CALIBRATION
FIELD OFTHE INVENTION
The present invention relates to a n active transceiver array and a method of calibrating
a n active transceiver array.
BACKGROUND
Wireless telecommunications systems are known. In a cellular system radio coverage is
provided in areas known as cells. A base station is located in each cell to provide radio
coverage. Traditional base stations provide coverage in relatively large geographical
areas and the cells are often referred to as macro cells.
A typical base station comprises: a control centre at ground level, a radio mast and a n
array of antenna located on the mast. The array of antenna operates to provide radio
coverage to end users within the cell by transmitting signals to, and receiving signals
from, end users within the cell. In such a typical base station the base station control
centre contains, amongst other things: a data management unit, a digital to analogue
converter, a filter and a power amplifier. Housing those items in the control centre at
ground level allows those components of the system to be substantially protected from
prevailing environmental conditions, such as precipitation or temperature fluctuation.
Analogue radio frequency (RF) signals generated in the control centre at ground level
are communicated to each antenna element making up the antenna array via
coaxial cable extending from the control centre at the base of the radio mast to the
antenna array provided near the top of the radio mast. In a "passive" array the RF
signal supplied to each antenna element is substantially identical; provision may be
made in the antenna array for a fixed phase shift across the array to be introduced to
the signal to be transmitted by the array. Such a phase shift across the array allows
redirection of wave fronts produced by the array, also known as "beam forming". An
array which performs such a fixed phase shift, irrespective of signal, may be known as a
"passive" system. That phase shift may be introduced by a physical tilt or other
appropriate arrangement of each of the antenna elements comprising the array.
A passive antenna array introduced a fixed phase difference across a n antenna array
to all radio frequency signals conveyed by that array to a n end user. In contrast a n
"active" antenna array is able to introduce a phase shift across the antenna array
appropriate to the location one or more end users receiving a signal. An active
antenna array is able to simultaneously shift different signals for different end users by
introducing different phase shifts. In particular, an active antenna array is able to make
use of linear superposition of radio waves to transmit, for example, a high power signal
to an end user a large distance from the array and a lower power signal to an end user
closer to the array simultaneously by introducing different phase shifts across the array
for each of those signals.
An active antenna array can thus be understood to function as a set of several parallel
lower power transceiver chains which operate in a parallel manner. The parallel
transceiver chains enable dynamic beam forming of transmissions to end users. It will
be understood that each transceiver chain or transceiver module within such a
transceiver array is directly fed with a signal according to the user equipment being
served by the base station.
In any antenna array system consisting of a multitude of transceiver chains, and
particularly when that array is used to generate a specific beam pattern, some degree
of calibration of each transceiver chain may be necessary. Calibration ensures that
the required phase and amplitude weighting of the set of signals to be transmitted to
user equipment across the array can be ensured. It will be understood that, as a result
of performance variations in components, temperature drift, ageing, and other
variations, each transceiver chains in a n array will induce a complex aberration in the
signal as it passes through the chain and that each transceiver chain in an array will
display non-linear behaviour. The signals at the output of each transceiver chain will
vary in phase and amplitude, even if the input signals at the individual transceiver
chains are identical. It is therefore required to calibrate such an array system.
It is desired to provide a calibration method for use in a n improved active transceiver
array.
SUMMARY
Accordingly, a first aspect provides an active transceiver array for a wireless
telecommunications network comprising:
a plurality of calibratable transceiver modules,
each transceiver module comprising:
a transceiver chain operable to process a primary signal and generate a processed
primary signal;
a comparator unit operable to compare said primary signal and said processed
primary signal to determine a transceiver chain error induced by said transceiver chain
in said processed primary signal; and
a correction unit which uses the transceiver error to correct said primary signal to be
processed by said transceiver chain.
An array system, comprising a multitude of transceivers and, in particular, a plurality of
antennas, may be used to generate a specific pattern and also to form beams. Beam
forming requires calibration of the array of transceivers to ensure that the required
phase and amplitude weighting of the signals can be ensured. There may be
performance variations in components, temperature drift, or ageing of each
transceiver chain and thus each transceiver chain may induce a different variation or
aberration in the signal to be transmitted. It will therefore be understood that a n array
may display non-linear behaviour across a set of transceiver chains.
As a result, the signals at the output of the transceiver chains transmitted by antenna
may vary in phase and amplitude in a manner which is not intended and that the
required beam forming may be not achieved in an optimum manner. In order to
ensure that an array operates in accordance with intention, an array system may be
calibrated.
Various calibration methods are possible. Possibilities in relation to the transmit path
include calibration by sending a test signal through each transmitter chain in an array
comparing the signals at the output of the transceiver chain with what is expected.
The comparison may be done in a number of ways. For example, a test signal may be
sent to each transceiver chain in an array in sequence and the signal transmitted over
air by each transceiver chain may be compared to an output signal of one of the other
transceiver chains (by an appropriate summation method). Alternatively, each output
signal may be compared to a known reference or "test" signal. One possible problem
associated with such calibration methods is that the array system must be taken offline
in order to be calibrated. The system may need to be taken offline because a test
signal may be sufficiently different to a typical operational signal, thereby causing
various operational limits to be exceeded (such a situation occurs mostly whilst
calibrating the transmission mode of an array). Another problem may be that the
injection of a test signal into a receiver may disable reception of a signal to be
received. Such is the intrusive effect of such calibration methods that they are primarily
applicable to systems which can successfully be taken offline or calibrated in a factory
environment, for example, phased arrays in radar systems. That approach is less
feasible in respect of active antenna systems used in mobile telecommunications,
where an antenna array is exposed to a constantly changing environmental condition.
for example, changes in temperature and changes in weather, and furthermore
wireless telecommunications systems typically require a high level of availability in order
to provide continuous service to customers.
A general calibration method for a transmission path may take the form of providing a
sampling receiver near to, or in, an antenna array, which determines phase and
amplitude of a test signal transmitted from each individual transceiver chain in an
array. A test signal is sent to each chain in the array in turn, and received by the
dedicated sampling receiver. Such a method uses the actual signal transmitted by a
chain over the air interface. The information relating to measured phase and
amplitude from the dedicated sampling receiver is sent to a central unit which
operates to send a phase and amplitude adjustment command to a digital processing
unit or an RF phase and amplitude adjustor in each individual transceiver chain. That
general process is repeated for all transmitters 1to N until phases and amplitudes of
each provided transceiver chain have reach required levels.
