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List of digital television broadcast standards
DVB family (Europe)
DVB-S (satellite)
DVB-T (terrestrial)
DVB-C (cable)
DVB-H (handheld)
ATSC family (North America)
ATSC (terrestrial/cable)
ATSC-M/H (mobile/handheld)
ISDB family (Japan/South America)
ISDB-S (satellite)
ISDB-T (terrestrial)
ISDB-C (cable)
SBTVD/ISDB-Tb (Brazil)
Chinese Digital Video Broadcasting standards
DMB-T/H (terrestrial/handheld)
ADTB-T (terrestrial)
CMMB (handheld)
DMB-T (terrestrial)
DMB Family (Korean handheld)
T-DMB (terrestrial)
S-DMB (satellite)
MediaFLO
Codecs
Video
Audio
Frequency bands
VHF
UHF
SHF

ATSC-M/H (Advanced Television Systems Committee - Mobile/Handheld) is a standard in the USA for mobile digital TV, that allows TV broadcasts to be received by mobile devices.[1] Its official appellation is A/153.

Just as the DVB-H and 1seg are mobile TV extensions to the DVB-T and ISDB-T terrestrial digital TV standards respectively, ATSC-M/H is an extension to the available digital TV broadcasting standard ATSC A/53. ATSC is optimized for a fixed reception in the typical North American environment and uses 8VSB modulation. The ATSC transmission scheme is not robust enough against doppler shift and multipath radio interference in mobile environments, and is designed for highly directional fixed antennas. To overcome these issues, additional channel coding mechanisms are introduced in ATSC-M/H to protect the signal.

Contents

Evolution of mobile TV standard

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Requirements

Several requirements of the new standard were fixed right from the beginning:

  • Completely backward compatible to ATSC (A/53),
  • Broadcaster can use their available license without additional restrictions and
  • Available legacy ATSC receivers can be used to receive the ATSC (A/53) standard without any modification.

Proposals

Ten systems from different companies were proposed, and two remaining systems were presented with transmitter and receiver prototypes:

To find the best solution, the Advanced Television Systems Committee assigned the Open Mobile Video Coalition (OMVC) to test both systems. The test report was presented on May 15, 2008. As a result of this detailed work by the OMVC, a final standard draft was designed by the Advanced Television Systems Committee, specialist group S-4. ATSC-M/H will be a hybrid. Basically the following components of the proposed systems are used:

  • RF-Layer from the MPH standard
  • Deterministic frame structure from A-VSB
  • Signaling of service designed on the base of the established mobile standards

Candidate standard

On December 1, 2008, the Advanced Television Systems Committee elevated its specification for Mobile Digital Television to Candidate Standard status. In the following six months, the industry will test the standard with their potential customers and start first product developments. Before it becomes an official standard, additional improvements will be proposed. ATSC members approved the ratified A/153 standard in October 2009.

Structure of mobile DTV standard

The ATSC Mobile DTV standard ATSC-M/H (A/153) is modular in concept, with the specifications for each of the modules contained separate Parts. The individual Parts of A/153 are as follows:

Principle

ATSC-M/H is a service for mobile TV receivers and partly uses the 19.39 Mbit/s ATSC 8VSB stream.

Block Diagram Headend

Technology

ATSC-M/H BW consumes fixed chunks of 917kbit/s out of the total ATSC Bandwidth. Each such chunk is called M/H Group. A data pipe called parade is a collection of one to eight M/H groups. A parade convey one or two ensembles which are logical pipes of IP datagrams. Those datagrams in turn carry TV services, System Signaling tables, OMA DRM key streams and Electronic Service Guide. ATSC-M/H improves design by detailed analyse of experience with other mobil DTV standards. The SMT table replaces PAT-PMT-INT and SDP of DVB-H which created huge complexity for the network and receivers. The time interleaving of ATSC-M/H is 1 second comparing to DVB-H 100-200msec.

ATSC-M/H Layer Model

Protocol stack

ATSC-M/H protocol stack is mainly an umbrella protocol that uses OMA ESG, OMA DRM, MPEG-4 in addition to many IETF RFCs.

Transport stream data structure

The ATSC-M/H standard defines a fixed transport stream structure, based on M/H Frames, which establishes the location of M/H content within the VSB Frames and allows for easier processing by an M/H receiver. This is contrary to the legacy ATSC transport stream, defined in A/53, in which there is no fixed structure to establish the phase of the data relative to VSB Frames.

One M/H Frame is equivalent in size to 20 VSB Frames and has an offset of 37 transport stream (TS) packets relative to the beginning of the VSB Frame. Each M/H Frame, which has a fixed duration of 968 ms, is divided into five M/H sub-frames and each sub-frame is further subdivided into sixteen M/H Slots. Each slot is the equivalent amount of time needed to transmit 156 TS packets. A slot may either carry all main ATSC data (A/53) or 118 packets of M/H data and 38 packets of main data. The collection of 118 M/H packets transmitted within a slot is called an M/H Group. Each of the 118 M/H packets within an M/H Group are encapsulated inside a special TS packet, known as an MHE packet.

