PERFORMANCE ASSESSMENT OF A DVB-T TELEVISION SYSTEM

PERFORMANCE ASSESSMENT OF A DVB-T TELEVISION

SYSTEM

A. Morello, G. Blanchietti, C. Benzi, B. Sacco, M. Tabone

RAI Research Centre, Torino, Italy



ABSTRACT



In the framework of the DVB, RACE dTTb and ACTS VALIDATE European

projects, the RAI Research Centre has directly contributed to the definition and

validation of the DVB system for digital terrestrial television broadcasting (DVB-T).

The system, allowing fixed and (static) portable reception, is based on OFDM

modulation with a variety of modulation/channel coding configurations, and is

characterised by two operational modes, the first one with 2K carriers for conventional

multi-frequency networks (MFN), the second one with 8K carriers to cover also

single frequency networks (SFN).

This contribution presents the results of the VALIDATE laboratory tests carried out by

RAI in winter and spring 1996-1997 on the second dTTb demonstrator, developed in

compliance with the DVB-T specification and including both the 2K and the 8K

options. The system under test included the complete transmission and reception

television delivery chain: picture / sound coding, multiplexing, OFDM modulation,

High Power Amplifier, semi-consumer tuner, coherent OFDM demodulator, video

decoding and display.

The tests investigated the ruggedness of the OFDM DVB system in presence of a

variety of typical channel distortions, such as Gaussian and impulsive noise, tuner

phase noise, echoes with various delays and amplitudes, Doppler frequency shifts,

interference from an other DVB-T signal and from analogue PAL/SECAM TV signals,

transmitter non-linearity.

Even if the equipment being tested at the moment is only a first generation of

equipment, the test results have been largely in line with the performance predicted

by computer simulations, apart from few exceptions, and allow to gain an important

insight on the characteristics of the sophisticated modulation, channel coding and

equalisation techniques of the DVB-T system.

For the sake of conciseness it was not possible to include a description of the DVB-T

system, nor a tutorial on its technical background, but the reader can refer to [2] and

[3] to better understand the measurement results.



1. INTRODUCTION



Taking into account the rapid progress of digital television technologies in production,

transmission and emission and the new commercial requirements identified by the

broadcasters and the consumer industry, the DVB Project has started its activity in

1993, aiming at the harmonisation of the strategies for the introduction of digital

television broadcasting in Europe on the various delivery media. The first significant

success of the Technical Module of the DVB project has been the definition at the

end of 1993 of the DVB-S system for direct-to-home satellite broadcasting of multi-

programme TV, followed by the DVB-C system for cable networks. These two

systems were adopted also for TV distribution on satellite master antenna systems

(SMATV) and on microwave point-to-multipoint links (MMDS). Due to the higher

hostility of the terrestrial VHF/UHF propagation environment and to the longer terms

introductory plans, the system for digital terrestrial broadcasting, named DVB-T, has

been agreed only in November 1995, and approved as an ETSI Standard in February

1997 [1]. The RACE dTTb Project, in co-operation with other national projects as HD-

DIVINE and HDTVT, have studied, designed and developed the DVB-T system

taking into account the service requirements defined by DVB; among these, the most

technically demanding are:

 the possibility to obtain (stationary) portable reception with omnidirectional

antennas in addition to fixed reception with directive roof-top antennas

 the possibility to operate on single frequency networks (SFN), where the coverage

of large areas (a region or even a country) is achieved by synchronised

transmitters operating on the same RF channel, with significant advantages in

terms of spectrum efficiency.



