Patent Description:
Satellite communications systems provide a means by which data, including audio, video and various other sorts of data can be communicated from one location to another. The use of such satellite communications systems has gained in popularity as the need for broadband communications has grown. Accordingly, the need for greater capacity over each satellite is growing.

In satellite systems, information originates at a station (which in some instances is a landbased, but which may be airborne, seaborne, etc.) referred to here as a Satellite Access Node (SAN) and is transmitted up to a satellite. In some embodiments, the satellite is a geostationary satellite. Geostationary satellites have orbits that are synchronized to the rotation of the Earth, keeping the satellite essentially stationary with respect to the Earth. Alternatively, the satellite is in an orbit about the Earth that causes the footprint of the satellite to move over the surface of the Earth as the satellite traverses its orbital path.

Information received by the satellite is retransmitted to a user beam coverage area on Earth where it is received by a second station (such as a user terminal). The communication can either be uni-directional (e.g., from the SAN to the user terminal), or bi-directional (i.e., originating in both the SAN and the user terminal and traversing the path through the satellite to the other). By providing a relatively large number of SANs and spot beams and establishing a frequency re-use plan that allows a satellite to communicate on the same frequency with several different SANs, it may be possible to increase the capacity of the system. User spot beams are antenna patterns that direct signals to a particular user coverage area (e.g., a multi beam antenna in which multiple feeds illuminate a common reflector, wherein each feed produces a different spot beam). However, each SAN is expensive to build and to maintain. Therefore, finding techniques that can provide high capacity with few such SANs is desirable.

Furthermore, as the capacity of a satellite communication system increases, a variety of problems are encountered. For example, while spot beams can allow for increased frequency reuse (and thus increased capacity), spot beams may not provide a good match to the actual need for capacity, with some spot beams being oversubscribed and other spot beams being undersubscribed. Increased capacity also tends to result in increased need for feeder link bandwidth. However, bandwidth allocated to feeder links may reduce bandwidth available for user links. Accordingly, improved techniques for providing high capacity broadband satellite systems are desirable.

<CIT> discloses a system and method of free-space optical satellite communications that includes a ground station and transceiver for transmitting and receiving an optical communications signal.

<CIT> discloses an antenna system and method for transmitting or receiving a plurality of pixel beams of satellite communication signals.

<CIT> discloses a method and system of communicating in free space using an optical communication system, such as for intersatellite and satellite-to-ground communications.

<CIT> discloses a satellite telecommunication system with multibeam coverage and frequency reusing.

The disclosed techniques, in accordance with one or more various embodiments, is described with reference to the following figures. The drawings are provided for purposes of illustration only and merely depict examples of some embodiments of the disclosed techniques. These drawings are provided to facilitate the reader's understanding of the disclosed techniques. They should not be considered to limit the breadth, scope, or applicability of the claimed invention. It should be noted that for clarity and ease of illustration these drawings are not necessarily made to scale.

The figures are not intended to be exhaustive or to limit the claimed invention to the precise form disclosed. It should be understood that the disclosed techniques can be practiced with modification and alteration, and that the invention should be limited only by the claims and the equivalents thereof.

Initially, a system that uses radio frequency (RF) communication links between satellite access nodes (SANs) and a satellite is discussed. Following this introduction is a discussion of several optical transmission techniques for broadband capacity satellites. Following an introductory discussion of systems having an optical feeder link, three techniques are discussed for modulating signals on an optical feeder link. In addition, three architectures are provided for implementing the techniques.

<FIG> is an illustration of a satellite communications system <NUM> in which a relatively large number of stations (referred to herein as "SANs", also referred to as "gateways") <NUM> communicate with a satellite <NUM> using RF signals on both feeder and user links to create a relatively large capacity system <NUM>. Information is transmitted from the SANs <NUM> over the satellite <NUM> to a user beam coverage area in which a plurality of user terminals <NUM> may reside. In some embodiments, the system <NUM> includes thousands of user terminals <NUM>. In some such embodiments, each of the SANs <NUM> is capable of establishing a feeder uplink <NUM> to the satellite <NUM> and receiving a feeder downlink <NUM> from the satellite <NUM>. In some embodiments, feeder uplinks <NUM> from the SAN <NUM> to the satellite <NUM> have a bandwidth of <NUM>. In some embodiments, the feeder uplink signal can be modulated using <NUM> quadrature amplitude modulation (QAM). Use of <NUM> QAM modulation yields about <NUM> bits per second per Hertz. By using <NUM> bandwidth per spot beam, each spot beam can provide about <NUM>-<NUM> Gbps of capacity. By using <NUM> SANs, each capable of transmitting a <NUM> bandwidth signal, the system has approximately a <NUM> bandwidth or a capacity of about <NUM> Gbps (i.e., <NUM> Tbps).

<FIG> is an illustration of a simplified satellite, according to embodiment useful for understanding the invention, that can be used in the system of <FIG>, wherein the satellite uses RF signals to communicate with SANs. <FIG> is a simplified illustration of the repeaters <NUM> used on the forward link (i.e., receiving the RF feeder uplink and transmitting the RF user downlink) in the satellite of <FIG>. A feed <NUM> within the feeder link antenna (not shown) of the satellite <NUM> receives an RF signal from a SAN <NUM>. Although not shown in detail, the user link antenna can be any of: one or more multi beam antenna array (e.g., multiple feeds illuminate a shared reflector), direct radiating feeds, or other suitable configurations. Moreover, user and feeder link antennas can share feeds (e.g., using dual-band combined transmit, receive), reflectors, or both. In one embodiment, the feed <NUM> can receive signals on two orthogonal polarizations (i.e., right-hand circular polarization (RHCP) and left-hand circular polarization (LHCP) or alternatively, horizontal and vertical polarizations). In one such embodiment, the output <NUM> from one polarization (e.g., the RHCP) is provided to a first repeater <NUM>. The output is coupled to the input of a Low noise amplifier (LNA) <NUM> (see <FIG>). The output of the LNA <NUM> is coupled to the input of a diplexer <NUM>. The diplexer splits the signal into a first output signal <NUM> and second output signal <NUM>. The first output signal <NUM> is at a first RF frequency. The second output signal <NUM> is at a second RF frequency. Each of the output signals <NUM>, <NUM> are coupled to a frequency converter <NUM>, <NUM>. A local oscillator (LO) <NUM> is also coupled to each of the frequency converters <NUM>, <NUM>. The frequency converters shift the frequency of the output signals to a user downlink transmission frequency. In some embodiments, the same LO frequency is applied to both frequency converters <NUM>, <NUM>. The output of the frequency converters <NUM>, <NUM> is coupled through a channel filter <NUM>, <NUM> to a hybrid <NUM>. The hybrid <NUM> combines the output of the two channel filters <NUM>, <NUM> and couples the combined signal to a linearizing channel amplifier <NUM>.

Combining the signals within the hybrid <NUM> allows the signals to be amplified by one traveling wave tube amplifier (TWTA) <NUM>. The output of the linearizing channel amplifier <NUM> is coupled to the TWTA <NUM>. The TWTA <NUM> amplifies the signal and couples the amplified output to the input of a high pass filter and diplexer <NUM>. The high pass filter and diplexer <NUM> split the signal back into two outputs based on the frequency of the signals, with a higher frequency portion of the signal being coupled to a first antenna feed <NUM> and a lower frequency portion of the signal being coupled to a second antenna feed <NUM>. The first antenna feed <NUM> transmits a user downlink beam to a first user beam coverage area U1. The second antenna feed <NUM> transmits a user downlink beam to a second user beam coverage area U3.

The output <NUM> of the feed <NUM> from the second polarization (e.g., LHCP) is coupled to a second arm <NUM> of the repeater. The second arm <NUM> functions in a manner similar to the first <NUM>, however the output frequencies transmitted to the user beam coverage areas U2 and U4 will be different from the frequencies transmitted to the user beam coverage areas U1 and U3.

In the following embodiments, an optical link is used to increase the bandwidth of the feeder uplink <NUM> from each SAN <NUM> to the satellite <NUM> and the feeder downlink <NUM> from the satellite to each SAN <NUM>. This can provide numerous benefits, including making more spectrum available for the user links. Furthermore, by increasing the bandwidth of the feeder links <NUM>, <NUM>, the number of SANs <NUM> can be reduced. Reducing the number of SANs <NUM> by increasing the bandwidth of each feeder link to/from each SAN <NUM> reduces the overall cost of the system without reducing the system capacity. However, one of the challenges associated with the use of optical transmission signals is that optical signals are subject to attenuation when passing through the atmosphere. In particular, if the sky is not clear along the path from the satellite to the SANs, the optical signal will experience significant propagation loss due to the attenuation of the signals.

In addition to attenuation due to reduced visibility, scintillation occurs under adverse atmospheric conditions. Therefore, techniques can be used to mitigate against the effects of fading of the optical signal due to atmospheric conditions. In particular, as will be discussed in greater detail below, the lenses on board the satellite used to receive the optical signals and the lasers on board the satellite used to transmit optical signals can be directed to one of several SANs. The SANs are dispersed over the Earth so that they tend to experience poor atmospheric conditions at different times (i.e., when fading is likely on the path between the satellite and a particular SAN, it will be relatively unlikely on the path between the satellite and each of the other SANs).

By taking into account the differences in atmospheric conditions in different parts of the country, the decision can be made when the atmosphere between the satellite and a particular SAN is unfavorable to the transmission of an optical signal, to use a different SAN to which the atmospheric conditions are more favorable. For example, the southwest of the continental United States has relatively clear skies. Accordingly, SANs can be located in these clear locations in the country to provide a portal for data that would otherwise be sent through SANs in other parts of the country when the sky between those SANs and the satellite is obstructed.

In addition to directing the satellite to communicate with those SANs that have a favorable atmospheric path to/from the satellite, signals that are received/transmitted by the satellite through one of several optical receivers/transmitters can be directed to one of several antennas for transmission to a selected user beam coverage area. The combination of flexibility in determining the source from which optical signals can be received on the optical uplink and the ability to select the particular antenna through which signals received from the source will be transmitted allows the system to mitigate the negative impact of the variable atmospheric conditions between the SANs and the satellite.

As disclosed herein, at least three different techniques that can be used to communicate information from SANs through a satellite to user beam coverage areas in which user terminals may reside. Three such techniques will now be described. A very brief summary of each is provided, followed by a more detailed disclosure of each architecture.

Briefly, the first technique uses a binary modulated optical signal on the uplink. Several SANs each receive information to be transmitted to user terminals that reside within user beam coverage areas. The optical signal is modulated with digital information. In some embodiments, each SAN transmits such a binary modulated optical signal to the satellite. The digital information may be a representation of information intended to be transmitted to a user beam coverage area in which user terminals may reside. The signal is detected in the satellite using an optical detector, such as a photodiode. In some embodiments, the resulting digital signal is then used to provide binary encoding, such as binary phase shift keying (BPSK) modulate an intermediate frequency (IF) signal. The IF signal is then upconverted to a satellite RF downlink carrier frequency. Modulating the RF signal with BPSK can be done relatively simply where the size, power, and thermal accommodation on the satellite is small. However, using BPSK as the baseband modulation for the RF signal on the user downlink <NUM> may not provide the maximum capacity of the system. That is, the full potential of the RF user downlink <NUM> is reduced from what it may be possible if a denser modulation scheme is used, such as <NUM> QAM instead of BPSK on the RF user downlink <NUM>.

