11.1. Introduction

LTE (Long-Term Evolution) radio access was initially designed for high-speed communications, taking into account high mobility and low latency for data transport.

Using LTE radio access for connected objects exposes different constraints such as battery life and terminal cost, and decreases the speed and latency requirements.

The reduction of the radio channel bandwidth is one of the factors making it possible to keep the objectives fixed for the connected objects:

• – the bandwidth is reduced to 1.4 MHz or 5 MHz for the LTE-M radio interface;
• – the bandwidth is reduced to 180 kHz for the NB-IoT (NarrowBand Internet of Things) radio interface.

The reduction of the modulation index and the absence of a MIMO (Multiple Input Multiple Output) mechanism also contribute to reducing energy consumption and lowering terminal costs.

The power saving mode (PSM), on the one hand, and the extended discontinuous reception (eDRX), on the other hand, are complementary mechanisms that reduce the terminal power consumption during standby or when the terminal radio interface and the functions relating to radio resource control (RRC) are switched off.

11.2. Special features

11.2.1. PSM feature

The PSM feature is designed to help terminals to reduce battery consumption and potentially reach 10 years of battery life.

The PSM feature is similar to a power off, but the terminal maintains the context, which avoids turning off the terminal and reconnecting to the network when restarting.

When a device starts the PSM procedure, it provides two timers (T3324 and T3412). The switch-off time of the radio interface and the functions associated with the RRC protocol is the time difference between these two timers (Figure 11.1).

During the activation of timer T3324, the terminal in the idle state listens for paging. When timer T3324 expires, the terminal enters PSM until timer T3412 expires. The terminal is no longer reachable by the network but can at any time request the restoration of a session.

The network can accept these values or define others. The network then keeps the context of the terminal and the terminal remains attached. If a terminal wakes up and sends data before timer T3412 expires, no attachment procedure is necessary.

The terminal cannot be contacted by the paging procedure when it is off. The network may choose to store the incoming data for transfer to the terminal once it wakes up.

The maximum time that a terminal can be accessed for is 186 minutes (timer T3324). The maximum time that a terminal can be powered down for is approximately 413 days (timers T3412 and T3324).

11.2.2. eDRX feature

The extended discontinuous reception (eDRX) is an extension of an existing feature, which can be used by terminals to reduce power consumption. The eDRX configuration can be used without PSM or in combination for additional energy savings.

The eDRX feature extends the time interval during which the terminal does not listen to the network. It may be entirely acceptable that the terminal will not be reachable for the cycle duration, between 20.48 and 10 485.76 seconds for the NB-IoT radio interface and between 5.12 and 2621.44 seconds for the LTE-M radio interface.

11.2.3. Coverage extension

Coverage extension (CE) improves coverage by using repetition techniques for physical channels relating to the user and control planes.

Mode CE A (respectively CE B) supports up to 32 repetitions (respectively 2048 repetitions). This mode (respectively CE B) improves the maximum coupling loss (MCL) by 5 dB (respectively 15 dB).

Mode CE A is the default operating mode. The difference in coverage between a terminal Cat. M1 operating in mode CE A and a terminal Cat. 1 lies in the fact that the terminal Cat. M1 has only one receiver and a reduced transmission power. This difference is compensated for by using a small number of repetitions.

Mode CE B is an optional extension offering a greater coverage improvement at the expense of capacity and latency. It was primarily designed to provide indoor coverage. For this reason, it is intended for applications that require limited data rates or volumes.

11.3. LTE-M interface

11.3.1. Radio channel

The LTE-M radio interface uses the narrow bandwidth concept to assign sub-carriers of the LTE radio channel. Each narrow bandwidth comprises six consecutive physical resource blocks (PRBs).

The LTE-M interface uses the concept of wide bandwidth to allocate a higher bandwidth, consisting of four narrow bandwidths, when possible, or one to three narrow bandwidths when it is not possible.

11.3.2. Guard time

Although a terminal has a bandwidth of 1.4 MHz, it can access different narrow bands for each sub-frame. If the transmission involves repetition over multiple sub-frames, a frequency hopping between the sub-frames can be applied. Frequency hopping takes place between different narrow bands and in blocks of 1 to 16 sub-frames depending on the CE mode.

Two OFDM (Orthogonal Frequency-Division Multiplexing) symbols are pre-empted to form the guard time when frequency hopping occurs. Figure 11.2 describes the different scenarios for the uplink.

When both sub-frames carry the same physical channel (PUSCH or PUCCH), each sub-frame is pre-empted with an OFDM symbol.

