# Hybrid Multi-Antenna Techniques for V2X Communications--Prototyping and Experimentation
## 1 Introduction  
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, 3]. 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 theother 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, 11, 12, 13, 14, 15, 16]. 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.