Power-adaptation strategies for DS/CDMA communications with successive interference cancellation in Nakagami-fading channels

In this paper, power adaptation for direct-sequence code-division multiple-access communications that employs a successive interference cancellation (SIC) receiver is considered. The transmission power is adapted so that, with the channel variations, the received power levels of each user have appropriate disparities. Under the constraint of average transmission power, we consider two strategies in adjusting the disparity between received signal powers. With the first strategy, the average bit-error rate (BER) for a given user averaged over channel fading statistics is minimized, while with the other, the instantaneous BER is equal for all users. We find that the performance difference between the two strategies becomes negligible as the average transmission power or line-of-sight component increases. We also discuss the impact of appropriate disparity in received power levels on the BER performance of SIC receivers.     

Implementation aspects for successive interference cancellation in DS/CDMA systems

This paper addresses the practical implementation of Successive Interference Cancellation (SIC) for DS/CDMA systems. The practical aspects of such an implementation are discussed and a convenient pipelined architecture is presented. An important aspect is the integration of power control (PC) and interference cancellation which has synergistic effects of optimizing the SIC, reducing intercell interference, and facilitating the implementation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Parallel interference cancellation employing RAKE receiver with selection diversity for multiuser asynchronous DS/CDMA detectors in multipath Rayleigh fading channels

In this paper, we propose and analyse parallel CCI multistage cancellation by combining RAKE and selection diversity. In order to account for channel variations, adaptive implementation of decision thresholds at the RAKE output is suggested. It is shown to provide significant improvement over either hard or soft decision techniques especially in the near‐far situation. Investigation of the system robustness to imperfect channel parameter estimation is also presented. The communication channel is modelled as slowly varying Rayleigh fading discrete multipath channel. Copyright © 2002 John Wiley & Sons, Ltd.     

Successive interference cancellation for multiuser asynchronousDS/CDMA detectors in multipath fading links

There has been an increasing interest in the use of code-division multiple access (CDMA) in cellular mobile and wireless personal communications. The choice of such multiaccess technique is attractive because of its potential capacity increases and other technical factors such as privacy and multipath rejection capabilities. However, it is well known that the performance of CDMA can be significantly degraded due to cochannel interference (CI) and the near-far effects. We consider the performance of direct-sequence (DS)-based CDMA over fading channels that are modeled as slowly varying Rayleigh-fading discrete multipath channels. Specifically, we propose and analyze an adaptive multistage interference cancellation strategy for the demodulation of asynchronous DS spread-spectrum multiple-access signals. Numerical results show that the proposed multistage detector, which alleviates the detrimental effects of the near-far problem, can significantly improve the system performance     

Multistage interference cancellation with diversity reception forasynchronous QPSK DS/CDMA systems over multipath fading channels

This paper introduces a multistage interference cancellation (MIC) technique with diversity reception for quadrature phase shift keying (QPSK) asynchronous direct-sequence code division multiple access (DS/CDMA) systems over frequency-selective multipath Rayleigh fading channels. Unlike the previous MIC, which tries to remove the lump sum of the multiple-access interference (MAI) and self-interference (SI), this introduced MIC attempts to cancel only the MAI and part of the SI due to the intersymbol interference, while treating the remaining SI created by the current symbol as useful information for symbol decision. In this technique, the RAKE combining is used to collect signal replicas over multiple fading paths. Upper and lower bounds on the bit error probability are derived using a Gaussian approximation and the characteristic function method. Furthermore, effects of channel estimation error on the performance are studied. Analytical and simulation results show that the introduced MIC can provide a performance extremely close to that in an ideal single-user environment and outperforms the previous MIC even in the presence of channel estimation error     

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 of various multistage interference cancellation schemesfor asynchronous QPSK/DS/CDMA over multipath Rayleigh fading channels

The performance of multistage interference cancellation (MIC) and three combining techniques, i.e., multipath decorrelating (MIC-DECO), optimum combining (MIC-OPTM), and RAKE combining (MIC-RAKE) for asynchronous quadrature phase-shift keying/direct-sequence code-division multiple access over frequency-selective multipath Rayleigh fading channels is studied. The analytical bit-error probabilities of the MIC-DECO and MIC-OPTM are derived and shown to be in a good agreement with simulation results. Both analytical and simulation results show that the MIC-DECO, MIC-OPTM, and MIC-RAKE in a multiuser environment provide a good performance close to the ideal performance in a single-user system even in the presence of channel estimation error     

Analysis of a simple successive interference cancellation scheme ina DS/CDMA system

Compensating for near/far effects is critical for satisfactory performance of DS/CDMA systems. So far, practical systems have used power control to overcome fading and near/far effects. Another approach, which has a fundamental potential in not only eliminating near/far effects but also in substantially raising the capacity, is multiuser detection and interference cancellation. Various optimal and suboptimal schemes have been investigated. Most of these schemes, however, get too complex even for relatively simple systems and rely on good channel estimates. For interference cancellation, estimation of channel parameters (viz. received amplitude and phase) is important. We analyze a simple successive interference cancellation scheme for coherent BPSK modulation, where the parameter estimation is done using the output of a linear correlator. We then extend the analysis for a noncoherent modulation scheme, namely M-ary orthogonal modulation. For the noncoherent case, the needed information on both the amplitude and phase is obtained from the correlator output. The performance of the IC scheme along with multipath diversity combining is studied     

