nwen 439 - ecs.wgtn.ac.nz

NWEN 439Special Topic: Protocols and

Architecture for the Internet of Things

Winston [email protected]

Week 3 Lecture 2

IoT Wireless Technologies• Long-Range Low Power Wireless Technologies• LoRaWAN• Narrowband IoT (NB-IoT)• IEEE 802.11ah

• Short-Range Low Power Wireless Technologies• Bluetooth• IEEE 802.15.4• Z-Wave

NWEN 439 Protocols and Architectures for IoT 2

References

• Aamir Riaz, “Inter-communicating things – IoTs,” Workshop Pacific Radio-communication Workshop 2019 (PRW-19), Coral Coast, Fiji, 11-12 April 2019.• Sami Tabbane, “LTE-M and NB-IoT Networks,” CoE Training on

Traffic Engineering and advanced wireless network planning, Bangkok, Thailand, 30 September to 3 October, 2019.


Narrowband IoT

• 3GPP effort that standardized the communication technology for carrying IoT data over cellular networks.

• 3GPP Release 13, completed in June 2016

• Licensed band – proper regulation, and possibly data type prioritization policies

• Good for low end devices (low throughput, delay-tolerant use cases with low mobility support, e.g. smart meters, remote sensors, smart buildings, etc.)


2

II. TRANSMISSION SCHEMES AND DEPLOYMENT OPTIONS

A. Downlink transmission scheme The downlink of NB-IoT is based on OFDMA with the

same 15 kHz subcarrier spacing as LTE. Slot, subframe, and frame durations are 0.5 ms, 1 ms, and 10 ms, respectively, identical to those in LTE. Furthermore, slot format in terms of cyclic prefix (CP) duration and number of OFDM symbols per slot are also identical to those in LTE. In essence, an NB-IoT carrier uses one LTE PRB in the frequency domain, i.e. twelve 15 kHz subcarriers for a total of 180 kHz. Reusing the same OFDM numerology as LTE ensures the coexistence performance with LTE in the downlink. For example, when NB-IoT is deployed inside an LTE carrier, the orthogonality between the NB-IoT PRB and all the other LTE PRBs is preserved in the downlink.

B. Uplink transmission scheme The uplink of NB-IoT supports both multi-tone and single-

tone transmissions. Multi-tone transmission is based on SC-FDMA with the same 15 kHz subcarrier spacing, 0.5 ms slot, and 1 ms subframe as LTE. Single-tone transmission supports two numerologies, 15 kHz and 3.75 kHz. The 15 kHz numerology is identical to LTE and thus achieves the best coexistence performance with LTE in the uplink. The 3.75 kHz single-tone numerology uses 2 ms slot duration. Like the downlink, an uplink NB-IoT carrier uses a total system bandwidth of 180 kHz.

C. Deployment options NB-IoT may be deployed as a stand-alone carrier using any

available spectrum exceeding 180 kHz. It may also be deployed within the LTE spectrum allocation, either inside an LTE carrier or in the guard band. These different deployment scenarios are illustrated in Fig. 1. The deployment scenario, stand-alone, in-band, or guard-band, however should be transparent to a user equipment (UE) when it is first turned on and searches for an NB-IoT carrier. Similar to existing LTE UEs, an NB-IoT UE is only required to search for a carrier on a 100 kHz raster. An NB-IoT carrier that is intended for facilitating UE initial synchronization is referred to as an anchor carrier. The 100 kHz UE search raster implies that for in-band deployments, an anchor carrier can only be placed in certain PRBs. For example, in a 10 MHz LTE carrier, the indexes of the PRBs that are best aligned with the 100 kHz

grid and can be used as an NB-IoT anchor carrier are 4, 9, 14, 19, 30, 35, 40, 45. The PRB indexing starts from index 0 for the PRB occupying the lowest frequency within the LTE system bandwidth.

