基于随机天线阵的物理层安全通信系统峰均功率比问题研究

采用多天线发射方式保障通信信息的安全传输是物理层安全通信研究领域中常采用的手段之一,但这种方式会导致发射信号高峰均功率比的缺点,从而影响合法用户的通信性能。该文从多天线发射的加权系数出发,定义了基于多天线发射方式的物理层安全通信信号峰均功率比(PAPR)概念,并且针对随机天线阵物理层安全通信系统提出一种基于部分传输序列的物理层安全通信信号PAPR抑制算法。仿真结果表明该文提出的PAPR抑制算法能在不降低发射信号安全性能的条件下有效地降低发射信号的PAPR值。     

PTS with Non-Uniform Phase Factors for PAPR Reduction in OFDM Systems

Partial transmit sequence (PTS) is one of the effective and uncomplicated PAPR reduction methods for OFDM systems. The phase factors applied in the weighting sequences are assumed to be uniformly distributed within [0, 2pi). By carefully investigating the phase distribution of OFDM signals, we proposed a modified PTS scheme outperforming the conventional PTS scheme in PAPR reduction performance. The process of the proposed scheme is the same as the conventional PTS, except for applying the new non-uniform phase factors. The required side information is the same as the conventional PTS applying uniform phase factors.     

PAPR Reduction in MIMO-OFDM Systems: Spatial and Temporal Processing

Multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) technology is a promising solution for next generation wireless communications, due to high bandwidth efficiency, resistance to RF interference, and robustness to multipath fading. A major drawback of OFDM is its high peak-to-average power ratio (PAPR) which results in non-linearities in the output signal. In this paper, two methods based on spatial/temporal processing are proposed to reduce the PAPR of MIMO-OFDM systems. These methods divide the OFDM block at each transmit antenna into some subblocks. Then, spatial and temporal processing in the form of circular shifting or interleaving are applied to generate different candidate sequences. Finally, for each transmit antenna the candidate sequence with the lowest PAPR is chosen for transmission. Compared to the conventional PAPR reduction schemes such as ordinary partial transmit sequences (O-PTS), the proposed methods require lower computational complexity and have superior PAPR reduction performance.     

Joint ICI Cancellation and PAPR Reduction in OFDM Systems Without Side Information

Intercarrier interference (ICI) self-cancellation, new ICI self-cancellation and conjugate cancellation schemes have been proposed in the literature to mitigate the effect of ICI. In this paper we have performed the mathematical analysis of PAPR performances for ICI self-cancellation, new ICI self-cancellation and ICI conjugate cancellation schemes and it is found that PAPR performance of these schemes are either very close to or poorer than the standard OFDM signal, which necessitates the requirement of PAPR reduction. After realizing the need of PAPR reduction in ICI cancellation schemes, we have proposed a joint scheme to reduce ICI and PAPR simultaneously. In this paper, we have proposed a multipoint partial transmit sequence (PTS) scheme, to improve the PAPR performance of ICI cancellation schemes. The proposed multipoint PTS based PAPR reduction scheme is coupled with ICI cancellation schemes in such a way that CIR performance of these schemes after coupling remains unchanged and no SI is required at the receiver to recover the original data signal. A comparison of CIR and PAPR performances for ICI cancellation schemes with and without PAPR reduction is also presented in this paper. The analytical results of CIR and PAPR performances for conventional ICI cancellation and joint ICI cancellation and PAPR reduction confirm the outperformance of the proposed scheme. We have also evaluated the SER performance of the joint schemes over additive white Gaussian noise and fading channels and presented a comparison with other existing schemes.     

