IEEE Transactions on Communications

Published by Institute of Electrical and Electronics Engineers
Online ISSN: 0090-6778
Publications
Article
We derive upper and lower bounds to the curve of expected delay versus throughput for the delay-enhanced version of the celebrated 0.487 multiple access contention resolution algorithm. The key to the derivation is a technique for generating upper and lower bounds to monotonic functions of the transmission lag via corresponding bounds on the kernel of the integral equation for the distribution of said lag. The separation between the upper and lower bounds is small, and the results agree well with simulations.
 
Article
The performance of binary partial response continuous phase modulation (with index 0.5) using a differentially coherent receiver depends on the choice of the receiver filter. An optimum MMSE design method for this filter is presented. The receiver filter is equivalent to the cascade of a matched filter and an equalizer in order to reduce inherent intersymbol interference (ISI). It is shown that performance degradation with respect to that of the differential binary phase shift keying (BPSK) system is due to inherent ISI contained in the signal and also to noise enhancement and correlation caused by the receiver filter. The bit error probability on the Gaussian channel is calculated by assuming that ISI is Gaussian. The Gaussian minimum shift keying (MSK) signal is used for illustration
 
Article
Frequency-shift-keying (FSK) signals with modulation index m = 0.5 have two significant properties. They have no discrete frequency components and nearly all the signal energy is contained within a narrow frequency region equal to frac{3}{2} of the bit speed even without any band limiting. Unfortunately, a complete mathematical description of FSK signals in general is difficult, because FSK is a nonlinear process. A well-known exception is FSK with m = 1 , because this signal can be described as the summation of two AM signals. This makes it easy to describe frequency- and time-domain properties. In this paper it is shown that by decomposing FSK signals with m = 0.5 into two signals with well-understood spectral properties, it is possible to describe time- and frequency-domain properties and to calculate the effects of restricted bandwidth and linear distortion on the signal-space diagrams and eye patterns. Finally it is shown that duobinary FM can be treated as a special case of an FSK ( m = 0.5 ) signal.
 
Article
In the above titled paper (ibid., vol. 55, no. 5, pp. 941-952, May 07), a change to equation (7), that was requested by the author, was omitted in the final version. The paragraph leading up to, and including the equation, is repeated here.
 
Article
In the above titled paper (ibid., vol. 55, no. 10, pp. 1837-1844, Oct. 09), there was a typographical error on page 1839. The incorrect text and the corrected text appear here.
 
Article
A modification to the access policy for ETHERNET<sup>TM</sup>Version 1.0 is proposed. By placing additional restrictions (compared with ETHERNET) on each station that is attempting to transmit, this access policy makes it more likely a successful message transmission will occur, and in less time, than the published Version 1.0. Theoretical analysis substantiates these claims.
 
Article
A bidirectional system using 1.3 μm lasers as a transmitter and a receiver are investigated. The lasers considered are commercially available Fabry-Perot and distributed-feedback lasers. It is found that the poor performance of a laser receiver, compared to a PIN receiver, stems from three factors: low responsivity, high capacitance, and polarization dependence of responsivity, high capacitance, and polarization dependence of responsivity. The effect of each factor is evaluated. It has been clarified that the polarization dependence of sensitivity originates from the TE and TM modes dependence of both mode confinement factor and reflection at a facet. Using the measured parameters, 1.3-μm laser receiver sensitivity for a 10<sup>-9</sup> error rate is estimated. Burst data transmission experiments show that estimated sensitivities agree well with the measured values. Optical ping-pong transmission distance of narrowband services is determined based on estimated sensitivity
 
Article
Interframe coder simulation results are described for standard-broadcast-rate monochrome television in applications where the television camera is largely stationary, such as TV-conferencing or video telephone. An algorithm for coding at 1.5 Mbits/s, whose parameters were optimized via trial and error, produces pictures with little resolution loss for low-to-moderate movement, but somewhat visible moving-area resolution loss for higher movements.
 
