Hossam M. H. Shalaby

Modified quadratic congruence codes for fiber Bragg-grating-based spectral-amplitude-coding optical CDMA systems

We have constructed a series of new code families for the spectral-amplitude-coding optical code-division multiple-access (CDMA) system, and proposed new transmitter and receiver structures based on tunable chirped fiber Bragg gratings (FBGs). The proposed system has been analyzed by taking into account the effects of phase-induced intensity noise, shot noise, and thermal noise. We have also compared the performance of this system with that of a former system where a Hadamard code is used. It has been shown that the new code families can suppress the intensity noise effectively and improve the system performance significantly. When the effective power is large (i.e., >-10 dBm), the intensity noise is the main factor that limits the system performance. When the effective power is not sufficiently large, thermal and shot noise sources become the main limiting factors and the effect of thermal noise is much larger than that of shot noise.

Chip-level detection in optical code division multiple access

A new detector for optical code-division multiple-access (CDMA) communication systems is proposed. This detector is called the chip-level receiver. Both ON-OFF keying (OOK) and pulse-position modulation (PPM) schemes, that utilize this receiver, are investigated in this paper. For OOK, an exact bit error rate is evaluated taking into account the effect of both multiple-user interference and receiver shot noise. An upper bound on the bit error probability for pulse-position modulation (PPM)-CDMA system is derived under the above considerations. The effect of both dark current and thermal noises is neglected in our analysis. Performance comparisons between chip-level, correlation, and optimum receivers are also presented. Both correlation receivers with and without an optical hardlimiter are considered. Our results demonstrate that significant improvement in the performance is gained when using the chip-level receiver in place of the correlation one. Moreover the performance of the chip-level receiver is asymptotically close to the optimum one. Nevertheless, the complexity of this receiver is independent of the number of users, and therefore, much more practical than the optimum receiver.

Performance analysis of optical synchronous CDMA communication systems with PPM signaling

Direct-detection optical synchronous code-division multiple-access (CDMA) systems with M-ary pulse-position modulation (PPM) signaling are investigated. Optical orthogonal codes are used as the signature sequences of our system. A union upper bound on the bit error rate is derived taking into account the effect of the background noise, multiple-user interference, and receiver shot noise. The performance characteristics are then discussed for a variety of system parameters. Another upper bound on the probability of error is also obtained (based on Chernoff inequality). This bound is utilized to derive achievable expressions for both the maximum number of users that can communicate simultaneously with asymptotically zero error rate and the channel capacity. Our results show that under average power and bit error rate constraints, there always exists a pulse position multiplicity that permit all the subscribers to communicate simultaneously. >

Multirate optical fast frequency hopping CDMA system using power control

This paper addresses the problem of real-time multimedia transmission in fiber-optic networks using code division multiple access (CDMA). We present a multirate optical fast frequency hopping CDMA (OFFH-CDMA) system architecture using fiber Bragg gratings (FBGs). In addition, we argue that, in multimedia applications, different services have different quality of service (QoS) requirements; hence, the user only needs to use the minimum required power to transmit the signal, such that the required signal-to-interference ratio (SIR) is met. We show that a variable bit rate optical communication system with variable QoS can be implemented by way of power control with great efficiency. Present-day multirate optical CDMA systems concentrate on finding the code structure that supports a variable rate system, neglecting the importance of the transmission power of active users on the multiple access interference (MAI) and, therefore, on the system capacity. We assign different power levels to each rate through a power control algorithm using variable optical attenuators, which minimizes the interference and, at the same time, provides variable QoS constraints for different traffic types. Although we are using a code family that preserves good correlation properties between codes of different lengths, simulations show a great improvement in the system capacity when power control is used.

New code families for fiber-Bragg-grating-based spectral-amplitude-coding optical CDMA systems

A series of new code families are constructed for spectral-amplitude-coding optical code division multiple access (CDMA) systems. Then structures of both the transmitter and the receiver in such a system are also proposed based on tunable chirped fiber Bragg gratings. Our analysis shows that the proposed new code families can suppress the intensity noise effectively and, hence, improve the overall system performance.

