Showing papers on "Communication channel published in 1996"

Variable-rate variable-power MQAM for fading channels

Abstract: We propose a variable-rate and variable-power MQAM modulation scheme for high-speed data transmission over fading channels. We first review results for the Shannon capacity of fading channels with channel side information, where capacity is achieved using adaptive transmission techniques. We then derive the spectral efficiency of our proposed modulation. We show that there is a constant power gap between the spectral efficiency of our proposed technique and the channel capacity, and this gap is a simple function of the required bit-error rate (BER). In addition, using just five or six different signal constellations, we achieve within 1-2 dB of the maximum efficiency using unrestricted constellation sets. We compute the rate at which the transmitter needs to update its power and rate as a function of the channel Doppler frequency for these constellation sets. We also obtain the exact efficiency loss for smaller constellation sets, which may be required if the transmitter adaptation rate is constrained by hardware limitations. Our modulation scheme exhibits a 5-10-dB power gain relative to variable-power fixed-rate transmission, and up to 20 dB of gain relative to nonadaptive transmission. We also determine the effect of channel estimation error and delay on the BER performance of our adaptive scheme. We conclude with a discussion of coding techniques and the relationship between our proposed modulation and Shannon capacity.

Channel assignment schemes for cellular mobile telecommunication systems: a comprehensive survey

Abstract: This article provides a detailed discussion of wireless resource and channel allocation schemes. The authors provide a survey of a large number of published papers in the area of fixed, dynamic, and hybrid allocation schemes and compare their trade-offs in terms of complexity and performance. We also investigate these channel allocation schemes based on other factors such as distributed/centralized control and adaptability to traffic conditions. Moreover, we provide a detailed discussion on reuse partitioning schemes, the effect of handoffs, and prioritization schemes. Finally, we discuss other important issues in resource allocation such as overlay cells, frequency planning, and power control.

Recent advances in high-speed underwater acoustic communications

Abstract: In recent years, underwater acoustic (UWA) communications have received much attention as their applications have begun to shift from military toward commercial. Digital communications through UWA channels differ substantially from those in other media, such as radio channels, due to severe signal degradations caused by multipath propagation and high temporal and spatial variability of the channel conditions. The design of underwater acoustic communication systems has until recently relied on the use of noncoherent modulation techniques. However, to achieve high data rates on the severely band-limited UWA channels, bandwidth-efficient modulation techniques must be considered, together with array processing for exploitation of spatial multipath diversity. The new generation of underwater communication systems, employing phase-coherent modulation techniques, has a potential of achieving at least an order of magnitude increase in data throughput. The emerging communication scenario in which the modern underwater acoustic systems mill operate is that of an underwater network consisting of stationary and mobile nodes. Current research focuses on the development of efficient signal processing algorithms, multiuser communications in the presence of interference, and design of efficient modulation and coding schemes. This paper presents a review of recent results and research problems in high-speed underwater acoustic communications, focusing on the bandwidth-efficient phase-coherent methods. Experimental results are included to illustrate the state-of-the-art coherent detection of digital signals transmitted at 30 and 40 kb/s through a rapidly varying one-mile shallow water channel.

On verifying the first-order Markovian assumption for a Rayleigh fading channel model

Abstract: The use of received signal-to-noise ratio (SNR) as the side information in communication systems has been widely accepted especially when the channel quality is time varying. On many occasions, this side information is treated as the received SNR of the current channel symbol or that of previous symbols. In particular, the first-order Markov channel provides a mathematically tractable model for time-varying channels and uses only the received SNR of the symbol immediately preceding the current one. With the first-order Markovian assumption, given the information of the symbol immediately preceding the current one, any other previous symbol should be independent of the current one. Although the experimental measurements confirm the usefulness of the first-order Markovian assumption, one may argue that second or higher-order Markov processes should provide a more accurate model. We answer this question by showing that given the information corresponding to the previous symbol, the amount of uncertainty remaining in the current symbol should be negligible.

Subspace-based channel estimation for code division multiple access communication systems

Abstract: We consider the estimation of channel parameters for code-division multiple access (CDMA) communication systems operating over channels with either single or multiple propagation paths. The multiuser channel estimation problem is decomposed into a series of single user problems through a subspace-based approach. By exploiting the eigenstructure of the received signal's sample correlation matrix, the observation space can be partitioned into a signal subspace and a noise subspace without prior knowledge of the unknown parameters. The channel estimate is formed by projecting a given user's spreading waveform into the estimated noise subspace and then either minimizing the likelihood or minimizing the Euclidean norm of this projection. Both of these approaches yield algorithms which are near-far resistant and do not require a preamble.

