Analysis of diffuse optical wireless channels employing spot-diffusing techniques, diversity receivers, and combining schemes

In this letter, the performance of an indoor optical wireless spot-diffusing system using various multibeam transmitter configurations, in association with direction diversity and combining techniques, is assessed and compared under the impact of multipath dispersion and ambient light noise through theoretical analysis and computer simulation. Computer simulation for three different multibeam transmitter configurations and a conventional diffuse transmitter is carried out. Diversity receiver and wide field-of-view (FOV) receiver configurations are evaluated in conjunction with the proposed configurations. For the diversity-detection case, a receiver comprising an array of narrow FOV detectors (three and seven segments) oriented in different directions is used to maximize the collected signals and minimize noise. A novel line-strip multibeam system (LSMS) is investigated with single and diversity receiver configurations, and is compared with other spot-diffusing methods. Combining schemes, including selection combining, maximum ratio combining, and equal gain combining are employed for the presented configurations. Our results indicate that the performance improvement obtained through the use of LSMS with a three-direction diversity receiver is about 20 dB signal-to-noise ratio enhancement over the conventional diffuse system, and 26 dB when combining techniques are used. Root mean square delay-spread performance for the proposed configurations, at different positions on the communication floor, are also evaluated and compared.     

Imaging diversity receivers for high-speed infrared wirelesscommunication

We discuss two modifications to the design of wireless infrared links that can yield significant performance improvements, albeit at the price of increased complexity. In line-of-sight and non-line-of-sight links, replacement of a single-element receiver by one employing an imaging light concentrator and a segmented photodetector can reduce received ambient light noise and multipath distortion. For a fixed receiver entrance area, such an imaging receiver can reduce transmit power requirements by as much as about 14 dB, depending on the link design and the number of photodetector segments. Imaging receivers also reduce co-channel interference, and may therefore enable infrared wireless networks to employ space-division multiplexing, wherein several transmitters located in close proximity can transmit simultaneously at the same wavelength. In nondirected non-line-of-sight links, replacement of the diffuse transmitter by one that projects multiple narrow beams can reduce the path loss, further reducing the transmit power requirement by several decibels. We describe the design of an experimental 100 Mb/s infrared wireless link employing a multibeam transmitter and a 37-pixel imaging receiver     

MMSE techniques for space diversity receivers in OFDM-based wireless LANs

This paper studies the application of the minimum mean square error (MMSE) beamformer to orthogonal frequency division multiplexing (OFDM)-based wireless local area networks. The questions here addressed are mainly the design with finite-length data and the choice of the OFDM signal domain where the beamformer is applied, either frequency or time. As OFDM signals need more samples than other modulations to stabilize the estimation of the signal statistics, how to exploit the finite-length training sequence provided for the design of equalizers becomes an important issue. The paper also shows that the usual frequency processing in OFDM is not always the best choice for the spatial beamforming, mainly for channels with a very high delay spread. Then, time processing turns out to be the best suited approach in terms of the tradeoff between performance and complexity. Additionally, novel modifications of the MMSE spatial filter are proposed to improve the raw bit-error rate performance: 1) a temporal semiblind approach that exploits the cyclic prefix and 2) windowing in the frequency domain.     

Angle diversity for nondirected wireless infrared communication

We outline the benefits and challenges of using angle diversity in nondirected wireless infrared (IR) communications systems. Multiple transmitter beams and multiple narrow field-of view receivers reduce the path loss, multipath distortion, and background noise of the channel, which leads to improved range. We also discuss practical considerations for multielement angle diversity systems, including channel characterization and suboptimal detection techniques. Maximal-ratio combining provides nearly optimal performance up to 100 Mb/s for the angle diversity systems considered. The design and performance of a prototype angle diversity IR communication system are discussed. The prototype can maintain 70 Mb/s at a P_e of 10^-9 over a 4-m range     

A new family of soft-output adaptive receivers exploiting nonlinear MMSE estimates for TDMA-based wireless links

