• Journal of the Institute of Electronics Engineers of Korea TC
• /
• v.48 no.9
• /
• pp.27-34
• /
• 2011

Cognitive Radio, which is proposed to improve the efficiency of wireless communication systems is the system to share primary user's frequency bands with a secondary user. In this situation, a secondary user uses NC-OFDM (Non-contiguous Orthogonal Frequency Division Multiplexing) to transmit data effectively without interfering with the primary user's spectrum. However, NC-OFDM, in common with contiguous OFDM, degrades the performance of the system by generating high PAPR (Peak-to-Average Power Ratio). In this paper, firstly, we analyse PAPR corresponding to the distribution of subcarriers in NC-OFDM. Then the PAPR reductions that employ the PTS (Partial Transmit Sequence) and SLM (Selective Mapping) are evaluated. Finally, the computational complexities of the PTS and SLM adopting pruned-FFT are compared with conventional PTS and SLM. Further, it is shown that the NC-OFDM with pruned-FFT is more efficient than the contiguous OFDM in terms of computational complexity and PAPR reduction performance.

• ETRI Journal
• /
• v.43 no.1
• /
• pp.7-16
• /
• 2021

Noncontiguous orthogonal frequency division multiplexing (NC-OFDM)-based cognitive radio (CR) systems achieve highly efficient spectrum utilization by transmitting unlicensed users' data on subcarriers of licensed users' data when they are free. However, there are two disadvantages to the NC-OFDM system: out-of-band power (OBP) and a high peak-to-average power ratio (PAPR). OBP arises due to side lobes of an NC-OFDM signal in the frequency domain, and it interferes with the spectrum for unlicensed users. A high PAPR occurs due to the inverse fast Fourier transform (IFFT) block used in an NC-OFDM system, and it induces nonlinear effects in power amplifiers. In this study, we propose an algorithm called "Alternative Projections onto Convex and Non-Convex Sets" that reduces the OBP and PAPR simultaneously. The alternate projections are performed onto these sets to form an iteration, and it converges to the specified limits of in-band-power, peak amplitude, and OBP. Furthermore, simulations show that the bit error rate performance is not degraded while reducing OBP and PAPR.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
• /
• v.21 no.9
• /
• pp.965-970
• /
• 2010

Because of a large number of subcarriers, the high PAPR(Peak-to-Average Power Ratio) is the major drawback of NC-OFDM system used for wireless communication system. Comb type pilot assignment is more efficient and lower computational complexity for the channel estimation than the block type pilot. However, even if the CAZAC(Constant Amplitude Zero Autocorrelation) matrix transform is used for the PAPR reduction of the data symbols, PAPR increases when the pilot is inserted in comb type with the data symbols. Therefore, in this paper, we additionally use a new SLM technique in order to lower the PAPR again even in the comb type pilot. Also, a new SLM technique suggested in this paper does not need any additional bandwidth for sending selection information for SLM. This combined method has good PAPR reduction performance and efficient data transmission.

• Journal of the Institute of Electronics Engineers of Korea TC
• /
• v.48 no.6
• /
• pp.28-33
• /
• 2011

Recently, the investigation of the cognitive radio (CR) system is actively progressed as one of the methods for using the frequency resources more efficiently. In CR systems, when the frequency band allocated to the incumbent user is not used, the unused frequency band is assigned to the secondary user. Thus, the FFT input signals corresponding to the actually used frequency band by the incumbent user are assigned as '0'. In this paper, based on the fact that there are many '0' input signals in CR systems, a low-power FFT design method for NC-OFDM is proposed. An efficient zero flag generation technique for each stage is first presented. Then, to increase the utility of the zero flag signals, modified architectures for memory and arithmetic circuits are presented. To verify the performance of the proposed algorithm, 2048 point FFT with radix-24SDFstructureisdesignedusingVerilog HDL. The simulation results show that the power consumption of FFT is reduced considerably by the proposed algorithm.

• Proceedings of the IEEK Conference
• /
• 1998.10a
• /
• pp.191-194
• /
• 1998

본 논문에서는 OFDM 시스템에서 사용되는 부 반송파의 수와 동일한 길이의 complementary code를 사용하여 각 부 반송파 데이터를 frequency 영역에서 scrambling하는 PAPR 감소 방안을 제안한다. 이 방법은 부 반송파의 수와 동일한 길이의 complementary code 집합을 이용하여 주어진 데이터에 부과해서 PAPR 값이 가장 적은 complementary code를 사용하여 scrambled된 데이터를 송신함으로서 PAPR 값을 감소시키는 방법이다. 수신기에서의 복호를 위하여 Nc개의 complementary code에 해당하는 log2Nc 비트의 code 인덱스를 데이터와 함께 전송한다. 이때 code 인덱스 비트가 PAPR 값에 영향을 주지 않도록 하기 위하여 CODE인덱스와 데이터가 함께 complementary code로 scrambling되어진다. 제안된 이 방법을 사용하면 주파수 대역효율을 높이면서 PAPR 값을 크게 줄일 수 있다.

• Journal of Communications and Networks
• /
• v.16 no.2
• /
• pp.238-244
• /
• 2014

A key technical challenge in TV white spaces is the efficient spectrum usage without interfering with primary users. This paper considers available spectrum discovery scheme using in-band sensing signal to support super Wi-Fi services effectively. The proposed scheme in this paper adopts non-contiguous orthogonal frequency-division multiplexing (NC-OFDM) to utilize the fragmented channel in TV white space due to microphones while this channel cannot be used in IEEE 802.11af. The proposed solution is a novel available spectrum discovery scheme by exploiting the advantages of a sensing signaling. The proposed method achieves considerable improvement in throughput and delay time. The proposed method can use more subcarriers for transmission by applying NC-OFDM in contrast with the conventional IEEE 802.11af standard. Moreover, the increased number of wireless microphones (WMs) hardly affects the throughput of the proposed method because our proposal only excludes some subcarriers used by WMs. Additionally, the proposed method can cut discovery time down to under 10 ms because it can find available channels in real time by exchanging sensing signal without interference to the WM.