Frequency Planning Guidelines

Frequency Planning GuidelinesT-Mobile USA

Frequency Planning Guidelines

Confidential and Proprietary T-Mobile Document Ref: ENG / RF / TGU - document.doc


Document Title: xxxx_Frequency Planning

Document Subject: Frequency Planning Guidelines

Document Author: Matthieu Marescaux

Author’s Manager: Yasmin Karimli

Company: T-Mobile

Document Category: RF Standards

Document Keywords: ENG / RF / TGU

Document Comments: Release for Comments and Approval

Document Web Location: http://rf.eng.voicestream.com/Library/.doc

Date: August 30th, 2002

Document Approved: Mark Cosgrove, Dir. RF Systems Engineering

Document Authorized: Mark Cosgrove



Table of Contents

1 ScopeThis document outlines the Radio Frequency (RF) channel assignment strategy for

existing and new markets. Reference material and background information has been included as an informative annex.

2 Introduction

Each market has available a set band of frequencies as defined by the relevant FCC license. The licensed frequency band is used to support discrete 200KHz wide channels. GSM operates on a predefined numbering scheme such that each 200KHz channel has a specific channel number assigned. The defined channels and band information for GSM – North American (GSM-NA) is shown in table 1.

FCC PCS Band

B/W (Uplink and Downlink)

MS TX Band (MHz)

BTS TX (MHz)

GSM CH

A 30 MHz 1850.0 1930.0 512-586D 10 MHz 1865.0 1945.0 587-611B 30 MHz 1880.0 1960.0 612- 686E 10 MHz 1885.0 1965.0 687-711F 10 MHz 1890.0 1970.0 712-736C 30 MHz 1895.0 1975.0 737-811

Table 1 FCC bands and GSM Channel Allocations

GSM requires that each cell (a site being made up of one or more sectored cells) have one frequency that is used to broadcast network and cell control information and act as a pilot frequency. This frequency is defined as the Broadcast Common Control Channel (BCCH) frequency. Timeslot 0 of the BCCH frequency carries the logical BCCH channel and additional channels that are used for paging, synchronization, and initial system access. The BCCH frequency is required to transmit constantly at a set frequency and at full power. We will explain why the 7/21 re-use pattern is the strategy we recommend to use for the BCCH layer

Non-BCCH frequencies can adapt on a timeslot-by-timeslot basis, being able to change frequency (Called hopping) and transmit power levels (Power control). The Non-BCCH channels are therefore able to achieve better performance in terms of tolerance of interference and noise than the BCCH frequency. This better performance is exploited by reusing non-BCCH frequencies more often within a given area such that the traffic supported per MHz of frequency is increased. This document is aimed at promoting the use of frequency hopping 1/1



strategy for markets with challenging terrain and 1/3 for markets in which the site are spread out with regular azimuth.

This document discusses the fundamental concepts of frequency reuse, various techniques for maximizing capacity, spectrum partitioning schemes, and finally to provide recommendations for partitioning of 5MHz, 10MHz, 15MHz and 20MHz spectrum (understand 5,10,15 or 20MHz in uplink band and 5,10,15 or 20 MHz in downlink band). The Houston trial results are provided in Appendix.

We recommend to local RF team to use this document as a frequency planning strategy guide. The recommended frequency planning strategies are summarized in the recommendation part located at the end of this document.

This document is intended for networks that have implemented GSM and GPRS technologies, as it is the present case for all Voicestream markets.



3 Frequency re-use theory

Frequency reuse, as the name suggests, involves reusing the same frequency repeatedly in a cellular network. It is this fundamental concept that enable cellular systems to provide the necessary traffic carrying capacity to support its subscriber base.

3.1 Cellular network conceptsIn a cellular network, a group of contiguous cells all using different frequencies are

grouped into clusters. A term commonly used to describe a frequency re-use scheme is N, where N denotes the number of cells in a cluster, and thus the total number unique frequencies.

Here is how a cellular pattern, here with N=7 (i=2 and j=1) is displayed:

As shown on this figure the re-use pattern has to follow the 2 arrows directions (or one arrow if j=0) to be regular and this is the reason why N has to verify that . Where i and j are integers.

That implies 1, 3, 4, 7, 9, 12, 13, 16, 19, 21, 27 are usually the values that we usually consider in GSM.

For example, N=3, denotes a 3 sites cluster, each cell with a unique frequency group. Repeating this cluster over the geographic area of coverage forms the cellular network. In a sectorized reuse scheme, a site number / cluster size convention is commonly used to denote the reuse pattern. For instant 3/9, denotes a 3 site / 9 cell cluster (3 sectors per site).

The frequency-repeat pattern determines the maximum number of radios that can be deployed in each cell, thus the maximum amount of traffic carried.

A cellular network may consist of omni sites or sectorized sites or a combination of both. Given the same total number of channels, the capacity of a sectorized site is less than the capacity of an omni site, as the example below illustrates.

