Effect Of Multiuser Interference Research Articles

A Comprehensive Investigation on Multi-User Interference Effects in Vehicular Visible Light Communications.

Vehicular visible light communications (VLC) are considered a suitable technology for vehicular platooning applications. Nevertheless, this domain imposes strict performance requirements. Although numerous works have shown that VLC technology is compatible with platooning applications, existing studies are mainly focused on the physical layer performances, mostly ignoring the disruptive effects generated by neighboring vehicular VLC links. Nevertheless, the 5.9 GHz Dedicated Short Range Communications (DSRC) experience has shown that mutual interference can significantly affect the packed delivery ratio, pointing out that these effects should be analyzed for vehicular VLC networks as well. In this context, this article provides a comprehensive investigation focused on the effects of mutual interference generated by neighboring vehicle-to-vehicle (V2V) VLC links. Therefore, this work provides an intensive analytical investigation based on simulation and also on experimental results that demonstrate that although ignored, the influence of mutual interference is highly disruptive in vehicular VLC applications. Hence, it has been shown that without preventive measures, the Packet Delivery Ratio (PDR) can decrease below the imposed 90% limit for almost the entire service area. The results have also shown that although less aggressive, multi-user interference affects V2V links even in short-distance conditions. Therefore, this article has the merit of emphasizing a new challenge for vehicular VLC links and points out the importance of multiple-access techniques integration.

Combined Effects of Multi-User Interference and Correlated Fading on MIMO Interference-Unaware Transceiver

ABSTRACT The combined effects of multi-user interference and correlated fading on the interference-unaware transceiver (IUT), which performs transformations in both the transmitter and the receiver without the interference knowledge, are studied. We show that for IUT under multi-user interference, receive correlated fading can have no impact on the achievable rate and the achievable rate can be increased with the increase in the correlations of the interference’s transmit correlated fading. Furthermore, we show that even if there is transmit correlated fading, the achievable rate of IURT can converge to the interference-free MIMO channel capacity as the number of receive antennas increases and the required number of receive antennas for becoming interference free can be decreased by transmit correlated fading.

Simulative Comprehensive Analysis of 2D Algebraically Constructed Codes for Optical CDMA

In this manuscript, comprehensive analysis of distinct optical codes belonging to family of algebraically constructed codes has been demonstrated. Many optical codes belonging to algebraically constructed family such as Extended Reed-solomon codes (E-RS), Multi-level prime codes (MPC), Extended-multi level prime codes (E-MPC), and Variable-weight quadratic congruence codes (VWQCC), Bipolar/Unipolar codes are taken into consideration. The performance characteristics of these codes are examined through Hard-error limiting probability (HEP) equations. Investigations revealed that Bipolar/Unipolar code is the one algebraically developed code that has outperformed all the codes considered here even in the presence of Multi-user interference effect. It has been observed that this optical 2D Bipolar/Unipolar code shows minimum BER value with increasing user count. Hence Bipolar/Unipolar code is robust to multiple user interference affected OCDMA systems.

Multi-user High Data Rate Indoor VLC Systems

In this paper, we propose, design and evaluate the performance of an angle diversity transmitter (ADT) to achieve high data rate multi-user indoor visible light communication (VLC) systems. Our proposed systems are examined with three different receivers: a wide-field of view (W-FOV) receiver, an angle diversity receiver (ADR) and an imaging diversity receiver (IMDR). A novel algorithm is introduced also to eliminate the multi-user interference that is caused by line of sight (LOS) components, since these have a great impact on VLC systems performance. The effect of multi-user interference, mobility and diffuse reflections are considered in this work. We also investigate the effect of an increase in the number of the users on our proposed systems while achieving a high data rate for each user. Our proposed systems achieve 1 Gb/s for ADT with W-FOV receiver, 2 Gb/s for ADT with ADR and 4 Gb/s for ADT with IMDR with a simple modulation technique (on–off-keying, OOK).

