Chi Square Random Variable

While trying to derive the theoretical bit error rate (BER) for BPSK modulation in a Rayleigh fading channel, I realized that I need to discuss chi square random variable prior.

What is chi-square random variable?

Let there be independent and identically distributed Gaussian random variables with mean 0 and variance and we form a new random variable,

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Then is a chi square random variable with degrees of freedom.

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