On The Coherence Time of Fiber-Based Distributed Antenna Systems

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The possibility of using radio over fiber (RoF) analog transmissions to characterize the wireless propagation channel of distributed antenna systems is investigated. In particular, the coherence time of kilometer-long, fiber-based networks is analyzed. Joint measurements of the phase noise of parallel links to the same site (intra-site) and to different sites (inter-site) in such a network are presented. The coherence time of the phase noise and phase noise difference between links is evaluated by means of the Allan time deviation. Measurements are performed at a frequency of 2.3 GHz under both stationary and non-stationary temperature conditions, for a duration of ten hours.In the setup analyzed, the phase noise was found to be white, with a phase drift component that determines the coherence time of the link. Coherence times between two to six minutes have been measured for a phase offset of 10 degrees Celcius. Values of the correlation coefficient r for intra-site phase noise processes higher than 0.99 have been measured, while correlation coefficients for inter-site links exhibit lower values, r >0.87. All phase noise processes display a strong, negative correlation with the local environment temperature, with r <-0.75 and temperature drift coefficients in the range of -20$ degrees to -30 degrees per Celcius degree. On the other hand, phase noise difference between links showed weak correlation with temperature, where coherence times were in the order of 15 minutes for inter-site links and 60 minutes for intra-site links. Moreover, it was found that hardware imbalances between links, as well as temperature gradients of several Celcius degrees per hour can considerably reduce coherence times.The results of this investigation suggest that, when parallel fiber optic links are used, the phase coherence of the measurement system might prevent the double-directional characterization of the wireless propagation channel, unless care is taken to minimize variations in the (distributed) temperature environment. Alternatively, phase offsets can be measured several times per hour.
StatusPublished - 2014
EvenemangCOST IC1004, 2014 - Aalborg, Danmark
Varaktighet: 2014 maj 262014 maj 28


KonferensCOST IC1004, 2014

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