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Mobile TV with China's DTV standard is easier than in U.S. and Europe

Mobile DTTB enables not only the transmission and reception of conventional TV programs to mobile and handheld devices. It also opens the business to many interactive and customized programs.

By Raj Karamchedu Legend Silicon Corporation

Broadcast television is rapidly evolving from analog to digital around the world. Digital television deployment -- combined with rapid growth in the personal media market -- is creating opportunities for many broadcasters to deliver television content not only to large screen displays, but also to the mobile and portable devices. Table 1 outlines when various regions transition from analog signals to digital television (DTV) broadcasts, as well as the DTV standards in use.

Table 1: Digital conversion around the world

While these terrestrial DTV standards are designed to provide high definition television (HDTV) services with fixed antenna reception, most are not well-suited to meet the stringent low-power requirements of mobile and portable devices. As a result, broadcasters continue to face challenges in determining which technology and broadcast mechanism can be used to reach the fast growing market of mobile and portable users.

The one exception is the standard developed in China. The China Digital Television Terrestrial Broadcasting (DTTB) System Standard, also known as GB20600-2006, became the mandatory national DTTB standard in August 2007.

GB20600-2006 was designed to deliver a consistent, high-quality digital TV viewing experience no matter where consumers are sitting: in their living room watching television or on a high-speed train watching shows on their cell phones. The technology can broadcast audio and video at transmission rates of greater than 24 Mbps to consumer devices. Because the mobile reception capability is inherently built into the standard, these consumer devices now have a mobile TV feature that works not only when stationary, but even while traveling at speeds greater than 200 km per hour.

The China television market is in the midst of a broadcast revolution because of this new free-to-air terrestrial DTV standard. GB20600-2006 is spurring station owners to broadcast HDTV signals to TVs and set-top boxes, creating a market opportunity that is larger than any other in the world. With 380 million television households, China is home to more televisions than any other country in the world. And nearly 70 percent of those households receive their programming via roof-top antenna.

At the same time, the GB20600-2006 standard is creating a significant new market for mobile TV services. There are more than 600 million cell phone subscribers in China and nearly seven million new mobile phones are purchased each month. Now that the free-to-air HDTV broadcast signal has become a reality, manufacturers of cell phones and other handheld mobile devices are rushing to incorporate mobile TV reception into their products.

What makes the China GB20600-2006 terrestrial DTV standard suitable for mobile services? In this article we answer this question in two ways. First, we review the basic GB20600-2006 air interface standard and highlight the features of the standard that were designed for mobile capabilities.

Second, we provide a detailed overview of a television service provider's broadcast system using the GB20600-2006 standard and discuss the suitability of such a system for mobile services when a multi-protocol encapsulation (MPE) service is overlaid on top of it. Additionally, MPE with GB20600-2006 will enable many of the existing and new receiver designers to develop solutions that provide reception of added programs along with free-to-air programs without added complexity. An MPE-based GB20600-2006 system is capable of broadcasting 20 to 30 mobile DTTB programs in an 8MHz channel.

Next: GB20600-2006 broadcast to mobile devices

GB20600-2006 broadcast to mobile devices
Receiving DTV on mobile devices requires consumer terminals that operate on lower power and robust signal reception. Typically mobile devices do not need high-resolution video images because the screens are much smaller than that of fixed receivers. For example, a regular DTV at home or laptop may have a screen resolution of 720p, but a mobile device can show good quality video with a resolution of QVGA (320 x 240 pixels).

Mobile TV receivers require vastly more complicated signal processing algorithms at their core because, at high speeds, they have to estimate the channel sufficiently and frequently. It is this processing of GB20600-2006's time domain synchronous - orthogonal frequency division multiplexing (TDS-OFDM) technology that can do better than both the 8-Vestigial Side Band (8-VSB) technology in the United States' Advanced Television Systems Committee (ATSC) standard and the coded orthogonal frequency division multiplexing (COFDM) technology in Europe's digital video broadcast (DVB) standard.

