1. Wireless Communication Cellular concepts
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2. Overview of the Previous Lecture
Comparison of Fixed Telephone versus Cellular Call
The Cellular Concept
Frequency Reuse
New Topics
Channel Allocation Schemes
Hand off Strategies
Interference and System Capacity
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3. An Example of a Cellular Cluster
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4. Capacity versus interference for same size cell
Decrease N for More Capacity: If Cluster Size, N is decreased while cell size remains fixed, more clusters are required to cover the area (M increases). Therefore, Capacity increases.
Increase N for Less Interference: On the other hand, if N is increased (large cluster size) means that co-channels are now farther than before, and hence we will have less interference.
Value of N is a function of how much interference a mobile or a base station can tolerate.
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Frequency Reuse
Geometry of the hexagonal cells is such that to cover adjacent areas completely, N can have only some of the values.
N should satisfy the relationship,
where i and j are positive integers. Typical values of N are 4,7,12 etc.
To draw the cell pattern given i and j:
Move i cells across hexagons.
Turn 60º CCW and move j cells.
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6. Frequency Reuse (N=7, i=2, j=1)
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7. Method of locating co-channel cells in a cellular system
Example:
N=19
i=3
j=2
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8. Channel Assignment Strategies
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9. Channel Allocation Techniques
To satisfy the user, a channel needs to be available on request.
Reasonable probability of call blockage.
Three types of algorithms for channel allocation:
Fixed channel allocation (FCA)
FCA with Channel Borrowing
Dynamic channel allocation (DCA)
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10. SNSCT/IV ECE A/WC/UNIT II
Hand Off Strategies
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11. SNSCT/IV ECE A/WC/UNIT II
HAND OFF:
“When a mobile moves into a different cell while a conversation is in progress, MSC transfers the call to a new channel belonging to the new base station”. (Hard handoff)
In modern wireless systems, there may be no change in the actual channel; only the base station changes (Soft Hand off).
Mobile Switching Center needs to:
Identify the new base station
Allocate new Voice and Control channels associated with the new base station.
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Desired Behavior:
Prioritize Hand off to new call initiation
Successful transfer of call
Hand off should be as infrequent as possible
Hand off should be imperceptible (very slight)
Hand off Threshold Power Level:
There is a minimum power level of the signal at BS for acceptable voice quality. (-90 to –100 dBm).
Hand off Threshold is defined D dBm above this level, so that the system has time to process hand off.
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13. Margin D needs to be neither too small, nor too large
D too small  too short time to perform handoff, The call may be dropped in the meanwhile.
D too large  unnecessary handoff burden. Hand off becomes more frequent, and occurs even when Power is at the acceptable level.
Call will be dropped if
D is too small,
Delay in MSC processing because of:
High Traffic hours, New channel not available
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14. SNSCT/IV ECE A/WC/UNIT II
Handoffs – the basics
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15. When to Hand off? MSC needs to ensure:
that the power drop is not temporary: because of FADING.
that MS is actually moving away from the current BS.
MSC measures signal strength for some time (average received signal power). These calculations are performed to:
Avoid the unnecessary hand off,
Complete the necessary hand off before call drops
The time available to decide hand off depends on the subscriber’s speed.
Speed of the user can be estimated from the slope of the short term average Power. Steep curve Quick Hand off
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16. First Generation Systems:
Signal Strength Measurements are made by base stations and supervised by MSC.
Each base station constantly monitors power levels of its reverse voice channels, which tells the relative location of the user.
Reverse Signal Strength Indicators: Power levels of all the calls in a cell.
Locator Receiver: Power levels of Users of neighboring cells are also measured.
Both these pieces of information are passed to MSC for hand off decision.
In 1G: Hand off processing takes almost 10sec: that requires
D ~= 6-12 dB
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17. Second Generation Systems: Digital TDMA
Concept of Mobile Assisted Hand Off (MAHO)
Mobile station continually measures power levels of surrounding Base Stations and reports these measurements to the serving Base Station.
