A Software Radio Modem Approach Based on a DSP.

Transcription

1 245 Encuentro de Investigación en Ingeniería Eléctrica Zacatecas, Zac, Marzo 17 18, 2005 A Software Radio Modem Approach Based on a DSP. J. González Villarruel, Profesor Investigador, Escuela de Graduados en Ingeniería y Ciencias del Tecnológico de Monterrey, Departamento de Electrica y Electrónica, Campus Estado de México, Estado de México CP TEL: +(55) , ext. 2479, correo-e: gjavier@itesm.mx D. Santana Gómez, Asistente Investigador, Escuela de Graduados en Ingeniería y Ciencias del Tecnológico de Monterrey, Departamento de Electrica y Electrónica, Campus Estado de México, Estado de México CP TEL: +(55) , ext. 2479, correo-e: dsantana@itesm.mx Resumen Los modems inalámbricos son una parte clave para los dispositivos móviles para adecuar la información al canal. Para adaptar los datos a los diferentes estándares inalámbricos, los radios del futuro necesitarán ser implantados en forma de software. Gracias a las tecnologías DSP e inalámbricas, hoy en día los radio modems con software para 2 y 2.5 G ya han sido implantados [1]. En un Radio-Software, intentamos combinar varios protocolos de comunicación inalámbrica y reducir las etapas de procesamiento de RF. Aquí, hemos analizado el diseño un software para modem sobre un DSP como parte de un demostrador de radio inalámbrico. Abstract --- Wireless modems are key part for mobile devices to adequate the information to the channel. In order to adapt data to different wireless standard, future radios will need to be implemented on software form. Thanks to DSP and wireless technologies, today software modem radios for 2 and 2.5 G have already been implemented [1]. In a Software Radio, we aim to combine several wireless communications protocols and reduce the RF processing stages. Here, we have analyzed the design of a software modem on a DSP as part of a wireless radio demonstrator. Key Words Radio modem, Software radio, wireless comunications. D I. INTRODUCTION ue to the appearance of multiple mobile communications standards, the design and implementation of wireless radios has been orientated to software implementation. In this case, the mobile device is able to switch from one air-interface standard to another. The idea is to use the same device for different services as GPS for location from cellular networks, access to Internet from wireless LANs and access to printer from a Bluetooth air interface. This paper presents a software radio approach based on DSP implementation. This paper will be presented in four main sections, first section is an introduction, second section presents a brief description of the software radio approach, third section describes the

2 246 analysis of a digital modulator, fourth describes the design of a software modem, fifth section presents the software radio challenges, and finally, the conclusions are presented in section seven. II. SOFTWARE RADIO There are three main facts that involve the Software Radio Paradigm [2]: a) Move Analog to Digital conversion as close to the receiving antenna as possible. b) Replace software for hardware processing. c) Facilitates a transition from dedicated to general-purpose hardware. Figure 1, shows a General Software Radio where the analog RF section must be simple and robust, D/A section must be as close as possible to the antenna and be able to processes high dynamic signals with low power consumption, and finally the signal processing implemented in the software section. In this paper we will be focused in the Modulator and Demodulator as part of the software implementation. Figure1. General Software Radio. III. DIGITAL MODULATION TECHNIQUES A modulated signal may be represented by the equation 1. The linearity and non-linearity of the modulation form depends on the relation between the input and the output of the system [3]. v(t) = Re g(t)e jw c { t } v(t) = x(t)cosw c t y(t)sinw c t where : and g(t) = x(t) + jy(t) x(t) = Re{ g(t) }= R(t)cosθ(t) y(t) = Im{ g(t) }= R(t)sinθ(t) (1) Linear Modulation: The input-output relation of modulator satisfies the Principle of Superposition.

