GSM Basic Introduction

MS Team Damcon Engineering Islamabad (p.v.t)

GSM

GSM is the most popular method that uses all over the world for mobile communication. Before going into the details of the GSM System it is worthy to have some understanding about the history of GSM.

History of GSM in brief

• In 1982-85 Conférence Européenne des Postes et Télécommunications (CEPT) began specifying a European digital telecommunications standard in the 900 MHz frequency band. This standard later became known as Global System for Mobile communication (GSM).

• In 1986 field tests were held in Paris to select which digital transmission technology to use. The choice was Time Division Multiple Access (TDMA) or Frequency Division Multiple Access (FDMA).

• In 1987 a combination of TDMA and FDMA was selected as the transmission technology for GSM.

• In 1989 European Telecommunication Standards Institute (ETSI) took over responsibility for GSM specification.

• In 1991 the GSM 1800 standard was released. • In 1992 first commercial Phase 1 GSM networks were launched.

BASE STATION SUBSYSTEM (BSS)


The Base Station Subsystem of a GSM network contains the Base Transceiver Station (BTS), the Base

Station Controller (BSC), and the Transcoder Controller (TRC). A figure of GSM network components is

given bellow.

GSM Network architecture

Base Transceiver Station (BTS)

A BTS acts as the interface between MS’s (Mobile Station) and the network, by providing radio coverage

functions from their antennae. The channel concept used in the communication between BTS and MS will

be described in section 4.2.2.

Base Station Controller (BSC)

The BSC controls a major part of the radio network. Its most important task is to ensure the highest

possible utilization of the radio resources. The main functional areas of the BSC are:

• Radio Network Management

• BTS Management

• TRC Handling

• Transmission Network Management

• Internal BSC Operation and Maintenance

• Handling of MS connections


BSC handles MS connections during a call setup and during a call.

Call set up involves the following processes:

Paging: the BSC sends paging messages to the BTS’s defined within the desired LA (Location Area). The

load situation in the BSC is checked before the paging command is sent to the BTS.

Signaling set-up: during call set-up, the MS connection is transferred to an SDCCH (Stand alone Dedicated Control Channel) allocated by the BSC. If the MS initiated the connection, the BSC checks its processor load before the request is further processed.

Assignment of traffic channel: after SDCCH assignment, the call set-up procedure continues with the

assignment of a TCH (Traffic Channel) by the BSC. As this takes place, the radio channel supervision

functions in the BSC are informed that the MS has been ordered to change channels. If all TCHs in the

cell are occupied an attempt can be made to utilize a TCH in a neighboring cell.

The main BSC functions during a call are:

Dynamic power control in MS and BTS: the BSC calculates adequate MS and BTS output power based on

the received measurements of the uplink and downlink. This is sent to the BTS and the MS every 480 ms

to maintain good connection quality.

Locating: This function continuously evaluates the radio connection to the MS, and, if necessary,

suggests a handover to another cell. This suggestion includes a list of handover candidate cells. The

decision is based on measurement results from the MS and BTS. The locating process is being executed in

the BSC.

Handover: if the locating function proposes that a handover take place, the BSC then decides which cell

to handover to and begins the handover process. If the cell belongs to another BSC, the MSC/VLR (Mobile

Switching Centre / Visitor Location Register) must be involved in the handover. However, in a handover,

the MSC/VLR is controlled by the BSC. No decision making is performed in the MSC because it has no real

time information about the connection.

Transcoder Controller (TRC)

The primary functions of a TRC are to perform transcoding and to perform rate adaptation. The function

of converting from the PCM (Pulse Code Modulation) coder information to the GSM speech coder

information is called transcoding. This function is present in both the MS and the BSS.

Rate adaptation involves the conversion of information arriving from the MSC/VLR at a rate of 64kbits/s


to a rate of 16kbits/s, or transmission to a BSC (for a full rate call). This 16kbits/s contains 13kbits/s of

traffic and 3kbits/s of inband signaling information.

This is an important function. Without rate adaptation the links to BSC’ would require four times the

data rate capabilities. Such transmission capabilities form an expensive part of the network. By reducing

the rate to 16kbits/s, it is possible to use one quarter of the transmission links and equipment.

In Ericsson’s GSM systems, the TRC contains units, which perform transcoding and rate adaptation. These

hardware units are called Transcoder and Rate Adaptation Units (TRAUs). All TRAUs are pooled, meaning

that any BSC connected to the TRC can request the use of one of the TRAUs for a particular call.

The TRC also supports discontinuous transmission. If pauses in speech are detected, comfort noise is

generated by the TRAU in the direction of the MSC/VLR.

Data rates for a single call on GSM links

OMC-R (Operations & Maintenance Center-Radio)

OMC-R is the interface between the BSS and the human working in the system. The operations and

maintenance center (OMC) is connected to all equipment in the switching system and to the BSC. OMC

is that connected to the BSC. Thus the operations and maintenance needs of the Network Operations

division is provided by the OMC-R, while the OMC-S connected to the switching system is used for

controlling the switch operations. The OMC is the functional entity from which the network operator

monitors and controls the system. The purpose of OMC is to offer the user cost-effective support for

centralized, regional and local operational and maintenance activities that are required for a GSM

network. An important function of OMC-R is to provide a network overview and support the maintenance

activities of different operation and maintenance organizations.