Compressed digital-data interactive program system

Abstract: An interactive cable television system is disclosed which utilizes a standard cable television distribution network for simultaneously providing a plurality of viewers with an interactive television program comprising a plurality of signals related in time and content. Video signals are transmitted in a digital format, more than one signal being multiplexed onto a data stream on a single channel. The video signals may be compressed for efficiency. A receiver, in conjunction with a signal selector, selects a particular NTSC channel for playback, then selects a particular video signal from the data stream, and decompresses the video signal for playback. Seamless switching between video signals on different channels is provided. An alternative embodiment is disclosed wherein the various signals which comprise the interactive program are switched at the head end rather than at the receiver. The multiple choice control unit selects a desired signal by relaying the multiple choice selections of the user through a relay box back to a remotely located switching station. The switching station routes the correct video signal down the appropriate cable channel for the particular user.

Method for the distribution of code and data updates

Abstract: A system and method for distributing software applications and data to many thousands of clients over a network. The applications are called "channels", the server is called the "transmitter", and the client is called the "tuner". The use of channels is based on subscription. The end-user needs to subscribe to channel before it can be executed. When the end-user subscribes to a channel the associated code and data is downloaded to the local hard-disk, and once downloaded the channel can be executed many times without requiring further network access. Channels can be updated automatically at regular intervals by the tuner, and as a result the end-user is no longer required to manually install software updates, instead these software and data updates are automatically downloaded and installed in the background. This method of automatic downloading of updates achieves for the client the same result as the broadcast distribution of software over a connection based network, but wherein the client initiates each update request without requiring any special broadcast networking infra structure.

A rate-splitting approach to the Gaussian multiple-access channel

Abstract: It is shown that any point in the capacity region of a Gaussian multiple-access channel is achievable by single-user coding without requiring synchronization among users, provided that each user "splits" data and signal into two parts. Based on this result, a new multiple-access technique called rate-splitting multiple accessing (RSMA) is proposed. RSMA is a code-division multiple-access scheme for the M-user Gaussian multiple-access channel for which the effort of finding the codes for the M users, of encoding, and of decoding is that of at most 2M-1 independent point-to-point Gaussian channels. The effects of bursty sources, multipath fading, and inter-cell interference are discussed and directions for further research are indicated.

Comparison of diversity combining techniques for Rayleigh-fading channels

Abstract: Several methods of diversity combining for a Rayleigh-faded channel are evaluated and compared. The methods considered are, for coherent reception, maximal ratio combining (MRC), selection combining (SC), and a generalization of SC, whereby the two (three) signals with the two (three) largest amplitudes are coherently combined. We will call this method second (third) order SC, and denote it SC2 (SC3). Similar techniques are also investigated for noncoherent reception, with equal gain combining (EGC) replacing MRC, and noncoherent versions of SC2 and SC3. Numerical results indicate that SC2 and SC3 significantly enhances the bit-error rate (BER) performance relative to that achievable with SC, and under certain conditions approaches the performance achieved by MRC or EGG. The performance enhancement of SC2 and SC3 is especially noticable for noncoherent reception, where EGC is seen to provide the best performance only for low BER values. In fact, when the BER is 10/sup -3/ or greater, SC2 and SC3 performed comparably to EGG, and in some cases performed better than EGC.

Code division multiple access (CDMA) communication system

Abstract: A subscriber unit for use in a multiple access spread-spectrum communication system includes a spread spectrum radio interface, responsive to a rate function signal from a base station, and first and second despreaders. The base station assigns the rate function spread-spectrum message channels and the first despreader recovers and modifies an information signal one of the spread spectrum message channels. The information channel mode is then modified for processing by the second despreader, with the second despreader supporting a different information signal rate. The subscriber unit has a capability of communicating with a dynamically changing a transmission rate of an information signal which includes multiple spread spectrum message channels. The system includes a closed loop power control system for maintaining a minimum system transmit power level for a radio carrier station and the subscriber units, and system capacity management for maintaining a maximum number of active subscriber units for improved system performance.

Enhancing throughput over wireless LANs using channel state dependent packet scheduling

Abstract: Unlike wired networks, packets transmitted on wireless channels are often subject to burst errors which cause back to back packet losses. Most wireless LAN link layer protocols recover from packet losses by retransmitting lost segments. When the wireless channel is in a burst error state, most retransmission attempts fail thereby causing poor utilization of the wireless channel. Furthermore, in the event of multiple sessions sharing a wireless link, FIFO packet scheduling can cause the HOL blocking effect, resulting in unfair sharing of the bandwidth. This observation leads to a new class of packet dispatching methods which explicitly take the wireless channel characteristics into consideration in making packet dispatching decisions. We compare a variety of channel state dependent packet (CSDP) scheduling methods with a view towards enhancing the performance of the transport layer sessions. Our results indicate that by employing a CSDP scheduler at the wireless LAN device driver level, significant improvement in the channel utilization can be achieved in typical wireless LAN configurations.

