WO2001013543A1

WO2001013543A1 - Satellite communication system using wide fixed beams and narrow steerable beams

Info

Publication number: WO2001013543A1
Application number: PCT/US2000/022732
Authority: WO
Inventors: Leonard N. Schiff
Original assignee: Qualcomm Incorporated
Priority date: 1999-08-19
Filing date: 2000-08-16
Publication date: 2001-02-22
Also published as: AU6916500A

Abstract

A system and method for adjusting a phased array antenna (408) on a satellite (102) for satellite-to-ground communications to produce composite beam (302) having a wide beam (602) for transmitting signals to all users within the service region, and one or more narrow, high gain beams (604A, 604B) for transmitting signals to (and receiving signals from) areas of concentrated user traffic. This composite beam (302) can be formed by adjusting a first set of phased array coefficients (702) to produce a wide beam (602), and then adjusting one or more additional sets of phased array coefficients (704) to create the narrow beams (604A, 604B). Using narrow, high gain beams to illuminate areas of concentrated user traffic reduces satellite power whenever an appreciable fraction of the total beam traffic resides within the concentrated areas. This reduction is achieved because the power required to handle the traffic in these concentrated areas is drastically reduced.

Description

SATELLITE COMMUNICATION SYSTEM USING WIDE FIXED BEAMS AND NARROW STEERABLE BEAMS

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates generally to satellite communications, and more particularly to using a satellite antenna to produce wide fixed beams and narrow steerable beams for satellite-to-ground communications.

II. Related Art

Today, competing satellite communication systems offer mobile and fixed satellite-based voice and data services that reach the most remote areas of the world at a competitive cost. Users having specially equipped communication devices communicate directly with a satellite orbiting overhead, rather than, for example, with a terrestrial base station in a conventional cellular network. The satellite (and in some cases, more than one satellite) receives, amplifies, and retransmits these signals to an Earth station, referred to herein as a gateway or hub. The gateway then transfers the information to an existing communications network (for example, a Public Switched Telephone Network (PSTN), a Public Land Mobile Network (PLMN), or another gateway for transfer to other signal recipients. Similarly, calls originating from an existing communications network pass through the gateway, up to the satellite, and then down to the destination user. Networks of satellites orbiting the earth can provide coverage for vast geographical areas.

Satellite communication systems provide economical long distance transmission (cost is not necessarily a function of distance), broad service coverage, freedom from man-made restrictions or natural barriers such as geopolitical boundaries, capability of covering undeveloped areas without having to add a costly land telephone infrastructure, and new service capabilities such as position location.

As these satellite communication systems gain users even more users want to make use of such systems and capacity becomes an increasingly important issue. Existing systems employ one or more channelization techniques to increase capacity, such as frequency division multiple access (FDMA), code division multiple access (CDMA), time division multiple access (TDMA), spatial channelization (sometimes called frequency reuse), and polarization multiplexing schemes. However, future capacity requirements could demand even more system access than these conventional techniques can provide.

Spatial channelization can be employed to divide the terrestrial area serviced by a satellite, referred to herein as the satellite's footprint, into two or more service regions. Spatial channelization is made possible by various techniques for producing beams, that is, the directional transmission of electromagnetic energy. A beam is formed for each region, such that most of the energy transmitted using a beam illuminates the ground within the service region. This allows a small set of frequencies to be reused in known patterns across the footprint. CDMA communications systems allow the same set of frequencies to be used in adjacent regions. It is understood that similar techniques are used for receiving signals that are transmitted by users within the service region at the satellite.

A description of satellite communications systems employing code division multiple access (CDMA) spread-spectrum signals, 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 ' and U. S Patent No. 5,691,974, which issued November 25, 1997, entitled "Method and Apparatus for Using Full Spectrum Transmitted Power in a Spread Spectrum Communication System for Tracking Individual Recipient Phase Time and Energy," both of which are assigned to the assignee of the present invention, and are incorporated herein by reference. On the forward link, information is transmitted from a gateway to a user terminal over one or more beams. These beams often comprise a number of so-called subbeams (also referred to as frequency division multiple access (FDMA) channels, or CDMA channels) covering a common geographic area, each occupying a different frequency band. More specifically, in a conventional spread-spectrum communication system, one or more preselected pseudorandom noise (PN) code sequences are used to modulate or "spread" user information signals over a predetermined spectral band prior to modulation onto a carrier signal for transmission as communication signals. PN spreading is a method of spread-spectrum transmission that is well known in the art, and produces a communication signal with a bandwidth much greater than that of the data signal. On the forward link, PN spreading codes are generally shared by all communication signals within a given subbeam. In a conventional CDMA spread-spectrum communication system,

"channelizing" codes are used to discriminate between different user terminals within a satellite sub-beam on a forward link. The channelizing codes form orthogonal channels in a subbeam over which communication signals are transferred. That is, each user terminal has its own orthogonal channel provided on the forward link by using a unique channelizing orthogonal code. Walsh functions are generally used to implement the channelizing codes, also known as Walsh codes or Walsh sequences, and create what are known as Walsh channels. A typical orthogonal code length is 64 code chips for terrestrial systems and 128 code chips for satellite systems. Existing spatial channelization schemes for satellite communication systems employ beams designed to provide a substantially isoflux pattern on the surface of the Earth within the service region. The isoflux pattern is obtained by shaping the beam so that the gain at the edge of coverage is higher than at the beam center. This compensates for the difference in losses due to a roll-off in signal power at the beam edges due to the curvature of the Earth's surface, and longer distances from the source at beam edges. As a result of the isoflux patterns, users within a given service region receive satellite transmissions having approximately the same power regardless of position within the region.

