Post on 13-Jul-2016
description
GSM Fundamentals & RF
What you will learn here?• GSM Network Architecture• GSM channel Architecture• GSM Optimization Steps ( Performance, Drive testing &
Benchmarking)• Parameters Introduction• Case Study
Page3
Radio InterfaceAnother MSC
HLR/AUCSMC
PSTN ISDN
OMC
MS
Um
MS
A-bis
BSC
A
BTS
MSC/VLR
Power
Frequency
Time
FDMA
Frequency
Power Time
TDMA
FrequencyCDMA
PowerTime
Multiple Access Methods
Frequency Reuse Pattern
Three types of frequency reuse patterns
• 7 Cell reuse pattern
• 4 cell reuse pattern
• 3 cell reuse pattern
3 Site Reuse Patternc2
c1c3
a1a2
a3
b1
b2
b3
c1c2
c3Cell Re-use
Channels in GSM Air interface
Logical Channels
Control Channels Traffic Channels
Full rate => Used for speech at 13 Kbits/s
or sending data at 9.6 Kbits/s
Half rate => Used for speech at 6.5 Kbits/s or sending data at 4.8 Kbits/s
Enhanced Full rate => Used for speech at 13 Kbits/s
or sending data at 9.6 Kbits/s but with almost Land line quality
TCH = TRAFFIC CHANNEL
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Broadcast control channel (BCH)
Control channelCommon control channel (CCCH)
Voice channel(TCH)
FCCH SCH BCCH(system information)
TCH/FAGCH RACH SDCCH FACCH
SACCH
TCH/HPCH
Common channel (CCH)
Dedicated channel (DCH)
Logical channel
Logical Channel Type• GSM900 and DCS1800 have the same logical channel category
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FCCHSCHBCCH
PCH
AGCH
BCCH
CCCH
Common Channel
SDCCH
SACCH
FACCH
TCH/F
TCH/H
DCCH
TCH
Dedicated Channel
Downlink Logical Channel
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RACH CCCH
Common channel
SDCCH
SACCH
FACCH
TCH/F
TCH/H
DCCH
TCH
Dedicated channel
Uplink Logical Channel
Page12
Use of Logical Channels
FCCH
Allocate signaling channel
Power-on Search for frequency correction burst
Search for synchronous burst
Listen to the system information
Monitor paging message
Send access burst
Set up the call
Allocate voice channel
Conversation
Release the call
Idle mode
SCHBCCH
PCH
RACHAGCHSDCCHSDCCHTCHFACCH
Dedicated mode
Idle mode
Page13
Network Identity Parameter — CGI
• Network identity parameters mainly include cell global identity (CGI) and base station identity code (BSIC).
CGI=MCC+MNC+LAC+CI
• Once MS receives SYS INFO, it decodes the CGI information, and decides whether it can stay in the cell according to the MCC and MNC indicated by CGI. At the same time, it judges whether the current location area is changed, so as to decide whether to execute location update. During the location update process, MS will report the new LAI to the network, so that the network can know the LA in which MS is currently located.
Page14
Mobile Country Code ( MCC)
• Definition– MCC consists of 3 decimal numbers. It
indicates the home country of the mobile subscriber.
• Format– MCC is composed of 3 decimal numbers. The
coding range is decimal 000-999.
Page15
Mobile Network Code ( MNC)
• Definition– MNC is used to uniquely identify a specific
GSM PLMN network in a certain country (decided by MCC).
• Format– MNC is composed of two decimal numbers.
The coding range is decimal 00-99.
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Location Area Code ( LAC)• Definition
– To locate the location of MS, the whole area covered by each GSM PLMN is divided into different location areas. LAC is used to identify different location areas.
• Format– LAI contains LAC, which is composed of two bytes. LAC
adopts hexadecimal coding. The available range is from 0001H to FFFEH. The code 0000H and FFFFH cannot be used (please refer to specification GSM0303, 0408, and 1111). One location area can contain one or more cells.
Page17
Cell Identity ( CI)• Definition
– To uniquely identify each cell in the GSM PLMN, the network operator needs to allocate one code for each cell, which is the cell identity (CI). Cell identity, together with LAI, is used for identity of each cell in the world. (specification 0303).
