Digital Cellular Radio Research Articles

Steven V. Abramson: An OLED Unicorn Setting a Shining Example for the Industry

Steven V. Abramson: An OLED Unicorn Setting a Shining Example for the Industry

Performance analysis of decision-feedback equalisation for cellular mobile radio with cochannel interference and fading

The error probability of minimum-mean-square-error decision-feedback equalisation (MMSE-DFE) is evaluated for digital cellular mobile radio systems in the presence of cochannel interference (CCI) and is compared with linear equalisation (LE). The main contribution of the paper is that this analysis accounts for pulse waveform, modulation and fading of the signal of interest, as well as the CCI. Quadrature-amplitude modulation (QAM) signalling in frequency-selective and quasi-static channels is considered. The CCI is treated as a stationary process, when caused by random phase and symbol-timing offsets relative to the signal of interest. Analysis includes techniques combining antenna diversity. The performance improvement as a function of taps in both feedforward and feedback filters is quantified. Owing to residual intersymbol interference (ISI) and CCI, the evaluation of the error probability is extremely complicated and time consuming in simulation. To overcome this issue, an efficient method based upon Gauss quadrature rules (GQR) is presented to compute the error probability. The method is not limited due to interference statistics and it yields remarkable advantages compared with other methods. The convergence of finite-length results to their infinite-length counterparts is also provided. Unlike the case of white noise, the simulations reveal that with the same finite length the DFE is unable to outperform the linear equaliser in a CCI-dominated channel if the feedback filter is of insufficient length.

Average error probability of digital cellular radio systems using mrc diversity in the presence of multiple interferers

The performance of digital cellular radio systems employing maximal ratio combining diversity is analyzed in a flat-fading channel with cochannel interference and additive white Gaussian noise. It is assumed that the desired signal may experience Rice fading (due to the presence of a line-of-sight component), while the interferers are Rayleigh-faded and may have similar or dissimilar average powers. Exact expressions are derived for the average symbol-error probability of M-ary phase-shift keying modulation in the presence of multiple independent Rayleigh-faded interferers.

Linear and non‐linear space diversity combining algorithms over fading channels in TDMA systems

AbstractThis paper investigates the performances of various adaptive algorithms for space diversity combining in time division multiple access (TDMA) digital cellular mobile radio systems. Two linear adaptive algorithms are investigated, the least mean square (LMS) and the square root Kalman (SRK) algorithm. These algorithms are based on the minimization of the mean‐square error. However, the optimal performance can only be obtained using algorithms satisfying the minimum bit error rate (BER) criterion. This criterion can be satisfied using non‐linear signal processing techniques such as artificial neural networks. An artificial neural network combiner model is developed, based on the recurrent neural network (RNN) structure, trained using the real‐time recurrent learning (RTRL) algorithm. It is shown that, for channels characterized by Rician fading, the artificial neural network combiners based on the RNN structure are able to provide significant improvements in the BER performance in comparison with the linear techniques. In particular, improvements are evident in time‐varying channels dominated by inter‐symbol interference. Copyright © 2001 John Wiley & Sons, Ltd.

Outage probability of cellular radio systems usingmaximal ratio combining in Rayleigh fading channel with multiple interferers

The performance of digital cellular radio systems with maximal ratio combining (MRC) diversity is analysed in a flat-fading Rayleigh channel with cochannel interference and additive white Gaussian noise (AWGN). Closed-form expressions are derived for the outage probability in the presence of multiple independent Rayleigh faded interferers.

Performance analysis of maximal ratio combining and comparison with optimum combining for mobile radio communications with cochannel interference

The performance of maximal ratio combining for space diversity reception in digital cellular mobile radio systems is studied for communications in the presence of multiple cochannel interference (CCI) sources and is compared to optimum combining. The main contribution of the paper is that the analysis accounts for fading of the signal of interest (SOI) as well as the cochannel interference (CCI). The paper considers BPSK signalling in flat, quasi-static channels. Rayleigh or Rice fading is assumed for the SOI, while CCI is assumed subject to Rayleigh fading. Channels associated with interference sources are assumed independent and identically distributed. Using a multivariate statistical analysis approach and assuming equal-power interference sources, analytical expressions are derived for the density function of the array output signal-to-interference ratio (SIR), the outage probability, and the average probability of bit error with maximal ratio combining. Earlier results obtained for optimum combining and Rayleigh fading are extended to the case when the SOI is subject to Rice fading. A limited analysis of the equal gain combiner is also presented.

