Using Modern Design Tools To Evaluate Complex Communication Systems: A Case Study on QAM, FSK and OFDM Transceiver Design SOTIRIS H. KARABETSOS, SPYROS H. EVAGGELATOS, SOFIA E. KONTAKI, EVAGGELOS C. PICASIS, GIANNIS K. KARAMPETSOS, and ATHANASE A. NASSIOPOULOS. RDTL Laboratory Department of Electronics Technological Educational Institute (TEI) of Athens Egaleo, Athens GREECE Abstract: - This paper describes and demonstrates the use of modern design tools on the design and implementation of contemporary communication systems for educational or research purposes. Specifically, a case study on the design, simulation and implementation of QAM (Quadrature Amplitude Modulation), FSK (Frequency Shift Keying) and OFDM (Orthogonal Frequency Division Multiplexing) transceivers is presented. This is achieved through the integration of Matlab/Simulink tools with Texas Instrument DSP (Digital Signal Processors) development kits. An approach like this, recommends a fully compatible and flexible design environment, which can be easily adopted for educational purposes. Key-Words: - QAM, FSK, OFDM, System Level Design, Communication Systems, DSP. 1 Introduction The study, design and prototyping of contemporary communication systems involves several and often very complicated tasks that have to be solved. Specifically, definition of specifications and requirements, algorithmic design, system level design, simulation, implementation and finally verification are some of the steps that have to be followed. An example of such modern systems is the adopted standards for wireless local area networks (WLAN) and digital video broadcasting (DVB-T) [1][2]. Consequently, this methodology has to be entailed in the educational process so as students to be able to appreciate the whole concept, besides studying the fundamental principles. Of course, it is apparent that such complicated projects, demand the use of specialized and sophisticated hardware and software equipment. On the other hand, advanced software packages and hardware platforms allow the prospect of the before mentioned approach to be feasible. This work demonstrates the system level design, simulation and implementation of a QAM, an FSK and an OFDM transceiver, with the aid of such modern tools. The design and simulation was made using Matlab/Simulink from Mathworks Inc. while the implementation is done using the TMS320C6713 DSK development kit from Texas Instrument Inc. The cooperation of Matlab IDE and Code Composer IDE, allows for direct download of Simulink models to DSP code. It is interesting to note the importance of the above combinatorial use, since it facilitates for a real-life implementation rather than simulation only [3]. Additionally, an approach like this, sets the scene for the implementation of a complete experimental framework, where students or researchers can study and experiment on the whole range of digital modulations or communications in general. The paper is organized as follows. Section 2, presents the design of baseband QAM, FSK and OFDM transceiver. The general structure regarding several requirements is given while specific tasks and specifications are discussed and analyzed. In section 3, experimental results concerning the evaluation of the implemented systems, are presented. Finally, section 4, deals with conclusions and further work. 2 QAM, FSK and OFDM system level design
imaginary part (Q). Then, these two signals are used as input to an I-Q modulator whose carrier Fig. 1: General communication system design framework. The purpose of the presented designs is a pedagogical approach to previously mentioned complicated issues, using state of the art design tools. Model development is parameterized in such a way to easily study and correlate the results for either educational or research purposes. Based on that, the models consist of a combination of either integrated software blocks or newly created ones. The general structure of the experimental framework is depicted in Fig. 1. The framework consists of the following basic parts: a) two personal computers which are being used in order to transmit and receive data, respectively, b) two DSP boards that invoke the communication between the computers and c) an oscilloscope for signal observation. Additionally, the implemented systems are capable of transmitting raw text data, audio files and pictures. Moreover, we notice that due to hardware limitations and personal computer compatibility the operating frequency range of practical implementations is bounded by a sampling frequency of Fs=44.1KHz. 2.1 QAM transceiver Fig. 2 illustrates a 4-QAM-transceiver model that consists of the following building blocks: a) RTDX (Real Time Data exchange) input channels used as data generators, b) QAM modulator, c) QAM demodulator, d) Decoding level and e) DAC or RTDX output channel for data recovery. Initially, coded text is received via two RTDX input channels. Prior to this, ASCII to QAM symbols conversion is done. The first input channel (ichan1) receives the real part (I) and the other (ichan2) the Fig 2: QAM Transmitter, QAM Receiver frequency is set to 10 khz. At the receiver, coherent demodulation is used using two sixth order Butterworth IIR lowpass filters. Moreover, attenuation and delay due to the presence of the filters is confronted using amplification and rate transition respectively. Decoding follows demodulation and the resulted signal either enters the RTDX output channel for data recovery on a Personal Computer, or a DAC for signal observation via oscilloscope. 