gsm theories, network analayzation & microwave transmission

GSM Theories, Network Analayzation & Microwave Transmission

National Diploma In Engineering Sciences General Industrial Tanning - 1

Global System for

Mobile

Communications



Global System for Mobile Communications (GSM)

GSM is a digital, mobile; radio standard developed for mobile, wireless, voice communications.

GSM uses a combination of both the time division multiple access (TDMA) and frequency division

multiple access (FDMA). With this combination, more channels of communications are available,

and all channels are digital.

The GSM service is available in following frequency bands:

• 900-MHz & 900 E

• 1800-MHz

• 1900-MHz

GSM Network Elements

A GSM network consists of the following network components:

• Mobile station (MS)

• Base transceiver station (BTS)

• Base station controller (BSC)

• Mobile switching center (MSC)

• Authentication center (AuC)

• Home location registers (HLR)

• Visitor location registers (VLR)

Mobile Station;

The mobile station (MS) is the starting point of a mobile wireless network. The MS can contain the

following components:

• Mobile terminal (MT)—GSM cellular handset

• Terminal equipment (TE)—PC or personal digital assistant (PDA)

The MS can be two interconnected physical devices (MT and TE) with a point-to-point interface or a

single device with both functions integrated

Base Transceiver Station;

When a subscriber uses the MS to make a call in the network, the MS transmits the call request to the

base transceiver station (BTS). The BTS includes all the radio necessary for radio transmission

within a geographical area called a cell. The BTS is responsible for establishing the link to the

MS and for modulating and demodulating radio signals between the MS and the BTS.

Base Station Controller;

The base station controller (BSC) is the controlling component of the radio network, and it

manages the BTSs. The BSC reserves radio frequencies for communications and handles the

handoff between BTSs when an MS roams from one cell to another. The BSC is responsible for

paging the MS for incoming calls.

Mobile Switching Center;

The mobile switching center (MSC) is a digital ISDN switch that sets up connections to other

MSCs and to the BSCs. The MSCs form the wired (fixed) backbone of a GSM network and can



switch calls to the public switched telecommunications network (PSTN). An MSC can connect to a

large number of BSCs.

Architecture of the GSM Network

GSM Channel Concept

The carrier separation is 200 kHz. This provide

• 124 carriers in GSM 900 band

• 374 carriers in GSM 1800band

• 299 carriers in GSM 1900 band

Each carrier is shared by 8 Time Slots (TS). So the total numbers of channels;

• 124 x 8 = 992 channels in GSM 900

• 374 x 8 = 2992 channels in GSM 1800

• 299 x 8 = 2392 channels in GSM 1900

Frequency Allocation

GSM Physical Channels

UP – LINK DOWN - LINK UP-LINK + 45 MHz

GSM 900 890 – 915 MHz 935 – 960 MHz

UP-LINK + 45 MHz

GSM 900

G-BAND

880 – 890 MHz 925 -935 MHz

UP-LINK + 95 MHz

GSM 1800 1710 – 1785 MHz 1805 – 1880 MHz

UP-LINK + 95 MHz

GSM 1900 1850 – 1910 MHz 1930 – 1990 MHz



The up and down-link frequency channel pair allocation has been arranged such that the two

frequencies comprising a channel pair are 45 MHz apart.

Each of these frequency pairs is identified by an Absolute Radio Frequency Carrier Number

(ARFCN).

GSM Physical Channel Structure

Absolute Radio Frequency Channel Number (ARFCN)

Within the spectrum allocated for cellular mobile communications, the radio channels are identified

by ARFCN. When the system is operating in Frequency Division Duplex (FDD) mode then the

channel number is associated with both the uplink and downlink radio channels.

ARFCN Frequency (MHz) ARFCN Frequency (MHz)

975 925.2 976 925.4

977 925.6 978 925.8

979 926 980 926.2

981 926.4 982 926.6

983 926.8 984 927

985 927.2 986 927.4

987 927.6 988 927.8

989 928 990 928.2

991 928.4 992 928.6

993 928.8 994 929

995 929.2 996 929.4

997 929.6 998 929.8

999 930 1000 930.2

1001 930.4 1002 930.6

1003 930.8 1004 931

1005 931.2 1006 931.4

1007 931.6

1008 931.8



1009 932 1010 932.2

1011 932.4 1012 932.6

1013 932.8 1014 933

1015 933.2 1016 933.4

1017 933.6 1018 933.8

1019 934 1020 934.2

1021 934.4 1022 934.6

1023 934.8

GSM Logical Channels

Traffic Channels (TCH)

A traffic channel (TCH) is used to carry speech and data traffic. Traffic channels are defined using a

26-frame multi frame, or group of 26 TDMA frames. The length of a 26-frame multi frame is

120ms. Out of the 26 frames, 24 are used for traffic, 1 is used for the slow associated control

channel (SACCH) and 1 is currently unused.

Full Rate & Half Rate TCH

They can be defined as full-rate TCHs (TCH/F, 22.8 kbps) and half-rate TCHs (TCH/H, 11.4

kbps). Half-rate TCHs double the capacity of a system effectively by making it possible to transmit

two calls in a single channel. If a TCH/F is used for data communications, the usable data rate drops

to 9.6 kbps (in TCH/H: max. 4.8 kbps) due to the enhanced security algorithms. Eighth-rate TCHs

are also specified, and are used for signaling. In the GSM Recommendations, they are called stand-

alone dedicated control channels (SDCCH).



Signaling channels

The signaling channels on the air interface are used for call establishment, paging, call maintenance,

synchronization, etc.

There are three type of signaling channels

1. Broadcast Channels

2. Common Control Channels

3. Dedicated Control Channel

Broadcast Channels (BCH)

Carry only downlink information and are responsible mainly for synchronization and frequency

correction. This is the only channel type enabling point-to-multipoint communications in which short

messages are simultaneously transmitted to several mobiles

BCH Characteristics

• Each cell has a designated BCH carrier

• All BCH timeslots transmit continuously on full power

• TS 0 contains logical control channels

• TS1-7 optionally carries traffic

• BCCH block occur once each 51-frame multiframe

• Each block comprises 4 frames carrying 1 message

The BCHs include the following channels;

1. Broadcast Control Channel (BCCH): General information, cell specific (local area code

(LAC), network operator, access parameters, list of neighboring cells, etc). The MS receives

signals via the BCCH from many BTSs within the same network and/or different networks.

2. Frequency Correction Channel (FCCH): Downlink only; correction of MS frequencies;

transmission of frequency standard to MS; it is also used for synchronization of an

acquisition by providing the boundaries between timeslots and the position of the first

timeslot of a TDMA frame.

3. Synchronization Channel (SCH): Downlink only; frame synchronization (TDMA frame

number) and identification of base station. The valid reception of one SCH burst will provide

the MS with all the information needed to synchronize with a BTS

Common Control Channels (CCCH)

A group of uplink and downlink channels between the MS and the BTS. These channels are used to

convey information from the network to MSs and provide access to the network. The CCCHs include

the following channels;

1. Paging Channel (PCH): Downlink only; the MS is informed by the BTS for incoming

calls via the PCH

2. Access Grant Channel (AGCH): Downlink only, BTS allocates a TCH or SDCCH to the

MS, thus allowing the MS access to the network.

3. Random Access Channel (RACH): Uplink only, allows the MS to request an SDCCH in

response to a page or due to a call; the MS chooses a random time to send on this channel.

