Comparison between adaptive OFDM and single carrier modulation with frequency domain equalization

The performance of multicarrier and single carrier modulation schemes is compared for wideband mobile radio communication systems. In both cases the fast Fourier transform (FFT) and its inverse are utilized. The transmission systems are simulated with time-variant transfer functions measured with a wideband channel sounder. In case of OFDM, the individual subcarriers are modulated with fixed and adaptive signal alphabets. In particular, the systems are compared with respect to the degradation due to pilot-aided channel estimation and carrier frequency synchronization.

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.

Multiple-input multiple-output (MIMO) signal processing combined with orthogonal frequency division multiplexing (OFDM) is a widely accepted solution to achieve the high bit rates that new communication standards require in frequency-selective wireless channels. In recent years, a new frequency-domain equalized (FDE) cyclic-prefixed single-carrier (CPSC) block transmission system has been proven to achieve similar performance to OFDM for coded systems with the same overall complexity, avoiding its main drawbacks and becoming a candidate for future wireless standards. A FDE CPSC spatial multiplexing system is analyzed and compared to OFDM. The implementation of different frequency-domain MIMO equalization and detection schemes is evaluated. Simulation-based performance results prove the efficiency of FDE CPSC based spatial multiplexing systems and highlight that these transmission schemes can outperform OFDM in uncoded and high coding rate MIMO systems, as it has been shown in the literature for the SISO case.

Burts-by-burst (BbB) adaptive high-speed down-link packet access (HSDPA) style multicarrier systems are reviewed, identifying their most critical design aspects. These systems exhibit numerous attractive features, rendering them eminently eligible for employment in next-generation wireless systems. It is argued that BbB-adaptive or symbol-by-symbol adaptive orthogonal frequency division multiplex (OFDM) modems counteract the near instantaneous channel quality variations and hence attain an increased throughput or robustness in comparison to their fixed-mode counterparts. Although they act quite differently, various diversity techniques, such as Rake receivers and space-time block coding (STBC) are also capable of mitigating the channel quality variations in their effort to reduce the bit error ratio (BER), provided that the individual antenna elements experience independent fading. By contrast, in the presence of correlated fading imposed by shadowing or time-variant multiuser interference, the benefits of space-time coding erode and it is unrealistic to expect that a fixed-mode space-time coded system remains capable of maintaining a near-constant BER. Following a basic portrayal of adaptive OFDM, the paper investigates a combined system constituted by a constant-power adaptive modem employing space-time coded diversity techniques in the context of both OFDM and multicarrier code division multiple access (MC-CDMA). The combined system is configured to produce a constant uncoded BER and exhibits virtually error-free performance when a turbo convolutional code is concatenated with a space-time block code. Again, it was found that the advantage of rendering the multiple-input multiple-output (MIMO)-aided modem BbB-adaptive erodes, when the affordable system complexity facilitates the implementation of multiple transmitters and receivers, if it can be ensured that no shadowing and no multiuser interference fluctuation is experienced. By contrast, in the presence of these impairments, only BbB-adaptive transceivers can provide a near-constant BER.

The paper studies the cyclostationarity of an orthogonal frequency division multiplexing (OFDM) with a view to recognizing OFDM against single carrier linear digital (SCLD) modulations. The analytical expressions for the nth-order cyclic cumulants (CCs) and cycle frequencies of an OFDM signal embedded in additive white Gaussian noise and subject to phase, frequency and timing offsets are derived An algorithm based on a second-order CC is proposed to recognize OFDM against SCLD modulations. The recognition algorithm of the authors obviates the need for preprocessing tasks, such as symbol timing estimation, carrier and waveform recovery, and signal and noise power estimation. The results of simulation experiments confirm the theoretical analysis.