Article
This paper describes the interframe coding system (TRIDEC 1.5) used for NTSC color television signal transmission at a 1.5 Mbit/s rate. TRIDEC 1.5 is developed with the intention of making video teleconferencing service more economical. The system utilizes the following highly efficient coding techniques: 1) interframe combinational difference coding 2) temporal subsample coding 3) noise reduction 4) demand refreshing to protect against channel errors, which makes periodic forced refreshing unnecessary. In addition, new coding parameter control methods have been devised to prevent buffer memory overflow. The subjective evaluation test results make it clear that the encoded picture quality is good enough for teleconferencing use.
 
Article
Methods are presented for the transmission of wideband digital signals above and below the frequency division multiplexing (FDM) baseband spectrum of analog microwave systems. In particular, it is shown that a Data Above Voice (DAV) system employing a 1.544 Mbit/s data channel above FDM channels and a Data Under Voice (DUV) system employing 1.544 Mbit/s channel below 720 FDM channels, are practical hybrid systems. Theoretical models of hybrid transmission are disucssed and the DAV/DUV methods are compared. A conceptual hybrid system design procedure is introduced and field measurement results on an 8 hop 6 GHz system are presented, comparing favorably with theoretical predictions.
 
Article
As a powerful tool for economizing on the digital transmission of videotelephone signals, interframe coding techniques have drawn increasing attention, and various interframe coding schemes have been proposed recently. However, many problems remain to be studied in establishing technical feasibility for these techniques and schemes. This concise paper reports on field trial in which a 1.544-Mbits/s digital transmission of videotelephone signals was conducted over Nippon Telegraph and Telephone Public Corporation's (NTT's) PCM100M digital repeatered line between Tokyo and Yokohama, Japan. The experimental interframe codec is based on the well-understood conditional picture element replenishment algorithm. The object of the field trial was to demonstrate, not the validity of the algorithm itself, but the technical feasibility of the interframe coding system as an entity. Experimental results are satisfactory enough to assure us of the system's practicality in the near future.
 
Article
A theoretical analysis is presented for a multi-hop relay system composed of QPSK-DPR's. A Direct Phase Regenerator (DPR) can regenerate a QPSK signal at carrier frequency. Special attention is given to jitter generation and accumulation. Circuit parameters characterizing jitter generation are clarified and obtained quantitatively. The required upper limit for jitter per DPR is also obtained from alloted C/N degradation in a multi-hop system. Simulation experiments were carried out using a novel 400 Mbit/s QPSK-DPR with less than 1 dB inherent equivalent C/N degradation. The results Show that a hybrid system, consisting of 4 to 9 DPR's and a base band regenerator, is feasible.
 
Article
We have transferred the principle of the Costas-type nonlinear phase-locked loop to the lambda = 10.6 mu m wavelength of CO 2 lasers. The ability of the optical Costas loop to regenerate the carrier of a binary phase-shift-keyed input signal and to perform coherent demodulation was demonstrated at a data rate of 20 Mbits/s.
 
Article
The technology advances made in the development of an engineering feasibility model of a Nd:YAG laser communication system for space test are discussed and details of evaulations presented. The model includes the baseplate, optomechanical structure, 19-cm (7.5-in) diameter telescope and prototype space qualifiable components. The main technological achievements are in the areas of laser and its operation, the modulator and the digital format, and the integration of the various optical components into one optomechanical structure. These as well as other technology advances will be discussed and the system performance capabilities indicated.
 
Article
In this paper, two algebraic decoders for the (103, 52, 19) and (113, 57, 15) quadratic residue codes, which have lengths greater than 100, are presented. The results have been verified by software simulation that programs in C++ language have been executed to check possible error patterns of both quadratic residue codes.
 
