Question

Wireless transmission

1) What are the main problems of signal propagation?

2) What is the main advantage of hierarchical 64 QAM?

Answer 1

Wireless transmission

Wirless transmission mean while transmission over different wires typically does not cause interface.this is important in wireless transmission.the frequencies used for transmission are all regulated.multiplexing is important topics in the context because the medium is always sheared.multiplexing scheme have to ensure low interface between different sender.

1.what is main problem of signal propagation

Most of wirless transmission take place through atmosphere that means that signals can travel through air,rain dust etc.while the path loss or attenuation does not cause too much trouble for short distance for example contain Lans.the atmosphere heavily influence transmission over long distance for example satellite transmission.even mobile phone system are influenced by weather conditions such as heavy rains.rains can absorb too much radiated energy of antenna .so communication links may break down as soon as rains set in.depending on the on the frequency radio waves also penetrates objects.an extreme form of attenuation is blocking or shadowing of radio signal due to learge obstacle.the higher the frequency of singnal the more it behave like light even small obstacles like a small wall or trees in an alley may block the singnal.another problem contain that reflections of the signals if an object is learge compared to wavelengths of the singnal example huge bungalow or mountains the signal is reflected. The reflected signal is not as strong as original as object can absorb some of signal power singnal transmitted from a sender may bounce off the walls of building sevral time before they reach receivers.another problem is refraction this effect because the velocity of electromagnetic wave depend on the density of medium through which it travels if the size of obstacles is in the order of wavelength or less than .wave can scattered.another problem is diffraction effect is very similar to scattering.wavws will be defletrfat an edge and propagate at different direction..now consider on sender side both impulses are separated at the receiver both impulses interface that is overlap in time.

Question 2.

A 64 Ram can code 6 bit oer symbol.there two most significant bits are used for the work singnal embedded in the qam signal.if the reception of the signal is good the entire qam constellation can be resolved .under poor reception condition for example with moving receivers only the qpsk portion can be resolved.a high priority data atram in DVB-T is encoded with QPSK using the two most significant bits.the remains four bits represt low priority data for tv this could means that standard resolution data stream is coded with the high priority the high resolution with low priority.if the signal is distorted at least standard tv resolution can be received. Although QAM appears to increase the effency of the transmission for communication system by utilizing both amplitude and phase variation.the other benefit 64 QAM is variants is efficient usage if bandwidth this due to the fact the QAM represents more number of bits per carrier that 64 QAM maps 6 bit per carrier