Scott L. Miller

An adaptive direct-sequence code-division multiple-access receiver for multiuser interference rejection

An adaptive receiver is considered for use in combating the near-far problem in direct-sequence code-division multiple-access (CDMA) communication networks. The focus of the paper is on the multiuser interference rejecting capability of the receiver. The receiver uses a chip matched filter followed by an adaptive equalizer structure to perform the despreading operation. This adaptive structure allows the receiver to adjust to the prevailing interference and noise environment. An analysis of the receiver giving the form of the equalizer coefficients and the mean squared error in steady state is given. From these results the capacity of a CDMA network using this receiver structure is obtained and compared with a system using conventional receivers. The receiver is shown to be immune to the near-far problem in the sense that the performance without any power control is nearly identical to the performance with perfect power control. The receiver is also shown to offer a two-fold increase in capacity relative to a conventional receiver with perfect power control. >

Propagation delay estimation in asynchronous direct-sequence code-division multiple access systems

In an asynchronous direct-sequence code-division multiple access (DS-CDMA) communication system, the parameter estimation problem, i.e., estimating the propagation delay, attenuation and phase shift of each users transmitted signal, may be complicated by the so-called near-far problem. The near-far problem occurs when the amplitudes of the users received signals are very dissimilar, as the case might be in many important applications. In particular, the standard method for estimating the propagation delays will fail in a near-far situation. Several new estimators, the maximum likelihood, an approximative maximum likelihood and a subspace-based estimator, are therefore proposed and are shown to be robust against the near-far problem. No knowledge of the transmitted bits is assumed, and the proposed estimators can thus be used for both acquisition and tracking. In addition, the Cramer-Rao bound is derived for the parameter estimation problem.

Peak power and bandwidth efficient linear modulation

In portable wireless communication systems, power consumption is of major concern. Traditional modulation and coding schemes have been designed from the standpoint of minimizing average power. However, for linear power amplifiers needed for spectrally efficient modulation formats, amplifier efficiency and hence power consumption are determined by the peak power of the transmitted signal. This paper looks into modulation formats which minimize peak power and retain high spectral efficiency. Peak power is broken into a sum (in terms of decibels) of average power and a peak-to-average power ratio, and a variety of modulation formats are evaluated in terms of peak power efficiency in both a Gaussian noise and Rayleigh fading channel. A generalized phase shift keying (PSK) modulation format is developed and shown to offer superior peak power efficiency relative to that of commonly used linear modulation formats. Two schemes are presented for reducing the peak-to-average power ratio of various modulation formats. First, data translation codes are used to avoid data sequences which cause large peaks in the transmitted signal. This approach was found to be most productive in quadrature amplitude modulation (QAM) formats. Finally, an adaptive peak suppression algorithm is presented which further reduces the peak-to-average power ratios of the PSK and generalized PSK formats. The peak suppression algorithm is also applicable to /spl pi//4-QPSK and was found to improve peak power efficiency of that format by about 1.25 dB over a Rayleigh fading channel.

An efficient code-timing estimator for DS-CDMA signals

We present an efficient algorithm for estimating the code timing of a known training sequence in an asynchronous direct-sequence code division multiple access (DS-CDMA) system. The algorithm is a large sample maximum likelihood (LSML) estimator that is derived by modeling the known training sequence as the desired signal and all other signals including the interfering signals and thermal noise as unknown colored Gaussian noise that is uncorrelated with the desired signal. The LSML estimator is shown to be robust against the near-far problem and is also compared with several other code timing estimators via numerical examples. It is found that the LSML approach can offer noticeable performance improvement, especially when the loading of the system is heavy.

Adaptive detection of DS/CDMA signals in fading channels

This paper examines the behavior of the minimum mean-squared error (MMSE) receiver in frequency-nonselective-fading channels. It is noted that the MMSE receiver will often lose phase lock on the desired signal when the desired signal dips into a deep fade. A modification to the MMSE receiver is presented which is demonstrated to function quite nicely in flat-fading channels. Analytical results for the modified MMSE receiver are presented and found to agree very well with simulation results. These analytical results are then compared to the theoretical performance of the conventional (i.e., correlator) receiver in terms of both bit-error rate (BER) and capacity. As expected, the modified MMSE receiver was found to offer a substantial improvement in both BER and capacity. Finally, a simple empirically derived formula is given which will give a good approximation to the BER of the modified MMSE receiver in a Rayleigh-fading environment. This formula can also be used to determine the number of users a given system can support. It is noted that as E/sub b//N/sub 0/ grows, it is quite feasible to approach 100% channel utilization with the MMSE receiver, whereas a conventional receiver is typically limited to a utilization of 10%-20%.

