## 8 Limitation and Future Research Directions  
The following is by no means an exhaustive account of the challenges and future research directions in the field of NB-IoT and LoRaWAN. Nonetheless, we focused on realising the above results of the performance of LoRaWAN and NB-IoT using one eNB and two UEs, as the same approach could applied to cellular LTE-M and other unlicensed LPWANs. LoRaWAN and NB-IoT were selected since they have both gained research and industrial interest with their wide coverage, low-power consumption, good security, and low cost. By introducing additional UEs and eNBs to our testbeds, a better understanding of a larger IoT network performance can be achieved, but we will explore this further in our future work. Although the NB-IoT experiment was performed based on 3 GPP release 13, new features have been introduced to the standard release 16, as discussed in Section 5.2.1. The UE used can only support standard release 13 due to a gap between NB-IoT standardisation and manufacturing. Similarly, LoRaWAN was performed in TTN version 2, latency, and other QoS presented could be enhanced when performed in TTN version 3.  
As an improvement to this study, packet losses due to signal propagation challenges should be considered when planning IoT networks, especially in challenging environments. Link quality is essential in this type of wireless network to determine the type of service deployed in a particular environment and the packet losses. Refer to [25] for the propagation losses' comparison of NB-IoT and LoRaWAN due to transmissions in underwater, underground, and metallic surfaces. NB-IoT is also faced with roaming problems when the service is to be deployed on a global scale. This means that deploying NB-IoT in more than one country will require the ability to establish international commercial roaming agreements with local carriers, produce NB-IoT carrier-specific modules, and create robust SIM and network management systems to provide cross-border IoT services. Based on the Deutsche Telekom and Vodafone NB-IoT roaming trial, it remains challenging to take an NB-IoT module with Deutsche Telekom SIM and use it in another country's NB-IoT carrier, such as T-Mobile in the United States [80].  
Since NB-IoT and LoRaWAN will be deployed in challenging environments, continuous improvements will also be required in data security, self-configuration/organising/healing, cloud and edge computing capabilities, resource mapping and air interface. NB-IoT services are primarily provided by the mobile network operators (MNOs) using their available infrastructure with a decision on the business implications and technical alterations to be made at the base station. However, it is also possible to provide clean slate NB-IoT services but at a higher cost. The air interface is one of the optimisation challenges in the in-band deployment of NB-IoT in an LTE network. It makes use of the core LTE design featuressuch as channel coding, uplink and downlink frequency division access, data rate matching, interleaving, etc. For the efficient coexistence of NB-IoT inside an LTE carrier, orthogonality to the LTE signal must be protected when mapping NB-IoT to LTE resources. The UE needs to be designed to automatically detect the deployment mode and subsequently identify LTE resource blocks. Similarly, to achieve scalability in LPWANs, new LPWAN modules should incorporate adequate memory, processing power, and bandwidth.