Performance of multicarrier CDMA with successive interference cancellation in a multipath fading channel

A high capacity, low complexity, and robust system design for a successive interference cancellation (SIC) system is developed and analyzed. Multicarrier code-division multiple access (MC-CDMA) is used to suppress multipath and to overcome the multipath channel estimation problem in single-carrier SIC systems. In addition, an optimal power control algorithm for MC-CDMA with SIC is derived, allowing analytical bit-error rate expressions to be found for an uncoded system. Low-rate forward error-correcting codes are then added to the system to achieve robustness. It is found that the capacity of the coded system approaches the additive white Gaussian noise capacity for SIC, even in a fading multipath channel with channel estimation error. This indicates that MC-CDMA is very attractive for systems employing SIC.     

Performance of multicarrier CDMA with DPSK modulation anddifferential detection in fading multipath channels

Multicarrier (MC) direct sequence (DS) code division multiple access (CDMA) with differential phase-shift keying (DPSK) modulation and differential detection is proposed. Transmitted data bits are differentially encoded after serial-to-parallel conversion to a number of parallel streams. On each branch, encoded bits are direct sequence spread spectrum (SS) modulated and transmitted using different carriers. The system is analyzed with a differential detector in static Rayleigh fading multipath channel, in fast Rayleigh fading multipath channel and for variable overlapping between carrier spectra in static fading channel. Closed-form expressions are derived for the error probability and evaluated for many cases. The performance is compared to that of a system using phase-shift keying (PSK) with conventional matched filter (CMF) coherent receiver. For static fading channel, the error probability performance of the differential detector is close to that of CMF receiver. For fast fading, the performance degrades slightly with increasing fading rate. Finally; successive carriers of the system are allowed to overlap with various overlapping percentages. The condition of a single path can be achieved by increasing both the number of carriers and the separation between successive carriers. Also, for each number of carriers, there exists an optimum overlapping percentage at which the system performance is optimized. The performance of the proposed DPSK with differential detection system is close to that of PSK with CMF receiver, but the former is simpler to implement     

Performance of asynchronous orthogonal multicarrier CDMA system infrequency selective fading channel

An asynchronous multicarrier (MC) direct-sequence (DS) code-division multiple-access (CDMA) scheme for the uplink of the mobile communication system operating in a frequency selective fading channel is analyzed. The bit error rate performance of the system with either equal gain combining or maximum-ratio combining is obtained. Numerical results indicate that the system performs better than that of the conventional DS-CDMA system and another MC-DS-CDMA system     

Extended orthogonal polyphase codes for multicarrier CDMA system

The Complete Complementary (CC) codes have been able to improve the performance of multicarrier CDMA system for both downlink and uplink transmission. However, the number of users supported by CC code design is limited as its family size is relatively small. In this letter, an extension to CC codes using Zadoff-Chu sequence is proposed. The proposed codes are able to support large number of users and have perfect auto- and cross-correlation function just like CC codes.     

Performance of multicarrier DS CDMA systems

In this paper, we apply a multicarrier signaling technique to a direct-sequence CDMA system, where a data sequence multiplied by a spreading sequence modulates multiple carriers, rather than a single carrier. The receiver provides a correlator for each carrier, and the outputs of the correlators are combined with a maximal-ratio combiner. This type of signaling has the desirable properties of exhibiting a narrowband interference suppression effect, along with robustness to fading, without requiring the use of either an explicit RAKE structure or an interference suppression filter. We use bandlimited spreading waveforms to prevent self-interference, and we evaluate system performance over a frequency selective Rayleigh channel in the presence of partial band interference; we also compare system performance with that of a single-carrier RAKE system     

Cyclic shifted orthogonal complementary codes for multicarrier CDMA systems

In this letter, we present a set of cyclic shifted orthogonal complementary (cyclic-OC) codes for multicarrier CDMA systems. The conventional OC codes are not adequate for the system because they are able to support only the limited number of users. By applying the proposed cyclic-OC codes for the inter-symbol interference (ISI)-free multicarrier systems, the number of users can be extended to the size of code length. We validate the effectiveness of the proposed scheme through simulations.     

