35805776 freequency hopping

8/2/2019 35805776 Freequency Hopping

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1 Introduction

GSM networks today are under mounting pressure to provide users with good quality

communication equivalent to wireline networks while meeting increasing traffic load as a result of

subscriber growth. In fact, many users use their phones as their main instrument for

communication with friends, colleagues and clients.

As networks complete their macrocellular layer, and are in the process of planning or rolling out a

microcellular layer, subscriber growth continues unabated. If the network is to survive and satisfy

its paying users, new solutions to capacity and quality have to be implemented immediately. In

addition, any new capacity or quality solution should meet the following criteria:

Increases Network Quality

Increases Network Capacity

Ease of Feature Rollout

Utilizes Existing Network Infrastructure

The best feature available today that meets the mentioned criteria is frequency hopping.

Frequency hopping until recently is an underutilized feature in most GSM networks worldwide. The

lack of use is mainly due to not understanding the planning rules, or not appreciating the capacity

increase that frequency hopping can bring to the network. In the past, most networks were able tohandle growth because additional spectrum was available, and macrocell sites could still be added

to handle new subscribers. So there was not a great amount of interest to use this inherent

feature of GSM to add capacity to the network.

Today Motorola has extensive experience in planning, implementing, and optimizing frequency

hopping in networks throughout the world. In fact, frequency hopping in the upcoming years will

be the norm in networks and not the curiosity that it was in the past.

2 GSM Basics

2.1 Speech Coding in GSM

The GSM speech coder breaks up human voice into 20ms blocks that are transmitted over eight

consecutive TDMA frames. Speech data is divided into three different bit classes in the following

manner:

Class Ia: 50 bits Block + Convolution Coded

Class Ib: 132 bits Convolution Coded

Class II: 78 bits Unprotected (no additional coding)

Class 1a

50 bits

Class 1b132 bits

Class 278 bits

260 bits

50 bits3

132 bits4

378 bits 78 bits

Tail

Bits

Parity

Convolutional Code

456 bits

57 bitsEven

57 bits

Odd

57 bits

Even

57 bits

Odd

57 bits

Even

57 bits

Odd

57 bits

Even

57 bits

Odd

DiagonalInterleaving

8 consecutive TDMA burst over the Umair interface


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To protect data further so that the speech coder can correct for lost air bursts, the bits are

reordered through diagonal interleaving for transmission. After coding the total number of bits is

equal to 456 bits. These 456 bits are divided into eight sub-blocks. These sub-blocks are divided

into even numbered bits and odd numbered bits, four even and four odd. This process divides up

the bits for transmission over the air interface and enables the coder to interpolate or fill in missing

or corrupted bursts when reassembling received speech frames.

A non-hopping call does benefit from coding and interleaving as the designers of the air interface

intended since it is quite common to expect to lose air bursts in a real world radio environment.

The problem for a non-hopping call is that speech bursts lost to signal fading or interference tend

to corrupt too many consecutive air bursts. Since the call is tied to a single frequency, it does not

have the ability to move to a better frequency unless a handover is triggered from sufficient

interference or a stronger neighbor.

In the case of a hopping call, signal fading and interference is combated by switching from a

deficient frequency to better one. In this manner, the chance for a series of corrupted bursts from

a poor frequency can be avoided by spreading the time between bad bursts on that frequency thus

utilizing the benefit of interleaving and allowing the speech to be decoded into a good speech

frame.

2.2 Error Measurement in GSM

2.2.1 Bit Error Rate (BER)

BER estimation in GSM.

BER in GSM is calculated as shown in the above block diagram. The received frames are convolutionally decoded and re-coded again to compare with the original received input. The resultant BER is calculated based on the difference. Thefollowing scale ofBER is defined in GSM as RxQual:

RxQual BER Range (%) Assigned BER (%)0 < 0.2 0.14

1 0.2 0.4 0.28

2 0.4 0.8 0.57

3 0.8 1.6 1.13

4 1.6 3.2 2.26

5 3.2 6.4 4.53

6 6.4 12.8 9.05

7 > 12.8 18.1

2.2.2 Frame Erasure Rate (FER)

It is important to realise that the raw BER explained in the previous section is not a direct representation of perceived speechquality, although both BER and speech quality are loosely correlated. 2 calls having the same BER (RxQual) may presentdifferent speech quality to the listeners. This is quite evident considering the effect, on the speech quality, of a short but deep

fading and a constant low BER. The average BER may be the same but the recovered speech will be different.Speech quality in a GSM network is directly related to the integrity of the recovered speech frames, after decoding and de-interleaving, which is measured by FER.

