Interim Optimization Module:
CDMA Optimization Principles
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Optimization Chapter Objectives
• Preparatory knowledge
• Recognize key CDMA system parameters and their typical settings,
ranges, effects of adjustment
• Understand the basic nature of CDMA messaging to facilitate
optimization activities
• Recognize the CDMA messages sent on the various CDMA forward and
reverse channels
• Consider a limited selection of optimization tools and techniques
currently in use, including their basic structure, features, and typical
applications
• Review certain key system problems, their root causes, and effects of
specific parameter and configuration changes on them
Cellular System Performance Criteria - Voice Quality
Performance criteria
Voice quality
Service quality
Special features
Mean Opinion Score
MOS
5 (Excellent)
4 (Good)
3
2
1
•
There are three categories for specifying cellular system performance criteria
– voice quality given by MOS-parameter (Mean Opinion Score)
– service quality presented by the following parameters
• RF coverage in terms of percentage of area, time, and subscribers
• grade of service (GOS) in terms of blockage probability
• dropped call rate (DCR) in terms of probability of a dropped call
– special services
Cellular System Performance Criteria - Voice
Quality, continued
•
•
•
•
•
•
Voice quality is one of three criteria for specifying cellular system performance
Voice quality evaluation is based on averaging of subjective users opinions which are expressed as a
set value X at which Y percent of service subscribers rating voice quality as good or excellent
The average of the scores obtained from a selected poll of customers is called Mean Opinion Score
(MOS); the desirable MOS= 4-5
The relationship between MOS and voice quality scale is as follows
– MOS=5 Excellent (speech perfectly understandable)
– MOS=4 Good (speech easily understandable, some noise exist)
– MOS=3 Fair (speech understandable with slight effort and repetitions)
– MOS=2 Poor (speech understandable only with frequent repetitions)
– MOS=1 Unsatisfactory (speech not understandable)
In designing a cellular system, the voice quality is specified with respect to what maximum value of
C/I the speech coder/decoder can tolerate
The C/I ratio of 18 dB (AMPS systems) is the required level for getting a desirable voice quality
Cellular System Performance Criteria - RF Coverage
Performance criteria
Voice quality
Service quality
Special features
RF coverage
Grade of Service
Dropped Call Rate
• In cellular systems with irregular terrain, it is not practical to engineer the
system to equally cover 100% of service area because of
– it will require increased transmit power and other techniques to illuminate
weak spots with sufficient field strength
– it will cause complications with interference control
Cellular System Performance Criteria - RF
Coverage, continued
•
•
•
•
•
•
•
Cellular system designers usually state the following objectives in terms of percentage of
area and customers depending on terrain configuration
– to cover 90 percent of flat terrain area providing MOS between 4 and 5 for 75
percent of users
– to cover 75 percent of hilly terrain area providing MOS between 4 and 5 for 90
percent of users
A cellular system operator can set lower percentage values for a low-performance and
low-cost start-up system configuration
In some sources, the coverage objective is defined in terms of percentages of area and
time - 90 percent of the service area covered for 90 percent of the time
In planning cell coverage, RF engineer should provide field strength coverage over
densely populated areas and exclude the zones of uninhabited swamps, wilderness and
terrain
Typically, suburban area coverage is engineered to a mean RSS of -93 dBm which
corresponds to the FCC 39-dBuV/m contour
Coverage in rural areas is usually engineered to a mean RSS of -100 dBm which
corresponds to the FCC 32-dBuV/m contour
Handheld coverage in urban areas is usually engineered to a mean RSS of -78 dBm
which corresponds to the 54-dBuV/m contour
Cellular System Performance Criteria - Grade Of
Service (GOS)
Performance criteria
Voice quality
Service quality
Special features
RF coverage
Grade of Service
Dropped Call Rate
•
•
The Grade of Service is usually presented by the blocking probability parameter
For a start-up system configuration, the GOS parameter is specified as 0.02 which means
that 2 percent of call attempts are blocked by the system at the busy hour; this value is
averaged across all cells in the system
Cellular System Performance Criteria - Grade Of
Service (GOS), continued
• Actual blocking probability depends on such factors as traffic distribution, cell
site locations, number of radios at each cell and system parameters setting
• Actual blocking probability at each cell may fluctuate during day time and can
be affected by weather and traffic conditions
• To decrease the blocking probability at a busy?cell near freeway or near
entertainment place requires a good engineering of system resources
• High actual blocking probability at particular cell(s) shows that traffic load
balancing is required; this is the most complex task of system optimization
because it involves cell acquisition, antenna pointing and handoff thresholds
Cellular System Performance Criteria - Dropped
Call Rate (DCR)
Performance criteria
Voice quality
Service quality
Special features
RF coverage
Grade of Service
Dropped Call Rate
•
•
•
The dropped call is defined as an established call which leaves the system before it is
normally terminated
The Dropped Call Rate (DCR) parameter represents what percentage of all established
calls is dropped during a specified time period
The DCR and voice quality are inversely proportional and high DCR may indicate
Cellular System Performance Criteria - Dropped
Call Rate (DCR), continued
• RF engineer should be aware, that subscriber´s perception of dropped call rate
usually higher than actual measured value due to the following
– subscriber unit is not functioning properly
– the user is operating a portable in vehicle
• The commonly used formula for calculation of DCR suggested by Dr. W.C.Y.
