HDTV Mobile Reception in Automobiles

Transcription

1 HDTV Mobile Reception in Automobiles NOBUO ITOH AND KENICHI TSUCHIDA Invited Paper Mobile reception of digital terrestrial broadcasting carrying an 18-Mb/s digital HDTV signals is achieved. The effect of diversity reception in moving automobiles for a 64 quadrature amplitude modulation orthogonal frequency division multiplexing (QAM-OFDM) signal is investigated by prototype hardware. There are two methods for diversity reception of OFDM signals. The first is the Doppler compensation directivity control system. For this method, high-performance on-glass antennas and new diversity reception systems for OFDM reception have been developed to verify HDTV service availability in mobile reception environments. Novel horizontally polarized on-glass antennas suitable for DTV were developed. The antenna elements were printed on the inside surface of the rear window glass of a passenger van. OFDM signals received by the four antenna elements were weighted and combined using maximal ratio combining (MRC). The experiments were conducted in urban areas and they showed that employing diversity techniques would make HDTV mobile reception possible in many areas. The other method is post-fft diversity. In the receiver, MRC is performed after an FFT operation on each branch signal. Experimental results show that accurate HDTV mobile reception can be achieved by using a four-branch MRC system. Also, the minimum usable electric field strength could be reduced compared with that of a single dipole antenna. Keywords Adaptive array antenna, digital terrestrial television broadcasting (DTTB), diversity reception, high definition television (HDTV), ISDB-T, mobile reception, OFDM. I. INTRODUCTION Many HDTV programs are broadcast on each 6-MHz channel in Japan. Mobile receivers are being developed so that viewers can enjoy HDTV programs anywhere they go. Up to now, it has been a big challenge to make mobile receivers that can display HDTV programs in a car. A directional antenna with high gain and a narrow beam width is used for fixed reception. Moreover, it is assumed that the antenna is usually set about 10 m above the ground Manuscript received March 11, 2005; revised August 25, N. Itoh is with Toyota Central R&D Laboratories Inc., Aichi , Japan ( n-itoh@mosk.tytlabs.co.jp). K. Tsuchida is with NHK (Japan Broadcasting Corporation), Tokyo , Japan ( tsuchida.k-fy@nhk.or.jp). Digital Object Identifier /JPROC on a house rooftop. On the other hand, mobile receivers usually use an omnidirectional antenna, which has smaller gain and is set no more than a few meters above the ground. Besides the consequent decrease in signal-to-noise ratio and increase in multipath effects, mobile reception conditions change according to the movement of receiver. Although such conditions make HDTV reception in a mobile environment difficult, diversity reception techniques can improve the quality of mobile reception. There are two methods for diversity reception of orthogonal frequency division multiplexing (OFDM) signals. The first one is Doppler compensation directivity control. For this method, high-performance on-glass antennas and new diversity reception systems for OFDM reception have been developed to verify HDTV service availability in mobile reception environments. In particular, novel horizontally polarized on-glass antennas suitable for digital television were developed. Four antenna elements were printed on the inside surface of the rear window glass of a passenger van. OFDM signals received by the antenna elements were weighted and combined using maximal ratio combining (MRC). The experiments were conducted in urban areas, and they showed that employing diversity techniques would make HDTV mobile reception possible in many areas. The other method is post-fft diversity. In the receiver, MRC is performed after an FFT operation on each branch signal. Experimental results show that accurate HDTV mobile reception can be achieved by using a four-branch MRC system. Also, the minimum usable electric field strength could be reduced in comparison with that of a single dipole antenna. This paper first describes the problems concerning mobile reception and then discusses the two technologies developed for improving reception characteristics. II. PROBLEMS OF MOBILE RECEPTION A. Difference Between Received Electric Field Strengths Due to Lower Receiving Antenna Height As shown in Fig. 1, compared with an antenna for fixed reception in a typical household, the antenna for mobile /$ IEEE 274 PROCEEDINGS OF THE IEEE, VOL. 94, NO. 1, JANUARY 2006

