Antenna Correlation Research Articles

LAS Detector with Soft-Output MMSE Initialization Under Imperfect Channel Estimation and Channel Correlation

This paper proposes a modified likelihood ascent search (LAS) algorithm for multiple-input multiple-output (MIMO) systems under correlated channel and imperfect channel state information. We assume that the LAS detector is initialized by the minimum mean square error (MMSE) filter. We propose a new maximum-likelihood (ML) decoding metric by considering the soft-output of the MMSE filter. Our proposed LAS takes both channel estimation errors and spatial correlation of antennas into account when computing the ML metric. We show that, while increasing the computational complexity, the proposed LAS remains suitable for massive MIMO systems. Simulation results show that the proposed LAS outperforms the conventional LAS detector, specially when the transmitter correlation increases.

Media‐based single‐symbol generalized spatial modulation

SummarySingle‐symbol generalized spatial modulation (GSM) improves upon the limitation of spatial modulation (SM) by reducing the number of required transmit antennas to achieve high data rates. In this paper, we investigate the application of media‐based modulation (MBM) based on radio frequency mirrors to single‐symbol GSM, with the aim of improving error performance. The theoretical average bit error probability of the proposed scheme is derived, employing a lower bound approach and used to validate the Monte Carlo simulation results. Finally, the effect of optimal and suboptimal mirror activation pattern selection employing Euclidean distance and channel amplitude coupled with antenna correlation is investigated for the proposed media‐based single‐symbol GSM system. The Monte Carlo simulation results obtained demonstrate a significant improvement over the conventional SM, GSM, and media‐based SM (MB‐SM) schemes in terms of error performance and spectral efficiency.

Asymptotic Analysis of RZF Over Double Scattering Channels With MMSE Estimation

This paper studies the ergodic sum rate performance of regularized zero-forcing (RZF) precoding in the downlink (DL) of a multiuser multiple-input single-output (MISO) system, where the channel between the base station (BS) and each user is modeled by the double scattering model. This non-Gaussian channel model is a function of both the antenna correlation and the structure of scattering in the propagation environment. This paper first makes the preliminary contribution of deriving the minimum-mean-square-error (MMSE) channel estimate for this model. The system model accounts for channel estimation errors and per-group channel correlation matrices for the users. The analysis assumes that the number of BS antennas, the number of single-antenna users and the number of scatterers in each group grow large while their ratio remains bounded. Under this setting, this paper derives deterministic equivalents of the signal-to-interference plus noise ratio (SINR) and the sum rate which are almost surely tight in the large system limit. The derived approximations are expressed in a closed-form for the special case of multi-keyhole channel. We show that the performance of a massive MIMO system does not scale linearly with the number of antennas under uncorrelated channel conditions and is actually limited by the number of scatterers in the propagation environment. Simulation results confirm the close match provided by the asymptotic analysis for moderate system dimensions.

Asymptotic Performance Analysis of Massive MIMO Relay Systems With Multi-Pair Devices Over Correlated Fading Channels

In this paper, we investigate a relay system with multi-pair single-antenna devices, where massive antenna array (also known as massive multiple-input-multiple-output (MIMO) technology) is deployed at the relay with low-complexity maximum-ratio combining/maximum-ratio transmission precoding scheme. By combining massive MIMO and relay cooperation, the performance of relay-assisted communication systems is predicted to be dramatically improved in many areas, e.g., spectral efficiency, energy efficiency, inter-user interference cancelation, and so on, by exploring the abundant spatial degree of freedom of massive MIMO. However, a large number of antennas employed at relay node inevitably leads to antenna correlation because of small antenna spacing subjected to the limited size of the relay node, which may affect the system performance and has not yet been compressively studied. Thus, we focus on analyzing the effect of antenna correlation on the asymptotic performance of system ergodic rate by using a general channel correlation model. First, by means of the deterministic equivalent technology, the analytical expression of the ergodic rate with arbitrary channel correlations is derived, which presents the quantitative relations among system parameters, i.e., the numbers of relay antennas and device-pairs, the channel spatial correlations at the relay's receiver side and the transmitter side, and the transmit powers of the source devices and the relay. By using a general scaling model for system parameters with respect to the relay antenna number, the asymptotic performance of the ergodic rate under spatially correlated channel and the corresponding scaling laws of the system parameters are obtained, which provides useful guidelines to trade off the transmit power and the ergodic rate as well as the number of the served device-pairs. It is further revealed that the influence degrees of the antenna correlations at the relay's receiver and the transmitter on the ergodic rate is nearly the same. Finally, the analytical conclusions are justified through the numerical simulations.

