Interference Rejection Combining Research Articles

Walsh–Hadamard Transform Based Non-Orthogonal Multiple Access (NOMA) and Interference Rejection Combining in Next-Generation HetNets

In heterogeneous networks (HetNets), non-orthogonal multiple access (NOMA) has recently been proposed for hybrid-access small-cells, promising a manifold network capacity compared to OMA. One of the major issues with the installation of a hybrid-access mechanism in small-cells is the cross-tier interference (intercell interference (ICI)) caused by the macrocell users (MUs) that are unable to establish a connection to the small-cell base station (SBS). In this paper, a joint strategy is proposed for hybrid-access small-cells using the Walsh–Hadamard transform (WHT) with NOMA and interference rejection combining (IRC) to achieve high performance gains and mitigate intercell interference (ICI), respectively. WHT is applied mathematically as an orthogonal variable spreading factor (OVSF) to achieve diversity in communication systems. When applied jointly with NOMA, it ensures better performance gains than the conventional NOMA. It reduces the bit error rate (BER) and enhances subsequent throughput performance of the system. IRC is used at the receiver side for managing the cross-tier interference caused by MUs that are unable to connect to the small-cell base station (SBS) for hybrid-access. The work considers both ideal and nonideal successive interference cancellation (SIC) conditions for NOMA. Mathematical modeling is provided for the proposed joint strategy for HetNets and the results validate it in terms of BER and subsequent user throughput performance, compared to the conventional NOMA approach.

Partial FFT Demodulation With IRC in MIMO-SC-FDE Communication Over Doppler Distorted Underwater Acoustic Channels

Multi-input multi-output single carrier frequency domain equalization (MIMO-SC-FDE) has been extensively researched in the past years as a technique to realize high data-rate underwater acoustic (UWA) communication. However, it will suffer from severe inter-symbol interference(ISI), frequency offset and co-channel interference (CoI) over Doppler distorted UWA channels. In order to solve the above problems introduced by UWA channels in MIMO communication scenario, partial FFT demodulation with interference rejection combining (IRC) algorithm is proposed in this letter. Compared with previous works, the proposed algorithm has the ability to simultaneously suppress signal distortion brought by ISI, Doppler shift and CoI by frequency domain processing. Numerical simulation results show that the BER performance of MIMO-SC-FDE in Doppler distorted UWA channel environment is improved with acceptable increase of computational complexity.

Centralized UL/DL Resource Allocation for Flexible TDD Systems With Interference Cancellation

Flexible time division duplex (TDD) is expected to be one of the key technologies for 5G flexible air interface. The main benefit of flexible TDD is to allow for better radio resource utilization based on instantaneous user traffic demands. To boost network capacity, one could thus, utilize flexible TDD along with a dense deployment of small cells. Flexible TDD systems can be designed to benefit from strong-interference cancellation of the cross-link interference along with advanced inter-cell coordination. In this paper, we propose a centralized coordination scheme with joint uplink/downlink (UL/DL) user scheduling, rate, and MIMO rank adaptation. To reduce the complexity of coordinated scheduling, we propose a cluster-based scheduling scheme where joint UL/DL selection and joint user scheduling are performed within a cluster. MIMO rank and rate adaptation are performed across the clusters using inter-cluster rate and rank adaptation. Simulation based results are evaluated in an outdoor urban micro scenario with bursty traffic and assuming realistic channel estimation. It is observed that the proposed flexible TDD scheme with joint UL/DL scheduling, interference cancellation, and coordination provides over 120% gain in the fifth percentile and over 29% gain in the average end-to-end UL throughput as compared to flexible TDD standalone scheduling without coordination. Our results show that successive interference cancellation receivers provide over 20% UL throughput gain for flexible TDD systems as compared to Interference Rejection Combining (IRC) receivers. We also find that fully flexible TDD with interference cancellation significantly outperforms fully synchronized TDD for different cell sizes.

5G 통신 시스템을 위한 NSC 데이터 검출 기법

급증하는 데이터 사용량에 대비하고 신규 응용 서비스를 창출하기 위해 3GPP (3rd Generation Partnership Project)에서는 기존 LTE (Long Term Evolution) 규격에서 크게 진화한 5G 용도의 NR (New Radio) 규격을 개발하고 있다. 본 논문에서는 5G 요구사항 중 높은 전송률 및 고신뢰성을 만족시키기 위해 256 QAM 변조 방식과 LDPC (Low Density Parity Check) 코드를 적용한다. 더불어, 데이터 검출 기법인 IRC (Interference Rejection Combining)와 SIC (Successive Interference Cancellation)를 결합한 NSC (Novel Suppression and Cancellation)를 제안하여 신뢰성을 더 높이고자 한다. 모의실험 결과 제안한 NSC가 높은 신뢰성과 데이터 전송률을 갖는 것을 알 수 있다.

