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Joe Kwak InterDigital Communications Corporation

November 2003. doc: IEEE 802.11-03/898r0. PSNI: New PHY Measurement for Link Quality comparative measurements of receiver output quality to support network management. Joe Kwak InterDigital Communications Corporation. Outline. Need for new PHY measurements

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Joe Kwak InterDigital Communications Corporation

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  1. November 2003 doc: IEEE 802.11-03/898r0 PSNI: New PHY Measurement for Link Qualitycomparative measurements of receiver output qualityto support network management Joe Kwak InterDigital Communications Corporation Joe Kwak, InterDigital

  2. Outline • Need for new PHY measurements • RCPI and PSNI Relation to SNR in Demodulator • Perceived Signal-to-Noise-plus-interference Indicator (PSNI) Definition • PSNI Analysis: Relation to EbNo, SNR and BER • Use of variance to align fading channels with AWGN • Required measurement sample size • Motions to incorporate PSNI Joe Kwak, InterDigital

  3. Need for New PHY Measurements • RSSI is defined at antenna input connector but is not fully specified: no unit definitions, no performance requirements (accuracy, testability). • Since so little about RSSI is specified, it must be assumed that widely variant implementations already exist. It is not possible to compare RSSIs from different STAs and perhaps not even from different channels/PHYs within same STA. • RSSI may have limited use for evaluating AP options within a STA and within a given PHY, but not between PHYs. RSSI is rescaled between DSSS and OFDM PHYs. • RSSI is clearly not useable by network management for handoff or load balancing. RSSI from one STA does not relate to RSSI from any other STA. • In high interference environments, RSSI is not an adequate indicator of desired signal quality, since it indicates the sum of desired signal + noise + interference powers. • Proposed RCPI provides quantized, objective input power measure (S+N+I). • Proposed PSNI provides quantized, comparative measure of received signal quality [observed S/(N+I)] for all channels/rates and among all PHYs and between all STAs. Joe Kwak, InterDigital

  4. PHY Measurement Architecture • RCPI measures total RF Power at antenna input connector A. • PSNI measures observed S/(N+I) within demodulator but normalizes measurement for FER at E. AGC B C D Demodulator and tracking loops (PHY specific) E Radio front end FEC Decoder (optional) Frame Check (CRC) A/D A A: Total RF power, RF S/(N+I) from each AP C&D: Bit Error Rate (BER) @each data rate from each AP B: BB S/(N+I) from each AP (BB power constant by AGC) E: Frame Error Rate (FER) @each data rate from each AP Joe Kwak, InterDigital

  5. Measure PHY Demod Input (power) and Output (QOS) • Accurate S/(N+I) measurement at A is interesting but because RF/demod implementations vary widely, it cannot be used comparatively between STAs to evaluate delivered signal quality. • Accurate FER measurement at E is ideal quality measure, but cannot be measured frame by frame. FER can only be accurately measured over 100s-1000s of frames. Also, FERs are comparable only at same frame size and data rate. Good STA 10E-5 -80dbm Good STA 10E-5 -80dbm E (FER) A (dBm) E (FER) A (dBm) Med STA 10E-5 Med STA 10E-4 -78dbm -80dbm Marginal STA Marginal STA 10E-5 10E-2 -75dbm -80dbm Signal at same objective SNR Signal at same subjective SNR Measure RCPI power at A. Measure PSNI quality in middle, but specify PSNI with FER at E. Joe Kwak, InterDigital

  6. Received Channel Power Level dBm (S + N + I) Operating Margin Observed Digital SNIR Ratio (PSNI in demodulator) Required Min RSPL Level (RCPI at antenna connector) Theoretical SNR for required BER Observed Analog SNIR Ratio Desired Signal Power dBm FEC Decoder Loss, if any Total Modem Implementation Losses (TML) Input Analog SNIR Ratio Input SNR Ratio Demodulator Loss Rx Amp Noise Figure + IM Distortion Channel Impairments (CI) (fading + multipath + etc, = 0 in AWGN) Total Channel Condition Losses o dBm Antenna Connector: Input Power Level (S+ N + I) Interference Power at Input dBm Thermal Input Noise Level (-100dBm) Temp = 290K = 24.6dB NBW = 22MHz = 73.4dB Boltzman’s C (-198dBm/Hz/K) RCPI and PSNI Relation to SNR in Demodulator Joe Kwak, InterDigital

