Digital Transmission of TV Omid Reza Marouzi 1Shahrood University of Technology

Bandpass Digital Modulation Modulation for Data Communication – The four main types of modulation used in modern modems are: 1.Frequency-shift keying (FSK) 2.Phase-shift keying (PSK) 3.Quadrature amplitude modulation (QAM) 4.Orthogonal frequency division multiplexing (OFDM) 2Shahrood University of Technology

FSK Modulation for Data Communication: Frequency- Shift Keying (FSK) – Frequency-shift keying (FSK) is the oldest and simplest form of modulation used in modems. – In FSK, two sine-wave frequencies are used to represent binary 0s and 1s. – A binary 0, usually called a space, has a frequency of 1070 Hz. – A binary 1, referred to as a mark, is 1270 Hz. – These two frequencies are alternately transmitted to create the serial binary data. 3Shahrood University of Technology

FSK Figure 1: Frequency-shift keying. (a) Binary signal. (b) FSK signal. 4Shahrood University of Technology

PSK Modulation for Data Communication: Phase-Shift Keying – In phase-shift keying (PSK), the binary signal to be transmitted changes the phase shift of a sine-wave character depending upon whether a binary 0 or binary 1 is to be transmitted. – A phase shift of 180°, the maximum phase difference that can occur, is known as a phase reversal, or phase inversion. – During the time that a binary 0 occurs, the carrier is transmitted with one phase; when a binary 1 occurs, the carrier is transmitted with a 180° phase shift. 5Shahrood University of Technology

PSK Figure 2: Binary phase-shift keying. 6Shahrood University of Technology

QPSK Modulation for Data Communication: QPSK – One way to increase the binary data rate while not increasing the bandwidth required for the signal transmission is to encode more than 1 bit per phase change. – In the system known as quadrature, quarternary, or quadra phase PSK (QPSK or 4-PSK), more bits per baud are encoded, the bit rate of data transfer can be higher than the baud rate, yet the signal will not take up additional bandwidth. – In QPSK, each pair of successive digital bits in the transmitted word is assigned a particular phase. – Each pair of serial bits, called a dibit, is represented by a specific phase. 7Shahrood University of Technology

QPSK Figure 3: Quadrature PSK modulation. (a) Phase angle of carrier for different pairs of bits. (b) Phasor representation of carrier sine wave. (c) Constellation diagram of QPSK. 8Shahrood University of Technology

QPSK Modulator Modulation for Data Communication: QPSK – The QPSK modulator consists of a 2-bit shift register implemented with flip-flops, commonly known as a bit splitter. – The serial binary data train is shifted through the register. – The bits from the flip-flops are applied to balanced modulators. – The carrier oscillator is applied to one balanced modulator and through a 90° phase shifter to another balanced modulator. – The outputs of the balanced modulators are linearly mixed to produce the QPSK signal. 9Shahrood University of Technology

QPSK Modulator Figure 4: A QPSK modulator. 10Shahrood University of Technology

QAM Modulation for Data Communication: QAM – One of the most popular modulation techniques used in modems for increasing the number of bits per baud is quadrature amplitude modulation (QAM). – QAM uses both amplitude and phase modulation of a carrier. – In 8-QAM, there are four possible phase shifts and two different carrier amplitudes. – Eight different states can be transmitted. – With eight states, 3 bits can be encoded for each baud or symbol transmitted. – Each 3-bit binary word transmitted uses a different phase-amplitude combination. 11Shahrood University of Technology

8-QAM Figure 5: A constellation diagram of a 8-QAM signal. 12Shahrood University of Technology

Spectral Efficiency – Spectral efficiency is a measure of how fast data can be transmitted in a given bandwidth (bps/Hz). – Different modulation methods give different efficiencies. ModulationSpectral efficiency, bps/Hz FSK<1 GMSK1.35 BPSK1 QPSK2 8-PSK3 16-QAM4 13Shahrood University of Technology

Noise & BER Spectral Efficiency and Noise – The signal-to-noise (S/N) ratio clearly influences the spectral efficiency. – The greater the noise, the greater the number of bit errors. – The number of errors that occur in a given time is called the bit error rate (BER). – The BER is the ratio of the number of errors that occur to the number of bits that occur in a one second interval. 14Shahrood University of Technology

OFDM Shahrood University of Technology15 Orthogonal Frequency-Division Multiplexing (OFDM) – A wideband modulation method called OFDM is growing in popularity. – OFDM is also known as multicarrier modulation (MCM). – Although OFDM is known as a modulation method, the term frequency-division multiplexing is appropriate because the method transmits data by simultaneously modulating segments of the high- speed serial bit stream onto multiple carriers spaced throughout the channel bandwidth.

