2
Microwaves UCL
Outline – GSM System
History
Configuration of the GSM network
GSM signals
Performances of the GSM system
...
3
Microwaves UCL
History
First European cellular radio system installed in
Scandinavia in 1981
Other systems installed but...
4
Microwaves UCL
History
New system installed in 1992
GSM=Global System for Mobile communications
A new study group starte...
5
Microwaves UCL
Configuration of the GSM network
Frequency allocation:
890 – 915 MHz mobile to base station
935 – 960 MHz...
6
Microwaves UCL
Configuration of the GSM network
Structure of the GSM network
MSC
BTS
BTS
BSC
BTS=base station transmitte...
7
Microwaves UCL
Configuration of the GSM network
Ground coverage performed by the base station (BTS) supervised by
the co...
8
Microwaves UCL
Configuration of the GSM network
Mobility management:
– Dynamic data bases
– Each user is registered in t...
9
Microwaves UCL
GSM signals
Total band available 25 MHz
FDMA 124 channels of 200 kHz
TDMA each 200 kHz channel sends an i...
10
Microwaves UCL
GSM signals
A “Traffic channel” contains 26 frames (120 ms=26x4.615ms), 24 for
the voice, 1 for control ...
11
Microwaves UCL
GSM signals
260 bits for 20 ms speech
50 bits 132 bits 78 bits
50 3 parity 132 4 initialisation of the d...
12
Microwaves UCL
GSM signals
26 training symbols for the evaluation of the channel response
(channel response, phase vari...
13
Microwaves UCL
GSM signals
Problem of propagation delay
– Max radius of the cell: 35 km, this makes a max delay of 233....
14
Microwaves UCL
Performances of the GSM system
Degradation of the signal to noise ratio (SNR) due to noise
and interfere...
15
Microwaves UCL
Performances of the GSM system
Co-channel interferences
Frequency reuse increases spectral efficiency bu...
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Microwaves UCL
Performances of the GSM system
1
2
3
4
1
1
1
2
2
3
3
4
4
1
1
1
1
1
1
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4 cells reuse pattern
7 cells re...
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Microwaves UCL
Performances of the GSM system
1
3
2
4
7
5
8
9
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12 cells reuse pattern 19 cells reuse pattern
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Microwaves UCL
Performances of the GSM system
The minimum distance depends
on
– The number of cells at the
same frequen...
19
Microwaves UCL
Performances of the GSM system
7.55 R19
6 R12
4.6 R7
3.46 R4
DK
When K increases, D increases also and t...
20
Microwaves UCL
Performances of the GSM system
21
Microwaves UCL
Performances of the GSM system
∑+
=
+ 6
100 'iIN
C
IN
C
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Microwaves UCL
Performances of the GSM system
Co-channel interference is function of the parameter
a=D/R
D can be calcu...
23
Microwaves UCL
Performances of the GSM system
24
Microwaves UCL
Performances of the GSM system
Adjacent channel interference
– Due to the adjacent channel
– Due to anot...
25
Microwaves UCL
Performances of the GSM system
Near-far effect
– 2 mobiles send their signal to the base station, one be...
26
Microwaves UCL
Performances of the GSM system
Example
2 mobiles
d1=16 km
d2=0.8 km
Attenuation of 40dB/dec: 40 log
d2/d...
27
Microwaves UCL
Performances of the GSM system
How to reduce interferences?
– Good frequency repartition scheme (! Non l...
28
Microwaves UCL
Design of the base station
Parameters available:
– Position of the antennas
– Height of the antennas
– T...
29
Microwaves UCL
Design of the base station
Choice of the power level received at the limit of the cells (receiver
and pe...
30
Microwaves UCL
Design of the base station
31
Microwaves UCL
Design of the base station
32
Microwaves UCL
Design of the base station
33
Microwaves UCL
Design of the base station
Space diversity at the base station for increasing the
performances:
Boresigh...
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Gsm system

  1. 1. 2 Microwaves UCL Outline – GSM System History Configuration of the GSM network GSM signals Performances of the GSM system Design of the base station
  2. 2. 3 Microwaves UCL History First European cellular radio system installed in Scandinavia in 1981 Other systems installed but incompatibility between the standards and impossibility to use the same equipment across the borders (Analogue FM, SCPC/FDMA, …) CEPT (Conférence Européenne des Postes et Télécommunications) installs the GSM (Groupe Spécial Mobile) in 1982 in view of specifying a global European system at 900 MHz. Trade off has to be achieved between spectral efficiency, voice quality, cost of the receiver, portability, cost of the base station, etc.
