A DS-CDMA system using despreading sequences weighted by adjustable chip waveforms

This paper evaluates the performance of a direct-sequence code-division multiple-access system using coherent receivers in which the despreading sequences are weighted by adjustable chip waveforms. The chip weighting waveforms under consideration are designed for multiple-access interference (MAI) rejection. Assuming that the received chip waveforms are rectangular, new expressions for the signal-to-interference-plus-noise ratio (SINR) of the decision variable are derived when different weighted despreading sequences (WDS's) are used in the receiver. The novelty of the derived expressions is that each of the expressions, when the system parameters are given, is determined only by one parameter of the adjustable chip waveforms employed. As a result, we can simply tune the parameter to its optimal value in real-time for MAI rejection without knowing the other users' spreading codes, timing, and phase. The criterion for tuning the parameter is to maximize the SINR of the decision variable based on the relative strength between the additive Gaussian white noise and the MAI. Numerical results show that when the multiple-access interference is significant, the receivers using WDS's outperform significantly the conventional receiver using a rectangular despreading sequence. Brief analysis for band-limited spreading signals is also provided to reveal the practical implications of the proposed technique. © 1999 IEEE.published_or_final_versio

Effects of of weighted despreading sequence on CDMA performance

This paper received the best paper award in the conferenceWaves of the Year 2000This paper addresses the effects of a weighted despreading sequence on the performance of a coherent receiver in a DS-CDMA system. The chip weighting waveforms used in the receiver are designed for the purpose of multiple access interference rejection by emphasizing the transitions of the received signal of interest. Since the resulting weighted despreading sequence is determined only by one parameter, this leads to easy tuning of the weighted despreading sequence in practice to achieve the best performance. The criterion of tuning the parameter is to maximize the derived signal to interference plus noise ratio of the decision variable based on the relative strength between white noise and multiple access interference. As a result, according to the proposed approach, we can reject the multiple access interference without knowing the co-user's spreading codes, timing, and phase since maximizing the signal to interference plus noise ratio in Gaussian noise minimizes the probability of error of the receiver.published_or_final_versio

DS-CDMA with power control error using weighted despreading sequences over a multipath rayleigh fading channel

In this paper, closed-form solutions for the average bit error rate (BER) performance of a direct-sequence codedivision multiple-access system with imperfect power control are derived for both coherent and noncoherent receptions operating over a multipath Rayleigh fading channel. The RAKE structure receivers under consideration employ despreading sequences weighted by adjustable exponential chip waveforms optimized for multiple-access interference rejection. The chip-weighting waveforms employed are determined only by one parameter γ which leads to easy tuning of the waveforms in practice to achieve the best performance. The results indicate that the number of active users supported at a given BER for the case of γ tuned to maximize the average signal to interference plus noise ratio H is much larger than the case of γ = 0 (fixed or rectangular despreading sequence). It is shown that imperfect power control affects the irreducible BER for the case of γ = 0. On the other hand, the effect of imperfect power control on BER performance for the case of γ tuned to maximize Ĥ is equivalent to a reduction in the average signal-to-noise ratio, and, hence, system performance can be compensated by increasing the transmitter power. It is further shown that the effect due to imperfect power control on BER performance is significant while that on the maximum value of Ĥ obtained by tuning γ is rather insignificant. Index Terms-Code-division multiple access, RAKE receivers, spread-spectrum communication. © 1999 IEEE Publisher Item Identifier S 0018-9545(99)05722-9.published_or_final_versio

A CDMA Receiver Using Exponentially Weighted Despreading Waveforms

This paper presents a DS-CDMA receiver using exponentially weighted despreading waveforms (WDW). The chip weighting waveforms are designed for the purpose of multiple access interference (MAI) rejection by emphasizing the transitions of the received signal of interest. The WDW is determined only by one parameter, which leads to easy tuning of the WDW in practice to achieve the best performance. As a result, we show that the proposed receiver can reject MAI without knowing co-user's spreading codes, timing, and phase, and hence increase system capacity. Analysis and numerical results show that the proposed receiver outperforms the conventional receiver especially when MAI is significant. Finally, a discussion on the effect of bandlimited spreading signals is also given on the practical implications of the proposed technique.published_or_final_versio

PN code tracking for exponentially weighted chip despreading sequence

This paper presents the code tracking performance on the use of weighted chip despreading PN sequence in a coherent delay locked tracking scheme for direct-sequence spread-spectrum (DS/SS) systems. In the delay locked loop (DLL), the conventional rectangular chip waveform for early and late despreading PN sequences is replaced by weighted exponential chip waveform. The chip weight (γ) is tuned to optimize the DLL performance. The early-late spacing is considered as one chip time. This scheme is analyzed for both ideal and band limited received signals in the presence of additive white Gaussian noise.published_or_final_versio

Coherent DS-SS receiver using exponentially weighted despreading function with imperfect power control in multipath Rayleigh fading environment

