U.S. patent application number 10/521744 was filed with the patent office on 2005-12-08 for wideband antenna and receiver calibration.
Invention is credited to Richardson, Michael Richard.
Application Number | 20050272392 10/521744 |
Document ID | / |
Family ID | 29764162 |
Filed Date | 2005-12-08 |
United States Patent
Application |
20050272392 |
Kind Code |
A1 |
Richardson, Michael
Richard |
December 8, 2005 |
Wideband antenna and receiver calibration
Abstract
A method is provided for calibrating an antenna and receiver
system having multiple channels, each channel having an antenna,
cable feed and associated receiver components, the method
comprising the steps of: (i) applying a wideband calibration signal
to each antenna feed, the wideband calibration signal having
similar characteristics to an operational signal; (ii) measuring a
correlation response across a plurality of said channels; (iii)
deriving an estimate of signal transfer response for each of said
plurality of channels based on the correlation response; and (iv)
applying compensation factors for each of said plurality of
channels derived from the estimate of signal transfer response.
Inventors: |
Richardson, Michael Richard;
(Romsey Hampshire, GB) |
Correspondence
Address: |
Crowell & Moring
PO Box 14300
Washington
DC
20044-4300
US
|
Family ID: |
29764162 |
Appl. No.: |
10/521744 |
Filed: |
January 19, 2005 |
PCT Filed: |
November 19, 2004 |
PCT NO: |
PCT/GB04/04896 |
Current U.S.
Class: |
455/272 |
Current CPC
Class: |
H04L 27/2624 20130101;
H04B 7/0837 20130101; H04L 7/08 20130101; H04J 3/0608 20130101;
H04B 17/10 20150115; H01Q 3/267 20130101 |
Class at
Publication: |
455/272 |
International
Class: |
H04B 001/06 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 21, 2003 |
GB |
0327041.0 |
Claims
1. A method of calibrating an antenna and receiver system having
multiple channels, each channel comprising an antenna, feed cable
and associated receiver components, the method comprising the steps
of: (i) applying a wideband calibration signal to each antenna
feed, the wideband calibration signal having similar
characteristics to an operational signal; (ii) measuring a
correlation response across a plurality of said channels; (iii)
deriving an estimate of signal transfer response for each of said
plurality of channels based on the correlation response; and (iv)
applying compensation factors for each of said plurality of
channels derived from the estimate of signal transfer response.
2. A method according to claim 1, wherein said plurality of
channels comprise all said multiple channels and wherein, at step
(ii), the correlation response is measured with reference to said
wideband calibration signal.
3. A method according to claim 1, wherein said plurality of
channels comprise all but a selected one of said multiple channels
and wherein, at step (ii), the correlation response is measured
with reference to said selected one channel.
4. A method according to claim 1, wherein said wideband calibration
signal comprises a pseudo-random binary sequence modulated
according to a modulation scheme providing similar modulation and
bandwidth characteristics to those of the operational signal.
5. A method according to claim 1, wherein, at step (iii), deriving
said estimate of signal transfer response comprises determining the
delay through the respective channel.
6. A method according to claim 1, wherein, at step (iii), deriving
said estimate of signal transfer response further comprises
deriving phase characteristics of the respective channel.
7. A method according to claim 1, wherein, at step (iii), deriving
said estimate of signal transfer response further comprises
deriving amplitude characteristics of the respective channel.
8. A method according to claim 1 further comprising the step of:
(v) repeating steps (i) to (iv) to compensate for changes in signal
transfer response over one or more of said plurality of channels.
Description
[0001] The present invention relates to antenna calibration and
more particularly, but not exclusively, to a method of calibrating
a wideband antenna and receiver system. The method of calibration
is particularly, though not exclusively, suitable for use with
phased array antennae, particularly of the type found in mobile
cellular telephone systems used for the reception of wideband
signals and employing digital receivers and digital signal
processing techniques.
[0002] Mobile cellular telephone systems usually comprise a network
of base stations operable to communicate with mobile handsets of
users (subscribers) in order to provide telephone and other
services.
