Medium access control for cellular-based vehicular ad-hoc networks

Gallo, Laurent

Vehicle to Vehicle (V2V) communications represent a critical enabler for safety of life applications such as Highly Autonomous Driving (HAD), thus are sub- ject to stringent reliability constraints. 802.11p is the current de-facto standard for V2V, formalized by European Telecommunications Standard Institute (ETSI) in the EU and the Federal Communication Commission (FCC) in the USA. Fast evolving cellular technologies such as 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) are also working towards V2V support. Differently from 802.11p, however, LTE has a strict centralized communication scheduling, which is a limiting factor for the specific traffic patterns, network topology, and safety criticality requirements of V2V. However, very recent evolutions of the LTE standard, enabling direct Device-to-Device (D2D) operations, open the path towards making it a viable candidate to coexist with 802.11p, thus providing a technological redundancy desirable for safety-of-life systems. D2D LTE, currently referred to as the Proximity Services (ProSe), still retains a strong centralized structure, with unsupervised operations being en- abled only for Public Safety UEs. None of the currently available technologies has been specifically designed for the traffic patterns and requirements of V2V communications. V2V over LTE D2D, specifically, requires careful analysis, as many questions need to be answered: how can the control of the transmissions shift from a central entity (basestation) to many, distributed, actors? How it is possible to exploit LTE's channel structure to enable broadcast, cross-cell, pan-operator communications?

This thesis shows that unsupervised operations, currently loosely specified in ProSe, are essential for V2V. A proposition is made to address V2V needs, by splitting Medium Access Control (MAC) layer analysis into two separate enti- ties: the resource reservation and the distributed channel access, of which only the former is performed by the network. A periodical, semi static-resource reservation scheme, based on a shared resource pool, was identified as the key to address the communication pattern requirements and to minimize the network involvement. A slotted and periodical channel organization pattern is proposed, which allows the scheduling to be treated as a TDMA-like system, wherein slots are distributed in both time and frequency. Two different approaches are then considered for distributed channel access: blind, based on Optical Orthogonal Codes (OOC), and aware of concurrent users' transmission patterns, Self-Organizing TDMA (STDMA). Their performance are analytically evaluated and benchmarked against 802.11. The effect of the proposed channel configuration is evaluated analytically and by means of simulation.

The cellular V2V mechanism proposed in this thesis allows for cross-cell broadcast, in-band as well as out-of-band deployment. The flexible channel structure enables coexistence with 802.11p, which is however challenging due to the substantially different channel access techniques. It is shown that OOC can outperform 802.11p while only being marginally affected by the half du- plex impairment introduced by the time / frequency disposition of transmis- sion slots, but its performance degrades with increasing channel load because of its blind access. STDMA, on the other hand, allows for more stable performance, but its re-reservation mechanism is more significantly affected by losses due to half duplex, thus requiring modifications to adapt to the proposed channel structure. 

Systèmes de Communication
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