Spectral Efficiency Enhancement Research Articles

Spectral and power efficiency trade‐offs for multiple‐antenna reception techniques under co‐channel interference

ABSTRACTSpatial receive diversity and beamforming are two competing multiple‐antenna reception strategies for enhancement of spectral and power efficiency of wireless communication systems because they both use the same resource, that is the spatial degrees of freedom. We analyse in this paper ergodic, block‐fading outage and delay‐limited outage spectral efficiencies as well as average bit error rates and service outage probabilities of a hybrid diversity‐beamforming receiver with antenna partitioning over flat Rayleigh‐fading uplink channels under co‐channel interference. The results validated by Monte Carlo simulations decipher the trade‐offs in terms of spectral and power efficiencies in between all‐diversity, hybrid diversity‐beamforming and all‐beamforming reception for different scenarios represented by average signal‐tonoise ratio per path and desired signal‐to‐interference ratio for using these multiple‐antenna reception techniques in interference‐limited wireless communication systems. Copyright © 2011 John Wiley & Sons, Ltd.

Enhancing spectral efficiency and capacity in synchronous OCDMA by transposed-MPC

This paper investigates a newly proposed spreading code-set based on the prime code (PC) families referred to as ‘transposed modified prime codes (T-MPC)’ for the enhancement of spectral efficiency of synchronous incoherent optical code-division multiple-access (OCDMA) networks. This code family increased the code-set cardinality up to twice of existing PC families’ size. This also implies that a greater number of users can be accommodated by the network. Since there is no longer a time-shift feature in T-MPC like in conventional modified prime codes (MPC), the code is not predictable and thus even more secure. Since the code structure is similar to MPC, its deployment in a system/network already running MPC will not require hardware modification. Due to the higher code utilization factor of T-MPC, a greater number of users are accommodated under certain bit-error rate (BER) resulting in remarkable improvement in the spectral efficiency (SE) and capacity. The T-MPC compatibility with low-weight energy-efficient MPC construction is also investigated. The BER and SE performances are analyzed and compared with existing code families. The results indicated that the T-MPC employment can improve up to 50% higher spectral efficiency.

Offset-QAM based coherent WDM for spectral efficiency enhancement

Optically multiplexed multi-carrier systems with channel spacing reduced to the symbol rate per carrier are highly susceptible to inter-channel crosstalk, which places stringent requirements for the specifications of system components and hinders the use of high-level formats. In this paper, we investigate the performance benefits of using offset 4-, 16-, and 64-quadrature amplitude modulation (QAM) in coherent wavelength division multiplexing (CoWDM). We compare this system with recently reported Nyquist WDM and no-guard-interval optical coherent orthogonal frequency division multiplexing, and show that the presented system greatly relaxes the requirements for device specifications and enhances the spectral efficiency by enabling the use of high-level QAM. The achieved performance can approach the theoretical limits using practical components.

Reduced Complexity Subcarrier Allocation Schemes for DFT-Precoded OFDMA Uplink Systems

A resource allocation problem of maximizing total achievable rates for multi-user DFT-precoded orthogonal frequency division multiplexing (OFDM) uplink systems is investigated in this paper. A suboptimal subcarrier allocation scheme is proposed, which uses a proposed spectral efficiency enhancement parameter as an index to assign subcarriers to users. The proposed scheme has a lower computational complexity than that of the existing scheme which is shown to be close to the multiuser waterfilling solution. The simulation results show the proposed suboptimal scheme outperforms the existing scheme. Besides, an iterative subcarrier allocation scheme is proposed to improve the performance.

