IEEE Transactions on Communications

The commenters offer a counterexample to the results presented by W. Yue (see ibid., vol.39, no.6, p.818-22, June 1991) concerning the effect of capture on multichannel slotted ALOHA. A simplified deterministic capture is assumed by Yue. Numerical results show that the actual average throughput is much higher than that obtained by Yue for the same parameters.<> View full abstract»

Modeling alternatives for a fast packet switching system are analyzed. A nonblocking switch fabric that runs at the same speed as the input/output links is considered. The performance of the considered approaches are derived by theoretical analysis and computer simulations. Performance comparison between input queueing approaches with different selection policies are presented. Novel input and out... View full abstract»

Importance sampling is recognized as a potentially powerful method for reducing simulation runtimes when estimating the bit error rate (BER) of communications systems using Monte Carlo simulation. Analytically, minimizing the variance of the importance sampling (IS) estimator with respect to the biasing parameters has typically yielded solutions for systems for which the BER could be found analyti... View full abstract»

Design and performance analyses of a fast frequency-hopped (FFH) spread spectrum communication system employing differential binary phase shift keying (DBPSK) modulation and differentially coherent demodulation are presented. The receiver utilizes a hop timing tracking loop to lock the hop clock. Through this tracking loop, the differentially coherent demodulation of an FFH signal is made possible... View full abstract»

A precoding scheme for noise whitening on intersymbol interference (ISI) channels is presented. This scheme is compatible with trellis-coded modulation and, unlike Tomlinson precoding, allows constellation shaping. It can be used with almost any shaping scheme, including the optimal SVQ shaping, as opposed to trellis precoding, which can only be used with trellis shaping. The implementation comple... View full abstract»

The authors consider the reconstruction of images from partial coefficients in block transform coders and its application to packet loss recovery in image transmission over asynchronous transfer mode (ATM) networks. The proposed algorithm uses the smoothness property of common image signals and produces a maximally smooth image among all those with the same coefficients and boundary conditions. It... View full abstract»

A new, relatively simple architecture for generating continuous phase modulation (CPM) signals is presented. It can produce arbitrary frequency pulses for binary full-response signals with arbitrary modulation indexes. It can also produce partial-response signals or M -ary signals with reasonable accuracy. The method filters and then hard limits the switched and summed outputs of a 90°... View full abstract»

The authors suggest two interpolative block truncation coding (BTC) image coding schemes with vector quantization and median filters as the interpolator. The first scheme is based on quincunx subsampling and the second one on every-other-row-and-every-other-column subsampling. It is shown that the schemes yield a significant reduction in bit rate at only a small performance degradation and, in gen... View full abstract»

An L-frequency trellis coding scheme designed for fading channels that provides diversity up to order L is described. L-frequency coding is advantageous over channel coding schemes that achieve implicit time-diversity through trellis coding in conjunction with time-interleaving because it requires negligible end-to-end delay. The authors derive an upper-bound for the pro... View full abstract»

A coefficient-by-coefficient adaptive discrete cosine transform (DCT) is investigated. The interframe DCT has been known to possess an adverse effect in that it generates mosquito and blocking noise. By analyzing DCT for the intraframe and the interframe video signals, a new adaptive intra/interframe DCT is developed. Cases without and with the movement-compensation are discussed separately. Throu... View full abstract»

The tandem-type go-back-N (GBN) scheme has been proposed for satellite communications to reduce the long roundtrip delay using the onboard processing of satellites. The author considers the exact analysis of the throughput performance of this scheme and its application to a broadcast communications system such as a point-to-multipoint file transfer system. First, the throughput expression for the ... View full abstract»

The authors model the delay-throughput performance of a class of multiple-access circuit-switched ring local area networks that allow multiple messages to be transferred concurrently on the ring. In particular, they model analytically the PLAYTHROUGH protocol under assumptions of uniform and symmetric traffic, infinite buffers, and a first-in, first-out (FIFO) queueing discipline at each station t... View full abstract»

A distributed topology-control algorithm has been developed for each node in a packet radio network (PRN) to control its transmitting power and logical neighbors for a reliable high-throughput topology. The algorithm first constructs a planar triangulation from locations of all nodes as a starting topology. Then, the minimum angles of all triangles in the planar triangulation are maximized by mean... View full abstract»

A phase modulator, constructed with three Armstrong modulators, is presented. In this modulator, the maximum phase deviation can be extended up to π without frequency multiplication. Numerical results of the signal distortion are given for several values of the maximum phase deviation and compared to other Armstrong type phase modulators View full abstract»

It is always desirable to maintain communications in difficult situations, even though fewer messages can get across. The author has developed such capabilities for one-way broadcast media, such as the envisioned terrestrial broadcasting of digital high-definition television signals. In this television broadcasting, the data from video source encoders are not equally important. It is desirable tha... View full abstract»

A method extending the Kirchhoff classical approach for mobile maritime satellite communications is presented. Carrier to specular and carrier to multipath ratios are evaluated from scattering by the rough sea surface calculations using the boundary perturbation method. They are compared with experimental results and with other experimental models. The results illustrate the limited effects of the... View full abstract»

The uplink and downlink performance of a digital cellular radio system that uses direct sequence code division multiple access is evaluated. Approximate expressions are derived for the area averaged bit error probability while accounting for the effects of path loss, log-normal shadowing, multipath-fading, multiple-access interference, and background noise. Three differentially coherent receivers ... View full abstract»

Methods for determining the network reliability of a multihop packet radio network in the presence of hostile jammers are discussed. A new connectivity parameter called radio connectivity is defined as the maximum number of disjoint communication paths that are still usable between given nodes s and d after the jammer is on or, more generally, the minimum number of jammers needed... View full abstract»