gbo_002_e1_1 gsm radio interface technology-31

7/27/2019 GBO_002_E1_1 GSM Radio Interface Technology-31

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GSM Radio Interface Technology

ZTE University


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Objectives

At the end of this course, you will be able to: Understand GSM speech transmission process

Understand GSM radio key technology

Grasp the function of various logical channels

Describe the channel mapping pattern


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Content

Voice transmission process

Radio key technology

Radio channel


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Voice Processing in the GSM System

A/D

D/A

Voice

codingChannel

coding

Interle

avingEncryption

Burst pulse

formingModulation

Voice

decoding

Channel

decodingDeinterleaving Decryption

Burst pulse

disassembleDemodulation

260bit/20ms 456bit/20ms 33.8kbit/s 270kbit/s


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Voice encoding

RPE-LTP (Rule Pulse Excitation-Long TermPrediction )

260bit/20ms=13Kbit/s voice block

Parameter encoding


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Channel coding

Channel coding serves to improve transmissionquality and overcome the negative impact of

interferences on signals.

Commonly used channel coding methods are:

Convolution coding

Fire coding

Parity check coding.


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Channel coding

50 bit (Ia) 132 bit

(Ib)

78 bit (Ic)

260 bits /20ms

50 132 783 4

Fire

Code

Protection

bits

136 bit53 bit

189

bit

CONVOLUTIONAL

CODER (1:2)

378

bit

78 bit

456

bit


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Channel Interleving

Target: Change continuous bit errors into

discontinuous bit errors which can be corrected by

channel coding

3 41 2 3 41 2 3 41 2 3 41 2

3 41 3 41 2 3 41 2 3 41 2

3 41 2 3 41 2 3 41 2 3 41 2

2

Message block

Interleving

After

interleving

Error bits


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Two stage interleving

456 bit

0 1 2 3 4 5 6 7

8 9 10 11 12 13 14 15

1 2 3 4 5 6 7 8

456 bit

0 1 2 3 4 5 6 7

8 9 10 11 12 13 14 15

1 2 3 4 5 6 7 8

456 bit

0 1 2 3 4 5 6 7

8 9 10 11 12 13 14 15

1 2 3 4 5 6 7 8

456 bit

0 1 2 3 4 5 6 7

8 9 10 11 12 13 14 15

1 2 3 4 5 6 7 8

57 1 57 1 57 1 57 1 57 1 57 1 57 1 57 1

Block A Block B Block A+1 Block B+1

116-bit block 116-bit block 116-bit block 116-bit block

Odd N+4 Odd N+5Odd N+6

Odd N+7Even N Even N+1 Even N+2 Even N+3


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Encryption/Decryption

Encryption is accomplished by exclusive oroperation of an encryption sequence (computed

by A5 encryption algorithm via key Kc and frame

number) and 114 information bits on a normal

burst.


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Modulation and Demodulation

GMSK is a special digital FM modulation mode The modulation rate is 270.833 kilobauds.

The Frequency Shift Keying (FSK) modulation

with bit rate four times of frequency offset (67.708

kHz )is called MSK (Minimum Shift-frequency

Keying).

Gaussian demodulation filter is used to further

reduce the modulation spectrum.


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The GMSK can be expressed by a I/Q diagram


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Content

Voice transmission process

Radio key technology

Radio channel


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Discontinuous TransmissionDTX

Two aims can be achieved by adopting DTX mode. Lower the total interference level in the air

Save transmitter power.

The DTX mode and the normal mode are optional,

since the former will slightly lower the transmissionquality.


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Frequency Hopping (FH)

Type Baseband

Synthesized

Function Frequency diversity

Interference

diversity

Parameters HSN

MAIO


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Diversity receiving

The diversity reception technology is usually introduced tothe GSM system to receive on several tributaries the

signals with little relativity but carrying the same

information and then output the signals after they are

combined.

In this way, the impact of fading on the stability of receiving

signals can be decreased.

