Chaotic Free-Space Laser Communication over a Turbulent Channel

The dynamics of errors caused by atmospheric turbulence in a self-synchronizing chaos-based communication system that stably transmits information over a $\sim 5\mathrm{k}\mathrm{m}$ free-space laser link is studied experimentally. Binary information is transmitted using a chaotic sequence of short-term pulses as a carrier. The information signal slightly shifts the chaotic time position of each pulse depending on the information bit. We report the results of an experimental analysis of the atmospheric turbulence in the channel and the impact of turbulence on the bit-error-rate performance of this chaos-based communication system.

Figure 1

Schematic for the free-space laser communication system based on the chaotic pulse position modulation transceiver. See text.

Figure 2

Fluctuations of the received power $P(t)$ in the experiment with a nonmodulated laser generating constant output intensity (10 mW). Normalized received power $P_R(t)=P(t)/⟨P(t)⟩$ measured at the photodetector output (a), and corresponding averaged power spectrum of $P_R(t)$ (b) illustrate the presence of strong laser beam intensity scintillations.

Figure 3

Histogram of the probability distribution for the random variable ${\xi }_P=\ln (P/⟨P⟩)$ measured by the PIN photodetector. This histogram, $N({\xi }_P)/N$, is computed using $N={10}^5$ consecutive samples of the data $P(t)$, a 20 sec fragment of which is shown in Fig. 2.

Figure 4

Fluctuations of the CPPM pulses of light intensity after traveling through atmospheric turbulence. Pulse amplitude $A_p$ measured in volts at the output of amplifier (a). Propagation times ${\tau }_m$ (b) and ${\tau }_t$ (c) in $\mu \sec $. The pulse propagation times are computed from data acquired simultaneously at the output of CPPM Tx and output of amplifier (SR560) at a sampling rate of $5\times 106$ samples per sec.

Figure 5

Typical structure of errors shown in 20 consecutive measured data streams each of length $\sim 170\mathrm{m}\sec $ transmitted at $\sim 2\min $ intervals. Each strip presents 10 000 bits which are transmitted with the CPPM method. White intervals of the strips mark blocks of data received without errors. Narrow black ribbons in the middle of the strips mark the blocks of data received with errors. The gray background shows the blocks of the dropped-out data caused by the loss of CPPM pulses due to fading instances.