THE NETWORK AND SWITCHING SUBSYSTEM(NSS)


The GSM network and protocols

Mobile Services Switching Center (MSC)

The primary node in a GSM network is the MSC. It is the node, which

controls calls both to MS’s and from MS’s. The primary functions of an MSC

include the following:

Switching and call routing MSC interact with other nodes to successfully establish a call. During a call it

involves in handovers from one BSC to another and inter MSC handover. Charging MSC contains functions for charging mobile calls and information about the

particular charge rates to apply to a call at any given time or for a given

destination. During a call it records this information (Call Data Record-CDR)

and sends it to the billing center. Service provisioning Supplementary services are provided and managed by a MSC. In addition,

the SMS service is handled by MSC’s


Communication with HLR & VLR MSC is communicating with HLR & VLR when call setup and release and get

subscription information. Communication with other MSCs MSC’s communicate with each other during call setup or handovers between

cells belonging to different MSC’s. Control of the connected BSCs An MSC may communicate with its BSC’s during; for example, call set-up and

handovers between two BSC’s.

Visitor Location Register (VLR)

The role of a VLR in a GSM network is to act as a temporary storage location

for subscription information for MSs which are within a particular MSC

service area. Thus, there is one VLR for each MSC service area. This means

that the MSC does not have to contact the HLR every time the subscriber

uses a service or changes its status.

VLR contains following data.

• Identity numbers for the subscriber

• Supplementary service information (e.g. whether the subscriber

has call forwarding on busy activated or not)

• Activity of MS (e.g. idle)

• Current LA of MS

Home Location Register

The HLR is a centralized network database that stores and manages all

mobile subscriptions belonging to a specific operator. It acts as a permanent

store for a person's subscription information until that subscription is

cancelled. The information stored includes:


• Subscriber identity (i.e. IMSI, MSISDN)

• Subscriber supplementary services

• Subscriber location information (i.e. MSC service area)

• Subscriber authentication information

Authentication Center (AUC)

The primary function of an AUC is to provide information, which is then used

by an MSC/VLR to perform subscriber authentication and to, establish

ciphering procedures on the radio link between the network and MS’s. The

information provided is called a triplet and consists of:

• A non predictable Random number (RAND)

• A Signed Response (SRES)

• A ciphering Key (Kc)

Equipment Identity Register

The equipment identification procedure uses the identity of the equipment itself (IMEI) to ensure that the MS terminal equipment is valid.

GSM CELLS

There are two main types of cells:

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).

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.

Typically, omni-directional cells are used to gain coverage, whereas sector cells are used to gain capacity.

Cell types

Reuse of frequency is an important factor when planning a GSM network. The frequency 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.

Frequency reuse patterns


FREQUENCY HOPPING

Multi wave propagation varies the signal strength which causes fading dips.

This is known as Rayleigh fading which is frequency dependent. Frequency

Hopping changes the frequency with time to eliminate frequency dependent

Rayleigh fading. Frequency Hopping reduces the effects of interferences and

gives a great support to cater the capacity upgrading as well.

There are 2 types of frequency hopping, Base Band Hopping (BBH) and

Synthesized Frequency Hopping (SFH)

Base Band Hopping (BBH)

Base band hopping

In this type of hopping, only frequencies used by the TRXs can be allocated

to the FHS (Frequency Hopping Sequence).The BCCH supporting Time Slot

cannot hop. TS 0 of the BCCH TRX is always transmitting on the BCCH

frequency. Other timeslots can use other frequencies unless the BCCH

frequency is transmitted by any other TRX at the same time.

The major advantage of the BBH is the time TS 1 to TS 7 of the BCCH

frequency containing TRX is allowed to perform hopping. But Frequency

Hopping performs best with at least 4 hopping frequencies. So cells need at

least 4 TRXs and it is the disadvantage of this system.

Synthesized Frequency Hopping (SFH)


Synthesized Frequency Hopping

In SFH each frame unit is connected to one carrier unit. Hopping is

performed by changing the carrier unit by changing a synthesizer. As the

communication is not hopping between the carrier units but the carrier unit

frequency itself is hopping. So many frequencies can be used as hopping

frequencies. In SFH, TRX returns in every time slot to a different frequency.

Thus the TCH remains on the same TRX but the frequencies of that TCH

hops.

FREQUENCY PLANNING PARAMETERS

BCCH – Broadcast Control Channel

BCCH frequency planning is done for each sector in the site individually.

The BCCH frequency must be determined so that the same frequency of

some other site does not interfere with the planning site (Co-channel

interference) and the adjacent higher frequency and the lower frequency

also should not be used in a near site that can be in a position to interfere

the given site (Adjacent channel interference).

HSN – Hopping Sequence Number

It is one of 4 input parameters to the GSM hopping sequence generator

algorithm.Range of HSN is 0 to 63. HSN = 0 means the hopping is in cyclic

mode.