Analysis of a pipelined successive interference cancellation schemefor a DS/CDMA system

The bit error rate (BER) expression for a proposed pipelined successive interference cancellation (PSIC) scheme in a direct sequence/code division multiple access (DS/CDMA) system is derived analytically. The proposed PSIC scheme is a pipelined modification of a successive interference cancellation (SIC) scheme. The numerical results show that the proposed PSIC scheme outperforms the SIC scheme in viewpoints of the BER and the decoding delay performances     

Successive interference cancellation in multicarrier DS/CDMA

In this paper, we present a successive interference cancellation (SIC) scheme for a multicarrier (MC) direct-sequence code-division multiple-access (CDMA) system, using band-limited spreading waveforms to prevent self-interference. In every subband, the SIC receiver successively detects the interferers' signals and substracts them from the user-of-interest. A comparison is made among SIC, a minimum mean-squared error (MMSE) receiver, and matched filter (MF) detection with maximal-ratio combining (MRC). We also consider suboptimal combining using pilot symbol-assisted modulation (PSAM) to make the system more realistic. Analytic bit-error probabilities for SIC, MMSE, and MF in correlated Rayleigh fading channels are derived. The theoretical results for SIC, MMSE, and MF are shown to agree well with simulations. In particular, SIC and MMSE are shown to achieve better performance than MF with MRC; when the number of users is small, SIC provides better performance than does MMSE. Further, the correlation among different subcarriers is studied, and only large subcarrier correlation coefficients result in an obvious worsening of performance. Finally, we derive results for the performance degradation that an SIC scheme experiences in MC CDMA due to phase and timing tracking errors. It is shown that SIC can still retain a performance advantage over MF-MRC, although the advantage of SIC decreases with increasing tracking errors, especially when subcarrier correlation coefficients are small     

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.     

Symbol-by-symbol based adaptive interference canceller forasynchronous DS/CDMA systems in multipath fading channels

The authors propose a new interference cancellation scheme called the symbol-by-symbol based adaptive interference canceller (SAIC), which adaptively estimates and removes both multiple access interference (MAI) and intersymbol interference (ISI) at the output of the Rake receiver. The SAIC is considerably simpler to implement than existing techniques; computer simulation results demonstrate that it can perform much better than conventional Rake receivers     

Iterative power control for imperfect successive interference cancellation

Successive interference cancellation (SIC) is a technique for increasing the capacity of cellular code-division multiple-access (CDMA) systems. To be successful, SIC systems require a specific distribution of the users' received powers, especially in the inevitable event of imperfect interference cancellation. This apparent complication of standard CDMA power control has been frequently cited as a major drawback of SIC. In this paper, it is shown that surprisingly, these "complications" come with no additional complexity. It is shown that 1-bit UP/DOWN power control-like that used in commercial systems-monotonically converges to the optimal power distribution for SIC with cancellation error. The convergence is proven to within a discrete step-size in both signal-to-noise plus interference ratio and power. Additionally, the algorithm is applicable to multipath and fading channels and can overcome channel estimation error with a standard outer power control loop.     

Partial successive interference cancellation in hybrid DS/FHspread-spectrum multiple-access systems

Partial successive interference cancellation is considered in hybrid DS/FH spread-spectrum multiple-access (SSMA) systems. We first show that the lowest BER attained by employing full interference cancellation in DS/SSMA systems can almost be achieved by employing a partial interference cancellation in hybrid DS/FH systems. The reduction in the number of cancellations translates into an alleviation of correlator speed requirements and a reduction in delays incurred in interference cancellations. The optimal number of frequency slots that minimizes the BER is investigated as a function of the number of interference cancellations. The effect of imperfect power control on the BER is investigated     

Iterative space-time processing for multiuser detection in multipath CDMA channels

Space-time processing and multiuser detection are two promising techniques for combating multipath distortion and multiple-access interference in code division multiple access (CDMA) systems. To overcome the computational burden that rises very quickly with increasing numbers of users and receive antennas in applying such techniques, iterative implementations of several space-time multiuser detection algorithms are considered here. These algorithms include iterative linear space-time multiuser detection, Cholesky iterative decorrelating decision-feedback space-time multiuser detection, multistage interference canceling space-time multiuser detection, and expectation-maximization (EM)-based iterative space-time multiuser detection. A new space-time multiuser receiver structure that allows for efficient implementation of iterative processing is also introduced. Fully exploiting various types of diversity through joint space-time processing and multiuser detection brings substantial gain over single-receiver-antenna or single-user-based methods. It is shown that iterative implementation of linear and nonlinear space-time multiuser detection schemes discussed in this paper realizes this substantial gain and approaches the optimum performance with reasonable complexity. Among the iterative space-time multiuser receivers considered in this paper, the EM-based (SAGE) iterative space-time multiuser receiver introduced here achieves the best performance with excellent convergence properties.     

