Comments on "A noncoherent tracking loop with diversity and multipath interference cancellation for direct-sequence spread-spectrum systems"

For original paper see IEEE Trans. Commun., vol.46, p.1516-24 (1998 November). In the original paper, the authors presented a noncoherent tracking loop with diversity and multipath interference cancellation to improve the loop performance on frequency-selective fading channels. However, the effects of imperfections in multipath interference cancellation have been dropped out in the statistical analyses, and only evaluated by using computer simulations. The present authors show that such effects can change the error characteristic, at least in the mean sense, and thus make the analyses of jitter performance improper.     

Cyclostationarity-based filtering for narrowband interferencesuppression in direct-sequence spread-spectrum systems

This paper addresses the problem of narrowband interference suppression in direct-sequence spread-spectrum (DS/SS) techniques, which have been adopted to implement code division multiple access (CDMA) systems for wireless mobile communications. The theory of cyclic Wiener filtering, based on the cyclostationarity assumption for the signals involved in the reception problem, is applied to design single-channel adaptive frequency-shift filters which exploit both temporal and spectral correlation properties, i.e., the correlation between time- and frequency-shifted versions of the received signal. The numerical results show that receiving structures based on the proposed cyclostationarity-based interference suppression schemes largely outperform receivers that utilize conventional linear time-invariant suppressors, when they operate in highly contaminated interference environments     

Optimal signature sets for oversaturated quasi-scalable direct-sequence spread-spectrum systems

Signature sets that maximize the sum capacity of an oversaturated code-division multiple-access (CDMA) system are derived, under the constraint that the signature sets have to be quasi-scalable. It turns out that the optimal signature set is composed of orthogonal signature sets, with at most one incomplete orthogonal set. As compared to the Gaussian multiple-access channel (GMAC), the loss in spectral efficiency for this multiple-orthogonal CDMA (OCDMA) system remains very low.     

A convex projections method for improved narrow-band interferencerejection in direct-sequence spread-spectrum systems

A method is presented for enhancing the narrow-band interference rejection capability of direct-sequence spread-spectrum systems employing an adaptive notch filter. The method, based on projection onto convex sets, restores that part of the spread-spectrum signal distorted by the filter. Simulation results are presented which show the output bit-error rate (BER) improvement gained by using the signal restoration scheme     

Time tracking error in direct-sequence spread-spectrum networks dueto coherence among signals

Data are presented which reveal a time synchronization error in CDMA (code division multiple access) spread-spectrum networks where mutual signal phase coherence is high among communicators. The error is not white noise but is, in fact, periodic with an amplitude, phase, and mean offset which appears to be related to a mixture of the carrier frequency, chip rate, and code frame rate. This periodicity is more evident as carrier and code phase coherence increases among participants in a network and high tracking accuracy is the goal. This has implications for future high-rate data communications and navigation networks in which errors may be attributed to local oscillator instabilities more than a transmission link. The data using a commercial modem suggest that improvements in local oscillator stabilities will improve coherence among network participants but will not necessarily reduce time synchronization errors     

MLSE receiver for direct-sequence spread-spectrum systems on a multipath fading channel

To accommodate high-speed data transmissions, it may be necessary to substantially reduce the processing gain of a direct-sequence spread-spectrum (DSSS) system. As a result, intersymbol interference effects may become more severe. In this paper, we present a new structure for maximum-likelihood sequence estimation equalization of DSSS signals on a multipath fading channel that performs the function of despreading and equalization simultaneously. Analytical upper bounds are derived for the bit-error probability when random spreading sequences are used, and comparisons to simulation results show that the bounds are quite accurate. The results also show that significant performance improvement over the conventional RAKE receiver is obtained.     

