Patent Publication Number: US-11641236-B2

Title: Broadband satellite communication system using optical feeder links

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 16/865,520, filed May 4, 2020, which is a continuation of U.S. application Ser. No. 16/547,081, filed Aug. 21, 2019, now U.S. Pat. No. 10,735,089, which is a division of U.S. application Ser. No. 16/023,320, filed Jun. 29, 2018, now U.S. Pat. No. 10,454,570, which is a continuation of PCT Application No. PCT/US2016/069628, filed Dec. 30, 2016, which claims benefit of U.S. Provisional Application No. 62/273,730, filed Dec. 31, 2015, the disclosure of each of which is hereby incorporated by reference in its entirety for all purposes. 
    
    
     TECHNICAL FIELD 
     The disclosed techniques relates to broadband satellite communications links and more specifically to satellites using optical links for broadband communication between satellite access nodes and the satellites. 
     BACKGROUND 
     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 land-based, 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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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&#39;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. 
         FIG.  1    is an illustration of an example of a satellite communications system using radio frequency signals to communicate with the satellite and having a relatively large number of satellite access nodes (“SANs”, also known as “gateways”) to create a high capacity system. 
         FIG.  2    is an illustration of a simplified satellite that uses RF signals to communicate with SANs. 
         FIG.  3    is a simplified illustration of an example of the repeaters used on the forward link. 
         FIG.  4    is a simplified schematic of an example of a first of the three system architectures in which an optical link is used to communication on the feeder link. 
         FIG.  5    shows an example of the relationship of IF signals, optical channels and optical bands used by the system in some embodiments. 
         FIG.  6    shows an example of an optical transmitter used to perform the optical modulation of the binary data stream onto the optical signals. 
         FIG.  7    is an illustration of an example of the return path for the system of  FIG.  4   . 
         FIG.  8    is a simplified schematic of an example of a third system architecture in which an optical link is used to communicate on the feeder link. 
         FIG.  9    is an illustration of an example of the relationship between sub-channels, carriers and optical signals within the system of  FIG.  8   . 
         FIG.  10    is a simplified illustration of an example of a SAN. 
         FIG.  11    is an illustration of an example of the return link for the system of  FIG.  8   . 
         FIG.  12    is a simplified schematic of an example of a system architecture in which a satellite has on-board beamforming. 
         FIG.  13    is a simplified block diagram of an example of a weight/combiner module. 
         FIG.  14    is a simplified schematic of an example of a system architecture in which an optical signal is RF modulated at a SAN and sent to a satellite that has on-board beamforming capability. 
         FIG.  15    is an illustration of an example of a forward link of a satellite communications system using ground-based beamforming and including an optical forward uplink and a radio frequency forward downlink. 
         FIG.  16    is an example of a forward beamformer used in a system performing ground-based beamforming. 
         FIG.  17    is a more detailed illustration of an example of the return link components within the example  FIG.  18    is a simplified illustration of components of a satellite used for receiving and transmitting the forward link of an example system using ground-based beamforming. 
         FIG.  18    shows of an example of the components of a satellite in greater detail. 
         FIG.  19    is an illustration of an example of user beam coverage areas formed over the continental United States. 
         FIG.  20    is an illustration of an example of an optical transmitter having a timing module for adjusting the timing of the beam element signals and the timing pilot signal. 
         FIG.  21    is a system in which each of the K forward beam input signals contain S 500 MHz wide sub-channels. 
         FIG.  22    is a simplified block diagram of an example of a beamformer. 
         FIG.  23    is an illustration of an example of a SAN. 
         FIG.  24    is an illustration of an example of a return link for a system having ground-based beamforming. 
         FIG.  25    is an illustration of an example of one of the SANs in the return link. 
         FIG.  26    is an example of an illustration of an example return beamformer. 
     
    
    
     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. 
     DETAILED DESCRIPTION 
     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.  1    is an illustration of a satellite communications system  100  in which a relatively large number of stations (referred to herein as “SANs”, also referred to as “gateways”)  102  communicate with a satellite  104  using RF signals on both feeder and user links to create a relatively large capacity system  100 . Information is transmitted from the SANs  102  over the satellite  104  to a user beam coverage area in which a plurality of user terminals  106  may reside. In some embodiments, the system  100  includes thousands of user terminals  106 . In some such embodiments, each of the SANs  102  is capable of establishing a feeder uplink  108  to the satellite  104  and receiving a feeder downlink  110  from the satellite  104 . In some embodiments, feeder uplinks  108  from the SAN  102  to the satellite  104  have a bandwidth of 3.5 GHz. In some embodiments, the feeder uplink signal can be modulated using 16 quadrature amplitude modulation (QAM). Use of 16 QAM modulation yields about 3 bits per second per Hertz. By using 3.5 GHz bandwidth per spot beam, each spot beam can provide about 10-12 Gbps of capacity. By using 88 SANs, each capable of transmitting a 3.5 GHz bandwidth signal, the system has approximately a 308 GHz bandwidth or a capacity of about 1000 Gbps (i.e., 1 Tbps). 
       FIG.  2    is an illustration of a simplified satellite that can be used in the system of  FIG.  1   , wherein the satellite uses RF signals to communicate with SANs.  FIG.  3    is a simplified illustration of the repeaters  201  used on the forward link (i.e., receiving the RF feeder uplink and transmitting the RF user downlink) in the satellite of  FIG.  2   . A feed  202  within the feeder link antenna (not shown) of the satellite  104  receives an RF signal from a SAN  102 . 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  202  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 203 from one polarization (e.g., the RHCP) is provided to a first repeater  201 . The output is coupled to the input of a Low noise amplifier (LNA)  304  (see  FIG.  3   ). The output of the LNA  304  is coupled to the input of a diplexer  306 . The diplexer splits the signal into a first output signal  308  and second output signal  310 . The first output signal  308  is at a first RF frequency. The second output signal  310  is at a second RF frequency. Each of the output signals  308 ,  310  are coupled to a frequency converter  312 ,  314 . A local oscillator (LO)  315  is also coupled to each of the frequency converters  312 ,  314 . 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  312 ,  314 . The output of the frequency converters  312 ,  314  is coupled through a channel filter  316 ,  318  to a hybrid  320 . The hybrid  320  combines the output of the two channel filters  316 ,  318  and couples the combined signal to a linearizing channel amplifier  322 . 
     Combining the signals within the hybrid  320  allows the signals to be amplified by one traveling wave tube amplifier (TWTA)  324 . The output of the linearizing channel amplifier  322  is coupled to the TWTA  324 . The TWTA  324  amplifies the signal and couples the amplified output to the input of a high pass filter and diplexer  326 . The high pass filter and diplexer  326  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  328  and a lower frequency portion of the signal being coupled to a second antenna feed  330 . The first antenna feed  328  transmits a user downlink beam to a first user beam coverage area U. The second antenna feed  330  transmits a user downlink beam to a second user beam coverage area U3. 
     The output 331 of the feed  202  from the second polarization (e.g., LHCP) is coupled to a second arm  332  of the repeater. The second arm  332  functions in a manner similar to the first  201 , 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 some embodiments, an optical link can be used to increase the bandwidth of the feeder uplink  108  from each SAN  102  to the satellite  104  and the feeder downlink  110  from the satellite to each SAN  102 . This can provide numerous benefits, including making more spectrum available for the user links. Furthermore, by increasing the bandwidth of the feeder links  108 ,  110 , the number of SANs  102  can be reduced. Reducing the number of SANs  102  by increasing the bandwidth of each feeder link to/from each SAN  102  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  114  may not provide the maximum capacity of the system. That is, the full potential of the RF user downlink  114  is reduced from what it may be possible if a denser modulation scheme is used, such as 16 QAM instead of BPSK on the RF user downlink  114 . 
