Chapter 5

LTE-M

Abstract

In this chapter, we describe the Long-Term Evolution for Machine-Type Communications (LTE-M) technology with an emphasis on how it is designed to fulfill the objectives that LTE-M targets, namely achieving low device cost, deep coverage and long battery lifetime while maintaining capacity for a large number of devices per cell, with performance and functionality suitable for both low-end and mid-range applications for the Internet of Things.

Section 5.1 describes the background behind the introduction of LTE-M in the Third Generation Partnership Project (3GPP) specifications and the design principles of the technology. Section 5.2 focuses on the physical channels with an emphasis on how these channels were designed to fulfill the objectives that LTE-M was intended to achieve. Section 5.3 covers LTE-M procedures in idle and connected mode, including all activities from initial cell selection to completing a data transfer. The idle mode procedures include the initial cell selection, which is the procedure that a device has to go through when it is first switched on or is attempting to select a new cell to camp on. Idle mode activities also include acquisition of system information, paging, random access, and multicast. Descriptions of some fundamental connected mode procedures include scheduling, power control, mobility, and positioning. Both the fundamental functionality introduced in 3GPP Release 13 and the improvements introduced in Release 14 and Release 15 are covered. Finally, coexistence between LTE-M and Fifth Generation (5G) New Radio (NR) is presented in Section 5.4. The performance of LTE-M including its fulfillment of 5G mMTC requirements is covered in Chapter 6

Keywords

5G; 5G migration; Access barring; Blind decoding; Cell selection; Channel coding; Coexistence; Connected mode; Early data transmission; Extended coverage; Frame structure; Frequency hopping; Idle mode; LTE-M; Master information block; Mobility; Modulation; MPDCCH; New radio; Paging; PBCH; PDSCH; Power control; Power head room; PRACH; PSS; PUCCH; PUSCH; Random access; Scheduling; Search space; Synchronization; SSS; System information block; Wake-up signal

1. 5.1 Background 
   1. 5.1.1 3GPP standardization 
   2. 5.1.2 Radio Access Design Principles 
      1. 5.1.2.1 Low device complexity and cost 
      2. 5.1.2.2 Coverage enhancement 
      3. 5.1.2.3 Long device battery lifetime 
      4. 5.1.2.4 Support of massive number of devices 
      5. 5.1.2.5 Deployment flexibility 
      6. 5.1.2.6 Coexistence with LTE 
2. 5.2 Physical layer 
   1. 5.2.1 Physical resources 
      1. 5.2.1.1 Channel raster 
      2. 5.2.1.2 Frame structure 
      3. 5.2.1.3 Resource grid 
   2. 5.2.2 Transmission schemes 
      1. 5.2.2.1 Duplex modes 
      2. 5.2.2.2 Narrowband and wideband operation 
      3. 5.2.2.3 Coverage enhancement modes 
   3. 5.2.3 Device categories and capabilities 
   4. 5.2.4 Downlink physical channels and signals 
      1. 5.2.4.1 Downlink subframes 
      2. 5.2.4.2 Synchronization signals 
      3. 5.2.4.3 Downlink reference signals 
      4. 5.2.4.4 PBCH 
      5. 5.2.4.5 MWUS 
      6. 5.2.4.6 MPDCCH 
      7. 5.2.4.7 PDSCH 
   5. 5.2.5 Uplink physical channels and signals 
      1. 5.2.5.1 Uplink subframes 
      2. 5.2.5.2 PRACH 
      3. 5.2.5.3 Uplink reference signals 
      4. 5.2.5.4 PUSCH 
      5. 5.2.5.5 PUCCH 
3. 5.3 Idle and connected mode procedures 
   1. 5.3.1 Idle mode procedures 
      1. 5.3.1.1 Cell selection 

      5.3.1.2 System Information acquisition 

      1. 5.3.1.3 Cell reselection 
      2. 5.3.1.4 Paging, DRX and eDRX 
      3. 5.3.1.5 Power Saving Mode 
      4. 5.3.1.6 Random access in idle mode 
      5. 5.3.1.7 Connection establishment 
      6. 5.3.1.8 Access control 
      7. 5.3.1.9 Multicast 
   2. 5.3.2 Connected mode procedures 
      1. 5.3.2.1 Scheduling 
      2. 5.3.2.2 Channel quality reporting 
      3. 5.3.2.3 Random access in connected mode 
      4. 5.3.2.4 Power control 
      5. 5.3.2.5 Mobility support 
      6. 5.3.2.6 Positioning 
   3. 5.3.3 Procedures common for idle and connected mode 
      1. 5.3.3.1 MPDCCH search spaces 
      2. 5.3.3.2 Frequency hopping 
4. 5.4 NR and LTE-M coexistence 
5. References 