When both sub-frames carry different physical channels (PUCCH and PUSCH), pre-emption only takes place on the physical uplink shared channel (PUSCH).

11.3.3. Physical channels

11.3.3.1. PBCH

The LTE-M radio interface uses the same physical broadcast channel (PBCH) as the LTE radio interface, with optional repetitions.

The PBCH is divided into two parts:

• – the basic physical channel, which is the channel of the LTE radio interface. This channel is transmitted in the first sub-frame of each frame with a periodicity of 40 ms;
• – the physical repetition channel for which the OFDM symbols of the slot are repeated up to five times, depending on the duplex mode and the length of the cyclic prefix.

Repetitions occur in sub-frame 9 of the previous frame and in sub-frame 0 of the current frame for the frequency-division duplex (FDD) (Figure 11.3). For time-division duplex (TDD), repetitions occur in sub-frames 0 and 5 of the same radio frame.

For the FDD mode, all symbols of the basic physical channel are repeated four times if the cyclic prefix is normal and three times if the cyclic prefix is extended. For the TDD mode, if the cyclic prefix is extended, all symbols are repeated three times, and if the cyclic prefix is normal, symbols 0 and 1 are repeated five times and symbols 2 and 3 are repeated three times.

11.3.3.2. MPDCCH

The MTC (Mobile Type Communication) physical downlink control channel (MPDCCH) carries specific downlink control information (DCI):

• – format 6-0 is used to allocate a resource to the mobile for the uplink on the PUSCH;
• – format 6-1 is used to allocate a resource to the mobile for the downlink on the physical downlink shared channel (PDSCH);
• – format 6-2 is used to indicate the presence of paging in the PDSCH.

In the case of mode CE A, the power control of the PUCCH and PUSCH is provided by formats 3 and 3A. In the case of mode CE B, the transmission power is set to the maximum value.

The processing done for the MPDCCH and ePDCCH (enhanced PDCCH) is very similar, the main differences being repetitions and frequency hopping.

The MPDCCH occupies in two, four or six PRBs transmitted in a narrow bandwidth and starts after the symbols assigned to the PDCCH.

The number of repetitions is defined by a combination of static configuration and dynamic selection for a given transmission.

11.3.3.3. PDSCH

The LTE-M radio interface uses the same PDSCH as the LTE radio interface, with optional repetitions and frequency hopping.

The transmission modes are mode 1 (single antenna), mode 2 (transmission diversity), mode 6 (precoding based on a closed-loop codebook) and mode 9 (precoding based on a codebook, one layer). Mode 6 is only supported for mode CE A, whereas modes 1, 2 and 9 are supported for both modes CE A and CE B.

The LTE-M radio interface does not support the MIMO mechanism because most terminals are low cost and have a single receiving antenna.

The PDSCH is transmitted in consecutive sub-frames. These sub-frames are transmitted in blocks whose number is equal to:

• – 1 in mode FDD and in mode CE A;
• – 4 in mode FDD and in mode CE B;
• – 1 in mode TDD and in mode CE A;
• – 10 in mode TDD and in mode CE B.

If the PDSCH is repeated over several sub-frames, frequency hopping between sub-frames may optionally be applied.

The number of repetitions is a combination of a static configuration (four values for mode CE A, eight values for mode CE B) and a dynamic selection of the value for a given transmission.

The system information block 1 (SIB1), transported in the PDSCH, has no fixed location in the time domain. Two narrow bandwidths are selected based on the physical-layer cell identity (PCI).

The SIB1 is transmitted with a periodicity of eight frames in the time domain. The location of the SIB1, corresponding to the frame number and the sub-frame number, depends on the number of repetitions and the value of the PCI.

11.3.3.4. PUSCH

The LTE-M radio interface uses the same PUSCH as the LTE radio interface, with optional repetitions and frequency hopping.

As for the PDSCH, the number of repetitions is defined by a combination of static configuration and dynamic selection for a given transmission.

11.3.3.5. PUCCH

As with the LTE radio interface, the physical uplink control channel (PUCCH) carries uplink control information (UCI), with a limitation of the format type, given the limitation of the transmission modes:

• – format 1 supports the scheduling request (SR) for which no bit is transmitted, the evolved node base station (eNB) detecting only the presence of energy in the PUCCH;
• – format 1A supports the information HARQ (Hybrid Automatic Repeat reQuest) indicator (HI) corresponding to a positive or negative acknowledgment bit of a transport block received on the PDSCH;
• – format 2 supports channel state information (CSI) relating to the PDSCH, corresponding to 20 bits.

The PUCCH is transmitted in blocks of sub-frames, each block being the subject of a frequency hopping. Frequency hopping is not used if the number of repetitions is smaller than the number of sub-frames in a block.

