Multicarrier DS-CDMA System Research Articles

Performance of Generalized Receiver Employed by Broadband Multicarrier DS-CDMA System Using Space-Time Spreading-Assisted Transmit Diversity

In this paper the multicarrier direct-sequence code-division multiple access (MC DS-CDMA) using space-time spreading assisted transmit diversity is investigated in the context of broadband wireless communi-cations systems constructed based on the generalized approach to signal processing in noise over frequency-se-lective Rayleigh fading channels. We consider the issue of parameter design for the sake of achieving high-ef-ficiency communications in various dispersive environments. In contrast to the conventional MC DS-CDMA wireless communication system employing the time (T)-domain spreading only, in the present paper the broad-band wireless MC DS-CDMA wireless communication schemes employ both the time (T)-domain and frequen-cy (F)-domain spreading, i.e., employ the TF-domain spreading. The bit-error rate (BER) performance of the space-time spreading assisted broadband MC DS-CDMA wireless communications system is investigated for down-link transmissions associated with the single user and multiuser generalized detectors and is compared with that of the single user correlation detector and the multiuser decorrelating detector. Our study demonstra-tes that with appropriately selecting the system parameters, the broadband MC DS-CDMA wireless communi-cation system using the space-time spreading assisted transmit diversity constitutes a promising downlink tran-smission scheme. This scheme is capable to support ubiquitous communications over diverse communication environments without the BER performance degradation

Quasi-synchronous multicarrier DS-CDMA using Z-Complementary sequences

In this paper, a multicarrier DS-CDMA system which employs Z-Complementary Sequences (ZCS) as spreading code is investigated. The new system can not only eliminate multipath interference and Multiple Access Interference (MAI), but also support flexible number of users compared with system using Orthogonal Complementary (OC) codes. Multicarrier DS-CDMA using OC codes can be regarded as a special case of our proposed system. Zero Correlation Zone (ZCZ) of ZCS can be flexibly adjusted to meet the requirements on the user number and maximum multipath and inter-user delay, in this way, more users can be supported without losing interference-free property. The effectiveness of the proposed system is validated through theoretical analysis and simulation results.

Impact analysis of array weight error on the antenna array aided multicarrier DS-CDMA with synchronous uplink transmissions

In this letter, we analyze the effects of array weight error (AWE) on the antenna array (AA) aided uplink synchronous multicarrier DS-CDMA system. The bit error rate performance is evaluated in terms of the important factors such as the number of antennas, the variance of AWE, and the number of users. Furthermore, the scenario of synchronous uplink with AA is compared with asynchronous scheme. Numerical results show that the performance employing synchronous transmission with AA in uplink can be further improved, even at high level of AWE.

PERFORMANCE OF MULTIBAND COMPLEX WAVELET BASED MULTICARRIER DS-CDMA SYSTEM WITH MULTI-ANTENNA RECEIVER OVER NAKAGAMI-m FADING CHANNEL

On the basis of analyzing the principle of multicarrier DS-CDMA, we propose a novel multiband complex wavelet based multicarrier DS-CDMA system in this paper by using the optimized multiband complex wavelet as multicarrier basis function. The system bit error rate (BER) performance is investigated over Nakagami- m fading channel. Without any cyclic preflx (CP), the proposed system can avoid the decrease of spectrum e-ciency and data rate of conventional multicarrier DS-CDMA with CP. Meanwhile, the space diversity combining (SDC) technique based on multi-antenna receiver is employed to improve the system performance further. By the mathematical derivation, the BER analysis of the system with or without SDC is given in detail. Theoretical analysis and simulation results show that the proposed multicarrier system outperforms the conventional multicarrier DS-CDMA system and real wavelet packet based multicarrier DS-CDMA system due to the superior properties of the optimized multiband complex wavelet. Especially, the application of SDC technique can efiectively improve the system ability against spatial fading and interferences, and thus the superior performance is obtained.

