Orthogonal Frequency Domain Multiplexing Research Articles

Joint Channel Estimation and Synchronization Techniques for Time-Interleaved Block-Windowed Burst OFDM

From a conceptual perspective, beyond-5G technologies promise to deliver very low latency, even higher data rates, and ultrareliable connections for future generations of communication systems. Modulation schemes based on orthogonal frequency-domain multiplexing (OFDM) can accommodate these requirements for wireless systems. Several hybrid OFDM-based systems, such as the time-interleaved block-windowed burst–OFDM (TIBWB–OFDM), are capable of achieving even better spectral confinement and power efficiency. This paper addresses the implementation of the TIBWB–OFDM system in more realistic and practical wireless link scenarios by addressing the challenges of proper and reliable channel estimation and frame synchronization. We propose to incorporate a preamble formed by optimal correlation training sequences such as the Zadoff–Chu (ZC) sequences. The added ZC preamble sequence is used to jointly estimate the frame beginning through signal-correlation strategies and a threshold decision device, and acquire channel-state information (CSI) by employing estimators on the basis of the preamble sequence and transmitted data. The employed receiver estimators show that it is possible to detect the TIBWB–OFDM frame beginning and highlight the robustness of the TIBWB–OFDM technique to imperfect channel estimations by showing that it can provide comparatively close BER performance to the one where the CSI is perfectly known.

Testing different Channel Estimation Techniques in Real-Time Software Defined Radio Environment

In modern wireless communication to maximize spectral efficiency and to minimize the bit error rate OFDM (Orthogonal frequency-domain multiplexing) is used. OFDM is used broadly in networks using various protocols, including wireless vehicular environment IEEE 802.11p, IEEE 802.16d/e Wireless Metropolitan Area Networks, Long-Term Evolution 3GPP networks and IEEE 802.11a/g/n Wireless Local Area Networks. The main challenges involved when using OFDM for wireless communications are short channel-coherence bandwidth and the narrow coherence time, and both have a major effect on the reliability and latency of data packet communication. These properties increase the difficulty of channel equalization because the channel may change drastically over the period of a single packet. Spectral Temporal Averaging is an enhanced decision-directed channel equalization technique that improves communication performance (as far as the frame delivery ratio (FDR) and throughput) in typical channel conditions. This paper reports tests of Spectral Temporal Averaging channel equalization in an IEEE 802.11a network, compared with other channel equalization techniques in terms of the FDR in a real-time environment. Herein, a software defined Radio (SDR) platform was used for estimating the channel. This proves that the system can provide over 90% of delivery ratio at 25 db of Signal to Noise Ratio (SNR) for various digital modulation techniques. For this purpose, an experimental setup consisting of software-defined radio, Universal Software Radio Peripheral (USRP) N210 along with wide bandwidth daughter board as hardware and GNU radio is used.

Analysis of effects of multipath signal with nonuniform Doppler shift on vertical underwater acoustic communication

In this research, effects of a multipath signal with a nonuniform Doppler shift on underwater acoustic communication between surface and underwater vehicles are investigated by simulation. Single-carrier modulation (SCM) and orthogonal frequency domain multiplexing (OFDM) are utilized and the Doppler shift is caused by the roll motion of the surface vehicle. In SCM communication, it is revealed that a decision feedback equalizer suppresses effects of the phase shift of the multipath signal caused by the nonuniform Doppler shift. On the other hand, a resampling method is proposed and utilized to evaluate the performance of OFDM demodulation after removing the effects of the nonuniform Doppler shift of the direct signal. Consequently, it is shown that the Doppler shift of the multipath signal causes intercarrier interferences even with the method. Furthermore, it is demonstrated that OFDM communication is more sensitive to the Doppler shift of the multipath signal than SCM communication.