In TDS-OFDM, a pseudo-nose (PN) sequence is used for channel estimation, while in COFDM a set of pilot tones is used for channel estimation. The innovation behind TDS-OFDM is that the PN sequence is in time-domain, which dramatically accelerates the channel estimation process and enables TDS-OFDM to achieve high-speed channel estimation. For COFDM, a certain minimum number of symbols are required to complete the channel estimation fully, whereas in TDS-OFDM, only one symbol is required to achieve the same. The 8-VSB technology is even worse because while the preamble sequence, which is used to estimate the channel, is every 0.5ms in TDS-OFDM, it is at every 24ms for the 8-VSB technology in ATSC. As a result, it takes even longer time for the channel estimation to be complete in ATSC.

Figure 1: Cyclic-prefix OFDM Symbol

Figure 2: TDS-OFDM symbol

From a digital communication viewpoint, the OFDM signals (which are the basis for the COFDM signaling), using cyclic prefix (CP) and zero-padding prefix are equivalent, as shown in Figures 1 and 2. The primary variation between these two schemes lies only in the use of different prefix signals.

The insight that drove the TDS-OFDM performance is that the superposition of a PN to the prefix will not destroy the orthogonal properties because its effects can be removed. Moreover, the addition of a PN signal brings about a number of benefits, such as fast synchronization, accurate channel estimation and high spectral efficiency, which are vital for mobility.


Table 2: Performance Comparison of DVB-H, ATSC and GB20600-2006

In the development of GB20600-2006, researchers considered other DTV standards -- including Europe's DVB and the U.S. ATSC standards. Chinese researchers then adopted the best features of each and improved upon them further. Table 2 compares performance of the three standards, including the handheld version of the DVB standard (DVB-H).

Next: Features of China DTV transmission standard

Features of China DTV transmission standard
The block diagram of a China DTV transmitter is presented in Figure 3. The transmitter includes randomizer, forward error correction (FEC) encoder, mapping and interleaving, system information generation, multiplexing, frame data processing, frame header generation, framing, base-band processing and up-conversion.

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Figure 3: Block diagram of China DTV transmitter

The input transport stream (TS) stream is randomized and then sent to the FEC encoder bock. The FEC encoder is the concatenated code of Bose and Ray-Chaudhuri (BCH) and low-density parity-check code (LDPC) code. The outer code is the BCH (752,762) block code. The inner code is the LDPC code. There are three different codes with similar structure, which are LDPC (7493, 3048), LDPC (7493, 4572) and LDPC (7493, 6912). The encoded bit stream is mapped to the constellation and interleaved by a convolutional time-domain interleaver and a frequency-domain block interleaver. The time interleaver has two modes M=240 and M=720. The frequency-domain interleaver applies only for multi-carrier. The modulation schemes supported by the standard are 4QAM-NR, 4QAM, 16QAM, 32QAM and 64QAM.

The modulation scheme, LDPC rate and time interleaver mode are referred to as system information (SI). The SI is encoded using a 32-bit Walsh code. The multiplexing block combines 3744 data symbols and 36 information symbols into one frame with 3780 symbols. These 3780 symbols are processed by the frame body processing block. In the case of multi-carrier mode (C=3780), these 3780 symbols are converted to the orthogonal frequency division multiplexing (OFDM) signal using a 3780-point inverse discrete fourier transform (IDFT). In the case of the single-carrier mode (C=1), 3780-point IDFT is by-passed. The 3780 symbols form one frame body.

Table 3: China DTV Key Parameters

The key parameters of the China DTV standard is summarized in Table 3.

Next: Combination of time-domain and frequency-domain signal processing

Combination of time-domain and frequency-domain signal processing
In most previously adopted DTV transmission standards, the signal is either processed in the time-domain (U.S. ATSC standard) or in the frequency-domain (European digital video broadcast DVB standard). However, the China DTV standard in the case of C=3780 combines both the time-domain signal and frequency-domain signal.

