A communication architecture based on backscattering radio and software definition is proposed for WSNs which require low bit-rate and ultra-low cost sensors. It consists of a central Hub for RF carrier generation, homodyne detection and complex information processing, and sensors with carrier modulation function. In the former, a software-defined transceiver is constructed to transmit the carrier and receive the reflection from various sensors, and extract and process their transmitted messages. In the latter, the transmitter for each sensor is simplified as a transistor connected to an antenna and each sensor is designed around a low-power micro-controller driving a low-power RF switch, and the information from each sensor is modulated onto its unique subcarrier. Thus, a complete backscattering radio link communication system is established to achieve communication between each sensor and the center Hub. The simulation results based on the theory and the experimental results from the prototype show that the proposed WSN communication system not only achieves a wide range of reliable communication and data transmission and processing, but also has low complexity and ultra-low cost.