Orthogonal Frequency Division Multiplexing Symbol Length Research Articles

Time-Domain Channel Estimation Scheme for OFDM over Fast Fading Channels

In high-mobility scenarios, the time variation of mobile radio channels leads to a loss of orthogonality among subcarriers in orthogonal frequency division multiplexing (OFDM) systems, resulting in intercarrier interference (ICI) and performance deterioration. Conventional channel estimation schemes are usually based on pilot tones, which are distributed in each OFDM symbol to estimate the channel variation. Hence, the channel estimator itself suffers from ICI. In this study, a new estimation scheme, which does not suffer from ICI, is proposed to estimate the channel variation within OFDM symbols. The main idea is to zero-pad (ZP) the OFDM symbol in the time domain. Then, in the middle of the ZP interval, an impulse signal is inserted as a pilot sample, which is used to estimate the channel at the pilot signal in the OFDM symbol. Finally, a linear model is used to estimate the channel variation over an OFDM symbol. Additionally, we derive the mean squared error (MSE) of the proposed estimation technique under the constraint that the channel varies linearly within OFDM symbols. Simulation results show that our scheme can achieve a substantial improvement in the bit error rate (BER) performance of OFDM, in spite of the OFDM symbol length being increased. Moreover, in many cases, the new scheme can achieve the same BER performance as the perfect knowledge of channel state information (CSI). Theoretical analysis and numerical simulations show that our scheme achieves excellent performance with much lower computational complexity.

Symbol error rate on fading self-interference channel in full-duplex

In-band full-duplex (FD) is being considered as a promising technology for the next generation wireless communication systems. In this paper, the performance of orthogonal frequency division multiplexing (OFDM) modulation system with different symbol duration and the code spreading system with different spreading sequence lengths under time-varying self-interference (SI) channel in FD mode is investigated respectively. Typically, the SI channel is estimated during the SI cancellation duration and used for SI suppression in the whole data transmission duration. First, the expressions of the residual SI power during the data transmission duration are derived under the classical, the uniform, and the two-way Doppler SI channels. Second, the signal-to-interference-and-noise-ratio after the SI mitigation is obtained. Third, the symbol error rates for the OFDM modulation and the code spreading systems are given. Simulation results show that OFDM symbol length should be selected longer when the symbol duration is significantly lower than the SI coherent time while the length of the coding spreading system should be chosen shorter.

On Performance of Vector OFDM With Linear Receivers

Vector Orthogonal Frequency Division Multiplexing (OFDM) for single transmit antenna systems is a general transmission scheme, where OFDM and Single-Carrier Frequency Domain Equalization (SC-FDE) can be treated as two special/extreme cases. Due to its flexibility, it has drawn more and more attention recently. So far, all the studies about Vector OFDM assume the Maximum Likelihood (ML) receiver. In this paper, we investigate the performance of Vector OFDM with linear receivers, i.e., the Zero-Forcing (ZF) and Minimum Mean Square Error (MMSE) receivers. We first show that the detection SNR gap between the MMSE and ZF receivers increases with both channel SNR and the vector blocks (VB) size defined in Vector OFDM. Then, it is proved that for both ZF and MMSE receivers, all the transmitted symbols have equal performance. This is different from the Vector OFDM with ML receiver, where different VBs may have different coding gain, and thus may have different performances. We analyze the diversity order for Vector OFDM with MMSE receiver, and show that, regardless of the Vector OFDM symbol length <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> , the diversity order can be represented as min{[ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-R</sup> ], <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</i> }+1, where <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> is the VB size, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> is the spectrum efficiency in bits/symbol, and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</i> is the maximum delay of the multipath channel. For Vector OFDM with ZF receiver, we show that the diversity order equals 1 and the performance is the same as the conventional OFDM at high SNR.

