Showing papers on "Communication channel published in 1995"

Information capacity and power control in single-cell multiuser communications

Abstract: We consider a power control scheme for maximizing the information capacity of the uplink in single-cell multiuser communications with frequency-flat fading, under the assumption that the users attenuations are measured perfectly. Its main characteristics are that only one user transmits over the entire bandwidth at any particular time instant and that the users are allocated more power when their channels are good, and less when they are bad. Moreover, these features are independent of the statistics of the fading. Numerical results are presented for the case of single-path Rayleigh fading. We show that an increase in capacity over a perfectly-power controlled (Gaussian) channel can be achieved, especially if the number of users is large. By examining the bit error-rate with antipodal signalling, we show the inherent diversity in multiuser communications over fading channels.

Finite-state Markov channel-a useful model for radio communication channels

Abstract: The authors first study the behavior of a finite-state channel where a binary symmetric channel is associated with each state and Markov transitions between states are assumed. Such a channel is referred to as a finite-state Markov channel (FSMC). By partitioning the range of the received signal-to-noise ratio into a finite number of intervals, FSMC models can be constructed for Rayleigh fading channels. A theoretical approach is conducted to show the usefulness of FSMCs compared to that of two-state Gilbert-Elliott channels. The crossover probabilities of the binary symmetric channels associated with its states are calculated. The authors use the second-order statistics of the received SNR to approximate the Markov transition probabilities. The validity and accuracy of the model are confirmed by the state equilibrium equations and computer simulation. >

On channel estimation in OFDM systems

Abstract: The use of multi-amplitude signaling schemes in wireless OFDM systems requires the tracking of the fading radio channel. The paper addresses channel estimation based on time-domain channel statistics. Using a general model for a slowly fading channel, the authors present the MMSE and LS estimators and a method for modifications compromising between complexity and performance. The symbol error rate for a 18-QAM system is presented by means of simulation results. Depending upon estimator complexity, up to 4 dB in SNR can be gained over the LS estimator.

Subspace methods for the blind identification of multichannel FIR filters

Abstract: This paper addresses a problem arising in a context of digital communications. A digital source is transmitted through a continuous channel (the propagation medium), and several measurements are performed at the receiver, either by means of several sensors, or by oversampling the received signal compared to the emission rate. Given only these observations, the baseband equivalents of the corresponding channels have to be recovered. An orthogonality property between "signal" and "noise" subspaces is exploited to build some quadratic form whose minimization yields the desired estimates up to a scale factor. This is in the same spirit as recent works by Tong et al. (see Proc. 25th Asilomar Conf., p.856-860, 1991) but requires fewer computations. Numerical simulations demonstrate the performance of the proposed methods in a channel identification context. >

A least-squares approach to blind channel identification

Abstract: Conventional blind channel identification algorithms are based on channel outputs and knowledge of the probabilistic model of channel input. In some practical applications, however, the input statistical model may not be known, or there may not be sufficient data to obtain accurate enough estimates of certain statistics. In this paper, we consider the system input to be an unknown deterministic signal and study the problem of blind identification of multichannel FIR systems without requiring the knowledge of the input statistical model. A new blind identification algorithm based solely on the system outputs is proposed. Necessary and sufficient identifiability conditions in terms of the multichannel systems and the deterministic input signal are also presented.

Wireless on-premises video distribution using digital multiplexing

Abstract: A digital network delivers multiplexed channels to a customer premises. Each multiplexed channel contains a digitally multiplexed data stream including digitized broadband information relating to a number of programs. At the customer premise, a shared processing system includes several channel selectors and program selectors. Each channel selector selects one of the multiplexed channels, and each program selector selects digitized broadband information relating to a selected program from a selected channel. A multiplexer combines the selected digitized broadband information from the program selectors into a transport stream. A transmitter system, for example comprising a digital modulator, a spread spectrum modulator and a broadcast antenna, provides a wireless broadcast of the digital transport stream throughout the customer premise and possibly one or more near by premises. Terminal devices within range of the broadcast receive the wireless broadcast and process selected digitized broadband information from the transport stream to present information relating to a selected program, e.g. on an associated television set.

