CDMA Signals Research Articles

MC CDMA Papr Reduction Techniques Using Discrete Transforms And Companding

High Peak to Average Power Ratio (PAPR) of the transmitted signal is a serious problem in multicarrier modulation systems. In this paper a new technique for reduction in PAPR of the Multicarrier Code Division Multiple Access (MC CDMA) signals based on combining the Discrete Transform either Discrete Cosine Transform (DCT) or multi-resolution Discrete Wavelet Transform (DWT) with companding is proposed. It is analyzed and implemented using MATLAB. Simulation results of reduction in PAPR and power Spectral Density (PSD) of the MC CDMA with companding and without companding are compared with the MC CDMA with DCT and companding, DWT and companding systems. The new technique proposed is to make use of multi-resolution DWT in combination with companding in order to achieve a very substantial reduction in PAPR of the MC CDMA signal.

Predistortion of a 100‐W base station high‐power amplifiers

AbstractThis article presents the design of a fifth‐order RF predistorter for use in a base‐station high‐power amplifier (HPA).The predistorter uses two intermodulation (IM) signal generators, with identical configuration, to individually generate the third‐ and fifth‐order inband intermodulation (IM3 and IM5) signals for cancellation of the third‐ and fifth‐order intermodulation distortion products (IMDP3 and IMDP5) at the output of the HPA. The IM signal generators are constructed using two Schottky diodes as non‐linear devices to generate the inband IM3 and IM5 signals. The proposed predistorter is designed and optimized using computer simulation. A hardware prototype is built and tested in a practical 100‐W HPA using a standard IS‐95 CDMA signal at 2.2 GHz. Measured results show that the spectral regrowth of the signal at the HPA output can be significantly suppressed over a wide range of output power levels. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:861–867, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.25855

An Adaptive Multiuser Detection Algorithm for CDMA Systems Using Antenna Diversity

Based on the minimum mean squared error (MMSE) between the data stream and the linear combiner output, a new multiuser detection (MUD) algorithm that combines space–time (ST) processing and antenna array on direct-sequence CDMA signals is proposed. The proposed ST-MUD algorithm is proved to be equivalent to two existing MMSE-based ST-MUD algorithms, and the theoretical BER performances for all the three algorithms are the same. The most attractive feature of the new ST-MUD algorithm is based on the fact that the new method does not require explicit estimation of channel and signaling information. This avoids any channel estimation error, and the method is thus more robust and more accurate than the other two ST-MUD algorithms in practical implementation. Adaptation of the proposed ST-MUD algorithm is implemented by using training sequences. Performance of this new multiuser detector is compared with that of two existing MMSE multiuser detectors and the conventional single-user space–time rake receiver through simulations. The proposed ST-MUD algorithm provides a performance better than existing algorithms and is especially suitable for practical CDMA systems.

Application of Blind Adaptive LMS Algorithm for Detection of CDMA Signals in Frequency Selective Fading Channels

Application of Blind Adaptive LMS Algorithm for Detection of CDMA Signals in Frequency Selective Fading Channels

Analysis of BER performance in presence of nonlinear distortion due to PD-HPA in downlink DS-CDMA signals

A predistorter-high power amplifier (PD-HPA) pair has become a common practice in wireless communication to compensate for nonlinear distortion due to HPA. However, the PD-HPA pair still produces severe signal distortion when the input signal exceeds the PD-HPA's saturation level. The effects of such distortion on bit error rate (BER) degradation in downlink direct sequence-code division multiple access signals (DS-CDMA) are analyzed. We establish which signal characteristics at the HPA input are the factors contributing to BER. Assuming that the baseband CDMA signal is characterized as a complex Gaussian process, we develop analytic expressions for the BER and the contributing factors to BER.

Drain efficiency amplifier of 50% with optimized power range for CDMA base station

AbstractA novel high‐efficiency Doherty amplifier is presented here.A carrier amplifier and a peak amplifier are set to asymmetrical drain voltages to extend the power range where a high efficiency of the amplifier is maintained. In Doherty amplifier, matching circuits of the carrier amplifier and peak amplifier are designed for each drain voltage so that the efficiency and signal linearity of the amplifier at a 7.2 dB back‐off point from its saturated output power become higher than those of a conventional amplifier. A Doherty amplifier containing laterally‐diffused metal‐oxide semiconductor field‐effect transistors achieves an adjacent channel leakage power ratio of −30.89 dBc and a drain efficiency of 50% at an output power of 49.3 dBm. This is the highest drain efficiency of a Doherty amplifier for a CDMA signal to the best of the authors' knowledge. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 1244–1246, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25195

