Performance of optimum wavelet waveform for DS‐CDMA chip waveform over QS‐AWGN channel

In this paper, we search for a better chip waveform based on orthogonal wavelets for direct sequence‐code division multiple access (DS‐CDMA) signals to improve the probability of error (P_e) performance with minimal signal bandwidth variations. First, we derive the P_e expression over a quasi‐synchronous additive white Gaussian noise channel for DS‐CDMA signals, which use various pulse shaping waveforms including orthogonal wavelets as chip waveforms. It is observed that this expression depends on the chip waveform. Then, we design an optimum wavelet by using the relationship between wavelets and filter coefficients to reduce the probability of error. The DS‐CDMA system using the optimum wavelet waveform results in a lower probability of error than those using the conventional chip waveforms such as raised cosine, half‐sine and rectangular waveforms. Especially, the P_e of the optimum wavelet‐based scheme becomes significantly better than those of the conventional chip waveforms‐based schemes under the heavy loading that is the case for commercial wireless systems. When the systems work with full load (i.e. the number of users equals the processing gain), the optimum wavelet‐based system results in 0.5, 2.1 and 4 dB better SNR values than those of the raised cosine, half‐sine and rectangular‐based systems, respectively, for a P_e value of 10^?3. Copyright © 2005 John Wiley & Sons, Ltd.     

Performance of optimum wavelet for DS‐CDMA chip waveform over multi‐path Rayleigh fading channels

In this paper, the probability of error (P_e) expression of asynchronous direct sequence‐code division multiple access (DS‐CDMA) signals using band‐limited chip waveforms is derived over multi‐path Rayleigh fading channels. In receiver, a matched filter‐based rake receiver in conjunction with maximum ratio combiner (MRC) is considered. Numerical values for the P_e are calculated for various chip waveforms including an optimum wavelet waveform. Analytical results are verified by conducting simulations. Results show that the optimum wavelet‐based scheme outperforms time‐limited raised cosine, half sine, rectangular and band‐limited square‐root raised cosine chip waveform‐based schemes in terms of the P_e and the capacity defined as the number of users per Hertz for a same P_e level. Copyright © 2006 John Wiley & Sons, Ltd.     

Multi‐carrier platform for wireless communications. Part 1: High‐performance,high‐throughput TDMA and DS‐CDMA via multi‐carrier implementations

Multi‐carrier technologies in general, and OFDM and MC‐CDMA in particular, are quickly becoming an integral part of the wireless landscape. In this first of a two‐part survey, the authors present the innovative transmit/receive multi‐carrier implementation of TDMA and DS‐CDMA systems. Specifically, at the transmit side, the pulse shape (in TDMA) and the chip shape (in DS‐CDMA) corresponds to a linear combining of in‐phase harmonics (called a CI signal). At the receiver side, traditional time‐domain processing (equalization in TDMA and RAKE reception in DS‐CDMA) is replaced by innovative frequency based processing. Here, receivers decompose pulse (or chip) shapes into carrier subcomponents and recombine these in a manner achieving both high frequency diversity gain and low MAI. The resulting system outperforms traditional TDMA and DS‐CDMA systems by 10–14 dB at typical BERs, and, by application of pseudo‐orthogonal pulse shapes (chip shapes), is able to double system throughput while maintaining performance gains of up to 8 dB. Copyright © 2002 John Wiley & Sons, Ltd.     

Performance Investigation of Space‐Time Block Coded Multicarrier DS‐CDMA in Time‐Varying Channels

In this letter, we evaluate the system performance of a space‐time block coded (STBC) multicarrier (MC) DS‐CDMA system over a time selective fading channel, with imperfect channel knowledge. The average bit error rate impairment due to imperfect channel information is investigated by taking into account the effect of the STBC position. We consider two schemes: STBC after spreading and STBC before spreading in the MC DS‐CDMA system. In the scheme with STBC after spreading, STBC is performed at the chip level; in the scheme with STBC before spreading, STBC is performed at the symbol level. We found that these two schemes have various channel estimation errors, and that the system with STBC before spreading is more sensitive to channel estimation than the system with STBC after spreading. Furthermore, derived results prove that a high spreading factor (SF) in the MC DS‐CDMA system with STBC before spreading leads to high channel estimation error, whereas for a system with STBC after spreading this statement is not true.     

