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22/6/27 第 1 第 Frequency Reuse
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Page 1: 6.freq reuse and network planning

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Frequency Reuse

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GSM Frequency BandGSM900

Upward 890 915 MHzDownward 935 960 MHzTx-Rx difference is 45MHz , carrier space is 200KHz 。

GSM1800Upward 1710 1785 MHzDownward 1805 1880 MHzTx-Rx difference is 95MHz , carrier space is 200KHz 。

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GSM Channel Numbering

GSM900 GSM900 : FU(n) = 890 + 0.2n MHz

FD(n) = Fu(n) + 45 MHz , 1 n 124

E-GSM900 : FU(n) = 890 + 0.2(n-1024) , 974 n 1023

FD(n) = Fu(n) + 45 MHz

n is called Absolutely Radio Frequency Channel Number. GSM1800

Fu(n) = 1710.2 + 0.2(n-512) MHz

FD(n) = Fl(n) + 95 MHz , 512 n 885

374 channels

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Frequency ReuseFrequency reuse is referred to as the same frequency is

reused after a certain distance in cellular wireless system. Generally, a limited frequency band is divided into many groups, each containing a few carriers, which are in turn assigned to adjacent cells.

Usually, frequency reuse patterns include:

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Frequency Reuse

Ordinary frequency reuse: “43”, “33” and more close “23” and “13”.

Dual frequency reuse: BCCH and TCH adopt different frequency reuse patterns respectively.

Concentric: the Underlay and Overlay adopt different frequency reuse patterns respectively.

MRP: different layers adopt different reuse patterns.

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M

U

L

T

I

P

L

E

L

A

Y

E

R

BCCH

TCH1

TCH2

TCHn 层

TCHn+1 层

TCHn+2 层

900 micro cell

Conventional

Superlayer

FH Group

FH Group

MRP

900macro cell 1800 macro cell

1800 microcell

Dual band

Network Topology

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43 Frequency Reuse

“4 3” frequency reuse pattern is that each site is divided into 3 sector. 12 frequencies form a group which are distributed to 4 different sites. Each site owns 3 frequencies.

A3

D2B1

C3

B2D1

D3

A2C1

B3

C2A1

B3

C2A1

A3

A1B1

D1

D3D2

C3

B2A1

C3D2

C3

C1

D2B1C2A1

A2C1

D3

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33 Frequency Reuse

“3 3” frequency reuse pattern is that each site is divided into 3 sector. 9 frequencies form a group which are distributed to 3 different sites. Each site owns 3 frequencies.

A3

C2B1

B3

A2C1

C3

B2A1

A3

C2B1

B3

A2C1

B3

A1C1

A1

A3A2

C3

B2A1

A3A3

C3

C1

B2A1B2A1

A2C1

B3

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33 Frequency Reuse

A1 B1 C1 A2 B2 C2 A3 B3 C3

1 1 2 3 4 5 6 7 8 9

2 10 11 12 13 14 15 16 17 18

3 19 20 21 22 23 24 25 26 27

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Frequency Reuse Distance

The following equation is used to estimate frequency reuse distance:

D —— frequency reuse distance R —— cell radius N —— frequency reuse factor. N=9 for “33”;

N=12 for “4 3”For “3/9” frequency reuse, D=5.2RFor “4/12” frequency reuse, D=6R

RND *3

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Definition of C/I and C/A

Co-channel Interference C/I:C/I refers to the interference of another cell using the same frequency to the current cell. The ratio of carrier to interference is called C/I.

GSM specification regulates that C/I >9dB. In implementing, it requires C/I>12dB.

Adjacent channel interference C/A

C/A refers to interference of adjacent cell to the current cell. The ratio is called C/A. The GSM specification regulates that C/A>-9dB.

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Calculation of C/I

Where, Pown_cell is the signal strength of current cell; Pi_BCCH is BCCH signal strength of interfering cell i measured by MS.

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22

2

2

2

2

2 2 22

22

2

2

2

Concentric

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Concentric( 2)

The principle of ordinary concentric is that a cell is divided into underlay and overlay, both having the same site and sharing the same TRXs, antenna and BCCH. The coverage of overlay is the same as that of ordinary cell, while the underlay use small transmitting power and thus has smaller coverage. The frequency reuse factor of overlay differs from that of underlay. The former adopts frequency reuse pattern of 4 3, while the latter adopts closer pattern of 3 3, or 2 3. The BCCH and SDCCH are used by overlay, in which the call will be set up. As a cell is subdivided into two layers, a brand new switching algorithm should be added. Inter-cell hand-over is conducted in overlay,while overlay-underlay and intra-underlay is added to intra-cell hand-over.