2 Fig. 1. Radio wave environment for mobile reception. Fig. 2. Example of field strength variation in mobile reception. reception is lower and, as a result, the received electric field strength is lower. According to ITU-R Recommendation P.1546 [1], the correction value for received electric field strengths in the UHF band for reception at 10 m and 1.5 m above ground is 16 db. Moreover, experimental tests [2] using a Yagi antenna set 10 m above the ground and a standard dipole antenna set at 1.2 m have shown that the average difference in the received electric field is 11.2 db with a standard deviation of 3.4 db. B. Variation in Received Electric Field Strength Fig. 2 shows an example of the measured variation in electric field strength during mobile reception. An OFDM signal at 509 MHz (bandwidth: about 430 khz) was measured by a receiver moving at 36 km/h. The figure shows that electric field strength instantaneously drops many times during a short traveling distance of 10 m. C. Frequency Shift Due to Doppler Effect During mobile reception, the reception frequency is shifted by the Doppler effect. If a radio wave signal is received from one direction, the received frequency can be controlled in accordance with the frequency shift of the received signal. In contrast, if radio wave signals are received from many directions owing to reflections from buildings and so on, the amount of received frequency shift depends on the direction from which the signal came, and the combination and demodulation of these signals with various Fig. 3. Antenna directivity control technology. frequency shifts will degrade the reception characteristics. In addition, as the transmission frequency and traveling speed of the mobile receiver increase, the frequency shift increases. For example, at the highest frequency in the UHF band (i.e., 767 MHz), the maximum Doppler shift is about 70 Hz at 100 km/h. This shift in the UHF band at high traveling speeds is too big to be neglected. III. MOBILE-RECEPTION TECHNOLOGIES To solve the problems described above, the input level of a receiver must be increased. The following sections describe two methods for doing this. A. Doppler Compensation Directivity Control System [3], [4] We have developed a mobile reception technique combining adaptive directional control technology with Doppler compensation technology to stably receive HDTV programs with the quality of fixed reception while traveling in a highspeed automobile. 1) Directional Control Technology [5]: As an automobile moves, the conditions of incoming radio waves, interception, and reflections from nearby buildings are all subject to change. To deal with these changes, a control system was ITOH AND TSUCHIDA: HDTV MOBILE RECEPTION IN AUTOMOBILES 275

3 Fig. 4. Block diagram of antenna directivity control. Fig. 5. Doppler compensation technology. Fig. 7. High-speed mobile receiving system. Fig. 6. Doppler compensation using two antennas spaced closely. developed to continuously optimize the sensitivity of the antenna to momentary changes in the arrival direction of radio waves (Fig. 3). The control was achieved electronically by processing the received signals from an array of multiple antennas. Fig. 4 shows the basic algorithm. The phase difference was calculated using the complex correlation coefficients between the received signals at each antenna and the combined signal, after which the complex coefficients for each multiplier were determined so as to sequentially compensate for the difference. Besides working effectively in a low CN ratio environment, this algorithm can suppress problematic multipaths in mobile reception by improving the antenna sensitivity to the desired wave direction. 2) Doppler Compensation Technology: During high-speed driving, Doppler shifts occur in proportion to the vehicle speed. In particular, high speed in urban areas causes multiple Doppler shifts in the incoming waves reflected from various directions. This effect has made it impossible to receive broadcasts while traveling at high speeds in such areas. However, if the reception point remains stationary, a Doppler shift does not occur. The Doppler shifts can be compensated as follows. Let us assume that a car equipped with a two-element antenna travels as indicated in Fig. 5. The signals from the stationary point on the ground are estimated from the signals received by two antennas. The 276 PROCEEDINGS OF THE IEEE, VOL. 94, NO. 1, JANUARY 2006

