Note: Descriptions are shown in the official language in which they were submitted.
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DUAL DISTRIBUTED ANTENNA SYSTEM
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to communication systems,
particularly indoor communication systems including cellular
telephones, personal communication services (PCS), wireless
private branch exchange (PBX) and wireless local loop telephone
systems. More specifically, the present invention relates to a
novel and improved distributed antenna system for microcellular
communication systems to facilitate indoor communications using
spread spectrum signals.
II. Description of the Related Art
The use of code division multiple access (CDMA)
modulation techniques is one of several techniques for
facilitating communications in which a large number of system
users are present. Other multiple access communication system
techniques, such as frequency hopping spread spectrum, time
division multiple access (TDMA), frequency division multiple
access (FDMA) and amplitude modulation schemes such as amplitude
companded single sideband (ACSSB) are known in the art. However
the spread spectrum modulation technique of CDMA has significant
advantages over these modulation techniques for multiple access
communication systems. The use of CDMA techniques in a multiple
access communication system is disclosed in U.S. Patent No.
4,901,307, issued February 13, 1990, entitled "SPREAD SPECTRUM
MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR
TERRESTRIAL REPEATERS", assigned to the assignee of the present
invention.
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In the just mentioned patent, a multiple access technique is disclosed
where a large number of mobile telephone system users each having a
transceiver communicate through satellite repeaters or terrestrial base
stations (also referred to as cell-sites stations, cell-sites, or for short,
cells)
using code division multiple access (CDMA) spread spectrum
communication signals. In using CDMA communications, the frequency
spectrum can be reused multiple times thus permitting an increase in
system user capacity. The use of CDMA results in a much higher spectral
efficiency than can be achieved using other multiple access techniques.
The terrestrial channel experiences signal fading that is characterized
by Rayleigh fading. The Rayleigh fading characteristic in the terrestrial
channel signal is caused by the signal being reflected from many different
features of the physical environment. As a result, a signal arrives at a
mobile unit receiver from many directions with different transmission
delays. At the UHF frequency bands usually employed for mobile radio
communications, including those of cellular mobile telephone systems,
significant phase differences in signals traveling on different paths may
occur. The possibility for destructive summation of the signals may result,
with on occasion deep fades occurring.
Terrestrial channel fading is a very strong function of the physical
position of the mobile unit. A small change in position of the mobile unit
changes the physical delays of all the signal propagation paths, which
further results in a different phase for each path. Thus, the motion of the
mobile unit through the environment can result in a quite rapid fading
process. For example, in the 850 MHz cellular radio frequency band, this
fading can typically be as fast as one fade per second per mile per hour of
vehicle speed. Fading this severe can be extremely disruptive to signals in
the terrestrial channel resulting in poor communication quality.
Additional transmitter power can be used to overcome the problem of
fading. However, such power increases effect both the user by excessive
power consumption, and the system by increased interference.
The direct sequence spread spectrum CDMA modulation techniques
disclosed in LT.S. Patent No. 4,901,307 offer many advantages over narrow
band modulation techniques used in communication systems employing
satellite or terrestrial repeaters. The terrestrial channel poses special
problems to any communication system particularly with respect to
multipath signals. The use of CDMA techniques permit the special
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problems of the terrestrial channel to be overcome by mitigating the adverse
effect of multipath, e.g. fading, while also exploiting the advantages
thereof.
In a CDMA communication system, the same wideband frequency
channel can be used for communication by all base stations. Typically a
FDMA scheme is used where one frequency band is used for
communications from the base stations to the remote or mobile stations
(forward link) and another for communications from the remote or mobile
stations to the base stations (reverse link). The CDMA waveform properties
that provide processing gain are also used to discriminate between signals
that occupy the same frequency band. Furthermore the high speed
pseudonoise (PN) modulation allows many different propagation paths to
be separated, provided the difference in path delays exceed the PN chip
duration, i.e. 1 /bandwidth. If a PN chip rate of approximately 1 MHz is
employed in a CDMA system, the full spread spectrum processing gain,
equal to the ratio of the spread bandwidth to system data rate, can be
employed to discriminate against paths that differ by more than one
microsecond in path delay from each other. A one microsecond path delay
differential corresponds to differential path distance of approximately 1,000
feet. The urban environment typically provides differential path delays in
excess of one microsecond, and up to 10-20 microseconds are reported in
some areas.
In narrow band modulation systems such as the analog FM
modulation employed by conventional telephone systems, the existence of
multiple paths results in severe multipath fading. With wideband CDMA
modulation, however, the different paths may be discriminated against in
the demodulation process. This discrimination greatly reduces the severity
of multipath fading. Multipath fading is not totally eliminated in using
CDMA discrimination techniques because there occasionally exists paths
with delayed differentials of less than the PN chip duration for the
particular system. Signals having path delays on this order cannot be
discriminated against in the demodulator, resulting in some degree of
fading.
It is therefore desirable in the such communication systems that
' some form of diversity be provided which would permit a system to reduce
fading. Diversity is one approach for mitigating the deleterious effects of
fading. Three major types of diversity exist: time diversity, frequency
diversity, and space diversity.
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Time diversity can best be obtained by the use of repetition, time
interleaving, and error detection and correction coding which is a form of
repetition. The present invention employs each of these techniques as a
form of time diversity.
CDMA by its inherent nature of being a wideband signal offers a form
of frequency diversity by spreading the signal energy over a wide bandwidth.
Therefore, frequency selective fading affects only a small part of the CDMA
signal bandwidth.
Space or path diversity is obtained by providing multiple signal paths
through simultaneous links from a mobile user through two or more base
station. Furthermore, path diversity may be obtained by exploiting the
multipath environment through spread spectrum processing by allowing a
signal arriving with different propagation delays to be received and
processed separately. Examples of path diversity are illustrated in U.S.
Patent No. 5,101,501, issued March 31, 1992, entitled "SOFT HANDOFF IN A
CDMA CELLULAR TELEPHONE SYSTEM", and U.S. Patent No. 5,109,390,
issued April 28, 1992, entitled "DIVERSITY RECEIVER IN A CDMA
CELLULAR TELEPHONE SYSTEM", both assigned to the assignee of the
present invention.
The deleterious effects of fading can be further controlled to a certain
extent in a CDMA system by controlling transmitter power. A fade which
decreases the power received by the base station from the mobile unit can be
compensated for by increasing the power transmitted by the mobile station.
The power control function operates in accordance with a time constant.
Depending on the time constant of the power control loop and the length of
time of a fade, the system may compensate for the fade by increasing
transmit power of the mobile unit. A system for base station and mobile
unit power control is disclosed in copending U.S. Patent No. 5,056,109,
issued October 8, 1991, entitled "METHOD AND APPARATUS FOR
CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR
MOBILE TELEPHONE SYSTEM", also assigned to the assignee of the present
invention.
The existence of multipath can provide path diversity to a wideband
PN CDMA system. If t'vo or more paths are available with differential path
3~ delay greater than one chip duration two or more PN receivers can be
employed to separately receive these signals at a single base station or
mobile unit. Since these signals typically exhibit independence in
multipath fading, i.e., they usually do not fade together, the outputs of the
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two receivers can be diversity combined. Therefore a loss in performance
only occurs when both receivers experience fades at the same time. Hence,
one aspect of the present invention is the provision of two or more PN
receivers in combination with a diversity combiner. In order to exploit the
5 existence of multipath signals, to overcome fading, it is necessary to
utilize a
waveform that permits path diversity combining operations to be
performed.