Lee
– in a noise-limited case
N



^n

P a   n  1
1
where






n 0
Pa - is the probability of a dropped call
n - is the weighted value for those calls having n handoffs
- is the probability that RSS is below the specified threshold
(RSS< -102 dBm when C/N =18dB, C =-102 dBm, N =-120 dBm)
Cellular System Performance Criteria - Special
Features
Performance criteria
Voice quality
•
•
Service quality
Special features
Cellular system operator is interested in providing to subscribers many special features in
addition to basic telephone service
– call forwarding
– call waiting
– voice mail box
– automatic roaming
Some customers may not be willing to pay extra charges for special services
Cellular System Performance Evaluation - Metrics Set
Measurement Data
System Design
System Performance
Analysis
Cellular System
Operation
System Parameters
•
•
•
System
Optimization
Networking Design
Performance Metrics
Almost all cellular systems have dedicated resources and procedures for system performance
evaluation
Various measurement data logs (for instance, established calls, dropped calls, completed hand-offs,
blocked call attempts etc.) are reported to the switch for collecting, processing, and generating of
performance metrics
RF engineer should be familiar with system performance metrics and be able to use them for system
performance optimization (system parameters tuning)
Cellular System Performance Evaluation Metrics Set, continued
•
•
•
•
Metric: Established Calls
– defined as the percent of all attempts that could result in an answered call
– the real world range is (80 -85) % per cell site
Metric: Handoff Failures
– defined as the portion of handoff initiations that where aborted due to a failure during
handoff process; initiations and failures are taken for the same cell
– the real world range is (0-5)%
Metric: Cell Blocking
– defined as the portion of origination seizures (setups) that are denied due to a lack of
available radios or trunks at Base Station
– the real world range is (2-5)%
Metric: Cell Trunk Group Utilization
– defined as the ratio of a measured carried traffic to engineered traffic capacity based
upon a selected Erlang model
– the real world range is (70-90) %
This Evolving Optimization Chapter:
What is Included Today
•
•
As this chapter goes to press, CDMA systems with large numbers of BTSs and
subscribers are not yet available for use as examples. Therefore, this chapter does
not include all of the optimization material which ultimately will be included.
This chapter includes the following:
– detailed listings of all important CDMA system parameters, including ranges,
recommended values, and clarifying details (see also accompanying
documentation)
– a survey of the optimization tools currently in use, including their basic
structure, features, and typical applications
– summary of CDMA messages sent on the various forward and reverse
channels
– discussion of system problems, root causes, and effects of specific parameter
and configuration changes on them
This Evolving Optimization Chapter:
What is Not Available Yet
• Forthcoming revisions of this chapter will include the following, as they
become available:
– peg count examples from live systems
– actual file and screen captures from live optimization tools
– actual OM examples of system problems showing effects of specific
parameter and configuration changes
– MTX parameter details
• Overviews of the functions and features of commercial optimization tools
from SAFCO, Grayson, and any others which become available
• Registration, hard-handoff, call/handoff blocking parameters are not yet
addressed in this document.
Families of Issues in the Real World
RF Coverage
Slow Handoff
RF Components
Hard Handoff
Forward Interference
Microwave
Interference
Frequency
Coordination
PN Plan
Access Failures
Excessive Handoff
Neighbor Window
Neighbor Lists
Handoff Boundary
Balancing
Link Balance
Power Control
• All of the issues at left can
cause system performance
problems.
• Issues shaded green are
related to optimizable CDMA
system parameters and are
discussed in this course
• Issues shaded red involve RF
solutions or design attention
such as during site selection
• Issues shaded blue involve
external factors requiring
cooperation or legal means
Optimization Phases
• The RF optimization process progresses in stages as a network evolves:
• Before Commercial In-Service:
– System/cluster shakedown/de-bugging
– Dropped call rate.
– Access success rate
– Search window settings
– Neighbor lists
– RF coverage/handoff control
– Hard handoff success rate
– The system/cluster may be operated on a simulated load for these
tests
Optimization Phases (continued)
• At Beginning of Commercial Service
– Dropped call rate
– Access success rate
– Hard handoff success rate
– Capacity
• During Growth and System Expansion
– Introduction of new sites
– Dropped call rate
– Access success rate
– RF coverage/handoff control
– Hard handoff success rate
– Capacity
Pre-Conditions for Optimization
• To be ready for optimization, the system must already have reached a
basic threshold of airworthiness. The following steps should already be
complete before serious optimization begins:
– BTSs calibrated
– Network stable: all required sectors on the air and able to carry calls
– BSC support is functional: selector logging, parameter changes,
enabling/disabling OCNS, wilting/blossoming sectors
– area maps available
– site database exists in Planet (or other propagation prediction tool)
– vehicles and analytical tools are available
– PN plan has been developed and applied
– first cut?neighbor lists have been developed and datafilled
– conflicting spectrum users have been cleared
Typical Optimization Sequence
•
•
•
Prior to Commercial Launch:
– datafill audit
– shakedown
– set simulated load
1st full drive tests
– dropped call analysis using analysis tools (cdmastat, dropseek)
– failed access analysis
– spot check drives to recreate problems and validate changes
– srch_win_a (mje analysis), srch_win_n/r (TA messages)
– automated neighbor list audit
– RF coverage (best server, n-way, TXpower)
– drive benchmark route several times
– per-site stats to pinpoint problems (TXGA, FER)
– baseline for performance/trend analysis (sum files)
Make changes, 2nd full drive, performance/trend analysis
Exit Criteria: When to stop Optimizing?
• Optimization is an ongoing process which is reapplied at intervals through
the entire service life of a system.
• However, specific optimization sequences are normally performed to
reach definable goals. Typical objectives are:
– Achieving a target dropped call rate.
– Achieving a target access success rate.
– Achieving coverage over a specified geographical area.
• The results of optimization are recorded and ongoing system performance
is tracked against the optimized results. When performance or
circumstances warrant, major optimization activities are resumed.
