Number Of Simultaneous Active Users Research Articles

Interference Characterization in Underlay Cognitive Networks With Intra-Network and Inter-Network Dependence

Interference modeling in cognitive radio network is important to ensure adequate coverage in the network. A reliable interference model, however, depends on accurately characterizing the distribution of users. In this paper, the dependence between primary and secondary networks is examined in order to capture more system parameters related to system characterization. Hence, two cases are considered - primary user (PU) interference control and PU with secondary user (SU) interference control mechanisms. Under PU interference control, distributions of PUs follow the Matern hard core process while the distribution of SUs follow the Poisson hole process (PHP). However, under PU with SU interference control, the distribution of active SUs follow a modified PHP. Bound and approximate expressions were derived for coverage probability at both primary and secondary networks, while simple yet accurate expressions were obtained to depict the number of simultaneous active users supported for the two cases. The tight closeness between the bound and the approximate expressions shows the reliability of the presented theoretical analysis. Furthermore, the bipolar network model assumption was relaxed while the case of independence assumption among users was also considered. Numerical result showed close tightness when the bipolar network model assumption was relaxed while the independence assumption was shown to overestimate users' coverage probability.

Experiment of wireless optical communication applied to hybrid SAC/PDM OCDMA scheme with FBG-based fiber lasers

In this study, hybrid spectral amplitude coding and polarization division multiplexing based optical code-division multiple access (SAC/PDM-based OCDMA) was proposed for transmitting an optical-coded and polarized signal over a wireless transmission channel. The experimental results showed that the desired signal easily identifies either a horizontal or a vertical polarized state and then extracts the M-sequence of the signature address code correctly. Hence, compared with SAC-OCDMA schemes, the spectral efficiency and number of simultaneous active users of the proposed hybrid SAC/PDM-based OCDMA scheme are twice those of the conventional SAC-OCDMA scheme.

Fairness and Rate Coverage of Symmetric Transmission over Heterogeneous Cellular Networks under Diverse Coupling and Association Criteria

Human communication, received through physical senses, is often a symmetric service. On other occasions, only certain feedback is needed for multisensory communication but even in these cases a minimum throughput is needed in the uplink for signaling purposes. In this paper, the joint uplink (UL) and downlink (DL) binary rate that can be achieved by realistic spectral efficiencies as signaled by channel quality indicators in LTE have been studied. Specifically, symmetric binary rate gathers both uplink and downlink capacities while forcing them to be reached simultaneously in order to carry the bidirectional information flow. We assume a two-tier heterogeneous cellular network with macro and pico Base Stations (BS) and User Equipments (UE) uniformly distributed. Downlink transmission to a UE could be associated to its nearest base station or to that BS from which the average received power is maximum. In the later case, certain bias could be added to the power received from the small cells in order to offload users from macro to pico BSs. Realistic limits to users simultaneously active in a cell are included in the evaluation. Regarding uplink, it has been considered two options. As current networks, uplink association can be coupled to that in downlink. However, uplink association might also be decoupled from that of DL and UL transmission carried out over that path with minimum losses. This possibility would reduce interference after fractional power control (FPC). Results for rate coverage, spectral efficiency, and binary rate for marginal uplink, marginal downlink, and joint uplink/downlink has been obtained from simulations as a function of diverse system parameters. Besides averaging, fairness measurements have also been studied. Results show that taking into account limits on the maximum spectral efficiency and on the number of simultaneous active users within a cell strongly modifies results on joint binary rate. Although spectral efficiency of DL is maximum for average criteria, its DL binary rate is not necessarily the highest as many macro users get inactive. Moreover, for realistic 3GPP path loss models, decoupled DL/UL association does not improve results enough to be worthwhile.

Design and performance analysis in multiuser optical CDMA systems

The article presents comparative performance analysis of the proposed Optical CDMA system for 32 and 16 users with two dimensional codes. Numerical simulations have been done under interference significant environment, considering noise and dark current at data rates 2.5, 5, 7.5 and 10Gbps over single mode fiber for transmission distance of more than 270km. Results illustrate overall good performance, degraded with augment in bit rate and transmission distance, impervious with raise in number of simultaneous active users. Depicts significant performance improvement with inclusion of forward error correction RS (255,239), for low attenuation and chirp factor. It perceived, this is one of the efficient functional techniques for next generation broadband optical networks together with higher security owing to encoding and decoding, as it allow multiple users in the network to access the same fiber channel asynchronously.

