Note: Descriptions are shown in the official language in which they were submitted.
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TITI F OF THE INVENTION
Method for the distribution of digital radio broadcasting
signals to terrestrial retransmitters from either satellite or terrestrial
5 sources.
FIELD OF THE INVENTION
The present invention relates to the implementation of
10 on-channel retransmitters for the construction of Digital Radio
Broadcasting networks, in particular those based on the Eureka 147
system.
BACKGROUND OF THE INVENTION
Although the development of Digital Radio Broadcasting
is still in its infancy, it is already possible to identify prior art in this field for
the distribution of signals to a network of auxiliary transmitters.
The acronyms DAB, DSB, DRB, are generally used
interchangeably, and somewhat ambiguously, in the Digital Radio field,
but are all meant, in this document, to designate the Eureka 147 system,
used in either terrestrial or satellite modes.
The Eureka 147 DRB system, which is standardized by
the European Telecommunications Standards Institute (ETSI, "Radio
Broadcast Systems; Digital Audio Broadcasting to Mobile, poffable and
fixed receivers' pr ETS 300 401 (Final draft), November 1994), uses
Orthogonal Frequency Division Multiplexing (COFDM). The properties of
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this modulation method, in particular the power addition one, permits the
illumination of a region with a digital broadcasting signals from multiple
sites. The advantages brought about by this method are several, for
example:
1) the aggregated transmitter power of the multiple transmission sites
necessary to achieve a minimum field strength at the boundary of an area
is much lower than that of a single high-power transmitter because of
propagation properties; and
2) the probability of receiver fading from shadowing is much less, within
the coverage area, as it may be in simultaneous view of several
transmitters.
There is one main site from which the assembled
multiplex of program elements will be radiated. In the case of terrestrial
networks, this site is at an elevated favorable location, at which
conventional Television and Frequency Modulation (FM) broadcasting is
also often conducted.
Another mode of operation is to illuminate entire
continents, countries or regions directly from a geosynchronous satellite.
In that case, reception in rural areas will generally be adequate, because
of the relative rarity of obstacle. The reception of satellite signals in cities25 will on the on the other hand less reliable, because of the large amount
of shadowing and scattering which will affect more the generally weak
satellite signals than it would for stronger terrestrial ones. The
implementation of satellite Eureka 147 therefore calls for a concept of a
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Hybrid Terrestrial Satellite network, where urban coverage would be
assisted by a network of retransmitters.
Figure 1 illustrates a generic retransmitter setup, which
5 constitutes the prior art in this field.
A main transmitter site A, including a power amplifier 1
and an omnidirectional antenna 2, which could also be a geosynchronous
orbit satellite, illuminates a given region in which the receiver 6 is located.
In order to improve coverage in some circumstances, it
is desired to build an auxiliary retransmitter installation, which will have an
overlapping coverage. This installation can be called a Gap Filler, a
Coverage Extender or a Terrestrial Retransmitter B depending on the
15 context in which it is built. A Gap Filler is used to correct small shadows
within the planned coverage areas. A Coverage Extender prolongs the
planned coverage area. A Hybrid Terrestrial Satellite retransmitter (HTS
retransmitter) completes satellite coverage in difficult to reach areas, such
as heavily constructed urban regions.
The normal way of procuring a signal feed for the
operation of the retransmitter is to point a high gain, directive antenna 3
in the direction of the main transmitter, and filter and amplify (see
amplifier 4) the received signal before retransmitting the signal through an
25 antenna 5.
There are several issues concerning this mode of
distribution:
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1) The main challenge is the realization of a low mutual coupling between
the transmission antenna 5 and reception directional antenna 3. These
elements, along with the power amplifier 4 form a closed loop. The
Barkhausen criterion for oscillation will be met when loop gain exceeds
5 unity. Depending on the scenario, at 1.5GHz, the amplifier can have
anywhere from 60 to 120dB of gain. It is essential that antennas with
gains as high as possible are used, to reduce the amount of active gain
required. A singing and ripple margin should be added to the gain figure
to obtain the targeted mutual coupling. This coupling can be difficult to
10 realize in the face of environment reflections;
2) the pattern of the main antenna 2 is optimized for the illumination of
objects on the ground, and will generally have some amount of beam tilt.
The coverage extender B will tend to be on elevated sites, and may have
15 its receive antenna pointing in the rapid fall zone above the main lobe,
resulting in a large sensitivity to environment parameters. Modifying the
radiation pattern to present more field strength towards the horizon will
result in a loss of efficiency at the transmit site;
20 3) because of the main transmitters antenna 2 beam tilting, the
transmission path's Fresnel zone between the main and auxiliary site is
well illuminated, augmenting the number of rays contributing to fading and
echoes at the retransmitter site;
25 4) DRB networks should have its timing optimized in order to provide the
best guard time margins at the receiver. The off-air mode of distribution
implies that the signal radiated at the retransmitter can only be retarded
relative to the main signal. In the case of satellite broadcasting, this
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significantly limits the circumstances in which retransmitters may be
applied; and
5) because of the Rayleigh amplitude distribution of COFDM, the DRB
5 signal from the main site is somewhat distorted from the unavoidable
saturation distortion stemming from economical operation of the power
amplifier 1, and will have some internal noise contribution. Since this
mode of distribution is analogic, the signal-to-noisê ration will increase
with successive amplifications.
In practice, the off-air mode of signal distribution is likely
to be used most often with short-range gap-fillers.
An other way to distribute DRB signals would be to use
15 out-of-band channels, such as wideband analog or digital telephone
circuits, or private microwave radio systems. This will result in a
multiplication of costs which may render DRB prohibitively expensive, and
in a general waste of spectrum. If the main transmitter is a satellite, then
payload capacity is lost because of the requirement of providing a second
20 reflector antenna covering the same shaped beams as the main, large
reflector used at L or S band.
OBJECTS OF THE INVENTION
An object of the present invention is therefore to provide
an improved method for the distribution of digital radio broadcasting
signals to terrestrial retransmitters from either satellite or terrestrial
sources.
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SUMMARY OF THE INVENTION
More speciflcally, in accordance with the present
invention, there is provided a method for the distribution of digital radio
5 broadcasting signals to terrestrial retransmitters comprising the step of:
emitting a first signal at a predetermined frequency;
receiving said first signal at a retransmitter site;
retransmitting said received signal from said
retransmitter site; and
isolating a distribution function of said retransmitted
signal from the first signal, while retaining the use of the same allocated
frequency for both first and retransmitted signal through radiation
polarization discrimination.
The problem that the invention described herein
addresses is the distribution of signals from the main location, whether
terrestrial or satellite, to the retransmitter sites.
In the present text and in the appended claims, the term
20 "retransmitter" is given a broad meaning and can be construed
indifferently as a gap-filler, a coverage extender, or a terrestrial
retransmitter. The nuances between each type of retransmitters are
described in the International Telecom Union (ITU) Special Publication on
DRB (ITU-R Special Publication, "Terrestrial and Satellite Digital Sound
25 Broadcasting to Vehicular, Poffable and Fixed Receivers in the VHF/UHF
Bands' Radiocommunication Bureau, Genève, 1995). All three
categories of retransmitters share however the same basic
characteristics, which is to complete coverage in some fashion, and
require a mean of signal distribution.
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Other objects, advantages and features of the present
invention will become more apparent upon reading of the following non
restrictive description of preferred embodiments thereof, given by way of
5 example only with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the appended drawings:
Figure 1, which is labelled PRIOR ART, illustrates a
schematic representation of a conventional on-channel retransmitter;
Figure 2 represents a terrestrial DRB system using
15 leading time analog distribution, implemented with on-channel orthogonal
polarization feed, and a delay line delta which permits the auxiliary signal
to lead the main signal. The distribution channel is implemented with an
antenna whose polarization is orthogonal to that of the main transmitter.
The implementation of the retransmitter is similar to that of conventional
20 retransmitter;
Figure 3 represents a terrestrial DRB system using
digital distribution implemented with an on-channel orthogonal
polarization feed, and an auxiliary modulator for the distribution signal.
25 The retransmitter contains a demodulator, and a COFDM modulator;
Figure 4 represents a hybrid terrestrial satellite DRB
system using analog distribution implemented similarly as for the
terrestrial case illustrated in figure 2. The difference from the terrestrial
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case lies in the reuse of the main spacecraft's reflector. The simultaneous
illumination of a terrestrial receiver 8 by both terrestrial and satellite
signals is also shown; and
Figure 5 represents a hybrid terrestrial satellite DRB
system using digital distribution implement similarly as for the terrestrial
case illustrated in figure 3. The difference from the terrestrial case lies in
the reuse of the main spacecraft's reflector. The simultaneous illumination
of a terrestrial receiver 8 by both terrestrial and satellite signals is also
1 0 shown.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The present solution to the problems described in the
background of the invention is to isolate the retransmitted signal
distribution function from the coverage area illumination signal, while
retaining the use of the same allocated frequency for both through
radiation polarization discrimination.
There are four forms of the invention described herein.
Form 1: (Figure 2)
For a terrestrial network, use of orthogonal polarization
25 for the analog distribution of COFDM modulated signals, and sent in
synchronism or leading in time the signal radiated by the main broad
beamed antenna. The use of a COFDM format permits the use of
effective radiated power similar or greater than that of the main signal.
As can be seen from this figure, the main antenna 1 emits a vertically
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polarized signal while the directional antenna 3 emits a horizontally
polarized auxiliary distribution signal. Of course, these two signals are
amplified by respective power amplifiers 5 and 7 before being transmitted.
A delay 8 may be introduced between the two transmitted signals.
Form 2: (Figure 3)
For a terrestrial network, the auxiliary distribution signal
can also be modulated using a method other than COFDM, for example
10 MSK (Minimum Shift Keying), implemented by the modulator 9. In that
case, the digitally coded DRB data source is modulated upon an auxiliary
carrier, and the received auxiliary signal is demodulated by a canceller
demodulator 10 before being modulated by the COFDM modulator 11,
reamplified by the power amplifier 6 and retransmitted by the
15 retransmitter antenna 2.
Form 3: (Figure 4)
For a hybrid terrestrial-satellite network, form 1 can be
20 used. In that case however the auxiliary time-leading signal is expected
to be of a much lower power. The distinctive element here is that instead
of using two distinct antennas with very different radiation patterns, the
main reflector 1, which is already appropriately shaped, is used for both.
Affer all, in that case, the distribution network should have the same
25 coverage as the main signal. With an auxiliary signal in the order of about
20dB below the main signal, a consumer receiver will generally be
insensitive to that auxiliary signal, even with reflections and poor
consumer-grade antenna polarization discrimination. A terrestrial
receiving antenna 5, on the other hand, with its possibly excellent
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cross-polarization performance, and because it is pointed away from
potential scatterers, will be able to separate well both components, even
if the undesired one has a much larger field strength. The much lower
power of the auxiliary signal has a very small effect on the satellite's
5 power budget. The realization of a dual polarization antenna is very
simple with circular polarization, by using a quadrature coupler, which is
identified as a Polarization Combiner labelled 2 on figures 4 and 5. This
coupler could then couple orthogonal propagation modes in a circular
wave guide leading to the main reflector's 1 dual mode feed horn.
Form 4: (Figure 5)
Similarly to forms 2 and 3, for a hybrid terrestrial-satellite
network, a modulation method other can be used for signal distribution,
1~ but using the main spacecraft reflector 1. In that case, if a constant
envelope method is used, a slight power efficiency advantage is gained,
which is interesting on a spacecraft.
The main COFDM signal is retarded digitally. Since the
20 implementation of a delay in the digital realm is very inexpensive, a large
delay equivalent to several DRB symbols can be inserted, to permit great
latitude in the adjusting of the retransmitter network synchronisation
The main advantage of forms 2 and 4 is that an analog
25 path not longer exists between the retransmitter's receive and transmit
antennas, which eliminates the possibility of oscillation. Excessive mutual
antenna coupling will result in receiver blanking, but not in catastrophic
oscillation. Much higher levels of coupling can then be tolerated and
treated than in the analog case.
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The delay elements ~f forms 1 and 3, which are labelled
8 in figure 2 and 10 in figure 4 have two purposes:
1) To permit the adjustment, and generally permit the increase in the
5 distance between network stations without having to resort to a Eureka
147 mode with a longer guard interval, such as mode 1 or 4; and
2) To decorrelate the signal of the orthogonally polarized distribution
channel from the main coverage signal. This decorrelation permits the
10 identification and elimination of scattered signals. This depolarization,
identified on figure 2 as the parasitic cross-polarization component, stems
from intrinsic cross-polarization sensitivities of antennas 1 and 4 on figure
3, and also depolarization occurring from environment scattering.
For terrestrial networks, the maximum amount of delay
"delta" (~) is a function of the potential interaction of the signals at the
auxiliary and coverage beam, the expected environment scattering delay
spread, the receiver synchronization algorithm limitations, and the
network requirements in the placement of the retransmitters. In Eureka
147 mode 2, with a guard time of 62 microseconds, this delay can be
typically from 0 to about 50 microseconds.
For forms 3 and 4, the amount of delay "delta" (o) can
be much larger, because of the lower levels of the auxiliary signal,
projected to be about 20dB below the main signal. Thanks to the leading
auxiliary signal, the terrestrial receiving antenna needn't be anymore at
the retransmitter site, where mutual coupling is difficult to reduce, but
could be located at a well engineered site with good isolation from the
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terrestrial radiated element, and feeding a common urban network (not
shown).
The delay is realized by any common mean conceivable
5 for its implementation. Direct analog retardation at 1.5GHz can be
accomplished using a low-loss single-mode fiber optic delay line, or a
Surface Acoustic Wave device. An other possibility is to use digital
baseband techniques and separate Radio-Frequency (RF) modulators.
The higher Effective Isotropic Radiated Powers (EiRPs)
of the first form is not very detrimental to the operation of receivers placed
in the swath of the auxiliary distribution beam. The reason is that the
signal is also COFDM, and will contribute to the receiver's performance.
Also, the receive antenna will have EiRPs higher than that of the main
15 coverage beam will permit the improvement of antenna mutual coupling
margins at the retransmitter site.
The regenerated data clock from the demodulated
auxiliary signal can be used for the frequency synchronization of the
20 retransmitter signal. Such synchronization is crucial in single frequency
networks, and can be implemented either using plesiochronous or
phase-lock techniques.
The polarizations illustrated were chosen to be
25 representative of what will typically be used in the proposed systems.
Present thinking favors vertical polarization for terrestrially located
transmitters at L-band, whereas the use of a single circular polarization
is planned for satellite broadcasting. What is simply required are
polarizations at antipodal positions on the Poincare sphere, such as
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Vertical and Horizontal polarizations, or LHCP (Left Hand Circular
Polarization) and RHCP (Right Hand Circular Polarization).
There is however a possible exception to this in the
5 Hybrid Terrestrial Case. The polarization of the auxiliary feed could be
chosen to be somewhat elliptic or linear, selected as to optimize the
receiver's behaviour in the presence of reflections.
In forms 1 and 3, if the delay delta is zero, then the
10 auxiliary Directional Antenna 3 could be fed from the output of th~ main
Power Amplifier 5 through a power divider or directional coupler, which is
not illustrated.
Although the present invention has been described
15 hereinabove by way of preferred embodiments thereof, it can be modified,
without departing from the spirit and nature of the subject invention as
defined in the appended claims.
