Base stations for use in cellular communications systems

Details Base stations for use in cellular communications systems Base stations for use in cellular communications systems

1997-11-17

2002-10-08

Hunter, Daniel (Department: 2685)

Telecommunications

Transmitter and receiver at same station

Radiotelephone equipment detail

C342S368000, C342S371000, C342S372000

Reexamination Certificate

active

06463301

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to base stations for use in cellular communications systems.
BACKGROUND OF THE INVENTION
Cellular communications systems are currently in use providing radio telecommunications to mobile users. Such systems divide a geographic area into cells, each cell being served by a base station through which subscriber stations communicate. Cells are often divided into sectors with each sector being served by an antenna arrangement mounted at the base station. Sectored systems can provide increased capacity and reduced interference compared with non-sectored systems.
FIG. 1
shows a typical array of cells
10
, each cell being divided into three sectors
11
,
12
,
13
and served by a base station
14
.
To meet increasing demand for mobile communications services there is interest in further improving the capacity of systems.
One known technique for improving the capacity or coverage on the uplink path of a cell site is to form fixed receive beams at the base station such that each cell sector is covered by a number of beams rather than just a single beam. By narrowing an antenna's beam pattern in azimuth, the antenna gives increased gain in the boresight direction. For example, increasing the number of beams in a 120° sector from 1 to N (N=4 is a suitable example), allows one to design beams giving approx. 10 log
10
(N) dB of gain in their boresight direction. This narrowing of the beam pattern also improves spatial filtering by rejecting interference caused by other users within the same sector (but not in the beam direction) and from users in neighbouring cells.
The combination of increased gain and reduced interference level allows for a greater path loss figure in the power budget for the uplink, and hence a greater cell range. Alternatively, for a given cell radius it is possible to increase capacity. In a typical mobile Code Division Multiple Access (CDMA) system, forming extra beams on the uplink is effectively equivalent to increasing the sectorisation factor. As an example, providing four beams per uplink sector in a tri-sectored cell gives equivalent performance gains to using cells which are divided into twelve sectors.
The simplest way to form these beams is by using separate antennas, one for each beam. Each beam is constructed as a separate antenna, such as a flat plate antenna construction with printed elements and appropriate phasing connections to provide the required directivity and hence gain. Base station antennas are normally constructed with a narrow gain pattern in elevation. This would require a tall antenna of the order of 10 to 20 wavelengths in height. Forming beams with individual passive antennas is attractive because it allows the gain pattern to be tailored to requirements. However, a beam pattern which is narrow in azimuth also requires a wide antenna aperture of several wavelengths in width. This may lead to antennas which are excessively heavy and which have a high wind loading.
An alternative technique for generating N beams with full sector coverage is to generate orthogonal beam outputs from the same aperture. The beams are orthogonal in the sense that there is zero mutual coupling between beam ports, and the average value of the cross-product of the radiation pattern of one beam with the conjugate of any other beam is zero. As an example, four beams can be generated from four radiating elements, and it is only required to support a single such antenna for each sector because the set of beams use a single common antenna aperture. A common technique for doing this beamforming is to pass antenna element outputs through passive phase shifters to create beamformed outputs in the frequency band on which the signals are received (i.e. ‘at RF’). One such implementation is known as the ‘Butler Matrix’. In order to ensure the full gain (approx. 10 log
10
(N) dB) at the beam peaks, phase shifters with zero attenuation (a so-called ‘uniform aperture distribution’) are used. This gives a number of beams with approximately a ‘sinx/x’ gain profile.
FIG. 2
shows a typical coverage pattern for this type of antenna structure.
Four individual beams
101
,
102
,
103
,
104
area shown by dashed lines. The maximum gain (approx. 10 log
10
(N)) occurs at the beam peaks
110
. The problem is that the gain of neighbouring beams has dropped by 4dB at the beam crossovers
115
. These beam crossovers are halfway in angle to the first null. This is because for orthogonal beams the boresight of one beam corresponds to the null of another. These crossover points are often referred to as ‘cusps’.
Cusps cause problems when attempting to provide an even cellular coverage over a certain geographical area. Mapping the locus of the cell edge, i.e. the locus of points with, on average, equal quality of service, gives the sort of ‘flower petal’ arrangement shown in FIG.
2
. This diagram represents a single 120° sector of a tri-sectored cell site, with 4 orthogonal beams in the sector. The cusp depth
130
in terms of power in this example is 4 dB. The geographical distance this represents i.e. the difference in cell radius between beam peak and beam cusp depends on the propagation law which in turn depends on such factors as carrier frequency and antenna heights. For a typical propagation law of 35 dB increase in path loss per decade of range increase, and for a typical cell radius (at the beam peak) of 5 km, this represents a reduction in radius at the beam cusps of around 1.2 km, giving a cell radius of 3.84 km at the cusps.
It is not simple to tessellate such cells to allow the beam peaks from one cell to coincide with the cusps from another. If the cells are tessellated as if they were circular with a 5 km radius, then there will be areas of poor availability, where the received signal quality is likely to be poor. An alternative is to treat the cells as being circular with the lesser 3.84 km radius at the cusps. This improves availability but makes inefficient use of base stations, requiring almost 70% more base stations than for 5 km radius cells to cover a given geographical area. Operators may be tempted to tessellate bases with a cell radius somewhere between 3.84 km and 5 km, but this would lead to some areas on the cell edge of above-average availability, and other areas with below-average availability.
One solution to the cusping problem is described in European Patent Application EP 0 647 978 A2. An output of a transceiver is split into two signals which are fed to two adjacent beams. This application also describes how ripple in the inter-facet region of the radiation pattern of a muti-faceted antenna can be minimised by varying the relative phase of the facets.
The present invention seeks to minimise the effects of cusping in cellular radio systems.
SUMMARY OF THE INVENTION
A first aspect of the present invention provides a method of operating a base station of a cellular communications system comprising:
forming a plurality of adjacent beams in azimuth across a coverage area, and
varying the position of the plurality of beams in unison whereby to provide a mean antenna gain in all azimuthal directions across the coverage area.
Varying the position of the beams has the effect of varying the position of the cusped regions of the beam pattern thereby reducing the effects of cusping loss across the coverage area. The position of the beams can be varied by a movement in azimuth over one half, or multiples of one half, of the angular separation of the formed beams.
Preferably there are a plurality of base stations in the system, each of whose plurality of beams are varied in position independently of the other base stations. Independently steering the beam pattern of each base station has the advantage that there is minimal correlation between the gain profile of signals received by a subscriber from adjacent base stations, or in signals received by adjacent base stations from a particular subscriber. This further minimises the effects of cusping loss.
The position of the plurality of beams can be varied by mechan

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