Università degli studi di Roma Tor Vergata Facoltà di Ingegneria - Corso di Laurea in Ingegneria delle Telecomunicazioni Telecomunicazioni Spaziali

Telecomunicazioni Spaziali

Università degli studi di Roma “Tor Vergata”Facoltà di Ingegneria - Corso di Laurea in Ingegneria delle Telecomunicazioni



History of Television (1 / 2) • During the 1870's Sir William Crookes constructed a vacuum tube , this produced a

beam of electrons. • In 1873 Joseph May, a telegraph operator discovered photoelectric effect. • In 1875 George Carey proposed using a system where all points in an image were

simultaneously scanned and transmitted. • In 1881, Constantin Senlecq proposed converting the picture into a number of elements

which could then be transmitted in series. • A mechanical scanning system was found by Paul Nipkow in 1884. This involved a

spinning disk at either end of transmission. • By 1911 A A Cambell Swinton suggested making use of the cathode ray tube in a

television system. • In 1930 interlaced scanning was introduced in order to improve frame rate without

otherwise altering equipment• As early as 1928, John Logie Baird was able to demonstrate a working system for colour

television which is the basis for the current system. • In 1938, Georges Valensi suggested that it should be possible to achieve dual

compatibility; colour programmes should be transmitted such that viewing on a black and white television was possible.



History of Television (2 / 2)• In 1940, Peter Goldmark demonstrated a colour system involving the three primaries

being transmitted sequentially.

• In America the NTSC (National Television System Committee) colour system was developed and launched in 1954.

• In 1961 the SECAM (Sequential Couleur á Memoire) system was developed in France, followed by its variant the PAL (phase alternation by line) system developed in Germany in 1963 which was launched in the UK in 1967.

• in 1980, Nippon Hoso Kyotai (NHK) published a standard for HDTV system

• In 1986 the USA joined Japan in pushing for the NHK system within a few months a group of companies and broadcasters from Europe known as 'Eureka EU95' had begun to develop its own HDTV standard.

• In 1993 a new group was formed, initially called the European Launching Group and later the DVB (Digital Video Broadcasting) project.

• In 1994, the leaders of the Eureka EU95 project began research to convert their analogue HDTV system to using digital technologies.

• The American ATSC (Advanced Television Systems Comittee) system, based on the Grand Alliance work was also launched towards the end of 1998.



Analog TV standards comparing

The first three systems have the following similarities:* Despite the encoding differing, all nevertheless use a similar type of

colour system, involving a luminance and two chrominance signals based on red, green and blue primaries.

* All use interlaced scanning.

* All use two separate carrier signals, one FM modulated for sound and one AM modulated for picture.

PAL SECAM NTSC NHK HDTV EU95 HDTV

Lines per frame 625 625 525 1125 1250

Field rate 50 50 60 60 50

Interlace ratio 2:1 2:1 2:1 2:1 1:1

Aspect Ratio 4:3 4:3 4:3 16:9 16:9 Bandwidth /MHz

6,7 or 8 8 6 25.5 or 27 n/a*

Viewing angle 13º 13º 11º 30º 30º

HDTV standards are not compatible with the old standards.



Advantages of digital television

• More digital channel in the same band of an analog channel thanks to MPEG-2 compression system

• CD quality stereo sound thanks to MPEG Layer II (Musicam)• Service information, such as on-screen programme guide • Near Video On-Demand (NVOD)• Simulcasting (HDTV & SDTV)• Adaptivity to the channel• QEF• Wideband Internet Now



Structure of the DVB project

The project was officially started in 1993, all members pay an annual membership fee. Commercial Modules have the task to formulate requirements on the systems, these form the basis for the work in the Technical Module, ad-hoc groups works on special subjects. After the completion of development work the commercial working groups verify the specifications for the new systems and forward them, if necessary, to the 'Steering Board' for final decision. Promotion Module is a group whose task it is to look after interested persons in all parts of the world.



