hello friends today how can we stream a full HD  movie seamlessly which seemed impossible in legacy  

networks so friends in this video we have shown  what are the limitation of wireless channel how  

wide bands single channel are inefficient for  transmission to overcome how wide bands single  

channel is divided into small sub carriers and  increase efficiency by reducing delays pred how  

concept of orthogonality is introduced and FDMA  and achieves multi fold throughput hi everyone  

welcome back to the world of 4G previously we have  shown how mobility and global connectivity was  

achieved by introduction of wireless communication  but there are several complexities associated with  

Wireless channels multipath fading unlike a wired  channel which uses a fixed path the signals in a  

wireless channel can reach a user using multiple  paths all these signals known as multipath  

components may have different channel gain and  time delay this combined effect causes what  

we know as multiple failing delays Perret as a  consequence of multipath propagation the duration  

of a symbol gets extended this may interfere  with the next symbol this is called inter  

symbol interference or crosstalk guard periods  are introduced to avoid crosstalk frequency  

selective fading signals having bandwidth higher  than the coherence bandwidth of the channel faces  

variable attenuation at different frequencies  this ultimately distorts the signal and giving  

rise to frequency selective fading complex channel  equalization techniques are employed to reduce it  

inter channel interference often signal bandwidth  of adjacent carrier frequencies overlap with each  

other giving rise to inter channel interference  guard bands were introduced to avoid inter channel  

interference all these limitations compounded  with the scarcity of bandwidth gave rise to  

multiple access technique OFDM a before diving  into OFDM a let us understand multi-carrier  

wireless transmission system suppose a signal is  to be transmitted over a bandwidth fee and carrier  

frequency FC then symbol time for this would be t  is equal to 1 by B for a single carrier wideband  

channel of let's say 1 megahertz the symbol time  will be 1 microsecond given that the delay spread  

of the channel is of 2 microseconds the combined  symbol time would be 3 micro seconds which means  

delay spread occupies 66 percent of the combined  symbol time thus reducing the efficiency of the  

channel by one-third as delay spread is difficult  to control the effect of delay spread can be  

minimized by using multiple cell carriers of  lesser bandwidth so instead of having a single  

carrier of 1 megahertz we divided into 100 sub  carriers of 10 kilo Hertz each having a symbol  

duration of 100 microseconds so a delay spread of  2 microseconds will have a negligible effect over  

the channel efficiency this concept is used enough  DMA which uses slowly modulating sub carriers of  

higher symbol duration as these sub carriers are  modulated with data they gain bandwidth centered  

around the sub carriers frequencies guard bands  are used to separate them in frequency domain  

we can represent this transmitted signal in the  equation form as shown here where summation of  

individual symbol multiplied with different  carrier frequencies and transmitted at radio  

frequencies OFDM a orthogonal frequency-division  multiple access is a special case of this DMA  

where users are provided a set of sub carriers  overlapping in frequency domain however these sub  

carriers are specially designed to be orthogonal  to each other which allows them to occupy the same  

bandwidth without any interference this in turn  negates the use of guard bands as a result the  

subcarriers can be closely packed to increase  channel efficiency now before looking at the  

multiple access part let's understand how OFDM  more loggin or frequency division multiplexing  

works and how it's used in long term evolution in  OFDM high-speed data streams of large bandwidth  

are split into parallel slower sub streams of  lower bandwidth called sub carriers these sub  

carriers are centered around frequencies in  multiples of 15 kilo Hertz on both sides of  

DC as the lowest subcarrier is of 15 kilo Hertz  symbol duration TS is equal to 1 by 15 kilo Hertz  

or 66.7 microseconds consequently a 1 subcarrier  can provide a symbol rate of 15 kill symbol per  

second which is analogous to having a symbol  rate of one symbol per second for 1 Hertz of  

bandwidth or half a NIC west rate unlike in GSM  or UMTS LTE supports variable bandwidth as shown  

in the table as the bandwidth increases so does  the number of subcarriers in it let's take for  

example LTE band width of 20 megahertz which has  1200 sub carriers and thus subcarrier bandwidth  

of 18 mega Hertz 2 mega Hertz used as guard band  here we have 600 sub carriers on both side of the  

DC frequency all these carrier frequencies are  harmonics of 15 kilo Hertz varying from minus 9  

mega Hertz to 9 mega Hertz in time domain these  sub carriers will be represented as everlasting  

sinusoids at these carrier frequencies as shown  however in order to transmit data over these sub  

carriers they are loaded with modulation symbols  that represent the constellation points of digital  

modulation schemes like QPSK and Thor 2 columns  also the symbol duration for each of these cell  

carriers is always equal to sixty six point  seven microseconds which means that all these  

sub carriers have a whole number of cycles in  one symbol duration as we know that a rectangular  

function can be represented in frequency domain  as a sinc function which is centered around DC  

when multiplying a signal to a carrier frequency  and time domain signal will be shifts in frequency  

domain by the same amount of carrier frequency  thus we can represent these modulated sub carriers  

in frequency domain as a series of sync waves  centered around the carrier frequencies basically  

we are having 1200 such sync waves there are two  points to be noted here firstly the sub carriers  

are overlapping in frequency domain as we can  see the sub carriers are placed in a manner  

that all the other sub carriers have a zero  component at the peak of one sub carrier such  

sub carriers are called orthogonal as a result a  mobile can sample the frequency and face without  

any interference from neighboring sub carriers  orthogonality is achieved by ensuring that all  

the sub carriers have same symbol duration TS  and the subcarrier spacing is maintained at  

Delta F equal to one by TS secondly presence of  negative frequencies it can be explained by the  

fact that the sub carriers are transmitted below  the radio carrier so friends today we learn how  

information is split into smaller sub carriers  in multi carrier modulation and how efficiency  

was increased by placing new words all going in  our next video we will show how data starts its  

journey as a stream of fifths and are finally  transmitted as radio signal so friends don't  

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