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General overview of GSM cell design and frequency reuse concept. fading and frequency hopping network design
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CELL PLANNING AND FREQUENCY REUSE
Overview
Conventional Cell and Frequency Planning
Radio Network Features and their impact
Real” Cell and Frequency Planning
Site selection
Parameters to cell planning
Spectrum availableTraffic
Cost
GoS
QoS
Quality
Coverage
Speech Quality
C/I
Algorithms
Lee’s model and other empirical models
Ploss = PR1 + g10log(d / d1) + n10 log( f / f0) - a0
PR1 is the reference loss at d1(normally 1 mile) (e.g. -84dBm in a city like Tokyo and -
49dBm for open areas) g depends on the type of terrain (value between 2 and 4) n is between 2 and 3
Co-channel interferenceInfobits
Codingbits
CodeRate
Max data rate(kbs) /TS
Required C/I (dB)
modulation
GSM 260 196 0.5 13.3 9 GMSK
CS-1 181 275 0.45 9.05 9 GMSK
CS-2 268 188 0.65 13.4 13 GMSK
CS-3 312 144 0.75 15.6 15 GMSK
CS-4 428 28 21.4 23 GMSK
MCS-1 176 0.53 8.4 9 GMSK
MCS-2 224 0.69 11.2 13 GMSK
MCS-3
296 0.89 14.8 15 GMSK
MCS-4 352 1 16.8 23 GMSK
MCS-5 448 0.38 22.4 14.5 8PSK
MCS-6 592 0.5 29.6 17 8PSK
MCS-7 896 0.78 44.8 23.5 8PSK
MCS-8 1088 0.92 54.4 29 8PSK
MCS-9 1184 1 59.2 32 8PSK
Adjacent Channel interference
for co-channel interference C/Ic=9 Db
for adjacent (200 kHz) interference C/Ia1=-9 dB
for adjacent (400 kHz) interference C/Ia2=-41 dB
for adjacent (600 kHz) interference C/Ia3=-49 dB
GSM Cell
GSM cell are basically geographical areaCovered by radio frequencies of BTS antennasThe hypothetical shape of a GSM cell is a Hexagon.
Two types of GSM cell
Omni directional
Sector cell
The Hexagon
dc
a e
f
b
R
d
Area A=3(3)½R²/2
Distance between centersof two adjacent cells:d = (3)½R
Omni directional Cell
An omni-directional cell (or omnicell) is served by a BTS with an antenna which transmits equally in all directions (360 degrees).
BTS
Sector cell
A sector cell is the area of coverage from an antenna, which transmits, in a given direction only. For example, this may be equal to 120 degrees or 180 degrees of an equivalent omni- directional cell. One BTS can serve one of these sector cells with a collection of BTS’s at a site serving more than one, leading to terms such as two-sectored sites and more commonly, three-sectored sites.Typically, omni-directional cells are used to gain coverage, whereas sector cells are used to gain capacity.
Hexagonal or sectored cell
BTS
GSM Frequency reuse
• The frequency reuse concept is to use same frequency channel with another cell within the same GSM cell cluster.
• Since cells are more or less close to each other there is not perfect solution.
• Cells will always interfere with each other even if they are not immediate neighbors.
• How many cells must we consider when we do our planning? how many closest neighbors do we have?
• The frequency re-use patterns recommended for GSM are the 4/12 and the 3/9 pattern.
The only (almost) noise we have to consider comes from interfering base stations.If we want to reduce interference from neighbors we need to increase the D/R ratio.If D = R sqr(3k) then sqr(3k) should be big, that is increase k!If we need a C/I ratio of 18dB then we need k > 6.
In GSM networks a frequency reuse pattern with k = 3, 7 or 12.In the 900-band, which is 2x25MHz wide, we can have 124 carriers. If these are divided into groups of 12 frequencies we can have 10 groups.One cell can thus be covered by 10 carriers.Each carrier can have 8 connections thus amaximum of 80 calls in an area covered by a cell.
