A composite Gauss–Legendre quadrature method for the Q-function approximation and its application in 6-bit symbol sequence modulation over generalised fading distributions

This article introduces a 6-bit Symbol Sequence Modulation (6-SSM) scheme developed to enhance communication security and efficiency through sequence-based modulation. The 6-SSM is designed to exploit symbol sequences, enabling improved secrecy performance through time-slot diversity. To analytically evaluate the performance of the proposed 6-SSM system, an exponent-based Q-function approximation (QFA) is also developed. This QFA employs a composite Gauss–Legendre quadrature applied to the polar form of the Q function. By dividing the integration range into intervals N N and using n n nodes per interval, the approximation error is significantly reduced to a factor of 1N²ⁿ 1 N 2 n . A practical configuration with two intervals and two nodes yields a four-exponent QFA that offers high accuracy, low relative error, and reduced complexity compared to existing approximations. The QFA is used to derive closed-form expressions for the bit error rate (BER) of the 6-SSM scheme under generalised fading channels, including Nakagami- m , κ κ – μ μ , and η η – μ μ distributions. The analytical results match closely Monte Carlo simulations, confirming the precision of the proposed QFA. Additionally, BER expressions are employed to analyse the secrecy rate, revealing that higher-order SSM offers improved physical layer security.