GB2595691A

GB2595691A - Antenna array

Info

Publication number: GB2595691A
Application number: GB2008358.0A
Authority: GB
Inventors: Murray Edwards Fraser
Original assignee: Cambridge Consultants Ltd
Current assignee: Cambridge Consultants Ltd
Priority date: 2020-06-03
Filing date: 2020-06-03
Publication date: 2021-12-08
Also published as: GB202008358D0

Abstract

An antenna array, or a method of its operation, comprises: transmitting a first beam with a first set of signals by a first sub-array of antenna elements 7, based on a communication signal and transmitting a second beam with a second set of signals by a second sub-array of antenna elements 9, based on the communication signal. The first set of signals are out-phased by a first phase shift, and the second sets of signals are out-phased by a second phase shift, relative to that of the original communication signal. The antenna array elements 7, 9 are configured to form the first and second beams with aligned phase centres. Also disclosed is an antenna array calibration method, comprising: performing measurements of a test signal to obtain respective values indicative of attenuation between first and second transceivers and a common transceiver, at first and second frequencies; performing test measurements, at the second frequency, between the respective transmitters of the first and second transceivers and the receiver part of the common transceiver and obtaining a ratio of the measurement results; performing test measurements, at the first frequency, between the respective receivers of the first and second transceivers and the transmitting part of the common transceiver and obtaining a ratio of the measurement results; and calibrating the first and second transceivers according to the attenuation and ratio measurements obtained.

Description

Antenna Array The present invention relates to an antenna array (also referred to as a phased array). The invention has particular although not exclusive relevance to antenna arrays for advanced cellular telecommunication systems, for example 5G systems (and beyond) although it will be appreciated that such arrays find application elsewhere.

As part of any communications system, signals carrying information are transmitted between nodes of the system. For example, in a cellular telecommunications system, mobile telephones (referred to as user equipment or simply UEs) transmit signals to, and receive signals from, base stations such that mobile telephones can receive data from and transmit data to the base station.

The wireless signals being transmitted and received by the mobile telephone/base station have phase and amplitude variations with respect to time, and are modulated onto carrier waves in a predefined way such that there is a known relationship between the information that the signal is to carry and the instantaneous values of that signals' phase and amplitude. This enables the mobile telephone and the base station to demodulate and interpret the received signals in a mutually comprehensible manner, thereby facilitating communication between the respective devices.

As part of the transmission procedure between the respective devices, the original signal generated by each device is amplified, using a power amplifier, prior to its transmission via that device's antenna. Because of the amplitude variation of the signal being transmitted, typical signal amplification techniques introduce spectral regrowth, and thus unwanted adjacent channel leakage, which in turn limits operation in adjacent channels. As a result, the operation of the power amplifier is not particularly efficient. Moreover, the scarcity and cost of radiofrequency spectrum resources represent additional factors which need to be considered from the perspective of optimal operation of a power amplifier.

Array antennas, also sometimes referred to interchangeably as antenna arrays or phased arrays, have seen wide use in many communication systems, as well as finding applications in radar systems.

An array antenna comprises, in essence, multiple antenna elements which effectively work in tandem with one another such that the array can act as a single antenna. Advanced telecommunications systems can use array antennas for a variety of useful purposes. For example, array antennas enable the telecommunications system to benefit from beamforming techniques.

Array antennas are typically (but not always) split into two categories: digital arrays and non-digital arrays. Digital arrays have a complete transceiver chain connected to each array element which means that an array can create many beams using appropriate beamforming techniques, and non-digital arrays usually have a phase and amplitude shifter directly in the RF sections of the transmitter/receiver paths whereby only one beam can be formed (as opposed to digital arrays where the phase and amplitude is applied in the digital domain and thus multiple beams can be formed). In typical beamforming, the phase and amplitude of each signal transmitted by a respective array element is adjusted appropriately to form the signals being transmitted by the array into a discrete beam in a given direction. The beam formed is thus electronically (rather than mechanically) steerable, effectively allowing communication resources not only to be divided in frequency and in time, but also in space, thereby increasing capacity of the communication system: a key target capability of 50 systems. However, the presence of a large number of separate antenna elements, each having respective power amplification requirements, compounds the issues associated with power amplifier efficiency, thereby negating another key target capability of 5G systems -i.e. energy efficiency.

Accordingly, preferred embodiments of the present invention aim to provide methods and apparatus which address or at least partially deal with the above issues.

The present invention is set out in the appended independent claims. Optional features are set out in the appended dependent claims.

According to a first example, there is disclosed an array antenna for transmitting a communication signal, the array antenna comprising: a first array of antenna elements arranged for transmitting a first set of signals to form a first beam based on the communication signal; and a second array of antenna elements for transmitting a corresponding second set of signals to form a second respective beam based on the communication signal; wherein the first set of signals are out-phased with a first phase shift relative to a phase of the communication signal and the second set of signals are out-phased with a second phase shift relative to a phase of the communication signal, whereby the first and second beam are correspondingly out-phased; and wherein the first array of antenna elements and the second array of antenna elements are mutually configured to respectively form the first beam and the second beam to have phase centres that are substantially aligned with one another.

The first array of antenna elements and the second array of antenna elements may be mutually configured to respectively form the first beam and the second beam to have phase centres that are aligned with one another to within a wavelength of a carrier frequency used for communication signal transmission from the antenna array.

The antenna elements of the first array may be respectively interlaced with the antenna elements of the second array.

The antenna elements of at least the first array may be spaced with centres at a distance between adjacent antenna elements of half a wavelength of a carrier frequency used for communication signal transmission from the antenna array.

The antenna elements of the first array may be spaced with centres at distance from adjacent antenna elements of the second array of a quarter of a wavelength of a carrier frequency used for communication signal transmission from the antenna array.

The antenna elements of the first and second array may be mutually arranged to form a combined array having a generally polygonal or circular configuration, optionally a square, rectangular, or other quadrilateral configuration.

The antenna elements of the first and second array may be mutually arranged to form a combined two-dimensional array having an odd number of elements in at least one of the two dimensions.

Also disclosed is the array antenna of the first example, wherein the first array may have at least one more antenna element than the second array.

Transceiver apparatus comprising driver circuitry and an array antenna as mentioned above is disclosed, wherein the driver circuitry is configured to receive an input signal based on the communication signal and to generate the first set of out-phased signals for transmission from the first array of antenna elements and the second set of out-phased signals for transmission from the second array of antenna elements.

The driver circuitry of this transceiver apparatus may optionally comprise first power amplifier circuitry configured to amplify a first set of incoming signals derived from said input signal to generate the first set of out-phased signals, and second power amplifier circuitry configured to amplify a second set of incoming signals derived from said input signal to generate the second set of out-phased signals.