The Core Philosophy of Optimization
• Identify problems and troublesome performance issues
• Measure how often and how severely they occur
• Observe their actual occurrence
– capture the messaging on both forward and reverse links
– capture the parameters
– capture the locations where they occurred
– capture any other relevant circumstances
• Analyze your observations to determine causes
• Vary parameters and configuration seeking improvement
• Observe again and Benchmark the results
CDMA Forward Channel Messages
Pilot Channel
Sync Channel
No Messages
Sync Channel Msg
Paging Channel
Forward Traffic Channel
Order Msg
Authentication Challenge
Flash with Information Msg
TMSI Assignment Msg
System Parameter Msg
Status Request Msg
Alert with Information Msg
SSD Update Msg
Access Parameter Msg
Service Redirection Msg
Data Burst Msg
Status Request Msg
CDMA Channel List Msg
Authentication Challenge
Handoff Direction Msg
Service Request Msg
Page Msg
SSD Update Msg
Extended Handoff
Direction Msg
Service Response Msg
Slotted Page Msg
TMSI Assignment Msg
Analog Handoff Direction
General Page Msg
Null Msg
Neighbor List Update Msg
Service Option Control
Msg
Order Msg
Extended System
Parameter Msg
Retrieve Parameter Msg
In-Traffic System
Parameter Msg
Channel Assignment Msg
Neighbor List Msg
Data Burst Msg
Extended Neighbor List
Msg
Global Service
Redirection Msg
Feature Notification Msg
Maintenance Required
Order
Lock until
Power-Cycled Order
Service Connect Msg
Set Parameter Msg
Mobile Station
Registered Msg
Power Control
Parameter Msg
Send Burst DTMF Msg
Base Station Challenge
Confirmation Order
Parameter Update Order
Message Encryption
Mode Order
CDMA Reverse Channel Messages
Reverse Traffic Channel
Access Channel
Registration Msg
Registration Msg
Connect Order
Origination Msg
Authentication Challenge
Response Msg
Origination Continuation
Msg
Flash With Information
Msg.
Page Response Msg
Base Station Challenge
Order
Authentication Challenge
Response Msg
Data Burst Msg
Status Message
Pilot Strength
Measurement Msg.
Handoff Completion Msg
Base Station Challenge
Order
SSD Update
Confirmation Order
Long Code Transition
Request Order
Status Message
Initial Datafill
• Initial Datafill Values
• Since a significant portion of an optimization effort is devoted to system
parameters, every effort should be made to begin with a datafill that
incorporates the experiences gained from multiple systems.
• The datafill given in the following slides is typical of current systems and
can serve as a reliable starting point for an optimization effort in a new
system. 800/1900 MHz. parameters should be adjusted appropriately.
• BTS Calibration-Related Datafill
• Many of the datafill values are outputs from the BTS calibration process
(e.g. TXAttenNormal). Ensure that these are present in the datafill before
optimization begins.
CDMA Channel Parameters: Typical Values
Parameter Name
Range
Recommended
Remarks
Value
1. CDMA Channel Parameters
System Determination and Acquisition
0-1
CDMA_ AVAIL
0 - 2047
CDMA_CHAN
0 - 31
BAND_CLASS
IS95A+TSB74 or J-STD-008
OTAProtocolId
System Acquisition (Sync channel Information)
0-255
P_REV
0-255
MIN_P_REV
0 - 32,767
SID
0 - 65,535
NID
0 - 511
PILOT_PN
TFU_1 or TFU_2
SYS_TIME
0-3
PRAT
0-255
LP_SEC
LTM_OFF
0-63
DAYLT
0-1
1
283 or 384
IS95A+TSB74
283 for A band and 384 for B band
0 800 MHz
800 MHz
3 for 800 MHz
1 for 800 MHz
As determined by the local Operator
See Remarks
As determined by the local Operator
See Remarks
As determined by the local Operator
See Remarks
As detetermined by the System time
See Remarks
1 Half Rate (4800)
13
Offset of the local time in units of 0.5hr from
the System time (GPS)
Operator specific
Depending on whether Daylight saving is
On/Off
0 or 1
CDMA Access Parameters: Typical Values
2. Access Parameters
Request Response Parameters
PSIST(0-9)
0 - 63
0 ACCOLC(0 -9) are all permitted to transmit
PSIST(10-15)
MAX_CAP_SZ
PAM_SZ
REG_PSIST
MSG_PSIST
PROBE_PN_RAN
ACC_CHAN
ACC_TMO
PROBE_BKOFF
BKOFF
MAX_REQ_SEQ
MAX_RSP_SEQ
AUTH
RAND
0 ACCOLC(10 -15) are all permitted to transmit
2 5 Frames message
3 4 Frames preamble
0
0
0
0 1 Access channel
3 (2+3), 400 ms
0 (0 + 1) slot delay
1 (1 + 1) slot delay
2 Two probe sequences
2 Two probe sequences
0 No standard Authentication
0 Not applicable without Authentication
0-7
0-7
0 - 15
0-7
0-7
0 - 15
0 - 31
0 - 15 (x80 ms)
0 - 15
0 - 15
0 - 15
0 - 15
0-3
0 - (232-1)
CDMA Access Parameters: Typical Values
2. Access Parameters
Request Response Parameters
PSIST(0-9)
0 - 63
0 ACCOLC(0 -9) are all permitted to transmit
PSIST(10-15)
MAX_CAP_SZ
PAM_SZ
REG_PSIST
MSG_PSIST
PROBE_PN_RAN
ACC_CHAN
ACC_TMO
PROBE_BKOFF
BKOFF
MAX_REQ_SEQ
MAX_RSP_SEQ
AUTH
RAND
0 ACCOLC(10 -15) are all permitted to transmit
2 5 Frames message
3 4 Frames preamble
0
0
0
0 1 Access channel
3 (2+3), 400 ms
0 (0 + 1) slot delay
1 (1 + 1) slot delay
2 Two probe sequences
2 Two probe sequences
0 No standard Authentication
0 Not applicable without Authentication
0-7
0-7
0 - 15
0-7
0-7
0 - 15
0 - 31
0 - 15 (x80 ms)
0 - 15
0 - 15
0 - 15
0 - 15
0-3
0 - (232-1)
CDMA Handoff Parameters: Typical Values