Interference suppression algorithm based R-TDD for multi-user MIMO heterogeneous network

Aiming at the problem of uplink co-channel interference in multi-user multi-input multi-output (MIMO) heterogeneous network, a reversed time division duplex (R-TDD)-based method is used to improve the performance of interference rejection combining (IRC) algorithm. This method overcomes the difficulty of IRC algorithm in obtaining the interference noise covariance matrix by utilizing the R-TDD transmission scheme. On the premise that the spectrum efficiency is guaranteed, through combining R-TDD with spatial modulation, the transmission complexity is significantly reduced and the uplink co-channel interference is effectively controlled, so as to improve the system performance. Simulation results demonstrate that the R-TDD-based scheme can improve the performance of IRC algorithm in terms of spectrum efficiency.

大规模MIMO系统的干扰抑制合并技术研究

为了解决大规模MIMO系统中由于导频污染引起的严重的小区间干扰，对大规模MIMO系统中的干扰抑制合并(IRC)技术进行研究。该技术可利用小区间干扰和信道估计的相关性来抑制干扰。在非理想信道状态信息(CSI)情况下，分别分析了基站天线数、功率收缩因子和干扰因子对IRC方法系统性能的影响，并和最大比合并(MRC)和最小均方误差(MMSE)方法进行仿真对比。仿真结果表明，IRC系统性能明显优于MRC和MMSE，并且在基站天线数低于500时，功率收缩因子可以大于1/2。 In order to solve the serious inter-cell interference caused by the pilot contamination in massive MIMO, the interference rejection combining (IRC) technique is studied. The IRC technique can exploit the correlation between channel estimates and interference to suppress interference. In the case with imperfect channel state information (CSI), the IRC system performance is separately analyzed in the regimes of different numbers of base station (BS) antennas, power scaling factors and interference factors, and simulation comparison with maximum ratio combining (MRC) and minimum mean-square-error (MMSE) methods is made. Simulation results show that IRC has obviously better performance than MMSE and MRC, and the power scaling factor can be greater than 1/2 when the number of BS antennas is less than 500.

Selective Interference Rejection based Antenna Selection for MIMO over LTE Advanced Networks

An important goal for long term evolution advanced (LTE-A) network is to enhance the cell edge user throughput while achieving the link reliability and high data rate communication. However, the cell edge users’ throughput is limited by inter-cell interference. To suppress the interference signal, interference rejection combining (IRC) receiver is an effective solution for Release 10/11. In Multiple-Input Multiple-Output (MIMO) technique, the IRC receiver utilizes multiple antennas, larger the number of antennas, the more interfering signals can be suppressed. Due to practical constraints of increase in radio-frequency units impose a limit in the number of antennas. In this paper, a new selective interference rejection based antenna selection (SIR-AS) scheme is proposed to perform the better selection of transmitting antenna for cell edge MIMO user and enhance the system throughput. The selective interference rejection is performed by incorporating the IRC receiver, where the covariance matrix (CM) of the received interference signals is estimated and the minimum of CM is selected. The corresponding antenna set with minimum of CM is selected for transmission. Simulation results show that the proposed algorithm can effectively detect intercellular interference at cell boundaries and select the optimal antenna to reduce block error rate (BLER) and improve throughput performance compared to ideal IRC and maximum ratio combining algorithm, making it suitable for LTE-A downlink receiver in multi-cell MIMO systems.