In this paper, a closed-form solution for the average bit error rate (BER) performance of a coherent DS-CDMA system with imperfect power control is derived for a multipath Rayleigh fading channel. The RAKE receiver under consideration employs exponentially weighted despreading function with an adjustable parameter γ optimized for multiple access interference rejection in multipath environment. The results indicate that the number of active users supported at a given BER for the case of γ tuned to maximize the average signal to interference plus noise ratio SINR̄ is much larger than commonly used rectangular despreading function (γ = 0). It is shown that imperfect power control affects the irreducible BER for the case of γ = 0. On the other hand, the effect of imperfect power control on BER performance for γ tuned to maximize SINR̄ is equivalent to a reduction in the average signal to white noise ratio γ̄b and hence system performance can be compensated by increasing the transmitter power.published_or_final_versio

Half-sine and triangular despreading chip waveforms for coherent delay-locked tracking in DS/SS systems

The performance of a coherent delay-locked tracking scheme for direct-sequence/spread-spectrum systems using half-sine or triangular chip waveforms for early and late despreading sequences is analyzed. The effect of band-limiting on the received signals is considered. Mean time to lose lock (MTLL) and root mean square (rms) tracking error of the delay-locked loop (DLL) are compared with that of a conventional DLL which uses rectangular chip despreading waveforms. Linear and nonlinear (based on the renewal process approach) analyses are employed to evaluate the performance of the DLL. Results show that the use of either the half-sine or triangular chip waveform reduces the rms tracking error and increases the MTLL considerably when the early-late spacing is approximately between 0.7-1.3 chip times. The results apply in particular to the commonly used DLL using one chip early-late spacing. Computer simulation for band-limited signals confirms the analytical results. The use of these despreading chip waveforms also reduces tracking offset in multipath environments.published_or_final_versio

Capacity enhancement of band-limited DS-CDMA system using weighted despreading function

This paper addresses capacity enhancement of a band-limited direct-sequence code-division multiple-access system by using a weighted despreading function (WDF) in the receiver. An ideal Gaussian channel with perfect power control is assumed. The system performance is measured by the signal-to-interference-plus-noise ratio of the decision variable derived in the frequency domain, the bandwidth efficiency factor, the capacity enhancement factor, and the bit-error rate. It is shown that tuning a parameter of the WDF employed helps to partially flatten the in-band cross-spectrum of a pair of spreading and despreading functions. Numerical results show that the capacity of the proposed system improves over the conventional system using rectangular despreading function. To assess practical implications of the WDF receiver, the sensitivity to timing error is also analyzed.published_or_final_versio

Performance of Two-Hop DS-CDMA Systems Using Amplify-and-Forward Protocol over Different Fading Channels

This study analyses the performance of directsequence code division multiple access (DS-CDMA) based on two-hop amplify-and-forward protocol over Weibull symmetric fading channels as well as Rayleigh/Rician, Rician/Rayleigh asymmetric fading phenomenas. We investigate the bit-error rate (BER) of the considered system using multiple relays by considering the effect of Weibull fading parameter and Rician K factor on the system performance. Our simulation results demonstrate the positive impacts of the value of fading parameter, Rician K factor and increasing number of relay nodes on BER performance. It is also confirmed that the Rician K factor is more effective on the system performance over Rician/Rayleigh fading channels in comparison with Rayleigh/Rician fading environment

A Direct Sequence Code-Division Multiple-Access Local Area Network Model

The United States Air Force relies heavily on computer networks for every-day operations. The medium access control (MAC) protocol currently used by most local area (LAN) permits a single station to access the network at a time (e.g. CSMA/CD or Ethernet). This limits network throughput to, at most, the maximum transmission rate of a single node with overhead neglected. Significant delays are observed when a LAN is overloaded by multiple users attempting to access the common medium. In CSMA/CD, collisions are detected and the data sent by the nodes involved are delayed and transmitted at a later time. The retransmission time is determined with a binary exponential back-off-algorithm. Code Division Multiple Access (CDMA) is a technique that increases channel capacity by allowing multiple signals to occupy the same bandwidth simultaneously. Each signal is spread through multiplication with a unique pseudo-random code that distinguishes it from all other signals. Upon reception, the signal of interest is despread and separated from other incoming signals by multiplying it with the same exact code. With this technique, it is possible for multiple stations to transmit simultaneously with minimal ill effects. A simulation model is developed for a direct sequence spread spectrum CDMA (DS/CDMA) channel that incorporates the effects of multiple access interferers (MAI) having spreading codes from the same or different code families. The model introduces cross-correlation coefficients to calculate the signal-to-interference ratio and determine channel bit error performance. Transmission media attenuation and the near-far effects are accounted for in the model design. The model utility is demonstrated by determining the loss characteristics of a coaxial spread spectrum network. Due to the modular design, other transmission media characteristic can be easily incorporated. A bus network topology is simulated using 10Base2 coaxial cable. The model is compared and validated against a spread spectrum local area network hardware test bed