  7. PSNI: Demodulator-specific, Post-processing Estimator of Observed S/(N+I) and BER/FER. • All digital demodulators use tracking loops and complex post-processing to demodulate received symbols. Many internal demodulator metrics are proportional to perceived S/(N+I). Examples: • PSK: baseband phase jitter and received Error Vector Magnitude (EVM) • DSSS: spreading code correlation quality • OFDM: frequency tracking and channel tracking stability • OFDM: EVM on pilot channels (cf. Johnson, 802.11-03/844r0) • All FEC modes: corrected bit rate in FEC decoder • All modes: EVM on data symbols (cf. Kwak, 802.11-03/773r2) • Demodulator internal metrics are available on a frame-by-frame basis. • Demodulator metrics proportional to S/(N+I) are available at all data rates. • Demodulator internal metrics may be calibrated with respect to actual FER performance to accurately indicate perceived or observed S/(N+I) in controlled environment with AWGN. • Such demodulator internal metrics are fast estimators of S/(N+I) in both interference environments and interference-free (noise only) environments. • TGK need not specify which demodulator metrics to use, but needs only to specify how the quantized PSNI indicator relates to S/(N+I) and FER. Joe Kwak, InterDigital

  8. PSNI Concept: Measure Output Signal Quality • Specified like RSSI: 8-bit unsigned value, monotonically increasing with increasing S/(N+I). • PSNI shall be logarithmically scaled to perceived S/(N+I) which relates directly to FER performance. • Specify PSNI output value for each data rate using FER points: first point to “anchor” indicator, additional points to quantize and scale indicator slope and range of values. • Specify accuracy of PSNI in AWGN to be +/- 2.0dB in AWGN and +/- 4.0dB in fading channels. • PSNI range shall span the lower 43 dB portion of the operating range of S/(N+I) to cover high FERs at data rates from 1 to 54 Mbps. Joe Kwak, InterDigital

  9. PSNI Normative Specification Text • The PSNI indicator is a measure of the perceived, post-processing signal-to-noise-plus-interference (S/(N+I)) ratio in the demodulator. The allowed values for the Perceived Signal to Noise Indicator (PSNI) parameter shall be an 8 bit value in the range from 0 through 255. This parameter shall be a measure by the PHY sublayer of the perceived signal link quality observed after RF downconversion and is derived from internal digital signal processing metrics of the demodulator used to receive frames on that link. PSNI shall be measured over the PLCP preamble and over the entire received frame. PSNI is intended to be used in a relative manner, and it shall be a monotonically increasing, logarithmic function of the observed link S/(N+I). Specified PSNI performance shall be measured over no less than 1000 PPDUs from the same transmitter. PSNI accuracy and range shall be specified in AWGN and fading for each data rate as follows: Theoretical FEC coding gain assumed in FER calculations: R = 1/2, 5.4dB gain R = 2/3, 4.7dB gain R = 3/4, 4.4dB gain PSNI SPECIAL VALUE: “0” shall indicate inability to measure PSNI When PSNI exceeds high end of measurable range for a given data rate, maximum PSNI for that rate shall be reported. Fading channel model is IEEE exponential ray decay with 50nsec decay time. Joe Kwak, InterDigital

  10. PSNI specified on BER/FER curves Joe Kwak, InterDigital

  11. Data Rate/Modulation Adjustments (DRMx) Used to Offset BER Curves Table 1: DRM Rate/Modulation Adjustments Joe Kwak, InterDigital

  12. Example: PSNI = 76 • BERs vary based on FEC coding used at each data rate. FERs vary based on BER and PPDU length. • Note: in any STA, PSNI will vary only as a result of changing Channel Conditions or changing received Desired Signal Power Level. • Note: for efficiency, all STAs should operate at highest data rate possible while maintaining acceptable FER (QOS). Any STAs at these op points will report PSNI = 76 PSNI = 76 is equivalent to Observed SNIR = 3.7dB Joe Kwak, InterDigital