MAIO - Mobile Allocated Index Offset


The Mobile Allocated Index allows determining the correct line in the

Mobile Allocation look up table to find the corresponding ARFCN .The MAIO

is selectable for each timeslot and each TRX separately. It is constant on

the TRX but it changes between the frame units.

BSIC – Base Station Identification Code

BSIC Channels are to identify each station and sub stations. It can be

represented in 2 codes.

NCC (Network Color Code) – Is represented in 3 bit binary form.

Therefore it the numbers are from 0 to 7.

BCC (Base station Color Code) – It’s also in 3 bit form. Can be used 0 to

7.

CHANNEL CONCEPT

Each timeslot on a TDMA (Time Division Multiple Access) frame is called a physical

channel. Therefore, there are 8 physical channels per carrier frequency in GSM.

Physical channels can be used to transmit speech, data or signaling information. A

physical channel may carry different messages, depending on the information that is

to be sent. These messages are called logical channels. Information is transmitted via

physical channels as the form of bursts. The relationship between bursts and logical

channels is shown in the figure below.


The relationship between bursts and logical channels

The Control channels are control the communication (signaling) of the mobile station

with the base transceiver station while the Traffic channels bares the voice

information. Logical channels are transmitted on physical channels. The method of

placing logical channels on physical channels is called mapping. While most logical

channels take only one time slot to transmit, some take more. If so, the logical

channel information is carried in the same physical channel time slot on consecutive

TDMA frames. Because logical channels are short, several logical channels can share

the same physical channel, making the use of time slots more efficient.

Most of the cells are having 4 simultaneous carriers (or TRXs). The figure below shows

the carrier frequencies for a sample cell, including an additional allocation of a time

slot for DCCH (Dedicated Control Channel) in TRX1 due to a high call set-up load in

the cell. Otherwise one DCCH is enough.


The relationship between physical channels and logical channels

Time slot 0 of the first carrier frequency in a cell is always reserved for

signaling purposes. In this way, when an MS is determining whether a carrier

frequency is a BCCH (Broadcast Control Channel) carrier, it knows where to look.

8 SDCCH's and 4 SACCH's all can share the same physical channel (Channel D in the

figure above). This means that 8 calls can be set-up simultaneously on one physical

channel.

All time slots in a cell other than those assigned for signaling information are used for

traffic, i.e. speech or data. Logical channel TCH is used. In addition, at regular

intervals during a call, an MS transmits to the BTS measurements it has made about

signal strength and quality. Logical channel SACCH is used for this, replacing one TCH

time slot at a time.

AUTHENTICATION PROCESS OF A SUBSCRIBER


Subscriber authentication process

At subscription time, each subscriber is assigned a subscriber authentication Key (Ki). Ki is stored in the AUC (Authentication Center) along with the subscriber’s IMSI (International Mobile Subscriber Identity). Both are used in the process of providing a triplet. The same Ki and IMSI are also stored in the SIM.

1. A non-predictable random number, RAND, is generated at the Authentication Center.

2. RAND and Ki are used to calculate Signed Response (SRES) and Kc, using two different algorithms, A3 and A8 respectively.

3. RAND, SRES and Kc are delivered together to the HLR as a triplet.

4. The MSC/VLR transmits the RAND to the MS.

5. The MS computes the signature SRES using RAND and the subscriber authentication key (Ki) through the A3 algorithm.

6. The MS computes the Kc by using Ki and RAND through A8 algorithm. Kc will thereafter be used for ciphering and deciphering in MS.


7. The signature SRES is sent back to MSC/VLR, which performs authentication, by checking whether, the SRES from the MS and the SRES from the AUC match. If so, the subscriber is permitted to use the network. If not, the subscriber is barred from network access.

GSM NETWORK IDENTITIES

Network identities are numbers that a GSM network uses to locate a mobile

subscriber when it is establishing a call to that subscriber. As the network

relies on these identities to route calls to subscribers, it is important that

each identity is unique and correct.

Mobile Station ISDN number (MSISDN)

The Mobile Station ISDN number (MSISDN) uniquely identifies a mobile

telephone subscription in the PSTN numbering plan. This is the number

dialed when calling a mobile subscriber.

MSISDN

CC Country Code NDC National Destination Code SN Subscriber Number International Mobile Subscriber Identity (IMSI)

The International Mobile Subscriber Identity (IMSI) is a unique identity

allocated to each subscriber that facilitates correct subscriber


identification over the radio path and through the network. It is used for

all signaling in the PLMN.

IMSI

MCC Mobile Country Code MNC Mobile Network Code MSIN Mobile Station Identification Number Temporary Mobile Subscriber Identity (TMSI)

The Temporary Mobile Subscriber Identity (TMSI) is a temporary IMSI

number made known to an MS at registration. It is used to protect the

subscriber's identity on the air interface. The TMSI has local significance

only (that is, within the MSC/VLR area) and is changed at time intervals or

when certain events occur such as location updating

International Mobile Equipment Identity (IMEI)

The International Mobile Equipment Identity (IMEI) is used to uniquely

identify MS equipment to the network. The IMEI is used for security

procedures such as identifying stolen equipment and preventing

unauthorized access to the network.

