A PLC network system and method operative with OFDM for generating MIMO frames with suitable preamble portions configured to provide backward compatibility with legacy PLC devices and facilitate different receiver tasks such as frame detection and symbol timing, channel estimation and automatic gain control (AGC), including robust preamble detection in the presence of impulsive noise and frequency-selective channels of the PLC network. A MIMO PLC transmitter device may selectively vary amounts of cyclic shift (CS) for different transmit phases based on whether an initial CS vector elicits an ACK signal back from a PLC receiver within a timeout period.