Meeting mobile's demands with multicarrier systems

With the increasing requirements for future wireless applications, OFDM, MC-CDMA, and MC-DS-CDMA have all been considered for 4G wireless systems. These systems have the ability to incorporate very large band widths without sacrificing equalization complexity. The long symbol duration is effective at mitigating ISI, and adaptive modulation or frequency diversity can be used to provide protection against destructive fades. The benefit of MC CDMA is that it experiences frequency diversity because each bit is transmitted over several independently faded subcarriers. If some subcarriers experience destructive fades, diversity combining can be used at the receiver to recover the data. This improves the BER performance over OFDM, and this improvement is more significant as the number of subcarriers is increased. The draw back of MC-CDMA is that it may experience high levels of multiuser access interference (MAI) when the channel is heavily loaded. This occurs because each chip of the PN sequence experiences independent fading, which tends to destroy the orthogonality between spreading sequences. This increases the MAI and degrades the BER performance. Although OFDM, MC-CDMA, and MC-DS-CDMA signals experience a high PAPR, synchronization issues, and ICI, the benefits greatly outweigh these disadvantages.     

Carrier Frequency Offset in VSF-OFCDM Systems with Subcarrier Grouping: Analysis,Estimation and Correction

We investigate the effect of carrier frequency offset (CFO) on the performance of downlink variable spreading factor (VSF) orthogonal frequency and code division multiple access (OFCDM) systems when subcarrier grouping is used. An analytic expression of the signal to interference and noise ratio (SINR) is derived for the VSF-OFCDM with CFO for the case of maximal ratio combining (MRC) and equal gain combining (EGC) receivers. Numerical results show that, when the total spreading factor is fixed, a system with higher frequency domain spreading factor is more sensitive to CFO than that with lower frequency domain spreading factor. Also, for high E _b /N _o , EGC has better bit error rate performance than MRC due to the greater interference amplification present in MRC which compounds the effect of the loss of orthogonality. Due to the adverse impact of the CFO on VSF-OFCDM systems, we propose a correction scheme based on the maximum likelihood principle. We then use a gradient algorithm to estimate and minimize the effect of CFO in a tracking mode. Our results show that the BER performance in the low SINR environment can be improved significantly with few number of iterations for different spreading factors.     

On the Throughput Capacity of Cooperative Multi-hop Wireless Ad hoc Networks with Multi-flow

Cooperative relaying techniques are known to provide spatial diversity for wireless fading channels. In contrast to non-cooperative transmission (direct transmission), they increase link reliability, provide higher capacity, reduce transmit power, and extend transmission range. Mostly the gains of cooperative relaying are shown for single flow scenarios in the absence of inter-flow interference. In this paper we study the effect of inter-flow interference on the capacity of cooperative networks in multi-hop multi-flow settings. We used the conflict graph to model the interference and find the additional constraints introduced due to cooperative transmission by using the concept of cliques on the conflict graph, which can be used to capture the interference relation among links. We formulated the multi-commodity flow problem for network capacity using linear programming, and employed a clique based analysis of the conflict graph to compute interference constraints. It is observed that the throughput drops significantly when cooperative transmissions are used in the network. We also found that the hop counts increase when cooperative links are used that is due to avoiding interfering links, which results in losing the benefits of shortest-path routing.     

Multi-hop routing with cooperative transmission: a cross-layer approach

Cooperative diversity techniques have received a lot of attention recently due to their ability to provide spatial diversity in fading wireless environment without the requirement of implementing multiple antenna on the same device. It increases link reliability, provides higher capacity, reduces transmit power, and extends transmission range for the same level of performance and modulation rate. In this paper, we study the achievable gain of cooperative communications from a wireless cross-layer point of view in multi hop networks. We propose two routing algorithms applicable for wireless ad hoc networks. First, we propose an edge node based on a greedy cooperative routing (ENBGCR) algorithm, where we modify the geographic routing algorithm to incorporate the cooperative transmission and extend the coverage range of the nodes. The main objective of ENBGCR algorithm is to minimize the number of hops that messages transverse to reach their destination. Then the energy-efficient cooperative routing algorithm is proposed to minimize the end-to-end total transmission power subject to end-to-end target data rate. Simulation results for both algorithms show that the proposed strategies have great improvement in terms of delay and power saving respectively for the same quality of service requirement as compared to traditional algorithms.     

Swarm Intelligence Based Resource Management for Cooperative Cognitive Radio Network in Smart Hospitals

Wireless Personal Communications - Intelligent and efficient wireless sensor devices (IEWSD) can greatly facilitate the working of paramedic staff in next generation health care facilities. The...     

Resource Allocation Schemes in D2D Communications: Overview,Classification, and Challenges

Device-to-Device (D2D) communications is predicted to be a major part of 5G wireless communications due to its benefits such as flexible connectivity and unloading of traffic burden off the cellular networks. However, interference scenarios in D2D communications are usually more complicated than conventional wireless communications, e.g., spectrum reuse is commonly involved in D2D communications, especially when it works as an underlay. Therefore, how to coordinate the challenges of significant interference with the demand of higher data rates under the constraints of efficient energy consumption and spectrum utilization has become a haunting problem in our way to the ideal performance of wireless communications system. The solution resides in multiple resource allocation techniques in D2D communications as each of them attempts to solve or optimize one or several essential elements in the system. In this paper, a survey of resource allocation schemes in D2D communications is presented. We discuss the optimization classification including objectives, constraints, problem types and solutions. This paper also highlights system characteristics. Finally, future research challenges are outlined.     

