165PSA的话，第n个月的CPR = 6% * min(1, n/30) * 165，200PSA乘200

CPR折成monthly prerepayment rate = 1 - (1 - CPR) ^ (1/12)

假设你说的是sequential-pay CMO，本金部分（包括regular principal和prepayment）先付tranche A，全部付完了再tranche B，以此类推，利息部分按每个tranche的本金余额每个tranche每期都有，直到那个tranche的本金全部付完了

先把术语解释一下。麻烦。

CPR: Conditional Prepayment Rate

SMM: single-monthly mortality rate = 1 - (1 - CPR)^(1/12)

PSA: The Public Securities Association

The base PSA curve, or 100% PSA, assumes the prepayment rate starts at 0% CPR at age 0 and increases by 0.2% CPR per month until month 30, after which the speed is a constant 6% CPR. A rate of 50% PSA means that the CPR in any month is half that implied by 100% PSA. A rate of 200% PSA means that the CPR in any month is twice that implied by 100% PSA, and so on.