This application is the U.S. national phase of International Application No. PCT/SE2006/001170, filed 16 Oct. 2006, the entire contents of which is hereby incorporated by reference.
The technology disclosed herein relates to a system for adapting the beam-shape of an antenna in a wireless communication network.
Variable beam tilt is an important tool for optimizing radio access networks for cellular telephony and data communications. By varying the main beam pointing direction of the base station antenna, both interference environment and cell coverage area can be controlled.
Variable electrical beam tilt is conventionally performed by adding a variable linear phase shift to the excitation of the antenna elements, or groups of elements, by means of some phase-shifting device. For cost reasons, this phase-shifting device should be as simple and contain as few components as possible. It is therefore often realized using some kinds of variable delay lines. In the description, the terms “linear” and “non-linear” should be understood to refer to relative phase over multiple secondary ports of a multiport phase shifting network, and not the time or phase behaviour of a port in itself.
Conventional multi-port phase shifters, with one primary port and a number N (N>1) secondary ports, are implemented with linear progressive variable phase taper over the secondary ports. In addition to the linear progressive phase taper, fixed amplitude and phase tapers are often used as a means for generating a tapered nominal secondary port distribution.
a and 1b illustrate a conventional phase shifter 10, with one primary port 11, and the phase shifter generates in down-link linear progressive phase shifts over four secondary ports 121-124. A variable-angle “delay board” 13 has multiple trombone lines 14, one for each secondary port 121-124. The trombones lines 14 are arranged at linearly progressive radii. By a proper choice of junction configurations, line lengths, and line impedance values, the nominal phase and amplitude taper of the phase shifter can be controlled, for example to achieve uniform phase over the secondary ports as indicated by “0” in
The use of non-linear phase-shifting devices for controlling electrical down tilt has been contemplated, such as mentioned in U.S. Pat. No. 5,798,675, by Drach, U.S. Pat. No. 5,801,600, by Butland et al.
A system for tilt-dependent beam shaping using conventional linear phase shifters is disclosed in JP 2004 229220. The system has different beam width depending on the tilt angle, but this is achieved by a tilt angle control section (41) in combination with a vertical beam width control section (42) in the base station controller (4), see FIG. 6 in JP 2004 229220.
Traditionally, base station antennas have had a variable beam tilt range of approximately one beamwidth. This together with the fact that most mobile connections today are circuit switched voice with a fixed requirement on bit-rates, has not triggered any interest in improving the Signal-to-interference+noise ratio (SINR) close to the antenna. Normally it is good enough.
For particular cell configurations, e.g. highly placed antennas in combination with small cells, the need for using antennas with large beam tilt is greater. For antennas with conventional narrow elevation beam radiation patterns, the large beam tilt causes users close to the base station to experience a lower path gain than users close to the cell border, since the difference in path loss for the near and far users is smaller than the difference in directive antenna gain. For packet-based data communication this is not optimal usage of the available power. Therefore, for antennas with large beam tilt, some degree of radiation pattern null-fill below the main beam, or even some cosec-like beam-shaping is desirable.
In large cells, on the other hand, when no or small beam tilt is employed, the antenna pattern should be optimized for maximum peak gain. The path gain for the users at the cell border will anyway be smaller than for users closer to the base station because the path loss varies rapidly with vertical observation angle in the case of large cells and observation angles close to the horizon.
The technology disclosed herein provides a system that allows a radiation pattern of an antenna to be optimized both for high maximum gain at small tilt angles, and high degree of null filling below the main beam at large tilt angles.
The technology disclosed herein provides a system for changing the beam shape of an antenna, preferably having multiple antenna elements arranged in an array, in dependency of a tilt angle. Electric tilting is achieved by including a phase-shifting device that will provide phase shifts over secondary ports from the phase-shifting device. A phase-taper device provides changed phase taper over the antenna elements with tilt angle.
An advantage with the technology disclosed herein is that a single antenna may be used in an adaptive system, to fulfil the need for increasing the quality of a communication link and thus increase the bit rate associated with one or more simultaneous users, by maintaining an optimal antenna pattern, which depends on the distance to the base station.
Further objects and advantages will become apparent for a skilled person from the detailed description.
a and 1b show a linear phase shifter.
a and 2b show a first embodiment of a non-linear phase shifter.
a and 3b show diagrams illustrating phase shifts from the linear and non-linear phase shifters.
a-7d show elevation radiation patterns utilizing the technology disclosed herein.
