A Comparison of Frequency-Domain Block MIMO Transmission Systems

Block transmission techniques, with appropriate cyclic prefix and frequency-domain processing schemes, have been shown to be excellent candidates for digital transmission over severely time-dispersive channels, allowing good performance with implementation complexity that is much lower than traditional time-domain processing schemes. Orthogonal frequency-division multiplexing (OFDM) modulation is the most popular block transmission technique. Single-carrier (SC) modulation using frequency-domain equalization (FDE) is an attractive alternative approach based on this principle. In this paper, we propose two new receiver structures for multiple-input-multiple-output (MIMO) channels employing SC (MIMO-SC) modulation and FDE schemes. These receivers have a hybrid structure with frequency-domain feedforward and time-domain feedback filters for intersymbol interference (ISI) and interference cancellation. The proposed schemes are compared with different MIMO systems employing OFDM modulation (MIMO-OFDM) receivers in terms of performance [bit error rate (BER) and throughput] and complexity. Our performance results show the superiority of MIMO-SC approaches relative to MIMO-OFDM in terms of the BER performance for the simulated scenarios. Also, the simulation results show that the proposed hybrid MIMO-SC receivers yield a higher throughput than a MIMO-OFDM system.     

Overlapped universal filtered multicarrier system for uplink wireless communication

Universal filtered multicarrier (UFMC) is a promising new waveform that eliminates cyclic prefix and performs filtering on a chunk of successive subcarriers. Therefore, it is suitable for applications with requirements on high spectral efficiency and low latency. However, the limited bandwidth may be an obstacle for reliable communication in frequency selective channels. This paper proposes a new transmitter structure for uplink wireless communication system based on UFMC waveform. The basic idea is to allow a user to transmit its information on more than one subband simultaneously, increasing its robustness against frequency selectivity. To eliminate the multiuser interference brought by the overlapped transmission strategy, an iterative receiver with parallel interference cancellation is designed. Rate analysis is provided to reveal the rationale of the proposed scheme. Simulations on the symbol error rate performance are conducted to validate the transceiver design. It is found that the proposed scheme outperforms existing UFMC design without sacrificing any other performance metrics with only moderately increased complexity.     

CDMA系统上行链路的分数间隔频域均衡接收方案

提出一种应用于单载波CDMA系统异步上行链路的分数间隔频域均衡接收方案。针对异步上行链路各用户到达基站的信号具有任意随机时延的特性,为了提高CDMA系统上行链路的抗干扰能力,降低接收机检测复杂度。同时与现有单载波系统空中接口兼容,接收方案采用基于重叠截取技术的单载波分数间隔频域均衡器对抗频率选择性衰落所引起的各种干扰;用时域多阶并行干扰消除器减小用户间严重的多址干扰。仿真结果表明,这种分数间隔频域均衡接收方案比传统的码片级均衡接收方案在性能上有很大的提高,并且复杂度也比较低。     

Protocol design and data detection for two‐way relay networks with fractional asynchronous delays

In wireless two‐way relay systems, it is difficult to achieve perfect timing synchronization among different nodes. In this paper, we investigate relaying protocol design and data detect schemes for asynchronous two‐way relaying systems to combat the intersymbol interference caused by asynchronous transmission. We consider fractional asynchronous delays and two schemes are proposed based on cyclic prefixed single carrier block transmission, namely, the receiver frequency domain equalization scheme and relay synchronization and network coding (RSNC) scheme. In the receiver frequency domain equalization scheme, the relay simply amplifies the received signal and forwards to the two source nodes, and fractionally spaced frequency domain equalizer (FS‐FDE) is employed at the receiver to recover the transmit data. In the RSNC scheme, the asynchronous signals are resynchronized with an FS‐FDE at the relay node. The output signals of FS‐FDE are then demodulated and network coded before forwarding to the two source nodes. In this RSNC scheme, data detection at the source nodes is the same as that in synchronous networks because the asynchronous signals have already been synchronized at the relay node. Simulation results show that the performance of both schemes is almost the same as in the perfect synchronized two‐way relaying systems. Copyright © 2012 John Wiley & Sons, Ltd.     

