5.1 Introduction
The transmission of data in digital form has long been known to offer many advantages over analogue transmission. Digital modulation is an outgrowth of the more familiar methods of analogue modulation such as amplitude, frequency and phase modulation. Recommendation ITU-R BT. 1306. lists the important parameters of a DTTB modulation system and provides parameter values or ranges of values for those parameters. The Recommendation allows the system designer to tailor system performance to meet a variety of different design constraints. The modulation techniques proposed in the Recommendation are for either single or multiple carrier modulation methods and for different channel bandwidths - 6, 7 and 8 MHz options are available. This chapter explains some of the issues involved in selecting a suitable modulation system for a given application.
5.2 Spectral efficiency
It is generally agreed that to provide a DTTB service that can deliver HDTV or multi-programme SDTV services a bit rate of about 20 Mbit/s (or more) is required. To accommodate such a data rate requires an effective spectrum efficiency of 4 bit/s/Hz for a national 6 MHz system, or 3 bits/s/Hz for national 7 or 8 MHz systems.
Theoretical spectral efficiencies of up to 4 bits/s/Hz can be achieved by 16 QAM, 4 VSB or 16 PSK systems. These modulation methods could be applied either to modulation of a single carrier with a high data rate signal or to modulation of a large number of carriers with low rate data signals. Either single carrier or multi-carrier modulation could be the basis for a worldwide transmission standard.
However, the error statistics of practical terrestrial transmission channels are such as to require the inclusion of forward error correction coding in a practical transmission/modulation system. Considerations related to filter implementation may further reduce effective data rates in practical systems. The result of these considerations is that the net data rate will be less than that predicted from a simple consideration based on theoretical spectrum efficiency and channel bandwidth. With the use of two stage channel codes, a practical implementation can lead to a substantial reduction from gross to net channel data rate. For example, a coding scheme based on use of a 2/3 Trellis Code concatenated with a Reed-Solomon (207,187) code results in a net data-rate only 60% of the gross data-rate.
This has prompted consideration of more complex modulation systems. The added complexity being justified because of the opportunity to provide a required net data rate within a highly error protected channel. As a result designers have been investigating the performance of higher order modulation systems such as 64 QAM or 8 VSB.
Spectral efficiency results not only from the fundamental spectral information "density" in bit/s/Hz of the modulation system within any given channel, but is also influenced very much by the spectrum re-use characteristics of a particular digital system.
Factors affecting spectrum re-use in a given system include:
5.3 Modulation Techniques
5.3.1 General Considerations
Of the generic modulation systems (m-VSB, m-QAM, m-PSK, m-DAPSK) m-PSK requires higher transmission power (which may exacerbate channel planning problems) and is therefore not preferred. QAM and VSB modulation systems have similar power requirements and noise performance.
These modulation systems can be applied to either a single carrier modulated at a high data rate or to a large number of carriers modulated at relatively low rates - the multi-carrier approach. Currently most research efforts for DTTB are focused on either a single carrier system using 8 VSB and multi-carrier systems using 16 QAM, 64 QAM or even 256 QAM.
In both cases, the research work builds on experience gained in other fields. Experience with single carrier QAM and QPSK systems has come from applications in the fields of terrestrial microwave and satellite transmission. While experience with multi-carrier systems has come from high frequency modems designed for military and telephone applications but is now being supplemented by experience gained in the development of a digital audio broadcasting system in Europe.
Because of the severe channel impairments that can occur in the VHF and UHF television bands, transmission conditions for DTTB are likely to be significantly more difficult than for satellite or cable transmission.
5.3.2 Single-Carrier Modulation (SCM)
The modulation method for the single-carrier system proposed for the USA’s ATV service, 8-VSB (vestigial sideband), was chosen after comparative testing with QAM since, on an overall technical basis, it provided better service characteristics in a sharing environment with analogue television. This modulation method provides a means for the transmission of a high bit rate eight level baseband signal. In SCM, the effects of multipath are handled by the receiving system, often with an adaptive equalizer, as discussed below.
5.3.2.1 8-VSB modulation
The 8-VSB single-carrier modulation system is as follows: 19.29 Mbits/s are delivered in a 6 MHz channel.
The serial data stream is comprised of 188-byte MPEG-compatible data packets. Following randomization and forward error correction processing, the data packets are formatted into Data Frames for transmission and Data Segment Sync and Data Field Sync are added.
Each Data Frame consists of two Data Fields, each containing 313 Data Segments. The first Data Segment of each Data Field is a synchronizing signal, which includes the training sequence used by the equalizer in the receiver. The remaining 312 Data Segments each carry the equivalent of the data from one 188-byte transport packet plus its associated FEC overhead.
Each Data Segment consists of 832 symbols. The first four symbols are transmitted in binary form and provide segment synchronization. This Data Segment Sync signal also represents the sync byte of the 188-byte MPEG-2 compatible transport packet. The remaining 828 symbols of each Data Segment carry data equivalent to the remaining 187 bytes of a transport packet and its associated FEC overhead. These 828 symbols are transmitted as 8-level signals and therefore carry three bits per symbol. The symbol rate is 10.76 Msymbols/sec and the Data Frame rate is 20.66 frames/sec.
To assist receiver operation a pilot-carrier is included at approximately 310 kHz from the lower band edge.
System performance is based on the concept of a linear-phase, raised-cosine Nyquist filter response in the concatenated transmitter and receiver. The system filter response is essentially flat across the entire band, except for the transition regions at each end of the band. Due to the vestigial sideband nature of the transmitted signal, the same skirt selectivity on both sides is not required, although this parameter value must be implemented consistently since the receiver must match the transmitter. The roll-off in the transmitter has the response of a linear-phase, root-raised cosine filter.
Additional adjacent channel suppression (beyond that achieved by sideband cancellation) may be performed by a linear phase, flat amplitude response SAW filter. Adjacent channel energy spillage at the IF output needs to be at least 57 dB down from the desired ATV signal power.
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