  This paper presents new low-complexity lattice-decoding algorithms for
noncoherent block detection of QAM and PAM signals over complex-valued fading
channels. The algorithms are optimal in terms of the generalized likelihood
ratio test (GLRT). The computational complexity is polynomial in the block
length; making GLRT-optimal noncoherent detection feasible for implementation.
We also provide even lower complexity suboptimal algorithms. Simulations show
that the suboptimal algorithms have performance indistinguishable from the
optimal algorithms. Finally, we consider block based transmission, and propose
to use noncoherent detection as an alternative to pilot assisted transmission
(PAT). The new technique is shown to outperform PAT.