• Format– CI is composed of 16 bits, The available range is
0~65535.
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Network Identity Parameter — BSIC
• BSIC=NCC+BCC• In GSM system, each BTS is allocated with a color code, which is
called BSIC. MS can identify two cells with the same BCCH by the help of BSIC. In network planning, effort should be made to make sure that BCCH of neighbor cells are different from the serving cell’s BCCH to reduce the interference. Practically it is still possible that a same BCCH is re-used in the surrounding cells. For cells using the same BCCH in a relevant near distance, their BSIC must be different so that MS can identify two neighbor cells with same BCCH.
Page19
Network Color Code ( NCC)• Definition
– NCC is a part of BSIC. MS uses it to distinguish adjacent BTS that belong to different GSM PLMN.
• Format– NCC is composed of 3 bits, with the range of 0
to 7.
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BTS Color Code ( BCC)• Definition
– BCC is a part of BSIC. For its function, please refer to above sections.
• Format– BCC is composed of 3 bits. The available
range is from 0 to 7.
Mobile Maximum Range
• Range = (Timing advance * bit period * velocity) / 2
• Range = (63 * (3.693 * 10e-6) * (3 * 10e5))/2
• So range is 34.9 Kms.
Example: Impact of PC & DTX
1.00
1.50
2.00
2.50
3.00
3.50
1.00000% 2.00000% 3.00000% 4.00000% 5.00000% 6.00000% 7.00000% 8.00000%
Effective Frequency Load
DC
R (%
)
Non BCCH RF Hopping 2/2
Non BCCH RF Hopping 2/2 DLPC
Non BCCH RF Hopping 2/2 DLPC DTX
No Power Control no DTX
Power ControlPower Control & DTX
100% more speech traffic with the same DCR
Real Netw
ork
Hopping principlesHopping principles
Frequency
F3
F2
F1
RANDOM
TDMA frame
CYCLIC
cycle
Call is transmitted through several frequencies in order to
• average the interference (interference diversity)
• minimise the impact of fading (frequency diversity)
1 2 3 4 5 6 etc.
BB-FH vs. RF-FH
Flexibility• hopping with small configurations and small BW is possible
Maximum FH gain or easy allocation possible• hopping enabled over large number of frequencies
Number of antennas• in large configurations the number of antennas increases if AFE used
HW dependencies• old BTS generations do not support RF-FH• wideband antenna coupling equipment needed (AFEs)
Less HW restrictions• supported by all BTS generations• all antenna combining methods feasible (RTC or AFE)
Minimum number of antennas requiredEasy to Implement (switch on)
•existing planning tools•the same interferers and frequency plan than with IUO
Limitations with small configurations (<3 TRX) and small BW• hopping may not be possible on both layers (IFH)
DL PC• limited PC range in downlink (mobile AGC problems)
BB-FH
RF-FH
No simple answer, selection on case by case basis
BB Hopping ManagementBB Hopping Management
BCCH 1 2 3 764
0 1 2 3 764
0 1 2 3 764
0 1 2 3 764
TRX-1
TRX-2
TRX-3
TRX-4
RTSL-0 RTSL-2RTSL-1 RTSL-4RTSL-3 RTSL-5 RTSL-6 RTSL-7
f1
f2
f3
f4
BCCH timeslot, does not hop.
5
5
5
5
Timeslot 0 of TRXs 2-4 hop over MA(f2,f3,f4).This hopping group uses HSN-1
All timeslots 1-7 hop over MA(f1,f2,f3,f4).This hopping group uses HSN-2
Multipath Fading results in variations in signal strength which is known as Rayleigh Fading.
Rayleigh Fading phenomenon is dependent on path difference and hence frequency of reception.
A fast moving mobile may not experience severe effect of this fading since the path difference is continuously changing.
A slow moving mobile ( or a halted mobile ) may experience severe deterioration in quality.
But, if the frequency of reception is changed when this problem occurs, could solve it.
The fading phenomenon is fast and almost continuos, this means the frequency change should also be continuos.
This process of continuously changing frequency is known as Frequency Hopping.
