Digital Signal Processing Reference
In-Depth Information
make things worse, multiple communication devices present at the same ge-
ographic location will try to claim the same frequency band. The mac layer
of 802
.
11-based wlan's has anticipated on such occasions and provides a
semi-orchestrated channel access method. The contention-based random ac-
cess method (csma/ca
17
) gives all devices equal opportunities to acquire ac-
cess to the wireless channel. Unfortunately, the same frequency spectrum is
also in use by other competing networking technologies. The mac layer of
Bluetooth devices, for example, is kept simple and does not bother that much
about the potential presence of other devices that operate in the same 2
.
4 GHz
frequency band. The overall conclusion here is that dsss modulation is not
well suited for use in high-throughput wireless communication networks. The
low spreading factor results in a relatively simple, but also a very vulnerable
modulation method if deployed in a hostile multipath environment.
On the other hand, a single qpsk-modulated carrier has one competitive ad-
vantage over the ofdm modulation method discussed in Section 2.5: in a mul-
ticarrier system, subcarriers align in phase every once in a while, resulting in a
high peak signal value. In a single-carrier system, the time-domain signal has a
much lower papr value. As a result, the rf power amplifier of a 802
.
11b trans-
mitter needs a much lower back-off value from the 1 dB compression point, so
the pae
18
will be considerably better than for 802
.
11a/g devices.
2.7
Conclusions on single- and multicarrier systems
As a conclusion to this section, some of the problems that must be tackled in
a wideband wireless channel are summarized. Both the advantages and disad-
vantages of single- and multicarrier systems are considered here. From this,
the properties of an ideal (fictive) transceiver system are determined.
First of all, multipath is a severe problem, since it can cause isi for high symbol
rates. In an ofdm-based system, multipath reflections are dealt with by split-
ting up the symbol stream over multiple carriers, each of which is modulated at
only a fraction of the cumulative symbol rate. In a single-carrier system, some
tolerance against multipath reflections can be obtained by exploiting diversity
in one way or another. For example, in a dsss-based architecture, diversity of
coding is used to differentiate the signal-of-interest from unwanted reflections
and interference.
Unfortunately, as pointed out above, the resistance of dsss against frequency-
selective fading and narrowband interference depends heavily on the frequency
spreading factor. For high-throughput applications, the practically achievable
17
csma/ca: carrier sense multiple access with collision avoidance.
18
pae: Power-added efficiency.
