103,080 research outputs found
Multi-group frequency hopping OFDMA based on statistical multiplexing
No full textIn this paper, the multi-group frequency hopping OFDMA (MG-FH OFDMA) based on the statistical multiplexing is proposed for the downlink cellular system. Compared with the existed random frequency hopping OFDMA (RFH-OFDMA) system utilizing the statistical multiplexing, the proposed MG-FH OFDMA invokes the deterministic hopping pattern to reduce the number of subcarrier collisions. By dividing all users into different groups, the subcarriers are utilized sufficiently. Latin Square hopping pattern and user index updating scheme are applied to randomize the subcarrier collisions among users. The user capacity, defined as the maximum number of users served with a basic data-rate in a cell, is calculated with the consideration of intra-cell capacity and the other cell interference (OCI). Results show that the proposed MG-FH OFDMA achieves higher user capacity than that of the RFH-OFDMA
Bit error simulation of DQPSK for a slow frequency hopping CDMA system in mobile radio communications
No full textSpread spectrum (SS) techniques seem to be an attractive alternative to conventional narrow-band modulation. In this paper a frequency hopping (FH) system model based on slow FH (SFH) is proposed. The hopping pattern is in a pseudo-random manner through a set of completely independent channels. The simulated results of the bit error rate (BER) performance of the system for π/4 DQPSK modulation in the presence of additive white Gaussian noise (AWGN) and Rayleigh fading, together with Bose-Chaudhuri-Hocquenghem (BCH) coding and interleaving are obtained. Consideration is given to the optimum interleaving size, and the system performance in presence of both synchronous and asynchronous interferers
Slow Frequency-Hopping Multicarrier DS-CDMA for Transmission over Nakagami Multipath Fading Channels
No full textA novel multiple access scheme based on slow frequency hopping multicarrier direct-sequence code division multiple access (SFH/MC DS-CDMA) is proposed and investigated, which can be rendered compatible with the existing second-generation narrowband CDMA and third-generation wideband CDMA systems. The frequency hopping patterns are controlled by a set of constant-weight codes. Consequently, multirate communications can be implemented by selecting the corresponding sets of constant-weight codes having the required weights controlling the SFH patterns invoked. Two FH schemes, namely random and uniform FH, are considered and their advantages as well as disadvantages are investigated. We assume that the system operates in a multipath fading environment and a RAKE receiver structure with maximum ratio combining (MRC) is used for demodulation. The system’s performance is evaluated over the range of multipath Nakagami fading channels, under the assumption that the receiver has an explicit knowledge of the associated frequency-hopping (FH) patterns invoked. Furthermore, the performance of the SFH/MC DS-CDMA system is compared to that of the conventional single-carrier (SC) DS-CDMA system and that of the conventional MC DS-CDMA system, under the assumptions of constant system bandwidth and of constant transmitted signal power. Index Terms—Code division multiple access, constant-weight code, frequency hopping, Nakagami fading, orthogonal frequency division multiplexing
Blind Joint Soft-Detection Assisted Slow Frequency-Hopping Multicarrier DS-CDMA
No full textA novel multiple-access scheme based on slow frequency-hopping multicarrier direct-sequence, code-division multiple access (SFH/MC DS-CDMA) is proposed and investigated, which can be rendered compatible with the existing second-generation narrow-band CDMA and third-generation wide-band CDMA systems. Blind joint soft-detection of the SFH/MC DS-CDMA signals is investigated, assuming that the receiver has no knowledge of the associated frequency-hopping (FH) patterns invoked. The system’s performance is evaluated over the range of Nakagami multipath fading channels. The results show that blind joint soft-detection achieves the required bit-error rate performance, while blindly acquiring the FH patterns employed. This is advantageous during the commencement of communications or during soft handover. Index Terms—Blind detection, code-division multiple access, constant-weight codes, frequency-hopping, orthogonal frequency-division multiplexing
Time-Hopping Multicarrier Code-Division Multiple-Access
No full textA time-hopping multicarrier code-division multiple-access (TH/MC-CDMA) scheme is proposed and investigated. In the proposed TH/MC-CDMA each information symbol is transmitted by a number of time-domain pulses with each time-domain pulse modulating a subcarrier. The transmitted information at the receiver is extracted from one of the, say , possible time-slot positions, i.e., assuming that -ary pulse position modulation is employed. Specifically, in this contribution we concentrate on the scenarios such as system design, power spectral density (PSD) and single-user based signal detection. The error performance of the TH/MC-CDMA system is investigated, when each subcarrier signal experiences flat Nakagami- fading in addition to additive white Gaussian noise (AWGN). According to our analysis and results, it can be shown that the TH/MC-CDMA signal is capable of providing a near ideal PSD, which is flat over the system bandwidth available, while decreases rapidly beyond that bandwidth. Explicitly, signals having this type of PSD is beneficial to both broadband and ultra-wide bandwidth (UWB) communications. Furthermore, our results show that, when optimum user address codes are employed, the single-user detector considered is near-far resistant, provided that the number of users supported by the system is lower than the number of subcarriers used for conveying an information symbol
Production and performance of thin and thick film NTCR thermistors based on NiMn(_2)O(_4)+δ
