A Novel On-Channel Repeater for Terrestrial-Digital Multimedia Broadcasting System of Korea

Transcription

1 A Novel On-Channel Repeater for Terrestrial-Digital Multimedia Broadcasting System of Korea Sung Ik Park, Heung Mook Kim, So Ra Park, Yong-Tae Lee, and Jong Soo Lim Broadcasting Research Group Electronics and Telecommunications Research Institute Daejeon, Korea Abstract This paper presents an equalization on-channel repeater (OCR) for terrestrial-digital multimedia broadcasting (T-DMB) system of Korea. The proposed OCR uses a timedomain equalizer based on adaptive complex FIR filtering to remove a feedback signal due to incomplete antenna isolation and multipath signal existing between a main transmitter and the OCR. This makes it possible to emit a high-power errorcorrected signal from the OCR. Keywords-T-DMB;;OCR; Equalization I. INTRODUCTION In the terrestrial digital multimedia broadcasting (T-DMB) system, the mobile broadcasting standard in South Korea based on the Eureka-147 digital audio broadcasting (DAB) system [1], the orthogonal frequency division multiplexing (OFDM) has been adopted for signal transmission [2]. The T-DMB focuses on the broadcasting of moving pictures and their reception in harsh conditions such as places surrounded by high-rise buildings and highways where vehicles are moving at very high speed. Especially, for stable reception of the T-DMB in mobile condition, designing single frequency network () is required as well as maintaining certain level of channel power. In general broadcasting systems, s can be implemented with distributed transmitters (DTxTs) that use the same frequency among a number of transmitters by using global positioning system (GPS), and/or with on-channel repeaters (OCRs) that use the same frequency between transmitters and repeaters. In T-DMB networks, while the DTxTs can transmit high power signals with good quality, distance between the DTxTs is restricted by the length of guard interval of the OFDM system and the cost for their setup and maintenance is relatively high. In case of the OCRs, the installation and maintenance can be done efficiently, however, the transmitting power is limited by a feedback signal due to imperfect antenna isolation and the quality of transmitting signal is not reliable. As one of the core technologies to implement s, an OCR system enables the efficient use of frequencies, but system delay in the OCR can adversely affect performance of T-DMB receivers. In general, the permitted system delay in an OCR is three tenths of guard interval, which is approximately 72 microseconds [3]. In the terrestrial advanced television systems committee (ATSC) DTV system, based on single carrier, the equalization This work was supported by the IT R&D program of MIC/IITA. [2006- S006-01, Development of On-Channel RF repeating technology based on OFDM modulation] digital on-channel repeater (EDOCR), which overcomes the disadvantages of the conventional OCR, has been proposed and is currently operating in practice [4][5]. The digital part of the EDOCR consists of demodulation, equalization, and remodulation, and each part was designed to minimize system delay while reducing performance degradation. In the T-DMB system, based on multi-carrier, such demodulation, equalization, and re-modulation cannot be used since the OFDM system uses the fast Fourier transform (FFT) for demodulation and the inverse FFT (IFFT) for re-modulation, which cause milliseconds unit of system delay. This paper considers technical requirements of the OCRs to broadcast T-DMB signals in environment and proposes the configuration of the equalization OCR that meets such requirements. The proposed equalization OCR is analyzed by computer simulations, and it is also verified by laboratory tests. II. AND OCR FOR T-DMB SYSTEM As shown in Figure 1, the for T-DMB systems can be implemented by DTxTs and OCRs. The DTxTs receive T- DMB signals from broadcaster through studio to transmitter link (STL) and transmit with high power to provide stable signals to service areas. In order to design using DTxTs, the following matters should be considered. Since the guard interval of a T-DMB system is about 246 us, the maximum distance between two transmitters is about 74 Km. In the area where transmitting signals from multiple DTxTs are overlapped, the transmitting time of the DTxTs can be adjusted to minimize performance degradation of T-DMB receivers. The OCRs are installed to extend service coverage and fill gaps in the areas where broadcasting signals cannot reach such as tunnel, basement, and shielded areas, and the areas where DTxTs cannot be installed due to the distance limitation among DTxTs or economical inefficiency. The installed OCR receives T-DMB signals from a DTxT and re-transmits them on the same frequency as the received signals. The well-designed OCRs should meet the following requirements. The OCRs should maintain frequency synchronization between the received and transmitted signals. Any /08/$ IEEE 1826