Performance of multicode DS/CDMA with noncoherent M-ary orthogonal Modulation in multipath fading channels

This paper presents the performance analysis and simulation of a multicode direct-sequence code-division multiple-access system with noncoherent M-ary modulation, in a multipath fading environment. This type of transceiver is specified for the reverse link of the IS-95B and cdma2000 (radio configurations 1 and 2) systems and is intended to serve high-rate applications such as data transfer and video communications. While previous studies considered the analytical error performance of coherent multicode systems, little attention has been devoted in the literature to the noncoherent case. We provide concise and useful expressions for the interference terms as a function of the commonly used aperiodic cross correlation functions. After a statistical characterization of these terms, we make use of the Gaussian approximation (GA) in order to obtain the bit-error rate (BER). However, unlike some other analyses (for coherent detection) relying on the GA, in our derivation, we take into account the fact that all the codes transmitted by a mobile user fade in unison. As demonstrated via computer simulations, this fact is crucial to obtain a reliable estimate of the BER, especially when equal-gain combining (EGC) is used at the receiver. The analysis is also extended to include a simple closed-loop power control algorithm and hard handoff between multiple cells. In particular, we verify-for the multicode case-previous observations that the use of EGC allows improvement only for a certain range of values of the total interference seen at the receiver: When either the number of interfering users is too large, or too many codes are assigned to the high-rate user, the noncoherent combining loss becomes such that the use of many diversity branches can decrease the performance as compared to a system with little or no diversity.     

Group-orthogonal multicarrier CDMA

In the presence of frequency-selective multipath fading channels, code-division multiple access (CDMA) suffers from multiuser interference (MUI) and intersymbol interference (ISI); but when properly designed, it enjoys multipath diversity. Orthogonal frequency-division multiple access (OFDMA) is MUI-free, but it does not enable the available channel diversity without employing error-control coding. On the other hand, coded OFDMA may achieve lower diversity than a CDMA system employing the same error-control codes. In this paper, we merge the advantages of OFDMA and CDMA to minimize MUI effects, and also enable the maximum available diversity for every user. In our group orthogonal multicarrier CDMA (GO-MC-CDMA) scheme, groups of users share a set of subcarriers. By judiciously choosing group subcarriers, we guarantee that every user transmits with maximum diversity. MUI is only present among users in the same group, and is suppressed via multiuser detection, which becomes practically feasible because we assign a small number of users per group. Performance is analyzed, and simulations are carried out to illustrate the merits of GO-MC-CDMA relative to existing alternatives.     

Overview of multicarrier CDMA

The authors present an overview of new multiple access schemes based on a combination of code division and multicarrier techniques, such as multicarrier code-division multiple access (MC-CDMA), multicarrier direct sequence CDMA (multicarrier DS-CDMA), and multitone CDMA (MT-CDMA)     

An iterative maximum SINR receiver for multicarrier CDMA systems over a multipath fading channel with frequency offset

A robust iterative multicarrier code-division multiple-access (MC-CDMA) receiver with adaptive multiple-access interference (MAI) suppression is proposed for a pilot symbols assisted system over a multipath fading channel with frequency offset. The design of the receiver involves a two-stage procedure. First, an adaptive filter based on the generalized sidelobe canceller (GSC) technique is constructed at each finger to perform despreading and suppression of MAI. Second, pilot symbols assisted frequency offset estimation, channel estimation and a RAKE combining give the estimate of signal symbols. In order to enhance the convergence behavior of the GSC adaptive filters, a decisions-aided scheme is proposed, in which the signal waveform is first reconstructed and then subtracted from the input data of the adaptive filters. With signal subtraction, the proposed MC-CDMA receiver can achieve nearly the performance of the ideal maximum signal-to-interference-plus noise ratio receiver assuming perfect channel and frequency offset information. Finally, a low-complexity partially adaptive (PA) realization of the GSC adaptive filters is presented as an alternative to the conventional multiuser detectors. The new PA receiver is shown to be robust to multiuser channel estimation errors and offer nearly the same performance of the fully adaptive receiver.     

