Transcript doc.: IEEE 802.15-04-0xxx-00-004a

November 2004
doc.: IEEE 802.15-04-0xxx-00-004a
Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (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.
Submission
Slide 1
Welborn, Freescale
November 2004
doc.: IEEE 802.15-04-0xxx-00-004a
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
Submission
Slide 2
Welborn, Freescale
November 2004
doc.: IEEE 802.15-04-0xxx-00-004a
Issues for Low Power & Cost TG4a UWB
• Bandwidth
– Transmit power
– Ranging & complexity
– Performance
• Pulse rate
– Effects on efficiency & implementation
• Data Rate
• Interoperability & Coexistence
Submission
Slide 3
Welborn, Freescale
November 2004
doc.: IEEE 802.15-04-0xxx-00-004a
UWB Signal Bandwidth
• Transmit power spectrum density is limited to
-41.3 dBm/MHz (in the US) – power depends
on bandwidth
– Transmit power will 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
• Hardware complexity can also depend on
signal bandwidth
– Highly dependant on implementation, analog vs.
digital, sample rates, etc.
Submission
Slide 4
Welborn, Freescale
November 2004
doc.: IEEE 802.15-04-0xxx-00-004a
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 (1520 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 is
very low
– Tx power for UWB (~ -10 dBm = 0.1 mW) is a very small
fraction of radio power consumption
– Narrowband Tx power of ~5 dBm is only 3 mW – still a small
fraction of total radio power consumption
Submission
Slide 5
Welborn, Freescale
November 2004
doc.: IEEE 802.15-04-0xxx-00-004a
UWB Pulse Rate
• “Impulse radio” 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
• Low pulse rate means higher power per pulse and therefore
higher peak power (and voltage)
Submission
Slide 6
Welborn, Freescale
November 2004
doc.: IEEE 802.15-04-0xxx-00-004a
Data Rate Considerations
• Lowest PHY data rate does not necessarily mean lowest energy
consumption
• In fact, a fast radio can be more 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!
– Notice, transmit power is a small fraction of the total power budget
• Assumes that 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
Submission
Slide 7
Welborn, Freescale
November 2004
doc.: IEEE 802.15-04-0xxx-00-004a
Operating Frequency
• Multiple operating channels with different center
frequencies will have different performance
– Path loss includes 20 Log10(Fc) term
• Complexity of generating the reference frequency
depends on the specific frequency
• Acquisition at longer range requires longer integration
and therefore more accurate reference frequency
– Example: low cost, high quality crystals are available at 26
MHz (widely used in cell phones)
Submission
Slide 8
Welborn, Freescale