Evaluation of a DS/CDMA multiuser receiver employing a hybrid formof interference cancellation in Rayleigh-fading channels

This paper assesses the performance of a multiuser detection DS/CDMA receiver based on a hybrid scheme of successive interference cancellation (SIC) and parallel interference cancellation (PIC). Two configurations of the proposed hybrid IC are presented and compared with existing SIC and PIC schemes. The performance criteria used for comparison are complexity, delay, power control requirements, and average bit-error rate (BER) performance obtained by simulation in Rayleigh-fading channels with additive white Gaussian noise (AWGN). The suggested hybrid IC combines good average BER performance, short delay, acceptable complexity, and also operates under slow power control     

Performance and optimization of multistage partial parallel interference cancellation for wideband CDMA systems in multipath fading channels

In this paper, a closed-form bit error rate (BER) of hard-decision multistage partial parallel interference cancellation (PPIC) having perfect and imperfect channel estimation for complex spreading code-division multiple access (CDMA) over multipath Rayleigh channels was derived. The effects of receiving BER, power ratio of pilot to data channels, multiple-access interference (MAI) and multipath interference, additive white Gaussian noise, and other factors affecting channel estimation error are analyzed. The expression determining the optimal interference cancellation weight at path level for PPIC is first derived. Then, the oscillatory behavior of user-level-optimized PIC and the convergence of perfect channel estimation PIC and path-level-optimized PIC are validated. Results indicate that all factors determining the optimal weight certainly affect the BER performance of PPIC and that the lower BER limit of multistage PPIC depends on the adjustable factors, which include power ratio, observation length, and signal-to-noise ratio, and an unchangeable factor, i.e., number of users. In addition, the variance of MAI for complex spreading sequence is obtained, and the analytical expression can also be employed to predict the performance of multicode CDMA.     

Parallel interference cancellation for uplink multirate overlay CDMA channels

To provide new and/or higher rate wireless services with limited spectrum resources, frequency overlay has been naturally proposed to accommodate the new and legacy systems in a common band. We address the multiuser detection problem for overlaid code-division multiple-access (CDMA) scenarios. However, in contrast to the well-studied conventional single-rate CDMA, miscellaneous systems overlay almost always indicates the presence of multirate traffic that introduces an additional degree of freedom in receiver design-i.e., differences in the symbol rates. We concentrate on receiver design for multirate traffic, while assuming a lack of information exchange between the constituent (new and legacy) systems, as is commonplace in practice. We propose a receiver architecture based on linear parallel interference cancellation where the out-of-rate intersystem interference is estimated and subtracted by means of its characteristic subspace, thereby avoiding the need for the exact knowledge of signature waveforms of the interfering system. Simulation results validate our solution and show that the proposed receiver has better performance in various aspects than several other solutions for the same purpose.     

Iterative MMSE receivers for space-time trellis-coded CDMA systems in multipath fading channels

We explore code-division multiple-access (CDMA) systems with multiple transmit and receive antennas combined with space-time trellis codes over a frequency-selective channel. The conventional noniterative multiuser minimum mean square error (NIMU-mmse) detector is generalized to accommodate multiple antennas and multiple paths and then extended to include the turbo principle in an iterative fashion, allowing interference regeneration and cancellation at the receiver. Iterative multiuser mmse (IMU-mmse) receivers employing chip- and symbol-level detectors are derived and their equivalence is demonstrated. Computer simulations show that the proposed iterative mmse equalizers completely remove the interference of the other users in a multiantenna environment; they provide a significant improvement over the NIMU-mmse detector and they effectively achieve the single-user performance, even in a fully loaded system. Two suboptimal iterative mmse detectors, which allow a computational complexity reduction of up to three orders of magnitude compared to the IMU-mmse and still outperform the NIMU-mmse detector, are introduced. The proposed iterative mmse equalizers are analyzed and supported by extensive computer simulations.     

Proposal of an interference cancellation receiver with lowcomplexity for DS/CDMA mobile communication systems

The main targets of any direct-sequence code division multiple-access (DS/CDMA) mobile communication system are to overcome the multipath fading influences as well as the near/far effect and to increase the capacity. Many optimal and suboptimal detection approaches have been proposed and analyzed in the literature. Unfortunately, most of them share the drawback of requiring a complex implementation and do not represent a practical solution. This paper proposes a simple interference cancellation receiver for applications in DS/CDMA uplink communications. This receiver allows users to overcome the near/far effect and to enhance the system capacity. Differently from previous methods, the interference cancellation is performed on a one-shot basis. The performance of the proposed interference cancellation receiver is derived through computer simulations. However, a suitable analytical approach is also presented in the appendix in order to evaluate the bit error rate (BER) performance in the particular case of synchronous users and the transmission channel being affected only by additive white Gaussian noise (AWGN). The good behavior of the proposed approach is demonstrated by means of comparisons in terms of the BER performance and implementation complexity with the classical RAKE receiver and different multiuser receivers previously proposed in the literature on this subject