# On Analyzing Beamforming Implementation in O-RAN 5G
## 3 Challenges of Implementation: Splitting Options  
3 GPP defined and proposed eight possible splitting options. Splitting options are all the possibilities of separating modules in the RAN (between distributed and central units) so that each option defines (and standardizes) specific interfaces with different functionalities between CU and DU, as shown in Figure 3. These splits provide high flexibility to implementation since they allow the coordination between performance features, load management, and real-time performance optimization. In addition, they allow underlying technologies (SDN/NFV) and adaptation to various use cases by utilizing the configurable functional splits (e.g., variable latency on transport applications). The various splits supported new services, deployed on general-purpose or specialized hardware, with functions ideally placed to maximize scalability as well as spectrum and energy efficiency. The option adopted by O-RAN Alliance is Split Option 7 and, specifically, Split Option 7-2x [26; 27; 28; 29]. Split Option 7 assumes splitting at the physical layer function while maintaining radio functions (RF) at the DU. One of the key techniques in this spirit is the compression technique, which allows the reduction in transport bandwidth requirements between the DU and CU; furthermore, it allows the possibility of multiple configurations, which are known as options 7-1, 7-2, and 7-3 [30]. In Option 7-1, the FFT, cyclic-prefix (CP) removal, and possibly PRAC filtering function, resides in the DU within the uplink block diagram; the rest of the PHY functions reside in the CU. In the downlink block diagram, inverse fast Fourier transform (IFFT) and CP addition functions reside in the DU, and the rest of the PHY functions reside in the CU. The main benefit of this option is that it allows the implementation of advanced receivers. In Option 7-2, the FFT, CP removal, resource de-mapping, and possibly pre-filtering functions reside in the DU within the uplink block diagram; the rest of the PHY functions reside in the CU. In the downlink block diagram, inverse FFT, CP addition, resource mapping, and precoding functions reside in the DU, and the rest of the PHY functions reside in the CU. Overall, this option has some drawbacks, such as the high bandwidth requirements, relatively high latency requirements, and complex timing for the RU and  
Figure 3: Splitting options in 3 GPP specification (Source: 3 GPP). CU/DU link. However, it has been modified to be better than Split Option 7-1 in the case of high bandwidth requirements [31]. In Option 7-3 (only for downlink), only the encoder resides in the CU, and the rest of the PHY functions reside in the DU. The main benefit is that it reduces the front-haul requirements in terms of throughput to the baseband bitrates, as the payload for Option 7-3 is encoded data. This split has a high bandwidth requirement, relatively high latency requirements, and complex timing for the RU and CU/DU link. The O-RAN specification adapts Split Option 7-2x for front-haul specificities. The main goal is to reduce O-RU OPEX and maintenance costs. In comparison with Split 7-2, Split 7-2x has a simplified interface, an open interface protocol specifically designed to enable interoperability between RUs and DUs from different vendors, and there is not complex timing for the RU and CU/DU link. In Split 7-2x, the downlink and uplink processes flows are shown in Figure 4. The user bit sequence received from the MAC layer to PHY-High is processed into the O-DU. The O-DU operates through encoding, scrambling, modulation layer mapping, precoding (precoding can be paced in O-DU or O-RU), and resource element mapping (antenna mapping). Then, the outcome results need to be operated by the PHY-Low layer in O-RU through the IQ sampling sequence of an OFDM signal in the frequency domain. This sequence is then subjected to IFFT processing, converted to an OFDM signal in the time domain, and converted to an analog signal.