1. January 2003 Joe Kwak InterDigital Communications Corporation
IEEE /100r1
doc: IEEE /218r1
March 2003
PSNI: New PHY Measurement a post-processing, comparative measurement to support network management
Joe Kwak
InterDigital Communications Corporation
Submission
Joe Kwak, InterDigital

2. Outline Need for new PHY measurement
January 2003
IEEE /100r1
Outline
Need for new PHY measurement
Difficulties with objective measurements
RSSI Limitations
Perceived Post-Processing S/(N+I) Indicator (PSNI)
PSNI Specification details
Tasks for next meeting
Joe Kwak, InterDigital
Submission

3. 802.11 defined PHY Measurements
TGK is tasked with providing new interfaces, new measurements, and mechanisms to support higher layer functions for efficient network management.
currently has several PHY measurements, none of which seem suitable for handoff or network management purposes.
RSSI: reportable parameter for each received frame, but unquantized and not fully specified. Can be used to compare frames from different AP sources.
SQ: another unquantized indicator of code sync, but applicable only to DSSS PHY.
RPI histogram: quantized and specified, but measures channel power from all sources: x, radars, and other interferers. Cannot target measurement on any AP using particular PHY .
Joe Kwak, InterDigital
Submission

4. Network Management Needs Comparative PHY (sects
Network Management Needs Comparative PHY (sects. 15, 17, 18, 19) Measurement
1. To compare AP signals on same channel, same PHY, in same STA
2. To compare AP signals on same channel, same PHY, in different STAs
3. To compare AP signals on different channels, same PHY, in same STA
4. To compare AP signals on different channels, same PHY, in different STAs
5. To compare AP signals on different PHYs in different STAs
6. To compare AP signals on different PHYs in same STA
Comparative measurements are crucial to QOS and handoff decisions for Network Management, as discussed at Tutorial at last meeting.
Comparative measurements are important for load management and interference mitigation.
RSSI, as currently defined, only addresses needs #1 and #3.
Joe Kwak, InterDigital
Submission

5. RSSI definitions in
PMD_RSSI.indicate(RSSI): The RSSI shall be a measure of the RF energy received by the DSSS PHY. RSSI indications of up to 8 bits (256 levels) are supported.
RXVECTOR RSSI: The allowed values for the receive signal strength indicator (RSSI) parameter are in the range from 0 through RSSI maximum. This parameter is a measure by the PHY sublayer of the energy observed at the antenna used to receive the current PPDU. RSSI shall be measured during the reception of the PLCP preamble. RSSI is intended to be used in a relative manner, and it shall be a monotonically increasing function of the received power.
PMD_RSSI.indicate: This primitive shall provide the following parameters: PMD_RSSI.indicate(RSSI) The RSSI shall be a measure of the RF energy received by the OFDM PHY. RSSI indications of up to eight bits (256 levels) are supported.
PMD_RSSI.indicate: This primitive provides RSSI parameter as 0–8 bits of RSSI. The RSSI is a measure of the RF energy received by the High Rate PHY.
19.2 RSSI: The RSSI is a measure of the RF energy received by the ERP. The value depends on the value of the MODULATION parameter:
CCK, ER-PBCC: the 8 bit value of RSSI as described in
ERP-OFDM, DSSS-OFDM, the 8 bit value is in the range of 0 to RSSI maximum as described in
Joe Kwak, InterDigital
Submission

6. RSSI Limitations
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.
RSSI is not fully specified: no unit definitions, no performance requirements (accuracy, fidelity, 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.
Proposed PSNI overcomes these limitations by providing a quantized, comparative measure of S/(N+I) for all channels/rates and among all PHYs and STAs.
Joe Kwak, InterDigital
Submission

7. Options for PHY measurements
Desired Signal RF Power level and resulting Signal-to-Noise ratio (S/N+I) at antenna input are both inversely related to FER at output.
AGC B C D
Demodulator and
tracking loops
(PHY specific) E
Radio
front end
FEC
Decoder
(optional)
Frame
Check
(CRC)
A/D A
A: RF power,
RF S/(N+I) from each AP
C&D: Bit Error Rate (BER)
@each data rate
B: BB S/(N+I) from each AP,
(BB power constant by AGC)
E: Frame Error Rate (FER)
@each data rate
Joe Kwak, InterDigital
Submission

