The effect of nonlinear amplification on the analog TV signals caused by the terrestrial digital TV broadcast signals. Keisuke MUTO*, Akira OGAWA**

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1 The effect of nonlinear amplification on the analog TV signals caused by the terrestrial digital TV broadcast signals Keisuke MUTO*, Akira OGAWA** Department of Information Sciences, Graduate school of Science and Technology, Meijo University 1-51 Shiogamaguchi Tenpaku-ku Nagoya, JAPAN Phone: , FAX: Abstract In this paper, the degree of the influence of the terrestrial digital TV signals to the terrestrial analog TV signals caused by the nonlinear amplification is evaluated through the computer simulation. The effectiveness due to a simple filtering to suppress the digital TV signals is also verified. 1. Introduction The terrestrial digital TV broadcastings have been in service for three metropolitan areas in Japan since December 23. The simulcasts of digital and analog versions will continue to be on air by about 211 when the analog TV broadcastings are scheduled to close their services. The number of carriers within UHF-band increases greatly for this period, and it is likely for amplifiers used for the analog TV reception in many homes to be driven to the nonlinear regions, resulting in some amount of degradation in the received quality. This paper discusses modeling of the simulcast situation with a nonlinear amplification and the way of computer simulation to estimate the performance degradation of analog TV signals received in the presence of the nonlinearity and additive white Gaussian noise (AWGN). This paper also describes the simulated results in terms of the degradations in signal-to-noise ratio. Moreover, to reduce the influence of the digital TV signals on the analog TV signals, the suppression of the digital TV signals by means of a high-pass filter is considered. This paper shows the effectiveness due to this filtering Digital TV Carriers Analog TV Carriers Filter IFFT Parallel to Serial Conversion 2 I x + x 2 Q AWGN Nonlinear Amplification Serial to Parallel Conversion FFT Spectral Analysis Fig.1 Simulation model Output x-ax 3 r Fig.2 Nonlinear characteristic of the amplifier r Input

2 Digital TV Analog TV 2.3. Thermal noise ch MHz Fig.3 Example for frequency allocation in the simulation 2. Simulation condition 2.1. Simulation model IFFT of 124 points is performed to all spectra of digital and analog TV signals as shown in Fig.1. The carriers for analog TV are assumed to be of no modulation with the same level. Because the digital TV signals are OFDM with about 56 sub-carriers, in-phase (x i ) and quadratic (x Q ) components can be assumed to be Gaussian-distributed with zero mean and variance σ 2. The output data from IFFT are converted from parallel streams to a serial stream, and its amplitude is obtained. AWGN is applied to the signal and the data are input to a nonlinear amplifier shown in Fig 2. The saturated level r is defined by the function of x-ax 3. Referring to the characteristics of home-used amplifiers, the value of r is set at 76dBµV. To obtain the output spectrum, the amplified data are passed through a serial-to-parallel converter, and applied to an FFT circuit. In this paper, the allocation of TV carriers is assumed so that there are seven digital TV carriers and two analog TV carriers as shown in Fig.3, which simulates the situation in Nagoya area in Japan Filter A high-pass filter with a simple structure is desirable to suppress some amount of the digital TV signals. Then, the Butterworth filters with a simple structure are assumed to be used. The filters are characterized by amplitude response with maximally flat in the pass-band and monotonic through the frequency range, at the cost of roll-off steepness. To treat the thermal noise as AWGN on the simulation, in-phase (n i ) and quadratic (n o ) components are assumed to be Gaussian-distributed with zero mean and variance σ n 2. Available power of the thermal noise is irrelevant to the resistance that exists in an arbitrary circuit and proportional to the absolute temperature. If the frequency bandwidth of an arbitrary circuit is B Hz, the available noise power at the output is: N = ktb [W] (1) where k is the Boltzman constant ( [J/K]), T is the absolute temperature [K]. 3. Simulation 3.1. Simulation parameters The simulation parameters are shown in Table 1 and the amplitude characteristic of the filter is shown in Figure 4. The simulation is repeated 5 times. Because the bandwidth of one channel is 6MHz in the terrestrial TV Magnitude (db) Table 1 Simulation parameter Parameters Value FFT-IFFT points Simulation iteration Thermal noise Bandwidth of the amplifier Room temperature Type of high-pass filter Order Cutoff frequency AWGN 6MHz 3K Butterworth 2 29ch TV channel (ch) Fig.4 The amplitude characteristic of filter

