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Positioning for the Internet of Things: A 3GPP Perspective

Xingqin Lin, Johan Bergman, Fredrik Gunnarsson, Olof Liberg, Sara Modarres Razavi, Hazhir Shokri Razaghi, Henrik Rydén, Yutao Sui

arXiv:1705.04269v2cs.NI

TL;DR

GNSS may be unsuitable for massive IoT because of chip power and cost, and some devices may be in locations it does not cover. The paper reviews Release 14 positioning support for LTE-M and NB-IoT and simulates OTDOA positioning accuracy against Rel-9 LTE. LTE provided the best positioning accuracy, and LTE-M outperformed NB-IoT under the simulation setup; bandwidth reduction was not fully compensated by longer PRS transmission.

  • Problem

    GNSS may be unsuitable for massive IoT because of chip power and cost, and some devices may be in locations it does not cover.

    Source: §I · p. 1

  • Method

    The paper reviews Release 14 positioning support for LTE-M and NB-IoT and simulates OTDOA positioning accuracy against Rel-9 LTE.

    Source: §I · p. 1, §VI · p. 7

  • Results

    LTE provided the best positioning accuracy, and LTE-M outperformed NB-IoT under the simulation setup; bandwidth reduction was not fully compensated by longer PRS transmission.

    Source: §VI · p. 7

  • Takeaways & Limitations

    OTDOA accuracy in LTE-M and NB-IoT can help enable location-based services in many IoT use cases.

    Source: §VI · p. 7

    The simulations focus on positioning accuracy and leave other performance aspects, such as energy consumption, to future work.

    Source: §VI · p. 7

Abstract

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Many use cases in the Internet of Things (IoT) will require or benefit from location information, making positioning a vital dimension of the IoT. The 3rd Generation Partnership Project (3GPP) has dedicated a significant effort during its Release 14 to enhance positioning support for its IoT technologies to further improve the 3GPP-based IoT eco-system. In this article, we identify the design challenges of positioning support in Long-Term Evolution Machine Type Communication (LTE-M) and Narrowband IoT (NB-IoT), and overview the 3GPP's work in enhancing the positioning support for LTE-M and NB-IoT. We focus on Observed Time Difference of Arrival (OTDOA), which is a downlink based positioning method. We provide an overview of the OTDOA architecture and protocols, summarize the designs of OTDOA positioning reference signals, and present simulation results to illustrate the positioning performance.

I. INTRODUCTION

Many IoT applications need or benefit from location information, but GNSS can be unsuitable for massive IoT because of power and cost, and because devices may be outside its coverage. Release 14 work on LTE-M and NB-IoT positioning addresses limited earlier support and challenges from device capability and coverage extension.

I. INTRODUCTION

  • IoT positioning serves use cases including utility metering, environmental monitoring, asset tracking, and municipal services.The article frames MTC as lower-data-rate than mobile broadband, with demands for low-cost devices, better coverage, and years of battery operation.Source: §I · p. 1
  • GNSS may be unsuitable for massive IoT because of chip power and cost, while indoor, encapsulated, or underground devices may lack GNSS coverage.Terrestrial cellular networks are presented as a promising alternative.Source: §I · p. 1
  • Release 13 offered only limited LTE-M and NB-IoT positioning support, motivating Release 14 efforts to improve it.The stated ambition was to further increase the market impact of 3GPP MTC technologies.Source: §I · p. 1

II. USE CASES AND CHALLENGES OF IOT POSITIONING

  • Positioning accuracy requirements range from a few meters to hundreds of meters, with 50 m horizontal accuracy used as an implicit design target for LTE-M and NB-IoT.The paper says this target was not explicitly specified.Source: §II · p. 1
  • LTE-M and NB-IoT positioning must address limited device capability and coverage-extension conditions.Low-complexity bandwidth is 1.4 MHz for LTE-M and 180 kHz for NB-IoT, limiting timing resolution; extreme-coverage signal-to-noise ratios are usually below -10 dB.Source: §II · p. 2

III. PRELIMINARIES OF OTDOA ARCHITECTURE AND PROTOCOLS

In OTDOA, a UE derives time-difference measurements from positioning reference signals received from multiple cells. The LTE architecture and protocols route requests and measurements through the E-SMLC, while PRS design is central to enhancing LTE-M support and enabling NB-IoT support.

III. PRELIMINARIES OF OTDOA ARCHITECTURE AND PROTOCOLS

  • A UE forms reference signal time difference (RSTD) measurements by subtracting a reference-cell TOA from TOAs measured from multiple cells.Each TDOA constrains the UE position to a hyperbola; without noise and interference, the hyperbolas intersect at the UE position.Source: §III · p. 2
  • The MME initiates or receives a location request, asks the E-SMLC to position the UE, and may forward the estimated position to the UE or GMLC.The E-SMLC requests RSTD measurements from the UE and estimates its position after receiving them.Source: §III · p. 2
  • LPP supports signaling between the E-SMLC and UE, with LPPa and RRC supporting related interactions with the eNB.The LPP procedures transfer UE capabilities, PRS assistance data, and location information.Source: §III · p. 2
  • PRS design is central to enhancing LTE-M OTDOA support and enabling NB-IoT support, and LTE PRS separates cell signals through frequency, time, and code domains.The three mechanisms are frequency reuse, muting, and cell-specific sequences.Source: §III · p. 3

A. PRS Sequence and Mapping to Resource Elements

LTE-M PRS reuses LTE's pseudo-random QPSK sequence and maps it to resource elements in a cell-specific diagonal pattern. Frequency shifts provide six orthogonal mappings, while some symbols are reserved for other signals.

