The standardization of the digital TV broadcasting transmitter features

By A. A. Artamonov, K. N. Kopiev, Yu. N. Pogoreltsev, L. N. Protopopov

Review contents:

Digit is on a horizon The standardization of the digital TV broadcasting transmitter features Digital and digital-analog (hybrid) TV-transmitters Transmitters for digital terrestrial TV broadcasting Low-power DVB-repeaters from ELTI Company Spot: filling gaps in DVB-T networks with digital repeaters NEC transmitters for digital TV Transmitters of Rohde & Schwarz for broadcasting Radio broadcasting transmitters of Technosystem and Screen Service Italia Transmitters of Thales Broadcast & Multimedia for digital television broadcasting Measurement of path parameter of digital TV radio transmitters

In this paper the main particularities of transmitters for digital TV broadcasting are considered and the list of specific features is determined. On a base of common requirements to standardization method of the qualitative features of complex units a list of the features is offered and these features, on our glance, are possessed by characteristics, which are necessary for practical use. First of all, such features are the following: a frequency and spectral characteristics of the emitted oscillation, a level of out-of-band components in spectrum, as well as a value of the equivalent power losses. Correctness of the chosen features application for quality determination of the oscillation radiated was examined with the help of a mathematical model and as well in process of the commissioning tests of “ONEGA-0,5C” and “ONEGA-1,0C” transmitters. All original premises were confirmed by the results of the checking and allow to reckon on successful feature data using in the future.

Introducing the system of air digital television broadcasting has required the revision of the requirements to the complex of the equipment and, including, to transmitting unit. Specificity of the new requirements is conditioned on the characteristics of the used modulation method and corresponding features of the oscillation radiated.

In this article the results of consideration of the particularities of the main distortion type influence during the forming and reinforcement of the oscillation, intended for digital message transmitting, are presented.

The original materials for determination of the list of standardization features and requirements to them were the following: the European EN 300744 standard for television broadcasting on air, the reports on seminar and recommendations of the different commissions of European countries, have started the systematic digital broadcasting, as well as domestic standards on analog TV broadcasting systems. The following revision of the requirements was based on results of our theoretical and experimental studies, submitted for some international conferences, as well as experimental results of the studies of the digital television broadcasting systems in experimental zones of Nizhny Novgorod and Saint Petersburg cities.

Some features of radio transmitter were considered as original features.

Nominal average power

Unlike any analog system, in output oscillation of which a deterministic component with pronounced typical characteristics (sync consequence) is kept, the structure of the output oscillation of the digital system is practically does not differ from casual process. So the intensity of this oscillation it is necessary to value by the same method, as the intensity of the casual process that is to say by calculations to its dispersion or average value.

Deflections of the carrying frequency from nominal value

The possible deflections of the carrying frequency are defined by possibilities of the receiver synchronizing system to restore the original value of the frequency of the emitted oscillation. On estimations, provided in description of the experimental studies, and on recommendations on design of the broadcasting systems, the possible deflection of the frequency is ±500 Hz.

Differential phase

The phase changes, accompanying the instant change of the envelope of reinforcing oscillation, are equivalent to the appearance of the nonlinear distortions and bring to the same results. The results of the study of the quantitative estimation of the influence of this effect on level of out-of-band spectrum components, got by mathematical modeling, have shown that from this point of view the requirements to possible phase distortion value is approximately the same, as it is in analog system. Besides, the influence of this effect brings to the signal structure distortion and to change the mutual distances between symbols, which are perceived by receiver demodulator as reduction of the signal/noise ratio. The quantitative estimation of the influence of this mechanism of the influence of the differential phase is not known. The reason, obstructing the reception of this information, is a participation in final result forming the wide range of factors, including system of the receiving unit frequency automatic adjusting. In our appearance, the decision of this question can be received only by experimental checking with special stand, which include the device for imitation of the phase distortions and some the mostly wide-spread receiving units.

