1. Frequency Planning
Frequency Planning
Abstract
This is a technical document detailing a typical approach to Frequency Planning Process.
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2. Frequency Planning
CONTENTS
Frequency Planning
(1.0) Introduction Page 3
(2.0) Frequency Re-use Page 4
(3.0) Co-channel Interference and System Capacity Page 5
(4.0) Design Criterion Page 6
(4.1) Example Page 7
(5.0) Frequency Channel Allocation Page 7
(5.1) Example Page 7
(6.0) BSIC Planning Page 8
(6.1) Example Page 8
(7.0) Automatic Frequency Planning Page 9
(8.0) Frequency Hopping Page 9
(8.1) Frequency Hopping Techniques Page 10
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3. Frequency Planning
Frequency Planning
(1.0) Introduction:
The Cellular concept is a system with many low power transmitters, each providing coverage
to only a small portion of the service area. Each base station is allocated a portion of the total
number of channels available to the entire system, and nearby base station are assigned
different group of channels so that the interference between base stations is minimised. The
channels assignment in case of GSM900, E-GSM900 and DCS1800 (or GSM1800) is as
shown in Figure-(1.1) below,
45 MHz
45 MHz
880 890 915 925 935 960
GSM900 GSM900
UPLINK DOWNLINK
E-GSM900 E-GSM900
UPLINK Guard Band DOWNLINK
95 MHz
1710 1785 1805 1880
DCS1800 DCS1800
UPLINK DOWNLINK
Fig.- (1.1) Channels Assignment
As shown the Uplink and Downlink band are separated by 20 MHz of guard band in case of
GSM and DCS and 10 MHz in case of E-GSM. The channel separation between Uplink and
Downlink is 45 MHz in case of GSM and E-GSM and is 95MHz in case of DCS network. Each
channel(carrier) in GSM system is of 200 KHz bandwidth, which are designated by Absolute
Radio Frequency Channel Number (ARFCN). If we call Fl(n) the frequency value of the carrier
ARFCN n in the lower band(Uplink), and Fu(n) the corresponding frequency value in the
upper band (Downlink), we have:
GSM 900 Fl(n) = 890 + 0.2*n 1 ≤ n ≤ 124 Fu(n) Fu(n) = Fl(n) + 45
E-GSM 900 Fl(n) = 890 + 0.2*n 0 ≤ n ≤ 124 Fu(n) Fu(n) = Fl(n) + 45
Fl(n) = 890 + 0.2*(n-1024) 975 ≤ n ≤ 1023
DCS 1800 Fl(n) = 1710.2 + 0.2*(n-512) 512 ≤ n ≤ 885 Fu(n) = Fl(n) + 95
Table (1.1) ARFCN
Hence we have 124 channels in GSM900, 174 channels in E-GSM900 and 374 channels in
DCS1800.
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4. Frequency Planning
(2.0) Frequency Re-use:
One important characteristic of GSM networks is frequency planning wherein given the limited
frequency spectrum available, the re-use of frequencies in different cells is to be planned
such that high capacity can be achieved keeping the interference under a specific level.
A cell in a GSM system may be omni-directional or sectored represented by hexagons. In
GSM system a tri-sectored cell is assumed and the frequency plan is made accordingly. To
understand the frequency re-use planning, consider a GSM system having S channels
(ARFCN’s) allocated, wherein each cell (sector) is allocated k channels, assuming that all
three sectors have same number of k channels. If the S channels are divided among N base
stations each having three sectored cell, then the total number of available radio channels
can be expressed as,
S = 3kN
This explains N base stations each having three sectors and each sector having k channels.
The N base stations, which collectively use the complete set of available frequencies, in
which each frequency is used exactly once is called a Cluster. If the cluster is replicated M
times then the total number of channels, C, can be used as measure of capacity and is given
by,
C = M3kN = MS
The Cluster size N is typically equal to 3, 4, 7, or 12. Deciding a cluster size posses a
compromise between capacity, spectrum allocated and interference. A cluster size of 7 or 12
gives least interference frequency plan but as the cluster size is big enough hence re-use at
far away distance hence lesser capacity and would also require bigger frequency spectrum.
Consider an example where k equals 1 that is one frequency per sector. With a cluster size of
7 would require minimum spectrum of,
S = 3 x 1 x 7 = 21 ARFCN
or
21 x 0.2 MHz = 4.2 MHz of spectrum that is about 16% of total available spectrum in
GSM900.
Adding one more frequency per sector would take the requirement to 42 ARFCN or 33% of
total spectrum. On the other hand a cluster size of 3 would require (k = 1),
S = 3 x 1 x 3 = 9 ARFCN
or
9 x 0.2 MHz = 1.8 MHZ which is about 7% of total spectrum available.
Addition of one more frequency still results in about only 14% of spectrum required. But here
a big compromise is made on interference, as the cells are quite closely located hence re-use
would pose a major problem. Studies have revealed that cluster size of 4 gives the best
balance between capacity & interference, with k equal to 2 meaning two frequencies per
sector gives,
S = 3 x 2 x 4 = 24
or
24 x 0.2 MHz = 4.8 MHz that is about 19% of total spectrum available.