Fig. 1 illustrates the deployment options of NB-IoT with a 10 MHz LTE carrier. The PRB right above the DC subcarrier, i.e., PRB #25, is centered at 97.5 kHz (i.e. a spacing of 6.5 subcarriers) above the DC subcarrier. Since the LTE DC subcarrier is placed on the 100 kHz raster, the center of PRB#25 is 2.5 kHz from the nearest 100 kHz grid. The spacing between the centers of two neighboring PRBs above the DC subcarrier is 180 kHz. Thus, PRB #30, #35, #40, and #45 are all centered at 2.5 kHz from the nearest 100 kHz grid. It can be shown that for LTE carriers of 10 MHz and 20 MHz, there exists a set of PRB indexes that are all centered at 2.5 kHz from the nearest 100 kHz grid, whereas for LTE carriers of 3 MHz, 5 MHz, and 15 MHz bandwidth, the PRB indexes are centered at least 7.5 kHz away from the 100 kHz raster. Further, an NB-IoT anchor carrier should not be any of the middle 6 PRBs of the LTE carrier (e.g. PRB#25 of 10 MHz LTE, although its center is 2.5 kHz from the nearest 100 kHz raster). This is due to that LTE synchronization and broadcast channels occupy many resource elements in the middle 6 PRBs, making it difficult to use these PRBs for NB-IoT.

Similar to the in-band deployment, an NB-IoT anchor carrier in the guard-band deployment needs to have center frequency no more than 7.5 kHz from the 100 kHz raster. NB-IoT cell search and initial acquisition are designed for a UE to be able to synchronize to the network in the presence of a raster offset up to 7.5 kHz.

Multi-carrier operation of NB-IoT is supported. Since it suffices to have one NB-IoT anchor carrier for facilitating UE initial synchronization, the additional carriers do not need to be near the 100 kHz raster grid. These additional carriers are referred to as secondary carriers.

III. PHYSICAL CHANNELS NB-IoT physical channels are designed based on legacy LTE to a large extent. In this section, we provide an overview of them with a focus on aspects that are different from legacy LTE.

A. Downlink NB-IoT provides the following physical signals and

Fig. 1. Examples of NB-IoT stand-alone deployment and LTE in-band and guard-band deployments in the downlink.

LTE P

RB

#0LTE

PR

B #1

LTE DC subcarrier

LTE P

RB

#48LTE

PR

B #49

LTE P

RB

#24

LTE P

RB

#25

NB

-IoT

10 MHz LTE carrierLTE Guard-band

NB

-IoT

NB

-IoT

stand-aloneguard-bandin-band

LTE Guard-band

NB-IoT Deployment

•Standalone – new or reuse GSM channels• In-band – allocating a portion of LTE physical layer

resources, i.e. one physical resource block (PRB) to carry NB-IoT data•Guard band


4�STANDARDIZING NARROWBAND IoT

4 E R I C S S O N T E C H N O L O G Y R E V I E W �4��A P R I L 22 , 2016

have been included as key considerations in the speci!cation of NB-IoT. To future-proof the technology, its design exploits synergies with LTE by reusing the higher layers (RLC, MAC, and RRC), for example, and by aligning numerology (the foundation of the physical layer) in both the uplink and downlink. However, the access procedures and control channels for NB-IoT are new.

Prior to NB-IoT speci!cation, work had already begun on the design of another radio access for massive MTC to support Cat-M1 – a new UE category. With completion also targeted for release 13, the resulting standardization work item – eMTC – covers bitrates, for example, ranging from hundreds of kbps to 1Mbps. These requirements are broader than NB-IoT, which has been streamlined for applications with widely varying deployment characteristics, lower data rates, and operation with simpli!ed low-cost devices.

With a carrier bandwidth of just 200kHz (the equivalent of a GSM carrier), an NB-IoT carrier can be deployed within an LTE carrier, or in an LTE or WCDMA guard band*. The link budget of NB-IoT has a 20dB improvement over LTE Advanced. In the uplink, the speci!cation of NB-IoT allows for many devices to send small amounts of data in parallel.