3 247 No Linear Modulation: Input-Output relation of modulator does not (partially or fully) satisfies the principle of superposition In other terms, in a linear modulation the data is encoded in amplitude (PAM), phase (PSK) or both (MQAM) with no constant envelope. In Nonlinear Modulations, the data are encoded in frequency (FSK) with constant resulting in a less susceptible to amplitude and phase nonlinearities introduced by the channel and/or hardware [4]. Non-linear modulations are easily to implement but not bandwidth efficiently. Contrary, linear modulations are more difficult to implement and bandwidth efficient. The objective of a Software Radio is to have all these modulations in a single platform based in software programming. IV. SOFTWARE MODEM. A Generalized Software Radio Modulator is shown in Figure 2. Figure2. Software Radio Modulator. The main idea is that all the baseband process is done by a DSP, leaving to the RF stage the smallest processing as possible. The timing and frequency response for a software based FSK (equation 2) modulator obtained from a sine table in memory are showed in Figure 3 and reported in [5]. s 1 (t) = Acos(2πf 1 t + φ) kt t (k + 1)T, for 1 s 2 (t) = Acos(2πf 2 t + φ) kt t (k + 1)T, for 0 (2)

4 248 Figure 3. DSP implemented software FSK. A) Timing response, B) Frequency response. Then from the same table is possible to generate a PSK (equation 3) modulation only changing the polarity of the reading step [5]. Figure 4 shows the result of this technique. s 0 (t) = Acos2πtf c t 0 t T for 1 s 1 (t) = Acos2πtf c t 0 t T for 0 (3) Figure 4. DSP implemented software PSK. Software demodulation process can be done coherently and non-coherently. Coherent detection process receives a signal with a local carrier of same frequency and phase. Non-coherent. Software Radio receivers tendency is to reduce the RF complexity aim the homodyne topology by down converting the desired signal to a low-if and then processed in baseband by a DSP. This is showed in Figure 5. Figure5. Software Receiver In the DSP, both coherent and non-coherent demodulation could be implemented. Figure 6 shows the results of a non-coherent FSK demodulator reported on [5].

5 249 Figure 6. DSP implemented non-coherent FSK demodulator. The tendency is to have a coherent demodulator adapted to various modulations schemes by software. Next the decision algorithms that will improve the performance of the demodulator [6]. These decision algorithms like Integrate and Dump and the feed-forward and feed-back (DFF-DFE) algorithm are easier to implement by software than by hardware [7]. It will be impact in a better performance of the receiver. Besides, depending on the capacity of the DSP other functions like source encoder-decoder, channel encoder-decoder and error coding can be implemented. The idea is to have the complete communication system implemented in software. V. SOFTWARE RADIO CHALLENGES The main Software Radio Challenges is to have a Digital to Analog and Analog to Digital converters suitable for this processing. Since the RF desired signals and spourius require, the higher sampling rate and the higher resolution. In other hand the DSPs evolution must be aimed to the pipeline and frame based processing in order to achieve faster processing. Another challenge is the amplification in the antenna in order to have a linear process. Figure 7 shows the proposed DSP software radio setup. Figure7. DSP software radio setup. VI. CONCLUSIONS Software Radio techniques implies the placement of the Analog to Digital conversion stage closer to the antenna, reducing the RF processing stages. This involves the use of general-purpose processors and the development of software techniques for the different stages in a communication system. In this paper we had analyzed the techniques for the modulation and demodulation functions. Finally it was analyzed the challenges in Software Radio design. The next step in this work is to implement a DSP software radio

6 250 setup considering the RF impairments as non linear amplification and wireless channel impairments as fading, multiple reflections and time spread. REFERENCES [1] Jeffrey H. Reed, Software Radio. A modern approach to radio engineering, Prentice Hall, USA, [2] William Lehr, Fuencisla Merinao, Sharon Gillet, Software Radio:Implications for Wireless Services, Industry Structure, and Public Policy, Massachusetts Institute of Technology USA, [3] Leon W. Couch II, Modern Communication Systems. Principles and Applications, Prentice Hall, [4] Dr. Mike Fitton, Telecommunications Research Lab, Toshiba Research Europe Limited Report.. [5] Daniel Santana, Javier González, Luis González, Andrés García, Digital Modulation DSP Analysis and implementation based on integer k-sampling, presented in ICED 2004, Internacional Conference on Electronic Design, Veracrúz, Veracrúz, México. [6] Daniel Santana, Javier González, Analysis of a software Modem based on a DSP implementation, CONIELECOMP 2005, Congreso Internacional de Electrónica, Comunicaciones y Computadoras, Cholula, Puebla, Accepted IEEE Computer Society Press in the proceedings of the IEEE Computer Society Press [7] Schiphorst Roel, Hoeksema Fokke, Slump Kess, Bluetooth demodulation algorithms and their performance, University of Twente, Department of Electrical Engineer.