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7/27/2019 Interpreting CDMA Measurements
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Interpreting CDMAMeasurements
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Spectrum Measurements: Reading Poweron a Spectrum Analyzer
u Barthead
l Shaped by the IS-95 filter
l Filter has substantial ripple
u
Power is distributed over a 1.23MHz BW
l Noise like spectral distribution
l Level seen on a spectrumanalyzer depends onresolution bandwidth
Frequency Domain
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Spectrum Measurements: Reading Poweron a Spectrum Analyzer
u Example: IS-95 spectrum 5 MHz
span, 30 kHz resolution
bandwidth
l Approximate correction formarker reading 10 log(1.23
MHz/30kHz)=16.1 dBl The true power level is
- 48.21 + 16.1 or -32.2 dBm
u Crest factor must be considered
in rangingl Up to 12 dB above true
average power
l Peak power is -20 dBm in theexample above
Frequency Domain
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Non Linearity in the Frequency Domain
u Non Linearity Cause
Intermodulation
l Shoulders on Bart
l Power in adjacent frequencychannels
u Causes of Intermodulation
l Overdriven power amplifiers
l Mixers
Frequency Domain
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Non Linearity in the Frequency Domain
u IS-95 CDMA Signals have High
Crest Factors
l In excess of 12 dB
l Example: A 10 Watt averagepower transmitter needs to
have an amplifier with enoughoverhead to produce 158Watts Peak!
Frequency Domain
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Power Measurements
u IS-95 signals have substantial power variability
l High and variable crest factor
l Up to 12 dB for forward link signals
u
Reverse linkl Fast power control (800 Hz rate)
u Forward link
l Power depends on traffic
l Forward link power control
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Power Measurements
u Need true average power measurements
l Peak reading power meters can have errors
u It is useful to trigger measurements to frame clocks or power
control groups
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Power Measurement ExampleBroadband Power Meters
u Thermal power meter (HP 438 Power Meter)
u + 3.59 dBm
u
Average Power Meter (E6380A)l Trigger on frame clock
l Average for one complete frame
l +3.61 dBm
u RMS scaled peak reading power meter (HP 8920A)l Designed for FM signals
l +5.9 dBm
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Narrow-band Power Measurements:Channel Power
u Broadband power meters respond to all
signals present at the measurement port
l Highly accurate
l Limited to relatively high levels
u Frequency selective power meters are
needed to measure one signal in the
presence of others
u Channel power measures power in a 1.23
MHz BW
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Narrow-band Power Measurements:Channel Power
u Time domain method
l Apply a 1.23 MHz wide filter
l Measure power after filter
u
Frequency domain methodl Integrate power spectral density over a
1.23 MHz BW
u Good for low level measurements
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Modulation Accuracy
u RHO () Measurement is the fraction of power in a real signal
that correlates with a mathematically ideal signal.
l Time offsets are removed and displayed separately
l Frequency errors are removed and displayed separately
l Magnitude is normalized
l Phase is compensated
u Forward Link RHO is defined for Pilot only transmission
l Reverse Link RHO is defined for an arbitrary mobiletransmission
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Modulation Accuracy
u Intuitive approach: (assumes uncorrelated errors)
l Think of a CDMA signal as the sum of an ideal signal andan error signal
l RHO is the ratio of the ideal power to the total power
ErrorPowerIdealPower
IdealPower
ErrorPowerIdealPowerTotalPower
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RHO Example: Forward Link Pilot Only
u Upper Plot
l All parameters well in spec
u Lower Plot
l Time offset, frequencyerror, LO Feedthrough havebeen degraded
u Notes:
l Time offset and frequencyerror do not degrade RHO
l LO feedthrough doesdegrade RHO
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Notes on CDMA Frequency
u IS-95 CDMA signal envelopes are very variable
l Frequency counters do not yield the correct value. Countersusually read low.
l Frequency should be computed using a parameterestimation technique.
u Frequency is defined as the center of the ideal CDMA
spectrum.
l CDMA signals look like band limited noise.
l Measured with digital signal processing (DSP) techniques.
u Example:
l Real frequency = 881.520 MHz
l Counter reading = 746.823 MHz
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Walsh Code Domain Power
Frequency Domain
Walsh Code Domain
7/27/2019 Interpreting CDMA Measurements
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Walsh Code Domain Power
u Equivalent to spectrum analysis
for IS-95 CDMA FWD link
u Shows the fraction of total
power in each Walsh Code
Channel
l Walsh code number 0 - 63displayed horizontally
l dB displayed vertically
u Can be used to set power
levels in Pilot, sync, Paging,
Traffic
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Walsh Code Domain Power
u Code Domain noise floor is an
indicator of signal imperfections
l Non linearity
l Interference
l Spurious
l Noise
u Noise floor spec: < -27 dB for all
unused Walsh Codes
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Estimating RHO from Walsh Code DomainPower
u Assume:
l Error energy is distributedequally among all WalshCodes
u Ideal signal is distributed
among the active Walsh
channels
u Applicable for
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Estimating RHO from Walsh Code DomainPower
u = Code Domain Power
coefficient for the ith Walsh
Code. There are N activeWalsh Codes
ErrorPowerIdealPower
TotalPower
activej
jactive
i N
N
641
activej
jactive
i
1
1
N
N
N active i
6464
64
1
0 = -2.94 dB = .5081 = -7.88 dB = .162
17= -8.32 dB= .147
32= -7.84 dB =.164
= .980
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Non Linearity in the Walsh Code Domain:Walsh Code Mixing
u Non-Linearity can cause Walsh
Code Mixing
u Upper plot shows the Code
Domain power display for a
CDMA signal in a linear system
u Lower plot shows the same
signal through an amplifier
driven into compression
Mixing
Products
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Non Linearity in the Walsh Code Domain:Walsh Code Mixing
u Non-linearity causes power
from one Walsh code to bleed
into others.
