GEN04-4: Improved Runlength Control When Using Low-Density Parity-Check Codes

In high-density magnetic recording systems, a runlength constraint (typically (0, k)) is imposed to aid then timing recovery system. However, the traditional encoding structure where the runlength limited (RLL) encoder follows the ECC encoder is not a good fit when using LDPC codes. The main difficulty is that the LDPC decoder requires soft information, which is incompatible the typical RLL decoders which output hard decoding decisions. In this paper, we propose a structure so that soft information can be delivered directly to the LDPC decoder while the runlength constraint is satisfied. By integrating the bit-flipping scheme proposed in [Vasic and Pedagani, IEEE Trans. Magn, vol. 40, no. 5, pp. 1738-1743, May 2004] and an interleaver into the system, we are able to take advantage of the near-capacity performance of the LDPC code given the runlength limit. The simulations show that the BER performance loss of the LDPC code (rate 8/9, length 4932) due to the RLL constraint is less than 0.1 dB. The error floor introduced by bit-flipping scheme is limited to bit error rate (BER) levels lower than 10^-7.