  Integrated sensing and communications (ISAC) is widely recognized as a
pivotal and emerging technology for the next-generation mobile communication
systems. However, how to optimize the time-frequency domain radio resource
distribution for both communications and sensing, especially in scenarios where
conflicting priorities emerge, becomes a crucial and challenging issue. In
response to this problem, we first formulate the theoretical relationship
between frequency domain subcarrier distribution and the range Cram\'er-Rao
bound (CRB), and time domain sensing symbol distribution and the velocity CRB,
as well as between subcarrier distribution and achievable communication rates
in narrowband systems. Based on the derived range and velocity CRB expressions,
the subcarrier and sensing symbol distribution schemes with the optimal and the
worst sensing performance are respectively identified under both single-user
equipment (single-UE) and multi-UE orthogonal frequency-division multiple
access (OFDMA) ISAC systems. Furthermore, it is demonstrated that the impact of
subcarrier distribution on achievable communication rates in synchronous
narrowband OFDMA ISAC systems is marginal. This insight reveals that the
constraints associated with subcarrier distribution optimization for achievable
rates can be released. To substantiate our analysis, we present simulation
results that demonstrate the performance advantages of the proposed
distribution schemes.
