Lecture11upload.ppt

8/10/2019 Lecture11upload.ppt

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GSM Frame Structure

Lecture 11


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Various GSM Bursts All burst types specified for GSM follow a similar pattern:

Each burst always begins with tail bits, which are necessary tosynchronize the recipient. Tail bits are, except for the access burst,

always coded as 000. The tail bits are followed by data bits, which differ in format for the

various burst types.

Each burst is terminated by another set of tail bits and the so-called guard period.

This guard period is required for the sender to physically reduce

the transmission power.

The guard period is particularly long for the access burst, to allowmobile stations that are far from a BTS and hence experiencepropagation delays to also access the BTS (TA)


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Functional differences

between the five bursts Normal burst.

The normal burst is used for almost every kindof data transmissionon all channel types. The only exceptionsto that rule are the initialchannel requestfrom the mobile station sent in an access burst

and the transmission of the synchronizationdata of a BTS that isdone via the synchronization burst. All other data transfer on alltraffic channels, dedicated control channels (DCCHs) and commoncontrol channels (CCHs) in uplink and downlink directions are donein normal bursts.

Every normal burst contains 114 bits of useful data that are sent intwo packets of 57 bits each. The so-called training sequence is

placed between the two packets. There is a stealing flag between the training sequence and each

data packet, which indicates to the recipient whether a 57-bitpacket actually contains user data or FACCH information.


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between the five bursts Synchronization burst

The synchronization burst is used to transmit synchronizationchannel information (SCH)

It uses a format similar to that of the normal burst In both cases, there are two data packets, left and right, from the

training sequence. However, for the synchronization burst, eachpacket contains only a 39-bit payload, because the trainingsequence is 64 bits long.

Training sequence for the synchronization channel is identical forall BTSs and therefore allows a mobile station to easily distinguishan accessible GSM-BTS from any other radio system that

accidentally works at the same frequency. Therefore, the trainingsequence in the synchronization channel serves two purposes:

It allows the mobile station to determine if there might have beentransmission errors, and

it allows the mobile station to distinguish a GSM source from othertransmission systems on the same frequency


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between the five bursts Access burst

In contrast to the bursts describedso far, the access burst comes in arather unique format because of itsspecial tasks

A mobile station uses the accessburst only for the initial access to aBTS, which applies in two cases:

for a connection setup starting fromthe idle state and

for handover

In both cases the MS does not knowthe current distance to the BTS and,hence, the propagation delay for thesignal

As long as the propagation delay isnot known to the MS, the MSassumes it is zero. Therefore, itgenerally is uncertain if the accessburst arrives within the receiverwindow of a BTS and how big theoverlap is (Figure G.11).


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between the five bursts That is the reason for the lesser length of an access burst

and the longer duration of the guard period. To ensure thatan access burst arrives at the BTS during the proper timeperiod the number of bits for the access burst was set toonly 88 bits. The maximum distance between BTS and MS is,with this timing, about 35 km.The normal burst would not fitinto the receiver window if the unknown propagation delaywas greater than zero

That is the reason why the normal burst is used only after

the distance of the MS from the BTS is determined, and theMS is able to adjust its transmission accordingly.

The adjustment parameter is called offset time and iscalculated fairly simply


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between the five bursts The BTS knows format and length of an access burst and is able to

determine the actual propagation delay from when the signalarrives back at the BTS after being relayed by the MS

That also allows calculation of the distance of an MS from the BTSThe BTS provides the offset time to the MS, which in turn transmitsits signal earlier, exactly by that time period

The format of an access burst is also different from the otherbursts. The access burst begins with 8 tail bits, rather than 3 as inthe case of the other bursts

The tail bits, together with the following 41- bit synchronizationsequence which also always carries the same value, allows the BTS

to distinguish the access burst from error signals or interferingsignals. Hence, the access burst serves on the uplink a similarpurpose as the synchronization burst does on the downlink


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between the five bursts Frequency correction burst

The most simple format of all the bursts is used for thefrequency correction burst, which is transmitted only in thefrequency correction channel (FCCH)

All 148 bits (142 bits + 6 tail bits) are coded with 0. Asequence of zeros at the input of a GMSK modulatorproduces a constant transmitter frequency which is exactly67.7 kHz above the BCCH median frequency.

Therefore, the frequency of the FCCH is always 67.7 kHzabove the frequency that is advertised as the downlink

frequency. This constant transmission frequency allows anMS to fine-tune its frequency to the BCCH frequency, tosubsequently be able to read the data within thesynchronization burst.


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between the five bursts Dummy burst

When the MS powers up, it checks the power level of the BCCH frequenciesof the cells (BTSs) nearby to determine which BTS to use as a serving cell

Similarly, when the MS is active, that is, involved in a call, the power levelof the BCCH frequencies of the neighbor cells serve as basis for a possible

handover decision. To be useful as a reference, the BCCH frequency has to be transmitted with

a constant power level. Thus, all time slots have to be occupied, and it isnot allowed to apply power control on the downlink.

For this purpose, the dummy burst was defined. These dummy bursts are inserted into otherwise empty time slots on the

BCCH frequency.

