Course 312
Propagation
Propagation Drive-Testing:
Drive-Testing:
Analyzing
Analyzing Data
Data and
and Results
Results
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 1
Outline
■ Introduction: Purposes of Drive Testing
■ Practical Considerations
• How much driving is necessary?
• Is the drive test equipment link budget critical?
• Pitfalls in conducting the drive tests and gathering data
• Data file formats
■ Introduction to Propagation Models
■ Analyzing Drive Test Data for tuning Propagation Models
• Manual analysis for model tuning
• Automated analysis and model tuning in prediction tools
■ Considerations for Choosing between Competing Sites
■ System Design Topics
• Statistical Probability of Service, Fade Margins, Building
Penetration, Link Budgets
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 2
Introduction to Drive Testing
Drive testing is normally conducted for two related but distinct
purposes:
PROPAGATION MODEL DEVELOPMENT
Early in the design of a system, it is necessary to determine the
required number of sites and to identify reasonable height and
antenna parameters which will become the default standards
for most sites. During this time, test runs are made on several
dozen typical locations to get enough data to build propagation
models. If some of these sites later are found to be usable,
that’s great, but that’s not the main concern at this early stage.
SELECTION OF SPECIFIC COMPETING SITES
During actual construction of the system, and during any future
expansions, each site will have from one to several possible
locations. These must be compared so that the best sites are
selected for actual use, and any coverage and interference
problems identified and dealt with before committing to
construct.
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 3
Planning Drive Test Routes
■ The total linear distance which should be driven depends on how
accurately the cell’s size is desired to be known.
• If a 10% uncertainty in the radius of coverage is acceptable,
the total drive test route length should be approximately 10
times the anticipated radius of coverage of the cell. If a 20%
uncertainty in cell radius is acceptable, you need drive only
about 5 times the estimated cell radius in miles.
• If multipath fading is mild, the required distance is reduced by
perhaps another 20%, and if severe, it is increased.
• The drive test route shape is not highly critical, but should
obviously include all routes which are expected to be major
traffic origination points.
■ For a highly detailed analysis of drive route planning, see “On the
Sampling Requirements of a Cellular Drive Test” by Bernardin,
Yee, and Ellis, contained in IEEE Transactions on Antennas and
Propagation Special Issue on Wireless Communications (1997).
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 4
Coverage Parity and Link Budget Issues
■ Although the parameters of the test transmitter, receiver, and
antennas are not required to exactly mimic the actual wireless BTS
and subscriber sets, it is required that the overall link budget (and
therefore the overall coverage obtainable with the test setup)
should be the same or larger than the wireless system.
• This is easily achieved with the Grayson Electronics system.
• Special consideration is practically needed only when the
wireless system will use extremely directional antennas. In this
case, special attention is required with two objectives:
– to be sure the selected wireless antennas do not have
patterns so sharp that signal levels will be degraded in
parts of the desired coverage area, and
– to be sure the selected test transmitter antenna has
enough gain to keep the test link budget appropriately
large.
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 5
Pitfalls in Test Site Selection and Setup
■ Practical Logistics
• Accessibility: Setup, Teardown, and Verification when needed
– can you get in when you wish? delays?
• Reliability of Power
– inadvertent disconnections or failures?
• Physical Security of Antenna, Transmitter
– can anything fall, blow away, or be stolen?
■ Safety
• Electrical Safety: Power lines, etc.
• Physical Obstacles, tripping, and Falling Equipment
■ Representativeness
• Is the test antenna location truly representative of the
performance expected in a permanent installation?
■ Antenna Location Issues (obstructions, near-field problems)
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 6
Near-Field/Far-Field Considerations
■ Antenna behavior is very different close-in and far out
■ Near-field region: the area within about 10 times the
spacing between antenna’s internal elements
• Inside this region, the signal behaves as
independent fields from each element of the
antenna, with their individual directivity
Near-field
■ Far-field region: the area beyond roughly 10 times the
spacing between the antenna’s internal elements
• In this region, the antenna seems to be a
point-source and the contributions of the
individual elements are indistinguishable
• The pattern is the composite of the array
■ Obstructions in the near-field can dramatically alter the
antenna performance
When choosing a rooftop location for a test antenna,
ensure that there are no major obstructions or
reflecting objects in the near-field of the antenna
in directions of significant radiation
Course Series 300 -- CDMA Drive Test & System Optimization
Far-field
Issue 0.01 (April 7, 1997) Page 7
Local Blockage And Obstruction At A Site
■ Obstructions near the site are
sometimes unavoidable
■ Near-field obstructions can
seriously alter pattern shape
■ More distant local obstructions
can cause severe blockage, as
for example roof edge in the
figure at right
• Knife-edge diffraction
analysis can help
estimate diffraction loss in
these situations
• Explore other antenna
mounting positions
Course Series 300 -- CDMA Drive Test & System Optimization
Local obstruction example
Diffraction
over
obstructing
edge
Issue 0.01 (April 7, 1997) Page 8
SpectrumTracker Log File Format Example
D199511018
m580949RCRD000001000000001000000000200300000.10000401
R58096809314375121120118
G580968-079.251374+37.3693301959130465
R58101609314375121120119
R595960093143751211201 19
G595960-079.251374+37.3693302001030465
D1995110119
R00000809314375121120119
R00005609314375121120119
G000056-079.251374+37.3693302001040465
R03524009314375121 120118
G035240-079.251374+37.3693302004560465
R03528809314375121120119
m035313IDLE000001000000001000000000200300000.10000401
M050053N000000000
M050069N000000001
Course Series 300 -- CDMA Drive Test & System Optimization
SpectrumTracker log files
use an ASCII format with
each line representing a
record of some type of
information. The first
character of each line is a
code identifying the record
type and field structure for
the rest of the line.
