Frequency Hopping Concept

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)

THANK YOU