Site type Total TCH Traffic Capacity (2% blocking)


i

j

A

AA

AAA

Ai

j


availableOmni 45 35.6 Erlangs3 Sector 45 3 x 9.01 = 27.0 Erlangs

However, sectorization allows higher frequency re-use with smaller number of sites as each site contains 3 cells instead of one, and thus higher overall network capacity and that is making much more sense economically, therefore all GSM networks use sectorized sites. For both site types, several frequency re-use schemes are possible with varying levels of carrier to interference ratio (C/I).

For any re-use pattern, the ratio of co-channel cell site to the cell radius is:

This comes from the fact that we have, here shown for N=4:

Let’s call R’ the radius from the center of the hexagon to the middle of a side of the hexagon. We have:

The distance D between the middle of and hexagon and the middle of the next one that uses the same frequency verifies:

Therefore:

From this value we can estimate the theoretical interference created by the first ring of frequency re-use. The hexagon grid implies that six first ring cells always surround a cell (One for each side of the hexagon). Let’s illustrate how this work in the case of omni-directional site

If we estimate that the propagation of the signal is proportional to the distance power the attenuation factor –n, we have at the edge of the cell (i.e. worst case C/I):

So in dB we have:


R

D


We usually assume a value of n=3.5 for the attenuation.

3.2 Examples GSM frequency re-use patterns

Let’s illustrate this theory with few examples of plan that can be used for TCH and/or BCCH. But we should notice that any BCCH plan may be implemented in TCH but the reverse is not true:

(1) 1/1 frequency reuse

Minimum reuse distance to cell radius ratio; Co-channel interference; worst case C/Ic = 0dB Each neighboring cell use the same frequency group, this can only be used with

frequency hopping in GSM

Figure 3.1: 1/1 frequency reuse pattern

The example below illustrates the 3 re-use schemes for sectorized sites, and the approximate C/I ratio that can be achieved with a homogeneous network of cells using a 120 degree beam width antenna.


Minimum reuse distance to cell radius ratio; D/R = 1.732, Co-channel interference; worst case C/Ic = 5.5dB Adjacent channel interference; every neighboring cell uses an adjacent channel, this

can only be used with frequency hopping in GSM


1

3 2

1

3 2

1

3 21

3 2

1

3 21

3 2

1

3 2

1

1 1

1

1 1

1

1 11

1 1

1

1 11

1 1

1

1 1




Minimum reuse distance to cell radius ratio; D/R = 5.196 Co-channel interference; worst case C/Ic = 10.9dB, as the theoretical minimum C/I in

GSM is 9dB this re-use pattern is the smallest that can be used with non-hopping channels

Adjacent channel interference; 2 out of 9 cells have adjacent channel neighbor



Minimum reuse distance to cell radius ratio; D/R = 6.0 Co-channel interference; worst case C/Ic = 12.0dB Adjacent channel interference; 2 out of 12 cells have adjacent channels with

neighboring cells, worst case C/Ia = 0dB

Alternatively by swapping the allocation for 1 of the 4 sites, e.g. swapping cell using carrier 8 and 12 in the Figure 3. would yield:

Co-channel interference; worst case C/Ic = 12.0dB (for 2 out of 12 cells) Adjacent channel interference; no adjacent channel neighbor, worst case C/Ia = 5.1dB

(for 4 out of 12 cells)


3

9 6

2

8 5

1

7 4

3

9 6

2

8 5

1

7 4

3

9 6

2

8 5

1

7 4

3

9 6

2

8 5

1

7 4

3

9 6

2

8 5

1

7 4

3

9 6

2

8 5

1

7 4

3

9 6

2

8 5

1

7 4


Figure 3.4: 4/12 frequency reuse pattern – 2 adjacent channel neighbors

Figure 3.5: 4/12 frequency reuse pattern – no adjacent channel neighbor

From the above example, we can see that interference reduces as N increases; this is directly connected to the theoretical C/I calculation illustrated before in the omni site case. But on another hand the number of frequencies available in each group also decreases as N increases.

Although these grid patterns are often used for initial site planning, in practice however, it is the site acquisition process, which would ultimately determine the cellular pattern. There are many factors, which would significantly influence the network topology, these include:

Terrain (hilly or flat) Large water bodies Budgetary constraint


3

11 7 1

9 5

4

12 8

2

10 6

3

11 7 1

9 5

4

12 8

2

10 6

3

11 7 1

9 5

4

12 8

2

10 6

3

11 7 1

9 5

4

12 8

2

10 6

3

11 7 1

9 5

4

12 8

2

10 6

3

11 7 1

9 5

4

12 8

2

10 6

3

11 7 1

9 5

4

12 8

2

10 6

3

11 7 1

9 5

4

12 8

2

10 63

11 7 1

9 5

4

12 8

2

10 6

3

11 7 1

9 5

4

128

2

10 6

3

11 7 1

9 5

2

10 6 4

128

3

11 7 1

9 5

2

10 6 4

128

3

11 7 1

9 5

2

10 6 4

128

3

11 7 1

9 5

2

10 6 4

128

3

11 7 1

9 5

2

10 6 4

128

3

11 7 1

9 5

2

10 6 4

128

3

11 7 1

9 5

2

10 6 4

128

3

11 7 1

9 5

2

10 6 4

128


Site placement constraints

All these factors make it difficult to achieve the ideal network topology.