Successive fields of the
line can be interpreted and
decoded using the tables
in the following slides.
Issue 0.01 (April 7, 1997) Page 9
Decoding Log File Contents
First
Char.
Field Description (length)
Example
R
Time (6), Frequency (8),
RSSIMIN (3),RSSIAVG (3),RSSIMAX (3)
R23451209319375125...9980
r
Time (6), Frequency (8),
Instantaneous RSSI (3)
r02597619000300 78
M
Time (6), Marker Type (1),
Marker Description (9)
M005632N000000001
G
Time (6), Longitude (11),
Latitude (10), UTC (6), Altitude (4)
G023355-098.989889
+45.1234340423560400
D
Date and Hour (10)
D1994120901
m
Time (6), Mode (4), Dwell time (9),
m48427IDLE000005000000000020
Wait time (9), RSSI time (9),
000000010200040.00000201
RSSI type (1), Distance (8),
wavelengths (5), GPS stamp (1)
4-character Mode Codes
RCRD Record
CUST Custom Scan
BAND Band Scan
IDLE
■ In a SpectrumTracker file, the first character
of each line identifies the type of record and
the field structure as shown in the table
above.
Idle
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 10
Field Codes and Formats
Data Field
Length
Template
Time
6
mmsshh
Frequency
6
gmmmkkkhhh
RSSI
6
rrr
Marker type
6
-
Latitude
6
+/-dddmmmmmm +/- = N/S, ddd=degrees, m=decimal fraction
Longitude
6
+/-ddmmmmmm +/- = E/W, dd=degrees, m=decimal fraction
UTC
6
hhmmss
hh = hours, mm = minutes, ss = seconds
Altitude
6
mmmm
mmmm - meters 0000-9999
Date
6
yyyymmddhh
Mode
6
-
Dwell, Wait, or RSSI
Time
6
sssssssss
RSSI Type
6
-
Distance
Number of
wavelengths
GPS Stamp
6
mmmmm.mm
6
wwwww
6
-
Course Series 300 -- CDMA Drive Test & System Optimization
Description
mm=minutes, ss=seconds, hh=hundredths
g=GHz, mmm=MHz, kkk=kHz, h=100 Hz digit
rrr = RSSI in negative dBm
N = numeric, other types reserved
yyyy = year, mm = month, dd = day, hh = hour
(see separate mode table)
sssssssss = user-input seconds. 999999999 = inf.
0=dB/log avg, 2=cont 400Hz, 3=wave/dist avg.
Distance in meters if RSSI type is 3
Number of wavelengths if RSSI type is 3
0=time-based, 1=RSSI-based GPS stamping
Issue 0.01 (April 7, 1997) Page 11
Converting and Importing Drive Test Data for
Analysis
■ SpectrumTracker provides a graphic mode for display of collected
date in either time-domain or frequency-domain.
■ SpectrumTracker data can be externally “post-processed” for
other analytical purposes:
• propagation model calibration
• coverage or interference estimation and mapping
■ Post-Processing Techniques
• importing into Excel for manual analysis, manipulation
• manipulation in text editors, Word, etc.
• script files, basic programs, etc., for automatic manipulation
• conversion programs to proprietary formats
• “model fit” analysis features provided in some propagation
prediction tools
• importing to mapping tools for display (MapInfo, others)
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 12
Introduction to Propagation Models
-50
+90
-60
+80
-70
+70
-80
+60
Field
Strength,
+50 dBµV/m
RSSI,
dBm -90
-100
+40
-110
+30
-120
0
3
6
9 12 15 18 21 24 27 30 33
Distance from Cell Site, km
+20
■ Green Trace shows actual measured signal
strengths on a drive test radial, as determined by
real-world physics.
■ Red Trace shows the Okumura-Hata prediction for
the same radial. The smooth curve is a good “fit”
for real data. However, the signal strength at a
specific location on the radial may be much higher
or much lower than the simple prediction.
■ Basic “Area” Models
• mimic behavior of general propagation in an overall area
• no consideration of individual paths, obstructions, reflections
• based on curve-fitting to large amounts of measured data
• Examples: Okumura-Hata, COST-231
■ More Advanced Path-Specific Models
• mimic behavior of propagation between specific points
• include estimates of effects of obstructions, reflections, etc.
• usually implemented in elaborate software for system design
• may use area model techniques, but with additional code to mimic
mechanisms of obstruction, diffraction, and reflection using terrain and
clutter databases and geometric “ray-tracing” principles
• Examples: Planet General Model; NBS101 TIREM, etc.