On the Effect of Blockage Objects in Dense MIMO SWIPT Networks

Simultaneous wireless information and power transfer (SWIPT) is characterized by the ambiguous role of multi-user interference. In short, the beneficial effect of multi-user interference on RF energy harvesting is obtained at the price of a reduced link capacity, thus originating nontrivial tradeoffs between the achievable information rate and the harvestable energy. Arguably, in indoor environments, this tradeoff might be affected by the propagation loss due to blockage objects like walls. Hence, a couple of fundamental questions arise. How much must the network elements be densified to counteract the blockage attenuation? Is blockage always detrimental on the achievable rate-energy tradeoff? In this paper, we analyze the performance of an indoor multiple-input multiple-output SWIPT-enabled network in the attempt to shed a light of those questions. The effects of the obstacles are examined with the help of a stochastic geometry approach in which the locations of energy sources (also referred to as power heads) are distributed by using a Poisson point process and walls are generated through a Manhattan Poisson line process. The stochastic behavior of the signal attenuation and the multi-user interference is studied to obtain the joint complementary cumulative distribution function of information rate and harvested power. Theoretical results are validated through Monte Carlo simulations. Eventually, the rate-energy tradeoff is presented as a function of the density of walls to emphasize the cross-dependences between the deployment of the network elements and the macro parameters characterizing the topology of the venue.

Performance analysis of 2-D optical prime codes for optical-CDMA

In this paper, comparative analysis of code performance of distinct optical 2-D codes belonging to the Prime code family have been examined. Various optical 2-D Prime codes such as Low-weight Prime sequence code (LW-PSC), Modified prime-hop code (MPHC), Extended prime-hop code (E-CHPC), Transposed-modified prime code (T-MPC), Transposed-sparse padded modified prime code (T-SPMPC) are considered. Hard-limiting Error Probability (HEP) equations of these codes are used to scrutinize the performance of these codes. Performance of one of the 2-D Prime codes, more specifically MPHC performs best in OCDMA systems where only Multi-user interference effect is considered. This optical 2-D MPHC code possesses very low BER value as compared to universally accepted standard with the increasing user count. MPHC provides robustness to multiple user interference affected OCDMA therefore very much apt for applications where simultaneous access of immense number of active user is required.

Performance of image transmission over MC-CDMA based on super resolution technique

Multi-carrier code-division-multiple-access (MC-CDMA) is a promising technique to obtain high data rate in wireless communication. Contemplation of medical image transmission for various patient’s information using MC-CDMA system through wireless media is discussed. Arnold cat map encryption algorithm is ensured to enhance the image transmission securely. At each mobile station, transmitted bit stream of encrypted images is estimated using minimum-mean-square error (MMSE) algorithm. The picture quality is ensured at the receiver by adopting super resolution through sparse technique. The performance of considered system through stanford channel model specification is illustrated. PSNR metric is also considered for image transmission. From the analysis, it is observed that considered MC-CDMA system with encryption algorithm and sparse based super resolution technique mitigates multi-user interference effects while retaining the quality of individual medical images. Further, it is witnessed that the channel encoded proposed system’s performance achieves the promising bit-error-rate for MMSE receiver. Furthermore, from the simulation it is discerned that the computational time for super resolution is minimized using considered sparse technique.