Decision-directed adaptive receivers suffer performance degradation on time varying and intersymbol interference-impaired links because of two major problems: the use of predicted channel estimates due to the unavoidable decision delay of any detector, and the unreliability of hard decisions used for channel estimation and tracking. It is shown here that combining a recursive nonlinear symbol estimator with a channel estimator with a low prediction order may alleviate this performance degradation. In particular, it is here proposed to employ the nonlinear minimum mean-square error (NL-MMSE) filtered and fixed-lag smoothed estimates of the transmitted symbols in place of the usual hard decisions for channel estimation and tracking. It is also shown that these NL-MMSE estimates can be recursively computed on the basis of a linear transformation of the vector of the a posteriori probabilities (APPs) of the states of the channel. This approach allows the prediction order of the channel estimates to be limited and, at the same time, limits the performance degradation due to erroneous hard decisions. Another result presented here is that the use of NL-MMSE estimates in place of hard decisions is not based on mere intuition only, but is a straightforward consequence of the statement of the problem of MMSE channel estimation when the overly optimistic assumption of correct decisions is dropped. On the basis of this novel approach, a new family of soft-output adaptive receivers is presented for time-division multiple-access-based radio communications. The proposed family of adaptive receivers is based on an APP-computer and exploits the APPs for both channel estimation and detection. The versatility of the APPs ensures that the architecture of the proposed receiver is flexible, so that several estimators and detectors can be embedded in it.     

Improving PPM schemes in wireless infrared links at high bit rates

The authors conclude that pulse-position modulation (PPM) with Gaussian pulses provides better performance than classic PPM schemes with rectangular pulses over unguided optical links operating at high bit rates. In this fashion, both intersymbol interference (ISI), induced by multipath propagation, and the phase distortion effects due to the high-pass filter whose function is to suppress interference, are minimized. However, relevant improvement is not achieved if compared with the OOK format with Gaussian pulses and low duty cycle. The above conclusion is validated by the corresponding bit-error rate (BER) computation using Monte Carlo simulations     

Analysis of multiple-antenna wireless links at low SNR

Wireless channels with multiple transmit/receive antennas are known to provide a high spectral efficiency both when the channel is known to the receiver, and when the channel is not known to the receiver if the signal-to-noise ratio (SNR) is high. Here we analyze such systems at low SNR, which may find application in sensor networks and other low-power devices. The key point is that, since channel estimates are not reliable, it is often not reasonable to assume that the channel is known at the receiver at low SNR. In this unknown channel case, we show that for sensible input distributions, in particular all practical modulation schemes, the capacity is asymptotically quadratic in the SNR, /spl rho/, and thus much less than the known channel case where it exhibits a linear growth in /spl rho/. We show that under various signaling constraints, e.g., Gaussian modulation, unitary space-time modulation, and peak constraints, that mutual information is maximized by using a single transmit antenna. We also show that at low SNR, sending training symbols leads to a rate reduction in proportion to the fraction of training duration time so that it is best not to perform training. Furthermore, we show that the per-channel use mutual information is linear in both the number of receive antennas and the channel coherence interval.     

Analysis of polarization diversity at digital TV indoor receivers

The reduction of signal fading by using polarization diversity at a digital terrestrial TV receiver is evaluated. Two different diversity techniques, one operating at carrier (TV channel) level and the other at subcarrier level, are compared. A measurement campaign has been carried out in three different indoor environments using two antennas with orthogonal linear polarizations at the receiver. The polarization diversity gain has been calculated when the maximum selection technique is used. As expected, the level crossing rate and the average fade duration are considerably reduced by the diversity system     

Optimum order of angle diversity with equal-gain combining receivers for broad-band indoor optical wireless communications

Angle diversity is an effective technique to compensate for multipath temporal dispersion in a wireless infrared environment. Diversity is accomplished by using a multibranch receiver capable of resolving multipath. The goal of this paper is to illustrate the effect that the increasing diversity order of a receiver has on the performance of a link in a multispot-diffusing configuration with equal-gain combining outage probability based on probability of bit error is adopted as a performance measure. It is shown that there is an optimal number of branches, which minimizes outage probability. Increasing diversity order beyond this optimal number degrades the performance and increases the receiver complexity, cost, and susceptibility to any shadowing effects.     

