Frequency Hopping Network

Telecom Tutorials
Monday, June 03, 2013www.tempustelcosys.com
Introduction to Radio Link Control for
Real Frequency Hopping Networks

Radio Link Control Options:
Frequency Hopping, Power Control and DTX
Real network simulation environment
Simulation results
Capacity gains vs. re-use
Homogeneous vs. real network layouts
Different hopping modes
Recommendations with respect to operator’s bandwidth
Conclusions

Power Control (PC)
Discontinuous Transmission (DTX)
Frequency Hopping (FH)
Interference increase by tighter frequency re-use
can 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

FH, 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)

The 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

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
|H(f)| [dB]
ARFCN
0
-10
-20
-30
n n+1 n+2 n+3 n+4 ...

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
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

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

 In the non-hopping case, on all bursts the same
interferer occurs, i.e. no interference diversity
Interfering Cell TRX 1
1 1 1 1 1 1
2 2 2 2 2 2
3 3 3 3 3 3
Reference Cell TRX 1
1 1 1 1 1 1
TDMA frame # n n+1 n+2 n+3 n+4 n+5
4 4 4 4 4 4
TDMA frame # m m+1 m+2 m+3 m+4 m+5

Even in the cyclic FH, on all bursts the same interferer occurs, i.e. no
interference diversity
1 2 3 4 1 2
3 4 1 2 3 4
4 1 2 3 4 1
1 2 3 4 1 2
2 3 4 1 2 3

In the random FH case, from burst to burst different interferers occur
randomly, i.e. interference diversity
3 1 3 2 2 4
3 2 4 4 1 4
4 3 1 1 2 1
1 4 2 2 3 2
2 1 3 3 4 3

 With FH:  C/I decreases, raw BER and RXQUAL get worse
 But:  Voice quality (FER) improves
 Simulations can evaluate FH gains
FER [%]
probability 2% FER
C/I [dB]
per location
probability
Cyclic FH
Random FH
no FH
10%

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

Radio Network Planning (Tornado)
• network configuration
• pathloss predictions
• frequency plan
Real Network System Level Simulator
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
• quality metrics, e.g. FER
• planning guidelines
• parameter settings

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
0
20
40
60
80
100
120
140
21 14 9.3 7 4 1x3 1x1
Erl/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
0
20
40
60
80
100
120
140
21 14 9.3 7 4 1x3 1x1
Erl/Site
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
Operator Bandwidth: 8.6 MHz, i.e. 43 channels (15 BCCHs + 28 TCHs)
FH, PC and DTX used

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

5 hopping frequencies,
re-use 7 (frequency planning)
27 hopping frequencies,
re-use 1x1
CH profits from better frequency
diversity
Interference diversity from individual
freq. sets per cell
0
10
20
30
40
50
60
70
80
Erl/Site
PC DTX PC & DTX
RH
CH
FH only
0
10
20
30
40
50
60
70
80
Erl/Site
PC DTX PC & DTXFH only

Real Network, Co- and Adj. Interference
0
20
40
60
80
100
120
140
21 14 9.3 7 4 1x3 1x1
Log-Normal
Fading
Erl/Site
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 like
sigma of log normal fading,
QoS requirements etc.
Relative comparisons of optimum assignments vs. cluster 1x3 and 1x1 hold
irrespective of log normal fading

0
2
4
6
8
10
12
14
6 12 18 24 30 36 TCH freq.
Erl/Site/MHz
= 7 dB
0
2
4
6
8
10
12
14
6 12 18 24 30 36 TCH freq.
Erl/Site/MHz
= 5 dB
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

Significant capacity gains can be achieved
by FH, PC and DTX in dedicated TCH and
BCCH bands
Capacity and quality are determined by
a 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

Frequency Hopping Network

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Frequency Hopping Network