http://www.hindawi.comThe latest articles from Hindawi Publishing Corporation© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/291365This paper addresses the transmission of medical and context-aware data from mobile patients to healthcare centers over heterogeneous wireless networks. A handheld device, called personal wireless hub (PWH), of each mobile patient first gathers and aggregates the vital sign and context-aware data for various telemedicine applications. PWH transmits the aggregated data to the remote healthcare center over multiple wireless interfaces such as cellular, WLAN, and WiMAX. The aggregated data contain both periodic data and those nonperiodic unpredictable emergency messages that are sporadic and delayintolerant. This paper addresses the problem of providing QoS (e.g., minimum delay, sufficient data rate, acceptable blocking, and/or dropping rate) by designing a packet scheduling and channel/network allocation algorithm over wireless networks. The proposed resource-efficient QoS mechanism is simple and collaborates with an adaptive security algorithm. The QoS and security are achieved mainly with the collaboration of differentiator, delay monitor, data classifier, and scheduler modules within the PWH. This paper also discusses secure data transmission over body sensor networks by introducing key establishment and management algorithms. Simulation results show that the proposed framework achieves low-blocking probability, meets delay requirements, and provides energy-efficient secure communication for the combination of vital signs and context-aware data.Narasimha Challa, Hasan Çam, and Madhur Sikri© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/153469Natural disasters and terrorist acts have significant potential to disrupt emergency communication systems. These emergency communication networks include first-responder, cellular, landline, and emergency answering services such as 911, 112, or 999. Without these essential emergency communications capabilities, search, rescue, and recovery operations during a catastrophic event will be severely debilitated. High altitude platforms could be fitted with telecommunications equipment and used to support these critical communications missions once the catastrophic event occurs. With the ability to be continuously on station, HAPs provide excellent options for providing emergency coverage over high-risk areas before catastrophic incidents occur. HAPs could also provide enhanced 911 capabilities using either GPS or reference stations. This paper proposes potential emergency communications architecture and presents a method for estimating emergency communications systems traffic patterns for a catastrophic event.Juan D. Deaton© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/852397As a class of pseudorandom error correcting codes, generalized low-density (GLD) codes exhibit excellent performance over both additive white Gaussian noise (AWGN) and Rayleigh fading channels. In this paper, distributed GLD codes are proposed for multiple relay cooperative communications. Specifically, using the partial error detecting and error correcting capabilities of the GLD code, each relay node decodes and forwards some of the constituent codes of the GLD code to cooperatively form a distributed GLD code, which can work effectively and keep a fixed overall code rate when the number of relay nodes varies. Also, at relay nodes, a progressive processing procedure is proposed to reduce the complexity and adapt to the source-relay channel variations. At the destination, the soft information from different paths is combined for the GLD decoder thus diversity gain and coding gain are achieved simultaneously. Simulation results verify that distributed GLD codes with various number of relay nodes can obtain significant performance gains in quasistatic fading channels compared with the strategy without relays and the performance is further improved when more relays are employed.Changcai Han and Weiling Wu© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/259310The aim of this paper is to perform an overall comparison of different network layouts for CDMA-based cellular radio access. Cellular network layout, including base station site locations and theoretical azimuth directions of antennas, can be defined by tessellations in order to achieve a continuous coverage of the radio network. Different tessellation types—triangle, square, and hexagon—result in different carrier-to-interference scenarios, and thus will provide nonequal system-level performance. This performance of a cellular network is strongly related to configuration parameters as base station antenna height, beamwidth, and sectoring. In this paper, a theoretical model is defined for the assessment, which includes numerical analysis and system-level simulations. A numerical analysis was performed first, and then system-level Monte-Carlo simulations were conducted to verify and to extend numerical results. The obtained results of