     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 2 bits per symbol, including for example quadrature phase shift keying (QPSK), offset QPSK, 8-ary phase shift keying, 16-ary QAM, 32-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  114  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.  4    is a simplified schematic of a first of the three techniques noted above. A system  600  for implementing the first technique includes a plurality of SANs  602 , a satellite  604  with at least one single-feed per beam antenna  638 ,  640  and a plurality of user terminals  606  within user beam coverage areas  1801  (see  FIG.  19   ). 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  638 ,  640  may be a direct-radiating array or pail of a reflector/antenna system. In some embodiments, the system  600  has M SANs  602 . In the example system  600  and for each of the example systems discussed throughout this disclosure, M=8. However, none of the systems disclosed here should be limited to this number. M=8 is merely a convenient example, and in other embodiments, M can be equal to 2, 4, 10, 12, 16, 20, 32, 40, or any other suitable value. In some embodiments, the SANs  602  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  602  from the core node can be provided in any form that allows for efficient communication of the data to the SAN  602 , 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  1801 . 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  601 . 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.  5    shows the relationship of IF signals  903 , optical channels  915  and optical bands  907 ,  909 ,  911 ,  913  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 3.5 GHz wide binary modulated IF signals (e.g. 64)  903  carry binary data to be transmitted in one user spot beam. Examples of other bandwidths that can be used include 500 MHz, 900 MHz, 1.4 GHz, 1.5 GHz, 1.9 GHz, 2.4 GHz, or any other suitable bandwidth. 
     The binary (i.e., digital) content modulated onto each 3.5 GHz wide binary modulated IF signal  903  is used to perform binary intensity modulation of one of 16 optical channels within one of 4 optical bands  905 . In some embodiments, the four bands  907 ,  909 ,  911 ,  913  of the optical spectrum are 1100 nm, 1300 nm, 1550 nm and 2100 nm. 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 0.8 nm apart (i.e., 100 GHz wide). While the RF signal  903  that is modulated onto the optical channel is only 3.5 GHz wide, the spacing allows the optical signals to be efficiently demultiplexed. In some embodiments, each SAN  602  wavelength division multiplexes (WDM) several (e.g., 64) such 3.5 GHz optical signals  903  (i.e., 4×16) together onto an optical output signal. Accordingly, the digital content of 64 optical channels can be sent from one SAN  602 . 
       FIG.  6    shows an optical transmitter  607  used to perform the optical modulation of the binary data stream  601  onto the optical signals. In accordance with the embodiment that implements the scheme shown in  FIG.  5   , the optical transmitter  607  includes four optical band modules  608   a - 608   d  (two shown for simplicity) and an optical combiner  609 . Each of the 4 optical band modules  608  include 16 optical modulators  611  (two shown for simplicity) for a total of 64 modulators  611 . Each of the 64 modulators  611  output an optical signal that resides in one of 64 optical channels  915  (see  FIG.  5   ). The channels are divided into 4 optical bands  907 ,  909 ,  911 ,  913 . 
     The modulator  611  determines the optical channel  915  based on the wavelength λ 1 of a light source  654  that produces an optical signal. An MZM  652  intensity modulates the output of the first light source  654  with an intensity proportional to the amplitude of the binary data stream  601 . The binary data stream  601  is summed with a DC bias in a summer  656 . Since the binary data stream  610  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  652  is coupled to an optical combiner  609 . For a system using a modulation scheme such as the one illustrated in  FIG.  5   , each of the 16 light sources  654  that reside within the same optical band module  608  output an optical signal at one of 16 different wavelengths λ1. The 16 wavelengths correspond to the 16 optical channels  915  within the first optical band  907 . Likewise, the light sources  654  in the optical modulators  611  in each other optical band module  608  output an optical signal having a wavelength of XI equal to the wavelength of the channels in the corresponding optical band  909 ,  911 ,  913 . Accordingly, the 64 optical outputs 915 from the four optical band modules  608   a - 608   d  each have a different wavelength and fall within the 16 optical channels of the four bands that are defined by the wavelengths λ1 of signals generated by the 64 light source  654 . The optical combiner  609  outputs a wavelength division multiplexed (WDM) optical signal  660  that is the composite of each signal  915 . 
     The SAN  602  sends the optical signal  660  to the satellite  604  over an optical feeder uplink  108  (see  FIG.  4   ). The optical signal emitted by the optical transmitter  607  is received by a lens  610  in the satellite  604 . In some embodiments, a lens  610  is part of a telescope within the optical receiver  622 . In some embodiments, the lens  610  is steerable (i.e., can be directed to point at any one of several SANs  602  within the system or any one from within a subset). By allowing the lenses  610  to be pointed to more than one of the SANs  602 , the lens  610  can be pointed to a SAN  602  having an optical path to the satellite that is not currently subject to signal fading. The lens  610  may be pointed using mechanical 2-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  610  at the desired SAN  602 . 
     The optical receiver  622  further includes an optical demultiplexer  650 , such as a filter or prism. The optical receiver  622  has a plurality of outputs, each output corresponding with an optical wavelength. As shown in  FIG.  4   , the optical receiver  622  has 64 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  622 , are provided herein merely as an example and are not intended to limit the systems, such as system  600 , to a particular number. 
     In some embodiments, each wavelength resides within one of the four optical bands  907 ,  909 ,  911 ,  913 . Each optical wavelength is at the center of an optical channel. Optical channels within one band are spaced approximately 0.8 nm (i.e., 100 GHz) apart. Making the optical channels spacing wide makes it easier to provide an optical demultiplexer  650  that can demultiplex the optical signal to provide each of the 64 optical channels on a separate output. In some embodiments, an additional lens  613  is provided to focus the output of the optical demultiplexer  650  into the input of an optical detector, such as a photodiode  612 . The photodiode  612  generates an electrical signal by detecting the intensity envelope of the optical signal  660  presented at an optical input to the photo diode. In some embodiments in which the optical signal  660  was intensity modulated to one of two intensity levels, the first intensity level representing a logical “1” results in an electrical signal having a first amplitude which also represents a logical “1”. A second intensity level representing a logical “0” results in an electrical signal an amplitude representing a logical “0”. Therefore, the electrical signal is placed in a first state when the intensity of the optical signal  660  is in a state representing a logical “1” and placed in a second state when the intensity of the optical signal  660  is in a state representing a logical “0”. Accordingly, the optical receiver has a plurality of digital outputs  615 . The electrical signal output from the digital output  615  of the photodiode  612  is coupled to a modulator  614 , such as a bi-phase modulator. In some embodiments, such as the embodiment of  FIG.  4   , an LNA  617  is provided between the photo diode  612  and the bi-phase modulator  614 . The output of the bi-phase modulator  614  is a BPSK modulated IF signal (i.e., analog signal) having two phases. The BPSK modulator  614  outputs a signal having a first phase representing a logical “1” in response to the electrical input signal at the first amplitude (i.e., in the first state). When the input to the modulator  614  has an amplitude representing a logical “0” (i.e., the second state), the phase of the output of the BPSK modulator  614  is shifted to a second phase different from the first phase. The output of the modulator  614  is coupled to the input of a switch matrix  616 . 
     In the simplified schematic of  FIG.  4   , a second SAN  602 , lens  610 , optical receiver  622  and plurality of bi-phase modulators  614  (i.e.,  64 ) are coupled to the switch matrix  616 . While only two SANs  602  are shown in  FIG.  4   , it should be understood that the satellite may receive optical signals from several SANs  602  (e.g., 8). 
     In some embodiments, the switch matrix  616  shown in  FIG.  4    has a plurality of (e.g., 64) inputs for each lens  610 . That is, if the satellite  604  has 8 lenses  610 , the matrix switch  616  has 512 inputs, each coupled to one of the modulators  614 . The switch matrix  616  allows signals at the outputs of the switch matrix  616  to be selectively coupled to inputs of the switch matrix  616 . 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  616  at the expense of reduced flexibility in the system. 