However, using an isoflux pattern for satellite transmission and reception operates most efficiently where the users are uniformly distributed across the service region. While this may be the case in some regions, very often user communication traffic tends to be concentrated in relatively small areas during certain periods of the day based on certain societal activities. For instance, service regions that include heavily populated urban areas may experience heavy user traffic in these areas near certain roads or business centers during morning and evening commute hours when users make calls from their vehicles. User traffic density patterns, therefore, vary by both time of day and relative location. As a result, satellite communication systems that use time invariant, isoflux antenna beam configurations often operate in an inefficient manner, wasting power or failing to have sufficient resources to service some users.

A need, therefore, exists for an improved system and method for producing an antenna beam pattern for satellite-to-ground communications that more accurately reflects user traffic distributions and system access patterns.

SUMMARY OF THE INVENTION

A first preferred embodiment of the present invention is directed to a system and method for adjusting the radiation or beam patterns of a beamforming antenna, such as a phased array antenna, on a satellite used to communicate with terrestrial user terminals. A first plurality of phased array coefficients is adjusted to produce a wide beam. A second plurality of phased array coefficients is adjusted to produce at least one pencil beam, where the pencil beam illuminates a hot spot within the wide beam. The first and second plurality of phased array coefficients are then applied to the phased array antenna. For the forward link communications, the feeder link is demultiplexed into one or more channels and one or more subchannels. For each channel, a plurality of phased array coefficients is adjusted to produce a wide beam. For each subchannel, a plurality of phased array coefficients is adjusted to produce a pencil beam that illuminates a hot spot within at least one of the wide beams. The phased array coefficients are all then applied to the phased array antenna.

For the reverse link communications, one or more channels, beams, transmitted from the ground to the satellite are multiplexed into a single reverse feeder link. For each of these channels, a first plurality of phased array coefficients is adjusted to produce a wide receiving beam. A second plurality of phased array coefficients is adjusted to produce at least one pencil receiving beam, where each of the pencil beams illuminates a hot spot within the wide beam. The phased array coefficients for each of said channels are then applied to the phased array antenna.

In an alternative embodiment, a plurality of phased array coefficients is adjusted to produce a composite beam having a wide beam and one or more pencil beams, where each of the pencil beams illuminates a hot spot within the wide beam. The plurality of phased array coefficients is then applied to the phased array antenna.

For the forward link communications, a forward feeder link is transmitted from ground to satellite. The feeder link is demultiplexed into one or more channels. For each of these channels, a plurality of phased array coefficients is adjusted to produce the composite beam having a wide beam and one or more pencil beams, where each of the pencil beams illuminates a hot spot within the wide beam. The phased array coefficients for each of said channels are then applied to the phased array antenna.

For the reverse link communications, one or more channels, beams, transmitted from the ground to the satellite are multiplexed into a single reverse feeder link. For each of these channels, a plurality of phased array coefficients is adjusted to produce the composite receiving beam having a wide beam and one or more pencil beams, where each of the pencil beams illuminates a hot spot within the wide beam. The phased array coefficients for each of said channels are then applied to the phased array antenna.

Using narrow, high gain beams (referred to herein as "pencil beams") to illuminate areas of concentrated user traffic reduces satellite power whenever an appreciable fraction of the total beam traffic resides within the concentrated areas. This reduction is achieved because the power required to handle the traffic in these concentrated areas is drastically reduced.

The number of pencil beams can be dynamic and vary over time to account for the time varying nature of user communication traffic density. Pencil beams can be created and dropped or de-activated as areas of concentrated user traffic appear and disappear. The present invention, therefore, provides for efficient satellite-to-ground communication for a wide variety of service regions.

In addition, while the wide beams move over the Earth's surface as the satellite moves in orbit, for non-geosynchronous satellites, the narrow beams can be made to service a substantially fixed area by constantly, or at least periodically, changing the phased array coefficients.

The shape of each pencil beam can be matched, up to limits determined by the number of available antenna elements, system characteristics, or antenna design, to the area of concentrated user traffic that it illuminates or is desired to cover. Further, the gain of each pencil beam can be matched to the user traffic density. This feature further increases satellite- to-ground communication efficiency.

Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described with reference to the accompanying drawings. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.

FIG. 1 depicts a satellite communications environment within which the present invention is used; FIG. 2 depicts a frequency plan for the forward direction feeder link;

FIG. 3A depicts a preferred spatial channelization of the forward satellite user link having a footprint divided into beams;

FIG. 3B depicts a preferred spatial channelization of the reverse satellite user link having a footprint divided into beams FIG. 4 depicts forward direction satellite transponder apparatus according to a first preferred embodiment of the present invention;

FIG. 5 depicts an example beam in greater detail, including two example hot spots;

FIG. 6A depicts a contour plot of a beam, where the interior lines denote contours of substantially constant gain;

FIG. 6B depicts the beam of FIG. 6A projected along an x-axis;

FIG. 7 depicts a flowchart that describes a preferred method of producing a composite beam according to the present invention;

FIG. 8 depicts an example beamformer; FIG. 9A depicts an example phased array having time invariant coefficients;

FIG. 9B depicts an example phased array having time variant coefficients;

FIG. 10 depicts a flowchart that describes an alternative method of producing a composite beam according to the present invention;

FIG. 11 depicts a forward direction satellite transponder according to a second preferred embodiment of the present invention; and

FIG. 12 depicts a hot spot moving in relation to two contiguous beams. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

I. Overview of the Environment

Briefly stated, the present invention is directed to a system and method for adjusting beam patterns produced by a phased array antenna on a satellite for satellite-to-ground communications. A composite beam is produced having a wide beam for transmitting signals to (and receiving signals from) users within the service region with substantially uniform power, and one or more narrow, high gain beams for transmitting signals to (and receiving signals from) areas of concentrated user traffic. This composite beam can be formed by adjusting a first set of phased array coefficients to produce a wide beam, and then adjusting one or more additional sets of phased array coefficients to produce the narrow beams.

