## 6 Future Research Areas
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Issue** & **Reference** & **Section Summary** \\ \hline QoS & [11; 13; 15; 16] & In this Section 3.1, QoS requirements of URLLC (latency and reliability) and factors, which are a hindrance in achieving the desired QoS for URLLC, are discussed. \\ \multirow{4}{*}{Coexistence with eMBB} & [17; 18; 19] & In the 5G networks, many different applications with diverse requirements will exist in the same physical medium. Such a coexistence of services will raise many challenges for telecom companies. In Section 3.2, the problems with the coexistence of eMBB and URLLC with different service requirements are discussed. \\ \multirow{4}{*}{URLLC Packet Design} & [15; 20] & Packet design plays a vital role in achieving low latency. Minimizing the packet processing time will be a key factor in enabling low latency for URLLC. Packet structure proposed by LTE and NR to achieve low latency is discussed in Section 3.3. \\ \multirow{4}{*}{URLLC Scheduling} & [20] & Because of the unpredictable packet generation of URLLC, scheduling is a challenging task. In Section 3.4, some of the proposed scheduling schemes for URLLC and issues with those schemes are discussed. \\ \multirow{4}{*}{Energy issues for UE} & [21; 56] & To keep up with the latency requirement of URLLC, UEs are forced to perform extra tasks, which can result in low battery life for the UEs. \\ \multirow{4}{*}{Handover issues for URLLC} & [22; 23; 24; 25] & Providing uninterrupted services to a mobile user is the most significant facility of any telecom infrastructure. Providing such an uninterrupted service to a user using URLLC based services is quite difficult. Issues related to handover when it comes to strict latency are discussed in Section 3.6. \\ \multirow{4}{*}{Error Handling} & [17; 26; 27] & Wireless services are prone to many challenges, and providing highly reliable service in wireless communication is quite a tough task. The issues related to the handling of error packets and retransmission are covered in Section 3.7. \\ \multirow{4}{*}{Role of URLLC in operating IoT} & [33; 34] & IoT will play a major role in the coming era of technology. URLLC will play a vital role in supporting IoT services. In Section 4, the importance of URLLC to operate IoT is discussed. \\ \multirow{4}{*}{URLLC and Massive device connectivity} & [35; 36] & Although URLLC fulfills the basic requirement of reliability and latency for mission-critical IoT, it is a challenge for URLLC to provide simultaneous services to a vast number of devices. Section 4.1 covers the issues that URLLC brings in operating massive IoT devices. \\ \multirow{4}{*}{On-device AI and URLLC} & [21; 36; 37; 38] & In earlier sections importance of URLLC for time-critical applications is highlighted. However, the provision of low latency service to massive devices is also a challenge, as cited in Section 4.1. It is provoking researchers to seek new solutions to achieve low latency with high reliability. Among such solutions developing intelligent machines is quite prominent. In Section 4.2, issues related to AI/ML-based machines and relying on URLLC services for such machines are discussed. \\ \multirow{4}{*}{URLLC and V2V} & [39; 40] & An automated vehicle is one of the most anticipated services of the upcoming era. However, providing highly reliable and time-critical connectivity is still a challenge for URLLC. V2V connectivity opens a whole new level of disputes. Among them, some issues are discussed in Section 4.3. \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Summary of issues discussed in the paper. AI, artificial intelligence; ML, machine learning.