Hysteretic HNN MAI-Cancellation for CDMA Signals in Multipath Fading Channels

Multiple-access interference cancellation using hysteretic Hopfield neural network receiver for direct sequence code-division multiple access (DS-CDMA) in multipath fading channels is investigated. It has been shown that by applying the phenomenon of "hysteresis" to the Hopfield neural network (HNN) detector, performance of this detector may be enhanced in all near-far situations for different number of multipath rays. Introducing the concept of Hysteresis into HNN has made this suboptimum CDMA detector even closer to the optimum multiuser CDMA detector. As shown by simulation results, the bit-error rate performance achieved by the Hysteretic Hopfield Neural Network detector outperforms the classical HNN detector with a good margin and is promising.

Iterative Algorithm for Reducing the Peak-to-Average Power Ratio of Feedback-Controlled Multitone-Hopping CDMA Signals

A novel peak-to-average power ratio (PAR) control algorithm for feedback-controlled multitone-hopping code-division multiple access (FC/MH-CDMA) signals is proposed. In FC/MH-CDMA, since each chip consists of plural tones, the energy consumption due to a large PAR is not negligible at the transmitter. The proposed PAR control algorithm iteratively constructs a time-frequency code that achieves a preset, target PAR under the condition that all signals are asynchronously transmitted. A PAR of 1dB is shown to be achievable, and the bit-error rate performance is shown to be only slightly influenced if the target PAR is set to be larger than 3dB. The influence of quantization is also discussed in terms of its application to limited feedback channels.

A compact nonlinear fiber-based optical autocorrelation peak discriminator

We experimentally demonstrate a nonlinear fiber-based optical autocorrelation peak discriminator. The approach exploits four-wave mixing in a 37-cm highly-nonlinear bismuth-oxide fiber that provides a passive and compact means for rejecting cross-correlation peaks. The autocorrelation peak discriminator plays an important role in improving the detection of optical CDMA signals. Eye diagrams and bit-error rates are measured at different power ratios. Significant receiver sensitivity improvements are obtained and error-floors are removed. The experimental results show that the autocorrelation peak discriminator works well even when the amplitudes of individual cross-correlation peaks are higher than that of the autocorrelation peak.

Performance of Adaptive MMSE Receivers Using Aided Tentative Coefficients in Flat Fading Channels

The conventional matched filter (MF) receiver is considered the optimum filter to recover the CDMA signals. One of the problems of the MF is that its performance is significantly degraded due to the channel impairments and the increase of the multiple access interference (MAI). In this paper, an adaptive minimum mean square error-maximum likelihood (MMSE-ML) receiver is introduced to overcome this problem. This receiver uses two sets of adaptive coefficients to increase the ability of tracking the time variations of the channel. The performance of the receiver is measured in terms of bit error rate (BER) and compared with other receivers over flat fading channel. It is found that the performance of theadaptive MMSE-ML receiver is much better than the performance of the other receivers in the flat fading channel.

Novel probabilistic bounds on power level profile of spectrally-encoded spread-time CDMA signals

In this letter we introduce an effective tool to demonstrate signal power behavior of a typical spectrally encoded spread-time (SE/ST) CDMA system using probabilistic approach. By defining power level profile (PLP), as the probability of stretching instantaneous power from a pre-specified threshold, we can find novel upper bounds on the signal power profile. We discuss various properties owing to PLP and elaborate on the main parameters affecting the PLP value. We present three theorems, propose some simple upper bounds on the PLP, and give an insight on the main contributions of the results. We introduce a fundamental parameter, namely ß, as a key design parameter which relates the most important quantities affecting the PLP behavior. Finally, we propose a corollary demonstrating an approach to guarantee that the PLP value is set at zero. It is shown that using suitable distributions, we can also obtain some distribution gain, thereby enhancing the overall system performance.

Differential decorrelator: a new approach for designing CDMA linear detector

The systems that are designed based on CDMA are suffering from a multiple access interference problem. The decorrelator CDMA detector is a linear detector that can cancel the MAI but with channel noise enhancement. The complexity of the decorrelator is a linear function in the number of the system's users. In this research, a new detector is proposed that can cancel the MAI in the received CDMA signal with a complexity that is independent on the number of the system's users. The new detector does not need to know the users' signature codes. Also, it consists of two matched filter only. No correlation matrix is required. This simple structure reduces the complexity of the proposed CDMA detector if it is compared with the conventional decorrelator detector. The new detector is based on some mathematical operations on the output signals from two different matched filters. The detector idea is based on the symmetry property of the signatures' codes correlation matrix however; it does not need to know this matrix. The algorithm is valid as long as the correlation matrix is symmetry. It can work with synchronous and asynchronous system models.

