Network Coding for Road Traffic Safety Communication

Unlike ARQ, FEC or HARQ communication techniques, the NC technique is known to significantly improve the rate of packet dissemination without increasing the message communication overhead. Following the pioneer work in [3], NC has been seen as an innovative concept that is capable of unlocking the full potential of classical error recovery mechanisms such as ARQ, FEC or HARQ to achieve reliable packets broadcast in vehicular networks. Although several research studies have applied the NC concept to different classical error recovery methods for vehicular communication services, the literature on its application in road traffic safety communication to guarantee reliable and fast transmission of safety packets in vehicular environment is still quite scanty. More so, to the best of our knowledge, no study has been reported on the application of the combination of wireless communication techniques such as NC and cooperative communication, node clustering concept and classical error recovery approaches for timely and error-free safety packets broadcast in road traffic safety communication (this is studied in Chapter 3 and 4 of this thesis). However, Zhang et al. [224], Wu et al. [200], and another group of researchers at the University of Michigan – Dearborn [208], [201] have recently studied the potentials of applying the NC concept to relay based data loss recovery for reliable safety packets transmission in vehicular networks.

Based on the work reported in [209] on relay based data loss recovery, Yang and Guo [208] investigated the benefit of combining the NC technique and relay based data loss recovery for safety packets transmission reliability in vehicular networks. With the network coded relay based error recovery protocol proposed in [208], safety packets separately received from two different vehicles are combined into one packet by the use of the NC technique and relayed in order to widen the transmission coverage and improve the safety message transmission reliability. Any vehicle that receives the relayed coded

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packets will be able to decode the coded packets if the vehicle has at least one of the encoded packets. Furthermore, in [201], an improved version of the protocol is proposed, which increases the chances of decoding each received coded packet by encoding one packet from each of the relay vehicles in a typical linear motor road topology. Similarly, Zhang et al. [224] improved upon the work reported in [223], and proposed a novel cooperative forwarding solution, referred to as Cooperative Positive Orthogonal Codes (POC) based Forwarding (CPF) scheme to extend the POC-MAC for multi-hop communication in a typical highway vehicular networks. In [223], the dissemination of a data packet at each hop is retransmitted within a transmission frame of 𝐿 time-slots, but the proposed CPF protocol deterministically schedules the retransmission of each data packet according to the POC code-words. Furthermore, the CPF exploits the spatial diversity by distributing the multiple forwarding transmissions among all the cooperating relay vehicles at each hop as clearly depicted in Fig. 2.1. More so, in order to ensure proper penetration of the coded packet using the multi-hop dissemination, each of the multiple relay vehicles is assigned additional transmission opportunities (TOs) by the CPF protocol for each multi-hop flow to guarantee efficient data forwarding across the network. In Fig. 2.1, the dashed arrows represent the TOs assignment to the virtual relay members, while TOs correspond to POC code-words. Although the results reported in their published study show that CPF protocol outperforms other similar existing solutions, the authors did not specify how the multiple relay vehicles are selected, or the metrics used in the process to ensure that only suitable relay candidates that will guarantee optimal results are selected to perform the relaying functions.

Fig. 2.1: Reliable cooperative coded packets forwarding using multiple vehicles as virtual relays [224].

The overall performance of the relay based data loss recovery schemes proposed in [208] and [201] in terms of the rate of coded packets transmission reliability is compared with the performance of the repetition based protocol proposed by Xu et al. [207].

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Although no study has investigated the reliability performance comparison between conventional relay based error recovery protocol and network coded relay based error recovery protocol, it is obvious that the NC enabled relaying approach will certainly outperform the conventional relaying approach. This is because the NC technique enriches the content of each packet transmission by combining two or more packets into one prior to dissemination and subsequent multiple relays. Upon careful examination of the results of simulation experiments reported in [207], [208] and [201], it is obvious that the simulation settings and parameters of the three studies are the same apart from the fact that the vehicular densities used in [207] and [208] are slightly smaller than the vehicular density used in [201]. Hence, it is possible to directly compare the performance of the three studies using the results of the simulation experiments presented in the papers. Finally, the comparison of their results show that relay based protocols in [208] and [201] outperform the repetition based scheme in [207]. Furthermore, it is noteworthy to mention that the results of simulation experiments in [225] clearly show that the NC aided relay based scheme offers the highest performance in terms of transmission reliability of the coded safety packets.

2.3.4 Application of Random Network Coding, Vehicle Clustering, and