METHOD AND APPARATUS FOR PROVIDING FORWARD LINK SOFTER HANDOFF IN A CODE DIVISION MULTIPLE ACCESS COMMUNICATION SYSTEM

Abstract: A method and apparatus for providing forward link softer handoff in a code division multiple access communication system comprises utilizing spatially diverse antenna arrays and rerouting a softer handoff talk channel from a radiation pattern (415) of a first antenna array (404) into a radiation pattern (417) of a second antenna array (406).

Description: METHOD AND APPARATUS FOR PROVIDING FORWARD LINK SOFTER HANDOFF IN A CODE DIVISION MULTIPLE ACCESS COMMUNICATION SYSTEM Field of the Invention The present invention relates generally to communication systems, and more particularly, to a method and apparatus for providing forward link softer handoff in a code division multiple access (CDMA) communication system with antenna arrays, including narrow beam antennas. Background of the Invention A cellular communication system typically includes one or more mobile stations or units, one or more base stations, and a mobile telephone switching office. Although only three cells 14 and two mobile stations 11 are shown in FIG. 1, a typical cellular network may comprise hundreds of base stations, thousands of mobile stations, and more than one mobile telephone switching office. Each cell will have allocated to it one or more dedicated control channels and one or more voice channels. In rural areas, the antenna towers 12 are commonly located at the center of a cell 14, thereby providing omni-directional coverage. In an omnidirectional cell, the control channels and the active voice channels are broadcast in all areas of the cell, usually from a single antenna. Where base stations are more densely located, a sectorized antenna system may be employed as in the prior art, and shown by the schematic diagram of FIG. 2. Sectorization requires directional antennas 20 having, for example, a 120 degree radiation pattern as illustrated in FIG. 2. Each sector 22 is itself a cell having its own control channels and traffic channels. Note that "channel" may refer to a specific carrier frequency in an analog system, to a specific carrier /slot combination in a hybrid TDMA/FDMA system such as IS-54 and GSM, or to a specific PN code in a CDMA system such as IS-95. Existing narrow beam antenna arrays, which may comprise uniform linear arrays including dipoles or patch elements and passive beam forming networks including Butler Matrices, typically produce several beams covering a sector. For example, a four beam pattern which includes four 30 degree beams covering a 120 degree sector or region is not uncommon. These beams typically have beam crossovers at approximately minus 4 dB, whereas sector antennas typically have beam crossovers at approximately minus 8 dB. Thus, beam-to-beam overlap within a narrow beam antenna pattern is significantly greater than sector-to-sector overlap within a sectorized antenna pattern, or even the overlap of beams at the edge of two narrow beam antennas. In a CDMA system with narrow beam antennas, the forward link of adjacent beams produces interference in the beam overlap region. The beam overlap region of available narrow beam antenna panels is greater than the beam overlap region of available discrete antennas, because there is less control over the beams when using passive RF beam forming networks such as Butler Matrices. As stated above, a typical beam-to-beam crossover in a narrow beam antenna pattern is approximately minus 4 dB, while sector antennas can be made narrower than the sectors they occupy, and crossover at approximately minus 8 dB. With more overlap, there is more degradation. In addition, if narrow beam antennas are used for separate CDMA PN codes (i.e., one beam used for a current sector or PN), the additional overlap means more degradation on the forward link unless softer handoff is used. One solution to the problem of increased degradation is to put the mobile station into softer handoff by making an assignment and transmitting the talk channel on both beams. Softer handoff then adds some path diversity, improving the mobile reception, and allowing a reduced transmit power allocation. With sector antennas, the larger spatial separation decorrelates the two paths and the diversity gain compensates for the overlap degradation. However, if the two beams originate in a single narrow beam antenna panel instead of the spatially separated sector antennas of a conventional system, the beams will have a higher correlation, producing less path diversity benefit, and resulting in a higher required transmit power allocation. Therefore, a need exists for an improved method and apparatus for providing forward link