LTE-M and NB-IoT Definitions
The 3GPP Release 13 standard definitions: category M1 (LTE-M) and category NB1 (NB-IoT)
- Cat-M1: also known as LTE-M, defines a 1.4 MHz channel size and about 375 kbps of throughput. It is optimized for cost, coverage and long battery life, outperforming 2G MTC, while being completely backward compatible with previous LTE standards. In the specifications, these devices are mentioned as band limited / coverage enhanced (BL/CE).3
- Cat-NB1: also known as NB-IoT, defines a 200 kHz channel size and merely 10s of kbps of throughput. It is optimized for ultra low throughput and specifically designed for IoT devices of very low cost and very long battery life. Although NB-IoT shares most features of LTE, it is not backward compatible with previous LTE definitions. Three modes of operation are defined: 1) in-band, 2) in the guard band, or 3) out of band (e.g., in dedicated 200 kHz carriers, when it is possible to reuse 2G carriers).4
kbps of throughput and 1.4 MHz channel size with LTE-M
kbps of throughput and 200 kHz channel size with NB-IoT
One of the key drivers of growth in cellular IoT is continued innovation towards reducing terminal complexity. Device costs must be low enough for the technology to be incorporated into a wide range of applications. The 3GPP definitions provide for some immediate cost savings. For example, the reduced feature set reduces the size of the software stack and therefore the size of the on-volatile memory required. Moreover, category M1 and NB1 terminals require only a single receiver in instead of at least two that are required by higher category devices. 3GPP has also defined a new access scheme for FDD bands called half-duplex FDD (HD-FDD) where transmit and receive data is sent in a shared channel, but in different time slots. This removes the need for expensive duplexers, which in turn helps improve terminal power efficiency because duplexers typically have high insertion loss.
While proprietary solutions such as SigFox and LoRa operate in unlicensed bands, cellular IoT operates in licensed bands, which ensures quality of service and security. As such, cellular IoT terminals will typically have to support many more frequency bands than their non-cellular equivalents. 3GPP currently lists 25 distinct operating bands for category M1 or NB1. More will likely be added in the future. Operation in these licensed bands has traditionally been subject to much more stringent emission requirements as well as rigorous testing of resilience against jammers. Both transmit emission and receive blocking specifications have been defined for architectures using band-specific transmit and receive filters.
Figure 3: Evolution of Front-end Architecture from High-end LTE Device to Sequans’ SAW-less Design
In Figure 3 (b) illustrates a typical category M1 or NB1 architecture, which includes SAW filters in both of the transmit and receive chains for three low bands (699 MHz to 960 MHz) and two mid bands (1710 MHz to 2200 MHz). To share power amplifiers, switches must also be added to route the transmit signals through the correct filter for each operating band. Similarly, if the transceiver device has a limited number of input ports, switches are also needed on the receiver side. It is clear that when several bands are to be supported, the cost of the RF front-end can quickly grow to become the significant portion of overall product cost.
Sequans’ Monarch solution has been specifically designed to eliminate the requirement of using SAW filters in either the transmit or receive chains. This has been achieved by designing the transceiver with very low transmit noise and the receiver with exceptionally good linearity.
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