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 13
Structure of the Okumura Model
Path Loss [dB] = LFS + Amu(f,d) - G(Hb) - G(Hm) - Garea
Morphology Gain
0 dense urban
5 urban
10 suburban
17 rural
Free-Space
Path Loss
Amu(f,d) Additional Median Loss
from Okumura’s Curves
35
Correction factor, Garea (dB)
Urban Area
100
80
50
70
d, km
Median Attenuation A(f,d), dB
70
40
30
26
5
Open
25
area
si o
Qu a
pen
area
20
15
a
ur b
Sub
10
r ea
na
5
2
1
10
30
Frequency f, MHz
100
500
850 MHz
850
3000
Mobile Station
Height Gain
= 10 x Log (Hm/3)
100
200
300
500 700
1000 2000 3000
Frequency f, (MHz)
Base Station
Height Gain
= 20 x Log (Hb/200)
■ The Okumura Model uses a combination of terms from basic physical
mechanisms and arbitrary factors to fit 1960-1970 Tokyo drive test data
■ Later researchers (HATA, COST231, others) have expressed Okumura’s
curves as formulas and automated the computation
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 14
Okumura-Hata and Euro-COST231 Models
Okumura-Hata Model for 800 MHz.
AdB = 69.55 + 26.16 log(F) – 13.82 log(Hb) + ((44.9 – 6.55 log(Hb)) log D) + C
Euro-COST231 Model for 1900 MHz.
AdB = 46.3 + 33.9 log(F) – 13.82 log(Hb) + ((44.9 – 6.55*log(Hb)) log D) + C
Definitions
A = Path loss
F = Frequency in MHz
D = Distance between base station and terminal in km
H = Effective height of base station antenna in m
C = Environment correction factor
Course Series 300 -- CDMA Drive Test & System Optimization
Environmental Factor C
800 1900
0
-2
dense urban
-5
-5
urban
-10 -10 suburban
-17 -26 rural
Issue 0.01 (April 7, 1997) Page 15
The MSI Planet General Model
Pr = Pt + K1 + k2 log(d) + k3 log(Hb) + K4 DL + K5 log(Hb) log(d)
+ K6 log (Hm) + Kc + Ko
Pr - received power (dBm)
Pt - transmit ERP (dBm)
Hb - base station effective antenna height
Hm - mobile station effective antenna height
DL - diffraction loss (dB)
K2 - slope
K1 - intercept
K3 - correction factor for base station antenna height gain
K4 - correction factor for diffraction loss (accounts for clutter heights)
K5 - Okumura-Hata correction factor for antenna height and distance
K6 - correction factor for mobile station antenna height gain
Kc - correction factor due to clutter at mobile station location
Ko - correction factor for street orientation
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 16
Manual Model Tuning
■ Drive test data can be imported into Excel or read by custom
software to perform statistical evaluations for propagation model
tuning.
■ In general, the approach is to compute the sum-squared
differences between the measured data and the signal levels
predicted by the model. The model parameters (slopes,
intercepts, and other parameters if used) are adjusted individually
and in tandem through some search routine, seeking the lowest
possible total sum-squared error. When this is achieved, the
model is considered the best it can be.
■ A good indication of the quality of a model is the standard
deviation of the errors observed. If the standard deviation is better
(lower) than about 8 db., the model is better than most, or the
conditions are unusually “tame”. If the standard deviation is higher
than about 15 db., analyze possible causes before relying on it.
■ Chapter 3 of Rappaport (see bibliography) contains more detail.
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 17
Tuning Propagation Models in PlaNET
■ Parameters of propagation
models should be adjusted for
best fit to actual drive-test
measured data in the area
where the model will be used
■ The figure at right shows drivetest signal strengths obtained
using a test transmitter at an
actual test site in the demo
design area.
■ Planet automates the process
of comparing the measured
data with its own predictions,
and deriving error statistics
■ Prediction model parameters
can be “tuned” to minimize this
error
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 18
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 19
PlaNET’s Log(D) vs. Signal Display
■ Is the propagation model approximately correct?
• Is the data scatter small enough to justify use of a model?
• correct slope to match data
• correct position up/down on Y-axis?
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 20
Histogram Analysis of Differences
■ Planet produces histograms showing the distribution of the
differences between measured and predicted values
■ The mean of the difference between predicted and measured is a
very important quantity. It should be small (on the order of a few
dB).