IMEI


TAC Type Approval Code, determined by a central GSM body FAC Final Assembly Code, identifies the manufacturer

SNR Serial Number, an individual serial number of six digits

uniquely identifies all equipment within each TAC and FAC spare A spare digit for future use. When transmitted by the MS this

digit should always be zero Mobile Station Roaming Number (MSRN)

The Mobile Station Roaming Number (MSRN) is a temporary network

identity which is assigned during the establishment of a call to a roaming

subscriber.

Location Area Identity (LAI)

The Location Area Identity (LAI) is a temporary network identity, which is

also required for routing. The two main purposes of the LAI are, paging,

which is used to inform the MSC of the LA in which the MS is currently

situated and location updating of mobile subscribers.

Cell Global Identity (CGI)

The Cell Global Identity (CGI) is used for identifying individual cells within

a LA. Cell identification is achieved by adding a Cell Identity (CI) to the LAI

components.

Base Station Identity Code (BSIC)

The Base Station Identity Code (BSIC) enables MS's to distinguish between

different base stations sending on the same frequency.

BSIC


NCC Network Color Code (3 bits) identifies the PLMN. It does not

uniquely identify the operator. NCC is primarily used to

distinguish between operators on each side of a border

BCC Base Station Color Code (3 bits) identifies the Base Station to

help distinguish between BTS using the same control frequencies

MOBILE ORIGINATING CALL

Mobile originating call

A call is originated from a MS as follows,

1. The MS uses RACH (Random Access Channel) to ask for a signaling channel.

2. The BSC/TRC allocates a signaling channel, using AGCH

(Access Grant Channel).

3. The MS sends a call set-up request via SDCCH (Stand alone Dedicated Control

Channel) to the MSC/VLR. Over SDCCH all signaling preceding a call takes

place. This includes:

• Marking the MS as “active” in the VLR

• The authentication procedure


• Start ciphering

• Equipment identification

• Sending the B-subscriber’s number to the network

• Checking if the subscriber has the service “Barring of outgoing calls” activated

4. The MSC/VLR instructs the BSC/TRC to allocate an idle TCH. The BTS and MS are told to tune to the TCH.

5. The MSC/VLR forwards the B–number to an exchange in the PSTN, which establishes a connection to the subscriber.

6. If the B-subscriber answers, the connection is established.

MICROWAVE LINK

MAIN COMPONENTS OF A MICROWAVE LINK

Components of a Microwave link

The radio equipment configuration of a microwave link consists of,


• Indoor Unit (IDU) • Outdoor Unit (ODU) • Antenna Unit • ODU to IDU cabling (IF cable)

• Digital Distribution Frame (DDF)

Indoor Unit (IDU)

The IDU contains much of the intelligence of the system. Main functions of

the IDU include Providing the Data interface, Error correction, Modulation

and Demodulation, Alarm status monitoring and Site-to-site

communications. An IDU located in an equipment shelter to interface with

the operator interface and is connected to a close coupled ODU Antenna

assembly on the tower by a single coaxial cable. The IDU is independent of

any frequency band and will operate over all frequency ODUs. But it is

capacity dependant and there are separate modules capable of handling

different capacity inputs. Configuring and monitoring of link performance

can be done through the IDU front panel. Display of Local & Remote radios

and Alarm Relay Status LEDs are there in the IDU front panel for fault

identification. Memory backup inside the IDU ensures that it maintains its

configuration in the event of a DC power loss.

DMC XP4 IDU

Outdoor Unit (ODU)


The ODU converts data from the IDU into an RF signal for transmission. It

also converts the RF signal from the far end to suitable data to transmit to

the IDU. ODUs are weatherproofed units that are mounted on top of a

tower either directly connected to a microwave antenna or connected to it

through a wave guide. Here ODUs are full duplex configured. The ODU

receives its power from the IDU through acoaxial cable. ODU parameters

are configured and monitored through the IDU. Each ODU is designed to

operate over a predefined frequency band. For example 21.2 - 23.6GHz for

a 23GHz system, 17.7 - 19.7GHz for a 18GHz system and 24.5 - 26.5GHz for

a 26GHz system as for DMC XP4 ODUs. Suitable ground wire should be

connected to the ODU ground lug to an appropriate ground point on the

antenna mounting or tower for lightning protection. It should be noted

that this unit is electronically controlled. Transmitted power is controlled

by adjusting a value on the IDU which instructs the ODU to adjust the drive

voltage on its Tx PIN diode attenuator. The configuration of a DMC XP4

ODU is shown below.

DMC XP4 ODU

The polarization of the transmitted signal is determined by the position of

the polarization rotator fitted within the ODU mounting collar. The rotator

is an integral part of the antenna-mounting bracket. The default setting is

for vertical polarization. The ODU must be mounted on the collar to match

the chosen polarization.


Adjustments for polarization

Antenna Unit

Antenna unit maintains the connection to the other end via a microwave

signal path, provided that the line of sight condition is satisfied. In order

to have an uninterrupted communication, the two antennas should be

placed face to face, without any obstruction between.

