Abstract
Frequency diversity is a technique for improving link reliability by transmitting the same signal over multiple frequency channels. In practice, a transmitter sends redundant copies of the data on different carrier frequencies or spreads the signal over a wideband channel. Because multipath fading is typically frequency selective, different frequencies will experience deep fades at different times. Thus, if one copy of the signal is severely faded, another at a different frequency will likely be received correctly. This diversity in the frequency domain exploits the fact that channel impairments are uncorrelated across sufficiently separated frequencies. In other words, fading on one frequency will “miss” another frequency that can still carry the information. Wireless communication is a cornerstone of modern connectivity, enabling everything from mobile phone calls to global navigation systems. However, unlike wired communication, wireless systems face a major challenge: fading. Fading refers to the fluctuation of signal strength at the receiver due to multiple propagation paths, known as multipath fading. As signals reflect off buildings, terrain, or moving objects, they arrive at the receiver with different delays, phases, and amplitudes. These signals can interfere destructively, resulting in poor reception. Multipath fading encountered in time-varying channel renders wireless communications highly non-reliable. To obtain an average bit-error rate of 10−3 using BPSK modulation with coherent detection, performance degradation due to Rayleigh fading can account for a SNR of 15 dB higher than in AWGN . This paper is limited to the review of several study cases of MISO configuration where suitable coding or signal processing techniques are exploited to allow the extraction of transmit diversity without channel knowledge at the receiver. If the sub channels associated with transmit antennas have independent fades, the order of diversity is proven to be equal to the number of transmit antennas. This approach is attractive in public broadcasting systems such as cellular (for voice) or broadband wireless access (for data communications) to keep the subscriber-side equipment cost down with simpler hardware requirement and more compact form factor by avoiding the implementation of several receive antennas. To mitigate the effects of fading and enhance communication reliability, engineers employ a powerful set of techniques known as diversity. Diversity exploits the fact that different copies of the same signal transmitted over independently fading channels are unlikely to experience deep fades simultaneously. By combining these signal copies, the probability of complete signal loss is drastically reduced. This article explores the five principal types of diversity used in wireless systems: frequency diversity, time diversity, space diversity, polarization diversity, and angle diversity.
1. Frequency Diversity
frequency diversity is exploited to achieve passive localization with a limited number of hydrophones and address the challenge of multipath propagation. Specifically, the received signals of both hydrophones are decomposed in the frequency domain, respectively, at first. Then, a set of differential transmission-loss model-based equations between the two hydrophones are established on multiple frequencies. Based on these equations, the passive localization is modeled as a multivariate optimization problem. Meanwhile, the nonline of sight (NLOS)-related parameters are also involved in the optimization problem. Therefore, accurate localization and NLOS mitigation can be accomplished simultaneously by solving the optimization problem. To simplify the multivariate optimization problem for a reliable solution, a cepstrum-autocorrelation based multipath estimation algorithm is proposed, by which all the NLOS paths can be represented by an equivalent NLOS path. Consequently, the simplified optimization problem concerns only the desired coordinate of the source and a single NLOS-related parameter. Finally, a differential evolution algorithm is employed to solve the simplified optimization problem. Both simulation and lake trial results corroborate the effectiveness and the robustness of the proposed method. Determination of indoor position based on fine time measurement (FTM) of the round trip time (RTT) of a signal between an initiator (smartphone) and a responder (Wi-Fi access point) enables a number of applications. However, the accuracy currently attainable—standard deviations of 1–2 m in distance measurement under favorable circumstances—limits the range of possible applications. An emergency worker, for example, may not be able to unequivocally determine on which floor someone in need of help is in a multi-story building. The error in position depends on several factors, including the bandwidth of the RF signal, delay of the signal due to the high relative permittivity of construction materials, and the geometry-dependent “noise gain” of position determination. Errors in distance measurements have unusal properties that are exposed here. Improvements in accuracy depend on understanding all of these error sources. This paper introduces “frequency diversity,” a method for doubling the accuracy of indoor position determination using weighted averages of measurements with uncorrelated errors obtained in different channels. The properties of this method are verified experimentally with a range of responders. Finally, different ways of using the distance measurements to determine indoor position are discussed and the Bayesian grid update method shown to be more useful than others, given the non-Gaussian nature of the measurement errors.
2. Time Diversity
Time Diversity is a technique used to combat fading by transmitting the same data multiple times at different moments. Since the wireless channel varies over time, it is unlikely that the channel will be in a deep fade at all those times. By leveraging timedomain variations in the channel, time diversity ensures that at least one copy of the transmitted signal is likely to be received under better conditions. In wireless communication systems, fading—the fluctuation of signal strength due to multipath propagation and environmental factors—is a major challenge that degrades performance. To mitigate the effects of fading, engineers use various diversity techniques. Among them, Time Diversity is one of the simplest yet effective methods. This article explores the concept of time diversity, its working principles, implementation techniques, advantages, limitations, and real-world applications. 10 Gb/s free space optical (FSO) wiretap channel using optical code division multiple access (OCDMA) timediversity reception is designed and demonstrated for the first time. Reliability and security are investigated under weak and strong turbulence conditions. Two-dimensional optical encoder and decoder are constructed by wavelength selective switches and optical delay lines, and one-dimensional optical encoder and decoder are constructed by couplers and tunable optical delay lines. Experimental results show that reliability and security can be enhanced simultaneously using OCDMA time-diversity reception. At the received power of 2.58 dBm in strong turbulence, compared to non-diversity system, bit error rate (BER) of legitimate user in OCDMA time-diversity system is reduced from 3.9 × 10−7 to 6.73 × 10−8. Furthermore, the reliability improvement is more obvious when the correlation between the two signals of OCDMA time-diversity is smaller. The secrecy capacity can be increased from 0.546 bit/symbol to 0.665 bit/symbol in strong turbulence when received power is 2.28 dBm. On the other hand, the secrecy capacity of strong turbulence is about 15% higher than that of weak turbulence.
