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1.1 INTRODUCTION
Orthogonal Frequency Division Multiplexing is a field that is in vogue and the latest mobile communication technologies have exploited the advantages of the technique. There have been a lot of architectures that have been presented to improve the performance of the OFDM system. The OFDM transceivers employ both Inverse Fast Fourier Transform and Fast Fourier Transform blocks. A variety of architectures have been presented for improved performance in OFDM transceivers. Specifically FFT architectures which improve on the throughput and latency have also been extensively researched over the last two decades.Hence, a lot of research works concentrate on realization of FFT/IFFT for OFDM System.In particular, the pipelined FFT architectures have mainly been adopted to address the difficulties due to their attractive properties, such as small chip area, high and low power consumption. To the best to our knowledge, two types of pipelined FFT architectures exist.They are Delay Feedback (DF) and Delay Commutator (DC). Further, according to the number of input data stream paths, they can be classified into Multiple-path (M) or Single-path (S) architectures. Single-path architectures could be appropriate in cases when the system cannot ensure concurrent operations. Because in SDC radix-2 pipelined FFT architecture, which can achieve 100% multiplier utilization by reordering the inner data sequence. FFT For instance, wireless communication (Mobile Ad-hoc Network (MANET), space and telecommunication) application require the high speed OFDM for data transmission whereas wired communication application require the lower power and less area OFDM System for sharing the information.
CHAPTER 2
LITERATURE SURVEY
2.1 REFERENCE PAPERS
2.1.1VLSI Based Adaptive FFT Model for OFDM Application
INFERENCE
In this article, the structure of adaptive Fast Fourier Transformation (FFT) is designed for OFDM System through Very Large Scale Integration (VLSI) System design environment.Both R2SDF FFT and R2MDC FFT have different advantages in terms of VLSI concerns.From this paper, we get a knowledge about novel FFT architecture for OFDM transceiver System that exploits correlation between Signal to Noise Ratio (SNR) information.
2.1.2 Design of Optimized Complex Multiplier for R2MDC FFT
INFERENCE
In this paper, low complexity Radix-2 Multi-path Delay Commutator (R2MDC) FFT frequency transformation technique is developed through Very Large Scale Integration (VLSI) System design environment.From this paper we get a ideathat complex multiplier structure of R2MDC FFT requires more hardware utilization than other structure. In order to overcome this problem, complex multiplier architecture of R2MDC FFT is effectively optimized. Proposed optimized complex multiplier architecture consumes less hardware complexity than existing one.
3Complex-Multiplier Implementation for Pipelined FFTs in FPGAs
INFERNECE
Different approaches for implementing a complex multiplier in pipelined FFT are considered and implemented to find an efficient one in this paper.From this paper we get an idea of the some approaches resulted in the reduced number of DSP blocks and others resulted in reduced number of LUTs. Analysis of Synthesis results is performed for different widths (bit lengths) of complex multiplier approaches.
2.1.4 A Combined SDC-SDF Architecture for Normal I/O Pipelined Radix-2 FFT
INFERENCE
In this paper, an efficient combined Single-path Delay Commutator-Feedback (SDC-SDF) radix-2 pipelined fast Fourier transform architecture, which includes log2 N − 1 SDC stages, and 1 SDF stage. The SDC processing engine is proposed to achieve 100% hardware resource utilization.
INTRODUCTION TO OFDM
OFDM is a Frequency Division Multiplexing 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
OFDM Block Diagram
OFDM is a specialized FDM, the additional constraint being all the carrier signals are orthogonal to each other.The sub-carrier frequencies are chosen so that the sub-carriers are orthogonal to each other meaning that cross-talk between the sub-channels is eliminated and inter-carrier guard bands are not required. This greatly simplifies the design of both the transmitter and the receiver, unlike conventional FDM, a separate filter for each sub-channel is not required.
3.2 OFDM TRANSMITTER
OFDM Transmitter consists of channel encoder,modulation technique,frequency transmission.
CHANNEL ENCODER
Error control is accomplished by the channel coding operation that consists of systematically adding extra bits to the output of the source coder. These extra bits do not convey any information but helps the receiver to detect and / or correct some of the errors in the information bearing bits. There are two methods of channel coding:
1. Block Coding: The encoder takes a block of ‘k’ information bits from the source encoder and adds ‘r’ error control bits, where ‘r’ is dependent on ‘k’ and error control capabilities desired.
2. Convolution Coding: The information bearing message stream is encoded in a continuous fashion by continuously interleaving information bits and error control bits.Here the hamming encoder and decoder is used for better performance.
HAMMING ENCODER
In coding theory, Hamming(7,4) is a linear error-correcting code that encodes four bits of data into seven bits by adding three parity bits. It is a member of a larger family of Hamming codes, but the term Hamming code often refers to this specific code that Richard W. Hamming introduced in 1950. At the time, Hamming worked at Bell Telephone Laboratories and was frustrated with the error-prone punched card reader, which is why he started working on error-correcting codes.
The Hamming code adds three additional check bits to every four data bits of the message. Hamming's (7,4) algorithm can correct any single-bit error, or detect all single-bit and two-bit errors. In other words, the minimal Hamming distance between any two correct codewords is 3, and received words can be correctly decoded if they are at a distance of at most one from thecodeword that was transmitted by the sender. This means that for transmission medium situations where burst errors do not occur, Hamming's (7,4) code is effective.
The goal of Hamming codes is to create a set of parity bits that overlap such that a single-bit error (the bit is logically flipped in value) in a data bit or a parity bit can be detected and corrected,While multiple overlaps can be created, the general method is presented in Hamming codes.
