Digital Signal Processing Reference
In-Depth Information
by spreading the information over multiple symbols. Even better, not only the
speed at which symbols are pushed through the modulator can be controlled,
but also the smoothness in which the transition between two consecutive sym-
bols takes place and even which particular symbol transitions are (or are not)
allowed.
In the example of the gsm transceiver, digital information is flattened to a
stream of single-bit symbols. These symbols are subsequently applied to
a minimum-shift keyin (msk) modulator.
6
The advantage of msk is that it
produces an
angle-modulated signal
, with a constant amplitude. This has a
tremendous impact on the power consumption of the receiver: nonlinearities
in the front-end of the transmitter will not result in in-band distortion. A non-
linear, but power efficient class-c amplifier can thus be used for constant-
envelope signaling. In addition, the gsm modulator employs a very specific
type of msk, called Gaussian-shaped msk (gmsk). Since the main spec-
tral lobe of msk is fairly wide, the incoming symbols are passed through a
Gaussian pulse-shaping filter before they are applied to the modulator. The
smoother transitions between symbols at the output of the filter results in a
compaction of the power spectrum, and a more efficient use of the frequency
bands allocated to the gsm operator.
1.1
Wideband radio: spectral and spatial efficiency
The previous findings bring us one step closer to the real topic of this chapter:
having defeated the major shortcomings of pure analog modulation techniques,
our attention can be narrowed to the real problem of radio communication: the
limitations imposed by the wireless channel. Back in the good old days, most
wireless communication was of the long-distance point-to-multipoint type. To-
day, the same wireless channel must host a multi-user environment with heavy,
per-user traffic demands. It is evident that in a multi-user network, the number
of simultaneous point-to-point connections in a certain area is at least as impor-
tant as the spectral efficiency of a single device or the mobility of a single user.
This concept is covered by the term
spatial efficiency
, which is expressed in
units of bits per second per hertz per square meter (bits/s/Hz/m
2
). Spatial effi-
ciency is an increasingly important parameter for high-bandwidth data applica-
tions, such as wireless lan networks. What was already being done at a larger
scale, has now become common practice: the radio channel is spatially divided
in increasingly smaller cells, allowing for a higher reuse rate of the scarce spec-
tral resources offered by the wireless channel. For every multi-user application
it is important to consider the trade-off between data rate and mobility.
6
Minimum-shift keying is a subtype of continuous-phase fsk, where the frequency spacing between the
two fsk frequencies is minimized [Pas79].