Outcome-Based and Behavior-Based Coordination Efforts in Channel Relationships:

Abstract: Supplier personnel often emphasize outcomes and behaviors in personal communications with distributors, thereby signaling important product-market objectives and advocating specific distributor act...

Differential chaos shift keying : A robust coding for chaotic communication

Abstract: The statistical attributes of the received chaotic signal are evaluated in the noncoherent chaos shift keying (CSK) modulation scheme in order to perform the demodulation. This approach offers a very simple circuit configuration, but like the other solutions proposed before, it is sensitive to the channel noise and imperfections. In the differential chaos shift keying (DCSK) approach every incoming information bit is encoded into two bits. The first signal serves as a reference, while the second one carries the information. If the binary information to be transmitted is ”1” then first the reference signal is transmitted and after it that signal is repeated. For a ”0”, the inverted version of the reference signal is transmitted. The receiver has a storage capability and the demodulation is performed by evaluating the cross-correlation of the two signals. The effect of noise is reduced by averaging and proper design of the channel filter. The effect of channel imperfections is also reduced, because both the signal and its reference are sent via the same channel. The DCSK approach offers a very robust solution for the chaos communication and it can be implemented with very simple circuitry even in the microwave frequency region.

Device and method for sealing puncture wounds

Abstract: A device is proposed for inserting hemostatic material through a tissue channel and against the outside wall of a blood vessel of a patient, wherein the blood vessel wall has a puncture therein adjacent the tissue channel. The device includes a charge of hemostatic material and a hollow sheath adapted to pass through the tissue channel, the sheath having a cross sectional profile larger than the puncture. The device places the hemostatic material in the hollow sheath and advances the hemostatic material through the sheath to the outside of the vessel wall around the puncture.

Typing and subtyping for mobile processes

Abstract: The π-calculus is a process algebra that supports mobility by focusing on the communication of channels. Milner's presentation of the π-calculus includes a type system assigning arities to channels and enforcing a corresponding discipline in their use. We extend Milner's language of types by distinguishing between the ability to read from a channel, the ability to write to a channel, and the ability both to read and to write. This refinement gives rise to a natural subtype relation similar to those studied in typed λ-calculi. The greater precision of our type discipline yields stronger versions of standard theorems on the π-calculus. These can be used, for example, to obtain the validity of β-reduction for the more efficient of Milner's encodings of the call-by-value λ-calculus, which fails in the ordinary π-calculus. We define the syntax, typing, subtyping, and operational semantics of our calculus, prove that the typing rules are sound, apply the system to Milner's λ-calculus encodings, and sketch extensions to higher-order process calculi and polymorphic typing.

A channel access scheme for large dense packet radio networks

Abstract: Prior work in the field of packet radio networks has often assumed a simple success-if-exclusive model of successful reception. This simple model is insufficient to model interference in large dense packet radio networks accurately. In this paper we present a model that more closely approximates communication theory and the underlying physics of radio communication. Using this model we present a decentralized channel access scheme for scalable packet radio networks that is free of packet loss due to collisions and that at each hop requires no per-packet transmissions other than the single transmission used to convey the packet to the next-hop station. We also show that with a modest fraction of the radio spectrum, pessimistic assumptions about propagation resulting in maximum-possible self-interference, and an optimistic view of future signal processing capabilities that a self-organizing packet radio network may scale to millions of stations within a metro area with raw per-station rates in the hundreds of megabits per second.

Adaptive OFDM for wideband radio channels

Abstract: An OFDM (orthogonal frequency division multiplexing) transmission system is simulated with time-variant transfer functions measured with a wideband channel sounder. The individual subcarriers are modulated with fixed and adaptive signal alphabets. Furthermore, a frequency-independent as well as the optimum power distribution are used. The simulations show that with adaptive OFDM, the required signal power for an error probability of 10/sup -3/ can be reduced by 5...15 dB compared with fixed OFDM. The fraction of channel capacity which can be achieved with adaptive OFDM depends on the average signal-to-noise ratio and the propagation scenario.