The second technique also modulates the optical signal on the uplink using a binary modulation scheme. The modulated optical signal is detected by a photodiode. The resulting digital signal is coupled to a modem. The modem encodes the digital information onto an IF signal using a relatively bandwidth efficient modulation scheme, such as quadrature amplitude modulation (QAM). QAM is used herein to refer to modulation formats than encode more than <NUM> bits per symbol, including for example quadrature phase shift keying (QPSK), offset QPSK, <NUM>-ary phase shift keying, <NUM>-ary QAM, <NUM>-ary QAM, amplitude phase shift keying (APSK), and related modulation formats. While the use of the denser QAM scheme provides a more efficient use of the RF user link, using such encoding on the RF user downlink <NUM> requires a relatively complex digital/intermediate frequency (IF) conversion block (e.g., modem). Such complexity increases the size, mass, cost, power consumption and heat to be dissipated.

The third technique uses an RF modulated optical signal (as opposed to the binary modulated optical signals of the first two techniques). In this embodiment, rather than modulating the optical signal with digital information to be transmitted to the user beam coverage area, an RF signal is directly modulated (i.e., intensity modulated) on to the optical carrier. The satellite then need only detect the RF modulated signal from the optical signal (i.e., detect the intensity envelope of the optical signal) and frequency upconvert that signal to the user downlink frequency, relieving the satellite of the need for a complex modem. The use of an RF modulated optical signal increases the overall capacity of the communications system by allowing a denser modulation of the user link RF signal, while reducing the complexity of the satellite. Due to the available bandwidth in the optical signal, many RF carriers can be multiplexed onto an optical carrier. However, optical signals that are intensity modulated with an RF signal are susceptible to errors due to several factors, including fading of the optical signal.

Each of these three techniques suffer from the fact that there is an unreliable optical channel from the SANs to the satellite. Therefore, three system architectures are discussed to mitigate the problems of unreliable optical feeder link channels. In each configuration, additional SANs are used to offset the inherent unreliability of the optical links to the satellite. Signals can be routed from any of the SANs to any of the user beam coverage areas. Using additional SANs ensures that a desired number of SANs that have a high quality optical link to the satellite are available. Furthermore, flexibility in the routing through the satellite (i.e., referred to herein as "feeder link diversity") allows data to be transmitted from those SANs that have the desired quality optical channel to the satellite on the feeder link and to user spot beams on the user link in a flexible way.

Each of these three techniques will now be discussed in detail. Each of these techniques are discussed in the context of embodiments that have a particular number of components (i.e., SANs, lasers per SAN, transponders within the satellite, etc.). However, such specific embodiments are provided merely for clarity and ease of the discussion. Furthermore, a wide range of IF and/or RF frequencies, optical wavelengths, numbers of SANs, numbers of transponders on the satellite, etc. are within the scope of the disclosed embodiments. Therefore, the particular frequencies, wavelengths, antenna array elements, and numbers of similar parallel channels, components, devices, user beam coverage areas, etc. should not be taken as a limitation on the manner in which the disclosed systems can be implemented, except where expressly limited by the claims appended hereto.

<FIG> is a simplified schematic of a first of the three techniques noted above according to an embodiment useful for understanding the invention. A system <NUM> for implementing the first technique includes a plurality of SANs <NUM>, a satellite <NUM> with at least one single-feed per beam antenna <NUM>, <NUM> and a plurality of user terminals <NUM> within user beam coverage areas <NUM> (see <FIG>). Alternatively, any antenna can be used in which the antenna has multiple inputs, each of which can receive a signal that can be transmitted in a user spot beam to a user beam coverage area, such as direct radiating antennas, etc. The antennas <NUM>, <NUM> may be a direct-radiating array or part of a reflector/antenna system. In some embodiments, the system <NUM> has M SANs <NUM>. In the example system <NUM> and for each of the example systems discussed throughout this disclosure, M = <NUM>. However, none of the systems disclosed here should be limited to this number. M = <NUM> is merely a convenient example, and in other embodiments, M can be equal to <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or any other suitable value. In some embodiments, the SANs <NUM> receive "forward traffic" to be communicated through the system from a source (such as a core node, not shown), which may receive information from an information network (e.g., the Internet). The data communicated to a SAN <NUM> from the core node can be provided in any form that allows for efficient communication of the data to the SAN <NUM>, including as a binary data stream. In some embodiments, data is provided as a binary data stream modulated on an optical signal and transmitted to the SAN on an optical fiber. Forward traffic is received in streams that are identified with a particular user beam coverage area <NUM>. In some embodiments, the data may also be associated with a particular user terminal or group of user terminals to which the data is to be transmitted. In some embodiments, the data is associated with a terminal based on the frequency and/or timing of the signal that carriers the data. Alternatively, a data header or other identifier may be provided with the data or included in the data or in the data.

Once received, the forward traffic is a binary data stream <NUM>. That is, in some embodiments, the forward traffic is a binary representation, such as an intensity modulated or phase modulated optical signal. In alternative embodiments, the forward traffic can be decoded into any other binary representation.

<FIG> shows the relationship of IF signals <NUM>, optical channels <NUM> and optical bands <NUM>, <NUM>, <NUM>, <NUM> used by the system in some embodiments. The particular selection of bandwidths, frequencies, quantities of channels and wavelengths are merely examples provided to make disclosure of the concepts easier. Alternative modulation schemes can be used, as well as other optical wavelengths, quantities of channels and other RF and/or IF bandwidths and frequencies. The scheme shown is merely provided to illustrate one particular scheme that might be used. As shown, a plurality of <NUM> wide binary modulated IF signals (e.g. <NUM>) <NUM> carry binary data to be transmitted in one user spot beam. Examples of other bandwidths that can be used include <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or any other suitable bandwidth.

The binary (i.e., digital) content modulated onto each <NUM> wide binary modulated IF signal <NUM> is used to perform binary intensity modulation of one of <NUM> optical channels within one of <NUM> optical bands <NUM>. In some embodiments, the four bands <NUM>, <NUM>, <NUM>, <NUM> of the optical spectrum are <NUM>, <NUM>, <NUM> and <NUM>. However, bands may be selected that lie anywhere in the useful optical spectrum (i.e., that portion of the optical spectrum that is available at least minimally without excessive attenuation through the atmosphere). In general, optical bands are selected that have no more attenuation than bands that are not selected. That is, several optical bands may have less attenuation then the rest. In such embodiments, a subset of those optical bands are selected. Several of those selected bands may exhibit very similar attenuation.

In one example, each optical channel is defined by the wavelength at the center of the channel and each optical channel is spaced approximately <NUM> apart (i.e., <NUM> wide). While the RF signal <NUM> that is modulated onto the optical channel is only <NUM> wide, the spacing allows the optical signals to be efficiently demultiplexed. In some embodiments, each SAN <NUM> wavelength division multiplexes (WDM) several (e.g., <NUM>) such <NUM> optical signals <NUM> (i.e., <NUM> x <NUM>) together onto an optical output signal. Accordingly, the digital content of <NUM> optical channels can be sent from one SAN <NUM>.

<FIG> shows an optical transmitter <NUM> used to perform the optical modulation of the binary data stream <NUM> onto the optical signals. In accordance with the embodiment that implements the scheme shown in <FIG>, the optical transmitter <NUM> includes four optical band modules 608a - 608d (two shown for simplicity) and an optical combiner <NUM>. Each of the <NUM> optical band modules <NUM> include <NUM> optical modulators <NUM> (two shown for simplicity) for a total of <NUM> modulators <NUM>. Each of the <NUM> modulators <NUM> output an optical signal that resides in one of <NUM> optical channels <NUM> (see <FIG>). The channels are divided into <NUM> optical bands <NUM>, <NUM>, <NUM>, <NUM>.

The modulator <NUM> determines the optical channel <NUM> based on the wavelength λ <NUM> of a light source <NUM> that produces an optical signal. An MZM <NUM> intensity modulates the output of the first light source <NUM> with an intensity proportional to the amplitude of the binary data stream <NUM>. The binary data stream <NUM> is summed with a DC bias in a summer <NUM>. Since the binary data stream <NUM> is a digital signal (i.e., having only two amplitudes), the resulting optical signal is a binary modulated optical signal. The modulated optical output from the MZM modulator <NUM> is coupled to an optical combiner <NUM>. For a system using a modulation scheme such as the one illustrated in <FIG>, each of the <NUM> light sources <NUM> that reside within the same optical band module <NUM> output an optical signal at one of <NUM> different wavelengths λ1. The <NUM> wavelengths correspond to the <NUM> optical channels <NUM> within the first optical band <NUM>. Likewise, the light sources <NUM> in the optical modulators <NUM> in each other optical band module <NUM> output an optical signal having a wavelength of λ1 equal to the wavelength of the channels in the corresponding optical band <NUM>, <NUM>, <NUM>. Accordingly, the <NUM> optical outputs <NUM> from the four optical band modules 608a - 608d each have a different wavelength and fall within the <NUM> optical channels of the four bands that are defined by the wavelengths λ1 of signals generated by the <NUM> light source <NUM>. The optical combiner <NUM> outputs a wavelength division multiplexed (WDM) optical signal <NUM> that is the composite of each signal <NUM>.

The SAN <NUM> sends the optical signal <NUM> to the satellite <NUM> over an optical feeder uplink <NUM> (see <FIG>). The optical signal emitted by the optical transmitter <NUM> is received by a lens <NUM> in the satellite <NUM>. In some embodiments, a lens <NUM> is part of a telescope within the optical receiver <NUM>. In some embodiments, the lens <NUM> is steerable (i.e., can be directed to point at any one of several SANs <NUM> within the system or any one from within a subset). By allowing the lenses <NUM> to be pointed to more than one of the SANs <NUM>, the lens <NUM> can be pointed to a SAN <NUM> having an optical path to the satellite that is not currently subject to signal fading. The lens <NUM> may be pointed using mechanical <NUM>-axis positioning mechanisms. Pointing of the lens may be accomplished by measuring the receive signal strength of a signal transmitted over the optical channel and using the signal strength to identify when the lens is pointed at a SAN with an optical link of sufficient quality (i.e., above a desired quality threshold). Either ground commands or on-board processing may provide directions to the lens positioning mechanisms to correctly point the lens <NUM> at the desired SAN <NUM>.

The optical receiver <NUM> further includes an optical demultiplexer <NUM>, such as a filter or prism. The optical receiver <NUM> has a plurality of outputs, each output corresponding with an optical wavelength. As shown in <FIG>, the optical receiver <NUM> has <NUM> outputs. However, as noted above, the particular frequency, number of optical bands and wavelength selection, and thus the number of outputs from the optical receiver <NUM>, are provided herein merely as an example and are not intended to limit the systems, such as system <NUM>, to a particular number.

