High capacity communications satellite

A high capacity communication satellite uses a large number of parallel beams and optical processing to effect a fully interactive, high bandwidth, high channel capacity fully switched communication system. The satellite reuses its assigned bandwidth in each of the multiple beams. The beams are formed by either RF or optical means. The specific users in each beam are then separated optically using spatial light modulation (SLM) array correlation techniques. A single large SLM, or multiple smaller SLMs in combination, may be used. The individual customers are then repositioned in the array by optical SLM mixing and recorrelation. The result is then remodulated by another SLM array used as a mixer, and then recombined to reform the appropriate outgoing beams. The entire system then becomes a fully switched, high bandwidth, high channel capacity communications network on a single satellite.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to communication satellites, and in 
particular, to the provision of a high bandwidth, high channel capacity, 
fully switched, fully interactive communication network in a single 
satellite. The inventive system is a high capacity communications 
satellite, or HCCS. 
2. Description of the Related Art 
Satellites have been used for communication for years. One common use of 
satellites involves distributed transmission, like the C and Ku band 
TeleSat, direct broadcast satellites which have one or two beams. These 
satellites, which are in geosynchronous orbit (i.e. their orbital speed 
and altitude are such that they seem to hover over a particular position 
on the earth's surface,) broadcast a series of simultaneous "programs" in 
one direction to a large number of individual ground stations. These are 
not point to point or interactive (2-way) satellites. However, they do 
have a fairly wide bandwidth (typically 100-500 MHz). 
Another use for communications satellites is a so-called point to point 
gateway type use, in which a receiving beam is pointed at a large sending 
dish (for example, in Europe) and a corresponding transmitting beam is 
pointed at a receiving dish in the U.S. (for example, Intelsat). This 
system also is geosynchronous and wideband (100-500 MHz), but has a 
limited number of beams (for example, eight beams would be a large number 
for such a system). Also, these systems cover only limited areas, allow 
only limited switching, if any, and handle very few communication 
channels. 
Some newer system designs (Iridium, Ellipsat, Calling Communications) 
involve a large number (66 to 840) of low orbit satellites that pass 
messages among themselves to create a fully interactive network. These are 
very complex, expensive systems limited to low bandwidth (.apprxeq.10 KHz 
or less) and low capacity (50-200 channels in the overall system). 
Typical satellite communication systems have been limited by low bandwidth 
(e.g. 50-500 MHz would handle only 50-500 channels); switching networks, 
using standard video switching capable of inclusion in a satellite, would 
handle only 10-100 switched channels. Even the present nationwide 
telephone system handles only audio, which has a much lower bandwidth 
(.apprxeq.10 KHz), to switch about one million customers simultaneously. 
The ground telephone system contains 10,000-20,000 switching buildings, at 
a cost of over $100 billion. 
It would be desirable to provide a satellite system having a large number 
of channels and high bandwidth, while providing a fully switched, 
interactive system. While optical-based spatial light modulator (SLM) 
technology is known, and can be used for transmission through the air, as 
evidenced for example in copending Appln. No. 08/133,879, filed in the 
name of the present inventor, the application of SLM technology to provide 
high capacity satellite communications has not been known, so far as the 
present inventor is aware. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to create a communication 
satellite which does not suffer from the above drawbacks. 
It is a specific object of the invention to provide a system which combines 
large number of multiple antenna beams and a novel optical processing and 
switching system utilizing SLM technology. 
By utilizing a large number of parallel beams (100 to 4,000) and spatial 
light modulator (SLM) based optical processing to distinguish customers 
within each beam (100 to 1,000 customers/beam) and shift the individual 
customers to the appropriate output beam and output frequency, the present 
invention allows simultaneous switching of up to one million simultaneous 
1 MHz (full video) signals, thus yielding a fully interactive video 
network. 
The bandwidth achieved by the invention is 100 times the bandwidth of the 
expensive low earth orbit systems, and handles five to 20 times the number 
of simultaneous customers in a single satellite, in contrast to the 66 to 
840 satellites required at present. As a result, the inventive system is 
relatively quite low in cost. The SLMs and beamforming devices are fairly 
inexpensive single integrated circuits, enabling a reduction in satellite 
weight to be less than half of that of present satellite designs. 
