Showing papers by "Codex Corporation published in 1989"

Packet reservation multiple access for local wireless communications

Abstract: Simulation work is reported indicating that packet reservation multiple access (PRMA) allows a variety of information sources to share the same wireless access channel. Some of the sources, such as speech terminals, are classified as periodic and others, such as signaling, are classified as random. Packets from all sources contend for access to channel time slots. When a periodic information terminal succeeds in gaining access, it reserves subsequent time slots for uncontested transmission. Both computer simulations and a listening test reveal that PRMA achieves a promising combination of voice quality and bandwidth efficiency. >

Multidimensional constellations. I. Introduction, figures of merit, and generalized cross constellations

Abstract: The authors discuss the major attributes desired in signal constellations, such as signal-to-noise ratio (SNR) efficiency, simplicity of mapping bits to points and vice versa, compatibility with coded modulation schemes, and compatibility with quadrature amplitude modulation (QAM). The capability of supporting a so-called opportunistic secondary channel, often used for internal control signaling, is considered. The gain in SNR efficiency of a multidimensional constellation (lattice code) consisting of the points from a lattice Lambda within a region R compared to a cubic constellation is shown to be approximately separable into the coding gain of Lambda and the shape gain of R, for large constellations. Similarly, the expansion of the associated constituent 2-D constellation is shown to be approximately separable into a constellation expansion ratio (CER) coding component CER/sub c/( Lambda ) and a shaping component CER/sub s/(R). The N sphere is the region R with the best shape gain, but N also has large constellation expansion. Bounds for the best possible shape gain versus CER/sub s/(R) or peak-to-average-power ratio (PAR) are given. Generalized cross constellations are discussed. These constellations yield a modest shape gain with very low CER/sub s/(R) or PAR, are easily implemented, are well suited for use with coded QAM modems, and can be readily adapted to support an opportunistic secondary channel. >

The LMS algorithm with delayed coefficient adaptation

Abstract: The behavior of the delayed least-mean-square (DLMS) algorithm is studied. It is found that the step size in the coefficient update plays a key role in the convergence and stability of the algorithm. An upper bound for the step size is derived that ensures the stability of the DLMS. The relationship between the step size and the convergence speed, and the effect of the delay on the convergence speed, are also studied. The analytical results are supported by computer simulations. >

Reduced-state sequence estimation for coded modulation of intersymbol interference channels

Abstract: The authors investigated the detection of trellis codes designed for channels that are intersymbol interference free when they operate in the presence of intersymbol interference. A well-structured reduced-state sequence estimation (RSSE) algorithm is described which can achieve the performance of maximum-likelihood sequence estimation (MLSE) with drastically reduced complexity. Well-defined reduced-state trellises are first constructed by merging the states of the ML supertrellis using set partitioning principles. Then the Viterbi algorithm is used to search these trellises. A special case of RSSE, called parallel decision-feedback decoding, uses the encoder trellis, yet on channels with large attenuation distortion it can provide a significantly better performance than linear equalization. The performance of RSSE is examined analytically and through simulation, and then compared to that of MLSE and ideal decision-feedback equalization. It is noted that the performance advantage of RSSE can be obtained without significantly increasing the decoding delay or complicating an adaptive implementation. >

Multidimensional constellations. II. Voronoi constellations

Abstract: For pt.I see ibid., vol.7, no.6, p.877-92 (1989). Voronoi constellations, also called Voronoi codes are implementable N-dimensional constellations based on partitions of N-dimensional lattices ( Lambda ) that can achieve good shape gains and that are inherently suited for use with coded modulation. Two methods are given for specifying Voronoi constellations on the basis of arbitrary lattice partitions Lambda / Lambda /sub s/, where Lambda /sub s/, the shaping lattice, is an N-dimensional sublattice of Lambda . One of the methods is conjectured to be optimum, and the other has desirable symmetries and naturally supports opportunistic secondary channels. When Lambda and Lambda /sub s/ are 2-D-symmetric, the constituent 2-D constellation is itself a Voronoi constellation. The shaping constellation expansion ratio and peak-to-average-power ratio are determined in general and for various Lambda /sub s/. Methods for labeling Voronoi constellations are given. Their complexity is shown to be dominated by that of decoding Lambda /sub s/. It is also shown that coding and shaping are separable and dual. Bounds on the shape gain of Voronoi constellations are given that depend on the depth and normalized informativity of Lambda /sub s/. These bounds suggest the use of lattices Lambda with depth 2 and normalized informativity less than 1, which can achieve near-optimal shape gains with reduced constellation expansion and implementation complexity. >

