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A real-time testbed based on the technology of software radio is adopted to efficiently evaluate cutting-edge technologies in wireless communications, and thus becomes a valuable tool for both academic research and system prototyping. In this article we describe a software radio testbed, established in the software radio laboratory at Georgia Institute of Technology, used to implement a physical layer similar to IEEE 802.11a space-time coded orthogonal frequency-division multiplexing. The testbed consists of a 2 x 2 multiple-input multiple-output configuration with powerful digital signal processor chains, high-speed data exchange interfaces, and many advanced subsystem modules, such as high-speed high-resolution analog-to-digital and digital-to-analog converters, digital up/downconverters, and wideband RF transmit and receive front-ends with synchronous channels and programmable settings. The design methodology and implementation for key algorithms, such as time, sampling, and frequency synchronization, and channel estimation and compensation are discussed. The experimental data obtained from a typical indoor environment demonstrates that the prototype is capable of providing 30 Mb/s peak data rate, operating at the central frequency of 2.435 GHz with a spectral occupancy of 6.25 MHz. Spatial-temporal diversity gain associated with space-time coding is verified by the experimental results.