Performance analysis for an adaptive filter code-tracking techniquein direct-sequence spread-spectrum systems

This paper investigates tracking of direct-sequence spread-spectrum (DS/SS) signals based on an adaptive filtering technique. It is shown that a previously proposed hardware for code acquisition is also capable of code-tracking, and, hence, by performing both acquisition and tracking with the same circuitry, a significant simplification in the overall DS/SS receiver structure is gained. Analytical results show that the proposed scheme has a good tracking performance, as measured by the hold-in time and the false alarm penalty time, and is less sensitive to variations in the signal-to-noise ratio (SNR) compared to conventional delay-locked loops (DLLs). Moreover, simulation results show that the proposed adaptive filter tracking scheme has a smaller residual tracking error than that produced by a conventional maximum-likelihood estimator (MLE)     

New superregenerative architectures for direct-sequence spread-spectrum communications

The superregenerative receiver has been used for many decades as a low-cost and low-power receiver in short-range narrow-band communications. In this paper, we present two new architectures that make use of the superregeneration principle to achieve noncoherent detection of direct-sequence spread-spectrum signals. The local pseudorandom code generator is clocked by the quench oscillator, making the quench frequency equal to the chip rate. Under this condition, it is possible to take advantage of the characteristic broad reception bandwidth and the pulsating nature of the receiver to filter and despread the signal. The two superregenerative architectures, operating under periodic and pseudorandom quench, respectively, are analyzed and compared. Theoretical predictions are confirmed by experimental results in the ISM band of 2.4 GHz.     

A direct-sequence spread-spectrum communication system for integrated sensor microsystems.

Some of the most important challenges in health-care technologies have been identified to be development of noninvasive systems and miniaturization. In developing the core technologies, progress is required in pushing the limits of miniaturization, minimizing the costs and power consumption of microsystems components, developing mobile/wireless communication infrastructures and computing technologies that are reliable. The implementation of such miniaturized systems has become feasible by the advent of system-on-chip technology, which enables us to integrate most of the components of a system on to a single chip. One of the most important tasks in such a system is to convey information reliably on a multiple-access-based environment. When considering the design of telecommunication system for such a network, the receiver is the key performance critical block. The paper describes the application environment, the choice of the communication protocol, the implementation of the transmitter and receiver circuitry, and research work carried out on studying the impact of input data characteristics and internal data path complexity on area and power performance of the receiver. We provide results using a test data recorded from a pH sensor. The results demonstrate satisfying functionality, area, and power constraints even when a degree of programmability is incorporated in the system.     

Effects of cell correlations in a matched-filter PN codeacquisition for direct-sequence spread-spectrum systems

Rapid pseudonoise (PN) code acquisition with matched-filter correlators has been very popular in direct-sequence (DS) spread-spectrum systems. Conventionally, the analysis of this acquisition method is based on the assumption that the detections among cells are independent. However, there may be strong correlations among cell detections for the case that the cell size is less than a chip duration. In this paper, the mean acquisition time performance of the acquisition method is analyzed with the cell correlations being taken into account. Numerical results show that depending on the threshold value and other system parameters, the effect of cell correlations may be over 20% of the mean acquisition time for signal-to-noise ratios (SNR) of practical interest. The analytical results are substantiated by computer simulations     

An adaptive direct-sequence spread-spectrum receiver for burst typeinterference

The hybrid system is an extension of the state-space system model which allows for random and abrupt changes in the model parameters. In this paper we use the hybrid system to model burst type interference in a direct-sequence spread-spectrum (DS/SS) communication system. The resulting optimum receiver is easily derived but too computationally complex to be practical. Two suboptimal but more practical receivers are presented. In an example, the hybrid system is used to model pulsed narrowband interference. The performance of each suboptimal receiver is evaluated using computer simulations     

A protocol for adaptive transmission in direct-sequence spread-spectrum packet radio networks

Wireless communication channels may change greatly from one transmission to the next, due to variations in propagation loss and interference. The use of fixed transmission parameters for such channels results in wasted energy when channel conditions are good. Adaptation of the power, code rate, and symbol rate reduces energy consumption and interference caused to other systems. Such adaptation requires information about the characteristics of the channel, which is more difficult to obtain in a packet radio network (PRN) or other mobile ad hoc network than in a typical cellular communication system. We develop methods for providing partial information about the channel state from three statistics that are derived by different subsystems in the receiving terminals of a direct-sequence spread-spectrum PRN. We present and evaluate a protocol that uses this information to adapt the transmission parameters in response to changes in interference and propagation conditions in the network. The performance of the new adaptive-transmission protocol is compared with a system with fixed transmission parameters and with an adaptive protocol that is furnished with perfect knowledge of the channel state at the completion of each transmission.     

