EFFECT OF MULTIPATH AND DOPPLER IN WIRELESS MEDIUM
Due to the mobility of mobile terminal the received signal is rapidly fluctuating causing changes in the multiple signal component arriving via different path. And the fluctuation of signal amplitude is referred to as small scale fading. Here we are going to see effect of Multipath and doppler in wireless medium.
Rapid fluctuation of signal amplitude causes two effects. The first effect is caused due to the movement of the mobile terminal towards or away from the base station transmitter. It is called as the Doppler Effect.
And the second effect is caused by the addition of the signals arriving via different paths is called as multipath effect.
MODELING OF MULTIPATH FADING
The propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths in the wireless medium and the phenomenon is known as multipath propagation
Fluctuation of the signal amplitude because of the addition of the signal arriving with different phase resulting in multipath fading. Signal have travelled different distances by travelling along different paths due to this there is a phase difference. The phase of the arriving path is changing rapidly so, the received signal amplitude undergoes the rapid fluctuation that is modeled as a random variable with a particular distribution.
RAYLEIGH DISTRIBUTION is the most commonly used distribution for multipath fading. Destructive and constructive interference and the phase shifting of the signal are the effect of multipath.
It is the spectrum of the fluctuation of the received signal strength. It is also called as the classical Doppler spectrum and commonly used in mobile radio modeling. For the indoor application there is another model for Doppler spectrum that is uniform distribution. The fade rate and fade duration for a given mobile velocity is obtained from rms Doppler spread. Then these values can be used to design appropriate coding and interleaving technique for mitigating the effect of fading. The effects of fast is overcome by diversity technique by providing multiple copies of the signal at receiver.
CHANNEL MEASURMENT AND MODELLING TECHNIQUE
There is need to perform different type of measurement to understand and model the radio propagation characteristics. It is very difficult to analytically derive the expression how a radio signal may propagate through complex environment. Measurement of the radio channel are obtained where the wireless network is required to be setup. The measurement can be narrow band or wide band or both. For the simulation of environment the result of measurement could be directly used. All the data are included in a database. And these data are used to construct the empirical model for e.g. path loss model. These models are usually dependent on the frequency and the type of measurement taken and are site specific.
There are some difficulties in wireless channel modeling because of the complex propagation process on the basis of a wireless channel includes scattering, diffraction, reflection, and transmission through a large number of irregular object.
For the SIMO and MIMO type of application recently 3D measurement and modeling of the radio channel characteristics which includes the angle of arrival as well as the delay of arrival of the paths have become popular.
SIMO channel models:
It is a single input multiple output radio channel . in this model a typical cellular environment is considered where it is assumed that the base station can have a complex receiver and mobile transmitter are relatively simple .
MIMO channel model:
It is a multiple input multiple output channel model.
An extension of the SIMO model where there are M base station antenna element and N mobile antenna element is called MIMO channel model.
QUESTION & ANSWER
- What is the effect of multipath propagation??
The effect of multipath are
- Phase shifting of the signal
- Constructive inteference
- Destructive interference
2. What are the causes of multipath??
There are several causes of multipath:
Ionospheric reflection, refraction, atmospheric ducting, and reflection from the terrestrial object and water bodies like mountain and building.
3. What is the effect of multipath in digital radio communication and How it can be solved??
Multipath causes the error and affect the quality of communication in digital radio communication. These errors are caused due to the Intersymbol interference .
To solve the error Equalisers are used.
4. Explain SIMO and MIMO channel model??
SIMO channel model:
It is a single input multiple output channel model. In this model a typical cellular environment model is considered where it is assumed that the base station can have a complex receiver and mobile station is relatively simple.
MIMO channel model:
It is a multiple input multiple output channel model. And it is an extension of SIMO model where there are M base station antenna element and N mobile antenna element.