A method and system for constructing PN sequences that provide
orthogonality between the users so that mutual interference is reduced is
disclosed in U.S. Patent No. 5,103,459, issued April 7, 1992, entitled
"SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN
A CDMA CELLULAR MOBILE TELEPHONE SYSTEM", also assigned to the
assignee of the present invention. Using these techniques in reducing
mutual interference allowing higher system user capacity and better link
performance. With orthogonal PN codes, the cross-correlation is zero over
a predetermined time interval, resulting in no interference between the
orthogonal codes, provided only that the code time frames are time aligned
with each other.
The above mentioned patents and patent applications disclose a
novel multiple access technique wherein a large number of mobile unit
telephone system users communicate through satellite repeaters or
terrestrial base stations using code division multiple access spread spectrum
modulation that allows the spectrum to be used multiple times. The
resulting system design has a much higher spectral efficiency than can be
achieved using previous multiple access techniques.
In cellular telephone systems, a large geographic area is provided
with mobile telephone service by installing a number of base stations, each
positioned to cover a cell, and the set of cells situated so as to provide
coverage of the entire geographic area. If service demand exceeds the
capacity that can be provided by a set of base stations providing coverage
over a certain area, the cells are subdivided into smaller cells and more base
stations are added. This process has been carried out to the extent that some
major metropolitan areas have nearly 400 base stations.
In a further development of the cellular telephone idea, it is desired
to provide a number of verv small cells, called microcells, which would
provide coverage of a very limited geographic area. LTSUaIIv, it is considered
that such areas are limited to a single floor of an office building and the
mobile telephone service can be viewed as a cordless telephone system that
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may or may not be compatible with the mobile cellular telephone system.
The rationale for providing such a service is similar to the reasoning for use
of Private Branch Exchange (PBX) systems in business offices. Such systems
provide for low cost phone service . for a large number of calls between
phones within the business while providing simplified dialing for internal
phone numbers. A few lines are also provided to connect the PBX system to
the public telephone system, allowing calls to be made and received between
telephones in the PBX system and telephones located elsewhere. It is
desirable for the microcell system to provide a similar level of service but
with the added feature of cordless operation anywhere within the service
area of the PBX.
In the indoor communication system environment, path delays are
typically much shorter in duration than experienced in the outdoor
communication system environment. In buildings and other indoor
environments where indoor communication systems are used, it is
necessary to provide a form of diversity which enables discrimination
between multipath signals.
The primary problem solved by the disclosed invention is the
provision of a simple antenna system that provides high capacity, simple
installation, good coverage and excellent performance. Another problem
solved by the present invention is that it achieves the above coverage while
maintaining compatibility with the mobile cellular system and while taking
a negligible amount of capacity away from the mobile system. This is
achieved in the disclosed invention by combining the capacity properties of
CDMA with a new distributed antenna design that confines the radiation to
a very limited and carefully controlled area.
The implementation of spread spectrum communication techniques,
particularly CDMA techniques, in an indoor environment provides features
which vastly enhance system reliability and capacity over other
communication systems. CDMA techniques as previously mentioned
further enable problems such as fading and interference to be readily
overcome. Accordingly, CDMA techniques further promote greater
frequency reuse, thus enabling a substantial increase in the number of
system users.
3~
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SUMMARY OF THE INVENTION
A key aspect in implementing an indoor communication system is
the usage of the dual set of distributed antennas of the present invention.
In this concept, two sets of antennas are fed by a common signal with only
time delay processing to distinguish signals. The transmit output of the
base station is fed to a string of antenna elements for example with a coaxial
cable. The antenna elements connect to the cable using power splitters. The
resulting signals, amplified and frequency converted as necessary, are fed to
the antennas. The salient features of this distributed antenna concept are as
follows: (1) simple and inexpensive dual antenna node design; (2)
neighboring antennas have time delays inserted in feed structure so signals
received and transmitted from neighboring antennas are distinguishable by
PN temporal processing; (3) exploitation of direct sequence CDMA's ability
to discriminate against multipath; and (4) creation of deliberate multipath
that satisfies discrimination criteria.
In the present invention, two sets of antenna cables are positioned in
parallel thus creating a series of nodes comprised of two antenna elements.
Signals transmitted from antennas of different antenna elements at a
common node are provided different delay paths between the base station
and the antenna. The antenna elements may comprise down conversion
circuitry thus reducing the cabling path loss between the antenna elements
and the base station and allowing the use of readily available SAW devices
as delay elements.
Another advantage is that little site specific engineering is required
for installation. Normally, antenna placement is determined only by
physical constraints, together with the requirement that every location
desiring service must be covered by a set of two antennas. There is no
concern for the overlapping of antenna patterns. In fact, overlapping
coverage is desirable in that it provides diversity operation to all terminals
in the overlap area. Overlap is, however, not required.
. The advantages of the distributed antenna concept are clear when
considering the inherent simplicity of the base station equipment required
to support indoor communications of the type such as cellular telephone,
3~ PCS, wireless PBX, wireless local loop or wireless home extension
telephone.
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The invention may be summarized according to one
aspect as in a digital communication system in which at
least one remote terminal communicates with other terminals
through a base station using digitally modulated
communication signals, said base station having an antenna
system comprising: a first set of spaced apart antennas; a
second set of spaced apart antennas, each antenna of said
second set of antennas corresponding in location to a
corresponding antenna of said first set of antennas; signal
distribution means for coupling said communication signals
between said base station and said antennas of said first
and second sets; and delay means operatively coupled to said
antennas of said first set and second set and said signal
distribution means for providing a predetermined delay in
said communication signals coupled between said base station
and said antennas.
According to another aspect the invention provides
in a communication system in which system users communicate
through a base station with remote system users, said remote
system users communicating through said base station via a
radio link therewith, said base station comprising:
communication terminal means for spread spectrum modulating
a remote user directed information signal and providing a
spread spectrum modulated remote user directed information
signal, and for receiving and separately demodulating a
first and second aggregate spread spectrum modulated remote
system user information signal; first antenna means for
receiving a plurality of spread spectrum modulated remote
system user information signals, each of said spread
spectrum modulated remote system user information signals of
a predetermined time delay with respect to one another,
combining said plurality of spread spectrum modulated remote
system user information signals to form said first aggregate
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spread spectrum modulated remote system user information
signal and providing said first aggregate spread spectrum
modulated remote system user information signals to said
communication terminal means; and second antenna means for
receiving a plurality of spread spectrum modulated remote
system user information signals, each of said spread
spectrum modulated remote system user information signals of
a predetermined time delay with respect to one another,
combining said plurality of spread spectrum modulated remote
system user information signals to form said second
aggregate spread spectrum modulated remote system user
information signal and providing said second aggregate
spread spectrum modulated remote system user information
signals to said communication terminal means.
According to another aspect the invention provides
a communication system for facilitating the communication of
information signals between users of said communication
system, and between users of said communication system and
users of an external network, wherein certain users of said
communication system use remote terminals to communicate
within said communication system, via a radio link with a
base station, using code division multiple access
communication signals, said system comprising: communication
terminal means for receiving and direct sequence spread
spectrum modulating an information signal intended for a
recipient remote terminal user with a pseudorandom noise
(PN) spreading code comprised of a predetermined sequence of
binary chips each of a predetermined chip duration time;
first antenna means comprising multiple elements for
providing multiple radiations of said spread spectrum
modulated information signal with each radiation of said
spread spectrum modulated information signal delayed in time
with respect to one another by at least one chip duration
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time and for receiving said spread spectrum modulated
information signal; and second antenna means comprising
multiple elements for providing multiple radiations of said
spread spectrum modulated information signal with each
radiation of said spread spectrum modulated information
signal delayed in time with respect to one another by at
least one chip duration time and for receiving said spread
spectrum modulated information signal wherein each of said
elements of said first antenna means is collocated with an
element of said second antenna means forming a node.