No full textIn this study reliable film type NTCR thermistors based on NiMn(_2)O(_4)+δ were produced and their electrical properties were studied in detail. Electron-beam evaporation procedures have been applied to produce thin film NTCR thermistors. Phase pure NiMn(_2)O(_4)+δ target material was produced via a traditional ceramic precursor oxide route and thin films were deposited in an optimised procedure. The thickness distribution of evaporated films showed good agreement with a theoretical model, derived from evaporation theory and the sticking coefficient of the vapour on the substrates was approximately 80% ± 1.5%. The composition of electron-beam evaporated films was found to be not controllable in terms of the phase purity and the Ni : Mn ratio. In order to avoid these problems thick film NiMn(_2)O(_4)+δ NTCR thermistors were developed using direct screen-printing techniques. Detailed Rietveld refinement analysis was carried out for the source powder used for screen-printing. The main focus of the work was the measurement of resistance-temperature (R-T) characteristics of thin and thick films and pellets. In the temperature range of concern (77 K -550 K) conduction was found to be by variable-range hopping (VRH) and nearest-neighbour hopping (NNH); R ~ exp (TʆT)(^p), where the index p depends on the mode of hopping. Detailed analysis of R-T data showed that screen-printed films and pellets exhibited a p-value of 0.5, which was identified with VRH with a parabolic density of states (DOS) with an exponential dependence of resistance: R ~ exp (TʆT)(^0.5). For electron-beam evaporated films the mechanisms detected were NNH: R ~ exp (TʆT); and VRH with a constant DOS {p = 0.25) following: R ~ exp (TʆT)(_0.25). For screen-printed films with incorporated glass phase the electrical conduction mechanism was analysed using a.c. impedance spectroscopy and at low frequencies the hopping conduction was in agreement with the d.c. behaviour. The time constant of this mechanism could be described by an equivalent circuit containing a RC element. For higher frequencies a second mechanism was found, best described by a CRL element
Joshua Davis: Author of Spare Parts
No full textCitation: K-State First (2016). Joshua Davis: Author of Spare Parts [Flier]. Manhattan, Kansas: K-State First.Flyer advertising Joshua Davis's author talk at Kansas State University
Multistage Frequency-Hopping Assisted Ultra-Wideband Multiple-Access Communications
No full textUltra-WideBand (UWB) systems having a bandwidth on the order of a Gigahertz have recently received wide attention both in the USA and in Europe. In this contribution a novel UWB multiple access scheme based on the well-known family of Frequency-Hopping Multiple Access (FHMA) techniques is proposed, where the associated FH is implemented using MultiStage Frequency Hopping Multiple Access, or MS/UWB FHMA. We highlight the principles of the proposed MS/UWB FHMA communication system, investigate the associated spectrum assignment, the FH strategy, the applicable signal detection schemes as well as the achievable Bit Error Rate (BER) performance. Our study shows that MS/UWB FHMA is capable of supporting an extremely high number of users, while employing relatively simple receivers. PSfrag replacement
Low-Complexity Reduced-Rank Adaptive Detection in Hybrid Direct-Sequence Time-Hopping Ultrawide Bandwidth Systems
No full textAbstract — In this contribution reduced-rank adaptive minimum mean-square error multiuser detector (MMSE-MUD) is proposed and investi-gated for the hybrid direct-sequence time-hopping ultrawide bandwidth (DS-TH UWB) systems. The adaptive MMSE-MUD is operated based on the normalised least mean-square (NLMS) principles associated with using Taylor polynomial approximation (TPA)-assisted reduced-rank technique. It can be shown that the reduced-rank adaptive technique is beneficial to achieving low-complexity, high convergence speed and robust detection in hybrid DS-TH UWB systems. In this contribution bit-error-rate (BER) performance of the hybrid DS-TH UWB systems using proposed reduced-rank adaptive MMSE-MUD is investigated, when communicating over UWB channels modelled by the Saleh-Valenzuela (S-V) channel model. Our simulation results show that the TPA-assisted reduced-rank adaptive MMSE-MUD is capable of achieving a similar BER performance as that of the full-rank adaptive MMSE-MUD but with significantly lower detection complexity. I
Slow Frequency Hopping Assisted MC DS-CDMA using Large Area Synchronised Spreading Sequences
No full textThe family of Multi-Carrier Direct-Sequence CDMA (MC DS- CDMA) systems exhibits numerous attractive properties, which render them attractive candidates for next-generation wireless communications. We demonstrate that spreading codes exhibiting a so-called interference-free window (IFW) are capable of outperforming classic spreading codes, when the interfering multi-user and multipath components arrive within this IFW. The best possible quasi-synchronous timing of the spreading sequences has to be adjusted with the aid of accurate adaptive timing advance control, which has to be significantly more accurate than that used in the lower-bit-rate second-generation GSM system. Fortunately, the IFW duration may be extended with the advent of multi-carrier DS-CDMA proportionately to the number of subcarriers. Hence the resultant MC DS-CDMA system is capable of exhibiting a near-single-user performance without employing a multi-user detector. A deficiency of the resultant system is that the number of spreading codes exhibiting a certain IFW is limited and so is the IFW duration. This contribution sets out to mitigate the above-mentioned shortcomings so that when the users' delays are in the range of the IFW, we separate them with the aid of the unique, user-specific LAS spreading codes. By contrast, when the users roam at a high distance from the base-station and hence their received signal arrive outside the range of the IFW, we separate them using their unique frequency hopping patterns
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