Analysis of the Parameters of I/Q Signals in Digital Television Systems

Abstract

Algorithms for estimating the parameters of I/Q-signals are described, and the necessary sequence for measuring the parameters of the constellation diagram is investigated. This enables the tolerance on the error of the position of points of the constellation, the amplitude disbalance, the quadrature errors, the relative modulation error, and the phase jitter to be established independently of one another.Digital signals with OFDM modulation have found wide application in modern communications and broadcasting systems [1]. They are used, in particular, in the first and second generation digital terrestrial television broadcasting systems DVB-T and DVB-T2 [2, 3]. An important component of measurements in these systems is an analysis of the I/Qsignal parameters [4]. An I/Q-signal is a signal with digital modulation, represented in the form of a signal constellation in I (the in-phase component) and Q (the quadrature component) coordinates. An analysis of these signals can be applied both to a single carrier signal with OFDM modulation, and to groups of carriers. If we consider groups of carriers, one can obtain a single generation constellation diagram by superimposing all the symbols of a given group. Since several forms of carriers with different modulation systems (information carrying, pilot carrying and transmission parameter carrying) are usually present in a signal with OFDM modulation, it is recommended that the parameters of each form of carrier should be analyzed separately [4].One can measure the following parameters using stellar diagrams: the displacement of the center of the constellationthe vector of the systematic error of each symbol in a cell, the mean error in the position of points of the constellation (the System Target Error Mean, STEM), over all the symbols and its spread (the System Target Error Deviation, STED), the amplitude imbalance (AI), the quadrature error (QE), the relative modulation error (the Modulation Error Ratio, MER), the error vector (the Error Vector Magnitude, EVM), and the phase jitter (PJ).The errors in measuring all these parameters are determined by the measuring systems employed, and depend on the number of points of the constellation and the requirements imposed on the corresponding television-radio broadcasting systems. It is assumed that the clouds in each cell of the stellar diagram are due not only to Gaussian noise, but also to the effects of some other forms of interference, such as low coherent interference or residual channel correction errors. It should be noted that the use of algorithms for estimating these parameters [1] hampers the specification of the requirements imposed on the errors in measuring them, since the results of measurements are independent of one another.