the effect of nonlinear amplification on the analog …asahi/ogawa/publish/2004_ncsp2004...the...

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

1-501 Shiogamaguchi Tenpaku-ku Nagoya, JAPAN Phone: +81-52-838-2388, FAX: +81-52-832-1298

E-mail: *[email protected], **[email protected]

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 2003. The simulcasts of digital and analog versions will continue to be on air by about 2011 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

22QI xx +

Nonlinear Amplification

FFT

Serial to Parallel Conversion

Spectral Analysis

AWGN

Fig.1 Simulation model

Fig.2 Nonlinear characteristic of the amplifier

r

r

Input

Output

x-ax3

2. Simulation condition

2.1. Simulation model

IFFT of 1024 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 5600 sub-carriers, in-phase (xi) and quadratic (xQ) components can be assumed to be Gaussian-distributed with zero mean and variance σ2. 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-ax3. 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. 2.2. 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.

2.3. Thermal noise

To treat the thermal noise as AWGN on the simulation, in-phase (ni) and quadratic (no) components are assumed to be Gaussian-distributed with zero mean and variance σn

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:

kTBN = [W] (１)

where k is the Boltzman constant (1.3805×10-23[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 5000 times. Because the bandwidth of one channel is 6MHz in the terrestrial TV

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

1024 5000

AWGN 6MHｚ 300K

Butterworth 2

29ｃｈ

0 10 20 30 40 50-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

TV channe l (ch )

Mag

nitude

(dB

)

Fig.4 The amplitude characteristic of filter

ch.13 18 19 20 21 22 23 25 35 473 MHｚ 503 509 515 521 527 533 545 605

Fig.3 Example for frequency allocation in the simulation

Digital TV Analog TV

Fig.5 Power reduction (Ch.25)

Fig.6 Power reduction (Ch.35)

Fig.7 CNIR(Ch.25)

Fig.8 CNIR(Ch.35)

Fig.9 Power reduction when the digital TV carriers are filtered (Ch.25)

Fig.10 Power reduction when the digital TV carriers are filtered (Ch.35)

Fig.11 CNIR when the digital TV carriers are filtered (Ch.25)

Fig.12 CNIR when the digital TV carriers are filtered (Ch.35)

-6

-4

-2

0

2

4

6

8

10

35 40 45 50 55 60 65 70level of one digital TV carrier[dBμV]

pow

er

redu

ction [

dB]

ＡＴＶ：40[dBμV]







-6

-5

-4

-3

-2

-1

0


pow

er

redu

ction [

dB]








0

10

20

30

40

50

60

70

80

35 40 45 50 55 60 65 70level of one digital TV carrier [dBμV]

CN

IR[d

B]








-6

-5

-4

-3

-2

-1

0


pow

er

redu

ction [

dB]








-6

-5

-4

-3

-2

-1

0


pow

er

redu

ction [

dB]








0

10

20

30

40

50

60

70

80


CN

IR[d

B]

ＡＴＶ：40[dBμV] ＡＴＶ：45[dBμV]




0

10

20

30

40

50

60

70

80


CN

IR[d

B]




ＡＴＶ：70[dBμV]0

10

20

30

40

50

60

70

80


CN

IR[d

B]








broadcasting, the frequency bandwidth of the amplifier is set to 6MHz. The room temperature at the input to the amplifier is assumed 300 K(27℃). The adopted filter is 2nd-order high-pass Butterworth type.

3.2. 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 60dBµV 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 30dB in CNR. When the level of digital TV carrier is about 60dBµV, 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. 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 9-12.

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 30dB and over in CNIR at all levels of analog TV where the level of one digital TV carrier is 60dBµV.

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 60dBµV 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 60dBµV of the level of digital TV carriers, CNIR can be of about 30dB, 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

The effect of nonlinear amplification The effect of nonlinear amplification

on the analog TV signals caused by on the analog TV signals caused by

the terrestrial digital TV broadcast signalsthe terrestrial digital TV broadcast signals

Keisuke MUTO and Akira OGAWA

Meijo University

JAPAN

BackgroundBackground

In Japan, the terrestrial digital TV

broadcastings have been in service for three

metropolitan areas since December, 2003.

The provision of simulcastsimulcast has been made

for smooth transition from analog TV to

digital TV.

The period for the simulcastsimulcast will continue by

around 2011.

BackgroundBackground

Analog TV Analog TV carrierscarriers

Digital TV Digital TV carrierscarriers

UHFBooster

TV

The number of carriers

within UHF-band increases

greatly during the simulcast

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.

