Showing papers on "Spectral efficiency published in 1984"

High-Level Modulation and Coding for Nonlinear Satellite Channels

Abstract: Currently, 4-PSK is the prevalent modulation format in use for digital satellite communications. To improve bandwidth efficiency, 8PSK could be used instead, but a higher power would be needed; to improve power efficiency, error-correcting codes could be used, but at the expense of a larger bandwidth. Recently, Ungerboeck [1] has proposed a class of codes in which a constellation of 2M signals is used to transmit information at the rate of log_{2} M bits per symbol, and has shown that coding gains of up to several decibels can be achieved on the additive white Gaussian noise (AWGN) channel with no increase in bandwidth occupancy and a relatively small added complexity. Thus, these codes seem to be particularly attractive for application in the band-limited environment typical of satellite communication systems, provided that the performance gain that they provide on the AWGN channel is not lost over a satellite channel. The goal of this work is to assess the performance of this class of codes when used to transmit 3 information bits per symbol on a band-limited, nonlinear satellite channel. Three modulation formats are considered, namely 16-PSK, 16-QAM, and a 16-ary amplitude-phase keying scheme with two amplitude levels. It is found that 16-PSK outperforms the two other modulation formats, and that the use of these codes can provide a substantial performance improvement even on a satellite channel.

Throughput Analysis for Code Division Multiple Accessing of the Spread Spectrum Channel

Abstract: We consider the use of error correction codes of rate r on top of pseudonoise (PN) sequence coding for code division multiple accessing of the spread spectrum channel. The channel is found to have a maximum throughput of 0.72 and 0.36 based on the evaluation of channel capacity and cutoff rate, respectively. More generally, these two values are derived for given bandwidth expanding n/r versus n/N where n is the length of the PN sequence and N is the number of simultaneous users. It is found that to achieve the maximum throughput, n should be small. This implies that coding schemes with short PN sequences and low rate codes are superior in terms of throughput or antijam capability. The extreme case of n = 1 corresponds to using a very low rate code with no PN sequence coding. Convolutional codes are recommended and analyzed for their error rate and decoding complexity.

Rate 3/4 Convolutional Coding of 16-PSK: Code Design and Performance Study

Abstract: 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.

Four-dimensional modulation and coding - An alternate to frequency-reuse

Abstract: Four dimensional modulation as a means of improving communication efficiency on the band-limited Gaussian channel, with the four dimensions of signal space constituted by phase orthogonal carriers (cos omega sub c t and sin omega sub c t) simultaneously on space orthogonal electromagnetic waves are discussed. 'Frequency reuse' techniques use such polarization orthogonality to reuse the same frequency slot, but the modulation is not treated as four dimensional, rather a product of two-D modulations, e.g., QPSK. It is well known that, higher dimensionality signalling affords possible improvements in the power bandwidth sense. Four-D modulations based upon subsets of lattice-packings in four-D, which afford simplification of encoding and decoding are described. Sets of up to 1024 signals are constructed in four-D, providing a (Nyquist) spectral efficiency of up to 10 bps/Hz. Energy gains over the reuse technique are in the one to three dB range t equal bandwidth.

A comparison of Gaussian minimum shift keying to frequency shift keying for land mobile radio

Abstract: Data is presented comparing Gaussian Minimum Shift Keying (GMSK) to Frequency Shift Keying (FSK) for use in the UHF land mobile radio band at 800 MHz. The FSK system uses a Gaussian premodulation filter set at one half the bit Pate. Deviation is set at 0.41 times the bit rate. The FSK receiver has an IF bandwidth of 1.0 times the bit rate and uses discriminator detection. The GMSK system uses coherent detection. Data is presented on the effect of premodulation cutoff frequency and receiver bandwidth on system sensitivity and spectral efficiency. From this data a relationship between channel spacing, frequency stability, IF bandwidth, and transmitter spectrum is derived. Additional data on the performance of the FSK and GMSK systems in Rayleigh fading is presented.