백홀 시스템에서 시스템 레벨 시뮬레이션 기반 하이브리드 수신기 설계 및 성능 분석

폭발적으로 증가하고 있는 모바일 데이터 트래픽을 수용하기 위하여 셀 간 간섭을 제어할 수 있는 향상된 수신기에 대한 연구가 필요하다. 이를 위해 3GPP(3rd Generation Partnership Project) 에서는 인접 셀의 간섭 신호를 억제 및 제거하여 수신기의 SINR(Signal-to-Noise-plus-Interference Ratio)을 향상시켜 수신 성능을 올리는 NAICS(Network Assisted Interference Cancellation and Suppression)가 표준에서 논의되고 있다. 본 논문에서는 이웃셀로 부터 간섭의 영향을 줄이기 위하여 연판정 기반의 향상된 수신기를 제안한다. 제안한 수신기는 MMSE (Minimum Mean Square Error) 또는 IRC (Interference Rejection Combing) 수신기를 통해 간섭 신호의 불편 추정 (unbiased estimation) 값을 계산 하고 간섭 신호를 억제 및 제거한다. 간섭 신호는 LLR (Log-Likelihood Ratio)로 표현되는 소프트 정보를 통해 업데이트 한다. 시스템 레벨 시뮬레이션은 3GPP LTE-Advanced 하향링크 시스템의 20㎒ 대역을 기반으로 이루어 졌으며, 시뮬레이션 결과 제안한 수신기를 통해 기존 수신기보다 SINR, 전송률 및 스펙트럼 효율 측면에서 성능 향상을 가져다주는 것을 확인하였다.

Interference-Robust Air Interface for 5G Ultra-dense Small Cells

An ultra-dense deployment of small cells is foreseen as the solution to cope with the exponential increase of the data rate demand targeted by the 5th Generation (5G) radio access technology. In this article, we propose an interference-robust air interface built upon the usage of advanced receivers as main interference mitigation technique. Both Interference Rejection Combining (IRC) and Successive Interference Cancellation (SIC) principles are considered. An efficient usage of such receivers is ensured by a proper frame structure design and system assumptions. Different approaches for the rank adaptation are also investigated. Simulation results show that proposed air interface built upon advanced receiver can be a feasible alternative to traditional solutions based on frequency reuse planning. Open issues and further challenges are also addressed.

사이드홀 시스템에서 안테나 배열 설계 및 하이브리드 수신기

Recently, 3rd Generation Partnership Project (3GPP) has developed sidehaul system to cope with the explosively increasing mobile data traffic. The sidehaul system is based on single carrier-frequency division multiple access (SC-FMDA) due to its low peak to average power ratio (PAPR). Also, antenna array is designed to support multiple input multiple output (MIMO) in a restricted space. In this paper, we design the antenna array about uniform linear array (ULA), uniform circular array (UCA) and uniform planar array (UPA), and analysis the performance in sidehaul system. In addition, we propose the novel hybrid receiver full suppression cancellation (FSC) to reduce the interference from neighbor cell in sidehaul system. The proposed receiver can suppress and cancel the interference by combining interference rejection combining (IRC) with successive interference cancellation (SIC).

셀룰러 네트워크에서 D2D 통신 향상을 위한 FSC 수신기 설계 및 성능 분석

Cellular Network assisted device-to-device (D2D) communication has been growing to reduce the overload of eNodeB and mitigate the frequency shortage. However, by sharing the uplink frequency resource with the cellular network, the interference between cellular and D2D is increased. In this paper, we propose an advanced receiver for full suppression cancellation (FSC) to reduce the interference between cellular and D2D. The proposed receiver can suppress and cancel the interference by integrating the interference rejection combining (IRC) technique with successive interference cancellation (SIC). We perform a system level simulation based on the 20-MHz bandwidth of the 3GPP LTE-A system. Simulation results show that the proposed receiver can improve SINR, throughput and spectral efficiency compared to conventional receivers.

Linear Interference Suppression With Covariance Mismatches in MIMO-OFDM Systems

Interference cancellation is a key design concern for next-generation communication systems. One practical approach is the interference rejection combining (IRC) scheme, which is already being considered as the base receiver for next-generation 3GPP specifications. IRC treats interference as a stationary Gaussian process to facilitate simple interference suppression. However, this stationary assumption does not hold in practice; for example, the variances of the pilot signal and of the data signal from the interfering base station (eNB) are quite different. This considerably impacts the possible improvements of an interference-aware receiver. In this paper, novel interference suppression schemes are proposed, which handle separately the interfering pilot and the interfering data signals. The proposed schemes take into account channel estimation errors and the errors in estimating the covariance of interference plus noise. The performance of the proposed schemes is validated by using realistic channel models and asynchronous scenarios for MIMO-OFDM systems.

A dynamic clustering algorithm for downlink CoMP systems with multiple antenna UEs

Coordinated multi-point (CoMP) schemes have been widely studied in the recent years to tackle the inter-cell interference. In practice, latency and throughput constraints on the backhaul allow the organization of only small clusters of base stations (BSs) where joint processing (JP) can be implemented. In this work we focus on downlink CoMP-JP with multiple antenna user equipments (UEs) and propose a novel dynamic clustering algorithm. The additional degrees of freedom at the UE can be used to suppress the residual interference by using an interference rejection combiner (IRC) and allow a multistream transmission. In our proposal we first define a set of candidate clusters depending on long-term channel conditions. Then, in each time block, we develop a resource allocation scheme by jointly optimizing transmitter and receiver where: a) within each candidate cluster a weighted sum rate is estimated and then b) a set of clusters is scheduled in order to maximize the system weighted sum rate. Numerical results show that much higher rates are achieved when UEs are equipped with multiple antennas. Moreover, as this performance improvement is mainly due to the IRC, the gain achieved by the proposed approach with respect to the non-cooperative scheme decreases by increasing the number of UE antennas.