■ The standard deviation of the difference also should be small. If it is
substantially larger than 8 dB., then either:
• the environment is very diverse
(perhaps it should be broken
into pieces with separate
models for better fit) or
• the slope of the model is
significantly different than the
observed slope of the
measurements (review the Sig.
vs. Dist. graph)
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 21
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 22
PlaNET’s Log(d) vs. Error Display
■ This view helps identify situations where the model slope is wrong
or there are major errors far-out or close-in. In such cases, the
error slope will be non-horizontal and clumps of error will be seen
at specific distances
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 23
Analyzing Error Distribution by Location
■ Suppose a major hill blocked
the signal in one direction, or
the antenna pattern had an
unexpected minimum in that
direction
■ This would cause the data in
the shadowed region to differ
substantially from data in all
remaining directions
■ Planet can display the error
values on a map like the one at
right, to provide quick visual
evidence for identifying this
type of problem
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 24
Drive Testing to Evaluate Specific Sites
■ The last several slides deal with how to optimize propagation
models using drive test data from typical sites. This is the main
use of drive test data during the pre-construction period in most
wireless systems. However, when the system is finally under
construction, and afterwards as new sites are being added to the
existing system, propagation models are no longer the main focus
of the drive test analysis. Instead, we want to know just what a
specific site will cover, what it will not cover, and whether it will
deliver troublesome interference to other areas of the system. The
same data can be used but the analysis takes on a decidedly more
practical tone.
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 25
CDMA Cell Design Philosophy
■ IS-95/J-Std008 CDMA technology exploits soft handoff for diversity gain
and performance improvement. However, the rake receiver in the
subscriber unit has only three “fingers”. This implies that if more than
three comparable RF signals are available, only the best three can be
“harvested” simultaneously (even under ideal conditions), and the rest will
be interferors causing potentially severe degradation of performance.
■ CDMA systems should be engineered to provide a dominant serving
signal from ONE sector, with signals from up to TWO additional sectors
tolerable so long as they are at least a few dB. below the dominant server.
■ Every contemplated site should be considered from this perspective.
• The site is desired to cover its assigned “footprint”, providing a signal
strong enough overcome obstacles and to penetrate buildings and
vehicles. Its coverage area must meet adjoining sites at a usable
signal level.
• The site is desired not to achieve “best server” status outside its
intended footprint. In particular, areas beyond the immediately
surrounding tier of adjoining sites should be almost completely free
from “hot spots” of this site.
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 26
Comparing and Evaluating Competing Sites
■ Tools and Techniques for Comparative Analysis of Test Data
• “Best Server” Plots comparing the competing sites
• “Delta” plots of signal strength comparison
• C/I plots including adjoining sites
■ When comparing competing sites for selection, the following
concerns must be considered:
• Specific dead-spot and penetration problem areas
• Specific hot-spot interference potential by the site as an
aggressor
• Specific interference areas from adjoining sites to the
contemplated site as a victim
• Site cost, aesthetics, availability, accessability, maintainability
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 27
System Design Considerations
Statistical
Statistical Probability
Probability of
of Service
Service
Penetration
Penetration Losses
Losses &
& Fade
Fade Margins
Margins
Link
Link Budgets
Budgets
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 28
Statistical Probability of Service
■ Propagation models and prediction tools usually give “average” or
“most probable” signal strength values as their customary outputs
■ Customers and system operators are more concerned to achieve a
high reliability of service -- a high statistical probability that the
signal at a customer’s location will be equal to or greater than a
defined minimum acceptable value
■ If the statistical variation of measured signals is known compared
against predicted signal levels, reliability of service can be
estimated and even controlled using statistical techniques
■ In practice, an additional margin of signal is allowed to overcome
the statistical likelihood of fading (“fade margin”)
• the margin can include allowances for variations in many
factors -- outdoor loss, penetration loss, multipath cancellation,
etc.
• the margin is applied during system design in the link budget
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 29
Statistical Techniques
Practical Application Of Distribution Statistics
Percentage of locations where
observed RSSI exceeds predicted
RSSI
■ Technique
• Use a model to predict RSSI
• Compare measurements with
model
– obtain median signal strength M
– obtain standard deviation σ
– now apply correction factor to
obtain field strength required for
desired probability of service
10% of locations exceed
this RSSI
50%
RSSI,
dBm
90%
■ Applications: Given
• A desired outdoor signal level
• The observed standard
deviation σ from signal strength
measurements
• A desired percentage of
locations which must receive
that signal level
• Compute a “cushion” in dB
which will yield that % coverage
confidence
Course Series 300 -- CDMA Drive Test & System Optimization
Distance
Occurrences
Median
Signal
Strength
Normal
Distribution
RSSI
σ,
dB
Issue 0.01 (April 7, 1997) Page 30
Area Availability And Probability Of Service
Cell Edge vs. Total Cell Coverage Area
Statistical View of
Cell Coverage
75%
■ Overall probability of service is best close
to the BTS, and decreases with
increasing distance away from BTS
■ For overall 90% location probability within
cell coverage area, probability will be
approximately 75% at cell edge
90%
Area Availability:
90% overall within area
75%at edge of area
Course Series 300 -- CDMA Drive Test & System Optimization
• This result is derived theoretically,
confirmed in modeling with
propagation tools, and observed
from measurements
• Valid if path loss variations are
log-normally distributed around
predicted median values, as in
the mobile environment
• 90%/75% is a commonly-used
wireless numerical coverage
objective
Issue 0.01 (April 7, 1997) Page 31
Application Of Distribution Statistics: Example
■ Let’s design a cell to deliver at least -95
dBm to at least 75% of the locations at
Cumulative Normal Distribution
the cell edge
(This will be 90% of total locations within
100%
the cell)
90%
■ Assume that measurements you have
80%
made show a 10 dB standard deviation
75%
70%
σ
■ On the chart:
60%
50%
40%
30%
20%
0.675σ
10%
0%
-3 -2.5 -2 -1.5 -1 -0.5 0
0.5 1
1.5 2
2.5 3
Standard Deviations from
Median (Average) Signal Strength
Course Series 300 -- CDMA Drive Test & System Optimization
• To serve 75% of locations at the
cell edge , we must deliver a
median signal strength which is
.675 times σ stronger than -95
dBm
• Calculate:
- 95 dBm + ( .675 x 10 dB )
= - 88 dBm
• So, design for a median signal
strength of -88 dBm!
