# Priority-Based Machine-To-Machine Overlay Network over LTE for a Smart City
## 1 Introduction  
In many smart city application scenarios--from building monitoring and healthcare monitoring, through smart parking and smart city lighting, to crowd sensing and vehicular safety applications--a large number of smart devices send their messages to appropriate servers for further analysis and actions, as Figure 1 schematically shows. As monitoring and transmission oftentimes do not require human intervention, such communications are referred to as Machine-to-Machine (M2M) or Machine-Type Communications (MTC) [1]. Networks that support M2M traffic must provide broad coverage, but also message reliability and limited delay. In densely-populated urban areas, such requirements can be achieved through WiFi networks, but in sparsely-populated rural areas and along the highways, one must rely on cellular networks such as LTE/LTE-A (Long-Term Evolution and Long-Term Evolution-Advanced) [2]. In some M2M scenarios, messages arrive regularly with approximately constant inter-arrival times and thus can be transmitted using some kind of scheduled access; this is the case, for example,for healthcare applications, building monitoring and smart city lighting. In other cases such as crowd sensing and vehicular safety, messages arrive randomly and thus may be serviced through contention-based access. In both cases, messages are short, and typical interarrival periods are low when compared to other traffic such as video and data transmitted over LTE. As a result, M2M messages can be transmitted using LTE's Physical Random Access Channel (PRACH), which was originally intended to be used for initial access or area tracking by a terminal (User Equipment (UE)) that is not connected to the base station (eNodeB), for uplink synchronization of a UE that is connected to eNodeB, when a connected UE has to transmit uplink data or to acknowledge received data or when a UE needs to perform a handoff to the target cell [2]. The LTE standard prescribes that PRACH access be performed using a four-way handshake, which is contention-based (the details of random access are presented further in Section 2 below). If PRACH is congested due to a large number of M2M and/or H2H terminals attempting access concurrently, the SINR observed at the receiver (i.e., eNodeB) may be reduced to the extent that messages cannot be detected, and consequently, many of the access attempts will fail; this is denoted as the outage condition. In the past several years, a number of techniques to improve the performance of M2M traffic by alleviating outage due to congestion have been suggested:  
1. UEs may be grouped into traffic classes for which random access may be delayed or temporarily blocked. This technique is known as access class barring (which is already supported for H2H calls) and extended access class barring [3]. 2. Differentiation among traffic classes can be achieved by allowing different backoff windows for different classes of UEs [4]. 3. Further differentiation among the classes can be achieved by allowing members of the traffic class to attempt access only in predefined time slots within specific LTE frames, using base station scheduling [5]. 4. The base station can apply a suitable polling scheme (also known as pull-based scheme) to differentiate between UEs. In this scheme, M2M terminals initiate random access only after being paged by the eNodeB [6]. 5. PRACH resource separation and dynamic PRACH resource allocation schemes allocate different PRACH resources such as preamble sequences and random access slots to different types of traffic (i.e., H2H and M2M) in a dynamic manner [7]. 6. Traffic classes can be differentiated by using different power levels rather than using power ramping based on the age of the attempt [8]. 7. Failed calls may be allocated a specific set of preambles to use for repeated access attempts [9]. Recently, an overlay network for M2M traffic was proposed that dedicates a portion of PRACH resources to M2M traffic [10].