DVB-T2 is an abbreviation for Digital Video Broadcasting – Second Generation Terrestrial; it is the extension of the television standard DVB-T, issued by the consortium DVB, devised for the broadcast transmission of digital terrestrial television. This system transmits compressed digital audio, video, and other data in "physical layer pipes" (PLPs), using OFDM modulation with concatenated channel coding and interleaving. It is currently broadcasting in parts of the UK under the brand name Freeview HD.

History

In March 2006 DVB decided to study options for an upgraded DVB-T standard. In June 2006, a formal study group named TM-T2 (Technical Module on Next Generation DVB-T) was established by the DVB Group to develop an advanced modulation scheme that could be adopted by a second generation digital terrestrial television standard, to be named DVB-T2.^[1]

According to the commercial requirements and call for technologies ^[2] issued in April 2007, the first phase of DVB-T2 would be devoted to provide optimum reception for stationary (fixed) and portable receivers (i.e., units which can be nomadic, but not fully mobile) using existing aerials, whereas a second and third phase would study methods to deliver higher payloads (with new aerials) and the mobile reception issue. The novel system should provide a minimum 30% increase in payload, under similar channel conditions already used for DVB-T.

The BBC, ITV, Channel 4 and Five agreed with the regulator Ofcom to convert one UK multiplex (B, or PSB3) to DVB-T2 to increase capacity for HDTV via DTT.^[3] They expected the first TV region to use the new standard would be Granada in November 2009 (with existing switched over regions being changed at the same time). It was expected that over time there would be enough DVB-T2 receivers sold to switch all DTT transmissions to DVB-T2, and H.264.

Ofcom published its final decision on April 3, 2008 for HDTV using DVB-T2 and H.264 ^[4]: BBC HD would have one HD slot after DSO at Granada. ITV and C4 had, as expected, applied to Ofcom for the 2 additional HD slots available from 2009 to 2012.^[5]

Ofcom indicated that it found an unused channel covering 3.7 million households in London, which could be used to broadcast the DVB-T2 HD multiplex from 2010, i.e., before DSO in London. Ofcom indicated that they would look for more unused UHF channels in other parts of the UK, that can be used for the DVB-T2 HD multiplex from 2010 until DSO.^[6]

The DVB-T2 specification

DVB-T2 test modulator developed by BBC Research & Innovation.

The DVB-T2 draft standard was ratified by the DVB Steering Board on June 26, 2008,^[7] and published on the DVB homepage as DVB-T2 standard BlueBook,^[8]. It was handed over to the European Telecommunications Standards Institute (ETSI) by DVB.ORG on June 20, 2008.^[9] The ETSI process resulted in the DVB-T2 standard being adopted on September 9, 2009.^[10] The ETSI process had several phases, but the only changes were text clarifications.^[11] Since the DVB-T2 physical layer specification was complete, and there would be no further technical enhancements, receiver VLSI chip design started with confidence in stability of specification. A draft PSI/SI (program and system information) specification document was also agreed with the DVB-TM-GBS group.

Tests

Prototype receivers were shown in September IBC 2008 and more recent version at the IBC 2009 in Amsterdam. A number of other manufacturers demonstrated DVB-T2 at IBC 2009 including Albis Technologies, Arqiva, DekTec, Enensys, Harris, Pace, Rohde & Schwarz, Tandberg, and TeamCast. Other manufacturers planning DVB-T2 equipment launches include CellMetric, Cisco, Digital TV Labs, Humax, NXP Semiconductors, Panasonic, ProTelevision Technologies, Screen Service, SIDSA, Sony, ST Microelectronics and T-VIPS.^[11] The first test from a real TV transmitter was performed by the BBC Research & Innovation in the last weeks of June 2008^[12] using channel 53 from the Guildford transmitter, southwest of London: BBC had developed and built the modulator/demodulator prototype in parallel with the DVB-T2 standard being drafted.

