3.4  Normal Frequency Reuse Technology

3.4.1  C/I under 4 x 3 Frequency Reuse Pattern

The spectrum utilization ratio can be expressed by frequency reuse degree, which reveals the aggressiveness of the frequency reuse. The frequency reuse degree can be expressed by the following equation: freuse=NARFCN / NTRX

Here NARFCN is the total number of the available channel numbers, and NTRX is the number of TRXs configured for the cell.

For the n x m frequency reuse pattern, "n" indicates the number of the base stations in the reuse clusters, and "m" indicates the number of the cells under each base station. In this case, the frequency reuse degree can be expressed by the following equation:

freuse= n x m

In actual planning, however, the allocated number of channel numbers will be greater than n x m, so the actual  freuse  is usually greater than n x m. Therefore, the smaller the freuse, the more aggressive the frequency is reused and the higher the frequency utilization ratio is. As the aggressiveness of the frequency reuse grows, however, it will bring greater interference to the network. In this case, you must enable the technologies, including DTX and power control, to solve this problem. The more aggressive the frequency is reused, the lower the spectrum utilization ratio is, but the conversation quality is better at this time.

 
The purpose the frequency planning is to reach a balance between the frequency utilization ratio and the network capacity. Based on the assurance of the network quality, you must take measures to maximize the network capacity.

In the GSM system, the 4 x 3 frequency reuse pattern is in basic use. Here "4" indicates 4 base stations (each base station consists of 3 cells), and "3" indicates the 3 cells under the control of each base station. Therefore, there are 12 sectors are available. And the 12 sectors makes up of a frequency reuse cluster, but the frequency in the same cluster cannot be reused.

For the 4 x 3 frequency reuse pattern, the intra-frequency spacing is great, so it can meet GSM system's requirement on the intra-frequency interference protection ratio and adjacent frequency interference protection ratio. As a result, this frequency reuse pattern is good for the network quality and security. Under the 4 x 3 frequency reuse pattern, the frequency reuse aggressiveness is 12.

For the aggressive reuse introduced hereunder, because the BCCH plays an important role in the network and you cannot use the apply the anti-interference measures, such as downlink power control and DTX, to the BCCH, you must apply the 4 x 3 frequency reuse pattern or looser reuse patterns to the BCCH carriers.

 Normal 4 x 3 frequency reuse pattern.

3.4.2  10MHz Bandwidth 4 x 3 Frequency Reuse

Hereunder are several assumptions:

l           The available bandwidth is 10MHz.

l           The channel number is 45–94.

l           If the channel numbers ranging from 81–94 (14 channel numbers in total) are allocated to the BCCH, and the other channel numbers are allocated to TCH.

If the previous assumptions are present, the frequency planning under 4 x 3 frequency reuse pattern is provided in Table:

 Frequency planning under 4 x 3 frequency reuse pattern   (a)

Frequency group number

A1

B1

C1

D1

A2

B2

C2

D2

A3

B3

C3

D3

Channel number of each frequency group

94

93

92

91

90

89

88

87

86

85

84

83

80

79

78

77

76

75

74

73

72

71

70

69

68

67

66

65

64

63

62

61

60

59

58

57

56

55

54

53

52

51

50

49

48

47

46

45

 

According to this table, the channel numbers in the first line are BCCH numbers, in which the channel numbers 81 and 82 are standby channel numbers. The channel number of BCCH of the cell A1 is 94. It is 80, 68 and 56 for other carriers, and so on.

In a cluster which contains 12 cells, the frequency group for base station A is {A1, A2, and A3}; the frequency group for base station B is {B1, B2, and B3}; the frequency group for base station C is {C1, C2, and C3}; and the frequency group for base station D is {D1, D2, and D3}.

Therefore, as listed in this table, no channel number is reused within a cluster. In addition, the intra-frequency and adjacent frequency are not available for the adjacent cells and the same cell.

However, the drawbacks of this frequency reuse pattern are that the frequency reuse ratio is low and the capacity expansion needs a great amount of the frequency resources. Therefore, this reuse pattern is not used in the areas where the network capacity needs to be constantly expanded.

