JOURNAL OF RESEARCH IN NATIONAL DEVELOPMENT VOLUME 8 NO 2, DECEMBER, 2010
IMPROVED SIGNAL RECEPTION USING SPACE DIVERSITY
TECHNIQUE IN MOBILE COMMUNICATION SYSTEMS
J.J. Biebuma, S.I. Orakwe, C.C. Ejidike
Department of Electrical/Electronics Engineering,
University of Port Harcourt, Port Harcourt
Email: christabel4love2003@yahoo.co.uk
Abstract
This paper
presents an improved signal reception in mobile
communication system using space diversity
technique. Using two transmit antennas and one
receive antenna the scheme provides the same
diversity order as maximalratio receiver
combining (MRRC) with one transmit antenna, and
two receive antennas. It is also shown that the
scheme may easily be generalized to two transmit
antennas and M receive antennas to provide a
diversity order of 2M.
Keywords:
Diversity, maximal ratio combining, bit error
rate, signal to noise ratio, wireless
communications.
Introduction
The nextgeneration wireless systems are required
to have high voice quality as compared to current
cellular mobile radio standards and provide high
bit rate data services (up to 2Mbits/s). At the
same time, the remote units are supposed to be
small lightweight pocket communicators. In other
words, the next generation systems are supposed to
have better quality and coverage, good power and
bandwidth improved utilization, and be deployed in
diverse environments.
Figure 1:
Multipath propagation
Maximal ratio combining is known to be optimal in
the sense that it yields the best statistical
reduction of fading of any linear diversity
combiner. This work is only concentrating on
Maximal ratio combining method of achieving
diversity. The two parameters considered in this
work are signal to noise ratio (SNR) and bit error
rate (BER).
Bit error ratio is a performance
measurement that specifies the number of bit
corrupted or destroyed as they are transmitted
from its source to its destination (Song et.al,
1999). BER can be expressed mathematically as:
BER =
(1)
Signal to noise ratio is defined as the ratio of a
signal power to noise power and it is normally
expressed in decibel (dB) and can be
mathematically expressed as:
(2)
In the next generation of mobile communication
systems there is a need for higher performance of
signal reception, accomplished by increasing
capacity or reducing multipath interference.
Multipath interference in the mobile
communication is anything which alters, modifies,
or disrupts a signal as it travels along a channel
between a transmitter and a receiver. This paper
focuses on solving the following problem:
·
Rayleigh fading or multipath fading involving
irregular signal strength variations.
·
Path loss which increases with distance between
base station and mobile.
·
Performance degradation shown in the difference
between the mean signal  to  noise ratio (SNR)
and non  fading signal.
Signal fading due to the multipath propagation
can be reduced using space diversity techniques
considering signal to noise ratio (SNR) and bit
error rate (BER) as parameters.
When the mechanisms that caused fading in
communication channels were first modelleled in
the 1950s and 1960s, the principles developed were
primarily applied to over the horizon
communication covering a wide range of frequency
bands. This paper emphasizes on Rayleigh fading,
primarily in the UHF band that affects mobile
systems, such as cellular and personal
communication systems (PCS). In the analysis of
communication systems performance, the classical
(ideal) additivewhiteGaussianNoise (AWGN)
channel, with statistically independent Gaussian
noise samples corrupting data samples free of
intersymbol interference (ISI), is the usual
starting point for developing basic performance
results (Sklar, 2002). In a wireless mobile
communication system, a signal can travel from
transmitter to receiver over multiple reflective
paths. This phenomenon, referred to as multipath
propagation, can cause fluctuations in the
received signals amplitude, phase and angle of
arrival, giving rise to the terminology multipath
fading.
Diversity schemes
Diversity is a commonly used technique in mobile
radio systems to combat signal fading. The basic
principle of diversity is as follows. If several
replicas of the same informationcarrying signal
are received over multiple channels with
comparable strengths, which exhibit independent
fading, then there is a good likelihood that at
least one or more of these received signals will
not be in a fade at any given instant in time,
thus making it possible to deliver adequate signal
level to the receiver.
There are several techniques for obtaining
diversity branches, the most important of these
are: Space, frequency, polarization and time
diversity.
Space diversity is the most common form of
diversity technique in mobile radio base stations.
It is easy to implement and does not require
additional frequency spectrum resources. Space
diversity is exploited on the reverse link at the
base station receiver by spacing antennas apart so
