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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 l.jpg

Call Admission Control Schemes in UMTS

Kamala Subramaniam


Dr. Arne A. Nilsson

Outline l.jpg

  • Overview of UMTS

  • Rationale behind CAC schemes

  • Prevalent CAC Schemes

  • Conclusions

What is umts l.jpg
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 l.jpg

  • 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 l.jpg
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

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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.

Outline7 l.jpg


  • Overview of UMTS

  • Rationale behind CAC schemes

  • Prevalent CAC Schemes

  • Conclusions

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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

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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.

Outline10 l.jpg




  • Overview of UMTS

  • Rationale behind CAC schemes

  • Prevalent CAC Schemes

  • Conclusions

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CAC Schemes

  • Capacity Based Schemes

  • Mobility Based Schemes

  • Interference Based Schemes

  • Adaptive Call Admission Control (ACAC)

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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}

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Results: Fixed Guard Scheme policy

Blocking and Dropping Probabilities with no Guard Channels implemented

Blocking and Dropping Probabilities with 25% Guard Channels

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Results: Fixed Guard Scheme policy

Blocking Probabilities Vs Guard Channels

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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.

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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

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“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 l.jpg
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

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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:

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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

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Mobility Based Schemes

  • New Call Bounding Scheme

  • Hybrid Scheme

Results mobility based schemes l.jpg
Results: Mobility Based Schemes

Handoff Call Blocking Probability New Call Blocking Probability

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Interference Based Schemes

  • Admit user into system only if Interference threshold not passed

  • CAC scheme: guarantee dropping probability below threshold at high offered loads.

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Interference Based Schemes

  • Wideband Power-Based Admission Control Strategy

    uplinkadmission criterion: Itotal_old + I > Ithreshold

    downlink admission criterion: Ptotal_old + Ptotal > Pthreshold

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Interference Based Schemes

  • Throughput Based Admission Control Strategy

    Uplink criterion: UL + L > UL_threshold

    Downlink criterion: DL +L > DL_threshold

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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 l.jpg
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.

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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:

Slide29 l.jpg

  • 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:

Conclusions l.jpg

  • 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