19-06-2012, 02:19 PM
Propogation effects and parameters
Propogation effects and parameters.pptx (Size: 126.6 KB / Downloads: 36)
Transmitted signal (from transmitting antenna which will be received at the receiving antenna) travels a complex path
propagation occurs in 3 mechanism- reflection, diffraction and scattering (Due to exposure of signals to natural and man-made structures,)
Buildings, mountains, etc (All these factors contribute to variation in the signal level, thereby varying the signal coverage and quality in the network)
Large Scale Fading
(The variation of the mean received signal strength for larger distance (several hundreds or thousands of meters) between an arbitrary transmitter-receiver (T-R) is called large scale fading. The variation in the received signal strength occurs because of shadowing of the signal by buildings, mountains etc. Propagation models that characterize the mean signal strength are useful in estimating the radio coverage area of a transmitter. )
Frequency Selective Fading
A transmitted signal undergoes frequency selective fading if the channel possesses a constant-gain and linear phase response over a bandwidth which is smaller than the bandwidth of the transmitted signal. This is the case for wideband systems. (In other words, the spectral components of the transmitted signal are affected by different amplitude gains and phase shifts.)
Diversity techniques
(The communication process across the wireless channel is severely degraded due to multipath fading. Thus ) To improve the strength of the received signal by reducing the multipath fading effect, diversity technique is used.
Diversity improvement is achieved with no increase in the transmission power or bandwidth but at the expense of increased system complexity and moderate increase in receiver power consumption. (Diversity combining consists of receiving redundant information-bearing signals over two or more fading channels, thus combining these multiple replicas at the receiver improves the overall received SNR.)
. Selection Combiner
In selection combining (SC), the combiner outputs the signal on the branch with the highest SNR. With SC the branch output from the combiner has an SNR equal to the maximum SNR of all the branches.
( Since only one branch output is used, co-phasing of multiple branches is not required, hence) this technique can be used with either coherent or differential modulation.
(This technique is simple but ineffective because)all the antenna branches are not used effectively, hence wastage of hardware.
Maximal-Ratio Combiner
In Maximal-ratio combining (MRC), the signals from all the branches are co-phased and weighted accordingly depending on the strength of the received signal to produce the output. Thus the limitation of the selection combining is overcome by the maximal-ratio combiner. This is an optimal receiver but the complexity lies in the estimation of channel weights and phases.
Switched Combiner
Only one branch is connected to the receiver at any time.
The receiver selects a particular branch until its SNR drops below a predetermined threshold.
When this happens the receiver switches to another branch.
There are 2 types namely switch and stay combiner and switch and examine combiner.