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V2X Meets NOMA: Non-Orthogonal Multiple Access for 5G Enabled Vehicular Networks
Boya Di, Lingyang Song, Yonghui Li, Zhu Han
TL;DR
Massive V2X connectivity creates access collisions and distributed interference-management challenges. This paper introduces NOMA into LTE-based vehicular networks, designs scheduling and resource-allocation mechanisms, and extends them to safety-critical broadcast; the proposed NOMA-MCD scheme performs better than OMA and reduces resource collisions.
Problem
Massive connectivity creates access collisions, while distributed interference management remains challenging for users.
Method
The paper introduces NOMA into LTE-based vehicular networks and designs multiplexing, scheduling, resource-allocation, and network-control mechanisms, extending NOMA-MCD to safety-critical V2X broadcasting.
Results
The NOMA-MCD scheme performs better than the OMA-based scheme and efficiently reduces resource collisions.
Takeaways & Limitations
NOMA shows potential to enhance cellular V2X services and support massive connectivity through power-domain or code-domain multiplexing.
Abstract
from arXiv · showhide
Benefited from the widely deployed infrastructure, the LTE network has recently been considered as a promising candidate to support the vehicle-to-everything (V2X) services. However, with a massive number of devices accessing the V2X network in the future, the conventional OFDM-based LTE network faces the congestion issues due to its low efficiency of orthogonal access, resulting in significant access delay and posing a great challenge especially to safety-critical applications. The non-orthogonal multiple access (NOMA) technique has been well recognized as an effective solution for the future 5G cellular networks to provide broadband communications and massive connectivity. In this article, we investigate the applicability of NOMA in supporting cellular V2X services to achieve low latency and high reliability. Starting with a basic V2X unicast system, a novel NOMA-based scheme is proposed to tackle the technical hurdles in designing high spectral efficient scheduling and resource allocation schemes in the ultra dense topology. We then extend it to a more general V2X broadcasting system. Other NOMA-based extended V2X applications and some open issues are also discussed.
I. INTRODUCTION
LTE-based V2X offers broad coverage and controllable latency, but orthogonal access becomes congested in dense networks. The paper examines NOMA to support massive connectivity, reduce collisions, and improve V2X latency and reliability.
- LTE is considered promising for V2X because it offers large coverage, controllable latency, and high data rates.
- OMA-based LTE suffers resource collisions, difficult access-rate guarantees, and severe packet loss in dense V2X topologies.
- NOMA allows multiple users to share channels through power- or code-domain multiplexing, improving spectrum efficiency and alleviating congestion.
- NOMA-based V2X introduces distinct scheduling, spectrum-management, power-control, and signaling challenges because of mobility, interference, and CSI requirements.
- The article studies NOMA-based V2X unicast systems, proposes a mixed centralized/distributed scheme, and discusses extensions including broadcasting, uplink V2I, and multi-operator V2V.
A. LTE-supported V2X Services
LTE-supported V2X uses direct and cellular communication modes for safety-critical and traffic-efficient services. Dynamic and semi-persistent scheduling address different latency and traffic patterns, while mobility and dense topology create interference and scheduling challenges.
- LTE-supported V2X includes LTE-D for direct V2V communication and cellular uplink/downlink for vehicle-to-infrastructure communication.
- LTE-D can reduce end-to-end latency by bypassing the base station, but moving vehicles and dense topology create difficult cross interference.
- Cellular unicast downlink may waste licensed-spectrum resources, whereas MBMS broadcasting or multicasting can improve cell-edge performance and reduce latency.
- Dynamic scheduling allocates resources per packet using real-time CSI and suits sudden, frequently varying transmissions.
- Semi-persistent scheduling assigns predefined resource sets periodically, reducing signaling latency for periodic short messages with fixed packet sizes.
B. Non-Orthogonal Multiple Access Technique
NOMA supports overloaded transmission by multiplexing users in the power or code domain. The paper describes PD-NOMA with SIC and CD-NOMA with sparse codebooks and spreading sequences.
- NOMA provides two principal approaches: power-domain multiplexing and code-domain multiplexing.
- PD-NOMA lets multiple users share a channel simultaneously and uses successive interference cancellation to decode co-channel signals.
- PD-NOMA exploits differences in received power levels to obtain higher spectrum efficiency than OMA.
- CD-NOMA, also called SCMA, spreads sparse codewords across multiple subchannels to realize overloaded transmission.
- In CD-NOMA, user bit streams map to codewords, which are multiplexed over subchannels and detected using the codeword sparsity.
III. NOMA APPLICABILITY TO CELLULAR V2X
The paper applies PD-NOMA to cellular V2X, beginning with a unicast model and addressing interference, mobility, scheduling, power control, and signaling. It proposes a mixed centralized/distributed design that combines BS scheduling with distributed user power control.
- The proposed NOMA-MCD approach targets resource-collision reduction and low latency in cellular V2X, then extends toward broadcast scenarios.
- NOMA-based V2X Unicast System Model: In the unicast model, multiple V2X pairs share subchannels, while SIC decodes conflicting receivers according to transmitters’ decreasing channel gains.
- NOMA-based V2X Unicast System Model: The optimization objective is to maximize successfully decoded packets whose target-signal data rates exceed a given threshold.
- Key Problems and Solutions of Resource Allocation and Signaling Control: Centralized dynamic allocation can incur delay from per-packet resource requests, while distributed contention-based access makes interference management costly and difficult.
- Key Problems and Solutions of Resource Allocation and Signaling Control: Full centralized SPS can reduce delay but may cause resource collisions because it cannot track rapidly changing CSI during vehicle mobility.
