In this paper we study the opportunistic use of microwave satellite signals in the Ku and Ka bands to detect the presence of rain events and to estimate their intensity. This approach, based on measuring the excess signal attenuation due to rain, has the advantage that the potential number of sensors over the territory could be increased dramatically also allowing prospectively the realization of accurate regional rain maps. Moreover, employing the same satellite network whose signals are opportunistically used, the measurements can be easily fed back and universally distributed, contributing to the effective implementation of a nowcasting platform, which could help the timely detection of catastrophic rain events. Results are very promising and show that the already good estimates obtained by using signals in the Ku band can be further improved by using signals in the Ka band. Unfortunately, as of today, the frequency with which the receivers on the Ka band provide signal measurements is too low for fast changing rain events and how to address this problem is the object of future research.

This paper focuses on the issue of carrier frequency synchronization for filtered multitone wireless transmission over time-frequency selective fading channels. Rather than either relying only on known pilot symbols multiplexed within the transmitted burst or exploiting the specific signal structure in a blind mode, the estimation algorithm we pursue is derived from the maximum likelihood principle and takes advantage of both pilot symbols and also the unknown information-bearing symbols through specific differential decision-directed processing. When compared to conventional pilot-based methods, the proposed approach improves the frequency acquisition range without degrading estimation accuracy at even lower cost in terms of computational complexity.

This paper deals with carrier frequency offset recovery for burst-mode filter-bank multicarrier transmission over channels affected by severe time-frequency selective fading. Unlike previously proposed algorithms, whereby frequency is recovered either relying on known pilot symbols multiplexed with the data stream (pilot-aided, or PA, approach), or exploiting specific properties of the multicarrier signal structure in a non-pilot-aided (NPA) fashion, here we present and discuss an algorithm based on the maximum likelihood principle, which can be qualified as combined PA/NPA since it takes advantage both of pilot symbols and also indirectly of data symbols through knowledge and exploitation of their specific modulation format. The algorithm requires the availability of the statistical properties of channel fading up to second-order moments. It is shown that the above approach allows to improve on both estimation accuracy and frequency acquisition range of previously published schemes.

In this paper we deal with carrier frequency estimation for filter-bank multicarrier burst transmission over time-frequency selective fading channels. Three open-loop algorithms are discussed, all based on the use of known pilot symbols. The first scheme is derived from the maximum likelihood principle, while the second one consists of its low-SNR approximation, and both of them require knowledge of the fading covariance matrix. The third scheme is ad-hoc and is based on the well-known delay-multiply concept. Theoretical analysis and simulation results are employed to assess the above algorithms in a time-frequency selective mobile wireless scenario.

This paper deals with receiver design issues for filter-bank multicarrier burst transmission over time- and frequency selective fading channels, with emphasis on synchronization and channel estimation aspects. Specifically, the proposed symbol timing recovery algorithm relies on the transmission of a short known preamble, whilst carrier synchronization is implicitly carried out by a properly modified Bayesian channel estimation algorithm relying on pilot symbols uniformly spaced throughout the subcarriers. The above schemes are assessed and their impact on the receiver performance in terms of frame error rate is evaluated in typical mobile wireless channel conditions. In particular, the sensitivity of the receiver to a frequency offset affecting the input signal is investigated.

The TETRA enhanced data service (TEDS) standard has been developed within the Technical Committee TETRA of the European Telecommunications Standards Institute as a major upgrade to the existing narrow-band ETSI TETRA system to supply professional users with high-speed IP packet data services over wireless mobile channels. TEDS includes a number of innovative key features such as multicarrier-based modulation format, powerful payload and header encoding, link adaptation and others. The aim of this paper is to give a comprehensive overview of the TEDS standard with particular emphasis to the physical layer features. Upper layer protocols are treated in a companion paper. Representative link performance results obtained via computer simulations are given for typical wireless environment setups.

