  In millimeter-wave (mmWave) channels, to overcome the high path loss,
beamforming is required. Hence, the spatial representation of the channel is
essential. Further, for accurate beam alignment and minimizing the outages,
inter-beam interferences, etc., cluster-level spatial modeling is also
necessary. Since, statistical channel models fail to reproduce the
intra-cluster parameters due to the site-specific nature of the mmWave channel,
in this paper, we propose a ray tracing intra-cluster model (RT-ICM) for mmWave
channels. The model considers only the first-order reflection; thereby reducing
the computation load while capturing most of the energy in a large number of
important cases. The model accounts for diffuse scattering as it contributes
significantly to the received power. Finally, since the clusters are spatially
well-separated due to the sparsity of first-order reflectors, we generalize the
intra-cluster model to the mmWave channel model via replication. Since narrow
beamwidth increases the number of single-order clusters, we show that the
proposed model suits well to MIMO and massive MIMO applications. We illustrate
that the model gives matching results with published measurements made in a
classroom at 60 GHz. For this specific implementation, while the maximum
cluster angle of arrival (AoA) error is 1 degree, mean angle spread error is 9
degrees. The RMS error for the cluster peak power is found to be 2.2 dB.