TRANSMISSION PLANNING

FRESNEL ZONE

The line of site condition must be there to correct operation of

a microwave link. The area that the signal spreads out from line of site is

called the Fresnel zone. If there is an obstacle in the Fresnel zone, part of

the radio signal will be diffracted or bent away from the straight-line path.

The practical effect is that on a point-to-point radio link, this refraction will

reduce the amount of RF energy reaching the receive antenna. The

thickness or radius of the Fresnel zone depends on the frequency of the

signal — the higher the frequency, the smaller the Fresnel zone.


Microwave lin

The radius of the Fresnel zone can be calculated by the formula bellow.

Fn = The nth Fresnel Zone radius in meter

d1 = The distance of P from one end in meter

d2 = The distance of P from the other end in meter

λ = The wavelength of the transmitted signal in meter

The direct path between the transmitter and the receiver needs a clearance

above ground of at least 60% of the radius of the first Fresnel zone to

achieve free space propagation conditions.

FREE SPACE LOSS

As signals spread out from a radiating source, the energy is spread out over

a largersurface area. As this occurs, the strength of that signal gets weaker.

Free space loss (FSL), measured in dB specifies how much the signal has

weakened over a given distance.

FSL = 92.4 + 20 log10 D + 20 log10 F

FSL = Free Space Loss in dB

D = Path Length in kilometers


F = Radio Frequency in Gigahertz

LINK BUDGET

Link budget is the itemized list of all system losses and gains in dB from the

transmitter on one end to the receiver at the other end.

Radio path link budget

FADING AND FADE MARGINS

Multipath Fading is the dominant fading mechanism for frequencies lower

than 10GHz. A reflected wave causes a multipath, i.e. when a reflected

wave reaches the receiver as the direct wave that travels in a straight line

from the transmitter. As a thumb rule, multipath fading, for radio links

having bandwidths less than 40MHz and path lengths less than 30Km is

described as flat instead of frequency selective.

Flat fading

A fade where all frequencies in the channel are equally affected called

Flat fading. There is barely noticeable variation of the amplitude of the

signal across the channel bandwidth.

Frequency-selective fading


In Frequency-selective fading there are amplitude and group delay

distortions across the channel bandwidth.

Rain fading

Rain attenuates the signal caused by the scattering and absorption of

electromagnetic waves by rain drops and it is significant for long paths

(>10Km). Vertical polarization is far less susceptible to rainfall attenuation

than are horizontal polarization frequencies.

Fade margin

Fade margin is the difference (measured in dB) between the nominal signal

level received at one end of a radio link and the signal level required by

that radio to assure that a packet of data is decoded without error. In

other words, fade margin is the difference between the signal received

and the radio’s specified receiver’s sensitivity. (Figure 11.3)

Fade margin = Signal level received – Sensitivity of the receiver

ADDING REDUNDANCY

There are two modes of system installations that are widely used; protected

and unprotected systems.

Unprotected system

In the unprotected system there’s only one ODU and due to any reason if a unit fails

the communication will fail.


Protected system

Here there are two ODUs. While in operation only one ODU is performing the transmission while other one always keeps track of the operating one and both do the receiving. At any instant if the active transmitter faces any problem, the other one will detect that and will automatically take over the functioning in a smooth manner. For critical links this configuration will ensure a more reliable performance.

ADDING DIVERSITY

There are 2 diversity methods are basically used in the microwave links,

Space diversity and Frequency diversity to improve the availability of the

link.

Space diversity

This configuration caters the problems that can occur due to multi path

fading. Here the receiving end has two antennas. When a signal is being

sent from another station, it is being received from both the antennas and

automatically one signal is taken in for processing. The best one is chosen

based on BER and signal strength measurements.


System with space diversity

Frequency Diversity

This configuration caters the problems that can occur due to interferences.

Here the receiver as well as the transmitter is dual systems, I.e. they can

operate in two frequencies. At any instance only one frequency is actively

operating while the other is in a stand-by mode. At any instance if the

receiving signal’s quality fades then the receiving end informs the

transmitter about the issue and both switches to the other frequency.

System with frequency diversity

TOWERS AND ANTENNAS

TRANSMISSION TOWERS

The construction of new towers, BTS cabins and other civil engineering related applications come under the infrastructure development. There are mainly three types of towers as shown in the figures.


Monopole Self Support Guyed

Monopoles consist of tapered steel tubes that fit over each other to form astable pole

Self-supporting towers are having free-standing lattice structures

Guyed towers are stabilized by tethered wires

ANTENNAS

There are several types of antennas are being used to meet the requirement of the network. General BTS location includes GSM (900MHz band) antennas and DCS (1800MHz band) antennas to provide the GSM coverage. Sometimes dual band antennas that are having both GSM and DCS facility are used rather than having 2 separate antennas. In addition to that some Sites have 3G antennas and WiMax antennas.

Antenna tilting is used to adjust the coverage area of an antenna. There are two ways of antenna tilt adjustment mechanisms.