Performance analysis of multi-carrier IDMA system for co-operative networks

In this paper, we explicate error-rate (ER) results of transmitter pre-processing (TP) aided coded multi-carrier (MC)-interleave-division-multiple-access (IDMA) scheme for co-operative downlink (DL) communication. The base station (BS) transmits the coded symbols to the relays during the first time interval. The relay decodes the symbol and then re-encodes the information so that error free communication can be achieved. Then, the coded bit stream is transmitted during the second time interval. Finally, we will obtain the user’s information at the destination using iterative decoding algorithm and achieve better ER with lower signal-to-noise ratio (SNR). We assume that each user is reinforced by at least single relay. We consider MC modulation which convert frequency-selective channel into flat-fading channel. We realize TP technique on both the BS and the intermediate relays, so that the effects of Multi-user interference and Inter-relay-interference are suppressed. To be specific, we evaluate ER performance of MC-IDMA system using TP for co-operative network with rich scattering effects based on LTE-vehicular channel environments considering both BS to relay and relay to mobile stations (MS). From our simulation results, we observe that turbo-coded co-operative aided MC-IDMA DL system with MUTP shows a substantial improvement in its ER performance with reduced amount of transmit power in terms of SNR.

Performance analysis of new optical matrices for constructing two-dimensional temporal/spatial optical code-division multiple-access networks

This study presents designs for developing two-dimensional optical matrices exhibiting low cross correlation and excellent orthogonality. The key scheme is to use original one-dimensional temporal codes and a particular mapping technique. A two-dimensional temporal/spatial optical code-division multiple-access network using the proposed optical matrices was also constructed. This study subsequently analyzed the effect of multiuser interference on code performance. The analysis result reveals that some families of optical codes applied to the network theoretically provided a favorable code performance level.

Triple-Polarized Multi-User Mimo-Idma System under Correlated Fading Channel

A multiple-input multiple-output interleave division multiple access (MIMO-IDMA) system with Triple Polarized Division Multiplexing (TPDM) is presented in this paper. The present methodology replaces three independent linearly polarized antennas with a single triple polarized antenna at both the transmitter and receiver. The users in the communication link are accommodated and separated using a user-specific interleaver combined with low rate spreading sequence. To eliminate the effects of multi-stream interference (MSI), minimum mean square error (MMSE) algorithm based on successive interference cancellation (SIC) Multi-user detection (MUD) technique is employed at the receiver. Furthermore, log-maximum a posteriori probability (MAPP) decoding algorithm is implemented at the mobile stations (MSs) to overcome the effects of multi-user interference (MUI) effects. The paper also evaluates the effects of coded MIMO-IDMA in the downlink communication by adopting the Stanford University Interim (SUI) and Long-term Evolution (LTE)channel model specifications. In comparison with the traditional uncoded system, the present solution considering turbo coded triple-polarized MIMO-IDMA system with iterative decoding algorithm provides better bit error rate (BER) with reduced signal to noise ratio (SNR). The simulation results also show that though the SNR requirement is higher for the proposed technique compared to the conventional uni-polarized antenna based MIMO-IDMA system, it gives the advantages of achieving higher data rate with reduced cost and space requirements in the context of a downlink (DL).

Fast frequency hopping codes applied to SAC optical CDMA network

This study designed a fast frequency hopping (FFH) code family suitable for application in spectral-amplitude-coding (SAC) optical code-division multiple-access (CDMA) networks. The FFH code family can effectively suppress the effects of multiuser interference and had its origin in the frequency hopping code family. Additional codes were developed as secure codewords for enhancing the security of the network. In considering the system cost and flexibility, simple optical encoders/decoders using fiber Bragg gratings (FBGs) and a set of optical securers using two arrayed-waveguide grating (AWG) demultiplexers (DeMUXs) were also constructed. Based on a Gaussian approximation, expressions for evaluating the bit error rate (BER) and spectral efficiency (SE) of SAC optical CDMA networks are presented. The results indicated that the proposed SAC optical CDMA network exhibited favorable performance.

Tomlinson–Harashima Precoding for Multiuser MIMO Systems With Quantized CSI Feedback and User Scheduling

This paper studies the sum rate performance of a low complexity quantized CSI-based Tomlinson-Harashima (TH) precoding scheme for downlink multiuser MIMO transmission, employing greedy user selection. The asymptotic distribution of the output-signal-to-interference-plus-noise ratio of each selected user and the asymptotic sum rate as the number of users K grows large are derived by using extreme value theory. For fixed finite signal-to-noise ratios and a finite number of transmit antennas n <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> , we prove that as K grows large, the proposed approach can achieve the optimal sum rate scaling of the MIMO broadcast channel. We also prove that, if we ignore the precoding loss, the average sum rate of this approach converges to the average sum capacity of the MIMO broadcast channel. Our results provide insights into the effect of multiuser interference caused by quantized CSI on the multiuser diversity gain.