GA-PTS Using Novel Mapping Scheme for PAPR Reduction of OFDM Signals Without SI

In recent time orthogonal frequency division multiplexing (OFDM) has proved its mettle as a preferred choice for high speed transmission in wireless applications due its efficient mechanism to combat the inter symbol interference. However sudden high peaks in OFDM signal envelope lead to high peak to average power ratio (PAPR). Enhanced values of PAPR result into complex RF amplifier circuit with reduced efficiency. There are various methods available for PAPR reduction, out of them partial transmit sequence (PTS) is most effective proven choice for PAPR reduction. However PTS technique requires cumbersome searching of all possible phase factors to find optimal phase factor which produces lowest PAPR, the information regarding optimal phase set used at the transmitter need to be sent to the receiver as side information (SI) for decoding purpose, however transmitting SI requires additional transmission bandwidth thus reducing overall bandwidth efficiency. To reduce the exhaustive searching genetic algorithm (GA) could be used with PTS leading to GA-PTS system. In this paper a GA-PTS system is proposed which uses novel octagonal geometry for constellation extension purpose. This scheme does not require transmission of any side information for decoding purpose at receiver and at the same time GA-PTS system reduces the required number of searches. Simulations are presented to show that proposed scheme provides similar PAPR performance as conventional PTS but without need of SI transmission at reduced number of searches.     

A simple transmit diversity technique for wireless communications

This paper presents a simple two-branch transmit diversity scheme. Using two transmit antennas and one receive antenna the scheme provides the same diversity order as maximal-ratio receiver combining (MRRC) with one transmit antenna, and two receive antennas. It is also shown that the scheme may easily be generalized to two transmit antennas and M receive antennas to provide a diversity order of 2M. The new scheme does not require any bandwidth expansion or any feedback from the receiver to the transmitter and its computation complexity is similar to MRRC     

Merging multicarrier CDMA and oscillating-beam smart antenna arrays: exploiting directionality, transmit diversity, and frequency diversity

In this paper, a novel merger of multicarrier code-division multiple access (MC-CDMA) and smart antenna arrays is introduced. Here, a group of Q carriers in the MC-CDMA system is applied to its own M-element smart antenna array at the base station (BS). The smart antennas are located in close proximity to one another. We generate a transmit diversity gain at the receiver by carefully moving (oscillating) the antenna array's pattern. The pattern oscillation is achieved by applying appropriate time-varying phases to array elements of each smart antenna. The beam pattern oscillation ensures a mainlobe at the position of the intended user and small oscillations in the beam pattern. This beam pattern oscillation leads to a time-varying channel with a controllable coherence time; hence, a transmit diversity benefit, in the form of a time diversity benefit, is available at the receiver. Employing MC-CDMA with the proposed smart antenna at the BS, we achieve: 1) directionality which creates high network capacity via space-division multiple access; 2) a transmit diversity gain which supports high performance at the receiver in the mobile unit; and 3) increased capacity and performance via MC-CDMA's ability to support both CDMA and frequency diversity benefits, respectively.     

Low Complexity Partial Transmit Sequence with Complex Gain Memory Predistortion in OFDM Systems

In this paper the peak to average power ratio (PAPR) reduction and digital predistortion effects in orthogonal frequency division multiplexing (OFDM) systems are investigated. By applying a predistortion technique called complex gain memory predistortion (CGMP), power amplifier works at higher power efficiency. The proposed enhanced partial transmit sequence scheme is applied for PAPR reduction and integration with CGMP technique results in increasing in OFDM system efficiency and prolonged battery life. Simulation and results are examined with actual power amplifier and OFDM signal with quadrature phase shift keying (QPSK) modulation.     

Peak Power Reduction in Closed-Loop MIMO-OFDM Systems via Mode Reservation

This letter studies peak-to-average power ratio (PAPR) reduction for orthogonal frequency-division multiplexing (OFDM) signals in eigenbeamformed multiple antenna systems. The weak eigenmodes abandoned by the waterfilling allocation are intentionally filled in order to offset signal peaks in the time domain.Numerical results indicate that this method can significantly increase the efficiency of peak power constrained systems with no modification of the receiver required.     

Rapid slot synchronization in the presence of large frequency offset and Doppler spread in WCDMA systems

Slot synchronization is the most challenging step for rapid cell search in intercell asynchronous code division multiple access systems such as wide-band code division multiple access (WCDMA). For rapid cell search, it is desirable to design the receiver robust to initial frequency offset and Doppler spread. In this letter, we consider combining schemes for rapid slot synchronization of WCDMA signals in such channel impairments. We propose an inner-slot differential combining scheme that exploits partial correlation of slot synchronization code, making the receiver tolerable to a large amount of initial frequency offset and Doppler spread. Unlike conventional differential combining schemes, the proposed scheme is also applicable to the use of transmit antenna diversity. The detection performance of the proposed combining scheme is analyzed with the use of transmit antenna diversity. Finally, the analytic results are verified by computer simulation.     

