International Journal of Engineering Trends and Technology (IJETT) – Volume 31 Number 4- January 2016
Implementation of RSS based CDMA handoff
Algorithm for estimation of Optimal Hadd and
Hdrop values
S. Neeraja.1, G. Sasibhushana Rao2
1
2
Assistant Professor, Dept of ECE, GIT, GITAM University, Andhra Pradesh, India
Professor, Department of ECE, College of Engineering, Andhra University, Visakhapatnam, India
Abstract
Proper design of handoff is one of the main
challenges in cellular networks, since it has a great
impact on the system performance and capacity. Soft
Handoff is used in CDMA and WCDMA systems
provide a smoother transition and improve the
perceived Quality of Service (QoS) by proper
selection of Hysteresis add (Hadd) and Hysteresis
drop (Hdrop) values. In this paper, optimum Hadd
and Hdrop values are estimated by using new
Received Signal Strength (RSS) based CDMA
handoff Algorithm. In this paper, first the analysis is
done for variable base station (BS) and mobile
station (MS) antenna heights with fixed Hadd and
Hdrop values (12 dB and 4 dB) for estimating the
practical BS and MS antenna heights at minimum
RSS but in this analysis the soft handoff distance is
more (approx. 500m) which causes forward link
interference. In the next step, with the estimated BS
and MS antenna heights (35m, 1.5m) are taken into
consideration and the effect of variable Hadd,
Hdrop values on soft handoff distance is evaluated.
By this analysis the optimum Hadd, Hdrop values
(3dB to 5dB) are estimated for a CDMA cellular
system with moderate soft handoff distance which
improves the quality of service.
Keywords: Handoff, Soft Handoff, RSS, Hadd,
Hdrop, CDMA
1. Introduction
The CDMA and WCDMA systems are more popular
because of soft handoff mechanism which provides
seamless call transmission. The 2G TDMA based
GSM cellular system provides the hard handoff
mechanism which is “break before make’ approach.
In this, MS breaks the connection to the old BS and
then make the communication with the new BS
which causes the “Ping –Pong effect” and
communication quality is degraded [1], whereas, in
2G CDMA and 3G WCDMA cellular systems allow
“soft handoff” between the cells because of its
unique universal frequency reuse feature[2]. Soft
handoff allows the mobile stations to communicate
with two or more base stations at the same time. The
mobile station does not interrupt the communication
with old base station until a new connection is
ISSN: 2231-5381
obtained with the new base station and eliminates
“Ping-Pong effect” and lower switching load on the
network signaling and provides smooth call
transmission; together with power control and also
used as an interference-reduction mechanism [3]-[5].
The mobile stations are separated by unique pseudo random sequences and multiple data flows are
transmitted simultaneously via radio interface.
Consequently, due to the usage of soft handoff
mobile station (MS) is able to get benefit from
macro diversity thereby enhancing both quality of
service and capacity. Proper design of soft handoff is
one of the main challenges in 2G/3G cellular
networks, since it has a great impact on the cellular
system performance and capacity [6]. So, in this
paper, a soft handoff algorithm is implemented for
improving the CDMA cellular system performance
by estimating the Hysteresis add and Hysteresis drop
with practical BS and MS antenna heights.
II. CDMA Cellular System
CDMA cellular technology is initially known as IS95 (Interim Standard - 95) and is competing with
GSM technology for dominance in the cellular world.
The CDMA cellular system is based on Code
Division Multiple Access (CDMA) scheme which
was demonstrated in 1998. It operates in 800 MHz
and 1.9 GHz PCS bands [7]. Compared to GSM
cellular systems, CDMA requires fewer cell towers
and provides a calling capacity improved by five
times. CDMA also provides more than 10 times the
voice traffic of earlier analog system (1G cellular
system).
In CDMA cellular system, unique spreading codes
are used to spread the base band data before
transmission [8]-[9]. CDMA assigns a unique code
sequence to each user that is used to code data
before transmission. If a receiver knows the code
sequence related to a user, it is able to decode the
received data. The codes are shared by the mobile
station and the base station. These codes are called
“Pseudo Random Sequences”. Using these
sequences, the entire spectrum is utilized by all the
users at all times.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 31 Number 4- January 2016
Handoff Mechanism
Mobility is the most important feature of a cellular
and mobile communication system. Even though the
mobile phone users are moving inside or outside the
cells, they are able to communicate with each other.
