Differential Orthogonal Frequency Division Multiplexing Research Articles

Error Probability Analysis for Rectangular Differential OFDM With Index Modulation Over Dispersive Channels

In this letter, a novel forgetting factor aided rectangular differential (RD) orthogonal frequency division multiplexing (OFDM) with index modulation (IM) is proposed for dispersive channels, which amalgamates the concept of RD coding for exploiting the benefits of OFDM-IM with the absence of channel state information (CSI). To be more specific, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${N}$ </tex-math></inline-formula> subcarriers are partitioned into <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${G}$ </tex-math></inline-formula> subblocks for IM mapping. By employing RD coding during two adjacent subblocks, non-coherent detection without CSI can be carried out at the receiver. To further improve the performance, a novel forgetting factor is exploited at the receiver, which can be optimized via a closed form without extra complexity. Based on the derived forgetting factor, the upper bound of the average bit error probability (ABEP) of the proposed RD-OFDM-IM scheme is derived and is validated by the simulation results. Both the theoretical and simulation results indicate that the proposed RD-OFDM-IM scheme is capable of providing a considerable performance gain over its conventional differential OFDM (D-OFDM) counterpart.

Differential orthogonal frequency division multiplexing communication in water pipeline channels.

The problem of information transmission through water pipelines is addressed and a class of methods based on differentially encoded orthogonal frequency division multiplexing (OFDM) is proposed. Specifically, two methods are investigated; one based on conventional differential encoding with an estimation of the average time of arrival and another based on double encoding. Results show that the first approach improves performance in the low signal to noise ratio (SNR) region, while the second is better suited at high SNR. Adopting these techniques closes the performance gap between differential OFDM systems and coherent OFDM systems while retaining the benefits of computational simplicity.

Eigendecomposition-Based Partial FFT Demodulation for Differential OFDM in Underwater Acoustic Communications

Differential orthogonal frequency division multiplexing (OFDM) is practically attractive for underwater acoustic communications since it has the potential to obviate channel estimation. However, similar to coherent OFDM, it may suffer from severe inter-carrier interference over time-varying channels. To alleviate the induced performance degradation, we adopt the newly emerging partial fast Fourier transform (FFT) demodulation technique in this paper and propose an eigendecomposition-based algorithm to compute the combining weights. Compared to existing adaptive methods, the new algorithm can avoid error propagation and eliminate the need for parameter tuning. Moreover, it guarantees global optimality under the narrowband Doppler assumption, with the optimal weight vector of partial FFT demodulation achieved by the eigenvector associated with the smallest eigenvalue of the pilot detection error matrix. Finally, the algorithm can also be extended straightforwardly to perform subband-wise computation to counteract wideband Doppler effects.

A Novel Differential Detection for Differential OFDM Systems With High Mobility

In conventional differential detection (CDD), the previous received symbol is used as the phase reference for the current received symbol, therefore the channel variation due to fast fading causes an error floor. We propose a novel iterative decision-directed differential detection (iD4) algorithm for orthogonal frequency division multiplexing (OFDM) systems employing differential PSK modulation in each subcarrier. The core of iD4 is a low complexity time and frequency 2-D estimator which can effectively estimate the channel variation experienced by adjacent time domain symbols. This estimated channel variation is then compensated in the differential detection process. Both theoretical analysis and computer simulations show that significant performance improvement can be achieved using iD4 over the CDD for OFDM receivers in fast fading channels.

Experimental demonstration of differential OFDM underwater acoustic communication with acoustic vector sensor

A new Orthogonal Frequency Division Multiplexing (OFDM) based underwater acoustic communication scheme utilizing acoustic vector sensor is presented. The acoustic channel assumed has the slow time-varying characteristic and the neighboring signals can be considered as transmitted through the same channel. The differential modulation encodes information through the relative phase changes between adjacent subcarriers. Therefore, instead of adding pilots to monitor the channel, the differential demodulation is used to reconstructs transmitted information without necessity of channel estimation. This increases the transmission rate and reduce the complexity of processor at the receiver. An acoustic vector sensor is used in the differential OFDM system to further improve the performance of the system with 4.8dB gains that can be obtained by combined processing pressure and velocity signals from the acoustic vector sensor. To validate the capability of the proposed scheme, an experiment with picture and voice source information was conducted in a lake. The results obtained at different distances demonstrate the proposed system offers a significant advantage.

Robust differential modulations for asynchronous cooperative systems

Accurate channel state information and time/frequency synchronization are challenging to acquire in mobile ad hoc cooperative set-ups, where resource-constrained relaying terminals rapidly (dis)join cooperation. This paper derives low-complexity differential modulations and cooperative transmission schemes so that detection at the receiver is feasible without channel knowledge or synchronization information. A simple distributed doubly-differential (DD) time-division multiplexing (TDM) scheme using diagonal space–time (ST) unitary codes is derived to bypass multiple carrier frequency offsets (CFOs) and collect space diversity. Precoded differential orthogonal frequency division multiplexing (OFDM) transmissions are derived to additionally bypass timing offsets and collect multipath diversity using either DD diagonal or single-differential (SD) orthogonal ST block codes (OSTBCs). DD diagonal mappings suffer from considerable coding gain and signal-to-noise ratio (SNR) loss due to unitary constellation constraints and noise enhancement at the receiver. SD-OSTBCs achieve higher performance with reduced decoding complexity, at the cost of reduced CFO mitigation range. Simple and robust non-coherent selective and adaptive transmissions are also included, showing that full space and multipath diversity can still be achieved even with decoding errors at relays. Simulations corroborate theoretical claims.

