CGLS Method for Efficient Equalization of OFDM Systems Under Doubly Dispersive Fading Channels with an Application Into 6G Communications in Smart Overcrowded

Orthogonal frequency division multiplexing (OFDM) is proposed as a convenient digital modulation scheme to transmit data encoded by means of turbo trellis coded modulation (TTCM). The resulting transmission system is described and the advantages of using OFDM to transmit TTCM encoded data are highlighted. The system is investigated by means of simulations in order to assess its performance over dispersive deterministic channels and over dispersive Rayleigh fading channels. As a benchmark, a system having a similar computational complexity, but employing classical trellis coded modulation (TCM) to encode the data is also investigated.

Recently developed subspace techniques for blind adaptive multiuser detection are briefly reviewed first. In particular, blind methods based on signal subspace tracking for adapting linear multiuser detectors in AWGN CDMA channels are considered, as well as extensions of these techniques to frequency selective fading channels, dispersive channels, and antenna array spatial processing. In addition, subspacedbased nonlinear adaptive techniques for robust blind multiuser detection in nondGaussian ambient noise channels are also described. Several new techniques are then developed within the subspace framework for blind joint channel estimation and multiuser detection, under some specific channel conditions. These include l1r an adaptive receiver structure for joint multiuser detection and equalization in dispersive CDMA channels, l2r a subspace method for joint multiuser detection and equalization in unknown correlated noise, and l3r a method for joint interference suppression and channel tracking in timedvarying fading channels.

We propose a novel space-time coding-assisted double-spread RAKE receiver (STC-DS-RR) based CDMA scheme for downlink (base to mobile) transmissions over dispersive Rayleigh fading channels. The STC-DS-RR scheme invokes turbo convolutional (TC) channel coding and space-time block coding (STBC) for achieving both time and space diversity gains in addition to the path diversity gain provided by the dispersive Rayleigh fading environments. The double-spreading mechanism of the STC-DS-RR scheme is capable of detecting the wanted user's channel-impaired wideband signals with the aid of a low-complexity RAKE receiver, without resorting to complex multi-user detection. The performance of the proposed scheme was investigated using quadrature-phase-shift-keying (QPSK), when communicating over dispersive channels. It was shown that the wideband scheme advocated is capable of achieving E/sub b//N/sub 0/ gains up to 35 dB in comparison to an uncoded narrowband single-transmitter scheme.

Cognitive radio technology, which helps to alleviate spectrum scarcity problem, plays an important role in future wireless communication systems. Spectrum sensing is the key technique of cognitive radio. The upcoming fifth generation (5G) communications need to deal with fast time-varying channels (i.e., channel state is non-static within a sensing window), while existing sensing methods are based mainly on conventional quasi-static channels. Consequently, traditional sensing methods may not work well in 5G communications. This paper aims to propose a sliced sensing technique, which can support 5G cognitive radio applications working in fast time-varying channels. We analyze the impact of non-static channels on the performance of traditional sensing methods and disclose a negative factor, i.e., out-of-phase feature distortion effect. Accordingly, we propose a sliced spectrum sensing scheme and probe its feasibility via several relevant trials. Based on the aforementioned works, a complete version of the sliced sensing technique is designed, which can adapt to channel variation and choose the optimal slicing strategy. Simulation results manifest that the sliced sensing technique outperforms traditional sensing methods in both Rayleigh fading and 3GPP spatial channel models.

The matched-filter bound (MFB) for continuously dispersive Rayleigh fading channels with receive diversity is derived. The treatment is quite general, as the case of discrete multipath channels with correlated fading is included in our model as a special case, and we impose no constraints with regard to the selectivity of the channel. It is assumed that a statistical characterization of the channel is available through its direction-delay-Doppler power spectral density. The Karhunen-Loe/spl grave/ve expansion for vector processes is used to describe the received signal. The MFB is analyzed with respect to several parameters characterizing the mobile radio channel, the waveform, and the antenna configuration.

