Upload
dinhtu
View
238
Download
6
Embed Size (px)
Citation preview
UTRAN Radio Resource Management
u Introductionu Handover Control
u Soft/Softer Handoveru Inter Frequency Handover
u Power Controlu Closed Loop Power Controlu Open Loop Power Control
u Interference Management
u Load Control
u Call Admission Controlu Congestion Control
u Packet Data Transmission
u Packet Data Controlu Dynamic Scheduling
BTS 1
BTS 3
BTS 2UE
UMTS Networks 2Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
References
H. Holma, A. Toskala (Ed.), “WCDMA for UMTS”, Wiley, 5th edition, Wiley, 2010H. Kaaranen, et.al., “UMTS Networks: Architecture, Mobility and Services”,
Wiley, 2001. (see chapter 4)
A. Viterbi: “CDMA: Principles of Spread Spectrum Communications,” AddisonWesley, 1995.
J. Laiho, A. Wacker, T. Novosad (ed.): “Radio Network Planning andOptimisation for UMTS,“ Wiley, 2001
T. Ojanperä, R. Prasad, “Wideband CDMA for Third Generation MobileCommunication”, Artech House, 1998.
R. Prasad, W. Mohr, W. Konhäuser, “Third Generation Mobile CommunicationsSystems”, Artech House, March 2000.
3GPP standards:
u TS 25.214: “Physical Layer Procedures“ (esp. power control)u TR 25.922: “Radio Resource Management Strategies“u TR 25.942: “RF System Scenarios“
UMTS Networks 3Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
u Efficient use of limited radio resources (spectrum, power, code space)u Minimizing interferenceu Flexibility regarding services (Quality of Service, user behaviour)u Simple algorithms requiring small signalling overhead onlyu Stability and overload protectionu Self adaptive in varying environmentsu Allow interoperability in multi-vendor environments
Radio Resource Managementalgorithms control theefficient use of resourceswith respect tointerdependent objectives:– cell coverage– cell capacity– quality of service
RRM – High-Level Requirements
UMTS Networks 4Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
RRM – Components
Radio Resource Management
MediumAccessControl
Packet DataControl
LoadControl
HandoverControl
PowerControl
Core Network/ other RNCs
Physical layer
typically inRNC
typically inNodeB
UMTS Networks 5Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
Handover Control: Basics
General: mechanism of changing a cell or base station during a call or session
Handover in UMTS:u UE may have active radio links to more than one Node Bu Mobile-assisted & network-based handover in UMTS:
u UE reports measurements to UTRAN if reporting criteria (which are set bythe UTRAN) are met
u UTRAN then decides to dynamically add or delete radio links dependingon the measurement results
Types of Handover:u Soft/Softer Handover (dedicated channels)u Hard Handover (shared channels)u Inter Frequency (Hard) Handoveru Inter System Handover (e.g. UMTS-GSM)u Cell selection/re-selection (inactive or idle)
All handover types require heavy support from the UMTS network infrastructure!
UMTS Networks 6Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
Macro Diversity & Soft Handover (Wrap-Up)
Downlink: combining in the mobile stationUplink: combining in the base station and/or radio network controller
NodeB 1NodeB 2
UE
UMTS Networks 7Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
Soft/ Softer Handover
u In soft/softer handover the UE maintains active radio links to more than oneNode B
u Combination of the signals from multiple active radio links is necessary
Soft Handoveru The mobile is connected to (at least) two cells belonging to different NodeBsu In uplink, the signals are combined in the RNC,
e.g. by means of selection combining using CRCSofter Handover
u The mobile is connected to two sectors within one NodeBu More efficient combining in the uplink is possible like
maximum ratio combining (MRC) in the NodeB instead of RNC
Note:u In uplink no additional signal is transmitted, while in downlink each new link
causes interference to other users, therefore:u Uplink: HO general increase performanceu Downlink: Trade-off
UMTS Networks 8Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
Soft and Softer Handover in Practice
UMTS Networks 9Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
Soft Handover Control
• Measurement quantity, e.g.EC/I0 on CPICH
• Relative thresholds dadd &ddrop for adding & dropping
• Preservation time Tlink toavoid “ping-pong” effects
• Event triggered measurementreporting to decreasesignalling load
MeasurementQuantity
Link to 1 Link to 1 & 2 Link to 2 time
CPICH 1
CPICH 2
ddrop
NodeB 1 NodeB 2
dadd Tlink
UE
soft handoverarea
UMTS Networks 10Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
Soft Handover – Simulation Results
Soft handover significantly improves the performance, but …
0%
5%
10%
15%
20%
25%
5 15 25 35 45 55
Offered Traffic [Erlang per site]
Out
age
Pro
babi
lity
(Blo
ckin
gan
dD
ropp
ing)
1 linkmax 2 SHO linksmax 4 SHO linksmax 6 SHO links
UMTS Networks 11Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
Soft Handover – Simulation Results II
… the overhead due to simultaneous connections becomes higher!