softer handoff in a CDMA communication system with antenna arrays. Brief Description of the Drawings The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objects, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: FIG. 1 is a schematic plan view illustrating an omni-directional cellular pattern as in the prior art; FIG. 2 is a schematic plan view illustrating a sectorized cellular pattern as in the prior art; FIG. 3 is a schematic view illustrating a talk channel in softer handoff with two adjacent beams of a narrow beam antenna as in the prior art; FIG. 4 is a schematic view illustrating the utilization of spatially diverse narrow beam antennas with routing of the softer handoff talk channel into different diversity channels of adjacent beams in accordance with the present invention; FIGS. 5A-5B are schematic views illustrating the utilization of discrete directive antennas or an array of antenna elements forming a phased array as an alternative to narrow beam antennas; and FIG. 6 illustrates a logical flowchart of the method of providing forward link softer handoff in a CDMA communication system with spatially diverse antennas according to the method and system of the present invention. Detailed Description of the Invention With reference now to the figures, and in particular with reference to FIG. 3, there is depicted a talk channel 302 in softer handoff with two adjacent beams; beams A and B of narrow beam antenna 304 as in the prior art. Narrow beam antenna 304 comprises Butler matrix 316 and a uniform linear array of antenna elements 320-326. As seen in FIG. 3, talk channel 302 is connected to switch 306, which is connected to transmitters 308-314, respectively. As will be understood by those skilled in the art, an antenna pattern is formed by routing a signal from one of the transmitters 308-314 to Butler matrix 316, where it is thereafter distributed to antenna elements 320-326. For example, signals transmitted by transmitters 308 and 310 are phased and distributed by Butler matrix 316 and antenna elements 320-326 such that the patterns produced will be beams A and B, respectively. Each of the other beams (C and D) are formed in a similar fashion. When a mobile station in the beam overlap region between two adjacent beams, for example beams A and B, requires softer handoff to a second beam, the talk channel 302 is routed via switch 306 to transmitters 308 and 310, which are coupled to the narrow beam antenna 304, producing beam patterns A and B, thereby placing the mobile station within two adjacent beams of the narrow beam antenna 304. However, since beams A and B originate in a single narrow beam antenna panel, the beams have a higher correlation, thereby producing less path diversity benefit and resulting in a higher required transmit power allocation. In accordance with a preferred embodiment of the present invention, FIG. 4 illustrates the utilization of spatially diverse antenna arrays 404 and 406 including rerouting the softer handoff talk channel 402 from a radiation pattern 415 of the first antenna array 404 into a radiation pattern 417 of the second antenna array 406. As seen in FIG. 4, talk channel 402 is coupled to softer handoff router 430, which is coupled to first antenna array transmitters 440-446 and to second antenna array transmitters 448-454, respectively. Router 430 includes first switching device 432 and second switching device 434. When a mobile station in the beam overlap region between two beams of the same array, for example adjacent beams A and B of first antenna array, requires softer handoff to a second beam, the talk channel 402 is rerouted via router 430 to transmitter 440 of first antenna array and to transmitter 450 of second antenna array, which are coupled to antenna elements 420-423 with appropriate phasing from beam former 416, and to antenna elements 424-427 with appropriate phasing from beam former 419, respectively, thereby effectively rerouting the softer handoff talk channel 402 from radiation pattern B of the first antenna array 404 into radiation pattern B of the second antenna array 406. By utilizing radiation patterns, in this example adjacent beams, from alternate, spatially separated antenna arrays for softer handoff rather than radiation patterns from the same array, the two diversity paths are less correlated, thereby improving the diversity gain. As seen in FIG. 4, the first antenna array 404 is adapted to transmit a first plurality of radiation patterns 415 (A, B, C, D) defining a first coverage area. Preferably, first antenna array 404 comprises a passive beam forming network 416 and antenna elements 420-423 of the uniform linear array 418. In the preferred embodiment, antenna elements 