Showing papers on "Spectral efficiency published in 1990"

The performance enhancement of multibeam adaptive base-station antennas for cellular land mobile radio systems

Abstract: The problem of meeting the proliferating demands for mobile telephony within the confinement of the limited radio spectrum allocated to these services is addressed. A multiple-beam adaptive base-station antenna is proposed as a major system component in an attempt to solve this problem. The approach is demonstrated by employing an antenna array capable of resolving the angular distribution of the mobile users as seen at the base-station site, and then using this information to direct beams toward either lone mobiles, or groupings of mobiles, for both transmit and receive modes of operation. The energy associated with each mobile is thus confined within the addressed volume, greatly reducing the amount of cochannel interference experienced from and by neighboring cochannel cells. To ascertain the benefits of such an antenna, the conventional and proposed antenna systems are modeled in a typical mobile radio environment. For a given performance criterion, the theoretical results indicate that a significant increase in the spectral efficiency, or capacity, of the network is obtainable with the proposed antenna. >

Multimedia traffic management principles for guaranteed ATM network performance

Abstract: A framework is presented for the specification of performance requirements, and several high-level design principles for performance-driven ATM traffic controls based on resource reservation are developed. Simulation results illustrate that to guarantee performance levels, bursty traffic should be statistically multiplexed only if connection peak rates are low relative to the network link speed or burst durations are short. A pragmatic traffic management approach that favors simplicity and robustness in the traffic control design rather than optimizing bandwidth efficiency is stressed. >

Spread-spectrum multiple-access performance of orthogonal codes for indoor radio communications

Abstract: Direct-sequence spread spectrum, with its inherent resistance to multipath, is a promising technique for indoor wireless communication. To allow multiple users within a limited bandwidth, code division multiple access (CDMA) is needed. The bandwidth efficiency of M-ary CDMA systems in fading multipath indoor radio channels is analyzed. It is shown that M-ary signaling improves the bandwidth efficiency significantly when compared to binary signaling. >

On the spectral efficiency of CPM systems over real channel in the presence of adjacent channel and cochannel interference: a comparison between partial and full response systems

Abstract: In all the new radio systems to be designed-for example, local radio networks or portable communication systems-a crucial point is determination of the actual spectral efficiency obtainable. To this end, the fundamental role played by adjacent and cochannel interference must be considered. The computer program implemented allows general analysis of continuous phase modulation (CPM) systems with limiter-discriminator detection and symbol-by-symbol regeneration, the combined effects of intersymbol interference, noise, and adjacent channel or cochannel interference have been tested to give the design criteria of the transmission system. To emphasize the role played by interference, an ideal multipath-free channel has been considered: with reference to a binary case, Gaussian minimum-shift keying (GMSK) systems have been investigated and compared with full-response CPM systems to obtain a suitable tradeoff between spectral efficiency (connected to channel spacing at radio frequency) and signal-to-noise ratio with fixed bit-error probability. The numerical results presented emphasize the performance obtainable with full- and partial-response techniques for varying system parameters such as phase deviation index, receiver filter bandwidth, and spectral efficiency. >

Control channel structure for TDMA mobile radio systems

Abstract: A control channel structure for time-division multiple access (TDMA) mobile radio systems that must offer high system capacity and ISDN services is described. This structure overcomes the difficulties in random access efficiency and the application of a layered protocol. A functional channel definition and layered signaling frame format are described. Functional channel mapping on physical channels is then described. Several methods for the frequency assignment of control channels are compared from the points of radio channel efficiency and functional ability. The optimum method is determined. Mapping information is broadcast to realize the control channel structure needed. An improved random access scheme for the common control channel is proposed. This scheme obtains good throughput performance, and realizes the application of data link layer protocols. Thus, efficient and flexible signaling for radio link control and data services is achieved. >

Channel coding with quadrature-quadrature phase shift-keying (Q2PSK) signals

Abstract: Channel coding due to treillis modulation has been proven to be useful for band-limited channels. However, these modulations, mostly designed with n-use of 2-D signals, are primarily aimed at coding gain only. This paper attempts to point out that utilization of all available signal space dimensions, which is limited by the time-bandwidth product, may improve the bandwidth efficiency and simultaneously bring an additional coding gain.