Low-Overhead Cyclic Reference Signals for Channel Estimation in FDD Massive MIMO

Massive multiple input multiple output (MIMO) transmission and coordinated multipoint transmission are candidate technologies for increasing data throughput in evolving 5G standards. Frequency division duplex (FDD) is likely to remain predominant in large parts of the spectrum below 6 GHz for future 5G systems. Therefore, it is important to estimate the downlink FDD channels from a very large number of antennas, while avoiding an excessive downlink reference signal overhead. We here propose and investigate a three part solution. First, massive MIMO downlinks use a fixed grid of beams. For each user, only a subset of beams will then be relevant, and require estimation. Second, sets of coded reference signal sequences, with cyclic patterns over time, are used. Third, each terminal estimates its most relevant channels. We here propose and compare a linear mean square estimation and a Kalman estimation. Both utilize frequency and antenna correlation, and the later also utilizes temporal correlation. In extensive simulations, this scheme provides channel estimates that lead to an insignificant beamforming performance degradation as compared to full channel knowledge. The cyclic pattern of coded reference signals is found to be important for reliable channel estimation, without having to adjust the reference signals to specific users.

Optimal Transmission in MIMO Channels With Multiuser Interference

Cochannel interference is one of the inevitable deleterious components in designing and analyzing of a wireless network. The use of multiple antennas at both transmitting and receiving nodes is a promising technique to suppress and/or alleviate the effect of cochannel interference on capacity. In this paper, we assess the effects of both antenna correlation and cochannel interference on the ergodic capacity of multiple-input multiple-output channels with covariance feedback. In particular, we consider a general family of spatial fading correlation model-called unitary-independent-unitary -which encompasses most of zero-mean channels with arbitrary fading profiles including the popular separable correlation channel models. We derive the average minimum mean-square error and signal-to-interference-plus-noise ratio of the parallel spatial streams using Berezin’s supermathematics. We then put forth the structure of optimal input covariance matrix maximizing the mutual information connected with the necessary and sufficient conditions as a generalization of the noise-limited case, which is tested by a simple iterative algorithm. Together with the powerful supermathematical framework, the result in the paper enables us to quantify the multiuser MIMO interference effects on the capacity in terms of spatial correlation and interference power heterogeneity.

Effect of estimation error on dual MRC receiver over Hoyt fading channels

ABSTRACTThe effect of antenna correlation and the estimation error on the dual maximal ratio combining receiver over Hoyt fading channels has been analysed in this paper. We have derived the expressions of bit error rate and outage probability to observe the quantum of performance degradation due to estimation error along with correlation. From our analysis, we have concluded that the error in envelope estimation degrades the performance of the receiver up to a great extent than the phase estimation error. Since both antenna correlation and estimation error are unavoidable in practical application, the authors feel that the provided results will assist the design engineers to sustain the quality of service in a wireless receiver.

A compact triple-band dual-element MIMO antenna with high port-to-port isolation for wireless applications

In this literature, a compact planar triple-band multiple input multiple output (MIMO) antenna with two monopole elements is presented. Each element includes a defective complementary open-loop resonator (COLR), two slots on both sides of the feedline, two stepped stubs and a monopole ground plane. This structure operates in the frequency bands of 2.22–2.54 GHz, 3.14–3.9 GHz and 5.3–5.7 GHz for WiFi, WiMAX and WLAN applications, respectively. The antenna is not based on a common ground plane, so antenna is low cost and compact. High measured isolation is achieved which is better than 34 dB in all operating bands without using additional decoupling structure. An equivalent circuit model is proposed to investigate the behaviour of the antenna. The envelope correlation coefficient of antenna is less than 0.001 and diversity gain is about 10. The proposed MIMO antenna is fabricated and there is a good agreement between the experimental measurements with the simulation results.

