Note: Descriptions are shown in the official language in which they were submitted.
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USING MULTIPLE ANTENNAS TO MITIGATE
SPECULAR REFLECTION
BACKGROUND OF THE INVENTION
L Field of the Invention
The present invention relates generally to diversity processing in
spread-spectrum satellite communication systems. More specifically, the
present invention relates to using multiple antennas to mitigate the effects
of specular reflection on signal reception.
II. Description of the Related Art
A typical satellite-based communication system comprises at least one
terrestrial base station, central station, or hub (hereinafter referred to as
a
gateway); at least one user terminal, remote station, or mobile station (for
example, a mobile telephone); and at least one satellite for relaying
communications signals between the gateway and the user terminal. The
gateway provides links from one or more user terminals to other user
terminals or linked communication systems, such as a terrestrial telephone
system.
A variety of multiple access communications systems have been
developed for transferring information among a large number of system
users. These techniques include time division multiple access (TDMA),
frequency division multiple access (FDMA), and code division multiple
access (CDMA) spread-spectrum techniques, the basics of which are well
known in the art. The use of CDMA techniques in a multiple access
communication system is disclosed in U.S. Patent No. 4,901,307, which
issued February 13, 1990, entitled "Spread Spectrum Multiple Access
Communication System Using Satellite Or Terrestrial Repeaters," and U.S.
Patent Application Serial No. 08/368,570, filed January 4, 1995, entitled
"Method And Apparatus For Using Full Spectrum Transmitted Power In A
Spread Spectrum Communication System For Tracking Individual
Recipient Phase Time and Energy," which are both assigned to the assignee
of the present invention, and are incorporated herein by reference.
The above-mentioned patent documents disclose multiple access
communication systems in which a large number of generally mobile or
remote system users employ user terminals to communicate with other
system users or users of other connected systems, such as a public telephone
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switching network. The user terminals communicate through gateways and
satellites using CDMA spread-spectrum type communication signals.
Communication satellites form beams which illuminate a "spot"
produced by projecting satellite communication signals onto the Earth's
surface. A typical satellite beam pattern for a spot comprises a number of
beams arranged in a predetermined coverage pattern. Typically, each beam
comprises a number of CDMA channels or so-called sub-beams, covering a
common geographic area, each occupying a different frequency band.
In a typical spread-spectrum communication system, a set of
preselected pseudorandom noise (PN) code sequences is used to modulate
{i.e., "spread") information signals over a predetermined spectral band prior
to modulation onto a carrier signal for transmission as communication
signals. PN spreading, a method of spread-spectrum transmission that is
well known in the art, produces a signal for transmission that has a
bandwidth much greater than that of the underlying data signal. In a
forward communication link (that is, in a communication link originating
at a gateway and terminating at a user terminal), PN spreading codes or
binary sequences are used to discriminate between signals transmitted by a
gateway over different beams, and their timing is used to discriminate
between multipath signals. These PN codes are shared by all
communication signals within a given beam, and typically consist of 28 to 2Is
code chips with preselected chip periods or chipping rates on the order of
1.22 Mhz, although other code lengths and rates are well known.
In a typical CDMA spread-spectrum system, channelizing codes are
used to discriminate between signals intended for particular user terminals
or wireless receivers transmitted within a beam or CDMA channel on the
forward link. That is, a unique orthogonal channel is provided for each user
terminal on the forward link by using a unique "channelizing" orthogonal
code. Walsh functions are generally used to implement the channelizing
codes, with a typical length being on the order of 64 code chips for
terrestrial
systems and 128 code chips for satellite systems. However, other types of
orthogonal functions can be employed as desired.
Typical CDMA spread-spectrum communication systems, such as
disclosed in U.S. Patent No. 4,901,307, contemplate the use of coherent
modulation and demodulation for forward link user terminal
communications. In communication systems using this approach, a "pilot"
carrier signal (hereinafter referred to as a "pilot signal") is used as a
coherent
phase reference for forward links. That is, a pilot signal, which contains no
data modulation, is transmitted by a gateway throughout a region of
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coverage. A single pilot signal is typically transmitted by each gateway for
each beam used for each frequency used, that is, CDMA channel. These pilot
signals are shared by all user terminals receiving signals from the gateway.
