Post on 10-Apr-2018
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Logical channels, Bursts
Thorough GSM fundamentals
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PHYSICAL CHANNELS:-
Time Division Multiple Access (TDMA) divides one radio frequency channel into
consecutive periods of time, each one called a "TDMA frame". Each TDMA frame contains eight
shorter periods of time known as "timeslots". These timeslots can be compared to the seats in the
vehicle. The TDMA timeslots are called "PHYSICAL CHANNELS", as they are used tophysically move information from one place to another
LOGICAL CHANNELS:-
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Broadcast Channels (BCH)
FrequencyCorrection Channel (FCCH)
Downlink Channel
BTS: Transmits a carrier frequency (Pure sine wave of 67.7 Khz)
This Solve 2 purpose :
a> Make sure that this is BCCH Carrier
b> To allow the MS to synchronize to the frequency
MS: After Switch on MS Scan for this channel, since it has no informati
on
to which frequency to use. FCCH carrier enables a mobile to tune its
frequency to that being broadcast by the BTS.
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Synchronization Channel (SCH)
Downlink Channels
BTS: Transmits TDMA Frame number + Base Station
Identity Code (BSIC= NCC + BCC )
MS: MS decodes the BSIC if the chosen BTS is GSM
Base station within a cell
Broadcast Channels (BCH)
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Broadcast Control Channel (BCCH)
Downlink Channels
BCCH contains the detailed Network and cell specificinformationsuch as :
Frequency used by Cell and its Neighboring cells.
Frequency HSN
Paging Groups
LAI
Max output power allowed in the cell
Broadcast Channels (BCH)
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Paging Channel (PCH)
Downlink Channels
BTS: Broadcast the paging message to indicate the IncomingCalls or Incoming SMS. Paging message also includesthe MSs identity number IMSI/TMSI
MS: MS listens to the PCH. If it identifies its own mobilesubscriber identity number on the PCH, it will respond.
Common Control Channels (CCCH)
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Random Access Channel (RACH)
RACH is transmitted Uplink only
When mobile is paged , it replies on RACH requesting asignaling channel.
RACH can also used if the MS wants to make a contact the NW/
Originating calls
Common Control Channels (CCCH)
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Access Grant Channel (AGCH)
Downlink channel
AGCH is answer to the RACH
NW assigns a signaling channel (SDCCH) to the MS. This
assignment is performed on the AGCH
Common Control Channels (CCCH)
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Dedicated Control Channels (DCCH)
Stand Alone Dedicated Control Channel (SDCCH)
SDCCH is bi-directional Channel
System Signaling
Call Setup
Authentication
Location Update
Assignment ofTraffic channels and Transmission of Short
messages
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Dedicated Control Channels (DCCH)
Slow Associated Control Channel (SACCH)
SACCH is transmitted in both Uplink and Downlink directions
SACCH is associated with each SDCCH and also with TCH
Uplink : MS Sends the averaged measurement on its own B
TS and
neighboring BTSs
Downlink: MS receives information regarding information
concerningTransmit power to use
Instructions on Timing Advance
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Fast Associated Control Channel (FACCH)
While Calls in progress and HO is required FACCH is used
FACCH works in Stealing mode meaning that one 20ms
segment of speech is exchanged for signaling information
necessary for the HO
Dedicated Control Channels (DCCH)
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Traffic channels are Bi-directional logical channel that transfer the user
speech or data.
Full Rate TCH ( TCH/F) :
This channel carries information at a gross rate at 13Kbit/s
HalfRate TCH (TCH/H) :
This Channel carried information at a gross rate at 6.5Kbit/s
Enhance Full Rate :
The speech coding in EFR is still done at 13Kbit/s, but the
coding mechanism is different that is used for normal FR.EFRgives better speech quality at the same bit rate than
normal FR.
Traffic Channels (TCH)
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Call to MS
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FDMA/TDMA structure of GSM
In GSM, every impulse on a particular frequency is called a burst and every burst
corresponds to a time slot (TS). Eight bursts orTSs numbered from 0 through 7 forms a
TDMA frame.
