Peak-to-average power ratio in high-order OFDM

The problem of peak-to-average power ratio (PAPR) of high-order orthogonal frequency-division modulation (OFDM) is considered. Using results on level crossing of random processes, an upper bound on the probability that the PAPR of an OFDM signal will exceed a given value is derived. Numerical computations are used to show that this bound is tight for low-pass OFDM systems. The central limit theorem is used to find an asymptotic expression for the bound when the number of carriers N grows to infinity. The central limit theorem is also used to find an asymptotic expression for another bound that is based on the envelope of the OFDM signal, and is tighter for bandpass systems. It is shown that, effectively, the PAPR grows as 2lnN and not linearly with N, and by developing a lower bound on the probability that the PAPR of an OFDM signal will exceed a given value, it is shown that asymptotically most OFDM symbols have a PAPR close to 2lnN. Some approaches to coping with the PAPR problem are discussed in light of the obtained results     

多载波DS-CDMA系统抗干扰能力分析

结合并行转输和多载波调制技术，提出了一种多载波直接序列码分多址模型，对系统在存在窄带干扰的Rayleigh衰落信道下的性能进行了理论分析，并同Rake接收的单载波DS CDMA系统进行了比较。分析和仿真表明：该系统具有较强的抑制窄带及多址干扰的能力，同单载波DS CDMA系统相比，能支持更多的用户数。     

Peak-to-average power ratio and power efficiency considerations in MIMO-OFDM systems

Peak-to-average power ratio (PAR) reduction is a powerful tool that can increase power efficiency and reduce distortion noise in MIMO-OFDM systems. Various PAR metrics have been used in the MIMO-OFDM literature without justifications regarding the physical mechanisms that support those PAR definitions. In this paper, we show that in order to deliver high power efficiency, two MIMO-OFDM linear-scaling schemes are possible: one that scales all MIMO branches with the same factor and another that uses a different scaling factor for each branch. For each system, we derive a meaningful PAR metric that should be used.     

Peak-to-average power ratio in MIMO-OFDM systems using selective mapping

This letter evaluates the peak-to-average power ratio (PAR) performance in a space-time block coded (STBC) multi-input multi-output orthogonal frequency-division multiplexing (MIMO-OFDM) system using the selective mapping (SLM) approach. The investigated SLM-based MIMO-OFDM system selects the transmitted sequence with the lowest average PAR over all transmit antennas and retrieves the side information very accurately at the expense of only 0.5-dB degradation of the PAR, which can improve the overall detection performance of the MIMO-OFDM system with erroneous side information, compared to the individual SLM approach.     

Combined power control and error-control coding in multicarrier DS-CDMA systems

We propose truncating the transmission power (allocating no power) for symbols with low channel gain, and tagging erasures on the corresponding symbols at the receiver. The motivation is that symbols with low channel gain are highly likely to be in error and yet, if transmitted, consume the energy resource and generate interference to other users. Truncating the power for those symbols has the effect of reducing the interference to other users and allocating more power on symbols with high channel gain (thereby reducing the error probability). Since block codes can correct twice as many erasures as errors, the coded performance can be improved by properly combining the power control with the error-control coding. In this letter, we analyze the performance of the Reed-Solomon-coded multicarrier direct-sequence code-division multiple-access systems with two power-control schemes. We show that the probability of incorrect decoding can be significantly improved by properly combining the power control with the error control coding.     

降低OFDM系统峰均功率比的方法

针对正交频分复用系统,提出了一种复杂度较低的降低峰均功率比的方法.该方法首先实施时域过采样,再采用功率归一化和限幅,最后对限幅后的信号进行低通滤波.此外,从信噪比和信号噪声失真比等角度分析了限幅对过采样OFDM信号的影响.仿真结果表明,与传统的限幅滤波方法对比,该方法能有效抑制滤波后系统峰均功率比的回升.     

Peak-to-average power ratio reduction in CDMA systems using metric-based symbol predistortion

In this paper, we propose a simple peak-to-average power ratio (PAPR) reduction algorithm for code-division multiple access (CDMA) systems. The described algorithm is based on predistorting a set of symbols using an appropriate metric and does not need sending any side information to the receiver. Using binary phase-shift keying (BPSK) modulation, we show that performance of this algorithm approaches that of the optimum PAPR reduction procedure while avoiding its excessive complexity.     

