Nakagami Distribution

Generalization of the chi distribution. Shape parameter is $\nu>0.$ The support is $x\geq0.$

$$\begin{eqnarray*} f\left(x;\nu\right) & = & \frac{2\nu^{\nu}}{\Gamma\left(\nu\right)}x^{2\nu-1}\exp\left(-\nu x^{2}\right)\\ F\left(x;\nu\right) & = & \frac{\gamma\left(\nu,\nu x^{2}\right)}{\Gamma\left(\nu\right)}\\ G\left(q;\nu\right) & = & \sqrt{\frac{1}{\nu}\gamma^{-1}\left(\nu,q{\Gamma\left(\nu\right)}\right)} \end{eqnarray*}$$

where $\gamma$ is the lower incomplete gamma function, $\gamma\left(\nu, x\right) = \int_0^x t^{\nu-1} e^{-t} dt$.

$$\begin{eqnarray*} \mu & = & \frac{\Gamma\left(\nu+\frac{1}{2}\right)}{\sqrt{\nu}\Gamma\left(\nu\right)}\\ \mu_{2} & = & \left[1-\mu^{2}\right]\\ \gamma_{1} & = & \frac{\mu\left(1-4v\mu_{2}\right)}{2\nu\mu_{2}^{3/2}}\\ \gamma_{2} & = & \frac{-6\mu^{4}\nu+\left(8\nu-2\right)\mu^{2}-2\nu+1}{\nu\mu_{2}^{2}} \end{eqnarray*}$$

Implementation: scipy.stats.nakagami

MLE of the Nakagami Distribution in SciPy (nakagami.fit)

The probability density function of the nakagami distribution in SciPy is

$$\begin{equation} f(x; \nu, \mu, \sigma) = 2 \frac{\nu^\nu}{ \sigma \Gamma(\nu)}\left(\frac{x-\mu}{\sigma}\right)^{2\nu - 1} \exp\left(-\nu \left(\frac{x-\mu}{\sigma}\right)^2 \right),\tag{1} \end{equation}$$

for $x$ such that $\frac{x-\mu}{\sigma} \geq 0$, where $\nu \geq \frac{1}{2}$ is the shape parameter, $\mu$ is the location, and $\sigma$ is the scale.

The log-likelihood function is therefore

$$\begin{equation} l(\nu, \mu, \sigma) = \sum_{i = 1}^{N} \log \left( 2 \frac{\nu^\nu}{ \sigma\Gamma(\nu)}\left(\frac{x_i-\mu}{\sigma}\right)^{2\nu - 1} \exp\left(-\nu \left(\frac{x_i-\mu}{\sigma}\right)^2 \right) \right),\tag{2} \end{equation}$$

which can be expanded as

$$\begin{equation} l(\nu, \mu, \sigma) = N \log(2) + N\nu \log(\nu) - N\log\left(\Gamma(\nu)\right) - 2N \nu \log(\sigma) + \left(2 \nu - 1 \right) \sum_{i=1}^N \log(x_i - \mu) - \nu \sigma^{-2} \sum_{i=1}^N \left(x_i-\mu\right)^2, \tag{3} \end{equation}$$

Leaving supports constraints out, the first-order condition for optimality on the likelihood derivatives gives estimates of parameters:

$$\begin{align} \frac{\partial l}{\partial \nu}(\nu, \mu, \sigma) &= N\left(1 + \log(\nu) - \psi^{(0)}(\nu)\right) + 2 \sum_{i=1}^N \log \left( \frac{x_i - \mu}{\sigma} \right) - \sum_{i=1}^N \left( \frac{x_i - \mu}{\sigma} \right)^2 = 0 \text{,} \tag{4}\\ \frac{\partial l}{\partial \mu}(\nu, \mu, \sigma) &= (1 - 2 \nu) \sum_{i=1}^N \frac{1}{x_i-\mu} + \frac{2\nu}{\sigma^2} \sum_{i=1}^N x_i-\mu = 0 \text{, and} \tag{5}\\ \frac{\partial l}{\partial \sigma}(\nu, \mu, \sigma) &= -2 N \nu \frac{1}{\sigma} + 2 \nu \sigma^{-3} \sum_{i=1}^N \left(x_i-\mu\right)^2 = 0 \text{,}\tag{6} \end{align}$$

where $\psi^{(0)}$ is the polygamma function of order $0$; i.e. $\psi^{(0)}(\nu) = \frac{d}{d\nu} \log \Gamma(\nu)$.

However, the support of the distribution is the values of $x$ for which $\frac{x-\mu}{\sigma} \geq 0$, and this provides an additional constraint that

$$\begin{equation} \mu \leq \min_i{x_i}.\tag{7} \end{equation}$$

For $\nu = \frac{1}{2}$, the partial derivative of the log-likelihood with respect to $\mu$ reduces to:

$$\begin{equation} \frac{\partial l}{\partial \mu}(\nu, \mu, \sigma) = {\sigma^2} \sum_{i=1}^N (x_i-\mu), \end{equation}$$

which is positive when the support constraint is satisfied. Because the partial derivative with respect to $\mu$ is positive, increasing $\mu$ increases the log-likelihood, and therefore the constraint is active at the maximum likelihood estimate for $\mu$

$$\begin{equation} \mu = \min_i{x_i}, \quad \nu = \frac{1}{2}. \tag{8} \end{equation}$$

For $\nu$ sufficiently greater than $\frac{1}{2}$, the likelihood equation $\frac{\partial l}{\partial \mu}(\nu, \mu, \sigma)=0$ has a solution, and this solution provides the maximum likelihood estimate for $\mu$. In either case, however, the condition $\mu = \min_i{x_i}$ provides a reasonable initial guess for numerical optimization.

Furthermore, the likelihood equation for $\sigma$ can be solved explicitly, and it provides the maximum likelihood estimate

$$\begin{equation} \sigma = \sqrt{ \frac{\sum_{i=1}^N \left(x_i-\mu\right)^2}{N}}. \tag{9} \end{equation}$$

Hence, the _fitstart method for nakagami uses

$$\begin{align} \mu_0 &= \min_i{x_i} \, \text{and} \\ \sigma_0 &= \sqrt{ \frac{\sum_{i=1}^N \left(x_i-\mu_0\right)^2}{N}} \end{align}$$

as initial guesses for numerical optimization accordingly.