# 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
This cell-splitting architecture has several open problems that need to be solved in the future, such as the dynamic capability and flexibility of the model, the complex inference model, security risks, the requirement of extreme data rate, and the interoperability of multi-vendor elements. Near-RT RIC's current implementation of the TS xApp relies on the currently available AD and QP xApps. TS is critical in the RAN. Better ML models with higher accuracy should be applied in the AD and QP xApps. This will improve the overall TS xApp performance. Another problem arises from the design of the present O-RAN RIC. RIC cannot understand the connections between the same subscriber or other UE. Instead, RIC only owns high-level insights in a connection. This increases the risk of false-positive alarms/actions, especially without IMSI/IMEI correlation across connections. Some more suggestions are also raised by [72] to improve xApp and RIC platform. These suggestions include a smaller xApp package, support for time-series handling, support for the message queue, scaling out method, more AI technologies (online training, reinforcement learning (RL), and federated learning), and hardware acceleration. O-RAN's RIC consumes a non-negligible amount of resources. Additionally, O-RAN's RIC has forward compatibility limitations because it is coupled to specific implementations, such as Redis or Prometheus databases. As a result, applications must poll these databases frequently to find new agents. The work in [73] introduces FlexRIC to answer these problems. FlexRIC is a software development kit developed by OAI to build service-oriented controllers. It is made up of a server library and an agent library. FlexRIC has two innovative service models with proof-of-concept prototypes for RAN slicing and traffic control. Results show that FlexRIC uses 83%. There are several existing research relating to Near-RT RIC xApps. A method that enables fault-tolerant xApps in the RIC platform, known as RIC fault-tolerant (RFT), is proposed by [74]. The RFT is deployed so that the Near-RT RIC is more tolerant towards faults, thus preserving high scalability. Results show that the RFT can meet latency and throughput requirements as replicas increase [74]. Another xApp called NexRAN is proposed by [75]. NexRAN is an open-source xApp that can perform closed-loop controlled resource slicing in the O-RAN ecosystem. NexRAN is executed on O-RAN Alliance's RIC to control the RAN slicing in srsRAN. NexRAN testing is performed in the POWDER research platform. Testing results show that NexRAN can successfully perform the RAN slicing use case. An RRM xApp based on RL is proposed by [76]. The xApp dynamically adapts the per-flow resource allocation, modulation, and coding scheme to meet the traffic flow KPI requirements based on the network status. The xApp testing is performed on OAI LTE RAN in a small laboratory setup. It is necessary to conduct future testing to determine the performance in large-scale, heterogeneous, and non-stationary scenarios. O-DU or the DU generally is one of the most discussed topics by wireless mobile communications enthusiasts. In Section 5, we have mentioned that O-DU is responsible for L2 and some L1 processing. It is also noted that accelerators are included in the O-DU specifications. Back to Section 2, we also have pointed out that through NFV, the operator can use a virtual DU (vDU). However, there is currently a problem with the DU implementation. Since vDU is deployed in a COTS server, most vDUs rely on x86 architecture. This is because most COTS servers still use the x86 architecture. The problem is that x86 architecture cannot meet the O-DU requirements under special cases like meeting the numerology 4's 1-microsecond latency requirement and handling heavy UE traffic load. In these cases, the x86-based processing can get extra support from accelerators. Regarding accelerators, O-RAN Alliance has already defined some specifications for the use of accelerators in the new documentation.