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LTE/LTE-A Random Access for Massive Machine-Type Communications in Smart Cities

Md Shipon Ali, Ekram Hossain, Dong In Kim

arXiv:1608.03042v1cs.NIcs.IT

TL;DR

Massive MTC creates severe LTE/LTE-A random-access congestion because bursty access attempts increase preamble collisions. This paper reviews congestion-control proposals and develops a collision-resolution model using contention-tree splitting; simulations report improved success and reduced outage relative to slotted-Aloha-based access.

  • Problem

    Massive and bursty MTC access increases preamble collisions and sharply degrades LTE/LTE-A random-access performance.

  • Method

    The paper reviews LTE/LTE-A congestion-control proposals and develops a collision-resolution model based on m-ary contention-tree splitting.

  • Results

    The proposed CRB-RA model requires fewer retransmissions and substantially reduces outage compared with standard slotted-Aloha-based random access.

  • Takeaways & Limitations

    Resolving collided preambles can support more reliable and time-efficient network access for massive MTC while coexisting with existing LTE/LTE-A MAC operation.

  • Takeaways & Limitations

    With 754 attempts over 200 ms, estimated preamble collision probability reaches 30% with 10800 opportunities per second and 69% when only 30% are available for low-rate MTCDs.

Abstract

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Massive Machine-Type Communications (MTC) over cellular networks is expected to be an integral part of wireless "Smart City" applications. The Long Term Evolution (LTE)/LTE-Advanced (LTE-A) technology is a major candidate for provisioning of MTC applications. However, due to the diverse characteristics of payload size, transmission periodicity, power efficiency, and quality of service (QoS) requirement, MTC poses huge challenges to LTE/LTE-A technologies. In particular, efficient management of massive random access is one of the most critical challenges. In case of massive random access attempts, the probability of preamble collision drastically increases, thus the performance of LTE/LTE-A random access degrades sharply. In this context, this article reviews the current state-of-the-art proposals to control massive random access of MTC devices in LTE/LTE-A networks. The proposals are compared in terms of five major metrics, namely, access delay, access success rate, power efficiency, QoS guarantee, and the effect on Human-Type Communications (HTC). To this end, we propose a novel collision resolution random access model for massive MTC over LTE/LTE-A. Our proposed model basically resolves the preamble collisions instead of avoidance, and targets to manage massive and bursty access attempts. Simulations of our proposed model show huge improvements in random access success rate compared to the standard slotted-Aloha-based models. The new model can also coexist with existing LTE/LTE-A Medium Access Control (MAC) protocol, and ensure high reliability and time-efficient network access.

INTRODUCTION

Smart-city MTC requires cellular connectivity for many autonomous devices, but LTE/LTE-A random access faces severe congestion from irregular and bursty transmissions. The article reviews congestion-control proposals and introduces collision resolution as an alternative to collision avoidance.

  • INTRODUCTION: Smart-city applications use large populations of autonomous devices whose communications constitute Machine-Type Communications.These applications include utilities, e-health, surveillance, environmental monitoring, and connected vehicles.
  • INTRODUCTION: LTE/LTE-A is positioned as a major cellular technology for supporting MTC applications requiring mobility, coverage, security, and diverse QoS.
  • INTRODUCTION: Random access initiates LTE data transfer, but slotted-Aloha operation makes massive, irregular, and bursty MTC access prone to severe congestion.
  • INTRODUCTION: Existing proposals are compared using access delay, success rate, QoS guarantee, energy efficiency, and impact on HTC traffic.
  • INTRODUCTION: The article reviews LTE/LTE-A random-access congestion solutions and develops a collision-resolution approach based on m-ary contention-tree splitting.The paper distinguishes this approach from collision avoidance, which restricts access-attempt arrival rates and can increase delay.

Random Access Preamble

LTE random access uses contention-based preambles mapped onto PRACH resources, with configuration and access parameters supplied through SIB2. The section introduces the physical resource structure and the information UEs use before transmitting.

  • Random Access Preamble: LTE random-access preambles are orthogonal digital signatures divided between contention-free and contention-based access.The eNB reserves some preambles for assigned UEs and leaves the remainder for randomly selected contention-based access.
  • Random Access Preamble: An FDD RA slot occupies six frequency-domain resource blocks, while its duration depends on the preamble format.All 64 preambles map into 839 RACH subcarriers, with 25 guard subcarriers.
  • Random Access Preamble: UEs select among four FDD preamble formats according to distance, delay spread, and required transmission resources.The number of RA slots per radio frame is determined by the preamble configuration index.
  • Random Access Preamble: After synchronizing and decoding the MIB, a UE obtains RA parameters from SIB2 before generating a contention-based access attempt.

