Problems Associated with Long Path Communication Links in Tropical Areas

We provide a compact characterization of the outage rates for wideband wireless communication, quantifying the effects of frequency and spatial diversity. Our information-theoretic computations yield the following findings. (a) Complex models typically employed in simulations can be replaced by simple, bandwidth-dependent, tap delay line models without loss of generality. (b) The spectral efficiency (i.e., the achievable rate, divided by the bandwidth) is well approximated as a Gaussian random variable, so that it is only necessary to specify its mean and variance to obtain outage rates. (c) The mean spectral efficiency depends only on the spatial diversity, while the variance depends on the spatial and frequency diversity via a product. Our definitions of frequency and spatial diversity have physically motivated interpretations, and are not based on high signal-to-noise ratio (SNR) asymptotics as in prior work.

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The most frustrating and troublesome issue in wireless communication is fading. Estimating the fading occurrence for the design of greatly reliable communication link is crucial. In this paper, we present a novel mathematical method for predicting the future signal fading on the basis of current and past data. The application of a Semi Markov Model as a generalization of the Markov Model is discussed for predicting the deep fading occurrence probabilities of the received envelope in wireless communications channels. This flexible model is given for assessing the system performance with the envelope correlation. 142 deep fading data whose amplitudes are lower than a mean of the amplitudes which occurred in a typical wireless system with the Jakes filtering are considered. The transition probability matrix and the holding time mass functions are calculated for the next 1 to 21 unit times. One unit time is regarded as the inverse of sampling frequency; moreover, the core matrix and the cumulative probability distribution of the waiting time are obtained. Calculating the interval transition probabilities for Amplitude to Amplitude transition for these deep fades demonstrates the forecasting occurrence probabilities in the future and the possibility of forecasting the fading occurrences in dimensions of time and amplitude.

In this chapter we look at various basic issues that arise in communication over fading channels. We start by analyzing uncoded transmission in a narrowband fading channel. We study both coherent and non-coherent detection. In both cases the error probability is much higher than in a non-faded AWGN channel. The reason is that there is a significant probability that the channel is in a deep fade. This motivates us to investigate various diversity techniques that improve the performance. The diversity techniques operate over time, frequency or space, but the basic idea is the same. By sending signals that carry the same information through different paths, multiple independently faded replicas of data symbols are obtained at the receiver end and more reliable detection can be achieved. The simplest diversity schemes use repetition coding . More sophisticated schemes exploit channel diversity and, at the same time, efficiently use the degrees of freedom in the channel. Compared to repetition coding, they provide coding gains in addition to diversity gains . In space diversity, we look at both transmit and receive diversity schemes. In frequency diversity, we look at three approaches: single-carrier with inter-symbol interference equalization, direct-sequence spread-spectrum, orthogonal frequency division multiplexing. Finally, we study the impact of channel uncertainty on the performance of diversity combining schemes. We will see that, in some cases, having too many diversity paths can have an adverse effect due to channel uncertainty.

In this paper, we propose a space-time coding and carrier block spreading based multiple-input-multiple-output (MIMO) orthogonal-frequency-division-multiplexing (OFDM) system. The proposed system exploits the potential spatial and frequency diversity of MIMO systems in frequency-selective fading environments through a two-stage procedure. In the first stage, we use space-time coding to exploit the spatial diversity, and in the second stage, we adopt block spreading among blocks of OFDM carriers to obtain frequency diversity. Moreover, by using rotated Hadamard matrix or rotated discrete Fourier transform (DFT) matrix as the carrier spreading matrix, the system can also possess a certain degree of modulation diversity. Our proposed system enjoys the advantage of high flexibility and low complexity. It allows the use of any existing space-time coding scheme as its component space-time code, and the diversity gains are obtained with only moderate transmit and receive processing complexity. Simulation results show that our proposed system can achieve desired spatial and frequency diversity with low detection complexity. Comparing to some existing space-frequency coding schemes, our proposed system enjoys the merit of low complexity, although it could suffer from some performance degradation, which is caused by the separation of space-time coding and frequency-domain spreading process.

