Wireless Curriculum Development SectionWireless Curriculum Development SectionWireless Curriculum Development SectionWireless Curriculum Development Section
ISSUEISSUE
OWJ103104 WCDMA Load OWJ103104 WCDMA Load Control Control
OWJ103104 WCDMA Load OWJ103104 WCDMA Load Control Control
1.11.1
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The ContentThe Content
The Load Concept
The Load Control Measures
Admission Control
Load Balance
Congestion Control
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Load MeasurementLoad Measurement
Load Concept
Uplink Load
Downlink Load
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Load ConceptLoad Concept
Load:
The occupancy of capacity
TDMA and FDMA system: the capacity is hard
limited.
Example: In GSM, the cell capacity (channel
number) is fixed.
CDMA system:
soft capacity
Hard capacity
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Load ConceptLoad Concept
Time
Frequency
FDMA
User#1
User#2
User#3
User#1User#2
User#3Frequency
TimeTDMA
Frequency
Time
CodeCDMA
User#1User#2
User#3
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Capacity of CDMA systemCapacity of CDMA system
Soft capacity:
Interference (UL)
Power (DL)
Hard capacity
Code
Hard ware resource
Transport resource, NodeB processing capability.
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System loadSystem load
Uplink : RTWP (Received Total Wide band Power)
Downlink : TCP (Transmitted Carrier Power)
RTWP
noiseBackgroudUL 1
The RTWP has an unique corresponding with the uplink load
The TCP has no unique corresponding with the actual downlink load factor. And because the actual downlink load factor is difficult to measure, we use the TCP instead.
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Activity factor : The percentage of time that the data
transmission is active.
The UL and DL activity factor may be different for dedicated
channel.
Different service type has different activity factor.
Activity factorActivity factor
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Eb/No
j
j
b
PRTWP
P
R
W
powerceInterferen
powerSignalgaingprocesNE
sin/ 0
RTWP
NE
RWP
b
j
0/
)/(1
1
0/
)/(1
1
NE
RWL
b
j
celltheinuserallj
jUL Lf )1(
Received power Pj
Define load factor for user j
UL load factor
Definition of UL load factorDefinition of UL load factor
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Noise rise : defined as the ratio of RTWP and background
noise.
NP
RTWPrisenoise
UL
risenoise
1
1
Relationship between UL load factor and the noise rise.
risenoise
risenoiseUL
1
and
Noise rise & UL load factorNoise rise & UL load factor
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Pj can be written as
erferencebase
ULNjjj
P
PLRTWPLP
int
1
1
Received power if there is no interference.
Increment due to the interference.
Meaning of the UL load factor
Meaning of UL load factorMeaning of UL load factor
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Stability and noise riseStability and noise rise
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Eb/No
N
miiN
ji
itotal
jm
total
jm
j
ob
PL
P
LP
L
p
R
WNE
,1 ,
,
,
,/
is the non-orthogonal factor
is the interference of own celljm
total
L
P
,
is the interference of neighbor cells
N
mii ji
itotal
L
P
,1 ,
,
is the pathlossL
Definition of DL load factorDefinition of DL load factor
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Let’s suppose for simply, and base on , we can getitotaltotal PP ,
j
jtotal PP
)()/(
)/(1
)/(
)/(
,1 ,
,
,
N
mii ji
jm
j j
job
jmj j
jobN
total
L
L
RW
NE
LRW
NEP
P
Defined as the DL load factor DL
Base power due to the pass loss.
Increment due to the interference.
Definition of DL load factorDefinition of DL load factor
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Non-orthogonal factor and interference from other cells
are various with position, and are difficult to measure.
N
mii ji
jm
L
L
,1 ,
,
The same DL load factor may correspond to different transmitted
carrier power due to the different pass loss (cell radius).
This is why the TCP will not correspond uniquely with the DL
load factor, and why we use TCP in the DL CAC instead.
Transmitted power & DL load factorTransmitted power & DL load factor
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The Load controlsThe Load controls
CAC: Call Admission Control Used to admit/reject the resource request to keep
the system work in stable state.Load Balance:
To balance load between cells to maximize the system capacity
Congestion Control: To make the system come back from congestion
state to the stable state.
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The ContentThe Content
The Load Concept
The Load Control Measures
Admission Control
Load Balance
Congestion Control
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Load control - CACLoad control - CAC
Admission Control
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Call Admission Control (CAC)Call Admission Control (CAC)
What is CAC?
Why we need CAC?
How the CAC been implemented.
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What is CAC?
