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The Performance of NMSK Signals in Nonlinear Channels
Toma JAVORNIK Gorazd KANDUS Alister BURR
Publication
IEICE TRANSACTIONS on Communications
Vol.E85B
No.7
pp.12651275 Publication Date: 2002/07/01 Online ISSN:
DOI: Print ISSN: 09168516 Type of Manuscript: PAPER Category: Wireless Communication Technology Keyword: nonlinear channel, NMSK, MAMSK, power spectrum, BER,
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Summary:
We consider bandwidthefficient modulation schemes for use on nonlinear channels, such as that due to a nonlinear high power amplifier (HPA) in a wireless system. Continuous Phase Modulation (CPM) schemes are known to perform well on such channels, because they have constant amplitude, but their bandwidth efficiency is low. NMSK improves this by superposing two or more such signals, but this results in a nonconstant amplitude. In this paper we investigate the performance of NMSK on a nonlinear channel, modelled using a travelling wave tube (TWT) nonlinearity. We first consider the spectrum and the BER performance of NMSK on a nonlinear channel, making use of the Euclidean distance spectrum of the modulated signal. We then consider the effect of nonlinear amplification on these properties. Signal spectrum was determined by simulation, since no closedform expression is available when the effect of AMPM conversion are included. We find that the spectrum is remarkably little affected, being only slightly broadened. BER is also evaluated by direct simulation, as well as from the Euclidean distance spectrum. The latter now exhibits a series of clusters, instead of discrete lines, and we find that at least the whole of the first cluster must be considered in calculating the BER, not just the minimum distance or the centroid of the cluster. The detector used in the simulation applies an inverse distortion function, then uses maximumlikelihood sequence estimation (MLSE) set up for the linear channel. This is no longer optimum, because the noise is distorted, and therefore it is also compared with a true MLSE detector. We find that the BER performance is, however, somewhat degraded compared to the linear channel. We determine backoff levels from saturation to optimise overall power efficiency.


