ofdm in gnu radio

8/9/2019 Ofdm in Gnu Radio

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2015

Author: Rakesh

Email id: [email protected]

Orthogonal Frequency-Division Multiplexing (OFDM)


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#9/3, Shree Lakshmi Complex, 2nd Floor, Opp. To Vivekananda Park, Girinagar, Bangalore - 560085.

Ph. No: 080-26722726, Website:www.tenettech.com,Email: [email protected]

What is Gnu radio?

GNU Radio is a software library, which can be used to develop complete applications for radio

engineering and signal processing.

Introduction

GNU Radio is a free and open-source software development toolkit that provides signal

processing blocks to implement software radios. It can be used with readily-available low-cost

external RF hardware to create software-defined radios, or without hardware in a simulation-

like environment.

GNU Radio is licensed under the GNU General Public License (GPL) version 3. All of thecode is copyright of the Free Software Foundation. While all the applications are implemented

using python language while critical signal processing path is done using C++ language.

Idea behind GNURADIO

The goal is to give ordinary software people the ability to 'hack' the electromagnetic spectrum,

i.e. to understand the radio spectrum and think of clever ways to use it.

Why GNURADIO

Instead of purchasing multiple expensive radios, a single generic radio can be implemented

using gnu radio software and with support of minimal hardware to receive and transmit

processed signal at required frequencies and any data type can be passed from one block to

another i.e.it can be in bits, bytes, vectors, bursts or more complex data types

Since the performance critical blocks are implemented in C++ using processor floating point

extensions the developers are able to implement real-time, high-throughput radio systems in a

simple-to-use, rapid-application-development environment.

One can use it to write applications to receive data out of digital streams or to send data into

digital streams, which is then transmitted using hardware. GNU Radio has filters, channel
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codes, synchronisation elements, equalizers, demodulators, vocoders, decoders, and many

other elements which are called as blocks that are typically found in radio systems.

Features

The main features of gnu radio are flexibilityand configurability.

Extending GNU Radio is also quite easy, if you find a specific block that is missing you can

quickly create and add it.


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What is OFDM?

Cyclic prefix insertion is commonly used in orthogonal frequency division multiplexing

(OFDM) systems as a way to mitigate the effects of intersymbol-interference (ISI). It

copies the end section of an inverse fast Fourier transform (IFFT) packet to the beginning

of an OFDM symbol. Usually the length of the cyclic prefix is longer than the length of

the dispersive channel to completely remove ISI. OFDM modulation therefore mostly

revolves around cyclic prefix: OFDM modulation includes IFFT operation and cyclic

prefix insertion; OFDM demodulation includes cyclic prefix removal and FFT operation.

Modern communications systems feature highly dynamic scalability, which often

requires changing system parameters on-the-fly based on channel conditions and user

quality of service (QoS) requirements. This design example demonstrates cyclic prefix

insertion and removal for a reconfigurable OFDM system using the Altera FFT

MegaCore function. It supports run-time reconfiguration of FFT size and cyclic prefix

size. You can apply such designs to many different systems, such as OFDM-based

wireless communications systems, including WLAN, WiMAX, and 3GPP2 long term

evolution (LTE) systems, as well as digital video broadcast (DVB) systems.

Orthogonal frequency-division multiplexing (OFDM) is a method of encoding digital

data on multiple carrier frequencies. OFDM has developed into a popular scheme for

wideband digital communication, whether wireless or over copper wires, used in

applications such as digital television and audio broadcasting, DSL Internet access,

wireless networks, power line networks, and 4G mobile communications.

OFDM is a frequency-division multiplexing (FDM) scheme used as a digital multi-

carrier modulation method. A large number of closely spaced orthogonal sub-carrier

signals are used to carry data on several parallel data streams or channels. Each sub-

carrier is modulated with a conventional modulation scheme (such as quadrature

amplitude modulation or phase-shift keying) at a low symbol rate, maintaining total data

rates similar to conventional single-carrier modulation schemes in the same bandwidth.


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The primary advantage of OFDM over single-carrier schemes is its ability to cope with

severe channel conditions (for example, attenuation of high frequencies in a long copper

wire, narrowband interference and frequency-selective fading due to multipath) without

complex equalization filters. Channel equalization is simplified because OFDM may be

viewed as using many slowly modulated narrowband signals rather than one rapidly

modulated wideband signal. The low symbol rate makes the use of a guard interval

between symbols affordable, making it possible to eliminate intersymbol interference

(ISI) and utilize echoes and time-spreading (on analogue TV these are visible as ghosting

and blurring, respectively) to achieve a diversity gain, i.e. a signal-to-noise ratio

improvement. This mechanism also facilitates the design of single frequency networks

(SFNs), where several adjacent transmitters send the same signal simultaneously at the

same frequency, as the signals from multiple distant transmitters may be combined

constructively, rather than interfering as would typically occur in a traditional single-

carrier system.


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1) Implementation of Orthogonal frequency-division multiplexing

The GNUradio Companion with basic blocks (Options block and Variable block)

Options blockdefines the ID, Title and Description of the experiment.


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Variable Blockdefines to declare variables globally. This block maps a value to a unique

variable. This variable block has no graphical representation. The variable can be referenced

(by ID) from other blocks in the flowgraph.

o ID = samp_rate

o Value = 32000Hz

Random source: Generates samples of random number of [min,max) repeat samples if

specified.


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o ID:analog_random_source_x_0

o Output type: Byte

o Number of samples: 1000

SHORT to FLOAT: Convert stream of short to stream of float.

ID: blocks_short_to_float_0

VEC Length: 1

OFDM Mod: Modulates an OFDM stream. Based on the options FFT length,occupied tones

and cp length this block creates OFDM symbols using a specified modulation option.


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o ID:digital_ofdm_mod_0

o Input type: Byte

o Modulation: BPSK

o FFT length: 512

Throttel block: Throttel is a device that control the flow of samples such that average rate does

not exceeds samples/sec.Throttle is used because no hardware interface has done.


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o ID: blocks_throttel_0

o Type: complex

o Vec length: 1

WX GUI FFT Sink: Used to control how the computed FFT is scaled and fit to the available

plot area. Grid position is used to position the graphical element in the window. Notebook paramis used to place the graphical element inside of a notebook page.

o ID: wxgui_fftsink2_0

o Type: complex


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The Complete block representation

Genrated output

ofdm in gnu radio

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