2015 Orthogonal Frequency-Division Multiplexing (OFDM)
Author: Rakesh Rakesh Email id:
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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 simulationlike environment. GNU Radio is licensed under the GNU General Public License (GPL) version 3. All of the code 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 flexibility and 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 multicarrier 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 subcarrier 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 singlecarrier system.
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1) Implementation of Orthogonal frequency-division multiplexing The GNUradio Companion with basic blocks (Options block and Variable block)
defines the ID, Title and Description of the experiment. Options block defines
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defines to declare variables globally. This block maps a value to a unique u nique Variable Block defines variable. This variable block has no graphical representation. The variable can be referenced (by ID) from other blocks in the flowgraph. ID = samp_rate o Value = 32000Hz o
Random source: Generates samples of random number of [min,max) repeat samples if specified.
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ID:analog_random_source_x_0 Output type: Byte o 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 o
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 o o o
ID:digital_ofdm_mod_0 Input type: Byte Modulation: BPSK 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 samples/sec. Throttle is used because no hardware interface has done.
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o o o
ID: blocks_throttel_0 Type: complex 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 param is used to place the graphical element inside of a notebook page. ID: wxgui_fftsink2_0 o Type: complex o
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The Complete block representation
Genrated output
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