Application of orthogonal frequency division multiplexing in cognitive underwater communication

With the development of underwater acoustic communication in military and commercial field and the urgent need for underwater wireless Ad Hoc networks, developing an intelligent and high-bit-rate underwater communication system is imminent. OFDM(Orthogonal Frequency Division Multiplexing) technology could be a good platform for cognitive underwater communication, which has advantages including high efficiency in spectrum utilization, faster communication speed and flexibility in choosing frequencies. A design scheme of the OFDM based cognitive underwater communication and block diagram are given. The system can intelligently choose NC-OFDM (Non-Contiguous OFDM), DOFDM (Differential OFDM) or Pilot-added OFDM communication schemes in order to meet different channel conditions and different rate requirements and to overcome the problem of data conflict and the waste of spectrum resources in multi-users’ competitive communication. Meanwhile, the system also can intelligently choose parameters in each scheme, such as sub-channel, pilot interval and error-correcting codes. The simulation results prove the feasibility and effectiveness of the OFDM based cognitive underwater communication.

Differential OFDM for acoustic communications

High-rate acoustic communication typically rely on coherent detection which requires sophisticated channel estimation, and may in turn suffer a penalty in performance when channel tracking is less than ideal (a situation that is often inevitable on time-varying channels). To improve the robustness of signal detection, orthogonal frequency division multiplexing (OFDM) is considered with differentially coherent detection. The resulting receiver has very low computational requirements, and a potential to outperform coherent OFDM detection when channel tracking becomes difficult. Differential encoding is applied in the frequency domain (across carriers) so that it does not require the channel to remain constant over consecutive blocks in time. Instead, it requires only that the channel transfer function change slowly between adjacent carriers, but this requirement coincides with the basic premises of OFDM system design. At the same time, closely spaced carriers promote bandwidth-efficiency. For extreme situations, when close carrier separation leads to insufficient temporal coherence within each OFDM block, a method of partial FFT demodulation can be used with differentially coherent detection. The ensuing receiver algorithm is cast into the multi-channel (spatial diversity) framework, and its performance is illustrated using synthetic, as well as experimental data.

CFO estimation schemes for differential OFDM systems

This paper proposes two blind carrier frequency offset (CFO) estimation schemes for differentially modulated orthogonal frequency division multiplexing (OFDM) systems. The proposed schemes estimate the fractional part of the CFO with only two consecutive OFDM blocks, and they exploit two implicit properties associated with differentially modulated OFDM (DOFDM) systems, i.e., the channel keeps constant over two consecutive OFDM blocks, and the DOFDM systems employ an M-ary phase-shift keying constellation. One of the schemes is based on the finite alphabet (FA) constraint and the other one is based on the constant modulus (CM) constraint. They provide a trade-off between the performance and computational complexity. The constrained Cramer-Rao lower bound is also derived. Several numerical examples are presented to validate the efficacy of the proposed schemes.

Coherent and Differential ICI Cancellation for Mobile OFDM with Application to DVB-H

We develop a reduced-complexity hybrid frequency/time-domain orthogonal frequency division multiplexing (OFDM) channel estimation algorithm for high-mobility scenarios where the channel varies significantly within each OFDM block, resulting in severe intercarrier interference (ICI). The algorithm exploits the banded and symmetric structure of the channel matrix in the frequency domain and its sparse structure in the time-domain to achieve significant complexity reductions, which we quantify for the Digital Video Broadcasting-Handheld (DVB-H) system. Furthermore, we compare coherent and differential mobile OFDM detection for DVB-H.

Performance of Noncoherent Maximum-Likelihood Sequence Detection for Differential OFDM Systems With Diversity Reception

For the single-carrier M-ary differential phase-shift keying (MDPSK), the multiple-symbol differential detector, or the noncoherent maximum-likelihood sequence detector (NSD), and its three special cases, namely, the noncoherent one-shot detector, the linearly predictive decision-feedback (DF) detector, and the linearly predictive Viterbi receiver are reviewed based on a hierarchical interpretation. For the multicarrier transmission, the differential orthogonal frequency division multiplexing (OFDM) systems with diversity reception are discussed. It is well known that there are two types of differential OFDM systems, namely, the time domain differential OFDM (TD-OFDM) and the frequency domain differential OFDM (FD-OFDM). In this paper, the NSD and its special cases are incorporated to the differential OFDM systems. Furthermore, we provide a simple closed-form bit-error-rate (BER) expression for the differential OFDM systems utilizing the noncoherent one-shot detector with diversity reception in the time-varying multipath Rayleigh fading channels. Numerical results have revealed that, with multi-antenna diversity reception, the performance of the noncoherent one-shot detector is improved significantly. However, when only one or two receive antennas are available, the implementation of the linearly predictive DF detector or the linearly predictive Viterbi receiver is necessary for achieving better and satisfactory performance.

Frequency offset estimation for differential OFDM

This paper proposes a novel frequency offset (FO) estimator for differential orthogonal frequency division multiplexing. By separating the normalized FO into integral and fractional parts, our FO estimation is carried out in two successive stages. In the first stage, we take advantage of differential coding and the finiteness of signal constellation to derive a channel-independent cost function whose minimization leads to an accurate estimate of the fractional part. It is proved that under certain mild assumptions, the identifiability of the fractional part can be guaranteed with probability one. After compensating the fractional part, the second stage estimates the remaining integral part with the aid of a judiciously designed initial block of differential coding. Combining these two stages, the proposed estimator is shown capable of offering full-range FO estimation without relying on any training or virtual subcarriers. Furthermore, unlike some existing estimators, the proposed estimator is applicable in both frequency-flat and frequency-selective fading channels. Another salient feature of the proposed estimator is that accurate FO estimation can be achieved with two received data blocks only, making it very attractive especially for short-burst communications. The merits of our FO estimator have been confirmed by theoretical analysis as well as corroborating simulations.