Radio Path Channels Bursts

8/10/2019 Radio Path Channels Bursts

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Properties of Radio Path


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4. Radio Path

Module Objectives

This module describes the GSM Radio Path.

After studying this material you should be

able to:

Explain at least one of the problems in the air interface and

its solution in the GSM

Name at least three logical channels in the air interface and

explain their functions without using any reference.


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Fast Fading (Rayleigh Fading)

-Caused by multipath propogation. Signal received is the vector

sum of original transmitted signal.

-Typical in PLMN, in city areas where many reflectors exist, tallbuildings and mountains.

Fast Fading

-Selective Fading (time dispersion)

-Flat Fading


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Selective Fading-Reflected signal coming from objects that are far away (1-5km)

-Bit Rate in GSM is 270kbit/s.Time corresponds to 1.1km bitlength.This distance&longer distances causes problems.

-Typical in areas of mountainous terrain or areas with large

expansions of water or both.

-Result is a problem called ISI (Inter Symbol Interference)

BTS


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Selective Fading-Solution-We model the air interface and treat it as a filter.

-26 bits are added into the middle of every burst called the training

sequence code.

-After receiving a burst, we pass the burst through a filter in

MS/BTS.

-When we find the best possible filter we use this filter to interpretthe data/speech bits.

-Not all possible filters are tried but only the ones that are the most

likely to reduce time used in filtering.

-We use Viterbi Equalizers.

Air-Interface FilterSignal awaited

to be received


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Properties of Radio Path Flat Fading

-Caused by the Vector Summation of signals from near objects.-Summation can be beneficial but can also be zero and cause

fading dips.

-In GSM 900/DCS 1800 dips occur approximately 17/8.5cm.

RX sensitivity

Approx.

17cm

Fading dips

-Fading dips are most likely to occur at every half wave length and

therefore are frequency dependent.

-Faster the MS moves, less information is lost because of fading dips.


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Flat Fading-Solutions

-Frequency Hopping

-Channel Coding/Speech Coding

-Interleaving

-Antenna/Receiver Diversity



Properties of Radio Path Frequency Hopping

F2

F1

F3

F4

Time

-Dips are frequency dependent so by changing the transmitted

frequency we can move the dips to different locations.

-Beneficial especially to slow moving MSs

-Optional in the BTS, but obligatory in MS

-For each cell(sector)the hopping sequence is the same.

-Information of the sequence is broadcast on the control channels.

-Hopping sequence must be different on each BTS site.

-Frequency that sends the control channels must remain on the same

frequency and power level.


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Channel Coding(Block Coding and Convolutional Coding)

Block Coding

-Used to detect errors

-Adds additional bits into the information sent(check-sum bits, convolutional-coder reset bits)

Convolutional Coding

-Used to detect and correct errors detected

-Doubles the amount of bits coded



SPEECH

SEGMEN-TATION

SPEECH

CODING

BLOCK

CODING

CONVOL-TUTIONAL

CODING

INTER-

LEAVING

CIPHERING

BURST

FORMAT-

TING

TRXMODULA-

TION

33.8 Kbits

900/1800 MHz

20 ms 13 Kbits/sec

= 260 bits22.8 Kbits= 456 bits

Channel coding





Received signal

RXRX

Signal

Processing

Antennas

Antenna/Receiver Diversity

-Two receiver aerials-Signal received seperately and strongest is selected by

DSPU.

-Aerials six/three meters apart, thus giving 6db gain in signal strength.

-GSM 900 approximately 6 meters,GSM 1800 approximately 3 meters.




Solution-Adaptive power control, transmission power of BTS/MSchanges-Adaptive power control is active on all speech and commonchannels but not on broadcast channels

BTS

Slow Fading

-Caused by hills and other objects on the signal way



Channel Organization

There are 2 different types of channels in GSM/DCS Physical Channel

-Single TSL on a single frequency

-8 physical channels per frequency (TDMA frame)

-Information sent on 1 channel is termed a burst Logical Channel

-Within a burst

-Information sent is of particular type

(speech/signalling/data/sms)-There are 11 logical channels

-Logical channels are mapped so that they take minimum

space in order to maximize amount of traffic channels.

