Mortgage-Backed Securities: Products, Structuring, and Analytical Techniques, 2nd Edition

CHAPTER 7

Fundamental MBS Structuring Techniques

Divisions of Interest

As with the division and structuring of principal cash flows, interest cash flows can also be altered and redirected within a structure. The factors driving the structuring of interest are identical to those in principal structuring discussed in the preceding chapter. The goal is to create bonds that, by appealing to different segments of the fixed income investment community, maximize the proceeds generated by creating and selling the deal. By redistributing interest within the deal, bonds can be created with different coupon levels and structures, thus allowing for different degrees of exposure to interest rates and prepayment speeds. As examples, the following bonds can all be structured off either a pool of collateral or a parent tranche:

• Floaters. Bonds where the interest adjusts (or “floats”) with a benchmark index. They have minimal exposure to changes in interest rates (i.e., their durations are very short), and are tailored to investors looking to reap a spread over short-term funding levels.
• Principal-only (PO) tranches. Bonds that pay no coupon interest to the holder. Sold at a deep discount to investors, its returns are based on the rate at which principal is returned to the investor. These bonds typically have very long and volatile durations, since their prices are affected not only from the impact of a different discount rate on the cash flow's present value, but by changing prepayment speeds. ...