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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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General Handoff Procedure Network Assisted
(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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General Handoff Procedure Network Assisted
(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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General Handoff Procedure Network Assisted
(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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General Handoff Procedure Network Assisted
(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
General Handoff Procedure Network Assisted
(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
General Handoff Procedure Network Assisted
(Step 7)
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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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Practical Handoff Considerations (cont.)
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Figure 3.4 The umbrella cell approach.
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Practical Handoff Considerations (cont.)
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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Practical Handoff Considerations (cont.)
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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