Communications and Network, 2010, 2, 246-250
doi:10.4236/cn.2010.24035 Published Online November 2010 (http://www.SciRP.org/journal/cn)
USING Hybrid Adaptive Techniques to Reduce
Multipath Effects in S-PCN Mobile Terminals
Sunday E. Iwasokun, Michael O. Kolawole
Department of Electrical and Electronics Engineering, School of Engineering and Engineering
Technology, Federal University of Technology, Akure, Nigeria
Received April 24, 2010; revised June, 16, 2010; accepted August 2, 2010
Abstract
Multipath signal processing is a promising technique for increasing the capacity of downlink frequency of
satellite communication networks (S-PCN). The paper presents an approach to processing and reducing mul-
tipath signals received from S-PCN typified of mobile terminal users in clustered or mountainous environ-
ment. Use of hybrid linear adaptive antenna array technique and adaptive filtering technique provides im-
proved performance by eliminating uncorrelated signal residing in antenna sidelobes.
Keywords: Multipath Signal, S-PCN, Hybrid Adaptive Technique, Adaptive Antenna Array, Mobile Termi-
nal
1. Introduction
Satellite personal communication networks (S-PCN) has
been improved and touted to provide communication
networks to vast region of the earth. S-PCN faces multi-
ple of challenges particularly those related to user termi-
nals as well as the space segment regulatory challenges
[1], and technical issues including topography and multi-
ple access [2], link diversity and traffic allocation [3],
and delayed signal component [4]. S-PCNs are subject to
multipath propagation caused by scattering from objects
in the vicinity of the satellite footprints and mobile ter-
minal. In mountainous terrain, the mountain may be visi-
ble (line-of-sight) to both the satellite and the handheld
terminal and act as large reflectors.
Some channels in S-PCN have one or more clusters of
propagation paths for which each cluster has large num-
ber of paths with small differential delays. Some of these
multipath signals may act as interference to one or more
of better signal components; implying that for each clus-
ter, the differences among the path delays are small on
the adaptive antenna array. In any case, the position
given by a satellite-based system relies on propagation
time measurement. Hence, the environment of the an-
tenna will play a major role because the propagation
phenomena caused by the environment induce indirect
propagation paths that introduce delays [5].
In this paper, the alternate path (multipath) reception is
used as the only major signal available on mobile termi-
nal. Most mobile stations transceivers are ideally circu-
larly polarized and isotropic in nature, with a single low
gain antenna element. However, these single element
units are particularly more susceptible to noise interfer-
ence because they receive signals from all directions.
The resolution for a channel in which the multipath
arises as a result of specular reflections off a number of
objects and each individual multipath component is not
dispersed in time [6]. The output of a specular multipath
channel consists of the sum of a number of attenuated
time-delayed versions of the transmitted signal from sat-
ellite, each of which arrives at the mobile terminal with-
out distortion [7]. The adaptive antenna has been recog-
nized as way to enhance capacity and coverage of the
system [8]. Adaptive antenna array mounted on the mo-
bile terminal is an approach suitable to combating un-
wanted signals in the sidelobes of antenna pattern as
communication satellite traverses the sky, which this
study attempts to investigate.
In some cases, line-of-sight (direct) and multipath (in-
direct) signals may be available to handheld terminal, the
use of switched antenna diversity may assist the hand-
held terminal to determine which of the signals received
is stronger in term of transmitted power component, and
then to switch to this antenna for a given time interval [9].
In the case where there is no line-of-sight signal from the
satellite but numerous multipath signals might be inci-

S. E. IWASOKUN ET AL.
247
dent on the handheld terminal unit, the antenna with the
strongest incident signal will be selected to receive the
signal.
2. Array Geometry
An adaptive array antenna unit is designed, as in Figure
1, bringing all the signals received by the various ele-
ments from a particular source into phase for further
processing, as well as achieving some desired perform-
ance, such as maximizing the received signal-to-noise
ratio (SNR). Drawing from [10] and [11], we analyse the
antenna array assuming that 1) there is a direct path for
the signal from the satellite and interfering signal is un-
correlated with the desired signal, 2) all the array ele-
ments are placed closely enough such that there is no
significant amplitude variation due to the difference in
propagation path length for any two elements, and 3)
there is no significant difference in – the direc-
tion-of-arrival (DOA) – of a particular plane wave at any
two elements.
For N elements array of identical characteristics, the
first pulse is taken as the phase centre, f(t- τ). The second
pulse is advanced by τ and denoted by f(t- τ), the third
pulse is advanced by 2τ and denoted by f(t- 2τ), and so
on. For convenience, five elements are used, so






 
12
34
2
34
To
ftaf taftaf t
af taf t





 

(1)
where 01
…5 are the weights/gains of each ele-
ment of the array. In adaptive antenna array, the
weights/gains of each element are taken to be the same
(i.e., 01 4
,aa
aa
a
... a

 ), conforming to practical situation,
and the elements to be equally spaced. Had the weights
variable, there is possibility they may modulate the de-
sired signal. Now, if the first pulse is Fourier transformed
and is represented by()f

, then






 
2
01 2
34
34
jj
T
jj
fafafeafe
af eaf e
 
 



 






(2)
By multiplying (2) by
j
e

  and then subtracting the
Figure 1. Hybrid adaptive processing system for S-PCN mobile terminal.