Performance analysis of digital cellular radio systems in Nakagami fading and correlated shadowing environment

The bit error rate (BER) performance of digital cellular radio systems was investigated in a Nakagami (1960) fading, correlated lognormal shadowing and additive white Gaussian noise (AWGN) environment for noncoherent differential phase shift keying (DPSK) modulation. Two models were used to determine the BER; the first one is based on the cumulative power levels of cochannel interferers while the second one is based on instantaneous cochannel interference power. The relative advantages of the two models were presented for various design parameters. The effects of bit energy to noise ratio, frequency reuse distance, cluster size, correlation coefficient, shadow spread, and fading parameter were studied. The BER was observed to be lower in a correlated shadowing environment compared with the uncorrelated case. The near-far effect was studied by assuming that both the desired and interfering mobiles are randomly located in their corresponding cells.

Separation of cochannel GSM signals using an adaptive array

The Global System for Mobile communications (GSM) is a digital cellular radio network that employs time division multiple access (TDMA). In such a cellular system, frequencies are reused in different regions for spectral efficiency, and thus, the transmissions in a given cell can interfere with those in distant cells. This cochannel interference can be a major impairment to the signal of interest. In this paper, we describe a beamformer and equalizer system that is capable of separating and demodulating several cochannel GSM signals. The signal model includes intersymbol interference (ISI) induced by the Gaussian transmit filter, and the channel model incorporates multipath propagation and additive white Gaussian noise. The GSM synchronization sequences are used to compute the beamformer weights and achieve frame synchronization simultaneously. Decision-feedback equalization is employed to compensate for the transmit filter ISI and to demodulate the data.

Diversity transmission and adaptive MLSE for digital cellular radio

Adaptive equalization at the mobile in conjunction with diversity transmission at the base station can combat the adverse effects of both frequency-selective and frequency-nonselective fading at the mobile receiver. This is suggested as an alternative to diversity combining at the mobile. Thus, a substantial saving in the mobile receiver's implementation cost and power consumption is expected. In this paper, a time-domain multiple access system with two transmitting antennas at the base station and maximum-likelihood sequence estimation at the mobile is considered. Per-survivor processing (PSP), a combined channel-tracking and sequence-estimation algorithm, has been used for improving the system performance. System performance results in the presence of severe Doppler frequency shifts and additive white Gaussian noise in a flat fading channel are presented. The computational complexity and acquisition capabilities of the PSP algorithm are also presented.

Communications [Technology 1998 analysis and forecast

The communications industry is rampaging ahead and metamorphosing so rapidly as to defy comprehension. Wireless technology is all over the place-from globe-spanning satellite systems to local-area networks. The Internet, and above all the World Wide Web, race ahead exponentially, burdening the circuit-switched public voice telephone network and stirring a hunger for faster public data networks. Two other areas where events are challenging assumptions fundamental to current nationwide systems are deregulation and privatization. Immense business deals are being negotiated and then renegotiated as competing visions of the future conduce to differing notions of corporate valuation. And pundits can be found all over the media explaining to each other why everyone else has failed to grasp the true meaning and vast implications of the communications revolution. Constantly cited as a major force behind the excitement is the need for multimedia, though it is not always clear what it is, who needs it, and what for. In the communications context, it is simply a way of describing a broadband system that is good at handling both bursty and constant-bit-rate data. Global wireless access, digital cellular radio, a satellite network for Internet access in space, wireless access for fixed subscribers, and making local access easier are discussed.

Communications — Summary

The liberalisation of telecoms in the UK has had great benefits but BT is prevented from connecting optical fibre to each house. Digital cellular radio can deliver 1000 bits/s. Information Communications Systems include data, cabling and equipment to process voice, text, data and graphics. By the year 2005 optical fibre will be affordable in the workplace removing the current limitations on bandwidth. The BRICC project recommended more cooperative working. Business, ICS and manufacturing are likely to converge in the future.

