Roll Bscsf08 m 37 Roll No 27

8/3/2019 Roll Bscsf08 m 37 Roll No 27

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Roll No.

BCSF08-M27

BCSF08-M37

Name.

Asad Abbas

Hussam Ali


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Coding/

Digitizing

channel

coding

Bit inter-

leaving

CIPHERING/

ENCRYPTIO

N

Burst

Formation

MODULATION&

TRANSMISSION

Transmitter Receiver

Speech/

analogue

CHANNEL

DE CODING

Bit inter-

leaving

DECIPHERING/

DECRYPTION

BURSTDeformation

Equalization

Delay

DEMODULATION

Radio

waves

De Coding


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Transforms the analog signal (voice) into a digital representation

GSM speech coder is based on RELP (Residual-excited linearprediction)

And enhanced with GSM RPE-LTP (Regular Pulse Excited -Long Term Prediction) with Linear Predictive Coding to turn ouranalog voice into a compressed digital equivalent.

Takes advantage of the fact that in normal conversation a personspeaks on average for less than 40% of time

By incorporating a VAD (voice activity detector ) in the speechcoder the GSM system will operate in a discontinuoustransmission mode (DTX) which provide a battery life to andreduce instantaneous radio interface GSM transmitter is not activeduring silent periods


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In order to reduce the amount of data needed to represent

the sound wave, the analog signal is first inputted into

a band pass filter. Band pass means that the filter only

allows signal that fall within a certain frequency range to

pass through it, and all other signals are cut off,

orattenuated. The BP filter only allows frequencies

between 300Hz and 3.4 kHz to pass through it. This limits

the amount of data that the Analog/Digital Converter is

required to process.


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Speech Coding

The LPC/RPE sequence is then fed into the Long-Term Prediction

(LTP) Analysis function. The LTP function compares the sequence it

receives with earlier sequences stored in its memory and selects

the sequence that most resembles the current sequence. The LTP

function then calculates the difference between the two sequences.

Now the LTP function only has to translate the difference value as

well as a pointer indicating which earlier sequence it used for

comparison. By doing this is prevents encoding redundant data.


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Coding/

Digitizing

channel

coding

Bit inter-

leaving

CIPHERING/

ENCRYPTIO

N

Burst

Formation

MODULATION&

TRANSMISSION


Speech/

analogue

CHANNEL

CODING

Bit inter-

leaving

DECIPHERING/

DECRYPTION

BURSTDeformation

Equalization

Delay

DEMODULATION

Radio

waves

De Coding


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As we all know, the audio signal must betransmitted across a radio link from thehandset to the Base Station Transceiver(BTS). The signal on this radio link is subject toatmospherics and fading which results in alarge amount of data loss and degrades theaudio.

In order to prevent degradation of audio, thedata stream is put through a series of error

detection and error correction procedurescalled channelcoding. The first phase ofchannel coding is called blockcoding.


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single 260-bit (20ms) audio block is delivered to the block-coder. The 260 bits are divided up into classes according totheir importance in reconstructing the audio. Class I are thebits that are most important in reconstructing the audio. Theclass II bits are the less important bits. Class I bits arefurther divided into two categories, Ia and Ib.


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Class Ia - 50 bits (most sensitive to biterrors)Class Ib - 132 bits (moderately sensitiveto bit errors)Class II - 78 bits (least sensitive to error)


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This 267-bit block is then inputted into a convolutional code.Convolutional coding allows errors to be detected and to be correctedto a limited degree. The class I "protected" bits are inputted into acomplex convolutional code that outputs 2 bits for every bit that entersit. The second bit that is produced is known as a redundancy bit. Thenumber of class I bits is doubled from 189 to 378.

This coding uses 5 consecutive bits to calculate the redundancy bit,this is why there are 4 bits added to the class I bits when the cycliccode was calculated. When the last data bit enters the register, it usesthe remaining four bits to calculate the redundancy bit for the last databit. The class II bits are not run through the convolutional code. Afterconvolutional coding, the audio block is 456 bits


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Coding/

Digitizing

channel

coding

Bit inter-

leaving

CIPHERING/

ENCRYPTIO

N

Burst

Formation

MODULATION&

TRANSMISSION


Speech/

analogue

CHANNEL

CODING

Bit inter-

leaving

DECIPHERING/

DECRYPTION

BURSTDeformation

Equalization

Delay

DEMODULATION

Radio

waves

De Coding


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Now, one problem remains. All of this error detection and error correction coding will notdo any good if the entire 456-bit block is lost or garbled. In order to alleviate this, the bitsare reordered and partioned onto eight separate sub-blocks. If one sub-block is lost thenonly one-eighth of the data for each audio block is lost and those bits can be recovered

using the convolutional code on the receiving end. This is known as interleaving.Each 456-bit block is reordered and partitioned into 8 sub-blocks of 57 bits each.These eight 57-bit sub-blocks are then interleaved onto 8 separate bursts. As youremember from the TDMA Tutorial, each burst is composed of two 57-bit data blocks, for atotal data payload of 114 bits.