Pilot signals are used by user terminals to obtain initial system
synchronization and timing, frequency, and phase tracking of other signals
transmitted by the gateway. Phase information obtained from tracking a
pilot signal carrier is used as a carrier phase reference for coherent
demodulation of other system signals or traffic signals. Many traffic signals
can share a common pilot signal as a phase reference, providing for a less
costly and more efficient phase tracking mechanism.
When a user terminal is not involved in a communications session
(that is, the user terminal is not receiving or transmitting traffic signals),
the
gateway can convey information to that particular user terminal using a
signal known as a paging signal. For example, when a call has been placed to
a particular mobile phone, the gateway alerts the mobile phone by means of
a paging signal. Paging signals are also used to distribute traffic channel
assignments, access channel assignments, and certain system overhead
information.
For satellite systems, user terminal receivers generally experience a
reasonably low amount of signal degradation due to generalized multipath
signal reflections. Satellite signals tend to arrive at sufficiently steep
angles
so as to avoid many obstructions, mostly buildings, that create multipath
signals in terrestrial cellular systems. However, the user terminals are
susceptible to a problem or variety of multipath signals known as specular
reflection.
Specular reflection occurs when a component of a received signal is
scattered from a surface, such as the ground, at an angle equal to the
incident
angle of the received signal. The characteristics of the reflected component
(termed the "specular" component) are a function of the incident angle and
the electrical properties, roughness, and homogeneity of the impinging
surface. Specular reflection occurs a significant amount of the time for
many satellite systems. The scattered signals are directed into receivers or
receive antennas positioned just above ground level.
For a receiver antenna at a certain height above the ground the
specular component can add with the direct signal component to cause
significant degradation in the signal level. Due to phase variations between
the direct and specular signal components, they can destructively or
constructively interfere with each other. This can result in a large
oscillation in the signal level or energy, as a user terminal moves or as the
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satellite changes elevation angle relative to the receiver antenna (as in low
earth orbiting satellite systems). In addition, the signal level may also fall
below a required level for adequate reception or modulation. In the case of
pilot signals, they may not function properly as phase references, also
preventing proper signal reception or demoduiation. In the case of paging
signals, necessary information may not be imparted to allow a user terminal
to detect incoming calls or to select proper access channels:
A typical satellite signal receive antenna exhibits gain which decreases
or "rolls off' in value as the elevation angle for received signals approaches
zero and goes negative, or below the local horizon for the antenna. Specular
radiation is reflected by the ground, or other smooth surface, and enters or
is
intercepted by the antenna at negative elevation, and, therefore, lower gain
being applied.
If the antenna gain experienced by a direct signal is much larger than
for the specular reflection, then the direct signal strength dominates and
little or no degradation is observed. This is generally the situation for
direct
signals received at higher elevation angles. They are arriving in a higher
gain region for the antenna while the specular component arrives in the
negative gain region. However, antenna gain typically decreases slowly with
lower elevation. Therefore, direct and specular signals received at a lower
elevation experience similar gain. In this situation, the specular radiation
is
closer to the direct signal in strength and causes greater interference and
signal degradation. That is, interference from specular radiation tends to be
more significant for lower elevation angles.
What is needed is a way to mitigate the effects of specular reflection,
and maintain or improve communication signal reception, especially for
satellite communication systems.
SUMMARY OF THE INVENTION
The present invention is a system and method for using multiple
antennas in a satellite communication system receiver to mitigate the effects
of specular reflection. In one embodiment of the present invention, the
system includes a first antenna for receiving a satellite communication
signal along first direct and specular propagation paths; a second antenna,
displaced from the first antenna by a predetermined distance, for receiving
the satellite communication signal along second direct and specular
propagation paths; and combiner or means for combining the signals
received by the first and second antennas so as to maximize the signal-to-
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noise ratio of the resultant combined signal. The means for combining can
be a digital maximal ratio combiner that weights each signal based on its
individual signal-to-noise ratio prior to combining with the other signal.
