Using Fixed-Leg BPS to Solve for a Swap’s Fair Rate

# Why QuantLib computes the fixed-leg swap rate by this formula?


# Why QuantLib computes the fixed-leg swap rate by this formula?












I'm trying to understand how QuantLib creates (bootstraps) a yield curve from a vanilla swap at the source level. I have the following test code:

```
void testYieldFromIRSwap()
{
Settings::instance().evaluationDate() = Date(1, Jan, 2015);

auto dc = Actual360();

auto h1 = boost::shared_ptr<RateHelper>(new DepositRateHelper
                                               (0.03, Period(1, Years), 0.0, UnitedStates(), Following, false, Actual360()));
auto h2 = boost::shared_ptr<RateHelper>(new DepositRateHelper
                                               (0.04, Period(2, Years), 0.0, UnitedStates(), Following, false, Actual360()));

auto index = boost::shared_ptr<IborIndex>(new EURLibor1Y());
auto h3 = boost::shared_ptr<RateHelper>(
                new SwapRateHelper(0.05, Period(3, Years), UnitedStates(), Annual,
                                   Following, Actual360(), index));

std::vector<boost::shared_ptr<RateHelper>> helpers;
helpers.push_back(h1);
helpers.push_back(h2);
helpers.push_back(h3);

boost::shared_ptr<YieldTermStructure> yield(
                new PiecewiseYieldCurve<Discount,LogLinear>(
                    Settings::instance().evaluationDate(), helpers,
                        Actual360()));

const auto t1 = dc.yearFraction(Date(1, Jan, 2015), Date(1, Jan, 2016)); // 1.014
const auto t2 = dc.yearFraction(Date(1, Jan, 2015), Date(1, Jan, 2017)); // 2.031
const auto t3 = dc.yearFraction(Date(1, Jan, 2015), Date(1, Jan, 2018)); // 3.044

std::cout << yield->discount(0)  << std::endl; // Must be 1
std::cout << yield->discount(t1) << std::endl; // 1/((1+0.03) ^ 1.014) = 0.9704721
std::cout << yield->discount(t2) << std::endl; // 1/(1+0.04) ^ 2.031   = 0.9234328
std::cout << yield->discount(t3) << std::endl;
}
```

This is obviously a toy example. I have two deposits that gives the discount rates for the first two years. My swap is an annual 3y swap, I expect the swap gives me the discount rate for the the third year. I do get the value but I don't understand how it's computed.

The code for bootstrapping a discount factor from a swap is:

```
Real SwapRateHelper::impliedQuote() const {
    QL_REQUIRE(termStructure_ != 0, "term structure not set");
    // we didn't register as observers - force calculation
    swap_->recalculate();
    // weak implementation... to be improved
    static const Spread basisPoint = 1.0e-4;
    Real floatingLegNPV = swap_->floatingLegNPV();
    Spread spread = spread_.empty() ? 0.0 : spread_->value();
    Real spreadNPV = swap_->floatingLegBPS()/basisPoint*spread;
    Real totNPV = - (floatingLegNPV+spreadNPV);
    Real result = totNPV/(swap_->fixedLegBPS()/basisPoint);
    return result;
}
```

Everytime when QuantLib guesses a new discount factor (i.e. iteratively bootstrapping), this function computes a new NPV for the floating leg. I'd expect this NPV be set to equal to the NPV for the fixed leg, from which a new swap rate can be computed.

What I don't understand is this particular line:

```
Real result = totNPV/(swap_->fixedLegBPS()/basisPoint);
```

I think BPS is the sum of accurate interest and is computed by:

```
bps += cp->nominal() * cp->accrualPeriod() * df;
```

Question:

Why do we need to divide the new fixed-leg NPV by BPS and basisPoint? Why basisPoint is set to 1.0e-4 (1 basis point)? What's the point of doing it?

## Answer by Luigi Ballabio (score 6, accepted)

https://quant.stackexchange.com/a/20624

`fixedLegBPS` is the basis-point sensitivity of the fixed leg, that is, how much its NPV changes when the fixed rate changes by one basis point: it's calculated as the NPV corresponding to a fixed rate of 1 bps.

Since the NPV of the fixed leg is linearly proportional to the fixed rate, you can write the equation

```
targetNPV : fixedRate = BPS : 1 basis point
```

The line you highlight simply solves it for the fixed rate.

## Answer by Giancarlo Giuffra (score 0)

https://quant.stackexchange.com/a/20623

it seems the method fixedLegBPS() returns the sum of the discount factors associated to the fixed leg multiplied by 1 basisPoint. So what SwapRateHelper::impliedQuote() returns is actually the new fair rate of the swap to be used in the minimization algorithm.

I have tried to find the implementation of the calculate() method from class VanillaSwap::engine (VanillaSwap.recalculate() calls calculate() which calls its engine calculate() method ) but I haven't been successful. I think you can find confirmation of the behavior of the method fixedLegBPS() there since it calls calculate() and then returns legBPS_[0].

Hope it helps. (and let me know where this implementation is if you find it)