FIG. 1 depicts a satellite communications environment 100 within which the present invention is useful. A satellite 102, in orbit around Earth 104, communicates with a user terminal 106 using a user link 110. Satellite 102 communicates with user terminals 106 within a footprint 114, using a transmit antenna 116 and a receive antenna 118. Satellite 102 also communicates with a gateway 108 via a feeder link 112.

Satellite 102 is preferably a simple, low cost satellite designed to minimize both production costs and launch costs. Satellite 102 is preferably in a low earth orbit, which permits communication with low power user terminals 106 (for example, wireless devices such as cellular phones). However, those skilled in the art will recognize that the principles described herein apply to satellites of varying sophistication and orbit, as well as a variety of wireless devices.

Transmit antenna 116 is generally a phased array antenna that is controlled by adjusting one or more sets of phased array coefficients, where each coefficient has a phase component. When reduced costs or complexity are not a concern, the phased array coefficients may also have a gain component, although not required. Receive antenna 118 is also a phased array antenna controlled by one or more sets of phased array coefficients. In a preferred embodiment, transmit antenna 116 and receive antenna 118 are implemented as two separate two-dimensional antenna arrays. However, those skilled in the art will recognize that, with the appropriate switching, alternative embodiments could employ the same antenna elements for both transmitting and receiving signals. Other alternative embodiments could employ one-dimensional antenna arrays, or three-dimensional antenna arrays, as desired. User terminal 106 can represent many different communication devices such as, but not limited to, a wireless telephone, a data transceiver, or a paging or position determination receiver, and can be hand-held, mobile or vehicle mounted, or fixed station units, as desired. A typical hand held unit is similar in design to a conventional cellular phone. In a preferred embodiment, the hand held unit can communicate with terrestrial cellular networks as well as with satellite 102. A typical mobile user terminal includes a hand set and a car kit. The car kit provides battery power, a higher RF power output and a higher gain antenna. Fixed station units communicate with satellite 102, but typically not with other terrestrial cellular networks. The fixed station units are generally used to serve areas that are not served by terrestrial cellular or wireline networks. They are fixed installations with a primary power source, higher RF power and a fixed high gain antenna.

Gateway 108 connects user terminal 106 through satellite 102 to other terrestrial communication networks (for example, cellular systems, conventional phone networks, satellite systems) and to other gateways. For example, gateway 108 receives telephone calls from terrestrial switching equipment (not shown) and transmits the calls to the appropriate user terminals 106 using feeder link 112 and user link 110. In the return direction, user terminal 106 transmits to gateway 108 through user link 110 and feeder link 112. Gateway 108 then connects the communication link or call to the terrestrial switching equipment which can then connect to other desired signal recipients through the standard telephone system. Connections can also be made to terrestrial cellular subscribers or to another user terminal 106. In a preferred embodiment, gateway 108 employs one or more parabolic antennas 120 to establish feeder link 112. Gateway 108 supports voice communications, paging, and data transmissions.

Feeder link 112 represents the two-way communication between satellite 102 and gateway 108. In a preferred embodiment, feeder link 112 includes all communications between user terminals 106 within footprint 114, and the terrestrial communication networks connected to gateway 108. Feeder link 112 also includes other communications, such as telemetry and control commands transferred between gateway 108 and satellite 102 or user terminals 106.

FIG. 2 depicts a frequency plan for the forward direction feeder link 112 (that is, from gateway 108 to satellite 102). Communications between gateway 108 and satellite 102 employ FDMA, CDMA, and polarization multiplexing schemes to make efficient use of the available bandwidth. As shown in FIG. 2, the frequency band of feeder link 112 is divided into two or more channels 202 corresponding to user links (110). In a preferred embodiment, the band is divided into eight channels 202 (I, K, M, O, H, C, G, and D) employing right hand circular polarization (RHCP) and eight channels 202 (L, N, P, J, A, F, E, and B) employing left hand circular polarization (LHCP).

In a typical system design, each channel 202 preferably covers a 16.5 MHz wide bandwidth, spaced 19.38 MHz apart between 5091 MHz and 5250 MHz. Channel 202 can include many different types of information, including, but not limited to, voice and data. These individual channels 202 are in turn made up of two or more (preferably 13) FDMA subchannels (not shown), also referred to as CDMA channels in CDMA communication systems. These FDMA subchannels are further divided into two or more orthogonal code channels typically implemented using Walsh code sequences (preferably up to 128 of length 128 in satellite communication systems). Those skilled in the art will recognize that other techniques are available for efficient spectrum utilization, such as TDMA. The general concepts described herein apply, regardless of the multiple access scheme employed.

Feeder link 112 also includes a command channel 204 that carries command information between satellite 102 and gateway 108. This information can be stored in one or more memory elements or circuits on the satellite and retrieved as needed, or used substantially as it is received. Alternatively, the information on command channel 204 may not be the final coefficients but be used to compute the coefficients.