Re-use distance
reuse distance D
k = 6number of cells inpattern
U
V
j
i30º
Re-use distance
D = (i2 + ij + j2)½2Rcos 30°D = (i2 + ij + j2)½ (3) ½ RNumber of cells in there-use patternN = i2 + ij + j2i in (1,2,3,4 …..)j in (0,1,2,3,4 …..)D/R = (3N)½
Hierarchical Cells
Umbrella Cell:
Macro Cell: Antenna above average rooftop height
Micro Cell: Antenna below average rooftop height
Pico Cell: Indoors
DTX- DiscontinuousTransmission
Average Voice activity is around 50%
DTX is a feature that allows to be transmitted only
when there is something to be transmitted
Uses VAD (Voice Activity Detector)
It safes on battery power
Improves the overall network quality by reducing
unnecessary interference
Dynamic Power Control
This enable the BTS and the Mobile to transmit
only the power necessary for effective
communications
Power Control Commands are via the SACCH
This improves the battery live of Mobile Phones
And it improve the overall network quality by
reducing unnecessary interference
Base Band Frequency Hopping Number of frequencies equal to number of transceiivers
ControllerCall 1
ControllerCall 2
ControllerCall 3
ControllerCall 4
Tx and Rx on f0
Tx and Rx on f1
Tx and Rx on f2
Tx and Rx on f3
COMBINER
f2 f3 f0 f1
f3 f0 f1 f2
f1 f2 f3 f0
f0 f1 f2 f3
Synthesised Hopping Number of frequencies more or equal to number of transceiivers
ControllerCall 1
ControllerCall 2
ControllerCall 3
ControllerCall 4
Tx and Rx hopping
Tx and Rx hopping
Tx and Rx hopping
Tx and Rx hopping
f2 f3 f0 f1
f3 f0 f1 f2
f1 f2 f3 f0
f0 f1 f2 f3
Frequency Diversity
Raleigh fading is frequency dependent
Diversity: combining two or more uncorrelated versions
of the same signal
For “conventional” frequency diversity the info is sent on
two different frequencies at the same time.
To be uncorrelated the two frequencies should be more
than 1/(multi-path spread), where the multi-path spread is
dependent on the environment.
For urban areas the frequencies should be more than
600kHz apart
Why does hopping work?
Review interleaving
If one timeslot gets completely lost during
transmission 1/8 of two speech frames are lost.
At the receiver the speech frames are de-interleaved
The channel coding can recover from the 12.5% BER.
Interleaving and Channel Coding is part and parcel of
the GSM standard - it works even without hopping.
Interference Diversity
Extent of Interference diversity depends on: Interference load (DTX and Power Control) Frequency reuse: low re-use -> low gain; dependent on area type. Number of Frequencies (less -> less gain) Cyclic or Random
Interference diversity gain reached with 25% load, 12 frequencies in Urban area with random hopping is 2.5dB - mostly it is less.
Planning for FH network
Use separate frequency blocks for TCH and BCCH
BCCH frequency channel must be Always On No
hopping over BCCH.
Plan TCH layer:
MAL : Mobile radio frequency channel Allocation List
HSN: Hopping sequence number
MAIO: Mobile Allocation Index Offset
MAI: Mobile Allocation Index
Selecting a BCCH block
Why a BCCH block? Identifying the source of interference Re-evaluation of the neighbor list For collecting data for a measurement based plan Optimum size? Where a change in a BCCH carrier will on
average make the same difference as a change in a TCH carrier in the optimized plan
Selecting a BCCH block
Block Size BCCH =
Total Number of Carriers Available/(Average Traffic on TCH layer per cell/8)*Scaling (DTX.PC) + 1
TCH layer
MAIO MA
MAI 0 2 1A 2A 3A 1B 2B 3B 1C 2C 3C
1 1 3 1 2 3 4 5 6 7 8 9
2 2 4 10 11 12 13 14 15 16 17 18
3 3 1 19 20 21 22 23 24 25 26 27
4 4 2 28 29 30 31 32 33 34 35 36
HSN = X 4 1 2 3 2 4 3 1TRX1 ON 1A has MAIO = 0 28 1 10 19 10 28 19 1TRX1 ON 1A has MAIO = 2 10 19 28 1 28 10 1 19
THANK YOU
SHASHANK ASTHANA