Homodyne OFDM with Optical Injection Locking for Carrier Recovery

Homodyne detection provides the simplest digital signal processing (DSP) solution to optical coherent detection and minimizes the receiver bandwidth requirements. These features make it promising for high spectrally efficient formats such as optical orthogonal frequency domain multiplexing (OFDM), which has a flat optical spectrum and which is thus inherently sensitive to high-frequency distortions, e.g., due to limited detector bandwidth. The key to homodyne detection is recovery of the carrier from the received signal all optically (as opposed to frequency offset compensation via DSP. Herein, we use optical injection locking (OIL) in conjunction with carrier tone-assisted OFDM to achieve this. In contrast to previous reports, we show that OIL carrier recovery with subsequent homodyne detection can operate without the need for any optical prefiltering. First, we evaluate the performance as a function of the carrier tone guardband bandwidth. Further, we improve the robustness of this technique using a slow-phase lock loop that compensates for drift in the laser's temperature/current control electronics. Using this improved setup, we compare our all-optical-carrier-recovered homodyne and the “traditional” DSP-assisted intradyne detection for the case of OFDM-16QAM signals. Finally, we compare the computing complexity necessary for both approaches and estimate the intradyne performance limitations due to the carrier-local oscillator frequency offset.

Performance Analysis of VBLAST Based MIMO OFDM System in Vehicular Channel

Vehicular communication is emerging as an important ingredient for successful implement of Intelligent Transportation Systems(ITS). But the development of suitable communications systems plays an important role in mobility condition. The desired system should support dynamic wireless exchange of data between nearby vehicles or road side infrastructure. In this regards multiple input and multiple output (MIMO) orthogonal frequency domain multiplexing (OFDM) system is possibly the best solution. This paper deals with the performance analysis of the MIMO OFDM system in Nakagami-m channel with Doppler shift and also partial hardware implementation of baseband signal processing.

Ultrawideband Signals in Medicine [Life Sciences

In the last decade, the utilization of radio technology for a variety of clinical and medical applications has increased dramatically. Advances in microelectronics have enabled the miniaturization and integration of biomedical sensors and radio transceivers into single units. Such wireless sensors can be worn or implanted, and they facilitate the continuous collection and transmission of physiological signals. The centrally coordinated or distributed interconnection of wireless biomedical sensors, referred to as a body area network (BAN), has already been standardized in IEEE Standard 802.15.6-2012. Besides narrowband (NB), IEEE Standard 802.15.6-2012 supports the use of ultrawideband (UWB) radio interfaces for wearable sensors. The U.S. Federal Communications Commission (FCC) defines UWB signals to be those with fractional bandwidth exceeding 20% of the center frequency, or alternatively, a bandwidth greater than 500 MHz. A number of techniques can be used to generate UWB signals; the most widely used is the one referred to as impulse radio (IR) and consists of transmitting very narrow pulses in the time domain, commonly in the order of a few nanoseconds with fast rise times reaching 50 ps [1]. Another common approach to generate UWB signals is subcarrier aggregation in orthogonal frequency-domain multiplexing (OFDM), which is generally used for short-range nonmedical indoor communications. The very large bandwidth enables high-data-rate communications. In the presence of noise and power constraints, UWB allows trading a section of the ?bandwidth for power according to the Shannon?Hartley theorem. This means UWB communication systems can operate with ultralow power and low signal-to-noise ratio (SNR) using different modulation and coding strategies. UWB signals have other inherent characteristics that make them suitable for the wireless interface of wearable biomedical sensors. Noiselike behavior due to the extremely low maximal effective isotropically radiated power (EIRP) spectral density of ?41.3 dBm/MHz makes UWB signals difficult to detect by NB systems and robust against jamming, potentially rescinding the need for complex encryption algorithms. Additionally, UWB signals do not represent a threat to patients? safety [2]. Much research has been done toward the use of BAN technology for ubiquitous monitoring of patients suffering from chronic diseases. Nevertheless, the commercial use of UWB for BAN has been limited in part by the FCC regulations, which restrict the communication applications to 3.1?10.6 GHz where propagation conditions in the body environment are rather unfavorable.

