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Block diagram of the MC-CDMA/TDD transmission system between the base station and mobile terminal i (K users).
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The paper investigates channel pre-equalization for the uplink of multi-carrier code division multiple access (MC-CDMA) mobile cellular systems. This technique solves the crucial problem of uplink channel estimation by keeping the high spectral efficiency of MC-CDMA with no need of pilot symbols. We study an optimisation criterion based on the maxi...
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... That was possible thanks to the technique of spectral spreading, in condition that the emitted signals by different users have some proprieties allowing them to separate. In opposition of the other techniques of multi-access such frequency division multiple access (FDMA) and time division multiple access (TDMA), where the capacity of the number of users is limited by the frequency and time resources, respectively, the number of users in CDMA is fixed by the proprieties of used spreading codes [18]. That is why the CDMA is an alternative to the others multiplexing techniques to increase the re-use frequency factor and eventually the spectral efficiency of communication systems. ...
A scheme of chaotic spreading sequence for multicarrier code division multiple access system (MC-CDMA) is proposed to estimate the transmission channel. This system spreads spectrum and identifies the channel, simultaneously. The proposed scheme uses a chaotic sequence generated by a logistic map as a training signal and estimate channel parameters according to dynamics of the chaotic sequence. Encoding data by chaotic sequences is first built and then the orthogonal codes are used to spread the encrypted data for multiusers scheme. At the reception, first the channel parameters are identified using a training chaotic sequence in order to equalize the received data, and then the encrypted information is decoded for the desired user. The studies reveal that the proposed system (chaos plus orthogonal codes) significantly outperforms the Walsh- Hadamard code spreading in MC-CDMA system. The performance of the systemis considered in the multiuser case by means of simulation. The simulation result shows that the proposed chaotic code spreading approach for channel identification achieves significant improvement in the channel identification, comparing to using others training sequence or the least square method.
... The second technology is called as the combined equalization of selection diversity (SD) that is the optimal single-user combined equalization technique to minimize Bit Error Rate. The third technique is in fact a type of technique that represents a specific compromise between a combined equalization of MRT-MRC and SD [17][18][19][20]. ...
... q K ], is K, which can be N I ). In order to avoid this problem, in this section, we consider pre-equalization in TDD mode [15]- [17], which can eliminate the need of the channel estimation at the AP by exploiting the channel reciprocity. ...
... Note that in (5), H H k is pre-multiplied to q k s k;t for preequalization at each active device. Thus, at the AP, the channel estimation is not required, which is a salient feature of TDDbased multiple access schemes [15]- [17]. Substituting (5) into (1), the received signal vector at the AP is written as ...
... which becomes (14) after some manipulations. Substituting (31) into (12), we have P j = L−1 l=0 ω j,lâ 2 j,l (λ), which becomes (15). This completes the proof. ...
In this paper, we study pre-equalization for sparse index multiple access (SIMA), which was proposed to enable one-shot identification and detection in random access, over frequency-selective fading channels in time division duplexing (TDD) mode. Each device is to decide a transmit vector to adapt to its channel for pre-equalization based on the channel reciprocity. Due to pre-equalization, the receiver does not need to estimate the channel state information (CSI) of active devices. We further modify pre-equalization with multiple precoding matrices for a better performance with a lower transmission power. For low signaling overhead in random access, we also consider one-shot identification and detection to perform active device identification and their signal detection simultaneously with a low-complexity algorithm by exploiting the sparsity of device activity as well as unique identification sequences.
... To reduce this factor, the transmission energy have to be carefully selected in order to manage efficiently the mobile resources and to avoid increasing the interference, or alternatively pre-equalization strategies with energy constraints can be applied at the transmitter to achieve the desired QoS [3], [5], [6], [28]. Usually the QoS of the received information is evaluated through the signal to interference-noise ratio (SINR) of the estimated symbols, which can be linked to the BER or FSR [9], [21]. In fact, the complexity at the receiver is another key issue, specially at the downlink, since the mobile units (MUs) have limited computational resources. ...
... Moreover, for j-th user, the optimal minimum-norm solution p o j [k] is given by (21), where the coefficients (β j 1 , . . . , β j U ) can be obtained by ...
... As a result, from (21), the optimal solution can be expressed as ...
