Name: 4 - OFDMA/SC-FDMA 第2部 4G（LTE）の基礎知識 (4 - OFDMA/SC-FDMA Part 2 - Fundamentals of 4G (LTE))
Uploaded: 2021-01-14T07:03:01.000Z
Duration: 7 min 58 s
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：

Streams of data, in the form of bits reaches
network gateway from various sources.

The Gateway forwards it to the eNodeB. But how
does the eNodeB converts them into radio

signals and transmit them to the user over
the air interface? How does the user equipment

communicate back with eNodeB?
Hello Friends. Welcome back to the world of 4G.

Today we will discuss uplink and downlink
data transmission in the network.

As discussed earlier, Data is transmitted
in OFDMA over parallel subcarriers of 15 Kilo hertz.

The subcarriers are further divided
on the time axis as 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 8 bits

of data to be transmitted using QPSK Modulation.
The data will be divided into four parallel

streams of 2 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 data
stream. Mathematically, it means multiplying

the complex modulation symbol, to the corresponding
subcarrier frequency.

Thus, in a 20 MHz channel, all the 1200 resource
elements carrying 1200 symbols over 1200 subcarriers

are modulated with appropriate modulation
scheme.

Now, Since the data is modulated over 1200
subcarriers, A transmitter will require 1200

oscillators for its generation, and another
1200 will be required by the receiver, for

proper Demodulation. 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 at Transmitter side.
all we need to do, is to send these time domain

As the data is being sampled by IFFT, Samples
must be taken above the Nyquist rate for faithful

reproduction at the receiver. For an LTE spectrum
of 20 MHz, the highest frequency component

is 9MHz 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 in LTE is taken as multiple
of 3.84 Mega samples per second. Thus for

20 MHz Spectrum which has a sampling rate
of 30.72 mega samples per second, there are

The FFT size of the IFFT processor thus depends
on the LTE bandwidth as shown.

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 it's effect can

be reduced by increasing the symbol duration.
Also, how guard period were used to eliminate

it. But abrupt changes in time domain during
transition period, from symbol to guard period

causes inter carrier interference in frequency
domain, and disturbs the orthogonality between subcarriers

So In O FDMA, We employ a more
complex kind of a guard period, called Cyclic prefix,

in which end part of a symbol is transmitted
in the preceding guard period. It also ensures

orthogonality between the sub-carriers, by
keeping the OFDM symbol periodic over the

extended symbol duration and therefore, avoiding
Inter-carrier and inter channel Interference simultaneously.

since O FDMA uses composite
IFFT samples, cyclic prefix is added by taking

some samples from the end of a symbol period,
and placing them at the beginning.

Now This Time sampled signal, is converted
into an analog wave by a Digital to Analog

Converter. Further composite waveform is modulated
at the desired radio frequency for transmission.

For eg. For an operator having license for
2320 Mhz to 2340 Mhz. The 18 Mhz band will be

mapped from 2321 Mhz to 2339 Mhz using analog
modulation. And finally the EnodeB transmits

ENB 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 many positives in O-FDMA,
but IFFT summation, of multiple parallel subcarrier

results in high Peak to Average power ratio.
High PAPR results in high power consumption

for signal generation. But handheld devices
have limited power capacity. This makes OFDMA

unfavourable for uplink transmission.
In order to overcome this, LTE uses SC-FDMA

In O FDMA, we have one to one mapping between
symbol and subcarrier, but SC-FDMA allows

a symbol to be transmitted in parts, over
multiple subcarriers. For example, in O FDMA

one symbol occupies one subcarrier of 15 kilo
hertz, but in SCFDMA, same symbol is distributed

among multiple subcarriers of 15 Kilo hertz.
In short, SC-FDMA behaves like a single carrier

system with short symbol duration compared
to OFDMA.

To achieve this, SC-FDMA introduces a N point
FFT block right after the serial to parallel

converter in the O FDMA structure. The FFT
block converts parallel sequence of symbols

in time domain to different frequency points.
Now, Peak to average power Ratio is proportional

to the square of number Of carriers involved.
SC-FDMA 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

in the uplink direction is same as used in
Downlink Direction.

In our next video, we will be talking about
Physical, Transport and Logical channels used

in Long Term Evolution. So friends, Don’t
forget to subscribe to our channel, like our

videos and comments your views or suggestions.