Frequency Hopping - Basics

s

Introductionof

Radio Link Control Featuresin

GSM NetworksU. Rehfuess and K. Ivanov, Siemens AG, Mobile Radio

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Outline Capacity Enhancement Radio Link Control Options:

- Frequency Hopping (FH), Power Control (PC), Discontinuous Transmission (DTX)

Diversity Effects of Frequency Hopping- Frequency Diversity

- Interference Diversity

Real Network Simulation Investigations - Capacity gains vs. re-use

- Homogeneous vs. real network layouts

- Different hopping modes

- Recommendations with respect to operator’s bandwidth

Conclusions

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General Methods for Capacity Enhancement

GSM

Frequencyreuse

BSdensity

channel usage

8 for FR16 for HR

Spectrumf. operator

5 per MHz(200 kHz)

traffic

area

traffic

channel

channels

carrier

carriers

bandwidth

1

cluster sizebandwidth

sites

area

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Capacity Enhancement by Radio Link Control OptionsPower Control (PC)

Discontinuous Transmission (DTX)

Frequency Hopping (FH)

Interference increase by tighter frequency re-usecan be compensated for by combination of FH, PC and DTX

reduces interference due to minimum transmission power

reduces interference due to no transmission during silence periods

mitigates frequency selective Rayleigh fading for slow MSs averages interference due to interference diversity

Tight frequency re-use yields capacity gain in existing sites at moderate cost? How far shall re-use be tightened for optimum performance?

Planned re-use down to 4 ? Cluster 1x3 ? Cluster 1x1

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Radio Link Control Options in the GSM SpecsFH, PC and DTX are mandatory (for MS) GSM Phase 1 features

FH: GSM 05.02

PC, DTX: GSM 05.05 and 05.08

PC dynamic range MS (GSM 05.05):

GSM 900 phase1: 39 dBm (33 dBm typ.) - 13 dBm 8 W (2 W typ.) - 20 mW

GSM 900 phase2: 39 dBm (33 dBm typ.) - 5 dBm 8 W (2 W typ.) - 3 mW

GSM 1800/1900: 36 dBm (30 dBm typ.) - 0 dBm 4 W (1 W typ.) - 1 mW

PC dynamic range BS (GSM 05.05):

TRX Power class (GSM 900: 320 .. 2.5 W, GSM 900 Micro 250 mW .. 25 mW)

Static RF power step: 0 .. -12dB (2dB steps)

Dynamic RF power control: 0 .. -30 dB (2dB steps)

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Diversity Effects of Frequency HoppingThe information of one GSM speech frame is spread over

8 successive bursts

20 ms speech frame

TDMA frame

0 1 2 3 4 5 6 7

channel coding & interleaving

Isolated corrupted bursts can be compensated by a strong forward error correction by convolutional channel coding

Soft decoding exploits mix of “good” and “bad” bursts

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Diversity Effects of Frequency HoppingFrequency Diversity

Due to multi-path fading, the radio channel is frequency selective

Changing the transmission frequency from burst to burst leads to individual propagation conditions for each burst

F3

F2

F1

MS Location Distance

SignalLevel

s

Diversity Effects of Frequency HoppingFrequency Diversity and Velocity

Wavelength: 900MHz ~ 30 cm, 1800MHz ~ 15 cm MS movement within one Speech Frame vs. SACCH period

3.6 km/h (1 m/s) 50 km/h (~14 m/s)TCH/FS 20ms 2 cm << 28 cm ~ SACCH 480ms 48 cm > 670 cm >>

TCH/FS performance strongly depends on FH at low speed SACCH perf. (radio link timeout!) fairly independent of FH

TDMA frame

n n+1 n+2 n+6n+3 n+4 n+5 n+7

25 7751 103

SACCH period: 480 ms

Speech Frame period: 20 ms

TDMA frame

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Diversity Effects of Frequency Hopping Frequency Diversity Gains

Cyclic FH reaches max. gain of e.g. 5 dB at 8 frequencies Random FH reaches max. gain of e.g. 5 dB at 64 frequencies

S/N gains by FH for TU3 (3km/h)

cyclic FHrandom FH

Frequency diversity gains are limited by the number of repetitions of frequencies within the interleaving depth, e.g. 8 for TCH/FS

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Diversity Effects of Frequency Hopping Interference Diversity - no FH

In the non-hopping case, on all bursts the same interferer occurs, i.e. no interference diversity

Interfering Cell TRX 11 1 1 1 1 1



Reference Cell TRX 11 1 1 1 1 1

TDMA frame # n n+1 n+2 n+3 n+4 n+5


TDMA frame # m m+1 m+2 m+3 m+4 m+5

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Diversity Effects of Frequency Hopping Interference Diversity - cyclic FH