Frequency Hopping
RF Hopping ManagementRF Hopping Management
BCCH 1 2 3 764
0 1 2 3 764
0 1 2 3 764
0 1 2 3 764
TRX-1
TRX-2
TRX-3
TRX-4
RTSL-0 RTSL-2RTSL-1 RTSL-4RTSL-3 RTSL-5RTSL-6 RTSL-7
f1
BCCH TRX does not hop.
5
5
5
5
MAIOs are differentfor different TRXswithin the samehopping group-> no collisions.
MA1 = {f2, f3, f4,..}
HSN-1
AMR INTRODUCTION
AMR Introduction• AMR (Adaptive Multi Rate) consists of a family of codecs with
different bit-rates operating in GSM FR and HR• The aim is to improve channel (FR/HR) quality by adapting the most
appropriate channel codec based on current radio condition• Codec mode adaptation (link adaptation) is based on received channel
quality in both MS and BTS (the codec adaptation possible each 40ms)• The basic AMR codec mode sets for MS and BTS are provided by BSC
via layer 3 signalling• MS shall support all speech codec modes, although only a set of up to 4
speech codec modes are used during a call• AMR Capacity benefits:
– Improved robustness in Full Rate allows tightening of re-use patterns - increased spectral efficiency
– Operation of Half Rate channels will free available capacity for data traffic - reduced blocking
• AMR Coverage benefits:– Improved performance at cell edge
AMR Speech Codec
Voice quality
Robustness
AMR codecs:8 for Full rate and 6 for Half Rate:
1.0
2.0
3.0
4.0
5.0
No Errors 16 dB C/I 13 dB C/I 10 dB C/I 7 dB C/I 4 dB C/I
MOS (Mean Opinion Score)
EFRAMR FR
AMR Full Rate performance compared to Full Rate EFR in Clean Speech
1.0
2.0
3.0
4.0
5.0
No Errors 19 dB C/I 16 dB C/I 13 dB C/I 10 dB C/I 7 dB C/I 4 dB C/I
FRAMR HR
MOS (Mean Opinion Score)
AMR Half Rate performance compared toFull Rate in Clean Speech
Full Rate Half rateEFR 12.2
10.27.957.95 (*)
IS 136 7.47.46.76.75.95.95.155.154.754.75Speech bit rate
(*) Requires 16 kbit/s TRAUNot supported by Nokia BSS
Adaptive Multi Rate• Channel cross bit-rate is 22.8 kbit/s in GSM FR/EFR: 13 kbit/s speech
coding and 9.8 kbit/channel coding (HR channel gross bit rate 11.4 kbit/s)• In the AMR case, codec mode can be changed and more error correction
bits can be used whenever channel requires
0
5
10
15
20
25
FR12.2
FR10.2
FR7.95
FR 7.4 FR 6.7 FR 5.9 FR5.15
FR4.75
HR7.95
HR 7.4 HR 6.7 HR 5.9 HR5.15
HR4.75
AMR codec mode
Cha
nnel
bit-
rate
(kbi
t/s)
Channel codingSpeech coding
AMR Codec Modes
Channel mode
Channel codec Mode
Source coding bit-rate, speech
Net bit-rate, in-band channel
Channel coding bit-rate, speech
Channel coding bit-rate, in-band
CH0-FS 12.20kbit/s (GSMEFR) 0.10 kbit/s 10.20 kbit/s 0.30 kbit/s CH1-FS 10.20 kbit/s 0.10 kbit/s 12.20 kbit/s 0.30 kbit/s CH2-FS 7.95 kbit/s 0.10 kbit/s 14.45 kbit/s 0.30 kbit/s
TCH/FR CH3-FS 7.40 kbit/s (IS-641) 0.10 kbit/s 15.00 kbit/s 0.30 kbit/s CH4-FS 6.70 kbit/s 0.10 kbit/s 15.70 kbit/s 0.30 kbit/s CH5-FS 5.90 kbit/s 0.10 kbit/s 16.50 kbit/s 0.30 kbit/s CH6-FS 5.15 kbit/s 0.10 kbit/s 17.25 kbit/s 0.30 kbit/s CH7-FS 4.75 kbit/s 0.10 kbit/s 17.65 kbit/s 0.30 kbit/s
CH8-HS 7.95 kbit/s (*) 0.10 kbit/s 3.25 kbit/s 0.10 kbit/s TCH/HR CH9-HS 7.40 kbit/s (IS-641) 0.10 kbit/s 3.80 kbit/s 0.10 kbit/s
CH10-HS 6.70 kbit/s 0.10 kbit/s 4.50 kbit/s 0.10 kbit/s CH11-HS 5.90 kbit/s 0.10 kbit/s 5.30 kbit/s 0.10 kbit/s CH12-HS 5.15 kbit/s 0.10 kbit/s 6.05 kbit/s 0.10 kbit/s CH13-HS 4.75 kbit/s 0.10 kbit/s 6.45 kbit/s 0.10 kbit/s
Table: Channel and speech codec modes for AMR
(*) Requires 16 kbit/s TRAU. Therefore it is not seen as a feasible codec mode and will not be supported by Nokia.
