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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