OFDM Waveform Research Articles

Mobile Communications Beyond 52.6 GHz: Waveforms, Numerology, and Phase Noise Challenge

In this article, the 5G New Radio (NR) physical layer evolution to support beyond 52.6 GHz communications is addressed. The performance of both OFDM based and DFT-s-OFDM based networks are evaluated with special emphasis on the phase noise (PN) induced distortion. It is shown that DFT-s-OFDM is more robust against PN under 5G NR Release 15 assumptions, specifically regarding the supported phase tracking reference signal (PTRS) designs. To further improve the PN compensation capabilities, the PTRS design for DFT-s-OFDM is revised, while for the OFDM waveform a novel block PTRS structure is introduced, providing similar link performance as DFT-s-OFDM with enhanced PTRS design. We demonstrate that the existing 5G NR Release 15 solutions can be extended to support deployments at 60 GHz bands with the enhanced PTRS structures. In addition, DFT-s-OFDM based downlink for user data could be considered for beyond 52.6 GHz communications to further improve system power efficiency and performance with higher order modulation and coding schemes. Finally, network link budget and cell size considerations are provided, showing that at certain bands with specific transmit power regulation, the cell size can eventually be downlink limited.

FD-FBMC: A Solution for Multicarrier Full Duplex Cellular Systems

This letter reports on a critical issue with the application of OFDM to a full-duplex (FD) cellular system. With small frequency offsets of uplink signals, the downlink BER increases considerably. This is attributed to the high spectral-leakage-interference from the OFDM waveforms of geographically nearby uplink users. As an alternative, we propose applying the FBMC waveform to the same FD cellular system scenario. We calculate the precise spectral-leakage-interference for OFDM and FBMC, based on which, we set up a semi-analytic BER simulation framework. Analysis and numerical results confirm that spectral leakage interference from uplink signals is the critical performance limiting factor. Even with tiny frequency offsets among the users, the performance of FD with OFDM is severely degraded. Although the FDs with the other multicarrier waveforms having lower spectral leakage such as GFDM, UFMC, and f-OFDM overcome this issue to a certain extent, the degradations remain substantial. However, the FBMC-based FD cellular system achieves almost spectral-leakage-interference-free performance, and thus, it substantially outperforms the counterparts with other waveforms.

OFDM joint communication–radar with leaky‐wave antennas

In this Letter, the authors report on a 26 GHz joint communication–radar system. By exploiting the beam steering features of leaky-wave antennas, angle-of-arrival estimation and multi-user transmission are achieved. Furthermore, the Zadoff–Chu preamble of the OFDM waveform is used to provide a frequency-modulated continuous-wave radar signal. This way, the wide operational bandwidth is used for both, communications and radar, yielding high data throughput and precise user localisation. Experimentally, spectral-efficient 16-QAM multi-user 5 × 4 Gbit/s communications and user localisation with an accuracy of about 2 cm and ±1.5° is demonstrated.

Mutual-Information-Based OFDM Waveform Design for Integrated Radar-Communication System in Gaussian Mixture Clutter

Existing studies on joint radar and communication systems are mainly based on a common assumption: Gaussian environment. Nevertheless, this does not hold in many complicated scenarios. In this letter, mutual information (MI)-based waveform design method for the integrated orthogonal frequency division multiplexing radar-communication system in Gaussian mixture (GM) clutter is investigated. First, from the statistical perspective, the approximate differential entropy of received signal in GM clutter is derived. Subsequently, an adaptive waveform design method is proposed by maximizing the MI, subject to three metrics: communication channel capacity; subcarrier power ratio; and energy constraint. Simulation shows the tradeoff among the above metrics.

