The present invention relates generally to antennas for wireless data communications networks, and more specifically to beamforming antenna systems.
Modern wireless communications systems place great demands on the antennas used to transmit and receive signals, especially at cellular wireless base stations. Antennas are required to produce a carefully tailored radiation pattern with a defined beamwidth in azimuth, so that, for example, the wireless cellular coverage area has a controlled overlap with the coverage area of other antennas.
In addition to a defined azimuth beam, such antennas are also required to produce a precisely defined beam pattern in elevation; in fact the elevation beam is generally required to be narrower than the width of the azimuth beam.
It is conventional to construct such antennas as an array of antenna elements so as to form the required beam patterns. Such arrays require a feed network to split signals for transmission into components with the correct phase relationship to drive the antenna elements; when receiving, the feed network doubles as a combiner.
An array consisting of a single vertical column of antenna elements is commonly used as a building block at cellular radio base stations. Such a column antenna can be designed to produce the required narrow elevation beam, and will typically be designed to give azimuth coverage of a sector in a cellular wireless network. In a simple configuration, three such column antennas are deployed at a base station to give coverage to three sectors; this is a form of a spatial division multiple access (SDMA) system, in which the capacity of a cellular wireless system is enhanced by enabling a given frequency band to be used substantially independently by wireless links which are spatially separated.
The required azimuth beam patterns for a system such as that illustrated by
It may be convenient to locate the beamformer 8 close to the antenna elements 7a . . . 7d, which will typically be located on an antenna tower. It may also be advantageous in terms of cost, size and performance to integrate the beamformer with the antenna elements, contained within the same enclosure.
However, the integration of a beamformer with its associated antenna elements may present disadvantages in terms of potential upgrade strategies if such strategies require access to the individual antenna elements. In order to access the individual antenna elements, an operator would require to climb the tower and modify or replace the beamformer; in the case of an integrated system this may not be possible, necessitating the replacement of the integrated unit 6. The replacement and re-alignment of antenna elements may be costly; accordingly the lack of an economical upgrade path may limit the deployment of an otherwise attractive integrated beamformer and antenna system.
It is an object of the present invention to provide methods and apparatus which addresses these disadvantages.
In accordance with aspects of the present invention, there is provided methods and systems according to the appended claims.
More specifically, in one aspect there is provided a method of receiving signals from a first antenna element, said first antenna element providing input to a beamformer, the beamformer being arranged to receive input from at least one other antenna element and being arranged to generate at least two beams as output therefrom, the method comprising the steps of:
combining said at least two beam outputs at a connecting port such that said signals from said first antenna element are constructively combined at the connecting port; and
combining said at least two beam outputs at the connecting port such that signals from antenna elements other than the first antenna element providing input to the beamformer are destructively combined at the connecting port; and
configuring the connecting port so as to provide access to individual said signals received by said antenna elements.
The connecting port provides access to signals at an individual antenna element without the need to remove the beamformer, which may for example be beneficial in situations where access to the beamformer is difficult or costly or where the beamformer is physically integrated with the antenna elements.
Constructively combining signals is a process of combining signals substantially in phase so that the magnitude of the resultant signal is maximised. Destructively combining signals is a process of combining signals in such a way that they cancel, so that the magnitude of the resultant signal is minimised.
Preferably, the beams formed by the beamformer are orthogonal beams. The benefit of forming orthogonal beams is that the signal loss between the antenna element and the connecting point is minimised.
In one arrangement the beamformer can be arranged to form three output beams from a combination of input from three antenna elements, at least one of the antenna elements being said first antenna element, the method further comprising combining said three output beams at the connecting port. In such an arrangement the beamformer can be configured according to the following steps, so as to achieve the aforementioned constructive and destructive combining of signals at the connecting port:
combining signals received by the first antenna element with signals received by a third of said three antenna elements, in which combining comprises in-phase combining, to provide a third output beam;
combining signals received by the first antenna element with signals received by the third antenna element, in which said combining comprises anti-phase combining, so as to provide a first intermediate signal;
combining signals received by a second of the three antenna elements with the first intermediate signal such that the intermediate signal is at minus ninety degrees phase to the signals received by the second antenna element, so as to provide the first output beam; and
combining signals received by the second antenna element with the first intermediate signal such that the intermediate signal is at ninety degrees phase to the signals received by the second antenna element, to provide the second output beam.
