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Base Station Final Draft
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Maintaining and Troubleshootingcdma2000 Base Stations
Page 2
Module Objectives
At the end of this presentation you will be able to:• Understand why maintenance testing is important• Be familiar with the new variable length Walsh Code Display• Be familiar with key CDMA transmitter measurements• Be able to relate the measurements to solving network problems• Understand the differences in IS-95 and IS-2000 Base Station
measurements
Page 3
Why Do Maintenance Test?
• System performance is a big competitive issue• Periodic maintenance helps prevent shutdown• Equipment problems may only show up as reduced capacity• Monitoring for interference finds problems unrelated to network
equipment• Systems will be stressed as loading increases• Defective components may be hidden by CDMA's soft handoff, power
control, and error correction
Page 4
What We Test - The CDMA Network
Public SwitchedTelephoneNetwork(PSTN)
Mobile TelephoneSwitching Office
(MTSO)
BTS
BTSBTS
Backhaul (T1)
Base StationTransceiverSubsystem
(BTS)
Page 5
CDMA Site Block Diagram
SectorGamma
TxRxRx
SectorBeta
TxRxRx
CDMAController
ChannelElement
Morechannelelements
�D/A & A/DConversion
Modulate& Amplify
DigitalInterface
GPS Rx
System TimeReference
& Distribution
TXRXA
RXB
To BSCor MTSO
T1
Sector Alpha
Page 6
Base Station Parametric Measurements
• Total Power (Average Power & Channel Power) • Waveform Quality (Rho) • Carrier Feedthrough • Pilot Time Tolerance (Time Offset)• Frequency Tolerance (Frequency Error)• Code Domain Power • Antenna and site Measurements
Page 7
Base Station Measurement Choices
Connected Measurements Over-the-Air Measurements
Page 8
Transmitter Test Setup
Even Second Pulse*
Base Station TimingBTS RF SignalFrom BTS Antenna Port
* Only Required for Time Offset
Page 9
Why is Forward Link Power Management Important?
Power management is critical to maximizing the system’s capacity
• Network operators sometimes attempt to set power higher to extend coverage to reduce infrastructure cost; result can be pilot pollution problems
• Initial settings for the sites must be accurate to match settings specified by the RF engineering department
• More power is not necessarily better but can lead to interference and dropped calls
• Too little power for the site may result in dead spots between sites
Page 10
Why is Accurate Power Measurements Important
• An Inaccuracy of ± 1dB can• Equate to affecting power
output by 21% • Affect channel capacity• Interfere with cell site
planning• CDMA system is limited by
interference from other transmitters
Power Set too Low –Dead area
BTS
BTS
BTS
Power Set too High –Added interference
Page 11
CDMA Average Power Measurements
Input Detector A / DConversion
DSP
• Performing cdma power measurements requires a new measurement technique
• High/variable crest factor of cdma forward link makes peak power meters read high
• Base Station Test Set Power Meter works independent of crest factor
• Broadband measurement• Offers the best accuracy (7.5%)• Measurement is triggerable
Page 12
CDMA Channel Power Measurements
• Band-limited measurement (1.23 MHz bandwidth)• Useful when interference sources exist (including other cdma
channels)• Wider dynamic range than average power
• For low-level signals (down to -70 dBm)• Measurement is triggerable
Input A / DConversion
Band PassFilter (DSP) DSP
Page 13
Non-Linearity in the Frequency Domain
• Non-linearity causes intermodulation
• ‘Shoulders’ on the Waveform• Power in adjacent channels
• Causes of intermodulation• Overdrive Power Amplifiers• Bad Mixers
• CDMA signals have high crest factor
• In excess of 12 dB• EXAMPLE: 10 Watts average
power transmitter needs to have an amplifier with enough headroom to produce 158 Watts peak
Energy in Adjacent Channels
Shoulders
Page 14
Base Station Parametric Measurements
• Total Power (Average Power & Channel Power) • Waveform Quality (Rho) • Carrier Feedthrough • Pilot Time Tolerance (Time Offset)• Frequency Tolerance (Frequency Error)• Code Domain Power • Antenna and site Measurements
Page 15
Waveform Quality (Rho)
� Power that correlates with idealTotal Power
=
Signal PowerSignal Power + Error Power� =
� > 0.912
Page 16
Why is Rho Important?
