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System Designers' Guide to UWB Link Margins

Understand UWB link margins from a practical viewpoint, comparing different UWB methods and their link losses. Details the impact of various factors on link margin calculations.

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System Designers' Guide to UWB Link Margins

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  1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [Designer’s Guide to TG3a UWB Link Margins] Date Submitted: [13 January 2004] Source: [Kazimierz “Kai” Siwiak] Company [TimeDerivative, Inc.] Address [PO Box 772088, Coral Springs, FL 33071] Voice [+1-954-937-3288] E-Mail: [ k.siwiak@ieee.org ] Re:[Link Margins for UWB from the system designer’s point of view] Abstract: [This contribution describes UWB system link margins from a “customer’s” point of view, and contrasts those margins with the SG3a / TG3a selection criteria. ] Purpose: [UWB Link margins in the selection process were determined for the purpose of comparing the relative merits of various UWB approaches. While suitable for that purpose, the results are optimistic for practical system designs. This contribution documents areas of additional practical link losses, and is a first step in practical link design. The additional losses are slightly different for various UWB methods.] 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. K. Siwiak / TimeDerivative, Inc.

  2. System Designers’ Guide to UWB Link Margins Kai Siwiak IEEE Submission Vancouver IEEE January 2004 K. Siwiak / TimeDerivative, Inc.

  3. Introduction • The data rate and range capabilities of TG3a UWB PHYs is currently derived from selection criteria documents • The Selection Criteria are for PHY selection and are NOT useful for system designs • Several factors detracting from the link margin are presented here • Slightly different results are seen for different UWB PHYs K. Siwiak / TimeDerivative, Inc.

  4. Selection criteria is not a design tool • Optimistic in “free space” by 5 to 11 dB depending on the variety of UWB used • Optimistic by 11 to 17 dB in multipath • The link is margin-starved! • System Designers’ dilemma: “How good is the link, really?” K. Siwiak / TimeDerivative, Inc.

  5. The Selection Criteria • Selection criterion is a convenience • Was a suitable basis for 15.3a PAR • Calculation is almost “equal for all,” but artificial • Result is contrived, but generally adequate for PHY selection • Link margin for design must be more accurate • Noise BW error is corrected • EIRP is corrected based on FCC measurement method • Multipath propagation model included • Effects of multipath must be included K. Siwiak / TimeDerivative, Inc.

  6. Channel Noise BW • Rb term in 03/031r5 is throughput, not channel BW • True channel noise BW is Rb/(FEC Rate) • Effect is: link SNR overestimated by the amount of the FEC rate K. Siwiak / TimeDerivative, Inc.

  7. Effect of FEC in the Free Space [Selection Criteria Scenario] Eb/N0 pre-FEC operating point • In AWGN FEC can be a net loss at low Eb/N0 • In multipath... • Need Monte Carlo simulations • Tendency drives BER curve to the AWGN value with FEC BER BPSK or QPSK Eb/N0 K. Siwiak / TimeDerivative, Inc.

  8. EIRP: The FCC Way • Selection criterion uses -41.3 dBm/MHz • FCC says: • Derate full anechoic chamber results by 4.7dB [see FCC R&O 02-48] • Or, measure in semi-anechoic chamber or certified OATS • 4.7dB accounts for a constructive coherently adding ground reflection (in FCC semi-anechoic chamber) • The net effect similar for both systems because receiver is 1 MHz BW K. Siwiak / TimeDerivative, Inc.

  9. PSD Measurements on FCC EMI OATS 0 to 3 m: search for peak D= (h1-h2)2+d2 R= (h1+h2)2+d2 DUT D=R-D ED = “1” D D = 32+22 - 3 = 0.61 m if wavelet is shorter than about 0.61 m than the two paths add as “power,” otherwise, add as voltage: IF the test receiver BW is large enough! [It is NOT] ER = “0.718” 1 m metal ground plane R 3 m K. Siwiak / TimeDerivative, Inc.

  10. 2 1.5 2 M 1 i 1.5 0.5 S 1 0 i 0 20 40 60 80 100 120 140 i 0.5 0 0 20 40 60 80 100 120 140 i OFDM Signal “Spectrum Analyzer Signal” on FCC OATS 128 carriers in full anechoic chamber need to de-rate EIRP by this amount: 4.7 dB! 128 carriers on FCC semi-anechoic chamber K. Siwiak / TimeDerivative, Inc.

  11. 2 1.5 S 1 i 0.5 2 0 1.5 0 20 40 60 80 100 120 140 i S 1 i 0.5 0 0 20 40 60 80 100 120 140 i Moving the Sense Antenna moves around the Peaks ... a few cm up need to de-rate EIRP by this amount: 4.7 dB! a few cm down ... K. Siwiak / TimeDerivative, Inc.

  12. CDMA Signal ... • Coherence length is the chip length, however: • Test receiver BW is 1 MHz, hence “coherence length” is much larger than the chip length • Net result: reflection from OATS ground plane adds coherently, even for impulses • (Direct + reflected) components will behave like sinewave carriers – similar to OFDM! • Actual effect on wide-band victim receivers much more benign for CDMA vs. OFDM, but EIRP is affected K. Siwiak / TimeDerivative, Inc.

  13. Path Loss • TG3a channel model does not consider propagation attenuation • Median loss not taken into account • it is NOT 1/r2 at 10m • “Strongest path” breaks to 1/r3 near 3m • One model of the additional loss is: • L=10 log(1-e-dt/d); where dt=3 m, d=10 m • L=5.9 dB Ref: K. Siwiak, H. Bertoni, and S. Yano, “Relation between multipath and wave propagation attenuation,” Electronic Letters, Vol. 39, No. 1, Jan. 9, 2003, pp. 142-143. K. Siwiak / TimeDerivative, Inc.

  14. In Fading • Actual antenna gain: -1.8dB • Fading Effect on 10 m link • CDMA: -1.0 dB • OFDM: -6.0 dB* [ref: 15-03-0344-03-0003a] *[lesser values also claimed, ref. not available] K. Siwiak / TimeDerivative, Inc.

  15. Summary: Link Margin Effect * lesser values also claimed K. Siwiak / TimeDerivative, Inc.

  16. Conclusions • UWB link is MARGIN-STARVED • Other issues remain • NEED to review available improvements • Modulation efficiency (need better than BPSK/QPSK) • FCC emission measurement method • Cost of diversity improvements need to be explored • NEED to review the application space K. Siwiak / TimeDerivative, Inc.

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