OFDM Shahrood University of Technology16 Orthogonal Frequency-Division Multiplexing (OFDM) – The carriers are frequency-multiplexed in the channel. – The data rate on each channel is very low, making the symbol time much longer than predicted transmission delays. – This technique spreads the signals over a wide bandwidth, making them less sensitive to the noise, fading, reflections, and multipath transmission effects common in microwave communication.

OFDM Shahrood University of Technology17 Figure 6: Concept of OFDM.

OFDM Shahrood University of Technology18 Figure 7: Simplified processing scheme for OFDM in DSP.

Usage of OFDM Shahrood University of Technology19 OFDM is used (among others) in the following systems: IEEE a&g (WLAN) systems IEEE a (WiMAX) systems ADSL (DMT = Discrete MultiTone) systems DAB (Digital Audio Broadcasting) DVB-T (Digital Video Broadcasting) OFDM is spectral efficient, but not power efficient (due to linearity requirements of power amplifier). OFDM is primarily a modulation method; OFDMA is the corresponding multiple access scheme.

OFDM system block diagram Shahrood University of Technology20 IFFT Coding & Interl. Coding & Interl. Bit-to- symbol mapping Bit-to- symbol mapping S/P Add CP Add CP FFT P/S Sync Modu- lation Modu- lation Demod. Deinterl. & Decoding Deinterl. & Decoding Channel

Subcarrier modulation Shahrood University of Technology21 Modulation BPSK QPSK 16-QAM 64-QAM Bit rate 6 Mbit/s 9 Mbit/s 12 Mbit/s 18 Mbit/s 24 Mbit/s 36 Mbit/s 48 Mbit/s 54 Mbit/s BPSK = Binary Phase Shift Keying (PSK) QPSK = Quaternary PSK QAM = Quadrature Amplitude Modulation Re Im 16-QAM signal constellation in the complex plane

Why (for instance) 54 Mbit/s ? Shahrood University of Technology22 Symbol duration = 4  s Data-carrying subcarriers = 48 Bits / subchannel = 6 (64-QAM) Bits / OFDM symbol = 6 x 48 = 288 Channel coding: number reduced to 3/4 x 288 = 216 bits/symbol => Bit rate = 216 bits / 4  s = 54 Mbit/s Symbol duration = 4  s Data-carrying subcarriers = 48 Bits / subchannel = 6 (64-QAM) Bits / OFDM symbol = 6 x 48 = 288 Channel coding: number reduced to 3/4 x 288 = 216 bits/symbol => Bit rate = 216 bits / 4  s = 54 Mbit/s

Subcarrier modulation and coding Shahrood University of Technology23 N data subcarriers or subchannels carry N data symbols in parallel (= transmitted at the same time). A symbol carries 1 bit (BPSK), 2 bits (4-PSK), 4 bits (16-QAM), or 6 bits of user data (64-QAM). N data symbols in parallel form one OFDM symbol. For each modulation method, there are several coding options for FEC (Forward Error Control). They must be taken into account when calculating user data rates, as shown on the previous slide. Typical coding options: 1/2 (convolutional encoding), 2/3 and 3/4 (puncturing) coding rates.

Gray bit-to-symbol mapping in QAM Shahrood University of Technology24 Gray bit-to-symbol mapping is usually used in QAM systems. The reason: it is optimal in the sense that a symbol error (involving two adjacent symbols in the QAM signal constellation) results in a single bit error Example for 16-QAM

Subcarrier signal in time domain Shahrood University of Technology25 Time Guard time for preventing intersymbol interference In the receiver, FFT is calculated only over this time period Symbol duration Next symbol TGTG IEEE a&g: T G = 0.8  s, T FFT = 3.2  s IEEE a offers flexible bandwidth allocation (i.e. different symbol lengths) and T G choice: T G /T FFT = 1/4, 1/8, 1/16 or 1/32 T FFT

Orthogonality between subcarriers (1) Shahrood University of Technology26 Guard time Symbol part that is used for FFT calculation at receiver Subcarrier n Subcarrier n+1 Previous symbol Next symbol Orthogonality over this interval

Orthogonality between subcarriers (2) Shahrood University of Technology27 Guard time Symbol part that is used for FFT calculation at receiver Subcarrier n Subcarrier n+1 Previous symbol Next symbol Orthogonality over this interval Each subcarrier has an integer number of cycles in the FFT calculation interval (in our case 3 and 4 cycles). If this condition is valid, the spectrum of a subchannel contains spectral nulls at all other subcarrier frequencies.