• Key measure of modulation quality• Analogous to FM accuracy/distortion (AMPS) and EVM (TDMA
systems)• Rho performance affects site/sector coverage area and capacity in
the site/sector• Rho failures can indicate problems in:
• Compression in linear amplifiers• Magnitude and phase errors in the IQ modulator• Phase non-linearity (group delay)Spurious signals in the
transmission path• Carrier feedthrough
Page 17
Base Station Parametric Measurements
• Total Power (Average Power & Channel Power) • Waveform Quality (Rho) • Carrier Feedthrough • Pilot Time Tolerance (Time Offset)• Frequency Tolerance (Frequency Error)• Code Domain Power • Antenna and site Measurements
Page 18
Carrier Feedthrough
• Carrier feedthrough (origin offset)• Should be < -25 dBc• Carrier feedthrough in I/Q Domain and Frequency Domain
Page 19
Pilot Time Tolerance(Time Offset)• Measure of Short Code sequence timing vs. System time
• Checks the “start” of the PN offset as compared tothe even-second clock signal
110 … 011 ... 011 … 100 111 … 010 000 … 001 110 … 011101000..000 101000..000
110 … 011 ... 011 … 100 111 … 010 000 … 001 110 … 011101000..000
110 … 011 ... 011 … 100 111 … 010 000 … 001 110 … 011101000..000 101000..000
101000..000
PN Offset 0
PN Offset 1
PN Offset 2
32768 Chips
32768 Chips
15 Zero’s
64 chips 64 chips 64 chips
Even Second
32768 Chips
Page 20
What is Pilot Time Tolerance?
• Time offsets outside of specifications can affect handoffs between cells - the island effect
• Time offset is one of the parameters that will lead to errors inposition location with the introduction of E911 and network operator services
• Potential causes for failures of pilot time tolerance:• GPS receiver and timing distribution failures• Cells with a propagation delay greater than the PN Offset time period• The timing delay adjustment (used to compensate for time delays
through the sites cabling) may be off
Page 21
Frequency ErrorWhy is it important?• GPS drift or out-of-lock condition can create the island cell effect • Frequency drift can lead to site timing errors which will lead to
errors in position location with the introduction of E911 and network operator services
• Failures point to problems in GPS receiver and timing distribution
Note: This measurement cannot be made with a frequency counter frequency tolerance (Frequency Error) specifications: ±0.05 ppm PCS (99 Hz @ 1980 MHz)
Page 22
Base Station Parametric Measurements
• Total Power (Average Power & Channel Power) • Waveform Quality (Rho) • Carrier Feedthrough • Pilot Time Tolerance (Time Offset)• Frequency Tolerance (Frequency Error)• Code Domain Power • Antenna and site Measurements
Page 23
Code Domain Power is Important
• Analogous to spectrum analyzer on FDMA systems• Verifies network system settings
• Allocation of assigned power to pilot, sync, paging and traffic channels
• Specified: Code Domain noise floor must be at least <27 dB below total power in inactive channels, else capacity suffers
• Code Domain Power failures can indicate problems in• Linear Amplifiers• Channel elements / cards• Settings in network control software
Page 24
The CDMA Concept10 Khz BW 1.23 Mhz BW 1.23 Mhz BW 10 Khz BW
fcfc
CDMATransmitter
CDMAReceiver
BasebandData
0 0
fc fcfcExternal Interference
Interference Sources
fc
Other User Noise
Encoding &Interleaving
Walsh CodeSpreading
Walsh CodeCorrelator
BasebandData
Decode & De-Interleaving
1.23 Mhz BW1.23 Mhz BWSpurious Signals-113 dBm/1.23 Mhz
9.6 kbps 19.2 kbps 1228.8 kbps 9.6 kbps19.2 kbps1228.8 kbps
Background Noise Other Cell Interference
Page 25
Code Domain Power
1.2288 MHz
User1
User3
User2
Pilot Paging Synch0 1 2 3 4 5 6 7 8 9 32 40 63
PilotSynch Frequency DomainPaging
User #3User #2User #1
Code Domain
Walsh Code
Page 26
IS-2000 Code Domain Power ScreenSupplemental channels are clearly visible as wide blocks.
Supplemental Channel
Single SR1 (1xRTT) channel
Page 27
Cdma2000 Code Domain Power Screen Bit Reverse Ordering
Pilot is the same for IS-95 and 1XRTT channel
Marker Primary + Supplemental Channels
Primary channel Walsh 20
Current Display Bit Reverse order
Channel is 4 Codeswide
Page 28
Complex Power Code Domain power display
Q Code
I Code shows combined IS-95 and 1xRTT
Q Codes show 1XRTT only
I Code
Any Channel that is active in I Code and not active in Q Code means it is IS-95 only.
Page 29
Complex Power Code Domain power display
I Code shows combined IS-95 and 1XRTT
Q Codes show 1XRTT only
Any Channel that is active in I Code and not active in Q Code means it is IS-95 only.
