Showing papers in "IEEE Communications Magazine in 1983"

Vector quantization: A pattern-matching technique for speech coding

Abstract: V ECTOR QUANTIZATION (VQ), a new direction in source coding, has recently emerged as a powerful and widely applicable coding technique. I t was first applied to analysis/synthesis of speech, and has allowed Linear Predictive Coding (LPC) rates to be dramatically reduced to 800 b/s with very slight reduction in quality, and further compressed to rates as low as 150 b/s while retaining intelligibility [ 1,2]. More recently, the technique has found its way to waveform coding [3-51, where its applicability and effectiveness is less obvious and not widely known. There is currently a great need for a low-complexity speech coder at the rate of 16 kb/s which attains essentially “toll” quality, roughly equivalent to that of standard 64-kb/s log PCM codecs. Adaptive DPCM schemes can attain this quality with low complexity for the proposed 32 kb/s CCITT standard, but at 16 kb/s the quality of ADPCM or adaptive delta modulation schemes is inadequate. More powerful methods, such as subband coding or transform coding, are capable of producing acceptable speech quality at 16kb/s but have a much higher implementation complexity. The difficulty is further compounded by the need for a scheme that can handle both speech and voiceband data at the 16 kb/s rate. These two types of waveforms occupy the same bandwidth in the subscriber loop part of the telephone network, yet they have a widely different statistical character. Effective speech coding at this rate must be geared to the specific character of speech and must exploit our knowledge of human hearing. On the other hand, a waveform that carries data must be coded and later reconstructed so that a modem can still extract the data with an acceptably low error rate. This is purely a signal processing operation not involving human perception. Vector quantization appears to be a suitable coding technique which caters to this dual requirement. VQ may become the key to 16 kb/s coding; it may also lead to improved quality waveform coding at 8 or 9.6 kb/s. In this paper, we review recent results obtained in waveform coding of speech with vector quantization and

An introduction to spread spectrum

Abstract: M ODERN military communications systems are , increasingly adopting the digital method of transmitting information. In a digital communications (or data) link, the information to be sent is represented by a sequence of electronic pulses. In the simplest form of digital signal transmission, these pulses are referred to as binary digits, or “bits.” Each pulse, or bit, is the smallest amount of data that can be communicated, and the messages to be sent ‘are composed of larger sets of these bits. The manner in which message information is imparted to the data bits is the subject of information modulation, a topic well covered in most basic texts on communications.” The subject addressed here is the reliable detection, at the communications receiver, of the individual bits when interference is present, so that the information carried by the sequence of data bits can be recovered. The time duration of a data bit implies a minimum bandwidth capability for the communications link. Thus, it is important to understand the basic concept of bandwidth involved in any criteria for reliable reception.

A structured overview of digital communications--A tutorial review--Part II

Abstract: I N THE first part of this two-part paper, a structure and hierarchy were developed for tracing the key signal processing steps of a typical digital communications system. With the structure as a guide, formatting, source coding, and modulation transformations were examined. Also treated were potential trade-offs for power-limited and bandwidth-limited systems. In Part II, the signal processing overview continues with channel coding, multiplexing and multiple access, frequency spreading, encryption, and synchronization. To complete the overview, fundamental link analysis relationships are reviewed in the context of a satellite repeater channel.

Robust signal processing for communication systems

Abstract: S TATISTICAL models are pervasive in the characterization of the communication process because of the inherently random nature of information and because of the apparently random behavior of noise. Moreover, the use of statistical models for defining reasonable optimality criterla has proven to be a powerful means of designing effective signal processing procedures for communication systems. In order to obtain the signal processing scheme which is optimum under an appropriate performance criterion in a given application, some specific assumptions about the inputs to the signal processor are usually necessary. For example, the matched filter maximizing signal-to-noise ratio in the detection of a signal in additive noise requires specification of the signal shape and noise covariance function in order to be synthesized. When a signal processing procedure which has been developed to give optimum performance under specific conditions is used in a situation where these assumptions are not valid, its performance may degrade seriously from the predicted performance. Its performance may be quite sensitive to deviations of the actual characteristics encountered in its application from those assumed in its design. Unfortunately, .the existence of a truly accurate statistical model is rare in practice, and thus considerable study has been applied to the problem of designing systems which can perform well in uncertain statistical environments. Such systems are generally termed robust. In this article, we discuss several aspects of the concept of robustness as it applies to the signal processing elements of communication systems. Since the advent of statistical modeling for communication system analysis and design, it has been recognized that statistical models only approximately

