Rates of linear codes with low decoding error probability

Consider binary linear codes obtained from bipartite graphs as follows. There are k ≥ 1 left nodes each representing a message bit and there are m = m(k) right nodes each representing a parity bit, generated from the corresponding set of message node neighbours. Both the message and the parity bits are sent through a memoryless binary input channel that either retains, flips or erases each transmitted bit, independently. Based on the received set of symbols, the decoder at the receiver obtains an estimate of the original message sent. If the decoding error probability Pk −→ 0 and the average degree per parity node remains bounded as k → ∞, then the rate of the code k k+m −→ 0 as k → ∞.