Scalable, low-latency and accurate RFID counting algorithms have recently been proposed as a fundamental building block to support more complex query operations in a large-scale RFID system. One distinct feature of these algorithms is that they do not require explicit identification of individual tags and therefore can eliminate the latency bottleneck caused by serialization during multiple access control. However, these algorithms all assume reliable communications between the reader and the tags. While this assumption is also adopted by many tag-identification protocols in the current RFID standards, it is practically unachievable given the current technology and low-cost requirement of RFID tags. In fact, recent empirical studies have found that the communication between an RFID reader and a set of seemingly “in-range” tags are still unreliable and highly non-deterministic due to the varying channel conditions. In this paper, we discuss the design and performance analysis of a set of channel-oblivious RFID counting algorithms which can estimate the size of a tag-set of interest over unreliable wireless channels. The proposed schemes can provide accurate cardinality estimates without any prior knowledge of the channel parameters. We first propose a series of algorithms and analyze their performance under a simplified memoryless lossy channel model. We then extend them to handle the impact due to backscattering effects and correlated losses found in practical RFID systems. Our proposed designs only require simple modifications to standard RFID tags and readers and can be implemented using current technologies with minimal increase in tag/ reader cost. Our designs can also be extended to other RFID counting algorithms which assumed reliable communication channels.

Several new RFID counting algorithms have recently been proposed based on the probabilistic counting schemes introduced by Kodialam et al. These existing algorithms took into account the unreliability of the communication channels between the RFID reader and the tags, and are capable of providing accurate tag-count estimates. However, all algorithms were designed and evaluated based on a simplistic packet loss model. It assumes that the loss probability of a packet between the reader and the tag-set follows an independent, identical distribution. As presented by some empirical measurements, movements of personnel or equipments in a building can generate Doppler effect, which introduces time correlations to the fading signal. Thus, the realistic packet loss of the wireless channels is temporally correlated due to the frequent change of the nearby environment. Depending on specific implementations of each algorithm, temporally correlated packet loss might have significant impact on the tag-set cardinality estimation. In this paper, we evaluate the performance of the aforementioned RFID counting algorithms under a more sophisticated time-correlated channel fading model. In particular, we focus on investigating how temporal correlations would influence the accuracy of these existing algorithms. Based on the experimental statistics that characterized the indoor channels, we refine the channel model to describe the time-correlation. Comparisons of the performance of the counting schemes under the simplistic uncorrelated packet loss channel model and the refined correlated channel model are conducted. We also propose extensions for these RFID counting schemes to mitigate the estimation inaccuracy generated by the correlated packet loss.

Feedback Retransmission Approximation for Slotted ALOHA (FRASA) is proposed to study the stability region of finite-user slotted ALOHA under the collision channel. With FRASA, the stability region is derived in closed form for any number of users in the system. The result derived from FRASA is shown to be identical to the analytical result of finite-user slotted ALOHA when there are two users. It is shown that the stability region obtained from FRASA is a good approximation to the stability region of finite-user slotted ALOHA. The convex hull bound, which is convex, piecewise linear and outer bounds the stability region of FRASA, is provided.$p$-convexity, an essential property that the stability region of FRASA should have to ensure the convex hull bound is close to the boundary, is characterized. From these, it is derived that the stability region of FRASA can never be convex when there are more than two users. A separate convex and piecewise linear inner bound on the stability region of FRASA, the supporting hyperplane bound, is also given.

