04 mn1790 frequency planning ww fh

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Frequency Planning: ContentsFrequency Planning: Contents

• Interference

• Frequency Reuse and Reuse Patterns

• Cluster

• Cluster: Exercise

• Spectrum Efficiency

• Optimization of Spectrum Efficiency

• Interference Reduction

• Frequency Hopping

• Power Control

• VAD/DTX

• Interference Matrix

• Frequency Allocation Strategies

• Tool supported Frequency Allocation

• Interference Analysis

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Interference Interference

Following GSM 05.05:

Reference interference ratio for all BTS and MS types:

• For co-channel interference: C/Ic = 9 dB

• For (first) adjacent channel interference: C/Ia1 = - 9 dB

• For (second) adjacent channel interference: C/Ia2 = -41 dB

• For (third) adjacent channel interference: C/Ia3 = -49 dB

At these values, the so called reference interference performance in terms of (maximum) frame erasure rate, bit error rate or residual bit error rate must be met for the different type of channels (FACCH/H or F, SDCCH, BCCH, AGCH, PCH, SACCH, RACH, SCH, TCH/F9.6 or H4.8 or F4.8 or F2.4 or H2.4 or FS or HS) in different specified propagation conditions (TU3 no FH, TU3 ideal FH, TU50 no FH, TU50 ideal FH, RA250 no FH).

Inter system interference:

Interference from any other system

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The spectrum due to modulation for a GSM 900 MHz MS, taken from GSM05.05 Annex A:

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The spectrum due to modulation for a GSM 900 MHz BTS, taken from GSM05.05 Annex A:

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Frequency Reuse and Reuse Patterns Frequency Reuse and Reuse Patterns

Frequency reuse:With the help of the frequency reuse concept, high capacities can be achieved. However, care must be taken that the interference caused by the reuse cells is in tolerable limits.The following consideration allows an estimation of the minimum reuse distance D:

R

D

Co-channel reuse cell(Interferer I1 )

Co-channel reuse cell(Interferer I2 )

D1≈D

D2≈D

≈D

≈D

≈D≈D

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Frequency Reuse and Reuse Patterns Frequency Reuse and Reuse Patterns

Signal to interference ratio:

In general, for a mobile radio environment:

If the transmission power of all base stations has the same value and also the path loss exponent is uniform in the coverage area, then the signal to interference ratio can be rewritten as:

Di is the distance of the ith interferer from the MS and is assumed to be ≈D (see picture).(The cluster size K will be introduced and discussed below.)

∑=

=

m

iiI

SIS

1

ndd

r PP −= )(00

)log(1000 d

dr nLL −=

mK

mRD

D

RIS

nn

m

i

ni

n 3)/(

)(1

==∑

==

−

−

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ClusterCluster

ClusterA group of cells, where each frequency is exactly used once.

Cluster size (K):Assuming hexagonal shaped cells, due to the hexagon symmetry the number K of cells within a cluster is given by:

K = i2 + ij + j2 with i,j: integer (0,1,2,3,…..)

Exercise: Calculate the 8 smallest values of K.

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Cluster: Exercise Cluster: Exercise

Exercise: Use regular hexagons to cover the two dimensional plane. Name the outer cell radius R.• Express the inner cell radius r as function of R. • Express the reuse distance D as function of R, i and j.• Express the cluster size K as function of i and j. • Express the co-channel reuse ratio Q=D/R as function of K.

j

i

DRr

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Cluster Cluster

Example:

The following example shows an omni-cell configuration with cluster size 7.

1

23

45

6

7

1

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45

6

7

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45

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7

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7

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7

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56

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6 1

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7

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7

1

237

45

65

6

7

6

7

237 2

3

4

3

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Cluster: ExerciseCluster: Exercise

R

D

Co-channel reuse cell

Co-channel reuse cell

D-R

D

D-R

D

D+RD+R

Exercise:Consider a cluster of size K=7. Express the S/I ratio as function of the co-channel reuse ratio Q for the MS shown in the following picture which experiences worst case co-channel interference. Use the approximations displayed in the figure for the calculation of the S/I.