It is possible to sample a transmitted signal via a test antenna element. Such an
arrangement may not necessarily be performed during normal operation of an
antenna array since, if all elements from 1to N are fully operational it is not possible for
a receiver to determine the phase of a transmitted signal from a n individual antenna
element. That is to say, at test receiver may not necessarily be able to determine
between signals from different antenna elements. In such an arrangement a signal is
sent over the air, as in normal operation, and each element of a n array is configured
individually on the basis of a signal received by a test signal receiver. It will be
understood that an antenna array must be taken offline and no data traffic can be
handled during such a calibration method.
Rather than use transmission over the air, it is possible to calibrate an array using a
coupler and switch method. In such an arrangement, rather than use a signal
transmitted over the air by a n antenna, a coupler is used to sample the signal to be
sent by the antenna just before it is sent. The signal is therefore typically a radio
frequency analogue signal. In such an arrangement, using a common central unit,
and comparator, it is necessary to use a highly accurate switch. The phase and
accuracy of such a switch and of the lines between the coupler and the comparator
unit for each of the elements in an array are highly critical and the cost of accurate
cables and switches with multiple inputs for large arrays can be particularly expensive.
I† is possible to use a receiver provided in the far field in order to calibrate an array.
Such an approach allows for very precise phase and amplitude measurements since
no coupler or measurement lines are required. Such a process may, however, be
applied in a factory of laboratory environment, since provision of a sampling antenna in
the far field is required and an element by element calibration is necessary. Such a n
approach may not be applied cost effectively in a mass production environment.
Furthermore, a recurring calibration process in accordance with the far field method is
not possible once an antenna array has been deployed in the field.
Accordingly, the first aspect provides an active transceiver array in which each module
is provided with a comparator unit. Although it may at first appear that provision of a
comparator module at each module unnecessarily complicates the apparatus and
construction of a n array in comparison to a common comparator unit, such an
arrangement can offer significant constructional advantages. An arrangement in
accordance with the first aspect requires minimal hardware, for example, an RF
comparator device) and minimal processing resource compared to other calibration
schemes which may require dedicated receivers and/or signal processors and
expensive hardware and/or computational resources. Provision of a comparator at
each module reduces the need for a high number of phase stable distribution
arrangements, since the comparison is done close to each module and the
determined transceiver chain error calculated by the comparator device may then be
more reliably communicated between components of an array calibration control
system.
Furthermore, provision of a comparator unit in each module facilitates the use of a real
modulated data traffic signal for calibration purposes. As a result, each module, and
thus the array as a whole, may be calibrated during operation without the need to emit
a test signal. Such a dynamic and responsive calibration is advantageous in a wireless
telecommunications system, where compliance with standards and an extraordinary
high reliability and availability of an array system may be desired to ensure user
equipment service level can be maintained.
Furthermore, it will be understood that provision of a comparator unit in each module
allows a n array to be successfully scaled without undue burden, whilst still maintaining
calibrated operation. Such a possibility is not necessarily possible in a dedicated
common unit comparator calibration arrangement.
In one embodiment the transceiver array further comprises a primary signal generation
unit operable to generate a different primary signal for each of the plurality of
transceiver modules. Accordingly, each module in an array may be provided with a
different primary signal, thereby to achieve required beam forming.
In one embodiment the transceiver array further comprises a phase stable distribution
element operable to distribute the primary signal to each of the plurality of transceiver
modules. In one embodiment the phase stable distribution element is operable to
distribute the primary signal to the comparator unit of each of the plurality of
transceiver modules. Accordingly, by utilizing a phase stable distribution device it will
be appreciated that the signal distribution network does not itself contribute to
aberration in a manner which serves to affect comparator operation.
In one embodiment the phase stable distribution element is operable to distribute the
same said primary signal to said comparator unit of each of said plurality of transceiver
modules. Accordingly, whilst a different primary signal may be distributed to each of
the modules for transmission, the same signal may be transmitted to each of the
module comparator modules. A relevant comparison for calibration purposes may
then only be made in the module to which the primary signal distributed to module
comparators relates. Such an arrangement may simplify the hardware of an array
according to the first aspect.
In one embodiment the phase stable distribution network comprises a standing wave
line.
A standing wave line may comprises a feed arrangement including a waveguide of a
predetermined length, which is coupled to a reference signal source, and which is
terminated at one end in order to set up a standing wave system along its length, and
a plurality of coupling points at predetermined points along the length of the
waveguide, which are each coupled to a said comparison means of a respective said
radio element.
Accordingly, it is possible to provide an accurate distribution mechanism for phase and
amplitude reference signals for calibration of active antenna arrays for mobile
communications. Such a distribution mechanism is mechanically robust and costeffective.
In one embodiment a reference source signal of phase and/or amplitude is coupled to
a finite length of a transmission line, which is terminated such as to set up a standing
wave within the transmission line length. For a length of transmission line or other
waveguide terminated at one end with an impedance which substantially matches the
waveguide characteristic impedance, radiated travelling waves will progress along the
line and be absorbed in the terminating impedance. For other terminations however,
some radiation will not be absorbed, but be reflected from the end, and will set up a
standing wave system, where the resultant wave amplitude changes periodically along
the length of the waveguide (there will in addition be time variation of the voltage
value at each point along the line as a result of wave oscillation /phase rotation). The
amount reflected depends on the terminating impedance, and in the limiting cases of
short circuit and open circuit there will be a complete reflection. In other cases, there
will be partial reflection and partial absorption.
The standing wave signal may be sampled at predetermined tapping or coupling
points along the length of the line, which all have the same amplitude and phase
relationships, or at least a known relationship of phase and amplitude. Such coupling
points may occur at or adjacent voltage maxima/minima within the standing wave,
where the change of voltage with respect to line length is very small. Accordingly, the
requirement for mechanical accuracy in positioning of the coupling point can be
reduced compared with a star-distribution network arrangement.
In one embodiment, the coupling points are each connected by a respective flexible
short length of line of accurately known length to respective comparators in respective
transceiver elements (more generally radio elements). Short lengths of flexible cable,
all of the same length, may be formed very accurately ascompared with a stardistribution
network.
In one embodiment the waveguide may be formed as a plurality of sections of
waveguide of predetermined length, interconnected by releasable couplings: such an
arrangement permits scaling to any desired size of antenna.
A standing line distribution network is of particular use for frequencies of the order of
GHz, usually up to 5GHz, that is microwave frequencies in the mobile phone allocated
bands, where coaxial cable is generally used as a transmission line. However, it may
also be applicable to other frequencies, greater and smaller, and coaxial cable may
be replaced by other waveguide and transmission line constructions such as hollow
metallic waveguides, tracks on a printed circuit or any other construction.
