Network Planning

1. Introduction to GSM Network

GSM System Architecture

GSM Bandwidth

GSM 900 and GSM 1800 are twins 
                                        GSM 900                          GSM 1800
Frequency band             890...960 MHz                1710...1880 MHz
Number of channels       124                                  372
Channel spacing             200 kHz                          200 kHz
Access technique          TDMA                             TDMA
Mobile power              0,8 / 2 / 5 W                       0,25 / 1 W

There are no major differences between GSM 900 and GSM 1800
GSM 900 :

GSM 1800 :



Logical Channels

Same in GSM900 and GSM1800


Downlink Channels

       Common Channels

Dedicated Channels




Up-link Channels
       Common Channels





      Dedicated Channels

Use of Logical Channels

---> FCCH : Search for Frequency Correction Burst

---> SCH :Search for Synchronisation sequence
---> BCCH : Read System Informations
---> PCH: Listen for Paging

---> RACH: Send Access burst

---> AGCH: Wait for signalling channel allocation

---> SDCCH: Call setup

---> FACCH: Traffic channel is assigned

---> TCH: Conversation

---> FACCH: Call release

Mapping of Logical Channels
.Logical channels are mapped to physical channels
Signalling : sequences of 51 frames
Traffic : sequences of 26 frames

The Mobile Radio Link
     Radio Wave Propagation
  The theory of wave propagation is an exact science

Mobile Telecommunications
What is special about Mobile Communications ?
  -Multi-path propagation
radio path is a miserable propagation medium
  -Limited transmit energy
transmitting power of mobiles determines service range
battery life-time
  -Limited spectrum
sets upper limit for data rates  (Shannon´s theorem)
additional effort needed for channel coding
frequencies need to be re-used ==> self- interference
  -Many mobile users

Radio Channel

Multipath propagation

Shadowing

Terrain structures

Reflections

Interference

Reflections




Strong echoes can cause excessive propagation delay
uncritical, if within equalizer window
can cause severe (self-) interference if out of equalizer window

Fading(1)



Slow fading(Lognormal Fading)
shadowing due to large obstacles on the way

Fast fading (Rayleigh fading)
destructive interference of several signals
“fading dips”, “radio holes”

Fading(2)



Signal Variations




Propagation
   Free- space propagation

signal strength decreases exponentially with distance

     Reflection

specular R.

amplitude:  A --> α*A   (α < 1)

phase :  0   -->  - 0

polarization: material dependent phase shift

     Diffuse R.

amplitude:  A --> α*A   (α << 1)

phase :  0                -->  random phase

polarization :  random

     Absorption

heavy amplitude attenuation      
material dependent phase shifts depolarization

    Diffraction


wedge- model
knife edge
multiple knife edges

Propagation Model

Historical CCIR- Model for radio/ TV-stations

not very accurate nor serious

Okumura- Hata

empirical model

measured and estimated additional attenuations

estimations for larger distances (range: 5 .. 20km)

don´t use for small distances ( < 1km)

Hata’s Model

Adapted for 900 MHz, Europe, different land usage classes

                                              morpho : additional attenuation due
                                                             to land usage classes


with
f: frequency in MHz
h: BS antenna height [m]
a(h): function of MS antenna height
d: distance between BS and MS  [km]
and
A= 69.55,  B = 26.16 (for 150 .. 1000 MHz)
A= 46.3  ,  B = 33.9   (for 1000 ..2000MHz)


Land Usage Types
Urban:                     small cells, 40..50 dB/dec attenuation
Forest:                  heavy absorption; 30..40 dB/dec;
                                  differs with season (foliage losses)
Open, farmlands:      easy, smooth propagation conditions
Water:                     signal propagates very easily ==> dangerous !
Mountain faces:    strong reflections, long echoes
Glaciers:     very strong reflections; extreme delays
                     strong interference over long distance
Hilltops:     can be used as barriers between cells
              do NOT use as antenna sites locations

Walfish- Ikegami Model

Model for urban microcellular propagation

Assumes regular city layout (“Manhattan grid”)

Total path loss consists of three parts:

line-of-sight loss   LLOS

roof-to-street loss LRTS

mobile environment losses  LMS

Antenna Characteristics
Lobes
main lobes
side / back lobes
front-to-back ratio
Halfpower beam-width (3 dB- beam width)
Antenna downtilting
Polarization
Antenna bandwidth
Antenna impedance
Mechanical size
windload

Coupling Between Antennas

Horizontal separation

     needs approx. 5λ
     distance for sufficient decoupling

      antenna patterns superimposed if distance too close

Vertical separation

           distance of 1λ provides good        

           decoupling value

           good for RX /TX decoupling

Minimum coupling loss

Installation Examples

omnidirectional.: 5 .. 20m

directional : 1 ... 3m

Recommended decoupling

   TX - TX: ~20dB

   TX - RX: ~40dB

Horizontal decoupling 

distance depends on

      Antenna gain

      Horizontal rad. pattern

Omnidirectional antennas

       RX + TX with vertical separation 

       RX, RX div. , TX with vertical separation (“fork”)

Vertical decoupling is much more effective

 Directional antennas

sectorized sites

Antenna (down-) tilting


improve spot coverage

reduce interference

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