0
0,5
1
1,5
2
1 2 4 6
Max. Active Set Size
Mea
nN
umbe
rofA
ctiv
eLi
nks
UMTS Networks 12Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
Handover f1 Û f2 always neededbetween layers
Hierarchical cell structure (HCS)
Macro Micro Macro
f1
f2
f1
Handover f1 Û f2 neededsometimes at hot spot
Hot-spot
f1
f2f1 f1
Hot spot
Inter-Frequency Handover
u Hard handoveru Inter-frequency measurements of target cell needed in both scenariosu Mobile-assisted handover (MAHO)
u slotted (compressed) mode for inter-frequency measurements to find suitabletarget cell
u also supports GSM system measurementsu Database assisted handover (DAHO)
u no measurements performed on other frequencies or systemsu use cell mapping information stored in data base to identify the target cell
UMTS Networks 13Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
Cell Selection/Re-selection
u “Handover” when UE is idle or there is noactive connection between UE andUTRAN
u Goal: find a suitable cell to camp on
u The cell to camp on is chosen by the UEon measured link quality Q, e.g. EC/I0 onCPICH (after cell-search)
u Cell re-selection with hysteresis H toavoid “ping-pong” effects
u Additional offsets for Q on differentfrequencies, e.g. to support hierarchicalcells
u Mapping functions for Q between UMTSand GSM to support priorities
u Cell selection and re-selection mainlyperformed internally in UE, but controlledby UTRAN with broadcast of neighbourcell frequencies and control parameters(hysteresis, mapping, etc.)
NodeB 1NodeB 2
UE
Q1 > QminQ1 > Q2
Cell selection border
Q1Q2
UMTS Networks 14Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
Power Control: Basics
Controls the setting of the transmit power in order to:u Keep the QoS within the required limits, e.g. data rate, delay and BERu Minimise interference, i.e. the overall power consumption
Power control handles:u Path Loss (Near-Far-Problem), Shadowing (Log-Normal-Fading) and
Fast Fading (Rayleigh-, Ricean-Fading)u Environment (delay spread, UE speed, …) which implies different
performance of the de-interleaver and decoder
Uplink: per mobileDownlink: per physical channel
Three types of power control:u Inner loop power controlu Outer loop power control (SIR-target adjustment)u Open loop power control (power allocation)
Downlink power overload control to protect amplifieru Gain Clipping (GC)u Aggregated Overload Control (AOC)
UMTS Networks 15Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
Near-Far Problem:• Spreading sequences are not orthogonal
(multi-user interference)• Near mobile dominate• Signal to interference ratio is lower for far
mobiles and performance degrades
The problem can be resolved throughdynamic power control to equalize allreceived power levels
AND/OR
By means of joint multi-user detection
Near-Far Problem – Power Control (Wrap-Up)
NodeB
UE 1
UE 2
UMTS Networks 16Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
Closed Loop Power Control
u Closed loop power control is used on channels, which are established in bothdirections, such as DCH
u There are two partsu Inner Loop Power Control (ILPC): receiver generates up/ down
commands to incrementally adjust the senders transmit poweru Outer Loop Power Control (OLPC): readjusts the target settings of the
ILPC to cope with different fading performance
NodeB
UE
control command: Up/DownExample: Uplink Closed Loop Power Control
Inner Loop(1500 Hz)
SIR > SIRtarget ?
