Chapter6 Frequency Planning and Anti-Interference Technology

Wireless Network Planning Table of Contents

Table of Contents

Chapter 6 Frequency Planning and Anti-interference Technology.........................................6-1

6.1 Frequency planning.............................................................................................6-1

6.1.1 Frequency division and C/I requirements..............................................................6-1

6.1.2 Principles of the frequency planning......................................................................6-3

6.1.3 Basic frequency reuse...........................................................................................6-4

6.1.4 Compact frequency reuse.....................................................................................6-66.2 Anti-interference technology..........................................................................................6-16

6.2.1 Frequency hopping technology............................................................................6-16

6.2.2 Power control.......................................................................................................6-21

6.2.3 Discontinuous transmission.................................................................................6-23

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Wireless Network Planning Chapter 6 Frequency Plan and Anti-interference Technology

Chapter 6 Frequency Planning and Anti-interference Technology

6.1 Frequency Planning

Today, for the development of the cellular system, its capacity is limited by the given frequency bandwidth. The frequency may satisfy the requirements of the capacity in a certain area only by reusing. However, the frequency reuse, especially the compact frequency reuse pattern, will certainly cause the problem how to reduce the common adjacent frequency interference with which we must be faced: in the equivalent areas, the more incompact the frequency reuse interval is, the less the common adjacent frequency interference is, with less capacity; with more compact frequency reuse, the capacity is promoted to a certain content, but with the promotion of the common adjacent frequency interference. How to obtain the balance between the capacity and the voice quality is the problem that must be settled by the frequency plan, in other word, a good frequency plan may realize the promotion of the network capacity on the basis of maintaining a good voice quality.

6.1.1 Frequency Division and C/I Requirements

The cellular system is generally divided into the GSM900M and DCS1800M systems with the carrier frequency interval of 200 kHz, based on the frequencies. The division of its uplink and downlink frequencies is as follows:

Network type Frequency bandwidth (uplink/downlink)(MHz)GSM900 890～915/935～960DCS1800 1710～1785/1805～1880

I. GSM900

It has total 124 frequency bands, the sequence numbers (ARFCN) are 1-124, with 200kHz of protective band on each end. According to the national regulation, the Mobile occupies 890-909/935-954MHz, while Unicom occupies 909-915/954-960MHz. The relation between the frequency and the sequence number (n) is as follows:

Base station receiving: f1(n)＝890. 2＋(n-1)×0. 2 (MHz)

Base station sending: f2(n)＝f1(n)＋45 (MHz)

II. DCS1800

It has total 374 frequency bands, the sequence numbers (ARFCN) are 512-885. The relation between the frequency and the sequence number (n) is as follows:

Base station receiving: f1(n)＝1710. 2＋(n-512)×0. 2 (MHz)

Base station sending: f2(n)＝f1(n)＋95 (MHz)

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The Mobile occupies 1710MHz-1720MHz, with the corresponding frequency sequence numbers of 512-561; while the Unicom occupies 1745 MHz-1755MHz, with the corresponding sequence numbers of 687-736.

III. C/I requirements

In the cellular system, since the frequency resource is limited, the replicated use of the frequency is an effective approach for promoting the frequency availability. The replicated use of the frequency would cause the interference with each other, so-called common frequency interference. The closer the interval between both frequencies is, the higher the frequency availability, but with higher interference. Figure 6-1 describes the distribution of the interference. For the convenience of analysis, the cell is indicated by the regular hexagon. In the figure, D is the reuse interval, and [fn] is the reuse frequency. The strength of the interference power is dependent on the effective emission power, reuse interval and path fading. By the derivation, the reuse frequency cluster number K is obtained:

K=i2+ij+j2

where i, j are integers.

Figure 6-1 Interference schematic diagram

Let:

q DR 3 K (1)

then the analysis of the common frequency interference is:

CI C

Ik (2)

where Ik is number k interference signal, K=1,2,. . . N.

The above expression may also be:

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CI 1

(qk )

(3)

where qk is number K common frequency interference factor, is the path fading offset determined by the actual geographic environment, in the mobile environment,

the path fading value =3-5, generally it is 4.

For the base station with the omnidirectional antenna, the first level of the interference source includes six (since the interference over the second level is small, it may be omitted), if the conditions of the six interference cells are the same, the one with the largest interference is to be taken into consideration, then

q 1 (6 C

I )1

(4)

For the 120 degrees of directional base station, it is theoretically thought that there are two interference sources, however, considering the influences of the antenna side and back lobes, the interference sources are still calculated by six (the worst condition), it can be obtained from the expression (4)

CI (q 1 )

6 (5)

The relation between C/I and q can be obtained from the above expression, and further the relation between C/I and K. If the cellular arrangement is not proper, each base station will undertake more interference sources.

GSM is an interference limited system, according to the demodulation requirements of the signal in the air interface, GSM specifies that the common adjacent frequency protective ratio should meet the following requirements:

Common frequency C/I: C/I≥9dB; it has 3dB allowance in engineering, i. e. , C/I≥12dB;

Adjacent frequency suppression ratio: C/A≥-9dB; it has 3dB allowance in engineering, i. e. , C/A≥-6dB

Second adjacent suppression ratio: C/A2≥-41dB.