In one embodiment the transceiver chain comprises a digital to analogue converter
and an antenna element, and each module further comprises a coupling, operable to
couple the transceiver chain to the comparator unit, the coupling being provided
between the digital to analogue converter and the antenna element. Accordingly, by
providing a comparator unit for each module and by ensuring the required connector
between the transceiver chain and the comparator is of minimal length, the
contribution of that connector to the measured transceiver chain error may be
minimized. Placing the coupling between the analogue converter and the antenna
may allow the sampled signal sent to the comparator to be as similar as possible to the
signal that is actually transmitted by a n antenna element of a module. That
arrangement ensures that as much of the aberration induced by a transceiver chain
can be accounted for and that calibration can be a s full and accurate as possible.
In one embodiment, the correction unit comprises a digital signal modification unit. In
one embodiment, the correction unit comprises an RF phase and amplitude adjuster.
Accordingly, calibration steps may be taken whilst a signal is in a digital or analogue
phase. Furthermore, if both devices are provided, correction steps may be taken in a
combination of digital or analogue phases.
A second aspect provides a calibratable transceiver module forming part of an active
transceiver array in a wireless telecommunications network, comprising :
a transceiver chain operable to process a primary signal and generate a
processed primary signal;
a comparator unit operable to compare the primary signal and the processed
primary signal to determine a transceiver chain error induced by the transceiver chain
in the processed primary signal; and
a correction unit which uses the transceiver error to correct the primary signal to
be processed by the transceiver chain.
In one embodiment, the transceiver chain comprises a digital to analogue converter
and an antenna element and the module further comprises a coupling, operable to
couple said transceiver chain to the comparator unit, provided between the digital to
analogue converter and the antenna element.
In one embodiment, the transceiver chain comprises a digital to analogue converter
and an antenna element, and each module further comprises a coupling, operable to
couple the transceiver chain to the comparator unit, the coupling being provided
between the digital to analogue converter and the antenna element. Accordingly, by
providing a comparator unit for each module and by ensuring the required connector
between the transceiver chain and the comparator is of minimal length, the
contribution of that connector to the measured transceiver chain error may be
minimized. Placing the coupling between the analogue converter and the antenna
may allow the sampled signal sent to the comparator to be as similar as possible to the
signal that is actually transmitted by an antenna element of a module. That
arrangement ensures that as much of the aberration induced by a transceiver chain
can be accounted for and that calibration can be as full and accurate as possible.
In one embodiment, the correction unit comprises a digital signal modification unit. In
one embodiment, the correction unit comprises an RF phase and amplitude adjuster.
Accordingly, calibration steps may be taken whilst a signal is in a digital or analogue
phase. Furthermore, if both devices are provided, correction steps may be taken in a
combination of digital or analogue phases.
A third aspect provides a method of calibrating an active transceiver array in a wireless
telecommunications network comprising a plurality of transceiver modules,
the method comprising the steps, for each module, of:
processing a primary signal in a transceiver chain to generate a processed
primary signal;
comparing the primary signal with said processed primary signal using a
comparator unit to determine a transceiver chain error induced by the processing of
the primary signal by the transceiver chain; and
correcting the primary signal to be processed by the transceiver chain using the
determined transceiver chain error.
It will be appreciated, that all aspects, including the first and second aspects, may be
implemented in either the transmit path or reception path of a transceiver module
forming part of an active transceiver array. All aspects may be particularly useful and
applicable to the transmit path.
In one embodiment, the primary signal comprises a traffic signal. Accordingly, the
method may be implemented using a data traffic signal for transmission to user
equipment. As described above in relation to the first aspect, it will be appreciated
that use of a comparator unit in each module facilitates the use of a real modulated
data traffic signal for calibration purposes. As a result, each module, and thus the array
as a whole, may be calibrated during operation without the need to shut down
operation to emit a test signal. Such a dynamic and responsive calibration is
advantageous in a wireless telecommunications system, where compliance with
standards and an extraordinary high reliability and availability of a n array system may
be desired to ensure user equipment service level can be maintained.
In one embodiment, the method steps are repeated consecutively for each module
forming part of the transceiver array. Accordingly, one module forming part of the
array may be calibrated at a time by applying a calibration method to each module in
sequence. It may be possible, in one embodiment, to perform the method steps
concurrently for each module forming part of a n array. Furthermore, in one
embodiment, the calibration steps are periodically repeated . Periodic repetition may
allow dynamic or substantially continuous calibration of a n array, thereby allowing a
rapid response to environmental conditions affecting operation of the array.
In one embodiment, the primary signal comprises a test signal. Accordingly, it will be
appreciated that the array, module and method of the various aspects described may
be utilized in commissioning a system or in a factory calibration of a system. The test
signal may be a continuous wave, the continuous wave may be sinusoidal.
In one embodiment, the primary signal comprises a sinusoidal test signal, and the
method further comprises the steps of:
sweeping the sinusoidal test signal in frequency and, based on the determined
transceiver chain error, determining a phase length of the transceiver chain.
Accordingly, it will be appreciated that by calculating the deviation of phase change
over frequency it is possible to determine a group delay of the signal chain. Such a
calculation may be used to compensate for group delay in the digital domain and
thereby improve signal quality of the transmitted signal.
Further particular and preferred aspects of the present invention are set out in the
accompanying independent and dependent claims. Features of the dependent
claims may be combined with the features of the independent claims as appropriate,
and in combinations other than those explicitly set out in the claims.
BRIEF DESCRIPTION OFTHE DRAWINGS
Embodiments of the present invention will now be described further, with reference to
the accompanying drawings, in which:
Figure 1 illustrates schematically the main components of a wireless network base
station;
Figure 2 illustrates schematically the main components of an active antenna array for
use in a base station such as that shown in Figure 1;
Figure 3 illustrates schematically embodiments of a transmitter chain and a receiver
chain for use in an active array similar to that shown in Figure 2;
Figure 4 illustrates schematically an example comparator device for use in calibration
of a transceiver module;
Figure 5 illustrates schematically a calibratable transceiver module and active array
architecture according to one embodiment;
Figure 6 illustrates schematically an array of four transceiver modules and active array
architecture according to one embodiment;
Figure 7 is a schematic diagram of a means of distributing a reference signal to
respective transceivers of an active antenna array, incorporating a star-distribution
network;
Figure 8 is a schematic diagram of progression of a travelling electromagnetic wave
along a transmission line length, having its free end terminated with a matching
impedance;
Figure 9 is a schematic diagram of a standing electromagnetic wave along a
transmission line, which has its free end terminated with a short circuit;
Figures 10a, 10b, and 10c are diagrammatic views of a length of transmission line with
coupling points formed by capacitive coupling ports, for use in one embodiment;
Figure 11 is a schematic view of a feed arrangement of a reference signal to
transceiver elements of an active antenna, in accordance with one embodiment;
Figure 12 is a schematic block diagram of a means for phase and amplitude
adjustment within a transceiver element of the active array of Figure 11; and
Figure 13 is a schematic diagram of an alternative means of distributing a reference
signal to transceivers in an active antenna array embodiment, forming a distribution
arrangement for 2-D arrays.
