The issues of frequency offset and channel estimation are considered for the downlink of coordinated multiple-input multiple-output orthogonal frequency-division multiplexing systems. Multiple carrier frequency offsets exist in this coordinated system. Without implementing an appropriate compensation for these offsets, both inter-carrier and inter-cell interference will degrade the system performance. Here, we adopt a parallel interference cancelation strategy to iteratively mitigate inter-cell interference, and propose a frequency offset estimator, approximated by a Hadamard product and a Taylor series expansion, to eliminate the inter-carrier interference. Our scheme is significantly less complex than existing methods. Furthermore, the proposed channel estimator is robust to frequency offsets and performs comparably well to these conventional approaches.

We consider the issues of frequency offset and channel estimation in downlink coordinated multiple-input multiple-output orthogonal frequency-division multiplexing systems. Multiple carrier frequency offsets exist in this coordinated system. Without implementing an appropriate compensation for these offsets, both inter-carrier and inter-cell interference will degrade system performance. In response, we adopt parallel interference cancelation to iteratively mitigate inter-cell interference, and propose a frequency offset estimator, approximated by Hadamard product and Taylor expansion, to eliminate the inter-carrier interference. Our scheme is significantly less computational complex than the existing methods. Furthermore, the proposed channel estimator is robust to frequency offsets and performs comparably to conventional approaches.

In time-invariant (TI) channels, the maximum likelihood channel estimator (MLCE) needs no matrix inversion if equally spaced pilots or a training symbol is used in an OFDM system. This optimality vanishes in time-variant channels since CFR dispersion destroys the orthogonality between subcarriers. However, our analysis results show that TI-MLCE is still useful to provide complete CIRs under our low-complexity modification. We provide both analysis and simulation results to demonstrate our claim. Satisfactory performance is observed that more than 60% complexity is saved in the extension.

For orthogonal frequency division multiplexing (OFDM) systems, the bandwidth is divided into many subcarriers, requiring a highly accurate frequency offset estimator. In this paper, a novel frequency offset estimator utilizing the periodicity embedded in a training symbol is proposed. We generalize the well known correlation-based methods such that the estimation range of all the correlations embedded in a training symbol can be extended to the maximum. These correlations can thus be utilized freely without degrading the estimation range. Since only one training symbol is needed, the transmission efficiency is increased. The proposed method needs only the periodicity of the training symbol which leaves the design of pilot symbol for other considerations such as timing synchronization and channel estimation. To implement this estimator, a low-complexity algorithm is introduced. Performance analysis and simulation results are both given to compare with the previously proposed best linear unbiased estimator (BLUE), and the superiority of our proposal is witnessed.

In this paper, we propose a cross-layer error recovery scheme to counteract channel errors for wireless data communications. It is realized by integrating the multiple reject (MR) ARQ and packet-level coding techniques. Partial error recovery is functioned at the lower layer since the retransmissions are constrained by a timeout value set at the upper layer. The residual errors are expected to be dealt at the upper layer. Packet-level coding exploited at the upper layer may effectively reduce the upper layer retransmissions and thus enhance the system performance. Performance analysis is conducted to demonstrate that our proposed scheme shows substantial benefits especially as the channel becomes noisy.

In this paper, we consider orthogonal frequency division multiplexing (OFDM) systems with severe Doppler effect. In the frequency domain, this effect destroys the orthogonality between subcarriers and introduces interchannel interference (ICI). In the time domain, fast time-varying channels make channel estimation difficult and degrade the estimation accuracy. Thus ICI-mitigating and improved estimation techniques are desired. We proposed a new equalization technique to suppress ICI in linear minimum mean square error (LMMSE) sense and complexity can be greatly reduced. A novel channel estimation technique with low complexity and free of training symbol design is also proposed to estimate time-varying channels. Numerical results show that our proposal has significant performance improvement over traditional estimation and equalization techniques.

This letter treats the problem of m transmission slots sharing a pool of n automatic repeat request slots. Two cases are treated. In one, these m slots belong to the same base station-terminal pair, i.e., a frequency-division multiplexing channel. In the other, they are used by m different base station-terminal pairs, i.e., a time-division multiplexing channel. The smallest n to satisfy loss and delay requirement is determined.

