Upload
sammy-ulali
View
355
Download
0
Embed Size (px)
Citation preview
s
Introductionof
Radio Link Control Featuresin
GSM NetworksU. Rehfuess and K. Ivanov, Siemens AG, Mobile Radio
s
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
s
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
s
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
s
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)
s
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
s
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
s
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
s
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
Interfering Cell TRX 22 2 2 2 2 2
Interfering Cell TRX 33 3 3 3 3 3
Reference Cell TRX 11 1 1 1 1 1
TDMA frame # n n+1 n+2 n+3 n+4 n+5
Interfering Cell TRX 44 4 4 4 4 4
TDMA frame # m m+1 m+2 m+3 m+4 m+5
s
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
Reference Cell TRX 11 2 3 4 1 2
Interfering Cell TRX 13 4 1 2 3 4
Interfering Cell TRX 24 1 2 3 4 1
Interfering Cell TRX 31 2 3 4 1 2
Interfering Cell TRX 42 3 4 1 2 3
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
s
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
Reference Cell TRX 13 1 3 2 2 4
Interfering Cell TRX 13 2 4 4 1 4
Interfering Cell TRX 24 3 1 1 2 1
Interfering Cell TRX 31 4 2 2 3 2
Interfering Cell TRX 42 1 3 3 4 3
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
s
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%
s
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
s
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
s
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
s
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
s
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
s
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
s
Importing Simulation Results to Tornado - C/I in re-use 1x1
s
Importing Simulation Results to Tornado - FER in re-use 1x1
s
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
s
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%
s
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
s
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
s
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!
s
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.
s
s
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
s
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
s
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
s
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 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 7 freq.4 plan’d
RH 9 freq.3 plan’d
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]
s
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
Cyclic FH 4 Frequencies
0
1
2
3
4
5
6
7
0,1 1 10 100
FER@90% [%]
RX
QU
AL@
90%
2% FER
Cyclic FH 8 Frequencies
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
s
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!
s
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
s
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