DESCRIPTION OFTHE EMBODIMENTS
Figure 1 illustrates schematically the main components of a wireless communications
network base station . The base station 1comprises: a radio mast 2 and a data
management unit 3 which communicates with a "core network" of the wireless
communications network. The base station further comprises a processing unit 4 and
an active antenna 5 operable to transmit radio frequency signals to, and receive radio
frequency signals from, end users 6 in response to information received from the core
network.
Figure 2 illustrates schematically the main components of an active transceiver array for
use in a base station such as the one shown in Figure 1. The active transceiver array
may also be referred to as an "active antenna array". An active transceiver array 5
comprises a set of substantially identical transceiver chains 7. In Figure 2, four active
transceiver chains 7 are shown. Each active transceiver chain comprises transmission
and reception apparatus. In the active transceiver array shown, each transceiver
chain comprises: a radio transmitter 8 including a digital to analogue converter, a
power amplifier 9, a filter or diplexer 0 and an antenna element 11 in the transmission
chain. In the reception chain the transceiver comprises: an antenna element 11, a
diplexer or filter 12, a low noise amplifier 13 and an analogue to digital converter
forming part of a receiver.
A simplified explanation of the operation, in transmission mode, of a base station similar
to that shown in Figure 1follows. Although reference throughout is made primarily to a
transmit path, it will be appreciated that an analogous approach may be
implemented in respect of a reception path. A data management unit 3 receives
digital information relating to a signal to be transmitted to a user 6 from the core
network. The information received by the data management unit is transferred, via a
digital connector, to processing unit 4. Processing unit 4 acts to dynamically generate
signals to be transmitted by each of the active transceiver elements 7 forming the
active transceiver array 5. It is processing unit 4 that calculates, for each signal to be
transmitted to user 6, an appropriate phase shift to introduce across the active
transceiver array 5 to ensure correct resultant beam forming. A calculated digital
signal is generated by processing unit 4 for each element 7 and transmitted via a digital
connection to each of those elements.
The signal received by each element 7 is converted by radio transmitter 8 into a radio
frequency analogue signal. The radio frequency signal is fed to power amplifier 9 and
then filtered by a frequency filter 10 before being passed to the antenna element 11for
transmission to end user 6. In the arrangement shown schematically in Figure 2, it can
be seen that by introducing an incremental constant phase shift between the digital
signal transmitted by processing unit 4 to each of the four elements 7 forming the array
5, the resultant signal is electronically beam-formed and the array is operable to
transmit a signal having a tilted phase front 15.
It will be understood that in an array system capable of such beam forming, it is
necessary to calibrate each of the transceiver modules 7 to ensure that the phase and
amplitude weighting of the signal actually transmitted match the intended phase and
amplitude of the signal which it is intended to transmit. Furthermore, it will be
appreciated that, due to performance variations in components, temperature drift
and/or ageing, transceiver chains forming part of each element act to induce
aberration in a signal which are non-linear even if the input signals of the individual
transceiver chains are identical. To compensate for those differences, it is necessary to
calibrate the system. Once the aberration in each transmitted signal is known, it is
possible to adjust the system accordingly, to compensate for the aberration which is
induced by the transceiver chain, such that the resulting phase and amplitude of the
transmitted signal matches that which is intended to be transmitted.
Figure 3 illustrates schematically embodiments of a transmitter chain and a receiver
chain for use in an active array similar to that shown in Figure 2. Transmitter chain 20a
comprises a transceiver containing: a digital analogue converter, power amplifier and
diplexer filter (9, 10, 11 respectively) and antenna element 1 . Transmitter chain 20a
may further include means to adjust the phase and amplitude of the signal transmitted
by antenna element 11, that adjustment being for the purposes of calibration. The
adjustment can be achieved either by applying appropriate amplitude and phase
modification settings in digital signal processing 16 to a digital signal. Alternatively, or
additionally, adjustment may be applied whilst the signal is in a n analogue phase by
using a radio frequency phase and amplitude adjuster 17. A combination of digital
and analogue phase adjustment techniques may be applied as appropriate to
achieve calibration.
Receiver chain 20b comprises a receiver including a filter, low noise amplifier, a n
analogue to digital converter (12, 13, respectively) and antenna element 11. Again,
adjustment to the phase and amplitude of a signal may be may be achieved through
use of either one of, or a combination of, a radio frequency phase and amplitude
adjuster 17 provided for use while the signal is in a n analogue phase, or by appropriate
use of digital signal processing in a digital signal processor 6 provided once the signal
has been converted to the digital phase.
Figure 4 illustrates schematically an example comparator device for use in calibration
of a transceiver module. Calibration of transceiver chains maybe achieved by
implementing a method in which signal powers are nulled. This technique may result in
effective and accurate calibration of transceiver modules. It is possible to use such a
method and utilise actual data traffic signals transmitted or received by each individual
antenna element 11to calibrate each transceiver module in a n active array. Such a
method requires that an exact copy of the radio frequency output signal is provided
via a phase stable reference distribution to a phase and amplitude comparator such
as that shown in Figure 4. A phase and amplitude comparator 400 is operable to
compare a sample of the actual output radio frequency signal to be transmitted by
antenna element 11 (that signal being represented in Figure 4 as signal 410) with a
phase stable reference signal 420 provided directly to the comparator 400.
The result of the comparison carried out by comparator 400 is a delta output relating to
phase and a delta output relating to amplitude. The phase difference delta value is
illustrated as signal 430 in Figure 4. The amplitude difference is represented as signal 440.
It will be appreciated that the input to a transceiver chain can be adjusted using either
the radio frequency phase and amplitude adjuster 17 or digital signal processor 6
shown in Figure 3 until it is determined that the delta outputs for phase and amplitude
at the comparator 400 have reached zero. Once that occurs, the antenna will be
transmitting substantially what it is expected to transmit and thereby may achieve
substantially optimum operation.
It will be appreciated that use of a data traffic signal may allow for dynamic and
substantially continuous calibration of modules forming an array to be carried out.