The HCCS system uses from 100-4,000 simultaneous beams (the baseline design 
being 1,000). Since it is possible to reuse the full spectrum in each beam 
if the beams are coded properly, it is possible to handle 500 customers 
per beam (1 MHz/channel in a total bandwidth of 500 MHz), enabling total 
simultaneous usage by about 1 million customers. 
The remaining problem is how to switch the 500,000 outgoing channels. As 
mentioned above, the present phone system requires 10,000-20,000 buildings 
to switch the same number of much lower-bandwidth audio channels; 
moreover, the HCCS system must switch the same number of much 
higher-bandwidth (1 MHz) video channels within a fairly small satellite. 
Recent developments in SLM using quantum well technology have created the 
capability of 1024.times.1024 pixel arrays that can be driven at 1 GHz 
rates from full reflectance to almost zero reflectance (over 40 dB dynamic 
range). While arrays of this size would enable full implementation of the 
invention, and would be a preferred embodiment, as a practical matter at 
present only smaller SLMs are available in the necessary quantities and 
costs. It is within the contemplation of the invention to use a larger 
number of smaller SLMs (perhaps two, four, eight, or 16 or more as desired 
or necessary) in combination to provide performance comparable to that 
achieved by the larger SLMs. 
In accordance with a preferred embodiment of the inventive switching 
technique, first the 500 channels per beam are encoded by either frequency 
assignment or broadband coding. Then, a 1024.times.1024 SLM array (made up 
of either a single SLM or multiple smaller SLMs) mixes the incoming 
frequencies from the assigned frequencies to baseband where they are 
detected and bandpassed. Alternatively, in the broadband coded case, the 
decoding signals are multiplied by the input and are integrated to 
separate the 500 channels per beam. Once separated and detected, they are 
remodulated by another set of SLMs to either move them to the appropriate 
beam or create the appropriate wide bandwidth per output beam to 
retransmit the information. The entire system takes approximately seven to 
10 1024.times.1024 SLM arrays, a few detector arrays, and a few linear 
(1024.times.1) arrays with appropriate optics. 
Additional optics to redirect a portion of each beam back to its same area 
can also be added to handle the expected higher volume of local calls. 
It also is within the contemplation of the invention to add special 
circuitry as necessary to break some channels down further into over 100 
audio channels, or to combine a number of channels for high definition 
television (HDTV) transmission.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 depicts a high capacity communications satellite system in 
accordance with the present invention, in which N beams 2, with M 
simultaneous customers 1 per beam, are shown. These M customers would be 
only a small fraction of the total customers in the beam. However, since 
only approximately 1% of the customers utilize a two-way communication 
system at any one time, these M simultaneous users could represent as many 
as 100.times.M potential customers or terminals, each having a small 
antenna, a transceiver, and a video camera and TV player. 
The M simultaneous users (where M is from 100 to 4,000; typically 500) 
would have some low level of near-lossless video compression to compress 
each signal to a 1 MHz band (or digital equivalent), for a total of 500 
signals in a typical implementation. These 500 signals, each having a 
bandwidth of 1 MHz, are either frequency coded or digitally coded to 
discriminate them from each other. The signals are received by one of the 
N parallel receive beams 3 (typically N=1000) created by the multiple beam 
antenna receiving system on a geosynchronous satellite (not shown). 
On the satellite, the 1000 beams 3 (each containing 500 simultaneous users) 
are transmitted along N channels 4 to a multiple beamformer 5, using a 
1.times.N SLM illuminated by a laser to form M optical channels 6. Each 
optical channel 6 is then spread in one dimension using a diverging 
cylindrical lens to illuminate a N.times.M SLM array in an individual 
channel resolver 7. The N.times.M SLM array is driven by appropriate 
sinusoidal signals on its backplane in order to downconvert the desired 
individual channels to video. After appropriate detection and filtering, 
each channel of each beam is effectively decoded and its signal is 
isolated on one pixel of the N.times.M detector array, yielding N.times.M 
optical channels 8. 
