Comparison of coded orthogonal frequency division multiplexing and multicarrier code division multiple access systems for power line communications

Orthogonal frequency division multiplexing (OFDM) and multicarrier code division multiple access (MC‐CDMA) systems are comparatively evaluated for power line communications (PLC) in a frequency‐selective fading environment with additive coloured Gaussian noise which is used to model the actual in‐home power line channel. OFDM serves as a benchmark in order to measure the performance of various MC‐CDMA systems, since multicarrier modulation systems are considered the best candidate for this kind of channel. Both single‐user and multi‐user cases are taken into account, making use of the appropriate combiner schemes to take full advantage of each case. System efficiency is enhanced by the application of different coding techniques, a fact which shows that powerful coding can make the difference under such a hostile medium. The impact of block interleaving is investigated, while the simulation examines how different modulation schemes fair under the imposed channel conditions as well. The performance of the system is assessed by the commonly used bit error rate vs signal‐to‐noise ratio diagrams and there is also a comparison regarding throughput efficiency among all the tested systems. As stated in Section 4, a promising PLC application is attained. Copyright © 2004 John Wiley & Sons, Ltd.     

一种用于OFDM系统的频偏估计方案

针对时变多径的移动信道,提出了一种应用于OFDM系统的基于时频域联合导频的频偏估计方案。理论分析给出了该频偏估计方案的估计精度和适用范围,同时仿真验证了该方寒无论在高斯信道还是多径衰落信道下都有很好的性能。     

Peak‐to‐average power ratio reduction in space frequency block coding multi‐input multi‐output orthogonal frequency division multiplexing systems by tone reservation

In this paper, a tone reservation (TR) method is employed to reduce the peak‐to‐average power ratio (PAPR) in multi‐input multi‐output orthogonal frequency division multiplexing systems with space frequency block coding (SFBC). The key idea of the employed TR method is taking signals on multiple transmit antennas into account to design appropriate peak reduction symbols, which can significantly reduce the PAPR of SFBC multi‐input multi‐output orthogonal frequency division multiplexing signals. With the employed TR scheme, the SFBC structure can be maintained, whereas the traditional TR method would destroy it, resulting in the degradation of bit error rate (BER) performance. Simulation results demonstrate that the employed TR scheme can provide significantly better BER performance than the traditional TR method with slight PAPR reduction degradation. Copyright © 2013 John Wiley & Sons, Ltd.     

Robust spectrum sensing for orthogonal frequency division multiplexing signal without synchronization and prior noise knowledge

Spectrum sensing is defined as the task of detecting the presence of licensed users and is an essential prerequisite for opportunistic spectrum access in cognitive radio. Motivated by the infeasible assumptions of perfect synchronization and prior noise knowledge in most of the existing spectrum sensing algorithms, a robust orthogonal frequency division multiplexing (OFDM) signal sensing scheme, with the use of a noise power insensitive threshold, is investigated in this paper. Identification of primary OFDM signals is achieved by sliding the local pilot reference over the received signals and measuring the frequency domain correlations. The necessity of prior noise power knowledge for the sensing threshold determination is removed by employing the proposed interference insensitive test metric, which is a ratio of uniformly distorted correlations. As a result, no noise power information is required in the sensing process. In addition, the effects of both timing and frequency offsets are mitigated with a novel extended time domain segmentation as well as multiple frequency domain correlations via a frequency sliding window. Numerical results are provided to validate the theoretical analysis and estimate the performance of the proposed algorithm. Copyright © 2012 John Wiley & Sons, Ltd.     