Novel Transmit Diversity Scheme for TDS-OFDM System With Frequency-Shift m-Sequence Padding

This contribution proposes a design method for training sequence (TS) defined as frequency-shift m-sequence (FSMS) and a novel transmit diversity scheme with FSMS padding as guard interval (GI) for the further evolution of time domain synchronous orthogonal frequency domain multiplexing (TDS-OFDM) systems. The good auto-correlation characteristic of FSMS can facilitate synchronization and channel estimation at the receiver. Due to the cross-correlation characteristics of FSMSs, the accuracy of channel estimation is affected by the mutual interference among the transmit antennas. To further improve the accuracy of channel estimation, the channel impulse response (CIR) should be estimated in an iterative manner to suppress the effects of the mutual interference and additive white Gaussian noise. Compared with the transmit diversity schemes based on the space-frequency coded TS and space-time coded TS for TDS-OFDM system, the proposed transmit diversity scheme with FSMS padding can provide diversity gain and superior system performance over time-varying and/or frequency-selective propagation conditions.

Novel Channel Estimation Method Based on Training Sequence Cyclic Reconstruction for TDS-OFDM System

In this paper, a novel channel estimation method for time domain synchrotrons orthogonal frequency domain multiplexing (TDS-OFDM) based on training sequence cyclic reconstruction is proposed to eliminate residual inter-block interference (IBI); it estimates the channel impulse response (CIR) in an iterative manner. A simulation and analysis show that the proposed method can effectively perform the channel estimation over long-delay multipath channels with low complexity.

Novel overlay/underlay cognitive radio waveforms using SD-SMSE framework to enhance spectrum efficiency- part i: theoretical framework and analysis in AWGN channel

Recent studies suggest that spectrum congestion is primarily due to inefficient spectrum usage rather than spectrum availability. Dynamic spectrum access (DSA) and cognitive radio (CR) are two techniques being considered to improve spectrum efficiency and utilization. The advent of CR has created a paradigm shift in wireless communications and instigated a change in FCC policy towards spectrum regulations. Within the hierarchical DSA model, spectrum overlay and underlay techniques are employed to enable primary and secondary users to coexist while improving overall spectrum efficiency. As employed here, spectrum overlay exploits unused (white) spectral regions while spectrum underlay exploits underused (gray) spectral regions. In general, underlay approaches use more spectrum than overlay approaches and operate below the noise floor of primary users. Spectrally modulated, spectrally encoded (SMSE) signals, to include orthogonal frequency domain multiplexing (OFDM) and multi-carrier code division multiple access (MC-CDMA), are candidate CR waveforms. The SMSE structure supports and is well suited for CR-based software defined radio (SDR) applications. This paper provides a general soft decision SMSE (SDSMSE) framework that extends the original SMSE framework to achieve synergistic CR benefits of overlay and underlay techniques. This extended framework provides considerable flexibility to design overlay, underlay and hybrid overlay/underlay waveforms that are scenario dependent. Overlay/underlay framework flexibility is demonstrated herein for a family of SMSE signals, including OFDM and MC-CDMA. Analytic derivation of CR error probability for overlay and underlay applications is presented. Simulated performance analysis of overlay, underlay and hybrid overlay/underlay waveforms is also presented and benefits discussed, to include improved spectrum efficiency and channel capacity maximization. Performance analysis of overlay/underlay CR waveform in fading channels will be discussed in Part II of the paper.

Iterative cancellation of clipping noise in multilevel quadrature amplitude modulation multi-carrier CDMA system

An iterative reconstruction method that was proposed for clipping noise cancellation in orthogonal frequency domain multiplexing (OFDM) systems is applied to multilevel quadrature amplitude modulation-based multi-carrier (MC) code division multiple access signals in downlink. The iterative method uses a known distortion function, maybe a nonlinear one such as a successive clipping and filtering process, in the iterations to give an approximation of inverse of the distortion process and then the iterative method removes distortion under a convergence condition. The authors show that MC signal with properly chosen clipping threshold satisfies convergence conditions of the iterative method. In contrast to some of the other reconstruction-based techniques, this method requires no extra bandwidth and side information and it can be implemented with reasonable complexity. Furthermore, the authors show that the proposed iterative scheme can be enhanced by using an extra frequency bandwidth. Exploiting extra bandwidth improves the performance of the reconstruction-based methods in case of using successive clipping and filtering. Simulation results will be used to demonstrate achievable bit-error-rate improvement by the proposed enhanced iterative scheme.