Error performance improvement of QDPSK in the presence of cochannel or multipath interference using diversity

Abstract: The quadriphase PSK system is thought to show the highest spectrum efficiency among M-ary PSK systems for mobile communication purposes and is a noted digital radio system the diversity effect of the QDPSK system using differential detection has been analyzed approximately However, since no exact analysis has been done, approximation accuracy has not been clarified This paper first compares the exact analytical value under cochannel interference and the approximate formula As a result, it is shown by the method of least squares that the error rate is approximated very closely in precision with a simple formula Analysis of multipath interference, which simulates frequency selective fading, is carried and the performance difference from that under cochannel interference is discussed the diversity effect for multipath interference is lessened slightly compared with cochannel interference It is also indicated that when the average power of a first-coming wave is nearly equal to that of a delaying wave (often the case in multipath interference) and the error rate is degraded, the diversity effect cannot be expected practically Finally, the need for a technique to alleviate degradation caused by this difficult-to-control propagation characteristic is mentioned

The Combined Effects of Noise and Multipath Propagation in Multilevel PSK Radio Links

Abstract: A general analysis of the behavior of multilevel PSK radio links is carried out under multipath propagation conditions. A method is proposed to evaluate system unavailability due to the combined effects of noise and faded channel distortion. The "outage domain," having fixed the transmission channel, is calculated in the space of the variables which characterize any anomalous propagation condition. In particular, with reference to a "twopath model" (a "direct ray" and an "echo"), variations of carrier and symbol synchronisms due to the echo are taken into account and their importance is emphasized in realizing the numerical results. These are shown to be in very good agreement with the experimental ones which have appeared in the literature. An analytical realization of the results obtained is developed and leads to a simple asymptotic formula explaining outage probability as a function of some parameters of the echo statistics, the transmission channel adopted, and the number of modulation levels. Rapid evaluation of this formula allows us to plot outage probability as a function of spectral efficiency, measured in bits/s/Hz.

Distributive Demand-Assigned Packet Switching with Trailer Transmissions

Abstract: In a packet satellite system using ALOHA type random access techniques, channels and on-board processing may be effectively used to provide significant gains in throughput, delay, and bandwidth efficiency. Motivated by this conjecture, a distributive demand-assigned packet broadcast system through a processing satellite with multiple request channels and user buffers is first presented. This scheme is shown to be able to achieve the above goals except, as any other demand type scheme, it also exhibits relatively poor delay performance at low traffic when compared with pure random access schemes. An extended scheme using trailer transmission to improve delay performance at low traffic is then introduced. Two techniques for handling trailer transmissions are studied and compared. Both are shown to provide lower packet delay at light trafric. lay at light traffic.

CPM - The effect of filtering and hard limiting

Abstract: The attractive properties of the digital constant amplitude modulation scheme CPM (continuous phase modulation) have been demonstrated in a previous paper in this journal.1 This concerned both the power and spectral efficiency of the large family of CPM schemes over the Gaussian channel. In this paper the considered channel can be both bandlimited or band- and amplitude-limited at the same time. Analytical results as well as simulations are presented for a number of selected CPM schemes. In today's increasing demand for bandwidth, CPM is an attractive candidate for future satellite applications because of its spectral efficiency. In addition to this comes the fact that the transmitted signal has a constant amplitude and good detection performance when optimal coherent detection is performed.

Computer simulation and evaluation of mobile radio voice communication systems

Abstract: In urban and suburban areas, narrowband mobile radio voice transmission suffers from multipath fading phenomena, which are time-varying and related to geographic environment, carrier frequency, and vehicle speed. Traditionally, mobile radio voice communications have used narrowband frequency modulation (FM). An FM channel usually requires 25-30 kHz of bandwidth, and the performance is heavily affected by multipath. Much recent research effort has been directed toward finding better transmission schemes. On the one hand, advances in solid-state technology and the demand for spectral efficiency have led to the proposal of single-sideband amplitude modulation (SSBAM) with a pilot tone as a better alternative than FM. On the other hand, due to the increasing use of digital techniques and the possibility of more robust transmission, there is an increasing demand for consideration of digital techniques for mobile radio voice transmission. Our research evalutes the performance of various mobile radio voice processing and transmission methods, both analog and digital, using computer simulation. In the analog category, we compare amplitude-companded SSBAM (with pilot) with traditional FM, in the face of receiver front-end noise and co-channel interference. In the digital category, we compare a proposed speech waveform coder, called controlled adaptive prediction delta modulation (CAPDM), with traditional continuously variable slope delta modulation (CVSD); channel coding schemes and bit scrambling techniques are also evaluated. The analog and digital schemes are also compared. From our simulation, we find that SSBAM with two-stage amplitude companding and a pilot tone, where the pilot tone is used to combat fading through a technique called feedforward signal regeneration (FFSR), is more robust than traditional FM in the face of receiver front-end noise and multipath fading. In co-channel interference, however, we find that FM still has an advantage over SSBAM. In the digital category, assuming DPSK modulation at a transmission rate of 15 kbits/sec, we find that our CAPDM coder, which uses both adaptive companding and adaptive prediction, performs much better than the traditional CVSD coder in multipath fading, in both front-end noise and co-channel interference. (Abstract shortened with permission of author.)

