CHANNEL AWARE MEDIUM ACCESS CONTROL IN COGNITIVE RADIO NETWORKS

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 Radio spectrum is a limited resource

Has gone starved and is more scarce to get

Need to come up with solutions that would not only solve the problem of under usage of spectrum but also improve the current usage.

Cognitive radio is a novel method that might propose new solutions

Spectral usage in a urban area [1]

Cognitive Radios Some of the challenges to mention in this field are :

Identify frequency bands incumbent by licensed users and those that are vacant.

Unlicensed users exploit vacant bands also know as “spectrum holes”[2]

Medium access for unlicensed users is highly dynamic and opportunistic

Medium Access Control in CR Attracts attention of a larger research community

Focus on design of robust and effective spectrum access techniques for cognitive radio networks

Closely related to MAC development in multichannel wireless networks, but different

Development of CR-MAC protocols has often borrowed design ideas from existing protocols in multichannel wireless networks

CR – MAC protocols 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

IEEE 802.22[3]IEEE 802.22 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

Cognitive Radio MACControl 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

Various CR – MAC protocols [6]

Channel aware MAC schemes Channel Aware Schemes focus to exploit the Channel state information around a user

Can be used to better model the competitive scenario a user faces

Makes a user self 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]) Designed for competitive and distributed network case, where users negotiate communication over a single channel

Primarily a channel aware aloha scheme

Each user opportunistically contends for its success, directed by its own CSI and competition it faces in surroundings

Protocol selects the best links in terms of channel states and therefore achieves performance close to that of centralized schedulers

CAD-MAC[7] Outline Three step signaling before confirmation of the success of a link(RTS-CTS-ACK)

Each user maintains and modifies its own threshold values (derived from CSI)

Channels are modelled as Rayleigh fading

About 10% time of a Frame time is spent in resolution of contentions

Optimizes network throughput and minimizes interference between selected links

Time frame organization in CAD-MAC[7]

Optimization of ThresholdsOptimize for the opening Contention resolution slot – set the initial values

The values of optimal thresholds [1] are:

Where

is the distribution function for Rayleigh fading channels.

Optimization of Thresholds Update thereafter for every next threshold- whether to increase, decrease and keep the thresholds same

Increase if a collision of requests has happened

Decrease if no request was transmitted in the previous resolution slot, and receiver has invited requests

In other cases, keep the values same

Our Proposal We try to extend this scheme in first, multichannel distributed network case Also try to embed the following concepts to make it applicable for cognitive

radio networks - Spectrum mobility - Dynamic spectrum access - Channel Switching mechanism

Our proposal also uses a separate control channel and time synchronization of secondary users

Simulated the design over several test topologies that include the presence of both primary and secondary users

Our ProposalIntroduce some changes in control signaling mechanism

Different from Single band CAD-MAC, we have following changesSelection of best possible links for contentions, where we have to consider all

the opportunities that are available to users across the spectrumSwitching mechanism – failing to get success for data transmission on one

channel makes a user contend for another channelThreshold modification - need to consider the dynamic spectral environment

in calculation and optimization of these values

Example test case with presence of both primary and secondary users

Revised control signaling exchange

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.