A. PRS Sequence and Mapping to Resource Elements

  • LTE-M uses the same pseudo-random QPSK PRS sequence as LTE, initialized according to OFDM symbol index, slot index, and physical cell identity.The sequence varies with those indices and cell identity.Source: A. PRS Sequence and Mapping to… · pp. 3–4
  • PRS uses a diagonal resource-element mapping, with cell-specific frequency shifts producing six orthogonal mappings.Each resource block has 12 subcarriers, and PRS uses two resource elements in an OFDM symbol; the shift is physical cell identity mod 6.Source: A. PRS Sequence and Mapping to… · p. 4
  • The first three OFDM symbols of a subframe are excluded from PRS, and other unused symbols may carry LTE cell-specific reference signals.The first three symbols may carry LTE L1/L2 signals such as PDCCH.Source: A. PRS Sequence and Mapping to… · p. 4
  • LTE-M introduces PRS frequency hopping across two or four configured six-PRB bands to partly compensate for its reduced receive bandwidth.The first hopping location is fixed at the center of the LTE carrier.Source: A. PRS Sequence and Mapping to… · p. 4

B. PRS Subframe Configuration

LTE PRS configuration supports varying positioning-occasion lengths and periods, while LTE-M adds tailored transmissions and denser scheduling options. These mechanisms let narrowband devices receive longer or more frequent PRS transmissions while limiting resource overhead for wideband LTE devices.

B. PRS Subframe Configuration

  • LTE PRS positioning occasions contain 1, 2, 4, or 6 consecutive subframes and repeat every 160, 320, 640, or 1280 subframes.An eNB can configure the transmission offset relative to a system-frame cycle.Source: B. PRS Subframe Configuration · p. 4
  • Cells may schedule separate PRS configurations for LTE and LTE-M, giving LTE-M longer narrowband transmissions and LTE shorter wideband transmissions with less overhead than one over-dimensioned configuration.The design targets good performance for both device types while limiting PRS resource use.Source: B. PRS Subframe Configuration · p. 4
  • For denser LTE-M transmissions, the supported occasion lengths extend to 10–160 subframes, and multiple occasions may be configured within a legacy PRS period.The occasion interval may be 10, 20, 40, or 80 subframes.Source: B. PRS Subframe Configuration · p. 4
  • An example schedule pairs wideband, short PRS for LTE with narrowband, long PRS for LTE-M, including two LTE-M occasions per period.The schedules can overlap in time and frequency, facilitating PRS sharing and radio-resource usage.Source: B. PRS Subframe Configuration · p. 5

C. PRS Muting

PRS muting improves detection of weak-cell signals that share a frequency shift with strong-cell signals, trading interference mitigation against potentially longer positioning time. LTE-M reuses this mechanism while limiting assistance-data overhead by applying each muting bit across a legacy PRS period.

C. PRS Muting

  • Muting strong-cell PRS can improve detectability of weak-cell PRS sharing its frequency shift, at the cost of potentially increased time-to-fix.Networks choose the hearability/time-to-fix tradeoff according to deployment scenarios.Source: C. PRS Muting · p. 5
  • LTE-M applies each muting bit to all positioning occasions within a legacy PRS period, keeping the bit string at 2, 4, 8, or 16 bits.This avoids the assistance-data overhead that a 1024-bit string would cause.Source: C. PRS Muting · p. 5
  • For a two-bit string “01” applied to the Figure 3 configuration, the two narrow-band PRS positioning occasions in every other PRS period are muted.Source: C. PRS Muting · p. 5

V. NARROWBAND POSITIONING REFERENCE SIGNALS IN NB-IOT

NB-IoT NPRS configuration supports two complementary parts: a bitmap-based Part A and a periodic, consecutive-subframe Part B. Their combination determines which subframes contain NPRS.

