ccna lecture :-uploaded by xafran khan

7/29/2019 ccna lecture :-uploaded by xafran khan

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EEE464

Wireless Communication Systems

Lecture 3The Cellular Concept (cont.)

Lecturer: Asad Khan


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Lecture 2 Review

Frequency Reuse

Cluster

Cells Adjacent Channels

Co-Channels

Co-Channels reuse strategy


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Outline

Channel Assignment Strategies

Handoff and Roaming

Handoff Strategies

Handoff Management General Handoff Procedure

Generic Database Handoff Management

Prioritizing Handoffs

Practical Handoff Considerations

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Channel Assignment Strategies can be classified as:

1. Fixed CA

It is the optimum allocation strategy for uniform traffic across the

cells.

Each cell is allocated a predetermined set of voice channels.

Any call attempt within the cell can only be served by unused

channels in that particular cell.

If all the channels are occupied, the call is blocked and the

subscriber does not receive service.

Borrowing strategy: cell is allowed to borrow channels fromneighboring cells supervised by the respective MSC.

The MSC facilitates borrowing by making sure that the borrowing

does not disrupt or interfere with the ongoing calls in the donor

cell.

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Channel Assignment Strategies (cont.)

2. Dynamic CA

Voice channels are not allocated permanently.

Each time a call is made, the serving BS requests a channel from

MSC.

MSC only allocates unused frequency in the cell or any other

neighboring cells to avoid co-channel interference.

It reduces the likelihood of blocking.

Also requires MSC to collect real-time data on channel occupancy,

traffic distribution and radio signal strength indications (RSSI) ofall channels continuously.

DCA increases the storage and computational load on the system

but provides advantage of increased channel utilization decreases

probability of a blocked call.

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Handoff and Roaming

Handoff / Handover: Occurs when a mobile station using a

frequency channel needs to change it for another frequency

channel.

Intra-handoff: inside the same cell.

Inter-handoff: between two cells.

Roaming allows subscribers to initiate or receive calls when visiting

a different cellular network.

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Handoff Strategies

Must be performed successfully and as infrequently as possible, and be

unnoticeable or imperceptible to the users.

An optimum signal level is defined that initiates the handoffhandoff

threshold.

It is typically higher than the minimum signal level.

Margin is given by = Pr(handoff)Pr(min usable)

If is too large unnecessary handoffs, burden on MSC(Mobile

Switching center).

If is too small insufficient time to complete handoff before a call is

lost due to weak signal conditions. BS(Base Station) monitors the signal level for a certain period of time

before a handoff is initiated to be certain that the mobile is actually

moving away from the serving Base Station.

Dwell time: over which a call may be maintained within a cell, without

handoff. 7


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Handoff Strategies (cont.)

8Figure 3.1 Illustration of a handoff scenario at cell boundary.


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Handoff Management

Network Assisted Handoff (First Generation) In the first generation system signal strength measurement is done at

the base station.

Each base station continuously monitors the signal strength of all of itsreverse channels to measure the relative distance of each mobile userwith respect to the base station.

There is a locator receiver in each base station that monitors the powerlevel at the mobile nodes in the neighboring cells indicating the handoffsituation for those nodes .

Mobile Assisted Handoff (Second Generation)

Every mobile node measures the received power from surrounding basestations and continuously reports the signal strength to the serving basestation.

The handoff is initiated when the power received from the neighboringbase station exceeds the power received from the serving base stationby certain value and for certain time.

It is much faster than the network assisted.

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Sample RSS from two BSs as seen by the MS

travelling in a straight line

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distance between base stations in miles

RSS+uncorrelatedlognormalfadingin

decibels

BS1

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

-60

-70

-80

-90

-100

-110

-120

-130

-140

BS2


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General Handoff Procedure Network Assisted

(Step 1)

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MSC

BS1

BS2

MS

TCxy1- RSS became

below threshold

2- Send handoffrequest to MSC


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(Step 2)

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MSC

BS1

BS2

MS

TCxy1- Measure signal

level ofxyin

neighboring cells

2- Measure signal level

ofxyon request via

MSC


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(Step 3)

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MSC

BS1

BS2

MS

TCxy2- Compare results

and select best cell

for handoff

1- Send measurement

data of channelxyto

MSC

TCxy


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(Step 4)

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MSC

BS1

BS2

MS

TCxy

1- Cell 2 is selected;

2- Select a channel (TC wz)

in cell 2

3a- BS2: initiate TX via

channel wz

3b- Initiate TX in wz

and send signalling

FTC wz

4a- BS1: Initiate

handofforder

4b- Send handoff

order to MS (startlisten wz and wait

signalling in wz)


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(Step 5)

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MSC

BS1

BS2

MS

TCxy

FTC wz

1- Confirm handoff

orderreceipt; send

respective signalling;

listen to FTC wz

2- BS detects MS

answered to handoff

order

X


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MSC

BS1

BS2

MS

TCxy


(Step 6)

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1- Turns on RTC wz

and returns

connectivity signalling

2b- No more reception

in RTCxy, inform

MSC (channel is free)

X

TC wz

2a- Detects connectivity

signalling from MS

3-Handoffexecuted

BS1: Turn off TCxyX


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MSC

BS1

BS2

MS

Handoff complete


(Step 7)

17

TC wz


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Prioritizing Handoffs

Guard channel concept: A fraction of the total available channels in

a cell is reserved exclusively for handoff requests.

Advantage: efficient spectrum utilization when dynamic

channel assignment is used.

Disadvantage: reduces total carried traffic.

Queuing of handoff requests: To decrease probability of forced

termination.

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Practical Handoff Considerations (cont.)

Umbrella cell approach

Large umbrella cells co-located with a number of smaller cells

(micro-cells).

Large cell has a larger coverage area for high-speed users (e.g.

moving vehicles). It reduces the number of handoffs for high-speed users.

Micro cells are reserved for low-speed users (e.g. pedestrian).

The MSC(Mobile Switching Center) needs to estimate the user

speed and allocate channels in one or the cell type. Some low-speed users can also be allocated channels in the

large cell to avoid forced termination.

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Figure 3.4 The umbrella cell approach.


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Cell Dragging

In urban environment a slow user in a LOS(Line of sight) path

with the base station can continue receiving strong signal from

the base station even though it is deep in the neighboring cell.

This will cause disturbance in the traffic distribution .

The solution requires careful adjustments in the handoff

threshold and radio coverage parameters.

Handoff time window In the first generation system the handoff time is about 10

seconds.

In the second generation system (e.g. GSM) the handoff time is

improved to 1-2 seconds.21


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The newer cellular system makes handoff decision based on a

variety of metrics, not simply received signal strength.

e.g. co-channel and adjacent channel interference along with

the received signal strength.

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