Image Processing Reference
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interference frequency suffer degradation. Thus, filtering out the NBI before demodulation
is better than frequency excision. The FIC algorithm achieves the best result because there is
no spectrum leakage, as happens with frequency excision, and there is no amplitude and
phase distortion as seen in the case of adaptive complex filtering.
It should be noted that the adaptive filtering scheme and frequency cancellation scheme lead
to a degradation in the overall performance when SIR >0. This is due either to the amplitude
and phase distortion of the adaptive notch filter or to a wrong estimation of NBI parameters
during the identification. The degradation can be reduced by the implementation of a
higher-order notch filter or by using more sophisticated identification algorithms. The
degradation effect can be avoided by simply switching off the filtering when SIR > 0. Such a
scheme is easily realizable, as the amplitude of the NBI can be monitored at the BP output of
the filter (Fig. 8).
In Fig. 10b, the results of applying a combination of methods are presented. A multi-tone NBI
(an interfering signal composed of five sine-waves) is added to the OFDM signal. One of the
NBI tones is 10 dB stronger than the others. The NBI filter is adapted to track the strongest NBI
tone, thus preventing the loss of resolution and Automatic Gain Control (AGC) saturation. It can
be seen that the combination of FE and Adaptive Complex Filtering improves the
performance, and the combination of FIC with Adaptive Complex Filtering is even better.
Fig. 10c shows BER as a function of SIR for the CM3 channel when QPSK modulation is
used, the NBI being modelled as a complex sine wave. It is evident that the relative
performance of the different NBI suppression methods is similar to the one in Fig. 10a but
the BER is higher due to the fact that NBI is QPSK modulated.
The experimental results show that the FIC method achieves the highest performance. On
the other hand, the extremely high computational complexity limits its application in terms
of hardware resources. In this respect, Adaptive Complex Filtering turns out to be the
optimal NBI suppression scheme, as it offers very good performance and reasonable
complexity. The FE method shows relatively good results and its main advantage is its
computational efficiency. Therefore the complex DSP filtering technique offers a good
compromise between outstanding NBI suppression efficiency and computational
complexity.
2.2.2 RFI mitigation in GDSL MIMO systems
The Gigabit Digital Subscriber Line (GDSL) system is a cost-effective solution for existing
telecomunication networks as it makes use of the existing copper wires in the last
distribution area segment. Crosstalk, which is usually a problem in existing DSL systems,
actually becomes an enhancement in GDSL, as it allows the transmission rate to be extended
to its true limits (Lee et al, 2007). A symmetric data transmission rate in excess of 1 Gbps
using a set of 2 to 4 copper twisted pairs over a 300 m cable length is achievable using
vectored MIMO technology, and considerably faster speeds can be achieved over shorter
distances.
In order to maximize the amount of information handled by a MIMO cable channel via the
cable crosstalk phenomenon, most GDSL systems employ different types of precoding
algorithms, such as Orthogonal Space-Time Precoding (OSTP), Orthogonal Space-
Frequency Precoding (OSFP), Optimal Linear Precoding (OLP), etc. (Perez-Cruz et al, 2008).
GDSL systems use the leading modulation technology, Discrete Multi-Tone (DMT), also
known as OFDM, and are very sensitive to RFI. The presence of strong RFI causes nonlinear
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