  In this paper, we study and predict the charged-particle pseudorapidity
multiplicity density ($\frac{dN_{ch}}{d\eta}$), transverse momentum spectra of
identified particles and their ratios in relativistic heavy ion collisions at
the Large Hadron Collider (LHC), using the string-melting version of a
multi-phase transport (AMPT) model with improved quark coalescence method.
Results of the charged-particle pseudorapidity multiplicity density from AMPT
model calculations for Pb+Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV are
compared with the experimental data. Good agreements are generally found
between theoretical calculations and experimental data. We predict
$\frac{dN_{ch}}{d\eta}$ for Xe+Xe collisions at $\sqrt{s_{NN}}=5.44$ TeV at
different centralities, and $p_T$ spectra of charged pions, kaons and protons,
and their ratios $K/\pi$ and $p/\pi$ in Pb+Pb collisions at
$\sqrt{s_{NN}}=5.02$ TeV and Xe+Xe collisions at $\sqrt{s_{NN}}=5.44$ TeV that
are being studied at LHC. The $p_T$ spectra of identified particles in Pb+Pb
collisions from the improved AMPT model are compared and found to be consistent
with results from the iEBE-VISHNU hybrid model with TRENTo initial condition.
