1. A Survey of Spectrum Sensing Algorithm for Cognitive Radio Applications
YaGun Wu
netlab

2. Outline Introduction Challenges Spectrum Sensing Methods
Cooperative Sensing
Spectrum Sensing in Current Wireless Standards
Conclusion
1.目的在介紹spectrum sensing的方法
2.因為spectrum sensing是cognitive radio network中最有挑戰性的issue之一.

3. Introduction
Cognitive radio: A radio that can change its transmitter parameters based on interaction with the environment in which it operates.
Cognitive radio:就是能夠sense信號,並且動態的和自行調整無線電的參數或變數去達到最大的throughput,減低干擾,
一個重要的目的是去找尋沒有使用的頻譜以供使用.

4. Spectrum Holes Power Frequency Time Spectrum in use by Primary user
Main aspect: One main aspect of cognitive radio is related to autonomously exploiting locally unused spectrum to provide new paths to spectrum access.
Power
Time
Frequency
Spectrum in use by Primary user
Spectrum Hole
1.由於頻譜使用上,因為primary user並不是持續的使用,會產生一些空洞.
這些空洞就可以被其他使用者所使用.
2.而至於如何去得知這些資訊,就是sensing所要作的事情了.

5. Function on Cognitive radio
Functions
Spectrum sensing
Spectrum management
Spectrum sharing
Spectrum mobility
Cognitive Cycle
Spectrum analysis
Spectrum decision

6. Several Aspects 1.Multi-dimensional spectrum sensing concept
2.Challenges
3.Spectrum sensing methods
4.Cooperative sensing concept.

7. Multi-Dimensional Spectrum Awareness
Conventional sensing methods usually exploits three dimensions: frequency, time, and space.
Multi-dimensional spectrum awareness: include the process of identifying occupancy in all dimensions of the spectrum space and finding spectrum holes, or more precisely spectrum space holes.
1.考慮更多的dimensions能夠探測到更多的頻譜使用的機會....
但是相對的也會有更多的挑戰....與問題
2.multi-dimensional spectrum awareness包含幾個動作,在所有的dimensions中辨識出被佔用的spectrum space,並且精確的找出spectrum holes.

8. Multi-Dimensional Spectrum Awareness

9. Multi-Dimensional Spectrum Awareness
Code:在一段時間內,同時使用而不造成干擾是有可能的,

10. Challenges Hardware requirements Hidden primary user problem
Detecting spread spectrum primary users
Sensing duration and frequency
Decision fusion in cooperative sensing
在sensing 中會遇到的挑戰有....

11. Single-Radio and Dual-Radio
Two different architectures of sensing
single-radio: only a specific time slot is allocated for spectrum sensing.
dual-radio: one radio chain is dedicated for data transmission and reception while the other chain is dedicated for spectrum monitoring
Single-radio:只有在特定的時間內可以作sensing的動作, 但是spectrum的效率會減低,因為在一定的時間內做sensing的動作而不是資料傳輸.
Dual-radio:

12. Challenges Hidden primary user problem: many factors including severe
multipath fading or shadowing
observed by secondary users
while scanning for primary users’
transmissions.
Detecting spread spectrum primary users: The two major spread spectrum technologies are frequency hoping spread-spectrum (FHSS) and direct-sequence spread spectrum (DSSS).
1.Hidden primary user problem: cognitive radio user沒辦法探測到primary user的傳輸動作,因為primary user的傳輸動作並不在cognitive radio的範圍內.
2.展頻技術的偵測,FHSS是在數個狹窄的頻帶中改變頻率,而DSSS則是在單一的頻帶中改變能量.

13. Challenges
Sensing duration and frequency: In order to prevent interference to and from primary license owners, cognitive radio should be able to identify the presence of primary users as quickly as possible.
Decision fusion in cooperative sensing:
Security:
1.Sensing duration and frequency: If the statuses of primary users are known to change slowly, sensing frequency requirements can be relaxed draft standard是30秒. 干擾跟變動度影響sensing的週期. TV channel則因為變動很不頻繁. 2. 3.

14. Spectrum Sensing Methods
Energy Detector Based Sensing: The signal is detected by comparing the output of the energy detector with a threshold which depends on the noise floor.
Inability to differentiate interference from primary users and noise, and poor performance under low signal-to-noise ratio (SNR)values.
1.用一個metric值去跟threshold去做比較,metric值的由來與Threshold的由來,

15. Spectrum Sensing Methods
Waveform-Based Sensing: Known patterns are usually utilized in wireless systems to assist synchronization or for other purposes. Such patterns include preambles, midambles, regularly transmitted pilot patterns, spreading sequences etc.
Waveform-based sensing requires short measurement time.
1.利用已知的樣本去作比對跟收到的信號,
2.waveform-based sensing勝過energy detector based sensing.在信賴度和收斂的時間convergence time上(輻合的時間).
此方法用在802.11b和WiMAX介面中則是在引用的paper中有提出.

