Vehicle-to-everything (V2X) communications have gained a significant momentum globally, as the main enabler of intelligent transportation systems (ITSs) [1
]. The latter one includes novel applications, services and technologies that aim to improve transport safety and efficiency, driver/passenger experience, and environmental performance, leading towards connected vehicles and autonomous driving [2
]. The automotive sector is considered probably the most important vertical domain for 5G and beyond 5G communication networks, with both telecom and automotive industries competing for the exploitation of the new opportunities [4
]. In this concept, a fundamental novel approach is the support of the dynamically varying heterogeneous network structures and topologies. In particular, the mechanism of direct mobile-to-mobile communications is inherently integrated in 5G [5
]. Moreover, the 5G framework supports multiple radio access technologies enabling V2X communications—from existing (ITS-G5 [6
]) and new evolving technologies (LTE-V2X in LTE Rel. 14 [7
]) to an upcoming standard (eV2X)—yet to be defined, which will be included in future new radio (NR) releases [8
5G defines three basic objectives [9
]: (a) increased throughput via enhanced broadband, (b) massive machine-type communications (10x-100x more devices), and (c) ultra-reliable, low-latency communications. The first 5G release focuses on objective (a), while the second on (b). On the other hand, V2X and ITS impose significant requirements on (b) and (c), while self-driving cars stand in the conjunction of all objectives. This means that V2X will have a central role in the formation and evolution of 5G. Thus, a new flexible architecture that increases throughput and reliability, while it decreases bit error rate (BER) and latency, is going to be the new communication paradigm towards 5G. However, such an architecture should always satisfy the stringent constraints that exist in V2X communication environments regarding the low latency and reduced complexity.
The performance of V2X, as well as other systems that support direct and/or adhoc communication modes can be further improved with the application of sophisticated transmission/reception techniques, which have already been used in legacy cellular systems. However, the direct adaptation of techniques in adhoc or partially controlled networks -like V2X- with fast-varying topologies and heterogeneous applications and services is not possible. In this context, one of the most efficient technique for system performance improvement is diversity. The term diversity refers to a set of multi-antenna transmission/reception schemes used to improve the quality and reliability of a wireless link. While transmit diversity with the use of space-time/frequency codes is already scheduled for integration in V2X, the direct application of receive or closed-loop transmit diversity methods is not a straightforward task due to various constraints inherent in V2X communications, for example, adhoc establishment and resolution of links, support of various modes of services simultaneously (broadcast, multicast, unicast), small dimensions and limited space for antenna and radio frequency (RF) equipment, low complexity requirements, and limited signal processing capabilities. The situation worsens in V2X channels, where the dynamic channel decreases the diversity gain due to outdated channel state information and the Doppler shifts. In addition, the current standard versions do not implicitly support diversity in transmitter (Tx) or receiver (Rx), thus, the system designer should apply diversity on-top of the system standardized operation.
In the context of low complexity multi-channels techniques for direct type of communications, various approaches have been proposed, for example, References [10
]. For example, in Reference [13
], a new compact antenna module suitable for combined use with LTE and ITS-G5 was proposed and its performance was evaluated through simulation. Nevertheless, the integration of multiple antennas in real-world mobile radio transceivers imposes significant challenges rising from the variety of hardware and signal processing constraints as well as the peculiarities of the wireless medium. In this article, we present a novel hybrid diversity scheme that simultaneously supports multi-mode services, and has been developed and tested under real-world conditions in the context of H2020-ROADART project. The scheme exploits the capabilities of electronically switched parasitic array radiator (ESPAR) antennas, which demonstrate considerably reduced complexity and size, compared to conventional multi-antenna solutions. Capitalizing on the additional degrees of freedom offered by the ESPAR antennas, the design of a new diversity engine is presented. The engine implements a flexible, hybrid scheme that combines conventional diversity, that is, a maximal ratio combiner (MRC) and a pattern selection technique built on low-cost, compact ESPAR antennas. Finally, the new technique was integrated in a real-world testbed for truck-to-truck (T2T) communications and the results from various experimentation trials are presented. It is noteworthy that the new diversity engine is compatible with all current V2X technologies (NR, LTE or WiFi-based solutions) and new features can be easily integrated on it as new standards and technologies emerge.
The paper is organized as follows. In Section 2
, the basic principles of the ESPAR antennas, the diversity concept as well as the main steps that were followed for their configuration are presented. Moreover, in Section 3
, a detailed description for the hybrid diversity engine is presented, in which the operational algorithm and the reconfiguration procedure are analyzed. In Section 4
, various performance results are given, based on a real world vehicle-to-vehicle (V2V) communication testbed that has been built. Finally, in Section 5
, the concluding remarks are provided.
