# A Constrained Coding-Aware Routing Scheme in Wireless Ad-Hoc Networks
## 2 Related Work
Furthermore, most CAR and DCAR schemes are based on the idealized assumption that data flows have the same rate. Hence, some researchers focused on the problem of the multi-rate interference in the CAR schemes. The work in [32] proved that NC could improve network throughput in a rate-diversity scenario, and the authors used a new linear programming model to calculate the maximal throughput. By exploiting the spatial diversity, [PERSON] et al. proposed a cooperative network coding (CNC) mechanism to find coding opportunities in wireless multi-rate and -hop networks, and they provided a theoretical formulation to calculate the maximal throughput [33]. [PERSON] et al. concluded that the rate-matching problem in the coding nodes was among the main influential elements in wireless multi-rate networks [32]. However, these works are theoretical and not easily implemented. [PERSON] et al. proposed a simple implementation that used a weighting factor to react to flow intersection of different rates. The weighting factor consists of the counter reuse ratio and destination address reuse ratio [34]. Afterward, some works focused on the routing protocols considering the queue length in the coding nodes to solve the rate-matching problem [35]. Queue length can react to the node states and is also easy to implement. The DCAR scheme used a simple modified interference queue length (MIQ) to respond to the node state and incorporate traffic load, and the interference information will be gathered in a unified manner [10]. [PERSON] and [PERSON] used the queue management scheme to improve the TCP performance of CAR [36]. In our past work [37], we used a Markov chain to formulate the queue state of the coding nodes and proposed an OQMCAR algorithm based on the queue state and local topology to reduce the network end-to-end delay. The summarized view of the key related works is shown in Table 1.  
In light of the aforementioned works, there has been intense interest in avoiding congestion in wireless multi-flow networks and in theoretically analyzing the rate-matching problem in wireless multi-rate networks. However, the primary coding conditions that cause the congestion seldom change. In this work, we propose the constrained coding conditions to reduce the congestion and use a novel coding benefit function with consideration of the queue length to address the rate-matching problem easily. \begin{table}
\begin{tabular}{l l l l l l} \hline \hline
**NC-aware** & **Coding** & **Routing** & **Algorithm** & **Network Coding** & **Evaluation Scenario** \\
**Protocol** & **Type** & **Metric** & **Type** & **Range** & \\ \hline COPE [5] & Inter-flow & Coding gain & Centralized & Two-hop & Wireless multi-hop networks \\ CAOR [9] & Inter- and intra-flow & & & & \\ DCAR [10] & Inter-flow & CRM & Distributed & Multi-hop & Wireless ad hoc networks \\ FROM [12] & Inter-flow & FORM & Distributed & Multi-hop & Wireless mesh backbone networks \\ LFCR [13] & Inter-flow & RPC & Distributed & Multi-hop & Wireless multi-hop networks \\ CFCR [14] & Inter-flow & CDS & Distributed & Multi-hop & Wireless mesh networks \\ DGCDR [15] & Inter-flow & DGCDR & Distributed & Multi-hop & Wireless multi-flow networks \\ I2 NC [27] & Inter- and NUM & Centralized & Two-hop & Wireless mesh networks \\  & intra-flow & & & & \\ HyCare [34] & Inter-flow & N-ETOX & Distributed & Multi-hop & Wireless multi-hop networks \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Overview of the key NC-aware routing protocols. DCAR, distributed coding-aware routing; FROM, free-ride-oriented routing metric.