DVB standards

INTERACTIVE INTERACTIVE

DVB-S

DVB-MSDVB-MC

DVB-C

DVB-T

Integrated Receiver-Decoder (IRD)

B-ISDN, ASDL, PSTN, GSM

DVD DVC

PC

SDTV / EDTV / HDTV



DVB : wideband now

ServiceProvider

SatelliteOperator

InformationProvider

User PC

DVB Card

18" dish

Leased line

The Net

Delivery of web pagesat 6- to 33 Mbit/s



DVB-SI (1/2)

• NIT The Network Information Table groups together services belonging to a particular network provider (for example more satellites located at a single orbital position form a satellite network). It contains all the tuning information that might be used during the set-up of an IRD. It is also used to signal a change in the tuning information.

• SDT The Service Description Table lists the names and other parameters associated with each service in a particular MPEG multiplex. The receiver use SDT to

compile and display a list of available services. • EIT The Event Information Table provide information ( event name, start time,

duration,...) in chronological order regarding the events contained within each service.

• TDT The Time and Date Table carries time and data information in UTC format.

DVB-SI adds information that enables DVB IRDs to automatically tune to particular services and allows services to be grouped into categories with relevant schedule information, it is based on four tables. Each table contains descriptors outlining the characteristics of the services/event being described :



DVB-SI (2/2)

• BAT The Bouquet Association Table provides a means of grouping services that might be used as one way an IRD presents the available services to the viewer, for example “children’s channels” , “sports channels” , etc.

• RST The Running Status Table it’s used to rapidly update the running status of one or more events, this may be necessary when an event starts early or late due to scheduling change. The Running Status sections are sent out only once, at the time the status of an

event changes, unlike the other SI tables which are normally repetitively transmitted. • ST The Stuffing Tables may be used to replace or invalidate either sub-tables or

complete SI tables.

In addition there are three optional SI tables:



DVB-S

In satellite’s channels there are power limitation so the main design objective is for a sistem ruggedness against noise and interference, this is achieved by means of QPSK and concatenation of convolutional and RS codes that provide a BER of 10-11 this means an error every hour.



Framing structure



Randomization for energy dispersal

• Polinomial for PRBS shall be 1+X14+X15

• Sequence “100101010000000” loaded at start of every 8 packet• Sync byte not randomized • Also operating if there is no input stream• SYNC1 bit-wise inverted to initialize the descrambler

Randomizing is necessary to decorrelate the transmitted spectrum from the data content, this is achieved by means of a Pseudo Random Binary Sequence generator



Outer Coding (1/3)RS codes are a block coding technique, the data stream is broken up into blocks and redundant data is then added to each block, the data is further subdivided into a number of symbols. DVB uses a RS (204,188) code, utilizing sixteen check symbols per 188 information symbols for a total codeword length of 204 symbols. RS encoding then consists of the generation of these check symbols from the original data. The process is based upon finite field arithmetic so named because the result of any operation is still an element of the field. The field elements are all values from 0 to 2 m -1, where m is the number of bits per symbol. The field polynomial is used to determine the order of the elements in the finite field, DVB uses 8 bit symbols { m =8} and a field polynomial of 285 p(x) = x8+x4+x3+x2+1 .

The last item that needs to be known to generate a particular RS implementation is the generator polynomial starting root, DVB uses a generator polynomial starting at root zero

g(x) = (x+λ0) (x+λ1)… (x+λ15) con λ = 02HEX .



Outer Coding (2/3)• DVB RS [204,188] code can correct (204-188)/2 or 8 errors per 204-symbol

codeword. A burst error of 57 to 64 consecutive bits (dependent upon whether the error starts on a symbol boundary) can be corrected by that RS code. If these same errors are more evenly spaced within the codeword, however, it will require many more check symbols to correct all of the errors. For this reason, RS codes are generally combined with other coding methods such as Viterbi, which is more suited to correcting evenly distributed errors.

* The shortened RS code may be implemented by adding 51 bytes, all set to 0, before the information bytes at the input of a (255,239) encoder. After the RS coding procedure these null bytes shall be disharged.



Outer Coding (3/3)Input data stream is clocked back out of the function while being fed back into the check symbol generation circuitry. A series of finite field adds and multiplies results in each register containing one check symbol after the entire input data stream has been entered. Check symbols are shifted out at the end of the original message.

