NB-IoT and eMTC both belong to the cellular Internet of Things and share the “3C” characteristics of cellular IoT:
•Coverage Enhanced Coverage
•Consumption Low Power Consumption
•Cost Low Cost
To achieve the “3C” goals, NB-IoT and eMTC have different implementation methods, as follows:
1
Comparison of Key Technologies of NB-IoT and eMTC
Enhanced Coverage
The coverage target for NB-IoT is MCL 164dB, and its coverage enhancement is mainly achieved through increasing the uplink power spectral density and repeated transmissions.
The coverage target for eMTC is MCL 155.7dB, with the same power spectral density as LTE, and coverage enhancement is mainly achieved through repeated transmissions and frequency hopping.
MCL (Maximum Coupling Loss) refers to the path loss from the base station antenna port to the terminal antenna port. From the coverage target perspective, eMTC is about 8dB lower than NB-IoT.
How does repeated transmission enhance coverage?
Repeated transmission involves sending a transmission block across multiple subframes. Repetition Gain = 10log Repetition Times, meaning that retransmitting twice can enhance the signal by 3dB. NB-IoT can support up to 2048 downlink retransmissions and 128 uplink retransmissions.
Both NB-IoT and eMTC use repeated transmission to enhance coverage.
How does increasing uplink power spectral density enhance coverage?
Uplink control information and service information are sent in a narrower LTE bandwidth, resulting in greater PSD (Power Spectrum Density) gain at the same transmission power, thus reducing the demodulation requirements for the receiver.
In the downlink direction, if NB-IoT uses a standalone deployment model, the downlink transmission power can be independently configured, with power spectral density the same as GSM but about 14dB higher than LTE FDD.
In the uplink direction, since the minimum scheduling bandwidth for NB-IoT is 3.75K or 15K, the uplink power spectral density can be increased by up to 17dB. Considering that the maximum transmission power for GSM terminals can reach 33dBm, while for NB-IoT it is 23dBm, the actual power spectral density of NB-IoT terminals can be about 7dB higher than that of GSM terminals.
eMTC shares transmission power and system bandwidth with LTE, showing no enhancement in power spectral density, primarily relying on repeated transmissions and frequency hopping for coverage enhancement.
It is worth mentioning about NB-IoT:
•In the downlink direction, only the power in the standalone deployment can be independently configured, while the power in in-band and guard band deployment modes is limited by LTE power. Therefore, under in-band and guard band deployment modes, NB-IoT requires more retransmissions to achieve coverage levels comparable to those in standalone deployment mode.
•In the uplink direction, there is basically no difference among the three modes.
Low Power Consumption
For low power consumption, both NB-IoT and eMTC use the same technologies, including: PSM, eDRX, and extended cycle timers.
①PSM (Power Saving Mode)
Mobile phones need to remain on standby at all times, or else how can someone find you when they call? But this means the phone must periodically listen to the network, which consumes power.
However, IoT terminals are different from mobile phones; they spend most of their time in deep sleep, reporting only one or two messages a day or even a week, and then entering deep sleep after remaining idle for a while, without needing to listen for messages.
PSM allows IoT terminals to enter deep sleep mode after transmitting data, similar to shutting down, without any communication activity.
②eDRX
DRX (Discontinuous Reception) is a method of receiving signals intermittently to save power. eDRX is an extended version of discontinuous reception.
Mobile phones can receive signals intermittently to save power. NB-IoT and eMTC extend this discontinuous interval for greater power savings.
③Extended Cycle Timer
Flexibly configure long-cycle location update timers RAU/TAU to reduce wake-up frequency.
Low Cost
To reduce costs, strategies include decreasing protocol stack processing overhead, using single antennas and FDD half-duplex modes to lower RF costs, and low data rates and bandwidth inherently reduce chip processing complexity, among others.
For instance, FDD half-duplex mode means there is no need to process sending and receiving simultaneously, making it cheaper and more power-efficient compared to full-duplex.