I Code
Q Code
Page 30
IS-2000 SR1, RC3 (9.6 kbps)
Long CodeDecimator
Interleaver
38.4 ksps
1/4 Rate Conv.Encoder
38.4 ksps
9.6 kbps
38.4 kbps
Walsh 64Generator
1228.8 kcps
1228.8 kcps
1228.8 kbps
1228.8kbps
Q
I
S -PPC
Q
I
PCDec
1228.8 kcps
Q Short Code
I
Q
1228.8 kcps
ComplexScrambling
FIR
FIRI Short Code
OrthogonalSpreading
1228.8 kcps
1228.8 kcps
+
+
+
-
38.4 ksps
19.2 ksps
19.2 ksps
P.C. Bits
Gain
Gain
PunctureTiming
Add CRC andTail Bits
Short Code Scrambler
Walsh CodeGenerator
1228.8 kcps
Q Short Code
Q
I
FIR
FIR
I Short CodeIS-95B
Full RateData Bits Power
ControlPuncture
8.6 kbps
I
800 bps
OptionalCan be Carried by F-DCCH
1228.8 kbps
Long CodeGenerator
User LongCode Mask
800 bps
Q
Decimate by Walsh Length/2
Page 31
Non-Linearity in Walsh Code ChannelsMixing Products
• Non-Linearity can cause Walsh Code mixing
• Upper display shows Code Domain Power Display for a cdma signal in a linear system
• Lower Plot shows the same signal through an amplifier driven into compression
• Non-linearity causes power from one Walsh code to bleed into others.
• Walsh 1 mixed with Walsh channel 32, creating power in Walsh 33
• Walsh channel 17 mixes with Walsh 32 creating power in Walsh 49
MixingProducts
Page 32
CW Interference in the Code Domain
• PN spreading distributes CW power over all Walsh codes
• CW tones look like white noise in the Walsh code domain
• Example: CW spur with 200 kHz offset and the same signal level as the cdma signal
• Noise floor is –21 dBc. Standard calls for minimum of –27 dBc
• Spur shows up in Spectrum Analyzer trace in a single location while noise is evenly spread in Code Domain
Page 33
Noise in the Code Domain
• All of the power in White Gaussian Noise (WGN) that falls inside the 1.23 MHz BW becomes interference
• Contributes to the code domain noise floor
• WGN is Walsh code white or equally distributed over all 128 Walsh codes
• Example: AWGN with the same power spectral density as a cdma signal
• Equivalent to tone example• Code Domain floor at –21 dB
Page 34
AWGN in the Code Domain
• All sources of uncorrelated power behave similarly
• Example:• Signal power = -10 dBm/1.23 MHz• Noise power = - 13 dBm/1.23 MHz
• White noise over the 1.23 MHz BW shows up in the frequency domain
• White noise again raises the noise floor in the Code Domain
Page 35
Base Station Over-Air Measurements
Page 36
Pilot, Sync, and PagingCode Domain Display
Page 37
Quick Paging Channel in cdma2000
Page 38
Pilot Dominance Required for Over-Air Tests
Dominant Pilot
Page 39
Over-Air Base Station Measurement Statistics
Page 40
Base Station Troubleshooting
Page 41
Base Station Parametric Measurements
• Total Power (Average Power & Channel Power) • Waveform Quality (Rho) • Carrier Feedthrough • Pilot Time Tolerance (Time Offset)• Frequency Tolerance (Frequency Error)• Code Domain Power • Antenna and site Measurements
• Antenna Return Loss (VSWR)• Cable Fault Location• Insertion Loss• GPS Timing Distribution Measurements
Page 42
Antenna Return Loss
•Checks the health of the feedline and antenna network
•Excessive return loss results in decrease in transmitted power
•Antennas and cables are sometimes damaged due to vandalism and environment
•Test periodically and compare to benchmarks taken at installation time - can store the sweep results
Page 43
Antenna Return LossTest Set-up
Source Signal
Reflected Signal
Directional Coupler(Bridge)
Antenna, Feedline and Connectors
RF Source
SpectrumAnalyzer
Agilent Test Set
Page 44
Distance to Cable Fault Measurement Test Set-up
Agilent Test Set Fault Connector
RF Source
SpectrumAnalyzer
Cable under test
PowerDivider
Antenna orTermination
Results FaultIndication
Fau
lt M
a gn i
t ud e
Distance
Page 45
Cable Fault Measurements
• Cable Fault tests are a good complement to the return loss test (test SWR; if it fails, use cable fault to isolate the problem)
• Allows the technician to better isolate the fault• Can be used to determine if a “tower crew” is needed to fix the
problem• Accuracy: 4 feet for a 500’ cable• Maximum cable length: 500 feet
Page 46
Testing the GPS receiver and distribution
Spectrum Analyzer verifies presence and level of the 16X chip clock
• External GPS RX verifies frequency accuracy of 16X chip clock
Page 47
Summary
• Why we test• Why Power is a Critical Base Station Parameter• What Tests are Performed on a Base Station• What the Test Results mean