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Recent studies have shown that using space-time code is an effective approach to increase the data rate over wireless channels. Space-time turbo (ST-Turbo) codes formed by concatenating space-time codes with turbo codes, take advantage of both the high diversity order of space-time systems and the randomness of the turbo codes. In this paper, we compare two ST-Turbo codes, i.e., simple space-time turbo codes (SiSTT) and turbo trellis-coded modulation space-time block codes (TTCM-STBCs), and their approximate versions with respect to performance and energy consumption for both general-purpose processor and synthesized implementations. The approximations are aimed at reducing the computational complexity and include reduction in the number of paths, number of iterations, and datapath computations. Analysis of the simulation results show that SiSTT-based versions should be used for higher SNR applications where low energy consumption is the primary design objective, and TTCM-STBC-based versions should be used where performance is the primary design objective. Finally, four ST-Turbo algorithms (i.e., baseline SiSTT and its energy-efficient approximate version and the baseline TTCM-STBC and its energy-efficient approximate version) have been synthesized in 0.18-/spl mu/m CMOS technology and the implementations compared with respect to area, power, and latency.