Call Admission Control Schemes in UMTS

Kamala Subramaniam

Advisor

Dr. Arne A. Nilsson

Outline

Overview of UMTS

Rationale behind CAC schemes

Prevalent CAC Schemes

Conclusions

What is UMTS?

Universal Mobile Telecommunications Systems

Member if the 3G (3rd Generation) family

Developed by ETSI (European Telecommunications Standards Institute) within the ITU’s (International Telecommunication Union’s) IMT (International Mobile Telecommunications ) framework.

Why UMTS?

Today consumers use the Internet to access integrated services such as voice, data and multimedia.

Next logical step is to provide the same services with the added feature of mobility

UMTS provides data up to 2Mbps making portable videophones a reality

UMTS Quality of Service (QoS) Classes3GPP (3rd Generation Partnership Project) defines four classes for UMTS Conversation Class: Delay Constrained / Connection Oriented/ Constant Bit Rate Streaming Class: Delay Constrained / Connection Oriented / Variable Bit rate Interactive Class: Longer Delay Constraints / Connectionless Background Class: Best Effort Connectionless Services

Importance of Call Admission Control (CAC) Schemes in UMTS Need to admit calls selectively into the system minimizing call

dropping and call blocking Must make efficient use of Network’s Resources Must guarantee QoS. Typical QoS parameters maybe: Blocking Probabilities

Transmission RatesDelayReliability

Need to multiplex a non-homogeneous mix of traffic within a limited set of resources and various propagation characteristics.

Need to incorporate mobility complications and handoff procedures.

OutlineOutline

Overview of UMTS

Rationale behind CAC schemes

Prevalent CAC Schemes

Conclusions

Rationale behind CAC schemes

A UMTS network showing cellular architecture, where each cell is served by the Node-B and the Radio Network Controller (RNC) serving a bunch of Node-B’s

CAC Terminology

New Call: When a mobile user wants to communicate to another, the Mobile Terminal (MT) obtains a new channel from the Base Station (BS) it hears best. If a channel is available, the BS grants it and a new call originates

New Call Blocking Probability (or simply blocking probability): If all channels are busy, the MT is not granted the channel and the call is blocked.

Handoff Call: The procedure of moving between cells when a call is in progress is called a “handoff”. During handoff the MT requests resources from the BS in the cell it is moving to.

Handoff Call Dropping Probability (or simply dropping probability): When the MT is denied a channel in the cell it is moving to, the call is dropped.

Priority: Forced termination of a call in progress is more annoying than blocking of a new calling attempt from the users point of view. Clearly, handoff calls must be given a higher priority.

Cell Dwell Time: After entering a cell, the time a MT resides in it.

OutlineOutlineOutlineOutline

Overview of UMTS

Rationale behind CAC schemes

Prevalent CAC Schemes

Conclusions

CAC Schemes

Capacity Based Schemes

Mobility Based Schemes

Interference Based Schemes

Adaptive Call Admission Control (ACAC)

Capacity Based Schemes1. Fixed Guard Channel / Cutoff Priority Scheme.

C = CA + CH;C: Total Number of ChannelsCA: Channels allocated to handle admitted calls (handoff and new)CH: Guard channels allocated to handle handoff calls

New Call Admitted: if total number of calls (handoff and new) < CA

Handoff Call Admitted: if CA + CH < C

PA= number of on-going callsDN = number of rejected callsDH = number of rejected handoff calls

If handoff call request{ If PA < C, PA = PA + 1, and grant admission Otherwise, DH = DH + 1, and reject}If new call request{ If PA < C, then PA = PA + 1, and grant admissionOtherwise, DN = DN + 1, and reject}If a call is completed or handoff-ed to another cell{PA = PA – 1}

Results: Fixed Guard Scheme policy

Blocking and Dropping Probabilities with no Guard Channels implemented

Blocking and Dropping Probabilities with 25% Guard Channels

Hand off Blocking Probability

0

0.2

0.4

0.6

0.8

1

1.2

time (sec)

Call Blocking Probability

0

0.1

0.2

0.3

0.4

0.5

0.6

time (sec)

Hand off Blocking Probability

0

0.2

0.4

0.6

0.8

1

time (sec)

Call Blocking Probability

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

time (sec)

Results: Fixed Guard Scheme policy

Phb and Pnb vs # Guard channels

0

0.5

1

1.5

0 20 40 60

# Guard channels

Blo

ckin

g

Pro

bab

ility

: P

hb

an

d P

nb Handoff Blocking

Probability

New Call BlockingProbability

Blocking Probabilities Vs Guard Channels

Capacity Based Schemes2. Adaptive Fixed Guard Channel Scheme.

Dropping rate Increases, increase number of guard channels Keep Dropping rate below Threshold at all times

τ: Time period for updating measurementsH: handoff calls into cells (both rejected and admitted)DH: number of rejected handoff calls in the past τ secondsTH: threshold for handoff call dropping probability

If a handoff call is dropped andDH/H ≥ αuTH thenCH = min {CH + !, Cmax},where αu is the threshold chosen as, e.g. 0.9.

If DH/H <= αdTH for N consecutive handoff calls, thenCH = max {CH – 1, Cmin}, where αd is another threshold chosen as e.g., 0.6 and N is an integer

chosen as e.g.,10.