According to a second example, there is disclosed a method for transmitting a communication signal by an array antenna which comprises transmitting a first set of signals, by a first array of antenna elements, to form a first beam based on the communication signal; and transmitting a corresponding second set of signals, by a second array of antenna elements, to form a second respective beam based on the communication signal; wherein the first set of signals are out-phased with a first phase shift relative to a phase of the communication signal and the second set of signals are out-phased with a second phase shift relative to a phase of the communication signal, whereby the first and second beam are correspondingly out-phased; and forming the first beam and the second beam to have phase centres that are substantially aligned with one another.

According to a third example there is disclosed a method for calibrating transceivers for an array antenna, the array antenna comprising a first transceiver and a second transceiver of a pair of transceivers used for transmission and reception of communication signals, and a common transceiver; the method comprising: performing a first set of measurements of a test signal to obtain respective values indicative of attenuation between the first transceiver and the common transceiver respectively for a first frequency and for a second frequency; performing a second set of measurements of the test signal to obtain respective values indicative of attenuation between the second transceiver and the common transceiver respectively for the first frequency and for the second frequency; performing, at the second frequency, a third set of measurements of the test signal, respectively between a transmitter part of each of the first and second transceivers and a receiver part of the common transceiver to obtain a ratio between results of the third set of measurements; performing, at the first frequency, a fourth set of measurements of the test signal, respectively between a receiver part of each of the first and second transceivers and a transmitter part of the common transceiver to obtain a ratio between results of the third set of measurements; and calibrating the pair of transceivers used for signal transmission based on the obtained values indicative of attenuation and the obtained ratios.

The array antenna may continue to operate for communication purposes while the first transceiver and the second transceiver are taken out of service for performance of the calibration method.

The method may also comprise respectively repeating the first, second, third and fourth set of measurements iteratively for each of at least one further pair of transceivers used for transmission and reception of communication signals, with that at least one further pair of transceivers treated as the first transceiver and second transceiver, wherein at least one transceiver of the further pair of transceivers did not form part of the first pair of transceivers In the calibration method (A) the performing the first set of measurements may comprise, with the first transceiver as a current transceiver, performing the following steps: (i) tuning the current transceiver and the common transceiver to a current frequency equal to the first frequency; (ii) performing measurements at the current frequency comprising: a loopback measurement, between the transmitter part and the receiver part of the current transceiver, of a test signal; a loopback measurement, between a transmitter part and receiver part of the common transceiver, of the test signal; a forward measurement, between the transmitter part of the current transceiver and the receiver part of the common transceiver, of the test signal; a reverse measurement, between the receiver part of the current transceiver and the transmitter part of the common transceiver, of the test signal; obtaining, based on the performed measurements, a value indicative of attenuation between the first transceiver and the common transceiver at the current frequency; (iii) tuning the current transceiver and the common transceiver to the second frequency and repeating step (ii) with the second frequency as the current frequency; (B) the performing of the second set of measurements may comprise, with the second transceiver as the current transceiver, repeating steps (i) to (iii); (C) before performing the third and fourth set of measurements, transmitter parts of the first and second transceivers, and a receiver part of the common transceiver, may be tuned to the second frequency, and receiver parts of the first and second transceivers, and a transmitter part of the common transceiver, may be tuned to the first frequency; (D) the performing the third set of measurements may comprise performing, at the second frequency, a respective measurement of the test signal, between the transmitter part of each of the first and second transceivers and the receiver part of the common transceiver, and obtaining a ratio between results of the measurements between the transmitter part of each of the first and second transceivers and the receiver part of the common transceiver; (E) the performing the fourth set of measurements may comprise performing, at the first frequency, a respective measurement of the test signal between the receiver part of each of the first and second transceivers and the transmitter part of the common transceiver, and obtaining a ratio between results of the measurements between the receiver part of each of the first and second transceivers and the transmitter part of the common transceiver; and (F) the calibrating the pair of transceivers may comprise calibrating the transceivers used for signal transmission based on the values indicative of attenuation obtained in steps (A)(i) and (B)(i) and the ratios obtained in steps (D) and (E).

The method according to the above example may further comprise respectively repeating steps (A) to (F) iteratively for each of at least one further pair of transceivers used for transmission and reception of communication signals with that at least one further pair of transceivers treated as the first transceiver and second transceiver, wherein at least one transceiver of the further pair of transceivers did not form part of the first pair of transceivers.

Also disclosed is an antenna array comprising means for transmitting a first beam-35 formed out-phased signal and means for transmitting a second beam-formed out-phased signal that has a phase centre substantially aligned with (e.g. within a wavelength or so of) a phase centre of the first beam-formed out-phased signal.

Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently (or in combination with) any other disclosed and/or illustrated features. In particular but without limitation the features of any of the claims dependent from a particular independent claim may be introduced into that independent claim in any combination or individually.

Embodiments of the invention will now be described by way of example only with reference to the attached figures in which: Figure la is a simplified, plan view of an array antenna; Figure lb is a simplified, perspective view of an array antenna; Figure 2 schematically illustrates out-phasing signal transmission; Figure 3 is a simplified, plan view of an 8x8 array antenna; Figure 4 is a simplified, plan view of a linear array antenna receiving a plane wave; Figure 5 is a simplified, perspective view of a 2-D array antenna; Figure 6a shows a beam shape as a function of angle 0; Figure 6b shows a beam shape as a function of angle 0; Figure 7 is a simplified, plan view of a 7x7 array antenna; Figure 8 is a simplified, plan view of signal assignments for the array antenna of Figure 7; Figure 9 is an illustration of the beam shapes generated by the array antenna of Figure 7; Figure 10 is a simplified plan view of a basic architecture for transceiver calibration; Figure 11 is a simplified plan view of loopback measurements at two transceivers; Figure 12 is a simplified plan view of measurements between two transceivers; Figure 13 is a simplified plan view of measurements between two transceivers; Figure 14 is a simplified plan view of measurements between three transceivers; and Figure 15 is a simplified plan view of measurements between three transceivers. 5 Overview An example of an array antenna will now be described by way of example only with reference to Figures la and lb in overview.

Figure la illustrates, schematically, transceiver apparatus generally at 1. The transceiver apparatus comprises an array antenna 3 and driver circuitry 5 for generating the signals to be transmitted/received via the array antenna 3.