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Concentric( 3)

The absorbing of traffic by underlay is limited by traffic lay-out and coverage. It will increase the capacity by 10-30% and is related to traffic distribution. The underlay is more suitable for outdoor traffic concentrated in the vicinity of BTSs because of its smaller transmitting power.

More attention should be paid when network planning is carried out using concentric technology. On the condition that the network quality will not be affected, a closer frequency reuse pattern should be adopted. At the same time, DPC and DTX technology should be adopted to ensure network quality.

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C/I-based Concentric IUO

222

222

222

222

2

2

2

2

2

22

2

2

2

2

2

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IUO (2)

IUO has the same network structure as ordinary concentric, consisting of Overlay and Underlay. In general, the two layers share the same site and antenna. Similar to ordinary cell, the Overlay adopts 4*3 frequency reuse pattern, while the Underlay adopts closer pattern. Like general macro cell, the Overlay realize continuous covering mainly for MSs around boundaries of cells. The Underlay realize discontinuous covering, providing services mainly for MSs located nearby the sites and inside buildings with strong isolation. Like ordinary concentric, the control channel functions within Overlay where calls are established.

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IUO( 3)

Different from ordinary concentric, the Underlay and Overlay of IUO both use the same transmitting power. Because the Underlay adopts closer frequency reuse pattern, the chances of co-channel and adjacent channel interference increase. So, IUO adopts a handing over algorithm based on C/I to solve this problem. It functions as follows: the call is established within Overlay, the BSC continuously monitor the downward C/I in Underlay. When C/I reaches applicable threshold, the call will be handed over to Underlay. At the same time, the C/I is monitored continuously and the call will be handed over to Overlay if it deteriorates to inapplicable threshold.

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IUO( 4)

The implementation of IUO is easy. Little modification is needed and no special requirement to MSs. Because of adopting special algorithm on handing over, measurement and estimation on C/I, the speech quality can be guaranteed while increasing the system capacity. Because both Underlay and Overlay have the same transmitting power, it’s very suitable for absorbing traffic inside building which has a strong isolation capability to radio signal. When implementing the network, the C/I based handing over parameter should configured correctly. The DPC and DTX should also be considered.

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Capacity of C/I Based Concentric Technology

Capacity comparison with “4x3”pattern:Grade of Service (GOS): 2 %Per subscriber traffic at busy hour: 0.03 ErlBandwidth: 6MOverlay adopts “4x3” pattern

0 500 1000 1500 2000 2500 3000 3500 4000 4500

Standard 4X3

Underlay 3x3

Underlay 2x3

Underlay 1x3

Subsc./bts2TRXs Overlay1TRXs Overlay All overlay

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Set-forth of MRP(1)

The adoption of frequency reuse pattern depends mainly on the condition of sites. In reality, there are great differences in antenna height, earth shape and coverage. Moreover, different cells have different traffic requirement, the number of TRXs needed is also different. So, it’s unreasonable to adopts idealized and regular frequency reuse pattern indiscriminately, which will cause low frequency utilization rate and difficult to ensure the C/I requirement.

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Set-forth of MRP(2)

So, in regard to real wireless network situation, dynamic assigning frequency to cells according to their traffic is the developing trend of frequency reuse planing. Multiple frequency Reuse Pattern (MRP) is an irregular and dynamic frequency reuse mode designed according to GSM features.

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Principle of MRP

MRP is an irregular frequency reuse mode utilizing irregularity and frequency hopping technology of GSM system. That is, dividing frequency band into different combinations, each combination adopting different frequency reuse pattern. Each carrier of a cell may adopts different frequency reuse pattern.

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TCH2 FRF=6

BCCH FRF=12

TCH1 FRF=9

For Microcell

MRP

FRF:Frequency Reuse Factor

Bandwidth=6 MHz

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MRP Characters

Increase network capacity greatly and decrease average frequency reuse factor to 8, thus increasing frequency reuse efficiency greatly.

Flexible channel allocation. Different frequency reuse pattern may be adopted step by step according to capacity requirement.

Some carriers can be released for micro-cells. Should combine with FH, DPC and DTX to lower

interference. Especially for sites distributed unevenly.