Communications: directive radio wave systems and devices (e.g. – Directive – Including a steerable array
Patent
1997-05-22
1999-10-26
Blum, Theodore M.
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a steerable array
H01Q 322, H01Q 324, H01Q 326
Patent
active
059736410
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a base station arrangement as used in cellular radio communications systems and in particular relates to an antenna feed network arrangement having a null-free coverage and more particularly to an antenna arrangement having a null-free coverage and down-tilt capabilities.
Cellular radio systems are used to provide telecommunications to mobile users. In order to meet the capacity demand, within the available frequency band allocation, cellular radio systems divide a geographic area to be covered into cells. At the centre of each cell is a base station through which the mobile stations communicate with each other and with a fixed (wired) network. The available communication channels are divided between the cells such that the same group of channels are reused by certain cells. The distance between the reused cells is planned such that co-channel interference is maintained at a tolerable level.
When a new cellular radio system is initially deployed operators are often interested in maximising the uplink (mobile station to base station) and downlink (base station to mobile station) range. Any increase in range means that less cells are required to cover a given geographic area, hence reducing the number of base stations and associated infrastructure costs. The downlink range is primarily increased by increasing the radiated power from the base station. National regulations, which vary from country to country, set a maximum limit on the amount of effective isotropic radiated power (EIRP) which may be emitted from a particular type of antenna being used for a particular application. In Great Britain, for example, the EIRP limit for digital cellular systems is currently set at +56 dBm. Hence the operator is constrained and, in order to gain the maximum range allowable, must operate as close as possible to the EIRP limit, without exceeding it. In cellular radio base stations, the antennas are generally arranged to cover sectors, of typically 120.degree. in azimuth--for a trisectored base station. The antenna arrays comprise a number of vertically oriented layered antenna arrays to provide an M.times.N array to serve a sector. Each vertically oriented antenna array is positioned parallel with the other linear antenna arrays. The radiating antenna elements of a vertical array cooperate to provide a central narrow beam coverage in the elevation plane and broad coverage in azimuth, radiating normally in relation to the vertical plane of the antenna array. In the elevation plane the radiation pattern consists of a narrow "main" beam with the full gain of the antenna array, plus "side lobes" with lower gains. With a uniform phase excitation for the antenna array, there are deep "nulls" between the main lobe and the first side lobes on either side. These produce undesirable "holes" in the base station coverage.
Downtilt in the cellular radio environment is used to decrease cell size from a beam shape directed to the horizon to the periphery of the cell. This provides a reduction in beam coverage, yet allows a greater number of users to operate within a cell since there is a reduction in the number of interfering signals. The antennas used in a base station can be of a layered or tri-plate form and each antenna radiating element of an antenna array is formed on the same layer.
This tilt can be obtained by mechanically tilting the antenna array or by differences in the electrical feed network for all the antenna elements in the antenna array. Electrical downtilt can be used to controllably steer a radiation beam downwardly from an axis corresponding to a normal subtended by an array plane and results from a consecutive phase change in the signal fed to each antenna element in an antenna array. Mechanical downtilting is simple yet requires optimisation on site; electrical downtilting allows simple installation yet requires complex design. However, neither forms of downtilting compensate for nulls which are formed between lobes in the radiation pattern.
The present invention seeks to overcome or red
REFERENCES:
patent: 4045800 (1977-08-01), Tang et al.
patent: 4652883 (1987-03-01), Andricos
patent: 5414433 (1995-05-01), Chang
Adams David Neil
Smith Adrian David
Smith Martin Stevens
Blum Theodore M.
Northern Telecom Limited
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