Orthogonality between subcarriers (3) Shahrood University of Technology28 Orthogonality over the FFT interval: Phase shift in either subcarrier - orthogonality over the FFT interval is still retained:

Time vs. frequency domain Shahrood University of Technology29 TGTG T FFT Square-windowed sinusoid in time domain => "sinc" shaped subchannel spectrum in frequency domain

Subchannels in frequency domain Shahrood University of Technology30 Single subchannelOFDM spectrum Spectral nulls at other subcarrier frequencies Subcarrier spacing = 1/T FFT

Multipath effect on subcarrier n (1) Shahrood University of Technology31 Guard time Symbol part that is used for FFT calculation at receiver Subcarrier n Previous symbol Next symbol Delayed replicas of subcarrier n

Multipath effect on subcarrier n (2) Shahrood University of Technology32 Guard time Symbol part that is used for FFT calculation at receiver Subcarrier n Previous symbol Next symbol Delayed replicas of subcarrier n Guard time not exceeded: Delayed multipath replicas do not affect the orthogonality behavior of the subcarrier in frequency domain. There are still spectral nulls at other subcarrier frequencies.

Multipath effect on subcarrier n (3) Shahrood University of Technology33 Guard time Symbol part that is used for FFT calculation at receiver Subcarrier n Previous symbol Next symbol Delayed replicas of subcarrier n Mathematical explanation: Sum of sinusoids (with the same frequency but with different magnitudes and phases) = still a pure sinusoid with the same frequency (and with resultant magnitude and phase).

Multipath effect on subcarrier n (4) Shahrood University of Technology34 Guard time Symbol part that is used for FFT calculation at receiver Subcarrier n Previous symbol Next symbol Replicas with large delay

Multipath effect on subcarrier n (5) Shahrood University of Technology35 Guard time Symbol part that is used for FFT calculation at receiver Subcarrier n Previous symbol Next symbol Replicas with large delay Guard time exceeded: Delayed multipath replicas affect the orthogonality behavior of the subchannels in frequency domain. There are no more spectral nulls at other subcarrier frequencies => this causes inter-carrier interference.

Multipath effect on subcarrier n (6) Shahrood University of Technology36 Guard time Symbol part that is used for FFT calculation at receiver Subcarrier n Previous symbol Next symbol Replicas with large delay Mathematical explanation: Strongly delayed multipath replicas are no longer pure sinusoids!

Task of pilot subcarriers Shahrood University of Technology37 Pilot subcarriers contain signal values that are known in the receiver. These pilot signals are used in the receiver for correcting the magnitude (important in QAM) and phase shift offsets of the received symbols (see signal constellation example on the right). Re Im Received symbol Transmitted symbol

Transmitted and received subcarrier n Shahrood University of Technology38 Guard time Symbol part that is used for FFT calculation at receiver Transmitted subcarrier n Previous symbol Next symbol Received subcarrier n Magnitude error Phase error

Error Detection and Correction Shahrood University of Technology39 When high-speed binary data is transmitted over a communication link, whether it is a cable or radio, errors will occur. These errors are changes in the bit pattern caused by interference, noise, or equipment malfunctions. Such errors will cause incorrect data to be received. The number of bit errors that occur for a given number of bits transmitted is referred to as the bit error rate (BER).

Shahrood University of Technology40 Error Detection and Correction The process of error detection and correction involves adding extra bits to the data characters to be transmitted. This process is generally referred to as channel encoding. The data to be transmitted is processed in a way that creates the extra bits and adds them to the original data. At the receiver, these extra bits help in identifying any errors that occur in transmission caused by noise or other channel effects.

Shahrood University of Technology41 A key point about channel encoding is that it takes more time to transmit the data because of the extra bits. These extra bits are called overhead in that they extend the time of transmission. Channel encoding methods fall into to two separate categories, error detection codes and error correction codes. Error Detection and Correction

Parity Check Shahrood University of Technology42 Error Detection – Many different methods have been used to ensure reliable error detection: Redundancy is a method that ensures error-free transmission by sending each character or message multiple times until it is properly received. Encoding schemes like the RZ-AMI are used whereby successive binary 1 bits in the bit stream are transmitted with alternating polarity.

