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Wireless Link Budgets: Design Factors Analysis
Some of the factors that decide the performance of a wireless link include transmission power, transmission bandwidth, and antenna gain and signal to noise also increases and hence the SNR decreases. ratio. Let us discuss the performance of a wireless link in view of short range and long range communications separately. The ultimate aim is that our transmitted signal should combat the channel noise and successfully reach the receiver without fading out.
Design Factors – Transmission Power and Channel Noise
Transmission Power
In general, the power of the transmitted signal must be above a certain threshold to ensure that the signal doesn’t get buried inside the channel noise. Higher the signal power (or, the transmission power), higher is the chance that the signal level will make it through to its destination without getting much affected by the channel noise. The signal to noise ratio (SNR) also increases when the signal power increases.
$$SNR(dB)=10log_{10}(\frac{Signal\:Power}{Noise\:power})$$
$$Signal\:Power\:increases\Rightarrow\:SNR\:increases$$
If we maintain a high value of signal power at the transmitter, the signal can combat the channel noise and consequently be successfully detected and decoded by the receiver. However, we can’t keep increasing the signal power as this would potentially create interference with other channels.
There are limits posed on the permissible operating power levels by regulatory bodies such as Federal Communications Commission (FCC), International Telecommunication Union (ITU), Telecom Regulatory Authority of India (TRAI) and so on. These power limits varies for different applications.
Higher the transmission power, stronger the signal is and higher the coverage is. Hence, higher is the SNR. However, there is a limit to the levels of power within which one can operate.
The power requirements vary depending on the application. For marine radios, the operating power is in the order of watts while commercial AM and FM radio broadcasting operates in the order of few hundreds of watts to few tenths of kilowatts
Channel Noise
It is not possible to have any wireless communication system with absolutely zero noise level. Noise is additive and hence at the receiver, we have the signal that contains both the transmitted data and noise. The most commonly used noise model is the Additive White Gaussian Noise (AWGN). The noise follows Gaussian distribution. The Power Spectral Density (PSD) of white noise is uniform throughout the frequency range and it is independent of frequency.
The white noise is so named because of its similarity to white light which contains equal amounts or intensities of all frequencies of the visible spectrum.
Apart from white noise, we have flicker noise whose PSD is inversely proportional to signal frequency and Brownian noise who’s PSD is inversely proportional to the square of the signal frequency.
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