We show how two spiking neuron models encode continuous-time signals into spikes (action potentials, time-encoded pulses, or point processes) using a special form of sigma-delta modulation (SDM). In particular, we show that the well-known leaky integrate-and-fire (LIF) neuron and the simplified spike response model (SRM0) neuron encode the continuous-time signals into spikes via a proposed asynchronous pulse SDM (APSDM) scheme. The encoder is clock free using level-crossing sampling with a single-level quantizer, unipolar signaling, differential coding, and pulse-shaping filters. The decoder, in the form of a low-pass filter or bandpass smoothing filter, can be fed with the spikes to reconstruct an estimate of the signal. The density of the spikes reflects the amplitude of the encoded signal. Numerical examples illustrating the concepts and the signaling efficiency of APSDM vis-à-vis SDM for comparable reconstruction accuracies are presented. We anticipate these results will facilitate the design of spiking neurons and spiking neural networks as well as cross fertilizations between the fields of neural coding and the SDM.

This paper considers the application of distributed transmit diversity (TD) to digital broadcast networks. Distributed TD allows spatial encoding across transmit sectors (cell-sites or satellites) each with a single antenna. We focus on Alamouti space-frequency (SF) codes in particular and consider terrestrial systems based on cellular type networks. System coverage results are presented for a 1xEV-DO broadcast network based on a comprehensive evaluation methodology developed in 3GPP2 for system modeling and orthogonal frequency division modulation (OFDM). The results show that distributed transmit diversity can deliver significant coverage gains. For example, given a bandwidth of 1.25 MHz, a data rate of 1.2 Mbps and a minimum bit-error rate (BER) requirement of 0.1%, the broadcast coverage increases from roughly 40% in the absence of distributed TD to 70% with the Alamouti linear combiner and to 82% with a zero-forcing (ZF) spatial equalizer

Abstract This paper investigates the forward‐link peak and average data rates, throughput, and coverage of a cellular CDMA system for delivering high‐speed wireless data services. The analysis takes into account major aspects commonly found in the forward data channel and applies the generalized Shannon capacity formula for multi‐element antenna (MEA) systems. The study focuses on the physical layer and is flexible for various propagation environments, antenna configurations, multicode allocations, user distributions, and cell site configurations. Numerical results for various multicode allocations are presented for a system model with two‐tier interfering cells operating under a frequency selective slow fading channel with propagation environments specified in the Recommendation ITU‐R M.1225. Copyright © 2006 John Wiley & Sons, Ltd.

The use of rate-compatible punctured turbo and rate-compatible punctured convolutional (RCPT/RCPC) codes as channel codes in a direct-sequence code-division multiple-access system where the system bandwidth expansion is fixed is investigated. The best RCPC and RCPT code rate in terms of maximizing the system spectral efficiency and minimizing the optimal power allocation where the receiver is either a matched filter (MF) or a minimum mean-square error (MMSE) device is assessed. It is shown that for the MF receiver, the coding-spreading tradeoff favors a code-rate reduction. In the case of the MMSE receiver, when the E/sub b//N/sub 0/ value and the system load are increased, the best code rate also increases. By examining the slope of the performance curves, it is deduced that, under similar operating conditions, the best code rate of the RCPT codes is lower than that of the RCPC codes. Also, the best code rate for a Rayleigh fading channel is lower than that for an additive white Gaussian noise channel.

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.

The estimation of signal-to-noise ratio (SNR) for power control of the cdma2000 forward fundamental channel is investigated. Power estimation of the signal component is based on the power control (PC) subchannel symbol whose power is independent of the vocoder frame rate which can vary. An effective method of removing PC bit modulation and equalizing channel variation is proposed to coherently combine PC symbols. Power estimation of the noise component is accomplished by a differential approach to reduce the effect of errors introduced by fading and by increasing the noise sample size. Performance of the estimation algorithm is verified through computer simulations. Results show that the long term average frame error rate (FER) performance of the proposed algorithm is less than 1.5 dB from that of perfect SNR estimation.

This paper investigates the forward-link peak and average data rates, throughput, and coverage of a cellular CDMA system for delivering high-speed wireless data services. The analysis takes into account major aspects commonly found in the forward data channel and applies the generalized. Shannon capacity formula for multi-element antenna (MEA) systems. The study focuses on the physical layer and is flexible for various propagation environments, antenna configurations, multicode allocations, user distributions and cell site configurations. Numerical results for various multicode allocations are presented for a system model with two-tier interfering cells operating under a frequency selective slow fading channel with propagation environments specified in the recommendation ITU-RM.1225.