Even in the cyclic FH, on all bursts the same interferer occurs, i.e. no interference diversity








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Diversity Effects of Frequency Hopping Interference Diversity - random FH

In the random FH case, from burst to burst different interferers occur randomly, i.e. interference diversity








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System Quality in FH-GSMC/I, raw Bit Error Rate (BER) and Frame Erasure Rate (FER)

With FH: C/I decreases, raw BER and RXQUAL get worseBut: Voice quality (FER) improves Simulations can evaluate FH gains

FER [%]

probability 2% FER

C/I [dB]per location

probability

Cyclic FHRandom FHno FH

10%

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Homogeneous vs. Real World Network Structures

Ideal homogeneous cell layout• homogeneous propagation

conditions• homogeneous traffic

distribution etc. real world effects are

neglected

Real inhomogeneous cell layout• various propagation conditions,

depending on site position, topology, morphology, antennae ...

• inhomogeneous traffic distribution

real world effects are taken into account

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Investigated Network Structure

Network configuration:operator bandwidth : 8.6 MHz, i.e. 43 carrierscarriers per cell (incl. BCCH) : 2, 3, 4, 5, 8, 10, 28investigated TCH re-use factors : 21.5, 14, 9.3, 7, 4, 1x3, 1x1

237 cellssite to site 1 .. 3 kmsectorised (66° beam width @ 3dB)frequency assignment strategies:frequency groups / optimising assignmentscommon band / dedicated band (15 BCCHs) / mixed re-use schemearea of investigation:24 representative cells have been selected in downtown area

50km50km

50km50km

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The System Simulation Model “Real Network”

Radio Network Planning (Tornado)Radio Network Planning (Tornado)

• network configuration• pathloss predictions• frequency plan

Real Network System Level SimulatorReal Network System Level Simulator

Radio Network Model• Cell selection• MS positioning• implementation of FH,

PC, DTX and GSM multi-frame structure

• calculation of CIRburst

Radio Network Model• Cell selection• MS positioning• implementation of FH,

PC, DTX and GSM multi-frame structure

• calculation of CIRburst

CIRburst Statistical Radio Link Model• mapping of CIRburst onto

BER, FER, 1bRBER

Statistical Radio Link Model• mapping of CIRburst onto

BER, FER, 1bRBER

• quality metrics, e.g. FER• planning guidelines• parameter settings

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Radio Network Model

Best Server Selection Algorithm

0.0 x

ycoverage prediction for cell n

coverage prediction for cell 1

width of simulation areaheight ofsimulationarea

coverage prediction for cell 2

grid

min(PL1, PL2, .., PLn)

Best Server Plot

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Radio Network Model

Parameters:• log normal fading : 7

dB• handover margin: 5 dB • co + adj. ch.

interference• call duration: 24s • locations: 10000• mainly DL simulated• multi path propagation

profile: TU 3• FH: NH vs. RH vs. CH • FH: incl. vs. excl. BCCH • PC off vs. on• DTX off vs. on

Parameters:• log normal fading : 7

dB• handover margin: 5 dB • co + adj. ch.

interference• call duration: 24s • locations: 10000• mainly DL simulated• multi path propagation

profile: TU 3• FH: NH vs. RH vs. CH • FH: incl. vs. excl. BCCH • PC off vs. on• DTX off vs. on

Snap Shot SimulationSnap Shot Simulation

s

0

20

40

60

80

100

120

140

21 14 9.3 7 4 1x3 1x1

Erl

/ S

ite

mean TCH re-use, opt. assignment cluster

Ideal Homogeneous Network

2/2/2

3/3/3

4/4/4

5/5/5

Co-Channel Interference

Co- and Adj. Interference

Simulation Results: Capacity Gain from Radio Link Options

0

20

40

60

80

100

120

140

21 14 9.3 7 4 1x3 1x1E

rl /

Site

mean TCH re-use, opt. assignment cluster

Real Network

2/2/2

3/3/3

4/4/4

5/5/5

Co-Channel Interference

Co- and Adj. Interference

Capacity is limited by the minimum of hard blocking, e.g. fulfilling Erlang-B Table at 2% (red dashed line - - - ) soft blocking, e.g. fulfilling quality criterion FER 2% for 90% of the calls

Operator Bandwidth: 8.6 MHz, i.e. 43 channels (15 BCCHs + 28 TCHs)

FH, PC and DTX used

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Cyclic Hopping vs. Random Hopping

CH profits from better frequency diversity

Interference diversity from individual freq. sets per cell

5 hopping frequencies,re-use 7 (frequency planning)