the numerical analysis indicate that a hexagonal “clover-leaf” layout is superior, but the results of system-level simulation give similar performance for the triangular and square layouts. These results indicate also the importance of the antenna height optimization for all layouts. Moreover, the simulation results also pointed out that 6-sector configuration is superior both in coverage and in capacity compared to nominal 3-sector configuration that is typically preferred in coverage-related network deployments in practice.Jarkko Itkonen, Balazs Tuzson, and Jukka Lempiäinen© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/979813Ultrawideband (UWB) random noise signals provide secure communications because they cannot, in general, be detected using conventional receivers and are jam-resistant. We describe the theoretical underpinnings of a novel spread spectrum technique that can be used for covert communications using transmissions over orthogonal polarization channels. The noise key and the noise-like modulated signal are transmitted over orthogonal polarizations to mimic unpolarized noise. Since the transmitted signal is featureless and appears unpolarized and noise-like, linearly polarized receivers are unable to identify, detect, or otherwise extract useful information from the signal. The wide bandwidth of the transmitting signal provides significant immunity from interference. Dispersive effects caused by the atmosphere and other factors are significantly reduced since both polarization channels operate over the same frequency band. The received signals are mixed together to accomplish demodulation. Excellent bit error rate performance is achieved even under adverse propagation conditions.Jack Chuang, Matthew W. DeMay, and Ram M. Narayanan© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/893705Channel models for outdoor wireless systems usually assume two-dimensional (2D) random scattering media. In the practical outdoor wireless channels, the impact of the wave propagation in the third-dimension is definitely important; especially when the communication system efficiently exploits potentials of multiple antennas. In this paper, we propose a new model for multiple-input multiple-output (MIMO) multicarrier propagation channels in a three-dimensional (3D) environment. Specifically, the proposed model describes the cross-correlation function (CCF) between two subchannels of an outdoor MIMO channel employing directional antennas and in the presence of nonisotropic wave propagation in 3D space. The derived CCF consists of some correlation terms. Each correlation term is in the form of a linear series expansion of averaged Bessel functions of the first kind with different orders. In practice, each correlation term has a limited number of Bessel components. Our numerical evaluations show the impact of different parameters of the propagation environment as well as the employed antennas on the resulting CCF. Using the proposed CCF, we also establish simple formulas to approximate the coherence time, the coherence bandwidth and the spatial coherence of such channels. The numerical curve fitting results fit to the empirical results reported in the channel modeling literature.Hamidreza Saligheh Rad and Saeed Gazor© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/867465Wireless ad hoc networks allow anywhere, anytime network connectivity with complete lack of central control, ownership, and regulatory influence. Medium access control (MAC) in such networks poses extremely timely as well as important research and development challenges. Utilizing directional antennas in wireless ad hoc networks is anticipated to significantly improve the network performance due to the increased spatial reuse and the extended transmission range. Nevertheless, using directional antennas in wireless ad hoc networks introduces some serious challenges, the most critical of which are the deafness and hidden terminal problems. This paper thoroughly explores these problems, one of which is discovered and reported for the first time in this paper. This paper also proposes a new MAC scheme, namely, directional MAC with deafness avoidance and collision avoidance (DMAC-DACA), to address both problems. To study the performance of the proposed scheme, a complete directional communication extension to layers 1, 2, and 3 is incorporated in the ns2 simulator. The simulation results show that DMAC-DACA significantly enhances the performance and increases the network throughput. This paper also reveals that deafness has a greater impact on network performance than the hidden terminal problem.Yihu Li and Ahmed Safwat© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/691571Recently, wireless sensor networks have been used extensively in different domains. For example, if the wireless sensor node of a wireless sensor network is distributed in an insecure area, a secret key must be used to protect the transmission between the sensor nodes. Most of the existing methods consist