Spectrum sensing for cognitive VHF land mobile radio communication networks
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The 2015 migration from VHF Analog to Digital TV created plenty of white spaces in the entire VHF TV Band(174-230MHz). Spectrum bands once used for analogue TV broadcasting were completely cleared, leaving a space for deploying new wireless services, and, in another way, digital television technology geographically interleaved spectrum bands to avoid interference between neighboring stations, also leaving a space for deploying new unlicensed wireless services. These white spaces can be used by other wireless applications and internet services whose radio spectrum is already pushed to maximum utilization and is therefore scarce for emerging wireless applications. Such applications include IP TV, wireless internet, cellular telephony, Zigbee, WiMax-Advanced, LTE, UWB Network. In this study, we implemented a VHF LMRS that can also utilise the TVWS in the VHF TV band for mission critical voice transmissions. The utilisation of TVWS makes the solution fit in the interweave spectrum access model, where SUs have to utilize empty spectrum bands for transmissions. We detect VHF LMR transmissions in the TVWS using energy sensing technique, with the real-time energy detector developed on the Software Defined Radio (SDR) testbed composed of RTL-SDR device, VHF Radio and GNU Radio. The noise floor of the system is first analysed to determine the detection threshold, obtained using the empirical cumulative distribution method. Simulated energy detector using GNU Radio was used to set a benchmark. In both simulations and the real-time platform, a Narrow Band frequency Modulation(NBFM) was generated and transmitted through the TVWS. The performance of the implemented real-time energy detector compared to simulated one was lower, due to the noise distribution being not perfectly AWGN, and thermal noise from the RTL-SDR. Notably also the transmission in TVWS was generally of much signal energy compared to transmission in traditional LMR frequency(approximately 10% improvement) and thus improved penetration in remote areas and thick forests.This research proves that CR implementations for VHF LMRS can be implemented with TVWS on VHF Radio, RTL-SDR and the GNU Radio palatiform.