I wouldnt say pss is a frequency domain simulation. ADS is frequency domain simulation, which means that it converts every useful parameter into frequency domain variable, perform calculation in frequency domain, and convert it back to
time domain. pss is a simulation which finds a solution in time domain. but there is
a limitation. the solution must be periodically steady state. because of this, we need
beat frequency or beat time. pss can not find a solution if the solution of the circuit
is far from beat frequency or beat time. for example, you set a beat frequency of 1GHz, and the circuit oscillates around 650MHz, then you are not gonna have a useful
sim results from that. if you set a beat frequency of 1GHz, and the circuit oscillates
around 2GHz, then you will get meaningful solution assuming the number of harmonics in pss are correct.
if pss and trans sim are both time domain sim, why do we need both?
it's because of narrow band characteristic presented in RF signal. suppose you want
a simulation with a resolution of 10MHz while 1GHz and 1.001GHz signals coexists.
the trans sim will be long and painful to get any meaningful results from the above condition. but by using pss, you can get an answer relatively easy.
if you want to check any possible oscillation presented in the circuit, you need to
do trans sim first. find a oscillation frequency, and run a pss with that oscillation
frequency as a beat frequency, then you will get more accurate results