Abstract:
A system and method are disclosed for deploying a plurality of point-to-point receiver transmitter pairs such that the main beam of transmitting radio does not point directly at any receiving radio antenna other than the desired receiving antenna. A three dimensional lattice matrix deploying nodes in different horizontal planes and aligning the nodes such that nodes of a particular row of nodes do not point directly at one another is used. Accordingly, particular horizontal and vertical angles, ideally greater than ½ the main beamwidth, are used to align nodes of the lattice. Additionally, inter-nodal distances and polarization isolation are taught to improve mutual interference.

Description:
BACKGROUND OF THE INVENTION 
     Before wireless high frequency point-to-point communication service can be provided on a mass basis in an area there must exist a deployment scheme that can support the planned service. Such a scheme must be able to deploy a large number of radio links, i.e., two way radio communication established through main beams of both transmitting and receiving antennas where the antenna gain is at its maximum in a given area such that the individual radio links do not significantly interfere with one another. 
     There are a number of parameters that determine the magnitude of such interference, such as the antenna gain in the path of the interference, the “hop” distance between interfering and interfered, polarization isolation and frequency channel separation. For example, interference is worst case where the interfering transmitting main beam is directed towards the interfered receiving main beam, somewhat less when the interfering main beam is directed toward the interfered receiving sidelobe, and even less when the interfering transmitting sidelobe is directed toward the interfered receiving sidelobe. Additionally, the interference decreases the farther apart, i.e., the greater the “hop” distance between the interfering and the interfered. Likewise, orthogonal polarization and frequency separation provide radiation in transmitting to receiving interference. 
     A good example of a deployment scheme is the cell structure currently in use for cellular wireless service. The cellular cell structure provides a model to show that the interference is controllable by frequency reuse and sectorization. Typically, in a cellular network, each set of frequencies is reused in every seventh cell, with each cell divided into three sectors. 
     Cellular networks are broadcast based such that a transmitter sends out signals into a designated area and any receiver within that area can pick up the signals, if properly tuned. Point-to-point radios work at a frequency, typically above 18 GHZ, where the wavelengths are short so that for effective communication the transmitter and receiver must be pointing essentially directly at each other, i.e., line of site. Such narrow beam transmission implies that the transmitters and receivers are all in fixed positions with respect to each other where their density is not great. Thus, in contrast to cellular systems, there is no need in point-to-point systems to “blanket” a given area with transmitted signals. This line of sight requirement has allowed point-to-point systems to be constructed without regard to each other. However, as the demand for point-to-point systems increases (because of their inherent higher data carrying capacity), interference between discrete systems will result when a particular receiver is within the radiation pattern of more than one transmitter. 
     Thus, a need exists in the art for a system and method for developing a deployment pattern for transmitter/receiver pairs so as to minimize interference while maximizing the frequency reuse pattern. 
     A further need exists for such a deployment system which can be replicated from location to location. 
     A further need exists for such a system in which not all of the transmitter/receiver pairs need be deployed at any time, but which will accommodate growth in any direction throughout the deployment region on a pre-approved basis. 
     A need exists for a deployment standard for high frequency radio transmission systems which will allow any transmitter/receiver pair to be added by any user at certain calculable points within a geographic region while still maintaining maximum effective coverage within that region. 
     SUMMARY OF THE INVENTION 
     These and other objects, features and technical advantages are achieved by a system and method in which a network node deployment structure is utilized to improve the mutual interference associated with densely deployed radio systems as well as to provide a small exclusion zone for any single link. Accordingly, a lattice matrix structure is used to control interference between transmitter/receiver pairs. In the ideal case, the lattice nodes would be multi-level, i.e., at alternating heights, perhaps on top of tall and medium height buildings respectively. Therefore, the preferred embodiment of the present invention provides dense radio deployment where nodes are disposed such that interference is present to all, is through antenna sidelobes only, thus significantly improving mutual interference. 
     In the preferred embodiment of the present invention, the angle α is the angle in the horizontal plane between adjacent antenna nodes on different planes (the high (H) plane and the low (L) plane) and the angle β is the angle in the elevation or vertical plane between adjacent antenna nodes on different planes. The angle α may be equal to angle β to reduce the complexity of implementing the lattice structure of the present invention while providing interference avoidance. Ideally, both angles are smaller than 5 degrees. 
