Optical Wireless Systems Employing Adaptive Collaborative Transmitters in an Indoor Channel

We propose a novel optical wireless (OW) system based on a power adaptive multibeam spot-diffusing transmitter serving multiple seven-segment maximum ratio combining (MRC) angle diversity receivers. A feedback link is assumed between the transceivers so that each receiver conveys to the multibeam transmitter the new beams transmit power weights to be used to achieve the best signal quality at a given receiver location. Two cases involving three and five receivers are considered. Four different configurations for the placement of the three-receiver case in the room are also examined. The system's performance in each case is evaluated in terms of signal-to-noise ratio (SNR) and is compared with the single receiver scenario with and without power adaptation. In the presence of one receiver, the transmit spot powers can be adjusted for optimum performance at that receiver location. For multiple receivers, there is conflict, and we propose spot power adaptation based on the average requirements (power distribution in spots), i.e., transmit equal gain combining (EGC) of spot power or MRC of transmit spot powers. The results show that the three receivers benefit most from an adaptive transmitter when each is placed at a corner of the room. In this case, an SNR increase of as much as 2.6 dB is achieved for all three receivers at the corners by both MRC and EGC. Moreover, when all receivers are placed away from the line of diffusing spots, our proposed MRC collaborative approach is 1 dB better than the noncollaborative system. This gain reduces the difference from the upper bound set by the single receiver adaptation, which is 3 dB. For a mobile transmitter, MRC also significantly improved the SNR for the farthest receivers at the opposite end from the transmitter located near one room corner.      

Broadband Infrared Access with a Multi-Spot Diffusing Configuration: Performance

We evaluate transmission link performance for a Multi-Spot Diffusing Configuration (MSDC) for indoor wireless optical LANs. MSDC utilizes a multibeam transmitter and a composite receiver consisting of 7 narrow field-of-view (FOV) branches. Numerical evaluation is performed for two values of the receiver FOV corresponding to the cases when at least one or two diffusing spots are covered by a branch. Required optical power is used as a measure for MSDC link evaluation. The composite receiver provides angle diversity, which allows implementation of effective combining techniques. Selection Combining (SC) and Maximal Ratio Containing (MRC) methods have been analyzed. Our simulation results show that MSDC can reach much higher bit rates than a diffuse link can, without any channel equalization. MSDC link employing angle diversity receiver with larger FOV (each receiver branch capturing at least two diffusing spots) and using MRC method shows a promising performance for up to several hundreds of Mbps. System robustness against shadowing and blockage is also investigated. MSDC is more robust when an obstacle is located near the receiver, while this may cause severe problems in a diffuse link.     

Adaptive mobile spot diffusing angle diversity MC-CDMA optical wireless system in a real indoor environment

In this paper, we introduce a mobile optical wireless (OW) multicarrier - code division multiple access (MC-CDMA) system that employs a new adaptive line strip multibeam system (ALSMS) with diversity detection. Our results indicate that a significant improvement in the bit error rate (BER) can be obtained in the presence of very directive noise, multipath propagation and shadowing typical in a real indoor environment. With transmitter and/or receiver mobility, ALSMS can improve the BER performance by almost 10^-5 with 2 active users, and increase the signal-to-noise ratio (SNR) by more than 13 dB compared to the unadaptive LSMS.     

Line strip spot-diffusing transmitter configuration for optical wireless systems influenced by background noise and multipath dispersion

In this letter, we propose and evaluate a novel optical wireless configuration that employs a multibeam transmitter in conjunction with a narrow field-of-view direction diversity receiver. Such a configuration overcomes the drawbacks and combines the advantages of both types of optical wireless links, including line-of-sight and diffuse transmission. A multibeam transmitter placed on the communication floor was adopted to produce multiple diffusing spots on the middle of the ceiling in the form of a line strip. The design goal is to reduce the effect of intersymbol interference and to improve the signal-to-noise ratio (SNR) when the system operates under the constraints of background noise and multipath dispersion. Simulation results show that our line strip multibeam transmitter (LSMT) with only three branches diversity gives about 23 dB SNR improvement over the conventional system. The results also show that the multipath dispersion, which induces pulse spread, is significantly reduced when the LSMT with diversity detection is used.     