Research on uplink coordinated transmission schemes in LTE-advanced systems

The investigation of inter-cell interference mitigation techniques is a key area in wireless communications.Coordinated multiple points(CoMP) transmission/reception is a candidate technique for interference cancellation in 3GPP LTE-Advanced system.However,the coordination scheme in CoMP remains a key research problem to be solved,which will have a strong influence on the performance of CoMP.In this paper,a novel coordinated transmission scheme is proposed for the uplink LTE-Advanced system.In our scheme,several base transceiver stations(BTS) and users are selected as coordination partners which form a CoMP cluster.Joint processing is used at the receiver to mitigate interference.From the perspective of coordinated partner selection,our scheme can be divided into static and dynamic coordination which are both considered to fully exploit the throughput gain of CoMP.The proposed schemes are evaluated by system level simulation and compared with the conventional LTE system based upon single cell processing.Our simulation results demonstrate that the proposed schemes attain superior performance as opposed to the conventional system in terms of cell average and cell edge throughput.     

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     

Evaluation of a DS/CDMA multiuser receiver employing a hybrid formof interference cancellation in Rayleigh-fading channels

This paper assesses the performance of a multiuser detection DS/CDMA receiver based on a hybrid scheme of successive interference cancellation (SIC) and parallel interference cancellation (PIC). Two configurations of the proposed hybrid IC are presented and compared with existing SIC and PIC schemes. The performance criteria used for comparison are complexity, delay, power control requirements, and average bit-error rate (BER) performance obtained by simulation in Rayleigh-fading channels with additive white Gaussian noise (AWGN). The suggested hybrid IC combines good average BER performance, short delay, acceptable complexity, and also operates under slow power control     

Low‐complexity inter‐carrier interference cancelation scheme for SC‐IFDMA system

After the standardization of SCFDMA as the uplink transmission scheme for LTE, frequency synchronization and resulting interference cancelation received considerable attention. In this paper, mathematical modeling of uplink SCFDMA system with interleaved subcarrier assignment scheme (SC‐IFDMA) is carried out in the presence of carrier frequency offset, and the results were utilized in framing the concept of effective interference matrix, which efficiently represents the interference cancelation problem. We propose two schemes to mitigate the effects of interference and channel based on linear filtering approach, and the typical structure of effective interference and channel matrices in SC‐IFDMA were utilized in formulating a low‐complexity implementation model for the proposed compensation schemes. Even though many works have been reported in the field of interference cancelation of SCFDMA system, majority of them were extension of the interference compensation schemes proposed for OFDMA system, whereas schemes proposed in this paper utilize the typical characteristics of the SC‐IFDMA system. The proposed schemes were simulated using MATLAB, and the performance is compared with existing schemes. Copyright © 2015 John Wiley & Sons, Ltd.     

Space-Time Adaptive MMSE Multiuser Decision Feedback Detectors With Multiple-Feedback Interference Cancellation for CDMA Systems

In this paper, we propose a novel space-time minimum mean square error (MMSE) decision feedback (DF) detection scheme for direct-sequence code-division multiple access (DS-CDMA) systems with multiple receive antennas, which employs multiple-parallel-feedback (MPF) branches for interference cancellation. The proposed space-time receiver is then further combined with cascaded DF stages to mitigate the deleterious effects of error propagation for uncoded schemes. To adjust the parameters of the receiver, we also present modified adaptive stochastic gradient (SG) and recursive least squares (RLS) algorithms that automatically switch to the best-available interference cancellation feedback branch and jointly estimate the feedforward and feedback filters. The performance of the system with beamforming and diversity configurations is also considered. Simulation results for an uplink scenario with uncoded systems show that the proposed space-time MPF-DF detector outperforms existing schemes such as linear, parallel DF (P-DF), and successive DF (S-DF) receivers in terms of bit error rate (BER) and achieves a substantial capacity increase in terms of the number of users, compared with the existing schemes. We also derive the expressions for MMSE achieved by the analyzed DF structures, including the novel scheme, with imperfect and perfect feedback and expressions of signal-to-interference-plus-noise ratio (SINR) for the beamforming and diversity configurations with linear receivers.     

SC-FDE系统的一种新型判决反馈均衡器

针对单载波频域均衡系统MMSE均衡器存在残留码间干扰的缺点,提出MMSE-RISIC判决反馈均衡器消除残留码间干扰.MMSE-RISIC均衡器采用传统MMSE均衡后的判决数据,对残留码间干扰进行估计并消除.残留码间干扰的估计主要采用FFT和IFFT运算,与其他方法相比计算量较小.对该均衡器在不同信道下进行了计算机仿真,结果表明,在频率选择性衰落信道条件下,系统性能有了较为明显的提高.     