6.1.2 Principles of the Frequency Planning

A good network structure is the basis of a good frequency planning. When the frequency plan is carried out in a certain area, it is done in a geographic division mode, but some frequency bands must be reserved (when the frequency is sufficient for use) at the division junctures or the frequency bands are to be divided. The selection of the juncture should be away from the hotspot area or complicated networking area. Generally, the plan is carried out beginning from the place where the base stations are closely packed. Due to the irregularity of the site distribution, it is difficult to guarantee that the frequencies of the same level of carrier frequencies can be planed fully according to the common pattern of 4×3 or 3×3, it is needed to be adjusted according to the actual situations. Whatever patterns may be used for the frequency plan, the following principles should be observed:

(1) It is not allowed that there are cofrequency frequency bands in the same base station;

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(2) In the same cell, the frequency interval between BCCH and TCH is preferably over 400K;

(3) When the frequency hopping is not used, the frequency interval between TCHs in the same cell is preferably over 400K;

(4) In non 1×3 reuse pattern, the direct adjacent base stations should avoid cofrequency (even though the directions of their antenna main lobe are different, the influences of the side lobes and back lobes may be difficult to be estimated due to the reasons of the antenna and environment);

(5) Considering the complexity of the suspending antenna and propagation environment, the base stations with closer distance should prevent from the opposite cofrequency (including diagonal opposite) as possible;

(6) Generally, for the 1×3 reuse, it is ensured that the frequency hopping band should double the number of the hopping carrier frequencies or over;

(7) Pay attention to the cofrequency reuse, the case that there are the same BCCH and the same BSIC in the adjacent areas should be avoided.

(8) Enabling the PBGT handover, after the adjacent frequency suppression ratio is determined via the parameter adjusting, the adjacent frequency may be employed in the direct adjacent opposite cells. Since the PBGT handover algorithm is judged based on the BCCH field density (path dissipation), when the BCCH frequency and the TCH frequency are alternatively allocated, after the downlink power control is enabled, the new adjacent frequency interference problem may be occur: for example, when a mobile phone makes a call on the number 50 TCH of the cell A, the BCCH of its adjacent cell B is 51, and the PBGT handover may not be occur because the field density is less than that of the cell A; however, the downlink timeslot of the call on the number 50 of the cell A may be less than BCCH of its adjacent cell B because of the power control, which causes the new adjacent frequency interference. The solution is: respectively allocating the independent frequency range to the BCCH frequency and the TCH frequency without any interleaving as possible; reducing the overlapped coverage between the cells; properly increasing the expected level of the downlink power control.

6.1.3 Basic Frequency Reuse

The most basic frequency reuse pattern of GSM is the 4×3 frequency reuse, which is the basis of other frequency reuse patterns. "4" refers to 4 sites, "3" indicates that each site has 3 cells, total 12 cells are the frequency cluster. For various cells in the same cluster, their frequencies are different. Figure 6-2 shows the 4×3 frequency reuse cell cluster.

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Figure 6-2 Basic frequency reuse 4×3 reuse cluster

For the 12 cells shown in the figure, their frequencies are different, covering other cells in the figure, and reusing one grouplink of frequencies in the 12 frequency cluster. An example is used to describe the 4×3 frequency reuse. It is assumed that the available bandwidth is 12. 2MHz, the channel number is 34-95, the assignment of the 12 channel groups is shown in Table 6-1.

Table 6-1 4×3 frequency reuse assignment table

A1 B1 C1 D1 A2 B2 C2 D2 A3 B3 C3 D3

34 35 36 37 38 39

40 41 42 43 44 45 46 47 48 49 50 51

52 53 54 55 56 57 58 59 60 61 62 63

64 65 66 67 68 69 70 71 72 73 74 75

76 77 78 79 80 81 82 83 84 85 86 87

88 89 90 91 92 93 94 95

It can be seen from the table, in the case of 12. 2MHZ, the average largest site type is S5/5/5. When allocating the frequencies to the base stations, the rule {A1, A2, A3} or {B1, B2, B3} or {C1, C2, C3}, or {D1, D2, D3} is selected, the co-frequency or adjacent frequency may not occur in the same cell and the adjacent cell.

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6.1.4 Compact Frequency Reuse

When the network construction is developed to certain content, it will certainly bring along the rapid increase of the capacity, and put forward more strict requirements for the network construction in turn. With the application of cell splitting, micorcell and distributive antenna system, how to appropriately plan the frequencies becomes a challenging question, and various frequency compact reuse technologies emerge accordingly. Currently, the most used are MRP, IUO and FRACTIONAL REUSE (1×3) technologies.

Different frequency reuse technologies need the corresponding software and hardware technology supports, and subsequently the handover, power control, channel assignment algorithm and the functions such as DTX and frequency hopping are improved and developed; it is put forward more strict requirements for the selection of the antennas. In other words, as the frequency reuse is very compact, how to reduce the interference is the key to guarantee the system service quality. For the planning, it appears as the appropriate selection of the base station position and the cell direction, the appropriate setting of the height and declination angle of the antenna for reducing the cross-cell coverage; for the system function, it appears as the utilization of the DTX and frequency hopping technologies, and of the PBGT handover, uplink/ downlink power control, optimized channel assignment algorithm, and the like. For the parameter settings, it includes the appropriate settings of various thresholds of the base station static power and handover, and of the frequency hopping parameters for reducing the possibility of the collision of the common adjacent frequencies to the maximum extent; in addition, for the equipment, it puts forward more strict requirements with regard to the stray indexes of the base station equipment and the selective indexes of the adjacent channel.