The DVB-T system [1, 2] can transmit digital multi-programme, conventional-

definition television signals in MPEG-2 MP@ML format, but it is open to evolve

towards HDTV by using higher MPEG-2 levels and profiles. The transmission

scheme is based on a multi-carrier coded modulation named C-OFDM [3],

characterised by two operational modes, the first one with 2K carriers for conventional

multi-frequency networks (MFN), the second one with 8K carriers to cover also

single frequency networks (SFN). This modulation is particularly suitable to operate

on the terrestrial multipath propagation channel because of the narrow-band

characteristic of each data carrier and of the presence of a "guard interval" (duration

Tg) which separates adjacent symbols and avoids inter-symbol interference in the

presence of echoes. The DVB-T system offers a bit-rate capacity ranging from 5

Mbit/s to 31.5 Mbit/s depending on the chosen level of m-QAM modulation (m=4, 16,

64), the inner code rate (1/2, 2/3, 3/4, 5/6 or 7/8) and the guard interval duration

(Tg/Tu==1/4, 1/8, 1/16 or 1/32; Tu=useful symbol duration). It is optimised for 8

MHz channels (European UHF channellisation), but it can be easily adapted to 7 MHz

and 6 MHz channels by adjusting the receiver sampling frequency.

The system bit-rate capacity and the computer simulation performance over various

multipath channels are given in the Annex, which repeats the information of

Appendix A of the DVB-T specification.



The ACTS VALIDATE Project [4] has been set up at the end of 1995 by the

European Commission in order to validate the DVB-T specification in practical

situations, by means of interworking tests between different pieces of equipment,

extensive laboratory tests, real over-air transmissions using single frequency networks

or interleaved channels. In addition the project is investigating the transmitter and

domestic repeater technology and developing planning methods suitable for digital

terrestrial television broadcasting.



In the framework of the VALIDATE activity, a complete DVB-T chain was set-up at

the RAI Research Centre of Turin, including real time MPEG-2 encoders / decoders

(for three programmes), a Transport Stream multiplexer/demultiplexer, the dTTb C-

OFDM modem, an RF transmitter on UHF channel 28 (or 43), a “quasi-consumer”

tuner.

Among the DVB-T compliant pieces of equipment up to now available to the

members of the VALIDATE Project, the dTTb demonstrator is the first one

implementing not only the 2K, but also the 8K FFT mode, and therefore it can give

important confirmations on the specification performance in SFNs.

Table 1 lists the DVB-T system configurations which have been tested.



TABLE 1 - Tested Modes

Modulation Code Rate Carriers Ru [Mbps] Guard Interval 

QPSK 1/2 2K & 8K 5.53 1/8

16QAM 3/4 2K & 8K 18.10 1/32

64QAM 2/3 2K & 8K 19.91 1/4, (1/8 **)

Note (*): Tg/Tu, Tg=guard interval duration, Tu=useful symbol duration

Note (**): =1/4 unless otherwise explicitly indicated



The results presented in this contribution cover the system performance with

Additive White Gaussian Noise (AWGN), with impulsive noise, with static and time-

varying echoes and with non-linear distortions. These figures are important to verify

some of the key elements debated during the DVB-T standard definition: the system

sensitivity to phase noise in the 8K mode, the implementation margin, the possibility

to operate with high-level, long-delay echoes (e.g. 0 dB amplitude and 200 s delay,

for SFNs), the steepness of the failure characteristic with echoes outside the guard

interval, the possibility to serve static or even mobile portable receivers.





2. LABORATORY SET-UP AND BASIC TEST PROCEDURE



The laboratory performance evaluation of the DVB-T system is carried out by

changing the parameters of the channel impairments (noise, interference, multipath

echoes, phase noise, frequency off-sets, non-linearity) and measuring the BER at

the receiver side, after Viterbi error correction. Additional observations on the

decoded picture are carried out to verify if, as indicated by theory, BER=2 .10-4 after

Viterbi decoding allows to deliver good service quality (i.e., less than an error event

per hour), thanks to the powerful outer error correction by Reed-Solomon (188,204)

coding.

Figure 1 shows a simplified scheme of the RAI laboratory set-up, simulating the

channel impairments at the intermediate frequency (IF, 35.5 MHz), in order to exploit

the flexibility of the laboratory instruments and the availability of SAW filters at these

frequencies. Up and down frequency converters (quasi-transparent in terms of

system performance degradation), from IF to radio frequency (RF, UHF channels 28

or 43) and viceversa, allows to test the RF devices (transmitter and receiver).