Under Test Filtered-OFDM And UFMC 5g Waveform Using Cellular Network

In this study, filtered orthogonal frequency division multiplexing (F-OFDM) and universal filtered multicarrier (UFMC) were proposed for complexity reduction in the 5G waveform. Cyclic prefix orthogonal frequency division multiplexing (CP_OFDM) is well suited for 4G; however, the major problem of the 4G modulation methods is their susceptibility to high peak to average power ratio (PAPR). Another problem of OFDM is the issue of sideband leakage. The existing 4G systems mainly depend on the CP_OFDM waveform, which cannot support the host of applications provided by the 5G platform. 5G-generated traffic is likely to exhibit different features and requirements compared to the existing wireless technology. Consequently, investigations have been devoted to other multiple access schemes. The existing limitations of OFDM can be mitigated by using the UFMC technique. To ensure that the demands and requirements of the upcoming 5G cellular networks are satisfied, this study presents an enabler called filtered-OFDM (f-OFDM) for flexible waveform configurations. Contrarily, the assigned bandwidth in the f-OFDM is split into various sub-bands to accommodate different services in each sub-band using the most suited waveform, thereby enhancing the spectrum utilization using a different filter. Additionally, the advantages of F-OFDM and UFMC were portrayed via a wide comparison with the current 5G waveforms.

Performance of an Adaptive Compander in Wireless Mobile Communication Systems

The most popular technique used for high data rate services in 4G scenario is MIMO-OFDM. Space-time block codes have been recommended for high diversity gain in MIMO communication systems. Due to comparatively simpler design and less decoding complexity orthogonal STBCs are very attractive. In this paper, we propose a novel adaptive companding method for peak to average power ratio reduction in IEEE 802.16 standards used in 4G wireless mobile communications with orthogonal space time block coded MIMO-OFDM systems. This is obtained by fixing compander parameters independently for statistically unrelated symbols in OFDM waveform and according to the features of input symbols. The bit error rate performance analysis of the proposed system under Rayleigh fading MIMO channel conditions has also been presented. Simulation results showed that the proposed method performs better for a given PAPR preset value and a slight improvement in the BER performance was observed over fixed compander.

Efficient Multidimensional Wideband Parameter Estimation for OFDM Based Joint Radar and Communication Systems

In this paper, we propose a new pre-processing technique for efficient multidimensional wideband parameter estimation. One application is provided by an orthogonal frequency division multiplexing-(OFDM) based joint radar and communication system, which uses SIMO architecture. In this paper, the estimated parameters are given by the range (time delay), the relative velocity, and the direction of arrival (DoA) pairs of the dominant radar targets. Due to the wideband assumption, the received signals on different subcarriers are incoherent and, therefore, cannot fully exploit the frequency diversity of the OFDM waveform. To estimate the parameters jointly and coherently on different subcarriers, we propose an interpolation-based coherent multidimensional parameter estimation framework, where the wideband measurements are transformed into an equivalent narrowband system. Then, narrowband multidimensional parameter estimation algorithms can be applied. In particular, a wideband $R$ -D periodogram is introduced as a benchmark algorithm, and we develop the $R$ -D Wideband Unitary Tensor-ESPRIT algorithm. The simulations show that the proposed coherent parameter estimation method significantly outperforms the direct application of narrowband parameter estimation algorithms to the wideband measurements. If the fractional bandwidth is significant and the SNR is not too low, the estimates provided by the narrowband estimation algorithms can become inconsistent. Moreover, the interpolation order should be chosen according to the SNR regime. In the low SNR regime, interpolation with a lower-order (i.e., linear interpolation) is recommended. For higher SNRs, we propose an interpolation with higher-order polynomials, e.g., fourth-order (cubic splines) or even higher.

Adaptive Modulation and Coding with Selective Retransmission under OFDM signaling

In this paper, we present bandwidth efficient selective retransmission method in conjunction with adaptive modulation and coding (AMC) scheme for OFDM waveform. In the proposed method, when a packet failure occurs, receiver requests retransmission of information symbols prone to error corresponding to the low signal-to-noise ratio (SNR) sub-carriers of OFDM modulation. The selective retransmission avoids unnecessary retransmission and AMC chooses a proper modulation and coding scheme with an objective to maximize the throughput. Our method achieves higher throughput as compared to conventional retransmission methods such as Chase combining hybrid automatic repeat reQuest (CC-HARQ) and incremental redundancy hybrid automatic repeat reQuest (IR-HARQ). We also provide the throughput and delay analysis of the proposed method for non-truncated ARQ. The simulation results demonstrate throughput gain without significant impact on delay as compared to the conventional retransmission approaches.

