Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [ System Design Issue

1. 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. Welborn, Freescale

2. 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 • Bandwidth • Transmit power, ranging, complexity & performance • Pulse rate • Effects on efficiency & implementation • Data Rate & Frequency • Interoperability & Coexistence Welborn, Freescale

4. UWB Signal Bandwidth • Transmit power spectrum density is limited to -41.3 dBm/MHz (in the US) – power depends on bandwidth • Maximum transmit power will typically vary from -14 dBm (500 MHz BW) to -10 dBm or more (1300 MHz or more BW) • In general, time resolution is inversely proportional to signal bandwidth – better ranging resolution with more BW • At least for TOA-type approaches • 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 transmit power can offset the complexity of wider BW • E.g. could tolerate larger noise figure or less digital processing Welborn, Freescale

5. 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 4524 Frequency (MHz) 3100 5100 FCC Mask Welborn, Freescale

6. UWB Signal Bandwidth Total Radio Power • 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 • This potential advantage in fading performance can be offset by higher transmit power for narrowband systems • Higher transmit power may not impact total power much unless radio power consumption is very low • Transmit power for UWB (~ -10 dBm = 0.1 mW) is a small fraction of radio power consumption (< 1% ?) • Narrowband transmit power of ~5 dBm (?) (3 mW) – may still be a small fraction of total radio power consumption Welborn, Freescale

7. UWB takes full advantage of natural channel physics Narrow band systems have deeper fads and must compensate Low Fading for UWB 100 25% of Narrow Band Channels are Faded by 6 dB or more 25% P (Received Energy < x) DS-UWB 10-1 1.4 GHz BW 75 MHz BW 4 MHz BW Theoretical Rayleigh 10-2 -20 -15 -10 -5 0 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 Welborn, Freescale

8. 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 • This is 1-24 pulses per symbol for high rate, but could be 100s or 1000s of pulses per bit for lower rate radios • 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) Welborn, Freescale

9. 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 transmit power • Process technology can limit available peak voltage that can be achieved without an external power amplifier ___ • If pulse rate is 100x slower, then peak voltage is 100 = 10x higher Welborn, Freescale

10. Data Rate Considerations • Lowest PHY data rate does not necessarily mean lowest energy consumption • In fact, a fast radio can potentially be more energy efficient than a slow radio. Example: • Compare a 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%) • The largest power use is turning on the radio and processing signal Welborn, Freescale

11. Data Rate Considerations Lower radio power, but longer transmission time for data payload Power Preamble Time Higher peak power but shorter transmission time for same payload Power Preamble Time • Total energy use from battery is the “area” under the power vs. time curves shown above • Relative efficiency depends on power & duration (payload size) Welborn, Freescale

12. 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 • Precise ranging using TOA methods requires high precision time measurements over relatively long intervals (RTT) to determine small differences in signal propagation times • Simplified and improved with good reference clock Welborn, Freescale

13. 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 Welborn, Freescale

14. Summary: UWB for Low Rate Communications • UWB has great potential for low power communications • Defining a waveform that enables low-cost, low-power communications requires consideration of many implementation issues • Bandwidth • Pulse rate • Operating frequency • PHY data rate to support applications Welborn, Freescale