AMR Link adaptation
Codec mode adaptation• Codec mode changed according
to channel conditions (UL/DL C/I)
• Only up to four codecs can be used during a call
• Goal—the highest MOS (Mean Opinion Score)
• Mode indications inform the receiver about the currently applied codec mode
• Mode Command informs MS about the codec mode to be applied on the uplink
Channel mode adaptation
• FR <-> HR changed by handover (packing and unpacking)
• Based on BTS load (BSC level) and channel condition (RxQual)
AMR FR codec mode adaptation example
speech coding
chan. coding
0
5
10
15
20
25
30
0 5 10 15 20 25 30
Time[s]
[dB
]C/I EFR operation AMR mode
12.2 kbit/s
7.95 kbit/s
6.70 kbit/s
5.90 kbit/s
AMR Mode
C/I
AMRFR HR12.210.27.957.4 7.46.7 6.75.9 5.95.15 5.154.75 4.75
Benefits of AMR 1/2• Speech quality enhancement: AMR maintains good speech quality in
the situation where the connection faces low C/I or low signal level• Capacity and coverage gain: Link level simulation results illustrated
improvement in terms of TCH FER (up to 6dB at 1% FER in C/I)• Signalling channel performance: due to retransmissions schemes used
by these channels the probability of signalling success maintain very high even for very degraded conditions
• Improved BCCH plan: tighter frequency reuse or better quality with same frequency reuse, potentially releasing frequencies to be used on the non-BCCH layer. This is applicable when EFR legacy terminals do not represent a significant portion of the traffic.
Benefits of AMR 2/2• Mixed EFR – AMR traffic networks: use more aggressive power control for
AMR mobiles in order to decrease the average interference level in the network. Due to better error correction capability against the channel errors lower C/I target can be set for AMR mobiles hence lower PC thresholds can be used. Therefore, the overall interference decreases in the network (smaller average transmission power) and thus the quality of the existing EFR connections increase
• HR utilisation increases the hardware capacity of the cell since two half-rate connections can be allocated to fill only one timeslot.
• When compare AMR HR to previous GSM HR codec, it is noticed that AMR HR obtains remarkable better speech quality
DISCUSSION & QUESTIONS
Frequency Hopping is done in both Uplink and Downlink . Frequency is changed in every TDMA Frame Mobile can Hop on maximum 64 frequencies The sequence of Hopping can be Cyclic or Non-Cyclic There are 63 Non-Cyclic Hopping sequences possible Different Hopping sequence can be used in the same cell.
BCH Timeslot can never HOP, but the remaining Timeslots can very well hop.
Frequency Hopping
Reduction in Average Interference With Frequency Hopping consistent interference will become bursty. So even though, both the co-channel cells will be using the same set of ARFCN's for
Hopping, interference will not be continuos. This is because, GSM cells are not Frame synchronized, and change in frequency is
related to Frame nos. If same HSN is used in two cells, then either the interference will be nil , or if a phase
correlation exists then it will be continuos. So the two cells should preferably use different HSN's . Sectorial cells ( controlled by the same BTS) can use same HSN, since the sectors don't
come up at the same time. Cells if they are synchronized, can use same HSN, if each cell has an offset of some
TDMA frames. Offset of TDMA frames is also required to avoid SACCH occurring at the same time in all
synchronized cells, as they kills away the objective of DTX.