According to another aspect the invention provides
in a digital communication system where signals intended for
transfer to a receiving terminal are transmitted from a
transmitting terminal as digitally modulated signals,
I5 wherein said receiving terminal in receiving multipath
propagations of each digitally modulated signal requires a
minimum predetermined time difference between multipath
propagations of each digitally modulated signal as received
for demodulation thereof to provide said signals intended
for said receiving terminal, a method for creating multipath
propagations of said transmitted digitally modulated signals
wherein each multipath propagation is of at least said
minimum predetermined time difference with respect to
another upon reception at said receiving terminal, said
method comprising the steps of: providing a plurality of
spaced apart dual antenna elements; providing from said
transmitting terminal a communication signal to each of said
dual antenna elements; providing a different predetermined
delay in said communication signal as provided to each of
said dual antenna elements; and providing a different
predetermined delay in said communication signal as provided
to each antenna of said dual antenna elements.
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According to another aspect the invention provides
in a communications system in which an information signal is
modulated at a first station with a pseudorandom noise (PN)
code, comprised of a predetermined sequence of code chips
each of a predetermined time duration, with said PN
modulated noise signal modulated upon a carrier for
transmission, an antenna system comprising: a first
plurality of antennas coupled in series and coupled to said
first station: a plurality of delay elements each disposed
in series between adjacent coupled ones of said antennas of
said first plurality of antennas; a first station delay
coupled to said first station; a secondary plurality of
antennas coupled in series and coupled to said first station
delay, each of said second plurality of antennas positioned
with a corresponding one of said first plurality of
antennas; and a secondary plurality of delay elements each
disposed in series between adjacent coupled ones of said
antennas of said secondary plurality of antennas.
According to another aspect the invention provides
an antenna system for transmitting and/or receiving a spread
spectrum modulated information signal wherein said spread
spectrum modulated information signal is generated by spread
spectrum modulating an information signal with a
pseudorandom noise code comprised of a predetermined
sequence of code chips each of a predetermined chip
duration, said antenna system comprising: a first set of
serially coupled antenna elements; a second set of serially
coupled antenna elements, each antenna of said first set
collocated with an corresponding antenna of said second set;
a set of delay elements positioned such that at least one
delay element is interposed between each of said antenna
elements of said first and second sets; and an additional
delay element coupled in series with said second set of
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antenna elements such that every signal received and
transmitted by said second set of antenna elements is
delayed thereby.
According to another aspect the invention provides
an antenna system for transmitting and/or receiving a spread
spectrum modulated information signal wherein said spread
spectrum modulated information signal is generated by spread
spectrum modulating an information signal with a
pseudorandom noise code comprised of a predetermined
sequence of code chips each of a predetermined chip
duration, said antenna system comprising: a set of delay
elements, each delay element providing a delay time on the
order of a multiple of one chip duration in length; a set of
antennas; a system of cabling coupling said antennas and
said delay elements in a serial string, wherein each of said
delay elements is coupled between predetermined pairs of
said antennas; a second set of delay elements, each delay
element providing a delay time on the order of multiple of
one chip duration in length; a second set of antennas, each
antenna of said second set collocated with an antenna of
said first set; an initial delay element having a delay time
on the order of one chip duration in length; and a second
system of cabling coupling said second set of antennas and
each of said second set of delay elements in a serial
string, wherein each delay element of said second set of
delay elements is coupled between predetermined pairs of
said antennas and wherein said initial delay element is
coupled in series therewith.
According to another aspect the invention provides
an antenna system for transmitting and/or receiving a spread
spectrum modulated information signal coupled to a base
station wherein said spread spectrum modulated information
signal is generated by spread spectrum modulating an
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information signal with a pseudorandom noise code comprised
of a predetermined sequence of code chips each of a
predetermined chip duration, said antenna system comprising:
a set of delay elements, each delay element providing a
delay time on the order of a multiple of one chip duration
in length; a first set of antennas, each having an antenna
coverage area; a system of cabling coupling each of said
antennas of said first set to said base station and coupling
one said delay elements between said base station and at
least one antenna of said first set; a second set of delay
elements, each delay element providing a delay time on the
order of a multiple of one chip duration in length; a second
set of antennas, each having an antenna coverage area, and
wherein each particular antenna of said second set of
antennas corresponds to a particular antenna of said first
set in that the antenna coverage area of said particular
antenna of said first set and the antenna coverage area of
said particular antenna of said second set are substantially
the same; and a second system of cabling coupling each of
said antennas of said second set to said base station and
coupling one said second set of delay elements between said
base station and at least one antenna of said second set
such that each antenna of said second set has a different
delay with respect to said base station than said
corresponding antenna of said first set of antennas with
respect to said base station.
According to another aspect the invention provides
in a digital communication system in which at least one
remote terminal communicates with other terminals through a
base station using digitally modulated communication
signals, said base station having an antenna system
comprising: a first set of spaced apart antennas; a second
set of spaced apart antennas, each antenna of said second
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set of antennas corresponding in location to a corresponding
antenna of said first set of antennas; signal distribution
means for coupling said communication signals between said
base station and each antenna of said first and second sets;
and delay means operatively coupled in series with each of
said antennas of said first and second sets and said signal
distribution means for providing a predetermined delay in
said communication signals between said base station and
each antenna of said first and second set.
According to another aspect the invention provides
an apparatus in a digital communication system in which at
least one remote terminal communicates with a base station
using digitally modulated communication signals, said
apparatus comprising: a base station having at least two
independent receivers; a first set of spaced apart antennas;
a second set of spaced apart antennas, each antenna of said
second set of antennas corresponding in location to a
corresponding antenna of said first set of antennas thereby
forming a first aggregate antenna pattern; first signal
distribution means for coupling said communication signals
between a first one of said at least two independent
receivers of said base station and said first and second
sets of antennas; first delay means operatively coupled such
that each antenna of said first set of antennas and said
second set of antennas exhibits a different delay with
respect to said first one of said at least two independent
receivers of said base station; a third set of spaced apart
antennas; a fourth set of spaced apart antennas, each
antenna of said fourth set of antennas corresponding in
location to a corresponding antenna of said third set of
antennas thereby forming a second aggregate antenna pattern;
second signal distribution means for coupling said
communication signals between a second one of said at least
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two independent receivers of said base station and said
third and fourth sets of antennas; and second delay means
operatively coupled such that each antenna of said third set
of antennas and said fourth set of antennas exhibits a
different delay with respect to said second one of said at
least two independent receivers of said base station and
such that a particular antenna of said third set of antennas
and a particular corresponding antenna of said fourth set of
antennas exhibits a different delay with respect to said
base station than a particular antenna of said first set of
antennas and a particular corresponding antenna of second
set of antennas having a corresponding first aggregate
antenna pattern overlapping a corresponding second aggregate
antenna pattern of said particular antenna of said third set
of antennas and a particular corresponding antenna of said
fourth set of antennas.