End-to-end amplitude frequency characteristic

Amplitude frequency distortions in transmitter bring to appearance of additional unevenness of the sub-carrier amplitudes at input of the receiver. The effect of this is an appearance of the additional load on receiver equalizer that brings to reduction of signal/noise ratio during receiving. The approximate calculations and experimental testing have shown that at value of unevenness which is not more than 0.5 dB the influence of this factor is low.

Level of noise spectral components at emission of the harmonic oscillation

Noise spectral components in output oscillation are provided, basically, by presence of phase fluctuations in reference generator and unit of the working frequencies forming. These fluctuations bring to change of the mutual distances between symbols, which are perceived by demodulator of the receiver as reduction of signal/noise ratio. Our calculations and analysis of published data have shown that value of these noise components must not exceed 80…85 dB/Hz in the 0.1…10 kHz frequency range, and 110 dB/Hz in the 0.1…1 MHz frequency range.

Signal/noise ratio in band of the emitted oscillation

The appearance of the noise in band of the emitted oscillation is provided by the influence of non-linearity of the amplifying path, the fluctuations of the frequency phase and dynamic phase distortions. These noises bring to the raise of minimum possible signal/noise ration in channel that equivalent to the reduction of emitted power.

Level of the out-of-band spectrum components

The out-of-band spectrum components are provided by transformation of the working oscillation spectrum in power amplifiers due to the non-linearity of their amplitude feature and phase relationship from instant value of an envelope. At most possible level of out-of-band spectrum components are defined in accordance with requirements of the EN 300 744 standard of DVB-T system.

Irregularity of the group delay (GD)

GD irregularity brings to the situation in which the phases of the certain sub-carrier groups are shifted comparatively to their original value that brings to the change of the mutual distances between symbols, which are perceived by the receiver demodulator as reduction of signal/noise ratio. Our calculations and analysis of published data have shown that maximum GD irregularity in digital system must not exceed 250 ns. In analog system the GD irregularity is not standardized directly and its value is defined obliquely on base of the requirements to surge characteristic. In general case the GD type is formed in exciter path and units of the co-ordination of the transmitter output with antenna. As the GD irregularity influence, provided by concordance device features, does not withdraw at change of the “exciter-modulator” complex of analog system to corresponding complex of the digital system, so in digital system the possibility of GD irregularity correction should be provided. According to preliminary estimation the range of this correction should be not less than ±500 ns.

Value of the equivalent noise losses

The efficiency loss at radio transmitter of the digital system are provided by simultaneous influence by following factors: a non-linearity of amplitude characteristic, a relationship of the phase from the instant envelope value, the phase fluctuations, GD irregularity and irregularity of amplitude-frequency response. The total value of the power it is possible to define uniquely under the known values of each of these factors. However the sum of possible values for each of them (which are defined on the base of other requirements), can exceed the most possible noise value in band of the emitted oscillation. So it is introduced that this feature shall be independently controlled. At most available value of the total loss shall not exceed 0.5 dB.

Peak and average power ratio at transmitter output

The theoretical value of peak oscillation power in digital system is exceedingly high (in a thousands exceeds its average value), however in real situation the restriction of this value is existed. Such restriction is equivalent to presence of nonlinear distortion and inevitably is accompanied by increase of the out-of-band spectrum components level and noise level inside band of the emitted oscillation. To limit the influence of this factor, the average and peak power ratio should be specified.

The estimation of considered above features with standpoint of need and sufficiency of their inclusion in a range of standardized features has shown the following:

• Non-linearity of amplitude characteristic, influence of the differential phase, irregularity of the through amplitude-frequency response, noise and discrete spectral components at emission of the harmonic oscillation, as well as GD irregularity bring to the same effect, which is an increase the energy loss value;
• The requirements to the possible frequency deflection value, resulting from conditions of electromagnetic compatibility, are more hard in contrast with requirements, required for ensuring of the normal acceptance (100 Hz instead of ±500 Hz);
• The value of ratio of peak output power to its average value is an element of the compromise settlement, which will take by the developer of the power amplifier, and any version of this settlement will inevitably influence to the level of out-of-band spectrum components.