Types:

Space Diversity

Frequency Diversity

Time Diversity

Polarization Diversity


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Dynamic power control

Target: On the basis of certain service quality Lower the total interference level in the air

Save transmitter power

Power control command sent by SACCH

MS: Dedicated mode

Static 6 level, Dynamic 15 level

Step: 2dBm

A B


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Timing AdvanceTA

In the GSM, because TDMA is adopted in the airinterface, the MS must employ the TSs allocated

to it only, and remain inactive in other TSs.

Otherwise, it may affect the MSs using other TSs

on the same carrier.

It does not work,

Let MS send me

By one TA in advance

Time delayTA


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Timing AdvanceTA

BTS calculate TA Send by SACCH to MS

MS Dedicated mode

Range: 0-63Bits Distance: 0-35Km


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Content

Voice transmission process Radio key technology

Radio channel


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Frame1

1 2 3 4 5 760RF

Frame0

1 2 3 4 5 760

Frame N

1 2 3 4 5 760

Channel 0

Channel 1

Channel 7

Time Division Multiple Access (TDMA)


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GSM Frame structure


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Physical and logical channel

Physical Channels

the path used to carryinformation between an MS and a BTS

Logical Channels different information carried on

the physical channels


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Logical channels

Broadcast

Channels

LogicalChannels

ControlChannels

Common

ControlChannels

TrafficChannels

Dedicated

ControlChannels

FCCH

SCH

BCCH

RACH

AGCH

PCH

SDCCH

SACCH

FACCH


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FCCH

SCH TDMA#

BCCH

Hey. Im aGSM xmitter!

GSM?

GSM!!!

MCC...MNC...Cl

Okok

Broadcast Channel (BCH)


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PCH downlink only

Hello! You have a call.

RACH uplink only

Hello! I have to set

up a call.

I need SDCCH.

AGCH downlink only

Ok. Use SDCCH.

Common Control Channel (CCCH)


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SDCCH

SACCH/TCH

MS power

FACCH/TCHhandover

On SDCCH:

-call set up signaling

-SMS

-etc..

On SACCH

-SMS

-MS power

Dont shoutat me.

Dedicated Control Channel (DCCH)


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Channel Mapping

Item Configuration Combination

1 TCHFull TCH/F + FACCH/F + SACCH/TF

2 TCHHalf TCH/H)+FACCH/H+ SACCH/TH

3 TCHHalf2 TCH/H + FACCH/H+ SACCH/TH+ TCH/H

4 MainBCCH FCCH + SCH + BCCH + CCCH

5BCCHCombined

FCCH+SCH+BCCH+CCCH+SDCCH/4+

SACCH/C4

6 BCH BCCH + CCCH

7 SDCCH SDCCH/8+ SACCH/C8

8 BCCHwithCBCHFCCH+SCH+BCCH+CCCH+ SDCCH/4+

SACCH/C4 + CBCH

9 SDCCHwithCBCH SDCCH/8+SACCH/C8+CBCH


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Structure of 26-Frame Traffic Channel


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Structure of 51-Frame Control Channel

SF B C

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

51

SF C C SF C C SF C C I

R R R R R R R R R R

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

SF C C

R R R R R R R R R R

III

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

A1 A2 A3 III

A5 A6 A7 III

D0 D1 D2 D3 D4 D5 D6 D7 A0

D0 D1 D2 D3 D4 D5 D6 D7 A4

SF B C SF C C SF D0 D1 SF D2 D3 ISF A0 A1

SF B C SF C C SF D0 D1 SF D2 D3 ISF A2 A3

D3

D3

R R

R R

A2 A3

A0 A1

D2

D2

SF

SF

D0 D1

D0 D1

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 R R R R R R

FFCCH SSCHBBCCH CCCCHCCCH=PCH+AGCH+RACHRRACH DSDCCH

ASACCH/C Iidle

BCCH+CCCH

BCCH+CCCH

8 SDCCH/8

8 SDCCH/8

BCCH+CCCH+4SDCCH/4

BCCH+CCCH+4SDCCH/4

(a) FCCH+SCH+BCCH+CCCH

(b) SDCCH/8(0,...,7)+SACCH/C8(0,...,7)

(c) FCCH+SCH+CCCH+SDCCH/4(0,...,3)+SACCH/C4(0,...,3)

R R

R R


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