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BRIEF DESCRIPTION OF THE DRAWINGS
The features, objects, and advantages of the
present invention will become more apparent from the
detailed description set forth below when taken in
conjunction with the drawings:
Figure 1 is an illustration of an exemplary
antenna pattern for a distributed antenna system of the
present invention;
Figure 2 is a block diagram of a basic exemplary
dual element distributed antenna system and base station
interface;
Figure 3 is a block diagram of a basic exemplary
E-structure element distributed antenna system alternative
to Figure 2;
Figure 4 is a block diagram of an exemplary
implementation of a remote or mobile unit transceiver;
Figure 5 is a block diagram of a basic exemplary
dual element distributed antenna system with an alternative
base station interface;
Figure 6 is a block diagram of an exemplary
microcell base station;
Figure 7 is a block diagram of a dual element
distributed antenna system using an active element
structure;
Figure 8 is an alternative block diagram of a dual
element distributed antenna system using an active element
structure;
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Figure 9 is a block diagram of a basic exemplary
dual element distributed antenna system comprised of three
parallel arrays of antennas;
Figure 10 is a block diagram of an exemplary
placement of a dual element distributed antenna system
comprising a plurality of parallel arrays; and
Figure 11 is a block diagram of a dual element
distributed antenna system comprising a two parallel arrays
of antennas and comprising a base station having multiple
independent receivers.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A single set of antennas and delay elements
provides the most basic implementation of the distributed
antenna function. Details of the single set of antennas are
disclosed in U.S. Patent No. 5,280,472 issued January 18,
1994. However a system employing a single set of antennas
can experience service quality reductions that can be
alleviated by a dual set of antennas. A CDMA system, to
achieve high capacity, uses a strict power
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control mechanism. Each mobile unit transmits enough power to
communicate with the antenna with the lowest path loss to the mobile
unit. Communications with all other antennas thus have less than optimal
energy.
The effects of multipath fading may cause a momentary degradation
of service if a mobile unit is positioned in close proximity to a first
antenna
and positioned at some great distance from other antennas. Under this
condition the mobile unit transmits enough power to communicate with
the first antenna but not enough power to communicate reliably with the
distant antennas. If under this condition the mobile unit were to abruptly
experience a severe multipath fade with respect to the first antenna, the
reduced signal level at the first antenna and the low signal level at the
distant antennas may cause a degradation in service. Communication
between the base station and the mobile unit would be sub-optimal until
the power control loop increased the transmit power from the mobile unit
or until the mobile unit moved to alleviate the multipath fading.
The sub-optimal performance created as described above can be
mitigated by placing two antennas at each node. Thus, as opposed to evenly
distributing single antennas, the mobile unit in general has similar
distance, and hence path loss, between pairs of collocated antennas. If the
mobile unit is positioned in close proximity to a pair of collocated antennas
and positioned at some great distance from other antennas and the mobile
unit abruptly experiences a severe multipath fade with respect to the one of
the collocated antennas, the second collocated antenna should have
sufficient signal level to maintain communication with the mobile unit
without degradation.
To obtain maximum benefit from the present invention, the
collocated antennas should exhibit an independence in fading meaning that
the probability that a severe fade would occur for both antennas at the same
mobile unit location is small. To achieve independent fading, a degree of
diversity between the collocated antennas is required.
One way to achieve diversity in collocated antennas is to place the
antennas some distance apart. The distance should allow the two antennas
to have substantially the same coverage area while being spaced apart
enough to provide independent fading. Placing rivo antennas at one base
station to obtain diversity is common practice in macrocellular systems. In
a macrocellular svstem, two antennas having relatively large coverage
areas, generally on the order of several miles, are placed at one base
station.
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Typically the antennas are placed about 10 to 20 wavelengths (about 6 to 12
feet at the most common frequency used for cellular communications) apart
to obtain path diversity and thus independence in fading.
A second method to achieve diversity in collocated antennas is to
provide each antenna of a set of collocated antennas with a different
polarization, such as vertical and horizontal polarization. A standard
indoor environment is bounded in three dimensions. A mobile unit
within the three dimensional structure has a variety of signal paths to/from
a fixed antenna involving multiple reflections from the surfaces of the
structure. Depending on the angles involved, each reflection of a signal
may rotate the polarization of the reflected signal. Therefore two signals
having different polarization reflecting from the same set of surfaces form
two signal paths having different phase characteristics. Because the signals
have different phase characteristics, they also have different fading
characteristics. Due to this process, two collocated antennas where the
antennas have two different polarizations possess a high degree of
independence in fading even if the antennas are placed very close to one
another.
Figure 1 provides an illustration of an exemplary antenna pattern for
a dual set of antennas configured in accordance with the present invention.
The antenna pattern, as illustrated in Figure 1, is generated by two series of
omnidirectional antennas. Each set of antennas (30 and 35) defines an
antenna pattern 40A - 40J that is preferably overlapping with the pattern of
neighboring antennas. For example, antennas 30A and 35A define the
antenna pattern 40A. Neighboring antennas refer to antennas having
overlapping or contiguous antenna patterns which are not collocated
antennas at a common node. The overlapping of patterns provides
continuous antenna coverage for the desired area. The two sets of antennas
are coupled in series in an exemplary manner. The first set of antennas is
coupled as indicated by line 10. The second set of antennas is coupled as
indicated by line 20. The second set of antennas is approximately in parallel
with the first set of antennas such that each antenna of the first set is
collocated with an antenna of the second set.
As mentioned previously, control of signal power is an important
aspect of a CDMA telephone system in order to realize high user capacity.
conventional omnidirectional antenna radiates a signal roughly equally in
all directions. The signal strength is reduced with radial distance from the
antenna according to the propagation characteristics of the physical
WO 95!06365 PCT/US94109657
11
environment. The propagation characteristics may vary from inverse
second power to inverse 5.5 power law of the radial distance between the
mobile unit and the fixed antennas.
A base station that is designed to serve a certain radius must transmit
at a sufficient power level so that a mobile unit at the edge of the cell
covered by the base station receives an adequate signal power level. Mobile
units that are closer than the edge of the cell receive a greater than
adequate
signal level. Directional antennas beams can be formed using a variety of
techniques known in the art. However, the forming of directional beams
cannot alter the propagation law. Coverage of a desired area by a signal can
be obtained by a combination of antenna pattern, antenna placement, and
transmitter power.
The use of a distributed antenna system provides the desired antenna
pattern, such as coverage of a hallway of a building, where each antenna
element provides limited coverage. In providing limited antenna coverage
the power necessary to reach a mobile unit within the smaller coverage area
is correspondingly reduced since propagation loss is reduced.
However, there is a problem with multiple antennas all radiating the
same signal. There may be areas, particularly near points equidistant from
two or more of the antennas where the signals may be received from the
two antennas that cancel each other. Points where the signal may cancel are
separated by approximately one-half wavelength. At 850 MHz this is equal
to 17.6 cm or about 7 inches. If two signals arrive at the receive antenna at
equal strength but opposite in phase, then they may so cancel. Essentially,
this is man-made multipath fading. As with natural multipath fading,
diversity is the best method for mitigation of fading. The CDMA system
design provides several methods of diversity for mitigation of multipath
fading.
The above mentioned patents and copending patent applications
disclose a cellular telephone system that uses a CDMA modulation with
a 1.25 MHz bandwidth, multiple forms of diversity, and very careful
transmitter power control. One method of exploiting diversity is the
provision of a "rake" receiver architecture in which multiple receivers are
provided, each capable of receiving a signal that has traveled a different
path
3~ and therefore exhibits a different delay. Included is a separate searcher
receiver which continuously scans the time domain looking for the best
paths and assigning the multiple data receivers accordingly.
WO 95/06365 PCT/US94/09657
12
Another method of diversity is path diversity. In path diversity, the
signal is radiated from multiple antennas, providing more than one
propagation path. If two of more antennas can provide acceptable
communication paths to the mobile unit receiver then fading mitigation
through path diversity can be obtained.
In the microcell system it is desired to provide multiple antennas in
order to provide coverage in a desired coverage area but the capacity
demand for the system does not require that each antenna be supplied with
a separate set of signals as in the conventional cellular system. Instead, to
minimize the cost and complexity of the system, it is desired to feed some or
all of the antennas in the microcell system with the same RF signals. In
areas of the microcell system where good paths are possible to two or more
of the antennas then path diversity can be obtained.
What is desired is a simple, low cost way to distinguish the signals
feeding the different antennas without adding significantly to the
complexity of the system. The method for so doing in the present
invention is the addition of delay elements in the feeder lines between the
base station transceiver and the antennas elements within an array of
antennas.