4. Handoff Parameters
Pilot Search Parameters
PILOT_PN
SEARCH_WIN_A
SEARCH_WIN_N
SEARCH_WIN_R
NGHBR_MAX_AGE
PILOT_INC
NGHBR_CONFIG
0 - 511
0 - 15(4 - 452 PN Chps)
0 - 15(4 - 452 PN Chps)
0 - 15(4 - 452 PN Chps)
0 - 15
0 - 15
0-7
Pilot Strength Parameters
T_ADD
T_DROP
T_TDROP
T_COMP
0 - 63(-0.5x dB)
0 - 63(-0.5x dB)
0 - 15 (=< 0.1 - 319 sec)
0 - 15 (x0.5 dB)
See Remarks
B
B
As determined by the local Operator
6 28 PN chips
130 PN chips
130 PN chips
2
4
0
28
32
3
5
-14 dB
-16 dB
4 sec
2.5 dB
CDMA Registration Parameters: Typical Values
Registration Parameters
SID
NID
REG_ZONE
TOTAL_ZONES
ZONE_TIMER
0
0
0
0
0
MULTI_SIDS
0-1
MULTI_NIDS
0-1
BASE_ID
BASE_CLASS
PAGE_CHAN
MAX_SLOT_CYCLE_INDEX
BACAST_INDEX
HOME_REG
FOR_SID_REG
FOR_NID_NEG
POWER_UP_REG
POWER_DOWN_REG
PARAMETER_REG
0
0
0
0
0
0
0
0
0
0
0
REG_PRD
BASE_LAT
BASE_LONG
REG_DIST
RESCAN
PREF_MSID_TYPE
0 - 127
-1296000, +1296000
2592000, +2592000
IMSI_11_12
MCC
TMSI_ZONE_LEN
TMSI_ZONE
0 - 127
0- 1023
0 - 15
-
32,767
65,535
4095
7
7
- 65,535
- 15
-7
-7
-7
-1
-1
-1
-1
-1
-1
See Remarks
See Remarks
Operator specific
See Remarks
See Remarks
0-1
0-7
Country specific
As determined by the local Operator
As determined by the local Operator
0 Zone Registration not currently supported
0 Zone Registration not currently supported
0 Zone Registration not currently supported
1 If roaming is permitted, this should be set to 1
If roaming or more than one NID in the
1 Operator, set to 1
To be clarified whether this is same as the
CellId
0 Public macro cellular system
1 One paging channel
2
0 Broadcast paging not enabled (i.e. for SMS)
1
1
1
1
1
1
Periodic registration disabled (but can be set
0 as required)
As determined by the local Operator
As determined by the local Operator
0 No distance based registration
0
3 IMSI & ESN
As specified by the local Operator (MNC or
0 00)
Mobile country code of Canada
0 TMSI not currently supported
0 TMSI not currently supported
Pilot Data Base
PilotChannel
CDMA QC Pilot Database Parameters:
Typical Values
Derived by the
system
CDMACenterFrequency
ExtendedBaseId
Available
QuickRepeat
BlankAndBurst
ForwardGain
PilotGain
NeighborList
CellType
IntersystemCellId
MSCID
SyncChannel
SyncGain
PagingChannel
PagingGain
word32
0 -1
0 -1
0 -1
0 - 255
0 - 255
word32Array, up to 20 nieghbors
CELL_STANDARD,
CELL_PILOT_BEACON,
CELL_BORDER
WordArray
MarketId and SwitchNumber
As determined by the Preferred Channel Set
Derived by the system BandClass, CDMAFreq,BASE_ID,Sector
1
0
0 Not used
0
216
As determined by the RF design
See Remarks
CELL_STANDARD
If no HHO in the cell
Operator specific
Operator specific
CellId (BaseId) of the HHO target cell
Global switch ID of the HHO target cell
0 - 255
68 10 dB down from pilot
0 - 255
130 4.4 dB down from pilot
CDMA Wilting, Blossoming, Breathing Parameters
Typical Values
Wilting, Blossoming and Breathing Parameters
WiltBlossStepSize
1/16 - 255/16 dB
WiltBlossStepTime
WiltBlossEnabled
BreathingStepSize
1 - 20 (units of 100 ms)
0-1
1/16 - 255/16 dB
BreathingStepTime
BreathingDelta
BreathingEnabled
1 - 20 (units of 100 ms)
0/16 - 255/16 dB
0-1
RecPowerEstimationFilterRate
RecPowerDecayExponential
TXAttenNormal
TXPowerMax
TXAttenAntenna
MiniBTSToRFFETxCableAtten
TPEFilterDecayExponential
DigitalTxTotalPowerEstimation
Rate
ReverseLinkCapacityEstimatio
nPeriod
ForwardPowerEstimationEnabl
MinPilotToTotalPwrRatio
(ForwardLink)HandoffBlockingT
(ForwardLink)CallBlockingThre
(ReverseLink)HandoffBlocking
(ReverseLink)CallBlockingThre
TXAttenNormal
TXPowerMax
TXAttenAntenna
1 - 10 (units of 20 ms)
0 - 16
0/16 - 1120/16
528/16 - 736/16 dBm
0/16 - 96/16 dB
0/16 - 179/16 dB
0 - 16
192/16
See Remarks
688/16
See Remarks
See Remarks
1 - 200 sec
1 - 20 (units of 100 ms)
0-1
-255/16 to 0/16 dB
0 - 4194303 (inits of bits square)
0 - 4194303 (inits of bits square)
0 - 100 (%)
0 - 100 (%)
0/16 - 1120/16
528/16 - 736/16 dBm
0/16 - 96/16 dB
TBA
TBA
TBA
TBA
See Remarks
688/16
See Remarks
2 Rate should be 1 dB/sec
100 msec, TBC whether the step size unit is
1 100 or 5 ms
1
2 Rate should be 1 dB/sec
100 msec, TBC whether the step size unit is
1 100 or 5 ms
12 dB rise over the noise floor
0 Disabled
40 msec, TBC whether the step size unit is 20
2 or 5 ms
6
As given by the installtion & calibration
43 dBm(20 W)
As measured at the site
As measured at the site
5
This parameter is not currently settable, TBC
2 2 or 5
200 msec, TBC whether the step size unit is
2 100 or 5 ms
1 Enabled
-8 - 8 dB Ec/Ior
Not supported
Not supported
As given by the installtion & calibration
43 dBm(20 W), TBC
As measured at the site
Introduction to Types of Parameters
•
Types of Parameters
– IS-95/J-STD-008 vs. Qualcomm Specific
– Global/Sector Specific/BSC/BTS
The distinction between these definitions is important and not necessarily obvious:
• Global parameters apply to the whole system (one BSC/MTX)
• Sector specific parameters apply to specific sectors. If a mobile is in soft handoff
with two sectors containing different values for a given parameter, there are
parameter-specific rules for which value will be used:
– 1. For SRCH_WIN_A the mobile will use the widest value.