Issue 0.01 (April 7, 1997) Page 32
Statistical Techniques:
Normal Distribution Graph & Table For Convenient Reference
Cumulative Normal Distribution
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
-3
-2.5 -2
-1.5 -1
-0.5
0
0.5
1
1.5
2
Standard Deviation from Mean Signal Strength
Course Series 300 -- CDMA Drive Test & System Optimization
2.5
3
Standard
Deviation
-3.09
-2.32
-1.65
-1.28
-0.84
-0.52
0
0.52
0.675
0.84
1.28
1.65
2.35
3.09
3.72
4.27
Cumulative
Probability
0.1%
1%
5%
10%
20%
30%
50%
70%
75%
80%
90%
95%
99%
99.9%
99.99%
99.999%
Issue 0.01 (April 7, 1997) Page 33
In-Building Testing Perspective
■ In-building coverage originating from outdoor macrocells is usually
characterized in terms of additional “penetration loss” referenced
against the outdoor signal strength at ground level, and a standard
deviation of the penetration loss around the median value
■ A large number of measurement samples is required to adequately
characterize an individual building or class of buildings
■ Obtaining location data (coordinates, etc.) of indoor measurement
locations is more difficult than outdoors due to blockage of GPS
signals indoors, and indoor obstructions to motorized vehicles
• Most operators use walking measurement strategies to sample
virtually all likely customer locations within subject buildings,
and do not attempt to log individual measurement locations
• measurement data for each floor of the building is segregated
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 34
Composite Probability Of Service
Dealing with Effects of Multiple Attenuating Mechanisms
Building
Outdoor Loss + Penetration Loss
σCOMPOSITE = ((σOUTDOOR)2+(σ ENETRATION)2)1/2
P
LossCOMPOSITE = LossOUTDOOR+LossPENETRATION
■ For an in-building user, the actual signal level includes regular outdoor
path attenuation plus building penetration loss
■ Both outdoor and penetration losses have their own variabilities with their
own standard deviations
■ The user’s overall composite probability of service must include composite
median and standard deviation factors
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 35
Composite Probability of Service
Calculating Fade Margin For Link Budget
■ Example Case: Outdoor attenuation σ is 8 dB., and penetration loss σ
is 8 dB. Desired probability of service is 75% at the cell edge
■ What is the composite σ? How much fade margin is required?
σCOMPOSITE = ((σOUTDOOR)2+(σPENETRATION)2)1/2
= ((8)2+(8)2)1/2 =(64+64)1/2 =(128)1/2 = 11.31 dB
Cumulative Normal Distribution
On cumulative normal distribution curve, 75%
probability is 0.675 σ above median.
Fade Margin required =
(11.31) • (0.675) = 7.63 dB.
100%
Composite Probability of Service
90%
80%
75%
70%
60%
50%
40%
30%
20%
10%
0%
.675
-3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3
Standard Deviations from
Median (Average) Signal Strength
Calculating Required Fade Margin
Building
OutComposite
Penetration Door
Total
Environment
Type
Median Std.
Std.
Area
Fade
(“morphology”) Loss, Dev. Dev.
Availability
Margin
dB
Target, %
dB
σ, dB σ, dB
Dense Urban Bldg. 20
8
8
90%/75% @edge
7.6
Urban Bldg.
15
8
8
90%/75% @edge
7.6
Suburban Bldg.
10
8
8
90%/75% @edge
7.6
Rural Bldg.
10
8
8
90%/75% @edge
7.6
Typical Vehicle
8
4
8
90%/75% @edge
6.0
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 36
Link Budgets and System Design
The Link Budget Perspective:
We have available a fixed amount of transmitter power. At the other end fo
the link, the receiver requires a certain signal power level to acceptably
recover the information. The difference between these two power levels,
expressed in dB, is sometimes called by the slang term “link budget”. This
is the maximum amount of attenuation which can be tolerated between
transmitter and receiver. If the attenuation is larger, the link won’t work.
Of course, there are incidental gains and losses of antennas, transmission
lines, and other components in the transmission path; these use up some
of the otherwise-tolerable loss.
Allowances also must be made for fading (fade margins), various effects of
handoffs and traffic loading, and any other relevant factors.
Usually all of the incidental gains, losses, and other factors consume a small
fraction of the overall link budget. The largest single component in dB. is
the propagation loss itself.
The term “link budget” refers to a list or spreadsheet showing the calculation
of all the above. In addition, the term is sometimes used in a slang sense
to mean the actual value in db. available for propagation loss.
