Each base station in a mobile telecommunications system requires a certain coverage area, for instance .+-.60.degree.. By utilizing multi-beam antennas a mobile telecommunications system may gain both capacity and increased coverage. This is achieved by having a number of simultaneous narrow antenna beams from an antenna array illuminating the coverage area.
The following demands ought to be met for such a multi-beam antenna:
a) the antenna beams need to illuminate the entire intended coverage area;
b) a high antenna gain is aimed at, which results in narrow antenna beams. On the other hand the shape of the beams as well as side lobes is generally of less interest as long as the antenna gain is not influenced;
c) few receiver/transmitter channels is desired to reduce the system costs and complexity.
As is clear from the demands set forth above there is a contradiction when many narrow beams, covering a large area shall be accommodated within a few receiver/transmitter channels.
A standard method to obtain simultaneous narrow antenna beams from an antenna array normally utilizes a Blass or Butler matrix network for combining the individual antennas or antenna elements in an antenna array. In the literature can be found several methods utilizing a Butler matrix for feeding an antenna array having several antenna beams. In U.S. Pat. No. 4,231,040 to Motorola Inc., 1978, an apparatus and a method is disclosed for adjusting the position of radiated beams from a Butler matrix and combining portions of adjacent beams to provide resultant beams having an amplitude taper resulting in a predetermined amplitude of side lobes with a maximum of efficiency. This is achieved by first adjusting the direction of the beams by a set of fixed phase changers at the element ports of the Butler matrix. Two and two of adjacent beams are then combined by interconnections of the ports at the beam side of the Butler matrix. By this method 4 beams are achieved with an 8.times.8 matrix. However nothing is discussed about the coverage of the resulting beams.
Another document, U.S. Pat. No. 4,638,317 to Westinghouse, 1987, describes how the element ports of a Butler matrix fed array antenna are expanded to feed more elements than the basic matrix normally provides outputs for. By this distribution of power an amplitude weighting is achieved over the surface of the array antenna and the level of side-lobes is slightly reduced. In the present context this is of less relevance as such a device is intended as a component in a system for reduction of side-lobes. The number of beams is not changed. The coverage of the beams is shortly commented by casually. However the device will hardly be utilized as one single beam forming instrument.
Generally multiple beams from an antenna are usually achieved in a beam forming network, where transformations takes places between element and beam ports. Blass matrixes and Butler matrixes are examples of such transformations. The Butler matrix is interesting as it generates orthogonal beams, which results in low losses. FIG. 1 demonstrates, according to the state of the art, a Butler matrix with the two outer beam ports terminated to keep the number of receiver/transmitter channels down.
FIG. 2 demonstrates an example of a radiation pattern generated by such a beam forming matrix as illustrated in FIG. 1. The solid line beams are those connected to the four receiver/transmitter channels, while those with dashed lines are terminated and not being part of the system. As can be seen the coverage is not acceptable out at .+-.60.degree.. The dotted line marks an example of a desired output for a hexagonal coverage. Consequently this antenna has a poor coverage at large radiation angles.
Nor can traditional beam forming at the outermost beam be used, as the antenna gain then decreases too much.
Thus there are still problems to be solved to be able to present a well behaving antenna system having a limited number of receive/transmit channels for a base station in mobile communication systems.