2 divergence in frequency will cause the apparent echoes to have the characteristics of a Doppler shift. Such Doppler shifts place additional burdens on receivers. The OCRs should remove a feedback signal caused by low isolation between transmitting and receiving antennas. If there is not enough antenna isolation, the feedback signal from transmitting antenna may interfere a received signal. Such feedback signal can result in an oscillation of the power amplifier in the OCRs. The OCRs should have good quality of a transmitting signal. Although the received signal is distorted by multipath signal between a main transmitter and repeater, the OCRs have to be able to effectively recover the distortion of the received signal. If such distortion is not removed properly, the re-transmitting signal still remains to be distorted, resulting in the coverage reduction. The processing delay of the OCRs should be as short as possible. If the processing delay of the OCRs is longer than the guard interval, it causes pre-ghost with long time delay, resulting in the performance degradation of a receiver. Exte Gap Cover nd Filling age equalization OCR, which operates periodically (non-real time), estimates tap coefficients of the complex FIR filter in time domain and the complex FIR filter compensates channel distortion by using the estimated information. The equalization OCR can transmit with higher power than the conventional RF and IF OCRs because it has the capability of removing a feedback signal caused by incomplete antenna isolation. The equalization OCR has relatively short time delay since it has a simple structure of the complex FIR filter for channel compensation instead of using the FFT for demodulation and equalization, and the IFFT for re-modulation. The time delay of the proposed OCR is determined by the reference tap location of the FIR filter, so that the OCR with time delay shorter than the guard interval can be implemented by adjusting the reference tap location. As shown in Figure 2, the digital signal processing block of the equalization OCR consists of an inverse channel estimator that estimates inverse channel periodically (non-real time), and a complex FIR filter that compensates channel distortion caused by multipath and feedback signals using the estimated inverse channel. The inverse channel estimator comprises a demodulator, a channel estimator, and an inverse converter. DTxT #4 STL Link DTxT #1 OCR RF OCR Coverage Exte Cover Extension nd age Broadcaster DTxT #3 DTxT #2 Figure 1. configuration for T-DMB system III. THE PROPOSED OCR FOR T-DMB SYSTEM In this section we propose the equalization OCR shown in Figure 2, which meets the above requirements. The proposed OCR consists of a receiver, a digital signal processor, and a transmitter. The receiver consists of a pre-selector, a LNA, a frequency down converter and an ADC. The digital signal processing block consists of an inverse channel estimator and a complex FIR filter. The transmitter consists of a DAC, a frequency up converter, an HPA and a channel filter. The equalization OCR has the following characteristics. Because the equalization OCR removes multipath and compensates linear distortions existing between a main transmitter and the OCR, it can transmit good quality of output signal. The inverse channel estimator of the Figure 2. Structure of the equalization OCR Demodulator sends received phase reference symbol (PRS) signals for channel estimation to the channel estimator. In order to obtain the received PRS signals, the processes of frame synchronization, symbol synchronization, frequency synchronization, and demodulation are required. 1827

