With the rapid growth of the Internet of Things (IoT) devices, a lot of IoT malware has been created, and the security against IoT malware, especially the family classification, has become a more important issue. There exist three requirements which classification systems must achieve: detection of new families, precise classification for sequential inputs, and being independent of computer architectures. However, existing methods do not satisfy them simultaneously. In this paper, we propose a realtime IoT malware classification system based on pending samples. In order to detect new families and to classify sequential inputs precisely, we introduce the concept of “pending samples”. This concept is useful when heterogeneous inputs which are difficult to classify instantly come into the system. This is because the system can postpone classifying them until similar samples come. Once similar samples are gathered, we regard these samples as a new cluster, meaning that detecting new families is achieved. Moreover, we use printable strings to satisfy the requirement of being independent of architectures because strings are common among different architectures. Our results show the ability to detect new families demonstrated by finding new clusters after applying our algorithm to the initial clusters. Furthermore, our new clustering algorithms achieves a 0.130 higher V-measure compared to the k-means algorithm, which is the representative clustering algorithm.

Website Fingerprinting Attack (WFA) which identifies websites browsed on Android devices is extremely dangerous because it creates an opportunity for stealing private information. As the most feasible WFA method, we focus on a method that can identify a website by using the power consumption model restored from CPU data. However, that is not effective in a real situation where multiple background tasks run because CPU data which are unrelated to browsing are confused. Furthermore, the previous method cannot accurately identify simple websites that are subject to background tasks. Thus, a more feasible method is required to indicate the dangers. In this paper, we propose a website fingerprinting attack based on virtual memory of process on Android device. We focus on the fact that a specific process about browsing websites works when a website is browsed. Because each process has its virtual memory which is independent of each other, the useful feature of a task can be extracted from the virtual memory without noise. Therefore, the proposed method can precisely identify a browsed website by using the virtual memory-based features even if background tasks work. Furthermore, the proposed method can obtain effective information even for a simple website. By computer simulation with a real dataset, we demonstrate that the proposed method can improve up to 86%, 89%, and 82% in precision, recall, and F-measure, respectively for websites which the previous scheme cannot identify at all.

The development of deep learning (DL) techniques has enabled to crack traditional text-based CAPTCHA (Com-pletely Automated Public Turing test to tell Computers and Humans Apart), which results in new security issues. As a coun-termeasure against DL based attacks, the adversarial CAPTCHA is well suited since it can increase the difficulty of machine recognition while ensuring human readability. However, spatial smoothing can negate the effectiveness of adversarial CAPTCHAs because adversarial noises in them are subject to averaging pixels. As far as we know, there are no effective counters against spatial smoothing, whereas it is the critical problem which facilitates spreading automated attacks. Therefore, to address the unsolved problem, in this paper, we propose an adversarial text-based CAPTCHA generation method utilizing spatial smoothing. We focus on the fact that when spatial smoothing is applied to an image, the amount of information it carries decreases, making the whole image blurred. Spatial smoothing is only viable as an attack when the mitigation of the adversarial noise has a larger impact than the whole image getting blurred. Thus, when the degree of spatial smoothing exceeds a certain threshold, the impact of the two aspects reverses, and the difficulty of the recognition increase. By utilizing this phenomenon in the generation of CAPTCHAs, the proposed method can indirectly neutralize the intended effect of spatial smoothing by attackers, preventing the recognition rate from increasing. Our evaluation shows the proposed method can reduce the recognition rate by up to 34%, compared to the conventional method. Besides, an experiment on human recognition rates marked 73.67%, showing that human recognition is maintained at an acceptable level.

Detecting a Rogue Access Point (RAP) in Wi-Fi network is imperative. The previous scheme is user side detection focusing on two channels used by a RAP. That scheme can detect a RAP in stable traffic environment by revealing the channel used with a Legitimate Access Point (LAP) with intentional interference. However, the detection performance is degraded in the real environment where traffic is more unstable because it affects the traffic on the channel. Thus, it is necessary to design the scheme which is independent of such factors. In this paper, we propose RAP detection by using Address Resolution Protocol (ARP) failure under the Media Access Control (MAC) address duplication. Our main idea is that the traffic is relayed via a RAP and a LAP on the LAN path between a client and a gateway under the attack. This is because the RAP must be established between a client and a LAP to provide Internet connection. On the basis of this idea, the proposed scheme reveals that the Access Point (AP) with which a client connects is a RAP by discovering the MAC address of a LAP on the path. In order to find the MAC address, we leverage the phenomenon that a client cannot receive ARP reply packets in the situation where its MAC address and that of a AP are duplicated on the path. By doing this, the presence of a LAP is revealed, which can judge that the connected AP is a RAP. In our evaluation, the proposed scheme achieves accuracy of 96.5% even in unstable traffic environment. True positive rate and false positive rate are 31.0% higher and 9.0% lower than the previous scheme. Furthermore, the proposed scheme can detect RAPs accurately in real environment where the previous scheme cannot.

There exists a need of “image steganalysis” which reveals whether steganographic signals are embedded in an image to improve information security. Among various steganalysis, Convolutional Neural Networks (CNN) based steganalysis is promising since it can automatically learn the features of diverse steganographic algorithms. However, we discover the detection performance of CNN is degraded when an image is intentionally reduced by the nearest-neighbor interpolation before steganography. This is because spatial frequency in a reduced image gets high, which disturbs the training. In order to overcome this shortcoming, in this paper, we propose a preprocessing methodology by using additional steganography on CNN-based steganalysis. In the proposed preprocessing, steganographic signals are additionally embedded into both reduced original images and reduced steganographic ones since a difference of spatial frequencies between them gets obvious, which helps CNN learn features. Whenever reduced images are trained in CNN or inspected whether they are steganographic ones or not, steganography is applied to them once by the proposed preprocessing. Thus, an image is regarded as a steganographic one if the trained model judges steganography is applied to it twice; otherwise it is an original one. Since the proposed methodology is very simple, its computational cost is low. Our evaluation shows accuracy in a model with the proposed preprocessing is 10.6% higher than that in the conventional one. Besides, even in the situation where another steganography is additionally embedded, the proposed preprocessing yields 7% higher accuracy compared with the conventional one.

Recently, connected and automated vehicles have potential to mitigate traffic congestion and improve traffic efficiency at a highway on-ramp. Especially, vehicle to infrastructure communication based scheduling schemes are paid attention because a central control server can assign merging timing to automated vehicles to improve traffic efficiency on whole roads. Although several schemes calculate merging timing based on their criteria, these schemes cause traffic congestion when inflow traffic becomes high. In order to solve this problem, it is necessary to schedule merging timing of vehicles such that density of vehicles decreases on whole roads. In this paper, we propose a scheduling scheme for cooperative merging at a highway on-ramp with maximizing average speed of automated vehicles. The main idea of the proposed scheme is to utilize the traffic model that density and speed of vehicles have a negative correlation. By utilizing the traffic model, the proposed scheme preferentially assigns merging timing to vehicles which are on a congested road by maximizing average speed of all vehicles on whole roads. Simulation result shows that the proposed scheme decreases density of vehicles on whole roads even if inflow traffic becomes high. Furthermore, the proposed scheme improves outflow traffic from a highway on-ramp by 400 (veh/h) at most in comparison with related schemes.

In this paper, we propose a novel visual similarity-based phishing detection scheme using hue information with auto updating database. Since a PWS (Phishing Website) is created based on targeted legitimate website or other subspecies whose hue information is similar each other, many PWSs can be exhaustively detected by tracing similar colored subspecies. Based on this notion, the proposed scheme detects a new PWS which has similar hue information to already detected PWSs. By repeating this procedure, the detection scope can be effectively expanded. In order to avoid the misdetection of legitimate websites which have similar hue information to database's ones, the proposed scheme utilizes the fact that the combination of used colors is hard to be similar among legitimate websites and PWSs. By the computer simulation with real dataset, we demonstrate that the proposed scheme improves the detection performance as the number of detected PWSs increases.

To realize content distribution, NDN (Named Data Networking) is gathering attention. Since NDN is vulnerable to spreading fake contents, router based verification schemes are proposed to solve this problem. However, routers are vulnerable to the attack which puts a burden to them by verification of contents (verification attack). In order to detect it, the scheme leveraging the fact that the number of the request of unverified contents and the verification of them increase under the attack is proposed. While verification attack can be detected by that scheme, the attack has already occurred. In order to detect the attack before it occurs, in this paper, we propose a trust-based verification attack prevention scheme using tendency of contents request on NDN. We focus on the fact that the access interval to unverified contents tends to be short dramatically just before verification attack occurs. By leveraging this fact, the router determines that verification attack has occurred and restricts requests of all users temporarily. However, in this case, it is impossible to identify attackers, and the requests of legitimate users are also restricted. Therefore, we focus on the fact that legitimate users tend not to request contents in a cache in many cases. Meanwhile, in order to conduct verification attack, attackers need to request such contents for a short time. By giving low trust value to users requesting these contents, a router can identify attackers and restrict only attackers' requests. Our evaluation results show our scheme can detect verification attack before the attack. Furthermore, we clearly demonstrate that our scheme can restrict only attackers' requests.

Detecting Android malware is imperative. As a promising Android malware detection scheme, we focus on the scheme leveraging the differences of traffic patterns between benign apps and malware. Those differences can be captured even if the packet is encrypted. However, since such features are just statistic based ones, they cannot identify whether each traffic is malicious. Thus, it is necessary to design the scheme which is applicable to encrypted traffic data and supports identification of malicious traffic. In this paper, we propose an Android malware detection scheme based on the level of SSL server certificate. Attackers tend to use an untrusted certificate to encrypt malicious payloads in many cases because passing rigorous examination is required to get a trusted certificate. Thus, we utilize SSL server certificate based features for detection since their certificates tend to be untrusted. Furthermore, in order to obtain the more exact features, we introduce required permission based weight values because malware inevitably require permissions regarding malicious actions. By computer simulation with real dataset, we show our scheme achieves an accuracy of 92.7 %. True positive rate and false positive rate are 5.6% higher and 3.3% lower than the previous scheme, respectively. Our scheme can cope with encrypted malicious payloads and 89 malware which are not detected by the previous scheme.