This can be achieved by the process handoff.
Handoff is a mechanism of transferring an ongoing
active call from one cell to another or one base
station to another or one channel to another as a user
moves through the coverage area of a cellular system
[10]-[11]. The cellular handoff is not an easy process
to implement in practically, as the cellular network
needs to decide when to handoff and to which cell.
III. RSS Based CDMA Handoff Algorithm
Implementation for estimating optimum Hadd
and Hdrop values
CDMA systems support the soft handoff process.
Active set is defined as the set of base stations to
which the Mobile station (MS) or User Terminal
(UT) is simultaneously connected [12]-[13]. In soft
handoff region, UT is connected with more than one
base station. Initially when UT is very near to BS,
the active set consists of only one BS but as distance
from BS1 increases and when soft handoff region
starts then active set size changes with distance.
The primary objective of a soft handoff algorithm is
to provide good signal quality. Signal quality can be
improved by including more base stations in the
active set, but this comes at the cost of increased use
of system resources. For minimizing the active set
size, one alternative is to frequently update the
active set at each time instant, to maintain the
smallest active set with sufficient signal quality.
However, frequent updates or handoffs bring
switching costs with them[14]. A soft handover
algorithm is implemented to maintain the user’s
active set. The following parameters are needed to
describe the soft handoff algorithm which is shown
in Fig. 1.
RSS1
RSS2
Received
signal
strength at
UT
Hadd
Hdrop
BS1
BS1&BS2
BS2
Fig. 1 RSS based soft handoff algorithm
Hadd: Hysteresis for adding
Hdrop: Hysteresis for dropping
RSS1: Received Signal Strength from
BS1
RSS2: Received Signal Strength from
BS2
ISSN: 2231-5381
Hysteresis add, determines the received signal
strength level for which qualifying BSs are added to
the active set, whereas a hysteresis drop, determines
when the weak BSs are dropped from the active set.
RSS based CDMA Handoff Algorithm:
Active set is reorganized in following manner,
If active set consists of BS1 and
RSS1>RSS2 and |RSS1-RSS2| is greater
than Hadd
then active set consists of BS1 itself.
If
( RSS1
RSS2)
Hadd ,
active set consist of BS1 and BS2.
If active set consists of BS1 and BS2 and
( RSS1 RSS2 becomes greater than or
equal to Hdrop, active set consist of that
base station whose average signal strength
is higher. Base station with smaller signal
strength will be dropped from the active set.
Effect of Soft Handoff Distance and Soft Handoff
Parameters on CDMA Cellular System
The soft handoff distance is the difference between
the distance at which Hadd started and the distance
at which Hdrop assigned. The soft handoff distance
is a marker of how long a soft handoff will take
place. The soft handoff distance and Hadd and
Hdrop are very important parameters in determining
system performance and need to be carefully
optimized for a given situation.
A wider margin results in longer average soft
handoff duration. By changing the Hadd and
Hdrop the handoff region will be affected.
Reducing and expanding the cell boundaries,
increases the soft handoff region.
If the soft handoff distance is more, it allows
more MSs to be in soft handoff mode. It will
decrease reverse link interference and benefits
from macro diversity. That is, the UTs in
handoff are connected to the best available link
and therefore, do not transmit excessive power.
However, the increase in the number of UTs in
soft handoff and the increase in the average
handoff duration can increase system
complexity and tie up previous inadequate
system resources. In soft handoff, by allowing
multiple BSs to transmit to one UT the forward
link interference increases.
The challenge is to optimize the Hadd, Hdrop and
soft handoff distance so that the capacity and quality
of service requirements are satisfied, while keeping
the operational cost and system complexity reduced.
CDMA Cellular System Model
Consider, cellular network as shown in Fig.
2. A user terminal moves from the cell served by
base station1 (BS1) towards another with BS2 with
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International Journal of Engineering Trends and Technology (IJETT) – Volume 31 Number 4- January 2016
constant speed along a straight line. Dc is the
distance between the two base stations. Both of the
base stations are assumed to be located at the centre
of the respective cell and operating with the equal
transmitting power.
