  This letter analyzes the fundamental performance of integrated sensing and
communications (ISAC) systems. For downlink and uplink ISAC, the diversity
orders are analyzed to evaluate the communication rate (CR) and the high
signal-to-noise ratio (SNR) slopes are unveiled for the CR as well as the
sensing rate (SR). Furthermore, the achievable downlink and uplink CR-SR
regions are characterized. It is shown that ISAC can provide more degrees of
freedom for both the CR and the SR than conventional frequency-division sensing
and communications systems where isolated frequency bands are used for sensing
and communications, respectively.
