gulated spectrum at different times and places based on the national regulations and the commercial agreement. Three main frameworks have been considered for spectrum sharing. These frameworks are the citizens broadband radio service (CBRS), licensed shared access (LSA), and concurrent shared access [97]. The USA has dedicated the 3.5 GHz for CBRS, while Europe has considered the 2.3 GHz for LSA. On the other hand, the unlicensed spectrum has been investigated for LTE by considering three technologies as follows: 1. LTE-unlicensed (LTE-U) [98]: It has been standardized by LTE-U forum and considered in 3GPP releases 10, 11, and 12 [99]. It is based on carrier aggregation in order to boost the mobile network capacity. Accordingly, it enables the subscribers to access to the unlicensed 5 GHz band with an anchor carrier on M. Alfaqawi et al. the licensed bands. This technology has been proposed for countries that do not mandate to perform the listen-before-talk (LBT) protocol such as the case in the USA, but, instead, it is based on the duty cycle. 2. Licensed-assisted access (LAA) [100]: Unlike LTE-U, LAA mandates to perform LBT, and thereby it is adopted in Europe and Japan. It has been standardized by the 3GPP in releases 13, 14, and 15 [101]. LAA and enhanced-LAA (eLAA) are based on carrier aggregation. Accordingly, LAA enables a supplementary UL/DL carrier on the unlicensed 5 GHz band with an anchor carrier on the licensed bands. 3. MuLTEfire [53]: Unlike LTE-U and LAA, MuLTEfire operates on standalone mode on the unlicensed sub-1, 2.4, and 5 GHz bands. It has been standardized by MuLTEfire alliance [53, 95]. Similar to LAA, it mandates to perform LBT protocol before accessing to the unlicensed channel. Analogously with LTE-LAA and muLTEfire, anchored NR-U and standalone NR-U have been considered by 3GPP in releases 16 and 17 for 5G operating on unlicensed spectrum. NR-U in 3GPP release 16 operates on the unlicensed 2.4, 3.5, 5, and 6 GHz bands. These bands are extended in 3GPP release 17 for above- 6 GHz bands 