Two-level channel coding for cooperative wireless networks based on WiMAX LDPC codes

In this paper, a channel coding scheme is considered for a pair of cooperative broadcasting nodes in a cognitive wireless network. The scheme offers two levels of error protection using as components two low-density parity-check (LDPC) codes drawn from the IEEE 802.16e (WiMAX) specification. A modified version of the |u|u+v| code construction of Plotkin is employed, to obtain two values of error protection for two types messages, as follows. Each cooperating node transmits two sequences: For the first node, the first sequence is a modulated codeword m(v̅₁) from a code C₁, while the second node remains quiet. The second sequence sent by the first node is again m(v̅₁) while the second code sends a modulated codeword m(v̅₂) from a code C₂. At the receiving node antenna, the sequences are superposed to construct an “over-the-air” modulated |u|u + v| code. Messages with a higher degree of error protection are intended for receiving nodes physically located far from the pair of cooperating nodes. The performance is examined of two-level codes using both BPSK/QPSK mappings and two 4-PAM mappings with two different levels of average energy, over both AWGN and Rayleigh fading channels.