  We consider in this paper the design of full diversity and high rate
space-time codes with moderate decoding complexity for arbitrary number of
transmit and receive antennas and arbitrary input alphabets. We focus our
attention to codes from the threaded algebraic space-time (TAST) framework
since the latter includes most known full diversity space-time codes. We
propose a new construction of the component single-input single-output (SISO)
encoders such that the equivalent code matrix has an upper triangular form. We
accomplish this task by designing each SISO encoder to create an ISI-channel in
each thread. This, in turn, greatly simplifies the QR-decomposition of the
composite channel and code matrix, which is essential for optimal or
near-optimal tree search algorithms, such as the sequential decoder.