0

10

20

30

40

50

60

70

80

Erl /

Site

PC DTX PC & DTX

RHCH

FH only

27 hopping frequencies, re-use 1x1

CH cannot profit from PC and DTX due to missing interference diversity

0

10

20

30

40

50

60

70

80

Erl /

Site

PC DTX PC & DTXFH only

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Importing Simulation Results to Tornado - C/I in re-use 1x1

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Importing Simulation Results to Tornado - FER in re-use 1x1

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Effects of Simulation Assumptions on Capacity Gains

Real Network, Co- and Adj. Interference

Operator Bandwidth: 8.6 MHz, i.e. 43 channels (15 BCCHs + 28 TCHs)

FH, PC and DTX used

0

20

40

60

80

100

120

140

21 14 9.3 7 4 1x3 1x1

Log-Normal

Fading

Erl

/ S

ite

mean TCH re-use, optimum assignment cluster

2/2/2

3/3/3

4/4/4

5/5/5

= 3dB = 5dB = 7dB

Absolute Erl/Site values significantly depend on simulation assumptions likesigma of log normal fading, QoS requirements etc.

Relative comparisons of optimum assignments vs. cluster 1x3 and 1x1 hold irrespective of log normal fading

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Comparison between different Quality of Service Criteria

0

20

40

60

80

erl/site

21 (CH) 14 (CH) 9.3 (CH) 7 (CH) 4 (RH) 3 (RH) 1 (RH)

TCH reuse

98% calls with FER < 10% 95% calls with FER < 5% 90% calls with FER < 2%

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Spectral Efficiency vs. Operator Bandwidth

Limited spectrum: reuse 1x1 recommended due to higher FH gains

Sufficient spectrum: planned reuse (e.g. 6) recommended due to better C/I and sufficient FH gains

Planned re-use profits more on measures to achieve homogeneous network design

0

2

4

6

8

10

12

14

6 12 18 24 30 36 TCH freq.

Erl

/ Site

/ M

Hz

= 7 dB

0

2

4

6

8

10

12

14

6 12 18 24 30 36 TCH freq.

Erl

/ Site

/ M

Hz

= 5 dB

Reuse 1x1Reuse 6

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Conclusions Significant capacity gains can be achieved

by FH, PC and DTX in dedicated TCH and BCCH bands

Capacity and quality are determined bya trade-off between

local mean C/I in the network FH interference diversity gains

Two distinct ways can be chosen to maximise capacity: re-use 4 in random FH for good C/I and good interference

diversity re-use 1x1 with MAIO management in random FH for maximum

interference diversity Re-use 1x3 ignores 4 colour theorem leading to poor C/I and

insufficient FH gains in real networks (“bad compromise”) Depending on operator spectrum, re-use 1x1 is recommended for

limited spectrum and re-use 4 or higher for sufficient spectrum

0

20

40

60

80

100

120

140

21 14 9.3 7 4 1x3 1x1

Erl

/Sit

e

mean TCH re-use, optimum assignment cluster

2/2/2

3/3/3

4/4/4

5/5/5

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And who invented Frequency Hopping ???Patented Aug. 11, 1942

UNITED STATES PATENT OFFICE2,292,387

SECRET COMMUNICATION SYSTEMHedy Kiesler Markey, Los Angeles, and George

Antheil, Manhattan Beach, Calif.Application June 10, 1941, Serial No. 397,412

6 Claims. (Cl. 250-2)

This invention relates broadly to secret communication systems involving the use of carrier waves of different frequencies, and is especially useful in the remote control of dirigible craft, such as torpedoes.An object of the invention is to provide a method of secret communication which is relatively simple and reliable in operation, but at the same time is difficult to discover or decipher ...

Hedy Lamarr (Hedy Kiesler Markey)1913 - 2000, actress, dancer and - engineer!

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Additional Information

K. Ivanov et al: Frequency Hopping Spectral Capacity Enhancement of Cellular Networks. Proc. ISSSTA96, 1996, pp 1267-72.

U. Rehfuess, K. Ivanov, C. Lueders: A Novel Approach of Interfacing Link and System Level Simulations with Radio Network Planning. Proc. GLOBECOM 1998, pp 1503-08.

U. Rehfuess, K. Ivanov: Comparing Frequency Planning against 1x3 and 1x1 Re-Use in Real Frequency Hopping Networks. Proc. IEEE VTC‘99 Fall, Amsterdam, 1999, pp 1845-49.

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Simulation Results: Average C/I vs. Required C/ISoft blocking is determined by experienced C/I per location, e.g. C/I @ 10% outage required C/I for e.g. FER = 2%

Example: 16 busy timeslots (16 Erl) on TCH TRXs per sector on average:

-2

0

2

4

6

8

10

12

14

CH 3 freq.