420-423 comprise patch elements or dipoles. Although first antenna array 404 is described as comprising a narrow beam antenna array including a passive beam forming network and a uniform linear array, it will be appreciated by those skilled in the art that other configurations may readily be utilized without departing from the spirit and scope of the present invention. For example, as illustrated in FIG. 5A, a plurality of discrete directive antennas, such as sector antennas or dish antennas, may be used instead of the passive beam forming network /uniform linear array combination illustrated in FIG. 4. Similarly, the passive beam forming network 416 illustrated in FIG. 4 may be instead an active electronic gain and phase weighting and summing to a linear or circular array as illustrated in FIG. 5B. Referring back to FIG. 4, a second antenna array 406 is adapted to transmit a second plurality of radiation patterns 417 (A, B, C, D) defining a second coverage area. As discussed above with regard to the first antenna array, in the preferred embodiment, second antenna array 406 comprises passive beam forming network 419 and antenna elements 424-427 of the uniform linear array 428. In the preferred embodiment, the first and second coverage areas overlap, and first antenna array 404 is spatially separated from second antenna array 406 by a predetermined distance, which in the preferred embodiment is approximately 10 to 15 feet horizontally. In the preferred embodiment, passive beam forming network 419 includes a Butler Matrix, and uniform linear array 428 includes a plurality of antenna elements 420, which in the preferred embodiment comprise patch elements or dipoles. Although second antenna array 406 is described as comprising a narrow beam antenna array including a passive beam forming network and a uniform linear array, it will be appreciated by those skilled in the art that other configurations may readily be utilized without departing from the spirit and scope of the present invention. For example, as illustrated in FIG. 5A, a plurality of discrete directive antennas, such as sector antennas or dish antennas, may be used instead of the passive beam forming network /uniform linear array combination illustrated in FIG. 4. Similarly, the passive beam forming network 419 illustrated in FIG. 4 may be instead active electronic gain and phase weighting and summing to a linear or circular array as illustrated in FIG. 5B. In the preferred embodiment, each of the first and second antenna arrays comprises a narrow beam antenna and each of the first and second plurality of radiation patterns include a first plurality of adjacent beams and a second plurality of adjacent beams, respectively. However, it will be appreciated by those skilled in the art that other antenna array configurations, and other radiation patterns, may be utilized without departing from the spirit and scope of the present invention. In addition, as will be appreciated by those skilled in the art, when two or more carriers are deployed, alternate frequencies may be placed on alternate antenna panels, and a third carrier may be separated by two channel spacings, thereby allowing more efficient frequency combining. In so doing, the present invention still provides two antenna arrays covering the same region, thereby allowing frequencies fl, f3, f5, etc. to be placed on the main panel, and frequencies f2, f4, f6, etc. to be placed on the diversity panel. With reference now to FIG. 6, there is depicted a logical flowchart of the process of providing forward link softer handoff in a CDMA communication system with spatially diverse antennas according to the method and system of the present invention. As shown, the process begins at block 600, wherein the step of transmitting, from a first antenna array, a first plurality of radiation patterns defining a first coverage area is performed. Thereafter, as shown at block 602, the step of transmitting, from a second antenna array spatially separated from the first antenna array, a second plurality of radiation patterns defining a second coverage area, the first and second coverage areas overlapping, is performed. Thereafter, as shown at block 604, the step of rerouting a softer handoff talk channel from a radiation pattern of the first antenna array into a radiation pattern of the second antenna array is performed. The foregoing description of a preferred embodiment of the invention has been presented for the purpose of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiment was chosen and described to provide the best illustration of the principles of the invention and its practical application, and to enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.