Frequency Hopping
HandoversHard Handoff
Analog, TDMA and GSMSoft Handoff
CDMA
Break before Make Make before Break
Cleaner Handovers• The mobile measures up to 32 adjacent cells for
– Signal Strength (RxLevel)– Signal Quality (RxQual)– updated every 480 mS and sends to BTS
• Sophisticated Handover based on– RxLevel– Interference– RxQual– Timing Advance– Power Budget
BTS
BTS
BTS
BTS
BTS
BTS
BTS
BTS
BSC
VLR
HLR EI
R
OMC
SMSCB
C
AUC
VMSC
MSC
Abis
A
MS
BTS
BTS
BTS
BTS BTS
BTS
BTS
BTS
BSC
BSC
PSTN
VLR
TRAU HLREIR
OMC
SMSC
BC
AUC
VMSC
MSC
Abis
A
OML
GSM NETWORK ELEMENTS
GSM Network Components• Mobile Station consists of two parts-
– Mobile Equipment (ME)– Subscriber Identity Module (SIM)
• ME – Hardware e.g. Telephone, Fax Machine, Computer.
• SIM– Smart Card which plugs into the ME.
Channels On Air Interface• Physical Channel • Logical Channel
• Physical Channel– Physical channel is the medium over which the
information is carried.
• Logical Channel– Logical channels consists of the information carried
over the Physical Channel.
GSM Channels
Traffic Channel
TCH carries payload data - speech, fax, data
• Connection may be:
- Circuit Switched - voice or data or - Packet Switched – data
• TCH may be:
• Full Rate (TCH/F)
- one channel per user
- 13 kb/s voice, 9.6 kb/s data or
• Half Rate (TCH/H)
- one channel shared between two users
- 6.5 kb/s voice, 4.8 kb/s data
Traffic Channels
TCH/FFull rate 22.8kbits/s
TCH/HHalf rate 11.4 kbits/s
• Time is divided into discrete periods called “Timeslots”
Control Channel
DCCH(Dedicated Channels)Downlink & Uplink
CCCH(Common Control Chan)Downlink & Uplink
Synch.Channels
RACHRandom
Access ChannelCBCH
Cell Broadcast Channel
SDCCHStandalone dedicated
control channel
ACCHAssociated
Control Channels
SACCHSlow associated Control Channel
FACCHFast Associated
Control Channel
PCH/AGCH
Paging/Access grant
FCCHFrequency
Correction channel
Control Channels
BCH ( Broadcast channels )Downlink only
BCCHBroadcast
control channel
SCHSynchronization
channel
Broadcast Channels (BCH)
BCH channels are all downlink and are allocated to timeslot zero.
Channels are:
• FCCH: Frequency control channel sends the mobile a burst of all ‘0’ bits which allows it to fine tune to the downlink frequency
• SCH: Synchronization channel sends the absolute value of the frame number (FN), which is the internal clock of the BTS, together with the Base Station Identity Code (BSIC)
• BCCH: Broadcast Control Channel sends radio resource management and control messages, Location Area Code and so on.
Some messages go to all mobiles, others just to those that are in the idle state
Common Control Channels (CCCH)
CCCH contains all point to multi-point downlink channels (BTS to
several MSs) and the uplink Random Access Channel:
• CBCH: Cell Broadcast Channel is an optional channel for general information such as road traffic reports sent in the form of SMS
• PCH: Paging Channel sends paging signal to inform mobile of a call
• RACH: Random Access Channel is sent by the MS to request a channel from the BTS or accept a handover to another BTS.
A channel request is sent in response to a PCH message.
• AGCH: Access Grant Channel allocates a dedicated channel (SDCCH) to the mobile
• NCH: Notification Channel informs MS about incoming group or
broadcast calls
Dedicated Control Channels (DCCH)
SDCCH( Standalone Dedicated Control Channel )
Uplink and Downlink
Used for call setup, location update and SMS.
SACCH( Slow Associated Control Channel )
Used on Uplink and Downlink only in dedicated mode.
Uplink SACCH messages - Measurement reports.
Downlink SACCH messages - control info.
FACCH( Fast Associated Control Channel )
Uplink and Downlink.
