Adaptive Duplicated Filters and Interference Canceller for DS-CDMA Systems: Part I - Algorithm

The multiuser detection is a signal processing tech- nique which enhances the performance and capacity in DS- CDMA technology. Since its complexity issue was too expensive to handle, there was focus on the suboptimal detectors. In this paper we propose a hybrid method called adaptive differential interference cancellation method (ADIC) which is a hybrid of the differential interference cancellation method and which is then combined in a adaptive way to determine a weight matrix which will map the outcome of the differential interference cancellation method to come out with a better performance. This paper also looks into the computational cost of different methods using the floating point operation (flops) which is a good estimation of the comparative computational cost if not the exact one. I. INTRODUCTION The conventional DS-CDMA match filter detector fails to combat these issues due three prominent drawbacks: unrealis- tic ucorrelated code, near-far problem (even with strict power control (1)), and tough multiple access interference (MAI). Many advanced signal processing approaches have been made to improve the performances of these kinds of detectors. In this paper we will study the different approaches that were made in this field and implement these methods to reach a conclusion about the detection methods. As the name of the topic suggests the focus of our work will be study and propose cost effective better performing multiuser detection method. We will explore Multiuser Detection Techniques in Synchronous CDMA environment. Multi-user detection deals with the development and ap- plication of joint demodulation and interference cancellation cancellation techniques for improved detection of a desired set of digital signals in the presence of multiple access interfer- ence, intersymbol interference and noise. Since veterbi forward dynamic algorithm (2) is used in case of Optimum multiuser detection (3) the complexity grows in with the increase in the number of users. Hence the complexity is immense which increases exponentially with the increase in the number of users. Looking ahead for lower complexity hence lesser cost of implementation, the paper proposes the use of sub-optimal linear detectors (4). The sub-optimal detector the paper next focuses on is Decorrelator type of Detector (5) which is optimal in least squares. This detector out performs the optimal detector in many respects. Then the Minimum Mean Square Error (MMSE) linear detector is proposed which tries to minimize the squared error from the required transformation. It is one of the most popular detectors in use due to its easiness to implement and its lower complexity. But then in the above detection methods the decisions made were based on certain linear transformation criterion and there are missing efforts to check the performance in between the detection process to improvise the transformation method better detection. So then the paper proposes Adaptive multiuser detection. Then the paper focuses on Interference cancellation methods which utilize the feedback to reduce Multiple Access Interference (MAI) in the detected bits. Under the Interference Cancellation umbrella the Successive Interference Cancellation (SIC) (6), (7) method is explored and simulated to observe the results. Then the next method which performs better than SIC and more commonly used in real life, called Parallel Interference Cancellation (PIC) (6), (8) is also implemented. Before going ahead to implement the final proposed detection method, this paper implements Differential Interference Cancellation (DIC) (9) method. And then this paper finally proposes an Inter- ference Cancellation method called the Adaptive Differential Interference Cancellation Multiuser Detection method which is cost effective in comparison to other Interference Cancellation methods as we will require lesser hardware (multipliers as in the SIC or Parallel Interference Cancellation (PIC)) for that matter. More ever since we are using adaptive method the desired BER can be further reduced. This proposed method will be discussed in detail in the following section.

UMTS-FDD mode downlink system is in practice supposed to be partially loaded due to interference limitations. In other words, some of the orthogonal codes are not used. This indicates that the system is overdetermined and there exists a noise space which can be exploited to cancel the intracell and intercell interference. However, since the scramblers that differentiate the base stations are aperiodic, so are the noise spaces, which necessitates having receiver structures that vary by symbol period. In this work, interference cancellers are obtained by the interpretations of multistage multiuser detection (MUD) structures, whose time varying parts are limited to scramblers and descramblers. MUD equations are formulated from symbol level MMSE-ZF equalization followed by decorrelation among users via polynomial expansion. In the first order, a number of receiver components are replaced by their MMSE equivalents and supplements in order to obtain better SINR figures for the user of interest.

This paper develops an improved generalized multistage multiuser detector that uses the partial interference cancellation approach developed by Divsalar. The detector that is proposed for uplink code-division multiple access systems uses the generalized multistage detector to produce the estimates of the received data. The regenerated multiuser interference is then weighted proportionally to the reliabilities and subtracted. The performance of the matched filter, the conventional multistage multiuser detector, the partial parallel interference cancellation detector and the proposed new detector are evaluated in Rayleigh fading channels. Finally, bit error rate results are presented versus signal-to-noise ratio. It shows that considerable improvement can be achieved by the proposed scheme in heavily loaded systems.