This creates a possibility of collisions with transmissions from other MSs



Dedicated Control Channels (DCCH)

Responsible for roaming, handovers, encryption, etc. The DCCHs include the following channels;

1. Stand-alone Dedicated Control Channel (SDCCH); Communications channel between MS

and the BTS; signaling during call setup before a traffic channel (TCH) is allocated

2. Slow Associated Control Channel (SACCH); Transmits continuous measurement reports in

parallel to operation of a TCH or SDCCH 3. Fast Associated Control Channel (FACCH); Similar to the SDCCH, but used in parallel to

operation of the TCH; if the data rate of the SACCH is insufficient, “borrowing mode” is

used: Additional bandwidth is borrowed from the TCH; this happens for messages associated

with call establishment authentication of the subscriber, handover decisions, etc.



GSM Identities



GSM Identities

To switch a call to a mobile subscriber, the right identities need to be involved. It is therefore

important to address them correctly. Followings are those identities;

Mobile Station ISDN Number (MSISDN)

The MSISDN is a number, which uniquely identifies a mobile telephone subscription in the public

switched telephone network numbering plan. These are the digits dialed when calling a mobile

subscriber.

The MSISDN is consisted with followings;

• Country Code (CC)

• National Destination Code (NDC)

• Subscriber Number (SN)

MSISDN = CC + NDC + SN

Ex: - 94 77 1234567

International Mobile Subscriber Identity (IMSI)

The IMSI is a unique identity allocated to each subscriber to allow correct identification over the

radio path and through the network and is used for all signaling in the PLMN. All network-related

subscriber information is connected to the IMSI. The IMSI is stored in the SIM, as well as in the

HLR and in the serving VLR.

The IMSI is consisted with followings;

• Mobile Country Code (MCC)

• Mobile Network Code (MNC)

• Mobile Subscriber Identification Number (MSIN )

IMSI = MCC + MNC + MSIN

Ex: - 94 77 1234567890

Temporary Mobile Subscriber Identity (TMSI)

The TMSI is a temporary number used instead of IMSI to identify an MS. The TMSI is used for the

subscriber’s confidentiality on the air interface. The TMSI has only local significance (that is, within

the MSC/VLR area) and is changed at certain events or time intervals.

International Mobile Equipment Identity (IMEI)

The IMEI is used for equipment identification and uniquely identifies a MS as a piece or assembly of

equipment.

The IMEI is consisted with followings;

• Type Approval Code (TAC), determined by a central GSM body

• Final Assembly Code (FAC), identifies the manufacture

• Serial Number (SNR), uniquely identifies all equipment within each TAC & FAC

• Spare, a spare bit for future use.



IMEI = TAC + FAC + SNR + Spare

Mobile Station Roaming Number (MSRN)

A MSRN is used during the call setup phase for mobile terminating calls. Each mobile terminating

call enters the GMSC in the PLMN. The call is then re-routed by the GMSC, to the MSC where the

called mobile subscriber is located. For this purpose MSRN is allocated by the MSC and provided to

the GMSC.

The MSRN is consisted with followings;

• Country Code (CC)

• National Destination Code (NDC)

• Subscriber Number (SN)

MSRN = CC + NDC + SN

Location Area Identity (LAI)

The LAI is used for paging, to indicate to the MSC in which Location Area (LA) the MS is currently

situated and also for location updating of mobile subscribers.

The LAI is consisted with followings;



• Location Area Code (LAC)

LAI = MCC + MNC + LAC

Cell Global Identity (CGI)

Each cell is identified by cell identity (CI). A CI is unique within a location area (LA).

CGI is consisted with following;



• Location Area Code (LAC)

• Cell Identity (CI)

CGI = MCC + MNC + LAC + CI

Ex:- 413 05 10001 19102

Base Station Identification Code (BSIC)

In GSM, the mobile station uses BSIC to distinguish between neighboring base station.

The BSIC is consisted with

• Network Colour Code (NCC)

• Base Transceiver Colour Code (BCC).

BSIC = NCC + BCC



GSM Principles



Cells

A cell can be defined as an area of radio coverage from one BTS antenna system. It is the smallest

building block in a mobile network and a cell can be represented by a hexagon.

There are two types of cells;

1. Omni directional cell:

An omni-directional cell (or omnicell) is served by a BTS with an antenna which transmits

equally in all directions (360 degrees).

2. Sector cell: A sector cell is the area of coverage from an antenna, which transmits, in a given direction

only. For example, this may be equal to 120 degrees or 180 degrees of an equivalent omni-

directional cell. One BTS can serve one of these sector cells with a collection of BTS’s at a

site serving more than one, leading to terms such as two-sectored sites and more commonly,

three-sectored sites.

Omni-directional cells are used to gain coverage and sector cells are used to gain capacity. But in Sri

Lanka every service provides use sector cells to server their customers.

Clusters

Groups of frequencies can be placed together into patterns of cells called clusters. A cluster is a

group of cells in which all available frequencies have been used once and only once.

Since the same frequencies can be used in neighboring clusters, interference may become a problem.

Therefore, the frequency re-use distance must be kept as large as possible. However, to maximize

capacity the frequency re-use distance should be kept as low as possible.

The re-use patterns recommended for GSM are the 4/12 and the 3/9 pattern. 4/12 means that there

are four three-sector sites supporting twelve cells using twelve frequency groups.

In the 3/9 cell pattern there are always 9 channels separating each frequency in a cell. However,

when compared with the 4/12 pattern, cells A1 and C3 are neighbors and use adjacent frequencies.

Therefore, the C/A interference will increase. In this case, an operator may use frequency hopping

which, if planned correctly, could reduce the possibility of such adjacent channel interference.

Omni cell Sector cell



Example for a cluster – area inside black boarder

Cell Selection & Reselection

When the mobile (MS) is turned on, it begun to measure

all received power level from all cells within range. Then

the MS calculate the average power level received from

each cell and stored it in RXLEV parameters. Then the

C1 parameter is calculated for each cell based on RXLEV

parameter. MS compare the cell which gives positive C1

values and hold on to the cell which gives highest

positive C1 value.

Cell reselection is achieved by comparing current cell C1

value with neighboring cell C1 value

Based on the calculated C1 value, the MS decide which

cell to connect. This connection process called as “camps-on” to that cell.

Once camps-on the MS to cell, in idle mode the MS must measure the received power level

periodically from each BCCH frequencies of each cell and stored them for calculation of C1 value.

Any C1 values below 0 are ignored and keep best six neighboring cells.

In order to optimize cell reselection, additional cell reselection parameters can be optionally

broadcast on the BCCH of each cell. The cell reselection process can optionally employ a parameter

C2.

Path Loss Criterion (C1)

C1 = (RXLEV – RXLEV_ACCESS_MIN) – (MS_TXPWR_MAX_CCH – P)

Reselection Criterion (C2)

C2 = C1 + CELL_RESELECT_OFFSET – (TEMPORARY_OFFSET x H (PENELTY_TIME - T)

Calculated C

Values



• Cell Reselection Offset - This optional parameter is a positive or negative offset applied to

each cell to encourage or discourage MSs to reselect that cell.

• Penalty Time - When the MS places the cell on the list of the strongest carriers, it starts a

timer which expires after the PENALTY_TIME. This timer will be reset when the cell is

taken off the list. For the duration of this timer, C2 is given a negative offset. This will tend

to prevent fast moving MSs from selecting the cell

• Temporary Offset - This is the amount of the negative offset described in the ‘Penalty Time’

above. An infinite value can be applied, but a number of finite values are also possible.

Handover

Hanover is the process in which a cellular phone is handed from one cell to the next in order to

maintain a radio connection with the network.

