frequency hopping concept
TRANSCRIPT
FREQUENCY HOPPING CONCEPT
Presented By:Usman Javed Bhatti
Senior Engineer Radio Network PlanningUfone PTML
Each burst is transmitted on a different frequency Both mobile and base station follow the same
hopping sequence
The MS/BTS changes the
frequency from burst to burst
Frequency A
Frequency B
Frequency C
Frequency D
4.615millisec. (1 Frame = 8*577 microsec.)
Frame 0 Frame 1 Frame 2 Frame 3
Frequency Hopping Concept
1. Frequency Diversity Frequency hopping can reduce the effect of multipath fading. Multipath
fading is frequency and location dependent. With frequency hopping, a non-moving mobile will typically not remain in a specific fading dip longer than one TDMA frame. The low signal strength dips in
multipath fading are thus levelled out, and the mobile will perceive a more even radio environment. This is called frequency diversity
Capabilities of FH
Multipath fading occurs when signals arrive at thereceiver both directly from the transmitter, and,indirectly, due to propagation through objects or
reflection. These signals arrive at slightly differenttimes, with different amplitudes and phases. Theysum together constructively and also destructively(fading dips). The fading dips appear at different
spatial locations for different frequencies, i.e. they arefrequency and location dependent. This phenomenonis called multipath fading. Fading dips are separatedby approx. 17 cm for GSM 900, and approx. 8 cm for
GSM 1800 and GSM 1900.
Multipath Fading
Fading
Fading (cont…)
Non HoppingNon Hopping
Freq. f1Freq. f1
Freq. f1Freq. f1
Freq. f2Freq. f2
HoppingHopping
Corrupted BurstsCorrupted Bursts
Frequency Diversity
2. Interference DiversityInterference is dependent on time, frequency and mobile location.
Withoutfrequency hopping, some cell planning margins must be incorporated so
that sufficient service quality can still be provided in an interfered situation. By changing frequency on every TDMA frame, a mobile only experiences interference on a particular frequency once in a number of hops. Similarly, interference on a particular frequency will be spread
across many mobiles (i.e. averages out with other mobiles). This is called interference averaging and results in interference diversity. With
interference diversity, the perceived radio environment will be more even. As a result of frequency hopping, cell planning margins can be reduced
which makes it possible to implement a tighter frequency plan.
f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1
f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1 f1
Wanted Call (f1 fixed)Wanted Call (f1 fixed)
Interfering Call (f1 fixed)Interfering Call (f1 fixed)
Corrupted Bursts because of InterferenceCorrupted Bursts because of Interference
Non HoppingNon Hopping
f1 f3 f1 f2 f3 f1 f2 f1 f3 f1 f2 f3 f2 f1 f2 f1 f3 f2 f3 f1 f2 f1 f3 f1 f3 f2 f1
f3 f2 f1 f3 f1 f3 f2 f3 f1 f2 f1 f3 f1 f2 f3 f1 f2 f1 f3 f2 f1 f3 f1 f2 f1 f2 f3
Wanted Call (Hopping over f1, f2, f3)Wanted Call (Hopping over f1, f2, f3)
Interfering Call (Hopping over f1, f2, f3)Interfering Call (Hopping over f1, f2, f3)
Corrupted Bursts because of InterferenceCorrupted Bursts because of Interference
HoppingHopping
Interference Averaging
A
BC
DF
EG
HI
JL
K
4x3 Reuse Pattern
Efficient Reuse Pattern Optimize Control Over
Interference Better Call Quality
Objectives of Frequency Planning
Operators would like to use
from the Available Spectrum
in Every Sector
Utilize Maximum Capacity
Every Carrier Frequency
In An Ideal Scenario
INTERFERENCEINTERFERENCE
Results in
Frequency Hopping
How to remove Interference ?
Conservative frequency reuse pattern
Typically 4 site by 3 sector repeat
12 frequencies provide one carrier per sector
A
BC
DF
EG
HI
JL
K
Conventional Frequency Planning (4x3 Reuse)
High Interference -Likely To Drop Calls
Low Interference -Good Call Quality
Some Interference -Mildly Degraded Quality
Excess Interference -No Service
Coverage Plot for 4x3 Reuse Pattern (No Frequency Hopping)
XY
Z
XY
Z
XY
Z
XY
Z
Reuse frequencies more often
For example on a 1 site by 3 sector repeat
3 frequencies provide one carrier per sector
Aggressive Frequency Planning(1x3 Reuse)
High Interference -Likely To Drop Calls
Low Interference -Good Call Quality
Some Interference -Mildly Degraded Quality
Excess Interference -No Service
Coverage Plot for 1x3 Reuse Pattern (No Frequency Hopping)
High Interference -Likely To Drop Calls
Low Interference -Good Call Quality
Some Interference -Mildly Degraded Quality
Excess Interference -No Service
Coverage Plot for 1x3 Reuse Pattern (With Frequency Hopping)
From a subscriber point of view, frequency hopping gives an improved speech quality in many situations. From an operator point of view, the benefits are:
• tighter frequency reuse and increase in capacity,• a more robust radio environment,• a possibility to give subscribers a more uniform speech quality.