As seen in Figure I a, in this example, the array comprises two sets 7n,m, 9n,m of interlaced antenna elements 7, 9 represented by alternately filled patches. The respective antenna elements 7, 9 of each set of antenna elements 7n,,, 9n,rn in this embodiment have centres that are spaced from one another by half a wavelength (A/2) of an operating frequency of the antenna array 3 (e.g. corresponding to a carrier frequency used for communication via the array). The two sets of antenna elements 7,,m, 9n,m are offset from one another by half that spacing (i.e. A/4). Whilst the A/2 and A/4 spacings are particularly beneficial, it should be understood that other spacings may be utilised as required. It will be appreciated that the A/2 is the maximum spacing at which no grating lobes will appear in a beam shape for a scan angle between -900 and 900 0.e. for spacings no greater than A/2 there will be single main beam rather than a main beam plus one or more undesirably high level (similar to main beam level) grating lobes. Nevertheless, whilst it is beneficial for the array to have a A/2 spacing corresponding to the highest carrier frequency to be used this does not preclude successful operation at higher frequencies where grating lobes can be tolerated. For example, in multiple-input multiple-output (MIMO) antenna systems spacings of 0.7 lambda are sometimes used to deliberately create addition signal paths through the grating lobes The driver circuitry 5, which is generally controlled by a controller 12 comprising control logic, comprises signal processing circuitry 11 for generating the respective signals to be transmitted from each set of antenna elements 7n,,, 9n,m and a respective set of power amplifiers 13a, 13b for amplifying those generated signals for transmission. As the antenna elements 7, 9 can also receive signals, it will be understood that the driver circuitry 11 will also comprise circuitry for processing signals received at the array antenna 3. In effect the driver circuitry 11 encompasses multiple antenna element transceivers, each transceiver serving a different respective antenna element 7, 9 and at least one switch for switching between transmission and reception when the array is operating in Time Division Duplex (TDD) mode. Alternatively, the array may operate in Frequency Duplex Division (FDD) mode, and instead of the aforementioned at least one switch used for TDD operation, in the FDD mode at least one frequency duplexer is used instead. Each power amplifier 13a, 13b is arranged to drive a different respective antenna element 7, 9 of one of the two sets of antenna elements L1,, 9n,rn.

Beneficially, in the antenna array 3 shown generally in Figure la, the antenna array 3 and the driver circuitry 5 are mutually arranged to make advantageous use of the principle of out-phasing to increase operational efficiencies.

Out-phasing is a technique in which the signal to be transmitted, via an antenna, is decomposed into two signals with different phases but constant amplitude. As the two decomposed signals have a constant signal envelope, when the signals are amplified by respective power amplifiers prior to transmission, the amplification process does not introduce spectral regrowth into the signal and, accordingly, the operational efficiency of the power amplification procedure can be increased relative to a typical power amplification process (in which signals having a non-constant, variable, signal envelope are amplified, which problematically results in spectral regrowth). In traditional out-phasing the amplified signals are then recombined via summing at a power combiner, thereby recovering the original signal, which is then provided to subsequent transmission apparatus for transmission via the antenna.

As described in more detail below, in this example the driver circuitry 5 is arranged to drive each set of antenna elements 7,,m, 9n,n, with a different respective set of out-phasing signals (S1 n,m and S2n,m). Specifically, as shown in Figure la, an input signal representing the signal to be transmitted is received and decomposed by the signal processing circuitry 11, which outputs unamplified versions of each set of out-phasing signals (S1 n,m and S2n,m). The unamplified signals are then amplified by the power amplifiers 13a, 13b to produce the respective set of out-phasing signals (Sln,rn and S2n,m) for driving each set of antenna elements 7n,m, The signal processing circuitry 11 is configured to apply beamforming techniques to the signals that it generates such that the signals transmitted by each set of antenna elements 7n,m, 9n,, produce a different respective out-phasing beam 7-1, 9-1 as shown illustratively in Figure 1 b.

Beneficially, by virtue in particular of the interlaced arrangement of the antenna elements 7, 9, the antenna element sets 7,,m, 9n,, together with the driver circuitry 5 are mutually configured such that, in operation, the out-phasing beams 7-1, 9-1 formed by the different antenna element sets 7n,m, 9n,m share a common (or near common) beam centre and are, in effect, spatially superimposed (it will be understood that the slight offset between the illustrated beams in Figure lb is to allow the reader to distinguish between them). Accordingly, as those skilled in the art will appreciate, this arrangement allows the out-phasing technique to be used to produce well defined out-phasing beams that can be steered through a wide range of pointing angles without significant signal distortion.

Moreover, the antenna element 7, 9 arrangement advantageously allows the formation of an out-phased beam shape which can be scanned in more than one dimension. The antenna array 3 of this example also avoids the need for complicated switching arrangements which, in addition to introducing additional loss and expense, would only provide for coarse scanning in different dimensions.

Beneficially, in order to ensure that the circuitry used to drive the array antenna 3 produces signals for the different antenna elements that are correctly aligned in phase and amplitude, after power up, an improved calibration technique is used.

The calibration technique helps to ensure that the respective shape of each beam is accurate and that the antenna pattern sidelobe levels are kept under control.

The calibration technique described in more detail below beneficially calibrates the array antenna 3 to take account of differences between each transceiver chain for different respective antenna elements 7, 9 for the entire length of those transceiver chains. Moreover, the calibration technique also takes account of the effect of differences in the length of the feeder lines between the antenna elements 7, 9 and the switch for switching between transmission and reception.

The calibration technique allows calibration to be achieved using only a limited number of antenna elements at any one time (typically two) and thus can beneficially be used continuously, for example by operating on only two elements at a time. In a large array two elements being used for calibration purposes and thus being out of use for normal signal transmission will have little impact on antenna performance.

The calibration technique utilises a test signal and a "common transceiver" that is not used for transmission/reception of signals. Instead, this common transceiver is dedicated for calibration purposes, and is used in turn with different antenna element transceivers being calibrated, to determine a set of calibration parameters for achieving calibration between those antenna element transceivers.

Forward and reverse calibration measurements for a given antenna element transceiver are obtained by tuning that antenna element transceiver and common transceiver to a reception band (e.g. uplink), obtaining a set of measurements, and then re-tuning to a transmission band (e.g. downlink) and repeating the measurements. The set of measurements obtained in each direction typically include measurements for the antenna element transceiver, for the common transceiver, and between the antenna element transceiver and the common transceiver. This procedure can then be repeated for another antenna element transceiver, and the respective sets of measurements for each antenna element transceiver used to perform calibration adjustments for those antenna elements. In this way the effect of attenuation and phase caused by feedline length differences (and/or other transmission chain differences) between different antenna elements can be determined and hence, where the feedline length (and/or another transmission chain characteristic) differs between respective antenna elements, the effect of these differences can be taken into account by adjusting the calibration parameters (e.g. phase and/or amplitude) appropriately for those antenna elements.

This measurements technique also beneficially allows any errors associated with 30 the common transceiver to be cancelled out and the phase and amplitude of the antenna element transceivers to be aligned relatively to zero.