A comparator 400 such as that shown in Figure 4 typically receives two input signals
410, 420 which are each buffered and amplified in a log-mag-amplifier. Subsequently
a mixing process is performed, which results in a calculated phase difference between
signals 410 and 420 in the form of a DC output voltage which changes in magnitude
according to phase offset between the two signals. As a result of a summation of the
two signals 410, 420, the amplitude difference may also be represented by a DC
voltage 440. Such a process works for both continuous wave signals and also
modulated signals. As a result, if the signals 410, 420 are modulated but identical and
show a constant phase and amplitude offset, the offset in phase amplitude can be
represented by two constant DC output voltages.
The two independent output voltages 430, 440 representing phase and amplitude
differences can be used in a closed feedback loop to adjust the phase and amplitude
signal of a signal sent to each transceiver module until the actual transmitted signal has
the same phase and amplitude as is intended. It will, of course, be understood that
phase and amplitude comparison can be implemented in various other configurations,
for example, by sampling and digitising the two input signals 410, 420 and determining
the phase and amplitude offset in the digital domain.
Figure 5 illustrates schematically a calibratable transceiver module and active array
architecture according to one embodiment. Calibration apparatus 500 shown in
Figure 5 is operable to be calibrated using a replica of its own modulated signal during
operation. Calibration apparatus 500 comprises: processing unit 4, transceiver module
7, reference transmitter 510, splitter 520, and a phase table reference distribution
element 530. Central processing unit 4 comprises a base band processor 550, a field
programmable gate array (FPGA) 560 and a calibration processor 570. Transceiver
module 7 comprises transceiver chain 580, antenna element 11, and a phase and
amplitude comparator unit 400. Transceiver module 7 further comprises a signal
sampling coupler 590.
Base band processor 550 provided in central processing unit 4 operates to allocate
data to be transmitted according to available resources, for example, in accordance
with frequency band, code domain, and time slot, dependent upon chosen
transmission scheme, for example, W-CDMA. After performing that allocation, base
band processor 550 passes data to FPGA 560. FPGA 560 operates to generate, from
the base band data stream, a data stream for each individual transceiver module.
In Figure 5 only one transceiver module 7 is shown for the sake of clarity. It will be
appreciated there may be any number of transceiver modules 7 from one to N. FPGA
560 sends a data stream for transmission by transceiver module 7 to that transceiver
module. The data stream is shown schematically in Figure 5 as digital signal 565. Once
digital signal 565 reaches transceiver module 7 it is converted to an analogue radio
frequency signal by transceiver 580. The resulting radio frequency signal 565 is fed to
antenna element 11. Just before antenna element 1, a coupler 590 is provided.
Radio frequency transmission signal 585 is sampled by coupler 590. That sample is
passed to phase and amplitude comparator 400. It will be understood that the sample
signal is the first input 410 for the purposes of calibration.
Calibration processor 570 provided in central processing unit 4 determines which of the
provided transceiver modules is to be calibrated at any given moment. In the
arrangement shown in Figure 5, the calibration processor 570 chooses to calibrate
transceiver module 7 (also known as the first module in this particular arrangement).
The calibration processor 570 is in communication with FPGA 560 and, when calibration
processor 570 wishes to calibrate the first element, the FPGA is instructed to send an
exact digital copy of an unmodified signal to be sent to the first element to the
reference transmitter 510. The digital copy of the signal 565 is shown schematically in
Figure 5 as digital signal 566. Digital reference signal 566 is converted into a radio
frequency signal by reference transmitter 510. The analogue reference signal 567 is
divided in a splitter 520 and carried forward to each individual transceiver module by a
phase stable reference distribution 530. In the embodiment shown in Figure 5, the
phase stable reference distribution is a star network with N cables of equal length.
Analogue signal 567 is carried to transceiver module 7 by cable 531 . It will be
understood that cables 532, 535 and 534 of the star distribution network would feed to
transceiver modules 2, 3 and 4 respectively but that those modules are not shown in
Figure 5 for reasons of clarity.
It is possible to use various phase stable distribution elements, 530. In particular, in some
embodiments it is possible to use a phase stable distribution element as described in
more detail in relation to Figures 7 to 13.
Returning to the embodiment shown in Figure 5, signal 567 is used as the second input
420 to phase amplitude comparator 400 provided in transceiver module 7. Phase and
amplitude comparator 400 then operates as described in relation to Figure 4 to
generate phase difference and amplitude difference signals 430, 440. Those phase
difference signals are communicated to calibration processor 540. The phase
difference and amplitude difference signals are used by the calibration processor to
determine phase and amplitude offset to be applied to the signal 565 for transmission.
The offset values calculated by calibration processor 570 are communicated to FPGA
560 where a phase and amplitude offset of the output signal 565 are applied until the
measured phase and amplitude variation of the antenna module has reached the
desired threshold, typically zero.
It will be understood that calibration processor 570 may repeat the process for all
antenna modules forming part of the array. That process typically occurs for each
module consecutively.
Furthermore, it will be appreciated that all initial phase and amplitude offsets (due to
differences in the antenna connector or line lengths of the reference distribution 530)
can be determined via a factory calibration. Those factory calibration factors, which
will typically take the form of common offsets, may be stored inside the antenna array
processing unit 4 and can be fully taken in account when the FPGA calculates phase
and amplitude corrections to be applied due to variations in transceiver chains of the
transceiver modules.
One advantage of a method and apparatus as described in relation to Figure 5 is that
there are no switches required to sample the test signal. Furthermore, in the case
where an analogue phase and amplitude comparator is used, no dedicated test
receiver is required. As a result, phase and amplitude comparison may be achieved in
a cost effective manner using mixing and summation processes of two radio frequency
signals. A calibration processor and apparatus such as that shown in Figure 5 takes into
account all varying static and delay shifts in the entire digital to radio frequency upconversion
process into account. Even if no factory calibration is performed, the phase
and amplitude uncertainty is a result of the connection between the sample coupler
590 and the antenna element 11 and the variations in the reference signal distribution
530.
The calibration concept illustrated in Figure 5 is not only suited to perform
measurements with a broadband modulated signal and can, of course, be used with a
sinusoidal test signal generated by base band processor 550 and implemented by
FPGA 560. It will be understood that such a sinusoidal signal can be swept in frequency
and phase length of a transmission chain can be measured over the frequency. By
calculating deviation of phase change over a frequency sweep it is possible to
determine the group delay of the signal chain. Such a calculation can be used to
compensate for the group delay in the digital domain in order to improve signal quality
of the signal to be transmitted by the array.