In indoor visible light communication (VLC) systems, multiple optical transmitters and reflections will result in a dispersive multi-path channel. We propose a location-based equalization (LBE) method for indoor VLC using orthogonal frequency division multiplexing. With LBE, the location information of a VLC receiver, particularly in terms of coordinates, is utilized for channel estimation. The estimated channel parameters can be further used in a variety of ways for real-time mitigation of multi-path effects induced by a dispersive channel. As a result, the channel distortion can be predictable and thus VLC becomes a smart location-based value-added service. With different implementation schemes, extensive numerical simulations and comparisons are carried out to verify the feasibility of LBE. Results show that LBE can effectively improve system bit error rate (BER) performance at different indoor locations. Even in the presence of location error, link shadowing, or receiver tilt, the LBE design, especially based on only a line-of-sight channel, still shows good robustness, which can achieve reliable transmission quality with BER < 10−3 at various locations. With LBE, the horizontal and vertical tolerance against location error can be 0.28 m and 0.1 m, respectively, while the tolerance angle against receiver tilt is basically no less than 30°.

In this contribution we propose a novel space-time coding (STC) scheme for transmissions over dispersive Rayleigh fading channels, which invokes a joint-detection assisted minimum mean square error based decision feedback equalizer (JD-MMSE-DFE) for jointly detecting the channel-impaired wideband signals of multiple transmit antennas. The performance of the proposed scheme was investigated using quadrature phase shift keying (QPSK) when communicating over channels exhibiting multipath components. It was shown that the scheme advocated outperforms conventional STC due to benefitting from the multipath diversity of the dispersive channel.

Orthogonal frequency-division multiplexing (OFDM) is a promising technology for communication systems. However, the synchronization of OFDM over dispersive fading channels remains an important and challenging issue. In this paper, a synchronization algorithm for determining the symbol timing offset and the carrier frequency offset (CFO) in OFDM systems, based on the maximum-likelihood (ML) criterion, is described. The new ML approach considers time-dispersive fading channels and employs distinctive correlation characteristics of the cyclic prefix at each sampling time. The proposed symbol timing estimation is found to be a 2-D function of the symbol timing offset and channel length. When compared with previous ML approaches, the proposed likelihood function is optimized at each sampling time without requiring additional pilot symbols. To practically realize the proposed method, a suboptimum approach to the ML estimation is adopted, and an approximate but closed-form solution is presented. Nonlinear operations of the approximate solution can be implemented using a conventional lookup table to reduce the computational complexity. The proposed CFO estimation is also found to depend on the channel length. Unlike conventional schemes, the proposed method fully utilizes the delay spread of dispersive fading channels (which usually reduces the accuracy of estimations). Furthermore, the Cramer-Rao lower bound (CRLB) on the CFO estimate is analyzed, and simulations confirm the advantages of the proposed estimator.

A maximum likelihood sequence estimator for the dispersive Rayleigh fading channel is developed. Following previous work, the MLSE uses a Kalman based channel estimator to acquire the channel parameters necessary to formulate the maximum likelihood metric. However, unlike the MLSE receiver presented previously, the proposed receiver uses the f-power series channel model to formulate the ML metric and the state space representation of the channel and the received samples. For channels with small delay spreads, this approach is advantageous because only a small number of parameters are required to be estimated by the Kalman channel estimator. Simulation results are presented for various channel parameters.

This work combines the techniques of multiple-input multiple-output (MIMO) antennas and orthogonal frequency division multiplexing (OFDM) to deliver high-capacity high-data-rate wireless communications over broadcast time dispersive frequency selective fading channels. For broadcast channels, multiple communications can occur at the same frequency subcarrier and time slot from one base station (BS) to many mobile stations (MS). To realize this, we develop a so-called orthogonal space division multiplexing (OSDM) scheme to decompose per-subcarrier channel into many uncoupled single-user spatial modes. In particular, the scheme is modified to improve its robustness to the channel mismatches arising from pilot-aided channel estimation and Doppler spread. Simulation results reveal that following the ESTI HiperLAN II parameters; our proposed scheme can achieve good performance with four simultaneous downlink users at Doppler spread of 40 Hz, which corresponds to mobile moving at speed of 8.5 km/h communicating at 5 GHz.