Intercarrier interference suppression and channel estimation for OFDM systems in time-varying frequency-selective fading channels

In orthogonal frequency division multiplexing (OFDM) systems, channel variations within an OFDM block destroy orthogonality between subcarriers, resulting in intercarrier interference (ICl), which increases an error floor in proportional to normalized Doppler frequency. To mitigate the effects of channel variations, many schemes have been proposed, but they are computationally complex or at the price of sacrificing spectral efficiency. In this paper, we first provide an ICI analysis in both time- and frequency-domain while existing literatures analyze the ICI effects mainly in the frequency domain. Based on this analysis, we then propose a novel ICI suppression technique, which uses iterative ICI cancellation in time domain and has a computational complexity that is linear in the OFDM symbol length. Channel estimation in rapidly time-varying scenarios is critical for ICl suppression and coherent demodulation. We also propose a scheme for estimating the channel parameters varying within an OFDM block. Along with the channel estimation scheme, we investigate the issue of optimum pilot tone placement. Theoretical analysis and simulation results show that the proposed ICl suppression method and channel estimation scheme have low computational complexity and good performance for most Doppler frequency of practical importance.

Low-Complexity Equalization of OFDM in Doubly Selective Channels

Orthogonal frequency division multiplexing (OFDM) systems may experience significant inter-carrier interference (ICI) when used in time- and frequency-selective, or doubly selective, channels. In such cases, the classical symbol estimation schemes, e.g., minimum mean-squared error (MMSE) and zero-forcing (ZF) estimation, require matrix inversion that is prohibitively complex for large symbol lengths. An analysis of the ICI generation mechanism leads us to propose a novel two-stage equalizer whose complexity (apart from the FFT) is linear in the OFDM symbol length. The first stage applies optimal linear preprocessing to restrict ICI support, and the second stage uses iterative MMSE estimation to estimate finite-alphabet frequency-domain symbols. Simulation results indicate that our equalizer has significant performance and complexity advantages over the classical linear MMSE estimator in doubly selective channels.

ICI and ISI analysis and mitigation for OFDM systems with insufficient cyclic prefix in time-varying channels

Channel variation during an OFDM block leads to the loss of orthogonality among subcarriers, resulting in inter-carrier interference (ICI) in orthogonal frequency division multiplexing (OFDM) systems. Many schemes have been proposed to suppress ICI, but they are computationally complex or at the price of sacrificing bandwidth. In some cases, such as high-density television (HDTV) broadcasting and satellite OFDM systems, the very long delay spreads pose the possibility that the channel length exceeds that of the moderate cyclic prefix (CP), resulting in intersymbol interference (ISI) and intercarrier interference (ICI). A time-domain equalizer (TEQ) is usually used in the receiver to reduce the duration of the overall response of the transmission system, and therefore minimize the ISI and ICI. However, the optimum design of TEQ turns out to be a difficult task. The ICI's introduced by insufficient CP and channels variations are different and usually exploit different methods to mitigate them. In this paper, we provide an ICI and ISI analysis in time- and frequency- domain for OFDM systems with insufficient CP over time-variant channels. Basing on this analysis, we propose an iterative method for joint ICI and ISI cancellation whose complexity is linear in the OFDM symbol length. Theoretical analysis and simulation results indicate that the proposed method can effectively mitigate ICI and ISI introduced by channel variation and insufficient CP with high bandwidth efficiency.

Iterative decoding for reducing cyclic prefix requirement in OFDM modulation

Orthogonal frequency division multiplexing (OFDM) requires a cyclic prefix extension to combat the detrimental effects of excess delay in the channel. A cyclic prefix length of 25% of total OFDM symbol length is specified for next generation wireless local area network systems (IEEE standard 802.11a). Whilst combating multipath and enabling single tap equalisation in the frequency domain, the additional overhead has a detrimental effect on data throughput. Presented is the performance of low density parity check encoding at the transmitter and iterative (‘turbo’) decoding at the receiver to remove the cyclic prefix. It is shown that with an initial log likelihood operation at the receiver, iterative decoding is able to overcome the intersymbol interference introduced by the channel.