Low Rate Communications

• UWB has great potential for low power communications– Low fading margin can provide same range for lower

transmit power– Large (ultra-wide) bandwidth can provide fine time resolution

provides potential for accurate ranging

• Drawbacks due to regulations– Limited transmit power – how much is enough?

• Operation at long ranges is highly dependent on NLOS path loss characteristics

November 2004

Welborn, FreescaleSlide 3

doc.: IEEE 802.15-04-0626-02-004a

Submission

Issues for Low Power & Cost TG4a UWB

• Bandwidth– Transmit power, ranging, complexity & performance

• Pulse rate– Effects on efficiency & implementation

• Data Rate & Frequency• Interoperability & Coexistence

November 2004

Welborn, FreescaleSlide 4

doc.: IEEE 802.15-04-0626-02-004a

Submission

UWB Signal Bandwidth• Transmit power spectrum density is limited to -41.3 dBm/MHz

(in the US) – power depends on bandwidth– Transmit power will typically vary from -14 dBm (500 MHz BW) to -

10 dBm or more (1300 <Hz or more BW)• In general, time resolution is inversely proportional to signal

bandwidth – better resolution with more BW• Hardware complexity can also depend on signal bandwidth

– Highly dependant on implementation, analog vs. digital, sample rates, etc.

– Rake receiver complexity depends on signal bandwidth• Performance versus number of taps and tap bit width

• Higher Tx power can offset higher BW complexity

November 2004

Welborn, FreescaleSlide 5

doc.: IEEE 802.15-04-0626-02-004a

Submission

Possible Signal Bandwidth Options for Low Rate UWB

4524397834323100 5100

DS-UWB Low BandPulse Shape (RRC)

Possible Lower Rate Signaling Bands (500 MHz bandwidth)

FCC Mask

Frequency (MHz)

0-3

-20

RelativePSD (dB)

November 2004

Welborn, FreescaleSlide 6

doc.: IEEE 802.15-04-0626-02-004a

Submission

UWB Signal Bandwidth

• One of the primary advantages of UWB is the potential to significantly reduce multipath fading– Narrowband radios can suffer significant multipath fades

(15-20 dB or more)– UWB signals often fade only a few dB

• However, this 10-15 dB potential advantage in transmit power may not matter unless radio power consumption is very low– Tx power for UWB (~ -10 dBm = 0.1 mW) is a very small

fraction of radio power consumption (< 1%)– Narrowband Tx power of ~5 dBm is only 3 mW – may still be

a small fraction of total radio power consumption

November 2004

Welborn, FreescaleSlide 7

doc.: IEEE 802.15-04-0626-02-004a

Submission

Low Fading for UWB• UWB takes full

advantage of natural channel physics

• Narrow band systems have deeper fads and must compensate

-20 -15 -10 -5 0 510-2

10-1

100

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4 MHz B

W

75 M

Hz

BW

1.4

GH

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WThe

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al Ray

leigh

DS-UWB

25%

25% of Narrow Band Channels are Faded by 6 dB or more

• Large coherent relative BW enables radios with no fading– This is a first for wireless– Allows FEC to be turned off, or left out for short range apps

November 2004

Welborn, FreescaleSlide 8

doc.: IEEE 802.15-04-0626-02-004a

Submission

UWB Pulse Rate

• “Impulse radio” (IR) originally meant low pulse rate (10’s of M pulse/sec) using “time hopping” for multiple access and pulse position modulation (PPM)

• More generally, IR is just pulse-based spread spectrum with data modulation– Many choices for modulation (BPSK, PPM, OOK, etc.)– One or more pulses per data symbol

• Direct sequence UWB (DS-UWB) is simply high rate pulsed UWB with multiple pulses per symbol & BPSK– 1300-2600 M pulses/second

• Pulse rate does not fundamentally affect transmit power, signal bandwidth or system performance

• Pulse rate does affect energy per pulse and therefore peak power (and voltage)

November 2004

Welborn, FreescaleSlide 9

doc.: IEEE 802.15-04-0626-02-004a

Submission

Higher Pulse Rate = Lower Peak Power

• Lower pulse rate requires higher “energy per pulse” and therefore higher peak power (and voltage) for same power

• Process technology can limit available peak voltage that can be achieved without an external power amplifier

Higher peak power & voltage for same average power

November 2004

Welborn, FreescaleSlide 10

doc.: IEEE 802.15-04-0626-02-004a

Submission

Data Rate Considerations• Lowest PHY data rate does not necessarily mean lowest energy

consumption• In fact, a fast radio can be more energy efficient than a slow radio• Example:

– Compare: 1 Mbps radio at 100 mW versus 10 kbps radio at 10 mW– 32 kB @ 10 kbps = 0.256 mW*seconds– 32 kB @ 1 Mbps = 0.0256 mW*seconds – 1/10 of the energy per bit!

• Assumptions– Both radios achieve minimum range requirement for application – Minimum acquisition time is a function of SNR (range) not data rate – Requires fast wake-up and shut down of radio with aggressive power

management• Relative energy usage depends on packet size

– Fast radio advantage is higher for longer packets • Notice transmit power is a small fraction of the total power (<1%)

November 2004

Welborn, FreescaleSlide 11

doc.: IEEE 802.15-04-0626-02-004a

Submission

Operating Frequency

• Multiple operating channels with different center frequencies will have different performance– Path loss includes 20 Log10(Fc) term

• Cost of generating the reference frequency depends on the specific frequency – Example: low cost, high quality crystals are available at 26

MHz (widely used in cell phones)• Better frequency accuracy can relax other system

constraints– Acquisition at longer range requires longer integration and

therefore more accurate reference frequency– High accuracy clock can allow longer “sleep” time & better

power management

November 2004

Welborn, FreescaleSlide 12

doc.: IEEE 802.15-04-0626-02-004a

Submission

Interoperability & Coexistence

• Many type of UWB systems and waveforms will share the UWB bands– Interoperability between TG4a & higher rate systems could

enable improved coexistence

• Interoperation with higher rate systems could increase the utility of the TG4a standard– Interoperability of low cost sensor/RFID devices with nearby

UWB CE devices– Interoperability with DS-UWB could be quite simple if correct

parameters are chose for TG4a• Common reference frequency, codes & operating bands