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Live on Top


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GSM Transmission

GSM Signal Processing

GSM Air Interface


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GSM Transmission


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Effects on Radio Communication

Unlike fixed links, mobile radio transmissions suffer large fluctuation in both timeand space

During propagation, the radio signal experiences different effects (phenomenon)which affect the signal quality


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Signal degradation can be classified by type :

Path Loss

Due to distance covered by the radio signal,Free space path loss

LFS = 32.44 + 20 log F (MHz) +20 log d (Km)

Signal attenuation

Resulting from shadowing effects introduced by the obstacles between

transmitter and receiver

Fading of the signal

Caused by numerous effects all of which are related to the Radio

propagation phenomenon


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The Radio propagation phenomenon

Reflection

Propagating wave impinges on an object which is large compared to

wavelength E.g., the surface of the Earth, buildings, walls, etc.

Diffraction

Radio path between transmitter and receiver obstructed by surface with

sharp irregular edge

Waves bend around the obstacle, even when LOS does not exist

Scattering

Objects smaller than the wavelength of the propagating wave

E.g., foliage, street signs, lamp posts


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These phenomena's of the radio propagation causes multi kind of fading:

1. The normal fading:

2. The multi path fading:

Fading means that The signal strength received fluctuates around a mean

value while changing the mobile position resulting in undesirable beats in

the speech signal.


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Fading Problems

1. Shadowing (Normal fading):

The reason for shadowing is the presence of obstacles like largehills or

buildings in the path between the site and the mobile.

Multi path scattering from nearby objects. Attenuation effects

2. Rayleigh Fading (Multi-path Fading): Due to the reflected signals from the surrounding building.

So, a combination of direct and out-of-phase reflected waves at the receiver

yields attenuated signals (i.e. signal paths of different paths with differenttimes of arrival at the receiver)


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Fading Problems


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Fading Problems Solutions

1. Increase the fading Margin


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1. Increase the fading Margin:

We will define these parameters:

Global mean value: it is the supposed value of signal to be received my the

receiver Fading dip: the decrease happened due to the fading

Receiver sensitivity: the min. value the receiver can receive it and still detect the

signal The fading margin: the distance between the global mean value and the

receiver sensitivity

The solution is to increase the fading margin by:

increase the global mean value which is suppose to be received by the receiver

by make optimization for the network

increase the receiver sensitivity


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2. Antenna diversity (Space Diversity)

The cell transceiver will use two receiving antennas instead of one.

They will be separated by a distance of about (10* ), and they will

receive radio signals independently, so they will be affected differently bythe fading dips and the better signal received will then be selected.


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2. Antenna diversity (Space Diversity)

Received Signal Strength

Distance


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3. Frequency hopping (frequency Diversity)

Frequency

f 0

Frame

f 1

f 2

f 3

f 4

Time


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Time Delay problem

Transmission delay is unavoidable in the radio interface. If themobile stationmoves away from the base station during a call, the further distance the more

delay. The uplink is as the same.

If the delay is too high, the timeslots of the signal from a certain mobile station andthat of the next signal from another mobile station received by the base station will

overlap each other,


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Time Delay problem

BTS

F

2

1

2

T

1 2

S l i i d l bl


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Solution to time delay problem

To avoid this, during a call, the measurement report sent from the mobile station tothe base station carries a delay value. Moreover, the base station should monitor the

time when the call arrives and send an instruction to the mobilestation via the

downlink channel every 480ms so as to inform the mobile station the time of advance

transmission.

This time is the TA (timing advance), which ranges between 0~63 (0~233s ). The

TA value is limited by the timing advance code 0~63bit of the GSM system. Therefore,

the maximum coverage distance of the GSM is 35km. Its calculation is as follows:1/2 x (3.7s /bit) x 63bit x c = 35km

{In the formula, 3.7s /bit is the duration per bit (0.577msec/156.26bit); 63bit is the

maximum bit number of the time adjustment; c is the light speed (transmission speed

of the signal); and indicates that the go and return trip of the signal.}

According to the above description, the distance corresponding to 1bit period is 554m.

Influenced by the multi-path propagation and MS synchronization precision, the TA

error may reach up to about 3bit (1.6km).