     The outputs of the switch matrix  616  are each coupled to an upconverter  626 . The upconverter  626  upconverts the signal to the frequency of the user downlink carrier. For example, in some embodiments, the signal output from the switch matrix  616  is a 3.5 GHz wide IF signal. The 3.5 GHz wide IF signal is upconverted to an RF carrier having a 20 GHz center frequency. The output of each upconverter  626  is coupled to a corresponding power amplifier  630 . The output of each amplifier  630  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  638 ,  640 . Accordingly, each of the outputs of the switch matrix  616  is effectively coupled to a corresponding one of the antenna inputs. In some embodiments, each input of each antenna  638 ,  640  transmits a user spot beam to one user beam coverage area  1801  (see  FIG.  19   ). The switch matrix  616  is capable of selecting which input (i.e., bi-phase modulator  614 ) is coupled to which output (i.e., upconverter  626 ). Accordingly, when (or before) the signal from one of the SANs  602  fades and errors become intolerable, the switch matrix  616  can couple the input of the upconverter  626  (i.e., the associated antenna feed) to a SAN  602  that is sending an optical signal that is not experiencing significant fading. In some embodiments, the switch matrix  616  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  610  is receiving a signal from the SAN  602  to which it is pointing, each of the 64 outputs from the optical receiver  622  associated with that Lens  610  will have a signal. In the embodiment in which each antenna input to the antennas  638 ,  640  transmits a user spot beam to a particular user coverage area  1801 , all of the user coverage areas  1801  will receive a signal (assuming the switch matrix  616  is mapped to couple each input to one output). The switch matrix  616  selects which analog output from the bi-phase modulator  614  is to be coupled to each antenna input (e.g., transmitted to each feed of the single-feed per beam antenna  638 ,  640 ) (i.e., in each user spot beam). However, when the optical signal from a particular SAN  602  fades, a signal is still provided to all of the antenna inputs to ensure that no user coverage areas  1801  loses coverage. Time multiplexing the signals from one SAN to more than 64 antenna inputs allows one SAN  602  to provide signals to more than 64 user coverage areas  1801 . 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  610  that is directed to a SAN  602  that has a weak optical link is redirected to another SAN to which there is a stronger optical link. More generally, the matrix  616  can be used to time multiplex analog signals output from the optical receiver  622  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  610  is receiving a sufficiently strong optical signal, the switch matrix  616  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  616  is provided by a telemetry signal from a control station. In most embodiments, since all 64 of the IF signals from the same SAN  602  will degrade together, the switch matrix  616  need only be able to select between K/64 outputs, where K is the number of user spot beams and 64 is the number of photo diodes  612  in one optical receiver  622 . As noted above, the process of controlling the routing through the satellite to map SANs  602  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  604  has more antenna inputs than transponders (i.e., paths from the optical receiver to the switches  634 ,  636 ). That is, a limited number of transponders, which include power amplifiers (PAs)  630 , upconverters  626 , 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  612  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  604 . In this embodiment, RF switches  634  are used to direct the output of the PA  630  to different inputs of the one or both of the antennas  638 ,  640  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  634 ,  636  to direct the signal to a particular antenna  638 ,  640 , the signal received by each of the lenses  610  can be directed to a particular spot beam. This provides flexibly in dynamically allocating capacity of the system. 
     The switches  634 ,  636  direct the signal to inputs of any of the antennas  638 ,  640  mounted on the satellite. In some embodiments, the output from the switches  634 ,  636  may be directed to a subset of the antennas. Each antenna  638 ,  640  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  630  may be directly connected to the antenna inputs, with the matrix switch  616  determining which of the signals detected by each particular photo diodes  612  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 transponder and antenna inputs, having switches  634 ,  636  can reduce the complexity of the switch matrix  616 . That is, using a combination of the switch matrix  616  and switches  634 ,  636 , the switch matrix  616  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  634 ,  636  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  616  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  616  essentially remains set for relatively long periods of time. The configuration of the switch matrix  616  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  616  is used to time multiplex data between different forward downlink antenna inputs. Accordingly, the switch matrix  616  selects which inputs to couple to the output of the switch matrix  616 . 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  842 ,  844  is tolerable. In some such embodiments, time multiplexing the analog outputs of the optical receiver  622  to different antenna inputs allows one SAN  602  to service more than one user beam coverage area. During a first period of time, one or more signals output from an optical receiver  622  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  615  from the optical receiver  622  can be performed in response to one of the lens  610  of an optical receiver  622  pointing to a “weak” SAN  602  (i.e., a SAN  602  having an optical link that is below a quality threshold). In such a embodiment, a first data stream initially set to the weak SAN  602  can be redirected by the core node to a “strong” SAN  602  (i.e., a SAN  602  having an optical link that is above the quality threshold). The strong SAN  602  time multiplexes that information such that for a portion of the time the strong SAN  602  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  602  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  604  by the poor optical link between the weak SAN  602  and the satellite  604  can be transmitted to the satellite  604  through the strong SAN  602 . During this time, the lens  610  that is pointing at the weak SAN  602  can be redirected to point to a strong SAN  602  that is not already transmitting to the satellite  604 . 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  602  that is using the failing feeder uplink and to a SAN  602  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  602  can then be routed through the switch matrix  616  to the spot beam to which data is intended to be sent. 
     The system  600  has the advantage of being relatively simple to implement within the satellite  604 . Conversion of binary modulated optical data to a BPSK modulated IF signal using photodiodes  612  and bi-phase modulators  614  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  114  is not the most efficient use of the limited RF spectrum. That is, greater capacity of the RF user downlink  114  (see  FIG.  1   ) can be attained by using a denser modulation scheme, such as 16 QAM instead of BPSK on the RF user downlink  114 . 
     For example, in an alternative embodiment of the system  600  that implements the second of the three techniques noted above, the analog signal  618  that is to be transmitted on the user downlink is modulated with a denser modulation scheme. Generating the complex modulation on the analog signal  618  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  618  can be a high order modulation such as 64-QAM, 8 psk, 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  615  of the optical receiver  622  providing an electronic signal that has binary modulation representing the underlying content. In order to modulate the analog signal  618  with a more complex modulation scheme, such as 16 QAM, the modulator  614  is a QAM modulator and thus perform QAM modulation of the IF signal based on the digital content output from the photodiode  612 . 
     Accordingly, in some embodiments, the bi-phase modulator  614  of the system  600  is replaced with a QAM modulator  614  (i.e., a modulator in which each symbol represents more than 2 bits). Accordingly, rather than limiting the modulation of the IF signals  618  to a binary modulation scheme (i.e., two logical states), such as BPSK, the modulator  614  allows the IF signals  618  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  618  (QAM verses BPSK), it is more complex, requires more power, is heavier and more expensive than a bi-phase modulator. 
       FIG.  7    is an illustration of the return path for the system  600 . User terminals  606  transmit a binary modulated signal to the satellite  604 . Switches  402  coupled to each element of the antenna (e.g., single beam per feed antennas  404 ,  406 ) select between satellite transponders comprising a Low noise amplifier (LNA)  408 , frequency converter  409  and digital decoder  410 . The frequency converter  409  down converts the received signal from the user uplink frequency to IF. The decoders  410  decode the binary modulation on the received IF signal. Accordingly, the output of each decoder  410  is a digital signal. The digital decoders  410  are coupled to inputs to a switch matrix  416 . The switch matrix  416  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  602 ) depending upon whether there is significant fading on the optical downlink to each SAN  602 . The outputs of the switch matrix  416  are coupled to inputs to optical transmitters  607 . Each optical transmitter  607  is essentially identical to the optical transmitter  607  shown in  FIG.  6    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.  5   ), each of four optical band modules  608  receive 16 outputs from the matrix switch  416  for a total of 64 inputs to the optical transmitter  607 . In some embodiments in which the satellite can receive optical signals from 8 SANs  602 , there are 8 such optical transmitters  607  that can receive a total of 512 outputs from the switch matrix  416 . Each optical transmitter  607  outputs an optical signal  660 . The optical signal  660  is receive by a lens  412  within an optical receiver  414  in a SAN  602 . The optical receiver  414  and lens  412  are essentially identical to the optical receiver  622  and lens  610  within the satellite  604 , as described above with reference to  FIG.  4   . Accordingly, the output of the optical receiver  414  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  602 . 
