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EKT 450 Mobile Communication System

Chapter 6: The Cellular Concept

Dr. Azremi Abdullah Al-Hadi

School of Computer and Communication Engineering

[email protected]

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mailto:[email protected]

Introduction

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• Introduction to Cellular System

• Frequency Reuse

• Channel Assignment Strategies

• Handoff Strategies

• Interference + System Capacity

• Trunking + Grade of Service

• Improving Capacity in Cellular System

Introduction to Cellular System

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Traditional mobile service was structured in a fashion similar to television broadcasting: One very powerful transmitter located at the highest spot in an area would broadcast in a radius of up to 50 kilometers.

Drawbacks:

• High power consumption

• Impossible to reuse same frequencies throughout the system - Low capacity

Introduction to Cellular System

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• Solves the problem of spectral congestion and user capacity.

• Offer very high capacity in a limited spectrum without major technological changes.

• Reuse of radio channel in different cells. • Enable a fix number of channels to serve an arbitrarily

large number of users by reusing the channel throughout the coverage region.

Frequency Reuse

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• The design process of selecting and allocating channel groups for all the cellular base stations within a system is called frequency reuse or frequency planning.

• Each cellular base station is allocated a group of radio channels within a small geographic area called a cell.

• Neighboring cells are assigned different channel groups.

• By limiting the coverage area to within the boundary of the cell, the channel groups may be reused to cover different cells.

• Keep interference levels within tolerable limit.

Frequency Reuse

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• Seven groups of channel (into different cells) from A to G

• Actual radio coverage is called footprint determined from field measurements or propagation prediction models.

• Omni-directional antenna versus directional antenna.

Why Hexagonal Cell?

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• Hexagonal cell shape has been universally adopted, since it permits easy and manageable analysis of a cellular system.

• For a given distance between the center of a polygon and its farthest perimeter points, the hexagon has the largest area among other sensible geometric cell shapes.

• By using the hexagon geometry, the fewest number of cells can cover a geographic region, and the hexagon also closely approximates a circular radiation pattern which would occur for an omni-directional base station antenna and free space propagation.

What other sensible geometric cell shapes?

Why Hexagonal Cell?

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• When using hexagons to model coverage areas, base station transmitters are depicted as either being: o In the center of the cell (center-excited), or o On three of the six cell vertices (edge-excited).

• Normally: o Omni-directional antennas are used in center-

excited cells o Sectored directional antennas are used in corner-

excited cells.

Concept of Frequency Reuse

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• Consider a cellular system which has a total of S duplex channels.

• Each cell is allocated a group of channels, k (k < S). • The S channels are divided among N cells.

• The total number of available radio channels, S = kN

• The N cells which use the complete set of channels is called cluster.

• The cluster can be repeated M times within the system. The total number of channels, C, is used as a measure of capacity

MSMkNC ==


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• The capacity is directly proportional to the number of replication M.

• The cluster size, N, is typically equal to 4, 7, or 12.

• The frequency reuse factor is given by 1/N.

• Hexagonal geometry has:

• exactly six equidistance neighbors.

• the lines joining the centers of any cell and each of its neighbors are separated by multiples of 60 degrees.

How to maximize capacity? solution


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• The total number of channels, C, is used as a measure of capacity

• If k and N remain constant,

• If C and k remain constant,

MSMkNC ==

MC ∝

MN 1∝


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• Only certain cluster sizes and cell layout are possible.

• The number of cells per cluster, N, can only have values which satisfy:

where i and j are non-negative integers.

22 jijiN ++=


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To find the nearest co-channel of a neighboring cell: 1. Move i cells along any chain of hexagons. 2. Turn 60 degrees counter clockwise. 3. Move j cell.

Method of locating co-channel cells in a cellular system. In this example, N = 19 (i.e., i = 3, j = 2). (Adapted from [Oet83] IEEE.)

