Adaptive Multiplexing for Free-Space Optical Communication

NASA's Jet Propulsion Laboratory, Pasadena, California

"Laser-based adaptive wavelength division multiplexing and code division multiplexing" (LAWDM-CDM) is the name of a method that has been proposed to increase security, signal-to-noise ratios (SNRs), redundancy, and adaptability in free-space optical communications. The need for this or a similar method arises as follows:• In free-space optical communications, various phenomena can affect the propagation and reception of laser beams; these phenomena include jitter of transmitter and receiver platforms and atmospheric attenuation and distortion of laser beams. In a conventional free-space optical communication system, data are transmitted at a constant rate that is not changed to adapt to changing conditions; consequently, for example, a sudden change in weather can disable the free-space optical link, causing loss of data.

Free-space optical communications are not entirely secure because laser beams can propagate beyond intended receivers and therefore can be intercepted by unintended parties.

In addition to wavelength-division and code-division multiplexing, LAWDM-CDM would involve the use of multiple transmitting apertures, adaptive control of the data-transmission rate and the transmitter power, and other advanced techniques as described below.

Eavesdropping would be reduced by using adaptive optics to reduce atmosphere-induced beam spreading and by tailoring the transmitter power to be low enough to minimize propagation beyond the intended receiver yet high enough to reach the intended receiver with an acceptably high SNR. One could choose wavelengths with limited atmospheric transmission to reduce the range of the optical signal.
Wavelength hopping and code-division multiplexing would make it difficult for an unintended recipient to decode a signal.
Under adverse atmospheric conditions, the bit rate would be reduced to keep the bit-error rate below a specified maximum acceptable level.
At the transmitter, laser light back-scattered by the atmosphere would be detected in a lidarlike subsystem, the output of which would be utilized in an algorithm that would select the optimum bit rate and power level.
Multiple apertures at a transmitter would help to reduce the effect of the scintillation.
Time-spread code-division multiplexing would enhance reception.

The figure illustrates an example of a simple system that would utilize a combination of wavelength-division multiplexing and time-spread code-division multiplexing to increase security. In the transmitter, a code sequence comprising five parallel bits during each bit period would control the simultaneous generation (for "1") or nongeneration (for "0") of pulses by five laser diodes, each operating at a different one of five wavelengths (l₁ through l₅). Before being launched for propagation to the receiver, the signal in the ith wavelength channel would be delayed by an amount t_i, which would be a unique integer multiple of the bit period, t. The longest delay would be 4t for l₄.

In order to be able to decode the signal properly, the receiver would have to be equipped with delay lines complementary to those in the transmitter: In the receiver, the incoming signal would be demultiplexed into the five wavelength channels and the signal in the ith channel would be delayed by 4t-t_i, so that the total of transmitter and receiver delays in each channel would be 4t and, hence, the five signals would come out of the receiver simultaneously, just as the original five bits went into the transmitter simultaneously.

Provided that the differences among the five wavelengths were sufficiently large and the transmitter power were properly adjusted, this system would offer a security advantage in that it would be difficult for an unintended recipient to detect its operation. Moreover, eavesdropping on any single wavelength would not enable one to decode the message.