  It is expected that B5G/6G networks will exploit both terahertz (THz) and
millimetre wave (mmWave) frequency bands and will increase flexibility in user
equipment (UE)-cell association. In this paper, we introduce a novel stochastic
geometry-based framework for the analysis of the
signal-to-interference-plus-noise-ratio (SINR) and rate coverage in a
multi-tier hybrid mmWave and THz network, where each tier has a particular base
station (BS) density, transmit power, bandwidth, number of BS antennas, and
cell-association bias factor. The proposed framework incorporates the effects
of mmWave and THz channel characteristics, BS beamforming gain, and blockages.
We investigate the downlink (DL) and uplink (UL) decoupled cell-association
strategy and characterise the per-tier cell-association probability. Based on
that, we analytically derive the SINR and rate coverage probabilities of a
typical user for both DL and UL transmissions. The analytical results are
validated via extensive Monte Carlo simulations. Numerical results demonstrate
the superiority of the DL and UL decoupled cell-association strategy in terms
of SINR and rate coverage over its coupled counterpart. Moreover, we observe
that the superiority of using the DL and UL decoupled cell-association strategy
becomes more evident with the dense deployment of THz networks.