Figure 2 illustrates an embodiment using a dual set of antennas with
delay elements. Base station 100 provides signals to and accepts signals from
the antenna array comprising node 200A - 200N. Analog transmitter 120
produces the RF signal for transmission by the distributed antenna array.
The signal is split to form two signals to be transmitted by the parallel
paths
by splitter 160. The first transmit path is delayed by delay element 150 and
then coupled to the first receive path by combiner 140 that may be replaced
with a duplexer. The second transmit path is directly coupled to the second
receive path by combiner 170 that may also be replaced with a duplexer.
Combiner 180 sums the two receive paths, one of which has been delayed by
delay element 155, and analog receiver 110 accepts the combined incoming
RF signal for processing.
The combined receive and transmit signals are cabled via distribution
cables 130 and 132 to first node 200A comprising two distribution
elements 190. Each element 190 comprises coupler 192 for coupling a
portion of the signal between antenna 196 and a respective distribution
cable 130 or 132. Each element 190 also comprises delay element 194 for
delaying signals and providing diversity from other antenna elements on
distribution cable 130 or 132. Delay element 150 provides the diversity of
WO 95/06365 PCT/US94/09657
2~.~7~~~
13
signals of collocated antennas at a common node. Delay element 194
provides time diversity of signals from neighboring antennas. To retain
complete time diversity of each neighboring antenna, the delay time of
delay element 194 should be different than the delay time of delay
element 150. An exemplary relationship in the delay period of delay
elements 150 and 194 is such that the delay between the base station and
every antenna in the system is different by at least one chip duration. The
difference in delay can be achieved by selecting the delay time of
element 150 to be greater than the sum of the delays in a single path (e.g.
the
delay of element 150 is N times the delay of element 194). It can also be
achieved by selecting the delay time of element 150 to be an appropriate
submultiple of the delay of element 194 (e.g. the delay of element 150 is
equal to one chip duration and the delay of element 194 is two chip
durations). A second node 200B, containing like elements, is cascaded in
series with first node 200A. The set of antennas continues in this manner
for the length of the dual sets.
An alternative embodiment of Figure 2 is illustrated in Figure 3.
Figure 3 has an E-structure while performing the same function as the
parallel structure of Figure 2. Within base station 102, analog
transmitter 240 and analog receiver 250 are coupled by combiner 260 to
distribution cable 230. Each node in the E-structure comprises a first
coupler 212 that couples signals between distribution cable 230 and
antenna 218. A second coupler 214 couples signals between distribution
cable 230 and second antenna 222 via delay element 220. Delay element 220
is used to provide time diversity at node 210A between antennas 218
and 222. A second delay element 216 is positioned in series with cable 230
and provides diversity between nodes, e.g. between nodes 201A and 210B.
Components 212 - 222 can be rearranged within each node in a variety of
fashions to perform the same basic function.
If the multiple antenna systems described above are provided with
delay lines in the feeders such that the signal from each antenna is at least
one chip duration delayed from its neighbors, then the multiple receiver
architecture of the mobile units allows the signal from each antenna to be
received separately and to be coherently combined in such a way that
cancellation does not occur. Ln fact, fading due to other reflections in the
environment can be greatly mitigated by the disclosed technique because a
form of path diversity is provided.
WO 95/06365 ~~ ~~ i~ PCT/US94l09657
14
The mobile units contain one or more data receivers and a searcher
receiver. The searcher receiver scans the time domain determining what
paths exist and which are the strongest paths. The available data receivers
are then assigned to demodulate the signals traveling the strongest available
paths. The base station receivers contain a similar capability.
Figure 4 illustrates in block diagram form an exemplary mobile unit
CDMA telephone set. The mobile unit includes an antenna 300 which is
coupled through duplexer 302 to analog receiver 304 and transmit power
amplifier 306. _
Receiver 304 receives the RF frequency signals from duplexer 302 for
amplification and frequency downconversion. The signals are also filtered
and digitized for providing to digital data receivers 310A - 310N along with
searcher receiver 314. Further details of an exemplary embodiment of
receivers 304, 310A - 310N and 314 are illustrated in the above-mentioned
U.S. Patent Nos. 5,103,459 and 5,109,390.
Receiver 304 also performs a power control function for adjusting the
transmit power of the mobile unit. Receiver 304 generates an analog power
control signal that is provided to transmit power control circuitry 308.
The digitized signal at the output of analog receiver 304 may contain
the signals of many on-going calls together with the pilot carriers
transmitted by the current base station and all neighboring base station. The
function of the receivers 310A - 310N are to correlate the samples with the
proper PN sequence. This correlation process provides a property that is
well-known in the art as "processing gain" which enhances the signal-to
interference ratio of a signal matching the proper PN sequence while not
enhancing other signals. Correlation output is then synchronously detected
using the pilot carrier from the closest base station as a carrier phase
reference. The result of this detection process is a sequence of encoded data
symbols.
A property of the PN sequence as used in the present invention is
that discrimination is provided against multipath signals. When the signal
arrives at the mobile receiver after passing through more than one path, or
in the present invention through more than one, antenna, there is a
difference in the reception time of the signal. If this time difference
exceeds
one chip duration, then the correlation process discriminates between the
signals. The data receiver can track and demodulate either of the earlier or
later arriving signal. If rivo or more data receivers, typically three, are
WO 95/06365 PCT/US94/09657
provided then multiple independent paths can be tracked and processed in
parallel.
Searcher receiver 314, under control of control processor 316 is for
continuously scanning the time domain around the nominal time of a
5 received pilot signal of the base station for other multipath pilot signals.
Receiver 314 measures the strength of any reception of a desired waveform
at times other than the nominal time. Receiver 314 compares signal
strength in the received signals. Receiver 314 provides a signal strength
signal to control processor 316 indicative of the strongest signals.
10 Processor 316 provides control signals to data receivers 310A - 310N for
each
to process a different one of the strongest signals.
The outputs of receivers 310A - 310N are provided to diversity
combiner and decoder circuitry 318. The diversity combiner circuitry
contained within circuitry 318 adjusts the timing of the two streams of
15 received symbols into alignment and adds them together. This addition
process may be proceeded by multiplying the two streams by a number
corresponding to the relative signal strengths of the two streams. This
operation can be considered a maximal ratio diversity combiner. The
resulting combined signal stream is then decoded using a forward error
correction (FEC) decoder also contained within circuitry 3I8. The usual
digital baseband equipment is a digital vocoder system. The CDMA system
is designed to accommodate a variety of different vocoder designs.
Baseband circuitry 320 typically includes a digital vocoder (not shown)
which may be a variable rate type. Baseband circuitry 320 further serves as
an interface with a handset or any other type of peripheral device. Baseband
circuitry 320 provides output information signals to the user in accordance
with the information provided thereto from circuitry 318.
In the mobile unit-to-base station link (reverse link), user analog
voice signals are typically provided through a handset as an input to
baseband circuitry 320. Baseband circuitry 320 includes an analog to digital
(A/D) converter (not shown) which converts the analog signal to digital
form. The digital signal is provided to the digital vocoder where it is
encoded. The vocoder output is provided to a forward error correction
(FEC) encoding circuit (not shown). In the exemplary embodiment the error
correction encoding implemented is of a convolutional encoding scheme.
The digitized encoded signal is output from baseband circuitry 320 to
transmit modulator 322.
WO 95/06365 PCT/US94/09657
~,1
16
Transmit modulator 322 encodes the transmit data, which in the
exemplary embodiment is a 64-ary orthogonal signaling technique based
upon Walsh codes, and then modulates the encoded signal on a PN carrier
signal whose PN sequence is common amongst all mobile units, but is of a
different code phase offset assigned to the mobile station fbr the call. In
the
alternative the PN sequence may be chosen according to the assigned
address function for the call The PN sequence is determined by control
processor 316 from call setup information that is transmitted by the base
station and decoded by receivers 310A - 310N and control processor 316.