A similar frequency plan is preferably employed for reverse direction feeder link 112, with channels 202 spaced between 6875 MHz and 7075 MHz. Otherwise, reverse direction feeder link 112 is substantially identical to the forward link described with respect to FIG. 2. However, those skilled in the art will appreciate that both forward and reverse link directions can utilize other frequencies and channel spacing, and so forth. User link 110 represents the two-way communication link between satellite 102 and user terminals 106 within footprint 114. In a preferred embodiment, user link 110 employs spatial channelization to make efficient use of the available spectrum. Again, the general concepts described herein apply to other embodiments employing alternative techniques for increasing capacity. Note that footprint 114 can take one of many arbitrary or known shapes, such as a rectangle (as shown in FIG. 1) or a circle (as shown in FIGS. 3 A and 3B).

FIG. 3A depicts the spatial channelization of user link 110 with footprint 114 divided into beams 302 A through 302P, referred to collectively as an antenna beam configuration 300A. Antenna beam configuration 300A is preferably used in the forward direction (that is, from satellite 102 to user terminals 106) user link 110. Transmit antenna 116 is adjusted to produce beams 302A through 302P. At a minimum, each beam transmits signals with substantially uniform power density across the area defined for it in FIG. 3A, including an appropriate compensation for the curvature of the Earth (that is, an isoflux pattern). As will be discussed below, this uniform transmission is modified according to the present invention. Any number of conventional techniques for calculating phased array coefficients can be used to adjust transmit antenna 116 to produce desired beams 302 within antenna beam configuration 300A.

A variety of beam patterns are possible and it may be desirable to maintain different patterns for the feeder and user links. FIG. 3B depicts footprint 114 divided into an alternative antenna beam configuration 300B, which is preferably used in implementing reverse direction user link 110 (that is, for signals transferred from user terminals 106 to satellite 102). Comparing FIGS. 3A and 3B illustrates that the antenna beam configurations for transmit antenna 116 and receive antenna 118 are preferably different from one another. Other alternative embodiments use the same antenna beam configurations for both.

FIG. 4 depicts a forward direction satellite transponder 400A, residing in satellite 102, that demultiplexes the signals on feeder link 112 and performs the necessary signal processing to form antenna beam configuration 300 (300A, 300B). A demultiplexer 402 splits feeder link 112 into two or more channels 202 and at least one command channel 204, where used. All communications destined for those user terminals 106 within a given beam 302 are transmitted over a single channel 202. For example, communications for user terminals within beam 302A are transmitted over channel 202A, while communications for user terminals within beam 302E are transmitted over channel 202E, and so forth. The demultiplexed channels 202 and 204 are then fed into transmit antenna 116.

Transmit antenna 116 includes one or more beamformers 404, an adder 406, and an antenna array 408. Each beamformer 404 produces a different beam 302 in antenna beam configuration 300A. The pattern and power used to form each beam 302 is generally predetermined according to a desired fixed beam shape or position, and beamformers (404) can be preprogrammed or constructed to act accordingly without further input or control. The allocation of the feeder link channel signals among the various beams is also pre-selected. However, according to the present invention, the "wide" and "narrow" beams discussed below can have different shapes as desired. Generally, the wide beams have a fixed pattern, while the narrow beams are dynamically adjusted or adjustable. However, the wide beams can also be configured as adjustable, depending on the system design. Therefore, control information necessary for adjusting the beamformers can be transmitted over command channel 204, to provide the desired changes in beam shape or power. The output of each beamformer 404 is N lines wide, where N is the number of antenna elements in antenna array 408.

Adder 406 sums the signals from beamformers 404, producing an output N lines wide. The first output line of adder 406 is the sum of the first line from each beamformer 404, the second output line of adder 406 is the sum of the second line from each beamformer 404, and so on for each of the N output lines. Each of the N output lines from adder 406 is fed to an antenna element within antenna array 408. As discussed above with respect to transmit antenna 116, antenna array 408 is preferably a two-dimensional antenna array, but could alternatively be implemented as a one- or three- dimensional antenna array.

Each channel 202 is thereby transmitted to each of the user terminals 106 within a particular beam 302 (as in 202A→302A, 202B→302B ...

202L→302L ... 202P→302P). The individual user terminals 106 then receive a CDMA channel over the beam covering their respective area of location and choose an appropriate orthogonal code channel (paging, pilot, synch, or traffic) according to spread spectrum techniques known to those skilled in the art. Though not discussed herein, satellite 102 employs a return direction satellite transponder to route reverse user link 110 signals to gateway 108 through reverse feeder link 112. Skilled artisans will recognize that the principles discussed herein with respect to forward direction transponder 400A are easily applied to generate the appropriate beam patterns and signal paths for a return direction transponder.

A network of satellites 102 is preferably deployed having partially overlapping footprints 114. This network can provide service to user terminals 106 over large geographic areas. FIG. 5 depicts an example beam 302 in greater detail. The outline of beam 302 defines a geographic area on the ground within which the particular channel 202 assigned to that beam is transmitted. As discussed above, oftentimes user terminal traffic is distributed non-uniformly over the geographic area defined by beam 302. Assume for purposes of the following description that the region illuminated by beam 302 includes one or more hot spots 502 (502A, 502B), defined as areas within beam 302 where the density of user terminal traffic is higher than the density elsewhere in beam 302.

For example, hot spot 502A might represent a densely crowded urban area, or a major highway that experiences heavy traffic patterns at certain times during the day. Hot spots 502 can be very dynamic or transitory in nature appearing at certain times, and then disappearing at others. Hot spots 502 can, therefore, vary over time by size, shape, and user traffic density. As shown in FIG. 5, a hot spot 502A is of a different size and shape than a hot spot 502B.