Rayleigh衰落信道下多载波DS-CDMA系统性能分析

本文基于多载波传输技术，提出了一种多载波码分多址模型。并在Rayleigh衰落信道下对其误码性能进行了分析。分析及仿真结果表明，系统具有降低多址干扰的能力，同传统的单载波直接序列码分多址系统相比，能支持更大的用户容量。本系统能满足第三代移动通信系统对高速数据传输的要求。     

Quasi-synchronous multicarrier DS-CDMA using Z-Complementary sequences

In this paper, a multicarrier DS-CDMA system which employs Z-Complementary Se- quences (ZCS) as spreading code is investigated. The new system can not only eliminate multipath interference and Multiple Access Interference (MAI), but also support flexible number of users com- pared with system using Orthogonal Complementary (OC) codes. Multicarrier DS-CDMA using OC codes can be regarded as a special case of our proposed system. Zero Correlation Zone (ZCZ) of ZCS can be flexibly adjusted to meet the requireme...     

基于非对称限幅法降低OOFDM系统的PAPR

为降低OOFDM(光正交频分复用)系统中的高PAPR(峰均功率比),文章提出了一种基于非对称限幅法的抑制算法,其核心思想是通过门限电平去除信号负项并对信号峰值进行限幅,产生满足OOFDM系统要求的正、实基带信号,同时联合滤波技术避免较大PAPR的出现.仿真结果表明,该方法既能有效地改善PAPR特性,又能保证系统获得可靠...     

应用PTS降低MIMO-OFDM信号峰均比技术研究

多输入多输出正交频分复用（MIMO—OFDM）系统已成为新一代高速通信系统研究中的热点。而系统所产生的高峰均比（PAPR）信号是限制MIMO—OFDM技术实用化的主要障碍。在对部分传输序列（PTS）技术进行研究的基础上。本文提出了一种应用PTS降低MIMO—OFDM信号峰均比（PAPR）的方案一并行PTS。仿真结果表明。该方法能有效降低MIMO—OFDM信号峰均比。     

Subspace multiuser detection for multicarrier DS-CDMA

A subspace-based linear minimum mean-squared error (MMSE) multiuser detection scheme is proposed for a multicarrier direct-sequence code-division multiple-access (MC-DS-CDMA) system. Typically, a MC-DS-CDMA system employs a band-limited chip waveform. The band-limited nature of the chip waveform causes problem in applying standard subspace techniques because no non noise subspace can be formed. It is shown that channel and timing information needed for the construction of the linear MMSE detector can be identified by a multiple-signal-classification-like algorithm based on a finite-length truncation approximation of the chip waveform. In practice, since perturbed versions of the subspaces assumed in the finite-length truncation approximation are actually observed, and because of the band-limited property of the chip waveform, the accuracy of the channel estimation and, hence, the performance of the MMSE detector are degraded. This effect is investigated in this paper.     

Interference-free broadband single- and multicarrier DS-CDMA

The choice of the direct sequence spreading code in DS-CDMA predetermines the properties of the system. This contribution demonstrates that the family of codes exhibiting an interference-free window (IFW) outperforms classic spreading codes, provided that the interfering multi-user and multipath components arrive within this IFW, which may be ensured with the aid of quasi-synchronous adaptive timing advance control. It is demonstrated that the IFW duration may be extended with the advent of multicarrier DS-CDMA proportionately to the number of subcarriers. Hence, the resultant MC DS-CDMA system is capable of exhibiting near-single-user performance without employing a multi-user detector. A limitation of the system is that the number of spreading codes exhibiting a certain IFW is limited, although this problem may be mitigated with the aid of novel code design principles     

Software-defined-radio-assisted adaptive broadband frequencyhopping multicarrier DS-CDMA

Against the backdrop of the explosive expansion of the Internet and the continued dramatic increase in demand for high-speed multimedia wireless services, there is an urgent demand for flexible, bandwidth-efficient transceivers. Multistandard operation is also an important requirement for the future generations of wireless systems. This article presents a versatile broadband multiple access scheme, combining frequency hopping with multicarrier DS-CDMA (FH/MC DS-CDMA). In the short term, the proposed FH/MC DS-CDMA scheme could be employed in the existing second- and third-generation CDMA system bands. Hence, it has the potential of supporting compatibility with these existing systems. This article also shows that FH/MC DS-CDMA exhibits a high grade of flexibility in the context of system design and parameter reconfiguration. Therefore, in the long term, the proposed FH/MC DS-CDMA scheme is capable of meeting the requirements of future broadband mobile wireless systems, with the aid of introducing more advanced techniques facilitated by the employment of software-defined radios (SDR) and with the advent of efficient adaptive baseband algorithms     

Peak-to-average power ratio reduction of an OFDM signal usingpartial transmit sequences

