## Calibrating time-dependent volatility to swaption prices

We have seen in a previous post how to fit initial discount curves to swap rates in a model-independent way. What if we want to control the volatility parameter to match vanilla rates derivatives as well? Just as we found for vanilla calls and puts, we will need to chose a model, for example the Hull-White extended Vasicek (HWeV) model that we’ve seen before (there are a few reasons why this isn’t a great choice, discussed later).

Our next choice is which vanilla rates options we want to use for the calibration. A common choice is the interest rate swaption, which is the right to enter a swap at some future time with fixed payment dates and a strike . These are fairly liquid contracts so present a good choice for our calibration. A ‘payer swaption’ is one in which we pay fixed and recieve floating, and a ‘receiver swaption’ is the opposite. For simplicity, for the rest of this post we will assume all payments are annual, so year fractions are ignored.

A reciever swaption can be seen as a call option on a coupon-paying bond with fixed payments equal to at the same payment dates as the swap. To see this, consider the price of a swap discussed before:

where is the time now. This is exactly the same as the price of buying a unit of bond at time for $1, receiving the fixed coupons at each payment period, and receiving the original notional back at time . So, the price of a swaption is an option on receiving a portfolio of coupon payments, each of which can be thought of as a zero-coupon bond paid at that time, and the value of the swaption is the positive part of the expected value of these: where the cash payments have been replaced by the strike of the option (the only ‘notional’ payment that will be paid) This isn’t readily tractable, but we make use of Jamshidian’s decomposition (I won’t go further here – this is worth it’s own post!) to re-write this with the max inside the summation: where is the price of a ZCB at time expiring at if rates at make the value of the coupon-bearing bond equal to . Looking at this expression, we see that each term is simply the present value of an option to buy a ZCB at time that expires at one of the payment dates with strike . So the price of a swaption has been expressed entirely as the price of a portfolio of options on ZCBs! For the HWeV model, these are deterministic and depend only on the initial rate, and calibrated time dependent parameters in the model. Since rates are gaussian in HWeV this can be done analytically. Calculating these for time-varying parameters is algebra-intensive and I leave it for a later post, but for constant parameters the calculation is described in Brigo and Mercurio pg 75-76 and gives a price of where We can see how we could use the above to calibrate the volatility parameter to match a single market-observed swaption price. When several are visible, the challenge becomes to choose a piecewise continuous function to match several of them. In HWeV this can be done analytically, but for more general models some sort of optimisation would be required. Since these contracts have an exercise date when the swap starts and the swaps themselves will have another termination date which define a 2-dimensinal space , it will not be possible to fit all market-observable swaptions with a one factor model. Many alternatives are discussed in the literature to deal with this concern, but the general procedure is the same. Practically, we should choose the most liquid swaptions and bootstrap to these, and only a few (5Y, 10Y etc) will practically be tradable in any case. ## Fitting the initial discount curve in a stochastic rates model I’ve introduced the Vasicek stochastic rates model in an earlier post, and here I’m going to introduce a development of it called the Hull-White (or sometimes Hull-White extended Vasicek) model. The rates are modelled by a mean-reverting stochastic process which is similar to the Vasicek model, except that the term is now allowed to vary with time (in general and are too, but I’ll ignore those issues for today). The freedom to set theta as a deterministic function of time allows us to calibrate the model to match the initial discount curve, which means at least initially our model will give the right price for things like FRAs. Calibrating models to match market prices is one of the main