Performance analysis of multicell multiuser MIMO MC DS/CDMA system with MMSE user-ordered SIC technique

The conventional minimum mean squared error (MMSE) receiver which actively suppresses multiuser interference (MUI), outperforms the RAKE receiver for direct sequence code division multiple access (DS-CDMA) in multipath slow fading channels. However the performance gap between the conventional MMSE receiver and the theoretical bound is still large. We therefore propose an extended MMSE receiver which spans three symbol intervals the desired symbol interval as well as the previous and the next symbol interval. The extended MMSE receiver outperforms the conventional MMSE receiver especially for large system load and large number of propagation paths in the channel. Its computational complexity is approximately three times the complexity of the conventional MMSE receiver but still independent of the number of active users and the number of paths in the channel. As a consequence the extended MMSE receiver allows a low-complexity low-power implementation in the mobile user-terminal for communication over multipath slow fading channels in the forward link.

We present a successive interference cancellation (SIC) scheme for a multicarrier (MC) DS CDMA system. In every subband, the SIC receiver successively detects the interferers' signals and substracts them from the user-of-interest. A comparison is made among SIC, matched filter detection with an equal gain combiner (MF-EGC), matched filter followed by maximal-ratio combining (MF-MRC) and minimum mean-squared error (MMSE) receivers with optimal tap coefficients. For this latter receiver, we determine the performance both when the number of taps equals the processing gain, denoted by N, and when the number of taps is roughly one-half of the processing gain. By analysis and simulation, it is shown that the SIC has better ability to suppress multiple-access interference (MAI) than do MF and [N/2]-tap MMSE in single carrier (SC) CDMA, and MF-EGC, MF-MRC and [N/2]-tap MMSE in MC CDMA, and this translates to increased capacity and better fading resistance. Further, SIC and N-tap MMSE yield approximately the same performance in SC CDMA, but SIC has a poorer performance than does an N-tap MMSE in MC CDMA. In addition, we derive results for the performance degradation that a SIC scheme experiences in MC CDMA due to both phase and timing tracking errors. It is shown that with small tracking errors, SIC can still retain a performance advantage over MF-EGC and MF-MRC, although the advantage of SIC decreases with increasing tracking errors.

Orthogonal space-time block codes (STBC), and the Alamouti scheme in particular, are of particular interest in multiple-input multiple-output (MIMO) systems since they achieve full spatial diversity over fading channels and are decoded from linear processing at the receiver. Nevertheless, due to the expensive spectral resource, increasing network capacity requires the development of interference cancellation (IC) techniques allowing several users to share the same spectral resources without impacting the transmission quality. In this context several IC schemes have been developed during this last decade, where each user is equipped with multiple antennas and employs STBC at transmission. However, these IC techniques require multiple antennas at reception, which remains a challenge at the handset level due to cost and size limitations. For this reason, low complexity single antenna interference cancellation (SAIC) techniques, currently operational in GSM handsets, have been developed recently for single antenna users using real-valued modulations or complex filtering of real-valued modulations, by using a widely linear (WL) filtering at reception. Extension to multiple antennas at reception is called multiple antenna interference cancellation (MAIC) technique. The purpose of this paper is to extend the SAIC/MAIC technology to users using both real-valued constellations, such as amplitude shift keying (ASK) constellations, and the Alamouti scheme at transmission. A WL minimum mean square error (MMSE) receiver, completely new for IC purposes in the context of radio communications systems using the Alamouti scheme, is proposed and analyzed. This receiver, which corresponds to the maximum likelihood (ML) receiver for synchronous intranetwork interferences, is able to separate up to 2 <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> Alamouti users from <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> antennas at reception, hence SAIC capability for <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> = 1.