A new direct-sequence code-division multiple-access (DS-CDMA) system using hybrid spreading sequences in order to increase system capacity is proposed. This scheme permits accommodating more than N users, where N is the processing gain of the spreading sequences. Users are divided into two classes. The centerpiece idea is to synchronously assign N mutually orthogonal codes to class-1 users, and asynchronously assign quasi-orthogonal codes to class-2 users. The proposed architecture is realized through the DS-CDMA and multicode CDMA (MC-CDMA) techniques for class-1 and class-2 users, respectively, and the system capacity is considerably enhanced as compared with Sari et al.. The new approach still excels over Vanhaverbeke et al. when the number of class-1 users K/sub 1/ is large; but our desire is to expand system capacity. Compared with Sari et al. and Vanhaverbeke et al., the new design is able to support multirate applications, and is especially suitable for high-speed multimedia transmissions. Furthermore, the performance of the MC-CDMA system over an additive white Gaussian noise channel is evaluated for various primary codes and mutually orthogonal subcodes. It is shown that orthogonal Gold codes concatenated with Hadamard sequences yield the lowest bit-error rate when the signal-to-noise ratio lies between 0 and 30 dB.

We investigate the departure process of an ATM output buffer intaking asymptotically second-order self-similar traffic using matrix-analytic technique. The results obtained include the marginal and joint distribution of interdeparture times. We show that output and input processes asymptotically possess the same tail and smoothing effect takes place only for traffic with small Hurst parameters.

We study the problem of wireless access to asynchronous transfer modes (ATMs). We consider three classes of ATM sources: constant bit rate (CBR), variable bit rate (VBR), and available bit rate (ABR). We propose a polling scheme with nonpreemptive priority. Under such a scheme, we derive sufficient conditions such that all the CBR sources satisfy their jitter constraints and all the VBR sources satisfy their delay constraints. The remaining bandwidth is used by the ABR sources, for which we adapt a random access scheme proposed by Chen and Lee (1994). For this random access scheme, we derive the throughput-offer load characteristic, and thus the capacity. Based on this, we propose adaptive random access schemes that track the offer load to its optimal value. Our simulations show that our adaptive schemes maintain a high throughput with respect to the whole range of system load.

This paper examines the feasibility of adding a personal communication network (PCN) to a frequency band which is already allocated to fixed-service microwave systems. To achieve this goal, spread spectrum techniques are used to spread narrowband PCN signals into wideband. The forward and reverse link performance of PCN users under the influence of the microwave system and the influence of PCN users on the microwave system are both examined. It is proven that spectrum sharing between the PCN network and microwave systems is indeed feasible. Although our analysis is done based upon the consideration of only one microwave system, it can be easily extended to allow the appearance of two or more microwave systems.

A novel two-layer low Earth orbiting satellite network is proposed. Compared with a single layer global network, not only is the system performance improved but also the number of satellites can be reduced. The main ideas include (1) raising the altitude of the original global network, and (2) adding several satellites at a lower altitude to form a regional network to serve calls from the areas of heavy traffic. In order to fully utilize the benefits of the two-layer network, additional functions such as adjustable beams and dynamic channel management are added. The call blocking rate is derived. Numerical examples are provided to demonstrate several interesting phenomena. A network of multiple LEO satellites has also been proposed as a means to support global personal communications.

CDMA has been proven to be one of the promising mainstream multiaccessing techniques in future cellular mobile communication systems. It is offering attractions such as soft capacity, soft handoff, and antimultipathing. A quality-based birth-and-death queueing model is developed for the purpose of evaluating the performance of a CDMA cellular that supports two-rate transmissions.

A study on the use of direct sequence spread spectrum techniques to reduce the damage caused by multipath interference in random accessed mobile packet radio networks is conducted. Basically the reception of a packet is affected by three types of interference: channel noise, multipath interference, and multiuser interference. To reflect reality, signal capture due to near-far field effect shall be incorporated in the analysis. Two types of capture effect are considered. Our results show that spread spectrum greatly reduces the multipath and multiuser interference at the expense of channel bandwidth. Furthermore, the two types of capture effect behave quite differently. One of them seems too optimistic and behaves counter-intuitively.>

Security of mobile communication networks is always a great concern to both vendors and subscribers. User authentication is a very basic component in the issue of security. It is used to screen illegal access to the network. This paper investigates the effect of user authentication on the performance aspect of a cellular mobile communication network. The performance measures examined include channel utilization, call success rate, and call drop rate. It is observed through numerical examples that user authentication not only eliminates intruders but also offers better performance if a fast enough authentication processor is used. The results derived in this paper can be used in the design of a practical system.