Multispot diffusing configuration for wireless infrared access

In order to combine the advantages and to overcome the drawbacks of a direct line-of-sight or a diffuse configuration for wireless infrared access, a multispot diffusing concept utilizing a holographic spot array generator is presented. Simulation results are presented and compared with those for a pure diffuse configuration in terms of link characteristics, when a single-element or a multibranch composite receiver is employed. The multispot transmitter ensures a more uniform signal power distribution. Improvements of about 2 dBo (optical decibels) can be achieved compared to a Lambertian pattern illumination. The increased power path loss at the edges of the communication cell is accompanied with a decrease in the delay spread resulting in an extension of the coverage range. Utilization of angle diversity detection improves the signal-to-noise ratio by more than 7 dB when selecting the best receiver branch and more than 10.5 dB in the case of maximal-ratio combining. Use of a multibeam transmitter and an angle diversity receiver reduces the likelihood of shadowing of the receiver due to an obstacle standing along the path between the receiver and the transmitter     

Multiple spot diffusing geometries for indoor optical wireless communication systems

In order to improve the performance of indoor optical wireless communication links, two multispot diffusing geometries based on diamond and line strip spot distribution geometries are proposed, analysed and compared to the known uniform spot distribution. Such geometries combine the advantages of the diffuse and the line‐of‐sight systems, giving great robustness and ease of use. The novel line strip multibeam transmitter geometry has resulted in a receiver signal‐to‐noise ratio (SNR) improvement of about 4 dB compared to the conventional diffuse system as well as a significant reduction in the pulse spread. Simulation and comparison results for both the conventional diffuse system and the three multispot diffusing geometries are presented. Further, pulse responses, SNR, and delay spread results at various locations are presented. Copyright © 2003 John Wiley & Sons, Ltd.     

Adaptive mobile line strip multibeam MC-CDMA optical wireless system employing imaging detection in a real indoor environment

Three methods (transmit power adaptation, imaging reception, and Multicarrier Code Division Multiple Access (MCCDMA)) are introduced to the optical wireless (OW) system and a significant improvement is achieved in the presence of very directive noise, multipath propagation, mobility, and shadowing typical in a real indoor environment. In the absence of shadowing, replacing a single non-imaging receiver by an imaging receiver with maximal ratio combining (MRC) improves the signal-tonoise ratio (SNR) by 20 dB in a conventional diffuse system (CDS) operating at 30 Mbit/s at a transmitter-receiver separation of 6m in agreement with previous results in the field. Further SNR improvement of 24 dB is achieved when a line strip multi-beam system (LSMS) replaces the CDS when both systems employ an imaging MRC receiver. Furthermore, our new adaptive LSMS (ALSMS) system coupled with the imaging MRC receiver offers an SNR improvement of 23 dB over the imaging MRC LSMS illustrating the gain achieved through adaptation. The results also show that combining transmit power adaptation with spotdiffusing (i.e. ALSMS) coupled with an imaging receiver based on select best (SB) increases the bandwidth from 46.5 MHz (nonimaging CDS) to 7.53 GHz thus enabling the OW system to achieve higher data rates and provide multi-user capabilities in our case by employing a MC-CDMA scheme. In a 10 user MC-CDMA OW system, a bit error rate (BER) improvement from 4.9 × 10^?1 to 2.1 × 10^?5 is achieved when the imaging MRC ALSMS system replaces the imaging CDS in a shadowed environment.     

Performance of Indoor Infrared Wireless CDMA Systems with Angle Diversity

Indoor infrared communication systems is one of the possible ways of offering data rates in excess of 100 Mbit/s without the need for wiring. Multiple users can share an infrared channel by code division-multiple access (CDMA) techniques. However, the CDMA system performance is limited by both background noise and co-channel interference. In this paper we study the use of angle diversity for mitigating the effects of the noise and interference. The system considered uses on-off shift keying modulation with multibeam transmitters and imaging receivers. The overall system performance for different diversity combining techniques is evaluated and compared to a system without diversity. Numerical results for a 2-user CDMA system indicate that signal to noise and interference ratio (SNIR) improvement (over systems with no diversity) of 5 dB is obtained for at least 50% of an ensemble of 10000 sample evaluations. The generalized selection combining (GSC)--a new diversity technique yet to be implemented for infrared systems--offers the best performance even with its reduced complexity.     