Down Up

Transmitter Converter Converter Tuner

RF(tx) RF(rx)



OFDM Multipath OFDM

modulator receiver

Channel IF(rx)



IF(tx) BER

Spectrum

MPEG MUX interference Analyzer Meter

AWGN

& Coders

Figure 1 - Block diagram of the RAI laboratory set-up



The following basic test procedure allows to evaluate the DVB-T system performance

in the presence of a variety of different channel impairments, by measuring the noise

margin loss ###C/N [dB] (see Figures 1 and 2) at the reference BER*=2.10-4 after

Viterbi decoding:

 the channel impairment under consideration is switched off. The additive noise

level is increased until BER* is achieved. The corresponding carrier-to-noise ratio

at the receiver input (C/N*) is noted (measured in the receiver noise bandwidth of

7.6 MHz)

 the channel impairment (e.g. the interfering signal) is switched on. The additive

noise level N is reduced by ###C/N dB, corresponding to the noise margin loss.

The impairment level (e.g. the interference power I) is increased until the

reference bit-error ratio BER* (after Viterbi decoding) is obtained again. The

impairment level (e.g. I) is noted.

The procedure can set the noise margin loss in pre-determined steps (e.g., ###C/N =

1, 2, ... , ### dB), and identify the corresponding channel impairment level (e.g., I),

or viceversa. It should be noted that ###C/N = ### corresponds to the case without

additive noise, where the channel impairment produces itself the reference BER*.

BER







 C/N







-4

2 10 BER*





without with

impairment impairment



C/N

C/N*





Figure 2- Measurement of the noise margin loss ###C/N produced by a channel

impairment



3. SYSTEM IMPLEMENTATION LOSSES OVER AWGN CHANNEL

In order to identify the implementation losses introduced by the various elements of

the DVB-T chain, the BER versus C/N curves after Viterbi decoding have been

measured, and the noise margin losses have been identified as summarised in

Table 2. The laboratory set-up connections, indicated for example as IF(tx)- RF(rx),

make reference to Figure 1.

The basic modem implementation losses and the total chain degradation are referred

to the computer simulation figures (see Table A1 of the Annex, AWGN, ideal

receiver). Two figures are reported when different results were obtained on UHF

channel 28 (530 MHz) and on UHF channel 43 (650 MHz).



TABLE 2 - System implementation losses C/N

C/N [dB]

IF(tx)-IF(rx) IF(tx)-RF(rx) RF(tx)-RF(rx) (*)

Modulation  FFT Basic modem Tuner degradation Total chain

and coding implementation losses including phase noise degradation

ch. 28 ch.43 ch. 28 ch.43

QPSK 1/2 1/8 2K 1.7 0.0 1.7 1.8

8K 1.6 0.0 1.8 1.8

16QAM 3/4 1/32 2K 1.6 0.0 1.7 1.7

8K 1.5 0.1 1.7 1.7

64QAM 2/3 1/4 2K 2.4 0.2 0.5 2.8 3.1

8K 2.3 0.3 0.7 2.9 3.3

Note (*): HPA producing spectrum shoulders of the order of - 38 dB



It should be remarked that, to estimate the channel frequency response and equalise

the received constellations, the system under test makes use of time-domain and

frequency-domain interpolation (named 2-D algorithm) applied to the scattered pilots.

For =1/4 the residual noise on the estimated channel response produces a C/N

degradation of about 1.6 dB (with respect to the computer simulation figures of the

Annex), while for 210 >210 115 280 >425

(320*)

8K 24 50 75 >210 27 70 >425

16QAM 3/4 2K 23(**) 58 88 165 37 80 155

(=1/32) (120*)

8K 5 15 21 40 10 19 45

64QAM 2/3 2K (=1/4) 14 19 33 95 15 28 100

(120*)

8K (=1/8) 4 8 10 23 4 8.5 21

Note (*): computer simulation results

Note (**):=0.8 was used, since the system could not operate at =0.9 even with

fd=0



The performances with a single echo and with multiple echoes are comparable and,

as expected, the 2K configuration is about four times faster than the 8K configuration,

because of its shorter symbol duration. The maximum acceptable Doppler shift

depends heavily on the echo amplitudes.