11.3.3.6. PRACH

The physical random access channel (PRACH) transports the random access preamble, transmitted in six PRB resources.

The PRACH configuration is a combination of configurations indicated by the cell and parameters specific to the terminal.

11.4. NB-IoT interface

11.4.1. Radio channel

The NB-IoT interface occupies a frequency band of 180 kHz, which corresponds to 12 sub-carriers in the frequency domain, for example a PRB.

Three operation modes are defined (Figure 11.4):

• – the autonomous operation which can be deployed in the GSM radio channel, for example, and whose bandwidth is equal to 200 kHz;
• – the guard-band operation which can be deployed in the available guard band of the LTE radio channel;
• – the operation in the useful bandwidth of the LTE radio channel.

11.4.2. Resource block

The PRB, for the downlink, consists of 12 sub-carriers spaced by 15 kHz, in the frequency domain.

For the uplink, the eNB entity defines the spacing between the subcarriers (Figure 11.5):

• – a spacing of 15 kHz, as for the downlink;
• – a spacing of 3.75 kHz. The PRB then consists of 48 subcarriers.

The slot consists of seven OFDM symbols, whose duration depends on the spacing between the sub-carriers (Figure 11.5):

• – 0.5 ms in the case of a spacing of 15 kHz between the sub-carriers;
• – 2 ms in the case of a spacing of 3.75 kHz between the sub-carriers.

11.4.3. Physical signals and channels

11.4.3.1. NPSS and NSSS

The narrowband primary synchronization signal (NPSS) carries a Zadoff–Chu sequence of 11 OFDM symbols. This sequence is fixed and therefore contains no information on the cell. The sequence is transmitted in the sub-frame 5 of each frame, which allows the terminal to acquire the frame synchronization (Figure 11.6).

The narrowband secondary synchronization signal (NSSS) has a Zadoff–Chu sequence of 131 resource elements. Four types of sequence are used with a periodicity of 80 ms. The sequence also makes it possible to deduce the PCI. As for the LTE radio interface, 504 values are defined. The sequence is transmitted in the sub-frame 9 of each even frame (Figure 11.7).

The first three OFDM symbols are omitted because they can carry the PDCCH when the NB-IoT radio interface is used in the LTE radio channel band.

When the terminal synchronizes with NPSS and NSSS, it may not know the operation mode (1, 2 or 3 OFDM symbols for the PDCCH). Therefore, this restriction applies to all operation modes.

The two synchronization signals are punctured by the cell-specific reference signal (CRS), assuming the use of four antenna ports.

The PRB numbers of the LTE radio interface assigned to the NB-IoT radio interface for synchronization are provided in Table 11.1, in the case of operation in the LTE radio channel.

11.4.3.2. NRS

The narrowband reference signal (NRS) is transmitted in each sub-frame for one or two antenna ports. Its location is determined from the PCI parameter. When the NRS is transmitted on two antenna ports, the resource elements assigned to the antenna port 1 are set to zero in the resource blocks corresponding to the antenna port 0, and vice versa (Figure 11.8).

11.4.3.3. NPBCH

The narrowband PBCH (NPBCH) carries the master information block narrowband (MIB-NB) which contains the following information:

• – the data for the partial calculation of the frame and hyper-frame numbers;
• – the scheduling of the SIB1-NB;
• – the operating mode of the NB-IoT radio interface.

The NPBCH is transmitted in sub-frame 0 and distributed over eight blocks. The first block is repeated on eight consecutive sub-frames 0. The same operation is performed on the next seven blocks. Each block is transmitted over a period of 80 ms and the periodicity of the MIB-NB is 640 ms (Figure 11.9).

As for the NPSS and NSSS, the first three OFDM symbols are omitted and the NPBCH is punctured by the CRS. The NPBCH is also punctured by the NRS, for which it is assumed that two antenna ports are used (Figure 11.10).

The PRB numbers of the LTE radio interface assigned to the NB-IoT radio interface for the NPBCH are provided in Table 11.1.

11.4.3.4. NPDCCH

The narrowband PDCCH (NPDCCH) carries the DCI, for which three formats are defined:

• – format N0: the DCI indicates the resources that the terminal must use for uplink data transmission;
• – format N1: the DCI defines the location of the terminal data in the NPDSCH and the repetition frequency;
• – format N3: the DCI provides additional information, such as paging or changing system information.

For operation in the bandwidth of the LTE radio channel, the NPDCCH is punctured by the NRS and CRS. To avoid conflict with LTE radio control channels, the PDCCH start-up symbol is indicated by the SIB1-NB (Figure 11.11).