Convolutionally‐Coded and Spectrum‐Overlapped Multicarrier DS‐CDMA Systems in a Multipath Fading Channel

Multicarrier DS‐CDMA is an effective approach to combat fading and various kinds of interference. In this paper, we present an overlapped multicarrier DS‐CDMA system, wherein each of the rate 1/M convolutionally‐encoded symbols is also repetition coded and transmitted using overlapped multicarriers. However, since the frequency spectrums of successive carriers are allowed to overlap, the transmission bandwidth is more efficiently utilized. The effect of the overlapping percentage between successive carriers of a multicarrier DS‐CDMA system on the performance is investigated to determine the overlapping percentage showing the best performance. We suggest two methods for sub‐band overlapping variation. One is to allow variation of sub‐band overlapping percentage when the total number of subcarriers is fixed. The other is to increase the number of sub‐bands (the number of repetitions R) with fixed subband bandwidth. Given a total number of subcarriers MR, we show that the BER variation is highly dependent on the roll‐off factor β of a raised‐cosine chip wave‐shaping filter irrespective of convolutional encoding rate 1/M and repetition coding rate 1/R. We also analyze the possibility of reduction in total multi‐user interference by considering the variation of both the roll‐off factor (0 < β ≤ 1) and the sub‐band overlapping factor (0 < λ ≤ 2), and show that the proposed system may outperform the multicarrier DS‐CDMA system in [3].     

Multi‐carrier platform for wireless communications. Part 2: OFDM and MC‐CDMA systems with high‐performance,high‐throughput via innovations in spreading

Multi‐carrier technologies in general, and OFDM and MC‐CDMA in particular, are an integral part of the wireless landscape. In this second part of a two‐part survey, the authors present an innovative set of spreading codes known as CI codes, and demonstrate how these significantly increase performance and capacity in OFDM and MC‐CDMA systems, all the while eliminating PAPR concerns. Regarding OFDM: the spreading of each symbol over all N carriers using CI spreading codes (replacing the current one symbol per carrier strategy) are presented. CI codes are ideally suited for spreading OFDM since, when compared to traditional OFDM, CI‐based OFDM systems achieve the performance of coded OFDM (COFDM) while maintaining the throughput of uncoded OFDM, and, at the same time, eliminate PAPR concerns. When applied to MC‐CDMA, CI codes provide a simple means of supporting 2N users on N carriers while maintaining the performance of an N‐user Hadamard Walsh code MC‐CDMA system, i.e., CI codes double MC‐CDMA network capacity without loss in performance. The CI codes used in OFDM and MC‐CDMA systems are directly related to the CI pulse (chip) shapes used to enhance TDMA and DS‐CDMA (see part 1): hence, the CI approach provides a common hardware platform for today's multi‐carrier/multiple‐access technologies, enabling software radio applications. Copyright © 2002 John Wiley & Sons, Ltd.     

A comparison of system performance using two different chip pulsesin multiple-chip-rate DS/CDMA systems

This paper describes a comparison of system performance using two different chip waveforms of spreading sequences in multiple-chip-rate (MCR) direct-sequence (DS)/code-division multiple-access (CDMA) systems. The chip pulses used in this study are closely related to the characteristics of output filter employed at transmitter. In general, the chip waveform is an important factor to determine the link performance. The raised cosine chip pulse with a roll-off factor of α will be adopted for IMT-2000 systems in order to reduce both the intersymbol effect and the spectral width of the modulated signal. However, due to the complexity of obtaining quantitative results on the performance of MCR-DS/CDMA systems, rectangular chip pulses are mainly utilized in performance analysis. Therefore, it is necessary to investigate the effect of the chip pulses used, i.e., a rectangular and a raised cosine chip pulses on system performance in order to evaluate MCR-DS/CDMA systems accurately. Thus, the effect of the chip pulses used on the performance in MCR-DS/CDMA systems is investigated in terms of the system capacity and blocking probability. It is shown that the system using a raised cosine chip pulse (i.e., RC system) supports at least 80% more capacity and 57% more traffic than that using a rectangular chip pulse (i.e., R system)     

A near‐Nyquist rate transmission: A new minimum‐bandwidth direct‐sequence code division multiple access scheme

Bandlimited direct‐sequence code division multiple access (DS‐CDMA) attracts much attention for its compact spectrum and the ability to suppress inter‐symbol interference. Among the various bandlimited DS‐CDMA systems available, minimum‐bandwidth DS‐CDMA (MB‐DS‐CDMA) is the only realizable Nyquist rate transmission system. But, MB‐DS‐CDMA only applies to certain kinds of spreading codes. Accordingly, this study proposes a modified DS‐CDMA structure which extends the application of MB‐DS‐CDMA to all common spreading codes at the expense of a negligible reduction in the transmission rate. Additionally, the bit error rate of the proposed schemes adopting either single‐user or multi‐user detection receiver is analyzed and compared with that of the commonly‐used raised‐cosine‐pulsed DS‐CDMA over multipath fading channels. The numerical results show that given a sufficiently large number of users, the bit error rate performance of modified MB‐DS‐CDMA is comparable to that of the raised‐cosine‐pulsed DS‐CDMA scheme; meanwhile, the realizable modified MB‐DS‐CDMA approaches the ultimate transmission rate.     