TDMA frame & Bursts Concept
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TimeAllocation,TDMA frame structure
GSM is based on TDMA technology, which means that channels (for traffic and
signaling) are separated from each other by time. This means that in radio path between theantennas of a Mobile Station (MS) and a Base Station (BTS), every channel has a specific
time on each frequency during which it can act.
The basic division is that one frequency is divided into eight Time Slots and each of
these Time Slots is an individual channel. More precisely, each frequency has eight
channels, either traffic channels or signaling channels.T
hese eight channels have their own"time slots" related to the time for transmitting or receiving data. So, every channel has a
'right' to act every eighth time slot.
Each Time Slot is 0.577 ms and thus eight Time Slots last 4.615 ms.
Physically this TDMA Frame is called TRx.
Generally we can put maximum 12 TRx in one BTS
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BurstsNormalBursts: This burst is used to carry information on :
Traffic channel
SDCCH Channel
Broadcast Control Channel
Paging Channel
Access Grant Channel
SACCH & FACCH Channel
It is used for the traffic channels, stand alone dedicated channels, broadcast control
channel, paging channel, access grant channel, and slow and fast associated control
channels.
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Access burst:
It is used to send information on the Random Access Channel (RACH). This
burst contains the lowest number of bits. The purpose of this extra free space is to measure the
distance between the Mobile Station and the BTS at the beginning of a connection.
This process determines a parameter called "timing advance" which ensures
that the bursts from different mobile stations arrive at the correct time, even if the distances
between the various MSs and the BTS are different. This process is carried out in connection with
the first access request and after a handover. A maximum theoretical distance of about 35
kilometres is allowed between the BTS and the MS.
MOBILEMAXIMUM RANGE
RANGE =
TIMING ADVANCE = DELAY OFBITS (0-63)
BIT PERIOD= 577/156.25 = 3.693Qsec =3.693 * 10e-6 sec
VELOCITY= 3 * 10e5 Km/sec
RANGE= 34.9 Km
T
IMIMG ADVANCE *B
IT
PER
IOD* VELOCIT
Y2
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FrequencyBursts:
All 148 bits(142+6) are coded with 0. The output of GMSK Modulator is a fixedfrequency signal exactly 67.7 Khz above the BCCH carrier frequency.
Thus the MS on receiving this fixed frequency signal fine tunes to the BCCH frequency
and waits for the Sync burst to arrive after 1 TDMA Frame.
Bursts
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SynchronizationBursts:
This burst is used for time synchronization of the MS
39Bit x 2=78 Bits :Are decoded to arrive 25-SCH control bits and that contains
the information of the NCC ,BCC & TDMA FN
64 Bits : Long training seq. of 64 Bits are identical for all BTS
Bursts
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DummyBursts:
To enable the BCCH frequency to be transmitted with a constant power level, dummy
burst are inserted.
This burst is transmitted on when no other type of burst is to be sent.
Thus it makes possible for MS to perform the power measurement on the BTS in order
to determine which BTS to use for initial access or which to use for HO
CCCH is replaced by the dummy page, when there is no paging message to transmit.
Bursts
1 i l t 156. 5 bit ti (15/ 6 .577 )
TB
3
Mixed Bits Training Sequence TB Guard BandMixed Bits
58 26 3 8.2558
58Bits: Coded with pseudo random bit seq. to prevent confusion with Freq correction burst
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Relationship Between Burst & FrameBurst : Physical content of a TS is called Burst.There are 5 types of Bursts
each having 15/26 ms duration and 156.25 Bits.
Hyperframe: In GSM system every TDMA frame is assigned a fixed number,
which repeats itself in a time period of3 HOURS 28 MINUTES 53
SECONDS 760 MILLISECONDS. This time period is referred to
as Hyperframe.
Superframe : =51x26 Multiframes. So, Duration =51x26x8x15/26=6Sec 120ms
Multiframe :There are two types of multiframe.
26 TDMAFrame Multiframe :Used to carry TCH, SACCH and
FACCH
Duration =26 x 8 x 15/26 =120ms
y51TDMAFrame Multiframe : Used to carry BCCH,CCCH,SDCCH
and SACCH.
yDuration =51 x 8 x 15/26 =235.38ms
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THANK YOU