Minimum-shift keyed modem implementations for high data rates

Abstract: IGITAL communications links capable of conveying data at hundreds of megabits per second (Mb/s) are becoming increasingly important in various applications. An example is the use of time division multiple access (TDMA) transmission through a satellite relay wherein several data channels are routed through single uplink and downlink paths (antenna beams). Separation of the channels from each other is accomplished through interleaving and deinterleaving in time, with possible rerouting taking place in the satellite. When carrying traffic from large metropolitan areas, the required data rates can exceed 100 Mb/s. One such implementation, currently under study by NASA,' involves data transmission at rates of up to 550 Mb/s. Such applications require the use of modulation schemes that use the available bandwidth efficiently. Furthermore, because power is at a premium on board a satellite, the utmost in power efficiency is desired of the chosen modulation scheme. This implies the use of a constant-envelope modulation scheme since amplifiers, such as traveling wavetube amplifiers, are most efficient when operated near saturation. Three popular constant-envelope modulation schemes for efficient transmission of digital data are quadriphase-shift keying (QPSK), offset (or staggered) quadriphase-shift keying (OQPSK), and minimum-shift keying (MSK). Two excellent recent articles [1,2] in IEEE Communications Magazine have discussed their general properties and attributes as well as those of other modulation schemes with constant (or nearly constant) envelopes. Other recent papers [3-61 have presented analytical results and computer

Low probability of intercept

Abstract: LOW Probability of Intercept (LPI) analysis shows the effect of scenario-dependent parameters and detectability-threshold factors in jamming and nonjamming environments. The most significant improvement in LPI performance may be obtained by operating at Extremely High Frequency (EHF) and by maximizing the effective spread-spectrum processing gain and the communicator’s antenna discrimination to the jamming signal, and by minimizing the number of symbols in the message. To illustrate this effect, the LPI performance between an airborne command post and two advanced satellite-communications systems were analyzed in this paper. The two satellite systems analyzed operate at Super High Frequency (SHF) (8/7 GHz) and EHF (44/20 GHz). Although this analysis was made for the airborne command post/satellite scenario, it is directly applicable to other scenarios such as surface ships communicating to airborne relays or submarines communicating to satellites. The basic purpose of an LPI capability fpr a communications system is to prevent the enemy from locating our communications systems, which will decrease the effect of both electronic attack (jamming) and physical attack. In an electronic attack, LPI makes it difficult for the jammer to locate the communications channel; in a physical attack, the effectiveness of antiradiation missiles (ARM’S) may be significantly reduced. Additionally, the effectiveness of intelligence functions, such as electronic support measures (ESM), will be significantly reduced by LPI capability. Critical communications links for command and control (C*) are likely to be supported by either satellite communications or extended line-of-sight (ELOS) relayed communications. These links would be used to support essential information exchanges and would therefore be likely targets for enemy ESM interceptors.

Cellular mobile radio

Abstract: C ELLULAR MOBILE RADIO holds the promise of meeting the increasing demand for mobile telephone channels. In this paper, we describe the cellular mobile radio concept, with emphasis on the two key features of a cellular system: frequency reuse and cell splitting. W e also briefly discuss the history of mobile radio and recent FCC decisions resulting in the present situation. Finally, we describe two methods for achieving improved coverage of remote locations:, a satellite-augmented terrestrial radio system and an HF-augmented system. Mobile telephone service has traditionally been expensive and cumbersome, with a grade-of-service substantially 1,ower than that provided by fixed landlines. Despite these problems, the demand for service has far outstripped the available capacity in many major cities, resulting in long waiting lists for service. Modern technology has made possible a significant increase in the efficiency of mobile radio through frequency reuse and sophisticated system control. In this paper, we discuss recent FCC decisions that have changed the future of mobile radio. W e then describe the cellular concept in some detail and discuss some potential future applications.