Floor acquisition multiple access (FAMA) for packet-radio networks

Abstract: A family of medium access control protocols for single-channel packet radio networks is specified and analyzed. These protocols are based on a new channel access discipline called floor acquisition multiple access (FAMA), which consists of both carrier sensing and a collision-avoidance dialogue between a source and the intended receiver of a packet. Control of the channel (the floor) is assigned to at most one station in the network at any given time, and this station is guaranteed to be able to transmit one or more data packets to different destinations with no collision with transmissions from other stations. The minimum length needed in control packets to acquire the floor is specified as a function of the channel propagation time. The medium access collision avoidance (MACA) protocol proposed by Karn and variants of CSMA based on collision avoidance are shown to be variants of FAMA protocols when control packets last long enough compared to the channel propagation delay. The throughput of FAMA protocols is analyzed and compared with the throughput of non-persistent CSMA. This analysis shows that using carrier sensing as an integral part of the floor acquisition strategy provides the benefits of MACA in the presence of hidden terminals, and can provide a throughput comparable to, or better than, that of non-persistent CSMA when no hidden terminals exist.

Simulcasting digital video programs for broadcast and interactive services

Abstract: To provide interactivity, a public wireless packet data network is combined with a broadband digital broadcast network. In the preferred embodiment, the broadcast network utilizes multiple transmitters at separately located sites simultaneously broadcasting the same multi-channel, multi-program signal. Broadcast waves from the transmitters propagate throughout substantially overlapping portions of the service area. Customer premises receiving systems include a receiving antenna and one or more digital entertainment terminals. The terminal includes a channel selector and digital receiver for capturing a digital transport stream from a selected channel. A processor converts selected program information from the transport stream for presentation, e.g. via a television set. The terminal also includes a CPU controlling the operation of the channel selector and the processor in response to user inputs. The CPU also communicates signaling information for interactive services via an RF packet data modem included in the terminal and the public wireless packet data network. In addition to the signaling for interactive services, the packet data network provides transport for data communications between other data devices.

Program time guide

Abstract: A program time guide for an interactive viewing system allows a user to control the time and channels for which program information is displayed. The user can navigate through the program time guide to identify and select desired programs. The program time guide displays information for various types and sources of programming by assigning each program source a channel number. Selecting a program on a particular channel causes the system to tune to an associated frequency to receive video signals, launch a computer executed application which generates displayed information, or to perform some other activity associated with that channel or program.

Source-controlled channel decoding

Abstract: Source and channel coding have been treated separately in most cases. It can be observed that most source coding algorithms for voice, audio and images still have correlation in certain bits. Transmission errors in these bits usually account for the significant errors in the reconstructed source signal. This paper proposes a modification of the Viterbi decoding algorithm (VA) for binary trellises which uses a priori or a posteriori information about the source bit probability for better decoding in addition to soft inputs and channel state information. Analytical upper bounds for the BER of convolutional codes for this modified VA (APRI-VA) are given. The algorithm is combined with the soft output viterbi algorithm (SOVA) and an estimator for the residual correlation of the source bits to achieve source-controlled channel decoding for framed source bits. The description is simplified by an algebra for the log-likelihood ratio L(u)=log(P(u=+1)/P(u=-1)) which allows a clear definition of the "soft" values of source-, channel-, and decoded bits as well as a simplified description of the traceback version of the SOVA. Applications are given for PCM transmission and the full rate GSM speech codec. For an PCM coded oversampled bandlimited Gaussian source transmitted over Gaussian and Rayleigh channels with convolutional codes the decoding errors are reduced by a factor of 4 to 5 when the APRI-SOVA is used instead of the VA. A simple dynamic Markov correlation estimator is used. With these receiver-only modifications the channel SNR in a bad mobile environment can be lowered by 2 to 4 dB resulting in the same voice quality. Further applications are briefly discussed. >

