logging in or signing up Digital Broadcasting Ahmedmalaa Download Post to : URL : Related Presentations : Share Add to Flag Embed Email Send to Blogs and Networks Add to Channel Uploaded from authorPOINT lite Insert YouTube videos in PowerPont slides with aS Desktop Copy embed code: Embed: Flash iPad Dynamic Copy Does not support media & animations Automatically changes to Flash or non-Flash embed WordPress Embed Customize Embed URL: Copy Thumbnail: Copy The presentation is successfully added In Your Favorites. Views: 2150 Category: Science & Tech.. License: All Rights Reserved Like it (3) Dislike it (0) Added: August 27, 2010 This Presentation is Public Favorites: 3 Presentation Description No description available. Comments Posting comment... By: roshnik (21 month(s) ago) hello sir, I NEED DIS PRESENTATION FOR SOME RESERCH WORK. PLEASE MAIL ME DIS AT email@example.com. thank you Saving..... Post Reply Close Saving..... 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Multiplexing is applied to independent signals, but these signals are subset of one main signal ! In OFDM, the signal is itself split first into independent channels, modulated, and then re-multiplexed to compose the OFDM carrier In this sense, OFDM is a special case of FDM. If a FDM channel is a faucet with one water stream, then OFDM is a shower ! Slide 3: Ahmed M. Alaa (c) 2010 Orthogonal Frequency Division Multiplexing (OFDM) By Analogy with making a shipment via truck; we can imagine the case as one truck carrying an amount of data (FDM) and a set of trucks carrying the same amount of data (OFDM), in case of accident, only data will be partially affected in the OFDM case! The subsets of signals are carried via Sub-carriers, the sub-carriers must be orthogonal for the idea to work If the independent sub-channels are multiplexed by frequency: multi-carrier transmission, if multiplexed by code: multi-code transmission OFDM is multi-carrier FDM; each channel occupies sub-channels consisting of orthogonal carriers rather than a single carrier. Slide 4: Ahmed M. Alaa (c) 2010 Orthogonal Frequency Division Multiplexing (OFDM) What is the importance of carrier orthogonality? The sine and cosine carriers have their area zero through an integer number of periods, when multiplying two sinusoids having their frequencies be integer multiples of each other, the result has a zero area through a period. Thus sub-carriers are all orthogonal if they have their frequencies integer multiples. These frequencies are called harmonics Orthogonality Allows for the transmission on a lot of sub-carriers in a tight frequency space without interference from each other; in essence, this is similar to CDM. In FDM: Interference is avoided by keeping the channels apart by guard bands (10% of bandwidth), no care about orthogonality ! In OFDM: Interference is avoided by sub-carrier orthogonality, spectra of sub-channels are overlapped in the frequency space Slide 5: Ahmed M. Alaa (c) 2010 Orthogonal Frequency Division Multiplexing (OFDM) An example of OFDM using 4 sub-carriers: In OFDM, we have N Carriers, where N can range between any integer from 16 to 1024 in present technologies, it depends on the environment in which the system operates Assuming a system with the first few bits are: 1,1,-1,-1,1,1,1,-1,1,-1,-1,-1,-1,1,-1,-1,-1,1,… We split this main stream to sub-signals. The serial to parallel conversion is given in the following table, every sub-channel will have lesser bandwidth (1/4 of main stream Bandwidth). Slide 6: Ahmed M. Alaa (c) 2010 Orthogonal Frequency Division Multiplexing (OFDM) Now we want to know the frequency of the sub-carrier C1? Assuming the symbol rate is 1 and sampling frequency is 1. From the Nyquist theorem, the maximum frequency in the signal has to be half the sampling frequency. The information rate per carrier is ¼ or 1 symbol per second for the whole four carriers. The smallest frequency that carry a bit rate of ¼ is ½ Hz. We will pick 1 Hz for convenience as the carrier frequency. The minimum carrier frequency is the baseband bandwidth. Other frequencies