Blind carrier frequency offset estimation for tile‐based orthogonal frequency division multiple access uplink with multi‐antenna receiver

In this study, the authors propose a blind carrier frequency offset (CFO) estimation method for the tile structure orthogonal frequency division multiple access (OFDMA) uplink with multi-antenna receiver. They employ an iterative approach to extract the signal component for each user gradually and propose a carefully designed CFO estimator to update the CFO estimate during the iterative procedure. The key ingredient of the proposed method is using few subcarriers on both sides within each tile as the ‘guard subcarriers’, which can greatly mitigate the effect of multi-user interference. The proposed method supports not only fully loaded transmissions, but also the generalised assignment scheme that provides the flexibility for dynamical resource allocation. The numerical results are provided, which indicate that the proposed method can almost converge to the analytical lower bound within a few iterative cycles. It is seen that the proposed method also outperforms the existing competitor with multi-antenna receiver in terms of estimation performance, especially with few adopted blocks.

A Novel Multi User Interference Cancellation Scheme for Synchronous OCDMA Networks

In this paper we have proposed a novel multi-user interference (MUI) cancellation scheme for OCDMA systems employing prime code families. We have also discussed in detail the feasibility and implementation of this novel scheme. In addition, the performance analysis of a few prime code families are evaluated experimentally using this newly proposed MUI cancellation scheme. The results reveal that the proposed scheme is easy to implement and totally removes the MUI effect.

Two- and four-relay selection schemes for application in interference limited legacy networks

The limiting effect of multi-user interference from an adjacent cell upon the relays used for cooperative transmission is considered in the context of legacy networks which adopt max(min(·,·)) type relay selection policies. We extend previous work which considered single relay selection to the selection of two or four such relays, as is required in distributed space–time coding. We obtain new analytical expressions for outage probability over Rayleigh frequency flat fading channels for two signal-to-noise ratio regimes. We confirm by simulation that such a relay selection scheme has robustness to relay selection feedback error and outperforms a single relay selection scheme.

Performance Analysis of Cooperative Time Hopping UWB Systems with Multi-User Interference

In this letter, we propose an analytical model to study the performance of cooperative links for time hopping ultra wideband (UWB) systems under two relay models: amplify-and-forward (AF) and decode-and-forward (DF). The effects of multi-user interference (MUI) are considered when deriving closed-form expressions for outage probability and average bit error probability (BEP) over single-path model. Simulation results show that the presented analytical model can accurately predict the link performance.

Genetic algorithm-assisted joint quantised precoding and transmit antenna selection in multi-user multi-input multi-output systems

This study presents a simple and efficient genetic algorithm-assisted approach for joint quantised precoding and transmit antenna selection based on the criterion of maximum capacity. The objective is to alleviate the effect of multi-user interference and to reduce hardware costs, such as the cost of radio frequency chains associated with antennas in the downlink of multi-input multi-output systems with limited feedback. To avoid the enormous search effort required by existing approaches, the authors propose a novel variant of the conventional genetic algorithm, called the hybrid genetic algorithm, in which each chromosome is divided into a bit string for precoding vector selection and an integer string for transmit antenna selection. In addition, new crossover and mutation operations are employed to accommodate these new chromosomes. The results of simulations show that the performance of the proposed approach is close to that of the exhaustive search method, but its computational complexity is substantially lower.

