# A Survey on 5G Coverage Improvement Techniques: Issues and Future Challenges
## 1 Introduction
### Existing Surveys  
According to the author, there is no detailed review of coverage enhancement techniques in cellular networks. However, there are some papers that explore the challenges of the future cellular network requirements, including coverage as one of the subtopics. In 5G, coverage is the basic issue, and to overcome this issue, many authors have proposed various algorithms and methods. To improve coverage in cellular networks, heterogeneous networks (HetNets) and small cells are proposed [10; 11; 12]. The HetNets with less complex user association algorithms and small cells with interference mitigation algorithms are still required to improve the efficiency and reliability of the cellular system. In the literature, network coverage was improved using optimization techniques such as machine learning, linear programming, and mathematical analysis. The optimization of coverage capacity, deployment cost, and coverage area in 5G are explored [13; 14; 15; 16]. Along with the deploy \begin{table}
\begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline \hline
**Generation** & **Access Techniques** & **Data Rate** & **Frequency Bands** & **Applications** & **Key Parameters** \\ \hline
5G & NOMA, FBMC & 2.4 Kbps & 1.8 GHz, 2.4 GHz, 30–300 GHz & Voice, Data, Video calling, ultra HD video, virtual reality applications & Ultra-low latency, ultra-speed, and ultra-reliability \\ \hline
4G & LTEA, OFDMA, SCFDMA, WIMAX & 10 Kbps & 2.3 GHz, 2.5 GHz, 3.5 GHz & Voice, data, video calling, HD television, and online gaming & Faster broadband internet and lower latency \\ \hline
3G & WCDMA, UMTS, CDMA & 384 Kbps to 5 Mbps & 800 MHz, 900 MHz, 5 Mbps & & \\  & & & 850 MHz, 900 MHz, 1800 MHz, 1900 MHz, 2100 MHz & Voice, data, and video calling & Broadband internet and smart phones \\ \hline
2G & GSM, TDMA, CDMA & 100 Mbps to 200 Mbps & 800 MHz, 900 MHz, 1800 MHz, 1900 MHz & Voice and data & Digital \\ \hline
1G & FDMA, AMPS & 10 Gbps to 50 Gbps & 800 MHz & Voice & Mobility \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Evaluation of technology generations from 1G to 5G. ment cost and network capacity, it will provide the optimization algorithms for resource allocation, quality of service (QoS), spectral efficiency, and throughput. The conventional multiple access techniques will not fulfil the future challenges of 5G, such as high data rates, low latency, massive connectivity, and high spectral efficiency. In 5G, the increased number of cellular users increases the receiver's complexity. However, non-orthogonal multiple access (NOMA) techniques can be used to reduce receiver complexity [17; 18; 19]. This will allocate the multiple users in one resource block based on allocation techniques and decode the users at the receiver to reduce the system's complexity. This method will improve the bit error rate, throughput, and system capacity. Device-to-Device (D2D) technology is also used to reduce the system complexity, provide security from malicious users, and provide better reliability [20; 21; 22; 23; 24]. The bandwidth of the cellular network can be improved by using carrier aggregation (CA) techniques [25; 26; 27; 28; 29]. The opportunities and future challenges of massive MIMO in 5G and beyond are discussed in [29]. In the literature, there exists several techniques and technologies to enhance the network coverage, but not in an integrated manner. The authors focused on key parameters of the 5G networks and its implementations, deployments individually. The authors in Refs. [10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29], focused on one or two coverage enhancement techniques such as small cells, CA, D2D, NOMA, MIMO, or 5G optimisation techniques--but not all in one paper. Therefore, this survey aims to provide broad and complete information about all coverage enhancement techniques and the intelligent optimization techniques, their pros and cons to fulfil the existing gap in wireless cellular networks, and also future challenging issues.