Flexible optical wireless links and networks

The worldwide demand for broadband communications is being met in many places by installed single-mode fiber networks. However, there is still a significant "first-mile" problem, which seriously limits the availability of broadband Internet access. Free-space optical wireless communications has emerged as a viable technology for bridging gaps in existing high-data-rate communications networks, and as a temporary backbone for rapidly deployable mobile wireless communication infrastructure. In this article we describe research designed to improve the performance of such networks along terrestrial paths, including the effects of atmospheric turbulence, obscuration, transmitter and receiver design, and topology control.     

Analysis of equal gain diversity receivers in correlated Rayleigh fading channels

Utilizing a desirable exponential integral representation of Gaussian probability integral, this letter derives the average bit error rate (ABER) expressions for coherent binary signals that employ a dual branch equal gain combining receiver. Our numerical results reveal that the branch correlations do not affect the ABER significantly provided power correlation coefficient is less than 0.3 in Rayleigh fading.     

Performance evaluation of 2.5 Gbit/s and 5 Gbit/s optical wireless systems employing a two dimensional adaptive beam clustering method and imaging diversity detection

Previous indoor mobile optical wireless systems operated typically at 30 Mbit/s to 100 Mbit/s and here we report on systems that operate at 2.5 Gbit/s and 5 Gbit/s. We are able to achieve these improvements through the introduction of three new approaches: transmit beam power adaptation, a two dimensional beam clustering method (2DBCM), and diversity imaging. Through channel and noise modeling we evaluated the performance of our systems. The performance of a novel optical wireless (OW) configuration that employs a two dimensional adaptive beam clustering method (2DABCM) in conjunction with imaging diversity receivers is evaluated under multipath dispersion and background noise (BN) impairments. The new proposed system (2DABCM transmitter with imaging diversity receiver) can help reduce the effect of intersymbol interference and improve the signal-to-noise ratio (SNR) even at high bit rate. At a bit rate of 30 Mbit/s, previous work has shown that imaging conventional diffuse systems (CDS) with maximal ratio combining (MRC) offer 22 dB better SNR than the non-imaging CDS. Our results indicate that the 2DABCM system with an imaging diversity receiver provides an SNR improvement of 45 dB over the imaging CDS with MRC when both operate at 30 Mbit/s. In the CDS system, an increase in bandwidth from 38 MHz (non-imaging CDS) to 200 MHz approximately, is achieved when an imaging receiver is implemented. Furthermore, the three new methods introduced increase the bandwidth from 38 MHz to 5.56 GHz. At the least successful receiver locations, our 2.5 Gbit/s and 5 Gbit/s imaging 2DABCM systems with MRC offer significant SNR improvements, almost 26 dB and 19 dB respectively over the non-imaging CDS that operates at 30 Mbit/s.     

RF response of analog optical links employing optical phaseconjugation

The RF transfer function of analog optical links employing optical phase conjugation (OPC) for the compensation of the carrier suppression effect is investigated theoretically. It is well known that the ultimate performance of the OPC technique depends on several system parameters such as fiber span lengths, dispersion characteristics, input optical powers, or optical transmitter chirp. The influence of these parameters on the RF system response is studied by means of simulation, showing that the main degradation of the equalized response is due to the joint action of nonlinear effects and asymmetric optical power change along the fiber spans     

多天线无线数据通信系统中多用户分集的研究

研究当接收天线不少于发送天线时多输入多输出(MIMO)系统的多用户分集能力。首先从理论上分析了发送天线个数等于1和2时最大似然接收和迫零接收系统的平均吞吐量和调度增益,以及仿真分析了发送天线个数大于2时系统性能。理论分析和仿真表明：在多用户的MIMO系统中,接收的平均信噪比、用户个数、收发天线个数、接收机的结构等对于多用户分集有很大的影响。当发送天线个数为1时,接收天线较少(1,2,3)和平均信噪比为．10dB时调度增益很大,但调度增益随着天线个数和发送功率增大急剧下降。和最大似然接收相比,迫零接收具有更大的多用户分集增益,因此迫零接收机的吞吐量可以很容易超过最大似然接收机。     

Modeling and analysis of WAP performance over wireless links

In this paper, an analytical model for studying the performance behavior of wireless application protocol (WAP) over wireless links is proposed. A Rayleigh fading channel model is used to characterize the behavior of the wireless channel. Mathematical expressions that represent the performance of WAP as a function of the protocol and the channel parameters are derived. Computer simulation results are presented to validate the analytical results. It is shown that WAP performs better in a bursty error environment than in a random error environment. The goodput of WAP can be improved by increasing the WAP packet group size, but the significance of the improvement depends on the underlying channel condition.     

