Abstract:
A millimeter-wave point-to-multipoint LMDS radio system for broadband wireless access having frequency reuse that includes a cell area divided into an integer number k sub-sectors of equal angular arc 360/k degrees each; a hub having antennas each configured to generate an antenna beam that covers an integer number j sub-sectors, an angular gap i being defined between edges of coverage of one of the antennas where i is an integer number of sub-sectors; a total integer number of n channel sets each of at least one radio channel and each configured independent and free of interference from other ones of the channels in other ones of the channel sets, each of the channel sets having a number of uses defined by a quantity k/(i+j) that is an integer equal or greater than two, each channel set being deployed with a common re-use pattern with an angular stagger between antennas of (i+j)/n sub-sectors.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This is a continuation-in-part of U.S. Ser. No. 09/307,692, filed May 10, 1999, which is a utility patent application based on provisional patent application No. 60/085,351, filed May 13, 1998. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to antenna configurations in a sectorized radio system coverage area served by a central hub station that employs frequency re-use of a given channel in different sectors. 
     2. Discussion of Related Art 
     Fixed point-to-multipoint radio systems operate at millimeter-wave frequencies or microwave frequencies. Such systems that operate at 24-42 GHz are sometimes termed Local Multipoint Distribution Systems (LMDS) or Local Multipoint Communications Systems (LMCS). These systems could be either one-way broadcast type, or two-way systems. Such systems that operate at approximately 2.5 GHz are sometimes termed Multi-channel Multipoint Distribution Systems (MMDS). Traditional cellular radio systems that use antennas provide sectorization of a cell. 
     In all cellular-structured radio systems, there is frequency re-use. In VHF and UHF bands, this means the re-use of a given channel in a nearby cell. At higher frequencies, such as millimeter-wave bands from 24-42 GHz, and possibly at lower microwave frequencies such as 2.5 GHz, radio propagation is closer to straight-line and base station or “hub” antenna patterns are sharply defined. This opens the possibility of re-using a given channel within the same cell site in another sector. 
     An area to be served by a single hub is angularly-divided into k sub-sectors of equal angular arc, the sub-sectors radiating out from the hub. All antennas have identical sectoral radiation patterns approximately j sub-sectors wide. A first such antenna provides coverage of the first j sub-sectors using a first channel set. After a gap of the next i sub-sectors, the next j sub-sectors are covered by another antenna re-using the first channel set. This continues around 360 degrees of arc. The quantities k, i, j and k/(i+j) are integers. k/(i+j)&gt;=2 and is equal to the number of times the first channel set is reused in the cell. 
     A second channel set is used to provide additional beams. These are similarly spaced i+j sub-sectors apart, but the first such beam is angularly staggered from the beams using the first channel set by (i+j)/n sub-sectors, where n is the total number of channel sets available. The quantity (i+j)/n need not necessarily be an integer. There are also k/(i+j) re-uses of the second channel set. The other channel sets, up to a total of n channel sets, are similarly staggered. In total, there are kn/(i+j) antenna beams using n channel sets. For complete coverage of 360 degrees with no gaps, n&gt;1+i/j is required. 
     The traffic capacity of a cellular radio system is proportional to the number of channel sets in use. A normalized measure of capacity is therefore the number of beams per hub divided by the number of channel sets available to the system. By this measure, a conventional cellular radio system with an omnidirectional antenna radiating n channel sets would have a capacity of 1. 
     The capacity of conventional cellular radio systems may be increased with narrow-beam sector antennas at the hub. However, adding antennas to an existing hub requires replacement or re-orientation of the existing antennas. Also, existing hubs have no appreciable redundancy so that equipment failure results in loss of service until repairs are complete. Still, redundancy may be made available but it requires doubling the number of radios. 
     BRIEF SUMMARY OF THE INVENTION 
     One aspect of the present invention relates to an improved system of frequency reuse in millimeter-wave point-to-multipoint LMDS radio systems for broadband wireless access in which each channel can be reused several times within a single base station or hub in order to increase traffic capacity. 
