Silvano Pupolin

M-QAM-OFDM system performance in the presence of a nonlinear amplifier and phase noise

The orthogonal frequency-division multiplexing (OFDM) modulation format has been proposed in Europe as the standard for broadcasting both audio and television digital signals and for wide-band wireless communication systems (e.g., HIPERLAN II). The performance of the OFDM scheme is severely affected by the nonlinearity of the high-power amplifier at the transmitter end and by the phase noise of the oscillators. In this paper, we investigate the joint effects induced on the OFDM signal by the amplifier nonlinearity and by the phase noise. An accurate statistical description of each contribution to the signal distortion is provided. Theoretical results show perfect agreement with those obtained by simulation and they can be used to derive the OFDM system performance, without the need to run extensive simulations.

Impact of amplifier nonlinearities on OFDM transmission system performance

The power spectral density of an orthogonal frequency-division multiplexing (OFDM) signal after a saturated high-power amplifier (HPA) is analytically derived. The distortion of the HPA-processed OFDM signal is defined, and its power spectrum is computed. The spectra of the signal and of the distortion are used to get an accurate estimate of the bit-error rate of an OFDM transmission system and to derive compensation at the receiver, which leads to performance improvement.

Performance Analysis of Digital Radio Links with Nonlinear Transmit Amplifiers

This paper evaluates the effects of transmit amplifier nonlinearities on digital radio link performance. Special emphasis is given to links carrying 256-level modulations, for which the impact of even mild nonlinearities can be severe. Performance is measured in terms of the flat fade margin ( F ) normalized by its theoretical maximum (F max ). Curves of F / F_{max} versus amplifier input drive level are used to evaluate and compare numerous design strategies. A major contribution of this work is the development of a fast, accurate method of computer analysis, in contrast to the more cumbersome Monte Carlo simulations generally used for these assessments. Moreover, the new method is used to study a variety of techniques devised to combat the nonlinearity, particularly signal predistortion. It is also used to investigate various approaches to pulse shaping and filtering, to quantify the degradation in performance with increasing numbers of modulation levels, and to examine the possible benefits of linear equalization.

Optimum transmission ranges in multihop packet radio networks in the presence of fading

The evaluation of optimum transmission ranges in a packet radio network in a fading and shadowing environment is considered. It is shown that the optimal probability of transmission of each user is independent of the system model and is p/sub o//spl sime/0.271. The optimum range should be chosen so that on the average there are /spl chi/(G/b)/sup 2//spl eta// terminals closer to the transmitter than the receiver, where G is the spread spectrum processing gain, b is the outage signal-to-noise ratio threshold, /spl eta/ is the power loss factor and /spl chi/ depends on the system parameters and the propagation model. The performance index is given in terms of the optimal normalized expected progress per slot, given by /spl thetav/(G/b)/sup 1//spl eta// where /spl thetav/ is proportional to the square root of /spl chi/. A comparison with the results obtained by using deterministic propagation models shows, for typical values of fading and shadowing parameters, a reduction up to 40% of the performance index. >

Analysis of the uplink of an asynchronous multi-user DMT OFDMA system impaired by time offsets, frequency offsets, and multi-path fading

We study through analysis the joint effect of time offsets, frequency offsets, and multi-path fading in the uplink of an asynchronous multi-user system for wireless communications deploying discrete multi-tone modulation and demodulation. We derive analytical expressions for the multiple access interference, and we quantify its detrimental effects through the evaluation of both the average SINR, and the symbol-error-rate performance. As a result it is shown that the MAI strongly depends not only on the aforementioned impairments but also on the tone assignment algorithm used to multiplex the users. Based on our analysis, the insertion of appropriate time and frequency guard intervals and the accurate selection of the tone assignment algorithm effectively reduce the MAI, so that a proper trade off with spectral efficiency can be met to optimize system performance.

Nonlinearity compensation in digital radio systems

Digital radio links with bandlimited pulses exhibit a severe performance degradation when the transmitter high power amplifier operates near saturation. To cope with the increase of nonlinear intersymbol interference due to the amplifier nonlinearities, a discrete-time Volterra system can be used to process the transmitted data. We present an efficient technique for implementing adaptive data predistorters with memory based on a discrete-time Volterra system composed of digital linear filters and memoryless nonlinear devices working at the symbol rate. Third- and fifth-order structures are proposed and a system performance evaluation is presented for several realistic situations. >

Outage probability in multiple access packet radio networks in the presence of fading

The outage probability in a mobile communications environment, where the interferers are randomly distributed on the ground, taking into account the background noise, the cochannel interference and the random effects of propagation (log-normal shadowing and Rayleigh fading), is evaluated. Although not feasible in closed form, the computation can be performed numerically in a very fast and accurate manner. A comparison to the results reported in the recent literature is presented, along with a sensitivity analysis and an example of application. >

Power Control for Cognitive Radio Networks Under Channel Uncertainty

Cognitive radio (CR) networks can re-use the RF spectrum licensed to a primary user (PU) network, provided that the interference inflicted to the PUs is carefully controlled. However, due to lack of explicit cooperation between CR and PU systems, it is often difficult for CRs to acquire CR-to-PU channels accurately. In fact, if the PU receivers are off, the sensing algorithms cannot obtain the channels for the PU receivers, although they have to be protected nevertheless. In order to achieve aggressive spectrum re-use even in such challenging scenarios, power control algorithms that take channel uncertainty into account are developed. Both log-normal shadowing and small-scale fading effects are considered through suitable approximations. Accounting for the latter, centralized network utility maximization (NUM) problems are formulated, and their Karush-Kuhn-Tucker points are obtained via sequential geometric programming. For the case where CR-to-CR channels are also uncertain, a novel outage probability-based NUM formulation is proposed, and its solution method developed in a unified fashion. Numerical tests verify the performance merits of the novel design.

Slotted ALOHA for high-capacity voice cellular communications

Slotted ALOHA is proposed as a multiple access scheme for high capacity voice cellular communications in mobile radio environment. The performance of such a system, in the presence of fading and shadowing, is evaluated for both mobile-to-base and base-to-mobile links, in terms of number of supported conversations per cell, under some constraints on maximum tolerable delay. The numerical results show that a system of this sort can compete with other multiaccess schemes currently considered, such as TDMA, FDMA, and even CDMA. A heuristic stability analysis is also presented, showing that the proposed system does not suffer from instability problems. >

Performance of single user detectors in multitone multiple access asynchronous communications

We consider a multitone multiple access wireless system where users are multiplexed by the assignment of sub-sets of the available tones. The information signals of distinct users are multicarrier modulated and propagate through asynchronous frequency selective fading channels, i.e., uplink. In this scenario intersymbol, intercarrier, and multiple access interference arise. We consider simple single user detection and study its performance in terms of outage probability. In particular we investigate the effect on outage probability of the choice of the sub-channel transmit filters and the tone allocation strategy.