Computer-implemented inbuilding prediction modeling for cellular telephone systems

A computer-implemented modeling tool for cellular telephone systems predicts signal strength under real conditions within a building, by considering the effects of inter-building and intra-building structures on transmitted signals. The modeling tool gives more accurate predictions under line of sight conditions, when obstructions occur due to inter-building and intra-building structures.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a computer-implemented system for the 
design and development of cellular telephone systems or other wireless 
communication systems. In particular, the present invention discloses a 
modeling system integrated with a comprehensive set of software tools for 
the design, development and management of cellular telephone systems. 
2. Description of Related Art 
The capacity of cellular telephone systems in urban areas is typically its 
most precious commodity. The use of smaller cells, called "microcells," 
has been one of the solutions used to increase the capacity of cellular 
telephone systems. 
Because capacity is such a precious commodity, the design and management 
decisions made for cellular telephone systems are usually made to maximize 
the capacity of the system. For example, engineers must design the system 
to maximize the coverage of the geographic area with the minimum number of 
cell sites. In addition, interference problems must be studied so that 
their effect is minimized. Further, the blocking probability of each cell 
site must be analyzed to ensure proper call initiation. 
The design of a cellular telephone system or other wireless communications 
system is typically performed using modeling techniques before the system 
is placed in actual usage. The Lee model, described in "Mobile Cellular 
Telecommunications," by William C. Y. Lee, Second Edition, 1995, which is 
incorporated by reference herein, is the standard model for designing a 
cellular telephone system. The models and the investigations performed in 
this area concentrate on analyzing the propagation of electromagnetic 
waves under a line of sight analysis. 
However, calls that are generated from within buildings or that are 
directed toward cellular phones within buildings are generally not taken 
into account by system designers. The reflections or blocking of a direct 
line of sight transmission due to exterior and interior building walls are 
typically ignored or averaged out during the modeling process. Ignoring or 
averaging these effects on the transmission of cellular signals was 
assumed to be proper since cells are small. However, microcell antennas, 
once placed in operation, need to be adjusted in terms of placement, power 
output, and antenna beam patterns because the models used do not 
accurately predict the conditions experienced in actual use of the 
cellular telephone system. 
It can be seen, then, that there is a need for a better modeling tool to 
more accurately predict conditions present when the cellular telephone 
system is placed in operation. 
SUMMARY OF THE INVENTION 
To minimize the limitations in the prior art described above, and to 
minimize other limitations that will become apparent upon reading and 
understanding the present specification, the present invention discloses a 
method, apparatus, and article of manufacture for modeling cellular 
telephone systems to predict signal strength for cells within buildings 
under real conditions, by considering the effects of the inter-building 
and intra-building structures on the transmitted signals. 
One object of the present invention is to solve the above-described 
problems by using models of the buildings to more accurately predict the 
conditions that the cellular telephone system will be used under. Another 
object of the present invention is to increase capacity of the cellular 
telephone system. It is a further object of the present invention to more 
accurately model a cellular telephone system. It is a further object of 
the present invention to reduce the costs of implementing a cellular 
telephone system. 
For a better understanding of the invention, its advantages, and the 
objects obtained by its use, reference should be made to the drawings 
which form a further part hereof, and to accompanying descriptive matter, 
in which there are illustrated and described specific examples of an 
apparatus in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION 
In the following description of the preferred embodiment, reference is made 
to the accompanying drawings which form a part hereof, and in which is 
shown by way of illustration the specific embodiment in which the 
invention may be practiced. It is to be understood that other embodiments 
may be utilized as structural changes may be made without departing from 
the scope of the present invention. 
Overview 
The present invention provides methods for accurately determining the 
signal strength for transmitters and receivers that are within buildings. 
The method includes using typical line of sight calculations, as well as 
determining the inter-building and intra-building effects on the signal 
strength calculation. 
The features of buildings of modern construction that influence propagation 
between transmitter and receiver either inside the same building 
(intra-building) or from within the building to outside of the building 
(inter-building) affect the signal strength of the transmitted and 
received signals. Even on the same floor of a building, the signal between 
transmitter and receiver can be altered by items and people within the 
building. 
