Reduce the probability of blocking for handoff and calls in cellular systems based on fixed and dynamic channel assignment

TELKOMNIKA Telecommunication, Computing, Electronics and Control
Vol. 18, No. 4, August 2020, pp. 1731~1737
ISSN: 1693-6930, accredited First Grade by Kemenristekdikti, Decree No: 21/E/KPT/2018
DOI: 10.12928/TELKOMNIKA.v18i4.15729  1731
Journal homepage: http://journal.uad.ac.id/index.php/TELKOMNIKA
Reduce the probability of blocking for handoff and calls
in cellular systems based on fixed and dynamic
channel assignment
Shahir Fleyeh Nawaf, Mohammad Omar Salih, Mohammed Hassan Dervish
Department of Electrical Engineering, Tikrit University, Iraq
Article Info ABSTRACT
Article history:
Received Feb 2, 2020
Revised Feb 18, 2020
Accepted Mar 2,2020
In cellular systems the high probability of blocking represents a big problem
for users, the proposed solution by reducing the blocking probability
and investigation cellular systems by method channels assignment. The aim
from apaper is studying the effect the channel assignment on the value
of blocking probability. The results showed that the fixe channeld assignment
gives a large probability of blocking for high loads, while (FCA) reduce
probability of blocking for handoff and calls according to cluster size.
The cellular system representation in the case of (DCA), in (3-cell reuse) and
(7-cell reuse), the results showed the first best way to reduce blocking
probability and lead to reduce to approximately zero when loads that are less
than 200%. Increasing the cluster size causes to reduce blocking probability.
The results showed that the probability blocking for handoff less than from
probability of blocking for new calls.
Keywords:
Blocking probability
Dynamic channel assignment
Fixed channel assignment
Handoff
Traffic load
This is an open access article under the CC BY-SA license.
Corresponding Author:
Shahir Fleyeh Nawaf,
Department of Electrical Engineering,
Tkrit University,
Sallah-AL-Deen/Alala, Iraq.
Email: shahir735@gmail.com
1. INTRODUCTION
Cellular networks are based on dividing the area to be serviced into a number of cells. Cells in
cellular systems are divided into groups of each group consisting of a number of cells [1]. The channels
assigned to the cellular system are used entirely in the group and reused in another group in an orderly
manner to ensure that there is no overlap between similar channels in adjacent cells. In order to maximize
capacity using the same number of channels and to reduce overheating, several channel assignment strategies
have emerged. Call blocking occurs when a vacant channel is not available base station (BS) to be allocated
to the user [2]. There are two types of blocking calls; the first is to block new calls and the second blocking
the transfer of any continuous call because of the movement of the user and move from one cell to another or
from one section to another.
In 2001, the researchers [3] presented a study on call entry control methods. The importance
of these methods in wireless networks with a small cell size where handoffs are frequent. These methods
have demonstrated high efficiency in providing the best quality of service (QoS) for HO calls with little
and slight decrease in new calls. In 2003, the researchers [4] studied methods of determining the bandwidth
of a multi-service cellular network and the dynamic segmentation method, showing that both methods

 ISSN: 1693-6930
TELKOMNIKA Telecommun Comput El Control, Vol. 18, No. 4, August 2020: 1731 - 1737
1732
worked well with the correct dealings with the control parameters. The former provided (QoS) acceptable but
with more requirements. The second is higher channel channel efficiency.
In 2004, Markopoulo et al [5] introduced a set of algorithms for transport with the help of location
and safe space information to make correct transport decisions that were implemented in GSM systems.
Swing, in 2005, researchers [6] reviewed a theoretical summary of the initiation of the handoff and
the decision to carry out the discussion of the different types of handoff. In 2012, the researchers in [7]
studied a system for improving channel assignment in cellular communications. The study was about
the performance of the DCA distribution scheme for cellular networks (multi-agent system) compared to
the distribution of the FCA scheme, where simulations showed methods another is to understand the DCA
distribution scheme more efficiently than the FCA distribution scheme. In 2017, researchers in [8] studied
design complicated control systems using nano-based devices. Besides, since there is a critical manner
of temperature in QCA devices. In 2019, the researchers in [9] show that A number of case studies will be
present to demonstrate the efficiency and effectiveness of the system.