IMAGE TRANSMISSION THROUGH MC-CDMA CHANNEL: AN IMAGE QUALITY EVALUATION

The multicarrier code division multiple access (MC-CDMA) system is considered as an advancement in mobile communication system. Seen as the next generation communication technology after the 3G, signals can be easily transmitted and received using the Fast Fourier Transform (FFT) device without increasing the transmitter and receiver complexities. Besides, the MC-CDMA system provides good spectral efficiency. If the number of sub-carriers and the spacing between sub-carriers are chosen properly, it is unlikely that all the sub-carriers will be in deep fade. Thus, it provides frequency diversity. Frequency diversity means that frequencies separated by more than the coherence bandwidth of the channel will be uncorrelated and thus will not experience the same fades. Three types of MC-CDMA systems were proposed — MC-CDMA, multicarrier DS-CDMA and multitone CDMA. In MC-CDMA systems, the transmitter spreads the user data over different sub-carriers using spreading code (Hadamard Walsh code) in frequency domain. The multicarrier DS-CDMA system spreads the S/P converted data streams using spreading code in time domain. This scheme is proposed for uplink communication because this scheme can provide quasi-synchronization. In the MT-CDMA scheme, the S/P converted data stream is spread in time domain so that the spectrum of each sub-carrier prior to spreading operation can satisfy the orthogonality condition with minimum frequency separation. Thus, the spectrum of each sub-carrier no longer satisfies the orthogonality condition. MC-CDMA uses longer spreading code as compared to other schemes and thus accommodates more users. The JPEG2000 still image compression is a new image compression technique which is optimized not only for efficiency, but also for scalability and interoperability in network and mobile environment. This standard can provide superior low bit rate performance and continuous-tone and bi-level compression. Because of the robustness to bit errors, JPEG2000 is widely used in wireless communication channel.

Improved Linear Parallel Interference Cancellers

In this paper, taking the view that a linear parallel interference canceller (LPIC) can be seen as a linear matrix filter, we propose new linear matrix filters that can result in improved bit error performance compared to other LPICs in the literature. The motivation for the proposed filters arises from the possibility of avoiding the generation of certain interference and noise terms in a given stage that would have been present in a conventional LPIC (CLPIC). In the proposed filters, we achieve such avoidance of the generation of interference and noise terms in a given stage by simply making the diagonal elements of a certain matrix in that stage equal to zero. Hence, the proposed filters do not require additional complexity compared to the CLPIC, and they can allow achieving a certain error performance using fewer LPIC stages. We also extend the proposed matrix filter solutions to a multicarrier DS-CDMA system, where we consider two types of receivers. In one receiver (referred to as Type-I receiver), LPIC is performed on each subcarrier first, followed by multicarrier combining (MCC). In the other receiver (called Type-II receiver), MCC is performed first, followed by LPIC. We show that in both Type-I and Type-II receivers, the proposed matrix filters outperform other matrix filters. Also, Type-II receiver performs better than Type-I receiver because of enhanced accuracy of the interference estimates achieved due to frequency diversity offered by MCC.

Performance Investigation of Space-Time Block Coded Multicarrier DS-CDMA in Time-Varying Channels

In this letter, we evaluate the system performance of a space-time block coded (STBC) multicarrier (MC) DS-CDMA system over a time selective fading channel, with imperfect channel knowledge. The average bit error rate impairment due to imperfect channel information is investigated by taking into account the effect of the STBC position. We consider two schemes: STBC after spreading and STBC before spreading in the MC DS-CDMA system. In the scheme with STBC after spreading, STBC is performed at the chip level; in the scheme with STBC before spreading, STBC is performed at the symbol level. We found that these two schemes have various channel estimation errors, and that the system with STBC before spreading is more sensitive to channel estimation than the system with STBC after spreading. Furthermore, derived results prove that a high spreading factor (SF) in the MC DS-CDMA system with STBC before spreading leads to high channel estimation error, whereas for a system with STBC after spreading this statement is not true. Keywords ⎯Space-time block coding, MC DS-CDMA, channel estimation error.

Spectral efficiency of a single-cell multi-carrier DS-CDMA system in Rayleigh fading

The spectral efficiency of a multi-carrier direct-sequence code-division multiple-access (MC/DS-CDMA) system operating in a Rayleigh fading environment is investigated and evaluated in terms of the theoretically achievable channel capacity (in the Shannon sense) per user, estimated in an average sense. This short paper covers operation of the considered system over broadcast communication randomly time-varying channels as applicable to wireless radio networks and single-cell indoor mobile systems and leads to the derivation of a closed-form expression for the achieved spectral efficiency. Furthermore, the relation between the number of the employed sub-carriers and the achieved spectral efficiency is revealed.