In some embodiments, each wavelength resides within one of the four optical bands <NUM>, <NUM>, <NUM>, <NUM>. Each optical wavelength is at the center of an optical channel. Optical channels within one band are spaced approximately <NUM> (i.e., <NUM>) apart. Making the optical channels spacing wide makes it easier to provide an optical demultiplexer <NUM> that can demultiplex the optical signal to provide each of the <NUM> optical channels on a separate output. In some embodiments, an additional lens <NUM> is provided to focus the output of the optical demultiplexer <NUM> into the input of an optical detector, such as a photodiode <NUM>. The photodiode <NUM> generates an electrical signal by detecting the intensity envelope of the optical signal <NUM> presented at an optical input to the photo diode. In some embodiments in which the optical signal <NUM> was intensity modulated to one of two intensity levels, the first intensity level representing a logical "<NUM>" results in an electrical signal having a first amplitude which also represents a logical "<NUM>". A second intensity level representing a logical "<NUM>" results in an electrical signal an amplitude representing a logical "<NUM>". Therefore, the electrical signal is placed in a first state when the intensity of the optical signal <NUM> is in a state representing a logical "<NUM>" and placed in a second state when the intensity of the optical signal <NUM> is in a state representing a logical "<NUM>". Accordingly, the optical receiver has a plurality of digital outputs <NUM>. The electrical signal output from the digital output <NUM> of the photodiode <NUM> is coupled to a modulator <NUM>, such as a bi-phase modulator. In some embodiments, such as the embodiment of <FIG>, an LNA <NUM> is provided between the photo diode <NUM> and the bi-phase modulator <NUM>. The output of the bi-phase modulator <NUM> is a BPSK modulated IF signal (i.e., analog signal) having two phases. The BPSK modulator <NUM> outputs a signal having a first phase representing a logical "<NUM>" in response to the electrical input signal at the first amplitude (i.e., in the first state). When the input to the modulator <NUM> has an amplitude representing a logical "<NUM>" (i.e., the second state), the phase of the output of the BPSK modulator <NUM> is shifted to a second phase different from the first phase. The output of the modulator <NUM> is coupled to the input of a switch matrix <NUM>.

In the simplified schematic of <FIG>, a second SAN <NUM>, lens <NUM>, optical receiver <NUM> and plurality of bi-phase modulators <NUM> (i.e., <NUM>) are coupled to the switch matrix <NUM>. While only two SANs <NUM> are shown in <FIG>, it should be understood that the satellite may receive optical signals from several SANs <NUM> (e.g., <NUM>).

In some embodiments, the switch matrix <NUM> shown in <FIG> has a plurality of (e.g., <NUM>) inputs for each lens <NUM>. That is, if the satellite <NUM> has <NUM> lenses <NUM>, the matrix switch <NUM> has <NUM> inputs, each coupled to one of the modulators <NUM>. The switch matrix <NUM> allows signals at the outputs of the switch matrix <NUM> to be selectively coupled to inputs of the switch matrix <NUM>. In some embodiments, any input can be coupled to any output. However, in some embodiments, only one input can be coupled to any one output. Alternatively, the inputs and outputs are grouped together such that inputs can only be coupled to outputs within the same group. Restricting the number of outputs to which an input can be coupled reduces the complexity of the switch matrix <NUM> at the expense of reduced flexibility in the system.

The outputs of the switch matrix <NUM> are each coupled to an upconverter <NUM>. The upconverter <NUM> upconverts the signal to the frequency of the user downlink carrier. For example, in some embodiments, the signal output from the switch matrix <NUM> is a <NUM> wide IF signal. The <NUM> wide IF signal is upconverted to an RF carrier having a <NUM> center frequency. The output of each upconverter <NUM> is coupled to a corresponding power amplifier <NUM>. The output of each amplifier <NUM> is coupled to one of a plurality of antenna input, such as a inputs (e.g., antenna feeds not shown) of one of the antennas <NUM>, <NUM>. Accordingly, each of the outputs of the switch matrix <NUM> is effectively coupled to a corresponding one of the antenna inputs. In some embodiments, each input of each antenna <NUM>, <NUM> transmits a user spot beam to one user beam coverage area <NUM> (see <FIG>). The switch matrix <NUM> is capable of selecting which input (i.e., bi-phase modulator <NUM>) is coupled to which output (i.e., upconverter <NUM>). Accordingly, when (or before) the signal from one of the SANs <NUM> fades and errors become intolerable, the switch matrix <NUM> can couple the input of the upconverter <NUM> (i.e., the associated antenna feed) to a SAN <NUM> that is sending an optical signal that is not experiencing significant fading. In some embodiments, the switch matrix <NUM> allows the content that is provided to the antenna inputs to be time division multiplexed so that content from a particular SAN can be distributed to more than one user spot beam (i.e., antenna feed).

That is, when each lens <NUM> is receiving a signal from the SAN <NUM> to which it is pointing, each of the <NUM> outputs from the optical receiver <NUM> associated with that Lens <NUM> will have a signal. In the embodiment in which each antenna input to the antennas <NUM>, <NUM> transmits a user spot beam to a particular user coverage area <NUM>, all of the user coverage areas <NUM> will receive a signal (assuming the switch matrix <NUM> is mapped to couple each input to one output). The switch matrix <NUM> selects which analog output from the bi-phase modulator <NUM> is to be coupled to each antenna input (e.g., transmitted to each feed of the single-feed per beam antenna <NUM>, <NUM>) (i.e., in each user spot beam). However, when the optical signal from a particular SAN <NUM> fades, a signal is still provided to all of the antenna inputs to ensure that no user coverage areas <NUM> loses coverage. Time multiplexing the signals from one SAN to more than <NUM> antenna inputs allows one SAN <NUM> to provide signals to more than <NUM> user coverage areas <NUM>. While the total capacity of the system is reduced, the availability of the system to provide each user coverage area with content is enhanced. This is beneficial in a system with an optical feeder link. In some embodiments, such time multiplexing is done for a short time while the lens <NUM> that is directed to a SAN <NUM> that has a weak optical link is redirected to another SAN to which there is a stronger optical link. More generally, the matrix <NUM> can be used to time multiplex analog signals output from the optical receiver <NUM> to more than one user spot beam, such that during a first period of time the analog signal is coupled to a first antenna input (e.g., feed) transmitting a user spot beam directed to a first user beam coverage area. During a second period of time, the analog signal is coupled to a second antenna input (e.g., feed) transmitting a user spot beam directed to a second user beam coverage area.

Once each lens <NUM> is receiving a sufficiently strong optical signal, the switch matrix <NUM> can again map each output to a unique output in a one-to-one correspondence of input to output. In some such embodiments, control of the switch matrix <NUM> is provided by a telemetry signal from a control station. In most embodiments, since all <NUM> of the IF signals from the same SAN <NUM> will degrade together, the switch matrix <NUM> need only be able to select between K/<NUM> outputs, where K is the number of user spot beams and <NUM> is the number of photo diodes <NUM> in one optical receiver <NUM>. As noted above, the process of controlling the routing through the satellite to map SANs <NUM> to user spot beams is referred to herein as feeder link diversity. As will be discussed below, feeder link diversity can be provided in three different ways.

In some embodiments, the satellite <NUM> has more antenna inputs than transponders (i.e., paths from the optical receiver to the switches <NUM>, <NUM>). That is, a limited number of transponders, which include power amplifiers (PAs) <NUM>, upconverters <NUM>, etc., can be used to transmit signals to a relatively larger number of user beam coverage areas. By sharing transponders among antenna inputs, the output from each photo diode <NUM> can be time multiplexed to service a number of user beam coverage areas that is greater than the number of transponders provided on the satellite <NUM>. In this embodiment, RF switches <NUM> are used to direct the output of the PA <NUM> to different inputs of the one or both of the antennas <NUM>, <NUM> at different times. The times are coordinated so that the information present on the signal is intended to be transmitted to the user beam coverage area to which the input is directed (i.e., the feed is pointed). Accordingly, one transponder can be used to provide information to several user beam coverage areas in a time multiplexed fashion. By setting the switches <NUM>, <NUM> to direct the signal to a particular antenna <NUM>, <NUM>, the signal received by each of the lenses <NUM> can be directed to a particular spot beam. This provides flexibly in dynamically allocating capacity of the system.

The switches <NUM>, <NUM> direct the signal to inputs of any of the antennas <NUM>, <NUM> mounted on the satellite. In some embodiments, the output from the switches <NUM>, <NUM> may be directed to a subset of the antennas. Each antenna <NUM>, <NUM> is a single-feed per beam antenna directed to a particular user beam coverage area, thereby producing a spot beam. In alternative embodiments, the PAs <NUM> may be directly connected to the antenna inputs, with the matrix switch <NUM> determining which of the signals detected by each particular photo diodes <NUM> will be transmitted to which of the user beam coverage areas. In addition, even in embodiments in which there are an equal number of satellite transponders and antenna inputs, having switches <NUM>, <NUM> can reduce the complexity of the switch matrix <NUM>. That is, using a combination of the switch matrix <NUM> and switches <NUM>, <NUM>, the switch matrix <NUM> need not be capable of coupling each input to each output. Rather, the matrix inputs, outputs and antenna inputs can be grouped such that any input of a group can be coupled only to any output of that same group. The switches <NUM>, <NUM> can switch between antenna inputs (e.g., feeds) to allow outputs of one group to be coupled to an antenna input of another group.

The switch matrix <NUM> may be operated statically or in a dynamic time division multiple access mode. In the static mode of operation, the configuration of the paths through the switch matrix <NUM> essentially remains set for relatively long periods of time. The configuration of the switch matrix <NUM> is only changed in order to accommodate relatively long-term changes in the amount of traffic being transmitted, long term changes in the quality of a particular link, etc. In contrast, in a dynamic time division multiple access mode, the switch matrix <NUM> is used to time multiplex data between different forward downlink antenna inputs. Accordingly, the switch matrix <NUM> selects which inputs to couple to the output of the switch matrix <NUM>. This selection is based on whether the input signal is strong enough to ensure that the number of errors encountered during demodulation of the signal at the user terminal <NUM>, <NUM> is tolerable. In some such embodiments, time multiplexing the analog outputs of the optical receiver <NUM> to different antenna inputs allows one SAN <NUM> to service more than one user beam coverage area. During a first period of time, one or more signals output from an optical receiver <NUM> can each be coupled through to a unique one of a first set of antenna inputs (i.e., directed to a unique one of a first set of user beam coverage areas). During a second period of time, one or more of those same signals can be coupled through to different antenna inputs (i.e., different user beam coverage areas). Such time multiplexing of the analog outputs <NUM> from the optical receiver <NUM> can be performed in response to one of the lens <NUM> of an optical receiver <NUM> pointing to a "weak" SAN <NUM> (i.e., a SAN <NUM> having an optical link that is below a quality threshold). In such a embodiment, a first data stream initially set to the weak SAN <NUM> can be redirected by the core node to a "strong" SAN <NUM> (i.e., a SAN <NUM> having an optical link that is above the quality threshold). The strong SAN <NUM> time multiplexes that information such that for a portion of the time the strong SAN <NUM> transmits information directed to a first set of user beam coverage areas to which the first data stream is intended to be sent. During a second period of time, the strong SAN <NUM> transmits a second data stream directed to a second set of user beam coverage areas. Accordingly, during one period of time, information that would have been blocked from reaching the satellite <NUM> by the poor optical link between the weak SAN <NUM> and the satellite <NUM> can be transmitted to the satellite <NUM> through the strong SAN <NUM>. During this time, the lens <NUM> that is pointing at the weak SAN <NUM> can be redirected to point to a strong SAN <NUM> that is not already transmitting to the satellite <NUM>. As noted above, this process of redirecting information from a weak SAN to a strong SAN is an aspect of feeder link diversity.