Hand off is initiated when power level from BS of neighboring cell exceeds that of the current cell, for certain time, or by a certain level
MAHO method ensures faster hand off since burden of MSC is shared by Mobile Stations
MAHO is particularly suited for Micro-Cellular Systems.
In GSM: The time reduces to 1-2 seconds:  that needs
D ~= 0-6dB
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Inter-System Hand Off
Definition: “-----When a mobile moves from one cellular system to another, with different MSC”.
The situation occurs when
“Signal in the resident cell becomes weak, and no other cell within the system can take the call”.
Issues:
Local Call becomes long-distance call
Compatibility between two MSCs.
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Prioritizing Hand off
Systems differ in methods and policies of hand off:
Some systems take hand off like a new call initiation: user will be more annoyed in case of call drop than call blocking for some time.
Various Methods of Prioritizing Hand off have been devised and implemented.
Guard Channel Concept
Reserve some channels exclusively for hand off--- do not use them for call initiation.
Advantage is increased probability of successful hand off. Disadvantage is lower capacity because of less number of channels for call initiation.
No disadvantage in Dynamic Channel Allocation
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Queuing of Hand off:
Decreases the probability of forced termination of a call due to lack of available channels.
Trade off between the “probability of forced termination” and “total carried traffic”. That is,
“Probability of forced termination” decreases at the cost of reduced Total Carried Traffic.
Queuing is possible because of the time available between the Threshold power level and the Hand off power level.
Delay Time and Queue Size are related to service area.
Queuing doesn’t guarantee zero probability of call drop.
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21. Practical Hand Off Strategies:
High Speed Vehicle VS Pedestrians
In micro-cells for more capacity, MSC may be over-burdened in case high speed users changing the micro-cells quickly.
Obtaining a new Cell Site is not always practical: Zoning, Ordinances and Laws are barrier to new cell sites.
The UMBRELLA CELL Concept: Large and Small cells are co-located. Install small antennas etc for low-speed users for proper radio coverage. This guarantees large area coverage as well.
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22. SNSCT/IV ECE A/WC/UNIT II
The Umbrella Cell Approach
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23. Umbrella cell approach:
Minimizes the number of hand offs for high speed users,
Provides additional micro-cell users for pedestrians.
A High speed user converting into a low speed user may be switched to smaller micro-cell by the Base Station, without MSC intervention.
Speed Estimation can be performed by slope of short term average calculations of the received power, or by some more more sophisticated algorithms.
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Cell Dragging
Results from Pedestrian users who provide very strong signal to base station.
For example, in Line Of Sight (LOS) case, power does not drop to the hand off threshold level even when the user has moved deep into the neighboring cell.
Solution: Hand Off Threshold and Radio Coverage Parameters need to be adjusted.
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Faster Hand Off
Faster hand off facilitates call “rescue”, that is in need of a hand off
Hand off in modern systems is based on calculations of additional parameters including: co-channel and adjacent channel interference at Base Station or Mobile Station.
In CDMA systems: Same channel is used by all the cells. MSC decides from which BS to take the signal and pass to PSTN. (Soft Hand Off)
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26. Interference and System Capacity
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Interference
It is a major limiting factor in the performance of cellular radio systems. (In comparison with wired comm. Systems, the amount and sources of interferences in Wireless Systems are greater.)
Creates bottleneck in increasing capacity
Sources of interference are:
1. Mobile Stations
2. Neighboring Cells
3. The same frequency cells
4. Non-cellular signals in the same spectrum
Interference in Voice Channels: Cross-Talk
Interference in Control Channels: missed/blocked calls
Urban areas usually have more interference, because of:
a) Greater RF Noise Floor,
b) More Number of Mobiles
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Types
1) Co-Channel Interference (CCI)
2) Adjacent Channel Interference (ACI)
3) Other services: like a competitor cellular service in the same area
Co-Channel Interference and System Capacity
The cells that use the same set of frequencies are called co-channel cells.
The interference between signals from these cells is called Co-Channel Interference (CCI).
Cannot be controlled by increasing RF power. Rather, this will increase CCI.
Depends on minimum distance between co-channels.