GSM uses the speech coding algorithm as explained in section (???). Each Speech Frame is interleaved over 8 TrafficChannels (TCH) for Transmission. Resulting in an overall rate of one received speech frame over 4 consecutive TrafficChannels (TCH). TCHs are defined using a 26-frame multi-frame, which is about 120 ms. Out of the 26 frames, 24 are used

Convolutional (de)coder

456 bits

buffered

260 bits

buffered

Re-code

456 bits

CompareCalculate

BER

buffered

Re-coded data

Decode


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for traffic, one is used for the Slow Associated Control Channel (SACCH) and one is unused. This is shown in the followingfigure.

TCH frame = { 0, 1,2,11, 13, 14,25 }

For a recovered speech block to be discarded or erased, either the CRC check on the convolutionally encoded Class 1 bits failor the number of error bits in the whole block must exceed a certain level.Each TCH Multi-frame supports 6 speech frames. In each measurement report (480 ms) period there are 4 TCH multi-frames,so a total of 24 Speech frames are received during each measurement period. FER can be calculated from the recoveredSpeech Frames and is available for every measurement report period (480ms). FER has the range of 0 (best) to 24 (worst).

Although FER is a better representation of speech quality, it is not included as part of the measurement report in GSMrecommendations.

3 Frequency Hopping System

3.1 Some Basics

The MS/BTS operating in a frequency hopping system are able to Tx/Rx on different frequencies for

every TDMA burst ( 577s). GSM recommendation defines the following parameters for a

frequency hopping system and they are sent from the BTS to MS in the assignment messages

during call setup.

Mobile Allocation (MA): This is the set of frequencies the mobile/BTS are allowed to hopover. Two time-slots on a same transceiver of a cell may be configured to operate on differentMA. MA is the subset of the total allocated spectrum for the GSM operator and the maximumnumber of frequencies in a MA list is limited by GSM recommendation to 64.

Mobile Allocation Index Offset (MAIO): This is an integer offset that determines which

frequency within the MA will be the operating frequency. If there are N frequencies in the MAlist, then MAIO = {0, 1, 2, N-1}.

Hopping Sequence Number (HSN): This is an integer parameter that determines how thefrequencies within the MA list are arranged. There are 64 HSN defined by GSM. HSN = 0 sets acyclical hopping sequence where the frequencies within the MA list are repeated in a cyclicalmanner.

HSN = 1 to 63 will provide pseudo random hopping sequence. The pseudo random pattern will

repeat itself after every hyperframe, which is equal to 2,715,648 (26x51x2048) TDMA frames

or about 3 hours 28 minutes and 54 seconds.

Motorola defines a Frequency Hopping Indicator (FHI) that is made up of the above three GSM

defined parameters. Up to 4 different FHI can be defined for a cell in a Motorola BSS and every

time-slot on a transceiver can be assigned one of the defined FHI, independently. The tables in

Annex (B) shows the Mobile Allocation Index (MAI) of a frequency hopping system, for

different HSN & MAIO settings. MAI is an integer that points to the frequency within a MA list,

where MAI = 0 and MAI = N-1 being the lowest and highest frequencies in the MA list of N

frequencies. MAI is a function of the TDMA frame number (FN), HSN & MAIO of a frequency

Hopping System. The algorithm involved is documented in GSM 05.02 and it is included inAnnex

(A).

3.2 Frequency Hopping Implementation

There are 2 ways to implement frequency hopping at a BTS.

