Robert E. Swanson

Turbo decoding for PR4: parallel versus serial concatenation

Recent work on the application of turbo decoding techniques to partial response class 4 (PR4) channels has focused on parallel concatenation systems that require three a posteriori (APP) detectors. A simplified serial concatenation system is presented that uses as its outer code a single convolutional code and as its inner code the partial response channel. An extension of this serial concatenation system is also presented that combines a second code with the channel, forming a more powerful inner code. Both proposed systems require only two APP detectors, offering significant savings in complexity and computation time. These serial concatenation systems are shown to perform as well as the more complicated parallel concatenation systems, offering substantial gains over uncoded systems. Additionally, the effect of precoding is investigated. Simulation results comparing the parallel and serial concatenation systems are also presented.

Turbo decoding for partial response channels

The partial response channel can be viewed as a rate-1 encoder in which the output alphabet differs from the input alphabet. In serially concatenated coding schemes, the partial response channel can serve as the inner encoder. Previous work on the application of turbo decoding techniques to partial response channels has focused on using a parallel concatenation of convolutional encoders as the outer code and the partial response channel as the inner code. This system requires three a posteriori probability (APP) detectors-one matched to the channel and two matched to the constituent encoders. A simplified system is presented that uses as its outer code a single convolutional code and as its inner code the partial response channel. The simplified system requires only two APP detectors, offering significant savings in complexity and computation time. This single convolutional code system is shown to perform as well as the more complicated system, offering substantial gains over uncoded systems. Simulation results for three magnetic recording channel models are presented: a partial response channel with additive white Gaussian noise, an equalized Lorentzian channel model, and a media noise model called the microtrack model. Since the use of an outer Reed-Solomon code is anticipated in an actual system, the burst-error statistics are investigated. System performance with various interleaver designs and precoders is also investigated.

A new class of two-dimensional RLL recording codes

The benefits of using modulation codes in two dimensions in multichannel recording include improved clocking and improved ratios of user bits to recorded bits. Recent work has described a class of d/sub x/, k/sub y/ codes in two dimensions which satisfy the d/sub x/ constraint in one dimension and the k/sub y/ constraint in the other. In tape systems this method is extremely vulnerable to dropouts. A new class of two-dimensional run-length codes that operate using the usual d/sub x/, k/sub y/ constraint along the track is proposed, with an additional k/sub y/ constraint across the tracks. In this approach the horizontal d/sub x/, k/sub x/ code is allowed a much larger k/sub x/ constraint since it is no longer the sole carrier of clocking information. Capacities are calculated for a range of codes and number of channels, and an example of the construction of one such code is given. An extension of the codes that preserves clocking during channel loss is described. >

An alternative construction of interleaved RLL codes for partial response channels using multichannel methods

Current work in magnetic recording channels has demonstrated significant improvement in recording density using partial response communication methods along with maximum likelihood detection. The class IV partial response channel (PR4) behaves as the interleaving of two identical run-length limited (RLL) channels. Efficient recording codes for the PR4 channel have been made by considering a channel bit sequence to be composed of two interleaved subsequences. Finite state techniques are used to describe run-length constraints on each subsequence as well as a global run-length constraint on both sequences combined. A recent paper by Swanson and Wolf discusses multichannel run-length limited codes that turn out to be very similar to interleaved run-length limited codes. The present paper discusses the similarities between the code description of multichannel RLL codes and the interleaved RLL codes required by the PR4 channel. The extension of the techniques of multichannel RLL work to the interleaved problem is discussed. It was found that suitable code constructions are possible by viewing the problem as two distinct RLL codes that have individual maximum run-lengths onto which a global run-length is imposed. Two examples are developed to illustrate the ideas. >

Experimental analysis of the effects of tape thickness on magnetic recording

Advances in magnetic tape recording have produced media with magnetic layers as thin as 0.1 μm. In this article, a metal particulate tape with a magnetic layer thickness of ∼0.37 μm is compared to a standard thick media tape with a magnetic layer thickness of ∼4 μm. Measurements of the isolated pulse are made and shown to compare well with micromagnetic simulations. The replay voltage versus current is measured with a 4 μm track width, shielded magnetoresistive head at various densities. The thin tape shows better high density response at high currents than the thick tape. The simulations show that the transitions are sharper on the thin tape due to both the reduction in thickness, and an improvement in the particle orientation. The better oriented particles yield narrower pulses, thereby improving the high frequency response of the tape. Frequency response measurements are also taken with the magnetoresistive head, which yield a signal to noise ratio of ≳20 dB at 200 kfci. Last, overwrite performance ver...

Two-dimensional multichannel partial response class IV recording codes

Partial response coding has recently been used in the magnetic disk recording business to increase the attainable linear density. It is also being investigated for use in magnetic tape channels, many of which use simultaneous recording on multiple tracks. Since both disk and tape are two-dimensional recording surfaces, there is promise for improvement in capacity and clocking if both dimensions are used simultaneously. While two-dimensional recording is especially suited to the multichannel tape environment it may also be useful in future disk products. This paper examines the application of partial response class IV coding to the two-dimensional, multichannel environment. >