Release 13 not only includes standards for eMTC and NB-IoT, it also contains important re!nements, such as extended discontinuous reception (eDRX) and power save mode (PSM). PSM was completed in release 12 to ensure battery longevi&, and is complemented by eDRX for use cases involving devices that need to receive data more frequently.

Deployment flexibility and migration scenarios As a !nite and scarce natural resource, spectrum needs to be used as efficiently as possible. And so technologies that use spectrum tend to be designed to minimize usage. To achieve spectrum efficiency, NB-IoT has been designed with a number of deployment options for GSM, WCDMA, or LTE spectrum, which are illustrated in Figure 2.gg standalone – replacing a GSM carrier with an NB-IoT

carriergg in-band – through flexible use of part of an LTE carriergg guard band – either in WCDMA or LTE

Starting with standalone The standalone deployment is a good option for WCDMA or LTE ne(orks running in parallel with

GSM

LTE LTE

LTE

Standalone

200kHz

200kHz

200kHz

In-band

Guard band

Figure 2: Spectrum usage

deployment options

*Guard band is a thin band of spectrum between radio bands that is used to prevent interference.

NB-IoT within LTE Physical Layer


•an NB-IoT carrier uses one LTE PRB in the frequency domain (12 15kHz subcarriers) for a total of 180 kHz.•Only ONE channel

per subframe for NB-IoT

NB-IoT Features

• NB-IoT promises: Improved coverage, low power consumption, and massive connections (50K); simulations show support for up to 200K connections*• 180 kHz bandwidth: reduction of RF/baseband

complexity, costs and power consumption• Low power consumption, battery life à• Long range (<35km) & indoor/

underground penetration capabilities


* S. Landström, J. Bergström, E. Westerberg and D. Hammarwall, “NB-IoT: A Sustainable Technology For Connecting Billions Of Devices”, Ericsson Technology Review, April 22, 2016.

of 20 bytes application report, 65 bytes upper layer protocol header, and 15 bytes SNDCP/LLC/RLC/MAC/CRC overhead is assumed. The following steps are used in calculating the exception report latency –

• Synchronization • Master Information Block (MIB) acquisition • Random access including Msg 1-4. This also includes a

waiting time between reception of random access preamble and Random Access Response (RAR) transmission in case the eNB is in the middle of a downlink transmission.

• Uplink grant + data transfer (for 99% confidence level) Table VI shows the latency results. It can be seen that latency is below 10 seconds in both scenarios.

TABLE VI. LATENCY EVALUATION

Activity Stand-alone In-band

Synchronization 520 1110

MIB acquisition 640 1920

PRACH 1440 1440

Wait 572 1440

DCI + RAR 72 288

Msg3 349 349

DCI + Msg4 83 299

DCI (UL grant) 45 153 UL Data Tx (99% confidence) 2883 2883

Total Latency 6604 9882

D. Battery Life Analysis

For power consumption, the target is to minimize consumption to provide battery life of ten years with battery capacity of 5 Wh at 164-dB MCL. The analysis assumes periodic uplink reporting with the transactions during an uplink reporting event are shown in Fig. 6.

Fig. 6. Message exchange during an uplink reporting event.

The power consumption per state used the assumption in Table VII.

TABLE VII. CURRENT CONSUMPTION ASSUMPTIONS

Power [mW] Battery power during Tx (assuming 44% PA efficiency) 543

Battery power for Rx 90 Battery power when Idle but not in PSS 2.4

Battery power in Power Save State (PSS) 0.015

In Table VIII the estimated lifetime in years are presented for two different packet sizes, two reporting intervals and at 164-dB MCL coverage level.