Synthesiser Frequency Hopping (SFH)

Baseband Frequency Hopping (BBH)

Please note that the above 2 methods only refer to the radio transmitter of a BTS. The output signals from these methods areexactly identical on the air-interface. The mobile station and the BTS radio receiver will always use the retune method, i.e.SFH.

120 ms

unusedSACCH

25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0


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3.2.1 SFH

The transceiver unit re-tunes to a different operating frequency set (Tx & Rx) on each TDMA burst ( 577s). The re-tuning willfollow the sequence explained in the previous section. In theory, there is no restriction on the number of frequencies thetransceiver unit can hop on. However, GSM specifications limit the total number to 64 frequencies for a SFH transceiver unit.

3.2.2 BBH

In this method, the transceiver unit will always transmit at an assigned frequency. Frequency hopping is done by switching theinformation frame of one call from one transceiver to another within a cell, per TDMA burst ( 577s). The switching oftransceivers will follow the sequence defined in FHI, as explained in previous section. The resultant transmitted signal on theair-interface is identical to SFH. Please note that the uplink path will not use BBH and the transceiver on which the call isestablished will always receive the uplink signal from the MS. All the processing (e.g. coding, interleaving etc) will be carriedout by this transceiver and the processed information will be routed to different transceivers for transmission.

4 MOTOROLA Supported features

The following section outlines how frequency-hopping systems are configured in a Motorola BSS with software load 1510(GSR3) and beyond. These may only applicable in a Motorola BSS and have no direct equivalence in other suppliersequipment.Motorola offers friendly and highly flexible solutions in supporting Frequency Hopping System defined in ETSI GSMrecommendations. Both SFH and BBH can be enabled at different sectors within the same site.

All existing BSS hardware can be configured to support frequency hopping and all changes that are needed to configure afrequency hopping system can be carried out in dbase (soft) changes. The only exception is in BTS where a Remote Tuneable

Cavity Combiner (RTC) is used and this limitation will be discussed in the coming section.

Motorola defines a Frequency Hopping Indicator (FHI) that specifies a frequency hopping

system. Up to 4 (0 3) different FHI can be defined for a cell in a Motorola BSS dBase and every

time-slot on a transceiver can be assigned one of the defined FHI, independently. Each FHI

consists of the following parameters:

Hopping Support: defines the hopping system of a GSM cell, SFH or BBH.

Mobile Allocation (MA): The hopping frequency list. Valid values are 1 63 in SFH and itmust equal or less than the number of hopping carriers in BBH. (see section ??)

Hopping sequence number (HSN): Defines how the frequencies in the MA are hoppedthrough. Valid values are 1 63. (see section ??)

Each defined FHI can be modified on-line from the OMCR terminal or the BSC MMI without causinga reset to the affected sites. However, depending on the type of hopping system and parameters

being changed, active calls on the affected transceiver carriers may be lost. For details of this

operation, please refer to the associated technical manuals.

MAIO of a transceiver carrier is defined by the ARFCN assigned to it during RTF equipage.

Modification to the MAIO can be carried out by the command chg_rtf_freq, which changes the

ARFCN of the affected transceiver carrier. Please refer to the associated technical manuals for the

effect of this action to the BSS.

4.1 BBH

When a BBH carrier or time slot is activated or de-activated, other affected carriers or time slots

(hop over its frequency) must be reconfigured to include or exclude a channel in the operatingfrequency list. Intra-cell handover will be attempted to move all the active calls on these time slots

to unaffected time slots (e.g. non-hopping or MA list without this frequency) within the cell.

Unsuccessful intra-cell handover (e.g no time slots available) will cause the calls to be cleared.

In a BBH system, a parameter called hopping_ins_mode is used to determine whether a

previously inactive carrier would be brought into service as hopping or not, outside of site

initialization time (sysgen mode).

The number of channels in a MA list must be equal or smaller than the number of BBH carriers in a

cell. It is worth noting that the FHI assigned to a timeslot must be in accordance to the MA list of

the FHI. For example: In a cell with 3 BBH carriers (namely, A, B & C) and MA = {f1, f2, f3} is

defined in FHI 2. If FHI 2 is to be assigned to time slot 2 of carrier A, then time slot 2 of carrier B &

C must also be assigned FHI 2, to make the system work.