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The yellow cells use the same set of
frequency channels, and hence, interfere
with each other
In the cellular system there are 6 first-
layer co-channels
In constant cell size and RF power, CCI is a function of Distance between the co-channel cells(D), and the size of each cell (R).
Increasing ratio D/R, CCI decreases.
Define Channel Reuse Ratio = Q = D/R
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For hexagonal geometry, D/R can be calculated:
Smaller Q provides larger capacity, since that would mean smaller N. (Capacity ∝ 1/N).
Larger Q improves quality, owing to less CCI.
For N=3, Q=3,
N=7, Q=4.58,
N=12, Q=6,
N=13, Q=6.24
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Signal-to-interference ratio
S is the power of the signal of interest and is the power of kth interference.
The signal strength at distance d from a source is
That is, received signal power is inversely related to nth power of the distance.
where n = path loss exponent
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Then we can express the SIR in terms of distance
where the denominator represents the users in neighboring clusters using the same channel
Let be the distance between cell centers. Then
Note how C/I improves with the frequency reuse N.
Analog systems: U.S. AMPS required C/I ~= 18dB For n = 4, the reuse factor for AMPS is N ≥ 6.49, so N = 7.
Now, let us consider the worst case for a cluster size of N= 7. The mobile is at the edge of the cell. Express C/I as a function of actual distances.
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Worst Case Design
Worst case carrier-to-interference ratio
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Increasing N from 7 to 12, brings C/I above the 18dB level. However, the system capacity is decreased.
Reduction of capacity by 7/12 for taking care of the worst case situation when CIR ~= 17.4 dB is not justified because this situation will occur very rarely.
Conclusion: Co-Channel Interference controls the link performance which then decides Frequency Reuse Plan, and System Capacity.
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37. Channel Planning for Wireless Systems
Judicious assignment of the appropriate radio channels a more difficult problem in practice. Theoretical analysis provides a guideline.
In general 5% of the available spectrum is reserved for control channels
Frequency reuse of control channels more conservative.
In CDMA N=1, however, practical difficulties forces some kind of frequency planning.
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38. Adjacent Channel Interference
Interference from channels that are adjacent in frequency,
The primary reason for that is Imperfect Receive Filters which cause the adjacent channel energy to leak into your spectrum.
Problem is severer if the user of adjacent channel is in close proximity.  Near-Far Effect
Near-Far Effect: The other transmitter(who may or may not be of the same type) captures the receiver of the subscriber.
Also, when a Mobile Station close to the Base Station transmits on a channel close to the one being used by a weaker mobile: The BS faces difficulty in discriminating the desired mobile user from the “bleed over” of the adjacent channel mobile.
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Near-Far Effect: Case 1
The Mobile receiver is captured by the unintended, unknown transmitter, instead of the desired base station
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40. SNSCT/IV ECE A/WC/UNIT II
Near-Far Effect: Case 2
The Base Station faces difficulty in recognizing the actual mobile user, when the adjacent channel bleed over is too high.
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Minimization of ACI
(1) Careful Filtering ---- min. leakage or sharp transition
(2) Better Channel Assignment Strategy
Channels in a cell need not be adjacent: For channels within a cell, Keep frequency separation as large as possible.
Sequentially assigning cells the successive frequency channels.
Also, secondary level of interference can be reduced by not assigning adjacent channels to neighboring cells.
For tolerable ACI, we either need to increase the frequency separation or reduce the passband BW.
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Example 2.3
In the US APMS system the total number of channels is: 416
Out if these 395 are used as voice channels, whereas 21 are used a control channels.
Two operators share these 416 channels eqyally.
Channel allocation according to Table 3.2 (next slide). Using subsets iA+
For N=7, each cell in a cluster will have 1 control channel, neighboring clusters will have remaining 14 control channels.
Control channels will be thus be reused according to N=21 pattern.
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44. Power Control for Reducing Interference
Power levels transmitted by every subscriber unit are under constant control by the Base Station.
Mobile Station should transmit minimum power to maintain quality link on reverse channel.
This has benefits of longer Battery Life at the Mobile Station Reduced reverse SIR.
In CDMA systems, it is extremely important to control the power, as the neighboring cells are using the same channel.
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