fractional-T/E carrier (T1/E1), digital sub- scriber line (DSL), pseudo-wire, Ethernet, WDM, and gigabit passive optical network. The choice of wired backhaul is driven by the availability of physical media, cost, and capacity requirements. Wireless backhaul is needed when base stations do not have access to copper, HFC, or fiber transport. Wireless backhaul is also attractive when time to deployment is criti- cal or when leasing costs are prohibitive. It is estimated that nearly 70% of worldwide LTE base station installations use wireless backhaul [80]. The methods of delivering wireless back- haul transport are LoS microwave, LoS millimeter wave, non-LoS (NLoS) sub-6 GHz micro- wave, broadband satellite links, and in-band or out-of-band relay nodes. The HetNet architecture comprises four general classes of base station: (1) macro-cell, (2) metro-cell, (3) pico-cell, and (4) femto-cell. Table 1.2 compares the types of base station, deployment scenarios, and the set of possible wireless backhaul solutions. Wireless back- haul radios operate over a wide spectrum of licensed and unlicensed RF bands extending to 80 GHz (see Fig. 1.41). The RF spectrum for wireless backhaul ranges from sub-6 GHz NLoS to C/Ka/Ku-band microwave LoS, and Q/V/E-band mmWave LoS. Each RF band has spectrum restrictions, channel bandwidth limitations, and specific propagation character- istics. Channel bandwidth can vary from 5 to 160 MHz in NLoS systems; from 3.5 to 56 MHz in microwave LoS systems; or from 28 to 112 MHz and 250 MHz to 5 GHz in mmWave systems. All these specifications impact the type of modulation and carrier-to- noise ratio, and thereby mandating certain capacity trade-offs and maximum link distance. Table 1.2 shows that backhaul throughput for each base station class must match the respec- tive cell-site capacity. An optimal wireless backhaul solution further adapts the performance requirement with a particular deployment scenario. The method of wireless backhaul deter- mines frequency band operation, radio design specific