The flavour-changing neutral current decays are interesting probes for new physics searches. The angular distributions of the $\mathrm{B}^+ \to \mathrm{K}^{*+} \mu^ +\mu^-$ decay is studied using a sample of proton-proton collisions at $\sqrt{s} = 8~\mathrm{TeV}$ collected with the CMS detector at the LHC, corresponding to an integrated luminosity of $20.5~\mathrm{fb}^{-1}$. An angular analysis is performed to determine the longitudinal polarisation fraction and the muon forward-backward asymmetry, as functions of the dimuon invariant mass squared. All the measurements are consistent with the standard model predictions. In addition, the projections for the sensitivity of the measurement of the $P_5'$ parameter in the $\mathrm{B}^0 \to \mathrm{K}^{*0} \mu^ +\mu^-$ angular analysis that is foreseen to be performed on the data that CMS will collect at the High-Luminosity LHC is reported.