3 The channel estimator estimates the channel distortions (multi-path, linear distortion, and feedback signal) of received signals by using PRS signals, which are predefined pilot signals between a main transmitter and the OCR. The channel estimation is performed by comparing the extracted pilot signal from the demodulator with the predefined PRS signal in frequency domain. The inverse converter converts the inverse of the estimated channel distortions in frequency domain into the tap coefficients of the complex FIR filter in time domain while maintaining stability and causality. IV. TEST RESULTS Computer simulations and laboratory tests (with implemented OCR) were performed to verify the performance of the proposed OCR for T-DMB system. The channel model used for the simulations and laboratory tests was modified Brazil channel A with feedback signal at 10 us, which is shown Table 1. The complex FIR filter used 600 taps, in which the number of pre-taps was 20 and the number of post taps was 580. The detailed parameters for computer simulations are in Table 2. Figure 3 shows the input and output spectrums, signal constellations, and complex FIR filter coefficients of the equalization OCR. Figure 4 shows the symbol error rate (SER) of input and output signals of the equalization OCR after FFT in the modified Brazil channel A with a feedback signal. According to the Figures 3 and 4, a feedback signal and multipath were removed by the time domain equalizer of the equalization OCR. For the laboratory tests, the inverse channel estimator was implemented in DSP. In addition, the complex FIR filter used 200 taps in which the number of pre-taps was 20 and the number of post taps was 180. The system delay of the equalization OCR is determined by a number of pre-taps, and in this case the delay was approximately 10 us (=20 * 0.5 us). Figure 5 shows the input and output spectrums of the equalization OCR in the modified Brazil channel A including - 2 db and -5 db feedback signals with 30 db CNR. According to the Figure 5, the implemented OCR completely removes the feedback signals caused by low isolation between transmitting and receiving antennas and the multipath channel between a main transmitter and the equalization OCR. V. CONCLUSION This paper considered technical requirements of the OCRs to broadcast the T-DMB signals using for and proposed the configuration of the equalization OCR that meets such requirements. The proposed OCR uses a time-domain equalizer, which consists of the inverse channel estimator and the complex FIR filter, to compensate channel distortions such as multipath and feedback signals. By the computer simulation and laboratory test results, the proposed OCR shows high output power and good quality of output signal. The proposed OCR can be easily extended to other standards, such as the DVB-T/H, ISDB-T, and Wibro system etc. Moreover, to improve the capability of the feedback signal cancellation the additional feedback signal canceller in [3] can be combined with the proposed OCR. The use of the proposed OCR instead of the conventional OCRs leads us easy implementation and high performance of T-DMB networks with less frequency resources. TABLE I. 1.00E E E E E E-05 CHANNEL PROFILE OF MODIFIED BRAZIL CHANNEL A Delay [us] Amplitude [db] Main Signal Post-Ghost # Post-Ghost # Post-Ghost # Post-Ghost # Post-Ghost # Feedback Signal , -5, -10 TABLE II. Parameter IF Center Frequency CNR Number of Complex FIR Filter Taps System Delay Channel FB = -9 db [In] FB = -9 db [ Out ] FB = -5 db [In] FB = -5 db [ Out ] FB = -1 db [In] FB = -1 db [ Out ] PARAMETERS OF COMPUTER SIMULATIONS Specifications MHz 10 ~ 30 db 600 (pre-taps = 20, post-taps = 580) 10 us Single Feedback & Modified Brazil A 1.00E E E E E E-05 FB = -9 db [ In] FB = -9 db [ Out ] FB = -5 db [ In] FB = -5 db [ Out ] FB = -1 db [ In] FB = -1 db [ Out ] Figure 4. SER of equalization OCR input and output after FFT in single feedback signal and modified Brazil channel A (Left: SER in single feedback signal, Right: SER in modified Brazil A channel) REFERENCES [1] ETSI, Radio Broadcasting Systems; Digital Audio Broadcasting (DAB) to mobile, portable and fixed receivers, ETSI EN v1.3.3, Sept [2] S. Cho, G. Lee, G. Bae, K. Yang, C.-H. Ahn, S. I. Lee, and C. Ahn, System and Services of Terrestrial Digital Multimedia 1828

4 Broadcasting (T-DMB), IEEE Trans. on Broadcasting, Vol. 53, No. 1, March [3] A. Wiewiorka and P. N. Noss, Digital on-channel repeater for DAB, BBC R&D White Paper WHP120, BBC, Sept [4] S. W. Kim, Y.-T. Lee, S. I. Park, H. M. Eum, J. H. Seo, and H. M. Kim, Equalization Digital On-Channel Repeater in Single Frequency Networks, IEEE Trans. on Broadcasting, vol. 52, no. 2, June [5] Y.-T. Lee, S. I. Park, H. M Eum, J. H. Seo, H. M. Kim, S. W. Kim, and J. S. Seo, A Design of Equalization Digital On- Channel Repeater for Single Frequency Network ATSC System, IEEE Trans. on Broadcasting, vol. 53, no. 1, pp 23-37, Mar Figure 3. Input and output spectrums, signal constellations, estimated filter coefficients (absolute) of equalization OCR in modified Brazil channel A with 30 db CNR and -5 db feedback signal. 1829

5 (a) Input/output spectrums of equalization OCR in -2 db feedback signal (b) Input/output spectrums of equalization OCR in modified Brazil channel A with -5 db feedback signal Figure 5. Input/output spectrums of equalization OCR in -2 db feedback signal with 30 db CNR and modified Brazil channel A with -5 db feedback signal (left: input, right: ouput) 1830