- Key Problems and Solutions of Resource Allocation and Signaling Control: NOMA-MCD combines BS-controlled SPS and frequency allocation with distributed per-slot power control by transmitting users.
1) Centralized Spectrum Management of the BS:
The BS formulates centralized spectrum allocation using partial CSI and solves the resulting non-convex assignment through many-to-many swap matching.
- Unlike OMA, the allocation accounts for co-channel interference and uses an indicator variable x_j,k to denote whether subchannel k is assigned to Tx user j.
- The BS uses partial CSI containing path loss and shadowing because full CSI is costly and quickly outdated under vehicle mobility.
- The allocation objective models successful decoding through a logistic approximation based on rate thresholds, channel gains, and interference-related users.
- Binary allocation variables make the optimization non-convex, so it is converted into a many-to-many matching problem with externalities.
- A swap-matching algorithm repeatedly exchanges Tx-user and subchannel matches when the total utility improves, stopping when no blocking pair remains.
2) Distributed Power Control of the Users:
Users perform distributed power control through iterative Tx-Rx signaling, balancing target-link decoding against interference across neighboring receivers.
- Power control is difficult because each Tx affects multiple neighboring receivers, while each target receiver may receive superposed signals from multiple Tx users.
- Successful-interference modeling uses a logistic approximation, while SIC decoding relies on a higher-channel-gain signal being decoded first.
- Each transmission slot contains a control-signaling portion with multiple Tx-Rx iterations followed by data transmission.
- Tx users adjust power so reference signals reach target receivers while minimizing interference to other receivers, using feedback from neighboring users.
- If a Tx user’s co-channel interference exceeds a threshold, its transmit power is set to zero; otherwise, power is minimized subject to the direct-link rate threshold.
C. Performance Evaluation
The proposed NOMA-MCD scheme combines centralized resource allocation with distributed power control and is evaluated against an OMA-based LTE-D scheme.
- The NOMA-MCD scheme combines BS-controlled frequency allocation with distributed power control and Tx-Rx selection.
- The distributed control process completes within the control portion of a slot, after which receivers decode using obtained CSI.
- Performance is compared with OMA-based LTE-D using packet reception probability and latency performance.
- In the evaluated setting, 20% of vehicles are Tx users, 80% are Rx users, and at most two Tx users share a subchannel.
- The NOMA-MCD scheme performs better than the OMA-based scheme.
- For broadcasting, the extension lets one Rx user receive from multiple Tx users simultaneously, reducing resource collision and improving connectivity through power-domain multiplexing.
1) Tx-Rx selection and time-frequency resource allocation of the BS:
The paper considers BS scheduling and resource allocation for V2X broadcasting and uplink V2I, including SCMA-based grant-free access for dense connectivity.
- 1) Tx-Rx selection and time-frequency resource allocation of the BS:: Because half-duplex users cannot transmit simultaneously within one another’s communication range, the BS selects Tx/Rx subsets and allocates time-frequency resources.
- 1) Tx-Rx selection and time-frequency resource allocation of the BS:: The centralized broadcast allocation is formulated as a three-dimensional integer programming problem.
- 2) Power control strategy of each Tx user:: Broadcast Tx users adjust power to maximize successfully decoded neighboring Rx users, but dense environments make universal decoding difficult.
- B. NOMA-based Uplink V2I Networks: Contention-based SCMA is proposed as a candidate for low-latency uplink V2I access because short safety packets face LTE uplink bottlenecks.
- B. NOMA-based Uplink V2I Networks: Each CTU combines time, frequency, an SCMA codebook, and a pilot sequence; collisions are handled through random back-off and joint MPA decoding.
- B. NOMA-based Uplink V2I Networks: SCMA enables grant-free uplink transmission and system overload through multiplexing, but connectivity and latency require a trade-off in ultra-dense networks.
C. NOMA-based V2V Networks with Multiple Operators
The multiple-operator V2V setting shares carriers across vehicles subscribed to different operators, creating resource-collision and coordination challenges. NOMA-based joint transmission and reception is proposed to improve cell-edge spectrum efficiency, while mobility and imperfect beam fitting remain design concerns.
- Vehicles subscribed to different operators may transmit on the same carrier, so multiple cell-edge V2V pairs can contest shared frequencies and cause resource collisions.
- Cooperation among operators is needed for joint spectrum management and power control to support reliable direct communication.
- NOMA-based joint data transmission and reception assisted by operators is proposed to improve cell-edge users’ spectrum efficiency.
- Vehicle mobility requires dynamic cell hand-off in the proposed scheme.
- Precoding differs from the traditional single-cell case because separate antennas may not form physical beams that fit co-channel NOMA-user distributions.
V. CONCLUSIONS AND FUTURE OUTLOOK
The paper introduces NOMA into LTE-based vehicular networks to reduce resource collisions and support diverse V2X applications, extending a unicast scheme to safety-critical broadcasting. It reports reduced collisions versus OMA and identifies synchronization, coexistence, coordination, and mobility-related issues for future work.
- The paper introduces NOMA into LTE-based vehicular networks to support massive connectivity and high spectrum efficiency.
- The NOMA-MCD scheme is developed for V2X unicast and extended to a more general safety-critical V2X broadcast scenario.
- The proposed NOMA-MCD scheme efficiently reduces resource collision compared to the traditional OMA-based scheme.
- Open issues include synchronization under high mobility, LTE–WiFi coexistence, and coordination among services and devices with different requirements.
- NOMA applications discussed include cooperative V2X, cognitive V2X, networking NOMA, and contention-based SCMA.
- Cognitive NOMA lets vehicles opportunistically access cellular-user channels while protecting cellular services through coordinated power control and user coordination.