Irregularities of site coverage will increase the carrier to interference ratios. In general, areas consisting of hilly terrain and large water bodies pose the most difficult frequency planning problems.

The theoretical minimum C/I for which GSM is designed to work is 9 dB (GSM rec 05.05). However in reality it is not very efficient if we do not reach C/I = 12dB.

An N=12 might not allow the C/I requested to be sufficient. Moreover, to deal with the irregularities of site coverage, certain amount additional frequencies (i.e. the value of N) should be allowed. If

If the spectrum available allows it, we shall advise the implementation of N=21 (7/21 pattern) for non-hopping channels. The choice of this value of 7/21 is because it is the lowest value for which we have a regular pattern (N verifies ), where N is a multiple of 3 (tri-sectorial sites) and a sufficient theoretic C/I=15.56dB.

Let’s point out here that strategies such as 5/15, 6/18 or 8/24 that are often use do not respect a regular pattern.



4 Recommended Planning Rules

4.1 Frequency planning general rules

Based on industry technical reports, vendor discussions and field trial work carried out in the Houston market by Aerial (see Appendix I), the following recommendations have been devised for the application of frequency hopping to increase system capacity.

1. Separate frequency channel sets will be assigned to BCCH and non-BCCH usage, this is detailed in section 52. A fractional reuse pattern with soft loading shall be used. 3. For non-BCCH channels a 1/3 or 1/1 frequency hopping reuse is optimum, the frequency hopping benefits are detailed in section 4.34. Downlink Power control and DTX need to be active.5. Average loading of high traffic area should not be more than 40% for a 1/3 and 15% for a 1/16. The peak loading for some sectors will be 60% if the 1/3 is implemented and 30% if 1/1 is used7. Non-BCCH channels are chosen for voice in preference to the BCCH channels.8. GPRS (and future EDGE) traffic is assigned to the BCCH in preference to the Non-BCCH channels, this is detailed in section 7.

The Non-BCCH frequencies will be used as the first choice for carrying voice traffic. This ensures that mobiles achieve all the benefits of frequency hopping. To maximize the performance of the traffic channels it will be necessary to minimize the number of frequencies assigned to the BCCH plan. Although fractional reuse averages out interference from many sources, effectively eliminating the need for frequency planning on the non-BCCH channels, the actual peak and averaging loading will depend on the quality of the traditional frequency plan, i.e. the BCCH plan.

4.2 Frequency hopping benefits

4.2.1 Interference averaging concept

In traditional cell planning, where transmit powers are constant and frequencies are non-hopping, the downlink interference pattern is also constant and is worst at the cell edges where a low number of interferers dominate.

As the frequency reuse pattern is tightened the number of interferers increases, as surrounding sites transmissions are no longer suitably attenuated by distance, however some BTS continue to be lower interferers than others.

By letting sectors change frequency on a seemingly random basis the interference pattern at a single point is no-longer constant but changing. Frequency hopping exploits the



interleaving and coding protection of GSM such that short bursts of very poor interference can be tolerated.

Interference at a point is no longer defined by a low number of dominant interferers but is now defined by the average interference received from a large number of non-dominant interferers. This interference averaging is repeated across the sector for all mobiles.

Interference

F1

F2 F3

MS_1 MS_2 MS_3

No hopping Interference

F1

F2 F3

MS_1 MS_2 MS_3

With hopping

F1

F2

F3 F1

F2 F3

average

Figure 1 Interference Averaging

source: Nokia

4.2.2 Frequency hopping a good interference averaging technique

Frequency hopping, a standard feature of the GSM system, provides the most effective interference averaging technique. The term frequency hopping describes a technique where the base station and mobile station changes RF frequency between each burst of transmission. The number of RF frequencies over which to hop is called a hop-set.

By hopping over a number of frequencies, the interference created by each transceiver (TRX) is averaged across all frequencies in the hop-set. The impact on the overall interference level is additive but not focused on any one frequency or geographical area. Hopping over a sufficiently large frequency range will also average out the effect of frequency selective fading, which is a characteristic of multipath environments. This is known as frequency diversity. This benefit is particularly important for the downlink, as the mobile station does not have receiver diversity.



