Abstract:
A method for calculating path loss PL at a point P in a coverage area CA of an antenna at a cell site. The method comprises the steps: transmitting a signal from the antenna, the antenna having an effective antenna gain Gant, and measuring a received signal strength indicator RSSI of the transmitted signal by a receiving antenna at the point P. The method further comprises the steps: determining a dominant indirect radiation path between the antenna and the receiving antenna to establish the effective antenna gain Gant from the antenna in a direction of the dominant indirect radiation path, and calculating path loss PL at the point P using the established effective antenna gain Gant and measured received signal strength indicator RSSI. A method for mapping a coverage area, a method for simulating changes in a communication network and a system for cell planning.

Description:
TECHNICAL FIELD 
     The present invention relates to a method for calculating path loss to one or more points in a coverage area of an antenna at a cell site. 
     BACKGROUND 
     Cell planning within mobile communication systems, such as GSM is an important task for telephone operators to optimize the performance for mobile telephone users. The mobile communication system comprises a network containing a plurality of sites, each site have one or more antennas providing coverage for a cell coverage area. In a present cell planning tool, an operator provides measured data from selected points in the coverage area as input data to the cell planning tool. The input data is obtained by measuring the signal strength at the selected points. If a change occurs in the network during a network tuning stage, the measured signal strength cannot always be reused. This problem is solved by converting the measured signal strength at the selected points into path loss and use the path loss as input data in the cell planning tool. The path loss may be reused for all changes in the network, except for changes in the physical antenna position or frequency band changes. 
     The path loss is calculated by taking the transmitting power P tx  from the antenna and subtracting the known received signal strength indicator RSSI of a signal. The path loss equation may be expressed as:
 
Path loss[dB]= P   tx [dBm]−RSSI[dBm],  (1)
 
     The transmitting effective isotropically radiated power (EIRP) P tx  may be expressed as:
 
 P   tx [dBm]= P   in [dBm]+ G   ant [dB],  (2)
 
where P in  is the known input power to the antenna and G ant  is the effective antenna gain. The effective antenna gain G ant  is calculated by estimating the angles of direction  α  (which could be expressed in Cartesian coordinates x,y,z or azimuth α azimuth  and elevation α elevation ) between the transmitting and receiving antennas and mapping these angles to the 3-dimensional antenna pattern.
 
