crn - assisted multicasting-2011!12!20

8/3/2019 CRN - Assisted Multicasting-2011!12!20

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IOWA STATE UNIVERSITY LABORATORY FOR ADVANCED NETWORKS (LAN)

Efficient Multicasting in CognitiveRadio Networks Using Cooperation

and Channel Diversity

Ahmed E. Kamal

Dept. of Electrical & Computer EngineeringIowa State University


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IOWA STATE UNIVERSITY LABORATORY FOR ADVANCED NETWORKS (LAN) 22

Outline

Cognitive radio networks (CRN): Why?

What?

Multicasting problem in CRNs

Receiver assisted multicasting (RAM) for CR-WMNs: The single cell case

The multiple cell case (collision free schedules)

Summary and conclusions


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Motivation Behind Opportunistic Spectrum Access Initiative

The current spectrum allocationpolicy leads to:

Spectrum crowdedness

Spectrum underutilization

Opportunistic spectrum access(OSA)

Secondary users (SUs) getopportunistic access to idleunlicensed channels.

Secondary users must not causeharmful interference to licensedusers (Primary Users, or PUs)


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Opportunistic Spectrum Access (OSA)

OSA requires that SUs:

Monitor spectrum activity

Switch between frequency

channels, probably at a fine

granularity, as needed.

Software Defined Radios is the enabling

technology ofOSA.

Software defined radios are able to change

their transmission/reception parameters like:

Operating frequency

Transmission power

Modulation scheme

Cognitive Radio is a software defined radio radio that can change its operationalparameters in response to changes in the surrounding wireless environment.

[1] Cognitive Radio: An Integrated Agent Architecture for Software Defined Radio. Dissertation.

Doctor of Technology. Joseph Mitola III. Royal Institute of Technology (KTH), May2000

.


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A Cognitive Radio Network (CRN)

Is a wireless network that relies onO

SA for its operation. Research directions in CRN:

Sensing, and identifying idle spectrum channels.

Spectrum sharing and management

Networking protocols specific to CRNs.

After identifying idle spectrum channels, a CRN is amultichannel wireless networks, with different channelavailabilities at different locations, and different times

However, there are some major difference between CRNs andtraditional multichannel wireless networks.


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Assisted Multicast Scheduling


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Network Model: Cognitive Radio Wireless Mesh Network (CR-WMN)

CR operates as a Wireless Mesh Network

(WMN)

The CR-WMN consists of multiple meshrouters (MR):

Each MR manages a groups of mesh clients (MC).

MR and its clients MC form a cell

MRs in adjacent cells communicate The CR-WMN coexists with a primary

network, using one or more licensed channels

The CR-WMN opportunistically exploits theset of licensed channels, where the availability

of each of which is determined by a SpectrumSensing and Management Entity (SSME).

A gateway, or more, exits which connects theCR-WMN to the Internet.


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The Multicast Problem

We first consider the multicast scheduling problem within asingle cell of a CR-WMN

We then extend this to guarantee collision free scheduling acrossmultiple cells.

Mmulticast sessions exist in the cell, all of which originate from

the MR.

The network opportunistically utilizes a set Lof orthogonal

channels, with K=|L|

The nodes share a control channel or any of the alternativesproposed in literature for control information exchange.


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Multicasting in CRNs: Broadcast Deformation

Channel heterogeneity problem in CRNs

Broadcasting using a single transmission may not be possible inCRNs.

In the example below, SU A needs two transmissions to broadcast

the same data to the other three SUs.


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Proposed Solution

Cooperative multicasting: MCs which receive packets, assist MR in delivering packet to other

receivers (i.e., assisted multicast):

Within the same group (intra-group assistance)

Between different groups and using overhearing (inter-group assistance)

Network coding & packet overhearing (between multicast groups)


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Unassisted Multicast

- ato white nodes

- bto black nodes

Non-cooperativemulticast requires sixtime slots in this case.


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Assisted Multicast Within The Same Group (Intra-Group)


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Assisted Multicast across Multiple Groups (Inter-Group)


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Network Coding Assisted Multicast (Codeword Exchange)


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Assisted Multicast Scheduling Problem

The unassisted multicast scheduling problem (UMS) is similar to AMSexcept that no assistance or codeword exchange operations areallowed.

The UMS and AMS problems are NP-hard (even for a single group): By reduction from set cover problem

Two ILPs were introduced for the UMS and AMS problems with a

single multicast group.


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ILP of the AMS-Single Cell, Single Group Problem

Input: a single multicast group, channel availability at each member,connectivity information between members.

Objective: minimize the multicast period.

Constraints:

At most one transmission per node per time slot.

At most one transmission per channel per time slot.

A node can either receive or transmit in any time slot.

A node (except the MR) cannot transmit a packet at time t unless it has receivedit in [0,t-1].

Output:A multicast schedule in the form of:(node, codeword, channel, slot)

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UMS formulation is straight forward (set cover)

AMS-Multihop is too complicated (mainly because of the coding operating)


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Heuristic Assisted-Multicast Scheduling (HAMS) Algorithm

HAMS deals, iteratively, with each slot independently andtries to optimize the schedule within that slot.

Within each slot:

1. Scheduling the MR transmission (channel and codeword):

Select the codeword that covers maximum number of uncovered MCs.2. Scheduling the assistance operation (channel and codeword):

Select the (channel, codeword) pair that covers maximum number ofuncovered neighbors.