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4.2 SFH

If the BCCH frequency is included in the MA list, timeslot 1 to 7 of the BCCH carrier will not be able

to carry traffic. This is an inherent limitation of SFH and it is recommended that BCCH frequencies

should be excluded from the MA list whenever possible.

SFH cannot be implemented at a cell that uses narrow-band Tx combiner (e.g. RTC -Remote

Tunable Cavity Combiner). The reason is SFH requires the hopping carrier and associated Tx

combiner to retune to a new frequency every TDMA burst ( 577s). Since RTC re-tuning involves

mechanical movement, it is not possible to cope with the speed. As a result, only broadbandcombiner, e.g. hybrid combiner, can be used at a SFH cell.

4.3 Enhancement in Future Software Load (GSR4)

Below are the enhancements that are available in the coming software load (GSR4):

Frequency Redefinition: this is a procedure defined in GSM-phase 2 that can change theproperties of an active Frequency Hopping System, without affecting the active calls in a cell.

Improved TCH Control: This feature provides the operator the flexibility of prioritizing theassignment of TCH in a frequency-hopping cell. A hopping or non-hopping carrier may be givendifferent preferences in TCH assignment, depending on the operating RF environment of thecell.

Explicit SDDH Control: This feature allows the operator to choose the explicit carrier forSDCCH allocation. It may be desirable to place all SDCCH on the BCCH carrier to maximize theperformance of a hopping cell.

Different Quality Thresholds: In a frequency hopping system, a call can withstand higherRxQual (BER) than a non-hopping system. As a result, a RxQual threshold, for imperativehandovers, that is different from a non-hopping carrier will help to improve systemperformance.

5 How Frequency Hopping Improves Quality

Frequency Hopping can improve the radio air-interface quality of a cellular network in 2 ways:

Frequency Diversity. Interference Averaging.

5.1 Frequency Diversity

Quality is improved in the network by using frequency hopping to alleviate the effects of frequency

selective fading that is inherent in radio wave propagation in the GSM 900 band, and especially at

frequencies in the GSM1800/1900 band where environmental factors have a great effect on the

stability of radio signal levels.

Fixed frequency carriers, non-hopping, experience natural signal fading in the radio environment.

Generally, fading is not a great problem unless the mobile station is in an area of low signal

strength (i.e. indoors or at cell boundaries), or is in an area of no dominant server. In this case,

normal Rayleigh fading can cause disruptions to speech by inducing bit error that cannot be

corrected, since the receiver is getting too many consecutive corrupted speech bursts over the air

interface.

In a GSM, once a speech call is allocated to a channel, voice is transmitted over 8 consecutive

TDMA frames for every 20 ms of speech. If a speech call is placed on a fixed carrier, non-hopping,

then the call is tied to the fading profile of that frequency. So as a call experiences a slow fade the

BER becomes a problem and affects call quality. The GSM air interface is designed to handle some

degree of BER to counteract a reasonable amount of air interface corruption in the mobile

environment.

The same call on a frequency hopping system is moving from frequency to frequency every 4.62

ms, and can take advantage of the different fading profiles of each frequency in the allocated

hopping sequence. The greater the hopping frequencies are spaced, the greater the de-correlationbetween the fading profile of each frequency and the signal level. Field data shows that when calls

are made on a hopping and on a non-hopping carrier, hopping calls have far greater signal

stability. Frequency hopping averages out extremes in high signal levels and low signal levels.

Field data of calls hopping over as little as four frequencies show a pyramid shaped graph of0

5

10

15

20

25

0-2 2-4 4-6 6-8 8-10 10-12 12-14 14-16 16-18 18-63

RXLEV Range

P

ro

b

(%)

2

4

6

10

12

14

16

18

20

0-2 2-4 4-6 6-8 8-10 10-12 12-14 14-16 16-18 18-63

RXLEV Range

Pr

o

b

(%)

Non Hopping System Hopping System


6/13

receive signal level with more of the data points near the mean with a smaller standard deviation

than the graph for a fixed frequency, non-hopping, call. These are shown in the figures below:

The following figures illustrate the effect of hopping over 2 frequencies.