In the presence of 0 dB echoes, the 64QAM 2/3 8K mode can follow only slow

channel variations (few Hertz), while the QPSK 1/2 2K mode can track Dopplers

higher than 100 Hz, typical of mobile reception.

It should be noted that the simulation results predict Doppler tracking capabilities for

the DVB-T system which are 4 to 6 times higher than the measured results. This is

due to the difference of the algorithms implemented in the dTTb system, that must be

able to handle also other channel distortions (e.g. narrow band interferences) which

are not considered in the simulations. Further studies are ongoing to identify

optimised receiver algorithms.

6. PERFORMANCE WITH INTERFERENCES





4 Since the curves of C/N versus f are very steep, the choice DC/N=4 dB is not crucial for the results

d

of the tests.

6.1. DIGITAL TV UNWANTED

The co-channel (CCI) protection ratio PR is defined as the carrier-to-interference

power ratio producing a noise margin loss C/N=###. Table 7 gives the measured

co-channel Protection Ratios of the three modulations under test, in 2K, =1/4

mode, on UHF channel 28.

Since a single OFDM modulator was available at RAI, the interfering signal was

derived from the useful signal, delayed by 360 s (more than the total OFDM symbol

duration of 280 s) and with a frequency shift f (with respect to the centre of the RF

channel). The frequency shift was set at 17 different values (between -8 kHz and

+8kHz, step 1 kHz), and the worst PR was identified.

When other DVB-T modulators will become available, further tests will be carried out

with independent interfering signals, to validate and complete these preliminary

results.



TABLE 7: Co-channel Protection Ratios for DVB-T interfered by DVB-T

(2K mode, preliminary results)

Modulation Code rate PR

QPSK 1/2 5.1

16-QAM 3/4 14.5

64-QAM 2/3 19.5



The results show that the digital interference from an uncorrelated DVB-T signal has

similar effect as a Gaussian noise of equal power in the receiver bandwidth.



6.2. ANALOGUE TV UNWANTED

A number of co-channel protection ratio measurements with DVB-T interfered by

analogue TV have been carried out by BBC (PAL-I), by CCETT (L/SECAM) and by

RAI (PAL-G). The measured results should be regarded only as provisional figures at

the moment, as the equipment being tested is only first generation equipment.

Table 8 summarises the laboratory test results of co-channel protection ratios of the

DVB-T signals interfered by unwanted analogue TV signals, modulated by 75%

colour bars still picture. The measured protection ratios are the ratio of the mean

power of the wanted DVB-T signal to the peak sync. power of the unwanted analogue

signal. The results are measured without noise, and at a bit-error-rate of 2.10-4 after

the Viterbi decoder, except where otherwise indicated.

The columns L and U have the following meanings: L corresponds to the lower part

of the measurement envelope; U corresponds to the upper part of the measurement

envelope. It should be noted that the lower part of the measurement envelope

corresponds to better performance.

The following conditions apply to the respective unwanted analogue signals and

wanted DVB-T signals:

1. PAL-I, with FM sound unmodulated, with NICAM. DVB-T signal: guard interval

1/32

2. PAL-G, FM sound modulated by 1 kHz tone, with NICAM. DVB-T signal: guard

interval 1/4

3. L/SECAM, AM sound unmodulated, no NICAM. DVB-T signal: guard interval 1/4



Table 8 Co-channel protection ratio (dB) for DVB-T interfered by analogue TV

signals

Wanted Inter- DVB-T MODE

signal fering QPSK, r=1/2 16-QAM, r=3/4 64-QAM, r=2/3

signal 8K 2K 8K 2K 8K 2K

L U L U L U L U L U L U

BBC* PAL-I N/A N/A -11 -9 N/A N/A -4 -0.5 N/A N/A -1.5 +1

dTTb PAL-G -2 +4 -3 +4 -2 +4 -2 +5 +2 +6 +3 +6

dTTb SECAM N/A N/A N/A N/A N/A N/A N/A N/A -5 +5 +5 +8

Note (*): failure point measurements (probably these measurements give slightly

lower protection ratios than BER=2.10-4 measurements). The channel estimation in

the BBC modem uses temporal filtering, which gives better measurements on static

channels, but worse temporal performance.