2
Technical Parameter Comparison of NB-IoT and eMTC
The main differences between NB-IoT and eMTC are:
NB-IoT aims for the lowest cost, longest battery life, no mobility, and very low data rates. It is suitable for applications with no mobility, small data amounts, insensitivity to latency, and high sensitivity to cost, such as smart parking, smart lamp posts, and smart metering.
To meet more application scenarios and market demands, Re-14 and subsequent versions will enhance NB-IoT with a series of technologies, including added positioning and multicast capabilities, higher data rates, paging and random access on non-anchor carriers, enhanced mobility in connected states, and support for lower UE power levels.
eMTC supports voice, has faster transmission rates, and supports mobility, but the module cost is relatively higher, making it suitable for wearable devices, health monitoring, and indoor mobile applications.
3
Comparison of Deployment Methods for NB-IoT and eMTC
NB-IoT Deployment Methods
NB-IoT can be deployed in three ways: standalone deployment, guard band deployment, and in-band deployment.
Standalone deployment is suitable for reusing the GSM frequency band, where the GSM channel bandwidth is 200KHz, which exactly provides space for NB-IoT’s 180KHz bandwidth, with a 10KHz guard interval on both sides.
Guard band deployment utilizes unused 180KHz bandwidth resources in the LTE edge protection frequency band.
In-band deployment utilizes any resource block in the LTE carrier. However, in in-band deployment mode, some PRBs cannot be occupied by NB-IoT.
eMTC Deployment Methods
eMTC supports co-deployment with LTE and also supports standalone deployment.
It mainly uses LTE in-band deployment mode, supporting both TDD and FDD. eMTC and LTE operate in the same frequency band, with resource allocation managed by the base station, sharing some control channels. Therefore, operators can directly deploy eMTC within existing LTE frequency bands without needing to allocate separate spectrum.
4
Comparison of Physical Layer Technologies of NB-IoT and eMTC
4.1 Comparison of Time-Frequency Domain Structures
NB-IoT
Downlink:
NB-IoT downlink is consistent with LTE, using Orthogonal Frequency Division Multiple Access (OFDMA) technology, with a subcarrier spacing of 15kHz. The length of time slots, subframes, and radio frames are 0.5ms, 1ms, and 10ms, respectively, including the number of OFDM symbols per time slot and the cyclic prefix, which are the same as in LTE.
NB-IoT carrier bandwidth is 180KHz, equivalent to the bandwidth of one PRB (Physical Resource Block) in LTE, which is 12 subcarriers * 15KHz/subcarrier = 180KHz, ensuring compatibility with LTE downlink. For instance, when using LTE carrier in-band deployment, the downlink NB-IoT PRB can maintain orthogonality with other LTE PRBs.
Uplink:
NB-IoT uplink supports multi-tone transmission and single-tone transmission.
Multi-tone transmission is based on SC-FDMA, with a subcarrier spacing of 15kHz, 0.5ms time slots, and 1ms subframes (the same as LTE).
Single-tone transmission can have a subcarrier spacing of 15KHz and 3.75KHz, where 15KHz is the same as LTE to maintain compatibility in the uplink; when the subcarrier is 3.75KHz, one time slot in the frame structure is 2ms long (containing 7 symbols), and 15KHz is an integer multiple of 3.75KHz, resulting in minimal interference with the LTE system.
eMTC
eMTC is an evolved function of LTE, maintaining the frequency domain structure consistent with LTE. It is defined across TDD and FDD LTE systems with bandwidths of 1.4M~20MHz, but regardless of the bandwidth it operates in, eMTC’s maximum scheduling is 6RB. 3GPP defines the LTE system bandwidth as a series of 6RB narrowbands (NB), as shown in the figure below:
eMTC’s frame structure is consistent with LTE.
