Markus Antweiler

Efficient and portable SDR waveform development: The Nucleus concept

Future wireless communication systems should be flexible to support different waveforms (WFs) and be cognitive to sense the environment and tune themselves. This has lead to tremendous interest in software defined radios (SDRs). Constraints like throughput, latency and low energy demand high implementation efficiency. The tradeoff of going for a highly efficient WF implementation is the increase of porting effort to a new HW platform. In this paper, we propose a novel concept for WF development, the Nucleus concept, that exploits the common structure in various wireless signal processing algorithms and provides a way for efficient and portable implementation. Tool assisted WF mapping and exploration is done efficiently by propagating the implementation and interface properties of Nuclei. The Nucleus concept aims at providing software flexibility with high level programmability, but at the same time limiting HW flexibility to maximize area and energy efficiency.

2nd Order Cyclostationarity of OFDM Signals: Impact of Pilot Tones and Cyclic Prefix

This paper deals with 2nd order cyclostationarity of orthogonal frequency division multiplex (OFDM) signals. A new generalized formula for the spectral correlation density (SCD) function is derived. Compared to related work in the literature, our derivation is not restricted to the case that all sub-carriers of an OFDM signal carry statistically independent data. The reason for that is that correlated data in terms of pilot tones are typically introduced on different carriers for channel estimation and synchronization purposes. The new formula allows us to analyze the impact of such pilot tones on the SCD. In addition, it gives extra information about the impact of the cyclic prefix.

Implementations of Sorted-QR Decomposition for MIMO Receivers: Complexity, Reusability and Efficiency Analysis

Matrix decomposition of the channel matrix in the form of QR decomposition (QRD) is needed for advanced multiple input and multiple output (MIMO) demapping algorithms like sphere decoder. Due to the computation-intensive nature of the QRD, its implementation has to be highly efficient. Flexibility in several forms, e.g. support for different algorithms, reusability of wireless implementations, portability, etc. is highly sought in wireless devices. The contradictory nature of flexibility and efficiency requires tradeoffs to be made between them in system development. In this paper, we have analyzed such tradeoffs by implementing two minimum mean squared error-sorted QRD algorithms. The algorithms have been implemented in four different methods with varying degree of reusability and in five different forms of portability. The performance of the implementations is evaluated by using the real-time constraints from the LTE standard. For all the implementations, modular equations for accurately estimating the execution time are derived.

Efficient implementations from libraries: Analyzing the influence of configuration parameters on key performance properties

Library based waveform (WF) development approaches have the potential to address one of the key challenges in software defined radio technology, developing portable and at the same time implementation-efficient WFs. The term WF, in this paper, represents a complete wireless standard with several modes. Upon standardizing the library and the interfaces of its components, different vendors can provide Flavors, which are efficient implementations, for some/all components of a WF as a board-support-package. Flavors might provide different trade-offs with respect to key performance related properties like bit error rate, throughput, latency, area, etc. This paper analyzes such a scenario by using FFT as an example. We use 10 Flavors of FFT from 2 vendors for 3 processing elements to analyze the influence of configuration parameters like input data-width, scaling, etc. on the performance properties. Analysis has been performed by doing two tests: using a simple OFDM transceiver system, calculating root mean square error. Based on our analysis, we have identified the main parameters that influence the properties. Our investigations stress the need for modeling the differences in performance on key properties in terms of the influencing parameters. Such a model can assist in selecting a set of appropriate Flavors for implementing a WF and enable tool assisted WF-development. Furthermore, it is evident from our investigations that Flavors exhibit trade-offs in properties and pose tough constraints in several aspects.

Design and Analysis of Hierarchically Modulated BICM-ID Receivers With Low Inter-Layer Interferences

In this article, we present a novel methodology to optimize Hierarchically Modulated Bit-Interleaved Coded Modulation with Iterative Decoding (HM-BICM-ID). This methodology allows designing a receiver which supports several configurations. Each configuration is able to decode the same transmitted signal over the air with different fidelity. This concept permits using radios with varying processing capabilities, e.g. handheld radios, vehicular based radios etc. However, earlier simulation results have shown that HM-BICM-ID loses, if compared to non-hierarchical schemes, in Bit Error Rate (BER) performance due to Inter-Layer Interferences and design restrictions. Our proposed iterative tunable procedure optimizes hierarchical modulation schemes considering two criteria, the Harmonic Mean of the minimum squared Euclidean Distance and the bit error probability. The optimization is done by moving critical constellation points towards the optimal direction. A novel modulation scheme has been found and simulation results show an improved asymptotic BER performance in a wide range of channel conditions for an exemplary two-layered HM-BICM-ID. Finally, we present an analysis of HM-BICM-ID in context of Extrinsic Information Transfer Charts.

Analysis of BICM-ID Receivers Exploiting Transformations of Extrinsic Information

In this article we analyze a novel idea to increase the applicability of Bit Interleaved Coded Modulation with Iterative Decoding (BICM-ID) to legacy waveforms. One essential design parameter of BICM-ID receivers with respect to the error correcting capabilities is the symbol mapping of the digital modulation scheme. A so-called Semi-Set Partitioning (SSP) symbol mapping is well known to provide higher stepwise gains in robustness in every iteration than a Gray encoded symbol mapping. The novel approach is based on the idea to make in a first step of BICM-ID the deliberately false assumption that a well performing symbol mapping has been used at the transmitter, even though in reality a less powerful symbol mapping was applied. In a second step, the mismatch in both symbol mappings is compensated by an innovative Transformation of Extrinsic Information (TEI). After having reviewed the innovative TEI idea in more detail, we will discuss the fundamentals of the required signal processing. In addition, as a novelty of this article we will analyze three different ways to implement the TEI approach. Several simulation results will be shown which demonstrate the theoretically achievable performance gains.

On box-operations for L-Values and their applicability to soft-decision algorithms

Modern digital communication systems can benefit from soft-decision algorithms in receiver implementations. Such algorithms require reliability information about every data bit which is typically given in terms of Log-Likelihood Ratios (LLRs) or short L-Values. Several efficient receiver implementations based on L-Values have already been proposed in the literature. Many of them exploit the famous Box-Plus operation. Its inverse has also been introduced in terms of the Box-Minus operation. In this letter, at first the specific characteristics of both operations, the Box-Plus as well as the Box-Minus operation, will briefly be reviewed. Secondly, as a novelty we will define two more operators, the Box-Multiply and the Box-Divide operation. An illustrative example will demonstrate the necessity for these two new operators. Finally, we will propose a practical solution for eliminating a critical side constraint of the Box-Minus operation which was known to be a limiting factor so far.

Analysis of a modified Switchable Bayesian Learning Automaton for Cognitive Radio

One of the most important topics in Cognitive Radio communications is that all communication partners change to the same frequency at the same time. A critical aspect of this process is a powerful channel selection algorithm, because each channel switching process requires resources and bears the risk of connection loss. Therefore, it is important to choose the channel that can be expected to be available for the longest time. This requires collecting information about all usable channels and developing a selection strategy. In [1] a machine learning approach, the Switchable Bayesian Learning Automaton (SBLA), is proposed for this task. Recently, we have implemented that algorithm to our cognitive radio simulator, which was presented in [2]. This paper describes the implementation, points out the advantages and drawbacks of the algorithm, and introduces improvements for its use in real-time systems.