Frequency hopping benefits [4]: Better tolerance to low C/I: The GSM channel coding scheme is designed to recover lost information when the punctures are short in duration. Frequency hopping is more resistant to interference because of the convolutionnal decoding, this frequency diversity gain is detailed in part 4.2.3. Easier frequency planning: Once the spectrum has been partitioned and the hop-sets identified, TRX can be added to the system with relative ease by simply including them in the most appropriate hopping set for that cell ‘Soft’ capacity and gradual degradation: rather than a small number of users being victimized by co-channel in a small number of areas, all users on hopping channels will be degraded somewhat. This is called ‘soft capacity’ and is generally desirable in systems where large numbers of users share a limited common resource Frequency selective fading diversity: By FH over larger frequency ranges, the coherence bandwidth can be overcome and the depth of fast fades and BER/FER is consequently reduced. This can greatly improve channel conditions for stationary and slow moving mobile users in any environment. The overall benefits in terms of S/N ratio have been estimated to be around 2dB. Note that for users who are mobile, the effects of fast fading are taken care of by the interleaved channel coding error correction mechanisms

The disadvantage includes:

Synthesized frequency hopping requires a minimum of two TRX to implement, one for the BCCH, and the other for hopping TCH. Base-band hopping will require as many TRX as there are frequencies in the hop-set Quality is very traffic sensitive; the quality of the network degrades very rapidly with increase in traffic, but on the other hand a high traffic makes the frequency planning impossible if non-hopping strategy is implemented

4.2.3 Frequency Diversity Performance Gains

Rayleigh fading causes deep notches in the received signal strength due to active cancellation of the received signals that arrive at a mobile caused by the difference in path lengths. Rayleigh fading is most prominent in urban and suburban environments where most radio propagation paths are non-line-of-sight. The occurrence of these deep notches, both in space and time, are highly frequency dependent.

By changing the carrier frequency (frequency hopping) on a burst-by-burst basis, the occurrence of these notches are spread over several transmission bursts rather than effecting a group of consecutive bursts. Frequency hopping hence provides frequency diversity that has the effect of de-correlating the errors across the interleaved time-slots. To achieve de-correlation between hopping bursts frequency separations of 400KHz (2 GSM channels) is needed.

The speech codec of GSM delivers data rate to the channel codec of 13kbps. The channel coding applies convolution coding and parity protection for the more important speech bits. The 456 bits of the encoded speech are divided in to 8 groups of 57 bits. Therefore



individual encoded speech bursts are interleaved over eight time-slots, to ensure that errors caused by radio fading are as distributed through out the speech frame as much as possible. The maximum performance of the coding is achieved when any radio, induced errors are de-correlated across the interleaved bursts.

Moving mobiles benefit by increasing the time spread and hence de-correlation of transmission errors; above 35km/h a correlation envelope of 0.7 is achieved. Mobiles at slower speed therefore have relatively poor performance when compared to the high-speed mobiles. Frequency diversity is able to provide high levels of de-correlation. As high-speed mobiles already experience de-correlated bursts, the frequency diversity gain is only available to slow moving mobiles.

Several simulation studies have been performed on the gain attainable from FH in a noise-limited environment.

The COST 231 study “Performance of Slow Frequency Hopping in GSM, Poznan, September 1995) produced the following tables from link level simulations:

Number of Frequencies

Frequency Hopping Gain TU 3 Frequency Hopping Gain TU 50

Cyclic Hopping

Absolute Level (dB)

Relative Gain(dB)

Absolute Level (dB)

Relative Gain(dB)

1 11.5 0.0 6.5 0.02 8.5 3.0 6.0 0.53 7.5 4.0 6.0 0.54 6.5 5.0 6.0 0.58 5.5 6.0 6.0 0.5

Random Hopping

1 11.5 0.0 6.5 0.02 9.5 2.0 6.5 0.03 8.5 3.0 6.5 0.04 8.0 3.5 6.0 0.58 7.5 4.0 6.0 0.5

12 7.0 4.5 6.0 0.5

Table 2 Frequency Hopping Gains

The link simulation shows that the gain in a noise limited environment for mobiles at a medium to high speed is less than 0.5 dB even for hopping over 12 channels. For a slow moving mobile the gains are much higher; even for just two carrier hopping the gain is 2 to 3dB, for higher hopping sequences gains of up to 6dB can be attained. In an ideal hopping environment, slow moving mobiles are able to attain the same level of performance as fast moving mobiles.

In defining the link budgets for urban and suburban sites the performance differences between fast and slow moving mobiles is not taken in to account. Therefore hopping



effectively helps slow moving mobiles meet the link budget assumptions and does not offer an improvement over the link budget.

4.2.4 Baseband or synthesized frequency hopping?

Frequency hopping is implemented in 2 ways, the baseband and the synthesized frequency hopping.

Baseband hopping only allows the TCH (or SDCCH) to hop on as many frequencies as there are hopping TRX (e.g. if 3 sectors are hopping the frequency hopping would only contain 3 frequencies).