Comparison of coherent and differential detection of BPSK and QPSK in a quasistatic fading channel

Abstract: The advantage of coherent detection over differential detection for quadrature phase-shift keying (QPSK) and binary phase-shift keying (BPSK) in a flat-fading channel is evaluated based on receiver-sensitivity analysis using three performance criteria. QPSK is concentrated upon to perform simulations for a frequency-selective fading channel. Spectral efficiency improvement due to better frequency reuse for the more sensitive coherent detection is evaluated with a combination of analysis and simulation. It is found that coherent detection with perfect carrier recovery is superior to differential detection by at least 2.4 dB, which converts to about a 30% improvement in spectral efficiency in a two-dimensional, frequency reuse system with a fourth-power propagation exponent. This advantage is preserved even if diversity selection is used to mitigate fading. >

Bandwidth efficient transmission of 4 Gb/s on two microwave QPSK subcarriers over a 48 km optical link

Abstract: The use of two quadrature phase shift keying (QPSK)-modulated subcarriers over a wideband direct detection optical communications system with a RF bandwidth efficiency of 1 b/s/Hz is reported. It is shown that two subcarriers each carrying 2 Gb/s QPSK, compactly spaced at just twice the data rate, can achieve a receiver sensitivity of -19.5 dBm over 48 km without crosstalk between subcarriers. This represents a receiver sensitivity improvement of 5 dB over a 4-Gb/s single QPSK subcarrier system. >

Highly bandwidth efficient transmission through continuous polarisation modulation

Abstract: A binary coherent optical transmission scheme using continuous polarisation shift keying (CPOLSK) is proposed and analysed. Its bandwidth efficiency turns out to be better than that of MSK.

Cellular mobile communication systems and a channel assignment using neural networks

Abstract: The authors formulate a new dynamic channel assignment method as a coloring problem in a network with an edge weight and show that the problem is NP-complete. They propose an application of neural networks to the channel assignment problem in a cellular mobile radio communication system. The computer simulations show that the channel assignment method using a neural network gives a higher spectral efficiency than usual methods. >

Bandwidth-efficient trellis coded modulation schemes

Abstract: The author proposes a scheme that shapes the power density spectrum of the transmitted signal to reduce its bandwidth. The encoder is modeled as a finite-state machine consisting of a shift register and a mapper. A pulse-shaping filter generates time-limited or Nyquist pulses. The whole system is optimized with respect to bandwidth and power efficiency, where average-power and peak-power criteria are considered. The author presents the achievable data rates of systems with quaternary input symbols and an out-of-band power of -20 dB for the various mappers and pulse-shaping filters considered. It can be seen that bandlimited pulses allow roughly a doubling of the data rate with respect to time-limited pulses. The bandwidth gain that can be reached with proper coding is remarkable. The drawback is the increased receiver complexity needed to resolve the intersymbol interference. >

An energy- and bandwidth-efficient data transmission system for time-selective fading channels

Abstract: The author investigates the use of smearing/desmearing filters in combination with linear modulation to achieve time diversity without bandwidth expansion in digital mobile radio systems. A low-complexity receiver structure for smeared data transmission in time-selective fading is described. The receiver is analyzed for perfect and noisy channel estimation and perfect interleaving. Results on the bit error performance show that the proposed data transmission system is very robust to Rayleigh fading and noisy channel estimation. In the proposed system, time-selective fading merely introduces an additional stationary Gaussian noise component. This makes the combination with coding straightforward. The first results on the performance of smeared data transmission with trellis-coded 8-PSK (phase-shift keying) are given. >

On the performance of symbol- and fractionally-spaced equalization in digital magnetic recording

Abstract: Equalization is a mature technology in digital communications and is used in many applications to improve the spectral efficiency (bits/sec/Hz) of the channel and reduce sensitivity to channel variations. It has the same potential for digital magnetic recording systems and may be used to achieve similar benefits, namely, (1) higher storage density on available head/medium combinations, and (2) reduced performance sensitivity to variation in the readback signal resulting from manufacturing tolerances, etc. The purpose of this paper is to describe analytic and simulated results on the performance of digital equalization with partial response methods. Two partial response signals, namely, Class 4 (PR4) and extended-Class 4 (EPR4), are considered. The digital equalizer is configured as a transversal filter (also called finite impulse response, FIR, filter) that linearly combines a finite sequence of samples of the readback signal to form a desired output. The sampling interval equals either the symbol period or a fraction thereof, but the output is computed at the symbol rate, 1/T. The resulting approaches are referred to as synchronous and fractionally-spaced equalization (FSE) respectively.