Performance of a turbo coded multicarrier DS/CDMA system withnonuniform repetition coding

This paper presents a turbo coded multicarrier direct sequence code division multiple access (DS/CDMA) system, where the outputs of a turbo encoder are repetition coded at multiple rates and transmitted in parallel over a number of subchannels. A performance bound useful in the so-called error floor region is obtained for Rayleigh fading channels when different diversity orders are given to each turbo code symbol. Simulation results are also provided for the low signal-to-noise ratio (SNR) region where the bound is not applicable. It is observed that the error floor can be lowered, with some performance loss in the low SNR region, by applying nonuniform repetition coding to the turbo code symbols     

Performance comparisons of channel estimation techniques in multipath fading CDMA

The problem of pilot-symbol-aided estimation of multipath fading channels in up-link code-division multiple-access (CDMA) systems is considered. The transmitted symbol streams of each user are divided into time-slots; and each time-slot contains a number of pilot-symbols followed by information data symbols. Channel estimation is based on interpolation of the channel values corresponding to the pilot symbols in adjacent time-slots. Existing channel estimation techniques, including the weighted multislot average method and the wavelet expansion method, are studied. Two new channel estimation methods, namely, the robust channel interpolator, and the polynomial channel interpolator, are developed and are compared with these techniques. It is seen that the two new channel estimation methods significantly outperform the existing methods in multipath fading CDMA systems, for a wide range of Doppler values, and under various receiver schemes (with single or multiple receive antennas), such as the RAKE receiver, the interference cancellation receiver, and a receiver which performs iterative channel estimation and interference cancellation.     

Performance analysis of multicarrier code selective CDMA system with frequency repetition

In this paper, we present a new Multi-Carrier Code-selective Code Division Multiple Access System (MC CS-CDMA) with frequency repetition. The code selection process enables the proposed scheme to decrease the number of subcarriers for the same transmission data rate and total bandwidth as conventional MC DS-CDMA. The remaining bandwidth can be used for frequency repetition in order to achieve frequency diversity gain. Our results show that by maintaining a constant data rate, total bandwidth and PAPR, the proposed MC CS-CDMA system achieves a much greater performance improvement than conventional MC DS-CDMA.     

MMSE detection of multicarrier CDMA

Minimum mean-squared error (MMSE) detection of multicarrier code-division multiple-access (CDMA) signals is investigated. The theoretical performance of two different design strategies for MMSE detection are compared. In one case, the MMSE filters are designed separately for each carrier, while in the other case the optimization of the filters is done jointly. Naturally, the joint optimization produces a better receiver, but the difference in performance is shown to be substantial. The multicarrier CDMA performance is then compared to that of a single-carrier CDMA system on a frequency-selective fading channel. A mechanism is then developed to track the channel fading parameters for all the users' signals so that joint optimization of the receiver filters is possible in a time-varying channel. Simulation results show that the performance of this receiver is close to ideal theoretical results for moderate vehicle speeds. The performance begins to degrade when the normalized Doppler rate is higher than about 1%     

Spectral efficiency of multicarrier CDMA

We analyze the spectral efficiency (sum-rate per subcarrier) of randomly spread synchronous multicarrier code-division multiple access (MC-CDMA) subject to frequency-selective fading in the asymptotic regime of number of users and bandwidth going to infinity with a constant ratio. Both uplink and downlink are considered, either conditioned on the subcarrier fading coefficients (for nonergodic channels) or unconditioned thereon (for ergodic channels). The following receivers are analyzed: a) jointly optimum receiver, b) linear minimum mean-square error (MMSE) receiver, c) decorrelator, and d) single-user matched filter.     