Synthesized hopping allows the TCH to hop on the whole spectrum allocated for hopping with very little limitations (it is actually limited to 63 frequencies in a group and this limit is rarely reached as it corresponds to a 12.6 MHz spectrum).

Baseband hopping was actually implemented when GSM started, as some vendors were unable to propose synthesized hopping on their early equipments. As it is much easier to plan and reinforce the performance of frequency hopping, synthesized frequency hopping is the implementation recommended.

4.2.5 What frequency hopping pattern should be implemented?

The re-use patterns most commonly used in GSM for hopping are 1/1 and 1/3. In theory they are quite equivalent as you can reach a 16% load (20% if optimized) of the frequency group if using 1/1 and 50% load of the frequency group if using 1/3. But as the frequency group used in 1/1 (this group in that case represents the whole spectrum used for hopping channels) is three times larger than the frequency group used in 1/3 (a third of the spectrum allocated for hopping channels).

The theoretical advantage of the 1/3 reuse pattern is that the adjacent cells never use the same channels, but it is counterbalanced by the fact that collisions with the cell using the same group and which is often in dense urban areas quite close are very important. The 1/3 re-use pattern is very efficient if there is a regular pattern, typically this is true for flat areas with buildings of homogeneous sizes.

The 1/1 is the hopping strategy that we recommend migrating towards in cases of more challenging environment:

As it is impossible in the real world to have a 120 degrees angle between azimuths for all sites (building or terrain mask, unpopulated areas such as water or mountain covered if pattern is respected, etc…). And these sites that do not respect the pattern degrade the 1/3 hopping strategy. Whereas on the other hand 1/1 strategy does not suffer any degradation from pattern not respected.

We often have a number of hopping frequencies that is not divisible by 3 so hopping groups in 1/3 might unbalanced or get a reduced number of frequencies. 1/1 strategy of course maximizes the use of the bandwidth allocated for the frequency hopping



The 1/1 strategy also reduces interferences more easily in areas that would require a site that does not respect the pattern (bi-sectorial site to cover a highway, pico-cell in hot spots)

In case of a large spectrum allocation and a low usage other techniques such as 3*9, 3*3, 4*4, 4*12, and 5*5 may be used in frequency hopping with smaller groups of frequencies, therefore the interference is limited. The issue with these techniques is that a frequency plan must be performed; these techniques will not be studied further in this document.

Recommendations for different markets, according to their spectrum allocation, will be given in section 5.

4.3 Average and Peak Loading

Fractional loading performance is dependent on the interference averaging achieved by having more transmitting frequencies available to a sector than there are TRXs. The use of different hopping patterns on similar orientated sectors randomizes the possibility of collisions. At any one time only a fraction of the possible frequencies in a reuse pattern are being transmitted. For two sectors using the same MA list, but different HSN, the probability of the same frequency being transmitted at the same time is 1/(N*T), where N is the number of transmit frequencies available, T is the number of TRX.

To maintain speech quality and drop call performance this probability needs to be kept within certain limits. This is called soft loading. There is no automatic way of limiting the traffic such that the performance of the system is not impacted; therefore the loading of a network needs to be periodically monitored. The loading of the system is defined in two parameters:

Average Load Peak Load.

The Peak load is computed from the highest traffic cells in an area and is often set as a hard limit, i.e the number of available TRXs are set at the Peak load, such that the system reaches 2% blocking at the peak load.

The Average Load is computed by taking all cells in an area and computing the effective traffic loading and frequency reuse. There is no hard limit to the average load and hence the performance of a network must be monitored. Once the average load limit is reached cell splits will be needed. It is possible for a network to reach the average load limit before any one site has reached the peak load limit.

4.3.1 Process for Calculating Loading

Step 1. Measure BH Traffic for busiest cell and surrounding area, approximately 21 sites.

Step 2. Compute the peak cell traffic and the average cell traffic



Step 3. Convert peak and average traffic levels in to channel requirements using Erlang B tables and 0.1% blocking1

Step 4. Convert channel requirements in to TRX occupancy Step 5. Calculate the TRX loading.

Example

8 Sites, 24 sectors carrying 279 ErlangsPeak traffic is 26 Erlangs8 Frequencies per MA list

Peak AverageTraffic per cell 26 11.6TCH per cell @ 0.1% GOS 42 27Loaded Frequencies (8 TCH per frequency)

5.25 3.4

Loading (8 Frequencies per MA list)

65% 42%

9 freq per MA list 58% 37%

Table 3 Loading calculations

4.4 Frequency hopping implementation

4.4.1 Implementing the 1/1 frequency hoppingThe frequency hopping implementation requires three elements:

Frequency group definition HSN (hopping sequence number) planning MAIO (mobile allocation index offset) planning

The frequency group definition is quite simple in the case of 1/1, as it will be composed of all the frequencies available for TCH.

