A method for optimal adaptive setting of pulse-position-modulation pulse detection thresholds, which minimizes the total probability of error for the dynamically fading optical free space channel, is presented. The threshold's adaptive setting, in response to varying channel conditions, results in orders of magnitude improvement in probability of error, as compared to use of a fixed threshold. The adaptive threshold system itself is based on a robust channel identification system that uses average signal strengths to estimate the degree of fade and total attenuation in the channel, and a radial basis function network for estimating pulse spreads, all with excellent accuracy.

A novel connection request procedure for a land mobile satellite system (LMSS) is described. The procedure is based on a binary-feedback free-access collision resolution algorithm (CRA) operating over a number of parallel channels. This protocol is shown to achieve a much higher stable throughput than the previously recommended modified slotted ALOHA protocol for making connection requests. The complexity involved in implementing such a protocol is inherently low. Furthermore, this protocol is superior to other multiple-channel obvious blocked-access CRA protocols. By interleaving the time slots among individual channels, it can be applied to multiple-channel land mobile satellite type systems. Simulation results are obtained that demonstrate stable performance up to capacity. Moreover, the delay throughput characteristic is extremely flat over a wide range of traffic input rates.>

This paper describes and analyzes a pipeline protocol for the data message communications of MSAT-X, a proposed experimental satellite-based mobile communications network. A demand-assigned multiple access protocol using pure ALOHA for making reservation requests has been developed for MSAT-X under error-free assumptions. Preliminary propagation studies indicate that the shortterm bit error rate of satellite channels in a mobile environment can be as high as 10-3. Therefore, error-control schemes must be developed to ensure reliable transmissions. In this paper, we propose a retransmission scheme using selective repeat to minimize the end-to-end delay. We also use slotted ALOHA for making reservation requests to increase the overall system throughput. Since the number of channels available for reservation and data channels is essentially fixed for a given voice call blocking probability and a fixed call arrival rate, the analysis presented in this paper is also applicable to the integrated voice and data services of MSAT-X. Various operational scenarios have been investigated.

This paper describes an ALOHA random access protocol for fading communications channels. A two-state Markov model is used for the channel error process to account for the channel fading memory. The ALOHA protocol is modified to send multiple contiguous copies of a message at each transmission attempt. Both pure and slotted ALOHA channels are considered. The analysis is applicable to fading environments where the channel memory is .short compared to the propagation delay. It is shown that smaller delay may be achieved using replications and, in noisy conditions, can also improve throughput.

This paper describes a feasibility study of the adaptive mobile access protocol (AMAP) for MSAT-X, a proposed experimental mobile satellite communication network. The mobiles are dispersed over a wide geographical area and the channel data rate is limited due to the size and cost limitations of mobile antennas. AMAP is a reservation based multiple-access scheme. The available bandwidth is divided into subchannels, which are divided into reservation and message channels. The ALOHA multiple-access scheme is employed in the reservation channels, while the message channels are demand assigned. AMAP adaptively reallocates the reservation and message channels to optimize system performance. It has been shown that if messages are generated at a rate of one message per hour, AMAP can support approximately 2000 active users per 2400 bit/s channel with an average delay of 1.4 s.

FH/MFSK has been Proposed for a multiuser spread spectrum digital communication system to combat both self-jamming and intentional jamming. An independence assumption at the energy detector outputs is used to evaluate such a system for a digitized voice mobile radio system [11]. This paper presents a correlated model and compares its performance to an independence model under the symbol error probability criterion. A Gaussian process model is also developed for comparison. It is found that the conventional Gaussian approximation is inadequate in predicting the number of users that can be accommodated by an FH/MFSK system using the conventional receiver. Furthermore, for the cases we evaluated, the independence model provides an excellent approximation to the correlated model. Additionally, a conditional Chernoff bound is presented for the more general case of frequency-hopping systems with multiple hops per symbol.

An optimal linear mean-square estimation algorithm is derived under the constraint that the algorithm be multiplication-free. A classical linear estimation problem with block lengthNgenerally requiresN^{2}multiplications. For many on-line signal processing situations a large number of multiplications is objectionable. This class of estimation problems includes the classical linear filtering of a random signal in random noise, as well as the linear equalization of digital data over a dispersive channel with additive noise. Here we consider the linear estimation problem on a binary computer where the estimation parameters are constrained to be powers of two and thus all multiplications are replaced by shifts. Then the optimal constrained linear estimation problem resembles an integer-programming problem except that the allowable discrete points are nonintegers. The branch-and-bound principle is used to convert this minimization problem to a series of convex programming problems. An algorithm is given for the solution as well as numerical results for filtering and data equalization. These examples show that the multiplication-free constraint does not generally increase the mean-square error significantly compared with the classical optimal solution. Furthermore, the intuitive "round to the nearest power of two" procedure for the estimation parameters can be inferior to the optimal brunch-and-bound solution.