Coherent Free Space Optical Communication and Networks

The research addressed the feasibility of an optical wireless network over the atmosphere in the presence of turbulence and interference. These networks will have significant impact for over-land and over-ocean applications as well as links to aircraft and satellites. Data rates of over 100 Gbps can be supported but only with the right mitigation techniques against various impediments. The most significant challenge is to handle atmospheric turbulence/weather and intentional interference so network protocols can run over these links.

The program was conducted with the following two goals:

(1) Demonstrate the superiority of diversity coherent systems over direct detection systems in the mitigation of fading due to atmospheric turbulence and background noise rejection.

(2) Demonstrate the superior performance of coherent systems against jamming including using near-field transmitter and receiver phase and amplitude control with feedback

Diversity Systems -- Diversity is a sensible technique to mitigate fading experienced in free space optical communication because of the improvement in average fading statistics that it provides. Moreover, in the presence of background noise and an intelligent interferer, diversity coherent detection is preferable over diversity direct detection. This is due to coherent detection’s ability to limit the amount of unwanted background noise and interference detected.

The available diversity equals the product of the number of independent (in the sense of being in different phase coherence cells) transmitter elements and receiver elements. If a low rate (~10kbps) feedback link is available between the receiver and the transmitter, there is the possibility of transmitter phase pre-distortion to help focus the optical energy on the receiver array, enhancing energy delivery efficiency of the free space channel. Receiver phase tracking element by element either via local-oscillator (LO) tuning or signal path phase modulation is demonstrated in this research program. The data is fed back to the transmitter for phase and amplitude modulation of the individual array elements.

TCP Shortcomings in Free Space Optical (FSO) Networks and Modified TCP -- TCP’s congestion control which has been used in the Internet for roughly two decades has been successful in preventing congestion collapse. However, when data rates and geographic spans increase, and communication links without wires or cables (such as satellite and free-space optical links) are added into the network, TCP has performance issues leading to low throughput. This is partly due to the TCP sender’s limited rate of window increase. It is also due to the TCP sender unnecessarily reducing its window upon link losses (it assumes every packet loss is due to congestion and that it should thus reduce its window to relieve the congestion).

FSO optical links are different from fiber optic and radio frequency (RF) wireless links because they have long fades. It is typical for an outage to last tens of milliseconds even when reasonable amounts of link margin and diversity are used. These long link outages cause a large number of consecutive packets to be dropped and can cause the TCP sender’s retransmission timer (RTO) to expire thereby resulting in reduction of the sender’s window to one packet in flight without acknowledgement. In fiber optic and RF wireless links, the typical losses are of single packets which usually cause three duplicate ACKs and window halving rather than timeouts. The fact that FSO outages can cause timeouts is an observation that has not been considered before. When high bandwidth-delay product paths are combined with FSO links with atmospheric turbulence, the outages cause severe throughput degradation. The throughput degradation is severe because after a long outage occurs and the sender’s window timeout and is reduced to one packet in flight, it takes the sender a long time to increase the window to a size large enough to make good use of the available rate in the network.

Transient Analysis -- When a user sends a small or moderate sized file (for example <1 MB), TCP is often not in steady state for most of the file transfer, particularly if the round-trip distance is long. Thus, steady state throughput does not give an accurate estimate of the time it takes to send the file. Transient throughput of TCP and Modified TCP after session initiation is the critical performance metric.

The objective of the research is to develop a cost-effective low-power free space optical communication system architecture suitable for rapidly deployable, dynamic, high data rate communication applications. Any mobile platform requiring high data rate communication, such as a drone, could potentially be a direct beneficiary of this research. Additionally there are examples when a fixed platform would benefit from this project: a company may require extra communication capacity to support a short-term high speed connection. Atmospheric turbulence, can however cause severe fading leading to the loss of 109 consecutive bits of data. As a result, this program focuses on developing an architecture to overcome turbulence induced fading. In general, there are several techniques used to overcome turbulence induced fading. Some current systems use a single transmitter and single detector and simply use extra power to overcome the 20-30dB fades. This approach is prohibitively expensive due to the high cost of multi-watt 30dB amplifiers. Another approach, borrowed from astronomy, is to employ a large deformable mirror to compensate for the turbulence. This approach effectively uses spatial diversity to overcome the turbulence. Theoretically it performs very well, but is there a way to exploit spatial diversity without the need for large, expensive deformable mirrors? The answer is a sparse aperture system, which uses many small transmitters and receivers spaced far enough apart to achieve the same spatial diversity as the deformable mirror provides.

Sparse Aperture Communication Performance -- We found the spatial modulation that optimally mitigates the effects of the turbulent atmosphere under the assumption of independent control of the phase and magnitude of each transmitter. Through coherent detection, the receiver measures the phase and magnitude of the received wave. The architecture optimally allocates transmit power to the spatial modes with the smallest propagation losses to decrease bit error rate and mitigate turbulence-induced outages. We compare the predicted theoretic average bit-error-rate (lines) of the optimal spatial modulation with a simulation (points) of communication through turbulence.

Conclusion -- Optical communication over the turbulent atmosphere has the potential to provide reliable rapidly reconfigurable multi-gigabit class physical links over tens of kilometers. Such systems, however, are prone to long (up to 100ms) and deep (10-20dB) fades and are susceptible to interference. For this project, we have shown that a sparse aperture system with spatial mode control provides costeffective protection against fades compared with a single aperture system.

Optical communication generally provides such high data rates that the added complexity involved in implementing a system that communicates over multiple spatial modes simultaneously is not typically justified by the added rate. As such, this work has focused on metrics related to communicating on one spatial mode at any given instant, such as BER and outage probability.