Multicarrier systems are adopted in several standards for their ability to achieve optimal performance in very dispersive channels. In particular, orthogonal-frequency division multiplexing (OFDM) and filtered multitone (FMT) systems are two examples where the modulation filter has an ideal rectangular amplitude characteristic in time and frequency domains, respectively. In this letter, we propose new equalization schemes for FMT and compare their performances with OFDM. In general, FMT has a greater spectral efficiency than OFDM, due to the absence of the cyclic prefix and a reduced number of virtual carriers. However, it exhibits a higher distortion per subchannel, due to the imperfect equalization of the transmit filters. As a performance comparison, we considered both the achievable bit rate (ABR) and the bit error rate (BER) in a multipath Rayleigh fading channel. We note that while ABR gives a theoretical bound on the system bit rate, assuming the knowledge of the channel at the transmit side, the BER refers to an uncoiled system with a fixed modulation. Although FMT requires a fixed structure with a higher computational complexity than OFDM, it turns out that FMT, even with the simplest one tap per subchannel adaptive equalizer, yields a better performance than OFDM, both in terms of ABR and BER. Hence, FMT can be a valid alternative to OFDM for broadband wireless applications, also.

For the bridge located in deep valley, with high tower and long span structure, the traditional artificial crack detection can not meet the needs of today. In order to solve this problem, a new bridge crack detection equipment is designed based on quadrocopter unmanned aerial vehicle. According to this equipment, the developed system contains: reconnaissance unmanned aerial vehicle system, acquisition and transmission system, crack positioning system, and graphics operations system. Reconnaissance UAV system is improved based on quadrocoper UAV to meet the requirements of flight power, flight control, flight safety and signal stability. In the integrated transmission system, the designed UAV equipped with Hawkeye 5S motion camera, and 4G communication is used to complete the real-time image transmission; The crack location system uses BDS / GPS dual-mode combined single-point positioning module to realize the accurate location of bridge surface cracks; Graphics operations system, based on C++ and python language, designed a superpotent actual-time video streaming graphics operations method, and used median filter, image grayscale, histogram equalization, threshold segmentation and other methods to process the crack image. The designed bridge crack detection equipment can effectively solve the problem of low crack detection efficiency, and realize the intelligence and automation of crack identification. It has practical engineering application value and promotes the development of bridge crack detection technology.

An adaptive near-far resistant technique for the blind joint multiuser identification and detection in asynchronous CDMA systems is analyzed in fading and dispersive GSM channels.

Due to the bandwidth efficiency and high robustness in delay dispersive multipath fading channel, the OFDM system is most widely adopted by different digital communication systems, as their main modulation technique. The major problem in the OFDM system is its sensitivity to the channel conditions, hence leads to synchronization errors. The main problems are ISI and ICI, due to delay dispersion of multipath fading channel and Doppler shift during the transmission. So synchronization algorithms required to compensate the effect of channel variation. In this paper a robust and simple synchronization algorithm is presented, which mainly focuses on timing offset estimation for OFDM synchronization. The starting point of the received data is detected by reception of one training symbol. It is observed that the proposed algorithm is more robust than conventional Schmidl and Cox algorithm and Rathkanthiwar and Gandhi (RG) algorithm, despite being computationally simpler.

A deployment of cooperative multiple sensors offers considerable enhancement of the automatic modulation classification (AMC) performances, particularly in multipath fading environment. However, the recent studies showed that potential performance gains reported under the idealized conditions are not straightforwardly achievable in real-world application. Through a comprehensive analysis of cooperative AMC solutions using cumulants under the realistic conditions and for the different dispersive fading channels, we here show that these huge performance gains are practically achievable if the appropriate channel estimation methods are applied.