Efficient detectors for MIMO-OFDM systems under spatial correlation antenna arrays

This work analyzes the performance of the implementable detectors for multiple-input-multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) technique under specific and realistic operation system condi- tions, including antenna correlation and array configuration. Time-domain channel model has been used to evaluate the system performance under realistic communication channel and system scenarios, including different channel correlation, modulation order and antenna arrays configurations. A bunch of MIMO-OFDM detectors were analyzed for the purpose of achieve high performance combined with high capacity systems and manageable computational complexity. Numerical Monte-Carlo simulations (MCS) demonstrate the channel selectivity effect, while the impact of the number of antennas, adoption of linear against heuristic-based detection schemes, and the spatial correlation effect under linear and planar antenna arrays are analyzed in the MIMO-OFDM context.

On the performance of cooperative vehicular networks under antenna correlation at RSU

Vehicular communications is gradually becoming mature after decades of exciting developments and thriving advances. Resultantly, these advances have opened new possibilities for vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) communications to meet the requirements of safety applications and future self-driving technologies. While performance limits of single link vehicular communications have been well analyzed in the literature, only incremental growth has been shown in the domain of multi-antenna communications. Another major concern is that the existing works mostly assume independent fading at the antennas mounted on road side unit (RSU), thus neglecting the impact of channel correlation. Our work addresses this issue by evaluating packet error probability for two renowned antenna correlation models i.e., constant correlation (CC) and exponential correlation (EC), under Nakagami-m fading. We also consider cooperation between intermediate vehicles to ensure reliable communication from the source vehicle to the RSU. More specifically, we derive closed-form expressions of packet error probability for three cooperative techniques, namely, single helper selection (SHS), multi-hop cooperative selection (MCS) and multiple helper selection (MHS). We quantify the performance variations for different numbers of intermediate helper vehicles, and for varying values of fading parameter and correlation coefficients. Finally, we validate our mathematical derivations by performing extensive simulations in MATLAB.

Performance assessment of correlated Rayleigh‐inverse Gaussian fading channel over distributed MIMO systems with ZF detectors

Composite fading channel modelling of terrestrial wireless propagation is crucial for the design of communication system and its performance analysis. In this study, a systematic characterisation of Rayleigh-inverse Gaussian (RIG) distribution is provided for modelling the channel of distributed multiple-input multiple-output (MIMO) systems with zero-forcing (ZF) receiver in the presence of transmit antenna correlation. At the outset, the closed-form expression for probability density function of instantaneous signal-to-noise ratio (SNR) is derived followed by its associated moments and cumulative density function. In the subsequent analysis, exact closed-form analytical expression of ergodic capacity with its bounds are derived and their asymptotic expressions are obtained in high and low SNR regime. Then the closed-form expressions are deduced for average symbol error rate (ASER) and outage probability (OP). Additionally, the authors propose to formulate simplified expressions for asymptotic ASER as well as OP and assess their approximation accuracy. Numerical results are presented to compare the performance of RIG model with the existing generic models and illustrate that RIG is superior in terms of ergodic capacity, ASER and OP. Finally, the derived analytical expressions are validated through Monte Carlo simulations which demonstrate the effectiveness of RIG model in the diverse environmental scenario for various system and channel parameters.

Performance analysis of zero-forcing precoding with signal space diversity under antenna correlation

We consider a multiantenna transmission system where the transmitter and receiver respectively have M and two antennas. Specifically, zero-forcing precoding is employed at the transmitter based on a possibly imperfect channel state information (CSI). The channel estimation is carried out at the transmitter using pilot signals sent from the receiver. Adopting a dual-stream transmission structure, the error performance of the system is enhanced by resorting to signal space diversity (SSD). The performance of the proposed system with binary phase shift keying modulation is studied under a slow flat Rayleigh fading scenario with uncorrelated transmit antennas and correlated receive antennas. We separately examine two cases: the perfect CSI and imperfect CSI at the transmitter. In the former scenario, we provide an exact closed-form expression on the bit error probability (BEP) of the proposed approach. A tight approximation on the BEP of the proposed scheme is presented for the latter scenario. The case with distinct channel estimation qualities for the two subchannels is also considered in the second scenario. The proper rotation angles for the BPSK signal constellation are analytically obtained such that the derived expressions on the BEP are minimized under both cases. We show that the inclusion of SSD into the original scheme yields a noteworthy improvement on the BEP performance with only an insignificant increase in complexity and no additional use of bandwidth/time slots. It is also shown that the proposed technique is more immune against the channel estimation errors as compared to the original method.