Performance analysis of MMSE receivers for DS-CDMA in frequency-selective fading channels

The performance of the minimum mean-squared error (MMSE) receiver for the detection of direct sequence code division multiple access is considered in various fading channel models. Several modifications to the basic MMSE receiver structure which have been previously proposed for use on nonselective fading channels are reviewed and shown to represent different approximations to a single common form. The performance of this general structure is analyzed as well as various extensions suitable for frequency-selective fading channels. Particular attention is given to the performance advantage gained through knowledge of the fading parameters of the various transmission paths of each users signal. It is shown that having this knowledge is not particularly useful on a flat fading channel unless the loading is very heavy and even then the difference in performance is only minimal. On the other hand, having this knowledge is crucial in a multipath fading channel and the inability to learn the fading channel parameters will lead to substantial degradation in capacity. A heuristic explanation to support this result based on a dimensionality argument is also presented.

DS-CDMA synchronization in time-varying fading channels

The problem of estimating propagation delays of the transmitted signals in a direct-sequence code-division multiple-access (DS-CDMA) system operating over fading channels is considered. Even though this study is limited to the case when the propagation delays are fixed during the observation interval, the channel gain and phase are allowed to vary in time. Special attention is given to the near-far problem which is catastrophic for the standard acquisition algorithm. An estimator based on subspace identification techniques is proposed, and the Cramer-Rao bound, which serves as an optimality criterion, is derived. The Cramer-Rao bound is shown to be independent of the near-far problem, which implies that there is no fundamental reason for propagation delay estimators to be near-far limited. Furthermore, the proposed algorithm is experimentally shown to be robust against the near-far problem.

Sensitivity of wireless network simulations to a two-state Markov model channel approximation

In this paper, we investigate the appropriateness of wireless network simulations that employ a two-state Markov model to approximate a flat Rayleigh fading channel. Our approach first analyzes the statistical similarities and differences between block error processes generated by 1) an 802.11b defined communication scheme in the presence of flat Rayleigh fading and 2) a representative two-state Markov model. It is shown that, at low SNR, the Markov model does not generate an adequate frame error process. Furthermore, we quantify the effects of said statistical deviations through ns2 network simulations. This work further advances the understanding of the appropriate, and inappropriate, environments under which the two-state Markov model is a suitable approximation of a fading channel.

Training analysis of adaptive interference suppression for direct-sequence code-division multiple-access systems

This paper studies the transient behavior of an adaptive near-far resistant receiver for direct-sequence (DS) code-division multiple-access (CDMA) known as the minimum mean-squared error (MMSE) receiver. This receiver structure is known to be near-far resistant and yet does not require the large amounts of side information that are typically required for other near-far resistant receivers. In fact, this receiver only requires code timing on the one desired signal. The MMSE receiver uses an adaptive filter which is operated in a manner similar to adaptive equalizers. Initially there is a training period where the filter locks onto the signal that is sending a known training sequence. After training, the system can then switch to a decision-directed mode and send actual data. This work examines the length of the training period needed as a function of the number of interfering users and the severity of the near-far problem. A standard least mean-square (LMS) algorithm is used to adapt the filter and so the trade-off between convergence and excess mean-squared error is studied. It is found that in almost all cases a step size near 1.0/(total input power) gives the best speed of convergence with a reasonable excess mean-squared error. Also, it is shown that the MMSE receiver can tolerate a 30-40 dB near-far problem without excessively long convergence time.

Persistent inappropriate sinus tachycardia after radiofrequency current catheter modification of the atrioventricular node

Atrioventricular (AV) junctional reentrant tachycardia (JRT) is the most frequent cause of paroxysmal supraventricular tachycardia.1 Recently, radiofrequency current delivered through a transvenous catheter was used to modify AV nodal properties so that AVJRT is no longer inducible while preserving normal AV nodal conduction. 2–4 This technique has produced high success rates with relatively few complications. In this report, we present a previously unreported complication observed in 3 patients undergoing AV node modification using radiofrequency current: persistent, inappropriate and symptomatic sinus tachycardia.