2. PROBABILITY OF BLOCKING HANDOFF AND CALLS
Mobility is the most important feature in cellular communication systems and usually
the continuation of the service is accomplished by assigning delivery from one cell to another. Signal in
the communication channel, when new calls arrive, MS should contact the appropriate BS and also when
there are symptoms within the cell boundaries that require a transfer within the same cell of the acceptable
link quality so that it can interfere with adjacent and channels using the same frequency [10]. Cell handoff
intra is desirable when the connection to the BS server is affected by excessive interference while another
connection to the same BS can be equipped with better quality [11, 12]. The transfer process involves two
phases: the first is to assess the quality of communication and initiate the transfer and the second allocation of
radio network resources. [13] Cell systems that are smaller in cell sizes are faster and more evaluated for
reliable communication quality and transport algorithm. The amplitude is low due to excessive interference
in channels using the same frequency [14, 15].
2.1. Reason of handoff
The transfer process is initiated by the base station (BS) or (MS);
a. Type of Carriage The radio communication transport is caused by the mobile (MS) technology within
the coverage area [16].
- Number of MSS per cell.
- Number of new calls in the cell.
- The number of calls taken to the cell from neighboring cells as a result of the transfer.
- Number of incomplete calls.
- Dwell time on cell.
b. Network management works on the transfer in the event of an imbalance in the density of traffic between
neighboring cells, there is the best balance of channels and other resources required between those
cells [17].
c. Service-related transfers occur as a result of poor service.
2.2. Types of handoff
There are two types of handoff [18, 19];
- Hard handoff: means the connection before cutting any contact with the new cell before disconnecting
from the servant cell. The user can communicate with more than one cell during the transfer process and
complete when BS selects its target from the BSs. This transfer is important because the BS will select
the strongest signal from the existing BSs.
- Soft handoff: Disconnection before connecting means any disconnection from the servant cell before
contacting the neighboring cell, as the MS does not connect to more than one base station at any time
and is continuous and discontinuous. Actual because MS connects to only one BS at any time [20].
2.3. Handoff initiation
The transfer decision is based on the BS signal over time and based on the average measurements
of the received signal, ie the relative signal strength. If the signal is weak and below the threshold limit,
the two strongest BSs to be transmitted are compared at the point where the signals are equal if the threshold
is higher than this value (equal point) [21]. If the threshold limit is less than this value (T2), the MS will
delay the transfer until the signal level crosses the threshold at position B. In the case of T3, it may be
delayed so that the MS reaches a new cell. In case the received signal is strong and with the backend

TELKOMNIKA Telecommun Comput El Control 
Reduce the probability of blocking for handoff and calls in cellular systems on… (Shahir Fleyeh Nawaf)
1733
by the mobile station, the transfer occurs at point C, and this prophylaxis prevents the effect of frequent
swings and transfers between BSs caused by rapid fluctuations in the signal strength received from BSs.
If the received signal strength with the hysteresis is the threshold limit, then the transfer to a new cell is
performed only when the received signal from the servicing base station falls below the threshold limit
and the signal received from the MS to which the MS will move is greater than the received signal,
the handover will occur at point whene the MS reach to receved signal domaine [22, 23].
2.4. Blocked calls cleared
When the user requests the service arrives directly to the channel if there is an available channel
and the reverse of the call request is blocked directly and that the arrival of calls is calculated by the Bosnian
distribution (Poisson distribution) and assume that there is an unlimited number of users in addition to
follow-up, and there is a store does not store requests employed for all users includes. Blocking user requests
for the channel at any time and the probability of the channel operating for users follow the exponential
distribution and there are a limited number of channels available in systems containing the theory of
participation, and these systems use the Erlang B model to calculate the probability of blocking calls
and measure the degree of service [24, 25]. The probability of repelling is calculated under;
Pr [blocking] =
𝑨 𝒄
𝒄!
∑
𝑨 𝒌
𝒌!
𝒄
𝒌=𝟎
Pr: probability.
Pr: probability.
Ac: Traffic load per channel.
C: Number of channels per cells.