There are about 31 handover causes and they can be mainly categorized into two groups. Followings

are mostly considered.

Emergency Better Condition

Cause 2 Too low quality on uplink Cause 12 Power budget evaluation

Cause 3 Too low level on uplink Cause 20 Forced directed retry

Cause 4 Too low quality on downlink Cause 23 Traffic

Cause 5 Too low level on downlink Cause 24 General capture

Cause 6 Too long distance between MS

& BTS

Cause 27 AMR channel adaptation (FR

to HR)

Cause 15 High interference on uplink

(Intracell HO)

Cause 28 Fast traffic HO

Cause 16 High interference on downlink

(Intracell HO)

Cause 29 Tandem free HO

Cause 26 AMR channel adoption HO Cause 30 Move from PS to CS zone

Other handover causes

Cause 07: Consecutive bad SACCH frames received in a microcell

Cause 10: Too low level on the uplink in the inner zone

Cause 11: Too low level on the downlink in the inner zone

Cause 13: Outer zone level on uplink and downlink

Cause 14: High level in neighbor lower large cell for slow mobile

Cause 17: Too low level on the uplink in a microcell compared to a high threshold

Cause 18: Too low level on the downlink in a microcell compared to a high threshold

Cause 21: High level in neighbor cell in the preferred band

Cause 22: Too short MS – BTS

Cause 31: 2G to 3G handover



Co-channel Interference (C/I)

Cellular networks are more often limited by problems caused by interference rather than by signal

strength problems. Co channel interference is caused by the use of a frequency close to the exact

same frequency. The former will interfere with the latter,

leading to the terms interfering frequency (I) and carrier

frequency (C).

The GSM specification recommends that the carrier to

interference (C/I) ratio is greater than 9 decibels (dB).

This C/I ratio is affected by the following factors:

• The location of the MS

• Local geography and type of local scatters

• BTS antenna type, site elevation and position

Using TEMS investigation tool, the C/I ratio can be observed

and the highest show best values. Generally above 15.00 are

considered as best.

Adjacent channel interference (C/A)

Adjacent frequencies (A), that is frequencies shifted 200 kHz from the carrier frequency (C), must be

avoided in the same cell and preferably in neighboring cells also. Although adjacent frequencies are

at different frequencies to the carrier frequency they can still cause interference and quality

problems.

The GSM specification states that the carrier-to-adjacent ratio (C/A) must be larger than -9dB.

Frequency Hopping (FH)

Mobile radio carriers suffer from frequency-selective interferences, for example, fading due to the

multi path propagation phenomena. As the carrier signal attenuates with distance, frequency selective

interference can have an increasingly significant affect on the signal quality.

Frequency hopping (FH) employs a constantly changing transmission frequency on the radio carrier.

Therefore the effects of frequency selective interference will be reduced by producing an averaging

effect over the interference caused on each frequency employed within the FH sequence. This results

in an overall improvement in S/N ratio.

Type of Frequency Hopping

There are two types of FH

1. Base Band Hopping (BBH)

BBH can be employed if there are several TRXs. The call is switched to each TRX in

accordance with the assigned hopping sequence.

2. Synthesizer Frequency Hopping (SFH)

In SFH single TRX can be used. The call is switch in single TRX within its TCHs.

BBH happen as follow figure;

Each TRX has a dedicated BCCH. As an example, for each TRX has BCCH of 992, 994, and 996.

This method is hardy used, because each cell has more BCCH and effect on frequency re use.



BCCH SDCCH TCH TCH TCH TCH TCH TCH

TCH SDCCH TCH TCH TCH TCH TCH TCH

TCH SDCCH TCH TCH TCH TCH TCH TCH

SFH happen as following figure;

Each TRX has set of frequencies. Hopping is happening on same TRX among channels. Most of

time, SFH is used.

BCCH SDCCH TCH TCH TCH TCH TCH TCH

TCH TCH TCH TCH TCH TCH TCH TCH

TCH TCH TCH TCH TCH TCH TCH TCH

The BCCH carrier is not hopping. Only TCHs are hopping.

Hopping Sequence Number (HSN)

The pattern of hopping frequencies is sequential or random. For a GSM network, can be assigned

one cyclic hopping pattern or any one of 63 random cyclic hopping patterns. Each those patterns can

be defined by a Hopping Sequence Number (HSN) and the range of HSN is 0 - 63.

F1 F2 F3 F4 F1 F2 F3 F4 – Sequential

F1 F4 F3 F2 F1 F4 F3 F2 –Random

Mobile Allocation Index Offset (MAIO)

TCH carriers must start at different point in sequence to avoid co-channel (C/I) interference.

Hopping sequence for each TRX must be different or have a mobile allocation index offset (MAIO).

As a example, the BTS consisted with three TRXs and they all have same HSN of 39. But the MAIO

for each TRX is 0, 2 and 6

TEMS Investigation tool can be used to

find out the HSN and MAIO of TRX.

TRX_0

TRX_1

TRX_2

TRX_0

TRX_1

TRX_2

Hopping Frequencies,

MAIO and HSN



Performance

Management of a

Network



Performance Management of a Network

Performance management is the process and procedures to ensure that the network operates at

maximum efficiency and to the defined Quality of Service (QoS) levels throughout the lifetime.

Methods of Network Performance Measuring

• Drive Testing data

• OMC Statistical Testing

• Protocol Analyzer Testing

Drive Testing

Drive testing consists of test teams driving on pre-defined routes in a network region and

periodically initiating calls and measuring signal strength. The types of test data collected include

handovers failures, low-quality audio (SQI), GPRS and dropped calls etc. These data are transferred

from the MS to a dedicated PC which is included data collecting software (TEMS

INVESTIGATION by Ericsson).

Tems Tool

Then the engineers can use this data together with software to identify any problems in the network.

Drive testing is auditing method of network and helps to identify and resolve the radio related

problems.

Generally drive testing data are used to identify the followings;

• Coverage Gaps

• Abnormal interference levels

• Missing Neighbors relationships

• Messaging protocol performance

Drive Testing Equipments

The system generally consisted with;

• One or more test mobile phones (MS)

• GPS receiver

• Data collecting software (TEMS)



Additionally USB broadband dongles, USB hubs and Data cards are used.

GPS Receiver

Test Mobiles (MS) USB Broadband dongles

Data Collecting Software USB Hubs

GPS

Test Mobiles

PC/Laptop with

measuring software

Power Source (AC)



Test Mobile Data

• BCCH/BSIC/TCH Information

• Received Signal Level (RXLEV)

• Receive Quality Level (RXQUAL)

• Timing Advance (TA)

• Neighbor Data

• Layer 3 Messages



Many handover problems, dropped calls and so on are

due to incorrect neighbor lists.

Each cell should be knew its all neighboring cells. It

causes handover failures.

RxLev data can be displayed for the six best neighbor

cells and compared with the serving cell’s RxLev



GSM Line Chart ( RxLev & SQI)

Drive testing as a performance measurement method;

• Can provide data for limited geographical area.

• Can only provide snap shot of network characteristics.

• Can identify specific faults.

• Drive testing can be expensive in terms of cost, manpower and equipment resources.

Why Drive Testing Need;

The network can be monitored using the BSC static. Followings are the advantages of drive testing

over BSC static.

• BSCs provide counters with number of dropped calls but do not indicate why or where.

• BSCs do not collect information on poor downlink quality

• BSCs cannot give information on areas without network access from either poor signal or

quality

• BSCs do not store detailed information on cells

• Test mobiles are the only solution for diagnosing localized network performance issues.