In this paper, an enhanced scheme of Index Modulation for Orthogonal Frequency Division Multiplexing (IM-OFDM), called Repeated IM-OFDM with Diversity Reception (ReIM-OFDM) is proposed. ReIM-OFDM achieves performance improvement over the convetional IM-OFDM at the same spectral efficiency by providing additional space and frequency diversity. Similar to IM-OFDM, the proposed ReIM-OFDM also activates K out of total N subcarriers to convey information bits using both the active sub-carriers and their indices. However, it differs from IM-OFDM in that all activated sub-carriers are modulated by the same M-ary data symbol. For signal combining, either Maximal Ratio Combining (MRC) or Selection Combining (SC) can be used in the spatial and sub-carriers domain to help ReIM-OFDM achieve both space and frequency diversity gain. In order to analyze performance of the system, the Moment Generating Function (MGF) is used to obtain the closed-form expressions for the pairwise index error probability (PEP) and the symbol error probability (SEP) in both cases: ReIM-OFDM-MRC and ReIM-OFDM-SC. Effects of various system parameters on the SEP are analyzed to select the best ReIM-OFDM configuration with minimum error probability. Our analysis proves the effectiveness of ReIM-OFDM over IM-OFDM at the same spectral efficiency.

Since space diversity and frequency diversity are both efficient techniques in the radar applications, we propose in this paper a method for simultaneous usage of both techniques. On this basis, a multi-frequency array radar in which all frequencies are used simultaneously to increase the number of targets that can be uniquely identified, is introduced and analyzed. First, the signal model for such a system is derived and then, it is shown how the use of multi-frequency signal can noticeably improve the parameter identifiability. Specifically, we show the maximum number of targets that can be uniquely identified by the multi-frequency array radar is L times that of its single-frequency array counterpart, where L is the number of frequencies.

Ionosphere scintillation is a critical source of error that threatens the integrity and continuity of GNSS receiver measurements. In this paper, we examine the GNSS signal carrier phase behavior during deep fades caused by strong equatorial ionosphere scintillation (the so-called “canonical fading”). We begin with a discussion to clarify the relationship between conventional and diffraction-induced cycle slips, which include noise-induced cycle slips due to signal fading as well as phase transitions. We then investigate the occurrence of signal fading and phase behavior in triple-frequency GPS data gathered during a 2013 March 10 scintillation event near Ascension Island. We discuss standard detrending techniques as well as the application of geometry-ionosphere-free combination of GNSS phase measurements when analyzing the carrier phase behavior. We compare the behavior of the three carrier phases during deep fades in order to assess (when possible) which signals contain diffraction-induced cycle slips. We use the geometry-ionospherefree combination of phase measurements to supplement this analysis. Ultimately, we present useful examples that serve as a guide for future development of mitigation techniques for diffraction-induced cycle slips.

Literature indicates that frequency diversity can be utilized to compensate channel uncertainties such as multipath fading. Therefore, in this paper it is exploited for improving accuracy in locating stationary and mobile objects in the indoor environment. First, frequency diversity technique is introduced for small scale and temporal variation compensation of received signals and demonstrated analytically that it in fact enhances location accuracy. A novel metric is introduced in selection combining in order to achieve location accuracy through the addition of frequency diversity upon two of the available location determination schemes. The results are evaluated experimentally against the case where there is no frequency diversity for reception by using low cost wireless RF devices such as motes. An asset location tracking system is then devised to both improve accuracy and predict asset movement. Frequency diversity in terms of channel spacing of 55 MHz is evaluated and shown to provide a reduction in location determination error between 18 and 23% when compared to a system without frequency diversity. Finally, results from frequency diversity are compared against the spatial diversity techniques in terms of improvement in location accuracy, power consumption of the transmitter, and hardware and software costs.

In this paper, a planar multiple input multiple output MIMO UWB array is proposed for 3D microwave imaging applications. According to the virtual array concept, the advantage of the MIMO array lies on improving the angular resolution with a reduced number of antennas, and achieve the same resolution as a fully populated conventional arrays. In addition, the UWB systems provide a better range resolution thanks to their large bandwidth. The frequency diversity and its influence on the system's resolution is also studied. For the MIMO array, an optimal UWB antenna element is designed with more than 128% of fractional bandwidth with no considerable distortion of the transmitted impulse. For the imaging algorithm, the Back-projection algorithm is implemented and used for the reconstruction of the images. In order to study the performance of the designed system, the full-wave electromagnetic simulator CST MWS is used which allow the modelling in three-dimensions (3D). The resulting images of a different target materials and dielectric properties confirm the reliability of the proposed system and the impact of the spatial and frequency diversity in the resolution improvement while considerably reducing the cost of the MIMO UWB array.