Fading Profile of F1

Effect of deep fading to TDMA frames

Effect of deep fading in a hopping system to the TDMA frames

As can be observed, benefit of frequency diversity gained from frequency hopping is significant.Not only the total number of bad frames is reduced, more importantly the occurance of bad frames

in consecutive order is reduced as well. This will improve the speech quality as the lost bits have

higher probability to be recovered by the GSM decoding mechanism and hence a lower number of

erased speech frames. (refer section 2.2.2 regarding FER)

Signal

Level

Deep fade

Good Signal level

Threshold

Good frame

Bad frame

F1 on air

F2 on air

Good frame

Bad frame

F1 fading profile

F2 fading profile


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5.2 Interference Averaging

Interference protection is probably the biggest improvement that comes as a result of

implementing frequency hopping. Calls made on fixed frequency systems may suffer from

interference, which has a good chance of not diminishing in the lifetime of a call unless the

subscriber changes position, or the interfering channel is deactivated.

Either co-channel or adjacent channel interference hits fixed frequency calls normally at the cell

border. This type of interference is constant to the subscriber in the downlink direction. Usually,

interference found at the cell boundary cannot be escaped from unless a handover is made to aclean frequency. To avoid interference on fixed frequency systems larger separation between

reuse groups is used to lessen the chance of co-channel or adjacent channel interference from

degrading call quality. The cost of loose reuse schemes to the network is capacity.

Co-channel interference in non-hopping cells

A hopping system distributes interference throughout within the hopping cells, instead of

concentrating it in any particular area or bad spots. By sharing the channels continuously but not

necessarily simultaneously, hopping has the effect of eliminating or smoothing the C/I extremes at

very good and very bad spots. The spreading of interference creates interference diversity,

which reduces the probability of any one mobile being interfered for long duration. Call quality is

improved and the consistency of a GSM call is improved.

C/I plots of fixed and hopping systems

Trial data from urban cities has found that frequency hopping clearly improves received quality as

compared to a fixed frequency system when even one of three frequencies in a hopping sequence

is interfered. The following figure illustrates this:

f1

ff11

InterferenceInterference !!

f1

Cell A

Cell B

ff11

High InterferenceDropped Calls

Low InterferenceGood Quality

Medium InterferenceNoisy Calls

Very High InterferenceNo service

Fixed frequency4X3 reuse plan

Frequency hopping1X3 reuse plan

Prob RXQUAL 6/7 vs C/I

1 Freq. in the sequence affected by interference

0

5

10

15

20

25

30

6 8 10 12 14 16

C/I dB

Sequence of 3 freqs

+

Sequence of 4 freqs

Sequence of 5 freqs

Fixed freq

Cell A

Cell B

Resultant

frames of cell A

Good Frames

Idle time of Cell B

Potential bad

frames if C/I < 9dB


8/13

6 How Frequency Hopping Enhances Network Capacity

In principle, implementation of frequency hopping system will not add extra capacity to the

existing network. Frequency hopping when implemented will enable more aggressive frequency re-

use pattern that leads to better spectrum efficiency. This enables the network operator to add

more transceivers in existing sites while maintaining the network quality. In a congested network

with fixed frequency plan, adding transceivers would mean compromising the carrier interference

ratio (C/I), which may lead to unacceptable quality level that may eventually crash the network if

pushed to the limit. Thus, frequency hopping is effectively compressing the available spectrum to

make room for extra capacity, without degrading the average C/I as in a fixed frequency system.

In a cellular network, there is always a tradeoff between capacity & quality. Maintaining the current

capacity, implementing frequency hopping will improve overall quality. On the other hand, extra

capacity could be added by implementing frequency hopping while maintaining the current quality.

However, realizing maximum gains in both quality and capacity would not be achievable.

6.1 Frequency Plan

Frequency Planning is considered the most fundamental and important plan for any cellular

system. Limited spectrum is available and frequencies have to be re-used. An optimised frequency

re-use plan is crucial to the success of a cellular network in order to obtain maximum capacity from

limited bandwidth.