2.2 FSK transceiver Fig. 3, depicts the design of an FSK transceiver. The RTDX interface is providing the data that are about to be transmitted. The transmitter is illustrated in Fig. 3. A VCO (Voltage Control Oscillator) is responsible for FSK modulation. The DSP boards are wired connected, meaning that a cable is the communication channel of the system. Demodulation is illustrated in Fig. 3. A closed loop that consists of the VCO and the lowpass filter
represents the phase-locked loop (PLL), which is responsible for correct demodulation. From these demands, we specify the OFDM symbol duration, thus the spacing of OFDM subcarriers. TMS320C6713 DSK board DAC output Oscilloscope or/and Spectrum Analyzer TMS320C6713 DSK board ADC input Personal Computer (PC) Fig 4: a) OFDM transmitter architecture, b) OFDM transceiver signal processing. Fig 3: FSK Trans mitter, FSK Receiver 2.3 OFDM transceiver The general structure of the implemented OFDM transceiver is depicted in Fig. 4. The system level design was made using MATLAB and SIMULINK. The implementation is done using the TMS320C6713 DSK development kit. The model consists of the following basic building blocks: a) Data generator as the bit stream source, b) QAM/PSK modulator for mapping bits to QAM/PSK values, c) IFFT for OFDM modulation, d) Cyclic prefix insertion and e) DAC for digital to analog conversion. In the current stage, the transceiver is adjusted to process text as binary source data. The receiver consists of the reversed operation blocks along with synchronization and channel compensation blocks. The main design of an OFDM system involves specifying several parameters regarding certain requirements, set up by the propagation environment and the quality of service. Such requirements are the available bandwidth, the required bit rate and the tolerable delay spread. Further details can be found at [4][7]. The specifications for our model are summarized in Table 1. As we see some of them are tunable, providing flexibility for further development. Furthermore, the structure of an OFDM frame involves a preamble pilot OFDM symbol, appended at the start of the data OFDM symbols. This known symbol is used for OFDM symbol time detection and channel estimation. Experimentation revealed that such a symbol is sufficient for a wired static communication channel. The number of data OFDM symbols that constitute a frame is also tunable having a typical value between 400 to 500 OFDM symbols. Additionally, pilot subcarriers are embedded within the OFDM data symbols, providing the means for correct de-rotation of altered subcarriers due to synchronization Figure 5: Modulated and demodulated QAM signal Demodulated real and imaginary part
Figure 6: FSK modulation and demodulation impairments. The resulted output OFDM signal from the TMS320C6713 DSK board is observed by means of an oscilloscope and spectrum analyzer and stored to either a personal computer or a second DSP board for demodulation. The latter involves: a) detection of an incoming OFDM frame, b) estimation of OFDM symbol time, c) channel estimation, d) sampling frequency clock offset compensation, e) FFT demodulation and f) raw text data recovery. Figure 7: Case of OFDM unmodulated subcarrier generation, 4 subcarriers (not adjacent), 15 adjacent subcarriers. PARAMETER Data rates Modulation Number of subcarriers Sampling frequency (Fs) FFT/IFFT size for 44.1Ksps (N) OFDM useful symbol duration (IFFT/FFT interval) VALUE Tunable On-Off Keying and QPSK Tunable 44.1 KHz (Ksps) 128 N Fs 128 44100 2.9m se Figure 8: Case of OFDM On-Off Keying modulation, data stream = [1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0], data stream = [1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1]. Guard interval (T CP ) Tunable Subcarrier spacing Fs 44100 344.5Hz N 128 Signal bandwidth Tunable OFDM symbol 2.9msec+ T CP duration Table 1: OFDM system specifications. Figure 9: Case of OFDM QPSK modulation, data stream = [0 1 2 3 0 1 2 3 0 1 2 3 0 1 2 3], random data. 3 Experimental evaluation The following sections provide some indicative experimental results, concerning the evaluation of the described communication systems. Due to limited space only real measurements illustrations are given while simulation results are also available. 3.1 QAM transceiver Fig. 5 depicts an indicative instance of QAM modulation resulting from raw text transmission. The figure also depicts the real part (I) of the demodulated QAM symbol as well. Since the oscilloscope has two channels it is obvious that we cannot represent both real and imaginary part along with the modulated signal in the same graph.
