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

A receiver configured to approximate log-likelihood ratio metrics for a plurality of turbo encoded symbols, the plurality of turbo encoded symbols having been modulated with square quadrature amplitude modulation signal constellations having gray code labeling, the receiver comprising:

• a demodulator configured to extract a complex-valued modulation symbol soft decision on a received modulation symbol, the modulation symbol being associated with a plurality of turbo encoded symbols, the complex-valued modulation symbol soft decision having an in-phase component and a quadrature component;
• and
• a log-likelihood ratio computation module coupled to the demodulator and configured to receive the complex-valued modulation symbol soft decision from the demodulator, scale the complex-valued modulation symbol soft decision to obtain a log-likelihood ratio metric for a most-significant turbo encoded symbol of the modulation symbol, and apply a linear combination of a triangle function and a ramp function to the complex-valued modulation symbol soft decision to obtain log-likelihood ratio metrics for remaining turbo encoded symbols of the modulation symbol.

A receiver configured to approximate log-likelihood ratio metrics for a plurality of turbo encoded symbols, the plurality of turbo encoded symbols having been modulated with square quadrature amplitude modulation signal constellations having gray code labeling, the receiver comprising:

• a processor;
• and
• a processor-readable storage medium coupled to the processor and containing a set of instructions executable by the processor to extract a complex-valued modulation symbol soft decision on a received modulation symbol, the modulation symbol being associated with a plurality of turbo encoded symbols, the complex-valued modulation symbol soft decision having an in-phase component and a quadrature component, scale the complex-value modulation symbol soft decision to obtain a log-likelihood ratio metric for a most-significant turbo encoded symbol of the modulation symbol, and apply a linear combination of a triangle function and a ramp function to the complex-valued modulation symbol soft decision to obtain log-likelihood ratio metrics for remaining turbo encoded symbols of the modulation symbol.