Energy Efficiency Architecture Design for Heterogeneous Cellular Networks

Heterogeneous cellular networks (HetNets) have emerged as a new promising paradigm to further enhance capacity, where multiple types of low power smallcells are overlaid in a high power macrocell. They provide more opportunities to explore the potential cognition and cooperation diversities to improve the spectral efficiency. On the other hand, energy efficiency is a critical performance metric, which deserves more attention from academia, industry, and standardization, in particular, in scenarios where smallcells are densely deployed. In this paper, a systematic architecture is presented to efficiently utilize network resources and thus improve the overall energy efficiency. The architecture is referred to as OCRT because it combines a multi‐tier energy efficiency considerations of operators, core networks, radio access networks and terminals (e.g., OCRT). Furthermore, a corresponding triply‐cycle‐based functional structure is proposed for the OCRT to make various interactions between the corresponding functional entities clear. An implementation scheme of OCRT based on cognitive information interaction cycle and an energy efficiency‐aware protocol is presented. Finally, a use case of the presented OCRT green design is provided for energy efficiency optimization in a cognition‐and‐cooperation‐characterized HetNet. Copyright © 2015 John Wiley & Sons, Ltd.     

Characterization of tool wear when machining alloy 718 with high-pressure cooling using conventional and surface-modified WC–Co tools

Coolant supplied by high pressure into the cutting zone has shown the lower thermal loads on the tool when machining difficult-to-cut materials as the Alloy 718. In this study, we investigate how the combination of high-pressure cooling and tool–surface modifications can lead to further improvements regarding tool life. The general approach is to enhance the coolant–tool interaction by increasing the contact area. Therefore, we machined cooling features into flank and rake faces of commercially available cemented tungsten carbide inserts. In this way, the surface area was increased by ~ 12%. After the cutting tests, the tools were analyzed by scanning electron microscopy combined with energy-dispersive X-ray spectroscopy. Compared with conventional tools, the tool modifications reduced the flank wear by 45% for the investigated cutting parameters. Furthermore, we were able to significantly increase the cutting speed and feed rate without failure of the tool. The investigated surface modifications have great potential to enhance the productivity of metal cutting processes.     

Adaptive Subcarrier Allocation in Synchronous Reverse Links of a Multicarrier CDMA System with Time and Frequency Spreading

Multicarrier code-division multiple access (CDMA) with time and frequency spreading has been recently considered as a candidate for fourth-generation (4G) wireless systems. This signaling scheme simultaneously utilizes code spreading in the time and frequency domains to simultaneously improve frequency diversity and minimize multiuser access interference. As a result, it is capable of outperforming multicarrier CDMA systems that employ 1-D spreading. In this paper, a novel adaptive subcarrier allocation algorithm is developed for multicarrier CDMA with time and frequency spreading to improve the overall bit error rate (BER) performance for all spreading configurations. This algorithm assigns users to subcarrier groups that provide favorable fading characteristics while simultaneously reducing the amount of interference caused to other users. The proposed algorithm is shown to provide a performance improvement, ranging from 1.5 dB with 2times16 (time times frequency) and spreading to 7 dB with 16times2 (time times frequency) spreading. The algorithm is also shown to maintain or improve the BER floor for each spreading configuration. It is concluded that at higher and lower levels of E_b/N_o, a higher frequency- and time-domain spreading should be, respectively, employed to improve BER performance. Furthermore, the E_b/N_o threshold level to switch between time and frequency spreading for the analyzed system is found to be 2.5 dB.     

A new physics design of control safety rods for prototype fast breeder reactor

The absorber rods of 500 MWe prototype fast breeder reactor (PFBR), which is under construction at Kalpakkam, have been designed to provide sufficient shutdown margin during normal and accidental conditions for ensuring the safe shut down. There are nine control and safety rods (CSR) and 3 diverse safety rods (DSR). Absorber material used is initially 65% enriched B₄C. Based on the reported experiments in PHENIX reactor and design of absorber rods in SUPERPHENIX, the design of CSR is modified by introducing 20 cm length natural B₄C at the top and bottom of absorber column and maintaining the remaining portion with 65% enriched B₄C. This design ensures sufficient shutdown margin (SDM) during normal operation and also during the one stuck rod condition. For comparison of the above two designs, a CSR of 57% of enrichment was considered which gives the same worth as the revised CSR design with natural B₄C sections in top and bottom. There is significant savings in the initial inventory of enriched B₄C for CSR. The annual requirement of enriched boron also reduces. This new CSR can last for about 5 cycles, based on its clad life. But, it is planned to be replaced after every 3 cycles (1 cycle equals 180 efpd) of operation due to radiation damage effects in hexcan D9 steel. Use of ferritic steel for hexcan can extend the life of CSR to 5 cycles.