Multi-carrier modulations have generated strong interest recently because of the challenging characteristics of important contemporary channels. While the idea of partitioning spectrally using orthogonal frequency division multiplexing (OFDM) was developed over thirty years ago, its implementation as a practical solution to a communications system problem is much more recent due to advances in digital signal processing and VLSI. Implementation has brought about a simultaneous increase in developing analysis that allow the performance characteristics of the approach to be understood in practical channels. Example channels where OFDM is being strongly considered are data communications over CATV channels and wireless applications such as digital terrestrial broadcasting. Widespread consideration of OFDM as a primary transport mechanism has occurred to date only for high speed data over the twisted pair channel. In this application, the technology is referred to as the Asymmetric Digital Subscriber Line (ADSL). Unlike that application, however, CATV and wireless systems represent RF channels. As such, one important characteristic of each is the phase jittery nature of low-cost, high frequency synthesized carriers. The research will present unique results indicating how these impairments degrade the OFDM system. The approach taken is to model the untracked jitter variation as a lowpass process, note the breakdown of traditional assumptions associated with single carrier modulations, and proceed with analytical solutions from this practical model. The single carrier solutions required are developed, including solutions for slow and rapid phase noise for standard and offset 16-QAM. Results for 64-QAM are also developed, and can be easily extended to any rectangular constellation, such as 256-QAM or 1024-QAM. A comparison of the various simplifying analysis assumptions is made to contrast with exact performance curves when phase noisy RF channels are encountered. A straightforward technique to improve link margin on phase-noise limited, slow jitter channels is presented. Finally, system design guidelines are developed for the difficult moderately varying jitter situation, critical for multi-carrier systems that rely on multiple subchannels of individually low symbol rate.

This paper deals with the synchronization of single carrier transmission with frequency domain equalization (FDE) for broadband mobile radio applications (e.g. wireless ATM). For such applications the carrier frequency and temporal position of the FFT (fast Fourier transform) window have to be synchronized. Furthermore, an estimation of the channel transfer function is necessary. A synchronization concept is presented which uses only one pilot FFT symbol for all the required synchronization tasks.

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

Orthogonal Frequency-Division Multiplexing (OFDM) is different from a single-carrier modulation, which is a digital multi-carrier modulation scheme. It has proceeded in broadband wireless or broadband wired communication as an effective settlement with Inter-Symbol Interference which occurred due to a multipath fading. In the Light Fidelity Technique, the OFDM modulation is well-known and widely employed; however, the using of OFDM modulation with Optical Camera Communication is difficult to employ. To increase data rate, the researchers consider to use multi-LED to transmit data but the effect among LEDs needs to consider. This paper presents the Multiple Input Multiple Output OFDM OCC system according to the effect of the rolling shutter. The proposed scheme according to OFDM deploys a rolling-shutter effect to transmit signal at a high rate with more than 2 LEDs. Based on the theoretical of the Rolling OFDM scheme, we propose a novel scheme architecture with technical details of the system architecture.

OF THE DISSERTATION Adaptive OFDM Radar for Target Detection and Tracking by Satyabrata Sen Doctor of Philosophy in Electrical Engineering Washington University in St. Louis, December 2010 Research Advisor: Dr. Arye Nehorai We develop algorithms to detect and track targets by employing a wideband orthogonal frequency division multiplexing (OFDM) radar signal. The frequency diversity of the OFDM signal improves the sensing performance since the scattering centers of a target resonate variably at different frequencies. In addition, being a wideband signal, OFDM improves the range resolution and provides spectral efficiency. We first design the spectrum of the OFDM signal to improve the radar’s wideband ambiguity function. Our designed waveform enhances the range resolution and motivates us to use adaptive OFDM waveform in specific problems, such as the detection and tracking of targets. We develop methods for detecting a moving target in the presence of multipath, which exist, for example, in urban environments. We exploit the multipath reflections by utilizing different Doppler shifts. We analytically evaluate the asymptotic performance of the detector and adaptively design the OFDM waveform, by maximizing the noncentrality-parameter expression, to further improve the detection performance. ii Next, we transform the detection problem into the task of a sparse-signal estimation by making use of the sparsity of multiple paths. We propose an efficient sparse-recovery algorithm by employing a collection of multiple small Dantzig selectors, and analytically compute the reconstruction performance in terms of the `1-constrained minimal singular value. We solve a constrained multi-objective optimization algorithm to design the OFDM waveform and infer that the resultant signal-energy distribution is in proportion to the distribution of the target energy across different subcarriers. Then, we develop tracking methods for both a single and multiple targets. We propose a tracking method for a low-grazing angle target by realistically modeling different physical and statistical effects, such as the meteorological conditions in the troposphere, curved surface of the earth, and roughness of the sea-surface. To further enhance the tracking performance, we integrate a maximum mutual information based waveform design technique into the tracker. To track multiple targets, we exploit the inherent sparsity on the delay-Doppler plane to develop an computationally efficient procedure. For computational efficiency, we use more prior information to dynamically partition a small portion of the delay-Doppler plane. We utilize the block-sparsity property to propose a block version of the CoSaMP algorithm in the tracking filter.

