Cross Layer Analysis of Spectrum Sensing in the Cognitive Radio Paradigm

(1)

International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459,ISO 9001:2008 Certified Journal, Volume 5, Issue 4, April 2015)

225

Cross Layer Analysis of Spectrum Sensing in the Cognitive

Radio Paradigm

Anandteerth S Mathad

1

, Mrinal Sarvagya

2

1

Assistant Professor, New Horizon College of Engineering, Bangalore, India

2_{Professor, Reva University, Bangalore, India}

Abstract— In this paper, the performance of the carrier

sense multiple access with collision avoidance (CSMA/CA) protocol at MAC layer with sensing errors at physical layer is investigated. Here the sensing errors are captured by modifying the Markov chain model widely used for the CSMA/CA analysis called Bianchi’s model. We investigate the throughput as a function of carrier sensing error probabilities: probability of missed detection and probability of false alarm for given CSMA/CA parameters. The normalized saturation throughput of CSMA/CA networks under the presence of sensing error is obtained. With imperfect carrier sensing, the false alarm error has an effect of extending the contention window size, and the missed detection error results in effects like shrinking the contention window etc. We also emphasize on the packet length (L) on the performance of the system under the presence of these errors.

Keywords— Cognitive Radio, CSMA/CA, miss detection,

false alarm, packet length

I. INTRODUCTION

Wireless technology has been tremendously advancing in recent times. Novel researches are made overnight and fascinating results are accomplished. It is never surprising that such an immense technological growth in offers numerous challenges to the research community. The classical layered approach put a serious limit on the performance of wireless systems. The layer to layer communication sounds meaningful than each layer taking care of its own business. For improvement in the performance of wireless networks ‗cross-layer designing‘ (layer-layer interaction) was coined. Many issues arouse for cross layer design. The situation became even tougher with new paradigm ‗cognitive networks‘. Since cognitive networks, being intelligent systems, smart in understanding surroundings are limited in performance by challenges in spectrum sensing, sharing, mobility, end to end Qos etc, a cross-layer design is worth pursuing.

Here in our evaluation of this paper, we consider the model proposed in paper [1] which is a modified form of conventional Markov chain model for carrier sense multiple access with collision avoidance (CSMA/CA) proposed in paper [2] to incorporate the cross-layer scenario. Though the authors of paper [3] have devised a cross-layer design of carrier sensing multiple access with collision avoidance (CSMA/CA) at the medium access control (MAC) layer with spectrum sensing at the physical layer for cognitive radio networks, they have considered cross layer design rather than cross layer analysis.

We consider a slotted CSMA/CA system with time synchronization. It is assumed that transmitters in the network have only one back-off stage, a packet is made up of slots, and the CW size at the back-off stage is Wo slots. We also assume that all the nodes in the network are in saturated mode since saturated mode sounds more meaningful and posing more challenges. At the beginning of each slot, each transmitter senses the channel and the sensing outcome is used to determine the availability of the channel. If the level of the sensor's input signal is larger than a predetermined threshold, the transmitter determines that the channel is busy. Otherwise, it regards the channel as idle.

(2)

International Journal of Emerging Technology and Advanced Engineering

226

II. MODEL FOR EXAMINING THE CROSS LAYER IMPACT

[image:2.612.57.280.320.372.2]

Paper [1] proposes the model shown in fig 1 for the operation of each node capturing the false alarm and miss detection errors at the sensing stage, where the state transition occurs every slot. The chain is divided into the right and left sides with state So being the center. The right side corresponds to the states of decreasing the value of the back-off counter. Here, state Si represents that the node has the back-off counter value of i, and the node does not transmit a packet in these states (1 < i< Wo - 1). On the other hand, the left side corresponds to the states of transmitting a packet (-(L -1) < I < 0), and state Si in the left side represents that the CSMA/CA node is transmitting (-i + 1)th slot of a packet with the back-off counter value of 0.

Fig -1 Markov model for a node with sensing errors (0 < Pi, Pm < 1) [1]

In the states of the right side of the chain, there are two conditions under which the node decreases its back-off counter value; the node decreases its current back-off counter value by one 1) when the carrier sensor of the node determines that the channel is idle (with probability l - pf)

when it is actually idle (with probability 1 - α, where α is the channel activity factor), or 2) when the carrier sensor of the node declares that the channel is idle (with probability pm) when it is actually busy (with probability (α).

