Chapter 7 - Applications

Orthogonal Frequency Division Multiplexing (OFDM) is one of the promising contendents in the race of very high data rate delivery, for 4G wireless systems. The conventional OFDM systems, so far deployed make use of Fast Fourier Transform (FFT) as their basic building block. Besides the numerous advantages, the restriction over the data rate imposed by the FFT in the form of cyclic prefix may be intolerable for the next generation OFDM systems. So, researches are on the way to replace FFT with a more suitable transform which is redundant free. DWT OFDM is one such system in which FFT in the conventional OFDM is replaced with Discrete Wavelet Transform (DWT). Using simulations, already it has been established that in ideal conditions, the performance of DWT OFDM is better than that of FFT OFDM. As phase noise can be considered as the major RF impairment and one of the leading performance limiting factors of the wireless systems, this paper analyses the Bit Error Rate (BER) performance of the DWT OFDM in presence of phase noise. BER performances of both DWT OFDM and FFT OFDM are analysed and compared in presence of phase noise. For this, Matlab Simulink model have been developed for both types of OFDM, and simulation is carried over in presence of phase noise for different modulation schemes. In all cases the BER performances of DWT OFDM is found to be fairly ahead that of FFT OFDM.

Conventional orthogonal frequency division multiplexing (OFDM) systems use discrete Fourier transform (DFT) as the basis function. Recently, it was shown that it is possible to use other sinusoidal transforms, e.g. discrete cosine transform (DCT), as OFDM basis functions. However, relative little is known regarding the performance of these alternative basis functions and how they compare with traditional DFT. This paper investigates the performance of OFDM modems employing alternative basis functions under different fading channel models in both double sideband and single sideband operation modes. A semi-analytical approach is also developed to predict and compare the error performance between different basis functions. The predicted results are close to the simulations thus verifying the simulation model. It was found that in general DFT has a better error performance, but interspersed basis is capable of offering a higher net throughout under certain channel conditions.

Abstract. Today Discrete Fourier Transforms (DFTs) are applied in various radio standards based on OFDM (Orthogonal Frequency Division Multiplex). It is important to gain a fast computational speed for the DFT, which is usually achieved by using specialized Fast Fourier Transform (FFT) engines. However, in face of the Software Defined Radio (SDR) development, more general (parallel) processor architectures are often desirable, which are not tailored to FFT computations. Therefore, alternative approaches are required to reduce the complexity of the DFT. Starting from a matrix-vector based description of the FFT idea, we will present different factorizations of the DFT matrix, which allow a reduction of the complexity that lies between the original DFT and the minimum FFT complexity. The computational complexities of these factorizations and their suitability for implementation on different processor architectures are investigated.

Objectives: For Orthogonal Frequency Division Multiplexing (OFDM), Fast Fourier Transform (FFT) processoris required. So, in this work, a variable point Pipelined FFT processor is designed by utilizing Verilog language. Further transmitter and receiver blocks are designed to form OFDM system. Methods: Speed enhancement is the key area which is been looked into this project work. Pipelined Architecture that is going to be implemented is RS2DF (Radix-2 Single path Delay Feedback). Standard FPGA Flow is adapted to implement this project i.e., right from specification to bit file generation. Simulations and Synthesis will be done using Questasim and Xilinx ISE. Verilog Simulations will be compared with in-built MATLAB FFT Core. MATLAB is used for reference and verification. Findings: This Project clarified the usage of FFT IP and OFDM in Xilinx ISE focusing on a specific family of FPGA. The execution of the proposed algorithm ought to perform superior to anything the base algorithm with the all-out basic way getting additionally advanced along these lines expanding the speed of activity. maximum operation frequency achieved was 236.189MHz but in base paper maximum operation frequency was 30Mhz. The systems utilized are Interna1 and externa1 Pipe1ining of modules and Distributed LUT based idea, the target of the above methods is to lessen the basic way deferral and increment the general speed of activity of structure, the disadvantage is the increments in area and output latency. Area is not of a much issue as present day days FPGAs has immense measure of assets. Output 1atency should be taken consideration with additional rationale at whatever point FFT is coordinated with other system modules. Application: The project depicts technique for streamlining timing basic ways of FFT to work at higher-speeds, to be utilized as a feature of LTE Protoca1 or any wire1ess protoco1 such as WLAN and LDACS. Keywords: Fast Fourier Transform (FFT), Hardware Description Language (HDL), Orthogonal Frequency Division Multiplexing (OFDM), Pipeline, Verilog

