NTN Part 2 – NB-IoT over Satellite

In the previous post on the topic, I’ve been looking at how 3GPP has studied and then extended the LTE and 5G NR air interface so it can be used over a satellite link. For transmitting data to and from smartphones with omnidirectional antennas over satellites, 3GPP has extended the LTE Narrowband Internet of Things (NB-IoT) air interface. But why a narrowband technology for Internet of Things applications, all the hype is around satellite connectivity for smartphones these days!? Thanks to the experience gained with my Garmin InReach Mini 2, I think I have a pretty good idea why. So let’s have a look at the limitations of a satellite link to small mobile devices and which nifty features that are part of terrestrial NB-IoT can also be very useful for this type of satellite communication.

The Problem Space

When we think about smartphones today, we associate datarates of several hundreds megabits per second and network availability pretty much anywhere, i.e. both indoors and outdoors. All of that goes away when we talk about satellite communication to small devices with a unidirectional antenna. In practice, here is what happens:

  • Limited Datarate: Without a large parabolic antenna, the datarate is limited to a few kilobits per second. In other words, most applications that are used on smartphones won’t work over such a link. Also, this means that the signaling overhead for establishing and maintaining a connection must be as low as possible.
  • High Cost per Bit: Current satellite constellations like Iridium and Globalstar charge around $1 per kilobyte! Even if next generation satellite constellations would be an order of magnitude cheaper, data transmission would still be significantly more expensive than what we are used to from terrestrial networks today.
    So what about Starlink you might wonder? Dozens of megabits per second and 80$ a month are far away from $1 per kilobyte. True, but a large parabolic dish allows to use the air interface to and from a satellite much more efficiently, which significantly lowers cost. That’s perhaps not the only reason why Starlink is cheaper, but it definitely contributes a lot.
  • Outdoor Only: Data transmission is only possible outdoors or when the device is indoor and close to a window that doesn’t have an invisible heat insulating layer.
  • Limited Availability: Network availability depends on how much of the horizon and how much of the sky is visible. This means that in many outdoor scenarios, data transmission is only possible for a few seconds before the satellite disappears behind the horizon or an object blocks the transmission path. This can be somewhat improved by having more satellites in the sky. Iridium currently uses 66 satellites, which means that there is only one or two satellites over all of Europe, at any time. Starlink, on the other hand has over 3000 satellites in orbit today, so more of them could be visible, which would significantly improve availability.
  • Push vs. Pull: In many mobility scenarios it is often the case that the device will be unable to see a satellite for prolonged amounts of time. Incoming data might thus sometimes have to be buffered in the network for the downlink or in the device for the uplink for many minutes or hours.
  • Is IP the right protocol? Yes, terrestrial cellular networks pretty much use the Internet Protocol (IP) for everything. But for such slow, unreliable and often unavailable links, is it the right protocol? Even if only UDP is used?

How LTE NB-IoT Addresses These Limitations

The good news is that all of these limitations are also present in terrestrial networks when aiming to send small amounts of data to very power efficient Internet of Things (IoT) devices in places with marginal network coverage. NB-IoT has been around since at least 2016 and is deployed in many LTE networks around the globe, so it’s a proven and mature technology. And here’s how it addresses the issues mentioned above:

  • A narrow channel: Instead of channel bandwidths of between 5 and 100 MHz used in LTE and 5G networks today, NB-IoT channels are just 180 kHz wide. Like in a full LTE/5G channel, 15 kHz sub-carriers (‘tones’) are used, which allows to deploy NB-IoT inside normal LTE channels. Limiting the channel bandwidth to 180 kHz allows to focus the available transmission power in a much narrower channel, which improves range. This is not only very useful for terrestrial networks, but obviously also when communicating with a satellite in space.
  • Extended Idle Mode Discontinuous Reception (DRX): A typical RRC Idle state paging interval in terrestrial LTE networks is 1.28 seconds. This means that a mobile device has to wake up once a second to see if there is an incoming IP packet waiting. On the NB-IoT air interface, this interval can be extended to a maximum of around 43 minutes, which greatly helps to reduce power consumption. My Garmin inReach Mini 2 device that uses the Iridium network, for example, only checks once an hour for incoming messages. It’s not IP based and there is no paging, but it shows that such a long paging interval could indeed be very useful for satellite communication.
  • Extended Buffering of MT data: This is a feature that plays together with the Extended Idle Mode. If there are incoming packets arriving from an external network such as the Internet while the mobile is in extended Idle Mode DRX, the S-GW is asked by the MME to buffer the incoming packets until the paging can be sent.
  • Power Save Mode: A mobile device can request to completely power down its receiver after a configured paging availability time and not be available for further pagings until an agreed time. This way, a device can stay registered for many days without having to listen to incoming pagings or having to send occasional tracking area updates.
  • Use of the control plane for small amounts of data: NB-IoT has a mode in which small amounts of data can be included in RRC signaling messages. This significantly reduces the signaling overhead.
  • Non-IP Data Delivery (NIDD): For some applications, perhaps even for sending short text based messages to mobile devices via satellites, the Internet Protocol might be too much of an overhead. For such cases, data can be sent over an IP (UDP or TCP) connection to a gateway, which then removes the overhead and sends the user data over the MME to the mobile device. This setup pretty much resembles of how I think Garmin’s inReach service works over Iridium. And it looks like some operators support this data delivery method in their terrestrial LTE NB-IoT networks today. Verizon, for example, offers NIDD over the NB-IoT Control Plane as described here.
  • Resuming RRC Connections: For applications that occasionally send or receive data, another way to reduce overhead is to suspend the RRC signaling connection between the device and the network instead of terminating it. Signaling can then resume later without going through things like authentication, UE capability exchange, ciphering setup, etc.

So much for today. If you want to know more about those features, have a look at my NB-IoT posts I’ve written many years ago.