The protection ratios show a periodicity which corresponds to the DVB-T carrier

spacing (i.e. 4.4 kHz in the 2K mode and 1.1 kHz in the 8K mode). The amplitude of

this variation is indicated in the table by the columns “L” and “U” in the results. In

some cases a more random shape is observed. If in the future, with more mature

technology, the periodic shape is confirmed, it could be exploited to optimise the

required protection ratios in the service area, using precision offsets.

The figures for 2K and 8K are not significantly different, except for the SECAM

measurements. However in this case, the AM sound carrier was unmodulated, and

no NICAM signal was used.

In the dTTb demonstrator, the results are much less dependent on the DVB-T mode

than expected. The reason for this is still being investigated, but the performance in

the 16-QAM and QPSK modes could be expected to improve following further

hardware developments.

The 64-QAM mode is identical to the mode M3 in the draft ITU-R WP11C

recommendation [XYZ] (TG11-3). The measured protection ratios for this mode are

very close to the figures currently in this draft recommendation.



Very significant performance degradation has been observed when the analogue

vision carrier is close to a DVB-T continual pilot. With 64-QAM a degradation of 18-

21 dB has been observed when this effect occurs. The results in Table 1 have

excluded this effect, which is not thought to be fundamental to the DVB-T system.

However, receiver designers should make any effort to improve the equipment with

respect to these artefacts (5). In the Informative Annex some more technical

background to this problem is given.



7. SENSITIVITY TO TRANSMITTER NON-LINEARITY

The sensitivity of a digital system to non-linear distortions is a key parameter to

determine the possibility to exploit at best the RF power of the transmitters, which are

intrinsically non-linear. The OFDM modulation has a quasi-Gaussian amplitude

distribution, therefore a non-linear high power amplifier (HPA) causes peak clipping

and intermodulation among the various carriers, with the following effects:

 in-band interference generation, with noise-like distribution, affecting all the

OFDM constellations (while the constellation shape distortion, which takes place

in single carrier systems, does not occur)

 out-of-band spectrum "shoulders" regeneration, with interference effect on the

adjacent channels



While the levels of the OFDM spectrum shoulders are directly dependent on the

particular HPA technology, on its back-off (power reduction with respect to a

reference level, e.g., the saturation level) and on the use of HPA non-linearity pre-





5 Although the frequency position of the DVB-T signal could be carefully selected in order to avoid

interactions between the vision carrier and the continual pilots, this could be a significant constraint for

planning, and would not solve the problem of man-made CW interferences that can be occasionally

observed in the service area.

correctors, in first approximation the noise margin loss C/N only depends on the

spectrum shoulders and on the system ruggedness against noise. In order to

demonstrate this, the dTTb system has been tested at various levels of HPA out-put

power levels and C/N was measured versus the spectrum shoulders, defined as the

power density ratio [dB] between the in-band and out-of-band 6 maxima of the

spectrum. It should be noted that the evaluation of the shoulders on a spectrum

analyser is affected by an uncertainty of at least 1 dB.

Table 9 gives the test results achieved with a 50 Watt solid state HPA on UHF

channel 43, limited to the 2K mode, since the 8K mode gave the same results (within

0.1 dB). In the back-off determination, the reference HPA power is chosen arbitrarily

at the level corresponding to a shoulder level of about 20 dB.