Various compact reuse technologies utilize the multi-layer concept on the whole, planning the BCCH and TCH in multi-layer, various layers utilize different frequency reuse pattern. Considering that BCCH must at intervals send the system message for the detection and contact of the mobile phones, so that the mobile phones can decode correctly and the report correctness of the mobile phones may be increased, BCCH should be assigned with at least 12 frequency bands. During the actual assignment, considering the irregularity of the base station distribution and the change of the directional angles of the cells, over 14 of frequency bands are generally assigned to BCCH, and the frequency plan is to be made generally by utilizing the 4×3 or more incompact frequency reuse pattern. TCH frequency plan is made by utilizing patterns such as 3×3, 2×3, 1×3 and the like. It is to be mentioned here, the purpose for various carrier layers to utilizing different reuse degrees is to prevent the interference as possible, which is embodied in the following aspects:

In the case of the non-uniform network site types, as not each of cells is to utilize TRXs of the last layer or layers, TRXs of the last layer or layers of TRXs may realize the more compact reuse degree (even though in the case of no frequency hopping).

Since each carrier layer is to utilize the different reuse patterns as possible, the frequency bands of any two of the cells in the network are not the same, that is, there are no thoroughly co-frequency cells.

After the multiple frequency reuse is realized, while the interference is increased, the TRXs are also increased in the cell, so that the frequencies participated in the hopping are increased, increasing the gain.

If there are the frequency band with smaller interference and the frequency band with larger interference simultaneously in a cell, after

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the hopping technology is utilized, the frequency band with smaller interference and the frequency with larger interference will be merged. According to the characteristics of the Viterbi decoder, the interfered frequency band may still be utilized normally. Although the interference is variable with regard to each burst, the voice quality will be dependent on the average value of the interference with regard to a specific connection.

I. Multiple compact reuse pattern (MRP)

The multiple compact reuse technology is also called MRP (Multiple Reuse Pattern), allowing the simultaneous existence of several different frequency reuse patterns, which is put into operation, in the same GSM network. For example: BCCH utilizes the 4×3 reuse pattern, TCH utilizes 3×3 and 2×3 patterns, Figure 6-3 shows the schematic diagram of its structure.

Figure 6-3 Schematic diagram of multiple compact reuse

In the figure, the same color refers to the same grouplink of frequencies that are reused, the size of the circle refers to the coverage range. L1, L2, …, Lm refer to the frequency layers in the cell, it can be seen from the figure, the reuse of the layer that is closer to the top layer is more compact. In the case of the given frequency, comparing the multiple compact reuse with the same reuses in various layers, the number of the channels in the unit area will be increased significantly.

Essentially, MRP is a frequency planning method, without putting forward any special software/hardware requirements. It is established based on the concept of a carrier multi-layer. That is to say, all of available frequency bands are divided into several groups, each of which acts as a carrier layer. According to the compact reuse rules, the frequency bands arranged in each layer which are illustrated by way of the following example, conform to the following expression: n1≥n2≥n3≥n4≥. . . . . . ≥nm.

Layer Number of frequency band

BCCH n1

TCH1 n2

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TCH2 n3

. . . . . . . . . . . . . . . . . . . . .

TCHm-1 nm

For example, assuming that the available frequency bandwidth is 10MHz and the signal channel numbers are 46-94, the rules of the BCCH and TCH carrier layers may utilize the continuous grouping pattern. For the continuous grouping pattern, the BCCH frequency band is preferably added with 1-2 additional frequency bands for planning, i. e. , total 12-14 frequency bands for planning. Table 6-2 has no reserved frequency band.

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Table 6-2 Continuously assigning MRP

Carrier type Absolute carrier number of the available frequency band

Number of the available frequency bands

BCCH 46～57 12

TCH1 58～66 9

TCH2 67～74 8

TCH3 75～82 8

TCH4 83～88 6

TCH5 89～94 6

It can be seen from the above table, the above frequency bands are thus divided into 6 groups, the carrier layer where the broadcasting channel (BCCH) is located has 12 frequency bands for reusing, the service channel is divided into TCH1-TCH5, total 5 groups of the carrier layers, each grouplink is assigned with different numbers of frequency bands for reusing.

Thus, in the case of 10MHz of the bandwidth, the base station configuration is made to S6/6/6. According to the above assignment of the frequency bands for various carrier types, the frequency plan is carried out in the entire network. In the case of the traditional 4/12 reuse pattern, the maximum configuration of the base station can be made to S4/4/4.

With regarding to the continuous grouping pattern, there may be the cofrequency/adjacent interference in the base station frequency layer, and the interference between the base station frequency layers occurs at the frequency boundary point.

In addition to the continuous assignment, the interval assignment may be utilized, Figure 6-4 shows the schematic diagram of the interval assignment. In the figure, it is assumed that the frequencies that may be assigned to BCCH are 1, 3, 5, …, 37, from which, 12 frequency bands are obtained for BCCH, the rest frequencies are assigned to TCH1, THC2, THC3 and MICRO, each layer of frequencies are selected at regular intervals. In the case that there are the adjacent frequency interference within the layer instead of between the layers, when the traffic is not very busy, this pattern is useful for reducing the network interference.

