Orthogonal Frequency Division Multiplexing Demodulation Research Articles

On the Performance of OFDM Systems Over a Cartesian Clipping Channel: A Theoretical Approach

We introduce an accurate theoretical approach for computing the symbol-error rate (SER) of an M-ary quadrature amplitude modulation (M-QAM) orthogonal frequency division-multiplexing (OFDM) system in the Nyquist rate Cartesian clipping channel. The Cartesian clipper clips the high peak values of the Nyquist rate in-phase/quadrature (I/Q) components of the complex baseband OFDM signal separately. In contrast to previous works that approximate the nonlinear noise, in the frequency domain, as a Gaussian additive random process, an accurate expression is derived for the probability density function (pdf) of the clipping noise at the output of the OFDM demodulator on each subcarrier. The inverse Fourier transform of the characteristic function of the noise is used to derive this accurate pdf. Using this pdf, we can evaluate the performance of the OFDM system for each subcarrier with high accuracy, especially at high backoffs where the Gaussian approximation of the nonlinear noise is no longer valid. The proposed method has the accuracy and validity of the simulation while being comparatively much less time consuming.

Performance Analysis of Multiple-Symbol Differential Detection for OFDM over Both Time- and Frequency-Selective Rayleigh Fading Channels

The performance of orthogonal frequency-division multiplexing (OFDM) system with multiple-symbol differential detection (MSDD) is analyzed over both time- and frequency-selective Rayleigh fading channels. The optimal decision metrics of time-domain MSDD (TD-MSDD) and frequency-domain MSDD (FD-MSDD) are derived by calculating the exact covariance matrix under the assumption that the guard time is longer than the delay spread, thus causing no effective intersymbol interference (ISI). Since the complexity of calculating the exact covariance matrix turns out to be substantial for FD-MSDD, we also develop a sub-optimal metric based on the simplified covariance matrix. The comparative analysis between TD-MSDD and FD-MSDD suggests that the most significant improvement is achieved by the FD-MSDD with the optimal metric and a large symbol observation interval, since the time selectiveness of the channel has a dominant effect on the bit error rate of the OFDM system.

Blind and Semiblind Detections of OFDM Signals in Fading Channels

This paper considers the problem of blind joint channel estimation and data detection for orthogonal frequency-division multiplexing (OFDM) systems in a fading environment. Employing a regression model for a time-varying channel, we convert the problem into one that finds the data sequence x whose associated least-squares (LS) channel estimate z(x) is closest to the space of some regression curves (surfaces). We apply the branch-and-bound principle to solve the nonlinear integer programming problem associated with finding the curve that fits a subchannel in the LS sense. A recursive formula for fast metric update is obtained by exploiting the intrinsic characteristic of our objective function. The impacts of reordering the data sequence and selective detection are addressed. By employing a preferred order along with a selective detection method, we greatly reduce the detector complexity while giving up little performance loss. Both the complete and the reduced-complexity algorithms can be used for blind and semiblind detections of OFDM signals in a subchannel-by-subchannel manner. To further reduce the complexity and exploit the frequency-domain channel correlation, we suggest a two-stage approach that detects a few selected positions in some subchannels first, and then, treating the detected symbols as pilots, determines the remaining symbols within a properly chosen time-frequency block by a two-dimensional model-based pilot-assisted algorithm. The proposed methods do not require the information of the channel statistics like signal-to-noise ratio or channel correlation function. Performance of differential modulations like differential quaternary phase-shift keying and STAR 16-ary quadrature amplitude modulation are provided. Both blind and semiblind schemes yield satisfactory performance.

位相回転にトランスペアレントなトレリス符号化変調を用いたＯＦＤＭにおけるＰＴＳ付加情報の伝送

Orthogonal Frequency Division Multiplexing (OFDM) signals consist of many independent subcarriers and have a high peak power. The Partial Transmit Sequence (PTS) scheme has been proposed as an effective scheme for reducing the peak power. The scheme employs phase rotations of subcarriers, and a set of rotated angles corresponds to side information, which is transmitted through the same channel as data or another channel. In this paper a coding technique is considered to improve the performance of both data and side information in an OFDM system with PTS. Since the side information is used at the detection of OFDM, the side information should be recovered before the detection. This is, in general, against the causality, because the side information is not obtained before the detection. We proposed a new coding method based on a rotationally invariant trellis coded modulation for the data and side information. If we insert side information into the position that does not suffer from phase rotation, it becomes possible to correct the error due to rotations after decoding. Thus we can obtain coding gain and peak power reduction in the OFDM system using a PTS. As a result we show that the bit error rate of the encoder is at most twice worse than the normal.

Impulsive noise suppression in OFDM based communication systems

Orthogonal frequency division multiplexing (OFDM) is a technique used for terrestrial digital video broadcasting (DVB-T) and many other modern applications. The longer OFDM symbol duration provides an advantage in a presence of weak impulsive noise, because impulsive noise energy is spread among simultaneously transmitted OFDM sub-carriers. However, it has been recently recognized that this advantage turns into a disadvantage if the impulsive noise energy exceeds certain threshold. In this paper the algorithm for impulsive noise suppression in OFDM receivers is proposed and investigated. Whereas traditional methods for impulsive noise suppression are implemented in a time domain before OFDM demodulation, proposed algorithm compensates impulsive noise in a frequency domain after OFDM demodulation and channel equalization. The method is applied to DVB-T and its performance is studied by means of simulation.

Totally blind APP channel estimation for mobile OFDM systems

A new two-dimensional blind channel estimation scheme for coherent detection of orthogonal frequency-division multiplexing (OFDM) signals in a mobile environment is presented. The channel estimation is based on the a posteriori probability (APP) calculation algorithm. The time-variant channel transfer function is completely recovered without phase ambiguity with no need for any pilot or reference symbols, thus maximizing the spectral efficiency of the underlying OFDM system. The phase ambiguity problem is solved by using a 4-QAM (quadrature amplitude modulation) scheme with asymmetrical arrangement. The results clearly indicate that totally blind channel estimation is possible for virtually any realistic time-variant mobile channel.

0.5-μm CMOS circuits for demodulation and decoding of an OFDM-based digital TV signal conforming to the European DVB-T standard

In the context of digital terrestrial TV based on the DVB-T standard, four 0.5-/spl mu/m CMOS IC's (IC1-IC4) are presented. IC1 integrates an 8-K fast Fourier transform for orthogonal frequency division multiplexing demodulation, IC2 performs channel estimation/correction, and IC3 is a forward error corrector implementing a Viterbi and a Reed-Solomon decoder. IC4, which is based on a digital signal-processing core, performs the synchronization tasks of the complete receiver. These four chips have been designed and manufactured using a 0.5-/spl mu/m, 3,3-V, triple-metal CMOS process. Their global complexity is about 500 kgates of standard cells and 1.5 Mbits of memory, which represents a total die area of 435 mm/sup 2/ in 0.5 /spl mu/m. The total power dissipation is about 3.5 W when working at nominal frequency. More generally, these four IC's constitute the digital front-end part of a global chipset receiver (specified within the European project DVBird), also including an analog front end and a MPEG2 demultiplexer IC.