Current perspectives in digital speech

Abstract: THE field of digital speech is rapidly coming to fruition as an area of opportunity for commercial application. In this paper we attempt to give a broad tutorial overview of some of the key aspects of this technology. We briefly outline some of the basic properties of speech and the techniques of coding that use these properties and point to areas of current research and areas of practical hardware implementation. We also attempt to outline some of the potential application areas for this new technology.

Orbital efficiency through satellite digital switching

Abstract: T ELECOMMUNICATIONS traffic studies on communications satellites indicate a saturation of current C-band and Ku-band spectrums in the early 1990’s. Satellite switched time division multiple access (SS/TDMA) is a developing technology that will enhance the efficient utilization of frequency spectrum allocations and the geosynchronous orbital arc. This paper describes basic SS/TDMA concepts and esign considerations, and provides an overview of selected planned SS/TDMA systems.

Semiconductor optoelectronic devices for free-space optical communications

Abstract: LIGHT emitting devices based AlGaAs lasers are very useful radiation sources in free-space opticalcommunications systems. Following a brief review of the properties of individual injection lasers, more complex devices are described. These include (or are relevant to) monolithic integration configurations of the lasers with their electronic driving circuitry, power combining methods of semiconductor lasers, and electronic methods of steering the radiation patterns of semiconductor lasers and laser arrays. Fabrication of such devices is one of the major prerequisites for realizing fully the potential advantages of free-space optical communications.

A comparison between frequency hopping and direct spread pn as antijam techniques

Abstract: FREQUENCY Hopping (FH) and Direct Spread Pseudonoise (PN) are the signal processing techniques most often used to achieve an antijam capability. However, comparisons between the techniques in order to determine relative superiority have often been marred in the past by one-sided defenses of one technique or the other. In this paper, an unbiased and comprehensive comparison between the two techniques is presented. The comparison is based on the premise that neither technique is categorically superior, and compares the relative merit of the two techniques for each of a number of issues relevant to antijam systems. By noting both the important issues for a particular application and the relative performance of the two techniques for those important issues, support can be provided for the process of selecting the appropriate technique.

Digital switching in local area networks

Abstract: Until about 1960, the most prevalent digital switches were message switches. In a message switch, information is sent to one or more intermediate points where the message is stored (and possibly processed and regenerated) until the information is demanded, or until a link is freed up to finally forward the information to its destination. Message switches were classically digital since the information, such as telegraph data, was naturally digital and the processing speeds required were low [2]. However, most of the information in the world is transmitted not by message switches but by circuit switches, and it was only in the 1960’s that the digitization of circuit switches became practical. In a circuit switch, storage is not required as it is in message switches. Instead, an end-to-end path is established through intermediate switches. After this “call-set-up,” the path is dedicated to the users until their session is completed, after which the links are reallocated to other users. The classical domain of circuit switching is voice transmission, since a fairly long “call-set-up-time” is usually acceptable before communication takes place. However, the control of call-set-ups is complex, and voice digitization requires high speed digital circuits [3,4]. The late 1960’s and early 70’s saw increasing use of packet. switching, a new form of digital switching that combined some aspects of both circuit and message switching [5]. Packet switching is like message switching in that information is sent on a store-and-forward basis. However, in the case of packet switching, the messages are broken into packets of the order of 100 to 2000 bits, each of which is then sent on a store-and-forward basis and reassembled at the destination to form the original message. The advantage provided by packet switching is that for bursty type data traffic, long haul lines are occupied only for the amount of time it takes to transmit a short packet. Therefore, efficient utilization of resources can be achieved. The packet switch can accomplish some of the functions of a circuit switch by establishing a “virtual path,’’ that is, an end-to-end logical channel which can be allocated to a given pair of users. Thus, for interactive data, the packet switch achieves the advantage of the circuit switch in having a preassigned route, and achieves the advantage of the message switch in efficient utilization of resources by storing and forwarding when resources are available. Packet switching was developed to meet the needs of terminal and computer generated data, and was made possible by the emergence of the minicomputer. It is clear that each of these now classical switching methods was developed partly as a result of technological innovations, and partly because each was tailored to meet a given set of user requirements using new technology. Within the past ten years, a new technology and need have led to a new kind of digital switching, different from its predecessors. The new switch is called a Local Area Network (LAN). Although it may be novel to think of the LAN as a digital switch, there is no doubt it performs the functions of a digital switch. It digitally allocates resources ‘for the transmission of digital information. However, the switching functions are so pervasively distributed throughout the network that the switch is the entire network [6,7]. The technology which .has made the LAN possible is the microprocessor and other inexpensive Large Scale Integration (LSI) devices. The primary domain of LAN’s is the transmission of digital da ta in a limited geographic area, within a single facility or campus of facilities, to serve hosts, minis, micros, word processors, and other digital devices. Simply put, as the cost of digital processing has decreased, it has become feasible to distribute switching functions close to terminals. A revolution in digital switching, begun by packet switching and completed by the LAN, is the allocation of “intelligent” functions to the switch. The microprocessors in the LAN are being assigned more and more complex communications functions and are being used to offload terminals and computers of tasks that have been classically categorized as data processing. We shall see that there is a broad range of functions an LAN can perform in a variety of ways. For the moment, let us

