In this paper, we consider a new large-scale communication scheme where randomly distributed AmBC nodes are involved as secondary users to primary transmitter (PT) and primary receiver (PR) pairs. The secondary communication between a backscatter transmitter (BT) and a backscatter receiver (BR) is conducted by the BT's reflecting its corresponding PT's signal with different antenna impedances, which introduces additional double fading channels and potential inter-symbol-interference to the primary communications. Thus, at a typical PR, the backscatter signals are regarded as either decodable signals or interference. Assuming the locations of PTs form a Poisson point process and the locations of BTs form a Poisson cluster process, we derive the SINR and SIR based coverage probabilities for two network configuration scenarios. Numerical results on the coverage probabilities indicate the possibility to involve a large amount of AmBC nodes in existing wireless networks.