Effects of Nonlinear Power Amplifiers on the Spectrum of CPFSK Signals

Abstract: The nonlinear behaviour of high-power amplifiers (HPAs) constitutes a serious, conditioning performance impairment in digital radio, whenever great spectrum efficiency is required such as in modern satellite communications, terrestrial radio links or radio mobile systems. In fact, the effects of filtering followed by amplifying in transmission give rise to power spectrum spreading on account of nonlinear distortion, and consequently to an increase of adjacent channel interference. When the HPA must work at saturation in order to give out its maximum power, angle modulation systems, with constant envelope, seem to be more attractive than amplitude - and phase - modulated ones, which require linearization [1]. In this paper, the CPFSK signal with modulation pulses assumed to be rectangular, with a length ? less than the symbol time, T, is tested in the above-mentioned conditions. The power spectral density in the various points of the transmitter is displayed for fixed HPA characteristics (AM/AM and AM/PM conversions); its behaviour is analyzed for varying filter bandwidths and modulation parameters (duty-cycle of the modulation pulses, ?/T, and maximum angular frequency deviation, ? d ). The results obtained, experimentally checked, show that, unlike other angle modulation systems (PSK, offset PSK, MSK), in the CPFSK case the HPA output has a power spectrum which can be efficiently controlled by means of filtering at the intermediate frequency. For this purpose, only a suitable choiche of ?/T and ? d is required.

Protocol design of the EC 900 cellular radio system

Abstract: The Mobile Automatic Telephone System EC 900 is a high-capacity cellular system developed for operation in the CEPT 900 MHz-band. This contribution concentrates on the so-called "radio interface", i.e. signalling and protocols used on the radio path between mobile and fixed stations. In particular, the access, paging, and broadcast Functions handled on common control channels are explained in detail, which allow the implementation of off-air call set-up and call queueing in order to increase spectrum efficiency. Results of detailed system simulations are used to demonstrate system performance.

A cowarison of gaussian minimum sift keying to frequency shift keying for land mbiu radio

Abstract: Data is presented comparing Gaussian Minimum Shift Keying (GMSK) to Frequency Shift Keying (FSK) for use in the WF land mobile radio band at 800 MHz. The FSK system uses a Gaussian premodulation filter set at one half the bit rate. Deviation is set at 0.41 times the bit rate. The FSK receiver has an IF bandwidth of 1.0 times the bit rate and uses discriminator detection. The GMSK system uses coherent detection. Data is presented on the effect of premodulation cutoff frequency and receiver bandwidth on system sensitivity and spectral efficiency. From this data a relationship between channel spacing, frequency stability, IF bandwidth, and transmitter spectrum is derived. Additional data on the performance of the FSK and GMSK systems in Rayleigh fading is presented.

Evaluation of the Spectrum Efficiency of Direct Sequence Systems

Abstract: This paper reviews the concept of spectrum efficiency and discusses the factors affecting the spectrum efficiency of communication systems. However, major effort is devoted to the evaluation of the spectrum efficiency of direct sequence systems and to investigate the possibility of operating such systems over some existing frequency bands already assigned for narrowband systems. The results show that the spectrum efficiency of direct sequence systems may exceed those of narrowband systems under certain operating conditions. It is quite possible that the wideband systems share frequency bands with existing narrowband systems by careful planning.