Cellular IoT – Part 9 – 50.000 devices per cell

There is an interesting number floating around in various whitepapers and articles on IoT: NB-IoT supports 50.000 devices per sector per cell. So where does that number come from and is it realistic?

The number results from the system design and simulations with a huge number of assumed parameters in 3GPP TR 45.820. For Narrowband-IOT, Chapter 7.3.6 is the place to look for the capacity evaluation. For the 50.000 devices per sector per cell the following main parameters were used:

A single 200 kHz carrier with 5 uplink transmissions per hour of a single IP packet in each transmission with 20 bytes of user data (105 bytes including all overhead).

Yes, that sounds very little but a lot can be encoded in 20 bytes 5 times an hour. Taken together with a lot of parameters to describe the network setup, network access, uplink grants, downlink assignments, and radio conditions the simulation shows that 50.000 devices per sector are feasible with a single 200 kHz carrier.So let’s do a quick verification of that number via a totally different approach: A 200 kHz NB-IOT channel transfers 12 signals on the frequency axis and 14 signals in the time axis per millisecond. Let’s be optimistic and assume most uplink transmissions are QPSK modulated, which would be 12*14*2 bits/ms * 1000 1/2 = 336.000 bit/s. Reference signals, synchronization signals and other overhead such as system information broadcasts and redundancy need to be subtracted. 50.000 devices sending 105 bytes 5 times an hour produce 26.250.000 bytes of data per hour or, divided by 60 minutes, 60 seconds and multiplied 8 bits to the byte 58.333 bits/s. In other words, the amount of data fits well into the raw channel even if all other overhead mentioned above is added. Another interesting number is how often a random access request will occur with 50.000 devices in a sector communicating 5 times an hour. Multiplying the number of devices with 5 attempts an hour and dividing it by 60 minutes, 60 seconds there will be 70 RACH attempts per second, or one every 14 milliseconds. Again, that sounds manageable.
So yes, 50.000 devices per sector transferring only very little data seem to be possible even with only a single 200 kHz bearer.