Mobile

Resource Management

Presentation slides prepared for the course

CSE 6065 – Advanced Mobile Communication

Adapted from the slides of Dr. Md. Mahfuzul Islam, Associate Professor, BUET from the course “Wireless Resource Management” 1

Topics to discuss

 Wireless Resource  Resource Reservation  Resource Allocation and call admission Control 2

Resource Limitation in Wireless Media

Bandwidth in wired media 600 Mbps for 1 ATM line …………….

600 Mbps for 1 ATM line Bandwidth in wireless media

384 Kbps for cell radius 10 km ( 2G / 2.4G) / 2 Mbps for cell radius < 100 m (Pico-cell, 3G)

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Mobility in Wireless Communications

  When an active mobile moves from one cell to another ( handoff call ), then resources must be available that cell to continue the call, otherwise the call will dropped .

at From the users perspective,

“Dropping an ongoing call is more undesirable to the user than blocking a new call.” Reserving resources in neighbouring cells is the only solution to this problem.

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Channel Assignment Strategies

   For efficient spectrum utilization, frequency reuse scheme should be consistent with objectives   Increasing system capacity Minimizing interference Strategies can be classified as Fixed and Dynamic In Fixed Channel Assignment Strategy,  Each cell is allocated a predetermined set of voice channels.

  A call attempt can only be served if unused channel in that particular cell is available If all channels are occupied then the call is blocked 5



Channel Assignment Strategies (contd.)

 Several variation exist like borrowing strategy  A cell is allowed to borrow a channel from neighboring cell if all of its channels are occupied  A mobile switching center (MSC) supervises such procedures and ensures that borrowing of channel does not disrupt or interfere with any of the calls in progress in the donor cell Dynamic Channel Assignment Strategy    Voice channels are not allocated to cells permanently On each call request, the BS requests a channel from MSC.

MSC allocates a channel by taking into account   the likelihood of future blocking within the cell The frequency of use of the candidate channel , reuse distance 6

Channel Assignment Strategies (contd.)

   Hence, MSC only allocates a channel if that is not presently in use in the cell which falls within minimum restricted distance of frequency reuse .

It reduces the likelihood of the call blocking, increasing the trunking capacity of the system.

It requires MSC to collect real-time data on channel occupancy, traffic distribution and RSSI of all channels   This increases storage and computational load on the system But provides increased channel utilization and decreased call blocking 7

Handoff Strategies

   Handoff : a mobile user moves to a different cell while conversation is in progress, MSC transfers the call to a new BS.

  Identifying new BS New voice and control channels to be allocated Handoff must be performed    Successfully Infrequently Imperceptible To achieve this, designer must specify optimum signal level at which handoff initiates 8

Handoff Strategies (contd.)

  Once, a signal level is specified as minimum usable for acceptable voice quality   A slightly stronger signal level is used as threshold Normally taken between -90dBm and -100 dBm.

This margin ∆ = Pr_handoff – Pr_min , can not be too large or too small    If ∆ is too large, unnecessary handoffs, burden on MSC If ∆ is too small, insufficient time to complete a handoff before a call is lost due to weak signal ∆ should be chosen carefully to meet conflicting requirements 9

Handoff Strategies (contd.)

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Handoff Strategies (contd.)

  Call drops    Excessive delay by MSC due to high traffic load ∆ is set too small for handoff time No channels are available on any of nearby BS When to handoff ,  Drop in signal level is not due to momentary fading    Mobile is actually moving away from serving BS To ensure this,  BS monitors the signal level for certain period of time  The period depends on the vehicle speed If slope of average received signal level is steep, handoff is made quickly 11

Handoff Strategies (contd.)

 In 1G, signal level was measured by BS and supervised by MSC  Each BS constantly monitors the signal strength of all its reverse channels to determine relative location of each mobile user   In addition, the locator receiver (a spare receiver) is used to scan and measure RSSI of mobile users in neighboring cells and reports to MSC Based on these measurements, MSC decides if handoff is necessary 12

Mobile assisted handoff (MAHO)

 In 2G, handoff decisions are mobile assisted       Each mobile measures RSSI of all surrounding BS Reports to serving BS Handoff is initiated if power of serving BS is lesser than nearby BS by a certain level or for a certain period of time Enables calls to be handed over between Base Stations at much faster rate than in 1G MSC no longer constantly monitors RSSI.