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 37
CDMA Reverse Link Budget Model Example
Term or Factor
Given
Budget
Formula
MS EIRP (dBm)
23.0 dBm
A
MS EIRP (watts)
0.2 W
Fade Margin (dB)
-7.6 dB
B
Soft Handoff Gain (dB)
4.0 dB
C
Receiver Interference Margin (dB)
-3.0 dB
D
Building Penetration Loss (dB)
-20.0 dB
E
BTS RX antenna gain (dBi)
17.0 dBi
F
BTS cable loss (dB)
-3.0 dB
G
MS TX power (dBm)
23.0 dBm
MS TX power (watts)
0.2 W
MS antenna gain and body loss (dBi)
0.0 dBi
kTB (dBm/14.4 kHz)
-132.4
H
BTS noise figure (dB)
6.4 dB
I
Eb/Nt (dB)
6.2 dB
J
BTS RX Sensitivity
-119.8 dB
Uplink Path Loss (dB)
130.2 dB
Course Series 300 -- CDMA Drive Test & System Optimization
H+I+J
A+B+C+DE+F+G(H+I+J+K)
Issue 0.01 (April 7, 1997) Page 38
Reverse Link Budget Discussion
•
This link budget traces the reverse path, from handset to BTS.
•
The Mobile Station (handset) maximum power is 200 militates, which is 0.2 watts or +23 dBm.
•
Handset gain and body loss are zero.
•
The handset antenna has no significant gain, and is used in a cluttered environment where gain is of no value.
•
Body loss is attenuation due to blockage or absorption by the body of the user. This value is set to zero, consistent with holding
the handset alongside the head where it is essentially unobstructed.
•
The net mobile station Effective Isotropic Radiated Power (EIRP) is therefore the same as the handset power, 200 mw. which is 0
dBm. (We will explore isotropic and dipole antenna references in the antenna section to follow.)
•
The fade margin is -7.6 dB. Fade margin is an additional loss allowance reserved to account for the unavoidable excess path loss
which statistically occurs. It is calculated to take into account both fades in the outdoor environment, and fades due to excess building
penetration loss, as calculated in the examples earlier in this section. If we had no fade margin, the link budget would represent our
median case (half our users would do as well or better, and half worse). With the fade margin included in the link budget, we can be
confident that the specified probability of area coverage is achieved.
•
Soft Handoff Gain is the dynamic improvement which results because the handset is being monitored by multiple BTS when it is near
a cell edge. Frame-by-frame, 50 times a second, whichever BTS heard the handset best is used as the source for extracting the
user’s signal. When the handset is in a momentary fade on one BTS, it is highly likely it will not be in a fade on another. The value
from field trials is 4 dB. This factor is only needed at the cell edge, which is where it comes into play.
•
Receiver interference margin accounts for the presence of other users on the system. With a design loading percentage of 50%,
which represents about 16.5 users per sector, this value is -3.0 dB.
•
Building penetration loss is the median attenuation estimated for diffraction of RF into buildings (or vehicles), as described earlier in
this section.
•
BTS RX antenna gain is the gain (compared to an isotropic radiator) of the receiving antenna at the BTS. A common value is 17 dBi.
•
The following terms represent the noise/interference floor.
•
kTB is thermal noise in signal bandwidth. -132.4 dBm/14.4 KHz. @ ambient
•
BTS noise figure is further noise floor degradation due to the BTS receiver; 6.4 dB. is typical.
•
Eb/Nt is the energy per bit to noise spectral density of other users
•
BTS receive sensitivity is the specified minimum input required
•
Uplink path loss is the remaining permissible path loss available after all the above factors have been paid from the budget.
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 39
CDMA Forward Link Budget Model Example
Term or Factor
Budget
Formula
BTS EIRP/traffic channel (dBm)
44.0 dBm
A
BTS EIRP/traffic channel (watts)
25.1 W
Fade margin (dB)
-7.6 dB
B
Receiver interference margin (dB)
-3.0 dB
C
Building Penetration Loss
-20.0 dB
D
MS antenna gain and body loss (dBi)
0.0 dBi
E
-116.8 dBm
F
130.2 dB
A+B+C+D-E+F
BTS TX power (dBm)
BTS
% Power for traffic channels
No. of traffic channels in use (chs.)
Given
44.0 dBm
25.67 W
74%
19
BTS cable loss (dB)
-3.0 dB
BTS TX antenna gain (dBi)
17.0 dBi
MS RX sensitivity
(NF 10.5 dB, Eb/No 5 dB)
Downlink Path Loss (dB)
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 40
Forward Link Budget Discussion
This link budget traces the forward path, from BTS to handset.
•
Base Station (BTS) maximum power is 25.67 watts, which 44 dBm.
•
74% of total BTS power is set aside for the traffic channels. (The remaining 26% is used for Pilot, Sync, and Paging.)
•
A total of 19 traffic channels are assumed to be in use. This could represent 15 users using this cell alone and 4 users of other cells
presently in soft handoff.
•
Transmission line loss between BTS equipment and antenna is 3.0 dB.
•
BTS antenna gain is 17.0 dBi compared to an isotropic radiator.