Delay Tolerant Network (DTN) is characterized by a lack of end-to-end connectivity. Due to this, detecting flooding attack in DTN is a challenging and important task. Among several schemes against flooding attack in DTN, the scheme using Encounter Record (ER) that consists of past transmission history of each node is gathering attention. Since ER entries are exchanged between nodes, a node can detect an attacker whose transmission rate is too much. Although an attacker may falsify entry to pretend that his/her transmission rate is less than the actual value, it can also be detected through the contradiction between a falsified entry and another entry. However, since a node sends all ER entries in its own buffer regardless of whether an entry is helpful to detect an attacker or not, the energy consumption increases as the number of entries increases. In this paper, we propose an ER reduction scheme based on theoretical contact probability for flooding attack mitigation. We focus on the fact that if the falsified entry does not exist, corresponding entries are not helpful to detect an attacker. Since the falsified entries are propagated over the network with the lapse of time, the probability that there is no contradicting entries over the network gets higher if a node has not received any contradicting entries for a sufficient time. By removing such entries, the energy consumption can be reduced while the effectiveness of ER is kept. By computer simulation, we demonstrate our scheme successfully reduce the energy consumption while the same level of performance is achieved.

The main ideas of our scheme are that the storage nodes are selected according to the updating frequency of data for reducing the transmission and the delay, and the relay nodes can be acted as the owners of data instead of the real owner of data for protecting the user's privacy. Furthermore, by using the DIFF method, the user can effectively modify the stored data with the lower transmission cost and delay.

In order to fix a bug or install a new application, disseminating a code to specific target sensors is an important management task in wireless sensor networks. In order to disseminate a code to specific target sensors in an energy efficient manner, it is required to select the minimum required number of forwarders with a small number of control messages. In this paper, we propose an energy efficient target specific code dissemination scheme with a forwarder selection algorithm. The proposed scheme consists of two steps, (i) a forwarder selection algorithm with a control message suppression, (ii) unnecessary forwarders reduction with a sleep mechanism. In order to select the minimum required forwarders with a small number of control messages, we propose an algorithm to select a forwarder where more target sensors are reachable, through a single control message by leveraging overhearing of control messages. In addition, in order to reduce the number of forwarders, we propose a sleep mechanism to put unnecessary forwarders to sleep where any target sensor is not reachable. A performance analysis has shown that the number of control messages of the proposed scheme is always theoretically smaller than that of a previously published forwarder selection scheme under the same condition. In addition, we have shown that the proposed scheme reduces the energy consumption compared with the previous scheme.

On web application services, detecting obfuscated malicious JavaScript utilized for the attacks such as Drive-by-Download is an urgent demand. Obfuscation is a technique that modifies some elements of program codes and is used to evade the pattern matching of traditional anti-virus softwares. In particular, encode obfuscation is adopted in almost all malicious JavaScript codes as the most effective technique to hide their malicious intents. Therefore, many approaches focus on encode obfuscation to detect malicious JavaScript. However, we point out that malicious JavaScript obfuscated by the techniques except for encode obfuscation can easily evade those approaches. Motivated by the above, in this paper, we first investigated the malicious files that previous schemes cannot detect, and found that some files contain divided URL in their codes. In order to detect such JavaScript codes as malicious, we propose obfuscated malicious JavaScript detection scheme using the feature based on divided URL. We focus on the fact that the segments of URL are declared as variables and connected later. Our scheme stores variables and their contents in the dictionary type object and in the connection parts, verifies that malicious URL can be reconstructed. By the computer simulation with real dataset, we show that our scheme improves the detection effectiveness of the conventional scheme.

Recently, the detection of target link flooding attack which is a new type of DDoS (Distributed Denial of Service) is required. Target link flooding attack is used for disconnecting a specific area from the Internet. It is more difficult to detect and mitigate this attack than legacy DDoS since attacking flows do not reach the target region. Among several schemes for target link flooding attack, the scheme focusing on traceroute is gathering attention. The idea behind that is the attacker needs to send traceroute to investigate the topology around targeted region before attack starts. That scheme detects the attack by finding rapid increase of traceroute. However, it cannot work when attacker's traceroute ratio is low. In this paper, we propose traceroute-based target link flooding attack detection scheme by analyzing hop count to the destination. Since the attacker must choose the link flooded to disconnect the target area, the destinations of attacker's traceroutes are concentrated within several hops from the target link while legitimate user's ones are distributed uniformly. By analyzing the number of traceroutes as per hop counts, the change can be emphasized and the attack symptom might be more easily captured. By computer simulations, we first prove the above hypotheses and show that our scheme has more robustness compared with the conventional scheme.

The detection of phishing websites and identifying their target are imperative. Among several phishing detection schemes, the scheme using visual similarity is gathering attention. It takes a screenshot of website and stores it to the database. If the inputted website''s screenshot is similar to database''s one, it is judged as phishing. However, if multiple similar websites exist, the first inputted website is regarded as legitimate. As a result, it cannot correctly detect legitimate website and identifying phishing target becomes difficult. As a second shortcoming, if the screenshot of phishing website is locally different from ones in the database, false negative occurs. In this paper, we propose visual similarity-based phishing detection scheme using image and CSS with target website finder. To remedy first shortcoming, we focus on the fact that legitimate websites are often linked by other websites and regard such website as legitimate and store the screenshot and CSS in the database. Since CSS is a file which defines the websites visual contents, attackers often steal legitimate CSS to mimic the legitimate website. Thus, by detecting the website which plagiarizes appearance or CSS of legitimate website, we detect phishing website and its target simultaneously. Moreover, we can alleviate the second shortcoming by using CSS because it is probable that the websites which have locally different appearance use identical CSS. By computer simulation with real dataset, we demonstrate our scheme improves detection accuracy while finding phishing target.

In RFID-enabled supply chains, in order to securely deliver products, a secret sharing scheme has been proposed. An encryption key of tags is split into shares and the key can be recovered with the sufficient number of correct shares. The conventional scheme introduces sufficient number of dummy tags with bogus shares and prevents an attacker who reads all tags during shipping from obtaining tag information. The number of required dummy tags is decided so that the success probability of the attack becomes lower than that in the brute force manner. When the number of shipping products is small, too many dummy tags are required to prevent the attacker from selecting correct shares with a small number of trial. In this paper, we propose a practical scheme which reduces the number of tags by using extra legitimate tags. An encryption key and its share for legitimate tags are assigned to not only the conventional legitimate tags but also extra legitimate tags, and thus, the number of legitimate tags is virtually increased. By doing this, the number of required dummy tags can be reduced. Extra legitimate tags are separately composed from conventional legitimate tags and dummy tags, then they can be distinguished easily. We show the effectiveness of our scheme by computer simulation.

In WRSNs (Wireless Rechargeable Sensor Networks), the wireless energy transfer technology enables charging sensor batteries and maintaining perpetual operation of the network. The MC (mobile charger) is able to provide stable and reliable energy supply for sensor nodes. Although many researchers study the efficient scheduling of the MC, none of them considers the situation where both the static sink and the MC can collect packets. In this case, each sensor node should choose the appropriate destination, i.e., the static sink or the MC. In this paper, we propose closer destination selection scheme for mobile sink and charger enabled WRSNs where a static sink and an MC coexist. In order for each sensor node to send packets to the closer destination, i.e., the static sink or the MC, when an MC tours to charge sensor nodes, it constructs another temporal tree-based topology network by sending a route construction packet to sensor nodes within the limited area. Whenever the MC stays at a sensor node, it calculates the maximum number of hops that sensor nodes should switch to a temporal network. This temporal network that the MC creates does not disturb the original network. By constructing a temporal network, an MC relaxes the load of sensor nodes around the static sink without unnecessary path extension. We evaluate the performance of our scheme through the computer simulation. Results indicate that our scheme improves the packet arrival rate and energy efficiency compared to the conventional scheme.

The video streaming service over Wireless Mesh Networks (WMNs) is an attractive feature for live streaming and video conversation when disaster occurs. The path construction with the conventional metrics e.g., throughput, latency, or packet loss rate, suffers from the repeated waiting time due to the CSMA/CA since paths are concentrated on nodes with good metric. As a consequence, it brings about throughput fluctuation of video streaming service. We propose two metrics that reflect on the magnitude of CSMA/CA interference around a node in order to select a path that the throughput of video streaming session will get stable. The first metric is calculated from the number of received Request To Send (RTS)/Clear To Send (CTS) frames. Since the number of RTS/CTS reflects on the frequency of the packet transmission around a node, a source node can chooses a path which does not make a node waiting for constant time due to the CSMA/CA on the existing path that might use streaming service. The second metric uses the number of neighbor nodes and we set the larger metric value as more nodes exist. The idea comes from the fact that a node which has few neighbor nodes does not much interfere other established paths. The proposed metrics can be easily installed into Hybrid Wireless Mesh Protocol (HWMP) which is a de facto standard path selection protocol defined in IEEE802.11s. We show that the proposed metrics can achieve lower rate of throughput fluctuation than that of conventional metric by using the networks simulator Qualnet.

The IPv6 Routing Protocol for Low-power and Lossy networks (RPL) is a standard routing protocol to realize the Internet of Things (IoT). Since RPL is a tree-based topology network, an attacking node may falsely claim its rank towards neighbor nodes in order to be chosen as a parent of them and to collect more packets to tamper. In this paper, we propose a secure parent selection scheme so that each child node can select a legitimate node as its parent. In the proposed scheme, each node chooses a parent after excluding the best candidate if multiple parent candidates exist. Our scheme utilizes the fact that an attacking node claims falsely a lower rank than that of a legitimate nodes. We show that attacking nodes have no merits to claim lower ranks than true ones in a secure parent node selection scheme. By the computer simulation, we show that the proposed scheme reduces the total number of child nodes attached to attacking nodes in comparison with the conventional RPL scheme.

