Connected vehicles for safety and traffic efficient applications require device-to-device connections supporting one-to-many and many-to-many communication, precise absolute and relative positioning and distributed computing. Currently, the 5.9 GHz Dedicated Short Range Communications (DSRC) and 4G-Long-Term Evolution (LTE) are available for connected vehicle services. But both have limitations in reliability or latency over large scale field operational tests and deployment. This paper proposes the device-to-device (D2D) connectivity framework based on publish-subscribe architecture, with Message Queue Telemetry Transport (MQTT) protocol. With the publish-subscribe communication paradigm, road mobile users can exchange data and information in moderate latency and high reliability manner, having the potential to support many Vehicle to Everything (V2X) applications, including vehicle to vehicle (V2V), vehicle to roadside infrastructure (V2I), and vehicle to bicycle (V2B). The D2D data exchanges also facilitate computing for absolute and relative precise real-time kinematic (RTK) posi-tioning. Vehicular experiments were conducted to evaluate the performance of the proposed publish-subscribe MQTT protocols in term of latency and reliability. The latency of data exchanges is measured by One-trip-time (OTT) and the reliability is measured by the packet loss rate (PLR). Our results show that the latency of GNSS raw data exchanges between vehicles through 4G cellular networks at the rate of 10 Hz and the data rates of 10 kbps are less than 300 ms while the reliability is over 96%. Vehicular positioning experiments have also shown that vehicles can exchange raw GNSS data and complete mov-ing-base RTK positioning with the positioning availability of 98%.
A connected vehicle includes the different communication devices (embedded or portable) present in the vehicle, that enable in-car connectivity with other devices present in the vehicle and/or enable connection of the vehicle to external devices, networks, applications, and services. Commercial and consumer applications of connected vehicles include everything from fleet management, emergence assistance, traffic safety and efficiency, infotainment, parking assistance, roadside assistance, remote diagnostics, and telematics. Global Navigation Satellite Systems (GNSS)-based positioning and navigation and precise digital maps are essential parts of the connected vehicle services. In recent years, more professionally connected vehicles include the concepts of Vehicle-to-Everything (known as V2X) interactions, such as Vehicle-to-Vehicle (V2V), Vehicle-to-People (V2P) and Vehicle-to-Network (infrastructure) data or message exchanges. In the V2X scenarios, it requires device-to-device connections supporting one-to-many and many-to-many communications. The mobile devices/platforms often provide data streams and images to the servers or other devices instead of information or context-aware messages. The computation or decision making should be completed at the mobile device edge in real-time instead of at a server with a delay. V2X communications requires high timeliness, low round-trip time (RTT) latency, and high user scalability. In support of wireless connectivity among vehicle-based devices, and between fixed roadside devices and vehicle-based devices, the currently deployable technologies define the hardware and services operating from the application layer down to the physical layer. These include the 5.9 GHz Dedicated Short-Range Communications (DSRC)/IEEE Standard for Wireless Access in Vehicular Environments (WAVE), and 4G-Long-Term Evolution (LTE). The developing technologies include LTE-Direct and LTE-Vehicle and 5th generation cellular technology (5G) Ultra-reliable Machine-Type communications (uMTC), which are also hardware dependent.
The Internet-based vehicular connections have been implemented based on LTE mobile broadband services for the commercial and consumer purposes such as vehicle fleet tracking and management and route guidance. These connections are implemented in the application-layer, which is on top of given physical-layer and transport layers. There are a number of advantages of using application-layer solutions for vehicular connections. Firstly, it can get benefit from widely available mobile devices for implementations, for instance, smart-phone devices. Secondly, it is a software solution, which can be easily implemented and easily to updated, on top to hardware. Thirdly, the solution can be easily migrated from the current 4G LTE MBB services to the future 5G extreme MBB services. Overall, it has potential to support many connected vehicle applications. However, the existing connected vehicle for fleet management is mainly two-way connection between vehicle and a server and not for data exchanges between vehicles. For V2X applications, the main concern is about the performance of the application layer connections in terms of latency, reliability and scalability for connected vehicle applications. The consequent question is to what degree the application layer solution can support different V2X applications. Are there more effective application protocols for better performance? For instance, many Internet based connections for vehicles are based on the HTTP client-server architecture. The client-server architecture is a centralized model, in which clients’ requests are sent to the server to receive the information. Such client-server architecture performs well when data is stored in a central server, such as a file and a database server, or contents are accessible from third parties through the server. With the client-server model, a single server can accommodate various services to many client requests and the client can send requests to multiple servers through a network.