By determining when a feeder uplink signal is experiencing an unacceptable fade, data can be routed away from the SAN <NUM> that is using the failing feeder uplink and to a SAN <NUM> that has a feeder uplink signal that has an acceptable signal level. By the process of feeder link diversity, the signal transmitted through the selected SAN <NUM> can then be routed through the switch matrix <NUM> to the spot beam to which data is intended to be sent.

The system <NUM> has the advantage of being relatively simple to implement within the satellite <NUM>. Conversion of binary modulated optical data to a BPSK modulated IF signal using photodiodes <NUM> and bi-phase modulators <NUM> is relatively simple. Such bi-phase modulators are relatively easy and inexpensive to build, require relatively little power and can be made relatively small and lightweight. However, using BPSK modulation on the RF user downlink <NUM> is not the most efficient use of the limited RF spectrum. That is, greater capacity of the RF user downlink <NUM> (see <FIG>) can be attained by using a denser modulation scheme, such as <NUM> QAM instead of BPSK on the RF user downlink <NUM>.

For example, in an alternative embodiment of the system <NUM> that implements the second of the three techniques noted above, according to an embodiment useful for understanding the invention, the analog signal <NUM> that is to be transmitted on the user downlink is modulated with a denser modulation scheme. Generating the complex modulation on the analog signal <NUM> requires that the modulator be a very complex modulator that takes the digital data stream and converts the data stream to one or more complex modulated signals. The complex modulated signal <NUM> can be a high order modulation such as <NUM>-QAM, 8psk, QPSK for example. Alternatively, any other modulation scheme can be used that is capable of modulating symbols onto an IF carrier, where the symbols represent more than two logical states. That is, the binary intensity modulation of the optical signal results in the output <NUM> of the optical receiver <NUM> providing an electronic signal that has binary modulation representing the underlying content. In order to modulate the analog signal <NUM> with a more complex modulation scheme, such as <NUM> QAM, the modulator <NUM> is a QAM modulator and thus perform QAM modulation of the IF signal based on the digital content output from the photodiode <NUM>.

Accordingly, in some embodiments, the bi-phase modulator <NUM> of the system <NUM> is replaced with a QAM modulator <NUM> (i.e., a modulator in which each symbol represents more than <NUM> bits). Accordingly, rather than limiting the modulation of the IF signals <NUM> to a binary modulation scheme (i.e., two logical states), such as BPSK, the modulator <NUM> allows the IF signals <NUM> to be modulated with a denser modulation scheme (i.e., schemes in which symbols are capable of representing more than two values, such as QAM). While the more complex QAM modulator provides a more efficient modulation of the IF signals <NUM> (QAM verses BPSK), it is more complex, requires more power, is heavier and more expensive than a bi-phase modulator.

<FIG> is an illustration of the return path for the system <NUM>. User terminals <NUM> transmit a binary modulated signal to the satellite <NUM>. Switches <NUM> coupled to each element of the antenna (e. single beam per feed antennas <NUM>, <NUM>) select between satellite transponders comprising a Low noise amplifier (LNA) <NUM>, frequency converter <NUM> and digital decoder <NUM>. The frequency converter <NUM> down converts the received signal from the user uplink frequency to IF. The decoders <NUM> decode the binary modulation on the received IF signal. Accordingly, the output of each decoder <NUM> is a digital signal. The digital decoders <NUM> are coupled to inputs to a switch matrix <NUM>. The switch matrix <NUM> allows signals that are received over each of the user spot beams to be modulated on different optical links (i.e., transmitted to different SANs <NUM>) depending upon whether there is significant fading on the optical downlink to each SAN <NUM>. The outputs of the switch matrix <NUM> are coupled to inputs to optical transmitters <NUM>. Each optical transmitter <NUM> is essentially identical to the optical transmitter <NUM> shown in <FIG> and discussed above. In some embodiments in which the optical spectrum is used in essentially the same manner as used on the forward feeder link (see <FIG>), each of four optical band modules <NUM> receive <NUM> outputs from the matrix switch <NUM> for a total of <NUM> inputs to the optical transmitter <NUM>. In some embodiments in which the satellite can receive optical signals from <NUM> SANs <NUM>, there are <NUM> such optical transmitters <NUM> that can receive a total of <NUM> outputs from the switch matrix <NUM>. Each optical transmitter <NUM> outputs an optical signal <NUM>. The optical signal <NUM> is receive by a lens <NUM> within an optical receiver <NUM> in a SAN <NUM>. The optical receiver <NUM> and lens <NUM> are essentially identical to the optical receiver <NUM> and lens <NUM> within the satellite <NUM>, as described above with reference to <FIG>. Accordingly, the output of the optical receiver <NUM> is a binary data stream. The output of the optical receiver is sent to an information network, such as the network that provided forward traffic to the SAN <NUM>.

In an alternative embodiment, the return link for the system <NUM>, the modulation used on the return uplink from the user terminals <NUM> to the satellite <NUM> is a more efficient modulation scheme than binary modulation. Accordingly, the binary modulator <NUM> is a more complex modulator <NUM>. The binary data output from the demodulator <NUM> is the result of decoding the modulated symbols modulated onto the IF signal by the user terminal <NUM>. For example, if <NUM> QAM was used on the user uplink, then the signal output from the demodulator is a digital stream of values represented by <NUM> QAM symbol. The binary signal output from the converter <NUM> is coupled to an input to the switch matrix <NUM>. Both the binary demodulator and the complex demodulator <NUM> output a digital data stream to be used to perform binary modulation of the optical signal transmitted on the feeder downlink by the optical transmitter <NUM>.

<FIG> is a simplified schematic of a system <NUM> for implementing the third technique according to an embodiment useful for understanding the invention. In some embodiments of the system <NUM>, a SAN <NUM> receives the forward traffic as "baseband" signals <NUM> that are coupled to the inputs of a baseband to IF converter <NUM>. In some embodiments, seven <NUM> wide baseband sub-channels <NUM> are combined in a <NUM> wide IF signal <NUM>. Each of the <NUM> wide signals <NUM> is transmitted to one user coverage area <NUM>. <FIG> illustrates the relationship between baseband sub-channels <NUM>, IF signals <NUM> and optical signals within the system <NUM>.

Examples of other bandwidths that can be used include <NUM> (e.g., a single <NUM> sub-channel), <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or any other suitable bandwidth.

<FIG> is a simplified illustration of a SAN <NUM>, such as the SAN <NUM> shown in <FIG>. In some embodiments, there are <NUM> baseband to IF converters <NUM>, shown organized in four IF combiners <NUM>, each comprising <NUM> converters <NUM>. Grouping of the baseband to IF converters <NUM> within IF converters <NUM> is not shown in <FIG> for the sake of simplifying the figure. Each of the <NUM> baseband to IF converters <NUM> has S inputs, where S is the number of sub-channels <NUM>. In some embodiments in which the sub-channel <NUM> has a bandwidth of <NUM> and the signal <NUM> has a bandwidth of <NUM>, S equals <NUM>. Each input couples one of the sub-channels <NUM> to a corresponding frequency converter <NUM>. The frequency converters <NUM> provide a frequency offset to allow a subset (e.g., S = <NUM> in <FIG>) of the sub-channels <NUM> to be summed in a summer <NUM>. Accordingly, in some embodiments, such as the one illustrated in <FIG>, a SAN <NUM> processes <NUM> channels, each <NUM> wide. In some embodiments, the <NUM> wide signal can be centered at DC (i.e., using zero IF modulation). Alternatively, the signal <NUM> can be centered at a particular RF frequency. In one particular embodiment, an RF carrier <NUM> is centered at the RF downlink frequency (in which case the satellite will need no upconverters <NUM>, as described further below). The output <NUM> from each summing circuit <NUM> is an IF signal <NUM> that is coupled to one of <NUM> optical modulators <NUM>. The <NUM> optical modulators <NUM> are grouped into <NUM> optical band modules <NUM>. Each optical modulator <NUM> operates essentially the same as the optical modulator <NUM> shown in <FIG> and discussed above. However, since the input <NUM> to each optical modulator <NUM> is an analog signal, the optical signal output from each optical modulator <NUM> is an intensity modulated optical signal having an amplitude envelope that follows the amplitude of the IF signal <NUM>.

An optical combiner <NUM> combines the outputs from each of the <NUM> optical modulators <NUM> to generate a wavelength division multiplexed (WDM) composite optical signal <NUM>. The number of baseband to IF converters <NUM> and the number of optical modulators <NUM> in the optical band module <NUM> can vary. As shown in <FIG>, the four optical modulators <NUM> can be designed to output optical signals having wavelengths centered at <NUM> nanometers, <NUM> nanometers, <NUM> nanometers and <NUM> nanometers.

In the system <NUM>, the optical transmitter <NUM> (similar to the optical transmitter <NUM> of <FIG>) emits an RF modulated composite optical signal <NUM>. The RF modulated composite optical signal <NUM> is received within the satellite <NUM> by a lens <NUM> (see <FIG>). The lens <NUM> can be directed to any of a plurality of SANs <NUM> capable of transmitting an optical signal to the satellite <NUM>. The output of the lens <NUM> is coupled to the input of an optical detector, such as a photodiode <NUM> (e.g. a PIN diode). The photodiode <NUM> detects the envelope (i.e., the contour of the intensity) of the optical signal and converts the envelope of the optical signal to an electrical signal. Since the optical signal is intensity modulated with the IF signal <NUM>, the resulting electrical signal output from the photodiode <NUM> is essentially the same as the IF signal <NUM> that was modulated by the SAN <NUM> onto the composite optical signal <NUM>. The photodiode <NUM> is coupled to an amplifier <NUM>. The signal output from the amplifier <NUM> is then coupled to an input of a matrix switch <NUM>. The matrix switch <NUM> performs in the same way as the matrix switch <NUM> discussed with respect to <FIG> above. Accordingly, the switch matrix <NUM> selects which inputs to couple to the output of the switch matrix <NUM>. The output of the matrix switch <NUM> is handled the same as in the systems <NUM> described above in embodiments in which the signal <NUM> is at zero IF. In embodiments in which the signal <NUM> output from the baseband to IF module <NUM> within the SAN is at a frequency that is to be directly transmitted from the satellite <NUM>, then the handling will be the same, but for the fact that the upconverters <NUM> are not required.

<FIG> is an illustration of the return link for the system <NUM>. The return link for the system <NUM> is essentially the same as shown in <FIG>. However, rather than the user terminals <NUM> transmitting a signal having binary modulation, the user terminals <NUM> transmit a signal having a more efficient modulation (e.g., <NUM> QAM rather than QPSK). Accordingly, the output digital decoder <NUM> is not required. The downconverter <NUM> downconverts the RF frequency used on the user uplink to an appropriate IF frequency. In some embodiments, the IF frequency signal is a zero IF signal that is <NUM> wide. The output of each downconverter <NUM> is coupled to an input of the switch matrix <NUM>. Therefore, the inputs of the MZM modulator <NUM> (see <FIG>) receive an analog signal from the switch matrix <NUM>. Accordingly, the output of each optical modulator <NUM> is an intensity modulated optical signal in which the intensity envelope tracks the signal output from the downconverter <NUM>. In some embodiments, the optical modulator <NUM> directly modulates the RF user uplink frequency onto the optical signal. Accordingly, the frequency converter <NUM> is not required. In embodiments in which the downconverter <NUM> reduces the user uplink frequency to a zero IF signal, the combined optical signal <NUM> is handled in the same way as discussed with regard to <FIG>. In embodiments in which the optical signal is modulated with the user uplink frequency, a downconverter may be included within the modem <NUM> or prior to coupling the signal from the optical receiver <NUM> to the modem <NUM>.

