Massive Access

In contrast to LTE/4G, the 5th Generation of mobile networks (5G) considers a 100times increase in the number of connected devices and a variety of new device- and service classes like Machine-type Communication (MTC) and Internet-of-Things (IoT), with harsh requirements on the transmission regarding latency and reliability, specifically in synchronous operation. Therefore, within 5G research, the term “Massive Access” refers to the development of new access strategies to cope with the increased number of connections for typical use-cases.

One specific challenge on the random access (RA) is the initial detection of users with very small data packets and very low latency requirements, typically for MTC scenarios [1]. Due to the high signaling overhead of the RA procedure in 4G and the relative small packet size, LTE is not suitable to support low-latency applications for a massive number of devices. Therefore, “One-Shot-Transmission” concepts are under discussion for 5G [2] which considers transmission of control signaling and payload at the same time in the physical layer random access channel and reduces the signaling overhead – and hence the latency – dramatically.

Another challenge on the physical layer is the multiple-user multiplexing, i.e. the efficient sharing of the overall available bandwidth among different users. With 4G/LTE, orthogonal multiple access based on frequency division are implemented (OFDMA / SC-FDMA), which divides the physical resources in an orthogonal time-frequency grid. The allocation of orthogonal resources eases the radio resource management (RRM) within one cell, since the signals of multiple users signals are independent and do not interfere with each other. Hence, inter-cell interference management can be implemented easily. However, the limitation of an orthogonal multiple access (OMA) scheme is the stringent requirement of synchronization within the network, which is hard to achieve in a dense deployment with high velocity and asynchronous operation – typically for 5G. Further, any resource in the orthogonal grid can only be reused once within one cell, which limits the efficiency and number of connected devices in the cell. Therefore, for 3GPP Release-15, non-orthogonal multiple access (NOMA) schemes are discussed within RAN1 [3]. In contrast to orthogonal MA, individual user data are superimposed using spreading, coding, multi-dimensional modulation and/or power allocation – based pre-processing (see Figure 1) such that the physical resources within one cell can be reused multiple-times. Thus, the number of connections increases.

The structure of the received superposition of all users signals requires an iterative receive processing (MPA, SIC, etc.) in order to decode all independent data streams. Thus, receiver complexity is one of the limiting factors. Therefore, within the One5G – project [4], an integration of NOMA concepts into Massive – MIMO antenna systems is investigated, taking advantage of spatial multiplexing in order to reduce interference between co-scheduled users and to enable less complex receiver structures within a NOMA scheme.

References:

[1] G. Wunder, M. Kasparick and P. Jung, "Interference Analysis for 5G Random Access with Short Message Support," Proceedings of European Wireless 2015; 21th European Wireless Conference, Budapest, Hungary, 2015, pp. 1-6.

[2] C. Bockelmann et al., "Massive machine-type communications in 5g: physical and MAC-layer solutions," in IEEE Communications Magazine, vol. 54, no. 9, pp. 59-65, September 2016.

[3] RAN1 TR 38.802 v2.0.0 on Study on New Radio (NR) Access Technology; Physical Layer Aspects

[4] One5G, Projekt Webseite