1. January 2003 Joe Kwak InterDigital Communications Corporation
IEEE /100r1
doc: IEEE /898r0
November 2003
PSNI: New PHY Measurement for Link Quality comparative measurements of receiver output quality to support network management
Joe Kwak
InterDigital Communications Corporation
Submission
Joe Kwak, InterDigital

2. Outline Need for new PHY measurements
January 2003
IEEE /100r1
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
PSNI as “black box” performance specification
Motions to incorporate PSNI
Joe Kwak, InterDigital
Submission

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
Submission

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
Submission

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.
-80dbm
Good
STA
10E-5
-80dbm
Good
STA
10E-5
A (dBm)
E (FER)
A (dBm)
E (FER)
Med
STA
10E-4
Med
STA
10E-5
-78dbm
-80dbm
Marginal
STA
Marginal
STA
10E-2
-75dbm
10E-5
-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
Submission

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

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, /844r0)
All FEC modes: corrected bit rate in FEC decoder
All modes: EVM on data symbols (cf. Kwak, /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 output signal quality in AWGN, fading and with interference degradations.
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
Submission

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 single PSNI output value for each data rate using FER point: first point to “anchor” indicator, additional points to quantize and scale indicator slope and range of values.
PSNI output values reflect output FER and are specified in AWGN and in one representative fading channel.
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
Submission

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

10. 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
Submission

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

12. PSNI = 76 is equivalent to Observed OSNIR = 3.7dB
Example: PSNI = 76 = OSNIR
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.
Single STA switching data rates will report same PSNI at either rate.
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 OSNIR = 3.7dB
Joe Kwak, InterDigital
Submission

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
Submission

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 dB = EbNo -13.4dB
In general, SNR = EbNo -13.4dB + DRMx, with DRMx from Table 1.
and so EbNo = SNR dB - DRMx,
and OEbNo = OSNIR dB - DRMx
From page 15 we have:
PSNI = 6*[OEbNo - 4.4dB + DRMx - CFy], and substituting for OEbNo,
PSNI = 6*[(OSNIR dB - 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
Submission

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 in AWGN and fading. 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 /773r2.
Joe Kwak, InterDigital
Submission

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
Submission

17. Why Does Variance Measurement Permit Alignment with AWGN?
5. The variance of the measured parameter can be used to compute an adjustment 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
Submission

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

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

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

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

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

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

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
Submission

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

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

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
Submission

28. PSNI is Specified for “Black Box”
SET INPUTS
READ OUTPUT
Sig Level
Chan Fade
PSNI
STA RCVR
(at selected rate)
Noise
BER or
throughput
RCVR implementation may use:
-EVM on data symbols
-EVM on pilots
-other FEC decoder metrics
-FEC corrected bit rate
-spreading code correlation quality
-many others possible
Adjust sig, noise, fading for
desired BER or Throughput
for selected rate
Joe Kwak, InterDigital
Submission
RCVR implementation may use:
-EVM on data symbols
-EVM on pilots
-other FEC decoder metrics
-FEC corrected bit rate
-spreading code correlation quality
-many others possible

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
Submission

30. 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 accurately indicate observed link quality in fading channels. Need variance adjustment, as demonstrated.
PSNI premise has been validated. EVM on data symbols (with variance adjustment) is adequate basis for PSNI. But as Steve Pope, Carl Andren and others have indicated, other demod parameters may be preferred by certain manufacturers.
PSNI is the only link quality proposal in TGk which can indicate 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
Submission

31. Motion for PSNI normative text
Move to instruct the editor to incorporate the text from document r1-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
Submission