Gerd Vandersteen

A methodology for efficient high-level dataflow simulation of mixed-signal front-ends of digital telecom transceivers

The explosion of the telecommunications market requires miniaturization and cost-effective realization of the front-ends of transceivers for digital telecommunications. New architectures must therefore be simulated at high level. Current methodologies and corresponding tools suffer from common drawbacks, such as lower accuracy, slow simulation speed, etc. A new methodology has been developped for the efficient simulation, at the architectural level, of mixed-signal front-ends of digital telecom transceivers. The efficient execution is obtained using a multi-rate, multi-carrier signal representation together with a dataflow simulation scheme which switches dynamically towards the most efficient signal processing technique available. An implementation of this methodology shows both excellent runtimes and a high accuracy.

Nonparametric initial estimates for Wiener-Hammerstein systems

Abstract This paper studies the generation of nonparametric initial estimates for Wiener-Hammerstein systems. The kernel idea is to replace the original structure by an alternative representation that is easier to identify in a two step procedure. In the first step, the best linear approximation is estimated, in the second step a structure of the form GfG–1 is identified, where G is a linear dynamic system and f a static nonlinear system. By making a nonparametric approach it is possible to reduce the required user interaction, which strongly increases the user friendliness.

Dataflow simulation of mixed-signal communication circuits using a local multirate, multicarrier signal representation

The explosion of the telecommunications market requires miniaturization and cost-effective realization of the front-ends of transceivers for digital telecommunications. New architectures must therefore be simulated at a high level. Current methodologies and corresponding tools suffer from common drawbacks, such as low accuracy, slow simulation speed, etc. A new methodology has been developed for efficient simulation of mixed-signal front-ends of digital telecom transceivers at the architectural level. Efficient execution is obtained using a multirate, multicarrier signal representation together with a dataflow simulation scheme that switches dynamically to the most efficient signal processing technique available. An implementation of this methodology shows both excellent runtimes and a high accuracy for realistic front-end architectures.

CAD for RF circuits

Wireless transceivers for digital telecommunications are heterogeneous systems that combine digital hardware, software and analog circuitry. The pressure to miniaturization and lower power consumption for these transceivers imposes tight specifications on their analog RF parts. Many aspects of RF circuits cannot be simulated accurately and efficiently with a classical circuit-level SPICE approach. In this paper three important simulation problems for RF circuits are addressed: 1. High-level simulation of analog and RF blocks for the determination of the specifications of the circuits. 2. Accurate circuit-level simulation of nonlinear circuits with time constants that differ largely. 3. Efficient and accurate computation of phase noise in RF oscillators. For each of these problems, solutions are proposed. These solutions illustrate that accurate and efficient simulations of RF communication circuits need a heterogeneous variety of advanced algorithms.

Identifying the main nonlinear contributions: use of multisine excitations during circuit design

Modeling and understanding the nonlinear behavior of analog and RF circuits is essential for good design of telecommunication systems. Classical Volterra series give the designer this necessary insight, but they are only valid for weakly nonlinear systems and they are difficult to edit. To overcome these limitations, we developed a technique to identify, quantify and qualify the sources of nonlinear behavior in analog and RF circuits by combining the information obtained by a set of simulations that use periodic excitation signals with a given power spectrum and arbitrary phases. The paper describes and demonstrates this approach by the analysis of the cascade of a BiCMOS power preamplifier and power amplifier with adaptive biasing for 5 GHz wireless local area networks (WLAN). The approach is applicable to weakly and strongly nonlinear systems, which is demonstrated by pushing the amplifier into compression. Furthermore, it provides useful design information, such as the contribution of each subcircuit to the overall nonlinear behavior.

Broadband high frequency differential coupler

This paper describes the design methodology of a broadband, high frequency differential coupler. The output signal of this differential coupler is proportional to the difference of 2 input ports, i.e. proportional to the differential signal of the 2 inputs. This signal is made using a hybrid structure. The overall performance of this differential coupler and the hybrid structure is characterized by the differential coupling factor and by the common mode rejection ratio. A pair of broadband impedances are used to scale the 2 incoming signals with 2 complex coefficients. A symmetric structure of two identical broadband directional couplers and a symmetric power combiner are used afterwards to obtain the differential signal. The high bandwidth of both the terminations, the directional couplers and the power combiner make it possible to obtain a differential coupling factor and a common mode rejection ratio which is frequency insensitive over several decades. Measurements demonstrate a differential coupler with a bandwidth of more than 3 decades.

Efficient bit-error-rate estimation of multicarrier transceivers

Multicarrier modulation schemes are widely used in several digital telecommunication systems, such as Asymmetric Digital Subscriber Lines (ADSL) and Wireless Local Area Network (WLAN) based on Orthogonal Frequency Domain Multiplexing (OFDM). An estimate of the Bit-Error-Rate (BER) degradation due to non-idealities in the transceiver (e.g. nonlinear distortions in the analog front-ends, digital clipping,...) is much more complicated in a multicarrier system than in a single-carrier system due to the large number of carriers and the huge number of possible transmitted symbols. This paper proposes a method for estimating the BER of such OFDM modulation schemes in a CPU time that is two orders of magnitude smaller than a Monte-Carlo method, as confirmed by simulations on a 5 GHz IEEE 802.11 WLAN receiver front-end.

High-level simulation and modeling tools for mixed-signal front-ends of wireless systems

Wireless applications such as WLAN, GSM, DECT, GPS,... require low-cost and low-power transceivers. Moreover, a high flexibility is required when wireless terminals will have to cope simultaneously with several standards. To achieve this, while maintaining high performance, the possibilities of analog and digital signal processing need to be combined in an optimal way during the realization of a transceiver. This is only possible when system designers can efficiently study tradeoffs between analog and digital. Making such tradeoffs is too complicated for pen-and-paper analysis. Instead, efficient simulation of mixed-signal architectures with detailed models for the different building blocks is required. This paper discusses high-level modeling and simulation approaches for mixed-signal telecom front-ends. Comparisons to commercial high-level simulations show an important reduction of the CPU times of typical high-level simulations of telecom transceivers such as bit-error-rate computations. This efficient simulation approach together with the accurate modeling tools, that include substrate noise coupling, form an interesting suite of tools for advanced architectural studies of mixed-signal telecom systems.

Estimating Parameterized Scalable Models from the Best Linear Approximation of Nonlinear Systems for Accurate High-Level Simulations

System designers of communication systems need to compare the simulated behavior of a system with the linear and nonlinear specifications. They need high-level models to perform these simulations fast. The existing high-level models for nonlinear components do not scale smoothly with external parameters like the input power. To overcome this problem, a modeling technique based on the best linear approximation is developed. The parameterized models describe trajectories of the poles and zeros as a function of the input power. The resulting models accurately describe both the linear and nonlinear behavior of the system components. They can easily be implemented in modern simulators.

Designing power amplifiers? Use good excitation signals

The performance of any communication front-end depends largely on the efficiency and the linearity of the power amplifier. Power amplifiers are difficult to design since the available design techniques are not valid for the more complex modulation schemes used nowadays. Therefore, one needs to include information about these complex signals in the design strategy. But this information should be used such that it allows the designer to take good design steps. We reviewed the traditionally used load-pull design technique for power amplifier designs using multi-tone excitation signals to MIMIC the real excitation signals. This approach is illustrated on the loadpull characteristics of a 5 GHz power amplifier in 0.13 ¿m CMOS technology. This example shows the difference in design choices when using the correct excitation signals. Therefore, it illustrates the importance of these methods and the excitation signal used.