4 Fig. 8. Image comparison with digital high-definition television broadcasting and analog broadcasting. receiving point remains stationary on the ground, whereas from the viewpoint of the automobile, it moves backward with the same speed as the automobile. Fig. 6 shows an actual application. The signal received by the two antennas closely spaced on the side window glass is corrected by taking into account the driving speed of the automobile. This permits an estimate of the signal received at a fixed point. 3) High-Speed Mobile Receiving System: Fig. 7 shows an outline of the demonstration system. The antenna was made of transparent film, which did not obstruct the driver s view. Two elements were attached to each of the front, rear, and side windows of the bus, totaling eight elements. The two elements on each side were set close to each other in order to improve the reception characteristics during high-speed traveling. The radio waves received by these antennas were input into a signal processor for Doppler compensation and directivity control 4) Demonstration: Fig. 8 compares digital HDTV received by this high-speed reception system with analog TV images received by a conventional receiver in a bus. The analog broadcast images were distorted by the bus movement, even in areas where the electric field strength was high. On the other hand, the digital HDTV images received by the high-speed system showed no blurring, and a stable image could be continuously received. 5) Experimental Results: We measured the mobile reception characteristics while traveling at high speed on an expressway in Japan. The same bus equipped with the high-speed mobile reception system was used in this experiment. Fig. 9 illustrates the reception results, superimposed on a map of the Tokyo area where the experimental course was located. The image reception rates at each point (calculated in terms of TS packet errors) are shown in different colors, with the reception conditions being plotted on the map along the course of travel. Except in tunnels, most of the measured locations in downtown Tokyo had good reception characteristics (shown in blue) during high-speed driving. Departing from Tokyo Tower and driving southwest on the expressway, reception characteristics gradually deteriorated until it became impossible beyond the Yokohama/Machida Interchange. The boundary lines for the fixed reception area, provided by the Association for Promotion of Digital Broadcasting (D-PA), are also shown in the figure. The lines have been plotted on the map used for this experiment for comparison with the mobile reception area of the demonstration. Doing so reveals that the high-speed mobile reception system could receive terrestrial digital HDTV over almost the same area that D-PA quoted as the fixed reception area. In addition, since the experiments conducted on actual roads were limited, evaluations were also conducted in the laboratory. These bench experiments indicated that the Doppler compensation directivity control system can achieve stable reception under simulated circumstances of traveling at 180 km/h. B. Post-FFT Diversity System As a means to stably receive HDTV programs with the quality of fixed reception while traveling in a high-speed automobile, this mobile reception technique involves separating OFDM signals into individual carriers and selecting and combining signals according to carrier units. 1) Method [6]: We employ space diversity reception, which uses multiple receiving antennas and selects or combines the signals received by them. Fig. 10 shows the block diagram of the post-fft diversity reception system for an OFDM channel. There are four antenna branches ITOH AND TSUCHIDA: HDTV MOBILE RECEPTION IN AUTOMOBILES 277

5 Fig. 9. Experiment result of high-speed mobile reception. Fig. 10. Block diagram of post-fft diversity reception system. for combining the received signal. Assuming that each signal is statistically uncorrelated, combining the signals would enhance the carrier-to-noise ratio (CNR) of the OFDM channel. To maximize the CNR, MRC is performed after an FFT operation for the signals of each branch. The weighting factor for each carrier is derived from the frequency response, which is calculated from the scattered pilot (SP) signal of each branch signal. The weighting factor of l-branch MRC for the kth carrier follows: can be derived as (1) (2) 278 PROCEEDINGS OF THE IEEE, VOL. 94, NO. 1, JANUARY 2006