The HSN planning is using all HSN except 0 (use 1 to 63). Each site shall use the same HSN for all its cells as the cells are synchronized and therefore it allows avoiding any adjacent channel interference with the correct MAIO implementation. There is however an exception to this rule if the cells are collocated but not synchronized then the cells should use different HSN. This is true currently for any Nokia site larger than S666 and any Nortel site bigger than S888 and for Ericsson bigger than S11_11_10 (32 TRX).

The HSN planning must be performed so that the sites using the same HSN should be as far as possible from each other, because if two sites are using the same HSN they will interfere each other all the time, hence there will be no benefit from frequency hopping. If there are pico-cells using frequency hopping then the planning of the HSN of these cells must be performed after the macro layer has been done in order to prioritize the interference from the cells with longer coverage range.

The MAIO planning should be performed in order to avoid any adjacency between the different cells of the same site. This is achieved this way:

1 The measured BH traffic is carried traffic where as Erlang B tables uses offered traffic. By using a very low level of blocking the difference between offered and carried traffic channel requirements is minimized.



Frequency of hopping group

f1 f2 f3 f4 f5 f6 f7 f8 f9 f10 f11 f12 f13

Sector A MAIO

0 6 12

Sector B MAIO

2 8

Sector C MAIO

4 10

We can see from this table that using the MAIO 0,6,12,18… TRX 1,2,3,4 of sector A, MAIO 2,8,14,20… for TRX 1,2,3,4 of sector B and MAIO 4,10,16,22… for TRX 1,2,3,4 of sector C we are sure to avoid any adjacent channel and co-channel interference between the site’s different cell.

If we summarize graphically each site’s sector should look this way:

Sector A MAIO 0,6,12,18…

For all three sectors HSN=N with 1=<N<=63

Sector C MAIO 4,10,16,22… Sector B MAIO 2,8,14,20…

4.4.2 Implementing a 1/3 frequency hopping re-useThe same way as the 1/1 we have to define for the 1/3 the HSN and the MAIO. Unlike

for the 1/1, the frequency groups also have to be defined.There are two ways to set the hopping groups for the 1/3:

Contiguous groups: the first group (each group contains N frequencies) contains F1, F2, F3 … Fn, the second group contains Fn+1, Fn+2 … F2n and the third group contains F2n+1, F2n+2 … F3n

Interleaved groups: the first group (each group contains N frequencies) contains F1, F4, F7 … F3n-2, the second group contains F2, F5, F8 … F3n-1 and finally the third group contains F3, F6, F9 …F3n

The interleaved solution is better because: You can reach 50% frequency load without adjacent channel interference with a

correct MAIO strategy whereas even the most optimized MAIO strategy does not reach 50% load in case of contiguous groups

If you have to go over 50% load, in the case of contiguous groups you will have systematic intra cell adjacent channel interference whereas you will only have interference between different sectors in the case of interleaved groups



If a 1/3 strategy is used, the first group is used for each site’s sector A, the second group is used for each site’s sector B and the third for each sector C:

Sector A group F1, F4 …F3n-2

Sector C group F3, F5…F3n Sector B group F2, F5 …F3n-1

As for the 1/1, the HSN used should be the same for each sector as they are all synchronized and therefore we can avoid adjacent channel interference between sectors of the same cell.

The MAIO strategy should be implemented this way (frequencies in red mark the fact the above MAIO is used):

MAIO 0 1 2 3 4 5 6 7 8 9Group A F1 F4 F7 F10 F13 F16 F19 F22 F25 F28Group B F2 F5 F8 F11 F14 F17 F20 F23 F26 F29Group C F3 F6 F9 F12 F15 F18 F21 F24 F27 F30

From this table we can observe that there is no adjacent channel at the same time. So MAIO 0,2,4,6… should be used for TRX 1,2,3,4 of sector A, MAIO 1,3,5,7… used for TRX 1,2,3,4 of sector C

Graphically the HSN, MAIO implementation of 1/3 hopping is:

Sector A MAIO 0,2,4,6 …


Sector C MAIO 0,2,4,6 … Sector B MAIO 1,3,5,7…

4.4.3 Implementation of the 3/3 (also called 3/1) hopping strategy

The 3/3 implementation of frequency hopping (also called 3/1) is an implementation in which we have a re-use pattern of 3 and for which in each site all the cells are using the same frequency groups. Therefore there are 3 groups of frequencies.

It is better like in the 1/3 hopping case to chose interleaved groups, therefore as each cell in the site are using the same group, you are sure to avoid adjacent channel collision between different cells.

For the HSN, as all cells within the same site are synchronized it is advised to use the same HSN for each cell of the same site.

For the MAIO, as there is no adjacent channel problem, the only thing to worry about is to set different MAIO for any TRX of the site.



Here is one way to achieve this:

Sector A MAIO 0,3,6,9 …


Sector C MAIO 1,4,7,10 … Sector B MAIO 2,5,8,11…

4.5 Cell Split concept

Cell splitting involves adding sites to an existing plan such that the traffic on a particular sector is reduced. The new cell must be of smaller coverage area than the cell being split such that it does not simply overlap and allows the C/I ratio to be maintained.

