\begin{equation}
\boldsymbol{b}_l =
\begin{cases}
\hat{\boldsymbol{n}}_{i}^{(l)}, & \text{if} \,\,\, 1 \leq l \leq L,\\
\\
\widetilde{\boldsymbol{y}}_{l}, & \text{if} \,\,\, l = L + 1
\end{cases},
\nonumber
\end{equation}