Demodulation of real and imaginary parts is illustrated in Fig 5. Each pulse represents a letter of a word, which is cyclic repeated. Moreover, the transceiver s design approach allows for students to choose which multilevel (M-ary) QAM scheme is going to use. 3.2 FSK transceiver FSK modulation and demodulation from a periodic pulse signal that derives from the RTDX interface (simulating real data) channel is depicted in Fig. 6. The pulse was chosen to be periodic in order to illustrate carrier frequency changes. 3.3 OFDM transceiver Fig. 7 and depicts the case of 4 not adjacent and 15 adjacent unmodulated OFDM subcarrier generation respectively, while Fig. 8 and provides the modulation results when different data patterns of On-Off keying modulation per subcarrier is used. Additionally, besides the corresponding spectrum for each case, the In-phase (I) and Quadrature (Q) for the time domain OFDM signal is also given. Furthermore, Fig. 9 shows the case of QPSK modulation per subcarrier when a cyclically repeated data (QPSK symbol) pattern is used, while Fig. 9 depicts the OFDM signal and spectrum for random (e.g. text) data transmission. The number of subcarriers was chosen to be 16 for illustrative purposes. From the oscilloscope measurements we see that that the design specifications, as they are defined from Table 1, are exactly met. Education and Initial Vocational Training (O.P. Education) under the action: 2.2.2. Reformation of Undergraduate Studies Programs. References: [1] IEEE Std. 802.11a/D7.0-1999, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: High-speed Physical Layer in the 5GHz Band, IEEE 1999. [2] ETSI EN300-744 v1.2.1, Digital Video Broadcasting (DVB): Framing Structure, channel coding and modulation for digital terrestrial television, ETSI 1999-01. [3] The Mathwotks Inc., Link For Code Composer Studio Development Tools, User s Guide, Version 1. [4] R., Van Nee and R., Prasad, OFDM for wireless multimedia communications,, Artech House Publishers, 2000. [5] Juha Heiskala and John Terry, OFDM Wireless LANs: A theoretical and Practical Guide, Sams Publishing, 2002. [6] M., Engels, Wireless OFDM systems: How to make them work, Kluwer International Series In Engineering and Computer Science, Vol. 692, 2002. [7] S. Karabetsos, E.N. Zois, and A. Nassiopoulos, Baseband System Level Design, Simulation and Implementation Of An OFDM Transmitter Using The TMS320C6713 DSK, in Proc. of EDERS 2004 (The European DSP Education & Research Symposium), November 2004. 4 Conclusions In this paper we have presented an educational approach to the design, implementation and evaluation of contemporary communication systems. This was achieved through the design and implementation demonstration of complicated communication systems such as QAM, FSK and OFDM transceivers. In overall, we have shown that the use of modern design tools results a dynamic approach that can easily serve in the educational process. Future work involves the implementation of a complete experimental framework for studying and prototyping of communication systems. Acknowledgments This work and its dissemination efforts have been funded by the Greek Operational Programme for