Information and Communication Technology Seminar, Vol. 1 No. 1, August 2005
ISSN 1858-1633 2005 ICTS 105
CI PERFORMANCE COMPARISON OF AN ADAPTIVE AND SWITCHED BEAM IN THE GSM SYSTEMS EMPLOYING SMART ANTENNA
Tito Yuwono
1
, Mahamod Ismail
2
, Zuraidah bt Zainuddin
2
1
Department of Electrical Engineering Universitas Islam Indonesia UII Jogjakarta INDONESIA
2
Department of Electrical, Electronics and System Engineering Universiti Kebangsaan Malaysia, 43600 UKM Bangi Selangor, MALAYSIA
Email:titoyuwonoyahoo.com ,mahamod.ukm..my,
zuraidahyahoo.com
ABSTRACT
The continuous increment of mobile communication subscribers, require a higher capacity
and spectrum efficiency of the cellular system especially in an urban area where the traffic density is
high. In order to accommodate the increasing number of subscribers, smart antenna technology is used to
improve system’s performance, which could increase the number of call and data transmission as well as
simplifying the networking management. The basic principle of a smart antenna is to control and reduce
the effect of interference to provide the user with the highest quality of uplink and downlink signal. This
can be achieved by using narrow beams with signal processing capability at the base sites and to optimize
its radiation pattern, which automatically updated according to MS movement or as signals conditions
change. In this paper, two types of smart antenna were considered, which are the switched beam systems and
adaptive antenna systems. The significant difference between these two systems is from the beamforming
point of view. The key part of this study is to develop a simulator for adaptive antenna systems AAS and
switched beam systems SBS based on GSM 900 in order to determine the factors that could affect
network’s quality and capacity. Simulation results of different techniques are compared and the
performance evaluations of CI are discussed. Results show that the AAS gives a better performance
compared to SBS, with average improvement of CI around 5 to 10 dB.’
Keywords : CI, Smart Antenna, Adaptive, Swithched beam, GSM
1. INTRODUCTION
New technologies are being introduced into the market with the evolution of the mobile
communication system. Cellular phone companies are always trying to give the best services to the
customers through high quality coverage with more capacity.
Smart antenna has become another alternative in order to increase the capacity and efficiency of the
Fig.7 Graphical view of the simulation
Information and Communication Technology Seminar, Vol. 1 No. 1, August 2005
ISSN 1858-1633 2005 ICTS 106
system. It offers various methods to improve the performance of the wireless system. In general, smart
antenna has a potential to provide wider coverage area, increase the performance of the downlink and
uplink while improving the system’s capacity.
In this paper, research has been done based on two types of smart antenna, which are switched beam
system SBS and adaptive antenna system AAS that are used in base stations. The comparisons of these
two systems were done according to a few aspects such as system’s quality performance and system’s
capacity. The significant difference between these two systems is from the beam forming point of view in
order to serve MS[1]. Figure 1a shows the radiation pattern of the switched beam system while Figure 1b
shows the radiation pattern for adaptive antenna system. In Figure 1b, the main lobe is always
directed to the desired signal while at the same time it puts the null to the interference signal.
Figure 1 : Radiation pattern that might be chose or transmitted to serve target MS red and co-channel interference signal blue by
SBS and AAS.
2. CLASSIFICATION OF SMART ANTENNA
Smart antenna sometime also referred to as intelligent antenna, adaptive array or dynamic or
adaptive sectoring and can be implemented using either switched beam or adaptive beam technique.
Figure 1a and b shows a typical antenna pattern for a switched beam and an adaptive beam
implementation of a smart antenna.
2.1. Switched Beam
Switched beam or switched lobe antennas are directional antennas deployed at base stations of a cell.
They have only a basic switching function between separate directive antennas or predefined beams of an
array. The setting that gives the best performance, usually in terms of received power, is chosen. The
outputs of the various elements are sampled periodically to ascertain which has the best reception
beam. Because of the higher directivity compared to a conventional antenna, some gain is achieved. Such an
antenna is easier to implement in existing cell structures than the more sophisticated adaptive arrays,
but it gives a limited improvement.
The switched-beam system is achieved by utilizing multiple narrow fixed beams 15 to 30 degree
horizontal beamwidth in a single sector. Since the beam pattern in a sector is predetermined, the mobile
station is not always in the main lobe or center of the beam. The beam can also be “on” or “off” depending
on the spatial distribution of the mobile.
Switched-beam systems are more economical to implement, requiring only a static beamforming
network, RF switch and control. A separate beam selection is continuously made for each subscriber unit
as it moves through the cells coverage area. Commercial deployments of switched-beam systems
have demonstrated significant capacity improvements: over 100 capacity gain in analogue and GSM
networks. For example, in Metawave Communications field trial, a GSM smart antenna system significantly
reduced interference and achieved a 6 dB CIR improvement. In the same trial, the improvement in
CI led to a 50 reduction in dropped calls. Studies have also indicated that network capacity increases of
100 can be achieved with deployments at selected sites representing only 38 of the total number of
base sites in a network[2].