Outage predictions in digital cellular radio systems

Abstract: A model for predicting downlink outage due to cochannel interference in cellular radio systems is presented. The model accounts for path loss, shadowing, and Rayleigh fading. The effects of cell sectoring, power control, and hand-offs are discussed. Particular results are obtained for a cellular system that uses QDPSK with Reed-Solomon coding. Computation of the outage is described. Outage control techniques are discussed, and the impact of Reed-Solomon coding on spectral efficiency is evaluated. >

A satellite mobile communication system based on Band-Limited Quasi-Synchronous Code Division Multiple Access (BLQS-CDMA)

Abstract: Discussed here is a new approach to code division multiple access applied to a mobile system for voice (and data) services based on Band Limited Quasi Synchronous Code Division Multiple Access (BLQS-CDMA). The system requires users to be chip synchronized to reduce the contribution of self-interference and to make use of voice activation in order to increase the satellite power efficiency. In order to achieve spectral efficiency, Nyquist chip pulse shaping is used with no detection performance impairment. The synchronization problems are solved in the forward link by distributing a master code, whereas carrier forced activation and closed loop control techniques have been adopted in the return link. System performance sensitivity to nonlinear amplification and timing/frequency synchronization errors are analyzed.

Performance of trellis coded 16 QAM/TDMA system for land mobile communications

Abstract: The author proposes a six-channel TDMA (time-division multiple access) system using trellis-coded 16 QAM (quadrature amplitude modulation) to support a wide variety of services, including voice facsimile, and ISDN (integrated services digital network) terminals in land mobile communication systems. This system can transmit six voice channels (8 kb/s) per carrier with a carrier spacing of 25 kHz. A pilot-symbol insertion-type fading estimation method, space diversity, symbol interleaving, and two carrier frequency-hopping techniques are applied to this system for improving the transmission performance. Computer simulation confirms that the BER (bit-error-rate) performance and the spectral efficiency of the proposed TDMA system are superior to those of the pi /4-QPSK (quadrature phase-shift keying) system. >

Applications of frequency modulation interference cancellers to multiaccess communications systems

Abstract: Cancellation of interfering frequency-modulated (FM) signals is investigated with emphasis towards applications on the cellular telephone channel as an important example of a multiple access communications system. In order to fairly evaluate analog FM multiaccess systems with respect to more complex digital multiaccess systems, a serious attempt to mitigate interference in the FM systems must be made. Information-theoretic results in the field of interference channels are shown to motivate the estimation and subtraction of undesired interfering signals. This thesis briefly examines the relative optimality of the current FM techniques in known interference channels, before pursuing the estimation and subtracting of interfering FM signals. The capture-effect phenomenon of FM reception is exploited to produce simple interference-cancelling receivers with a cross-coupled topology. The use of phase-locked loop receivers cross-coupled with amplitude-tracking loops to estimate the FM signals is explored. The theory and function of these cross-coupled phase-locked loop (CCPLL) interference cancellers are examined. New interference cancellers inspired by optimal estimation and the CCPLL topology are developed, resulting in simpler receivers than those in prior art. Signal acquisition and capture effects in these complex dynamical systems are explained using the relationship of the dynamical systems to adaptive noise cancellers. FM interference-cancelling receivers are considered for increasing the frequency reuse in a cellular telephone system. Interference mitigation in the cellular environment is seen to require tracking of the desired signal during time intervals when it is not the strongest signal present. Use of interference cancelling in conjunction with dynamic frequency-allocation algorithms is viewed as a way of improving spectrum efficiency. Performance of interference cancellers indicates possibilities for greatly increased frequency reuse. The economics of receiver improvements in the cellular system is considered, including both the mobile subscriber equipment and the provider's tower (base station) equipment. The thesis is divided into four major parts and a summary: the introduction, motivations for the use of interference cancellation, examination of the CCPLL interference canceller, and applications to the cellular channel. The parts are dependent on each other and are meant to be read as a whole.

Integrated DSP/RF design for an MSAT transmitter

Abstract: In mobile radio systems, the relatively inefficient use of the spectrum by existing Frequency Modulation (FM) techniques is limiting the available channels. Linear modulation methods are thought to provide the necessary push into more efficient usage of the spectrum. The limitation, to date, of the linear modulation techniques is the fact that the nonlinear power amplifier tends to spread the spectrum and thus offset any spectrum efficiency advantage. Linearization techniques are presently being investigated, in the literature, in order to recover some of the lost spectrum, due to spreading. To this end, digital signal processing (DSP) techniques are thought to provide real time predistortion methods. Some of the complex modulation blocks, normally confined to analog circuitry, are being implemented using DSP circuitry. This paper will address the linearization techniques, for mobile satellite (MSAT) transceivers, that use an integrated DSP/radio frequency design.