The first four sub-blocks (0 through 3) are mapped onto the even bits of four consecutivebursts. The last four sub-blocks (4 through 7) are mapped onto the odd bits of the next 4consecutive bursts. So, the entire block is spread out across 8 separate bursts.

Taking a look at the diagram below we see three 456-bit blocks, labeled A, B, and C. Eachblock is sub-divided into eight sub-blocks numbered 0-7. Let's take a look at Block B. Wecan see that each sub-block is mapped to a burst on a single time-slot. Block B is mappedonto 8 separate bursts or time-slots. For illustrative purposes, the time-slots are labeled Sthrough Z.


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In the following diagram, we examine time-slot W. We see that bits from B4 are mapped onto the odd-number bits (bits1,3,5....109,111,113) and we would see bits from C1 mapped onto the even number bits (bits 0,2,4....108,110,112). This processcontinues indefinitely as data is transmitted. Time-slots W, X, Y, and Z would all be mapped identically. The next time-slot wouldhave data from Block C and Block D mapped onto it. This process continues for as long as there is data being generated.


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the process of interleaving effectively distributes a single 456 bit audio block over 8 separate bursts. Ifone burst is lost, only 1/8 of the data is lost, and the missing bits can be recovered using theconvolutional code.

Now, you might notice that the data it takes to represent a 20ms (456-bits) audio block is spread outacross 8 timeslots. If you remember that each TDMA frame is approximately 4.615ms, we can determinethat it takes about 37ms to transmit one single 456-bit block. It seems like transmitting 20ms worth ofaudio over a period of 37ms would not work. However, this is not what is truly happening. If you look at aseries of blocks as they are mapped onto time-slots you will notice that one sub-block ends every fourtime-slots, which is approximately 18ms. The only effect this has is that the audio stream is effectivelydelayed by 20ms, which is truly negligible.


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Coding/

Digitizing

channel

coding

Bit inter-

leaving

CIPHERING/

ENCRYPTIO

N

Burst

Formation

MODULATION&

TRANSMISSION


Speech/

analogue

CHANNEL

CODING

Bit inter-

leaving

DECIPHERING/

DECRYPTION

BURSTDeformation

Equalization

Delay

DEMODULATION

Radio

waves

De Coding


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Ref http://seussbeta.tripod.com/crypt.html

Hardware Based Encryption

Systems:

Digital Voice Protection: Digital Voice Protection (DVP) is a

proprietary speech encryption technique used by Motorola for their

higher-end secure communications products

Voice encryptions work by digitizing the conversation at thetelephone and applying a cryptographic technique to the resultingbit-stream. In order to decrypt the speech, the correct encryptionmethod and key must be used.

Software Based Encryption Systems:

STU III (Secure Telephone Unit,Generation III) used by USA

STEs (Secure Terminal Equipment)

For high bandwidth apps

PGPFone

Nautilus

Speak Freely


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Encryption

It is used to protect signaling and data. An encryption key is computed using:

algorithm A8 (stored on the SIM card),

the subscriber key

a random number (nonce) delivered by the network (same as the one used forauthentication).

A 114-bit sequence is produced using: the encryption key,

algorithm A5

the burst numbers.This bit sequence is then XORed with the two 57 bit blocks of data included in anormal burst. To decrypt correctly, the receiver has to use the same algorithm A5 forthe deciphering procedure.


20/35

Coding/

Digitizing

channel

coding

Bit inter-

leaving

CIPHERING/

ENCRYPTIO

N

Burst

Formation

MODULATION&

TRANSMISSION


Speech/

analogue

CHANNEL

CODING

Bit inter-

leaving

DECIPHERING/

DECRYPTION

BURSTDeformation

Equalization

Delay

DEMODULATION

Radio

waves

De Coding


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The burst assembling step manages thegrouping the bits into bursts.


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In the diagram below, we can see how this works. The diagram shows 16bursts. Remember that a burst occurs on a single time-slot and the duration ofa time-slot is 577 s. Eight time-slots make up a TDMA frame, which is4.615ms. Since a single resource is only given one time-slot in which totransmit, we only get to transmit once every TDMA frame. Therefore, we onlyget to transmit one burst every 4.615ms.


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During each time-slot, a burst is transmitted that carries data from two different 456-bit blocks.In the diagram below, Burst 1 carries data from A and B, burst 5 has B and C, burst 9 has Cand D, etc. Looking at the diagram, we can see that it does take approximately 37ms for Block

B to transmit all of its data, (bursts 1-8). However, in bursts 5-8, data from block C is also beingtransmitted. Once block B has finished transmitting all of its data (burst 8), block C has alreadytransmitted half of its data and only requires 4 more bursts to complete its data.