Associated with TCH only.
Call Scenarios
• Mobile to Mobile– Intra-city – Inter-city
• Mobile to Land– Intra-city – Inter-city
• Land to Mobile– Intra-city – Inter-city
Mobile To Land Sequence
1
3
CHANNEL REQUEST
DCCH ASSIGN
SIGNALLING LINKESTABLISHED
REQUEST FOR SERVICE
SET CIPHER MODE
SET-UP
EQUIPMENT ID REQUEST
AUTHENTICATION
MS BSS MSC VLR HLR PSTNEIR
RACH
AGCH
SDCCH
SDCCH
Call Info7
4
6
5
2CR
CC
8 COMPLELTE CALLCALL PROCEEDING
9 ASSIGNMENT COMMAND
INITIAL & FINALADDRESS (IFAM)
ASSIGNMENT COMPLETE (ACM)
10
ANSWER(ANS)
11
CONNECT ACKNOWLEDGE
SDCCH
SDCCH
ASSIGNMENT COMPLELTE
MS HEARS RINGTONEFROM LAND PHONE
ALTERING
RING TONESTOPS
CONNECT
(channel)
(TCH)
FACCH
FACCH
FACCH
TCH
(circuit)
FAACH
BILLING STARTS
Hello!
MS BSS MSC
VLR HLR PSTN EIR
Call Contt.
Customer..Expectation
• Good coverage – where ever he goes• Good quality• No blocking• Value added services
– SMS– Voice mail– MMS– Call forward/call waiting– Data/internet at high data rates– prepaid
Basic Network Design Objectives
The basic objectives of a wireless system are:– COVERAGE: provide sufficient cell sites to deliver RF coverage
of the entire desired area.– BUILDING/VEHICLE PENETRATION: deliver sufficient signal
levels to adequately penetrate buildings and vehicles where appropriate.
– TRAFFIC: ensure that no cell captures more traffic than it can handle at the desired grade of service (i.e., blocking percentage)
– PERFORMANCE: design, construct, and adjust the network to deliver reliable service free from excessive origination and call delivery failures, dropped calls, quality impairments, and service outages.
– ECONOMICS: provide return on investment sufficient to support operating and capital expenses, expand the network to take advantage of growth opportunities, and retire costs of construction prior to depreciation of the network equipment.
What is Performance Optimization?
• The words “performance optimization” mean different things to different people, viewed from the perspective of their own jobs
• System Performance Optimization includes many different smaller processes at many points during a system’s life– recognizing and resolving system-design-related issues (can’t
build a crucial site, too much overlap/soft handoff, coverage holes, etc.)
– “cluster testing” and “cell integration” to ensure that new base station hardware works and that call processing is normal
– “fine-tuning” system parameters to wring out the best possible call performance
– identifying causes of specific problems and customer complaints, and fixing them
– carefully watching system traffic growth and the problems it causes - implementing short-term fixes to ease “hot spots”, and recognizing problems before they become critical
Optimization• Optimisation is an ongoing process of analysing network performanceagainst Quality of Service targets:
Performance•Measurements of network performance cover:
• Traffic in erlangs
• TCH and SDCCH Grade of Service (Congestion)
• Call success rate
• Handover failure
• Coverage area
• Coverage quality
• Subscriber base and growth
• Key Performance Indicators (KPI) are measurable dynamic
parameters that help to target areas of concern
KPI’s• Appropriate KPIs to use depend on:
• The nature of the network• Data sources available• Measurement tools available• Ability of engineering team• Cost of network infrastructure
• Sources of data include:• Surveyed data - from drive tests• Network statistics - from OMC • Field engineer reports
Radio Interface Optimization• Power Control
• Diversity
• Frequency Hopping
• Antenna Parameters ( Height, Azimuth, Tilts )
Antenna Tilts
Antenna Tilts
Benchmarking•Surveyed data from test-mobile measurements can be used to
benchmark system performance against that of a competitor
• Problems that may be identified from surveyed data:
• Poor coverage
• Unexpected interference
• Missing handover definitions
• Installation problems at BTS
• Test-mobile measurements should include:
• continuous calls to test coverage
• repetitive short calls to test call-success