Introduction Related Work System Model Protocol Description Control Operation Incumbents’ Reclaiming Resolution and Spectrum Handoff Channel Access Delay in CN Window Conclusion

1. Wireless Pers Communication (2014) 74:803–821
Springer Science + Business Media New York 2013
Authors:
Gyanendra Prasad Joshi
Seung Yeob Nam
SungWon Kim
Presented By:
Iffat Anjum
Date : 22/03/2014

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3. • Cognitive radio (CR) is a form of wireless communication in
which a transceiver can intelligently detect which
communication channels are in use and which are not, and
instantly move into vacant channels while avoiding occupied
ones.
• Research Challenges:
▫ Spectrum scarcity.
▫ Expensive spectrum license.
▫ Determining spectrum opportunity.
▫ Ensuring incumbent licensees’ right.
▫ Computing complexity and flexibility.
▫ Traditional hidden node and exposed node problem.
▫ Multichannel hidden node problem.
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4. • PMAC is a Synchronization based multi-transceiver and
multichannel MAC protocol.
• Historical prediction method used for channel selection
algorithm.
• Distributed sensing for spectrum opportunity.
• Cooperative sensing.
• Utilization of Channel Negotiation(CN) window.
• Decentralized, flexible and low complexity.
• Spectrum hand-off method.
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5. • IEEE 802.22:
▫ Standardized a MAC layer based on CR[Cognitive Radio] for reusing
the spectrum allocated to the TV broadcast service.
▫ But, the architecture is centralized.
• DCSS (Distributed coordinated spectrum sharing MAC
protocol for cognitive radio)[Nan, H., Hyon,T.,&Yoo; IEEE symposium on new Frontiers in
DySpan 2007]
▫ RTS-CTS based data communication.
▫ Multiple antenna-based protocol.
▫ Negotiates for a data channel per data packet.
▫ Relies fully on a dedicated common CCC[Common Control Channel].
▫ All nodes need to contend for access to the control channel and data
channels remain underutilized.
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6. • MMAC-CR (A distributed multichannel MAC protocol for
multi-hop cognitive radio networks)[Timmers,M., Pollin, S., Dejonghe, A.,Van der
Perre, L.,&Catthoor, F. (2010); IEEE Transactions on Vehicular Technology, 59(1), 446–459.]
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▫ Time is divided into an ad hoc traffic indication message
(ATIM) window and data window.
▫ One handshake on the CCC is needed per connection during the
BI.
▫ Have to wait until the end of ATIM window to send and receive
data packets.
▫ Multiple channel switching overheads
▫ Multichannel hidden node problems.

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• OC-MAC (Cross-layer based opportunistic MAC protocols
for QoS provisioning over cognitive radio wireless
networks.)[Su, H., & Zhang, X. (2008); . IEEE Journal on Selected Areas in Communications, 26(1), 118–129 ]
▫ Two transceivers, one is for a dedicated control channel and the
other is for selected data channel.
▫ P-persistent carrier sense multiple access is used in the
negotiation phase for negotiation.
▫ But, it cannot guarantee fairness to the Su’s[Secondary Users].

8. • {Ch|Chi , i = 1, 2, . . . N} non-overlapping incumbent license
channels (LCs) conditionally and opportunistically
accessible by the SUs.
• Dedicated CCC available with all the required qualities for
reliable communication for all times.
• Each node is synchronized by periodic beacon transmission
to learn the network-wide spectral opportunities.
▫ Secondary Transmission are pushed on those times.
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Figure: Structure of the P-MAC (a view from time-channel
domain)

