Communications: radio wave antennas – Antennas – Plural antennas spaced a fractional or full wave length apart
Reexamination Certificate
2000-07-17
2001-11-27
Phan, Tho (Department: 2821)
Communications: radio wave antennas
Antennas
Plural antennas spaced a fractional or full wave length apart
C343S872000, C343S890000
Reexamination Certificate
active
06323823
ABSTRACT:
BACKGROUND
1. Field of Invention
The present invention relates to wireless information communications systems. More particularly, the present invention relates to an adaptive antenna array of a base station formed as a plurality of spaced apart clusters of antenna elements lying generally within a common horizontal plane.
2. Related Art
Wireless data and voice communications services are proliferating throughout the world. One popular service is the so-called “cellular” telephone service. In cellular telephone service, service areas are divided up into “cells”, where each cell covers a specific geographical area and services mobile units located in, or passing through, the service area. Typically, radio frequencies are used in the ultra high frequency spectrum, and more typically in the 800 MHz or higher frequency range. The nature of radio wave propagation at these relatively short wavelengths limits the maximum effective distance between the mobile and the base, frequently to several miles. This propagation limit enables reuse of the same frequencies or bands within non-adjacent cells of the cellular network. Since the service range of each base station is limited to a radius of e.g. several miles, it is necessary to provide a number of base stations within a service area in order to provide effective wireless service throughout the area.
One known way to increase the number of mobile stations that may be served within a cell is to divide the cell into sectors, such as three sectors, spaced apart by 120° about the compass rose. In such an arrangement, each sector is provided with its own 120°-wide transmit beam from the base station.
Further increase in the number of mobile stations that may be simultaneously served within a cell or sector is to employ base station antenna arrays having plural elements. Embedding adaptive antenna array technology into the existing cellular telephone infrastructure potentially provides very significant capacity increases. This technology offers the ability to eliminate same cell interference for mobile stations being served simultaneously. It offers the prospect of a reduction of inter-cell interference. It also increases the signal-to-noise ratio of a particular mobile station being served and therefore enables an increase in user data rate. These benefits and advantages result in either higher data throughputs, or the ability to service more mobile stations simultaneously, within a given cell or service infrastructure. With spatially separated elements, beamforming becomes practical for both transmit and receive modes. Focusing radiant energy in the direction of a mobile station reduces the amount of overall power needed to be generated by the base station in order to maintain a given service quality. Antenna array technology can be used to focus power coming from the mobile station to the base station via a reverse link or an uplink, as well as from the base station to the mobile station via a forward link or downlink.
Usually, during transmit mode, a wide transmit beam is desired so that the transmit beam, and its associated pilot, reaches all of the mobiles within the service area or sector, since the base station does not initially know where any particular mobile would be within that area. In transmit mode, relatively wide transmit beams may be formed by using phased antenna elements of an array wherein the elements are spaced relatively closely apart, with spacing between adjacent elements being on the order of between one half and one wavelength at the transmit frequencies. At the cellular frequency bands in the range of 800 MHz, one wavelength equals 0.375 meters, or 14.775 inches, with one half wavelength being half of these linear values. After a particular mobile station is located within the service area of the base station, narrower transmit beams may be employed to divide and concentrate limited base station power among all of the mobile stations being served simultaneously.
In base station receive mode, very narrow beams are highly desirable in order to provide multiple beam diversities and concentrate the signal energy from a particular one of the mobiles operating within a particular one of the available service channels and to exclude or reduce signal energies from other mobiles within the same service area using other ones of the available service channels. Beamforming narrow beams in receive mode requires that the phased receive antenna elements be placed relatively farther apart than the transmit elements. Phased adjacent receive elements are most preferably placed apart by approximately three wavelengths. At 800 MHz, three wavelengths equals 1.125 meters or 44.325 inches. From these desirable spacings, it becomes immediately apparent that base station receive mode antenna arrays may become relatively quite large and visually noticeable at the base station locations within the neighborhoods of the various cellular communications service areas. Since the highest service requirements occur in the most highly populated areas, large base station antenna arrays become the subject of observation and complaint by a relatively large part of the population as a whole. One popular misconception held by some members of the public at large is that the larger the antenna array, the greater will be the exposure level to electromagnetic radiation at the vicinity of the array. Also, members of the public may object to what is perceived to be a negative visual impact or blight upon the environment of a particular neighborhood presented by large antenna arrays providing wireless communications services.
For example,
FIG. 1
shows a conventional three-sector cellular antenna array
10
mounted at desired elevation above ambient terrain upon a triangular support tower
12
. A triangular support tower is frequently employed in wireless communications because it provides considerable strength with minimal material and takes advantage of the inherent strength of three-leg, triangle geometry in the horizontal plane and triangle bracing in the vertical planes of each tower face. The antenna array
10
is designed to serve three service sectors
14
,
16
and
18
. For sector
14
, a transmit-receive element
20
is located at one corner of the tower
12
, and a receive-only element
22
is located at another corner of the tower
12
at a spacing selected to enable effective diversity reception. The antenna elements
20
and
22
are enclosed and protected from the weather by radomes
24
, typically formed of radio-wave-transparent material such as molded fiberglass or plastic.
The transmit-receive element
20
is adapted to broadcast a service beam throughout the sector
14
, and the element
20
may also be simultaneously used to receive at a different frequency or band with the inclusion of conventionally available duplexer filter technology, or may be used in a time division multiplex arrangement, with one time increment operating in transmit mode and a next time increment operating in receive mode. The receive mode element
22
provides spatial diversity reception for signals arising within the service sector
14
. Similarly, the service sector
16
includes transmit-receive element
26
and receive-only element
28
, and the service sector
18
includes transmit-receive element
30
and receive-only element
32
. While the arrangement of antenna array
10
in
FIG. 1
enables some beamforming, very narrow receive-mode beams with additional array gains of about 5 dB with respect to a single antenna element are not achievable with only two spatially diverse receive antenna elements.
Narrow beamforming creating very narrow beams with high antenna array gains at the base station for both receive and transmit modes typically requires more antenna elements.
FIG. 2A
presents a more recently proposed antenna array for wireless cellular communications service which employs a relatively large multi-element receive antenna array
50
and a relatively small multi-element transmit antenna array
52
. While
FIG. 2A
shows the transmit array
52
to
Scherzer Shimon B.
Wong Piu Bill
Fulbright & Jaworski L.L.P.
Metawave Communications Corporation
Phan Tho
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