The client-server architecture consists of three major components including hardware, software, and communication middleware, which can communicate with each other. Each component of the client-server comprises: 1) hardware is related to the client and the server that client usually requests the services to the server, which is the computer that provides the services and the responses to every client requests; 2) software is used to response the requirement of users (clients); 3) communication middle-ware is used to transmit information among the client and the server across a network [
To overcome the limitations of the client-server architecture and difficulties in the current connected vehicle applications, we extend the current two-way communications to unidirectional communications to support time critical Device-to-Device (D2D) data exchange based on Internet connections. Specifically, the D2D connection is to introduce the publish-subscribe communication paradigm in the application layer to support the proposed D2D data exchanges with the mobile broadband services (MBB) under the current 4G-LTE networks and the future 5G extreme Mobile Broadband (eMBB) services. In the following, the D2D data exchanges based on the publish-subscribe paradigm is outlined, followed by several use cases and requirements for connected device communications. In Section 3, we will discuss about the D2D GNSS data exchanges and positioning performance with MQTT protocols and the experimental results are presented to demonstrate the performance in the vehicle connection case. Section 4 is the summary of the findings of the paper.
The term “Device-to-Device (D2D)” refers to direct communication between two mobile devices. D2D was initially proposed as a new paradigm in cellular networks. LTE-Direct is an innovative D2D technology that enables mobile devices and applications to passively discover and interact with the world around them in a privacy sensitive and power efficient manner [
• V2V and V2I safety message and location data exchanges for cooperative awareness: The data are required to be transmitted at the rate between 1 and 10 Hz. The payload of each message ranges from 60 to 1500 Bytes [
• V2N and V2I traffic data exchanges for traffic efficiency: This transmission does not have strict delay or reliability requirements, since there is no need for prompt action at the vehicle side. Each vehicle updates the Traffic Management server (uplink) every few seconds with location, status and road information, which are required for the more efficient route selection. The payload of this type of message is up to 1500 Bytes [
• V2P and P2V data exchanges for vulnerable road users (VRUs): This use case is similar to the cooperative awareness category (i.e., latency and reliability requirements) with the difference in that the destination device is a user equipment (e.g., smartphone), where the needed information is less, e.g., payload sizes from 60 to 120 Bytes [
• Location-based vehicle and personal tracking: This offers a tracking platform within a group of people and vehicles in the same company without a centralized Location-based services (LBS) server. In other words, a mobile app can be designed to track family members. The data exchange rate of 1 Hz is sufficiently frequent and the message size should be about 60 to 120 Bytes.
• Device to Server GNSS raw data collections and Server to Device distribution in RTCM formats: This is similar to the V2V RTCM data exchanges, but the data are collected or transmitted at the frequency of 1 Hz or lower. If the payload of each message includes all raw measurements from multiple constellations, the message size can be 1200 to 1500 Bytes.
Overall, in the above D2D use cases, the messages of 60 to 1500 Bytes are required to be transmitted at the frequency of up to 10 Hz. While the LTE Direct and 5G and uMTC technologies may support all use cases of D2D use cases in years of the future, many D2D use cases may also be deployed right now through the application protocols based on the publish-subscribe communication paradigm. Therefore, in the following subsections, we introduce the publish-subscribe communication paradigm, and propose the application-layer D2D framework, which can be implemented with the current Internet connections.
The publish-subscribe paradigm enables unidirectional communication from a publisher to one or more subscribers. In software architecture, publish-subscribe is a messaging pattern where senders of messages, are called publishers, and receivers are called subscribers. Publishers do not program the messages to be sent directly to specific subscribers, but categorize published messages into classes, in which subscribers express interest in one or more classes and receive messages that are of their interest. Publishers and subscribers communicate for the interested messages without knowledge of subscribers or publishers. The Publish-subscribe model enables event-driven architectures and asynchronous event notifications, while improving performance, reliability and scalability.
In the publish-subscribe model, subscribers typically receive only a subset of the total messages published. The process of selecting messages for reception and processing is called filtering. Three publish-subscribe schemes, including topic-based, content-based and type-based, have been researched to identify the events of interest [
MQTT is a message centric wire protocol designed for Machine-to-Machine (M2M) communications that provides lightweight, simple implementation, open standard, reliability, and efficiency with regards to processor, memory, and network resources. The publish-subscribe communication model is used to transfer the telemetry-data in the messages format from publisher (device), along restricted environments and unreliable networks, to brokers across TCP/IP. Recently, the two main versions of MQTT are mentioned as follows: 1) MQTT version 3.1 has been developed to transmit data over Transmission Control Protocol/Internet Protocol (TCP/IP) and it is defined as the basic transport and network service and 2) MQTT-SN which has been developed for transmitting data through User Datagram Protocol (UDP) over low-bandwidth wireless communication networks [
To verify the concept of the D2D publish-subscribe model, we perform the experiments with the MQTT protocol in device-to-device data exchange. This experiment is to compare the RTCM data exchange RTT and PLR between vehicle
and base station and between two vehicles, and their Real Time Kinematic positioning performance. The experiment and results are presented in the following subsection.
This experiment demonstrates V2V RTCM data exchanges and RTK positioning performance. One vehicle was used to host two U-Blox ZED-F9P receivers/laptops.
