M-ary optical data storage: fixed versus variable length modulation

We consider theoretical and practical aspects of an M-ary optical recording channel. Specifically, we address the performance tradeoffs between conventional pulse amplitude modulation (PAM) and M-ary runlength limited (RLL) coding in an optical disc system with compact disc parameters. Using a linear-time invariant channel model with white and nonwhite noise components, we see that, for signal-to-noise ratios found in typical CD systems, coded PAM outperforms M-ary RLL codes for a range of storage densities. In addition, this work also shows that for systems that require moderate-to-small values of M, M-ary RLL codes can provide significant advantages.