2.5. Blocked call delay
Waiting is used when there is no vacant channel. The service request is delayed until a vacant
channel is available. It measures the degree of service and defines the probability of blocking the call after
waiting for a specified time in the queue. The form for calculating the probability of delaying the call is
Erlang C as follows [26]:
Pr [delay > 0] =
𝐴 𝑐
𝑐!
[{1−
𝐴
𝐶
}𝐶! ∑
𝐴 𝑘
𝐾!
+𝐴 𝑐 ]𝐶−1
𝐾=0
If there is no channel available, the call is delayed and the probability of delaying the call lasts more than
t seconds according to the following [21]:
Pr [delay > 𝑡] = Pr [𝑑𝑒𝑙𝑎𝑦 > 0] ∗ Pr [𝑑𝑒𝑙𝑎𝑦 > 𝑡/𝑑𝑒𝑙𝑎𝑦 > 0]
3. CHANNELS ASSIGNMENT
There are several ways to assign channels when new calls arrive, or attempt to transmit.
The efficient channel assignment algorithm produces high spectrum efficiency for a specific service score
that includes call quality, the probability of blocking the new call and the probability of short termination.
Figure 1 shows the assignment of channels [27].
- Fixed chanel assignment: In this strategy, a set of voice channels is allocated to any communication
attempt within the cell that can be served by unused channels in the cell. If all channels in the cell are
busy, calls will be blocked. There are several variations of the fixed channel strategy and one method is
called a hypothetical strategy that allows the cell to borrow channels from the adjacent cell if
the channels are busy. The MSC supervises the lending method to avoid interference with any call in
the donor cell [28]. In the FCA system, a fixed set of channels is allocated and cannot be changed for
efficient operation and also reduces the interstitial haemorrhage that uses the same channel to reduce
the reuse distance. The problem with the FCA system is due to irregular traffic in the BSs network.
K users in the first cell will be blocked by calls while m channels in the second cell will not be used,
this method is applied at a widespread level [28, 29].
- Dynamic channel assignment: In this strategy, the frequency channels are not pre-assigned to the cell of
the cellular network and all frequency channels are kept in a central storage. When there is a channel
demand at one of the base stations, MSC selects the appropriate frequency channel that gives the highest
channel efficiency. The channel is allocated during the duration of the call and after the call is returned

 ISSN: 1693-6930
1734
to the central store of channels or allocated again to a user within the same cell that was controlled by
the channel before. This strategy reduces the problem mentioned above in the FCA when traffic is
irregular. In DCA systems, there is no relationship between channels and cells. When the channel is
required in the cell, it is allocated under the requirements of frequency reuse provided that it is not
violated. There are two problems in this type: first, this method contains a degree of random
participation between users, and the second that includes complex algorithms to determine the available
channel that is more efficient [30, 31].
- Hybrid Channel Assignment: This type mixes the two types above.
- Flexible Channel Assignment: each cell allocates a fixed set of channels, but the channel repository is
retained for flexible allocation. This is either specific or predictive. The specific allocation schemes
depend on the known change in the shape of traffic. Of cells according to measurements [32].
Figure 1. Channels assignment
4. SIMULATION AND RESULTS
A simulation model of a cellular system was constructed to study the effect of channel assignment
on the probability of repulsion in WCDMA. The mobile stations were represented in these cells and
distributed randomly on a regular basis. The number of mobile stations using communication channels in
each cell is variable depending on the instantaneous load but is less than or equal to the number of channels
in each cell (20) channels. The number of calls during the simulation period depends on the traffic load value
shown in the following equation:
Traffic Load = (new call arrived * holding time)/number of channels
This law shows the traffic load within one hour that the used parameters in the simulation are shown
in Table 1. For mobile stations, they move at a speed ranging from (0-20) m/sec with a uniform random
distribution, the speed and direction of users change with different probabilities for special angles
(45 and 90). This change in direction occurs periodically every 60 seconds. The probability of a change in
direction, stop, and speed change is in accordance with an approved movement model. Table 2 shows
the possibility of changing the direction and angles used in this paper through simulation, the location of
mobile stations is tracked periodically every 1 second and the location measurement is assumed to be already
known by GPS.
