 The air interface is the wireless link between the user device and the equipment at the cell site's tower or antenna. The base station at the tower or antenna contains a radio that receives and transmits mobile traffic and amplifies (strengthens) signals. It also transmits traffic to other antennas and performs the conversion of signals between over-the-air radio frequency formats such as LTE to those compatible www.TechnicalPDF.com with landline networks, and vice-versa on the traffic between the backhaul and the base station. REITs-A Way to Finance New Antennas and Towers Companies such as Crown Castle and American Tower specialize in building towers and antennas. They don't offer cellular service; they lease space on their towers to mobile carriers who use the land around the towers for cell site equipment. Instead of each mobile carrier owning their own tower, these third-party companies build towers and lease shared space on them. For example, a large operator might lease space on a tall water tower for its antenna and land surrounding the tower for ancillary equipment. Carriers often share space on these towers to save costs. Crown Castle and American Tower are structured as real estate investment firms (REITs). With REITS, outside investors underwrite tower construction by purchasing the land on which the towers and cell sites are located and leasing the space back to American Tower and Crown Castle. In return, Crown Castle and American Tower provide investors a stream of payments from their subsequent profits providing shared space on their towers to mobile carriers. Some mobile carriers including Verizon build their own towers or hire contractors to build them. Cell sites are the costliest part of mobile networks because there are large numbers of cell sites in 4G and especially in 5G networks. These steep costs www.TechnicalPDF.com make it challenging for all but the largest mobile providers to build out their networks for use with new generations of mobile protocols that require many more cell sites b