Control processor 316 provides the PN sequence information to transmit
modulator 322 and to receivers 310A - 310N for call decoding. As a further
detail an outer PN code may be used upon the PN spread signal. Further
details on data modulation are disclosed in U.S. Patent No. 5,103,459.
Transmit modulator 322 further converts the modulated signal to
analog form for modulating upon an IF carrier. The IF signal output from
transmit modulator 322 is provided to transmit power control circuitry 308.
In circuitry 308 transmission signal power is controlled by the analog power
control signal provided from receiver 304. Control bits transmitted by the
microcell base station in the form of power adjustment commands are
processed by data receivers 310A - 310N and provided to control
processor 316. These power adjustment commands are used by control
processor 316 in setting the power level in mobile unit transmission. In
response to these commands, control processor 316 generates a digital power
control signal that is provided to circuitry 308. Further information on the
relationship of receivers 310A - 310N and 314, control processor 316 and
transmit power control 308 with respect to power control is available in the
above-mentioned U.S. Patent No. 5,056,109.
Transmit power control circuitry 308 outputs the power controlled
modulated signal to transmit power amplifier circuitry 306. Circuitry 306
amplifies and converts the IF signal to an RF frequency. Circuitry 306
includes an amplifier which amplifies the power to a final output level.
The intended transmission signal is output from circuitry 306 to
duplexer 302. Duplexer 302 couples the signal to antenna 300 for
transmission to the microcell base station.
The base station structure is similar to the mobile unit structure of
Figure 4. The preferred embodiment of the base station described below
contains elements corresponding to Figure 5 which illustrates an alternative
base station structure embodiment of the structure of Figure 2. In Figure 5
WO 95/06365 PCT/US94/09657
~1~'~ 6
17
the mobile unit signals received by the base station for each of the parallel
paths are not combined at RF and instead are separately received and
demodulated in the base station and coherently combined as digital bits.
Separate demodulation of the two returning paths has several advantages
including increased signal to interference ratios due to the coherent
combining and less fluctuations in power control both of which lead to high
mobile unit-to-base station link capacity.
In Figure 5, the nodes and elements are identical to corresponding
elements in Figure 2. Base station 100' has a modified RF structure as
shown in Figure 5. Additional analog receiver 115 functions independently
of analog receiver 110 with each coupled different demodulators as further
illustrated in Figure 6. Combiner 180 and delay element 155 of Figure 2
have been eliminated since there is no need for these component in this
particular embodiment.
Figure 6 illustrates in block diagram form the exemplary embodiment
of a microcell base station. In Figure 6, the receiver system is comprised of
analog receiver 110 and optionally analog receiver 115 corresponding to like
components of Figures 2 and 5. The receiver system is further comprised of
independent searcher receiver 500 and digital data receivers 510A - 510N
associated with analog receiver 110; independent searcher receiver 515 and
digital data receivers 520A - 520N associated with analog receiver 115; and
diversity and combiner & decoder circuitry 530. It should be noted that for
the antenna implementation of Figure 2 the base station need not include
searcher receiver 515, digital data receivers 520A - 520N, and analog
receiver 115 The receiver system may also include associated with each
analog receiver 110 and 115 any number of digital data receivers. It should
be understood that as few as one digital data receiver (e.g. data
receiver S10A) associated with each analog receiver may be employed.
However in order to take full advantage of the functionality of a rake
receiver it is preferred that two or more data receivers, e.g. typically three
or
four, be employed for each antenna system. Further details of an exemplary
embodiment are provided in U.S. Patent Nos. 5,103,459 and 5, 109,390.
As illustrated in Figure 5 analog receivers 110 and 115 respectively
output a digitized version of composite signal formed from the
transmissions of one or more mobile units. Searcher receivers 500 and 515
each track the multipath propagations of an individual mobile unit's
transmissions. Data receivers 510A - 510N and 520A - 520V' each are
assigned to demodulate a particular multipath propagation of the
WO 95/06365 ~ c~ PCT/US94I09657
18
modulated data signal to extract the encoded message data. The composite
signal output from analog receivers 110 and 115 is also provided to other
sets of searcher receivers and corresponding data receivers (not shown)
which are identical in construction to searcher receivers 500 and 515 and
data receivers 510A - 510N and 520A - 520N for tracking and demodulating
signals transmitted by other mobile units.
The microcell base station of Figure 6 includes CDMA controller 540
which is coupled to data receivers S10A - 510N and 520A - 520N along with
searcher receivers 500 and 515. CDMA controller 540 provides Walsh
sequence and code assignment, signal processing, timing signal generation,
power control and various other related functions.
Signals received on one of the sets of antennas are provided to analog
receiver 110 and then to searcher receiver 500. Searcher receiver 500 is used
to scan the time domain about the received signal to ensure that digital data
receivers 510A - 510N are tracking and processing the strongest available
time domain signals associated with a particular mobile unit. Searcher
receiver 500 provides corresponding signals to CDMA controller 540 which
in response thereto generates and provides control signals to digital data
receivers 510A - 510N for selecting the appropriate received signal for
processing.
Signals received on the second of the set of distributed antennas, if
used, are provided to analog receiver 115 and then to searcher
receivers 520A - 520N. Searcher receivers 515 is also used to scan the time
domain about the received signal to ensure that digital data
receivers 520A - 520N are tracking and processing the strongest available
time domain signals associated with a particular mobile unit. Searcher
receiver 515 provides corresponding signals to CDMA controller 540 which
in response thereto generates and provides control signals to digital data
receivers 520A - 520N for selecting the appropriate received signal for
processing. The output signals from receivers 510A - 510N and 520A - 520N
are then processed for optimal performance by diversity combiner &
decoder 530.
The signal processing in the base station data receivers and searcher
receivers is different in several aspects than the signal processing by
similar
elements in the mobile unit. In the mobile unit-to-base station link
(reverse link) unlike the base station-to- mobile unit link (forward link),
the
mobile unit does not transmit a pilot signal that can be used for coherent
reference purposes in signal processing at the base station. The mobile
WO 95/06365 ~ ~ ~ PCT/US94/09657
19
unit-to-base station link is characterized by a non-coherent modulation and
demodulation scheme using 64-ary orthogonal signaling.
Referring again to Figure 6, searcher receiver 500 and digital data
receivers 510A - S10N, receive the composite signal output from analog
receiver 110. In order to decode the spread spectrum signals transmitted to
the particular base station receiver through which a single mobile unit
communicates, the proper PN sequences must be generated. Further details
on the generation of the mobile unit signals are in U.S. Patent No. 5,103,459.
Each data receiver tracks the timing of the received signal it is
receiving. This is accomplished by the well known technique of correlating
the received signal by a slightly early local reference PN and correlating the
received signal with a slightly late local reference PN. The difference
between these two correlations averages to zero if there is no timing error.
Conversely, if there is a timing error, then this difference will indicate the
magnitude and sign of the error and the receiver's timing is adjusted
accordingly.
Signals from an external or internal network, such as a private
branch exchange (PBX), are coupled to the appropriate transmit modulator
vocoder 555 under control of CDMA controller 540. Transmit
modulator 535 which under control of CDMA controller 540 spread
spectrum modulates the data for transmission to the intended recipient
mobile unit. Transmit modulator 535 is assigned to encode and modulate
data intended for transmission to the particular mobile unit for which
search receivers 500 and 515 along with data receivers 510A - 510N
and 520A - 520N are assigned. Transmit modulator 535 modulates the
vocoder data with an orthogonal code selected from a set of orthogonal
codes with the signal then modulated with a PN spreading code. The PN
spread signal is then converted to analog form and provided to transmit
power control circuitry 550.
