This paper proposes a combination of the Single Carrier (SC) block transmission and the Code Division Multiple Access (CDMA) technique. Performance comparisons with the traditional SC DS-CDMA block transmission system are presented. A key feature of the proposed transmission technique is that, unlike SC DS-CDMA transmissions, the original code orthogonality between users is preserved despite propagation through multipath frequency selective channels, thus ideally allowing the decoupling of the signals at the receiver side. Besides this advantage, and mostly due to it, the performance results indicate a clear superiority of the proposed system over the traditional SC DS-CDMA. The simulation results consider frequency domain MMSE (Minimum Mean Squared Error) equalization and assume perfect knowledge of the transmission channels.

In this paper, we propose a new detection technique for multiuser multiple-input multiple-output (MU-MIMO) systems. The proposed scheme combines a lattice reduction (LR) transformation, which makes the channel matrix nearly orthogonal, and then employs a multi-branch (MB) technique with successive interference cancellation (SIC). A single LR transformation is required for the receive filters of all branches in the scheme, which proposes a different ordering for each branch and generates a list of detection candidates. The best vector of estimated symbols is chosen according to the maximum likelihood (ML) selection criterion. Simulation results show that the proposed detection structure has a near-optimal performance while the computational complexity is much lower than that of the ML detector.Index Terms-Multiuser MIMO systems, lattice reduction, multi-branch detection, successive interference cancellation.

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Massive multiple-input multiple-output (MIMO) systems are strong candidates for future fifth-generation (5G) heterogeneous cellular networks. For 5G, a network densification with a high number of different classes of users and data service requirements is expected. Such a large number of connected devices needs to be separated in order to allow the detection of the transmitted signals according to different data requirements. In this paper, a decoupled signal detection (DSD) technique which allows the separation of the uplink signals, for each user class, at the base station (BS) is proposed for massive MIMO systems. A mathematical signal model for massive MIMO systems with centralized and distributed antennas in heterogeneous networks is also developed. The performance of the proposed algorithm is evaluated and compared with existing detection schemes in a realistic scenario with distributed antennas. A sum-rate analysis and a computational cost study for DSD are also presented. Simulation results show an excellent performance of the proposed algorithm when combined with linear and successive interference cancellation detection techniques.

In this paper a detailed study of an efficient transmission technique, recently proposed, which combines the single carrier (SC) block transmission, and the code division multiple access (CDMA), referred to as chip spread CDMA (CS-CDMA), is performed. The main feature of CS-CDMA is that, unlike the tradicional direct sequence (DS-CDMA) transmission system, the code orthogonality between users is maintained even when the transmissions, are made through a multipath frequency selective time-invariant channels. Thus, CS-CDMA allows the decoupling of user signals at the reception side. However, existing comparisons between CS-CDMA and DS-CDMA only consider the up-link scenario and particular conditions, which include orthogonal user codes and time invariant channels. In this scenario the CS-CDMA technique has shown a significant superiority when compared with the traditional DS-CDMA transmission. A detailed analisys of the CS-CDMA techique in more general conditions is presented in this work. We consider different environments such as, time invariant and time varying transmission channels, both orthogonal and non-orthogonal users' codes and up-link and down-link scenarios. We adopt frequency domain equalization with MMSE (Minimum Mean Squared Error) detection. Numerical results with performance comparisons indicate that, despite the effects of non-ideal conditions, CS-CDMA maintains a clear superiority over the DS-CDMA transmission.