ObjectivesObjectives

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.

Object for the simulation Object for the simulation UHFUHF--band allocation in Nagoya areaband allocation in Nagoya area

Analog TV : 25ch,35ch

Digital TV : 13ch,18ch 22ch Same levelSame level

Digital TV : 23ch 1/31/3

Digital TV Digital TV carrierscarriers

Analog TV Analog TV carrierscarriers

13ch 18 19 20 21 22 23 25 35

Evaluated items Evaluated items

Items to be evaluated for the quality are :

Power reductionPower reduction

CNIRCNIR(Carrier-to-Noise plus Intermodulation Ratio)

in analog TV carriers.

Simulation modelSimulation model

Digital TV Analog TV

Filter

IFFT

S/P

FFT

Spectral Analysis

Nonlinear

Amplifier

P/S

22

QI xx +

AWGN AWGN

Ix Qx

Conditions of TV carriersConditions of TV carriers

Analog TV carriers Analog TV carriers

No modulation

Same level between two-carriers

Digital TV carriers Digital TV carriers

OFDM with about 5600 subOFDM with about 5600 sub--carrierscarriers

xI : In-phase component

xQ : Quadratic component

GaussianGaussian--distributiondistribution(Zero mean ,Variance )

Setting of the filterSetting 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 possibleThe filter should be as simple as possible

2nd2nd--order order

highhigh--passpass

ButterworthButterworth

filterfilter0 50 100 150 200

-20

-15

-10

-5

0

Frequency (Hz)

Magnitude (dB)

The amplitude characteristic

The cut-off frequency

29ch

Setting of AWGNSetting of AWGN

Gaussian-distribution : zero mean

variance n2.

Available power of the AWGN

[W] kBTN =

k : Boltzman constant (1.3805 10-23 [J/K])

B : Bandwidth of one channel [Hz]

T : Absolute temperature [K]

Setting of nonlinear amplifierSetting of nonlinear amplifier

The saturated level r is defined by the function

of x-ax3.

Referring to the characteristics of home-used

amplifiers, the value of r is set at 76dB V.

x

x ax3

Input

Output

r

rx ax3 r

Simulation parametersSimulation parameters

1024

2.48 10-14 W

6MHz

300K

Butterworth

2

29ch

The number of FFT-IFFT points

AWGN

Bandwidth of the amplifier

Room temperature

Type of high-pass filter

Order

Cut-off frequency

ValueValueParametersParameters

Power reduction (35ch)Power reduction (35ch)

The case without filteringThe case without filtering The case with filteringThe case with filtering

-6

-5

-4

-3

-2

-1

0

35 40 45 50 55 60 65 70

level of one digital TV carrier[dBµV]

Received power[dB]

40[dBµV]

50[dBµV]

60[dBµV]

70[dBµV]

-6

-5

-4

-3

-2

-1

0

35 40 45 50 55 60 65 70


Received power[dB]

40[dBµV]

50[dBµV]

60[dBµV]

70[dBµV]

-6

-4

-2

0

2

4

6

8

10

35 40 45 50 55 60 65 70


Received power[dB]

40[dBµV]

50[dBµV]

60[dBµV]

70[dBµV]

Power reduction (25ch)Power reduction (25ch)

IntermodulationIntermodulation productsproducts

-6

-4

-2

0

2

4

6

8

10

35 40 45 50 55 60 65 70


Received power[dB] 40[dBµV]

50[dBµV]

60[dBµV]

70[dBµV]


0

10

20

30

40

50

60

70

80

35 40 45 50 55 60 65 70


CNIR[dB]

40[dBµV]

50[dBµV]

60[dBµV]

70[dBµV]

CNIR (35ch)CNIR (35ch)

Improvement by Improvement by

7 to 8dB7 to 8dB0

10

20

30

40

50

60

70

80

35 40 45 50 55 60 65 70


CNIR[dB]

40[dBµV]

50[dBµV]

60[dBµV]

70[dBµV]


0

10

20

30

40

50

60

70

80

35 40 45 50 55 60 65 70

level of one digital TV carrier[dBµV]CNIR[dB]

40[dBµV]

50[dBµV]

60[dBµV]

70[dBµV]

0

10

20

30

40

50

60

70

80

35 40 45 50 55 60 65 70


CNIR[dB]

40[dBµV]

50[dBµV]

60[dBµV]

70[dBµV]

CNIR (25ch)CNIR (25ch)


Conclusion 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 60dB 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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