Before describing the antenna array used in the example of Figure 1 in greater detail, the general principles of out-phasing signal amplification will be discussed, by way of example only, with reference to Figure 2 and the principles of array antennas will be discussed with reference to Figures 3 to 6.

Out-Phasing Principles Figure 2 schematically illustrates a simple out-phasing circuit.

As seen in Figure 2, the out-phasing circuit receives an input signal (s = r cos(p) having a phase and amplitude which vary with time, and hence having a variable signal envelope, is input into a signal processor (e.g. calculating engine 17). The signal processor is configured to decompose the input signal into two discrete out-phased signals (0.5 cos(p ± r) where ri = arcos(r)) in two separate paths with different phases but constant amplitude. Accordingly, the decomposed signals have a constant signal envelope, and are hence better suited for subsequent amplification. It will be appreciated that the symbols s, r, p and q are all functions of time (i.e. s(t), r(t), p(t) and q(t)) but that the t component is omitted for clarity as this is implicit throughout.

The two out-phased discrete signals are each amplified by a respective power amplifier 213a, 213b, and then recombined though signal addition by a summing unit 19, to recover the original signal prior to transmission via an antenna. The summing unit may be, for example, a Wilkinson signal combiner or a variant thereof.

In more detail, the out-phasing procedure can be considered mathematically as follows, starting with the original amplitude and phase modulated signal s(t) = r(t) cos(p(t)) written in simplified form (without the implied (t)) as: s = r cos(p) where r takes a value between 0 to 1 and p takes a value of -7 to +-rr Considering trigonometrical identities, this signal can also be written equivalently as: s = 0.5(cos(A) + cos(B)) From the trigonometrical identities we can then write: 5 = cos(0.5(A -B)) cos(0.5(A + B)) By equivalence we can then say: r = cos(0.5(A -B)) = 0.5(A + B) Thus: A -B = 2 arccos(r) A + B = 2 0 So: A = p arccos(r) B = [3-arccos(r) Typically, this is written as: A= n B= p -Wth: = arccos(r) Thus, from the amplitude value 'it the value of the out-phasing angle, 4), (corresponding to n in the above analysis) may be calculated and the equivalent signal may be constructed as: s = 0 5(cos(I3 + n) cos(I3 -n)) r cos(13) The efficiency of the out-phasing method is a function of the out-phasing angle, 4).

Compensation can be added to increase the efficiency (albeit at the potential expense of re-introduction of some spectral regrowth in the output signal).

Array Antenna Principles A conventional antenna array will now be described with reference to Figures 3 to 6.

Figure 3 is a schematic illustration of an exemplary square array antenna 21 in plan view.

As seen in Figure 3, the array antenna 21 comprises 64 separate antenna elements 23 arranged in the form an 8 x 8 element array. The round patches each represent the antenna elements themselves and the elements centres are shown as being separated A/2 in both the x and y directions (although other separations may be used).

Each of the antenna elements 23 may respectively transmit and/or receive signals and are driven by a respective transceiver forming pad of the array's (Tx/Rx) electronics (e.g. corresponding to the driver circuitry in Figure 1), which in practice are typically placed behind the plane of the array. In operation, therefore, when a signal is transmitted by the array 21, the array's radiation pattern will be emitted out from the plane of the array 21 (out of the plane of the page) and the radiation pattern of a received signal will be received into the plane of the array 21 (into the plane of the page).

Figure 4, is schematic illustration of an exemplary linear array receiving a planar wave for use in explaining the operational principals of array antennas. Figure 4 shows, in simplified cross-section, a view of a linear array antenna 25 comprising N antenna elements, n = 0... N-1, spaced X/2 apart. The linear array may, for example, represent one row of the array 21 shown in Figure 3.

A plane wave is shown approaching the linear array 25 at angle 8 (note that the analysis which follows is provided with respect to reception, though the same principles apply with respect to the transmit direction).

Noting that: a = d sin(e) AT= a/c k= 2 -mu c Where f is the frequency, c is the speed of light, -rr has its standard definition, k is the wavenumber and 0 can take a value from -90 to 90 degrees for this linear (1-dimensional) case.

The signal phase difference, I), between any two antenna elements may thus be represented as: = 2 -rrf AT = 2 -rrf a/c = kdsin(0) The planar wave signal, En, arriving at each antenna element, n = 0 N -1, of the linear array depicted in Figure 4 can therefore be written as: En (A / ro)e(- o e(-j n k d sin(0)) or: En (A / ro) eej k I0) e(-j 0 Rd sin(8)) Ei (A / ro) k re) e(-j 1 k d sin(e)) E2 (Al ro) e ro) e(-j2 etc...

EN-1 rc (A / ro) e(-; (N-1) k d sin(0)) As the distance between antenna elements, d, in this linear array 25 is equal to one half wavelength (A/2), the value of n represents the number of half wavelengths distant from the left-hand edge of the array (i.e. from n = 0) in this example. It will be understood that rather than the left-hand edge, this reference point may be elsewhere, e.g. the geometric centre of the array, as all distances are relative.

To create a receive beam, pointing in a specific direction, 00, the respective expression for the wave at each element is multiplied by ea n Rd sin(60)). For a signal arriving at the angle, OP, this will produce a maximum output.

When summing the elements over n for all values of 0 and for a given fixed value of eo, this gives: E(8) = (Al ro) eej k r0) Zn expa n Rd [sin(S0) -sin(E))]) E(0) = (A / ro) e(1 k ro) AF(0) Where the sum is over the n = 0 N -1 antenna elements (i.e. over the entirety of the linear array 25).

This summation term is called the Array Factor (AF) and this factor is responsible for the characteristic beam shape of an array antenna. The term shown before the summation represents the received signal as a function of range ro.

Referring to Figure 5 which illustrates beam formation for a two dimensional antenna array, the above-described calculation can be extended to two dimensions, e.g. to a square-type array antenna (or indeed an array of any shape, such as rectangular, oblong or circular), such as that shown in e.g. Figure 3, by defining a beam pointing as 130, 00 and scanning over a second angle 0 covering 0 to 360 degrees. The beam can then be formed from the Array Factor sum in both dimensions and multiplying them to form a 'composite' Array Factor.

Figure 5 provides a perspective view of a two-dimensional square-type array antenna 21 (without the patches shown to aid clarity), and defines the angles 00 and 00.

As seen in Figure 5, the schematic array of antenna elements occupies the x and y axis, and a resultant beam formed by the array emits out of the plane of the array into the z dimension.

In this example, the composite array factor (AF) becomes the sum over the n = 0..N -1 antenna elements in one dimension multiplied by the sum over the m = 0...M -1 antenna elements in the other (i.e. the antenna elements of the array in both the x and y axes): AF(e, z n n k d [sin(60)cos(o) -sin(G)cos3)]) x z m e0 m k d [sin(3o)sin(o) -sin(6)sin()]) The main beam direction is then given by the values of Bp and 00, which are then input into the calculation to provide the array antenna's beam shape.