Coherent DS-CDMA performance in Nakagami multipath fading

Abstract: The coherent reception of direct sequence-code division multiple access (DS-CDMA) signals in a multipath fading channel is considered. The channel model assumes independent paths with Nakagami fading statistics; this model includes the Rayleigh channel os a special case and is a reasonable model for a frequency-selective fading channel. The bit error rate (BER) performance of a RAKE receiver under various multipath fading conditions is derived and evaluated. The results indicate that over 50 users may access the channel (assuming 127 chips per bit) simultaneously under typical fading conditions, and can achieve an uncoded BER of 0.01 with an average received signal-to-noise ratio per bit of 10 dB. >

Multimedia channel formulation mechanism

Abstract: A communication network and process for communicating thereon is disclosed which can support multimedia communications. Communication channels are formulated using a two step process. In a first step, channel types and fixed attributes thereof are defined. When needed, one or more channels of the predefined types are subsequently allocated in a second step wherein user-definable parameters are specified. The user-definable parameters and fixed attributes of each allocated channel control the scheduling of transmission and receipt of information on each channel.

Apparatus and method for digital data transmission using orthogonal codes

Abstract: A method and apparatus for carrying out synchronous co-division multiple access (SCDMA) communication of multiple channels of digital data over a shared transmission media such as a cable television system coaxial cable, a fiber optic or copper conductor telephone link, terrestial microwave, satellite link, local or wide area network, wireless including cellur network or some combination of these media using suitable interface circuitry. The system includes modems at remote units and a central unit to receive time division multiplexed digital data arranged into timeslots or channels and uses orthogonal codes to encode each channel of multiple data and spread the energy of each channel data over a frame of data transmitted in the code domain. Spreading the data this way makes the system less susceptible to impulse noise. Frames are synchronized as between remote and central units using a ranging scheme which is also useful in any other system transmitting data by frames in a distributed system where synchronizing the frames as between all units regardless of differences in propagation delays is necessary. Each frame in the SCDMA modulation scheme includes a gap or guardband containing no other data. The ranging process involves training each remote unit to impose enough delay prior to re-transmission of a barker code received from the CU such that a barker code sent by the RU arrives at the CU during the gap. The process of setting the delay in each RU is a trial and error process, and each RU starts the ranging process asynchronously. Contention resolution protocols such that only one RU is aligning to the gap at any particular time are taught.

Experimental characterization of non-directed indoor infrared channels

Abstract: We have experimentally characterized nondirected indoor channels that use intensity modulation and direct detection of an infrared carrier at a wavelength of 832 nm. At several locations in each of five different rooms, we have studied line-of-sight and diffuse link configurations, with and without shadowing, amounting to a total of approximately 100 different channels. We have measured channel frequency responses over the 2-300 MHz range by using a swept-modulation frequency technique, and from these data, we have computed channel impulse responses, path losses and r.m.s. delay spreads. Using channel impulse responses, we have calculated power penalties induced by multipath intersymbol interference in baseband on-off-keyed links operating at bit rates of 10, 30 and 100 Mb/s, considering unequalized links and those employing zero-forcing decision-feedback equalization. Unshadowed line-of-sight configurations generally have smaller path losses, r.m.s. delay spreads and power penalties than their unshadowed diffuse counterparts. Shadowed line-of-sight configurations, however, generally exhibit larger values of all three parameters than the corresponding shadowed diffuse configurations. We show that among the channels measured here, there is a strong correspondence between channel r.m.s. delay spread and multipath power penalty. Finally, we provide an analysis indicating why non-directed infrared channels using intensity modulation and direct detection do not exhibit multipath fading, and justifying their representation as linear, time-invariant systems. >

A high-data-rate wireless local-area network

Abstract: An apparatus and method for communicating data between at least two data devices, suitable for use as a wireless local-area network, that provides robust data communication via a radio communications channel corrupted by multipath interference, particularly at high data rates. A preferred embodiment of the invention represents data as a sequence of Walsh-function waveforms encoded by pseudo-noise direct-sequence spread-spectrum modulation. Walsh-function-encoding of the data provides a long symbol duration, thereby allowing the spread-spectrum modulation to provide processing gain sufficient to substantially overcome multipath interference, while providing a high data rate. In another preferred embodiment, Walsh-function modulation is supplemented with various forms of phase modulation, such as coherent PSK for bi-orthogonal signalling, and DPSK between orthogonal symbols for noncoherent bi-orthogonal signalling, thereby further increasing data rate without reducing processing gain. In another preferred embodiment, Walsh-function modulation is supplemented with spectral shaping to allow increased bandwidth occupancy, thereby further increasing the processing gain without sacrificing data rate.