     In US 2002/0063656 A1, by Tanley J. Gutowski, a standard procedure for estimating the angles of directions is disclosed. A straight line is drawn from the transmitting antenna position to the receiving antenna position when calculating the path loss. This approach usually has enough accuracy but will produce erroneous results in specific environments with indirect signal paths, such as reflected signal paths in an area with high buildings. 
     SUMMARY 
     An object with the present invention is to provide a method for calculating the path loss in an antenna&#39;s coverage area at an antenna site with indirect signal paths, which calculated path loss may be reused when the radiation characteristics of the antenna at the antenna site is changed. 
     This object is achieved by determining a dominant radiation path for measurement points in the antenna&#39;s coverage area. If the dominant radiation path is an indirect path, e.g. reflected or diffracted path, correct angles of direction for a transmitted signal from the antenna are determined. An indicator for received signal strength is measured at each measurement point and a path loss is calculated using the correct angles of direction. Path loss and angles of direction characterize each measurement point and may be reused when the characteristics of the antenna are changed during operation. 
     Another object is to provide a method for mapping a coverage area, a method for simulating changes in a communication network and a system for cell planning, using the dominant radiation path between a receiver and a transmitting antenna to establish an effective antenna gain and thus a correct path loss and angles of direction at each measurement point in a coverage area. 
     An advantage with the present invention is that measured signal strength at points within a highly reflective environment will generate a more accurate path loss than prior art solutions, which path loss may be reused if the antenna characteristics are altered during operation. 
     A further advantage is that a suggested change in antenna characteristics may be simulated and a simulated coverage calculated based on measurements before the change is implemented in a communication network. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a top view of a metropolitan area illustrating direct and indirect radiation paths. 
         FIG. 2  shows a perspective view of the top view in  FIG. 1 . 
         FIG. 3  shows a flow chart for calculating path loss according to the invention. 
         FIG. 4  shows a flow chart for mapping a coverage area according to the invention. 
         FIG. 5  shows a flow chart for simulating changes in a communication network according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The basic principle of the present invention is to determine an actual transmitting direction from an antenna along a dominant radiation path to a receiver at a point within the coverage area of the antenna. Each antenna has an antenna gain G ant  which varies dependent on the angle of direction, as specified by the antenna manufacturer, i.e. in a three-dimensional antenna pattern. If the dominant radiation path between the antenna and the receiver is indirect, e.g. reflected or diffracted against buildings, then the angle of direction from the antenna along the radiation path differs from the estimated angle of directions according to prior art, i.e. using a straight line between the antenna and the receiver to estimate the angle of direction. 
     In a cell planning tool, used by telephone operators, measured data from a plurality of points within a coverage area of an antenna are stored. These data contain the position of each point and corresponding path loss. The path loss is derived from measured signal strength at each point, and previously stored path loss may normally be reused for all cell changes except physical antenna position change and frequency band change. 
     The standard procedure for estimating the angle of directions is to draw a straight line from the transmitting antenna position to the receiving antenna position, as discussed above. This approach usually has enough accuracy but has been seen to give erroneous results in a specific environment. If the dominant signal received by the mobile receiver has not taken the closest route, but has been reflected and/or diffracted along the signal path, the estimated angles of directions may be completely wrong based on the prior art approach.  FIGS. 1 and 2  illustrate this clearly. 
       FIG. 1  shows a top view of a metropolitan area with three buildings  1 - 3 , a base station antenna  4  mounted to a wall of building  1 , and three measurement points P 1 , P 2 , P 3  indicated on a street  5  along building  3 . A direct radiation path (illustrated by a solid line), and a dominant radiation path (illustrated by a dashed line), are shown between the antenna  4  and each point P 1 -P 3 . 
       FIG. 2  shows a perspective view of the same metropolitan area as in  FIG. 1 . Signals along the direct radiation paths (solid lines) for point P 1  and P 3  are illustrated to propagate through the obstacles, i.e. the buildings. 
     Point P 1  is positioned along street  5  between building  2  and  3  with no free line of sight. The direct radiation path, solid line  6   a  is blocked by building  2  and the dominant radiation path, dashed line  6   b , diffracts around the corner of building  2 , and the dominant radiation path is thus indirect. The angle of direction for the direct radiation path  6   a  and the dominant indirect radiation path  6   b  from the antenna  4  differs, which is illustrated by the angle α 1 . When measuring a received signal strength indicator (RSSI) at point P 1 , the received dominant signal has angles of direction from the transmitting antenna  4  that differs α 1  from the angle of direction for the direct radiation path  6   a . This information needs to be taken into account when establishing an effective antenna gain G ant  in the correct direction (i.e. along the indirect radiation path  6   b ) and subsequent calculation of path loss to point P 1 . 
     Point P 2  is positioned between points P 1  and P 3  and has a free line of sight to the base station antenna  4 . In this case the direct radiation path, solid line  7   a , coincides with the dominant radiation path, dashed line  7   b . Thus, the dominant radiation path is straight. The angle of direction from the antenna  4  is estimated, and path loss to point P 2  is calculated, in accordance with prior art techniques. 
