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We investigate the impact of the spreading sequences on the performance of forward link MC-CDMA systems. We show that between the extreme cases of uncorrelated and highly correlated channels, the spreading sequences have a great influence on the mutual interference power. This is true for Walsh-Hadamard and Fourier orthogonal sequences where the ap...
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... The advantage is that the MC-CDMA architecture is not modified and a minimum amount of computation, which can use simple single-user detection (SUD), is required at the receiver. The issue is then to find the best spreading code families in a statistical sense [12][13][14][15][16][17] and the optimal set of codes to assign to the active users for a given family [18][19][20][21][22][23][24][25]. ...
... Both methods improve the system performance, but they do not provide the optimal code sets. In [21], the authors got better results by assuming the FCF of the equalized channel is perfectly known and by using an iterative process for minimizing the MAI power, but they provided no analytical expression and no estimation method. ...
... Only the most effective techniques from previous publications are investigated. They include the methods proposed by Tsai et al. [22], Mourad et al. [21], Jao et al. [23], and our algorithm. For both methods in [21] and [23], we consider ideal and realistic conditions, that is, perfect and imperfect channel estimation. ...
In this paper, we address the problem of multiple access interference (MAI) in a downlink multicarrier code division multiple access system. Because in the realistic case of correlated faded subcarriers, MAI greatly depends on the codes assigned to users, one way of improving performance without increasing receiver complexity consists in an appropriate code selection. We propose a code allocation strategy with the following properties: possible use with any code sequence and equalizer, low complexity, and efficient management of load variations. The allocation problem is formulated as a minimization problem of a cost function related to MAI. First, we provide analytical expressions for the channel frequency correlation function after maximum ratio combining, equal gain combining, and minimum mean square error equalization and for the MAI power. Then, by approximating these expressions, we define a simple cost function and build an iterative algorithm on the basis of the minimum maximum criterion to select the spreading codes. Finally, a complete analysis of the allocation efficiency is provided versus key parameters, in particular, the degree of correlation between the faded subcarriers, the system load, and the equalization techniques.
... scenario implies the usage of orthogonal/ non-orthogonal spreading sequences. Spreading codes have a great influence on the mutual interference power and their selection reflects on the global performances of systems [7]. Two coding/decoding techniques are used: convolutional and turbo technique. ...
... The users have signals with different powers and the vector of amplitudes is given in (7). The orthogonal spreading sequences are illustrated in (1) and the corresponding correlation matrix in (3). ...
In this paper we present a comparative analysis of conventional & MMSE multiuser detectors performances in CDMA systems. At base station end the main reason that affects performances is multiple access interference. It can be caused by non-perfectly orthogonal spreading codes used to differentiate the users or by (im)perfect power conditions. The evaluation criterion for performances is BER versus SNR. Based on the obtained results several important conclusions for the usage of these detectors can be withdrawn.
... By exploiting the information from all users, they achieve better performance, but the main drawback is to strongly increase the receiver computational complexity, so the implementation in a mobile device is still a technical issue. Another approach for improving MC-CDMA performance without increasing complexity at the receiver side consists in selecting the set of spreading codes allocated to the active users, which has a great influence on the MAI level in a system transmitting through a realistic channel with correlated fading on subcarriers [8], [9]. The problem is then to define a code selection strategy with the following properties: ...
... To reduce the number of examined combinations examine, Mourad et al. [9] propose an iterative algorithm, but the power of the interference between users is calculated at each iteration and this calculation requires the knowledge of the channel FCF (Frequency Correlation Function) after equalization. The authors not provide any analytical expression, so an additional stage is needed to estimate the FCF, which introduces more complexity. ...
... To design an algorithm independently of the code family and of the equalization technique, the problem is formulated in a general way. It then consists in trying to find the optimal code sets that minimize a cost function related to the MAI affecting the active users of the MC-CDMA system, as in [9]- [11]. Since this optimization problem has no analytical solution and the exhaustive search suffers from a prohibitive computational cost, it is necessary to develop a faster and simpler solving technique. ...
In this paper, we address the problem of MAI (Multiple Access Interference) degrading the performance of a downlink MC-CDMA system in a frequency selective fading channel. Our objective is to develop a code allocation strategy subject to use any code family and any equalizer. This allocation problem is formulated as a minimization problem of a cost function related to MAI. Firstly, we build the cost function from an approximation of the analytical expressions of the MAI power for MRC, EGC and MMSE equalization. Then we propose an iterative algorithm based on the min-max criterion to select the spreading code of the active users. Simulation results show the efficiency of our allocation strategy versus key parameters, in particular the correlation degree between the faded subcarriers.
... As the spreading codes for MC-CDMA, Walsh codes, Gold and orthogonal Gold codes, polyphase Zadoff-Chu codes as well as complementary Golay codes are mostly employed (e.g. [2], [3], [4]). Then, in order to minimize the BER, the spreading codes jointly reducing PAPR and MAI have to be used. ...