Shared use video processing systems for distributing program signals from multiplexed digitized information signals

Abstract: A broadcast system supplies multiplexed channels to a plurality of receiving systems. The broadcast system preferably comprises a wireless digital simulcasting network. Each multiplexed channel carries multiplexed digital information signals relating to a group of programs. At least one of the receiving systems serves a plurality of living units. The multi-unit receiving system includes common equipment to perform as much of the processing of the digital information signals as is practical for the particular installation. The common equipment includes a digital receiver processing each multiplexed channel to capture a digital transport stream therefrom and a demultiplexer for separating out the digital information signals for each program from each digital transport stream. A switch routes selected information signals to individual drops or loops going to the living units. In one implementation, each digital information signal is modulated, and selected modulated signals are routed and transported over telephone wiring to terminals in the living units. The terminals demodulate and decode selected program signals for presentation on a television. In a second implementation, a decoder in the common equipment decodes each digital information signal, and the common equipment transmits an analog television channel carrying decoded signals for selected programs over coaxial cables to the living units. In this implementation, the added equipment required in a living unit includes only a remote control, an infrared receiver and a data device to transmit program request messages over coaxial cable to the common equipment.

Wavelet preprocessing for automated neural network detection of EEG spikes

Abstract: The purpose of this study was to investigate the feasibility of using a wavelet transforms (WT) as a preprocessor for an artificial neural network (ANN) based EEG spike detection system. The study aimed at decreasing the input size to the ANN detector, without decreasing the information content of the signal and degrading the detection performance. Since routine clinical EEG requires recordings from many channels (generally 32 or 64), input size becomes a critical design parameter for real-time multichannel spike detection systems. For a sliding window of 20 points, more than 600 input lines will be necessary for a 32-channel system, which is not easily manageable with current ANN technology. One approach to this problem is to use a single ANN module for each EEG channel and integrate the outputs across channel information with a second module. The authors have successfully developed a 16 channel prototype of such a system working in real-time. This system used a 20 point (100 ms) sliding time window and employed a floating point digital signal processor for real-time operation. Adding more channels to this system would be difficult for real-time operation. In addition, the authors' recent studies showed that the best detection performance is attained with a 30 point (150 ms) time window, further increasing the computational load. Thus, the development of a preprocessor to reduce the input size without significantly reducing the information content would be very helpful in developing large multichannel EEG spike detection systems. >

Coding for interactive communication

Abstract: Let the input to a computation problem be split between two processors connected by a communication link; and let an interactive protocol /spl pi/ be known by which, on any input, the processors can solve the problem using no more than T transmissions of bits between them, provided the channel is noiseless in each direction. We study the following question: if in fact the channel is noisy, what is the effect upon the number of transmissions needed in order to solve the computation problem reliably? Technologically this concern is motivated by the increasing importance of communication as a resource in computing, and by the tradeoff in communications equipment between bandwidth, reliability, and expense. We treat a model with random channel noise. We describe a deterministic method for simulating noiseless-channel protocols on noisy channels, with only a constant slowdown. This is an analog for general, interactive protocols of Shannon's coding theorem, which deals only with data transmission, i.e., one-way protocols. We cannot use Shannon's block coding method because the bits exchanged in the protocol are determined only one at a time, dynamically, in the course of the interaction. Instead, we describe a simulation protocol using a new kind of code, explicit tree codes.