5.1. Background

In this section we describe the introduction of Long-Term Evolution for Machine-Type Communications (LTE-M) into the 3GPP specifications and the design principles. The design principles were adopted in order to achieve the required low device complexity and cost, coverage enhancement, long device battery lifetime and support of massive number of devices for the massive Internet of Things (IoT) segment. In addition, the design was required to support adequate peak rates and mobility support to be able to address more demanding applications such as voice services, and ensuring high deployment flexibility and coexistence with ordinary LTE.

5.1.1. 3GPP standardization

LTE-M extends LTE with features for improved support for Machine-Type Communications (MTC) and IoT. These extensions have their origin in the 3GPP study item Study on provision of low-cost MTC User Equipments based on LTE [1], in this book referred to as the LTE-M study item.

Since the conclusion of the LTE-M study item, a number of related 3GPP work items have been completed, starting with an initial Release 12 work item that can be seen as a precursor for the more ambitious Release 13 work item:

1. • Release 12 work item Low cost and enhanced coverage MTC UE for LTE [2], sometimes referred to as the MTC work item, which introduced LTE device category 0 (Cat-0)
2. • Release 13 work item Further LTE Physical Layer Enhancements for MTC [3], sometimes referred to as the eMTC work item, which introduced the Coverage Enhancement (CE) modes A and B as well as LTE device category M1 (Cat-M1)

In this book we use the term LTE-M when we refer to the Cat-M device category series, the CE modes, and all functionality that can be supported by the Cat-M devices or the CE modes, such as the power consumption reduction techniques Power Saving Mode (PSM) and Extended Discontinuous Reception (eDRX). According to this definition, all LTE devices (including Cat-0 devices) that implement CE mode support are considered LTE-M devices, but if they do not implement CE mode support then they are not considered LTE-M devices. Cat-M devices have mandatory support for CE mode A and are always considered LTE-M devices.

Already, many LTE-M networks have been deployed and a device ecosystem has been established. The GSM Association (GSMA), which is an organization that represents the interests of mobile network operators worldwide, tracks the status of commercial launches of LTE-M. Since the completion of Release 13 LTE-M in March 2016, there had been more than 30 LTE-M launches in over 25 markets as of June 2019, according to GSM Association [4]. On the device side, the Global Mobile Suppliers Association (GSA) published a research report in 2018 [5] stating that as of August 2018, there were 101 modules supporting LTE-M, 48 of which also support NB-IoT.

Release 13 laid the foundation for LTE-M in the form of low-cost devices and coverage enhancements, but the LTE-M standard has been further evolved in Releases 14 and 15:

1. • Release 14 work item Further Enhanced MTC for LTE [6], sometimes referred to as the feMTC work item, which introduced various improvements for support of higher data rates, improved VoLTE support, improved positioning, multicast support, as well as the new LTE device category M2 (Cat-M2)
2. • Release 15 work item Even Further Enhanced MTC for LTE [7], sometimes referred to as the efeMTC work item, which introduced further improvements for reduced latency and power consumption, improved spectral efficiency, new use cases, and more

Table 5.1

New LTE-M features introduced in 3GPP Releases 14 and 15.

Release 14 (2017)	Section	Release 15 (2018)	Section
Support for higher data rates • New device category M2 • Higher uplink peak rate for Cat-M1 • Wider bandwidth in CE mode • More downlink HARQ processes in FDD • ACK/NACK bundling in HD-FDD • Faster frequency retuning	5.2.3 5.2.3 5.2.2.3 5.3.2.1.1 5.3.2.1.1 5.2.4.1	Support for new use cases • Support for higher device velocity • Lower device power class	5.3.2.5 5.1.2.1
VoLTE enhancements • New PUSCH repetition factors • Modulation scheme restriction • Dynamic ACK/NACK delays	5.2.5.4 5.2.5.4 5.3.2.1.1	Reduced latency • Resynchronization signal • Improved MIB/SIB performance • System info update indication	5.2.4.2.2 5.3.1.2.1 5.3.1.2.3
Coverage improvements • SRS coverage enhancement • Larger PUCCH repetition factors • Uplink transmit antenna selection	5.2.5.3.2 5.2.5.5 5.2.5.4	Reduced power consumption • Wake-up signals • Early data transmission • ACK/NACK feedback for uplink data • Relaxed monitoring for cell reselection	5.2.4.5 5.3.1.7.3 5.3.2.1.2 5.3.2.5
Multicast support	5.3.1.9	Increased spectral efficiency • Downlink 64QAM support • CQI table with large range • Uplink sub-PRB allocation • Flexible starting PRB • CRS muting	5.2.4.7 5.3.2.2 5.2.5.4 5.3.2.1 5.2.4.3.1
Improved positioning	5.3.2.6	Improved access control	5.3.1.8
Mobility enhancements	5.3.2.5