Article
The presence of both multiple-access interference (MAI) and intersymbol interference (ISI) constitutes a major impediment to reliable communications in multipath code-division multiple-access (CDMA) channels. In this paper, an iterative receiver structure is proposed for decoding multiuser information data in a convolutionally coded asynchronous multipath DS-CDMA system. The receiver performs two successive soft-output decisions, achieved by a soft-input soft-output (SISO) multiuser detector and a bank of single-user SISO channel decoders, through an iterative process. At each iteration, extrinsic information is extracted from detection and decoding stages and is then used as a priori information in the next iteration, just as in turbo decoding. Given the multipath CDMA channel model, a direct implementation of a sliding-window SISO multiuser detector has a prohibitive computational complexity. A low-complexity SISO multiuser detector is developed based on a novel nonlinear interference suppression technique, which makes use of both soft interference cancellation and instantaneous linear minimum mean-square error filtering. The properties of such a nonlinear interference suppressor are examined, and an efficient recursive implementation is derived. Simulation results demonstrate that the proposed low complexity iterative receiver structure for interference suppression and decoding offers significant performance gain over the traditional noniterative receiver structure. Moreover, at high signal-to-noise ratio, the detrimental effects of MAI and ISI in the channel can almost be completely overcome by iterative processing, and single-user performance can be approached
 
Article
The ever increasing demand for multimedia services has led to the extensive use of fixed wireless outdoor networks, where the transmitted information signals arrive to the final destination through a relay node, creating a dual-hop transmission. Due to the increased demand for bandwidth and the spectral congestion at low frequencies, operation at frequencies above 10GHz satisfying the Line-Of-Sight condition is required. At this frequency range, the dominant fading mechanism exposing spatial and temporal stochastic properties is rain attenuation, typically modeled as a lognormal flat fading channel when expressed in dB (log-lognormal in linear scale). In this paper, the outage performance of a dual hop system using several radio relay types is presented and physical prediction models with analytical formulas are proposed, employing spatially correlated lognormal fading channels. Extended numerical results investigate the impact of various operational, geometrical and geographical radio relay parameters, as well as the impact of the employment of a relay channel estimator, on the end-to-end performance.
 
Article
Recently, a new algebraic decoding algorithm for quadratic residue (QR) codes was proposed by Truong et al. Using that decoding scheme, we now develop three decoders for the QR codes with parameters (71, 36, 11), (79, 40, 15), and (97, 49, 15), which have not been decoded before. To confirm our results, an exhaustive computer simulation has been executed successfully.
 
Article
We discuss the system engineering features of an 11 GHz 140 Mbit/s system primarily in terms of the requirements to provide an overlay on the existing British Post Office microwave radio network. Significant influences on the system characteristics adopted are the relatively dense nature of the network, with a multiplicity of traffic nodes, and the digital transmission performance objectives. A description is given of the system, including some details of particular units and sub-systems, and results of measurements on a simulated hop. We describe a prototype system, whose installation is imminent, and give an account of the attributes to be evaluated. This prototype is a precursor to fully operational systems to be placed into service by early 1982.
 
Article
An investigation is conducted of the high-rate punctured convolutional codes suitable for Viterbi and sequential decoding. Results on known short-memory codes M &les;8 discovered by others are extended. Weight spectra and upper bounds on the bit error probability of the best known punctured codes having memory 2&les; M &les;8, and coding rates 2/3&les; R &les;7/8 are provided. Newly discovered rate-2/3 and -3/4 long-memory punctured convolutional codes with 9&les; M &les;23 are provided together with the leading terms of their weight spectra and their bit error performance bounds. Some results of simulation with sequential decoding are given
 
Article
An analytical study to predict multipath fading outages in terrestrial digital radio systems is described. The method, which is quite general, makes specific use of previously reported statistics on multipath fading. It is applied, in this study, to 6- and 11-GHz systems carrying 90 Mbits/s per RF channel. For purposes of comparing alternate approaches, three different modulations are considered, namely, (i) conventional QPSK at 45 Mbits/s, using two polarizations per channel; (ii) Offset-QPSK at 45 Mbits/s using two polarizations per channel; and (iii) 8-level PSK at 90 Mbits/s, using one polarization per channel. In all cases, a fourthorder Butterworth transmit filter is assumed which enforces FCC spectral emission requirements. The channel is treated as a two-path propagation medium, the receiver is assumed to be coherent, and various practical assumptions are invoked regarding the recovered carrier phase, timing phase, receive filter response, bit error rate objectives and fade margin. Results are given in terms of expected yearly fraction of multipath outage per repeater hop vs. hop length. We find that the choice of operating band has only a minor effect on the results; that OffsetQPSK systems suffer somewhat more multipath outage than conventional QPSK systems; that 8-level systems suffer more multipath outage than 4-level systems (although this difference may be offset by cross-polarization effects in the latter); and that all the approaches considered require repeater spacings of 15 mi or less in order to meet short-haul outage objectives over average terrain.
 