In a yet further arrangement, the beamformer can be arranged to form four output beams from a combination of input from four antenna elements, at least one of the antenna elements being the first antenna element, the method further comprising combining the four output beams at the connecting port.
Conveniently, a further beamformer can be used to combine the beams output from the beamformer. The further beamformer may be connected as an inverse beamformer, that is to say the input beam ports of the further beamformer are connected to the output beam ports of the beamformer, in which case an individual input port of the further beamformer provides the connecting port. The benefit of using a further beamformer is that a similar technology may be employed to that used to implement the beamformer, thereby potentially giving cost savings in design and construction.
Preferably, the further beamformer provides a further connecting port, thereby ensuring that a radio system that requires access to more than one individual antenna element may be provided with such access via a suitable further connecting port. Examples of such radio systems are multiple in multiple out (MIMO) systems, diversity combination and adaptive beamforming systems.
Conveniently the connecting port, or in the case of arrangements that include the further beamformer, the further connecting port will be connected to a receiver.
Advantageously, the further beamformer may be implemented by an array of weighting elements, which weight, in amplitude and phase, the beam outputs from the beamformer and combine the weighted components at a connecting port. The advantage of implementing the further beamformer by an array of weighting elements is that the implementation may be economical, for example the weighting elements and combination may be implemented in digital signal processing.
The beamformer arrangement can also be arranged to transmit signals from antenna elements; in one arrangement, this involves transmitting signals from a first antenna element, said first antenna element receiving input from a beamformer, the bcamformer being arranged to input signals to at least one other antenna element and comprising a set of beam ports for receiving signals to be transmitted via said antenna elements, the set of beam ports being connected to a connecting port external to said beamformer, the method comprising:
coupling signals to said beam ports of the beamformer such that signals received at the connecting port are constructively combined at said first antenna element; and
coupling signals to said beam ports such that signals received at the connecting port are destructively combined at antenna elements other than said first antenna element,
thereby enabling a transmitter connected to said connecting port to transmit from said first antenna element and not to transmit from the other antenna elements.
Further features and advantages of the invention will become apparent from the following description of preferred embodiments of the invention, given by way of example only, which is made with reference to the accompanying drawings.
In general, the present invention is directed to methods and apparatus that enhance the capacity of wireless communications between a base station and remote stations. The invention will be described in the context of a cellular wireless system, but it is to be understood that this example is chosen for illustration only and that other applications of the invention are possible.
Typically, the internal structure of the beamformer will produce the same number of beams as there are antenna element ports; if fewer beams are required for use, then the unwanted beam ports (11b) may be simply terminated with an appropriately matched impedance. Conventionally unwanted beam ports are terminated internally to the beamformer, as illustrated in
It should be noted that the system shown in
On receive, the system operates as follows: signals are received by antenna elements 7a . . . 7c and transformed to beams that are output via beam ports 11a, 11b, 11c, at the output of the bcamformer 8, each beam corresponding to a radiation pattern received by the antenna array 7a . . . 7c. The beam outputs are then transformed by the inverse beamformer 12 to represent the signals originally received by the antenna elements 7a . . . 7c. In the case of the system of
For many applications (such as MIMO) the relative phase of the signals on these outputs is not important; however, if the relative phases on the individual element ports are required to correspond with the relative phases of the signals at the antenna array, then this can be achieved by the addition of phase shifting elements in line with the individual element ports 25a, 25c. The system has the characteristic that signals received by an individual antenna element 7a are connected to a respective individual element output of the inverse beamformer 25a, whereas signal received by the other elements 7b, 7c in the array are not connected to that individual element output 25a. The combination of the beamformer 8 and the inverse beamformer 12 thus in effect gives access to signals received by the antenna elements 7a . . . 7c without the need to remove the beamformer 8 from the system. This is particularly advantageous in arrangements where the beamformer 8 and the antenna elements 7a . . . 7c are integrated into a unit such that removal of the beamformer unit may not be feasible.
