Performance of Fractionally Spread Multicarrier CDMA in AWGN as Well as Slow and Fast Nakagami-m Fading Channels

Abstract—In multicarrier code-division multiple-access (MCCDMA), the total system bandwidth is divided into a number of subbands, where each subband may use direct-sequence (DS) spreading and each subband signal is transmitted using a subcarrier frequency. In this paper, we divide the symbol duration into a number of fractional subsymbol durations also referred to here as fractions, in a manner analogous to subbands in MC-CDMA systems. In the proposed MC-CDMA scheme, the data streams are spread at both the symbol-fraction level and at the chip level by the transmitter, and hence the proposed scheme is referred to as the fractionally spread MC-CDMA arrangement, or FS MCCDMA. Furthermore, the FS MC-CDMA signal is additionally spread in the frequency (F)-domain using a spreading code with the aid of a number of subcarriers. In comparison to conventional MC-CDMA schemes, which are suitable for communications over frequency-selective fading channels, our study demonstrates that the proposed FS MC-CDMA is capable of efficiently exploiting both the frequency-selective and the time-selective characteristics of wireless channels. Index Terms—Broadband communications, code-division multiple access (CDMA), fractionally spreading, frequency-domain spreading, multicarrier modulation, Nakagami fading, timedomain spreading

Time-Selective Signaling and Reception for Communication over Multipath Fading Channels

Journal PaperThe mobile wireless channel affords inherent diversity to combat the effects of fading. Existing code division multiple access (CDMA) systems, by virtue of spread-spectrum signaling and RAKE reception, exploit only part of the channel diversity via multipath combination. Moreover, their performance degrades under fast fading commonly encountered in mobile scenarios. In this paper, we develop new signaling and reception techniques that maximally exploit channel diversity via joint multipath-Doppler processing. Our approach is based on a canonical representation of the wireless channel which leads to a time-frequency generalization of the RAKE receiver for diversity processing. Our signaling scheme facilitates joint multipath-Doppler diversity by spreading the symbol waveform beyond the inter-symbol duration to make the channel time-selective. A variety of detection schemes are developed to account for the inter-symbol interference (ISI) due to overlapping symbols. However, our results indicate that the effects of ISI are virtually negligible due to the excellent correlation properties of the pseudo-random codes. Performance analysis also shows that relatively small Doppler spreads can yield significant diversity gains. The inherently higher level of diversity achieved by time-selective signaling brings the fading channel closer to an additive white Gaussian noise channel, thereby facilitating the use of powerful existing coding techniques for Gaussian channels.Texas Advanced Technology ProgramNokiaNational Science Foundatio

Time-selective signaling and reception for communication over multipath fading channels

The mobile wireless channel offers inherent diversity by virtue of multipath and Doppler shifts. Multipath diversity is exploited via spread-spectrum signaling employed in code division multiple access (CDMA) systems. However, the RAKE receiver commonly used in CDMA systems exploits only multipath diversity, and consequently suffers from significant performance degradation under fast fading. We develop new signaling and reception schemes in the context of CDMA systems that fully exploit the channel via joint multipath-Doppler diversity. The signaling waveforms are spread in time and frequency. Receiver structures are developed to deal with the inter-symbol interference (ISI) introduced by overlapping successive symbols. Analytical and simulated performance results indicate that the effects of ISI are negligible due to the excellent correlation properties of the spreading codes. Moreover, even the small Doppler spreads encountered in practice can yield significant performance gains. Additionally, the time-selective signaling scheme allows for substantially higher level of diversity and thereby brings the fading channel closer to an additive white Gaussian noise channel. This facilitates the use of error control codes developed for the Gaussian channel

Time-Selective Signaling and Reception for Communication over Multipath Fading Channels

Masters ThesisThe mobile wireless channel offers inherent diversity by virtue of multipath and Doppler shifts. Multipath diversity is exploited via spread-spectrum signaling employed in code division multiple access (CDMA) systems. However, the RAKE receiver commonly used in CDMA systems exploits only multipath diversity, and consequently suffers from significant performance degradation under fast fading. We develop new signaling and reception schemes in the context of CDMA systems that fully exploit the channel via joint multipath-Doppler diversity. The signaling waveforms are spread in time and frequency. Receiver structures are developed to deal with the inter-symbol interference (ISI) introduced by overlapping successive symbols. Analytical and simulated performance results indicate that the effects of ISI are negligible due to the excellent correlation properties of the spreading codes. Moreover, even the small Doppler spreads encountered in practice can yield significant performance gains. Additionally, the time-selective signaling scheme allows for substantially higher level of diversity and thereby brings the fading channel closer to an additive white Gaussian noise channel. This facilitates the use of error control codes developed for the Gaussian channel.Texas Advanced Technology ProgramNokiaNational Science Foundatio

Blind Estimation of Multi-Path and Multi-User Spread Spectrum Channels and Jammer Excision via the Evolutionary Spectral Theory

Despite the significant advantages of direct sequence spreadspectrum communications, whenever the number of users increases orthe received signal is corrupted by an intentional jammer signal,it is necessary to model and estimate the channel effects in orderto equalize the received signal, as well as to excise the jammingsignals from it. Due to multi-path and Doppler effects in thetransmission channels, they are modeled as random, time-varyingsystems. Considering a wide sense stationary channel during thetransmission of a number of bits, a linear time-varying modelcharacterized by a random number of paths, each beingcharacterized by a delay, an attenuation factor and a Dopplerfrequency shift, is shown to be an appropriate channel model. Itis shown that the estimation of the parameters of such models ispossible by means of the spreading function, related to thetime-varying frequency response of the system and the associatedevolutionary kernels. Applying the time-frequency orfrequency-frequency discrete evolutionary transforms, we show thata blind estimation procedure is possible by computing thespreading function from the discrete evolutionary transform ofthe received signal. The estimation also requires the synchronizedpseudo-noise sequence for either of the users we are interestedin. The estimation procedure requires to adaptively implementingthe discrete evolutionary transform to estimate the spreadingfunction and determine the channel parameters. Once the number ofpaths, delays, Doppler frequencies and attenuations characterizingthe channel are found, a decision parameter can be obtained todetermine the transmitted bit. We will show also that ourestimation approach supports multiuser communication applicationssuch as uplink and downlink in wireless communicationtransmissions. In the case of an intentional jamming, common inmilitary applications, we consider a receiver based onnon-stationary Wiener masking that excises such jammer as well asinterference from other users. Both the mask and the optimalestimator are obtained from the discrete evolutionarytransformation. The estimated parameters from the computedspreading function, corresponding to the closest to the line ofsight signal path, provide an efficient detection scheme. Ourprocedures are illustrated with simulations, that display thebit-error rate for different levels of channel noise and jammersignals

Inter-carrier interference suppression in orthogonal frequency division multiple access (OFDMA) systems uplink

Ph.DDOCTOR OF PHILOSOPH