S. E. IWASOKUN ET AL.
248
resulting expression from (1), and rearranging ensuing
expression and using known geometric series expansion
we have


2
0
sin 5
() ()sin
jf
fafe f



 
















(3)
Typical S-PCN frequencies of 1.376 and 1.80 GHz [12,
13] are used to examine the antenna array’s behaviour
using (3). Resulting graphs are shown in Figure 2. The
sidelobes performance has an important impact on inter-
ference received from adjacent satellites (and ground
based signals operating on same bands), as well as in
determining antenna noise temperature. To successfully
eliminate, or reduce significantly, the effect of external
interference, the sidelobes have to be attenuated, if not
removed. The sidelobes are cancelled or removed in this
paper by the adaptive array process.
3. Processing Unit: Modelling Interferences
with Adaptive Filter
Our approach models the effect of interferers on the
S-PCN systems using interferences as recursive random
processes, with the array antenna. The input to
the processor in Figure 1 is the inverse Fourier
()
i
Nt


1(f

)


 of the array output, which now becomes X(t)
to the summer. The effect of noise from each antenna
element is factored in at the output; that is, for n ele-
ments with equal average signal-to-noise (SNR), the av-
erage SNR of the array combiner is 10 , which
is higher in dB than the average SNR of any one of the
10 log()n
elements [14]. Prior to processing, unit-step delay is in-
troduced into the incoming signals and then superimpose
on the signal of interest. The superposition allows the
system model of the handheld unit to determine which of
the arriving signals is stronger, and then switch to the
antenna for a given time interval before filtration to
eliminate or reduce interference.
Following Figure 1, the recursive-filtering algorithm
can be represented as a linear, discrete-time (t) model of
the form:




ˆˆ
UtXtNtWt

(4)


ˆ
Yt AUt

(5)
where


A
t is the system parameter; is opti-
mum adaptive but weighted filter and is additive,
uncorrelated system interference, assumed white, zero-
mean Gaussian and stochastic; and filter (or
measurement) output. The optimum interference

ˆ
Wt

Nt

Yt


ˆ
Wt
is defined as
  
0
ˆk
ith
i
WttNt i 



 (6)
where Nth is the acceptable interference threshold, and k
is the filter order. The proceeding coefficient of the filter
can be estimated from the present coefficient and other
thresholds [15]:




12()(
ii th
ttXtNt  )i

  (7)
where Nth is the convergence constant.
The adaptive filter adapts the filter coefficients to
achieve desired signal ensuring convergence; that is,
Figure 2. Antenna array ’s re sponse at typical S-PCN frequencies.

S. E. IWASOKUN ET AL.
249
Figure 3. Simulated received response of mobile terminal at typical S-PCN frequencies.
minimizing error

t

at each time index:

ˆ
() () tYtAUt   (8)
Ensuring fast convergence a local minimum is sought
leading to establishing threshold value; i.e. ,
 

1
0
k
tt
j
tjj

th






  (9)
where th

is the local minimum threshold. The adapta-
tion gain ()
g
m is introduced for coefficient updating
recursion for the period of the signal measurement:
0
ˆ()
() () ()
m
i
Ui
gm
X
iNi








(10)
where m is the period which the mobile terminal engages
the network.
As shown in Figure 3, application of adaptive filter in
conjunction with the antenna array processing shows
encouraging responses in terms of interference reduction.
The implication of our method is that the variation of the
filter’s weights, as a result of movement of the mobile
users, may affect the effectiveness of the system as the
satellite moves from orbit to another; low earth orbit
(LEO) through to geostationary orbit (GEO), for exam-
ple. However, a mobile user of any of the systems might
experience the same problem since the environment is
changing too rapidly for the weights to converge. This is
an area that is being investigated. Also, the relative mo-
tion between the user and the satellite reference may be
another source of error resulting in Doppler shift. Recent
work of [16] suggests that using frequency lock loop
could compensate for this error. Whilst our technique has
utilised uniform weight for the antenna array, this should
not a problem if variable weights were to be utilised [17].
4. Conclusions
The inherent communication capabilities of satellites
render them an attractive solution for the personal com-
munication in mountainous terrain. The paper has pre-
sented an antenna-array plus adaptive filtering model as
a way of processing multipath signals in satellite per-
sonal communication systems. This technique has shown
that it can eliminate an uncorrelated signals residing in
the antenna sidelobes successfully. This technique is
easily adaptable to S-PCN in LEO operational environ-
ment as a result of shorter time required by LEO satel-
lites to move across the sky.
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