Multi-user selection diversity for spread-spectrum multi-carrier multiple-access systems

Recently, Multi-User Selection Diversity (MUSDiv) for single-carrier systems has been under extensive study on account of the enhancement it provides to system performance with minimum feedback requirements. However, its application to multichannel systems is considered straightforward and thus, it has not been thoroughly examined. In this paper, the performance of MUSDiv is investigated when applied to the spread-spectrum multi-carrier multiple-access system, where the scheduling has to be performed for all the available channels and self interference must also be considered. Specifically, based on the absolute and normalized Signal-to-Noise-Ratio (SNR) scheduling algorithms, two algorithms are presented, modified and optimized, so that they can be applied on a subband instead of a single-channel basis. Moreover, we propose a new scheduling scheme which constitutes a trade-off between the previous schemes, concerning fairness and capacity performance. The new algorithms are related to the symbol Signal-to-Noise-plus-Interference-Ratio (SNIR) instead of SNR and two interference models were devised to this end. Closed-form expressions for the system capacity are extracted for each case, which are compared with simulation results. The research is also extended to the case of non-identical user power profiles among the available subcarriers. The channel state information required to utilize multi-user selection diversity is already necessary for the most common combining schemes, so no more feedback is actually needed.     

Frequency-hopped spread-spectrum transmission with band-efficientmodulations and simplified noncoherent sequence estimation

Frequency-hopped spread-spectrum transmission using band-efficient modulations that are phase-continuous during each hop, is presented. A range of system parameters is considered, including signaling spectrum, reception, system complexity, and performance in the presence of noise and jamming. The particular cases where the hopped modulation is minimum-shift-keying (MSK), duobinary minimum-shift-keying (DMSK), or tamed-FM (TFM), are studied in detail. Results are presented for various modulation indexes, rectangular and raised cosine pulse shapings, and a range of hop interval lengths. Sequence estimation on a hop-by-hop basis is considered. The noncoherent likelihood sequence receiver must keep all possible paths, and so the computation and complexity becomes large for system transmitting many bits per hop. Therefore, a simplified noncoherent Viterbi-like sequence estimation algorithm with reduced complexity is introduced. System performance has been evaluated in Gaussian noise, partial-band jamming and multiple-tone jamming, using bounds and a system simulation. The compact nature of the hopped spectrum, when a number of bits are transmitted per hop, offers greater spectrum utilization and the prospect of improved performance in the presence of multiple-tone jamming or interference     

Path diversity for FFH/PSK spread-spectrum communication systems

A fast frequency hopping (FFH) method which uses path-diversity combining is proposed. Diversity techniques are realized when a signal is received from diverse independent paths, each of which carries identical information but suffers from independent fading variations. The improvement of communication performance of FFH systems is possible as the delayed paths are used and path-diversity combination is realized. The advantages of this method, operating in Rayleigh fading channels are confirmed by theoretical analyses. The improvement is in order of 2~3 dB at bit error rate (BER) of 10^-3. This method can be also effective against interferences from other users in a code division multiple access (CDMA) environment. The performance of this system in a CDMA environment is evaluated by theoretical analyses and is shown to be superior to non-combining method. At BER of 10^-3 the required E_b/N₀ of the proposed system is 5 dB lower. If E_b/N_o is fixed, a greater number of users can be accommodated using the proposed system     

Reducing multipath tracking errors in spread-spectrum ranging systems

Analysis of coherent and noncoherent delay lock loops shows that a smaller early-late spacing than the commonly used one chip time reduces multipath tracking error considerably. The results are especially interesting for spread-spectrum ranging systems such as GPS.<>     

Mathematical modeling and performance analysis for a two-stageacquisition scheme for direct-sequence spread-spectrum CDMA