•
Considering all of the above, each traffic channel is effectively using a share of the total BTS Effective Isotropic Radiated Power
(EIRP). This per-channel portion is +44.0 dBm (25.1 watts).
•
The fade margin is -7.6 dB. Fade margin is an additional loss allowance reserved to account for the unavoidable excess path loss
which statistically occurs. It is calculated to take into account both fades in the outdoor environment, and fades due to excess building
penetration loss, as calculated in the examples earlier in this section. If we had no fade margin, the link budget would represent our
median case (half our users would do as well or better, and half worse). With the fade margin included in the link budget, we can be
confident that the specified probability of area coverage is achieved.
•
Receiver interference margin accounts for the presence of other users on the system. With a design loading percentage of 50%, this
value is -3.0 dB.
•
Building penetration loss is the median attenuation estimated for diffraction of RF into buildings (or vehicles), as described earlier in
this section.
•
Handset gain and body loss are zero.
•
The handset antenna has no significant gain, and is used in a cluttered environment where gain is of no value.
•
Body loss is attenuation due to blockage or absorption by the body of the user. This value is set to zero, consistent with holding
the handset alongside the head where it is essentially unobstructed.
•
MS receive sensitivity is the specified minimum input required to deliver specified performance. It is based on the MS having
somewhat poorer RF performance than the base station, as shown by a noise figure (NF) of 10.5 dB. The Eb/No is 5 dB., since on the
forward link the number of interferes and their behavior is much more civilized than on the reverse link. The overall value is -116.8
dBm, which is about 3 dB. poorer than the sensitivity of the BTS.
•
Downlink Path Loss (dB) is the remaining permissible path loss available after all the above factors have been paid from the budget.
Incidentally, the actual dynamic BTS output will be smaller than shown in this budget since we have neglected soft handoff effects which
give an improvement similar to the one seen on the reverse link.
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 41
CDMA Link Budget Conclusions
Forward (Downlink)
Reverse (Uplink)
Term or Factor
Given
MS TX power (dBm)
23.0 dBm
MS TX power (watts)
0.2 W
MS antenna gain and body loss (dBi)
Budget
Term or Factor
BTS TX power (dBm)
BTS
0.0 dBi
MS EIRP (dBm)
23.0 dBm
MS EIRP (watts)
0.2 W
% Power for traffic channels
No. of traffic channels in use (chs.)
Given
Budget
44.0 dBm
25.67 W
74%
19
Fade Margin (dB)
-7.6 dB
BTS cable loss (dB)
Soft Handoff Gain (dB)
4.0 dB
BTS TX antenna gain (dBi)
Receiver Interference Margin (dB)
-3.0 dB
BTS EIRP/traffic channel (dBm)
44.0 dBm
Building Penetration Loss (dB)
-20.0 dB
BTS EIRP/traffic channel (watts)
25.1 W
BTS RX antenna gain (dBi)
17.0 dBi
Fade margin (dB)
-7.6 dB
BTS cable loss (dB)
-3.0 dB
Receiver interference margin (dB)
-3.0 dB
Building Penetration Loss
-20.0 dB
kTB (dBm/14.4 KHz)
-132.4
BTS noise figure (dB)
6.4 dB
Eb/Nt (dB)
6.2 dB
BTS RX Sensitivity
Uplink Path Loss (dB)
MS antenna gain and body loss (dBi)
-119.8 dB
130.2 dB
-3.0 dB
17.0 dBi
0.0 dBi
MS RX sensitivity
(NF 10.5 dB, Eb/No 5 dB)
-116.8 dBm
Downlink Path Loss (dB)
130.2 dB
■ Forward and reverse links should be in gain balance. Excess gain on
just one link is no advantage during two-way communication
• Link balance adjustments are made by differential wilting or
blossoming of the BTS using BSM commands
■ The reverse link is usually the more difficult link due to interference and
power control issues of mobiles
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 42
Bibliography
“Wireless Communications Principles & Practice” by Theodore S. Rappaport. Street price, $71. 641 pp., 10 chapters, 7 appendices.
Prentice-Hall PTR, 1996, ISBN 0-13-375536-3. If you can only buy one book, buy this one. Comprehensive summary of wireless
technologies along with principles of real systems. Includes enough math for understanding and solving real problems. Good
coverage of system design principles.
“The Mobile Communications Handbook” edited by Jerry D. Gibson. 577 pp., 35 chapters. CRC Press/ IEEE Press 1996, ISBN 0-84930573-3. $89 If you can buy only two books, buy this second. Solid foundation of modulation schemes, digital processing theory, noise,
vocoding, forward error correction, excellent full-detailed expositions of every single wireless technology known today, RF propagation,
cell design, traffic engineering. Each chapter is written by an expert, and well-edited for readability. Clear-language explanations for
both engineers and technicians but also includes detailed mathematics for the research-inclined. Highly recommended.
“Mobile Communications” edited by Fraidoon Mazda. $29. 277 pp., Focal Press/Butterworth-Heinemann, Reed Educational &
Professional Publishing Ltd. ISBN 02405 1458 0. Good background summary of radio technologies and current industry practice, in
Radio Paging (UK), PMR and trunked radio systems, Cordless Communications, Cellular Radio Systems, PCN/PCS, and
Communication Satellite Systems. UK author and perspective, but excellent scope. See companion work below. Compact.