T i i M t i l


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4. Radio Path

4.2 Channel Organization

COMMON

CHANNELSCOMMON

CHANNELS

BROADCAST

CHANNELSBROADCAST

CHANNELS

COMMON

CONTROL

CHANNELS

COMMON

CONTROL

CHANNELS

DEDICATED

CONTROL

CHANNELS

DEDICATED

CONTROL

CHANNELS

TRAFFIC

CHANNELSTRAFFIC

CHANNELS

FCCHFCCH SCHSCH BCCHBCCH SDCCHSDCCH SACCHSACCH FACCHFACCH

PCHPCH RACHRACH AGCHAGCH TCH/FTCH/F TCH/HTCH/H TCH/EFRTCH/EFR

DEDICATED

CHANNELSDEDICATED

CHANNELS

LOGICALCHANNELSLOGICAL

CHANNELS

Training Material


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Broadcast channels

Frequency Correction Channel (FCCH) Pure sine wave.

The MS searches for this channels to switch on.

Downlink.

Synchronisation Channel (SCH) After locking to the frequency the MS synchronises with

the SCH.

The SCH contains the BSIC of the BTS and the TDMAframe number (used in encryption).

Broadcast Control Channel (BCCH) Common information about the BTS:

Used frequencies Frequency hopping sequence Channel combination Paging groups Surrounding cell information


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Dedicated channelsStand Alone Dedicated Control Channel (SDCCH)

Bi-directional channel. Used for call set-up procedures, e.g. authentication.

The traffic channel (TCH) is assigned by usingSDCCH.

Slow Associated Control Channel (SACCH)

Associated with SDCCH and TCH. Measurement reports.

MS power control.

Timing alignment.

Fast Associated Control Channel (FACCH)

Associated with TCH.

For quick control communication, e.g. handover.

Physically replaces 20 ms of speech, stealingmode


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Traffic channels

Full Rate Bi-directional channel.

Used for speech or datatransmission.

User data bit rate 13 kbit/s.

Half Rate Bi-directional channel.

Used for speech or datatransmission.

User data bit rate 6-7 kbit/s.

Enhanced Full Rate (EFR) Bi-directional channel.

Used for high quality speechtransmission.

User data bit rate 13 kbit/s.

4Training Material


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4. Radio PathTraining Material

Continuously transmits information

to the Mobile Stations

FCCH

Frequency Correction Channel

Pure sine wave carrier for easy

recognition by MS

SCH

nisatioSynchro n Channel

Immediately after FCCH, provides

BSIC and current TDMA frame no.

BCCH

Used for setting up signalling connections

between the MS and the BSS

PCH

Paging Channel

Downlink channel used to page

the MS for an incoming call

RACH

Random Access Channel

Uplink channel used by the MS to

request an SDCCH channel for call

setup or in response to a page

AGCH

Access Grant Channel

Downlink channel used to assign

an SDCCH to an MS

Stand-alone Dedicated Control Channel

Channels used for signalling

related to a particular call

Bi-directional channel used for call

setup procedures, eg authentication

Slow Associated Control Channel

Associated with SDCCH and TCH for

call control, eg power control,

timing advance, measurements

Fast Associated Control Channel

Steals a TCH for fast information

transfer in situations such as

handovers

Used for transmitting and receiving

speech and data

TCH/F

Full rate speech or

data channel

TCH/H

Half rate speech or

data channel

Broadcast Control Channel

Broadcasts general BTS information:

- used frequencies- frequency hopping sequence- channel combinations- paging groups- neighboring cell information

4.3 Logical

Channels

4 R di P thTraining Material


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4. Radio PathTraining MaterialTDMA Frames and Multiframes (Downlink)