C2, C3 and C4 are harmonics of C1. The harmonics on ½ Hz are: 1 Hz, 3/2 Hz and 2 Hz The harmonics on 1 Hz are: 2 Hz, 3 Hz and 4 Hz We will pick the BPSK as our modulation scheme. We can pick any modulation scheme with no limit: QPSK, 8-PSK, and 32-QAM. Slide 7: Ahmed M. Alaa (c) 2010 Orthogonal Frequency Division Multiplexing (OFDM) Carrier 1 We need to transmit: -1,1,-1,-1,1,-1. We have a carrier frequency 1 Hz. Carrier 2 We need to transmit: 1,1,-1,1,1,-1. We have a carrier frequency 2 Hz. Slide 8: Ahmed M. Alaa (c) 2010 Orthogonal Frequency Division Multiplexing (OFDM) Carrier 3 We need to transmit: -1,1,1,-1,-1,1. We have a carrier frequency 3 Hz. Carrier 4 We need to transmit: -1,-1,-1,-1,-1,1. We have a carrier frequency 4 Hz. Slide 9: Ahmed M. Alaa (c) 2010 Orthogonal Frequency Division Multiplexing (OFDM) What we have done is taken the bit stream , distributed the bits, one bit at a time to the four sub-carriers as shown Slide 10: Ahmed M. Alaa (c) 2010 Orthogonal Frequency Division Multiplexing (OFDM) What if we added the four parallel channel ? We have a parallel to serial conversion which is realized mathematically by the IFFT (inverse fast fourier transform). The generated OFDM signal is shown in the figure: We call an IFFT block with N carriers: N-point IFFT What is the physical meaning of this ? The row 1 of the table represents the amplitudes of a certain range of sinusoids! Thus, the IFFT would retrieve a time domain signal. For example, at the first N instants, we capture the amplitude of a low frequency (C1) and assign it to the carrier (C1), and so on. Slide 11: Ahmed M. Alaa (c) 2010 Orthogonal Frequency Division Multiplexing (OFDM) Every row in the table can be considered as a spectrum as shown below. Actually those rows are not spectra but the IFFT is a mathematical concept that doesn’t care what goes in and out ! Each row has only 4 frequencies. Each of these rows can be converted to a time domain signal. The input to IFFT is a time domain signal disguising as spectrum. The IFFT converts N points to a time domain signal correspond to a symbol that conveys four bits. Slide 12: Ahmed M. Alaa (c) 2010 Orthogonal Frequency Division Multiplexing (OFDM) Actually the IFFT block may be called FFT (They’ll produce same results), but in literature it is always called IFFT The functional diagram of an OFDM (transmitter and receiver) is shown below. What is the effect of fading on the OFDM system ? What modifications we have to do on the current scheme ? Slide 13: Ahmed M. Alaa (c) 2010 Orthogonal Frequency Division Multiplexing (OFDM) What is Fading ? If there are many paths between the transmitter and the receiver, the receiver will get many copies of the signal with different delays and different gains. Slide 14: Ahmed M. Alaa (c) 2010 Orthogonal Frequency Division Multiplexing (OFDM) Fading is a big problem for signals, the demodulator must have a way to handle this ! How would we handle it in a moving car, urban areas with tall buildings, and populated areas ? The maximum delay for a signal is called the spread delay, and it varies per environment. The response of a channel with fading: The Deep fades frequencies: are some frequencies in the channel’s band that aren’t allowed to carry any info. Slide 15: Ahmed M. Alaa (c) 2010 Orthogonal Frequency Division Multiplexing (OFDM) Frequency selective fading: The fading that occurs in a non-uniform way across the frequency band, it occurs at selected frequencies that are function of environment Rayleigh fading: There is no direct component (no Line of sight component!), all received components are reflected Flat fading: The delay spread is less than symbol duration The frequency selective fading occurs when the delay spread is much larger than the symbol duration ! What about OFDM? Because the information are split into many sub-carriers, only a small subset of data are affected (carriers at deep fading frequencies) Slide 16: Ahmed M. Alaa (c) 2010 Orthogonal Frequency Division Multiplexing (OFDM) An OFDM signal has an advantage in a channel with frequency