“Analytical Techniques in Telecommunications” edited by Fraidoon Mazda. $29. 310 pp., Focal Press/Butterworth-Heinemann, Reed
Educational & Professional Publishing Ltd. ISBN 02405 1451 3. Excellent compact softcover compendium of communications signal
theory. Key mathematical concepts and formulas well indexed for easy access. Statistical Analysis, Fourier Analysis, Queuing
Theory, Information Theory, Teletraffic Theory, Coding, Signals and Noise, plus good Calculus, Series & Transforms, Matrices and
Determinants, and Trigonometric/General summaries.
“Digital Communications: Fundamentals and Applications” by Bernard Sklar. 771 pp., Prentice Hall, 1988. $74 ISBN# 0-13-211939-0
Excellent in depth treatment of modulation schemes, digital processing theory, noise.
"Communication Electronics" by Louis E. Frenzel, 2nd. Ed., list price $54.95. Glencoe/MacMillan McGraw Hill, April, 1994, 428 pages
hardcover, ISBN 0028018427.
"Wireless Personal Communications Services" by Rajan Kuruppillai. 424 pp., 75 illus., McGraw-Hill # 036077-4, $55 Introduction to major
PCS technical standards, system/RF design principles and process, good technical reference
“Wireless and Personal Communications Systems” by Garg, Smolik & Wilkes. 445 pp., Prentice Hall, 1996, $68. ISBN 0-13-234-626-5 $68.
This is the little brother of “The Mobile Communications Handbook”. Good explanation of each technology for a technical newcomer to
wireless, but without quite as much authoritative math or deep theoretical insights. Still contains solid theory and discussion of practical
network architecture.
"Voice and Data Communications Handbook" by Bates and Gregory 699 pp, 360 illus., McGraw-Hill # 05147-X, $65 Good authoritative
reference on Wireless, Microwave, ATM, Sonet, ISDN, Video, Fax, LAN/WAN
"Mobile Cellular Telecommunications" by William C. Y. Lee. 664 pp., 232 illus., 038089-9, $60 A classic. Bill Lee’s current definitive
reference work on all the wireless technologies
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 43
Bibliography (concluded)
"Spread Spectrum Communications Handbook" by Simon, Omura, Scholtz, and Levitt. 1227 pp., 15 illus., McGraw-Hill # 057629-7, $99.50
Definitive technical reference on principles of Spread Spectrum including direct sequence as used in commercial IS-95/JStd008
CDMA. Heavy theory.
"Cellular Radio: Principles and Design" by Raymond C. V. Macario. 215 pp., 142 illus., 044301-7, $50 Good introduction to RF basics for
AMPS and TDMA technologies; but no CDMA coverage.
“Applications of CDMA in Wireless/Personal Communications” by Garg, Smolik & Wilkes. 360 pp., Prentice Hall, 1997, ISBN 0-13-572157-1
$65. Probably the best CDMA-specific book we’ve seen. Excellent treatment of IS-95/JStd. 008 as well as W-CDMA. More than just
theoretical text, includes chapters on IS-41 networking, radio engineering, and practical details of CDMA signaling, voice applications,
and data applications.
"CDMA: Principles of Spread Spectrum Communication" by Andrew J. Viterbi. 245 p. Addison-Wesley 1995. ISBN 0-201-63374-4, $65.
Definitive CDMA Theory. Valuable but dry reading for mere mortals. Much calculus.
"PCS Network Deployment" by John Tsakalakis. 350 pp, 70 illus., McGraw-Hill #0065342-9, $65 Tops-down view of the startup process in
a PCS network. Includes good traffic section.
"CDPD: An Introduction" by John Agosta. 256 pp, 100 illus, McGraw-Hill # 000600-8, $50. Detailed reference on how CDPD works in
Analog and TDMA systems.
"Cellular System Design & Optimization" by Clint Smith and Curt Gervelis. 448 pp, 110 illus., McG.-Hill #059273-X, $65 Optimization for
conventional cellular systems; implications for PCS but not CDMA-specific.
"The ARRL Handbook for Radio Amateurs (1997)" published by the American Radio Relay League (phone 800-594-0200). 1100+ page
softcopy ($44); useful exposure to nuts-and-bolts practical ideas for the RF-unfamiliar. Solid treatment of the practical side of
theoretical principles such as Ohm’s law, receiver and transmitter architecture and performance, basic antennas and transmission
lines, and modern circuit devices. Covers applicable technologies from HF to high microwaves. If you haven’t had much hands-on
experience with real RF hardware, or haven’t had a chance to see how the theory you learned in school fits with modern-day
communications equipment, this is valuable exposure to real-world issues. Even includes some spread-spectrum information in case
you’re inclined to play and experiment at home. At the very least, this book will make dealing with hardware more comfortable. At
best, it may motivate you to dig deeper into theory as you explore why things behave as they do.
Course Series 300 -- CDMA Drive Test & System Optimization
Issue 0.01 (April 7, 1997) Page 44