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GSM - RADIO INTERFACE
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GSM - RADIO INTERFACE
IN THIS PRESENTATION Radio Interface
Frequency Bands & Specifications
Multiple Access Method FDMA & TDMA
FDMA /TDMA Frame Representation
Logical Channels : Traffic & Control
Operational Concepts
Other Salient Features of RF I/F- DTX, Time Alignment
Diversity, Fr. Hopping,
Power Control.
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Communication - Mobile
TelephoneExchange
SubscriberLine(2W)
Inter-Exchange
Junction
Mobile SwitchingCentre (MSC)
BSC BTS
MS
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GSMRADIO INTERFACE
Most Important Interface
To increase spectral efficiency
-- Large number of simultaneous calls in agiven bandwidth
-- Frequency Reuse
-- Interference
-- Use of Interference Reduction Techniques
Full Compatibility between mobile stations ofvarious Manufacturers & Networks of differentvendors to help roaming
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GSMUplink & Downlink
Frequency Bands
GSM 900 Mhz
DCS 1800 MHz
BTS
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GSM Specifications
GSM 900
Mobile to BS (UP-LINK) - 890 to 915 MHz
BS to Mobile (DOWN -LINK) - 935 to 960 MHz
Bandwidth - 25 MHz
GSM 1800 ( DCS ) :
Mobile to Cell (UP-LINK) - 1710 to 1785 MHz
Cell to Mobile (DOWN -LINK) - 1805 to 1880 MHz
Bandwidth - 75 MHz
RF Spectrum :
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GSM Specifications
Carrier Separation - 200 kHz
No. of RF Carriers - 124Access Method - TDMA/FDMA
Modulation Method - GMSK
Transmission Rate - 270.833 Kbps
Speech Coding - Full rate 13 KbpsHalf rate 6.5 Kbps
Duplex Distance - 45 MHz
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GSM - MULTIPLE ACCESS GSM uses both FDMA & TDMA
Freq
Mhz.890.2
1
890.4
2
890.6
3
890.8
4
891.0
5 6
914.8
124
FDMA Access along Frequency axis
Each RF carrier 200khz apart
Total 124 RF Channels available.One or more carrier assigned to each base station
...
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Absolute Radio Freq Carrier Number (ARFCN) 1 and 124 not useduntil it is co-ordinated with Non -GSM operators in adjacent freq. bands.
Thus for practical purposes only 122 RF Carriers are available.
F up-link (n) = 890.2 +0.2* ( n-1 ) MHz
F down-link (n) = 935.2 +0.2* ( n-1 ) MHz
Frequency for any ARFCN ( n) can be calculated from :
Here 124.
GSM - MULTIPLE ACCESS
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GSMFDMA
25 MHz 25 MHz
Mobile to Base
0 1 2
890.2 890.4 890.6(MHz)
Base to Mobile
0 1 2
935.2 935.4 935.6
200 kHz45MHzChannel layout and frequency bands of operation
890 935 960915
200 kHz
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GSM
TDMA
87
65
43
21
87
65
43
21
45 MHz
Frequency
F2F1(Cell transmit)
F2F1(Cell Rx)
Amplitude
Typical TDMA/ FDMA frame structure
GSM
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GSMDigital Voice Transmission
- In GSM speech coding a block of 20 ms is encoded in one set of260 bits.
- This calculates as 50X 260 = 13 kbps. Thus GSM speech coderproduces a bit rate of 13 kbps per subscriber.
- This provides speech quality which is acceptable for mobile
telephony and comparable with wire-line PSTN phones.
Speech Coding
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01100011000111110011100
Speech Code
20 ms
Speech Signal
Speech Coding
Parameters like tone,length of tone,pitch are transmittedSampling=50 times/sec instead of 8000 of 260 bits each
overall bitrate= 50x260x8 subs=104kbps
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Channel Coding
Block
coder50 Very important bits
132 Important bits
78 Not so important bits
1:2Convolutional
Coder
456
4 Tail bits
53 bits 378 bits
3 parity bits260 bits
Detection & correction of errors
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GSMDigital Voice Transmission
- It uses 260 bits from speech coding as input and outputs 456encoded bits.
- In one burst one block of 57 bits from one sample and anotherblock from another sample are sent together.
- These 456 bits for every 20 ms of speech are interleaved formingeight blocks of 57 bits each.
Channel Coding
Interleaving
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GSMDigital Voice Transmission
To counteract the problems encountered in radio path:
Burst Formatting
- Additional bits as training sequence added to basic speech/data.
- Total of 136 bits added, bringing overall total to 592 bits.