Multitone spread spectrum multiple access communications system in a multipath Rician fading channel

Abstract: The paper proposes a new multiple access communications system based on the combination of multitone transmission and direct sequence spectrum spreading (DS/SS). Multitone transmission is proposed because the associated larger symbol duration is favorable to combat the effects of multipath propagation. The multiple access capability of the system is achieved by means of DS/SS which also helps to combat the presence of several paths. On the other hand, DS/SS is also attractive to decrease the multiple access interference. The performance of this system is investigated for a multipath, slowly Rician fading and frequency selective channel, and a suboptimal receiver based on matched filters. The multiple access interference is also taken into account, A model for the computation of the bit error probability is derived for QPSK modulation of the orthogonal tones by the spread symbols. The system performance is investigated for selection diversity and maximal ratio combining (RAKE reception). Then the influence of the number of tones is investigated for given bandwidth, bit rate and transmission power. The positive effect of a multitone spread spectrum transmission is demonstrated. >

Blind channel identification based on second-order statistics: a frequency-domain approach

Abstract: In this communication, necessary and sufficient conditions are presented for the unique blind identification of possibly nonminimum phase channels driven by cyclostationary processes. Using a frequencydomain formulation, it is first shown that a channel can be identified by the second-order statistics of the observation if and only if the channel transfer function does not have special uniformly spaced zeros. This condition leads to several necessary and sufficient conditions on the observation spectra and the channel impulse response. Based on the frequency-domain formulation, a new identification algorithm is proposed

Digital entertainment terminal with channel mapping

Abstract: Dynamic programming of a digital entertainment terminal (DET) facilitates operation of the terminal to offer a variety of functionally different broadband services. The terminal includes a network interface module which couples the terminal to a specific type of communication network for receiving a digital broadband channel. The DET downloads on a narrowband signaling channel a VIP program map that identifies the available video information service providers (VIPs) on the basis of the location of their corresponding software control signals. The software control signals are transmitted cyclically to enable access by a random DET at any time. A program memory captures the VIP program map and at least a portion of the software control signals received over the digital broadband channel as software executable by the control processor during turn-on of the DET. When a user presses a GUIDE button on a remote control service, the DET displays the available information service providers in accordance with the VIP program map, thereby enabling the user to conveniently access the broadband services offered by the selected information service provider.

Blind channel identification based on second-order statistics: a frequency-domain approach

Abstract: In this communication, necessary and sufficient conditions are presented for the unique blind identification of possibly nonminimum phase channels driven by cyclostationary processes. Using a frequency domain formulation, it is first shown that a channel can be identified by the second-order statistics of the observation if and only if the channel transfer function does not have special uniformly spaced zeros. This condition leads to several necessary and sufficient conditions on the observation spectra and the channel impulse response. Based on the frequency-domain formulation, a new identification algorithm is proposed. >

Advanced television systems for terrestrial broadcasting: Some problems and some proposed solutions

Abstract: The first part of this paper discusses the requirements that must be met by a new television broadcasting system to maximize its acceptability to the various stakeholders, including broadcasters, equipment manufacturers, program producers, regulatory authorities, and viewers. The most important performance factors are efficient use of over-the-air spectrum, coverage versus quality, cost, interoperability, and the existence of a practical transition scenario. It is concluded that all receivers need not have the same peformance, and that low-cost receivers must be available for noncritical locations in the home. If this variation in price and performance is made possible by appropriate system design, then interoperability is facilitated and nondisruptive improvement over time is made possible, as desired by the Federal Communications Commission. In the second part of the paper, techniques that may permit meeting these requirements are discussed. These include joint multiresolution source and channel coding, multicarrier modulation, and hybrid analog/digital coding and transmission. The analog transform coefficients are subjected to spread-spectrum processing, and coded orthogonal frequency-division multiplex (COFDM) is applied to the complex hybrid symbols to be transmitted through the channel. Various methods of equalization and of improving noise, interference, and multipath rejection are compared. Finally, an example is given of a system that meets the various requirements by making use of a number of the techniques discussed. The system provides extended coverage, albeit at lower quality than currently proposed all-digital systems, and equal or higher quality than such systems in much of their service area. It also features self-optimization at each receiver depending on signal quality and receiver characteristics, and facilitates the design of receivers of lower cost and performance for less-critical applications. >