Spurious-free Dynamic Range (SFDR) of frequency sources such as local oscillators in radio has a profound effect on the minimum signal strength that can be detected. Greater the level of unwanted spurs the more difficult it is to detect an incoming signal in the presence of existing interferers and minimum signal/noise ratio at the demodulator input. Taylor series based Direct Digital Synthesis (DDS) produces an SFDR of around 74dB. The proposed architectures in this paper attempt to improve the SFDR by using a Hartley Image suppressor and using an adaptive sinusoidal interference canceller. The resultant improvement in SFDR is around 45 dB by using the Hartley image suppressor and 40dB using the adaptive interference canceller. The two proposed spur suppression techniques are compared in terms of dynamic power, realized noise floor and hardware complexity.

We develop a novel multistage group-blind multiuser detection technique for reverse link in multi-cell direct sequence (DS) code-division multiple access (CDMA) systems. This approach fully utilizes all known users' information, performs interference cancellation (IC) to suppress the interferers within the cell, and exploits blind adaptive decision-directed (DD) MMSE detector to suppress the inter-cell interference. This process can be made iterative and in the multistage process, the tentative decision bits from DD-MMSE detectors can be fed back to the IC part to make interference regeneration more accurate. A simplified RLS (SRLS) algorithm is derived particularly for DD-MMSE detectors and steady-state SIR performance of the SRLS algorithm is analyzed. It is shown that the proposed detector with SRLS algorithm is sufficient to perform accurate multiuser detection and has significant performance gain over that of the pure blind detectors and the non-blind detectors for the given scenario. Moreover, compared with the conventional RLS algorithm, the SRLS algorithm has lower computational complexity and the convergence speed is faster. We also develop two group-blind channel estimation methods to estimate the effective signature waveforms in multipath fading channel. It is seen that the group-blind channel estimation methods using updated received signal correlation matrix can effectively combat the mismatch problem faced by pure blind detectors.

In this paper we present an autonomous wide band adaptive interference canceller solution that alleviates the problem of performance degradation of an electronic support system (ESS) operating in a high interference RF environment. The interference canceller improves the sensitivity and dynamic range of a complex receiver to allow the detection of signals of interest that are dominated by local high power interfering signals. It is demonstrated through simulation and prototype system testing that the interference canceller can enhance capability of a state of the art ESS in the presence of both saturating and non-saturating interfering signals, independent of modulation scheme. The results show a 15 – 25 dB cancellation improvement and a detection range improvement of up 16 times when compared with a system without cancellation within the radar horizon.

It is well-known that the capacity of a direct sequence CDMA system is multiple access interference limited. Adaptive antenna arrays and interference cancellation can significantly reduce the multiple access interference and can greatly enhance the system performance. This paper investigates the performance improvement that can be achieved by integrating adaptive antenna array processing and interference cancellation, where the receiver steers the beams of the adaptive antenna towards not only the desired user but the interferers as well to facilitate the interference cancellation process. We analyze how the use of an FFT beamformer preceding the 2-D RAKE receiver reduces the complexity of the receiver without severe degradation in the overall system performance.

In this paper, we propose an adaptive multistage parallel interference cancellation (PIC) structure for multipath fading channels. Similarly to the previous partial interference cancellation (IC) approach suggested in Divsalar et al. (1998), only part of the multiaccess interference (MAI) estimate is canceled at each stage. However, the weights employed in this proposed scheme are derived from an LMS algorithm which tries to minimize the mean-square error between the actual signal received and its replica. The complexity of the proposed adaptive multistage PIC scheme is much lower than that of linear multiuser detectors. Performance of the proposed structure in single-path Rician and multipath Rayleigh fading channels is evaluated by simulations. Our results show that the proposed technique outperforms the existing interference cancellation methods in multipath fading channels.

The performance of the SAGE algorithm in channel parameter estimation could be severely degraded by the error accumulated from the interference cancellation (IC) stage as each path in the channel is estimated sequentially. This is a direct result of removing inaccurate estimate of the interfering paths, and errors occurring at the IC stage for the first few paths will be accumulated and influence the process of the paths later on. Adaptive interference cancellation suggested by Xue [1] employs a set of weights to control the the amount of interferences removed from each interferer. We extend this idea to the SAGE algorithm, where the weights are obtained by minimising the mean square error between the received signal and its estimate through a least mean square (LMS) algorithm. Simulation results show the proposed scheme can outperform the standard SAGE algorithm in non-LOS environments.

One of the most important applications of adaptive filter is Interference or noise cancellation. The objective of adaptive interference cancellation is to obtain an estimate of the interfering signal and to subtract it from the corrupted signal and hence obtain a noise free signal. The tracking performances of the LMS and NLMS algorithms are compared when the input of the adaptive filter is no stationary For this purpose, the filter uses an adaptive algorithm to change the value of the filter coefficients, so that it acquires a better approximation of the signal after each iteration. The LMS (Least Mean Square), and its variant the NLMS (Normalized LMS) are two of the adaptive algorithms widely in use. This paper presents a comparative analysis of the LMS and the NLMS in case of interference cancellation from speech signals. For each algorithm, the effects of two parameters-filter length and step size have been analyzed. Finally, the performances of the two algorithms in different cases have been compared.

