owed to reuse traditional 4G bands such as 1.8 GHz, 2.3 GHz, and 2.6 GHz for 5G services. To address the long term increase in demand for bandwidth, the bands between 3-24 GHz are also allocated for 5G use and future expansion. High band: Currently 26 GHz, 28 GHz, and 40 GHz are being considered globally to support ultra-high-speed broadband use cases. To expand the capacity requirements, the millimeter wave (mmWave) bands have attracted significant interest for use in 5G networks. Although mm Wave, from 30-300 GHz, serves only a small coverage area, that short range also allows efficient reuse of the spectrum and limits the inter-cell interference. 5G was first deployed in South Korea by SK Telecom in April 2019 using 3.5 GHz spectrum. 10.2 Timing Requirements for Mobility Networks Traditionally, radio networks required strict frequency stability at the air interface to: minimize dis- turbance between adjacent radio base stations (BS); facilitate call handover; and fulfill regulatory requirements on interference. In addition to that, the back-haul transport methods (for example, E1/T1, SDH/SONET, or Ethernet) require tight frequency synchronization to allow accurate multi- plexing and de-multiplexing. To understand the impact of these requirements on the timing design, the following section covers the fundamentals of radio technology. The sections that follow cover each radio technology and de- scribe the reason why timing becomes important for that technology. 10.2.1 Multi-Access and Full-Duplex Techniques To distinguish and separate signals between different transmitters and receivers, 2G and 3G cellular networks adopted multi-access techniques such as FDMA, TDMA, and CDMA. Figure 10-2 illus- trates each of the techniques, which are further described in the list that follows. Frequency Frequency Frequency User n User 1 User 2 User n User 2 User 1 (a) FDMA (b) TDMA (c) CDMA Figure 10-2 Multi-Access Techniques FDMA is the process of dividing one frequency channel or bandwidth into multiple nonoverlap- ping subch