Having discussed the three different techniques for modulating signals on the feeder link, each of which use a first system architecture having a satellite that uses a matrix switch <NUM> to allow a flexible assignment of received carriers to user spot beams, a second and third system architectures are discussed. The second system architecture includes a satellite having on-board beam forming. The third system architecture uses ground-based beam forming.

<FIG> is a simplified schematic of a system <NUM> using the technique shown in <FIG> (i.e., modulating the optical feeder uplink with binary modulation and using that binary content to modulate an RF user downlink). However, the system <NUM> uses the second system architecture in which a satellite <NUM> is capable of performing on-board beamforming. The system <NUM> operates similarly to the system <NUM> described above. However, the IF output from each bi-phase modulator <NUM> is coupled to a weight/combiner module <NUM> rather than to the switch matrix <NUM>.

<FIG> is a simplified block diagram of a weight/combiner module <NUM> in which K forward beam signals <NUM> are received in the weight/combiner module <NUM> by a beamformer input module <NUM>. The K signals <NUM> are routed by the input module <NUM> to an N-way splitting module <NUM>. The N-way splitting module <NUM> splits each of the K signals <NUM> into N copies of each forward beam signal, where N is the number of elements in the antenna array that is to be used to form K user spot beams.

In the example of the system described above with respect to <FIG>, there are <NUM> active SANs, each transmitting an optical signal comprising <NUM> optical channels. Each of the <NUM> optical channels carries a <NUM> IF signal (i.e., forward beam signal). Therefore, there are <NUM> forward beam signals (i.e., <NUM> SANs x <NUM> IF signals). Accordingly K = <NUM>. In some embodiments, the satellite has an antenna array <NUM> having <NUM> array elements. Accordingly, N = <NUM>.

Each output from the N-way splitting module <NUM> is coupled to a corresponding input of one of <NUM> weighting and summing modules <NUM>. Each of the <NUM> weighting and summing modules <NUM> comprises <NUM> weighting circuits <NUM>. Each of the <NUM> weighting circuits <NUM> place a weight (i.e., amplify and phase shift) upon a corresponding one of <NUM> signals output from the N-way splitting module <NUM>. The weighted outputs from the weighting circuits <NUM> are summed by a summer <NUM> to form <NUM> beam element signals <NUM>. Each of the <NUM> beam element signals <NUM> is output through a beamformer output module <NUM>. Looking back at <FIG>, the <NUM> beam element signals <NUM> output from the weight/combiner module <NUM> are each coupled to a corresponding one of <NUM> upconverters <NUM>. The upconverters <NUM> are coupled to PAs <NUM>. The outputs of the PAs <NUM> are each coupled to a corresponding one of <NUM> antenna elements of the antenna array <NUM>. The antenna array can be any of: a direct radiating array (where each antenna element directly radiates in the desired direction), an array fed reflector (where each antenna element illuminates a reflector shared by all antenna elements), or any other suitable antenna configuration. The combination of the antenna array <NUM> and the weight combiner module <NUM> is also referred to as a phased array antenna.

The relative weights of the signals being applied to the elements at each of the locations within the phase array antenna <NUM> will result in the plurality of weighted signals superposing upon one another and thus coherently combining to form a user beam.

Accordingly, by applying desired weighting to the plurality of signals <NUM> to generate the beam element signals <NUM> output from the weight/combiner module <NUM>, a signal <NUM> applied to each input of the weight/combiner module <NUM> can be directed to one of the plurality of user beam coverage areas. Since the satellite <NUM> can use the weight/combiner module <NUM> and array antenna <NUM> to direct any of the received signals to any of the user beam coverage areas, information that would otherwise be transmitted over a particular feeder uplink that is experiencing intolerable fading can be routed to one of the other SANs. Accordingly, the information can be transmitted to the satellite <NUM> through a SAN <NUM> that is not experiencing intolerable fading to provide feed link diversity, as described above in the context of the matrix switch <NUM>. Similar time division multiplexing can be done to transmit signals received by one of the lenses <NUM> in several user spot beams as described above.

Using a satellite <NUM> that has on-board beamforming provides flexibility to allow feeder link diversity with regard to signals received from the plurality of SANs <NUM>. The use of on-board beam forming eliminates the need for the switch matrix <NUM> shown in <FIG>. A similar architecture can be employed on the return paths (i.e., the user uplink and the feeder downlink). That is, the user ground terminals <NUM> transmit an RF signal up to the satellite <NUM> on the user uplink. Receive elements in the antenna array <NUM> receive the RF signal. The weight/combiner module <NUM> weights the received signals received by each receive element of the antenna <NUM> to create a receive beam. The output from the weight/combiner module <NUM> is down converted from RF to IF.

In some embodiments, the upconverters <NUM> are placed at the input of the weight/combiner module <NUM>, rather than at the outputs. Therefore, RF signals (e.g., <NUM> signals) are weighted and summed. The beam element signals are then transmitted through each of the antenna array elements.

In some embodiments, the satellite has several weight/combiner modules (not shown for simplicity). The inputs to each weight/combiner module are coupled to one or more optical receivers <NUM>. In some embodiments, all of the outputs from one optical receiver <NUM> are coupled to the same weight/combiner module. Each weight/combiner module generates N outputs. The N outputs from each weight/combiner module are coupled one-to-one to elements of one N-element antenna array (only one shown for simplicity). Accordingly, there is a one-to-one relationship between the antenna arrays <NUM> and the weight/combiner modules <NUM>.

In some embodiments, the second architecture shown in <FIG> (i.e., on-board beam forming) is used with a QAM modulator <NUM>, similar to the system <NUM>. However, the satellite <NUM> has on-board beamforming.

<FIG> is a simplified schematic of a system <NUM> using the technique discussed with respect to <FIG> in which an optical signal is RF modulated at the SAN <NUM> according to an embodiment useful for understanding the invention. However, the satellite architecture is similar to that of <FIG> and <FIG> in which a satellite <NUM> has on-board beamforming capability. The SANs <NUM>, lenses <NUM>, optical detectors (such as photodiodes <NUM>), amplifiers <NUM> and upconverters <NUM> are all similar to those described with respect to <FIG>. However, the weight/combiner module <NUM> and array antenna <NUM> are similar to those described with respect to <FIG>. Similar to the architecture described in <FIG>, the weight/combiner <NUM> and array antenna <NUM> allow the satellite <NUM> to transmit the content of the signals received from one or more of the SANs <NUM> to any of the user beam coverage areas, thus providing feeder link diversity. Therefore, if one or more of the feeder uplinks from the SANs <NUM> to the satellite have an intolerable fade, the content that would otherwise be sent on that feeder uplink can instead be sent through one of the other SANs <NUM> using a feeder uplink that is not experiencing an intolerable fade.

<FIG> is an illustration of a forward link of a satellite communications system <NUM> using the third system architecture (i.e., ground-based beamforming) including an optical forward uplink <NUM> and a radio frequency forward downlink <NUM>. In some embodiments, the system <NUM> includes a forward link ground-based beamformer <NUM>, a satellite <NUM> and a relatively large number (M) of SANs <NUM> to create a relatively large capacity, high reliability system for communicating with user terminals <NUM> located within <NUM> user beam coverage areas <NUM> (see <FIG> discussed in detail below). Throughout the discussion of the system <NUM>, M = <NUM> SANs <NUM> are shown in the example. However, M = <NUM> is merely a convenient example and is not intended to limit the system disclosed, such as system <NUM>, to a particular number of SANs <NUM>. Similarly, <NUM> optical channels are shown in the example of the system <NUM>. Likewise, the antenna array is shown as having <NUM> elements. As noted above, the particular frequencies, wavelengths, antenna array elements, and numbers of similar parallel channels, components, devices, user beam coverage areas, etc. should not be taken as a limitation on the manner in which the disclosed systems can be implemented, except where expressly limited by the claims appended hereto.

Forward traffic (i.e., forward beam input signal <NUM>) to be communicated through the system <NUM> is initially provided to the beamformer <NUM> from a source, such as the Internet, through distribution equipment, such as a core node or similar entity (not shown). The distribution equipment may manage assignment of frequency and/or time slots for transmissions to individual user terminals and group together data destined for transmission to particular beams, in addition to performing other functions. Input signals <NUM> to the beamformer <NUM> (or some portion of the information carried by the forward beam input signal <NUM>) can represent data streams (or modulated data streams) directed to each of <NUM> user beams. In one embodiment, each of the <NUM> forward beam input signals <NUM> is a <NUM> wide IF signal. In some embodiments, the forward beam input signal <NUM> is a composite <NUM> wide carrier that is coupled to the input of the beamformer <NUM>.

Each of the forward beam input signals <NUM> is "directed" to a user beam coverage area <NUM> by the beamformer <NUM>. The beamformer <NUM> directs the forward beam input signal <NUM> to a particular user beam coverage area <NUM> by applying beam weights to the <NUM> forward beam input signals <NUM> to form a set of N beam element signals <NUM> (as further described below with respect to <FIG>). Generally, N is greater than or equal to K. In some embodiments, N = <NUM> and K = <NUM>. The <NUM> beam element signals <NUM> are amplified and frequency converted to form RF beam element signals <NUM>. Each is transmitted from an element of an N-element (i.e., <NUM>-element) antenna array <NUM>. The RF beam element signals <NUM> superpose on one another within the user beam coverage area <NUM>. The superposition of the transmitted RF beam element signals <NUM> form user beams within the user beam coverage areas <NUM>.

In some embodiments, the <NUM> beam element signals <NUM> are divided among several SANs <NUM>. Accordingly, a subset of the beam element signals <NUM> (e.g., <NUM>/<NUM>) are coupled to each SAN <NUM>, where <NUM> is the number of SANs <NUM>. Thus, the combination of <NUM> SANs <NUM> will transmit <NUM> beam element signals <NUM> from the beamformer <NUM> to the satellite <NUM>. In some embodiments, the beamformer <NUM> is co-located with one of the SANs <NUM>. Alternatively, the beamformer <NUM> is located at another site. Furthermore, in some embodiments, the beamformer <NUM> may be distributed among several sites. In one such embodiment, a portion of the beamformer <NUM> is co-located with each SAN <NUM>. Each such portion of the beamformer <NUM> receives all of the forward traffic <NUM>, but only applies beam weights to those <NUM> (i.e., <NUM>/<NUM>) signals <NUM> to be transmitted to the SAN <NUM> that is co-located with that portion of the beamformer <NUM>. In some embodiments, several beamformers are provided (not shown for simplicity). Each beamformer generates N outputs (i.e., beam element signals). The N beam element signals will be coupled one-to-one to elements of one N-element antenna array on the satellite <NUM> (only one shown for simplicity). Accordingly, there is a one-to-one relationship between the antenna arrays <NUM> and the beamformers <NUM>. In some embodiments in which all of the beam elements from one beamformer <NUM> are transmitted to the satellite <NUM> through one SAN <NUM>, there is no need to coordinate the timing of the transmissions from different SANs <NUM>. Alternatively, in embodiments in which beam elements output from the same beamformer <NUM> are transmitted to the satellite <NUM> through different SANs, the timing of the beam element signals is taken into consideration using timing controls as discussed further below.