The PN sequence for the frame header is processed in the time-domain, while the data is modulated using OFDM technology, which treats the original data as frequency-domain signal and coverts it to the time-domain signal using IDFT. The frame header PN sequence is kept in the time domain, which makes the channel estimation, synchronization and carrier recovery easier and faster.

PN guard interval, PN sequence
In order to overcome the inter-symbol-interference (ISI) problem in the OFDM system, a must be added as the guard interval. In TDS-OFDM technology, a PN sequence (pseudorandom number sequence) is used to separate two neighboring frame bodies. This PN sequence will function as the guard-interval. For this reason, the PN sequence is also referred to as a PN guard interval. At the receiver, once the channel impulse response is estimated, the received PN sequence can be as estimated well.

After removing the PN sequence from the received signal, the difference signal is the same as zero-padding guard interval. The advantage of the PN guard interval is the higher spectrum efficiency since the CP takes about 10 percent or more data rate. In the case of C=1 in GB20600-2006, the PN sequence can be used as a training sequence.

OFDM technology
The OFDM technology is well known for its ability to deal with ISI and the easy implementation of the frequency-domain equalizer. TDS-OFDM technology, on which the China standard is based, offers these advantages of OFDM technology. It uses 3780-point IDFT to obtain 3780 orthogonal sub-carriers. The block interleaving in the frequency domain helps in the implementation of inverse fast Fourier transform (IFFT).

Fast and accurate channel estimation
As mentioned above, the frame header PN sequence also is designed for the receiver to estimate the channel impulse response. Since PN sequence is transmitted in the time domain, the correlation between the received PN sequence and the original transmitted PN sequence can be a good estimation of the channel impulse response. Therefore, the channel response can be estimated quickly, and the estimated response reflects the actual channel during the period of PN duration. With channel response obtained from two neighboring PN sequences, the change of channel condition can be observed accurately. This feature allows the tolerance of Doppler frequency to reach as high as 110Hz, even for 64QAM.

Next: High performance of FEC encoder

High performance of FEC encoder
The FEC encoder is the concatenated with BCH code and LDPC code. The LDPC codes designed for TDS-OFDM are structure and irregular LDPC codes. The three codes have a quasi-cyclic structure with the same sub-block size. With optimization of their degree distributions of variable nodes and check nodes, these codes achieve good balance between the requirement for good performance at low signal-to-noise ratio (SNR) and the requirement of a very low error floor. Using the iterative decoding algorithm, the three LDPC codes not only converge fast but also achieve very low error floor. With the BCH code, the error floor can be below10^-12.

In the implementation of the LDPC decoder, the quasi-cyclic structure in the three LDPC codes makes the addressing of check nodes and bit nodes very simple, significantly reducing the complexity of the decoder design. The quasi-cyclic structure feature also exists in the generation matrices of the three LDPC codes. Therefore, the encoding of the three LDPC codes can be implemented using shift-registers.

Robust encoding of system information
The system information includes the modulation mode, LDPC rate and time interleaver mode. The system information also is used to indicate the first frame in each super-frame and whether the super-frame is the even or odd super-frame in a minute-frame. Each combination of the above listed information is mapped to one 32-bit Walsh code in the family of W(k,32) or 1-W(k,32) (k=0,1, , 31). Since all 32 vectors in W(k,32) are orthogonal, this system information can be obtained reliably at the receiver even in very bad and noisy channel conditions.

Time interleaver
The time interleaver is a convolutional interleaver. The number branch is B=52. The time interleaver modes are M=240 and M=720. The spreading of the time interleaver is 170 frames and 510 frames respectively. Time interleaver can significantly improve the tolerance to Doppler frequency and the single pulse interference.

It is clear from the preceding review of the air-interface specification that the GB20600-2006 standard accommodates significantly higher Doppler rates and faster channel estimation, which directly translate into higher mobile and vehicular speeds.