Code division multiple access (CDMA) and orthogonal frequency division multiplexing (OFDM) are two techniques that are being commonly used in most wireless communication systems. In this paper we examine the coexistence of the above schemes through a two user communication system, i.e., one user transmits using CDMA and the other transmits using OFDM. Using an OFDM based receiver, we derive the received symbols at the output of each branch of the OFDM demodulator. The symbols transmitted by the CDMA user are then detected using the minimum power distortionless response (MPDR) approach, an array processing technique commonly used in beamforming. Through simulations, we then show that the MPDR method is indeed very efficient in suppressing the OFDM interference

We propose an method of implementing simultaneously despreading operation for high speed downlink packet access (HSDPA) wideband code division multiple access (W-CDMA) signals using fast Fourier transform (FFT). HSDPSA is a part of the W-CDMA standard and it provides an access method for high speed data communications. The basic idea of the HSDPA is similar to that of the orthogonal frequency division multiplex (OFDM), namely, instead of using multiple frequency to transmit symbols, HSPDA uses multiple spreading codes to transmit symbols. Although the idea is same, they uses different transformation for multiplexing: OFDM uses the fast Fourier transformation (FFT) and, on the other hand, HSDPA (WCDMA) uses Walsh-Hadamard transformation (WHT). Therefore, it becomes hard to implement these two technologies into a single configuration. The proposed method implement the despreading process as a combination of an FFT and a post-processing. By computer simulation, it is shown that the proposed method provides same BER performance with that of the direct despreading calculation. Moreover, we show that, by generalizing the configuration of the proposed method, WCDMA can be expressed as a special case of the multicoded multicarrier (MC-MC) CDMA. MC-MC CDMA is a newly proposed multiplexing method and is a mixture of the OFDM and CDMA technologies. We show that all of OFDM, MC-MC CDMA and WCDMA can be implemented by a combination of an FFT and a multiplication of orthogonal matrix. Hence, using the proposed method, we can implement those three technologies into a single configuration.

Orthogonal Frequency Division Multiplexing (OFDM) is one of the most spectrally efficient methods so far, which helps to achieve high data rates and also mitigates the severe problem of the multipath propagation and Inter Symbol Interference. Many IEEE 802.11 standards have adopted OFDM in their implementations for wireless systems. The heart of the OFDM is the digital implementation of Discrete Fourier Transform (DFT). DFT is efficiently implemented by FFT and IFFT algorithms. Radix-2 FFT Cooley-Tukey algorithms are quite popular for FFT and IFFT [1] implementations. The objective of this paper is to propose a modified Cooley-Tukey Radix-2 Single-path delay feedback algorithm which is capable of computation of both FFT and IFFT for serial inputs with the help of same hardware rather than separate for each. The combined FFT/IFFT processor can find its applications in half-duplex OFDM-based communication systems. Due to the efficiency and simplicity of the hardware, this implementation is attractive for low-cost applications.

A precise method for calculating the bit error probability of a discrete cosine transform (DCT)-based orthogonal frequency-division multiplexing (OFDM) system on AWGN channels in the presence of frequency offset is derived. These accurate results are used to examine and compare the bit error probability performances of a DCT-OFDM system and the conventional discrete Fourier transform (DFT)-based OFDM system. Several signaling formats, such as binary phase shift keying (BPSK), quaternary phase shift keying (QPSK), and 16-ary quadrature amplitude modulation (16-QAM) are considered. Analysis and simulation results show that the DCT-OFDM system outperforms the DFT-OFDM system in the presence of carrier frequency offset.

Fractional Fourier domain equalization for DCT based OFDM system with CFO

OFDM (Orthogonal Frequency-Division Multiplexing) is one of the most used digital schemes at the moment. Conventionaly, OFDM uses inverse Fast Fourier transforms and Fast Fourier Transforms (IFFT/FFT) at the transmitter and receiver to modulate and demodulate data. This work analyses different alternatives to the use of these IFFT/FFT of OFDM systems by replacing them with Discrete Cosine Transform (DCT) blocks. This results in lower computational requirements, mainly due to the use of real arithmetic operations instead of complex operations, and higher information rates, shown in simulations.

The authors propose a novel localised discrete Fourier transform (DFT) spread M-ary amplitude shift keying (M-ASK) orthogonal frequency division multiplexing (OFDM) system with the Hermitian symmetry (DFT-spread M-ASK HS OFDM) to reduce the peak-to-average power ratio (PAPR). Compared with the conventional OFDM and localised DFT-spread OFDM, the proposed system has lower PAPR not only because of the DFT-spread scheme but also its lower modulation order than other OFDM systems. The lower modulation order of the proposed system can be achieved by using the Hermitian symmetry of DFT. The simulation result shows that the localised DFT-spread M-ASK HS OFDM provides a significant improvement in bit-error-rate and reduced PAPR.