The check symbols, which form the remainder in the encoder section, will cause the syndrome calculation to be zero in the case of no errors. If there are errors, the resulting polynomial is passed to the Euclid algorithm, where the factors of the remainder are found. The result is then evaluated for each of the incoming symbols

over many iterations, and any errors are found and corrected.

On receiver side the incoming symbols are divided into the generator polynomial in the Syndrome calculation block.



Convolutional interleavingIt is not economic to cover every code word against burst because they do not occur often enough. The solution is to use a technique known as interleaving.

Based on the Forney approach, it is composed of I=12 branches , each one of these contain a shift register whose dept depend on the branch. For syncronization purpose, the sync bytes shall always be routed in the branch “0” corrisponding to a null delay.



Inner codingIn DVB Viterbi convolutional coding may be used to prevent random errors from reducing the power of the interleave scheme.

Following interleave, the data are fed to a shift register. The contents of the shift register produce 2 outputs that represent different parity checks on the input data so that bit errors can be corrected.

There will be 2 output bits for every input bit therefore the coder is described as a ½ coder but any rate between 1 and ½ may be realized by means of puncturing , in this way it is possible to adjust the correcting power as a function of the link quality.



Baseband shaping & Modulation

Prior to modulation, the I and Q signals shall be square root raised cosine filter with a roll-off factor α that shall be 0,35 .

The system shall employ conventional Gray-coded QPSK modulation with assolute mapping (no differential coding).



DVB-C

The cable network system has the same core as the satellite system, but the modulation system is based on quadrature amplitude modulation (QAM) rather than QPSK, and no inner-code forward error-correction is needed. The system is centred on 64-QAM, but lower-level systems, such as I6-QAM and 32-QAM can also be used. In each case, the data capacity of the system is traded against robustness of the data. Higher-level systems, such as 128-QAM and 256-QAM may also become possible, but their use will depend on the capacity of the cable network to cope with the reduced decoding margin. In terms of capacity, an 8MHz channel can accommodate a payload capacity of 38.5 Mbit/s if 64-QAM is used, without spill-over into adjacent channels.



Byte to symbol mapping

After convolutional interleaving, an exact map of bytes into symbols shall be performed, the process is illustrated for the case of a 64-QAM.

The two MSB of each symbol shall then be differentially coded in order to obtain a π/2 rotation-invariant QAM constellation.



QAM constellations



DVB-T



Hierarchical modulation

• The LowPriority stream is of higher bitrate, but lower robustness than the HP one.

• For example in a 64QAM the 2 MSB could be used to map HP in a rugged QPSK while the other bits are used to map the LP with higher bit rate but lower robustness

• On LP & HP are applied different code rates.

The splitter produces two streams, HP & LP

Scope :• The HP stream is received also with really bad channel

• It is possible to receive the same signal with different quality receivers.



Inner interleaving

Hierarchical and non hierarchical inner interleaving provide mapping of input bit onto output modulation symbols in this case is shown 64-QAM.

For hierarchical mode, the two MSB are mapped in a QPSK .



OFDM : why ?Differently from satellite communication where we have one single direct path from transmitter to receiver in the classical terrestrial broadcasting scenario we have to deal with a multipath- channel: The transmitted signal arrives at the receiver in various paths of different length. Since multiple versions of the signal interfere with each other (inter symbol interference (ISI)) it becomes very hard to extract the original information.



OFDM : the concept

The common representation of the multipath channel is the channel impulse response (cir) of the channel which is the signal at the receiver if a single pulse is transmitted

Let's assume a system transmitting discrete information in time intervals T. The critical measure concerning the multipath-channel is the delay τmax of the longest path with respect to the earliest path. A received symbol can theoretically be influenced by (τmax / T) previous symbols.

