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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 ?. U niversal M obile T elecommunications S ystems

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call admission control schemes in umts

Call Admission Control Schemes in UMTS

Kamala Subramaniam


Dr. Arne A. Nilsson

  • Overview of UMTS
  • Rationale behind CAC schemes
  • Prevalent CAC Schemes
  • Conclusions
what is umts
What is UMTS?
  • Universal Mobile Telecommunications Systems
  • Member if the 3G (3rdGeneration) 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 q uality o f s ervice qos classes
UMTS Quality of Service (QoS) Classes

3GPP (3rdGeneration 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 c all a dmission c ontrol cac schemes in umts
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 Rates



  • 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.


  • Overview of UMTS
  • Rationale behind CAC schemes
  • Prevalent CAC Schemes
  • Conclusions
rationale behind cac schemes
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
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.




  • Overview of UMTS
  • Rationale behind CAC schemes
  • Prevalent CAC Schemes
  • Conclusions
cac schemes
CAC Schemes
  • Capacity Based Schemes
  • Mobility Based Schemes
  • Interference Based Schemes
  • Adaptive Call Admission Control (ACAC)
capacity based schemes
Capacity Based Schemes
  • Fixed Guard Channel / Cutoff Priority Scheme.

C = CA + CH;

C: Total Number of Channels

CA: 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 calls

DN = number of rejected calls

DH = 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 admission

Otherwise, DN = DN + 1, and reject}

If a call is completed or handoff-ed to another cell

{PA = PA – 1}

results fixed guard scheme policy
Results: Fixed Guard Scheme policy

Blocking and Dropping Probabilities with no Guard Channels implemented

Blocking and Dropping Probabilities with 25% Guard Channels

results fixed guard scheme policy14
Results: Fixed Guard Scheme policy

Blocking Probabilities Vs Guard Channels

capacity based schemes15
Capacity Based Schemes
  • 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 measurements

H: handoff calls into cells (both rejected and admitted)

DH: number of rejected handoff calls in the past τ seconds

TH: threshold for handoff call dropping probability

If a handoff call is dropped and

DH/H ≥ αuTH then

CH = min {CH + !, Cmax},

where αu is the threshold chosen as, e.g. 0.9.

If DH/H <= αdTH for N consecutive handoff calls, then

CH = 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 schemes16
Capacity Based Schemes
  • Fractional Guard Channel Policy

New calls accepted with probability = βi

Handoff 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:

results fractional guard channel policy
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
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:
mobility based schemes
Mobility Based Schemes
  • Integral MBCR


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

where RjI is the integral part and RjF is the fractional part

mobility based schemes21
Mobility Based Schemes
  • New Call Bounding Scheme
  • Hybrid Scheme
results mobility based schemes
Results: Mobility Based Schemes

Handoff Call Blocking Probability New Call Blocking Probability

interference based schemes
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 schemes24
Interference Based Schemes
  • Wideband Power-Based Admission Control Strategy

uplinkadmission criterion: Itotal_old + I > Ithreshold

downlink admission criterion: Ptotal_old + Ptotal > Pthreshold

interference based schemes25
Interference Based Schemes
  • Throughput Based Admission Control Strategy

Uplink criterion: UL + L > UL_threshold

Downlink criterion: DL +L > DL_threshold

interference based schemes26
Interference Based Schemes
  • 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:

results interference based schemes
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.

a daptive c all admission c ontrol acac
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:
  • 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:
  • 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