Digital private branch exchanges

Abstract: A digital private automatic branch exchange provides a plurality of ports which may comprise line trunks or operator circuits, the ports being grouped with each group being controlled by an individual microprocessor circuit which performs all real time control over the ports. Voice communication between ports is effected by time division multiplex in connection with a digital switch system forming part of a common control which is controlled by a central processing unit responsive to the microprocessors in each port group for assigning time slots to each interconnection channel. Isolation between the central processing unit and the rest of the system is provided by a peripheral bus to which the common control units and port groups are connected, which peripheral bus is connected to the CPU bus by way of an interface circuit, permitting the system to operate with various types of central processing units without redesign of the peripheral units. A conference circuit is also provided for making available a range of conference sizes by combining the available lines to the conference circuit into groups of a predetermined size which may be expanded by combining groups to form conferences of larger or intermediate size.

Switched digital capability: An overview

Abstract: SDC (Switched Digital Capability) allows end-to-end circuit-switched 56 kb/s data over much of the existing Bell System Network. Since SDC is based largely on the stored program-controlled space and time-division switches, 2-wire local loops, and T-carrier facilities that are already deployed in the network, it can be realized with minimum modification to the existing network. SDC is expected to play a major role in the evolution of the present network to the ISDN (Integrated Services Digital Network) of the future.

Filtering and equalization for digital transmission

Abstract: ICITAL transmission systems of all types are of great current interest as we embark on the information age. For any new transmission system, the topology, components, and system parameters are determined by many and various techniques. They are chosen not only by explicit, deductive technical reasoning, but also by the art of engineering, the desire for continued product line compatibility, patents possessed or not possessed, and new technology. Nonetheless, most digital transmission systems are made up using one or more of the following components: a bandpass filter, low-pass filter, Nyquist filter, slope equalizer, cable equalizer, and transversal equalizer. These items are the subject of this article. It is ironic that digital transmission systems have a preponderance of such components-all necessary for analog signal conditioning of the digitally transmitted signal. In the myriad of digital transmission systems, two distinct approaches to filtering and equalization predominate: one for baseband systems and the other for frequency translated, or modulated, signal systems. This division results from the need to have a competitive system transmission efficiency to cost ratio: where transmission efficiency is transmitted bit rate/channel bandwidth (BR/BW). For baseband systems, as might be expected, the difficulties, and thus cost, of filtering and equalization increase with increased transmission efficiency. To obtain a favorable BR/BW to cost ratio, the bandwidth, an available quantity, is usually sacrificed to keep system costs low. Signal modulation is required when it is desired to simultaneously transmit several baseband signals over common transmission facilities using a frequency division multiplex scheme. A modulated signal system is also necessary when the transmission vehicle has a significant low-frequency limitation; as does, for example, the general customer premises-to-customer premises telephone connection. In such systems, costs start off high because of the additional modulation circuitry. Money can then be further spent on filtering and equalization to allow for increased transmitted bit rate in order to provide a more reasonable BR/BW to cost ratio. Based upon the above premise, the text expands further on the differences between filters and equalizers used for baseband and modulated digital transmission systems.