6.2 Conventional re-use pattern

The most common conventional frequency plan of a cellular system is the 4X3 (3-sector 4-site) re-

use plan. This means that the re-use pattern is repeated every four 3-sector sites or every 12

sectors, as shown in the below figure:

Non Hopping Hopping Hopping Hopping

A

BC

FD

G E

HI JLK

A

BC

FD

G E

HI

JL

K


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For example, an uniform 2-2-2 site configuration would require 4X3X2, 24 channels for the

frequency plan. 4X3 re-use pattern is a good compromise between co-channel interference and

capacity. The typical carrier Interference ratio (C/I) is calculated to be about 13.6dB, which is

above GSM specified 9 dB.

6.3 Aggressive re-use in frequency hopping system

Re-use plan in a frequency hopping network is different and more aggressive than it is in a fixed

frequency network. There are also some general differences between BBH and SFH system.

6.3.1 SFH

Most of the SFH networks employ 2 different re-use plan for the BCCH and TCH layers. Since the

BCCH will not be hopping, conventional fixed frequency re-use plans such as 4X3 or 5X3 will be

used. It is always a design goal to have a best BCCH layer, within the resource of the network. As

for the TCH layer, the common methodology would be 1X3 (1 site 3-sector) re-use pattern. This is a

much more efficient spectrum utilization, which is not possible in a fixed frequency system as the

resultant C/I would be degraded badly beyond of the cell radius. An even more aggressive re-use

plan 1X1 (1 site 1 sector) is feasible in networks where the operating environment permits it. 1X1

is by far the most efficient and yet practical aggressive re-use plan tested and proposed by

Motorola. Nevertheless, careful planning has to be practiced to achieve good results. The guide

lines are outlined in the next section.

Loading Factor (or sometimes termed as Fractional load factor) is an important parameter in SFH

systems. It is calculated a

loading factor = (highest non BCCH transceiver count in

a cell)

(Number of hopping channels)

Since the number of frequency channels is always higher than the transceiver count in a cell, some

channels will be idle at one time. Thus, loading factor is equivalent to the maximum channel-

occupancy to total-channel ratio in a cell at any given instant. The lower the value the lower is the

channel loading, which indicates fewer collisions of frequencies and hence better quality.

A theoretical maximum of 50% is permitted in 1X3 SFH. Any value higher than 50% practically

results unacceptable quality. Some commonly used loading factor are 40%, 33%, 25% etc. In 1X1

SFH, a practical tested loading factor is 1/6 or 16.7%. For a rough comparison, this is about

equivalent to a 33% loading in 1X3 SFH or a well-planned 4X3Xn fixed re-use network, as far as

average quality is concerned. In terms of spectrum utilization or capacity, 1X1 at 16.6% loading is

equivalent to 1X3 at 50% loading.

6.3.2 BBH

Different re-use patterns are employed in BBH systems. Since the number of hopping frequencies

must equal or less than the number of transceivers in the cell, the quality gain of BBH is higher in

the cells with higher transceiver count. As a result, a progressive re-use pattern is usually used.

This is analogy to a layered cake with a loose BCCH plan at the base and progressively tighter planfor each subsequent transceiver added to the cell.

For example: BCCH 4X3 plan

1st TCH 3X3 plan

ACB

ACB

1X3 re-use

A

AA

AAA

1X1 re-use


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2nd TCH 2X3 plan and so on

Alternatively, a homogeneous re-use plan that is tighter than conventional 4X3 can be used. The

widely used pattern would be homogeneous 3X3 re-use plan, which yields comparative results as

in progressive re-use mentioned above.

6.3.3 Example in capacity gain

Take the case of an operator who has 7.2Mhz (or 36 GSM channels) of spectrum to use. The

following table compares the network capacities for different frequency re-use plans.

