Real World Exposure and CVA Simulation

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Real world exposure and CVA simulation
The risk-neutral approach assumes that asset prices follow stochastic process with drift coinciding with the short rate r(t) being risk-free interest rate. dS(t)=S(t)[rdt+σ(S(t),t) 〖dW〗^Q (t)]
Instead, in real-world measure they follow more complex process, which embodies time and risk aversion of investors, namely: dS(t)=S(t)[μ(S(t),t)dt+σ(S(t),t) 〖dW〗^R (t)] or, equivalently, a process with real-world stochastic discount factors which depend on risk-free interest rates but also on asset prices itself.
The form of this process with almost arbitrary process’s drift term complicates the implementation; for example, makes it difficult in practice to simulate asset prices through standard analytic or quasi-analytic approaches of measure transformation via Girsanov theorem. Let alone the case of imperfect replication with infinite number of no-arbitrage consistent measures, the absence of the unambiguous common real-world measure, as opposed to risk-neutral ones, make real-world simulation much more difficult to implement that risk-neutral one.
Having a limited supply of analytic or quasi-analytic shortcuts and unique no-arbitrage consistent measure, practitioners often choose a brute force approach for risk management purposes such as real-world PFE and CVA simulations, which both require full simulation of exposure distributions for any other-than-vanilla instrument. This brute force approach, known as nested Monte Carlo on Monte Carlo, comprises the following two steps repeated in a loop: drawing real-world yield curves and other relevant market factors at all forward model steps using explicit model assumptions (equivalently, assumptions on drift terms) for a subset of stochastic paths,
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...k-neutral market models.

Nested Monte Carlo schema requires calculation of instrument

There are a few shortcuts for vanilla instruments. One can omit next nested Monte Carlo step and just price vanilla instruments off the forward curves at each model step t_i. Similarly, one can reduce the pricing of vanilla caps or swaptions to interpolation mechanism of forward volatilities/prices, which is an effective but valid way to avoid building nested evolution model.

For weak path-dependent instruments such as Bermudan callable instruments, which permit representation of conditional exposures as nonlinear function of some market states or non-callable underlying exposures, there is a way to avoid by reusing standard risk-neutral American Monte Carlo results.

Works Cited

Solvency II and Nested Simulations - a Least-Squares Monte Carlo Approach
Staum
Riccardo Rebonato

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