## 2 Mobile M2M Literature Review
### M2M Support in Cellular Networks  
Nowadays cellular networks offer wide coverage areas, high data rate, and decreasing latency, and therefore they are a key enabler of M2M communications. The challenges associated with mass-scale M2M networks can be resumed to the multitude and diversity of devices, the scalable connectivity, and supporting of both legacy and new services and devices [25]. [PERSON] _et al_. [26] argue that, even in the presence of regular LTE traffic, mobile M2M traffic can not be considered negligible, and it can have a dramatic impact on the LTE network performance in terms of Quality of Service (QoS) and throughput. [PERSON] _et al_. [27] evaluate the performance of an LTE gateway using CoAP and representative M2M traffic patterns and network configurations through simulations. The authors argue that traffic patterns depend very much on the single application considered, and, therefore, do not describe or justify their choices. The scenario consists of a single LTE cell where the evolved NodeB (eNB), the only mandatory node in the radio access network (RAN), serves one LTE M2M gateway and a variable number of terminals with traditional Internet traffic, called H2H User Equipments (UEs). The LTE M2M gateway, in turn, serves a variable number of smart objects. The results showed that LTE is sensitive to both intra-UE and inter-UE signal interference, which results in a high delay or packet loss when the number of smart objects served is greater than a few tens or the cell throughput approaches its limits. [PERSON] _et al_. [28] describe algorithms for device management to mitigate interference and device co-existence in LTE. Similar to wireless networks, M2M communications for cellular networks can benefit from improvements on channel access or by introducing data aggregation techniques. A contention based uplink channel access for M2M in an LTE network is proposed by [PERSON] _et al_. [29]. With contention based access, UEs select resources randomly without indications from eNB, which saves signalling overhead and, hence, latency is reduced. Simulation results showed that a network coordinated random access stabilization scheme used to control the expected number of simultaneous access to a common random access channel (RACH) can effectively improve the access delay in LTE-Advanced [30]. [PERSON] _et al_. [31] study the impact of data aggregation in M2M on throughput and delay in a cellular network. They motivate the use of an M2M relay as an M2M data aggregator to improve uplink transmission efficiency in LTE-Advanced due its overheads. They propose a tunnel-based aggregation scheme in which only M2M data units destined to the same tunnel exit point are aggregated at the tunnel's entry point, according to priority classes. The results show a significant reduction in protocol overheads. Furthermore, the results show that aggregation, as expected, increases the delay per unit in the delivery, but the global delay can rapidly decrease with the increase of M2M devices. Transmission scheduling schemes can be introduced in mobile M2M communications to reduce delay or to achieve higher energy consumption efficiency. [PERSON] _et al_. [32] achieves a delay reduction in a remote monitoring system by using a transmission scheduling scheme used in an Evolution-Data Optimized (EVDO) and in a Wideband Code Division Multiple Access (W-CDMA) networks. They achieve a probability of sensor status not reaching a monitoring center within a delay of 6 s lower than \(10^{-6}\) compared to the probability of \(10^{-4}\) for a best-effort effort scheme. This transmission scheduling scheme achieved more than 85% of average throughput compared to the best-effort scheme for the W-CDMA network, while having a similar performance for the EVDO network. [PERSON] _et al_. [33] study the viability of using the General Packet Radio Service (GPRS) for a low data rate long-lasting battery powered operation of M2M devices.