On transmit, the system operates in the reverse manner of the operation on receive. Signals connected to individual element ports 25a, 25c are transformed to give inputs to the beam ports 11a . . . 11c of the beamformer 8 such that the outputs of the beamformer to antenna elements 7a . . . 7c correspond to the signals input to the respective individual element ports 25a, 25c. That is to say that a signal input to an individual element port 25a of the inverse beamformer will be transmitted from the respective antenna element 7a and not from the other antenna elements 7b, 7c. As in the case of receive, the relative phase of the signals transmitted from the antenna elements may not be the same as the relative phases between the signals connected to the respective individual elements ports of the inverse beamformer. If the relative phases on the individual element ports are required to correspond with the relative phases of the signals at the antenna array, then this can be achieved by the addition of phase shifting elements in line with the individual element ports 25a, 25c as was mentioned for the case of receive.
The operation of a four port hybrid coupler comprising two pairs of ports according to an embodiment of the invention will now be described; referring to
The signals on the two ports B, C between which the power is split differ in phase by 90 degrees; the port C which is unpaired with the input port A carries signals with a −90 degree phase difference to those received by the paired port B. In the convention adopted in
If the four port coupler 13 is used as a combiner, then signals present on each output of the coupler may be obtained by vector addition; if signals that are 180 degrees out of phase with each other but that are otherwise identical arrive at a given output port, then the signals will cancel so that the port appears isolated. Accordingly the signal power will be transmitted to another port at which the signals do not cancel.
The operation of the beamformer 8 of
The flow of signals from the first beam port as indicated by reference numeral 11a will now be described.
Signals entering at an amplitude of 1 into port A of the second four port coupler 13b are split into a component designated as a reference phase of 0 degrees at port B and a component at −90 degree phase at port C. The signal is split equally in power, that is to say the amplitude of each is half of the square root of two. Signals from port B of the second four port coupler 13b are connected to the second antenna element port 9b, and have an amplitude of half of the square root of two and at a phase of 0 degrees. Signals from port C of the second four port coupler 13b are connected to port A of the first four port coupler 13a and have an amplitude of half of the square root of two and a phase of −90 degrees. The signals leave port B of the first four port coupler 13a and have an amplitude of one half and a phase of −90 degrees; these signals are connected to the first antenna element port 9a. Signals leave port C of the first four port coupler 13a and have an amplitude of one half and a phase of −180 degrees; these signals arc phase shifted by a further −90 degrees by the phase shifter 16a and are then connected to the third antenna element port 9c, at an amplitude of one half and a relative phase of −270 degrees, that is to say equivalent to +90 degrees relative to signals entering port A of the first four-port coupler 13a.
As a result, the antenna array 7a . . . 7c to which the antenna ports 9a . . . 9c are connected is excited as follows: the phase on signals on the first 7a, second 7b and third 7c antenna elements respectively is −90, 0, +90 degrees and the amplitude is 0.5, 0.707, 0.5 respectively. If the antenna elements 7a . . . 7c are half a wavelength apart in the azimuth plane at the frequency of operation of the antennas, then the excitation of the antenna elements results in a beam at −30 degrees from boresight (that is, closer in angle to the line from the centre of the array to the first element than to the line from the centre of the array to the third element), where boresight is an angle perpendicular in azimuth to the array 7a . . . 7c. On receive, signals will be received from a similar beam.
The flow of signals from the second beam port as indicated by reference numeral 11b will now be described.