In the first configuration, the analysis is done for
constant soft handoff parameters (Hadd=12dB and
Hdrop=4dB) and for variable base station antenna
heights (150m, 100 m, 50m and 35m) and mobile
station antenna heights (3m and 1.5m), By this
analysis, practical base and mobile station heights
are designed for minimum received signal strength.
Cell
2
Fig. 2 CDMA cellular system model
Received signal strength (RSS) at user terminal is
affected by the pathloss. Path loss is the
deterministic component of RSS, which can be
evaluated by propagation path loss model which is
Cost 231 Hata model.
COST-231 Hata Model
To extend Hata-Okumura model for personal
communication system (PCS) applications operating
at 1800 to 2000 MHz, the European Co-operative for
Scientific and Technical Research (COST) came up
with COST-231 model [15]. This model is derived
from Hata model and depends upon four parameters
for prediction of propagation loss which include
frequency, height of mobile antenna, height of base
station and distance between base station and mobile
antenna. For CDMA and WCDMA cellular systems,
the Cost 231 model is most suitable for estimating
the path loss and RSS.
According to this model, the path loss in urban areas
is given by,
Pathloss for UT to BS1:
In the second configuration, with the estimated
practical base station and mobile station antenna
heights are taken into consideration and the effect of
different values of Hadd, Hdrop on soft handoff
distance is evaluated. By this analysis the optimum
Hadd, Hdrop values are estimated for a CDMA
cellular system which improves the quality of
service with moderate soft handoff distance.
Configuration 1
-75
-85
-90
-95
-100
-105
Hadd=12 dB
-115
(3)
PL= Path loss, in dB
hb1, hb2= Height of the base station in meters, 30m to
200m
hm= Height of the mobile station in meters, 1m to
10m
f= Carrier frequency in MHz=1800 MHz
d = Distance between base station and mobile station
in kilometers, 1Km to 2Km
Dc=distance between two base stations=2000m
Pt= Transmitter Power =43 dBm =13 dB
The RSS from BS1
RSS1=Pt-PL1
(4)
Received signal strength from BS2
RSS2=Pt-PL2
(5)
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BS1
0
0.2
0.4
BS1&BS2
0.6
0.8
1
distance km
BS2
1.2
1.4
1.6
1.8
2
Fig. 3 RSS based Soft handoff algorithm at Hadd
=12 dB, Hdrop =4 dB, hb1=hb2=150 m and hm=3 m
PL2(dB)=46.33+33.9log(f)-13.82log(hb2)a(hm)+[44.9-6.55log(hb2)]log(Dc-d) (2)
a(hm)in dB=[1.1log(f)-0.7]hm-[1.56log(f)-0.8]
Hdrop=4 dB
-110
-120
PL1(dB)=46.33+33.9log(f)-13.82log(hb1)a(hm)+[44.9-6.55log(hb1)]log(d) (1)
BS1
BS2
-80
dB
BS2
U
T
Cell
1
The RSS based CDMA handoff algorithm is
implemented for estimating optimum Hadd and
Hdrop values for 2G and 3G cellular systems.
received signal strength
BS1
IV. Results and Discussion
Fig. 3 represents the various received signal strength
variations of BS1 and BS2 with, hb1=hb2=150 m and
hm=3m and Hadd=12dB and Hdrop=4dB. It is
observed that, as the mobile is moving away from
the BS1, the RSS decreases i.e. at 100 m, the RSS1
is -78.599 dB, at 1000 m the RSS1 is -109.245 dB
whereas at 1900 m RSS1 is -117.788 dB. It can also
be observed that as the mobile is approaching the
BS2 the signal strength increases. The mobile is
entering into soft handoff region at d = 600 m and
switching on to BS2 at d = 1100 m and the signal
strength of BS1 at that time is -110.514 dB.
Therefore, the soft handoff distance is 500 m. The
received signal strengths are very high with these
antenna heights.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 31 Number 4- January 2016
-85
-75
-80
BS1
BS2
received signal strength dB
-85
-90
-90
BS1
BS2
-95
received signal strength dB
Considering, hb1 is 150m, hb2 is 100m and hm is 3m
at Hadd =12 dB and Hdrop =4 dB. Then, results are
shown in Fig. 4.