Figure 6-4 Discontinuous frequency assignment

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The reason that MRP may realize the frequency compact reuse layer by layer so as to realize the increase of TRX is: since not every cell needs the last layer of TRX during the initial stage, the last layer of TRX may realize the more compact reuse. Further, after MRP is utilized, while the interference is increased, TRXs are also increased in the cell, so that the frequencies participated in the hopping is increased, increasing the gains. If there are the frequency point with smaller interference and the frequency point with larger interference simultaneously in a cell, after the hopping technology is utilized, the frequency point with smaller interference and the frequency with larger interference will be merged. The Viterbi decoder can still demodulate the code element correctly. The interference appears in the concept of the average value, which does not affect the normal operation of the base station.

It should be mentioned here, when the MRP frequency assignment is carried out, the minimum frequency reuse degree of the TCH layer is recommended not less than 6; and the average frequency reuse degree of the TCH layer is at least between 7-8. If the frequency resources are available, in the initial frequency planning, it is very effective to reserve a given frequency bands for the microcell and for settling the troublesome problems in optimization.

II. Ordinary concentric circle technology

The ordinary concentric circle technology is to divide the ordinary cell into two service layers, i. e. , the great circle and the small circle, to divide the frequency bands into two parts, the TRX powers corresponding to some of the frequency bands are adjust down, so that two concentric circles with different coverage in the coverage range. For the mobile phones in the great circle, the frequency bands with more incompact frequency reuse such as the BCCH frequency band, as possible; for the mobile phones in the small circle, the frequency bands with the more compact frequency reuse such as the frequency bands except BCCH, as possible, the system capacity can be effectively increased by utilizing the compact frequency reuse pattern for the frequency bands across the small circle. It is seen from Figure 6-5, since the mobile phones in the small circle is far away from the interference source, the voice quality can still be guaranteed, even though the frequency with the compact reuse is used. Also, since the mobile phones in the great circle utilize the frequency with the incompact reuse, the voice quality can be guaranteed.

Figure 6-5 Schematic diagram of the compact frequency reuse in the concentric circle cell

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If the inner circle is overlapped with the excircle or inner circle of the other cell, the inner circle can be switched to another cell directly, which can effectively reduce the congestion condition in the excircle.

When the ordinary concentric circle technology is utilized, as the inner circle of the concentric circle can utilize the more compact frequency reuse pattern, comparing to MRP, it can increase the network capacity to the larger extent, and the network quality is also guaranteed. In some special case, for example, when the excircle of the concentric circle cell is configured with only one carrier BCCH, using 4×3 frequency reuse pattern, and other TCH carriers are all configured on the inner circle with the 1×3 frequency reuse pattern, the concentric circle cell is just the same as the 1×3, and the average frequency reuse degree is the same as the 1×3, therefore, the concentric circle in this case can effectively reduce the interference in the entire network without reducing the network capacity, so as to realize the network quality higher than 1×3.

The problem caused by the use of the ordinary concentric circle is that the traffic control, i. e. , the handover control, between the inner circle and the excircle. Based on the feature that the coverage ranges of the inner circle and excircle are different, the signal level threshold and TA value threshold are generally regarded as the handover basis; based on this feature, this technology may be used to guide the traffic in the cell in which the coverage ranges of some carriers are different.

III. Intelligent underlay/overlay (IUO)

To guarantee that the coverage ranges of all the carriers in the base station are identical, the intelligent underlay/overlay (IUO) technology is introduced, in which, all the TRXs in the cell are the same. The design philosophy of the IUO is shown in Figure 6-6.

Figure 6-6 Schematic diagram of the IUO structure

It is seen from the figure, the IUO philosophy is to divide the base station frequencies into two parts, or so-called two layers, one layer is called "REGULAR layer", and the other "SUPER layer". For "REGULAR layer", the interval of the frequency reuse is larger, utilizing the incompact frequency reuse pattern; for "SUPER layer", the interval of the frequency reuse is smaller, utilizing the compact reuse pattern. The frequency assignment of IUO is described by way of an example, assuming that the assignable frequency band is 10. 4MHz. Figure 6-7 shows the example of IUO frequency assignment.

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Figure 6-7 Example of IUO frequency assignment

BCCH reuse: 15

R TCH TRX reuse: 12

S TCH TRX reuse: 6

BCCH selects 15 frequency bands, utilizing the 4×3 reuse pattern. REGULAR layer utilizes 24 frequency bands. The 4×3 reuse pattern is utilized. SUPER utilizes 12 frequency bands and the 2×3 reuse pattern. After the IUO technology is utilized, the maximum site type is S5/5/5, if the 4×3 reuse pattern is utilized, the maximum site type is only S4/4/4.

The problem caused by the use of IUO is that SUPER interference is larger, the specific handover algorithm is needed to be provided by the equipment to judge the interference in the network; once it is found that C/I goes beyond a given standard, the system will switch the user to the REGULAR layer. Both of the judgment and handover are completed automatically. When C/I>good threshold, the frequencies of the S layer are used; when C/I<bad threshold, the frequencies of the R layer are used. For example: when the frequency hopping is not used, C/I>17db, the frequencies of the S layer are used, C/I<12db, the frequencies of the R layer are used; when the frequency hopping is used, C/I>11db, the frequencies of the S layer are used, C/I<7db, the frequencies of the R layer are used. The traffic of the R/S layers can be controlled by adjusting the threshold value.

C/I detection is based on the RXLEV and RXQUAL of the BCCH channel in the adjacent cell reported by MS, the cofrequency reuse cell is predefined in the system, forcing it as the adjacent cell. The mobile phones report the best and strongest signal of the six measurements, the base station judges the component of the cofrequency cell, calculates the C/I, and obtains the criterion for the R/S layer handover.