Benchmarking of GSM Network

In benchmarking, the relevant GSM network is compared with other GSM networks. Benchmarking

is one of the drive tests and it helps to identify networks’ weak point. Those are as follows;

• RxLev

• C/I

• RxQual

• SQI

• GPRS / EDGE

• Dropped calls

• Blocked Calls

• Handover failures

Method of benchmarking

For benchmarking, generally used two sets of equipments. One set for the relevant network and other

for all other networks which are compared with the relevant network.

For the first set of equipments;

• Laptop with data collecting software (TEMS INVESTIGATIONS)

• Four mobile stations (MS)

• GPS receiver

All mobile stations are connected to the laptop and software and arrange them as follow;

1. MS 1 for dedicated mode (long call )

2. MS 2 for idle mode

3. MS 3 for short call

4. MS 4 for GPRS / EDGE

5. GPS receiver as MS 5

For other set of equipment (consider three other GSM networks);

• Laptop with data collecting software

• Six mobile stations

• GPS receiver

All mobile stations are connected to the laptop and software and arrange the as follow;

1. MS 1 for netork_1 ideal

2. MS 2 for network_1 dedicated

3. MS 3 for network_2 ideal


5. MS 5 for network_3 ideal


7. GPS receiver as MS 7

The benchmarking is done on pre defined path. The data (logs) are recorded while drive and

analyzed and generate reports to identify bad and good issues of the network. These analyzed data

are help to improve the network performance.

The following figures are shown the receiving level (RxLev) of different three GSM networks.

Figure_1 is the relevant GSM network’s RxLev and figure_2 shows another two GSM networks’

RxLev.



Receiving levels of GSM networks

Figure_1 – Benchmarked Network

Figure_2 – Other two networks

The numbers (1 to 5) indicated the clusters in the area

Cluster_1, Cluster_2……etc.

As above, generate report for C/I, RxQual and SQI.



Single Site Verification (SSV)

Single side verification or SSV is done for a newly installed site (BTS) before it is on for commercial

air. In SSV, the site is tested for proper working of calls, SMS and GPRS (EDGE). Those test as

follows;

• Mobile Originated Call (MOC)

• Mobile Terminated Call (MTC)

• Short Message Service (SMS)

• Handover Intercell

• GPRS (EDGE)

Before the above test the site is tested for proper installation of hardware. Specially for followings;

• Connector of feeders tightness to BTS

• Cable swap

• Tilts of base station antennas

• Orientation of antennas

• Height of antennas

• Feeder size and feeder length

• Labeling

Above hardware inspection is very important, if wrong installation can cause power loss (VSWR),

frequency interference, overshoots, lack of coverage…etc.

Cable swap can be identified by a pre drive test. Before go to the site, pre drive is done and match

the CELL ID with planned BCCH of each cell.

The above mentioned tests are done with help of Mobile Station (MS), GPS and laptop.

Those tests have to be done for each cell by locking to relevant BCCH of cell.

The laptop is with measuring software (TEMS INVESTIGATIONS) and provides facility to lock or

target to relevant BCCH.

For a example;

Each cell has BCCH of 995, 986 and 999. For tests of cell 1, lock the MS to 995MHz frequency at

idle mode. But when call originating or terminating (Dedicated Mode), the cell of MS locked can be

changed according to the C1 value (Receiving Level).

Generally for a single BCCH only one time slot (TS) for GPRS and other for calls are allocated. But

for maximum performance of site, in SSV, TS of each cell increase to 1 to 4 by the BSC.

Finally the drive test for each cell is performed. This is very important and following are determined

from this drive test.

• Neighbor Definitions (for handovers)

• Blocked Call, Call Attempts, Dropped Calls, Handovers, Handover Failures….etc

• Speech Quality (SQI), Mobile Power Control…etc

• Rx Level with distance

• Actual site coverage



SSV on site check list

Procedure of SSV

1. Check for cable swap

2. Measure the all antennas height from the earth

3. Check the orientation (azimuth) of each antenna

4. Check the both mechanical and electrical tilts of each antenna

5. Adjusted above parameters if necessary

6. Check the tightness of connectors of feeder for each sector

7. Measure the total feeder length from BTS to antenna

8. Check for alarms (VSWR) ; not relevant for fiber optical

9. Confirm each cell for; MOC, MTC, SMS, EDGE (GPRS), and HANDOVERS

10. Taking pictures of each sector

11. Drive testing for each cell



Drive testing in SSV

When drive testing, three types of mobile stations (MS) are used for;

• Dedicated mode

• Locked idle mode

• Unlock idle mode

Today cellular networks are consisted with GSM 900 band or with both GSM 900 and DCS 1800

bands due to traffic. The drive testing method is different according to these bands.

Drive testing of GSM 900

In drive testing of GSM 900;

• Lock a MS to cell’s BCCH

• Make a long call from another MS

The drive is done for the direction of each cell; for example lock the idle phone to cell_1’s BCCH

and drive in the coverage area of cell_1. Then the MS is unlocked and come to the cell_1 again on

previous path. This is done for each three cell according to above procedure.

Drive Testing of DCS 1800

In this the drive testing is easier than GSM 900. One sector is consisted with two BCCH because

GSM and DCS both are included. So there are six BCCH. Lock the idle mode MS for all six BCCH

and drive for each sector.

In urban areas, the sites are installed more than in rural areas due to traffic. So drive testing in urban

area generally drives till find the next site. But in rural areas, generally drive till find the “No Service

Area”.



Data Analyzing

Statistical data is usually generated in the form of reports. These reports indicate the measurements

of specific pre-defined KPIs. Analysis should take two forms:

• Current Parameter Values.

Identifying unusual parameter values, for example, an unusually high number of dropped

calls over the reporting period.

• Comparison with Previous Reports.

Comparison of current parameter values with those acquired over a longer period may

identify patterns of problems or trends in potential degradation which could be addressed

before significantly affecting network performance.

Data can be analyzed under several headings;

1. Call Success – call attempt in terms of set-up time, clear down success, assignment success

etc.

2. Statistical Distribution – RxLev, RxQual

3. Handover analysis – success rate of attempt handovers

4. Neighbors – comparing neighbor cells found by signal level measurement with the neighbor

list in the site database.

5. Coverage Analysis - analyzing the coverage threshold levels using cell/neighbor cell

comparison to identify problem area.

6. Quality – it give a comparison of signal quality from serving and neighboring cells.

Results should also be compared with existing specific data sets such as demographic data,

marketing data, trends and traffic growth predictions.

Key Performance Indicators (KPIs)

KPIs are established by the operator as a benchmark against which to judge the performance of a

network.

KPIs are generally based on two sets of data;

• Network statistics from the OMC

• Drive test statistics using test mobile equipment

General Key Performance Indicators

KPI’S vary from operator to operator but the following are generally common to all KPI statistics;

1. TCH/SDCCH Usage Traffic Flow, Mean holding time, Congestion, Busy time, RF Losses.

2. Handover Statistics Success rate, Failure rate, Handovers due to poor quality, poor level, Power budget handovers

Handover due to interference

3. Connection Establishment

Successful immediate assignment procedure

Attempted immediate assignment procedure

Cell TCH assignments

Main KPI Definitions

There are many KPI definitions. Followings are some of them.



• Call Setup Success Rate (CSSR). Call setup attempts resulting in a successful TCH

assignment.

• Drop Call Rate (DCR). Successfully established calls that are terminated abnormally.

