Digital Signal Processing Reference
In-Depth Information
The total power that can be transmitted in compli-
ance with the Part 15 rules imposed by the fcc is re-
markably low: even when integrated over the entire
frequency range of 7
.
5 GHz, the maximum output
power is in the order of half a milliwatt (
PSD
[dBm/M
H
z]
-40
-50
-60
-70
-80
-90
0.96
1.6
2.73.1
4.2
4.8
6.0
8.5
10.6
frequency [GHz]
2
.
5dBm).
Compare this to the power levels that are employed by the 802
.
11a system
(
−
23 dBm and higher), and it should be clear that those two orders of mag-
nitude must have some repercussions regarding the signal-to-noise ratio of the
received signal. Also, as exemplified by the 802
.
11a system transmitting in the
middle of the uwb spectrum, there has never been any intention to allocate this
very large frequency band exclusively for low-psd wideband radio. The toler-
ance against in-band interference is a difficult design issue and at the same time
the most important performance measure of a wideband radio system. Maybe
the whole issue is best described by the label that is found on many Part 15
compliant devices:
+
CFR 47 part 15.19 labeling for license-free rf-generators:
Operation is subject to the following two conditions: (1) this device
may not cause interference, and (2) this device must accept any inter-
ference, including interference that may cause undesired operation of
the device.
Disadvantages of OFDM in wideband radio
In other words, a Part 15C intentional transceiver
system has no legal rights at all and should find its
own way around interference that suddenly turns up
in the middle of its frequency spectrum. A wide-
band radio system should dynamically react on
changing channel conditions and must be able to quickly reallocate resources
to frequency bands with an acceptable noise and interference level. This is
probably the main reason why the attention of the industry has progressively
shifted towards a multicarrier approach based on the ofdm system. It is be-
lieved that the flexibility of ofdm is the most reliable answer to unstable
parameters that come with a wideband channel such as narrowband interfer-
ence (nbi) and intersymbol interference (isi). ofdm - originally developed
for high-psd communication systems - is a system that entirely relies on dig-
ital signal processing, and while it is the most flexible system from a system
engineer's point of view, it entirely relies on a digital representation of the
baseband signal. For a radio system with a large spectral footprint, this im-
plies that the entire signal chain from antenna down to the digital back-end
must support this large bandwidth. Of course, with the obvious consequences
OFDM ?