16. Spectrum Sensing Methods
Cyclostationarity-Based Sensing: Cyclostationarity feature detection is a method for detecting primary user transmissions by exploiting the cyclostationarity features of the received signals.
The cyclostationarity based detection algorithms can differentiate noise from primary users’ signals.
1.定期的
2.能區分primary user的noise,原因是雜訊是廣義靜態分佈(WSS),

17. Spectrum Sensing Methods
Radio Identification Based Sensing:
Matched-Filtering: Matched-filtering is known as the optimum method for detection of primary users when the transmitted signal is known. It requires perfect knowledge of the primary users signaling features.
Other Sensing Methods:

18. Comparison of Various Sensing Methods
Waveform-based sensing is more robust than energy detector and cyclostationarity based methods.
Energy detector based sensing
is limited.
Cyclostationary-based methods
perform worse than energy
detector based sensing methods
when the noise is stationary.
1.Energy detector based sensing會因為noise的問題而失去正確性.
2.而當noise是穩定不動的狀態時,cyclostationary based sensing則效能比energy detector based sensing還差.

19. Cooperative Sensing
Cooperative sensing decreases the probabilities of mis-detection and false alarm considerably. It can solve hidden primary user problem and it can decrease sensing time.
Using control channel to share spectrum sensing result.
Collaborative spectrum sensing is most effective when collaborating cognitive radios observe independent fading or shadowing.
利用額外的頻段去作control channel傳送share spectrum sensing result.

20. Centralized ,Distributed Sensing and External Sensing
Centralized Sensing
In centralized sensing, a central unit collects sensing information from cognitive devices, identifies the available spectrum, and broadcasts this information to other cognitive radios or directly controls the cognitive radio traffic.
Only the cognitive radios with reliable information are allowed to report their decisions to the central unit.

21. Centralized ,Distributed Sensing and External Sensing
In the case of distributed sensing, cognitive nodes share information among each other but they make their own decisions as to which part of the spectrum they can use.
Only final decisions are shared in order to minimize the network overhead due to collaboration.
External Sensing
An external agent performs the sensing and broadcasts the channel occupancy information to cognitive radios.
1.Protocol的overhead是一個很大的問題,因為去減少overhead是值得討論的.
2.所以只有final decisions才會被shared出來.
External sensing則是利用外部的agent去作sensing的動作,而且cognitive radio也不需要再花時間去作sensing的動作.
此方法在802.22中被使用.

22. Spectrum Sensing in Current Wireless Standards
IEEE k: It is a standard for radio resource management. Some of the measurements include channel load report, noise histogram report and station statistic report.
Bluetooth: Adaptive frequency hopping (AFH), is introduced to the Bluetooth standard to reduce interference between wireless technologies sharing the 2.4GHz unlicensed radio spectrum.
在一個無線區域網內，每個設備通常連接到提供最強信號的接入點。這種管理有時可能導致對一個接入點過度需求並且會使其他接入點利用率降低，從而導致整個網路的性能降低，這主要是由接入用戶的數目及地理位置決定的。在一個遵守802.11k規範的網路中，如果具有最強信號的接入點以其最大容量載入，而一個無線設備連接到一個利用率較低的接入點，在這種情況下，即使其信號可能比較弱，但是總體吞吐量還是比較大的，這是因為這時網路資源得到了更加有效的利用。
因為802.11b/g和bluetooth都是使用2.4GHz的頻帶,所以bluetooth利用AFH技術,在使802.11b/g共存時,避開802.11b/g使用的頻率,以避免干擾的情形.
AFH會去偵測是否有人在ISM的頻帶中,是否去避免掉他們.

23. Spectrum Sensing in Current Wireless Standards
IEEE : It based wireless regional area network (WRAN) devices sense TV channels and identify transmission opportunities.
It is a practical example of centralized collaborative sensing.

24. Conclusion
Spectrum is a very valuable resource in wireless communication systems.
Cognitive radio is one of the efforts to utilize the available spectrum more efficiently through opportunistic spectrum usage.
One of the important elements of cognitive radio is sensing the available spectrum opportunities.

25. Thank you!!