     A key factor in increasing radio link density is to place radio links as closely together as possible, including the exclusion (radiation) zone of the transmitting antenna. The primary method for accomplishing the desired link density is to insure that the main beam of any transmitting radio antenna does not point directly at (or send its radiation pattern to) any receiving radio antenna other than the desired receiving antenna. Other factors, which are important in the placement transmitting and receiving radio antennas, are antenna polarization isolation and antenna sidelobe control. 
     The 3-dimensional lattice structure has the capability of keeping interfering radio links from pointing directly at one another. Thus, it is a technical advantage of this invention that the horizontal and vertical angles between antennas can be pre-calculated so as to allow for maximum density and minimum interference. 
     It is a further technical advantage of this invention that the angles, both horizontal and vertical, can be adjusted to accommodate various geometric constraints while still serving to maximize the coverage within a given geographic zone. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 shows a top down (horizontal plane) view of the lattice node structure of this invention; 
     FIG. 2 shows a side (vertical plane) view of the lattice node structure; 
     FIG. 3 shows the transmit signal zones of a typical node; 
     FIGS. 4A,  4 B and  5  illustrate how, with the inventive layout of nodes, interference will not occur; 
     FIG. 6 shows the polarization assignment for the nodes; and 
     FIG. 7 shows the node structure connected to a switch for intercommunication and for communication with the public switched (or private) communication network. 
    
    
     DESCRIPTION OF THE INVENTION 
     Turning now to FIG. 1, there is shown the top down view of a three-dimensional lattice of the present invention in which adjacent nodes alternate between disposition in a high plane (H) and a low plane (L). Accordingly, nodes  101 ,  103 ,  105 ,  107 ,  109  and  111  are in the L plane and nodes  102 ,  104 ,  106 ,  108 ,  110  and  112  are in the H plane. For example, node  105  is Low (L) and next adjacent nodes  102 ,  104 ,  106  and  108  are high (H), whereas diagonal nodes  101 ,  103 ,  107  and  109  are also Low (L). Ideally, all of the high nodes are on the same plane and all of the low nodes are on the same plane although different such planes may be used. Typically, the angle α is the horizontal angle offset between adjacent antenna nodes (as shown in FIG. 1) and the angle β is the vertical angle between adjacent antenna nodes on different planes (as shown in FIG.  2 ). Ideally, angle α is equal to angle β to simplify the implementation of the lattice structure while avoiding interference. 
     As discussed, in FIG. 1, the horizontal angle between adjacent nodes is the angle α while, as shown in FIG. 2, the angle between adjacent nodes in the vertical direction, such as between nodes  101  and  102 , is the angle β. 
     As with cellular radio networks, the number of radios in a point-to-point network that may be deployed in a given area is limited by interference from other radios operating on the same frequency. The point-to-point radio interference characteristic is as shown in FIG. 3, which shows radio A transmitting a signal to radio receiver B. Areas  301  and  302  are referred to as exclusion zones. For example, area  302  and  301  are where the interference from radio A would be too much for other receivers pointing in the direction of radio A and operating on the same frequency channel to function. 
     FIG. 3 also shows a typical relative size of the exclusion zones. The exclusion zones of FIG. 3 shows the worst case interference as the transmitting main beam to receiving main beam for a one frequency, same polarization, deployment. It should be appreciated that, in certain directions, zone  302  covers a very large area relative to the link dimension, L, between radios A and B and thus another receiver operating on a same channel and in line with radio A as receiver B should not be disposed in this transmission shadow. 
     The angles α and β are selected with reference to the beamwidth (w) of the transmission exclusion zone. Ideally, α and β should be larger than half the beamwidth of the antenna as measured in their respective planes. Normally beamwidth is measured at the 3 dB down points, however, the preferred embodiment of the present invention measures transmission null to null for determination of the angles α and β. Accordingly, if a is larger than half of the null to null beamwidth in the horizontal plane, there is no transmission horizontally to interfere with another node disposed in the same horizontal plane. Likewise, if β is larger than half of the null to null beamwidth in the vertical plane, there is no transmission horizontally to interfere with another node in the same horizontal plane. This relationship is described in further detail with reference to FIG. 4A below. 
