Reuse of GSM White Space Spectrum for Cognitive Femtocell Access

Nowadays, cyber-physical system (CPS) relies on wireless networks for devices control and information backhaul. But the mass deployment CPS devices make operators´ spectrum scarce situations even more worse. Hence, cellular network operators anticipate the Dynamic Spectrum Access (DSA) technology to solve the spectrum shortage problem in the context of cognitive radio (CR). Femtocells, acting as gateways in CPS, integrate CPS devices into cellular networks in a seamless manner. The concept of cognitive femtocell can solve the spectrum congestion problem even within a massive network on the CPS scale. However, in practical systems, cellular white space spectrum should be quantitatively measured to guide cognitive femtocell access algorithms design. We are the first to conduct a comprehensive measurement study for the purpose of measurement, discovery and model features of GSM white space spectrum. We evaluate availabilities of extra 21.4 MHz capacity in GSM white space spectrum as a reason of artificial GSM network spectrum planning. In our study, we find out that perfect results can hardly be obtained because of inherent measurement trade-offs, even when extremely high sweep speed of 16 GHz/s is applied at the receiver. Based on statistical analysis of real-scene traces, we propose an Efficient Duty Cycle (EDC) model to accurately characterize the white space in GSM network by considering miss-detection probabilities. Cross-validating evaluation results show that the EDC model can well decrease interference probabilities at high time-granularity measurement periodicity scenarios. Our results confirm the feasibility of cognitive femtocells access in an intra-operator scenario and can be applied to future wireless networks.