8. Best Choice for 802.11K PHYs? A E Signal at same objective S/N
Accurate S/N+I (or RSSI power) 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, 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
Med
Marginal A E
10db
10E-2
10E-4
10E-5
12db
15db
Signal at same objective S/N
Signal at same subjective S/N
Best choice is somewhere in the middle.
Joe Kwak, InterDigital
Submission

9. Proposal: Use a demodulator-specific, post-processing estimator of S/(N+I) to provide quantized BER/FER indication.
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
DSSS: spreading code correlation quality
OFDM: frequency tracking and channel tracking stability
Demodulator internal metrics are available on a frame-by-frame basis.
Demodulator metrics proportional to analog S/(N+I) are available at all data rates. Metric may be rate dependant or not.
Demodulator internal metrics may be calibrated with respect to actual FER performance (at any specified data rate/coding rate) in controlled environment to accurately indicate perceived or delivered S/(N+I).
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 indicator relates to S/(N+I) and FER
Joe Kwak, InterDigital
Submission

10. Perceived Signal to Noise Indication (PSNI)
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 indication value at two FER points: first point to “anchor” indicator, second point to quantize and scale indicator slope and range of values.
Specify accuracy of output indicator for the selected two FER points.
PSNI range shall span the lower 32 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

11. Draft Normative Text (add as para 18. 4. 5. 17 titled “PMD_PSNI
Draft Normative Text (add as para titled “PMD_PSNI.indicate”)
PSNI: 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 quality observed after RF downconversion and is derived from internal digital signal processing metrics of the demodulator used to receive the current frame. PSNI shall be measured over an entire received frame. PSNI is intended to be used in a relative manner, and it shall be a monotonically increasing, logarithmic function of the perceived S/(N+I). PSNI accuracy and range shall be specified at two different FER operating conditions for each PHY:
For DSSS PHYs at 1, 2, 5.5, 11, 22, and 33 Mpbs in AWGN attenuating channel:
1. At an FER of 10% (+/-0.2%) at a PSDU length of 1000 bytes, received at a data rate of 11 Mbps without FEC coding, PSNI shall indicate 125 +/- 8.
2. At an FER of 10% (+/-0.2%) at a PSDU length of 100 bytes, received at a data rate of 1 Mbps without FEC coding, PSNI shall indicate 32 +/- 8.
For OFDM PHYs at 6, 9, 12, 18, 24, 36, and 54 Mpbs in AWGN attenuating channel :
1. At an FER of 10% (+/-0.2%) at a PSDU length of 1000 bytes, received at a data rate of 54 Mbps using R=3/4 FEC coding, PSNI shall indicate 205 +/- 8.
2. At an FER of 10% (+/-0.2%) at a PSDU length of 1000 bytes, received at a data rate of 6 Mbps using R=1/2 FEC coding, PSNI shall indicate 60 +/- 8.
Joe Kwak, InterDigital
Submission

12. PSNI May Supercede RSSI in Existing Specs
The definition of PSNI meets the requirements for RSSI:
8-bit unsigned value
Monotonically increasing function of received signal power
TGK may specify that for STAs which comply with K, STAs may report PSNI (or RCPI) in any data field calling for RSSI. A new SME control variable could specify the use of the RSSI field: a) for PSNI, b) for RCPI, or c) for RSSI and so on.
This would make the PSNI, and other new indicators, broadly reported and broadly applicable as frame quality measurements useful for all PHYs and STAs.
Joe Kwak, InterDigital
Submission

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

14. BER Curve Adjustments for Rate/Modulation
Joe Kwak, InterDigital
Submission

15. Example: PSNI = 101
BERs vary based on FEC coding used at each data rate. FERs vary based on BER and PSDU length
Joe Kwak, InterDigital
Submission

16. Partial DSSS PSNI table
Joe Kwak, InterDigital
Submission

17. Partial OFDM PSNI table
Joe Kwak, InterDigital
Submission

18. Tasks for Next Meeting
Integrate normative text into new TGK draft spec.
Discuss any implementation issues with modem manufacturers.
Discuss PSNI accuracy spec: /- 8 units == +/- 1.0 dB.
Simulate effectiveness of EVM or other demodulator parameter as basis for PSNI in interference environments with indoor channel models.
Consider feedback, criticism and alternate draft specification wording.
Joe Kwak, InterDigital
Submission