Note that FIG. 5 depicts only a snapshot in time of communication signal patterns. In a preferred embodiment, satellite 102 moves with a velocity v with respect to the surface of the Earth. The areal mapping or regions covered by beams 302 making up antenna beam configuration 300 do not vary with time, that is, the area defined by footprint 114 moves with satellite 102 at a velocity v with respect to the ground. This is analogous to a light affixed to the bottom of satellite 102 that casts a beam onto the ground, illuminating the area beneath satellite 102 (footprint 114) as it moves in orbit with respect to the Earth. Since hot spots 502 are generally fixed geographic areas (though possibly time-varying themselves), they will appear to move within beam 302 at velocity v. Beam 302, therefore, appears to pass over hot spots 502, which effectively enter one side of beam 302 and exit the other. Hot spots can also move in more than one direction relative to the beam, and at different velocities where the hot spot itself also moves on the Earth's surface, and enter and emerge on various sides of beams. II. Overview of the Invention

FIGS. 6A and 6B depict an example beam 302 generated according to the present invention, given the example hot spots depicted in FIG. 5. FIG. 6A depicts beam 302 in two dimensions, where the interior lines denote contours of substantially constant gain. Beam 302 includes a wide beam 602 and one or more pencil beams 604. Each pencil beam 604 is a narrow beam located or projected within wide beam 602, that illuminates a particular hot spot 502. For instance, in FIG. 6A, pencil beam 604A illuminates hot spot 502A, and pencil beam 604B illuminates hot spot 502B. Note that although the following describes beam 302 in terms of transmitting signals using transmit antenna 116 (forward direction of user link 110), skilled artisans will recognize that the description also applies to receive antenna 118 producing beam 302 for signal reception (reverse direction of user link 110). FIG. 6B depicts beam 302 projected along an x-axis. Wide beam 602 transmits to user terminals within beam 302 in a substantially uniform or power adjusted manner, with a maximum gain g_wh. Note that beam shaping or power adjustment used for wide beam 602 near the edges creates a curve for beam strength versus lateral position that is preferably slightly concave to compensate for the curvature of the earth (the concavity is exaggerated in FIG. 6B for purposes of illustration), as is known in the art.

According to the present invention, pencil beams 604 are used to illuminate one or more hot spots 502 within beam 302. A pencil beam 604 has a higher gain than wide beam 602, and is narrower. For example, pencil beam 604A illuminates hot spot 502A and has a gain g _A that is higher than g_wb. As shown in FIGS. 6A and 6B, the shape of pencil beam 604 is preferably matched to the shape of the hot spot that it illuminates, to the extent possible given a finite number of antenna elements in transmit antenna 116 dedicated to forming beam 302. Further, the gain of pencil beam 604 is dependent upon the signals being transferred in accordance with user traffic density within the illuminated hot spot. In this preferred embodiment, each pencil beam 604 can, therefore, have a different gain and a different shape. As shown in FIGS. 6A and 6B, pencil beam 604A has a lower gain and is more narrow (at least when projected onto the x-axis) than pencil beam 604B. This is due to the difference in shape and user traffic density between hot spots 502A and 502B, that is, hot spot 502B is wider along the x- axis and has a greater user traffic density. In an alternative embodiment, a standard pencil beam having a fixed shape and gain is employed regardless of the properties of the hot spot area or region that is being illuminated.

HI. Operation According to a First Preferred Embodiment

A first preferred embodiment of the present invention is now described with reference to FIGS. 7 and 8. FIG. 7 depicts a flowchart that describes a preferred method of producing a composite beam 302 according to the present invention. FIG. 8 depicts an example beamformer 404 in detail. Beamformer 404 includes phased arrays for producing wide beam 602 and one or more pencil beams 604, and an adder 806. As will be clear to those skilled in the art, the various components of FIGS. 4 and 8 can be implemented in hardware, software, or a combination of both.

As is shown in FIG. 8, channel 202 is input to each phased array. Wide beam phased array 802 splits channel 202 into N lines and weights them according to phased array coefficients made available for this purpose. For example, when used, commands presented using command channel 204, or previously stored in a memory or storage location, or hardwired into the satellite are recalled. Alternatively, certain information provided on the command channel or in advance is used to compute the coefficients. Similarly, each pencil beam phased array 804 splits channel 202 into N lines and weights them according to phased array coefficients loaded using command channel 204, or otherwise made available. Adder 806 performs a function similar to adder 406, that is, the first output line of adder 806 is the sum of the first line from each phased array, the second output line of adder 806 is the sum of the second line from each phased array, and so on for each of the N output lines. As shown in FIG. 4, the N-line output of adder 806 is then fed into adder 406. Those skilled in the art will recognize that in an alternative embodiment, adders 406 and 806 can be combined into one functional unit.

In step 702, a first set of phased array coefficients is adjusted to produce wide beam 602. Many different conventional techniques known to those skilled in the art for determining phased array coefficients could be employed to produce the shape of wide beam 602 shown in FIGS. 6A and 6B. Different techniques optimize different characteristics of the beam pattern based on desired beam characteristics, such as side lobe height, pass-band ripple, or stop-band attenuation, as would be known to those skilled in the art.