Signals entering at an amplitude of I into port D of the second four port coupler 13b are split into a component designated as a reference phase of 0 degrees at port C and a component at −90 degree phase at port B. The signal is split equally in power, that is to say the amplitude of each is half of the square root of two. Signals from port B of the second four port coupler 13b are connected to the second antenna element port 9b, at an amplitude of half of the square root of two and at a phase of −90 degrees. Signals from port C of the second four port coupler 13b are connected to port A of the first four port coupler 13a with an amplitude of half of the square root of two and a phase of 0 degrees. The signals leave port B of the first four port coupler 13a with an amplitude of one half and a phase of 0 degrees; this signal is connected to the first antenna element port 9a. Signals leave port C of the first four port coupler 13a with an amplitude of one half and a phase of −90 degrees; this signal is phase shifted by a further −90 degrees by the phase shifter 16a and then connected to the third antenna element port 9c, at an amplitude of one half and a relative phase of −180 degrees.
As a result, the antenna array 7a . . . 7c is excited as follows: the phase on signals on the first 7a, second 7b and third 7c antenna elements respectively is 0, −90, −180 degrees and the amplitude is 0.5, 0.707, 0.5 respectively. If the antenna elements are half a wavelength apart in the azimuth plane at the frequency of operation of the antennas, then the excitation of the antenna elements results in a beam at 30 degrees from boresight. On receive, signals will be received from a similar beam.
Hence it can be seen that the signals output via the first and second beam ports 11a, 11b form a pair of beams, one at −30 degrees and the other at +30 degrees to boresight. It will be appreciated that this pair of beams is well suited to give coverage within a 120 degree cellular base station sector such as in the system illustrated by
The amplitude taper in the excitation across the array 7a . . . 7c, in which the centre element 7b has a higher amplitude than that of the end elements 7a, 7c, is beneficial in reducing sidelobe levels of the beams compared to the beam that would be formed if the elements were excited with equal amplitudes. Lower sidelobe levels in turn are beneficial in reducing interference between beams and therefore improving the capacity of a cellular wireless system.
It can be shown that the beams produced by the beamformer of
It should be noted that the amplitudes set out in the examples above are in arbitrary units and do not take account of implementation losses. Also, the phases quoted do not account for transmission delays through components and between components (except where specifically mentioned). A practical implementation would typically be laid out so that transmission paths are equalised in terms of delay from each beam port to each antenna port, as far as is possible. Techniques for the equalisation of transmission delays are well known in the art; for example, lengths of transmission line may be designed with the required delay characteristics.
The beamformer design need not necessarily be passive, as shown; instead, amplifiers may be inserted between stages if signal gain is required.
The third beam port 11c of the beamformer 8 illustrated by
The flow of signals from the third beam port as indicated by reference numeral 11c will now be described.
Signals entering at an amplitude of 1 into port D of the first four port coupler 13a leave port B of the first four port coupler 13a with an amplitude of half of the square root of two and a phase of −90 degrees; this signal is connected to the first antenna element port 9a. Signals leave port C of the first four port coupler 13a with an amplitude of half of the square root of two and a phase of 0 degrees; this signal is phase shifted by a further −90 degrees by the phase shifter 16a and then connected to the third antenna element port 9c, at an amplitude of half of the square root of two and a relative phase of −90 degrees.
As a result, the antenna array 7a . . . 7c is excited as follows: the phase on signals on the first 7a, second 7b and third 7c antenna elements respectively is −90, no signal, −90 degrees and the amplitude is 0.707, 0, 0.707 respectively. If the antenna elements are half a wavelength apart in the azimuth plane at the frequency of operation of the antennas, then the excitation of the antenna elements results in a beam at boresight, with additional lobes either side of boresight. On receive, signals will be received from a similar beam.
The operation of the inverse beamformer 12, having two four port couplers 13a, 13b, in conjunction with the beamformer 8 configured as described above with reference to
Signals entering the first individual element port 25a of the inverse beamformer 12 are connected to port A of the fourth four port coupler 13d. Signals leave port B of the fourth coupler 13d at a phase of 0 degrees and signals leave port C of the fourth coupler 13d at a phase of −90 degrees.
