US researchers have come up with a technique for improving the performance of free-space optical (FSO) communications.
FSO communications suffer from variable reliability as a result of varying channel conditions - for example water particles in cloud cause fading and dispersion. Combining FSO and RF transmission in hybrid systems...
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... leads to high availability systems, but the much reduced bandwidth of the RF link limits performance when conditions thwart the optical link.
By using ultra-short laser pulses (down to 20fs), and sending the same data in parallel at a range of different rates, Mohsen Kavehrad, professor of electrical engineering at Penn State University, showed that FSO communications could be made more reliable and support higher bit rates under adverse conditions.
"Every wireless channel has a so called 'coherence bandwidth' value," said Kavehrad. "So long as a digital signal transmission rate is below this value, the signal can get through, free of inter-symbol interference. So for multi-rate signals, the lower rates can survive."
The data stream is spread across the time-frequency plane (a technique known as fractal modulation), and the receiver retrieves the data stream (or streams) according to the received signal quality, giving preference to higher rate streams. Kavehrad and Belal used wavelets as the basic carrier.
"We use holography to generate and separate the wavelets," explained Belal. "You just generate the mother wavelet and then the others can be generated as a fraction of the transmission bit rate of the mother. They can all co-exist in the channel without interference."
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