of preselecting m keys from a key pool and forming a key chain. Then, the sensor nodes make use of the key chain to encrypt the data. However, while the secret key is being transmitted, it can easily be exposed during transmission. We propose a dynamic key management protocol, which can improve the security of the key juxtaposed to existing methods. Additionally, the dynamic update of the key can lower the probability of the key to being guessed correctly. In addition, with the new protocol, attacks on the wireless sensor network can be avoided.Chin-Ling Chen and Cheng-Ta Li© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/573785IEEE 802.16 standard suite defines the air interface specifications for fixed and mobile broadband access in wireless metropolitan area networks. Although the IEEE 802.16 MAC has been well defined by various bandwidth allocation and scheduling mechanisms to support QoS for different applications, efficient bandwidth allocation still remains as an open issue. We analyze and develop a mathematical model to evaluate the performance of the contention-based and delay-tolerant applications in IEEE 802.16 networks. We focus our attentions on allocating the uplink bandwidth efficiently, the basic goal is to optimize the performance with an optimal bandwidth allocation mechanism. The results of our analysis lay out clearly that a maximum uplink throughput and a minimum number of pending bandwidth request transmission can always be acquired by optimizing the contention period size in a frame. This optimal size is also influenced by the number of terminals in the network, which is also analyzed in the later part of the paper. Our results can be used for providing probabilistic throughput guarantee and determining the optimal contention period.Fei Yin and Guy Pujolle© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/540873Optimizing the balance between handoff quality and power consumption is a great challenge for seamless mobile communications in wireless networks. Traditional proactive schemes continuously monitor available access networks and exercise handoff. Although such schemes achieve good handoff quality, they consume much power because all interfaces must remain on all the time. To save power, the reactive schemes use fixed RSS thresholds to determine when to search for a new available access network. However, since they do not consider user motion, these approaches require that all interfaces be turned on even when a user is stationary, and they tend initiate excessive unnecessary handoffs. To address this problem, this research presents a novel motion-aware scheme called network discovery with motion detection (NDMD) to improve handoff quality and minimize power consumption. The NDMD first applies a moving average convergence divergence (MACD) scheme to analyze received signal strength (RSS) samples of the current active interface. These results are then used to estimate user's motion. The proposed NDMD scheme adds very little computing overhead to a mobile terminal (MT) and can be easily incorporated into existing schemes. The simulation results in this study showed that NDMD can quickly track user motion state without a positioning system and perform network discovery rapidly enough to achieve a much lower handoff-dropping rate with less power consumption.Yung-Mu Chen, Tein-Yaw Chung, Ming-Yen Lai, and Chih-Hung Hsu© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/897329As a follow-on to the 2002 digital television (DTV) broadcast demonstration from a solar-powered stratospheric flying wing, a prototype stratospheric airship was used for a more realistic DTV broadcast demonstration in 2004, albeit at a lower altitude. The DTV signal was occasionally lost at the receiver directly below the airship during the demonstration. Adverse antenna-vehicle integration effects were investigated using a commercially available antenna simulation software, because the radiation pattern of the antenna on the airship could not be measured directly. The ground handling bars on the airship gondola were found to introduce deep and sharp nulls into the radiation pattern of the broadcast antenna. Some mitigation techniques that would have fitted within the constraints of the time are discussed. Changing to nonconductive ground handling bars and a multiturn helical antenna would have avoided the problem, according to the simulation results.Derek Gray, Mamoru Nagatsuka, Mikio Suzuki, and Ryu Miura© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/950951Huaiyu Dai, Michael Gastpar, Nihar Jindal, and Liang-Liang Xie© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/390489This paper deals with multiuser detection through base station cooperation in an uplink, interference-limited, high frequency reuse scenario. Distributed iterative detection (DID) is an interference mitigation technique in which the base stations at different geographical locations exchange detected data iteratively while performing separate detection and