OVERLAY COGNITIVE RADIO OFDM SYSTEM FOR 4G CELLULAR NETWORKS. Presented by: Meenakshi Awasthi Sr. Asstt. Professor ECE Department

Transcription

1 OVERLAY COGNITIVE RADIO OFDM SYSTEM FOR 4G CELLULAR NETWORKS Presented by: Meenakshi Awasthi Sr. Asstt. Professor ECE Department

2 2 Contents Background Introduction Why Cognitive Radio Software Defined Radio Existing Cognitive Radio Technologies Overlay Cognitive Radio OFDM Conclusion Reference

3 3 Background Frequency spectrum scarcity is currently one of the most annoying bottlenecks for the deployment of 4G wireless networks. A 4G system requires a higher amount of bandwidth as it is not compatible with existing techniques. When there is no free spectrum available, no new broadband communication techniques can be put into operation. Although frequency spectrum is scarce, the utilization of assigned spectrum is not sufficient. Cognitive radio provides a promising solution to the spectrum scarcity problem. TV white space is the most expected spectrum in which CRs can be deployed.

4 4 Contd. When TV broadcasting systems switched from analog to digital broadcasting, around 100 MHz was freed up, depending on the country. This part of the spectrum has become known as the digital dividend, and specific reallocation of this spectrum to other services is a current process in many countries.

5 5 Introduction The term cognitive radio (CR) was coined by Dr.Joseph Mitola III in late 1990s. Cognitive Radio is a smart radio which autonomously detects and exploits empty spectrum to increase your file transfer rate. CR plays a pivotal role in fourth generation (4G) cellular networks, such as Long Term Evolution (LTE)-Advanced and WiMAX, to solve the spectrum scarcity problem. The main purpose of CR is to share the spectrum belonging to TV broadcasters with cellular networks at the same time.

6 6 Why cognitive radio? Mobile devices are dealing with an increasingly complicated wireless environment. In some heavily used frequencies, signal interference is a big problem. Today, regulators and users leave large amounts of frequency unused to avoid interference. Still we say we have scarce spectrum resource, it s just getting wasted.

7 7 Software Defined Radio A software-defined radio system, or SDR, is a radio communication system where components, that have been typically implemented in hardware, are instead implemented by means of software on a personal computer or embedded system. A basic SDR system may consist of a personal computer equipped with a sound card, or other analogto-digital converter, preceded by some form of RF front end. SDR provides an ideal platform for the realization of CR. It is expected that CR will evolve from current SDR radio by adding more and more cognition features.

8 8 Existing Cognitive Radio Techniques There are three types of CR paradigms: Interweave Underlay Overlay

9 9 Interweave It is based on the idea of opportunistic communication. It takes advantage of temporary frequency voids, referred to as spectrum holes, that are not in use by the licensed owners. In interweave CR, a secondary user periodically monitors the radio spectrum, intelligently detects occupancy in the different parts of the spectrum, and then opportunistically communicates over spectrum holes with minimal interference to active primary users.

10 10 Underlay In this system, secondary users are allowed to share the channel simultaneously with primary users. It protect primary users by enforcing a spectral mask on the secondary signals so that the interference generated by the secondary devices is below the acceptable noise floor for the primary users of the spectrum. The spectral mask constraints are compensated by the access to a wide bandwidth over which the secondary signal can be spread and despread to provide a useful signal-tonoise ratio (SNR) for secondary communications. The interference power constraints associated with underlay systems allow only short-range communication.

11 11 Overlay Overlay CR also allows concurrent primary and secondary transmissions. The premise for overlay systems is that the secondary users can use part of their power for secondary communication and the remainder of the power to assist (relay) primary transmissions. By careful choice of the power split, the increase in a primary user s SNR due to the assistance from secondary relaying is exactly offset by the decrease in the primary user s SNR due to the interference caused by the remainder of the secondary transmit power used for secondary communication.

12 12 Cntd. Underlay CR can only be used when there is a strong primary signal, whereas overlay CR is more appropriate for weak primary signals.

