Lim Nguyen

Optical spectral amplitude CDMA communication

We report the first demonstration of bipolar coding techniques in the optical spectral domain for incoherent optical code division multiple access (CDMA) communication. Based on the modulation and detection principles that we have developed, the power spectrum of an erbium-doped superfluorescent fiber source was encoded using bipolar codes and decoded using an optical bipolar correlator. A CDMA testbed consisting of two encoders and one decoder was implemented with bulk optics and free-space transmission. Our measurements verify the correlations between the bipolar codewords and demonstrate the rejection of multiple access interference.

Self-encoded spread spectrum communications

We present a novel spread spectrum communication system for the secure transmission of digital information. The approach is based on the unconventional self-encoding principles that we have developed at the University of Nebraska for the modulation and detection of spread spectrum signals. The proposed system does not use pseudo-random codes, and is unique in that traditional transmit and receive code generators based on maximal-length sequences or chaotic signals are not needed. In fact, the enhanced transmission security arises not only due to the spread-spectrum nature of the signal, but also from the true randomness of the unique spectrum spreading and de-spreading processes. This paper describes the proposed system and presents the theoretical and simulation results of the system performance in an additive white Gaussian noise channel.

MAI and ICI of synchronous downlink MC-CDMA with frequency offset

We analyze the performance of code division multiple access (CDMA) systems with multicarrier (MC) that employ random spreading sequences in downlink frequency selective fading channels. We obtain the probability density function (pdf) of the multiple access interference (MAI) of CDMA systems and extend the results to MC-CDMA systems to determine the pdf of MAI, inter-carrier interference (ICI) and noise in terms of the number of users, the spreading factor, the number of sub-carriers and frequency offset. The results show that the effects of frequency offset vary with system loading. We consider the synchronous downlink of cellular MC-CDMA and derive a Gaussian approximation of the MAI and ICI to reduce the computational complexity in calculating the bit error rate (BER). The accuracy of the Gaussian approximation is examined by computer simulations.

Chip-interleaved self-encoded multiple access with iterative detection in fading channels

We propose to apply chip interleaving and iterative detection to self-encoded multiple access (SEMA) communications. In SEMA, the spreading code is obtained from user bit information itself without using a pseudo noise code generator. The proposed scheme exploits the inherent diversity in self encoded spread spectrum signals. Chip interleaving not only increases the diversity gain, but also enhances the performance of iterative detection. We employ user-mask and interference cancellation to decouple self-encoded multiuser signals. This paper describes the proposed scheme and analyzes its performance. The analytical and simulation results show that the proposed system can achieve a 3 dB power gain and possess a diversity gain that can yield a significant performance improvement in both Rayleigh and multipath fading channels.

Delayed Self-Heterodyne Phase Noise Measurements With Coherent Phase Modulation Detection

We present coherent phase modulation detection for laser phase noise measurements with delayed self-heterodyne method. The technique is demonstrated for the first time with the distributed feedback laser and external cavity laser. The results are within 15% of self-homodyne measurements using an optical coherent receiver.

Self-encoded spread spectrum modulation with differential encoding

In this paper we analyze self-encoded spread spectrum (SESS) modulation with differential encoding and decoding. We show that this structure eliminates the BER effect of error propagation. We also investigate maximum likelihood sequence detection (MLSD) of SESS sequence using the Viterbi algorithm. The results show that MLSD outperforms the feedback detector in terms of the BER performance.

Effects of phase noise of monolithic tunable laser on coherent communication systems

We investigate the effects of different phase noise processes of SGDBR laser on coherent systems. The SGDBR device operated well with QPSK modulation at 5 Gbaud, while the performance of 16-QAM was significantly degraded due to excess noise. The white FM noise mainly defines the ultimate performance of coherent reception, but the low frequency excess noise can potentially degrade the performance of systems that employ 16-QAM format at 5 Gbaud.

Phase Noise Characterization of SGDBR Lasers Using Phase Modulation Detection Method With Delayed Self-Heterodyne Measurements

This study employed the coherent phase modulation (PM) detection method to characterize the phase noise of a Sampled-Grating Distributed Bragg Reflector (SGDBR) laser via delayed self-heterodyne (DSH) measurements. We derived the formula for phase-error variance from the FM noise spectrum of the laser and performed measurements on the Distributed Feedback (DFB) and SGDBR lasers. The results confirmed that the DFB laser phase noise is well described by white FM noise while the SGDBR laser has additional frequency fluctuations below 400 MHz. The measurement and simulation results also demonstrated that the low frequency excess noise significantly broadens the linewidth of SGDBR lasers and can degrade the performance of coherent optical communication systems.

Photoelectric current distribution and bit error rate in optical communication systems using a superfluorescent fiber source

In spectrum-sliced wavelength division multiple access (WDMA) optical communication systems, excess noise due to the thermal nature of the broadband source, such as the superfluorescent fiber source (SFS), limits the signal-to-noise ratio (SNR) at the photodetector output. If the statistics of the detected signal are Gaussian, a common assumption, then the bit-error rate (BER) performance of the system is predicted to reach a minimum that cannot be improved by increasing the received optical power. The actual performance, however, depends on the true detection noise statistics. We have measured the statistical distribution of the photoelectric current from a PIN photodiode illuminated by a spectrum-sliced, erbium-doped SFS. The histogram of the measurement data is best described by a negative binomial model for the corresponding photoelectron count with thermal light. Consequently, the BER performance of a spectrum-sliced WDMA system using such a superfluorescent fiber source is not limited by the SNR, a fundamentally different result from that predicted using the Gaussian assumption.

MAI and ICI of Asynchronous Uplink MC-CDMA With Frequency Offset

In this paper, we analyze the performance of random spreading code-division multiple-access (CDMA) systems with a multicarrier in asynchronous uplink channels. We first derive the probability density function (pdf) of the multiple-access interference (MAI) plus noise and then extend the results to orthogonal frequency division multiplexing (OFDM) with intercarrier interference (ICI) that is caused by a frequency offset. We obtain the pdf of the MAI and the ICI plus noise under Rayleigh and frequency-selective fading as a function of the number of users and the spreading factor, as well as the number of subcarriers and the frequency offset. The bit-error-rate (BER) analysis shows that the power penalty from the frequency offset increases with the system loading. We develop the Gaussian approximation that provides an accurate estimation of the BER with reduced computational complexity comparing to the direct calculations using the pdf.