The adjustments of step 702 are preferably calculated at gateway 108, using known processors or processing elements, and transmitted to satellite 102 over command channel 204. This allows for a less complex and less expensive satellite 102. However, an alternative embodiment employs a signal or control processor (not shown) in satellite 102 to make the necessary calculations, such as a small onboard computer or similar known processing element. Note that since wide beam 602 is preferably fixed with respect to satellite 102, these calculations need only be made once, though infrequent adjustment could be implemented for various reasons. In step 704, an z^'th set of phased array coefficients is adjusted to produce an z^'th pencil beam illuminating one hot spot 502 (if any are present within the geographic region defined by beam 302). In a preferred embodiment, those hot spots 502 within beam 302 are illuminated in order of user terminal traffic density, with resources being allocated to illuminate the most dense traffic area first, although this is not required by the invention. Other spot illumination selection or sorting arrangements are possible, such as illuminating in order of average strength of signal (that is, illuminating those hot spots first that require the most gain to achieve an acceptable signal level). Note that the phased array coefficients that produce pencil beams 604 should be able to vary with time in order that the pencil beam track hot spots 502 that are stationary with respect to the ground, but moving with respect to satellite 102, or that are moving with respect to both the ground and the satellite. Again, the adjustments of step 704 are preferably calculated at gateway 108 based on known satellite ephemeris data, beam patterns, and so forth, and transmitted using command channel 204.

The number of pencil beams can be dynamic and vary over time to account for the time varying nature of user communication traffic density. Pencil beams can be created and dropped or de-activated as areas of concentrated user traffic appear and disappear. The present invention, therefore, provides for efficient satellite-to-ground communication for a wide variety of service regions. In addition, while wide beams move over the Earth's surface in accordance with the satellite orbit, the narrow beams can be made to service a substantially fixed area by constantly, or at least periodically, changing the phased array coefficients.

In step 706, if there are no additional, or initial, hot spots that remain non-illuminated or unserviced by pencil beams within beam 302, then in step 710 the phased array coefficients forming the composite beam are loaded into phased arrays 802 and 804 through command channel 204. FIG. 9A depicts phased array 802 in greater detail. Channel 202 is split into N separate paths, one for each antenna element in antenna array 408. Each path is multiplied by a time-invariant complex gain coefficient CTI (labeled CTI-, through CTI_N). The complex gain coefficients, referred to collectively as a set of phased array coefficients, are loaded via command channel 204 and can have phase and amplitude components. In some configurations only phase will be adjusted, while in others both phase and signal strength (gain) will be adjusted, to form a desired beam pattern. The splitting and multiplication functions can be implemented in either hardware or software, and can take advantage of known processing elements using in the communication system, as would be clear to one skilled in the art.

FIG. 9B depicts phased array 804 in greater detail. Channel 202 is split into N separate paths, one for each antenna element in antenna array 408. Each path is multiplied by a time-variant complex gain coefficient CTV (labeled CTV_j through CTV_N). The set of time varying phased array coefficients are dynamically loaded using command channel 204 and can have phase and amplitude components. As with phased array 802, only phase may be used in some configurations, and the splitting and multiplication functions can be implemented in either hardware or software, as would be clear to one skilled in the art. Returning to step 708 of FIG. 7, if un-illuminated hot spots remain, and if another pencil beam 604 is available, another set of phased array coefficients is calculated in step 704. If there are no available pencil beams 604, then processing proceeds to step 710. The number of pencil beams 604 available to satellite 102 depends upon the processing power available (whether employed at gateway 108 or satellite 102), the number of antenna elements within antenna array 408, and various characteristics of wide beam 602 and pencil beams 604. Skilled artisans will recognize that forming a composite beam can be analyzed using linear filter theory, bringing to bear all the conventional techniques known to that art. The time varying complex gain coefficients in phased arrays 804 must be updated over time in order that pencil beams 604 continue to illuminate the stationary hot spots 502. As shown in FIG. 7, the preferred method repeats, beginning with step 704 (it generally is unnecessary to readjust the time-invariant wide beam 602). The phased array coefficients for each pencil beam 604 are adjusted to take into account the movement of satellite 102 in orbit.

FIG. 10 depicts a flowchart that describes an alternative method of producing a composite beam 302 according to the present invention. In step 1002, one or more hot spots 502 (if any are present within the geographic region defined by beam 302) are selected to be illuminated by a pencil beam (one pencil beam 604 per hot spot 502). As described above with respect to step 704, the hot spots 502 within beam 302 are preferably illuminated in order of user terminal traffic density, with the most dense being illuminated first. Again, other spot selection or sorting arrangements are possible, such as illuminating in order of average signal strength.

In step 1004, a set of phased array coefficients is adjusted to produce a composite beam having a wide beam 602 and one or more pencil beams 604, as determined in step 1002. Any one of a number of conventional techniques for spatial array processing can be used to determine the appropriate phased array coefficients, given the desired composite beam pattern. Skilled artisans will recognize that step 1004 is mathematically related to steps 702 and 704 in FIG. 7. Combining the set of phased array coefficients calculated in steps 702 and 704 into a single set of phased array coefficients produces a composite beam as in step 1004. Again, these calculations are preferably calculated at gateway 108 and transmitted to satellite 102 using command channel 204.

In step 1006, the phased array coefficients calculated in step 1004 are loaded into beamformers 404. In this embodiment, each beamformer 404 is preferably implemented as a single phased array as depicted in FIG. 9B. Given that a single set of phased array coefficients is calculated to produce the composite beam pattern, the need for multiple parallel phased arrays (as shown in FIG. 8) is obviated. The phased array coefficients are loaded into beamformers 404 via command channel 204.

Steps 1002 through 1006 are repeated over time in order that pencil beams 604 can be adjusted to track hot spots 502. Those skilled in the art will recognize that this update can be performed in an efficient manner where only the pencil beam portion of the composite beam is adjusted. In the above embodiment, beam 302 is formed as the composite of wide beam 602 and pencil beams 604. Because the integrated beam gain over 4π steradians must be unity, the gain of wide beam 602 is slightly lower in the composite beam than would be the case if beam 302 included only wide beam 602. This decrease in wide beam 602 gain requires that slightly more power be expended when transmitting signals to those user terminals only within wide beam 602, in order to achieve the same received signal power. However, if the total hot spot user terminal traffic in a beam is a significant fraction of the total beam traffic, total transmission power is reduced. This is caused by the high gain pencil beams 604 requiring much less power to transmit signals to the large number of user terminals 106 within those beams. Adjusting transmit antenna 116 to produce a composite beam including one or more pencil beams 604, therefore, reduces satellite 102 power (or increases capacity).