1) Preamble transmission from UE to eNB:

The contention-based LTE random-access procedure proceeds from UE preamble transmission through eNB detection and response, uplink connection request, and contention resolution. Colliding UEs retry when their requests do not receive successful resolution.

  • 1) Preamble transmission from UE to eNB:: A UE randomly selects a contention-based preamble and transmits it in the next available RACH slot using pathloss, delay spread, resource, and power considerations.
  • 1) Preamble transmission from UE to eNB:: The eNB detects active preambles from the power-delay profile and uses the RA-RNTI to identify their RA slots.
  • 1) Preamble transmission from UE to eNB:: The eNB sends an RAR containing timing advance, an uplink grant, and a temporary C-RNTI for the subsequent RRC request.
  • 1) Preamble transmission from UE to eNB:: The UE uses the assigned bandwidth to transmit an RRC connection request addressed to the temporary C-RNTI and carrying an existing or initial identity.
  • 1) Preamble transmission from UE to eNB:: During contention resolution, successful UEs receive an RRC contention setup, while collided UEs without proper timing advance receive no feedback and initiate another attempt.

MAJOR LIMITATIONS OF LTE RANDOM ACCESS

LTE random access has limited capacity for bursty massive-MTC events, producing high collision probabilities and instability. The reviewed congestion controls improve access success but can impose delay and may not handle massive bursts effectively.

  • MAJOR LIMITATIONS OF LTE RANDOM ACCESS: LTE provides 10800 RA opportunities per second, but simultaneous preamble access remains bounded, while massive events can trigger thousands of near-simultaneous attempts.The average slotted-Aloha RA success rate is reported as around 37%.
  • MAJOR LIMITATIONS OF LTE RANDOM ACCESS: 30% collision probability rises to 69% when 754 attempts use 30% of the contention-based opportunities, indicating unstable massive-MTC access.The values are reported for 10800 and 3240 RA opportunities per second, respectively, over 200 ms.
  • MAJOR LIMITATIONS OF LTE RANDOM ACCESS: The review organizes congestion solutions into 3GPP-specified and non-3GPP-specified classes using access delay, success rate, energy efficiency, QoS, and HTC impact.
  • MAJOR LIMITATIONS OF LTE RANDOM ACCESS: ACB reduces access arrival rates by probabilistically barring devices, with separate access classes available for MTCDs and QoS-based subgrouping.
  • MAJOR LIMITATIONS OF LTE RANDOM ACCESS: EAB dynamically bars and unbars low-priority MTCDs according to arrival rate to preserve timely access for delay-constrained devices.
  • MAJOR LIMITATIONS OF LTE RANDOM ACCESS: ACB can improve success rate but may require restrictive access probabilities, causing long delays and failing to handle massive bursty attempts.

6) MTC-specific backoff:

MTC-specific backoff increases the waiting time imposed on MTC devices after failed access attempts, distinguishing their treatment from human-type devices.

  • 6) MTC-specific backoff:: Backoff intervals discourage UEs from retrying immediately after a collision or channel-fading failure.A failed first attempt triggers a Backoff Interval (BI).
  • 6) MTC-specific backoff:: A second failed access attempt results in a larger Backoff Interval than the previous one.
  • 6) MTC-specific backoff:: MTC-specific backoff assigns MTCDs a larger Backoff Interval than HTCDs.

7) Dynamic resource allocation:

LTE random-access congestion can be addressed through resource allocation, slot assignment, resource separation, paging, and group paging, each with distinct capacity or delay trade-offs.

  • 7) Dynamic resource allocation:: Dynamic RACH allocation expands PRACH resources in time, frequency, or both according to the level of random-access congestion.The eNB can allocate up to ten subframes as PRACH or add 1.08 MHz of PRACH bandwidth.
  • 8) Slotted random access:: Slotted random access assigns each MTCD a dedicated access opportunity, but large RA cycles can create long access delays.The scheme can also produce collisions, uneven slot utilization, and underutilized resources.
  • 9) Separate RACH for MTC:: Separating RACH resources for HTC and MTC can reduce the negative impact of random-access congestion on HTCDs.Separation may use distinct RA slots or separate preamble subsets, while restricting MTCDs to their assigned resources.
  • 10) Pull-based random access:: Pull-based access lets the eNB control congestion by delaying paging, but paging many MTCDs requires extra control channels.Group paging reduces paging-channel usage, although simultaneous attempts remain bounded by available RACH resources.

Non-3GPP Random Access Solutions

Non-3GPP proposals combine resource separation, dynamic allocation, access barring, and prioritization to manage congestion and differentiate MTC service classes.