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call arrive
Get the service characteristic and the current load
Suppose the user is accepted, and calculate the predictedUL load factor. And then compared with the threshold.
admitted?
Suppose the user is accepted, andcalculate the predicted DL load factor.And then compared with the threshold.
admitted?
call admitted call rejected
end
n
y
y
n
Flow chart of CACFlow chart of CAC
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Principle of the CACPrinciple of the CAC
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Why we need CAC?
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Why we need CAC?Why we need CAC?
WCDMA is an interference limited system, after a new call
is admitted, the system load will be increased.
If a cell is high loaded, a new call will cause ongoing user
dropped.
We must keep the coverage planed by the Radio Network
Planning.
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Admission control scenariosAdmission control scenarios
CAC is needed under such scenarios
New call
Handover
New RAB(s) for ongoing call
Bandwidth increasing reconfiguration
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How the CAC been implemented.
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Only for dedicated channels, and reserve a certain resource for the
common channels.
UL and DL CAC algorithms are independently.
In principle, a request will be admitted only when UL and DL are
both admitted. But if UL or DL CAC switch is closed, only one
direction CAC also can be realized.
StrategiesStrategies
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Handover threshold, conversational threshold and other
services threshold are used to keep the priority of the handover
higher than the conversational, conversational than the Best
Effort services.
UL CAC is not need for intra-frequency handover because the
interference is already included in the target cell.
StrategiesStrategies
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Admission request
Smaller than thethreshold?
admitted
y
end of UL CAC
rejected
n
Get current RTWP, and calculate thecurrent load factor. RTWP
PNUL 1
Get the traffic characteristic, andestimate the increment of load factor.
RNEW
b
0
1
1
Calculate the predicted load factor.
0_ ULpredictedUL
UL Algorithm1UL Algorithm1
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Admission request
Smaller than thethreshold?
admitted
y
end of DL CAC
rejected
n
Get current TCP. )(NP
Get the traffic characteristic, andestimate the increment of tcp. P
Calculate the predicted tcp. commPPNP )(
DL Algorithm1DL Algorithm1
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Reference service : 12.2k AMR with activity factor = 1.
Normalized traffic factor of user i :
dardsc
ici NE
NEk
tan0
0
Normalized traffic factor(NTF) has the meaning of equivalent nu
mber of user i relatives to the 12.2k AMR.
UL and DL may have different normalized traffic factor.
No resource is reserved for common channels.
Normalized traffic factorNormalized traffic factor
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Admission request
Smaller than thethreshold?
admitted
y
end of UL/DL CAC
rejected
n
Get current total NTF.
N
iikNK
1
)(
Get the traffic characteristic, andestimate the increment of NTF.
dardsc
NcN NE
NEk
tan0
101
Calculate the predicted NTF. 1)()1( NkNKNK
UL & DL Algorithm2UL & DL Algorithm2
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Selected by algorithm switch
Auto switch from algorithm 1 to algorithm 2 when algorithm1 is
selected for UL/DL.
When RTWP and/or TCP measurement value are/is
invalid/unavailable, the CAC will change from algorithm 1 to 2
automatically.
When measurement are/is valid/available, the CAC will
change back to algorithm 1 automatically.
UL and DL are independently.
Switch between algorithm 1 &2Switch between algorithm 1 &2
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Load control - LBLoad control - LB
Load Balance
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Load Balancing (LDB)Load Balancing (LDB)
What is LDB?
Why do we need LDB?
How is the LDB implemented?
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What is LDB?
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Load decrease, and cell expand.
Load increase, and cell shrink.
Original
Intra-frequency LDB ( Cell Breathing)Intra-frequency LDB ( Cell Breathing)
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f1
Load level indication
For cells have different frequencies, and the same coverage.
f2f1 & f2
Result
Inter-frequency LDBInter-frequency LDB
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Why we need LDB?
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Why we need LDB?Why we need LDB?
Intra-frequency LDB
Inter-frequency LDB
To move calls from a heavily loaded cell to neighboring
cells.
To relieve calls in surrounding cells when lightly loaded.
To distribute calls from a heavily loaded cell to the
lightest loaded carrier co-located.
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How the LDB been implemented.
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Measure transmittedcarrier power Ptx
Ptx < Pexpand
&PCPICH < PCPICH,max
Increase the PCPICH
one step
y
Wait for next period
Ptx > Pshrink
&PCPICH > PCPICH,min
n
Decrease the PCPICH
one step
y
n
Intra-frequency LDBIntra-frequency LDB
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Measure transmittedcarrier power
Ptx(m),m=1,2,...M
Get Ptx,max and Ptx,min fromPtx(m)
Ptx,max > Padjust
&Ptx,max - Ptx,min >
Pdifference
y
Initialize the hardhandover attempt counter
i=0
i < N?