다이오드 직렬 궤환을 이용한 개별 차수 혼변조 발생기 및 응용

본 논문에서는 개별 차수 혼변조 신호 발생기를 이용한 전치 왜곡 선형화기 구조를 제안하였다 혼변조 신호 발생기는 공통 에미터 증폭기의 에미터 단자에 쇼트키 다이오드를 삽입하여, 3차와 5차 혼변조 신호를 다이오드 바이어스 전압에 따라서 발생시킨다. 이렇게 만들어진 혼변조 신호는 다이오드의 직렬 궤환을 이용한 것으로, 다이오드 바이어스 제어에 따라서 혼변조 신호의 크기와 위상을 변화시킬 수 있다. 이러한 혼변조 발생기를 전치 왜곡 선형화기로 사용했을 때, 반송파 2톤 신호 인가 시 3차와 5차 혼변조 신호는 각각 13.5 dB, 0.9 dB의 개선을 보였고, 광 대역폭을 갖는 CDMA IS-95 2FA 신호를 인가하였을 경우, 중심 주파수로부터 <TEX>${\pm}0.885$</TEX> MHz, <TEX>${\pm}1.23$</TEX> MHz 떨어진 지점에서 ACLR이 각각 2.3 dB, 2.5 dB의 개선됨을 확인하였다. This paper proposes an individual order predistortion linearizer using intermodulation distortion(IMD) generator for cancellation the third and the fifth IMD of power amplifier. The IMD generator for controlling the third and the fifth IMD consist of common Emitter amplifier and Schottky diode. These signals are generated by series feedback of Schottky diode to obtain the inverse AM/AM and AM/PM characteristics of power amplifier. The individual order predistorters are consisted of individual IMD generator, power splitter and combiner. The test results show that the third and the fifth IMD can be improved by a maximum 13.5 dB and 0.9 dB in case of CW 2-tone signals. Also, the Adjacent Channel Leakage Ratio(ACLR) can be improved 2.3 dB, 2.5 dB at <TEX>${\pm}0.885$</TEX> MHz, <TEX>${\pm}1.23$</TEX> MHz offset frequency for CD-MA IS-95 2FA signals.

Low‐complexity unambiguous acquisition methods for BOC‐modulated CDMA signals

AbstractThe new M‐code signals of GPS and the signals proposed for the future Galileo systems are of split‐spectrum type, where the pseudorandom (PRN) code is multiplied with rectangular sub‐carriers in one or several stages. Sine and cosine binary‐offset‐carrier (BOC) modulations are examples of modulations, which split the signal spectrum and create ambiguities in the envelope of the autocorrelation function (ACF) of the modulated signals. Thus, the acquisition of split‐spectrum signals, based on the ambiguous ACF, poses some challenges, which might be overcome at the expense of higher complexity (e.g. by decreasing the step in searching the timing hypotheses). Recently, two techniques that deal with the ambiguities of the ACF have been proposed, and they were referred to as ‘sideband (SB) techniques’ (by Betz, Fishman et al.) or ‘BPSK‐like’ techniques (by Martin, Heiries et al.), since they use SB correlation channels and the obtained ACF looks similar to the ACF of a BPSK‐modulated PRN code. These techniques allow the use of a higher search step compared with the ambiguous ACF situation. However, both these techniques use SB‐selection filters and modified reference PRN codes at the receivers, which affect the implementational complexity. Moreover, the ‘BPSK‐like’ techniques have been so far studied for even BOC‐modulation orders (i.e. integer ratio between the sub‐carrier frequency and the chip rate) and they fail to work for odd BOC‐modulation orders (or equivalently for split‐spectrum signals with significant zero‐frequency content). We propose here three reduced‐complexity methods that remove the ambiguities of the ACF of the split‐spectrum signals and work for both even and odd BOC‐modulation orders. Two of the proposed methods are extensions of the previously mentioned techniques, and the third one is introduced by the authors and called the unsuppressed adjacent lobes (UAL) technique. We argue via theoretical analysis the choice of the parameters of the proposed methods and we compare the alternative methods in terms of complexity and performance. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Statistical Analysis of Multipath Fading Channels Using Generalizations of Shot Noise

This paper provides a connection between the shot-noise analysis of Rice and the statistical analysis of multipath fading wireless channels when the received signals are a low-pass signal and a bandpass signal. Under certain conditions, explicit expressions are obtained for autocorrelation functions, power spectral densities, and moment-generating functions. In addition, a central limit theorem is derived identifying the mean and covariance of the received signals, which is a generalization of Campbell's theorem. The results are easily applicable to transmitted signals which are random and to CDMA signals.

