Carl R. Nassar

High-performance MC-CDMA via carrier interferometry codes

This paper introduces the principles of interferometry to multicarrier code division multiple access (MC-CDMA). Specifically, we propose the use of MC-CDMA with novel carrier interferometry (CI) complex spreading codes. The CI/MC-CDMA method, applied to mobile wireless communication systems, offers enhanced performance and flexibility relative to MC-CDMA with conventional spreading codes. Specifically, assuming a frequency selective Rayleigh-fading channel, CI/MC-CDMAs performance matches that of orthogonal MC-CDMA using Hadamard-Walsh codes up to the MC-CDMA N user limit; and, CI/MC-CDMA provides the added flexibility of going beyond N users, adding up to N-1 additional users with pseudo orthogonal positioning. When compared to MC-CDMA schemes capable of supporting greater than N users, CI/MC-CDMAs performance exceeds that of MC-CDMA. Additionally, this new system is analyzed in the presence of phase jitters and frequency offsets and is shown to be robust to both cases.

Generation of correlated Rayleigh fading envelopes for spread spectrum applications

A procedure for generating N Rayleigh fading envelopes with any desired covariance matrix is given. This method, numerical in nature, enables researchers to simulate correlated fading envelopes, for use in: (1) the study of the impact of correlation on diversity system performance and (2) the study of multicarrier CDMA (MC-CDMA), where the number of carriers notably exceeds the degree of system diversity.

High-throughput, high-performance OFDM via pseudo-orthogonal carrier interferometry spreading codes

The paper introduces to orthogonal frequency-division multiplexing (OFDM) systems a novel pseudo-orthogonal carrier interferometry spreading code which spreads each parallel data stream over all the OFDM carriers. Pseudo-orthogonal carrier interferometry (PO-CI) spreading codes are carefully selected to introduce the following benefits to OFDM: up to 2N parallel data streams can be coded onto N carriers, with little degradation in performance; when rate 1/2 channel coding is applied in addition to PO-CI spreading codes, the resulting binary phase-shift keying OFDM systems demonstrate the performance of coded OFDM and the throughput of uncoded OFDM; PO-CI codes are carefully selected to spread in a manner which eliminates the peak-to-average power ratio problems characteristic of traditional OFDM.

Overcoming peak-to-average power ratio issues in OFDM via carrier-interferometry codes

OFDM (orthogonal frequency division multiplexing) is susceptible to high peak-to-average power due to an unstable envelope. Many solutions have been utilized in order to decrease the high peaks that are possible, but in these cases complexity is also added to the system architecture. In our earlier work, we introduced CI codes as a powerful tool to increase OFDM performance. This paper shows how carrier interferometry phase coding eliminates peaks in the signal envelope and in effect the problems associated with large PAPR.

Introduction of carrier interference to spread spectrum multiple access

This paper introduces a new scheme for spread spectrum multiple access. Like MC-CDMA, this scheme accomplishes spectral spreading by transmission of identical data over N carriers simultaneously. However, unlike any existing CDMA technique to date, this method supports user orthogonality not through the use of spreading codes (based on PN sequences), but rather through multiple carrier interference. Specifically, in this novel method, aptly named CIMA (carrier interference multiple access), the interference of multiple carriers enables user orthogonality or pseudo orthogonality based on user positioning in time. It is shown that CIMA supports simplified receiver structures for AWGN channels, and offers performance benefits in AWGN and fading environments.

Large set of CI spreading codes for high-capacity MC-CDMA

In this letter, a large set of spreading codes that doubles capacity in multicarrier code-division multiple-access (MC-CDMA) systems without any cost in bandwidth and with negligible cost in performance is introduced. This large set is comprised of: 1) complex spreading codes instead of conventional real-valued spreading codes and 2) two sets, each made up of orthogonal complex spreading codes, with minimum cross correlation between sets. Simulations performed over Rayleigh fading channels demonstrate 100% gains in terms of MC-CDMA capacity with negligible loss in performance.

Generalized fading channel model with application to UWB

To date, a number of statistical UWB channel models have been proposed. However, when compared with measurement data, inconsistencies are observed between theoretical model and practical measurement. In this paper, a novel statistical UWB channel model is proposed based on current measurement results, one which overcomes the inconsistencies of previous modelings. Some key features of the proposed model include: (1) the arrival time of each resolvable path is calculated using a 2-state Markov model-each state of the model characterizes a different likelihood of arrival based on a unique Poisson distribution (this accounts for the clustering property of paths observed in UWB channel measurements), and (2) individual multipath components demonstrate powers that follow a gamma distribution with an average power determined by an exponential decay. Furthermore, the proposed channel model represents a generalized model. By simply updating the initial parameter setting, other commonly used channel models (both narrowband and wideband) can be derived from our channel model.

Throughput enhancement in TDMA through carrier interferometry pulse shaping

This paper introduces a novel TDMA scheme that provides enhanced throughput by employing carrier interferometry pulse shapes (CI pulse shapes). At the transmitter, CI pulse shapes are created from the superposition of N carriers, which generates a short mainlobe (pulse) in time. CI pulse shapes are positioned both orthogonally and pseudo-orthogonally in time, enabling the introduction of additional bits into a TDMA burst. Specifically, up to a 100% increase in throughput can be achieved. At the receiver, a novel TDMA detector is deployed where: the pulse shape is broken down into its frequency components and optimally recombined to create frequency diversity benefits. Simulations performed over hilly terrain (HT) and typical urban (TU) GSM channel models indicate that, with a 100% increase in throughput, the proposed system offers up to 6.5 dB performance gains at probability of error of 10/sup -2/ relative to a standard GSM system employing a decision feedback receiver.

Narrowband interference rejection in OFDM via carrier interferometry spreading codes

OFDM (orthogonal frequency division multiplexing) has gained a great deal of attention of late and is considered a strong candidate for many next generation wireless communication systems. However, OFDM (in its current implementation) does not demonstrate robustness to narrowband interference. In our earlier work, we showed how carrier interferometry (CI) spreading codes may be applied to spread OFDM symbols over all N subcarriers. This allows OFDM to exploit frequency diversity and improve performance (without loss in throughput). In this paper, we show that, by spreading OFDM symbols over all N subcarriers via CI spreading codes, the resulting CI/OFDM system is also capable of suppressing narrowband interference. Simulation results over AWGN and multi-path fading channels confirm CI/OFDMs robustness in the presence of narrowband interference.

Innovative pulse shaping for high-performance wireless TDMA

This letter introduces a novel pulse shaping scheme that enables receivers to demonstrate high performance in wireless fading environments. Called carrier interferometry pulse shaping, pulses are created by the superposition of N carriers. At the receiver, low probability-of-error performance is achieved by breaking the pulse into its frequency components and optimally recombining to create frequency diversity benefits. When implemented in a wireless TDMA system, simulations indicate 5-8-dB improvement at probability-of-error of 10/sup -2/ over traditional Gaussian pulse shaping with decision feedback equalization (DFE(6,4)) in hilly terrain (HT) and typical urban (TU) channels.