Peak-to-average ratio reduction scheme for multi-carrier systems with block-based channel estimation

A simple scheme is proposed to reduce the peak-to-average ratio (PAPR) for multi-carrier systems with block-based channel estimation, where each user simultaneously transmits multiple traffic blocks across the frequency, and the channel estimation is done within each block. Unlike most existing schemes, the proposed scheme does not require additional side information or estimation at the receiver. Results indicate significant improvements in both the PAPR and cubic metric reduction.     

Peak‐to‐average power ratio reduction in space–time coded MIMO‐OFDM via preprocessing

In this paper, we propose a trellis exploration algorithm based preprocessing strategy to lower the peak‐to‐average power ratio (PAPR) of precoded MIMO‐OFDM. We first illustrate the degradation in PAPR due to optimal linear precoding in MIMO‐OFDM systems. Then we propose two forms of multi‐layer precoding (MLP) schemes to reduce PAPR. In both schemes, the inner‐layer precoder is designed to optimize system capacity/BER performance. In the first MLP scheme (MLP‐I), a common outer‐layer polyphase precoding matrix is employed. In the second MLP scheme (MLP‐II), data stream corresponding to every transmit antenna is precoded with a different outer‐layer polyphase precoding matrix. Both outer‐layer precoders are custom designed using the trellis exploration algorithm by applying the aperiodic autocorrelation of OFDM data symbols as the metric to minimize. Simulation results indicate that both MLP schemes show superior PAPR performance over conventional MIMO‐OFDM with and without precoding. In addition, MLP better exploits frequency diversity resulting in BER performance gains in multi‐path environments. Copyright © 2010 John Wiley & Sons, Ltd.     

SFBC MIMO-OFDM peak-to-average power ratio reduction by polyphase interleaving and inversion

Many PAPR reduction schemes have been proposed for OFDM systems. Among these, the signal scrambling methods such as the partial transmit sequences (PTS) (S. H. Muller, et al., 1997) and selective mapping (SLM) (R. W. Bauml, et al., 1996) are attractive as they obtain better PAPR property by modifying OFDM signals without distortion. These schemes can also be applied to a SFBC MIMO-OFDM system, which is advantageous for dispersive channels, in a straightforward way by performing signal scrambling on data sequence before it is distributed to the transmit antennas according to employed encoding scheme. Note however that in the case of PTS PAPR reduction in the time domain is not possible, which leads to prohibitively large complexity of such scheme. In this letter, we introduce more effective approach, the polyphase interleaving and inversion (PII) PAPR scheme and its reduced complexity version (RC-PII), which is designed to suppress peaks in SFBC-OFDM, transmit diversity.     

Antenna combining for the MIMO downlink channel

A multiple antenna downlink channel where limited channel feedback is available to the transmitter is considered. In a vector downlink channel (single antenna at each receiver), the transmit antenna array can be used to transmit separate data streams to multiple receivers only if the transmitter has very accurate channel knowledge, i.e., if there is high-rate channel feedback from each receiver. In this work it is shown that channel feedback requirements can be significantly reduced if each receiver has a small number of antennas and appropriately combines its antenna outputs. A combining method that minimizes channel quantization error at each receiver, and thereby minimizes multi-user interference, is proposed and analyzed. This technique is shown to outperform traditional techniques such as maximum-ratio combining because minimization of interference power is more critical than maximization of signal power in the multiple antenna downlink. Analysis is provided to quantify the feedback savings, and the technique is seen to work well with user selection and is also robust to receiver estimation error.     

多路DFT扩频的低峰均比OSFBC传输方法

空频分组码（SFBC）可以为MIMO系统带来发射分集增益,而单载波频分多址（SC-FDMA）系统的发送信号具有低峰均比（PAPR）,但直接传输正交SFBC（OSFBC）会出现PAPR过大的问题。为此,本文以四发射天线为例针对OSFBC提出了一种基于DFT扩频的低PAPR传输方案,通过设置多个并行DFT单元并将得到的多组频域数据等间隔交叉排列映射到子载波上,根据频域循环移位和内插的性质使时域信号等效为多条单载波信号的叠加,PAPR与同点数SC-FDMA信号大致相当。同时,本文方案未带来误码率性能的下降,能够取得的分集增益与正交空时分组码（OSTBC）相同。      

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.     

