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

November 2004
doc.: IEEE 802.15-04-0626-02-004a
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 802.15-04-0626-02-004a
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 802.15-04-0626-02-004a
Issues for Low Power & Cost TG4a UWB
• Bandwidth
– Transmit power, ranging, complexity & performance
• Pulse rate
– Effects on efficiency & implementation
• Data Rate & Frequency
• Interoperability & Coexistence
Submission
Slide 3
Welborn, Freescale
November 2004
doc.: IEEE 802.15-04-0626-02-004a
UWB Signal Bandwidth
• Transmit power spectrum density is limited to -41.3 dBm/MHz
(in the US) – power depends on bandwidth
– Transmit power will 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
Submission
Slide 4
Welborn, Freescale
November 2004
doc.: IEEE 802.15-04-0626-02-004a
Possible Signal Bandwidth Options for Low Rate UWB
Relative
PSD (dB)
Possible Lower Rate
Signaling Bands
(500 MHz bandwidth)
DS-UWB Low Band
Pulse Shape (RRC)
0
-3
-20
3432
3978
3100
4524
5100
Frequency (MHz)
FCC Mask
Submission
Slide 5
Welborn, Freescale
November 2004
doc.: IEEE 802.15-04-0626-02-004a
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
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
Submission
Slide 6
Welborn, Freescale
November 2004
Low Fading for UWB
100
P (Received Energy < x)
doc.: IEEE 802.15-04-0626-02-004a
• UWB takes full
advantage of
natural channel
physics
25% of Narrow Band Channels
are Faded by 6 dB or more
25%
10-2
-20
DS-UWB
1.4 GHz BW
10-1
-15
-10
-5
0
• Narrow band
systems have
deeper fads and
must compensate
5
X (dB)
• 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
Submission
Slide 7
Welborn, Freescale
November 2004
doc.: IEEE 802.15-04-0626-02-004a
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)
Submission
Slide 8
Welborn, Freescale
November 2004
doc.: IEEE 802.15-04-0626-02-004a
Higher Pulse Rate = Lower Peak Power
Higher peak power & voltage
for same average 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
Submission
Slide 9
Welborn, Freescale
November 2004
doc.: IEEE 802.15-04-0626-02-004a
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%)
Submission
Slide 10
Welborn, Freescale
November 2004
doc.: IEEE 802.15-04-0626-02-004a
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
Submission
Slide 11
Welborn, Freescale
November 2004
doc.: IEEE 802.15-04-0626-02-004a
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
Submission
Slide 12
Welborn, Freescale