NORDIG published a DVB-T2 receiver specification and performance requirement on the 1 July 2009.^[13] In March 2009 the Digital TV Group (DTG), the industry association for digital TV in the UK, published the technical specification for high definition services on digital terrestrial television (Freeview) using the new DVB-T2 standard. The DTG's test house: DTG Testing are testing Freeview HD products against this specification.^{[citation needed]}

The standard

The following characteristics have been devised for the T2 standard:

• COFDM modulation with QPSK, 16-QAM, 64-QAM, or 256-QAM constellations.
• OFDM modes are 1k, 2k, 4k, 8k, 16k, and 32k. The symbol length for 32k mode is about 4 ms.
• Guard intervals are 1/128, 1/32, 1/16, 19/256, 1/8, 19/128, and 1/4. (For 32k mode, the maximum is 1/8.)
• FEC is concatenated LDPC and BCH codes (as in DVB-S2), with rates 1/2, 3/5, 2/3, 3/4, 4/5, and 5/6.
• There are fewer pilots, in 8 different pilot-patterns, and equalization can be based also on the RAI CD3 system.^[14]
• In the 32k mode, a larger part of the standard 8 MHz channel can be used, adding about 2% extra capacity.
• DVB-T2 is specified for 1.7, 5, 6, 7, 8, and 10 MHz channel bandwidth.
• MISO (Multiple-Input Single-Output) may be used (Alamouti scheme), but MIMO will not be used. Diversity receivers can be used (as they are with DVB-T).
• Bundling of more channels into a SuperMUX (called TFS) is not in the standard, but may be added later.

System differences with DVB-T

The following table reports a comparison of available modes in DVB-T and DVB-T2.^[15]

For instance, a UK MFN DVB-T profile (64-QAM, 2k mode, coding rate 2/3, guard interval 1/32) and a DVB-T2 equivalent (256-QAM, 32k, coding rate 3/5, guard interval 1/128) allows for an increase in bit rate from 24.13 Mbit/s to 35.4 Mbit/s (+46.5%). Another example, for an Italian SFN DVB-T profile (64-QAM, 8k, coding rate 2/3, guard interval 1/4) and a DVB-T2 equivalent (256-QAM, 32k, coding rate 3/5, guard interval 1/16), achieves an increase in bit rate from 19.91 Mbit/s to 33.3 Mbit/s (+67%).^[16]

Technical details

DVB-T2 at a glance.

Framing structure of DVB-T2.

The processing workflow is as follows:

• Input pre-processing
  • Physical Layer Pipe (PLP) creation: adaptation of Transport Stream (TS), Generic Stream Encapsulation (GSE), Generic Continuous Stream (GCS), or Generic Fixed-length Packetized Stream (GFPS)
• Input processing
  • Mode adaptation
    • Single PLP (mode 'A'): data are assembled in groups called BaseBand Frames (BBFRAMEs), with lengths of K_bch bits, defined by modulation and coding (MODCOD) parameters, in a 'normal' length or 'short' length version
      • Input interface
      • CRC-8 encoding
      • BaseBand (BB) header insertion
    • Multiple PLPs (mode 'B')
      • Input interface
      • Input stream synchronization
      • Delay compensation
      • Null packets deletion
      • CRC-8 encoding
      • BB header insertion
  • Stream adaptation
    • Single PLP (mode 'A')
      • Padding insertion
      • BB scrambling: a Pseudo Random Binary Sequence (PRBS) with generator 1 + x¹⁴ + x¹⁵ is used to scramble completely every BBFRAME
    • Multiple PLPs (mode 'B')
      • PLP scheduling
      • Frame delay
      • In-band signaling or padding insertion
      • BB scrambling
• Bit Interleaved Coding and Modulation (BICM)
  • Forward Error Correction (FEC) encoding: each BBFRAME is converted into a FECFRAME of N_ldpc bits, by adding parity data. Normal FECFRAMEs are 64,800 bits long, whereas short FECFRAMEs are 16,200 bits long. The effective code rates are 32,208/64,800 (1/2), 38,688/64,800 (3/5), 43,040/64,800 (2/3), 48,408/64,800 (3/4), 51,648/64,800 (4/5), 53,840/64,800 (5/6)
    • Outer encoding: a BCH code, capable to correct 10 or 12 errors per FECFRAME, is used to compute parity data for the information data field. The BCH generator polynomial is of the 160^th, 168^th, or 192^nd grade
    • Inner encoding: a Low Density Parity Check (LDPC) code is cascaded to the BCH
  • Bit interleaving
    • Parity bits block interleaving
    • Twist column interleaving
  • Bit demultiplexing to cell words
    