TABLE 9 -Noise margin loss C/N [dB] versus spectrum shoulders SS [dB]

(reference: C/N in IF(tx)- RF(rx) configuration)

Noise margin loss C/N [dB]

output back-off [dB] 5.9 5.2 4 3.1 2.4 1.7 1 0.2 0 (**)

SS[dB] 38 37 32 29 26 24 22.5 21 20

modulation & code

QPSK 1/2 (=1/8) 0 0 0 0.1 0.1 0.1 0.1 0.2 0.2

(0*) (0*)(0*) (0*) (0*) (0*) (0.1*) (0.1*) (0.1*)

16QAM 3/4 0 0 0 0.1 0.2 0.3 0.5 0.9 1.1

(=1/32) (0*) (0.1*)

(0*) (0.1*) (0.3*) (0.5*) (0.7*) (1.0*) (1.3*)

64QAM 2/3 (=1/4) 0.2 0.2 0.3 0.5 0.8 1.3 2.0 3.9 -

(0*) (0.1*) (0.2*) (0.5*) (1.0*) (1.7*) (2.6*) (4.5*)

Note (*) from approximated theory (see below); note (**): back-off reference arbitrarily

chosen



The results are in good agreement with the approximated theory which assumes that

the in-band intermodulation is Gaussian noise-like, with a C/I equal to the spectrum

shoulder.

Under this hypothesis, the degradation produced by the non-linearity is = -

1/(-1 --1), where  is the C/N required by the system without non-linear distortion

and  is the shoulder level (natural numbers). The theoretical results in Table 9 are

obtained by this formula, with C/N=4.8, 14.0 and 19.1 dB, for QPSK, 16QAM and

64QAM, respectively.



From these non-linearity tests it is possible to deduce that, to achieve a degradation

lower than 0.2 dB, the spectrum shoulders should be kept at least 13 dB over the

C/N required by the modulation/coding scheme to be adopted.



9. CONCLUSIONS



This contribution has reported the laboratory test results obtained by RAI on the dTTb

demonstrator, compliant with the DVB-T specification. The tests investigated the

ruggedness of the OFDM system in presence of a variety of typical channel

distortions, such as Gaussian and impulsive noise, tuner phase noise, echoes with

various delays and amplitudes, Doppler frequency shifts, interference from an other





6 measured around 500 kHz from the edge of the main lobe; the worst case between the upper and

lower shoulders is used

DVB-T signal and from analogue PAL/SECAM TV signals, transmitter non-linearity.

The test results have been largely in line with the performance predicted by computer

simulations, and allow to gain an important insight on the characteristics of the

sophisticated modulation, channel coding and equalisation techniques of the DVB-T

system. Nevertheless the equipment being tested at the moment is only a first

generation of equipment, and consequently the tests allowed to identify few channel

configurations in which the performance was not as good as expected. On the basis

to the test results, further activity is ongoing to improve the receiver algorithms. When

a variety of DVB-T compliant receivers will become available, the test results will be

compared to identify a “reference receiver model”, which should become the basis

for radio-frequency planning and service coverage predictions.



10. REFERENCES



1. ETSI, "Digital broadcasting systems for television, sound and data services;

framing structure, channel coding and modulation for digital terrestrial television",

ETS 300744, 1997



2. Moller: “COFDM and the choice of parameters for DVB-T”, Proceedings of the

20th International Television Symposium, Montreux 1997



3. Stott: “Explaining some of the magic of COFDM”, Proceedings of the 20th

International Television Symposium, Montreux 1997



4. Oliphant: “ VALIDATE - verifying the European specification for digital terrestrial

TV and preparing the launch of services”, Proceedings of the 20th International

Television Symposium, Montreux 1997



5. Morello, V.Mignone, M.Visintin “A High SFN coverage algorithm for DVB-T

receivers” to be published in the Proceedings of the International Broadcasting

Convention, IBC’97, Amsterdam



11. ANNEX 1: AN OVERVIEW OF THE DVB-T SYSTEM

The DVB systems designed for the various media show a great commonality of sub-

systems, including video and sound coding, multiplexing, error protection and

channel coding. The only sub-system which is optimised for each specific channel

characteristic (satellite, cable or terrestrial) is the “channel adapter”, including the

channel coder for error protection and the modulator. The main parameters of the

DVB-T system are described in [2], while the basic concepts of C-OFDM are

explained in [3].

The conceptual block diagram of the DVB systems is shown in Fig. A1.

