Abstract:
A cellular network includes first cell sites having a first layout and second cell sites, interspersed among the first cell sites, having a second layout. The layout refers to the orientation of the antennas in the cell sites. By making the second layout different than the first layout, the amount of interference in the network reduces and, thus, the capacity of the network increases.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to wireless communication systems and, more particularly, to a system and method for reducing interference and increasing the capacity of a cellular network. 
     BACKGROUND OF THE INVENTION 
     Cellular networks have remained relatively static since they were first designed in the late seventies. This original design included simple methods for assigning frequencies by sets and general rules regarding reuse distance, frequency assignments, and antenna orientation. 
     FIG. 1 is a diagram of a conventional three-sectored cell site deployment  100 . Each cell site  110  includes three antennas, for example, antenna  120  pointing north (0°), antenna  130  pointing southeast (120°), and antenna  140  pointing southwest (240°). Ideally, an antenna transmits signals at full power levels within its sector and no signals beyond the cross-over points between sectors. In practice, however, a gradual fall-off results with any angular deviation from the bore sight (i.e., the direction of the antenna). This fall-off has increased in recent years because cellular providers use narrower antennas to reduce interference between antennas in the same cell site. 
     FIG. 2 is a diagram of power levels of signals transmitted by a typical antenna deployed in a cellular network. In the figure, {circumflex over (1)} identifies the highest power levels along the bore sight and {circumflex over (2)} identifies the decreased power level at the cross-over points between sectors. The power loss at the cross-over points can be as much as 10 dB for very narrow beam antennas. 
     FIG. 3 is a diagram identifying power levels of the cell sites in the conventional cell site deployment of FIG.  1 . FIG. 3 identifies the power levels at the cell edges, using {circumflex over (1)} to identify the highest power level along the bore sight of an antenna and {circumflex over (2)} to identify the decreased power level at the cross-over points between sectors. Location  310  identifies a point of intersection along the bore sights of three antennas from the three closest cell sites. Location  320  identifies a point of intersection of six antennas, two from each of the three closest cell sites. 
     At points of intersection, such as location  320 , six potential signals of equal power levels exist from the three closest cell sites. In the case of Code Division Multiple Access (CDMA) systems, these signals include pilot and associated traffic channels. In the case of analog systems, such as Advanced Mobile Phone Systems (AMPS), these signals include analog channels. At all points, for example location  320 , the signals from the different antennas undergo log-normal shadowing, which is the variation of the average power level due to reflections from buildings and other structures. This can result in ping-ponging between the cell sites and increased interference, because a signal not used by a receiver constitutes interference. 
     FIG. 4 is a diagram showing the effect of interference caused by second tier cell sites in the conventional cell site deployment of FIG.  1 . Second tier cells sites include a second ring of cell sites with respect to a point of interest. In FIG. 4, cell sites  410 - 430  are second tier cell sites with respect to an intersection point at location  320 . 
     Cell sites  410 - 430  include antennas  415 - 435 , respectively, pointing along their bore sights to location  320 . At location  320 , the closest cell sites  440 - 460  transmit signals at low power while the second tier cell sites  410 - 430  transmit signals at full power (i.e., along the bore sights of their antennas) in this direction. The signals from the second tier cell sites  410 - 430  result in added interference at location  320 . 
     Typically, the path loss for a conventional cell site deployment, such as the one shown in FIG. 1, is 40 dB per decade. This results in three additional signals at location  320  that have power levels of 12 dB-γ below the power levels of the other signals at this location, where γ refers to the antenna loss at the cross-over point relative to the bore sight gain. Thus, if the antenna pattern drops by 6 dB at a deviation of 120° from its bore sight, then a total of nine signals within 6 dB of each other exist at location  320 . 
     In a propagation environment where the path loss exponent is much less than 40 dB per decade (e.g., microcell and other line-of-sight environments), the effect at intersection points, such as location  320 , can be even more pronounced. For example, in a line-of-sight environment, at least nine pilot channels of similar power levels could be received at location  320 . 
     A CDMA system configured in a manner consistent with the IS-95 standard can only decode three pilot and associated traffic channels at any one time. The rest of the signals appear as interference. The fading environment and shadowing of the existence of so many signals cause excess overhead messages between the base station and the mobile as the pilots enter and leave the active set. The IS-95B standard will better handle this situation by only reporting those situations where the system requires the adding or dropping of signals from the active set. This standard will not, however, reduce the amount of interference. 
     In the case of analog systems, the ratio of the channel power to the level of interference (C/I) can be relatively low at intersection points, such as location  320 , because of the presence of many interfering signals. This results in the effective reuse of analog channels being larger than necessary and dominated by locations, such as location  320 . The end result is the lowering of the cell site capacity because the reuse distance, from location  320 &#39;s point of view, becomes higher than necessary. 
     Therefore, a need exists to reduce the interference resulting from conventional cell site deployment to, thereby, increase the capacity of the cellular network. 
     SUMMARY OF THE INVENTION 
     Systems and methods consistent with the present invention address this need by reconfiguring the cell sites to increase the capacity of individual sectors and cell sites. 