3. Detailed Description of the Diversity Engine
Let’s consider two vehicles (A and B), which are equipped with the diversity engine depicted in Figure 6
. At the base of the engine, an MRC is employed, which combines the received signal-to-noise ratio (SNR) from the four independent RF chains. In the course of the research activities, other diversity schemes were also tested, that is, equal gain combining (EGC), antenna selection and minimum mean square error (MMSE). The MRC was selected as the one with the best performance, despite the fact that MMSE is theoretically optimal in terms of SNR. However, the MMSE requires knowledge of the noise and interference variance, thus, for its real-world implementation a noise power estimator was also used. Due to the estimation error and the non stationary nature of the noise+interference level in dynamic vehicular environments, the MMSE diversity performance diverged from its expected optimal behavior.
Uncorrelated signal reception is desired in order to maximize the diversity gain and this is ensured due to the increased distance among the ESPARs. In order to evaluate the received SNRs, the engine uses measurements of the error vector magnitude (EVM) at the received quadrature phase-shift keying (QPSK) constellation of pilot symbols, in conjunction with the automatic gain controller (AGC) values for each RF chain. It is also assumed that the system hosts two sets of applications. The first set contains services and applications in broadcast mode of operation (i.e., reception of cooperative awareness messages), while the second set involves the direct communication of A and B, implementing, for example, a platoon. Thus, in the investigated scenario, the two transceivers host two services, a single message service and an A-to-B unicast.
In order to describe the simple but yet efficient operation of the diversity engine, an exemplary use case is presented where A and B are communicating directly with the use of a V2X standard (ITS-G5 or C-V2X/PC5). Initially, it is assumed that B receives an ITS packet. The received packet is processed and if originated by A, it is forwarded to the diversity engine, which uses the selected metrics (EVM and AGC values) in order to decide whether to change the currently selected combination of patterns of the four ESPAR antennas. All engine decisions are taken during reception. During the transmission phase, B employs the combination of patterns selected during the receiving phase. Due to the reciprocity principle, it is expected that the pattern that optimizes reception from A, also offers increased power towards A. The same operation is also followed by vehicle A. Since many unicast, multicast, and broadcast applications are required to operate in parallel, functionalities that ensure coexistence of omni and directive patterns were also developed, as demonstrated below.
During the field trials of the ROADART project, the implemented diversity engine was able to support various modes of operation, including random selection of pattern combinations for each RF chain or user-defined manual pattern selection. Here, we focus on a simple, automated, standard-agnostic, and yet remarkably efficient approach presented in the flow graphs of Figure 6
and Figure 7
. During the field trials, the 3-element ESPAR with PIN diodes were used—providing three active patters per antenna (the notation used is 0: omni, 1: front, 2: back). As shown in Figure 6
, the basic operation of the diversity scheme depends on two thresholds (
) related with the two control parameters, that is, MRC SNR and a timer, respectively. Initially, all patterns are set to omni. The rationale of the diversity algorithm is the following:
The pattern reconfiguration for the RF chains depends on the MRC output value; if it falls below a predefined threshold (), the reconfiguration procedure will initiate. Therefore, the pattern combination will not change if the SNR of the current selection does not fall below , despite the fact that a different combination with better performance may exist. In this context, continuous unnecessary changes are avoided, which results to a complexity reduction and avoidance of synchronization problems. At the same time, important performance degradation is avoided, since the use of ensures that the MRC-SNR remains relatively high.
When on transmit-mode, the transceiver will select either the omni pattern for single message broadcast, or the pattern combination that was decided during reception in order to optimize the link between B and A.
On the other hand, if reconfigurations have not been performed for more than
received packets (parameter timer of Figure 6
, the diversity engine will attempt to reconfigure and search for a better pattern combination, despite the fact that the SNR threshold is not violated. The reconfiguration trigger through the timer is used in order to periodically force the system to search for better pattern combinations and improve performance, even if the SNR remains relatively high.
The reconfiguration procedure is presented in Figure 7
. It is performed in two stages, since there is no explicit channel state feedback mechanism defined in the standards. During the first stage (variable check = false in Figure 6
and Figure 7
), for each pair of antennas, that is, each side mirror in our implementation (defined by counter j), the engine estimates the received SNR. The main reconfiguration rule is that for each j, the RF chain with the minimum SNR is set to omni and the one with the maximum SNR is selected for reconfiguration and is denoted as “active-directive”. This policy will allow us to exploit maximally the directive gain of the “active-directive” antenna, while we maintain the use of an omni antenna for the exchange of broadcast messages through all directions.
For the case of the “active-directive” antenna, the first reconfiguration step is defined as follows: If the current pattern is directive (pattern 1 or 2), then the omni pattern, 0, is selected. Otherwise, directional patterns are selected in a random manner. The status of the previous pattern combination is saved and the same procedure is performed for the other pair of antennas (or all j’s existing in the system).