TABLE VIII. BATTERY LIFE (YEARS)

Packet size, reporting interval Stand-alone In-band

50 bytes, 2 hours 2.6 2.4

200 bytes, 2 hours 1.2 1.2

50 bytes, 1 day 18.0 16.8

200 bytes, 1 day 11.0 10.5

The energy consumption evaluation shows that the 10 year battery life target can be met or exceeded for “once per day” traffic scenarios.

IV. CONCLUSION

This paper provides an overview of NB-IoT and discusses the design targets of NB-IoT include low-cost devices, high coverage (20dB improvement over GPRS), long device battery life (more than 10 years), and massive capacity. Our results show that the targets can be achieved in all deployment scenarios.

REFERENCES [1] Ratasuk, R.; Mangalvedhe, N.; Ghosh, A., "Overview of LTE

enhancements for cellular IoT," PIMRC, Sept. 2015. [2] Ratasuk, R.; Prasad, A.; Zexian Li; Ghosh, A.; Uusitalo, M., "Recent

advancements in M2M communications in 4G networks and evolution towards 5G," ICIN, Feb. 2015.

[3] 3GPP TR 36.888, Study on provision of low-cost Machine-Type Communications (MTC) User Equipments (UEs) based on LTE, v.12.0.0, June 2013.

[4] TR 45.820, “Cellular System Support for Ultra Low Complexity and Low Throughput Internet of Things,” V2.1.0, August, 2015.

[5] RP-150492, “Revised WI: Further LTE Physical Layer Enhancements for MTC,” Ericsson, RAN#67, Shanghai, China.

[6] RP-152284, “Revised Work Item: Narrowband IoT (NB-IoT),” Huawei, HiSilicon, RAN#70, Sitges, Spain.

[7] R1-157248, “NB IoT – Capacity evaluation,” Nokia Networks, RAN1#83, Anaheim, USA, 2015.

[8] R1-157246, “Coverage Evaluation for Stand-alone Operation,” Nokia Networks, RAN1#83, Anaheim, USA, 2015.

[9] R1-157249, “Coverage Evaluation for Guard-band Operation,” Nokia Networks, RAN1#83, Anaheim, USA, 2015.

[10] R1-157252, “Coverage Evaluation for In-band Operation,” Nokia Networks, RAN1#83, Anaheim, USA, 2015.

Battery Life (years)

NB-IoT Data Rates and Latency

•Maximum transmission block sizes (TBS) : • Downlink: 680bits• Uplink: 1000bits

•Peak data rates at Layer 1: • Downlink: 226.7 kbps, using max TBS (680 bits) over 3ms• Uplink: 250 kbps, using max TBS (1000 bits) over 4ms

• Latency – up to 10s


Cell Search and Initial Access

When UE is powered on:1. Detects and picks a suitable cell to join2. Obtains the frame, subframe and symbol timings, and

synchronize to carrier frequency3. Initiates a random access procedure to join the network;

NB-IoT random access procedure is contention-based.


Random Access (RA) Procedure1. UE transmits a RA preamble, and starts the random

access response (RAR) window, W

2. Upon successful preamble reception, Base Station (BS) transmits an RAR containing info on uplink resources to use for next message transmission.

3. UE transmits its ID (Scheduled Transmission) & other key info [Msg3] then starts Contention Resolution Timer

4. BS performs contention resolution and informs successful UEs via the Contention Resolution Message

5. UEs that do not receive any response by the expiration of timers will restart process, after backoff time b, randomly chosen from [0,B]


Source: Y Sun, F Tong, Z Zhang and S He, “Throughput Modeling and Analysis of Random Access in Narrowband Internet of Things,” IEEE Internet of Things journal, vol 5, no 3, June 2018.

SUN et al.: THROUGHPUT MODELING AND ANALYSIS OF RANDOM ACCESS IN NB-IoT 1487

Fig. 1. Procedure of random access in NB-IoT.

is configured according to the coverage levels, includ-ing normal coverage, extended coverage, and extremecoverage. The corresponding MCL is 144/154/164 dB.Every preamble sequence has a cyclic prefix and fivesymbols 1. If the eNB does not receive the preamblesequences, UEs need to retransmit.