Experimentally Optimized Narrowband FM System for TDMA Operated Mobile Radio Environment

Abstract: Results of ;in experimental optimization of deviation setting, Iiandpziss filter (BPF) bandwidth, and spectral efficiency (7) of four lcvel Pulse Amplitiicle Modulated FM (4 PAM-FM) in the Time ilivisioii Mtiltiple Access (TDMA) burst-operated Caussim and Rayleigh failed environments are presented. Experimentally olitainecl bit-error rate (BER) cui~es and specti-uiii photograplis froin ii prototype hardware modem tlesigiied at UC Davis are presented. Specti-a1 splattering due to burst opct-:itioii is experimentally stiidied. The 4 PAM-FM systciii is ii vialilc altei-ii;itive ioi- inobile communications :i pp I ica t io iis.

Bandwidth efficient signal sets with partitioned equicorrelated properties for the AWGN channel

Abstract: Signal sets are considered which give rise to multitone M-ary frequency shift keying (MT-MFSK) modulation, which represents a bandwidth efficient alternative to optimum performance modulation schemes for the AWGN channel (simplex signaling for coherent detection and orthogonal signaling for noncoherent detection). The channel is examined in the power-limited region, and it is shown that MT-MFSK modulation exhibits optimum asymptotic performance with bandwidth efficiency gains of an order of magnitude over the orthogonal MFSK modulation technique for sets of finite size. MT-MFSK is viewed as a class of signal sets derived from combinatorial designs. The main properties and structure of such sets are presented and exploited to derive tight upper bounds for the error probability. >

Spectrum conservation techniques for future telecommunications

Abstract: The author presents a quantitative method for evaluating spectrum conservation techniques and an overview of several spectrum conserving technologies and policy/regulatory considerations related to implementing spectrum-efficient technologies in the future. It is found that there are many trade-offs in communication system design factors and technology options that have an impact on spectrum efficiency. The spectrum conserving potential of a system is a function of several design factors all of which must be taken into consideration when evaluating the system's spectrum utilization efficiency. Such factors include antennas, signal processing, and RF filters. It is noted that, to ensure effective and efficient use of the spectrum, regulatory and policy procedures must be established. However, because of the economic burden on the marketplace that can be associated with requirements for spectrum efficient communication techniques, mandated regulatory controls should only be implemented where necessary to ensure the continued availability of the spectrum. >

Multidimensional trellis coding for MPSK

Abstract: The use of multidimensional M-ary phase-shift keying (MPSK) signaling is considered for bandwidth and power efficient communication. Trellis-coded modulation is used to code N successive symbols (N-dimensional coding) of MPSK at one time to form a more complex signal than symbol-by-symbol coding, with the expectation of improved performance. In the general case, it is found that the allowed values of bandwidth efficiency (b.p.s./Hz or bits/symbol) are a discrete set, any integer plus J/N where J can have the integer values 0 through N-1. In all cases, the allowed values of M are smaller than those for symbol-by-symbol coding for the same bandwidth efficiency. Thus, the minimum distance between signal points is increased by N-dimensional coding, and this is an important factor in increasing the coding gain. A search of possible trellises and codes reveals that trellis coding of two 6 PSK symbols at a time for 2 b.p.s./Hz gives an asymptotic coding gain of 4.77 dB (as compared to uncoded 4 PSK) with a 16-state trellis. >

Cochannel performance of 16-level QAM with phase-locked TTIB/FFSR processing

Abstract: Transparent tone in band (TTIB) is a reference based channel sounding technique which can facilitate the transmission of data with coherent demodulation. By exploiting the power and bandwidth efficiency of multi-level modulation schemes such as 16-level quadrature amplitude modulation (QAM), high data rate transmission can be achieved over frequency congested mobile links. With the use of feed forward signal regeneration (FFSR), the channel can be sounded and the undesirable propagation effects, such as fading, can be reduced. Two important factors used in defining modulation efficiency are tolerance to adjacent and cochannel interference. Although TTIB offers a much reduced channel spacing over existing cellular schemes such as the Total Access Communication System (TACS) and digital Time Division Multiple Access (TDMA) system, this must be equated with the protection ratio associated with each modulation scheme. Simulation results of the cochannel performance of 16-level QAM/TTIB subject to fading conditions as would be experienced in land mobile channels are presented.