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5. Frequency Planning
Figure 1.2 illustrates the frequency reuse for cluster size of 4, where cells labelled with the
same letter use the same group of channels.
B1
B3
D1 A1
B2
D3 A3
B1 C1 B1
D2 A2
B3 C3 B3
D1 A1 D1
B2 C2 B2
D3 A3 D3
C1 B1 C1
D2 A2 D2
C3 B3 C3
A1 D1 A1
C2 B2 C2
A3 D3 A3
B1 C1 B1
A2 D2 A2
B3 C3 B3
D1 A1 D1
B2 C2 B2
D3 A3 D3
C1 C1
D2 A2 D2
C3 C3
C2 C2
Fig.- (1.2) 4 x 3 Re-use pattern
(3.0) Co-channel Interference and System capacity:
Frequency re-use implies that in a given coverage area there are several cells that uses the
same set of frequencies. These cells are called co-channel cells and the interference between
signals from these cells is called co-channel interference. Unlike thermal noise which, can be
overcome by increasing the S/N ratio, co-channel interference cannot be combated by simple
increase in carrier power. This is because an increase in carrier power increases the
interference to neighbouring co-channel cells. To reduce co-channel interference, co-channel
cells must be physically separated by a minimum distance in order to provide sufficient
isolation due to propagation.
In a cellular system where the size of each cell is approximately the same, co-channel
interference is independent of the transmitted power and becomes the function of the radius
of the cell (R), and the distance to the centre of the nearest co-channel cell (D). Figure- (1.3)
explains the relation between the cell radius R, cluster size N and the re-use distance D,
Here,
Outer Cell radius: R
Inner Cell radius: r = 0.5 x (3)1/2 x R.
Re-use distance: D = R x (3 x (i2 + j2 + ij))1/2
D/R = (3 x N)1/2
The Cluster size, N = (i2 + j2 + ij)
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6. Frequency Planning
D j
i D
j
R i
r
Fig.- (1.3) Re-use distance calculation.
Where i and j are non-negative numbers. To find the nearest co-channel neighbour of a
particular cell, one must do the following: (1) move i cells along any chain of hexagons and
then (2) turn 60 degrees counter-clockwise and move j cells. This is illustrated in the figure
above for i = 1 & j = 2 for a cluster size of 7.
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,
(D/R)γ = 6 (C/I)
(Note: C/I is in dB and should be converted to numeric values for calculation)
Here, γ is the propagation index or attenuation constant with values ranging between 2 to 4.
(4.0) Design Criterion:
An optimal frequency plan requires minimal interference between co-channel and adjacent
channel cells, GSM Rec. 05.05 has defined the interference ratios for co-channel and
adjacent channel cells. The actual interference ratio shall be less than a
specified limit, called the reference interference ratio. The reference interference ratio shall be
for base station and all types of MS,
- for cochannel interference : C/Ic = 9 dB
- for adjacent (200 kHz) interference : C/Ia1 = - 9 dB
- for adjacent (400 kHz) interference : C/Ia2 = - 41 dB
For the network planning purpose it is recommended that a value of C/I c ≥ 9 dB and the first
adjacent channel C/Ia ≥ -9 dB. This implies that the first adjacent channel should not be used
in the same sector cell or the same base station.
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7. Frequency Planning
(4.1) Example:
As an illustration let us consider that we require to design a system with C/I of 12 dB and we
have from field drive test results the value of γ as 3.5, inserting these values in equation
(D/R)γ = 6 (C/I) we have,
(D/R)3.5 = 6 x 10.78 = 64.75
3.5 Log(D/R) = Log(64.75) = 1.81
(D/R) = Antilog(1.81/3.5)
This gives (D/R) = 3.29.
With this we can back calculate the required cluster size from equation D/R = (3 N)1/2 as,
N = (3.29)2 / 3 = 3.61
Hence a cluster size of 4 will satisfy our required C/I criteria rather if we back calculate for
Cluster of size 4 then we get C/I of 19dB.
(5.0) Frequency Channel Allocation:
In GSM systems we divide the total allocated spectrum into two sub-groups one for Control
information with traffic referred to as BCCH frequency and other only for traffic referred to as
TCH (or non-BCCH) frequency. In case where the network has Microcells then the total band
allotted is divided for BCCH and TCH, wherein each band is further sub-divided for
Macrocellular & Microcellular applications. Figure (1.3) explains the concept,
TCH BCCH TCH
Macro Cell Micro Cell
Fig.- (1.3) Frequency band allocation.
The re-use may differ for both the groups, as little or no compromise is made for BCCH
frequency interference whereas certain compromise could be made for TCH frequency
interference. Typically a cluster size of 4 or 7 is considered for BCCH re-use whereas a
cluster size of 3 or 4 is used for TCH re-use. The number of channels in each group depends
on the spectrum allocated and C/I criteria for re-use in each case.