     In an alternative embodiment, the return link for the system  600 , the modulation used on the return uplink from the user terminals  606  to the satellite  604  is a more efficient modulation scheme than binary modulation. Accordingly, the binary modulate  410  is a more complex modulator  410 . The binary data output from the demodulator  410  is the result of decoding the modulated symbols modulated onto the IF signal by the user terminal  606 . For example, if 16 QAM was used on the user uplink, then the signal output from the demodulator is a digital stream of values represented by 16 QAM symbol. The binary signal output from the converter  502  is coupled to an input to the switch matrix  416 . Both the binary demodulator and the complex demodulator  410  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  607 . 
       FIG.  8    is a simplified schematic of a system  800  for implementing the third technique. In some embodiments of the system  800 , a SAN  802  receives the forward traffic as “baseband” signals  809  that are coupled to the inputs of a baseband to IF converter  1605 . In some embodiments, seven 500 MHz wide baseband sub-channels  809  are combined in a 3.5 GHz wide IF signal  811 . Each of the 3.5 GHz wide signals  811  is transmitted to one user coverage area  1801 .  FIG.  9    illustrates the relationship between baseband sub-channels  809 , IF signals  811  and optical signals within the system  800 .
         Examples of other bandwidths that can be used include 500 MHz (e.g., a single 500 MHz sub-channel), 900 MHz, 1.4 GHz, 1.5 GHz, 1.9 GHz, 2.4 GHz, or any other suitable bandwidth.       

       FIG.  10    is a simplified illustration of a SAN  802 , such as the SAN  802  shown in  FIG.  8   . In some embodiments, there are 64 baseband to IF converters  1605 , shown organized in four IF combiners  1602 , each comprising 16 converters  1605 . Grouping of the baseband to IF converters  1605  within IF converters  1602  is not shown in  FIG.  8    for the sake of simplifying the figure. Each of the 64 baseband to IF converters  1605  has S inputs, where S is the number of sub-channels  809 . In some embodiments in which the sub-channel  809  has a bandwidth of 500 MHz and the signal  811  has a bandwidth of 3.5 GHz. S equals 7. Each input couples one of the sub-channels  809  to a corresponding frequency converter  1606 . The frequency converters  1606  provide a frequency offset to allow a subset (e.g., S=7 in  FIG.  10   ) of the sub-channels  809  to be summed in a summer  1608 . Accordingly, in some embodiments, such as the one illustrated in  FIG.  10   , a SAN  802  processes 64 channels, each 3.5 GHz wide. In some embodiments, the 3.5 GHz wide signal can be centered at DC (i.e., using zero IF modulation). Alternatively, the signal  811  can be centered at a particular RF frequency. In one particular embodiment, an RF carrier  811  is centered at the RF downlink frequency (in which case the satellite will need no upconverters  626 , as described further below). The output  811  from each summing circuit  1608  is an IF signal  811  that is coupled to one of 64 optical modulators  611 . The 64 optical modulators  611  are grouped into 4 optical band modules  608 . Each optical modulator  611  operates essentially the same as the optical modulator  611  shown in  FIG.  6    and discussed above. However, since the input  811  to each optical modulator  608  is an analog signal, the optical signal output from each optical modulator  611  is an intensity modulated optical signal having an amplitude envelope that follows the amplitude of the IF signal  811 . 
     An optical combiner  609  combines the outputs from each of the 64 optical modulators  611  to generate a wavelength division multiplexed (WDM) composite optical signal  1624 . The number of baseband to IF converters  1605  and the number of optical modulators  611  in the optical band module  608  can vary. As shown in  FIG.  9   , the four optical modulators  611  can be designed to output optical signals having wavelengths centered at 1100 nanometers, 1300 nanometers, 1550 nanometers and 2100 nanometers. 
     In the system  800 , the optical transmitter  607  (similar to the optical transmitter  607  of  FIG.  4   ) emits an RF modulated composite optical signal  1624 . The RF modulated composite optical signal  1624  is received within the satellite  804  by a lens  610  (see  FIG.  8   ). The lens  610  can be directed to any of a plurality of SANs  802  capable of transmitting an optical signal to the satellite  804 . The output of the lens  610  is coupled to the input of an optical detector, such as a photodiode  612  (e.g. a PIN diode). The photodiode  612  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  811 , the resulting electrical signal output from the photodiode  612  is essentially the same as the IF signal  811  that was modulated by the SAN  802  onto the composite optical signal  1624 . The photodiode  612  is coupled to an amplifier  808 . The signal output from the amplifier  808  is then coupled to an input of a matrix switch  616 . The matrix switch  616  performs in the same way as the matrix switch  616  discussed with respect to  FIG.  4    above. Accordingly, the switch matrix  616  selects which inputs to couple to the output of the switch matrix  616 . The output of the matrix switch  616  is handled the same as in the systems  600  described above in embodiments in which the signal  811  is at zero IF. In embodiments in which the signal  811  output from the baseband to IF module  607  within the SAN is at a frequency that is to be directly transmitted from the satellite  804 , then the handling will be the same, but for the fact that the upconverters  626  are not required. 
       FIG.  11    is an illustration of the return link for the system  800 . The return link for the system  800  is essentially the same as shown in  FIG.  7   . However, rather than the user terminals  606  transmitting a signal having binary modulation, the user terminals  606  transmit a signal having a more efficient modulation (e.g., 16 QAM rather than QPSK). Accordingly, the output digital decoder  410  is not required. The downconverter  850  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 3.5 GHz wide. The output of each downconverter  850  is coupled to an input of the switch matrix  416 . Therefore, the inputs of the MZM modulator  652  (see  FIG.  6   ) receive an analog signal from the switch matrix  416 . Accordingly, the output of each optical modulator  611  is an intensity modulated optical signal in which the intensity envelope tracks the signal output from the downconverter  850 . In some embodiments, the optical modulator  611  directly modulates the RF user uplink frequency onto the optical signal. Accordingly, the frequency converter  850  is not required. In embodiments in which the downconverter  850  reduces the user uplink frequency to a zero IF signal, the combined optical signal  660  is handled in the same way as discussed with regard to  FIG.  7   . In embodiments in which the optical signal is modulated with the user uplink frequency, a downconverter may be included within the modem  418  or prior to coupling the signal from the optical receiver  414  to the modem  418 . 
     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  616  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.  12    is a simplified schematic of a system  1000  using the technique shown in  FIG.  4    (i.e., modulating the optical feeder uplink with binary modulation and using that binary content to modulate an RF user downlink). However, the system  1000  uses the second system architecture in which a satellite  1004  is capable of performing on-board beamforming. The system  1000  operates similarly to the system  600  described above. However, the IF output from each bi-phase modulator  614  is coupled to a weight/combiner module  1006  rather than to the switch matrix  616 . 
       FIG.  13    is a simplified block diagram of a weight/combiner module  1006  in which K forward beam signals  1002  are received in the weight/combiner module  1006  by a beamformer input module  1052 . The K signals  1002  are routed by the input module  1052  to an N-way splitting module  1054 . The N-way splitting module  1054  splits each of the K signals  1002  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.  4   , there are 8 active SANs, each transmitting an optical signal comprising 64 optical channels. Each of the 64 optical channels carries a 3.5 GHz IF signal (i.e., forward beam signal). Therefore, there are 512 forward beam signals (i.e., 8 SANs×64 IF signals). Accordingly K=512. In some embodiments, the satellite has an antenna array  1008  having 512 array elements. Accordingly, N=512. 