– 2. For SRCH_WIN_N the mobile will use the TBD value.
– 3. For SRCH_WIN_R the mobile will use the TBD value.
– 4. For T_ADD, T_DROP mobile will use highest (least negative).
– 5. For T_TDROP, the mobile will use the maximum value.
– 6. For T_COMP, the mobile will use the minimum value.
– 7. For ForwardGain, QuickRepeat the mobile will use TBD value
Types of Parameters: BTS, BSC
• Some values are repeated at the BTS and BSC:
– Values in the BTS datafill appear on the paging channel.
– Values at the BSC apply to the traffic channel. However, until the
Traffic System Parameters Message is implemented, it is not possible
to update the mobile settings during a call. Therefore, while
parameters such as search windows, T_ADD, T_DROP etc. may be
set per sector at the BSC, the mobile will currently only use the values
it gets from the paging channel. For example, a call originated in a
rural cell with a large SRCH_WIN_A that is subsequently carried into
a downtown cell with a smaller window setting will not update its
search window until the call is released and the mobile monitors the
paging channel again.
Types of Parameters:
Formation of Composite Neighbor Lists
• Neighbor lists are a special case. As with other parameters, the settings at
the BTS are used on the paging channel. Since the mobile is only locked
to one sector at a time during idle, its neighbor search procedure will use
only the neighbor list from that sector. Once on a traffic channel, the
mobile will generate a composite neighbor list (up to a maximum of 20
entries) with the following priorities:
– 1.
Any pilots that have recently been dropped from the active
list but have not yet exceeded NGHBR_MAX_AGE.
– 2.
The neighbor list as received on the most recent Neighbor
List Update Message from the BSC (although any pilots from 1.
above will not be repeated).
• Note that, even if the Neighbor List Update Message contains 20 entries,
the mobile will give priority to pilots defined in 1. above, so all 20 might
not be used.
Types of Parameters
Further Neighbor List Priorities
•
The neighbor list received from the BSC is itself a composite (up to a maximum
of 20 entries) of the neighbor lists of the sectors named in the most recent Handoff
Completion Message. The following priorities are used:
– 1.
The neighbor list of the first pilot in the Handoff Completion
Message.
– 2.
The neighbor list of the second pilot in the Handoff Completion
Message (although any pilots from 1. above will not be repeated).
– 3.
The neighbor list of the third pilot in the Handoff Completion
Message (although any pilots from 1. or 2. above will not be repeated).
– 4. ...and so on until all the pilots have been used or the list has the
maximum of 20 entries.
Setting Parameters
• Setting Parameters
• Some parameter changes may be made using
a set script while others require a download.
Beware that changes made earlier using a set
script will be lost each time a download is
performed. If the change needs to be made
permanent, the download files also must be
changed.
Generating Initial Neighbor Lists
Initial Neighbor List Generation
• The initial neighbor lists for a new system or portion of a system can be
generated as follows:
– 1.
In Planet, set the EIRP values in the site database to be
consistent with the pilot power and generate an equal power
boundaries plot.
– 2.
For each sector, create a neighbor list consisting of the
sectors with which it shares a common boundary.
– 3.
Prioritize the list according to the boundary length (longest
boundary first).
• Do not be tempted to add more distant sites to the neighbor list The
objective is to keep the neighbor lists to the minimum length and hence
reduce search times
PN Planning Issues Survey
Co-PN Interference
• PN Planning
– Before optimization begins, it is assumed that a PN plan has already
been set up. The focus now is on possible glitches and bugs in that
plan.
– The symptoms in the diagnostic data will be explained in a later
section. However, we will not look at some problem mechanisms.
• 1.Co-PN
– To avoid "Co-PN" interference with the serving cell, the minimum
cellsite spacing (in km) should be:
min spacing = (SRCH_WIN_A)/(2 x 3.3 x 1.2288) + 2R
– where R is average cell radius in region of interest and
SRCH_WIN_A is expressed in chips.
More Co-PN Interference Mechanisms
• 2. Another possibility for "Co-PN" interference is if PN1 is in the mobile's
neighbor list but some energy from a distant re-use of PN1 falls inside the
neighbor search window. The "correct" local PN1 will be put in the
active list. If the "false" PN1 subsequently becomes one of the 3 strongest
multipaths, the mobile will center an active search window on it and try to
demodulate it resulting in forward link interference.
• 3. A third "Co-PN" possibility is if "false" PN energy arrives at the mobile
that matches an active set PN that is not currently being demodulated. If
the "false" PN subsequently becomes one of the 3 strongest multipaths,
the mobile will center an active search window on it and try to
demodulate it resulting in forward link interference.
• 4.A fourth "Co-PN" possibility is if "false" PN energy arrives at the
mobile that matches an active set PN that is currently being demodulated.
This will only cause interference if the "false" PN energy falls inside the
active window for that PN.
Adjacent PN Interference
•
Adjacent PN (ie. PN-PILOT_INC)
– Problems first occur with cellsite spacings in the "ring" defined by the
following distances (in km). If any distant pilot is interpreted as one of the
pilots in the mobile's neighbor list, the local site will be added to the active
list, even if not required.
outer radius = ((PILOT_INC x 64)/(3.3 x 1.2288)) + ((SRCH_WIN_N)/(2
x 3.3 x 1.2288)) + 2R
inner radius = ((PILOT_INC x 64)/(3.3 x 1.2288)) - ((SRCH_WIN_N)/(2
x 3.3 x 1.2288))
– However, this will not cause demodulation problems unless the "false" PN
becomes one the strongest three multipaths and a rake finger is assigned to it
(note that, unless the mobile has already assigned a finger to that PN from the
(correct) local PN, it will center it's active window on the "false" PN.
Therefore, the equations above are correct in having the SRCH_WIN_N and
not SRCH_WIN_A).