selective fading ! Because only some sub-carriers are affected, instead of the whole symbol knocked out, we have a small subset of the bits are lost. With proper coding; this can be retrieved! The BER of OFDM in a fading channel is much better than QPSK/FDM which is a single carrier wideband signal. The advantage here is the diversity of the multi-carrier such that fading applies only to a small subset. The usage of Cyclic prefix To mitigate the delay spread. Assume you’re driving a car and the car in front of you splashes a water spot onto your car, what shall you do to avoid the water splash? Get farther from the car in front of you By equating the water splash with the delay spread, we consider the front symbol to splash water to the symbol in the back. In effect, these splashes are considered as noise and affect the beginning of the symbol next to the front symbol. Slide 17: Ahmed M. Alaa (c) 2010 Orthogonal Frequency Division Multiplexing (OFDM) The delayed symbol and the original symbol To mitigate the noise at the front of the symbol, we will move our symbol further away from the region of delay spread as shown below. A little of blank space has been added between symbols to catch with the delay spread But the blank spaces are not favorable for hardware that like to crank signals continuously, we have to fill blank spaces with something ! Slide 18: Ahmed M. Alaa (c) 2010 Orthogonal Frequency Division Multiplexing (OFDM) Choice 1: Let the symbol last longer ? If we just extended the symbol, the front of the symbol which is very important to figure out the phase of the symbol is now corrupted by a “splash” ! Choice 2: Moving the symbol back? If we move the symbol back, not only we’ll have a continuous signal, but we’ll also have one that can get corrupted and we don’t care because this part will be cut prior to demodulation Slide 19: Ahmed M. Alaa (c) 2010 Orthogonal Frequency Division Multiplexing (OFDM) Slide the symbol to start at the edge of the delay spread and fill the guard with a copy of the symbol that appears to be the tail of the symbol . We want: We will be extending the symbol 1.25 times of its duration, to do this, copy the back of the symbol and glue it in the front. We are just adjusting the starting phase and make the duration longer ! The cyclic prefix is the superfluous bit we add to the front Theoretically we have to do this for every sub-carrier but actually we do this for the overall OFDM signal We do the Cyclic prefix after the IFFT operation and remove it at receiver before demodulation Slide 20: Ahmed M. Alaa (c) 2010 Orthogonal Frequency Division Multiplexing (OFDM) For a 32 samples OFDM, 0.25 guard space is 8 samples, this applied to the overall OFDM as shown : The Cyclic prefix: mitigates the link fading and ISI, also OFDM generally tolerates delay spread because the symbol duration increases preventing deep fades. But it increases the bandwidth ! Slide 21: Ahmed M. Alaa (c) 2010 Orthogonal Frequency Division Multiplexing (OFDM) The overall scheme after adjustments is now robust to link fading and ISI OFDM Performance The performance of the OFDM system would be evaluated, the parameters discussed are: 1- Spectrum and performance 2- BER of OFDM 3- Synchronization 4- Coding 5- Parameters of real OFDM Slide 22: Ahmed M. Alaa (c) 2010 Orthogonal Frequency Division Multiplexing (OFDM) Spectrum and performance The unshaped QPSK produces a bandwidth of (1+α)Rs. In OFDM, the adjacent carriers are overlapping, the bandwidth approaches (N+1)/N bits per Hz. So the larger the number of carriers, the better. Note that without any pulse shaping, the out of band signal is 50 dB down ! Slide 23: Ahmed M. Alaa (c) 2010 Orthogonal Frequency Division Multiplexing (OFDM) By comparing this to the QPSK signal, we find that sidebands are much lower for OFDM and the OFDM spectrum is of less variance ! Slide 24: Ahmed M. Alaa (c) 2010 Orthogonal Frequency Division Multiplexing (OFDM) BER (Bit error rate) performance OFDM does not perform well in non-linear channels due to its amplitude variations (so they are not use