An example beam shape for an 8 x 8 phased array antenna, with to and 00 both equal to 0 degrees and 0 plotted as -90 to 90 degrees, is shown in Figure 6a which is essentially a 1 dimensional slice through a 2 dimensional beam -note that the main beam is clearly visible when 9 = 0 degrees.

Moreover, as mentioned previously, beams formed by array antennas may beneficially be electronically steered. For instance, by changing the values of ea 30 and to the beam can be steered to any angle.

In this regard, reference is made to Figure 6b which shows the beam of Figure 6a shifted to 60 = 30 degrees and cpo = 0 degrees. Electronic steering has particular advantages for advanced telecommunications systems, such as those operating in accordance with fifth generation (5G) telecommunications standards, which have particularly strict requirements with respect to device mobility and connection reliability, as the latency associated with typical mechanical steering of beams may not be able meet these demanding requirements.

Antenna Array using Dual-Beam Out-phasing An exemplary array antenna for use with the transceiver apparatus described with reference to Figure 1 will now be described, by way of example only, with reference to Figures 7 to 9. The principle and application of dual-beam out-phasing introduced with reference to Figure 1, will also be described in more detail.

Figure 7 illustrates, in a plan view, one possible design of an array antenna 27 that may be used with the transceiver apparatus 1 of Figure 1. As illustrated, the array antenna 27, in this example, comprises 49 antenna elements arranged to form an N x M two-dimensional array, where both N and M are equal (i.e. N=M=7).

The 49 antenna elements comprise a first antenna element set forming a first antenna sub-array 29 comprising comprising 25 antenna elements, and a second antenna element set forming a second antenna sub-array 31n,m comprising 24 antenna elements. The first and second sets 29n,m, 31n,m of the antenna elements are arranged in an interlaced pattern as indicated by the alternately shaded and spotted patches of the array 27 shown in Figure 7 (with the associated transmission/reception circuitry omitted from the schematic for clarity). The first set of antenna elements 29 comprises comprises four rows of four antenna elements 29, and three rows of three antenna elements 29, arranged alternately (i.e. in a four-three-four-three-four-three-four pattern). The second set of antenna elements 31n,m comprises four rows of three antenna elements 31, and three rows of four antenna elements 31, arranged alternately (i.e. in a three-four-three-four-three-four-three pattern).

The array comprising an odd number of antenna elements 29, 31 in each dimension (N and M) is particularly beneficial as it provides for a relatively simple arrangement of antenna elements that is particularly suitable for ensuring that the two sets of antenna elements produces beams having common, or near common, beam centres. Nevertheless, it will be appreciated that the array may use an even number of elements in one or both dimensions and still provide beams with beam centres sufficiently close to one another to provide significant benefits.

As shown in Figure 7, the centres of the antenna elements of the antenna array 27 are spaced A/4 apart, in the N ('x') and M ('y') dimensions, from one another. Whilst a A/4 spacing is particularly beneficial and results in the respective elements of each antenna sub-array 290,,, 31 nm being spaced at A/2 in the N dimension (in this example), it will be appreciated that other suitable spacings may be used, such as

A/2, for example.

The antenna array 27 according to this example is configured to use out-of-phase signal amplification. Accordingly, in this example in operation, a signal to be transmitted is decomposed by the signal processing circuitry (e.g. as illustrated in Figure 1) into two discrete signal sets (S1 n,m and S2n,m), having a different phase, which are then respectively amplified by power amplifiers, as discussed above with respect to Figure 1. The antenna elements 29 of the first sub-array 290,, are configured to transmit the first set of decomposed and subsequently amplified signals (corresponding to a first out-phasing signal 510,m) and the antenna elements 31 of the second array 31 n,m are configured to transmit the second set of decomposed and subsequently amplified signals (corresponding to a second out-phasing signal S20,m).

Therefore, it will be appreciated, in a manner analogous to that illustrated in Figure lb, that a first beam is formed by the first sub-array 290,0, being driven by the first set of out-phasing signals (S1 0m) and a second beam, out of phase relative to the first beam, is formed by the second sub-array 31 n,m by being driven by the second set of out-phasing signals (S2n.m).

In operation, both beams point in the same direction (which means both beams have the same values of 00 and 1)0) and, as a result of the interlaced pattern of antenna elements 29, 31 both beams also have substantially the same phase centre In other words, the two beams, when they are formed, are superimposed one on top of the other. It will be appreciated that whilst the beams ideally have the same phase centre benefits can be derived when the beams have substantially, but not exactly, the same phase centre (for example where the phase centres are within a wavelength or so of the carrier frequency used to communicate signals via the antenna array).

The two beams are essentially formed in the same way as for a single beam as described above, but using appropriate values of n and m corresponding to the distance (in numbers of half-wavelengths) from the centre of a given antenna element 29, 31 from the antenna sub-array's phase centre. The generation of the beams using appropriate values of n and m, which relate to ether the first beam (generated by antenna elements 29 of the first sub-array 29n,m) or the second beam (generated by antenna elements 31 of the second sub-array 31n,n,), will now be explained in more detail below by way of example only.

In more detail, consider that the antenna array 27 is configured to transmit an amplitude and phase modulated signal of the form: s = r e(1131 where r = 0 to 1, = --rr to -F-rr where n is defined as: n = arccos(r) The respective sets of out-phasing signals (S1n.rn and S2n,rn) are respectively sent to each antenna sub-array 29n,m, 31n,m, as discussed above.

The first set of out-phasing signals, 51n.rn, provided to the shaded antenna elements 29 of the first sub-array 29n,m, is calculated from: Si n,m = e(113+9) (apodisafionn,m / norm) eu n1 k d [sin(30)cos(410)]) x e(j m1 k d [sin(60)sin(I)0)]) The second out-phasing signal, S2n,m, provided to the spotted antenna elements 31 of the second sub-array 31 n,m is calculated from: S2n,m eo + eo n2 k d Nin(13)cos(I) )]) 0 m2 k d [sin(13)sin(1) )]) 0 0X e 0 0 Where: The n and m values for each beam set are illustrated, in detail with reference to Figure 8, and which respectively represent the number of half-wavelengths distant from the antenna's phase centre for each given antenna element.

The apodisationn.mc values are calculated to optimise a beam's ability to cancel for values of q approaching Tr/2.

The 'norm value adjusts the relative beam amplitude and is calculated, during calibration (see below) from: (1/No of elements in second sub-array) ZnEm apodisationn,m to also optimise the beam's ability to cancel for values of q approaching Tr/2.

The apodisation and norm values are themselves dependent on the specific number of antenna elements used in each dimension, and so will vary depending on the operational requirements of any given array antenna.

