Summary: This paper addresses turbo-encoder design for coding with high spectral efficiency using parallel concatenated trellis-coded modulation and symbol interleaving. The turbo-encoder design involves the constituent encoder design and the interleaver design. The constituent encoders are optimized for symbolwise effective free distance, and each has an infinite symbol-wise impulse response. We identify the canonical structures for the constituent encoder search space. In many cases of practical interest, the optimal structure for these constituent encoders connects the memory elements in a single row. This single row generally applies to turbo-code constituent encoders for parallel concatenation and is not restricted to symbol interleaving. To lower the error floor, a new semi-random interleaver design criteria and a construction method extends the spread-interleaver concept introduced by Divsalar and Pollara. Simulation results show that the proposed system employing symbol interleaving can converge at a lower signal-to-noise ratio than previously reported systems. We report simulation results between 0.5 and 0.6 dB from constrained capacity for rates of 2 and 4 bits/s/Hz.