h, side-lobes) play a critical role in characterizing delay spread of these deploy- ment scenarios. Similarly, Doppler spread due to multipath causes channel fluctuations, which in turn could be minimized by using highly directional antennas. Highly directional antennas, however, pose challenges in user tracking. 2.4 New Spectrum Models Over the past few decades, some regulators, for example, in North America and Europe, have moved toward technology-neutral regulations, that is, providing maximum flexibility to licensees in deploying their technology of choice within the allocation of the service as long as a set of least restrictive technical conditions is met. The two important elements to technology neutrality, that is, flexibility and least restrictive conditions, are related to each other in a dialectic way. The flexibility element enables licensees to make decisions on their technology4 of choice over the term of the license within the specified rules. The least 4 The duplexing mechanism is often considered a technology element. There are radio interfaces with more than one mode of duplexing where almost every other aspect of the interface remains the same. A full 2.4 New Spectrum Models restrictive element maintains that the flexibility given is within bounds, that is, only the minimum necessary for preventing harmful interference to other services in the band or in the adjacent bands. Technology neutrality has generally led to market-driven deployments and transition of 2G to 3G to 4G technologies in many countries without much need to repurpose, refarm, and reregulate cellular bands. As mentioned in the previous section, significant spectrum is likely to be allocated for 5G in many parts of the world. However, in some cases even this may not resolve the spectrum shortage completely. One reason for this could be that the largest chunk of 5G spectrum is likely to be in the mmWave range, which may not be suitable for certain applications due to propagation characteristics. Even though availability of enough