# A Survey on Open Radio Access Networks: Challenges, Research Directions, and Open Source Approaches
## 6 Challenges and Future Research Directions
### O-RAN Alliance Architecture Issues
MACsec is based on IEEE 802.1X and provides a simple and fast security solution compared to the internet protocol security (IPsec) standard in 3 GPP, which is not dedicated to speed. Theoretically, the throughput for small packets using MACsec is almost double that of IPsec. Future research could be directed toward proving this through field tests. Another option to be considered is using WireGuard. As mentioned, 3 GPP defines a flexible, functional split for DU and RU. O-RAN Alliance chooses the split 7.2 option in its architecture. Research by [85] shows that flexible, functional splits have more benefits than fixed split options. The research focused on placing RAN slices in a multi-tier 5G Open RAN architecture. The problem was formulated mathematically, considering different functional split options for each network slice separately. Results showed that flexible, functional split outperforms fixed functional split in utilizing physical network resources and satisfying different network slice requirements. Another work also suggested a dynamic functional split to solve the timing requirement of connecting an O-DU or O-RU with a DU or RU that does not follow O-RAN delay requirements [7]. Further research should be undetaken to determine whether the O-RAN Alliance should upgrade its architecture specification to support a flexible, functional split. O-RU has an issue related to the O-FHI. Various vendors have followed the O-RU specifications that O-RAN has supplied. Nevertheless, some of these claims are false. Despite claiming that their RU is O-RAN compliant, these vendors still design it with their proprietary standards. Some do not even use the O-FHI library that OSC has provided. It poses a huge problem because they will fail when the RUs want to be tested with the O-DU. As a solution, these vendors will modify existing or create new codes either on the DU or the RU side. While this method solves the problem, following the O-RU standard at the start of the RU designing process is more recommended. One interesting research direction for the O-RU is related to the beamforming process. The beamforming process has become one of the solutions to reduce total power consumption in joint optimization schemes [86]. It was made to decrease the number of BBUs required to handle network traffic, maximize the total input from users by defining physical resource blocks, enhance the precision of radio connection, and increase throughput and number of parallel connections in a given cell area [66; 86]. It can also minimize energy costs in mobile cloud and network considering QoS. Despite all these benefits of beamforming, this process still needs high cost and complex design when applied in a real environment. To overcome these problems, 3 GPP and O-RAN Alliance have developed several functional split options. As explained in Section 4, O-RAN's architecture uses the 7-2x split option, which can enhance flexibility, scalability, and interoperability within the network. In Section 4, we have also introduced several beamforming methods proposed by O-RAN Alliance. The last method proposed by the O-RAN Alliance--the channel information-based beamforming method--is a digital beamforming process and can be considered the best method out of these four methods. This method offers steerable beams using antennas with dedicated radio signals and radio paths, enabling it to provide a high number of beams that can be transmitted dynamically. O-RU generates these beams based on the channel information from O-DU. However, implementing the pre-coding in O-RU can produce inter-RU interference. To overcome this problem, a zero-forcing algorithm (ZF) is proposed [66]. ZF is an algorithm that can be implanted in the beamforming process and solves the inter-RU interference problem. The ZF beamforming is a beamforming method that can mainly be used in massive MIMO. This method allows transmitting and receiving data to specific users while eliminating interference from these users.