14

CH 4 freq.

9.3

CH 5 freq.

7

RH 7 freq.

4

RH 9 freq.

1x3

RH27 freq.

1x1hopping mode, # frequencies, TCH re-use

C/I

[dB

]

Real Network

-2

0

2

4

6

8

10

12

14

CH 3 freq.

14

CH 4 freq.

9.3

CH 5 freq.

7

RH 7 freq.

4

RH 9 freq.

1x3

RH27 freq.

1x1hopping mode, # frequencies, TCH re-use

C/I

[dB

]

Ideal Homogeneous Network C/I@10%req.C/I(2%FER)Soft blocked Soft capacity potential

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Simulation Results: C/I Distributions vs. Re-Use

cluster 1x1cluster 1x3random re-use 3mean re-use 4mean re-use 7mean re-use 9.3mean re-use 14

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4 Colour Theorem

Real networks have sites off grid, varying propagation conditions etc. Cluster 1x3 may lead to large areas which actually use re-use 1 resulting in

poor voice quality and handover problems Cluster 1x3 cannot address omni-sites

Mean Re-Use 4 Cluster 1x3

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Simulation Results: Optimum Tight Re-Use

Similar capacity can be achieved in planned re-use 4, planned re-use 3, random re-use 3 and re-use 1x1

Clusters 1x3 and 2x2 (Nokia) perform poor due to degradation in experienced C/I (violation of 4 colour theorem) and poor interference diversity (frequency groups)

hopping mode, # frequencies, TCH re-use

Capacity [Erl/Site]

0

10

20

30

40

50

60

70

80

RH 7 freq.4 plan’d


RH 9 freq.

3 random

RH 9 freq.

clust.1x3

RH14 freq.

clust. 2x2

RH27 freq.

clust.1x1

hopping mode, # frequencies, TCH re-use

Experienced C/I [dB] vs. Required C/I [dB]

0

1

2

3

4

5

6

7

8



RH 9 freq.

3 random

RH 9 freq.

clust. 1x3

RH14 freq.

clust. 2x2

RH27 freq.

clust. 1x1

req.C/I(2%FER) [dB]

C/I@10% [dB]

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RXQUAL vs. FER in FH NetworksNo Frequency Hopping

0

1

2

3

4

5

6

7

0,1 1 10 100

FER@90% [%]

RX

QU

AL@

90%

2% FER

Cyclic FH 2 Frequencies

0

1

2

3

4

5

6

7

0,1 1 10 100

FER@90% [%]

RX

QU

AL@

90%

2% FER


0

1

2

3

4

5

6

7

0,1 1 10 100

FER@90% [%]

RX

QU

AL@

90%

2% FER


0

1

2

3

4

5

6

7

0,1 1 10 100

FER@90% [%]

RX

QU

AL@

90%

2% FER

RXQUAL is used in HO and PC decisions RXQUAL thresholds have to be adapted for FH

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Frequency Hopping and Concentric Cells

In an interference limited scenario, performance of calls at cell boarder (low RXLEV) may be enhanced by allocating them on BCCH TSs.Calls closer to the BS are allocated on tight re-use hopping TCH TSs.

Concentric cell feature yields RXLEV dependent channel allocation:BCCH-TRX configured as “complete cell”hopping TCH-TRXs configured as “inner cell”

Setting of proper threshold on a per-cell basis causes effort!

10% value determines capacity limit (“90% calls shall be better than x dB”)

Therefore, measures to enhance performance for the worst 10% calls enhance overall capacity!

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Measuring FH improvements in the Field• Call drop rates cannot show full FH gains, since SACCH

performance is not strongly related to FH

• RXQUAL statistics for both uplink and downlink get worse with FH and need to be interpreted -> required RXQUAL

• Currently no vendor supports speech quality related FER measurements in the BSSfor downlink, no MS reporting is standardisedfor uplink, BS vendor specific implementations are feasible

• TEMS drive/walk test can show FH improvement on downlink speech quality

• BR6.0 will have measured FER statistics for the uplink and estimated FER statistics for the downlink

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Typical Frequency Hopping Gains

Typical frequency hopping gainThe following table shows the typical gain from frequency hopping in a GSM 900 network (example of the signal-to-noise ratio required to obtain 0.2% residual BER for class 1b bits):

Frequency hopping TU3 TU50 HT100

None 11.5 7.5 6.82 frequency 10.0 6.5 6.74 frequency 8.25 6.0 6.68 frequency 7.5 6.0 6.616 frequency 6.75 6.0 6.6

Source: SIEMENS TED-BSS

Frequency Hopping - Basics

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