Patent Publication Number: US-6338031-B1

Title: Computer-implemented inbuilding prediction modeling for cellular telephone systems

Description:
This application is a Continuation of application Ser. No. 08/904,309, filed Jul. 31, 1997, entitled ‘COMPUTER-IMPLEMENTED INBUILDING PREDICTION MODELING FOR CELLULAR TELEPHONE SYSTEMS’, which application is incorporated herein by reference. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. patent application Ser. No. 08/904,440, filed on same date herewith, by William C. Y. Lee, et al., and entitled “COMPUTER-IMPLEMENTED MICROCELL PREDICTION MODELING WITH TERRAIN ENHANCEMENT,” which application is incorporated by reference herein. 
     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 th e 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 i s 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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings in which like reference numbers represent corresponding parts throughout: 
     FIG. 1 illustrates an exemplary computer system  10  that could be used to implement the present invention; 
     FIG. 2 is a diagram that illustrates a microcell (or other cell) of a cellular telephone system contained within a building; 
     FIG. 3 is a simplified diagram providing a top view of the building layout shown in FIG. 2; 
     FIG. 4 is a simplified diagram providing a top view of the building layout shown in FIG. 2; 
     FIG. 5 is another simplified diagram providing a top view of the building layout shown in FIG. 2; 
     FIG. 6 is another simplified diagram providing a top view of the building layout shown in FIG. 2; 
     FIG. 7 is a flow chart illustrating the logic performed by the modeling tool according to the present invention; 
     FIG. 8 is a diagram that illustrates a microcell of a cellular telephone system that was measured and then modeled using the present invention; and 
     FIG. 9 is a graph of the predicted signal strength versus the measured signal strength using the present 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 λ/2, where λ 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 1    40 , segment d 2    42 , and segment d 3    44 . 
     Information is gathered on the building  28  layout to determine the lengths of segments d 1    40 , d 2    42 , and d 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 1    40  are calculated first. If segment d 2    42  exists for a given path  38 , then the losses due to segment d 2    42  are calculated. Finally, any losses due to segment d 3    44  are calculated. 
     Segment d 1    40  is the distance from transmitter antenna  30  to a first room, segment d 2    42  is the distance from the first room intersection to boundary  48 , and segment d 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 1    40 , and segments d 2    42  and d 3    44  are zero. 
     The line of sight losses (L los ) for segment d 1    40  is given by:          L   los     =       4      π                   d   1       λ                     
     where: 
     λ=the wavelength of the transmitted signal, and 
     d 1 =the length of segment d 1    40 . 
     If P t  is the power transmitted from the transmitter antenna  30 , the total power P r  at the receiver antenna  34  is given by: 
     
       
         P r =P t +G t −L los +G r   
       
     
     where: 
     G t =gain of transmitter antenna  30 , and 
     G 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:          L   los     =       4      π                 d     λ                     
     where: 
     L 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: 
     
       
         P r =P t +G t −L los +G 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 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 los  is calculated as described with respect to FIG.  4 :          L   los     =       4      π                   d   1       λ                     
     Segment d 2    42  is the distance from the standard room  54  wall intersection to the receiver antenna  34  along the segment d 2   42 . The loss L room due to segment d 2   42  is calculated as: 
      L room =m room  log d 2   
     where: 
     m room =the slope of the standard room  54 , and 
     d 2 =the length of segment d 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 room , 
     L 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 r ) received from the transmitter antenna  30  at the receiver antenna  34  is given by: 
     
       
         P r =P t +G t −L los −L room +G r   
       
     
     If the receiver antenna  34  is in a special room  56 , then the path losses due to segment d 2    42  is given by: 
     
       
         L SpecialRoom =m SpecialRoom  log d 2   
       
     
     where: 
     L SpecialRoom =the losses due to segment d 2    42 , and 
     m SpecialRoom =the slope of the special room. 
     The value of the slope for the special room  56 , m 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 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: 
     
       
         P r =P t +G t −L los −L SpecialRoom +G 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 1    40 , a loss due to segment d 2    42 , and a loss due to segment d 3    44 . 
     As discussed with respect to FIG. 5, segment d 1    40  would contribute a path loss of L los , and segment d 2    42  would contribute a path loss of L room . However, for a receiver antenna  34  that is positioned outside the building  28 , an additional path loss, L outside , is introduced by the existence of segment d 3    44 . L outside  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 1    40  is given by:          L   los     =       4      π                   d   1       λ                     
     The path loss contributed by segment d 2    42  is given by: 
     
       
         L room =m room  log d 2   
       
     
     The path loss contributed by segment d 3    44  is given by: 
     
       
         L outside =Δ+20 log d 3   
       
     
     where Δ is an additional space loss, typically 20 dB. 
     The power at the receiver antenna  34 , P r , is then given by: 
     
       
         P r =P t +G t −L los −L room −L outside +G 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 los ) for each segment d i :          L   los     =       4      π                   d   i       λ                     
     where: 
     L los =line of sight loss, 
     λ=the wavelength of the transmitted signal, and 
     d i =the length of segment i. Block  66  also represents the computer  12  accumulating the loss L los , for all segments i in L los . 
     If any segment i intersects a standard room, then Block  68  represents the computer  12  calculating the loss L room : 
     
       
         L room =m room  log d i   
       
     
     where: 
     L room =standard room loss (initialized to 0), 
     m room =the slope of the room, and 
     d i =the length of segment i. Block  68  also represents the computer  12  accumulating the loss L room  for all segments i in L room . 
     If any segment i intersects a special room, then Block  70  represents the computer  12  calculating the loss L specialroom : 
     
       
         L SpecialRoom =m SpecialRoom  log d i   
       
     
     where: 
     L SpecialRoom =special room loss (initialized to 0), 
     m SpecialRoom =the slope of the special room, and 
     d i =length of the segment i. Block  70  also represents the computer  12  accumulating the loss L SpecialRoom  for all segments i in L SpecialRoom . 
     If any segment i is outside the building, then Block  72  represents the computer  12  calculating the calculating the loss L outside : 
     
       
         L outside =Δ+20 log d i   
       
     
     where: 
     L outside =outside loss (initialized to 0), 
     d i =length of the segment I, and 
     Δ=additional loss, normally Δ=20. Block  72  also represents the computer  12  accumulating the loss L outside  for all segments i in L outside . 
     Finally, Block  74  represents the computer  12  calculating the power at the receiver antenna: 
     
       
         P r =P t +G t −L los −L room −L outside +G r   
       
     
     where: 
     P r =power at receiver antenna, 
     P t =power at transmitter antenna, 
     G t =gain of transmitter antenna, and 
     G r =gain of receiver antenna. 
     The values P r , P t , G t , and G 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 los , L room , and L 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  (node 1 , node 2 , and node 3 ) 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.