The phase relationship between each of the RF beam element signals <NUM> transmitted from each of the N elements of an antenna array <NUM> and the relative amplitude of each, determines whether the beam element signals will be properly superpose to form beams within the desired user beam coverage areas <NUM>. In some embodiments in which there are <NUM> SANs <NUM> (i.e., M = <NUM>) each SAN <NUM> receives <NUM> beam element signals <NUM>.

In order to maintain the phase and amplitude relationship of each of the <NUM> RF beam element signals <NUM> to one another, the beamformer <NUM> outputs <NUM> timing pilot signals <NUM>, one to each SAN <NUM>, in addition to the N beam element signals <NUM>. Each timing pilot signal <NUM> is aligned with the other timing pilot signals upon transmission from the beamformer <NUM> to each SAN <NUM>. In addition, the amplitude of each timing pilot signal <NUM> is made equal.

<FIG> is a detailed illustration of the forward beamformer <NUM>. The forward beamformer <NUM> receives <NUM> forward beam signals <NUM> representing the forward traffic to be sent through the system <NUM>. The signals <NUM> are received by a matrix multiplier <NUM>. The matrix multiplier <NUM> includes a beamformer input module <NUM>, a <NUM>-way splitting module <NUM> and <NUM> weighting and summing modules <NUM>. Other arrangements, implementations or configurations of a matrix multiplier can be used. Each of the <NUM> forward beam signals <NUM> is intended to be received within a corresponding one of <NUM> user beam coverage areas <NUM>. Accordingly, there is a one-to-one relationship between the <NUM> user beam coverage areas <NUM> and the <NUM> forward beam signals <NUM>. In some embodiments, the distribution equipment (e.g., the core node) that provides the forward traffic to the beamformer <NUM> ensures that information to be transmitted to a particular user beam coverage area <NUM> is included within the forward beam input signal <NUM> corresponding to that user beam coverage area <NUM>.

The <NUM>-way splitting module <NUM> splits each of the <NUM> forward beam signals <NUM> into <NUM> identical signals, resulting in <NUM> x <NUM> (i.e., N x K) signals being output from the <NUM>-way splitting module <NUM>. When N is equal to <NUM> and K is equal to <NUM>, the splitting module <NUM> outputs <NUM> x <NUM> = <NUM>,<NUM> signals. <NUM> unique signals output from the splitting module <NUM> are coupled to each of the <NUM> weighting and summing modules <NUM>. The signals coupled to each of the weighting and summing modules <NUM> are weighted (i.e., phase shifted and amplitude adjusted) in accordance with beam weights calculated by a forward beam weight generator <NUM>. Each of <NUM> weighted signals corresponding to the same array element N are summed in one of <NUM> summers <NUM>.

Since each group of <NUM> outputs from of the summers <NUM> will be coupled to, and transmitted by, a different one of the <NUM> SANs <NUM>, a timing module <NUM> is provided. The timing module <NUM> adjusts when the beam element signals <NUM> are sent from the beamformer to ensure that each group of <NUM> IF beam element signals <NUM> arrives at the user beam coverage area <NUM> at the appropriate time to ensure that the superposition of the signals <NUM> results in the proper formation of the user beam. Alternatively, the forward beam weights can be generated taking into account differences in lengths and characteristics of the paths from each SAN <NUM> to the satellite <NUM>. Accordingly, a signal <NUM> would be coupled to the forward beamformer <NUM>. In some embodiments, the timing module <NUM> generates the timing pilot signal <NUM> transmitted from the forward beamformer <NUM> to each SAN <NUM>. In some embodiments, one timing pilot signal <NUM> is generated and split into <NUM> copies of equal amplitude, one copy sent to each SAN <NUM>. Alternatively, the amplitude of the copies may be a predetermined ratio. As long as the ratio between timing pilot signals <NUM> is known, RF beam element signals <NUM> can be equalized to ensure that they will superpose with one another to form the desired user spot beams. In some embodiments in which the corrections to alignment are made in the timing module <NUM> within the beamformer <NUM>, each SAN <NUM> returns a signal <NUM> derived from the SAN timing correction signal <NUM> to a timing control input to the beamformer to allow the forward beamformer <NUM> to determine corrections to the alignment of the signals to each SAN <NUM>. In some embodiments, SAN timing correction signals <NUM> are then used by the timing module <NUM> to adjust the timing of the beam element signals <NUM>. In other embodiments, the SAN timing correction signal <NUM> are used by the forward beam weight generator <NUM> to adjust the beam weights to account for differences in the paths from the beamformer <NUM> through each of the SANs <NUM> to the satellite <NUM>. As noted above, corrections to the alignment can alternatively be made in each SAN <NUM>.

Once the beam element signals <NUM> have been properly weighted and any necessary timing adjustments made, each of the <NUM> signals <NUM> are coupled to one of the SANs <NUM>. That is, each of the <NUM> SANs <NUM> receives <NUM> beam element signals <NUM> (i.e., <NUM>/<NUM>) from the forward beamformer <NUM>. An optical transmitter <NUM> within each SAN <NUM> receives, multiplexes and modulates those <NUM> beam element signals <NUM> that it receives onto an optical carrier.

<FIG> is an illustration of an optical transmitter <NUM> used in some embodiments of the system <NUM>. The optical transmitter <NUM> is similar to the optical transmitter <NUM> discussed above with respect to <FIG>. However, the input signals <NUM> differ, since they are beam weighted by the beamformer <NUM>. Furthermore, the timing pilot signal <NUM> provided by the beamformer <NUM> is coupled to an optical modulator <NUM> and modulated onto an optical carrier within the same band as the band of other optical modulators <NUM> within the same optical band module <NUM>, as determined by the wavelength of the light source <NUM> within that optical modulator <NUM>. In some embodiments, each optical band module <NUM> is identical. However, modulating the timing pilot signal <NUM> need only be done in one such optical band module <NUM>. Alternatively, as shown in <FIG>, only one optical band module <NUM> is configured to modulate a timing pilot signal <NUM>. The other optical band modules <NUM> may be similar to the optical band module <NUM> show in <FIG> and described above. In either embodiment, in a system in which <NUM> SANs <NUM> each receive <NUM> beam element signals <NUM> and modulate them onto <NUM> optical channels within <NUM> different optical bands, as shown in <FIG>, there are four optical band modules within the optical transmitter <NUM> in each SAN <NUM>.

The timing pilot signal <NUM> follows the same path to the satellite as the IF beam element signals <NUM>. Therefore, by comparing the arrival time of the timing pilot signals sent from each SAN <NUM> at the satellite <NUM>, differences in the arrival times of the IF beam element signals can be determined and correction signals can be generated and transmitted to each SAN <NUM>. Similar to the optical transmitter <NUM>, the optical channels <NUM> output by each optical modulator <NUM> shown in <FIG> are combined in an optical combiner <NUM>. The composite optical signal <NUM> is emitted from an optical lens <NUM> within the optical transmitter <NUM>. The optical lens <NUM> operates as an optical signal transmitter capable of transmitting an optical signal to the satellite <NUM>.

A composite optical signal <NUM> from each of the SANs <NUM> with the <NUM> beam element signals <NUM> and the timing pilot signal <NUM> is transmitted to the satellite <NUM> on the optical forward uplink <NUM> and received by one of <NUM> optical receivers <NUM> within the satellite <NUM>. Each of the <NUM> optical receivers <NUM> within the satellite <NUM> demultiplexes the <NUM> optical channels <NUM> from the composite optical signal <NUM>.

<FIG> shows the components of a satellite <NUM> (see <FIG>) in greater detail. The Satellite <NUM> receives and transmits the forward link in accordance with some embodiments of a system using ground-based beamforming, as noted above with reference to <FIG>. The components of the forward link of the satellite <NUM> include <NUM> optical receivers <NUM>, <NUM> amplifier/converter modules <NUM> and a <NUM>-element antenna array <NUM>. In some embodiments of the system <NUM>, similar to the embodiments shown in <FIG>, <FIG> and <FIG>, in which there are <NUM> SANs (i.e., M = <NUM>), the received composite signal <NUM> includes <NUM> optical channels divided into <NUM> bands of <NUM> each, each of which carries a <NUM> wide IF channel. Furthermore, there are K = <NUM> user beam coverage areas <NUM> and N = <NUM> elements in the antenna array. As noted elsewhere in the present discussion, these numbers are provided merely as an example and for ease of discussion.

Each optical receiver <NUM> is associated with a corresponding amplifier/converter module <NUM>. The optical receivers <NUM> each include a lens module <NUM>, and a plurality of optical detectors, such as photodiodes <NUM>. The lens module <NUM> includes a lens <NUM> (which in some embodiments may be similar to the lens <NUM> described above with respect to <FIG>), an optical demultiplexer <NUM>, a plurality of optical demultiplexers <NUM> and a plurality of output lenses <NUM>.

In operation, the composite optical signal <NUM> is received from each of the <NUM> SANs <NUM>. A lens <NUM> is provided to receive each composite optical signal <NUM>. In some embodiments, the lenses <NUM> can be focused (in some embodiments, mechanically pointed) at a SAN <NUM> from which the lens <NUM> is to receive an composite optical signal <NUM>. The lens <NUM> can later be refocused to point to a different SAN <NUM>. Because the lenses <NUM> can be focused to receive composite optical signal <NUM> from one of several SANs <NUM>, the satellite <NUM> can receive signals from <NUM> SANs <NUM> selected from among a larger number <NUM> + X SANs <NUM>. In some embodiments X= <NUM>. Therefore, <NUM> different SANs <NUM> are capable of receiving information intended to be communicated to user beam coverage areas <NUM> in the system. However, only eight of the <NUM> SANs <NUM> are selected to have information that is transmitted be received by the satellite <NUM>.

The signal path of one of the composite optical signals <NUM> through the forward link of the satellite <NUM> is now described in detail. It should be understood that each of the <NUM> signal paths taken by the <NUM> received composite optical signals <NUM> through the forward link of the satellite <NUM> operate identically. The composite optical signal <NUM> that is received by the lens <NUM> is directed to an optical demultiplexer <NUM>. In a system using the modulation scheme illustrated in <FIG>, the optical demultiplexer <NUM> splits the composite optical signal <NUM> into the four bands <NUM>, <NUM>, <NUM>, <NUM> (see <FIG>). That is, the optical demultiplexer <NUM> splits the composite optical signal <NUM> into the four optical wave lengths onto which the beam element signals <NUM> were modulated by the SAN <NUM> that sent the composite optical signal <NUM>. Each of the optical outputs from the optical demultiplexer <NUM> is coupled to a corresponding optical demultiplexer <NUM>. Each of the four optical demultiplexers <NUM> output <NUM>/(<NUM> x <NUM>) optical signals for a total of <NUM> x (<NUM>/(<NUM> x <NUM>) = <NUM>/<NUM> = <NUM> optical signals. Each of the <NUM> optical signals output from the four optical demultiplexers <NUM> is directed to an output lens <NUM>. Each of the output lenses <NUM> focus the corresponding optical signal onto a corresponding photo detector, such as a photodiode <NUM>. Each photodiode <NUM> detects the amplitude envelope of the optical signal at its input and outputs an RF transmit beam element signal <NUM> corresponding to the detected amplitude envelope. Accordingly, the RF transmit beam element signals <NUM> output from the optical receivers <NUM> are essentially the beam element signals <NUM> that were modulated onto the optical signals by the SANs <NUM>.