IV. Fractional reuse technology (1×3 or 1×1 fractional reuse)

The fractional reuse technology and the 1×3 or 1×1 reuse technology are combined with each other; for the 1×3 or 1×1 reuse, the reuse interval is smaller and the interference is larger, the RF hopping technology should be utilized, the collection of the available hopping frequencies is far lager than the number of TRXs, and the MA, HSN, MAIO parameters should be set, so as to avoid the collision of the frequencies.

Assuming 10MHz bandwidth, 50 frequency bands, BCCH occupies 14 frequency bands, and TCH utilizes 36 frequency bands. If the 4×3 reuse pattern is utilized for planning, each cell is assigned with 3 TCH frequency bands, with the site type of S4/4/4. If the fractional reuse 1×3 is used, the TCH frequency bands that may be used by each cell are 12, and the actual available frequencies used by the cell are dependent on the fractional reuse ratio; specifically, if FR LOAD is 50%, then TRX=12×50%=6, with the maximum site type of S7/7/7.

According to the previously obtained relational expression of C/I and K and the relation between K and FR, the curve chart 6-8 of FR and C/I is obtained, from which it can be seen, the better C/I ratio (12dB) can be guaranteed when the average load factor is 0. 5, and C/I is unbearably deteriorated when the average load factor is 1, i. e. , the true 1X3 pattern; on the other hand, it is recommended that the actual

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average frequency reuse degree is not less than 6, which is the bottom line of the current frequency reuse degree.

In this reuse pattern, the interference may be expressed as the collision probability of the common adjacent frequencies; the result of the simulation shows that the collision probability is only relative to FR, independent from how much the available frequencies are and how much the available TRXs are.

Figure 6-8 Relation graph of FR and C/I

It should also be mentioned, when this pseudo spread spectrum pattern is used, if the initial plan is incorrectly made, comparing to the small traffic, the quality is seriously deteriorated when the traffic increases. The preferred method is to simulate the interference conditions with the large traffic by sending the idle Burst function when the network is initially established, and perform the adjustment for optimization.

Figure 6-9 1×3 fractional reuse frequency assignment

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Figure 6-9 shows the base station frequency arrangement in the case of the 1×3 fractional reuse pattern visually. Total N (>12) BCCH frequency bands and 18 TCH frequency bands. The frequency bands assigned in a cell is to be described now, TRX1 utilizes one of the N BCCH carriers, at a certain time, TRX2, TRX3 and TRX4 that are using the 1×3 pattern are assigned to the 3 frequency bands of the 6 TCH carriers. Each TRX (2-4) is configured with the same MA and HSN, but MAIO is different.

Now the specific application of the 1×3 reuse pattern will be described by an example of the Unicom GSM900 network somewhere.

Unicom 900 frequency band: 96-124

Carrier configuration: S3/3/3

BCCH carrier layer: 96-109 reuse pattern: 4×3

TCH carrier layer: 110-124 reuse pattern: 1×3

(1) Sequence grouping solution

TCH is grouped in sequence, the three cells in the same base station utilize the same HSN, different sites utilize different HSNs, all the carrier of the same layer in the network utilize the same MAIO. Assuming that the frequency hopping groups are allocated as follows:

Grouplink one: 110 111 112 113 114

Grouplink two: 115 116 117 118 119

Grouplink three: 120 121 122 123 124

HSN of the site A is 1, MAIOs of the two carriers TCH1 and TCH2 in each cell are 0 and 2 respectively, HSN of the site B is 2, MAIOs of the two carriers TCH1 and TCH2 are 0 and 2 respectively, and so on; in this way, the cofrequencies among the three different cells in the same site are avoided; comparing to the TCH interval grouping, the possible collision of the cofrequencies in the adjacent cells opposite to the different sites are reduced; however, comparing to the TCH interval grouping, the possible collision of the cofrequencies between the cells parallel to the different sites in direction is increased.

(2) Interval grouping solution

TCH utilizes the interval grouping, the three cells in the same base station utilize the same HSN, the different sites utilize the different HSNs, and the carriers of the same layer in the same base station utilize the different MAIO. Assuming that the frequency hopping groups are allocated as follows:

Grouplink one: 110113 116 119 122

Grouplink two: 111 114 117 120 123

Grouplink three: 112 115 118 121 124

HSN of the site A is 1, MAIOs of the two carriers TCH1 and TCH2 in the grouplink one cell are 0 and 1 respectively; MAIOs of the two carriers TCH1 and TCH2 in the grouplink two cell are 2 and 3 respectively, MAIOs of the two carriers TCH1 and TCH2 in the grouplink three cell are 4 and 0 respectively, HSN of the site B is 2, and so on. In this way, the cofrequencies among the three different cells in the same site are avoided; comparing to the TCH sequence grouping, the possible collision of the cofrequencies in the adjacent cells opposite to the different sites are increased;

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however, comparing to the TCH sequence grouping, the possible collision of the cofrequencies between the cells parallel to the different sites in direction is reduced.