Block A completes transmitting its data at the end of the fourth burst. Block B finishes in theeighth, block C, in the 12th, and block D in the 16th. Viewing it this way shows us that everyfourth burst completes the data for one block, which takes approximately 18ms.


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Coding/

Digitizing

channel

coding

Bit inter-

leaving

CIPHERING/

ENCRYPTIO

N

Burst

Formation

MODULATION&

TRANSMISSION


Speech/

analogue

CHANNEL

CODING

Bit inter-

leaving

DECIPHERING/

DECRYPTION

BURSTDeformation

Equalization

Delay

DEMODULATION

Radio

waves

De Coding


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The modulation chosen for the GSM system isthe Gaussian Minimum Shift Keying (GMSK).

The GMSK modulation has been chosen as acompromise between spectrum efficiency,complexity and low spurious radiations (thatreduce the possibilities of adjacent channelinterference). The GMSK modulation has a rate of270 5/6 kbauds and a BT product equal to 0.3.


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DTX is used to suspend the radio transmission during the silence periods. This

exploits the observation that only 40-50% during a conversation does the speakeractually talk. DTX helps also to reduce interference between different cells and toincrease system capacity. It prolongs battery charge life. The DTX function isperformed by means of:

Voice Activity Detection (VAD), which has to determine whether the soundrepresents speech or noise, even if the background noise is very important. If thevoice signal is considered as noise, the transmitter is turned offproducing then, an

unpleasant effect called clipping.

Comfort noise. A side-effect of the DTX function is that when the signal isconsidered as noise, the transmitter is turned off and therefore, a total silence isheard at the receiver. This can be very annoying to the receiving user since itappears as a dead connection. In order to overcome this problem, thereceivercreates a minimum of background noise called comfort noise. Comfortnoise eliminates the impression that the connection is dead.


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Coding/

Digitizing

channel

coding

Bit inter-

leaving

CIPHERING/

ENCRYPTIO

N

Burst

Formation

MODULATION&

TRANSMISSION


Speech/

analogue

CHANNEL

CODING

Bit inter-

leaving

DECIPHERING/

DECRYPTION

BURSTDeformation

Equalization

Delay

DEMODULATION

Radio

waves

De Coding


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The timing of the bursts transmissions is veryimportant. Mobiles are at different distances from the

BTS. Their delay depends, consequently, on their distance.

Timing advance allows signals coming from different

distances to arrive to the BTS at the right time. The latter

measures the timing delay of the MNs. If the burstscorresponding to an MN arrive too late and overlap with

other bursts, the BTS tells, the MN to advance the timing in

transmission of its bursts.


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The BTSs perform timing measurements; they also performmeasurements on the power level of the different mobile stations.

These power levels are adjusted so that the power is nearly the

same for each burst.

The BTS controls its power level. The MN measures the strength

and the quality of the signal between itself and the BTS. If themobile station does not receive correctly the signal, the BTS

changes its power level and retransmits.


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Coding/

Digitizing

channel

coding

Bit inter-

leavingCIPHERING/

ENCRYPTIO

N

Burst

FormationMODULATION

&TRANSMISSIO

N


Speech/

analogue

CHANNEL

CODING

Bit inter-

leaving

DECIPHERING/

DECRYPTION

BURSTDeformation

Equalization

Delay

DEMODULATION

Radio

waves

De Coding


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It is a method used to conserve the MN's power.

The paging channel is divided into sub

channels corresponding to single mobile stations.

Each MN 'listens' only to its sub channel while it

stays in sleep mode for the duration of the rest

sub channels of the paging channel.


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At the GSM frequency bands, radio waves reflect from buildings, cars,hills, etc. So not only the 'right' signal (the output signal of the emitter) isreceived by an antenna, but also many reflected signals, which corruptthe information, with different phases. An equalizer is in chargeofextracting the 'right' signal from the received signal. It estimatesthe channel impulse response of the GSM system and then constructsan inverse filter. The receiverknows which training sequence it mustwait for. By means of comparing the received training sequence withthe expected one, the receiver computes the coefficients of thechannel impulse response. In order to extract the 'right' signal, the

received signal is passed through the inverse filter.


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http://www.cs.ucl.ac.uk/staff/t.pagtzis/wireless/gsm/radio.ht

ml

http://www.gsmfordummies.com/encoding/encoding.

shtml

http://seussbeta.tripod.com/crypt.html

http://library2.usask.ca/theses/available/etd-12202003-

142739/unrestricted/0105Thesis.pdf

http://www.azizi.ca/gsm/coding/index.htmlWireless Communications Principles and Practice (2nd Edition) page 509

Documents

Roll Bscsf08 m 37 Roll No 27