More suitable for microcellular where HO is frequent 13

Handoff Strategies (contd.)

  Intersystem handoff  If a mobile moves from one cellular system to a different system controlled by a different MSC  Issues to be addressed  A local call becomes a long-distance call (roaming)  Compatibility between two MSC must be determined  Different systems have different policies and methods for managing handoff requests Prioritizing handoff  Call termination in middle of conversation is more annoying than being blocked on a new call attempt 14

Prioritizing Handoffs



Two methods of handoff prioritizing



Guard channel concept

   A fraction of available channels is reserved exclusively for handoff requests Has disadvantage of reducing total carried traffic Offers efficient spectrum utilization when dynamic channel assignment strategies by minimizing number of required guard channels 15

Guard Channel Schemes

Let C be total capacity of the cell and N number less than C .

be any  New call originates only when available Bandwidth is more than the threshold C-N .

 If available Bandwidth is less than C-N , then only priority calls (Handoff Calls) are permitted.

Hong et al, 1986

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Problems of Guard Channel Schemes

 

Poor Channel Utilization.

Call Blocking Rate (CBP) increases.

Advantage Call Dropping Rate (CDP) decreases.

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Prioritizing Handoffs (contd.)

 Queuing of handoff requests      Possible due to time interval elapsed when the signal level drops below to threshold until minimum signal level Decrease probability of forced termination due to lack of available channels Tradeoff between decrease in probability of forced termination and total traffic The delay time and queue size is determined from traffic pattern Queuing does not guarantee zero probability of call termination since large delays will signal level to drop min 18

Handoff Queue Method

 When resources are available , one of the calls in the handoff queue is served.

 If there is no available resource , call requests are being queued.

Gaasvik at el, 1991

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Problems of Handoff Queue Method

 Need Huge buffer for real time multimedia  Sophisticated Scheduling mechanism is needed for delay sensitive transmission-

Shouldn’t expire before transmission.

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Practical handoffs consideration

 Several problems arise to design a system for wide range of mobile velocities  High speed vehicles pass through a cell in a matter of seconds  With micro cells addition, the MSC can quickly become burdened   Pedestrian users may never need a handoff during a call Issues   Schemes to handle high speed and low speed users simultaneously Ability to obtain new cell sites 21

Practical handoffs consideration (contd.)

    Additional capacity is provided through addition of new cell sites, Difficult to obtain new cell sites Install additional channels and BS at same location of an existing cell By using different antenna heights and power levels, possible to provide large and small cells, which are co-located at single location called umbrella cell  Provide large coverage area to high speed users minimizing number of handoffs   Small coverage to slow speed users Speed can be estimated by BS or MSC by RSSI 22

Practical handoffs consideration (contd.)

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Some Existing Resource Reservation Schemes

     

Non-directional schemes reserve resources in all directions



Directional Schemes - Shadow Cluster Concept (Levine at el, 1997) - Predictive Mobility Support for QoS Provisioning (Aljadhai and Znati, 2001).



Non-directional Schemes - Guard Channel (Hong at el, 1986) - Hand off queue (Gaasvik at el, 1991) - Dynamic methods for QoS priority (Huan at el, 2002) - Borrowing Scheme (Kadi at el, 2002) - Implicit QoS Provisioning (Ganguly at el, 2002) Directional schemes reserve resources only in the direction of Mobility

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Resource (Bandwidth) Management

 Calculating

Cell Visiting Probability

(CVP) B A   Estimation of amount of resources to be reserved and time window for which reservation is required.

Bandwidth Allocation and

Call Admission Control

(CAC) 6,7 6 M. El-Kadi, S. Olariu and H. Wahab, “A Rate-based borrowing scheme for QoS Provisioning in Multimedia Wireless Communications”,

IEEE transactions on parallel and distributed systems

, vol. 13, no. 2, pp. 156-166, Feb 2002. 7 A. Malla, M. El-Kadi, S. Olariu and P. Todorova, “A Fair Resource Allocation Protocol for Multimedia Wireless Networks”,

transactions on parallel and distributed systems

, vol. 14, no. 1, pp. 63-71, Jan 2003.