Detecting SPIT (Spam over Internet Telephony) is an urgent demand with growing voice communication services. Chaisamran et al. proposed a trust-based SPIT detection scheme and it is superior to other trust-based schemes since it is valid for unknown users. However, this scheme might misdetect a call from low-frequent users as SPIT gradually in time, since they seldom call and thus their social trusts get decreased with time. In this paper, we propose a two-stage SPIT detection scheme using BC (Betweenness Centrality) and social trust to decrease misdetection of a call from low-frequent users as SPIT. BC indicates user's centrality in the entire network and the value of BC against legitimate users gradually increases with time even if users seldom call, whereas it does not increase against spammers since the connection between a legitimate caller and a spammer is hardly established. As a first stage, we use BC as a feature to correctly identify a call request from a low-frequent user. Then we judge whether a call is from a legitimate caller or a spammer by using social trust. By the computer simulation, we show that our scheme improves the false positive rate while maintaining high true positive rate.

In Wireless Sensor Networks (WSNs), Energy Harvesting (EH) is emerging as a promising solution to realize the energy-efficient green communications. Non-EH nodes can save the energy by using EH nodes as relaying nodes, and relaying with the better throughput is required to efficiently consume the precious energy. In most cases, the EH charging rate gets low since current charging techniques cannot convert the environmental energy well, so it is important to achieve high performance in low charging rate. In this paper, we propose an adaptive relay selection scheme based on the average charging rate to achieve better throughput even in low charging rate. In our scheme, each node adaptively changes the criterion to be a relaying candidate based on the current charging rate; an EH node whose charging rate is low raises the threshold to save the precious energy for better channel condition. By the computer simulation, we show that our proposed relay selection scheme achieves the better throughput than that of the conventional schemes in low charging rate, which is the more realistic condition.

With the recent growth of mobile communication, location-based services (LBSs) are getting much attention. While LBSs provide beneficial information about points of interest (POIs) such as restaurants or cafes near users, their current location could be revealed to the server. Lien et al. have recently proposed a privacy-preserving k-nearest neighbor (k-NN) search with additive homomorphic encryption. However, it requires heavy computation due to unnecessary multiplication in the encryption domain and this causes intolerant burden on the server. In this paper, we propose a lightweight private circular query protocol (LPCQP) for privacy-preserving k-NN search with additive and multiplicative homomorphism. Our proposed scheme divides a POI-table into some sub-tables and aggregates them with homomorphic cryptography in order to remove unnecessary POI information for the request user, and thus the computational cost required on the server is reduced. We evaluate the performance of our proposed scheme and show that our scheme reduces the computational cost on the LBS server while keeping high security and high accuracy.

The hybrid deployment of wireless sensors with static and mobile nodes in the monitoring area is an important issue to cover a maximum sensing area with a limited number of nodes. Furthermore, mobile sensor nodes can relocate themselves to improve coverage area in the network. In this paper, we propose a method that reduces the complexity on the relocation of initial deployment and coverage hole healing of mobile sensor nodes in the hybrid Wireless Sensor Networks (WSNs). Our method finds the best-ordered ways of mobile nodes' movement to give the shortest distance movements for mobile nodes in WSN. Adaptive threshold distance is used to eliminate the consideration of some mobile nodes, which are already occupied or situated within the threshold distance from optimal new positions. The simulation results show that the proposed methods can give much smaller calculation time to find the shortest paths for the movements of mobile nodes in WSN.

As VoIP (Voice over IP) grows rapidly, it is expected to prevail tremendous unsolicited advertisement calls, which type of calls is referred to SPIT (SPam over Internet Telephony). SPIT detection is more difficult to execute than email SPAM detection since the callee or SPIT detection system does not tell whether it is SPIT or legitimate call until he/she actually takes a call. Recently, many SPIT detection techniques are proposed by finding outliers of call patterns. However, most of these techniques suffer from setting a threshold to distinguish that the caller is legitimate or not and this could cause to high false negative rate or low true positive rate. This is because these techniques analyse call pattern by a single feature e.g. call frequency or average call duration. In this paper, we propose a multi-feature call pattern analysis with unsupervised Random Forests classifier, which is one of the excellent classification algorithms. We also propose two simple but helpful features for better classification. We show the effectiveness of Random Forests based classification without supervised training data and which features contribute to classification.

For cooperative wireless networks, we propose a medium access control (MAC) protocol with distributed relay selection using group-based probabilistic contention and reparticipation. The relay with the minimum outage probability is selected in a distributed way. Based on the achievable outage probability, relay candidates are divided into multiple groups, and each relay candidate in a group uses a probability to send the acknowledgement (ACK) packet back to the source to contend for being selected. Each group is defined by a specified range of the outage probability. Relays in a group with lower outage probability range contend earlier. In addition, the relay candidate that does not send the ACK packet in the current time slot is assigned with a higher probability to contend in the next time slot. Once a relay candidate survives, the contention process is terminated. Thus, transmitting the unnecessary ACK packets for contention is avoided. Simulation results show that compared to the conventional scheme, the proposed one has a better performance in terms of the outage probability, shortens the contention period, and reduces the number of ACK packets for contention.

We propose a grid-based routing protocol which divides the network area for large side length of square cells by using cell rotation to reduce the number of relay nodes between the source node and the mobile sink. The proposed scheme divides each cell into multiple sub-cells, and assumes one or two sub-cells to be active-cells. Then, the proposed scheme confines the existing area of active nodes to each active-cell. Because the maximum transmission range is a fixed value, the side length of square cells can be enlarged by confining the area where the active node exist in each cell. Therefore, the proposed scheme decreases the number of relay nodes between the source node and the mobile sink due to the large cells divided, and reduces the data packets delivery latency and the energy consumption. The simulation results show that the proposed scheme improves the packets latency and the energy consumption efficiency.

We propose a primary traffic based multihop routing with the cooperative transmission (PTBMR-CT) algorithm that enlarges the hop transmission distances to reduce the number of cognitive receivers on the route from the cognitive source (CS) to the cognitive destination (CD). In each hop, from the cognitive nodes in a specified area depending on the status of the primary traffic, the cognitive node that is farthest away from the cognitive sender is selected as the cognitive receiver. However, when the primary source (PS) is transmitting the data to the primary destination (PD), another cognitive node is also selected to be the cognitive receiver for the cooperative transmission from the cognitive nodes that are in a specified area. The cooperative transmission is performed if the PS still transmits the data to the PD when the cognitive receiver of the next hop is being searched. The simulation results show that the average number of cognitive receivers is reduced by the PTBMR-CT compared to the conventional primary traffic based farthest neighbor routing (PTBFNR), and the conventional PTBFNR is outperformed by the PTBMR-CT in terms of the average end-to-end reliability, the average end-to-end throughput, and the average required transmission power of transmitting the data from the CS to the CD.

Frequent topological changes due to high mobility is one of the main issues in Vehicular Ad-hoc NETworks (VANETs). In this paper, we model transmission probabilities of 802.11p for VANETs and effect of these probabilities on average transmission time. To evaluate the effect of these probabilities of VANETs in routing protocols, we select Dynamic Source Routing (DSR), Fish-eye State Routing (FSR) and Optimized Link State Routing (OLSR). Framework of these protocols with respect to their packet cost is also presented in this work. A novel contribution of this work is enhancement of chosen protocols to obtain efficient behavior. Extensive simulation work is done to prove and compare the efficiency in terms of high throughput of enhanced versions with default versions of protocols in NS-2. For this comparison, we choose three performance metrics; throughput, End-to-End Delay (E2ED) and Normalized Routing Load (NRL) in different mobilities and scalabilities. Finally, we deduce that enhanced DSR (DSR-mod) outperforms other protocols by achieving 16% more packet delivery for all scalabilities and 28% more throughput in selected mobilities than original version of DSR (DSR-orig).

We propose a power saving localization scheme that reduces redundant localization packets by considering distances from neighbor sensor nodes and a maximum localization error of a sensor node while maintaing localization error at the same level as the conventional scheme. In our proposed scheme, we set the threshold based on the overlapped area and each sensor node inhibits transmitting its localization packet if a neighbor sensor node nearer than the threshold transmits a localization packet. In addition, in our proposed scheme, by allocating a backoff proportional to each sensor node's maximum localization error, the sensor node with larger localization error inhibits transmitting its localization packet if there is a neighbor sensor node with smaller localization error. By the simulation with NS-2, we evaluate the number of transmitting localization packets, the localization error and the total energy consumption of the conventional scheme and our proposed scheme. The simulation results show that our proposed scheme can reduce the number of transmitting localization packets and the total energy consumption while maintaining localization error at the same level as the conventional scheme.

In general, automobiles travel from the origin to the destination using a shortest route. However, the shortest route may not be a highest wireless connection-capacity route, because of availability of wireless services (base station and access points etc.) along the route. To the best of our knowledge, currently no algorithm exists for selecting a route that maximizes wireless connection-capacity, while keeping route length shortest and close to shortest. In this paper, we propose two modified version of Dijkstra route selection algorithms: one for selecting a maximum connection capacity shortest route, and the other is for discovering higher wireless connection-capacity routes; the length of the route could be larger than a shortest route, but no larger than predetermined bound. The second proposed algorithm exploits the state change of the intersection to broaden the search range of possible routes. Results from our extensive simulation for a Manhattan-street type grid network with the heterogeneous IEEE 802.11a wireless access, show that for a 50% increase in route length and 15 Access Points (APs), the proposed algorithm can increases wireless connection-capacity by 35.67% and 31.27% compared to the shortest and random route selection algorithms, respectively.