Joint optimization of PAPR reduction based on modified TR scheme for MIMO-OFDM radar

In this paper, we propose a joint optimization method to reduce peak-to-average power ratio (PAPR) for multiple-input multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) radar systems, based on the modified tone-reservation (TR) scheme which has sufficient degree of freedom to satisfy some special and meaningful waveform design criteria. In terms of applying independent or concurrent reserved subcarrier weights for transmit antennas, we provide two PAPR reduction schemes respectively, i.e., independent TR (ITR) and concurrent TR (CTR). It is demonstrated that the proposed joint optimization method can effectively reduce the PAPR to an acceptable threshold and is superior to the conventional methods. Note that the optimized OFDM waveforms for MIMO radar can avoid the grating-lobes in distance domain. Simulation results are presented to verify the derivations.

Low Probability of Intercept-Based Optimal OFDM Waveform Design Strategy for an Integrated Radar and Communications System

The integration of radar and communications systems can provide great advantages, such as enhanced efficiency, structure simplification, less occupied hardware resources, and interference mitigation, compared with traditional individual radar and communications applications. Extensive studies have presented achieving improved system performance, whereas the problem of low probability of intercept (LPI)-based waveform design for the integrated system is seldom discussed in the literature. In this paper, an LPI-based optimal orthogonal frequency multiplexing modulation (OFDM) waveform design strategy is developed for an integrated radar and communications system (IRCS). The dedicated transmitter in this system transmits integrated OFDM waveform for simultaneously target parameter estimation and downlink communications. The basis of the underlying strategy is to employ the optimization technique to design the integrated OFDM waveform of IRCS in order to minimize the total radiated power, while satisfying the specified requirements of target parameter estimation and data information rate. We analytically show that the resulting optimization problem is convex and can be solved by formulating the Karush–Kuhn–Tuckers optimality conditions. Numerical simulation results demonstrate that our proposed strategy can solve the waveform design problem in the IRCS with low complexity, and the LPI performance of the IRCS can efficiently be enhanced by utilizing the proposed integrated OFDM waveform design strategy.

Investigation of Secret Key Capacity in MIMO-OFDM Wireless Systems

MIMO and MIMO-OFDM systems are the enabling technologies for the next generation wireless systems. However, the radio signals transmitted are available for reception by a illegitimate user leading to eavesdropping. This raises severe concerns on the security of the transmission. Physical Layer security aims to exploit the wireless channel characteristics and in turn provide secure transmission robust against eavesdropping. In this paper, we investigate the Secret Key Capacity (SKC) in MIMO-OFDM systems. We derive expressions for SKC in MIMO systems and extend further to OFDM waveforms. Numerical simulations are performed for various transmit and receive antenna configurations without any estimation errors.

Closed-Form Expressions for ICI/ISI in Filtered OFDM Systems for Asynchronous 5G Uplink

One of the major purposes for fifth generation (5G) communications waveform design is to relax the synchronization requirements for supporting efficient massive machine type communications (MTCs). Polynomial cancellation coded orthogonal frequency-division multiplexing (PCC-OFDM) and universal filtered multi-carrier (UFMC) are designed to reduce the sidelobes of the OFDM waveform to protect against intercarrier interference (ICI) in the 5G uplink. To the best of our knowledge, there is no analysis of the effect of ICI for the UFMC system with time offset transmissions that many arise in MTC scenarios. Furthermore, there is no study on reducing the computational complexity of the UFMC system. This paper provides closed-form expressions for time offsets interference in such a case for OFDM, PCC-OFDM, and UFMC. This paper also presents theoretical analysis for the signal-to-interference-plus-noise ratio, achievable rate, and bit error ratio performance. The results show that PCC-OFDM significantly protects against ICI at the cost of halving the spectral efficiency. UFMC improves the ICI and intersymbol interference protection performance, especially when the length of time offset is very small, at the cost of significantly increasing the computational complexity. Finally, this paper proposes the overlap and adds UFMC and a variant of UFMC using infinite impulse response prototype filter banks to reduce the processing complexity.

