streams of data in the form of bits
reaches network gateway from various
sources the Gateway forwards it to the
email note bead but how does the egos be
convert them into radio signals and
transmit them to the user / the air
interface
how does the user equipment communicate
back with denote the hello friends
welcome back to normal apology to go to
discuss link and download data
transmission in the network as discussed
earlier data is transmitted to know fdma
over parallel so carriers of 15 kilo
hertz the subcarriers are further
divided on the time axis is blocks of
one symbol duration or 66.7 microseconds
this basic unit is known as resource
element each resource element carries
one symbol now in order to transmit data
over these resource elements first data
is modulated with specific modulation
scheme this scheme depends on the
physical channels mapped on the resource
grid suppose we have eight bits of data
to be transmitted using UPS came
legislation the data will be divided
into four parallel streams of two bits
phase and amplitude assignment is done
according to the qpsk constellation
finally data is placed over the resource
elements by adjusting the phase and
amplitude of subcarrier to those derived
for the datastream mathematically it
means multiplying the complex modulation
symbol to the corresponding subcarrier
frequency thus in a 20 megahertz channel
all the 1200 resource elements carrying
1200 symbols over 1,200 subcarriers are
modulated with appropriate modulation
scheme now since the data is modulated
over 1,200 subcarriers a transmitter
will require 1200 oscillators for its
generation and another 1,200 will be
required
either receiver for property modulation
the hardware complexity and sheer amount
of power consumption would have left
ofdm to theoretical idea and far from
implementation ofdm was made reality
with the advent of digital signal
processing techniques so instead of
using 1200 oscillators ifft solves this
problem by converting the parallel
frequency-domain signals into samples of
a composite time domain signal which are
much easier to generate a transmitter
side all we need to do is to send these
time-domain samples at radio frequencies
as the data is being sampled by ifft
samples must be taken above the new
quest rate for faithful reproduction at
the receiver for an LT spectrum of 20
megahertz the highest frequency
component is nine megahertz which means
the sampling rate should be greater than
18 mega samples per second
in other words 1200 samples per ofdm
symbol of 66.7 microseconds since UMTS
had a sampling rate of 3.84 mega samples
per second to achieve backwards
compatibility sampling rate and LT is
taken as multiple of 3.84 mega samples
per second that's for 20 megahertz
spectrum which has a sampling rate of
30.7 to mega samples per second
there are 2048 samples per symbol in
OFDM a the fft size of the ifft
processor this depends on the LTE
bandwidth show now
previously we have shown in wireless
channel due to multipath propagation we
face delay spread and inter symbol
interference
we have also discussed how its effect
can be reduced by increasing the symbol
duration also how guard . were used to
eliminate it but abrupt changes in time
domain during transition period from
symbol to guard . causes inter carrier
interference in
frequency domain and disturbs the
orthogonal ality between subcarriers so
no fdma we employ a more complex kind of
guard . called cyclic prefix in which
and part of his symbol is transmitted in
the preceding guard . it also ensures
orthogonal ality between the subcarriers
by keeping the ofdm symbol periodic
overextended symbol duration and
therefore avoiding intercarrier and
internet channel interference
simultaneously senso fdma uses
compositae f50 samples cyclic prefix is
added by taking some samples from the
end of a simple . and placing them at
the beginning now this time sampled
signal is converted into analog wave by
a digital-to-analog converter further
composite waveform is modulated at the
desired radio frequency for transmission
for an example in operator having
license for 2320 megahertz to 2340
megahertz the 18 megahertz and will be
mapped from 2321 megahertz to 2339
megahertz using analog modulation and
finally the a node be transmitted signal
over air in oadby informs the user about
the allocated subcarriers and the
corresponding modulation scheme so when
the RF signal reaches at the user's
terminal all these processes are
reversed and finally user is able to
receive the data intended for it
although there are positives in OFDM a
but ifft summation of multiple parallel
carrier results in high peak to average
power ratio
hi pap our results in high power
consumption for signal generation but
handheld devices have limited power
capacity this makes ofdm a
favorable for uplink transmission in
order to overcome this LTE uses as the
fda in a blink direction in OFDM a we
have one-to-one mapping between symbol
and sub carrier but has the FDA allows a
symbol to be transmitted in parts over
multiple subcarriers for example in OFDM
a one symbol occupies one sub carrier of
15 kilo hertz but in as the fdma same
symbol is distributed among multiple
subcarriers of 15 kilo hertz in short as
the fdma behaves like a single carrier
system with short symbol duration
compared to ofdm a to achieve this as
the FDA introduces the n point FFT block
right after the serial to parallel
converter in the ofdm a structure the
fft block converts parallel sequence of
symbols in time domain two different
frequency points now peak to average
power ratio is proportional to the
square of number of carriers involved as
the FDA reduces papr by reducing the
number of carriers obviously there are
problems in single carrier transmission
as we discussed earlier but the side
effects are reduced by ensuring that the
total bandwidth over which a symbol is
transmitted is still not too high
apart from this remaining block and the
uplink direction is same as used in
downlink direction in our next video we
will be talking about physical transport
in logical
animals used in long term evolution so
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