Mechanical tilt


Tilting the antenna using an adjustable joint that is fixing the antenna to the tower is mechanical tilting. (Figure A)

Electrical tilt

There is a screw at the bottom of the antenna. This screw is connected to a ferrous core, which is in inside of the antenna. When the screw is rotated the down tilt of the main lobe is changed. (Figure B)

Figure A: Mechanical tilt Figure B: Electrical tilt

VSWR MEASUREMENT

VSWR stands for Voltage Standing Wave Ratio. An impedance of exactly 50 Ohm can only be practically achieved at one frequency. The VSWR defines how far the impedance differs from 50 Ohm with a wide-band antenna. The power delivered from the transmitter can no longer be radiated without loss because of this incorrect compensation. Part of this power is reflected at the antenna and is returned to the transmitter. The forward and return power forms a standing wave with corresponding maximum and minimum voltages. This wave ratio defines the level of compensation of the antenna.

This test is performed to make sure the cables from BTS to antenna are installed properly. Readings are taken of the whole system of antenna, jumpers and cable up to the BTS. Analyzer is used to measure the VSWR and it draws a graph taking frequency to x-axis and VSWR to y-axis.

Calibration


Before any reading is taken, the very first thing is to calibrate the analyzer. Analyzer is switched on and cable is connected to the port. The desired frequency range is entered (890MHz – 960MHz for GSM and 1710MHz – 1880MHz for DCS). To the other end of the port, the ‘Open’ side of the dummy is connected and reading is taken. Then ‘Close’ side and ‘Load’ side of the dummy should be connected. The analyzer process data and if it gives a VSWR reading around 1 for the ‘Load’, then it is ready to take the readings.

Taking Measurements

Jumper cable to the BTS is removed from the BTS port and connected to the analyzer and a graph is obtained. The connectors should be well-tighten; otherwise a false reading may be given. The VSWR graph should be below 1.2 for an acceptable installation. If not, the next check points going in sequence are,

• Connection point of jumper and antenna cable. • Replace the antenna by the dummy load at the next end

jumper. • Replace the jumper cable and measure. • Worst case, replace the antenna cable.

Following are two graphs obtained from a VSWR test. The graph at top is one with the VSWR at an acceptable range and the bottom one is one which is not.


VSWR is at an acceptable range

VSWR is not at an acceptable range


INDOOR SOLUTIONS

When providing an indoor coverage, it is necessary to be aware of coverage

and interference. (i.e. signals from indoor cell must be strong enough to

guarantee a full coverage of the building and get free of interference from

the outdoor cells.) So the signals from indoor cell must be limited not to

interference with the outside network.

Before installing an indoor cell, it must be design a plan with antenna

locations, cable lengths, Splitter types, Tappers…etc by using the map of

the building and special software (e.g. RF UV). After finishing performance

tests, indoor site installations can be done. There are two types of indoor

antennas for the indoor solution process: Omni & panel.

Indoor antennas

By using Omni antennas, signal can be transmitted over an area of

hemisphere. So it is very useful for covering an open area (like a room, hall

or an auditorium). It is possible to give coverage for a long directional area

(i.e. corridor, stairs…etc) by using Directional antennas.

POWER SYSTEM OF A TYPICAL BTS SITE

The following figure shows the Power System of a typical BTS site.


Power System of a typical BTS site

A typical BTS site has a 2 way power supply, CEB and Generator power. The

ATS (Automatic Transfer switch) is there to select the power source. The

rectifier converts the AC supply voltage to a DC voltage of -48V. The battery

bank is there to supply the power during an emergency case that is when

both the CEB and Generator power fails. The HRC (High Rupture Capacity

Fuses) prevents the equipment from voltage spikes.

The BTS and modems operate in -48V DC. The rectifier unit converts the AC

supply into -48V DC. Battery bank is used as a back-up power at a mains

failure. The manufactures provide battery cells, the rack and other

necessary components for every battery bank.

UNINTERRUPTIBLE POWER SUPPLY (UPS)

An uninterruptible power supply (UPS), also known as a continuous power

supply (CPS) or a battery backup, is a device which maintains a continuous


supply of electric power to connected equipment by supplying power from a

separate source when utility power is not available. It differs from an

auxiliary power supply or standby generator, which does not provide instant

protection from a momentary power interruption, however could be used to

provide uninterrupted power to equipment for 1 - 20 minutes until a

generator can be turned on. The switch over time is stated by most

manufacturers as being less than 4 milliseconds, but typically can be as long

as 25 milliseconds depending on the amount of time it takes the Standby

UPS to detect the lost utility voltage. There are 2 types of UPS available.

Off-line UPS

This type of UPS remains idle until a power failure occurs, and then

switches from utility power to its own power source, almost

instantaneously.

Off-line UPS

On-line UPS

On-line UPS continuously powers the protected load from its energy

reserves stored in a lead-acid battery or flywheel, while simultaneously

replenishing the reserves from the AC power. It also provides protection

against all common power problems, and for this reason it is also known as

a power conditioner and a line conditioner.


On-line UPS

LIGHTNING PROTECTION SYSTEM (LPS)

A Lightning Protection System (LPS) provides a means by which a lightning

discharge may enter or leave earth without passing through and damaging

personnel, electrical equipment, and non-conducting structures such as

buildings. A Lightning Protection System does not prevent lightning from

striking; it provides a means for controlling it and prevents damage by

providing a low resistance path for the discharge of the lightning energy.

