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MULTI-BAND CSMA/CABASED COGNITIVE RADIO
NETWORKS
Jo Woon Chong, Youngchul Sung, and Dan Keun Sung
School of EECS KAIST
IWCMC 2009
Outline
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

Introduction
Proposed RawPEACH MAC Scheme
 For

Multi-band CSMA/CA System
Markov Chain Analysis
 For
The Operation of Secondary User in RAWPEACH
Networks


Numerical Results
Conclusion and Comment
Introduction
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


In IEEE 802.11e, QoS is classified according to various
classes of service.
However, the absolute guarantee of QoS for higher
priority users in not still provided due to its dependence
on contention-based collision resolution
Propose a new flexible MAC scheme based on
CSMA/CA
Provide strict QoS guarantee to certain high priority users
(primary users)
 Embedding channelization into the CSMA/CA operating in
multiple bands

Introduction (cont’d)
4

Random access protocol with PrE-Arbitrated
CHannelization (RawPEACH)
 The
evolution of the conventional CSMA/CA to provide
QoS guarantee to certain users in the network by
incorporating multi-band operation and channelization

Analyze the performance of the proposed mixed
MAC scheme
 Using
a new CSMA/CA model based on a Markov
chain capturing the primary user channel activity and
the number of bands
Proposed RawPEACH MAC Scheme
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Number of primary users: Np
Number of secondary users: Ns
RawPEACH – The Operation of PU
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
Orthogonal physical channel in multiple frequency
hopping sequences in multiple frequency bands
 Latin
square, which exploit frequency diversity to avoid
channel fading and interference.

In this scheme,
 Each
primary user transmits its frame over one primary
frame interval Tf,p
 and hops to a different band according to the assigned
hopping pattern for the next frame interval.
RawPEACH – The Operation of PU
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1.
2.
If the number, NP , of primary users is larger than that, NCH, of
frequency bands, the orthogonality among primary users cannot be
maintained
The RawPEACH protocol needs to employ admission control to
maintain the orthogonality among the primary users in the network
RawPEACH - The Operation of SU
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
Secondary users are provided only with best-effort
services
 be
required not to interfere with the operation of
primary users in cognitive radio networks
 Assume that all the secondary users sense all NCH
frequency bands at the beginning of every secondary
time slot with length Tf,s.
 When
a particular band is sensed to be used by a primary
user, the secondary users do not transmit their frames in the
band at that time.
RawPEACH - The Operation of SU
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
The performance of the secondary users is
controlled by various design parameters
 such
as the arbitrary inter-frame space (AIFS), the
contention windows (CW), the frame lengths of primary
and secondary users, etc.
RawPEACH - The Operation of SU
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 If
all the channels are busy due to the transmission of
primary users
 the
secondary users freeze their back-off counter, and wait
until at least one of the primary users ceases to transmit the
frame
 If
at least one among NCH channels is sensed to be idle
 all
the secondary users decrease the values of their backoff counters by one
 If
more than one secondary user transmit their frames
simultaneously on the same primary user-free channel
 the
transmission of the secondary users fails
RawPEACH
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
The overall operation can be viewed as a single
CSMA/CA protocol
 all
the users in the network sense the channel after AIFS
from the frame boundary of their own
Markov Chain analysis for SU
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1. State Si,j means “a secondary user is in the ith backoff stage and jth backoff counter
value”
2. Assume that the probability of each frequency band being busy due to the primary
user transmission is identical and independent, and is equal to Pp.
3. the probability Pb that all channels are busy is given by PpNCH, where NCH denotes the
number of channels
Tx successes
Freeze counter
Tx fails
Markov Chain analysis for SU
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 Calculate
the stationary probability(bij) and the transmission
probability(τ) to find the collision probability(p)
Markov Chain analysis for SU
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The collision probability(p) 
the probability that at least one other
secondary user transmits data
simultaneously with the given secondary
user.
the probability that the channel is busy
due to a primary user although other
secondary users do not transmit
N1chs is the number of secondary users in one free-channel
Markov Chain analysis for SU
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
Normalized throughput per band
 Calculate
the total system throughput is based on a
band-by-band approach.
 Successful
transmission
 Idle
 Collision
 The
normalized throughput S per band
Numerical Result
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Numerical Result (cont’d)
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Conclusion
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
Proposed a new combined MAC scheme
 Primary

Evaluate the throughput performance of the
proposed scheme
 Markov

user and secondary user
Chain
Future work
 OFDM/OFDMA
based WLAN standards
Comments
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
Modeling a system using Markov Chain
 Stochastic

Process
Introduce a new idea/model to a conventional
scheme