Random access communications channel for data services

Abstract: In a digital communication system for communicating digital information (100), the digital communication system having a forward link (120) and a reverse link (130), a system and method for communicating a data packet. The system comprises a communicating transceiver (202), from among a number of digital transceivers (102), for sending the data packet on a random access channel (208) over the reverse link and for receiving the digital information from the forward link. The system also comprises a base station (108) for receiving the data packet on the random access channel (208) from the reverse link (130) and for sending the digital information over the forward link (120). The digital transceivers (102) share the random access channel (208). The digital transceivers (102) have a bandwidth demand. The system (100) may also include a dedicated channel (214) for communicating the data packet between the communicating transceiver (202) and the base station (108) and a processor (212) for switching from the random access channel (208) to the dedicated channel (214) when the bandwidth demand exceeds a first threshold, and for switching from the dedicated channel (214) to the random access channel (208) when the bandwidth demand drops below a second threshold. The system is well suited for use in CDMA applications.

CD3-OFDM: a novel demodulation scheme for fixed and mobile receivers

Abstract: This paper describes a novel channel estimation scheme identified as coded decision directed demodulation (CD3) for coherent demodulation of orthogonal frequency division multiplex (OFDM) signals making use of any constellation format [e.g., quaternary phase shift keying (QPSK), 16-quadrature amplitude modulation (QAM), 64-QAM]. The structure of the CD3-OFDM demodulator is described, based on a new channel estimation loop exploiting the error correction capability of a forward error correction (FEC) decoder and frequency and time domain filtering to mitigate the effects of noise and residual errors. In contrast to the conventional coherent OFDM demodulation schemes, CD3-OFDM does not require the transmission of a comb of pilot tones for channel estimation and equalization, therefore yielding a significant improvement in spectrum efficiency (typically between 5-15%). The performance of the system with QPSK modulation is analyzed by computer simulations, on additive white Gaussian noise (AWGN) and frequency selective channels, under static and mobile reception conditions. For convolutional coding rate 1/2, the results indicate that CD3-OFDM allows one to achieve a very fast adaptation to the channel characteristics in a mobile environment (maximum tolerable Doppler shift of about 80 Hz for an OFDM symbol duration of 1 ms, as differential demodulation) and an E/sub b//N/sub 0/ performance similar to coherent demodulation (e.g., E/sub b//N/sub 0/=4.3 dB at bit-error rate (BER)=2/spl middot/10/sup -4/ on the AWGN channel). Therefore, CD3-OFDM can be suitable for digital sound and television broadcasting services over selective radio channels, addressed to fixed and vehicular receivers.

Spread spectrum transceiver module utilizing multiple mode transmission

Abstract: A data transceiver module for digital data communications in a portable hand-held data terminal has multiple data spread spectrum modes which include direct sequence and frequency function modulation algorithms. The transceiver module has multiple user or program configurable data rates, modulation, channelization and process gain in order to maximize the performance of radio data transmissions and to maximize interference immunity. Various module housings, which may be PCMCIA type, are able to be mated with a suitably designed data terminal. Media access control protocols and interfaces of multiple nominal operational frequencies are utilized. Wireless access devices in a cell based network each consider a variety of factors when choosing one of a plurality of modes of wireless operation and associated operating parameters. Such selection defines a communication channel to support wireless data, message and communication exchanges. In further embodiments, the wireless access devices also support a second channel, a busy/control channel, for managing communication on the main communication channel and to overcome roaming and hidden terminal problems. Roaming terminal devices are also configured to support the dual channel design. Such configuration in both circumstances may involve the use of a multimode radio that is timeshared between the two channels or two radios, one dedicated to each channel.

Routing and channel assignment for low power transmission in PCS

Abstract: There has been increased interest recently from military, civil, and commercial sectors in networks capable of self-organization. Routing and channel assignment for multi-hop communications in these networks are complex problems, given the interactions between the various transmissions, all of which must share some fixed bandwidth, and by the lack of a central controller. By enforcing a "reuse distance" similar to the frequency reuse factor in AMPS cellular service, we can route and assign channels to (place) arriving calls in a peer-to-peer network so as to significantly reduce power requirements and interference.

Hierarchical object-based stochastic modeling of fluvial reservoirs

Abstract: This paper describes a novel approach to modeling braided stream fluvial reservoirs. The approach is based on a hierarchical set of coordinate transformations involving relative straingraphic coordinates, translations, rotations, and straightening functions. The emphasis is placed on geologically sound geometric concepts and realistically-attainable conditioning statistics including areal and vertical facies proportions. Modeling proceeds in a hierarchical fashion, that is (1) a stratigraphic coordinate system is established for each reservoir layer, (2) a number of channel complexes are positioned within each layer, and then (3) channels are positioned within each channel complex. The geometric specification of each sand-filled channel within the background of floodplain shales is a marked point process. Each channel is marked with a starting location, size parameters, and sinuosity parameters. We present the hierarchy of eight coordinate transformations, introduce an analytical expression for the channel cross-section shape, describe the simulation algorithm, and demonstrate how the realizations are made to honor local conditioning data from wells and global conditioning data such as areal and vertical proportions.

