A novel CDMA multiuser interference cancellation receiver withreference symbol aided estimation of channel parameters

A novel multistage successive interference cancellation scheme is proposed which operates on the reverse code-division multiple-access (CDMA) link using binary phase shift keying (BPSK) modulation, coherent detection and reference symbols to obtain channel estimates. The results of a single cell multiuser investigation demonstrate increased traffic capacity. However, this increase is initially very moderate due to corruption of the channel estimates by interference from symbols not yet demodulated and cancelled. A modification in the transmitted signal structure addressing this problem is proposed. The results of a single cell analysis of the modified cancellation scheme demonstrate that the system's traffic capacity reaches approximately 80% of that of a multistage successive interference cancelling receiver operating with the perfect knowledge of channel parameters. A subsequent multicell investigation shows that for a hexagonal cell geometry with a path loss exponent of four and without any forward error correction (FEC) coding or cell sectorization, the system capacity compares very favorably with that of the IS-95 system, which employs powerful error control coding. Capacities with other path loss exponents and cell geometries are also investigated. The results show a substantial traffic capacity increase over that of a comparable receiver without interference cancellation     

Rate-Distortion Optimized Video Transmission Over DS-CDMA Channels with Auxiliary Vector Filter Single-User Multirate Detection

In this paper, we consider the rate-distortion optimized resource allocation for video transmission over multi-rate wireless direct-sequence code-division-multiple-access (DS-CDMA) channels. We consider the performance of transmitting scalable video over a multipath Rayleigh fading channel via a combination of multi-code multirate CDMA and variable sequence length multirate CDMA channel system. At the receiver, despreading is done using adaptive space-time auxiliary-vector (AV) filters. We propose a new interference cancelling design that uses just a single AV filter for single-user mutirate despreading. Our experimental results show that the proposed interference cancelling design has excellent performance in scalable video transmission over DS-CDMA systems that use a combination of multicode multirate and variable processing gain multirate CDMA. The proposed design takes advantage of the fact that single user's video data is transmitted using two spreading codes, one for the base layer and one for the enhancement layers, and of the fact that these spreading codes can have different processing gains. The proposed interference cancelling design is compared with two conventional single-user multirate CDMA receiver configurations, however now we use an AV filter rather than a simple matched filter. We also propose a resource allocation algorithm for the optimal determination of source coding rate, channel coding rate and processing gain for each scalable layer, in order to minimize the expected distortion at the receiver.     

Performance of a spatio-temporal matched filter receiver forDS/SSMA communications

A spatio-temporal matched filter receiver for direct-sequence spread-spectrum multiple-access (DS/SSMA) communications with aperiodic random quadriphase spreading sequences is derived, and the system performance is analyzed. It is shown with the method of characteristic functions that the cross-correlation coefficients between the desired user's and the interfering users' spreading sequences tend, in distribution, to independently and identically distributed circularly symmetric complex Gaussian random variables as the processing gain goes to infinity. Based on this Gaussian approximation, the structure of the spatio-temporal matched filter receiver is derived and a bit error rate formula is obtained. Using Monte-Carlo simulations, as well as analytical methods, it is shown that the spatio-temporal matched filter receiver achieves a significant performance improvement over the conventional, temporal, and spatial matched filter receivers by effectively suppressing the multiple access interference     

A CDMA interference canceling receiver with an adaptive blind array

Interference cancelling receivers have been suggested as low complexity multiuser receivers for code division multiple access (CDMA) systems. A multi-element interference cancelling receiver is proposed, and it is demonstrated that using spatial information about the users will improve the performance of the receiver. Two blind algorithms are suggested to adaptively combine the outputs of the antenna elements. The performances of these algorithms are compared, and it is shown that without requiring any additional information, the receiver can spatially discriminate between the users and improve the error performance     

Output MAI distributions of linear MMSE multiuser receivers inDS-CDMA systems

Multiple-access interference (MAI) in a code-division multiple-access (CDMA) system plays an important role in performance analysis and characterization of fundamental system limits. We study the behavior of the output MAI of the minimum mean-square error (MMSE) receiver employed in the uplink of a direct-sequence (DS)-CDMA system. We focus on imperfect power-controlled systems with random spreading, and establish that in a synchronous system (1) the output MAI of the MMSE receiver is asymptotically Gaussian, and (2) for almost every realization of the signatures and received powers, the conditional distribution of the output MAI converges weakly to the same Gaussian distribution as in the unconditional case. We also extend our study to asynchronous systems and establish the Gaussian nature of the output interference. These results indicate that in a large system the output interference is approximately Gaussian, and the performance of the MMSE receiver is robust to the randomness of the signatures and received powers. The Gaussianity justifies the use of single-user Gaussian codes for CDMA systems with linear MMSE receivers, and implies that from the viewpoints of detection and channel capacity, signal-to-interference ratio (SIR) is the key parameter that governs the performance of the MMSE receiver in a CDMA system     