Channel‐power profile estimation for orthogonal frequency‐division multiplexing systems

In this work, a channel‐power profile estimation for orthogonal frequency‐division multiplexing systems, based on the cyclic prefix (CP), is introduced. By knowing the delay of each path, the time‐dispersion information can be derived. The proposed method, considering long intersymbol interference (ISI) fading channels, requires only the coarse symbol timing information. More specifically, quasi‐stationary fading channels are considered. The basic contribution is to obtain the maximum‐likelihood estimation of the correlation coefficient based on the CP. Subsequently, the relationship between the correlation coefficient and the channel‐tap powers is explored. With the estimate of correlation coefficient, the least‐square solution of the channel‐tap powers can be determined. The proposed method is suitable for both short and long ISI channels. Furthermore, the Cramér–Rao lower bound of the channel‐power profile estimation is analyzed, and simulations confirm the advantages of the proposed estimator. Copyright © 2011 John Wiley & Sons, Ltd.     

Parallel relay‐assisted three‐phase MIMO space division multiple access transmission for multi‐hop throughput improvement

Multi‐hop communications equipped with parallel relay nodes is an emerging network scenario visible in environments with high node density. Conventional interference‐free medium access control (MAC) has little capability in utilizing such parallel relays because it essentially prohibits the existence of co‐channel interference and limits the feasibility of concurrent communications. This paper aims at presenting a cooperative multi‐input multi‐output (MIMO) space division multiple access (SDMA) design that uses each hop's parallel relay nodes to improve multi‐hop throughput performance. Specifically, we use MIMO and SDMA to enable concurrent transmissions (from multiple Tx nodes to single/multiple Rx nodes) and suppress simultaneous links' co‐channel interference. As a joint physical layer (MAC/PHY) solution, our design has multiple MAC modules including load balancing that uniformly splits traffic packets at parallel relay nodes and multi‐hop scheduling taking co‐channel interference into consideration. Meanwhile, our PHY layer modules include distributive channel sounding that exchanges channel information in a decentralized manner and link adaptation module estimating instantaneous link rate per time frame. Simulation results validate that compared with interference‐free MAC or existing Mitigating Interference using Multiple Antennas (MIMA‐MAC), our proposed design can improve end‐to‐end throughput by around 30% to 50%. In addition, we further discuss its application on extended multi‐hop topology. Copyright © 2012 John Wiley & Sons, Ltd.     

Optimal resource allocation for multiple network‐coded two‐way relay in orthogonal frequency division multiplexing systems

This paper studies optimal resource allocation for multiple network‐coded two‐way relay in orthogonal frequency division multiplexing systems. All the two‐way relay nodes adopt amplify‐and‐forward and operate with analog network coding protocol. A joint optimization problem considering power allocation, relay selection, and subcarrier pairing to maximize the sum capacity under individual power constraints at each transmitter or total network power constraint is first formulated. By applying dual method, we provide a unified optimization framework to solve this problem. With this framework, we further propose three low‐complexity suboptimal algorithms. The complexity of the proposed optimal resource allocation (ORA) algorithm and three suboptimal algorithms are analyzed, and it is shown that the complexity of ORA is only a polynomial function of the number of subcarriers and relay nodes under both individual and total power constraints. Simulation results demonstrate that the proposed ORA scheme yields substantial performance improvement over a baseline scheme, and suboptimal algorithms can achieve a trade‐off between performance and complexity. The results also indicate that with the same total network transmit power, the performance of ORA under total power constraint can outperform that under individual power constraints. Copyright © 2012 John Wiley & Sons, Ltd.     

Recursive maximum likelihood estimation of time‐varying carrier frequency offset for orthogonal frequency‐division multiplexing systems

A recursive maximum likelihood carrier frequency offset (CFO) estimator is proposed in this work, where redundancy information contained in the cyclic prefix of multiple consecutive orthogonal frequency‐division multiplexing (OFDM) symbols is exploited in an efficient recursive fashion. Because the estimator is based on multiple OFDM symbols, the time‐varying CFO must be considered. We investigate the effect of time‐varying CFO on the performance of the estimator and the trade‐off between fast tracking ability and low estimation variance. We show that, without channel noise, the mean squared error (MSE) of estimation due to CFO estimation variation increases approximately quadratically with n, where n is the number of OFDM symbols used for CFO estimation (estimation window size), whereas the MSE due to channel noise decreases proportionally to 1/n (approximately) if the CFO is constant. A closed‐form expression of the optimal estimation window size (approximately) is derived by minimizing the MSE caused by both time‐varying CFO and channel noise. For wireless systems with time‐varying rate of change for CFO, the proposed estimator can be implemented adaptively. In addition, typical optimal estimation window sizes for WiMAX, DVB‐SH and MediaFLO systems are evaluated as an example. Copyright © 2011 John Wiley & Sons, Ltd.     