     Each output from the N-way splitting module  1054  is coupled to a corresponding input of one of 512 weighting and summing modules  1056 . Each of the 512 weighting and summing modules  1056  comprises 512 weighting circuits  1058 . Each of the 512 weighting circuits  1058  place a weight (i.e., amplify and phase shift) upon a corresponding one of 512 signals output from the N-way splitting module  1054 . The weighted outputs from the weighting circuits  1058  are summed by a summer  1060  to form 512 beam element signals  1062 . Each of the 512 beam element signals  1062  is output through a beamformer output module  1064 . Looking back at  FIG.  12   , the 512 beam element signals  1062  output from the weight/combiner module  1006  are each coupled to a corresponding one of 512 upconverters  626 . The upconverters  626  are coupled to PAs  630 . The outputs of the PAs  630  are each coupled to a corresponding one of 512 antenna elements of the antenna array  1008 . 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  1008  and the weight combiner module  1006  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  1008  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  1002  to generate the beam element signals  1062  output from the weight/combiner module  1006 , a signal  1002  applied to each input of the weight/combiner module  1006  can be directed to one of the plurality of user beam coverage areas. Since the satellite  1004  can use the weight/combiner module  1006  and array antenna  1008  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  1004  through a SAN  602  that is not experiencing intolerable fading to provide feed link diversity, as described above in the context of the matrix switch  616 . Similar time division multiplexing can be done to transmit signals received by one of the lenses  610  in several user spot beams as described above. 
     Using a satellite  1004  that has on-board beamforming provides flexibility to allow feeder link diversity with regard to signals received from the plurality of SANs  602 . The use of on-board beam forming eliminates the need for the switch matrix  616  shown in  FIG.  4   . 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  606  transmit an RF signal up to the satellite  1004  on the user uplink. Receive elements in the antenna array  1008  receive the RF signal. The weight/combiner module  1006  weights the received signals received by each receive element of the antenna  1008  to create a receive beam. The output from the weight/combiner module  1006  is down converted from RF to IF. 
     In some embodiments, the upconverters  626  are placed at the input of the weight/combiner module  1006 , rather than at the outputs. Therefore, RF signals (e.g., 20 GHz 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  622 . In some embodiments, all of the outputs from one optical receiver  622  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  1008  and the weight/combiner modules  1006 . 
     In some embodiments, the second architecture shown in  FIG.  12    (i.e., on-board beam forming) is used with a QAM modulator  614 , similar to the system  600 . However, the satellite  1104  has on-board beamforming. 
       FIG.  14    is a simplified schematic of a system  1200  using the technique discussed with respect to  FIG.  8    in which an optical signal is RF modulated at the SAN  802 . However, the satellite architecture is similar to that of  FIGS.  12  and  11    in which a satellite  1204  has on-board beamforming capability. The SANs  802 , lenses  810 , optical detectors (such as photodiodes  812 ), amplifiers  613  and upconverters  626  are all similar to those described with respect to  FIG.  8   . However, the weight/combiner module  1006  and array antenna  1008  are similar to those described with respect to  FIGS.  10 ,  10 A and  11   . Similar to the architecture described in  FIG.  12   , the weight/combiner  1006  and array antenna  1008  allow the satellite  1004  to transmit the content of the signals received from one or more of the SANs  802  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  802  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  802  using a feeder uplink that is not experiencing an intolerable fade. 
       FIG.  15    is an illustration of a forward link of a satellite communications system  1400  using the third system architecture (i.e., ground-based beamforming) including an optical forward uplink  1402  and a radio frequency forward downlink  1404 . In some embodiments, the system  1400  includes a forward link ground-based beamformer  1406 , a satellite  1408  and a relatively large number (M) of SANs  1410  to create a relatively large capacity, high reliability system for communicating with user terminals  806  located within 512 user beam coverage areas  1801  (see  FIG.  19    discussed in detail below). Throughout the discussion of the system  1400 . M=8 SANs  1410  are shown in the example. However, M=8 is merely a convenient example and is not intended to limit the system disclosed, such as system  1400 , to a particular number of SANs  1410 . Similarly, 64 optical channels are shown in the example of the system  1400 . Likewise, the antenna array is shown as having 512 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  1407 ) to be communicated through the system  1400  is initially provided to the beamformer  1406  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  1407  to the beamformer  1406  (or some portion of the information carried by the forward beam input signal  1407 ) can represent data streams (or modulated data streams) directed to each of 512 user beams. In one embodiment, each of the 512 forward beam input signals  1407  is a 3.5 GHz wide IF signal. In some embodiments, the forward beam input signal  1407  is a composite 3.5 GHz wide carrier that is coupled to the input of the beamformer  1406 . 
     Each of the forward beam input signals  1407  is “directed” to a user beam coverage area  1801  by the beamformer  1406 . The beamformer  1406  directs the forward beam input signal  1407  to a particular user beam coverage area  1801  by applying beam weights to the 512 forward beam input signals  1407  to form a set of N beam element signals  1409  (as further described below with respect to  FIG.  16   ). Generally. N is greater than or equal to K. In some embodiments, N=512 and K=512. The 512 beam element signals  1409  are amplified and frequency converted to form RF beam element signals  1411 . Each is transmitted from an element of an N-element (i.e., 512-element) antenna array  1416 . The RF beam element signals  1411  superpose on one another within the user beam coverage area  1801 . The superposition of the transmitted RF beam element signals  1411  form user beams within the user beam coverage areas  1801 . 
     In some embodiments, the 512 beam element signals  1409  are divided among several SANs  1410 . Accordingly, a subset of the beam element signals  1409  (e.g., 512/8) are coupled to each SAN  1410 , where 8 is the number of SANs  1410 . Thus, the combination of 8 SANs  1410  will transmit 512 beam element signals  1409  from the beamformer  1406  to the satellite  1408 . In some embodiments, the beamformer  1406  is co-located with one of the SANs  1410 . Alternatively, the beamformer  1406  is located at another site. Furthermore, in some embodiments, the beamformer  1406  may be distributed among several sites. In one such embodiment, a portion of the beamformer  1406  is co-located with each SAN  1410 . Each such portion of the beamformer  1406  receives all of the forward traffic  1407 , but only applies beam weights to those 64 (i.e., 512/8) signals  1409  to be transmitted to the SAN  1410  that is co-located with that portion of the beamformer  1406 . 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  1408  (only one shown for simplicity). Accordingly, there is a one-to-one relationship between the antenna arrays  1416  and the beamformers  1406 . In some embodiments in which all of the beam elements from one beamformer  1406  are transmitted to the satellite  1408  through one SAN  1410 , there is no need to coordinate the timing of the transmissions from different SANs  1410 . Alternatively, in embodiments in which beam elements output from the same beamformer  1406  are transmitted to the satellite  1408  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  1411  transmitted from each of the N elements of an antenna array  1416  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  1801 . In some embodiments in which there are 8 SANs  1410  (i.e., M=8) each SAN  1410  receives 64 beam element signals  1409 . 
     In order to maintain the phase and amplitude relationship of each of the 512 RF beam element signals  1411  to one another, the beamformer  1406  outputs 8 timing pilot signals  1413 , one to each SAN  1410 , in addition to the N beam element signals  1409 . Each timing pilot signal  1413  is aligned with the other timing pilot signals upon transmission from the beamformer  1406  to each SAN  1410 . In addition, the amplitude of each timing pilot signal  1413  is made equal. 
       FIG.  16    is a detailed illustration of the forward beamformer  1406 . The forward beamformer  1406  receives 512 forward beam signals  1407  representing the forward traffic to be sent through the system  1400 . The signals  1407  are received by a matrix multiplier  1501 . The matrix multiplier  1501  includes a beamformer input module  1502 , a 512-way splitting module  1504  and 512 weighting and summing modules  1506 . Other arrangements, implementations or configurations of a matrix multiplier can be used. Each of the 512 forward beam signals  1407  is intended to be received within a corresponding one of 512 user beam coverage areas  1801 . Accordingly, there is a one-to-one relationship between the 512 user beam coverage areas  1801  and the 512 forward beam signals  1407 . In some embodiments, the distribution equipment (e.g., the core node) that provides the forward traffic to the beamformer  1406  ensures that information to be transmitted to a particular user beam coverage area  1801  is included within the forward beam input signal  1407  corresponding to that user beam coverage area  1801 . 