Hence, the transition probability from State Si, to State

Si-1 is given by (1 – α) pf+ (1 - α)(1 - pf) in the right side of

the chain. In addition, looping from State Si to State Si is

also possible when the carrier sensor declares the presence of a signal in the channel and the transition probability for this is given by (1 – α) pf + α(1 - pm). In the left side of the

chain, on the other hand, the CSMA/CA node is in the process of transmitting a packet with length of L slots. When the back-off counter value of the node reaches zero (State S0), the node starts to transmit a packet.

Whether the transmission of each slot of the packet is successful or not, the node continues transmitting the packet and the state of the node moves to the left until it ﬁnishes the transmission of the packet. If the transmission is not collided by the transmission of other nodes during the whole packet duration, the packet is successfully transmitted.

If at least one of the L slots of the packet is collided, on the other hand, we consider that the packet is collided and the transmission is not successful. Whether the transmission is successful or collided, the node selects a new value for the back-off counter randomly from [0, W0-

1] after the transmission. Thus, the B state transition probabilities for the chain are given by

P {Si-1│Si} = α pm + (1 - α) (l -pf), i [1, W0 – 1], P {Si│Si} = α (1 - pm) + (1 - α) pf, i [1, W0 – 1], P {Si-1│Si}=1, i [-(L – 2), 0],

P {Si│S-(L – 1)}= 1/W0, i [0, W0 – 1].

The normalized throughput S is given by

Where,

Ps = Probability of successful transmission

Pi = Probability of collision

Pc = Probability of idle channel

E{P} = average payload length

E{L}=average packet length. (In our analysis, we assume that E{P}= E{L}= L).

a) False alarm only (0 < Pf < 1, Pm = 0)

When the input SNR is high and the sensing threshold of the sensor is low, the sensor rarely determines that a channel is idle when it is actually busy (Pm= 0). In this case, however, sensor can determine that the channel is busy even if it is actually idle, and the false alarm probability takes a value in the entire range of [0, 1]. The effect of the false alarm error on the CSMA/CA is equivalent to extending the CW size Wo of the CSMA/CA. b) Miss detection only (Pf = 0,0<Pm < 1)

When the input SNR at the carrier sensor is high and the sensing threshold is set to a large value, the sensor seldom makes a false alarm error (Pf = 0) whereas the miss probability can range over [0,1]. This has a similar in impact of what we see as decreasing the CW size.

III. SIMULATION

(3)

International Journal of Emerging Technology and Advanced Engineering

[image:3.612.372.510.136.267.2]

227

Table -1:

Parameters for simulation

Parameters Value Description

L 1, 2, 3, 4, 5 Length of packet

Wo 4, 8, 16 CW size

Pm 0.2, 0.4, 0.6,

0.8, 1.0

Probability of miss detection

Pf 0.2, 0.4, 0.6,

0.8, 1.0

Probability of false alarm

Α 0.5 Channel activity

(busy/idle)

IV. RESULTS AND DISCUSSION

[image:3.612.43.297.157.366.2]

Fig a shows normalized throughput under false alarm only condition. Here probability of missed detection Pm is assumed to be zero. The throughput decreases as the false alarm probability increases since CSMA/CA transmitters become more conservative in channel use and lose the chances of successful transmissions. The fall of throughput curve is monotonous as it can be observed.

Fig a : Normalized throughput under false alarm only condition

Fig b: Normalized throughput for missed detection only condition

Fig b shows normalized throughput for missed detection only condition. Here the probability of false alarm Pf is assumed to be zero. We observe the following thing-

For small L, throughput is slightly increasing or approximately constant function of Pm which results in reduced CW value though channel is busy

For large L, throughput is a decreasing function of Pm since increased L results in noticeable collision in between

[image:3.612.97.234.466.603.2]

Fig c and d show the scenario of normal operating conditions in presence of Pf and Pm respectively. When L is small, throughput is insensitive to Pm but sensitive to Pf. For larger L, throughput is insensitive to Pf but sensitive to Pm.