Signals from port B of the fourth coupler 13d are connected to port D of the third four port coupler 13c. Signals leave port B of the third coupler 13c at a phase of −90 degrees and port C at a phase of 0 degrees, which then undergo a further −180 degree phase shift in the phase shifter 16b so that the signals are presented to port D of the second four port coupler 13b at −180 degrees phase, which is equivalent to 180 degrees phase, and leave port B of the second four port coupler 13b at 90 degrees phase and port C of the second four port coupler 13b at 180 degrees phase.
Signals entering the second four port coupler 13b from port A leave port B at −90 degrees and leave port C of the second four port coupler 13b at −180 degrees, that is equivalent to 180 degrees. Accordingly, the signals at port B of the second four port coupler 13b arriving from ports A and D of the second four port coupler 13b are equal amplitude and in anti-phase and cancel; no signals are thus transmitted to the second antenna element 9b. However, the signals at port C of the second port coupler 13b arriving from ports A and D of the second coupler 13b are in phase and reinforce at a phase of 180 degrees.
The final signal combination occurs in the first four port coupler 13a; it will be appreciated that the signals arriving at port A and port D of this first four port coupler 13a are at the same amplitude as each other. Signals arriving at port A of the first four port coupler 13a, at 180 degrees, leave port B 180 degrees and leave port C at 90 degrees. Signals arriving at port D of the first four port coupler 13a, at −90 degrees, leave port B at 180 degrees and leave port C at −90 degrees. Accordingly, the signals at port B of the first four port coupler 13a arriving from ports A and D are in phase and reinforce; the signals are thus transmitted to the first antenna element 9a. The signals at port C of the first four port coupler 13a arriving from ports A and D are equal amplitude and in anti-phase and cancel, so that no signals are transmitted to the third antenna port 9c.
Hence it can be seen that the first, second, third and fourth couplers 13a . . . 13d are arranged such that signals transmitted into the first individual element port 25a of the inverse beamformer 12 are transmitted to the first antenna element port 9a of the beamformer 8 and not to the other antenna element ports 9b, 9c.
Similarly, signals from each of the individual element ports 25a . . . 25c arrive only at the respective antenna element ports 9a . . . 9c, while signals received at each of the antenna element ports 9a . . . 9c arrive only at the respective individual element ports 25a . . . 25c.
The values of the weighting elements are arranged such that on receive, signals originating at the antenna element port 9a corresponding with the individual element port 25a are combined constructively at the respective individual element port 25a, whereas signals originating at the other antenna element ports 9b, 9c are combined destructively at the respective individual element port 25b, 25c.
On transmit, the same weighting values may be used as on receive to achieve the desired connection between an antenna element port 9a and a respective individual element port 25a.
The weighting network may be implemented via a physical device, which may be bi-directional, allowing the use of the inverse beamformer for transmit, receive, or for both. Alternatively, the weighting network may be implemented via digital signal processing components.
An example of the weighting values that could be used with the beamformer 8 design as shown in
It should be noted that a phase shift of 180 degrees has the same meaning in this context as a phase shift of −180 degrees. It should also be noted that the phase shifters could alternatively be placed in series with the first and second beam ports 11e, 11f of the second beamformer 8b.
The above embodiments are to be understood as illustrative examples of the invention. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.
This application is a continuation of U.S. patent application No. 13/231,360, filed Sep. 13, 2011, entitled “Antenna System”, invented by David Neil Adams et al., which is a continuation of U.S. patent application No. 12/145,753, filed Jun. 25, 2008, now U.S. Pat. No. 8,063,822. All of the above-identified application are hereby incorporated by reference in their entireties as though fully and completely set forth herein.
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Number | Date | Country | |
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20130113658 A1 | May 2013 | US |
Number | Date | Country | |
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Parent | 13231360 | Sep 2011 | US |
Child | 13722015 | US | |
Parent | 12145753 | Jun 2008 | US |
Child | 13231360 | US |