# Hybrid Multi-Antenna Techniques for V2X Communications--Prototyping and Experimentation
## 2 Distributed Multiple Antennas for V2X Communications
### Diversity Concept  
In Figure 4, a generic view of the block-diagram for an orthogonal frequency division multiplexing (OFDM)-based diversity communication system is presented. Such a transmission scheme is common for all recent V2X communication standards. The system can be divided into three main parts, the Tx, the Rx, and the channel. The basic blocks for the signal processing procedure at the Tx include: scrambling, encoding, modulation, and inverse fast Fourier transform (IFFT) implementation. The Tx employs a single-antenna, however, we have also included an optional space-time/frequency block encoder (STBC/SFBC) that enables Tx diversity (with the use of two antennas). It is noted that ITS-G5 does not support STBC or SFBC. The option is expected to be included in the future C-V2X versions, however, it was not included in the final version of the 3 GPP Rel. 15 standard.  
At the Rx, the diversity engine is located after the OFDM demodulator. The Rx also includes the complementary/inverse functions with respect to the Tx. Moreover, the main idea for the implementation of the diversity engine is depicted in Figure 5. Based on it, the Rx is composed by a set of RF modules and a communication unit. In our implementation the compact RF modules were also placed inside the truck mirrors--very close to the antenna. Each RF module contained two RF chains. The signal from the four RF chains is digitized and led to the communication unit, where the hybrid diversity scheme is applied in the baseband. According to the adopted concept, signal digitization is performed near the antennas, therefore, the cable length between the antennas and the communication unit does not introduce any extra attenuation. Thus, antennas can be mounted on the side mirrors (or any other place on the truck), while the communication unit resides into the vehicle with no signal quality degradation.  
The proposed diversity scheme is an ideal candidate for vehicular communications, since:  
* it provides increased MIMO support with a limited number of RF chains;
* it utilizes small, compact antennas able to be installed in many parts of the vehicle, which can improve links' quality (with other vehicles or the infrastructure);
* it is designed to minimize the need for extra cabling, especially regarding the rigid and lossy RF cables--a significant requirement imposed by the auto-manufacturers;
* it provides significant diversity and beam-tracking gain on-top (and without modification) of the generally simplistic V2X protocols that do not yet explicitly support multi-antenna configurations;
* it allows the simultaneous improved support for both broadcast and unicast ITS services. The latter ones consist of a set of broadcast single message services, but also a set of peer-to-peer  
Figure 4: Simplified block diagram for orthogonal frequency division multiplexing (OFDM)-based transmission/reception model with diversity.  
communication ones. Moreover, some types of future single message services (e.g., platooning) may benefit by beam steering/tracking optimizing transmission along the direction of the platoon movement.