Radio transmitters are evolving towards digitalintensive solutions to exploit reconfigurability and benefit from CMOS process scaling. Outphasing has been identified as a suitable candidate for digital wideband transmitters. However, with recent digital-intensive outphasing transmitters the achieved performance in terms of adjacent channel leakage ratio (ACLR) has been limited. This paper identifies the sampling images of the modulating phase signal as the main factor limiting the ACLR of digital outphasing transmitters. We present a new digital interpolating phase modulator architecture, capable of providing significantly better sampling image attenuation. When evaluated in outphasing configuration with a 100 MHz OFDM signal at the carrier frequency of 2.46 GHz, and 10-bit phase resolution, the proposed solution achieves an ACLR of-59 dBc, compared to-43 dBc achievable with the phase modulator architecture utilized in state-of-the-art digital outphasing transmitters. The proposed digital interpolating phase modulator is also capable of custom carrier generation, a straightforward method for generating an arbitrary carrier frequency up to 1.25 times the phase modulator sampling rate.

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This paper describes the first purely digital approach to reduce the receive band noise in digitally-intensive RF transmitters. The proposed solution applies bandpass deltasigma modulation and dynamic element matching (DEM) to the receive band (RX-band) instead of the transmit band. This enables selective attenuation of the noise originating from amplitude quantization and static mismatches of the digitalto-analog converter (DAC), which would otherwise reach the transmitter output almost unattenuated. A highly configurable 4 th-order noise transfer function is designed to achieve optimum attenuation in the programmable RX-band, while ensuring negligible degradation of the transmitted signal quality as well as stable operation of the tree structure DEM encoder. A general validation of DEM, independent from the duration of the DAC impulse response, is also presented. The proposed solution is verified through system-level simulations with LTE signals. In the presence of typical amplitude and timing mismatches, the RXband noise can be reduced below-160 dBc/Hz without filtering after the DAC, thus potentially enabling SAW-less operation of all-digital transmitters.

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We present the first all-digital LTE transmitter using programmable digital attenuation of RX-band noise. The system is architectured to fully exploit the speed and low cost of DSP logic in deep-submicron CMOS processes, without increasing at all the design effort of the RF circuitry. To achieve SAWless operation, the transmitter 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 noise, without need for digital predistortion, calibration or bulky analog filters. The transmitter was fabricated in 28nm CMOS, and occupies only 0.82 mm 2. Besides low outof-band noise, our system also demonstrates state-of-art linearity performance, with measured CIM3/CIM5 below-67 dBc, and ACLR of-61 dBc with LTE20 carrier. The circuit consumes 150 mW from 0.9/1.5V supplies at +3 dBm output power.

We present a prototype RF transmitter with an integrated multilevel class-D power amplifier (PA), implemented in 28-nm CMOS. The transmitter utilizes tri-phasing modulation, which combines three constant-envelope phase-modulated signals with coarse amplitude modulation in the PA. This new architecture achieves the back-off efficiency of multilevel outphasing, without linearity-degrading discontinuities in the RF output waveform. Because all signal processing is performed in the time domain up to the PA, the entire system is implemented with digital circuits and structures, thus also enabling the use of synthesis and place-and-route CAD tools for the RF front end. The effectiveness of the digital tri-phasing concept is supported by extensive measurement results. Improved wideband performance is validated through the transmission of orthogonal frequency-division multiplexing (OFDM) bandwidths up to 100 MHz. Enhanced reconfigurability is demonstrated with noncontiguous carrier aggregation and digital carrier generation between 1.5 and 1.9 GHz without a frequency synthesizer. For a 20-MHz 256-QAM OFDM signal at 3.5% error vector magnitude (EVM), the transmitter achieves 22.6-dBm output power and 14.6% PA efficiency. Thanks to the high linearity enabled by tri-phasing, no digital predistortion is needed for the PA.

This paper presents a digital interpolation chain for non-integer variable-ratio sampling rate conversion, targeted to 4G mobile applications. Such a system is needed in all-digital transmitters, where the sampling rate of the digital input to the RF front-end must be an integer fraction of the carrier frequency. A highly configurable architecture is proposed to cope with the flexibility needed in 4G applications. The system achieves excellent ACLR of 75 dB, EVM degradation of 0.05%, and RXband noise below -160 dBc/Hz. Digital synthesis of the circuit in a 40nm low-power CMOS process results in a core area of only 0.073 mm 2 . The estimated power consumption is between 6 and 29 mW, depending on channel bandwidth and transmission band.