in satellite links), but the system is exemplary in fading environments due to the diversity of multi-carrier sub-sets. That is why it is used for moving users. Synchronization Another problem is that tight synchronization is needed. So a pilot tone is added in the sub-carrier space to equalize the channel and lock the receiver. Coding The coding used is the Convolutional coding prior to OFDM, the coded version is called (Coded OFDM) or COFDM. Slide 25: Ahmed M. Alaa (c) 2010 Orthogonal Frequency Division Multiplexing (OFDM) OFDM is used in real-world in the following applications: Modem/ADSL applications, here it is called Discrete multi-tone or (DMT), it is also used in wireless internet modem (802.11a), the specs for 802.11a are: Data rates: 6 Mbps to 48 Mbps Modulation: BPSK, QPSK, 16 QAM and 64 QAM Coding: Convolutional and Reed Solomon FFT size: 64 with 52 sub-carriers, 48 for data and 4 for carriers FFT period/symbol period: 3.2 microseconds Guard duration: 0.25 of the symbol, 0.8 microseconds Symbol time: 4 microseconds Try simulating the BER for an OFDM system using MATLAB. OFDM is the transmission technology in which most digital broadcast technologies are based on Slide 26: Ahmed M. Alaa (c) 2010 Orthogonal Frequency Division Multiplexing (OFDM) MATLAB Simulation examples: BER for an OFDM system using MATLAB. Spectrum of an OFDM based DVB-T 2k system in a Rayleigh channel Slide 27: Ahmed M. Alaa (c) 2010 Orthogonal Frequency Division Multiplexing (OFDM) Final transceiver is given by the next figure: Note that the whole spectrum are raised on a common carrier at the RF front end Slide 28: Ahmed M. Alaa (c) 2010 Digital Broadcasting technologies and Mobile TV DVB Systems and Mobile TV Introduction Media FLO overview CMMB systems Design example: MediaFLO transmitter Slide 29: Ahmed M. Alaa (c) 2010 Introduction: Broadcast Mobile technologies and handovers Slide 30: Ahmed M. Alaa (c) 2010 Introduction: Broadcast Mobile technologies and handovers Main broadcast scenarios Slide 31: Ahmed M. Alaa (c) 2010 Introduction: Broadcast Mobile technologies and handovers What is Mobile TV? Mobile television usually means television watched on a small handheld device. It may be a pay TV service broadcast on mobile phone networks or received free-to-air via terrestrial television stations from either regular broadcast or a special mobile TV transmission format. Some mobile televisions can also download television shows from the internet, including recorded TV programs and podcasts which are downloaded and stored on the mobile device for later viewing. Mobile TV is a service which allows cell phone owners to watch television on their phones from a service provider. Television data can be obtained either through an existing cellular network or a propriety network. In South Korea, mobile TV is largely divided into satellite DMB (S-DMB) and terrestrial DMB (T-DMB). Although S-DMB initially had more content, T-DMB has gained much wider popularity because it is free and included as a feature in most mobile handsets sold in the country today. Slide 32: Ahmed M. Alaa (c) 2010 Introduction: Broadcast Mobile technologies and handovers Mobile TV standards are divided into Satellite and Terrestrial: Terrestrial DVB-H (Digital Video Broadcasting - Handheld) - Europe, Asia ATSC-M/H (ATSC Mobile/Handheld) - North America T-DMB (Terrestrial Digital Mulitmedia Broadcast) - South Korea 1seg (One Segment) - Mobile TV system on ISDB-T MediaFLO - launched in US, trialled in UK and Germany DMB-T/H - China and Hong Kong DAB-IP (Digital Audio Broadcast) - UK iMB (Integrated Mobile Broadcast, 3GPP MBMS) Satellite DVB-SH (Digital Video Broadcasting - Satellite for Handhelds) S-DMB (Satellite Digital Multimedia Broadcast) - South Korea CMMB (China Mobile Multimedia Broadcasting) - China The European Union adopted DVB-H/DVB-SH over other versions of the technology in 2008. Slide 33: Ahmed M. Alaa (c) 2010 Introduction: Broadcast Mobile technologies and handovers Challenges: Power consumption: Battery technology for mobile portable devices may be stuck in a