The paper makes a brief introduction of Multiple Carrier-Code Division Multiple Access (MC-CDMA) scheme based on the principles of Orthogonal Frequency Division Multiplexing (OFDM) and CDMA, and a mathematical model of the uplink is established in MC-CDMA system.The simulation research on different multi-user detection methods in MC-CDMA system is implemented by using MATLAB software, and it also compares the impacts to system performance by several different multi-user detection methods, such as De-Correlation (DEC) multi-user detection, Minimum Mean Square Error (MMSE) multi-user detection and Parallel Interference Cancellation (PIC) multi-user detection, based on understanding and analyzing the experimental results.In addition, the paper also focuses on the research of multi level PIC multi-user detection performance in different primary detection methods and different series detections.

System-on-a-chip (SoC) is now a trend in digital design because it gives a lot of advantages over discrete electronic based product such as higher speed, lower power consumption, smaller size, lower cost etc. Reconfigurable platforms such as FPGA, CPLD, and PLD are now being used for designing and implementing SoC due to their low cost, high capacity and tremendous speed. In this project single chip Orthogonal Frequency Division Multiplexing (OFDM) transmitter and receiver have been designed using Verilog HDL. OFDM is a multi carrier modulation technique used in the various digital communication systems like 3G GSM, WiMAX and LTE etc. The main advantage of this transmission technique is its robustness to channel fading in wireless communication environment. There are many applications of OFDM in communication such as digital audio broadcasting, Asynchronous Digital Subscriber Line (ADSL) and High bit-rate Digital Subscriber Line (HDSL) systems. In OFDM, two algorithms digital signal processing algorithm Fast Fourier transform (FFT) and Inverse Fast Fourier Transform (IFFT) are mainly involved. The 8-point IFFT/FFT Decimation-In-Frequency (DIF) with radix-2 algorithm has been analyzed and incorporated in the design. The design has been simulated on the FPGA platform with Altera's Quartus II simulator. Simulation results show that each of the modules of the proposed OFDM is working as desired. The test output achieved from the simulation result of the OFDM has been verified with that of the MATLAB output.

This paper introduces four block joint detection (JD) receivers which combining the orthogonal frequency division multiplexing (OFDM) and code division multiple access (CDMA) techniques, i.e., OFDM-CDMA receiver. The performance of the OFDM-CDMA receiver with orthogonal and non-orthogonal spreading sequences is investigated in mobile radio channels. The sequences considered as spreading codes are orthogonal Walsh-Hadamard, Orthogonal Gold, Gold and maximim length sequences or m - sequences. The JD algorithms for the OFDM- CDMA receiver are Zero Forcing Block Linear Equalizer (ZF- BLE), Minimum Mean Square Error Block Linear Equalizer (MMSE-BLE), with and without decision feedback (DF). Time- domain equalization does not require DFT or FFT operations and can be used to compensate the channel fading effect. Hence for hardware implementation, the time domain interpretation can lead to a more efficient structure when compared with the frequency domain approach. The bit error rate (BER) performance comparisons of different spreading codes and equalizers are shown in the simulations.

Severe time-dispersion of the channel is one of the limiting factors for high data rate acoustic communications in shallow water. To overcome the large time spread, the research community has devoted enormous effort in the design of high efficient, low complexity equalizers. The goal of these equalizers is to convert the time-dispersive channel into a unit-tap impulse function. In another research direction, code division multiple access (CDMA) and orthogonal frequency division multiplexing (OFDM) systems are designed to combat the multi-path effect. However, using OFDM or CDMA alone to combat the multi-path effect often forces system designers to reduce the data rate in such severe time-dispersive channels. In this paper, we propose to adopt a class of generalized equalizers to generate an effective channel with a desired length. These equalizers are referred to as channel-shortening equalizers. Then, other advanced modulation schemes, such as OFDM and CDMA, can be employed to overcome the remaining time-dispersion of the channel. Compared to the unit-tap equalization, channel-shortening equalizers can provide quality performance at low computation cost. Examples of current channel-shortening equalizers are based on minimum mean-squared error (MMSE) and maximum shortening signal-to-noise ratio (MSSNR) criteria. In this paper, these algorithms are compared in the context of underwater acoustic communications. Their performance, complexity, and implementation in acoustic communications are investigated. In the digital subscriber line (DSL) channel, where channel-shortening equalizers are often applied to, MSSNR algorithms perform better than MMSE algorithms. However, we show that the MMSE algorithm is more suitable for the underwater acoustic channels with lower complexity and robustness to noise. With the MMSE method, a channel-shortening equalizer with 20 taps can convert a typical 120-tap acoustic channel into a 33-tap shortened channel. Using data obtained from the Kauai experiment, June-July 2003, it shows that the mean of the effective signal-to-interference ratio (SIR) achieves 15.7 dB. For the same channel, the optimum unit-tap equalization needs more than 200 taps to achieve the same effective SIR