3. Space Diversity
Space Diversity involves the use of two or more antennas that are spatially separated to transmit or receive the same signal. Since the radio signal travels through different paths, fading will not affect all paths simultaneously and equally. By using multiple spatially separated antennas, a communication system can select or combine the best signal path, thereby reducing the effects of fading and improving signal reliability. The integration of visible light communications (VLC) in future generation of wireless communications leads to consider the deployment of multiple access points (APs) transmitting in the optical domain. Since each optical AP generates a small and confined coverage footprint, scenarios comprising multiple optical APs are subject to intercell interference. In this context, angle diversity receivers (ADRs) composed of multiple photodiodes pointing to distinct orientations each, have been proposed for mitigating the interference and blocking effects. The design of ADRs typically assumes that the field-of-view (FoV) generated by each photodiode does not overlap with the FoV of all other photodiodes. In this work, we propose the derivation of the theoretical expressions of the probability distribution function (PDF) and the cumulative distribution function (CDF) of the signalto-interference plus noise ratio (SINR) in multicell scenarios for ADRs in which the FoV generated by each photodiode may overlap with the FoV of the other photodiodes. Several geometrical conditions are proposed in order to derive the statistical characterization of photodiode combining schemes such as select best combining (SBC), equal gain combining (EGC) and maximum ratio combining (MRC). It is shown that the derived closed-form expressions obtain a similar performance as the results obtained through Monte Carlo simulations. Moreover, the SINR enhancement due to the use of the proposed ADR in comparison with single photodiode receivers is highlighted. Urbanization’s rapid progress presents an urgent challenge for developing a predictive, quantitative theory of “the death and life of cities” (a.k.a. “the essential diversity conditions for the urban built environment”). Despite the importance of activity diversity (i.e. serving different primary functions), existing works ignored that time diversity (i.e. attracting people at different times of the day) and space diversity (i.e. attracting people from different districts) also play important roles in promoting urban life in large cities. With assistance of human mobility and crowd sourcing data, this article thoroughly validates whether activity, time, and space diversity are essential and inseparable components of urban vitality in the Wuhan, China context. To achieve the goal, point of interest (POI) data are utilized to quantitatively measure activity diversity, human mobility data are adopted for building quantitative metrics of time diversity and space diversity, and a detailed urban perception map is crowd sourced as ground truth data for establishing a regression model between urban diversity indicators and urban vitality. The resultant regression model succeeds to decouple the relationship between population concentration, activity diversity, time diversity, space diversity, and urban vitality. It confirms that activity diversity together with time diversity and space diversity has stronger association with urban vitality than any single diversity indicator. Our contributions are threefold: (a) we provided a comprehensive collection of metrics for measuring urban diversity, (b) we confirmed that activity, time, and space diversity are essential components of urban vitality, and (c) our methodology can be replicated at scale to understand urban vitality under various geographic, societal, and economic contexts due to easy accessibility of similar datasets.
4. polarization Diversity
Both radiating elements of the MIMO antenna system are circularly polarized (CP) at the desired frequency band (915 MHz). In fact, Antenna- 1 operates at left-handed CP (LHCP) and Antenna- 2 operates at right-handed CP (RHCP), enabling polarization diversity at 915 MHz. Moreover, the radiating elements radiate in two opposite directions; thus, they enable pattern diversity. The performance of the proposed antenna system is analyzed in a realistic human phantom. Reactive slots in the patch antennas are incorporated to achieve compactness ( Ļ× (4.7) 2×0.13 = 9.01 mm3) and CP characteristics. Furthermore, a rectangular slot on the ground plane is introduced to reduce the mutual coupling between the radiating elements. The proposed antenna has compact dimensions of 9.01 mm3, a peak realized gain of −24.6 dBiC, and a high isolation level of 29.7 dB within the whole band. The specific absorption rate (SAR), link margin (LM) and main MIMO channel parameters are investigated which show good results. The key features of this antenna are its compact size, high isolation level between the antennas, and polarization and pattern diversity. Fiber-to-chip coupling is an essential issue for taking high-performance integrated photonic devices into practical applications. On a thin-film lithium niobate platform, such a high-performance coupler featuring low loss, large bandwidth, and polarization independence is highly desired. However, the mode hybridization induced by the birefringence of lithium niobate seriously restricts a polarization-independent coupling. Here, we propose and experimentally demonstrate a high-performance and polarization-diversity cantilever edge coupler (EC) with the assistance of a two-stage polarization splitter and rotator (PSR). The fabricated cantilever EC shows a minimal coupling loss of 1.06 dB/facet, and the fully etched PSR structure shows a low insertion loss (IL) of -0.62 dB. The whole polarization-diversity cantilever EC exhibits a low IL of -2.17 dB and -1.68 dB for TE0 and TM0 mode, respectively, as well as a small cross talk of <-15 dB covering the wavelength band from 1.5 Āµm to 1.6 Āµm. A polarization-dependent loss <0.5 dB over the same wavelength band is also obtained. The proposed fiber-to-waveguide coupler, compatible with the fabrication process of popular thin-film lithium niobate photonic devices, can work as a coupling scheme for on-chip polarization-diversity applications.
BIODATA 
: Ahmad Reyhan Al Hilmy
: 1D – JTD /02
: 244101060025
: 07/07/2005
: Jl. Brigjend katamso No. 81-83
: 085604335001
: reyhanalhilmy775@gmail.com
: Renang