Multiple trellis coded modulation using BFSK/MPSK modulation

Abstract: Multiple trellis-coded modulation (MTCM) is used in conjunction with a frequency/phase modulation scheme. While conventional 2D trellis coders output one channel symbol per trellis branch, multiple trellis coders output more than one channel symbol per trellis branch. It is shown that the combination of multiple trellis coding and 4D BFSK/MPSK modulation can yield substantial coding gains over the 2D multiple trellis coded modulation technique, while incurring a slight increase in transmitter bandwidth. Furthermore, 4D MTCM schemes which possess noninteger throughput have been designed. It is shown that these combined modulation/coding schemes offer a favorable tradeoff between coding gain and spectral efficiency. >

Interference effects in high spectrum efficiency M-QAM radio systems

Abstract: M-QAM radio systems have been investigated in the last few years with the aim of implementing ever increasing values of M. With a view to determining the actual spectral efficiency obtainable with an M-QAM system, adjacent and cochannel interference effects are investigated; consequently, degradations in signal-to-noise ratio required to guarantee a fixed error probability are evaluated. The algorithm proposed, which is also compared and checked with different, alternative methods already published, can achieve both accuracy of the results and efficiency in computing time. Thus it allows rapid system investigation whatever the value of spectrum efficiency, i.e. for any choice of radiofrequency channel spacing and any number of modulation states. The numerical results, obtained with different values of rolloff factor, emphasise the ever increasing sensitivity of system performance to adjacent and cochannel interference when M increases.

Miller coded pilot aided modulation schemes for digital mobile radio

Abstract: The performance of Miller coded pilot aided modulation systems is studied by computer-aided design techniques. It is shown that the use of this nonredundant code may improve the bit error rate (BER) performance of pilot aided modulation schemes over similar uncoded coherent and noncoherent systems. Miller coded random data have a spectral density about 12 dB lower than uncoded data at and around the unmodulated carrier frequency. The use of this code reduces the interference between the data spectrum and the pilot tone, and hence improves the BER performance. This improvement is significant, particularly in low bit-rate systems where a relatively wide bandpass filter is required for pilot extraction and successful compensation for fading and Doppler shift caused phase variations. This improved BER performance is obtained with no redundancy, but at the cost of some spectral efficiency. >

Evaluation of VHF FM, SSB, and ACSSB radio systems in the interference context of the land mobile bands

Abstract: Experimental evaluation to compare VHF land-mobile FM, adjacent channel single sideband (ACSSB), and SSB systems is conducted. Information such as adjacent and cochannel protection ratios, reuse distances, audio frequency responses, etc., are obtained, and the main factors influencing the calculation of spectrum efficiency in the land mobile bands are determined. In relation to protection ratio measurements, conventional objective and subjective evaluation methods and interference criteria are evaluated. An objective method that produces results similar to the subjective method is derived. Factors influencing the introduction of ACSSB, SSB, or other technologies in the land mobile bands are presented. >

Spectrum efficiency of multi-carrier switching TDMA microcellular communication systems

Abstract: The spectrum efficiency of microcellular digital systems under high-density traffic and indoor propagation environments is described. The number of necessary channels is evaluated for given traffic densities and several indoor propagation models related to room size. It is shown that more than one hundred channels are required for one system under heavy traffic density conditions, for example 20000 erl/km/sup 2/. Multicarrier switching time-division multiple access (TDMA) radio access, which shares carriers and timing as slots, is effective in interference limited conditions. The improved channel efficiency of carrier-switching TDMA, about double that of conventional TDMA, is confirmed by computer simulation. >

MSAT voice modulation considerations

Abstract: The challenge for Mobile satellite (MSAT) voice services is to provide near toll quality voice to the user, while minimizing the power and bandwidth resources of the satellite. The options for MSAT voice can be put into one of two groups: Analog and Digital. Analog, nominally narrowband single sideband techniques, have a shown robustness to the fading and shadowing environment. Digital techniques, a combination of low rate vocoders and bandwidth efficient modems, show the promise of enhanced fidelity, as well as easier networking to the emerging digital world. The problems and tradeoffs to designers are many, especially in the digital case. Processor speed vs. cost and MET power requirements, channel coding, bandwidth efficiency vs. power efficiency etc. While the list looks daunting, in fact an acceptable solution is well within the technology. The objectives are reviewed that the MSAT voice service must meet, along with the options that are seen for the future.