In Mobile IPv6(MIPv6), a Mobile Node(MN) communicating with the Correspondent Node(CN) cannot prove the ownership of the claimed IP address. If a malicious node impersonates victim's IP address, it could hijack the session or forward packets to non-existing destination or other nodes. Currently, in order for a MN to prove the ownership of its own IP address, it is considered to use Feige-Fiat-Shamir (FFS) identification scheme [1]. However, there is one serious problem. In this scheme, a CN has to verify all of the Binding Update requests, and this leads to DoS(Denial of Service) attack. This paper shows a method which mitigates the effect of the DoS attack by making the challenge twice in a transaction. We make the first challenge easily verified to exclude the malicious nodes and the second one much more difficult than the first one to avoid impersonation. This method can efficiently exclude the malicious nodes which do not have proper IP addresses by verifying the first challenge. Furthermore, by making the challenge twice, our scheme can decrease the probability of impersonation over the previous scheme for equivalent calculation amount. By the computer simulation, we show that the proposed scheme is efficient to decrease the effect of the DoS attack and the probability of impersonation compared to the previous scheme.

We propose the asynchronous receiver-initiated MAC protocol with the stair-like sleep that each node reduces its own sleep time by the sleep-change-rate depending on the number of hops from the source to the sink in wireless sensor networks (WSNs). Using the stair-like sleep, our protocol achieves the high delivery ratio, the low packet delay, and the high energy efficiency due to the reduction of the idle listening time. Our protocol can formulate the upper bound of the idle listening time because of the feature that the sleep time decreases in the geometric progression, and the reduction of the idle listening time is obtained by using the stair-like sleep. In our proposed scheme, the sink calculates the sleep change rate based on the number of hops from the source to the sink. By using the control packets which have the role of ACK, our proposed protocol can achieve the stair-like sleep by no additional control packets. In addition, even in the network condition that multi-targets are detected, and hops to the sink are changed frequently, our proposed protocol can change the sleep change rate adaptively because the sink can always obtain the number of hops from the source to the sink. Simulation results show that the proposed protocol can improve the performances in the packet delivery ratio, the packet delay, and the energy efficiency compared to the conventional receiver-initiated MAC (RI-MAC) protocol.

A queueing model is proposed for multiuser systems for increased spectrum efficiency and better QoS provisioning. Broadcast services are quickly finding their way in to the IP domain and service providers need to introduce different traffic servicing models to handle these bandwidth intensive multimedia services. In an Orthogonal- frequency-division-multiple-access system, using conventional subcarrier per-user resource allocation method is inefficient. Here we present a two-stage queueing model where broadcast streaming traffic are aggregated by separate queues and processed in batches in the first-stage before propagating to the root queue. Modeling the general user traffic as an exogenous traffic source is also considered, which provides more delay performance improvement. The queueing framework is mathematically analyzed for average delay performances. Simulation results validate that not only resource consumption is reduced, but system queue delays also show substantial reductions. The introduced queueing model provides a framework for handling broadcast traffic and to reduce its impact on total system throughput.

Quantization is an important operation in digital communications systems. Quantizer not only introduces quantization noise but it also changes the statistical properties of the quantized signal. Furthermore, quantization noise cannot be always considered as an additive source of Gaussian noise. It depends on the input signal probability density function. In an orthogonal-frequency-division- multiplexing transmission the signal undergoes different operations which change its statistical properties. In this paper we theoretically analyze the statistical transformations of the signal from the transmitter to the receiver and determine how these effect the quantization. The discussed process enables the design of a quantizer with an optimal resolution for the required performance metrics. Simulations relate the effect of quantizer resolution and the channel statistics to the bit error rate at the receiver.

In the bistatic radar, it is important to suppress the undesired signals such as the direct propagated signal from transmitter and its multipath components. Conventionally, the suppression method by subtracting the replicas of the undesired signals is often used. However, it requires the precise delay times of the undesired signals. In this paper we propose a new method to detect the target without any information on the undesired signals' delay times in digital terrestrial TV-based bistatic radar, the signal of which is based on orthogonal frequency division multiplexing(OFDM). This kind of the radar is called “passive bistatic radar.” We adapt a scheme based on maximum signal to noise ratio(MSN) algorithm, which makes signal to interference plus noise ratio(SINR) maximum for the certain delay component. The maximum sensitivity is steered along the delay which relates to the target position, as if the search beam is steered along the direction in array signal processing. The sensitivity is steered by the weight applied to the received signal in the frequency domain. We obtain the method to estimate the weight to achieve the maximum SINR in the delay estimation which also includes the compensation for the reduction of the weight's length caused by decorrelation among the delay components. The simulation results show our proposed method without any information on the undesired signal's delays provides the sufficient performance compared to the conventional one.

Kalman filters are effective channel estimators but they have the drawback of having heavy calculations when filtering needs to be done in each sample. In our paper we obtain the steady-state Kalman gain to estimate the channel state thus eliminating a larger portion of the calculation burden. Steady- state value is calculated by transforming the vector Kalman filtering in to scalar domain by exploiting the filter characteristics when pilot subcarriers are used for channel estimation. Kalman filters operate optimally in the steady-state condition. Therefore by avoiding the convergence period of the Kalman gain, the proposed scheme is able to perform better than the conventional method. Also, driving noise variance of the channel is difficult to obtain practical situations and accurate knowledge is important for the proper operation of the Kalman filter. Thus we extend our scheme to operate in the absence of the knowledge of driving noise variance by utilizing received signal-to-noise ratio (SNR). Simulation results show considerable estimator performance gain can be obtained compared to the conventional Kalman filter.

In the bistatic radar using broadcasting signals, the delays between signals propagated directly and scattered by the target are measured to estimate the target position with cross-correlation. This results in the delay profile with the width limited by the bandwidth of the signal. To get better resolution, larger bandwidth is required, however, the bandwidth of broadcasting signals can not be increased by the receiver. In this paper, we propose a new method of the moving target detection with MU-SIC (MUltiple SIgnal Classification), processed at only receiver. That offers much narrower width than that of cross-correlation. The proposed method can distinguish the moving targets from the stationary targets, though the conventional MUSIC can not. This is realized by the decorrelation of the time delay-derived phase rotation factors caused by moving targets. The performances using ISDB-T (Integrated Service Digital Broadcasting -Terrestrial), the bandwidth of which is 5.57 MHz, are evaluated by the computer simulations. The results show that two close moving targets, separated by 0.1 ¿s, with SNR=-10 dB each can be resolved even if the stationary target is located around the one moving target.

In orthogonal frequency division multiple access -code division multiplexing (OFDMA-CDM), rate adaptation is known as one of the key techniques to improve the reliability of the transmission signal. However, since the one subchannel size in OFDMA-CDM depends on the spreading code length, the fading shift which the transmission signal receives per one subchannel might depend on the spreading code length. If the influence of the fading shift can be suppressed according to the multiplexing ratio, the effect of the rate adaptation can be improved. In addition, by allocating the subchannel with multiuser diversity in OFDMA, the effect of the rate adaptation can be more improved. We propose a subchannel allocation scheme based on the maximum, minimum and average signal to interference plus noise ratio (SINR) and orthogonal variable spreading factor (OVSF) code allocation to despread in narrowband according to the multiplexing ratio. We demonstrate that the proposed scheme can improve the bit error rate (BER) performance.

In cluster-based multihop networks, the Ad Hoc Routing (AHR) scheme has been proposed to reduce the implementation complexity of the optimal routing scheme. However, this complexity reduction also causes performance loss. In addition, when the practical distributed relay selection is considered, further performance loss occurs due to the relay selection error. In this paper, we exploit cooperative diversity to compensate these performance losses, address the problem of the conventional distributed relay selection scheme, and then propose a novel Distributed Ad Hoc Cooperative Routing (DAHCR) scheme. In the proposed approach, the sender selects not only the receiver but also one relay to form a cooperative communication to reduce the power consumption. Simulation results reveal that the performance improvements achieved by the cooperative diversity and the proposed DAHCR scheme are the trade-offs of increased complexity.

Recently, there are many researches about a trace-back method which specifies the source of transmitted packets in mobile ad-hoc network. In ad-hoc network, since there is no fixed infrastructure, each node has to record the traceback information and reconstruct the trace-path by itself. In conventional researches, since traceback is handled by each node, the variation of network topology by mobility of nodes causes degradation of success rate of the trace-path reconstruction and increase of traffic by inquiring traceback information to the all nodes on the network. In this paper, to achieve a traceback method that some nodes act as units by using clustering technique, we propose the schemes to manage information about traceback in each cluster and reconstruct a trace-path efficiently, and a traceback method between each cluster head. In our scheme, we can avoid degradation of success rate of the trace-path reconstruction caused by mobility of each node and because traceback is handled by only cluster heads we can also reduce the traffic and time for traceback. By a computer simulation, we evaluate the number of queries and success rate of traceback and show that the proposed scheme can reduce the both number of queries and achieve high success rate of traceback.

This paper proposes an optimum mechanism to select appropriate gateway based on multiple node metrics: remaining energy, mobility, and number of hops, for interconnected mobile ad hoc network (MANET) with infrastructured network. To choose an optimum gateway node, we use the multiple criteria decision making (MCDM) method called simple additive weighting (SAW) to outrank the optimum gateway node. SAW method calculates the weights of gateway node by considering these three metrics. Node with the highest weight will be selected as a gateway. Simulation results show that our scheme can improve throughput performance, gateway lifetime, and packet delivery ratio of MANET and infrastructured network.

We propose a mitigation technique of high-level multiple access interference(MAI) for multimedia optical Code- Division Multiple-Access(CDMA) systems using the optical power selector. The proposed system can eliminate high-intensity MAI at the receiver for the low-priority users. Moreover, the proposed scheme can reduce by half the required number of code sequences compared to the conventional scheme. As a result, the proposed scheme can obtain higher code spreading gain due to the increase of number of weights at the same code-length. We analyze performances of the proposed scheme and show that both the high-priority users and low-priority users achieve lower bit error rates in comparison to the conventional scheme.