Table 5.1 provides a summary of the LTE-M enhancements introduced in 3GPP Releases 14 and 15, with references to the relevant book sections. All the Releases 14 and 15 features can be enabled through a software upgrade of the existing LTE network equipment. In many cases it may also be possible to upgrade the software/firmware in existing devices to support the new features.

In Release 15, 3GPP evaluated LTE-M against a set of agreed Fifth Generation (5G) performance requirements defined for the massive machine-type communications (mMTC) use case [8]. As shown in Chapter 6, LTE-M meets these requirements with margins and does in all relevant aspects qualify as a 5G mMTC technology. As we will see in Section 5.4, LTE-M is also able to coexist efficiently with the 5G New Radio (NR) air interface introduced in Release 15.

5.1.2. Radio Access Design Principles

5.1.2.1. Low device complexity and cost

During the LTE-M study item [1], various device cost reduction techniques were studied, with the objective to bring down the LTE device cost substantially to make LTE attractive for low-end MTC applications that have so far been adequately handled by GSM/GPRS. It was estimated that this would correspond to a device modem manufacturing cost in the order of 1/3 of that of the simplest LTE modem, which at that time was a single-band LTE device Cat-1 modem.

The study identified the following cost reduction techniques as most promising:

1. • Reduced peak rate
2. • Single receive antenna
3. • Half-duplex operation
4. • Reduced bandwidth
5. • Reduced maximum transmit power.

A first step was taken in Release 12 with the introduction of LTE device Cat-0 that supported a reduced peak rate for user data of 1   Mbps in downlink and uplink (instead of at least 10   Mbps in downlink and 5   Mbps in uplink for Cat-1 and higher categories), a single receive antenna (instead of at least two), and optionally half-duplex frequency-division duplex (HD-FDD) operation.

The next step was taken in Release 13 with LTE device Cat-M1 that includes all the cost reduction techniques of Cat-0, plus a reduced bandwidth of 1.4   MHz (instead of 20   MHz), and optionally a lower device power class with a maximum transmit power of 20   dBm (instead of 23   dBm). Release 15 introduces an even lower 14-dBm power class to enable implementation of devices with low power consumption and small form factor, see Section 5.2.3.

With the cost reduction techniques introduced in Release 13, the Bill of Material cost for the Cat-M1 modem was estimated to reach that of an Enhanced GPRS modem. For further information on LTE-M cost estimates, refer to Section 6.7.

5.1.2.2. Coverage enhancement

The LTE-M study item [1] also studied coverage enhancement (CE) techniques, with the objective to improve coverage of LTE networks at that time by 20 dB to provide coverage for devices with challenging coverage conditions, for example stationary utility metering devices located in basements.

The study identified various forms of prolonged transmission time as the most promising coverage enhancement techniques. The fact that many of the IoT applications of interest have very relaxed requirements on data rates and latency can be exploited to enhance the coverage through repetition or retransmission techniques. The study concluded that 20   dB coverage enhancement can be achieved using the identified techniques.

Release 13 standardized two CE modes: CE mode A, supporting up to 32 repetitions for the data channels, and CE mode B, supporting up to 2048 repetitions. Recent evaluations show that the initial coverage target of 20   dB can be reached using the repetitions available in CE mode B. For further information on LTE-M coverage and data rate estimates, refer to Sections 6.2 and 6.3.

In this book we refer to LTE devices with CE mode support as LTE-M devices. These devices may be low-cost Cat-M devices, or they may be higher LTE device categories configured in a CE mode. For more information on the CE modes refer to Section 5.2.2.3.

