Digital Fountain Codes System Model and Performance over AWGN and Rayleigh Fading Channels

Digital fountain codes were originally packet-level forward erasure codes, but researchers have found that they are good at correcting bit errors. Modern LDPC and turbo codes have been shown to approach the Shannon limit. However, they are fixed rate codes and cannot well satisfy the Internet services that may encounter variable loss rates or asynchronous data access. For both packet and bit levels, fountain codes share the same belief propagation decoding mechanism with LDPC and turbo codes. We propose a general fountain codes system model over noisy channels. Conventional LT codes work poorly on AWGN and Rayleigh fading channels but Raptor codes offers near capacity performance if their pre-codes are a WiMAX LDPC code or our left-regular LDPC code. The code performance can be approved if we select more reliable received symbols for decoding. Our simulation results support the model design.