     The cell area is divided into an integer number k sub-sectors of equal angular arc 360/k degrees each. Each antenna beam covers an integer number of j sub-sectors. Each channel set, out of a total integer of n sets, is re-used. The angular gap between edges of beams, which use the same channel set, is an integer number of i sub-sectors. The quantity k/(i+j), which is equal to the number of uses of each channel set, must be an integer greater or equal than 2. Each channel set is deployed with the same re-use pattern, but the channel sets are angularly staggered from each other by (i+j)/n sub-sectors, which is not necessarily an integer. To cover each part of the cell with a uniform number of channel sets using the minimum number of channel sets, then n=i+j, the angular stagger between antennas is one sub-sector, and there are k antennas. The number of channels within each channel set need not be equal. In total, the hub uses kn/(i+j) antenna beams, and the normalized traffic capacity is k/(i+j). 
     The invention may serve unanticipated locally higher capacity requirements in some portions of the cell by permitting an increase in capacity to be effected without the need to employ narrow-beam sector antennas at the hub. Additional antennas can be added to an existing hub without the need to replace or re-orient the existing antennas. Overlapping sector beams provide redundancy to increase reliability in the event of equipment failure. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     For a better understanding of the present invention, reference is made to the following description and accompanying drawings, while the scope of the invention is set forth in the appended claims. 
     FIG. 1 is a schematic representation of a circular coverage area of a cellular radio system in accordance with an embodiment of the invention that has sector antenna beams, each being 120 degrees wide and having an angular stagger between antennas that is an integer number of 60 degree sub-sectors. 
     FIG. 2 is a schematic representation of a circular coverage area of a cellular radio system in accordance with a further embodiment of the invention. This embodiment has sector antenna beams, each being 60 degrees wide and an angular stagger between antennas that is an integer number of 60 degree sub-sectors. 
     FIG. 3 is a schematic representation of a circular coverage area of a cellular radio system in accordance with another embodiment of the invention. This embodiment has sector antenna beams, each being 120 degrees wide and an angular stagger between antennas that is a non-integer number of 60 degree sub-sectors. 
     FIG. 4 is a schematic representation of a circular coverage area, which is used for a cellular radio system in accordance with yet a further embodiment of the invention. This embodiment has sector antenna beams, each being 90 degrees wide and having an angular stagger between stagger between antennas that is an integer number of 45 degree sub-sectors. 
     FIG. 5 is a flow chart of an algorithm used by a remote station to recover from equipment failure at the hub. 
     FIG. 6 is a flow chart of an algorithm used by a remote station to dynamically re-tune channels in response to load-balancing at the hub. 
     FIG. 7 is a schematic representation of a conventional three sectored cell. 
     FIG. 8 is a schematic representation of a conventional cell-splitting technique. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 depicts a circular coverage area or cell  101 , with a base station or hub  102  at its center, divided into k (k=6) equal 60 degree sub-sectors  111 - 116 . The hub  102  comprises backhaul facilities, such as optical fiber or leased lines, multiplexing equipment, radio modems, radios and antennas. The hub is capable of radiating and/or receiving one or more modulated radio channels through each antenna. 
     A first hub antenna  121  having a 120 degree radiation pattern (approximately pie-shaped) provides coverage to remote stations such as  141   a  and  141   b , located in j (j=2) sub-sectors  111  and  112 . Communications through antenna  121  make use of a first channel set  151 . 
     A channel set is a set of one or more radio channels, which are approximately “orthogonal”, i.e., independent and free from interference from all other channels in the other channel sets. Commonly, channel sets are comprised of a set of distinct radio carrier frequencies. However, channel sets could also be comprised of sets of independent time slots (in time-division multiple access systems) or substantially uncorrelated spreading codes (in code-division multiple access systems) or unique combinations of frequencies, time slots or spreading codes. Further, the use of a frequency in conjunction with horizontally polarized antennas in one channel set and vertically polarized antennas in another channel set allows the same frequency to be included in two different channel sets. 
     After a gap i (i=1) sub-sectors, (sub-sector  113 ), channel set  151  is re-used again, in this case in the beam of antenna  124 , which covers sub-sectors  114  and  115 . Thus, channel set  151  is reused 2 times in the cell, or k/(i+j) times in general. 
     A second channel set  152  is used for the beam of antenna  122 , which covers sub-sectors  112  and  113 . A third channel set  153  is used for the beam of antenna  123 , which covers sub-sectors  113  and  114 . The angular stagger, in sub-sectors between beams is one sub-sector, or in general, (i+j)/n, where n is the total number of channel sets to be used in the cell. 