     Point P 3  is positioned along street  5  between building  1  and  3  with no free line of sight. The direct radiation path, solid line  8   a  is blocked by building  1  and the dominant radiation path, dashed line  8   b , reflects against the walls of building  2  and  3 , and the dominant radiation path is thus indirect. The angle of direction for the direct radiation path  8   a  and the dominant indirect radiation path  8   b  from the antenna  4  differs, which is illustrated by the angle α 3 . When measuring a received signal strength indicator (RSSI) at point P 3 , the received dominant signal has angle of direction from the transmitting antenna  4  that differs α 3  from the angle of direction for the direct radiation path  8   a . This information needs to be taken into account when establishing an effective antenna gain G ant  in the correct direction (i.e. along the indirect radiation path  8   b ) and subsequent calculation of path loss at point P 3 . 
     As illustrated in  FIG. 1 , the difference in angle of direction between the direct signal and the diffracted or reflected signal may be significant, especially for point P 3 , and the effective antenna gain will most likely be quite different for the two signals. It should be noted that the illustrated difference in angles of direction α 1 , α 3  may be a function of both azimuth α azimuth  and elevation α elevation . 
     A combination of both reflection and diffraction may naturally also generate the dominant radiation path between the antenna  4  and a measurement point. 
       FIG. 3  shows a flow chart  10  for calculating path loss PL at one or more points within a coverage area of an antenna, as shown in  FIGS. 1 and 2 . The flow starts, step  11 , and proceeds to transmit a signal from the base station antenna, step  12 . An input power P in  is fed to the antenna, which has an effective antenna gain G ant  that depends on the angles of direction and a 3-dimensional antenna radiating pattern. Thus, the transmitting effective isotropically radiated power (EIRP) P tx  of the signal from the antenna depends on the angles of direction. 
     A counter k is set to one (k=1) in step  13 , where k is used as an index for points (i.e. positions) within the coverage area, and a received signal strength indicator RSSI for point P 1  is measured in step  14 . A dominant radiation path is determined for point P 1  in step  15 . If the dominant radiation path is an indirect, i.e. reflected or diffracted, the flow proceeds to determine angles of direction  α  for the dominant indirect radiation path in step  16 . This step is preferably performed by applying a propagation model, such as a Ray-Tracing propagation model, to calculate which received signal is the dominant signal at the receiver at point P 1 , and thereafter determine the angles of direction that the dominant signal left the transmitting antenna. On the other hand, if the dominant path is the direct radiation path, the flow proceeds to estimate angles of direction  α  for the dominant direct radiation path in step  17 , which is a straight line between the antenna and the receiver. 
     The angles of direction  α  for the dominant radiation path are used to establish the effective antenna gain G ant  in step  18 . The effective antenna gain is determined as a function of the direction the dominant signal left the transmitting antenna, i.e. the angles of direction estimated in step  17  or determined in step  16 . 
     Path loss is thereafter determined in step  19  using Eq. (1) and (2). The information regarding path loss and angles of direction for point P 1  is stored in a memory or database in step  20  for future use. A decision to proceed with measurement in more points within the coverage area is made in step  21 . If measurements should be performed in another point, k is increased by one (k=k+1) in step  22  and the flow is fed back to step  14  for a new measurement in point P 2 . Steps  14  to  22  are repeated until no measurement in an additional point should be made and the flow ends in step  23 . 
     As an alternatives the determination of angles of direction  α  for the dominant indirect radiation path in step  16  could be performed by measuring a receiving direction at the receiver for the dominant signal, and thereafter calculate the dominant radiation path using the receiving direction at the receiver as a starting point to determine the angles of direction the dominant signal left the transmitting antenna. 
       FIG. 4  shows a flow chart  30  for mapping a coverage area according to the invention. The flow starts, step  31 , and an integer n is set to 1 in step  32  to indicate the selected coverage area CA that should be mapped. A number of points in CA 1  is selected, step  33 , and the process of calculating path loss PL and angle of direction α for each selected point in CA 1  is performed in step  34  using the flow chart  10  described in connection with  FIG. 3 . 
     The coverage area CA 1  is mapped using the stored information regarding PL and α, step  35 , and if another coverage area is selected to be mapped; step  36 , the integer n is increased by one (n=n+1) in step  37  and fed back to step  33 . Steps  33  to  37  are repeated for selected coverage areas until no other area is selected and the flow ends, step  38 . 
       FIG. 5  shows a flow chart  40  for simulating changes in a communication network according to the invention. The flow starts, step  41 , and the selected coverage area is mapped, step  42 , using the flow chart  30  described in connection with  FIG. 4 . Changes in antenna properties, such as tilting, is simulated, step  43 , which will result in changes in the effective antenna gain G ant . The stored PL for each point in the selected coverage area are retrieved step  44 , from the memory (or database), and α is updated with the changed antenna properties using step  15 - 18  in  FIG. 3  to calculate new angles of direction α. A simulated coverage is calculated in step  45  using the new angles of direction α and applying this result to step  43  to calculate the effective antenna gain G ant  from step  43 . The flow ends, step  46 , when the simulated coverage has been calculated. 
     By using the flow described in connection with  FIG. 5 , it is possible to avoid making changes that unintentionally will reduce the coverage in a selected CA, and make sure that a suggested change in antenna properties, such as tilting, or changing the antenna input power P in  will be not seriously affect the coverage for the users of the communication network. 
     A system for cell planning is preferably implemented in a control unit, such as a computer, controlling several cell sites. Information regarding all transmitting antennas serving a coverage area which are associated with the cell sites is fed to the control unit. The control unit has access to the memory (or database) where information regarding path loss and angles of direction are stored. Mapping and simulation of coverage areas is performed by the control unit.