2) Random Access Response: UEs start a timer called ran-dom access response (RAR) window after transmittingthe preamble sequences to the eNB. If the UEs do notreceive a response during this time, the sequences needto be retransmitted. After the eNB receives the preamble,it sends the RAR to the UEs, which includes the ran-dom access radio network temporary identity, the timingadvance, a temporary cell radio network temporary iden-tifier and some Msg3 scheduling messages. When morethan one UE selects the same preamble sequences instep one, a collision happens. The conflicting UEs willreceive the same RAR from eNB.

3) Scheduled Transmission: The scheduled transmission,also called Msg3, contains the RRC connection, recon-figuration requests and data volume and power head-room report (DPR). DPR consists of power headroomreport and buffer status report. UEs can be allocatedwith the corresponding channel resources according toDPR. Unless the UEs receive any response from eNB,they will redo this step.

4) Contention Resolution: The contention resolution is alsocalled Msg4. The eNB randomly selects a UE that com-pletes step 3 to transmit Msg4. After receiving Msg4,the UE begins data transmission.

For ease of exposition, we make the following assumptionsfor the random access in NB-IoT.

1) In practice, when a collision happens, the UEs transmit-ting the same sequences in step 1 will receive the sameRAR. The contention will be solved completely afterstep 4. Here we assume that retransmission is necessarywhen there is a collision in step 1. We do not considerany power or physical loss in random access procedure.

TABLE IINOTATIONS

2) There is a cycle time T defined as the service timefrom the start of the current period to the start of thenext period, including the maximum backoff time andthe transmission time of a packet. The UEs that selectthe same delay parameter in backoff mechanism conflictwith each other and need to wait for a new cycle. Oncea UE occupies the channel, the other UEs have to waitfor a new cycle, select a new delay parameter in backoffmechanism and initiate a new random access procedure.

3) If more than one UE selects the same delay parameter,collision happens. Then, the UEs need to select a newdelay parameter randomly from 0 to the contention win-dow size for retransmission. The contention window isthe maximum delay in the backoff mechanism.

4) The length of a queue in NB-IoT is K, i.e., the queue canhold up to K packets. Different packet number representsdifferent queue state. When the queue is empty, the UEdoes not initiate a random access procedure, i.e., the UEdoes not conflict with other UEs. The packets’ arrivalsis a Poisson process with a rate of λ.

5) The probability that a UE successfully transmits a datapacket is equal to the probability that the UE occupies anavailable channel successfully. Once a UE successfullytransmits a packet, it releases the channel. If the queueis empty, the UE becomes idle. When there is a newpacket needs to be transmitted, the UE initiates a newrandom access procedure.

Some important notations used in our model are listed inTable II. Based on the assumptions above, we consider a fun-damental investigation of throughput modeling and analysis ofrandom access in NB-IoT under different parameters.

IV. THROUGHPUT MODELING AND ANALYSIS

In this section, we propose the system throughput modelingand analysis. In order to achieve the expression of system

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NB-IoT Assumptions

• IoT devices typically send small amounts of data.

• IoT devices tend to be deployed in signal-challenged locations like basements and remote rural areas.

•While massive in number, IoT devices communicate only intermittently, hence supporting 50K connections is viable.


NB-IoT vs LTE-M

LTE-M• Another option for IoT traffic in cellular networks

• Evolution of LTE optimized for Machine Type CommunicationsLow power consumption

• Long range ~ 11km

• Low data rate ~ 1 Mbps

• Low latency 100 ~ 150ms


LTE-M

• Bandwidth: 700-900 MHz for LTE• Some resource blocks reserved for IoT on LTE bands


Spectrum and access

• Licensed Spectrum

• Bandwidth: 700-900 MHz for LTE

• Some resource blocks allocated for IoT on LTE bands

18

NB-IoT vs LTE-M usage scenarios


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