An effective cross bar switch 9 is then applied to switch any individual 
channel to any desired output location. In its simplest embodiment, this 
would be done by encoding the signal at its source, on the ground to 
ensure that once detected, it will be in the desired column to be sent to 
the desired receive location. This would require no "intelligence" on the 
part of the satellite, and no changes in the satellite's operation. 
In a slightly more complex implementation, a "double hop" capability would 
be added, in which transceivers on the ground in selected (or in all) 
beams could receive a signal and re-route it to the desired end points. 
This allows for alternative routing, when needed. 
In a more general embodiment, selected pixels would be "remodulated" with 
arbitrary frequencies (or codes), the downconverting and detection process 
being repeated in either plane. The signal on any pixel could be moved to 
any other pixel, to permit fully random cross bar coupling. 
Once the signals have been decoded and detected, they are used to modulate 
another N.times.M SLM to create N.times.M optical signal paths 10. These 
are then provided to an individual channel modulator 11, which includes 
another N.times.M SLM whose backplane contains appropriate sinusoidal or 
code modulation to "fill" the bandwidth of the retransmitted beams. The 
signals output over the N.times.M optical channels 12 then are provided to 
a beam combiner 13 which includes a 1.times.N detector array and 
cylindrical optics, yielding N optical channels 14. Then, a multiple 
beamformer 15 is used to create the appropriate signals 16 to create in 
turn N retransmitted beams 17 which are coaxial with the N received beams. 
These beams (typically 1000) contain the 500 channels each completing the 
cross linking of full video, simultaneous communication of one million 
customers. 
An additional path is created from multiple beamformer 5 by subdividing K 
direct channels 18. This is done most easily if the channels are frequency 
encoded by a simple filter 19, such as a direct return filter, on each 
beam. The filtered channels are added along K direct channels 20 to 
multiple beamformer 15 to permit a large number of local video connections 
within each local beam. 
FIGS. 2A-2C show different methods of creating the "multiple antenna 
beams". FIG. 2A shows a standard multiple feed curved reflector design, 
commonly called a Gregorian fed multiple beam antenna. In that antenna, a 
series of actual RF feeds 21 are located at the focal plane of a curved 
reflector 22 so as to create a series of beams 23 that would cover a large 
area (like the U.S.) FIG. 2B shows an RF Luneburg lens, a technique that 
utilizes a dielectric sphere 24 that has a variable dielectric constant as 
a function of radius so as to focus any parallel rays to a point on the 
far side of the sphere. If M feeds were located on the appropriate 
locations 25, M beams 23 covering the desired area would be created. 
The above two techniques are well known to ordinarily skilled artisans in 
this technological field, and so need not be detailed any further here. 
However, these techniques do tend to be cumbersome when employed in a 
satellite system. A more volume efficient design is shown in FIG. 2C, 
which shows a Luneburg optical lens approach, in which M incoming beams 26 
are sampled by an RF multi-element array 27 of appropriate element number 
and spacing to create M beams, whose elements are connected in a pixel to 
array element manner to an N.times.M SLM 29. Prior to output to SLM 29, 
the output of array 27 is downconverted from RF to baseband in 
downconverter 28. A laser 30 illuminates the SLM 29 via appropriate 
cylindrical optics 31 and a half-mirror 32, and the output beam is focused 
onto the appropriate M detectors using a variable dielectric sphere 33 to 
sample the M beams. M feeds 34 (which can be diode lasers) are collocated 
to create the outgoing beams. As can be appreciated, the FIG. 2C 
embodiment would be quite a bit smaller than those of FIGS. 2A or 2B. 
FIG. 3 describes the internal processing from the output 6 of the incoming 
beamformer 5 (N optical channels), through the input 14 of the outgoing 
beamformer 15 (N optical channels) as shown in FIG. 1. Referring to FIG. 