3. Scheduling overhearing opportunities:

The codeword that is useful to maximum number of uncovered neighborsis overheard.


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Performance Evaluation

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Gain of Different Assistance Levels

Gain = 100*(Unassisted Assisted )/Unassisted


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HAMS Performance (Single-group)

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Effect of Channel Availability

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The Multiple Cell Case:Collision Avoidance


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Collision Free Scheduling across Cells

How to make sure that the schedule in one cell of the mesh network

has no conflict (collision) with the schedule of any of the adjacentcells?

Reactive: calculate the schedules and then resolve collisions Pros:

Each cell can make full use of the available channels, obtaining higher gainfrom the assistance mechanism.

Cons: Requires a collision resolution mechanism, and difficult to implement.

Proactive: proactively build collision free schedules. Pros:

Easier to implement, and requires no post-scheduling collision resolution.

Cons: A Cell may not be able to utilize the full set of available channels, and will

thus have a smaller assistance gain.


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Collision Free Scheduling across Cells: Reactive Approach

Calculate the schedules of all cells.Align the schedules such that there are no collisions

and the time span (time to schedule all cells) is

minimized (ILP formulation).

The alignment considers the following properties: Precedence property: an MC cannot transmit a codeword unless

it has already received it (or a combination that can produce it)

through an earlier transmission(s). Conflict property:

One transmission per node per slot.

One transmission per channel.

A node can either a transmitter or a receiver in a give time slot.


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Collision Free Scheduling across Cells: Proactive Approach

MRs exchange requests to start scheduling.

Priority mechanism is used to break ties in case ofconcurrent requests.

When the MR receives acknowledgments from all of its

neighboring MRs, it calculates its schedule and sends it to itsneighbors.

MRs will avoid using a particular channel, in a particularslots, if it is used in a neighboring cell at the same time.


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Proactive vs. Reactive


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Summary and conclusions

We proposed an assistance paradigm that relies on receivernodes to forward the multicast data to other receivers thathave not yet received their own data.

Network coding was also proposed as another assistance

technique that further reduced the total multicast period. Cooperative networking solutions are a must for networks

with heterogeneous and unguaranteed resources like CRNs.

Any networking solution with no mechanism to recover

from channel failures, caused by licensed users activity, isan incomplete solution in the case of CRNs.


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Acknowledgements

Dr. Hisham Almasaeid (currently with Amazon)

Financial support by the National Science Foundation, USA

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Thanks..Questions?


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Cognitive Radio Networks vs. TraditionalMultichannel Wireless Networks

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Heterogeneous Channel Availability, Intermittent Connectivity

Different SUs may observe

different sets of availablechannels depending on theactivity of primary, licensed,users (PUs) in their vicinity.

Channel availability can changespatially and temporally.

This is referred to as the channelavailability heterogeneity property.

Intermittent connectivity, evenfor stationary SUs.

Harmful


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Dynamic Topology

In traditional networks,

network topology is stable forstationary nodes. This may notbe the case in CRNs.

Rerouting can induce significant

overhead.


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Broadcast Deformation

Broadcasting using a single transmission may not be possible in

CRNs.

In the example below, SU A needs two transmissions to broadcast

the same data to the other three SUs.


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Heterogeneous Transmission Range

In some proposals, SUs can optimize their transmission powers to

make the most use of spectrum resources.

This leads to heterogeneity in the transmission range across different channelsfor the same SU.


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Communication Coordination

How to coordinate the communication and medium access

between SUs?

Common Control Channel

May not always exist

Subject to congestion

Source of system vulnerability, attacking this channel can cripple the network.


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Switching Latency

Cognitive radio nodes might have to switch between widely separated

channels. Channel switching latency becomes a key concern.

Cannot be ignored while making routing decisions.


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Failure Recovery

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Recovery From Failures

There is a need for a recovery mechanism to fix theschedule in case of any interruptions caused by PU activity.

If an MR transmission fails, it can retry it in the next timeframe or add extra slots to the time frame (on available

channels) How to recover MC transmissions?

Assume an MC zis supposed to transmitp1+p2 at slot t, giventhat it has receivedp1 at t-1 from xandp2 at t-2fromy. If either

of the transmissions scheduled at t-1 and t-2fails, the transmissionat twill be delayed.


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Queueing System at MCs

Input Queue (IQ): holds received codewords, indexed by frame ID.

Availability Queue (AQ): a virtual queue holding codewords which can bedecoded from the ones in IQ. This queue is indexed by frame ID.

Delayed Queue (DQ): a virtual queue holding all scheduled codewords which arenot yet in AQ. Indexed by slot ID.

Output Queue (OQ): holds scheduled codewords which are in AQ, but not yet

transmitted due to channel unavailability.


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Recovery Scheduling at the MR

MCs inform the MR about the size of their OQs.

Whenever there are some backlogged MCs, the MRcalculates a recovery schedule to run in the next frame.

The MR processes the backlogged MCs iteratively, giving

priority to the one with the maximum OQ size. It assigns recovery slots, if possible, for the selected transmission,

and moves to the next MC.

If no recovery slots are found, nothing is done. If the packet delay

exceeds a predetermined threshold, the MR recalculates the wholeschedule.


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Example: original schedule


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Example: recovery behavior

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