3 power reduction [db] ATV:4[dBμV] ATV:45[dBμV] ATV:5[dBμV] ATV:55[dBμV] ATV:6[dBμV] ATV:65[dBμV] ATV:7[dBμV] level of one digital TV carrier[dbμv] Fig.5 Power reduction (Ch.25) power reduction [db] ATV:4[dBμV] ATV:45[dBμV] ATV:5[dBμV] ATV:55[dBμV] ATV:6[dBμV] ATV:65[dBμV] ATV:7[dBμV] level of one digital TV carrier[dbμv] Fig.9 Power reduction when the digital TV carriers are filtered (Ch.25) power reduction [db] ATV:4[dBμV] ATV:45[dBμV] ATV:5[dBμV] ATV:55[dBμV] ATV:6[dBμV] ATV:65[dBμV] ATV:7[dBμV] level of one digital TV carrier[dbμv] Fig.6 Power reduction (Ch.35) power reduction [db] ATV:4[dBμV] ATV:45[dBμV] ATV:5[dBμV] ATV:55[dBμV] ATV:6[dBμV] ATV:65[dBμV] ATV:7[dBμV] level of one digital TV carrier[dbμv] Fig.1 Power reduction when the digital TV carriers are filtered (Ch.35) CNIR[dB] level of one digital TV carrier [dbμv] Fig.7 CNIR(Ch.25) ATV:4[dBμV] ATV:45[dBμV] ATV:5[dBμV] ATV:55[dBμV] ATV:6[dBμV] ATV:65[dBμV] ATV:7[dBμV] CNIR[dB] ATV:4[dBμV] ATV:45[dBμV] 2 ATV:5[dBμV] ATV:55[dBμV] ATV:6[dBμV] ATV:65[dBμV] 1 ATV:7[dBμV] level of one digital TV carrier[dbμv] Fig.11 CNIR when the digital TV carriers are filtered (Ch.25) CNIR[dB] Fig.8 CNIR(Ch.35) ATV:4[dBμV] ATV:45[dBμV] ATV:5[dBμV] ATV:55[dBμV] ATV:6[dBμV] ATV:65[dBμV] ATV:7[dBμV] level of one digital TV carrier[dbμv] CNIR[dB] ATV:4[dBμV] ATV:45[dBμV] 2 ATV:5[dBμV] ATV:55[dBμV] 1 ATV:6[dBμV] ATV:65[dBμV] ATV:7[dBμV] level of one digital TV carrier[dbμv] Fig.12 CNIR when the digital TV carriers are filtered (Ch.35)

4 broadcasting, the frequency bandwidth of the amplifier is set to 6MHz. The room temperature at the input to the amplifier is assumed 3 K(27 ). The adopted filter is 2nd-order high-pass Butterworth type Simulated results without filtering The simulated results are obtained in terms of power reduction and CNIR (Carrier to Noise + Intermodulation Ratio) of the analog TV carriers. As simulated results, the output power reductions for 25ch and 35ch are shown in Fig.5 and Fig.6, respectively, the CNIRs for 25ch and 35ch are shown in Fig.7 and Fig.8, respectively. In Fig.5, the power reduction begins at about 55dBµV of the level of each digital TV carrier. When the level of each digital TV carrier is 6dBµV or more, on the contrary, some increases in the level of analog TV carrier are observed. This is because the amount of intermodulation products may become significant. In Fig.7, in the case of analog TV of 25ch, the CNIR starts decreasing at about 45dBµV of the level of digital TV carrier. The lower limit of the quality for analog TV signals is generally 3dB in CNR. When the level of digital TV carrier is about 6dBµV, the CNIR is found to be below the above-mentioned quality. In case of the analog TV carrier of 35ch, which is relatively far from the digital TV group, the amount of power reduction is not so large. 4. Conclusion This paper discussed the influence on the analog TV signals when the amplifiers placed at the user premises for the purpose of amplifying analog TV signals are driven to the nonlinear zone. The received quality of analog TV signal of 25ch became poorer by the influence of nonlinear amplification when the level of each digital TV carrier is 6dBµV or more. Then, the high-pass filter is inserted at the input to the amplifier to suppress the digital TV signals. As a result, for 6dBµV of the level of digital TV carriers, CNIR can be of about 3dB, which is regarded as the lower limit in quality serviceable for analog TV. It was found that the filtering is effective for keeping the quality of analog television signals. Acknowledgment This work has been partly supported by the Ministry of Education, Science, Sports and Culture, Government of Japan, under Grant-in Aid for Scientific Research. Reference [1] Fumio Ikegami, "Electricity and electronic engineering basic course communication engineering", Rikogakusha 3.3. Simulated results with filtering In order to reduce the level of digital TV carriers and improve the performance, a high-pass filter is inserted at the input to the amplifier. The effects of this filtering are shown in Figures Power reductions of analog TV carrier of 25ch are about 5dB, which is almost constant over the range below 65dBµV of the level of the digital TV, while the power reductions for the analog TV of 35ch are about 2dB. This is because the analog TV carrier of 25ch is attenuated by virtue of the high-pass Butterworth filter. On the other hand, CNIR improved as a whole about 7-8dB compared with the time that had not been filtered. Analog TV signals 25ch can achieve 3dB and over in CNIR at all levels of analog TV where the level of one digital TV carrier is 6dBµV.