     Similar to channel set  151 , channel set  152  is re-used again, with a gap of 1 sub-sector between the edge of its beam and the beginning of the next beam which re-uses the same channel set. The second use of channel set  152  is from antenna  125 , which covers sub-sectors  115  and  116 . 
     Similar to channel sets  151  and  152 , channel set  153  is used twice, from antenna  123  which covers sub-sectors  113  and  114 , and from antenna  126  which covers sub-sectors  116  and  111 . 
     The complete system consists of 6 antennas using 3 channel sets. The system provides complete coverage to all portions of the cell. Two overlapping beams cover every cell portion. 
     Turning to FIG. 2, an embodiment is shown where j=1 and i=1, which means that k must be even. Where n=2, a pattern of non-overlapping sectors with channel sets alternating in adjacent sectors results. FIG. 2 depicts an embodiment where k=6, which means that sector antenna beams are 60 degrees wide. Antennas  221 - 226  cover sub-sectors  211 - 216  respectively. Antennas  221 ,  223  and  225  use the channel set  251 . Antennas  222 ,  224  and  226  use the channel set  252 . The normalized capacity of this system is 3, which is greater than the case of FIG.  1 . 
     Turning to FIG. 3, an embodiment is shown where k= 6  as in FIG. 2, except that not all areas are served by the same number of beams. In the embodiment of FIG. 3, n&lt;(i+j) but n&gt;=1+i/j so that there is complete coverage of the entire cell by at least one beam, but not all areas are served by the same number of beams. Here, j=2 and i=1 as in FIG. 1, but n=2, not 3 as in FIG.  1 . 
     In this particular case, the angular stagger between the two channel sets, (i+j)/n sub-sectors is a non-integer, 1.5 sub-sectors. Antennas  321 - 324  have 120 degree patterns, as in FIG.  1 . Antenna  321  covers sub-sectors  311  and  312  using channel set  351 . Antenna  322  covers sub-sector  313 , as well as half  312   b  of sub-sector  312  and half  314   a  of sub-sector  314  using channel set  352 . Antenna  323  covers sub-sectors  314  and  315  using channel set  351 . Antenna  324  covers sub-sector  316 , as well as half  315   b  of sub-sector  315  and half  311   a  of sub-sector  311  using channel set  352 . 
     Note that only ⅓ of the cell area is covered by two channel sets  351  and  352 ; the other ⅔ is covered by only 1 channel set. The only portions covered by two channel sets are half  311   a  of sub sector  311 , half  312   b  of sub-sector  312 , half  314   a  of sub-sector  314  and half  315   b  of sub-sector  315 . Average normalized traffic capacity across the entire cell is still 2, as in FIG. 1, however. This configuration is useful if there are not enough orthogonal channels available in the system to form 3 channel sets, or if j&gt;&gt;i and therefore the areas with lower capacity are a small fraction of the cell. Also, the stagger need not be exactly (i+j)/n sub-sectors in this case for the properties to hold. 
     Many LMDS systems are based on 4-sectored cells, so that the hubs may be laid out on a square grid with square coverage areas. The system of the present invention may be advantageously employed to split such cells without disturbing the original 4 antennas, as shown in the embodiment of FIG.  4 . 
     FIG. 4 depicts an 8-sectored system using 90 degree beams, which may be obtained by addition of four antennas to a pre-existing 4-sectored cell. In this case, k=8, j=2, i=2, n=4. Normalized capacity is 2. Antenna  421  covers sub-sectors  411  and  412  using channel set  451 . Antenna  422  covers sub-sectors  412  and  413  using channel set  452 . Antenna  423  covers sub-sectors  413  and  414  using channel set  453 . Antenna  424  covers sub-sectors  414  and  415  using channel set  454 . Antenna  425  covers sub-sectors  415  and  416  using channel set  451 . Antenna  426  covers sub-sectors  416  and  417  using channel set  452 . Antenna  427  covers sub-sectors  417  and  418  using channel set  453 . Antenna  428  covers sub-sectors  418  and  411  using channel set  454 . 