At the second stage (check = true), the selected pattern for the active RF chain of each antenna pair is evaluated and the system validates the reconfiguration procedure as successful or failed. If the measured SNR is increased compared to the previous formation, then the reconfiguration is assessed as successful. On the other hand, if the estimated SNR is decreased, the reconfiguration attempt is considered failed and two scenarios exist depending on the formation of the current pattern: if it is omni, then the previous selected pattern is restored, since it is assessed as the optimum for the current radio channel state and despite the fact that reconfiguration was triggered, no SNR improvement can be achieved; if it is directive, the opposite directive pattern is selected, since the current selection does not favor propagation towards the desired direction. In any case, at least two antennas are set to omni. This is necessary for the rapidly varying vehicular network, since the vehicle should be able to receive/transmit beacon and traffic messages from/to all directions as part of the safety-related, single message services of the ITS framework.
A far as the integration phase is concerned, the diversity engine was implemented as a software module in C++ and executed on the communication unit that is digitally connected to the RF modules, as shown in Figure 6
. Moreover, just like the RF modules, Linux operating system was also uploaded on the communication unit. The communication unit software is hosted on a Linux operating system.
Conceptualization, K.M., L.M., P.S.B., and A.G.K.; methodology, K.M., L.M., P.S.B., and A.G.K.; software, K.M. and L.M.; validation, K.M. and L.M.; investigation, K.M., L.M., and P.S.B.; writing—original draft preparation, K.M., L.M., and P.S.B.; writing—review and editing, K.M., L.M., P.S.B., and A.G.K.; visualization, K.M. and L.M.; supervision, A.G.K. All authors have read and agreed to the published version of the manuscript.
This research was funded by Horizon 2020 project ROADART. The research of K Maliatsos is funded by Greece and the European Union (European Social Fund- ESF) through the Operational Programme «Human Resources Development, Education and Lifelong Learning» in the context of the project “Reinforcement of Postdoctoral Researchers—2nd Cycle” (MIS-5033021), implemented by the State Scholarships Foundation (IKY).
This research has received funding from the European Union’s Horizon 2020 research and innovation programme under ROADART Grant Agreement No. 636565. The research of K. Maliatsos is supported by Greece and the European Union (European Social Fund- ESF) through the Operational Programme «Human Resources Development, Education and Lifelong Learning» in the context of the project “Reinforcement of Postdoctoral Researchers—2nd Cycle” (MIS-5033021), implemented by the State Scholarships Foundation (IKY).
Conflicts of Interest
The authors declare no conflict of interest.
The following abbreviations are used in this manuscript:
|AGC||Automatic gain controller|
|BER||Bit error rate|
|EGC||Equal gain combining|
|ESPAR||Electronically switched parasitic array radiator|
|EVM||Error vector magnitude|
|FFT||Fast Fourier transform|
|ITS||Intelligent transportation systems|
|IFFT||Inverse fast Fourier transform|
|KPI||Key performance indicators|
|LLR||Log likelihood ratio|
|MMSE||Minimum mean square error|
|MRC||Maximal ratio combiner|
|OFDM||Orthogonal frequency division multiplexing|
|PDF||Probability density function|
|QPSK||Quadrature phase-shift keying|
|RAN||Radio access network|
|SFBC||Space-frequency block coding|
|STBC||Space-time block coding|
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Installation of two reconfigurable electronically switched parasitic array radiator (ESPAR) antennas inside the truck side mirror casing.
The simulated 5.9 GHz H-plane radiation patterns of the 3-printed ESPAR antenna (blue) and the 5-wire monopole ESPAR antenna (red) for the two antenna states ((a) OFF-OFF, (b) ON-OFF).
The measured 5.9 GHz gain radiation patterns of the ESPAR antenna for the quasi-omni (left) and the directive states (right) at the H-plane (top) and the E-plane (bottom).
Simplified block diagram for orthogonal frequency division multiplexing (OFDM)-based transmission/reception model with diversity.
Diversity engine with two radio frequency (RF) modules (one per side mirror) with two RF chains and reconfigurable antennas per module. Simplified block diagram.
Diversity engine with two RF modules (one per side mirror) with two RF chains and reconfigurable antennas per module. Implemented diversity engine—algorithmic representation.
Reconfiguration function of the diversity engine.
Signal-to-noise ratio (SNR) vs time for diversity and SISO-OMNI.
Histograms of SNR for communications links (diversity vs. SISO-OMNI).
Achievable throughput at 10 MHz bandwidth (diversity vs. SISO-OMNI).
SNR gain of the diversity engine vs. SISO-OMNI (visualization with Grafana tool).
Coverage probability of the diversity engine vs. SISO-OMNI (visualization with Grafana tool).
Bit error rate using the diversity engine vs. SISO-OMNI (visualization with Grafana tool).
Packet error rate using the diversity engine vs. SISO-OMNI (visualization with Grafana tool).
Latency (Physical Layer) of the diversity engine vs. SISO-OMNI (visualization with Grafana tool).
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