Analysis and design of diversity schemes for ad hoc wireless networks

Diversity schemes permit efficient communication over fading channels but are often hard to analyze and design in networks with many nodes. For Rayleigh-fading channels, there exists an interesting relationship between resistive circuits and time and path diversity mechanisms in wireless ad hoc networks. A resistor-like network element, the erristor, representing the normalized noise-to-signal ratio, is introduced. Given an end-to-end packet delivery probability, the logarithmic mapping from link reception probabilities to erristor values greatly simplifies the problems of power allocation and the selection of time and path diversity schemes, which is illustrated in a number of examples. We focus on transmission strategies with selection combining and simple noncoherent "decode-and-forward" strategies, which is motivated by their practicality. Thanks to its conceptual simplicity, the formalism that is developed provides valuable insight into the benefits of diversity mechanisms.     

The application of complex quantized feedback in integrated wireless receivers

Integrated wireless receiver architectures, such as direct-conversion receivers, offer many advantages over the conventional heterodyne receivers including smaller size, lower cost, and reduced power consumption. However, the design of monolithic receivers, using direct-conversion, involves many challenges including dealing with low-frequency disturbances, namely, dc-offset and 1/f noise (especially in CMOS implementations), in-phase (I) and quadrature (Q) amplitude and phase mismatch, local oscillator (LO) leakage, and even-order distortions. A cost-effective method to minimize the low-frequency disturbances is to use ac-coupling in the baseband signal path. However, it results in baseline wander effects, especially in spectrally efficient modulation schemes such as quadrature amplitude modulation (QAM) where the baseband signal spectrum contains a significant amount of energy near dc. A system solution to mitigate the effects of low-frequency disturbance is presented in this paper. The quantized feedback (QFB) technique is used in conjunction with ac-coupling to minimize the baseline wander effects. A cross-coupled (CC) QFB extension to compensate for the receiver local oscillator phase error as well as the IQ mismatch is also described. Simulation results are presented to demonstrate the effectiveness of this complex QFB technique.     

New transmit schemes and simplified receivers for MIMO wireless communication systems

In this paper, optimized transmit schemes for multiple-input multiple-output (MIMO) systems with simplified receivers are proposed for the downlink of high-speed wireless communication systems. In these systems, MIMO signal preprocessing is performed at the transmitter or base station with the receiver at the mobile station having a simplified structure that requires only limited signal processing. An important potential application for our transmit MIMO techniques is in the downlink of high-speed wireless communication systems with Vertical Bell Laboratories Layered Space-Time (V-BLAST) or a similar technique utilized in the uplink, creating a high-speed duplex system with a simplified mobile station transceiver structure. Two approaches are introduced and these depend on whether or not receive diversity is employed at the receiver. Both methods require that channel state information be available at the transmitter. In addition, some important associated issues such as peak-to-average power ratio requirements at the transmitter and robustness to downlink channel errors are also investigated and various solutions are proposed. Simulation results are provided and these show that performance improvement can be achieved when compared with other MIMO transmit schemes.     

Modeling of nondirected wireless infrared channels

We show that realistic multipath infrared channels can be characterized well by only two parameters: optical path loss and RMS delay spread. Functional models for the impulse response, based on infrared reflection properties, are proposed and analyzed. Using the ceiling-bounce functional model, we develop a computationally efficient method to predict the path loss and multipath power requirement of diffuse links based on the locations of the transmitter and receiver within a room. Use of our model is a simple, yet accurate, alternative to the use of an ensemble of measured channel responses in evaluating the impact of multipath distortion