     Referring to FIG. 1, assume that there is a failure of the hub radio that serves antenna  121 . Remote station  141   b  loses communication with the hub. Following its pre-configured strategy, it would then re-tune to a channel within channel set  152 , as it is located in sub-sector  112 , which is also covered by channel set  152 . It could then re-establish communications with the hub via antenna  122 . Similarly, remote station  114   a  would re-tune to a channel within channel set  153 , as it is located in sub-sector  111 , which is also covered by antenna  126  using channel set  153 . Thus, the overlapping sector system can provide equipment redundancy without the need for additional radios, which increases the availability of the system in the face of equipment failure at the hub. 
     FIG. 5 depicts a flow chart of the algorithm used by a remote station such as  141   b  to recover from equipment failure at the hub. At the time of initializing a new subscriber station, step  710  is performed. Remote station  141   b  (FIG.  1 ), for example, would be loaded with a primary channel in channel set  151 , and a secondary channel in channel set  152 . At step  720  (FIG.  5 ), the remote station tunes to the primary channel and begins monitoring downstream signaling from hub  102  (FIG. 1) via antenna  121 . The remote station then enters a loop at step  730  (FIG. 5) in which it continually checks if communication with the hub on the primary channel has been lost. 
     For example, the remote station might expect a regular message from the hub and reset a timer each time the message is received. Expiry of the timer would indicate that the regular message was not received within a certain time interval and would be an indication of probable hub failure of the radio using antenna  121  (FIG.  1 ). This condition would cause the remote station, at step  740  (FIG.  5 ), to re-tune to its secondary channel, which would be in channel set  152 . As antenna  122  using channel set  152  covers the area of the cell where remote station  141   b  is located, the remote station would then be able to successfully re-establish communications with the hub. 
     The failure of a radio connected to antenna  121  would be expected to cause system alarms resulting in notification of the network operator and eventual replacement of the radio. When channel set  151  from antenna  121  becomes available again, the hub could send a notification message to remote station  141   b . This notification would be sent via antenna  122  on the channel within channel set  152  in use by remote station  141   b . At step  750 , this notification would be detected. The remote station would re-tune to the primary channel again, and re-establish communications with the hub via antenna  121 . 
     Also, the system of the present invention has an increased ability to serve local “hot spots” of traffic within the coverage area of the hub compared to conventional narrow-beam sectorization systems. As each beam covers a relatively broad area, the total traffic of high-traffic and low-traffic sub-areas within the covered sector may remain within the traffic limit that can be serviced by a channel set. 
     An additional advantage of this system accrues if the remote stations have the ability to dynamically re-tune from channels in one channel set to channels in another channel set. This re-tuning may preferably arise upon loss of communication or upon an indication from the hub of current traffic conditions. 
     For example, in FIG. 1, assume remote station  141   b  is in communication with the hub station via antenna  121  using a channel in channel set  151 . Remote station  141   b  could be pre-configured to re-tune to a certain channel within channel set  152  in the event it loses communication on the channel in channel set  151 . Furthermore, since each remote station is within the coverage area of two channel sets, the hub could load balance, or dynamically assign remote stations to one channel set or the other, depending on traffic conditions of the moment, in order to make full use of both channel sets. Alternatively, the hub could broadcast information about current traffic loading of each channel set, and the remote stations could autonomously choose which channel sets to use based on the information. 
     FIG. 6 depicts a flow chart of the algorithm used by a remote station, such as  141   b  (FIG.  1 ), to dynamically re-tune channels in response to load-balancing at the hub. At the time of initializing a new subscriber station, step  810  is performed. Remote station  141   b , for example, would be loaded with a primary channel in channel set  151 , and a secondary channel in channel set  152 . At step  820 , the remote station tunes to the primary channel and begins monitoring downstream signaling from hub  102  via antenna  121 . Note that these are the same steps required for the redundancy-switching algorithm described in FIG.  5 . The remote station then enters a loop at step  830  in which it continually checks if the hub has indicated the need to re-tune to another channel. This condition would cause the remote station, at step  840 , to re-tune to its secondary channel, which would be in channel set  152 . As antenna  122  using channel set  152  covers the area of the cell where remote station  141   b  is located, the remote station would then be able to successfully re-establish communications with the hub. The hub would request this channel change, for example, if channel set  151  is being used to full capacity by another remote station, whereas channel set  152  has spare capacity. 
