OFDM

INTRODUCTION

Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier modulation technique.

Multicarrier transmission is a method to deal with frequency selective channels. In frequency

selective channels different frequencies experience different degrees of fading. The problem of

variation in fading levels among different frequency components is especially forced for high

data rate systems due to the fact that in a typical single carrier transmission the occupied

bandwidth is inversely proportional to the symbol period. The basic principle of multicarrier

transmission is to translate high rate serial data stream into several slower parallel streams such

that the channel on each of slow parallel streams can be considered flat. Parallel streams are

modulated on subcarriers.

In addition to that, by making symbol period longer on parallel streams the effect of the delay

spread of the multipath channel, namely inter-symbol interference (ISI), is greatly reduced. In

multipath channels multiple copies of the transmitted signal with different delays, which depend

on characteristics of the material from which the transmitted signal has been reflected, are

received at the receiver. The delay spread of a channel is a measure of degree of multipath effect

-it is equal to the difference between arrival times of the first and the last multipath components.

Due to the fact the length of the symbol period of each parallel stream scales proportionally

to the number of subcarriers used the percentage of overlap between two adjacent symbols due

to delay spread and resulting from it inter-symbol interference (ISI) also decreases

proportionally to the number of subcarriers.

ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING (OFDM)

Advantages Of Orthogonal Frequency Division Multiplexing (Ofdm)

In early multicarrier transmission systems subcarriers were non-overlapping to prevent

inter-carrier interference which can greatly degrade performance of a system. Individual

subcarriers were separated by guard bands which constituted wasted bandwidth.

The reason why Orthogonal Frequency Division Multiplexing (OFDM) has become the

most popular technique of multicarrier transmission is that subcarriers overlap in

frequency and therefore bandwidth utilization increases by up to 50%. Overlapping

subcarriers is allowed because in OFDM modulation subcarriers are orthogonal to each

other. Moreover, OFDM modulation/demodulation takes form of inverse FFT/FFT which

can be efficiently implemented in hardware using Fast-Fourier Transform algorithm.

When we use a wireless device to communicate to another one using a access point, most

of the time we are interested in high speeds.

Moreover with large data transmissions we require a good speed to transmit the data.

OFDM increases the throughput by dividing the bandwidth into subcarriers and

performing parallel transmissions using these subcarriers, thereby multiplexing the data.

Example:

If there are 10,000 mails to deliver to the place through highway ‘x’. Assume

highway ‘x’ is a traditional one with only a single lane to drive. If I send a single

truck loaded with 10,000 mails together, there is possibility that you might not get

any mails if an accident had happened

Assume, the highway ‘y’ where there are 5 lanes to drive. Now I send the 5 trucks in

each lane, each carrying 2000 mails. This time, even if there is an accident with one

of the trucks, you would receive 8000 mails, which is better than losing all of them.

Latter one is the OFDM concept. In between the 5 lanes, we can assume that there

are railings separating each lane from interference. These railings relate to guard

bands in OFDM preventing from inter symbol interference.

Literature Review

With the rapid growth of digital communication in recent years, the need for high-speed data

transmission has increased. The mobile telecommunications industry faces the problem of

providing the technology that be able to support a variety of services ranging from voice

communication with a bit rate of a few kbps to wireless multimedia in which bit rate up to 2

Mbps. Many systems have been proposed and OFDM system based has gained much attention

for different reasons. Although OFDM was first developed in the 1960s, only recently has it been

recognized as an outstanding method for high-speed cellular data communication where its

Implementation relies on very high-speed digital signal processing, and this has only recently

become available with reasonable prices of hardware implementation.

Multichannel Transmission

OFDM started in the mid 60’s, Chang [2] proposed a method to synthesis band limited signals

for multi channel transmission. The idea is to transmit signals simultaneously through a linear

band limited channel without inter channel (ICI) an inter symbol interference (ISI).

After that, Saltzberg [3] performed an analysis based on Chang’s work and he conclude that the

focus to design a multi channel transmission must concentrate on reducing crosstalk between

adjacent channels rather than on perfecting the individual signals.

In 1971, Weinstein and Ebert [4] made an important contribution to OFDM. Discrete Fourier

transform (DFT) method was proposed to perform the base band modulation and demodulation.

DFT is an efficient signal processing algorithm. It eliminates the banks of sub carrier oscillators.

They used guard space between symbols to combat ICI and ISI problem. This system did not

obtain perfect orthogonality between sub carriers over a dispersive channel.

It was Peled and Ruiz [5] in 1980 who introduced cyclic prefix (CP) that solves the orthogonality

issue. They filled the guard space with a cyclic extension of the OFDM symbol. It is assume the

CP is longer than impulse response of the channel.

Block Diagram:

Figure 1 Simulink Block Diagram for MIMO-OFDM

MatLab Results:

Bit Error Rate of OFDM:

Each time we run a bit-error-rate simulation. We transmit and receive a fixed number of

bits. We determine how many of the received bits are in error, and then compute the bit-

error-rate as the number of bit errors divided by the total number of bits in the transmitted

signal.

Using Matlab, we compute the bit-error-rate, ‘ber’, as:

ber= te/length(tx),

where ‘te’ is the total number of bit errors, and ‘tx’ is the transmitted bit vector.

The total length of the data is 96.

BER vs SNR

Verilog results:

Convolution encoder:

References:

[2]R.W Chang, “Synthesis of Band limited Orthogonal Signals for Multichannel Data Transmission”, Bell System Tech. J., pp.1775-1776, Dec 1996.

[3] B. R. Saltzberg, “Performance of an Efficient Parallel DataTransmission

System”, IEEE Trans. Comm., pp. 805-811, Dec 1967.

[4] S. B Weinstein and P.M. Ebert, “Data Transmission by Frequency Division

Multiplexing Using the Discrete Fourier Transform”, IEEE Transactions on

Communication Technology”, Vol. COM-19, pp. 628-634, October 1971.

[5] A. Peled an A. Ruiz, “Frequency Domain Data Transmission using Reduced

Computational Complexity Algorithms”, In Proc. IEEE Int. conf. Acoust.,

Speech, Signal Processing, pp. 964-967, Denver, CO, 1980