Ubiquitous computing (ubicomp) is a computing para-digm which utilizes human-centric systems and applications. With the widespread use of information appliances, robots and sensors, the ubicomp paradigm is expected to become a reality in the near future. Because close interaction between a person and the computing environment is required for ubicomp, autonomous decentralized control will play an important role. In this paper, we discuss autonomous decentralized control in ubicomp from the viewpoint of typical ubicomp applications, smart environments and context-awareness.

Application-level multicast (ALM) is a feasible alternative to IP multicast. In ALM, multicast related features, such as group membership management, multicast routing and packet replication, are implemented at end-hosts instead of routers. A multicast distribution tree is constructed in the application layer, so all nodes in this tree are end-hosts. Packet transmission between end-hosts uses conventional IP unicast service. Therefore, all end-hosts can enjoy multicast communications without IP multicast service. However, ALM has a serious problem that the multicast distribution tree is intrinsically fragile and an end-host failure causes tree partitions. In this paper, to deal with this problem, we propose a new tree construction protocol which makes outdegrees of intermediate nodes be balanced. The degree-balanced distribution tree can reduce the average number of nodes decoupled by tree partitions. To investigate performance of our protocol, it is compared with an existing ALM protocol. Our simulation results show that our protocol outperforms the existing protocol from the viewpoints of robustness, loss probability and receiver-perceived delay.

In this paper, we propose multicast technique in order to reduce the required network bandwidth by n times, by merging the adjacent multicasts depending on the number of HENs (Head-End-Nodes) n that request the same video. Allowing new clients to immediately join an existing multicast through patching improves the efficiency of the multicast and offers services without any initial latency. A client might have to download data through two channels simultaneously, one for multicast and the other for patching. Each video stream is divided into blocks which are the same size of multicast grouping interval I_m. Blocks then are evenly distributed into different HENs according to their popularity and the order of requests. Only when the playback time exceeds the amount of cached video data, server generates new multicast channel. Since the interval of multicast can be dynamically expanded according to the popularity of videos, it can be reduced the server's workload and the network bandwidth. We adopt the cache replacement strategy as LFU (Least-Frequently-Used) for popular videos, LRU (Least-Recently-Used) for unpopular videos, and the method for replacing the first block of video last to reduce end-to-end latency. We perform simulations to compare its performance with that of conventional multicast. From simulation results, we confirm that the proposed multicast technique offers substantially better performance.

Application-level multicast (ALM) is a novel technology for multipoint applications, such as large scale file distribution, video and audio streaming, and video conferencing. Although many ALM mechanisms or algorithms have been proposed, all the multicast functions have been independently developed and integrated into individual applications. In such a situation, the development of ALM applications includes a lot of redundancy. Our goal is to improve the efficiency of developing ALM applications by reducing the development redundancy and to provide application developers with a middleware on which various ALM applications can be efficiently developed with minimum efforts. To this end, we develop a functional unit oriented ALM middleware, namely RelayCast. RelayCast provides a minimum but fundamental set of functionality as a functional unit, and constructs the basis on which additional and specific functions (i.e. codec, video capture, etc.) for each application are implemented. Some functional units contain several components with different algorithms, and RelayCast meets the requirements of various applications by choosing the appropriate component. In this paper, we propose RelayCast architecture, and present the implementation and experiments of a prototype.

This paper discusses multicast routing in ad hoc networks. In multicast routing, a node delivers the same message to the other nodes within a multicast group along with a multicast tree. Since nodes are moving around in ad hoc networks, the links between the nodes change frequently. However, the multicast tree must be maintained to deliver the messages regardless of the link changes. This paper gives a description of an autonomous clustering-based hierarchical multicast routing protocol in ad hoc networks. Since the autonomous clustering scheme is adaptive to the node movement, the proposed multicast routing can maintain the multicast tree in despite of link changes. This paper shows the effectiveness of autonomous clustering-based hierarchical multicast routing from the point of view of adaptability to link changes and scalability to multicast members.

This work deploys Autonomous Decentralized System (ADS) based formulation to cluster of networked visual sensors. The goal is to utilize and integrate the sensing and networking capabilities of the sensors with the systematic and autonomous features of ADS to perform visual surveillance through object detection in the covered areas of interest. In the proposed approach, several cells are distributed through an area of interest called Autonomous Observer Cell. The decentralized subsystems detect and track moving objects present on the scene by looking through a camera embedded in each sensor. These subsystems form a cluster and each cluster sends information to an Autonomous Analysis Cell that determines if an object of interest is present. The Autonomous Observer Cells share a common data field and a cluster-head works as a gateway between the cluster and the Autonomous Analysis Cell.

