Digital Signal Processing Reference
In-Depth Information
frequency
f
r < 300m
[dalwhinnie ~]$
f
[dalwhinnie ~]$
[dalwhinnie ~]$
[dalwhinnie ~]$
f
[dalwhinnie ~]$
[dalwhinnie ~]$
200kHz
wlan
gsm
20MHz
Figure 1.4.
The gsm network has a high spectral efficiency and mobility.
A wlan system has higher throughput but a small cell size.
In case of the gsm network, for example, the emphasis is on mobility. In or-
der to grant multiple concurrent users access to the gsm frequency band -
each of them with considerable transmission power - spectral efficiency was
one of the decisive factors in the choice of the modulation type. On the other
hand, for wlan systems, the emphasis is on throughput rather than on mobility
(Figure 1.4).
According to Shannon's theorem,
7
increasing the capacity of any commu-
nication system requires either a proportional increase of the transmission
bandwidth or an exponential increase in signal quality. For high-throughput
networks, it is thus more beneficial to think in terms of channel bandwidth,
rather than pursuing a higher transmission power. Moreover, increasing the
transmission power is not a viable option because it would imply larger cell
sizes and, as a consequence, less concurrent users.
1.2
Increasing the spectral bandwidth
It has become abundantly clear that a reduction of the cell radius and a large
spectral bandwidth are the key enablers for high-throughput, multi-user per-
sonal network applications. At the time of writing, the wireless lan
8
networks
are the most successful illustration of this statement. Besides a decreased ac-
tion radius, there are also a couple of less obvious disadvantages that come
with wideband systems, though. As most readers will already be aware of,
free space
isn't exactly the most accurate approximation of the environment
in which wireless applications are being operated. Whereas multipath delay
7
Channel capacity
=
bandwidth
×
log
2
(
1
+
snr
)
[Sha48].
8
IEEE802.11a/b/g [Gas05] and Hiperlan/2 [Eur01].
