CMC ICT3207 Call Flow

Associate Professor

Dept. of Electrical and Electronic Engineering

University of Dhaka

Dr.

Mohammad

J

unaebur

R

ashid (

JR

)

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ICT3207: Cellular and Mobile Communication (3.0 Cr)

Course Teacher

Bangladesh University of Professionals

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Lecture 14

• The mobility of a terminal (mobile unit) is a requirement of great importance, supported by a procedure known as handover.

• To maintain the connection between base station and mobile terminals is the key to customer satisfaction.

• In the 3G or 4G wireless environment, a mobile user is able to continue using the mobile device while moving from one point of attachment to another.

• Depending on the access network that each point of attachment belongs to, the handoff can be either horizontal or vertical.

Horizontal Handoff:

• A horizontal handoff or intra-system handoff takes place between PoA (Point of Access) supporting the same network technology, e.g., two geographically neighboring BSs of a 3G

cellular network. Intra-system HO is known as horizontal HO.

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Lecture 14

Vertical Handoff

• A vertical handoff or inter-system handoff occurs between PoA supporting different network technologies, e.g., an IEEE 802.11 AP and a 3G BS. Basically, vertical HO is known as

inter-system HO. Vertical refers to the overlapping of wireless networks.

• Horizontal HO is a symmetric process, whereas vertical HO is an asymmetric process in which the MS moves between two different networks with different characteristics

Handoff

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Lecture 14

Handoff

• Vertical handoffs are implemented across heterogeneous cells of access systems, which differ in several aspects such as bandwidth, data rate, frequency of operation, etc.

• The different characteristics of the networks involved make the implementation of vertical handoffs more challenging as compared to horizontal handoffs.

• Vertical handovers refer to the automatic fall over from one technology to another in order to maintain communication.

• A vertical handover involves changing the data link layer

technology used to access the network.

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Lecture 14

Handoff

The main capabilities of Vertical handovers over Horizontal handovers are:

1. Vertical handovers use different access technology.

2. Vertical handovers use multiple network interfaces. 3. Multiple IP addresses are used in Vertical handovers.

4. QoS parameters can be changed in Vertical handovers and multiple parameters are used. 5. Multiple network connections are used in Vertical handovers.

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Lecture 14

Decision Algorithm

• The requirements of the handoff algorithm in heterogeneous networks which should be considered in the design of the handoff algorithm are as follows :

(i) handoff should be done fast and its delay should be minimum;

(ii) the number of handoffs should be minimal since excessive handoff results in signal

quality degradation increased traffic dropping probability and additional loads on the network;

(iii) the handoff procedure should be reliable and successful;

(iv) when the traffic in the WLAN becomes too high and overflow occurs, the handoff to

WLAN should be avoided;

(v) fast MS should remain connected to GSM / CDMA and prevented from connecting to WLAN since the WLAN is designed for low-velocity MS and assumes a small coverage area

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Lecture 14

Decision Algorithm

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Lecture 14

TOPSIS: Technique for Order Preference by Similarity to Ideal Solution

TOPSIS method considers three types of priority in vertical handover decision: (i) equal priority, (ii) mobile priority, and (iii) network priority.

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Lecture 14

Decision Algorithm

• X_WLAN: Predefined

threshold value when the handoff is in

WLAN;

• V_T: Velocity threshold

whether a fast mobile station (MS) or a slow

MS.

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Lecture 14

Decision Algorithm

(1) X_{CDMA ,} X_WLAN : predefined

signal strength thresholds for

the handoff in the CDMA network and WLAN,

respectively;

(2) D_CDMA, D_WLAN : predefined

distance thresholds for the CDMA network and WLAN,

respectively;

(3) D_BS: current measured

distance between BS and MS.

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Lecture 14

Dropped Call Rate

• The definition of a dropped call is after the call is established but before it is properly terminated.

• The call is established means: the call is setup completely by the setup channel. If there is a possibility of a call drop due to no available voice channels, this is counted as a blocked call

not a dropped call.

• If there is a possibility that a call will drop due to the poor signal of the assigned voice channel, this is considered a dropped call. This case can happen when the mobile or portable

units are at a standstill and the radio carrier is changed from a strong setup channel to a weak voice channel due to the selective frequency fading phenomenon.

• The perception of dropped call rate by the subscribers can be higher due to: 1. The subscriber mobile unit not functioning properly (needs repair).

2. The use operating the portable unit in a vehicle (misused).

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Lecture 14

Dropped Call Rate

Consideration of dropped calls

• In principle, dropped call rate can be set very low if we, do not need to maintain the voice quality. The dropped call rate and the specified voice quality level are inversely proportional.

• In designing a commercial system, the specified voice quality level is given relating to how much C/I (or C/N) the speech coder can tolerate.

• By maintaining a certain voice quality level, the dropped call rate can be calculated by taking the following factors into consideration:

1) Provide signal coverage based on the percentage (say 90%) that all the received signal will be above a given signal level.

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Lecture 14

Dropped Call Rate

3) Since the performance of the call dropped rate is calculated as possible call dropping in

every stage from the radio link to the PSTN connection, the response time of the handoff in

the network will be a factor when the cell becomes small, the response time for a handoff request has to be shorter in order to reduce the call dropped rate.

4) The signaling of the handoff and the MAHO algorithm will also impact the call dropped rate.

5) The relationship among the voice quality, system capacity and call dropped rate can be

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Lecture 14

Dropped Call Rate

Relationship among capacity, voice quality, dropped call rate

• Radio Capacity m is expressed as follows:

where B_T/B_c is the total number of voice channels. B_T/B_c is a given number, and (C/I)_s is a

required C/I for designing a system.

• The above equation is obtained based on six co-channel interferers which occur in busy traffic, i.e., a worst case. In an interference limited system, the adjacent channel interference

has only a secondary effect.

• Eq. (9.10-1) can be changed to the following

form:-• Since the (C/I)_s is a required C/I for designing a system, the voice quality is based on the

(C/I)_s. When the specified (C/I)_s is reduced, the radio capacity is increased. When the

measured (C/I) is less than the specified (C/I)_s, both poor voice quality and dropped calls can

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Lecture 14

System Evaluation

Number of channels per cell

• The next factor to be determined is the number of channels per cell, which is a function of the total number of channels available (amount of available spectrum divided by channel

bandwidth) and the required carrier-to-interference ratio. The formula for this factor is

for M = mK total number of channels,

where m = number of channels per cell, also called radio capacity

K = number of frequency-reuse cells

B_t = total bandwidth (transmitted or received)

B_c = channel bandwidth