Example 1

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If a total of 33 MHz of bandwidth is allocated to a particular FDD cellular telephone system which uses two 25 kHz simplex channels to provide full duplex voice and control channels, compute the number of channels available per cell if a system uses: a. Four-cell reuse b. Seven-cell reuse c. Twelve-cell reuse

solution

Channel Assignment Strategies

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• The choice of channel assignment strategy impacts the performance of the system, particularly as to how cells are managed when a mobile user is handed off from one cell to another.

• Fixed channel assignment:

– Each cell is allocated a predetermined set of voice channels.

– Any call attempt within the cells can only be served by unused channels in that particular cell.

– If all the channels in the cell are occupied, the call is blocked and the subscriber does not receive service.

Channel Assignment Strategies

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• Dynamic channel assignment: - Voice channels are not allocated to the cells permanently - Each time a call request is made, the serving base station

requests a channel from the MSC. - It reduces the likelihood of blocking, which increases the

trunking capacity of the system, since all the available channels in a market are accessible to all of the calls.

- But, it requires the MSC to collect real-time data on channel occupancy, traffic distribution, and radio signal strength indications (RSSI) of all channels on a continuous basis.

- This increases the storage and computational load on the system but provides the advantage of increased channel utilization and decreased probability of a blocked call.

Handoff Strategies

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• When a mobile moves into a different cell while a conversation is in progress, the MSC automatically transfers the call to a new channel belonging to the new base station.

• Handoff operation:

– Identifying a new base station

– Re-allocating the voice and control channels with the new base station.

Handoff Strategies

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• Handoff Threshold:

– Minimum usable signal for acceptable voice quality (-90dBm to -100dBm)

– Handoff margin cannot be too large or too small.

– If Δ is too large, unnecessary handoffs burden the MSC

– If Δ is too small, there may be insufficient time to complete handoff before a call is lost.

usable minimum,, rhandoffr PP −=∆

Handoff Strategies

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Handoff Strategies

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• Handoff must ensure that the drop in the measured signal is not due to momentary fading and that the mobile is actually moving away from the serving base station.

• Running average measurement of signal strength should be optimized so that unnecessary handoffs are avoided.

– Depends on the speed at which the vehicle is moving.

– Steep short term average the hand off should be made quickly.

– The speed can be estimated from the statistics of the received short-term fading signal at the base station.

Handoff Strategies

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• Dwell time: the time over which a call may be maintained within a cell without handoff, depends on:

– propagation

– interference

– distance

– speed

Handoff Measurement

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• In the first generation (1G) analog cellular systems: o Signal strength measurements are made by the

base station to determine the relative location of each mobile user with respect to the base station.

o Additionally, a spare receiver in each base station, called the location receiver, is used to determine signal strengths of mobile users which are in neighboring cells (and appear to be in need of handoff.)

Handoff Measurement

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• In second generation systems (TDMA technology): o Handoff decisions are made mobile assisted

handoff (MAHO). o Every mobile station measures the received power

from surrounding base stations and continually reports the results of these measurements to the serving base station.

o A handoff is initiated when the power received from the base station of a neighboring cell begins to exceed the power received from the current base station by a certain level or for a certain period of time.

o The MAHO performs at a much faster rate, and is particularly suited for micro cellular environments.

Handoff Measurement

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• Intersystem handoff: o Moves from one cellular system to a

different cellular system controlled by a different MSC.

o It may become a long-distance call and a roamer.

o Compatibility between the two MSCs need to be determined.

• Handoff requests is much important than handling a new call.

Prioritizing Handoff

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• Guard channel concept: o In this a fraction of total available

channels in a cell is reserved exclusively for handoff requests from ongoing calls which may be handed off into the cell.

• Queuing of handoff requests

o To decrease the probability of forced termination of a call due to lack of available channels.

Practical Handoff Consideration

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• Different type of users:

– High speed users need frequent handoff during a call.

– Low speed users may never need a handoff during a call.

• Microcells to provide capacity, the MSC can become burdened if high speed users are constantly being passed between very small cells.