2.2. Adaptive Beam
The adaptive array system used a similar idea but different approach in allocating the beams with respect
to the user. It utilizes more complicated signal processing algorithms to allow the system to use a
varying beam pattern in response to the movement of the user and the changing environment.
The main idea behind such a system is its ability to allocate the main lobe of the antenna array to the users
and nulls to all the other interferers. This clearly optimized the performance of the wireless system.
However, it added much complexity to the systems, as algorithms to continuously distinguished between the
signals from the user, the interfering signals from other signals and multi-path signals are required. In
addition, the angle of arrival for the signal must be determined so as to “point” the main lobe exactly at
the desired signal.
Adaptive antenna are a set of antenna elements that can change their antenna radiation or reception pattern
dynamically to adjust to variations in channel noise and interference, in order to improve the SINR of a
desired signal. The name ‘smart’ refers to the signal processing power that is part and parcel of the
adaptive antenna. It controls the antenna pattern by updating a set of antenna weights.
It is possible for adaptive systems to achieve greater system performance than switched-beam
systems. Using array signal processing allows the antenna system to take advantage of path diversity by
combining coherent multipath components. The adaptive pattern that is created can be optimised to
enhance the desired users signal while simultaneously suppressing interfering signals. However, the
theoretical performance benefits achieved by an
CI Performance Comparison of an Adaptive and Switched Beam in the GSM Systems Employing Smart Antenna – Tito Yuwono, Mahamod Ismail, Zuraidah bt Zainuddin
ISSN 1858-1633 2005 ICTS 107
adaptive system may be offset by the cost and complexities encountered in implementation.
In addition, a number of technical issues complicate adaptive systems. Specifically, downlink
beamforming is difficult due to the presence of multipath and frequency duplexing. Since the goal is
to select the best single direction to transmit to the user, it is critical that the base station processor detect
changes in power along the subscribers path. However, as the subscriber moves quickly, this
becomes more difficult because the received signal from the subscriber may change very rapidly. Signal
degradation can occur if the downlink beamformer cannot track these changes. Further, the beams formed
on the downlink are determined by uplink signal measurements. Because all cellular systems are
frequency duplexed, errors in the downlink beamformer are introduced because of the different
uplink and downlink signal characteristics.
2.3. Beamforming Technique For Adaptive
The beam formation at every base station is based on the mobiles’ DOAs. Figure 2a shows the actual
parameters used in the algorithm. Given the jth base station and two mobiles, say the ith and the kth, the
angles of arrival are denoted a αij and αkj
respectively while the separation angle between the mobiles is
ik j
γ
. By setting a minimum separation angle between adjacent mobiles, say
γ, we are then in a position to narrow or widen antenna beamwidths
after all directions of arrivals have been sorted.
2a
.............
Beam1 Beam2
Beam3 Beam4
Beam5
α 1j
α α
α α
α α
α α
α 2j
3j 4j
5j 6j
7j 8j
9j ij
Direction of arrival
2b
.............
Beam1 Beam2
Beam3 Beam4
α 1j
α α
α α
α α
α α
α 2j
3j 4j
5j 6j
7j 8j
9j ij
Direction of arrival
α newj
2c
Figure 2: a Actual beam formation algorithm parameters, b Examples of beam formation and c Beam reformation to
accommodate a new call
Figure 2b shows an example of beam formation where
α2j-α1j γ and α3j-α2j γ, α4j-α3j γ and α5j-α4j γ,
α6j-α5j γ, α7j-α6j γ and
α8j-α7j γ Henceforth, beam number 1 and 2 respectively will
cater for a group of three and two mobiles, while beams numbers 3, 4 and 5 will cater each for a single
mobile. The beams sizes are given by αij-αpj + γ
where i and p represent the first and the last mobiles in the same beam respectively. If a new call arrived and
served by the same BSj such that, α2j-α1j γ and α3j-α2j γ,
α4j-α3j γ and α5j-α4j γ, α6j-α5j γ, αnewj - α6j γ, α7j-α6j
γ and α8j-α7j γ
then the new formation of beams are as shown in Figure 2c. It can be seen that the number of beam is
reduced from five to four and there are only two beams serving lone mobile instead of four.