Incorporating sensor nodes with data aggregation capability to transmit less data flow in wireless sensor networks could reduce the total energy consumption. This calls for the efficient and effective data-centric routing algorithm to facilitate this advantage. In the first part of this paper, we model the data-centric routing problem by rigorous mixed integer and linear mathematical formulation, where the objective function is to minimize the total transmission cost subject to multicast tree constraints. With the advancement of sensor network technology, sensor nodes with configurable transmission radius capability could further reduce energy consumption. The second part of this paper considers the transmission radius assignment of each sensor node and the data-centric routing assignment jointly. The objective function is to minimize the total power consumption together with consideration of construction of a data aggregation tree and sensor node transmission radius assignment. The solution approach is based on Lagrangean relaxation in conjunction with the novel optimization-based heuristics. From the computational experiments, it is shown that the proposed algorithms calculate better solution than other existing heuristics with improvement ratio up to 169% and 59% with respect to fixed transmission radius and configurable transmission radius for network with 300 random generated nodes.

The advent of mobile IP communication has opened up several new areas of mission critical communication applications. But the bandwidth and reliability constraints coupled with handover latency are posing some hurdles which need to be overcome before real world mobile IP applications, with low tolerance for data loss, can be deployed. In this paper, we analyze the unreliability of existing information collection methods in the real-world MobileIP environment. We focus on this problem and propose a novel network management model that anticipates the wireless mobile entities and uses SNMP. The key idea of this model is the introduction of a store-and-forward type Managed Object (MO). During the period of unreachability between the Manager and the agent, the data is cached at the agent until the connectivity recovers. In our experiment we used a prototype implementation in real-world wireless communication field, and showed the effectiveness of our proposed method.

This paper discusses effective configuration methods for peer-to-peer (P2P) network topologies within a mobile ad-hoc network. With recent progress in mobile ad-hoc network technology promoting the creation of new and attractive services, we are examining and developing P2P network systems for operation within ad-hoc networks. Our focus is on identifying methods of network-topology control that provide the best balance between performance and availability. We evaluate three methods through computer simulation and field trials from the viewpoints of resource consumption and network integrity, and clarify their domains of applicability. The results are expected to contribute to the design of future P2P networks for operation in mobile ad-hoc networks.

Long running software systems are known to experience an aging phenomenon called software aging, one in which the accumulation of errors during the execution of software leads to performance degradation and eventually results in failure. To counteract this phenomenon a proactive fault management approach, called software rejuvenation, is particularly useful. It essentially involves gracefully terminating an application or a system and restarting it in a clean internal state. In this paper, we evaluate dependability performance of a communication network system with the software rejuvenation under the assumption that the requests arrive according to a Markov modulated Poisson process (MMPP). Three dependability measures, steady-state availability, loss probability of requests and mean response time on tasks, are derived through the hidden Markovian analysis based on the time-based software rejuvenation scheme. In numerical examples, we investigate the sensitivity of some model parameters to the dependability measures.

In optical burst switching (OBS) networks, the contention of optical bursts is the most serious problem due to the lack of buffers within the networks. Various deflection routing schemes and a routing scheme based on pre-calculated multiple paths have been proposed to resolve the contention. The latter routing scheme can successfully maintain a relatively limited transfer delay of optical bursts. This paper proposes a new decentralized routing scheme based on multiple paths to effectively resolve the contention of optical bursts. In this scheme, each source node splits the traffic load into pre-calculated multiple paths adaptively according to the measured loss rate of the optical bursts transferred through each path. This scheme does not require frequent notification of the measured loss rate because each source node selects one of the multiple paths probabilistically. In the OBS networks, the average transfer delay in the multi-path routing always exceeds that in a single-path routing because alternative paths with a larger transfer delay are also utilized in the multi-path routing. Thus, this paper proposes an adaptive load splitting method in which load splitting ratios for the multiple paths are autonomously adjusted to minimize the average transfer delay based on the condition that the required loss rate of optical bursts is satisfied. The performance of the proposed scheme was evaluated by computer simulation and based on the evaluation results; the ability of the proposed scheme to adjust the load splitting ratios for the multiple paths autonomously and avoid the contention of optical bursts adaptively is clarified even if the traffic load applied to the OBS network changes.