things quants do. Of course, the market doesn’t really obey our model. this means that, in time, the market prices and the prices predicted by our model will drift apart, and the model will need to be re-calibrated. But the better a model captures the various risk factors in the market, the less often this procedure will be needed. Using the trick to re-express the equation and integrating gives where is the rate at . The observable quantities from the discount curve are the initial discount factors (or equivalently the initial forward rates) , where The rate is normally distributed, so the integral must be too. This is because an integral is essentially a sum, and a sum of normal distributions is also normally distributed. Applying the Ito isometry as discussed before, the expectation of this variable will come wholly from the deterministic terms and the variance will come entirly from the stochastic terms, giving where throughout and since we have Two differentiations of this expression give and combining these equations gives an expression for that exactly fits the initial discount curve for the given currency and since is simply the initial market observed forward rate to each time horizon coming from the discount curve, this can be compactly expressed as Today we’ve seen how a simple extension to the ‘basic’ Vasicek model allows us to match the initial discount curve seen in the market. Allowing the volatility parameter to vary will allow us to match market prices of other products such as swaptions (an option to enter into a swap), which I’ll discuss another time. But we’re gradually building up a suite of simple models that we can combine later to model much more complicated environments. ## Stochastic Rates Models In a previous post, I introduced the Black-Scholes model, in which the price of an underlying stock is modeled with a stochastic variable that changes unpredictably with time. I’ve also discussed the basic model-independent rates products whose value can be determined at the present time exactly. However, to progress further with interest rate derivatives, we’re going to need to model interest rates more carefully. We’ve assumed rates are deterministic so far, but of course this isn’t true – just like stocks, they change with time in an uncertain manner, so we need to allow them to become stochastic as well. One way of doing this is by analogy with the BS case, by allowing the short rate (which is the instantaneous risk-neutral interest rate ) to become stochastic as well, and then integrating it over the required period of time to calculate forward rates. A very basic example of this is the Vasicek Model. In this model the short rate is defined to be stochastic, with behaviour governed by the following SDE where and are constants and is the standard Wiener increment as described before. This is marginally more complicated than the BS model, but still belongs to the small family of SDEs that are analytically tractable. Unlike stock prices, we expect rates to be mean-reverting – stock price variance grows with time, but we expect the distribution of rates to be confined to a fairly narrow range by comparison. The term in square brackets above achieves this, since if is greater than then it will be negative and cause the rate to be pulled down, while if it is below the term will be positive and push the rate up towards . Solving the equation requires a trick, which is instead of thinking about the rate alone to think about the quantity . This is equal to , and substituting in the incremental rate term from the original equation we have note that the term in has been cancelled out, and the remaining terms can be integrated directly from a starting time to a finishing time where is the initial rate. This is simply a gaussian distribution with mean and variance given by Where the variance is calculated using the “Ito isometry as stated above. Note that this allows the possibility of rates going negative, which is generally considered to be a weakness of the model, but the chance is usually rather small. As we know the distribution of the short rate, we can calculate some other relevant quantities. Of primary importance are the Zero Coupon Bonds, which are required for calculation of forward interest rates. A ZCB is a derivative that pays$1 at a future time , and we can price this using the Risk-Neutral Valuation technique.