     The 512-way splitting module  1504  splits each of the 512 forward beam signals  1407  into 512 identical signals, resulting in 512×512 (i.e., N×K) signals being output from the 512-way splitting module  1504 . When N is equal to 512 and K is equal to 512, the splitting module  1504  outputs 512×512=524,288 signals, 512 unique signals output from the splitting module  1504  are coupled to each of the 512 weighting and summing modules  1506 . The signals coupled to each of the weighting and summing modules  1506  are weighted (i.e., phase shifted and amplitude adjusted) in accordance with beam weights calculated by a forward beam weight generator  1508 . Each of 512 weighted signals corresponding to the same array element N are summed in one of 512 summers  1512 . 
     Since each group of 64 outputs from of the summers  1512  will be coupled to, and transmitted by, a different one of the 8 SANs  1410 , a timing module  1514  is provided. The timing module  1514  adjusts when the beam element signals  1409  are sent from the beamformer to ensure that each group of 64 IF beam element signals  1409  arrives at the user beam coverage area  1801  at the appropriate time to ensure that the superposition of the signals  1409  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  1410  to the satellite  1408 . Accordingly, the signal  2122  would be coupled to the forward beam weight generator  1508 . In some embodiments, the timing module  1514  generates the timing pilot signal  1413  transmitted from the forward beamformer  1406  to each SAN  1410 . In some embodiments, one timing pilot signal  1413  is generated and split into 8 copies of equal amplitude, one copy sent to each SAN  1410 . Alternatively, the amplitude of the copies may be a predetermined ratio. As long as the ratio between timing pilot signals  1413  is known, RF beam element signals  1411  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  1514  within the beamformer  1406 , each SAN  1410  returns a signal  2122  derived from the SAN timing correction signal  1419  to a timing control input to the beamformer to allow the forward beamformer  1406  to determine corrections to the alignment of the signals to each SAN  1410 . In some embodiments. SAN timing correction signals  1419  are then used by the timing module  1514  to adjust the timing of the beam element signals  1409 . In other embodiments, the SAN timing correction signal  1419  are used by the forward beam weight generator  1508  to adjust the beam weights to account for differences in the paths from the beamformer  1406  through each of the SANs  1410  to the satellite  1408 . As noted above, corrections to the alignment can alternatively be made in each SAN  1410 . 
     Once the beam element signals  1409  have been properly weighted and any necessary timing adjustments made, each of the 512 signals  1409  are coupled to one of the SANs  1410 . That is, each of the 8 SANs  1410  receives 64 beam element signals  1409  (i.e., 512/8) from the forward beamformer  1406 . An optical transmitter  1401  within each SAN  1410  receives, multiplexes and modulates those 64 beam element signals  1409  that it receives onto an optical carrier. 
       FIG.  17    is an illustration of an optical transmitter  1401  used in some embodiments of the system  1400 . The optical transmitter  1401  is similar to the optical transmitter  607  discussed above with respect to  FIG.  10   . However, the input signals  1409  differ, since they are beam weighted by the beamformer  1406 . Furthermore, the timing pilot signal  1413  provided by the beamformer  1406  is coupled to an optical modulator  611  and modulated onto an optical carrier within the same band as the band of other optical modulators  611  within the same optical band module  1403 , as determined by the wavelength of the light source  654  within that optical modulator  608 . In some embodiments, each optical band module  1403  is identical. However, modulating the timing pilot signal  1413  need only be done in one such optical band module  1403 . Alternatively, as shown in  FIG.  17   , only one optical band module  1403  is configured to modulate a timing pilot signal  1413 . The other optical band modules  608  may be similar to the optical band module  608  show in  FIG.  6    and described above. In either embodiment, in a system in which 8 SANs  1410  each receive 64 beam element signals  1409  and modulate them onto 16 optical channels within 4 different optical bands, as shown in  FIG.  5   , there are four optical band modules within the optical transmitter  1401  in each SAN  1410 . 
     The timing pilot signal  1413  follows the same path to the satellite as the IF beam element signals  1409 . Therefore, by comparing the arrival time of the timing pilot signals sent from each SAN  1410  at the satellite  1408 , 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  1410 . Similar to the optical transmitter  607 , the optical channels  915  output by each optical modulator  611  shown in  FIG.  17    are combined in an optical combiner  609 . The composite optical signal  1624  is emitted from an optical lens  2002  within the optical transmitter  1401 . The optical lens  2002  operates as an optical signal transmitter capable of transmitting an optical signal to the satellite  1408 . 
     A composite optical signal  1624  from each of the SANs  1410  with the 64 beam element signals  1409  and the timing pilot signal  1413  is transmitted to the satellite  1408  on the optical forward uplink  1402  and received by one of 8 optical receivers  1412  within the satellite  1408 . Each of the 8 optical receivers  1412  within the satellite  1408  demultiplexes the 64 optical channels  915  from the composite optical signal  1624 . 
       FIG.  18    shows the components of a satellite  1408  (see  FIG.  15   ) in greater detail. The Satellite  1408  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.  15   . The components of the forward link of the satellite  1408  include 8 optical receivers  1412 . 8 amplifier/converter modules  1414  and a 512-element antenna array  1416 . In some embodiments of the system  1400 , similar to the embodiments shown in  FIGS.  9 ,  13  and  16   , in which there are 8 SANs (i.e., M=8), the received composite signal  1624  includes 64 optical channels divided into 4 bands of 16 each, each of which carries a 3.5 GHz wide IF channel. Furthermore, there are K=512 user beam coverage areas  1801  and N=512 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  1412  is associated with a corresponding amplifier/converter module  1414 . The optical receivers  1412  each include a lens module  1701 , and a plurality of optical detectors, such as photodiodes  1703 . The lens module  1701  includes a lens  1702  (which in some embodiments may be similar to the lens  610  described above with respect to  FIG.  4   ), an optical demultiplexer  1704 , a plurality of optical demultiplexers  1706  and a plurality of output lenses  1708 . 
     In operation, the composite optical signal  1624  is received from each of the 8 SANs  1410 . A lens  1702  is provided to receive each composite optical signal  1624 . In some embodiments, the lenses  1702  can be focused (in some embodiments, mechanically pointed) at a SAN  1410  from which the lens  1702  is to receive an composite optical signal  1624 . The lens  1702  can later be refocused to point to a different SAN  1410 . Because the lenses  1702  can be focused to receive composite optical signal  1624  from one of several SANs  1410 , the satellite  1408  can receive signals from 8 SANs  1410  selected from among a larger number 8+X SANs  1410 . In some embodiments X=24. Therefore, 32 different SANs  1410  are capable of receiving information intended to be communicated to user beam coverage areas  1801  in the system. However, only eight of the 32 SANs  1410  are selected to have information that is transmitted be received by the satellite  1408 . 
     The signal path of one of the composite optical signals  1624  through the forward link of the satellite  1408  is now described in detail. It should be understood that each of the 8 signal paths taken by the 8 received composite optical signals  1624  through the forward link of the satellite  1408  operate identically. The composite optical signal  1624  that is received by the lens  1702  is directed to an optical demultiplexer  1704 . In a system using the modulation scheme illustrated in  FIG.  9   , the optical demultiplexer  1702  splits the composite optical signal  1624  into the four bands  907 ,  909 ,  911 ,  913  (see  FIG.  9   ). That is, the optical demultiplexer  1704  splits the composite optical signal  1624  into the four optical wave lengths onto which the beam element signals  1407  were modulated by the SAN  1410  that sent the composite optical signal  1624 . Each of the optical outputs from the optical demultiplexer  1704  is coupled to a corresponding optical demultiplexer  1706 . Each of the four optical demultiplexers  1706  output 512/(4×8) optical signals for a total of 4×(512/(4×8)=512/8=64 optical signals. Each of the 16 optical signals output from the four optical demultiplexers  1706  is directed to an output lens  1708 . Each of the output lenses  1708  focus the corresponding optical signal onto a corresponding photo detector, such as a photodiode  1703 . Each photodiode  1703  detects the amplitude envelope of the optical signal at its input and outputs an RF transmit beam element signal  1418  corresponding to the detected amplitude envelope. Accordingly, the RF transmit beam element signals  1418  output from the optical receivers  1412  are essentially the beam element signals  1409  that were modulated onto the optical signals by the SANs  1410 . 