MIMA-MAC is the most attractive MAC protocol for MIMO ad hoc networks, which consist of nodes with multiple antennas. MIMA-MAC, however, uses only a half of antennas for data transmission regardless of the number of antennas or transmission demands of neighboring nodes. In this paper, we propose a MAC protocol for MIMO ad hoc networks with parallel RTS processing (PRP-MAC) which determines the maximum allowable number of data streams after receiving up to 2 RTS packets at a receiver. PRP-MAC can maximize the number of transmitted data streams regardless of collision of RTS packets, the number of transmission demands from neighboring nodes and the number of antennas at each node. Our simulation results show that PRP-MAC can enhance the network throughput, and decrease the amount of control bytes per data packet compared with MIMA-MAC. In addition, we also show the availability of PRP-MAC in antenna-heterogeneous environments.

A security operation center (SOC), which monitors network traffic on each domain, has been established to detect cyber attacks. However, there have been ever increasing worms and distributed denial of service (DDoS) attacks on the Internet and the number of unknown attacks is increasing day by day. It is hard to defend network infrastructure via the SOC, which is operated by an internet service provider (ISP). It is thus important to predict new security threats and share incidents that occur with related ISPs. In the case of Japan, the Telecom Information Sharing and Analysis Center (Telecom-ISAC) Japan is established for a federation scheme with ISP operators against serious security incidents. In this research, we design a federation SOC framework that monitors wide-area networks and analyzes multi-point traffic using statistical approaches. It can suggest anomalous ISPs and traffic parameters automatically. Moreover, we propose an attack forecast technique to ensure a swift response to regular and new attacks. The technique depicts an attack map and learns attack propagation patterns by using the Bayesian inference. We implement the system and evaluate integrated scale of the ISPs and forecast correct rate.

To achieve a high throughput performance constantly, a H-ARQ (hybrid-automatic repeat request) scheme which controls the retransmission size is important. RB-HARQ (reliability-based HARQ) has been proposed as the control scheme of the retransmission size. However, in RB-HARQ, simulations must be needed beforehand and a receiver must memorize results of simulations. In this paper, we propose a H-ARQ scheme with controlling retransmission size by BLER (block error rate) which can be obtained by results of CRC (cyclic redundancy check) decoding. We show that a throughput performance of the proposed scheme is equivalent to that of RB-HARQ and the proposed scheme does not need simulations which must be needed in RB-HARQ.

In this paper, we propose the energy efficient routing protocol adapted to mobile sensors for multiple sinks in wireless sensor networks. In the proposed protocol, a forwarding sensor node exchanges the location information of neighboring sensor nodes at only the forwarding node sending data in order to reduce energy consumption due to HELLO messages. Therefore, a forwarding sensor node does not periodically exchange HELLO messages. In addition, the proposed protocol does not construct multicast routes between a source and each sink. Therefore, a forwarding node decides a next hop node which the energy consumption between a source and each sink is minimum, and then sends data packets to the next hop node. That is, the selection of a next hop node and the data transmission are repeated, and then the data is sent to multiple sinks. The simulation results show that the proposed protocol can reduce the energy consumption compared with other energy efficient routing protocols in sensor networks.

In reliable multicast, feedback and recovery traffic limit the performance and scalability of the network. In this paper, we present an improvement to the many-to-many reliable multicast protocol, group-aided multicast protocol (GAM), with a local-group based recovery by making use of forward-error correction (FEC) for local recovery. In contrast to original GAM, which only makes use of NACK-based recovery, our scheme produces FEC packets within the scope of a local group, instead of by the original senders. By using this scheme, redundancy traffic can be localized within a multicast session, and the overall recovery traffic can be reduced.

In the conventional subcarrier and power allocation schemes in multi-input multi-output and orthogonal frequency division multiple access (MIMO-OFDMA) systems, only equal fairness among users has been considered, and no scheme for proportional data rate fairness is considered. In this paper, subcarrier and power allocation criterion is proposed to maximize the total throughput under the constraints of total power and proportional data rate fairness among users. In the proposed scheme, subchannel allocation is firstly performed by using singular value decomposition (SVD) of channel matrix under the constraint of users' data throughput requirements, and then adaptive bit and power loading is applied. Simulation results show that total throughput is increased as well as each throughput is proportionally distributed among users in MIMO-OFDMA systems.

In this paper, we consider packet scheduling for cellular networks with relaying in high speed downlink shared channel, where mobile users are equipped with not only cellular but also ad hoc radio interfaces. In order to improve the system throughput, we propose a centralized packet scheduling scheme by adjusting transmission rates dynamically in response to the variation of channel quality and packet delay. A base station selects the appropriate relaying based on users' channel quality, and schedules packets to the users by considering their achieved data rate, traffic congestion and proportional fairness. We demonstrate by numerical results that the proposed scheme can utilize the capacity of network more efficiently, and achieve proportional fairness and traffic balance to users.

In this paper, we propose a mitigation technique of multiple access interference (MAI) for tunable-transmitter/fixed-receiver (TTFR)-based double time-encoded optical code-division multiple-access (CDMA) systems. The proposed system assigns distinct power-levels to each weight of second code-sequence and transmits the pulse-sequence with multiple power-levels. Double optical hard-limiters (DOHLs) with high-level thresholds can effectively eliminate low-power MAI as long as the intensity of incoming optical pulse is lower than the threshold value. By means of theoretical analysis, we show that the proposed system can achieve lower bit error probabilities than the conventional system transmitting the pulse-sequence with only one power-level.

In this paper, we propose a spectral-slice scheme and a chip-level detection for the TTFR-based asynchronous SPECTS-OCDMA system combined with time-encoding. The spectral-slice scheme can reduce the interference probability because the MAI with the different frequency has no adverse effects on the channel collision rate. Moreover, the chip-level detection can enhance the tolerance of MAI because the channel collision does not occur as long as either of the detection time positions receives no MAI. As a result, these techniques mitigate channel collisions. We analyze the channel collision rate theoretically, and show that the proposed system can achieve a lower channel collision rate in comparison with the conventional systems.

The solution to TDOA (time difference of arrival) equation is obtained by linearizing the equations via a Taylor-series expansion. To achieve high accurate positioning, the first path timing should be estimated correctly. The path search algorithm has been proposed to estimate first path. However, the detection probability of the first path depends on the received power of the pilot symbols. In this paper we propose TDOA mobile terminal positioning with weight control based on received power of pilot symbol in Taylor-series estimation. Computer simulation results indicate the proposed system generally performs better than the conventional system, especially when the transmit power is not high

Next generation mobile networks are expected to provide seamless personal mobile communication and quality-of-service (QoS) guaranteed IP-based multimedia services. Providing seamless communication in mobile networks means that the networks have to be able to provide not only fast but also lossless handoff. This paper presents a two-layer downlink queuing model and a scheduling mechanism for providing lossless handoff and QoS in mobile networks, which exploit IP as a transport technology for transferring datagrams between base stations and the high-speed downlink packet access (HSDPA) at the radio layer. In order to reduce handoff packet dropping rate at the radio layer and packet forwarding rate at the IP layer and provide high system performance, e.g., downlink throughput, scheduling algorithms are performed at both IP and radio layers, which exploit handoff priority scheduling principles and take into account buffer occupancy and channel conditions. Performance results obtained by computer simulation show that, by exploiting the downlink queuing model and scheduling algorithms, the system is able to provide low handoff packet dropping rate, low packet forwarding rate, and high downlink throughput.

In optical atmospheric (OA) pulse position modulation (PPM) code division multiple access (CDMA) systems, the conventional scheme which applies the maximum-value detection has a serious problem that the multiple access interferences (MAIs) become large due to scintillations. In this paper, to mitigate both effects of scintillations and MAIs, we propose a symbol decision scheme by using the comparing detection. In our proposed scheme, the minimum output in all weighted positions is detected in each PPM slot. Then, the maximum-value out of the minimum outputs is detected, and, the symbol corresponding to the PPM slot in which the output has the maximum-value is decided. Since the received symbol is correctly decided as long as there is at least one weighted position without MAIs in other PPM slots, the proposed scheme can mitigate both effects of MAIs and scintillations. We show that the proposed scheme can improve the bit error rate performance

In optical code division multiple access (CDMA) networks, the system which equips tunable transmitters and fixed receivers (i.e. TTFR) achieves simple configuration and protocols. However, when there are other simultaneous users who transmits pulses with the same code, large amount of multiple access interferences (MAIs) cause serious performance degradation. In this paper, we propose second encoding scheme in which first-encoded optical pulses are further encoded by the second optical orthogonal code (OOC) assigned to the transmitter. The proposed system with second encoding scheme can achieve higher processing gain in the correlator than the conventional system without second encoding scheme because the number of weights increases due to the second encoding. This results in the mitigation of the effect of MAIs. We analyze performance of the proposed system and show that the proposed system with second coding scheme can achieve better bit error probability than the conventional system without second encoding scheme

In this paper, we propose an OVSF code allocation at a base station and a two-stage combining method at a receiver to reduce inter-code interference in OFCDM system. In the proposed OVSF code allocation, OVSF codes which have different ancestor codes are allocated to users. If allocated codes are divided into chips shorter than spreading factor, chips which are produced by each allocated codes are orthogonal. In the two-stage combining method, the allocated code is divided into chips and received symbols are despread using chips. Demodulated symbols are produced from the despreading, multiplied by weights, and combined. The combining method of the proposed scheme has two stages. The first stage is the despreading, and the second stage is the weight combining. The proposed scheme uses few sub-carriers among which a difference of amplitude levels is small at the despreading, and combines demodulated symbols at the weight combining using suitable weights. Thus, the proposed scheme can reduce inter-code interference. We show that the proposed scheme achieves better BER performance than the conventional scheme by computer simulations.

In this paper, we propose split multi-path routing protocol with load balancing policy (SMR-LB) to improve TCP performance in mobile ad hoc networks. In SMR-LB, each intermediate node records how many primary paths are attempted to construct as well as the source ID which attempts to construct the primary path. Each intermediate node decides which primary path should be constructed by using the primary path and the source ID information. As a result, SMR-LB can utilize the load balancing policy to reduce the probability of congestion and avoid the continuous link breakages between the specific source and destination pair. We show that SMR-LB can improve TCP performance compared with the conventional protocols by computer simulation.