• Minimize handoff intervention:

– Handle the simultaneous traffic of high speed and low speed users.

Practical Handoff Consideration

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• Using different antenna heights and different power levels it is possible to provide large and small cells which are co-located at a single location. This technique is called umbrella cell approach and is used to provide large area coverage to high speed users while providing small area coverage to users traveling at low speeds.

• The umbrella cell approach ensures that the number of handoffs in minimized for high speed users and provides additional microcell channels for pedestrian users.

Hard Handoff and Soft Handoff

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• Hard handoff: when the signal strength of a neighboring cell exceeds that of the current cell, plus a threshold, the mobile station is instructed to switch to a new frequency band that is within the allocation of the new cell assign different radio channels during a handoff.

• For 1st generation analog systems, if takes about 10

seconds and the value for Δ is on the order of 6 dB to 12 dB.

• For 2nd generation digital systems, typically requires only 1 or 2 seconds, and Δ usually is between 0 dB and 6 dB.

Hard Handoff and Soft Handoff

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• Soft handoff: a mobile station is temporarily connected to more than one base station simultaneously. A mobile unit may start out assigned to a single cell. If the unit enters a region in which the transmissions from two base stations are comparable (within some threshold of each other), the mobile unit enters the soft handoff state in which it is connected to the two base stations.

• Consequently, handoff does not mean a physical change in the assigned channel, rather that a different base station handles the radio communication task.

• By simultaneously evaluating the receiver signals from a single subscriber at several neighboring base stations, the MSC may actually decide which version of the user’s signal is best at any moment in time.

Interference and System Capacity

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• Sources of interference:

– another mobile in the same cell

– a call in progress in the neighboring cell

– other base stations operating in the same frequency band

– non-cellular system leaks energy into the cellular frequency band

• Two major cellular interference:

– co-channel interference

– adjacent channel interference

Co- and Adjacent-Channel Cells

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Co-channel cells

Adjacent-channel cells

Co-channel interference

Adjacent-channel interference

Interference and System Capacity

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• Interference on voice channels causes cross talk, where the subscriber hears interference in the background due to an undesired transmission.

• On control channels, interference leads to missed and blocked calls due to errors in the digital signaling.

• Interference is more severe in urban areas, due to the greater RF noise floor and the large number of base stations and mobiles.

• The interference are difficult to control in practice largely due to random propagation effects.

• Even more difficult to control is out-of-band interference mainly from the base stations of competing cellular carriers (locating their base stations in close proximity.)

Co-Channel Interference and System Capacity

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• Frequency reuse - there are several cells that use the same set of frequencies:

– co-channel cells

– co-channel interference

• To reduce co-channel interference, it cannot be combated by simply increasing the carrier power of a transmitter. Instead, co-channel cell must be separated by a minimum distance.


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• When the size of the cell is approximately the same

– co-channel interference is independent of the transmitted power

– co-channel interference is a function of

• R : Radius of the cell

• D : distance to the center of the nearest co-channel cell

• Q is called the co-channel reuse ratio

NRDQ 3==


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• A small value of Q (small N) provides large capacity

• A large value of Q (large N) improves the transmission quality - smaller level of co-channel interference

• A tradeoff must be made between these two objectives


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Example 2

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You are trying to design a cellular network that will cover an area of at least 2800 km2. There are 300 available voice channels. Your design is required to support at least 100 concurrent calls in each cell.

If the co-channel cell centre distance is required to be 9 km, how many base stations will you need in this network?

solution


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• Let i0 be the number of co-channel interfering cells. The signal-to-interference ratio (SIR) for a mobile receiver can be expressed as:

S : the desired signal power

Ii : interference power caused by the i-th interfering co-channel cell base station

• The average received power at a distance d from the transmitting antenna is approximated by

or

n is the path loss exponent which ranges between 2 and 4.

