Interference Free Window Research Articles

On the performance of the Code Division Duplex system using interference rejection codes

In this paper, we investigate the performance of the Code Division Duplex (CDD) system using the interference rejection codes, which exhibit zero correlation values on the relative delay-induced code offsets and can be applied to mitigate the mutual interference between the uplink and downlink. Specifically, we propose to employ loosely synchronous codes to effectively combat the interference through the interference-free window. Moreover, the simplified RAKE structure is proposed as the receiver of the CDD device, which can significantly reduce the complexity in the implementation. The simulation results demonstrate that our proposed method can achieve a near inference-free performance when the user load of the system is moderate.

Sequence Sets in Wireless Communication Systems: A Survey

Sequences and their properties have been studied widely in different research areas because they have useful characteristics which are used in many applications. Correlation functions such as Golay correlation functions are a measure of similarity among sequences and, besides, they are powerful functions used extensively in many research areas. It is known that unitary sequence sets with ideal autocorrelation and cross-correlation functions do not exist, and this represents the main problem to construct orthogonal sequences given that are limited by bounds. In wireless communication systems (WCSs), orthogonal sequences are used as access codes which play a very important role to increase capacity and to provide higher robustness to interference. This survey describes the properties of these sequences and their relationships that make them attractive to be used in communication systems. We consider the correlation properties to classify and describe four main families of sequences: 1) traditional; 2) complementary; 3) interference free window; and 4) complex. Finally, a discussion on their advantages and disadvantages is given considering the WCS scenario, and the discussion leads to some challenges for future applications which offer some ideas to motivate research to improve those properties.

Interference Free Window (IFW) in Las Cdma System and Comparison with Traditional CDMA

Large area synchronous code-division multiple access (LAS-CDMA) is a proposed fourth generation cellular standard. The feature that distinguishes LAS-CDMA from traditional CDMA is the new set of codes used to separate users. LAS code has zero cross-correlation in the Zero Interference Window and has only an impulse auto- correlation in the Zero Interference Window. These excellent features of the technique help to eliminate

Spectrum Efficiency Enhancement in Dynamic Space Coded Multiple Access (DSCMA) System

In cellular mobile communication systems using coded modulations, the spectrum efficiency of the system is related to the number of available codes. Recently, large area synchronous (LAS) CDMA codes, which exhibit a region called interference free window (IFW) within some delay-spread, have been accepted as one of fourth-generation (4G) wireless communication systems. However, the number of synthesized LAS codes is very low due to the low duty ratio of the sequence. In this paper, a dynamic space coded multiple access (DSCMA) scheme which utilizes the spatial diversity from smart antenna system is proposed to overcome the low spectrum efficiency in LAS CDMA system. In the DSCMA a modified version of LAS codes called LAS even ternary (LAS-ET) codes is proposed. These codes are used together with a novel algorithm called dynamic space code (DSC), which will decrease the code length so that the spectrum efficiency can be increased. By taking advantage of dynamic code reuse assignment in spatial diversity, the spectrum efficiency of the DSCMA can be further increased significantly.

Efficient Correlator for LS Codes Generated from Orthogonal CSS

This paper presents a novel method to correlate loosely synchronized (LS) codes generated from mutually orthogonal complementary sets of sequences (CSS). The proposed correlator significantly reduces the total number of multiplications and additions to be performed, in comparison with straightforward correlators. This fast correlator, together with the interference free window that LS codes exhibit, make them a good choice for next generation of Wireless communications.

Space-Time Complementary Coding MIMO with Joint Spatial Diversity and Multiplex Capability

This paper introduces a new space-time coding scheme, namely space-time complementary coding (STCC) scheme. With the help of pair-wise complementary codes, an STCC MIMO system can achieve both spatial diversity and multiplex transmission. The scheme requires neither bandwidth expansion nor feedback information from receiver, and it offers a nearly interference-free and multipath-resist performance due to a widely open interference-free window of the pair-wise complementary codes. The analytical results reveals that it provides a uniquely high diversity-multiplex gain product for its applications in flat or frequency-selective fading channel.