This paper investigates several downlink packet scheduling algorithms in OFDM based systems for space division multiplexing (SDM) with adaptive antenna array beamforming. The packet scheduling algorithm, which generates directional beams toward accessing users in accordance with the priority based on received signal-to-interference plus noise power ratio (SINR) have been proposed. However, the algorithm can generate few directional beams due to the increase of inter beam interference. As a result, the improvement of overall capacity is limited. In this paper, we propose an algorithm to increase total throughput by increasing the number of directional beams. In our algorithm, difference of angle and interference among directional beams are considered. It is shown that the proposed algorithm can increase the sector throughput by 13 % as compared to the conventional SINR-priority based algorithm. Moreover, we investigate the effect to take into account the channel quality and waiting time of each user in the proposed algorithm from the viewpoint of the sector throughput and the fairness of transmission slot assignment probability. Simulation results show that the proposed algorithm with taking into account the received SINR averaged over the scheduled users improves the sector throughput by 5-10% as compared to the original proposed algorithm. It is also shown that, additionally taking into account the waiting time of the users, although the sector throughput is degraded by 12%, the number of users who have the transmission slot assignment probability of less than 0.1 is reduced by 26% compared to the original proposed algorithm

First, in order to increase the number of transmitted bits per pulse in an optical CDMA system, we consider an M-ary PAM coded system. However, when a correlator is applied in the receiver, the multiple access interferences (MAIs) with high intensities deteriorate the system performance significantly, even if the number of MAIs is small. Consequently, we propose an optical CDMA system with M-ary PAM in conjunction with the chip-level receiver by M-level threshold detection instead of a correlator. The proposed system can obtain a synergistic effect between the PAM and the chip-level receiver, in which the chip-level receiver reduces MAIs more compared with the correlator and the PAM has a larger number of transmitted bits per frame than on-off keying (OOK), and achieves higher tolerance to MAIs than the OOK-CDMA system with a chip-level receiver at the same bit rate and number of subscribers. We show that the proposed PAM-CDMA system with the chip-level receiver achieves better bit error probability than the OOK-CDMA system with the chip-level receiver.

The use of optical code division multiple access (OCDMA) techniques for contention resolution in optical burst switching (OBS) is proposed. Further a service differentiation scheme is proposed for QoS support. This novel approach achieves wider range of differentiated services than the existing offset-time-based scheme, by assigning prioritized codewords which are embedded in the signature sequence used for OCDMA. Steady-state throughput is obtained and the numerical results show that the proposed scheme provides higher throughput corresponding to the priority class assigned to the burst.

In this paper, in order to avoid disconnection of a multicast path caused by the hand off of a mobile host (MH), we propose a multicast protocol in mobile Internet protocol (IP) environment. In the proposed protocol, when MH which receives multicast packets via mobile multicast gateway (MMG) from correspondent host (CH) hands off to other networks, MMG exchanges the multicast path to reduce disconnected time without increasing utilization of bandwidth. The simulation results show that the proposed protocol can reduce both utilization of bandwidth and disconnected time compared with the conventional protocols.

In order to realize low handoff latency without increasing overhead in wired channel, we propose a handoff scheme using positional information for mobile IP based networks. In the proposed scheme, foreign agents (FA) exchange IP addresses and positional information. When a distance between FA and mobile host (MH) exceeds a threshold, FA copies packets and forwards them only to MH's next FA. Moreover, in order for MH to receive first advertisement (ADV) message earlier, the transmission interval of ADV message is shortened in next FA. By shortening transmission interval of ADV message, MH can receive forwarded packets earlier. In addition, when new FA receives registration request (REQ) message from MH, FA replies tentative registration (TREG) message to MH. Adding tentative registration, MH can begin receiving forwarded packets without waiting for receiving registration reply (REP) message from home agent (HA). By performance evaluation using both theoretical analysis and computer simulations, we show that the proposed scheme can realize low handoff latency without increasing overhead in both wired and wireless channel.

In order to reduce the number of binding updates (BU) when the number of connecting mobile hosts (MH) is limited for higher level mobility anchor point (MAP), a scheme adopting distributed location management (DLM) to multilevel hierarchical mobile IPv6 (M-HMIPv6) is proposed. In the proposed scheme, to avoid extreme load for higher level MAP, a threshold of the number of connecting MHs is set. When a MAP receives a BU and the number of connecting MHs exceeds the threshold, the MAP forwards the BU to the next MAP candidate. In addition, to reduce the number of BUs and extend a MAP domain, DLM is adopted in M-HMIPv6. The usage of the distance field in MAP option is modified to distinguish the route redundancy and keep transparency to HMIPv6. Moreover, to realize load balancing when mobility deviation exists, a load balancing by average BU interval in both access router (AR) and MH is adopted. By classifying MHs roughly by the average BU interval in AR, relatively faster MHs select higher MAPs. By performance evaluation using computer simulations, we show that the proposed scheme transparent to HMIPv6 can reduce the number of BUs and realize load balancing without excessive route redundancy.

We propose a scheduling algorithm minimizing voids generated by arriving bursts in an optical burst switched WDM network. In the proposed scheduling algorithm, when the burst which has arrived at an optical core router at a certain time can be transmitted in some data channels by using unused data channel capacity (called a void), our proposed scheduling algorithm selects the data channel in which a void newly generated after the burst transmission becomes minimum. We compare, by computer simulations, the performance of our proposed scheduling algorithm with that of the conventional one with respect to burst loss ratio. It is shown that our proposed algorithm can improve burst loss ratio.

We propose multiple subcarrier modulation (MSM) for infrared wireless systems using punctured convolutional codes and variable amplitude block codes. The rate-compatible punctured convolutional (RCPC) code deletes the coded bits corresponding to zeros in the puncturing table and does not allocate them on amplitude of the subcarriers. On the other hand, the proposed system maps them to the amplitudes of subcarriers where the amplitudes are selected so that the required bias can be minimized. We propose two systems that use the block codes to map the coded bits corresponding to zeros in the puncturing table: one block code maps them to only zeros (proposed 1), and the other block code maps them to the appropriate values among 0, and /spl plusmn/1, so that the average optical power can be minimized (proposed 2). We show that the proposed 2 tan achieve the minimum required SNR at the same average optical power among all the systems.

We propose an embedded transmission method using pulse position modulation (PPM) signaling with symmetric error correcting codes for optical code division multiple access (CDMA). In the method, 2/sup J/ slots of 2/sup J/-ary PPM signaling are embedded in a user's signature sequence with 2/sup J/ weighted positions. Therefore, the proposed method increases the number of bits per signature sequence. This increase in the number of transmission bits allows us to adopt J error correcting codes without decreasing the information rate and expanding the system bandwidth. Consequently, the effect of multiaccess interference (MAI) is removed within the error correction capability. Considering the effect of MAI, we analyze the BER under Poisson photon counting process. Numerical results show that the proposed method has an advantage of good energy efficiency over the conventional PPM method in applying error correcting codes.

We propose a scheduling algorithm for a WDM single-hop network. In the algorithm, the transmission of multicast packets with more destination address overlap is postponed in order to improve average packet delay and throughput. By prioritizing the transmission of multicast packets with less destination address overlap, our algorithm can decrease the number of data packets prevented from transmission by the other multicast packets with more destination address overlap and can reduce average packet delay. We compare, by computer simulation, the performance of the proposed scheduling algorithm with that of the conventional one with respect to average packet delay and throughput. As a result, it is shown that the proposed scheduling algorithm can improve average packet delay and throughput performance.

In the MPLS (multiprotocol label switching), VC-merge (virtual circuit-merge) is known as a scalable technique to reduce the total number of VC. However, in the conventional VC-merge capable switch, the number of reassembly buffers greatly increases as the switch size grows large. We propose a VC-merge capable switch with less buffer requirement. In the proposed model, we can reduce the number of reassembly buffers and decrease the access speed of buffer memory since we can share reassembly buffers between the cells that arrived at all input ports by the controller. We compare the packet loss probability and the mean system delay performance of the proposed model with those of the conventional model by computer simulations. As a result, we show that the number of reassembly buffers decreases on the order of O(N) and the access speed of buffer memory becomes low. Therefore, the proposed model is effective in MPLS.

In order to reduce optical beat interference (OBI) in an optical frequency-hopping code division multiple access (FH-CDMA) network, we propose the application of coherence multiplexing (CM). Since the CM technique achieves multiplexing by using time delays exceeding the coherence time of the light sources, the crosstalk between CM channels depends on the space of the delay difference between CM channels. We analyze the crosstalk to signal ratio as a function of the space, and find a channel space that is enough to neglect the crosstalk from other CM channels. Using this channel space, we obtain the number of CM channels accommodated within one chip duration. Since the conventional FH-CDMA supports traffic by the code channels only, its capacity is limited by the OBI as the traffic increases. In addition to the code channel, the proposed network supports traffic by the CM channels. Since the addition of CM channels decreases the number of active code channels for uniform traffic, the OBI is reduced. Numerical results show that the proposed network attains a larger capacity than the conventional FH-CDMA.

We analyze the performance of indoor infrared wireless systems using pulse position modulation (PPM) code-division multiple-access (CDMA) on diffuse channels by taking the effects of intersymbol interference (ISI) of more than ten chips into account. Then we analyze the system with a hard-limiter (HL) and the system with a linealizer with dead-zone (LDZ) on diffuse channels. We evaluate the performance of the system with HL, the system with LDZ, and that of the system with the decision-feedback equalizer (DFE) on diffuse channels. We show that the bit error rate (BER) performance of the system using PPM CDMA is worse than that of the system using on-off keying (OOK) CDMA on diffuse channels. We also show that the BER performance of the system with the HL and that of the system with the LDZ are both better than that of the unequalized system on diffuse channels. We also show that the BER performance of the system with HL is almost identical to that of the system with the DFE on diffuse channels. We further show that when the desired code sequence is (0,1,5) and the effects of ISI is large, the BER performance of the system with the LDZ is superior to that of the system with the HL. However, in the average BER performance over each code sequence, the system with the HL is better.