A. NPRS Subframe Configuration

  • Part A uses a 10- or 40-bit bitmap to mark NPRS subframes within an occasion, whose length serves as the NPRS period.Unlike LTE or LTE-M positioning occasions, its NPRS subframes need not be consecutive; Release-13 devices are protected by marking NPRS subframes invalid in the valid-subframe bitmap.Source: A. NPRS Subframe Configuration · p. 5
  • Figure 4’s Part A example uses 10-bit valid-subframe and NPRS bitmaps, with NPRS in invalid subframes 1 and 2.Subframes 1, 2, and 3 are marked invalid in the valid-subframe bitmap.Source: A. NPRS Subframe Configuration · p. 6, Figure 4 · p. 6
  • Part B specifies occasion periodicity, offset, and consecutive NPRS-subframe count, allowing transmissions up to 1280 subframes.Long transmissions partially compensate for reduced NPRS bandwidth and address coverage in challenging deployment environments.Source: A. NPRS Subframe Configuration · p. 6
  • When Parts A and B are both configured, a subframe contains NPRS only when both configurations indicate NPRS there.Source: A. NPRS Subframe Configuration · p. 6

B. NPRS Sequence and Mapping to Resource Elements

NPRS reuses LTE PRS’s diagonal resource-element pattern and sequence, with mapping adapted to NB-IoT deployment configurations. Inband placement determines mapping and sequence choice, while standalone and guardband deployments can use more OFDM symbols.

B. NPRS Sequence and Mapping to Resource Elements

  • NPRS resource-element mapping follows a diagonal pattern with frequency reuse factor 6, like LTE PRS.For inband deployment, mapping depends on the configured NPRS subframes.Source: B. NPRS Sequence and Mapping to… · p. 6
  • In inband deployment, Part A mapping skips the first three OFDM symbols and symbols potentially used by CRS; Part B also skips the last two symbols to avoid NRS.Source: B. NPRS Sequence and Mapping to… · p. 6
  • Standalone and guardband deployments can map NPRS to more OFDM symbols because they need not avoid the legacy LTE PDCCH region and CRS.Part B still omits the last two symbols in each slot, as in inband deployment.Source: B. NPRS Sequence and Mapping to… · p. 6
  • NPRS reuses the LTE PRS pseudo-random QPSK sequence, but NB-IoT’s single resource block requires only two sequence elements.Standalone or guardband deployment uses the central two LTE PRS elements; inband sequence choice depends on carrier position and helps improve quasi-orthogonality across carriers.Source: B. NPRS Sequence and Mapping to… · pp. 6–7

C. NPRS Muting

NPRS muting supports weak-cell signal detection when signals share a frequency shift with a strong cell. Part A and Part B have separate muting patterns, and a subframe is muted if either pattern mutes it.

C. NPRS Muting

  • NPRS muting can improve detectability of weak-cell signals sharing a frequency shift with strong-cell signals.Source: C. NPRS Muting · p. 7
  • Each NB-IoT carrier may signal a separate 2-, 4-, 8-, or 16-bit muting pattern for configured Part A and Part B.Part A bits apply to consecutive groups of 10 subframes; Part B bits apply to positioning occasions.Source: C. NPRS Muting · p. 7
  • When both configuration parts are used, an NPRS subframe is muted if either Part A or Part B mutes it.Source: C. NPRS Muting · p. 7

VI. SIMULATION RESULTS AND DISCUSSIONS

Under the simulated setup, LTE provides the best OTDOA positioning accuracy, followed by LTE-M and then NB-IoT; reducing bandwidth cannot be fully offset by longer PRS transmission. The simulations compare accuracy only, while OTDOA’s shared configured PRS offers a scalability contrast with uplink-based positioning.

VI. SIMULATION RESULTS AND DISCUSSIONS

  • The simulations compare positioning accuracy for Rel-9 LTE, Rel-14 LTE-M, and NB-IoT, leaving energy consumption and other performance aspects for future work.They assume a 10 MHz LTE carrier; LTE and LTE-M Category M2 use 160 ms PRS periods, with 1 and 6 PRS subframes per occasion, respectively.Source: §VI · p. 7
  • LTE provides the best positioning accuracy, and LTE-M outperforms NB-IoT; longer PRS transmission cannot fully compensate for bandwidth reduction.The authors attribute the ordering to LTE having the largest signal bandwidth and NB-IoT the smallest.Source: §VI · p. 7
  • OTDOA’s configured PRS can serve all devices needing positioning, whereas uplink-based positioning requires each device to transmit, giving OTDOA better scalability as device numbers grow.The cited alternative uses uplink signals detected by multiple LoRa gateways; the passage says it may have lower device impact than OTDOA.Source: §VI · p. 7

VII. CONCLUSIONS AND FUTURE WORK

The article reviews 3GPP Release 14 enhancements to positioning support for LTE-M and NB-IoT, focusing on OTDOA. It concludes that this support will improve the 3GPP-based IoT ecosystem and identifies possible directions for future positioning evolution.

VII. CONCLUSIONS AND FUTURE WORK

  • The article overviews Release 14 enhancements to 3GPP positioning support for LTE-M and NB-IoT, with a focus on OTDOA.The authors state that the enhanced support will further improve the 3GPP-based IoT ecosystem and increase the market impact of 3GPP IoT technologies.Source: §VII · pp. 7–8
  • Future work could enhance uplink positioning for LTE-M and NB-IoT, investigate integrated OTDOA for advanced UEs, and design future 5G positioning with synergies with these technologies.The article notes that massive LTE-M and NB-IoT devices may have long lifecycles extending beyond network evolution toward 5G.Source: §VII · p. 8

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