Variable-rate CELP based on subband flatness

Code-excited linear prediction (CELP) is the predominant methodology for communications quality speech coding below 8 kbps, and several variable-rate CELP schemes have been discussed in the literature, including QCELP, the variable-rate wideband digital cellular mobile radio speech coding standard specified in IS-95. A key component of these speech coders is the detection and classification of speech activity, and several cues for rate variation have been studied, such as measuring the short-term speech energy, deciding whether the speech is voiced or unvoiced, or making more sophisticated phonetic classifications. We present a new method for rate variation based on a measure of subband spectral flatness, called spectral entropy. Spectral entropy is a normalized indicator of the texture of the input spectrum and is thus less dependent on speech and background noise energy variations. We present some results on the use of spectral entropy for voice activity detection across subbands and then evaluate using spectral entropy for deriving mode and rate allocation cues for a variable-rate CELP coder operating at an average rate of 2 kbps. To achieve communications quality speech at this rate, we develop a new split-band vector quantization (VQ) technique for representing the line spectral pairs and a multiple codebook approach for efficiently quantizing the coefficients of a three-tap pitch predictor, called lag-indexed VQ.

Feasibility of ReproSystem in Indonesia

This paper describes Indonesias telecommunications system and discusses the feasibility of adopting the computer-based system MomCare for extending reproductive health care expertise to remote sites. The paper was presented at a pre-congress seminar entitled Learning Without Walls that was held on October 13-14 1995 in Bali Indonesia in conjunction with the XVth Asian and Oceanic Congress of Obstetrics and Gynecology. A small group of decision makers was given the opportunity to update its knowledge of the information revolution and to explore the possible impact on health care education and training. The MomCare system allows experts at referral centers to work with clinicians at remote sites using real-time transfer of video voice images and computer files. Telemedicine transfers allow for video conferencing still or slow motion image transfer electronic mail text dialogue white boarding and file transfer functions. Indonesias communication system includes a local telephone network; a long-distance network linked by fiber optic cables and coaxial cables microwaves submarine cables and satellites; and an international network of over 3770 circuits. Other facilities include the Packet-Switched Data Network based on digital transmission cellular mobile radio services and radio trunking. State-of-the-art systems can transmit data at rates up to 2400 bps and achieve high-quality nonfading links over short distances and non-line-of-sight paths. Indonesia is continuing to expand links increase transmission speeds and use data compression techniques at lower bandwidths in order to facilitate communication exchanges. The Johns Hopkins University has already demonstrated the utility of a system of inexpensive workstations to provide medical care. MomCare is feasible in Indonesia. The only question is the extent and speed with which to transfer the technology.

Performance evaluation of non-linear transmit power amplifiers for North American digital cellular portables

This paper discusses the system performance degradation resulting from using a non-linear transmit power amplifier (PA) in a North American digital cellular (NADC) portable radio. The NADC portables require a linear power amplifier for the spectrally efficient linear modulation scheme, /spl pi//4-DQPSK. It is well known that these PA's can exhibit non-linear characteristics that degrade system performance. System degradation is measured using bit error rate (BER) as the figure of merit. The PA is modeled by its AM-AM and AM-PM characteristics; the receiver architecture contains the simple, yet robust, differential detector. Simulation results are presented for additive white Gaussian noise (AWGN), Rayleigh fading, and frequency selective fading environments, and compared to results for a truly linear PA. In addition, PA spectral regrowth is discussed as well as the potential use of a linearization technique to compensate for the PA effects.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Digital tanlock loop for tracking π/4-DQPSK signals in digital cellular radio

A digital tanlock loop (DTL) is a nonuniform sampling digital phase-locked loop (DPLL) whose phase error detector, using the tan/sup -1/[/spl middot/] function with in-phase and quadrature samples of incoming signal, has a linear characteristic with a period of 2/spl pi/. In this paper, we study the application of an N-phase DTL in digital cellular radio with /spl pi//4-DQPSK modulation. The performance degradation due to fading effect is investigated in terms of bit error rate and steady state phase error variance by computer simulation. It is shown that, in the digital cellular radio system, the performances on both bit error rate and phase error variance using DTL are far better than that using the traditional digital N-phase I-Q loop.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Pilot-symbol aided coherent M-ary PSK in frequency-selective fast Rayleigh fading channels