Combining queueing theory with information theory for multiaccess

Abstract: We develop and analyze a multiaccess communication model over the additive Gaussian noise channel. The framework is information-theoretic; nonetheless it also incorporates some queueing-theoretic aspects of the problem. >

EVAM: an eigenvector-based algorithm for multichannel blind deconvolution of input colored signals

Abstract: A new algorithm is proposed for the deconvolution of an unknown, possibly colored, Gaussian or nonstationary signal that is observed through two or more unknown channels described by rational system transfer functions. More specifically, not only the root (pole and zero) locations but also the orders of the channel transfer functions are unknown. It is assumed that the channel orders may be overestimated. The proposed algorithm estimates the orders and root locations of the channel transfer functions, therefore it can also be used in multichannel system identification problems. The input signal is allowed to be nonstationary and the channel transfer functions may be a nonminimum phase as well as noncausal, hence the proposed algorithm is particularly suitable for applications such as dereverberation of speech signals recorded through multiple microphones. Several experimental results indicate improvement compared to the existing methods in the literature. >

Virtual channels in subscriber interface units

Abstract: A subscriber interface unit for use in conjunction with a television and a remote cable network headend having a plurality of available programs. The available programs include broadcast television channels and executable applications. The executable applications are intended to be received and executed at the subscriber interface unit. The subscriber interface unit maintains a virtual channel table having entries for a plurality of virtual channel numbers. An entry for a particular virtual channel number includes a designation of an available program from the headend which is to be associated with the virtual channel number. The program can be a broadcast channel or an executable application such as an electronic programming guide. The subscriber interface unit has a channel selector which maintains a current virtual channel number and which is responsive to commands by a human viewer to change the current virtual channel number. When a virtual channel is selected, the subscriber interface unit requests associated program from the remote headend and presents it on the video display device. If the selected virtual channel is associated with an executable application, the subscriber interface unit downloads and executes the application.

WDM systems with unequally spaced channels

Abstract: Crosstalk due to four-wave mixing (FWM) is the dominant nonlinear effect in long-haul multichannel optical communication systems employing dispersion-shifted fiber. A method is discussed to find non-uniform channel separations for which no four-wave mixing product is superimposed on any of the transmitted channels, therefore suppressing FWM crosstalk. The residual crosstalk, due to channel power depletion only, is analytically evaluated for intensity-modulated repeaterless wavelength-division-multiplexed (WDM) systems and compared to experimental results. The theory includes the effect of the channel depletion on the amplitude of each phase-matched FWM wave. The probability of error is evaluated including the statistics of the pattern dependent channel depletion. The BER curve computed for an 8-channel WDM system is found to be in good agreement with experimental results. In the experiment, repeaterless transmission of eight 10 Gb/s WDM channels over 137 km (11 Tb/s-km) of dispersion-shifted fiber was demonstrated and error-free operation was achieved over a wide range of input powers using unequally spaced channels. The same system with equally spaced channels could not achieve a probability of error lower than 10/sup -6/. The use of unequal channel spacing allowed fiber input power to be increased by as much as 7 dB, which could be translated into a fivefold increase of the bit rate per channel (and therefore of the system capacity), or to an increase in the system length of about 30 km. >

Handover and dynamic channel allocation techniques in mobile cellular networks

Abstract: This paper deals with an efficient dynamic channel allocation (DCA) technique applicable to terrestrial mobile cellular networks. A channel (or resource) is a fixed frequency bandwidth (FDMA), a specific time-slot within a frame (TDMA), or a particular code (CDMA), depending on the multiple access technique used. A cost function has been defined by which the optimum channel to be assigned on demand can be selected. In addition, a suitable mobility model has been derived to determine the effects of handovers on network performance. The performance of the proposed DCA technique has been derived by computer simulations in terms of call blocking and handover failure probabilities. Comparisons with the classical fixed channel allocation (FCA) technique and other dynamic allocation algorithms recently proposed in the literature have been carried out to validate the proposed technique. >