1. CHANNEL AWARE MEDIUM ACCESS CONTROL IN COGNITIVE RADIO NETWORKS
PALASH KATIYAR(10475)
ELECTRICAL ENGINEERING
INDIAN INSTITUTE OF TECHNOLOGY
KANPUR

2. 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]

3. 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

4. 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

5. 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

6. 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

7. Cognitive Radio MAC
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 DCF [5] like architecture is also very common
Use of CCC

8. Various CR – MAC protocols [6]

9. 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 awareness and CCC can be a handy tool in designing such protocols

10. 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
Onr such protocol is CAD -MAc

11. 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
3 step signaling and threshold control

12. Time frame organization in CAD-MAC[7]

14. Optimization of Thresholds
Optimize for the opening Contention resolution slot – set the initial values
The values of optimal thresholds 𝐻 𝑖𝑗 [1] are:
𝐻 𝑖𝑗 1 = 𝐹 𝑖𝑗 −1 𝑌 𝑖𝑗 1
Where 𝑌 𝑖𝑗 1 = 1− 𝑇 𝑖 𝑆 𝑖 + 𝑚 ∈ 𝑁 𝑖 𝑆 𝑚 𝑇 𝑖
𝐹 𝑖𝑗 is the distribution function for Rayleigh fading channels.
|𝑇 𝑖 | 𝑖𝑠 𝑡ℎ𝑒 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑢𝑠𝑒𝑟𝑠 𝑟𝑒𝑐𝑒𝑖𝑣𝑖𝑛𝑔 𝑝𝑎𝑐𝑘𝑒𝑡𝑠 𝑓𝑟𝑜𝑚 𝑢𝑠𝑒𝑟 𝑖
|𝑆 𝑖 | 𝑖𝑠 𝑡ℎ𝑒 𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑢𝑠𝑒𝑟𝑠 𝑠𝑒𝑛𝑑𝑖𝑛𝑔 𝑝𝑎𝑐𝑘𝑒𝑡𝑠 𝑡𝑜 𝑢𝑠𝑒𝑟 𝑖
|𝑁 𝑖 | 𝑖𝑠 𝑡ℎ𝑒 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑢𝑠𝑒𝑟𝑠 𝑖𝑛 𝑟𝑎𝑛𝑔𝑒 𝑡𝑜 𝑢𝑠𝑒𝑟 𝑖

15. 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

16. 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

17. Our Proposal Introduce some changes in control signaling mechanism
Different from Single band CAD-MAC, we have following changes
Selection of best possible links for contentions, where we have to consider all the opportunities that are available to users across the spectrum
Switching mechanism – failing to get success for data transmission on one channel makes a user contend for another channel
Threshold modification - need to consider the dynamic spectral environment in calculation and optimization of these values

18. Example test case with presence of both primary and secondary users

19. Revised control signaling exchange

21. 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.

22. 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 : 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 , April 2006 (Invited Paper).
[4]. http ://en:wikipedia:org/wiki/CognitiveRadio
[5]. IEEE WG,\Part 11:Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specication, in IEEE Std , August 1999.

23. 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.