Narrowband internet of things (NB-IoT), a recent cellular radio access technology based on Long-Term Evolution (LTE) introduced by Third-Generation Partnership Project (3GPP) for Low-Power Wide-Area Networks (LPWAN). The main motive of NB-IoT is to support massive machine-type communication (mMTC) and enable low-power, low-cost, and low-data-rate communication.
Narrowband-IoT Standard and Releases
Early in 2014, the LPWAN market rapidly developed due to the emergence of IoT. Realizing the need and potential for new communication ways, there started a feasibility study on cellular system support for ultra-low complexity and low throughput IoT solution referred to as cellular IoT. In the hot summer of 2014, there was a proposal for Narrowband Machine to Machine (NB-M2M) as a study item to cope with the IoT market needs and later in the same year, it was also proposed for the narrowband orthogonal frequency division multiplexing (NB-OFDM). In May 2015, 3GPP merged the two proposals (i.e., NB-M2M and NB-OFDM) and formed Narrowband Cellular IoT (NB-CIoT). Eight months later, there was another proposal for Narrowband Long-Term Evolution NB-LTE. At the end of 2015, 3GPP included all proposals as a work item for Release 13. NB-CIoT and NB-LTE had a key difference which was the number of the reused legacy LTE network resources to support interoperability. In the mid of 2016 NB-IoT was recognized as a new clean slate RAT. Only a few more improvement changes were allowed and implemented thereafter.
NB-IoT Release 13
3GPP introduced the techniques mentioned below in NB-IoT Release 13 to enable cellular massive IoT deployment for diverse use cases with low power, low complexity, and low cost.
- Mode of Operation
- With limited bandwidth requirement, NB-IoT can be deployed in three different modes which are standalone, in-band, and guard-band. In in-band and guard-band modes, NB-IoT occupies one PRBs of 180 KHz in LTE spectrum both in the downlink and uplink. It can also be allocated as standalone where it occupies the 200 KHz bandwidth by “refarming” the GSM spectrum.
- Multi-Tone Transmission Support
- NB-IoT introduces the allocation of Resource Units (RU) to multiple User Equipment (UE) to reach the massive device deployment objective contrary to LTE where the whole resource block is allocated to a single UE in the uplink.
- Complexity and Cost Reduction Techniques
- NB-IoT must have low complexity to reach the low-cost objective to facilitate massive connections. The features which were implemented to reach this objective include relaxed base-band processing, low memory storage, and reduced radio-frequency (RF) components. NB-IoT uses the restricted BPSK and QPSK modulation scheme in uplink and downlink transmission just by using single antenna support.
- Power Reduction Method
- NB-IoT devices are intended to have 10 years’ battery life so that they can support massive deployment with limited human intervention. In this there are two features i.e., Power Saving Mode (PSM) and extended Discontinuous Reception (eDRx) were supported. In Release 13, the device can be in PSM mode for approx. up to about 413 days but in eDRX, the device is in an inactive mode just for a few minutes or a few hours only.
- Coverage Enhancement Method
- NB-IoT is designed in such a way that it enhances the coverage for the applications which are in hard-to-reach areas such as deep indoors and basements. To this problem, NB-IoT now delivers an additional coverage of 20 dB as compared to the legacy LTE system.
NB-IoT Release 14 Enhancements
After the implementation of Release 13 features, studies erupted along with field trials that revealed the need for further enhancements to improve the quality of service as well as user experience. In this regard, 3GPP introduced a new enhancement features to NB-IoT – Release 14.
- Improved Positioning Technique
- Release 14 has an indoor advanced positioning method for observed time difference of arrival (OTDOA) in NB-IoT to enhance UE position measurement of cell identity (CID). In OTDOA method, the UE measures the times of arrival (ToAs) of positioning reference signals (PRSs) received from different transmitters concerning a reference node’s PRS transmission to form the reference signal time difference (RSTD) measurements.
- Multicast Services
- In Release 14, Multimedia Broadcast Multicast Services (MBMS) is supported through single-cell point-to-multipoint (SC-PTM). SC-PTM uses NPDSCH by mapping Single-cell MBMS Control CHannel (SC-MCCH) and Single-Cell MBMS Traffic CHannel (SC-MTCH) that carry control and data traffic to the physical layer scheduled by using the downlink control information (DCI). It optimizes resources as well as transmission latency by addressing the data to a group of UEs at the same time rather than sending it multiple times to separate devices.
- A New Power Class for Narrowband-IoT User Equipment
- In Release 14, the maximum allowed device’s output power has been reduced to 14 dBm. It has led to coverage relaxation of 9 dB that corresponds to 155 dB MCL as compared to 164 dB MCL and hence reduces the drained current.