Transmit power control circuitry 550 under the control of CDMA
controller 540 controls the transmission power for the signal. The output of
circuitry 550 is provided to summer 560 where it is summed with the
output of transmit modulator/transmit power control circuits of other
channel units. The output of summer 560 is provided to the analog
transmit 120. Analog transmitter 120 amplifies the signal for output via the
distributed antenna for radiating to the mobile units within the base station
service area, Further details on exemplary transmitter circuitry of Figure 6
are illustrated in U.S. Patent No. 5,103,459.
WO 95/06365 PCT/US94/09657
Figure 6 further illustrates pilot/control channel generators and
transmit power control circuitry 545. Circuitry 545 under control of CDMA
controller 540 generates and controls the power of the pilot signal, the sync
channel, and the paging channel for coupling to analog transmitter 120.
5 It is implicit in the previously described embodiments of the
distributed antenna that most RF signal processing, including frequency
conversion, amplification, and filtering is performed by the analog receiver
and analog transmit circuitry within the base station. There are advantages
however to moving these functions to the antenna elements of each node
10 by creating an active antenna element.
Figure 7 illustrates an embodiment of the active elements. Base
station 600 comprises analog receivers 605 and 635 which receive signals at
an intermediate frequency (IF) from the array of distributed antennas or
nodes 720A - 720N each comprised of a pair of active elements 705. In this
15 particular arrangement analog receiver 605 receives signals along
distribution cable 720 from a first set of active elements 705 while analog
receiver 635 receives signals along distribution cable 725 from a second set
of
active elements 705. Analog transmitter 625 produces signals at an IF
frequency that is split by splitter 630 into two signals for transmission by
the
20 parallel paths. Delay element 620 delays the transmit IF signal provided
upon distribution cable 722 which is intended for a first one of the set of
active elements 705 contained within nodes 720A - 720N. The
corresponding non-delayed transmit IF signal from splitter 630 is provided
upon distribution cable 722 to the second one of the set of active
elements 705 contained within the nodes 720A - 720N.
Active elements 705 require both do power and a frequency reference
signal. These signal may be generated for individual active elements or the
pair of active elements in the node. A preferred method of supplying these
signals is to add them to the IF signals on distribution cables 720, 722, 725
and 727. Reference frequency sources 610 and 612 each produce a reference
frequency signal which is used in a phase lock loop within the
corresponding element. Preferably the reference frequency signal is within a
different frequency band than the receive IF signal to facilitate ease of
filtering at the base station and element. In the exemplary embodiment
adders 640 and 642 add the reference frequency signals for transmission
down cables 720 and 725. Likewise power supplies 615 and 617 supply do
power for the active elements via adders 645 and 647 and cables 722 and 727.
It should be understood that the reference frequency signal and do power
WO 95/06365 PCTIUS94l09657
21
may be provided on either the transmit or receive distribution cables, or
other various arrangements, in accordance with the connection of elements
in elements 705.
Each node of the distributed antenna is comprised of two active
elements 705. Since all elements 705 are identical, except for the cables to
which they are connected, only the function of a single element 705
connected to a pair distribution cables need be discussed. Element 705
receives the IF transmit signals on cable 722 and couples it through delay
element 650 which provides time diversity between neighboring antennas.
A portion of the IF transmit signal is coupled from the main path by
coupler 655. The coupled signal is upconverted by mixer 690 for
transmission at an appropriate RF frequency. The signal is coupled to
antenna 700 via duplexer 695.
Antenna 700 also receives the signal transmitted by the mobile unit
and couples the signal to the receive portion of the element via
duplexer 695. The received signal is down converted to an IF signal by
mixer 675 and coupled by coupler 660 to cable 720. The signals coupled by
coupler 660 upon cable 720 is combined with the signals received by other
node elements which are delayed by delay element 665. An actual
implementation of the element may also comprise gain stages in the receive
path, for example located between duplexer 695 and mixer 675, for noise
figure considerations. Likewise the transmit path might also comprise gain
stages to increase the level of the signal at the antenna. Also filter
elements
may be added to facilitate the signal processing
Mixers 675 and 690 within element 705 must be driven by local
oscillator (LO) at an appropriate frequency. In this embodiment the LO's are
created within the element. LO 680 is a phase lock loop (PLL) that provides
the driving LO for mixer 675 and LO 685 is a PLL that provides the driving
LO for mixer 690. The reference frequency is used to lock the PLL circuits to
a common phase and is coupled from the receive path by low pass filter
(LPF) 670 that low pass filters the signal on cable 720 to extract the
reference
frequency signal. Also do power is coupled (not shown) from cable 722 for
all active functions performed within the element. An alternate frequency
plan may facilitate the use of a single LO.
The advantages of the active elements are numerous and the cost of
the simple hardware is minimal. The active elements may be implemented
cvith readily available mobile unit technology. The IF signal experiences
less cable loss per foot of cable than the RF signal does and thus decreases
WO 95106365 ~~ ~~ PCT/US94109657
22
the need for amplification. The delay elements are inexpensive at IF
frequencies compared to RF frequencies. At IF the delay elements may be
SAW filters that provide delay with little phase error across the signal
bandwidth and provide filtering for unwanted signals. SAW filters cascade
in series easily while higher frequency elements can require a high isolation
to perform correctly.
The active element can also be implement without the frequency
conversion circuitry. Figure 8 illustrates an alternative ernbodiment of the
active element distributed antenna. In Figure 8 the active amplification
elements are added in the transmit and receive paths within the antenna
element.
In Figure 8, base station 800 comprises analog receivers 805 and 835
which receive signals from the array of distributed antennas or
nodes 920A - 920N each comprised of active elements 905. In this particular
arrangement analog receiver 805 receives signals along distribution cable
920 from a first set of active elements 905 while analog receiver 835 receives
signals along distribution cable 925 from a second set of active elements 905.
Analog transmitter 825 produces a signal that is split by splitter 830 into
two
signals for transmission by the parallel paths. Delay element 820 delays the
transmit signal provided upon distribution cable 922 which is intended for a
first one of the set of active elements 905 contained within
nodes 920A - 920N. The corresponding non-delayed transmit signal from
splitter 830 is provided upon distribution cable 927 to the second one of the
set of active elements 905 contained within the nodes 920A - 920N.
Again active elements 905 require do power to operate. As discussed
with reference to Figure 8, one method of supplying the signal is to add it to
the signals on the distribution cables. Power supply 815 supplies do power
for the active elements as added to cable 922 by adder 845. Similarly, power
supply 817 supplies do power for the active elements as added to cable 927 by
adder 847.
Each node of the distributed antenna is comprised of two active
elements 905. Element 905 receives the transmit signal on cable 922 and
couples it through delay element 750 which provides time diversity
bett~~een neighboring nodes. A portion of the transmit signal is coupled
from the main path by coupler 755. The coupled signal is amplified by
amp 790 for transmission at an appropriate level. The signal is coupled to
antenna 800 via duplexer 795.
WO 95/06365 ~ ~ ~ PCT/US94/09657
23
Antenna 800 also receives the signal transmitted by the mobile unit
and couples the signal to the receive portion of the element via
duplexer 795. The received signal is amplified by low noise amp 775 and
coupled by coupler 760 to the signals received by other element that have
been delayed by delay element 765. An actual implementation of the
element may also comprise filter elements to facilitate the signal processing.
DC power is coupled (not shown) from cable 922 for all active functions
performed within the element.