Wavelet-based multicarrier transmission over multipath wirelesschannels

The conventional Fourier-based complex exponential carriers of a multicarrier system are replaced with orthonormal wavelets in order to reduce the interference. The wavelets are derived from a multistage tree-structured Haar and Daubechies orthonormal quadrature mirror filter (QMF) bank. Compared with the conventional OFDM, it is found that these wavelets reduce the power of ISI and ICI     

A multicarrier CDMA architecture based on orthogonal complementarycodes for new generations of wideband wireless communications

This article is a review of our ongoing research effort to construct a new multicarrier CDMA architecture based on orthogonal complete complementary codes, characterized by its innovative spreading modulation scheme, uplink and downlink signaling design, and digital receiver implementation for multipath signal detection. There are several advantages of the proposed CDMA architecture compared to conventional CDMA systems pertinent to current 2G and 3G standards. First of all, it can achieve a spreading efficiency (SE) very close to one (the SE is defined as the amount of information bit(s) conveyed by each chip); whereas SEs of conventional CDMA systems equal 1/N, where N denotes the length of spreading codes. Second, it offers MAI-free operation in both upand downlink transmissions in an MAI-AWGN channel, which can significantly reduce the co-channel interference responsible for capacity decline of a CDMA system. Third, the proposed CDMA architecture is able to offer a high bandwidth efficiency due to the use of its unique spreading modulation scheme and orthogonal carriers. Lastly, the proposed CDMA architecture is particularly suited to multirate signal transmission due to the use of an offset stacked spreading modulation scheme, which simplifies the rate-matching algorithm relevant to multimedia services and facilitates asymmetric traffic in up- and downlink transmissions for IP-based applications. Based on the above characteristics and the obtained results, it is concluded that the proposed CDMA architecture has a great potential for applications in future wideband mobile communications beyond 3G, which is expected to offer a very high data rate in hostile mobile channels     

Spectral overlay of multicarrier CDMA on existing CDMA mobilesystems

A spectral overlay of a narrow-band code division multiple access (CDMA) system and a wide-band multicarrier CDMA system is considered as a means for existing CDMA mobile networks to evolve into a future CDMA network. The achievable joint capacity of the overlaid system is provided and compared with that of another network evolution strategy of spectrally overlaying wide-band single-carrier CDMA to the existing CDMA systems     

Performance of successive interference cancellation inconvolutionally coded multicarrier DS/CDMA systems

This article presents a successive interference cancellation (SIC) scheme for a multicarrier (MC) asynchronous DS/CDMA system, wherein the output of a convolutional encoder modulates bandlimited spreading waveforms at different subcarrier frequencies. In every subband, the SIC receiver successively detects the interferers' signals and subtracts them from that of the user-of-interest. The SIC receiver employs maximal-ratio combining (SIC-MRC) for detection of the desired user, and feeds a soft decision Viterbi decoder. A comparison is made among SIC-MRC, matched filter detection with MRC (MF-MRC), and N-tap minimum mean-squared error (MMSE) receivers with optimal tap coefficients, assuming a slowly varying, frequency selective, Rayleigh fading channel, where N is the processing gain. Analysis and simulation results show that the SIC-MRC can obtain performance close to that of N-tap MMSE receivers, and both of them have better ability to suppress multiple-access interference (MAI) than does MF-MRC. Finally, with timing or phase tracking errors, the results show that SIC-MRC can still retain a performance advantage over MF-MRC     

Performance analysis of coded multicarrier spread-spectrum systemsin the presence of multipath fading and nonlinearities

In this paper, we analyze the effects of a nonlinear amplifier on the performance of convolutionally coded multicarrier spread-spectrum systems in the presence of multipath fading. Two performance measures, bit-error rate (BER) and adjacent channel power ratio (ACPR) are considered to assess the effects of in-band interference and out-of-band interference (spectral regrowth) from the nonlinear amplifier, respectively. A memoryless polynomial model is used to represent the bandpass amplifier amplitude nonlinearities (AM/AM) and a slow frequency-nonselective Rayleigh fading channel is assumed for each modulated carrier. Analytical results for the BER for uncoded systems and upper bounds on the BER for coded systems are presented. The optimum amplifier output power backoffs for both systems, which yield the smallest system power consumption, are also determined. The obtained ACPR shows its slow (inverse linear) falloff with the amplifier output backoff