Source coders Programme M UX



Video (*) (**)

M UX Outer Inner

1 Inner

Adaptation Outer Inter- Inter-

Audio M UX Convolutional M odulator

2 & Code leaver leaver

M UX Coder

Energy RS(204,188)

Data (I=12) (1/2, ...,7/8)

n Dispersal

SI

Transport M UX (*) absent in DVB-C (**) only in DVB-T

Source coding M PEG-TS multiplexing Outer adapter Channel adapter

(common sub-systems) (common sub-systems) (optimised to specific channels)

Fig.A1: Basic block diagram of the DVB Systems



The DVB systems are based on MPEG-2 vision and sound coding. The MP@ML

(Main Profile at Main Level) image coding algorithm is adopted, operating at bit-rates

up to 15 Mbit/s, but the introduction of higher MPEG-2 profiles and levels potentially

could allow for future evolution towards HDTV. The MPEG-2 Transport Stream (TS)

Multiplexing is adopted to merge in a single transmission stream a large number of

video, audio and data services. The MPEG transport packets have 188 bytes length

and are delimited by a sync byte.

The outer adapter (Fig.A1), common to all the DVB systems, provides signal

randomisation and a basic level of error protection by a Reed-Solomon outer code

RS(204,188), with correcting capability of T=8 random byte-errors. This error

correction scheme provides, for an input BER of about 2 .10-4 (independent errors), a

Quasi Error Free (QEF) quality target, i.e., less than one error-event per transmission

hour at the input of the MPEG-2 demultiplexer in the receiver.

To overcome the problem of the burst error statistic after Viterbi decoding, a

convolutional interleaving process (depth I=12 bytes) is applied, which multiplies the

burst-error correcting capability of the RS code by a factor of 12.



The DVB-T channel adapter, providing convolutional inner coding, inner interleaving

and modulation, allows to adapt the digital signals to the terrestrial channel

characteristics. It is optimised for 8 MHz channels (European UHF channellisation),

but it can be easily adapted to 7 MHz and 6 MHz channels by adjusting the receiver

sampling frequency.

The DVB-T system has been designed in order to cope with short “natural” echoes

due to multipath propagation, as well as with relatively long “artificial” echoes due to

self-interference occurring in SFNs. The system also provides good protection against

high levels of interference emanating from PAL/SECAM TV services. These

characteristics are achieved by using an OFDM modulation system associated with

convolutional error correcting coding [3], and by separating adjacent OFDM symbols

by means of a “guard interval”. Two modes of operation are defined: a “2K mode” with

guard intervals up to 56 s and a “8K mode” with guard intervals up to 224 s. The

“2K mode” is suitable for single transmitter operation and for “dense” SFN networks

with limited transmitter distances, of the order of 10 to 20 Km. The “8K mode” can

be used both for single transmitter operation and for large SFN networks, with

transmitter distances of the order of 40 to 80 Km.

The system allows different levels of QAM modulation (4, 16 and 64) and different

convolutional code rates (1/2, 2/3, 3/4, 5/6 or 7/8) to be used to trade bit rate versus

ruggedness.

The system also allows two level hierarchical channel coding and modulation,

including uniform and multi-resolution constellations, to improve the ruggedness

against channel impairments of part of the transmitted bit-stream. A low-bit-rate

programme service can thus be received under severe reception conditions, while the

other programmes in the multiplex can be correctly decoded only under less critical

conditions.

The transmitted signal is organised in “frames” of 68 OFDM “symbols”. Each OFDM

symbol is constituted by a set of K carriers (1705 for 2K and 6817 for 8K) with a

minimum frequency separation to avoid inter-carrier interference (4464 Hz for 2K and

1116 Hz for 8K) and transmitted simultaneously with a symbol duration T s. The

symbol is composed of two parts: a “useful” part with duration T u (224 s for 2K,

896 s for 8K), and a “guard interval” with a duration T g (where Tg/Tu can be 1/4, 1/8,

1/16 or 1/32). Not all of the carriers are modulated with data, since some of them (the

“pilot carriers” or “pilots”) are used to transmit reference information required by the

receiver for synchronisation (frame, frequency, phase), channel estimation,

transmission mode identification. There are three types of pilots: scattered, continual,

TPS (transmission parameter signalling).