     In accordance with the purpose of the invention as embodied and broadly described herein, a system consistent with the present invention includes first cell sites having a first layout and second cell sites, interspersed among the first cell sites, having a second layout. The layout refers to the orientation of the antennas in the cell sites. By making the second layout different than the first layout, the amount of interference in the system reduces and, thus, the capacity of the system increases. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an embodiment of the invention and, together with the description, explain the invention. In the drawings, 
     FIG. 1 is a diagram of a conventional three-sectored cell site deployment; 
     FIG. 2 is a diagram of power levels of signals transmitted by a typical antenna deployed in a cellular network; 
     FIG. 3 is a diagram identifying power levels of the cell sites in the conventional cell site deployment of FIG. 1; 
     FIG. 4 is a diagram showing the effect of interference caused by second tier cell sites in the conventional cell site deployment of FIG. 1; 
     FIG. 5 is a diagram of three-sectored cell sites in an exemplary cell site deployment consistent with the present invention; 
     FIG. 6 is a diagram more fully showing the exemplary cell site deployment of FIG. 5; 
     FIG. 7A illustrates a distribution of pilot channels between cell sites for a conventional cell site deployment; 
     FIG. 7B illustrates CDMA coverage for the conventional cell site deployment shown in FIG. 7A; 
     FIG. 8A illustrates a distribution of pilot channels between cell sites for a cell site deployment consistent with the present invention; and 
     FIG. 8B illustrates CDMA coverage for the cell site deployment shown in FIG.  8 A. 
    
    
     DETAILED DESCRIPTION 
     The following detailed description of the invention refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. 
     Systems and methods consistent with the present invention reduce interference and, thus, increase the coverage and capacity of a cellular network by changing the direction of the antennas in various cell sites in the network. In this manner, the systems and methods redistribute the analog channels in the case of an analog network and the pilot and associated traffic channels in the case of a CDMA network. 
     Exemplary Cell Site Reconfiguration 
     FIG. 5 is a diagram of three-sectored cell sites in an exemplary cell site deployment  500  consistent with the present invention. Only three cell sites  510 - 530  are shown in the figure for simplicity. Cell sites  510  and  530  include antennas arranged in a manner consistent with the conventional cell site deployment described above with regard to FIG.  1 . In other words, cell site  510 , for example, includes antenna  512  pointing north (0°), antenna  514  pointing southeast (120°), and antenna  516  pointing southwest (240°). Cell site  520 , however, includes three antennas rotated by approximately 60 degrees with respect to the antennas in cell sites  510  and  530 . In other words, cell site  520  includes antenna  522  pointing northwest (60°), antenna  524  pointing south (180°), and antenna  526  pointing northwest (300°). 
     FIG. 5 also identifies the power levels at the cell edges, using {circumflex over (1)} to identify the highest power level along the bore sight of an antenna and {circumflex over (2)} to identify the decreased power level at the cross-over points. The cell site reconfiguration has the effect of reducing the number of major signals at location  540  (corresponding to location  320  in FIGS.  3  and  4 ), while sacrificing a portion of the performance at location  550  (corresponding to location  310  in FIGS.  3  and  4 ). 
     Applicant discovered that the performance of the network is limited by the performance at location  540 , not at location  550 . Therefore, the additional number of interfering signals at location  550  does not degrade the system. In fact, the power level of the two interfering signals at location  550  is down by γ dB with respect to the other signals. 
     FIG. 6 is a diagram more fully showing the exemplary cell site deployment of FIG.  5 . As the figure shows, the number of interfering signals from second tier cell sites have been reduced by the reorganization of the cell site structure. One less bore sight signal, but two more interfering signals with power levels reduced by γ dB below the conventional deployment, exist at location  540 . As shown by the experimental results described below, the cell site reconfiguration results in less interference at location  540  and, thus, increases the coverage and capacity of the cellular network. 
     Experimental Results 
     FIG. 7A illustrates a distribution of pilot channels between cell sites for a conventional cell site deployment. In FIG. 7A, only pilot channels above a threshold of Ec/Io&gt;−23 dB (Ec/Io represents the ratio of the energy of a pilot signal to the level of interference) are shown. The largest grouping of pilot channels occurs at crossover points, such as location  320  (FIG.  3 ). The exact number of pilot channels is a function of the transmitted power levels, the traffic (e.g., eight Erlangs per sector), and the chosen threshold (−23 dB in this case). 
     FIG. 7B illustrates CDMA coverage for the conventional cell site deployment of FIG.  7 A. As shown in the figure, coverage holes exist at locations served by signals originating from the antennas&#39; cross-over points, such as location  320 . 
     FIG. 8A illustrates a distribution of pilot channels between cell sites for a cell site deployment consistent with the present invention. Again, only pilot channels above a threshold of Ec/Io&gt;−23 dB are shown. As shown by the figure, the large grouping of pilot channels at cross-over points, such as location  320 , have been reduced. 
     FIG. 8B illustrates CDMA coverage for the cell site deployment of FIG.  8 A. As shown by the figure, the coverage holes at locations, such as location  320 , have been practically eliminated. 
     Similar results occur in analog systems. A cell site deployment consistent with the present invention redistributes information-bearing channels in an analog system to reduce overall interference levels and, thus, increase the capacity of the system. As a result, antennas can decrease the transmit power levels on their forward links to increase the system capacity. 
     The systems and methods consistent with the present invention reorganize the cell site layout to decrease interference at the edges of the cell sites and, thus, increase the capacity of the cellular network. 
     The foregoing description of preferred embodiments of the present invention provides illustration and description, but is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The scope of the invention is defined by the claims and their equivalents.