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Global 21-cm experiments are built to study the evolution of the Universebetween the cosmic dawn and the epoch of reionisation. FlexKnot is a functionparameterised by freely moving knots stringed together by splines. Adopting theFlexKnot function as the signal model has the potential to separate the global21-cm signal from the foregrounds and systematics while being capable ofrecovering the crucial features given by theoretical predictions. In thispaper, we implement the FlexKnot method by integrating twice over a function offreely moving knots interpolated linearly. The function is also constrained atthe lower frequencies corresponding to the dark ages by theoretical values. TheFlexKnot model is tested in the framework of the realistic data analysispipeline of the REACH global signal experiment using simulated antennatemperature data. We demonstrate that the FlexKnot model performs better thanexisting signal models, e.g. the Gaussian signal model, at reconstructing theshape of the true signals present in the simulated REACH data, especially forinjected signals with complex structures. The capabilities of the FlexKnotsignal model is also tested by introducing various systematics and simulatedglobal signals of different types. These tests show that four to five knots aresufficient to recover the general shape of most realistic injected signals,with or without sinusoidal systematics. We show that true signals whoseabsorption trough of amplitude between 120 to 450 mK can be well recovered withsystematics up to about 50 mK.

FlexKnot as a Generalised Model of the Sky-averaged 21-cm Signal at z ~ 6-30 in the Presence of Systematics