decoding of their received data streams. This paper explores possible DID receive strategies and proposes to exchange between base stations only the processed information for their associated mobile terminals. The resulting backhaul traffic is considerably lower than that of existing cooperative multiuser detection strategies. Single-antenna interference cancellation techniques are employed to generate local estimates of the dominant interferers at each base station, which are then combined with their independent received copies from other base stations, resulting in more effective interference suppression. Since hard information bits or quantized log-likelihood ratios (LLRs) are transferred, we investigate the effect of quantization of the LLR values with the objective of further reducing the backhaul traffic. Our findings show that schemes based on nonuniform quantization of the “soft bits” allow for reducing the backhaul to 1–2 exchanged bits/coded bit.Shahid Khattak, Wolfgang Rave, and Gerhard Fettweis© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/132729This paper presents adaptive predistortion techniques based on a feed-forward neural network (NN) to linearize power amplifiers such as those used in satellite communications. Indeed, it presents the suitable NN structures which give the best performances for three satellite down links. The first link is a stationary memoryless travelling wave tube amplifier (TWTA), the second one is a nonstationary memoryless TWT amplifier while the third is an amplifier with memory modeled by a memoryless amplifier followed by a linear filter. Equally important, it puts forward the studies concerning the application of different NN training algorithms in order to determine the most prefermant for adaptive predistortions. This comparison examined through computer simulation for 64 carriers and 16-QAM OFDM system, with a Saleh's TWT amplifier, is based on some quality measure (mean square error), the required training time to reach a particular quality level, and computation complexity. The chosen adaptive predistortions (NN structures associated with an adaptive algorithm) have a low complexity, fast convergence, and best performance.Rafik Zayani, Ridha Bouallegue, and Daniel Roviras© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/601346A suboptimal partial transmit sequence (PTS) based on particle swarm optimization (PSO) algorithm is presented for the low computation complexity and the reduction of the peak-to-average power ratio (PAPR) of an orthogonal frequency division multiplexing (OFDM) system. In general, PTS technique can improve the PAPR statistics of an OFDM system. However, it will come with an exhaustive search over all combinations of allowed phase weighting factors and the search complexity increasing exponentially with the number of subblocks. In this paper, we work around potentially computational intractability; the proposed PSO scheme exploits heuristics to search the optimal combination of phase factors with low complexity. Simulation results show that the new technique can effectively reduce the computation complexity and PAPR reduction.Jyh-Horng Wen, Shu-Hong Lee, Yung-Fa Huang, and Ho-Lung Hung© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/427352Results from a measurement campaign emulating the high altitude platform (HAP)-to-indoor communication channel at S-band are presented in this paper. A link was established between a transmitter carried by a helicopter, representing the HAP, and a receiver placed at several locations in different building types including an airport, an office building, a shopping mall, a residential house, and a skyscraper. A wideband, directive channel sounder was used to measure building entry loss and time delay spread. Results of the building entry loss are presented as a function of building type, elevation, and building entry angle. Results of delay spread for each building are also provided.F. Perez-Fontan, V. Hovinen, M. Schönhuber, R. Prieto-Cerdeira, J. A. Delgado-Penín, F. Teschl, J. Kyröläinen, and P. Valtr© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/278016Ivan Cosovic, Friedrich K. Jondral, Milind M. Buddhikot, and Ryuji Kohno© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/125892Transformed codebooks are obtained by a transformation of a given codebook to best match the statistical environment at hand. The procedure, though suboptimal, has recently been suggested for feedback of channel state information (CSI) in multiple antenna systems with correlated channels because of their simplicity and effectiveness. In this paper, we first consider the general distortion analysis of vector quantizers with transformed codebooks. Bounds on the average system distortion of this class of quantizers are provided. It exposes the effects of two kinds of suboptimality introduced by the transformed codebook, namely, the loss caused by suboptimal point density and the loss caused by mismatched Voronoi shape. We then focus our attention