Table 1. Used parameters on simulation
Value Parameter
2000 m Cell radius (R)
3 Number of cells
20 Number of channels per cell
3.84 M chip per sec. Bandwidth
[0- 20] m/sec Speed
1 sec. Period time of measurements
15% Soft handoff overhead
50 Number of simulations runs
5 hours Time of simulation
180 sec. Holding time
(60–150) Traffic load
Table 2. Probability of change direction and angle
Parameter Value
Probability with which a MS stops 10%
Probability of keeping original moving direction 60%
Probability of making a 45° turn 10%
Probability of making 90° turn 20%

1735
Through the location and speed of the mobile station, it is possible to know the arrival of the station
to the transport area. The simulation model of the cell system was represented for 6 hours as the simulation
time. A rate of 45 readings was taken because they represent randomly distributed results since the system is
basically represented by random distributions and the correct values can only be known after finding
the average of the results values.
4.1. Effect channels assignment on probability of blocking
The fixed assignment of channels was applied to the simulation model to calculate the ratio
for probability of blocking and the new call, in addition to the handoffs equipment and for different loads
ranging from 70% to 160%, as shown in the Figures 2 and 3. We note from the Figure 2 that the probability
of blocking increases with increasing load and is very little at loads close to 70%. We also note that
the probability of blocking new calls is greater than the rate of blocking the handoff due to the presence of
a priority for the transfer over new calls, if a new call comes, you look at the vacant channels If there is
a vacant channel, it gives to this call (the user), but if all the channels are busy, the call will be blocked,
as will the case for the handoff operations. It is known that the probability of blocking handoff should be less
than the possibility of blocking new calls, because for the subscription, the continuation of the call is
a priority over the new call. In Figure 3 we note that an increase in the load leads to an increase in the number
of handoffs, and this is a logical result. When the load increases, the incoming calls will increase and this
leads to an increase in the number of handoffs. The strategy of dynamic channel assignment was applied to
two types of cellular systems, the first of 3cells (3-cell reuse) and the second of 7 cells (7-cell reuse) to study
the effect of channel assignment on the probability of blocking.
In Figure 4, we see that the blocking probability is a very low value when the loads are less than
100% and when the traffic load increases, we increase the probability of blocking and reach a rate of
approximately 1.7% for new calls and 0.25% for the carriers when carrying 160%. These results show a large
reduction rate in probability of blocking compared to FCA strategy. As for the FCA strategy at 100% load,
it is close to 0% in this system, and the reason for this is that a single cluster that includes 3 cells will contain
a store of all channels so that they are available in the cluster, and for any request (for transfer or call) in any
cell. We also see in the figure above that the probability of blocking the carry is less than the possibility of
blocking the new call, because there is an advantage to carry over the new call.
Figure 2. Probability of blocking for fixed assignment
of channels vs load changing
Figure 3. Handoffs ratio vs fixed
assignment of channels
Figure 4. Probability of blocking vs traffic load

 ISSN: 1693-6930
1736
5. CONCLUSION
When the traffic load increases, the probability of blocking and the number of handoffs increases.
When comparing the strategies of FCA and DCA, we see that the probability of blocking in the use of DCA
is much lower than the probability of blocking in the case of FCA. The size of the cluster in the cellular
system affects the value of the probability of repulsion in the case of the use of DCA. The larger the cluster,
the lower the probability of repelling. In the DCA strategy, the probability of repelling in very few loads is
very low to zero. The probability of blocking a carriage is significantly lower than the probability of blocking
new calls in the case of DCA due to the priority of the carriage.
REFERENCES
[1] T. S. Rappaport, “Wireless Communication Principles & Practice,” Prentice Hall PTR, second edition, 2002.
[2] Jingao W., “Performance Analysis of Hybrid Reservation Handoff Scheme for Integrated Wireless Mobile
Network,” Msc Thesis, University of Cincinnati, 2001.
[3] J. Hou, and Y. Fang, “Mobility-Based Call Admission Control Schemes for Wireless Mobile Networks,” Wireless
Communications and Mobile Computing, vol. 1, no. 3, pp. 269-282, 2001.
[4] B. Li, L. Li, Boli, and X. R. Cao, “On Handoff Performance for an Integrated Voiice/Data Cellular System,”
Wireless Networks, vol. 1, no. 9, pp. 393-402, 2003.