  13. PSNI Analysis: Relation to Observed Eb/No • PSNI = 0 is selected for a post-processing, Observed Eb/No (OEbNo) equal to 4.4dB, for BPSK at 1Mbit/s data rate. • 6 units (steps) per dB is selected to provide 43 dB range in 8 bit PSNI value. • So for 1 Mbit/s BPSK operation, PSNI = 6*[OEbNo - 4.4dB]. • In general for all other data rates and modulations, PSNI = 6*[OEbNo - 4.4dB + DRMx - CFy] , where DRMx is an S/N adjustment unique for each data rate/demodulation combination. DRMx values are calculated in Table 1 , as shown on page 11, and where CFy is a hardware-specific factor used to account for implementation variances in each FEC decoder in the STA. CFy = CGtheo - CGact = actual FEC decoder loss, for each decoder at each specified FER point. CGtheo values are listed on page 11. When no FEC decoder is used CFy = 0. • This relation is the foundation of the PSNI measurement. . Joe Kwak, InterDigital

  14. PSNI Analysis: Relation to Input SNIR (ISNIR) • SNR = C / N, where Eb = C * Tb, N = No * NBW (noise BW) and DR = 1 / Tb • So SNR = -------------- = -------------- = EbNo * DR / NBW • In db: SNR = EbNo + DR - NBW , where EbNo is shorthand for Eb/No in dB. • For DR = 1 Mbit/s and NBW + = 22 MHz, SNR = EbNo + 60dB - 73.4dB = EbNo -13.4dB • In general, SNR = EbNo -13.4dB + DRMx, with DRMx from Table 1. and so EbNo = SNR + 13.4dB - DRMx, and OEbNo = OSNIR + 13.4dB - DRMx • From page 15 we have: PSNI = 6*[OEbNo - 4.4dB + DRMx - CFy], and substituting for OEbNo, PSNI = 6*[(OSNIR + 13.4dB - DRMx) - 4.4dB + DRMx - CFy], and PSNI = 6*[OSNIR + 9.0dB - CFy] • Since ISNIR = OSNIR + TML + CI, where TML is the modem implementation loss and CI is the sum of all channel impairments, we have PSNI = 6*[(ISNIR-TML-CI) + 9.0dB - CFy] Eb / Tb Eb * DR No * NBW No * NBW Joe Kwak, InterDigital

  15. PSNI Analysis: Relation to BER/FER • PSNI is a direct measure of observed SNIR considering all channel impairments and implementation losses measured at the demodulator. • PSNI is specified with respect to output FER, which considers all implementation losses including any FEC decoder implementation loss. • Each STA will measure PSNI using a correction factor Cfy to account for the actual coding gain (CGact) of each FEC decoder. • Any STA measuring PSNI on a frame using FEC will use CFy so that the reported PSNI from all STAs is normalised and assumes a theoretical coding gain. CFy = CGtheo - CGact = actual FEC decoder loss • Reported PSNI value may be used to estimate OEbNo and BER/FER (QOS) for the reporting STA for any data rate. OEbNo = (PSNI/6) + 4.4dB - DRMx : • For data rates without FEC decoder, OEbNo is used with the theoretical EbNo curve for that modulation to estimate BER. • For data rates with FEC decoder, OEbNo is used with the theoretical FEC EbNo curve for for that modulation to estimate BER. • Note: PSNI relation to BER is specified for AWGN. In fading channels, the mean of the measured parameter used as basis for PSNI is adjusted using the measured variance to align the mean in fading with the AWGN case, as demonstrated for EVM in the simulations described in 802.11-03/773r2. Joe Kwak, InterDigital

  16. Why Does Variance Measurement Permit Alignment with AWGN? 1. Fading channels produce dynamic variations in instantaneous SNR 2. Range of individual packet SNRs increases with increasing variance. 3. For same aggregate effect on BER, the mean SNR of a fading channel must be higher than AWGN for low BERs. 4. For same aggregate effect on BER, the mean SNR of a fading channel must be lower than AWGN for high BERs. Joe Kwak, InterDigital

  17. Why Does Variance Measurement Permit Alignment with AWGN? 5. The variance of the measured parameter can be used to compute a factor to align the mean values in fading channels to the mean value in AWGN. 6. In this way the PSNI indicator relates directly to the BER/FER performance curves for AWGN in all channel conditions. Joe Kwak, InterDigital