on the application of the proposed general framework to providing capacity analysis of a feedback-based MISO system over spatially correlated fading channels. In particular, with capacity loss as an objective function, upper and lower bounds on the average distortion of MISO systems with transformed codebooks are provided and compared to that of the optimal channel quantizers. The expressions are examined to provide interesting insights in the high and low SNR regime. Numerical and simulation results are presented which confirm the tightness of the distortion bounds.Jun Zheng and Bhaskar D. Rao© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/473796We investigate the performance of turbo interference cancellation receivers in the space time block coded (STBC) direct-sequence code division multiple access (DS-CDMA) system. Depending on the concatenation scheme used, we divide these receivers into the partitioned approach (PA) and the iterative approach (IA) receivers. The performance of both the PA and IA receivers is evaluated in Rayleigh fading channels for the uplink scenario. Numerical results show that the MMSE front-end turbo space-time iterative approach receiver (IA) effectively combats the mixture of MAI and intersymbol interference (ISI). To further investigate the possible achievable data rates in the turbo interference cancellation receivers, we introduce the puncturing of the turbo code through the use of rate compatible punctured turbo codes (RCPTCs). Simulation results suggest that combining interference cancellation, turbo decoding, STBC, and RCPTC can significantly improve the achievable data rates for a synchronous DS-CDMA system for the uplink in Rayleigh flat fading channels.Derrick B. Mashwama and Emmanuel Oluremi Bejide© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/702036We consider joint optimization of data routing and resource allocation in multicast multihop wireless networks where interference between links is taken into account. The use of network coding in such scenarios leads to a nonconvex optimization problem. By applying the probability collectives (PCs) technique the original problem is turned into a new problem which is convex over probability distributions. The resulting problem is then further decomposed into a data routing subproblem at network layer and a power allocation subproblem at physical layer in order to achieve a cross-layer distributed solution for the whole range of SINR values. The proposed approach is also extended to minimum cost multicast problems and routing problems based on multicommodity flow and single Steiner tree, resulting in new distributed algorithms for such problems.Mohammad H. Amerimehr, Babak H. Khalaj, and Pedro M. Crespo© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/890194We consider the decoding of wireless communication systems with both source coding in the application layer and channel coding in the physical layer for high-performance transmission over fading channels. Variable length error correcting codes (VLECs) and space time trellis codes (STTCs) are used to provide bandwidth efficient data compression as well as coding and diversity gains. At the receiver, an iterative joint source and space time decoding scheme are developed to utilize redundancy in both STTC and VLEC to improve overall decoding performance. Issues such as the inseparable systematic information in the symbol level, the asymmetric trellis structure of VLEC, and information exchange between bit and symbol domains have been considered in the maximum a posteriori probability (MAP) decoding algorithm. Simulation results indicate that the developed joint decoding scheme achieves a significant decoding gain over the separate decoding in fading channels, whether or not the channel information is perfectly known at the receiver. Furthermore, how rate allocation between STTC and VLEC affects the performance of the joint source and space-time decoder is investigated. Different systems with a fixed overall information rate are studied. It is shown that for a system with more redundancy dedicated to the source code and a higher order modulation of STTC, the joint decoding yields better performance, though with increased complexity.Huijun Chen and Lei Cao© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/895654Orthogonal frequency-division multiplexing (OFDM) with multiple transmit and multiple receive antennas (MIMO-OFDM) is considered a candidate for high-data rate communication in various existing and forthcoming system standards. To achieve the usually desired low frame and bit error rates, MIMO-OFDM should be combined with adaptive bit loading (ABL) and forward error correction (FEC) coding, where the former is particularly apt for moderate mobility as considered in, for example, IEEE 802.16e OFDM systems. In this paper, we investigate “simple” coding schemes and their combination with ABL for MIMO-OFDM. In particular, we consider wrapped space-frequency coding (WSFC) and coded V-BLAST