Outage performance of orthogonal space–time block code transmission in opportunistic decode‐and‐forward cooperative networks

Outage performance is analyzed for opportunistic decode‐and‐forward cooperative networks employing orthogonal space–time block codes. The closed‐form expressions of diversity order and the end‐to‐end outage probability at high signal‐to‐noise ratio regime are derived for arbitrary relay number (K) and antenna configuration (N antennas at the source and each relay, N_D antennas at the destination) under independent but not necessarily identical Rayleigh fading channels. The analysis is carried out in terms of the availability of the direct link between the source and the destination. It is demonstrated that the diversity order is min{N, N_D} ⋅ KN if the direct link is blocked, and if the direct link is available, the diversity order becomes min{N, N_D} ⋅ KN + NN_D. Simulation and numerical results verify the analysis well. Copyright © 2011 John Wiley & Sons, Ltd.     

Conception and Performance Analysis of Efficient CDMA‐Based Full‐Duplex Anti‐collision Scheme

Ultra‐high‐frequency radio‐frequency identification (UHF RFID) is widely applied in different industries. The Frame Slotted ALOHA in EPC C1G2 suffers severe collisions that limit the efficiency of tag recognition. An efficient full‐duplex anti‐collision scheme is proposed to reduce the rate of collision by coordinating the transmitting process of CDMA UWB uplink and UHF downlink. The relevant mathematical models are built to analyze the performance of the proposed scheme. Through simulation, some important findings are gained. The maximum number of identified tags in one slot is g/e (g is the number of PN codes and e is Euler's constant) when the number of tags is equal to mg (m is the number of slots). Unlike the Frame Slotted ALOHA, even if the frame size is small and the number of tags is large, there aren't too many collisions if the number of PN codes is large enough. Our approach with 7‐bit Gold codes, 15‐bit Gold codes, or 31‐bit Gold codes operates 1.4 times, 1.7 times, or 3 times faster than the CDMA Slotted ALOHA, respectively, and 14.5 times, 16.2 times, or 18.5 times faster than the EPC C1 G2 system, respectively. More than 2,000 tags can be processed within 300 ms in our approach.     

Bit error rate performance of Haar wavelet based scale‐code division multiple access (HW/S‐CDMA) over the asynchronous AWGN channel

In this paper, we study a recently proposed multirate system, called wavelet based scale‐code division multiple access (W/S‐CDMA). W/S‐CDMA depends on the code, time and scale orthogonality introduced by pseudo‐noise (PN) sequences, and wavelets. In this system, the channel is partitioned into different scales, and each scale into time slots. In addition, the PN sequences are used in each scale to identify multiple users. In W/S‐CDMA, each user is assigned a specific scale and PN sequence, and transmits its successive information symbols with its PN sequence and the wavelets in that scale. More symbols are transmitted in finer scales. We analyse the bit error rate performance of Haar wavelet based S‐CDMA (HW/S‐CDMA) over an asynchronous additive white Gaussian noise (AWGN) channel by using a conventional detector for deterministic PN sequences. The performance of the system is compared to that of an equivalent multirate CDMA (MR‐CDMA) system for Gold and Kasami PN sequences. Results show that HW/S‐CDMA outperforms MR‐CDMA. In addition, because of its suitable format HW/S‐CDMA is also capable of employing the optimal PN sequence families with limited number of sequences such as Kasami, Bent, etc. repeatedly in different scales. Copyright © 2006 John Wiley & Sons, Ltd.     

On feasibility examination of DS‐CDMA systems with imperfect successive interference cancellation

In DS‐CDMA systems with the successive interference cancellation (SIC) technique, there are K! possible decoding orders for K active users and the decoding order has considerable impact on system performance. Once the constraints on the received powers of mobile stations and the bit‐energy‐to‐interference‐power‐spectral‐density ratio requirements are satisfied under some decoding order of SIC, the system is feasible. Otherwise, if the constraints are violated under all possible decoding orders, the system is infeasible. It is highly time‐consuming to examine the system feasibility directly by using the usual exhaustive search method (ESM) for a system with even moderate number of users. In this paper, we propose an efficient approach for examining the feasibility of DS‐CDMA systems with imperfect SIC. The proposed approach has significantly lower computational complexity than that of ESM and thus benefits the quick decisions of admission control and/or scheduling, which are essential for Quality of Service provisioning in DS‐CDMA systems. Furthermore, we theoretically prove that the system under the resultant decoding order obtained by the proposed approach is able to achieve the lowest outage probability among all possible decoding orders. We conduct extensive simulation experiments, and the numerical results validate our analysis and demonstrate the effectiveness of our approach. Copyright © 2010 John Wiley & Sons, Ltd.     

