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Extremely Large-scale Array (ELAA) promises to deliver ultra-high data rateswith increased antenna elements. However, increasing antenna elements leads toa wider realm of near-field, which challenges the traditional design ofcodebooks. In this paper, we propose novel near-field codebook schemes based onthe fitting formula of codewords' quantization performance. First, we analyzethe quantization performance properties of uniform linear array (ULA) anduniform planar array (UPA) codewords. Our findings reveal an intriguingproperty: the correlation formula for ULA codewords can be represented by theelliptic formula, while the correlation formula for UPA codewords can beapproximated using the ellipsoid formula. Building on this insight, we proposea ULA uniform codebook that maximizes the minimum correlation based on thederived formula. Moreover, we introduce a ULA dislocation codebook to furtherreduce quantization overhead. Continuing our exploration, we propose UPAuniform and dislocation codebook schemes. Our investigation demonstrates thatoversampling in the angular domain offers distinct advantages, achievingheightened accuracy while minimizing overhead in quantifying near-fieldchannels. Numerical results demonstrate the appealing advantages of theproposed codebook over existing methods in decreasing quantization overhead andincreasing quantization accuracy.

Extremely Large-scale Array Systems: Near-Field Codebook Design and Performance Analysis