... In this case, all M-ary QAM symbols located on the same circle will be mapped by nonlinear HPA following (3) from the original circle into another circle called an image circle. Because the nonlinear HPA will cause also a phase distortion, the particular symbols at the receiver side will be rotated along the image circle in an angle given by (4). It follows from (3), that the image circle radius (i.e. the magnitudes of the symbols located on the image circles) depends on the particular symbols and IBO parameter expressing of HPA operating point. ...
Performance evaluation and comparison of multi-carrier code division multiple access (MC-CDMA) system model for different spreading sequences at the presence of Saleh and Rapp model of high power amplifier (HPA) is investigated. Nonlinear amplification introduces degradation of bit error performance and destroys the orthogonality among subcarriers. In order to avoid performance degradation without requiring extremely large backoffs in the transmitter amplifier, it becomes convenient to use nonlinear multi-user detection techniques at the receiver side. In order to illustrate this fact, microstatistic multi-user receiver (MSF-MUD) and conventional minimum mean square error receiver (MMSE-MUD) are considered and mutually compared. The results of our analyses based on computer simulations will show very clearly, that the application of nonlinear MSF-MUD in combination with Golay codes can provide significantly better results than the other tested spreading codes and receivers. Besides this fact, a failure of Walsh codes especially at the Saleh model of HPA will be outlined by using constellation diagram.
The MC-CDMA multi-carrier spread spectrum scheme is a potential candidate for the air interfaces of future mobile radio systems. This scheme benefits from the robustness of OFDM multi-carrier transmission to multi-path propagation on one hand and on the other hand from the flexibility of CDMA code division multiple access technique. This thesis investigates the system level performance of MC-CDMA scheme in the downlink. The goal is to validate at the system level the concepts and algorithms of MC-CDMA physical layer.
The thesis addresses two connected themes. The first theme concerns the abstraction of the physical layer at the system level. This theme is also referred to as the link to system level interfacing. The main objective of the physical layer abstraction is to define an adequate measure that allows to predict the transmission quality at the system level. In this thesis, we consider the physical layer abstraction with respect to two time scales. The first time scale is macroscopic and averages the transmission of several data frames experiencing independent channel fading realizations. The second time scale is microscopic and includes only one data frame experiencing a particular channel fading realization. The macroscopic abstraction is appropriate for evaluating the system level performance through static system level simulations, whereas the microscopic abstraction is more appropriate for more accurate evaluation of the system level performance and for proper investigation of link adaptation algorithms through dynamic system level simulations. In our study, we analyze the problems raised by the two types of abstraction and propose novel and appropriate solutions.
The second theme concerns the evaluation of the cellular capacity performances of the MC-CDMA physical layer for different algorithms and configurations. Using the macroscopic abstraction, we develop a semi-analytical methodology for evaluating the cellular capacity. The capacity is evaluated at both the link level through a novel capacity indicator and at the system level through Monte Carlo static system simulations. The methodology developed here is a pragmatic and efficient tool to identify the physical layer configurations that are best suited for a given environment.
Keywords - MC-CDMA, interference modelling, link to system interface, cellular capacity, system level simulations.
The number of spreading sequences required for direct-sequence code-division multiple-access (DS-CDMA) systems depends on the number of simultaneous users in the system. Often a sequence family provides more sequences than are required; in many cases the selection of the employed sequences is a computationally intensive task. This selection is a key consideration, as the properties of the sequences assigned affect the error performance in the system. In this paper, a branch and bound algorithm is presented to perform this selection based on two different cost functions. Numerical results are presented to demonstrate the improved performance of this algorithm over previous work.
In this paper, an approximate analytical expression for the peak-to-average power ratio (PAPR) of a Multi-Carrier Code Division Multiple Access (MC-CDMA) signal is derived. Then, it is demonstrated that the PAPR of a MC-CDMA system employing Walsh-Hadamard (WH) codes can be suitably reduced by resorting to a judicious strategy for the allocation of the spreading signature codes. Eventually, a low-complexity implementation of the proposed strategy is presented and the relevant benefits, in terms of peak signal occurrence and robustness against nonlinear distortions, is numerically assessed over conventional random allocation strategy.
In this paper, we investigate the impact of Walsh-Hadamard spreading code allocation on the performance of a downlink MC-CDMA system in a time varying frequency selective channel. The analysis shows that this impact is important on the multiple access interference and the inter-carrier interference power levels. We propose a code allocation strategy that minimizes the global interference power and significantly improves the performance of the MC-CDMA system
Multi-carrier (MC) code division multiple access (CDMA) is able to take the advantages of OFDM and CDMA and is a potential technique for future wireless communications. For an uplink MC-CDMA system, the symbols of different users are spread in the frequency domain. However, the frequency-selective fading of wireless channels destroys the othogonality of the spreading codes for different users and causes multiple access interference (MAI), especially for a network with full load. To reduce the impact of MAI, we adaptively assign spreading codes according to channel state information and MAI environments. Since it requires high computational complexity to find an optimal set of spreading codes for all active users, we develop several simplified approaches to search the suboptimal spreading code sets. It is demonstrated by the computer simulation that the adaptive spreading code assignment, even though suboptimal, can significantly improve the performance of MC-CDMA systems.