One feature is the clear space between ceiling and furniture or floor that 
results in excess attenuation of signal. A second feature is the 
reflection and transmission qualities at interior and exterior walls of 
the building. Yet another feature is to provide different propagation 
formula for the different types of rooms within a building. The present 
invention identifies these components and uses them to predict the signal 
strength for an in-building microcell of a cellular telephone system. 
The resulting multipath structure causes the received signal to exhibit 
strong variations as either the transmitter or receiver antenna is moved 
over a distance on the order of .lambda./2, where .lambda. is the 
wavelength of the transmitted signal. For propagation inside buildings, it 
is impossible to account for every interaction as a radio signal 
propagates through the building, or to model the signal variation on a 
wavelength scale. 
Traditional practice has been to average the signal by moving the 
transmitter antenna or the receiver antenna over a spatial area having 
linear dimensions of 10 to 20 wavelengths (often in a circular path) to 
remove the rapid variation. The result has been referred to as the sector 
average. 
By considering the actual path that the signal takes between transmitter 
antenna and receiver antenna, including inter-building and intra-building 
effects, systems designers can construct a more accurate model of the 
conditions that the system must operate under. This enhanced modeling 
technique make cellular telephone systems easier to design and cheaper to 
place in the field. 
Hardware Environment 
FIG. 1 illustrates an exemplary computer system 10 that could be used to 
implement the present invention. The computer 12 comprises a processor 14 
and random access memory (RAM) 16. The computer 12 may be coupled to other 
devices, such as a monitor, a keyboard, a mouse device, a printer, etc. Of 
course, those skilled in the art will recognize that any combination of 
the above components, or any number of different components, peripherals, 
and other devices, may be used with the computer 12. 
Generally, the computer 12 operates under control of an operating system 
18. The present invention is preferably implemented using one or more 
computer programs 20 and data structures 22 operating under the control of 
the operating system 18. More specifically, the present invention 
comprises a modeling tool 20 and modeling data 22 that model a cellular 
telephone system, including the signal strength at points within a 
specified volume of space of the system, given certain constrictions and 
conditions within that space. The modeling tool 20 outputs the results of 
these operations as characters, text, and graphics on a monitor, printer, 
or other device attached to the computer 12. 
In the preferred embodiment, the operating system 18, the modeling tool 20, 
and the modeling data 22 are tangibly embodied in a computer-readable 
medium, e.g., data storage device 24, which could include one or more 
fixed or removable data storage devices, such as a removable cartridge 
drive, floppy disc drive, hard drive, CD-ROM drive, tape drive, etc. 
Further, the operating system 18 and the modeling tool 20 are comprised of 
instructions which, when read and executed by the computer 12, causes the 
computer 12 to perform the steps necessary to implement and/or use the 
present invention. Of course, those skilled in the art will recognize many 
modifications may be made to this configuration without departing from the 
scope of the present invention. 
Microcells 
FIG. 2 is a diagram that illustrates a microcell (or other cell) of a 
cellular telephone system contained within a building 28. Only a single 
floor is represented in the illustration, although those skilled in the 
art understand that the present invention will work for more than one 
floor. 
A transmitter antenna 30 is located at position 32 within the building 28, 
although those skilled in the art will recognize that position 32 could be 
outside the building 28. A receiver antenna 34 is located at position 36 
outside the building 28, although those skilled in the art will recognize 
that position 36 could be within building 28. 
A line of sight path 38 is shown between transmitter antenna 30 and 
receiver antenna 34. The path 38 has three components: segment d.sub.1 40, 
segment d.sub.2 42, and segment d.sub.3 44. 
Information is gathered on the building 28 layout to determine the lengths 
of segments d.sub.1 40, d.sub.2 42, and d.sub.3 44. The building 28 layout 
information includes the boundaries 46, 48, 50, and 52 of the building 28, 
the size, shape, and location of standard rooms 54 within the building 28, 
and the size, shape and location of any special rooms 56 within the 
building 28. Special rooms 56 include elevators, utility rooms, rooms with 
special wall construction such as safes or secure rooms, or other rooms 
that require different loss calculations from standard rooms 54. 
