We provide a general and flexible approach to LIBOR modeling based on the class of affine factor processes. Our approach respects the basic economic requirement that LIBOR rates are nonnegative, and the basic requirement from mathematical finance that LIBOR rates are analytically tractable martingales with respect to their own forward measure. Additionally, and most importantly, our approach also leads to analytically tractable expressions of multi-LIBOR payoffs. This approach unifies therefore the advantages of well-known forward price models with those of classical LIBOR rate models. Several examples are added and prototypical volatility smiles are shown. We believe that the CIR process-based LIBOR model might be of particular interest for applications, since closed form valuation formulas for caps and swaptions are derived. KEY WORDS: LIBOR rate models, forward price models, affine processes, analytically tractable models. 627 628 M. KELLER-RESSEL, A. PAPAPANTOLEON, AND J. TEICHMANN THE AFFINE LIBOR MODELS 629
AXIOMSLet us denote by L(t, T) the time-t forward LIBOR rate that is settled at time T and received at time T + δ; here T denotes some finite time horizon. The LIBOR rate is related to the prices of zero coupon bonds, denoted by B(t, T), and the forward price, denoted by F(t, T, T + δ), by the following equations:One postulates that the LIBOR rate should satisfy the following axioms, motivated by economic theory, arbitrage pricing theory, and applications.AXIOM 2.1. The LIBOR rate should be nonnegative, i.e., L(t, T) ≥ 0 for all 0 ≤ t ≤ T.AXIOM 2.2. The LIBOR rate process should be a martingale under the corresponding forward measure, i.e., L(·, T) ∈ M(P T+δ ). AXIOM 2.3. The LIBOR rate process, i.e., the (multivariate) collection of all LIBOR rates, should be analytically tractable with respect to as many forward measures as possible. Minimally, closed-form or semi-analytic valuation formulas should be available for the most liquid interest rate derivatives, i.e., caps and swaptions, so that the model can be calibrated to market data in reasonable time.Furthermore we wish to have rich structural properties: that is, the model should be able to reproduce the observed phenomena in interest rate markets, e.g., the shape of the implied volatility surface in cap markets or the implied correlation structure in swaption markets.
EXISTING APPROACHESThere are several approaches to LIBOR modeling developed in the literature attempting to fulfill the axioms and practical requirements discussed in the previous section. We describe later the two main approaches, namely the LIBOR market models and the forward price model, and comment on their ability to fulfill them. We also briefly discuss Markov-functional models. forward LIBOR rate is modeled as an exponential Brownian motion under its corresponding forward measure. This model provides a theoretical justification for the common market practice of pricing caplets according to Black's futures formula (Black 1976), i.e., assuming that the forward LIBOR rate is log-no...