In pure peer-to-peer (P2P) file sharing applications and protocols using a flooding-based query algorithm, a large number of control packets (query packets) are transmitted on the network to search for target files. This clearly leads to a degradation of communication quality on the network and terminals as the number of users of the application increases. To solve such problems, this paper proposes: (1) a unified framework to describe a wide variety of query algorithms for pure P2P and (2) a new query algorithm based on this framework. Our framework determines the number of destinations for query packets based on the hop value recorded in received query packets. Simulation results revealed that the proposed query algorithm can reduce the overhead in the flooding-based query algorithm and k-random walks without decreasing the success rate of retrieval regardless of the density of target files in the network.

Current TCP-friendly congestion control mechanisms adjust the packet rate in order to adapt to wired network conditions and obtain a throughput not exceeding that of a TCP connection operating under the same conditions. However, these mechanisms can not be directly applicable to wireless network because there is no way to distinguish congestion losses from wireless channel losses. In this letter, a new loss differentiation algorithm for wired-to-wireless streaming service is described. The approach does not only adjust the sending rate according to the network status, but also provide the useful feedback to the video encoder.

This paper tackles the problem of detecting a random signal embedded in additive white noise. Although the likelihood ratio test (LRT) is the well-known optimum detector for this problem, it may not be easily realized in applications such as radar, sonar, seismic, digital communications, speech analysis and automatic fault detection in machinery, for which suboptimal quadratic detectors have been extensively employed. In this paper, the relationships between four suboptimal quadratic detection schemes, namely, the energy, matched subspace, maximum deflection ratio as well as spectrum matching detectors, and the LRT are studied. In particular, we show that each of those suboptimal detectors can approach the optimal LRT under certain operating conditions. These results are verified via Monte Carlo simulations.

Bluetooth is reputed as a wireless networking technology capable of forming ad-hoc networks between digital devices. In particular, the Bluetooth scatternet will be an essential part of the fully distributed ad-hoc networks. However, scatternet is not fully described in the Bluetooth specification. This has been the topic of discussion among researches in relation to the formation algorithm, scheduling scheme, etc. Most of the proposed algorithms reported in past researches on scatternet formation are too large and complex to be implemented in a real commercial Bluetooth hardware. Therefore, the verifications of the proposed algorithms reported in past researches were done through only simulations. In addition, the formation process takes too long and these past researches had been conducted only in static environment where no node enters or leaves the scatternet. In this paper, therefore, we propose a new scatternet formation algorithm called Node Ring Scatternet (NRS), emphasizing on two aspects, i.e. implementation and dynamic property of the algorithm. The algorithm is very simple and compact and is verified to be easily implementable in a real commercial Bluetooth device. For the dynamic properties, the NRS entails relatively short formation delay and a reformation algorithm in a dynamic environment was designed. Therefore, the network of the NRS can be scalable and flexible. In addition, a new protocol called SFMP (Scatternet Formation & Management Protocol) was designed and is presented herein. Using this protocol, the NRS algorithm was implemented in a real Bluetooth device, and the performance was verified through hardware experiments. Based on the experimental results, it was found that the NRS composed of up to 20 nodes is formed and the proposed algorithm has shown improvement in terms of formation delay, formation probability and reformation.

In conventional OFDM systems, the effect of inter-block-interference (IBI) can be completely removed by inserting sufficient redundant symbols between successive transmission blocks. In this paper, based on the reformulated received block symbols of the discrete multirate filterbanks model, a new transceiver model for the cyclic prefix (CP) OFDM systems is proposed, associated with the oblique projector technique (view as the pre-processor for achieving IBI-free). Consequently, a novel ISI-free receiver with the zero-order FIR zero-forcing (ZF) filterbanks equalizer can be devised, under noise-free environment. For performance comparison the bit-error-rate (BER) is investigated for the cases of noisy and noise-free channels. In all cases, viz., the length of CP is shorter or longer than the order of the channel impulse response, we show that the same BER performance compared with the one suggested in [3] can be achieved, under the same assumptions and conditions. Since a simple cascade configuration of the IBI cancellation using the oblique projector followed by the ISI cancellation using the zero-order FIR ZF filterbanks equalizer can be realized for OFDM systems with sufficient or insufficient CP, the complexity of transceiver design can be reduced.