IEEE

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Call Admission Control

 A New Call originates only when the required bandwidth is available.

 When a user moves from one cell to another ( Handoff Call ), there must have Bandwidth available at that cell, otherwise Handoff Call terminates  Handoff Calls should have more priority that New Calls.

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Some popular Call Admission Control and Bandwidth Allocation Strategies

 Guard Channel Schemes (Hong at el, 1986)  Handoff Queue Method (Gaasvik at el, 1991)  Rate based Borrowing Schemes (Kadi at el, 2002)  Max-min Fairness Scheme (Malla at el, 2003)  Min-Max Fairness Scheme (Islam & Murshed, 2006) 27

Rate Based Borrowing Scheme

No BW Minimum BW Avg/Expected BW Equal Share Max/Desired BW Equal share  ABB  ABB (Actual Borrowable BW) BLT (BW Loss Tolerance)

New Call Admission: 1.

2.

If Requested BW > free BW then borrowing level L increases to L+1 . if already L=



, the call is blocked .

If borrowing 1 level is not sufficient, then the call is blocked .

Handoff Call Class I call ( real-time multimedia traffic ) Class II call ( non real time data traffic )

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Rate Based Borrowing Scheme Cont…

Class I handoff Call Admission 1.

2.

If free BW + reserved BW > = min BW , then call is accepted with free BW + reserved BW.

If call is not accepted with free BW + reserved BW then it tries to accept the call with min BW borrowing one share from each existing call.

Class II handoff Call Admission 1.

2.

If free BW > 0, then call is accepted with free BW.

If call is not accepted with free BW then it tries to accept the call borrowing one share from each existing call.

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Criticisms of Rate Based Borrowing Scheme Cont… Problem 1 : (Example) There are four calls currently in the cell A-60 (60-10), B-10 (10-10), C-5 (5-4) and D-4 (4-1) and free 1 Mbps

When a new call E (10-5) arrives

A(60M-10M) B(10M-10M) C(5M-4M) D(4M-1M) E(10M-5M) 60 3.285

56.715

10 0 10 5 0.788

4.212

4 2.956

1.044

10 1.971

8.029

-9 unit shortage Amt borrowed Final BW Problem 2: The scheme tries to ensure average/Expected BW existing calls which is the violates the definition of average .

for new call and If we always ensure average, then average will go up.

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Max Min Fairness Scheme

(a) (c) (b) (d) 31

Max Min Fairness Scheme Cont..

Starting equal share  total BW No.

of Connection s Admission condition for new Call  Expected BW <= equal Share  Expected BW <= free BW + ABB Admission condition for handoff Class I Call  Expected BW <= free BW + ABB+ reserved BW Admission condition for handoff Class I Call  Free BW + ABB > 0 32

Criticism of Max Min Fairness Scheme

Problem 1 : (Example) There are four calls currently in the cell A-10 (10-6), B-10 (10-10), C-5 (5-4) and D-4 (4-1)

(a) When free BW = 1 and a new call E (6-4) arrives

A(10-6) 6 7.5

B(10-10) 6 7.5

C(5-4) 6(1) 5 D(4-1) 6(2) 4 E(6-4) 6 6 3

(b) When free BW = 11 and a new call E (11-4) arrives if the call has expected BW = 9, Call is rejected since avg (40/8) < 9, although only free BW can be used to accept it.

Unfairness due to ensuring average is also still present in this scheme 33

Max Min Fairness Scheme

A(10-6) 6 7.25

7.333

B(10-10) 10 10 10 C(5-4) 4 5.25 (0.25) 5 D(4-1) 1 2.25

2.333

E(6-4) 4 5.25

5.333

5 unit extra 0.25 unit extra  It is assumed that Minimum BW for all class II calls are zero .

 There is no difference between class II new call and handoff call .

 Class II uses unutilized Reserve BW since we can borrow total bandwidth allocated for class II when needed.  Reserved BW used by class II call is never given to new class I call .

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Min-Max Fairness Scheme

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Thanks

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