race condition. Improved battery life can be used up by the upgraded mobile content and enhanced functions Memory: To support the high buffer requirements of mobile TV. Current memory capabilities available will not be suited for long hours of mobile TV User interface design: A large number of mobile phones do not support mobile TV; users have to purchase new handsets with improved LCD display and user interface that support mobile TV viewing Processing power: Device manufacturers should improve the processing power significantly to support a MIPS (micro instructions per second) intensive application like mobile TV. Slide 34: Ahmed M. Alaa (c) 2010 DVB systems and Mobile TV Reference: Dr. Jens Johann (Deutsche Telekom) E-Mail: firstname.lastname@example.org IEEE 802 Meeting, Denver, 14th July '08 Slide 35: Ahmed M. Alaa (c) 2010 DVB systems and Mobile TV Candidates for handover Activities: DVB-T DVB-H DVB-SH DVB-IPTV It was founded in 1993 with office based in Switzerland, in an effort to convert the analogue TV to digital TV 270 member organizations Today we have 58 standards and specifications, there are over 200 million decoders all over the world The DVB aims to provide the standard and specifications for digital TV by whatever means: Satellite, cable, terrestrial, microwave, DSL,…etc Slide 36: Ahmed M. Alaa (c) 2010 DVB systems and Mobile TV: Handovers Terrestrial transmission in the UHF and VHF bands, support of several channel bandwidths, optimized for fixed reception, but also usable for portable and mobile operation Terrestrial transmission to battery powered handhelds, uses the physical layer of DVB-. Access to mobile networks is possible Hybrid network of fixed and satellite networks, a single frequency Network , coverage of large areas Support of interactive services Digital TV using IP over bidirectional fixed broadband access Slide 37: Ahmed M. Alaa (c) 2010 DVB systems and Mobile TV Technical details OFDM modulation with 2k or 8k carriers (1705 and 6817 carriers or typically 2048 and 8192 N-point FFT) Selectable Guard Interval to fight multi-path propagation Robust channel encoding by concatenating Reed-Solomon and Convolutional encoding Typical user data in a 8 MHz channel is: 20 Mbps On the horizon: DVB-T2 Higher user data rate aiming at terrestrial HDTV transmission Support of data broadcast Improved channel encoding algorithms DVB-T: Terrestrial Slide 38: Ahmed M. Alaa (c) 2010 DVB systems and Mobile TV A Typical portable DVB-T receiver Slide 39: Ahmed M. Alaa (c) 2010 DVB systems and Mobile TV Technical details Additional OFDM mode: 4k carriers The input data is formatted as IP packets Data is encapsulated into MPEG stream Typical DVB-H devices has built-in antennas Battery powered devices: Time slicing reduce battery consumption by burst transmission DVB-H: DVB to Handheld Slide 40: Ahmed M. Alaa (c) 2010 DVB systems and Mobile TV DVB-H: Power saving by Time slicing DVB-H is 25% payload in a DVB-T channel DVB-H: DVB to Handheld Slide 41: Ahmed M. Alaa (c) 2010 DVB systems and Mobile TV Technical details DVB-SH combines satellite and terrestrial transmission Two architectures: DVB-SH-A uses OFDM on both, the satellite link and the terrestrial link whereas DVB-SH-B uses TDM on the satellite link and OFDM on the terrestrial link Preferred frequency bands: 1…3 MHz Supported bandwidths: 1.75 MHz, 5/6/7/8 MHz OFDM sizes: 1k / 2k / 4k / 8k Modulations: OFDM: QPSK, 16 QAM TDM: QPSK, 8 PSK, 16APSK DVB-SH targets the S-band (2.2 GHz), 50% higher than L-Band and 4 times than L-Band. DVB-SH: Satellite to Handheld Slide 42: Ahmed M. Alaa (c) 2010 DVB systems and Mobile TV DVB-SH: Satellite to Handheld Slide 43: Ahmed M. Alaa (c) 2010 DVB systems and Mobile TV DVB-IPTV It is the collective name for a set of technical specifications , that facilitate the delivery of digital TV using internet protocol over bi-directional fixed broadband Mobile networks are suited to support Mobile TV but they have limited resources , broadcast systems are available to help out Transmitters are following standards and regulations while