The active elements of Figure 8 may be used in an outdoor
environment incorporating a distributed antenna. For instance, in
downtown areas having tall building spaced closely together, a single
antenna base station may not sufficiently provide consistent signal levels
over the desired coverage area. An array of antennas could be used to cover
problem areas. In such a scenario, the nodes of the distributed antennas
would be placed in close proximity to each other and natural propagation
paths may not provide the necessary delay time required for separate
demodulation of the multipath signals. A distributed antenna of the
present invention would be the ideal solution in such a scenario. The
increased distance between nodes in an outdoor environment, and the
sequent higher power requirement for transmit and increase in cable loss
for receive, would necessitate the use of active elements. In particular the
configuration of Figure 8 is a realistic implementation of the system.
The present invention has been presented in the exemplary
embodiments as nodes coupled in series to form an array. In the event of a
node failure or a fault in the interconnect cabling, nodes located in the
series
connection beyond the failure or fault, with respect to the base station could
be rendered useless in the antenna system. In order to overcome this
potential drawback nodes may be coupled in parallel or series/parallel
combination to provide continued coverage in the event of a node failure
or interconnect cabling fault. A series/parallel combination of nodes is
illustrated in Figure 9 which represents a modified embodiment of the
exemplary embodiment of Figure 2. New elements 930, 932, 934, and 936,
which do not correspond to elements in Figure 2, are shown external to base
station 101 however these elements could function equivalently if
incorporated ~n~ithin the base station. New elements 930 and 934 are splitters
which couple the two antenna arrays to base station 101. The first antenna
array comprised of nodes 200 A' - 200" receives and supplies signals on
distribution cables 130' and 132'. The second parallel antenna array
WO 95/06365 PCTIUS94/09657
~ 24
comprised of nodes 200A" - 200N" receives and su lies si nals on
PP g
distribution cable 130" and 132" though added delay elements 932 and 936
respectively. The delay value of delay elements 932 and 936 is ideally
chosen such that each antenna in the system exhibits a different delay with
respect to the base station.
The topology presented in Figure 9 can take on a variety of
alternative forms. The nodes and elements of Figure 9 could be replaced
with the nodes and elements of Figure 3, 5, 7, or 8. Splitters 930 and 934
could couple more than two arrays to the base station. In fact in a model
parallel topology, each node of the system could be independently connected
to the base station. The base station topology also could take on a variety of
alternative embodiments. Base station 101 could be altered to contain
analog receiver 115 making this topology similar to that of Figure 5.
The placement of antennas in a system comprising a series/parallel
combination of nodes could take on a variety of forms. One placement
topology is illustrated in Figure 10. Figure 10 comprises a base station and
three parallel sets of series arrays. The topology of base station 940 is
arbitrary in this example and could be a simple variation of any base station
described herein. Each antenna node, 950A - 950N, 950A' - 950N', and
950A" - 950N", is a dual antenna node in accordance with the present
invention. Antenna nodes 950A - 950N comprise the first array. Antenna
nodes 950A' - 950N' comprise the second array. Antenna nodes
950A" - 950N" comprise the third array. Ideally every antenna node of
Figure 10 has a different delay with respect to the base station. The
placement of the antenna nodes in Figure 10 is illustrative of a distribution
of antennas which would provide a good deal of protection against a fault.
Instead of placing the second array after the first array and the third array
after the second, the nodes of each array are interspersed with nodes of the
other arrays. In this configuration, a fault in one array does not necessarily
cause a complete lack of service at any point in the coverage area of the base
station. Instead of creating a location where no service is provided, a fault
condition produces a soft failure of the system that may lower the
performance of the entire system
For a high capacity areas, a parallel or series/parallel combination of
nodes has additional advantages over the simple series connection of an
entire array. A CDMA link is limited as to a maximum number of separate
signals that it can efficiently combine in a given communication channel. If
more than the maximum number of signals are present, the system capacity
WO 95/06365 PCT/US94/09657
2S
is exceeded and overall system quality is degraded. Once the signals have
been summed together, as in the case of a single series of nodes, there is no
way to separate the signals such that they can be sent to multiple receivers
and individually demodulated. The base station circuitry needs to limit the
number of signals that are transmitted to the mobile units within the range
of its antennas. Each signal transmitted by the base station increases the
noise level at every mobile unit which is not the intended destination of
the signal. By using a parallel or series/parallel combination of nodes and a
plurality of receivers and transmitters, the signal handling capability of a
single base station can be increased.
To realize an increase in system capacity at a single base station, a
system is designed in which there are at least two independent arrays. In
this case, an independent array is defined as any set of nodes where each
node has a different delay with respect to the base station and where each
node in the system can belong to only one array. In this scheme there is no
disadvantage to having two nodes with the same delay with respect to the
base station as long as the two nodes belong to different independent arrays.
The independent arrays are placed such that there is some area which is
covered by only one independent array. Each independent array is supplied
a transmit signal from a dedicated transmitter and supplies a receive signal
to a dedicated receiver. When a mobile unit is positioned in the coverage
area of only one of the independent arrays, the transmitter corresponding to
the arrays to which the mobile is not communicating may stop transmitting
signals for that mobile station and thus reduce the interference to the other
mobile units. Likewise, when a mobile unit is positioned in the coverage
area of only one of the independent arrays, the receivers corresponding to
arrays to which the mobile is not communicating are free from interference
from that mobile unit. When a mobile unit is positioned in the coverage
area of two independent arrays, the two transmitters supply the same
information signal to the mobile unit but each transmitter uses a different
spreading sequence to modulate the information signal thereby increasing
the total signal received at the mobile unit while decreasing the chance of
destructive summation. Likewise when a mobile unit is positioned in the
coverage area of tivo independent arrays, the rivo receivers can individually
receive the signal and can then combine the energy from each path in the
demodulation process to provide a total increased signal level. (This
process is very similar to the process which is employed by base stations
comprised of multiple sectors in a standard CDMA cellular system.) Note
WO 95/06365 PCT/US94/09657
26
that the topology of Figure 9 comprising analog receiver 115 does not
function to increase the capacity of the base station 101 since receiver 115
receives an input from each node of the two parallel arrays shown.
An exemplary embodiment of this concept is shown in Figure 11
which is based upon the configuration of Figure 8. The first antenna array
comprised of nodes 920A' - 920N' is coupled analog transmitter 825 through
cable 927', adder 847, and splitter 830. The first antenna array is also
coupled
to transmitter 825 through cable 922', adder 845, delay 820, and sputter 830.
The second antenna array comprised of nodes 920A" - 920N" is coupled to
analog transmitter 825" through cable 927", adder 847", and splitter 830".
The second antenna array is also coupled to analog transmitter 835 through
cable 922", adder 845", delay 820", and splitter 830".
Use of the dual sets of parallel antennas affords advantages to the
system during general operation as well as reducing the worst case effects of
multipath fading. The coherent combining of the separate paths within the
base station increases the signal-to-noise ratio on the mobile unit-to-base
station link. The use of the present invention also decreases power control
fluctuations of the mobile unit. Both of these factors lead to higher capacity
and improved system performance. The advantages of the collocated
antennas are much greater than the advantages gained by merely uniformly
placing twice as many antennas in series.
There are many obvious variations of the embodiments of
Figures 2, 3, 5, 7, 8, 9, 10, and 11 including the simple rearrangement of
components within each element. Actual implementation of these
embodiments may require power splitting, gain, and filtering among other
functions. The previous description of the preferred embodiments is
provided to enable any person skilled in the art to make or use the present
invention. The various modifications to these embodiments will be readily
apparent to those skilled in the art, and the generic principles defined
herein may be applied to other embodiments without the use of the
inventive faculty. Thus, the present invention is not intended to be limited
to the embodiments shown herein but is to be accorded the widest scope
consistent with the principles and novel features disclosed herein.
WE CLAIM:
