Method for secure PPM-based laser communications

• US 5,206,909 A
• Filed: 01/18/1991
• Issued: 04/27/1993
• Est. Priority Date: 01/18/1991
• Status: Expired due to Fees

• Current Assignee: GTE Government Systems Corporation (General Dynamics Corporation)
• Original Assignee: GTE Government Systems Corporation (General Dynamics Corporation)
• Inventors: Gates, John A.
• Primary Examiner(s): CANGIALOSI, SALVATORE A
• Application Number:
  US07/643,250
• Time in Patent Office: 830 Days
• Field of Search: 380/59, 375/23, 359/152, 359/186
• US Class Current: 380/59
• CPC Class Codes: H04J 3/1676 Time-division multiplex with pulse-position, pulse-interval, or pulse-width modulation
• CPC Class Codes: H04L 25/493 by transition coding, i.e. the time-position or direction of a transition being encoded before transmission

Abstract

A system for secure communication between transmitting and receiving devices in a laser communications system using pulse position modulation, has a transmitter means to modulate and a receiver means to demodulate a transmission utilizing a protocol in which a number N (N≧1) of optical pulses represents each symbol to be transmitted. A transmission is initiated with a trigger event. Thereafter, a multi-pulse synchronization signal is transmitted as the first symbol of each transmission, the first pulse of said synchronization symbol being transmitted within a first time frame after said trigger. A buffer in said receiver receives and stores the first of said N frames of said synchronization symbol. Each subsequently received frame of said N frames of said synchronization symbol is added to data previously stored in said buffer. A frame template is calculated from the stored pulse data of said first N pulses in said buffer and from said synchronization symbol protocol. The calculated frame template is used for transmission of subsequent message symbols, which are then transmitted in N frames per symbol. The system includes an algorithm for the synchronization symbol format which pseudo-randomly positions each of said N-1 pulses after said first pulse within its frame by offsetting the start time of its frame by a predetermined amount of time, with reference to the start time of said first frame. The system also includes an algorithm for the message symbol format which pseudo-randomly positions each of said N-1 pulses after said first pulse within its frame by offsetting its pulse position from that of said first frame such that the separations of said pulses vary from frame to frame within a symbol, and said algorithmically determined position of any pulse falls within the live time of its frame.

Claims

1. A system for secure communication between transmitting and receiving devices in a laser communications system using pulse position modulation, comprising:

means to modulate and demodulate a transmission utilizing a protocol in which a number N (N≧

1) of pulses represents each symbol to be transmitted;

means to initiate a transmission with a trigger event;

means to transmit a multi-pulse synchronization signal as the first symbol of each transmission, the first pulse of said synchronization symbol being transmitted within a first time frame after said trigger;

buffer means in said receiver to receive and store the first of said N frames of said synchronization symbol;

means to add each subsequently received frame of said N frames of said synchronization symbol to data previously stored in said buffer;

means to calculate a frame template from the stored pulse data of said first N pulses in said buffer and from said synchronization symbol protocol, for transmission of subsequent message symbols;

means to transmit each subsequent message symbol in N frames per symbol, in said calculated frame template, the first of said frames of each message symbol being stored in said buffer, and subsequent frames being added to said stored data, such that the summed data in said buffer after receipt of said Nth frame of each message symbol represents the message symbol.

2. The system of claim 1 wherein said synchronization symbol protocol further comprises:

a synchronization symbol format known to both transmitter and receiver; and

an algorithm for pseudo-randomly positioning each of said N-1 pulses after said first pulse within its frame by offsetting the start time of its frame by a predetermined amount of time, with reference to the start time of said first frame.

3. The system of claim 1 wherein said message symbol protocol further comprises:

a message symbol format known to both transmitter and receiver; and

an algorithm for pseudo-randomly positioning each of said N-1 pulses after said first pulse within its frame by offsetting its pulse position from that of said first frame such that the separations of said pulses vary from frame to frame within a symbol, and said algorithmically determined position of any pulse falls within the live time of its frame.

4. The system of claim 1 wherein said means to transmit said symbols further comprises:

means to reduce transmitter power so that the SNR of transmitted pulses approximates the background noise of the communication channel.

5. A method for secure communication of data between transmitting and receiving devices in a laser communications system using pulse position modulation, comprising the steps of:

selecting a number N (N≧

1) of pulses to represent each symbol to be transmitted;

selecting a pulse position format for each pulse of said N pulses of a synchronization symbol, such that each of said N pulses appears to be pseudo-randomly positioned within its frame;

selecting a pulse position format for each pulse of said N pulses of each message symbol, such that each of said N pulses is pseudo-randomly positioned within its frame;

selecting a trigger event to signal the start of a transmission;

said number of pulses, symbol formats, and trigger event being known by both transmitter and receiver;

initializing a transmission with a trigger signal;

transmitting a synchronization symbol having N pulses, in N sequential frames, one pulse per frame, to a receiving device immediately after a trigger event;

receiving and storing the first frame of pulse data of said synchronization symbol in a buffer of said receiving device;

adding each subsequently received frame of pulse data of said synchronization symbol to pulse data previously stored in said buffer, such that the sum of said N frames is said synchronization pulse;

calculating a frame template, from the stored pulse data of said first N pulses in said buffer and from said selected synchronization symbol format, for subsequent message symbols; and

transmitting each subsequent message symbol in N frames per symbol, in said calculated frame template, the first of said frames of each message symbol being stored in said buffer, and subsequent frames being added to said stored data, such that the summed data in said buffer after receipt of said Nth frame of each message symbol represents the message symbol.

6. The method of claim 5, wherein the step of selecting said synchronization symbol format further comprises:

selecting an algorithm for pseudo-randomly positioning each of said N-1 pulses after said first pulse within its frame by offsetting the start time of its frame by an algorithmically determined amount of time, with reference to the start time of said first frame.

7. The method of claim 5, wherein the step of selecting said message symbol format further comprises:

selecting an algorithm for pseudo-randomly positioning each of said N-1 pulses after said first pulse within its frame by offsetting its pulse position from that of said first frame such that the separations of said pulses vary from frame to frame within a symbol by an algorithmically determined pattern, and said algorithmically determined position of any pulse falls within the live time of its frame.

8. The method of claim 5, wherein the step of initializing a transmission with a trigger signal further comprises the steps of:

recognizing said trigger signal at both ends of the communications link;

initiating the transmission of a synchronization signal by the transmitter within the first time frame following said trigger signal; and

receiving and storing in a buffer at the receiver all signals within said first frame following said trigger symbol.

9. The method of claim 5, wherein the step of transmitting a synchronization symbol further comprises the step of:

reducing the transmitter power so that the SNR of the transmitted pulse approximates the background noise of the communication channel.