A Cyclodespreader Architecture for Detecting Multiple Downlink Aperiodic CDMA Signals

Although a code-division multiple-access (CDMA) system is spectrally efficient and has some immunity against intentional reception, its capacity and performance are generally limited by multiple access interference (MAI) caused by other users. For an aperiodic CDMA system, the spreading sequences span multiple symbol intervals, and the cross-correlations between these sequences, which are a measure of the MAI, are periodic over multiple symbols. In this paper, we present a receiver with a parallel architecture that converts an aperiodic CDMA sequence into a piecewise periodic sequence in each arm. We refer to this receiver as a cyclodespreader because the despreader exploits the cyclostationary property of an aperiodic signal. As a result, the transmitted data can be detected separately in each arm of the receiver using low-complexity conventional algorithms proposed for periodic CDMA systems. The goal of the receiver is to decode several signals received from different cochannel base stations. The performance of the system is evaluated using real aperiodic CDMA signals, and it is compared to that of a conventional matched filter (MF) receiver using the number of correctly decoded messages as the performance measure.     

Adaptive array processing MLSE receivers for TDMA digitalcellular/PCS communications

Array processing is a promising approach for improving quality, coverage, and capacity in digital cellular communication systems. By combining array processing with maximum likelihood sequence estimation (MLSE), intersymbol interference (ISI) introduced by multipath propagation can be mitigated as well. Novel symbol-spaced and fractionally spaced adaptive array processing MLSE receivers are developed for both diversity and phased array antenna configurations. The practical issues of synchronization and channel estimation are addressed. A novel approach to automatic frequency error correction (AFC) is proposed and is shown to be critical when cancelling cochannel interference. Performance is evaluated for the reverse link of the IS-136 TDMA-based digital cellular system. Substantial improvements are obtained over conventional antenna configurations for receiver sensitivity (2.5-4 dB) and over traditional antenna combining when cochannel interference is present (0.5-25 dB)     

Analysis of a partial decorrelator in a multicell DS-CDMA system

For a multicell code-division multiple-access (CDMA) system, we propose a partial decorrelator that decodes a user by suppressing only the in-cell interferers. As a result, each user suffers only from other-cell interference and enhanced receiver noise. By analysis, we show that in random CDMA systems, the partial decorrelator outperforms the conventional receiver, within the operating regime of the conventional receiver. In simulation, we observe that when users have equal received powers at their respective receivers, a multicell system with partial decorrelator receivers yields roughly 1.5 times the capacity of the conventional system.     

Performance analysis of a linear MMSE receiver for bandlimited random-CDMA using quadriphase spreading over multipath channels

This paper analyzes the bit-error-rate (P/sub e/) performance of a linear minimum mean-square error (LMMSE) receiver for bandlimited direct-sequence code-division multiple-access systems which use quadriphase spreading with aperiodic pseudonoise (PN) sequences. The analysis is based on the improved Gaussian approximation (IGA) with focus on chip pulse shaping. It shows that the IGA reduces to the standard Gaussian approximation (SGA) if 1) random quadriphase spreading is employed, 2) the spreading factor takes moderate to large values, and 3) the chip pulse excess bandwidth (BW) is zero. Hence, the SGA, known for its inaccuracy in low regions of P/sub e/, remains an accurate approximation even when the number of active users in the system is small as long as the aforementioned conditions are met. The analysis holds for either matched or different transmit and receive filters. Consequently, closed form conditional P/sub e/ expressions are derived for the coherent selective RAKE and the LMMSE receivers and verified with Monte Carlo simulations. Numerical results are presented to illustrate the performance improvement achieved by the LMMSE receiver which, in contrast to the coherent selective RAKE receiver, not only suppresses interference when the excess BW of chip pulse is nonzero, but also harnesses the energy of all paths of the desired user. Under the examined scenarios tailored toward current narrowband system settings, the LMMSE receiver achieves 60% gain in capacity over the selective RAKE receiver. A third of the gain is due to interference suppression capability of the receiver while the rest is credited to its ability to collect the energy of the desired user diversified to many paths. Future wideband systems will yield an ever larger gain.     

