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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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Properties of Radio Path
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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Properties of Radio Path
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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Properties of Radio Path
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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Properties of Radio Path
Flat Fading-Solutions
-Frequency Hopping
-Channel Coding/Speech Coding
-Interleaving
-Antenna/Receiver Diversity
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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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Properties of Radio Path
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
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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
Properties of Radio Path
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Properties of Radio Path
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.
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Properties of Radio Path
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
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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