     It shall be appreciated that the exact value of angles α and β can be adjusted for sidelobe radiation patterns. Moreover, the angles α and β may not be consistent throughout the lattice, such as to adjust for differences in height and spacing between adjacent nodes due to terrain or for presence of signal anomalies, such as multipath or shadows. Since the radiation pattern of antenna may not be the same in the horizontal plane as it is in the vertical plane the angle α might be different from the angle β. 
     In better understanding the lattice structure of the present invention, the first step is to analyze the lattice in a horizontal plane. Shown in FIG. 4A, having the same lattice structure of FIG. 1 extended to include additional nodes, are two adjacent rows of nodes in a horizontal plane. It shall be appreciated from a review of FIG. 4A that avoidance of interference according to the present invention is two phase: angular and spatial. Accordingly, the angle α in the horizontal plane is selected not only such that only a desired adjacent node is within the antenna beam, but also such that a distance sufficient to avoid undesired interference is provided between any other nodes of the lattice which may fall along a vector associated with this antenna beam. 
     It shall be appreciated that the inter-nodal distance  1 , in combination with the angle α, affects this relationship. Accordingly, selection of the inter-nodal distance  1  may be utilized in combination with the selection of the angle α according to the present invention. For example, as shown above in FIG. 3, the transmission shadow of a particular point-to-point transmitter is typically thirty-one times the separation distance of the transmitter/receiver pair (this is typically because the transmitter power is adjusted to be sufficient to provide an acceptable signal at the receiver even during signal attenuating events such as rain fall). Accordingly, in an alternative embodiment, such as where the vertical angle β is small or null, the angle α and the inter-nodal distance  1  are selected such that a second receiver is not disposed along a vector of a first transmitter/receiver link not associated with the second receiver within a distance  31 (1) of the first transmitter. 
     The relationship of selection of the angle α and the inter-nodal distance  1  is better understood with reference to the nodes illustrated in FIG.  4 A. Link ab from node a to node b will be used for interference analysis. Line  1  is a vector extending from the link ab. Where the angle α is properly selected, only the desired two main antenna beams, shown here as transmitter/receiver pair a/b, will point at each other within a selected distance of link ab. Specifically, the main antenna beams of nodes of a same row or column, as illustrated in FIG. 4A, will not point at each other as long as the angle from the center of the main beam to the first sidelobe is less than α. 
     Additionally, by applying the angle α to direct the vector associated therewith in alternate directions for ones of the nodes, an alternating pattern, as shown in FIG. 4A, is produced. Accordingly, although one node of the transmitter/receiver pair c/d is disposed along the vector of link ab, line  1 , the transmitter/receiver pair c/d utilize a link cd (from node c to node d) having an angle  2 α from line  1  because of the angle α associated with link ab and the angle α associated with link cd. It should be appreciated from a review of FIG. 4 that this non-alignment of antenna beams through alternate application of the angle α holds true for all H to L links, i.e., no H node to L node link aligns with another H node to L node link, and for all L to H links, i.e., no L node to H node link aligns with another L node to H node link. 
     It shall be appreciated that, although transmitter/receiver pair d/e are disposed along the vector of link ab, line  1 , the d/e transmitter/receiver pair are disposed a substantial distance from the a/b transmitter/receiver pair. Accordingly, through proper selection of the angle α and inter-nodal distance  1 , no other transmitter/receiver pair is in the exclusion pattern of the ab link, nor will the a/b pair be within the exclusion zone of a third beam. Moreover, as will be fully appreciated from the discussion below, link de is a L to H link whereas link ab is a H to L link. 
     The lattice structure of the present invention provides for the use of the angle β in the vertical plane to provide nodes alternating between a L and H horizontal plane. Accordingly, line  1 , emanating from node a in the H plane and directed to node b in the L plane (therefore directed down as shown in FIG.  5 ), would not even intersect node d disposed in the L plane or node e disposed in the H plane, as long as β is selected such that no portion of the antenna main beam transmission is at the horizontal level from a to d or e. Therefore, although link ab appears to have main beam interference with link de in the horizontal plane (provided the transmission shadow of node a were sufficiently long), there is no such interference because of the alternate application of the angle β in the vertical plane. Accordingly, discrimination, in addition to the distance between transmitter/receiver pairs a/b and d/e, is provided according to the present invention. 