5.1.2.3. Long device battery lifetime

Support for a device battery lifetime of many years, potentially decades, has been introduced in a first step in the form of the PSM in Release 12 and in a second step in the form of the eDRX in Release 13. These features are supported for LTE-M devices and also for other 3GPP radio access technologies.

These techniques reduce the power consumption primarily by minimizing any unnecessary “on” time for the receiver and the transmitter in the device. Compared to ordinary LTE devices, LTE-M devices can have a further reduced power consumption during their “on” time mainly thanks to the reduced transmit and receive bandwidths.

PSM and eDRX are described in Sections 2.2.3, 5.3.1.4, and 5.3.1.5 , and the battery lifetime for LTE-M is evaluated in Section 6.5.

5.1.2.4. Support of massive number of devices

The handling of massive numbers of devices in LTE was improved already in Releases 10 and 11, for example in the form of access class barring (ACB) and overload control, as discussed in Section 2.2.1. Further improvements have been introduced later on, for example in the form of the Radio Resource Control (RRC) Suspend/Resume mechanism described in Section 2.2.2, which helps reduce the required signaling when resuming an RRC connection after a period of inactivity as long as the device has not left the cell in the meanwhile.

For more information on LTE-M capacity estimates, refer to Section 6.6.

5.1.2.5. Deployment flexibility

LTE-M can be deployed in a wide range of frequency bands, as can be seen from the list of supported bands in Table 5.2. Both paired bands for frequency-division duplex (FDD) operation and unpaired bands for time-division duplex (TDD) operation are supported, and new bands have been added in every release. Even though the simplest LTE-M devices (i.e. the Cat-M devices) only support a reduced bandwidth, LTE-M supports the same system bandwidths at the network side as LTE (1.4, 3, 5, 10, 15, and 20   MHz).

5.1.2.6. Coexistence with LTE

LTE-M extends the LTE physical layer with features for improved support for MTC. The LTE-M design therefore builds on the solutions already available in LTE.

The fundamental downlink and uplink transmission schemes are the same as in LTE, meaning Orthogonal Frequency-Division Multiplexing (OFDM) in downlink and Single-Carrier Frequency-Division Multiple-Access (SC-FDMA) in uplink, with the same numerologies (channel raster, subcarrier spacing, cyclic prefix (CP) lengths, resource grid, frame structure, etc.). This means that LTE-M transmissions and LTE transmissions related to, for example, smartphones and mobile broadband modems can coexist in the same LTE cell on the same LTE carrier and the resources can be shared dynamically between LTE-M users and ordinary LTE users.

If an operator has a large spectrum allocation for LTE, then there is also a large bandwidth available for LTE-M traffic. The downlink and uplink resources on an LTE carrier can serve as a resource pool that can be fully dynamically shared between LTE traffic and LTE-M traffic. It may furthermore be possible to schedule delay-tolerant LTE-M traffic during periods when the ordinary LTE users are less active, thereby minimizing the performance impact from the LTE-M traffic on the LTE traffic.

Table 5.2

Frequency bands defined for Cat-M1/M2 as of Release 15 [9].

Band	Duplex mode	Uplink [MHz]	Downlink [MHz]
1	FDD	1920–1980	2110–2170
2	FDD	1850–1910	1930–1990
3	FDD	1710–1785	1805–1880
4	FDD	1710–1755	2110–2155
5	FDD	824–849	869–894
7	FDD	2500–2570	2620–2690
8	FDD	880–915	925–960
11	FDD	1427.9–1447.9	1475.9–1495.9
12	FDD	699–716	729–746
13	FDD	777–787	746–756
14	FDD	788–798	758–768
18	FDD	815–830	860–875
19	FDD	830–845	875–890
20	FDD	832–862	791–821
21	FDD	1447.9–1462.9	1495.9–1510.9
25	FDD	1850–1915	1930–1995
26	FDD	814–849	859–894
27	FDD	807–824	852–869
28	FDD	703–748	758–803
31	FDD	452.5–457.5	462.5–467.5
39	TDD	1880–1920	1880–1920
40	TDD	2300–2400	2300–2400
41	TDD	2496–2690	2496–2690
66	FDD	1710–1780	2110–2200
71	FDD	636–698	617–652
72	FDD	451–456	461–466
73	FDD	450–455	460–465
74	FDD	1427–1470	1475–1518
85	FDD	698–716	728–746