This is our biweekly update for the comparison of performance, features, and resource allocation mechanisms for these three V2X communication technologies: IEEE 802.11p, LTE-V2X, and NR-V2X. The update is based on 4 papers, and we focus on their performance, and resource allocation strategies for vehicular networks.

IEEE 802.11p

• Communication Approach: Use the DSRC which is the Dedicated Short-Range Communication. And it is using the 5.9GHz spectrum.

• Advantages:

• It has little end-to-end latency compared to others in short-range communication.

• It has in vehicular ad hoc networks (VANETs) which is Well-established.

• It uses the EDCA which is the enhanced distributed channel access based on the CSMA/CA to handle medium access.

• Challenges:

• Because IEEE 802.11p has contention-based access, it has limitations in scalability.

• For the high vehicle density scenarios, it might cause congestion in that case.

• It is hard to reach the stringent latency demands and reliability demands for the next-generation AV applications.

LTE-V2X

• Communication Approach: It is based on the 4G cellular networks. It can support both Mode 3, which is scheduled by eNodeB. And Mode 4, which is autonomous resource selection by vehicles without infrastructure dependence.

• Advantages:

• It has a larger coverage than 802.11p. The large range can ensure reliable communication for longer distances, for example, the distance is larger than several hundred meters..

• Since it has semi-persistent scheduling (SPS), it can be more efficient for  high-load case. 

• It can supports V2V and V2I communications for cooperative awareness, while other technologies cannot support both of them.

• Challenges:

• Because of the resource allocation is decentralized, the Mode 4 performance might be impacted by collisions and some other interference problems. 

• Mode 3 depends on the network coverage, thus it is not effective for the weak cellular area.

NR-V2X (5G-V2X)

• Communication Approach: It is defined in 3GPP Release 16, NR-V2X introduces advanced features beyond LTE-V2X.

• Key Enhancements:

• It can support for new numerologies: Flexible subcarrier spacing (SCS) which can be 15 kHz, 30 kHz, 60 kHz, or 120 kHz. Thus it can reduce the latency. Besides, it can improve the time-sensitive applications reliability.

• It can reduce latency by multiplexing of Control and Data Channels in the Time Domain.

• For the new Unicast and Groupcast Modes, it can increase the efficiency of applications for resource utilization.

• The physical Sidelink Feedback Channel (PSFCH) can increase the group reliability and unicast communications.

• Challenges:

• We do not have enough NR-V2X research for the efficient resource allocation strategies.

• It requires seamless mode switching between LTE-V2X and NR-V2X for smooth performance, which has not been solved yet.

LTE-V2X Modes (Mode 3 and Mode 4) :

• Mode 3 (Centralized Control by eNB) :

• Mode3 is efficient. However, it depends on cellular infrastructure availability.

• We have limited research efforts for it.

• Mode 4 (Decentralized SPS Algorithm) :

• Vehicles can autonomously to select resources by using the sensing-based Semi-Persistent Scheduling (SPS).

• There might be high interference and congestion when there are too many vehicles on the road.

NR-V2X Resource Allocation (Mode 1 and Mode 2)

• Mode 1 (gNB-Controlled Resource Allocation) :

• Mode 1 can use the network slicing techniques to dynamically allocate resources for V2X applications.

• It can support Ultra-Reliable Low-Latency Communications (URLLC).

• Mode 2 (Autonomous Resource Selection by Vehicles, Similar to LTE-V2X Mode 4) :

• Mode 2 is more flexible, but it depends on novel interference mitigation strategies.

• IEEE 802.11p, LTE-V2X, and NR-V2X are at different stages of adoption in vehicular networks. For 802.11p, it is mature enough for short-range cases. And LTE-V2X is better for longer-range applications and  more congestion situations. NR-V2X is good for reliability, and it has lower latency, and efficient resource allocation.