logical channels, bursts

8/8/2019 Logical Channels, Bursts

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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



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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



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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



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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



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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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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



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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

logical channels, bursts

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