This paper presents an analytical method to investigate the impact of component crosstalk on the spectral efficiency (SE) of generalized quadrature phase shift keying (QPSK) modulation format by computing its bit error rate (BER) over additive white Gaussian noise (AWGN) channel. For this purpose we consider a wavelength division multiplexed (WDM) receiver system and the desired signal is coherently detected in presence of any arbitrary number of independent and identical (i.i.d.) crosstalk sources. The analysis exploits characteristic function method and examines the bandwidth expenses of the system while subjected to crosstalk sources. The theory is verified for single interferer case with the previous work reported in literature. The approach is useful for accurate and time-saving BER (thus SE) assessment and monitoring in the network/system, which can improve the reach and efficiency of the WDM system. It is observed from the result that crosstalk level and number of active interferers perform a vital role to determine the spectral efficiency of the system. In this context the study of the protection ratio i.e. the minimum crosstalk level required to obtain a fixed BER, is essential for any particular signal to noise ratio (SNR). The power budgeting of the network under component crosstalk is also discussed in terms of spectral efficiency and interference penalty.