Many frequency-recovery algorithms for orthogonal frequency-division multiplexing systems are geared to recover offsets up to half the distance of the subcarriers. If the error is larger, an ambiguity of an integer number of subcarrier spacings is left. This paper develops a blind algorithm for ambiguity resolution that works effectively over frequency-selective radio channels. Simulation results show a sizable improvement over existing techniques, expecially at high signal-to-noise ratios.

This paper introduces a blind (i.e., data independent) algorithm for carrier frequency offset recovery in an orthogonal frequency-division multiplexing (OFDM) receiver operating over frequency-selective fading channels. The main idea behind this algorithm is to exploit the time-frequency-domain exchange inherent to the modulation scheme. Due to this feature, a carrier frequency offset has a similar impact on OFDM as a clock timing offset has in a quadrature amplitude modulation (QAM) system. The scheme we propose is a variant of Oerder-Meyr's (1988) feedforward clock recovery. Its performance is assessed by simulation, and the results are compared to those obtained from Van de Beek-Sandell-Borjesson's (1997) frequency synchronizer, which bears comparable complexity. The new scheme is shown to outperform the latter over frequency-selective fading channels, notably at medium to high signal-to-noise ratios. We also evaluated the efficiency of two different (time domain and frequency domain) offset correction strategies embedded in a particular OFDM receiver.

This paper deals with the analysis and design of efficient non-data-aided clock and carrier (frequency/phase) synchronization algorithms intended for use in satellite digital video broadcasting systems employing turbo-coding techniques to enhance the power efficiency. The above issue appears quite challenging in view of both the extremely low signal-to-noise ratio, typical of turbo-codes operation, and the very short time allocated to the acquisition of synchronization parameters. These constraints rule out most conventional clock/carrier recovery schemes and demand a careful search for specific, highly efficient algorithms. We propose and analyze a clock/carrier synchronization scheme capable of operating at values of E/sub b//N/sub o/ as low as 1 dB with lock-in delay not exceeding 50 ms.

One key requirement for radio access in advanced, third generation (3G) mobile communication systems is the ability to accommodate a variety of services via a flexible and efficient medium access control (MAC) protocol. The paper presents and evaluates a new multiple access protocol termed CRDA (collision resolution and dynamic allocation), which has the potential to meet the above requirement. CRDA is basically a slotted packet-reservation multiple access technique with dedicated reservation slots, which allows the main shortcoming of previous PRMA schemes, i.e., contention, to be overcome through the integration in the MAC protocol of a code-division multiple-access (CDMA) transmission mode used to access the reservation slots. This prevents collisions during the reservation phase and enhances channel throughput, notably in the case of mixed voice/data traffic. Our simulations of the CRDA MAC integrate voice channels with data sources, generating what we call advanced data traffic, which has a very similar shape to the actual traffic generated by World Wide Web (WWW) applications. The standard assumption of Poissonian data traffic is also considered. Our conclusion is that the CRDA MAC protocol satisfactorily accommodates both types of traffic.

The linearization technique known as amplitude and phase (A&P) predistortion, proposed by D'Andrea and Lottici, is applied to the orthogonal frequency-division multiplexing (OFDM) transmission context with nonlinear radio-frequency high-power amplification. The above technique is shown to provide a major enhancement in power efficiency in comparison with the unprotected system, as well as a nonnegligible gain over an alternative linearization strategy, identified as minimum mean-square-error (MMSE) predistortion, presented in the literature for application to OFDM. The relative performance of the A&P and the MMSE predistorter schemes is assessed over the additive white Gaussian noise channel and also in a frequency-selective fading environment. The impact of adjacent channel interference is also discussed.

The linearization technique known as amplitude and phase (A&P) predistortion, proposed by D'Andea et al. (1996), is applied to the OFDM transmission context with nonlinear RF high-power amplification. The above technique is shown to provide a major enhancement in power efficiency in comparison with the unprotected system, as well as a nonnegligible gain over an alternative linearization strategy, identified as minimum mean square error (MMSE) predistortion, presented by Di Benedetto and Mandarini (1995) for application to OFDM (Benedetto and Mandarino 1996). The relative performance of the A&P and the MMSE PD schemes is assessed over the AWGN channel and also in a frequency-selective fading environment.