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Cluster: ExerciseCluster: Exercise

Exercise:

Assume a carrier to interference ratio of 15 dB being required for a satisfactory performance and a path loss exponent of n=4 respectively n=3. Consider 6 co-channel cells and for simplicity assume that all of them have the same distance form the MS. Calculate the frequency reuse factor and the cluster size that should be used in both cases to realize maximum capacity.

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Spectrum Efficiency Spectrum Efficiency

Spectrum efficiency:

The spectrum efficiency is defined as the traffic which can be handled with a given frequency range in a certain area and therefore measures how efficient a given frequency spectrum is used.

umUsedSpectrAreaTrafficficiencySpectrumEf ∗=

The unit of the spectrum efficiency is [ERL / km2 x MHz] .

Taking also into account the site density, the spectrum efficiency can be used to compare thenetwork design of different operators.

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Optimization of Spectrum Efficiency Optimization of Spectrum Efficiency

Omni- or sector cell configurations?

In general, sector cell configurations allow tighter frequency re-use but offer lower trunking gain.

Exercise:

• Consider a GSM network operator having 60 GSM carriers using 3 sector sites with a 21

reuse for the BCCH and a tighter reuse of 12 for the TCH.

What is the spectrum efficiency for this configuration (assume a regular configuration)?

• Consider an alternative configuration using omni sites with a 12 reuse for the BCCH and a

6 reuse for the TCH.

What is the spectrum efficiency for this alternative configuration?

• Perform the same calculations assuming the operator has only 20 carriers.

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Interference ReductionInterference Reduction

1) Via implementation of selected procedures

• Frequency hopping

• (Dynamic) Power Control

• VAD / DTX

2) Via optimization of physical and database parameters

• Selection of suitable base station sites (already during planning phase)

• Antenna fine tuning (height, tilt and azimuth)

• Change of antenna type (beam width reduction)

• Optimization of parameters limiting output power

• Optimization of parameters controlling handover regions

• Optimization of frequency plan

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Frequency HoppingFrequency Hopping

Frequency hopping is not timeslot hopping!

A connection is not transmitted using only one frequency in a cell, but bursts of consecutive TDMA frames of that connection are transmitted using certain frequencies of a specified frequency set (defined by the parameter MA, the so called Mobile Allocation).Without using frequency hopping, the speech quality of a connection may either be good or bad. There are calls in a cell which will suffer under bad speech quality, other connections will have a quite good speech quality.

The reason is that: Rayleigh fading (short term fading) is different for different frequencies,the interference level is different on different frequencies.

Frequency hopping will average the quality for all connections:For a certain connection, the link quality may now change from burst to burst. Nevertheless, due to interleaving, not only 1 but 8 consecutive bursts as a whole must be successfully decoded. Therefore, even if there are some bad quality bursts in these 8 consecutive bursts a speech frame may be still successfully decoded.

Frequency hopping is most effective in case of slow moving or static mobile stations.

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time

TS 0 1 2 3 4 5 6 7

TRX-0, f0

TRX-1, f1

TRX-2, f2

TRX-3, f3

1 TDMA-frame


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Without frequency hopping:

time

TS 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2

TRX-0, f0

TRX-1, f1

TRX-2, f2

TRX-3, f3


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Example of baseband frequency hopping:

time

TS 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2

TRX-0, f0

TRX-1, f1

TRX-2, f2

TRX-3, f3

BCCH Channel, FHSYID=0

TCH Channel, FHSYID=1 (HSN=0,MA=f0&f1&f2&f3),MAIO= 0




SDCCH Channel, FHSYID=1 (HSN=0,MA=f0&f1&f2&f3),MAIO= 0

TCH Channel, FHSYID=2 (HSN=0,MA=f1&f2&f3),MAIO= 0


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Example of synthesizer frequency hopping:

BCCH Channel, FHSYID=0

TCH Channel, FHSYID=0

TCH Channel, FHSYID=1 (HSN=0,MA=f1&f2&f3&f4&f5&f6), MAIO= 1



SDCCH Channel, FHSYID=0

TCH Channel, FHSYID=1 (HSN=0,MA=f1&f2&f3&f4&f5&f6),MAIO= 0

time

TS 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2

f0

f1

f2

f3

TRX-0

TRX-1

TRX-2

TRX-3

f0 f0

f1 f2

f0 f0 f0

f3

f0f0

f2

f3

f4

f0

f3

f4

f5


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There are different ways of hopping (hopping patterns) foreseen by GSM (see GSM 05.02): The hopping pattern is defined by the parameter HSN (Hopping Sequence (Generator) number, value range: 0 - 63).

HSN = 0: cyclic hopping: optimum averaging of Rayleigh fading.

HSN > 0: non cyclic (quasi random) hopping: optimum averaging of (co-channel) interference.

The frequency hopping used in GSM is so called Slow Frequency Hopping since the frequency is not changed during a whole burst.

From MS point of view the frequency is changed from TDMA frame to TDMA frame (1 TDMA frame having a duration of 8 x 577µs).

The Base station must be able to change the frequency from burst to burst. Two principle possibilities are distinguished:

• Baseband hopping

• Synthesizer hopping

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Handling of bursts in BTS for baseband hopping:

ControllerTRX-0

ControllerTRX-1

ControllerTRX-2

ControllerTRX-3

Transmitterf0

Transmitterf1

Transmitterf2

Transmitterf3

Combiner(e.g. filter combiner)

bus system


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Handling of bursts in BTS for synthesizer hopping:

ControllerTRX-0

ControllerTRX-1

ControllerTRX-2

ControllerTRX-3

Transmitterf0...fn

Transmitterf0...fn

Transmitterf0...fn

Transmitterf0...fn

Combiner (e.g. duplex

combiner)

bus system


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Parameters, mentioned in GSM (see GSM 05.02):

A value between 0 and 63 to control the hopping generator. Sent in the information element Mobile Allocation in the Assignment Command or the Handover Command message.

Hopping Sequence Number

HSN

Value range between 0 and 63. Mobiles, using the same Mobile Allocation and the same timeslot in a TDMA-frame must have different MAIOs. As a result all these mobiles are distributed on the available frequencies. Sent in the information element Channel Description contained for example in the Assignment Command message.

Mobile Allocation Index Offset

MAIO

List of frequencies to be used in the mobiles hopping sequence Max. 64 radio frequency channels Sent in the information element Mobile Allocation in the Assignment Command or the Handover Command message.

MobileAllocation

MA

Will be calculated from the reduced TDMA frame number T1, T2, T3’ which is broadcasted on the Synchronisation Channel FN = 51((T3 - T2) mod (26)) + T3 + 51x26xT1 T3 = (10xT3’)+1 Value range of FN: 0 to (26x51x2048)-1 = 2715647 (Compare: GSM 05.10)

(TDMA) Frame Number

FN

List of ARFCNs (max. 64) used in the cell. Broadcasted on BCCH, System Information Type 1 The MA must be a subset of the CA

Cell AllocationCA

RemarksFull name Abbre viation

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Power ControlPower Control

1.) Dynamic MS Power Control (UL Power Control)

2.) Dynamic BS Power Control (DL Power Control)

Stepwise increase or decrease of the MS / BS transmission power based on evaluation of RXQUAL and RXLEV measurements.

Ideas behind the usage of dynamic power control:

• to reduce the overall interference,

• to reduce MS battery consumption,

• and to reduce also the risk of a bad speech quality due to saturation of the BTS receiver.