• Traffic Channel Congestion (TCH Cong.). Attempts to allocate a TCH that are blocked due

to lack of TCH resources

• Handover Success Rate (HOSR). It measures the propagation of total HO attempts resulting

in a successfully completed HO, including inter-cell, intra-BSS and inter-BSS HO.

• SDCCH Congestion (SD_Cong). SDCCH assignment attempts that are block due to lack of

SDCCH channels.

• Traffic Channel Assignment Failure Rate (TCH_Ass_Flr). TCH assignment attempt that fail

after TCH allocation and before completed the call.

• SDCCH Access Success Rate (SD_Acc_Suc). Immediate assignment attempts that result in

successful assignment to SDCCH, for all call setup reasons including location update, IMSI

detach etc.

The data can be collected using TEMS Tool. After the data are analyzed make reports as follow for

each one. Following figures show dropped call and block call area in a cluster.



For dropped call area;

• Red color dots and numbers indicate call dropped area

For blocked call area;

• Purple color dots indicate call blocked areas



Antenna

Configuration



Antenna Configuration

Following antenna configuration issues can affect coverage performance of a new installed site

• Antenna Alignment

• Tilting Configuration

• Use of Antenna Diversity

Antenna Alignment

The azimuth (orientation) direction of the antenna can have a significant effect on interference with

other cells. This interference can result in shrinkage of cell size and quality of service degradation.

The sectored antenna must be aligned according to the cell plane of the cluster; otherwise the wrong

alignment of antenna may interfere to each other and reduce the quality of the network.

Unlike omni directional antennas, directional sectored antennas reduce the interference, and allow

frequency reuse.

But it increases the core channel interference (C/I).

As an example; considering above figure, if the cell A and cell B has same (core) or adjacent

channel, cell A cause much core or adjacent interference on cell B and cell B cause less interference

on cell A.

Antenna Tilt

Modern antennas are consisted with mechanical tilt or with both mechanical and electrical tilts.

Antenna tilting can be categorized as up tilt and down tilt. But up tilt is not good practically and it

cause shrinkage of coverage area. Generally do down tilt.

Down tilt is done to achieve following terms;

• Reduce interference

• Adjust cell size

• Direct coverage e.g. into a building

Mechanical tilt;

set by operator

distorts azimuth (H plane) radiation pattern

Electrical tilt;

set by manufacturer

reduces radiation H plane pattern equally

in all directions, without distortion

Arrow are indicated the

direction of antenna.

Hexagons are indicated a

cell

A B



In mechanical down tilt, the coverage area get reduce causing the shape of radio pattern change

asynchronous.

In electrical down tilt, the coverage area get reduce causing the shape of radio pattern change

synchronous.

Mechanical tilt can be done adjusting the scale



Electrical tilt can be adjusting the above knobs. For a dual band (GSM 900 & DCS 1800), there are

two knobs for each band.

Use of Antenna Diversity

Antenna diversity is a technique that can be used to improve the reception characteristics of a

BTS. It normally comprises two antenna elements separated by a pre-determined distance, each

receiving the same signal. The two signals can then be adjusted in phase to ensure they combine to

enhance the overall received signal strength. The decision whether or not to provide uplink diversity

is connected with the issue of link balancing.

Three forms of diversity reception often employed are;

1. Space Diversity

2. Frequency Diversity

3. Polarization Diversity

Space Diversity, two antennas are used at base station. They are installed far apart so the reception

signal is independent of each other. Generally the space between antennas is 3m for GSM 900 (10 x

Wave length).



Following figure shows a space diversity system

Frequency hopping is sometimes referred as Frequency Diversity Antenna Systems

Polarization Diversity, the dipoles crossed at 45 degrees. As the radio signal undergoes multiple

reflections and scattering, the plane of polarization is rotated randomly. This can be used to provide

diversity reception by designing antennas with dipoles crossed to receive different components of the

polarization. This gives good coverage of vertical polarization and strong components of rotated

signals.

Diversity is used when need to balance the system. Diversity helps to improve the uplink, but not

to downlink. It allows the BTS to operate at higher power and greater coverage to be achieved.

Handover algorithms used by the BSS take uplink and downlink power and quality levels into

account. If the system were unbalanced, the information would be inconsistent and handovers

would not be correctly implemented.



Optimization



Optimization

Optimization is conducted to ensure the network is operating at optimum efficiency and within the

defined quality of service constraints. It is basically the way to keep track of the network by looking

deep into statistics and collecting/analyzing drive test data.

Successful optimization requires;

• Recognition and understanding of common reasons for call failure

• Capture of RF and digital parameters of the call prior to drop

• Analysis of call flow, checking messages on the call messages on both forward and reverse

links to establish what happened, where and why.

Purpose of Optimization

The optimization is to intend providing the best network quality using available spectrum as

efficiently as possible. The purposes as below;

• Finding and correcting and existing problems after site implementation and integration.

• Meeting the network quality criteria agreed in the contract.

• Optimization will be continuous and iterative process of improving overall network quality.

• Optimization can not reduce the performance of the rest of the network.

• Area of interest is divided in smaller areas called clusters to make optimization and follow up

process easier to handle.

Optimization Process

1. Problem analysis; Check BSC performance

Coverage, analyze previous drive testing data

Check for customer complains.

2. Drive testing;

Define drive testing routes

Collecting LOG files

Re-driving on questionable areas.

3. Subject to investigate;

Non working sites/TRX, radio link failure, overshooting, coverage holes

C/I & C/A analyze, interference, drop calls

Missing neighbors, ping-pong handovers

Equipment’s performance

4. After the test;

Post processing of data, plotting RxLev and RxQual information for picture of driven area,

reporting urgent problems for immediate action

Analyzing network feature performance after new implementation

Transferring comments on parameter implementation after new change

5. Recommendation;

Defining missing neighbor, adjust Handover margin,

Propose new sites, antenna tilt change, antenna type change, TMA adding,

BCCH, BSIC change,

6. Tracking;

Re-driving area areas after implementing recommendation,

Create tracking file to continue implementations of recommendation



Network Audit Process

Network auditing can be done analyzed following parameters;

1. Network performance statistics (OMC)

2. A-interface performance statistical analysis

3. Call trace analysis

4. Alarm and events

5. GSM/GPRS performance drive test and benchmarking

OMC Statistics;

• Call success rate, Call setup rate

• TCH blocking, TCH assignment failure

• SDCCH assignment failure

• Dropped calls

• Handover success, Handover failures

A-Interface Analysis;

• Call setup failures

• Location update success rate

• Hanover causes

Call Trace Analysis;

• Downlink receive level and BTS power

• Uplink receive level and MS transmit power

• RXQual of uplink and downlink

Event & Alarms;

• Types of alarms generated

• Relevance of alarms

Drive Testing;

• Route – coverage, quality, MS tx power, SQI

• Events – dropped call, calla setup failure, HO failure, success HO

• GPRS – RLC throughput, LLC throughput, coding scheme usage……

• PDP context loss (GPRS call drop)

Evaluating Performance Data

The network is capable of generating a huge amount of statistical data. Therefore it is important to

categories the data in terms of important the data in terms of important and relevance. To achieve

this, performance counters has divided into group.

Main QoS Parameters

There are three main parameters that quantify network quality as perceived by the subscriber, those

are;

• Dropped call rate – indicate the number of premature call terminations. As an overall

network parameter the dropped call rate should be below 2%



• Blocking – percentage of calls that fail to connect due to network problems. In busy

hours, the rate is high.

• Traffic Throughput – Erlang is the unit of measure for describing the amount of

traffic throughput across the network. It can be expressed as the sum of the duration

of the calls divided by the measurement time.