The present invention can be extended to use more than two receive
5 antennas, as would be apparent to one skilled in the relevant art.
In one embodiment, the received signals are reduced to digital
baseband signals prior to combining. In an alternative embodiment, the
received signals are first combined at either RF or IF frequency. A time delay
is introduced into one of the signals prior to this initial combining. A rake
receiver is used to distinguish the signals received by the first and second
antennas at baseband, based on the time delay imposed. The baseband
digital signals are then combined so as to maximize the signal-to-noise ratio.
One advantage of the present invention is that it permits the signal
to-noise ratio of a received signal to be increased, in the absence of
multipath
signals.
Another advantage of the present invention is that it permits
mitigation of path blockages and multipath fading for vehicle-mounted
receivers.
BRIEF DESCRIPTION OF THE DRAWINGS
The features and advantages of the present invention will become
more apparent from the detailed description set forth below when taken i n
conjunction with the drawings in which like reference numbers indicate
identical or functionally similar elements. Additionally, the left-most digit
of a reference number identifies the drawing in which the reference number
first appears.
FIG.1 depicts a typical satellite communication system;
FIGS. 2a and 2b depicts the general geometric relationship between the
direct and specular components of a forward link satellite communication
signal;
FIG.3 depicts a plot for normalized signal-to-noise ratio (SNR),
measured in dB, versus elevation angle yl, measured in degrees, for a
receiver using a single antenna;
FIG. 4a and 4b depicts the geometric relationships of FIGS. 2a and Zb
for a two-antenna assembly;
FIG.5 depicts a circuit block diagram of a user terminal receiver
suitable for implementing one embodiment of the present invention;
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FIG. 6 presents a plot of normalized SNR versus elevation angle for
the embodiment of FIG. 5;
FIG.7 depicts a circuit block diagram of a receiver suitable for
implementing an alternative embodiment of the present invention;
FIG. 8 presents a plot of normalized SNR versus elevation angle for
an alternative embodiment of the present invention; and
FIG.9 depicts a circuit block diagram of a receiver suitable for
implementing another alternative embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED
EMBODIMENTS
I. Introduction
The present invention is an apparatus and method for using multiple
antennas in a satellite communication system receiver to mitigate the effects
of specular reflection. The preferred embodiment of the invention is
discussed in detail below. While specific steps, configurations and
arrangements are discussed, it should be understood that this is done for
illustrative purposes only. A person skilled in the relevant art will
recognize that other steps, configurations and arrangements can be used
without departing from the spirit and scope of the present invention.
The present invention will be described in five parts. First, a typical
satellite communication system is described. Second, the characteristics of
specular reflection are explained. Third, a digital combining solution is
presented. Fourth, an analog combining solution is presented. Finally,
other applications of the present invention are described.
II. A Typical Satellite Communication System
An exemplary wireless communication system 100 using satellites
and gateways or base stations is depicted in FIG.1. In a preferred
embodiment, communication system 100 is a CDMA spread spectrum
satellite communication system, but this is not required by the present
invention. Communication system 100 comprises one or more gateways 102
(102A,102B), satellites 104 (104A, 104B), and user terminals 106 (106A, I06B,
106C).
User terminals 106 each have or comprise a wireless communication device
such as, but not limited to, a wireless telephone, although data transfer
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devices (e.g., portable computers, personal data assistants, modems) are also
contemplated. User terminals 106 are generally of three types: portable user
terminals 106A, which are typically hand-held; mobile user terminals 106B,
which are typically mounted in vehicles; and fixed user terminals 106C
which are typically mounted in or on permanent structures. User terminals
are also sometimes referred to as subscriber units, mobile stations, or simply
"users" or "subscribers" in some communication systems, depending on
preference.
