Young C. Yoon

Optimum multi-user detection in ultra-wideband (UWB) multiple-access communication systems

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.

A simple and accurate method of probability of bit error analysis for asynchronous band-limited DS-CDMA systems

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.

An improved Gaussian approximation for probability of bit-error analysis of asynchronous bandlimited DS-CDMA systems with BPSK spreading

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.

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.

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.

A combined link adaptation and incremental redundancy protocol for enhanced data transmission

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.

A blind adaptive receiver for interference suppression and multipath reception in long-code DS-CDMA

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.

Coding-spreading tradeoff analysis for DS-CDMA systems

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/.

Linear MMSE interference suppression in asynchronous random-CDMA

This paper considers interference suppression, based on linear minimum mean square error (MMSE) filtering, in random code-division multiple-access (random-CDMA) systems such as IS-95 and wideband-CDMA using long pseudo-noise (PN) sequences. It examines the design of the MMSE receiver and its performance in terms of probability of bit error (P/sub e/). Expressions for P/sub e/ are derived for systems with multiple bit rates (and spreading factors), variable signal powers and chip pulse shaping. Numerical examples show substantial P/sub e/ gains in systems with one to two high-powered users and square-root raised cosine (Sqrt-RC) pulse shaping with 100% excess bandwidth.

Performance comparison of linear MMSE and coherent RAKE receivers in multipath asynchronous random-CDMA

This paper investigates the design of a linear minimum mean square error (MMSE) receiver for interference suppression in asynchronous random code-division multiple-access (random-CDMA) systems, such as IS-95 and wideband-CDMA (WCDMA), operating in multipath environments. Its performance is compared with that of the coherent RAKE receiver. The figure of merit is average output signal-to-noise-ratio (SNR). The proposed receiver maximizes SNR and considers all the desired users paths unlike the standard RAKE receiver which selects a subset of them. The performance improvement is supported and quantified by numerical examples. The achieved gain is shown to be substantial in nonuniform power conditions (typical of next-generation CDMA systems) and dense multipath environments where the signals are received via many paths. The chip pulse shape is the square-root raised cosine (Sqrt-RC) pulse with 22% excess bandwidth.