Performance instabilities due to the fading are vital subjects for long distance free space optical communication systems in which the signal lights travel through the atmosphere. The straightforward approach is to introduce the combinations of optical transmitters and receivers that can operate in various modulation speeds, and the operator selects the speed from the candidates in order to manipulate photons per bit according to the link conditions. However, the realization of such functions with single hardware may result in the halfway capabilities. For these problems, our group has been proposing the use of the technologies that we call Adaptive Distributed Frame Repetition (ADFR), in which a high-speed link is divided in time, and sending node replicates frames and transmits over a single link with the proper time intervals without concerning whether each frame arrives at the destination or not. Since the line rate is always constant, and functions equipped in the receiving node are simple, settings at the transmitting node relating to retransmission can be switched seamlessly without any negotiations with the opposite. In this time, the main functions of ADFR are implemented on the computers to be experimentally evaluated at 10 Gbps. In addition to the basic results about static and dynamic performances of ADFR, the combined performance with TCP assumed to be introduced in the GEO-Ground system will be discussed in the paper.
In order to stably achieve high throughputs in the satellite-to-ground free space optical communication systems, suppression of the influence of the fading due to atmospheric turbulence is an important technical subject to be overcome. Our group has proposed the use of Adaptive Distributed Frame Repetition (ADFR) as a countermeasure of this problem, which enables the seamless switching between the features of high-speed transmission and high tolerant link connection in response to the transmission link conditions. We have developed the test equipment of the functions whose interface was 10GbE, and demonstrated the improvements of the packet error ratio and the throughput against the varieties of the fading patterns. The settings of the ADFR should be carefully determined with the considerations of several trade-offs. The efficient suppression of PER enabled by the excess repetition and interval sacrifice the transmission bandwidth and the system latency, respectively. In this time, we develop a function that extracts the fading characteristics of the moment from the decimated signals after transmission, and, by using this information, realize the continuous optimizations of the ADFR settings to the time-changing fading patterns. The details of the estimation method and the demonstrations with this technique will be presented at the conference.
We proposed and evaluated the effectiveness of a simple rate control technique of Adaptive Distributed Frame Repetition (ADFR), which achieves both high speed link in the stable condition and suppresses the influence of turbulence-induced fading in the unstable condition. In order to estimate its performance in the LEO-to-Ground link with 10-Gb/s modulation speed, we developed a numerical simulator which enables us to calculate the frame error rate and the throughput under the specific turbulence condition emulated by random phase screens. We confirmed that ADFR can increase the link duration and the capacity by increasing the tolerance to the fading especially in the region of lower elevation angle. In addition, we evaluated its effectiveness through the comparison with the general adaptive rate control, in which the receiver sensitivity can be manipulated in accordance with the link conditions. The results show that ADFR provides similar performances with the sufficient received optical power.
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