Note: Descriptions are shown in the official language in which they were submitted.
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ANTENNA SYSTEM FOR ENHANCING THE COVERAGE AREA,
RANGE AND RELIABILITY OF WIRELESS BASE STATIONS
~'~~~UND OF TU~ TNV~ QN
FT~t-n OF TU~ TNVTN~ION
This invention relates to an antenna system and, more
specifically, to an antenna system primarily for use in conjunction
with base station in mobile communications systems.
~T~ n~Qr~TTON OF l~r~ ~TOR '~T
Mobile communication systems generally include a base station
for recei~ing and transmittin~ electromagnetic radiations with the
mobile terminal disposed within the coverage area of the base
station for transmitting electromagnetic radiations to and
receiving such radiations from the base station and where several
such base stations are generally linked together through base
station controllers (BSCs) and master station controllers (MSCs) to
provide a seamless communication link between a mobile terminal and
its calling party.
Mobile comm~nications are typically embodied within two bands.
Those systems between approximately 850 and 950 MHZ are referred to
as cellular and those systems between approximately 1.8 and 2.0 GHz
are referred as personal communication systems (PCSs). The total
mobile applications which together cover both bands are often
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referred t~ as personal communication networks (PCNs). This
invention relates to all PCN systems, i.e. systems which operate in
either or both frequency bands listed above.
The area, range and reliability of base stations are generally
limited in their coverage area by the base station receive noise
figure and the transmit effective isotropic radiated power ~EIRP).
The presently used PCN base station architecture utilizes a
vertical column array comprising a plurality of spaced apart
radiating elements for transmission and a separate such plurality
of radiating elements for reception. The antenna elements are
generally disposed in a vertical straight line on a support, the
distance between extreme antenna elements often being quite large,
often a few meters.
To improve performance, the receive antenna configuration
generally comprises two widely separated columns to provide spatial
diversity or a single orthogonally polarized column comprising two
orthogonal polarization outputs to provide polarization diversity.
The radiating elements are generally electrically conductive
members disposed on a support and are generally spaced between
three fourths and one wavelength apart. The antenna elements are
generally connected to a combiner via short transmission lines.
For the transmit configuration, the radiating element is fed
by a ground based high power amplifier through a long cable,
typically between 50 and 200 feet. The placement of the power
amplifiers within the base station also requires increased power
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from the amplifiers to overcome the insertion loss from the feed
cable as well as the combiner.
For the receive configuration, the combiner output is fed to
a filter/low noise amplifier (LNA) combination through either a
short transmission line (as in PCS systems for mast mounted LNAs)
or through a long cable (as in cellular systems for base station
integrated LNAs. Dual redundant amplifiers are typically provided
when mast mounted electronics are used to improve reliability at
the expense of complexity. For the receive configuration, the
effective noise increase contributed by the ohmic losses in the
array combiner and feed cable tdePending upon its length) are
amplified by the low noise amplifier and thus contribute to the
increase in the system noise figure.
There has been a constant desire to improve the range and
coverage area capability for individual base stations used in
wireless communication systems of the type described above. One
way to solve this problem has been to place a redundant pair of
LNAs on top of the tower and connect the LNAs to a passive
(i.e.antenna elements with a combiner) antenna column with a short
transmission line and a switch. The combiner loss and the switch
loss still limits the receive system noise figure.
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~3n~'~Y OF T ~ T~nn~N~ION
The above noted problem is minimized and there is provided an
improvement in the coverage area, range and reliability of the base
station by improving the transmit efficiency and receive
sensitivity of the base station without sacrificing reliability.
These improvements enable extension of the comml~nication coverage
area for a given base station.
Briefly, the above improvement is accomplished by substituting
for the presently existing antenna designs for use in conjunction
with mobile communication systems an active phased array antenna.
An active phased array antenna approach in accordance with the
present invention incorporates a low power amplifier (for transmit)
and/or a low noise amplifier ~for receive) as closely adjacent as
p~ssible to each element of an zrray, generally spaced by a few
centimeters or less from the associated element. The antenna
elements are generally disposed in a column. As an alternative, a
filter and amplifier can be coupled to a subgroup of the elements
of the array, though this arrangement would provide less
versatility as will be evident from the discussion hereinbelow.