Path 38 losses are then computed by segment. The losses due to segment 
d.sub.1 40 are calculated first. If segment d.sub.2 42 exists for a given 
path 38, then the losses due to segment d.sub.2 42 are calculated. 
Finally, any losses due to segment d.sub.3 44 are calculated. 
Segment d.sub.1 40 is the distance from transmitter antenna 30 to a first 
room, segment d.sub.2 42 is the distance from the first room intersection 
to boundary 48, and segment d.sub.3 44 is the distance from boundary 48 to 
the receiver antenna 34. Any of the segments 40, 42, or 44 can be zero 
length for a given signal strength calculation. 
FIG. 3 is a simplified diagram providing a top view of the building 28 
layout shown in FIG. 2. In this diagram, the transmitter antenna 30 and 
receiver antenna 34 are placed on a path 38 that has no obstructions. This 
is called a direct line of sight path 38. In this case, the path 38 
consists only of segment d.sub.1 40, and segments d.sub.2 42 and d.sub.3 
44 are zero. 
The line of sight losses (L.sub.los) for segment d.sub.1 40 is given by: 
##EQU1## 
where: .lambda.=the wavelength of the transmitted signal, and 
d.sub.1 =the length of segment d.sub.1 40. 
If P.sub.t is the power transmitted from the transmitter antenna 30, the 
total power P.sub.r at the receiver antenna 34 is given by: 
EQU P.sub.r =P.sub.t +G.sub.t -L.sub.los +G.sub.r 
where: 
G.sub.t =gain of transmitter antenna 30, and 
G.sub.r =gain of receiver antenna 34. 
FIG. 4 is a simplified diagram providing a top view of the building 28 
layout shown in FIG. 2. As shown, the transmitter antenna 30 and receiver 
antenna 34 can be moved around the building 28, and can sometimes be 
positioned within standard rooms 54 or special rooms 56 of the building 
28. Further, the transmitter antenna 30 and receiver antenna 34 can be 
very close to each other, which means that the antenna patterns of 
transmitter antenna 30 and receiver antenna 34 interfere. The area within 
which this interference takes place is called the Fresnel zone. In this 
diagram, the transmitter antenna 30 and receiver antenna 34 are placed on 
a path 38 that includes obstructions, where the receiver antenna 34 is 
within the Fresnel zone of the transmitter antenna 30, as well as the 
transmitted signal being obstructed by a room 54. 
When the receiver antenna 34 is within the Fresnel zone of the transmitter 
antenna 30, the path 38 losses are calculated by: 
##EQU2## 
where: L.sub.los =path 38 losses between transmitter antenna 30 and 
receiver antenna 34, and 
d=length of path 38. 
The power of the received signal at receiver antenna 34 is then given by: 
EQU P.sub.r =P.sub.t +G.sub.t -L.sub.los +G.sub.r 
FIG. 5 is another simplified diagram providing a top view of the building 
28 layout as shown in FIG. 2. In this diagram, the transmitter antenna 30 
and receiver antenna 34 are placed on a path 38 that includes 
obstructions, where the receiver antenna 34 is outside the Fresnel zone of 
the transmitter antenna 30, and the transmitted signal is obstructed by a 
standard room 54. In this situation, the path 38 losses are calculated 
using two components. 
Segment d.sub.1 40 is the direct line of sight path from the transmitter 
antenna 30 to the standard room 54 wall intersection, and the path loss 
L.sub.los is calculated as described with respect to FIG. 4: 
##EQU3## 
Segment d.sub.2 42 is the distance from the standard room 54 wall 
intersection to the receiver antenna 34 along the segment d2 42. The loss 
L.sub.room due to segment d2 42 is calculated as: 
EQU L.sub.room =m.sub.room log d.sub.2 
where: 
m.sub.room =the slope of the standard room 54, and 
d.sub.2 =the length of segment d.sub.2 42. 
The room 54 slope is typically 40, but can be other values, as measured or 
empirically determined. Once a room 54 slope has been measured, this value 
is substituted into the calculation to determine L.sub.room. 