This paper examines a blind adaptive implementation of a recently proposed linear minimum mean square error (LMMSE) receiver for code-division multiple-access (CDMA) systems with aperiodic spreading sequences in multipath channels. The receiver has been previously shown to perform multiple-access interference (MAI) suppression and multipath diversity combining. The adaptive implementation is based on a fractionally spaced equalizer (FSE) whose taps are updated by the leaky constant modulus algorithm (LCMA) in the cold start and the decision-directed least-mean-square (DD-LMS) algorithm when the channel eye is opened. The LCMA adds a quadratic constraint, defined as the squared norm of the FSE weight vector, to the constant modulus (CM) cost function. Simulation results show that the LCMA with a uniform initialization strategy (all taps equally set to a small non-zero value) acquires all paths associated with the desired user, suppresses MAI, and opens the channel eye for the DD-LMS algorithm to converge to the proximity of the MMSE solution.

This paper investigates multi-user detection in multiple-access communication systems based on ultrawideband (UWB) technology. As user numbers increase and the bandwidth to pulse repetition frequency (PRF) decreases, multiple-access interference (MAI) is expected to adversely affect system capacity and performance. The paper presents a framework for the design of multi-user detectors and proposes the optimum multi-user detector for UWB. Numerical examples illustrate the performance of the optimum detector versus that of the conventional single-user UWB detector.

This paper considers probability of bit error (P/sub e/) analysis in asynchronous band-limited direct-sequence code-division multiple-access (DS-CDMA) systems. It presents a simple and accurate method of P/sub e/ analysis. The proposed method can serve as an attractive alternative to the only two techniques currently available for band-limited systems: the standard Gaussian approximation (SGA) and the characteristic function method. The former is prone to inaccuracy while the latter, large computational complexity. The method generalizes the simplified improved Gaussian approximation (SIGA) derived previously for rectangular pulses. This paper also outlines a generalization of another method referred to as the improved Gaussian approximation (IGA). Numerical examples demonstrate the far greater accuracy of the generalized SIGA with respect to the SGA. The examples consider the IS-95 and square-root raised cosine (Sqrt-RC) pulses as well as uniform and nonuniform received power conditions.

This paper analyzes probability of bit-error (P/sub e/) performance of asynchronous bandlimited direct-sequence code-division multiple-access systems with binary phase-shift keying spreading. The two present methods of P/sub e/ analysis under bandwidth-efficient pulse shaping: the often-cited standard Gaussian approximation and the characteristic function (CF) method suffer from either a low accuracy in regions of low P/sub e/ (< 10/sup -3/) or a prohibitively large computational complexity. The paper presents an alternate method of P/sub e/ analysis with moderate computational complexity and high accuracy based on a key observation. A sequence of chip decision statistics (whose sum yields a bit statistic) forms a stationary, m-dependent sequence when conditioned on the chip delay and phase offset of each interfering signal. This observation permits the generalization of the improved Gaussian approximation previously derived for the rectangular pulse and the derivation of a numerically efficient approximation based on the CF method. Numerical examples of systems using the square-root raised-cosine and IS-95 pulses illustrate THE P/sub e/ performance, user capacity and the accuracy of the proposed method.

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.

The variability of the wireless channel requires adaptive error control. Conventional approaches use either link adaptation (LA), which adjusts the coding scheme based on the estimated channel condition, or incremental redundancy (IR), which adjusts the code rate by incrementally transmitting redundancy until decoding is successful. IR usually achieves a higher effective throughput than that of LA, but a larger delay compared to LA. A protocol which combines the advantages of LA and IR to enhance data transmission is proposed. In contrast to the conventional IR with a highest starting code rate, the proposed approach adaptively adjusts the starting code rate based on the current channel condition. Three schemes are proposed to accomplish adaptation. Simulation results show that the average number of transmissions is significantly reduced, with a small sacrifice in the average throughput compared with those of IR.