用于移动通信系统的发送分集技术

在分析接收分集技术最大比率接收合并(MRRC)方案的基础上，介绍了一种两分支发送分集方案。该方案采用2个发射天线、1个接收天线，可提供与1个发送天线、2个接收天线情况下的MRRC方案相同的分集增益。由于发送符号采用了正交性设计，该发送分集方案在接收端可以将不同的发送符号分离开来，分别进行最大似然检测。仿真结果表明，运用分集技术可大大改善无线通信系统的性能，且该方案和一发两收的MRRC方案性能相似、计算复杂性相同。该发送分集方案能更好地应用于移动通信系统。     

Space-time multiplexing for mimo multiuser downlink channels

In this paper, we study the downlink of a multiuser system, in which antenna arrays are employed at both the transmitter (base station) and the receivers (clients). A space-time modulation technique that can be seen as two-dimensional spreading is introduced. It provides full transmit diversity for every user, and accommodates N_t times the number of users as a single-antenna code-division multi-access (CDMA) scheme, where N_t is the number of transmit antennas. Thus multiple access is provided through spatial as well as code dimensions. In addition, the scheme forms groups of users that are orthogonal to each other. This feature translates into simplified detection strategies without loss of performance. The main detector structure of interest is a two-stage interference canceller because of its low complexity compared to other joint detectors. We will demonstrate that in conjunction with an unequal power allocation scheme, this receiver provides full diversity and suffers from only a small performance loss compared to the full-complexity maximum likelihood (ML) receiver. In a single-user multiple antenna system, the same spreading scheme and unequal power allocation yields a new approach to designing full-rate, full-diversity space-time codes having good performance with successive interference cancellers     

Multiple trellis coded orthogonal transmit scheme for multiple antenna systems

In this paper, a novel multiple trellis coded orthogonal transmit scheme is proposed to exploit transmit diversity in fading channels. In this scheme, a unique vector from a set of orthogonal vectors is assigned to each transmit antenna. Each of the output symbols from the multiple trellis encoder is multiplied with one of these orthogonal vectors and transmitted from corresponding transmit antennas. By correlating with corresponding orthogonal vectors, the receiver separates symbols transmitted from different transmit antennas. This scheme can be adopted in coherent/differential systems with any number of transmit antennas. It is shown that the proposed scheme encompasses the conventional trellis coded unitary space-time modulation based on the optimal cyclic group codes as a special case. We also propose two better designs over the conventional trellis coded unitary space-time modulation. The first design uses 8 Phase Shift Keying （8-PSK） constellations instead of 16 Phase Shift Keying （16-PSK） constellations in the conventional trellis coded unitary space-time modulation. As a result, the product distance of this new design is much larger than that of the conventional trellis coded unitary space-time modulation. The second design introduces constellations with multiple levels of amplitudes into the design of the multiple trellis coded orthogonal transmit scheme. For both designs, simulations show that multiple trellis coded orthogonal transmit schemes can achieve better performance than the conventional trellis coded unitarv space-time schemes.     

Antiphase Tones Across Transmitting Antennas: A Spectrum Sharing Technique for Cognitive CO-STFC MB-OFDM UWB System

This paper proposes a spectrum sharing technique for coexistence of multiband orthogonal frequency division multiplexing ultra wideband (UWB) system with other primary wireless services. Existing technique like tone nulling do not utilize subcarriers within victim band, hence affects the throughput of the system. The same nulling effect can be produced by applying antiphase tones across transmitting antennas and simultaneously utilizing the victim band to transmit the data. This scheme can be applied without any impact on the current specifications of the system. In this paper complex orthogonal space time frequency code is implemented to achieve diversity in multiple antenna system. UWB receiver processing is described for both victim and unprotected band. A less computationally complex active interference cancellation scheme is also applied to improve notch depth. The proposed scheme is implemented for fixed frequency interleaving operation and discussed for time frequency interleaving with special cases. The proposed scheme is very simple to implement and provide flexibility in terms of notch width and depth.