Electrical and Computer Engineering

This paper proposes an all-fiber fast optical frequency-hop code division multiple access (FFH-CDMA) for high-bandwidth communications. The system does not require an optical frequency synthesizer allowing high communication bit rates. Encoding and decoding are passively achieved by Bragg gratings, Multiple Bragg gratings replace a frequency synthesizer, achieving a hopping rate in tens of GHz. A main lobe sine apodization can be used in writing the gratings to enhance the system capacity and the spectrum efficiency. All network users can use the same tunable encoder/decoder design. The simultaneous utilization of the time and frequency domains offers notable flexibility in code selection. Simulations show that the encoder efficiently performs the FFH spread spectrum signal generation and that the receiver easily extracts the desired signal from a received signal for several multiple access interference scenarios. We measure the system performance in terms of bit error rate, as well as auto-to cross-correlation contrast. A transmission rate of 500 Mb/s per user is supported in a system with up to 30 simultaneous users at 10-9 bit error rate. We compare FFH-CDMA to several direct sequence-CDMA systems in terms of bit error rate versus the number of simultaneous users. We show that an optical FFH-CDMA system requires new design criteria for code families, as optical device technology differs significantly from that of radio frequency communications

We propose fast optical frequency-hop code division multiple access (FH-CDMA) for high bandwidth local area networks. Encoding and decoding are achieved by tunable fiber Bragg gratings. Frequency hopping offers many advantages compared to the previously proposed optical CDMA multiple access techniques. The simultaneous utilization of the time and frequency domains offers notable flexibility in the selection of codes, however code families previously developed for radio frequency (RF) communications are not directly applicable to an optical FH-CDMA system. We propose codes to meet the special constraints imposed by our encoding device and present theoretical and simulation results for their performance

We propose a new multirate optical communication system using optical fast frequency hopping CDMA (OFFH-CDMA) for multimedia applications in which different quality of services (QoS) are required. In this system, each user needs only to transmit the minimum required power to achieve a desired signal to interference ratio (SIR). We assign different power levels to each rate through an average interference-based power control algorithm using variable optical attenuators. Such an approach minimizes interference and at the same time provides variable QoS constraints for different traffic types. The simulation shows a great improvement in the system capacity

We demonstrate the effectiveness of multiuser detection for an ultra-wideband (UWB) pulse based direct sequence spread spectrum system using code division multiple access. Extensive simulations were run using channel soundings of the 2-8 GHz band collected in a residential setting and characterized by a high level of multipath fragmentation. We show that adaptive minimum mean square error (MMSE) multiuser detection (MUD) receivers are able to gather multipath energy and reject intersymbol and interchip interference for these channels to a much greater extent than RAKE receivers with 4 and 8 arms. We also demonstrate that the adaptive MMSE is able to reject an IEEE 802.11a OFDM interferer, even for SIR as severe as -30 dB. The adaptive MMSE exhibits only a 6 dB penalty in the 16-user case relative to the single user case. The practical RAKE receivers were incapable of effectively rejecting either strong narrowband interference or heavily loaded wideband interference. Even more moderate levels of interference caused significant performance degradation of RAKE receivers.