The RF output signals are then coupled to the amplifier/converter module <NUM>. The amplifier/converter module <NUM> includes <NUM>/<NUM> signal paths. In some embodiments, each signal path includes a Low noise amplifier (LNA) <NUM>, frequency converter <NUM> and PA <NUM>. In other embodiments, the signal path includes only the frequency converter <NUM> and the PA <NUM>. In yet other embodiments, the signal path includes only the PA <NUM> (the frequency converter <NUM> can be omitted if the feed signals produced by the SANs are already at the desired forward downlink frequency). The frequency converter <NUM> frequency converts the RF transmit beam element signals <NUM> to the forward downlink carrier frequency. In some embodiments, the output of each upconverter <NUM> is an RF carrier at a center frequency of <NUM>. Each of the <NUM> outputs from the <NUM> amplifier/converter modules <NUM> is coupled to a corresponding one of the <NUM> elements of the <NUM>-element antenna array <NUM>. Therefore, the antenna array <NUM> transmits the <NUM> forward downlink beam element signals <NUM>.

<FIG> is an illustration of user beam coverage areas <NUM> formed over the continental United States in accordance with some embodiments. In other embodiments, the user beam coverage areas may be located in different locations and with different spacing and patterns. In some embodiments, such as the embodiments shown in <FIG>, <FIG> and <FIG>, each feed of an antenna is focused to direct a user spot beam to one user beam coverage area. In other embodiments, such as shown in <FIG>, <FIG>, <FIG>, <FIG> and <FIG>, the <NUM> forward downlink beam element signals <NUM> are superposed on one another to form user beams directed to user beam coverage areas <NUM>. As shown in <FIG>, user beam coverage areas are distributed over a satellite service area that is substantially larger than the user beam coverage areas <NUM>. The <NUM> element antenna array <NUM> transmits the RF beam element signals <NUM> over the forward downlink <NUM> to each of the <NUM> user beam coverage areas <NUM>. User terminals <NUM> within each user beam coverage area <NUM> receive the user beam directed to that particular user beam coverage area <NUM> by virtue of the superposition of the RF beam element signals <NUM> transmitted from each of the <NUM> elements of the <NUM> element antenna array <NUM>.

In addition to the IF beam element signals <NUM> output from each optical receiver <NUM>, each optical receiver <NUM> demultiplexes a satellite timing signal <NUM> from the composite optical signal <NUM>. A satellite timing signal <NUM> is output from each receiver <NUM> and coupled the corresponding amp/converter module <NUM>. An LNA <NUM> within the amp/converter module <NUM> amplifies the satellite timing signal <NUM>. The output <NUM> of the LNA <NUM> is coupled to a satellite timing module <NUM>. In some embodiments, the satellite timing module <NUM> compares the satellite timing signal <NUM> received by each optical receiver <NUM> to determine whether they are aligned. The satellite timing module <NUM> outputs <NUM> SAN timing correction signals <NUM>, one to be returned to each of the <NUM> SANs <NUM>. In some embodiments, each SAN timing correction signal <NUM> is coupled to an input to a return amp/converter module <NUM> (see <FIG>). Each SAN timing correction signal <NUM> is amplified, frequency converted to the forward downlink frequency and coupled to an input to one of <NUM> optical transmitters <NUM> within the satellite <NUM>, similar to the optical transmitter <NUM> provided in the SAN <NUM>. In some embodiments, one of the eight is a reference for the other seven. Accordingly, no correction is made to the timing of the signals transmitted from the SAN <NUM> from which the reference satellite timing signal was sent. Therefore, no SAN timing correction signal <NUM> is sent for that SAN <NUM>. The SAN timing correction signal <NUM> is modulated onto each composite optical signal transmitted by the satellite <NUM> to each SAN <NUM>.

Each SAN timing correction signal <NUM> provides timing alignment information indicating how far out of alignment the timing pilot signal <NUM> is with respect to the other timing pilot signals (e.g., the reference satellite timing signal <NUM>). In some embodiments, the timing information is transmitted through the SANs <NUM> to a timing module <NUM> (see <FIG>) in the beamformer <NUM>. The timing module <NUM> adjusts the alignment of the beam elements prior to sending them to each SAN <NUM>. Alternatively, the timing alignment information is used by each SAN <NUM> to adjust the timing of the transmissions from the SAN <NUM> to ensure that the RF beam element signals <NUM> from each SAN <NUM> arrive at the satellite <NUM> in alignment. <FIG> is an illustration of an optical transmitter <NUM> having a timing module <NUM> for adjusting the timing of the beam element signals <NUM> and the timing pilot signal <NUM>. The timing module <NUM> receives a timing correction signal <NUM> from satellite <NUM> over the return downlink (discussed in further below). The timing module applies an appropriate delay to the signals <NUM>, <NUM> to bring the signals transmitted by the SAN <NUM> into alignment with the signals transmitted by the other SANs <NUM> of the system <NUM>.

In an alternative embodiment, timing adjustments can be made to the RF beam element signals <NUM> within the satellite based on control signals generated by the satellite timing module <NUM>. In some such embodiments, the control signals control programmable delays placed in the signal path between the optical receiver <NUM> and the antenna array <NUM> for each RF beam element signal <NUM>.

In an alternative embodiment, at least two of the satellite timing signals <NUM> are transmitted from the satellite back to each SAN <NUM>. The first is a common satellite timing signal <NUM> that is transmitted back to all of the SANs. That is, one of the received satellite timing signals <NUM> is selected as the standard to which all others will be aligned. The second is a loop back of the satellite timing signal <NUM>. By comparing the common satellite timing signal <NUM> with the loop back satellite timing signal <NUM>, each SAN <NUM> can determine the amount of adjustment needed to align the two signals and thus to align the IF beam element signals <NUM> from each SAN <NUM> within the satellite <NUM>.

<FIG> is a system <NUM> in which each of the K forward beam input signals <NUM> contain S <NUM> wide sub-channels. In some embodiments, K = <NUM> and S = <NUM>. For example, in some embodiments, seven <NUM> wide sub-channels are transmitted to one user coverage area <NUM>. <FIG> is an illustration of a beamformer <NUM> in which forward beam input signals <NUM> comprise seven <NUM> wide sub-channels, each coupled to a unique input to the beamformer <NUM>. Accordingly, as noted above, the sub-channels can be beamformed after being combined into an IF carrier, as shown in <FIG>, <FIG>. Alternatively, as shown in <FIG>, <FIG>, the sub-channels <NUM> can be beamformed before being combined using the beamformer <NUM>. Accordingly, the beamformer <NUM> outputs S x N beam element signals, with (S x N)/M such beam element signals being sent to each SAN <NUM>. In the example system <NUM>, S = <NUM>, N = <NUM> and M = <NUM>. As noted above, these numbers are provided as a convenient example and are not intended to limit the systems, such as the system <NUM>, to these particular values.

<FIG> is a simplified block diagram of a beamformer <NUM> in which each carrier comprises S sub-channels <NUM>, where S = <NUM>. Each of the sub-channels <NUM> is provided as independent input to a matrix multiplier <NUM> within the beamformer <NUM>. Therefore, <NUM> x <NUM> sub-channels <NUM> are input to the matrix multiplier <NUM>, where there are <NUM> user spot beams to be formed and <NUM> is the number of sub-channels in each carrier; that is, K = <NUM> and S = <NUM>. The <NUM>-way splitter <NUM> receives each of the <NUM> x <NUM> sub-channels <NUM>, where <NUM> is the number of elements in the antenna array <NUM>. Alternatively, N may be any number of antenna elements. Each sub-channel <NUM> is split <NUM> ways. Accordingly, <NUM> x <NUM> x <NUM> signals are output from the splitter <NUM> in a three-dimensional matrix. The signals <NUM>, <NUM>, <NUM> through <NUM>, K, <NUM> (i.e., <NUM>, <NUM>, <NUM> where K = <NUM>) are weighted and summed in a weighting and summing module <NUM>. Likewise, the signals <NUM>, <NUM>, <NUM> through <NUM>, <NUM>, <NUM> are weighted and summed in a weighting and summing module <NUM>. In similar fashion, each of other weighting and summing modules weight receive outputs from the splitter <NUM>, and weight and sum the outputs. The <NUM> x <NUM> outputs from the weighting and summing modules <NUM>, <NUM> are coupled to the inputs of a timing module <NUM>. The timing module functions essentially the same as the timing module <NUM> of the beamformer <NUM> discussed above. The beamformer <NUM> outputs <NUM> x <NUM> beam element signals <NUM> to the SANs <NUM>. Each of the <NUM> SANs <NUM> comprises an IF combiner <NUM>.

<FIG> is an illustration of a SAN <NUM> of system <NUM>. In some embodiments, a first baseband to IF converter <NUM> operates in similar fashion to the baseband to IF converter <NUM> discussed above with respect to <FIG>. The converter <NUM> outputs a signal <NUM> that is a combination of seven <NUM> beam element signals <NUM>. In addition, in some embodiments, at least one of the baseband to IF converters <NUM> includes an additional frequency converter <NUM>. The additional frequency converter <NUM> receives the timing pilot signal <NUM> from the beamformer <NUM>. The timing pilot signal <NUM> is combined with the beam element sub-channels <NUM> and coupled to the optical transmitter <NUM>. Each of the IF signals <NUM> coupled to the optical transmitter <NUM> are combined in the optical combiners <NUM> of each SAN <NUM> to form the transmitted composite optical signal <NUM>. The timing pilot signal <NUM> is coupled to the input of a frequency converter <NUM>. The frequency converter <NUM> places the timing pilot signal at a frequency that allows it to be summed with the beam element signals <NUM> by the summer <NUM>. Alternatively, the timing pilot signal <NUM> can be directly coupled to an additional optical modulator <NUM> dedicated to modulating the timing pilot signal <NUM>. The output of the additional modulator <NUM> is coupled to the combiner <NUM> and combined with the other signals on a unique optical channel dedicated to the timing pilot signal.