MODULATION TECHNIQUE
In OFDM ,the modulation technique used is QAM(Quadrature Amplitude Modulation).Quadrature Amplitude Modulation or QAM is a form of modulation which is widely used for modulating data signals onto a carrier used for radio communications.Quadrature Amplitude Modulation appears to increase the efficiency of transmission for radio communications systems by utilizing both amplitude and phase variations, QAM is a signal in which two carriers shifted in phase by 90 degrees are modulated and the resultant output consists of both amplitude and phase variations. In view of the fact that both amplitude and phase variations are present it may also be considered as a mixture of amplitude phase modulation.So,the QAM modulation isprefered.
The modulator are used to encode the signal, often data, onto the radio frequency carrier that is to be transmitted. The QAM modulator essentially follows the idea that can be seen from the basic QAM theory where there are two carrier signals with a phase shift of 90° between them.
These are then amplitude modulated with the two data streams known as the I or In-phase and the Q or quadrature data streams. These are generated in the baseband processing area.The two resultant signals are summed and then processed as required in the RF signal chain, typically converting them in frequency to the required final frequency and amplifying them as required.
It is worth noting that as the amplitude of the signal varies any RF amplifiers must be linear to preserve the integrity of the signal. Any non-linearities will alter the relative levels of the signals and alter the phase difference, thereby distorting he signal and introducing the possibility of data errors.
IFFT(Inverse Fast Fourier Transform)
OFDM is a frequency-division multiplexing technique using multiple orthogonal carriers. Each channel uses a Quadrature Amplitude Modulation (QAM) technique. In the transmitter the components of the complex input of each channel are lowpass filtered with pulse-shaping filters, multiplied respectively by a cosine and sine carrier, and added together. Alternatively the complex filtered input can be multiplied by a complex carrier. In other words instead of using cos(jωc t) and sine(jωc t) it uses exp(jωc t), where ωc is the carrier angular frequency of the channel.Basically it performs the operation (I+jQ) exp(jωct).However this is the typical operation performed by an FFT or IFFT. In the IFFT the complex conjugate of the product is used. The OFDM uses the very efficient algorithm of the FFT to perform the QAM modulation (in the transmitter) and demodulation (in the receiver) of the channels. By convention they decided to use the IFFT in the transmitter and the FFT in the receiver. In the receiver the complex conjugate version of the transmitter carrier is used.
CYCLIC PREFIX
The cyclic prefix used in Orthogonal Frequency Division Multiplexing provides an essential element of the overall signal acting as a guard band between each OFDM symbol.
Use of cyclic prefix is a key element of enabling the OFDM signal to operate reliably .
The cyclic prefix acts as a buffer region or guard interval to protect the OFDM signals from intersymbol interference. This can be an issue in some circumstances even with the much lower data rates that are transmitted in the multicarrier OFDM signal.
4Cyclic Prefix diagram
The basic concept behind the OFDM cyclic prefix is quite straightforward.The cyclic prefix is created so that each OFDM symbol is preceded by a copy of the end part of that same symbol.Different OFDM cyclic prefix lengths are available in various systems.
For example within LTE a normal length and an extended length are available and after Release 8 a third extended length is also included, although not normally used.
There are several advantages and disadvantages attached to the use for the cyclic prefix within OFDM
ADVANTAGES
o Provides robustness:The addition of the cyclic prefix adds robustness to the OFDM signal. The data that is retransmitted can be used if required.
o Reduces inter-symbol interference: The guard interval introduced by the cyclic prefix enables the effects of inter-symbol interference to be reduced
DISADVANTAGES
o Reduces data capacity:As the cyclic prefix re-transmits data that is already being transmitted, it takes up system capacity and reduces the overall data rate.
o Frequency Offset:
• Sub-carriers are very close and overlapping.
• Even a small frequency offset will result in ISI
• Causes of frequency offset:
Doppler shift
Phase noise caused in the channel
3.3 CHANNEL
The connection between transmitter and receiver is established through a communication channel.The communication can take place through wireline,wireless or fibre optic channels.The other media such as optical disks,magnetic tapes and disk etcmay also be called as a communication channel since they can also carry data through them.In digital communication the wireless channels are used.it eliminates the use of wires but the chance of error occurrence is more.However ,it may be noted that each and every communication channel has some inherent problems.These are :
1.Signal Attenuation: The signal attenuation in channel occurs due to the internal resistance of the channel and fading of the signal.
2.Amplitude and phase distortion:The transmitted signal is distorted in amplitude and phase due to the non linear characteristics of the communication channel .
3.Additive noise interference:Additive noise interference is produced due to internal solid state device and resistors etc., used to implement a communication system.
4.Mulipath distortion:The multipath distortion occurs mostly in wireless communication channel.
3.4 OFDM RECIEVER
REMOVE CYCLIC PREFIX
The addition of the cyclic prefixin the transmitter block removed in the receiver side.
FFT(Fast Fourier Transform)
In general, FFT is used to convert the time domain representation of sampled signal into frequency domain representation of sampled one. Two types of FFT structures are available for performing those transformations are (1) Decimation in Time (DIT) - FFT and (2) Decimation in Frequency (DIF) - FFT.The Radix-2 FFT architecture is based on Cooley – Tukey algorithm and it’s also called the butterfly diagram. Pipelined architecture is the best choice for high throughput applications.Several pipelined architectures have been developed, such as Multi-path Delay Commutator (MDC), Single-path Delay Feedback (SDF) and Single-path Delay Commutator (SDC) architecture was taken as a basis for implementation. Based on the radix-2/4/8 algorithm, an improved SDF architecture is designed.
QAM DEMODULATOR
The QAM demodulator is very much the reverse of the QAM modulator.The signals enter the system, they are split and each side is applied to a mixer. One half has the in-phase local oscillator applied and the other half has the quadrature oscillator signal applied.