3, the signals from the incoming beamformer 5 are constituted by N antenna 
beam signals on separate signal paths 100 (typically 1000 paths) each 
containing M frequency or digital coded simultaneous signals (typically 
M=500). These signal paths are connected to a 1.times.N SLM array 101. The 
array is illuminated by a laser 105 through a collimating lens 104 and a 
half mirror 103, the output of the laser 105 then being focused onto the 
line array 101 by a cylindrical lens 102. The lens 102 also spreads each 
combined beam reflected signal to cover a complete row of another SLM 
array 106. This array has each column hardwired together and modulated by 
the same signal within individual channel resolver 107. The first column 
is modulated by a frequency f.sub.1, the second by a frequency 2f.sub.1, 
the third by a frequency 3f.sub.1, etc. until the last column is modulated 
by a frequency Mf.sub.1. Thus the beam, which contains all frequencies 
from f.sub.1 to Mf.sub.1, is then multiplied by the reflectance of each 
pixel which also is modulated by f.sub.1 to Mf.sub.1 according to its 
position in the row. (The foregoing procedure actually is carried out in 
in-phase (I) and quadrature (Q) steps to cover both dimensions.) Thus the 
frequency effectively is "shifted" such that the desired channel is 
shifted or down converted to video. The array of signals is then bounced 
off half mirror 103 and focused by the collimating lens 108 onto 
detector/accumulator array 109. This procedure effectively detects the 
signal and low pass filters the desired signal for each pixel. 
The detector/accumulator array 109 is connected on a pixel by pixel basis 
to another SLM array 110 which is illuminated by laser 113 through 
collimating lens 112 and half mirror 111. At this point, each individual 
channel has been fully detected and its signal located on one of the 
N.times.M pixels of the SLM array 110. The image then is reflected off SLM 
array 114 which "remodulates" the individual signals to "fill" the 
outgoing beams. At this point, the incoming N beams are still spread 
across the rows where beam 1 is row 1, beam 2 is row 2, etc. The columns 
now represent the individual customers inside the beam, column 1 
representing customer 1, column 2 representing customer 2, etc. Individual 
channel modulator SLM 115, which in this embodiment is identical to SLM 
array 106 but rotated by 90.degree., takes this demodulated array and 
remodulates the signal corresponding to customer 1, beam 1 to frequency 
f.sub.1 ; customer 1, beam 2 to frequency 2f.sub.1, etc. As with SLM array 
106, the procedure is carried out in in-phase (I) and quadrature (Q) 
steps. Then, after the signals are reflected off the half mirror 111, they 
are compressed by cylindrical lens 116 into a single pixel which becomes 
outgoing beam 1. Each of the beams would be compressed in this manner, and 
the beams would be output via 1.times.N detector array 117 to the N 
antenna feeds 118. This is possible since the remodulation has the effect 
of modulating each "customer 1" with a different frequency, allowing 
receiving customers to differentiate their respective calls. 
Thus, each customer J from all N beams is remodulated so as to be separated 
in frequency and combined optically to create a new output beam J. 
For 1000 simultaneous customers per beam, and 1000 beams, this 
just-described embodiment would allow one customer from each beam to call 
customers in each of the other beams. Though the system's capability 
obviously would be quite large (1 million simultaneous video circuits), it 
would not match typical communication usage very well. This is because 
typically, a large number of calls are local and non-local calls, and tend 
to cluster into high density areas (e.g., New York City, Washington D.C.) 
One technique to alleviate the call density problem would be to place 
repeaters in a large number of suspected under-utilized regions. These 
repeaters could use beam K as a stopover between the original point and 
the desired destination. While this approach would use up some of the 
capacity of area K served by beam K, it also would provide significant 
system flexibility. 
FIGS. 4A and 4B describe two mechanisms for increasing the available number 
of local (i.e. within beam) calls by dedicating frequencies f.sub.1 to 
f.sub.k as "local" calls. This can be done on a beam-to-beam basis by 
direct filtering--a technique which will be described with reference to 
FIG. 4A--or by filtering all f.sub.1 to f.sub.k signals after they have 
been filtered spatially--a technique which will be described with 
reference to FIG. 4B. FIG. 4A describes an electronic (signal filter 
bypass) solution, while FIG. 4B describes an optical solution, involving 
an optical alteration of N.times.M array 115 to accomplish a partial 
bandwidth bypass. In FIG. 4A, an incoming signal 200 is divided into two 
signals by divider 201. One of the signals continues onto the 1.times.N 
SLM array 101 for processing as described before. The other channel is 
filtered in bandpass filter 202 and combined directly with the output 
signal coming from the 1.times.N detector array 117. These signals are 
summed in summer 203 to provide a summed signal, which is used to drive 
the output beam 204 corresponding to the same input beam. 