  • Non-3GPP Random Access Solutions: The surveyed non-3GPP proposals include approaches with distinct characteristics and reported improvements over specified 3GPP solutions.
  • Self-optimization overload control: SOOC combines RA resource separation, dynamic RA resource allocation, and dynamic access barring for MTC applications.It further divides MTCDs into high-priority and low-priority groups.
  • Prioritized RA: Prioritized RA divides applications into five classes and separates available RACH resources into three virtual groups.Its prioritized access algorithm is designed to provide QoS guarantees across application classes and virtual groups.

13) Group-based random access:

The reviewed approaches use grouped access, spatial reuse, and proactive load estimation to reduce contention or adapt access resources for massive and bursty MTC traffic.

  • 13) Group-based random access:: Group-based RA organizes MTCDs into access groups using criteria such as server, device specifications, QoS, or location.The approach extends pull-based group paging and assumes group members are sufficiently close for shared timing estimation.
  • 13) Group-based random access:: Each access group uses one preamble, while a group delegate communicates with the eNB on behalf of the group.The eNB selects the delegate using metrics such as channel condition or transmission power.
  • Code-expanded RA: Code-expanded RA represents an access attempt with multiple preambles transmitted across a predefined set of RA slots.The resulting sequence is treated as an RA codeword that the eNB identifies at reception.
  • Proactive load estimation: Proactive load estimation addresses bursty traffic by estimating contention users and distributing serving-phase RA slots according to QoS requirements.The estimation phase assigns different preambles to QoS sub-groups before the serving-phase allocation.

17) Non-Aloha-based random access:

The proposed collision resolution random access model resolves MTC preamble collisions through reserved preamble sets and a dynamically adjusted contention tree, while separating MTC and HTC handling.

  • Collision resolution: The CRB-RA model reserves new preamble sets for collided MTC devices and repeats this process until each preamble is decoded with an individual UE-ID.Collided MTC devices receive retransmission instructions in the RAR message, while collided HTC devices restart random access without that feedback.
  • Contention-tree design: A binary splitting-tree variant assigns two reserved preambles at each contention level for every collision detected at the root.The root contains all contention-based preambles, and each tree level is resolved in a separate virtual random-access frame.
  • Adaptive configuration: The number of reserved preambles is dynamically adjusted according to collision rate, trading shorter trees against longer per-level resolution time.Access delay depends mainly on selecting the appropriate value of m.
  • Algorithm: The algorithm selects m and additional random-access slots from collision thresholds, reserves preambles for each collision, and broadcasts updated resources through SIB2.The procedure sends RAR messages to collided MTC devices for reattempts.

Performance Analysis

The proposed CRB-RA model is evaluated against standard slotted-Aloha random access under massive simultaneous attempts using retransmission and outage metrics. It requires fewer retransmissions and substantially reduces access outage, supporting lower power use and delay.

  • Retransmissions: The CRB-RA model maintains access within a limited number of retransmissions, whereas slotted-Aloha requires many retransmissions as simultaneous attempts increase.The comparison includes 2-, 3-, 5-, and 10-ary CRB-RA variants and slotted-Aloha configurations with different numbers of RACH slots.
  • Retransmissions: More than 30 retransmissions are required by slotted-Aloha with peak preamble configuration when simultaneous attempts reach 3200 or more, while CRB-RA uses two RA slots per radio frame.The reported comparison concerns attempts needed for one successful access.
  • Outage probability: Around 70% average outage occurs for slotted-Aloha with two RA slots at 500 simultaneous attempts and with the maximum RA-slot configuration at 2500 attempts.The proposed model can still have nonzero outage under very large loads, but optimizing the contention-slot configuration reduces outage significantly.
  • Implications: The CRB-RA model’s lower retransmission requirement makes it efficient for power-constrained MTC applications and is expected to reduce access delay relative to slotted-Aloha models.These conclusions are based on the retransmission comparison and its stated implications for energy and delay.

CONCLUSION

The paper reviews LTE MAC congestion-control approaches for massive MTC and proposes a collision-resolution random-access model for massive, bursty requests. Simulations report reliable, time-efficient access that coexists with LTE MAC, while future optimization remains multidimensional.

  • Conclusion: The proposed collision-resolution model is designed to manage massive random-access requests and coexist with the existing LTE MAC protocol without modification.The authors report reliable and time-efficient access performance in simulation.
  • Conclusion: The model was simulated with a fixed reserved-preamble-set size, although its parameters can be optimized using collision rate, radio resources, and delay constraints.The optimization includes preambles per contention-tree slot and the size and duration of the virtual RA frame.
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