## 3 Related Work
### Efficient Selection of Modulation Coding Scheme (MCS)  
The design of an energy-efficient channel coding scheme for NB-IoT is directly linked to the selection of an appropriate modulation coding scheme (MCS). In order to achieve long-range communication, some work on efficient NB-IoT designs found in literature [3; 21; 45; 46] have proposed efficient techniques for modulation scheme selection. The common idea behind most proposed approaches consists of trading off high data rates for higher energy in each transmitted bit (or symbol) at the physical layer (PHY). This design technique allows for a signal that is more immune and that can travel longer transmission distances. Therefore, in general, the identified aim of most designs is to achieve a link budget of \(150\pm 10\) dB which can translate into a few kilometers and tens of kilometers in urban and rural areas respectively [3]. The reason why a value of \(150\) dB is used because it represents the lowest possible detectable radio signal power by the most sensitive possible radio receivers with noise sensitivity levels as low as \(-150\) dBm which translates to 10-6 pico-watt reception power as per the following mathematical deduction,  
\[\begin{cases}-150=10\times log(\frac{P_{Rx}}{\Gamma\ mW})\\ \frac{P_{Rx}}{10^{-3}}=10^{-15}\\ P_{Rx}=10^{-18}\ Watts=10^{-6}\ picoW\end{cases} \tag{22}\]  
These types of radio receivers are often used in the most sophisticated applications such as telemetry applications for example [47].  
Encoding more energy into signal's bits (or symbols) results in very high decoding reliability on the receiver side. Typical receiver sensitivities could, therefore, be as low as \(-130\) dBm. Modulation techniques used for most LPWAN technologies can be classified into two main categories namely narrowband techniques and spread spectrum techniques. Spread spectrum techniques spread a narrowband signal over a wider frequency band but with the same power density. The actual transmission is a noise-like signal that is harder to detect by an eavesdropper, more resilient to interference, and robust to jamming attacks (secure) [48].  
As opposed to other LPWAN technologies such as the LTE Cat-M1, which mainly uses spread spectrum modulation techniques, most work on NB-IoT designs found in literature [3; 45; 48], propose the use of narrowband modulation techniques. In general, narrowband modulation techniques provide a high link budget which is often less than 25 kHz. They are very efficient at frequency spectrum sharing between multiple links and they experience very low noise levels within each narrow band. To further reduce the experienced noise, some LPWAN technologies such as SIGFOX, WEIGHTLESS-N, and TELENSA, use ultra narrowband (UNB) of width as short as 100 Hz [45]. They are, therefore, susceptible to achieving longer transmission ranges.  
One of the major differences between narrowband modulation techniques and spread spectrum techniques remains that spread spectrum techniques often require greater processing gain on the receiver side to decode the received signal(below the noise floor) while no processing gain is required to decode the signal at the receiver through frequency de-spreading for the case of narrowbandmodulation techniques. This results in simpler and less expensive transceiver designs. Different variants of spread spectrum techniques are used by existing standard LPWAN technologies such as Chirp Spread Spectrum (CSS) and Direct Sequence Spread Spectrum (DSSS).