# Energy Efficiency in Short and Wide-Area IoT Technologies--A Survey
## 2 IoT Technologies
### Low-Power Wide-Area Networks (LPWANs)
#### 2.2.2 Cellular IoT
LTE-M  
LTE-M is a 3 GPP-standardized LPWAN technology, allowing a full reuse of the cellular infrastructure. LTE-M coexists with 2G, 3G, and 4G mobile networks, and benefits from all LTE-native security and privacy features, such as, support for user and mobile equipment authentication and confidentiality, and data integrity [57; 65]. LTE-M devices are best suited to mission-critical applications requiring real-time data transfer, for example, emergency communications in smart cities. LTE-M supports both TDD and Frequency Division Duplex (FDD) modes, using a sub frame structure of 1 ms, making it possible to achieve low latency data exchange. LTE-M offers extended and improved coverage, also for indoor scenarios, and is able to support Voice over LTE with device costs comparable to GSM, ultimately being attractive for different IoT applications. The LTE-M signal bandwidth was initially limited to 1.4 MHz, and the main supported features were frequency hopping and subframe repetitions. Release 14 introduced a 5 MHz bandwidth option, offering peak data rates of 384 kbps for downlink, and up to 1 Mbps for uplink, with a latency between 50 and 100 ms. Being an LTE extension, several LTE features are inherited in LTE-M, for example, Orthogonal Frequency Division Multiple Access (OFDMA) as downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) as uplink access schemes, with equivalent settings in terms of subcarrier spacing, cyclic prefix (CP) lengths, resource grid, and frame structure, to mention a few [58]. This results in LTE-M and LTE transmissions coexisting in the same LTE cell, on the same LTE carrier, and a possible dynamic sharing of resources. LTE-M uses the same channels as LTE and LTE-M physical channels and signals are transmitted using six Physical Resource Blocks (PRBs). The LTE-M frame consists of one hyperfine cycle, which has 1024 hyperframes, each hyperfine containing 1024 frames. One frame consists of 10 subframes, each divided into two slots of 0.5 ms, where each slot is divided into 7 OFDM symbols when we have a normal CP length, and 6 symbols in the case of extended CP length [56]. The physical layer provides services for data transport to the higher layers, using the transport channels via the MAC layer, which in turn transports data through the use of logical channels. NB-IoT  
NB-IoT is another 3 GPP-standardized technology for cellular LPWAN, mainly devoted to uplink transmission of small amounts of infrequent information by a massive amount of devices, for example, for smart agriculture and smart city scenarios [59]. Differently from LTE-M, NB-IoT is a brand new radio technology, and for this reason it is not fully backward compatible with existing LTE devices, even if it is designed to achieve high coexistence with LTE, GSM, and General Packet Radio Service (GPRS) technologies. With respect to LTE, new physical layer signals and channels are designed, in order to meet the demanding requirements in terms of extended coverage and low device complexity [57; 60; 61]. NB-IoT signals occupy a bandwidth of 180 kHz, that is a single LTE PRB, adopting a QPSK modulation over OFDMA, with 15 kHz sub-carrier spacing, in downlink, and BPSK or QPSK modulation over SC-FDMA in uplink, with configurable sub-carrier spacing of either 3.75 kHz or 15 kHz. NB-IoT supports FDD transmission mode since Rel-13, with TDD introduced in Rel-14. With respect to the spectrum occupied within a LTE carrier, three different deployments are possible:  
* _Stand-alone:_ One or more GSM carriers are used to carry NB-IoT traffic, making possible for the operators to ensure a smooth transition for mMTC services;
* _Guard-band:_ NB-IoT devices occupy guard bands of LTE carriers, thus avoiding interference and coexistence issue;
* _In-band:_ NB-IoT devices operate within a single dedicated PRB of an LTE carrier, thus requiring coordination with LTE.