Lec4_8Mar2014. It contain information about frequency reuse

8/12/2019 Lec4_8Mar2014. It contain information about frequency reuse

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Mobile Communication

Lec 4: Basic Aspects of Frequency

Planning in GSM


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Frequency Resource of GSM System

GSM 900 :

GSM 1800 :1710 1785 1805 1880

Duplex distance : 95 MHz

890 915 935 960

Duplex distance : 45 MHz


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Frequency Band Configuration

GSM900:

BTS receiver (uplink ): f1 (n) =890.2+ (n-1)*0.2 MHz

BTS transmitter (downlink ): f2 (n) =f1 (n) +45 MHz

GSM1800:

BTS receiver (uplink ): f1 (n) =1710.2 + (n-1) * 0.2 MHz

BTS transmitter (downlink ): f2 (n) =f1 (n) +95 MHz


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Concept of Frequency Reuse

{fi,fj..fk}

{fi,fj..fk} {fi,fj..fk} {fi,fj..fk}.. ..

One Large Cell

dMicro-cell system

Frequency resource is limited. If there is 8MHz frequency resource, 8 MHz = 40 channels * 8

timeslots = 320

Max. 320 users can access the network at the same time.

If every frequency is reused N times

Max. 320*N uses can access the network at the same time.


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Requirement for Interference and Carrier-to-InterferenceRatio

All useful signals carrier

All useless signals interference=

GSM standard: C / I >= 9 dB

In practical projects: C / I >= 12dB

Useful signal Noise from environment

Other signals

C/I =


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Interference in a GSM Network

Interference is a major limiting factor in the performance of cellular systems. It causes degradation of

signal quality and introduces bit errors in the received signal. Bit errors are partly recoverable by means ofchannel coding and error correction mechanisms.

Sources of Interference are a call in progress in the neighboring cell, other base stations operating on the

same frequency and any non-cellular system which leaks energy into the cellular frequency band (external

interference).

The interference situation is not reciprocal in the uplink and downlink direction. Mobile stations and base

stations are exposed to different interference situation.

There are two types of system generated interference

Co-channel interference

Adjacent channel interference


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This type of interference is the due to frequency reuse, i.e. several cells use the same set of frequency.

These cells are called co-channel cells.

Co-channel interference cannot be combated by increasing the power of the transmitter. This is because

an increase in carrier transmit power increases the interference to neighboring co-channel cells.

To reduce co-channel interference, co-channel cells must be physically separated by a minimum distance

to provide sufficient isolation due to propagation or reduce the footprint of the cell.

Some factors other than reuse distance that influence co-channel interference are antenna type,

directionality, height, site position etc.

GSM specifies C/I > 9dB.

Co-Channel Interference


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In a cellular system, when the size of each cell is approximately the same, co-channel interference is

independent of the transmitted power and becomes a function of cell radius(R) and the distance to the

center of the nearest co-channel cell (D).

Where iandjare non-negative numbers. To find the nearest co-channel neighbour of a particular cell, one must do the following:

(1) move icells along any chain of hexagons and then (2) turn 60 degrees counter-clockwise and movejcells. This is illustrated in

the figure above for i = 1&j = 2for a cluster size of 7.

Co-Channel Interference

Re-use distance: D = R x (3 x (i2 + j2 + ij)

Co-channel re-use Ratio: D/R = (3 x N)1/2

Cluster Size: N = (i2 + j2 + ij)

(D/R) = 6 (C/I)

By increasing the ratio of D/R, the spatial separation between co-channel cells relative to the

coverage distance of a cell is increased. Thus interference is reduced due to improved isolation

from the co-channel cells. The relation between the re-use distance ratio D/R and the co-

channel interference ratio C/I is as below,

Where is the propagation index or attenuation constant with values ranging between 2 to 4.


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Looser reuse

Higher frequency reuse

efficiency, but interference

is serious.

Tighter reuse

0 12 20

Little interference, but frequency

reuse efficiency is low.

Reuse Density

Reuse density (n*m) is the number of cells in a basic reuse cluster.

4*3, 5*3 & 6*3 etc

n: BTS number in a basic reuse cluster

m: Cell number per BTS in a basic reuse cluster


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Frequency re-use is alternatively represented as;

No of Sites (3 cells per site) in a Cluster /No of Frequency Groups

Usually 7/21, 4/12, 3/9 re-use patterns are used;

Reuse Density


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Interference resulting from signals which are adjacent in frequency to the desired signal is called adjacent

channel interference.

Adjacent channel interference results from imperfect receiver filters which allow nearby adjacent

frequencies to leak into the passband.

Adjacent channel interference can be minimized through careful filtering and channel assignments.

By keeping the frequency seperation between each channel in a given cell as large as possible the adjacent

channel interference may be reduced considerably.

GSM specifications state that:

For adjacent (200 kHz) interference: C/A200kHz= -9 dB



Adjacent Channel Interference


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Multi-layer Reuse Pattern


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Frequency Planning Principle

There should be no co-channel frequency carriers in one BTS. The frequency separation between BCCH and TCH in the same cell should be not

less than 400K.