Exploitation of the spatial dimension in MIMO systems enables co-channel concurrent transmissions between the nodes, thus increases the capacity of the network. A major challenge in the implementation of concurrent transmissions is the mitigation of the interference without causing excessive communication overhead in the network. In this paper, we present a low overhead Concurrent Communications scheme based on Adaptive Interference Cancellation (CC-AIC), which does not require any explicit channel estimations between the nodes in the network. By using the adaptive RLS algorithm for interference cancellation and PN sequence based preamble, we address the channel estimation and communication overhead problems of the previous Space Division-Carrier Sense Multiple Access (SD-CSMA) based protocols. Simulation results show that the proposed scheme effectively cancels the interference caused by other concurrent communications and delivers an aggregate network throughput that is 29%–78% higher than that delivered by the existing SD-CSMA based protocol. In addition, we present experimental results on the interference cancellation performance of the proposed method to prove the practicality of our method.

In space-limited co-located communication systems, wideband transmitters and receivers are deployed for various applications. The strong coupling between transmitters and receivers becomes a major concern. It is crucial to suppress the high power self-interference (SI) to prevent the receiver from working in the nonlinear area, or even saturating. As an effective approach to achieve this, the adaptive interference cancellation (AIC) technology is proposed in the context of a complex high power amplitude modulation radios. In this article, the system-level performance including the interference cancellation ratio requirements and the impact of the AIC on the receiving system is comprehensively studied for the first time. The operating mechanism of a single channel AIC system (AICS) and the configuration of a multichannel AICS are discussed. The performance analysis of the receiving system when implementing the AICS is performed, in terms of the noise figure and the sensitivity. By employing the proposed AICS, very limited effect is observed on the receivers, while the SI is successfully reduced with a high ICR, which fully satisfies the requirements for AM systems. The realization of the multichannel AICS as well as the measured cancellation performance are presented, verifying the proposed concept.

Dual Polarization Frequency Reuse (DPFR) effectively doubles the available frequency spectrum by simultaneously transmitting two independent signals over the same frequency band by using orthogonal polarizations. The result is a communication link whose capacity is essentially doubled. Such system gain does not come without its potential impairments. At higher frequencies, depolarization can occur due to several factors such as rain or poor antenna axial ratio thereby inducing interference and reducing link availability. For satellite-to-ground links, Adaptive Cross-Polarization Interference Cancellation (AXPIC) can be employed to mitigate the effects of depolarization and maximize link availability. Using the framework of a modem to be employed in a satellite downlink, AXPIC design trades and their system impacts are discussed. Laboratory tests producing cross-polarization channels are described along with performance test results for a minimum mean-square error (MMSE) AXPIC. As a benchmark with respect to un-restricted system complexity, modulation constrained information rate analysis for DPFR with cross-polarization interference is derived. Information theoretic analysis for a MMSE AXPIC is also presented with laboratory test results showing performance gains consistent with analysis.

We investigate the performance of adaptive interference cancellation receivers for the wideband code division multiple access (WCDMA) uplink physical data channel. For the WCDMA uplink waveform, we derive a blind adaptive receiver (BAR) based an the constant modulus algorithm (CMA), and an multistage adaptive parallel interference cancellation (APIC) receiver. In order to improve the performance of the APIC receiver, we propose a hybrid APIC (H-APIC) receiver structure which uses the CMA based BAR as the first stage of the APIC. We evaluate and compare the performance of the above receivers in a near-far scenario and show that the proposed H-APIC receiver performs better than the APIC receiver.

This paper proposes a new application of fuzzy inference mechanism to the interference cancellation in a direct-sequence code-division multiple-access (DS/CDMA) communication system over a non-perfect power control channel. Multistage parallel interference cancellation (PIC) is an effective method to cancel the multiple access interference (MAI) for equal-power channels; nevertheless, there is a significant degradation in performance for non-equal power ones. The effectiveness of interference cancellation is related to the reliability of the tentative decision involved in the interference estimates. We established a fuzzy logic system to adapt the weight of each interference cancellation path. According to the received signal's reliability, the weights of interfering users can be estimated by the fuzzy inference mechanism, so that the error propagation would be minimized. The simulation results show that the proposed adaptive fuzzy PIC scheme is robust to the interference and channel noise and is superior to the conventional PIC and constant weight PIC schemes under a severe near-far environment.