Abstract
A transmitter for an optical free-beam communication system includes two light transmitters for the optical transmission of a data signal using one single-sideband modulation, wherein each light transmitter emits a side of the band modulation so that a light signal arriving at a receiver corresponds to a double-sideband modulation.
Claims
1. A transmitter for an optical free-beam communication system comprising: two light transmitters configured to optically transmit a data signal using one single-sideband modulation, wherein each light transmitter emits a side of the band modulation so that a light signal arriving at a receiver corresponds to a double-sideband modulation.
2. The transmitter it claim 1, wherein the two light transmitters are spaced apart from each other by a distance that is greater than the-structure sizes of turbulence cells in an optical free-space transmission, so that the data signal is transmitted via different atmospheric paths and added up at the receiver, so that scintillation is halved.
3. A receiver terminal exclusively configured for demodulation of a double-sideband modulation signal.
4. An optical free-beam communication system, comprising: the transmitter of claim 1; and the receiver terminal of claim 3.
Description
[0032] Preferred embodiments of the invention will be explained below with reference to Figures.
[0033] In the Figures:
[0034] FIG. 1 illustrates the basic functionality of a transmitter diversity,
[0035] FIG. 2 shows an exemplary received power vector received at the satellite,
[0036] FIG. 3 shows interference effects by overlap of the signal spectrums in the frequency range,
[0037] FIG. 4 shows a schematic Illustration of a first embodiment of the transmitter according to the invention,
[0038] FIGS. 5 and 6 illustrate refraction index turbulences of the transmitter of the present invention compared with prior art.
[0039] FIGS. 1 and 2 were already discussed in the context of prior art.
[0040] FIG. 3 shows interference effects by overlap of the signal spectrums in the frequency range. On the left-hand side, a slight overlap of two signal spectrums is illustrated. On the right-hand side, a simulated effect in the time domain is illustrated for a large overlap. It is discernible that beat effects reduce the eye opening of the digital signal stream.
[0041] FIG. 4 is a block diagram of an embodiment of the transmitter according to the invention (top) and the receiver (bottom). It is obvious that SSB signals are generated for each aperture. Channel A and channel B lead to different channels. The receiver receives the overlap of the two signals and thus only has to demodulate a double-sideband modulation. The transmitter diversity has the same bandwidth as a double-sideband modulation.
[0042] FIGS. 5 and 6 illustrate two simulations using the transmitter of the present invention. The scintillation parameter scint represents the standardized variance of the received power. The two graphs in the upper portion of the Figure illustrate the measured power vectors in a GEO uplink laser transmission (see publication [2]). The measured signal is shown on the left and the probability density function is shown on the right. In the bottom portion of FIG. 5, the transmitter diversity method is simulated by overlapping the measured signal with a time-shifted version of itself, so that each channel receives a non-correlated version. The modulated data are included in the real portion of the received signal.
[0043] FIG. 5 illustrates a strong refraction index turbulence, wherein the measured uplink signal in a GEO satellite with the use of a transmitter is illustrated top left, while the probability density function of the measured power is illustrated top right in this scenario. The probability density function of a simulated SSB double transmitter diversity calculated from the measured signal is illustrated bottom left. The simulated signal with SSB transmitter diversity calculated from the measured signal is illustrated bottom right.
[0044] FIG. 6 corresponds to FIG. 5, wherein a weak refraction index turbulence is illustrated.
[0045] Due to the relation SI(n)=SI(1)/n.sub.Tx it is expected that the scintillation will halve when two transmitters are used. The result of the simulation illustrated in FIG. 5 and FIG. 6 meets this expectation.
