Efficient ISI cancellation for STBC OFDM systems using successive interference cancellation

In this letter, we propose an efficient detection scheme for space-time block coded (STBC) orthogonal frequency-division multiplexing (OFDM) with insufficient cyclic prefix (CP). The proposed scheme employs successive interference cancellation (SIC) and cyclic prefix reconstruction (CPR) concepts. Simulation results present that the proposed scheme outperforms the conventional scheme for STBC OFDM systems.     

Space-time turbo multiuser detection for coded MC-CDMA

The system capacity and performance of multicarrier code-division multiple-access (MC-CDMA) communication systems can be significantly enhanced by jointly employing MAP-based multiuser detection (MUD) and channel decoding techniques. In this paper, a group-oriented soft iterative MUD based on the combination of smart antennas and iterative MAP-based MUD is presented. The proposed method is featured as a novel technique for further increasing the system capacity and performance. In this method, all the users are first grouped into several groups according to their impinging direction of arrivals (DOAs). All users with similar DOAs are classified into the same group and then low-complexity MAP-based iterative MUD is employed in each group. Because spatial filtering cannot suppress all the interference between the groups, interference cancellation among the groups is used prior to MUD within each group. It is shown that the proposed group-oriented soft iterative MUD algorithm can significantly reduce the computational complexity compared with the conventional optimal MAP-based MUD schemes. It is also demonstrated that the performance of the proposed algorithm can approach that of a single-user coded MC-CDMA system with an antenna array in additive white Gaussian noise and frequency selective fading channels.     

MIMO Frequency Hopping Spread Spectrum Multi-Carrier Multiple Access: A Novel Uplink System for B3G Cellular Networks

In this paper, we propose and give the performance of a novel uplink system based on the combination of multi-carrier (MC), code division multiple access (CDMA) and multiple input multiple output (MIMO) techniques. First, we describe the interests of spread-spectrum multi-carrier multiple access (SS-MC-MA) scheme for uplink, especially compared to MC-CDMA. Classically, with SS-MC-MA, each user spreads its data symbols on a specific subset of adjacent or multiplexed subcarriers, to facilitate the channel estimation and reduce complexity at the reception. In order to compensate for the lack of frequency diversity of SS-MC-MA with adjacent subcarriers, we first combine it with an orthogonal space-time block code (STBC) and demonstrate the resulting spatial diversity gain. Then, we propose to allocate the subsets to the different users by applying a frequency hopping pattern (FH). In that case, each user benefits from the frequency diversity linked to the total bandwidth as with the multiplexed subcarriers solution, while keeping the advantages of the adjacent subcarriers solution. The gain provided by the use of the frequency hopping is stressed on. Finally, the performance of this scheme is evaluated over realistic MIMO channel with channel turbo coding for systems offering asymptotic spectrum efficiency of 1, 2, 3 and 4.5 bit/s/Hz. Thus, the efficiency of the novel proposed STBC FH SS-MC-MA system as a very promising multiple access and modulation scheme for the uplink of the future wideband wireless networks is successfully demonstrated.     

A New ML Based Interference Cancellation Technique for Layered Space-Time Codes

This paper presents a new and simple decoding algorithm for layered space time block codes such as the two independent Alamouti's codes which are also called the double space-time transmit diversity (DSTTD) system. By using group interference suppression and successive interference cancellation, we can treat DSTTD as two independent space-time block codes (STBC). We can then decode both of these STBC's through a simple maximum likelihood (ML) detector with null space-based interference cancellation. We also compare the proposed interference cancellation (IC) scheme with the conventional MMSE IC scheme. The performance of the proposed IC scheme is comparable to that of the MMSE IC scheme while the complexity reduction factor of the proposed scheme can be up to 5 compared to the MMSE IC scheme.     

MVDR‐combining‐aided diversity and spatial multiplexing for MIMO multi‐cellular networks with cochannel interference

Two multiple‐input multiple‐output (MIMO) schemes (a diversity scheme and a spatial multiplexing scheme) that employ the minimum variance distortionless response (MVDR) combining are proposed for multi‐cellular networks with cochannel interference. With the receive diversity provided by the MVDR combining, the proposed diversity scheme can be benefited by both the transmit diversity and the receive diversity, also, the proposed spatial multiplexing scheme can be benefited by both the receive diversity and the spatial multiplexing. The proposed MIMO schemes do not require the space‐time coding or the successive interference cancellation, thus they can result in less computational complexity than space‐time block code (STBC) and vertical‐Bell Labs layered space‐time (V‐BLAST). We show that the capacity of the proposed diversity scheme is close to or larger than that of STBC for the noise‐corrupted case and is much larger than that of STBC for the interference‐corrupted case. We also show that the capacity of the proposed spatial multiplexing scheme can be much larger than that of V‐BLAST for the interference‐corrupted case and the noise‐corrupted case, and the proposed spatial multiplexing scheme can achieve good compromise between diversity and spatial multiplexing. Copyright © 2011 John Wiley & Sons, Ltd.     

