Digital Signal Processing Reference
In-Depth Information
1.4
Interference immunity issues of wideband radio
As briefly touched upon in the previous section, high-powered narrowband in-
terference can be a serious issue in wideband radio receivers. Due to their large
modulation bandwidth, the possibility of an unwanted interferer signal falling
within the passband of a wideband receiver is much higher than for its nar-
rowband counterpart. In a narrowband receiver system, such as gsm, adjacent-
channel interference and the mirror frequency signal can be suppressed very
effectively using the two-step rf-to-if and if-to-baseband conversion process.
High-Q fixed-frequency ceramic filter sections at the intermediate frequency
(if) stage are very effective in suppressing blocker signals. When those filters
are located in front of the variable gain amplifier and the ad converters, the
linearity and dynamic range requirements of the subsequent circuitry is greatly
relaxed which guarantees a good sensitivity of the receiver in the presence of
strong blockers. In other words, narrowband receivers use their selectivity in
the frequency domain as the principal weapon against interference.
Now let's do the same reasoning for a wideband receiver, for example the
802
.
11g system in the 2
.
4 GHz band. The best approach would be to remove
in-band narrowband interferers before the vga and ad converters. However,
the number of interferers and their exact location in the passband of the re-
ceiver is unknown during design time. Suppressing unwanted signals in this
way would require a series of tunable notch-filters in the analog front-end. It is
clear that such a front-end would become impractically complex, so in practice
the responsibility for a dynamically adjustable filter is deferred to the digital
back-end.
Case study: OFDM modulation
The ofdm modulation method used by the 802
.
11a/g system provides op-
portunities for the transmitter to control how information is split up over the
frequency spectrum and offers a more flexible replacement for the notch-filter
bank in the analog domain.
13
Of course, all this comes at the expense of in-
creased dynamic range requirements in the entire analog signal chain. In order
to get some quantitative feeling for the impact of interference on the front-end
of a receiver, consider the case where only one unintended narrowband inter-
ferer emerges at some location within the 802
.
11a/g channel. For the sake of
clarity, the general outlines of the front-end of the ofdm receiver under con-
sideration are first sketched below.
13
Adaptive loading is not included in the 802
.
11a/g standard. For the reason behind this, see Section 4.2.