4.2 Comparison of Physical Channels
Physical Channels of NB-IoT
Downlink:
For the downlink, NB-IoT defines three physical channels:
①NPBCH, Narrowband Physical Broadcast Channel
②NPDCCH, Narrowband Physical Downlink Control Channel
③NPDSCH, Narrowband Physical Downlink Shared Channel
It also defines two physical signals:
①NRS, Narrowband Reference Signal
②NPSS and NSSS, Primary Synchronization Signal and Secondary Synchronization Signal
Unlike LTE, since the frequency bandwidth of NB-IoT is at most one PRB, these downlink physical channels use time division multiplexing, meaning they appear alternately at different times.
▲Time Division Multiplexing Between NB-IoT Downlink Physical Channels and Signals
As shown in the figure, the NB-IoT subframe is allocated to different physical channels and signals, with each NB-IoT subframe in the frequency domain being one PRB (12 subcarriers) and in the time domain being 1ms.
NPBCH
NPBCH channel differs from LTE’s PBCH, with a broadcast period of 640ms, repeated 8 times, as shown in the figure below. The terminal receives several subframe signals for demodulation.
NPBCH is located in subframe #0 of each radio frame, carrying MIB-NB (Narrowband Master Information Block), while other system information such as SIB1-NB is carried on NPDSCH.
NPDCCH
NPDCCH carries scheduling information for uplink and downlink data channels, including HARQ acknowledgment information for uplink data channels, paging indications, random access response scheduling information, data information from higher layers, paging messages, system messages, and random access response messages.
Unlike LTE’s PDCCH, which uses the first few symbols of the subframe, NPDCCH differs significantly, occupying 6 subcarriers in the frequency domain as NCCE (Narrowband Control Channel Element).
In standalone and guard band modes, all OFDM symbols can be used, while in in-band mode, NPDCCH is staggered with LTE control symbol positions. NPDCCH has two formats:
•NPDCCH format 0 has an aggregation level of 1, occupying NCCE0 or NCCE1
•NPDCCH format 1 has an aggregation level of 2, occupying NCCE0 and NCCE1.
The maximum repetition count for NPDCCH can be configured, with values ranging from {1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048}.
NPDSCH
NPDSCH occupies frequency domain resources across 12 subcarriers. In standalone and guard band modes, it uses all OFDM symbols. In in-band mode, it must stagger with LTE control domain symbols. Since SIB1-NB indicates the number of control domain symbols, if NPDSCH is used in SIB1-NB, the first three symbols are fixedly staggered.
NPDSCH uses QPSK modulation, with MCS ranging from 0 to 12. The repetition counts are {1, 2, 4, 8, 16, 32, 64, 128, 192, 256, 384, 512, 768, 1024, 1536, 2048}.
NRS
NRS (Narrowband Reference Signal), also known as pilot signal, primarily serves to measure channel quality in the downlink, used for coherent detection and demodulation by the terminal. NRS must be transmitted in all downlink subframes for broadcast and dedicated downlink channels, regardless of whether data is being transmitted.
NRS is time-frequency multiplexed with the information-carrying symbols in the subframes carrying NPBCH, NPDCCH, and NPDSCH, using 8 RE per subframe for each antenna port.
NPSS and NSSS
NPSS and NSSS are used for NB-IoT terminals to perform cell search, including time and frequency synchronization and detecting Cell ID. Since LTE’s synchronization sequence occupies 6 PRBs, NB-IoT cannot occupy these 6 PRBs. To avoid conflicts, NB-IoT requires redesigning.
NPSS is located in subframe #5 of every 10ms radio frame, with a period of 10ms, using the last 11 OFDM symbols in each subframe (as shown in the figure below).
For NB-IoT terminals, executing NPSS detection is a computationally complex process, conflicting with its design goal of simplification. Therefore, NPSS is designed as a short ZC (Zadoff-Chu) sequence.