Computationally efficient block transmission systems with and without guard periods

Block transmission systems using OFDM (orthogonal frequency domain multiplexing) are known for their computational efficiency in dealing with time dispersive, wireless channels. The efficiency relies primarily on data blocking by cyclic prefixing and on the exploitation of the eigenvalue decomposition of circulant channel matrices. Based on other blocking techniques and on other efficient matrix decompositions, further computationally efficient block transmission systems can be established. However, all these block transmission systems, including OFDM based systems, require the insertion of guard periods at the transmitter side, which unfavorably affects the spectral efficiency and the transmission power. We present overlapping methods that can efficiently avoid the insertion of guard periods in those systems. The measured bit error rates of the block transmission systems without guard periods are over a wide range of E B / N 0 values similar to the respective systems with guard periods. Additionally, the systems with guard period require more transmission energy to reach a similar E B / N 0 value at the receiver. The block processing is very similar for transmissions with and without guard periods, which can be reasonably exploited in a transceiver, capable to transmit and receive data blocks with and without guard periods.

Channel Estimation in Comb-Type Pilot Arrangements for OFDM Systems with Null Subcarriers

This letter analyzes the degradation effect of null subcarriers in orthogonal frequency domain multiplexing (OFDM) systems on the time-domain maximum likelihood (ML) estimation performance. The analysis is used as the basis for a proposal for a channel estimation method that can overcome performance degradation caused by null subcarriers. The accuracy of the proposed method is confirmed by the numerical analysis.

Nonlinear optimization for energy efficiency in IEEE 802.11a wireless LANs

Link-adaptation scheme has been recognized as an effective way to improve the performance of energy efficiency of the IEEE 802.11a wireless local-area networks (WLANs). In this paper, we propose a new link-adaptation scheme for energy efficiency by dynamically adjusting the data transmission rate according to network load and channel quality. We first give an overview of the IEEE 802.11a orthogonal frequency domain multiplexing (OFDM) physical layer and the distributed coordination function (DCF) of the IEEE 802.11 MAC. Then, based on the theoretical goodput analysis, we proposed a new method to enhance the energy efficiency via optimizing the goodput performance using nonlinear optimization with constraints. Despite the nonlinearity in the objective function and the constraints, we show that the energy efficiency optimization can be efficiently solved for global optimality through a convex optimization technique. Our simulation results show that our link-adaptation scheme enhances the energy efficiency as well as the goodput performance of an IEEE 802.11a wireless LAN through dynamic frame length control, contention window (CW) selection, and PHY rate selection, depending on the wireless channel condition. Moreover, the proposed scheme can be extended to address some similar problems in other kinds of networks, such as congestion control, admission control, and power control.

Blind channel estimation in MIMO OFDM systems with multiuser interference

This paper proposes a blind channel estimation method in the context of a multiuser orthogonal-frequency-domain-multiplexing (OFDM) system, where each user transmits utilizing all available subcarriers. A linear nonredundant block precoding scheme is applied at the input of the OFDM system. The precoding spreads the symbols of each user over all subcarriers, thus increasing multipath diversity. At the same time, it introduces a structure to the transmitted symbols, which is exploited at the receiver to estimate the channel in a blind fashion. The proposed channel estimation approach employs computationally simple cross-correlation operations and yields the channel up to a diagonal ambiguity. It does not require channel length information and is not sensitive to additive stationary noise. The precoding does not increase transmission power and maintains even distribution of power between OFDM blocks. A general description of precoding matrices is provided as well as analytical expressions of symbol error probability and signal-to-interference ratio, which could be used to obtain optimum precoding schemes.