Design of a Low-Profile Antenna for Use with 698-2,700 MHz Femtocell Base Stations [Antenna Applications Corner

A low-profile multiple-input, multiple-output (MIMO) antenna for long-term evolution (LTE) femtocell base stations (BSs) is presented in this article. The antenna element consists of a modified bowtie loaded by three arcs with different lengths that extend the lowest frequency to 698 MHz, a microstrip feeding line, and a planar reflector. It has dimensions of 260 mm t 150 mm t 50 mm and a bandwidth of 698n2,700 MHz. Four of the antenna elements are used to form a four-port MIMO antenna in a novel cross shape. The resulting MIMO antenna has a compact size (480 mm t 480 mm t 50 mm), reasonably low mutual couplings, and extremely low antenna correlations. The MIMO capacity of the proposed antenna is evaluated via numerical simulations, and good MIMO performance is observed.

Optimizing channel capacity in a MIMO–PSK wireless rayleigh fading channel

Due to the continual advancement of future generation communication systems, channel capacity is considered to be an important parameter for achieving greater signal strength by means of faster data transmission speed. Usually in multiple input multiple output (MIMO) systems, the diversity technique lessens the propagation speed significantly than the channel capacity. Using MIMO in combination with phase shift keying (PSK) modulation jointly known as MIMO–PSK systems, the present paper focuses on a novel distribution method in order to optimize the channel capacity over a Rayleigh fading channel. First, ergodic channel capacity and distribution capacity in terms of outage capacity is simulated for different MIMO–PSK configurations as well as channel conditions. Later, the channel performance of MIMO–PSK for the novel distribution conditions with antenna correlation effect is evaluated and finally the optimal channel capacity is determined.

Error performance of Uncoded Space Time Labelling Diversity in spatially correlated Nakagami‐q channels

SummaryGreater spectral efficiency has recently been achieved for Uncoded Space Time Labelling Diversity (USTLD) systems by increasing the number of antennas in the transmit antenna array. However, due to constrained physical space in hardware, the use of more antennas can lead to degradation in error performance due to correlation. Thus, this paper studies the effects of spatial correlation on the error performance of USTLD systems. The union bound approach, along with the Kronecker correlation model, is used to derive an analytical expression for the average bit error probability (ABEP) in the presence of Nakagami‐q fading. This expression is validated by the results of Monte Carlo simulations, which shows a tight fit in the high signal‐to‐noise ratio (SNR) region. The degradation in error performance due to transmit and receive antenna correlation is investigated independently. Results indicate that transmit antenna correlation in the USTLD systems investigated (3 × 3 8PSK, 2 × 4 16PSK, 2 × 4 16QAM, and 2 × 4 64QAM) causes a greater degradation in error performance than receive antenna correlation. It is also shown that 2 × 4 USTLD systems are more susceptible to correlation than comparable space‐time block coded systems for 8PSK, 16PSK, 16QAM, and 64QAM.

Impact of Antenna Correlation on Full-Duplex Two-Way Massive MIMO Relaying Systems

This paper investigates the impact of antenna correlation on full duplex massive multiple-input multiple-output (MIMO) two-way relaying systems. To be general, all nodes in the system are equipped with multiple antennas and arbitrary antenna correlation is considered. Deterministic equivalent of the achievable sum-rate is first given, which enables the derivation of the asymptotic sum-rate when the number of relay antennas is large. The impact of antenna correlation on the asymptotic sum-rate is then thoroughly analyzed. The results reveal that with a large number of relay antennas, the asymptotic sum-rate is independent of the antenna correlation at the relay’s transmitter side, however it is Schur-concave with respect to the eigenvalue vectors of the involved antenna correlation matrices at both the users’ transceiver sides and the relay’s receiver side. In other words, the antenna correlations at both the users’ transceiver sides and the relay’s receiver side have detrimental effects on the sum-rate. Moreover, the antenna correlation at the users is more significant to the asymptotic sum-rate than that at the relay. The analysis is general and can be easily reduced to consider half-duplex and/or one-way massive MIMO relaying systems. Numerical results are finally provided to validate the analysis. It is also shown that a larger number of user antennas may not always lead to a higher sum-rate, and thus proper selection of the number of user antennas is necessary to maximize the sum-rate.

