Algorithm 1: Select candidate relay nodes

Procedure: Find candidate relay nodes Input: $h_{k,d}$: Channel coefficient between sink and node, $h_{i,j}$: Channel coefficient between nodes $i$ and $j$; Output: $R$: A set of candidate relay nodes; For $i=1,2,\cdots ,K$ do |  Regard node $i$ as a relay node$r$ |  Get $h_{k,r}$ (channel coefficient between relay $r$ and node $k$); |  Calculate the difference of channel gain to relay and sink, $D=\left\{\right|h_{k,d}\left|-|h_{k,r}\right|\}$; |  Sort $D$ in the ascending order; |  For $j=1,2,\cdots ,K,j\ne i$ do |  |  Assume top j nodes in $D$ use the relay node; |  |  Minimize MSE of CRP, with power constraint for all nodes including relay; |  |  Calculate transmission power ($P_j^i$) of all nodes; |  End |  Find optimal number of nodes that can use node $i$ as relay and record ${\mathrm{MSE}}_i$ End Sort ${\mathrm{MSE}}_i$ in the ascending order; Select a number of top nodes as relay candidates $R$ based on ${\mathrm{MSE}}_i$; Return $R$