receivers are kept for designers to allow for the competition between design manufacturers. Slide 44: Ahmed M. Alaa (c) 2010 DVB systems and Mobile TV Case Study: NMI 310 DVB-T/H receiver from NMI A block diagram for the product is given by: Slide 45: Ahmed M. Alaa (c) 2010 DVB systems and Mobile TV Technical features of NMI 310 Low power consumption, less than 27 mW in time slice mode Integrated peripherals: wideband LNA’s, crystal oscillators, VCO, and PLL loop filters RAM: MPE-FEC RAM integrated Bands: VHF, UHF and L-band supported Extremely low noise: 3 dB system NF Doppler performance: 100 Hz for 16 QAM Slide 46: Ahmed M. Alaa (c) 2010 Media FLO overview Slide 47: Ahmed M. Alaa (c) 2010 Media FLO overview FLO technology was designed by Qualcomm FLO technology was designed specifically for the efficient and economical distribution of the same multimedia content to millions of wireless subscribers simultaneously. MediaFLO system architecture A MediaFLO system is comprised of four sub-systems: 1- The Network Operation Center (which consists of a National Operations Center and one or more Local Operation Centers) 2- FLO Transmitters 3- 3G Network 4- FLO-enabled devices (also known as FLO Handsets). Slide 48: Ahmed M. Alaa (c) 2010 Media FLO overview MediaFLO architecture Slide 49: Ahmed M. Alaa (c) 2010 Media FLO overview FLO Air Interface Fundamentals OFDM Modulation The FLO technology utilizes Orthogonal Frequency Division Multiplexing (OFDM) which is also utilized by Digital Audio Broadcasting (DAB), Terrestrial Digital Video Broadcasting (DVB-T) , and Terrestrial Integrated Services Digital Broadcasting (ISDB-T) OFDM can handle long delays from multiple transmitters with an appropriate length of cyclic prefix; a guard interval added to the front of the symbol (which is a copy of the last portion of the data symbol) ensures orthogonality and prevents inter-carrier interference. As long as the length of this interval is greater than the maximum channel delay, all reflections of previous symbols are removed and the orthogonality is preserved. The most fundamental tradeoff is the basic sub-carrier, or tone characteristics, which involves selection of the number of tones, as well as the cyclic prefix duration. Slide 50: Ahmed M. Alaa (c) 2010 Media FLO overview Design Key factor: Size of the transform The FLO physical layer uses a 4K mode (yielding a transform size of 4096 sub-carriers), providing superior mobile performance com-pared to an 8K mode, Robust performance can then be maintained to greater than 200 km/hour. Beyond 200 km/hour, degradation is graceful, creating minimal impact to the overall performance. This is supported by the FLO pilot structure (used for channel estimation), which enables receivers to handle delay spreads greater than the cyclic prefix. Either quadrature phase shift keying (QPSK)10 or quadrature amplitude modulation (QAM)11 is typically employed. The FLO air interface supports the use of QPSK, 16-QAM12 and layered modulation techniques. Non-uniform 16-QAM constellations (two layers of QPSK signals) with 2 bits applied per layer are utilized in layered modulation. Slide 51: Ahmed M. Alaa (c) 2010 Media FLO overview Bandwidth Requirements The FLO air interface is designed to support frequency bandwidths of 5, 6, 7, and 8 MHz. A highly desirable service offering can be achieved with a single Radio Frequency channel. In some regions, the 5 MHz allocations provided for Time Division Duplex (TDD) applications may also be applied to mobile media distribution. FLO’s air interface supports a broad range of data rates, ranging from .47 to 1.87 bits per second per hertz. In a 6 MHz channel, the FLO physical layer can achieve up to 11.2 Mbps at this bandwidth. The different data rates available enable tradeoffs between coverage and throughput. Slide 52: Ahmed M. Alaa (c) 2010 Media FLO overview Comparison with other Mobile Multicast technologies Slide 53: Ahmed M. Alaa (c) 2010 Media FLO overview Comparison with other Mobile Multicast technologies Slide 54: Ahmed M. Alaa (c) 2010 MediaFLO products Newport