  3. 3. 4 Microwaves UCL History New system installed in 1992 GSM=Global System for Mobile communications A new study group started to work on the next system: UMTS (Universal Mobile Telecommunications System) GSM used in Europe, South Africa, most Asian countries, Australia
  4. 4. 5 Microwaves UCL Configuration of the GSM network Frequency allocation: 890 – 915 MHz mobile to base station 935 – 960 MHz base station to mobile FDD system (frequency division duplex) Choice of digital signals – Integration of voice, data and signalling – Spectral efficiency – High quality – Low cost terminals
  5. 5. 6 Microwaves UCL Configuration of the GSM network Structure of the GSM network MSC BTS BTS BSC BTS=base station transmitters BSC= base station controller MSC=mobile switching center Switching center To the network HLR VLR HLR=home location register VLR=visitor location register cell
  6. 6. 7 Microwaves UCL Configuration of the GSM network Ground coverage performed by the base station (BTS) supervised by the controllers (BSC) which ensure a good quality of the link The mobile has a Subscriber Identity Module (SIM card) which is an international mobile subscriber identity + key BSC is responsible for power control and hand-over, channel allocation, signalling transmission and messages MSC is responsible for the traffic control – Start and end of the calls – Routing – Cost of the calls – Statistics – Hand-over procedures intra-cells and inter-cells – Connexion to the cable network – Management of the mobility and authentication
  7. 7. 8 Microwaves UCL Configuration of the GSM network Mobility management: – Dynamic data bases – Each user is registered in the HLR of his network, which knows the identity of the VLR regularly visited, to speed up routing
  8. 8. 9 Microwaves UCL GSM signals Total band available 25 MHz FDMA 124 channels of 200 kHz TDMA each 200 kHz channel sends an impulse of 577 µs in a frame of 4.615 ms (8 slots of 577 µs) In a BTS: different channel transmitters, in non adjacent bands (combined FDMA/TDMA) GSM channel = 1 burst in a TDMA frame, in a channel Channel 1 (200 kHz) Channel 124 (200 kHz) 890 MHz 915 935 960uplink downlink FDD 45 MHz 25 MHz
  9. 9. 10 Microwaves UCL GSM signals A “Traffic channel” contains 26 frames (120 ms=26x4.615ms), 24 for the voice, 1 for control and 1 unused Multiplexing: – 124 channels (FDMA) – 8 slots (TDMA) – Total number of channels: 992 Sampling and modulation – Quantization: 13 bits (8192 levels) – Sampling frequency: 8 kHz This makes 104 kbits/sec – Coding + compression (Regular Pulse Excited Linear Predictive Speech Codec RPE-LPC) reduces to 13 kbits/sec – Divided in binary blocs of 260 bits (20 ms speech) – Adding protection and Error Correcting Codes (bloc codes + CC (2,1,5)) makes 271 kbits/sec – This makes 156,25 bits in 0.577 ms (burst) – 1 bit=3.69 µs
  10. 10. 11 Microwaves UCL GSM signals 260 bits for 20 ms speech 50 bits 132 bits 78 bits 50 3 parity 132 4 initialisation of the decoder 189 bits 378 bits Convolutional code r=1/2, k=5 Total: 456 bits/20 ms 78 bits 8 blocs of 57 bits 57 57 57 57 57 57 h1 h2 Control messages Interburst interleaving
  11. 11. 12 Microwaves UCL GSM signals 26 training symbols for the evaluation of the channel response (channel response, phase variation from Doppler, delay correction for synchronisation) GMSK modulation Max delay of 85 ms in processing (not detectable by the user wrt 240 ms for geostationary satellite communication) Delay equaliser up to 16 µs Slow frequency hopping 217/s (frequency diversity) Encryption 58 (encrypt.)3 58 (encrypt.) 26 (training) 1 burst of 0.577 µs 156,25 bits 3 8,25
  12. 12. 13 Microwaves UCL GSM signals Problem of propagation delay – Max radius of the cell: 35 km, this makes a max delay of 233.3 µs for 70 km – No overlap between signals arriving at the base station, so we need a guard period of 252 µs: 68,25 bits in the burst for the first access!!! – First synchronisation sequence sent=41 bits + long guard period. The base station calculates the propagation delay and a timing advance is sent to the mobile (64 bits, precision of 3.69 µs). – The guard period is reduced to about 30 µs, which makes 8.25 bits
  13. 13. 14 Microwaves UCL Performances of the GSM system Degradation of the signal to noise ratio (SNR) due to noise and interferences Noise: AWGN Additive White Gaussian Noise Interferences: – Co-channel interferences – Adjacent channel interferences – Near-end far-end effect