Re-use planConfiguration

Capacity persite (Erlang)

Capacity gainover fixed plan

Fixedfrequency

4X3 3-3-3 44.7 -

BBH 3X3 4-4-4 65.7 47%SFH(37.5% loading)

4X3 (BCCH)1X3 (TCH)

4-4-4 65.7 47%

SFH(50% loading)

4X3 (BCCH)1X3 (TCH)

5-5-5 87.6 97%

SFH(16.7% loading)

4X3 (BCCH)1X1 (TCH)

5-5-5 87.6 97%

The above calculations are based on:

Erlang B table at 2.0% blocking rate.

2 time-slots are used for control channels in each sector, for all re-use plans.

It is worth noting that above are theoretical figures that may be different in an actual network and the operating environmentmay restrict direct implementation of the mentioned re-use plan. Nevertheless, it serves as a good example in demonstratingthe capacity gain with efficient spectrum usage.

7 Planning Guide

The ultimate goal of frequency planning in a GSM network is attaining and maintaining a highest

possible C/I ratio every where within the network coverage area. A general requirement is at least

12dB C/I, allowing tolerance in signal fading above the 9dB specification of GSM.

The actual plan of a real network is a function of its operating environment (geography, RF etc) and

there is no universal textbook plan that suits every network. Nevertheless, some practical guide

lines gathered from experience can help to reduce the planning cycle time.

7.1 Rules for SFH

As the BCCH carrier is not hopping, it is strongly recommended to separate bands for BCCH and

TCH. This has the benefits of:

Making planning simpler,

Better control of interference.

If micro cells are included in the frequency plan, the below band usage is suggested.

BCCH (4X3)

1st

TCH (3X3)

2nd

TCH (2X3)

Progressive re-use

1st

TCH (3X3)

BCCH (3X3)

2nd

TCH (3X3)

Homogeneous re-use

n channels m channels

BCCH TCH

Guard band

Macro BCCH

Micro TCH

Macro TCH

(SFH)

Micro

BCCH


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Macro BCCH

Micro TCH

Macro TCH

(SFH)

Micro

BCCH

12 channels 27 channels

8 channels

Practical rules for 1X3

BCCH re-use plan: 4X3 or 5X3, depending on the bandwidth available and operatingenvironment.

Divide the dedicated band for TCH into 3 groups with equal number of frequencies (N). Thesefrequencies will be the ARFCN equipped in the MA list of a Hopping system (FHI).

Use equal number of frequencies in all cells within the hopping area. The allocation offrequencies to each sector is recommended to be in a regular or continuous sequence. (seeplanning example)

Number of frequencies (N) in each group is determined by the design loading factor (orcarrier-to-frequency ratio). A theoretical maximum of 50% is permitted in 1X3 SFH. Any valuehigher than 50% would practically result unacceptable quality. Loading factor (sometimestermed as fractional load factor) represents the Some commonly used loading factor are 40%,33%, 25% etc. As a general guide-line,

N = (highest non BCCH transceiver count in a cell)

(loading factor)

For example: mixture of 4-4-4 and 5-5-5 site configurations and loading factor of 33%. Then N =

5/(0.33) = 15 frequencies in the MA list. As loading factor has direct effect on the overall network

quality and its setting is highly dependent on the RF environment, a smaller scale trial is

recommended to obtain the necessary data and experience before larger scale deployment. As a

general rule, SFH with 33% loading is equivalent to a well-planned 4X3 fixed frequency system.

Use same HSN for sectors within the same site. Use different HSN for different sites. This willhelp to randomize the co channel interference level between the sites.

Use different MAIO to control adjacent channel interference between the sectors within a site.

The following example illustrates the above planning guide.Bandwidth : 10 Mhz

Site configuration : mixture of 2-2-2, 3-3-3 & 4-4-4.

Loading factor : 33%Multi layer environment (micro & macro co-exist)

The spectrum is split as shown:

A total of 49 channels are available and the 1st and last one are reserved as guard band. Thus,

there are 47 usable channels. 12 channels are used in the BCCH layer with 4X3 re-use pattern.

Based on 33% loading and 4-4-4 configuration, N is calculated as N = 3 / 0.33 = 9 hopping

frequencies per cell. Thus, a total of 27 channels are required for the hopping TCH layer. The

remaining 8 channels are used in the micro layer as BCCH.