For each sub-frame, two narrowband control channel elements (NCCE) are defined, NCCE0 and NCCE1. Two formats are defined to use them:

• – format 0 occupies one NCCE;
• – format 1 occupies both NCCE.

For a standalone or guard-band operation, the NPDCCH is only punctured by the NRS. The size of the reserved area for the control channels of the LTE radio interface is void.

In order for the terminal to find the control information with reasonable decoding complexity, the NPDCCH defines the following search spaces:

• – type 1: the common search space for paging;
• – type 2: the common search space for random access;
• – the search space dedicated to the terminal.

The location of the NPDCCH for the type-1 search space is determined from the candidate sub-frames.

The parameters for calculating the location of the NPDCCH for the type-2 search space are provided in the SIB2-NB.

The parameters for calculating the location of the NPDCCH for the dedicated search space are provided in the message RRC ConnectionSetup.

The NPDCCH can operate in the bands used for mobile synchronization, defined in Table 11.1, or in dedicated bands.

11.4.3.5. NPDSCH

The narrowband PDSCH (NPDSCH) carries, on the one hand, the different RRC messages corresponding to the SIB-NB, paging, common messages and dedicated messages, and, on the other hand, the terminal traffic (IP packets or non-IP packets).

The NB-IoT interface supports, for the downlink, a transmission on two antenna ports, for SFBC (Space Frequency Block Coding) diversity.

Dedicated messages may encapsulate NAS (Non-Access Stratum) messages exchanged between the terminal and the mobility management entity (MME). NAS messages can carry terminal traffic.

For the different RRC messages, with the exception of the system information, and for the terminal traffic, the sub-frame structure assigned to the NPDSCH has the same characteristics as the NPDCCH.

The SIB1-NB provides the following information:

• – cell access information (country code, operator code, location code, cell identity);
• – information relating to the selection of the cell (minimum level of reception);
• – information relating to the scheduling of other SIB-NB.

The SIB1-NB is transmitted with a periodicity of 256 frames and a repetition of 4, 8 or 16 times. The size of the transport block and the number of repetitions are indicated in the MIB-NB.

The SIB1-NB is transmitted in the sub-frame 4. The frame on which the SIB1-NB system information starts is determined by the number of repetitions and the PCI parameter.

11.4.3.6. NPUSCH

Two formats are defined on the narrowband PUSCH (NPUSCH):

• – format 1 relates to transport channel data with transport blocks not exceeding 1000 bits;
• – format 2 contains UCI which are limited to an acknowledgment (HARQ) of the data received on the NPDSCH.

The resource unit (RU) is the smallest unit for mapping a transport block. Its structure depends on the format and spacing of the sub-carriers:

• – for the sub-carrier spacing of 3.75 kHz and for format 1, the resource unit consists of a sub-carrier in the frequency domain and 16 slots in the time domain;
• – for the sub-carrier spacing of 15 kHz and for format 1, there are four options listed in Table 11.2;
• – for format 2 and for the sub-carrier spacing of 3.75 kHz or 15 kHz, the resource unit consists of a sub-carrier and four time slots.

11.4.3.7. DMRS

The demodulation reference signal (DMRS) is multiplexed with the NPUSCH. The resource elements assigned to the DMRS depend on the format and spacing between the sub-carriers (Figure 11.12).

11.4.3.8. NPRACH

The preamble, transmitted on the Narrowband PRACH (NPRACH), is based on symbol groups on a single sub-carrier. Each symbol group has a cyclic prefix followed by five symbols (Figure 11.13).

Two preamble formats are defined, format 0 and format 1, which differ in length. The five symbols have a duration of T_SEQ = 1.333 ms, with a cyclic prefix of T_CP = 67 μs for format 0 and 267 μs for format 1, and a total length of 1.4 ms and 1.6 ms, respectively (Figure 11.13). The preamble format to be used is broadcasted in the SIB2-NB system information.

The preamble is composed of four groups of consecutive symbols. Frequency hopping is applied for each group of symbols, transmitted on a different subcarrier. The repetition number, 1, 2, 4, 8, 16, 32, 64 or 128 times, is indicated by the SIB2-NB. The same transmission power is used at each repetition.

The resources of the NPRACH are produced with periodicities between 40 ms and 2.56 s. The beginning of the period is provided in the SIB2-NB. The number of repetitions and the format of the preamble determine the end of each period.

In the frequency domain, a sub-carrier spacing of 3.75 kHz is applied. The NPRACH resources occupy a contiguous set of 12, 24, 36 or 48 subcarriers (Figure 11.13).