IV. Operation According to a Second Preferred Embodiment

FIG. 11 depicts a forward direction satellite transponder 400B according to a second preferred embodiment of the present invention. As described with respect to transponder 400A in FIG. 4, transponder 400B includes demultiplexer 402 and transmit antenna 116. Transmit antenna 116 includes adder 406 and N-element antenna array 408. Here, however, feeder link 112 includes not only channels 202A through 202P and command channel 204, but also includes one or more subchannels 1102. Demultiplexer 402 therefore splits feeder link 112 into channels 202, subchannels 1102, and command channel 204. Transmit antenna 116 also includes one or more wide beam beamformers 1104 and one or more pencil beam beamformers 1106. Each channel 202 is fed into a wide beam beamformer 1104, and each subchannel 1102 is fed into a pencil beam beamformer 1106. Adder 406 sums the beamformer signals, producing an output N lines wide (as described above). In the first preferred embodiment, all communications between gateway 108 and those user terminals 106 within a given beam 302 are transmitted over a single channel 202. Within channel 202, each user terminal 106 is assigned a separate code from a family of orthogonal codes, according to a CDMA channelization scheme. As described above, the entire channel 202 is transmitted using both wide beam 602 and one or more pencil beams 604, to each user terminal 106 within that particular beam.

By comparison, in the second preferred embodiment, a subset of those user terminals within a given beam 302 are assigned to one or more subchannels 1102. In the context of the present invention, all user terminals 106 within a given hot spot 502 (502A, 502B, ...) are preferably assigned to a single subchannel 1102 corresponding or assigned to that hot spot (1102A→502A, 1102B→502B, ... 1102N→502N). All communications with those user terminals 106 not within a hot spot continue to be transmitted over a particular channel 202 to all user terminals 106 within beam 302 via one wide beam beamformer 1104, for example, phased array 802 in FIG. 9A. Here, however, all communications between gateway 108 and those user terminals within a hot spot are transmitted over a particular subchannel 1102 via one pencil beam beamformer 1106, for example, phased array 804 in FIG. 9B. In a preferred embodiment, orthogonality is preserved between the CDMA codes assigned to user terminals 106 within a subchannel, and the codes assigned to those user terminals 106 within the surrounding wide beam. This is accomplished by using the same PN code or code set and chip timing as used in the surrounding wide beam, and a different set of orthogonal codes within the narrow pencil beam, and preferably making use of an auxiliary pilot, such as described in the "cdma2000 ITU-R RTT Candidate Submission (0.18)" material submitted in response to the International Telecommunications Union (ITU) request for submission of candidate Radio Transmission Technologies (RTTs) for the

IMT2000/FPLMTS Radio Interface, which is incorporated herein by reference.

This arrangement reduces the total power required for transmission because the signals intended for user terminals 106 within pencil beams 604 aren't also transmitted to other uninterested user terminals outside of the pencil beam, and vice versa.

V. Handling User Traffic at Beam Boundaries

FIG. 12 depicts an example of two contiguous beams, 302A and 302F, from antenna beam configuration 300A. An example hot spot 502 is also depicted, moving in the direction shown at velocity v, relative to the beams (or beams moving relative to spot). As described above, hot spots 502 in general are fixed geographically whereas footprint 114 (and the corresponding antenna beam configuration) moves with satellite 102 with a velocity v in relation to the ground. FIG. 12 depicts a snapshot in time where hot spot 502 has moved across beam 302F, partially crossing into beam 302A. Assume for purposes of this discussion that a pencil beam 604 illuminates hot spot 502.

At a time just prior to the view shown in FIG. 12 (when hot spot 502 is entirely within beam 302F), the user terminals within hot spot 502 are all assigned CDMA codes that are orthogonal to the other user terminals 106 within beam 302F. As hot spot 502 moves into beam 302A, those user terminals 106 within hot spot 502 that are also within beam 302 A must be switched over to a CDMA code that is orthogonal to the user terminals within beam 302A. This operation is referred to herein as a handoff. The manner in which a handoff is accomplished differs from the first to the second preferred embodiments described above. In the first preferred embodiment, communications with the user terminals within hot spot 502 are transmitted over channel 202F. As hot spot 502 passes into beam 302A, the user terminals are handed off from channel 202F to channel 202A. Composite beam 302A may or may not focus a pencil beam 604 on the portion of hot spot 502 within its bounds, depending upon the user traffic density here as compared to the rest of the beam.

Similarly, in the second preferred embodiment, communications with the user terminals within hot spot 502 are transmitted using, for example, subchannel 1102D. As hot spot 502 passes into beam 302A, the user terminals are handed off from channel subchannel 1102D to channel 202 A. Alternatively, if the user traffic density is sufficiently high, a new pencil beam 604 and subchannel 1102 (for example, 1102E) could be created using CDMA codes that are orthogonal to the codes used within beam 302A. Here, the user terminals are handed off from subchannel 1102D to subchannel 1102E. VI. 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 only, and not limitation. Thus, the breadth and scope of 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 is claimed is:

Claims

1. A method for adjusting a phased array antenna on a satellite that communicates with terrestrial user terminals, comprising the steps of: adjusting a first plurality of phased array coefficients to produce a wide beam; adjusting a second plurality of phased array coefficients to produce a narrower pencil beam, wherein said pencil beam illuminates a hot spot within said wide beam; and applying said first and second plurality of phased array coefficients to the phased array antenna.