The spacing between first and last carriers of the spectrum is 7.61 MHz,

approximately corresponding also to the total spectrum occupation because of the

steep roll-off of the OFDM signals.

11.1. System capacity and Simulated performance



In principle the C/N required by the DVB-T system is a random variable depending

on the channel response and on the adopted transmission mode (coding rate,

modulation, guard interval). Since a statistical characterisation of the system in the

various reception environments is too complex, only two “representative” channels

have been chosen in the specification (see Annex A and B of the ETSI specification)

for computer simulations, one for the fixed reception with directive antenna (F1,

Ricean channel) and one for portable reception (P1, Rayleigh channel). It should be

taken into account that these channels are not a worse case, since a single 0 dB

echo introduces higher degradations. It should also be noted that these channels

include only relatively short natural echoes (up to 5.4 s), well within the guard

interval, and do not represent a SFN situation. When an echo delay exceeds the

correct equalisation interval TF (corresponding usually to Tg), a steep transition

occurs and the echo effect becomes similar to that of an uncorrelated Gaussian noise

interference.

The required C/N (computer simulation results, ideal synchronisation and channel

estimation, excluding any implementation margin) for non-hierarchical transmission

for all combinations of coding rates and modulation types is given in Table A1, taken

from Annex A of the DVB-T specification [1]. The net bit rates after Reed-Solomon

decoder are also listed.



TABLE A1

REQUIRED C/N FOR NON-HIERARCHICAL TRANSMISSION TO ACHIEVE A BER = 2

10-4 AFTER THE VITERBI DECODER FOR ALL COMBINATIONS OF CODING RATES AND

MODULATION TYPES. THE NET BITRATES AFTER THE REED-SOLOMON DECODER

ARE ALSO LISTED.

Required C/N for

. -4

BER=2 10 after Viterbi Bit rate (Mbit/s)

QEF after Reed-Solomon

(IDEAL RECEIVER)

Modu- Code Gaussian Ricean Rayleigh

lation rate channel channel channel Tg/Tu = 1/4 Tg/Tu = 1/8 Tg/Tu = 1/16 Tg/Tu = 1/32

(F1) (P1)

QPSK 1/2 3.1 3.6 5.4 4.98 5.53 5.85 6.03

QPSK 2/3 4.9 5.7 8.4 6.64 7.37 7.81 8.04

QPSK 3/4 5.9 6.8 10.7 7.46 8.29 8.78 9.05

QPSK 5/6 6.9 8.0 13.1 8.29 9.22 9.76 10.05

QPSK 7/8 7.7 8.7 16.3 8.71 9.68 10.25 10.56

16-QAM 1/2 8.8 9.6 11.2 9.95 11.06 11.71 12.06

16-QAM 2/3 11.1 11.6 14.2 13.27 14.75 15.61 16.09

16-QAM 3/4 12.5 13.0 16.7 14.93 16.59 17.56 18.10

16-QAM 5/6 13.5 14.4 19.3 16.59 18.43 19.52 20.11

16-QAM 7/8 13.9 15.0 22.8 17.42 19.35 20.49 21.11

64-QAM 1/2 14.4 14.7 16.0 14.93 16.59 17.56 18.10

64-QAM 2/3 16.5 17.1 19.3 19.91 22.12 23.42 24.13

64-QAM 3/4 18.0 18.6 21.7 22.39 24.88 26.35 27.14

64-QAM 5/6 19.3 20.0 25.3 24.88 27.65 29.27 30.16

64-QAM 7/8 20.1 21.0 27.9 26.13 29.03 30.74 31.67



As derived by a theoretical study, for T u/Tg=1/4 the system implementation loss due

to non-ideal channel estimation and equalisation in the receiver is in the range 1.6 to

2.1 dB (including 0.3 dB loss due to pilot boosting), depending on the adopted

algorithms (indicated as 2-D or 1-D). This C/N loss could be easily reduced by

averaging in time the channel estimation, in order to filter-out the noise components,

but this would cause a reduction of the channel tracking capability in the presence of

time varying channels.