So we offered to change all features, anyway influencing upon quality of the receiving picture, to single feature which is an equivalent noise losses, as well as to toughen the requirements to the possible frequency deflection value and to exclude from the standardized factors the feature, reflecting the ratio of peak power value to average power value.

During the listing of standardized features it was accepted in attention that each feature shall feature meet the following requirements:

• The necessity and sufficiency (the necessity is provided that in the absence of given feature in the list of standardized features it is impossible to guarantee the system functioning with given quality, but sufficiency is provided the absence of need to attract the additional information for its role estimation);
• Single-definitiveness;
• Possibility of the undertaking the measurements by means of existed measuring equipment;
• It is advisable that numerical value of standardized feature allow determining the influence of corresponding phenomena on system quality.

In view of essential difference of the measurement method of the feature “equivalent noise degradation” from traditional measurement methods of the signal features, used in analog television broadcasting system, it is expedient to bring here more detailed description of the main correlations between this feature and the most important TV-broadcasting system features, as well as the results of the comparison its features with requirements stated above.

During the choice of the feature, characterizing the influence of the distortion in a transmitter on the quality of the TV-broadcasting system, we are taken into account that this influence can be is uniquely reflected by means of estimations of the error probability value on demodulator output (or one of the decoders). This position is based on the circumstance that system part quality from modulator input to digital demodulator output (inside which a transmitter is located) is defined by one single feature such as the probability of the wrong acceptance (P_error). Since the considered system is invariant to error stream nature, that to her is no difference, what factor that or other error is caused. So the influence of the transmitter distortion, bringing to increase of P_error, can be replaced by influence of “white” Gauss noise, which influence is accompanied by the same increasing of P_error. In literature, denoted to the description of the methods of the transmitter features estimation for system DVB-T, consequence from influence of the distortion is assigned by term “equivalent noise degradation” — END).

Necessity of feature measurement, characterizing the relationship of P_error from distortion value does not raise any doubts. If this chosen feature is uniquely connected with P_error value, that requirement of sufficiency will be also executed. From determination of the END feature, founded on substitution of distortion influence, follows that it is satisfies to the first condition which is necessity and sufficiency. For estimation of the END influence on the system features it is necessary to consider the main factors, from which the structure of the radio link energy potential is depended. With accuracy sufficient for practice it is possible to consider that the radio link energy potential is defined by loss values in transmitter, by signal fading at radio waves spreading, by noise of the airwaves, by noise of the receiver and loss, provided by signal distortion at demodulation. If as criterion quality of radio link a value is used, that natural measure of the losses in transmitter and receiver will be the values of equivalent noise. The mechanism of this noise influence is illustrated by formula 1 (is expected that all noise types are independent):

P_error = F ( P_s / (N_a+N_r1+ N_r2+ N_t) )

The dependence of the ENF-C/N from END

where
P_s is a power of the signal at the receiver input,
N_a is the noises of the airwaves,
N_r1 is the noises of the input circuits of the receiver,
N_r2 is an equivalent noises of the receiver, provided by demodulation distortions,
N_t — an equivalent noises of the transmitter,
F — a function, displaying the relationship of P_error from signal/noise ratio at the receiver input.

In general event of the correlation between noise values in formula 1 can be any. So for provision of the result certainty, got by means of proposed method, in some papers on this subject all calculation are executed provided that argument value in formula 1 is equal to the most possible signal/noise ratio (C/N) for given working mode, and on Viterbi decoder output the value of P_error = 2·10^-4. In this case in correct designed radio link the equivalent receiver and transmitter noises shall be approximately on one order less than a sum of air noise and noises of the receiver input circuits (N₀ = N_a + N_r1). For performing of accepted above condition P_error = 2·10^-4 (which must be executed in all cases) at presence of the losses in transmitter (N_t ≠ 0) N₀ value in formula 1 shall be reduced in contrast with ideal situation, corresponding to absence of these losses. In some materials for description of the mentioned phenomena the term “equivalent noise floor” (ENF) is accepted. If mark N₁ = N₀ - ENF₀ (here the index 0 beside ENF value means that there is its absolute value, rather then value in dB), then on the base of above-stated it is possible to take the following determination for END (the formula 2):

END,dB = P_c/N₁,dB - P_s/N₀,dB

This computable END value shows, insofar the energy potential of radio link as a result of distortion influences is decreased.

