Quadrature-Amplitude-Modulated Signalling over a Discrete-Multipath Linear Time-Variant Channel

Abstract

<p>In this thesis, a new paradigm is proposed for designing the transmitter and receiver for quadrature-amplitude-modulated signalling over a mobile radio channel. The new paradigm is based on a discrete-multipath linear time-variant model of the mobile radio channel, and hence the title of the thesis. The time-variant input-output relationship of the discrete multipath channel (DMC) is governed by a set of parameters which can be obtained in finite time by probing, that is, by transmitting a pre-assigned signal and then performing computations on the received signal. Therefore, once the parameters of the DMC's input-output relationship have been obtained in this manner, the receiver can, in principle, determine the subsequently transmitted data-carrying signal, or, the data itself, by performing computations on the received signal, which operation is referred to as signalling.</p> <p>Thus, the thesis proposes a philosophy of design based on alternate probing and signalling, and shows that when the transmitted signal is generated by quadrature amplitude modulation (QAM) the composition of QAM and DMC lends itself to this philosophy of design, even in the presence of intersymbol interference (lSI) and additive white Gaussian noise (AWGN).</p> <p>As regards probing, it is shown that by transmitting a suitable quadrature-amplitude. modulated signal all parameters of the DMC, or, rather, of the composition of QAM and DMC, can be estimated in the presence of AWGN. In particular, the maximum-likelihood method of estimation is shown to have the statistical properties needed to justify the philosophy of design.</p> <p>As regards signalling, based on the assumption that the parameters of the DMC, or, rather, of the composition of QAM and DMC, are known by the receiver, it is shown how the receiver may decide which data sequence was likely transmitted, taking into account lSI and AWGN according to some optimal rule. Motivated by the classical receiver design principals used for quadrature-amplitude-modulated signalling over a linear time-invariant channel in the presence of lSI and AWGN, namely.</p> <p>1. linear zero-forcing equalizer,</p> <p>2. decision-feedback zero-forcing equalizer,</p> <p>3. linear mean-square-error equalizer,</p> <p>4. decision-feedback mean-square-error equalizer,</p> <p>5. maximum-likelihood sequence estimator of the Forney-type,</p> <p>6. maximum·likelihood sequence estimator of the Ungerboeck type,</p> <p>the thesis shows how these principles can be generalized for quadrature-amplitude-modulated signalling over a DMC in the presence of lSI and AWGN. Despite the DMC's being time-variant, these generalized receivers can be implemented with a bank of continuous-time time-invariant filters at the front.</p> <p>The thesis, although mainly theoretical, illustrates some of the above methods through computer simulations. More specifically, numerical results are given for probing by maximum-likelihood method and signalling by a linear zero-forcing equalizer, under various system specifications and scenarios involving the geometry of propagation and speeds of movement.</p>