the radio interface

8/14/2019 The Radio Interface

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The Radio interfaceThe Radio interface

The mostimportantinterface in GSM

technology. i.e.

The GSM technology


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Why is it so important..?? It is universal media for transmission and

reception of signals.

Key element to enable mobility andwireless access and provide roamingfacility.

It is limited resource shared by all users.

It must be well defined to obtaincompatibility between MS and BTS.

The GSM technology


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The frequency spectrum is very

congested, with only narrow slots ofbandwidth allocated for cellularcommunications. Standard GSM has atotal of 124 frequencies available for use

in a network. Most network providers areunlikely to be able to use all of thesefrequencies and are generally allocated asmall subset of the 124.The spectral

efficiency depends on radio interface andtransmission. The spectrum efficiency canbe increased by decreasing theinterference . It includes :

-Frequency reuse-Sectorization. The GSM technology


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Frequency re-use

Three types of frequencyreuse patterns

7 Cell reuse pattern

4 cell reuse pattern

3 cell reuse pattern Frequency re-use increases

the capacity and hencespectrum efficiency. Cell Dia = R

7/21 cell cluster

1

23

4

5

6

7

D


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Sectorization

Omni directional antennasradiates same frequency in everydirection, thereby increasing thechances of interference and no. ofcells in a particular area too.

Sectorization splits a site into

number of cells ,which increasesthe capacity of the system. Directional antennas are used

which ensures that radio from onesector is concentrated in aparticular direction which :

--increases the signal strength.

--makes frequency reusepattern to be usedefficiently.

OMNI CELL

1 ANTENNA

b1

b2

b3

120O CELLS

3 ANTENNAS


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Access methods

Since radio frequency is limited, sothere is no dedicated channel toindividuals but it is provided on

demand. Radio channels are sharedby all users.

Both FDMA and TDMA methods helps

to provide radio access to subscribersand hence provides mobility.


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Tdma technologyTime division multiple access is a digital technology that

allows a number of users to access a single radiofrequency channel without interference by allocating aunique time slots to each user within the channel.

Each carrier frequency is subdivided in time domaininto 8 time slots

Each mobile transmits data in a frequency, in itsparticular time slot - Burst period = 0.577 milli secs.

8 time slots called a TDMA frame. Period is .577 * 8 =4.616 milli secs

0 1 2 3 4 5 6 7

4.616 ms

0.577 ms


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How it works?

Let us consider that four different conversationis going on simultaneously.

TDMA divides a single

RF carrier into 8 slots

and provides eachconversation a single

slot for very short duration. After the four slotsconversation has been transmitted, process isrepeated again.

A. Marry had a little lamb.

B. Ramu is a good boy.

C. There was an old man who lived

in a shoe.D.Jack and Jill went up the hill

Marry hada

Ramu is a There wasan

Jack and

Slot 1 Slot 2 Slot 3` Slot 4

RFchannelFreq. 1


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Tdma advantages

It can easily adapt to transmission of data as wellas voice communication.

ability to carry 6 kbps to 120 Mbps of data rateswhich allows services like fax, voice band data,and SMS as well as multimedia too.

Since TDMA technology separates users accordingto time, it ensures that there will be nointerference from simultaneous transmissions.

It provides users with an extended battery life,since it transmits only portion of the time duringconversations.

most cost effective technology to convert an

analog system to digital.


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Tdma disadvantages One major disadvantage using TDMA technology is

that the users has a predefined time slot. When

moving from one cell site to other, if all the timeslots in this cell are full the user might bedisconnected.

Likewise, if all the time slots in the cell in which theuser is currently in are already occupied, the userwill not receive a dial tone.

Another problem in TDMA is that it is subjected tomultipath distortion. A signal coming from a towerto a handset might come from any one of several

directions. It might have bounced off severaldifferent buildings before arriving which can causeinterference. to overcome this distortion, a timelimit can be used on the system. The system will bedesigned to receive ,treat and process that signalwithin a certain time limit. Once the time limit is

expired the signal is ignored.


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GSM utilizes two bands of 25 MHz. 890-915 MHzband is used for uplink while the 935-960 MHz isused for downlink.

The frequency bands are divided into 200 KHz

wide channels called ARFCNs (Absolute RadioFrequency Channel Numbers) i.e. there are 125ARFCNs out of which only 124 are used.

Each ARFCN supports 8 users with each usertransmitting / receiving on a particular time slot(TS).

124

123

.

2

1

124

123

.

2

1

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

Data burst = 156.25 bit periods = 576.9 s

960 MHz

959.8MHz

200KHz

935 MHz

935.2 Mhz

915 MHz

200KHz

45 MHz

Downlink (TDMA frame) = 8 TS

Uplink (TDMA frame)

TS: Time slot

914.8 MHz

890.2 MHz

890 MHz

UPLINK

DOWNLIN

K

Therefore 1 TDMA frame = 156.25 x 8 = 1250 bitsTherefore 1 TDMA frame = 156.25 x 8 = 1250 bitsand has a duration of 576.92and has a duration of 576.92 s x 8 = 4.615 mss x 8 = 4.615 ms


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frequency division multiple access(fdma)

Divide available frequency spectruminto channels each of the same bandwidth Channel separation achieved by filters:

Good selectivity Guard bands between channels Signaling channel required to allocate a trafficchannel to a user Only one user per frequency channel at any

time Used in analog systems, such as AMPS, TACS Limitations on: frequency re-use number of subscribers per area


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890 MHz 915 MHz935 MHz 960 MHz

Uplink Downlink

Example:Channel 48

0 124channel # 0 124channel #

Frequency Frequency

Duplex spacing = 45 MHzFrequency band spectrum = 2 x 25 MHzChannel spacing = 200 kHz

BTS

Uplink and downlink frequency distribution


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Contd

The bandwidth of 25 MHz is divided into equalparts of 200 kHz and this band of 200kHz iscalled as GUARD BAND.