Investigation on Interference Rejection Combining Receiver for Space–Frequency Block Code Transmit Diversity in LTE-Advanced Downlink

The interference rejection combining (IRC) receiver, which can strictly suppress intercell interference based on the minimum mean square error (MMSE) criteria, is effective in improving cell-edge user throughput. When assuming the Long Term Evolution (LTE) or LTE-Advanced downlink and open-loop transmit diversity employing the space-frequency block code (SFBC) using Alamouti coding, the IRC receiver must detect the Alamouti coded signals and suppress the interference signals using a couple of received signals in the frequency domain at the same time. To achieve this, the IRC receiver weight matrix, which consists of the channel matrix of the serving cell and the statistics of the covariance matrix, including the interference and thermal noise components, must be extended in the frequency domain, i.e., due to the effect of Alamouti coding, in addition to the spatial domain. These extended matrices can be estimated using the downlink reference signals (RSs) from the serving cell. However, some elements, including the effect of Alamouti coding in the extended covariance matrix, cannot be estimated using a practical estimation scheme that subtracts the replica symbols of the serving cell generated by the estimated channel matrix and the known RS sequence from the received RSs of the serving cell. This is because the RSs in LTE/LTE-Advanced are not transmitted using two adjacent subcarriers. This paper investigates the statistics of these unknown elements and proposes appropriate values, specifically inserting zero values, for these elements assuming the LTE/LTE-Advanced downlink. The results of simulations show that the IRC receiver using the proposed scheme, which has two receiver antenna branches, suppresses the intercell interference and improves the throughput by more than 10% compared with that for the conventional maximal ratio combining (MRC) receiver when a cell-edge environment is assumed.

Interference Aware Iterative Block Decision Feedback Equalizer for Single Carrier Transmission

The deployment of increasingly dense heterogeneous mobile networks can create high levels of interference among users that, combined with severely time-dispersive channels, can result in substantial performance degradation. To cope with both effects, in this paper, we propose an iterative block decision feedback equalizer (IBDFE) for single carrier (SC) transmissions that makes use of the correlation between the interference in the receiving antennas and minimizes the mean squared error (MSE) of the detected symbols. Our analytic and simulated performance results show that the proposed receiver can clearly outperform the conventional IBDFE and the linear interference rejection combining (IRC) detector in severely time-dispersive channels with strong cochannel interference.

Performance Evaluation of Interference Rejection Combining Receiver in Heterogeneous Networks for LTE-Advanced Downlink

The interference rejection combining (IRC) receiver effectively improves the cell-edge user throughput by suppressing interference from the surrounding cells. The work item (WI) for the specification of the IRC receiver is now ongoing for Release 11 Long-Term Evolution (LTE)-Advanced. Furthermore, heterogeneous networks where low power nodes such as picocells are overlaid onto macrocells are important to further improve the system throughput per unit area. In heterogeneous networks, to achieve an offloading gain from macrocells to picocells, cell range expansion (CRE) is applied. Additionally, inter-cell interference coordination (ICIC) is applied to reduce the severe inter-cell interference imposed from the macrocells onto the sets of user equipment (UEs) connected to picocells. In such cases, the interference statistics are completely different from traditional well-planned macrocell deployments, which have been investigated for the IRC receiver. This paper clarifies the effect of the IRC receiver in a heterogeneous network employing CRE and ICIC. Simulation results show that when both CRE and ICIC are applied, the effect of the IRC receiver becomes small due to a reduction in the severe inter-cell interference from ICIC. However, we clarify that the user throughput gain at the cumulative distribution function of 5% from the IRC receiver exceeding 10% is achieved compared to the conventional minimum mean square error (MMSE) receiver in a heterogeneous network regardless of the usage of ICIC. Furthermore, in heterogeneous networks employing CRE and ICIC, we clarify that an average user throughput gain exceeding 5% is achieved from the IRC receiver and the improvement in the average user throughput is high especially for the UEs connected to picocells compared to UEs connected to macrocells.

