CHANNEL AWARE MEDIUM ACCESS CONTROL IN COGNITIVE RADIO NETWORKS Supervisor – Dr. Ketan Rajawat

PA L A S H K AT I YA R ( 1 0 4 7 5 )

E L E C T R I C A L E N G I N E E R I N G

I N D I A N I N S T I T U T E O F T E C H N O L O G Y

K A N P U R

Cognitive Radios and MAC Designing Cognitive radio is a novel method that might propose new solutions for the problem of starved usage of spectrum Medium Access control designing is a challenging topic – A highly dynamic scenario A Cognitive radio medium access scheme should perform Channel Sensing Detection of primary users Dynamic channel selection Robust channel switching mechanism Improve overall usage in both licensed and unlicensed bands

Control information exchange in design of distributed MAC Overhead considerably higher than that in simple wireless networks Most of the schemes exploit out of band signaling Use of a dedicated control channel is quite popular – a widely accepted idea Use of IEEE 802.11 DCF [5] like architecture is also very common

Existing Approaches IEEE 802. 22 [3] WRAN standard Aimed at using CR techniques to allow sharing of unused spectrum allocated to television broadcast services [4]

What is WRAN - A WRAN network uses the white spaces in television broadcasts to facilitate broadband access. The point of a WRAN is to provide Internet access to areas that are difficult to reach by other means. The standard is centralized and does not serve distributed architecture No such standard exists for medium access in a distributed network cases

Channel aware MAC schemes Exploit the Channel state information around a user Better model the competitive scenario a user faces Makes a user aware of the surroundings it is operating in and enables it to make independent decisions Makes competition fair and ensure good throughput values, many of channel aware schemes have achieved throughput values comparable to centralized scheduling schemes Some example of prior works done in this fields are - CAD-MAC [7] and CAAC [8]

Channel Aware Distributed MAC(CAD-MAC[7]) Protocol selects the best links in terms of channel states and therefore achieves performance close to that of centralized schedulers

A threshold driven protocol, calculation and modification of thresholds define the performance

The skeleton for working of the protocols is as follows :

Time frame organization in CAD-MAC[7]

TIME FRAME COMPOSITION IN CAD-MAC

If Gains > Threshold Values, then only a user participates in channel contention

Performs signaling exchange with the receiver

A user that is active – Checks Thresholds Against Its Channel Gain Estimates

Leaves contention if successfully negotiated , or else starts again , updates threshold values

Protocol Working

Only the best links are selected in a neighborhood Unsuccessful transmitters change their behavior by updating the threshold values Upon collision of requests – transmitters react by increasing threshold values Upon reception of IDLE signals - transmitters react by decreasing threshold values Cycle continues for a maximum allotted time

Detailed Working procedure of the Single Channel CAD-MAC design

Work Done in Previous Semester Reproduced the results published in the research paper [7], related to the simulation of the single channel case of the proposed protocol Extended the protocol design for the multiband case Simulated for networks with 2 bands only and one primary user Followed threshold updating mechanism as used for single channel case for the multiband case also

Proposal For This Semester

- Updating threshold calculation mechanism for multiband case

- Testing the protocol on more number of Secondary users, more number of Bands and more number of Primary users and evaluate its performance

An Example – Setting Threshold Values Consider a general network , where User 1 communicates with one other user, out of other N users, all inside its transmission range Let the channels have uniformly distributed gains between 0 and 1 , i.e. let the pdf of channel gains be

= else Then the threshold values H for this user can be set as :

We propose the use only one threshold value to control the activity of a user in control signaling exchange

The spectral environment changes around a secondary user almost every CRS

Some users might win, some might give up and some may keep silent – all of this happens across the spectrum

Therefore , it will not be possible to maintain different thresholds for different bands

Also it is not possible for a transmitter to keep track of the reservations of all the channels and maintain individual thresholds

All the control signaling occurs on one channel

Channel gain between two users in such networks – General Case Let the channel gain between two users (i,j) be given as ‘h’, its mean(), PDF() and CDF () are known to transmitter, for some channel with identity ‘k’ IDEA - Select the channel in best state and then select the receiver For all comparisons we assume the channel gain between two users as

with the tied up CDF’s

Select that reciever ‘j’ , whose index maximizes the CDF values , i.e.

Actual threshold Calculation

The values of optimal thresholds [1] for first resolution slots are:

Where is the distribution function for Rayleigh fading channels.

Threshold value updating for the case where a user switches

Calculate the new CDF between users i and j ,without including the individual CDFs of the channels that are not free

Calculate new set of probabilities, with changed sets |N|, |T|, and |L|.

Update these values and follow routine procedure with changed values, until another switch occurs

Example test case with presence of both primary and secondary users

Revised control signaling exchange

Threshold Value behavior for a typical user in Multiband case , in presence of primary users

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Protocol Performance with one, two and three primary users, in environments with 4 and 6 free channels available to users

Results And Conclusions We are able to see that the application of a slotted aloha protocol for contention among users with varying spectral opportunities can produce good values of network throughput for secondary users. Not only our design is able to fairly choose interference free links among the contending users making it highly probable the selection of those links with comparably better channel states in a neighborhood We were able to implement the protocol for network cases with both primary and secondary users present.

References [1]. By Raghvendra Rao, Qi Cheng, Aditya Kelkar and Dhavel Chaudhary, in ICAST global community magazine

[2]. Ekram Hossain, Vijay K. Bhargava (2007) Cognitive Wireless Communication Networks, 1st edition, Springer, New York.

[3]. C. M. Cordeiro, K. Challapali, and D. Birru, \IEEE 802.22: An Introduction to the First Wireless Standard based on Cognitive Radios, J. Commun., Special Issue from selected papers from DySPAN 2005, vol. 1, no. 1, pp. 328337, April 2006 (Invited Paper).

[4]. http ://en:wikipedia:org/wiki/CognitiveRadio

[5]. IEEE 802.11 WG,\Part 11:Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specication, in IEEE Std 802.11-1999, August 1999.

References [6]. Antonio De Domenico, Emilio Calvanese Strinati, and Maria-Gabriella Di Benedetto,\A Survey on MAC Strategies for Cognitive Radio Networks,“ IEEE communications surveys and tutorials, Vol. 14, NO. 1, first quarter 2012

[7]. Guowang Miao, Ye(Georey)Li and Ananthram Swami,\Channel aware distributed Medium Access control", in IEEE/ACM transactions on networking, Vol. 20, No. 4,August 2012.

[8]. Bo Yang, Gang Feng, Yanyan Shen, Chengnian Long, Xinping Guan,\Channel-Aware Access for Cognitive Radio Networks", IEEE transactions on vehicular technology, VOL. 58, NO. 7, September 2009.