We present a successive interference cancellation (SIC) scheme for a multicarrier (MC) direct-sequence code-division multiple-access (CDMA) system, using band-limited spreading waveforms to prevent self-interference. In every subband, the SIC receiver successively detects the interferers' signals and substracts them from the user-of-interest. A comparison is made among SIC, a minimum mean-squared error (MMSE) receiver, and matched filter (MF) detection with maximal-ratio combining (MRC). We also consider suboptimal combining using pilot symbol-assisted modulation (PSAM) to make the system more realistic. Analytic bit-error probabilities for SIC, MMSE, and MF in correlated Rayleigh fading channels are derived. The theoretical results for SIC, MMSE, and MF are shown to agree well with simulations. In particular, SIC and MMSE are shown to achieve better performance than MF with MRC; when the number of users is small, SIC provides better performance than does MMSE. Further, the correlation among different subcarriers is studied, and only large subcarrier correlation coefficients result in an obvious worsening of performance. Finally, we derive results for the performance degradation that an SIC scheme experiences in MC CDMA due to phase and timing tracking errors. It is shown that SIC can still retain a performance advantage over MF-MRC, although the advantage of SIC decreases with increasing tracking errors, especially when subcarrier correlation coefficients are small.

In this paper, we present two layered multiuser detectors (MUDs) for MIMO frequency-time-domain (FT-domain) multi-carrier (MC) direct sequence code division multiple access (DS-CDMA) systems with an antenna array at the base station. We assume that multiple users are active and individually utilize multiple transmit antennas in the MC DS-CDMA system. The users are organized into groups, and each user is assigned a unique Time-domain (T-domain) signature code. Moreover, users in the same group share a unique F-domain signature code. As a result, they can exploit the T-domain and F-domain signature codes to spread their multiple symbols in parallel, and then transmit the FT-domain spread signals from the corresponding multiple antennas over the fading channels to the base station. However, because of the non-ideal channel effect and/or the use of non-orthogonal signature codes, the FT-domain spread MC DS-CDMA system is affected by multiple access interference (MAI) in the same way as CDMA-like systems. To mitigate the effects of MAI and improve the system’s performance, we propose two layered MUDs that exploit the layered soft decision errors. Specifically, in a trade-off between the performance and the computational complexity, only the soft decision errors of one user/one group are used in the proposed layered MUDs. The results of simulations and a complexity analysis demonstrate that the proposed layered MUDs outperform existing approaches and the computational complexity is modest.

In this contribution, we investigate the performance of dual-polarized multi input multi output (MIMO) multi-carrier interleave division multiple access system (MC-IDMA) over correlated frequency selective channels in the presence of channel state estimation error. In the code division multiple access (CDMA), orthogonal spreading sequence is used as user specific where as in IDMA, user specific interleaving pattern is used. In our considered work, Multi-carrier (MC) is realized by orthogonal frequency division multiplexing (OFDM). The channel is estimated at the mobile stations (MSs) by pilot assisted channel estimation (PACE) technique using minimum mean square error (MMSE) algorithm. Further, the dual-polarized MIMO system considered in our work which is realized with the aid of orthogonal polarizations, replaces two spatially separated uni-polarized antennas, thus resulting in polarization multiplexing. In particular, we evaluate the effects of three types of delay spread on MIMO MC-IDMA system in the context of downlink (DL) communications pertaining to the Standard University Interim (SUI) channel model. In a multi-user environment, users experience multi stream interference (MSI) in addition to the multiple access interference (MAI) and these interferences can degrade the system performance. Furthermore, we consider turbo code as channel encoder for MIMO MC-IDMA system to mitigate the effects of MAI and MSI. It is discerned from the simulation results that dual-polarized MIMO MC-IDMA system is capable of supporting many users and performs better in terms of achievable bit error rate (BER) in the context of DL communications with reduced number of antennas.

We propose a multistage joint adaptive array (AA) and spatial interference cancellation (IC) structure for a direct sequence code division multiple access (DS/CDMA) system. The structure utilizes both space and time processing to improve the performance of the base station receiver under the influence of multiple access interference (MAI) and in the absence of power control. The AA comprises a bank of temporal matched filters followed by a beamformer. IC is then used to remove the detected strong user from the received array input. This stage can be repeated for recover and remove more users. Computer simulation shows that, in the absence of power control, the proposed algorithm efficiently detects the weak user information at MAI levels of 30 dB where the conventional space-time structure fails.