as to obtain sufficient decorrelation. The key for
obtaining minimum uncorrelated fading of antenna
outputs is adequate spacing of the antennas. In
frequency diversity, signals are transmitted over
different frequencies. The frequency separation
between carriers should be larger than the
coherence bandwidth. They are not commonly used.
In Polarization diversity, two orthogonally
polarized (transmit or receive) antennas are used,
Orthogonal polarization exhibits uncorrelated
fading (scattering angle relative to each
polarization is random), only twobranch diversity
is possible. It is not commonly used. Time
diversity is the transmission of the same
information in time slots separated by channel
coherence time, has low efficiency and it is
useless for stationary users.
Several diversity combining methods are known. The
three main methods are: selection combining,
maximal ratio combining, and equal gain combining.
They can be used with each of the diversity
techniques discussed above.
In selection combining technique, one of the two
diversity branches with the highest
signaltonoise ratio (SNR) is connected to the
output of the receiver. The first signal above a
given threshold is used. The signal is used until
it falls below the threshold. At any time only one
signal branch is used and cophasing is
unnecessary. In Maximal Ratio Combining (MRC)
technique, the
M
diversity branches are first cophased and then
weighted proportionally to their signal level
before summing Fig. 3. The distribution of the
maximal ratio combiner has been shown to be
Prob
(3)
MRC is known to be optimal in the sense that it
yields the best statistical reduction of fading of
any linear diversity combiner, the mean SNR of the
combined signal may be easily shown to be
Mean =
=
M Γ
(4)
Therefore, combiner output mean varies linearly
with M. This confirms the intuitive result that
the output SNR averaged over fades should provide
gain proportional to the number of diversity
branches. In some applications of Equal Gain
Combining (EGC), it may be difficult to estimate
the amplitude accurately, the combining gains may
all be set to unity, and the diversity branches
merely summed after cophasing. The
distribution of equal gain combiner does not have
a neat expression and has been computed by
numerical evaluation. Its performance has been
shown to be very close to within a decibel to
maximal ratio combining (Goldsmith, 2005).
The effectiveness of diversity is usually
presented in terms of diversity gain (DG).
Diversity Gain can be defined as the improvement
in timeaveraged signaltonoise ratio (SNR) from
combined signals from a diversity antenna system,
relative to the SNR from one single antenna in the
system, preferably the best one.
(5)
Where the instantaneous SNR of the diversity in
combined signal is
is the mean
SNR of the combined signal,
is
the highest SNR of the diversity branch signals,
is the mean
value of
and
is a
threshold or reference level.
The probability P is dependent on the number of
branches M in the diversity system.
Maximal ratio combining
Maximal ratio combining (MRC) combines the
information from all the received branches in
order to maximize the ratio of signal to noise
power, which gives it its name. All branches are
weighted according to their individual voltage to
noise power ratios and then summed and the
weightings are designed so as to give maximum SNR
(Andrews, 2002).
Fig. 1 shows the baseband representation of the
classical twobranch classical maximalratio
receive combining (MRRC) scheme. At a given time,
a signal
is
sent from the transmitter. The
channel between the transmit antenna and the
receive antenna zero is denoted by
and between
the transmit antenna and the receive antenna one
is denoted by
where
=
(6)
Noise and
interference are added at the two receivers. The
received baseband signals are
(7)
Where
and
represent
complex noise and interference. Assuming
and
are Gaussian
distributed, the maximum likelihood decision rule
at the receiver for these received signals is to
choose signal
if
(8)
Where
is the
squared distance between signals
and
calculated by
the following expression:
(9)
The receiver combining scheme for twobranch MRRC
is as follows:
(10)
Expanding (8) and using (9) and (10) we get
(11)
Fig.2.
Twobranch MRRC.
(12)
Where
is the energy
of the signal. The maximalratio combiner may then
construct the signal
,
as shown in Fig. 1, so that the maximum likelihood
detector may produce
, which is
maximum likelihood estimate of
(Sklar,
2002).
Twobranch transmit diversity with
receivers
There may be applications where a higher order of
diversity is needed and multiple receive antennas
at the remote units are feasible. In such cases,
it is possible to provide a diversity order of 2
with two
transmit and
receive
antennas.
Fig.3.
the new twobranch transmit diversity scheme with
two receivers.
Table 1
The definition of channels between the transmit
and receive antennas