We propose a shared knockout ATM switch under non-uniform traffic with variable hot-spot ports in order to handle flexibly when the number and port of the hot-spot change. In the proposed model systems share a batcher network in order to decrease the total number of switch elements. Moreover, the proposed model has a function that the maximum cell transmission number called a speedup factor L/sub 2/ is set to be larger than the lower limit of the number of arrival cells L/sub 1/, i.e, L/sub 1/

We propose a new channel reservation protocol using a counter for detection of a source conflict in a WDM single-hop network with non-equivalent propagation delay. A source conflict occurs when a source node has the right to transmit more than two messages to their destination nodes using different wavelengths in the same time slot. In the proposed protocol, a source node can detect a source conflict before the assignment of wavelengths by investigating information about the final message which has succeeded in reservation. We approximately analyze the throughput considering the effect of a source conflict. Also, we show by computer simulation that our proposed protocol can reduce mean message delay dramatically without degrading the throughput performance as the offered load becomes large.

We evaluate the performance of indoor infrared wireless systems using on-off keying code-division multiple-access (OOK-CDMA) systems with three kinds of equalizers: decision-feedback equalizer (DFE), decision-feedforward-and-feedback equalizer (DFFE), and feedforward equalizer (FE). To estimate the impulse response, we use the training sequence that alternates '1' and '0' sequentially. Among three kinds of equalizers, we show that the system with the DFFE can achieve the best performance at high bit rate. When the bit rate is low or the transmitted optical power is low, the system with the FE achieves the best performance.

To improve the bit error rate (BER) performance of the linearized constant power coded OFDM (LCP-COFDM),that is, the compensating technique used for nonlinear distortion, we propose a technique that combines the partial transmit sequence (PTS) technique with the LCP-COFDM technique. By means of computer simulation, we evaluate the BER and power spectrum density (PSD) performances of the proposed technique. In the BER performance evaluation, we consider both cases where the side information of the PTS technique is coded and uncoded by error correction codes. As a result, we show that the proposed technique can improve the BER performance, while keeping very low out-of-band emission.

We propose a wavelength assignment algorithm which can improve the mean packet delay performance in a WDM single-hop network using a master/slave scheduler with transceiver tuning latencies. Our proposed algorithm can mask the tuning time of a transceiver by assigning the identical wavelength to the data channel in which the node transmits the data packet to the same destination node or receives data packet from the same source node in continuous time slots. We compare the performance of the proposed algorithm with that of the conventional algorithm with respect to the mean packet delay and throughput by computer simulations. It is shown that our proposed algorithm can improve the mean packet delay performance.

We propose an interference cancellation technique using a reference signal for optical synchronous code-division multiple-access (CDMA) systems. In the proposed system, we use the signature code sequences composed of the group information codes and the modified prime code sequences. The group information codes are added in the forefront of the signature code sequences to estimate the amount of the multiple access interference (MAI). The proposed cancellation technique can be realized with a simpler structure than the conventional canceller using the time division reference signal, because it can reduce the number of optical correlators from P to two, where P is the prime number. Considering the effects of the MAI, avalanche photodiode (APD) noise, and thermal noise, we analyze the performance of the proposed system. We show that the proposed canceller achieves better bit error probability than the conventional canceller.

In the intranet, there are demands for making secure communication groups that protect significant information in departmental and/or individual bases. To satisfy the demands, encryption elements in the system need to have exact and complicated process information. In general, this information is generated in a management server and downloaded to the encryption elements, every time the system configuration changes. We a propose group search protocol, by which encryption elements generate the process information automatically from the logical definition of the secure communication groups for themselves. By this method, secure communication groups for intranet with location freedom is realized, that is to say, users who have the function of encryption elements can move anywhere in the intranet freely.

In tracking systems using a phased array antenna, the adaptive-gain /spl alpha/-/spl beta/ tracker combined with circular prediction has been proposed for manoeuvring targets. However, the tracking quality of the circular prediction filter degrades for highly manoeuvring targets that continue to change the turning plane, since the circular prediction is calculated on the assumption that a target flies on the same plane of the previous three measured positions. In this paper, we extend the circular prediction to three-dimensional space and propose an adaptive-gain /spl alpha/-/spl beta/ tracker combined with three-dimensional circular prediction using estimation of the plane state to improve tracking quality.

Maneuvering targets are difficult to track with the Kalman filter since the target model of the tracking filter might not fit the real target trajectory and the statistical characteristics of the target maneuver are unknown in advance. In order to track such a heavy maneuvering target, the estimation of the target turn-direction is necessary. A two-stage estimator using advanced circular prediction which considers the target turn-direction is proposed for maneuvering target tracking. Simulation results are given for a comparison of the performance of our proposed scheme with that of conventional tracking filters.

For local wireless communication, the reservation code multiple access (RCMA) protocol has been proposed as a promising protocol, which allows all terminals to share a group of spreading codes on a contention basis and facilitates introducing voice/data integrated services into spread-spectrum systems. However, the voice packet dropping probability of RCMA is degraded in the heavy load condition because of the contention mechanism of random access. We propose the use of the control code for the voice terminal with the RCMA protocol to improve the voice packet dropping probability. In the proposed model, by assigning a minislot in the call set-up for each voice terminal, a voice terminal can reserve a spreading code efficiently without the packet collision by transmitting a mini-packet to a minislot while a data terminal has to contend for a code for each packet transmission. We show that the proposed model attains lower voice packet dropping probability than the conventional RCMA while the data packet average delay is almost the same as that of conventional RCMA by theoretical analysis and the computer simulation.

The bit error rate (BER) performance of a DS-CDMA receiver which consists of a RAKE receiver followed an adaptive MLSE diversity receiver is proposed to simultaneously suppress multi user interference and multi path fading interference. A new adaptive equalization in the channel estimator and predictor of the MLSE receiver is introduced. The RLS algorithm is used for the adaptive channel estimator. The Euclidean distance of the branch metric of the adaptive MLSE is computed by using the estimated channel impulse response and the channel input sequence. The results show that, the proposed model is useful to improve the performance of the DS-CDMA system in a fading channel and the capacity of the DS-CDMA system can be increased.

We propose a rate-adaptive indoor infrared wireless communication systems using punctured convolutional codes and adaptive pulse position modulation (PPM) to achieve high bit rate and realize communications even under bad channel conditions with limited transmitter power. The proposed system varies the code rate of the punctured convolutional codes and the modulation order L of L-PPM adaptively according to the channel conditions; when the channel condition is good, low-order PPM and high-rate codes are selected, while high-order PPM and low-rate codes are selected when the channel condition is bad. We analyze the performance of the proposed system on hybrid line-of-sight (LOS) channels. Our results show that the proposed system can achieve high bit rate and realize communications even under bad channel conditions with limited transmitter power by varying the code rate and the modulation order adaptively according to the signal-to-noise ratio (SNR).

For local wireless communications, the packet reservation multiple access (PRMA) protocol, which combines random access with time division access, has been proposed and analyzed. The study results shows that the performance of PRMA is degraded in the heavy load condition because of the contention mechanism of random access. We propose the use of an adaptive array antenna with the PRMA protocol to improve the performance. In the proposed model, the adaptive array can acquire a packet by pointing the peak antenna pattern toward that packet even when a collision occurs. We evaluate the throughput performance by a theoretical analysis. We also evaluate the throughput and the packet dropping probability by computer simulation. It is shown that the proposed model attains a higher throughput and lower packet dropping probability than the conventional PRMA.

We propose the multicarrier trellis coded slow FH/16QAM with an interference immunity decoding method to improve the bit error rate (BER) performance in digital land mobile communications. In our scheme, we estimate the power of the co-channel interference (CCI) using space symbols inserted in several slots to obtain the weight value per carrier. We carry out the immunity decoding method considering the amplitude of the interference symbols in calculating the branch metric. The BER performance of the system is investigated by computer simulation. It is shown that the performance of the proposed system is affected by the CCI per carrier. This scheme is more effective in restraining the effects of CCI especially for the uplink of cellular systems and in the case where the processing delay time is restricted, such as in mobile speech communications, compared with traditionally decoding.

The bit error rate (BER) performance of trellis coded M-ary PSK (TC-MPSK) in the presence of undesired multiple co-channel interference (CCI) and fading is investigated. The fading statistic for desired signal is Rician or Nakagami fading and the undesired interference signals are characteristic by Rayleigh fading. We assume all the interferers are unmodulated because most of errors are produced by Rayleigh fading itself rather than the modulating sequence, and thus, it can be treated as complex Gaussian noise and all the interferers have the same power. We use the error property and error bound based on transfer functions where the number of error states equals to the number of trellis states to evaluate the BER performance. It is shown that BER performance of TC 8PSK is improved as increasing either the power of desired signal or the value of signal to interference ratio (SIR).

We propose the wireless integrated multiple access (WIMA) protocol under fixed contention non-interleaved-frame fixed-length (F-NIFFL) scheme, in which hybrid contention/reservation access is used for data. In this scheme, some of data packets are first transmitted in the contention-based slots, and the packets which collided or not transmitted in these slots are made reservation and are transmitted in the later reserved slots. A two-dimensional first-order Markov chain model is used for our analysis, and we show that our protocols can decrease the data-packet delay in the conventional WIMA under pure reservation NIFFL (PR-NIFFL). Especially in satellite networks, because of large round-trip propagation delay, our protocol is more effective.

An optical code-division multiple-access (CDMA) system with double optical hard-limiters is proposed where the optical hard-limiters are placed before and after an optical correlator. Moreover, the-effect of the optical hard-limiter on the performance of the optical synchronous CDMA systems using modified prime sequence codes as signature codes is analyzed under the assumption of a Poisson shot noise model for the receiver photodetector where the noise due to the detector dark currents exists. We evaluate the performance under average power and bit rate constraints. Our results show that using the single optical hard-limiter slightly degrades the performance of the optical CDMA systems under the assumption of Poisson shot noise model for the receiver photodetector where the noise due to the detector dark currents exists. Moreover, we show that the optical CDMA systems with double optical hard-limiters have better performance than other conventional CDMA systems with and without the optical hard-limiter when the number of simultaneous users is not so large.

