NB-IoT, the cellular Narrowband IoT protocol was standardized for the first time in the 3GPP Release 13, and enhanced in the 3GPP Release 14. Here is a deep-dive into the 3GPP R14 NB-IoT protocol enhancements!
NB-IOT 3GPP R13
The 3GPP Release 13 specification standardized the NB-IoT protocol for providing narrowband wide-area connectivity for massive machine-type communications for IoT. The first NB-IoT specification provided the underlying air interface standard for an ultra-low complexity NB1 device class with a long battery life.
Overview of the 3GPP release 14 enhanced NB-IoT
The 3GPP Release 14 specification enhances the NB-IoT protocol in several ways:
- Increases NB-IoT positioning accuracy
- Introduces NB-IoT Multicast
- Enhances device mobility
- Increases peak data rates
- Introduces NB-IoT Multi-carrier operation
- Adds a lower device power class
- Allocates new NB-IoT frequency bands
The 3GPP Release 14 devices are referred to as Cat NB2 or Cat N2.
The main enhancements of the NB-IoT 3GPP R14 are described in the following.
NB-IoT release 14 positioning
Positioning capabilities have been one of the weak links in the NB-IoT protocol, and the 3GPP Release 14 introduces improvements to support applications that need accurate device positioning.
In 3GPP Release 14, Location and Positioning Protocol (LPP) signaling protocol is introduced for NB-IoT. While this protocol supports several positioning methods such as A-GNSS and WiFi; the OTDOA and E-CID positioning methods are specified in the 3GPP Release 14 together with the new Narrowband Positioning Reference signals (NPRS) and support for idle mode positioning.
What is OTDOA positioning?
OTDOA is an abbreviation for Observed Time Difference of Arrival, and it is a downlink based multilateration positioning method introduced in the LTE 3GPP Release 9.
How does OTDOA NB-IoT positioning work?
In OTDOA, an IoT device measures the time difference between specific signals from several the LTE base stations (eNodeB) and reports these time differences to a network system, Evolved Serving Mobile Location Center (ESMLC), which can derive the location of the device based on the time differences and knowledge of the eNodeBs’ locations.
The ESMLC sends an OTDOA measurement request to an IoT device through the LPP protocol. This measurement request contains additional assistance data including a list of cells, or eNodeBs with their Positioning Reference Signal (PRS) parameters and more.
LTE PRS is a well-established signal for Time of Arrival (TOA) measurements. The NB-IoT 3GPP Release 14 introduces a new PRS signal referred to as NPRS, i.e., Narrowband Positioning Reference Signal.
The IoT device then proceeds to carry out a set of Reference Signal Time Difference (RSTD) measurements during a given period. These measurements consist of estimating the exact time offsets between the Positioning Reference Signals received from the neighboring cells.
The device reports these time difference measurements together with an estimate of the measurement quality to the ESMLC, which uses them, and the knowledge of the cell locations to derive the position of the device.
Another new OTDOA functionality in the 3GPP R14 specification is the possibility of multiple PRS transmission configurations to enable higher positioning accuracy.
How does E-CID positioning works in NB-IoT?
Another method for NB-IoT positioning in 3GPP R14 is Enhanced Cell ID (E-CID), based on Cell of Origin (COO). The position of the device is estimated using the knowledge of the geographical coordinates of its serving LTE base station, the eNodeB.
In the simplest level of Cell ID positioning, accuracy is linked to the cell size, as the location server is only aware that a particular base station serves the device. This method would lack accuracy for some regulatory requirements.
However, E-CID positioning provides more advanced alternatives. The location of a device can be estimated more accurately by combining the cell location information with distance and angle-of-arrival measurements on radio signals.
The E-CID positioning accuracy can be enhanced with the estimation of the device’s distance from one, or three base stations based on the Narrowband Reference Signal Received Power (NRSRP) measurement, a standard quality measurement (NRSRQ); or TDOA and the measurement of the Timing Advance (TADV), or Round Trip Time (RTT). The device takes the measurements.
Another way to increase E-CID positioning accuracy is by combining the cell location information by measuring the Angle-of-Arrival (AoA) from two or three base stations. In this alternative, the base station takes the measurements.
The highest accuracy in E-CID can be derived when more than one base station is used in the calculations.
The 3GPP Release 14 introduces Multicast communication mode into the NB-IoT protocol.
With the introduction of the Multicast group communications functionality, NB-IoT becomes a more efficient protocol for massive IoT solutions with large fleets of simple devices such as smart light bulbs, smart-meters, sensors, actuators, switches.
Multicast enables efficient distribution of firmware, software, and task updates, or commands at once to a large group of IoT devices. Thus, it simplifies managing and maintaining large amounts of IoT devices running the same software and performing the same tasks. All IoT devices receiving the same content simultaneously save network resources – instead of being served one-by-one in a unicast mode. Additionally, NB-IoT Multicast enables synchronous control of things such as streetlights.
In the 3GPP Release 14 specification, NB-IoT Multicast is based on the Single Cell-Point to Multipoint (SC-PTM) protocol, with certain simplifications to support the lower device complexity, and stringent power efficiency requirements of NB-IoT.
How NB-IoT multicast works?
In current LTE networks, group communication is provided by the Multimedia Broadcast Multicast Service (MBMS) protocol, which is based on a subscription-based approach. However, MBMS multicast is inefficient in terms of resource utilization and energy consumption, and hence poorly suited for NB-IoT.
To keep device complexity and power consumption low, a simplified version of the SC-PTM is introduced for NB-IoT Multicast.