<FIG> is an illustration of a return link for the system <NUM> having ground-based beamforming. User terminals <NUM> located within a plurality of <NUM> user beam coverage areas <NUM> transmit RF signals to the satellite <NUM>. An <NUM>-element antenna array <NUM> on the satellite <NUM> (which may or may not be the same as the antenna array <NUM>) receives the RF signals from the user terminals <NUM>. <NUM>/<NUM> outputs from the <NUM>-element antenna array <NUM> are coupled to each of the <NUM> amplifier/converter modules <NUM>. That is, each element of the antenna array <NUM> is coupled to one LNA <NUM> within one of the amplifier/converter modules <NUM>. The output of each LNA <NUM> is coupled to the input to a frequency converter <NUM> and a pre-amplifier <NUM>. The amplified output of the LNA <NUM> frequency down-converted from RF user uplink frequency to IF. In some embodiments, the IF signal has a bandwidth of <NUM>. In some embodiments, the pre-amp <NUM> provides additional gain prior to modulation onto an optical carrier. The outputs of each amplifier/converter modules <NUM> are coupled to corresponding inputs to one of <NUM> optical transmitters <NUM>, similar to the optical transmitter <NUM> of <FIG>. Each of <NUM> optical transmitters <NUM> outputs and transmits an optical signal to a corresponding SAN <NUM>. The SAN <NUM> receives the optical signal. The SAN <NUM> outputs <NUM>/<NUM> return beam element signals <NUM> to a downlink beamformer <NUM>. The downlink beamformer <NUM> processes the return beam element signals <NUM> and outputs <NUM> beam signals <NUM>, each corresponding with one of <NUM> user beam coverage areas <NUM>.

The IF signals provided to the optical transmitter <NUM> from the amplifier/converter module <NUM> are each coupled to one of <NUM>/<NUM> optical modulators <NUM>. For example, if there are <NUM> elements in the antenna array <NUM> (i.e., N = <NUM>) and there are <NUM> SANs <NUM> in the system <NUM>, then <NUM>/<NUM> = <NUM>. In a system in which the IF signals are modulated onto wavelengths divided into <NUM> bands, such as shown in <FIG>, the optical modulators <NUM> are grouped together in optical band module <NUM> having <NUM>/(<NUM> x <NUM>) optical modulators <NUM>.

Each optical modulator <NUM> is essentially the same as the uplink optical modules <NUM> of the SAN <NUM> shown in <FIG>, described above. Each optical modulator <NUM> within the same optical band module <NUM> has a light source <NUM> that produces an optical signal having one of <NUM> wavelengths λ. Accordingly, the output of each optical modulator <NUM> will be at a different wavelength. Those optical signals generated within the same optical band module <NUM> will have wavelengths that are in the same optical band (i.e., in the case shown in <FIG>, for example, the optical bands are <NUM>, <NUM>, <NUM> and <NUM>). Each of those optical signals will be in one of <NUM> optical channels within the band based on the wavelengths λ <NUM>. The optical outputs from each optical modulator <NUM> are coupled to an optical combiner <NUM>. The output of the optical combiner <NUM> is a composite optical signal that is transmitted through an optical lens <NUM> to one of the SANs <NUM>. The optical lens <NUM> can be directed to one of several SANs <NUM>. Accordingly, the <NUM> optical transmitters each transmit one of <NUM> optical signals to one of <NUM> SANs <NUM>. The particular set of <NUM> SANs can be selected from a larger group of candidate SANs depending upon the quality of the optical link between the satellite and each candidate SAN.

<FIG> is an illustration of one of the SANs <NUM> in the return link. An optical receiver <NUM> comprises lens <NUM> that receives optical signals directed to the SAN <NUM> from the satellite by the lens <NUM>. An optical band demultiplexer <NUM> separates the optical signals into optical bands. For example, in some embodiments in which there are four such bands, each of the four optical outputs <NUM> are coupled to an optical channel demultiplexer <NUM>. The optical channel demultiplexer <NUM> separates the <NUM>/(<NUM> x <NUM>) signals that were combined in the satellite <NUM>. Each of the outputs from the four optical channel demultiplexers <NUM> are coupled to a corresponding lens <NUM> that focuses the optical output of the optical channel demultiplexers <NUM> onto an optical detector, such as a photodiode <NUM>. Output signals <NUM> from the photodiodes <NUM> are each coupled to one of <NUM>/<NUM> LNAs <NUM>. The output from each LNA <NUM> is coupled to the return link beamformer <NUM> (see <FIG>). In addition, one channel output from the optical receiver <NUM> outputs a timing correction signal <NUM> that is essentially the SAN timing correction signal <NUM> (see <FIG>) that was provided by the satellite timing module to the return amplifier/converter module <NUM>. In some embodiments, the timing correction signal <NUM> is coupled to a timing pilot modem <NUM>. The timing pilot modem outputs a signal <NUM> that is sent to the forward beamformer <NUM>. In other embodiments, the timing correction signal <NUM> is coupled to a timing control input of the timing module <NUM> (see <FIG>) discussed above.

<FIG> illustrates in greater detail, a return beamformer <NUM> in accordance with some embodiments of the disclosed techniques. Each of the <NUM> outputs signals <NUM> is received by the return beamformer <NUM> from each of the SANs <NUM>. The return beamformer comprises a beamforming input module <NUM>, a timing module <NUM>, matrix multiplier <NUM> and a beamformer output module <NUM>. The matrix multiplier <NUM> includes a K-way splitting module <NUM> and <NUM> weighting and summing modules <NUM>. The matrix multiplier <NUM> multiplies a vector of beam signals by a weight matrix. Other arrangements, implementations or configurations of a matrix multiplier <NUM> can be used. Each signal <NUM> is received by the beamformer <NUM> in the beamformer input module <NUM> and coupled to the timing module <NUM>. The timing module <NUM> ensures that any differences in the length and characteristics of the path from the satellite to the SAN <NUM> and from the SAN <NUM> to the return beamformer <NUM> is accounted for. In some embodiments, this may be done by transmitting one pilot signal from the return beamformer <NUM> to each SAN <NUM>, up to the satellite and retransmitting the pilot signal back through the SAN <NUM> to the return beamformer <NUM>. Differences in the paths between the return beamformer <NUM> and the satellite can be measured and accounted for.

The output of the timing module is coupled to a K-way splitter <NUM> that splits each signal into <NUM> identical signals. <NUM> unique signals are applied to each of <NUM> weighting and summing circuits <NUM>. Each of the <NUM> unique signals is weighted (i.e., the phase and amplitude are adjusted) within a weighting circuit <NUM>, such that when summed in a summing circuit <NUM> with each of the <NUM> other weighted signals, a return link user beam is formed at the output of the return beamformer.

Each of the architectures described above are shown for an optical uplink to the satellite. In addition, an optical downlink from the satellite to SANs on Earth operates essentially the reverse of the optical uplinks described. For example, with regard to the architecture shown in <FIG>, an optical downlink from the satellite <NUM> to the SAN <NUM> provides a broadband downlink. Rather than lenses <NUM> for receiving the optical uplink, lasers are provided for transmitting an optical downlink. Furthermore, rather than the bi-phase modulator <NUM> generating a BPSK modulated signal to be transmitted on an RF carrier, the bi-phase modulator modulates the optical signal using an optical binary modulation scheme. Similarly, an optical downlink can be provided using an architecture similar to that shown in <FIG>. In this embodiment, the modulator <NUM> would instead be a QAM demodulator that receives a QAM modulated RF or IF signal and demodulates the bits of each symbol and using binary optical modulation of an optical signal for transmission on the optical downlink. In the embodiment of the architecture shown in <FIG>, a similar architecture can be used in which the feeder downlink from the satellite to the SAN is optical, the received RF signals from the user terminals <NUM>, <NUM> are directed by a matrix switch to a laser pointed at the particular SAN selected to receive the signal. The RF signal is RF modulated onto the optical signal similar to the way the feeder uplink optical signal is RF modulated by the baseband/RF modem <NUM> in the SAN <NUM>.

In some embodiments, the lasers used to transmit an optical feeder downlink signal are pointed to one of several SANs. The SANs are selected based upon the amount of signal fade in the optical path from the satellite to each available SAN, similar to the manner in which the SANs of <FIG>, <FIG> and <FIG> are selected.

Although the disclosed techniques are described above in terms of various examples of embodiments and implementations, it should be understood that the particular features, aspects and functionality described in one or more of the individual embodiments are not limited in their applicability to the particular embodiment with which they are described. Thus, the scope of the claimed invention should not be limited by any of the examples provided in describing the above disclosed embodiments but by the appended set of claims.

Terms and phrases used in this document, and variations thereof, unless otherwise expressly stated, should be construed as open ended as opposed to limiting. As examples of the foregoing: the term "including" should be read as meaning "including, without limitation" or the like; the term "example" is used to provide examples of instances of the item in discussion, not an exhaustive or limiting list thereof; the terms "a" or "an" should be read as meaning "at least one," "one or more" or the like; and adjectives such as "conventional," "traditional," "normal," "standard," "known" and terms of similar meaning should not be construed as limiting the item described to a given time period or to an item available as of a given time, but instead should be read to encompass conventional, traditional, normal, or standard technologies that may be available or known now or at any time in the future. Likewise, where this document refers to technologies that would be apparent or known to one of ordinary skill in the art, such technologies encompass those apparent or known to the skilled artisan now or at any time in the future.

A group of items linked with the conjunction "and" should not be read as requiring that each and every one of those items be present in the grouping, but rather should be read as "and/or" unless expressly stated otherwise. Similarly, a group of items linked with the conjunction "or" should not be read as requiring mutual exclusivity among that group, but rather should also be read as "and/or" unless expressly stated otherwise. Furthermore, although items, elements or components of the disclosed techniques may be described or claimed in the singular, the plural is contemplated to be within the scope thereof unless limitation to the singular is explicitly stated.

The presence of broadening words and phrases such as "one or more," "at least," "but not limited to" or other like phrases in some instances shall not be read to mean that the narrower case is intended or required in instances where such broadening phrases may be absent. The use of the term "module" does not imply that the components or functionality described or claimed as part of the module are all configured in a common package. Indeed, any or all of the various components of a module, whether control logic or other components, can be combined in a single package or separately maintained and can further be distributed in multiple groupings or packages or across multiple locations.

Claim 1:
A satellite (<NUM>) comprising:
an optical receiver (<NUM>) having a plurality of radio frequency "RF" outputs (<NUM>), the optical receiver configured to receive a composite optical signal (<NUM>) comprising a plurality of beam weighted beam element signals (<NUM>), each RF output corresponding to a respective one of the beam weighted beam element signals, the optical receiver comprising:
an optical demultiplexer (<NUM>) having an input and a plurality of outputs, each output associated with a corresponding optical wavelength; and
a plurality of satellite receiver optical detectors (<NUM>) each having an RF output coupled to a corresponding RF output of the optical receiver, wherein the satellite receiver optical detectors are configured such that each RF signal output from the satellite receiver optical detectors has an amplitude that tracks the intensity of a corresponding optical signal applied to a corresponding satellite receiver optical detector input from a corresponding output of the optical demultiplexer;
a plurality of low noise amplifiers "LNA" (<NUM>), each having an LNA input and an LNA output, each LNA input coupled to a corresponding one of the optical receiver RF outputs;
a plurality of power amplifiers "PA" (<NUM>), each having a PA input and a PA output, each PA input coupled to a corresponding one of the LNA outputs, each PA output configured to output a respective forward downlink beam element signal (<NUM>) corresponding to a respective one of the beam weighted beam element signals; and
an antenna array (<NUM>) having a plurality of antenna elements, each antenna element having an antenna input coupled to a corresponding one of the PA outputs, wherein antenna patterns of at least some of the antenna elements overlap such that the forward downlink beam element signals transmitted therefrom will be superposed upon one another and thus coherently combine to form a user spot beam.