  To achieve ultra-high storage capacity, the data tracks are squeezed more and
more on the magnetic recording disks, causing severe intertrack interference
(ITI). The multihead multitrack (MHMT) detector is proposed to better combat
ITI. Such a detector, however, has prohibitive implementation complexity. In
this paper we propose to use the reduced-state sequence estimation (RSSE)
algorithm to significantly reduce the complexity, and render MHMT practical. We
first consider a commonly used symmetric two-head two-track (2H2T) channel
model. The effective distance between two input symbols is redefined. It
provides a better distance measure and naturally leads to an unbalanced set
partition tree. Different trellis configurations are obtained based on the
desired performance/complexity tradeoff. Simulation results show that the
reduced MHMT detector can achieve near maximum-likelihood (ML) performance with
a small fraction of the original number of trellis states. Error event analysis
is given to explain the behavior of RSSE algorithm on 2H2T channel. Search
results of dominant RSSE error events for different channel targets are
presented. We also study an asymmetric 2H2T system. The simulation results and
error event analysis show that RSSE is applicable to the asymmetric channel.