[0003] The operational range of base stations in mobile telephone
systems is typically of the order of 10-20 kilometres (km).
[0004] Typically, the power of signals transmitted by base stations
is of the order of a few tens or even hundreds of Watts. The
frequency of the signals is usually in the low microwave region of
the electromagnetic spectrum, typically around the 1-2 GHz range.
This range of frequencies is found in Global System for Mobile
(GSM) cellular mobile telephone networks in particular. Mobile
communications systems operating according to other mobile
communications standards, whether cellular or otherwise, have
corresponding operational frequency ranges. Examples of other
mobile communication systems include digital audio broadcasting
(DAB) systems, coded division multiple access (CDMA), Quadrature
Phase Shift Keying (QPSK) communication systems, High Performance
Radio Local Area Networks (HIPERLAN) and Universal Mobile
Telecommunication Systems (UMTS) telephone networks.
[0005] Signals generated in such systems occupy a wide bandwidth
and are often modulated with pseudo-random codes or sequences that
have a good auto correlation function. Correlation techniques are
widely used in GSM and similar communications systems to perform
signal synchronisation, detection and other signal processing
tasks.
[0006] Phased array antennae comprise a plurality of antenna
elements arranged in an array. Each individual antenna element in
the array has an associated feed cable and associated receiver
components. By processing and combining the signals received from
each of the elements in the array it is possible to control the
characteristics of the overall synthetic beam pattern (e.g. beam
shape, pointing direction, nulling of interference), for example by
applying different amplitude weightings, different phase shifts and
time delays to signals received from each of the antenna elements
before combining them. Amplitude weightings, phase shifts or time
delays may be applied to received signals in the analogue or
digital domain, as is well known.
[0007] Increasingly, phased array antennae are being employed in
cellular telephone networks, for purposes such as maximising
traffic capacity using spot beams, interference reduction and the
locating mobile handsets. For example, International Patent
Application number WO-A-9526116 (Ericsson GE Mobile Inc) describes
a phased array cellular base station that makes use of an number of
individual phased array antennae, each arranged to transmit a
different individual radio channel of a different frequency at any
one time. The power radiated by each phased antenna array is
selectively controllable to reduce possible interference while
maintaining communications with respective mobile units both near
and far from the base station.
[0008] Another example of a cellular base station that employs a
phased array antenna is described in United States Patent
US-A-2001027103 (Harris Corporation). In the system of
US-A-2001027103, each base station employs a phased array antenna
to enable the base station to define its antenna coverage pattern
with respect to any mobile transceiver so as to minimise
interference from one or more other transceivers, so enabling a
reduction in frequency reuse distance.
[0009] In practice, there are minor differences in the
characteristics of each of the cables, components and individual
antennae elements in a phased array antenna so that it is
necessary, or at least desirable, to calibrate the phased array
antenna and associated receiver system so that optimal performance
can be achieved and maintained. It is known to calibrate phased
array antennae, and associated receivers, by injecting a sinusoidal
(sine) wave signal into each of the antenna element feeds and
subsequently measuring the phase and amplitude variations through
each corresponding receiver channel. It has been found however that
such a technique is not optimal when used in certain types of
phased array antenna and receiver system and particularly those
which operate using digital receiver techniques and which are
required to receive wideband signals.
SUMMARY OF THE INVENTION
[0010] According to a first aspect of the present invention there
is provided a method of calibrating an antenna and receiver system
having multiple channels, each channel comprising an antenna, feed
cable and associated receiver components, the method comprising the
steps of:
[0011] (i) applying a wideband calibration signal to each antenna
feed, the wideband calibration signal having similar
characteristics to an operational signal;
[0012] (ii) measuring a correlation response across a plurality of
said channels;
[0013] (iii) deriving an estimate of signal transfer response for
each of said plurality of channels based on the correlation
response; and
[0014] (iv) applying compensation factors for each of said
plurality of channels derived from the estimate of signal transfer
response.
