Convexity Adjustment of Daily Compounded Swap under Hull-White Model

Question

I am working on a problem that deals OIS daily compounded swap under Hull-White 1-factor model. I am struggling with pricing the floating leg, on a delayed payment date:

$E^{T^p}_t[\prod_{i=0}^{n-1} (1 + \tau_i L_i)]$

where $L_i = L(t_i; t_{i+1}, t_{i+1})$ is the (daily) forward rate, $\tau_i$ is the accrual factor between the consecutive business days, and $T^p > t_{n}$ is the delayed payment date.

My thought was to derive the distribution of $\prod_{i=0}^{n-1} (1 + \tau_i L_i) = \prod_{i=0}^{n-1}\frac{1}{P(t_i, t_{i+1})}$ under $T^p$-measure, but the computation was too nasty, could anyone give me some hints/reference to do this? Thank you very much.

Answer 1

You can approximate the daily compounding of O/N rate with continuous one of the short rate: $$ \prod_{i=0}^{n-1} (1 + \tau L_i)= \prod_{i=0}^{n-1} \frac{1}{P(t_i,t_{i+1})} \approx \exp \left(\int_{t_0}^{t_n} r(u)du \right) $$

This is a reasonable approximation. If you need a justification or an assessment the error, you can start from the zero-coupon bond price expression under Hull-White of the form: $$P(t,t+\varepsilon) = \exp \left(A(t,t+\varepsilon) - r(t)B(t,t+\varepsilon) \right)$$ and look at what happens to $A$ and $B$ terms when: $\varepsilon \rightarrow 0$.

Next, you can derive the expression of $\int_{t_0}^{t_n} r(u)du$ under Hull-White model and under the $T_p$ measure. I think the easiest would be to work under the risk-neutral measure to get the expression of the integral of short rate from that of the short rate, and then use Girsanov to change measures to $T_p$.

Once you have this, it will be straightforward to get the desired expectation.

I didn't include any explicit derivations, rather hints as you asked. But let me know if you are stuck in one of the steps, and I might be able to help.

Answer 2

We know that $$ 1+\tau_iL_i=\frac{1}{P(t_i,t_{i+1})} $$ and, under the risk-neutral measure, $$ P(t,T)=P(0,T)\exp\left(\int_0^tr(s)\,ds+\int_0^t\sigma(s,T)\,dW_s-\frac{1}{2}\int_0^t\sigma^2(s,T)\,ds\right)\,.\quad\quad\quad (1) $$ Using $P(t_i,t_i)=1$ this implies \begin{align} P(t_i,t_{i+1})&=\frac{P(0,t_{i+1})}{P(0,t_i)}\exp\Bigg(\int_0^{t_i}\sigma(s,t_{i+1})-\sigma(s,t_i)\,dW_s\\ &\quad\quad-\frac{1}{2}\int_0^{t_i}\sigma^2(s,t_{i+1})-\sigma^2(s,t_i)\,ds \Bigg)\,. \end{align} Therefore, \begin{align} \prod_{i=0}^{n-1}1+\tau_iL_i&=\frac{P(0,t_0)}{P(0,t_n)}\exp\Bigg(\sum_{i=0}^{n-1}\int_0^{t_i}\sigma(s,t_i)-\sigma(s,t_{i+1})\,dW_s\\ &\quad\quad-\sum_{i=0}^{n-1}\frac{1}{2}\int_0^{t_i}\sigma^2(s,t_i)-\sigma^2(s,t_{i+1})\,ds \Bigg)\,. \end{align} Further, (1) implies with $P(T,T)=1$ that $$ \exp\left(-\int_0^{T}r(s)\,ds\right)=P(0,T)\exp\left(\int_0^T\sigma(s,T)\,dW_s-\frac{1}{2}\int_0^T\sigma^2(s,T)\,ds\right)\,. $$ So we have to calculate the expectation of \begin{align} \exp\left(-\int_0^{T}r(s)\,ds\right)\left(\prod_{i=0}^{n-1}1+\tau_iL_i\right) \end{align} which is $$ \frac{P(0,t_0)P(0,T)}{P(0,t_n)}\exp\left(-\sum_{i=0}^{n-1}\frac{1}{2}\int_0^{t_i}\sigma^2(s,t_i)-\sigma^2(s,t_{i+1})\,ds-\frac{1}{2}\int_0^T\sigma^2(s,T)\,ds\right) $$ times the expectation of the lognormal variable $$ e^Y:=\exp\left(\int_0^T\sigma(s,T)\,dW_s+\sum_{i=0}^{n-1}\int_0^{t_i}\sigma(s,t_i)-\sigma(s,t_{i+1})\,dW_s\right)\,. $$ To calculate that expectation you need to know only the variance of $Y$ which is a sum of integrals of the form \begin{align} &\int_0^{t_i}(\sigma(s,t_i)-\sigma(t_{i+1}))(\sigma(s,t_j)-\sigma(t_{j+1}))\,ds\quad\quad i\le j\\ &\int_0^{t_i}(\sigma(s,t_i)-\sigma(t_{i+1}))\sigma(s,T)\,ds\,. \end{align} This is as explicit as it gets. Even using the time dependent HW-form of $\sigma(t,T)\,,$ that is, $$ \sigma(t,T)=\int_t^T\sigma(s)\exp\left(-\int_s^T\lambda(u)\,du\right)\,ds $$ does not help much, unless you assume constant mean reversion $\lambda$ and constant volatility $\sigma$ of the short rate.

Answer 3

Why have you defined this valuation under the T-forward measure? If you consider the Hull-White model you can simply simulate the process under the risk-neutral measure and discount it with the money-savings account, $M(t)$. Every Libor rate, $L_i(t)$, will be simply defined in the terms of the short rate, $r(t)$.