Multiple access interference and multipath interference analysis of orthogonal complementary code‐based ultra‐wideband systems over multipath channels

In order to alleviate multiple access interference and multipath interference of ultra‐wideband (UWB) system, we propose the orthogonal complementary code (OCC)‐based direct‐sequence UWB system and offset‐stacking (OS)‐UWB system. OCC has perfect partial autocorrelation and cross‐correlation characteristics. With the application of OCC in UWB system, we can obtain better performance in multiple access interference and multipath interference. The proposed OS‐UWB structure can also achieve variable data rate transmission because of its innovative OS spreading technique. Theoretical analysis and simulation results show that the proposed UWB system can achieve excellent performance and outperform the unitary code‐based direct‐sequence UWB system. Copyright © 2013 John Wiley & Sons, Ltd.     

Rate adaptive resource allocation in orthogonal frequency division multiple access system using multi‐objective immune algorithm

Rate adaptive downlink resource allocation in orthogonal frequency division multiple access system is a constraint optimization problem, which is to maximize the minimum data rate of the user subject to constraint that total power cannot exceed a given value. How to handle the constraint is a key issue for constrained optimization problem. Different with the available schemes on constraint handling, the proposed algorithm converts the constraint into an objective. Then, the resource allocation is combined into a multi‐objective optimization problem. An improved multi‐objective optimization algorithm based on artificial immune system is proposed to solve it. The simulation results show that, compared with previous schemes, the proposed algorithm performs remarkable improvement in sum capacity. Copyright © 2013 John Wiley & Sons, Ltd.     

Noise‐based spreading in code division multiple access systems for secure communications

Binary spreading sequences or chaotic nonbinary sequences are used in code division multiple access systems. In contrast to those sequences, this paper presents the theory, simulation, and implementation in digital signal processor technology of a code division multiple access system that uses random spreading sequences. The expressions for probability of bit error are derived for the cases when the additive white Gaussian noise and fading are present in the channel. Because the overall transmitted signal is a sum of random signals, the security of signal transmission in this communication system can be enormously increased and will depend on the goodness of the random sequence generator. The developed theory was confirmed by a prototype of the system that was designed in digital signal processor technology. The prototype used spreading sequences that had statistical characteristics very close to the characteristics of theoretical random sequences. The main problem in the system is how to synchronize the received spread sequence with the locally generated sequence. Hence, a mathematical model of the synchronization block for the system has been separately designed and developed. The synchronization procedure has been demonstrated on the basis of application of a periodically repeated pilot sequence. Copyright © 2014 John Wiley & Sons, Ltd.     

Polyphase filter bank based multi‐band spectrum sensing in cognitive radio systems

Spectrum sensing has been identified as an essential enabling functionality for cognitive radio (CR) systems to guarantee that CR users could share the spectrum resource with licensed users on a non‐interfering basis. Recently, simultaneous sensing of multi‐band licensed user activity has been attracting more and more research interest. Generally, the multi‐band sensing is implemented through energy detection by estimating power spectral density. In this paper, we investigate a multi‐band energy detection architecture based on different polyphase filter banks (PFBs), which aim to reliably sense multiple active bands by exploiting the low power leakage property of PFB. We have theoretically derived the closed‐form expressions of detection probability and false alarm probability for PFBs and fast Fourier transform based detectors, respectively. Theoretical detection thresholds are therefore computed, which ensures a fair comparison for different detectors. Final experimental results are presented to verify our theoretical analysis and demonstrate that PFBs based sensing architecture exhibits better sensing performance than the conventional FFT.Copyright © 2014 John Wiley & Sons, Ltd.     