5 The effect of nonlinear amplification on the analog TV signals caused by the terrestrial digital TV broadcast signals Keisuke MUTO and Akira OGAWA Meijo University JAPAN

6 Background In Japan, the terrestrial digital TV broadcastings have been in service for three metropolitan areas since December, 23. The provision of simulcast has been made for smooth transition from analog TV to digital TV. The period for the simulcast will continue by around 211.

7 Background Digital TV carriers Analog TV carriers The number of carriers within UHF-band increases greatly during the simulcast UHF Booster TV There is a possibility that the boosters (home amplifiers) may be driven to nonlinear region. Some amount of degradation in the quality of analog TV signals is anticipated.

8 Objectives To evaluate the influence of the digital TV signals on the analog TV signals caused by the nonlinear amplification through the computer simulation. To estimate the effectiveness of filtering, which is placed for reducing the effect of the digital TV signals.

9 Object for the simulation UHF-band allocation in Nagoya area Analog TV : 25ch,35ch Digital TV : 13ch,18ch 22ch Same level Digital TV : 23ch 1/3 Digital TV carriers Analog TV carriers 13ch

10 Evaluated items Items to be evaluated for the quality are : Power reduction CNIR (Carrier-to-Noise plus Intermodulation Ratio) in analog TV carriers.

11 Simulation model Digital TV Analog TV Filter IFFT Spectral Analysis FFT AWGN xi P/S xq AWGN S/P Nonlinear Amplifier x I x Q

12 Conditions of TV carriers Analog TV carriers No modulation Same level between two-carriers Digital TV carriers OFDM with about 56 sub-carriers x I : In-phase component x Q : Quadratic component Gaussian-distribution (Zero mean,variance )

13 Setting of the filter To suppress the effect of the digital TV signals, a filter should be placed at the input to the amplifier. The filter should be as simple as possible 2nd-order high-pass Butterworth filter Magnitude (db) The amplitude characteristic The cut-off frequency 29ch Frequency (Hz)

14 Setting of AWGN Gaussian-distribution : zero mean variance n2. Available power of the AWGN N =kbt [W] k : Boltzman constant ( [J/K]) B : Bandwidth of one channel [Hz] T : Absolute temperature [K]

15 Setting of nonlinear amplifier The saturated level r is defined by the function of x-ax 3. Referring to the characteristics of home-used amplifiers, the value of r is set at 76dB V. Output r x ax 3 r x x ax 3 r Input

16 Simulation parameters Parameters The number of FFT-IFFT points AWGN Bandwidth of the amplifier Room temperature Type of high-pass filter Order Cut-off frequency Value W 6MHz 3K Butterworth 2 29ch

17 Power reduction (35ch) Received power[db] [dBµV] 5[dBµV] 6[dBµV] 7[dBµV] Received power[db] [dBµV] 5[dBµV] 6[dBµV] 7[dBµV] level of one digital TV carrier[dbµv] level of one digital TV carrier[dbµv] The case without filtering The case with filtering

18 Power reduction (25ch) Received power[db] Intermodulation products [dBµV] 5[dBµV] 6[dBµV] 7[dBµV] level of one digital TV carrier[dbµv] The case without filtering Received power[db] [dBµV] 5[dBµV] 6[dBµV] 7[dBµV] level of one digital TV carrier[dbµv] The case with filtering

19 CNIR (35ch) CNIR[dB] [dBµV] 5[dBµV] 6[dBµV] 7[dBµV] level of one digital TV carrier[dbµv] The case without filtering CNIR[dB] Improvement by 7 to 8dB 4[dBµV] 5[dBµV] 6[dBµV] 7[dBµV] level of one digital TV carrier[dbµv] The case with filtering

20 CNIR (25ch) CNIR[dB] [dBµV] 5[dBµV] 6[dBµV] 7[dBµV] level of one digital TV carrier[dbµv] The case without filtering CNIR[dB] 8 4[dBµV] 5[dBµV] 7 6[dBµV] 6 7[dBµV] level of one digital TV carrier[dbµv] The case with filtering

21 Conclusion The influence of digital TV signals on the analog TV signals is caused by the nonlinear amplification, and is evaluated through the simulation. The simulated results show the amount of the degradation may not be negligible for 6dB V or more of the level of a digital TV carrier. It is found that the filtering is effective for keeping the quality of the analog TV signals.

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