Coset codes for partial response channels; or, coset codes with spectral nulls

Abstract: Known coset codes are adapted for use on partial response channels or to generate signals with spectral nulls. By using coset precoding and running digital sum feedback, any desired tradeoff can be achieved between the power and spectra of the relevant sequences, up to the optimum tradeoff possible. A fundamental theorem specifying this optimum tradeoff is given. A maximum-likelihood-sequence-estimation (MLSE) decoder for the original code may be used for the adapted code, and such a decoder then attains the minimum squared distance of the original code. These methods sometimes generate codes with greater minimum squared distance than that of the original code; this distance can be attained by augmented decoders, although such decoders inherently require long decoding delays and may be subjected to quasi-catastrophic error propagation. The authors conclude that, at least for sequences supporting large numbers of bits per symbol, coset codes can be adapted to achieve effectively the same performance and complexity on partial response channels, or for sequences with spectral nulls, as they do in the ordinary memoryless case. >

Line probing modem

Abstract: A modem for receiving data sent from a remote device over a communication channel by using a single carrier modulated signal, the modem including a receiver for receiving the modulated signal and for receiving a line probing signal sent by the remote device over the channel, the receiver being capable of receiving the modulated signal over any one of a plurality of frequency bands; a line probing processor for measuring characteristics of the channel based upon the received line probing signal; and a selector for selecting one of the plurality of frequency bands, said selection being based upon the measured characteristics of the channel, said selected frequency band to be used for receiving the modulated signal from the remote device.

Distributed switching architecture for communication module redundancy

Abstract: Partner modules are assigned to a plurality of primary modules in a communications system, each partner module being adapted to route signals from the primary module to which it is assigned to a spare module in case the primary module fails. The partner modules preferrably also serve as primary modules for receiving signals from their associated communication lines. As a result, redundancy for a failed primary module is provided by a signal routing architecture that is distributed throughout the communications system.

Trellis shaping for modulation systems

Abstract: A digital data sequence is mapped into a signal point sequence for transmission, by selecting the signal point sequence from a subset of all possible signal point sequences based on the digital data sequence, all possible signal point sequences in the subset lying in a fundamental region of a trellis code, the fundamental region being other than a simple Cartesian product of finite-dimensional regions In another aspect, a digital data sequence is mapped into a sequence of signal points for transmission, by specifying a class of possible sequences based on the digital data, and selecting the signal point sequence from the class, the selection being based on the respective average powers of the possible sequences of the class, the selection being based on not only a fixed-length block of the digital data

A bounded-distance decoding algorithm for the Leech lattice, with generalizations

Abstract: An algorithm is given that decodes the Leech lattice with not much more than twice the complexity of soft-decision decoding of the Golay code. The algorithm has the same effective minimum distance as maximum-likelihood decoding and increases the effective error coefficient by less than a factor or two. The algorithm can be recognized as a member of the class of multistage algorithms that are applicable to hierarchical constructions. It is readily generalized to lattices that can be expressed in terms of binary code formulas, and in particular to construction B lattices. >

Adaptive rate control for echo cancelling modem

Abstract: Channel distortion and echo signals detrimentally affect data communication over a channel. The invention provides an echo cancellation modem for full-duplex communication which selects a modulation scheme prior to data communication, and requests changes to the level of modulation during data communication when channel quality has changed significantly. The modem includes an echo canceller (12, 14), a monitor (24) for determining the quality of the channel based on analysis of a residual echo component, and a controller (46) for selecting an acceptable modulation scheme based on the analysis by the monitor. The modem requests the remote device to operate at a selected rate, and operates at the selected rate only if the the remote device concurs. The modem uses a fall forward flag (68) to prevent contention between the modem and the remote device when the remote device cannot support a requested higher level modulation scheme. The modem includes re-initialization logic which selects appropriate monitor thresholds for coded and uncoded modulation options.

Queueing analysis of continuous bit-stream transport in packet networks

Abstract: The problem of transporting continuous bit-stream oriented (CBO) traffic through an all-packet network is examined. In the system considered, CBO traffic is packetized by collecting bits generated by a source during a fixed interval of time (packetization time) and packets from K such sources are multiplexed on a transmission link. The bit-streams are recreated at the receiving end by demultiplexing the packets and then playing out the packets of each bit-stream in sequence. The queuing system analyzed is a single-server queue with periodic arrivals and deterministic service times; the steady-state distributions of the queue length and delay are derived. The method of analysis is based on the ballot theorems and has a computational complexity of O(K), as compared to other proposed methods with complexities of O(K/sup 3/) or greater. It is shown that the delay, measured in units of the packetization time, decreases as the link bandwidth increases. Therefore, for the large capacity links, the contribution of the queuing delay to the total network delay is small compared to the packetization time. >

Synchronizing continuous bit stream oriented terminals in a communications network

Abstract: The frequency of a clock for a receiving terminal is controlled based on a predetermined clock frequency of a terminal that produces a continuous stream of data at a predetermined frequency for transmission to the receiving terminal over a communications network of the kind in which data is transmitted between the terminals in discrete packets that are delayed on the network by possibly different amounts Arrivals of packets that are sent to the receiving terminal are detected, time intervals between the arrivals of the successive packets are determined, and the time intervals are processed to generate an estimate that is related to the predetermined frequency The frequency of the receiving terminal clock is controlled in response to the estimate In one aspect, the time intervals are determined by measuring time differences of arrival between successive packets, and the measured time differences are filtered to generate the estimate In another aspect, the time intervals are determined by measuring phase differences between a reference signal (that indicates the arrival of each packet) and the receiving terminal clock, and the measured phase differences are filtered to generate the estimate