Complexities, error probabilities, and capacities of optical OOK-CDMA communication systems

The complexity in the hardware implementation of traditional optical code-division multiple-access correlation receivers with double optical hardlimiters is discussed. A comparison with the implementation of chip-level receivers is presented as well. In addition, the bit error probabilities and the throughput capacities for both chip-level and correlation systems (without hardlimiters) are derived and evaluated under code-correlation constraints equal to one and two. Our results reveal that chip-level receivers are much simpler and their performances are competitive with that of traditional correlation receivers with double optical hardlimiters. Further, the throughput capacity of chip-level systems can be increased by almost a factor of 3.4 when increasing the code-correlation constraint from one to two.

Optical CDMA random access protocols with and without pretransmission coordination

The link layer of an optical direct-detection code-division multiple-access (CDMA) packet network is considered. Two different protocols that need pretransmission coordination are proposed. A variation of the second protocol that does not need pretransmission coordination is discussed. Both system throughput and average packet delay are derived and investigated for two different receiver models: the correlation and chip-level receivers. Both multiple-access interference and the photodetectors shot noise are taken into account in the analysis. The case where the number of users exceeds the available number of CDMA codes is numerically investigated. Our results reveal that the proposed protocols yield competitive system throughputs when used with the correlation receivers. Further, significant improvement in the throughput is achieved when using chip-level receivers along with the second protocol.

Synchronous fiber-optic CDMA systems with interference estimators

Multiple-user interference estimation and cancellation techniques are proposed for synchronous optical code-division multiple-access (CDMA) communication systems. At the receiving end the multiple-user interference is estimated and is used to adapt a threshold level that is placed after an optical correlator. The special properties of the modified prime sequence codes are utilized in the estimation process. Four methods for adapting the threshold are proposed in this paper. The performance of the above system is analyzed and the bit error rates of the four adaptation schemes are compared to each other and to that of the system without cancellation. Our results reveal that the proposed system is very efficient in eliminating the effect of the multiple-user interference. Moreover, it is shown that the error floor which distinguishes the traditional systems is removed under the proposed schemes.

Performance analysis of an optical CDMA random access protocol

A random-access protocol for an optical direct-detection code-division-multiple-access (CDMA) packet network is proposed. This protocol is called a round-robin receiver/transmitter (R/sup 3/T) protocol. A mathematical description of the protocol is presented using a detailed state diagram. Several performance measures, namely, steady-state system throughput, protocol efficiency, and average packet delay, are derived based on the equilibrium point-analysis technique. The effect of several design parameters on the above performance measures have been examined with the aid of a set of numerical examples. In our numerical calculations chip-level models have been adopted at the receiver side. Our results reveal that an efficiency of more than 90% can be reached easily with mild system parameters. Further an asymptotic efficiency of /spl ap/ 100% is also achievable.

Direct detection optical overlapping PPM-CDMA communication systems with double optical hardlimiters

Direct detection optical code-division multiple-access (CDMA) communication systems involving overlapping pulse-position modulation (OPPM) is considered. Double optical hardlimiters placed before and after the correlator at the receiver side is proposed for this system. The performance (in terms of the bit error probability) of this system is evaluated taking into account the effect of both the multiple-user interference and the photodetector shot noise. Both the receiver dark current and thermal noise are ignored in our analysis since their effect is very minor. The performance of the above receiver is compared to that of the OPPM-CDMA correlator receiver without hardlimiters, OPPM-CDMA optimum receiver, and OOK-CDMA optimum receiver. Our results reveal that, for given pulsewidth and throughput constraints, significant improvement in the performance is acquired when adding double optical hardlimiters to the correlator of the OPPM-CDMA receiver. Moreover the performance of this system is asymptotically close to the optimum OPPM-CDMA system and is considerably superior to the optimum OOK-CDMA system. It is also reported that the capacity of the proposed system is about 5.3 times greater than that of the optimum OOK-CDMA system.