Gateways 102 (here 102A), also referred to as base stations, hubs, or
fixed stations in various systems, communicate with user terminals 106
through satellites 104 (104A and/or 104B). Generally, multiple satellites are
employed traversing different orbital planes such as in, but not limited to,
Low Earth Orbit (LEO) or Medium Earth Orbit (MEO). However, those
skilled in the art will readily understand how the present invention is
applicable to a variety of satellite system, gateway, or base station
configurations, or other moving non-satellite signal sources. Terrestrial
base stations 108 (also referred to as cell-sites or -stations) could be used
i n
some systems to communicate directly with user terminals 106. Typically,
such base stations (108) and satellites/gateways are components of separate
communication systems, referred to as being terrestrial and satellite based,
although this is not necessary. The total number of base stations, gateways,
and satellites in such systems depend on desired system capacity and other
factors well understood in the art. Gateways and base stations may also be
connected to one or more system controllers which provide them with
system-wide control or information, and can be connected to a public
switched telephone network (PSTN).
III. Specular Reflection
In a typical satellite communication system, the forward link (that is,
the communication link originating at satellite 104 and terminating at a user
terminal 106) typically experiences fading that is characterized as Rician.
Accordingly, the received signal consists of a direct component summed
with a multiply-reflected component having Rayleigh fading characteristics.
The power ratio between the direct and reflected components is typically on
the order of 6 to 10 dB, depending upon the characteristics of the user
terminal antennae and the environment surrounding the user terminal.
Significant degradation in the received signals and a resulting decrease in a
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user terminal receiver performance is caused by destructive interference
between such multipath signals.
Various approaches have been developed to mitigate the destructive
effects of the multiply reflected signal components. One such approach is
disclosed in commonly owned U.S. Patent No. 5,109,390 entitled "Diversity
Receiver In A CDMA Cellular Telephone System," issued April 28, 1992, the
disclosure of which is incorporated herein by reference. That patent
discloses a diversity receiver, also known as a "rake" receiver, for resisting
signal fading by coherently combining components of a multipath signal.
One particularly destructive multipath component, known as
"specular" reflection, is a multipath component reflected from the ground.
The general geometric relationship between the direct and specular
components of a forward link satellite communication signal are depicted in
FIG. 2 (2a and 2b). The relative angles of incidence and reflection are
exaggerated in size within FIG. 2 for purposes of illustrating the nature of
signal interaction and the problem being addressed.
In FIG. 2a, a portable user terminal 106A is equipped with an antenna
220, and in FIG. 2b, a mobile user terminal 106B is equipped with antenna
220. It will be readily apparent to those skilled in the art that the relative
vertical height and positions of user terminals changes from application to
application and terminal to terminal, and that the figures (Za, 2b, 4a, and
4b)
use a common label for the elevation angle for purposes of convenience in
discussion only, and not by way of limitation or to indicate that they are
identical in all applications.
Antenna 220 (both 2a and 2b) receives a direct signal component 202A
along a direct propagation path from satellite 104A. Antenna 220 also
receives a specular signal component 202B reflected from a large, relatively
smooth (at the frequencies of interest), planar object 204 such as the surface
of the Earth, location or area 206. Satellite 104A is at an elevation angle
yr.
Because signals being received by user terminals in such a system originate
at such great distances from the antenna, direct signal component 202A and
a direct component 202C to reflection spot 206 are nearly parallel. That is
the
two direct components are separated by an extremely small offset or angle.
The resulting incident and reflected angles for component 202C and specular
component 202B are both approximately fir. That is, the direct and specular
signal components are nearly parallel.
Interference between direct and specular components of the
communication signal can cause significant degradation. Depending on the
radiation pattern of receive antenna 220, such degradation can exceed a
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signal loss value of 6 dB. In low-power satellite communication systems,
such as currently planned spread spectrum systems, such degradation can be
very significant.