L.sub.room can vary for standard rooms 54 and special rooms 56. The room 
slopes can also be averaged for standard rooms 54 and special rooms 56, 
depending on the building 28 design. 
The power (P.sub.r) received from the transmitter antenna 30 at the 
receiver antenna 34 is given by: 
P.sub.r =P.sub.t +G.sub.t -L.sub.los.sup.- L.sub.room +G.sub.r 
If the receiver antenna 34 is in a special room 56, then the path losses 
due to segment d.sub.2 42 is given by: 
EQU L.sub.SpecialRoom =m.sub.SpecialRoom log d.sub.2 
where: 
L.sub.SpecialRoom =the losses due to segment d.sub.2 42, and 
m.sub.specialRoom =the slope of the special room. 
The value of the slope for the special room 56, m.sub.specialRoom, can be 
the same as a standard room 54 within the building 28, or can be different 
than a standard room 54. 
The power (P.sub.r) received from the transmitter antenna 30 at the 
receiver antenna 34, if the receiver antenna 34 is in a special room 56 is 
given by: 
EQU P.sub.r =P.sub.t +G.sub.t -L.sub.los -L.sub.SpecialRoom +G.sub.r 
FIG. 6 is another simplified diagram providing a top view of the building 
28 layout shown in FIG. 2. In this diagram, the transmitter antenna 30 is 
inside the building 28 and the receiver antenna 34 is outside the building 
28. The path losses now are comprised of three components: a loss due to 
segment d.sub.1 40, a loss due to segment d.sub.2 42, and a loss due to 
segment d.sub.3 44. 
As discussed with respect to FIG. 5, segment d.sub.1 40 would contribute a 
path loss of L.sub.los, and segment d.sub.2 42 would contribute a path 
loss of L.sub.room. However, for a receiver antenna 34 that is positioned 
outside the building 28, an additional path loss, L.sub.outside, is 
introduced by the existence of segment d.sub.3 44. Loutside can vary 
depending on the construction of the wall of the building 28, and can also 
vary depending on which wall 46, 48, 50, or 52 the path travels through. 
The path loss contributed by segment d.sub.1 40 is given by: 
##EQU4## 
The path loss contributed by segment d.sub.2 42 is given by: 
EQU L.sub.room =m.sub.room log d.sub.2 
The path loss contributed by segment d.sub.3 44 is given by: 
EQU L.sub.outside =.DELTA.+20 log d.sub.3 
where .DELTA. is an additional space loss, typically 20 dB. 
The power at the receiver antenna 34, P.sub.r, is then given by: 
EQU P.sub.r =P.sub.t +G.sub.t -L.sub.los -L.sub.room -L.sub.outside +G.sub.r 
Logic of the Modeling Tool 
FIG. 7 is a flow chart illustrating the logic performed by the modeling 
tool 20 according to the present invention. 
Block 62 represents the computer 12 accepting and storing modeling data 22 
in its memory 16, wherein the modeling data 22 includes the various 
measured values of the cellular telephone system, which are necessary to 
perform the computations indicated below. 
Blocks 64-74 represent the computer 12 computing a strength of a signal 
received at the receiver antenna from the transmitter antenna using the 
stored data. The strength of the signal is computed by determining a line 
of sight signal strength between the transmitter antenna and the receiver 
antenna, determining effects on the signal strength from one or more 
obstructions between the transmitter antenna and the receiver antenna, and 
modifying the line of sight signal strength using the determined effects. 
These steps are described in more detail below. 
Block 66 represents the computer 12 calculating line of sight losses 
(L.sub.los) for each segment d.sub.i : 
##EQU5## 
where: L.sub.los =line of sight loss, 
.lambda.=the wavelength of the transmitted signal, and 
d.sub.i =the length of segment i. Block 66 also represents the computer 12 
accumulating the loss L.sub.los for all segments i in L.sub.los. 
If any segment i intersects a standard room, then Block 68 represents the 
computer 12 calculating the loss L.sub.room : 
EQU L.sub.room =m.sub.room log d.sub.i 
where: 
L.sub.room =standard room loss (initialized to 0), 
m.sub.room =the slope of the room, and 
d.sub.i =the length of segment i. 