With respect to the question how to grouplink TCH so that the 1×3 frequency hopping interference is relatively smaller, both sequence grouping and interval grouping patterns have some defects; however, generally, the adjacent frequency influence of the adjacent cell opposite to the central area where the base stations are compact dense and are regularly distributed is larger than that of the adjacent cell parallel to the central area in direction, it is obviously advantageous to utilize the sequence grouping pattern; however, in the areas around the dense base stations, as the irregularity of the base station distribution, it is useful for homogenizing the influence caused by the interference by utilizing the interval grouping pattern. Therefore, which grouping pattern being utilized should be considered together with the actual environment situations. After the new channel allocation algorithm in the compact reuse pattern is realized, it is recommended that the sequence grouping solution is to be utilized, thus, the better guarantee of the service quality in the entire network will be actually implemented.

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6.2 Anti-interference Technology

6.2.1 Frequency hopping Technology

I. Several concepts

(1) Frequency hopping

Frequency hopping means that the carrier frequencies with useful information hop with the time sequence under the control of a sequence called hopping sequence number (HSN). A hopping sequence is an array that uniquely specifies all (N) frequencies, in the set of the frequencies (MA) including N frequencies, by the hopping sequence number (HSN), mobile allocation index offset (MAIO) and frame number(FN) through a given algorithm. N channels in the different timeslots may utilize the same hopping sequence, the different channels in the same timeslot in the same cell utilize the different mobile allocation index offset (MAIO).

(2) Hopping mode

Hopping mode is divided into the frame hopping and slot hopping in terms of the time domain, and into the RF hopping and baseband hopping in the carrier realization mode.

Frame hopping:hopping by the unite of TDMA frame, in this mode, each carrier may be regarded as a channel; the TCH on the TRX carrier where the BCCH is located during the frame hopping in a cell can not participate in the hopping, other different carriers should have different MAIOs, this is the special case of the slot hopping.

Slot hopping: every slot frequency band of every TDMA frame changes once, the TCH in the TRX where the BCCH is located during the slot hopping may participate in the hopping, however, it is currently realized only during the baseband hopping.

RF hopping: both of the transmitting TX and receiving RX of the TRX participate in the hopping. The number of the frequencies that participate in the hopping in a cell may be larger than the number of the TRXs in this cell.

Baseband hopping: each transmitter works at a fixed frequency, TX does not participate in the hopping, and the transmission hopping is implemented through the switching of the baseband signal, but RX must participate in the hopping. Therefore, the number of the hopping frequencies in a cell may not be larger than the number of TRXs in this cell.

(3) Frequency hopping algorithm

Now several parameters will be described first:

CA: cell allocation table, it is the set of the frequencies used in the cell;

FN: TDMA frame number, it is broadcast in the synchronous channel. BTS and MS are synchronized through FN (0-2715647);

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MA: a set of the wireless frequenc channels numbers used for the mobile station, it is a subset of CA. MA includes N frequency channel numbers, 1≤N≤64

MAIO: mobile allocation index offset (0-N-1); during the communication, the wireless frequency sequence number used in the air interface is one element of the set MA. MAI (mobile allocation index, 0-N-1) is used to determine one specific element of the set MA, in other words, the actually used frequencies are specified by the MAI. MAIO is a initial offset of the MAI, its purpose is to prevent multiple channels from scrambling for the same carrier in the same time.

HSN: hopping sequence (generator) number (0-63); it is the sequent hopping when HSN=0; and it is the random hopping when HSN≠0.

Only after the actual functions of various parameters in the hopping algorithm and the hopping mechanism are well understood, the relative parameters may be reasonably set, so as to put the system in the optimized operation. Figure 6-9 is the flow chart of calculating the actual operation frequency of the carrier at every hopping slot. Among them: MAI=(S+MAIO) MOD N,RFCHN=MA (MAI); S is obtained by calculating according to the frame number and hopping sequence number, and MAI is obtained from S plus S hopping offset moding the number of the carriers in the MA set.

17


Figure 6-10 Hopping algorithm

In Figure 6-10:

mod: mode

^: power

NBIN: INTEGER (log2N+1)

Table:

18


706510593394624211591100-109

1231799125110-113

9798422611197974916090-099

72120113535243904114117080-089

126514212166603712210877070-079

676292106532107405882060-069

4583133261106831987050-059

88112911271109387511155040-049

741210357896131273480030-039

254869656188511847101020-029

1001042312459768125640010-019

73941027895361639848000-009

ContentsAddress

706510593394624211591100-109

1231799125110-113

9798422611197974916090-099

72120113535243904114117080-089

126514212166603712210877070-079

676292106532107405882060-069

4583133261106831987050-059

88112911271109387511155040-049

741210357896131273480030-039

2548696

706510593394624211591100-109

1231799125110-113

9798422611197974916090-099

72120113535243904114117080-089

126514212166603712210877070-079

676292106532107405882060-069

4583133261106831987050-059

88112911271109387511155040-049

741210357896131273480030-039

254869656188511847101020-029

1001042312459768125640010-019

73941027895361639848000-009

ContentsAddress

(4) Concept of the synchronous cell

The concept of the synchronous cell is very important for the establishment of the hopping strategy and the effective reduction of the interference in the network. BTS and MS are synchronized through the appointment of the frame number. In the synchronous cells, since the frame number used by each TRX in various cells are the same, the same HSN may be used in various hopping group, and the MAIO is properly set, so as to avoid the collisions of the common or adjacent frequencies of various cells in the same base station or the collisions of the common adjacent frequencies in one cell.

II. Frequency hopping function

The frequency hopping is introduced in the GSM system, because the frequency hopping provides two functions: frequency diversity and interference averaging.