TSL0 TSL1 TSL2 TSL3 TSL4 TSL5 TSL6 TSL7

0 FCCH TCH1 SCH TCH2 BCCH TCH3 BCCH TCH4 BCCH TCH5 BCCH TCH6 CCCH TCH

7 CCCH TCH8 CCCH AGCH TCH

9 CCCH TCH10 FCCH TCH11 SCH TCH12 CCCH SACCH13 CCCH TCH14 CCCH PCH TCH15 CCCH TCH16 CCCH TCH17 CCCH TCH18 CCCH PCH TCH19 CCCH TCH

20 FCCH TCH21 SCH TCH22 SDCCH TCH23 SDCCH TCH

24 SDCCH TCH25 SDCCH IDLE26 SDCCH27 SDCCH28 SDCCH29 SDCCH30 FCCH31 SCH32 SDCCH33 SDCCH

34 SDCCH35 SDCCH36 SDCCH37 SDCCH38 SDCCH

39 SDCCH40 FCCH41 SCH42 SACCH43 SACCH44 SACCH45 SACCH46 SACCH47 SACCH48 SACCH49 SACCH50 IDLE51 FCCH52 SCH

MULTIFRAME,51TDMAFRAMES

MULTIFRAME,26TDMAFRAM

ES


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GSM burst types (1)

tailbits

3

fixed bits ("0")142

tailbits

3

guardperiod

8,25 bits

FREQUENCY CORRECTION BURST

tailbits

3

encrypted bits57

SB1

trainingsequence

26

SB1

encrypted bits57

tailbits

3

guardperiod

8,25 bits

NORMAL BURST

TDMA FRAME ~ 4.615 ms

576.9 s


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GSM burst types (2)

ext. tailbits

8

synchronisationsequence

41

encrypted bits36

tailbits

3

extended guard period68,25 bits

ACCESS BURST

tailbits3

encrypted bits39

extended trainingsequence64

encrypted bits39

tailbits3

guardperiod8,25bits

SYNCHRONISATION BURST

tailbits

3

mixed bits142

tailbits

3

guardperiod

8,25 bits

DUMMY BURST


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Burst types Frequency correction burst

Used to transmit the FCCH channel. No information.

Synchronisation burstUsed to transmit synchronisation information.

Access burstUsed to send RACH information.

RACH contains the first message from the MS to the BTS.

It has a long guard period to allow the BTS to calculate the MS distance

from the BTS and to provide timing advance information to the MS.

Normal burstUsed to send all other logical channel information.

Dummy burstUsed to fill up unused timeslots in the TRX, which transmits the BCCH

channel.

No real information.


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TIMESLOTS AND BURST

A TSL is a time interval of about 576.9 microseconds

us, that is 156.25 bit duration and its physical contentis called a burst. Its the formatted information

corresponding to one part of a TDMA channel. Five

different types of bursts exists in the system. Each of

these bursts contains particular information.


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Normal burst, this channel is used to carryinformation on traffic and control channels, expectRACH, SCH, FCCH

Frequency correction burst,is used forsynchronisation of the mobile

Synchronisation burst, This is used for timesynchronisation of the mobile

Access burts, This burst is used for randomaccess to provide for burst from a mobile whichdoes not know the timing advance at the firstaccess or after handover

Dummy burts, This is broadcast when no otherinformation is, as a fill in certain circumstances. Itcontains no information, but is of the same formatas a normal burst. Lets see contain of normalburst....


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O SS / G

TB Encrypted bits Training

sequence

Encrypted bits TB GB

3 58 26 58 3 8.25

It contains 116 encrypted bits, 114 of which are of

encrypted speech ora data, the two other bits being used

to indicate the burst is in steal mode for FACCH. It also

incudes a guard time of 8.25 bit(30.46 us) duration

during which the transmitter is powered down. Training

sequence is a fixed bit patternwhich the receiver uses

to create a model of the air interface. The three tailbits are

used to reset the convolutional coder, and also act as a

start and stop bit pattern

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Radio Path Channels Bursts