The multiuser detection is a signal processing tech- nique which enhances the performance and capacity in DS- CDMA technology. Since its complexity issue was too expensive to handle, there was focus on the suboptimal detectors. In this paper we propose a hybrid method called adaptive differential interference cancellation method (ADIC) which is a hybrid of the differential interference cancellation method and which is then combined in a adaptive way to determine a weight matrix which will map the outcome of the differential interference cancellation method to come out with a better performance. This paper also looks into the computational cost of different methods using the floating point operation (flops) which is a good estimation of the comparative computational cost if not the exact one. I. INTRODUCTION The conventional DS-CDMA match filter detector fails to combat these issues due three prominent drawbacks: unrealis- tic ucorrelated code, near-far problem (even with strict power control (1)), and tough multiple access interference (MAI). Many advanced signal processing approaches have been made to improve the performances of these kinds of detectors. In this paper we will study the different approaches that were made in this field and implement these methods to reach a conclusion about the detection methods. As the name of the topic suggests the focus of our work will be study and propose cost effective better performing multiuser detection method. We will explore Multiuser Detection Techniques in Synchronous CDMA environment. Multi-user detection deals with the development and ap- plication of joint demodulation and interference cancellation cancellation techniques for improved detection of a desired set of digital signals in the presence of multiple access interfer- ence, intersymbol interference and noise. Since veterbi forward dynamic algorithm (2) is used in case of Optimum multiuser detection (3) the complexity grows in with the increase in the number of users. Hence the complexity is immense which increases exponentially with the increase in the number of users. Looking ahead for lower complexity hence lesser cost of implementation, the paper proposes the use of sub-optimal linear detectors (4). The sub-optimal detector the paper next focuses on is Decorrelator type of Detector (5) which is optimal in least squares. This detector out performs the optimal detector in many respects. Then the Minimum Mean Square Error (MMSE) linear detector is proposed which tries to minimize the squared error from the required transformation. It is one of the most popular detectors in use due to its easiness to implement and its lower complexity. But then in the above detection methods the decisions made were based on certain linear transformation criterion and there are missing efforts to check the performance in between the detection process to improvise the transformation method better detection. So then the paper proposes Adaptive multiuser detection. Then the paper focuses on Interference cancellation methods which utilize the feedback to reduce Multiple Access Interference (MAI) in the detected bits. Under the Interference Cancellation umbrella the Successive Interference Cancellation (SIC) (6), (7) method is explored and simulated to observe the results. Then the next method which performs better than SIC and more commonly used in real life, called Parallel Interference Cancellation (PIC) (6), (8) is also implemented. Before going ahead to implement the final proposed detection method, this paper implements Differential Interference Cancellation (DIC) (9) method. And then this paper finally proposes an Inter- ference Cancellation method called the Adaptive Differential Interference Cancellation Multiuser Detection method which is cost effective in comparison to other Interference Cancellation methods as we will require lesser hardware (multipliers as in the SIC or Parallel Interference Cancellation (PIC)) for that matter. More ever since we are using adaptive method the desired BER can be further reduced. This proposed method will be discussed in detail in the following section.

The transmission scheme of multilayered space-time trellis codes (ML-STTC) achieves tradeoff between diversity gain and multiplexing gain by combining space-time trellis code (STTC) and layered space-time (LST) structure in multiple-input and multiple-output (MIMO) systems. In this paper, iterative minimum mean square error (MMSE) group detection and iterative MMSE interference cancellation (IC) group detection schemes are proposed to improve the performance of ML-STTC. Moreover, the iteration can reduce the influence of the detection order on performance, and compensate for the performance loss of the firstly detected layer instead of power allocation in the iterative MMSE IC group detection scheme.

The space-time coding can provide a diversity gain for the multiple-antenna system while spatial multiplexing provides higher data rates. In this paper we are combining space-time block code (STBC) with spatial multiplexing technique over space to take advantages of both schemes. The transmit antennas are divided into groups, with one of the groups transmitting STBC code given by Alamouti and other transmits spatial multiplexing data. At the receiver, a minimum mean square error (MMSE) detector with interference cancelation (IC) is integrated with the space-time decoder to reconstruct the desired signal effectively. MMSE decoder is used for compared to only MMSE method. STBC coded symbols (estimation of interfering terms and then space-time decoder is applied. 4×1 system gives slightly higher diversity gain with rate=3 as s1s2) are giving 8 dB gain as compared to purely orthogonal code (2×1 MMSE-SIC). Rate of 3×1 system is less but it is performing better in terms of reliability as compared to 4×1 system.