-100
-105
-110
-115
-120
Hadd=12dB
Hdrop=4dB
-125
-95
-130
-100
-135
-105
0
0.2
BS1
0.4
0.6
Hadd=12dB
Hdrop=4dB
-110
-120
BS1
0
0.2
0.6
0.8
1.4
1.6
1
distance km
1.2
1.4
1.6
1.8
2
Fig. 4 RSS based Soft handoff algorithm at Hadd = 12 dB,
Hdrop = 4dB, hb1 = 150 m, hb2=100m and hm=3m
For these considerations, the mobile is found to be in
the soft handoff region between 650 m and 1250m
and thus the soft handoff distance is 500m. It is also
observed that as the height of neighboring base
station (BS2) antenna decreases, the soft handoff
region moves towards the BS2.
2
Fig. 6 shows that as the antenna heights of BS1 and
BS2 both are reduced to 35m and for mobile antenna
height of 3m, the received signal strengths are found
to be less compared to the previous cases i.e. when
BS1 antenna heights are 150m, 100m and 50m. In
this case, the mobile is found to be in the soft
handoff region between 625m and 1125m and thus
the soft handoff distance is 500m.
-85
-90
BS1
BS2
received signal strength dB
-95
-70
BS1
BS2
-80
received signal strength dB
1.8
BS2
BS1&BS2
0.4
BS2
1.2
Fig. 6 RSS based soft handoff algorithm at Hadd
=12 dB, Hdrop =4 dB & hb1 =hb2 =35m and hm =3m
-115
-125
BS1&BS2
0.8
1
distance km
-90
-100
-105
-110
-115
-120
Hadd=12dB
Hdrop=4dB
-125
-130
-100
BS1
-135
-110
Hadd=12dB
BS1&BS2
BS1
0
0.2
0.4
0.6
0.8
0.4
0.6
BS1&BS2
0.8
1
distance km
BS2
1.2
1.4
1.6
1.8
2
Fig. 7 RSS based Soft handoff algorithm at Hadd=12
dB, Hdrop=4 dB, hb1= hb2=35m and hm=1.5m
1
distance km
1.2
BS2
1.4
1.6
1.8
2
Fig. 5 RSS based Soft handoff algorithm at Hadd=12 dB,
Hdrop=4 dB, hb1=150m, hb2=50m, and hm=3m.
Fig. 5 illustrates that as height of the BS2 decreases
to 50m, the soft handoff region moves towards BS2
and the MS is in the soft handoff region between
825m and 1325m. Thus, soft handoff distance is
equal to 500m and signal strength of BS1 at which
the mobile is completely handoff to BS2 is observed
as -112 dB at a distance of 1325m. As the mobile is
moving away from the BS1 the received signal
strength of mobile reduces. As the height of the
antenna reduces, the signal strengths of BS2 reduce
and the soft handoff region moves towards the BS2.
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0.2
Hdrop=4dB
-120
-130
0
Fig.7 shows that by considering antenna heights of
BS1 and BS2 as 35m and mobile antenna height as
1.5m, the received signal strengths are less
compared to previous case (Fig. 6), where the
mobile antenna height is 3m. The mobile is in the
soft handoff region between 625m and 1125m.
Therefore, soft handoff distance is 500m. The
corresponding values are represented in Table 1.
Table 1 Received signal strengths of BS1 and BS2 and
soft handoff performance at Hadd=12 dB, Hdrop=4 dB,
hb1= hb2=35m and hm=1.5m.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 31 Number 4- January 2016
Table 1 represents the received signal strengths of
the MS from BS1 and BS2 for equal base station
antenna heights of 35m and mobile station antenna
height of 1.5m. The RSS values obtained for these
considerations are vey less, for example at a distance
of 100m, the RSS observed from BS1 is -83.194 dB
while the RSS observed for the mobile station
antenna height of 3 m is found to be -87.515 dB and
thus the base station antenna heights of 35m and
mobile station antenna height of 1.5m are considered
to be the practical values for CDMA cellular system.
It is also observed that the soft handoff distance in
each case is observed as 500m which more distance
with the Hadd=12 dB and Hdrop is 4 dB.
With this first configuration practical base and
mobile antenna heights are realized for minimum
RSS and need to estimate the optimum Hadd and
Hdrop values for moderate soft handoff distance for
improving quality of service.
Configuration 2
RSS based CDMA hand algorithm is implemented
for practical antenna heights and estimated optimum
Hadd and Hdrop values by calculating soft handoff
distance with variable Hadd and Hdrop values.