- New Transport-Block-Size Support
- Release 14 added a new NB-IoT device category which supports the improved data rates by enhancing the Transport Block Size (TBS) to 2536 bits. These data rates can be reached thanks to the ability to support a second Hybrid Automatic Repeat Request (HARQ) for enhancing the reliability of the link for the UEs that experience favorable channel conditions.
- User Equipment Mobility Enhancement
- Release 14 has shown the possibility of Radio Resource Control (RRC) re-establishment for NB-IoT UE that supports data transfer via the control plane, i.e., the UE will try to re-establish the connection on that cell and resume the data transfer. This new RRC re-establishment feature hides the temporary loss of the radio interface to the upper layers.
NB-IoT Release 15 Enhancements
Of all the enhancements that were introduced in Releases 13 and 14, further improvements were added in Release 15 to satisfy the fast adoption of massive deployment which as follows.
- Latency Reduction
- In Release 15, NB-IoT supports a few new features to further reduce the transmission delay as well as to reduce the power consumption dissipated during long transmission requirements. The NB-IoT UE is now able to support the physical layer Scheduling Request (SR) which requests the network to send the access grant so that the UE can transmit the uplink data.
- Semi-Persistent Scheduling
- For better voice message support for the corresponding use cases, in Release 15, Semi-Persistent Scheduling (SPS) feature is introduced. The base station preconfigures the UE with the Radio Network Temporary Identifier (SPS-RNTI) which is used to specifically differentiate one NB-IoT UE from another or one radio channel from another. SPS enables the NB-IoT data reception at a regular configured periodicity.
- Small Cell Support
- For further improvement in the capacity as well as coverage, in Release 15, NB-IoT supports small cell deployments. NB-IoT UE restricts to allow transmit more power than the configured maximum power to avoid interference. On the other hand, to extend the IoT connectivity especially in remote and rural areas for use cases such as agriculture, logistics, and environmental monitoring, NB-IoT is now able to support up to 100 km range.
- Enhanced User Equipment Measurements
- UE measurements in Release 15 are improved in a way that only NSSS additionally to NRS is defined for radio resource management measurement enhancement. This means that NRS is determined by the resource elements that carry NSSS in the NSSS occasions that the UE measures in which the cell search and initial cell acquisition are improved.
- Time Division Duplex (TDD) Support
- In Release 15, a new feature of TDD support is introduced with a new TDD frame structure (type 2 To support some of the TDD configurations with few downlink subframes, few of the system information (SI) can be transmitted on non-anchor carriers. With that, UE will have reduced system information acquisition and search time, and hence reduced UE differentiation and access control.
NB-IoT Release 16 Enhancement Prospects
3GPP and many players in the telecom industry are involved in ongoing discussions for Release 16 enhancements. The agenda includes the following objectives with their corresponding solutions.
- Grant-Free Access
- Most of the power consumption takes place during the NB-IoT UE active time around Tx and Rx. In Release 16, the UE will be expected to transmit during RRC-Idle mode through Msg3 (RRC connection request) without access grant. A UE in RRC connected mode can transmit data without grant or with the simplified control-less grant.
- Simultaneous Multi-User Transmission
- The introduction of new schemes will enable simultaneous multi-user transmissions by using a shared resource in the time and frequency domains, such as Code division multiplexing (CDM), and multi-user multiple inputs multiple outputs (MU-MIMO), without increasing the number of antennae at the UE.
- Enhanced Group Message Mechanism
- In Release 16, there should be more enhancements to support downlink command between user groups and group RNTIs. This is due to MBMS which was proposed in Release 14 is only efficient for large size downlink command message transmission and requires many UEs to be deployed.
- Inter-RAT Idle-Mode Mobility
- For applications such as smart tracking of logistics that involve mobility, the NB-IoT UE may still need to be accessible even when moved to the area served by other base stations. 3GPP should introduce the new feature for NB-IoT UE support for inter-RAT mobility during idle mode because handover helps to reduce system information reading time.
- Network Management Tool Enhancement to Improve UE Differentiation
- NB-IoT UE is expected to be able to perform differentiation according to maximal tolerable delay per service to optimize the radio resource usage. As in the last release, the UE can be differentiated according to the traffic model (periodic communication indicator, periodic time, scheduled communication time, traffic profile) and battery indication.
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- Attach rate configuration
- NB1, NB2 and CAT-M support
- Coverage Enhancement
- CP control panel
- Sub Carrier Spacing
- Single Tone & Multi Tone
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