Principle steps for the power control procedure:

• Measurements

• Pre-processing (averaging) of the measurements (a not mandatory averaging procedure

is described in GSM 05.08)

• Decision based on comparison of averaged values with thresholds (a not mandatory

procedure is described in GSM 05.08)

• Power Control execution (power increase, decrease, or no change of transmission power)

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AV_RXQUALAV_RXLEV

MS transmission power

AV_RXQUALAV_RXLEV


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Path Loss


Maximum allowedMS transmission

power

Minimumtransmission

power

= min(MS_TXPWR_MAX,P)

POW_INCR_STEP_SIZE

POW_RED_STEP_SIZE

Power Regulation Area

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MS measures:DL-levelDL-qualityDL-levelof neighbor cells

MS measures:DL-levelDL-qualityDL-levelof neighbor cells

BTS measures:UL-levelUL-quality

Timing AdvanceIdle TCH

BTS measures:UL-levelUL-quality

Timing AdvanceIdle TCH

Measurement reports contain:

DL-levelDL-qualityDTX indicatorMax. 6 neighbor cell measurementsRXLEV_NCELLBSIC_NCELLBCCH_FREQ_NCELL

Measurement reports contain:

DL-levelDL-qualityDTX indicatorMax. 6 neighbor cell measurementsRXLEV_NCELLBSIC_NCELLBCCH_FREQ_NCELL

Measurement reports


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The following parameters are sent in the measurement reports:

Base Station Identification Code for neighbourBSIC_NCELL_(1-6)

Received level assessed on BCCH carrier in the neighbour cell, as indicated in the BCCH allocation

RXLEV_NCELL_(1-6)

Value of BA_IND for BCCH allocation usedBA_USED

The DTX indicator shows, whether or not the MS used DTX during the previous measurement period

DTX_USED

Received quality in the current serving cell, assessed over a subset of TDMA fames

RXQUAL_SUB_SERVING_CELL

Received quality in the current serving cell, assessed over all TDMA fames

RXQUAL_FULL_SERVING_CELL

Received level in the current serving cell, assessed over a subset of TDMA frames

RXLEV_SUB_SERVING_CELL

Received level in the current serving cell, assessed over all TDMA fames

RXLEV_FULL_SERVING_CELL

RemarksAbbreviation

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The following parameters and thresholds for the MS power control are mentioned in GSM 05.08:

Range: 2, 4 dBPOW_RED_STEP_SIZE

Range: 2, 4, 6 dBPOW_INCR_STEP_SIZE

Power Control Interval: Minimum time interval between changes in the transmission power level. Range: 0 – 30 s. Step size: 0.96 s (2 SACCH multiframes).

P_CON_INTERVAL

Maximum Transmission Power a MS may use in the adjacent (neighbour) cell number n. Range for a GSM 900 MS: 5 – 39 dBm. Range for a DCS 1800 MS: 0-30 dBm.

MS_TXPWR_MAX(n)

Maximum Transmission Power a MS may use in the serving cell. Range for a GSM 900 MS: 5 – 39 dBm. Range for a DCS 1800 MS: 0-30 dBm.

MS_TXPWR_MAX

(Upper) RXQUAL threshold on the uplink for power reductionU_RXQAUL_UL_P

(Lower) RXQUAL threshold on the uplink for power increaseL_RXQAUL_UL_P

(Upper) RXLEV threshold on the uplink for power reductionU_RXLEV_UL_P

(Lower) RXLEV threshold on the uplink for power increaseL_RXLEV_UL_P

RemarksAbbreviation

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RXQUAL

RXLEV

L_RXQUAL_XX_P

U_RXQUAL_XX_P

L_RXLEV_XX_P U_RXLEV_XX_P

00

7

63

Power Decrease concerning Level

Power Increase concerning Level

Power Decrease concerning Quality

Power Increase concerning Quality

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Power Decrease concerning Level

Power Increase concerning Level

Power Decrease concerning Quality

Power Increase concerning Quality

RXQUAL

RXLEV

L_RXQUAL_XX_P

U_RXQUAL_XX_P

L_RXLEV_XX_P U_RXLEV_XX_P

00

7

63

POW_RED_STEP_SIZE

Example solution

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VAD / DTXVAD / DTX

During a phone call, usually not both parties will speak simultaneously but only one side will speak. Therefore, in the UL as well as in the DL only about half of the conversation time speech information must really be transmitted over the air interface.

