Abstract:
A non-line of sight (NLOS) communications system and method are provided. An ensemble of photodetectors is used to collect the light, scattered in the sky being illuminated by initial pulsed laser beam carrying information. Each detector collects scattered light from one area in free space along the initial light propagation line. The same bit of information is detected multiple times on multiple detectors during the pulse transmission along its propagation path. Signals received by multiple detectors are synchronized and processed in a digital signal processing unit. Improved system sensitivity and reliability is achieved by multiple registration of the same bit of information. Special selection of the areas in free space ensures detection of a single bit of information during the time equal to a bit period.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present invention claims the benefit of U.S. Ser. No. 60/891,557 filed Feb. 26, 2007, which are fully incorporated herein by reference. 
     
    
     FIELD OF INVENTION 
       [0002]    This invention relates generally to the systems and methods for free-space optical communications, and more particularly to non-line of sight (NLOS) communications for military and civilian applications. This type of communications can provide a robust covert communication link where it is of vital importance such as military operations in urban terrain. 
       BACKGROUND OF THE INVENTION 
       [0003]    U.S. Pat. No. 5,301,051 by Geller discloses a covert communication system that uses ultraviolet light as a medium for communication. Suitable wavelengths are chosen by examining atmospheric penetration, attenuation by clouds, presence of interfering sources, and ease of generation and detection. 
         [0004]    It is well known that atmospheric gases such as ozone and oxygen strongly absorb light in the spectral range between 200 and 280 nm. It is called “solar blind” region of spectrum. It is beneficial to create a free-space communication link operating in this range since solar radiation will not interfere with the data transmission. Non-line of sight communication is based on the light scattering in atmosphere and detecting of at least some portion of the scattered light. Raleigh theory indicates a strong wavelength dependence of the scattering (˜λ −4 ) which means that blue light is scattered much more than red light. It is advantageous to use blue or UV light in NLOS communications since more light can be collected. 
         [0005]    An optical communications transceiver of U.S. Pat. No. 6,137,609 comprises a transmitter that sends out the same information simultaneously in two channels with different wavelengths and a receiver for detecting and comparing the received data. Additional reliability of the communications is achieved by the transmission doubling. 
         [0006]    Traditionally photomultipliers are used for UV light detection. Recently developed low noise high sensitive avalanche AlGaN photodiodes are compatible with the photomultiplier in their characteristics while providing setup compactness. US patent application No. 20050098844, which addresses manufacturing of such detectors, is incorporated herein by reference. 
         [0007]    There is still a need for improved light detection schematics to enhance sensitivity and reliability of non-line of sight UV optical communications. 
       SUMMARY OF THE INVENTION 
       [0008]    The system and method are disclosed for non-line of sight optical communications with improved sensitivity and reliability. The sensitivity improvement is achieved by implementation of a novel receiver, which comprises a series of photodetectors. An ensemble of photodetectors is used to collect the light, scattered in the sky being illuminated by initial laser beam carrying information. The preferred wavelength operation range is from 200 to 280 nm. Each detector collects scattered light from one area in free space along the light propagation. In the preferred embodiment the areas of light collection do not overlap. The output signals from the photodetectors impinge a time delay unit, which synchronizes signals from different detectors. Each time delay introduced by the delay unit to each detector output signal corresponds to the time of flight for the light pulse from one detection area to another. In real systems with an operation range from tenth of meters up to kilometers, each delay is in the range from 10 −10  to 10 −8  sec. A digital signal processing unit combines all synchronized signals, decodes and displays the information encoded in the initial light beam. In the preferred embodiment the information is encoded in Amplitude-Shift keying (ASK) format. 
         [0009]    In the preferred embodiment each detector collects light from an area in free space, which has essentially elliptical shape with major axis from 10 cm to 10 meters. The major axis of the elliptical area coincides with the direction of the initial light propagation. The length of the major axis is determined by the bit rate in the initial laser beam. 
         [0010]    In the preferred embodiment one-dimensional array of N photodetectors is used in the detection scheme, where N is integer. In another embodiment two-dimensional array of N photodetectors is used. In the preferred embodiment the photodetectors are avalanche photodiodes. In another embodiments an array of photomultipliers or solid state photodiodes or semiconductors detectors are employed. 
