Ulrich Berthold

Detection of Spectral Resources in Cognitive Radios Using Reinforcement Learning

Available spectrum for wireless communications is a limited resource which gains importance with the increasing demand for mobile communication services with high data rates. Measurements show that assigned frequency bands (FBs) are not used efficiently. One approach for increasing the efficiency in spectrum use is the concept of overlay systems, which can be seen as an enabling technology for cognitive radio and dynamic spectrum access by providing frequency agility. In this paper, we propose an approach for the detection of spectral resources based on reinforcement learning, allowing the cognitive radio to select the FBs with the most available resources.

OFDM-Based Overlay Systems: A Promising Approach for Enhancing Spectral Efficiency [Topics in Radio Communications]

Recently, several approaches for increasing spectral efficiency have been proposed to overcome the scarcity of spectral resources and to ensure that the ever-growing demand for high-data rate mobile communications can be met in the future. In this article, we focus on OFDM-based overlay systems that share a frequency band with already existing systems without degrading their performance. This is achieved by temporarily using those parts of the common frequency band that are currently not used by the licensed system. In particular, we focus on design issues for the physical layer as well as the medium access control layer and propose appropriate solutions for enabling coexistence. As a result, our investigations show that OFDM-based overlay systems are a promising approach for enhancing spectral efficiency.

Guidelines for designing OFDM overlay systems

Due to the increasing demand in mobile communications it is necessary to use the available spectrum as efficiently as possible. In this paper we focus on the concept of overlay scenarios where two different systems operate independently in the same frequency band. We describe a general set of characteristic parameters for licensed systems using FDMA (frequency division multiple access). In the context of these licensed systems especially OFDM (orthogonal frequency-division multiplexing) is a suitable technique to operate an additional system in a coexistence mode. Based on the parameter sets of both systems we derive constraints and guidelines on the design of OFDM overlay systems

Coordination of Allocation Measurements in OFDM Based Ad Hoc Overlay Systems

With an increasing demand for mobile communications an efficient usage of the available spectral resources gains importance, since suitable frequency bands are limited. Although nearly all frequency bands are assigned to specified services and users causing a frequency shortage, measurements show that nevertheless there are a lot of unused spectrum holes in the time frequency domain, leading to an overall low spectral efficiency (M.A. McHenry et al., 2006). In this paper we consider an overlay scenario with an OFDM (orthogonal frequency division multiple access) based additional ad hoc overlay system operating in the same frequency band as the licensed system and exploiting the spectrum holes. Since spectrum sensing is a key issue for the coexistence of both systems we propose a method in the MAC (medium access control) layer for frame synchronization, enabling the coordination of allocation measurements. Furthermore, we point out some signal processing aspects regarding overlay systems.

Spectral partitioning for modular software defined radio

Guidelines for designing and operating modular software defined radio (Mod-SDR) devices gain in importance as the interworking of different radio technologies emerges as a potential solution to achieving short time-to-market and to offering flexible services to mobile users. We continue to study the partitioning and scheduling of software modules in multiprocessor devices. In particular, we focus on spectral partitioning, which is known to yield good partitions for general-purpose parallel computing applications. We discuss a spectral partitioning algorithm in the context of Mod-SDR, comparing it to implicit partitioning results produced by a variant of Hus algorithm under realistic operating conditions. Test settings include two identical processors connected through either one or two data buses, and pipelining as a signal processing option for delay-insensitive transmission modes. Furthermore, we show how close both approaches come to the theoretical upper bound on two-processor speedup. Based on computer simulations, we can conclude on an enhanced set of guidelines for operating a modular software defined radio.

OFDM based overlay Systems - Design Challenges and Solutions

With respect to the ever-growing demand for mobile communications on the one hand and spectral scarcity on the other hand it is necessary to increase spectral efficiency. In this paper, we focus on OFDM (Orthogonal Frequency-Division Multiplex) based overlay systems, which share a frequency band with already existing systems by adaptively filling the spectral holes. In order to enable such a co-existence, on the physical (PHY) layer promising methods for minimising mutual interference between the overlay system and the licensed systems are presented. The medium-access control (MAC) layer protocols are designed such as to be able to efficiently signal the available subcarriers as well as to handle the changing available bandwidth. A third issue is the joint optimisation of PHY and MAC parameters which is essential in order to optimally exploit the available spectrum with minimum signalling overhead.

Distributed Detection in OFDM Based Ad Hoc Overlay Systems

Overlay systems are a feasible approach to increase the efficiency of spectrum use. An independent secondary user (SU) system operates in the same frequency band as a primary user (PU) system. In order to avoid mutual interference, an important task of every SU system is to periodically monitor the PU systems allocation. In this paper, we investigate the question how many SUs have to be deployed in the system area of an ad hoc network in order to meet a given network wide detection probability and how this effects the false alarm probability (and thus the performance of the SU system). This is achieved with the help of geometric random graph theory and results from a distributed detection approach. In our investigations we consider a scenario with an infinite system area, and therefore neglecting border effects, as well as a scenario including border effects.

A pipelining approach to operating modular software defined radio

Interworking between different radio technologies emerges as a potential solution to offering flexible services to mobile users. Modular software defined radio (Mod-SDR) devices are key to achieving short time-to-market under these conditions. Hence, guidelines for designing and operating such systems gain in importance. In this paper we continue to study the partitioning and scheduling of software modules on system-on-chip hardware for Mod-SDR. In particular, we focus on an efficient pipelining method, which we have named graph duplication pipelining (GDP). We discuss the GDP approach in the context of Mod-SDR, taking a UTRA FDD 64 kbps uplink transmitter as an example. We compare GDP to both nonpipelined and extremely pipelined signal processing, while observing realistic operating conditions. We show how close these approaches come to the theoretical upper bound on two-processor speedup. Furthermore, we examine the resulting depth of the radio frame pipeline which is important for delay-sensitive applications, where we discuss both circuit-switched and packet-switched services. Based on computer simulations we conclude on guidelines for operating modular software defined radio.