Block 68 also represents the computer 12 accumulating the loss L.sub.room 
for all segments i in L.sub.room. 
If any segment i intersects a special room, then Block 70 represents the 
computer 12 calculating the loss L.sub.Specialroom : 
EQU L.sub.SpecialRoom =m.sub.SpecialRoom log d.sub.i 
where: 
L.sub.SpecialRoom =special room loss (initialized to 0), 
m.sub.SpecialRoom =the slope of the special room, and 
d.sub.i =length of the segment i. 
Block 70 also represents the computer 12 accumulating the loss 
L.sub.SpecialRoom for all segments i in L.sub.SpecialRoom. 
If any segment i is outside the building, then Block 72 represents the 
computer 12 calculating the calculating the loss L.sub.outside : 
EQU L.sub.outside =.DELTA.+20 log d.sub.i 
where: 
L.sub.outside =outside loss (initialized to 0), 
d.sub.i =length of the segment I, and 
.DELTA.=additional loss, normally .DELTA.=20. 
Block 72 also represents the computer 12 accumulating the loss 
L.sub.outside for all segments i in L.sub.outside. 
Finally, Block 74 represents the computer 12 calculating the power at the 
receiver antenna: 
EQU P.sub.r =P.sub.t +G.sub.t -L.sub.los -L.sub.room -L.sub.outside +G.sub.r 
where: 
P.sub.r =power at receiver antenna, 
P.sub.t =power at transmitter antenna, 
G.sub.t =gain of transmitter antenna, and 
G.sub.r =gain of receiver antenna. 
The values P.sub.r, P.sub.t, G.sub.t, and G.sub.r are all directly 
measurable quantities of the cellular telephone system that are entered 
into the computer 12 and stored as the modeling data 22. The remaining 
values L.sub.los, L.sub.room, and L.sub.outside are computed, as indicated 
in Blocks 64-74, from directly measurable values of the cellular telephone 
system that are entered into the computer 12 and stored as the modeling 
data 22. 
Finally, block 76 represents the computer 12 outputting one or more reports 
as represented in FIG. 9. These reports are then used in the practical 
application of constructing cells for a cellular telephone system, or for 
optimizing cells already present in a cellular telephone system. 
Output of the Modeling Tool 
FIG. 8 is a diagram that illustrates a microcell of a cellular telephone 
system that was measured and then modeled using the present invention. In 
this measurement, a transmitter antenna 30 was first placed at one 
position 32 (node 1), and measurements were made of the signal strength at 
the receiver antenna 34, at various locations in various rooms as shown by 
the dots on FIG. 8. Transmitter antenna 30 was then placed at a different 
position 32 (node 2) and measurements were again made of the signal 
strength at the receiver antenna 34 at various locations in various rooms 
as shown by the dots in FIG. 8. Transmitter antenna 30 was finally placed 
at a third position 32 (node 3) and again measurements were made of the 
signal strength at receiver antenna 34 at various locations in various 
rooms 54. The three positions 32 (nodel, node2, and node3) were chosen to 
test some special cases for the transmitter antenna 30, such as 
transmitting through line of sight conditions, non-line of sight 
conditions, Fresnel zone conditions, through standard rooms, and through 
special rooms. 
FIG. 9 is a graph of the predicted signal strength versus the measured 
signal strength using the present invention. The graph shows the 
differences between the predicted signal strength for the three positions 
32 used in FIG. 8, and the measured signal strength for the three 
positions 32 as a function of distance from the transmitter antenna 30. 
The slopes of the standard rooms 54 and special rooms 56 were derived and 
applied to the building 28 layout to arrive at the predicted values. The 
mean delta between the predicted and measured values was 2.13 dB and more 
than 85 percent of the predicted values were within a delta of 4 dB. 
Conclusion 
The foregoing description of the preferred embodiment of the invention has 
been presented for the purposes of illustration and description. It is not 
intended to be exhaustive or to limit the invention to the precise form 
disclosed. Many modifications and variations are possible in light of the 
above teaching. It is intended that the scope of the invention not be 
limited with this detailed description.