With the help of discontinuous transmission (DTX; GSM 05.08 and GSM 06.31) the radio transmitter can now be switched off most of the time during speech pauses. To detect these speech pauses, DTX requires a voice activity detector (VAD; GSM 06.32) in the transmit side. DTX can be used in UL as well as in the DL direction.

Main reasons for the usage of DTX:

• To save power in the MS.

• To reduce the overall interference on the air interface in the network.

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VAD / DTXVAD / DTX

Transmitting side:

The TX DTX handler passes the traffic frames to the radio subsystem. Each frame is marked individually by a SP flag (Speech Indicator). SP flag = 1 indicates a speech frame, SP flag = 0 indicates a silence description (SID) frame which contains information on the acoustic background noise which is needed for the comfort-noise-synthesiser located in the receive side.

Speech frames will be sent each 20 ms.

SID frames will be sent each 480 ms.

Receiving side:

The RX DTX handler handles the DTX on the receive side.

Good speech frames (SID flag = 0, BFI flag = 0) are directly passed to the speech decoder.

Valid SID frames (SID flag = 2, BFI flag = 0) result in comfort noise (see GSM 06.12) generation until the next valid SID frame is detected or a good speech frame is detected.

(The SID flag is calculated from the SID frame detector (which is located in the receive side) based on the number of bit deviations within the SID code word.)

With DTX, only the transmission on air interface is interrupted. The transmission between TRAU and BTS is filled up with idle speech frames.

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VAD / DTXVAD / DTX

Transmit Side

TX Radio Subsystem

Speech encoder

TX DTX handler

Voice activity detection

Comfort Noise computation

Channel encoding

SP flag monitoring

Information bits

260

SP flag

1

SP=1: speech

SP=0: SID frame

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VAD / DTXVAD / DTX

Receive Side

RX Radio Subsystem

Speech decoder

RX DTX handler

Comfort Noise Synthesizer

Error correction &

Detection

SID frame detection

260 information bits

BFI=0, SID=0

(good speech frame)

(BFI=0, SID=2)

(valid SID frame)

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VAD / DTXVAD / DTX

DTX not applied:

4 out of 104 slots: idle

DTX applied:

12 = 8+4 out of 104 slots: not idle

Only 1 block of 456 bits (after channel decoding) is sent each 480 ms. Due to interleaving and depending on channel type, this SID information is sent in different TDMA frames using different numbers of bursts; see GSM 05.08)

Not idle slots: SACCH: 13, 39, 65, 91

SID: 52, 53, 54, 55, 56, 57, 58, 59

TDMA-frame number (FN) modulo 104

To assess quality and signal level during DTX

T T T T T T T T T T T T A T T T T T T T T T T T T -

26 TDMA frame = 120 ms

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Interference MatrixInterference Matrix

The interference matrix cij (1 ≤ i,,j ≤ N) is an NxN matrix which contains information about the minimal frequency separation required between corresponding cells i and j.

If cij = 0: co-channel reuse is allowed

If cij = 1: adjacent channel reuse is allowed

If cij = 2: neither co- nor adjacent channel reuse is allowed

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Frequency Allocation StrategiesFrequency Allocation Strategies

From mathematical point of view, frequency allocation is an optimization problem:

For a certain number of cells, frequencies have to be assigned in such a way that not only interference is avoided but also the total number of frequencies needed is as low as possible.

The effectiveness of a particular assignment (frequency plan) can be expressed by a so called cost value, a value which is tried to be minimized during allocation procedure.

