In vehicle-to-vehicle (V2V) communications, reliable channel estimation is critical for the system performance due to the extremely time-varying characteristic of V2V channels. In this article, we present a survey on the current channel estimation techniques for the IEEE 802.11p standard. According to deficiencies of the current schemes and considering characteristics of V2V channels, we propose a novel channel estimation scheme, which utilizes the data symbols to construct pilots as well as the correlation characteristics between channels within two adjacent symbols. Analysis and simulation results demonstrate that the proposed scheme outperforms currently widely-used schemes especially in high signal-to-noise ratio regime. At the end, some open issues for the future work conclude this article.

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Channel Estimation Schemes for IEEE 802.11p Standard

Fast and accurate frequency estimation of a complex sinusoidal plays an important role in many applications, including communications, radar, and sonar. This paper presents two methods for the frequency estimation of a complex sinusoidal in noisy environments. The proposed methods interpolate on shifted DFT coefficients to acquire the frequency estimates iteratively. The first method interpolates on the $q$ -shifted DFT coefficients of the signal, whose optimum iteration number is found to be a logarithmic function of the signal length. Furthermore, we also show that by appropriately selecting the value of the shift $q$ , the proposed method becomes asymptotically efficient. In the second method, we use hybrid half-shifted and $q$ -shifted DFT interpolators together, which converges in only two iterations. It is shown that both of the estimators are asymptotically unbiased with their mean square errors performing on the Cramer–Rao lower bound. Theoretical results are validated by extensive computer simulations.

Fast and Efficient Sinusoidal Frequency Estimation by Using the DFT Coefficients

This paper deals with timing and frequency synchronization in orthogonal frequency-division multiplexing (OFDM) systems. A robust multistage scheme that works in the time domain, independent of the preamble structure, is proposed. After coarse-timing estimation, joint timing and integer frequency synchronization is performed. Then, fractional frequency correction is carried out, and finally, fine-timing estimation completes the synchronization process. The new timing estimation method is flexible and can be adjusted according to the degree of channel distortion. Furthermore, frequency synchronization is efficiently accomplished with an estimation range that is as large as the bandwidth of the OFDM signal. The performance of the proposed method is evaluated in terms of the mean square error. The results indicate that the new method significantly improves performance compared with the previous methods.

Robust Timing and Frequency Synchronization for OFDM Systems

In multiuser MIMO (MU-MIMO) networks, the optimal bit rate of a user is highly dynamic and changes from one packet to the next. This breaks traditional bit rate adaptation algorithms, which rely on recent history to predict the best bit rate for the next packet. To address this problem, we introduce TurboRate, a rate adaptation scheme for MU-MIMO LANs. TurboRate shows that clients in a MU-MIMO LAN can adapt their bit rate on a per-packet basis if each client learns two variables: its SNR when it transmits alone to the access point, and the direction along which its signal is received at the AP. TurboRate also shows that each client can compute these two variables passively without exchanging control frames with the access point. A TurboRate client then annotates its packets with these variables to enable other clients to pick the optimal bit rate and transmit concurrently to the AP. A prototype implementation in USRP-N200 shows that traditional rate adaptation does not deliver the gains of MU-MIMO WLANs, and can interact negatively with MU-MIMO, leading to low throughput. In contrast, enabling MU-MIMO with TurboRate provides a mean throughput gain of 1.7x and 2.3x, for 2-antenna and 3-antenna APs respectively.

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Rate adaptation for 802.11 multiuser mimo networks

The Internet of Things (IoT) is expected to bring new opportunities for improving several services for the Society, from transportation to agriculture, from smart cities to fleet management. In this framework, massive connectivity represents one of the key issues. This is especially relevant when IoT systems are expected to cover a large geographical area or a region not reached by terrestrial network connections. In such scenarios, the usage of satellites might represent a viable solution for providing wide area coverage and connectivity in a flexible and affordable manner. Our paper presents a survey on current solutions for the deployment of IoT services in remote/rural areas by exploiting satellites. Several architectures and technical solutions are analyzed, underlining their features and limitations, and real test cases are presented. It has been highlighted that low-orbit satellites offer an efficient solution to support long-range IoT services, with a good trade-off in terms of coverage and latency. Moreover, open issues, new challenges, and innovative technologies have been focused, carefully considering the perimeter that current IoT standardization framework will impose to the practical implementation of future satellite based IoT systems.