An illustration is given in Figure 9, for the above described 7 x 7 array 27, showing an unadulterated beam and a beam set to give a -25dB out-phasing level It will be appreciated that the antenna array and associated signal processing circuitry are also mutually configured for operation as a receiver which is capable of receiving, combining and processing signals received, via similar out-phased beams, from other sources.

It can be seen, therefore, that the exemplary antenna array 27 described with reference to Figures 7 to 9 provides a particularly beneficial implementation which allows dual beam out-phasing to be performed without significant deleterious effects arising, for example, beam distortion or the like.

It will also be appreciated that whilst the antenna array described is for the provision of dual beam out-phasing, the method described can be extended for the generation of multiple simultaneous beams depending on requirements.

Calibration of the System As shown in the above examples, the transmitter and receiver form part of the overall array antenna and maintaining phase and amplitude coherence, element to element, is important for maintaining high accuracy beam shapes.

To ensure such coherence, and as discussed in detail below, the signal processing circuitry is configured for implementing a calibration method for calibrating a relative ratio in phase and gain of any pair of transmitter chains and similarly a relative ratio in phase and gain difference between any pair of receiver chains. Such a calibration may, for example, involve calibrating a transceiver chain of one antenna element relative either to the transceiver chain of its neighbouring antenna element or to the transceiver chain of any other antenna element of the array. This is achieved by calibrating each transceiver chain of the pair of transceiver chains with reference to a common transceiver that is only used for calibration purposes.

Once all the transceiver chains are calibrated relative to one another the absolute level of all chains can be adjusted against a known pre-calibrated value of gain and phase.

Calibration Method The calibration method is used, to align the transceivers used in the antenna array in phase and amplitude, after power up of the antenna array 27. This helps to ensure that the antenna array's beam shapes are accurate and its antenna pattern sidelobe levels are kept under control. Moreover, it will be appreciated that in an antenna array the associated antenna feed line lengths may be different for each of the different antenna elements which form part of the array -resulting in complex attenuations along and between respective transceiver chains. The calibration method also allows errors associated with these differences to be removed.

The calibration method calibrates for both differences in the entire transceiver chain and for the effect of the feedline up to the antenna transmit/receive switch.

Moreover, this calibration method may be used continuously by only operating on two antenna elements in the array at any one time. In a large array, two elements not being used for actual signal transmission at any one time will not impact on the performance of the antenna array in any notable way.

Referring to Figure 10, which schematically outlines the basic architecture used for the calibration procedure, operational transceivers 0 to N (e.g. transceivers which are used during the operation of the antenna array for signal transmission/reception) are connected to a non-operational common transceiver (transceiver c), which is used only used for calibration and not for signal transmission Initially, measurement values representing the effect of differences in the track (i.e. differences in the respective antenna feedline lengths) represented generally as ON in Figure 10, are obtained through loopback measurements between the receiver and transmitter of a given transceiver and forward and reverse measurements between that transceiver and the common transceiver.

In more detail, in Figure 10, transceivers 0 to N may be connected back on themselves (i.e. transmitter to receiver, Tx to Rx) to perform a loopback measurement, and can also be connected by a common connection to transceiver c for performing the forward and reverse measurements.

During calibration, the inputs to the transmitters 0 to N and c are fed with the same test signal, s, where s is of the general form: S = A(t)0('8) The values of ON may represent complex attenuations, which are not known, and are therefore established prior to the ratio calibration, where C is of the form: C= The following steps set out how the values of ON are calculated, with reference to Figure 10.

Step 1 The first step is to tune transceiver 0, TxRxo, and the common transceiver, TxRxG, to an uplink frequency (in this example a frequency which is defined as one of a telecommunications system's receive bands, and denoted in the following equations as a lowercase u) and then, in succession, measure the following four values: YCU S(aCUbCU) zcu = s(a0"bc"AAotie-7°00u) xth, = s(a,"b0.6110"e--000.) The signal s is available clock synchronous at the operational transceivers (i.e. transceivers 0 to N) and at the calibration transceiver. Accordingly, the signal cs' can be removed from the calculation by normalising as follows: s(a0121,00 f Thus, by normalising all the measurements in this way the following values are obtained: wou = (aoubou) YCU = (aCU bell) z" = (ao,b"AAoue-1A0ou) xo, = (aoub0,4Aoue -iAcku) The ratio of the measurement between transceiver 0 and transceiver c, Zou, to the loopback measurement at transceiver 0, Wou, may then be formed: aoubotAA0"e-1°00. bo"

AAo e-l'Ou. u bOu The ratio of the measurement between transceiver c and transceiver 0, x0o, to the loopback measurement at transceiver c, you, may also be formed: rOu acubouAiloue-Wou bou aAoue-f4" Multiplying the two ratios then gives: zo" xou bou anA A e b" AA ec JA0"" = AAou2 e-2;Acorn, ou ou u *Jou Then: Ycu ctoubo" ?Jou pAou I Z" Xou I I WOu Ycu I acpou = -2argiz11 Accordingly, the value of the attenuation of the connecting track between transceiver 0 (TxRx0) and the common transceiver c (TxRx,) at the uplink frequency, Con, is now known.

The squaring of the term e-M00,, introduces an ambiguity of -rr radians. There are thus two possible solutions that may be derived from the calculation. A simple way of resolving this is to have a pre-stored (approximate) static value for the phase shift of the track at the uplink (and downlink) frequencies. The solution of the two possible solutions that coincides (to within some accuracy) of this pre-stored value can then be selected.

Step 2 In the second step, TxRxo and TxRx0 are re-tuned to the downlink frequency On this example a frequency which is defined as one of a telecommunications system's transmit bands) denoted below as a lowercase d), and the process given for step 1 repeated.

The value of the attenuation of the connecting track between TxRxo and TxRx, at the downlink frequency, Cod, can thus be obtained.

Step 3 In step 3, steps 1 and 2 are repeated, with the next transceiver (TxRxi) and the common transceiver (TxRxe).

The values of the attenuation of the connecting track between TxRxi and TxRx, at the uplink frequency, Clo, and at the downlink frequency, Cid, can thus be obtained.

Step 4 In step 4, the first transceiver (TxRx0) and second transceiver (TxRxi) are tuned to the desired uplink and downlink frequencies, as determined by the telecommunications systems requirements. The common transceiver (TxRxe) is tuned to the reverse frequencies, so that the receiver (Rx,) of the common transceiver is configured to receive signals using the downlink frequency and its corresponding transmitter (Txo) is configured to transmit signals using the uplink frequency.