Figure 6 illustrates schematically an array of four transceiver modules and active array
architecture according to one embodiment. It will be appreciated that many of the
features shown in Figure 5 are also present in the embodiment of Figure 6 and that
identical reference numerals have been utilised where appropriate. When a
component is provided in each of the four modules shown in Figure 6, the reference
numerals have been amended and now include a suffix a, b, c as appropriate. For
example, the transceiver unit of the first module is referred to as 580, the transceiver unit
of the second module as 580a, the third module as 580b, and the final module as 580c.
Furthermore, the particular architecture shown in Figure 6 and the nature of the signals
being transmitted around the array are indicated in accordance with the following
code: a solid line indicates an analogue line, transmitting an analogue signal. In the
particular embodiment shown that analogue signal will typically be a radio frequency
signal. The dashed line of Figure 6 represents digital lines carrying a digital signal. The
dotted line indicates a digital signal and, in particular, in the embodiment shown in
Figure 6, the dotted line indicates a digital feedback path which leads from phase and
amplitude comparator devices 400, 400a, 400b, 400c to calibration processor unit 570.
Figure 6 illustrates a particular schematic construction of FPGA unit 560 in more detail.
FPGA unit 560 comprises a signal generation unit 56 and a signal modification unit 562.
It will be understood that signal generation unit 561 acts to generate a signal to be
transmitted by each module and it is this signal 566 that is sent to reference signal
generator and that is compared with the signal to be transmitted by each transceiver
chain by antenna elements 11 and 1l a to 1lc. Calibration processor 570 sends
relevant calibration messages to signal modification unit 562 and that signal
modification unit operates to change the signal generated by the signal generation
unit to compensate for aberrations induced by the transceiver chain of each module.
It will be appreciated that a particular advantage of an arrangement similar to that
shown in Figures 5 and 6, is that a real modulated signal may be used as a reference,
that is to say, a data traffic signal either received or transmitted may be used as a
reference in such a way that each transceiver module and the array of transceiver
modules can be calibrated during operation and without need to transmit or emit a
particular test signal. It will be appreciated that use of a test signal is possible but that
use of a test signal in addition to usual data traffic may compromise the standard
compliance of a transmitted signal, or act to impair and disturb signal quality for end
users.
It will be appreciated that use of a real modulated traffic signal is particularly
advantageous in relation to a mobile telecommunications system in which standard
compliance and extraordinarily high reliability and availability of an antenna system is
required to maintain service to user equipment in a cell. A calibration scheme
according to that described in relation to Figures 5 and 6 has the advantage that there
is less impact on availability of the system and no downtime of the active antenna
array is required in order to achieve calibration.
Furthermore, it will be appreciated that the necessary hardware effort for calibration is
minimal. In the embodiments shown a minimum amount of F hardware is required
(namely a radio frequency comparator unit 400), as opposed to some other calibration
techniques which require dedicated receiver and/or signal processor which typically
require expensive additional hardware and computational resources. It will be
appreciated that in some calibration schemes a receiver tuned to a particular
transmission frequency maybe required.
A method as described in relation to the embodiments does not require the use of
highly calibrated switches to sample or inject a test signal. Avoidance of the use of a
switch can increase method accuracy since the phase accuracy of such switches
involving four eight or more ports directly impacts measurement accuracy. Avoidance
of the use of a switch can also assist to keep system costs down since accurate
switches are expensive components.
Calibration system described in embodiments will be understood to be particularly
scalable since each transceiver module contains its own dedicated calibration
hardware with a minimal common unit.
Referring to Figure 7, this shows a means of distributing a reference signal of phase and
amplitude to the individual transceivers of a n active antenna array. A centrally
generated reference signal 1020 (VCO PLL) is split in a n N-way-power divider 1022 ( 1:Nsplitter)
and connected to the reference input of each transceiver unit 1024 by
respective transmission lines 1026 of equal length I. Length I is nominally equal to half
the length of the array IA. This forms a star-distribution network, and any change of the
line length results in a change of the phase length, giving rise to disadvantages. This is
due to the travelling nature of the wave propagation on the line: the phase change
is proportional to the length which the wave travels along the line: =
(360/), where is the wavelength of the radiation in the transmission line. If one
looks at a travelling wave at a certain snap-shot in time, the phase changes with the
position along the transmission line, as indicated in Figure 8 . In Figure 8, voltage values
are shown existing along the line at time intervals †i - † . As is well known the measured
voltage value is dependent on the amplitude A and phase of the electromagnetic
wave, and in the travelling wave of Figure 8, the measured voltage will vary, with time,
at each point on the line between +A and -A. In Figure 8, the line length is terminated
with the matching impedance of the transmission line, so that all the energy of the
travelling wave is absorbed. If, however, a line length is terminated with an impedance
other than a matching impedance, then a standing wave system may be set up.
A standing wave arrangement is shown in Figure 9 . Such a standing wave can be
generated along a line 1040 by feeding it with a signal 1042 from one end and shorting
the signal at the other end 1044. This short enforces a voltage-null at the end of the line.
The same energy that travels along the line is fully reflected at the short and travels
backwards towards the source. If the line is lossless (or reasonable low loss), this leads to
a standing wave on the line. Thus, the voltage value at any point along the line now
depends on time, but the phase of the wave does not vary along the line, rather the
amplitude A of the electromagnetic wave varies cyclically along the length of the line,
between maxima and minima, (positive and negative peaks), the maxima being
spaced apart one wavelength of the wave, as shown. The first minimum occurs at a
distance of /4 from the shorted end. At any given point along the line e.g. xl and x2
the amplitude is different. The maximum voltage occurs at the same point in time as
the minimum.
If the voltage on the line is now sampled by couplers 1046 with a low coupling
coefficient in order not to interfere with the standing wave, then the maximum at each
coupler output occurs at the same time (even they may differ in amplitude). If it
ensured that each coupler is spaced in a distance of 1, where is the wavelength of
the radiation in the transmission line, then it is also ensured, that the amplitude at each
coupler output is equal. If different amplitudes are desired, not necessarily equal, other
distances than can be chosen.
Such an arrangement of couplers attached to a line having a standing wave, may be
used to transmit an amplitude and phase reference signal to the individual antenna
elements of an active array system. Each coupler is attached to a respective
transceiver by a short length of cable, of accurately known length. One advantage of
such an arrangement is that it avoids the strict requirements of mechanical accuracy
of the star distribution arrangement of Figure 7. To minimize the amplitude difference
between coupling or tapping points, the couplings may be spaced in a distance of d=
(+/4) from the shorted end; this places each coupling in a voltage-peak of the
standing wave. Since the voltage distribution along the line follows a sinusoidal
function, and the derivative of the sinusoidal function near the maximum/minimum
value is zero, the sensitivity of the amplitude of the coupled signal to the physical
position of the coupling point is minimal.
