Signal Propagation Propagation: How the Signal are spreading from the receiver to sender. Transmitted to the Receiver in the spherical shape. sender When the signal transmit from sender to receiver , the character of signal may not be changed in the wired network. But wired signal there is a change in character, like packet loss, external factors .etc,.

Signal Propagation Ranges Transmission range communication possible low error rate Detection range detection of the signal possible no communication possible Interference range signal may not be detected signal adds to the background noise sender transmission distance detection interference

Path Loss of Radio Signals When Propagation in free space always like light (straight line). The signal passes sender to receiver in a straight line is called Line-of-sight (LOS) If there is a vacuum between sender receiver , it’s a free space loss. Received power by the receiver is P. Distance between sender and receiver is d. The received power by the receiver Pr is proportional to 1/d2. Pr α 1/d2. The sender emits signals with certain energy. If there is a vacuum space, the sends at the speed of light with spherical shape. If there is no obstacle continuously grows the surface of sphere (s) and it moves to the particular distance d. According to the equation s=4πd2

Path Loss of Radio Signals The additional parameters are important . The receiver power depends on the wavelength and gain of receiver and transmitter. The external factors are affects the signals . The path loss not occures in short distance. The atmosphere influences in long distance travel signal. Heavy Rain - Mobile Phone Depending upon the frequency , it penetrate objects A short wave length with lower frequencies penetrate easily A long wave length with higher frequencies can be blocked by external factors

Fundamental Propagation Behaviors Ground Wave < 2 MHz . LF and MF Low frequency follow the earth surface Used in Submarine Communication and AM Radio Sky Wave 2 to 20 MHz Waves bounce from the ionosphere to earth. The short waves are reflected Used in International broadcasts

Signal propagation Depending on the density of a medium Shadowing Making the signal weak Reflection at large obstacles Refraction Depending on the density of a medium Changing in Direction Scattering at small obstacles The signal meets one point and scatter as many Diffraction at edges

Multipath propagation Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction. i.e radio signals reaching the receiving antenna by two or more paths The signals travelling along different path, different length and different time. This effects is delay spread

A Real Example of Multiplexing How ???

Multiplexing Transmission of multiple data communication sessions over a common wire or medium Minimum of interference and a maximum of medium utilization.

Space Division Multiplexing The channels k1 to k3 can be mapped onto the three ‘spaces’ s1 to s3 Separate the channels and prevent the interference ranges from overlapping channels ki k1 k2 k3 k4 k5 k6 c k4 to k6 is allocated for some others. t c s1 Analog Telephone t s2 f f c t s3 f

Frequency Division Multiplexing The total bandwidth available in a communication medium is divided into a series of non-overlapping frequency sub-bands Each of which is used to carry a separate signal

Frequency Division Multiplex Separation of the whole spectrum into smaller frequency bands Advantages: no dynamic coordination necessary works also for analog signals Disadvantages: waste of bandwidth if the traffic is distributed unevenly inflexible guard spaces E.g FM Radio k1 k2 k3 k4 k5 k6 c f t

Time Division Multiplexing The time domain is divided into several recurrent time slots of fixed length A sample byte sub-channel 1 is transmitted during time slot 1, sub- channel 2 during time slot 2, etc

Time Division Multiplex A channel gets the whole spectrum for a certain amount of time Advantages: only one carrier in the medium at any time throughput high even for many users Mobile Phone for a Time k1 k2 k3 k4 k5 k6 c f t

Time and frequency multiplex Combination of both methods A channel gets a certain frequency band for a certain amount of time Example: GSM Advantages: better protection against tapping protection against frequency selective interference higher data rates compared to code multiplex but: precise coordination required k1 k2 k3 k4 k5 k6 c f t

Code Division Multiplexing (CDM) Also known as code division multiple access (CDMA) An advanced technique that allows multiple devices to transmit on the same frequencies at the same time using different codes Used for mobile communications. Each mobile device is assigned a unique 64-bit code (chip spreading code) To send a binary 1, mobile device transmits the unique code To send a binary 0, mobile device transmits the inverse of code

Code Division Multiplex Each channel has a unique code All channels use the same spectrum at the same time Advantages: bandwidth efficient no coordination and synchronization necessary good protection against interference and tapping Disadvantages: lower user data rates more complex signal regeneration Implemented using spread spectrum technology k1 k2 k3 k4 k5 k6 c f t