In this paper, a multiple-pulse signaling format for M-ary equicorrelated modulation (ECM) is proposed to enable the noncoherent detection on a multiple-symbol basis. Several time-limited and band-limited basis waveform sets are designed to embody the multiple-pulse ECM signals and explored to determine the spectral performance characteristics. Based on the maximum-likelihood decision principle, a block receiver is developed for noncoherently demodulating multiple-pulse ECM signals on additive white Gaussian noise channels. Tight upper and approximate bounds are derived and verified by simulation to evaluate the bit and symbol error probability characteristics of the developed ECM block receiver. It is analytically shown that the noncoherent M-ary ECM block receiver with a small-sized blocklength offers comparable performance to the ideal coherent M-ary simplex receiver when the pairwise signal correlation is appropriately chosen. In particular, the proposed noncoherent nonbinary simplex modulation is found to strongly outperform the conventional noncoherent nonbinary orthogonal modulation in terms of both power and spectral efficiencies.

We propose a multiple dwell serial search (MDSS) code acquisition for optical code-division multiple-access (O-CDMA) systems and theoretically analyze its performance. The search/lock strategy (SLS) is used as verification scheme for the multiple dwell detector. The operation of SLS is modeled by finite Markov chain to analyze the performance of the proposed system. Effect of system parameters, such as number of users, threshold and mean photon count per chip, on the performance of the proposed system is investigated. The theoretical result shows that the performance of the proposed system is less sensitive to parameter settings than the conventional single dwell serial search (SDSS) code acquisition system is. In addition, the proposed MDSS code acquisition system offers shorter mean acquisition time than that of conventional SDSS system.

The shuffle-like network (SL-Net) is known as a logical topology for WDM-based multihop packet-switched networks. Even if we fix the logical topology to an SL-Net, we can still reposition nodes in the SL-Net by re-tuning wavelengths of transmitters and/or receivers. In conventional node placement algorithms, routes between nodes are assumed to be given. In this paper, we propose two heuristic node placement algorithms for the SL-Net to decrease the average end-to-end packet transmission delay under a given traffic matrix in the case that routes are design variables. The principal idea is to prevent too many traffic flows from overlapping on any link. To attain the idea, in one of the algorithms, a node is selected one by one in a decreasing order of the sums of sending and receiving traffic requirements in nodes, and its placement and routes between the node and all the nodes already placed are simultaneously decided so that the maximum of the amounts of traffic on links at the moment is minimum. In the other algorithm, a node is selected in the same way, and first it is placed so that the average distance between the node and all the nodes already placed is as large as possible, and then routes between the node and all the nodes already placed are decided so that the maximum of the amounts of traffic on links at the moment is minimum. Numerical results for four typical traffic matrices show that either of the proposed algorithms has better performance than conventional algorithms for each matrix, and show that the proposed algorithms, which are based on a jointed optimization approach of node placement and routing, are superior to algorithms which execute node placement and routing as two isolated phases.

Since mobile ad-hoc networks have certain constraints, including power shortages, an unstable wireless environment, and node mobility, more power-efficient and reliable routing protocols are needed. Accordingly, this paper propose a new routing protocol, PERRA (Power Efficient Reliable Routing protocol for mobile Ad-hoc networks), that includes the advantages of on-demand protocols, while also providing power- efficient and reliable packet transmission. PERRA uses a new cost function to select the optimum path based on considering the minimum residual energy of the nodes on a path, the total energy consumed by a path to transmit and process a packet, and the path's stability in accordance with the node mobility. As a result, the proposed method increases the power efficiency, decreases the route-reconstructions due to residual power shortages and node mobility, and provides effective route maintenance mechanisms. The performance of PERRA is evaluated by simulations under various conditions.

Content Distribution Networks (CDNs) are highly advanced architectures for networks on the Internet, providing low latency, scalability, fault tolerance, and load balancing. One of the most important issues to realize these advantages of CDNs is dynamic content allocation to deal with temporal load fluctuation, which provides mirroring of content files in order to distribute user accesses. Since user accesses for content files change over time, the content files need to be reallocated appropriately. In this paper, we propose a cost-effective content migration method called the Step-by-Step (SxS) Migration Algorithm for CDNs, which can dynamically relocate content files while reducing transmission cost. We show that our method maintains sufficient performance while reducing cost in comparison to the conventional shortest-path migration method. Furthermore, we present six life cycle models of content to consider realistic traffic patterns in our simulation experiments. Finally, we evaluate the effectiveness of our SxS Migration Algorithm for dynamic content reconfiguration across time.

