## 2 Design Rules of Small Cell Enhancement Systems
### Basic Theory  
Based on CoMP scenarios, a diagram of the conceptual structure of radio frequency over SCE fiber optical repeater operation is shown in Figure 4. The fiber optical repeater consists of two modules. The first, which is the main distribution point, is the main unit (MU). The MU converts radio frequency signals in downlink (DL) to optical signals and converts optical signals to radio frequency signals in uplink (UL). The second module is the remote unit (RU), which features RRHs and detects downlink signals and uplink signals. A structure diagram of the MDP [13] system is illustrated in Figure 5. The radio frequency signals coming from the eNB's downlink are roughly +13 dBm [14]; therefore, the signal level should be attenuated for optical module receiving. The optical signals are separated into multimode multi-channel signals by RUs. Consequently, the multimode radio frequency downlink consists of a splitter and an autotune-adjustable digital attenuator. As the MDP receives optical signals through optical modules and directs them towards the RU, the conversion of radio frequency signals occurs via the uplink signal path's radio frequency modules. Consequently, the radio frequency multimode consists of a combiner and an autotune-adjustable digital attenuator. Its purpose is to distribute the uplink signal's power, which manifests as an attenuated radio frequency signal level in the eNB (where the noise floor is \(\leqq-120\) dBm). The eNB caters to multiple cellular phones or IoT devices, thereby increasing the input noise to the signal level, which can surpass \(-120\) dBm, subsequently disrupting the communication of weak-signal cell phones and impeding the quality of calls. A structure diagram for a RRH [15] is displayed in Figure 6. The remote optical unit, housed within a wireless terminal, serves the cellular phone. Figure 4: SCE structure diagram. Figure 5: MDP structure diagram. In the downlink signal path, the RRH's optical module receives an optical signal from the MDP and converts it into a radio frequency signal. This radio frequency signal is then amplified by a power amplifier and transmitted through the antenna. Consequently, the fundamental downlink architecture of the RRH is composed of an optical module, IF filtering, a high-frequency conversion module, and a power amplifier module. Similarly, in the UL part, the RRH's antenna receives a mobile station signal, which is then modulated to an optical signal through an independent fiber and sent back to the MDP. Next, the MU functions of the UL of the optical signal from the optical module convert this signal into a radio frequency signal, which is sent back to the base station. Thus, the basic UL architecture of the RRH comprises a low-noise amplifier module, IF filtering, a frequency conversion module, and an optical module. Finally, this study investigates SCE systems applied as multiple distribution points for remote radio head technology. Additionally, it examines experiences in designing, implementing, and managing multi-operator solutions and determines whether the operators' concerns about sharing can be alleviated to the satisfaction of all participants and whether the cost/benefit ratio and profitability that results from sharing with other operators outweighs any limitations. This has largely been the case among DAS vendors that can support multiple services over novel high bandwidth cellular-over-fiber optic indoor transition systems, as shown in Figure 7.  
The introduction of indoor multi-operator systems may satisfy many groups a with tested interest, such as the public, building owners, and various government authorities, as well as satisfy the common interest of mobile operators as it could reduce costs, enhance coverage, and ensure the rapid roll out of new services. Figure 6: RRH structure diagram. Figure 7: DAS support by novel cellular-over-fiber optic indoor transition systems.