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distortion in AGC and Analogue-to-Digital Converter (ADC) functional blocks, as well as
spectral leakage in the DFT process. Many DMT tones, if they are located close to the
interference frequency, will suffer serious SNR degradation. Therefore, RFI suppression is of
primary importance for all types of DSL communications, including GDSL.
k=1
k=2
k=3
k=4
k=5
k=6
k=7
k=1
k=2
k=3
k=4
k=5
k=6
k=7
FEXT NEXT
Pair 1
Z S
Z L
Pair 2
Pair 3
Pair 4
s ( k,n )
x ( k,n )
Transmitter
Receiver
Transmission cable
Fig. 11. MIMO GDSL Common Mode system model
The present section considers a MIMO GDSL Common Mode system, with a typical MIMO
DMT receiver, using vectored MIMO DSL technology (Fig. 11) (Poulkov et al, 2009).
To achieve the outstanding data-rate of 1 Gbps, the GDSL system requires both source and
load to be excited in Common Mode (Starr et al, 2003). The model of a MIMO GDSL channel
depicted in Fig. 11 includes 8 wires that create k =7 channels all with the 0 wire as reference.
Z S and Z L denote the source and load impedance matrices respectively; s ( k,n ) is the n -th
sample of k -th transmitted output, whilst x ( k,n ) is the n -th sample of k -th received input.
Wide-scale frequency variations together with standard statistics determined from
measured actual Far End Crosstalk (FEXT) and Near End Crosstalk (NEXT) power transfer
functions are also considered and OLP, 64-QAM demodulation and Error Correction
Decoding are implemented (ITU-T Recommendation G.993.2, 2006), (ITU-T Recommenda-
tion G.996.1, 2006). As well as OLP, three major types of general RFI mitigation approaches
are proposed.
The first one concerns various FE methods, whereby the affected frequency bins of the DMT
symbol are excised or their use avoided. The frequency excision is applied to the MIMO
GDSL signal with a complex RFI at each input of the receiver. The signal is converted into
the frequency domain by applying an FFT at each input, oversampled by 8, and the noise
peaks in the spectra are limited to the pre-determined threshold. After that, the signal is
converted back to the time domain and applied to the input of the corresponding DMT
demodulator. The higher the order of the FFT, the more precise the frequency excision
achieved.
The second approach is related to the so-called Cancellation Methods, aimed at the
elimination or mitigation of the effect of the RFI on the received DMT signal. In most cases,
when the SIR is less than 0 dB, the degradation in a MIMO DSL receiver is beyond the reach
of the FE method. Thus, mitigation techniques employing Cancellation Methods, one of
which is the RFI FIC method, are recommended as a promising alternative (Juang et
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