Such an arrangement may overcome some shortcomings of a star-distribution
arrangement, since the reduced dependence of the phase reference on the physical
location of the coupling point along the line reduces the manufacturing cost and
increases the accuracy of a system using a standing wave line ascompared to a starnetwork.
The signal may be transported from the coupling port to the reference
comparator in the respective transceiver by a much shorter cable (e.g. in the order of
several cm instead of several ten cms of a star network) and therefore be
manufactured much more precisely. Due to the shorter cable lengths, the costs of the
cables/line between the reference-line and the comparator are also reduced. The
dependence of the amplitude of the coupled signal is minimized by placing the
coupling ports a †distances d=(NA+A/4). For example, at 2GHz and a Teflon filled line, a
misplacement of the coupling point from the voltage maximum of +/-5mm corresponds
to a shift of 16.8°. With cos(16.8°)=0.95 this reduces the coupled amplitude by
20*log(0.95)=0.38dB, which is about half of the permitted tolerance in amplitude
accuracy for mobile communication antennas. Therefore the required mechanical
accuracy has been reduced from a sub-mm-level tolerance to a level of several mm
tolerance. It is much easier to achieve a sub-mm- or mm-accuracy on a short
connection line between the standing wave line and the transceiver than on a line
which is orders of magnitude longer, as in a star-network.
In Figures 10a, 10b, and 10c a form of coaxial line is shown, which is incorporated a
distribution arrangement for amplitude and phase reference signals. In Figure 10a, a
transmission line, which is a coaxial line 1050 with a shorted free end 1052, is coupled to
a reference source 1054. The line has a series of spaced capacitive coupled coaxial
coupling or tapping ports 1056. A perspective view of a coupling port is shown in
Figure 10b. In Figure 10c, a part-sectional view of a physical implementation of the
transmission line is shown, comprising a length of air-filled coaxial line 1060, which has a
length equal to one wavelength of the transmission signal (a 2Ghz signal has a
wavelength of the order of 15 cm in free space). One end has a male coupling
connector 1062, and the other end a female coupling 1064, for coupling to identical
sections of coaxial line, in order to provide a composite line of desired length. The
length 1060 has a capacitive coupling port 1066, having a n electrode pin 1068 which is
adjustable in its spacing from a central conductor 1070. The coupling coefficient can
be tuned to a desired value by the length of the coupling pin protruding into the
standing wave line.
In the illustrated case of the standing wave line filled with air, the distance between the
ports 1056 is A0=c0/f with O being the wavelength in free space. In antenna arrays the
distance of antenna elements is usually between 0.5 AO and 1A0, so that no gratings
lobes occur in the array-pattern. In mobile communication antenna arrays this distance
is usually in the order of -0.9 AO. It is beneficial, that the distance between the couplingports
for the reference signal matches the element distance, so the length of the wave
guide that connects the coupling ports with the comparator-input is minimized. This is
possible with the invention, by adapting the effective dielectric permittivity eeff used in
the standing wave line such, that the physical length lc between the couplings equals
approximately the element distance d between the antenna elements: 0.9 A0=d
A0/(square root(Eeff)). This is possible by using e.g. foam-material in the coaxial line as a
dielectric and adjusting the dielectric permittivity by the density of the foam.
Figure 1 shows an embodiment of a distribution arrangement for reference signals of
amplitude and phase to a n active antenna system. The embodiment incorporates the
coaxial line of Figures 10, and similar parts to those of earlier Figures are denoted by the
same reference numeral. In this embodiment the coupling or coupling ports 1056 are
separated by a n effective distance of 0.9 , and each coupling port 1056 is connected
by a short (of the order of a few cms, and short in relation to the length of line 1050)
flexible coaxial cable 1072 to a respective transceiver (radio) element 4, which includes
a comparator 10100 and which is coupled to a n antenna element 1012. The lengths of
the cables 1072 are precisely manufactured to be equal.
The arrangement for processing the phase and amplitude reference signal within a
transceiver (radio) element is shown in Figure 2 . A Digital baseband unit 1080 provides
signals, which include digital adjustment data, to a DAC 1081, which provides a
transmission signal for up-conversion in a n arrangement comprising low-pass filters 1082,
VCO 1084, mixer 1086, and passband filter 1088. The up-converted signal is amplified
by power amplifier 1090, filtered at 1092, and fed to antenna element 1094 via a n SMA
connector 1096. To achieve phase calibration and adjustment, a directional coupler
1098 senses the phase and amplitude , ί the output signal. This is compared in a
comparator 10100 with phase and amplitude references Aref, )ret \ 10102, to provide
an adjustment value 10104 to base band unit 1080. Alternatively, if analog adjustment
is required, a vector modulation unit 10106 is provided in the transmission path. Thus,
the comparator output 10104 is fed back either to a digital phase shifter and adjustable
gain block 1080 or a n analog phase shifter and gain block 10106, to adjust the phase
and amplitude of the transmitted signal until its phase and amplitude matches the
reference value.
The arrangement of capacitive coupling points of Figure 10, that is simple envelope
detectors for the standing wave detection, may leave a 180° phase ambiguity. This
ambiguity may be resolved by employing two similar standing wave lines, working with
same frequency signals, but fed with, e.g., 90° phase difference Q.e.. T/4 time
difference). Then, detection can comprise using two detectors against ground, or using
one detector between the two lines.
An advantage of the distribution means of embodiments of the standing wave line is
that it is scalable: the line can be designed as a single mechanical entity, or a s a
modular system, which is composed of several similar elements, which can be
connected to each other. If more coupling points are required, the line length is
increased by simply adding more segments.
In a modification, a distribution system for 2-dimensional arrays is provided. This is shown
in Figure 3, where a first line 101 10, as shown in Figures 10, is coupled at each coupling
point 101 12 to further coaxial lines 101 14, each line 101 14 being disposed at right angles
to line 101 10, and each line 10 14 being as shown in Figures 0 and having further
coupling points 101 16. Coupling points 101 16 are connected to respective transceiver
elements of a two dimensional active array.
In a further modification, by choosing a symmetrical implementation of the coupling
points about the mid-point of the standing wave line, the accuracy can be improved
further. Any error occurring in phase or amplitude is now symmetrical about the center
of the array. If any phase or amplitude error occurs now along the reference coupling
points (e.g. due to aging effects of the line), the symmetry of the generated beam is
nevertheless ensured and no unwanted beam tilt effect occurs. Further, a temperature
gradient along a n active antenna array does not affect phase accuracy of the signals
distributed to the respective antenna radiator modules. In practical operation, the
uppermost antenna can easily experience a n ambient temperature 20-30 degrees
higher than the one of the lowest element. This can cause a few electrical degrees
phase shift difference in a coaxial cable.