2‐D DOA estimation for local scattered CDMA signals by modified Fastica in colored non‐Gaussian noise

This paper deals with DOA estimation based on the modified fast independent component analysis (FastICA) technique with robust capabilities. The proposed method with linear generalized circular‐type array manifold model can mitigate estimate sensitivity to unknown additive colored non‐Gaussian noise for local scattered CDMA signals. ICA utilizes an approximation of negentropy based on the maximum entropy principle to measure non‐Gaussianity of the data sequences. Several computer simulation results demonstrate the proposed method significantly outperforms MUSIC and signal subspace‐based sequential one‐dimensional searching methods under the scenario of low signal‐to‐noise ratio and colored non‐Gaussian noise.

Simulation study of a successive interference cancellation receiver for signal monitoring on a CDMA downlink

AbstractA successive interference cancellation (SIC) algorithm is developed for the interception of signals transmitted on a CDMA forward link. The system is modelled after the IS‐95 standard but the results are also applicable to other CDMA systems such as the wideband CDMA (W‐CDMA) and IS‐2000 standards. In realistic systems, the different forward link channels are de‐orthogonalised, leading to performance degradation in conventional matched filter (CMF)‐type receivers. Simulation results indicate that the proposed SIC receiver ameliorates the detrimental effects of de‐orthogonalisation, interference and multipath, exhibiting significant gains in traffic capacity over the capacity of a CMF. The numerical simulations also show that the proposed receiver is relatively insensitive to system imperfections. The techniques developed in this paper can be used for interception, surveillance, reconnaissance or monitoring of CDMA signals; to relax the strict requirements on power control; and to improve the capacity of CDMA systems. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Applications of Adaptive LMS Algorithm for Detection of CDMA Signals

Applications of Adaptive LMS Algorithm for Detection of CDMA Signals

A 5 GHz high-temperature superconducting reaction-type transmitting filter based uponsplit open-ring resonators

A new kind of high-temperature superconducting (HTS) transmitting filter based on areaction-type resonator is presented. The purpose of an HTS reaction-type filter(HTS-RTF) is to eliminate the intermodulation distortion noise generated by microwavepower amplifiers such as those employed in mobile base stations. An HTS-RTFenables both higher power handling capability and sharper cutoff characteristicscompared to existing planar-type HTS transmitting filters, since a reaction-typeresonator does not resonate with high power fundamental signals. To achieve steepskirt characteristics and high power handling capability simultaneously, a 5 GHzthree-pole HTS-RTF using a split open-ring resonator is designed. This splitopen-ring resonator offers low maximum current densities and a high-unloadedQ factor with low radiation. The designed prototype filter has Chebyshev characteristics with a centrefrequency of 4.95 GHz and a bandwidth of 1.5 MHz. The HTS-RTF is fabricated using a double-sidedYBa2C3O7−δ thin film deposited on a 0.5 mm thick MgO substrate. The measured filter shows aninsertion loss of less than 0.1 dB and a third intermodulation distortion value of−56.7 dBc for a 40 dBm passband signal. In addition, adjacent channel leakage power ratio(ACLR) measurements using an actual wideband CDMA signal confirm an ACLRimprovement of about 10 dB for a four-carrier signal with power up to 40 dBm.

Adaptive multiuser receiver with joint channel and time delay estimation of CDMA signals based on the square-root unscented filter

The paper discusses an adaptive multiuser receiver for CDMA systems in which the scaled unscented filter (SUF) and the square root unscented filter (SURF) are used for joint estimation and tracking of the code delays and multipath coefficients of the received CDMA signals. The proposed channel estimators are more near-far resistant than the conventional extended Kalman filter (EKF) and present lower complexity than the conventional particle filter (PF) based methods. To present meaningful performance measures, the modified Cramer-Rao lower bound (CRLB) and computational complexity metrics are derived for the proposed and existing channel estimators. Computer simulation results demonstrate the superior performance of the proposed channel estimators. The proposed estimators are also shown to exhibit lower complexity relative to the PF.