    

    Constellation map of the rotated 256-QAM modulation (tilt angle is 3.57 degrees).
  • Gray mapping of cell words to constellations: either QPSK (4-QAM), 16-QAM, 64-QAM, or 256-QAM maps are used
  • Constellation rotation and cyclic quadrature (Q) delay: optionally, the constellations may be tilted counterclockwise by an amount of up to 30 degrees. Furthermore, the quadrature (imaginary) part of the cells is cyclically shifted by one cell
  • Cell interleaving
  • Time interleaving
• Frame building: the transmitted stream is organized in super frames, which are composed by T2 frames and FEF (Future Extension Frame) parts
  • Cell mapping: cells are mapped to OFDM symbols. A T2 frame is composed by a P1 symbol, one or more P2 symbols, regular data symbols, and a Frame Closing symbol (for certain configuration parameters). The P1 symbol is used for synchronization purposes, the P2 symbols convey L1 parameter configuration signaling, whereas the data symbols carry PLP data (there are three types: common PLPs, type 1 PLPs, and type 2 PLPs), auxiliary streams, and dummy symbols used as space filler
  • Frequency interleaving: random interleaving is done on every OFDM symbol (except P1)
• OFDM generation
  • Multiple-Input Single-Output (MISO) processing: Alamouti pre-processing is optionally applied to pairs of OFDM symbol cells. Given a_i the input cells, View formula on Wikipedia and View formula on Wikipedia transmitter group 1 and 2 cells, the mapping is done as View formula on Wikipedia and View formula on Wikipedia for group 1, and as View formula on Wikipedia and View formula on Wikipedia for group 2
  • Pilot insertion and dummy tone reservation: three classes of pilot tones are added. They are either continual (fixed position), scattered (cyclically moving position), or edge (boundary positions). There are 8 different configuration for scattered pilots (PP1 ... PP8). Moreover, a number of dummy carriers are not modulated and reserved to reduce the dynamic range of the DVB-T2 output signal (it helps to combat nonlinear phenomena in power amplifiers during broadcast).
  • Inverse Fast Fourier Transform (IFFT): classic IFFT is used to switch from the frequency domain into the time domain, after having adjusted carrier position relevant to the central transmit frequency. 1k (1024) to 32k (32768) carriers are available. There is also an extended mode, which allows to fill more data in the available bandwidth, using more active carriers and reducing the number of guard band (null) carriers.
  • Peak-to-Average-Power-Ratio (PAPR) reduction
  • Guard interval insertion: a cyclic prefix is inserted before the IFFT symbol, to recover from transmit channel echoes (multipath). Lengths from 1/128 to 1/4 of the IFFT length are allowed.
  • P1 symbol insertion: the P1 symbol is a particularly crafted 1k OFDM symbol, always inserted at the head of a T2 frame. It conveys few bits of information (spread, scrambled and DBPSK modulated), as it is mainly dedicated to fast synchronization (both in time and in frequency) at the receiver side. It is prepended and postpended by frequency shifted repetitions of itself, to ease receiver lock even if the nominal center frequency of the T2 signal is up to 500 kHz off.
  • Digital-to-Analog Conversion (DAC): the T2 samples are converted into an analog BB complex (I&Q) signal at a sample rate that depends on the channelization bandwidth. For instance, in 8 MHz wide channels, the complex sample time is 7/64 μs.

Usage of DVB-T2 in other countries

DVB-T2 has been trialled in other countries such as Spain^[17] and Germany. In Italy, Europa 7 HD multiplex will adopt it also.^[18]

See also

• OFDM system comparison table

Notes

References

• DVB document A122, Frame structure channel coding and modulation for a second generation digital terrestrial television broadcasting system (DVB-T2) ,
• DVB document A133 Implementation guidelines for a second generation digital terrestrial television broadcasting system (DVB-T2) and
• DVB document A136 Modulator Interface (T2-MI) for a second generation digital terrestrial television broadcasting system (DVB-T2) are available at the dvb.org website.
• DVB-T2 Fact Sheet, March 2010
• Freeview UK

External links

• Website of the DVB Project
• DigiTAG handbook on DVB-T2
• OFCOM DTT future
• OFCOM Pilots
• Overview of DVB-T2 by Enensys