WO1999017466A9

WO1999017466A9 - Using multiple antennas to mitigate specular reflection

Info

Publication number: WO1999017466A9
Application number: PCT/US1998/020076
Authority: WO
Inventors: James H Thompson; William R Panton; Mohammad Ali Tassoudji
Original assignee: Qualcomm Inc
Priority date: 1997-09-29
Filing date: 1998-09-25
Publication date: 1999-06-24
Also published as: WO1999017466A1; CN1115800C; JP2001518736A; CN1281603A; KR20010030790A; CA2303540A1; EP1020042A1; AU1185299A

Abstract

Apparatus and method for using multiple receive antennas (420A, 420B) in a satellite communication system (100) receiver (500, 700, 900) to mitigate the effects of specular reflection (202B, 402B) of a received signal (202A, 402A). The system includes first and second antennas (420A, 420B), for receiving a satellite communication signal along first and second direct (202A, 402A) and specular propagation paths (202B, 402B), respectively, and a digital maximal ration combiner (520) for combining the signals (202A, 202B, 402A, 402B, 420A, 420B) so as to maximize the signal-to-noise of the resultant combined signal.

Description

USING MULTIPLE ANTENNAS TO MITIGATE SPECULAR REFLECTION

BACKGROUND OF THE INVENTION

I. 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.

π. 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 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 2 to 2 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 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 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 demodulation. 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- 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 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 in 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 ψ, measured in degrees, for a receiver using a single antenna; FIG. 4a and 4b depicts the geometric relationships of FIGS. 2a and 2b 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; 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.

π. 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, 106B, 106C).

User terminals 106 each have or comprise a wireless communication device such as, but not limited to, a wireless telephone, although data transfer 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 102 A), 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 in 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 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 (2a, 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 ψ. 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 ψ. 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 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 ψ, 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 small 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 420 A 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 ψ.

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, 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, transverse electric or perpendicular polarization, the incident electric vector E is perpendicular to the plane of incidence and the reflection coefficient p_h is defined as the ratio of reflected to incident electric fields, or:

ήnψ+ lε_c-co - ψ

where 8c is the relative complex permitivity and Ψ 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 p_v is defined as the ratio of reflected to incident magnetic fields, or:

ε_r sin ψ- ε_^-cos^ ψ

P_v = ^C ,- = • (2) ε_c sm ψ+ ^c-cos-^ ψ

At the surface of a perfect conductor ε_c → ∞, p_h → -1, and p_v → 1.

For a left-hand circularly polarized (LCP) wave arriving at antenna 420B from a satellite 104A at an elevation angle of Ψ 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,„(ψ) = A(h+ jv) l sf2 (3)

τ E<_«(ψ _x) = - ^^ e jk(2hsmψ ^ψ) hp_h + j .^Λv p_v .) ( .4..)

where h and v are the horizontal and vertical unit vectors, respectively, and fc=2π/λ. The magnitude of the total electric field received by an antenna with a field gain function of g is given by the relationship:

^Etotal = ^Ψ) - ~E_inc(ψ)+g-(-ψ) . E_{ref ψ)} (5) Λ where g(ψ) i - = h+ g v and g(-ψ) = g_h h+ g_h v

Thus,

E jk(2hsinψ) total J∑ S h + Jg+ + (g_hP_h + Jg_v P_v )e (6)

This latter relationship can be divided into direct (E_d) and specular (E_s) terms for the respective components, which leads to:

total (7)

Note that the phase factor in the specular term causes Ε_total to vary as a function of Ψ. Because this variation has the shape of cos(λ:/zsinΨ), 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 420B, the total received electric field can be expressed as:

'total j£ (8)

This expression (8) can be divided into direct and specular terms according to the relationship:

E — _eM>>- ')s.mψ total (E_d + E_s) (9)

Note that the direct components have a phase difference of k(h - h')sin ψ which will cause cancellation when this quantity is equal to (2n+l)π. By vertically separating the two antennas by a predetermined distance h - h', SNR degradation can be minimized for a predetermined elevation angle Ψ_c, as shown by the relationship:

The predetermined elevation angle Ψ_c is selected according to various factors known in the relevant arts. In a preferred embodiment, the predetermined elevation angle Ψ_c is approximately 15°. 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. 5. 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 Ψ for Ψ_c=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 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 combiner 520 so as to maximize the signal-to-noise ratio of composite output signal 530. FIG. 8 presents a plot of normalized SNR versus elevation angle Ψ for Ψ_c=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 multipath 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. 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 using multiple antennas and digital combining. Referring again to FIG. 5, if the signals received by antennas 420 A and 420B are the same signal, the SNR 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 in 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:

Claims

1. A receiver in a satellite communication system, comprising: a first antenna for receiving a satellite communication signal along a first direct propagation path and a first specular propagation path; a second antenna, displaced from said first antenna by a predetermined distance, for receiving said satellite communication signal along a second direct propagation path and a second specular propagation path; and means for combining said signals received by said first and second antennas to maximize the signal-to-noise ratio of the resultant combined signal.

2. The receiver of claim 1, wherein said means for combining comprises means for weighting each signal based on the signal-to-noise ratio of that signal prior to combining.

3. The receiver of claim 2, further comprising: a signal delay unit having an input port and an output port, said input port electrically coupled to said second antenna; an analog combiner having two input ports and an output port, a first one of said input ports electrically coupled to said first antenna, a second one of said input ports electrically coupled to said output port of said signal delay unit; and searcher receiver means, electrically coupled to said output port of said analog combiner, for distinguishing said signal received by said first antenna from said signal received by said second antenna based on a time delay imposed by said signal delay unit.

4. The receiver of claim 3, wherein said communication signal is a code division spread spectrum type signal and the time delay is configured to be greater than one chip time.

5. The receiver of claim 4, further comprising analog receiver means electrically coupled between said analog combiner and said searcher receiver means.

6. The receiver of claim 5 further comprising digital receiver means electrically coupled to said analog receiver means, said searcher receiver means, and said means for combining.

7. The receiver of claim 1, wherein said second antenna is displaced vertically from said first antenna.

8. The receiver of claim 7, wherein said predetermined distance is selected to minimize interference between direct and specular components of said signal for a predetermined elevation angle.

9. The receiver of claim 8, wherein said predetermined elevation angle is approximately 15┬░.

10. A method for using multiple antennas to mitigate specular reflection in a satellite communication system, comprising the steps of: receiving a satellite communication signal along a first direct propagation path and a first specular propagation path at a first antenna; receiving said satellite communication signal along a second direct propagation path and a second specular propagation path at a second antenna, said second antenna displaced from said first antenna by a predetermined distance; and combining said signals received by said first and second antennas to maximize the signal-to-noise ratio of the resultant combined signal.

11. The method of claim 10, wherein said combining step comprises the step of weighting each signal based on the signal-to-noise ratio of that signal prior to combining.

12. The method of claim 11, further comprising the steps of: delaying said signal received at said second antenna by a predetermined time delay; combining said delayed signal and said signal received at said first antenna; and distinguishing said signal received by said first antenna from said signal received by said second antenna based on said predetermined time delay.

13. The method of claim 10, further comprising the step of displacing said second antenna vertically from said first antenna.

14. The method of claim 13, further comprising the step of selecting said predetermined distance to minimize interference between direct and specular components of said signal for a predetermined elevation angle.

15. The method of claim 14, wherein said predetermined elevation angle is approximately 15┬░.