Manual frequency allocation:

• In general, a manual frequency allocation is not easy.

• During manual allocation, often the available frequency band is grouped into so called

frequency groups.

• These different frequency groups are assigned to the different cells of a regular cluster.

Note: Real cell structures are mostly not regular.

☺ Advantages of the frequency group concept:

It allows an easy future addition of further TRX(s) i.e. a future capacity increase does not

require a complete re-planning of the area.

L Disadvantages of the frequency group concept:

It offers less flexibility concerning radio network optimization.

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Frequency Allocation Strategies - Examples Frequency Allocation Strategies - Examples

1. example: 4/12 reuse pattern

ð per cluster: 4 sites (with 3 sectors each)

ð 12 frequency groups: A1, B1, C1, D1, A2, B2, C2, D2, A3, B3, C3, D3

using the frequencies: 1 2 3 4 5 6 7 8 9 10 11 12

13 14 15 16 17 18 19 20 21 22 23 24

25 26 27 28 29 30 31 32 …..

A1A2

A3

B1B3

B2 C2

C3

D1D2

D3

C1

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ð per cluster: 1 site (with 3 sectors)

ð 3 frequency groups: 1 2 3

using the frequencies: 1 2 3

4 5 6

7 8 9

10 11 …

1

32

1

32

12

31

23

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ð per cluster: 1 site (with 3 sectors)

ð only 1 frequency group: 1

using the frequencies: 1 2 3 4 5 …

1

11

1

11

11

11

11

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Tool supported Frequency AllocationTool supported Frequency Allocation

Since real cell structures are mostly not regular, other approaches than assigning frequency

groups (manually) to regular clusters must be used:

Tool supported frequency allocation:

Required input data:

• Interference (frequency separation) matrix

• Number of Transceivers per cell (derived from a traffic analysis)

• Frequencies which shall not be used for assignment

Algorithms, used for frequency assignment:

• Graph colouring heuristics

• Randomized saturation degree heuristic and local search

• Neural network algorithm

• Two-phase algorithm

• (Intelligent) local search algorithm (based on combinatorial optimization theory)

• …

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The algorithms, used in the different frequency planning tools, often contain random processes which lead to slightly different results (a few percent) in case that these algorithms will be run several times.

ðA criterion is needed, which shows how good a specific assignment is. The lower bound is such a criterion. It can also be used to compare different assignment algorithms.

Tool supported Frequency AllocationTool supported Frequency Allocation

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Interference Analysis Interference Analysis

Manual interference analysis:Switching off base stations temporarily to find out interferer(s)• Absolutely not easy• In general very time consuming• In difficult situations nearly impossible to find out the interferer(s)

Interference analysis using special measurement equipment:The high performance measurement equipment allow a data capturing even during driving. The analysis of the data can be done on- or off-line. The measurement principle is based for example on the non-synchronicity of the 51 multiframes coming from different BTS. The time-offset of the different 51 multiframes coming from the different BTS is used as a fingerprint to find out the interferers.

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The 51 multiframe:

The following figure shows the 51 multiframe for a combined BCCH.

F S B C F S C C F S D0

D1

F S D2

D3

F S A0

A1

I

51 TDMA fames = 235.4 ms

D3

R R A2A3

R R R R R RR R R RR R R RR RR R R RR R R R RD0

D1

D2

D3

R R A0 A1

R R R R R RR R R RR R R RR RR R R RR R R R RD0

D1

D2

F S B C F S C C F S D0

D1

F S D2

D3

F S A2

A3

I

UL

DL

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BTS-7

BTS-5BTS-11

51 multiframe of BTS-5



T51 start time0 235400

T51

,BTS

-5

T51

,BTS

-11

T51

,BTS

-7

04 mn1790 frequency planning ww fh

Documents

path loss exponent

handover command message

fn transmitter f0

current serving cell

sector cell configurations

ma f0

adjacent channel reuse

ma f1