Video coding solutions for VANETs

Introduction The impending widespread deployment of Vehicular Ad Hoc Networks (VANETs) has created an opportunity for multimedia communication not only as part of vehicle safety provision, traffic management, and emergency response but also in the value-added ‘infotainment’ domain. The key feature of video distribution to the passengers of vehicles or crew members of emergency vehicles in the VANET is robustness and reliability, as the environment is highly error prone. As detailed below, complex movement patterns of vehicles and non-line-of-sight wireless propagation add to the challenge of multi-hop routing. One response to this challenge is through the error-resilience features of the H.264/AVC (Advanced Video Coding) codec, which together with multipath streaming, including peer-to-peer streaming [1], provide source coding solutions. Application-layer forward error correction (AP-FEC) as a general solution may replicate physical layer channel coding (unless the AP-FEC acts as a concatenated code), while error control through ARQ, apart from the additional latency introduced over multi-hop network paths, is unreliable when network links are constantly being broken. In retrospect, three events may be singled out in the rise of VANET multimedia communication. Firstly in 1999, the US FCC allocated 75 MHz bandwidth of the 5.9 GHz spectrum to Dedicated Short Range Communication (DSRC), essentially for wireless communication between vehicles and from a vehicle to the network infrastructure, normally via roadside units. Secondly, the term Vehicular Ad Hoc Network (VANET) was first applied in the 2004 ACM international workshop of that name and since then academic activity has burgeoned. And thirdly, in 2005 video streaming over VANETs was suggested as a way of reporting traffic congestion and accidents [2], as captured by roadside cameras. In 2005 also, an early feasibility study [3] in Japan was testing streaming video between two vehicles. Imaginative ways of responding to urban emergencies [4] by streaming video to responding vehicles are one of a number of VANET initiatives by Mario Gerla’s research group at the University of California, Los Angeles. Multi-wireless-interface vehicles placed within cellular networks are already proposed [5] as a way to relieve congested cells. As new ‘push’ multimedia services are introduced into 3G cellular networks, the same services may be extended into VANETs. VANETs may also support the exchange or sharing of personal video clips (as occurs in social networks). Roadside sources of multimedia content [6], possibly linked in a backbone network, can disseminate preencoded video or serve to notify the passengers of a passing vehicle of available video sequences in circulation within the VANET.