Reducing code wastage in orthogonal variable spreading factor‐based wideband code division multiple access networks

Most third‐generation and beyond wideband code division multiple access networks use the orthogonal variable spreading factor code tree for channelization codes. The codes in this code tree are limited and the performance of a wireless network depends upon the code assignment for new calls. In this paper, we introduce a term called ‘wastage capacity’, which gives us the amount of wastage caused when a code (single or multiple) with a data rate higher than the rate of the incoming call is assigned to it. We suggest two methods to keep wastage capacity below an arbitrary threshold value or zero. In the first method, we devised an algorithm in which wastage up to a certain threshold would be tolerated and the minimum rakes to get this wastage capacity were identified. In the second approach, we reduced the wastage capacity to zero irrespective of the number of rakes at the expense of higher cost and complexity. Copyright © 2011 John Wiley & Sons, Ltd.     

Bit error rate performance of orthogonal frequency‐division multiplexing relaying systems with high power amplifiers and Doppler effects

This paper analyzes the bit error rate performance of orthogonal frequency‐division multiplexing systems in mobile multihop relaying channels. We considered the uplink scenario and quantified the effects of mobile channel impairments such as Doppler shift due to user mobility per hop, high power amplifier distortions when amplifying the transmitted/relayed orthogonal frequency‐division multiplexing symbol per hop, and the cumulative effects of these impairments on multihop relaying channels. It was shown that the resulting intercarrier interference due to the cumulative effects of the phase noise generated by these impairments per hop becomes very significant in a multihop relaying communication system and severely degrades the bit error rate performance of the system. Simulation results agree well with and validate the analysis. Copyright © 2011 John Wiley & Sons, Ltd.     

Progressive intercarrier and co‐channel interference mitigation for underwater acoustic multi‐input multi‐output orthogonal frequency‐division multiplexing

Multi‐input multi‐output orthogonal frequency‐division multiplexing (MIMO‐OFDM) has been actively studied for high data rate communications over the bandwidth‐limited underwater acoustic (UWA) channels. Unlike existing receivers that treat the intercarrier interference (ICI) as additive noise, in this paper, the proposed receiver considers ICI explicitly together with the co‐channel interference (CCI) due to parallel transmissions in MIMO‐OFDM. Using a recently developed progressive receiver framework, the proposed receiver starts with low‐complexity ICI‐ignorant processing and then progresses to ICI‐aware processing with increasing ICI levels. The key components of the proposed receiver include the following: (1) compressed sensing‐based sparse channel estimation, (2) soft‐input soft‐output minimum mean square error/Markov chain Monte Carlo detector for interference mitigation, and (3) soft nonbinary low‐density parity check decoding. In addition to simulation, we use real data from the Surface Processes and Acoustic Communications Experiment 2008 (SPACE08) and the Mobile Acoustic Communications Experiment 2010 (MACE10) to verify the system performance, where the transmitter in SPACE08 was stationary and that in MACE10 was slowly moving. Simulation and experimental results show that explicitly addressing ICI and CCI significantly improves the performance of MIMO‐OFDM in UWA systems. Copyright © 2012 John Wiley & Sons, Ltd.     

Performance analysis of cooperative spectrum sharing using non‐orthogonal multiple access

This paper presents a cooperative spectrum sharing protocol using non‐orthogonal multiple access in cognitive radio networks. A 2‐phase protocol comprising of a primary transmitter‐receiver pair and a secondary transmitter‐receiver pair is considered. In the proposed protocol, 3 data symbols can be transmitted during the 2 phases; this is unlike the traditional decode‐and‐forward relaying where 1 data symbol can be transmitted and the conventional superposition coding–based overlay spectrum sharing and the cooperative relaying system using non‐orthogonal multiple access where 2 data symbols can be transmitted, under a single‐relay scenario. We have investigated performance of our proposed protocol in terms of ergodic sum capacity and outage probability along with analytical derivations over independent Rayleigh fading channels. We also compared our proposed protocol with the traditional decode‐and‐forward relaying, conventional superposition coding–based overlay spectrum sharing, and the cooperative relaying system using non‐orthogonal multiple access schemes to demonstrate efficacy of the proposed protocol. The simulation and analytical results are presented to confirm efficiency of the proposed spectrum sharing protocol.     

