logies for improved road safety in the EU; December 2017. [27] Qualcomm Technologies Inc. Accelerating C-V2X commercialization; 2017. [28] Gaurang Naik et al., IEEE 802.11bd & 5G NR V2X: Evolution of Radio Access Technologies for V2X Communications, Cornell University Online Library, March 2019. CHAPTER 8 Operation in Unlicensed and Shared Spectrum The RF spectrum is divided into licensed and license-exempt/unlicensed bands. The tradi- tional cellular communications systems used to exclusively operate in licensed spectrum. However, due to the insufficiency and cost of licensed spectrum below 6 GHz, the telecom- munication industry has shifted attention to the unlicensed bands to deploy supplementary uplink/downlink carriers or standalone systems. The use of unlicensed spectrum has been increasingly considered by cellular operators as a complementary mechanism to offload traffic from licensed-band RF carriers to increase the overall system throughput. Various approaches to cellular operation in unlicensed spectrum have been investigated and trialed in the past few years. Two practical cellular technologies for communication in unlicensed spectrum, the LTE- U and the LTE-based license-assisted access (LAA), have been investigated by LTE-U Forum and 3GPP. The use of Wi-Fi as a complementary carrier to offload cellular networks was stud- ied earlier and specified by 3GPP under LTE-Wi-Fi link aggregation (LWA) work item [12]. Operation in unlicensed spectrum is subject to various limitations and restrictions which are regional and band specific. The typical limits are in terms of total transmit power, power spectral density (PSD), carrier bandwidth, and duty cycle that each device can use. In addi- tion, sharing protocols may also be specified in some bands to protect other systems in the band or to allow efficient sharing. As an example, dynamic frequency selection (DFS) aims to protect radar systems and listen-before-talk (LBT) allows efficient spectrum sharing by minimizing inter-user interference in unlicensed spe