Algorithm 1: EERS pseudo-code for reference node selection (sink node only)
$Initial:,CoverNodesFlag\leftarrow false,$$cnt \leftarrow 0, k\leftarrow 0$, $SeqN\leftarrow 0$
1.	$s\leftarrow sinknode$
2.	$RefNodes\left(1\right)\leftarrow s$
3.	$Schslot\left(1\right)\leftarrow 0$
4.	$CoverNodesFlag\left(s\right)\leftarrow true$
5.	$\mathrm{Compute}\mathrm{the}\mathrm{hop}\mathrm{distance}\mathrm{from}\mathrm{s}\mathrm{and}\mathrm{the}\mathrm{level}\mathrm{to}\mathrm{every}\mathrm{node}\left(\mathrm{i}.\mathrm{e}.,\mathrm{using}\mathrm{BFS}\right)$
6.	$while \left(k !=LevelMax-1\right) do$//$LevelMax$ is equal to the level of the farthest node from sink node
7.	$for \left(every edge \left(i,j\right)\in \mathit{E}, \mathit{i}\in N_k,\mathit{j}\in N_{k+1} \right) do$
8.	$\mathrm{detrmine} d_{i,j}$
9.	$end for$
10.	$while \left(CoverNodesFlag\left(N_{k+1}\right) !=true\right) do$
11.	$R\leftarrow N_k\left(\max \left(d_{i,j}\right)\right)$
12.	$RefNodes\left(cnt\right)\leftarrow R$
13.	$SchSlots\left(cnt\right)\leftarrow cnt$
14.	$CoverNodesFlag\left(m\right) \leftarrow true$ // $\left(m\subseteq N_{k+1}\right)$ $\mathrm{are}\mathrm{the}\mathrm{neighbor}\mathrm{nodes}\mathrm{of} R$
15.	$cnt\leftarrow cnt+1$
16.	$end while$
17.	$k\leftarrow k+1$
18.	$end while$
19.	$SeqN\leftarrow SeqN+1$
20.	$\mathrm{brodcast}\mathrm{refernces}\mathrm{scheduling}\mathrm{message}$<$RefNodes,$$SchSlots, SeqN$>