The scenario we are dealing with in DVB-T is characterized by the following conditions: Transmission Rate : R = 1/T = 7.4Msym/sMaximum channel delay : τmax = 224μsFor the single carrier system this results in an ISI of τmax / T = 1600Instead in the case that the original data stream of rate R is multiplexed into N parallel data streams of rate Rrnc = 1/Trnc = R/N the ISI will be τmax / Trnc = τmax / (N T) = 0.2 if N = 8192



OFDM : the orthogonality principle (1/2) In OFDM the subcarrier pulse used for transmission is chosen to be rectangular. This has the advantage that the task of pulse forming and modulation can be performed by a simple Inverse Discrete Fourier Transform (IDFT) which can be implemented very efficiently as a I Fast Fourier Transform (IFFT). Accordingly in the receiver we only need a FFT to reverse this operation. According to the theorems of the Fourier Transform the rectangular pulse shape will lead to a sin(x)/x type of spectrum of the subcarriers



OFDM : the orthogonality principle (2/2)Obviously the spectrums of the subcarriers are not separated but overlap. The reason why the information transmitted over the carriers can still be separated is the so called orthogonality relation giving the method its name. By using an IFFT for modulation we implicitly chose the spacing of the subcarriers in such a way that at the frequency where we evaluate the received signal (indicated as arrows) all other signals are zero. In order for this orthogonality to be preserved the following must be true:

1.The receiver and the transmitter must be perfectly synchronized. 2.The analog components, part of transmitter and receiver, must be of very high quality. 3.There should be no multipath channel.

In particular the last point is quite a pity, since we have chosen this approach to combat the multipath channel. Fortunately there's an easy solution for this problem: The OFDM symbols are artificially prolonged by periodically repeating the 'tail' of the symbol and precede the symbol with it.

At the receiver this so called guard interval is removed again. As long as the length of this interval Δ is longer than the maximum channel delay τmax all reflections of previous symbols are removed and the orthogonality is preserved.



OFDM : frame structure (1/2)

• Each symbol is constituited by a set of 6817 carriers in the 8K mode or 1705 carriers in the 2K mode, it is transmitted with a duration TS = TU+Δ .

• Each trame has a duration og TF and consist of 68 symbol.

• Four frame constitute one super-frame.

• Scattered pilot cells • Continual pilot carriers• TPS carriers

In addition to the transmitted data an OFDM frame contain also :

The number of useful data carriers is 6048 for 8K mode and 1512 for 2K mode.



OFDM : frame structure (2/2)

• The scattered pilots use some carriers inside the symbol and this information is transmitted at boosted power level

• There are 177 continual pilot for the 8K mode and 45 continual pilot for the 2K mode.

Pilots can be used for syncronization and channel estimation , are modulated according to a PRBS corrisponding to their respective carrier index :

TPS sygnal is referred to an OFDM frame constituited of 68 symbols, each one of this convey one TPS bit so we have 68 bits so used :



DVB-CA (1/2)

A conditional access system makes use of both scrambling and encryption techniques in order to prevent reception by unauthorised persons. An encrypted message, known as an Entitlement Control Message (ECM) or multi-session key is used to send coded keys, known as control words to the receiver. This gives it the information necessary to descramble the signal. However, this will only occur if authority is given by an Entitlement Management Message (EMM). The control word is frequently changed (several times a minute) while the ECM is changed far less often, about once a month.



DVB-CA (2/2)

SimulcryptWith the use of MPEG-2 multiplexing of multiple signals into a single transmission, it is possible to have many separate ECM's which are sent together with the program information. This allows several different CA systems to operate together within a single scrambled broadcast. This requires a Common Scrambling Algorithm throughout the CA systems involved.

MulticryptIn a Common Interface environment, it is possible for a user to receive a service from a different CA system provider (which does not make use of Simulcrypt with other CA systems) by the addition of a module to the interface. This would allow the receiving of multiple signals from different sources, all with different CA systems, providing all of the different modules were available. This does however require the modules to be made for each of the CA systems in question. This is unlikely to occur given the cheaper alternative of Simulcrypt.

After early attempts to create a single standard encryption system failed to provide the level of security required and different systems began to emerge, actually on the market are used :



DVB against all …

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Università degli studi di Roma Tor Vergata Facoltà di Ingegneria - Corso di Laurea in Ingegneria delle Telecomunicazioni Telecomunicazioni Spaziali