We study the downlink scheduling performance of an IEEE 802.11-based roadside Access Point(AP) serving a number of moving vehicles. For determining the scheduling order, throughput and Fairness are two important considerations. If system throughput Maximization is the sole consideration, some vehicles can always get resource from the AP whereas some are starved. On the other hand, if fairness is the sole consideration, resource should be allocated among all the contending vehicles regardless of individual vehicle's channel conditions and the system utilization should not be affected greatly. In this paper, we propose a novel scheduling algorithm called VEhicular Context-Aware Downstream Scheduling (VECADS). By exploiting the real-time vehicular context including the position, speed and cumulative data received by each vehicle, the scheduler can get up-to-date information to determine an appropriate scheduling order. The design objective of VECADS is to strike a better balance between system throughput and fairness among heterogeneous drive-thru vehicles. The performance of VECADS is evaluated via extensive ns2 simulations using real-world vehicular traffic traces. We show that VECADS outperforms MV-MAX, the state-of-the-art scheduling scheme for Drive-thru networks, in terms of system throughput, by 7 % while eliminating the bandwidth starvation problem of ``weak" vehicles under MV-MAX for a Jain's Index of 0.895 vs. 0.727.

In order to spread malware more effectively, hackers have started to target popular social networking services (SNS) due to the inherent trust-relationship between the SNS users and the interactive nature of the services. A common attacking approach is for a malware to automatically login using stolen SNS user credentials and then deliver malicious weblinks (Uniform Resource Locators (URLs)) to the people on the contact/friend-list of the stolen user account by embedding them in some short messages. The victim then gets infected by clicking on the links thought to be delivered by their friends. However, for this approach to be effective, the malware has to mimic human-like behavior which can be quite challenging for anything beyond one or two-liner conversations. In this paper, we introduce Hitchbot, which uses a stealthier way to deliver malicious URLs by hitch-hiking on legitimate conversations among SNS users. In particular, when a SNS user sends a web-link/URL to his/her friends, Hitchbot will quietly replace it with a similar-looking, but malicious one by intercepting the link at one of the several possible points along the interactive input/output chain of the system. Since the malicious link is delivered within some proper conversation context between the legitimate users, this makes it much more difficult for the victim (as well as the innocent spreader) to realize the attack and thus can increase the conversion rate while reducing the rate of being detected substantially. The social hitch-hiking approach also enables Hitchbot to bypass most existing defense schemes which mainly rely on anomaly detection for user- behavior or application/network traffic pattern. As a proof of concept, we have implemented Hitchbot as a client-based module to hitch-hike on common social networking services including the Yahoo and Microsoft Messaging clients and other web-browser-based social networking services such as Facebook and Myspace. To quantify the effectiveness of Hitchbot, we have conducted experiments to measure the behavior of users in exchanging, handling and operating on URLs. Possible defense schemes for detecting social hitch-hiking attacks are also discussed.

Drive-thru Internet systems are multiple-access wireless networks in which users in moving vehicles can connect to a roadside access point (AP) to obtain Internet connectivity for some period of time as the vehicles pass through the AP's coverage range. In order to evaluate the type of communication services and the quality-of-service that these systems can provide, in this paper, we investigate the data communication performance of a vehicle in Drive-thru Internet systems. In particular, we derive analytical models with tractable solutions to characterize the average and the distribution of the number of bytes downloaded by a vehicle by the end of its sojourn through an AP's coverage range, in the presence of other vehicles contending for the same AP's resources. Our models are able to quantify the impact of road traffic density, vehicle speed, service penetration rate, AP's transmission range and the corresponding bit rate, on the amount of data downloaded by an individual vehicle. In terms of analysis technique, we map the study of our vehicular data downloading process into the transient analysis of a series of Markov reward processes. Our use of Markov reward model is novel in the sense that we only select from the corresponding Markov chain, a subset of relevant sample paths that matches the required behavior of our vehicular flow model. We also validate our proposed analytical models through extensive simulations, driven by empirical vehicular traffic traces. We believe our work offers a unique analytical framework based on which the interplay between vehicular traffic parameters and a vehicle's data communication performance in a Drive-thru Internet system can be studied and optimized in a systematic, quantitative manner.