10. • Each CR[cognitive radio] device is equipped with two half-duplex
transceivers
▫ Control transceiver.
 Wakes up before fast sensing ends, at middle of data window.
 Enters into the doze state after the CN window.
▫ Data transceiver.
 Capable of switching multiple types of frequencies.
 Send data and receive ACK.
 Responsible for fine sensing and fast sensing.
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11. • Incumbent Detection or Sensing:
▫ Fast sensing (time < 1ms/channel)
 gives three results:
 channel is busy,
 channel is idle and
 Uncertain.
▫ Fine sensing
 Provide sensing accuracy but at the cost of time.
 Fine sensing gives two results:
▫ channel is busy.
▫ channel is idle.
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12. • Distributed Spectrum Sensing:
▫ P-MAC employs distributed spectrum sensing (DSS) to
prevent incumbent-SU collision.
▫ At the very beginning,
 the nodes wait for a synchronization signal.
 When a node receives information about the next BI, it selects a
channel randomly for fast sensing (FS).
 From the next BI, as a node receives information of the other
channels from distributed sensing
 it selects a channel with the least nodes performing FS.
▫ After sensing,
 each SU updates the current channel status by its own
sensing result &
 overhearing the neighbors’ CNs on the CCC.
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Table : Channel status table (CST) CUL :channel usage list
SNR :signal to noise ratio
The SUs maintain channel ranking according to the channel selection
factor (CSF)

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Control Operation
• Nodes has a minimum back-off of tm,
tm = tf + tb
tf -> time for fast sensing,
tb -> random back-off time, 0 < tb ≤ CW.
• Each node maintains its own level for PCL [preferred channel list]
▫ Send it with the CN packet, or with the CN-ACK.
• The PCL is derived from the CSF[channel selection factor].
Ut (Chi )-> Utilization of
channel i by the incumbent
SNRmin->threshold for the
minimum SNR required
SNR(Chi ) ->SNR of channel i

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Control Operation
• P-MAC uses linear HPM[Historical Prediction Model] to predict the
incumbent’s arrival.
Ut−1 (Chi ) ->channel
utilization in the last time slot
γ ->smoothing factor
ˆU (Chi ) ->average
utilization of the time slot of
channel i in the past
η-> number of records kept so
far
• After receiving the CN message from any sender that is destined
to the receiver,
▫ It selects the channel common to both and with the best PCL level.
▫ Sends CN-ACK including RCPCL[receiver’s common PCL] to the sender.

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Control Operation
• The sender sends a CN reservation packet (CN-RES) to confirm
the channel reservation after receiving the RCPCL.
• Neighboring nodes update their channel status.
• The random back-off time (RBT) is calculated,
 If the current packet has its first transmission, CW is set to W.
CW is doubled after each collision with this packet, until it
reaches Wm.
• The CN/CN-ACK packet contains network allocation vector
(NAV) information to avoid the hidden terminal problem.

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Data Operation
• In PMAC, the nodes do not wait to send data until the start of
the data window.
▫ Maximum achievement in
each BI with this strategy is,
Prs ->success probability of a CR
node for channel access
CNW->size of CN window
CN-> number of available data
channels
• If there is no data in the transmission queue and no packets to
receive,
▫ It begins fine sensing (FiS) after the CN window.
▫ The node enters the doze state after fine sensing.

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Incumbents’ Reclaiming Resolution and Spectrum Handoff
• When the incumbents reclaim the band, the affected
communication links of the SUs will be lost.
• On the other hand, if the SUs can sense idle sub-bands, they
can reconstruct the communication links to them.
Channel Access Delay in CN Window
• The channel access delay[Contention Delay] is the time a node
spends to obtain access to the channel.
• The contention window size Wi in the back-off stage i:

19. • X denote the MAC layer access delay.
• The expectation of X can be expressed as:
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Channel Access Delay in CN Window

20. • The simulation results show that the P-MAC protocol
increases the good-put and decreases the delay.
• Although P-MAC uses two transceivers, it turns off the
transceivers to conserve energy without decreasing SU
performance.
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• The analysis results show that the CN window becomes a
bottleneck as the number of active nodes increases. More
study will be needed to address the CCC bottleneck problem.

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