Hardware Considerations for Digital Audio Broadcasting System Nariman Moezzi-Madani, Ehsan Rohani and S. Mehdi Fakhraie {n.moezi, e.rohani}@ece.ut.ac.ir, fakhraie@ut.ac.ir Silicon Intelligence and VLSI Signal Processing Laboratory ECE Department, University of Tehran, Tehran, IRAN. Abstract-In recent years, several implementations have been reported for Digital Audio Broadcasting (DAB) systems. Normally, implementation parameters of these systems are extracted from extensive system level simulations to adjust various parameters while maintaining the required performance. In this paper, the bit-true model of a DAB system is extracted and an accurate model simulation for the system is performed to find the word lengths of various parameters to approach the best trade off between performance and hardware cost. Here, the decimation-in-time algorithm for FFT/IFFT and adaptive LMS algorithm for time equalizer is adopted. Index Terms—Digital Audio Broadcasting (DAB), Bit-true, Word-length, OFDM, Equalizer. I. INTRODUCTION The Digital Audio Broadcasting (DAB) system, described in the European Eureka-147 standard [1], offers high-quality audio services, supports multimedia data to mobile reception and is likely to replace the traditional radio systems. Fig. 1 shows the block diagram of a conceptual DAB signal transmitter. The source encoder for the DAB system is the MPEG Audio Layer II encoder with restrictions on some parameters and some additional protection against transmission errors. The MPEG II audio signal and the other data are the input services to DAB transmitter. Each service signal is coded individually at source level in the transmitter, error protected, and then time interleaved in the channel coder. Each service is independently error protected with a coding overhead ranging from about 25% to 300%, the amount of which depends on the requirements of the broadcasters (transmitter coverage and reception quality).Then the services are multiplexed in the Main Service Channel (MSC), according to a predetermined and adjustable, multiplex configuration. Finally, Orthogonal Frequency Division Multiplexing (OFDM) is applied to the signal to shape the DAB signal which consists of a large number of sub-carriers [1]. The OFDM signal generation involves the processes of Quadrature Phase Shift Keying (QPSK) mapping of arrival signals, frequency interleaving, and differentially modulating. When the signal is generated, it is defined in the frequency domain, and is then transformed into its time domain representation by Inverse Fast Fourier Transformer (IFFT). The last complex output values of the IFFT are copied to the front of the symbol to add the guard interval (cyclic prefix). The guard interval is used to avoid inter symbol interference (ISI). The generated OFDM signal is amplified and sent to the RF block. The transmission frame consists of a sequence of three groups of OFDM symbols: synchronization channel symbols, Fast Information Channel (FIC) symbols and Main Service Channel (MSC) symbols. The synchronization channel symbols are comprised of the null symbol and the phase reference symbol [1]. The DAB system operates in 4 modes, and each mode has its specific characteristics, as shown in table I. Audio Encoder Channel Coder Channel Coder MUX OFDM Mod. & Freq. Inter. Transmitter Time- Interleaving Time Interleaving Audio Services Data Services Packet Mux. Channel Coder Channel Coder Time- Interleaving Time Interleaving Trans- mission Frame Mux. Channel Coder Channel Coder FIC Block Assmbler FIC Block Assmbler Service Information & Multiplex Information MSC Fig. 1: Conceptual block diagram of DAB transmitter [1]. TABLE I Different operation modes for DAB system [1]. System Parameter I II III IV Frame Duration 96 ms 24ms 24ms 48ms Null Symbol Duration 1297us 324us 168us 648us Guard Interval Duration 246us 62us 31us 123us Transmitter Separation 96km 24km 12km 48km Frequency Range 375MHz 1.5GHz 3GHz 1.5GHz Useful Sym. Duration 1ms 250us 125us 500us Total Symbol Duration 1246us 312us 156us 623us No. of Sub-Carriers 1536 384 192 768 As shown in Table I, DAB system works in three frequency modes: 375 MHz, 1.5 and 3 GHz. For the nominal frequency ranges, the transmission modes have been designed to suffer neither from Doppler spread nor from delay spread, while both of these exist in mobile reception with multipath propagation. In our simulations, we have considered mode I that system works in 375 MHz frequency band. This mode is most suitable for a terrestrial Single-Frequency Network in the VHF range, because it allows the greatest transmitter separation (96 Km) [1]. The spectrum of the signal is approximately rectangular, Gaussian noise-like, and occupies a bandwidth of 1.536 MHz. 0-7803-9262-0/05/$20.00©2005 IEEE 209