We focus on the use of Cramer-Rao-like bounds for the estimation of a deterministic parameter in the presence of random nuisance parameters. As a working example, we consider the problem of estimating the carrier frequency offset affecting a linearly modulated waveform received through a Rician fading channel. We calculate the relevant Cramer-Rao bound (CRB) and also a set of Cramer-Rao-like bounds proposed in the literature, among which we mention the hybrid CRB, the modified CRB and the Miller-Chang (1978) bound. An extension of the latter bound is also provided. The relative merits of these bounds are discussed, both in terms of their tightness and ease of calculation. A few useful inequalities involving the above bounds are also established.

This paper deals with symbol timing recovery for multirate transmission systems employing wavelets as signaling pulses either at baseband or with linear bandpass modulation. Our analysis is twofold. First, we investigate the impact of clock errors on the bit-error rate of the optimum receiver for a wavelet-based multirate signal. Second, we consider a nondata-aided maximum-likelihood clock synchronization scheme implemented as a multiple delay-locked loop. We derive a linearized loop model and assess its performance in terms of tracking jitter variance. The analytical results are contrasted with those obtained by simulation and with the relevant modified Cramer-Rao bound.

This paper is concerned with the estimation of the rate-of-change of the instantaneous frequency of an unmodulated carrier. Such an issue arises, for instance, in satellite communication systems based on a low-Earth-orbit constellation, wherein the speed of each satellite relative to earth is rapidly time-varying, and consequently, the received carrier is affected by nonnegligible Doppler shift and Doppler rate. First, we present simple yet efficient estimators of the Doppler rate derived from maximum-likelihood estimation theory. Next, we investigate their performance in terms of bias and estimation error variance. Finally, we compare our variance results with the relevant modified Cramer-Rao bound.

A novel detection algorithm for orthogonal frequency division multiplexing (OFDM) signals transmitted over frequency-selective channels is introduced and analyzed. Simulation results show that the proposed detection strategy, implemented via a standard Viterbi algorithm, provides improved performance over differential detection with a moderate increase in receiver complexity, and without requiring the periodic transmission of a training block.

A novel equalization/detection algorithm for orthogonal frequency division multiplexing (OFDM) signals transmitted over frequency-selective channels is introduced and investigated. The algorithm stems from the recognition that the Fourier transform processing inherent in OFDM turns a single wideband frequency-selective channel into a set of correlated narrowband frequency-flat fading channels. This suggests that sequence detection techniques, such as those discussed by Vitetta et al. (see IEEE Trans. Commun., vol.43, p.2750-8, 1995, IEEE Trans. Commun., vol.43, pt.II, p.1256-9, 1995, and Proc. IEEE Commun. Theory Mini-Conf (Globecom '96), London, UK, p.153-7, 1996), for time-selective flat-fading channels, can be also profitably utilized for joint equalization and decoding of OFDM signals in the frequency domain. Simulation results show that the proposed detection strategy, implemented via a standard Viterbi algorithm, provides improved performance over differential detection, with a moderate increase in receiver complexity and without requiring the periodic transmission of training blocks.

The context of this paper is parameter estimation for linearly modulated digital data signals observed on a frequency-flat time-selective fading channel affected by additive white Gaussian noise. The aim is the derivation of Cramer-Rao lower bounds for the joint estimation of all those channel parameters that impact signal detection, namely, carrier phase, carrier frequency offset (Doppler shift), frequency rate of change (Doppler rate), signal amplitude, fading power, and Gaussian noise power. Time-selective frequency-flat fading is modeled as a low-pass autoregressive multiplicative distortion process. In particular, the important case of "slow" fading, with the multiplicative process remaining constant over the whole data burst, is specifically discussed. Asymptotic expressions of the bounds, valid for a large observed sample or for high signal-to-noise ratio (SNR), are also derived in closed form. A few charts with numerical results are finally reported to highlight the dependence of the bounds on channel status (SNR, fading bandwidth, etc.).

