Backscatter communications can enable the proliferation of Zero-energy (ZE) Internet of Things (IoT) services in 6G, ushering in a new era in green communications. To achieve a widescale adoption, backscatter communications needs to be integrated into the cellular ecosystem. This motivates the need to design a bistatic backscatter modulation technique which is not only compatible with the 3GPP New Radio (NR) physical layer but also robust to direct link interference. In this paper, a non-ambient bistatic backscatter communications system consisting of a cellular carrier emitter (CE), a backscatter device (BD) and a cellular reader (RD) is investigated. An analytical model is presented to explain the design rationale for an orthogonal frequency division multiplexing (OFDM) based physical layer for the considered system. Specifically, to achieve orthogonality between the CE and BD signals arriving concurrently at the RD, a comb-like OFDM subcarrier allocation is leveraged at the CE while a specially designed impedance switching sequence is applied at the BD to generate OFDM-compatible backscatter symbols with a binary or higher-order phase shift keying modulation. The link performance of the proposed system is evaluated in terms of the achievable bit error rate versus the signal-to-noise ratio at the RD for the CE-RD link and the BD-RD link under direct link interference. Simulation results confirm the viability of the proposed design for a cellular ZE-IoT system.