PIMBIT Testing Recommendations

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PIMBIT Testing Recommendations

• Orville Nyhus
• January 13, 2009

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Fundamentals

• Primary objective is to determine possible
  channel degradation due to PIM interference
• This is not a directly measurable parameter
• A ratio of (IMCWN)/N can be measured
• Channel degradation can be computed from
  (IMCWN)/N
• This test requires no knowledge of LNA gain or
  system RF cable loss factors
• Measurement bandwidth is a user selectable
  parameter
• Measurements can be made with a variety of
  equipment, e.g., SDU receiver or spectrum
  analyzer, at manufacturers discretion

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CW Carrier PIMBIT Concept

• Two-Tone CW Carrier Transmission Test
• Point HGA away from satellite orbit
• Generally North in Northern Hemisphere
• Generally South in Southern Hemisphere
• Transmit frequencies for PIM test
• Two test transmit frequencies to be provided by
  Inmarsat
• Impacted victim receiver frequencies also
  provided by Inmarsat
• Measure and record noise level N on victim
  channel while no transmissions are in progress
• Transmit two CW carriers at recommended EIRP
• Measure and record IMCWN level on victim channel
• Form (IMCWN)/N ratio from recorded data
• Compute channel degradation from measured
  (IMCWN)/N ratio data

4
Channel Degradation

• Most Likely Interference Scenario
• High-Speed channel IM interfering with an active
  receive channel
• High-Speed channels operate at higher power than
  narrower slower-speed channels and hence the
  higher-power channels generate much stronger IM
• Narrower channels suffer from essentially white
  added noise of a high-speed IM product over the
  narrow channel bandwidth
• Receive channel Eb/(N0?N0) is degraded by
  increase in noise (?N0) due to IM power spectral
  density increasing the total channel noise above
  the normal channel AWGN (N0)
• Degradation can be expressed in terms of ?T/T
  ratio where ?T represents the added noise
  temperature due to the IM product where ?N0 k?T
  and k is Boltzmanns constant
• The channel degradation due to increased noise
  is 10 Log10(T?T)/T 10 Log101 (?T/T) in
  decibels
• ?T/T can be calculated from a measured (IMCWN)/N
  ratio

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Knowledge of IM Bandwidth is Required

• IM Bandwidth is needed to Relate to Measured CW
  IM Level
• Relative to an un-modulated CW carrier of the
  same total power the power spectral density of a
  modulated PSK signal is inversely proportional to
  the PSK symbol rate
• Signal bandwidth is proportional to symbol rate
• The two-sided effective RMS IM bandwidth of a
  root-raised-cosine filtered PSK signal is
  spread as
• Rsym is symbol rate
• P is IM product order
• Alpha is root-raised-cosine excess bandwidth
  factor
• Then IMCW BIM?N0 kBIM?T where k is
  Boltzmanns constant

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Knowledge of Detection Bandwidth

• Knowledge of the Detection Bandwidth used to
  Measure (IMCWN)/N is needed to Convert to ?T/T
• Measured noise is N BdetN0 kBdetT
• Where
• k is Boltzmanns constant
• Bdet is the receiver detection bandwidth
• N0 is noise power spectral density
• T is fundamental system noise temperature

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Computation of Channel Degradation

• Based on Measured (IMCWN)/N and IM Product
  Bandwidth and Detection Bandwidth Used One can
  Compute ?T/T and Channel Degradation
• With and
  we have
• And, therefore,
• Note that these factors are numerical, not in
  decibels
• With ?T/T derived from known and measured
  quantities the channel degradation due to
  increased noise on-channel can be computed as
  (refer to Slide 4)

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Example for Ninth-Order PIM for SBB

• Symbol Rate 151,200 sps
• Alpha 0.25
• BIM 257,135 Hz (P 9)
• Bdet 1 kHz (for this example)
• Recommended PIMBIT Failure Threshold

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Typical PIM Measurement Example (SA)

• See Next Slide - Spectrum Analyzer (SA) Display
• fTX1 1627.0 MHz
• fTX2 1650.0 MHz
• EIRP (each CW channel) 20 dBW
• 9th Order IM Frequency 1535.0 MHz
• (IMCW N)/N ?MKR 25.84 dB
• Bdet 1 kHz (Resolution Bandwidth)
• Calculations for SBB
• Symbol Rate 151,200 sps
• Alpha 0.25
• BIM 257,135 Hz

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Spectrum Analyzer Display - 9th Order PIM
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