Demodulation Problems from
Adjacent-PN Interference
– For the mobile to actually try and demodulate a
distant "false PN" that is already being
demodulated correctly from a local site, that site
would have to fall inside the active search
window. When considering the serving cell, the
distances become:
outer radius = ((PILOT_INC x 64)/(3.3 x 1.2288)) +
((SRCH_WIN_A)/(2 x 3.3 x 1.2288)) + 2R
inner radius = ((PILOT_INC x 64)/(3.3 x 1.2288)) ((SRCH_WIN_A)/(2 x 3.3 x 1.2288))
Categorizing System Access Failures
•
If the radio link fails prior to the mobile sending the Service Connect Complete
Message then it is considered a failed access attempt. Failures can be classified
into one of the following categories:
– 1) Access probes exhausted (not received by system)
– 2) Access probes exhausted (seen by system but can not reaching mobile)
– 3) Ack received by mobile but Channel Assignment Message not seen
– 4) Channel Assignment Message seen at mobile but mobile does not acquire
forward traffic channel
– 5) Mobile acquires forward traffic channel but system does not acquire
reverse tch
– 6) System acquires reverse traffic channel but Service Connect Message is
not seen at mobile.
Causes of Access Failures
•
•
Category 1. is likely due to coverage problems while the remainder are likely due
to the mobile being restricted to one pilot during access (any other pilots are
effectively interferers).
– Create a file containing the latitude, longitude and some dummy value for all
of the category 1. failures. Import this file into Planet as a survey and overlay
on a coverage prediction. Check that this type of access failure is not
happening in areas of solid coverage (suspect a problem at one of the sites if
this appears to be the case).
– Create a second file containing the latitude, longitude and some dummy value
for the rest of the failures. Import this file into Planet as a survey and overlay
on an equal power boundaries prediction. This type of access failure is likely
to happen in the handoff regions at the cell boundaries. If it seems to be
happening in isolated coverage, suspect a problem at the site.
Access Parameters can be changed to minimize access failures
– (INIT_PWR, MAX_REQ_SEQ, MAX_RSP_SEQ, PWR_STEP, NUM_STEP,
MAX_CAP_SZ, PROBE BKOFF, BKOFF)
Categorizing Dropped Calls
Dropped call analysis can consume a considerable amount of time. Using the
dropseek and cdmastat tools, the root cause of some of the drops can be
determined without the need to use the selector logs. However, most will
require deeper investigation. There is no substitute for thorough
knowledge of the air interface and IS-95.
• If the radio link fails after the mobile sends the Service Connect Complete
Message then it is considered a dropped call. Using the symptoms
described in later, separate the dropped calls into the following categories:
– 1.
Coverage related
– 2.
Forward link interference
– 3.
0ptimizable e.g. slow handoff, search window, coverage
control, PN plan related.
– 4.
Other.
Causes of Dropped Calls
•
•
•
Create a file containing the latitude, longitude and some dummy value for all of
the category 1. failures. Import this file into Planet as a survey and overlay on a
coverage prediction. Check that this type of dropped call is not happening in areas
of solid coverage (suspect a problem at one of the sites if this appears to be the
case).
Create a second file containing the latitude, longitude and some dummy value for
the category 2 failures. Import this file into Planet as a survey. Assess the call drop
locations against possible interference sources (e.g. other CDMA systems on the
same frequency, intermodulation from non-co-located AMPS sites, co-channel
interference from AMPS sites in the CDMA channel, microwave radio links,
cellsites for other PCS technologies).
For category 3, apply the solutions described elsewhere in this chapter for the
different types of dropped calls.
Handoff Problems
Role of RF Coverage
• Controlling the coverage area of individual cells is crucial to good CDMA
performance. Antenna patterns, orientations and tilts should be used to
confine cells to their intended coverage area. If this is not done, the
system will be prone to the following problems:
• Excessive Handoff:
– 4, 5 and 6 way handoff can be beneficial with many pilots but no
dominant server, or in a rapidly changing environment. The pilots not
currently being demodulated serve as hot standbys to which rake
fingers can be assigned very quickly. However, in order not to
compromise capacity (both forward and reverse link), this should be
the exception rather than the rule.
Handoff Problems
Additional Failure Mechanisms
• Window dropped calls: Pilots seen some distance outside their intended
coverage area may cause dropped calls for two reasons:
– 1) If distant pilot is strong enough to cause interference to a call in
progress, but mobile’s timing reference is from a local cell, the
distant pilot will be outside SRCH_WIN_N and hence it will not be
recognized and added to the active set. The call may drop.
– 2) If the mobile originates on the distant site and hence uses it as its
timing reference, the local sites will be invisible if they are outside
SRCH_WIN_N and the call will drop as soon as one of the local sites
becomes strong and causes interference.
• slow handoff dropped calls: Since, for a given SRCH_WIN_A setting, the
number of active pilots has a big influence on search time, there is an
increased chance of a new, strong pilot not being detected quickly enough.
Finding Causes of Excessive Handoff:
Generating Plots to Identify Sources
•
Survey data can be loaded into Planet to generate plots that will indicate which
sites are candidates for antenna configuration changes. The source data used
should represent one entire drive of the system/cluster:
– 1.Use the multimfa and mfa2bsv scripts to generate a strongest finger file for
every sector in the system/cluster. Each of these files contains the lat/long of
every location in the system/cluster where the mobile’s strongest rake finger
was assigned to that sector. The value to be plotted is the Ec/Io of that finger.
Use contour settings of -16 to -8 with a step of 2 and generate plots for one
sector at a time.
– 2.Use the multitx script to extract the lat/long and mobile transmit power
from the eva files. Use contour settings of -12 to +18 with a step of 10 and
generate one plot for the entire system/cluster.
– 3.The multistat script will generate files containing lat/long and handoff state.
Use contour settings of 1 to 5 with a step of 1 and generate one plot for the
entire system/cluster.
Finding Causes of Excessive Handoff:
Analyzing Plots to Identify Sources
•
Examine the plot for each sector in turn and determine whether the coverage is
excessive by drawing a curve joining the surrounding sites and assessing how
often the site is seen as the strongest server outside this line.
– If more than approximately 15% of the points are outside the line, a change is
required.
– If the problems occur along the main beam of the antenna, a downtilt alone
should be sufficient.
– If there are problems along the edge of the antenna beam, also consider a
narrower beamwidth antenna and/or a re-orientation. Do not try to remove
signal from areas where the mobile transmit power is above 18dBm.
