nd (of course) three sectors. Note how the cell-edge user's communication path to each of the three base stations is done on a separate frequency band, as indicated by the different colored bands. A variation of this scheme is called fractional frequency reuse which improves the spatial frequency bandwidth efficiency by reusing a large portion of the available bandwidth for their inner cell users while assigning the rest to the borderline users. Chapter 1 Open radio access network overview 27 Fronthaul DL Throughput for MUMIMO and MMIMO (MUMIMO: 4+4 layer, 4 antenna; MMIMO: 4+4 layer, 16 antenna) Option 8 Option 7-1 Option 7-2 Option 7-3 Option 6 Option 2 MU-MIMO MMIMO FIGURE 1-23 Fronthaul downlink throughputs for common functional split options. FIGURE 1-24 Single-frequency band operation. Frequency reuse as described above is widely deployed in macro base stations today. Its main drawback is the available spectral bandwidth and the static nature of the spectral division into smaller bands. In addition, note that the base station needs to support three sets of antennas and associated electronic components, making it complex to scale this solution down to very low cost. A more flexible approach is shown in the figure given next, which is still using frequency reuse to mitigate interference, but making the reuse scheme fully flexible by allowing each of 28 Open Radio Access Network (O-RAN) Systems Architecture and Design FIGURE 1-25 Sectorized frequency reuse. the three shown base stations to transmit/receive on the complete available spectrum (sin- gle-frequency network) as shown in the left-hand side of Fig. 1-26. In this scheme, the allocation of spectrum to each of the base stations is done dynami- cally, based on traffic load, user bandwidth requirements, and so on. The example on the right-hand side shows-as an example-two-third of the spectrum allocated to one base sta- tion, one-third to a second, and none to the third. Note that the 3GPP standard allows for flexibility in bandwidth allocation through mec