Fading is defined as any variation of phase , polarization , and or cedar power that is received on time. The most basic definition of fading in the mechanism of propagation includes refraction , reflection , diffraction , scattering , attenuation and radio gel guidance. Fading is caused by field geometry and conditions meteorology who do not need to be mutually related . All 0.3 – 300 GHz radio frequency transmission systems can experience fading
Fading mitigation can be done by adjusting the height of the antenna
fading effect , we use diversity techniques . Diversity is a powerful communication receiver technique that is used to improve the fading performance of radio channels
the costs are relatively low . In the technique of diversity , many copies of the information that the same is transmitted to the receiver in the path or channel that is independent in the statistics . As a result , the probability that all the replicas of the signal will fade in the same
reduced considerably .
TYPES OF DIVERSITY
The diversity of micro and diversity of the macro :
According to two types of fading; scale small faded and scale large fading , two diversity that appropriate techniques are implemented . To prevent fading in , the technique diversity of microscopic applied to reduce ketch l scale effects of fading by selecting all signals are optimized time . On the other hand , macroscopic diversity techniques are applied in the case of large- scale fading to increase the average SNR on the forward link by selecting a base station that is not overshadowed when others are located .
Space Diversity :
The diversity of spatial is the form of the oldest and most simple difference . This is achieved by using multiple antennas at the base station or in the station cellular or at both ends , which is separated by physical ( ideally separated by half or more long- wave ), so it receives a copy of which is different from the signal experiencing fading independently .
The diversity of the room , which has a lot of exploitation in Indonesia communication wireless to combat fading channels , promises speeds of data is high and the range of the network which is great .
Frequency Diversity :
In engineering the diversity of this , the signal information that is the same is transmitted on a carrier frequency that is different also, the separation frequency between them at least koherensibandwidth , (Δƒ) c, so that a copy of which is different from the signal experiencing fading independently .
Time Diversity :
The diversity of time is achieved by transmitting the case that each bit of information repeated at intervals of time that is short . The separation between delivery times must be greater than
time coherence , so that the copy ‘s different from the symbol that same experience fading independent . The time interval depends on the level of fading, and increases with the level decreases
Polarization diversity :
In the diversity of polarizations , many versions of the signal are transmitted and received through antennas with different polarizations , namely the electric and magnetic fields of the signal that carry modified information and many more signals are used to send the same information . So , orthogonal Polarization type is obtained . The mechanism of diversity is very practical because of the size of the antenna that is very small it can be used .
Angular diversity :
The system diversity is in need of some direction antenna which responded to the wave is independent of propagation . In terms of al cell , the diversity of angles can be achieved by using two antennas Directi onal Omni which act as elements of parasites of each other to change the pattern they manage the reception signal at the corner that is different .
Mechanical diversity is used to provide the receiver with multiple copies of the signal are the same . Diversity techniques are used to improve the performance of radio channels without increasing the transmitted power . Various types of diversity schemes have been discussed at the top , which has a scheme of its own advantages and drawbacks . Depending on the application and work environment , the most suitable diversity scheme is chosen . Also , we have been seeing a variety of Diversity Combining techniques are used to reduce the effects of fading on signal . Combining the schema is also application and environment dependent . In the combination of the different techniques , MRC has the performance of the best and tertinggikompleksitas , EGC is more easily implemented than with MRC, but its performance is approximately 1 dB more worse than MRC, and Selection Techniques has a performance that is bad but the complexity of the bit . So by using multiple sending and receiving antennas ,
diversity can be achieved to improve the performance of radio wireless communication channels , reducing fading
• Large Scale Fading , refers to the path loss caused by the effect of the transmitted signal over a large area . The phenomenon is influenced by the contours of the terrain that stand out ( hills , forests , billboards , clumps of buildings , etc .) Between the transmitter and receiver . Path loss is characterized by mean loss and variation around mean loss.
• Small Scale Fading , on small scale fading dramatic changes in the amplitude and phase of the signal that can be experienced as a result of small changes in the spatial separation between the receiver and transmitter . Small Scale Fading occurs if the object in the path of the signal change . Many factors physical that lead to small scale fading among propagation Multipath, movements relative between the transmitter and receiver , the speed of the object in the vicinity and the bandwidth of the transmission around the signal . Multipath propagation occurs as a result of shadows , diffraction , reflection , and refraction . When the receiver moves over a large area it experiences both types of fading.
If all the frequency components of the signal will experience fading with the same magnitude , then it is called frequency flat fading . Frequency flat fading occurs bandwidth coherence if the channel is greater than the signal bandwidth . That is ,
𝐵 𝑆 < 𝐵 𝐶
In which , 𝐵 𝑆 is the bandwidth of the signal and 𝐵 𝐶 is bandwidth coherence .
On the side of the other , if all the components of the frequency of the signal that is transmitted is influenced by a variety of amplitude and phase of which is shifted then fading to say as a frequency selective . Frequency selective fading occurs when bandwidth signal that is transmitted over a large of bandwidth coherence channel .
That is ,
𝐵 𝑆 > 𝐵 𝐶
The difference between slow and fast fading is very important for modeling mathematics fading channels and to evaluate the performance of systems of communication that operates on top of the channel is . The terms that refer to slow and fast fading are where the magnitude and change of the phase of the channel in the signal changes .
Fading is said to be slow if the time duration symbol 𝑇𝑠 is smaller than the channel coherence time 𝑇𝑐 .
Namely , 𝑇𝑠 < 𝑇 .
If the time duration symbol 𝑇𝑠 is greater than the channel coherence time 𝑇𝑐 it is called fast fading. In the channel fast fading, response channels impulses change with rapidly in the symbol duration of the signal .
Because it is a signal to experience fast fading if 𝑇𝑠 > 𝑇𝑐 .
Channel encoding is to add redundancy to the signal which is transmitted – in the case of this so that errors are caused by noise during transmission can be corrected in the receiver . The coding process for protection against channel errors is called error-control coding. Error-control codes are used in various applications , including communications satellites , communication in space , communications radio mobile , and network computers .
Equalization is a technique prosesan signal at the transmitter to make the signal more robust against delay spread. An equalizer channel is an important component of a communication system and is used to reduce ISI (inter symbol interference). The equalizer depends on the characteristics of the channel . In wireless channels , due to multipath fading, channel characteristics change with time . So it may be necessary to channel equalizer to keep track of time which varies the channel to provide performance that is reasonable .