NSSS is located in subframe #9, with a period of 20ms, appearing only in even frames, also using the last 11 OFDM symbols in each subframe.
NPSS provides time and frequency synchronization reference signals for NB-IoT terminals. Unlike LTE, NPSS does not carry any cell information, while NSSS carries PCI.
Uplink:
For the uplink, NB-IoT defines two physical channels:
①NPUSCH, Narrowband Physical Uplink Shared Channel.
②NPRACH, Narrowband Physical Random Access Channel.
Additionally, DMRS serves as the uplink demodulation reference signal.
NPRACH
Since LTE’s PRACH channel bandwidth is 1.08MHz, which is far greater than NB-IoT’s uplink bandwidth, it needs to be redesigned.
Unlike LTE’s Random Access Preamble, NB-IoT’s Random Access Preamble is single-tone transmission, with a subcarrier spacing of 3.75kHz, occupying one subcarrier. There are two formats: Preamble format0 and format1, corresponding to CP lengths of 66.7us and 266.7us, respectively, for different cell radii.
A single transmission of the Random Access Preamble contains four Symbol Groups, forming one NPRACH channel, with each Symbol Group comprising 5 Symbols and 1 CP (as shown in the figure below).
When the CP length is 66.67s (Format 0), the cell coverage radius reaches 10 kilometers. When the CP length is 266.7s (Format 1), the coverage radius reaches 40 kilometers. To extend coverage, the NPRACH channel can enhance coverage through repetition, with repetition counts of {1, 2, 4, 8, 16, 32, 64, 128}.
NPUSCH
NPUSCH is used to transmit uplink data and uplink control information. The uplink subcarrier spacing can be either 3.75KHz or 15KHz. There are two transmission methods for uplink: single-carrier transmission (Single tone) and multi-carrier transmission (Multi-tone), where Single tone subcarrier bandwidth includes both 3.75KHz and 15KHz, and Multi-tone has a subcarrier spacing of 15KHz, supporting 3, 6, or 12 subcarriers for transmission.
NPUSCH defines two formats: Format 1 and Format 2.
Format 1 is designed for uplink channel data on UL-SCH, using the same Turbo coding error correction as LTE, with resource block sizes far smaller than LTE, not exceeding 1000 bits.
Format 2 is used for HARQ acknowledgment signaling on NPDSCH, transmitting uplink control information (UCI), using repetition codes for error correction.
The smallest unit mapped to transmission blocks is called a resource unit (RU). It is determined by the NPUSCH format and subcarrier spacing. Uplink transmission resources are allocated on a RU (Resource Unit) basis, with Single tone and Multi-tone RU units defined as follows, and the number of scheduled RUs can be {1, 2, 3, 4, 5, 6, 8, 10}, as indicated in NPDCCH N0.
Unlike the basic unit of resource allocation in LTE systems, which is a subframe, NB-IoT uses the number of subcarriers and time slots as the basic unit for resource allocation:
For NPUSCH format 1,
When the subcarrier spacing is 3.75 kHz, only single-tone transmission is supported, with one RU in the frequency domain containing 1 subcarrier and 16 time slots, thus, the length of one RU is 32ms.
When the subcarrier spacing is 15kHz, both single-tone and multi-tone transmissions are supported, with one RU containing 1 subcarrier and 16 time slots, having a length of 8ms; when one RU contains 12 subcarriers, it has a time length of 2 time slots, which is 1ms, and this resource unit corresponds exactly to one subframe in LTE systems. The time length of the resource unit is designed as a power of 2 to utilize resources more effectively, avoiding resource gaps that lead to wastage.
For NPUSCH format 2,
RU always consists of 1 subcarrier and 4 time slots, so when the subcarrier spacing is 3.75 kHz, one RU lasts 8ms; when the subcarrier spacing is 15kHz, one RU lasts 2ms.
NPUSCH uses low-order modulation coding schemes MCS 0 to 11, with repetition counts of {1, 2, 4, 8, 16, 32, 64, 128}.
