The physics involved
The physics behind transmitters and receivers in a wireless network rests on balancing data rate and range. Such physics govern what is practical in real-world applications. To illustrate this, we can turn the previous equation into a logarithmic form, so multiplications become additions, and the divisions become subtractions. Unpacking each element will show how it contributes towards calculating what is possible within a wireless system and help reveal the secret to range.
The allowable link loss equation:
We start with the ratio of the gain of the transmitter and receiver antennas. Users normally want handheld devices that work in all directions with no focused transmission which requires omnidirectional antennas. Both and then need to be zero and can in effect be forgotten in the calculation.
We then add the transmit power, a key element governing how loud the transmitter can ‘shout’. If extending wireless range was simply a case of turning up the volume to whatever level was required a designer’s job would be much easier. However, the transmit power of any wireless communications system is limited by legal regulations. Licensing arrangements by regulatory bodies mean that the device or network is not allowed to transmit more than one hundred milliwatts for some ISM (Industrial, Scientific and Medical) frequency bands. These are very small amounts but increasing the transmit power higher will break legal regulations in the common 2.4Ghz band.
Lastly we subtract the minimum allowed received power. P_R is the section in curly brackets and dictates how sensitive the receiver can be. It is the sum of a number of key elements:
R is the data rate, or the rate at which the information is sent. The minimum power required is dependent on the data rate and, unlike other elements that are out of their hands, designers can choose what value this is. They know from the equation that a higher data rate is going to lower the amount of link loss you can have. So, for a higher data rate (often needed for large amounts of information) it is advisable to have a shorter physical range between transmitter and receiver. If only a low data rate is required, the distance can be extended significantly, even when the transmission power is very low. Herein lies a clue to extending wireless range.
This symbolises the signal to noise ratio and is normally expressed in decibels (dB). Essentially this determines how much noise (other signals) is allowed to get to the receiver before communications becomes difficult. Since much of the dominant noise in a wireless system is actually created inside the receiver itself it is possible to reduce this minimum signal-noise level to about zero dB by processing the data using error detection and correction techniques that enable reconstruction of the original data. For many systems the minimum signal to noise ratio is around 10dB. Increasing system complexity by using error-correction or advanced demodulation techniques directly influences the minimum required signal to noise ratio and is the only other element wireless designers can significantly influence in the equation. Some high-capacity, low bandwidth systems squeeze data more closely and need to have a higher signal to noise ration to work well.
Boltzmann’s constant, which is a scientific fundamental relating particle level energy to temperature and designers can’t change that.
The effective noise temperature of the radio receiver is T in Kelvin, a measure of the amount of internal noise it generates. It is not normally worth reducing this to lower than the natural noise temperature of the earth (about 290K) unless the receiver is only looking out to (colder) space..