On The Potential And Research Of ZP-OFDM For Cognitive Radio And Cooperative Communications

The electromagnetic radio spectrum, a scarce natural resource, is becoming more and more congested to accommodate diverse types of air interfaces in next-generation wireless networks. CR has emerged as a promising technology to revolutionize the spectrum utilization. CR is an intelligent wireless communication system that is aware of its surrounding environment and uses that understanding by building to learn from the environment and adapt its internal states to the variations in the incoming RF stimuli by changing its operating parameters in such a way that highly reliable communication and efficient utilization of the radio system can be achieved. The whole idea behind CR is that it should prompt effective spectrum usage, by intelligently aiding the radio system to effectively access the spectrum, reducing the mutual interference between CR-based rental (unlicensed) users (RUs) and licensed users (LUs) and providing coexistence between them. According to the Federal Communication Commission (FCC), CR is a radio system that continuously performs spectrum sensing, dynamically identifies the unused spectrum, and then operates in those spectrum holes where the licensed (primary) radio systems are idle.The favourite air-interface for CR is based on OFDM. This is due to OFDM's inherent flexibility in allocating power and bit rate among subcarriers which are orthogonal to each other. In particular, thanks to the inverse fast Fourier transform (IFFT) precoding and the insertion of the so-called Cyclic Prefix (CP) at the transmitter, the OFDM signals are not only orthogonal in the frequency domain, but also temporally orthogonal to each other. At the receiver end, the CP is discarded to avoid inter-symbol interference ISI; each truncated OFDM symbol is Fast Fourier Transformed (FFT)—an operation converting the frequency-selective channel into parallel flat-faded independent subchannels. Since the subchannel gains equal to the channel's frequency response value on the FFT grid, each subchannel can be easily equalized by a single-tap equalizer using scalar division.Most conventional OFDM-based wireless communication systems append CP to provide robustness against multipath effect. However, the same multipath robustness can be obtained by adopting zero-padded (ZP) prefix instead of CP. If the length of the zero-padding equals the length of CP, then the ZP-OFDM will achieve the same spectrum efficiency as CP-OFDM. However, unlike the need for a bandwidth-consuming channel coding in CP-OFDM, ZP-OFDM technique possesses two remarkable advantages for the CR wireless network. First, In ZP-OFDM, full symbol recovery is guaranteed and equalization can be carried out by finite impulse response (FIR) filter regardless of the channel zeros on or close to carrier frequencies. Second, when the ZP prefix replaces the CP in OFDM symbols, the ripples in the power spectral density (PSD) can be reduced significantly. Sidelobe level is a prominent advantage in the CR context, which can be used to minimize the interference and power leakage to other used spectra. Furthermore, the peak-to-average-power-ratio (PAPR) reduction can also be considered as a merit of ZP-OFDM.Cooperative communication and networking is a new communication paradigm that promises significant capacity and multiplexing gain increase in wireless networks. It realizes a new form of space diversity to combat the detrimental effects of severe fading by mimicing the multiple-input multiple-output (MIMO), while gets rid of the drawbacks of MIMO, such as size limitation and correlated channels. There are mainly three relaying protocols: amplified-and-forward (AF), decoded-and-forward (DF) and compress-and-forward (CF). In AF, the received signal is amplified and retransmitted to the destination. The advantage of this protocol is its simplicity and low cost implementation. However, the noise is also amplified at the relay. In DF, the relay attempts to decode the received signals. If successful, it re-encodes the information and retransmits it. Lastly, CF attempts to generate an estimate of the received signal. This is then compressed, encoded, and transmitted with the hope that the estimated value may assist in decoding the original codeword at the destination.Cooperative techniques have already been considered for wireless and mobile broadband radio, Cognitive Radio (CR) and also have been under investigation in various IEEE 802 standards. A recent evolution of IEEE 802.11 using mesh networking, i.e.,802.11s considers the update of 802.11 MAC layer operations to self-configuration and multihop topologies. As an amendment of 802.16 networks, IEEE 802.16j is concerned with multihop relay to enhance coverage, throughput, and system capacityIn this paper, the potentials of ZP-OFDM for CR is studied and compared to its counterpart CP-OFDM. ZP-OFDM outperforms CP-OFDM in many aspects, such as better sidelobe level and lower PAPR, better BER performance, easier blind channel estimation and blind symbol synchronization, and is more proper to be the modulation technique for CR. Better sidelobe level and lower PAPR due to linear structure of the ZP-OFDM symbol are strong merits for CR application. ZP-OFDM offers guaranteed symbol recovery regardless of where channel fades may appear. Low complexity in the blind channel estimation and blind symbol synchronization is also a major advantage of ZP-OFDM. In addition to that, a ZP-OFDM can be recast as a CP-OFDM by appropriately overlapping and adding, and appears to be more flexible than CP. In conclusion, the results of this study demonstrate a promising landscape for ZP-OFDM technique in CR and in blind channel estimation and in symbol synchronization area. Then, in this paper, we also investigate some potential applicable aspects of ZP-OFDM in the cooperative communications, by combining these two technologies together, the unique linear structure of the ZP-OFDM plays an important role in the cooperative system as well, we can dramatically enlarge the communication coverage. Transmission performance is improved by the diversity gains from cooperative ZP-OFDM. Capacity is enhanced by the cooperation between nodes and forms a virtual MIMO array system.