Dynamic Spectrum Access (DSA) networks can achieve higher spectrum efficiency by exploring the unused spectrum in licensed band. Most of the existing work focuses on maximizing the spectrum utilization while ignoring the immediate influence from the primary (licensed) users to the DSA traffic flows generated by secondary (often unlicensed) users. In this paper, we focus on providing reliability (in terms of a probabilistic life-time guarantee) to the DSA flows. We first propose 3 protection schemes, which provide different levels of pre-planned reliability to an end-to-end DSA flow. We will quantify the lifetime distributions of an end-to- end path in a DSA network under the aforementioned protection schemes. Based on our analysis of the end-to-end path lifetime distribution, various route selection algorithms are proposed to find paths with long lifetime under the corresponding protection scheme. Through simulations, we quantify the tradeoffs between required network capacity and uninterrupted call duration of DSA flows under different route selection algorithms.

Web access logs contain information which can be converted to represent the access history of individual users. A large number of essential attributes can be extracted from the access history. For example, the access counts of each webpage, the occurrence of different webpage access sequences and the time spent between consecutive accesses. Each of the above attributes represents a dimension in the feature space, and all the attributes together form a very high dimension space. Diffusion Wavelets can efficiently project the high dimensional data onto a low-dimensional space according to the correlations between various attributes, so that common anomaly detection algorithms can be applied. In this paper, we propose a system which leverages this technique to differentiate web-access requests generated by Denial of Service (DoS) attacks from legitimate ones. We demonstrate the effectiveness of the proposed system via simulation studies using real-world web access logs. For a simulated HTTP flooding attack which creates a 1000% overload at the web-server, the proposed scheme can reduce the ratio of the attack-to-legitimate requests admitted by the server from 200:1 to 30:1 so that more than 55% of the legitimate requests can still receive proper services under such a severe DoS attack.

A fast RFID counting algorithm with performance guarantee can be used as a fundamental building block for other more sophisticated RFID query protocols and operations. Recently, Kodialam et. al. propose various low-latency RFID counting schemes with accuracy guarantees based on a probabilistic counting approach which does not require explicit identification of individual tags. However, the proposed schemes all assume a perfect communication channel between the reader and the tags which is unlikely to be true in practice. On the contrary, as demonstrated by recent empirical measurement studies, the radio communications between an RFID reader and a set of seemingly "in-range" tags are rather non-deterministic and can even be unreliable at times due to varying radio conditions. In this paper, we extend the algorithms in by taking into account the effects of radio channel unreliability. By modeling the spatial distribution of tags and the corresponding channel fading effects, we analyze the new requirements on the algorithm parameters used in (e.g. number of reader polling cycles, frame-size and persistent probability) in order to achieve a desired level of estimation accuracy. Another key observation is that, unlike the perfect channel case where one can indefinitely reduce the estimation error by increasing the number of reader polling cycles, with an unreliable radio channel, there is a lower-bound on the estimation error due to the inherent variation in the spatial distribution of the tags and the radio channel conditions. Towards this end, we have derived an expression for this lower-bound. We also demonstrate the efficacy of our analytical results and their corresponding guarantees in estimation accuracy via an simulation study.

Opportunistic routing significantly improves the average progress per transmission over unicast routing by leveraging the opportunistic receptions of multiple potential forwarders in wireless mesh networks. Prior studies mainly focus on networks with omni-directional antenna only. Our previous work suggests that not every node contributes equally in a transmission. By concentrating the beam energy at a particular direction, directional antennas may further improve the performance of opportunistic routing in multi-hop wireless networks. In this paper, we derive an analytical model which allows the incorporation of various node distribution models, radio channel models and antenna models to evaluate the average progress per transmission. It is found that a directional antenna with high directivity does not always improve the performance of opportunistic routing and an optimal beamwidth exists for each particular network. When compared to the case of omni-directional antenna, a directional antenna with optimal beamwidth and direction settings can achieve 30 to 50% performance gain, in terms of average progress per transmission, under typical network configurations. Moreover, such performance gain can be as high as 100% for radio propagation environments where the packet reception probabilities fall off slowly with distance.