When frequency hopping is not used, the separation of TCH in the same cell

should be not less than 400K.

In non-1*3 & 1*1 reuse mode, co-channel should be avoided between theimmediately neighbor BTS.

Neighbor BTS should not have co-channels facing each other directly.

Normally, with 1*3 & 1*1 reuse, the number of the hopping frequencies should be

not less than twice of the number of frequency hopping TRX in the same cell.

Pay close attention to co-channel reuse, avoiding the situation that the sameBCCH has the same BSIC in adjacent area.


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Contents

1. Frequency Planning

2. Tight Frequency Reuse

3. Frequency Hopping


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Content of Frequency

Hopping

Advantages of hopping

Types of Hopping.

Parameter of hopping Collocation of hopping data


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Frequency Hopping


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Advantages of Hopping

Get an agreeable radio environment.

Provide a similar communication quality for every user.

Tighter reuse patterns are possible to be used for larger capacity.

Hopping helps in combating Radio Propagation effects like

Multipath fading & Shadowing etc.

Interference Diversity by averaging the interference for all radioconnections.


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MULTIPATH FADING & LOG NORMAL

FADING Rayleigh Fading is a fading when the signal is not

received directly from the transmitter, but from manydifferent locations where it has bounced. The receivedsignal is the sum of several identical signals which differsin phase. The sum maybe close to zero which will resultin a fading dip or signal cancellation and drops. Commonin urban areas.

Log Normal Fading is fading caused by obstacles such as

hills or buildings and give rise to a shadowing effect. Thisresults in decrease of signal strength which will vary whenthe mobile station is moving.


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Smoothen the rapid fading (Rayleigh fading)

Frequency Diversity of

Hopping


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Interference Diversity

Interference Dependent on

TIME

FREQUENCY

LOCATION OF MOBILE

By changing frequency on every TDMA frame, a mobile only experiencesinterference on a particular frequency once in a number of hops. Similarly,interference on a particular frequency will be spread across many mobiles (i.e.averages out with other mobiles).---called Interference Averaging

Interference Diversity - evens out the perceived radio environment

Dependent on the mode of hopping, cyclic or random, and the type offrequency hopping used, baseband and synthesizer hopping


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Smoothen and average the interference

Interference Diversity of

Hopping


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FREQUENCY HOPPING GAIN

The gain from frequency hopping

depends on factors such aspropagation environment, number

of hopping f requenc ies and

i n te r fe rence charac te r i s t i cs

The number of hopping frequencies

a f fec ts the ga in f r om bo th

f requency and i n te r fe rence

diversity

The hopping gain increases with

the number of hopping frequencies

C/I gain (dB)

2 3 4 5 6 7 8 9 10 11

# Frequenciesx

x

x

xxxx

x

x

Frequency Hoping

- Interference averaging gain- Frequency diversity gain

(typical, simulation)


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Types of Hopping

Hopping can be implemented in twoways

Base-band hopping

RF hopping


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Base Band Hopping Principle

FH bus


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Base Band Hopping

Baseband hopping : Each transmitter transmits on a fixed

frequency. The bursts from the transceiver controller are

routed to the different transmitters by a bus.

+ A narrow-band filter combiner can be used. To this

combiner it is possible to connect up to 16 TRXs without more

than 3dB combiner loss.

- It is impossible to hop on more frequencies than there are

TXs.


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RF Hopping Principle


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RF Hopping

Synthesizer hopping: The transmitters change frequency for

every burst.

+ It is possible to hop on more frequencies than there are

transmitters.

- Hybrid combiners must be used. When connecting many

transmitters the loss will be big.


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Frequency Hopping TCH Re-use Patterns

Various Re-use Pattern can be used for TCH

Hopping:

FLP1/1.

FLP1/3.

FLP3/3,3/9

MRP (Multiple Re-use Patterns).


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Available ARFCN:BCCH14+TCH36

1BCCH+3TCH

1BCCH+3TCH 1BCCH+3TCH

1BCCH+12TCH


4*3 Non-

Hopping TCH

1*3

1*3 or 1*1Reuse Patterns

1BCCH+36TCH


1*1


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Example of 1*3 Frequency

Reuse Suppose 900 band: 96 to124

BTS configuration: S3/3/3

BCCH layer: 96 to109 reuse pattern: 4*3

TCH layer: 110 to124 reuse pattern: 1*3


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Group 1 (MA1): 110 111 112 113 114 Cell1

Group 2 (MA2): 115 116 117 118 119 Cell2

Group 3 (MA3): 120 121 122 123 124 Cell3

TCH Consecutive Allocation

Scheme


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TCH Interval Allocation

Scheme

Group 1 (MA1): 110 113 116 119 122 Cell1

Group 2 (MA2): 111 114 117 120 123 Cell2

Group 3 (MA3): 112 115 118 121 124 Cell3


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Comparison Between Multi-

layer Reuse and 1*3 & 1*1 For Multi-layer reuse pattern, either Base band hopping or RF hopping can be

used. But for 1x3 & 1x1 reuse, only RF hopping can be used.

The frequency planning for the 1x3 & 1x1 mode is simple and it is easy to plan the

frequency for new added BTS.