The 5G wireless communication requires transmission of information with high data rate, channel capacity, QoS in addition to less PAPR. The modulation scheme is performed in the physical layer of the network. Multi Carrier Modulation (MCM) schemes are widely preferred over single carrier modulation. Orthogonal Frequency Division Multiplexing (OFDM) is a very competent modulation method used in 4G wireless communication systems. This paper concentrates on the principle, analysis and comparison of Cyclic Prefix OFDM (CP-OFDM) with the Filtered OFDM (F-OFDM) at a frequency of 15 MHz, SNR of 18dB for various modulations like QPSK, 16QAM, 64QAM and 256QAM. F-OFDM is a flexible form of OFDM technique with lesser value of the peak to average power ratio (PAPR) allowing it to be used in the 5G wireless applications.

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.

An OFDMA based channel access scheme is proposed for dynamic spectrum access to utilize frequency spectrum efficiently. Though the OFDMA based scheme is flexible enough to change the bandwidth and channel of the transmitted signals, the OFDMA signal has large PAPR (Peak to Average Power Ratio). In addition, if the OFDMA receiver does not use a filter to extract sub-carriers before FFT (Fast Fourier Transform) processing, the designated sub-carriers suffer large interference from the adjacent channel signals in the FFT processing on the receiving side. To solve the problems such as PAPR and adjacent channel interference encountered in the OFDMA based scheme, this paper proposes a novel dynamic channel access scheme using overlap FFT filter-bank based on single carrier modulation. It also shows performance evaluation results of the proposed scheme by computer simulation.

We propose a fixed-power adaptive turbo coded modulation scheme for orthogonal frequency division multiplexing (OFDM). In this paper, we first observe that turbo coded modulation provides more than 15 dB signal-to-noise-ratio (SNR) gain on uncoded OFDM in fading channels. Then we conclude that when the transmission rate is adapted to the channel adequately, adapting the power as well yields a negligible capacity gain for adaptive OFDM from numerical simulation results. An adaptive OFDM scheme based on subband adaptation is given and mode allocation algorithm based on the capacity evaluation of current subband is presented. Simulation results show that our adaptive OFDM scheme exhibits more than 7.5 dB SNR gain relative to non-adaptive turbo coded modulation. We also discuss the effect of feedback delay and channel estimation error on bit-error-rate (BER) performance of our adaptive scheme.

We develop methods for detecting a moving target in the presence of multipath reflections, which exist, for example, in urban environments. We take advantage of the multipath propagation that increases the spatial diversity of the radar system and provides different Doppler shifts over different paths. We employ a broadband orthogonal frequency division multiplexing (OFDM) signal to increase the frequency diversity of the system as different scattering centers of a target resonate variably at different frequencies. To overcome the peak-to-average power ratio (PAPR) problem of the conventional OFDM, we also use constant-envelope OFDM (CE-OFDM) signaling scheme. First, we consider a simple scenario in which the radar receives only a finite number of specularly reflected multipath signals. We develop parametric measurement models, for both the OFDM and CE-OFDM signaling methods, under the generalized multivariate analysis of variance (GMANOVA) framework and employ the generalized likelihood ratio (GLR) tests to decide about the presence of a target in a particular range cell. Then, we propose an algorithm to optimally design the parameters of the OFDM transmitting waveform for the next coherent processing interval. In addition, we extend our models to study the aspects of temporal correlations in the measurement noise. We provide a few numerical examples to illustrate the performance characteristics of the proposed detectors and demonstrate the achieved performance improvement due to adaptive OFDM waveform design.