with ABL and optimize both schemes to mitigate error propagation inherent in the detection process. Simulation results show that bit-loaded WSFC and V-BLAST optimized for coded MIMO-OFDM achieve excellent error rate performances, close to that of quasioptimal MIMO-OFDM based on singular value decomposition of the channel, while their feedback requirements for loading are low.Harry Z. B. Chen, Lutz Lampe, and Robert Schober© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/735732The channel estimation and frequency offset estimation scheme for future generation orthogonal frequency division multiplexing (OFDM-) based intelligent packet communication systems are proposed. In the channel estimation scheme, we use additional 8 short training symbols besides 2 long training symbols for intelligently improving estimation performance. In the proposed frequency offset estimation scheme, we allocate intelligently different powers to the short and long training symbols while maintaining average power of overall preamble sequence. The preamble structure considered is based on the preamble specified in standardization group of IEEE802.11a for wireless local area network (WLAN) and IEEE802.11p for intelligent transportation systems (ITSs). From the simulation results, it is shown that the proposed intelligent estimation schemes can achieve better mean squared error (MSE) performance for channel and frequency offset estimation error than the conventional scheme. The proposed schemes can be used in designing for enhancing the performance of OFDM-based future generation intelligent communication network systems.Jaemin Kwak, Sungeon Cho, Kitaeg Lim, Pusik Park, Daekyo Shin, and Jongchan Choi© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/596383HAPS is a promising technology capable of providing broadband multimedia services as an alternative to the satellite communication system or terrestrial network. In this study, economic aspects of HAPS service are analyzed, HAPS services are defined, and revenues from the defined services are forecasted assuming nine scenarios. Capital expenditure as well as operating expenditure is estimated. To evaluate the profitability of HAPS service, the net present value (NPV), payback period, and the internal rate of return (IRR) are calculated. The results show that HAPS is economically justifiable in all the scenarios. Assuming that ARPU for the service is $35 per month in the average scenario, NPV is calculated as $2964 million, IRR becomes 31.9%, and payback occurs in 2017, which implies that HAPS service is profitable in Korea. In addition, the results of the sensitivity analysis show that the results are fairly robust.Jonghwa Kim, Deokjoo Lee, Jaekyoung Ahn, Do-Seob Ahn, and Bon-Jun Ku© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/970842Relying on a simple flag-assisted mechanism, a multigroup priority queueing (MGPQ) medium access control (MAC) protocol is proposed for the wireless networks with multipacket reception (MPR). The proposed MGPQ scheme is capable of overcoming two major performance bottlenecks inherent in the existing MPR MAC protocols. First, the proposed solution can automatically produce the list of active users by observing the network traffic conditions, remove the need of active user estimation algorithm, and thus can largely reduce the algorithm complexity. Second, the packet blocking constraint imposed on the active users for keeping compliant with prediction is relaxed. As a result, the proposed MGPQ is not only applicable to both homogeneous and heterogeneous cases, but also outperforms the existing MPR MAC protocols. Simulation results show that the network throughput can be improved by 40% maximum and 14% average as compared with the well-known dynamic queue (DQ) MAC protocol.Wen-Fang Yang, Jwo-Yuh Wu, Li-Chun Wang, and Ta-Sung Lee© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/865393The International Telecommunication Union-Radiocommunication sector (ITU-R) has conducted frequency sharing studies between fixed services (FSs) using a high altitude platform station (HAPS) and fixed-satellite services (FSSs). In particular, ITU-R has investigated the power limitations related to HAPS user terminals (HUTs) to facilitate frequency sharing with space station receivers. To reduce the level of interference from the HUTs that can harm a geostationary earth orbit (GEO) satellite receiver in a space station, previous studies have taken two approaches: frequency sharing using a separated distance (FSSD) and frequency sharing using power control (FSPC). In this paper, various performance evaluation results of interference mitigation schemes are presented. The results include performance evaluations using a new interference mitigation approach as well as conventional approaches. An adaptive beamforming scheme (ABS) is introduced as a new scheme for efficient frequency sharing, and the interference mitigation effect on the ABS is examined considering pointing mismatch errors. The results