Periodic Counters

The periodicity of capturing measurement statistics depends on the function of the element being

tested and the level at which it is situated in the network.

• Daily Cell Measurement - Call setup success rate, TCH crop call rate, Handover

success rate, TCH congestion, SDCCH Congestion.

• Weekly BSS Measurements - Traffic trends, Cell retaining, Cell accessibility.

• Monthly BSC Measurements - BSC processor load, BSC call setup rate, BSC

handover success rate.

Optimization Activities

The network should be adjusted to improve the performance by adjusting performance parameters.

BSS Configuration Parameter Review

The review includes;

• Handover parameters, timers, thresholds and margins

• Power control thresholds

• Voting and average mechanisms for handover and power control

• Call setup parameters to maximize resource utilization

• C1/C2 cell reselection parameters

• Any vendor-specific advance traffic management algorithms

Identify and Fix Hardware Problems

The BSS hardware problems can be identified though the analysis of OMC statistical data, drive test

data, A-Bis and A-Interface logs. ‘Worst Performing Cells’ can be identified through analysis of

established KPI’s.

Followings are typical hardware problems;

Wrong antenna alignment, wrong connected feeders, poor transmission, incorrectly configured

combiners/duplexers etc.

Identify and Fix Neighbor Problems

In GSM, several neighbor cells can be defined for a serving cell. Usually, handovers should be made

to the strongest neighbor, but in some cases frequent handovers to this best neighbor can result in

congestion in the neighbor cell, affecting the users initiating calls from that cell.

The situation can also occur in reverse, when a handover required to the best neighbor can result in a

rejection due to unavailability of resources, causing the handover to be attempted to the next best

neighbor, which can delay the process and deteriorate the quality further.



Under certain circumstances, it may be necessary to remove a potential neighbor from the neighbor

list and provide alternatives. Usually, such decisions are made using demographic considerations.

Neighbor list can be optimized by;

• Analyze neighbor performance form statistics.

• Utilize automated neighbor detection

• Identify inconsistent neighbor profiles

• Modify appropriate neighbor lists

Identify and Fix Frequency Plan Problems

Frequency optimization activities involve identification of interference issues due to a poorly-

maintained frequency plan, and rectification through a process of coverage optimization and

frequency plan modifications.

This includes firstly ensuring the issues have been identified;

• Identify interference issues

• Identify coverage optimization issues impacting frequency plan quality (excessive coverage

overlaps, coverage splashes, etc.)

Followings are recommend appropriate frequency optimization techniques;

• BCCH and TCH Frequency Plans

• Frequency Diversity (space, polarity)

• Frequency Hopping

• DTX

• Antenna Down tilting



Digital Microwave

Transmission



Digital Microwave Transmission Overview

Definition of microwave

Microwave is a kind of electromagnetic wave. The frequency range of microwave is 300 MHz to 300

GHz. But in microwave communication, the frequency range is generally from 3 GHz to 30 GHz.

accordingly, the wavelength is between 1 decimeter and 1 centimeter, so microwave is also called

“centimeter wave”. According to the characteristics of microwave propagation, microwave can be

considered as plane wave. The plane wave has no electric field and magnetic field longitudinal

components along the propagation direction. The electric field and magnetic field components are

vertical to the propagation direction. Therefore, it is called “transverse electromagnetic wave” or

TEM for short.

Digital microwave communication is a way of transmitting digital information in atmosphere on

microwave or radio frequency (RF). Digital microwave communication refers to the microwave

communication that adopts the digital modulation scheme. The baseband signal is processed in the

Intermediate Frequency (IF) unit. Then the signal is converted into the microwave frequency band

through frequency conversion. Modulation can be done within the microwave frequency band

directly, but only for the Phase Shift Keying (PSK) modulation scheme.

Applications of Digital Microwave

• Used for Complementary networks to optical networks

• BTS backhaul transmission

• Redundancy backup of important links

• VIP customer access

• Emergency communications

• Special transmission conditions

Microwave frequency band selection

Generally-used frequency bands in digital microwave transmission include:

7GHz / 8GHz / 11GHz / 13GHz / 15GHz / 18GHz / 23GHz /26GHz / 32GHz / 38GHz

Every frequency band is identified by its central frequency, for example, the central frequency of the

7 GHz frequency band is 7575 MHz.

As shown in the figure, the rough principle of microwave band application is that low-band

microwave is usually used for large-capacity. But the network sometimes uses the lowest frequency



band. The lower frequency band has the longer wavelength and the stronger diffraction ability. There

are several rules on the use of frequency bands based on practical application experience,

1. For long-haul PDH microwave links (distance between stations is generally longer than 15

km), 8 GHz frequency band is recommended. If the distance between stations is not longer

than 25 km, 11 GHz frequency band can also be used. The specific frequency band shall be

determined based on the local climate conditions and microwave transmission cross-section.

2. For short-haul PDH microwave links (generally used in the access layer and the distance

between stations is shorter than 10 km), 11G / 13G / 14G / 15G / 18G frequency bands are

recommended.

3. For long-haul SDH microwave links (distance between stations is generally longer than 15

km), 5G / 6G / 7G / 8G frequency bands are recommended. If the distance between stations is

not longer than 20 km, 11 GHz frequency band can also be used. The specific frequency band

shall be determined based on the local climate conditions and microwave transmission cross-

section.

RF channel configuration

There are some important parameters, which are considered in common RF channel configuration.

• Frequency Range

• Central Frequency

• Transmit / Receive Spacing

• Channel Spacing

The central frequency indicates the specific frequency band. A frequency band is divided by the

central frequency into low bands and high bands. There is protection spacing between two frequency

bands. It prevents interference between frequency bands. After selecting the microwave frequency

band, RF channels should be configured. In this scheme frequency band are divided into several

smaller sub-bands to provide the spectrum required by the transmitter. These sub-bands are called as

“Channels”. These channels are usually indicated by their center frequencies and sequence number.

To avoid interference between the transmit frequency and the receive frequency; there must be a

certain spacing between them. This spacing is called “Transmit/receive spacing”. The transmit

frequency and the receive frequency always come in pairs, one in a low band and the other in a high

band.

As an example, let’s take a common RF channel configuration scheme for the 7 GHz. In this

configuration scheme, the frequency range is from 7425 MHz to 7725 MHz and the central

frequency is 7575 MHz. The central frequency of each wave channel varies with the configuration

of the specific transmit/receive spacing and channel spacing. When these parameters are fixed, the

central frequency of each channel is determined. It is calculated based on a certain formula.



BBU

MSTU

SCSU

BBIU

Modulation schemes used in digital microwave communication

The baseband signal cannot be directly transmitted over microwave radio channels but must be

converted into frequency band signal in order to implement microwave transmission. Generally, the

digital baseband signal is the service signal to be transmitted. After the digital baseband signal is

modulated into the IF signal, it still cannot be directly transmitted over the air link but must be

converted once more into the signal of a higher frequency.

By the parameters used to modulate the carrier signal with the digital baseband signal, there can be

various modulation methods

1. ASK (Amplitude Shift Keying): Use the digital baseband signal to change the carrier

amplitude.

2. FSK (Frequency Shift Keying): Use the digital baseband signal to change the carrier

frequency.

3. PSK (Phase Shift Keying): Use the digital baseband signal to change the carrier phase.

4. QAM (Quadrature Amplitude Modulation): Use the digital baseband signal to change the

carrier phase and amplitude.

At present, PSK and QAM are commonly used in digital microwave communication.