A circuit-switched network consisting of multiple low Earth orbiting satellites and three geostationary satellites is considered. Crosslinks among low-orbit satellites are used as the major communication channels, while calls are routed via geostationary satellites only when low-orbit routes exceed or equal to a hop-count threshold. Simulation results are used to illustrate the network throughput under various traffic conditions and different link capacities. The impact of satellite failures on the network performance is also investigated.>

This paper analyzes the end-to-end delay of a selective-repeat ARQ protocol which tries to adaptively use the channel idle time. The end-to-end delay consists of three parts: queueing delay at the transmitter, transmission delay over the channel, and resequencing delay at the receiver. Although this paper succeeds only in obtaining approximate end-to-end delay, the agreement with simulation is generally good.>

An ATM call interworking with the public switched telephone network may experience echo problems due to impedance mismatch. The authors propose two echo control designs to assure the quality of voice services by reducing round-trip echo delay. Performance of these two echo control designs is then analyzed.>

The performance of stop-and-wait, go-back-N, and selective-repeat ARQ protocols is examined for a multichannel system. The channels can either operate independently or jointly. For each protocol, the results obtained include throughput efficiency and delay. The results are also validated by computer simulations. The performance is compared to that of the single-channel system under the constraint of fixed total capacity. It is observed that the multichannel system has a chance to outperform the single-channel system.>

The performance of automatic-repeat-request (ARQ) protocols which are affected by nonindependent channel errors is studied. The protocols are the three basic ARQ protocols: stop-and-wait, go-back-N, and selective-repeat. Two types of nonindependent channel errors are considered: the kth-order Markovian errors and the gap errors. Both throughput efficiency and packet delay are obtained. The validity of analysis is verified by computer simulations. One key discovery is that when channel errors are dependent, selective-repeat ARQ achieves the same throughput efficiency as that in independent channel errors.>

A collision resolution algorithm for variable length messages is proposed and studied. Both throughput and delay are obtained. The delay analysis is done at both the packet and message level. Due to reduced contention, this protocol can achieve very high throughput. The effect of channel errors is also studied.>

This paper presents an efficient algorithm to deal with inexact reasoning, where fuzzy production rules are used for knowledge representation and these rules are represented by an augmented fuzzy rule matrix F. The algorithm performs inexact reasoning via repeated transformations of augmented truth state vectors by the transitive closure Q of the augmented fuzzy rule matrix F. Given the truth values of some conditions, the algorithm can perform inexact reasoning to evaluate the truth values of other conditions automatically. The time complexity of the algorithm is 0(k(m+k)2), where m is the number of conditions and k is the number of compound conditions appearing in the antecedent portion of the fuzzy production rules.

The protocol allows a 1.0 transmission probability for newly transmitted packets, while p for retransmitted packets. Compared to the protocol in which each packet is always transmitted with the same probability p, it shows better performance, especially when the channel traffic is light, as demonstrated by numerical results.>

A system composed of two queues that accepts correlated inputs and receives services from a synchronous server is studied. An exhaustive service discipline is adopted in serving a queue. Nonzero switchover time for the server to change service from one queue to the other is assumed. A closed-form expression for mean waiting time is obtained. The validity of the analysis if verified using the results of a computer simulation.>

A study is made of collision resolution algorithms for code-division multiple-access systems. Both random and fixed code assignment are considered. Different codes are orthogonal. Two types of multiuser interference are considered. Either the well-known 0.487 algorithm or a windowed version of Capetanakis' collision resolution algorithm is used in resolving collisions. Throughput and delay are derived for each of the protocols considered. Computer simulations are provided to verify the validity of the analysis.>

The effect of prioritization on the behavior of a concentrator which accepts packetized messages and receives a service from a transmitter operating in synchronous mode is studied. The messages arrive according to a Poisson process, and are divided into two classes: ordinary and priority. A buffer is shared between ordinary and priority packets. Two buffer-sharing strategies are considered: sharing with maximal length, and complete sharing subject to purgation. Upon the arrival of a message, if its size exceeds the remaining buffer space, then the message will be completely rejected. For each buffer-sharing strategy, results obtained include state transition probability, state probability, blocking probability, throughput, and delay. Computer simulations are used to verify the validity of results.>

The effect of multipath interference on the performance of packet radios is investigated. The protocols considered include pure ALOHA and slotted ALOHA. Signal reception with and without capture effect is considered in the analysis. Mathematical expressions of system throughput and packet delay have been successfully obtained for each protocol. Numerical calculations are used to demonstrate when the multipath interference can be ignored and when it can be disastrous.>

A fuzzy Petri net model (FPN) is presented to represent the fuzzy production rule of a rule-based system in which a fuzzy production rule describes the fuzzy relation between two propositions. Based on the fuzzy Petri net model, an efficient algorithm is proposed to perform fuzzy reasoning automatically. It can determine whether an antecedent-consequence relationship exists from proposition d/sub s/ to proposition d/sub j/, where d/sub s/ not=d/sub j/. If the degree of truth of proposition d/sub s/ is given, then the degrees of truth of proposition d/sub j/ can be evaluated. The formal description of the model and the fuzzy reasoning algorithm are shown in detail. The upper bound of the time complexity of the fuzzy reasoning algorithm is O(nm), where n is the number of places and m is the number of transitions. Its execution time is proportional to the number of nodes in a sprouting tree generated by the algorithm only generates necessary reasoning paths from a starting place to a goal place, it can be executed very efficiently.>

