Authors:Mishra Vishram,

Lau Chiew Tong, and

Chan Syin

Presented By:Iffat Anjum

Date: 20-11-14

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The recent outburst of high definition content delivery over wireless network

exist a paradox between the shortage of the frequency spectrum and its underutilization

Cognitive radio based dynamic spectrum access technique has been proposed to alleviate such paradox by opportunistically utilizing the spectral holes in the channels

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In MAC protocols for CRN

The idea is to develop a mechanism sharing the media (channel) such that:

interference is minimized and

CR nodes can communicate with each other effectively.

MAC scheme requires exchange of control signals

Over a common signaling channel

control channel saturation problem, can create a bottleneck for the communication

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hopping based control channel less protocol can be used

which utilizes rendezvous channel as a common channel for information exchange.

But, it requires a tight synchronization among all nodes

ensuring a grade of QoS

traffic from different application need to be prioritized in order to mitigate the detrimental effects of uniform resource allocation.

service latency, signal-to-noise ratio, response time, bandwidth etc, should be maintained too.

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In DC-MAC

a control channel less, non-channel-hopping MAC protocol for single-hop cognitive radio network

distributes the contention for channel access onto respective channel.

A time slotted beacon based structure is utilized on each channel to distribute the contention

solving the problem of control channel saturation.

Four priorities of data type is supported by utilizing different inter-frame spaces

ensuring of QoS provisioning.

Four way handshakes are utilized

mitigating the problem of hidden terminals (if any)

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Distributed ad-hoc cognitive radio network comprising of N CR nodes and Z licensed users

Each CR user equipped with a single half-duplex transceiver for communication purpose.

A rendezvous channel (RC) selected similar process of C-MAC [Cordeiro C., Challapali K., "C-MAC: A Cognitive MAC Protocol for Multi-Channel Wireless Networks,"IEEE DySPAN 2007, pp. 147-157]

Four priority classes

[WMM: Wi-Fi Alliance. "Wi-fi certified for wmm - support for multimedia applications with Quality of Service in Wi-Fi networks," Technical report, Wi-Fi Alliance, 2004.

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The CR network is non-hopping network

a CR node in the network stay on the selected channel until the channel ceases to be free or the node has to move to other channel for some reason.

The channel on which a node resides is called as home channel while all other channels w.r.t. it are termed as foreign channels.

Each channel has a representative which broadcasts the information about the nodes residing on the home channel onto RC.

The representative node (RN) is selected periodically

Periodic Quiet periods are used for the purpose of sensing an incumbent user on a channel.

A node can perform in-band sensing or out-of-band sensing.

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9Figure: Superframe structures for channels in DC-MAC

The superframe of RC is composed of three separate intervals:

Rendezvous channel beacon (RCB),

Data transfer and

Quiet period.

A RCB has time-slotted architecture, where the size of RCB depends on

the number of channels used in the system+2 (C+2)

During RCB, RN from all channels transmit their beacons on their respective slots

During quiet period of RC, all nodes which have home channel as RC stop their communication and perform in-band sensing.

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A superframe of data channel is composed of four separate intervals:

Home channel beacon (HCB),

Home channel information beacon (HCI),

data transfer and

quiet period.

A home channel beacon has a time-slotted architecture

size of HCB depends upon the number of nodes on home channel.

number of time-slot = number of nodes on home channel + 1

RN distributes the information gained from RCB

all the nodes on a home channel know about the information about the home channels of all other nodes in the network

distribution of quiet period

channel conditions

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A superframe of data channel is composed of four separate intervals:

Home channel beacon (HCB),

Home channel information beacon (HCI),

data transfer and

quiet period.

A home channel beacon has a time-slotted architecture

size of HCB depends upon the number of nodes on home channel.

The number of time-slot equals the number of nodes on home channel + 1.

RN distributes the information gained from RCB,

all the nodes on a home channel know about the information about the home channels of all other nodes in the network including the distribution of quiet period and channel conditions.

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A HCI is used to elect a RN from the set of all home nodes.

The home node which has the least amount of load is elected as a RN.

Assume, the load metric to be weighted average of packets sent/received in last 5 frames.

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Ni: represents the number of packets sent/received in last ith frame

Rendezvous Channel (RC) is established

all nodes join through Rendezvous Channel Beacon (RCB)

Initially nodes on RC perform out-of-band sensing to gather information about other channels and this information is used for migration purpose.

If no node exists on a channel, then a new node migrates from RC to that channel and starts its own beacon so that other nodes can follow up (migrate).

Assume

all node uniformly select a channel as home channel.

average N/C CR users on each channel.

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Network Initialization

The data transfer on RC and data channel follows RTS/CTS mechanism similar to 802.11.

Here a four way handshake is employed to handle the multi-channel hidden terminal problem.

RTS/CTS packet contains network allocation vector (NAV) to indicate the duration of transmission.

The size of NAV can be calculated as:

This allows other nodes in the network to go into doze mode for power saving

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DC-MAC Working

The system employs different set of inter-frame spaces for the purpose of provisioning of QoS.

An IFS is a random duration of time defined in [0, interval]

Before transmitting any message a node perform sensing of the channel

decreases its counter at each step when the channel is sensed free;

if the channel is sensed busy the counter is freezed.

sends the packet when the counter has reached zero.16

DC-MAC Working

Depending upon the home channels of transmitter and receiver

the communication may take place on either of the home channels of participating node.

If the specified channel characteristics are not available on any of the home channels,

then the communication may take place over a foreign channel

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DC-MAC Working

If both transmitter and receiver are on the same channel

then communication takes place using RTS/CTS/DATA/ACK mechanism, after IFS.

All nodes which overhear the transmission

Back-off their own transmission and goes into doze mode until the current communication is finished.

If the destination node lies on a channel other than that of the home channel of sender

the sender moves to the home channel of destination node

Once the communication is finished or HCI of the home channel of the sender starts, the sender moves back to its own home channel.

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Data Transfer on Home Channel

A sender first moves to the home channel of the destination node

sends MOV packet which contains the destination ID along with the channel the channel to which the destination node should move on.

If the receiver agrees, it replies with an acknowledgement.

Immediately after acknowledgement, both transmitter and receiver moves to the foreign channel

Once the communication is finished or the HCI of the home channel of the nodes starts, the nodes move back to their home channels.

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Data Transfer on a Foreign Channel

A sender first moves to the home channel of the destination node

sends MOV packet which contains the destination ID along with the channel info

If the receiver agrees, it replies with an acknowledgement.

Immediately after acknowledgement, both transmitter and receiver moves to the foreign channel

Once the communication is finished or the HCI of the home channel of the nodes starts, the nodes move back to their home channels.

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Data Transfer on a Foreign Channel

All C channels are modeled as ON-OFF source depending upon the presence or absence of the PU respectively.

The probability that a given channel will be available is calculated by,

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Performance evaluation of the system done in a discrete event simulator in MATLAB which simulates the system up to MAC frame level.

The proposed scheme is compared against similar literature work (C-MAC) done in [5].

proposed a distributed contention based MAC scheme for ad-hoc cognitive radio network with QoS provisioning.

distributes the contention of channel access onto different channels

reduces the control channel saturation problem.

simple temporal synchronization across all channels with non-hopping nodes in the network.

four different access categories to data packets based on different priorities.

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