Gsm Frequency Planning
- 1. Frequency Planning Frequency Planning Abstract This is a technical document detailing a typical approach to Frequency Planning Process. Page 1 of 10
- 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 Page 2 of 10
- 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. Page 3 of 10
- 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. Page 4 of 10
- 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) Page 5 of 10
- 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. Page 6 of 10
- 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, Page 7 of 10
- 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, Page 8 of 10
- 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. Page 9 of 10
- 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. Page 10 of 10