Abstract
Hybrid-free space optical and radio frequency wireless links are a way of providing reliable transport of real-time traffic in outdoor wireless environments. We consider a link layer protocol that assigns packets to each physical channel of such a hybrid link, which first attempts to send each packet over one of the links (the main link) and, if unsuccessful, sends the packet over the other link (the backup link). The hybrid link processes high-priority traffic by using the link layer protocol and additional (background) traffic at low priority over the backup link. In this setting, high-priority traffic can be transmitted at a rate as high as the maximum capacity of the main link, assuming that the backup link can compensate for main link capacity deterioration, with no need for reconfigurations aimed at adapting to changes in weather conditions, which is an advantage over other approaches. From the perspective of link availability for high-priority traffic, we compare our approach to using another protocol that does not require reconfigurations, which could be employed if the backup link is expected to have a constant transmission rate during the time interval of interest. For situations where both links can be represented by finite-state Markov models with states corresponding to channel bit error rates, as has been done in previous literature for radio frequency links and for free space optical links affected by strong atmospheric turbulence and Gaussian noise, we give a way to provide probabilistic quality of service guarantees for background traffic assuming that the high-priority traffic is insured to not exceed a given constant rate.
Highlights
Introduction and Related WorkHybrid wireless free space optical (FSO) and radio frequency (RF) links are a way of providing reliable transport for critical real-time traffic in an outdoor wireless environment, as weather conditions such as fog affect FSO links much more than RF links, while rain affects RF links (with a frequency of at least 300 MHz) much more than it affects FSO links
Introduction and Related WorkHybrid wireless free space optical (FSO) and radio frequency (RF) links are a way of providing reliable transport for critical real-time traffic in an outdoor wireless environment, as weather conditions such as fog affect FSO links much more than RF links, while rain affects RF links much more than it affects FSO links.Challenges in the usage of FSO links are that the receiver must be in the path of the transmission, which usually has a very small angle and that significant attenuation occurs due to weather conditions such as fog
Approaches to widen the applicability and improve the performance of FSO links have been taken in the following directions: adjusting signal intensity by increasing its power in case there are adverse weather conditions [1, 2], studying and finding ways to increase the transmission angle so that reception is less of a problem, using systems with multiple receivers and multiple transmitters (MIMO) [3, 4], and using hybrid FSO and RF links [1, 5,6,7,8,9] and combinations of two or more of these approaches
Summary
Hybrid wireless free space optical (FSO) and radio frequency (RF) links are a way of providing reliable transport for critical real-time traffic in an outdoor wireless environment, as weather conditions such as fog affect FSO links much more than RF links, while rain affects RF links (with a frequency of at least 300 MHz) much more than it affects FSO links. The average values of the measured parameters would not catch the short-term changes, and if a reconfiguration is triggered the change might reverse immediately after the reconfiguration took place To effectively cover such conditions, in [17] it was proposed to use a link layer protocol to distribute the load among the two underlying links of a hybrid FSO/RF link as an alternative. We first assume that in these time intervals one link (the main link) is a Rayleigh fading channel and that its average bit error rate varies according to a Markov process as was done, for example, in [18], while the backup link has constant capacity.
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