CRC Shahrood University of Technology43 Error Detection One of the most widely used systems is known as parity, in which each character transmitted contains one additional bit, known as a parity bit. The cyclical redundancy check (CRC) is a mathematical technique used in synchronous data transmission that effectively catches 99.9 percent or more of transmission errors.

FEC Shahrood University of Technology44 Error Correction – A number of efficient error-correction schemes have been devised to complement error detection methods. – The process of detecting and correcting errors at the receiver so that retransmission is not necessary is called forward error correction (FEC). – There are two basic types of FEC: block codes and convolutional codes.

Block Codes Shahrood University of Technology45 Error Correction: Block-Check Character – The block check character (BCC) is also known as a horizontal or longitudinal redundancy check (LRC). – It is the process of logically adding, by exclusive- ORing, all the characters in a specific block of transmitted data. – The final bit value for each horizontal row becomes one bit in a character known as the block-check character (BCC), or the block-check sequence (BCS).

Reed Solomon Shahrood University of Technology46 Error Correction: Block-Check Character – The most popular FEC codes are the Hamming and Reed Solomon codes. – These codes add extra parity bits to a transmitted word, process them using unique algorithms, and detect and correct bit errors. – Interleaving is a method used in wireless systems to reduce the effects of burst errors.

Convolutional Coding Shahrood University of Technology47 Error Correction: Convolutional Codes – Convolutional encoding creates additional bits from the data as do Hamming and Reed Solomon codes, but the encoded output is a function of not only the current data bits but also previously occurring data bits. – Convolutional codes pass the data to be transmitted through a special shift register. – As the serial data is shifted through the shift register flip-flops, some of the flip-flop outputs are XORed together to form two outputs.

Convolutional Coding Shahrood University of Technology48 Error Correction: Convolutional Codes – These two outputs are the convolutional code, and this is what is transmitted. – The original data itself is not transmitted. – Instead, two separate streams of continuously encoded data are sent. – Since each output code is different, the original data can more likely be recovered at the receiver by an inverse process.

Convolutional Coding Shahrood University of Technology49 Figure 8: Convolutional encoding uses a shift register with exclusive-OR gates to create the output.

DVB Shahrood University of Technology50  DVB Project is an industry-led consortium of over 300 companies  The DVB Project was launched on 10 th September, 1993  In 1995 it was basically finished and became operational  There are several sub-standards of the DVB standard  DVB-S (Satellite) – using QPSK – 40 Mb/s  DVB-T (Terrestrial) – using QAM – 50 Mb/s  DVB-C (Cable) – using OFDM – 24 Mb/s  These three sub-standards basically differ only in the specifications to the physical representation, modulation, transmission and reception of the signal

DVB Shahrood University of Technology51  The DVB stream consists of a series of fixed length packets which make up a Transport Stream (TS).  The packets support ‘streams’ or ‘data sections’.  Streams carry higher layer packets derived from an MPEG stream.  Data sections are blocks of data carrying signaling and control data.  DVB is actually a support mechanism for MPEG.  One MPEG stream needing higher instantaneous data can ‘steal’ capacity from another with spare capacity.

DVB Stream Structure Shahrood University of Technology52 MPEG-2 Transport MUX Packet Data structure after outer interleaving: Interleaving Depth I=12 Bytes Randomized transport packet: Sync Bytes and randomized Data byte Reed-Solomon RS(204,188,8) Error Protected packets.

Applied Process to Data Stream Shahrood University of Technology53 Transport multiplex adaptation and randomization for energy dispersal; Outer coding (i.e. Reed-solomon code); Outer interleaving (i.e. Convolutional interleaving); Inner coding (i.e. Punctured convolutional code); Inner interleaving (bit-wise or symbol interleaving are block based); Mapping and modulation; Orthogonal frequency division multiplexing (OFDM) transmission.

DVB Packet Shahrood University of Technology54 The DVB Transport stream consists of a series of packets 204 bytes long, 188 bytes carry information and the other 16 bytes carry an outer Reed-Solomon code. The packet is short and can survive a noisy channel subject to interference. 188 Bytes16 Bytes Information Reed Solomon Parity block

DVB Functional Block Diagram Shahrood University of Technology55