If the bandwidth is 10MHz, the maximum base station configuration is S4/4/4 under the normal 4 x 3 frequency reuse pattern, and the frequency reuse degree is 12.5 (50/4 = 12.5).

&  Note:

The maximum base station type mentioned in the chapter refers to the configuration type that most continuous base stations can reach. It does not include standalone base station.

 

3.4.3  19MHz Bandwidth 4 x 3 Frequency Reuse

For the 19MHz frequency (1 to 94) used by China Mobile, the 4 x 3 frequency reuse pattern are used for the frequency planning. The channel numbers ranging from 79 to 94 (16 channel numbers in total) are allocated to the BCCH, and other channel numbers are allocated to TCH. No channel number is reserved for micro cells. In this case, the frequency planning solution is provided in Table:

  Frequency planning under 4 x 3 frequency reuse pattern   (b)

Frequency group number

A1

B1

C1

D1

A2

B2

C2

D2

A3

B3

C3

D3

Channel number of each frequency group

94

93

92

91

90

89

88

87

86

85

84

83

78

77

76

75

74

73

72

71

70

69

68

67

66

65

64

63

62

61

60

59

58

57

56

55

54

53

52

51

50

49

48

47

46

45

44

43

42

41

40

39

38

37

36

35

34

33

32

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

 

 

 

 

 

 

 

As listed in this table, the channel numbers ranging from 79 to 82 are standby channel numbers. For the 19MHz bandwidth, the maximum base station type can be S8/7/7 under 4 x 3 frequency reuse pattern. The frequency reuse degrees are 11.75, 13.43, and 13.43, so the average value is 12.87.

3.4.4  6MHz Bandwidth 4 x 3 Frequency Reuse

For the 6MHz frequency (96 to 124) used by China Unicom, the 4 x 3 frequency reuse pattern is used for the frequency planning. The channel numbers ranging from 111 to 124 (14 channel numbers in total) are allocated to the BCCH, and other channel numbers are allocated to TCH. No channel number is reserved for micro cells. In this case, the frequency planning solution is provided in:

  Frequency planning under 4 x 3 frequency reuse pattern   (c)

Frequency group number

A1

B1

C1

D1

A2

B2

C2

D2

A3

B3

C3

D3

Channel number of each frequency group

124

123

122

121

120

119

118

117

116

115

114

113

110

109

108

107

106

105

104

103

102

101

100

99

98

97

96

 

 

 

 

 

 

 

 

 

 

As listed in this table, the channel numbers ranging from 111 to 112 are standby channel numbers. For the 6MHz bandwidth, the maximum base station type can be S3/2/2 under 4 x 3 frequency reuse pattern. The frequency reuse degrees are 9.67, 13.5, and 13.5, so the average value is 12.22.

3.4.5  4 x 3 Frequency Reuse Conclusion

The 4 x 3 frequency reuse pattern is a basic technology applied in frequency planning. It is applicable to other frequency aggressive reuse technologies that are used for the BCCH.

Theoretical analysis shows that when the base stations are regularly distributed and azimuths of the cells are consistent with each other, the interference can be reduced to the minimum. Therefore, if you intend to expand the network capacity, you can keep the base stations to be distributed as regular as possible and plan the azimuths of the cells along the same direction. In addition, you can also maintain the antennas at a similar height. However, sometimes you need to adjust the azimuth of the antenna to improve the coverage, which seems contradicts to the capacity expansion. Therefore, sometimes you must make find a balance between the coverage and capacity.

If the network capacity needs to be further expanded, you can take the following measures:

l           Split a cell into smaller cells. At present, however, the average coverage radius of the macro cell base stations in urban areas is already shorter than 500m, so further cell splitting will meet difficulty in cost and technology.

l           Utilize new frequency resources. For example, you can employ the 1800MHz band to establish a DSC 1800MHz network.

l           Under the current 900MHz network, use more aggressive frequency reuse technology to expand the network capacity.

At present, the aggressive frequency reuse technology works as the most economical and convenient way to expand the network capacity, so it is also the most popular with carriers.

The typical frequency reuse technology includes 3 x 3, 2 x 6, 2 x 3, 1 x 3, and 1 x 1.

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