Outer Loop(£ 100Hz)
target adjustmentBLERtarget
RNC
UMTS Networks 17Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
Impact of Power Control
-8
-6
-4
-2
0
2
4
6
0 0.2 0.4 0.6 0.8 1
pow
er/
fadi
ng[d
B]
time [sec]
2
3
4
5
6
7
8
0 0.2 0.4 0.6 0.8 1
E b/N
0[d
B]
speed = 3 km/h
Example: UMTS Closed Loop Power Control in the slow fading channel
UMTS Networks 18Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
Power Control Performance
5
5.5
6
6.5
7
7.5
0 20 40 60 80 100 120Velocity [km/h]
Requ
ired
UL
SIR
[dB]
PedAVehA
SIR requirement strongly depends on the environment (due to differentfast fading conditions – Jakes models)Þ outer loop power control needed to adapt SIR
UMTS Networks 19Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
Open Loop Power Control
u Open loop power control is used on channels that cannot apply closedloop power control, e.g. RACH, FACH
u The transmitter power is determined on the basis of a path loss estimatefrom the received power measure of the opposite direction
u To avoid excessive interference, probes with incremental power stepsuntil a response is obtained: “power ramping”
NodeB
UE
Open Loop Power Control on RACH
UMTS Networks 20Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
CDMA Overload
• CDMA systems tend tobecome unstable– More traffic increases the
interference– More interference requires
higher power– More power increases the
interference …
• Methods are required to limitthe system load– Restrict the access to the
system– Overcome overload
situations
UMTS Networks 21Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
Interference in CDMA Networks
u Frequency reuse factor is oneu CDMA is subject to high multiple access interferenceu Soft capacity: CDMA capacity (e.g. number of users) determined by the
interference is softu Handling of interference is the main challenge in designing CDMA networks
Interference Problem
Inter-Symbol Interference (ISI) Delayed components from thesame user signal interfere due tomultipath propagation
Multiple Acces Interference – MAI Different user signals interferedependent on the access scheme
Intra-Cell Interference Interference caused by the usersbelonging to same cell
Inter-Cell Interference Interference caused by the usersbelonging to neighbor cells.
UMTS Networks 22Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
Cell Breathing
u CDMA systems: cell size depends on the actual loadingu Additional traffic will cause more interferenceu If the interference becomes too strong, users at the cell edge can no
more communicate with the basestationu CDMA interference management
u Restriction of the users access necessaryu Cell breathing makes network planning difficult
Example: cell brething with increasing traffic
UMTS Networks 23Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
Cell Breathing (contd.)
coveragelow load
coveragemedium load
coveragehigh load
Coverage depending on load: load causes interference, which reduces thearea where a SIR sufficient for communication can be provided
shadowed area: connection maybe lost
UMTS Networks 24Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
Coverage vs. Capacity
u Capacity depends on:u QoS of the users (data rate, error performance (bit-error-rate))u User behaviour (activity)u Interference (out of cell)u Number of carriers/ sectors
u Coverage (service area) depends on:u Interference (intra- & inter-cell) + noiseu Pathloss (propagation conditions)u QoS of the users (data rate, error performance (bit-error-rate))
u Thus, trade-off between capacity and coverage
UMTS Networks 25Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
Coverage vs. Capacity
u Downlink limits capacity while uplink limits coverageu Downlink depends more on the load (users share total transmit BS power)
0 20 40 60 80 100 120 140 160 180 2000
0.5
1
1.5
2
2.5
3
3.5
¬Downlink
Uplink®
Erlangs (2% GOS)
Max
imum
cell
radi
us(k
m)
13kbps circuit switched service capacity versus maximum cell radius
UMTS Networks 26Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
Example of Coverage and Best Server Map
Application: RF engineering (cell layout)Legend: violet indicates high signal level, yellowindicates low level
cove
rage
map
Application: HO decisionLegend: color indicates cell with bestCPICH in area
best
serv
erm
ap
UMTS Networks 27Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
Load Control: Basics
u Main objective:u Avoid overload situations by controlling system loadu Monitor and controls radio resources of users
u Call Admission Control (CAC)u Admit or deny new users, new radio access bearers or new radio