Allocation policies of quasi-orthogonal codes on the forward link of a multicarrier DS-CDMA system

Abstract-Alternative code assignment policies for the forward link of a multicarrier DS-CDMA system employing the recently proposed quasi-orthogonal sequences are put forth in this letter. It is demonstrated how different allocation strategies of quasi-orthogonal codes alter the interference level suffered by active units; it is also shown how to shape the rate of interference growth as a function of the number of active users within the reference cell, implementing some specific code allocation patterns. The performance achieved by various schemes is assessed in a frequency selective fading environment, in terms of average bit error probability.

Timing-Free Blind Multiuser Detection for Multicarrier DS/CDMA Systems with Multiple Antennae

The problem of blind multiuser detection for an asynchronous multicarrier DS-CDMA system employing multiple transmit and receive antennae over a Rayleigh fading channel is considered in this paper. The solutions that we develop require prior knowledge of the spreading code of the user to be decoded only, while no further information either on the user to be decoded or on the other active users is required. Several combining rules for the observables at the output of each receive antenna are proposed and assessed, and the implications of the different options are studied in depth in terms of both detection performance and computational complexity. A closed form expression is also derived for the conditional error probability and a lower bound for the near-far resistance is provided. Results confirm that the proposed blind receivers can cope with both multiple access interference suppression and channel estimation at the price of a limited performance loss as compared to the ideal linear receivers which assume perfect channel state information.

Performance of multicarrier DS-CDMA systems with time-limited chip waveforms

This paper studies the effect of time-limited chip waveform shaping on the performance of multicarrier direct-sequence code division multiple access (MC-DS-CDMA) systems. The performance criterion is the average multiple access interference (MAI) at the output of a correlation receiver. Comparison among different chip waveforms is made based on a fixed bit rate and a fixed total bandwidth, where the fractional out-of-bandpower (FOBP) definition is used. It is demonstrated that, compared to the performance of single-carrier DS-CDMA (SC-DS-CDMA) systems, the performance of MC-DS-CDMA systems is less sensitive to the chip waveform shaping. Moreover, the relative performance of MC-DS-CDMA systems using different time-limited chip waveforms depends heavily on the bandwidth definition and is also very different compared to that of SC-DS-CDMA systems.

Multicarrier DS-CDMA with adaptive modulation and power allocation

Adaptive modulation and power allocation is introduced into the multicarrier DS-CDMA system to improve the system performance and bandwidth efficiency. First, the system design appropriate for adaptive modulation and power allocation is given, then the algorithm of adaptive modulation and power allocation is applied. Simulation results demonstrate great performance improvement compared with the fixed modulated one.

Capacity analysis of an uplink synchronized multicarrier DS-CDMA system

An uplink synchronous transmission scheme has been proposed for DS-CDMA systems to increase the uplink capacity through transmission timing control and a proper code usage. Since the scheme suppresses the interference along the first paths of other users, it can achieve a significant capacity increase when combined with a multicarrier (MC) technique. This article mathematically compares the proposed uplink synchronized MC DS-CDMA system with conventional DS-CDMA and MC DS-CDMA systems in terms of the maximum number of users.

Families of ternary sequences with aperiodic zero correlation zones for MC-DS-CDMA

A new method for generating ternary spreading sequences with aperiodic zero correlation zones and large family sizes is presented. The sequences are proposed as spreading sequences to provide high capacity and cancel multipath interference in a multi-carrier DS-CDMA system.

Convolutionally-Coded and Spectrum-Overlapped Multicarrier DS-CDMA Systems

Multicarrier DS-CDMA is an effective approach to combat fading and various kinds of interference. In this paper, we present an overlapped multicarrier DS-CDMA system, wherein each of the rate 1/M convolutionally-encoded symbols is also repetition coded and transmitted using overlapped multicarriers. However, since the frequency spectrums of successive carriers are allowed to overlap, the transmission bandwidth is more efficiently utilized. The effect of the overlapping percentage between successive carriers of a multicarrier DS-CDMA system on the performance is investigated to determine the overlapping percentage showing the best performance. We suggest two methods for sub-band overlapping variation. One is to allow variation of sub-band overlapping percentage when the total number of subcarriers is fixed. The other is to increase the number of sub-bands (the number of repetitions R) with fixed subband bandwidth. Given a total number of subcarriers MR, we show that the BER variation is highly dependent on the roll-off factor β of a raised-cosine chip wave-shaping filter irrespective of convolutional encoding rate 1/M and repetition coding rate 1/R. We also analyze the possibility of reduction in total multi-user interference by considering the variation of both the roll-off factor (0 < β ≤ 1) and the sub-band overlapping factor (0 < λ ≤ 2), and show that the proposed system may outperform the multicarrier DS-CDMA system in [3].

