4th Generation (4G)

4G is the name given to the fourth generation of mobile phone mobile communication technology standards. It is a successor of the third generation (3G) standards.It provides mobile ultra-broadband Internet access.

SWITCHING BETWEEN MODULATION SCHEMES

if the performance of the modulation schemes as the figure, Switching between modulation schemes is done to maintain a BER less than a certain BER the next figure as the desired BER less than 10^-3.

CHARACTERISTICS OF WIRELESS CHANNEL

The wireless radio channel poses a severe challenge as a medium for reliable high-speed communication. It is not only susceptible to noise, interference, and other channel impediments, but these impediments change over time in unpredictable ways due to user movement.

Adaptive or Fixed Modulation

This experiment to made to know which is the better adaptive modulation or fixed modulation The experiment is applicable to both TDD and FDD systems,provided that accurate predictions of the channel conditions exist.For each prediction of the SNR at the receiver, the modulation scheme is selected for 512 consecutive symbols.

Diversity

The diversity is a technique that combating the fading by ensuring that there will be many copies of the transmitted signal which effected with different fading over time, frequency or space.

Adaptive Modulation

 Adaptive Modulation

  AMC

 

 I-The Use of Adaptive Modulation Not Fixed    Modulation

The reason is as the Fading channels confront us with 
the problem of lost Packets and the need for frequent
retransmissions and Wireless communication systems 
 .are affected by changing weather conditions
And if those effects not handled correctly, such systems
may not be able to cope with issues such as signal
degradation thereby seriously endangering a Carrier’s
,quality of service obligations.To overcome this effects
making coding and interleaving to reduce the effect of
channel and weather effects but it reduces the throughput 
 of  the channel, but we can “Dynamic Adaptive Modulation”
which ensures maximum bandwidth most of the time with
.guaranteed critical services all the time

 :II- Idea of Adaptive Modulation

The basic idea of Adaptive Modulation is to adapt different
 order modulations that allow sending more bits per symbol
 and thus achieving higher throughputs or better spectral 
efficiencies However, it must also be noted that when using
a modulation technique such as 64-QAM, better signal-to-noise
 ratios (SNRs) are needed to overcome any interference and 
 .(maintain a certain bit error ratio (BER
 The use of adaptive modulation allows a wireless system to
.choose the highest order modulation depending on
,the channel conditions then when
 The channel condition is good use 16-QAM or 64-QAM* 
Bad channel condition use QPSK*
The channel is very bad do not send the data or use BPSK*

In one word we can define Adaptive Modulation as 

the automatic modulation adjustment that a wireless system
can make to prevent weather-related fading from causing 
.communication on the link to be disrupted
From the figure, knowing the mean of the adaptive modulation
As the channel changes the modulation schemes also changes
(to maintain a constant BER (here its 10^-5
 The adaptive modulation try to use the time -variation in 
 channel conditions to increase the capacity of the system
.not try to fight this effects only


 or can be  obvious from the following figures
that the modulation scheme changes due to weather changes or the distance form the antenna

:The Block diagram of adaptive modulation 



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