NARROWBAND
INTERFERENCE
CANCELLATION IN
MULTIBAND OFDM SYSTEMS
Z. Nikolova, V. Poulkov, G. Iliev, G. Stoyanov
Dept. of Telecommunications
Technical University of Sofia
BULGARIA
e-mail: zvv@tu-sofia.bg
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Multiband OFDM (UWB):
Technology for short range high data rate
communications, combining OFDM and frequency
hopping.
 Occupies a very wide frequency band and low
transmission power;
 UWB systems are subject to different types of
narrowband interferences, which could deteriorate
strongly and even block communications;
 NBI suppression is of primary importance for these
systems.
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Multiband OFDM
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NBI mitigation
 NBI avoidance methods. Based on avoiding
the transmission over frequencies with strong
narrowband interferers;
 Cancellation – suppression methods. Aim at
eliminating the effect of NBI on the received
UWB signal.
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Scheme for the suppression of NBI
How?
Via adaptive complex filtering using the LMS
algorithm to adapt to the central frequency of the NBI.
Why is possible?
 Compared with the desired wideband signal the
interference occupies a much narrower frequency
band, but with a higher power spectral density;
 UWB signal has autocorrelation properties quite
similar to that of AWGN, so filtering in the frequency
domain could be applied.
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Scheme for the suppression of NBI
cos(2fct)
Pre-filter
To AGC
I
LPF
VGA
ADC
From
SC
LNA
eR(n)
xR(n)
Q
LPF
VGA
ADC
F
ACF
eI(n)
xI(n)
yR(n)
sin(2fct)
( )2
NBI
F
T
yI(n)
( )2
To SC
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Variable complex
digital filter circuit derivation
1
(
1

z
)
LP
H LS
(
z
)

b
1
1  (1  2b) z 1
z
1
z 1  

1  z 1
The composite multiplier,
containing b is derived:

f
bˆ
b
+
1
(
1

z
)
1
1
LP V
z

z
(cos jsin)
H LS 1 ( z )  bˆ
1
ˆ
1  (1  2b) z
First-order
complex coefficient
transfer function
Second-order real
coefficient
transfer functions



Re
Im
H ( z )  H LS 1 ( z )  jH LS 1 ( z )


CV
LS 1
All of them of BP type, describing a complex digital filter section, which is variable with
respect to both the BW (by changing of ) and the central frequency (by changing of ).
H
Re
LS 1
ˆ cosθ z 1 (2βˆ  1) z 2
1

2
β
( z )  H RR ( z ) H II ( z )  βˆ
1  2(2βˆ  1) cosθz 1 (2βˆ  1) 2 z  2
H
Im
LS 1
ˆ ) sin z 1
2
(
1

b
( z )  H RI ( z )   H IR ( z )  bˆ
.
1
2 2
ˆ
ˆ
1  2(2b  1) cosz  (2b  1) z
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Variable complex
digital filter circuit derivation
Complex second-order
digital filter section
z 1 
z 
1  z 1
b
+
cos 
sin 
+
+
+
Real first-order
digital filter section
Out Re
In
+
b
+
z -1
z 1  z 1 (cos jsin)
z -1
Out Im
In Re
+
1
sin 
cos 
+
b
z -1
+
In Im
+
+
Out
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Variable complex second-order filter
Magnitude and group-delay responses of variable BP complex secondorder filter for different values of the central frequency
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Variable complex second-order filter
Magnitude, phase and group-delay responses of variable BP complex
second-order filter for different values of the bandwidth
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Adaptive complex second-order filter
The cost-function is the power of BS filter
output signal:
Block-diagram of a versatile adaptive
complex narrowband filter
[e(n)e (n)]
ADAPTIVE
ALGORITHM
xR(n)
+
eR(n)
SECOND-ORDER
COMPLEX
FILTER
yR(n)
yI(n)
xI(n)
+
eI(n)
For the BP filter we have the following real
and imaginary outputs:
y (n) y (n) y (n) and y (n) y (n) y (n)
R
R1
R2
I
I1
I2
yR1 (n); y I 1 (n) - when the input signal is xR (n)
yR 2 (n); y I 2 (n) - when the input signal is xI (n)
For the BS filter we have - real and imaginary
outputs:
e (n)x (n) y (n) and e (n)x (n) y (n)
R
R
R
I
I
where
e(n)eR (n) jeI (n)
The Least Mean Squares (LMS) algorithm is
applied to update the filter coefficient responsible
for the central frequency as follows:

(n1)(n)mRe[ e(n) y ' (n)]
m
is the step size controlling the speed of the
convergence;
(*)
denotes complex-conjugate;
y(n) is the derivative of y(n) with respect to
the coefficient subject of adaptation.
I
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Performance evaluation of the NBI suppression
scheme via simulations relative to baseband:
Assuming standard MF-OFDM receiver with 3 subbands
(as proposed MB-OFDM in the 3.1 – 4.8 GHz band) and IEEE
802.15 3a Channel Model 1.
 The NBI interference is modulated with a random frequency
appearing in one subband;
 Soft decision 4-bit Viterbi decoding was used, thus
recovering some of the lost data due to the frequency domain
excision.
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Results:
 The simulations showed that for one and the same BER
such NBI cancellation scheme gives more than 2 dB
improvement for signal-to-interference ratios below 8 dB.
Drawback of the scheme:
 Frequency excision is performed over all OFDM symbols
(dehopped over all subbands), nevertheless the fact that the
NBI appears only in one.
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Better if:
Frequency excision is performed not over all symbols but
only on those affected by NBI.
Proposed solution:
 Instead of one complex adaptive filter section a filter bank
with corresponding switching capabilities and a number of
ACF equal to the number of subbands is implemented.
 Corresponding ACF to be switched accordingly with the
hopping of the carrier frequencies and will process only the
OFDM symbols appearing in one and the same frequency
sub-band.
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Analysis:
Behavior of an ACF, composed of three second-order
complex filter sections.
 Input interfering signal is composed of three complex
sine-signals with frequencies f1=0.25, f2=0.2 and f3=0.15;
 Learning curves show that our filter bank is able to
detect and to track the input complex sinusoids and can be
successfully used for their cancellation.
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Adaptive complex second-order filter
Learning curves for second-order adaptive complex filter applying
different step size
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Adaptive complex second-order filter
Learning curves of an ACFB consisting of three second-order complex
filter sections
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Advantages of the ACF:
 Low computational complexity;
 Fast convergence (less than 100 iterations in the
example);
 Convenience for implementation with CORDIC
processors;
 The very low sensitivity of the initial LP section
ensures a high tuning accuracy.
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Another important advantage:
The proposed NBI cancellation scheme is that
the adaptive complex notch filter section has also a
bandpass output used for monitoring the NBI
and switching of the ACF in cases when the NBI
vanishes or is reduced to a predetermined level.
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THANK YOU!
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