FIG. 4B describes an optical solution to the same problem. The signal 
coming through half mirror 111 is partially interrupted by a full mirror 
205 oriented at 45.degree. which reflects off the vertical mirror 206 and 
another 45.degree. mirror 207 to image what is in region a to region b. 
Note that region b is rotated by 90.degree. with respect to region a. 
After appropriate modulation, the output beam contains frequencies f.sub.1 
to f.sub.k that are identical to the frequencies f.sub.1 to f.sub.k sent 
up in the same beam. 
FIG. 5 describes an alterative embodiment that replaces the downconversion 
SLM 106 and the remodulation SLM 115 with digital code multiplication. The 
nomenclature used in this Figure indicates that different codes can be 
used in communicating in the two directions. As shown, frequency f.sub.1 
is replaced with code K+1, frequency f.sub.2 is replaced with code K+2, 
and so on for the downconversion process, and frequency f.sub.1 is 
replaced with code 3, frequency f.sub.2 is replaced with code 2, and so on 
for the remodulation process. The reflective signal is then integrated to 
decode the desired signals. This technique will allow for many more 
channels to be contained in a given bandwidth, as is conventional in code 
division multiple access (CDMA) systems. 
The simplest embodiment, even with the inbeam repeaters and the Partial 
Bandwidth Bypass to increase the available local calls, would have 
difficulty handling a large number of calls between two separate beams. 
Using the repeater technique uses up one additional channel per extra 
call. Thus, for example, 10 calls between Beam 10 (Los Angeles) and Beam 
342 (Washington, D.C.) would take 19 total channels. A fully arbitrary 
cross bar switch, an embodiment of which is shown in FIG. 6, would handle 
that problem easily. 
The arbitrary cross bar switch implementation of FIG. 6 includes all of the 
structure of FIG. 3, but adds optical elements between N.times.M SLM array 
110 and an additional detector/accumulator 109 and SLM array 110. The 
first detector/accumulator 109 and SLM array 110 identify each incoming 
customer by column and each beam by row. The optical signal out of SLM 110 
is diverted by a half mirror 300 through half mirror 301, and is focused 
by lens 302 onto an N.times.M arbitrary modulator SLM array 303 (arbitrary 
modulator #1). This array 303 is a complex N.times.M array that allows for 
any frequency f.sub.1 -Mf.sub.1 to modulate any pixel in the N.times.M 
array. With the arbitrary modulator #1, each pixel can be multiplied by an 
arbitrary Kf.sub.1 that can be different for each pixel. 
The reflected arbitrarily modulated signal from array 303 then is focused 
to a line by the first cylindrical lens 304 and is spread by the second 
cylindrical lens 304 through half mirror 305 to another SLM array 306 
which downconverts each pixel to its f.sub.1 -Mf.sub.1 position. The image 
output by array 306 then is reflected by half mirror 305 through half 
mirror 307 to a second N.times.M arbitrary modulator SLM array 308 
(arbitrary modulator #2) which multiplies each pixel by an arbitrary value 
Lf.sub.1 which is different for each pixel. The output of SLM array 308 is 
reflected off half mirror 307 and passed through first and second 
cylindrical lenses 309, 309, similarly to the handling of the output of 
SLM array 303. Thus each pixel is downconverted to its f.sub.1 to f.sub.N 
position onto SLM array 311. 
The first SLM array 306 effectively moves the signal in plane #1-the second 
SLM array 311 effectively moves the signal in plane #2, which is 
orthogonal to plane #1. The signal is then re-detected (as done by a 
detector/accumulator 109) and used to modulate another SLM array (like SLM 
array 110) and combined with the original signal from SLM array 110. Thus 
any signal from any beam can be moved to be like any other signal from any 
other beam, yielding a great deal more flexibility. 
While the invention has been described in detail with reference to 
preferred embodiments, various changes and modifications within the scope 
and spirit of the invention will be apparent to those of working skill in 
this technological field. Thus, the invention is to be considered as 
limited only by the scope of the appended claims.