l Walsh 1 mixed with Walshchannel 32, creating power
in Walsh channel 33l Walsh channel 17 mixes
with Walsh channel 32,creating power in Walshchannel 49
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CW Interference in the Code Domain
u PN spreading distributes CW power
over all Walsh codes
l CW tones look like white noise in theWalsh Code domain
u Example: CW spur with 200 kHz
offset and the same level as the
CDMA signal
For the ith Walsh Code:
WW
WW
INTj
j
j
64
int
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CW Interference in the Code Domain
For unused Walsh Codes :
WW
W
INTj
j
64
int
dBW
W
INTj
212
64
int
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Noise in the Walsh Code Domain
u All of the power in WhiteGaussian Noise (WGN) that falls
inside the 1.23 MHz BW
becomes interference.
l Contributes to the code
domain noise floorl WGN is Walsh code white or
equally distributed over all 64Walsh codes
u
Example: AWGN with the samepower spectral density as a
CDMA signal.
l Equivalent to tone example
l Code domain floor at -21 dB
21 dB
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AWGN in the Code Domain
u All sources of uncorrelated power
behave similarly
u Example:
l Signal power = -10 dBm/1.23 MHzl Noise Power = - 13 dBm/1.23
MHz
l White over the 1.23 MHz BW
For the ith Walsh Code
22.8 dB
WW
WW
noisej
noise
j
j
64
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AWGN in the Code Domain
For unused Walsh Codes Wj =0
22.8 dB
WW
W
noisej
noise
j
64
dBW
W
noise
noise
j8.22
3
64
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Code Domain Power as an AccurateMeasurement of Eb/Nt
u This analysis ignores the power control sub-channel
u Let i = Code Domain power reading in active Walsh
Code i
u Let j = Average of the Code Domain power reading in
inactive Walsh Codes
u Let Wi = power in the ith Walsh Channel
WW
WW
noisei
noise
i
i
64
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Code Domain Power as an AccurateMeasurement of Eb/Nt
WW
noise
i
j
i64
9.6 KBPS 14.4 KBPS
12
j
i
noise
ib
BWNt
Eb
WWT
13.1
j
i
noise
ib
BWNt
Eb
WWT
WW
W
noisej
noise
j
64
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Eb/Nt Measurement Example
u Eb/Nt for the traffic channel on Walsh Code 42
l 9.6 KBPS
I = -13.15 dB = 0.0484
I = -22.50 dB = 0.0056
l Correction for the power control sub-channel requiresknowledge of the fraction of the total power in the WalshCode that is dedicated to traffic
12
j
i
Nt
Eb
dB
Nt
Eb85.11
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Walsh Code Domain Timing
u Measures the time alignment
between each Walsh Code and
the Pilot.
u Measured by a parameter
estimation method
u Walsh Codes are Orthogonal
l Only if time aligned
l IS-95 spec:< 50 nSec
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Walsh Code Domain Timing
u Misaligned Walsh codes can
interfere with each other
u Causes of Code Domain Timing
Errors
l TX register settings in CSMASIC (TX_Phase, Sn_TXCHIPX2_ADV. etc.)
l Errors or interferers that arenot power, timing or phase will
distribute themselves amongall three parameters
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Walsh Code Domain Phase
u Measures the phase difference
between each Walsh Code and
the Pilot
l IS-95 spec:< 50 mRad
l Measured by a parameter
estimation method
u The mobile receiver assumes that
all Walsh channels are phase
aligned with the pilot.
l Phase offsets cause cross talkbetween the I and Q
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Walsh Code Domain Phase
u Caused by baseband and RF
processing errors
u Caused by cross talk between I
and Q in the transmitter
u Caused by intermodulation
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Notes on Parameter Estimation
u Code Domain Measurements findthe best fit signal level, timing and
phase for each of the active
Walsh Codes
l Minimize the squared error
u Errors other than the above will
map into level, timing and phase
l Upper Plot : Single CDMAsignal
l Lower Plot: -20 dB, 600 nSecdelayed pilot added to thesignal
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Notes on Parameter Estimation
u This is an error that is not a truetiming or phase error. The
parameter estimator must apply
the error energy somewhere. It
distributes the errors among the
parameters that it can optimize.
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PN Offset Domain Measurements
u PN offset is what distinguishesbetween forward link transmitters
l Sectors
l Base Stations
u PN Offset or delay displayed
horizontally
u Ec/Io displayed vertically
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PN Offset Domain Measurements
u Upper plot
l Full PN display shows peakPilot Ec/Io for each PN offset.Shows PN offsets from 0 to 511
l Zoom display shows multipath
components
u Lower Plot
l 5 strongest pilots in descendingorder
l PN offset and delay relative toGPS time are also displayed
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PN Offset Domain Measurements
u Useful in measuring coverageparameters
l What a mobile sees in thefield
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Conclusion
u CDMA signals have unique characteristicsl Spread spectrum
l Code channels
l Noise like characteristics
l Require knowledge of signal properties for interpretation.
u Unique measurements/methodsl RHO, Pilot time offset
l Frequency
l Walsh Code domain power, timing and phase
l PN offset domain measurements
l Parameter estimation
u Applications
l RHO from Code Domain Power
l Measurement of Eb/Nt
l Coverage parameters