Modelling and equalization of rapidly fading channels

Abstract: Basis expansion ideas are employed in order to equalize frequency-selective, rapidly fading channels. For certain multipath fading channels (e.g. the mobile radio channel), the time variations of the coefficients can be modelled as a combination of a small number of complex exponentials, under the assumption of linearly changing path delays. Based on this observation, novel adaptive and decision feedback algorithms are derived which exploit such an explicit modelling of the channel's variations. The problem of estimating the frequencies of the exponentials is also addressed using second- and higher-order cyclic statistics. By integrating ideas from cyclostationary signal analysis, both batch and recursive methods are developed. Finally, some illustrative simulations are presented.

Effects of antenna directivity and polarization on indoor multipath propagation characteristics at 60 GHz

Abstract: In millimeter-wave indoor communications systems, the radiation patterns and polarizations of the antennas at base stations and remote terminals have a significant influence on channel characteristics. The work reported in this paper investigated the effects of the radiation patterns of the antennas at remote terminals on multipath propagation characteristics. These effects were investigated by indoor propagation measurements at 60 GHz conducted in a modern office room and by ray-tracing simulations based on geometrical optics. Multipath channel characteristics are compared in terms of impulse responses and their root-mean-square (rms) delay spreads for an omnidirectional antenna and for three directive antennas with different beam widths. From the results of measurements and ray-tracing simulations, the use of a directive antenna at the remote terminal is demonstrated to be an effective method of reducing the effects of multipath propagation. Further reduction in the multipath effects is found to be achieved by the use of circular polarization instead of linear polarization with the directive antennas.

Interactive special events video signal navigation system

Abstract: A system interactively controlled by a TV viewer remote control transmitter displays current or future programming information superimposed over the normal programming displayed on any channel accessible to the viewer's display screen. Identification signal data are accessed and generated by the computer for durations defined in the identification signal data either automatically or in response to the TV viewer remote. Information signal data are accessed and generated by the computer in response to the TV viewer remote. A signal combiner superimposes the identification signal or the information signal over the viewer usable signal of any selected channel from the tuner. The identification signal may include data defining an icon or textual matter alerting a viewer to the availability of information data. The information signal may include program descriptive data, schedule data, pricing data and/or payment authorization and recording prompt data and the like sequenced for interactive control of the computer by the viewer. The identification signal may be specially dedicated to a normal programming information channel so that the viewer can interact with respect to specific programs identified without changing from the programming information channel. Thus, the viewer is able to receive an automatic alert to availability of current and future programming and to elect to interactively exercise a variety of options with respect to such current and future programming without interruption of presently viewed programming.

Decoder for compressed and multiplexed video and audio data

Abstract: An apparatus for filtering TS packets multiplexed with a plurality of programs and sending the filtered packets to decoders. A packet landing buffer is provided in a RAM used for a microprocessor for system control. After a channel is selected, the microprocessor filters video and audio data and performs a value added service process.

Logical and composite channel mapping in an mpeg network

Abstract: The Logical Channel Table (LCT), utilizing simple conventional channels or Composite Channels, provides mapping between a Logical Channel Number (LCN) representing a service and the transport stream (TSID) / program number (PN) on which the service can be found. The LCT entry contains a Composite Channel Indicator (CCI), which when set to '1', indicates that the selected channel is a composite channel thereby indicating the home channel of the Composite Channel. The LCT provides the Composite Channel Table (CCT) to the decoder (10) so that the tuner (12) can be returned to the actual program designated in the CCT for current viewing. Each entry in the CCT associates a Composite Channel Number (CCN) with a LCN and represents the 'present' definition for the composite channel. As time progresses, the CCN is used as a 'pointer' to the LCN which is the currently active service for the composite channel. These tables work with conventional MPEG-2 service definitions to decode multi-service transport streams.

Spatio-temporal coding for wireless communications

Abstract: A compact model is derived for the multiple-input multiple output (MIMO), dispersive, spatially selective wireless communication channel. The multivariate information capacity behavior is then analyzed for various input and output antenna configurations. For high SNR conditions, the MIMO channel can exhibit a capacity slope in bits per dB of power increase that is proportional to the minimum of the number multipath components, the number of input antennas, or the number of output antennas. The discrete matrix multitone (DMMT) is proposed as an asymptotically optimal, practical spatio-temporal coding structure. Experimental examples that support the theoretical results are presented.