Digital Microwave Communication Equipment

Digital Microwave can be classified as follow;

MUX / DEMUX Mode PDH SDH

Capacity Small & Medium Capacity

2 – 16 E1, 34 E1

Large Capacity

STM -0, STM-1,2xSTM1

Trunk Radio

Split-Mount Radio Structure

All Outdoor Radio

Trunk Microwave Communication Equipments

Trunk microwave equipment means that all the units including RF, IF, signal processing and

multiplexing/demultiplexing are placed indoors and only the antenna feeder system is outdoor. The

indoor part and the outdoor part are connected by a waveguide tube.

The figure on the right shows the structure of trunk microwave equipment.

1. Branch RF Unit (BRU), which completes the function of

combining and splitting RF signals

2. Main Signal Transmission Unit (MSTU), which completes signal

sending, receiving and modulation/demodulation, SDH electrical

signal access and lossless switching

3. Supervision, Control and Switching Unit (SCSU), which

supervises and controls the entire system

4. Baseband Interface Unit (BBIU), which implements the access of

STM-1 optical signals and C3 PDH signals

Trunk microwave equipment features high cost, large transmission capacity and more stable

performance.

It is suitable for long-haul transmission and trunk transmission.



All Outdoor Microwave Equipment

The equipment is consisted of four parts;

1. Outdoor part including the antenna and RF processing unit. The antenna completes

directional transmitting and convergence receiving of RF signals and enlarges the

transmission distance. The RF processing unit transmits and receives RF signals and converts

RF signals into IF signals.

2. Intermediate frequency (IF) cable, which connects the RF processing unit with the IF and

baseband processes unit and supplies power to the RF processing unit.

3. IF and baseband processing unit, which processes IF and baseband signals.

4. Service and power cable, which completes service access and supplies power to the whole

equipment.

The installation is easy and the equipment room is saved, but transmission capacity is generally

small.

Split-Mount Microwave Equipment

The equipment is composed of four parts;

1. Antenna; Focuses the RF signals transmitted by

ODUs and increases the signal gain

2. Outdoor Unit (ODU); RF processing to realize

IF/RF conversion of signals

3. IF cable; Transmits IF signals and IDU/ODU

communication signals and also supplies power to

ODUs

4. Indoor Unit (IDU); Performs access, grooming,

multiplexing/demultiplexing and

modulation/demodulation of services.

The ODU can either be directly mounted onto the antenna or connected to the antenna through a

short soft waveguide to avoid feeder loss.

The split-mount microwave equipment generally used in small capacity, easy installation and

maintenance, and fast network construction. It is currently the most widely applied microwave

equipment



IDU is generally installed in a cabinet. Outdoor installation includes installing the antenna and ODU.

There are two types, each having two methods.

1. One ODU using one antenna

• Directly mounted to the antenna.

• Separately mounted to the antenna using a soft flexible wave guide

2. Two ODUs sharing one antenna – For protection of link

• Directly mounted to the antenna.

• Separately mounted to the antenna using a soft flexible wave guide

In this configuration antenna is coupled to the two ODUs using a Hybrid Coupler. The Hybrid

Coupler is a passive device used in radio and telecommunications. It is a type of directional coupler

where the input power is equally divided between two output ports.

In Direct mounting method avoid the loss caused by the soft waveguide. Separately Mounted method

is used when there is little installation space on the tower.

Microwave Antenna

Hybrid

Coupler

Flexible

Wave Guide

Two ODUs

ODU

Flexible

Wave Guide



Antennas are used to send out the electric wave energy launched by transmitters directionally or send

the electric wave energy received into receivers. There are two common types of microwave

antennas.

1. Parabolic Antennas

2. Cassegrainian Antennas

The two types of antennas have the same reflection plane but different feeder sources

.

Parabolic antenna has the feeder source in front of it. Generally small and can only transmit a short

distance.

Cassegrainian antenna has the feeder source behind it. Generally big and can transmit a long distance

The microwave antenna diameter generally varies according to the local climate condition, frequency

and distance between stations. Sometimes according to the operator.

Generally following diameter antennas are used for Microwave Link

0.3 m for , 0.6 m, 1.2 m, 1.8 m, 2.0 m, 2.4 m, 3.0m, 3.2 m, etc

Following parameters are used by the BHARTI AIRTELL LANKA.

0.3m 100m – 2.5km 23GHz

0.6m 2.5km – 7km 18GHz

1.2m 6km – 15km 15GHz

1.8m 15km – 35km 13GHz

2.0m >20m 6 – 8 GHz

Radiated Power Distribution of an Antenna

The radiated power of an antenna is basically distributed to the main lobe, the side lobe and the tail

lobe. The main lobe has the highest power, which extends to the two sides. The position where the

power decreases to half of the maximum power is called the “half-power point” and the angle

between the two half-power points is called the “half-power angle”.

Antenna Adjustment

The objective of antenna adjustment is to align the main lobe of the near end antenna to the main

lobe of the far end antenna. Generally the ODU has an interface called RSSI. Using this interface,

can be determined the receiving level of antenna.



During antenna adjustment, change the direction vertically or horizontally. Meanwhile, use a

multimeter to test RSSI at the receiving end. The peak point of the voltage wave indicates the main

lobe position in the vertical or horizontal direction. When antennas are poorly aligned, only a small

voltage may be detected in one direction.

The wrong alignment cause

Low receiving level.

Cannot meet the required RF level

The correct alignment cause good.

Receiving level without interference

Block Diagram of the IDU



The IDU implements the functions including service access, service dispatching,

multiplexing/demultiplexing, and modulation/demodulation. Thus the IDU is the main part of a set

of microwave equipment. The function modules of an IDU are shown in the figure. In the transmit

direction, the tributary unit or line unit completes service access, the service configuration is

completed via cross-connection, and the signal after service dispatching then undergoes microwave

frame multiplexing to form the complete microwave frame structure (but the signal is still digital

baseband signal). After being modulated, the signal is converted into the IF signal and then

transmitted via the IF cable to the ODU. In the receive direction, the process is just completely

reverse. Moreover, the IDU is monitored and controlled via the O&M interface and the system is

supplied power via the power interface

ODU

NEC IDU



Digital Microwave Networking

Common Networking Modes of Digital Microwave

Types of Digital Microwave Stations

Digital microwave stations can be classified as follows;

1. Pivotal stations - A station located in the backbone link to communicate with other stations

in various directions.

2. Add/drop relay stations - A station located in the middle of the link to add/drop tributaries

and communicate with the two stations in two directions of the backbone link.

3. Relay stations - A station located in the middle of the link without adding/dropping voice

channels.

4. Terminal stations - A station located at either end of the link or at the endpoint of a tributary

link.

Microwave Propagation

Fresnel Zone

The first Fresnel zone is the region where the microwave transmission energy is the most intense.

The obstruction in the Fresnel zone should be as little as possible



Due to obstruction of the first Fresnel Zone, cause link failures. When planning a new link try to

maintain good clearance between two stations, avoiding obstruct the first Fresnel zone.

As shown in figure, the first Fresnel zone is

obstructed. When the height of the obstacle

enters the first Fresnel zone, additional loss

might be added. As a result, the received level

is decreased and the transmission quality id

affected.