     It shall be appreciated that the use of the three dimensional lattice, including the α and β angles described above, allows for reuse of a transmitter/receiver pair operating on a same channel with substantially less separation there between than the full length of the transmission shadow. Moreover, through the use of different frequencies, i.e., assigning different channels to ones of the transmitter/receiver pair, in combination with the above described lattice structure utilizing the angles α and β allows for transmitter/receiver pairs utilizing different ones of the channels to be interleaved in order to provide denser coverage and/or more desirable signal conditions (better signal to noise, reduced co-channel interference, etc.). 
     An additional reduction in interference is possible using both vertical and horizontal (or other orthogonal or substantially orthogonal) polarization in the propagation for the reason that each antenna designed to receive one polarization will receive less energy having a different polarization. FIG. 6 shows a preferred embodiment alternating the transmitter polarization assignment (note that the transmitter and intended receiver parts are to always employ the same polarization). H and V represent the horizontal and vertical polarization respectively. Mapping the polarization assignment on FIG. 4A as shown in FIG. 4B, the interference node a to node f must be from a sidelobe of node a having a vertical polarization transmitter to a sidelobe of node f having a horizontal polarization receiver. This alternating polarization further reduces the mutual interference experienced between the two nodes. Node a and node e might interfere with each other through main beams when node a and node e are on a two-dimensional antenna placement and their power/gain are sufficient to communicate over the “hop” distance. However, the alternating polarization will provide the receiver at node e with a different polarization than that of node a&#39;s transmitter. So additional mutual isolation is also provided between these nodes which might otherwise experience undesirable levels of interference through polarization. 
     The above described implementation of the lattice of the present invention ensures the sum of interference from all radios in the network as perceived by any one of the radios is low enough to permit its functioning properly. This result is due substantially to the ability of the three dimensional lattice of the present invention deploying transmitter/receiver pairs such that the main-beam of other such transmitter/receiver pairs cannot directly interfere. 
     Excluding main-beam, interference has another effect of simplifying point-to-point radio network deployment. The optimization of interference and signal path can be done independently. For example, the signal path of every link can be optimized substantially without the concern of interference, i.e., the transmit power of a particular transmitter may be increased to provide desired signal conditions at the associated receiver without concern of substantially increasing the interference at other such receivers. Antenna sidelobe magnitude, polarization isolation and radio separation in distance are the design parameters for interference control, however the lattice structure of the present invention has already taken these parameters into consideration for the particular radiation characteristics of the transmitters and receivers. 
     Directing attention to FIG. 7, a further advantage of the lattice structure of the present invention may be seen. Here switch  701  is coupled to nodes of the lattice. Switch  701  may be a public switched network (PSN), a fibre optic backbone, or other such link providing intercommunication between nodes of the lattice and nodes external thereto (not shown). Moreover, switch  701  may provide communication between ones of the nodes of the lattice, such as to provide additional bandwidth where point-to-point links there between are at or near capacity, or where a link or links have failed, such as where a particular node is out of service. Regardless of the use to which switch  701  is put, it shall be appreciated that nodes of the lattice may establish communication with, or through, switch  701  through at most two point-to-point links. Moreover, through coupling switch  701  to slightly more nodes of the lattice, each node may establish communication with, or through, switch  701  through at most one point-to-point link. Accordingly, resources may be made available to nodes of the lattice very efficiently as independent connection to such resources is not required. 
     Although described above as a single angle α, associated with the horizontal plane, and a single angle β, associated with the vertical plane, the lattice of the present invention may utilize a plurality of either of such angles to provide flexibility in deploying the structure in environments including terrain differences or other obstacles such as buildings or to provide more dense deployment of transmitter/receiver pairs in one area of the lattice as compared to that of another part of the lattice. For example, in a metropolitan area where the nodes of the lattice are desired to be more densely deployed, i.e., inter-nodal distance  1  is small, the angle β may be selected to be more acute for this area while being less acute for a more suburban area. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.