Thus a standing wave distribution network may provide the following advantages:
Scalability D and 2D). It may therefore be ideal for the design of antenna arrays of
varying sizes, depending on the required gain, output power and beam width of the
system.
The required mechanical accuracy may be reduced theoretically completely if it is
used for phase reference distribution. In cases where it is used also as a n amplitude
reference, the required mechanical accuracy is decreased from a sub-mm-level to a
level of several mm.
The cost, weight and volume of such a reference distribution is reduced.
A person of skill in the art would readily recognize that steps of various above-described
methods can be performed by programmed computers. Herein, some embodiments
are also intended to cover program storage devices, e.g., digital data storage media.
which are machine or computer readable and encode machine-executable or
computer-executable programs of instructions, wherein said instructions perform some
or all of the steps of said above-described methods. The program storage devices may
be, e.g., digital memories, magnetic storage media such as a magnetic disks and
magnetic tapes, hard drives, or optically readable digital data storage media. The
embodiments are also intended to cover computers programmed to perform said
steps of the above-described methods.
The functions of the various elements shown in the Figures, including any functional
blocks labelled as "processors" or "logic", may be provided through the use of
dedicated hardware a s well as hardware capable of executing software in association
with appropriate software. When provided by a processor, the functions may be
provided by a single dedicated processor, by a single shared processor, or by a
plurality of individual processors, some of which may be shared. Moreover, explicit use
of the term "processor" or "controller" or "logic" should not be construed to refer
exclusively to hardware capable of executing software, and may implicitly include,
without limitation, digital signal processor (DSP) hardware, network processor,
application specific integrated circuit (ASIC), field programmable gate array (FPGA),
read only memory (ROM) for storing software, random access memory (RAM), and non
volatile storage. Other hardware, conventional and/or custom, may also be included.
Similarly, any switches shown in the Figures are conceptual only. Their function may be
carried out through the operation of program logic, through dedicated logic, through
the interaction of program control and dedicated logic, or even manually, the
particular technique being selectable by the implementer as more specifically
understood from the context.
It should be appreciated by those skilled in the art that any block diagrams herein
represent conceptual views of illustrative circuitry embodying the principles of the
invention. Similarly, it will be appreciated that any flow charts, flow diagrams, state
transition diagrams, pseudo code, and the like represent various processes which may
be substantially represented in computer readable medium and so executed by a
computer or processor, whether or not such computer or processor is explicitly shown.
The description and drawings merely illustrate the principles of the invention. It will thus
be appreciated that those skilled in the art will be able to devise various arrangements
that, although not explicitly described or shown herein, embody the principles of the
invention and are included within its spirit and scope. Furthermore, all examples recited
herein are principally intended expressly to be only for pedagogical purposes to aid the
reader in understanding the principles of the invention and the concepts contributed
by the inven†or(s) to furthering the art, and are to be construed as being without
limitation to such specifically recited examples and conditions. Moreover, all statements
herein reciting principles, aspects, and embodiments of the invention, as well as
specific examples thereof, are intended to encompass equivalents thereof.
CLAIMS
1. An active transceiver array for a wireless telecommunications network
comprising:
a plurality of calibratable transceiver modules,
each transceiver module comprising:
a transceiver chain operable to process a primary signal and generate a
processed primary signal;
a comparator unit operable to compare said primary signal and said processed
primary signal to determine a transceiver chain error induced by said transceiver chain
in said processed primary signal; and
a correction unit which uses the transceiver error to correct said primary signal
to be processed by said transceiver chain.
2 . An active transceiver array according to claim 1, further comprising a primary
signal generation unit operable to generate a different primary signal for each of said
plurality of transceiver modules.
3 . An active transceiver array according to claim 1 or claim 2, further comprising a
phase stable distribution element operable to distribute said primary signal to each of
said plurality of transceiver modules.
4 . An active transceiver array according to claim 3, wherein said phase stable
distribution element is operable to distribute said primary signal to said comparator unit
of each of said plurality of transceiver modules.
5. An active transceiver array according to claim 3 or claim 4, wherein said phase
stable distribution element is operable to distribute the same said primary signal to said
comparator unit of each of said plurality of transceiver modules.
6. An active transceiver array according to any one of claims 3 to 5, wherein said
phase stable distribution network comprises a standing wave line.
7 . An active transceiver array according to any preceding claim, wherein each
said transceiver chain comprises a digital to analogue converter and an antenna
element and wherein each said module further comprises a coupling, operable to
couple said transceiver chain to said comparator unit, said coupling being provided
between said digital to analogue converter and said antenna element.
8. An active transceiver array according to any preceding claim, wherein said
correction unit comprises a digital signal modification unit.
9. An active transceiver array according to any preceding claim, wherein said
correction unit comprises an RF phase and amplitude adjuster.
10 . A calibratable transceiver module for use in an active transceiver array in a
wireless telecommunications network, comprising :
a transceiver chain operable to process a primary signal and generate a
processed primary signal;
a comparator unit operable to compare said primary signal and said processed
primary signal to determine a transceiver chain error induced by said transceiver chain
in said processed primary signal; and
a correction unit which uses the transceiver error to correct said primary signal
to be processed by said transceiver chain.
11. A calibratable transceiver module according to claim 10, wherein said
transceiver chain comprises a digital to analogue converter and an antenna element,
and wherein said module further comprises a coupling, operable to couple said
transceiver chain to said comparator unit, provided between said digital to analogue
converter and said antenna element.
12 . A method of calibrating an active transceiver array in a wireless
telecommunications network comprising a plurality of transceiver modules,
said method comprising the steps, for each module, of:
processing a primary signal in a transceiver chain to generate a processed
primary signal;
comparing said primary signal with said processed primary signal using a
comparator unit to determine a transceiver chain error induced by said processing of
said primary signal by said transceiver chain; and
correcting said primary signal to be processed by said transceiver chain using
said determined transceiver chain error.
13. A method according to claim 12, wherein said primary signal comprises a traffic
signal.
14. A method according to claim 12 or 13, wherein the steps are performed
consecutively for each module forming part of the transceiver array.
15 . A method according to claim 12 or claim 14, wherein said primary signal
comprises a sinusoidal test signal and said method further comprises the steps of:
sweeping said sinusoidal test signal in frequency and, based on said determined
transceiver chain error,
determining a phase length of said transceiver chain.