If the network is built using narrow beam-width antennas (60 to 65 degree) then the new cell will be placed equidistant between the existing cells, at the edge of the cell to be split. Each sector of the new site will be designed to cover only 25% of the area of the existing cells. Operating the new cell at half power, building a lower height cell or using high levels of antenna tilt can achieve this.

Figure 2 Cell Split process for narrow beam systems

The “split” cell will be reduced in area by 50% and can be left operating at full power. However with a 1/3 reuse pattern a more balanced interference environment is required. This is achieved by reducing the power on the split cell by 50% and adding two additional cell split sites.

For narrow beam systems ideal cell splitting requires a 3:1 increase in sites.



For systems built using wide beam antennas (90 to 120 degree beamwidth), the process is the same but the site placement is different.

For 90 degree systems the new site is still placed at an equidistance between the existing sites, but in this case it is not at the limit of the cell but off to one side. If the cell split is placed at the farthest cell edge the grid array is broken and further cell splits will require the removal of the split cell.

Figure 3 Cell Split Process for Wide beam Systems

The initial addition of a cell split site reduces the area of the target cell by 38% and requires that the split site must be operated at full power. The addition of a second new site allows the split site to be reduced in power to balance the area and allow the coverage to be reduced to 25%.

For 90 degree systems an ideal cell split requires a 2:1 increase in sites.

In ideal systems the cell split process focuses on balancing the coverage area of the sites, which in turn balances, the interference.

In practice, traffic is not evenly distributed within a cell and additional information is needed such that the cell split is successful in reducing the traffic levels on split sectors.

When planning a cell split, it is necessary to gather information on traffic distributions within a cell. Timing Advance (TA) information will provide information as to the distribution of traffic. Each TA2 unit corresponds to 0.5km; for example all mobiles reporting a TA of 0 are with 0.5 km of the site, a TA of 1 between 0.5 to 1km etc.

A distribution of TA can be collected from the OMC-R by running Cell Trace. TA should be collected for the Busy Hour, the distribution calculated and the results compared to either the

2 Note that the TA is calculated based on the roundtrip delay of the radio signals and hence is the radio path length. Where strong multipath effects exist the radio path may be significantly longer than the true distance from the site.



predicted or known coverage area of the cell. A traditional cell split will offload the traffic in the outer 50% of the cell. If the TA distribution shows that the traffic is concentrated in the inner 50% of the sector than the cell split will be ineffective and another method of increasing capacity should be considered.

4.6 BCCH Layer Constraints

The GSM system requires that the BCCH carriers must be transmitted at constant output power, at all times, for all 8-time slots on the carrier. In addition, the BCCH carrier must be static, which implies it cannot frequency hop. This precludes the use of enhanced interference management techniques, as well as standard power control and DTX. For these reason, the BCCH layer is subject to a high interference level and as this TRX is essential for the mobile access and selection of the cell, therefore this layer shall be protected as much as possible. Note that 6 or 7 timeslots on BCCH carrier are used to carrier traffic.

The reuse schemes employed for the BCCH layer should be sufficiently relaxed to ensure acceptable QoS. Typically it is recommended if the spectrum allows it to use a 7/21 for the BCCH pattern to ensure a good quality of service. If the allocated spectrum is reduced, a 4/12 pattern is the minimal re-use that should be considered to guarantee a minimal access to the network.

4.7 BSIC Planning

The BSIC (base station identity code) is the combination of the NCC (network color code) and the BCC (base color code). The BCC and NCC take values between 0 and 7. Usually the NCC is unique to a market and should not be changed from cell to cell. The BCC on the other hand should be dealt with carefully. The rule should be to re-use the same couples BCCH frequency-BSIC as far apart as possible. The reason is that the couple BCCH-BSIC is used to recognize neighbors. Therefore if two cells quite close use the same couple it will confuse the handover process. Furthermore, the OMC will prevent the creation of two neighbors using same BCCH-BSIC couple.

4.8 Preparing the future migration plans

Until recently as Hardware limits of the older Ericsson and Nokia equipment is 6 TRX, older Nortel equipment is limited to 8 TRX. Therefore we were usually limiting the BTS expansion to S666 in the first two case and S888 for Nortel, with the hardware being the limiting factor.

As now all vendors offer (by end of 2002) high capacity base stations supporting 12 TRX per cabinet, it opens to Voicestream new possibilities for expansion. This enhancement will be highly needed for dense areas with a high subscriber base as well as it is required to support bandwidth consuming data technologies such as GPRS and EDGE.

In order to support the High Capacity Base Stations markets will need to tighten the current frequency assignment rules and take full advantage of fractional loading. In addition



data will eventually require separate channels as usage increases. Ultimately high data rates may require the deployment of 3G or similar technologies that will require spectrum to be set aside.