Article
Measurements of linear cross polarization and attenuation on a terrestrial link in the U.K. are reported and correlated with rainfall information. The cross polarization measured during rainfall is compared with theoretically predicted values and monthly cross-polar statistics are presented for a 16-month period.
 
DIMENSIONS OF THE ROOMS USED FOR MEASUREMENTS
Article
Results from propagation measurements, conducted in an indoor office environment at 2.4, 4.75, and 11.5 GHz, are presented. The data were obtained in small clusters of six measurements, using a coherent wideband measurement system. The channel characteristics for the three frequencies are compared by evaluating path loss, rms delay spread, and coherence bandwidth. An analytical model for evaluation of the bit-error rate (BER) of the stationary frequency selective indoor channel is developed for a coherent binary phase shift keying (BPSK) receiver, based on the complex impulse response of the channel. Computational BER results are obtained for data rates up to 50 Mb/s, using the measured multipath channel impulse responses. The BER results for a number of clusters are presented and compared for the maximum reliable data rate as inferred by the measured rms delay spread of the channel
 
Article
Depolarization caused by rain at 11.7 GHz is due to both differential phase shift and attenuation. In dual polarized communications systems it gives rise to interference which may be severe enough to cause an outage even though the allotted fade margin has not been exceeded. Described is a depolarization compensation experiment over a 10 km path with near circular polarization at 11.7 GHz. It made use of a turnstile junction feed system under computer control. Data presented demonstrate the stable performance of the method used during several rain events. At fades below about 15 dB significant isolation improvements were achieved, compared to a fixed cross-polarization channel. Finally, a method using the turnstile is suggested to receive two nonorthogonal signals without interference.
 
Article
A 120-channel transmultiplexer has been developed which interfaces two FDM basic supergroup signals (2 x 60 channels) directly to the digital time division switch or digital transmission systems. The equiplnent exploits a block processing digital SSB-FDM multiplex/demultiplex scheme, and has been built using newly developed high-speed CMOS pipeline multipliers and variable length shift registers. System design considerations and hardware implementation techniques are described in detail as well as the measured performance.
 
Article
Two new digital transmultiplexers intended for commercial use have been developed. One transmultiplexer performs a bilateral conversion between two 12-channel FDM group signals and a 24-channel PCM carrier signal. The other mutually connects two 60Channel FDM supergroup signals and five 24-channel or four 30channel PCM signals. Both exploit a block processing digital SSBFDM multiplex/demultiplex scheme employing a cascade of an FFT processor and a set of complex coefficient digital filters. They have been built using newly developed high-level DSP LSI chips. Algorithmic considerations, developed LSI architecture, and equipment configuration are described as well as digital processor design details and measured performance.
 
Lumped OA-link model (Tx to Rx)  
Distributed γ-link model
Article
This paper develops a Nash game formulation for optical signal-to-noise ratio (OSNR) in distributed optical links. The starting point is a recent network OSNR model developed for optically amplified links whereby channel powers are adjusted independently only at transmitter sites. A more general case is considered here where channel powers are also adjustable at intermediary dynamic sites, specific to optical networks. For this inherent distributed configuration a nested Nash game is formulated towards minimizing channel OSNR degradation along the link. Existence and uniqueness of the Nash equilibrium solution is shown and a recursive procedure for constructing it is given. Based on this, an iterative algorithm that is distributed with respect to both channels and -spans is proposed.
 
