Interpreting CDMA Measurements

7/27/2019 Interpreting CDMA Measurements

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October 13HP Proprietary - Not to be

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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


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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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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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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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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

Documents

Interpreting CDMA Measurements