[5] Markopoulos, P. Pissaris, S. Kyriazakos, A. Koutsorodi, “Performance Analysis of Cellular Networks
by Simulating Location Aided Handover Algorithms,” 5th European conference; Mobile &Wireless Systems
Beyond 3G, 2004.
[6] N. Ekiz., T. Salih, S. Kucukoner, and K. F. inanboylu, “An Overview of Handoff Techniques in Cellular
Networks,” International Journal of Information Technology, vol. 2, no. 2, pp. 215-221, 2005.
[7] R. R. Sedamkar, Swati Sonavane, “Improved Channel Assignment Scheme in Cellular Mobile Communication,”
Journalof Engineering and Techology, vol. 1, no. 3, pp. 415-423, 2012.
[8] Mohammad Mahdi Abdollahi, Mohammad Tehrani, “Designing a Novel Reversible Systolic Array Using QCA,”
Italian Journal of Science & Engineering, vol. 1, no. 3, pp. 158-166, 2017.
[9] Shujun Zhang, Michael Clark, Xuelei Liu, Donghui Chen, “The Effects of Bio-inspired Electromagnetic Fields on
Healthy Enhancement with Case Studies,” Emerging Science Journal, vol. 3, no. 6, pp. 369-381, 2019.
[10] C. Odimegwu, I. Zakaria, M. M. Abood, C. B. K. Nketsiah, M. Ahmad, “Review on Different Beneficial Ways of
Applying Alum Sludge in a Sustainable Disposal Manner,” Engineering Journal, vol. 4, no. 9, pp. 2230-2241, 2018.
[11] M. H. Chiu and M. A. Bassiouni, “Predictive Schemes for Handoff Prioritization in Cellular Networks Based on
Mobile Positioning,” IEEE Journal on Selected Areas in Communications, vol. 18, no. 3, pp. 510-522, 2000.
[12] Subramanya S. R., Yi B. K., “Mobile communications-an overview,” IEEE Potentials, vol. 24, no. 5, pp. 36-40, 2005.
[13] Sarddar D., Chatterjee S., Mazumder S. P., Mukherjee M., Mallik M. S., Biswas U., Naskar M. K., “Minimization
of call blocking probability using auxiliary stations,” International Journal of Computer Applications, vol. 25,
no. 7, pp. 50-56, 2011.
[14] Shahir Fleyeh Nawaf, “Channel Capacity Improvement of MIMO Communication Systems using Different
Techniques,” Tikrit Journal of Engineering Sciences, vol. 25, no. 1, pp. 35-40, 2018.
[15] Y. Zhao, “Standardization of Mobile Phone Positioning for 3G Systems,” IEEE Communications Magazine,
vol. 40, no. 7, pp. 108-116, July 2002.
[16] B. O. Omijeh, M. I. Udoh, “Performance Analysis of the WCDMA End-To-End Physical Layer,” International
Journal of Engineering and Technology, vol. 5, no. 12, pp. 623-635, 2015.
[17] Katzela and M. Naghshineh, “Channel Assignment Schemes for Cellular Telecommunication System:
A Comprehensive Survey,” IEEE Personal Communication, vol. 3, no. 2, pp. 10-31, 2000.
[18] Y. Park and F. Adachi, “Enhanced Radio Access Technologies for Next Generation Mobile Communication,”
Springer Kindle Edition, 2007.
[19] Sami A. Mawjoud, Farhad E. Mahmood, Sana S. M. Tahir, “Investigation of Macrocell/Microcell Channels
Selection In Multitier Cellular Networks,” Al-Raﬁdain Engineering Journal, vol. 18, no. 4, pp. 40-47, 2010.
[20] Lujain S. Abdulla, Sami A. Mawjoud, “Predictive Handoff Based on Mobile Positioning in CDMA System,”
Al-Rafidain Engineering journal, vol. 20, no. 3, pp.56-62, 2012.
[21] Mishra M. P., Saxena P. C., “Survey of channel allocation algorithms research for cellular systems,” International
Journal of Networks and Communications, vol. 2, no. 5, pp. 57-85, 2012.
[22] Cheng-Gang L., Zhen-Hong J., Xi-Zhong Q., Lei S. and Li Chen, “Improvement in the Priority Handoff Scheme
for MultiService Wireless Mobile Networks,” Journal of networks, vol.9, no. 1, pp.50-57, 2014.