An NB2 device is required to receive SC-PTM signaling messages only when in the idle mode (RRC_IDLE), and the device does not have to process the Multicast Control Channel (SC-MCCH) signal at the same time as MBMS Traffic Channel (SC-MTCH), nor any SC-PTM transmission when paging or Random Access Response (RAR) is in progress.
Additionally, in contrast to LTE, there is no Single Cell Notification Radio Network Temporary Identifier (SC-N-RNTI) in NB-IoT Multicast. Instead, notification of SC-MCCH change is indicated directly in the Downlink Control Indicators (DCI) to avoid the need to send a Multicast notification separately.
NB-IoT non-anchor carrier operation
An NB-IoT anchor carrier is used for the initial synchronization of a device.
Since it is enough with one anchor carrier for the device initial synchronization, the additional carriers possibly allocated for NB-IoT are not required to be close to the anchor carrier, spectrum-wise. These additional carriers are referred to as non-anchor carriers, and they merely add physical resources to the network enabling a higher density of devices.
In 3GPP Release 14, NB-IoT supports a multi-carrier mode of up to 15 non-anchor carriers, in addition to the anchor carriers for Physical Random Access Channel (PRACH) procedures. With the additional carriers providing extra Physical Resource Blocks under the same network configuration, this setup can allow up to 1,000,000 IoT devices deployed per square kilometer.
Therefore, in NB-IoT multi-carrier mode, a specific anchor carrier is configured for initial connection setup and data transfer. The secondary non-anchor carriers are configured only for unicast data transmission.
NB-IoT mobility in 3GPP R14
NB-IoT mobility capabilities as defined in the 3GPP R13 version are limited. As an example, an NB1 device can begin a reconnection procedure in case of a radio link outage, or similar, only when in the idle mode. This is inefficient for instance in radio conditions with frequent connection breaks.
The NB-IoT 3GPP Release 14 specification addresses this shortcoming by extending the possibility of attempting a reconnection of a lost radio link also when an NB2 device is in the connected mode, instead of having to wait for the idle mode.
To conclude: the enhanced mobility in 3GPP R14 does not enhance NB-IoT with full mobility and handovers. However, the ability to recover a lost connection quicker increases the robustness and efficiency of NB-IoT especially in volatile radio environments, where radio conditions are generally good, but disturbances can occur frequently.
How does the enhanced NB-IoT mobility work?
The NB-IoT 3GPP R14 introduces two new signaling procedures in the LTE Radio Resource Control (RRC) protocol to implement the enhanced mobility in NB-IoT: Connection Re-establishment and S1 eNodeB Control Plane Relocation Indication procedures.
They enable the connection and data retransmissions on the S1 interface between the Mobility Management Entity (MME), and the IoT device maintained even in case of a radio link failure.
A security token is included in the RRC Connection Re-establishment Request and RRC Connection Re-establishment messages so that the MME can authenticate the device, and the device can authenticate the LTE base station.
In case of successful device authentication, the MME can instruct the old eNodeB to return non-delivered packets to the MME, and after which it can release the old connection on the S1-interface between the device and the old eNodeB.
Increased NB-IoT data rates and reduced latency
The NB-IoT 3GPP Release 14 specification introduces higher data rates by allowing larger blocks of data to be carried in each transmission, and by increasing the number of Hybrid Automatic Repeat Request (HARQ) processes to enable several parallel outstanding transmissions while waiting for feedback.
The range of Transport Block Sizes (TBS) an NB-IoT NB2 device can transmit and receive is increased to 2,536 bits for downlink and uplink directions.
Additionally, an NB2 device can have up to two HARQ processes for uplink and downlink (maximum of 1352 and 1800 bits respectively) compared to only one in each direction in NB-IoT in 3GPP Release 13.
The higher amount of HARQ processes allows further peak rate increases because the time-space between transmissions is reduced while if the device decoding capability is increased.
NB-IoT data rates
This table gives you maximum NB-IoT data rates for NB1 and NB2 devices.
|NB1 – Release 13||NB2 – Release 14|
|Max downlink TBS||680 bits||2536 bits|
|Max downlink data rate||~26 kbps||~80/127 kbps (1HARQ/2HARQ)|
|Max uplink TBS||1000 bits||2536 bits|
|Max uplink data rate||~62 kbps||~105/159 kbps (1HARQ/2HARQ)|
New NB-IoT power class in 3GPP R14
The 3GPP Release 14 introduces new, third power class of 14 dBm for NB-IoT devices. Now the NB2 devices can be designed in three power classes: 14, 20 and 23 dBm. The purpose of the lower power class is to allow the use of smaller batteries and to support devices with a small form factor.
The new lower power class of 14 dBm reduces the Maximum Coupling Loss (MCL) of NB-IoT to 155 dB, which is roughly the same as with LTE Cat M1. According to 3GPP, the MCL for NB-IoT was 164 dB previously.
WHAT’S MAXIMUM COUPLING LOSS (MCL)?
MCL is defined as the maximum total channel loss between a device and LTE base station antenna at which the data service can still be delivered. The higher the MCL, the more robust the link is.
New NB-IoT frequency bands in 3GPP release 14
The 3GPP R14 specification introduces five new frequency bands for the NB-IoT protocol: 11, 21, 25, 31, and 70. With the enhancements of 3GPP Release 14, NB-IoT continues to be an FDD-only IoT protocol.
NB-IoT R14 is more robust and efficient!
NB-IoT has quickly gained a reputation as the de-facto cellular LPWA IoT protocol. With the enhancements in NB-IoT 3GPP Release 14 specification, the LTE Cat NB2 protocol becomes more robust, faster and power-efficient wireless protocol; device makers can fit NB-IoT into smaller devices, it enables efficient firmware and software updates for massive IoT device fleets and supports applications which need synchronous group communication and accurate positioning.