The delay performance of delay tolerant networks (DTN) can be improved by adding or replacing mobile nodes with higher mobility or transmit power. In this paper, we examine the design trade-offs in heterogeneous DTNs with two types of mobile relay nodes: normal and super. First we present the range of parameters in the Random Direction (RD) mobility model in which we have validated the Markovian assumption on the node inter-encounter intervals. Next, we describe the two-dimensional continuous time Markov chain (CTMC) model with absorption state, used for evaluating the performance of the heterogeneous DTNs. We demonstrate that the performance improvement of adding super nodes is not linear. For example, replacing 10% of the normal nodes with super nodes ones can achieve 40% of the delay reduction versus replacing all of them. Finally, Fluid Flow Approximation (FFA) and Moment Closure Methods for solving the CTMC with various error rates (about 10%) were developed to allow faster analysis of networks with large number of nodes.

Cognitive radio (CR) technology can achieve higher spectrum efficiency by exploring the unused spectrum in licensed band. Most of the existing work focuses on maximizing the spectrum utilization but ignores the immediate influence from primary users to network throughput. In this paper, we investigate the importance of planned link restoration in cognitive radio networks. We formulate the link restoration problem as an integer programming problem. By considering both channel assignment and interference between links, the link through-put can be guaranteed even when primary users appear, and therefore can provide the needed reliability for real-time wireless applications. We consider a link failure model which captures the induced link failures from multiple primary users operating on one frequency channel. Under this failure model, our algorithm explores the sharing of backup capacity. We compare our algorithm to two baseline restoration schemes. Our algorithm performs very well in terms of capacity usage and throughput reliability. By reserving 24.8% of network capacity, our algorithm meets the guarantee requirement in all simulation cases, while "no restoration" just meets the guarantee requirement in 50.8% of all simulation cases.

Recently, the idea of opportunistic routing has been widely explored to improve the performance of multi-hop wireless mesh networks. Most of the previous studies use simulations or empirical measurements to evaluate the performance gain of opportunistic routing and therefore are limited to relatively few types of scenarios. In this paper, we take an analytical approach to study the potential gain of opportunistic routing in multi- hop wireless networks. Unlike other analytical studies which use a deterministic channel model, our approach captures the key characteristics of opportunistic routing, i.e. its ability to take advantage of the numerous, yet unreliable wireless links in the network in a probabilistic manner and study the effectiveness of opportunistic routing under diverse radio propagation environment using lognormal shadowing and Rayleigh fading models. Our results show that, under typical network configurations and neglect overhead, the average progress per transmission of opportunistic routing in lognormal shadowing (Rayleigh fading) environment is about 3 (1.5) times higher than that of traditional unicast routing. Finally, we also demonstrate the potential benefits of using different forwarding regions and directional antennas in opportunistic routing.

In evaluating the performance of a wireless network, the interference between wireless links plays a key role. In previous works, interference was assumed to be a binary phenomenon, i.e., either the links mutually interfere with each other, or they do not interfere. However, there were experimental results contradicting this binary assumption. We term this aspartialinterference. In this paper, we present an analytical framework to characterize partial interference in a single-channel wireless network under unsaturated traffic conditions, and use 802.11b with basic access scheme and differential binary phase shift keying as an illustration. An analogy is drawn between partial interference and code division multiple access to demonstrate their similarities. The gain in capacity across unit cut by exploiting partial interference can be as high as 67% under scheduling in a modified Manhattan network.

Real-time traffic has stringent delay requirement. However, bandwidth limited and error-prone wireless networks often show significant delay jitter. Traditional packet discarding schemes designed for high speed wired networks, such as random early detection (RED), are inadequate for wireless networks with high latency and delay variability. This paper describes an analytical framework for designing lifetime packet discard policies and proposes the channel state aware packet discard on expiration likelihood (CAPEL) algorithm, which only admits packets with expiration likelihood below a predefined requirement. Its effectiveness for improving goodput is quantified under a time varying channel modeled by the Markov-modulated Poisson process (MMPP). In addition, we use Gamma distribution to approximate the conditional system time distribution under the MMPP channel. It is verified to be accurate by simulation.