Article
We present and analyze a method of interpolation that improves the amplitude resolution of an analog-to-digital converter. The technique requires feedback around a quantizer that operates at high speed and digital accumulation of its quantized values to provide a PCM output. We show that use of appropriate weights in the accumulation has important advantages for providing finer resoution, less spectral distortion, and white quantization noise. The theoretical discussion is supplemented by the report of a practical converter designed especially to show up the strengths and weaknesses of the technique. This converter comprises a sigma-delta modulator operating at 8 MHz and an accumulation of the 1-bit code with triangularly distributed weights. 13-bit resolution at 8 kwords/s is realized by periodically dumping the accumulation to the output. We present a practical method for overcoming a thresholding action that distorts low-amplitude input signals.
 
Article
The design and optimization of multicarrier communications systems often involve a maximization of the total throughput subject to system resource constraints. The optimization problem is numerically difficult to solve when the problem does not have a convexity structure. This paper makes progress toward solving optimization problems of this type by showing that under a certain condition called the time-sharing condition, the duality gap of the optimization problem is always zero, regardless of the convexity of the objective function. Further, we show that the time-sharing condition is satisfied for practical multiuser spectrum optimization problems in multicarrier systems in the limit as the number of carriers goes to infinity. This result leads to efficient numerical algorithms that solve the nonconvex problem in the dual domain. We show that the recently proposed optimal spectrum balancing algorithm for digital subscriber lines can be interpreted as a dual algorithm. This new interpretation gives rise to more efficient dual update methods. It also suggests ways in which the dual objective may be evaluated approximately, further improving the numerical efficiency of the algorithm. We propose a low-complexity iterative spectrum balancing algorithm based on these ideas, and show that the new algorithm achieves near-optimal performance in many practical situations
 
Article
The author considers an asynchronous time-division-multiplexing system with a finite number of identical users generating data only during their so-called active periods, which alternate with so-called passive periods. Assuming geometric distributions for the active and passive periods of the users, the author derives a functional equation which describes the full steady-state buffer behavior of the multiplexer. The buffer occupancy at various moments can be derived from this equation and explicit functions of the parameters of the system. The results indicate a strong dependency of the mean buffer occupancy on the actual lengths of the active and passive periods of the users, even for a given mean user activity
 
Article
We study the application of the eigencanceler, a reduced-rank method based on the eigendecomposition of the estimated covariance matrix, to the wireless communication problem. Simple closed-form bounds are obtained for the bit-error rate of binary phase-shift keying modulation in the presence of cochannel interference in systems using sample matrix inversion (SMI) and the eigencanceler. The application of SMI and the eigencanceler to a flat fading time-division multiple-access system is studied in the context of the IS-54/IS-136 standard. It is shown that adaptive antennas in conjunction with reduced-rank processing can be used to increase capacity of such systems by reducing the frequency reuse factor from 7 to 1
 
Article
A microwave line-of-sight propagation experiment is carried out in Denmark at frequencies around 14 GHz. Results from long term measurements of multipath propagation are presented. The multipath fade durations are shown to be log-normally distributed. The level dependence of the probability of fading, P , the average duration of fades, bar{t} , and the number of fades, N 0 , is investigated. Results show that P propto L^{2} , bar{t} propto L^{2/3} and N_{0} propto L^{4/3} . This differs from previously published results, where proportionalities of L<sup>2</sup>, L and L , respectively, have been proposed. Statistical results on enhancements above free space level are also presented. The results presented are believed to enable an improved description of the effects of multipath propagation.
 
Article
This correspondence presents results related to measurmg the error performance of an experimental 12/14 GHz satellite link established between Carleton University, Ottawa, Canada, and NASA Ames Research Center, California, in 1976-1977. The measurements were performed on a 9.6 kbit/s voiceband data channel which supported the continuous synchronous transmission of fixed length frames (composite packets). The results include measurements of packet error rate versus uplink power settings under various atmospheric conditions, densities of packet error-free interval lengths, within-packet error statistics, and propagation delay variations.
 