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Frame Rate = 270.8 Kb/s

One BIT Duration = 3.7 sec

Bit stream is moving with the velocity of light which equals 3 x105 Km/sec

Then, when bit 2 is transmitted, bit one will cut a distance =

3.7 x 10 -6 x 3 x 10 5 = 1.1 km

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

270.8 Kb/s

Time dispersion problem

270.8 Kb/s 270.8 Kb/s

Ti di i bl


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Time dispersion problem

Directray1.1Km

1.1Km

1.1Km

There would be an interferencebetween the bit in the reflected ray

and 1 bit later in the direct way .

Ti di i bl S l ti


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Time dispersion problem Solution

1. Increase the Carrier to reflection ratio The C/I ratio is defined as the difference in signal strength between the signal

received from the BTS and the strongest reflected signal .

C/I Relative position to the BTS

Result

Planner should choose the proper position of the site to make the C/Imaximum everywhere in the coverage area of the site.

Ti di i bl S l ti


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Time dispersion problem Solution

2. Use Viterbi Equalizer

Choose xxx so that the

difference between T and

T* is the minimum

Probable

received

bit pattern

Received

Bits

Correlator

Channel

Model

Data T* Data

xxx T xxx

Compare

VITERBI

DiscontinuousTransmission


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Discontinuous Transmission

DTX: Discontinuous Transmission Shut off the transmission at voice intervals;

Only transmit necessary frames

The transcoder at the RX terminal produces comfortable noise.

Prolong batterylife and reduce

interference

DiscontinuousTransmission


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Discontinuous Transmission

DISCONTINUOUS TRANSMISSION:

DTX increases the efficiency of the system through a decrease in the possible

radio transmission interference level.

It does this by ensuring that the Mobile Station does not transmit unnecessarymessage data.

DTX can be implemented, as necessary on a call, its' effects will be most

noticeable in communications between two Mobile Stations.

When implemented at the mobile station DTX also result in considerable

power saving. If the mobile does not transmit during 'silences' there is a

reduction in the overall power output requirement.

Voice Activity Detection


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Voice Activity Detection

VOICE ACTIVITY DETECTION: VAD is a mechanism whereby the source transmitter equipment identifies the

presence or absence of speech.

VAD implementation is effected in speech mode by encoding the speechpattern silences at a rate of 500 bit/s rather than the full 13 Kbit/s.

This results in a data transmission rate for background noise, which is

regenerated in the receiver, known as "comfort" noise. Comfort noise is

necessary. Because without "comfort" noise the total silence between the

speech would be considered to be disturbing by the listener.

Implemented by the transcoder.

Power Control


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Power Control

Prolong battery life Reduce network interference

Include both uplink power control and downlink power control

Level and quality are taken into account BSC is the final adjudicator

BCCH Carrier is not

involved in power

control.

Time

Signal level

Target level value:

e.g. -85 dm

Power Control


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Power Control

During the process of radio transmission of signals, to reduce the interference, toincrease the utilization efficiency of the frequency spectrum, and to prolong the

battery life, the transmission power can be adjusted, that is called power control.

More specifically, the power control refers to adjust the transmission power of themobile station or base station in the radio mode within a certain range.

Its objective is the same as that of the DTX. When the receiving level and quality

is rather strong, the transmission power at the TX terminal canbe reduced

appropriately so that the communication can be kept at a certain level. In this way,the interference on other calls around can be reduced.

GSMSignalProcessing


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GSM Signal Processing

GSMTransmissionProcess


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GSM Transmission Process

Segmentation

Speech Coding

Channel Coding

Interleaving

Ciphering/Encryption

A/D Conversion

Burst Formatting

Modulation and

Transmission

Analog toDigital Conversion


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Analog to Digital Conversion

Analog to digital conversion takes

place in 3 steps:

1. Sampling

2. Quantization

3. Coding

1. Sampling

Telecommunication systems use Sampling rate = 8 Kbit/s



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2. Quantization

Within common telephony, 256 levels are used while in GSM 8192 levels are

used.




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Coding involves converting the quantized values into binary.

Every value is represented by a binary code of 13 bits (2 13 = 8192).

1 2 3 4 5 6 7 8 9 10 11 12 13 0 1 2 3 4 5 6 7 8 9 10 11 12 13

3. Coding




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The output rate of the A/D Conversion process is: 8000 Samples/Sec x 13 bits/Sample = 104 Kb/s

If one frequency will be used for 8 calls, then the bit rate will be:

8 x 104 kb/s = 832 kb/s this will not fit in the 200 KHz channelallocated for one frequency.