We propose FRASA, Feedback Retransmission Approximation for Slotted ALOHA, to study the stability region of finite-user slotted ALOHA under collision channel. With FRASA, we derive in closed form the boundary of the stability region for any number of users in the system, which is shown to be accurate via simulations. We use convex hulls and supporting hyperplanes to construct convex and piece-wise linear outer and inner bounds on the stability region of FRASA respectively to facilitate network optimization. We hope the analytical findings with FRASA can provide more insights on the characterization of the capacity region of other types of wireless random access networks, and enable traffic engineering with linear constraints in the design of wireless mesh networks.

We study the stability of finite-user infinite-buffer slotted ALOHA with partial interference. For the case of two users, there is a gradual, transition from the collision channel to the orthogonal channel when the link separation increases. The stability region can be either convex or nonconvex, depending on the link separation and the transmission probability vector. A partial characterization on the boundary of the stability region in closed form for the case of general number of users is also given. We hope this work can provide insight in designing traffic engineering algorithms in wireless mesh networks with practical random access protocols like 802.11.

Disruption tolerant network (DTN) refers to the type of sparse mobile ad hoc network where the nodes are connected intermittently. A common strategy to cope with intermittent network connectivity is to use multiple-copy routing for message delivery. However, the resultant replicates of messages incur significant burden on the bandwidth and storage requirements of each node. In this paper, we investigate the effect of excessive message-replications in multiple-copy routing in DTNs under communication bandwidth and buffer constraints. By modeling the message delivery process as a Markov chain, we first analytically derive the delivery latency as a function of message-replication limit for the single-message-delivery case. The performance of the multiple-message-multiple-flow case is then evaluated via extensive simulations. For the latter, we observe that there is an optimal value for the message-replication limit of each message beyond which network performance will degrade. Finally, we propose an alternative forwarding and message-dropping strategy to address the problem of unfairness found in the basic FIFO-with-blocking strategy. Our results show that the average message delivery delay can be reduced by as much as 25% with the proposed scheme.

This paper proposes an energy-efficient and receiver-driven, TDMA-based MAC protocol (RMAC) for wireless sensor networks. By placing the ownership of the timeslots in the hands of the receiver nodes and letting the receiver nodes assign the timeslots to their neighboring sender nodes, RMAC not only eliminates the need for the sender nodes to explicitly wake-up a receiver node for data transmission, but also eliminates any collision or contention overhead among the sender nodes. Our simulation results show that RMAC outperforms other sender- driven, TDMA-based MAC protocols in terms of the packet latency and power consumption. We also devised a mechanism to enable unused timeslots to be "stolen" by other sender nodes and show that in the case of uniform timeslots assignment among the sender nodes, "timeslots stealing" increases the network capacity by as much as 200%. In addition, we propose a simple timeslots reassignment procedure to allow the receiver nodes to redistribute the timeslots among the sender nodes according to their offered traffic load, and show that it enables the network to efficiently reduce the packet latency in the face of asymmetric, bursty traffic patterns.

With layer 2 and 3 handover algorithms, wireless LAN can allow users to roam between different access points without breaking the connection to the internet. However, one of the biggest problems in roaming is the latency in the handoff process. Recently, Ramani and Savage [1] have proposed a new scheme called SyncScan which can reduce the overall handoff latency in layer 2 by more than 80%. However, this scheme assumes that the beacon transmission schedule among the access points is known a priori, and the beacons are transmitted on time. In this paper, we will propose a self-organized access point coordination system to generate and maintain a conflict-free beacon transmission schedule. Also, we will present a performance evaluation of the SyncScan scheme by studying the effect of beacon transmission delay due to data traffic, and mobility on handover performance.