(1) Frequency diversity

The frequency hopping can reduce the influence of signal strength variations caused by the multipath fading, this function may be equivalent to the frequency diversity. In the mobile communication, due to the influence of the Rayleigh fading, the wireless transmission signal may change rapidly in the large amplitude, and this change is relative to the frequency. As the difference between the frequencies is larger, the fading will be more independent; for the mobile communication band, 200KHz frequency interval basically guarantee the noncorrelation of the fading characteristics between the frequencies, and 1MHz can thoroughly guarantee this noncorrelation. By the hopping, all of the burst pulses containing the same voice frame code word will not be damaged by the Rayleigh fading in a manner, as shown in Figure 6-11.

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Received signal level

Distance


Distance


Distance

Figure 6-11 Fading

The statistics shows that the frequency hopping gain is relative to the environment factor, especially to the movement speed of the mobile station. When MS moves in a high speed, the position change of two burst pulses in the same channel will be subjected to the influence of other fading; the higher the speed is, the lower the gain is. However, for the numerous users who move slowly with the mobile phones, the frequency diversity is advantageous.

Further, the hopping gain is also relative to the number of the frequencies , when the number of the frequencies is reduced, the gain is reduced, too. The relation between the number of the frequencies and the hopping gain in such a way that the hopping is the pseudo spread spectrum, the obtained gain is the processing gain obtained from the useful signal spread transmission frequency band. The basic method for actually measuring the hopping gain is that, on the prerequisite that the same FER is required, the receiver will require for different C/Is at various numbers of the frequency hopping bands, and the difference between these C/I is the gain obtained from the hopping.

Some documents list the relation between the number of the frequency hopping bands and the hopping gain (the actual gain will be subjected to the environmental influence):

The number of the carriers that participate in the hopping Frequency diversity gain

〈=1 02 33 44 55 5. 56 67 6. 38 6. 59 6. 810 6. 9

>=11 7

(2) Interference averaging

The frequency hopping provides the difference of the interference in the transmission path, so that all the burst pulses that contain a part of the code words may not be

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damaged by the interference in the same manner, and the original data can be recovered from other part of the receiving stream through the correction coding and interleaving of the system. Obviously, the frequency hopping may obtain a certain gain only when the interference is distributed in a narrow band; if the interference is distributed in a wide band, all the burst pulses will be damaged, and the original data can not be recovered, so that no gain can be obtained. In the actual network, the interference is generally distributed in a narrow band.

In the state of the hopping, it is found that the error bit ratio tends to upward in the test, however, people subjectively feel that the voice quality is improved. The reason is that, though the error bit ratio is increased, but the index of the voice frame erase ratio (FER) is improved, in the view of the voice communication, it is understood that the voice quality is improved; however, in the view of the data service, it may have some defects, especially, when the data speed rate is very high, the frequency hopping becomes harmful. This result will be seen from the simulation of the GPRS later.

6.2.2 Power Control

I. Mobile station power control

The mobile station power control is divided into two adjusting stages, i. e. , the stable adjusting stage and the initial adjusting stage. The stable adjusting is the normal method for performing the power control algorithm, while the initial adjusting is used in the time when the call connection is initially started. When a connection is performed, MS is output as the nominal power of the cell where it is located (the nominal power indicates that the MS transmitting power is the MS maximum transmitting power MS_TXPWR_MAX_CCH in the broadcast system messages on the BCCH channel of the cell where it is located. If MS does not support this power class, the supported power class that is nearest to it will be utilized, such as the maximum output power class supported by the reported MS Classmark in the establishment indication message). However, since BTS may simultaneously support multiple calls, the receiving signal intensity should be reduced in a new connection as quick as possible, otherwise, the quality of other call supported by this BTS may be deteriorated due to the saturation of the BTS multi-coupler, and the call quality of other cells may be affected due to the high interference. Therefore, the purpose of the initial stage power control adjusting is to reduce the MS transmitting power as quick as possible until the stable measurement report is obtained, so that the MS can be adjusted according to the stable power control algorithm.

The parameters that must be selected in the uplink power control, such as the expected desirable uplink receiving level, desirable uplink receiving quality, etc. , are all set by the O&M data management console, the data configuration can be dynamically carried out according to the actual situations of the cell. After a given number of the uplink measurement reports is received, by the processing methods such as interpolation and filtering, the actual uplink receiving level and the receiving quality are obtained, then they are compared with the desirable uplink receiving level and the receiving quality, with the power control algorithm, the power class to which the MS should be adjusted is calculated; if it is different from the current MS output power class and meets a given application restricted conditions (such as the power adjusting step length restriction, MS output power range restriction), the power adjusting command is sent. The essence of the uplink power control adjusting is to enable the actual uplink receiving level and receiving quality obtained from interpolation and filtering to progressively approach the desirable uplink receiving level and receiving quality set by O&M. The purpose for the interpolation and filtering of the measurement reports is to process the lost measurement report, clear the

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temporary nature (spilliness), so as to ensure the stability of the power control algorithm.

The difference between the initial stage power control and the stable stage power control is that the expected uplink receiving level and receiving quality in the initial stage is different from that in the stable stage, the length of the filters are also different, and only the downward adjustment is performed in the initial stage.

II. Base station power control

The base station power control is an optional function. The base station power control is basically identical to the MS power control, except that the base power control utilizes only the stable power control algorithm. The parameters that must be selected in the power control include the receiving level threshold (lower limitation) to be performed the power control and the receivable maximum sending level threshold (upper limitation). The receiving level RXLEV is divided into 64 classes, with numbers from 0 to 63, class 0 of the receiving level is the lowest, while the class 63 of the receiving level is the highest.

