Diversity and its Types
Diversity Reception is generally known as a method or way of combining multiple copies of the same message signal that is received by the receiver from different channels. Basically, this is done to lessen or eliminate the effect of fading.
In wireless communication technology, diversity reception improves the reception of radio frequency signals. In simplest terminology, diversity is known to be a powerful communication receiver technique by implementing which the efficiency of wireless links can be improved.
Need of Diversity
We all have the basic idea regarding radio wave propagation that when the signal is transmitted from one end to another than it undergoes fading. Basically, there exists numerous buildings or other structures that act as obstacles in the transmission path that behaves as scatterers of the transmitted signal.
When the transmitted signal which is in the form of waves interacts with these obstacles in the path then the signal which gets received at the receiving antenna is subjected to fading.
The extent of fading depends on how heavily built-up the area is where transmission is taking place. Meaning that in a heavily cluttered region like the city center, fading is quite severe. However, with the decrease in the level of urbanization, fading also reduces. Not only this, in rural areas, no large buildings are present and so in that region only trees act as obstacles in the path of wave propagation. Thus, if no trees are present in the wave propagation path, then fading will be the least.
Doppler spread is inseparably associated with fading.
Now, the question comes to our mind – what is Doppler spread?
So, doppler spread is defined as the broadening of the narrow-band signal spectrum at the time of transmission when multipath propagation takes place.
One should note here that the fading rate varies in proportion to the speed and frequency of transmission. Also, as the signals are coming from different paths thus, doppler spread is also noticed in the received signal.
Hence, it is not wrong to say that both fading and doppler spread are two manifestations of the same process in the time domain and in frequency domain.
The effect of fading on the transmitted signal can be reduced to a large extent by diversity reception. Basically, the technique of diversity reception is applied at the base or mobile station.
Diversity reception is the name given to a technique where multiple independent samples of a message signal are transmitted and collected. As the copies of the same signal are transmitted through different paths then definitely the fading that they will undergo will be uncorrelated.
More simply, we can say that during transmission each copy of the signal will get faded in a different manner as each individual copy will meet up with different obstacles in the propagating path.
Thus, there is a very low chance that each received copy of the signal will be specified below a certain level. However, there is quite a high chance that a specific number of copies will undergo fading below that certain level.
For instance, if there are n copies of a message signal that are to be transmitted and p is the probability of a single copy faded below a certain level, then the probability of all samples faded below that certain level is pn.
Thus, one can conclude that a signal which is obtained from a suitable combination of various samples will exhibit less severe fading than the one individual same collected alone.
It is to be noted here that only the receiver makes the diversity decision and the work of transmitted is only to transmit the same signal through different channels.
Now, the question arises of how to get the independent samples of the same message signal. So, this is answered under the following subhead.
Types of Diversity
The various types of diversity are discussed below:
1. Frequency Diversity: Here the name itself is indicating that each sample of the message signal is transmitted over a different carrier frequency. This makes the received signals uncorrelated from each other. However, it is to be noted here that the carrier frequencies must exhibit such separation that is more than the coherence bandwidth of the radio frequency channel.
The major drawback associated with this technique is that the receiving unit must be tuned to the multiple carrier frequencies in order for the signals to get received. In general, it is not common that the same message signal is modulated with different carriers as this leads to a reduction in spectral efficiency. This technique makes use of a single antenna.
2. Time Diversity: In the time diversity approach, the same copies of the signal are transmitted over the channel at different instants of time. When the channel environment exhibits time-varying nature meaning that at one instant, the signal undergoes severe fading and at other instant, the signal undergoes less fading then, this approach is used.
The conclusion of this approach is that the channel behaves differently at different time instants then the information can be recovered at the receiver in a proper manner. TDMA-based system like GSM is based on this approach.
3. Space Diversity: This type is associated with different antenna units that are present at distant locations from each other in order to show separability. Basically, here signals are obtained from multiple antennas, and therefore it is sometimes also known as antenna diversity.
Till today, it is known to be the most convenient method of diversity reception in mobile radio communication. Generally, here the physical distance of separation of two antennas must be greater than λ/2. This type of diversity is very much popular at use in MIMO-based systems like LTE.
4. Polarization Diversity: In this type of diversity, signals achieved at the receiver must be of different polarization. Here multiple transmitting antennas with different polarization are required. Basically, when the same transmitted wave is polarized in a different manner then it will be scattered differently by the obstacles present in the path and so fading will be different.
In this technique, the power used is half as the whole power gets divided between two differently polarized antennas. It is believed that in order to have good performance, orthogonally polarized antennas are to be used. The reason behind this is that with this, one can get 30 dB difference in the strength of the signal at the receiving end.
Hence from the above discussion, we can conclude that diversity reception is quite a useful technique that improves channel efficiency by reducing the effects of fading.