Claims: Claims What is claimed is: 1. An apparatus for providing forward link softer handoff in a code division multiple access cellular communication system, comprising: a first antenna array adapted to transmit a first plurality of radiation patterns defining a first coverage area; a second antenna array adapted to transmit a second plurality of radiation patterns defining a second coverage area, the first and second coverage areas overlapping, the first and second antenna arrays being spatially separated; and a softer handoff router coupled to the first antenna array and to the second antenna array, the softer handoff router adapted to reroute a softer handoff talk channel from a radiation pattern of the first antenna array into a radiation pattern of the second antenna array. 2. An apparatus as recited in claim 1, wherein at least one of the first antenna array and the second antenna array comprises a plurality of antenna elements and a beam former adapted to produce a plurality of adjacent beams. 3. An apparatus as recited in claim 2, wherein the softer handoff router is adapted to route the softer handoff talk channel from a radiation pattern of the first antenna array into an adjacent beam of the second antenna array. 4. An apparatus as recited in claim 2, wherein the beam former comprises a butler matrix. 5. An apparatus as recited in claim 2, wherein the plurality of antenna elements further comprise a uniform linear array. 6. An apparatus as recited in claim 5, wherein the uniform linear array comprises one of a plurality of patch elements and a plurality of dipoles. 7. An apparatus as recited in claim 1, wherein at least one of the first antenna array and the second antenna array comprises a plurality of directive antennas. 8. A method for providing forward link softer handoff in a code division multiple access cellular communication system, the method comprising the steps of: transmitting, from a first antenna array, a first plurality of radiation patterns defining a first coverage area; transmitting, from a second antenna array spatially separated from the first antenna array, a second plurality of radiation patterns defining a second coverage area, the first and second coverage areas overlapping; and rerouting a softer handoff talk channel from a radiation pattern of the first antenna array into a radiation pattern of the second antenna array. 9. A method as recited in claim 8, wherein the step of transmitting a first plurality of radiation patterns includes transmitting a first plurality of adjacent beams and the step of transmitting a second plurality of radiation patterns includes transmitting a second plurality of adjacent beams. 10. A method as recited in claim 9, wherein the step of rerouting includes rerouting the softer handoff talk channel from a radiation pattern of the first antenna array into an adjacent beam of the second antenna array. 11. A method as recited in claim 8, including the step of placing alternate frequencies on alternate arrays when two or more carriers are deployed. 12. A method as recited in claim 11, including the step of separating a third carrier by two channel spacings when a third carrier is deployed. 13. An apparatus for providing forward link softer handoff in a code division multiple access cellular communication system, comprising: a first antenna array adapted to transmit a first plurality of radiation patterns defining a first coverage area, the first plurality of radiation patterns including a first plurality of adjacent beams; a second antenna array adapted to transmit a second plurality of radiation patterns defining a second coverage area, the second plurality of radiation patterns including a second plurality of adjacent beams, the first and second coverage areas overlapping, the first and second antenna arrays being spatially separated; and a softer handoff router coupled to the first and second antenna arrays, the softer handoff router adapted to reroute a softer handoff talk channel from a beam of the first antenna array into an adjacent beam of the second antenna array. 14. An apparatus as recited in claim 13, wherein at least one of the first antenna array and the second antenna array comprises a plurality of antenna elements and a beam former adapted to produce the plurality of adjacent beams. 15. An apparatus as recited in claim 14, wherein the beam former comprises a butler matrix. 16. An apparatus as recited in claim 14, wherein the plurality of antenna elements further comprises a uniform linear array. 17. An apparatus as recited in claim 16, wherein the uniform linear array comprises one of a plurality of patch elements and a plurality of dipoles. 18. An apparatus as recited in claim 13, wherein at least one of the first antenna array and the second antenna array comprises a plurality of directive antennas.

Inventors: ROHANI KAMYAR, SCHORMAN ERIC REED, ROHANI, KAMYAR, SCHORMAN, ERIC, REED
Applicants: MOTOROLA INC, MOTOROLA, INC.
Application year: 2000
Main classification: H04Q7
IPC codes: H04W16, H04W36