Interference Rejection Combining in LTE Networks

Long Term Evolution (LTE) provides high spectral efficiency within one cell, but it is shown to be highly vulnerable to inter-cell interference. While LTE as well as Wideband Code Division Multiple Access (WCDMA) suffers uplink interference especially with a frequency reuse factor of one, WCDMA has some inherent layer 1 mechanisms to alleviate these effects, namely macro-diversity and spreading codes. In contrast, LTE user data are more localized in the time/frequency domain, calling for different mechanisms for interference reduction in the receiver. For the higher layers, techniques such as fractional power control or fractional frequency reuse can be used to mitigate the interference but are shown to be sub-optimal. For layer 1, interference rejection combining (IRC) is described as an efficient alternative to increase uplink bit rates in areas where cells overlap. IRC principles in the presence of radio fading channels are detailed, while strategies for both interference suppression and signal-to-interference-plus-noise ratio (SINR) maximization are considered. Taking a didactic approach, we compare multiple antenna receiver techniques (maximal ratio combining (MRC), IRC, multiple input multiple output (MIMO)), analyze the key factors of IRC efficiency (the number of receiving antennas and interferers, time synchronization, interference over thermal, and scheduling decisions), and provide link level performance comparisons. We also provide implementation considerations for integrating the IRC in the current eNodeB receiver. As the purpose of this paper is to give a basic level introduction to IRC applied to the LTE uplink and highlight the respective surroundings, the IRC approach will be covered rather intuitively by illustrations without a great deal of theoretical detail. © 2012 Alcatel-Lucent.

Design and Evaluation of Antenna Selection Methods for Interference Rejection Combining

Co-channel interference is one of the major factors limiting the capacity in modern cellular networks. Interference rejection combining (IRC) is an effective technique to suppress co-channel interference, requiring receivers with multiple antennas. For terminals using IRC, the larger the number of available antennas, the more interfering signals that can be suppressed. However, production cost, size and energy consumption of the required radio-frequency units impose a limit in the number of practical antenna elements. In this paper, we investigate the combination of IRC with antenna selection (AS), as a cost-effective approach to improve the interference suppression performance of wireless receivers, and present different low-complexity AS algorithms. In addition, we propose algorithmic modifications in case the estimates of interfering channels are unknown at the receiver, and an estimate of the interference-plus-noise covariance matrix (CM) has to be used. It is found that, in the presence of channel estimation errors, AS techniques using the CM approach are able to provide better performance than techniques using estimates of interfering channels. These findings and the effectiveness of the proposed algorithms are validated by both computer simulations and experimental testing using a hardware prototype.

Impact on Inter-Cell Interference of Reference Signal for Interference Rejection Combining Receiver in LTE-Advanced Downlink

This paper investigates the dominant impact on the interference rejection combining (IRC) receiver due to the downlink reference signal (RS) based covariance matrix estimation scheme. When the transmission modes using the cell-specific RS (CRS) in LTE/LTE-Advanced are assumed, the property of the non-precoded CRS is different from that of the data signals. This difference poses two problems to the IRC receiver. First, it results in different levels of accuracy for the RS based covariance matrix estimation. Second, assuming the case where the CRS from the interfering cell collides with the desired data signals of the serving cell, the IRC receiver cannot perfectly suppress this CRS interference. The results of simulations assuming two transmitter and receiver antenna branches show that the impact of the CRS-to-CRS collision among cells is greater than that for the CRS interference on the desired data signals especially in closed-loop multiple-input multiple-output (MIMO) systems, from the viewpoint of the output signal-to-interference-plus-noise power ratio (SINR). However, the IRC receiver improves the user throughput by more than 20% compared to the conventional maximal ratio combining (MRC) receiver under the simulation assumptions made in this paper even when the CRS-to-CRS collision is assumed. Furthermore, the results verify the observations made in regard to the impact of inter-cell interference of the CRS for various average received signal-to-noise power ratio (SNR) and signal-to-interference power ratio (SIR) environments. key words: LTE-Advanced, interference rejection combining, inter-cell interference, reference signal, turbo equalizer

On the Equivalence of MMSE and IRC Receiver in MU-MIMO Systems

In order to combat the residual interference in multiuser multiple-input and multiple-output systems (MU-MIMO), a number of low-complexity interference aware type of receivers have been considered. In this letter, single layer Minimum Mean Square Error (MMSE) and Interference Rejection Combiner (IRC) are applied in MU-MIMO transmission. Based on some very simple but extremely useful matrix manipulations, their equivalence is proven analytically and demonstrated via Monte Carlo simulations.