The single radio frequency is called as

ABSOLUTE RADIO FREQUENCY CHANNELNUMBER (ARFCN).

Frequency allotment is always done in pairsi.e. if 890.2 MHz is allocated as transmissionfrequency then 935.2 MHz will be allocated tothe same operator as receiving frequency.

The spacing between trans and receive

frequency is called as DUPLEX SPACING and it isequal to 45 MHz


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The GSM technology

From Speech to Radio Transmission

Blah Blah Blah...Blah... Blah... Blah...Digitizing andSource Coding

Channel Coding

Interleaving

Ciphering

Burst Formatting

Modulating Demodulating

Burst De-formatting

Deciphering

De-interleaving

Channel Decoding

Source Decoding


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channelshannelsThere are two types of channels- physical and logical channels.

Each timeslot on a TDMA frame is called a physical channel. Therefore,

there are 8 physical channels per carrier frequency in GSM. Physical

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

890MHz 915 MHz

0 1 2 3 4 5 6 7

Physical channels

Physical channels


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Logical channelsogical channelsLogical channels are multiplexed into physicalchannels. They are laid over the grid of physical

channels. Each logical channel performs a specificfunction. It is of two types:+ traffic channels (TCHs)+ control channels (CCHs)Traffic channels carry user information such as encoded

speech or user data.Two general forms are defined:Full rate traffic channel at a gross bit rate of 22.8kbps.Half rate traffic channel at a gross bit rate of 11.4kbps.

Control channels carry system signalling and

synchronisation data for control procedures Such aslocation registration mobile station synchronisation,paging, random access etc. between base station andmobile Station . It is of three types:Broadcast

Commondedicated


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LOGICAL CHANNELS

TRAFFIC CONTROL

FULL RATEBm 22.8 Kb/S

HALF RATELm 11.4 Kb/S

BROADCAST COMMON CONTROL DEDICATED CONTROL

FCCH SCH BCCH

PCHRACH

AGCH

SDCCH SACCH FACCH

CH -- FREQUENCY CORRECTION CHANNELH -- SYNCHRONISATION CHANNELCH -- BROADCAST CONTROL CHANNELH -- PAGING CHANNELCH -- RANDOM ACCESS CHANNELCH -- ACCESS GRANTED CHANNELCCH -- STAND ALONE DEDICATED CONTROL CHANNELCCH -- SLOW ASSOCIATED CONTROL CHANNEL

CCH -- FAST ASSOCIATED CONTROL CHANNEL

DOWN LINK ONLY

UPLINK ONLY

BOTH UP &

DOWNLINKS


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Broadcast Channel -roadcast Channel -BCHCH Broadcast control channel (BCCH) is a

base to mobile channel which providesgeneral information about the network,the cell in which the mobile is currentlylocated and the adjacent cells.

Frequency correction channel (FCCH) is abase to mobile channel which providesinformation for carrier synchronization.

Synchronization channel (SCH) is a baseto mobile channel which carriesinformation for frame synchronizationand identification of the base stationtransceiver


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Common control Channel -ommon control Channel -CCHCH Paging channel (PCH) is a base to mobile

channel used to alert a mobile to a calloriginating from the network.

Random access channel (RACH) is amobile to base channel used to requestfor dedicated resources.

Access grant channel (AGCH) is a base tomobile which is used to assign dedicatedresources (SDCCH or TCH)


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Dedicated Control Channel -edicated Control Channel -DCCHCCH Slow associated control channel (SACCH) is

a bi-directional channel used for exchangingcontrol information between base and amobile during the progress of a call set up

procedure. The SACCH is associated with aparticular traffic channel or stand alonededicated control channel

Fast associated control channel (FACCH) is abi-directional channel which is used forexchange of time critical informationbetween mobile and base station during theprogress of a call. The FACCH transmitscontrol information by stealing capacity

from the associated TCH

L ti d t f th bil


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Location update from the mobileLocation update from the mobile

RACH send channel request

AGCH receive SDCCH

SDCCH authenticate

SDCCH switch to cipher mode

SDCCH request for location updating

SDCCH authenticate response

SDCCH cipher mode acknowledge

SDCCH allocate TMSI

SDCCH acknowledge new TMSI

SDCCH switch idle update modeSDCCH switch idle update mode

MSS BTSTS

C ll t bli h t f th bil


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Call establishment from the mobileCall establishment from the mobile

RACH send channel request

AGCH receive SDCCH

SDCCH do the authentication and TMSI allocation

SDCCH require traffic channel assignment

SDCCH send call establishment request

SDCCH send the setup message and desired number

FACCH switch to traffic chnl & send back (steal bits)

FACCH receive alert signal ringing sound

FACCH acknowledge connect message and use TCH

TCH conversation continues

FACCH receive connect messageMSS BTSTS


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Call establishment to a mobileall establishment to a mobile

Receive signaling channel SDCCH on AGCH

Receive alert signal and generate ringing on FACCH

Receive authentication request on SDCCH

Generate Channel Request on RACH

Answer paging message on SDCCH

Authenticate on SDCCH

Receive setup message on SDCCH

FACCH acknowledge message and switch to TCH

Receive connect message on FACCH

Receive traffic channel assignment on SDCCH

Mobile receives paging message on PCH

FACCH switch to traffic chnel & send back (steal bits)

MSS BTSTS

the radio interface

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