One of the possible frequency plan and parameter settings are outlined in the below table:

ARFCN HSN MAIO

Sector A 21,24,27,30,33,36,39,42,45 Any from {1,2,63} 0, 2, 4

Sector B 22,25,28,31,34,37,40,43,46 Same as above 1, 3, 5

Sector C 23,26,29,32,35,38,41,44,47 Same as above 0, 2, 4


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The above MAIO setting will avoid all possible adjacent channel interference among sectors within

the same site. The interference (co or adjacent channel) between sites will still exist but they arereduced by the randomization effect of the different HSN. (Annex B)

Practical rules for 1X1

1X1 is usually practical in rural area of low traffic density, where the average occupancy of thehopping frequencies is low. With careful planning, it can be used in high traffic area as well.

BCCH re-use plan: 4X3 or 5X3, depending on the bandwidth available and operatingenvironment.

The allocation of TCH frequencies to each sector is recommended to be in a regular orcontinuos sequence.

Use different HSN to reduce interference (co and adjacent channel) between the sites.

Use same HSN for all carriers within a site and use MAIO to avoid adjacent and co-channel

interference between the carriers. Repeated or adjacent MAIO are not to be used within thesame site to avoid co-channel and adjacent channel interference respectively.

Maximum loading factor of 1/6 or 16.7% is inherent in a continuous sequence of frequencyallocation. Since adjacent MAIO is restricted, the maximum number ofMAIO permitted wouldbe:

In a 3-cell site configuration, the logical maximum loading factor would be 1/6 or 16.7%.

7.2 Rules for BBH

All the rules outlined for SFH are generally applicable in BBH. As the BCCH is in the hopping frequency list, a dedicated bandseparated from TCH may not be essential. An example of spectrum allocation is shown below:

BBH channels & micro TCH

Micro BCCH

HSN = 1

HSN = 1

Different MAIO to

avoid co-channel

Non adjacent

MAIO to avoid

adjacent-channel

HSN = 1

Max MAIO = x (Total allocated

channel)


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8 Optimisation

Any major modifications made in a cellular network are initially accompanied by performance

change. Implementing frequency hopping in a planned and optimized network would certainly shift

its overall performance, usually away from the better side. This is why post implementation

optimization is always crucial and important.

There are several means of measuring the network performance and comparing them before and

after implementation:

OMCR statistics include key, cell level and carrier/time-slot statistics.

Drive test drive around the test area and monitor the RF environment with a test phone.

Call tracing trace every nth call in a cell or BSS and process the result with analyzing softwaretools.

Speech quality assess the received speech quality with subjective scores.

8.1 Performance Monitoring

8.1.1 OMCR statistics

The following table summarizes the important statistics for general performance measurements.

Statistics that are not listed may be used to calculate other performance parameters.

8.1.2 Drive Test

Drive test the test area before and after frequency hopping implementation to compare the results.

The routes have to be defined and followed in both cases for consistency. A test phone with file

logging capability is essential. Ericsoft TEMS is the most accepted industrial standard and is

strongly recommended. Using GPS for location logging is an added advantage and further analysis

with propagation tools (e.g. NetPlan) is possible.

Downlink RF characteristics (e.g. RxLev, RxQual, FER, MS_Tx_pwr etc) and call related parameters

(e.g. Handover, call-setup etc) along the drive-test routes are logged to files and available for post

drive analysis. Obvious observations such as poor voice quality, high BER, dropped calls, handover

failures etc should be noted during the drive and file marks should be inserted accordingly.

Statistics Group Statistics name To monitor

Key RF_LOSS_RATE RF LossesTCH_RF_LOSS_RATE RF LossesSDCCH_RF_LOSS_RATE RF LossesHANDOVER_SUCCESS_RATE

Handover

HANDOVER_FAILURE_RATE HandoverTCH_ASSIGN_SUCCESS_RATE

TCH assignment

Cell OUT_HO_CAUSE_ATMPT Handover cause

Carrier/Time-slot INTF_ON_IDLE InterferenceBER Interference

35805776 freequency hopping

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