## 2 Broadband System
### Worldwide Interoperability for Microwave Access (WiMAX)
#### 4.2.2 TV White Space: Broadcast to Enhanced Mobile Broadband  
The growing increase in the various multimedia applications and services has been contributing significantly to the radio spectrum scarcity, which is one of the key bottlenecks in wireless communications. In recognition of this, the Federal Communications Commission (FCC) has observed that despite the increase in demand the spectrum access, the allocated licensed spectrum bands including the bands for TV broadcasting, have not been efficiently utilized at certain times and locations. In this regard, a substantial portion of allocated very high-frequency (VHF) and ultra-high-frequency (UHF) radio spectrum channels in different geographic areas may not be used for the intended broad  
Figure 7: Direct broadcast satellite architecture. cast services by the TV stations (primary network) owing to the probability of instigating interference. This implies that there is a high potential for unlicensed cognitive devices (secondary users) to operate in such unoccupied spectrum bands known as TV White Space (TVWS), alleviating the global radio spectrum congestion and improving the spectrum efficiency. The TVWS signifies the TV channels that are unused or underutilized by any licensed services at a certain location and a specific period. It is normally employed to define contiguous or interleaved unused portions of the spectrum in the broadcast TV frequency bands (VHF and UHF) that exist among various assigned channels to prevent interference. Based on this, the FCC has authorized the license-exempt use of TVWS for the new wireless applications/services support. This is also facilitated by the cognitive radio systems. Although TVWS offers an attractive spectrum opportunity for wireless broadband services, its exploitation is based on the condition that there should be no interference to the incumbent users [187; 188]. Moreover, the transition from analog to digital TV has brought about the local accessibility of benign wireless communication channels in the TVWS spectrum. Consequently, reallocation of unused broadcast TV spectrum to broadband and other wireless services enables unlicensed access for fixed and mobile applications [187]. Furthermore, it has been demonstrated that the unused spectrum can be exploited to deliver broadband Internet access while functioning amicably with the neighboring TV channels. Note that the TVWS spectrum can penetrate obstacles such as trees, doors, and walls. These features make it highly attractive for various applications such as cost-effective Internet services beyond the Wi-Fi range and surveillance purposes in areas with challenging topography/during harsh weather [189]. Besides, opportunistic access to TVWS can offer low energy consumption and cost-effective spectrum access for a number of outdoor and indoor communication systems, including home networking, rural broadband, and device-to-device communication via efficient radio spectrum utilization, making it a good candidate for the 5G FWA [190]. Note that there are a set of requirements regarding frequency agility and strict spectral mask that have to be met by potential white space devices (WSDs) for effective wireless transmission over TVWS. For instance, based on their frequency agility, they should be able to select and change channels considering their geolocation. Furthermore, they have to strictly adhere to the imposed spectral masks to guard the incumbent users [191]. Consequently, one of the main problems of dynamic spectrum sharing in TVWS is the management of the interference. This will help in protecting both the incumbents and the cognitive users from a variety of interference sources. Likewise, to facilitate interference-free coexistence between heterogeneous unlicensed networks, the geolocation-based approach and spectrum sensing techniques are normally implemented for spectrum sharing in the TVWS [190].