Power saving in different wireless (WLAN, WPAN and sensor) networks is very crucial since most of the wireless devices are battery-operated. However, putting the devices into the sleep state will result in increased message latency. In our previous paper, we used vacation model from queuing theory to model synchronous wakeup patterns of networked nodes with two different service disciplines: exhaustive and gated. According to the analytical results, the design tradeoffs between latency and power consumption for these two patterns were compared. In this paper, we first present an approximate analysis of the 802.11PSM based on the gated discipline. Then we propose a new 802.11PSM algorithm which combines the advantages of both exhaustive and gated designs. Finally performance comparisons with the current 802.11PSM are included.

Data reliability and real-time delivery of critical information are very crucial in many wireless sensor network applications such as intruder detection and surveillance. In this paper, we explore different mechanisms to provide differentiated services for time-critical information flows. We compare their performances in terms of the packet delivery ratio and end-to-end latency for a high-priority information flow. Our simulation and delay analysis results show that an approach where the wireless medium is reserved along the path from the source to the sink for a high-priority information flow, can provide guaranteed and low latency delivery for a high-priority flow, with acceptable delay impact to the low-priority flows.

Real-time packets are not useful to the receiver if they cannot arrive at the destination within their lifetimes. Discarding packets that are likely to expire after being delivered over the wireless link can increase network efficiency. However, due to delay and bit rate variability of 3G networks, a fixed threshold for packet lifetime discard is not adequate to achieve good throughput under dynamic channel conditions. This paper proposes a channel state dependent (CSD) lifetime packet discard policy. Considering ARQ retransmission as the dominant cause of delay variation and bit rate transitions, a four-state Markov model is used to study the performance of the CSD discard scheme. It is shown that the CSD approach yields better goodput than the channel state indifferent (CSI) approach. Since the scheme relies on accurate estimation of the channel state, impact of delayed knowledge of the channel state is also discussed.

UMTS radio link control (RLC) contains a suite of features and options that make it a challenging task for performance evaluation. We develop a periodic-polling-based retransmission model to analyze the RLC protocol. We derive the evolution of the queue length distribution that can be used to construct a Markov chain. While the task of obtaining a closed-form solution for this Markov chain given a general set of system parameters seems to be formidable, we have been able to obtain a closed-form solution for the average system occupancy for some interesting special cases. To validate the efficacy of the analytical model, we perform numerical calculation on the upper and lower bounds of the average packet delay and compare it with the simulation results. The results show that the lower bound derived is very tight.

UMTS is a third generation radio access network which supports multimedia-capable mobile communication such as packet data and Internet access. In order to provide end-to-end quality of service (QoS) in the UMTS network, certain features need to be incorporated into the MAC design. One possible way of providing different QoS is via providing priority mechanisms. The priority mechanism can be implemented in terms of access priority, service priority and/or buffer management schemes. In this paper, we three access priority schemes based on the random access channel structure. The three schemes are: random chip delay access (RCDAP), random backoff based access (RBBAP), and variable logical channel based access priority (VLCAP). We discuss scenarios where such access priority schemes may be useful. Via simulations, we evaluated and compared these three access priority schemes, Our results indicate that all three schemes provide delay differentiation. However, only the RBBAP scheme provides delay differentiation among different priority classes over a wide range of access request rates without impacting the throughput of the access requests. The other two schemes may cause the throughput of access requests from the lower priority classes to drop apart from forcing them to have a higher access delay.

In the design of wireless communication systems, there are usually multiple objectives such as low cost, small size, high reliability and large capacity. The system architect would need to study tradeoffs among different combinations of modulation, coding, multiple access and antenna techniques to determine the best design. We describe a graphical system design method for evaluating tradeoffs between user bandwidth and multi-user interference to maximize system capacity.