Enhancement of spectral stability and efficiency on blue light-emitters via introducing dibenzothiophene- S, S-dioxide isomers into polyfluorene backbone

Dibenzothiophene- S, S-dioxide isomer (FSO) monomers were copolymerized with extensively employed 9,9-dioctyl-2,7-fluorene monomer by Suzuki polycondensation. By introducing FSO unit into polyfluorene backbone, the spectral stability and efficiency of the blue-emitting polyfluorenes are significantly improved. All of the obtained copolymers and their devices exhibit stable photoluminescence (PL) and electroluminescence (EL) spectra, respectively, upon change of thermal annealing temperature and current densities. Furthermore green emission which is usually associated with excimer/aggregation or defects is absent in PL and EL spectra. In fact, the FSO unit acts as a deep trap for electron in the polyfluorene backbone, which could suppress the long wavelength emission. Moreover, the FSO unit lowers LUMO energy levels of the polyfluorenes, thus can maintain a balanced charge carrier injection and transport in the device based on the polymers, and therefore improves the device efficiencies. The device based on PF-2,8FSO1 and PF-3,7FSO5 show an external quantum efficiency (EQE) of 3.6% and a luminance efficiency (LE) of 3.7 cd/A with CIE coordinate of (0.16, 0.07), and an EQE of 3.8% and a LE of 4.6 cd/A with CIE coordinate of (0.15, 0.12), respectively. The results indicate that poly (dibenzothiophene- S, S-dioxide -co-9,9-dioctyl-2,7-fluorene)s could be a promising candidate for blue-emitting polymers with spectral stability and high efficiency.

Spectral Efficiency Enhancement in Multi-Channel Systems Using Redundant Transmission and Diversity Reception

We first consider conventional multi-channel systems with fixed-power variable-rate transmission of independent data streams as a function of channel variations. In these conventional systems, no data is transmitted through the sub-channels whose received signal strength is below a certain cutoff threshold. We then propose new hybrid adaptive modulation and diversity schemes for multi-channel transmission systems. The proposed hybrid schemes attempt to increase the overall spectral efficiency by (i) transmitting redundant information symbols through the deeply faded sub-channels that are not to be used in conventional multi-channel systems and then (ii) diversity combining these replicas at the receiver end. We evaluate the performance of the newly proposed hybrid schemes and compare their spectral efficiency with that of (i) the conventional scheme and (ii) a benchmark scheme which maximizes the overall spectral efficiency but is too complex for practical implementation. We show using some selected numerical examples that the proposed hybrid schemes offer spectral efficiency very close to that of the optimal scheme with much reduced complexity.

OFDM-Based Overlay Systems: A Promising Approach for Enhancing Spectral Efficiency [Topics in Radio Communications

Recently, several approaches for increasing spectral efficiency have been proposed to overcome the scarcity of spectral resources and to ensure that the ever-growing demand for high-data rate mobile communications can be met in the future. In this article, we focus on OFDM-based overlay systems that share a frequency band with already existing systems without degrading their performance. This is achieved by temporarily using those parts of the common frequency band that are currently not used by the licensed system. In particular, we focus on design issues for the physical layer as well as the medium access control layer and propose appropriate solutions for enabling coexistence. As a result, our investigations show that OFDM-based overlay systems are a promising approach for enhancing spectral efficiency.

Enhancing spectral efficiency of binary NRZ optical networks with electronic signal processing

We have analyzed a scheme using electrical prefiltering at the transmitter and electronic signal processing at the receiver to enhance the spectral efficiency of binary non-return-to-zero (NRZ) wavelength-division-multiplexed (WDM) networks consisting of typical terrestrial dispersion maps employing standard single-mode fiber (SSMF) and erbium-doped fiber amplifiers (EDFAs). Our analysis shows that up to 1.6 bits/s/Hz of spectral efficiency is achievable using this technique with orthogonal polarization launch of alternate channels in 10.7 Gb/s binary NRZ systems for transmission distances up to 800 km typically used in metropolitan optical networks.

Spectral efficiency of direct‐sequence code‐division multiple‐access in cellular mobile radio

In this paper, the spectral efficiency achieved by a direct-sequence code-division multiple-access (DS-CDMA) system operating in a cellular mobile radio environment is discussed. In contrast to previous works, here, spectral efficiency is examined in terms of channel capacity per user, estimated in an average sense, and the system's inherent diversity potential is considered. The analysis covers only the forward link, assuming a static model of operation for the considered DS-CDMA system. In addition, it is shown that the application of a power control scheme leads to spectral efficiency enhancement due to cochannel interference reduction. Hence, the derived expressions provide a means for quantitative evaluation of cellular DS-CDMA spectral efficiency with respect to multiple-access and cochannel interference. © 1997 John Wiley & Sons, Ltd.