Article
The design and performance aspects of Canada's first commercial 14/12 GHz ANIK C earth stations are reviewed, with particular reference to the satellite link degradations of the 90 Mbit/s digital message service. Following design considerations based on computer simulations, extensive test data are presented with respect to BER, jitter, and other performance characteristics of the loop closed via. an ANIK C satellite simulator (SS). An analysis of the effects of linear and nonlinear distortions is applied to the formulation of a model that provides insight into the practical quantitative assessment of field tolerance and maintenance limits and of tradeoffs for new digital transmission systems.
 
Article
This paper presents the performance of a pseudoerror monitoring technique for a 16 QAM 140 Mbit/s digital radio in presence of multipath dispersive fading. The so-called pseudoerrors, generated by means of a threshold modification of two, namely, secondary receivers are entered into an extrapolating function to obtain a fast bit error ratio (BER) calculation. A counting time of 10 ms was retained in order to follow fading depth changes up to 100 dB/s and fading notch speeds up to 300 MHz/s approximately. We have considered three structures for the receiver: without equalization, with IF amplitude equalization, and decision feedback equalization (DFE). The results obtained show the estimated and real BER within a margin that includes the two recommended CCIR values: 10<sup>-3</sup>and10<sup>-6</sup>. In particular, the estimated signature obtained in the case of using IF equalization reveals that this fast BER calculation could be an effective choice to control a frequency diversity switch, even in the presence of a fading activity with rapid variations.
 
Article
This paper describes an optical fiber system developed at the British Post Office (BPO) Research Centre, Martlesham Heath, which provides telephone circuits in an 8 Mbit/s junction between the Centre and Ipswich Telephone Exchange via 13 km of BICC cable incorporating Corning fibers. In parallel, a 140 Mbit/s system operating over 8 km of cable has been demonstrated. The systems were constructed in accordance with standard BPO equipment practice and the cables were installed by local area staff in existing ducts.
 
Article
A comparison between different line codes suitable for transmission of 140 Mbit/s digital signals over 1.2/4.4 mm coaxial cable has been made using the computer optimization program, "OPUT." One alternative, the two level partial response class 1 (PR1), is chosen and a laboratory system has been built and tested. A block diagram of the system followed by a description is given. Performance of the system is presented and commented on. It is concluded that a PR1 system can be implemented with reasonable noise margin with a repeater spacing of 2.15 km for a 1.2/4.4 mm coaxial cable.
 
Article
Novel blind channel estimators based on the finite alphabet property of information symbols are derived in this paper for OFDM and related multicarrier code-division multiple access (MC-CDMA) systems. The resulting algorithms are applicable not only to standard OFDM transmitters with cyclic prefix, but also to the zero padded OFDM transmissions that improve symbol recovery at the expense of altering the transmitter and complicating the equalizer. Based on FFT-processed received data, channel identifiability is guaranteed regardless of channel zero locations and various channel estimation algorithms become available by trading on the complexity for performance. Unlike existing blind channel estimators, the proposed alternatives require short data records especially for PSK transmissions. The inherent scalar ambiguity is easily resolved because it has unit amplitude and phase values drawn from a finite set. Decoupling channel from symbol estimation enables a phase-directed operation that improves upon decision-directed schemes that are known to suffer from error propagation. Practical issues are also addressed including the presence of frequency guard intervals, constellation and power loading, various frame designs, coded transmissions as well as semi-blind and online implementations for systems with training sequences. The algorithms are tested with simulations and also compared with existing alternatives in a realistic HIPERLAN/2 setting
 
Article
The traffic on a 10-Mb/s Ethernet local-area network that connects diskless workstations to file servers in a university environment is analyzed. The traffic is substantially heavier than has been recorded in previous studies; measured over 1-s intervals, it frequently exceeds 30% of the network bandwidth. The distribution of packet lengths and the patterns of packet interarrival times are interpreted for the three protocols that carry significant traffic: the transmission control protocol (character traffic), the network disk protocol (paging traffic), and the network file system protocol (remote file access traffic). The two latter protocols account for 68% of the packets and 94% of the data bytes on the network. File access to a remote file server generates bursts of traffic that can last several seconds and that demand bandwidths on the order of 120 kB/s, or about 10% of the Ethernet bandwidth
 