Rx antenna 0 
Rx antenna 1 
Tx antenna 0 


Tx antenna 1 


Table 2
The notation for the received signals at the two
receive antennas

Rx antenna 0 
Rx antenna 1 
Time



Time



Fig. 3
shows the baseband representations of the new
scheme with two transmit and two receive antennas.
Table 1 defines the channels between transmit and
receive antennas, and Table 2 defines the notation
for the received signal at the two receive
antennas. Where
are receivers
at different points and
are complex
random variables representing receiver thermal
noise and interference. The combiner in Fig. 3
builds the following two signals that are sent to
the maximum likelihood detector:
(13)
Substituting
the appropriate equations we have:
(14)
We may
hence conclude that, using two transmit and
receive
antennas, we can use the combiner for each receive
antenna and then simply add the combined signals
from all the receive antennas to obtain the same
diversity order as
–branch
MRRC.
Simulations result
The diversity gain is a function of many
parameters, including the modulation scheme and
FEC coding.
Fig:
4
Fig:
5
Fig. 4 and 5: The BER performance comparison of
coherent BPSK with MRRC and twobranch transmit
diversity in Rayleigh fading.
Fig. 4 and
5 shows the BER performance of uncoded coherent
BPSK for MRRC and the new transmit diversity
scheme in Rayleigh fading. It is assumed that the
total transmit power from the two antennas for the
new scheme is the same as the transmit power from
the single transmit antenna for MRRC. It is also
assumed that the amplitudes of fading from each
transmit antenna to each receive antenna are
mutually uncorrelated Rayleigh distributed and
that the average signal powers at each receive
antenna from each transmit antenna are the same.
Further, we assume that the receiver has perfect
knowledge of the channel. Although the assumptions
in the simulations may seem highly unrealistic,
they provide reference performance curves for
comparison with known techniques. As shown in Fig.
4 and 5, the performance of the new scheme with
two transmitters and a single receiver is 3 dB
worse than twobranch MRRC. The 3dB penalty is
incurred because the simulations assume that each
transmit antenna radiates half the energy in order
to ensure the same total radiated power as with
one transmit antenna. If each transmit antenna in
the new scheme was to radiate the same energy as
the single transmit antenna for MRRC, however, the
performance would be identical (Goldsmith, 2005).
In other words, if the BER was drawn against the
average SNR per transmit antenna, then the
performance curves for the new scheme would shift
3 dB to the left and overlap with the MRRC curves.
Nevertheless, even with the equal total radiated
power assumption, the diversity gain for the new
scheme with one receive antenna at a BER of 104
is about 15 dB. As stated before, these
performance curves are simple reference
illustrations. The important conclusion is that
the new scheme provides similar performance to
MRRC, regardless of the employed coding and
modulation schemes.
Conclusions and discussions
An improved signal reception in mobile
communication system has been presented. It is
shown that, using two transmit antennas and one
receive antenna, the signal reception provides the
same diversity order as MRRC with one transmit and
two receive antennas. It is further shown that the
technique may easily be generalized to two
transmit antennas and M receive antennas to
provide a diversity order of 2M. An obvious
application of the technique is to provide
diversity improvement at all the remote units in a
wireless system, using two transmit antennas at
the base stations instead of two receive antennas
at all the remote terminals (Cavers, 1991). The
scheme does not require any feedback from the
receiver to the transmitter and its computation
complexity is similar to MRRC. When compared with
MRRC, if the total radiated power is to remain the
same, the transmit diversity scheme has a 3dB
disadvantage because of the simultaneous
transmission of two distinct symbols from two
antennas. Otherwise, if the total radiated power
is doubled, then its performance is identical to
MRRC. Moreover, assuming equal radiated power, the
scheme requires two halfpower amplifiers compared
to one full power amplifier for MRRC, which may be
advantageous for system implementation. The scheme
also requires twice the number of pilot symbols
for channel estimation when pilot insertion and
extraction is used (Alamouti, 1998).
References
Alamouti S. M. (1998)
A simple
transmit diversity technique for wireless
communications. IEEE
Journal on Sel. Areas in Communications,
16(8):1451 – 1458, Oct..
Andrews J.
G., Choi W., and Heath R. W. (2002) Overcoming
interference in multiantenna cellular networks,
Under Revision,
IEEE
Wireless Communications Magazine.
Bernard Sklar (2002)
"Digital
Communications: Fundamentals and Applications",
PrenticeHall, 2nd Edition, pp. 3033.
Cavers J.
K. (1991) An analysis of pilot symbol assisted
modulation for Rayleigh fading channels, IEEE Trans., Veh, Technol., vol. 40, pp.
686–693.
Goldsmith A. J. (2005)
Wireless
Communications. Cambridge University Press.
Minyan S., Yang X., Joachim H. (1999) “High Data
Rate Wireless System”, IEEE, pp.13441350.