Effects of carrier frequency offset on the average effective SNR and the average BER for asynchronous MC‐CDMA uplink systems with a guard period

In this paper, a novel blind carrier frequency offset (CFO) estimation is proposed on the basis of the linearly constrained optimization for the uplink transmission of space-time block-coded multicarrier code division multiple access systems. First, the full-dimensional spatial-and-temporal data are formed to avoid violation of the second-order statistics in the conventional receiver design. A set of weight vectors is then provided for acquiring each multipath signal from the desired user while the others get rejected. Finally, the estimated CFO is obtained in accordance with maximizing the well-defined measurement function, which is formulated by collecting all the output powers of the receiver. A space-time averaging technique is also proposed to enhance the robustness to the finite sample effect. Simulation study confirms that with the proposed CFO estimator used in the preceding, the receiver can successfully achieve the same performance of the optimal receiver working in the absence of CFO.Because the erroneous second-order statistics caused by CFO leads to failure in tracking the signalplus-noise subspace, these detectors exhibit severe performance degradation in the CFO estimation. To avoid this phenomenon, a two-level CFO estimator [20] was designed to maximize the figure of merit for an MC-CDMA downlink system incorporated with STBC, in which all users' CFOs are supposed to be aligned. However, in uplink transmission, this is typically impossible to achieve. The misaligned CFOs from different users destroy the convexity of the estimation merit figure and result in failure to accurately determine the CFO. As a remedy, an iterative estimation [21] without the time-consuming peak search was designed for a quasiorthogonal STBC uplink system [22,23]. In spite of the effectiveness in dealing with misaligned CFOs, this approach does not consider the relation between the consecutive time slot signals and works with the received array data associated with each time slot instead of the full-dimensional spatial-and-temporal data. This means that there is no combining gain at the receiver when constructing the measurement function. To the authors' knowledge, no prior works have been devoted to representing the blind CFO estimation using the full-dimensional spatial-and-temporal data for an uplink transmission of STBC MC-CDMA systems.In this paper, a novel blind CFO estimation is developed via the following implementation procedure. To avoid phase inversion, we first form the full-dimensional spatial-and-temporal data without taking the complex conjugate operation on the received data associated with the second time slot. A set of weight vectors is obtained on the basis of the linearly constrained optimization and is exploited to acquire each multipath signal from the desired user while the others get rejected [24]. Relying on point searching, we determine the estimated CFO by maximizing a well-defined measurement function, which is constructed by collecting all the output power of the receiver. On the basis of the S...

Section: Space-time Averaging Techniquementioning

confidence: 99%

“…which is similar to (16) except that the phase compensated data vector is replaced by Q J T augˆH aug . /y.i/.…”

Section: Space-time Averaging Techniquementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Blind carrier frequency offset mitigation in space–time block‐coded multicarrier code division multiple access uplink transmission

Chen

Lin

2014

“…The derivation of (6) is given in Appendix A. The time domain samples in (6) can be represented in an M 1 vector y .n/ D OEy 0 .n/, y 1 .n/, : : : , y M 1 .n/ T as follows:…”

Section: System Modelmentioning

confidence: 99%

A novel multicarrier CDMA transmission scheme for cognitive radios with sidelobe suppression

Rajabzadeh

Khoshbin

2012

SUMMARYNoncontiguous multicarrier code division multiple access (MC‐CDMA) is an effective multiple access method for cognitive radio systems that can deploy the noncontiguous vacant parts of a certain spectrum shared with the users of a primary system. However, the large spectral sidelobes of the Fourier transform based implementation of the MC‐CDMA interfere with the adjacent primary transmission. In this paper, a novel complex signature sequence set is proposed for synchronous downlink MC‐CDMA based cognitive radio networks to suppress the sidelobes. To do so, the minimization of the sidelobes power is developed as an eigen‐optimization problem that is optimally solved by using the eigenvalue decomposition method. The optimal complex signature sequences are chosen as the eigenvectors attained by the eigenvalue decomposition of a symmetric matrix that is dependent of the spectral characteristics of the primary users. Simulation results show that by a slight decrease in the maximum number of active users compared with full load, the sidelobes can be considerably suppressed. The effect of cyclic prefix length, the bandwidth of the primary system and the number of active users on the sidelobe suppression performance is analyzed through numerical simulations. Copyright © 2012 John Wiley & Sons, Ltd.

“…In contrast to ordinary CDMA systems based on application of binary sequences [1,[6][7][8][9][10], sequences with a zero-correlation zone [11], and the systems based on applications of chaotic and Gold sequences [12][13][14][15][16][17][18], this paper presents a theoretical model of a noise-based phase shift keying (NPSK) CDMA system that uses random sequences as the carriers of users' information, which allow a higher level of security in signal transmission. Related to this aspect, the system proposed is substantially different from the systems described in literature.…”

Section: Introductionmentioning

confidence: 99%

Noise‐based spreading in code division multiple access systems for secure communications

Berber

2014

Binary spreading sequences or chaotic nonbinary sequences are used in code division multiple access systems. In contrast to those sequences, this paper presents the theory, simulation, and implementation in digital signal processor technology of a code division multiple access system that uses random spreading sequences. The expressions for probability of bit error are derived for the cases when the additive white Gaussian noise and fading are present in the channel. Because the overall transmitted signal is a sum of random signals, the security of signal transmission in this communication system can be enormously increased and will depend on the goodness of the random sequence generator. The developed theory was confirmed by a prototype of the system that was designed in digital signal processor technology. The prototype used spreading sequences that had statistical characteristics very close to the characteristics of theoretical random sequences. The main problem in the system is how to synchronize the received spread sequence with the locally generated sequence. Hence, a mathematical model of the synchronization block for the system has been separately designed and developed. The synchronization procedure has been demonstrated on the basis of application of a periodically repeated pilot sequence. erates a multiuser signal that has characteristics of a white Gaussian noise signal because the spreading is performed using Gaussian random number generator. The system is supposed to have the best possible masking property of the transmitted information contents and the best possible protection of information contents, which depends mostly on the random characteristics of spreading sequences.In practice, it is hard to design a pure random number generator. In order to confirm our theoretical model, we generated Gaussian sequences in the digital signal processor (DSP) prototype by using the random sequences generated by nonlinear feedback shift registers and then transforming them using the Box-Muller method [12]. For this purpose, a prototype of the complete CDMA communication system is developed including noise and fading generators by using TigerSHARC Analog Devices processor as a hardware platform.The theoretical model of the system is developed for the case when the transmitter generates a multiuser signal, composed of N single-user random noise-like signals. The expression for the probability of error is derived, and the characteristics of the system are analyzed, assuming that additive white Gaussian noise (AWGN) and fading are present in the channel [19,20]. A CDMA system with impulse noise in the channel and diversity reception is presented in [21]. The uplink system is analyzed assuming a wide-band multipath channel represented by the delay tapped lines [22].In this system, the received CDMA signal is a mixture of Gaussian sequences, which increases security in signal transmission because it is hard to distinguish and separate message signals from the channel noise. The noise-like random nature of the trans...