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

Abstract

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.

A voltage-mode RF DAC for massive MIMO system-on-chip digital transmitters

“…The frequency response of the RF DAC will have high energy lobe at f s or a multiple of f s depending on the oscillating pulse duration. The higher Nyquist images of the digital data at f s (or a multiple of f s ), will be the RF output signal of the RF DAC [5,7,12,19].…”

“…The digital data is processed in the digital blocks and the sample rate is increased to the update rate of the RF DAC. In general two approaches exist for implementing a mixing RF DAC depending on how the mixing operation in each cell of the DAC is performed, as discussed in [19]: at the data path as ''mixing logic'' or modulating the tail current as ''series mixing'' and therefore various oscillating PAM signals have been used for RF DAC implementation such as continues sinusoidal [7,12], discrete oscillating PAM [20], or a bipolar rectangular [5,19]. To implement the RF DAC in a process that is compatible with co-integration by digital ASIC, mixing logic approach is preferred due to the voltage headroom limitations and also the ''digital-in-nature'' characteristic of this method, so that the RF DAC actually benefits from process scaling.…”

Studies on Selected Topics in Radio Frequency Digital-to-Analog Converters

“…In an attempt to "digitalize" the IQ transceiver, direct-digital-to-RF transmitters have been suggested [73,60,36,4,6,5,71,3,19,39,40,100,81,107].…”

“…The pulse in subplot (5) in Fig. 2.7, has been very popular in recent RF DACs [39,40,101,100,81] mainly due to its simplicity in implementation and being hardware efficient (comparing to other oscillating PAMs). This oscillating pulse is referred to as bipolar rectangular zero-order-hold PAM or simply bipolar PAM.…”

Power-Scalable Dynamic Element Matching for a 3.4-GHz 9-bit $\Delta\Sigma$ RF-DAC in 16-nm FinFET

“…One important application of high-performance DEM DACs is in radio transmitters for wireless communications [3]- [5], where the aforementioned digitalization trend has strongly emerged during the past two decades. Modern radio standards utilize complex modulation schemes, wide channel bandwidths and stringent out-of-band (OOB) emission limits, thus imposing formidable speed and linearity requirements on the D/A conversion.…”