Acquisition of synchronism is considered for DS/SS CDMA systems. For large systems with large timing uncertainties, it has been shown previously that acquisition in the presence of multiple-access interference may impose a significant limitation on capacity. This leads the authors to consider a system in which timing uncertainties are relatively small and to propose an acquisition scheme which exploits this to reduce complexity and acquisition overhead. The proposal may be appropriate for a microcellular environment for personal communications in which CDMA packet transmission is employed for both voice and data. Packetized transmission would imply that the overhead available for acquisition is small, and the large number of microcells would restrict the cost of the acquisition scheme used in the receiver in each microcell. The acquisition time required for a simple serial search scheme may therefore be unacceptably large. On the other hand, while acquisition using a passive matched filter is fast, the filter length required for reliable acquisition is liable to be excessive in terms of cost and complexity. Motivated by these considerations, the authors propose a two-stage acquisition scheme which employs a short programmable matched filter for initial detection, followed by a correlator for verification. Numerical results based on an approximate analysis of acquisition performance in the presence of multiple access interference are employed to compare the scheme with conventional acquisition schemes     

Accurate evaluation for MDPSK with noncoherent diversity

The error probability analysis of M-ary differential phase-shift keying signals with noncoherent diversity combining over general fading channels is not available in the literature except for some simple cases. The difficulty lies in the philosophy which attempts to explicitly determine the phase distribution expressions of the received signal, and this often leads to a mathematically intractable issue. In this paper, we take a novel approach by formulating the phase distribution in terms of joint moment generating functions of the real and imaginary parts of the decision variable at the receiver output. We further derive fast convergent techniques for two-dimensional (2-D) inverse Laplace transform enabling us to accurately evaluate the phase distribution. The error probability formulas thus obtained involve a twofold integral, which can be efficiently evaluated by using our algorithms developed on the basis of the 2-D trapezoidal summation and Gauss-Chebyshev quadrature. The new technique is very general, taking into account the effects of arbitrary diversity order, symbol alphabet size M, and arbitrary diversity branches correlation. Numerical results are also presented for illustration.     

非相干扩频接收机伪码跟踪环设计及FPGA实现

伪码跟踪环的设计是实现非相干扩频接收机的关键环节。为了实现非相干扩频接收机的伪码跟踪,设计了能量归一化的延迟锁定跟踪环,给出了环路的实现结构及环路参数的计算方法。分析了非相干扩频的特点,指出环路设计的关键点,在此基础上阐述了码环鉴别器、环路滤波器、超前滞后码发生器的设计及实现方法,并给出一套具体的实现参数。Modelsim仿真结果及FPGA实测数据表明所设计的环路能对伪码进行精确跟踪。     

Performance of noncoherent maximum-likelihood sequence detection for differential OFDM systems with diversity reception

For the single-carrier M-ary differential phase-shift keying (MDPSK), the multiple-symbol differential detector, or the noncoherent maximum-likelihood sequence detector (NSD), and its three special cases, namely, the noncoherent one-shot detector, the linearly predictive decision-feedback (DF) detector, and the linearly predictive Viterbi receiver are reviewed based on a hierarchical interpretation. For the multicarrier transmission, the differential orthogonal frequency division multiplexing (OFDM) systems with diversity reception are discussed. It is well known that there are two types of differential OFDM systems, namely, the time domain differential OFDM (TD-OFDM) and the frequency domain differential OFDM (FD-OFDM). In this paper, the NSD and its special cases are incorporated to the differential OFDM systems. Furthermore, we provide a simple closed-form bit-error-rate (BER) expression for the differential OFDM systems utilizing the noncoherent one-shot detector with diversity reception in the time-varying multipath Rayleigh fading channels. Numerical results have revealed that, with multi-antenna diversity reception, the performance of the noncoherent one-shot detector is improved significantly. However, when only one or two receive antennas are available, the implementation of the linearly predictive DF detector or the linearly predictive Viterbi receiver is necessary for achieving better and satisfactory performance.