The components of a typical Lightning Protection System (LPS) are as

follows:

Air Terminal

The Air Terminal is the part of the LPS that is intended to intercept

lightning flashes. The Air Terminal intercepts the downward-moving

stepped leader of the lightning strike, by launching an upward-going

attachment spark. Once the attachment is achieved, the bulk of the

lightning current follows the ionized path. In this way, an Air Terminal

diverts the lightning away from personnel and electronic equipment.

Down Conductor


The Down-Conductor is that part of the external Lightning Protection

System (LPS) that conducts lightning current from the Air Terminal system

to the Earth Termination system.

Outdoor Units Grounding

The Outdoor Unit, consisting of a Radio Frequency Unit and Antenna, must

include a grounding point for connection to the grounding.

Earth Termination System

The Earth Termination System is that part of external LPS that is intended

to conduct and disperse lightning current to earth.

Lightning Protectors

Lightning Protectors provide an additional protection to the equipment

embedded protectors, in places where lightning occurs with a high

probability.


Lightning Protection System

SURGE PROTECTOR

A surge protector is an appliance designed to protect electrical devices from

voltage spikes. A surge protector attempts to regulate the voltage supplied

to an electric device by either blocking or by shorting to ground voltages


above a safe threshold. Some parameters regarding surge protectors are

given bellow.

Clamping voltage

This is also known as the let-through voltage. This specifies what voltage

will cause the metal oxide varistors (MOVs) inside a protector to conduct

electricity to the ground line. A lower clamping voltage indicates better

protection, but a shorter life expectancy.

Surge Protector

Joules

This number defines how much energy the surge protector can absorb

without failure. A higher number indicates greater protection because the

device will divert more energy elsewhere and will absorb less energy

resulting in a lower voltage spike. Generally, 200 joules is undersized


protection since harmful voltage spikes are significantly larger than 200

joules. Better protectors start at 1000 joules and 50,000 amperes. If

properly installed, for every joule absorbed by a protector, another 4 or 30

joules may be dissipated harmlessly into ground.

Response time

Surge protectors do not kick in immediately; a slight delay exists. The

longer the response time the longer the connected equipment will be

exposed to the surge. However, surges don't happen immediately either.

Surges usually take around a few microseconds to reach their peak voltage

and a surge protector with a nanosecond response time would kick in fast

enough to suppress the most damaging portion of the spike.

WCDMA TECHNOLOGY

By definition, the bandwidth of a WCDMA (Wideband Code Division Multiple

Access) system is 5 MHz or more, and this 5 MHz is also the nominal

bandwidth of all 3G WCDMA proposals.

This bandwidth was chosen because,

• It is enough to provide data rates of 144 and 384 Kbps (these

were 3G targets), and even 2 Mbps in good conditions.

• Bandwidth is always scarce, and the smallest possible allocation

should be used, especially if the system must use frequency bands

already occupied by existing 2G systems.

• This bandwidth can resolve more multipaths than narrower

bandwidths, thus improving performance.


3G Network Architecture

In a CDMA system all users occupy the same frequency at the same time,

no time scheduling is applied, and their signals are separated from each

other by means of orthogonal codes. Each user is assigned a code applied

as a secondary modulation, which is used to transform a user’s signals into

a spread spectrum-coded version of the user’s data stream.

Tree of orthogonal codes

The receiver then uses the same spreading code to transform the spread-

spectrum signal back into the original user’s data stream. These codes are


chosen so that they have low cross-correlation with other codes. This means

that correlating the received spread-spectrum signal with the assigned code

despreads only the signal that was spread using the same code. All other

signals remain spread over a large bandwidth. That is, only the receiver

knowing the right spreading code can extract the original signal from the

received spread-spectrum signal. The figure above shows the tree of

orthogonal codes.

Power Control in WCDMA

Power control in uplink must make signal powers from different users

nearly equal in order to maximize the total capacity in the cell.

In downlink the power control must keep the signal at minimal required

level in order to decrease the interference to users in other cells.

Received power at the base station

PDH and SDH

PLESIOCHRONOUS DIGITAL HIERARCHY (PDH)

There are two types of multiplexing hierarchies of Time Division Multiplexing

(TDM), used in digital transmission. Traditionally,

digital transmission systems and hierarchies have been based on

multiplexing signals which are plesiochronous. To recover a 64kbps channel


from a 140Mbps PDH signal it is necessary to demultiplex the signal all the

way down to the 2 Mbps E1 level before the location of the 64 kbps channel

can be identified. PDH requires multiplexing steps of 2-8, 8-34, 34-140 to

add or demultiplexing steps of 140-34, 34-8, 8-2 to drop out an individual

speech or data channel.

Plesiochronous Digital Hierarchy

Signal Digital bit rate / (kb/s) Channels

E0 64 1 E0

E1 2048 32 E0

E2 8448 4 E1

E3 34368 16 E1

E4 139264 64 E1

SYNCHRONOUS DIGITAL HIERARCHY (SDH)

Synchronous Digital Hierarchy is a standard for telecommunications

transport formulated by the International Telecommunication Union (ITU).