The active antenna approach thus involves distributing a plurality
of amplifiers and filters, when required, across the array
aperture. The advantages of the active antenna system for transmit
and receive configurations are briefly discussed below.
For receive configurations, by maintaining the low noise
amplifiers close to the radiating elements, system noise is reduced
by as much as 4.5 d~ over the conventional approach in which the
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LNA is integrated with the base station and by as much as 1.5 dB
where an LNA is integrated with the passive antenna column at the
tower top or the mast of the antenna system.
Since a significant percentage of the effective noise
degradation in the antenna system is a result of the loss in the
feed cables and power combiners, it can be seen that, in the case
of the receiving section of the active antenna system, the noise
picked up by the feed cables is never amplified whereas this noise
is amplified in the prior art system. Accordingly, a much smaller
effective noise element arrives at the base station relative to the
prior art system described above.
Similarly, for transmit configurations, the low power
amplifiers, when integrated with the radiating elements, increase
the EIRP by as much as 4.5 dB (for the same amplifier power output)
over the conventional approach where the power amplifiers are
integrated with the base station.
The distributed nature of the amplifiers also improves
reliability since the system can be designed to be fully compliant
even after failure of one or more of the receive or transmit
amplifiers. Even when sufficient failures occur to reduce overall
system performance, the performance degradation is graceful rather
than catastrophic.
In the transmit configuration, using active antenna systems,
noise in the feed cable amplified by the amplifiers is of no
concern, since the signal power is several tens of dB higher than
the noise power. The main advantage of active antenna systems for
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the transmit case is that, since the power amplifiers are placed
between the antenna elements and the feed cable which is connected
to the base station driver amplifier, there is no reduction in the
amplified signal power due to the cable and combiner loss as is the
case in the existing PCN base stations.
Another advantage of distributed power amplifiers is that the
amplifiers operate in a reduced thermal density environment,
yielding enhanced reliability. The reduced thermal density results
from the fact that the heat is distributed across the full area
occupied by the antenna elements in the array rather than in a
concentrated small area as in the case of unitary high power
amplifiers used in the existing PCN base stations.
Since the impact of lossy elements following low noise
amplifiers or preceding low power amplifiers on the system noise
figure or EIRP, respectively, is small, the active antenna system
is capable of a great deal of versatility in that variable
attenuators and/or phase shifters can be placed in the signal path
of each antenna element. As is well known in the art, these
variable attenuators and phase shifters can operate to provide
electronic beam shaping and pointing capability.
The active antenna systems of the present invention can be
used for receive systems using either spatial or polarization
diversity and in fact are valid independent of the diversity
approach employed. Therefore, all of the advantages claimed for
active antennas apply to all diversity receive systems.
Furthermore, in a polarization diverse receive system, both
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orthogonal polarizations are implemented with each polarization
port fed to its own filter/LNA network and combiner.
~T~R n~C~TPTTON QF TU~ n~WuNas
FIGURE 1 is a diagram of a prior art PCN base station
architecture for an antenna system for both transmission and
reception of electromagnetic radiations;
FlGURE 2 is a diagram of an active antenna system architecture
in accordance with a first embodiment of the present invention; and
FIGURE 3 is a diagram of an active antenna system architecture
in accordance with a second embodiment of the present invention.
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~L ~~Tp~TON OF T~ E~ E~SBODTNCEXrrg
Referring first to FIGURE 1, there is shown a system utilizing
the prior art PCN base station antenna architecture. FIGURE 1 is
typical of the existing state of the art for PCS antenna systems.
However, the discussion that follows applies to existing cellular
systems as well. The system includes a transmit antenna system 1
and a receive antenna system 7. The transmit antenna system 1
includes a support 3 having thereon a plurality of radiating
antenna elements 5 disposed in a straight line, the distance from
the topmost element to the bottommost element being a few meters.