Waveform and Numerology to Support 5G Services and Requirements

The standardization of the next generation 5G radio access technology has just started in 3GPP with the ambition of making it commercially available by 2020. There are a number of features that are unique for 5G radio access compared to the previous generations such as a wide range of carrier frequencies and deployment options, diverse use cases with very different user requirements, small-size base stations, self-backhaul, massive MIMO, and large channel bandwidths. In this article, we propose a flexible physical layer for the NR to meet the 5G requirements. A symmetric physical layer design with OFDM is proposed for all link types, including uplink, downlink, device-to-device, and backhaul. A scalable OFDM waveform is proposed to handle the wide range of carrier frequencies and deployments.

Experimental characterization of an all-optical wavelength converter of OFDM signals using two-mode injection-locking in a Fabry-Pérot laser.

While optical OFDM has been demonstrated for superior transmission performance, its analogue waveform in the time domain challenges many conventional all-optical wavelength converters (AOWC) that are needed for future flexible optical networks. There only exist a few reports on AOWC of OFDM signals, which are mainly based on the low-efficient four-wave mixing. In this paper, we propose an AOWC for OFDM signals by using two-mode injection-locking in a low-cost Fabry-Pérot laser. The control signal and the probe signal at a milliwatt power level are combined and injected into the FP laser. By a proper control, they can be injection-locked to two longitudinal modes in the FP laser and subsequently, the transmission of the probe signal is conditioned by the control signal. We conduct an experimental study on various aspects of this AOWC. Despite a vendor-specified electrical-to-optical (E/O) modulation bandwidth of 2.5 GHz, we find that the optical-to-optical (O/O) modulation bandwidth of AOWC is free from this limit and can be much wider. We examine the linear transfer curve of the AOWC by simply using the OFDM waveforms as the stimulus. The performance tolerance to the wavelength detuning and injected power ratio is also measured. The proposed AOWC can provide a linear transfer function from the control signal to the probe signal to support the random-fluctuated OFDM waveform. We also investigate the maximum capacity of the AOWC by using the adaptive bit-loading OFDM. Finally, we measure the power penalty after the AOWC at two different bit rates to show the tradeoff between the penalty and capacity.

IRCI free colocated mimo radar based on sufficient cyclic prefix OFDM waveforms

In this paper, we propose a cyclic prefix (CP) based MIMO-OFDM range reconstruction method and its corresponding MIMO-OFDM waveform design for co-located MIMO radar systems. Our proposed MIMO-OFDM waveform design achieves the maximum signal-to-noise ratio (SNR) gain after the range reconstruction and its peak-to-average power ratio (PAPR) in the discrete time domain is also optimal, i.e., 0dB, when Zadoff-Chu sequences are used in the discrete frequency domain as the weighting coefficients for the subcarriers. We also investigate the performance when there are transmit and receive digital beamforming (DBF) pointing errors. It is shown that our proposed CP based MIMO-OFDM range reconstruction is inter-range-cell interference (IRCI) free no matter whether there are transmit and receive DBF pointing errors or not. Simulation results are presented to verify the theory and compare it with the conventional OFDM and LFM co-located MIMO radars.