Carrier Frequency Offset Compensation with Successive Cancellation in Uplink OFDMA Systems

Similar to OFDM systems, OFDMA systems also suffer from frequency mismatches between the receiver and the transmitter. However, the fact that each uplink user has a different frequency offset makes the compensation more challenging than that of OFDM systems. This letter proposes successive interference cancellation (SIC) for compensating the frequency offset in the uplink OFDMA systems. A decorrelator is used to remove the inter-carrier interference (ICI) within a user's signal and successive cancellation is applied to mitigate the multi access interference (MAI) arising due to the frequency difference among uplink users. The proposed algorithm is shown to eliminate the interference and has a manageable complexity.     

Minimum Mean-Squared Error Iterative Successive Parallel Arbitrated Decision Feedback Detectors for DS-CDMA Systems

In this paper we propose minimum mean squared error (MMSE) iterative successive parallel arbitrated decision feedback (DF) receivers for direct sequence code division multiple access (DS-CDMA) systems. We describe the MMSE design criterion for DF multiuser detectors along with successive, parallel and iterative interference cancellation structures. A novel efficient DF structure that employs successive cancellation with parallel arbitrated branches and a near-optimal low complexity user ordering algorithm are presented. The proposed DF receiver structure and the ordering algorithm are then combined with iterative cascaded DF stages for mitigating the deleterious effects of error propagation for convolutionally encoded systems with both Viterbi and turbo decoding as well as for uncoded schemes. We mathematically study the relations between the MMSE achieved by the analyzed DF structures, including the novel scheme, with imperfect and perfect feedback. Simulation results for an uplink scenario assess the new iterative DF detectors against linear receivers and evaluate the effects of error propagation of the new cancellation methods against existing ones.     

用于LTE接收机的MMSE—FDE均衡算法

根据LTE上行SIMO接收机特点,对MMSE—RISIC均衡器进行简化修改,得到MMSE—FDE均衡器。该均衡器所有步骤全在频域进行,大大降低了均衡算法的复杂度。将该均衡器进行计算机仿真,其结果表明,该均衡器在3GPP LTE协议最常用的EPA和EVA信道中应用,能比较有效地消除MMSE均衡残留的码间干扰,使系统性能明显提高。     

一种线性化的全双工MIMO收发器设计

针对全双工MIMO收发器发射通道非线性以及接收通道存在强烈自干扰的问题,该文提出一种使发射通道线性化并通过射频多抽头重建与数字重建消除自干扰的具有较低硬件成本与软件复杂度的设计方案:(1)基于改进的串扰消除和数字预失真(CTC-DPD)算法并复用反馈通道进行去耦合和数字预失真使发射通道线性化、等增益;(2)在接收通道加入可调衰减器并用多维梯度下降法基于接收的残留自干扰功率最小原则调整抽头参数;(3)基于频域信道估计进行数字自干扰重建。实现的20 MHz带宽LTE全双工22 MIMO通信样机,发射通道经过线性化后带内更平坦,而带外噪声抑制了约30 dB。射频和数字消除一轮调整共耗时约0.17 ms,总消除能力约75 dB。16QAM映射时全双工双向数据速率总和220 Mbps,相对单向时的110 Mbps实现了频谱效率的翻倍。通信样机证明了该方案的可行性。     

Parallel interference cancellation for carrier frequency offset in OFDM assisted with recursive least squares algorithm

Parallel interference cancellation (PIC) assisted with recursive least squares (RLS) algorithm is proposed to cancel the interference due to the carrier frequency offset (CFO) in orthogonal frequency division multiplexing (OFDM) system. The proposed algorithm is composed of two stages, which are RLS scheme and PIC scheme. RLS scheme is selected to compensate the frequency offset in the time domain in the first stage, and the interference induced by residual frequency offset is canceled by the PIC scheme in the frequency domain in the second stage. The result of bit error rate (BER) shows that its performance is robust for cancellation as comparison criteria, even though the frequency offset is 0.45. The 16QAM constellation is also simulated to observe the improvements from the proposed suppression schemes.     