Assessment of an Optical Burst Switched Core Node With a Restricted Number of Simultaneous Active Users Using the Burst Error Loss Rate

The technical superiority of spectral-amplitude coding optical code-division multiple-access (SAC-OCDMA) systems over traditional wavelength division multiplexing (WDM) systems in optical burst switched (OBS) networks is mainly attributed to the former’s better medium access control (MAC) layer performance. Nevertheless, in order to conduct an accurate comparison, a thorough study of the physical layer performance should be involved, especially because in many cases the physical layer noise would affect the maximum achievable number of simultaneously active users. Hence, in this work, we develop a novel assessment approach that combines both MAC and physical layer capabilities by introducing a new burst error loss rate parameter. In particular, the approach targets cases with limitations on the number of simultaneous active users. Next, as an example of a noisy physical layer, the effect of phase-induced intensity noise on the number of active users in OBS/SAC-OCDMA systems is analyzed. Our analysis shows that this effect introduces a burst error rate (BurstER) in the multiple-access interference cancellation operation (not investigated before). This BurstER is an increasing function of the number of active users and hence would suppress the system MAC layer performance. Finally, assuming an ideal WDM physical layer, we employ the developed approach to present an illustrative performance comparison between OBS/SAC-OCDMA and OBS/WDM systems. The results show that OBS/SAC-OCDMA performance outperforms that of OBS/WDM when the number of tolerated bits in error per burst exceeds a certain value.

An efficient MAI cancellation technique in Optical CDMA

Optical code division multiple access (OCDMA) is a promising operational system for future backhaul optical networks especially for switching and optical signal multiplexing. However, OCDMA operation is restricted by multiple access interference (MAI) due to the asynchronous nature of transmission. In this study, we discuss an efficient MAI technique for OCDMA systems that depends not only on using hybrid frequency shift keying (FSK) modulation but also on employing the double padded modified prime code (DPMPC) as a signature sequence. MAI cancellation is performed by subtracting a reference signal from the received signal of the desired user. The simulation results show that the performance of the FSK-OCDMA technique is superior to the performance of the PPM-OCDMA technique. Furthermore, the obtained results illustrate that the FSK-OCDMA technique is able to accommodate a large number of simultaneous active users with low error-rate.

Performance Analysis of Hybrid OCDMA/WDM System for Metro Area Network

In this study a hybrid spectral amplitude coding for optical code division multiple access (i.e., called OCDMA) over wavelength division multiplexing (WDM) network is proposed to support large number of subscribers. The proposed system combines two different techniques, namely OCDMA and WDM. The modified double weight (MDW) code is used as a signature address for designing the system. This code has numerous advantages such as this code can accommodate more number of simultaneous active users under considerable standard bit error rate (e.g. ≤ 10-9) and shorter code length. The proposed system can enhance the network capability to support high number of users. Therefore, this system can be considered as a promising solution for Metro Area Network (MAN).

Multiple access interference cancelation technique in optical CDMA systems

This paper presents a simple and efficient multiple access interference (MAI) cancelation technique in optical code division multiple access (OCDMA) system. The proposed technique is based on hybrid frequency shift keying (FSK) with an enhanced modified prime code as a signature sequence for coding techniques. Coherent FSK modulation along with incoherent demodulation using Arrayed-Waveguide Grating has been examined in the transceiver structure. In the proposed technique, a reference signal is constructed by using one of the addressed spreading sequences, and MAI cancelation is performed by subtracting the reference signal from the received signal of the desired user. The performance of the proposed FSK-OCDMA system is compared with the performance of the existing pulse position modulation (PPM)---OCDMA system. The simulation results reveal that the bit-error rate performance of the proposed technique is superior to the performance of the pulse position modulation (PPM) technique. Also, the results indicate that the proposed technique is very power efficient, and when the bit rate is constant, the network capacity can be expanded to accommodate a large number of simultaneous active users with low error rate. Moreover, the proposed technique simplifies the hardware of the receiver design.