[0015] In order to optimise performance and increase flexibility,
preferred embodiments of the present invention are adapted for use
with a phased array antenna-system incorporating digital receivers
for synthetic beam/null generation and steering. Such arrays
comprise a number of antenna elements, with parallel receiver
channels comprising cabling, analogue-to digital converters (ADCs)
and digital signal processing systems. In normal operation, the
outputs of these channels are suitably combined in delay, amplitude
and phase to create the desired synthetic antenna beam patterns.
This process can only be performed successfully if the signal
transfer characteristics of each of the channels are known.
[0016] In a preferred embodiment of the present invention, said
plurality of channels comprise all said multiple channels and, at
step (ii), the correlation response is measured with reference to
said wideband calibration signal. Alternatively, said plurality of
channels comprise all but a selected one of said multiple channels
and wherein, at step (ii), the correlation response is measured
with reference to said selected one channel.
[0017] Preferably, the wideband calibration signal comprises a
pseudo-random binary sequence modulated according to a modulation
scheme providing similar modulation characteristics to those of the
operational signal.
[0018] If a phased array antenna and receiver system which is
adapted for use with wideband signals is calibrated using a
mono-frequency sine wave, as with known calibration arrangements,
then the resultant calibration factors which are obtained may only
be applicable to a narrow frequency band around the frequency of
the sine wave and not to the full bandwidth of the operational
signal, as is possible through use of the present invention,
because the receiver system may exhibit different characteristics
outside this narrow frequency band.
[0019] Furthermore with a digital receiver system, differential
delays between channels may be introduced due to effects such as
ADC clock phasing and skew, latency of data stream processing,
storage and buffering activities and interface protocols etc.
Preferred embodiments of the present invention are particularly
well suited to taking account of such effects when used to
calibrate antenna systems employing digital receiver
techniques.
[0020] In a preferred embodiment of the present invention, either a
pseudo-random binary sequence, or a symbol sequence with the
characteristics of a typical GSM signal is readily and easily
generated by test equipment. Both types of signal possess good
auto-correlation properties. GMSK modulation is then preferably
applied and the result is up-converted to an operational receiver
frequency to provide the calibration signal.
[0021] After processing through the antenna feed and receiver
system the calibration signal in each channel is cross-correlated
with the original calibration signal, or with the output of one of
the receiver channels, to obtain the delay, amplitude and phase
characteristics of each receiver channel.
[0022] Preferred embodiments of the present invention will now be
described by way of example only, and with reference to the
accompanying drawings, of which:
[0023] FIG. 1 is a diagram showing a phased antenna array
calibration arrangement according to a first embodiment of the
present invention; and
[0024] FIG. 2 is a diagram showing a phased antenna array
calibration arrangement according to a second embodiment of the
present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
[0025] An arrangement for calibrating a phased antenna array and
receiver system will now be described with reference to FIG. 1,
according to a first preferred embodiment of the present
invention.
[0026] Referring to the FIG. 1, a portion of a phased antenna array
and receiver system is shown, with four receiver channels
illustrated, in which each of the antenna elements in the array of
antenna elements 10 are linked by means of switches 12, 13, 14, 15
and respective antenna feeds 17, 18, 19 and 20 to digital receivers
22, 23, 24 and 25 respectively. When the switches 12-15 are set to
a first position, signals received at the antenna elements 10 are
routed to the digital receivers 22-25 in the normal way. However,
when the switches 12-15 are set to a second position--a calibration
mode--a calibration signal generated by a signal generator 35 is
input to each of the antenna feeds 17-20 and hence to the digital
receivers 22-25. An output from each of the digital receivers 22-25
is connected a respective complex (I, Q) correlator 27, 28, 29 and
30. The calibration signal generated by the signal generator 35 is
also input directly to each of the complex correlators 27-30 so
that they can each perform a complex correlation of the calibration
signal received over their respective antenna channel with the
directly supplied calibration signal. Example outputs of the
correlators 27-30 for this first preferred embodiment are shown
(32) in FIG. 1.
[0027] As mentioned above, known calibration methods for use in
phased array antennae make use of a sine wave calibration signal or
other narrow band signal when attempting to measure a signal
transfer response for each of the channels of an antenna array.