Conclusions
1. Base Band Hopping (BBH) The TRXs transmit always the same frequency Number of frequencies for hopping = Number
of carriers
Types of Frequency Hopping
2. Synthesiser Frequency Hopping (SFH) The TRXs change (retune) the frequency every
burst Number of frequencies for hopping > Number
of carriers TRXs can hop over a range of 64 different
frequencies
The TRXs always transmit a fixed frequency.
The call “hops” over the TRXs, maintaining the same timeslot, on a per burst basis.
In reception the call is always processed by the same TRX (the one where the call started).
The number of frequencies to hop over is limited by the number of TRX equipped in the cell.
The BCCH carrier can hop in timeslots 1 to 7 (without power control/DTX).
Base Band Hopping
f1 f4f3f2
Base Band Hopping (cont…)
TRX 0
TRX 1
TRX 2
TRX 3
TRX 4
Mobile A
Mobile B
Mobile C
Mobile D
Mobile E
NNerer Hopping Frequencies = N Hopping Frequencies = Nerer TRXs in service TRXs in service
Base Band Hopping (cont…)
Call assigned to TRX 3 Timeslot 5 at the set up
7 4 f1 1 4 f1 3
... ... ... ... ... ... ...
TCH
TRX
3
TCH
TRX
4
TCH
TRX
2
BCCH
TRX
1
f4 f3 f2 f1
Cyclic
TRANSMISSION RECEPTION
BURST # Timeslot Frequency TRX Timeslot Frequency TRX
1 4 f3 3 4 f3 3
2 4 f4 4 4 f4 3
3 4 f1 1 4 f1 3
4 4 f2 2 4 f2 3
5 4 f3 3 4 f3 3
6 4 f4 4 4 f4 3
Base Band Hopping (cont…)
The TRXs are able to retune to a new frequency each burst.
The call always stays in the same TRX. One TRX can hop up to over 64 different
frequencies. Wide-band combining devices (hybrids) are
required in the base station (Cavity Combiners can not be used with SFH).
The BCCH frequency can be included in the hopping sequence, but in practice, BCCH carrier never hops, and carries traffic on timeslots 1 to 7.
Synthesiser Frequency Hopping (SFH)
Hopping over 8 frequencies
BCCH f1…f8 f1…f8 f1…f8 f1…f8
Synthesiser Frequency Hopping (cont…)
Types of Synthesiser Frequency Hopping
1. 1 X 3 SFH Three Sector Sites Three Sets of ARFCNs Each Sector uses different MA
1. 1 X 1 SFH One Single set of ARFCNs Each Sector uses same MA
Specific Amount Of Spectrum
Specific NumberOf Sites
ScenarioRF Capacity
Subscribers
Specific Amount Of Spectrum
Specific NumberOf Sites
Scenario (cont…)
Subscribers
RF Capacity
Scenario (cont…)
Subscribers
RF Capacity
Operator Concerns
QualityMore Spectrum New Sites
Scenario (cont…)
Subscribers
RF Capacity
Traditional Ways1. Allocate More Spectrum2. Increase Spectral Efficiency
• Smaller Cells• Tighter Frequency Reuse
Scenario (cont…)
Traditional Ways1. Allocate More Spectrum2. Increase Spectral Efficiency
• Smaller Cells• Tighter Frequency Reuse
Subscribers
RF Capacity
Scenario (cont…)Alternate Technique
Subscribers
RF Capacity
FH
Subscribers
RF Capacity
FH
Scenario (cont…)Alternate Technique
QUALITY IMPROVEMENT
Reduction in FER
Improves Voice Quality
Reduction in Drop Calls
Increase Call Success Rate
Advantages of Frequency Hopping
"For the same capacity FH improves the quality, and for a given average quality FH makes possible increase the capacity".
NON HOPPING
SYSTEM
C
A
P
Q
U
A
HOPPING SYSTEM
C
A
P
Q
U
A
HOPPING SYSTEM
C
A
PQ
U
A
OR
HOPPING SYSTEM
C
A
P
Q
U
A
OR
Quality – Capacity Trade Off
INVESTMENT REDUCTIONWith Respect to the Fixed System Case:
Reduction in Number of Sites Required, for the Same Capacity and Quality
Reduction in Time and Work Required for Planning
Reduction in Time and Work Required for Optimisation
Advantages of Frequency Hopping (cont…)
Timeslot 0 of BCCH carrier can not hop
Any timeslot configured as CCCH can not hop
The BCCH carrier can not hop with SFH
If SFH uses the BCCH frequency, the BCCH carrier can not carry traffic
The BCCH carrier can hop with BBH
With BBH, the hopping system assigned to timeslot 0 of Non-BCCH carriers can not include the BCCH frequency
Frequency Hopping Limitations(BBH and SFH)
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