         [0011]    In another embodiment of the present invention a non-line of sight communications system is disclosed thai transmits information in two directions each having its azimuth and elevation angle. The information transmission in each direction can be a Wavelength Division Multiplexed (WDM) transmission, where each wavelength represents a separate information channel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0012]      FIG. 1 . A block diagram of a non-line of sight communications system with a receiver having multiple detectors. 
           [0013]      FIG. 2 . An illustration of initial pulse propagation. 
           [0014]      FIG. 3 . (a) A linear array of photodetectors, and (b) a two-dimensional arrangement of detectors. 
           [0015]      FIG. 4 . An optical receiver for non-line of sight communication system. 
           [0016]      FIG. 5 . An optical receiver with detectors having different apertures (a) and the same apertures (b). 
           [0017]      FIG. 6 . A block diagram of non-line of sight communications system with an initial beam split into two beams directed along the different azimuths and having different elevation angles. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]      FIG. 1  illustrates the basic concept of the non-line of sight communications according to the present invention. Light source  1  irradiates an initial beam  2 , which propagates at an elevation angle B 1 . In the preferred embodiment the light source generates pulsed ultraviolet light in the range from 200 to 280 nm. Laser AVIA 266-3 from Coherent, Inc. located in Santa Clara, Calif. can be used as a light source. In the preferred embodiment the initial beam transmits a signal in Amplitude Shift Keying (ASK) format. A receiver  3  includes a number of photodetectors  4 - 7 .  FIG. 1  shows four photodetectors as an example, however the photodetector system may include any N number of units, where N≧2. The function of the detector system is to collect light being scattered by the atmospheric inhomogeneities along the initial beam propagation and to convert the light into electrical signals. Each photodetector collects light from the area along the light beam  2 . In the preferred embodiment each photodetector collects light from an essentially elliptical area. A first and a second elliptical areas O 4  and O 3  with corresponding major axes DE and DC are shown in  FIG. 1 . The major axes of the areas coincide with the direction of the initial beam propagation. In the particular example shown in  FIG. 1  the photodetector  7  collects light  11  scattered along the light beam  2  from the area with the major axis DE. Similarly other photodetectors  4 - 6  collect scattered light from their areas with major axes AB, BC, and CD correspondingly. The present invention discloses a multi-detector signal registration, where the same pulse  12  is detected several times along its propagation path. It is detected by the photodetector  4  on the AB cut, by the photodetector  5  on BC cut, by the photodetector  6  on CD cut, and by the photodetector  7  on DE cut. The photodetectors  4 - 7  output electrical signals  14 - 17 . 
         [0019]      FIG. 2  illustrates the pulse  12  transmission along the propagation direction. The signal, detected by the photodetector  7 , is delayed relative to the signal, detected by the photodetector  6 , by the time of the light propagation from CD area to DE area τ 1  combined with the difference in optical paths τ 2  cause by the initial beam elevation. In our system the time τ 1  is a one bit period of the transmitted signal. Accordingly, the length CD (the major axis of the area) is a bit distance, which is defined as a product of V and τ 1 , where V is a speed of light in air. In the preferred embodiment the length of the major axis is from 10 cm to 10 meters for each area. These numbers correspond to the optical transmission in the range from tenth of meters to kilometers. 
         [0020]    Returning back to  FIG. 1 , a time delay unit  13  introduces different delays in signals  14 - 16  in order to synchronize them with the signal  17 . The time delay unit outputs delayed electrical signals  14   a - 16   a . Each of the signals  14   a - 16   a  is delayed relative to the signal  17  by the time delay being equal to the time difference in light propagation from the laser light source to the corresponding detector as shown in  FIG. 2 . In the preferred embodiment the first time delay is from 10 −10  to 10 −8  sec, each other delay is a multiple of the first time delay. These numbers correspond to the optical transmission in the range from tenth of meters to kilometers. Such delay duration can be provided by the digital delay unit SY89296U from company Micrel, Calif. or similar device. 