Successive measurements are then made at the downlink frequency from transceiver O's transmitter (Txo) and transceiver l's transmitter (Txi) to the receiver (Rxe) of the common transceiver: zoca = s(aoa Coaka) zica = s(aia bca) As set out in step 1, these equations can be normalised by diving by 's', as s is distributed both to the transceivers to be calibrated and to the common transceiver: s (aod Coabcd) (aodCOdbcd) s(aidCiabcd) (aidCidbcd) The effect of the receiver (Rx,) of the common transceiver is then removed by forming the ratio: Zud (aidClcibcd) zoed (aod Cod bed) aot/COti Since the values of Cid and Cod are known, from steps 1 to 3, the ratios of the downlink transmitters can be calculated and adjusted to their correct relative value: aid _ ziod Cod aoa zoca Cia Successive measurements are then made at the uplink frequency (the uplink components are already tuned to the uplink frequency) and normalised with '5': xon, = (ac.Coubou) x"" = (aClubitt) The effect of the transmitter (Tx) of the common transceiver is then removed by forming the ratio: Xicu (acuClublu) bluClu xocu (acucubOu) COu Since the values of Cl. and Co. are also known, from steps 1 to 3, the ratio of the uplink receivers is now also known independent of the common transmitter.

blu Xlcuclu bou XauClu Accordingly, values corresponding to significant errors in the system are known, and therefore associated errors in a given transceiver's phase and amplitude may be calibrated out by the signal processing circuitry.

As shown in the example set out in Figure 10, all transceivers are connected to the 20 common transceiver. However, more than one common transceiver could be used, as long as it is connected to two transceivers, which are being calibrated, at once.

It will be appreciated that the local oscillators in the transceiver devices will change to a random phase when any new frequency is programmed in.

Advantageously, therefore, the sequencing of the calibration steps takes this into account to avoid introducing random phase undesirably.

Specifically, measurement of a single value of CNu or CNo, as described in steps 1 and 2, can be made without changing the frequency (and thus introducing an unknown phase) of any of the other transceivers. When making comparative measurements (e.g. those of steps 3 or 4) once the comparison is made the frequency is not changed (because doing so would introduce a random phase).

Continuous Sequential Calibration Advantageously, it is possible to sequentially calibrate around the complete array of transceivers, whilst the antenna array 27 is operational, by taking only one transceiver out of service at a time, thereby materially maintaining the performance of the array.

Specifically, once the array has been fully calibrated, is in operation, and the values of all the C. and Cd are nominally correct, a given transceiver, say transceiver n, may be selected for recalibration and taken out of service. Steps 1 and 2, as set out above, can be performed to obtain updated values of CNd and CNd. A relative calibration of transceiver chain n against another transceiver chain (e.g. for transceiver n-1) may be performed, and then the phase and amplitude of transceiver chain n adjusted to be in line with that of chain n-1, because the phases of transceiver chain n will be affected by the updating of the CNu and CNd values. Recalibrated transceiver n may then be placed back into service, and the method can then be applied to subsequent transceivers.

Exemplary Implementation of the Method for Two Transceivers An exemplary implementation of a method of receiver (Rx) calibration will now be described, by way of example only, with reference to Figures 11 to 15. Figure 11 shows the general architecture between an operational transceiver used for signal transmission/reception (transceiver #n) and a non-operational ('common') transceiver only used for calibration (transceiver #c). In this example the transceiver #n is configured to operate on two radiofrequency bands, bands 1 and 7. Accordingly, transceiver #n comprises hardware dedicated for transmission and reception on these bands -i.e. PM (power amplifier 1) and LNA1 (low noise amplifier 1) are dedicated for processing signalling on band 1, whilst PA2 and LNA2 are dedicated for processing signalling on band 7.

To measure the C values, in a manner similar to that described above, involves 8 separate measurements being made when calibrating the receiver with respect to downlink and to uplink frequencies.

Firstly, with reference to Figure 11, loopback measurements are made for transceiver #n (TxRx #n) and for the transceiver #c (TxRx #c), at both downlink and uplink frequencies -i.e. four separate measurements in total. The paths being measured are illustrated with emboldened lines, relative to the other lines of the drawing. In the following, all paths being measured are similarly illustrated with emboldened lines.

Then, measurements are made between TxRx #n's transmitter to TxRx #c's receiver, again at both downlink and uplink frequencies -i.e. a total of two separate measurements.

The corresponding measurement is then made from the common transceiver #c to transmitter #n, by measuring TxRx #c's transmitter to TxRx #n's receiver at both downlink and uplink frequencies; again, a total of two separate measurements.

With these 8 measurements, the values of CNd and CNu can be calculated, as shown per steps 1 and 2 above.

The transmitter chain relative calibration can then be made, in a manner analogous to steps 3 and 4 above, thus requiring a further two measurements as depicted in Figure 14.

Finally, the corresponding the receiver chain relative calibration measurements are made, i.e. the last two measurements, as shown in Figure 15.

Receiver-side Processing The decomposed amplified signals S1 n.m and S2n,m are transmitted by the array 27 towards a corresponding receiving party, such as a mobile telephone or other user equipment. The receiving party receives these signals and, in a manner analogous to Figure 2, recombines these signals by summing to obtain the original signal input into the signal processing circuitry 11 of the array 27.

Once the original signal has been recovered at the receiver-side, control logic of the receiving party may then, along with its associated hardware, demodulate the original signal, as recovered, in the usual way.

Modifications and Alternatives Detailed examples have been described above. As those skilled in the art will appreciate, a number of modifications and alternatives can be made to the above examples whilst still benefiting from the inventions embodied therein.

It will be appreciated, for example, that whilst the antenna array 27 was illustrated as a square 7 x 7 arrangement of antenna elements, it should be appreciated that other geometries of antenna elements may form an array which utilises the principles of out-phasing described herein, such as a rectangular 5 x 31 array or a rectangular 9 x 3 array. Other arrangements may also be utilised depending on system requirements, such as linear arrays.

The individual characteristics of antenna array 27, or any other such similar array which follow the above-described principles of out-phasing, may be modified for instance by discrete apodisation of a given antenna element or elements, e.g. by exciting individual antenna element(s) in the antenna array with different voltage amplitudes when providing signals to the antenna element, to control array side-lobe levels as required It should be understood that multiple transmission beams can be created and transmitted by a digital antenna array using the above-described out-phasing principles, for instance by combining signals with different values of eci and to.

The calibration method may beneficially be applied either to two transceivers which physically reside side-by-side on the array antenna, or to two transceivers which are not physically located next to one another, and which may be separated by the maximum extremity of the system.

Various other modifications will be apparent to those skilled in the art and will not be described in further detail here.