Performance analysis of multi‐stage interference cancellation for ultra‐wide band multiple‐access communication systems

In this contribution, a novel particle swarm optimization (PSO)‐based multi‐user detector (MUD) aided time‐hopping ultra‐wide band (TH‐UWB) system has been investigated in the multi‐path channel model. In this approach, the PSO‐based MUD employs the output of the Rake receiver as its initial value to search for the best solution which results in a formulated optimization mechanism. By taking advantage of the heuristic values and the collective intelligence of PSO technique, the proposed detector offers almost the same bit error rate (BER) performance as the full‐search‐based optimum MUD does, while greatly reducing the potentially computational complexity. Simulation results have been provided to examine the evolutionary behavior and the detection performance of the proposed PSO‐based MUD in both the additive white Gaussian noise and the multi‐path fading channel. Copyright © 2011 John Wiley & Sons, Ltd.     

Resource allocation strategies for full frequency reuse in tri‐sectorized multi‐cell orthogonal frequency division multiple access systems

In this paper, we propose several frequency reuse coordination schemes for interference management in orthogonal frequency division multiple access. The aim of these schemes, working together with time and frequency domain packet scheduling, is to achieve reuse of 1 at sector level in a tri‐sectorized base station scenario. Inter‐sector interference is strong at the frontier between sectors, and a tight coordination scheme needs be applied. To support this coordination scheme, the users' location information is then essential to determine if a user is prone to suffer strong inter‐sector interference. The performance of these resource allocation algorithms is compared with schemes based on fractional frequency reuse, where reuse of 1 at cell level is assumed. Copyright © 2012 John Wiley & Sons, Ltd.     

Integrating dynamic spectrum access and device‐to‐device via cloud radio access networks and cognitive radio

Dynamic Spectrum Access (DSA) can be integrated with Device‐to‐Device (D2D) communications to enable the exploitation of unused spectrum portions and to address the spectrum scarcity problem. Spectrum management mechanisms integrated into DSA and D2D allow low‐power communications between User Equipments without interfering with licensed primary users. However, these mechanisms tend to be energy and processing intensive, being unfeasible to implement in User Equipments with strict battery and processing limitations. On the other hand, Cloud Radio Access Networks already leverage the virtually unlimited computing capacity of clouds for baseband processing functions. Thus, in this article, we propose the Cognitive Radio Device‐to‐Device (CRD2D) approach aiming to offload spectrum management functionality to the cloud taking advantage of Cloud Radio Access Networks architecture to support the integration of DSA and D2D.     

Cross‐entropy‐based joint carrier frequency offset estimation and multiuser detection for multicarrier code division multiple access systems

This paper presents a joint carrier frequency offset estimation and multiuser detection based on a maximum likelihood approach in multicarrier code division multiple access systems. With the definition of a score function based on the log‐likelihood, the joint carrier frequency offset estimation and multiuser detection can be formulated as a nonlinear optimization problem over the joint of a multidimensional real space and a multidimensional discrete space. To reduce the computational complexity required by the joint decision statistic, while still obtaining a desirable performance, a new method using cross‐entropy optimization is proposed to solve the nonlinear optimization problem. Because of the robustness of the cross‐entropy optimization, the joint decision statistic can be efficiently solved, and, as shown by the furnished simulation results, the proposed approach can achieve satisfactory performance in various scenarios. Copyright © 2011 John Wiley & Sons, Ltd.