Media Inc. MediaFLO: NMI700 A block diagram for the product is given by: Slide 55: Ahmed M. Alaa (c) 2010 CMMB systems Slide 56: Ahmed M. Alaa (c) 2010 CMMB systems In order to create a uniform standard for mobile TV reception in China, the Chinese broadcasting authority SARFT (State Administration of Radio Film and Television) has required mobile radio operators to use a uniform standard China has standardized its own development for mobile television broadcasting. The standard is called CMMB (Chinese Mobile Multimedia Broadcasting) and was formerly also known as STiMi (Satellite Terrestrial Interactive Multi-service Infrastructure). The use of orthogonal frequency division multiplex (OFDM) with 4k/1k mode in 8 MHz/2 MHz channels and efficient error-protection mechanisms make the use of CMMB as a transmission standard ideal for mobile applications. Satellite transmission in the S band as well as terrestrial transmission are specified in accordance with the standard. Slide 57: Ahmed M. Alaa (c) 2010 CMMB systems CMMB and its characteristics are designed to enable both mobile and stationary digital TV reception with combined satellite and terrestrial broadcasting. For this purpose, a nationwide CMMB network is to be set up in China; the network was already in operation in several cities during the 2008 Olympics. The receivers can receive the terrestrial CMMB signal that is broadcast via re-transmitters as well as the direct satellite signal. Technical details Frequency: Terrestrial: UHF , Terrestrial: L Band , and Satellite: 2635 MHz - 2660 MHz Modulation: COFDM Sub-carrier Modulation: BPSK, QPSK, 16QAM Channel Bandwidth: UHF: 8 MHz (4k mode), L Band: 2 MHz (1k mode), Satellite: 25 MHz Video Source Coding: H.264 Slide 58: Ahmed M. Alaa (c) 2010 Design Example: MediaFLO physical layer using MATLAB Slide 59: Ahmed M. Alaa (c) 2010 Design Example: MediaFLO transmitter The basic structure of any transmitter Source Coding Channel Coding Interleaving Burst formatting Modulation MPEG2 Video coding (compression) Reed Solomon Assume 1 level for simplicity FLO Super frame structure OFDM Slide 60: Ahmed M. Alaa (c) 2010 Design Example: MediaFLO transmitter 1- MPEG2 Video compression Algorithm MPEG-2 is widely used as the format of digital television signals that are broadcast by terrestrial (over-the-air), cable, and direct broadcast satellite TV systems. It also specifies the format of movies and other programs that are distributed on DVD and similar discs. As such, TV stations, TV receivers, DVD players, and other equipment are often designed to this standard. MPEG-2 was the second of several standards developed by the Moving Pictures Expert Group (MPEG) and is an international standard (ISO/IEC 13818). Source Coding: MPEG-2 Video File/stream Compressed Stream Slide 61: Ahmed M. Alaa (c) 2010 Design Example: MediaFLO transmitter Video Compression review A great deal of research and development has resulted in methods to reduce the digital bandwidth required for video transmitter An uncompressed digital video signal: 150 Mbps of digital bandwidth Early compression reached 45 Mbps MPEG: Motion picture experts group The concept of digital video compression is based on the fact that very few bits change frame to frame. If only changes are transmitted, the transmission bandwidth would decrease Compression allows for: Reducing bandwidth and reducing the storage space. So the channel capacity increases Two types: MPEG-1: video storage on CD’s MPEG-2: HDTV quality transmission, with 50:1 compression ratio Slide 62: Ahmed M. Alaa (c) 2010 Design Example: MediaFLO transmitter Video Compression review (cont’d) The basis for video compression is to remove redundancy in the video signal stream For example: a car moving in a background, background is only fixed and the car is moving Video compression: Starts with an encoder, which converts analog video signal to digital format on a pixel-by-pixel basis. Each video frames is divided to 8x8 pixel blocks to determine which to transmit Two stages: Motion and Compensation: identify areas that involve