Estimation of Antenna Correlation Coefficient of N -Port Lossy MIMO Array

This paper proposes a simple yet accurate method for estimating the antenna correlation coefficient (ACC) of a high-order multiple-input multiple-output (MIMO) antenna. The conventional method employed to obtain the ACC from three-dimensional radiation patterns is costly and difficult to measure. An alternate method is to use the S-parameters, which can be easily measured using a network analyzer. However, this method assumes that the antennas are highly efficient, and it is therefore not suitable for lossy MIMO antenna arrays. To overcome this limitation, we define and utilize the non-coupled radiation efficiency in the S-parameter-based ACC formula. The accuracy of the proposed method is verified by the simulation results of a 4-port highly coupled lossy MIMO array. Further, the proposed method can be applied to N-port arrays by expanding the calculation matrix.

Performance Analysis of Massive MIMO Two-Way Relay Networks With Pilot Contamination, Imperfect CSI, and Antenna Correlation

We consider a multicell two-way relay network consisting of single-antenna user nodes and amplify-and-forward relay nodes having very large antenna arrays. We investigate the combined impact of co-channel interference (CCI), imperfect channel state information (CSI), pilot contamination, and the antenna correlation at the massive multiple-input multiple-output (MIMO) node. By using a large number of antennas at the relay, we can completely mitigate the effect of CCI. However, the effects of imperfect CSI and pilot contamination degrade the performance even with a large antenna array. Yet, the use of massive MIMO allows power scaling at the user nodes and relay, and thus, even with channel imperfections, the benefits of employing a massive-MIMO-enabled relay on transmit power savings are significant. Furthermore, we derive closed-form approximations for the sum rate when CCI and pilot contamination are absent and CSI is perfect. This result helps to decide the required number of relay antennas to obtain a certain percentage of the asymptotic sum rate. Also, our analysis of antenna correlation shows that it can be mitigated by using a large antenna array. We also find the optimal pilot sequence length to maximize the sum rate of the system.

Spectral efficiency analysis and pilot reuse factor optimisation for multi‐cell massive SC‐SM MIMO

As a combination of spatial modulation (SM) system and massive multiple-input multiple-output (MIMO) system, massive SM aided MIMO (SM-MIMO) system is recently proposed. In broadband scenarios, single-carrier (SC) modulation is introduced to massive SM-MIMO system, thus massive SC-SM MIMO system is proposed for uplink multi-user transmission over frequency-selective fading channels. In this study, the uplink spectral efficiency (SE) of multi-cell massive SC-SM MIMO system is analysed, meanwhile the pilot contamination effect is taken into consideration. A tight SE lower bound of multi-cell massive SC-SM MIMO system is proposed with maximum ratio (MR) combining, which also takes into account the imperfect channel estimation, transmit antenna correlation and path loss. The tightness of the authors' proposed closed-form SE lower bound is shown via simulation results. The optimal pilot reuse factor can be determined with different system configurations by simulations, and the pilot reuse factor that is larger than one is more suitable for less TAs and user equipments.

Uplink spectral efficiency analysis of multi-cell multi-user massive MIMO over correlated Ricean channel

In this paper, the performance of uplink spectral efficiency in massive multiple input multiple output (MIMO) over spacially correlated Ricean fading channel is presented. The maximum ratio combining (MRC) receiver is employed at the base station (BS) for two different methods of channel estimation. The first method is based on pilot-assisted least minimum mean square error (LMMSE) estimation, the second one is based on line-of-sight (LOS) part. The respective analytical expressions of uplink data rate are given for these two methods. Due to the existence of pilot contamination, the uplink data rate of pilot-assisted LMMSE estimation method approaches to a finite value (we name it as asymptotic rate in the paper) when the BS antenna number is very large. However, the data rate of LOS method goes linearly with the number of BS antennas. The expression of the uplink rate of LOS method also show that for Ricean channel, the spacial correlation between the BS antennas may not only decrease the rate, but also increase the rate, which depends on the locations of the users. This conclusion explains why the spacial correlation may increase, rather than decrease the data rate of pilot-assisted LMMSE. We also discuss the power scaling law of the two methods, and the asymptotic expressions of the two methods are the same and both independent of the antenna correlation.