Guangxi Zhu

Nonlinear companding transform for reducing peak-to-average power ratio of OFDM signals

A new nonlinear companding transform scheme is proposed to reduce the peak-to-average ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) signals. By exploiting statistical distribution of transmitted OFDM signals, the proposed scheme effectively reduces the PAPR by compressing the peak signals and expanding the small signals, while maintaining the average power unchanged by properly choosing transform parameters. The fact that the proposed companding scheme is described by a single-valued function allows to be transformed before amplification and to be restored the signals at the receiver. The proposed scheme can be applicable with any modulation format and subcarriers. Our simulations results confirm that the suggested scheme exhibits a good ability to reduce PAPR and a good BER performance with a solid state power amplifier (SSPA) in an additive white Gaussian noise (AWGN) channel.

PAPR Reduction of OFDM Signals Using Partial Transmit Sequences With Low Computational Complexity

Partial transmit sequences (PTS) is one of the attractive techniques to reduce the peak-to-average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) system. As conventional PTS technique requires an exhaustive searching over all the combinations of the given phase factors, which results in the computational complexity increases exponentially with the number of the sub-blocks. In this paper, we aim to obtain the desirable PAPR reduction with the low computational complexity. Since the process of searching the optimal phase factors can be categorized as combinatorial optimization with some variables and constraints, we propose a novel scheme, which is based on a nonlinear optimization approach named as simulated annealing (SA), to search the optimal combination of phase factors with low complexity. To validate the analytical results, extensive simulations have been conducted, showing that the proposed schemes can achieve significant reduction in computational complexity while keeping good PAPR reduction.

Complement block coding for reduction in peak-to-average power ratio of OFDM signals

In this article a new coding scheme, complement block coding (CBC), is proposed to reduce the PAPR of OFDM signals. This method utilizes the complement bits that are added to the original information bits, which can effectively reduce the PAPR of OFDM signals with random frame size N and the coding rate R /spl les/ (N - k)/N, where k is a positive integer and k /spl les/ N/2. The performance results obtained with CBC are given and compared with that of some well known schemes, such as simple block coding, modified simple block coding, simple odd parity code, and cyclic coding, for the same purpose. The results show that at the same coding rate 3/4, CBC can achieve almost the same performance as SBC and MSBC, but with lower complexity, and the same performance can be obtained with a higher coding rate using CBC. The PAPR reductions of CBC with coding rate (N - 1)/N are almost the same as with a coding rate less than (N - 1)/N, but almost the twice as these of SOPC when N /spl ges/ 16. Moreover, we can find that PAPR is the lowest for all block codes using CBC with coding rate 3/4. So modified CBC (MCBC) is also proposed and analyzed, combined with the subblock processing technique to make CBC effective for OFDM systems with large frame sizes. The flexibility in coding rate choice and low complexity make the proposed CBC more suitable for random frame size with high coding rate and can also provide error detection.

Cognitive femtocell networks: an opportunistic spectrum access for future indoor wireless coverage

Femtocells have emerged as a promising solution to provide wireless broadband access coverage in cellular dead zones and indoor environments. Compared with other techniques for indoor coverage, femtocells achieve better user experience with less capital expenditure and maintenance cost. However, co-channel deployments of closed subscriber group femtocells cause coverage holes in macrocells due to co-channel interference. To address this problem, cognitive radio technology has been integrated with femtocells. CR-enabled femtocells can actively sense their environment and exploit the network side information obtained from sensing to adaptively mitigate interference. We investigate three CRenabled interference mitigation techniques, including opportunistic interference avoidance, interference cancellation, and interference alignment. Macrocell activities can be obtained without significant overhead in femtocells. In this article, we present a joint opportunistic interference avoidance scheme with Gale-Shapley spectrum sharing (GSOIA) based on the interweave paradigm to mitigate both tier interferences in macro/femto heterogeneous networks. In this scheme, cognitive femtocells opportunistically communicate over available spectrum with minimal interference to macrocells; different femtocells are assigned orthogonal spectrum resources with a one-to-one matching policy to avoid intratier interference. Our simulations show considerable performance improvement of the GSOIA scheme and validate the potential benefits of CRenabled femtocells for in-home coverage.

Adaptive fuzzy switching filter for images corrupted by impulse noise

An adaptive fuzzy switching filter is presented that adopts a fuzzy logic approach for the enhancement of images corrupted by impulse noise. In order to achieve optimal detail preservation, we develop the maximum-minimum exclusive median method to estimate the current pixel. Simulation results indicate that the proposed filter impressively outperforms other techniques in terms of noise suppression and detail preservation across a wide range of impulse noise corruption, ranging from 1% to 90%.