So, Coding should be used to reduce the rate.

Segmentation


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1 2 3 4 5 6 7 8 9 10 11 12 13 0 1 2 3 4 5 6 7 8 9 10

1 2 3 4 . . . . . . . . . . . . . . . 160

160 sample in 20 ms = 1 Segment

Segmentation

GSM Speech Coding


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GS Speec Cod g

The voice compression coding technique is widely used in the modern digitalcommunication systems.

In this technique, a voice coder is used to set up a model to simulate the voice

and noise produced by human vocal organs.

The parameters to form the model will be transmitted through theTCH channels.

The voice coder is based on the residual excited linear prediction (REIP) coder.Moreover, the long term predictor (LTP) is used to enhance the compression

effect. LTP can make the coding of residual data more advantageous by removing

the vowels from the voice.

With 20ms as the unit, the voice coder outputs 260bits after compressed coding.

Therefore, the code rate is 13kbps.

GSM Speech Coding


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p g

Many types of speech coders are available. Some offer better speechquality, at the expense of a higher bit rate (waveform coders). Others use

lower bit rates, at the expense of lower speech quality (vocoders).

The hybrid coder used by GSM provides good speech quality with a low bitrate, at the expense of speech coder complexity.

Channel Coding


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g

To check and correct errors during the transmission, redundancy data and theinformation calculated from the source data are added to the stream so as to

increase the bit rate.

For the voice, the length of these codes is 456 bits every 20ms.

Inputis260bitsrepresenting

1segment

Interleaving


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g

If the voice signal is modulated and transmitted directly after channel coding, dueto the condition changes in mobile communication channel, a deepof the fading

will influence a successive string of bits and cause high bit error rate.

If the bits of a successive string are interfered or lost, the other end of thecommunication can not decode the interfered or lost bits.

To solve this problem, some technique or method to separate the successive bits

are required.

In this way, even if errors occur, the errors are only concernedwith a single or

very short bit stream, which will not lead to that the whole burst or the whole

message block cannot be decoded.

In this case, the channel coding will take effect and recover the bit errors

Interleaving


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g

First Level Interleaving1 2 3 4 5 6 7 8

9 10 11 12 13 14 15 16

17 18 19 20 21 22 23 24

25 26 27 28 .. .. .. ..

.. .. .. .. .. .. .. ..

.. .. .. .. .. .. .. ..

.. .. .. .. .. .. .. ..

.. .. .. .. .. .. .. ..

429 430 431 432

433 434 435 436 437 438 439 440

441 442 443 444 445 446 447 448

449 450 451 452 453 454 455 456

5

7Bits

8 Groups

Interleaving


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Second Level Interleaving1 A T 5 D

2 A T 6 D

3 A T 7 D

4 A T 8 D

1 B T 5 A

2 B T 6 A

3 B T 7 A

4 B T 8 A

1 C T 5 B

2 C T 6 B

3 C T 7 B

4 C T 8 B

1 D T 5 C

2 D T 6 C

3 D T 7 C

4 D T 8 C

1

2

3

4

5

6

7

8

20 ms Block A

1

2

3

4

5

6

7

8

20 ms Block A

1

2

3

4

5

6

7

8

20 ms Block D

1

2

3

4

5

6

7

8

20 ms Block D

1

2

3

4

5

6

7

8

20 ms Block c

1

2

3

4

5

6

7

8

20 ms Block c

1

2

3

4

5

6

7

8

20 ms Block B

1

2

3

4

5

6

7

8

20 ms Block B

Interleaving


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Ciphering and Encryption


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The purpose of ciphering is to encode the burst so that it cannot be interpreted byany other device than the intended receiver.

The ciphering algorithm in GSM is called the A5 algorithm.

It does not add bits to the burst, meaning that the input and output to theciphering process is the same as the input: 456 bits per 20 ms.

Burst Formatting


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Every transmission from an MS/BTS must include some extra information suchas the training sequence.

The process of burst formatting is to add these bits (along withsome others such

as tail bits) to the basic speech/data being sent.

In GSM, the input to burst formatting is the 456 bits received from ciphering. Burst

formatting adds a total of 136 bitsper block of 20 ms, bringing the overall total to592.