A novel technique to increase the capacity of code division multiple access system using scrambled spreading sequences

A multistage Linear Parallel Interference Cancellation (LPIC) approach is presented to mitigate the effect of multiuser interference in uplink Direct Sequence Code Division Multiple Access (DS CDMA) system. The system is overloaded to accommodate more number of users than the processing gain of the system. The performance of this overloading scheme is evaluated by assigning one set of orthogonal Gold codes to the first N users and another set of orthogonal Gold codes to extra users. Both the sets are overlaid by a common pseudo noise scrambling sequence. This paper proposes a novel technique of using Weighted Linear Parallel Interference Cancellation (WLPIC) for overloaded CDMA system resulting in bandwidth saving, better performance and increase in the speed of interference cancellation. Also this technique considerably reduces the multipleaccess interference, since set-1 users suffer from interference of the set-2 users only, while the set-2 users suffer from interference of the set-1 users. This provides a way for accommodating additional users in the same available bandwidth. Considerable conservation of time is achieved by avoiding the usage of conventional successive Interference Cancellation (IC) for scrambled CDMA system. The empirical results show that this scheme provides 50% overloading with three-stage weighted linear parallel interference cancellation (WLPIC) technique for N = 64 at a Bit Error Rate (BER) of 10-3 and 75% overloading at a BER of 10-2. The three-stage WLPIC scheme clearly outperforms matched filter detector, Conventional LPIC and the twostage WLPIC on Additive White Gaussian Noise (AWGN) channel.

Estimation and equalization of fiber-wireless uplink for multiuser CDMA 4G networks

Fiber-wireless (Fi-Wi) access fronts can support 100s of Mb/s envisioned by 4G networks. However, a major issue associated with Fi-Wi links is the nonlinear distortion of the radio-over-fiber (ROF) link coupled with the multipath dispersion of the wireless channel. Estimation and subsequent equalization of the concatenated fiber-wireless channel needs to be done, especially at high bit rates. The uplink is severely affected due to large fluctuations in the radio signal. This paper proposes an estimation and subsequent equalization algorithm for the Fi-Wi CDMA uplink. The estimation employs the properties of pseudo noise (PN) sequences and the equalization uses a novel Hammerstein type decision feedback equalizer (HDFE). The estimation and equalization are performed in the presence of multiple access interference (MAI) and wireless and optical channel noise. The cumulative effects of multiuser interference, multipath dispersion, nonlinear distortion, and noise are all considered in our analysis. Correlation properties of white-noise like PN sequences enable decoupling of the linear (wireless) and nonlinear (optical) channel portions. Furthermore, we propose a unique algorithm to mitigate MAI. Numerical evaluations show a good estimation and equalization of both the linear and nonlinear channels. Bit error rate (BER) simulations show that this algorithm leaves only small residual MAI.

An Enhanced Embedded-Pilot Channel Estimation Architecture for MIMO MC-CDMA Systems

In this work an enhanced semi-blind channel estimator is presented for multicarrier CDMA systems with multiple transmit and receive antennas. The novel scheme is based on embedded pilots that are characterized as virtual users and their number is determined in a dynamic fashion according to the propagation channel conditions. The superimposed code-multiplexed pilots are used initially for channel estimation before the despreading operation, thus achieving per subcarrier estimation using the known unique multi-layer pattern of the virtual users. Afterwards, the estimation is enhanced by using the despread pilots. Furthermore, an iterative scheme is incorporated in channel estimation process minimizing the effect of multiuser interference and operates either in interference cancellation, or in virtual user mode. The proposed architecture is evaluated for spatial diversity systems (space- frequency block coded) with two transmit and up to four receive antennas operating in realistic propagation channels for various modulation and coding schemes. The results demonstrate a significant performance improvement especially as the channel is dominated by its time variant characteristics. The dynamic assignment of spreading codes in the payload and the pilots offers to the system realistic flexibility and adaptivity in demanding channels achieving concurrently maximization of the real overall system throughput, while maintaining low overhead and complexity burden.