According to the fundamental theorem of asset pricing, the current price of a derivative divided by our choice of numeraire must be equal to it’s future expected price at any time divided by the value of the numeraire at that time, with the expectation taken in the probability measure corresponding to the choice of numeraire.

In the risk-neutral measure, the numeraire is just a unit of currency, initially worth $1 but continually re-invested at the instantaneous short rate, so that its price at time is$1 . Now, the price of the ZCB is given by

Although the RHS is true at any time, we only know the value of the ZCB exactly at a single time at the moment – the expiry date , at which it is worth exactly $1. Plugging in these values we have So the ZCB is given by the expectation of the integral of the rate over a period of time. Since the rate is itself gaussian, and an integral is the limit of a sum, it’s not surprising that this quantity is also gaussian (it’s effectively the sum of many correlated gaussians, which is also gaussian, as discussed before), but it’s rather tricky to calculate, I’ve included the derivation at the bottom of the post to same space here. The mean and variance are given by where The ZCB is given by the expectation of the exponential of a gaussian variable – and we’ve seen on several occasions that So the ZCB prices are with as defined above and and as these expressions only depend on the rates at the initial time, we can calculate ZCB bond prices and hence forward rates for any future expiry dates at any given time if we have the current instantaneous rate. Although we can calculate a discount curve for a given set of parameters, the Vasicek model can’t calibrate to an initial ZCB curve taken from the market, which is a serious disadvantage. There are more advanced generalisations which can, and I’ll discuss some soon, but they will use all of the same tricks and algebra that I’ve covered here. I’ve written enough for one day here – in later posts I’ll discuss changing to the t-forward measure, in which the ZCB forms the numeraire instead of a unit of currency, which simplifies many calculations, and I’ll use it to price caplets under stochastic rates, and show that these are equivalent to european options on ZCBs. An alternate approach to the short-rate model approach discussed today which is very popular these days is the Libor Market Model (LMM) approach, in which instead of simulating the short rate and calculating the required forwards, the different forwards required are instead computed directly and in tandem – I’ll look further at this approach in another post. Here is the calculation of the distribution of the integral of the instantaneous rate over the period to : and splitting this into the terms that contribute to the expectation and the variance we have to calculate the variance, we first need to deal with the following term we use stochastic integration by parts and we’re now in a position to try and find the variance of the integral where Ito’s isometry has been used again, and several more lines of routine algebra leads to the result ## Bootstrapping the Discount Curve from Swap Rates Today’s post will be a short one about calculation of discount curves from swap rates. I’ve discussed both swaps and discount curves in previous posts, you should read those before this one or it might not make much sense! Although bonds can be used to calculate discount bond prices, typically swaps are the most liquid products on the market and will go to the longest expiry times (often 80+ years for major currencies), so these are used to calculate many of the points on the discount curve [and often both of these can be done simultaneously to give better reliability]. In the previous post on swaps, I calculated the swap rate that makes swaps zero-valued at the current time where the ‘s here represent the fixing dates of the swap (although payment is made at the beginning of the following period, so the ‘th period is received at . Consider the sequence of times for which a sequence of swaps are quoted on the markets, with swap rates for the swap running from up to . We can back out the discount factor at each time as follows: and we can see from this the general procedure, calculating another ZCB from each successive swap rate using the expression These swaps and ZCBs are called co-initial because they both started at the same time . Now imagine that instead the swaps have the first fixing at time and their final fixing at time for – such swaps are called co-terminal swaps as they start at different times but finish at the same one. Once again we can calculate the discount factors up to a constant factor, this time by working backwards: and so on, the dcfs can be backed out. To specify the exact values of co-terminal swaps, we need to know at least one dcf exactly. In general the co-initial case will also require this – I implicitly assumed that they started fixing at where we know , but for general co-initial swaps we would also have this issue. ## Forward Rate Agreements and Swaps For calibration of discount curves from swap rates, see my post on Bootstrapping the Discount Curve from Swap Rates. In this post I’m going to introduce two of the fundamental interest rate products, Forward Rate Agreements (FRAs) and Swaps. FRAs allow us to ‘lock in’ a specified interest rate for borrowing between two future times, and Swaps are agreements to exchange a future stream of fixed interest payments for floating ones, or visa-versa. Both are model independent, which means we can statically hedge them today using just Zero Coupon Bonds (ZCBs) – so their prices won’t depend on our models for interest rates or underlying prices etc. ZCBs really are fundamental here – if you haven’t read my earlier post on them yet I recommend starting there! First, a note about conventions. In other contexts, I’ve used r to be the continuously-compounding rate, such that if I start with$1, by time it will be worth  . In the rates world, this would be very confusing, and rates are always quoted as simple annualised rates, so that $1 lent for half a year at 5% would return$1.025 exactly in half a year’s time (note the additional factor of 0.5 coming from the year-fraction of the deposit), and this rates convention will be used throughout this post.

Each ZCB gives us a rate at which we can put money on deposit now for expiry at a specific time, and we can construct a discount curve from the collection of all ZCBs that we have available to us. In the case of a ZCB, we deposit the money now and receive it back at expiry. A Forward Rate Agreement extends the idea of putting money on deposit now for a fixed period of time to putting it on deposit at a future date for a specific period of time. We’ve picked up an extra variable here – our rate for deposit starting now depends only on time of expiry , while the FRA rate will depend on the time that we put the money on deposit  as well as the time of expiry .

An FRA, just like a deposit, involves two cash flows. We pay the counterparty a notional at time , and receive our notional plus interest of at time ; where the first term is the Year Fraction, the second is the Forward Rate, and the final is the Notional. Since f will be fixed when we sign the contract, we can hedge these two cash flows exactly at using ZCBs. The value of the FRA is the value of receiving the second sum minus the cost of making the first payment:

This agreement is at ‘fair value’ if the forward rate makes , and re-arranging gives

An FRA allows us to ‘lock-in’ a particular interest rate for some time in the future – this is analogous in rates markets to the forward price of a stock or commodity for future delivery, which was discussed in an earlier post. Note that the price of all FRAs is uniquely determined from the discount curve [although in reality our discount curve will be limited in both temporal resolution and maximum date by the ZCBs or other products available on the market which we can use to build it].

