Daniel Markert

An All-Digital, Single-Bit RF Transmitter for Massive MIMO

This paper presents an all-digital transmitter solution particularly suited for Massive multiple-input and multiple-output (MIMO) systems for mobile communications. Massive MIMO is a key candidate to address the challenges of future mobile communication standards, especially to provide higher capacity in dense urban scenarios. While the required communication theory is elaborated to a great extend, the transceiver hardware complexity remains a potential economical show-stopper. This paper demonstrates that all-digital transmitters can be employed to reduce size, cost, and engineering effort of heavily parallelized transmit architectures. Therefore, today’s all-digital transmitter concepts are analyzed and improvements are suggested to increase performance and feasibility. The realized setups prove that the key specifications of mobile communication standards can be met utilizing dedicated integrated circuits or even by using off-the-shelf FPGAs and their high-speed interfaces. We show that we can generate 8 parallel 5 MHz LTE signals at 2.6 GHz out of a single FPGA with an ACPR of 48 dB with a coding efficiency of 50% using only binary waveforms.

New efficient architectures for RF pulse width modulators

This paper presents new architectures for RF pulse width modulation (RF-PWM). They allow for very efficient and simple implementation of this class of modulators. Even for the 2.6-Ghz band, the modulator can now be built with standard components. The new concepts support binary and M-ary output alphabets. One variant is capable of generating M-ary RF-PWM signals by only deploying one switching device. The new architectures are derived analytically and are illustrated by simulation results. A recent hardware implementation of this concept proofs the efficiency of this approach.

Coding efficiency of RF pulse-width-modulation for mobile communications

This paper discusses RF-PWM for mobile communication applications focusing on maximal achievable coding efficiency. Coding efficiency is one of the most crucial metrics in evaluating modulation concepts for switch mode amplification. It is shown that the achievable efficiency is mostly dependent on the chosen modulator waveform and the magnitude probability density function of the wanted signal. We determine theoretical limits and unveil unexpected benefits of unipolar waveforms. The results are applied to realistic communication test signals with various system parameter sets.

Analysis of the impact of phase- and amplitude distortions on the beam accuracy of active antenna arrays for mobile communication

In this paper a statistical analysis of the impact of phase- and amplitude-distortions on the performance of active antenna arrays for mobile communication base stations is presented. The cause and influence of different contributors to these distortions is described and the allowable phase- and amplitude-margins that need to be reached for mobile communication base station applications are derived. Finally a test setup of a distributed active antenna is implemented and it is shown, that the required accuracy can be achieved with standard RF-circuit design and components.

Compensation of imperfections caused by digital pulse width modulation using baseband injection

All-digital transmitter is a hot topic since a few years. One of the key elements of all-digital transmitters is the modulator, which modulates the baseband IQ signal onto a carrier frequency in the form of digital binary signals. The combination of delta-sigma modulation and pulse width modulation is a very strong candidate. However, the regular sampled pulse width modulator introduces non-linear distortions and images also known as aliasing effects. Due to these non-linear distortions and images, the signal integrity is violated especially when the power spectral density of the baseband input signal is not symmetrical around DC. In this paper, we propose a baseband injection method with low complexity to pre-compensate these unwanted emissions. This novel method does not have to run at the operating frequency of pulse width modulator but can run at a much lower frequency. Moreover, each term of the non-linear distortions and images can be individually compensated without mutual influence. In Matlab simulations we show that all the compensated non-linear distortions and images are suppressed by over 20 dB and disappear below the noise floor of delta-sigma modulator.