WIRELESS COMMUNICATION

MOBILE COMMUNICATIONS

LECTURE 5

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Tanvir Ahmad NiaziTanvir.niazi@mail.au.edu.pk

Air University, Islamabad

Overview of the Previous Lecture

Comparison of Fixed Telephone versus Cellular Call The Cellular Concept Frequency Reuse

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New TopicsChannel Allocation SchemesHand off StrategiesInterference and System Capacity

An Example of a Cellular Cluster

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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:1. Move i cells across hexagons.

2. Turn 60º CCW and move j cells.

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Frequency Reuse (N=7, i=2, j=1)

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Method of locating co-channel cellsin a cellular system

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Example:N=19i=3j=2

Channel AssignmentStrategies

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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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Hand Off Strategies

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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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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 1. D is too small,2. Delay in MSC processing because of:

High Traffic hours, New channel not available

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Handoffs – the basics

When to Hand off? MSC needs to ensure:

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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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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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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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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The Umbrella Cell Approach

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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Interference and SystemCapacity

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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 Stations2. Neighboring Cells3. The same frequency cells4. 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

(1) 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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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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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

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 DesignWorst 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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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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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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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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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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