A unified methodology using signal flow graphs is proposed and used to analyze several existing ARQ (automatic repeat request) protocols. The methodology offers a systematic and efficient way of obtaining throughput and delay. It also offers additional advantages in obtaining results such as the mean response time, and in analyzing the ARQ protocols in which the feedback channel is not error-free.>

A hybrid contention-based time-division multiaccess (TDMA) protocol is proposed and studied. Channel time is organized into frames, each consisting of reservation, fixed, and variable subframes. Users are divided into groups. Each group is given a data slot in the fixed subframe that is open to users in that group on the basis of contention. Each user can also place his demand in the reservation subframe. The number of slots assigned to a particular user is based on the information gathered from the other two subframes. Performance analyses, both transient and steady-state, are successfully carried out. Computer simulations verify the analyses.>

This paper studies the effect of routing on the behavior of a finite queue which accepts batch Poisson inputs and receives service from multiple synchronous servers. Upon the arrival of a group of customers idle or pseudoidle servers will be considered first in routing decisions. Otherwise routing will be determined randomly in accordance with a preset probability distribution. Results obtained include state probability, blocking probability, delay, and throughput. Validity of analysis has been verified by computer simulations. These results can he used in evaluating the performance of a computer communication network.

The performance of a differential PCM type adaptive reservation multiaccess technique is studied in this correspondence. The frame structure of the time-axis remains identical to that in [1]. However, channel reservation is handled differently. Analyses, both transient and steady-state, have been successfully completed. Results obtained include average frame length, queue length, average delay, etc. The validity of analysis is also verified by computer simulations. Through examples we witness that this new protocol generally has better performance than [1].

This correspondence reports the performance of a multibeam packet satellite using ALOHA type random access techniques. The satellite is assumed to provide an infinite buffer for successful packets on the uplinks of various beam zones. The transition overhead of a downlink transponder can be arbitrary. A TDMA frame is organized according to the traffic demand of each zone. This work is an extension of [1].

This paper studies the behavior of a packet switch which provides finite waiting space and receives packetized messages. The arrivals of the messages constitute a Poisson process. Each message consists of a random number of packets. The number of packets contained in a message is assumed to be an integer-valued random variable which may follow any arbitrary probability distribution. All packets residing in the buffer receive service from a single output transmitter operating synchronously at a constant rate. Each packet receives the same fixed service time from the transmitter and then leaves the system. Upon the arrival of a message, if the remaining buffer space is not enough to accommodate all packets of the message, then the entire message is completely rejected. Results such as message blocking probability, packet blocking probability, throughput, and mean delay have been obtained. Two different approaches, minislot approximation and the application of the residue theorem, are used to obtain these results. Especially, this combinatorially very complex problem is successfully solved by the residue theorem in a recursive manner. These results are useful in evaluating the performance of a packet switch. They are also useful for design purposes.

The performance of a multibeam packet satellite using ALOHA type random access techniques is studied. The satellite provides either no buffer or an infinite buffer for successful packets on the uplink channels. A TDMA frame is organized in accordance with the traffic demand from each area. System throughput and packet delay have been successfully obtained. Aiming at improving the performance of the system, three different protocols are introduced and studied in the zero buffer case. It is demonstrated, through examples, that satisfactory system performance can be obtained using an appropriate protocol. However, system performance can also be severely degraded if the transition time for the satellite transponder to switch from one zone to another is not negligible. In this case, the performance can be restored via the use of a buffer of sufficiently large size.

The performance of a packet satellite system with multiple uplink channels using priority service discipline for successful packets on the uplinks is analyzed in this paper. The satellite is assumed to provide either no on-board storage or an infinite buffer for successful packets on the uplinks. The delay-throughput characteristics for each priority as well as the entire system have been obtained. Expression for the z-transform of the system at equilibrium has been derived for the case of infinite buffer. As expected, it is demonstrated through examples that packets with lower priorities have lower throughput and suffer longer delay.

We analyze the performance of a one-bit digital matched filter (DMF) responding to binary signaling through a noisy multipath channel. The output signal-to-noise ratio(SNR_{o})plays a key role in the analysis. The SNRoperformance is fully studied for a two-path channel, both fading and non-fading. For the generalN-path channel, results are obtained only for the fading case. All results are conditioned on the knowledge of the time delays of all paths, and are valid only for small input signal-to-noise ratio(SNR_{i). Most of the results have been verified by computer simulations.