The widespread deployment of WiFi infrastructure has facilitated the development of Time-of-Flight (ToF) based sensing applications. ToF estimation, however, is a challenging task due to the complexity of multipath effect. In this paper, we propose a phase difference based method for ToF estimation and uncover the potential of signal cancellation to mitigate the impact of multipath and noise on phase differences among subcarriers. To separate the moving target path from the complex multipath for ToF estimation, we suggest employing specific elimination methods tailored to the characteristics of different signal components. For dynamic multipath, we observe that when a given subcarrier propagates along two paths to the receiver, with path lengths differing by half a wavelength, the phase difference introduced by these two paths cancels each other out. Therefore, we propose two metrics to identify signals that satisfy this condition, utilizing both frequency diversity and spatial diversity. Additionally, we propose leveraging time diversity to eliminate the static multipath component and reduce the impact of noise. We implemented the methods with off-the-shelf WiFi devices and achieved mean errors of 15.36 cm and 21.05 cm for distance estimation in outdoor and indoor scenarios, outperforming state-of-the-art ToF estimation method by 50% error reduction.

This paper deals with the study of received signal strength (RSS) as an input reference used in Bluetooth Low Energy (BLE) Indoor Positioning System (IPS) for location estimation. In the IPS where there are fixed scanners and portable advertisers which are sending no additional data, the RSS is the only reference for location process. Therefore, it is important to suppress RSS fluctuation caused by fading. The proposed method uses spatial diversity (SIMO approach) in combination with frequency diversity. Two different distances between four antennas were tested and for a quarter of the wavelength additional arrangement using back-side reflector was taken.

Orthogonal frequency and code division multiplexing (OFCDM) has been spotlighted as a next generation communication system. Cooperative OFCDM relaying system has advantages that provide the high reliability while maintaining high data rate because of getting the frequency diversity and the spatial diversity. However, this system decreases a throughput performance when it employs many relays. Therefore, in this letter, we present that the proposed system combines CDD due to complement throughput performance problem. In addition to that, the proposed system based on orthogonal frequency and code division multiple access (OFCDMA) which efficiently communicates each other through assigning the subcarrier and the channel code according to channel state or other environments.

Discrete Hartley transform (DHT) based orthogonal frequency division multiplexing (OFDM) outperforms the OFDM based on discrete Fourier transform (DFT) for its intrinsic frequency diversity. However, the space-frequency block coding (SFBC) cannot be applied since the symbols in Hartley domain are coupled. In this paper, by using the properties of DHT and DFT, we propose a DHT based SFBC-OFDM system design with better performance than and similar complexity to the DFT counterpart. Moreover, a low complexity algorithm is derived based on the properties of DHT to get the time domain signals of the DHT based SFBC code words with about halved complexity. Finally, analysis and simulations are provided to validate its feasibility and ability to exploit both the frequency and spatial diversity, as a promising alternative to the DFT counterpart.

Air-to-Ground (A2G) wireless communications using Unmanned Aerial Vehicles (UAVs) as Low Altitude Platforms (LAPs) have become a hot topic in recent decades. Among all the A2G channel models that can be used for A2G channel characterization, the two-ray ground reflection path loss model can be used for environments with fewer scatterers such as over water, suburban and mountainous. In such environments, signal down-fades appear due to the destructive combination of the direct component and the ground reflected component. This paper presents a frequency diversity approach to improve the coverage in such an environment. Our results show that frequency diversity can improve coverage at least by 17% when two frequencies are used assuming the threshold to be −80 dBm. Moreover, this work analyses the coverage improvement that can be obtained through polarization diversity. Our results show that having both horizontal and vertical polarizations can improve the coverage by 10%95%, assuming the transmission frequency to be 700MHz and UAV altitude is between 100 m300 m.