The basic format of an SDH signal allows it to carry many different services

in its Virtual Container (VC) because it is bandwidth-flexible. This capability

allows for such things as thetransmission of high speed packet switched

services, ATM. However SDH still permits transport and networking at the

2Mbps, 34Mbps, and 140Mbps levels accommodating the existing PDH signals.

In addition SDH supports the transport of signals based on the 1.544Mbps

hierarchy used in America.

Synchronous Digital Hierarchy

Bit rate / (Mb/s) Abbreviated SDH SDH Capacity

51.84 51 Mb/s STM-0 21 E1

155.52 155 Mb/s STM-1 63 E1

622.08 622 Mb/s STM-4 4 E4

2488.32 2.4 Gb/s STM-16 16 E4


9953.28 10 Gb/s STM-64 64 E4

39813.12 40 Gb/s STM-256 256 E4

SIGNALING SYSTEM NO.7 (SS7)

Signaling System No. 7 (SS7 or C7) is a global standard for

telecommunication defined by the International Telecommunication Union

(ITU) Telecommunication Standardization Sector (ITU-T). The standard

defines the procedures and protocol by which network elements in the

public switched telephone network (PSTN) exchange information over a

digital signaling network to effect wireless (cellular) and wire line call

setup, routing and control.

Each signaling point in the SS7 network is uniquely identified by a numeric

point code. Point codes are carried in signaling messages exchanged

between signaling points to identify the source and destination of each

message. Each signaling point uses a routing table to select the appropriate

signaling path for each message. There are three kinds of signaling points in

the SS7 network.

SSP (Service Switching Point)

SSPs are switches that originate, terminate or tandem calls. An SSP sends

signaling messages to other SSPs to setup, manage and release voice

circuits required to complete a call.

STP (Signal Transfer Point)

Network traffic between signaling points may be routed via a packet

switch called an STP. An STP routes each incoming message to an outgoing

signaling link based on routing information contained in the SS7 message.

Because it acts as a network hub, an STP provides improved utilization of

the SS7 network by eliminating the need for direct links between signaling

points.


SCP (Service Control Point)

A SCP is a centralized database that contains the information about how to

route a call.

A figure of the SS7 Protocol Stack is shown below.

The SS7 Protocol Stack

Message Transfer Part (MTP)

The Message Transfer Part (MTP) is divided into three levels:

• MTP Level 1

The lowest level, MTP Level 1, is equivalent to the OSI Physical Layer.

MTP Level 1 defines the physical, electrical and functional

characteristics of the digital signaling link.

• MTP Level 2

MTP Level 2 ensures accurate end-to-end transmission of a message

cross a signaling link. Level 2 implements flow control, message

sequence validation and error checking. When an error occurs on a

signaling link, the message (or set of messages) is retransmitted. MTP

Level 2 is equivalent to the OSI Data Link Layer.


• MTP Level 3

MTP Level 3 provides message routing between signaling points in the

SS7 network. MTP Level 3 reroutes traffic away from failed links and

signaling points and controls traffic when congestion occurs. MTP Level 3

is equivalent to the OSI Network Layer.

Signaling Connection Control Part (SCCP)

SCCP provides connectionless and connection-oriented network services

and global title translation (GTT) capabilities above MTP Level 3. A global

title is an address that is translated by SCCP into a destination point code

and subsystem number. A subsystem number uniquely identifies an

application at the destination signaling point. SCCP is used as the transport

layer for TCAP-based services.

Transaction Capabilities Applications Part (TCAP)

TCAP supports the exchange of non-circuit related data between

applications across the SS7 network using the SCCP connectionless service.

Queries and responses sent between SSPs and SCPs are carried in TCAP

messages. For example, an SSP sends a TCAP query to determine the

routing number associated with a dialed number and to check the personal

identification number (PIN) of a calling card user. In mobile networks (IS-

41 and GSM) TCAP carries Mobile Application Part (MAP) messages sent

between mobile switches and databases to support user authentication,

equipment identification and roaming.

ISDN User Part (ISUP)

The ISDN User Part (ISUP) defines the protocol used to set-up, manage and

release trunk circuits that carry voice and data between terminating line

exchanges (e.g., between a calling party and a called party). ISUP is used

for both ISDN and non-ISDN calls. However, calls that originate and

terminate at the same switch do not use ISUP signaling.


MOBILE INTELLIGENT NETWORK (MIN OR IN)

A Mobile Intelligent Network (MIN) is a telecommunications concept that

meets the market demand for advanced services within the existing

telephony network, from both the network operator’s and service provider’s

perspective. The intelligence in the MIN is realized in computer

software and data. The ultimate objective of MIN is to increase revenue for

the network operator and the service provider.

IN Network Architecture

IN Network Architecture

Advantages of MIN

• Increased subscriber numbers due to more attractive services • Increased revenue due to use of services • Increased subscriber loyalty • Increased flexibility in deploying services in a network • Decreased development time for services • Reusability of service modules


MIN services

• Personal Number • Pre-Paid SIM Card • Cellular Virtual Private Network (CVPN) • Information and business (I&B)

GSM Basic Introduction

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