Figure 6 provides a perspective on the M2M requirements by plotting the data rate vs. the
device population size. The figure illustrates where systems currently stand and how the research efforts are expanding them. Area R1 reflects the operating range of today’s systems, outlining the fact that the device data rate decreases as its population increases. In turn, R2 is the region that reflects current research aimed at improving the spectral efficiency. Finally, R5 indicates the region where operation is not feasible due to fundamental physical and information-theoretical limits. Regions R3 and R4 correspond to the emerging services discussed in this section:
• R3 refers to massive M2M communication where each connected machine or sensor transmits small data blocks sporadically. Current systems are not designed to simultaneously serve the aggregated traffic accrued from a large number of such devices. For instance, a current system could easily serve 5 devices at 2 Mb/s each, but not 10,000 devices each requiring 1 kb/s.
• R4 demarks the operation of systems that require high reliability and/or low latency,
but with a relatively low average rate per device. The complete description of this region requires additional dimensions related to reliability and latency. There are services that simultaneously pose more than one of the above requirements, but the common point is that the data size of each individual transmission is small, going down to several bytes. This profoundly changes the communication paradigm for the following reasons:
• Existing coding methods that rely on long code words are not applicable to very short
data blocks.
• Short data blocks also exacerbate the inefficiencies associated with control and channel
estimation overheads. Currently, the control plane is robust but sub optimal as it represents
only a modest fraction of the payload data; the most sophisticated signal processing is reserved for payload data transmission. An optimized design should aim at much tighter coupling between the data and control planes.