Filter bank‐based multiple access in next‐generation satellite uplinks: A DVB‐RCS2‐based experimental study

In the context of the on‐going evolution of satellite communications (SatCom) systems to their next generation, and in the direction of their integration with fifth generation (5G) terrestrial networks, it is of interest to study in depth the applicability in realistic SatCom of waveforms that have shown promise to meet the 5G requirements. This paper presents a comparative study, based on total degradation (TD) over a range of output back‐off (OBO) values, on out‐of‐band emission and spectral efficiency, of frequency division multiple access (FDMA) schemes employing offset quadrature amplitude modulation‐based filter bank multi‐carrier (FBMC /OQAM), classical orthogonal FDMA (OFDMA), and their single‐carrier counterparts to illustrate the potential gains from the integration of the FBMC waveforms in the satellite context and standards. The air interface simulated follows the digital video broadcasting (DVB) family of standards for the satellite uplink, considering both time and frequency synchronization impairments and two typical input constellations. Our results confirm the superiority of the single‐carrier (SC) schemes in such a nonlinear environment. The SC‐FBMC waveform is shown to be the most practical candidate since it is shown to attain a TD performance similar to that of SC‐OFDM at absolutely no cost in spectral efficiency.     

Non‐linear chirp based UWB waveform design for suppression of NBI

In order to alleviate the narrowband interference (NBI) to ultra wideband (UWB) systems, we propose two non‐linear UWB chirp waveforms based on the arctrigonometric and archyperbolic function in this paper. The proposed UWB pulses can obtain good performance in NBI suppression. Both of the two chirp pulses require only the time domain processing because of the inherent relationship between the frequency domain and the time domain. Theoretical analysis and simulation results show that the direct sequence pulse binary amplitude modulation (DS‐BPAM) UWB systems with the proposed chirp waveforms can achieve excellent NBI suppression performance and outperform the linear chirp waveform based UWB system significantly. Copyright © 2010 John Wiley & Sons, Ltd.     

Initial Timing Acquisition for Binary Phase‐Shift Keying Direct Sequence Ultra‐wideband Transmission

This paper presents a parallel processing searcher structure for the initial synchronization of a direct sequence ultra‐wideband (DS‐UWB) system, which is suitable for the digital implementation of baseband functionalities with a 1.32 Gsample/s chip rate analog‐to‐digital converter. An initial timing acquisition algorithm and a data demodulation method are also studied. The proposed searcher effectively acquires initial symbol and frame timing during the preamble transmission period. A hardware efficient receiver structure using 24 parallel digital correlators for binary phase‐shift keying DS‐UWB transmission is presented. The proposed correlator structure operating at 55 MHz is shared for correlation operations in a searcher, a channel estimator, and the demodulator of a RAKE receiver. We also present a pseudo‐random noise sequence generated with a primitive polynomial, 1+x²+x⁵, for packet detection, automatic gain control, and initial timing acquisition. Simulation results show that the performance of the proposed parallel processing searcher employing the presented pseudo‐random noise sequence outperforms that employing a preamble sequence in the IEEE 802.15.3a DS‐UWB proposal.     

A multicarrier CDMA system with adaptive subchannel allocation forforward links

Multicarrier transmission schemes have been introduced into code-division multiple access (CDMA) systems to gain advantages for high data rate transmission. One of the methods is to transmit identical narrowband direct-sequence (DS) waveforms in parallel over a number of subchannels using frequency diversity. In this paper, we propose a multicarrier CDMA system with an adaptive subchannel allocation method for forward links. In the proposed system, instead of identical DS waveforms being transmitted over a number of subchannels in parallel, each user's DS waveform is transmitted over the user's favourite subchannel which has the largest fading amplitude among all the subchannels. We analyze the performance characteristics of the system when orthogonal and random signature sequences are used. The proposed system is shown to have performance gain over the conventional multicarrier CDMA system. We also investigate how the performance is influenced when the signal is not perfectly allocated into the best subchannel     

Full Energy Spectra of Interface State Densities for n‐ and p‐type MoS2 Field‐Effect Transistors