     The RF output signals are then coupled to the amplifier/converter module  1414 . The amplifier/converter module  1414  includes 512/8 signal paths. In some embodiments, each signal path includes a Low noise amplifier (LNA)  1710 , frequency converter  1712  and PA  1714 . In other embodiments, the signal path includes only the frequency converter  1712  and the PA  1714 . In yet other embodiments, the signal path includes only the PA  1714  (the frequency converter  1712  can be omitted if the feed signals produced by the SANs are already at the desired forward downlink frequency). The frequency converter  1712  frequency converts the RF transmit beam element signals  1418  to the forward downlink carrier frequency. In some embodiments, the output of each upconverter  1712  is an RF carrier at a center frequency of 20 GHz. Each of the 512 outputs from the 8 amplifier/converter modules  1414  is coupled to a corresponding one of the 512 elements of the 512-element antenna array  1416 . Therefore, the antenna array  1416  transmits the 512 forward downlink beam element signals  1718 . 
       FIG.  19    is an illustration of user beam coverage areas  1801  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  FIGS.  4 ,  8  and  12   , 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  FIGS.  10 ,  11 ,  12 ,  14  and  14 A , the 512 forward downlink beam element signals  1718  are superposed on one another to form user beams directed to user beam coverage areas  1801 . As shown in  FIG.  19   , user beam coverage areas are distributed over a satellite service area that is substantially larger than the user beam coverage areas  1801 . The 512 element antenna array  1416  transmits the RF beam element signals  1411  over the forward downlink  1404  to each of the 512 user beam coverage areas  1801 . User terminals  806  within each user beam coverage area  1801  receive the user beam directed to that particular user beam coverage area  1801  by virtue of the superposition of the RF beam element signals  1411  transmitted from each of the 512 elements of the 512 element antenna array  1416 . 
     In addition to the IF beam element signals  1418  output from each optical receiver  1412 , each optical receiver  1412  demultiplexes a satellite timing signal  1415  from the composite optical signal  1624 . A satellite timing signal  1415  is output from each receiver  1412  and coupled the corresponding amp/converter module  1414 . An LNA  1710  within the amp/converter module  1414  amplifies the satellite timing signal  1415 . The output  1416  of the LNA  1710  is coupled to a satellite timing module  1417 . In some embodiments, the satellite timing module  1417  compares the satellite timing signal  1415  received by each optical receiver  1412  to determine whether they are aligned. The satellite timing module  1417  outputs 8 SAN timing correction signals  1419 , one to be returned to each of the 8 SANs  1410 . In some embodiments, each SAN timing correction signal  1419  is coupled to an input to a return amp/converter module  1904  (see  FIG.  24   ). Each SAN timing correction signal  1419  is amplified, frequency converted to the forward downlink frequency and coupled to an input to one of 8 optical transmitters  1401  within the satellite  1408 , similar to the optical transmitter  1401  provided in the SAN  1410 . 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  1410  from which the reference satellite timing signal was sent. Therefore, no SAN timing correction signal  1419  is sent for that SAN  1410 . The SAN timing correction signal  1419  is modulated onto each composite optical signal transmitted by the satellite  1408  to each SAN  1410 . 
     Each SAN timing correction signal  1419  provides timing alignment information indicating how far out of alignment the timing pilot signal  1413  is with respect to the other timing pilot signals (e.g., the reference satellite timing signal  1415 ). In some embodiments, the timing information is transmitted through the SANs  1410  to a timing module  1514  (see  FIG.  16   ) in the beamformer  1406 . The timing module  1514  adjusts the alignment of the beam elements prior to sending them to each SAN  1410 . Alternatively, the timing alignment information is used by each SAN  1410  to adjust the timing of the transmissions from the SAN  1410  to ensure that the RF beam element signals  1411  from each SAN  1410  arrive at the satellite  1408  in alignment.  FIG.  20    is an illustration of an optical transmitter  1460  having a timing module  1462  for adjusting the timing of the beam element signals  1409  and the timing pilot signal  1413 . The timing module  1462  receives a timing control signal  1464  from satellite  1408  over the return downlink (discussed in further below). The timing module applies an appropriate delay to the signals  1409 ,  1413  to bring the signals transmitted by the SAN  1410  into alignment with the signals transmitted by the other SANs  1410  of the system  1400 . 
     In an alternative embodiment, timing adjustments can be made to the RF beam element signals  1411  within the satellite based on control signals generated by the satellite timing module  1417 . In some such embodiments, the control signals control programmable delays placed in the signal path between the optical receiver  1412  and the antenna array  1416  for each RF beam element signal  1411 . 
     In an alternative embodiment, at least two of the satellite timing signals  1415  are transmitted from the satellite back to each SAN  1410 . The rust is a common satellite timing signal  1415  that is transmitted back to all of the SANs. That is, one of the received satellite timing signals  1415  is selected as the standard to which all others will be aligned. The second is a loop back of the satellite timing signal  1415 . By comparing the common satellite timing signal  1415  with the loop back satellite timing signal  1415 , each SAN  1410  can determine the amount of adjustment needed to align the two signals and thus to align the IF beam element signals  1418  from each SAN  1410  within the satellite  1410 . 
       FIG.  21    is a system  1450  in which each of the K forward beam input signals  1452  contain S 500 MHz wide sub-channels. In some embodiments, K=512 and S=7. For example, in some embodiments, seven 500 MHz wide sub-channels are transmitted to one user coverage area  1801 .  FIG.  22    is an illustration of a beamformer  1300  in which forward beam input signals  1452  comprise seven 500 MHz wide sub-channels, each coupled to a unique input to the beamformer  1300 . Accordingly, as noted above, the sub-channels can be beamformed after being combined into an IF carrier, as shown in  FIGS.  14 ,  15   . Alternatively, as shown in  FIGS.  14 A,  13   , the sub-channels  1452  can be beamformed before being combined using the beamformer  1300 . Accordingly, the beamformer  1300  outputs S×N beam element signals, with (S×N)/M such beam element signals being sent to each SAN  1410 . In the example system  1450 , S=7, N=512 and M=8. As noted above, these numbers are provided as a convenient example and are not intended to limit the systems, such as the system  1450 , to these particular values. 
       FIG.  22    is a simplified block diagram of a beamformer  1300  in which each carrier comprises S sub-channels  1452 , where S=7. Each of the sub-channels  1452  is provided as independent input to a matrix multiplier  1301  within the beamformer  1300 . Therefore, 512×7 sub-channels  1452  are input to the matrix multiplier  1301 , where there are 512 user spot beams to be formed and 7 is the number of sub-channels in each carrier; that is, K=512 and S=7. The 512-way splitter  1304  receives each of the 512×7 sub-channels  1407 , where 512 is the number of elements in the antenna array  1416 . Alternatively, N may be any number of antenna elements. Each sub-channel  1452  is split 512 ways. Accordingly, 512×512×7 signals are output from the splitter  1304  in a three-dimensional matrix. The signals 1, 1, 1 through 1. K, 1 (i.e., 1.512, 1 where K=512) are weighted and summed in a weighting and summing module  1306 . Likewise, the signals 1, 1, 7 through 1.512, 7 are weighted and summed in a weighting and summing module  1313 . In similar fashion, each of other weighting and summing modules weight receive outputs from the splitter  1304 , and weight and sum the outputs. The 512×7 outputs from the weighting and summing modules  1306 ,  1313  are coupled to the inputs of a timing module  1514 . The timing module functions essentially the same as the timing module  1514  of the beamformer  1406  discussed above. The beamformer  1300  outputs 512×7 beam element signals  1454  to the SANs  1410 . Each of the 8 SANs  1410  comprises an IF combiner  1602 . 