One of important properties of a multihop network is the mean internodal distance for evaluating transmission delay. The connective semi-random network achieves a smaller mean internodal distance than other networks. However, the results have only been obtained by computer simulation and no theoretical analysis has been performed. In this paper, we theoretically analyze the mean internodal distance of a connective semi-random network. Moreover, we also theoretically analyze a restricted connective semi-random network whose network connective probability is larger than that of a conventional connective semi-random network. It is shown that the theoretical analyzed results agree well with the simulated results in a conventional model and our model with small restriction. The influence of restriction on the mean internodal distance becomes small as the number of outgoing links per node becomes large.

The great interest in the implementation of local radio networks for indoor communications has stimulated much research activity. The idle signal multiple access (ISMA) system is known to obtain high throughput in the situation where a number of terminals transmit data packets to a base station over a common radio channel. In the system, the base station broadcasts signals which inform all the terminals of the idle or busy channel status. Each terminal is allowed to transmit a packet only when it receives an idle signal. However, the capture effect and the propagation delay cause an unfairness of the local throughput depending on the distance from the base station. We propose the ISMA which broadcasts multiple idle signals to decrease the unfairness of the local throughput without power control. The performance of the local throughput and total throughput are analyzed. It is shown that the unfairness of the local throughput due to the distance decreases without reducing the total throughput.

This paper investigates the performance of distributed dynamic channel assignment (DDCA) strategies in sector cell layout systems using directional antennas. Simulations are done under the same propagation conditions and with the same system parameters. They show that DDCA strategies in sector cell layout systems gain 2.3/spl sim/8.2 times higher capacity than do the fixed channel assignment (FCA) strategy in conventional cellular systems using omnidirectional antennas and 1.5/spl sim/2.5 times higher than do DDCA strategies in conventional systems.

We propose a slotted nonpersistent CSMA with an adaptive array to improve throughput performance. First, we analyze the performance of the throughput S and the average number of transmissions and schedulings G/S. We also evaluate S and G/S by the computer simulation in order to consider the hidden-terminal problem in the CSMA mode. It is found that the theoretical results of the proposed model agree well with the computer simulated results, and the proposed model attains higher S and smaller G/S than the conventional slotted nonpersistent CSMA. It is also shown that the performance of the proposed model with an adaptive array due to hidden terminals degradation is smaller compared to the conventional CSMA.

Weak signal detection is a basic and important problem in radar systems. The detectable range of a radar is determined by the radar equation. This equation shows that the performance of a radar is improved by increasing the receiver output SNR. For the purpose of improving the SNR, the process called "integration of pulses" has been used. We propose new methods based on the wavelet transform for the improvement of the SNR. The wavelet transform is a tool for time-frequency domain signal analysis and is sensitive to the transient signals. By computer simulations, we show that the receiver output SNR after the wavelet transform is applied becomes about 8 dB greater than that of the original signals. This value corresponds to a 60 percent improvement of the radar detectable range.>

In conventional trellis coded modulation (TCM), a higher-ary modulation scheme combining with a convolutional code is employed not to expand the transmitted bandwidth. This forces the system to be attended by signal constellation expansion and increasing the average signal power. As solutions to avoid signal constellation expansion, TCM systems using totally overlapped signal sets (TO-TCM and RU-TCM) were proposed. These schemes can realize a coded modulation system with no signal constellation expansion and achieve more coding gain than conventional TCM. However, a problem that the systems with totally overlapped signal sets might be catastrophic has been remained. The authors propose a novel TCM system using partially overlapped signal sets of nonequiprobable signaling (PO-TCM-NE). This scheme employs the partially overlapped signal constellation to control increasing signal points, and to avoid catastrophic error propagation. The nonequiprobable signaling is employed to reduce average signal power. Coding gain of proposed PO-TCM-NE is considerably improved; in consequence the average signal power is reduced much lower than other TCM systems with equiprobable signaling.

Traffic congestion often occurs on highways and results in a heavy call traffic load that impairs the performance of dynamic channel assignment (DCA) strategies. The authors therefore propose a channel framework in which all the available channels are divided into two subsets according to the directions in which the mobile stations move. The decision of the division ratio depends on the offered load of each direction. The authors investigate, through simulations, DCA performances with channel framework for one-sided and both-sided traffic congestion and compare these performances with that of DCA without the framework. They show that the channel framework increases call blocking but considerably reduces the occurrence of forced call termination and of channel changing during a call.

The performance of trellis coded hybrid frequency and phase shift modulation (TC HFPSK) with expurgated phase code and asymmetric signal constellation is investigated by using minimum squared free Euclidean distance d/sub free//sup 2/ and bit error rate (BER). It is found that TC HFPSK with expurgated phase code shows a larger d/sub free//sup 2/ than the corresponding TC HFPSK with the complete phase code for varying the angle /spl phi/ that determines the asymmetric signal constellation. The maximum value of d/sub free//sup 2/ of TC HFPSK with expurgated phase code is obtained when the signal constellation is symmetric. The BER performance is analyzed in additive white Gaussian noise (AWGN) and fading channels by using uniform error property (UEP) and error bounds based on the transfer function. It is found that the performance improvements of TC HFPSK are 2.1 dB and 5.0 dB in AWGN and fading channels respectively, at BER=10/sup -4/, compared to uncoded HFPSK.

The resequencing delay for a queueing system with multiple servers under threshold-type scheduling is analyzed. The general expressions for the average total delay, which is the sum of resequencing delay and queuing delay, are derived. The expressions are also applicable to the conventional M/M/n queuing system as the special case of zero-threshold policy. Next, the numerical calculation is carried out for a queuing system with three servers under a threshold-type schedule to consider the impact of the multiple thresholds on the average resequencing delay. It is found that the resequencing delay as well as the queuing delay for a queuing system with multiple servers of different service rates can be reduced under threshold-type scheduling by selecting the threshold values appropriately. Threshold-type scheduling is more effective in reducing the resequencing delay compared to the queuing delay, especially in the region of modest traffic intensity and as the difference in the service rates becomes larger.>

A carrier slot setting plan is proposed to reduce intermodulation (IM) effects for satellite single-channel-per-carrier (SCPC) systems. Carrier slots are divided into blocks and slight frequency gaps are set among blocks. This setting introduces frequency offset between the center frequencies of carrier slots and the center frequencies of the IM components. A method of deriving optimum division in order to derive the upper bound of improvement of the C/IM ratio in the worst channel is developed. The results show that the setting plan achieves significant improvement of IM effects at a cost of only slight bandwidth expansion, even in fully loaded SCPC systems.>

An analysis is made of the performance of three types of quadrature partial response signaling (QPRS) systems using complex one-sided and two-sided transversal filters with additional decision-feedback (DF) taps in the presence of single continuous wave (CW) interference and additive white Gaussian noise. It is found that both DF filtered noise which is necessarily colored by passing through the QPRS receive filter, while decoding binary data signals from ternary QPRS signals. It is also shown that the duobinary system has the best performance of the three types of QPRS systems when the frequency of CW interference is low.>

The effect of an arbitrary power-series envelope detector on signal-to-noise ratio (SNR) for non-Gaussian noise is considered. It is found that the insertion of the region of saturation in the input-output characteristic of the power-law envelope detector results in the improvement on the output SNR when input SNR is large.

To enable differential detection of coincident, hard-limited, and reduced sidelobe QPSK type of signals, we introduce asymmetrical pulse shapes. This asymmetry in theIandQbaseband channels leads to coincident (unstaggered) QPSK system applications. We consider the bit error probability performance of these modulation schemes on a hardlimited channel in the presence of uplink and downlink additive Gaussian noise. It is found that the unstaggered QPSK modulation with the asymmetrical pulse has better bit error probability performance than QPSK and staggered QORC.

This correspondence describes staggered quadrature overlapped squared raised-cosine (QOSRC) modulation. The power spectrum of QOSRC attains good spectral characteristics over a bandpass hardlimited channel compared to quadrature overlapped raised-cosine (QORC) modulation because of less fluctuation of the signal envelope. It is found that QOSRC has better bit error probability performance than QORC.

This correspondence describes modified QORC (MQORC) modulation. The power spectral density of MQORC is shown to take on the form of the product of the QPSK and SFSK spectra, which attains good spectral characteristics. We show that the bandpass hard-limited staggered MQORC tends to keep its sidelobes suppressed, and therefore, its sidelobes drop off faster compared to staggered QORC.

The effects of bandpass limiters onMth phase tracking systems in the presence of multiple sinusoidal interferences and additive Gaussian noise are considered. We derive general expressions for the ratio of the output to the input SNR (the SNR ratio) for anMth-power tracking loop with and without a bandpass limiter. The numerical calculations for the squaring limited and fourth-power limited PLL's in the presence of multiple sinusoidal interferences and noise are carried out, and the results are compared to those for similar loops without limiters. It is found that sinusoidal interference causes the limit of the SNR ratio for infinite carrier-to-noise ratio (CNR) to be no longer consistent with that for only Gaussian noise. The SNR ratio is degraded more as the carrier-to-interference ratio (CIR) decreases and the number of sinusoidal interferences increases. Furthermore, it is found that for the fourth-power PLL, the limiter does not always enhance the output SNR at moderate to strong input SNR's in the presence of sinusoidal interference.

The effects of thenth law amplifier on signal-to-noise ratio has not been represented as a function of the input signal-to-noise ratio when the input of the nonlinear device contains nonGaussian noise. In this paper, we analyze these effects and determine the improvement factor represented by the ratio of the output SNR to the input SNR. The results are illustrated by the calculation of the improvement factor for the cases where the input noise amplitude distributions are the uniform distribution, the triangle distribution, and the Rician distribution.