Analysis of the Effect of the I/Q Base-Band Filter Mismatch in an OFDM Modem

In digital communication modems in which a very high rate system clock is used, it is necessary to use analog base-band shaping filters in the inphase (I) and quadrature (Q) paths of the modulator. However, this type of implementation inherently produces a mismatch of the I and Q paths. In the present paper, results of the analysis of the transmitter (TX) I/Q mismatch in an Orthogonal Frequency Division Multiplexing (OFDM) system with Differential Coherent Quadrature Phase Shift Keying (DQPSK) modulation is presented. Theoretical analysis shows that the Signal-to-Noise (SNR) degradation due to the I/Q mismatch can be represented by a mismatch transfer function on the basis of which one can compute the maximum affordable amplitude and phase mismatch of the TX filters transfer functions.     

Mesures en impulsions pour faisceaux hertziens en visibilité directe

This work is providing an experimental tool for investigating the transmission characteristics of the propagation path of line-of-sight radio relay links in the 6 GHz band, resorting to a direct observation of its impulse response. Two circuit models are respectively representing the propagation characteristics (multipath scintillation) and the action upon transmitted signals. The timevariant impulse response is the most fitting system function for both propagation anomalies identification and message impairment’s evaluation. A measuring equipment for direct observation of the impulse response in the signal bandwidth is described. The data processing procedures leading to relative accuracies which are indicated satisfactory in all practical cases. Some experimental results are shown.     

Some Theoretic Results on a Digital EMG Signal Processor

As is well known, a general EMG signal processing scheme includes a prewhitening filter, a v-order detector, a smoothing filter, and a relinearizer. In this paper, a theoretical analysis of such a scheme is carried out, including a detector characteristic which presents a logarithmic behavior.     

Analysis of phase noise effects in OFDM modems

The effects of the phase noise (PN) on orthogonal frequency-division multiplex modems are evaluated. Three receivers are studied: a coherent receiver, a common phase error correction receiver (which is a receiver specially designed to combat PN) and a differential receiver. The impact of the PN on the decision signal-to-noise ratio (SNR) of each of these receivers is computed as a function of the PN spectrum. The resulting formulas are extremely simple. The theory is applicable to a wide range of PN models, and unifies and extends previous results on the topic. The conditions under which the decision SNR yields correct symbol error rate predictions are discussed. Simulations are reported that confirm the results.     

Orthogonal frequency division multiplex synchronization techniquesfor frequency-selective fading channels

The effect of time-domain and frequency-domain synchronization errors is quantified in the context of various coherently and noncoherently detected 1, 2, and 4 bits/symbol OFDM constellations, in order to demonstrate the wide applicability of the techniques proposed for mitigating the bit error rate (BER) performance degradations inflicted. A reference symbol is proposed and a range of correlation techniques are suggested for coarse and fine synchronization. Their performance is studied over time-dispersive Rayleigh fading channels, with the conclusion that the proposed synchronization techniques result in virtually unimpaired BERs over the range of wideband channels investigated in comparison to a perfectly synchronized system     

An application of MMSE predistortion to OFDM systems

Orthogonal frequency division multiplexing (OFDM) particularly suffers from the presence of nonlinearities since the signal amplitude is Rayleigh distributed. The degradation introduced by the nonlinear amplifier in the transmitter can be significantly reduced by using an analog cubic predistorter minimum mean square error (MMSE) predistorter proposed by the authors (see ibid., vol.43, no.12, p.2966, 1995)     

A new analog predistortion criterion with application to highefficiency digital radio links

Describes a new approach for designing analog cubic predistorters in view of minimizing nonlinear distortion introduced by the high power amplifier (HPA) in microwave radio links. The approach is based on a global mean squared error minimization between the input to the predistorter and the output of the power amplifier, rather than on the elimination of the cubic term. In this manner, two advantages are obtained. Firstly, the off-band distortion terms are reduced so that it is possible to eliminate the HF transmission filter. Secondly, the interference produced by the nonlinear distortion can be uniformly distributed over the entire constellation so that it is possible to reduce the back-off value of the HPA. Results of the application of the method proposed to some high spectrally efficient QAM systems, such as the 512-QAM system, are presented