tors. For low- and mid-band scenarios, FHBW using Option 7/8 might still be manageable considering the bandwidth and coverage in those bands. In addition to fronthaul transport network throughput limitations, which impose restric- tions on NG-RAN architectures an operator can use, latency is also a factor that needs to be considered. Table 7.3 shows an example of typical fronthaul latency requirements for various protocol split options. The latency requirements for lower-layer split Options 7 and 8 are limited by Hybrid ARQ (HARQ) loop and coordinated processing requirements and therefore are tighter, while the higher-layer split option is more relaxed in terms of latency, which is limited only by end-to-end performance requirements. To view the impact of fronthaul latency requirements on NG-RAN deployment archi- tecture, consider an example circular deployment area of 100 km radius and assume that DUs can be placed anywhere within this area to provide the required coverage and capacity. The maximum distance from a CU to DUs (or RUs) is limited by the corresponding latency requirement. Assuming an optical fiber deployment for fronthaul network and assuming speed of light in fiber to be X 105 km/s (two thirds of the speed of light in vacuum), a rough estimate of the number of CUs required for the deployment area for different CU-DU round-trip latency requirements is shown in Figure 7.2. As the round-trip latency requirements become less stringent, the number of CUs required reduces significantly, for example, in Option 2. On the other hand, the lower-layer split Options 7/8 that have tight latency requirements would need a larger number of centralized sites (i.e. CUs). Table 7.3 Latency requirements for various split options. Split Round-trip latency Comment Option 2 5-10 ms Limited by end-to-end performance Option 5 0.5-3 ms Limited by real-time Radio Link Control functionality Options 7-8 ~50 us Limited by Hybrid ARQ loop and coordinated processing requirements 2 Note that the FHBW requirements stated below are 