Next we look at a practical implementation of a television service provider's broadcast system that uses the GB20600-2006 standard for a fixed television application. When a MPE service is overlaid on top of such GB20600-2006 broadcast, the service can be extended to a mobile consumer terminal. By deploying an MPE-enabled GB20600-2006 broadcast system, several mobile-specific requirements such as lower mobile terminal power, and the provision of a variety of text, audio and video services to mobile consumers can be fulfilled.

First, let us review the block diagram of the standard GB20600-2006 broadcast system that a local broadcaster in a China city would typically deploy to transmit to a fixed television market.

Next: Standard GB20600-2006 broadcast system

Standard GB20600-2006 broadcast system
A standard broadcasting system targeted for fixed terrestrial television applications consists of the block elements as shown in Figure 4. The broadcast system can transmit multiple programs over a single channel bandwidth. Though these programs can be received by mobile handheld devices, they do not offer power savings and higher content resolution.

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Figure 4: Standard GB20600-2006 broadcast system

A program video source here refers to any source of video and audio, such as a DVD player, a video CD player or a video camera. Essentially it is the program the broadcaster intends to transmit to the receivers. The output from this video source should be raw video data. The raw video data can be large amount of information that requires large bandwidth and is not an ideal format for transmission.

To make the data suitable for transmission, the data is processed through the MPEG-2 encoder. The MPEG-2 encoder is a system that takes the raw video stream and compresses it into the MPEG-2 format. After compression, the size of the data is optimized for storage and transmission. The data used for storage is referred to as "program stream," while the data for transmission is referred to as "transport stream." The encoder then puts out packets of data called the MPEG transport stream for transmission.

The transport stream multiplex (TS-MUX) has the ability to take multiple MPEG transport streams that come into the MPEG2 encoders and combine them into a single MPEG transport stream. In common broadcast systems, multiple programs need to be broadcast and, in many cases there is only a single channel spectrum. As a result, it is crucial to have a TS-MUX that can merge all the different programs into a single transport stream, which then can be modulated and broadcast utilizing this single channel spectrum.

The MIP inserter coordinates the synchronized and simultaneous operation of all transmitters within a single frequency network (SFN) to ensure good quality of coverage across large areas, both indoors and outdoors. Finally the modulator performs the necessary baseband to radio frequency conversion before broadcasting the signal into the air via the transmitter tower.

MPE broadcast with GB20600-2006
A key enabler for the GB20600-2006 China DTV standard for mobile broadcasts is multi-protocol encapsulation (MPE).

In a standard broadcast, the encoded audio and video data is in an MPEG transport stream. The MPE is a mechanism for transporting Internet protocol (IP) data on top of this MPEG transport stream. In other words, the MPE overlay allows the broadcast of the IP data along with the transport stream.

Basic ingredients for GB20600-2006 broadcasting with MPE are:

• Time slicing
• Encapsulation of mobile services
• FEC

By leveraging these basic ingredients of the MPE overlay, a mobile, handheld-friendly television broadcast can be achieved, thereby benefiting broadcasters, consumers and manufacturers of handheld devices.

Next: MPE time slicing with GB20600-2006

MPE time slicing with GB20600-2006
A mobile receiver that receives MPE GB20600-2006 takes advantage of time slicing to conserve power in the mobile device. The power savings comes mostly from turning off the demodulator while the required service is not being transmitted.

In the time slicing technique, the broadcaster sends bursts of information using a higher instant bit rate, rather than a constant bit rate over a prolonged period of time. The Figure 5 illustrates this concept.

Figure 5: A DTV broadcast system with MPE

A constant broadcast stream without time slicing would have a constant bit rate continuously in time. But in a time-sliced broadcast, this stream arrives at the mobile receiver at a relatively higher bit rate and within a fixed burst time duration. The Figure 6 shows different services being time sliced, with the red color indicating an example Service 1.

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Figure 6: GB20600-2006 Broadcast Service with MPE

In this example, while the viewer is watching Service 1, the demodulator of the receiver is turned off whenever there is no MPE GB20600-2006 Service 1, thereby saving power.