A hybrid Interference cancellation (HIC) technique employing both Parallel and Successive Interference Cancellation (PIC/SIC) is proposed for Overloaded Direct Sequence Code Division Multiple Access (OCDMA) system under imperfect synchronization. Two types of receivers are proposed to efficiently mitigate the multiuser interference (MUI). In type I receiver, the MUI within group-1 users are first cancelled using Parallel Interference Cancellation (PIC). Then the multistage Successive Interference Cancellation (m-SIC) is performed on the resultant output of PIC to efficiently cancel the interference of group-2 users on group-1 users. The type II receiver is designed by performing PIC immediately followed by SIC and this combined process is repeated till a major chunk of multi user interference is cancelled. The BER performance of type II receiver is proved to be better than other cancellation techniques such as PIC, SIC and the proposed type I receiver.

The capacity and error probability of space-time block codes (STBC) are considered for a DS-CDMA system with PAM/PSK/QAM modulation in fading channels. The approach is based on an equivalent scalar A WON (additive white Gaussian noise) channel with a channel gain proportional to the Frobenius norm of the matrix channel for the STBC. Using this effective channel, capacity and probability of error expressions are derived for PSK/PAM/QAM modulation with STBC. Rayleigh, Ricean and Nakagami fading channels are considered. These results are extended to obtain the capacity and probability of error for a multiuser direct sequence code division multiple access (DS-CDMA) system employing STBC.

Consider a wireless network, in which multiple users exchange data with each other via a multi-antenna relay. The users transmit to the relay simultaneously. Then the relay regenerates the transmitted signal and precodes it before broadcasting to the users. In particular, we propose a minimum mean square error (MMSE) based precoder, which takes the relay detection errors into account. We indicate that this precoder can improve the bit error performance when uplink channel is in a bad condition. Afterwards, we deploy the ordered MMSE successive interference cancellation (MMSE-SIC) receiver, which exploits temporal diversity gains over the multiple downlink slots. Simulation results show that the ordered MMSE-SIC receiver can achieve sufficient diversity gains over traditional MMSE receiver.

A randomly spread direct sequence code division multiple access (DS-CDMA) system operating in flat fading is considered. Users are assumed to employ equal rates and transmit powers. The receiver ranks the users according to their received powers, and decodes only the subset that can be reliably decoded. Confining the discussion to asymptotic analysis (in terms of number of users and processing gain), we study the matched-filter detector, the linear minimum mean squared error (MMSE) detector, and the optimum detector under the above strongest-users decoding scheme. The total capacities under constraint, derived as functions of the fraction of users that cannot be reliably decoded (equivalent to the outage probability), are analyzed and compared.

The rapid growth of user density and heterogeneous service requirements in 5G and emerging 6G networks has intensified multi-user interference, particularly in power-domain non-orthogonal multiple access (PD-NOMA) systems. Conventional successive interference cancellation (SIC) techniques suffer from error propagation and performance degradation under imperfect channel conditions, limiting their reliability in practical deployments. This paper proposes a deep learning–based interference cancellation framework that combines Convolutional Neural Networks (CNNs) and Bidirectional Long Short-Term Memory (BiLSTM) networks for end-to-end signal detection in downlink NOMA systems. The proposed model exploits CNN layers to extract spatial interference features from complex-valued received signals, while BiLSTM layers capture bidirectional temporal dependencies caused by channel fading and symbol correlation. Unlike traditional SIC-based receivers, DL-NOMA-CancelNet directly reconstructs user signals without explicit channel estimation or manual interference subtraction. Simulation results obtained using MATLAB under Rayleigh fading and AWGN conditions demonstrate that our framework significantly outperforms conventional Least Squares (LS), Minimum Mean Square Error (MMSE), and single-network deep learning detectors. Specifically, this method achieves up to an 8–10× reduction in bit error rate (BER) across a wide signal-to-noise ratio (SNR) range, while maintaining low inference latency and moderate model complexity. These results confirm that our work provides a robust and scalable solution for interference mitigation in future 5G/6G downlink NOMA systems, particularly for ultra-reliable low-latency communications (URLLC) and massive machine-type communications (mMTC).