Adaptive frequency‐hopping schemes for CR‐based multi‐link satellite networks

In this paper, we study two dynamic frequency hopping (DFH)–based interference mitigation approaches for satellite communications. These techniques exploit the sensing capabilities of a cognitive radio to predict future interference on the upcoming frequency hops. We consider a topology where multiple low Earth orbit satellites transmit packets to a common geostationary equatorial orbit satellite. The FH sequence of each low Earth orbit–geostationary equatorial orbit link is adjusted according to the outcome of out‐of‐band proactive sensing scheme, performed by a cognitive radio module in the geostationary equatorial orbit satellite. On the basis of sensing results, new frequency assignments are made for the upcoming slots, taking into account the transmit powers, achievable rates, and overhead of modifying the FH sequences. In addition, we ensure that all satellite links are assigned channels such that their minimum signal‐to‐interference‐plus‐noise ratio requirements are met, if such an assignment is possible. We formulate two multi‐objective optimization problems: DFH‐Power and DFH‐Rate. Discrete‐time Markov chain analysis is used to predict future channel conditions, where the number of states are inferred using k‐means clustering, and the state transition probabilities are computed using maximum likelihood estimation. Finally, simulation results are presented to evaluate the effects of different system parameters on the performance of the proposed designs.     

Analysis of multi‐user detection of multi‐rate transmissions in multi‐cellular CDMA

In this paper, we address the issue of multi‐user receiver design in realistic multi‐cellular and multi‐rate CDMA systems based on performance analysis. We consider the multi‐user detection (MUD) technique, denoted interference subspace rejection (ISR), because it offers a wide range of canonic suppression modes that range in performance and complexity between interference cancellers and linear receivers. To further broaden our study, we propose a modified ISR scheme called hybrid ISR to cope better with multi‐rate transmissions. The performance analysis, which is based on the Gaussian assumption (GA) and validated by simulations, takes into account data estimation errors, carrier frequency mismatch, imperfect power control, identification errors of time‐varying multipath Rayleigh channels and intercell interference. This analysis enables us to optimize the selection of the MUD mode for multi‐rate transmissions in different operating conditions. The effectiveness of interference cancellation is indeed investigated under different mobile speeds, numbers of receiving antennas, near‐far situations, channel estimation errors, and out‐cell to in‐cell interference ratios. This investigation suggests that the out‐of‐cell interference, the residual in‐cell interference, the noise enhancement as well as low mobility favor the simplest MUD modes as they offer the best performance/complexity tradeoffs. Copyright © 2008 John Wiley & Sons, Ltd.     

Hybrid SNR-Adaptive Multiuser Detectors for SDMA-OFDM Systems

Multiuser detection (MUD) and channel estimation techniques in space-division multiple-access aided orthogonal frequency-division multiplexing systems recently has received intensive interest in receiver design technologies. The maximum likelihood (ML) MUD that provides optimal performance has the cost of a dramatically increased computational complexity. The minimum mean-squared error (MMSE) MUD exhibits poor performance, although it achieves lower computational complexity. With almost the same complexity, an MMSE with successive interference cancellation (SIC) scheme achieves a better bit error rate performance than a linear MMSE multiuser detector. In this paper, hybrid ML-MMSE with SIC adaptive multiuser detection based on the joint channel estimation method is suggested for signal detection. The simulation results show that the proposed method achieves good performance close to the optimal ML performance at low SNR values and a low computational complexity at high SNR values.     

Semidefinite programming detection for OvHDM signal

Non-orthogonal time-frequency division multiplexing （NTFDM） transmission scheme has been proposed to further improve the bandwidth efficiency and overcome the drawbacks of the conventional orthogonal frequency division multiplexing （OFDM） method. Based on such approach, the fast signal detection algorithm, semidefinite programming （SDP） detection, has been studied. As the coefficient matrix tends to be ill conditioned, the modified SDP algorithm combined with successive interference cancellation （SIC） has been developed. The improved algorithm is a good tradeoff between performance and detection complexity. Simulation results show that the proposed algorithm can achieve better performance than cutting plane aided SDP method.     