DMRS
DMRS is used for channel estimation. NPUSCH Format 1 structure is the same as LTE PUSCH time slot structure, with 7 OFDM symbols per time slot, and one symbol in the middle serves as DMRS. Format 2 also consists of 7 OFDM symbols per time slot, but uses three symbols in the middle as DMRS.
Physical Channels of eMTC
The subframe structure of eMTC is the same as LTE. Compared to LTE, eMTC downlink PSS/SSS and CRS are consistent with LTE, while PCFICH and PHICH channels are removed. It is compatible with LTE PBCH, adding repeated transmissions to enhance coverage, and MPDCCH is designed based on LTE’s EPDCCH, supporting repeated transmissions, while PDSCH adopts cross-subframe scheduling. Uplink PRACH, PUSCH, and PUCCH are similar to existing LTE structures.
eMTC can define up to 4 coverage levels, each with different configurable PRACH parameters for different repetition counts. Based on repetition counts, eMTC is divided into Mode A and Mode B, with Mode A having no repetitions or fewer repetitions, while Mode B has more repetitions.
▲Maximum Retransmission Counts of Different eMTC Channels Under Mode A and Mode B
Downlink:
PBCH
eMTC PBCH is fully compatible with LTE systems, with a cycle of 40ms, supporting eMTC’s cell indication fields. It employs repeated transmissions to enhance coverage, with each transmission repeating up to 5 times.
MPDCCH
MPDCCH (MTC Physical Downlink Control Channel) is used to send scheduling information, designed based on LTE R11’s EPDCCH. Terminals receive control information based on DMRS, supporting functions such as control information precoding and beamforming. An EPDCCH transmits one or multiple ECCEs (Enhanced Control Channel Element), with aggregation levels of {1, 2, 4, 8, 16, 32}, each ECCE consisting of multiple EREGs (Enhanced Resource Element Group).
The maximum repetition count for MPDCCH can be configured, with values ranging from {1, 2, 4, 8, 16, 32, 64, 128, 256}.
PDSCH
eMTC PDSCH is fundamentally similar to LTE PDSCH channels, but adds repetition and narrowband interleaving to improve PDSCH channel coverage and interference averaging. eMTC terminals can operate in both Mode A and Mode B:
• In Mode A, the maximum number of uplink and downlink HARQ processes is 8, with PDSCH repetition counts of {1, 4, 16, 32}
• In Mode B, the maximum number of uplink and downlink HARQ processes is 2, with PDSCH repetition counts of {4, 16, 64, 128, 256, 512, 1024, 2048}
Uplink:
PRACH
The time-frequency resource configuration of eMTC’s PRACH follows LTE’s design, supporting formats 0, 1, 2, and 3. It occupies 6 PRB resources, with different repetition counts supporting narrowband interleaving. Each coverage level can configure different PRACH parameters.
PRACH channels enhance coverage through repetition, with repetition counts of {1, 2, 4, 8, 16, 32, 64, 128, 256}.
PUCCH
PUCCH frequency domain resource formats are the same as LTE, supporting interleaving and repeated transmissions.
Mode A supports HARQ-ACK/NACK, SR, CSI on PUCCH, namely supporting PUCCH formats 1/1a/2/2a, with repetition counts of {1, 2, 4, 8}; Mode B does not support CSI feedback, only supporting PUCCH formats 1/1a, with repetition counts of {4, 8, 16, 32}.
PUSCH
PUSCH is the same as LTE, but the maximum number of RBs that can be scheduled is limited to 6. Both Mode A and Mode B are supported, with Mode A repetition counts of {8, 16, 32}, supporting up to 8 processes with higher rates; Mode B covers longer distances, with repetition counts of {192, 256, 384, 512, 768, 1024, 1536, 2048}, supporting a maximum of 2 HARQ processes in the uplink.
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