In this paper we extend the scalar modified Cramer-Rao bound (MCRB) to the estimation of a vector of nonrandom parameters in the presence of nuisance parameters. The resulting bound is denoted with the acronym MCRVB, where "V" stands for "vector". As with the scalar bound, the MCRVB is generally looser than the conventional CRVB, but the two bounds are shown to coincide in some situations of practical interest. The MCRVB is applied to the joint estimation of carrier frequency, phase, and symbol epoch of a linearly modulated waveform corrupted by correlated impulsive noise (encompassing white Gaussian noise as a particular case), wherein data symbols and noise power are regarded as nuisance parameters. In this situation, calculation of the conventional CRVB is infeasible, while application of the MCRVB leads to simple useful expressions with moderate analytical effort. When specialized to the case of white Gaussian noise, the MCRVB yields results already available in the literature in fragmentary form and simplified contexts.

We analyze a cellular asynchronous code-division multiple access system operating in the 63-64 GHz band for short-range communications. A simple propagation model for the 63-64 GHz band is first introduced and discussed. The system capacity, expressed as the maximum number of users per cell, is then analytically derived and favorably compared with other CDMA schemes operating in conventional frequency bands (i.e. below 2 GHz). Also, simple expressions for the system bit error rate are analytically derived for different cases and validated through computer simulations. Finally, power requirements are derived and the impact of atmospheric conditions is evaluated. It turns out that the use of 63-64 GHz band significantly increases the capacity of A-CDMA, and proves to be suitable for cellular systems and personal communication networks (PCNs) provided that adequate power margins are considered.

This paper is concerned with the design and performance evaluation of a direct-sequence spread-spectrum transmission and signal-detection technique for application to code-division multiple-access mobile radio networks. Unlike the conventional differential encoding/detection of data symbols, such a technique envisages differential encoding/detection of the spreading code chips to counteract the fast time-selective fading encountered in mobile radio transmissions. Binary PSK modulation on a Rician frequency-flat-fading channel is assumed, with Gaussian and Rayleigh channels as particular cases. The bit-error rate performance of the receiver is analytically evaluated as a function of the relevant system parameters, and the impact on the receiver performance of a carrier frequency offset is also investigated.

We present a fundamental lower bound to the mean-square error of estimators of the phase of a sine wave passed through a Rician-fading channel. Explicit expressions for the bound with Gaussian and Rayleigh channels are provided. The bound is derived by analyzing the performance of the Bayes minimum mean-square error phase estimator, and therefore represents the best performance attainable by physically realizable phase estimators over a Rician channel.

This paper presents a novel technique for power amplifier linearization in digital microwave radio systems. The proposed technique is based on the use of a predistortion circuit, whose AM/AM and AM/PM responses are separately implemented as polynomial approximations of the respective responses of the ideal linearizer. The proposed scheme is shown to attain superior performance in comparison with other well-known predistortion structures, such as those based on the cancellation of third or fifth order distortion, with no substantial aggravation in implementation complexity.

We present and analyze a technique for fast acquisition and accurate tracking of the carrier frequency in orthogonal frequency division multiplexing (OFDM) receivers. The scheme is based on a data-aided frequency estimation algorithm. The presence of known symbol sequences periodically inserted in the OFDM frame allows the data demodulator to rapidly lock onto the carrier frequency during the acquisition phase, even in the presence of frequency offsets up to a few tenths of the overall signaling rate. Once acquisition is over, the circuit switches to a decision-directed mode to perform fine frequency tracking for reliable data demodulation. The algorithm performance is analyzed in terms of width of the lock-in frequency range and of lock-in probability in the acquisition mode, and of mean-square frequency estimation error in the tracking mode. Since OFDM is known to be extremely sensitive to carrier frequency errors, the impact of the carrier frequency synchronizer on the receiver error rate is also investigated.