2D materials are promising to overcome the scaling limit of Si field‐effect transistors (FETs). However, the insulator/2D channel interface severely degrades the performance of 2D FETs, and the origin of the degradation remains largely unexplored. Here, the full energy spectra of the interface state densities (D_it) are presented for both n‐ and p‐ MoS₂ FETs, based on the comprehensive and systematic studies, i.e., full rage of channel thickness and various gate stack structures with h‐BN as well as high‐k oxides. For n‐MoS₂, D_it around the mid‐gap is drastically reduced to 5 × 10¹¹ cm^?2 eV^?1 for the heterostructure FET with h‐BN from 5 × 10¹² cm^?2 eV^?1 for the high‐k top‐gate. On the other hand, D_it remains high, ≈ 10¹³ cm^?2 eV^?1, even for the heterostructure FET for p‐MoS₂. The systematic study elucidates that the strain induced externally through the substrate surface roughness and high‐k deposition process is the origin for the interface degradation on conduction band side, while sulfur‐vacancy‐induced defect states dominate the interface degradation on valance band side. The present understanding of the interface properties provides the key to further improving the performance of 2D FETs.     

Dependence of Transformation Temperatures of NiTi‐based Shape‐Memory Alloys on the Number and Concentration of Valence Electrons

Dependence of transformation temperatures of ternary and quaternary NiTi‐based shape memory alloys on the number (e_v/a) and concentration (c_v) of valence electrons is investigated. Two distinct trends of transformation temperatures with respect to the number of valence electrons per atom are found depending on whether e_v/a = 7 or e_v/a ≠ 7. Clear correlations between transformation temperatures and c_v exist. M_s and A_s decrease consistently from 900 to ?100 °C, and 950 to ?30 °C, respectively, with increasing c_v from 0.145 to 0.296. The relationship of electron concentration on the elastic moduli of the NiTi‐based alloys is discussed. The possible influence of the atomic size of alloying elements on transformation hysteresis is introduced.     

Solution‐Processed,Alkali Metal‐Salt‐Doped,Electron‐Transport Layers for High‐Performance Phosphorescent Organic Light‐Emitting Diodes

High‐performance, blue, phosphorescent organic light‐emitting diodes (PhOLEDs) are achieved by orthogonal solution‐processing of small‐molecule electron‐transport material doped with an alkali metal salt, including cesium carbonate (Cs₂CO₃) or lithium carbonate (Li₂CO₃). Blue PhOLEDs with solution‐processed 4,7‐diphenyl‐1,10‐phenanthroline (BPhen) electron‐transport layer (ETL) doped with Cs₂CO₃ show a luminous efficiency (LE) of 35.1 cd A^?1 with an external quantum efficiency (EQE) of 17.9%, which are two‐fold higher efficiency than a BPhen ETL without a dopant. These solution‐processed blue PhOLEDs are much superior compared to devices with vacuum‐deposited BPhen ETL/alkali metal salt cathode interfacial layer. Blue PhOLEDs with solution‐processed 1,3,5‐tris(m‐pyrid‐3‐yl‐phenyl)benzene (TmPyPB) ETL doped with Cs₂CO₃ have a luminous efficiency of 37.7 cd A^?1 with an EQE of 19.0%, which is the best performance observed to date in all‐solution‐processed blue PhOLEDs. The results show that a small‐molecule ETL doped with alkali metal salt can be realized by solution‐processing to enhance overall device performance. The solution‐processed metal salt‐doped ETLs exhibit a unique rough surface morphology that facilitates enhanced charge‐injection and transport in the devices. These results demonstrate that orthogonal solution‐processing of metal salt‐doped electron‐transport materials is a promising strategy for applications in various solution‐processed multilayered organic electronic devices.     

High voltage photovoltaic mini‐modules

This work describes the design, simulation, fabrication process, and characterization of high voltage photovoltaic mini‐modules using silicon on insulator (SOI) wafers. The mini‐modules are made of a number of small area photovoltaic cells (<1 mm²) monolithically connected in series. Isolation between cells is performed by means of anisotropic etching of the active layer of the SOI wafer. Measurements using standard sunlight (AM1·5 100 mW/cm²) confirm the viability of this technology to fabricate small area arrays showing open circuit voltages, V _oc, between 620 mV and 660 mV and photocurrent densities up to 22·3 mA/cm² for single cells of 0·225 mm² area and 10 µm active film thickness. Series connection scales up V _oc and the maximum power, P _m, from 625 mV and 21·2 µW, respectively, in a single cell to 103 V and 3·2 mW when 169 cells are connected in series in a 0·42 cm² module total area. Copyright © 2008 John Wiley & Sons, Ltd.