Low‐complexity linear group‐SIC detectors

In this paper, we consider a linear group polynomial expansion successive interference cancellation (GPE‐SIC) detector in a synchronous CDMA system. It is a hybrid detector, which combines parallel and successive cancellation techniques in order to extract the advantages of both schemes. We use the fact that even if the cross‐correlation matrix of the system is not diagonal dominant, the sub‐matrices corresponding to different groups can be forced to be diagonal dominant by suitable grouping of users to approximate the decorrelator/MMSE detector by a low‐complexity polynomial expansion detector. The latter is computationally very efficient if the cross‐correlation matrix of users within the same group is diagonal dominant. Simulation results showed that the (GPE‐SIC) detector has the same performance as the linear group decorrelator successive interference cancellation (GDEC‐SIC) detector but with low‐computational complexity. Copyright © 2006 John Wiley & Sons, Ltd.     

Coherent frequency hopping multiple access (CFHMA) with multiuserdetection for mobile communication systems

This paper proposes a new multiple-access (MA) scheme, coherent frequency hopping MA (CFHMA) with multiuser detection, which effectively obtains the diversity effect for realizing spectrally efficient mobile communications, CFHMA is fundamentally equivalent to a coherent fast frequency hopping frequency-division MA (FDMA) that incorporates low spreading factor direct-sequence (DS) code-division MA (CDMA). Multiple users occupy the same frequency band, and they are simultaneously detected by multiuser detection. There are several implementations of CFHMA. Two major techniques are introduced: coherent hybrid DS fast FH (FFH) (CHYB-DS-FFH) and interleaved FH (IFH). This paper focuses on IFH and discusses its principle of operation, transmission system configuration, and performance aspects. The results suggest the feasibility of a spectrally efficient mobile radio system     

跳频信号盲检测的时频语义对消方法

复杂电磁环境下精准检测跳频信号是实施跳频信号侦查的先决条件。针对复杂干扰下跳频信号难以检测的问题,本文提出一种名为时频语义对消的方法。该方法设计了一种具有自注意力和图注意力机制的暹罗嵌套UNet,并根据该网络提取包含跳频信号、干扰信号和噪声的时频图语义信息。将得到的结果与不包含跳频信号时频图的语义信息相消就可以得到仅包含跳频信号的时频图,实现对跳频信号的检测。仿真结果表明所提方法可以在复杂干扰下实现对跳频信号的参数估计与盲检测,在信噪比高于-5 dB和信干比高于0 dB时,虚警概率与漏警概率低于1‰。在信号时频范围检测中,对比实验表明语义对消检测方法比语义分割检测方法交并比分数提升0.31,消融实验表明注意力模块对交并比分数提升为0.022。同时分析了所提网络的复杂度,结果表明该方案具有较小的参数量以及较快的处理速度,可以运用于跳频信号的实际检测当中。      

Power domain cyclic spread multiple access: An interference‐resistant mixed NOMA strategy

The next generation wireless access technology highly relies on nonorthogonal multiple access (NOMA) technique. This paper proposes a novel power domain cyclic spread multiple access (PDCSMA) scheme for the design of NOMA system with power domain superposition coding (SC) and cyclic spreading at the transmitter concurrent with symbol level successive interference cancellation (SL‐SIC) at the receiver. Based on acceptable difference in channel gain, the users are grouped together to form PDCSMA clusters, and the unique power is allotted to each user in a cluster. The user with good channel condition is allotted less power, and the user with poor channel condition is allotted more power. Each PDCSMA cluster has its own spreading code, and the data of every user in a cluster are cyclic spread with the same code. Each cluster supports the number of multipath components equivalent to the length of the spreading code. The use of cyclic spreading makes the signal suffered by multipath fading less prone to intra cluster interference. The user signal is decoded by minimum mean square error‐frequency domain equalization (MMSE‐FDE) or maximal ratio combining (MRC)–based receiver in which weak user is detected with hard decision, and strong user is detected with SIC. Compared with conventional power domain NOMA (PDNOMA) and interleaved NOMA, the proposed PDCSMA achieves better bit error rate (BER) performance and assures guaranteed detection.     

Robust Cognitive‐Radio‐Based OFDM Architecture with Adaptive Traffic Allocation in Time and Frequency

Cognitive radio (CR) has been proposed as an effective technology for flexible use of the radio spectrum. The interference between primary users and CR users, however, becomes a critical problem when they are using adjacent frequency channels with different transmission power levels. In this paper, a robust CR orthogonal frequency division multiplexing (OFDM) architecture, which can effectively suppress interference to nearby primary users and overcome adjacent channel interference (ACI) to the CR user, is proposed. This new approach is characterized by adaptive data repetition for subcarriers under heavy ACI, and adaptive time spreading for subcarriers near the borders of the CR user's spectrum. The data repetition scheme provides extra power gain against the ACI coming from primary users. Time spreading guarantees an acceptable interference level to nearby primary users. By computer simulation, we demonstrate that, under a CR environment, the proposed CR OFDM architecture outperforms conventional OFDM systems in terms of throughput and BER performance.     