A Swap is an agreement to exchange two cash flows coming from assets, but not the assets themselves. By far the most common is the Interest Rate Swap, in which two parties agree to swap a stream of fixed rate interest rate payments on a notional M of cash for a stream of floating rate payments on the same notional. Although the notional might be quite large, usually only the differences between the payments at each time are exchanged, so the actual payments will be very much smaller. The mechanics are probably best demonstrated by example:

A swap is written on a notional $100Mn, with periods starting in a year and continuing for three years and with payments at the end of each three month period; to pay fixed annualised 5% payments, and floating payments at the three-month deposit rate (fixed at the beginning of each period). What payments actually get made? The swap starts in a year’s time, but the first payment is made at the end of the first 3-month period, in 15 months time. At this time, the fixed payment will be . The floating payment will be whatever the three-month deposit rate was at 1 year, multiplied by the same prefactors, – let’s say that is 4% (although of course we won’t know what it is until the beginning of each period, and the payment will be delayed until the end of the period) – then the net cash flow will be$250,000, paid by the person paying fixed to the person paying floating.

The same calculation happens at each time, and a payment is made equal to the difference between the fixed and the floating leg cash flow. Although the notional is huge, we can see that the actual payments are much, much smaller [be alert for newspapers quoting ‘outstanding notional’ to make positions seem large and unsteady!]. The convention for naming Swaps is that if we are receiving fixed payments, we have a receiver’s swap; and if we are receiving floating payments, it is a payer’s swap.

What is the point of this product? Well, if we have a loan on which we are having to pay floating rate interest, using a swap we can exchange that to a fixed rate of interest by making fixed rate payments to the counterparty and receiving floating rate payments back, which match the payments that we’re making on the loan. This is of use to companies, who need to handle interest rate risk and might not want to be exposed to rates rising heavily on money they’re borrowing. A bank holding a large portfolio of fixed rate mortgage loans but required to pay interest to a central bank at floating rate might engage in a swap in the reverse direction to hedge it’s exposure.

How can we price this product? It’s easy to price the fixed payments – since each is known exactly in advance, we can hedge these out using ZCBs. The value of the sum of the fixed payments is

where is the fixed rate (5% in our example above), and  is the relevant year-fraction for each payment (0.25 in our example about). What about the floating payments? We have to think a little harder here, but it turns out we can use FRA agreements to hedge these exactly as well. The problem is that we don’t know what the three-month deposit rate will be in a year’s time. But we can replicate it: if we borrow an amount in a year’s time, and put that on deposit for three months, we’ll receive back  in 15 months. We know from our discussion above that we can enter an FRA to borrow in a year’s time, and we’ll need to pay in 15 months time – so we can guarantee to match the payment profile of the floating leg using the forward rates for the periods in question. The value of the floating payments is therefore

Since we can fix the price of the two legs exactly by arbitrage right now, we can value the swap by comparing the present value of each leg

As with FRAs, swaps are said to be at fair value when the values of the fixed and the floating rate match, and the overall value is zero. This is the fixed rate at which we can enter a Swap for free, and occurs when such that

This is called the Swap Rate. It is fully determined by the discount curve, and as we shall see the reverse is also true – the Swap Rate is in 1-to-1 correspondence with discount curve. Of course, after a swap is issued the Swap Rate will change constantly, in which case the actual fixed payment will no longer match and the swap will have non-zero value. If is the swap fixed coupon payment and is the current swap rate, then

where is called the annuity of the swap. The value is proportional to the difference between the swap rate and the swap fixed coupon.

Because of the number of institutions that want to handle interest rate risk resulting from loans, IR Swaps are one of the most liquidly traded financial products. Although we’ve derived their price here from the Discount Curve, in practice it is often done the other way around – Swaps often exist up to much higher maturity dates than other products, and Discount Curves at long maturities are instead constructed from the swap rates quoted on the market at these dates. This is a very important procedure, but financially rather trivial. I’m not going to cover it here but will probably come back to it in a short post in the near future.

As well as being important in their own right, FRAs and Swaps (along with ZCBs, of course) are the foundation of the rich field of interest rate derivatives. The right (but not obligation) to enter into an FRA – a call option on an FRA – is called a caplet, and a portfolio of such options on FRAs across different time periods is called a cap, since you have guaranteed a cap on the maximum interest you will have to pay over the whole period to borrow money (a put on an FRA is called a floorlet, and a sequence of these forms a floor, for similar reasons). An option to enter a swap is called a swaption, and these are also heavily traded wherever a borrower might want to re-finance a loan at a later date if interest rates move sharply. The pricing of these products becomes dependent on the underlying model that we assume for interest rates, and I’ll start to deal with them in later posts.