Two new analytic approximations are given for the mean delay in the timed token bus network specified in MAP (manufacturing automation protocol). Each station on the network has two kinds of timer for controlling the maximum amount of time for data transfer before giving up the token. High-priority stations would set large values in the token holding timer (THT) and transmit until the timer expires. Low-priority stations compare the elapsed time since the last token arrival (measured cycle time) to the token rotation timer (TRT) setting, and transmit only for the time left in the timer. The first approximation, based on Fuhrmann's bound for the limited-k service discipline, is for the symmetric case where all the stations have the same traffic load and the identical THT or TRT settings. The approximation has been compared to simulation results, and found to be quite accurate for a representation factory network, with THT or TRT set at two and five times the average total walk time. The second approximation is for the asymmetric case where there is one saturated low-priority (limited-TRT) station, and the rest of the stations in the network have high priority with limited-THT service. These two formulas will be useful in the planning and operation of MAP networks by providing guidance in the choice of timer settings for meeting different priority service requirements.>

Expressions are obtained for the mean excess token holding time for general service time distributions. They were used in the mean message delay approximations for both homogeneous token holding timer (THT) networks and nonhomogeneous THT and token rotating timer networks. Comparisons with simulation results indicated that these approximate formulas are quite accurate. The nonhomogeneous case considered here is only one of many possible mixes of low- and high-priority stations in a network. However, for the purpose of planning manufacturing automation protocol networks, only approximate and worst-case analyses are needed to determine the size of the network and the traffic characteristics for certain performance specifications. The formulas derived here can be used to provide guidance in the choice of timer settings for meeting various priority service requirements.>

The frequency-hopping multiple-access (FHMA) approach to digital mobile radio provides diversity gain against frequency selective fading. Optimum combining methods, in the maximum likelihood sense, are derived for FHMA systems with either noncoherent or differentially coherent detection. In the optimum combining structure, the detector outputs are first passed through a nonlinearity and then a linear combiner.

Higher picture quality and better link utilization can be obtained using time compression multiplexing of color television signals in satellite channels instead of the conventional approach of frequency-division multiplexing. A time compression/expansion technique based on chirp transforms is proposed for such an application. The real-time chirp transform needed is realizable by surface acoustic wave devices. The processing itself introduces truncation distortion on the signal, but the distortion can be controlled to a negligible level by proper design of the chirp filter. Preliminary results are encouraging and indicate that the method is promising for future applications.

In comparing two proposed frequency-hopping mobile radio systems for digitized speech, Goodman et al. have observed that the system employing multiple-frequency-shift-keying (MFSK) can accommodate many more users than the ones with differential-phaseshift-keying (DPSK). Besides the difference in modulation methods, the MFSK system uses hard-limited combining while the DPSK one uses linear combining. In this paper, we show that using linear combining in the MFSK system causes the performance to degrade considerably.

Previous results on the spread-spectrum system for mobile communication proposed by Cooper and Nettleton have been obtained by computer simulation. In this paper, closed-form bounds on the bit error rate are derived for flat, Rayleigh, and Rician fading conditions. For more general types of fading, the saddle point method is used. Since the bounds are easily computed, they are more useful than computer simulations for analyzing system performance.

An easily computed, closed form expression in terms of the saddle point location is used to approximate the error probability, and tight bounds are derived. For the Chebyshev pulse with 10 interferers, the approximation was within 1% for error Probabilities<10^{-6}. With 80 interferers, upperbound/lower bound was about 1.1 at SNR = 16 dB.

Determination of the amplitude distribution and density of shot processes is a problem of longstanding interest. In this paper we present approximations for these distributions and densities whose salient features are accuracy and ease of computation. The parameters that characterize the approximations are determined by the lowerorder moments or cumulants of the shot process. To illustrate the accuracy of the approximations, they are compared with the exact functions in three cases. A logarithmic error measure is introduced and a simple approximation for it is given.

Microwave imaging of anomalies, such as voids or discontinuities, in optically opaque regions is described. Images were reconstructed with two techniques. The experimental technique utilized laser light for reconstructions from detour phase holograms that encoded data measured in the region of Fresnel diffraction. Examples of images are presented for smooth and rough interfaces. The numerical reconstruction technique was based on the angular spectrum technique, and an approximate propagator was derived to describe propagation through two, homogeneous dielectric layers to one anomaly. Image quality was acceptable for either reconstruction method, but the optically reconstructed images are rather small because scale reduction is limited in practice.