All-Digital LTE SAW-Less Transmitter With DSP-Based Programming of RX-Band Noise

We present the first all-digital LTE transmitter (TX) using programmable digital attenuation of receive band (RX-band) noise. The system is architectured to fully exploit the speed and low cost of DSP logic in deep-submicrometer CMOS processes, without increasing at all the design effort of the RF circuitry. To achieve operation without surface acoustic wave filter, the TX uses digital bandpass delta-sigma modulation and mismatch-shaping to attenuate the DAC noise at a programmable duplex distance. These functions can be implemented entirely within DSP, thus taking advantage of the standard digital design methodology. Furthermore, the fully digital RX-band noise shaping significantly relaxes the performance requirements on the RF front-end. Therefore, 10 bits of resolution for the D/A conversion are sufficient to achieve -160 dBc/Hz out-of-band (OOB) noise, without need for digital predistortion, calibration, or bulky analog filters. The TX was fabricated in 28-nm CMOS, and occupies only 0.82 mmsuperscript2. Besides low OOB noise, our system also demonstrates state-of-art linearity performance, with measured CIM3/CIM5 below -67 dBc, and adjacent-channel leakage ratio of -61 dBc with LTE20 carrier. The circuit consumes 150 mW from 0.9-/1.5-V supplies at +3 dBm output power.