In this study the authors present an in-depth study, analysis and discussion on maximum likelihood (ML)-based receiver for a typical spectrally encoded spread-time CDMA in the presence of multiple narrowband interference (NBI) signals in an additive white Gaussian noise channel. Furthermore, the authors demonstrate that by combining useful properties of ML-based receiver and three-level codes, that is, codes with values of −1, 0, +1, the authors can introduce a new strategy in which superior performance with respect to previous receiver structure based on two-level codes, that is, codes with values of −1, +1, can be attained. With the help of an example, the authors drive, first, the required signal-to-noise+interference power ratio (SNR) in a multiple access communication environment with multiple NBIs and, second, SNR attenuating parameter owing to multiple NBIs for two- and three-level codes. Finally, the authors relate the above parameters to a number of simple descriptive factors such as bit error rate. The results are validated by an extensive simulation on the aforementioned system.

In this letter we introduce an effective tool to demonstrate signal power behavior of a typical spectrally encoded spread-time (SE/ST) CDMA system using probabilistic approach. By defining power level profile (PLP), as the probability of stretching instantaneous power from a pre-specified threshold, we can find novel upper bounds on the signal power profile. We discuss various properties owing to PLP and elaborate on the main parameters affecting the PLP value. We present three theorems, propose some simple upper bounds on the PLP, and give an insight on the main contributions of the results. We introduce a fundamental parameter, namely ß, as a key design parameter which relates the most important quantities affecting the PLP behavior. Finally, we propose a corollary demonstrating an approach to guarantee that the PLP value is set at zero. It is shown that using suitable distributions, we can also obtain some distribution gain, thereby enhancing the overall system performance.

In this letter we consider optimum code structure for positive optical code division multiple-access (optical CDMA) systems. Positive systems are a class of systems that operate with positive real numbers only. We consider the effect of multipleaccess interference in our model and show that code design for both On-Off Keying (OOK) and Binary PPM optical CDMA systems results in the same solutions. Furthermore, we show that a class of codes known as optical orthogonal codes (OOCs) are the best possible positive codes. In obtaining the results we define normalized divergence based on signal-to-multipleaccess interference ratio (SIR) for a multiple-access system in a useful manner and use it as our criterion to maximize the multiple-access capability of the codes. Finally, we demonstrate that BPPM/OOC can be considered as the closest counterpart of plusmn1 pseudorandom sequence in radio CDMA communication systems.

In this paper we study and analyze the performance of the RAKE receiver for a typical ultrawideband (UWB) spectrally-encoded spread-time (ST) CDMA in the presence of multiple Gaussian interference (GI) signals. We demonstrate that by combining useful properties of three-level codes, i.e., codes with values of -1, 0, +1, and RAKE receiver, we can introduce a new strategy in which it can result in superior performance when compared to previous results. In particular, following a discussion on three-level codes we can show that RAKE receiver with optimum multiplicative weights achieves a better performance when compared with simple two-level codes. With the help of an example we show various properties of RAKE receiver as well as three-level codes and relate these properties to a number of simple descriptive parameters of the communication media. Simulation results obtained in this paper indicate the precision with which the analytical results have been obtained in modeling the above system.

In this paper, we present a deep insight into the behavior of optical code-division multiple-access (CDMA) systems based on an incoherent, intensity encoding/decoding technique using a well-known class of codes, namely, optical orthogonal codes (OOCs). As opposed to parts I and II of this paper, where OOCs with cross-correlation$lambda = 1$were considered, we consider generalized OOCs with$1 leq lambda leq w$, where$w$is the weight of the corresponding codes. To enhance the performance of such systems, we propose that use of optical AND gate receiver, which, in an ideal case, e.g., in the absence of any noise source except the optical multiple-access noise, is optimum. Using some basic laws on probability, we present direct and exact solutions for OOCs with$lambda = 1,2,3,ldots,w$, with optical AND gate as receiver. Using the exact solution, we obtain empirical solutions that can be easily used in optimizing the design criteria of such systems. From our optimization scheme, we obtain some fresh insight into the performance of OOCs with$lambdageq 1$. In particular, we can obtain some simple relations between$ P_ e min$(minimum error rate),$L_min$(minimum required OOC length), and$N_max$(maximum number of interfering users to be supported), which are the most desired parameters for any optical CDMA system design. Furthermore, we show that in most practical cases, OOCs with$lambda = 2$or$3$perform better than OOCs with$lambda = 1$, while having a much bigger cardinality. Finally, we show that an upper bound on the maximum weight of OOCs are on the order of$sqrt2lambda L$where L is the length of the OOCs used in systems.

In this paper, we present a deep insight into the behavior of optical code-division multiple-access (OCDMA) systems based on an incoherent, intensity encoding/decoding technique using a well-known class of codes, namely, optical orthogonal codes (OOCs). As opposed to parts I and II of this paper, where OOCs with cross-correlation lambda=1 were considered, we consider generalized OOCs with 1leslambdalesw, where w is the weight of the corresponding codes. To enhance the performance of such systems, we propose the use of an optical and logic gate receiver, which, in an ideal case, e.g., in the absence of any noise source, except the optical multiple-access interference, is optimum. Using some basic laws on probability, we present direct and exact solutions for OOCs with lambda=1,2,3,...,w, with the optical and logic gate as receiver. Using the exact solution, we obtain empirical solutions that can be easily used in optimizing the design criteria of such systems. From our optimization scheme, we obtain some fresh insight into the performance of OOCs with lambdages1. In particular, we can obtain some simple relations between Pemin(minimum error rate), Lmin(minimum required OOC length), and Nmax(maximum number of interfering users to be supported), which are the most desired parameters for any OCDMA system design. Furthermore, we show that in most practical cases, OOCs with lambda=2,3 perform better than OOCs with lambda=1, while having a much bigger cardinality. Finally, we show that an upper bound on the maximum weight of OOCs are on the order of radic2lambdaL where L is the length of the OOCs