Multiuser subcarrier and bit allocation for MIMO-OFDM systems with perfect and partial channel information

Subcarrier and bit allocation schemes for SISO-OFDM systems in multiuser downlink scenario are well-documented. In this paper, we extend it to MIMO-OFDM systems and design a transmit scheme as a concatenation of dynamic subcarrier assignment, adaptive modulation and beamforming. With the goal of minimizing the overall transmit power required to meet the target BER, we develop an adaptive OFDM subcarrier allocation approach based on perfect and partial channel information. Simulation results show that our proposed algorithm outperforms the multiuser MIMO-OFDM with static FDMA technique considerably, and can lessen the performance loss caused by feedback delays. We have also evaluated the performance of the presented system under various number of transmit- and receive-antennas.

Radio-Resource Management and Access-Control Mechanism Based on a Novel Economic Model in Heterogeneous Wireless Networks

In this paper, we first investigate the radio-resource management and network-selection schemes in heterogeneous wireless networks. Then, we propose a novel economic model to allocate radio resources for both code-division multiple-access (CDMA) networks and wireless local area networks (WLANs). Particularly, the resource-usage constraint is deduced in the CDMA uplink, where the radio resource is allocated with the aim of maximizing the total welfare of the CDMA network while satisfying the signal quality requirement of all mobile users. Moreover, we formulate and solve the optimization problem in the WLAN to maximize the network welfare when the maximum throughput in the WLAN is achieved by controlling the optimum collision probability. An approximated closed-form expression for the network welfare is also given when the assigned bandwidth for each user is close to the bandwidth request. Finally, we design the joint access-control mechanism under different load conditions in coupled heterogeneous networks. Numerical results show that the proposed scheme can achieve approximately 20% more users and 10% more network social welfare, which outperforms the utility-function-based access selection and single network access-control mechanisms in the literature for heterogeneous wireless networks.

Differential space-time block codes from amicable orthogonal designs

In this paper, we propose an amicable-orthogonal-design-based differential space-time block code (ADSTBC) for multiple antennas. Compared with the existing differential modulation designs, our scheme imposes no restrictions on underlying signal constellation, and therefore can improve the spectral efficiency by exploiting efficient modulation techniques such as QAM, APSK, etc. We then derive a non-coherent maximum-likelihood decoder (MLD) with linear complexity for flat Rayleigh fading channels. Particularly, when the QAM constellation is used in ADSTBC, MLD can be further simplified to independently detect the real and imaginary parts of each modulated data symbol, and accordingly the implement cost can be greatly reduced; moreover, the computational amount for detecting single modulated data symbol is kept constant in MLD while the order of the QAM constellation increases.

OFDM peak-to-average power ratio, reduction by complement block coding scheme and its modified version

A new scheme is proposed to reduce the peak-to-average power ratio of OFDM signals and also to detect transmission errors. The proposed scheme is termed sub-block complement coding (SBCC). Performance results obtained with SBCC are given and compared with that of the standard complement block coding (CBC), cyclic coding (CC), simple block coding (SBC), modified simple block coding (MSBC) and simple odd parity code (SOPC) for the same purpose. The results show that, at the same coding rate 3/4, the proposed scheme can achieve almost the same performance as the cyclic code but with lower complexity. However, under the same coding rate, the PAPR reduction obtained by using SBCC is better than the rest schemes. The flexibility on choosing the coding rate and low complexity makes the proposed scheme SBCC more suitable for the large frame size with high coding rate and can provide error detection.

Reducing the peak-to-average power ratio using unitary matrix transformation

A theoretical analysis is presented to show that in orthogonal frequency division multiplexing systems, the peak-to-average power ratio (PAPR) can be reduced by performing a unitary matrix transformation on the input vector of N symbols. The authors also prove that this transformation does not degrade the bit error rate (BER) or power spectral density (PSD) performance. Based on this, the inverse discrete Fourier transform matrix is proposed as the unitary matrix to reduce the PAPR. The simulation results show that the proposed scheme can obtain significant PAPR reduction while maintaining good performance in the BER and the PSD. To further evaluate the performance of the proposed scheme, the authors compare it with some well known PAPR reduction techniques by simulations. It is demonstrated that the proposed scheme can offer better system performance and achieve a better compromise with regard to the PAPR reduction, BER, spectral efficiency and computational complexity.