To determine the data exchange latency One-Trip Time (OTT) from one vehicle/Laptop to another, a timestamp is added to the data message with the same GNSS time tag obtained from own receiver/laptop and received the other receiver/laptop. For any time epoch t, we denote GNSS time tag as GT(t), the timestamp of the data message of GT(t) from receiver A/Laptop A as TSa, and the timestamp of the data message of GT(t) from the receiver B/Laptop B as TSb. The time offsets between GNSS time and the Laptop time are given as follows:
Δ t a = T S a − G T ( t ) (1)
Δ t b = T S b − G T ( t ) (2)
When the Laptop A’s data message of GT(t) arrives in Laptop B, there is a timestamp TSab. The latency of the data message from the Laptop A is given as follows:
d t a b = T S a b − T S b (3)
The offsets ∆ta and ∆tb are not necessary constant over a period of time due to delays from application layers to hardware clock in the user end and computing delays.
offsets and delays and PLR for the delays over 300 ms. The results show that the mean OTT latency of RTCM data exchanges from one vehicle to another is less than 200 ms, and PLR for the delays over 300 ms is 4% and delays beyond one second are less than 1%. It also is observed that the variation of time offsets and delays are notably large due to the use of the Laptop-receiver connection settings for the experiments. This indicates the importance of time synchronization between two mobile terminals as the vehicle states must be predicted from the last observed update to the current time update for making collision avoidance decisions. Referring to the work [
FREQa | Data message obtained /received | Average time offset/delay (ms) | Standard deviation (ms) | Delayed message rate beyond 300 ms | Total message transmitted | PLRb (>300 ms) | PLR (>1000 ms) | |
---|---|---|---|---|---|---|---|---|
∆ta | 10 Hz | NMEA | 7710.00 | 32.66 | NA | NA | NAc | NA |
∆tb | 10 Hz | NMEA | 7960.00 | 43.04 | NA | NA | NA | NA |
dtab | 10 Hz | RTCM | 192.72 | 101.99 | 839 | 18,992 | 4.00% | 0.29% |
dtba | 10 Hz | NMEA | 123.98 | 62.45 | 191 | 18,992 | 1.01% | 0.02% |
a. FREQ = Frequency. b. PLR (Packet loss rate) >300, >1000 = The delay message is greater than 300 and 1000 milliseconds. c. NA = Not applicable.
Stationary Base (SB) | Moving Base (MB) | |||||
---|---|---|---|---|---|---|
Mean (cm) | STD (cm) | Availability | Mean (cm) | STD (cm) | Availability | |
Up-difference | 0.640 | 1.381 | 91.12% | 1.120 | 1.976 | 98.01% |
Distance | −0.027 | 1.969 | 96.99% | 0.164 | 0.689 | 98.38% |
RTK results in high availability or shorter solution outages, showing some advantage of moving base RTK. Detection and identification of RTK solution outages are the integrity determination issue, which is important for connected vehicle safety applications [
Internet-based vehicle connections can address many V2X applications. But the standard client-server architecture is tightly coupling with space, structure and time constraints and has limitations in support of Device to Device (D2D) applications in terms of latency, reliability and scalability performance. These are important requirements for high-demanding connected vehicle applications, such as Vehicle-to-Everything (V2X) applications. Currently, the dedicated short-range communication (DSRC) and 4G-LTE are two widely used candidate schemes for connected vehicle applications. However, the recent 4G-LTE experimental results have shown that the average RTTs are 300 to 400 ms for the vehicle speeds from 60 to 120 Km/h. DSRC latency can be well within the lowest requirement of 100 ms, but their PLRs are significantly degraded when the inter-vehicle distances are over 200 m. In this contribution, a D2D data exchange framework based on publish-subscribe architecture has been proposed. Publishers and subscribers communicate data of interest or messages without knowledge of subscribers or publishers. Publish-subscribe model enables event-driven architectures and asynchronous event notifications, while improving performance, reliability and scalability. The publish-subscribe model can support many D2D deployments as a device can be both a publisher and subscriber, and can communicate with each other through a publisher-subscriber server. The paper also introduced the well-established publish-subscribe application protocol: Message Queue Telemetry Transport (MQTT) for implementation of the proposed D2D based on the publish-subscribe model.
To demonstrate the performance of D2D GNSS RTCM data exchanges and RTK positioning performance, we have used two separate sets of GNSS receivers on the same testing vehicle. This setting allows for the assessment of relative positioning performance with constant antenna distance and constant (or zero) height difference with relative RTK results. The results have shown the mean OTT latency of RTCM data exchanges from one vehicle to another is less than 200 ms, and PLR for the delays over 300 ms is 4%. It is also observed that latency measurements have an uncertainty of tens of milliseconds due to the cross layer hardware connections and computing time variations. Relative positioning results have shown both SB and MB RTK solution accuracy of centimeters and availability of over 98% in the testing routes in a residential area. Results also indicate the needs for detection and identifications of RTK solution outages which should be excluded or bridged for connected vehicle safety applications.
The authors declare no conflicts of interest regarding the publication of this paper.
Sonklin, K., Wang, C., Jayalath, D. and Feng, Y.M. (2019) Connected-Vehicle Data Exchanges and Positioning Computing Based on the Publish-Subscribe Paradigm. Journal of Computer and Communications, 7, 82-93. https://doi.org/10.4236/jcc.2019.710008