- Each TS of TDMA frame is 0.577 ms long and during this time156.25 bits are transmitted.
- One burst contains only 148 bits. Rest of the space, 8.25 bitstime, is empty and is called Guard Period ( GP ).
- GP enables MS/BTS to ramp up and ramp down.
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8.253571261573
1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
D D D D DDD D D D D D DDD D
456 bits
Sample 1
456 bits
Sample 2
Normal Burst
Stream of Time Slots
Interleaving & Burst Formatting
1st Sample of 20 ms speech 2nd Sample of 20 ms speech
GS
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GSMSpeech to Radio waves
Speech Coding
Channel Coding
Interleaving
Burst formatting
Ciphering
Burst formatting
Deciphering
Analog
Modulation
200kHz BW
Speech Decoding
Analog
200kHz BW
Demodulation
De-interleaving
Channel Decoding
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FDMA/TDMA Scheme
BP1
BP2
BP3
BP4
BP5
BP6
BP7
BP8
BP1
BP2
TIME
890.0890.2
890.4890.6
890.8891.0
891.2 915.8
FREQ
MHz
BURSTF
R
A
M
E
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CM
CC SMS SS
Normal Burst
3
T
57
Encrypted
1
S
26
Training
1
S
57
Encrypted
3
T
8.25
GP
CM
CC SMS SS3
T
142
Fixed Bits
3
T
8.25
GP
FCCH Burst
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Other Salient Features Of GSM RF INTERFACE:
GSM RF INTERFACE
- Control of Transmitted Power.
- Timing Advance.
-Discontinuous Transmission.
- Diversity.
- Frequency Hopping.
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POWER CONTROL
To minimize co-channel interference and toconserve power, both the Mobile and BTSoperate at the lowest power level that will
maintain an acceptable signal quality.
Mobile decides that power level is acceptableusing bit errors ratio.
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DISCONTINUOUS TRANSMISSION(DTX)
The idea is based on the fact that a personspeaks less than 40% of time in normalconversation, so turning the transmitter off cansave power.
In order to distinguish voice and backgroundnoise, very accurate Voice Activity Detectorshould be used.
While transmitter is off, the receiving end will heara total silence, i.e. due to digital transmission.
To avoid this, comfort noise is generated trying tomatch the characteristics of background noise.
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DISCONTINUOUS RECEPTION (DRX)
While being in Idle Mobile Station has to listenonly to Paging Channel, that uses almost no
power.
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DISCONTINUOUS TRANSMISSION(DTX)
Speech activity only 40% of time.
Needs Voice activity detection.
Determination of voice threshold vis--vis noise.
Annoying clicks/inefficient DTX.
Generation of Comfort Noise at receiver to avoidthe feeling of the set being dead.
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Timing Alignment :
GSM RF Interface
- Large distance between BTS and MS causes the problem.
- Each MS on call is allocated a timeslot on TDMA frame.
- The problem occurs when the information transmitted by MS does not
reach BTS on allocated timeslot.
0 1 2 3 4 5 6 7
BTS
TDMA Frame
A on TS3
B
on TS2TS3
TS2
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Timing Advance : ( To counteract problem of Time Alignment )
- MS instructed to do its transmission certain bit-times earlier or laterto reach its timeslot at BTS in right time.
- In GSM systems maximum 63 bit-times can be used.
- This limits the GSM cell size to 35 Km radius.
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
Time
StartSending
GSM RF Interface
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Antenna Diversity :
GSM RF Interface
- Mounting two receiver antenna physically separated a distance.
- At 900 MHz with antenna spacing of 5-6 m we get 3 db gain.
Space Diversity
- Probability of both of them being affected by a deep fading dip
at same time is low.
No Diversit Antenna Diversit
Tx Rx Rx (A) Rx ( B)Tx
Polarization Diversity -
- Dual polarized antenna vertical and horizontal arrays.
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FREQUENCY HOPPING
Change of frequency after every frame in apre-determined manner
SFH improves performance in multi-path fading
Decreases required C/I
Mandatory for MS when requested by BS
FCCH ,SCH ,BCCH are not hopped Algorithm : Cyclic or pseudorandom
Provides interference diversity
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SUMMARY
GSM - RADIO INTERFACE
Radio Interface Frequency Bands & Specifications
Multiple Access Method FDMA & TDMA
FDMA /TDMA Frame Representation
Logical Channels Traffic & Control
Operational Concepts
Other Salient Features of RF I/F- DTX, Time AlignmentDiversity, Fr. Hopping,
Power Control.