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A Survey on Technologies, Standards and Open Challenges in Satellite IoT

We propose a new SNR estimation method based on the preamble for OFDM systems in frequency selective channels. The OFDM training symbols in the preamble are equalized by the known data in frequency domain and employed to estimate the noise variance. The second order moments of the received symbols are used to estimate the signal plus noise power in the OFDM packets. The SNRs on the subchannels and the average SNR of the packets can all be estimated. Simulation results show that the proposed method is robust to frequency selectivity in wireless channels, and its performance is considerably improved compared with the available methods.

SNR estimation algorithm based on the preamble for OFDM systems in frequency selective channels

An efficient frequency offset estimation method is proposed for wireless orthogonal frequency-division multiplexing (OFDM) systems. In the proposed method, the received samples of the training symbol are processed by a novel envelop equalized processing method, and the estimation of the frequency offset in the OFDM systems turns out to be the frequency estimation of a complex tone. A novel frequency estimation method is employed to estimate the frequency offset. The estimation range of the proposed method is as large as the bandwidth of the OFDM signal, and the performance of the proposed method is independent of the structure of the training symbol.

An Efficient Frequency Offset Estimation Method With a Large Range for Wireless OFDM Systems

A semi-blind selected mapping (SLM) scheme with low-complexity transceiver is addressed for peak-to-average power ratio (PAPR) reduction in orthogonal frequency-division multiplexing (OFDM) systems. In the proposed scheme, the phase rotation vectors for the data vector and those for the pilot vector in an OFDM symbol are separately designed, which enables both low computational complexity and embedding of the side information (SI) into the pilot subcarriers at the transmitter. To implement data detection at the receiver, pilot-aided embedded SI detection algorithm is introduced, and a low-complexity suboptimal SI detection algorithm is also proposed. Analysis and simulation results show that the proposed scheme can achieve much lower computational complexity than that of the available semi-blind SLM schemes at both the transmitter and receiver, whereas it has slightly worse PAPR reduction performance and almost the same bit error rate (BER) performance, even with the suboptimal SI detector, as compared with the conventional SLM (C-SLM) with perfect SI at the receiver.

A Semi-Blind SLM Scheme for PAPR Reduction in OFDM Systems With Low-Complexity Transceiver

To avoid the excess bandwidth and noise enhancement penalty involved in the pulse-shaping method for peak-to-average power ratio (PAPR) reduction of single-carrier frequency-division multiple access (SC-FDMA) signals, we propose a novel alternative method, which is called probabilistic pulse shaping . A set of weighting windows with zero excess bandwidth is designed to weight the outputs of the discrete Fourier transform precoder of a block of SC-FDMA symbols in a slot. Then, the candidate signal blocks with different weighting windows for each block are generated with the designed low-complexity implementation scheme, and the signal block with the lowest PAPR is selected to be transmitted. At the receiver, the received data can be detected in the same way as that of the conventional SC-FDMA using the defined equivalent channel without side information. It is shown by simulations that the proposed method achieves considerable improvements over the conventional SC-FDMA system in terms of both PAPR and bit-error-rate (BER) performance with low additional complexity.

PAPR Reduction of SC-FDMA Signals Via Probabilistic Pulse Shaping

We propose a simple and flexible peak-to-average power ratio (PAPR) reduction scheme for coded single carrier frequency division multiple access (SC-FDMA) signals in the uplink of the Long Term Evolution (LTE). The proposed scheme is based on the introduction of few bit errors to modify the few complex modulated symbols of each data SC-FDMA symbol in a sub-frame, which cause peaks of the output signal samples to be larger than a predetermined threshold value. In addition, the effect on the bit error rate (BER) performance of the few deliberately corrupted bits can be greatly mitigated by using the channel decoding at the receiver. Computer simulations show that the proposed scheme can reduce the PAPR of SC-FDMA signals effectively with almost the same BER as the conventional SC-FDMA signals when the signal-to-noise ratio (SNR) is below 35 dB.

Huining Zhang

Papers

SNR estimation algorithm based on the preamble for OFDM systems in frequency selective channels

An Efficient Frequency Offset Estimation Method With a Large Range for Wireless OFDM Systems

A Semi-Blind SLM Scheme for PAPR Reduction in OFDM Systems With Low-Complexity Transceiver

PAPR Reduction of SC-FDMA Signals Via Probabilistic Pulse Shaping

PAPR Reduction in Coded SC-FDMA Systems via Introducing Few Bit Errors