Orthogonal frequency-division multiplexing (OFDM) is an attractive technique for achieving high-bit-rate wireless data transmission. However, the potentially large peak-to-average power ratio (PAP) has limited its application. Recently, two promising techniques for improving the PAP statistics of an OFDM signal have been proposed: the selective mapping and partial transmit sequence approaches. Here, we present suboptimal strategies for combining partial transmit sequences that achieve similar performance but with reduced complexity     

Analysis of a multicarrier DS-CDMA code-acquisition system

We present two code-acquisition schemes for a multicarrier direct-sequence code-division multiple-access system, one that uses equal gain combining and the other that uses selection combining. The code-acquisition performance of the two multicarrier systems, as well as that of a single-carrier system, are analyzed in both nonfading and Rayleigh fading channels under the assumption that the receiver is chip-synchronized; the effect of partial-band interference (PBI) on the performance is also included. It is demonstrated that in an additive white Gaussian noise channel, the single-carrier system has a better code-acquisition performance than both multicarrier systems. However, in a Rayleigh fading channel, the code-acquisition performance of a multicarrier system with equal gain combining is better than that of the single-carrier system, while a multicarrier system with selection combining has the same performance as the single-carrier system. Further, the presence of PBI more severely affects the code-acquisition performance of the single-carrier system than those of both multicarrier systems. Finally, the code-acquisition performance of a multicarrier system with equal gain combining is always better than that of the selection combining system     

On the performance of multicarrier DS-CDMA with imperfect power control and variable spreading factors

Multicarrier direct-sequence code-division multiple access (MC-DS-CDMA) becomes an attractive technique for the future fourth-generation (4G) wireless system because it can flexibly adapt transmission rates by changing both time and frequency spreading factors and possesses many physical-layer advantages in dispersive fading channels. However, power control errors (PCE) and the complete multiple access interference (MAI) from all the intersubcarriers may significantly degrade the performance of the MC-DS-CDMA system. In this paper, we propose an analytical method to evaluate the joint effects of the PCE and the complete MAI on the multirate MC-DS-CDMA system. From analysis and simulation, we obtain some important insights into the performance issues of the MC-DS-CDMA system. First, the effect of PCE can exacerbate the impact of the complete MAI on the MC-DS-CDMA system, or vice versa . For BER=10/sup -3/ in a considered case, the joint effect of the complete MAI and PCE further degrades the performance by 2.1 dB compared with the sum of the degradation from the complete MAI and the PCE individually. Second, increasing frequency or time-domain spreading gain can improve the performance of the MC-DS-CDMA system, but the system also becomes more sensitive to power control errors. Third, a larger PCE can possibly make the frequency-domain diversity diminish faster than the gain obtained from the time-domain spreading although an MC-DS-CDMA system with a larger frequency-domain spreading gain (M) is usually better than that with a larger time-domain spreading gain (G/sub o/). In our example, for the standard deviation of PCE (/spl sigma//sub e/) equal to 0 dB, the BERs with (M,G/sub o/)= (4, 16) and (16, 4) are 9.3/spl times/10/sup -4/ and 3.7/spl times/10/sup -5/, respectively, while for /spl sigma//sub e/=4 dB, the BER performances of the two cases are all in the order of 10/sup -3/.     

Reducing the peak-to-average power ratio of multicarrier modulationby selected mapping

The authors propose a method for the reduction of peak-to-average transmit power ratio of multicarrier modulation systems, called selected mapping, is presented, which is appropriate for a wide range of applications. Significant gains can be achieved by selected mapping whereas complexity remains quite moderate     

Channel coding scheme for orthogonal multicarrier DS-CDMA system

The scheme does not reduce the transmission rate of information nor increase the transmitted signal bandwidth of the uncoded multicarrier system. Simulation results for the suggested scheme in a multipath fading channel show considerable coding gain over the uncoded case     

Performance of generalized multicarrier DS-CDMA over Nakagami-mfading channels

A class of generalized multicarrier direct sequence code-division multiple-access (MC DS-CDMA) schemes is defined and its performance is considered over multipath Nakagami-m fading channels. The spacing between two adjacent subcarriers of the generalized MC DS-CDMA is a variable, allowing us to gain insight into the effects of the spacing on the bit error rate (BER) performance of MC DS-CDMA systems. This generalized MC DS-CDMA scheme includes the subclasses of multitone DS-CDMA and orthogonal MC DS-CDMA as special cases. We present a unified analytical framework for determining the exact average BER of the generalized MC DS-CDMA system over generalized multipath Nakagami-m (1960) fading channels. The optimum spacing of the MC DS-CDMA system required for achieving the minimum BER is investigated and the BER performance of the system having optimum spacing is evaluated. The resultant BER is compared with that of both multitone DS-CDMA and orthogonal MC DS-CDMA