Efficient least-squares lattice algorithms based on Givens rotation with systolic array implementations

Abstract: LS (least squares) lattice algorithms based on Givens rotation, called Givens-lattice algorithms, are presented. They are derived by exploiting the relationship between the Givens algorithms and the RMGS (recursive modified Gram-Schmidt) algorithm. It is shown that the Givens-lattice algorithms are suitable for LS estimation of time-series signals and are computationally more efficient than the recently derived fast QR algorithm for the same purpose. Systolic array implementation of Givens-lattice algorithms using the same basic processing cells in the systolic arrays for the original Givens algorithm is discussed. Computer simulation results are given. >

Trellis shaping

Abstract: The author discusses trellis shaping, a method of selecting a minimum-weight sequence from an equivalence class of possible transmitted sequences by a search through the trellis diagram of a shaping convolutional code C/sub s/. Shaping gains on the order of 1 dB may be obtained with simple four-state shaping codes and with moderate constellation expansion. The shaping gains obtained with more complicated codes approach the ultimate shaping gain of 1.53 dB. With a feedback-free syndrome-former for C/sub s/, transmitted data can be recovered without catastrophic error propagation. Constellation expansion and peak-to-average energy ratio may be effectively limited by peak constraints. With lattice-theoretic constellations, the shaping operation may be characterized as a decoding of an initial sequence in a channel trellis code by a minimum-distance decoder for a shaping trellis code based on the shaping convolutional code, and the set of possible transmitted sequences is then the set of code sequences in the channel trellis code that lie in the Voronoi region of the trellis shaping code. >

On training fractionally spaced equalizers using intersymbol interpolation

Abstract: The use of an intersymbol interpolation method in training fractionally spaced equalizers (FSEs) is investigated. It is shown that the optimal interpolation filter depends on the amplitude frequency response of the transmitter filter and the channel. Using a nonoptimal interpolation filter increases the stead-state mean-square error of the FSE. An interpolated complex FSE (CFSE) using a stochastic gradient, or LMS, adaptive algorithm has very little advantage over an LMS CFSE with symbol-rate updating. However, an interpolated LMS phase-splitting FSE (PS-FSE) has a convergence speed that is twice as fast as a conventional PS-FSE. Special precautions for evaluating the performance of interpolated FSEs are discussed, and a novel evaluation scheme is proposed. >

Combinational static CMOS logic circuit

Abstract: A combinational static CMOS logic circuit for providing a plurality of basic two-input logic functions with reduced complexity and integrated circuit area. The combinational static CMOS logic circuit provides either a NAND or an XOR output at a first output terminal and a NOR output at a second output terminal. A configuration input terminal is utilized for selecting between the NAND or the XOR output being provided at the first output terminal. In an alternate configuration, the combinational static CMOS logic circuit provides either a NOR or an XNOR output at a first output terminal and a NAND output at a second output terminal.

Dual tone multi-frequency detector

Abstract: An apparatus for identifying a first tone and a second tone present in a dual-tone signal, the first tone being selected from a group of tones located within a low band and the second tone being selected from a second group of tones located within a high band, the low band and the high band each spanning a frequency range no greater than f0 wide, the apparatus including a first filter for isolating and downconverting the portion of the dual-tone signal falling within the low band so that it lies within a low frequency range extending from -f0 to F0 ; and a second filter for isolating and downconverting the portion of the dual-tone signal falling within the high band so that it lies within the low frequency range.

A rapid acquisition of PN sequences for DS/SS systems based on a truncated SPRT

Abstract: The authors study a rapid coherent acquisition scheme, using a truncated sequential-probability-ratio test (TSPRT), for direct-sequence-spread spectrum (DS/SS) systems. Since the partial correlation of pseudonoise (PN) sequences is difficult to characterize, the worst-case partial correlation is considered. Linearized bounds of the partial correlation are used for designing the TSPRT so that the resulting test can achieve the prescribed acquisition condition. The design parameters of the TSPRT are chosen so that the average sample size (ASN) is minimized while keeping the maximum ASN smaller than that of the fixed dwell scheme with similar false alarm and miss probabilities. Some simulation results are obtained, and they agree well with analytic results. >

Arbitration scheme for resolving access to a shared resource between a plurality of requestors

Abstract: A technique is described for arbitrating access to a resource among a number of requestors that share the resource according to a predetermined access priority. Each requestor is assigned an impedance in a voltage divider that comprises a string of interconnected impedances, each impedance being positioned in the voltage divider in accordance with the access priority of the requestor to which it is assigned. Each requestor contends for access to the shared resource by driving one side of its assigned impedance to a predetermined (e.g., ground) potential and detecting the resultant potential present at the other side of the assigned impedance to determine if the requestor has won the arbitration for access to the resource.