[23] R. R. Sedmkar, “Improved Channel Assignment Scheme in Cellular Mobile Communication,” Journal
of engineering and technology, vol. 1, no. 3, pp.70-77, 2012.
[24] Pramukantoro E. S., Luckies M., Bakhtiar F. A., “Bridging IoT infrastructure and cloud application using cellular-
based internet gateway device,” TELKOMNIKA Telecommunication Computing Electronics and Control, vol. 17,
no. 3, pp. 1439-1446, 2019.
[25] Salam Al-Khammasi, Dheyaa Alhelal, Nabeel Salih Ali, “Energy Efficient Cluster Based Routing Protocol
for Dynamic and Static Nodes in Wireless Sensor Network,” TELKOMNIKA Telecommunication Computing
Electronics and Control, vol. 16, no. 5, pp.1974-1981, 2018.

1737
[26] R. Shankar, P. Dananjayan, “Quality of Service in bandwidth adapted hybrid UMTS/WLAN interworking network,”
TELKOMNIKA Telecommunication Computing Electronics and Control, vol. 17, no. 6, pp. 2803-2811, 2019.
[27] Goldsmiths, “Wireless Communications,” Cambridge University Press, 2005.
[28] L. S. Abdulla, “The Predictive Schemes for Handoff Prioritization in Cellular Network Based on Mobile Position,”
Thesis, Mosul University, 2010.
[29] M. H. Chiu and M. A. Bassiouni, "Predictive Schemes for Handoff Prioritization in Cellular Networks Based on
Mobile Positioning,” IEEE Journal on Selected Areas in Communications, vol. 18, no. 3, pp. 510-522, 2000.
[30] Seah W. K., Eu Z. A., Tan H. P., “Wireless sensor networks powered by ambient energy harvesting (WSN-HEAP)-
Survey and challenges,” Conference: Wireless Communication, Vehicular Technology, Information Theory
and Aerospace & Electronic Systems Technology, 2009.
[31] F. I. Anyasi and Uzairue S. I., “Determination of GSM Signal Strength in Some Selected Location in EKPOMA:
IOSR,” Journal of Electronics and Communication Engineering (IOSR-JECE), vol. 9, no. 2, pp. 08-15, 2014.
[32] Wang X. G., Min G., Mellor J., Al-Begain K., “A QoS based bandwidth management scheme in heterogeneous
wireless networks,” International Journal of simulation, vol. 5, no. 1-2, pp. 9-17, 2004.
BIOGRAPHY OF AUTHORS
Shahir Fleyeh Nawaf was born in Mosul, Iraq 1961. He received B.Sc. Degree in
electrical and electronic engineering from Sarajevo University 1985. M.Sc. Degree in
electronic and communication engineering from University of Belgrade, Serbia 1987. In
1989 he had worked at air force communication department Baghdad Iraq. In 1992 he
worked at communication repairing factory in electronic department Baghdad Iraq.
In 1999 he worked at Al-Fatah company for manufacturing the communications
equipment. In 2006 he had worked as a lecturer in Tikrit University faculty of
engineering electrical department Iraq & currently he still. He has many research papers
interests’ electronics & communications field.
Mohammad Omar Salih was born in Tikrit Iraq, 1963. He received B.Sc. degree in
electrical and electronic engineering from Sarajevo University 1985. M.Sc. degree in
electronic and communication engineering from University of Belgrade, Serbia 1987.
In 1989 he had worked at air force communication department Baghdad Iraq. In 1992
he worked at communication repairing factory in electronic department Baghdad, Iraq. In
2006 he had worked as a lecturer in Tikrit University faculty of engineering electrical
department Iraq and currently he still. He has many research papers interests’ electronics
and communications field.
Mohammed Hassan Dervish was born in Tikrit Iraq 1962. He received B.Sc. degree in
electrical and electronic engineering from Sarajevo University 1984. M.Sc. Degree in
electronic and communication engineering from University of Belgrade Serbia 1986.
In 1993 he had worked at air force communication department Baghdad, Iraq. In 2006
he had worked as a lecturer in Tikrit University faculty of engineering electrical
department Iraq and currently he still there. He has many research papers interests
electronics and communications field.

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