Analytical bit error rate performance of DS-CDMA ad hoc networks using large area synchronous spreading sequences

The performance of large area synchronous (LAS) direct sequence code division multiple access (DS-CDMA) assisted ad hoc networks is investigated in the context of a single-hop infinite mesh of rectilinearly located ad hoc nodes. It is shown that LAS DS-CDMA exhibits a significantly better performance than the family of traditional spreading sequences used in a quasisynchronous DS-CDMA scenario having a low number of resolvable multipath components and a sufficiently high number of RAKE receiver branches. The benefits of LAS codes in ad hoc networks are multifold: (i) Their performance is noise-limited, rather than interference-limited, provided that the multipath and multi-user interference arrives within their interference free window. (ii) Under the same conditions LAS codes are robust against the ‘near–far’ effects imposed by ad hoc networks operating without base-station-aided power control, without accurate synchronisation and without implementationally complex interference cancellers.

On the uplink performance of LAS-CDMA

In this paper large area synchronized (LAS)-CDMA is investigated, which exhibits a significantly better performance than the traditional random code based DS-CDMA system. Closed-form formulas are derived for characterizing the BER performance of LAS-CDMA as a function of the number of resolvable paths L/sub p/, the maximum delay difference T/sub max/ and the number of users K when communicating over a Nakagami-m fading channel. Moreover, the limited number of available LAS codes having a certain interference free window width suggests that the employment of LAS-CDMA is beneficial in a low-user-load scenario, where a near-single-user performance can be achieved without a multiuser detector.

Design of spreading codes for quasi-synchronous CDMA with intercell interference

The recently proposed loosely synchronized (LS) spreading code can in principle realize an intracell-interference-free quasi-synchronous code-division multiple-access (QS-CDMA) system by creating a wide enough interference-free window (IFW). However, the problem of minimizing intercell interference (ICI) in a cellular QS-CDMA system remains an open issue. Addressing the problem from a sequence design viewpoint, the key challenge is how to generalize the known construction of a single LS code to the design of many families of generalized LS (GLS) codes so that a desirable code family can be selected for the realization of a low-ICI cellular QS-CDMA system. Our main contribution is a systematic construction of new families of GLS codes with favorable intercode cross-correlation properties within a certain window, while maintaining the desirable IFW property. Many such code families can be obtained from our construction by choosing different Hadamard matrices and different uncorrelated complementary pairs. Their effectiveness with respect to some meaningful evaluation criteria are compared. In particular, by a simplified system bit-error rate analysis, it is demonstrated that a new GLS code family significantly outperforms the conventional scrambled LS codes.

Interference-free broadband single- and multicarrier DS-CDMA

The choice of the direct sequence spreading code in DS-CDMA predetermines the properties of the system. This contribution demonstrates that the family of codes exhibiting an interference-free window (IFW) outperforms classic spreading codes, provided that the interfering multi-user and multipath components arrive within this IFW, which may be ensured with the aid of quasi-synchronous adaptive timing advance control. It is demonstrated that the IFW duration may be extended with the advent of multicarrier DS-CDMA proportionately to the number of subcarriers. Hence, the resultant MC DS-CDMA system is capable of exhibiting near-single-user performance without employing a multi-user detector. A limitation of the system is that the number of spreading codes exhibiting a certain IFW is limited, although this problem may be mitigated with the aid of novel code design principles

Spectral efficiency of distributed MIMO code division multiple access systems over multipath fading channels

AbstractDistributed multiple‐input multiple‐output (MIMO) system is a new architecture for next generation wireless communication systems with distributed multiple antennas. In this paper, an uplink model for multiuser distributed MIMO systems with wideband code‐division multiple‐access (CDMA) technology is presented. We focus on the case where there is no channel state information at the transmitter, but channel state information is assumed at the receiver. The spectral efficiency of the uplink system with single‐user detectors at the base stations under multipath fading channels is investigated carefully. Two cases with different assumptions of interference knowledge are studied in detail. The spreading sequences of the CDMA systems are assumed to employ loosely synchronized (LS) spreading sequences and random spreading sequences respectively. LS spreading sequences exhibit a so‐called interference free window (IFW), where both the auto‐correlation and cross‐correlation of the codes become zero. Therefore LS sequences have the promise of mitigating the effects of inter symbol interference (ISI) and multiple access interference (MAI) in multipath fading channels. It is shown by numerical results that the uplink spectral efficiency of the distributed MIMO‐CDMA system using LS sequences can significantly outperform that of the system using random spreading sequences. Copyright © 2005 John Wiley & Sons, Ltd.