Pilot-symbol aided coherent M-ary PSK modems in digital cellular mobile radio systems are analyzed theoretically. The error-floors caused by the Doppler spread in a fast fading channel are removed in both flat and selective fading channels. However, the error-floors caused by the delay spread are lower-bounded by those that exist in the ideal coherent detection. The systems are modeled as frequency-selective fast Rayleigh fading channels, corrupted by co-channel interference (CCI) and additive white Gaussian noise (AWGN). In the proposed scheme, pilot symbols are inserted periodically to monitor the channel characteristics. The fading processes experienced by the pilot symbols are used to estimate those suffered by the data symbols using interpolation or filtering. The estimated fade characteristics are used to compensate the random phase variation caused by the Doppler spread, so that the signals can be demodulated coherently. The theoretical performances of the fade compensated coherent modems are evaluated. The results show that the fade compensated coherent demodulation with the least redundancy achieves the same performance as the ideal differential detection in a fading channel. The performance approaches that of the ideal coherent demodulation as more redundancy is allowed. The pilot-symbol-insertion (PSI) scheme is also applicable to M-ary QAM modems and Rician channels. The residual frequency offset can also be compensated by the PSI technique.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Forward-error-control (FEC)-assisted adaptive equalization for digital cellular mobile radio

An adaptive equalization technique using forward error control coding is described and evaluated. Computer simulation is used to compare the FEC-assisted adaptive equalization to conventional adaptive equalization. The system model employed is based on the proposed digital cellular communication standard for North America. The FEC-assisted adaptive equalization technique performs better than the conventional equalization method at low and moderate Doppler frequencies with the same level of redundancy in the transmitted data. For systems employing both FEC coding and adaptive equalization, the FEC-assisted adaptive equalization method successfully combines these two functions while using the redundancy required for only one of them. In other words, training required for adaptive equalization can be provided at no extra cost in terms of data redundancy in a system using FEC coding and vice versa.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

MMSE diversity combining for wide-band digital cellular radio

A study of minimum mean-square error (MMSE) diversity combining for wide-band digital cellular radio, designed to combat intersymbol interference (ISI) caused by frequency selective fading is outlined. The systems analyzed use binary phase-shift keying (BPSK), quarternary phase-shift keying (QPSK) or 16-level quadrature amplitude modulation (16-QAM) with cosine rolloff spectral shaping, and space diversity with selection, maximal ratio or MMSE combining. A set of performance curves is presented for selected combining schemes showing the influence of the following system parameters: the diversity order (1 to 4); the cosine rolloff factor; the power delay spectrum (with its associated delay spread); the signal-to-interference ratio; and the number of modulation levels (2, 4 and 16). >

A power-efficient channel coder/decoder chip for GSM terminals

A compact power- and computing-delay-efficient channel codec chip for the Pan-European digital cellular radio (GSM) system is presented. This key component for the hand-portable mobile station, mainly implementing GSM Recommendation 5.03 on a full duplex basis, is accomplished through a dedicated architecture and application tailored memories. An important effort was made to increase the testability of the design; the sequentiality, the low pin count, and the presence of embedded macro functions implied the need for internal scan and BIST techniques. Full scan design and self-test facilities, supported by automatic test pattern generating software, resulted in time- and coverage-efficient testing. The chip is fabricated in a double-metal 1.2- mu m CMOS technology, using a cell-based design approach incorporating memory and programmable array macro blocks. A full-rate speech channel block is decoded in less than 1.8 ms and typical average in-system power consumption does not exceed 10 mW.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Dual diversity combining and equalization in digital cellular mobile radio

The performance of digital data transmission over frequency-selective fading channels is investigated. For statistically independent diversity paths, estimates of average attainable error rates and outage probabilities as functions of system parameters are provided. The dependences among the important system parameters are exhibited graphically for several examples, including quaternary phase-shift keying (QPSK). In the optimized uncoded QPSK with 1.5 b/s/Hz, two orders of magnitude in outage probability can be gained by diversity reception. When one compares the uncoded average probability of error for the optimized mean squared error (MSE) systems one finds at most an order-of-magnitude difference among the different equalizers investigated except for the zero-forcing equalizer, whose performance is drastically inferior to the others. Again, dual diversity can provide two orders of magnitude improvement in the average error rate or in outage probability for the uncoded optimized systems.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>