An M/H Parade is a collection of M/H Groups and can carry one or two M/H Ensembles. These Ensembles are logical pipes for IP datagrams. Those datagrams in turn carry TV services and the signaling of mobile content. The M/H Groups from a single Parade are placed within M/H Slots according to an algorithm defined in A/153 Part 2. The Number of Groups per M/H Sub-Frame (NoG) for an M/H Parade ranges from 1 to 8 and therefore the number of Groups per an M/H Frame for a Parade ranges from 5 to 40 with a step of 5. The data of a Parade are channel coded and distributed by an interleaver during an M/H Frame.

Mobile Data are protected by an additional FEC, as Interleaving and Convolutional Codes. To improve the reception in the receiver, training sequences are introduced into the ATSC-M/H signal to allow channel estimation on the receiver side.

Time slicing is a technique used by ATSC-M/H to provide power savings on receivers. It is based on the time-multiplexed transmission of different services.

Error protection

ATSC-M/H combines multiple error protection mechanisms for added robustness. One is an outer Reed Solomon Code which corrects defective Bytes after decoding the outer convolutional code in the receiver. The correction is improved by an additional CRC checksum since Bytes can be marked as defective before they are decoded (Erasure decoding)

The number of RS parity symbols can represent 24, 36 or 48. The symbols and the additional checksum form the outer elements of a data matrix which is allocated by the payload of the M/H Ensemble. The number of lines is fixed and the number of columns is variable according to how many slots per Subframe are occupied.

The RS Frame is then partitioned into several segments of different sizes and assigned to specified regions. The M/H data in these regions are protected by an SCCC (Series Concatenated Convolutional Code), incorporating a code rate of 1/2 or 1/4, and is specific to each region in a group. A 1/4 rate PCCC (Parallel Concatenated Code) is also employed as an inner code for the M/H signaling channel, which includes FIC (Fast Information Channel) and TPC (Transmission Parameter Channel). The TPC carries various FEC modes and M/H Frame information. Once the TPC is extracted, the receiver then knows the code rates being employed and can decode each region at its specified rate.

A modified trellis encoder is also employed for backwards compatibility with legacy A/53 receivers.

Signaling

ATSC M/H Signaling and Announcement defines three different layers of signalling. The layers are organized hierarchically and optimized to characteristics of the transmission layer.

  • Transmission Signaling System is the lowest layer and use the Transmission Parameter Channel (TPC). It provides information for the receiver needed to decode the signal
  • Transport Signaling System is the second layer it use the Fast Information Channel (FIC) in combination with the Service Signaling Channel (SSC). The main purpose of the FIC is to deliver essential information to allow rapid service acquisition by the receiver. The Service Signaling Channel (SSC), consists different signaling tables. The information carried within these tables can be compared to the PSIP information of ATSC. The SSC provides mainly the basic information, the logical structure of the transmitted services and the decoding parameters for video and audio
  • Announcement / Electronic Service Guide (ESG) is the highest layer of signaling. It use the Open Mobile Alliance (OMA) Broadcast Service Enabler Suite (OMA BCAST) Electronic Service Guide (ESG).An ESG is delivered as a file data session File Delivery over Unidirectional Transport (FLUTE) is used as delivery protocol. The ESG consists of several XML sections. With this structure, a program guide and enabled interactive services can be realized.

Signaling of video- and audio coding

Each video- or audio decoder needs information about the used coding parameters, for instance resolution, frame rate and IDR (Random Access Point) repetition rate. In MPEG-4/AVC, mobile TV systems the receiver uses information from the Session Description Protocol File (SDP-File). The SDP-file is a format which describes streaming media initialization parameters. In ATSC-M/H, the SDP-File is transmitted within the SMT-Table. Most of the information are coded as binary values and some are coded in the origin ASCII text format. In case of signaling with ESG, the complete SDP-File is transmitted

Service protection

Note that while it is intended to be free to air, its spec defines service protection, therefore it may or may not be free to air based on the actual business model which does not exist yet.

Single-frequency network (SFN)

In an SFN, two or more transmitters with an overlapping coverage send the same program content simultaneously on the same frequency. The 8VSB modulation used by ATSC allows SFN transmissions.[citation needed] To allow regular channel approximation, ATSC-M/H provides additional training sequences. ATSC A/110 defines a method to synchronize the ATSC modulator as part of the transmitter. The A/110 standard sets up the Trellis coder in a pre-calculated way to all transmitters of the SFN. In such an SFN, the ATSC-M/H multiplexer and the ATSC-M/H transmitter are synchronized by a GPS reference. The ATSC-M/H multiplexer operates as a network adapter and inserts time stamps in the MPEG transport stream. The transmitter analyzes the time stamp, delays the transport stream before it is modulated and transmitted. Eventually, all SFN transmitters generate a synchronized signal.

Other mobile standards

While MediaFLO has become available in parts of the U.S., it is a premium service that requires subscription. ATSC-M/H would be free to air, as are regular broadcast signals.

References

External links


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