Article
Optimal binary cyclic redundancy-check codes with 16 parity bits (CRC-16 codes) are presented and compared to those in existing standards for minimum-distance, undetected-error probability on binary symmetric channels (BSCs) and properness. The codes in several cases are seen to be superior at block lengths of practical interest when they are used on low-noise BSCs. The optimum minimum distance obtainable by some CRC-16 codes is determined for all block lengths. For several typical low-noise BSCs the minimum undetected error probability achievable with some CRC-16 codes is given for all block lengths
 
Article
The true Cramer-Rao lower bound (CRLB) is derived and evaluated for the estimation of carrier phase of Star 16-quadrature amplitude modulation (QAM) and can be simply applied to carrier frequency estimation. Different geometries are investigated by varying the ring ratio (RR). For signal-to-noise ratios (SNRs) between 6-15 dB, the CRLB with RR=3 is lower than that of Square 16-QAM. A modified phase estimator is presented, which closely follows the new CRLB. Investigation of symbol error performance in short packet length reveals Star 16-QAM to be superior to Square 16-QAM for SNR<13 dB, which is a reasonable operating range for a coded system. Although Square 16-QAM and Star RR=1.8 are optimum for a perfect receiver, when the effect of phase estimation is considered, we find Star RR=3 to be better for SNR below 10 dB.
 
Article
A novel long haul 5 GHz 16 QAM digital radio system, which has 200 Mbit/s transmission capacity within the 40 MHz interleaved channel allocation, is proposed and described. It is designed to be overbuilt on existing FDM-FM routes with an approximately 50 km repeater spacing. To achieve the 5 bit/s/Hz RF spectral efficiency, the 16 QAM modulation and Nyquist cosine roll-off spectral shaping techniques ( alpha = 0.5 ) are investigated. Then a new signal shaping filter, differential encoding and carrier recovery techniques are presented. Finally, the effects of TWT amplifier nonlinearity on a 16 QAM signal are experimentally investigated.
 
Article
In this paper we study the effect of intersymbol and quadrature channel interference on the average error probability of 16-ary offset quadrature amplitude modulation with rectangular or sinusoidal shaping functions. We present numerical results assuming that the transmitter and receiver filters are identical Butterworth filters of order N = 1,2,..., 6, and the decision rule is either fixed or adaptive.
 
Article
Vector quantization, in its simplest form, may be regarded as a generalization of PCM (independent quantization of each sample of a waveform) to what might be called "vector PCM," where a block of consecutive samples, a vector, is simultaneously quantized as one unit. In theory, a performance arbitrarily close to the ultimate rate-distortion limit is achievable with waveform vector quantization if the dimension of the vector, k , is large enough. The main obstacle in effectively using vector quantization is complexity. A vector quantizer of dimension k operating at a rate of r bits/sample requires a number of computations on the order of k2^{kr} and a memory of the same order. However, a low-dimensional vector quantizer (dimensions 4-8) achieves a remarkable improvement over scalar quantization (PCM). Consequently, using the vector quantizer as a building block and imbedding it with other waveform data compression techniques may lead to the development of a new and powerful class of waveform coding systems. This paper proposes and analyzes a waveform coding system, adaptive vector predictive coding (AVPC), in which a low-dimensionality vector quantizer is used in an adaptive predictive coding scheme. In the encoding process, a locally generated prediction of the current input vector is subtracted from the current vector, and the resulting error vector is coded by a vector quantizer. Each frame consisting of many vectors is classified into one of m statistical types. This classification determines which one of m fixed predictors and of m vector quantizers will be used for encoding the current frame.
 
