in legacy networks? So friends, In this video we have shown what are the limitation of wireless

channel, how wideband single channel are inefficient for transmission. To overcome, how wideband

single channel is divided into small subcarriers and increase efficiency by reducing delay

spread. How concept of orthogonality is introduced in FDMA and achieved multifold 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 path. All these signals known as multipath

components may have different channel gain and time delay. This combined effect causes

what we know as multipath fading.

Delay spread- As a consequence of multi path propagation the duration of a symbol gets

extended. This may interfere with the next symbol. This is called Inter Symbol Interference(ISI)

or Cross-talk. Guard periods are introduced to avoid cross-talk.

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 interchannel interference. Guard bands were introduced to avoid interchannel

interference.

All these limitations compounded with the scarcity of bandwidth gave rise to Multiple access technique. OFDMA

Before diving into OFDMA let us understand multi carrier wireless transmission

system.

Suppose a signal is to be transmitted over a bandwidth B, and carrier frequency Fc. Then

symbol time for this would be TS equal to 1/B.

For a single carrier wideband channel of let’s say 1 MHz, 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 microseconds. Which means delay spread occupies 66% of the combined

symbol time. Thus reducing the efficiency of the channel by 1/3.

As delay spread is difficult to control, the effect of delay spread can be minimised by

using multiple sub carriers of lesser bandwidth. So instead of having a single carrier of 1 MHz,

We divide it into 100 sub carriers of 10 Kilohertz. 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 in FDMA. Which uses slowly modulating subcarriers of higher symbol duration.

As these subcarriers are modulated with data, they gain bandwidth centred around the subcarriers

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. OFDMA or Orthogonal Frequency division Multiple

Access, is a special case of FDMA, where users are provided a set of subcarriers overlapping

in frequency domain. However, these subcarriers 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 or orthogonal

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

substreams of lower bandwidth called sub carriers. These subcarriers are centered around frequencies

in multiples of 15 Kilohertz on both sides of D.C.

As the lowest subcarrier is of 15 kilohertz. symbol duration, TS is equal to 1/15 kilohertz

or 66.7 microseconds. Consequently a 1 subcarrier can provide a symbol rate of 15 Kilo symbol

per second. Which is analogous to having a symbol rate of one symbol/second/Hertz of

bandwidth or half the Nyquist 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 bandwidth of 20 MHz, which has 1200 subcarriers and thus a

subcarrier bandwidth of 18 MHz. 2 MHz used as guard band.

Here we have 600 subcarriers on both side of the DC frequency. All these carrier frequencies

are harmonics of 15 Kilo hertz, varying from -9 MHz to +9 MHz.

In time domain, these subcarriers will be represented as everlasting sinusoids at these

carrier frequencies as shown. However in order to transmit data over these

subcarriers, they are loaded with modulation symbols, that represent the constellation

points of digital modulation schemes, like QPSK and Nth order QAMs. Also the symbol duration

for each of these subcarriers is always equal to 66.7 microseconds, which means that all

these subcarriers 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 centred around DC. When multiplying a signal to a carrier frequency

in time domain, signal will be shifts in frequency domain by the same amount of carrier frequency.

Thus we can represent this modulating subcarriers in frequency domain as a series of sinc wave

centered around the carrier frequencies. Basically we are having 1200 such sinc waves.

There are two points to be noted here, Firstly, the subcarriers are overlapping in

frequency domain. As we can see, the subcarriers are placed

in a manner that all the other subcarriers have a zero component at the peak of one subcarrier.

Such subcarriers are called orthogonal. As a result, a mobile can sample the frequency

and phase, without any interference from neighbouring subcarriers.

Orthogonality is achieved by ensuring that all the subcarriers have same symbol duration

TS, and the subcarrier spacing is maintained at delta F = 1/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 learnt how information is

split into smaller sub-carriers in multi carrier modulation, and how efficiency was increased

by placing them orthogonally. In our next video we will show how data starts its journey

as stream of bits and are finally transmitted as radio signal.

So friends, Don’t forget to subscribe to our channel, like our videos and comments

your views or suggestions.HAPPY LEARNING.