On the performance of multicarrier RAKE systems

In this paper, we integrate the multicarrier signaling technique and the RAKE receiver to design a DS-CDMA system with the capability to increase the data rate, mitigate the effect of correlation among the various subcarriers, and suppress partial-band interference. In the proposed system, a data sequence is serial-to-parallel converted, and multicarrier DS-CDMA is used on each of the parallel data streams. The receiver provides a RAKE for each subcarrier, and the outputs of the RAKEs are combined by a maximal-ratio combiner. We employ a correlated subcarrier and frequency-selective fading channel model to derive the average probability of error of the system and compare the results with those of both a single carrier RAKE system and a multicarrier DS-CDMA system in a frequency selective Rayleigh fading channel with an exponential multipath intensity profile, when multiple access interference and partial-band interference are present.

The Performance Improvement of a Multicarrier DS-CDMA System Using both Time-Diversity and Frequency Offset

In this paper, we show the robustness of a proposed system using both a frequency diversity and a time diversity effect when the channel assumptions are relaxed such that all subbands are not subject to independent fading. We take time-repetition coding with interleaving such that the repetition code symbols are spaced sufficiently far apart to guarantee time diversity. We also incorporate offset multicarriers into a proposed multicarrier DS-CDMA system in a different way compared to that of [10] to reduce the total multiple access interference. When we take a convolutional encoder instead of a time-repetition encoder, and again use both time diversity and frequency offset, then we can obtain the significant performance gain.

Channel coding scheme for orthogonal multicarrierDS-CDMA system

A channel coding scheme for an orthogonal multicarrier DS-CDMA system is suggested. The scheme does not reduce the transmission rate of information nor increase the transmitted signal bandwidth of the uncoded multicarrier system. Simulation results for the suggested scheme in a multipath fading channel show considerable coding gain over the uncoded case.

Performance of a Modified Multicarrier Direct Sequence CDMA System

In this paper, we present an improved multicarrier direct sequence (DS) code division multiple access (CDMA) scheme by modifying the system originally proposed by Kondo and Milstein [13]. In this modified system, different spreading sequences multiplied by a data sequence modulate different carriers. This is to prevent the multiple access capability from reducing when the fading characteristics of different carrier frequencies are highly correlated. We have derived a formula which determines the mean values of the relative received signal strength in a single carrier DS CDMA rake system and in a multicarrier DS-CDMA system. We present results on the comparison of the bit error rate (BER) performance of the two systems including the effect of correlation between fading characteristics of different frequencies under various multipath fading conditions. The results indicate that with 50 users the modified multicarrier DS CDMA system can achieve an uncoded irreducible BER of 1.7 × 10−−3 with an average received signal-to-noise ratio per bit of 10 dB, which is better than 3.0 × 10−−3 achieved by the single carrier DS CDMA rake system, and also show that if multicarrier CDMA system is used with respect to single carrier CDMA system, the SNR gain is up to 4.5 dB for the uncode BER of 10−−3 being achieved.

Performance of a coded multi-carrier DS-CDMA system in multi-path fading channels

Multi-carrier DS-CDMA has been considered as an effective scheme for reducing multiple access interference in quasi-synchronous transmission. The scheme allows the reduction of multiple access interference by transferring the orthogonality property of the signals into the frequency domain where the orthogonality property is robust to relative chip offsets between the spreading codes of the various users. However in multi-path channels, the multi-carrier technique results in frequency non-selective fading in the sub-channels, due to the narrower bandwidth, hence a reduction of the capability of the spread spectrum signal to mitigate the effect of multi-path propagation. In this paper, we consider the use of a Reed-Muller code with soft decision decoding to regain the corresponding loss in performance, and compare the resulting system with a single carrier DS-CDMA system. The effect of system parameters such as the number of sub-channels is investigated through numerical calculation and simulation, from which a number of system design criteria are arrived at.