The effects of specular reflection are presented with reference to
graphical plots depicting the results of computer simulations. FIG. 3 depicts
such a plot for normalized signal-to-noise ratio (SNR), measured in dB,
versus elevation angle yr, measured in degrees. The plot depicts two curves:
E_direct and E total. The E_direct curve, depicted as a solid line, represents
the magnitude of the electric field of the direct component 202A of the
forward link communication signal at antenna 220. The E_total curve,
depicted as a dash-dot line, represents the magnitude of the total electric
field, including direct and specular components, at antenna 220. As shown
by the E_total curve, the SNR degradation caused by the specular
component at low elevation angles is significant.
One approach to addressing this problem is to design an antenna that
has high gain at positive elevation angles and low gain at negative elevation
angles. Unfortunately, it is impractical to design a smali antenna with large
gain variations, as would be required for handheld or mobile wireless
devices, such as telephones. However, the inventors have found that, by
using two antennas vertically displaced by a known distance, and by
combining the signals received by the two antennas as described below, SNR
degradation caused by specular reflection at low elevation angles can be
mitigated.
FIGS. 4a and 4b depict the geometric relationships of FIGS. 2a and 2b,
respectively, but using a two-element antenna assembly or system. In
FIGS. 4a and 4b, the single antennas 220 of user terminals 106A and 106B
have been replaced by an antennas having two elements, 420A and 420B.
Element 420A is at a height h' above the ground 204, and antenna 420B is at
a height h above the ground 204. Antenna 420A receives a direct signal
component 402A and specular component 402B of the forward link
communication signal originating at satellite 104A as signal 402C, which
relfects at spot 206'. Antenna 420B receives a direct component 202A and a
specular component 202B of the forward link communication signal
originating at satellite 104A. As before, the incident and reflected angles of
specular components 202B and 402B (which are substantially parallel)
arriving from satellite 104A are approximately t~.
Although antennas 420A and 420B are shown as two elements within
the same antenna structure, other configurations are possible without
departing from the spirit and scope of the present invention. For example,
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the antennas can be mounted on two spaced apart separate supports or
masts, as long as the desired vertical height relationship is maintained.
The specular components can be characterized using horizontal and
vertical components with different reflection coefficients. For horizontal,
5 transverse electric or perpendicular polarization, the incident electric
vector
E is perpendicular to the plane of incidence and the reflection coefficient ph
is defined as the ratio of reflected to incident electric fields, or:
p sinyr-~/E~-cos2 yi (1)
h simy+~/P~ cost yi
where ~ is the relative complex permitivity and W is the elevation angle.
For vertical, transverse magnetic or parallel polarization, the incident E
vector is parallel to the plane of incidence and the reflection coefficient pv
is
defined as the ratio of reflected to incident magnetic fields, or:
~~ sin yi- ~E~-cost W
Py = . (2)
E~sinty+~~~-cos2 yi
At the surface of a perfect conductor ~ -> ~, ph -> -1, and pv ~ 1.
For a left-hand circularly polarized (LCP) wave arriving at antenna
420B from a satellite 104A at an elevation angle of V~ and an antenna height
of h, the vectors for the incident and reflected electric fields at the
receiver
antenna are given by the relationships:
E~(Yi)=A(h+.Iv)~~2 (3)
E~ ( ~V ) _ ~ e~k~2hsin yi) (h ph + ~ v pv) (4)
where h and v are the horizontal and vertical unit vectors, respectively, and
k=2n/~,. The magnitude of the total electric field received by an antenna
with a field gain function of g is given by the relationship:
E~~ =8(Y~)~E~(V~)+S(-~V)~Eref(Y~) . (5)
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where g(yi}= gh h+ gh v and g(-yr} = gh h+ gh v .
Thus,
Etotal = ~'[gh + Jgv + (gh ph + .Igv Pv )e~k(2hsin ty}~ (6)
This latter relationship can be divided into direct (Ed) and specular (ES)
terms
for the respective components, which leads to:
Etotal = Ed + ES {7}
Note that the phase factor in the specular term causes E total to vary as a
function of W. Because this variation has the shape of cos(khsinVl), the
number of oscillations in E-total decreases as the elevation angle increases.