Burst Formatting


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Now, the 592 bits will be sent on 4 bursts, each containing 2 x 57 bits + 136 / 4 =148 bits in each burst.

However, each time slot on a TDMA frame is 0.577 ms long.

This provides enough time for 156.25 bits to be transmitted (each bit takes 3.7

us),

The rest of the space, 8.25 bit times, is empty and is called the Guard Period

(GP). This time is used to enable the MS/BTS ramp up and ramp down.

Burst Formatting


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To ramp up means to get power from the battery/power supply for transmission.

Ramping down is performed after each transmission to ensure that the MS is not

transmitting during time slots allocated to other MSs.

The output of burst formatting is a burst of 156.25 bits per oneburst or 625 bits

per 20 ms.

When it is considered that there are 8 subscriber per TDMA frame, the overall bit

rate for GSM can be calculated to be 270.9 kbits/s.

Modulation Techniques


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The modulation technique used in GSM is Gaussian Minimum Shift Keying(GMSK).

GMSK enables the transmission of 270kbit/s within a 200kHz channel. This gives

a bit-rate of 1.3 bit/s per Hz.

This is rather low bit-rate but acceptable as the channel used has high

interference level in the air.

Modulation and Transmission


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GSM Air Interface


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Type of Communication Channel


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ApplicationsPropertiesType of Channel

FM radio, televisionOne-way onlySimplex

Police radioTwo-way, only one at a timeHalf duplex

Mobile systemsTwo-way, both at the same timeFull duplex

A duplex channel, such as that used during a mobile call, uses twofrequencies: one to the MS and one from the MS. The direction from the MS

to the network is referred to as uplink. The direction from the network to the

MS is referred to as downlink

Because it requires less power to transmit a lower frequency over a givendistance, uplink frequencies in mobile systems are always the lower band of

frequencies this saves valuable battery power of the MSs.

Frequency allocation


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GSM 1900GSM 1800GSM 900

1850-1910 MHz1710-1785 MHz890-915 MHzUplink

1930-1990 MHz1805-1880 MHz935-960 MHzDownlink

~16 cm~17 cm~33 cmWavelength

80MHz95MHz45MHzDuplex Distance

200KHz200KHz200KHzCarrier Separation

299374124Radio Channels

Frequency

system

space

FDMA in GSM


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Separation between carriers must be sufficient to eliminate interference betweenadjacent channels.

Where The more the separation the less the co-channel interference but the less

the available channels suited in the bandwidth.

It is found that a200 kHzchannel separation is suitable for all systems.

Spectrum Allocation (GSM 900)


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GSM 900 Frequency Allocation

F (MHz)915890

Uplink1 2 3 4 121 122 123 124

F (MHz)

Downlink

960935

1 2 3 4 121 122 123 124

890.2

890.4

890.6

935.2

935.4

935.6

200 KHz

1

1

121

121

Downlink 935 960 MHz

Uplink 890 915 MHz

TDMA in GSM


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With TDMA, one carrier is used to carry a number of calls, each call using thatcarrier at designated periods in time.

These periods of time are referred to as time slots.

Each MS on a call is assigned one time slot on the uplink frequency and one onthe downlink frequency

It is found that a8 Time Slotsper carrier is suitable for all systems each time slot

is0.577 msec.

Information sent during one time slot is called aburst.

GSM air interface


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GSM air interface is represented by all types of channels used in thecommunication between the base station and mobile subscribers.

GSM900 band is divided into124 RF channels,

Each channel is divided into8 time slots using TDMA. These time slots are called

physical channels. A physical channel may carry different messages, depending on the information

that is to be sent. These messages are called logical channels.

Physical Channels


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GSM900 band is divided into 124 RF channels, and each channel isdivided into 8time slots using TDMA.

These time slots are called physical channels.

Logical Channels


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Logical Channels

Control ChannelsTraffic Channels

Half Rate Full Rate

Synchronization CHannel

Broadcast Control CHannel

Frequency Correction Control CHannel

Random Access CHannel

Access Grant CHannel

Standalone Dedicated Control CHannel

Slow Associated Control CHannel

Fast Associated Control CHannel

Cell Broadcast Control CHannel

BroadcastCommon Dedicated

Paging CHannel

Traffic Channel


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Carries either encoded speech or user data up and down link between a singlemobile and a single BTS.