  18. Variance changes slope of BER curve [Base chart from Brian Johnson, 802.11-03/682r0] Joe Kwak, InterDigital

  19. Variance changes slope of BER curve Joe Kwak, InterDigital

  20. BPSK, R=1/2, AWGN Variance effects Data BER vs IEVM (with FEC) 0 -1.00 -5.00 0.00E 5.00E 1.00E 1.50E 2.00E 2.50E -1 E+01 E+00 +00 +00 +01 +01 +01 +01 -2 -3 BER(out of FEC decoder) (10-x) -4 -5 -6 -7 -8 20*log10(1/EVM) Joe Kwak, InterDigital

  21. EVM variance aligns fading BER with AWGN BER Joe Kwak, InterDigital

  22. Variance factor can align fading with AWGN Joe Kwak, InterDigital

  23. Variance factor can align fading with AWGN Joe Kwak, InterDigital

  24. Study of Sample Size for Error Analysis • In fading channels, IEVM varies significantly from packet to packet. • How many packets need to be measured to get a useful IEVMmean and IEVMsd? • 50nsec fading channel was simulated for all OFDM rates using different sample sizes of 10, 100, 1000 packets per measurement Joe Kwak, InterDigital

  25. Sample Size Study Results Partial results for BPSK, R=1/2 Joe Kwak, InterDigital

  26. Sample Size Study Results (cont) Std Dev of IEVMmod measurements Joe Kwak, InterDigital

  27. Sampling error for IEVM • Based on these results we see that normal sample error may have significant effect on IEVMmod. • In AWGN-dominated channels, a single EVM measurement is adequate. • In fading channels where EVM variance is high, a large number of measurements are required to achieve accurate results: • When measured over 100 packets, IEVMmod results may vary over a +/- 2db range. • When measured over 1000 packets, IEVMmod results may vary over +/- .4dB range. • IEVMavg, IEVMsd and adequate sample size are all needed for meaningful measurement. Joe Kwak, InterDigital

  28. Conclusions • RSSI is inadequate indicator of link quality. • RCPI quantifies power levels at input to receiver. • PSNI quantifies observed link quality at output of receiver. • EVM or EVMavg, without knowledge of channel condition, cannot indicate observed link quality in terms of BER/FER . • PSNI premise has been validated. EVM on data symbols (with variance adjustment) is adequate basis for PSNI. But as Steve Pope, Brian Johnson and others have indicated, other demod parameters may be preferred by certain manufacturers. • PSNI is the only link quality proposal in TGk which indicates output quality (BER/FER) for all rates and channel conditions. • PSNI does not constrain manufacturers to particular implementation. • Without a quantified link quality measurement, TGk’s work is not finished. • PSNI is mature enough (10 months) for inclusion into TGk draft. Joe Kwak, InterDigital

  29. PSNI Normative Specification Text • The PSNI indicator is a measure of the perceived, post-processing signal-to-noise-plus-interference (S/(N+I)) ratio in the demodulator. The allowed values for the Perceived Signal to Noise Indicator (PSNI) parameter shall be an 8 bit value in the range from 0 through 255. This parameter shall be a measure by the PHY sublayer of the perceived signal link quality observed after RF downconversion and is derived from internal digital signal processing metrics of the demodulator used to receive frames on that link. PSNI shall be measured over the PLCP preamble and over the entire received frame. PSNI is intended to be used in a relative manner, and it shall be a monotonically increasing, logarithmic function of the observed link S/(N+I). Specified PSNI performance shall be measured over no less than 1000 PPDUs from the same transmitter. PSNI accuracy and range shall be specified in AWGN and fading for each data rate as follows: Theoretical FEC coding gain assumed in FER calculations: R = 1/2, 5.4dB gain R = 2/3, 4.7dB gain R = 3/4, 4.4dB gain PSNI SPECIAL VALUE: “0” shall indicate inability to measure PSNI When PSNI exceeds high end of measurable range for a given data rate, maximum PSNI for that rate shall be reported. Fading channel model is IEEE exponential ray decay with 50nsec decay time. Joe Kwak, InterDigital

  30. Motion for PSNI normative text • Move to instruct the editor to incorporate the text from document 11-03-316r2-K-PSNI_NormText.doc into the TGk draft specification document • Moved by Joe Kwak • Seconded by: _______________ • Vote YEA _______ • Vote NEA _______ • ABSTAIN _______ • Motion Passes/Fails at ___% Joe Kwak, InterDigital

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