     When traffic conditions make it advantageous for the hub to move remote station  141   b  back to a channel in channel set  151 , the hub could send a notification message to the remote station at step  850 . The remote station would re-tune to the primary channel again, and re-establish communications with the hub via antenna  121 . 
     Even without dynamic re-tuning, the system of the present invention provides an improved ability to serve local “hot spots” of traffic, which may not be predicted at the time of deployment. Each antenna covers a relatively wide area of two sub-sectors, as compared to a system using 60 degree antennas. However, what counts is the total traffic seen from all remote stations in a 120 degree area. Therefore, high traffic from a few users in one 60 degree sub-sector may be balanced by low traffic from other users in the adjacent 60 degree sub-sector covered by the same antenna. As a result, the total traffic is still below the average traffic per sub-sector that the system is designed to serve. 
     For example, assume that the capacity of each channel set is 1 Mbit/s and each remote station presents a demand of 0.1 Mb/s. Thus, each channel set can support 10 remote stations. In the conventional 6-sectored arrangement of FIG. 8, each 60 degree sector can contain 10 remote stations, and the overall cell capacity is 60 remote stations. In the arrangement of FIG. 1, according to the present invention, there is also a total of 6 channel sets in use. This results in a total cell capacity of 60 remote stations; however, the arrangement is more tolerant of localized “hot spots” containing more than 10 remote stations in a given 60 degree sub-sector. For example, suppose sub-sector  111  contains 12 remote stations and sub-sector  112  contains 8 remote stations. In this case, 10 remote stations in sub-sector  111  can be served using channel set  151  from antenna  121 . The other 2 remote stations in sub-sector  111  could be served using channel set  152  from antenna  122 , along with the 8 remote stations in sub-sector  112 . 
     When j=1, there is no overlap of sectoral beams. When j&gt;=2, the sectoral beams overlap. When j&gt;=2 and in addition, n=i+j, then every portion of the coverage area is served by j beams and capacity is uniform throughout the cell. The system of this invention provides a normalized capacity of k/(i+j), which is always at least 2, therefore providing capacity gain over a conventional system. 
     Furthermore, by using overlapped sector beams, the system of this invention provides capacity gain without the need to employ sector antennas with coverage angles as narrow as a sub-sector. This may be advantageous, for example, in that a radio network operator can make use of one relatively broad-beamed antenna type to provide maximum coverage with minimum number of sector antennas in some cells, yet the operator can also configure high-capacity cells using the same part. 
     In addition, an operator can use the system of this invention to provide additional traffic capacity in an existing hub by the addition of new overlapping beams to the existing beams. This has an advantage over conventional cell splitting in that the existing antennas need not be re-oriented or replaced by narrower-beam antennas. 
     For instance, consider that the capacity of the cell in FIG. 1 is 6 channel sets that are being used. The normalized capacity is therefore  2  (6 channel sets used divided by 3 channel sets available). If only antennas  121 ,  123  and  125  are provided, using channel sets  151 ,  153  and  152  respectively, then the result is a conventional 3-sectored cell arrangement. Thus, the addition of antennas  152 ,  154  and  156  to a conventional 3-sectored cell doubles the capacity without any need to employ antenna types other than those with 120 degree beams. 
     The original three antennas need not be replaced or re-aligned. FIG. 7 depicts a conventional 3-sectored cell. The arrangement of FIG. 7 can be changed into that of FIG. 1 without changing the original three antennas or three channel sets used by those original three antennas. This has the advantage that service to the existing subscribers served by antennas  121 ,  123  and  125  need not be disrupted while the new configuration is being installed. By comparison, traditional cell-splitting techniques used to increase capacity in sectorized cellular radio systems, shown in FIG. 8 would involve replacement of the original  3  antennas  121 ,  123  and  125  by 6 new antennas  221 - 226 , each covering only 60 degrees. Note also, that conventional cell-splitting techniques would involve the use of six channel sets  251 - 256  for the six sectors created. 
     While the foregoing description and drawings represent the preferred embodiments of the present invention, it will be understood that various changes and modifications may be made without departing from the spirit and scope of the present invention.