GPS-aided inter-microcell interference avoidance for request-transmission splitting slotted ALOHA-based scheme in smart cities with connected vehicles
Introduction
Disseminating messages about traffic conditions among vehicles via vehicular communications is a promising solution for improving the efficiency and safety of traffic in a smart city [1], [2], [3]. With the increasing number of vehicles on the road, designing a vehicular network with a large access capacity to support the significant scale communication demand is an urgent need. For that reason, the United States Federal Communication Commission (FCC) especially allocates a bandwidth with 75 MHz on the 5.9-GHz band for vehicular communications. This bandwidth is composed of one control channel (CCH) and six service channels (SCHs), in which the CCH is used to transmit safety-related messages [4]; while the SCHs are used to transmit a wide variety of service data messages [5], [6]. Due to providing a bounded access delay, the slotted ALOHA-based access scheme on the CCH is meaningful, and various schemes are designed. Among them, the slotted ALOHA-based access scheme with infrastructure coordination has an outstanding performance in reducing scheduling overhead and increasing the access capacity and thus has attracted considerable attention [7], [8], [9], [10].
In the slotted ALOHA-based access schemes, the communication time is divided into equally spaced frames, and each frame is further divided into a fixed number of time slots. In the Adaptive Collision-Free MAC (ACFM) scheme [7], each vehicle obtains a time slot from a set of available time slots via a competitive access manner. The set of available time slots and the confirmation of successful access are determined by the packet broadcasted during the exclusive time slot of the roadside unit (RSU). To reduce the probability of competitive collisions in the ACFM scheme, the Adaptive Time Division Multiple Access-based MAC (VAT-MAC) scheme [8] was proposed, in which an RSU can estimate the number of nodes in its coverage and adjust the ratio of the number of time slots between the contention-free part and the contention access part within a fixed-length frame, so that the throughput performance can be improved.
Different from these schemes, in our previous work, aninfrastructure-coordinated request and transmission splitting slotted ALOHA-based (RTS-SA) scheme is proposed to enhance the access capacity [9], [10]. Each frame is divided into three phases: a contention access phase (CAP), a broadcast feedback phase (BFP), and a contention-free transmission phase (CTP). In the CAP, each vehicle repeats sending its request signals to an RSU. After receiving all signals, the RSU implement multiuser detection by employing the successive interference cancellation (SIC) technique. Then, the detected vehicles occupy the time slots, which are assigned by the RSU, to transmit without conflicts in the CTP. Compared to the no-infrastructure-coordinated access schemes, e.g., the VeMAC scheme [11], the RTS-SA can improve the successful access performance of vehicles.
However, the VAT-MAC and RTS-SA schemes only consider the access problem of vehicles aided with a single RSU. With the rapid development of the narrow-band IoT (NB-IoT) technology, the high-density deployment of micro base stations (mBSs) makes them infrastructures in vehicular networks rather than RSUs [12], [13], [14], [15]. In addition, mobile edge computing (MEC) offers cloud capacities at the mBSs [16], [17], [18]. These characteristics can support the implementation of the RTS-SA scheme, and the interference problem of the RTS-SA scheme in multiple mBSs scenario should be considered.
To this end, we present an improved RTS-SA, referred to as enhanced RTS-SA (eRTS-SA), to solve the interference problem. In the CAP, each vehicle utilizes its global positioning system (GPS) information and the location of mBSs to determine whether it is in the overlapping area of the adjacent microcells for avoiding interference at the both mBSs. In addition to the requested information, each vehicle reports its location to the mBS. To reduce the overhead, the road between any two adjacent mBSs is divided into segments, and each vehicle utilizes the segment number (only a few bits) to replace its location information. After performing the multiuser detection, the mBS sorts the detected vehicles in ascending order according to the segment numbers they transmitted and allocates them time slots sequentially. This time slot allocation strategy ensures that the same time slot would be assigned to vehicles that are geographically far apart, and a minimum mutual interference could be achieved in the CTP. Finally, the duration of a BFP is doubled and divided into two equal parts for these two adjacent mBSs to distinguish their broadcasted signals. In conclusion, the main contributions of this paper are summarized as follows:
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We analyze the causes of the interference problems in each phase of the RTS-SA scheme within a multiple synchronous mBSs scenario, and then propose the strategies to avoid interference occurring in each phase.
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We develop a practical method to realize the new time slot allocation strategy, in which each vehicle utilizes only a few bits to represent its location information and the mBS can make use of the location information of the detected vehicles when allocating time slots.