We propose a method of dimensioning and managing the bandwidth of a link on which flows with heterogeneous access-link bandwidths are aggregated. We use a processor-sharing queue model to develop a formula approximating the mean TCP file-transfer time of flows on an access link in such a situation. This only requires the bandwidth of the access link carrying the flows on which we are focusing and the bandwidth and utilization of the aggregation link, each of which is easy to set or measure. We then extend the approximation to handle various factors affecting actual TCP behavior, such as the round-trip time and restrictions other than the access-link bandwidth and the congestion of the aggregation link. To do this, we define the virtual access-link bandwidth as the file-transfer speed of a flow when the utilization of the aggregation link is negligibly small. We apply the virtual access-link bandwidth in our approximation to estimate the TCP performance of a flow with increasing utilization of the aggregation link. This method of estimation is used as the basis for a method of dimensioning the bandwidth of a link such that the TCP performance is maintained, and for a method of managing the bandwidth by comparing the measured link utilization with an estimated threshold indicating degradation of the TCP performance. The accuracy of the estimates produced by our method is estimated through both computer simulation and actual measurement.

In OFDM systems, employing a cyclic prefix (CP) as the guard interval is a simple way to combat the inter-symbol interference (ISI) and the inter-carrier interference (ICI), however it reduces the transmission efficiency of the system, especially for some channels with a very long delay spread. In this paper, we consider the OFDM system with insufficient CP, much more efficient than conventional OFDM systems. First, we present the system mathematical model and give the ISI and ICI analysis. Then the signal-to-interference power ratio (SIR) performance is presented. To reduce the ISI and ICI due to the insufficient CP, we develop a minimum-mean-square-error decision feedback equalizer (MMSE_DFE). Based on the MMSE criterion, the optimum feedforward and feedback filter coefficients are derived. For time-varying channel, to avoid brute force matrix inversion in conventional schemes, we propose an adaptive LMS based solution to update the filtering coefficients by tracing the channel variation. Since the high complexity of MMSE_DFE, a reduced complexity scheme, ordered successive partial interference cancellation DFE (OSPIC_DFE), is developed. From the performance comparison between the MMSE_DFE and the OSPIC_DFE, we see that the latter is very near to the former. Finally the simulation shows these proposed methods are highly effective in combating ISI and ICI with low complexity.

The IEEE 802.11 distributed coordination function (DCF) provides a contention-based distribution channel access mechanism for stations to share the wireless medium. However, performance of the DCF drops dramatically in terms of throughput, delay and jitter as the number of active stations becomes large. In this paper, we propose a simple and effective scheme, called distributed coordination function with virtual group (DCF/VG), for improving the performance of the IEEE 802.11 DCF mechanism. In this scheme, each station independently decides a virtual group cycle taking into account the current contention level. The virtual group cycle consists of one or more virtual groups and a virtual group includes an idle and a busy period. Each station chooses one virtual group and operates only in the chosen group of the cycle. In other words, each station decreases its backoff counter and tries to transmit its packet during the period of the chosen virtual group like in the IEEE 802.11 DCF. Performance of the proposed scheme is investigated by numerical analysis and simulation. Our results show that the proposed scheme is very effective and has high throughput and low delay and jitter behaviors under a wide range of contention levels.

A simple method for interleaving operation suitable for paralleled converter system is proposed. This method automatically detects the number of converters and adjusts phases between converter modules equally for any number of modules in the system. The method is realized by simple analog circuit which is easily implemented as conventional PWM controller IC. Principle of multiphase controlling circuit is introduced, and the influence of non-ideal circuit parameters on interleaving operation are discussed. A compensator for reducing phase error is also proposed to achieve wide-use application. Experimental and analytical results confirm the effectiveness of the proposed method.

Signal detectors generally utilize nonlinear statistics of an original observation rather than the original observation as it is. The sign statistic, a typical example of the nonlinear statistics, is the sign information of an observation and the sign detector relies only on the sign statistic. Since either detector might be of a better performance depending on the situation, it is quite important to determine which is the best performer among the detectors, based on the given situational information about noise and signal strength. In this letter, a qualitative analysis is presented that the correlation coefficients between the statistics and original observation can be used to predict the asymptotic performance of a detector utilizing one of the statistics, relative to the other detectors.

Although the multiuser detection scheme based on Kalman filtering (K-MUD) proposed by Zhang and Wei, is referred to as a "blind" algorithm, in fact it is not really blind because it is conditioned on perfect knowledge of system parameter, power of the desired user. This paper derives an algorithm to estimate the power of the user of interest, and proposes a completely blind multiuser detection. Computer simulations show that the proposed parameter estimation scheme obtains excellent effect, and that the new detection scheme has nearly the same performance as the K-MUD, there is only slight degradation at very low input signal-to-interference ratios (SIR).

