Any wireless communication system encounters 3 main problems due to signal motion in free space. These problems are:
1) Path loss.
2) Shadowing fading.
3) Multipath effect.
Each of these problems will be discussed in details in this article.
Path loss effect:
It is the effect of signal absorption due to signal travel in free space, which results in signal’s weakness. Path loss effect increases by increasing the distance over which the signal will travel.
So, this effect can cause a problem only over long distances which are called “large scale variations”.
We can approximately calculate the received signal power for unobstructed LOS (line of sight) signal as follow:
Pr =Pt(√Gr ×λ ÷(4Πd) )
Pr: is the received signal power.
Pt: is the transmitted signal power.
Gr : is the product of the transmit and receive antenna field radiation patterns .
λ: wavelength (λ = speed of light / frequency).
d: is the distance between the transmitter and the receiver
But, what if the environment is more complicated and we can’t obtain LOS signal?
Scientists tried to make more complex models such as:
1) Maxwell’s equations: which are complex and impractical
2) Free space path loss model: This is too simple model.
3) Ray tracing models: Requires site-specific Information.
4) Empirical models: is not general for all environments.
In addition to path loss, a signal will typically experience random variation due to blockage from objects in the signal path, giving rise to a random variation about the path loss at a given distance.
To reduce the shadowing effects:
1) Increase the transmitted signal power.
But, increasing transmitted signal power has a disadvantage that this increase may cause interference between the users due to high power.
Moreover, as the transmitted power increase, the cost also increases.
2) Macroscopic diversity: will be discussed in details in this article.
3) Power control:
Power control means, to increase or decrease power as necessary.
For example user s near to the base station shouldn’t transmit high power so as to avoid interference with other users. Also, far user should transmit high power so as to reach to the base station.
The conclusion of power control is to avoid near unwanted power and far needed power.
Multipath propagation effects:
Multipath means that the signal will travel in more than one path due to diffraction and scattering which result in a number of copies of the original transmitted signal.
These signals will interfere with each other, which result in distortion in the received signal.
To recover the original signal correctly and mitigate the effect of multipathing, the solution is to use rake receiver.
Rake receiver is simply consists of several sub-receivers, each sub-receiver is called “finger”.
Rake receiver is used to mitigate the effect of multipath.
Multipath channel consists of LOS (Line Of Sight) signal plus several multipath components.
These multipath components are delayed copies of the transmitted signal. This delay depends on the path of the signal. Without rake receiver these multipath components will cause self interference to occur.
The basic idea of rake receiver is to make use of these multipath components.
This can be done by adding all these components with each others after delaying each component. The delay should be equal to the maximum delay (the delay of the last received component) .This is a good idea because all the components contain the same information.
The result of this addition is the increase occurs in SNR (signal to noise ratio).
Rake receiver consists of the stages shown in the following figure:
1) Demodulation: of the received signal, since the transmitted signal should be digital (spreaded signal) but we can’t transmit digital signal in the channel because this requires wide bandwidth.
2) Despreading: to recover the original data.
3) Bit detection: for correlation.
4) Switch: to adjust the bit duration.
5) Hold: for all components as discussed above.
6) Summation: to increase SNR.
7) Decision: to determine the acceptable SNR.
Rake receiver is one type of diversity types.
Before discussing diversity types, we should know first what is meant by diversity:
Diversity: Same information is sent over independent fading paths, these signals are combined to mitigate the effect of fading.
It uses the fact that independent signal paths have a low probability to experience deep fades simultaneously.
Types of diversities:
1) Micro diversity: To mitigate the effect of multipath fading.
2) Macro diversity: To mitigate the effect of shadowing.
It is generally implemented by combining signals received by several base stations.
a) Space diversity:
Uses multiple of transmit or receive antennas (antenna array), where each element of the array is separated by decorrelation distance (which equals wavelength divided by two).
Advantage: No need for additional transmitted power or bandwidth.
Disadvantage: More expensive.
b) Frequency diversity :
Transmits the same narrow band signal at different carrier frequencies, where carriers are separated by the coherence bandwidth of the channel.
Advantage: more efficient.
Disadvantages: Require additional power and bandwidth.
c) Polarization diversity:
Use either two transmit antennas or two receive antennas with different polarization (vertically and horizontally polarized waves).
1) You have at most two diversity branches corresponding to the two types of polarization.
2) Lose half of the power, since the transmitted or received signal power is divided between the two differently polarized antennas.
Advantage: less cost we need only 2 antennas.
d) Time Diversity:
Transmit the same signal at different times, where the time difference is greater than the coherence time of the channel.
Advantage: No need to increase the transmitted power.
1) Decreasing of data rate, since data is repeated in the diversity time slots rather than sending new data in these time slots.
2) Useless for stationary users, because the channel coherence time is infinite and thus fading is highly correlated over time.
e) Angle of arrival: Use large number of antennas, in which each one covers part of the other areas due to scattering.
Diversity combining techniques:
1) Selection combining: The strongest signal is selected.
2) Threshold combining: Scan all the branches in sequential order and outputting the 1st signal with SNR above the given threshold.
3) Maximal ratio combining: Multiple the branches which has the highest SNR by the highest constant and sum all the branches.
4) Equal ratio combining: Same as Maximal ratio combining but multiple all the branches by the same constant .