-85
BS1
-90
BS2
received signal strength dB
-95
-100
-105
Hadd
a=1dB
b=2dB
c=3dB
d=4dB
e=5dB
f=6dB
g=8dB
h=10dB
i=12 dB
-110
-115
-120
-125
ji
x
h
g
f ed
pqr s
cba
tu
v
w
Hdrop
Hadd
Hdrop
p=1dB
q=2dB
r=3dB
s=4dB
t=5dB
u=6dB
v=8dB
w=10dB
x=12 dB
-130
-135
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
distance km
Fig. 8 RSS based Soft handoff algorithm at different
Hadd and Hdrop values at hb1= hb2=35m and
hm=1.5m.
Fig. 8 shows various H add and Hdrop values which
are indicated by different colours. From the figure it
ISSN: 2231-5381
is observed that if Hadd and Hdrop are at 12 dB, the
soft handoff distance is more and if Hadd and Hdrop
values are at 2 dB the soft handoff distance is less.
Fig.8 shows that greater value of drop hysteresis will
increase active set size for larger area and hence soft
handover region will increase. Large soft handover
region will increase interference which is not
desirable. Therefore, Hdrop should neither be very
large nor very small. This figure shows that, the
variations of soft handoff region with Hadd and
Hdrop i.e. as Hadd and Hdrop are increased, soft
handoff region increases rapidly. Large soft
handover region increases forward interference and
hence capacity decreases. Very small soft handoff
region may increase the probability of outage. This
is an interesting result and should be taken into
consideration while designing soft handoff algorithm.
Table 2 Soft handoff algorithm at different hysteresis add
and hysteresis drop and its observations
Table 2 represents that the received signal strengths,
call quality observations for different values of soft
handoff parameters (Hadd, Hdrop). If Hadd and
Hdrop are at 12dB, the received signal strength
during handoff is found to be -85 dBm and handoff
distance is very large i.e. 740m. The signal strength
is still sufficient to maintain the call. Also for a fixed
hysteresis add and variable hysteresis drop also the
soft handoff distance increases and vice versa. Large
Hadd and Hdrop values create large soft handoff
distance which results in increase of downlink
interference. As the soft handoff distance decreases,
the uplink interference increases. So, the design of
soft handoff parameters is very important for
improving the quality of service. From this Table 2,
it is observed that the Hadd, Hdrop values from 3 dB
to 5 dB range can give optimum performance with
moderate soft handoff distance (200m to 320m).
V. Conclusions
CDMA cellular system is the most popular system in
2G/3G generations. Most attractive characteristic of
CDMA cellular system is frequency reuse which
allows the soft handoff between the cells which
increases the cellular system capacity by 4 to 5 times
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International Journal of Engineering Trends and Technology (IJETT) – Volume 31 Number 4- January 2016
the capacity of the GSM cellular system and also
provides best QoS with seamless connection. In this
paper, a new RSS based CDMA handoff algorithm
is developed. The effect of various base station
antenna heights, mobile antenna heights on the
CDMA cellular system performance is investigated
with the help of soft handoff algorithm. First
configuration, fixed hysteresis add (12 dB)
hysteresis drop (4 dB) and base station antenna
height of 35 m, mobile antenna height of 1.5 m the
soft handoff distance observed is more which is
approximately 500 m but the received signal
strength is less. Therefore, it is concluded that base
station antenna height of 35 m and mobile antenna
height of 1.5 m are practical values for obtaining
minimum received signal strength. The effect of soft
handoff parameters (hysteresis add and hysteresis
drop) on CDMA cellular system using the soft
handoff algorithm is also analyzed in this paper. Soft
handoff performance is measured in terms of active
set size and soft handover distance. Simulation
results confirmed that the soft handoff parameters
setting has a vital effect on the handoff performance.
As the Hadd increases, the soft handoff distance
increases for a fixed Hdrop and vice versa. By
careful tuning and setting appropriate values for the
handoff parameters, a higher system performance
can be achieved. Finally, it is concluded that the
optimum hysteresis add and hysteresis drop to obtain
the best QoS for improving the CDMA cellular
system performance are in the range of 3 dB to 5dB.
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Suman Das, William Michael MacDonald, and Harish
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Rajat Prakash and Venugopal V. Veeravalli Locally
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