Method and apparatus for computing soft decision input metrics to a turbo decoder

A method of estimating log-likelihood (LLR) decoder metrics from a soft decision having an in-phase, component and a quadrature component, the soft decision having been demodulated in accordance with an eight-ary phase shift keyed (8PSK) modulation scheme, the method comprising the steps of:

• multiplying the in-phase component by a first constant value to obtain a first LLR metric;
• multiplying the quadrature component by the first constant value to obtain a second LLR metric;
• multiplying a subtraction value equal to a difference between an absolute value of the in-phase component and an absolute value of the quadrature component by an addition value equal to a sum of the absolute value of the in phase component and the absolute value of the quadrature component to obtain an intermediate value;
• and
• multiplying the intermediate value by a division value equal to a quotient of a second constant value and a square root of a sum of a square of the in-phase component and a square of the quadrature component to obtain a third LLR metric.

A device for estimating log-likelihood (LLR) decoder metrics from a soft decision having an in-phase component and a quadrature component, the soft decision having been demodulated in accordance with an eight-ary phase shift keyed (8PSK) modulation scheme, the device comprising:

• means for multiplying the in-phase component by a first constant value to obtain a first LLR metric;
• means for multiplying the quadrature component by the first constant value to obtain a second LLR metric;
• means for multiplying a subtraction value equal to a difference between an absolute value of the in-phase component and an absolute value of the quadrature component by an addition value equal to a sum of the absolute value of the in-phase component and the absolute value of the quadrature component to obtain-an intermediate-value;
• and
• means for multiplying the intermediate value by a division value equal to a quotient of a second constant value and a square root of a sum of a square of the in-phase component and a square of the quadrature component to obtain a third LLR metric.