Claims

Claims 1. An array antenna for transmitting a communication signal, the array antenna comprising: a first array of antenna elements arranged for transmitting a first set of signals to form a first beam based on the communication signal; and a second array of antenna elements for transmitting a corresponding second set of signals to form a second respective beam based on the communication signal; wherein the first set of signals are out-phased with a first phase shift relative to a phase of the communication signal and the second set of signals are out-phased with a second phase shift relative to a phase of the communication signal, whereby the first and second beam are correspondingly out-phased; and wherein the first array of antenna elements and the second array of antenna elements are mutually configured to respectively form the first beam and the second beam to have phase centres that are substantially aligned with one another.
2. The array antenna of claim 1, wherein the first array of antenna elements and the second array of antenna elements are mutually configured to respectively form the first beam and the second beam to have phase centres that are aligned with one another to within a wavelength of a carrier frequency used for communication signal transmission from the antenna array.
3. The array antenna of claim 1 or claim 2, wherein the antenna elements of the first array are respectively interlaced with the antenna elements of the second array.
4. The array antenna of any preceding claim, wherein the antenna elements of at least the first array are spaced with centres at a distance between adjacent antenna elements of half a wavelength of a carrier frequency used for communication signal transmission from the antenna array.
5. The array antenna of claim 4, wherein the antenna elements of the first array are spaced with centres at distance from adjacent antenna elements of the second array of a quarter of a wavelength of a carrier frequency used for communication signal transmission from the antenna array.
6. The array antenna according to any preceding claim, wherein the antenna elements of the first and second array are mutually arranged to form a combined array having a generally polygonal or circular configuration, optionally a square, rectangular, or other quadrilateral configuration.
7. The array antenna according to any preceding claim, wherein the antenna elements of the first and second array are mutually arranged to form a combined two-dimensional array having an odd number of elements in at least one of the two dimensions.
8. The array antenna according to any preceding claim, wherein the first array has at least one more antenna element than the second array.
9. Transceiver apparatus comprising driver circuitry and an array antenna according to any of claims 1 to 8, wherein the driver circuitry is configured to receive an input signal based on the communication signal and to generate the first set of out-phased signals for transmission from the first array of antenna elements and the second set of out-phased signals for transmission from the second array of antenna elements.
10. The transceiver apparatus according to claim 9, wherein the driver circuitry comprises first power amplifier circuitry configured to amplify a first set of incoming signals derived from said input signal to generate the first set of out-phased signals, and second power amplifier circuitry configured to amplify a second set of incoming signals derived from said input signal to generate the second set of out-phased signals.
11. A method for transmitting a communication signal by an array antenna, the method comprising: transmitting a first set of signals, by a first array of antenna elements, to form a first beam based on the communication signal; and transmitting a corresponding second set of signals, by a second array of antenna elements, to form a second respective beam based on the communication signal; wherein the first set of signals are out-phased with a first phase shift relative to a phase of the communication signal and the second set of signals are out-phased with a second phase shift relative to a phase of the communication signal, whereby the first and second beam are correspondingly out-phased; and forming the first beam and the second beam to have phase centres that are substantially aligned with one another.
12. A method for calibrating transceivers for an array antenna, the array antenna comprising a first transceiver and a second transceiver of a pair of transceivers used for transmission and reception of communication signals, and a common transceiver; the method comprising: performing a first set of measurements of a test signal to obtain respective values indicative of attenuation between the first transceiver and the common transceiver respectively for a first frequency and for a second frequency; performing a second set of measurements of the test signal to obtain respective values indicative of attenuation between the second transceiver and the common transceiver respectively for the first frequency and for the second frequency; performing, at the second frequency, a third set of measurements of the test signal, respectively between a transmitter part of each of the first and second transceivers and a receiver pad of the common transceiver to obtain a ratio between results of the third set of measurements; performing, at the first frequency, a fourth set of measurements of the test signal, respectively between a receiver part of each of the first and second transceivers and a transmitter part of the common transceiver to obtain a ratio between results of the third set of measurements; and calibrating the pair of transceivers used for signal transmission based on the obtained values indicative of attenuation and the obtained ratios.
13. The method according to claim 12, wherein the array antenna continues to operate for communication purposes while the first transceiver and the second transceiver are taken out of service for performance of said calibration.
14. The method according to claim 12 or 13, further comprising respectively repeating the first, second, third and fourth set of measurements iteratively for each of at least one further pair of transceivers used for transmission and reception of communication signals, with that at least one further pair of transceivers treated as the first transceiver and second transceiver, wherein at least one transceiver of the further pair of transceivers did not form part of the first pair of transceivers.
15. The method according to claim 12, 13 or 14, wherein: (A) the performing the first set of measurements comprises, with the first transceiver as a current transceiver, performing the following steps: (i) tuning the current transceiver and the common transceiver to a current frequency equal to the first frequency; (ii) performing measurements at the current frequency comprising: a loopback measurement, between the transmitter part and the receiver part of the current transceiver, of a test signal; a loopback measurement, between a transmitter part and receiver part of the common transceiver, of the test signal; a forward measurement, between the transmitter part of the current transceiver and the receiver part of the common transceiver, of the test signal; a reverse measurement, between the receiver part of the current transceiver and the transmitter part of the common transceiver, of the test signal; obtaining, based on the performed measurements, a value indicative of attenuation between the first transceiver and the common transceiver at the current frequency; (iii) tuning the current transceiver and the common transceiver to the second frequency and repeating step (ii) with the second frequency as the current frequency; (B) the performing of the second set of measurements comprises, with the second transceiver as the current transceiver, repeating steps (i) to (iii); (C) the method comprises, before performing the third and fourth set of measurements, tuning transmitter parts of the first and second transceivers, and a receiver part of the common transceiver, to the second frequency, and tuning receiver parts of the first and second transceivers, and a transmitter part of the common transceiver, to the first frequency; (D) the performing the third set of measurements comprises performing, at the second frequency, a respective measurement of the test signal, between the transmitter part of each of the first and second transceivers and the receiver part of the common transceiver, and obtaining a ratio between results of the measurements between the transmitter part of each of the first and second transceivers and the receiver part of the common transceiver; (E) the performing the fourth set of measurements comprises performing, at the first frequency, a respective measurement of the test signal between the receiver part of each of the first and second transceivers and the transmitter part of the common transceiver, and obtaining a ratio between results of the measurements between the receiver part of each of the first and second transceivers and the transmitter part of the common transceiver; and (F) the calibrating the pair of transceivers comprises calibrating the transceivers used for signal transmission based on the values indicative of attenuation obtained in steps (A)(i) and (B)(i) and the ratios obtained in steps (D) and (E).
16. The method according to claim 15, further comprising: respectively repeating steps (A) to (F) iteratively for each of at least one further pair of transceivers used for transmission and reception of communication signals with that at least one further pair of transceivers treated as the first transceiver and second transceiver, wherein at least one transceiver of the further pair of transceivers did not form part of the first pair of transceivers.