6 Table 2 Propagation Model (GSM/Typical Urban Area) Table 3 Maximum Receivable Doppler Frequency Fig. 11. Correct reception rate versus field strength as a function of the number of branches. Table 1 Required Field Strength for 50% and 90% Correct Reception Rate where is the frequency response of the channel, is the carrier, and is the output of MRC when l-branch diversity is used. 2) Experimental Results (Field Trials) [7]: We investigated the performance of the system utilizing the post-fft diversity reception in a car traveling in an urban and suburban area in Tokyo. The highest speed during the experiment was around 60 km/h. The reception antennas were horizontally nondirectional cross dipoles installed on the car s roof. Bit error rate (BER) performance and received electric field strength were measured at one-second intervals. Here, the correct reception rate (CRR) is defined as follows: diversity receiver. A Rayleigh fading environment was given independently for each branch, and the maximum receivable Doppler frequency was measured. The GSM typical urban area model [8] was used as the propagation model (see Table 2). Table 3 shows measurement results relative to the Doppler frequency shift. The measured maximum receivable Doppler frequency converted into corresponding traveling speeds for the transmission frequency of 600 MHz are 36 km/h for one branch, 117 km/h for two branches, 162 km/h for three branches, and 180 km/h for four branches. IV. CONCLUSION It is considered that HDTV mobile reception services will be possible on public transport like buses and trains and for passengers in the rear seats of private vehicles. In other words, it will become possible to provide new services hitherto unavailable by means of terrestrial analog or digital broadcasting. We anticipate that the popularity of mobile reception will spread as a wide variety of HDTV programs become available. CRR and BER where is electric field strength db V/m and is the electric field strength of a data sample. Fig. 11 shows the CRR versus electric field strength as a function of the number of branches. Table 1 shows the received electric field strengths for CRRs of 50% and 90%. The required electric field strength to obtain 90% CRR is 48 db V/m for four branches and 65 db V/m for one branch. The difference between four-branch and one-branch amounts to an improvement of 17 db. We also found that almost 100% correct reception was possible for four-branch diversity reception when the received electric field strength is higher than 50 db V/m. 3) Tolerance Against Frequency Shift Due to Doppler Effects: To evaluate the tolerance against Doppler frequency shift, a laboratory test was conducted with the prototype (3) REFERENCES [1] Method for point-to-area predictions for terrestrial services in the frequency range 30 MHz to 3000 MHz, ITU-R, Recommendation P [2] T. Taniguchi et al., Portable reception experiments of digital terrestrial television broadcasting the comparison with reception characteristics under fixed receptions, (in Japanese, ) ITE Tech. Rep. vol. 26, no. 53, pp , Jul. 2002, BCS [3] N. Itoh, Receiving over the air HDTV broadcasts in a moving car, (in Japanese, ) NikkeiBP AD Info no. 856, pp , Sep [4] N. Itoh, Receiving over the air HDTV broadcasts in a moving car, (in Japanese, ) NikkeiBP AD Info no. 857, pp , Sep [5] T. Shibata et al., Receiving HDTV on automobile, presented at the 11th World Congr. ITS, Nagoya, Japan, 2004, [6] H. Hamazumi et al., Characteristics of broad-band signal mobilereception use band-division-type diversity combination reception system-an example of improving characteristics of OFDM mobile reception, (in Japanese, ) IEICE Trans., vol. J80-B-2, no. 6, pp , [7] K. Kambara et al., Mobile reception of digital terrestrial broadcasting with 4 branches diversity receiver, presented at the 2003 ITE Annu. Conv., (in Japanese, ) [8] Recommendation 05.05, Global Systems for Mobile Communication (GSM). ITOH AND TSUCHIDA: HDTV MOBILE RECEPTION IN AUTOMOBILES 279

7 Nobuo Itoh received the B.E. and M.E. degrees from Fukui University, Fukui, Japan, in 1980 and 1982 and the Ph.D. degree from Gifu University Gifu, Japan, in He joined Sanyo Electric Co., Ltd in There he engaged in research of digital signal processing for hi-fi audio recording systems, HDTV transmission systems, and video image processing systems. During this period, he proposed a new digital modulation method for high-density recording and a new waveform equalization method for HDTV transmission. In 1998, he joined Toyota Central R&D Labs. Inc., Aichi, Japan, where he is a Laboratory Manager responsible for mobile communication systems. The main activities were a mobile reception system of terrestrial digital HDTV broadcasting and a high-reliability ITS communication system. He proposed an adaptive array antenna system to realize HDTV high-speed mobile reception. Dr. Itoh is a member of the Institute of Electronics, Information and Communication Engineers of Japan and the Institute of Image, Information and Television Engineers of Japan. Kenichi Tsuchida received the B.E. and M.E. degrees from Tohoku University, Sendai, Japan in 1988 and 1990, respectively. He joined NHK, Tokyo, Japan, in 1990 and has been with NHK Science and Technical Research Laboratories since He has studied digital terrestrial broadcasting systems and outside broadcasting link systems. His recent research interests include reception technique for digital terrestrial broadcasting in mobile situation. He studies the receiver s algorithms (channel estimation, channel correction, etc.) and diversity techniques for OFDM demodulation. 280 PROCEEDINGS OF THE IEEE, VOL. 94, NO. 1, JANUARY 2006