The term Internet of Things was first coined 18 years ago. Today, it is a global phenomenon at the center of the next generation of digital innovation. But its future is not yet assured as IoT needs a robust infrastructure and backend applications able to support the billions of devices that are being connected to the internet.
The majority of IoT devices are simple; equipped with one or more sensors, and a battery, radio, processor and maybe a camera. Many have limited power requirements and can operate for years in remote, harsh environments. Most of these devices are occasionally required to exchange infinitesimal amounts of downstream and upstream data.
Managing the communication needs of IoT devices is a work in progress. There are a number of short-range and long-range wireless network protocols enabling various types of IoT deployments, each with specific use cases. The list includes Bluetooth, BLE, Zigbee, LoRaWAN and many other similar protocols. Even though these protocols have found a market in IoT deployments, they all have limitations, especially when it comes to long-range and mobile use cases.
On the other hand, cellular networks are striving to evolve next-generation networks that provide the infrastructure to support higher bandwidth and lower latency. This makes them best suited for data-intensive and time-sensitive applications but inefficient for IoT applications that require lower bandwidths, lower data rates and reduced transmit power.
Taking note of these limitations, 3GPP introduced two new standards in its LTE Release 13: Enhanced Machine-Type Communication (eMTC) and NarrowBand-IoT (NB-IoT). Both are intended to cater to the requirements of IoT devices but are fundamentally different in approach. What’s more, both come with their own set of pros and cons. This blog focuses on the eMTC standard.
eMTC has other names. It is referred to as BL/CE where BL stands for bandwidth reduced, low complexity and CE stands for coverage enhancement. It is also referred to as LTE-M1 or CAT-M1.
LTE-M1 suggests some fundamental changes and improvements to eNodeB functionality for it to be able to support user equipment (UE) operating at significantly reduced power in poor coverage areas. Some of the primary enhancements include support for narrowband, cross-subframe scheduling, half duplex FDD support, multiple repetitions of data and control information, and more. It is expected that an eNode may support both LTE and eMTC UEs while sharing the same system bandwidth.
Narrowband (Reduced Bandwidth)
A narrowband is defined by 3GPP as six non-overlapping consecutive physical resource blocks in the frequency domain. An eMTC UE is only required to monitor a specific narrowband for upload and download (UL/DL) transmissions as against the complete system bandwidth in traditional LTE. For eNodeB, multiple narrowbands can be supported simultaneously and used to broadcast system information or schedule dedicated channels. eNodeB may also choose to schedule transmissions across narrowbands to attain better coverage and signal quality for all the UEs being served. However, the same UE is not expected to be transmitting or receiving on more than one narrowband or across multiple narrowbands at any given time.
Half Duplex FDD
When operating in Frequency Division Duplex (FDD), LTE-M1 also supports Half-Duplex mode, which means a UE shall either transmit or receive at a given time; there are no simultaneous UL/DL transmissions.
MPDCCH: The new control channel
Since LTE-M1 UEs only care about a narrowband of six RBs at a time, the traditional LTE channels that are carried in whole-system bandwidth (PDCCH, PHICH, PCFICH) cannot be reused for eMTC. This necessitates a need for another mechanism to send control information to UEs. Also, since eNodeB is expected to support both LTE and eMTC devices simultaneously, the new channel for eMTC should not interfere with LTE operations. A new channel called MPDCCH has been introduced for eMTC, which uses the resource blocks from within the PDSCH segment of regular LTE. (See Figure 1.)
Figure 1: MPDCCH is a new channel for eMTC
Several new DCI formats have been introduced to carry the control information for LTE-M1 UEs. DCI6-0A and DCI6-0B are used for the uplink grant when CAT-M1 UE is running in CE Mode A and CE Mode B, respectively. DCI6-1A and DCI6-1B are used for downlink assignments when CAT-M1 UE is running in CE Mode A and CE Mode B, respectively. The DCI6-2 format is used to carry the control information for paging messages specifically meant for Cat-M1 UEs.
Cross-subframe scheduling is the process of scheduling data and the corresponding control information for a UE in different subframes. This is contrary to the legacy LTE scheduling where data for a given UE is scheduled in the same subframe in which the control information is sent.
Since there are only six RBs available in a narrowband in one subframe, these aren’t always enough to accommodate both PDSCH and MPDCCH. Cross-subframe scheduling also gives UEs enough time to decode DCI information carried in MPDCCH and to prepare to receive PDSCH before the data arrives in subsequent subframes. (See Figure 2.)
Figure 2: Cross-subframe scheduling in LTE-M1
Coverage enhancements (CEs) are required to support IoT devices deployed in remote areas with limited coverage. The CE is typically achieved through repeating the transmissions. In typical LTE operations, each transmission is carried in a 1-millisecond span, but in eMTC, each transmission can be repeated tens, hundreds or even thousands of times based on the CE modes to improve the chances of a successful transmission.
The LTE-M1 specification has defined two CE modes: Mode A and Mode B. (See Figure 3.)
Figure 3: Coverage enhancement modes for LTE-M1
While CE Mode A provides moderate coverage enhancements and is mandatory for eMTC support, CE Mode B enables deep coverage but is optional. Also, CE Mode A supports functions and features such as connected mode mobility and multiple transmission modes.
An eMTC UE is required to support CE Mode A. It may or may not support CE Mode B. The UE indicates a CE level (from 0 to 3) during RACH procedure, based on which eNodeB initially decides the mode for that UE. eNodeB can subsequently select an appropriate mode for a UE based on the signal quality periodically reported by the latter. Typically, a UE is kept in CE Mode A unless it enters a significantly bad coverage area.
While eMTC has competition from both proprietary and standardized solutions, it carries the potential to emerge as the undisputed winner and the choice for IoT network solutions.
Aricent’s LTE software framework solutions (eNB or CN) are widely deployed by customers around the world. Aricent’s LTE eNodeB stack software is a well-known and proven base framework for building LTE eNodeB products. It comes with out-of-box support for many LTE advanced features and is highly flexible in terms of supporting varied deployment architectures including various Cloud-RAN split architectures.
The Aricent’s eNodeB software framework now also supports the LTE Release 13 eMTC feature. Aricent has licensed its eMTC software (L2 and L3+, both individually and together) to OEMs around the world. Aricent also specializes in providing framework-specific services, which includes integration, porting, customization and testing of integrated software framework components on the client’s product platforms.