 Layer processing to an accelerator to stick more to the "Software Defined Radio" mantra (Fig. 4-15): Radio unit The Radio Unit (RU) performance range runs wide. Low-end implementations can have similar radio frequency (RF) specifications as the Integrated Small Cell (ISC) with a 2-4 antenna, <6 GHz, <250-500 mW/output port. These systems are very cost sensitive. Chapter 4 Hardware architecture choices 155 Host Processor Layer 2/Stack Physical Layer Accel. Fronthaul Termination/Switch Time/Freq eCPRI Accel. M x 10/25G mid/ N X 10/25G mid/ backhaul backhaul FIGURE 4-14 Disaggregated approach. High-end implementations that have limited antenna count high output power of 10s of Watt, such as used in large, outdoor applications. Defined here as a 2. .8-antenna system targeting the <6 GHz 3GPP Radio Access Network market. These systems typically employ digital beamforming in the RU to direct energy towards the User Equipment (UE). Given the high output power, power amplifier (PA) efficiency is key. Massive Multime Input Output (M-MIMO) systems that employ tens of antennas (32...64) in the <6 GHz market. The output power from each antenna is in the 5 Watt range. The main challenge in these RUs is scalability and cost: how to efficiently build a system that can scale to "any" antenna count. 156 Open Radio Access Network (O-RAN) Systems Architecture and Design Host Processor Layer 2/Stack Physical Layer Accel. M X 10/25G mid/ N X 10/25G mid/ backhaul backhaul FIGURE 4-15 Software defined approach. In the mmWave space, scalability can also be from the very low to the very high-end: Low-end implementations can have similar RF specifications as the Integrated Small Cell (ISC) with 16 or 32 Antenna elements and output power (EIRP) like that of a handset (30-40 dBm). Higher-end implementations can support multiple antenna panels, with 128 or more Antenna Elements each and a large DC power budget (Fig. 4-16). Key components in the RU include: 1. Fronthaul and eCPRI Termination a. This entails termination of the Ethernet/CPRI b