motion and transmit the magnitude and direction of the displacement to a predictor in a decoder. Frame difference is called: Residual Transform residual on a block by block basis The encoded residual signal is transformed to more compact form by DCT (discrete cosine transform) represents each pixel by normalized values: Slide 63: Ahmed M. Alaa (c) 2010 Design Example: MediaFLO transmitter 2- Reed-Solomon channel coding Reed-Solomon codes are block-based error correcting codes with a wide range of applications in digital communications and storage. Reed-Solomon codes are used to correct errors in many systems including: Storage devices (including tape, Compact Disk, DVD, barcodes, etc) Wireless or mobile communications (including cellular telephones, microwave links, etc) Satellite communications Digital television / DVB High-speed modems such as ADSL, xDSL, etc. The Reed-Solomon encoder takes a block of digital data and adds extra "redundant" bits. Errors occur during transmission or storage for a number of reasons (for example noise or interference, scratches on a CD, etc). The Reed-Solomon decoder processes each block and attempts to correct errors and recover the original data. The number and type of errors that can be corrected depends on the characteristics of the Reed-Solomon code. Slide 64: Ahmed M. Alaa (c) 2010 Design Example: MediaFLO transmitter Reed Solomon codes are a subset of BCH codes and are linear block codes. A Reed-Solomon code is specified as RS(n,k) with s-bit symbols This means that the encoder takes k data symbols of s bits each and adds parity symbols to make an n symbol codeword. There are n-k parity symbols of s bits each. A Reed-Solomon decoder can correct up to t symbols that contain errors in a codeword, where 2t = n-k Example: A popular Reed-Solomon code is RS(255,223) with 8-bit symbols. Each codeword contains 255 code word bytes, of which 223 bytes are data and 32 bytes are parity. For this code: n = 255, k = 223, s = 8 2t = 32, t = 16 The decoder can correct any 16 symbol errors in the code word: i.e. errors in up to 16 bytes anywhere in the codeword can be automatically corrected. Slide 65: Ahmed M. Alaa (c) 2010 Design Example: MediaFLO transmitter The maximum length of a code with 8-bit symbols (s=8) is 255 bytes RS(255,223) can correct 16 symbol errors. In the worst case, 16 bit errors may occur, each in a separate symbol (byte) so that the decoder corrects 16 bit errors. In the best case, 16 complete byte errors occur so that the decoder corrects 16 x 8 bit errors Reed-Solomon codes are particularly well suited to correcting burst errors (where a series of bits in the codeword are received in error). Reed-Solomon encoding and decoding can be carried out in software or in special-purpose hardware. The Architecture depends on: Finite (Galois) field Arithmetic or Generator polynomial. MATLAB has predefined functions for Reed Solomon coding ! Slide 66: Ahmed M. Alaa (c) 2010 Design Example: MediaFLO transmitter 3-Interleaving Each channel coding technique has a limited error correction capability, so we need to scatter the error on distinct packets as channel errors usually occur in consecutive packages If one packet is corrupted, we lose a whole packet. But interleaving scatters this packet to distinct packets to correct its entire errors and then recover the correct packet through de-interleaving 2 3 10 9 20 13 11 1 12 6 7 8 15 16 17 17 Without Interleaving 2 20 12 15 3 13 6 16 10 11 7 17 9 1 8 17 With Interleaving Slide 67: Ahmed M. Alaa (c) 2010 Design Example: MediaFLO transmitter 4- Burst Formatting Slide 68: Ahmed M. Alaa (c) 2010 Design Example: MediaFLO transmitter Spectrum of the OFDM signal fo MediaFLO The Spectrum via MATLAB Welch tool before applying to the D/A Conversion. A 5 frame AVI video was applied to quantization, Reed-Solomon coding, OFDM modulation and simulated for an AWGN Rayleigh fading channel. Slide 69: Ahmed M. Alaa (c) 2010 Thank You ! You do not have the permission to view this presentation. In order to view it, please contact the author of the presentation.