Factors Affecting in Electric Wave Propagation

Terrain

When the electric wave is propagated in

the atmosphere, the reflected wave from

the ground surface is the major factor that

affects the received level. Smooth ground

or water surface can reflect a part of the

signal energy transmitted by the antenna

to the receiving antenna and cause

interference to the main wave (direct

wave). The vector sum of the reflected

wave and main wave increases or

decreases the composite wave. As a result, the transmission becomes unstable. Therefore, during the

microwave link design, avoid reflected waves as much as possible. If reflection is inevitable, make

use of the terrain ups and downs to block the reflected waves

Different reflection conditions of different terrains have different effects on electric wave

propagation. Terrains are classified into the following four types:

1. Mountains (or cities with dense buildings)

2. Hills

3. Plain

4. Large-area water surface

The reflection coefficient of mountains is the smallest, and thus the mountain terrain is most

suitable for microwave transmission. The hill terrain is less suitable. When designing the circuits,

avoid smooth plane such as water surface.

Generally there are two methods, which can be used to reduce the influence of ground reflection.

• Avoid smooth plane such as water surface during circuit design. This reduces the influence of

ground reflection.

• Make use of the terrain ups and downs to block the reflected waves if reflection is inevitable.

Atmosphere

Microwave is propagated in atmosphere, so atmosphere will have a big impact on microwave

propagation. Troposphere is the low altitude atmosphere within 10 km from the ground. Microwave

antennas will not be higher than troposphere, so the electric wave propagation in aerosphere can

be narrowed down to that in troposphere.

The main effects of troposphere on electric wave propagation are listed below

Direct Wave Reflected Wave



• Absorption caused by gas resonance. This type of absorption can affect the microwave at 12

GHz or higher.

• Absorption and scattering caused by rain, fog and snow. This type of absorption can affect

the microwave at 10 GHz or higher. Refraction, absorption, reflection and scattering caused

by in homogeneity of atmosphere.

• Refraction is the most significant impact to microwave propagation.

Fading

Fading means random variation of the received level. By the mechanism of fading, fading may fall

into;

• Duct type fading - Due to the effects of the meteorological conditions such as ground

cooling at night, burnt warm by the sun in the morning, smooth sea surface and anticyclone,

a non-uniform structure is formed in atmosphere.

• K-type fading

• Scintillation fading - When the dielectric constant of local atmosphere is different from the

ambient due to the particle clusters formed under different pressure, temperature, and

humidity conditions, scattering occurs to the electric wave.

• Rain fading, absorption fading

• Free space propagation fading.

Anti-fading Technologies for Digital Microwave System

In digital microwave propagation, various fading will occur due to factors such as atmosphere,

ground, and climate. This will cause receive power decrease or waveform distortion and thus further

result in circuit performance degradation. Therefore, anti-fading measures are used to improve the

performance of the transmission circuit system.

Fading factors can cause waveform distortion or power decrease. The anti-fading technologies can be

classified according to equipment -level countermeasures and system-level countermeasures. Each

anti-fading technology can only solve limited problems.

The equipment-level countermeasures include adaptive equalization, automatic transmit power

control (ATPC) and forward error correction (FEC). Adaptive equalization can reduce the influence

of waveform distortion, whereas ATPC and FEC can reduce the influence of power decrease.

The system-level countermeasures include the diversity receiving technology, which can reduce the

influence of both power decrease and waveform distortion.



Automatic Transmit Power Control (ATPC)

The purpose of ATPC is to enable the output power of the transmitter to automatically trace and

change with the received level of the receiver within the control range of ATPC. When propagation

fading occurs and the receiver detects that the propagation fading is lower than the minimum

received level specified by ATPC, the transmitter raise the transmit power.

Working principle of ATPC

The effects of ATPC

• Reduce the interference to adjacent systems and over-reach interference,

• Reduce up fading,

• Reduce DC power consumption

• Improve residual BER

Diversity Technology

Diversity means two or multiple transmission paths are used to transmit the same information and

the receiver output signals are selected or combined to reduce the effect of fading.

Diversity falls into;

1. Space diversity.

2. Frequency diversity.

3. Polarization diversity.

4. Angle diversity.

Space diversity and frequency diversity are more frequently used. Space diversity is economical and

has a good effect.

Frequency diversity is often applied to multi-channel systems as it requires a wide bandwidth.

Space Diversity

In space diversity, signals have different multi

path effect over different paths. Accordingly,

two or more suites of antennas at different

altitude levels may be used to receive the

signals at the same.

The merit of space diversity is that frequency resources are saved. The disadvantage is that the

equipment is complicated and two or more suites of antennas are needed.

The distance between the diversity antennas is 100 to 200 times the wavelength in frequently used

frequency bands.

Space diversity can effectively solve the K-type fading caused by the interference of ground-

reflective wave and direct wave, and the interference fading caused by troposphere reflection.



Frequency Diversity

In frequency diversity, signals at

different frequencies have different

fading characteristics in space

transmission. Two or more microwave

frequencies at certain frequency

spacing may be used to transmit and

receive the same information. Only

one suite of antenna is required. The disadvantage is that the utilization rate of frequency bands

is low.

Protection Modes for Digital Microwave Equipment

FD = Frequency Diversity

SD = Space Diversity

1 + 0 Configuration

ODU

IDU

ODU

IDU

Transmitter Receiver



1 + 1 Configuration (FD)

1 + 1 Configuration (SD)

ODU – 1

Switch

ODU - 2

ODU - 1

Switch

ODU - 2

IDU – 2

Switch

IDU - 1

IDU – 2

Switch

IDU - 1

IDU – 2

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Designing Microwave Transmission Links

When designing a microwave link, following four aspects should be covered;

1. Requirement on the point-to-point line-of-sight communication

2. Objective of designing a microwave transmission line

3. Transmission clearance

4. Meanings of K value in the microwave transmission planning.

Because the microwave is a short wave and has weak ability of diffraction, the normal

communication can be realized only in the line-of-sight transmission without obstacles.

The transmit power is very small as compared with radar. The only way to implement long-haul

communication is to use the antenna of a greater diameter or increase the transmit power of the

antenna.

In common geographical conditions, it is

recommended that there should no be obstacles

within the first Fresnel zone if K-factor is

equal to 4/3.

Steps to Design a Microwave Transmission Link

1. Determine the route according to the engineering map

2. Select the site of the microwave station

3. Draw the cross-sectional chart of the terrain

4. Calculate the parameters for site construction

As the first step, the area should be selected, which is curly as much as possible, such as the hilly

area. Also the water surface and the flat and wide area should be avoided, because that is not

suitable for the transmission of the electric wave. In this way, the strong reflection signal and the

accordingly caused deep fading can be avoided. The line should avoid crossing through or

penetrating into the mountainous area. The line should go along the railway, road and other areas

with convenient transportation

As the second step, the site of the microwave station should be selected. The distance between two

sites should not be too long. The distance between two relay stations should be equal, and each

relay section should have proper clearance. Avoid the interference from other radio services, such as

the satellite communication system, radar site, TV station, and broadcast station.

As the third step, cross-sectional chart of the terrain should be drawn. Calculate the antenna height

and transmission situation of each site. For the line that has strong reflection, adjust the mounting

height of the antenna to block the reflected wave, or have the reflection point fall on the earth surface

with small reflection factor. Consider the path clearance. The clearance in the plain area should

not be too big, and that in the mountainous area should not be too small.

As the fourth step, the parameters for site construction should be calculated. Calculate the terrain

parameters when the route and the site are already determined. Calculate the azimuth and the



elevation angles of the antenna, distance between sites, free space transmission loss & received

level, rain fading index, line interruption probability, and allocated values and margin of the

line index. When the margin of the line index is eligible, plan the equipment and frequencies, make

the approximate budget, and deliver the construction drawings.

gsm theories, network analayzation & microwave transmission

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