Markets should develop three to four year migration plans that systematically tighten the reuse for the BCCH channels and allow for higher fractional loading on the hopping traffic channels. Markets should aim to increase the TRX per sector to the levels shown for Advanced Network Designs. In many cases levels higher than the loadings shown may be obtained depending on the quality of the design. The ability to meet the Advanced Network Design levels is dependent on the ability to contain interference on the BCCH plan.

This can be attained by:

Consistent height of sites Use of Electrical down tilt and narrow beam antennas Use of Static Power control Maintaining consistent antenna orientation Gird like pattern of site placement

Advanced features such as concentric cells and layered networks will increase the capacity of the system allowing lower spectrum allocations to meet the 12 TRX limit, or alternatively allow for more spectrum to be set aside for data services. Such techniques are for further study.

Both the average and peak traffic levels represent a soft limit. This limit implies that although capacity is available in an area the performance of the system will be impacted by increasing the loading of the system beyond an average of 40%. If the soft limit of the system is exceeded then the average TRX per sector with in a given area needs to be reduced by adding additional sites, i.e. cell splitting.



5 Spectrum Partitioning

The question here is, for a given spectral bandwidth, how many frequency channels (ARFCN) should be used as the BCCH, the remainder will thus be used as TCH? Further, what re-use schemes should be deployed for the TCH carriers to support the projected traffic load?

5.1 Comparison of block and interleave partition

There are two basic methods for allocation of BCCH carriers, block partition and interleaved partition. In block partition, a block of frequencies is reserved for used as BCCH carriers, and the rest as TCH carriers. In interleaved partition, every 2nd or 3rd frequency is reserved for use as BCCH carrier. Refer to the diagram below.

BCCH TCH

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Block

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Interleave

Figure 5.4: Example of block and interleave spectrum partition

Full powerBCCH carrier

Down poweredTCH carrier

Figure 5.5: Interleave spectrum partition prevents downlink power control

The interleaved partition approach is not practical since it prevents the use of downlink power control, therefore the BCCH and TCH spectrum should be separated and not interleaved. As illustrated in Figure 5.5, when the TCH is power down, there is strong adjacent channel interference from the BCCH carrier, which is transmitted at full power.

5.2 Comparison between contiguous and split TCH

As illustrated in the diagram below, the BCCH frequencies block can be placed in the center of the available spectrum, thus splitting the TCH into 2 blocks of frequencies. This provides larger frequency hopping range.



However, the split TCH results in two adjacent TCH carriers. Thus increasing the potential for adjacent interference. Using guard bands on either side of the BCCH spectrum will solve this. It is recommended to use a guard band between TCH and BCCH band.

BCCH TCH

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Central

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Bottom

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Top

Figure 5.6: Example of side and central spectrum partition

Alternatively, placing the BCCH at the bottom or top of the available spectrum reduces the number of adjacent channel TCH to one, and provides a contiguous TCH block.

The penalty of this approach is that it reduces the ability of frequency hopping to combat frequency selective fading, however it does not seem to compensate for the loss of a frequency except in the case of very important spectrum allocation (typically 20 MHz uplink and downlink of spectrum or more). Therefore we recommend in most cases to use contiguous TCH and to have the BCCH located on top or bottom of the spectrum.



6 Market application

6.1 Frequency planning strategy suggested for a 5 MHz market (5 MHz bandwidth uplink and downlink)

For a 5MHz market (5MHz downlink, 5MHz uplink), there is a total of 25 ARFCNs (absolute radio frequency channel number). Of these 25, there is 1 frequency block guard band, the 25th ARFCN, and 2 low power guard bands (quarter watts, 24dBm maximum), the 1st

and the 24th. Therefore it leaves only 22 ARFCN available, which is very reduced.

In order to realistically provide a sufficient quality for the signaling a 4/12 BCCH reuse is inevitable. As we have to leave a guard channel between the TCH and BCCH band, this will leave only 9 channels to use for the TCH, implemented in 1/1 hopping, which allows us to go up to S333 configuration the frequency load being 2/9=22% quite high but achievable. Other strategies (non-hopping, 1/3) would probably be limited to lower configuration and in the case of non-hopping create a very complicated cell planning.

The cell split criteria would be to split at S333 configuration in a 5 MHz market.Here is the suggested frequency planning for a 5MHz market:

Block guard ¼ Power guard BCCH TCH guard band bet TCH&BCCH

The frequency groups would be set this way for the BCCH (group 1 to 4):

sector Group1 Group2 Group3 Group4A 2 3 4 5B 6 7 8 9C 10 11 12 13

As for the TCH the repartition will be the following:

Group 1/1 15 16 17 18 19 20 21 22 23

The MAIO strategy used is going to be:

Sector MAIO TRX1 MAIO TRX2A 0 6B 2 8C 4 7 (8 or 6 if the first or second cell has only 2

TRX)


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Frequency Planning Guidelines

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