Direct digital transmitters pave the way for 5G

In a modern digital transmitter, the digital input data is converted into two digital signals. These are used to modulate two parameters of the transmitted RF signal in order to increase the spectral efficiency. The input data can, therefore, be mapped on a 2D constellation diagram, which can be represented in a Cartesian or a polar system.

Based on these coordinate systems, there are two general transmitter (TX) architectures: Cartesian TX and polar TX (Figure 1). Cartesian TX combines two orthogonal RF signals (with a 90-degree phase difference) whose amplitudes are modulated by the real (I) and imaginary (Q) parts of the input data, respectively. Polar TX uses a single carrier signal where the amplitude (AM) and phase (Φ) are modulated by the amplitude and phase of the input data. The multiplication of AM and Φ can be done directly by the output power amplifier.