In this study, the problem of optimal synthesis of pre-equalisation factors for a multi-carrier code division multipleaccess wireless network is studied under different quality of service (QoS) restrictions and performance objectives. The downlink is particularly addressed, where the QoS is evaluated by the signal-to-interference-noise ratio (SINR) after signal detection by matched filtering. First, the minimum energy formulation under inequality QoS constraints is analysed, and it was concluded that the optimal performance is always achieved at the lower bounds. Next, the minimisation of the peak magnitude of the pre-equalisation factors was addressed by an optimal synthesis scheme through the infinity norm, where a linear programming strategy is proposed by using an upper bound. Finally, by combining the minimum energy and minimum peak strategies, the minimisation of an upper bound on the peak-to-average-power ratio (PAPR) is suggested, which can be solved through quadratic programming by incorporating the QoS constraints. The new synthesis schemes take into account two important transmitter-based properties: peak magnitude and PAPR. These ideas were extensively validated in simulation, where the synthesis schemes were compared by Monte Carlo tests at different load and noise levels, and objective SINRs.
... Page 4 of 14 http://jwcn.eurasipjournals.com/content/2013/1/138 is preferred over SUD and MUD techniques in terms of system complexity232425. Equalization may be linear or nonlinear [26]. ...
This research addresses the issue of average transmit optical power reduction in multi-carrier code division multiple access (MC-CDMA)-based indoor optical wireless communications employing intensity modulation with direct detection. The problem is treated in a novel way by investigating pre- and post-equalization-based subcarrier selection for transmit power reduction in downlink transmissions. Analytical expressions are derived for upper bounds of the required fixed DC bias for both cases. The fixed DC bias is used to reduce the system complexity on one hand and to devise optimal subcarrier selection criteria on the other. Simulation results based on the proposed subcarrier selection reveal significant power reduction subject to the 10 −4 bit error rate (BER) requirement for 10-Mbps 64-subcarrier MC-CDMA-based indoor optical wireless communication systems. In addition, the BER performance obtained from pre-equalization is shown to be no higher than that obtained from post-equalization for the same transmit power.
... is important to mention that previous pre-equalization schemes [4], [5], [6] are essentially open-loop strategies that do not take into account the roundtrip delay in their formulation, so a comparison with our proposed closed-loop structure is not feasible. ...
This paper addresses the dynamic pre-equalization problem in the downlink and uplink of MC-CDMA systems by following a distributed feedback perspective, and including explicit quality-of-service (QoS) restrictions. The proposals for the downlink and uplink are synthesized taking into account implementation considerations. The QoS requirements for each active user are translated into a prescribed signal to interference-noise ratio (SINR) after detection at the receiver. In the downlink, an independent pre-equalization factor per subcarrier is suggested which induces diversity in the problem formulation, where a simple matched-filter at the mobile units is assumed for signal detection at the mobile units (MU). In this formulation, the pre-equalization factors that minimize the transmission power and achieve the objective SINR can be accomplished. Meanwhile, in the uplink, a uniform pre-equalization scaling for all subcarriers is proposed, where now multiuser detectors are considered at the base station (BS) in order to achieve the objective SINR's. Moreover, a general closed-loop control structure is addressed which could simultaneously consider our proposals in the downlink and uplink. This framework allows to use distributed control algorithm previously proposed for power control in CDMA systems. Also, stability conditions are analytically deduced to guarantee convergence of the iterative algorithms. An extensive simulation evaluation is presented which shows that the QoS requirements are always satisfied for different load conditions in the MC-CDMA system.
... That was possible thanks to the technique of spectral spreading, in condition that the emitted signals by different users have some proprieties allowing them to separate. In opposition of the other techniques of multi-access such frequency division multiple access (FDMA) and time division multiple access (TDMA), where the capacity of the number of users is limited by the frequency and time resources, respectively, the number of users in CDMA is fixed by the proprieties of used spreading codes [18]. That is why the CDMA is an alternative to the others multiplexing techniques to increase the re-use frequency factor and eventually the spectral efficiency of communication systems. ...
A scheme of chaotic spreading sequence for multi carrier code division multiple access system (MC-CDMA) is proposed to estimate the transmission channel. This system spreads spectrum and identifies the channel, simultaneously. The proposed scheme uses a chaotic sequence generated by a logistic map as a training signal and estimate channel parameters according to dynamics of the chaotic sequence. Encoding data by chaotic sequences is first built and then the orthogonal codes are used to spread the encrypted data for multiuser scheme. At the reception, first the channel parameters are identified using a training chaotic sequence in order to equalize the received data, and then the encrypted information is decoded for the desired user. The studies reveal that the proposed system (chaos plus orthogonal codes) significantly outperforms the Walsh-Hadamard code spreading in MC-CDMA system. The performance of the system is considered in the multiuser case by means of simulation. The simulation result shows that the proposed chaotic code spreading approach for channel identification achieves significant improvement in the channel identification, comparing to using others training sequence or the least square method.