Dynamic signature assignment for direct sequence CDMA systems

Multiple access interference (MAI) is one of the major impediments in the reverse link of wide band CDMA systems, due to the synergy of the users' spreading codes, transmission delays and the channel characteristics. We propose to dynamically assign the users' spreading codes and transmission delays, i.e., to assign the user signature, in order to minimize mutual cross-correlations at the receiver. This dynamic signature assignment (DSA) approach helps to avoid (as opposed to combating) MAI at the base station. In assigning the signatures, we present a low-complexity iterative algorithm which utilizes channel information and is able to minimize signal-to-interference-plus-noise (SINR) ratio at receivers. Computer simulation results are presented which show a potential 2-3 fold capacity increase over conventional systems     

An Efficient Multipath Detection Scheme for CDMA Systems

In conventional CDMA receivers, the detection of multipath components and RAKE finger management is normally based on the received signal energy per path. These schemes essentially overlook the interference component contaminating the total received power. Consequently, they exhibit poor multipath detection capability especially at low signal-to-interference-plus-noise ratio (SINR). In this paper, we present a new scheme for multipath detection that takes into consideration the interference level in each resolved path individually. Specifically, the proposed scheme is devised to estimate and cancel the interference per path before detection. To account for the hardware limitations of the receiver, we propose a low complexity version of the above scheme which can be easily incorporated into the receiver structure. Our results show that the proposed scheme provides significant improvements in the detection probability of multipath components over the energy-based schemes.     

CDMA系统中的2-D Rake接收技术

给出了DS-CDMA(直接序列扩频-码分多址)系统中的接收信号模型,在对时域处理和传统Rake接收机分析的基础上,阐述了智能天线和Rake接收机相结合的2-D Rake接收机的概念和原理,并对两种接收机的输出信噪比进行了比较,结果表明在CDMA系统中采用时空信号综合处理的2-D Rake接收机容量和性能将有大幅提高.     

Adjacent-Cell Interference in Direct-Sequence CDMA Forward Traffic Channels

Each transmission from a base station in a mobile cellular direct-sequence CDMA network is a source of interference for the receivers in the mobile handsets that are operating in adjacent cells. This interference can limit the capacity of the forward traffic channels. The effect of adjacent-cell interference on the performance of the handset receivers is evaluated for a mobile cellular CDMA network that employs quadriphase-shift-key spreading, convolutional coding, and soft-decision decoding. It is demonstrated that acceptable performance may not be possible for a fully loaded cellular network. Of particular interest in this paper are cellular networks in which the base stations are mobile and must be interconnected by wireless communication links. Such networks are important for military applications and certain civilian emergency communications services.     

An adaptive CMMSE receiver for space-time block-coded CDMA systems in frequency-selective fading channels

A technique that can suppress multiple-access interference (MAI) in space-time block-coded (STBC) multiple-input-multiple-output (MIMO) code-division multiple-access (CDMA) systems is developed. The proposed scheme, called a constrained minimum mean square error (CMMSE) receiver, is an extension of the CMMSE receiver for a single-input-single-output system to MIMO systems. It is shown that the complexity of the proposed CMMSE receiver is almost independent of the number of transmitter antennas. The advantage of the proposed receiver over the existing receivers for STBC CDMA systems is demonstrated by comparing the closed-form expressions of the signal-to-interference plus noise ratio and simulated bit error rates. The results indicate that the proposed CMMSE receiver can provide a significant performance improvement over the conventional receivers and that the gain achieved by suppressing the MAI can be larger than that from increasing the transmitter or receiver diversity.     

Pilot interference cancellation technology for CDMA cellular networks

This paper considers the link-level and network-level performance of code division multiple access (CDMA) pilot interference cancellation (pilot IC) technology, a low-complexity advanced receiver technology being considered for use in commercial third generation (3G) CDMA cellular systems. The concept behind this technology is to estimate and cancel at the handset receiver the interference effects associated with CDMA downlink pilot signals broadcast from the base stations of the network. The canceling of interference at the receiver improves the signal-to-interference/noise ratio (SINR), which enables increased cell capacity or throughput. In this paper, we derive SINR expressions for evaluating the probability of error performance of both the RAKE and pilot IC handset receivers, under conventional random spreading code assumptions. The approach can easily and accurately model a wide variety of transmitter, channel, and receiver conditions, including the effects of channel estimation. We also utilize radio network simulations to illustrate and quantify the capacity gains available for 3G CDMA networks through the use of pilot IC handsets. Network simulations are also used to examine the reduced level of soft-handoff found to be possible in pilot IC-based networks and the increased flexibility available in setting pilot power levels. We further consider the impact of using stronger pilot signals for improving the demodulation performance of sensitive higher-order modulation constellations that are needed to support spectrally efficient high-rate data services.     

