1. doc.: IEEE 802.15-<doc#>
<month year>
doc.: IEEE <doc#>
November 2004
Project: IEEE P 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:[ ], FAX: [], 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 IEEE a standard.]
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
Welborn, Freescale
<author>, <company>

2. UWB for Low Rate Communications
November 2004
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
Welborn, Freescale

3. Issues for Low Power & Cost TG4a UWB
November 2004
Issues for Low Power & Cost TG4a UWB
Bandwidth
Transmit power
Ranging & complexity
Performance
Pulse rate
Effects on efficiency & implementation
Data Rate
Interoperability & Coexistence
Welborn, Freescale

4. Hardware complexity can also depend on signal bandwidth
November 2004
UWB Signal Bandwidth
Transmit power spectrum density is limited to 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.
Welborn, Freescale

5. November 2004
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 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
Welborn, Freescale

6. November 2004
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
M pulses/second
Low pulse rate means higher power per pulse and therefore higher peak power (and voltage)
Welborn, Freescale

7. Data Rate Considerations
November 2004
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 10 kbps = mW*seconds
32 1 Mbps = 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
Welborn, Freescale

8. November 2004
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)
Welborn, Freescale