(5.1) Example:
As an example consider C/I criteria of 12 dB for BCCH then the cluster size of 4 gives the
better result whereas if the C/I criteria is 9 dB for TCH, gives the cluster size of 3. The figure
(1.1) illustrates the case 4 x 3 re-use pattern for BCCH and the figure (1.4) below illustrates
the case of 3 x 3 re-use pattern for TCH,
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8. Frequency Planning
A1 A1 A1
A3 A3 A3
C1 C1 C1
A2 A2 A2
C3 C3 C3
B1 B1 B1
C2 C2 C2
B3 B3 B3
A1 A1 A1
B2 B2 B2
A3 A3 A3
C1 C1 C1
A2 A2 A2
C3 C3 C3
C2 C2 C2
Fig.- (1.4) 3 x 3 re-use pattern.
A1 B1 C1 D1 A2 B2 C2 D2 A3 B3 C3 D3
BCCH 1 2 3 4 5 6 7 8 9 10 11 12
A1 B1 C1 A2 B2 C2 A3 B3 C3
TCH 13 14 15 16 17 18 19 20 21
For DCS1800 planning with cluster size of 7 the frequency grouping is as follows,
In case of DCS1800 where a large band of spectrum is available the BCCH and TCH re-use
can be kept the same.
Set A1 B1 C1 D1 E1 F1 G1 A2 B2 C2 D2 E2 F2 G2 A3 B3 C3 D3 E3 F3 G3
BCCH 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
TCH1 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42
TCH2 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63
(6.0) BSIC Planning:
In addition to the assignment of frequency group to a cell, a Base Station Identity Code
(BSIC) must be assigned in association with the frequency group. This will eliminate the
possibility of incorrect cell identification and will allow the evolution to future cell architecture.
The BSIC is a two-digit code wherein the first digit is indicates NCC (Network Colour Code)
and the second digit indicates BCC (Base Station Colour Code). The NCC and BCC have
values ranging from 0 to 7, where the NCC is fixed for an operator, signifying at any given
point there can be maximum of 8 operators in an area. The BCC defines the cluster number
which means a group of 8 clusters carry unique identity which are re-used for another group
of 8 clusters and so on. The principal for allocation of the BSIC is the same as for the RF
carriers but at cluster level rather than cell level. The concept can be understood in the
following example,
(6.1) Example:
Assume a network with 100 base stations each having three sectors. The BCCH and TCH
share the same re-use plan 4 x 3. Which means we have cluster of 4 base stations, and in all
we have 100/4 = 25 clusters. Assume NCC code allocated is 6, which gives us clusters
starting from number 61 to 67. Hence seven clusters form a group and hence we have 25/7
that is 3 groups of 7 clusters plus additional 4 clusters which form part of the 4 th group. The
reuse of these 7 clusters group for BSIC numbered from 61 to 67 is shown in the figure (1.5)
below,
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9. Frequency Planning
Represent a
cluster of 4 sites
each having 3
sectors
62
62 67 63
67 63 61
61 66 64
66 64 65
65 62
62 67 63
63 61
61 66 64
64 65
Fig.-(1.5) BSIC 7 re-use cluster plan.
It should be noted that since BSIC are defined at cell (sector) level, hence there are every
possible chances that the three sectors within the same site can have different BSIC. The
reason being as BSIC is used for cell identification hence cells with same BCCH frequency
but different BSIC can be easily discriminated by the MS.
(7.0) Automatic Frequency Planning:
Automatic frequency planning is an feature offered by the planning tools to speed up the work
of channel assignment and presents more reliable frequency assignment to sites. AFP
(Automatic frequency planning) works on complex algorithm whose calculations are based on
the interference table data, field strength grids and an optional demand density grid (or traffic
distribution table). It allows human interaction at certain points such as assigning penalties to
different clutter types or allowing interference results to be neglected especially in coverage
boundaries of the network. AFP is of immense help and provides guidelines in the cases
where frequency assignment is required for big complex network. Basic frequency planning
tool is a standard feature of all available planning tools, however the advanced AFP tool
based on complex algorithm is provided as an optional feature.
(8.0) Frequency Hopping
The principle of Frequency Hopping used within GSM is that successive TDMA bursts of a
connection are transmitted via different frequencies-the frequencies belonging to the
respective cell according to network planning. This method is called Slow Frequency Hopping
(SFH) since the transmission frequency remains constant during one burst. In contrast to Fast
Frequency Hopping (FFH) where the transmission frequency changes within one burst.
The effect of frequency hopping is that link quality may change from burst to burst, ie a burst
of high BER may be followed by a burst of low BER, since
• Short term fading is different on different frequencies,
• The interference level is different on different frequencies.
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10. Frequency Planning
The results of frequency hopping are improvement in the received quality in fading situation
and interference averaging.
(8.1) Frequency Hopping Techniques:
The hopping techniques can be broadly classified into two main categories. They are,
• Base band Hopping
• Synthesised Hopping
As Frequency Hopping is a subject in it self, a separate document will be written
concentrating on Frequency Hopping Techniques in near future.
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