1x3 &1x1 mode requires a rather regular BTS location distribution (uniform cell

Plan, Antenna Azimuths & heights is a must).

For the cells with fixed number of TRX, when the traffic is heavy, the 1x3 & 1x1

provides higher service quality than that of Multi-layer reuse pattern.

TRX can be easily added to the 1x3 & 1x1 network, but number of hopping TRX

should not exceed half of the allocated hopping frequency number and the max

RF load desired for acceptable quality.


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Parameters used for Hopping


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HSNHopping Sequence NumberDefinition

The parameter that determines the type of hopping pattern to be used

Values ranges from 0-63

Two Types of HSN

CYCLIC Hopping - the frequencies are changed for every TDMAframe in a consecutive order . (i. e HSN = 0)

PSEUDO Random Hopping - is implemented as a pseudo-random

sequence. The sequence is stored in a look-up table in the mobile as

well as in the base stations. 63 independent sequences are defined .

Which of the 63 sequences to be used is specified with parameter. (i. e HSN = 1 63).


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HOPPING CYCLIC vs PSEUDO

RANDOM________________________________

... , f 4, f 1, f 2, f 3, f 4, f 1, f 2, f 3, f 4, f 1, f 2, ...________________________________

________________________________... , f 1, f 4, f 4, f 3, f 1, f 2, f 4, f 1, f 3, f 3, f 2, ...

CYCLIC

PSEUDO RANDOM


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Orthogonal Hopping Sequences

________________________________... , f 1, f 4, f 4, f 3, f 1, f 2, f 4, f 1, f 3, f 3, f 2, ...... , f 2, f 1, f 1, f 4, f 2, f 3, f 1, f 2, f 4, f 4, f 3, ...

... , f 3, f 2, f 2, f 1, f 3, f 4, f 2, f 3, f 1, f 1, f 4, ...... , f 4, f 3, f 3, f 2, f 4, f 1, f 3, f 4, f 2, f 2, f 1

Same Cell same channel groupeach transceiverassigned with same HSN

Introduction of MAIO

MAIO = is used to ensure that each TRX uses always an unique frequency.


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Load & Reuse Definitions for Spectrum Efficiency

of Frequency Plan

Frequency load, based upon actual traffic load. For example a

frequency load of 12% per cell means that each frequency is

used 12% of the time for a cell.

Hardware load, based upon number of TRXs. For example

20% HW load means e.g. 10 frequencies hopping on twoTRXs.

Frequency utilization. This is a measure of the spectrum

efficiency related to the actual traffic (very similar to

Frequency load measure).


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Frequency Load

The frequency load, which is a specific measure for 1/1 planned networks, reflects how effective the

hopping frequencies are used in a cell. This is done by measuring the actual traffic load and compare it

with the number of assigned frequencies. The upper limit of this measure is decided by the quality

standards for the network.

The frequency load measure is defined by first dividing the average full rate traffic load per cell by 8 to get

the average timeslot traffic. This is then divided by the number of hopping frequencies to get the average

traffic load per timeslot and frequency. This measure can be written as:

FREQ Load= Erlang cell/ 8* (Freq cell).

The average traffic load per cell can be calculated by measuring the average traffic in an area and divide

this by the number of cells in the area.

Fractional Load Btw 14-16 % has already been trialed in network with acceptable KPIs.


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Hardware Load

The HW load reflects the number of installed TRXs per cell if the spectrum allocation (number of

frequencies) for TCH frequencies is given. This makes the HW load very quick to use and easy to

understand. The hardware load is defined as the average number of hopping TRXs per cell divided by the number of

hopping frequencies per cell and can be expressed as:

HW Load= TRX cell/ Freq cell. the HW load gives a non-comparable measure for different networks since the HW utilization is different

for different networks (trunking gain is not taken in account).The HW load is NOT recommended to use

because this measure differ depending on the number of TRXs per cell (different carried traffic per

timeslot at 2% GOS). In order to make the HW load a bit more reliable the GoS compensated HW load can be calculated. The

real traffic is then taken into account and its expressed as:

GOS CompensatedHW Load= (TRX cell/ Freq cell) x (Erlang cell/ Erlang @

2% GOS cell).


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Frequency Utilization

The Frequency Utilization offers a measure of the true spectrum efficiency, based upon the actual traffic

and reuse in the network.

The Frequency Utilization is a way of describing the traffic load per frequency and its defined accordingly:FREQ Utilization= Erlang cell/ # Freq area.

The higher the figure of Frequency Utilization, the more spectrum efficient the plan is. Note that the total

number of frequencies used in the area should be used.

Example Case: lets assume 2 TRX /cell, Erlang per cell 9 & total available frequencies is 36.

FOR a BB Hopping network with 4/12 pattern 12 BCCH & 24 TCH frequencies are used thus:

FREQ Utilization: 9/36= 0.25 Erl/Freq.

FOR a 1/1 Planned network 12BCCH & TCH reduced from 24 to 15 thus:

FREQ Utilization: 9/27= 0.33 Erl/Freq.


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Documents

Lec4_8Mar2014. It contain information about frequency reuse