Combined multiuser detection and diversity reception for wirelessCDMA systems

Code division multiple-access (CDMA) techniques using interference cancellation are being explored for the capacity increase in third-generation universal mobile telecommunications systems. However, multipath fading is a major constraint on the performance of wireless CDMA systems, with multipath propagation worsening the effects of multiple-access interference, and fading on propagation paths leading to the near far problem. Multiuser detection, exploiting the knowledge of other users to cancel multiple-access interference, has the capability of eliminating the near far problem and providing a significant capacity increase in CDMA systems. On the other hand, diversity techniques effectively combat the fading effects of the channel. This paper investigates multiuser receivers that combine explicit antenna diversity, RAKE multipath diversity, and multipath decorrelating detection. Both coherent reception with maximal-ratio combining and differentially coherent reception with equal-gain combining are analyzed. The results demonstrate a significant increase in up-link capacity over the conventional RAKE receiver, at the expense of complexity. In the case of limited receiver complexity, where the number of correlators is less than the number of resolvable paths at the RAKE front-end, antenna diversity is shown to be effective in reducing residual multiple-access interference     

Forward-link performance of satellite CDMA with linear interference suppression and one-step power control

Wideband direct-sequence (DS)-code-division multiple-access (CDMA) is a strong candidate for both terrestrial and satellite components of UMTS. The forward-link capacity of a satellite DS-CDMA system with a conventional matched filter (MF) receiver is limited by interference from adjacent beams and possibly overlapping beams from multiple satellites. In this paper, we study the performance of the linear minimum mean squared error (MMSE) receiver for the satellite forward link. System constraints are long propagation delay, which prevents accurate closed-loop power control, and low on-board power consumption, which implies a low received bit energy to noise density ratio at the mobile receiver. We consider a "one-step" power adjustment algorithm which attempts to compensate for random shadowing and path loss, and compare the associated performance of the MMSE and MF receivers. Dual-satellite diversity is also considered. The effect of code rate on performance is studied through the use of punctured convolutional codes and the evaluation of random coding bounds. Our results indicate that linear MMSE interference suppression can improve the quality of service and increase system capacity significantly.     

Differential and coherent decorrelating multiuser receivers for space-time-coded CDMA systems

Previously, we proposed a differential space-code modulation (DSCM) scheme that integrates the strength of differential space-time coding and spreading to achieve interference suppression and resistance to time-varying channel fading in single-user environments. In this paper, we consider the problem of multiuser receiver design for code-division multiple-access (CDMA) systems that utilize DSCM for transmission. In particular, we propose two differential receivers for such systems. These differential receivers do not require the channel state information (CSI) for detection and, still, are resistant to multiuser interference (MUI) and time-varying channel fading. We also propose a coherent receiver that requires only the CSI of the desired user for detection. The coherent receiver yields improved performance over the differential receivers when reliable channel estimates are available (e.g., in slowly fading channels). The proposed differential/coherent receivers are decorrelative schemes that decouple the detection of different users. Both long and short spreading codes can be employed in these schemes. Numerical examples are presented to demonstrate the effectiveness of the proposed receivers.     

Quadriphase DS-CDMA with pulse shaping and the accuracy of the Gaussian approximation for matched filter receiver performance analysis

Probability of bit-error (P/sub e/) performance of asynchronous direct-sequence code-division multiple-access (DS-CDMA) systems is analyzed. In particular, the effects of pulse shaping, quadriphase (or direct-sequence quadriphase shift keying (DS-QPSK)) spreading, aperiodic spreading sequences and the coherent correlator or, equivalently, the matched filter (MF) receiver are considered. An exact P/sub e/ expression and several approximations: one using the characteristic function (CF) method, a simplified expression for the improved Gaussian approximation (IGA) and the simplified improved Gaussian approximation (SIGA) are derived. Two main results are presented. Under conditions typically satisfied in practice and even with a small number of interferers, the standard Gaussian approximation (SGA) for the multiple-access interference component of the MF statistic and for MF P/sub e/ performance is shown to be accurate. Moreover, the IGA is shown to reduce to the SGA for pulses with zero excess bandwidth. Second, the P/sub e/ performance of quadriphase DS-CDMA is shown to be superior or equivalent to that of biphase DS-CDMA. Numerical examples with Monte Carlo simulation are presented to illustrate P/sub e/ performance for square-root raised-cosine (Sqrt-RC) pulses and spreading factors of moderate to large values.     

Symbol-level Rake MMSE receiver for asynchronous DS/CDMA systems inmultipath fading channels

A new nonlinear adaptive minimum mean squared error (MMSE) receiver performing a successful cancellation of multiple access interference in multipath fading channels is proposed. It is observed that the proposed receiver could achieve a significant performance gain over any currently used adaptive MMSE receivers, at the cost of a relatively small increase in complexity and modification of the conventional DS/CDMA system