Capacity Based Schemes3. Fractional Guard Channel Policy

New calls accepted with probability = βiHandoff Calls accepted with probability = 1

where i is the state of the system

“Hot” Vs “Cold”

Define threshold h >0, e.g., 0.2,0.25 and 0.3 dc » h, “cold cell”: lots of available channels, βi = 1

number of available channels for new calls = (n - g) – i i » H, “hot cell”: lower resources, βi = 0

where i : state of the system g: number of guard channels

n: total number of channels H = (1 – h) n - g

New Call Acceptance Probability:

in total channels ofnumber

channels available ofnumber

cd

g-n i 0

g-niH )/)(85.0(

Hi 1

kniHei

Results: Fractional Guard Channel Policy

Blocking Probability of new calls as a function Dropping Probability of handoff calls of the offered traffic load as a function of the

offered traffic load

Rationale: Mobility Based Schemes

Users of two types: Low Speed (Pedestrian) users and High Speed (Vehicular) users

Cell Dwell Times = F (elapsed time in cell, velocity class)

Pr (call will request a handoff sometime after T) = Lh (t,T) for high-speed ; Ll (t,T) for low-speed Directional Factor: ; Ni is the set of neighboring cells to cell i

Influence curves:

Total Influence that all ongoing calls exert on cell j:

At time T, cell j needs to reserve:

call speed low afor ),(

call speedhigh afor ),(),,,(

TtL

TtLTtjiI

lij

hij

||

1

iij N

Sk

kijij TtLI ),(

jNi

ijj RR

Mobility Based Schemes1. Integral MBCR

Variations Conservative: Ceiling value of Rj; may waste resources Aggressive: Floor value of Rj; may increase dropping rate .

2. Fractional MBCR

where RjI is the integral part and Rj

F is the fractional part

newBjRCusedB

newBjRCusedB

newP ~ 0

~ 1

B - R - C B 0

B - R - C B 1

1 - B - R - C B 1

newI

jused

newI

jused

newI

jused

Fjnew RP

Mobility Based Schemes

3. New Call Bounding Scheme

3. Hybrid Scheme

otherwise 0

& 1 newBCused

Bbnd

Nusednew

BnewP

otherwise 0bnd

N usednew

B & newB - j

R~

- C usednew

B 1newP

Results: Mobility Based Schemes

Handoff Call Blocking Probability New Call Blocking Probability

Interference Based Schemes

Admit user into system only if Interference threshold not passed CAC scheme: guarantee dropping probability below threshold at high offered

loads.

Interference Based Schemes1. Wideband Power-Based Admission Control Strategy

uplink admission criterion: Itotal_old + I > Ithreshold

downlink admission criterion: Ptotal_old + Ptotal > Pthreshold curve load the of DerivationL

II

Ul

total

1

Density spectral Noise /bit perenergy Signal /NE

factor activity voice υ user, Jth of rate Bit R Rate, Chip W

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ij

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1).1(

1

N

j

iiob

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RNEW

i

RNE

WL

ob ./.1

1

Interference Based Schemes2. Throughput Based Admission Control Strategy

Uplink criterion: UL + L > UL_threshold

Downlink criterion: DL +L > DL_threshold

Density spectral Noise /bit perenergy Signal /NE

factor activity voice υ user, Jth of rate Bit R Rate, Chip W

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ij

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NER

Interference Based Schemes3. CAC Based on Signal to Noise Interference Ratio

uplink algorithm:

M-1 users in system, Mth user requesting access, minimum required power for new user is:

downlink algorithm:

power with which the ith user channel is received at the ith MT:

estimation of needed received power for Mth MT:

i.user by the dexperience nceinterefere : C - I I Gain; Processing : PG

)(/)(

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Results: Interference Based Schemes

Power-based CAC, downlink, homogeneous traffic distribution: offered traffic vs. accepted traffic and

maximum dropping probability for different values of the ratio Pthr/Pmax.

Interference-based CAC, uplink, homogeneous traffic distribution: offered traffic vs. accepted traffic and maximum dropping probability for different values of the threshold level.

Adaptive Call Admission Control (ACAC) Limit on acceptable interference threshold ↔ number of users of each service class in local

and neighboring cells

Obtain tradeoff between the number of voice and data users according to outage/blocking probability.

Outage Probability: P[C ≥ W] = δ

Acceptable Interference level:

Total interference plus noise power received at the BS:

L)1,....., (k .

.

oI

k)

b(E

k

kk RI

WS

L

kooutkk WnSSNI

1

ACAC

Constraint on the number of users:

where η = upper bound on the total received interference (0.1 < η < 0.25)

Bandwidth utilized by a user of class k:

kcco N

ikckckic

L

kc

L

kkokoko WRRN

11

6

11

)1(

kcco N

ikckckic

L

k

L

k ckokoko RRNC

111

6

1

Conclusions

Summarized UMTS CAC schemes from open literature

CAC schemes classified as capacity based, interference based, mobility based and adaptive

CAC schemes efficiently utilize system resources in order to:

Guarantee QoS

Minimize Blocking/Dropping Probabilities

Minimize Interference

Provide priority to Handoff Calls

Handle Mobility

Adaptive CAC’s which may be a combination of the above CAC’s are best for a system design