linksu Avoid overload situations, e.g. by means of blocking the requestu Decisions are based on interference and resource measurements
u Congestion Control (ConC)u Monitor, detect and handle overload situations with the already
connected usersu Bring the system back to a stable state, e.g. by means of
dropping an existing call
UMTS Networks 28Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
Resource Consumption
• Service/BLER-dependentresource consumption
• Uplink example:– Service I: Voice
Rb = 12.2kbps, Eb/Nt = 5dBaI = 0.99%
– Service II: DataRb = 144kbps, Eb/Nt = 3.1dBaII = 7.11%
• In downlink there is additionaldependency on the locationof the user– Cell center ® low
consumption– Cell edge ® high
consumption
a
UMTS Networks 29Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
Admission/ Congestion Control
Basic algorithm• Admission control is triggered
when load ³ thr_CAC– New users are blocked– Existing users are not
affected as long as load <thr_ConC
• Congestion Control istriggered when load ³thr_ConC– Reduce consumption of one
or several users– Simple action: drop the user– Repeat until load <
thr_ConC
UMTS Networks 30Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
50%
5 15 25 35 45 55
Offered Traffic [Erlang per s ite ]
Blo
ckin
gPr
obab
ility
thr_CAC = 50%thr_CAC = 75%thr_CAC = 90%
0%
2%
4%
6%
8%
10%
12%
14%
16%
18%
20%
5 15 25 35 45 55
Offered Traffic [Erlang per s ite]
Dro
ppin
gPr
obab
ility
thr_CAC = 50%thr_CAC = 75%thr_CAC = 90%
Tradeoff between blocking and droppingExample: 64k per user, urban
Call Admission Control: Simulation Results I
UMTS Networks 31Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
5 15 25 35 45 55
Offered Traffic [Erlang per site]
Cell
Load
ing
thr_CAC = 50%thr_CAC = 75%thr_CAC = 90%
Cell load depending on CAC thresholdExample: 64k per user, urban
Call Admission Control: Simulation Results II
UMTS Networks 32Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
Packet Data Control: Channel Switching
u Flexibility of packet servicesu Asymmetrical data ratesu Very low to very high data ratesu Control information/user information
u Efficient transmission making good use of CDMA characteristicsu Dedicated channel (DCH)
u Minimise transmission power by closed-loop power controlu Independence between uplink and downlink capacity
u Common channelu Random access in the uplink (RACH)u Dynamic scheduling in the downlink (FACH)
u Adaptive channel usage depending on traffic characteristicsu Infrequent or short packets Þ Common channel (Cell_FACH)u Frequent or large packets Þ Dedicated channel (Cell_DCH)u No packet transmission Þ UE “stand by” modus (URA_PCH)
UMTS Networks 33Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
CELL_DCH CELL_FACH CELL_DCH
Page Download Time Reading Time
DCH Active Time“Chatty Applications”
Channel Switching – Example
u Example: Web serviceu Chatty apps.: keep alive message, stock tickers, etc.
(e.g. 100 bytes every 15 sec)u Second stage: when no activity in CELL_FACH then switch to URA_PCH
UMTS Networks 34Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
· Power Control:– Balances user received quality (BLER, SIR)– Users at cell center get less share of BTS
transmit power assigned than at cell edge– Occurrence of power overload
· Rate Adaptation:– Transmit power ~ data rate– Users at cell edge get lower data rate assigned
than at cell center– Reduces also power overload
· On DCH combination of power control andrate adaptation
– Rate assignment at begin of a transmissionbased on load and user location
– Rate adaptation when ongoing transmissionaccording to power consumption and overload
– Based on RRC-signaling (time horizon:100msec … 10sec)
NodeB
UE 1
UE 2low data rate
area
high data ratearea
Power Control vs. Rate Adaptation
UMTS Networks 35Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
Rate Adaptation Performance
Rate adaptation significantly improves the RRM performance on DCH.
UMTS_urban, 50 kByte
0%
5%
10%
15%
20%
25%
30%
35%
40%
200 300 400 500 600
Cell Throughput [kBit/sec]
Out
age
Prob
abili
ty
384k64kadaptive
UMTS_urban, 50 kByte
0
1
2
3
4
5
6
7
8
200 300 400 500 600
Cell Throughput [kBit/sec]M
ean
Del
ay[s
ec]
384k64kadaptive
UMTS Networks 36Andreas Mitschele-Thiel, Jens Mueckenheim Oct. 2012
Dynamic Scheduling
• “Statistical multiplexing” of datapackets from different dataflows on one shared medium,e.g. on DSCH or HSDPA
• Scheduling with time-horizon of2msec … 1sec
• Optimised usage of radioresources
• Exploitation of the short-termvariations on the radiochannels (opportunisticscheduling)
• Can provide certain degree ofQoS
NodeB
Sample Flow
Flow #1
Flow #3
UE 3
UE 2
UE 1
Flow #2