A base station, including an antenna with multiple antenna elements, is arranged within a cell, where the characteristics of the antenna determine the size of the cell and the cell coverage area all else being equal. To accomplish the same signal strength in the entire cell, independent of the distance to the base station, the antenna gain G(θ) divided by the path loss L(θ) should be constant in the cell, as a function of observation angle θ:
However, the constant C changes with cell configuration, i.e. antenna installation height and cell size, which in turn means that the optimal antenna radiation pattern changes with beam tilt angle, as illustrated in
The terms “phase shift” and “time delay” are used interchangeably in the following description and it should be understood that these terms refer to equivalent properties in the present context, except if otherwise noted.
An essential part of the technology disclosed herein is to provide non-linear phase taper over the secondary ports of a phase shifter network. A method for achieving this is to use a multi-secondary port true time delay network in which the relative delay line lengths are, in general, non-linearly progressive. A true time delay network generates frequency-dependent phase shifts, a property which makes it particularly suitable for antenna applications, such as beam-steering.
The principle idea of a first embodiment of a non-linear phase shifter 20, in down-link, is illustrated in
As a non-limiting example, the phase-shifts from a linear and a non-linear true time delay network in down-link are compared in
Δφn=(n−1)Δφ=(n−1)kα,
where n is the secondary port number, α is the board rotation angle, and k is a constant that depends on implementation aspects, for example wave number of transmission lines and radial separation of the trombones 14.
The non-linear phase advance (relative phase) over the secondary ports 121-124 of a non-linear true time delay network is illustrated in
φn−φn−1≠φn+1−φn, for at least one n, nε{2,N−1}
wherein N is the number of delay branches. In
For beam tilt angles between 0° and 9°, the phase excitation is found by a linear interpolation of the phase excitations at 0° and 9°. Some of these radiation patterns 70 are shown in
The technology disclosed herein is not limited to the example with constant cell illumination described above, but is applicable in all cases where it is desirable, for one reason or another, to have a radiation pattern that changes with beam tilt angle. Furthermore, the technology disclosed herein is not limited to linear antenna arrays, but may also be implemented in a base station having a non-linear antenna array.
The technology disclosed herein allows the antenna pattern to be optimized both for high maximum gain at small tilt angles, and for good coverage (high degree of null filling) close to the antenna at large tilt angles θtilt.
The telecommunication system 80 also includes a second base station BS2. The second base station BS2 is connected via a second base station controller BSC2 to the core network 81. A non-uniform linear antenna array 88 comprises in this embodiment four antenna elements 84, not necessarily cross polarized as illustrated. Secondary ports 12 of a linear phase shifter 10 (prior art) are connected, via a phase-taper device 87 that changes the phase taper over the antenna elements with tilt angle θtilt, to each antenna elements 84 of the non-linear antenna array 88. A primary port 11 of the phase shifter 10 is connected to the second base station BS2. The second base station controller BSC2 controls the variable beam tilt by changing the position of a linear delay board, as previously described in connection with
It should be noted that the antenna array may have uniformly, or non-uniformly, arranged antenna elements 84, and cross polarized antenna elements are only shown as a non-limiting example and other types of antenna elements may naturally be used without deviating from the scope of the invention. Furthermore, antenna elements operating in different frequency bands may be interleaved without departing from the scope of the claims.
The illustrated telecommunication system (GSM) should be considered as a non-limiting example, and other wireless telecommunication standards, such as WCDMA, WiMAX, WiBro, CDMA2000, etc. may implement the described technology disclosed herein without deviating from the scope of the technology disclosed herein. Some of the described parts of the GSM system, e.g. base station controller BSCi and BSC2 may be omitted in certain telecommunication standards, which is obvious for a skilled person in the art.
Although the technology disclosed herein has been described in detail using down-link, the skilled person in the art may readily adapt the teachings for up-link, as is mentioned above.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/SE2006/001170 | 10/16/2006 | WO | 00 | 1/12/2010 |
Publishing Document | Publishing Date | Country | Kind |
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WO2008/048149 | 4/24/2008 | WO | A |
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Number | Date | Country | |
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20100134359 A1 | Jun 2010 | US |