An optical communications system for coding, transmitting, receiving, and coding digital message signals is described. A beam from a coherent optical source, such as a laser, is transmitted through an aperture in which there is disposed a random phase screen, e.g., ground glass. One or more isometric bar gratings containing the coded message data in a particular amplitude distribution pattern are disposed adjacent to the ground glass in the optical transmission path. The ground glass, phase screen, introduces a random coding to the message bit. At the receiver a plurality of point detectors sense the signals to facilitate subsequent decoding of the message.
This disclosure relates to a laser communication system for transmitting intelligence by means of partially coherent optical energy. The system provides improved transmission of intelligence from a transmitter site through non-confined free space and receiving said intelligence at a receiver site remote from and physically separate from said transmitter by non-confined free space. The system has a optical source at the transmitter site for producing a beam of spatially-coherent monochromatic, aperture limited electromagnetic optical energy as well as a modulator for modulating said beam with intelligence-bearing information to develop wavefronts of mutually-aligned orientation which beam is thereafter modified to partial coherence and a receiver site for both detecting said information in said partially coherent beam and deriving said demodulated information. The partially coherent beam has an a of preferably from 0.05 to 0.5 whereby improved reception is realized from optical communication over free-space of limited distances. Partial coherence of a coherent beam is achieved by numerous ways including passing it through a phase screen, or reflecting it against a rough surface, multimode laser operation or transmission through multimode optical fibers.
A signal transmission system impresses phase-modulated signals on to a multimode optical fibre waveguide 1. In order to overcome the effects of fading caused by small random changes in the parameters of the fibre the signals from different modes are all combined to derive a resulting signal which is detected in a common detector 23. The signals may be combined by a vibrating mirror 21 which sweeps the emergent beam across detector 23. Alternatively the tip of fibre 1 may be vibrated to achieve a similar effect.
An interference device for discriminating between radiation sources of differing coherence length comprises means to divide received radiation from a source into two components. A path difference, defining a coherence length cut-off, is introduced into the path of one component and the components are brought together for interference. The recombined light passes through a reticle with alternate opaque and tranparent bars and an optical band-pass filter to a detector. Interference fringes present in the plane of the reticle are swept across the reticle by the action of the collection optical system of the device which includes a scanning rotating mirror. Two similar devices can be arranged for band-pass coherence length filtering and when used in conjunction with a light soruce whose coherence is modulated the device can be used for signalling.
An improved radio paging system providing automatic acknowledgment of message delivery, including a base station with spatially directive reception means and a pager capable of transmitting acknowledgment signals. The spatially directive reception means at the base station enhances the reception quality of the acknowledgment signals transmitted by the pager over that obtainable with conventional omnidirectional reception means, thereby compensating for the disparity in the base station and pager transmission powers.
A method and apparatus for increasing the capacity and quality of wireless communication between a plurality of remote users and a base station is disclosed. Using measurements from an array of receiving antennas at the base station, parameters of multiple signals transmitted to the base station from a plurality of users in the same channel are calculated and used to obtain the positions and velocities of the users. The locations and other related signal parameters are used to calculate appropriate spatial demultiplexing strategies, reconstructing the individual transmitted signals from the receiver measurements and reducing interference to acceptable levels. This heretofore unavailable location information is used in solving the hand-off problem. This information is also used to calculate an appropriate spatial multiplexing strategy for simultaneous transmission of signals to users in the same channel. This can be the same as or distinct from the aforementioned receive channel. In combination, the transmit and receive systems establish multiple full-duplex links in the same channel by directively receiving and transmitting signals at the base stations only. This invention can also be implemented at the mobile sites to improve signal quality and to establish point-to-point communication links between multiple mobile users. Furthermore, implementation of both transmit and receive functions at the base station is not required. A receive-only system base station will still improve received signal quality and therefore capacity in addition to mitigating the hand-off problem.
