Overload control of massive random access for machine-type communications
Introduction
Cellular systems are expected to play an important role in the deployment of machine-type communications (MTC) or machine-to-machine (M2M) devices due to a widely deployed infrastructure and the support of device mobility. However, the cellular systems may not be optimal to support MTC devices because a large number of MTC devices are expected to be deployed in a specific area and most MTC devices transmit and receive a small amount of data. The massive MTC devices would generate a huge amount of information flows that cause contention for radio resources or congestion in the radio access network (RAN).
The 3rd Generation Partnership Project (3GPP) has classified MTC as an important area for the Long-Term Evolution (LTE) and LTE-Advanced systems (3GPP TR, 2011). It also has discussed the traffic characteristics of MTC applications and possible RAN improvements to support MTC. The first priority area for the RAN improvements is the RAN overload control that handles the random access (RA) of massive MTC devices. 3GPP has identified several improvements to control RA overload: access class barring, RA resource separation, dynamic allocation of RA resources, MTC-specific backoff, slotted access and pull-based schemes (3GPP TR 37.868, Laya, Alonso, Alonso-Zarate, 2013, Lin, Lee, Cheng, Chen, 2014). The access class barring can reduce the RA load by preventing MTC devices from initiating RA, but may increase the RA delay unnecessarily (Laya et al., 2013). The RA resource separation in Lin, Lee, Cheng, and Chen (2014) gives a higher priority to the non-MTC devices by allocating RA resources differently to MTC and non-MTC devices. However, it may provide limited benefits due to the reduced RA resources and a low priority for MTC devices.
In this paper, we focus on the fact that a retransmission mechanism of the LTE RA may lead to performance degradation because retransmitted RA attempts increase the possibility of collision, especially in an overload situation. This paper presents two possible solutions that can relieve the RA traffic overload. First, since the probability of collision is closely related with the number of simultaneous RA attempts, it is reasonable to evaluate the impact of the number of RA retransmissions on the RA performance. Second, this paper proposes a resource separation scheme where, unlike the conventional RA resource separation between MTC and non-MTC devices, RA resources for MTC devices are separated into two subsets that MTC devices can access according to the number of consecutive RA failures. The two solutions are analyzed by a mathematical model and a more realistic environment is considered in simulations.
Section snippets
Random access procedure
In LTE, the RA is an important procedure to establish wireless links between the user equipment (UE) and the network (Dahlman, Parkvall, & Skold, 2011). When a UE has packets to transmit, it performs the RA through the physical random access channel (PRACH) in an allowable access slot. The RA process is triggered by a UE for several purposes, including (1) to establish a radio link for initial access to the network, (2) to re-establish a radio link after radio-link failure, (3) to support
Overload control for massive random access
The LTE RA suffers from congestion when a large number of UEs try to access the network at the same time because the collision probability rapidly grows with the number of contending UEs. 3GPP has identified several improvements to deal with the network overload (3GPP TR, 2011) and a comprehensive survey for overload control has been summarized in Laya et al. (2013).
In LTE, all UEs are members of one out of 10 randomly allocated categories defined as access classes (ACs) 0–9, and some UEs may
Motivation and overload control solutions
3GPP has defined two traffic models to evaluate the performance of overload control schemes under different MTC access intensities. In traffic model 1, MTC devices access the network uniformly over s. In traffic model 2, MTC devices access the network following a beta distribution over s. Traffic model 2 is considered as an extreme scenario in which a large number of MTC devices access the network in a highly synchronized manner, for example, after a power outage (3GPP TR, 2011). The
Analysis model for random access
We derive RA channel efficiency and success probability for the LTE and resource separation schemes. Since the entire RA procedure is too complex to be formulated, we assume that there are N MTC devices and the RA is successful if a unique preamble is selected by a single MTC device.
Fig. 4 shows the assumed RA model for the LTE and resource separation schemes. Each MTC device in an idle state begins a new RA with probability p in each RA slot and selects one of the available RA preambles. If
Simulations
Although the simplified analysis model can offer a meaningful insight into the performance of the LTE and resource separation schemes, it is worthwhile to consider a more realistic environment by means of the protocol-level simulator recommended in 3GPP TR (2011). Of the two RA traffic models defined by 3GPP, traffic model 1 assumes that MTC devices access the network uniformly over a distribution period of s, whereas traffic model 2 assumes that a large number of MTC devices access the
Conclusions
This paper has proposed two RA overload control solutions for the massive machine-type communications. Since the retransmission mechanism of the LTE RA may lead to performance degradation due to the high possibility of collision in an overload situation, the proposed solutions were devised to reduce the number of simultaneous RA attempts. The first solution is a parameter optimization scheme that adjusts the maximum number of RA attempts. The second solution is a resource separation scheme, in
Acknowledgments
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2013R1A1A2010496). This research was also supported by SK Telecom.
References (11)
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