       FIG.  23    is an illustration of a SAN  1456  of system  1450 . In some embodiments, a first baseband to IF converter  805  operates in similar fashion to the baseband to IF converter  805  discussed above with respect to  FIG.  10   . The converter  805  outputs a signal  811  that is a combination of seven 500 MHz beam element signals  1454 . In addition, in some embodiments, at least one of the baseband to IF converters  1605  includes an additional frequency converter  1607 . The additional frequency converter  1607  receives the timing pilot signal  1413  from the beamformer  1300 . The timing pilot signal  1413  is combined with the beam element sub-channels  1452  and coupled to the optical transmitter  607 . Each of the IF signals  811  coupled to the optical transmitter  607  are combined in the optical combiners  609  of each SAN  1410  to form the transmitted composite optical signal  1624 . The timing pilot signal  1413  is coupled to the input of a frequency converter  1607 . The frequency converter  1607  places the timing pilot signal at a frequency that allows it to be summed with the beam element signals  1454  by the summer  1608 . Alternatively, the timing pilot signal  1413  can be directly coupled to an additional optical modulator  1610  dedicated to modulating the timing pilot signal  1413 . The output of the additional modulator  1610  is coupled to the combiner  609  and combined with the other signals on a unique optical channel dedicated to the timing pilot signal. 
       FIG.  24    is an illustration of a return link for the system  1400  having ground-based beamforming. User terminals  806  located within a plurality of 512 user beam coverage areas  1801  transmit RF signals to the satellite  1408 . An 512-element antenna array  1902  on the satellite  1408  (which may or may not be the same as the antenna array  1416 ) receives the RF signals from the user terminals  806 , 512/8 outputs from the 512-element antenna array  1902  are coupled to each of the 8 amplifier/converter modules  1904 . That is, each element of the antenna array  1902  is coupled to one LNA  1906  within one of the amplifier/converter modules  1904 . The output of each LNA  1906  is coupled to the input to a frequency converter  1908  and a pre-amplifier  1910 . The amplified output of the LNA  1906  frequency down-converted from RF user uplink frequency to IF. In some embodiments, the IF signal has a bandwidth of 3.5 GHz. In some embodiments, the pre-amp  1910  provides additional gain prior to modulation onto an optical carrier. The outputs of each amplifier/converter modules  1904  are coupled to corresponding inputs to one of 8 optical transmitters  1401 , similar to the optical transmitter  607  of  FIG.  4   . Each of 8 optical transmitters  1401  outputs and transmits an optical signal to a corresponding SAN  1410 . The SAN  1410  receives the optical signal. The SAN  1410  outputs 512/8 return beam element signals  1914  to a downlink beamformer  1916 . The downlink beamformer  1916  processes the return beam element signals  1914  and outputs 512 beam signals  1918 , each corresponding with one of 512 user beam coverage areas  1801 . 
     The IF signals provided to the optical transmitter  607  from the amplifier/converter module  1904  are each coupled to one of 512/8 optical modulators  611 . For example, if there are 512 elements in the antenna array  1902  (i.e., N=512) and there are 8 SANs  1410  in the system  1900 , then 512/8=64. In a system in which the IF signals are modulated onto wavelengths divided into 4 bands, such as shown in  FIG.  9   , the optical modulators  611  are grouped together in optical band module  608  having 512/(4×8) optical modulators  611 . 
     Each optical modulator  611  is essentially the same as the uplink optical modules  611  of the SAN  1410  shown in  FIG.  10   , described above. Each optical modulator  611  within the same optical band module  608  has a light source  654  that produces an optical signal having one of 16 wavelengths λ. Accordingly, the output of each optical modulator  611  will be at a different wavelength. Those optical signals generated within the same optical band module  608  will have wavelengths that are in the same optical band (i.e., in the case shown in  FIG.  9   , for example, the optical bands are 1100 nm, 1300 nm, 1550 nm and 2100 nm). Each of those optical signals will be in one of 16 optical channels within the band based on the wavelengths λ 2. The optical outputs from each optical modulator  611  are coupled to an optical combiner  609 . The output of the optical combiner  609  is a composite optical signal that is transmitted through an optical lens  2016  to one of the SANs  1410 . The optical lens  2016  can be directed to one of several SANs  1410 . Accordingly, the 8 optical transmitters each transmit one of 8 optical signals to one of 8 SANs  1410 . The particular set of 8 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.  25    is an illustration of one of the SANs  1410  in the return link. An optical receiver  622  comprises lens  2102  that receives optical signals directed to the SAN  1410  from the satellite by the lens  2016 . An optical band demultiplexer  2104  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  2106  are coupled to an optical channel demultiplexer  2108 . The optical channel demultiplexer  2108  separates the 512/(4×8) signals that were combined in the satellite  1408 . Each of the outputs from the four optical channel demultiplexers  2108  are coupled to a corresponding lens  2110  that focuses the optical output of the optical channel demultiplexers  2108  onto an optical detector, such as a photodiode  2112 . Output signals  2116  from the photodiodes  2112  are each coupled to one of 512/8 LNAs  2114 . The output from each LNA  2114  is coupled to the return link beamformer  1916  (see  FIG.  24   ). In addition, one channel output from the optical receiver  622  outputs a timing correction signal  1464  that is essentially the SAN timing correction signal  1419  (see  FIG.  18   ) that was provided by the satellite timing module to the return amplifier/converter module  1414 . In some embodiments, the timing correction signal  1464  is coupled to a timing pilot modem  2120 . The timing pilot modem outputs a signal  2122  that is sent to the forward beamformer  1406 . In other embodiments, the timing correction signal  1464  is coupled to a timing control input of the timing module  1462  (see  FIG.  20   ) discussed above. 
       FIG.  26    illustrates in greater detail, a return beamformer  1916  in accordance with some embodiments of the disclosed techniques. Each of the 512 outputs signals  2116  is received by the return beamformer  1916  from each of the SANs  1410 . The return beamformer comprises a beamforming input module  2203 , a timing module  2201 , matrix multiplier  2200  and a beamformer output module  2205 . The matrix multiplier  2200  includes a K-way splitting module  2202  and 512 weighting and summing modules  2204 . The matrix multiplier  2200  multiplies a vector of beam signals by a weight matrix. Other arrangements, implementations or configurations of a matrix multiplier  2200  can be used. Each signal  2116  is received by the beamformer  1916  in the beamformer input module  2203  and coupled to the timing module  2201 . The timing module  2201  ensures that any differences in the length and characteristics of the path from the satellite to the SAN  1410  and from the SAN  1410  to the return beamformer  1916  is accounted for. In some embodiments, this may be done by transmitting one pilot signal from the return beamformer  1916  to each SAN  1410 , up to the satellite and retransmitting the pilot signal back through the SAN  1410  to the return beamformer  1916 . Differences in the paths between the return beamformer  1916  and the satellite can be measured and accounted for. 
     The output of the timing module is coupled to a K-way splitter  2202  that splits each signal into 512 identical signals, 512 unique signals are applied to each of 512 weighting and summing circuits  2204 . Each of the 512 unique signals is weighted (i.e., the phase and amplitude are adjusted) within a weighting circuit  2206 , such that when summed in a summing circuit  2208  with each of the 512 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.  4   , an optical downlink from the satellite  602  to the SAN  604  provides a broadband downlink. Rather than lenses  610  for receiving the optical uplink, lasers are provided for transmitting an optical downlink. Furthermore, rather than the bi-phase modulator  614  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.  4   . In this embodiment, the modulator  614  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.  8   , 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  842 ,  844  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  811  in the SAN  802 . 
     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  FIGS.  4 ,  8  and  12    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 breadth and scope of the claimed invention should not be limited by any of the examples provided in describing the above disclosed embodiments. 
     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. 
     Additionally, the various embodiments set forth herein are described with the aid of block diagrams, flow charts and other illustrations. As will become apparent to one of ordinary skill in the art after reading this document, the illustrated embodiments and their various alternatives can be implemented without confinement to the illustrated examples. For example, block diagrams and their accompanying description should not be construed as mandating a particular architecture or configuration.