2. The method of claim 1, wherein said first plurality of phased array coefficients are time invariant and said second plurality of phased array coefficients are time variant, and wherein said pencil beam remains fixed on said hot spot.

3. The method of claim 1, wherein said first plurality of phased array coefficients are time variant at one rate and said second plurality of phased array coefficients are time variant at a second different rate.

4. The method of claim 1, wherein said phased array antenna comprises a transmit antenna and a receive antenna.

5. The method of claim 1, wherein each of said phased array coefficients comprises a phase component.

6. The system of claim 5, wherein each of said phased array coefficients comprises a gain component.

7. The method of claim 1, wherein the shape of said pencil beam approximates the shape of said hot spot.

8. The method of claim 1, wherein the gain of said pencil beam depends on the user traffic density of said hot spot.

9. The method of claim 1, wherein said wide beam is an isoflux beampattern.

10. A method for adjusting a phased array antenna on a satellite, wherein the satellite communicates with terrestrial user terminals, comprising the steps of: (a) adjusting a plurality of phased array coefficients to produce a composite beam having a wide beam and one or more pencil beams, wherein each of said pencil beams illuminates a hot spot within said wide beam; and

(b) applying said plurality of phased array coefficients to the phased array antenna.

11. A method for satellite communications using a phased array antenna, comprising the steps of:

(a) transmitting a ground-to-satellite feeder link; (b) demultiplexing said feeder link into one or more channels;

(c) adjusting a plurality of phased array coefficients for each of said channels, wherein each of said plurality of phased array coefficients produces a composite beam having a wide beam and one or more pencil beams, wherein each of said pencil beams illuminates a hot spot within said wide beam; and (d) applying said phased array coefficients for each of said channels to the phased array antenna.

12. A method for satellite communications using a phased array antenna, comprising the steps of:

(a) transmitting a ground-to-satellite feeder link; (b) demultiplexing said feeder link into one or more channels and one or more subchannels; (c) adjusting a first plurality of phased array coefficients for each of said channels, wherein each of said first plurality of phased array coefficients produces a wide beam;

(d) adjusting a second plurality of phased array coefficients for each of said subchannels, wherein each of said second plurality of phased array coefficients produces a pencil beam that illuminates a hot spot within at least one of said wide beams; and

(e) applying said phased array coefficients for each of said channels and for each of said subchannels to the phased array antenna.

13. The method of claim 12, wherein said first plurality of phased array coefficients are time invariant and said second plurality of phased array coefficients are time variant, and wherein said pencil beam remains fixed on said hot spot.

14. The method of claim 12, wherein said first plurality of phased array coefficients are time variant at one rate and said second plurality of phased array coefficients are time variant at a second different rate.

15. A system for adjusting a phased array antenna on a satellite, wherein the satellite communicates with terrestrial user terminals, comprising:

(a) means for adjusting a first plurality of phased array coefficients to produce a wide beam;

(b) means for adjusting a second plurality of phased array coefficients to produce a pencil beam, wherein said pencil beam illuminates a hot spot within said wide beam; and (c) means for applying said first and second plurality of phased array coefficients to the phased array antenna.

16. The system of claim 15, wherein said first plurality of phased array coefficients are time invariant and said second plurality of phased array coefficients are time variant, and wherein said pencil beam remains fixed on said hot spot.

17. The method of claim 15, wherein said first plurality of phased array coefficients are time variant at one rate and said second plurality of phased array coefficients are time variant at a second different rate.

18. The system of claim 15, wherein said phased array antenna comprises a transmit antenna and a receive antenna.

19. The system of claim 15, wherein each of said phased array coefficients comprises a phase component.

20. The system of claim 19, wherein each of said phased array coefficients comprises a gain component.

21. The system of claim 15, wherein the shape of said pencil beam approximates the shape of said hot spot.

22. The system of claim 15, wherein the gain of said pencil beam depends on the user traffic density of said hot spot.

23. The system of claim 15, wherein said wide beam is a substantially isoflux beampattern.

24. A system for adjusting a phased array antenna on a satellite, wherein the satellite communicates with terrestrial user terminals, comprising the steps of: means for adjusting a plurality of phased array coefficients to produce a composite beam having a wide beam and one or more pencil beams, wherein each of said pencil beams illuminates a hot spot within said wide beam; and means for applying said plurality of phased array coefficients to the phased array antenna.

25. A satellite communication system, comprising: a gateway; a satellite having an antenna array and a transponder, wherein said satellite is communicatively coupled to said gateway via a feeder link, wherein said transponder includes: a demultiplexer, coupled to said feeder link, wherein said demultiplexer outputs one or more channels; and one or more beamformers, each coupled to one of said channels and to said antenna array, and each configured to produce a composite beam having a wide beam and one or more pencil beams, wherein each of said pencil beams illuminates a hot spot within said wide beam.

26. A satellite communication system, comprising: at least one gateway; at least one satellite having an antenna array and a transponder, wherein said satellite is communicatively coupled to said gateway via a feeder link, wherein said transponder includes: a demultiplexer, coupled to said feeder link, wherein said demultiplexer outputs one or more channels and one or more subchannels, one or more wide beamformers, each coupled to one of said channels and to said antenna array, and each configured to produce a wide beam, and one or more narrow beamformers, each coupled to one of said subchannels and to said antenna array, and each configured to produce a pencil beam that illuminates a hot spot within at least one of said wide beams.