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Simulation results show that the throughput of the eRTS-SA scheme is only 4.8% lower than the theoretical maximum throughput, while 44% and 154% higher than that of the RTS-SA scheme and the VeMAC scheme, respectively.
The remainder of this paper is organized as follows. Section 2 describes the system model. Section 3 introduces the RTS-SA scheme and its interference problems in the multiple mBSs scenario. Section 4 describes the proposed scheme. Section 5 presents the simulation results. Finally, we conclude this paper in Section 6.
Section snippets
System model
We consider a network, illustrated in Fig. 1, where each mBS is equipped with MEC servers to serve vehicles in their microcells and vehicles move on a two-way highway with no on- and off-ramp. Because this paper considers replacing RSUs with mBSs as the infrastructures for vehicular networks, it is assumed that mBSs are uniformly deployed along the road. We refer to the th mBS by and the th vehicle by .
To quantitatively describe the position of nodes (vehicles or mBSs), we regard the
Basics of the RTS-SA scheme
In the RTS-SA scheme, the communication time is partitioned into frames of equal duration . Each frame is composed of three phases: (1) the CAP of duration , (2) the BFP of duration , and (3) the CTP of duration , as shown in Fig. 2. Furthermore, the CAP is divided into equally spaced mini-slots of durations , i.e., . The BFP of duration is used by mBSs to broadcast time slot allocation information. The CTP is divided into equally spaced time slots of duration . All
Proposed scheme
In this section, we first describe how the proposed eRTS-SA scheme avoids the interference problem of vehicles in overlapping areas of adjacent microcells and their interference impact assessment. To solve the problem that two adjacent mBSs occupy the same time resource, we redesign the frame structure. Then, to solve the hidden terminal problems in the CTP, we design a theoretically optimal time slot allocation strategy based on location information of vehicles. Finally, to reduce the request
Simulations
In this section, we evaluate an appropriate number of bits to represent a vehicle’s location information to achieve high throughput without introducing excessive overhead. Besides, we present simulation results to compare the performance of the eRTS-SA scheme with that of the RTS-SA scheme and the VeMAC scheme in a highway scenario.
Conclusion
Considering a more practical scenario where widely deployed mBSs in a smart city, an enhanced RTS-SA scheme was proposed in this paper to address the inter-microcell interference problems, resulting in the eRTS-SA scheme. In the CAP, except for the request information, each vehicle reported its location to mBS with the help of the GPS. We found that the interference effect of vehicles in the overlapping area between any two adjacent mBSs on other vehicles could be ignored, thus these vehicles
CRediT authorship contribution statement
Shenglong Peng: Conceptualization, Methodology, Software, Validation, Writing - review & editing. Liang Zhou: Methodology, Supervision, Writing - review & editing. Xuan He: Conceptualization, Methodology, Software, Writing - original draft. Junyi Du: Conceptualization, Writing - original draft.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Shenglong Peng received the B.E. degree in information engineering from Chengdu University of Technology, China, in 2011. He is pursuing his Ph.D. degree in National Key Laboratory of Science and Technology on Communications at University of Electronic Science and Technology of China. His research interests include access schemes and interference avoidance strategies in V2X networks.
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Cited by (1)
Shenglong Peng received the B.E. degree in information engineering from Chengdu University of Technology, China, in 2011. He is pursuing his Ph.D. degree in National Key Laboratory of Science and Technology on Communications at University of Electronic Science and Technology of China. His research interests include access schemes and interference avoidance strategies in V2X networks.
Liang Zhou is currently a Professor with the University of Electronic Science and Technology of China, China. His research interests include wireless communications and networking, error-control coding, information system engineering, and communication and cyberspace security.
Xuan He received the B.E., M.E., and Ph.D. degrees in communication and information engineering from the University of Electronic Science and Technology of China (UESTC), Chengdu, China, in 2011, 2013, and 2018, respectively. From Oct. 2016 to Sep. 2017, he was a Visiting Student sponsored by the China Scholarship Council (CSC) with the University of Waterloo, ON, Canada. He is now a research fellow with the Singapore University of Technology and Design (SUTD), Singapore. His current research interests include information theory, channel coding, and DNA-based data storage.
Junyi Du received the B.E. and Ph.D. degrees in communication and information engineering from the University of Electronic Science and Technology of China (UESTC), Chengdu, China, in 2011 and 2017, respectively. During Sep. 2015 to Sep. 2016, He was sponsored by the China Scholarship Council (CSC) as a visiting student with the University of New South Wales, Sydney, Australia. He is currently a researcher in the Southwest China Institute of Electronic Technology. His research interests include error correction coding, coded modulation and software defined network.