For a second antenna 420A positioned at a height h' with the same
gain function as first antenna 4208, the total received electric field can be
expressed as:
jk(h-H)ainyrC + + - jk(2h'sinyi}~
Etotal = ~ a gh + ~gv + (gh Ph + ~gv pv )e {8)
This expression (8} can be divided into direct and specular terms according to
the relationship:
=eJk(h-h~)any~ ~,d +~'s
total ~ } {
Note that the direct components have a phase difference of k(h - h')sin yr
which will cause cancellation when this quantity is equal to (2n+1)~. By
vertically separating the two antennas by a predetermined distance h - h',
SNR degradation can be minimized for a predetermined elevation angle Vl~,
as shown by the relationship:
2n+1
yi~ = sin-' 4 ,-hl~ ~ (10)
The predetermined elevation angle W~ is selected according to various
factors known in the relevant arts. In a preferred embodiment, the
predetermined elevation angle W~ is approximately 15°.
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IV. Digital Combining Solution
In one embodiment of the invention, the signal received by each
antenna is converted to a digital baseband signal before combining. A circuit
block diagram of a user terminal receiver 500 suitable for implementing this
embodiment of the invention is depicted in FIG. S. Here, receiver 500
includes two antennas 420A and 420B for receiving communication signals
from one or more satellites 104 (104A, 104B), and preferably uses a separate
receiver chain for each antenna. Receiver system 500 can include more than
two antennas and receiver chains, as would be apparent to one skilled in the
relevant art.
Receiver 500 also includes a digital maximal ratio combiner 520 for
combining the digital signals produced by each receiver chain to produce a
combined output signal 530. Output signal 530 is a digital data signal, which
can be transferred to vocoders and other known circuits or devices for
further processing, as would be apparent to one skilled in the relevant art.
Digital maximal ratio combiner 520 weights each digital signal based on its
SNR, prior to combining signals so as to maximize the SNR of output signal
530.
Each receiver chain includes a low noise amplifier (LNA) 504, a mixer
506, an analog receiver 508, and a digital receiver 510. Mixer 506 combines
the amplified signal produced by LNA 504 with a local oscillator signal to
downconvert the received signal from RF to IF frequencies. Analog receiver
508 includes a downconverter to reduce the frequency of the IF signal to
baseband. Analog receiver 508 also includes an analog-to-digital converter
to convert the analog baseband signal to a digital signal. Digital receiver
510
despreads and demodulates the digital signal, as necessary, and provides
error correction and other known signal processing operations. The output
of digital receiver 510 is a digital data signal. The digital data signals
produced by digital receivers 510 are then coherently combined by digital
maximal ratio combiner 520 so as to maximize the signal-to-noise ratio of
composite output signal 530.
FIG. 6 presents a plot of normalized SNR versus elevation angle W for
Wc=15° for two curves: E single and E_combined. The E single
curve,
represented by the dotted line in FIG. 6, represents the magnitude of the
total
received electric field for a single-antenna case, and corresponds to the
E_total curve of FIG. 3. The E combined curve, represented by a solid line
in FIG. 6, represents the magnitude of the electric field for the digitally
combined solution. As is apparent from the plot, the digital combining
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solution results in a significant SNR gain, not only at the predetermined
elevation angle of 15°, but also over the entire range of elevation
angles.
V . Analog Combining Solution
In an alternative embodiment of the present invention, the signals
received by. antennas 420 are initially combined prior to downconverting, so
only one receiver chain is needed. A time delay is imposed on one of the
received signals prior to initial combining so that the received signals can
be
distinguished by a rake receiver. The two digital data signals produced by
the rake receiver are then combined by a digital maximal ratio combiner, as
in the digital combining solution.
FIG. 7 depicts a circuit block diagram of a receiver 700 suitable for
implementing this embodiment. Receiver 700 includes a delay unit 712, a
combiner 714, a searcher receiver 716, digital receivers 510 and a digital
maximal ratio combiner 520. Searcher receiver 716 and digital receivers 510
form a rake receiver, such as that disclosed in commonly owned U.S. Patent
No. 5,109,390 entitled "Diversity Receiver In A CDMA Cellular Telephone
System," issued April 28, 1992, the disclosure of which is incorporated
herein by reference.