The transmit antenna system 1 has a high power amplifier (HPA) 13
and a filter 15 disposed at the base station 17 with the
amplifier/filter being connected to each of the radiating antenna
elements 5 via a combiner 54 and a feed cable 31 with the length of
the feed cable 31 varying anywhere between 15 and 70 meters, in
general. As can be seen, the long cable 31 leads to a loss of 2 to
3 dB of power transmitted by the ground based power amplifier.
The receive antenna system 7 includes a support 9 thereon a
plurality of radiating antenna elements 11 disposed in a straight
line with ~im~ions as in the transmit antenna system. The
receive antenna system 7 has a pair of filters 19, 21, filter 19
coupled to a pair of amplifiers 23 and 25 and filter 21 coupled to
a pair of amplifiers 27 and 29. Each radiating antenna element 11
is coupled to each of the filters 19 and 21 via power combiners S5
and 56 via feed cable 33 to provide vertical and horizontal
outputs. Although the em~odiment is shown for a receiving system
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with polarization diversity, the same discussion applies to the
receiving system utilizing spatial diversity. For the purpose of
illustration,, the filter/LNA combination is shown mounted on the
mast, as is the common practice for PCS base stations. As can be
seen, the ohmic losses in the combiners and short transmission
lines between combiners and the filter/LNA combination contribute
to the degradation in the system noise figure.
Referring to FIGVRE 2, there is shown an active antenna
architecture in accordance with the present invention. The antenna
is similar to that of FIGURE l except that all of the radiating
antenna elements 5 and ll are on the same support 35 and, rather
than having a single filter lS and amplifier 13 for the transmit
section and the filter l9, 21 and associated amplifiers 23, 25, 27,
29 for the receive section as shown in FIGURE l, each radiating
antenna element has its own filter and amplifier positioned as
closely adjacent to the antenna element as possible. This is shown
in FIGURE 2 wherein, for the transmit portion, each antenna e~ement
5 is connected to its own filter 37 and amplifier 39 with the feed
cable 41 extending from the base station 43 to each of the
amplifiers 39 through power divider 54. In the receive portion,
each radiating antenna element ll has two orthogonally polarized
outputs as in FIGURE l with each output have its own filter 45 and
amplifier pair 47 and 49. The outputs of the amplifiers 47 and 49
are combined in the power combiners 55 and 56 and are fed to the
base station via feed cables Sl and 53 respectively. The receive
configuration with polarization diversity is shown here ~or the
.. . ...
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purpose of illustration. However, the claim of active antenna
application holds as well for the receive configuration with
spatial or any other diversity as well.
For the receive configuration, it should be understood that
the positions of the amplifiers and associated filters can be
reversed where amplification can take place prior to filtering or
filtering can take place prior to amplification. When the filter
is placed ahead of the amplifier, the impact of the filter loss on
the system noise figure is reduced, but the amplifier stage must be
designed to have high dynamic range, so that any expected
interference can be passed by the amplifier to the filter without
generating any significant intermodulation products. When the
amplifier is placed ahead of the filter to provide out-of-band
rejection of signals prior to in-band signal amplification (as
shown in FIGURE 2), the filter must have low power loss with
implementation preferably performed in waveguides.
Referring now to FIGURE 3, there is shown a variation of the
architecture of FIGURE 2 wherein a variable phase shifter 51 and a
variable attenuator 53 are placed in series with each combination
of amplifier and filter of FIGURE 2. By varying the phase and
amplitude, there is provided the ability to electronically tilt the
beam and shape the beam in elevation, depending upon the traffic
patterns and the topography of the cell being served. The
elevation beam shaping and switching can be controlled dynamically
by the service provider through a remote controller.
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A second embodiment of the invention comprises only the
receive portion of the above described structure and a third
embodiment of the invention comprises only the transmit portion of
the above described structure.
Though the invention has been described with respect to
specific preferred embodiments thereof, many variations and
modifications will immediately become apparent to those skilled in
the art. It is therefore the intention that the appended claims be
interpreted as broadly as possible in view of the prior art to
include all such variations and modifications.
. . .
