In part 1 on this topic I’ve been giving a quick overview of Machine Type Communication (MTC) and Internet of Things (IoT) from a cellular network operator and 3GPP point of view. In this post I’ll jump right to the first couple of enhancements made the 3GPP LTE specifications over the years to show which enhancement could be used for which kind of MTC and IoT usage scenario and devices.
LTE Cat-1 Devices
Perhaps this is surprising but even the very first version of the 3GPP LTE (EUTRAN) specification (3GPP Release 8) contains device Category 1 (Cat-1) for simpler and more power efficient devices which are only required to support a maximum throughput of 10 Mbit/s. To drive down complexity, Cat-1 was the only device category back in 2008 that was allowed to have a single antenna, i.e. it is not MIMO (Multiple Input Multiple Output) capable. This is the theory, but in the last 8 years this device type has not been very popular and I haven’t seen any embedded devices that use such a modem yet.
LTE Cat-0 Devices and PSM
3GPP then went ahead many years later in Release 12 and defined LTE Device Category 0 (Cat-0). Devices can be further stripped down by limiting the supported data rate to 1 Mbit/s. Half-duplex transmission and reception, which is optional, can additionally reduce cost, complexity and power consumption by replacing duplex filters with a transmit/receive switch, i.e. a device can not send and receive at the same time.
In addition, Power Save Mode (PSM) mode was specified. It extends the LTE specifications with an additional radio interface state. Previously a device could either have a radio connection established to the network (RRC connected) or it could not be connected physically (RRC Idle) while remaining logically connected an keeping its IP address. Even if not connected a device can still receive data as the network sends a paging message to wake it up. PSM extends this scheme by allowing the device to keep its IP address but to stop listening for incoming paging requests for very long durations. The maximum time specified is 12.1 days. While in PSM state a device does not even have to send periodic Tracking Area Updates. The downside is that the device can’t be reached from the outside once it is in PSM state. As different applications require different PSM times, the device requests a timer value that controls how long the device remains reachable before it enters PSM state for power saving but without being reachable. A second timer value sent in the LTE Attach Message is how long the device would like to be in PSM state before it wakes up again. The network can then confirm and modify those values in the Attach Accept message.
While PSM does not require physical layer changes on the radio interface, NAS changes are required to exchange timer values and to make the core network aware which devices are reachable when incoming data packets arrive and which are not. Unlike for Cat-1 devices which will work in any LTE network today, Cat-0 devices were only specified in 3GPP Release 12, and therefore software updates on the network side are required for supporting them.
So much for today. In part 3 and 4, I’ll have a closer look at CAT-M1 and Cat-M2 (NB-IOT) extensions of 3GPP for LTE.