∑=

=0

1

i

iiI

SIS

n

r ddPP

−

=

00

−=

00 log10)dBm()dBm(

ddnPPr Receiver

d0

P0 : Measured power


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• When the transmission power of each base station is equal and the path loss exponent same throughout the coverage area, SIR for a mobile can be approximated as

Example: for N=7, the first layer of interfering cell, i0 = 6.

• For simplification, assume all interferers have equidistance,

( )∑=

−

−

=0

1

i

i

ni

n

D

RIS

( )00

3)/(iN

iRD

IS

nn

==which relates S/I to the cluster size, and in turn determines the overall capacity of the system

Adjacent-Channel Interference

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• Adjacent channel interference: interference from adjacent in frequency to the desired signal.

– Imperfect receiver filters allow nearby frequencies to leak into the pass-band

– Performance degrade seriously due to near-far effect.

desired signal

receiving filter response

desired signalinterference

interference

signal on adjacent channelsignal on adjacent channel

FILTER

Adjacent-Channel Interference

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• Adjacent channel interference can be minimized through :

o careful filtering and channel assignment.

o Keep the frequency separation between each channel in a given cell as large as possible

o A channel separation greater than six is needed to bring the adjacent channel interference to an acceptable level.

Example 3

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If a signal-to-interference ratio of 15 dB is required for satisfactory forward channel performance of a cellular system, what is the frequency reuse factor and cluster size that should be used for maximum capacity if the path loss exponent is (a) n = 4, (b) n = 3 ?

Assume that there are six co-channel cells in the first tier, and all of them are at the same distance from the mobile. Use suitable approximations.

solution

Power Control for Reducing Interference

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• In practical cellular radio and personal communication systems, the power levels transmitted by every mobile unit are under constant control by the serving base stations.

• This is done to ensure that each mobile transmits the smallest power necessary on the reverse channel.

• Power control not only helps prolong battery life, also reduces the interference on the reverse channel.

• It is especially important for CDMA systems, because every user in every cell share the same radio channel. (to reduce the co-channel interference.)

Power Control for Reducing Interference

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Need for power control ?

Trunking and Grade of Service

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• Trunking exploits the statistical behavior of users so that a fixed number of channels or circuits may accommodate a large, random user community.

• It allows a large number of users to share the

relatively small number of channels in a cell by providing access to each user, on demand, from a pool of available channels.

• The measure of traffic intensity, namely Erlang. For example:

0.5 Erlangs of traffic = a radio channel that is occupied for 30 minutes during an hour.


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The Grade of Service (GOS):

• A measure of the ability of a user to access a trunked system during the busiest hour.

• It is typically given as the likelihood that a call is blocked, or the likelihood of a call experiencing a delay greater than a certain queuing time.


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• The traffic intensity generated by each user Where H is the average duration of a call, λ is the average number of call requests per unit time for each user.

• The total offered traffic intensity for U users

• In a C channel trunked system, if the traffic is equally distributed among the channels, the traffic intensity per channel is

HAu λ=

CUAA u

C =

uUAA =


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• AMPS cellular system is designed for a GOS of 2% blocking.

• This implies that the channel allocations for cell sites

are designed so that 2 out of 100 calls will be blocked due to channel occupancy during the busiest hour.


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Two types of trunked systems:

• Blocked Calls Cleared trunking :

– Offers no queuing for call requests.

– Calls arrive as determined by a Poisson distribution.

– There are an infinite number of users.

– There are memoryless arrivals of requests.

– The probability of a user occupying a channel is exponentially distributed.

– A finite number of channels available.

– This is known as an M/M/m queue.

a memoryless poisson arrivals an exponential service time

the number of trunked channels


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This leads to the derivation of the Erlang B formula: where C is the number of trunked channels, A is the total offered traffic.

GOS

!

!][

0

==

∑=

C

k

k

c

r

kA

CA

blockingP


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• Blocked Calls Delayed trunking :

– Queuing is provided to hold calls which are blocked.

– If no channel available, the call request may be delayed until a channel becomes available.

– Measure of GOS : probability that a call is blocked after waiting a specific length of time.