Types of traffic channel:

Full rate (TCH) Transmits full rate speech (13 Kbits/s). A full rate TCH occupies one physical

channel.

Half rate (TCH/2)

Transmits half rate speech (6.5 Kbits/s).

Two half rate TCHscan share one physical channel, thus doubling the capacity

of a cell.

Control Channels


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These are used to carry signaling or synchronization data.

They are divided into three types:

Broadcast CHannels (BCH)

Common Control CHannels (CCCH)

Dedicated Control CHannels (DCCHs)

1.Broadcast Channels


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From Single BTS to all the mobiles in the area

Frequency Correction Control CHannel (FCCH) Carries information for frequency correction of the mobile

Synchronization CHannel (SCH) Carries 2 important pieces of information

TDMA frame number (max = 2715684 )

Base station identity Code (BSIC) Broadcast Control CHannel (BCCH)

Broadcasts some general cell information such as:

1. Location Area Identity (LAI),2. maximum output power allowed in the cell

3. and the identity of BCCH carriers for neighboring cells.

2.Common Control Channels


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To or from a certain BTS to a single mobile Paging CHannel (PCH)

BTS Transmits a paging message to indicate an incoming call or short

message. The paging message contains the identity number of the mobilesubscriber that the network wishes to contact.

Random Access CHannel (RACH)

MS Answers paging message on the RACH by requesting a signaling channelof SDCCH.

Access Grant CHannel (AGCH)

Assigns a signaling channel (SDCCH) to the MS.

3.Dedicated Control Channels


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Stand alone Dedicated Control Channel (SDCCH)

The BTS switches to the assigned SDCCH. The call set-up procedure is

performed in idle mode. The BSC assigns a TCH (carrier and time slot)

and the MS switches to the assigned TCH.

SDCCH is also used to Registration & Authentication

3.Dedicated Control Channels


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Slow Associated Control Channel (SACCH) Down Link: BTS Instructs the MS the transmitting power to use and gives

instructions on timing advance (TA).

Up Link: MS Sends averaged measurements on its own BTS (signalstrength and quality) and neighboring BTSs (signal strength). The MS

continues to use SACCH for this purpose during a call.

Fast Associated Control Channel (FACCH) Transmits handover order

Transmits necessary handover information

Normal Burst Structure


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Tail

Bits Encrypted Bits

Training

Sequence Encrypted Bits

Tail

Bits

Guard

Period

3 57 1 26 1 57 3 8.25

0 1 2 3 4 5 6 7

TDMA Frame

156.25 bits in 577 u Sec

The tail bits help the equalizer to determine the start and stop points

of the transmitted bits. They are three bits at the beginning and at

the end of the burst and they are always zeros



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Tail

Bits Encrypted Bits

Training


Tail

Bits

Guard

Period

3 57 1 26 1 57 3 8.25

0 1 2 3 4 5 6 7

TDMA Frame


The subscriber speech or data is encrypted into 57 bit blocks. Each

burst will contain two 57 bits blocks from two different speech

segments.



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Tail

Bits Encrypted Bits

Training


Tail

Bits

Guard

Period

3 57 1 26 1 57 3 8.25

0 1 2 3 4 5 6 7

TDMA Frame


One bit stealing flag will be added to each block to indicate whether

the burst is stolen for the FACCH signaling or used as a normal traffic

channel



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Tail

Bits Encrypted Bits

Training


Tail

Bits

Guard

Period

3 57 1 26 1 57 3 8.25

0 1 2 3 4 5 6 7

TDMA Frame


The guard period of 8.25 bits length, which is equivalent to about

30 s, is left at the end of each burst, to prevent overlapping

between consecutive bursts and to facilitate burst synchronization



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It carries information of all logical channels except RACH, SCH

and FCCH

Tail

Bits Encrypted Bits

Training


Tail

Bits

Guard

Period

3 57 1 26 1 57 3 8.25

0 1 2 3 4 5 6 7

TDMA Frame


Frequency Correction Burst Structure


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Tail

Bits Fixed Bits

Tail

Bits

Guard

Period

3 142 3 8.25

This is the one used by the channel (FCH) for frequency correctionof the mobile. It consists of a long sequence of bits called the fixed

bits which are all equal to zeros, leading to a constant frequency

output from the GMSK modulator

0 1 2 3 4 5 6 7

TDMA Frame


Synchronization Burst Structure


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0 1 2 3 4 5 6 7

TDMA Frame


Tail

Bits Encrypted Bits Synchronization Sequence Encrypted Bits

Tail

Bits

Guard

Period

3 39 64 39 3 8.25

The SCH burst consists of a long synchronization sequence along with the

important information being encrypted and divided into two blocks. The

TDMA frame number is sent on the SCH channel, which carries also the

Base station Identity code (BSIC). The TDMA frame number is used by the

mobile to determine which control channels will be transmitted on that frame.