... That was possible thanks to the technique of spectral spreading, in condition that the emitted signals by different users have some proprieties allowing them to separate. In opposition of the other techniques of multi-access such frequency division multiple access (FDMA) and time division multiple access (TDMA), where the capacity of the number of users is limited by the frequency and time resources, respectively, the number of users in CDMA is fixed by the proprieties of used spreading codes [18]. That is why the CDMA is an alternative to the others multiplexing techniques to increase the re-use frequency factor and eventually the spectral efficiency of communication systems. ...
A scheme of chaotic spreading sequence for multi-carrier code division multiple access system (MC-CDMA) is proposed to estimate the transmission channel. This system spreads spectrum and identifies the channel, simultaneously. The proposed scheme uses a chaotic sequence generated by a logistic map as a training signal and estimate channel pa-rameters according to dynamics of the chaotic sequence. En-coding data by chaotic sequences is first built and then the orthogonal codes are used to spread the encrypted data for multiusers scheme. At the reception, first the channel pa-rameters are identified using a training chaotic sequence in order to equalize the received data, and then the encrypted information is decoded for the desired user. The studies re-veal that the proposed system (chaos plus orthogonal codes) significantly outperforms the Walsh-Hadamard code spread-ing in MC-CDMA system. The performance of the system is considered in the multiuser case by means of simulation. The simulation result shows that the proposed chaotic code spreading approach for channel identification achieves signifi-cant improvement in the channel identification, comparing to using others training sequence or the least square method.
... The use of FDE at a receiver (called FDE reception) is an effective technique for improving the SC signal transmission performance in a frequency-selective channel [11]. Another FDE technique is frequency-domain pre-equalization (pre-FDE) transmission [74], [75]. References [76] and [77] introduce pre-FDE into the SC signal transmission. ...
Recently, frequency-domain equalization (FDE) has been attracting much
attention as a way to improve single-carrier (SC) signal transmission in
a frequency-selective wireless channel. Since the SC signal spectrum is
spread over the entire signal bandwidth, FDE can take advantage of
channel frequency-selectivity and achieve the frequency diversity gain.
SC with FDE is a promising wireless signal transmission technique. In
this article, we review the pioneering research done on SC with FDE. The
principles of simple one-tap FDE, channel estimation, and residual
inter-symbol interference (ISI) cancellation are presented.
Multi-input/multi-output (MIMO) is an important technique to improve the
transmission performance. Some of the studies on MIMO/SC with FDE are
introduced.
... In conventional MC-CDMA systems, MAI mitigation is accomplished at the receiver using well-known single-user or multiuser detection schemes [2]. Alternatively, prefiltering techniques based on the zero-forcing (ZF) or minimum-mean-square-error (MMSE) criterion can be employed to mitigate MAI and channel distortions in uplink transmissions [3]- [5]. The idea behind prefiltering is to vary the complex gain assigned to each subcarrier so that interference is reduced and the signal at the receiver appears undistorted. ...
... Our first objective is the design of the parameter that determines the prefiltering coefficients in (3). As a design criterion, we choose the minimization of the average bit-error rate (BER) of the active users. ...
Combined equalization has recently been proposed to enhance the error rate performance of conventional multicarrier code-division multiple-access (MC-CDMA) systems. This technique applies pre-equalization at the transmitter in conjunction with post-equalization at the receiver, thereby splitting the overall equalization process into two separate parts. In this way, efficient power allocation over the available subcarriers is possible at the transmitter, while leaving the interference cancellation task at the receiver. In this paper, we consider the uplink of an MC-CDMA system employing combined equalization. As the users transmit from different locations, the uplink signals arrive at the base station after passing through different multipath channels and the goal is to estimate the pre-equalized channel frequency response of each user. This is pursued following two different approaches. The first operates in the frequency-domain and treats the channel gains over adjacent subcarriers as independent unknown parameters. The second operates in the time-domain and achieves better performance by reducing the number of unknown parameters. Both schemes are based on maximum-likelihood reasoning and require knowledge of the transmitted symbols. Numerical examples are given to highlight the effectiveness of the proposed methods.
