System and method for improving receiver sensitivity of a DD-OFDM system without using frequency guard band

A system for improving receiver sensitivity of an optical Direct Detection-Orthogonal frequency division modulation (DD-OFDM) system without using frequency guard band, comprising:

• an interleaver configured to interleave input data to the DD-OFDM system to generate interleaved data;
• a first recursive systematic convolutional encoder configured to encode the input data with a first recursive systematic convolutional code to generate a first recursive systematic convolutional encoded data;
• a second recursive systematic convolutional encoder configured to encode the interleaved data with a second recursive systematic convolutional code to generate a second recursive systematic convolutional encoded data;
• a puncturing device configured to puncture the first recursive systematic convolutional encoded data and the second recursive systematic convolutional encoded data to generate a parity sequence;
• and
• a combiner configured to combine the input data with the parity sequence to generate coded DD-OFDM data;
• wherein the puncturing device is configured to use different puncturing rates for different OFDM subcarriers to generate the parity sequence, so as to obtain higher spectral efficiency.