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PSNI: New PHY Measurement a subjective, comparative measurement to support network management

January 2003. doc: IEEE 802.11-03/100r3. PSNI: New PHY Measurement a subjective, comparative measurement to support network management. Joe Kwak InterDigital Communications Corporation. Outline. Need for new PHY measurement Difficulties with objective measurements RSSI Limitations

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PSNI: New PHY Measurement a subjective, comparative measurement to support network management

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  1. January 2003 doc: IEEE 802.11-03/100r3 PSNI: New PHY Measurementa subjective, comparative measurement to support network management Joe Kwak InterDigital Communications Corporation

  2. 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

  3. 802.11K PHY Measurements • TGK is tasked with providing interfaces, measurements, and mechanisms to support higher layer functions for efficient network management. • 802.11 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: 802.11x, radars, and other interferers. Cannot target measurement on any AP using particular PHY . Joe Kwak, InterDigital

  4. 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

  5. RSSI definitions in 802.11 • 15.4.5.10.2 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. • 17.2.3.2 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. • 17.5.5.7.2 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. • 18.4.5.11.2 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. • 19.2RSSI: 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 18.4.5.11. • ERP-OFDM, DSSS-OFDM, the 8 bit value is in the range of 0 to RSSI maximum as described in 17.2.3.2. Joe Kwak, InterDigital

  6. RSSI Limitations • RSSI is a monotonic, relative indicator of power at the antenna connector, which indicates the sum of desired signal + noise + interference powers. • In high interference environments, RSSI is not an adequate indicator of desired signal quality. • 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/bands 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 load balancing or load shifting. RSSI from one STA does not relate to RSSI from any other STA. Joe Kwak, InterDigital

  7. Options for PHY measurements • Desired Signal RF Power level and resulting Signal-to-Noise ratio (S/N+I) at antenna input are both directly proportional 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

  8. 10db 10db Good STA Good STA 10E-5 10E-5 A A E E Med STA Med STA 10E-4 10E-5 10db 12db Marginal STA Marginal STA 10db 15db 10E-2 10E-5 Signal at same objective S/N Signal at same subjective S/N Best Choice for 802.11K PHYs? • 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. Best choice is somewhere in the middle. Joe Kwak, InterDigital

  9. Proposal: Use a demodulator-specific, subjective estimator of S/(N+I) to provide quantized 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

  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 range of values. • Specify accuracy of output indicator for the selected two FER points. • PSNI range shall span the lower 40 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

  11. First Draft Spec Wording for PSNI • 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, and 11Mpbs: 1. At an FER of 10% for 1000 octet frames received at an uncoded data rate of 11Mbps, PSNI shall indicate 128 +/- 7. 2. At an FER of 10% for 100 octet frames received at an uncoded data rate of 11Mbps, PSNI shall indicate 121 +/- 7. For OFDM PHYs at 6, 9, 12, 18, 24, 36, and 54Mpbs: 1. At an FER of 10% for 1000 octet frames received at a data rate of 12Mbps using R=1/2 coding, PSNI shall indicate TBD. 2. At an FER of 10% for 100 octet frames received at a data rate of 12Mbps using R=1/2 coding, PSNI shall indicate TBD. Joe Kwak, InterDigital

  12. PSNI Supercedes RSSI in Existing Specs • The definition of PSNI meets the requirements for RSSI: • 8-bit unsigned value (for DSSS PHYs) • Proportional to received signal power • TGK can specify that for STAs which comply with 802.11K, STAs shall report PSNI in any data field calling for RSSI. • This makes the PSNI indicator broadly applicable as a frame quality measurement useful for all PHYs and STAs. • RSSI(PSNI) elements and reporting/posting may further be mandated in the MIB by 802.11K to make the PSNI improvements available to higher layers. Joe Kwak, InterDigital

  13. PSNI specified on BER/FER curves For 11 Mbps For 2 Mbps For 5.5 Mbps For 1 Mbps STA front-end losses, antenna configs, or demod losses do not affect PSNI measure. 0 PSNI 121 128 Two specified points on logarithmic scale: Joe Kwak, InterDigital

  14. Partial DSSS PSNI table Joe Kwak, InterDigital

  15. Need to place OFDM curves on PSNI scale Figure 3: BER Curves for Coded and Uncoded OFDM Joe Kwak, InterDigital

  16. Tasks for Next Meeting • Complete OFDM mapping onto PSNI scale. • Complete PSNI table for OFDM • Verify PSNI scale range. • Discuss ease of implementation with modem manufacturers. • Discuss PSNI accuracy spec: +/- 7 units == +/- 1.1 dB. • Discuss PSNI accuracy, for full range of log scale, when specified at only two points. • Consider feedback, criticism and alternate approaches. Joe Kwak, InterDigital

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