The best tradeoff between coding and spreading in a single-cell direct-sequence code division multiple access (DS-CDMA) system is investigated. The best code rate in terms of the system spectral efficiency for a single-class system and the optimal power allocation for a multi-class system is analyzed by applying both a matched filter (MF) receiver and a minimum mean square error (MMSE) receiver. It is shown that for the MF receiver, the coding-spreading tradeoff favors a code rate reduction. In the case of the MMSE receiver, the spectral efficiency vs. code rate curve is convex, so there is a best code rate corresponding to a given E/sub b//N/sub 0/ specification. Numerical results show that the best code rate is a function of the system load, the required bit error rate, and the steepness of the required SIR vs. the code rate curve, i.e., the error correction capability of the applied coding codes. The best code rate to maximize the spectral efficiency is further related to the system design parameter E/sub b//N/sub 0/.

In this paper, highly reliable data transmission using concatenated Reed-Solomon (RS) /convolutional coding for a direct-sequence code division multiple access system is studied. The study is based on the analysis of the tradeoff between coding and spreading, and on the compromise between RS outer code and convolutional inner code under the constraint of a fixed bandwidth expansion for each service class. Some insightful observations about the mechanism of the concatenated coding scheme and their explanations are presented. Numerical results show that a best outer/inner code rate pair exists. By applying this best code rate pair over a slow Rayleigh fading channel, depending on the system load and the constraint length of the inner code, a gain of 2dB to more than 9dB in received power reduction for each user can be achieved, compared to the conventionally used code rate pair (outer/inner code rate = 0.8/0.5).

This paper considers an improved single-user detection technique for asynchronous direct-sequence code-division multiple-access (DS-CDMA) systems using long sequence spreading (random-CDMA) Most of the known detection schemes for DS-CDMA suffer from either poor performance under power-imbalance (near-far like) conditions, excessive complexity, or incompatibility with systems employing long sequence spreading. To address these problems, this paper considers a signal-to-noise ratio maximizing linear time-invariant filter for one-shot bit symbol detection exploiting some information about the interferers. This filter, referred to as the chip-delay locked matched filter (CLMF), exploits the cyclostationarity in multiple-access interference, and it can offer good near-far resistance while remaining suitable for systems with long sequence spreading. The CLMF requires knowledge of interferers chip delays and signal powers; however, knowledge of their pseudonoise sequences is unnecessary. This paper also demonstrates the improvement in performances offered by the CLMF over other single-user receivers such as the conventional matched filter and noise-whitening matched filter performance is evaluated in terms of probability of outage for single-rate and dual-rate DS-CDMA systems using bandwidth-efficient chip pulses, over a single-path additive white Gaussian noise channel. Errors in the interferer chip delay estimates degrade the CLMF performance. However, if the root-mean-square value of these errors is less than 5% of the chip interval, then this degradation is small.

This paper considers the design of near-far resistant matched filters (MFs) for direct-sequence code-division multiaccess (DS-CDMA) systems. We show how additional information on the individual powers and chip delays of locked users, that is available at the base station, can be exploited for the design of these MFs. The proposed receiver can specifically reduce the effects of the locked interferers at the expense of a partial increase in complexity over the conventional MF. Furthermore, this detection technique is a link between the linear minimum mean squared error (MMSE) centralized multiuser detector and the decentralized noise-whitening MF. Numerical results for the rectangular chip pulse illustrate the signal-to-noise ratio (SNR), near-far resistance, and bit-error rate (BER) improvement that the proposed MF yields over the conventional MF and noise-whitening MF.

Based on the criterion of maximizing signal to noise ratio (SNR), the optimum matched filter is, in general, the solution to a real integral equation. With regards to the detection of narrowband, bandpass signals, we present the full form of the integral equation based upon complex envelopes. The correct form of the integral equation includes a complementary or pseudo-autocorrelation function which can be ignored for most matched filtering applications. However, in the presence of improper complex noise, overlooking the function leads to a "non-optimal" matched filter. To illustrate the concept, we apply the theory to a simple direct-sequence code-division multiple-access (DS-CDMA) system.

The authors propose and analyze a direct-sequence spread-spectrum multiaccess (DS/SSMA) receiver that employs a cascade of cochannel interference (CCl) cancellers for communication over multipath fading channels. The receiver first coherently demodulates and despreads the received signal to produce correlator outputs and initial data estimates. Based on these estimates, the cancellation scheme essentially creates replicas of the contributions of the CCl embedded in the correlator outputs and removes them for a second improved hard data decision. By repeating this operation over and over, a cascade of CCl cancellers is derived. Through theoretical analysis and simulation, the authors investigate the canceller's bit error rate (BER) performance in both the absence and presence of errors in the amplitude and phase estimates of each user's received signal. Numerical results show the considerably large improvement in performance that can be attained by the cancellation scheme, even under partially degraded estimates.>