To transmit a signal over wireless body area network, the signal is transmitted over a fading channel with additive white Gaussian noise. The channel model can be obtained from the IEEE standards. My question is about the range of values of the SNR for the additive noise. In some papers, I found that simulation is done for SNRs ranging from zero db to about 20 db. Is this a suitable range? Do we need to simulate negative values for the SNR?