Fig c: Throughput versus Pf

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0

0.02 0.04 0.06 0.08 0.1 0.12

Pf

th

ro

u

g

h

p

u

t

Throughput under false alarm only case (Pm=0)

L=1 L=2 L=3 L=4 L=5

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.02

0.04 0.06 0.08 0.1 0.12 0.14

Pm

th

ro

u

g

h

p

u

t

Throughput under missed detection only case (Pf=0)

L=1 L=2 L=3 L=4 L=5

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0

0.02 0.04 0.06 0.08 0.1 0.12

Pf

th

ro

u

g

h

p

u

t

Throughput under normal operationg condition

[image:3.612.361.518.472.618.2]

(4)

International Journal of Emerging Technology and Advanced Engineering

[image:4.612.371.512.136.270.2] [image:4.612.85.249.136.475.2]

228

Fig d: Throughput versus Pm

fig e: Normal conditions with W=8 (Pf considered)

fig f: Normal conditions with W=4 (Pf considered)

fig g: Normal conditions with W=8 (Pm considered)

fig h: Normal conditions with W=4 (Pm considered)

V. CONCLUSION

It is observed that the sensing threshold should be chosen on the trade-off between large and small L values. Small L results in shrinkage of contention window there by back-off counter when actually the channel is busy. Large L will result in collisions, besides, the ratio L/W should be given concentration and an intermediate value has to be considered to better suppress the physical impacts Pf and Pm i.e L/W is neither too large nor too small.

REFERENCES

[1] Chong, J.W., Sung, D.K., Sung, Y.: ‗Cross-Layer Performance

Analysis for CSMA/CA Protocols: Impact of Imperfect Sensing‘, IEEE Trans. Veh. Tech., vol. 59, no. 3, March 2010.

[2] G. Bianchi, ―Performance analysis of the IEEE 802.11 distributed

coordination function,‖ Selected Areas in Communications, IEEE Journal on, vol. 18, no. 3, pp. 535–547, 2000.

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 Pm th ro u g h p u t

Throughput under normal operationg condition

L=1 L=2 L=3 L=4 L=5

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.05 0.1 0.15 0.2 0.25 Pf th ro u g h p u t

Throughput under normal operationg condition With W=8

L=1 L=2 L=3 L=4 L=5

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0 0.05 0.1 0.15 0.2 0.25 Pf th ro u g h p u t

Throughput under normal operationg condition with W=4

L=1 L=2 L=3 L=4 L=5

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 Pm th ro u g h p u t

Throughput under normal operationg condition w=8

L=1 L=2 L=3 L=4 L=5

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Pm th ro u g h p u t

Throughput under normal operationg condition w=4

[image:4.612.366.514.300.445.2] [image:4.612.92.238.499.639.2]

(5)

International Journal of Emerging Technology and Advanced Engineering

229

[3] Fotis Foukalas, George T. Karetsos , Periklis Chatzimisios ―

Cross-layer Design of CSMA/CA with Spectrum Sensing for Cognitive Radio Networks‖ , 2012

[4] Haitao Wu, Shiduan Cheng, Yong Peng, Keping Long, Jian Ma

―IEEE 802.11 Distributed Coordination Function(DCF): Analysis and Enhancement‖

[5] Yucek, T., Arslan, H.: ‗A Survey of Spectrum Sensing Algorithms

for cognitive radio applications‘, IEEE Comm. Surveys & Tutorials, vol. 11, no. 1, pp. 116-130, 1st 2009.

[6] Cormio, C., Chowdhury, K.R.: ‗A Survey on MAC protocols for

cognitive radio networks‘, Elsevier, Ad-Hoc Netw.7, 2009, pp. 1315– 1329.

[7] Chen, Y., Zhao, Q., Swami, A.: ‗Joint Design and Separation

Principle for Opportunistic Spectrum Access in the Presence of Sensing Errors‘, IEEE Trans. Inf. Theory, 2008, 54, (5), pp. 2053-2071.

[8] Cognitive Wireless RAN Medium Access Control (MAC) and

Physical Layer (PHY) Specifications: Policies and Procedures for Operation in the TV Bands, IEEE P802.22/D0.1 Draft Standard for Wireless Regional Area Networks - Part 22.

[9] Liu, Q., Zhou, S., Giannakis, G.B.: ‗Queuing with adaptive