Select one suitable userin the highest loaded
cell, and attempt to behandover to the lowest
loaded cell.
y
Successful?
i + 1
n
Wait for next period
y
n
Inter-frequency LDBInter-frequency LDB
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Compute the normalized traffic factor (NTF) for each of the UEs
belongs to the Maximum Transmitted Carrier Power carrier.
Rank the UEs by their NTF in descending order, and remove those
UEs whose NTF larger than a pre-set threshold. And mark all the
UEs in the list unselected.
Select the UE has the largest NTF and marked with unselected. At
the same time, mark it with selected.
W
RNE
W
RNE
ireference
referenceob
iiiob
)/(
)/(NTF
How to select user?How to select user?
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Load control - LCCLoad control - LCC
Congestion Control
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Why We Need Congestion Control ?Why We Need Congestion Control ?
To receive information correctly, signal in receiver
must be above certain S/I (Signal/Interference )
threshold.
In CDMA system, all users in a cell use the same
frequency, so one customer’s signal will be an
interference to others.
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Why We Need Congestion Control ?Why We Need Congestion Control ?
To overcome the interference, transmitter must
increase signal power, that is, it need more MS or
BTS power.
MS or BTS power is limited. If interference keeps
rising, S/I requirement can not be achieved at a
certain degree, quality of services will deteriorate
and some users will drop.
Congestion occurs.
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Why We Need Congestion Control ?Why We Need Congestion Control ?
To guarantee the network’s stability and custo
mers’ QoS (Quality of Service), we should try
our best to avoid congestion.
I.e., some actions should be taken to keep the
whole system away from congestion or quickly
recover from congestion!
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Congestion Control GeneralCongestion Control General
Abbr. LCC (Load Congestion Control)
Object: All BE (Best Effort) services.
Range: Downlink only. (Optimization is ongoing.)
Abstract: When congestion occurs, MAC limits the
valid max TF number, which pulls down the bandwidth
of all BE services, resulting in the reduction of load.
Once congestion relieves, MAC automatically recovers
the bandwidth of all BE services step by step slowly.
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Congestion Control DescriptionCongestion Control Description
Monitor Cell Carrier PowerPeriodically or by event
If Carrier Power >Congestion threshold, or
receive a event
Congestion Detectedand inform MAC
Congestion Relievedand infrom MAC
MAC decrease themax TF number
Start a Adjust Timer
expi res
Kill the Adjust Timer
MAC increase themax TF number
Start a Recover Timer
expi res
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Congestion Control DescriptionCongestion Control Description
LCC will monitor the cell’s downlink transmission carrier power
periodically or by event. When LCC finds the cell’s downlink load
rises above a certain threshold or receives a event claiming
congestion, it will inform MAC layer to limit the maximum
Transport Format(TF) of the BE service to a small value in order
to quickly pull down the bit-rate.
Congestion is not a long-term status, so after a certain moment,
congestion will relieve. Then MAC will recover the maximum TF
slowly. But if the first reduction of the maximum TF can’t control
congestion effectively, LCC will continue reduction until all BE
services’ bit-rate are reduced to a non-zero minimum value.
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Congestion Control ExampleCongestion Control Example
For a 144kbps background user: TB size is 336bits, max valid
TF number is 9.
When congestion is detected, MAC will compute: 9*50%≈4, so
the adjusted max valid TF number is 4. The bandwidth halves.
Note: 50% is a parameter named Adjust Coefficient.
MAC starts a Adjust Timer, and then computes at the timer’s
expiry: 4*50%≈2, so the adjusted max valid TF number is 2.
If MAC get informed that the congestion is relieved before the
Adjust Timer’s expiry, MAC begin the recovering process.
Otherwise, the user’s max valid TF number will be reduced to 1
finally.
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Congestion Control ExampleCongestion Control Example
When congestion relieves, MAC will be informed. So
MAC will begin recovering process.
Recovering Process: Assumed that the user’s max valid
TF number has been reduced to 1 during the congestion.
MAC first computes: 1+1=2. So the recovered max valid
TF number is 2. Note: 2 should be in the TFS which MAC
can use.
Then MAC starts a recovery timer, if it expires, MAC
increase the max valid TF number from 2 to 3……
Until the user’s max valid TF number is recovered to 9.
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