confirm that the application of ABS enables frequency sharing between two systems with a smaller power reduction of HUTs in a cocoverage area compared to this reduction when conventional schemes are utilized. In addition, the analysis results provide the proper amount of modification at the transmitting power level of the HUT required for the suitable frequency sharing.Bon-Jun Ku, Do-Seob Ahn, and Nam Kim© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/586878We study the potential benefits of base-station (BS) cooperation for downlink transmission in multicell networks. Based on a modified Wyner-type model with users clustered at the cell-edges, we analyze the dirty-paper-coding (DPC) precoder and several linear precoding schemes, including cophasing, zero-forcing (ZF), and MMSE precoders. For the nonfading scenario with random phases, we obtain analytical performance expressions for each scheme. In particular, we characterize the high signal-to-noise ratio (SNR) performance gap between the DPC and ZF precoders in large networks, which indicates a singularity problem in certain network settings. Moreover, we demonstrate that the MMSE precoder does not completely resolve the singularity problem. However, by incorporating path gain fading, we numerically show that the singularity problem can be eased by linear precoding techniques aided with multiuser selection. By extending our network model to include cell-interior users, we determine the capacity regions of the two classes of users for various cooperative strategies. In addition to an outer bound and a baseline scheme, we also consider several locally cooperative transmission approaches. The resulting capacity regions show the tradeoff between the performance improvement and the requirement for BS cooperation, signal processing complexity, and channel state information at the transmitter (CSIT).Sheng Jing, David N. C. Tse, Joseph B. Soriaga, Jilei Hou, John E. Smee, and Roberto Padovani© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/598401This paper presents a new class of low-density parity-check (LDPC) codes over ℤ2a represented by regular, structured Tanner graphs. These graphs are constructed using Latin squares defined over a multiplicative group of a Galois ring, rather than a finite field. Our approach yields codes for a wide range of code rates and more importantly, codes whose minimum pseudocodeword weights equal their minimum Hamming distances. Simulation studies show that these structured codes, when transmitted using matched signal sets over an additive-white-Gaussian-noise channel, can outperform their random counterparts of similar length and rate.Elisa Mo and Marc A. Armand© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/149803Orthogonal frequency division multiplexing- (OFDM-) based amplify-and-forward (AF) cooperative communication is an effective way for single-antenna systems to exploit the spatial diversity gains in frequency-selective fading channels, but the receiver usually requires the knowledge of the channel state information to recover the transmitted signals. In this paper, a training-sequences-aided linear minimum mean square error (LMMSE) channel estimation method is proposed for OFDM-based cooperative diversity systems with multiple AF relays over frequency-selective fading channels. The mean square error (MSE) bound on the proposed method is derived and the optimal training scheme with respect to this bound is also given. By exploiting the optimal training scheme, an optimal low-rank LMMSE channel estimator is introduced to reduce the computational complexity of the proposed method via singular value decomposition. Furthermore, the Chu sequence is employed as the training sequence to implement the optimal training scheme with easy realization at the source terminal and reduced computational complexity at the relay terminals. The performance of the proposed low-complexity channel estimation method and the superiority of the derived optimal training scheme are verified through simulation results.Kai Yan, Sheng Ding, Yunzhou Qiu, Yingguan Wang, and Haitao Liu© 2008, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/271989Group key exchange (GKE) is one of the basic building blocks in securing group communication. A number of solutions to GKE problem have been proposed, but most of them are not scalable and require a number of rounds linear with the number of group members. We present a method of constructing constant-round and identity-based protocol via secret sharing for GKE within universally composability (UC) framework. The resultant protocol focuses on round efficiency and three rounds of communication are required. The protocol allows the batch verification of messages signed by all other group participants. Moreover, compared with other identity-based protocols, the key generation center (KGC) in our protocol is not always online.Chunjie Cao, Chao Yang, Jianfeng Ma, and Sangjae Moon© 2008, Hindawi Publishing Corporation. All rights reserved.