Loosely synchronised spreading code aided network performance of quasi-synchronous UTRA-like TDD and FDD CDMA systems

It is demonstrated that loosely synchronised (LS) spreading codes exhibit a so-called interference-free window, where both the autocorrelation and cross-correlation of the codes become zero. Therefore LS codes have the promise of mitigating the effects of both inter-symbol-interference and multiple-access-interference in time dispersive channels. Hence, LS codes have the potential of increasing the capacity of CDMA networks. The work reported has studied the achievable network performance and compared it to that of a UTRA-like time division duplex (TDD) and frequency division duplex (FDD) CDMA system using orthogonal variable rate spreading factor codes.

Mutually orthogonal sets of ZCZ sequences

In the construction of zero correlation zone (ZCZ) sequences, as the length of the interference free window (IFW) increases, fewer sequences are available and hence their utility in multiuser systems diminishes. To address this issue, it is shown that, for a given IFW, it is possible to design multiple sets of ZCZ sequences which are mutually orthogonal.

Analysis of barbiturates in human serum and urine by high-performance capillary electrophoresis—micellar electrokinetic capillary chromatography with on-column multi-wavelength detection

The analysis of barbiturates in human serum (or plasma) and urine by high-performance capillary electrophoresis-electrokinetic capillary chromatography with on-column fast-scanning multi-wavelength detection is discussed. The use of a buffer of ca. pH 8 and containing sodium dodecyl sulphate provides a medium suitable for fast and high-resolution separations of barbiturates. Seven barbiturates are characterized by their retention and absorption spectra between 195 and 320 nm. Comparison of these computer-stored data with those of unknown samples is shown to allow the identification of barbiturates in samples of patients undergoing pharmacotherapy and in toxicological urine and serum specimens. Three-dimensional electropherograms provide reliable information on the requirement and suitability of sample pretreatment procedures. With urine, extraction of barbiturates prior to analysis is necessary. With human serum several barbiturates, including phenobarbital, are shown to elute in an interference-free window in front of uric acid and the proteins, allowing these substances to be determined by direct sample injection. The need for multi-wavelength detection over a relatively wide wavelength range as a means of peak confirmation in electrokinetic capillary analyses is demonstrated and limitations of this technique for compounds with similar retention behaviour and absorption spectra are discussed.

Determination of lead-210 in admixture with bismuth-210 and polonium-210 in quenched samples by liquid scintillation counting

An aqueous acidic mixture of /sup 210/Pb, /sup 210/Bi, /sup 210/Po, in secular equilibrium, was separated into its components by using a series of sequential complexometric extractions. A multi-channel pulse height analyzer was used to determine the effects of varying amounts of a chemical quenching agent on the liquid scintillation spectrum of each of the separated fractions. The /sup 210/Pb fraction was free of contaminating /sup 210/Bi. A mathematical algorithm was developed to predict the percent of /sup 210/Pb activity (0.61% of the initial /sup 210/Bi activity) in the bismuth fraction by using Cerenkov counting, which is specific for /sup 210/Bi. The approach taken was to relate the slope of the linear relationship between the logarithmically transformed theoretical decay curve for pure /sup 210/Bi and the transformed theoretical decay curve for /sup 210/Bi, /sup 210/Pb mixtures to the percent of contaminating /sup 210/Pb in the mixture. Equations were also developed (1) to predict the liquid scintillation counting efficiency of /sup 210/Pb (relative to an unquenched sample) by relating the level of quenching to a spectral quench parameter (SPQ-(E)) as determined by counting an external (/sup 226/Ra) and (2) to correct for the presence of /sup 210/Bi counts in the /supmore » 210/Pb window by relating the /sup 210/Bi count rate in the /sup 210/Pb window to the ratio of the count rate in a /sup 210/Bi interference free window over the SPQ(E). When combined, these expressions resulted in an algorithm that can be used to correct for variable amounts of chemical quenching and for the presence of variable amounts of /sup 210/Bi in the /sup 210/Pb-containing sample.« less