Article
A computational method allowing the calculation of bit error rate in the presence of filtering and some other impairments is described for 16 QAM modulation; a breadboard working at a bit rate of 140 Mbits/s has been implemented and experimental results are compared with calculated values. The possible use of this modulation type for a high capacity digital radio-relay system is considered. Some parameters are introduced for this purpose, especially the net fade margin parameter. In the case of the 140 Mbit/s system in the 10.7-11.7 GHz frequency band, 4 PSK and 8 PSK modulation types are compared with 16 QAM. System gain, frequency arrangement, nodal capacity and outage performances are evaluated.
 
Article
Recently, Cao and Beaulieu presented an insightful analysis on the bit-error performance of 16-quadrature amplitude modulation (QAM) scheme on Rayleigh fading channels with maximal ratio combining diversity and channel estimation errors. Unfortunately, although exact, the results presented in that paper are based on a complicated parabolic cylinder function, and a two-dimensional numerical integration has to be performed to evaluate the average probability of bit error. In this letter, we present a surprisingly simple expression for the average probability of error in a well-known closed form. We point out that the error rates for any N-ary square/rectangular QAM constellation with Gray mapping can also be expressed in this form. Furthermore, we show that the presented results can also be modified, in a simple fashion, for a generalized selection combining scheme in the presence of channel estimation errors.
 
Article
The performance of new bandwidth compressive 16 SQAM (Superposed-QAM) modems, operating in a highly power-efficient saturation mode, is analyzed. Nonlinearly amplified 16 SQAM retains the out-of-band energy much lower than 4-level rectangular (i.e., 16 QAM) or sinusoidal shaped (i.e., MAMSK) QAM, and operates within 0.7 dB of theoretical (i.e., 16 QAM in Nyquist linear channel) performance at P(e) = 10^{-6} using simple 4th-order Butterworth LPF's. Effects of baseband pulse shaping, receive filter BT s , power level variations and propagation time difference (or static phase shift) of transmit HPA's are analyzed in terms of BER.
 
Article
Convolutional coding coupled with 16-PSK modulation is investigated for bandwidth efficient transmission. Rate 3/4, small memory codes are found which are optimized in the free-distance sense on the Gaussian channel. These codes provide up to 4.8 dB of coding gain with 32 states over uncoded 8-PSK, a scheme having the same spectral efficiency as the codes described. The performance is compared with earlier findings of Ungerboeck and some recent results on R = 2/3 coded 8-PSK. In addition, we present results of a channel transmission study to assess the performance of the four-state code on the band-limited nonlinear channel, and find that performance of the coded scheme degrades comparably with uncoded 8-PSK, i.e., coding gain is roughly preserved.
 
Article
The need for a 16-kb/s speech coding algorithm that has very low coding delay while achieving essentially the same high quality as the 32-kb/s adaptive differential pulse code modulation (ADPCM) standard G.721 is addressed. The authors describe low-delay vector excitation coding (LD-VXC), a new coding algorithm which provides high quality with less than 2 ms of coding delay and is robust to transmission errors. The algorithm combines techniques such as vector quantization, analysis-by-synthesis, and perceptual weighting together with backward adaptive linear predictive encoding, and uses a novel long-term predictor employing backward adaptive pitch tracking. Perceptually based nose shaping and postfiltering contribute to the masking of audible quantization noise
 
Article
The bit-error rate (BER) performance of multilevel quadrature amplitude modulation with pilot-symbol-assisted modulation channel estimation in static and Rayleigh fading channels is derived, both for single branch reception and maximal ratio combining diversity receiver systems. The effects of noise and estimator decorrelation on the received BER are examined. The high sensitivity of diversity systems to channel estimation error is investigated and quantified. The influence of the pilot-symbol interpolation filter windowing is also considered.
 
Top-cited authors
Behnaam Aazhang
  • Rice University
Robert M Gray
  • Stanford University
H. Vincent Poor
  • Princeton University
L. Kleinrock
  • University of California, Los Angeles
Norman Beaulieu
  • Beijing University of Posts and Telecommunications