Digital Signal Processing Reference
In-Depth Information
Today, one of the most vivid examples of the active participation of digital pro-
cessing in the front-end of a transceiver is found in the gsm mobile phone sys-
tem. The modulation method used in gsm is Gaussian minimum-shift keying
(gmsk), a derivative of frequency-shift keying. The exact features of gmsk
modulation will be withheld from this discussion, but the gsm system has a
few remarkable details that are a perfect demonstration of the benefits of digi-
tal processing in wireless communications.
First of all, and most importantly, gsm is a true digital communication sys-
tem. Starting from the very beginning, the intention of the gsm standard was
to transport digital data. This in contrast to the analog-intended communica-
tion systems which were adapted later on, only to end up as the most horribly
inefficient transport vehicles for digital data.
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In the gsm system, (Figure 1.3) things are turned upside down: the ana-
log speech channel is immediately converted in a 13 kbit/s data stream by
the analog-to-digital converter and the rpe-ltp speech codec [Eur90]. The
biggest advantage of a digital representation is that the manipulation of data
becomes very transparent: error correction information can be easily inter-
leaved with the original raw data stream and data words can be reshaped into
symbols of arbitrary size. The latter finding opens up a whole new realm of
possibilities for the communications design engineer, who is no longer tied
to the nature and quirks of the analog incoming signal. Before modulation, a
fixed block of information bits can be restacked into new data
symbols
.Data
can be spread over symbols vertically, by choosing the number of amplitude
or phase/frequency quantization steps, and horizontally over a period of time,
13 bit PCM
8k samples/s
4kHz
13kbit/s
RPE-LTP
speech coder
voice
ADC
burst
modulation
error coding
200kHz
22.8kbit/s
Figure 1.3.
Manipulation of digital data in a gsm transceiver.
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Best possible proof: phone line dial-up modem.
