modate the activation/de-acti- vation procedures while controlling the data transmission delay. Because the anchor carrier in license band is always available, the supplemental downlink carrier in the unlicensed band can be used opportunistically. When the downlink traffic of the small cell exceeds a certain threshold and there are active users within the unlicensed band coverage area, the supplemen- tary downlink (SDL) carrier can be turned on for offloading the traffic. When the traffic load can be managed by the primary carrier or there is no user within the unlicensed band cover- age area, the SDL carrier is turned off. Opportunistic SDL mitigates the interference from LTE continuous cell-specific reference signal transmission in an unlicensed channel. MulteFire was proposed by the MulteFire Alliance in late 2015 as a standalone version of LTE for small cells, which operates in the unlicensed spectrum and can provide service to users with or without a universal subscriber identity module card (i.e., an operator subscrip- tion). It combines the advantages of LTE technology and the simplicity of Wi-Fi deploy- ments and can be deployed either by traditional mobile operators or neutral hosts. It specifies two different architectures: (1) a PLMN access mode, which allows mobile net- work operators to extend their coverage into the unlicensed band, especially in the case where a licensed spectrum is not available at certain locations; and (2) a neutral host net- work access mode, which is similar to Wi-Fi, a self-contained network deployment that pro- vides access to the Internet. Because of the nature of transmission in the unlicensed band and the need to adhere to the LBT requirements, MulteFire has introduced some modifica- tions to the radio interface of LTE [16]. 8.1.4.4 NR-Unlicensed The NR-U study item in Rel-16 is identifying and evaluating solutions for the NR when operating in the unlicensed spectrum. 3GPP will further investigate the coexistence with NR-based systems in the licensed and unlicensed bands 