## 3 Related Work
### Existing NB-IoT Energy-Efficient Channel Coding (CC) Approaches  
From the survey of the literature, the following main approaches provide a summary of the approaches that have been selected based on their relevance,  
1. **Automatic Repeat Request (ARQ) Approaches** In this category of approaches, the receiver requests re-transmission of data packets if errors are detected, using some error detection mechanisms. Authors in [27] proposes an open-loop forward error correction technique for NB-IoT networks to optimize ARQ signaling. In this approach, signaling only needs to indicate the DL data transfer completion, and does not have to be specific on which particular Packet Data Units (PDU) are lost during the transmission. This allows for the reduction of the simplicity of the channel coding approach, and therefore, allows the saving on computational energy consumption. This approach has been demonstrated to be efficient in enhancing the data rate performance in the Downlink (DL) of the NB-IoT network. Due to its low complexity, this approach has been further proven to also enhance the energy efficiency of the NB-IoT network as it considerably reduces the computational energy consumption due to data reception on the transceiver of the NB-IoT node. An approach proposed in [28] considers using a hybrid automatic repeat request (HARQ) process in scenarios where the NB-IoT network can only support half-duplex operations. The HARQ approach has been demonstrated to be capable of reducing the processing time at the NB-IoT node. The obtained results in [28] demonstrate that the use of the HARQ approach can lead to a saving of up to 20% on the overall energy consumption of the network. This energy efficiency performance has also shown that it is not significantly affected by the increase in the scalability of the NB-IoT network when the HARQ approach is used. The authors in [29] propose a hybrid channel coding approach. It consists of signaling hybrid automatic repeat request (HARQ) acknowledgments for narrowband physical downlink shared channel (NPDSCH) and uses a repetition code for error correction. In this case, the UserEquipment (UE) can be allocated with 12, 6, or 3 tones. However, the 6 and 3 tone formats are introduced for NB-IoT devices which due to coverage limitations cannot benefit from the higher device bandwidth allocation and results in higher energy consumption performance. 2. **Forward Error Correction (FEC) Approaches** Recent research ([30; 31; 32]) has investigated the efficiency of different re-transmission and FEC techniques in NB-IoT systems. Several researchers have quantified the effect of several network parameters on the efficiency of error correction techniques (and their associated network costs). However, no effort has yet been made to unify these studies into a systematic approach that could help with the selection of the most effective technique given certain network conditions. The authors in [33] have proposed an improved error correction algorithm for multicast over the LTE network and, by extension, over the Narrowband IoT network. The model used assumes a random distribution of packet losses and a constant loss rate in each scenario. The model can be expanded to include different error distributions and varying loss conditions during a series of NB-IoT downlink transmissions. The results obtained demonstrate that the use of a hybrid approach (combined HARQ and FEC) outperforms both the HARQ method as well as the FEC approach when used individually, in terms of energy efficiency. The authors in [34] proposed the use of an open-loop forward error correction technique as a mechanism to not only enhance the energy efficiency of the NB-IoT network but also to concurrently achieve efficient downlink data rate performance. The benefit of this approach lies in the fact that it enables extremely reliable firmware downloads which is an important feature in IoT in a number of applications such as wireless sensor network applications.