VSB modulation

The VSB modulation derived from English vestigial sideband modulation or digital vestigial sideband modulation called, is a modulation technique in the digital signal processing , a amplitude modulation combined with the vestigial sideband technique. To avoid confusion with the analog SSB modulation, it is referred to as 4-VSB, 8-VSB or 16-VSB, where the digits express the number of symbols available .

description

Modulation methods QAM and VSB for comparison and their magnitude spectra

A 2 n -VSB is closely related with a 2 2n - quadrature amplitude modulation (QAM). A carrier with the frequency F is used for information transmission - the complex signal in baseband position is expressed by two signal components which are designated as I ( real part ) and Q ( imaginary part ).

With VSB, n bits are transmitted per symbol , with QAM 2 · n bits are assigned. The double number of bits in QAM results from the fact that the I or Q signal can take up n bits of information per symbol independently of one another. With VSB, however, the Q signal is directly dependent on the I signal.

With VSM, to remove a sideband, an analytical signal in the baseband position (I and Q) is first generated with the help of the Hilbert transformation . So both I and Q carry the same information. In the bandpass position , after multiplication by the carrier frequency F , the two components are added as in QAM, whereby a sideband, for example the negative frequency components , disappear. As a result, only half the bandwidth is required compared to a 2 2n QAM.

The spectral efficiency is thus the same as a QAM. With the same signal-to-noise ratio (SNR) and the same symbol rate , half the data rate of a QAM is achieved with half the bandwidth. For demodulation, as with QAM - in contrast to z. B. the amplitude modulation (AM) also used in broadcasting - the carrier signal must be in the same phase as the transmitter; it must be possible to reconstruct it from the received signal - which must be taken into account when encoding the transmitted signals.

Main differences

Constellation diagram of a 16-QAM compared to a 4-VSB in the complex IQ level

A constellation diagram of a 16-QAM and a directly comparable 4-VSB modulation is shown on the left. With 16-QAM, 4 bits can be represented per symbol, which corresponds to the four levels or two bits each on the I or Q axis. For 4-VSB, the information is located at the receiver before only on one axis, for example in the direction of Q . The deflection on I is then irrelevant and can represent a strip. With four levels or “stripes” formed in this way, two bits per symbol can be represented in the complex plane, which, with an identical symbol rate, expresses half the data throughput of the VSB compared to QAM.

If the phase position in the receiver is not set correctly, the constellation diagram will rotate and the associated errors will result. Furthermore, compared to QAM, the VSB involves a slightly higher effort for modulation, but somewhat less effort for demodulation. This is the case because the Hilbert transform is required in the modulator instead of in the demodulator. - at least in the basic implementation of a coder and decoder.

In contrast to QAM, VSB modulation is in practice not combined with multi-carrier methods such as orthogonal frequency division multiplexing (OFDM).

use

The terrestrial digital television in North America according to the ATSC standard uses VSB and not OFDM in combination with QAM as standard, as is the case with the DVB-T common in Europe and large parts of Asia . The ATSC RF bandwidth is based on the NTSC channel grid and is 6 MHz, the symbol rate 10.76 MSymbol / s. 8-VSB is used, which results in a gross data rate of 32.28 Mbit / s. The most common net data rate given is around 60% of this value.

For higher data rates in the cable, not 16-VSB, but 64-QAM and 256-QAM is provided.

• 64-QAM: Symbol rate: 5.056941 MSymbol / s, gross data rate: 30.34165 Mbit / s
• 256-QAM: symbol rate: 5.360537 MSymbole / s, gross data rate: 42.88430 Mbit / s