To prevent accidental confusion with frequency correction bursts, thedummy burst is coded with a pseudorandom bit sequence predefined byGSM


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TA

Timing advance In GSM MS to send its data three time slots after it received the data from the

BTS. The BTS then expects the bursts from the MS in a well-defined time frame. This prevents collision with data from other mobile stations. The mechanism works fine as long as the distance between MS and BTS is

rather small. Increasing distance requires taking into account the propagation delay of

downlink bursts and uplink bursts. Consequently, the mobile station needs totransmit earlier than defined by the three time slots delay rule.

The information about how much earlier a burst has to be sent is conveyed tothe mobile station by the TA.

The TA is dynamic and changes in time.

Its current value is sent to the mobile station within SACCH. In the opposite direction, the BTS sends the current value for TA to the BSC

(e.g., for handover consideration). The farther the MS is away from the BTS, the larger is the required TA.


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TA

Timing advance Using the TA allows the BTS to receive

the bursts from a particular MS in theproper receiver window.

The BTS calculates the first TA whenreceiving a RACH and reports the valueto the BSC.

TA can take any value between 0 and 63,which relates to a distance between 0 kmand 35 km.

The steps are about 550 m (35 km/63 550 m).

Figure illustrates the effect of TA by anexample in which a connection is activeon TS 1.


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PROPAGATION DELAYS

MS2MS1d1>>d2d2

BTS Frame reference

MSs transmit

Propagation Delay tp

TS0 TS1 TS2 TS3 TS4 TS6TS5 TS7

Bits Overlapping


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TX BTS CAN WHAT GSM HOW WHEN WHAT

RX BTS yes the ms-isdn

RX MS1 CAN

TX MS1 yes

RX MS2 WHAT

TX MS2 the

RX MS3 GSM

TX MS3 ms-isdn

RX MS4 HOW

TX MS4

RX MS5 WHEN

TX MS5

RX MS6 WHATTX MS6

RX MS7

TX MS7

RX MS8

TX MS8

PROPAGATIONDELAY

D

D

+3TS

TA

PROPAGATION DELAY


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TX BTS CAN WHAT GSM HOW WHEN WHAT

RX BTS ms-isdn

RX MS1 CAN

TX MS1

RX MS2 WHAT

TX MS2

RX MS3 GSM

TX MS3 ms-isdn

RX MS4 HOW

TX MS4

RX MS5 WHEN

TX MS5

RX MS6 WHAT

TX MS6

RX MS7

TX MS7

RX MS8

TX MS8

yes

the

PROPAGATIONDELAY

D

D

+3TS - TA

yes the

TIMING ADVANCE = 2 * PROPAGATION DELAY

TIMING DV NCE


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Physical & Logical Channels Physical Channel is a combination consists of a TS

number and an ARFCN Different logical channels can be mapped onto a

physical channel GSM defines a wide variety of logical channels

To transmit user data To Provide signalling and control data of the

network So each specific TS or frame may be dedicated to

either handling traffic data, signalling data (requiredfor internal working of GSM system) or controlchannel data (from MSC, BS or mobile user)


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GSM Channels

Control ChannelsTraffic Channels

(TCHs)

Full

rate

Half

rate

Dedicated ControlChannels

(DCCHs)

SlowFast

Downlink

BroadcastChannels

(BCHs)

Common ControlChannels

(CCCHs)

Downlink Uplink

TCH /F TCH /H FCCH SCH BCCH PCH CBCH RACHAGCH SDCCH SACCHFACCH

Traffic Multiframing Signaling Multiframing Traffic Multiframing

(down uplink)

CONTROL CHANNELS


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GSM Channel Types

Two types of GSM logical channels Traffic channels (TCHs)

carry digitally encoded user speech or user data and

have identical functions and formats on bothfwd and rev link

six different types of TCHs in GSM

Control channels (CCHs)

carry signalling and

synchronizing commands between the basestation and the mobile station

a large no of CCHs in GSM


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GSM Traffic Channels

May be either full-rate or half-rate In full-rate, user data contained within one TS per frame In half-rate, user date mapped onto the same TS, but sent in

alternate frames. So two half-rate channel users share the same TS, but

alternately transmit during every other frame TCH data may not be sent in TS 0 of certain ARFCNs as they

may be used by broadcast channels (BCH) TS 0 reserved for control channel bursts in almost every frame


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Full-Rate TCH Full-Rate Speech Channel (TCH/FS)

Carries user speech digitized at a raw data rate of

13kbps With GSM channel coding added to the digitized speech,

the full-rate speech channel carries 22.8kbps

Full-Rate data channel for 9600bps(TCH/F9.6) Carries raw user data which is sent at 9600 bps With additional error correction coding by the GSM

standard, the 9600 bps data is sent at 22.8kbps


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Full-Rate Data Channel for 4800bps (TCH/F4.8) Carries raw user data which is sent at 4800bps

With additional error correction coding by the GSM standard, the 4800bps is sent at 22.8 kbps

Full-Rate Data Channel for 2400 bps (TCH/F2.4) Carries raw user data which is sent at 2400bps

With additional error correction coding by the GSM standard, the 2400bps is sent at 22.8 kbps

Almost similar Half-Rate TCHs for speech and data

with little variations


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Frames of TCH data brokenup every 13thframe by either SACCH data

or Idle frames

26thframe contains Idle bits when full-rate TCHs

used and

SACCH data when half-rateTCHs used

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