  14. 14. 15 Microwaves UCL Performances of the GSM system Co-channel interferences Frequency reuse increases spectral efficiency but produces interferences. R RD 2 cells with the same frequency f1 at a distance D. Key parameter: a=D/R
  15. 15. 16 Microwaves UCL Performances of the GSM system 1 2 3 4 1 1 1 2 2 3 3 4 4 1 1 1 1 1 1 17 4 cells reuse pattern 7 cells reuse pattern
  16. 16. 17 Microwaves UCL Performances of the GSM system 1 3 2 4 7 5 8 9 10 11 12 12 cells reuse pattern 19 cells reuse pattern
  17. 17. 18 Microwaves UCL Performances of the GSM system The minimum distance depends on – The number of cells at the same frequency f1, surrounding the cell – The shape of the cell (relief) – The height of the emitting antenna For ideal hexagonal cells RKD 3= K= number of different frequencies used by the base station D=reuse distance (2 cells with the same frequency)
  18. 18. 19 Microwaves UCL Performances of the GSM system 7.55 R19 6 R12 4.6 R7 3.46 R4 DK When K increases, D increases also and the interferences are reduced. The total number of channels being fixed, if K increases, there are less channels for one carrier and the efficiency decreases.
  19. 19. 20 Microwaves UCL Performances of the GSM system
  20. 20. 21 Microwaves UCL Performances of the GSM system ∑+ = + 6 100 'iIN C IN C
  21. 21. 22 Microwaves UCL Performances of the GSM system Co-channel interference is function of the parameter a=D/R D can be calculated for a given frequency reuse scheme and a given C/N, as C/I is often of the order of magnitude of 18 dB (USA). ∑ = − − iK kD R I C 1 γ γ Γis the slope coefficient of the propagation model Ki is the number of interfering cells at the 1st tier
  22. 22. 23 Microwaves UCL Performances of the GSM system
  23. 23. 24 Microwaves UCL Performances of the GSM system Adjacent channel interference – Due to the adjacent channel – Due to another channel farther – Due to another system (another country) – Caused by another cell or control signals – May be caused by non-linearities in the system if all channels are used.
  24. 24. 25 Microwaves UCL Performances of the GSM system Near-far effect – 2 mobiles send their signal to the base station, one being very close to the base station, the second one being very far (limit of the cell). Interference at the base station is highly probable – Solutions: » Separation of the channels (depends on the slope of the input filter to separate the signals) » Power reduction from the base station
  25. 25. 26 Microwaves UCL Performances of the GSM system Example 2 mobiles d1=16 km d2=0.8 km Attenuation of 40dB/dec: 40 log d2/d1= 52 dB Farthest mobile is 52 dB lower than the closest Assume an input filter of 12dB/octave or 40 dB/decade 2010;10;52log40 log40 3.1 1 24052 1 2 1 2 1 2 ==== f f f f f f f f 2010 3.1 1 2 == f f
  26. 26. 27 Microwaves UCL Performances of the GSM system How to reduce interferences? – Good frequency repartition scheme (! Non linearities) – Choice of the channel given to the mobile vs its quality – Choice of the radiating pattern of the antennas – Choice of the height of the antenna – Power levelling How to increase traffic capacity of the system – Reduce the size of the cells – Increase the number of channels in each cell – Dynamic assignment of the channels – Hand over
  27. 27. 28 Microwaves UCL Design of the base station Parameters available: – Position of the antennas – Height of the antennas – Type of antennas + diversity Position of the antennas – Irregular illumination zone due to terrain irregularities – Avoid interferences (take into account other emitters) It is important to optimise the whole system
  28. 28. 29 Microwaves UCL Design of the base station Choice of the power level received at the limit of the cells (receiver and performances expected), then first choice of the following parameters: – Type of the zone (γ attenuation coefficient) – Power emitted by the base station – Height of the antenna – Antenna gain and radiating pattern – Size of the cell Choice of the position of the antenna Evaluation of the power received on the ground with a software (see channel modelling) Check the global coverage and the possible interferences
  29. 29. 30 Microwaves UCL Design of the base station
  30. 30. 31 Microwaves UCL Design of the base station
  31. 31. 32 Microwaves UCL Design of the base station
  32. 32. 33 Microwaves UCL Design of the base station Space diversity at the base station for increasing the performances: Boresight In line Better performances for boresight situation, so better 3 antennas:

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