Conventional methods of calibration thus take little account of the
broadband signal reception characteristics of the antenna and
receiver components and do not allow delays due to latency or
analogue to digital converter clock skew which may be present in
digital receiver systems, to be measured.
[0028] In this first preferred embodiment, the signal generator 35
is arranged to generate a calibration signal having characteristics
of a typical GSM signal. However, the signal generator 35 may be
arranged to generate calibration signals typical of the particular
mobile communications systems in which the phase array antenna is
deployed, if other than a GSM system. In particular, the signal
generator 35 operates by applying GMSK modulation to a preselected
pseudo-random binary sequence having good auto-correlation
properties, and up-converting the modulated signal to the required
channel calibration frequency to produce the required calibration
signal.
[0029] In this first preferred embodiment, each of the complex
correlators 27-30 perform a complex correlation of the calibration
signals received over the respective receiver channel with the
calibration signal supplied directly from the signal generator 35.
The outputs from each correlator 27-30 are analysed in a
calibration processor (not shown in FIG. 1) in order to determine
the transfer characteristics of each channel, advantageously based
upon bandwidth and modulation characteristics similar to those
encountered in normal operation of the antenna and receiver system.
In particular, the calibration processor is arranged to search
across an appropriate range of signal delays for each receiver
channel to find the position of peak response in each correlator
output, which identifies the channel delay. As a consequence of the
low sidelobes typically associated with the autocorrelation
functions of the signal types of interest in GSM systems for
example, the position of the correlation peak corresponding to the
channel delay is easy to identify. The calibration process operates
in the complex signal domain and hence "In Phase" (I) and
"Quadrature" (Q) output values are produced. For the peak response
delay the calibration process may derive the amplitude response for
the respective channel using the value of
(I.sup.2+Q.sup.2).sup.1/2. Furthermore the channel phase shift (p
is derived from tan.sup.-1 (-Q/I). Therefore only three
measurements per channel (delay, amplitude and phase) need to be
made by the calibration processor in order to obtain an appropriate
set of compensation parameters to enable the antenna feed and
receiver system to be calibrated.
[0030] The calibration process may be repeated as required so as to
compensate for drifts or errors that may be introduced, for example
due to changes in ambient conditions or changes in clock signal
phasing after each system power up.
[0031] Thus it can be seen that invention also overcomes problems
often associated particularly with digital systems, namely
differential delays caused by factors such as analogue to digital
conversion clock phasing and skew, data stream processing,
buffering and storage.
[0032] Referring to FIG. 2, in a second preferred embodiment of the
present invention, rather than supply the calibration signal
directly to the complex correlators 27-30, it may be simpler in
practice to use the output of one of the receiver channels as the
calibration signal source for cross correlation purposes. This
alternative embodiment, which exploits the same principles as the
first embodiment, may be adequate for many applications and may be
simpler to implement. In this alternative embodiment the
calibration processor (not shown in FIG. 2) measures the amplitude,
phase shift and delay of each antenna receiver channel relative to
the channel selected as the calibration signal source.
[0033] There are a number of advantages shared by preferred
embodiments of the present invention over known antenna and
receiver system calibration arrangements.
[0034] Firstly, since the correlation can be performed over a wide
symbol span, there is obtained considerable processing gain against
noise, thus enhancing the accuracy of the calibration. Secondly the
calibration signal has the same or similar spectral and modulation
characteristics as a normal GSM signal, so that the effects of
filter responses, amplifier group delays etc. will be accurately
reproduced in the calibration. Thirdly, the amplitude and phase
characteristics of each channel are simply obtained from the real
and imaginary results of complex correlations. Fourthly, the
position of the easily identified correlation peak permits the
delay through each channel to be measured. This may be important
when digital receiver techniques are employed, since the latency
through digital systems may be significant and may in addition be
different at each power up. This can occur, for example, if the DSP
or interface clocks are derived from dividing down a high frequency
master clock, so that each logic block in the system then has the
choice of selecting at random from one of a number of possible
clock phases at power up.
[0035] It will be appreciated that the invention has been described
by way of example only and that variation to the above described
embodiments may be made without departing form the scope of the
invention.
* * * * *