– A call not having immediate access to a channel is determined by Erlang C:

∑−

=

−+=> 1

0 !)1(!

]0[ C

k

kc

c

r

kA

CACA

AdelayP


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The GOS of a trunked system where blocked calls are delayed : The average delay for all calls in a queued system

)/)(exp(]0[ ]0|[]0[][

HtACdelayPdelaytdelayPdelayPtdelayP

r

rrr

−−>=>>>=>

ACHdelayPD r −

>= ]0[


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The Erlang B formula plotted in graphical form


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The Erlang C formula plotted in graphical form

Example 4

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How many users can be supported for 0.5% blocking probability for the following number of trunked channels in a blocked calls cleared system?

(a) 1, (b) 5, (c) 10, (d) 20 and (e) 100

Assume each user generates 0.1 Erlangs of traffic.

solution

Example 5

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Pauh has an area of 1300 square miles and is covered by CELCOM using a seven-cell reuse pattern. Each cell has a radius of four miles and the city is allocated 40 MHz of spectrum with a full duplex channel bandwidth of 60 kHz. Assume a GOS of 2% for an Erlang B system is specified. If the offered traffic per user is 0.03 Erlangs, compute:

(a) The number of cells in the service area,

(b) The number of channels per cell,

(c) Traffic intensity of each cell,

(d) The maximum carried traffic,

(e) The total number of users that can be served for 2% GOS

(f) The number of mobiles per unique channel (where it is understood that channels are reused)

(g) The theoretical maximum number of users that could be served at one time by CELCOM.

solution

Example 6

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An urban area has a population of two million residents. Three competing trunked mobile networks (CELCOM, MAXIS and DIGI) provide cellular service in this area.

CELCOM has 394 cells with 19 channels each, MAXIS has 98 cells with 57 channels each, and DIGI has 49 cells, each with 100 channels.

Find number of users that can be supported at 2% blocking if each user averages two calls per hour at an average call duration of three minutes. Assume that all three trunked systems are operated at maximum capacity, compute the percentage market penetration of each cellular provider.

solution

Improving Capacity in Cellular System

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• Methods for improving capacity in cellular systems:

– Cell Splitting :

o Subdividing a congested cell into smaller cells.

o Allows an orderly growth of the cellular system

– Sectoring :

o Directional antennas to control the interference and frequency reuse of channels.

– Microcell Zone Concept :

o Distributing the coverage of a cell and extends the cell boundary to hard-to-reach place.

– Repeaters for range extension.

– More bandwidth – standards, country regulation etc.

– Borrow channel from nearby cells.


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• Cell Splitting : Split congested cell into smaller cells.

– Preserve frequency reuse plan.

– Reduce transmission power.


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microcell

Reduce R to R/2


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• Transmission power reduction from to

• Examining the receiving power at the new and old cell boundary

• If we take n = 4 and set the received power equal to each other

• The transmit power must be reduced by 12 dB in order to fill in the original coverage area.

• Problem: if only part of the cells are splitted:

– Different cell sizes will exist simultaneously

• Handoff issues - high speed and low speed traffic can be simultaneously accommodated

1tP 2tP

ntr RPP −∝ 1]boundary cell oldat [

ntr RPP −∝ )2/(]boundary cellnew at [ 2

161

2t

tPP =


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• Sectoring : Decrease the co-channel interference and keep the cell radius R unchanged

–Replacing single omni-directional antenna by several directional antennas.

–Radiating within a specified sector.


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67

Base Station Antennas

Omnidirectional : broadcast 3600

Sector : broadcasts 600 / 900 / 1200

Panel / Dish : Point to point

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600 Sectoring

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1200 Sectoring


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Microcell Zone Concept :

• Antennas are placed at the outer edges of the cell

• Any channel may be assigned to any zone by the base station

• Mobile is served by the zone with the strongest signal.

• Handoff within a cell

– No channel re-assignment

– Switch the channel to a different zone site

• Reduce interference

– Low power transmitters are employed


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• Repeaters for range extension:

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