Next: Encapsulating mobile services, such as ESG

Encapsulating mobile services -- ESG
The MPE GB20600-2006 electronic service guide (ESG) is used for announcement of the MPE GB20600-2006 services to the end user. The ESG and the service are both transmitted through the IP stream. This method is used for the announcement of the available MPE services in the MPE GB20600-2006 system. The MPE GB20600-2006 program content, typically encoded either in China's audio video standard (AVS) or H.264, is delivered as IP streams. The middleware in the mobile device extracts the IP address of the MPE GB20600-2006 service from the session description protocol (SDP) that is available in the ESG database.

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Figure 7: Mobile IP Services with MPE

Next, the middleware maps the IP address to MPE packet identifier (PID) through a program association table (PAT), IP notification table (INT) and program map table (PMT). The middleware then requests the GB20600-2006 demodulator driver for the MPE GB20600-2006 service with the MPE PID as the input. The driver receives the MPE GB20600-2006 data and extracts the IP packets, puts these IP packets into the TCP/IP stack (stage 9 in Figure 7). The media player on the mobile handheld device then receives these IP packets through a socket from the TCP/IP, decodes the stream and displays the audio and video content of the MPE GB20600-2006 service.

The MPE GB20600-2006 service and the ESG, both of which are in the form of IP streams, are combined in a MPE GB20600-2006 IP encapsulator, and an MPEG2 transport stream is generated and broadcast.

The receiver extracts the IP packets from the MPE frame, which provides the ESG tables and determines which services are available and at what time. Because the data packets are in IP format, any information that can be received over Internet can be delivered as a service.

Forward Error Correction
The third critical ingredient in the MPE overlay is the FEC. The mobile receiver is expected to receive information from the transmitters located several miles away. This distance can cause the signal to fade. There are many types of fading that can take place in a mobile environment, including Doppler and carrier-to-noise (C/N) losses.

As a result, the detection and correction of the errors becomes mandatory in the case of the mobile environment. MPE-FEC is a common way in the MPE GB20600-2006 technique to ensure the data link layer is protected against usual transmission errors. The MPE-FEC is critical to efficient recovery of the transmitted signal in a mobile system.

Next: Broadcast system with MPE GB20600-2006

Broadcast system with MPE GB20600-2006
Figure 8 depicts a MPE GB20600-2006 broadcast system. The path in red indicates the services targeted for the mobile devices and the blue indicates the standard DTTB broadcast.

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Figure 8: Standard GB20600-2006 broadcast system with MPE

Conclusion
The GB20600-2006 standard is inherently a very robust technology that is being deployed at a rapid rate for fixed applications. With a low added cost for MPE on the head-end side of the system, many more interactive and specialized programs and services can be broadcast for mobile audiences.

Additionally, MPE with GB20600-2006 will enable many of the existing and new receiver manufacturers to provide reception of added programs along with free-to-air programs without added complexity. It also will enable up to 30 mobile DTTB programs to be broadcast in an 8MHz channel.

Mobile DTTB enables not only the transmission and reception of conventional TV programs to mobile and handheld devices. It also opens the business to many interactive and customized programs. Using MPE along with the GB20600-2006 broadcast standard will open the doors to many new and different business opportunities for mobile broadcast services.

About the Author
Raj Karamchedu is the Director of Product Marketing at Legend Silicon. He has been a high technology product and business management professional since early 1994, having started his career at Cadence Design Systems as a signal processing/digital communication system design engineer. After forming a technology start-up, Karamchedu worked as a senior product marketing manager for software tools at Chameleon Systems, a reconfigurable processor startup. This was followed by a WLAN semiconductor product marketing management position at Systemonic, Inc. a startup acquired by Philips Semiconductors in 2003. Until July '05, Karamchedu was a senior product marketing manager for HDMI products at Silicon Image, a consumer electronics fab-less semiconductor company. He can be reached at rkaramchedu@legendsilicon.com.

(Source: http://www.dspdesignline.com)