         [0021]    A digital signal processing (DSP) unit  18  receives the signals  14   a ,  15   a ,  16   a ,  17  and recovers transmitted information. The unit  18  outputs a signal  19 , which can be displayed or further transformed for audio or video presentation. In the preferred embodiment the signal is encoded using Amplitude Shift Keying (ASK) format, however any other format may be used such as Phase Shift Keying (PSK), Frequency Shift Keying (FSK), Pulse Position Modulation (PPM), Mark-space format or another. In the preferred embodiment each of the ASK modulated signals  14   a - 17  is analyzed in the DSP unit on the presence of an information bit within the predetermined time equal to the one bit period. Since the same pulse is detected N times (in our particular example four times) using N detectors, signal-to-noise ratio increases in √{square root over (N)} times assuming that the noise is stochastic. Improvement of signal-to-noise ratio in the signal detection corresponds to the increased sensitivity and reliability of the detection. 
         [0022]    The array of the photodetectors may be one-dimensional as shown in  FIG. 3  ( a ). Alternatively, two-dimensional arrangement can be used as shown in  FIG. 3  ( b ). Each photodetector in two-dimensional arrangement may be used to detect light scattered by independent areas along the initial beam propagation path. Alternatively, a group of photodetectors may detect the signal from the same area. In yet another embodiment the photodetectors may receive signals from overlapping areas. In the preferred embodiment the photodetectors  4 ,  5 ,  6  and  7  are avalanche diodes as described in US Patent Application No. 20050098844 by Sandvik, incorporated herein by reference. Alternatively any other type of solid state photodetector, semiconductor photodetector or photomultiplier can be used. Hamamatsu R928 Photomultiplier with a UV filter was used in the experimental testing of the present invention. 
         [0023]    In the preferred embodiment the receiver  3  includes focusing element. It may be a multiple aperture element  21  as shown in  FIG. 4 , which comprises a set of optical elements  21   a - 21   d . Collective optics is an important part of the receiver which allows to gather more energy on the photodetectors and to increase the system sensitivity. Different delay lines τ 4 , τ 5 , τ 6  shown in  FIG. 4  are chosen in a way to synchronize signals  14 - 17 . Each of the different time delays τ 4 , τ 5  is a multiple of the first time delay τ 6 . Output delayed signals  14   a - 16   a  and  17  enter the DSP unit  18  for data processing, information recovery and results displaying. 
         [0024]    Optionally the receiver  3  may include a filter or a set of filters  25  to select a particular wavelength from incoming radiation. The filter  25  may serve as a shield from ambient light. Alternatively, when the initial beam is a wavelength division multiplexed (WDM) beam, the filter  25  may select a particular wavelength out of WDM signal. 
         [0025]    In the preferred embodiment the photodetectors  4 ,  5 ,  6  and  7  have different apertures as shown in  FIG. 5  ( a ). If the detectors have the same apertures θ, the size of the areas, from which the scattered light is detected, will be different as shown in  FIG. 5  ( b ). In the present invention the length of the areas is equal to the bit distance, which defined as a product of the one bit period by the speed of light. The bit distance is the same along the initial beam propagation direction, and therefore the detector apertures need to be selected to meet this requirement. 
         [0026]    In one embodiment of the invention the initial optical beam consists of series of optical beams, each directed along its azimuth and has its own elevation angle.  FIG. 6  shows the initial beam being split into two secondary initial beams  2 A and  2 B. The first part of the initial optical beam  2 A is directed along an azimuth A 1  towards the sky at an elevation angle B 1  above the horizon. The Sight beam  2 A is scattered on the atmosphere inhomogeneities in a free space along its transmission path, portions of the initial optical beam forming scattered light segments O 1  and O 2 . A receiver  3 A comprises a set of photodetectors and delay line units; it recovers information encoded in  2 A. The receiver  3 A may have a structure as shown in  FIG. 4 . Another part of the initial beam  2 B transmits information in the similar manner, and this information is detected and recovered by a receiver  3 B, the receiver  3 B may have a structure as shown in  FIG. 4 . In general case, the initial beam can be split in any number of secondary initial beams, each of them carrying independent information. The information transmission along each direction can be a WDM transmission with a number of frequency separated channels. 
         [0027]    In the preferred embodiment the receivers  3 A and  3 B comprise N detectors and a delay unit providing N delay lines to synchronize the detected signals. This provides √{square root over (N)} times improvement in the detection sensitivity and reliability as discussed above. 
         [0028]    The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in the light, of the above teaching. The described embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.