LSTBC+OFDM分层方法的一种改进

基于OFDMA(Orthogonal Frequency-Division Multiple Access)中不同用户的子载波选择矩阵相互正交,提出了频率选择性衰落信道下分层结构的空时分组编码LSTBC(Layered Space-Time Block Code)的一种改进的分层方法.发射端采用分层结构的空时分组编码结合OFDM技术,在接收端提出了利用OFDM中子载波选择矩阵进行分组干扰抑制,然后对LSTBC的每层进行空时分组码的传统解码.相对于基于奇异值分解的分组干扰抑制方法,该方法只需要一根接收天线,降低了系统接收机设计的复杂度.仿真结果验证该方法的有效性.     

Robust Multiuser Interference Cancellation for OFDM Systems With Frequency Offset

In orthogonal frequency division multiple access systems clusters of subcarriers are assigned to different users for parallel data transmissions. The subcarriers are overlapped, but orthogonal to each other such that there is no intercarrier interference (ICI). However, synchronization errors among users cause the loss of the orthogonality and introduce ICI resulting in multiple-access interference. Synchronization between users is particularly difficult in the uplink channel where the user signals are potentially asynchronous and affected by different frequency offsets due to misalignment in carrier frequencies and Doppler shifts. This paper proposes a method to lower the effects of different frequency offsets among user signals in an OFDMA uplink system. The multiple access interference due to the user frequency misalignments is reduced by reconstructing and removing the interfering signals in the frequency domain. An approach based on the selective cancellation method, is proposed and its performance is analyzed by means of theoretical analysis and computer simulations. The effectiveness of the proposed system has been evaluated in the case of ideal and no-ideal frequency offset estimation and has been compared with that of the classical successive and parallel cancellation schemes. Simulation results show that the proposed approach allows performance close to the ideal case, i.e., with ideal frequency synchronization among users, with a low increase of the implementation complexity. Moreover, it is also highlighted here, that the successive cancellation method slightly outperforms the selective scheme, at the expense of a higher computational complexity and processing delay     

Simplified group interference cancelling for asynchronous DS‐CDMA

A simplified group interference cancelling (IC) approach is investigated for asynchronous direct‐sequence code‐division multiple access on flat fading channels. The technique employs grouping by estimated signal‐to‐noise‐plus‐interference ratio (SNIR), and interference cancellation is performed blockwise, for a subset of the total number of users. We consider long random spreading codes, and include the effects of imperfect amplitude, carrier phase, and delay estimation. Performance of the technique shows SNIR gains of several dB, and concomitant improvements in error probability, with lower computational complexity than that of parallel or serial interference cancelling techniques. We also show that our SNIR expressions are applicable to both the AWGN and flat fading channels, and for moderate near–far conditions. In addition, we determine optimal group sizes for our technique, where optimality is in terms of average error probability over all users. Copyright © 2006 John Wiley & Sons, Ltd.     

上行链路抗远近效应的半盲MUD

本文提出了一种新的用于CDMA系统上行链路,且具有较强抗远近效应性能的半盲自适应多用户检测器(MUD)。借鉴串行干扰抵消(SIC)的思想,该方法利用一种新的自适应幅度估计器进行小区内用户信号的幅度估计,然后使用一种基于快速子空间跟踪算法的盲MUD,按照幅度从大到小的顺序依次检测出小区内的用户。这样,既通过盲方法抑制了来自小区外的干扰,又充分利用小区内用户的信息,提高了检测性能,特别是对弱信号的检测性能,具有较强的抗远近效应的性能。     

一种时频双选信道下OFDM系统的低复杂度迭代干扰抵消算法

为了消除正交频分复用(OFDM)系统在时频双选信道条件下由于信道快时变带来的载波间干扰,本文提出了一种新的低复杂度迭代干扰抵消方法.为了消除载波间干扰,已经提出了不少方法,但这些方法要么因为矩阵求逆运算带来了高计算复杂度,要么以性能退化为代价换来低计算复杂度.利用并行迭代干扰抵消技术,本文算法可以明显改善系统性能,同时借助带状矩阵近似和最小二乘QR分解(LSQR)迭代计算的特点来降低计算复杂度.仿真结果表明,在高信噪比条件下,随着迭代次数的增加,本文所提出的算法可以有效的减少"地板效应",在系统性能和计算复杂度之间取得更好的折中.     

混合DS／SFH CDMA扩频通信系统中的自适应跳频

文章基于进一步提高跳扩频通信系统的抗干扰性能，提出了在混合直接序列扩频（DS／SFHCDMA）通信系统中实现自适应跳频（AFH）方式，给出了基于优化考虑的自适应跳频地址编码方法，在给出信道仿真参数的前提下，对自适应跳频系统的误码性能进行了分析。结果表明自适应方式优于传统方式。