Effect of Variable Cyclic Prefix Length on OFDM System Performance over Different Wireless Channel Models

A Cyclic Prefix (CP) is a significant feature of an OFDM waveform. It is used to completely eliminate both Inter-Symbol Interference (ISI) and Inter-Carrier Interference (ICI) as long as the CP length is greater than the channel delay spread. By eliminating both ISI and ICI, the CP compensates for the effect of the multipath dispersion; but it consumes a considerable amount of the scarce spectrum and the power. Conventional OFDM uses a fixed and large CP length to overcome the ISI caused by channel delay spread under wireless mobile environment. This approach may degrade the overall spectral efficiency as well as consumes relatively more transmitter energy. Therefore, there is a need to adopt the CP length according to instantaneous channel parameters. In this paper, we presented a method for investigating the effect of varying the CP length on OFDM system performance over different multipath channel models. We estimated the variable CP length based on the RMS delay spread of the channels' power delay profile (PDP). According to this method, the estimated CP length optimizes the system capacity and improves the overall system performance. We showed that this approach could lead to a mathematical formula presenting the relationship between the CP length and the RMS delay spread of the channels.

Optimal Waveforms for Efficient Wireless Power Transmission

This work shows how using specifically selected or designed waveforms in wireless power transfer (WPT) systems can lead to improved RF to dc conversion efficiency in rectifier circuits in the receiving end of these systems. Signals with different time domain waveforms are considered such as OFDM, white noise and chaotic waveforms, and the performance of a rectifier circuit operating at 433 MHz is evaluated when using these signals in comparison to a single carrier constant envelope signal. The performed experiments show that selecting high peak to average power ratio (PAPR) signals lead to improved RF-DC conversion efficiency in rectifier circuits.

The Imaging of SAR Based on OFDM Waveforms Signal with the Compensation of Coding Term

该文探讨了正交频分复用(OFDM)波形在SAR中可能的应用，重点讨论了基于该信号体制的SAR成像。OFDM波形改变了传统线性调频信号平坦的频谱，给高分辨成像带来了新的问题。该文深入分析了OFDM波形的信号特点，推导了其2维频谱，提出了一种适合OFDM SAR精确成像的方法。该成像算法利用对编码项的补偿，实现了OFDM波形下的距离迁徙校正，有效降低了编码信号对旁瓣的影响，充分提高了OFDM波形的成像性能。仿真结果验证了所提方法的有效性。

Mutual Information Analysis of OFDM Radio Link Under Phase Noise, IQ Imbalance and Frequency-Selective Fading Channel

OFDM and other multicarrier waveforms are in general very sensitive to RF non-idealities, such as phase noise and IQ imbalance, of transmitting and receiving devices. Extensive work has been carried out in the open literature in analyzing the performance of OFDM radio link under such RF impairments in terms of detection error rate and mostly concentrating on one impairment at a time. However, there is only very limited work on analytical investigations of mutual information and rate loss expressions, the heart of communication theory, as functions of RF impairment levels. In this article, we derive two closed-form mutual information expressions, in the form of infinite series representation, for an arbitrary subcarrier of a general OFDM radio link impaired with transceiver phase noise and IQ imbalance in frequency-selective Rayleigh distributed block-fading radio channel, covering both uncorrelated as well as fully correlated mirror subcarrier scenarios. We also show that the mutual information saturates to a finite value due to the inherent RF impairments even in the case that the symbol-to-noise ratio approaches infinity. Extensive comparisons with results obtained from full OFDM radio link simulations are also provided to illustrate and verify the accurate match between analytical and simulated mutual information behavior.

Power Spectral Density Analysis of WLAN based OFDM System

is a technique used in modern broadband wireless communications systems. OFDM has been adopted for wireless standards such as IEEE 802.11a. In this paper, different Power Spectral Density (PSD) curves of OFDM signal based on WLAN with various pulse shapes are presented. OFDM waveform is analyzed in frequency domain response for different modulation techniques such as BPSK, QPSK, 16-QAM, 64-QAM and different block sizes such as 50 and 2048. Power spectral density increases with increase in levels of modulation. For high power spectral density of transmitter higher levels of QAM modulation techniques are preferred.