13 13 OVERLAY COGNITIVE RADIO OFDM

14 14 HIERARCHICAL MODULATION AND TRANSMISSION IN OFDM OFDM has been considered one of the most effective multicarrier techniques for broadband wireless communications. Its promising advantages are- robustness against multipath fading, high spectral efficiency, dynamic use, simplicity in channel equalization, and multiple access mechanism. OFDM has been widely adopted in many wireless communication standards, such as WiMAX and LTE. OFDM also appears to be a good modulation for CR, as it is easy to turn on/off subcarriers in accordance to available sensed spectrum.

15 15 HIERARCHICAL MODULATION

16 16 Contd. Hierarchical modulation (HM) is a technique to multiplex and modulate multiple data streams into one single symbol stream, where those multiple symbols are superimposed together before transmission. Hierarchical modulation is usually implemented by means of hierarchical constellation construction. In OFDM, HM can be applied in a frequency domain (i.e., separately to each data subcarrier). In contrast to HM, a time domain approach, hierarchical transmission (HT) can also be applied, in which two or more OFDM time domain signals are superimposed on each other before being transmitted.

17 17 Contd.

18 18 Frequency Domain Hierarchal Modulation for CR OFDM

19 19 Contd. In overlay CR OFDM, the secondary transmitter transmits a combination of layer 1 (primary DTV) and layer 2 (secondary cellular) signals. In this design, the layer 2 signal could be superimposed on the layer 1 signal in either the time or frequency domain. Overlay CR has a prerequisite that the secondary signal does not interfere with the primary signal. Therefore, it is logical to decode the primary signal first and then cancel it from the overall received signal to decode the secondary signal.

20 20 Contd. In this approach the layer 2 signal is superimposed on the layer 1 signal using frequency domain HM at the transmitter side. The superimposed signal is constrained by an embedding factor so that it does not interfere with layer 1. Figure also illustrates that the cognitive receiver first decodes the layer 1 signal in the frequency domain using a conventional method. After that, the receiver feeds back the layer 1 decision to perform OFDM frequency domain cancellation. The layer 2 signal is then demodulated based on the cancelled signal. The frequency domain HM approach leads to easy decoding of the second layer signal with less cancellation complexity and error. Unlike the time domain approach, it can effectively avoid the possible error introduced by time domain signal restoration procedures, such as CP reconstruction.

21 21 Contd. Since the signal superimposition is performed in the frequency domain before IFFT operation, the first and second layers have to share the same modulation parameters, including time and frequency synchronization, CP, pilots, and so on. As a cost of high performance, the two proposed schemes above increase the computational complexity at both the transmitter and receiver sides. It becomes even more difficult for implementation, since a single mobile device has to operate on a variety of possible CR transmission configurations. The trade-off between cost and performance, therefore, must be addressed in practice.

22 22 Conclusion An overlay CR OFDM system using either time domain hierarchical transmission or frequency domain hierarchical modulation is defined. To share TV band spectrum, there are two layers of OFDM signals, DTV and 4G signals, superimposed on each other. With the ability to detect and adapt to the surrounding environment, CR has become one of the widely recognized features of 4G wireless communication systems. The main purpose of CR is to share the spectrum belonging to TV broadcasters with cellular networks at the same time.

23 23 References 1. Songlin Sun, Vanhong Ju, Overlay Cognitive Radio OFDM System for 4g Cellular Networks,IEEE Wireless Communication April 2013, pp Y. Ma, D. I. Kim, and Zhiqiang Wu, Optimization of OFDMA-Based Cellular Cognitive Radio Networks, IEEE Trans. Commun., vol. 58, no. 8, Aug. 2010, pp C. Hausl and J. Hagenauer, I Relay Communication with Hierarchical Modulation, IEEE Commun. Letters, vol. 11, no. 1, Jan. 2007, pp \My Documents\Downloads\OVERLAY CR.pdf

24 24 Thank You