Hybrid OCDMA over WDM System Using Modified Double Weight (MDW) Code for Optical Access Network

In this paper a hybrid optical code division multiple access (OCDMA) over wavelength division multiplexing (WDM) network is proposed to support large number of subscribers. The system combines two different techniques, namely OCDMA and WDM for supporting many subscribers simultaneously. We used modified double weight (MDW) codes as a signature address in designing the system because this code can accommodate more number of simultaneous active users under considerable standard bit error rate (e.g. ≤ 10-9). The induced MDW code for hybrid system can eliminate multiuser interference fully and increase the bit-error-rate performance compared to other codes at the same time. We ascertained by simulation results that the proposed scheme provides very good performance and enhance the network capability. Therefore, this system can be considered as a promising solution for the next generation optical access network.

Design and Cost Performance of Decoding Technique for Hybrid Subcarrier Spectral Amplitude Coding-Optical Code Division Multiple Access System

Problem statement: A hybrid technique combining subcarrier multiplexed and Spectral Amplitude Coding-Optical Code Division Multiple Access (SAC-OCDMA) deploying the Khazani-Syed code was proposed. It was proposed as a mean of increasing the maximum number of simultaneous active users by increasing the subcarrier and/or the SAC-OCDMA code word. Multiple Access Interference (MAI) is the most dominant cause of performance degradation in SACOCDMA systems. Approach: A decoding technique which was simple and easy to build named Spectral Direct Decoding (SDD) technique was proposed for the hybrid system of subcarrier SACOCDMA. Results: The comparison between the complementary subtraction and proposed SDD techniques for hybrid subcarrier SAC-OCDMA system in terms of cost performance was presented here. Results showed that the cost performance of the SDD technique offers a significant improvement over the complementary subtraction detection technique. Conclusion: It is shown that the cost performance of the hybrid system using SDD technique is significantly better than using the Complementary subtraction technique.

Improved phase noise performance using orthogonal ternary codes over spectral polarization and amplitude coding networks

This study proposes an improved hybrid scheme of spectral polarization coding SPC and spectral amplitude coding SAC based on a specified orthogonal ternary sequence. In a previous SPC scheme, the entire wavelength was assigned either a vertical or a horizontal state of polarization. However, this study assigns positive, negative, or null chip values to individual wavelengths to create an orthogonal ternary sequence based on the bipolar Walsh-Hadamard matrix. This approach reduces the number of wavelength collisions at the photodetector and hence reduces the phase-induced intensity noise PIIN. Neglecting the effects of shot noise and thermal noise, we consider only PIIN with a degree of polarization P=0. The proposed scheme improves the bit error rate by 40% and 100% over SPC and complementary unipolar SAC schemes. The number of simultaneous active users supported by the hybrid scheme for the worst case P=1 is improved by 41.3% over the complementary unipolar SAC coding scheme for the ideal case P=0. © 2007 Society of Photo-Optical Instrumentation Engineers. DOI: 10.1117/1.2432883 Subject terms: optical code-division multiple-access; hybrid spectral polarization and amplitude coding; fiber Bragg grating; polarization beamsplitter; multiple- access interference; phase-induced intensity noise.

Complementary bipolar spectral polarization coding over fiber-grating-based differential photodetectors

This study proposes a spectral polarization coding scheme for the implementation of complementary bipolar optical correlation in a noncoherent optical code-division multiple-access network. The coder- decoders are implemented over differential photodetectors based on fi- ber Bragg gratings and polarization beamsplitters. Traditionally, comple- mentary unipolar spectral amplitude coding SAC schemes are used to transmit the specific codeword on data bit 1 and its complementary codeword on data bit 0. In the proposed scheme, the spectral amplitude is incorporated with polarization coding as the specific signature address code. A Walsh-Hadamard code is employed as the signature address to allocate to each specified wavelength to an individual vertical or horizon- tal state of polarization. The results indicate that multiple-access interfer- ence can theoretically be eliminated by correlation subtraction at the differential photodetectors. Neglecting the effects of shot noise and ther- mal noise, and considering only phase-induced intensity noise with a degree-of-polarization setting of P=0 for the ideal case, the bit error rate BER versus the number of simultaneous active users is improved by 43% in the proposed scheme over the complementary unipolar SAC scheme for a 10 9 error probability. Setting P=1 for the worst case, the BER performance of the proposed scheme matches that of the comple- mentary unipolar SAC scheme. © 2006 Society of Photo-Optical Instrumentation