Energy-efficient realization of soft-output signal detection is of great importance in emerging high-speed multiple-input multiple-output (MIMO) wireless communication systems. This paper presents three algorithm-level complexity-reduction techniques for soft-output detector design to achieve significant energy savings. To demonstrate their effectiveness, we designed a soft-output detector for 4×4 MIMO with 64-QAM using 65nm CMOS technology. While achieving near-optimum detection performance, the detector can support over 100Mbps throughput with only 0.24mm2 silicon area and 11mw power, leading to a ×10 improvement over the state of the art. Categories and Subject Descriptors B.4.1 [Input/Output and Data Communications]: Data Communications Devices General Terms Algorithms, Design Keywords MIMO, detection, spatial multiplexing, low power, VLSI