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

samples at radio frequencies.

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

2048 samples per symbol in O FDMA.

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

signal over air.

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 uplink direction.

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.