In this paper, we consider the architecture of an all-digital modem for band-limited spread-spectrum radio communication networks employing code-division multiple access with generalized MSK modulated signals. After a general description of the modem scheme, we focus on the problem of spreading code synchronization, presenting three different chip timing recovery loops and analyzing their relative performance in terms of RMS tracking error.>

Reliable data detection in time division multiple access (TDMA) communication systems strictly depends on the availability of accurate estimates of the synchronization parameters of the received signal, i.e., carrier frequency/phase and symbol timing, which must be derived from the burst preamble. The authors focus on the carrier frequency estimation aspect, and present a fast, open-loop, all-digital frequency offset estimation technique, whose performance is assessed in two different communication scenarios: a TDMA satellite link employing standard modulation and burst formats, and a mobile cellular terrestrial radio system with signal and channel characteristics obeying the pan-European Group Special Mobile (GSM) recommendations. The use of the algorithm as a frequency error detector (discriminator) in a recursive ("closed-loop") frequency offset estimator is also discussed, and some results concerning both the transient and the steady-state behavior of such a scheme are presented. Finally, the impact of the algorithm on the receiver BER is briefly analyzed.>

We introduce the modified Cramer-Rao bound (CRB) which, like the true CRB, is a lower bound to the error variance of any parameter estimator. The modified CRB proves useful when, in addition to the parameter to be estimated, the observed data also depend on other unwanted parameters. The relationship between the modified and true CRB is established and applications are discussed regarding the estimation of carrier-frequency offset, carrier phase, and timing epoch in linearly modulated signals. Modified CRBs for phase and timing estimation have been already discussed in previous works where it is shown that several practical carrier-phase and clock recovery circuits do attain such bounds. Frequency discrimination, instead, is not so well-represented in the literature and a significant contribution of this paper is the calculation of the modified CRB for frequency estimation. This bound is compared with the performance of some frequency detectors and it is concluded that further work is needed in search of more efficient frequency discrimination methods.>

This paper addresses the problem of coherent data detection over Rayleigh flat-fading channels. Minimum shift keying and Gaussian minimum shift keying are assumed as modulation formats. After baseband conversion and filtering, the received waveform is processed, in an all-digital fashion, by means of a decision-directed algorithm aimed at estimating and removing the propagation-induced distortion from the signal, so as to allow symbol detection to be effected via conventional coherent techniques. Two different algorithms are presented and their relative performance is assessed, assuming the bit error rate as figure of merit. The effect of stationary clock errors on the receiver performance is also investigated. The results extend and integrate those already available in the literature for linear, nonstaggered modulations.>

The problem of combined coding and modulation is addressed for M-ary continuous-phase frequency-shift-keying (CPFSK) with a redundant symbol alphabet. It is shown that in many cases of interest, the use of a nonuniform mapping between coded bits and phase increments at the CPFSK modulator allows for a significant increase of free Euclidean distance in the coder/modulator state trellis as compared to the uniform alphabet case. A systematic search for optimum encoder/mapper/modulator structures is carried out for short constraint length rate 1/2 and 2/3 codes combined with 4-ary and 8-ary CPFSK, respectively, and for a relevant set of rational modulation indexes. The power-bandwidth efficiency of the optimum schemes is discussed and compared with that exhibited by the best coder/modulator structures previously proposed in the literature.>

This paper presents an analysis of a synchronization circuit to be used for carrier-phase and symbol-timing recovery in continuous phase modulation systems.M-ary modulation formats with arbitrary pulse shaping and rational modulation indexes are assumed. Circuit performance is expressed in terms of variances of phase and timing errors. Numerical results are provided for some important cases, including minimum shift keying and tamed frequency modulation. Theoretical results are found in good agreement with computer simulations.