Digital Signal Processing Reference
In-Depth Information
Audio signals are coded as a protective measure against errors (convolutional coding and
VITERBI decoding). Another effective protective measure is interleaving.Time-inter-
leaving facilitates the restructuring of bits of a program that originally succeeded each
other using a fixed pattern to interleave them in such a way that they are wide apart in
time. Frequency interleaving means that the data of different programs are also restruc-
tured and allocated to the 1536 carrier frequencies in such a way that the data of a program
are wide apart in the spectrum.
The audio signals of a DAB signal are literally “ripped apart” in
the time and frequency domains. The DAB signal and its neigh-
bours therefore appear almost like wideband white noise.
If a narrow frequency band is disturbed by fading only a tiny
proportion of the signals is generally corrupted and can be
recognized and corrected in the decoding process.
Global System for Mobile Communications (GSM)
GSM is also a joint European development, in this case for mobile telephony. In the
D-network GSM uses the 890
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915 MHz frequency range for the uplink and the 925
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960 MHz range for the downlink. In the E-network the uplink is in the 1760.2
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1775
MHz range and the downlink in the 1855.2
1870 MHz range. Each of these bands is
divided into channels with a width of 200 kHz. Owing to uplink and downlink two
frequencies or channels always correspond to each other (duplex). This means that GSM
also uses FDMA (see Illustration 278).
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The objectives of GSM are protection against misuse and tapping (cryptology), high user
capacity, high utilization of bandwidth, transitions to fixed-line telephony, optimizing
telephone services, high data quality and low bit error probability, and integrated data-
and supplementary services.
GSM uses TDMA to separate users of a (local) cell. 8 users of a base station share one
carrier frequency.
The combination of TDMA and FDMA results in 400 transmission paths in the
D-network with 50 duplex channels of 200 kHz each and 8 users per channel. The bit rate
of a channel is 271 kBit/s at a bandwidth of 200 kHz. Again, the 4-DPSK process is used
for modulation. The bit rate per call is 13 kBit/s. The remaining transmission capacity is
used for data protection. Convolutional coding and interleaving is used for this purpose.
Asymmetric Digital Subscriber Line (ADSL)
ADSL (Asymmetric Digital Subscriber Line) procedures have already been described at
the beginning of Chapter 11. It is a special variation of DMT for cable sections. By
optimizing all the procedures of coding and modulation described so far it is possible to
transmit, alongside ISDN, up to 8 Mbit/s additionally via a regular 0.6 mm Cu-wire for
telephony.