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Dummy Burst Structure


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0 1 2 3 4 5 6 7

TDMA Frame


Tail

Bits Encrypted Bits Training


Tail

Bits

Guard

Period

3 58 26 58 3 8.25

The dummy burst is sent from the BTS when there is nothing elseto be sent. It carries no information and it has the same structure of

a normal burst with the encrypted bits replaced by a known bit

pattern to the mobile

Logical Channels onto timeslot 0 (Downlink)


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CBBBBSF

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 38 40 42 4436 46 48 50

F S F S F S F S F S I

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 39 41 4337 45 47 4935 51

B C C C C C C C C C

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 63 42 7

Frame 1 Frame 2 Frame 3 Frame 4 Frame 5 Frame 6 Frame 7

Mapping of the 51 burst consumes 51 TDMA frame

51 TDMA Frame = 1 Multi-frame

Logical Channels onto timeslot 0 (Uplink)


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RRRRRRR

R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R RR R R R R R R RR R

Time slot 0 in the uplink direction is reserved for the RACH channel which

is used by the mobiles to make random access request to the system

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 63 42 7


Logical Channels onto timeslot 1 (Downlink)


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D1D1D1D0D0D0D0

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 38 40 42 4436 46 48 50

I I I

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 39 41 4337 45 47 4935 51

D0 D1 D2 D3 D4 D5 D6 D7 A0 A1 A2 A3

I I I

52 102

D0 D1 D2 D3 D4 D5 D6 D7 A4 A5 A6 A7

Frame 1 Frame 2 Frame 3 Frame 4 Frame 5 Frame 6 Frame 70 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 63 42 7


Logical Channels onto timeslot 1 (Uplink)


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A6A6A6A5A5A5A5

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 38 40 42 4436 46 48 50

I I I

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 39 41 4337 45 47 4935 51

A5 A6 A7 D0 D1 D2 D3 D4 D5 D6 D7 A0

I I I

52 102

A1 A2 A3 D0 D1 D2 D3 D4 D5 D6 D7 A4


0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 63 42 7

Logical Channels onto timeslots 2 / 7


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TTTTTTT

2 4 6 8 10 12 14 16 18 20 22 24 261 3 5 7 9 11 13 15 17 19 21 23 25

A (SACCH)

This contains the control signaling, an example of this is in order to change

output power.

26 TDMA Frames

=

1 Traffic Multi-frame

Frame 1 Frame 2 Frame 3 Frame 4 Frame 5 Frame 6 Frame 70 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 63 42 7

T T T T T T T T T T T T A T T T T T T T T T T T T I

Timeslots and TDMA Frames


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Higher Capacity Cell

Broadcast TrafficDedicated

Traffic

00 1 2 3 4 5 6 7

00 1 2 3 4 5 6 7

Low Capacity Cell

Combined Traffic

00 1 2 3 4 5 6 7

Structure of Combined BCCH

1 multi-frame (51TDMA Frames) 235.38 ms Downlink


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Group Group1 Group2 Group3 Group4 Grpup5

Channel F S B4 C4 F S C4 C4 F S D04 D14 F S D24 D34 F S A04 A14 I

Channel F S B4 C4 F S C4 C4 F S D04 D14 F S D24 D34 F S A24 A34 I

Frame

Number0 1 2-5 6-9 10 11 12-15

16-

1920 21 22-25 26-29 30 31 32-35 36-39 40 41

42-

4546-49 50

47-50464541-4437-4014-3610-136-9540-3Frame Number

D2

4RRD14D04RRA14A04RRD04Channel

D2

4RRD14D04RRA34A24RRD34Channel

1 multi-frame (51TDMA Frames) 235.38ms Uplink

F:FCCH; S:SCH; B:BCCH; C:CCCH; I:IDLE; R:RACH

3.GSM Air Interfacegvvvvvvvvvvvvvvv

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