– To decide on the exact changes, use single cell signal plots in Planet and
experiment with antenna changes. A signal level reduction of around 8dB in
offending areas is the minimum that should be considered before
implementing the changes and re-driving the area.
– Beware that, as unwanted pilots are removed from an area, the Io is being
reduced and so the next drive of the area may reveal new problem pilots,
making this somewhat of an iterative process.
Soft Handoff Percentage: Some Precautions
•
•
T_ADD/T_DROP parameters are usually set -14 to -16; use extreme caution if
trying to reduce soft handoff percentage by changing these
How much soft handoff is normal? 35% rule-of-thumb?
– If the reverse link coverage has been designed assuming 4dB soft handoff
gain, we can calculate (for a propagation slope of 35 dB/decade) that the last
4dB of the cell radius represents an area of 42%. If this is true, roughly 42%
of the cell area needs to be in soft handoff to ensure complete reverse link
coverage.
– Forcibly restricting soft handoff to 35% using the handoff threshold(s)
(which are forward link parameters and hence somewhat independent of
reverse link coverage) could reduce reverse link performance
– This is a very simplistic calculation; the cell overlap which always occurs in
the real world will help the reverse link; but it illustrates the problems that
might result if soft handoff percentage is arbitrarily forced down using just
T_ADD and T_DROP without watching other consequences.
Search Windows
•
•
The search sequence for mobile that has two
active pilots and a neighbor list of three PNs is as
follows:
– A1, A2, N1, A1, A2, N2, A1, A2, N3, R, A1,
A2, N1,
The worst case search time to find a new neighbor
therefore can be generalized as:
– Search Time = (NN x (TN + (NA x TA))) + TR
– Where NA, NN are the number of actives and
neighbors respectively. TA, TN and TR are the
search times for the SRCH_WIN_A/N/R
windows. Minimizing the search time is
crucial to CDMA performance, both to avoid
dropped calls and to maximize capacity. Next,
we show how to establish a minimum search
window that is consistent with the
propagation environment within the
system/cluster.
SEARCH TIME AS A FUNCTION
OF WINDOW SIZE
Window
Datafill Max Delay Search Time
Size (Chips) Value
(uS)
(mS)
14 (7)
4
5.7
19
20 (10)
5
8.1
15
40 (20)
7
16.3
12
60 (30)
8
24.4
18
80 (40)
9
32.6
19
100 (50)
a
40.7
25
130 (65)
b
52.9
30
160 (80)
c
65.1
40
226 (113)
d
92
54
Notice that the curve fluctuates at the lower
settings and 28 chips is the smallest
reasonable setting.
Setting Search Window Size: SRCH_WIN_A
•
Setting SRCH_WIN_A
– Collected .mje files contain a histogram of the maximum mobile finger
separation for fingers locked to one pilot only. This is the basis for setting the
optimum value for SRCH_WIN_A.
– Gather all the .mje files for one entire network/cluster run.
– Classify the areas by average cell size (two regions should be sufficient i.e.
small?(dense) urban cells and average suburban/rural cells).
– Use one of the tools to generate an overall combined histogram. Evaluate the
histogram against the max Delay column in the above table and choose a
window size that will capture 99.9% of the finger separations.
– Check the shape of the histogram to ensure that the existing window setting is
not already too small (i.e. is there a sharp cutoff at the current window
setting). In this case, more data will have to be collected with a wider window
setting before a proper judgment can be made.
Setting Search Window Size for other Sets
• Setting SRCH_WIN_N
– This setting should be based on the spread observed in Neighbor set
records and will be much more dependent on cell size
– Three settings likely will be needed for downtown/suburban/rural
areas. A good starting point would be 80/130/226 chips
• Setting SRCH_WIN_R
– This setting should be based on the spread observed in Remaining set
records and will be much more dependent on cell size
– Three settings likely will be needed for downtown/suburban/rural
areas. A good starting point would be 80/130/226 chips
Neighbor List Considerations
• Use the program to generate a per-site histogram of all the handoff
transitions requested by the drive test mobiles over a complete drive of
the system/cluster.
• For each sector, examine the statistics in conjunction with the Planet equal
power boundaries plot. Consider removing any pilots that are currently in
the neighbor list but have less than 1% of the handoff transitions.
However, make sure that is not a consequence of the drive routes (for
example, do not remove adjacent sectors of a sectored site).
• Consider adding pilots that are not currently in the neighbor list but have
greater than 5% of the handoff transitions. Remember, though, that the
goal is to keep neighbor lists to a minimum (see below) so avoid adding
sites that are obviously not immediate neighbors of the serving cell (i.e.
try to make use of the composite neighbor list as much as possible).
Neighbor List Example
•
•
Referring to the figure, even if cell C appears as a handoff
candidate in the right half of cell A (shaded), it would not
be put in the neighbor list.
– The assumption is that, if C is a handoff candidate then
B certainly will be (if this is not true, there are bigger
problems to be addressed first).
– C will be in B neighbor list and so the composite
neighbor list sent to the mobile while in the right half
of A will contain C. The advantage occurs when the
mobile is in the left half of cell A since the mobile will
not waste search time looking for cell C.
Obviously the above diagram is very idealistic. Due to site
selection issues and the topology of a real environment,
many unusual cell arrangements will be found in the field.
However, it illustrates the principle that, in order to reduce
the search time to find new pilots, the aim should be to
minimize neighbor lists.
A
B
C
Per-Site Analysis
•
•
Analysis of data while locked to a single pilot can reveal configuration or
performance issues of a particular site or sector.
The script per_cell_data calls several awk scripts that extract per-site data and
outputs two files:
– A file containing the average transmit gain adjust on a per-site basis for all
sites in a run. This will highlight any sites having a significantly different link
balance. The file contains both the average and the number of points over
which that average was calculated so that many such files can be
concatenated , imported into Excel and a numerically-correct average recalculated over many runs. A site with a high transmit gain adjust may be
suffering from a poor receiver noise figure or may be in a location where
forward link interference is prevalent. A site with an abnormally low transmit
gain adjust may have a low transmit power.