Figure 3 shows flow diagram for the execution of the simulation process
3. RESULTS AND DISCUSSION
Based on the results of the simulations, the adaptive antenna and switched beam system’s
performance were analyzed. The value of CI depicts the signal quality in the cell, whereas the higher the
CI value, the quality of the signal received by MS is better. In the simulation, the evaluation of the
system’s performance was done by using CI as the measuring standard. Factors that are studied are:
1. Minimum 3-dB beamwidth antenna factor, β.
From Figure 4, the mean value of CI for the switched beam system is much higher than the mean
value of CI for adaptive antenna system. This phenomenon occurs because the beams that are
formed in the adaptive antenna system consist of various sizes, depending on the minimum separating
angle between adjacent MS. For a beam with small
β, the CI achieved is high and for greater
β, the value of CI achieved is low. Thus, the mean value of CI for
Information and Communication Technology Seminar, Vol. 1 No. 1, August 2005
ISSN 1858-1633 2005 ICTS 108
switched beam system is much lower. In the switched beam system, MS receives the same signal strength
because the entire beam has the same β size.
Figure 3: Flow diagram for the execution of the simulation process
Mean value of CI versus minimum 3-dB beamwidt β,
40 45
50 55
60 65
70
6 12
18 24
30 Minimum 3-dB beamwidth,
β [degree]
M e
a n
v a
lu e
o f C
I [d
B ]
AAS SBS
Figure 4: Mean value of CI versus minimum 3-dB beamwidth, β
2. Co-channel interference that affects the MS in the serving BS.
In figure 5, it is shown that when the interference increases, the value of CI will decrease, but the
quality of the signal received is still in good condition. This is due to the fact that a good quality cell for the
GSM system has a mean value of CI that exceeds the CI threshold value, which is 12dB[3].
Mean value of CI versus no of co-channel interference
40 45
50 55
60 65
70
1 2
3 4
5 6
No of co-channel interference
M ean
v a
lue of
C I
[ d
B ]
AAS SBS
Figure 5: Mean value of CI versus number of co-channel interference
3. Distance between MS and serving BS. When MS is located further from the Serving BS,
the strength of the signal received would decrease. As MS moves away from serving BS, it will come closer
to the interferer BS, this will increase the interference thus degraded the mean value of CI that can be
achieved. Figure 6 depicts that performance of serving BS with adaptive antenna system always has a better
performance than the switched beam system in term of efficiency of receiving the signal versus distance
factor.
Determine models and parameters for the simulation
Generate BS and cells; determine the coordinate for each BS
Generate MS randomly
Calculate path loss
Calculate signal strength of serving BS, interference CI value that received by
MS
Data acquisition and conclusion is made based on the results of simulation.
Determine angle of arrival AOA of each
MS from serving co- channel BS.
Calculate distance between MS
serving BS co- channel BS
Form beams based on the angle of arrival AOA
attained.
Calculate gain for each MS
CI Performance Comparison of an Adaptive and Switched Beam in the GSM Systems Employing Smart Antenna – Tito Yuwono, Mahamod Ismail, Zuraidah bt Zainuddin
ISSN 1858-1633 2005 ICTS 109
Mean value of CI versus distance between MS and BS 1
20 40
60 80
100 120
1 2
3 4
5 6
7 8
9
Distance between MS and serving BS [km]
Mean v a
lue of C I [
d B
]
Adaptiv e antenna sy stem AAS
Switched beam sy stem SBS
Figure 6: Mean value of CI versus distance between MS and serving BS.
4. Number of MS in a cell. Based on the graphs attained in Figure 7, it can be
concluded that if there is a small number of MS in a cell, performance of adaptive antenna system is better
compared to switched beam system. The performance degrades as the number of MS to be served increase.
Degradation of performance is due to the separating angle between adjacent MS. As the number
of MS increase, the separating angle between adjacent MS will decrease and become smaller than the
minimum separating angle that had been chosen. Hence, more MS will be served in the same beam and
the size of 3-dB beamwidth will be wider. Therefore, radiated power from the serving BS cannot be focused
to desired MS and the value of CI will decrease.
0,2 0,4
0,6 0,8
1 1,2
15 30
45 60
75 90
105 120
CI [dB]
Prob CI = abcissa AAS
MS=10 SBS
MS=10
a
0,2 0,4
0,6 0,8
1 1,2
15 30
45 60
75 90
10 5
12 CI [dB]
Prob C I
= abc is
s a
AAS MS=20
SBS MS=20
b
0,2 0,4
0,6 0,8
1 1,2
15 30
45 60
75 90 10
5 12
13 5
CI [dB] Prob CI = abcissa
AAS MS=30
SBS MS=30
c
0,2 0,4
0,6 0,8
1 1,2
20 40
60 80 10
12 14
CI [dB]
Prob CI = abcissa AAS
MS=40 SBS
MS=40
d
Figure 7: Distribution of CI values for adaptive antenna system and switched beam system for a No. of MS=10, b No. of MS=20, c
No. of MS=30 and d No. of MS=40.
Information and Communication Technology Seminar, Vol. 1 No. 1, August 2005
ISSN 1858-1633 2005 ICTS 110
4. CONCLUSION