The base station power control is divided into the static power control and the dynamic power control, the later is the fine adjusting based on the former. The 0505 protocol specification specifies that the base station static power class is divided into 6 (2dB/per class), when the maximum power output by the base station is 46dBm (40W), the class 6 is 34 dBm. The static power level is defined in the cell attribute table of the data management console, i. e., the maximum output power value Pn of the current dynamic power control is specified. As the dynamic power control classes are set to 15, the range of the dynamic power control is Pn-Pn-30dB. If the requirements cannot be satisfied when the dynamic power control reaches its maximum value, the static power control classes should be adjusted to increase the maximum output power value Pn of the dynamic power control.

III. Power control process

(1) Interpolation of the measurement report

The actual measurement report (MR) will be lost, which is divided into several cases. First, the MR message numbers reported by the BTS are discontinuous; second, the MR message optional items reported by the BTS will cause the discontinuity of some measurement reports no longer; third, the MR message is lost due to the error of the MR message format reported by the BTS. In engineering, the first order interpolation formula is used to estimate the lost measurement report, the purpose for doing so is to avoid the call loss due to the lower power.

(2) Filtering of the measurement report

The purpose for the filtering of the measurement report is to clear the temporary nature, so as to ensure the stability of the algorithm.

(3) Power control adjusting

The power adjusting calculation is made according to the difference degree between the current receipt conditions and the expected value, so as to determine the power value to be adjusted to.

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6.2.3 Discontinuous Transmission

During a communication process, the mobile users are making calls only in 40% of times, in most of times, no voice message is transmitted, thus, the system resources are wasted greatly. With respect to this case, the discontinuous DTX mechanism is introduced into the GSM, it prohibits the wireless signals that is thought to be unwanted by the users to reduce the interference level, so as to increase the efficiency of the system.

Whether the downlink DTX in the network is used is to be set by the network operator in the switching side, generally, it is controlled taking BSC as the unit, the control message is transmitted to the base station baseband processing part through the special signaling channel, and then to TC through the inband signaling of the TRAU frame, notify whether the downlink DTX is used. Whether the downlink DTXs of some manufacturers are used may also be set taking the cell as the unit.

The uplink DTX is set by the network operator in the wireless side, i. e. , setting the DTX parameter in the system message, this parameter is composed of 2 bits, its coding mode is shown as the follows:

DTX Meaning0 The mobile station may use DTX1 The mobile station must use DTX10 The mobile station is not allowed to use DTX11 Reserved

The parameter DTX is included in the information unit "Cell options", and transmitted regularly in the system message of each cell broadcast, the mobile phone is to determine whether the uplink DTX function is enabled according to this message.

To implement this mechanism of the DTX, the source must be able to indicate when the transmission is required and when is not. When the DTX mode is activated, the voice encoder must detect it is either voice or noise, which uses the voice detection VAD technology. By calculating some signal parameters and according to some thresholds, VAD can determine whether the receiving signal is either voice or noise. This judgment is based on a energy law: the energy of the noise is less than that of the voice. The VAD technology is to generate a group of thresholds in every 20ms voice block time, determining whether the next 20ms voice block is either voice or noise. However, when the background noise is very high, the noise signal will be regarded as the voice by the VAD and be encoded for sending. The downlink VAD is in TC, while the uplink VAD is in the mobile phone.

DTX may be used in both uplink and down link, but they are two programs that is no relative to each other. They may be activated by the system parameter according to the respective situations, whether the other party activates this function. There are two measuring methods in the GSM: one is called global measurement, which is to average the level and quality in the 104 slots of the entire measuring period (26 multi-frames of 4 TCHs); the other is called the local measurement, which is to measure and average the level and quality of the 12 slots, including the 8 continuous TCH burst pulses (for the TCH/F channel, 0-103 TDMA frames are taken as a circulation, these 8 burst pulse frame numbers are 52, 53, 54, 55, 56, 57, 58, 59 respectively; when no voice and signaling are transmitted, they have the description information of the comfortable noise, called SID) and 4 SACCH burst pulses carrying the measurement report (0-103 TDMA frames are taken as a circulation, these 4 burst pulse frame numbers are 12, 38 , 64, 90 respectively). For the conformity, whether the uplink/downlink of the system activate the DTX function, the base station and mobile station will complete both measuring methods; and whether the discontinuous

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transmission mode is utilized during the last measurement report period is indicated in every SACCH measurement report of the BTS ad mobile station, according to this indication, BSC is to decide that either global measuring or local measuring is used for judging.

The discontinuous transmission is applicable for the voice as well as the opaque data transmission, however, the carrier where the BCCH is located does not use this technology. DTX should be realized in every cell.

The main functions of the DTX technology in the uplink/downlink are: the uplink can save the mobile phone battery and reduce the interference in the system; the downlink can reduce the power consumption of the base station, reduce the interference and reduce the crosstalk in the base station.

When the downlink DTX is utilized together with the uplink DTX, the C/I cofrequency interference ratio of the system will be improved. This improvement may be applied to the cell planning with the compact frequency reuse, especially, when it is used together with the frequency hopping, larger system capacity may be obtained.

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Documents

Chapter6 Frequency Planning and Anti-Interference Technology