– A file containing all lines from the eva file for which the number of pilots
locked = 1. A column containing the PN is also added. This will allow
additional performance analysis on a per-site basis e.g. FER, traffic channel
gain, Ew/No setpoint, finger separation.
Successful Call Records
•
•
Characteristics of data if phone originates successfully, remains in service area,
completes a handoff, makes normal release:
– If only mobile data is available, the eva files will show:
• Receive power > -100dBm; Transmit power < +20dBm
– Normal Transmit Gain (actual value due to configuration & settings)
– Low forward FER, Good Ec/Io (> -10dB)
Messaging will be reliable. In LPAR output, a basic call (originated and released
from the mobile) will contain the following elements :
– Origination message (sent to MTX on Access Channel).
– Channel Assignment (received from MTX on Paging Channel).
– Service Connect Message (received from SBS on traffic channel).
– Service Connect Complete (to SBS - if exists, origination successful)
– Pilot Strength Measurement Message (sent to SBS - initiates handoff).
– Extended Handoff Direction Message (from SBS - directs handoff )
– Handoff Completion Message (sent to SBS - confirms receipt EHDM)
– Neighbor List Update Message (from SBS - new comp. Neighbor List)
– Release Order (sent to SBS, or received from SBS)
Successful Call
Analysis with Selector Logs
• Using the full eva files, the following can be established:
– Receive power > -100dBm, Transmit power < +20dBm
– Normal Transmit Gain Adjust (actual value depends on site
configurations, loading, NOM_PWR setting)
– Ew/No setpoint below maximum (13dB at time of writing)
– Traffic Channel gain below maximum (192 (-1dB pilot) for rate set 2
at time of writing)
– Low forward FER, Low reverse FER (either full or all rate)
– Good Ec/Io (> -12dB)
• The sorted message list for the same basic call will contain the following
IS-95 and NOIS messages:
– Message Type
Source Destination Logged at Purpose
– Origination
IS95
Mobile MTX
Mobile
Loss of Coverage: Call Drop
Mobile Data Characteristics
• Characteristics of data when phone is taken out of the service area or
taken into a coverage hole.
• If only mobile data is available, the eva files will show:
– Low receive power (<-100dBm), High transmit power (> +20dBm)
– Higher than normal Transmit Gain Adjust (actual value depends on
site configurations, loading, NOM_PWR setting)
– High forward FER, Low Ec/Io (< -10dB)
Loss of Coverage: Call Drop
Messaging Data Characteristics
•
Under these conditions, messaging will be unreliable at best and will likely be the
actual cause of the drop. The LPAR output may show some or all of the following:
– Repeats of the same message (check msg seq and ack seq #s)
• If a reverse message is repeated because an ack is not received, either it is
not getting to the selector (reverse link is worse) or the fwd ack is not
reaching the mobile (fwd link is worse). The eva files may show which
but ideally selector logs are required.
– If a fwd message is repeated then the rvs ack is not reaching the selector
(reverse link is worse).
– If the mobile repeats a message 3 times without seeing the ack, it will tear the
call down and will go to the sync channel.
– If the selector repeats a message 5 times without seeing the ack, it will send a
forward release and the call will be torn down. If the mobile sees the release,
it will respond with a reverse release and stop transmitting. Otherwise, it will
timeout when no good frames are received for TBD secs.
Forward Link Interference
Analysis with Mobile Data Only
• Characteristics of data for phone experiencing forward link interference
from a source outside the CDMA system:
– Some likely sources of interference are: intermodulation from an
AMPS-only BTS, co-channel interference from an AMPS BTS that
has not been cleared from the CDMA channel, an adjacent CDMA
operator using the same channel (this will remain until inter-system
soft handoff is available), a microwave link, raised noise floor due to
the transmitter spectrum of another CDMA operator in the same
market but on a different channel.
• If only mobile data is available, the eva files will show:
– Good receive power (> -100dBm), Normal transmit power (<
+20dBm)
– Higher than normal Transmit Gain Adjust (actual value depends on
site configurations, loading, NOM_PWR setting)
– High forward FER, Low Ec/Io (< -10dB)
Forward Link Interference
Analysis of Messaging
• Under these conditions, forward link messaging will be unreliable at best
and may be the actual cause of the drop. The LPAR output may show
some or all of the following:
– Repeats of the same message (check that the msg seq and ack seq
numbers are the same to ensure it is the same message).
– If a reverse message is repeated because an ack is not received, either
it is not getting to the selector (reverse link is worse) or the fwd ack is
not reaching the mobile (fwd link is worse). The eva files may
indicate which is happening but ideally selector logs are required.
– If a fwd message is repeated then the rvs ack is not reaching the
selector (reverse link is worse).
– If the mobile repeats a message 3 times without seeing the ack, it will
tear the call down and will go to the sync channel.
– If the selector repeats a message 5 times without seeing the ack, it will
send a forward release and the call will be torn down. If the mobile
sees the release, it will respond with a reverse release and stop
transmitting. Otherwise, it will timeout when no good frames are
received for TBD secs.
CDMA Default Channel Allocation Order
Allocation
Order
800 MHz
System A
800 MHz
System B
1900 MHz 1900 MHz 1900 MHz 1900 MHz 1900 MHz 1900 MHz
System A System B System C System D System E System F
1
283
384
25
425
925
325
725
825
2
242
425
50
450
950
350
750
850
3
201
466
75
475
975
375
775
875
4
160
507
100
500
1000
n/a
n/a
n/a
5
119
548
125
525
1025
n/a
n/a
n/a
6
78
589
150
550
1050
n/a
n/a
n/a
7
37
630
175
575
1075
n/a
n/a
n/a
8
1019*
777*
200
600
1100
n/a
n/a
n/a
9
691
736*
225
625
1125
n/a
n/a
n/a
10
n/a
n/a
250
650
1150
n/a
n/a
n/a
11
n/a
n/a
275
675
1175
n/a
n/a
n/a
CDMA 800 MHz. channel numbers are same frequencies as AMPS/D-AMPS channel numbers.
CDMA 1900 MHz. channel numbers are spaced 50 KHz. apart starting at the lower edge of the PCS
band. Channel 0 is 1850.0 MHz. on uplink, 1930.0 MHz. on downlink.