This letter presents a method of finding the optimal quantization stepsize that minimizes quantization loss and maximizes coded BER performance. We define 'Information Error Rate'(IER) and obtain the equation of the modified constraint length (K_m) to obtain an upper bound of coded BER performance of a l bit quantized soft decision Viterbi decoder. Using IER and K_m, we determine the optimal quantization stepsize of a 2 bit and 3 bit quantized soft decision decoding system in an AWGN channl with respect to SNR, and verify our strategies by simulation results.

Orthogonal frequency division multiplexing (OFDM) has been adopted in the physical layer of IEEE802.11a WLAN standard. In this Letter, a high-resolution frequency-offset estimation scheme is presented for the OFDM-based WLAN. The scheme efficiently exploits the features of the 802.11a preamble for high-resolution estimation of frequency-offset. Simulation results indicate that the algorithm is much superior to the conventional scheme for frequency-offset estimation in the 802.11a WLAN.

An efficient void filling (VF) algorithm is proposed for wavelength division multiplexing (WDM) optical packet switches (OPSes) handling variable-packet-length self-similar traffic. The computation complexity of the proposed algorithm is extremely low. We further compare the switching performance of the proposed algorithm with that of the conventional one. We demonstrate that the proposed algorithm offers significantly lower computation complexity with adequate performance.

NAT-PT and DSTM are becoming more widespread as de-facto standards for IPv6 dominant network deployment. But few researchers have empirically evaluated their performance aspects. In this paper, we compared the performance of NAT-PT and DSTM with IPv4-only and IPv6-only networks on user applications using metrics such as throughput, CPU utilization, round-trip time, and connect/request/response transaction rate.

A new channel estimator that does not require a separate frequency offset estimator is proposed. The new algorithm has low complexity and low latency compared to the well-known weighted multi-slot averaging algorithm. The simulation results demonstrate the improved resistance to high Doppler frequency and high frequency offset.

In this letter, a new channel-adaptive beamforming method is proposed for OFDMA systems with smart antennas. In the method, the size of a cluster for resource unit is determined adaptively according to a region-splitting criterion. It is shown by simulations that the proposed method shows good performance in both frequency-flat and frequency-selective channels.

In this letter, new results on the BER performance of multitone DS-CDMA systems for transmissions over Nakagami-m fading channels with exponentially decaying multipath intensity profile are presented. The results show that, in viewpoint of the BER performance, there is a critical relation between the number of resolvable paths and the effect of the rate of average power decay.

This paper presents a new on-off beamforming algorithm to provide an optimal weight vector. The proposed algorithm is based on the eigen-space on-off algorithm for exploiting a diversity gain that is available in the signal environments of wide angle spread. The proposed technique utilizes the primary and secondary eigenvectors to form the optimal weight vector, which is robust to the angle spread. It is confirmed in this paper that the proposed algorithm shows excellent performance compared to some typical beamforming methods which degrade the receiving performance as the angle spread increases. It is also shown that the proposed algorithm has a reasonable computational complexity of O(14N) and fast convergent speed, where N is the number of antenna elements.

In this paper, a new algorithm for the optimal training sequence with respect to sequence length in 1-dimensional cluster-based sequence equalizers (1-D CBSE) is presented. The proposed method not only removes the step of random training sequence selection but also shortens the length of the selected training sequences. The superiority of the new method is demonstrated by presenting several simulation results of quadrature phase shift keying (QPSK) signaling schemes and related analyses.

In this paper, we propose an efficient rate and power allocation scheme for multiuser OFDM systems to minimize the total transmit power under the given QoS requirements. We deduce the optimal solution of transmit power minimization problem and develop a suboptimal algorithm with low complexity based on the theoretical analysis. Because of the avoidance of iterative procedure, it is less complex than the existing schemes. The simulation results show that our proposal outperforms the existing schemes and it is very close to the optimal solution.

In medium access control (MAC) protocols for wireless local area networks (WLANs), the Round-Robin scheme is the general polling scheme. A major drawback of this scheme is that it is inefficient when only a small number of stations have packets to transmit. This inefficiency is caused by polling stations that have no packets to transmit. This paper proposes an effective and simple polling scheme to reduce the number of polling times for a station with no packets to transmit. For example, the simulation result shows that the throughput increases by 35.8% when fifteen stations out of thirty stations in the polling list have packets to transmit at IEEE 802.11a 54 Mbps rate.