Interference-Resilient Block-Spreading CDMA With Minimum-MAI Sequence Design

Code-division multiple-access (CDMA) schemes based on block spreading implement the spreading of entire data blocks rather than single symbols, thus achieving a higher robustness against the frequency selectivity of the channel and allowing the use of efficient modulation/equalization schemes operating in the frequency domain (FD). In this paper, we present a new block CDMA (B-CDMA) system where a single cyclic prefix (CP) is used at the end of each spread block. This provides a higher spectral efficiency with respect to existing schemes. By observing that complete orthogonality among users is achievable only for half-loaded systems on dispersive channels, we introduce new criteria for the design of spreading and despreading sequences, which aim at minimizing the mean-square error at the output of the despreader. For the equalization of the received signal, we propose an iterative block decision feedback equalizer, which iterates between equalization and decoding. Equalization filters are designed to minimize the mean-square error and take into account the residual interference due to the nonorthogonality of the spreading sequences. The performance of B-CDMA is evaluated in an uplink wireless scenario and compared to existing CDMA schemes.     

Channel‐estimate‐based frequency‐domain equalization (CE‐FDE) for broadband single‐carrier transmission

A channel‐estimate‐based frequency‐domain equalization (CE‐FDE) scheme for wireless broadband single‐carrier communications over time‐varying frequency‐selective fading channels is proposed. Adaptive updating of the FDE coefficients are based on the timely estimate of channel impulse response (CIR) to avoid error propagation that is a major source of performance degradation in adaptive equalizers using least mean square (LMS) or recursive least square (RLS) algorithms. Various time‐domain and frequency‐domain techniques for initial channel estimation and adaptive updating are discussed and evaluated in terms of performance and complexity. Performance of uncoded and coded systems using the proposed CE‐FDE with diversity combining in different time‐varying, multi‐path fading channels is evaluated. Analytical and simulation results show the good performance of the proposed scheme suitable for broadband wireless communications. For channels with high‐Doppler frequency, diversity combining substantially improves the system performance. For channels with sparse multi‐path propagation, a tap‐selection strategy used with the CE‐FDE systems can significantly reduce the complexity without sacrificing the performance. Copyright © 2004 John Wiley & Sons, Ltd.     

一种消除TD-SCDMA系统小区间干扰的方法

时分-同步码分多址(TD-SCDMA)系统采用的扩频码码长最大为16,当邻小区用户处于2小区边缘时会对本小区造成较大的干扰。针对上行链路提出了一种新的干扰消除方法:串行干扰消除-联合检测(SIC-JD)。该方法采用复杂低的SIC对上行链路中的小区间干扰进行消除,对于小区内干扰和残留的小区间干扰采用联合检测进行消除。3GPP case1信道下的仿真结果显示了此方法能够有效消除小区间和小区内干扰,提高接收机性能。     

Approximate Minimum BER Power Allocation for MIMO Spatial Multiplexing Systems

Precoding for multiple-input multiple-output (MIMO) spatial multiplexing generally requires high feedback overhead and/or high-complexity processing. Simultaneous reduction in transmitter complexity and feedback overhead is proposed by imposing a diagonal structural constraint to precoding, i.e., power allocation. Minimum bit-error rate (MBER) is employed as the optimization criterion, and an approximate MBER (AMBER) power-allocation algorithm is proposed for a variety of receivers, including zero-forcing (ZF), successive interference cancellation (SIC), and ordered SIC (OSIC). While previously proposed precoding schemes either require ZF equalization for MBER, or use a minimum mean-squared error (MMSE) criterion, we provide a unified MBER solution to power allocation for ZF, SIC, and OSIC receiver structures. Improved error-rate performance is shown both analytically and by simulation. Simulation results also indicate that SIC and OSIC with AMBER power allocation offer superior performance over previously proposed MBER precoding with ZF equalization, as well as over MMSE precoding/decoding. Performance under noisy channels and power feedback is analyzed. A modified AMBER algorithm that mitigates error propagation in interference cancellation is developed. Compared with existing precoding methods, the proposed schemes significantly reduce both transmit processing complexity and feedback overhead, and improve error-rate performance     

Performance Analysis of the Downlink CP-WCDMA System with Turbo Code in Frequency Domain

Modern wireless communications require an efficient spectrum usage and high channel capacity and high throughput. Turbo code, linear equalizers, multi-user detection and wideband code-division multiple access (WCDMA) are possible solutions to achieve spectral efficiency, high channel capacity, eliminate MAI, eliminate ISI and robustness against frequency selective fading. In this paper, we combine all these techniques and investigate BER performance. We propose a low complexity receiver structure for Single-input Single-output downlink cyclic prefix CP-WCDMA systems. It employs frequency domain interference cancellation schemes to mitigate the interference caused by the multipath fading channel. Also, the proposed scheme is developed for the downlink Turbo code CP-WCDMA system to maximize the throughput of the proposed system.