Delay unit 712 imposes a time delay on the signal received by antenna
420B so that the signals received by antennas 420A and 420B can be
distinguished by the rake receiver. In a preferred embodiment, the
magnitude of the time delay is greater than one chip time. A similar delay
approach is disclosed in commonly owned co-pending application Serial No.
08/855,242 (Attorney docket No. PA415) filed May 13, 1997, entitled
"Multiple Antenna Detecting And Selecting," which is incorporated herein
by reference.
Combiner 714 combines the two received signals, in a manner that
would be apparent to one skilled in the relevant art. Mixer 506
downconverts the combined signal, as described above. Analog receiver 508
downconverts the IF signal to a digital baseband signal, also as described
above. Searcher receiver 716 distinguishes the signals received by the two
antennas based on the time delay and passes each signal to a different digital
receiver 510. Digital receivers 510 despread and demodulate the received
signals, and the like, as described above. The digital data signals produced
by
digital receivers 510 are then coherently combined by digital maximal ratio
cornbiner 520 so as to maximize the signal-to-noise ratio of composite
output signal 530.
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FIG. 8 presents a plot of normalized SNR versus elevation angle W for
W~=15° for two curves: E_single and E combined. The E_single
curve,
represented by the dotted line in FIG. 8, represents the magnitude of the
total
received electric field for a single-antenna case, and corresponds to the
E_total curve of FIG. 3. The E combined curve, represented by a solid line
in FIG.6, represents the magnitude of the electric field for the analog
combining solution of the present invention. As is apparent from the plot,
the analog combining solution also results in SNR gains.
In an alternative implementation of the analog combining
embodiment, the received signals can . be delayed and combined after
downconverting, as shown in FIG. 9. This permits a delay unit 912 and
combiner 714 to be implemented at intermediate frequencies, rather than at
the higher communication signal RF frequencies. Since such elements
which are more easily manufactured, this results in significant cost
reductions in. As would be apparent to one skilled in the relevant art, other
implementations are possible without departing from the spirit and scope of
the present invention.
VI. Other Applications
Application of the present invention is not limited to mitigation of
the effects of specular reflection. Embodiments of the present invention are
also well suited to at least two alternative applications, which are described
below.
In one embodiment, the present invention is used to mitigate path
blockages and multipath fading for vehicle-mounted user terminals, such as
mobile user terminal 106B. Portable and mobile user terminals sometimes
encounter path blockage due to structures and foliage nearby. These
blockages become time-varying when the user terminal is in motion.
Similarly, there may be situations in which muitipath signals are generated
from structures or foliage.
In one embodiment of the present invention, receive antennas are
positioned on the vehicle such that the shadowed area due to small or thin
obstructions does not encompass all antennas simultaneously. Similarly,
the likelihood of destructive multipath interference at all of the antennas
simultaneously is less than the likelihood of destructive multipath
interference at a single antenna.
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In another embodiment, the digital combining solution of the present
invention is used to improve signal-to-noise ratios (SNR) for non-
multipath signals in an environment without multipath interference. Even
in the absence of multipath signals, receiver performance can be improved
5 using multiple antennas and digital combining. Referring again to FIG. 5, if
the signals received by antennas 420A and 420B are the same signal, the S N R
of output signal 530 is twice that of the single-antenna case. This principle
can be extended to larger numbers of antenna elements, as would be
apparent to one skilled in the relevant arts.
VII. Conclusion
While various embodiments of the present invention have been
described above, it should be understood that they have been presented by
way of example, and not limitation. It will be apparent to persons skilled i n
the relevant art that various changes in form and detail can be made therein
without departing from the spirit and scope of the invention. Thus the
present invention should not be limited by any of the above-described
exemplary embodiments, but should be defined only in accordance with the
following claims and their equivalents.
What we claim as our invention is:
