A DSP-Based Impulsive Noise Generator for Test ... - Semantic Scholar

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 54, NO. 6, DECEMBER 2007

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A DSP-Based Impulsive Noise Generator for Test Applications Ramón Martínez Rodríguez-Osorio, Member, IEEE, Leandro de Haro Ariet, Member, IEEE, Álvaro D. Castro Urbina, and Miguel Calvo Ramón, Member, IEEE

Abstract—Impulsive noise is known to be one of the most damaging types of wideband interference present in wireless communications systems, such as Universal Mobile Telecommunications Systems, Bluetooth, and Digital Video Broadcasting–Terrestrial. In order to quantify how impulsive noise degrades the quality of service (QoS) of a particular system and to evaluate the electromagnetic compatibility (EMC) requirements of a practical receiver under realistic impulsive noise conditions, it is necessary to use a flexible and modular impulsive noise generator. The primary objective of this paper is to present a low-complexity and highly flexible impulsive noise generator whose software module is implemented in a digital signal processor. In contrast to existing commercial equipment, the software implementation preserves the statistical characteristics of the measured impulsive noise parameters. In the proposed implementation, the generator presents two levels of flexibility that make it especially suitable for performing QoS tests and EMC conformance testing in realistic impulsive noise scenarios. First, the application is independent of the noise source under study. Second, the modularity of the testbed makes the generation of impulsive interference in different frequency bands possible. Experimental results confirm the feasibility and achievable performance of the proposed testbed. Index Terms—Digital signal processor (DSP), electromagnetic compatibility (EMC), impulsive noise, non-Gaussian noise, quality of service (QoS), testbed.

I. I NTRODUCTION

A

WIRELESS communications system may experience several kinds of noise, and the effect of each on system performance can be quite different. Thermal noise, which is Gaussian in nature and arises from the receiver, degrades system performance when the signal-to-noise ratio is below a predefined threshold [1], [2]. Impulsive noise, which is commonly referred to as shot or man-made noise, is artificial in nature and can be produced by a wide variety of sources such as power lines, electronic equipment, cars, and motorcycles [3]–[5]. Digital radio communications systems may be very sensitive to impulsive noise; this noise may produce synchronism or connection loosening, or may reduce system capacity as in code-division multiple-access systems. However, compar-

Manuscript received April 5, 2006; revised May 23, 2007. The authors are with Escuela Técnica Superior De Ingenieros De Telecomunicación, Departamento de Señales, Sistemas y Radiocomunicaciones, Universidad Politécnica de Madrid, 28040 Madrid, Spain (e-mail: ramon@ gr.ssr.upm.es). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIE.2007.903956

atively few research studies have investigated quality of service (QoS) degradation due to impulsive noise. Impulsive noise, in contrast to Gaussian noise, appears as bursts of peak signals with high amplitude and short duration. Because of its wideband nature, impulsive interference may significantly degrade the QoS offered by new broadband third-generation communication systems [6], Bluetooth [7], and Digital Video Broadcasting-Terrestrial (DVB-T) [8]. However, many practical receivers have not been tested for adherence to electromagnetic compatibility (EMC) requirements under realistic operational conditions because generators for realistic impulsive noise interference have not been available. Due to the diversity of man-made noise sources, extensive measurement campaigns have been carried out with the aim of deriving sophisticated statistical impulsive noise models in different frequency bands and environments [3], [8]–[11]. However, these models have not been used to study the performance degradation caused by realistic impulsive noise interference. Research on the degradation caused by impulsive noise has traditionally been done through simulation, providing results in terms of bit error rate [6], [9] or system throughput [7]. However, the study of QoS degradation for a particular application under realistic impulsive noise conditions makes the implementation of some kind of testbed or channel emulator mandatory. The aim of a digital signal processor (DSP)-based testbed is to provide a flexible platform, which is able to produce actual impulsive noise with different characteristics over a wide range of radio frequencies (RFs). However, and to the authors’ knowledge, available commercial impulsive noise generators for QoS testing are either focused on audio applications [12] or designed for a specific frequency range or system [13]. In addition to flexibility, the testbed architecture must be sufficiently modular to enable one to generate impulsive noise over a wide range of RFs. The contribution of this paper is the proposal of a flexible and modular DSP-based implementation of an impulsive noise generator. The prototype overcomes previous limitations by introducing two levels of flexibility, i.e., in the software and hardware sections. The impulsive noise generator can be used for different applications, such as the analysis of QoS or the realization of EMC acceptance tests in the presence of impulsive interference. The proposed system can also be used as a channel emulator for hardware-based simulations. This paper is organized as follows: In Section II, the methodology used for generating impulsive noise signals from measurement data is presented. Section III describes the

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implementation aspects of the testbed. Experimental results are presented in Section IV in order to show the feasibility of the prototype, and conclusions are provided in Section V. II. S YNTHESIS OF I MPULSIVE N OISE S IGNALS W ITH A PPROPRIATE S TATISTICAL D ISTRIBUTIONS One of the primary challenges in developing the prototype is to generate impulsive noise signals whose characteristics comply with the measured statistical distributions. Simple mathematical models do not represent measured impulsive noise well, while complex models preclude the noise generator from operating in real time. In contrast to the deterministic pure impulsive noise model used in [6], the measured impulsive noise must be characterized by three random variables, i.e., pulse amplitude ai , pulse duration wi , and temporal separation between pulses ti . Therefore, the impulsive noise signal envelope r(t) can be represented as a summation of pulses, i.e.,
ai gwi (t) ∗ δ(t − ti ) (1) r(t) = i

where gwi (t) represents the pulse waveform with duration wi , and δ(t) is the unit impulse function. In order to generate a consistent impulsive noise signal, an adequate characterization of the three parameters is required. Although theoretical probability density functions are commonly applied for statistical characterization (Gamma, Rayleigh, Beta, etc.), results rarely fit the measured time series [8]. We overcome this difficulty by extracting the probability distribution functions for the three parameters from experimental data. In particular, we make use of impulsive noise measured in urban environments in the Universal Mobile Telecommunications Systems (UMTS) band [3]. Following this methodology, the measured pulse amplitude, pulse duration, and pulse interval distribution functions are approximated through piecewise-linear functions, obtaining the so-called empirical distributions for the three parameters [14]. Fig. 1 compares the empirical and measured distribution functions for pulse amplitude and duration. Finally, the required random variables for {ai , wi , ti } are generated by applying the inverse transform method to the empirical distributions [14]. The values are stored in three different files that are subsequently transferred to the DSP application. III. I MPLEMENTATION As shown in Fig. 2, the testbed implementation consists of the following modules: 1) a host PC (for generating impulsive noise samples and controlling the DSP application); 2) a DSP platform for generating the digital impulsive noise samples [implemented with a floating-point DSP starter kit (DSK)];1 3) a digital-to-analog (D/A) converter evaluation board for generating the baseband noise waveforms; and 4) the RF front end, which consists of an RF modulator connected to a voltagecontrolled oscillator (VCO) tuned to the corresponding carrier 1 Texas

Instruments, TMS30C6711 DSK http://www.ti.com.

Fig. 1. Measured and approximated cumulative distribution functions for (a) pulse amplitude and (b) pulse duration (measurements taken from [3]).

frequency. In order to carry out QoS tests and EMC measurements on a wireless communication system, an antenna must be connected to the RF output of the modulator board. The measurement setup for analyzing the synthesized impulsive noise is shown in dotted lines. A DSP-based implementation has a number of advantages in terms of cost and flexibility. A DSP allows the resolution of complex mathematical and feedback problems in real time. Industrial applications based on DSP architectures using TMS30C6711 can be found for resolving nonlinear problems in network controllers for robotics [15] and also for controlling the operation of nonlinear dc–dc converters in the presence of disturbances [16]. The host PC is used to control the operation of the system and to store the noise pulse information. Before transferring these data to the DSP application, amplitude and temporal parameters are adapted to the requirements of the hardware modules. In

RODRÍGUEZ-OSORIO et al.: DSP-BASED IMPULSIVE NOISE GENERATOR FOR TEST APPLICATIONS

Fig. 2.

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Block diagram of the impulsive noise generation testbed (measurement setup in dotted lines).

particular, pulse amplitudes ranging from 0 to 0.35 mV must be adapted to the 12-bit resolution of the D/A converter. Baseband in-phase and quadrature components have been sampled at 20 Msa/s (megasamples per second), so the temporal resolution of the measurement system is 0.05 µs [3]. However, the temporal resolution of the generator imposed by the DSP platform is around 1 µs (as described in Section V). Therefore, the values for duration and separation between pulses must be adapted to the features of the prototype. This practical limitation will impose a reduction in the bandwidth of the generated impulsive noise signal. The primary objective of the DSP section is to generate a digital representation of the processed impulsive noise sample and to transfer this digital sample to the input of the D/A converter in an appropriate format. Two tasks must be completed to achieve this objective: first, the amplitude and temporal characteristics of the impulsive noise signal must be read from the input data files; and second, digital samples must be addressed and transferred to the corresponding data bus during the appropriate time intervals. The method for generating the train of noise pulses makes use of interrupts. Using available software utilities, a periodic hardware interrupt (HWI) mechanism is used to control the time between pulses without the need to write “zeros” in the data bus when there is no pulse present. In this way, the memory requirements of the system are significantly reduced. HWIs occur periodically, with a minimum period Tint of 10 µs. The value for Tint is imposed by two factors: 1) the hostto-DSK data transfer speed through the parallel port interface and 2) the fact that the DSP is not capable of executing any instruction if a lower interrupt period is used (i.e., the DSP is blocked). It is important to emphasize that the DSP application is independent of the impulsive noise characteristics. This implies that for generating impulsive interference of different characteristics, only the three input data files must be replaced in the host PC; there is no need to modify the software implementation. The DSK and the D/A converter board are connected using a printed circuit board interface board. Data, clock (CLK), and output enable (OE) signals are transferred from the DSP to control the D/A converter. The CLK frequency is 100 MHz. The OE is activated each time a new pulse is generated.

Fig. 3. Laboratory impulsive noise generation testbed.

The final stage of the prototype corresponds to the RF front end. A direct I/Q modulation scheme for the 2.4-GHz Industrial, Scientific and Medical band has been used. Evaluation kits for both the direct I/Q modulator with variable gain amplifiers and the VCO have been utilized.2 As a result of its modular architecture, only the RF front end (I/Q modulator and VCO) needs to be replaced for the generation of impulsive interference in any other RF band. IV. E XPERIMENTAL R ESULTS The laboratory setup is shown in Fig. 3. Two sets of performance measurements have been carried out. The first set of measurements was carried out by generating a periodic train of square pulses with the minimum available pulse duration without interruption threads. Due to hardware constraints, the minimum pulse duration that can be processed by the generator is 0.85 µs. In Fig. 4, two snapshots of the baseband impulsive noise signals are shown. Note that the synthesized signal has a significant impulsive characteristic. The waveforms also change from one snapshot to another due to the random and aperiodic nature of the impulsive noise signal. 2 Maxim IC, part numbers MAX2721 and MAX2750, http://www.maximic.com.

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Fig. 4. Baseband impulsive noise signals captured from the oscilloscope display. (a) 200 µs/div. (b) 50 µs/div.

data. This technique has been used to implement a real-time low-complexity and highly flexible DSP-based testbed for the generation of RF impulsive noise. The flexible and modular architecture of the implementation overcomes the constraints imposed by commercial impulsive noise generators and allows the synthesis of a wide variety of impulsive noise signals in different frequency bands. Furthermore, the software module allows for the generation of realistic impulsive noise, as it is based on the statistical properties of measured impulsive noise interference. The hardware components of the impulsive noise generator are commercially available, and only the interface board for connecting DSP and D/A converter boards has been custom designed. Experimental results have been validated by analyzing the RF spectrum of the synthesized modulated impulsive noise signal, which confirms that its characteristics conform to the synthesized impulsive interference. However, the prototype presents some limitations in performance because the minimum pulse duration that can be achieved with the DSP platform is 0.85 µs, whereas in a real environment, the pulse duration can be much shorter. This fact limits the bandwidth of the synthesized signal to about 1 MHz. The use of more powerful DSP or field-programmable gate array platforms and programming instructions in assembly language are potential solutions for increasing the bandwidth of the generated signal. Finally, test campaigns for evaluating the QoS degradation and EMC requirements under impulsive noise conditions will be carried out using the testbed as the impulsive interference source. As a result of its flexible and modular architecture, QoS tests and EMC measurements of practical receivers for UMTS, Bluetooth, or DVB-T can be performed using the same platform for the generation of impulsive noise interference. ACKNOWLEDGMENT The authors would like to thank Prof. M. G. Sánchez and Prof. I. C. Gómez of Universidad de Vigo for their help in the adaptation of the impulsive noise measurements to the adequate format. R EFERENCES

Fig. 5. RF spectrum of the modulated impulsive noise signal.

The spectrum of the modulated impulsive noise signal has also been analyzed. Fig. 5 shows a snapshot of the modulated signal from the spectrum analyzer display. Note that the spectrum reaches its maximum value in the carrier frequency and then decays toward the extremes of the band. V. C ONCLUSION We have presented a novel technique, which is both simple and reliable (as compared with actual interference), for synthesizing the impulsive noise interference from measured

[1] J. D. Parsons, The Mobile Radio Propagation Channel, 1st ed. London, U. K.: Pentech, 1992. [2] D. Middleton, “Statistical–physical models of electromagnetic interference,” IEEE Trans. Electromagn. Compat., vol. EMC-19, no. 3, pp. 106– 126, Aug. 1977. [3] M. G. Sánchez, I. Cuiñas, and A. Vázquez, “Shot noise in actual urban and industrial environments,” Microw. Opt. Technol. Lett., vol. 34, no. 2, pp. 112–115, Jul. 2002. [4] W. R. Lauber and J. M. Bertrand, “Statistics of motor ignition noise at VHF/UHF,” IEEE Trans. Electromagn. Compat., vol. 41, no. 3, pp. 257– 259, Aug. 1999. [5] E. N. Skomal, Man-Made Radio Noise. New York: Van Nostrand Reinhold, 1978. [6] E. Kudoh and F. Adachi, “Analysis of DS-CDMA transmission performance in the presence of pure impulsive noise over frequency selective fading,” IEICE Trans. Commun., vol. E85-B, no. 11, pp. 2395–2404, Nov. 2002. [7] D.-G. Kim, J.-S. Roh, S.-G. Cho, and J.-S. Kim, “Effects of impulsive noise and self co-channel interference on the Bluetooth scatternet,” IEICE Trans. Commun., vol. E85-B, no. 10, pp. 2002–2198, Oct. 2002. [8] M. G. Sánchez, L. de Haro, M. C. Ramon, A. Mansilla, C. M. Ortega, and D. Oliver, “Impulsive noise measurements and characterization in a UHF

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Ramón Martínez Rodríguez-Osorio (A’04–M’04) was born in Madrid, Spain, in 1975. He received the Ingeniero de Telecomunicación degree and the Ph.D. degree in electrical engineering from Universidad Politécnica de Madrid, Madrid, in 1999 and 2004, respectively. He is currently an Assistant Professor and works with the Grupo de Radiación, Universidad Politécnica de Madrid. He has published several papers in international conferences and journals, and has coauthored three books on simulation of communication systems. He has worked in satellite communications, antenna design, and powerline communications (PLC) systems. He has participated in a number of European Projects of FP6 and has an active participation in the Smart Antennas activity within ACE and also in the workpackage of field trials and integration of PLC systems within the OPERA project. His main research areas are smart antennas for mobile communication systems and the implementation of software-defined radio prototypes.

Leandro de Haro Ariet (S’90–M’90) received the Ingeniero de Telecomunicación degree and the Doctor Ingeniero de Telecomunicación degree (Apto cum laude) from Escuela Técnica Superior De Ingenieros De Telecomunicación (E.T.S.I. Telecomunicación), Universidad Politécnica de Madrid (UPM), Madrid, Spain, in 1986 and 1992, respectively. Since 1990, he has developed his professional career with E.T.S.I. Telecomunicación, UPM, as Profesor Titular de Universidad in the signal theory and communications area. He has been actively involved in several official projects and with private companies (national and international). He has also been involved in several European projects (RACE, ACTS, COST). The results of his research activity may be found in several presentations in national and international conferences as in published papers. His research activity cover the following topics: 1) antenna design for satellite communications (earth stations and satellite on board); 2) study and design of satellite communication systems; and 3) study and design of digital TV communication systems.

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Álvaro D. Castro Urbina was born in 1975. He received the Ingeniero de Telecomunicación degree from Universidad Politécnica de Madrid, Madrid, Spain, in 2003. During 2002 and 2003, he was a Hardware Engineer with the Grupo de Radiación, Departamento de Señales, Sistemas y Radiocomunicaciones, Universidad Politécnica de Madrid. He is currently with Escuela Técnica Superior De Ingenieros De Telecomunicación, Universidad Politécnica de Madrid. He has worked in the areas of digital signal processing, DSP programming, and mobile communication systems. Mr. Castro Urbina received the prize for the best Ms.C. thesis in UMTS systems from Telefónica Móviles in 2003.

Miguel Calvo Ramón (S’71–M’82) was born in Pueyo de Jaca, Huesca, Spain, on June 10, 1949. He received the Ingeniero de Telecomunicación degree and the Doctor Ingeniero de Telecomunicación degree from Escuela Técnica Superior De Ingenieros De Telecomunicación (E.T.S.I. Telecomunicación), Universidad Politécnica de Madrid (UPM), Madrid, Spain, in 1974 and 1979, respectively. Since 1974, he has been with the UPM, where he has worked in a number of projects related with numerical methods in electromagnetics, electromagnetic compatibility, and antennas for satellite communications. In 1983, he was a Research Visitor with Queen Mary College, London University, London, U.K. Since 1986, he has been a Catedrático (Full Professor) with the Señales Sistemas y Radiocomunicaciones (Signals, Systems, and Radiocommunications), UPM. From 1988 to 1989, he worked in the coordination procedures of the Spanish HISPASAT satellite system with INTELSAT. In 1993, he was a Technical Visitor with Nichols Centre, Kansas University, Lawrence. He has also been a part-time Technical Director with the Space Division, RYMSA. He has been involved in projects related to communication systems such as the development of a testbed simulator for the HISPASAT satellite system services, modeling of a satellite link for EUREKA-95, and development of a traffic simulator for interactive services in ACTS DIGISAT. He has also worked as an Auditor for the ACTS Programme, and has been Evaluator of Proposals in the framework of the IST (Information Society Technologies) program. He has coauthored a number of papers in technical reviews and contributed in a number of international conferences. He wrote a chapter in the book Reflector and Lens Antennas entitled “Analysis and Design Using Personal Computers” (Artech House, 1988).