The measurement of the END feature is carried out in accordance with formula 2 by substitution method, that is to say is it in the beginning the threshold value of signal/noise ratio (there is in view of noise N₁) at the receiver input for distorted signal is determined, and then the measurements are repeated under non-distorted signal. The END feature measured by such method will be to characterize the distortions, appearing in testing unit. These measurements can be executed either by means of measuring receiver EFA from R&S company, or by means of the adjustable noise generator, by the integrator and digital signal receiver, in which the achievement of the threshold signal/noise ratio is registered on criterion of the blackout.

According to formula 2 the ENF value is connected with END feature as follows (the formula 3):

ENF,dB = 10·lg(P_s/N₀) - 10·lg(10^END/10 - 1) + END

Since ENF₀<<N₀ then at additive nature of ENF components, the END components (expressed in dB) it is possible also to consider in the first approximation as additive. Then follows that END values, in dB, provided by distortions in different radio link elements, can be assembled. This position is confirmed by the result of the studies. This feature allows to track the level of the distortion of each type on the general factor which is an power potential of radio link. Thereby, the proposed feature satisfies to all above-stated requirements.

As total alternative feature some researchers are offered to use the MER feature. On our opinion such decision is inadvisable in view of ambiguity of relationship between values of MER and P_error. This ambiguity is provided by absence of weighing at summation of the deflections of the transmitting symbol vector from theoretical position for different point of the constellation, relationship between deflections for different points of the constellation, different type of the distribution laws of the deflections for different outraging factor and a error at changing of the nonlinear to linear averaging. Single dignity of this feature (in contrast with END) is a presence of the possibility of the direct measurement of the output distortion on an input of some unit without using of reference signal. This circumstance allows to use the MER feature for estimation of the modulator quality. The influence of ambiguity at this measurement will be slight, since the value of the distortion in modulator, as a rule, it is enough small so even relatively great measurement error will not bring to essential change the END value estimation for transmitter as a whole. Below on the figure the results of mathematical modeling of MER relationship from distortion value of different type, and on the next figure a relationship between MER value and END at small distortion are given. The analysis of these results has confirmed the above-stated suggestions about the nature of ambiguous relationship between MER value and P_error.

The relationship between MER and END values at influence of different distortion

The relationship between MER and END values under small distortions

On figure the following indications are used:
Clip is a nonlinear distortion of the amplitude,
NF is a dynamic phase distortion,
FaseConst is a phase noises,
FL3 is an irregularity of GD (the third order of the approximating polynomial).

As it was already mentioned above, a quality of the digital TV-broadcasting system area from modulator input to digital demodulator output is characterized by P_error, uniquely connected with signal/noise ratio, part of which is a “contribution”, provided by the distortion of the signal in a transmitter. Herewith naturally to use as measure of the influence of the distortion in transmitter such physical value, which is used for estimation of the general energy potential of the broadcasting system. In this instance to this wish in equal degree the values of the equivalent noise degradation (END) and equivalent noise floor (ENF) are satisfied. The END value will herewith correspond to value of the equivalent losses of the system energy potential, caused by distortion in transmitter, and ENF value — to equivalent noise value.

Since END and ENF features are hardly bound between itself, that choice one of them as measure of the distortion in transmitter amplifying path depends only from method of using of this measure quantitative estimation. For developer of transmitting unit is the ENF feature is more preferred, but for designer of the broadcasting system the END feature is more preferred.