室内MIMO ACO-OFDM可见光通信系统接收机设计

设计了一种在室内可见光MIMO通信系统(MIMO-VLC)中使用具有两个不同视场角(FOV)的光电二极管(PD)的角度分集光接收机(2FOV-ADR),其兼具两个不同视场角的接收机(2-FOV)和传统角度分集接收机(ADR)的优点,实现了更优的接收性能。对将LED灯用作数据发射器的典型室内可见光通信场景进行仿真,结果表明,2FOV-ADR均衡器输出端的最小信噪比(minSNR)要高于2-FOV接收机和传统ADR,实现了室内97%的位置的minSNR在45 dB以上,相比于前两种接收机,这一比例分别提高了96%和32%。最后,对使用非对称限幅光正交频分复用(ACO-OFDM)作为调制方案的系统,计算总误码率(BER),给出了迫零和最小均方误差均衡器的结果。结果表明,对于所考虑的室内位置,2FOV-ADR都具有最低的误码率。     

Polarization-insensitive frequency-shift-keying optical heterodynereceiver using discriminator demodulation

A polarization-state-independent binary frequency-shift-keying optical heterodyne receiver, achieved by splitting the received signal between two orthogonal polarization axes and combining the signals after demodulation, is described. The authors consider various receiver configurations, using a discriminator with a limiter for applications in which received-signal-envelope fluctuations cannot be ignored, and without a limiter when the fluctuations are negligible. For applications that require the limiter, a diversity method or a variable-gain limiter that improves the performance of the limiter/discriminator receiver is proposed. Numerical results indicate that with diversity and a limiter/discriminator structure, polarization independence can be achieved with a system performance which is nearly that of an ideal receiver. Using a discriminator without a limiter, when there is no source of envelope fluctuations other than those due to Gaussian noise, the receiver performance degradation compared to the ideal baseline receiver is 0.5 dB     

Performance evaluation of a triangular pyramidal fly-eye diversity detector for optical wireless communications

Background noise and multipath propagation are the major impairments in indoor optical wireless links. They can introduce heavy distortion in the received optical signal and can degrade the system performance. An investigation into the optical wireless system performance has been carried out for two configurations: a hybrid system, and a diffuse system with a single detector and a triangular pyramidal fly-eye diversity receiver (PFDR). Original results for both systems that employ a PFDR antenna, under different fields of view (FOVs), are presented. The design goal is to reduce the effect of signal spread and improve the signal-to-noise ratio when the system operates under the constraints of background noise and multipath dispersion. It is demonstrated that through PFDR FOV optimization the directional background interference can be reduced and the received pulse shape improved.     

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.     

Design and Experimental Results for a Direct-Sequence Spread-Spectrum Radio Using Differential Phase-Shift Keying Modulation for Indoor, Wireless Communications

We report on our design and measurements that have been made for a direct-sequence spread-spectrum radio using differential phase-shift keying modulation for a wireless PBX. We describe the design and implementation of a transmitter and a receiver using a surface acoustic wave (SAW) filter matching the spread-spectrum code of a user. The receiver performance is within 1 dB of the theoretical performance of a differential phase-shift keying (DPSK) receiver in the presence of additive white Gaussian noise. We also show receiver performance in a multipath fading indoor environment with multipath fade notches of up to 50 dB depth. The indoor channel multipath fading can be overcome by using an equal gain diversity combiner which is suitable when DPSK modulation is used. We confirm that the indoor mean power level attenuation follows the inverse fourth power of the distance. Also, we investigate the multiple-access capability of the system by introducing an interfering transmitter with a different spread-spectrum code sequence.     

Lightpath routing and spectrum allocation over elastic optical networks in content provisioning with cloud migration

In this paper, an improved receiver based on diversity combining is proposed to improve the bit error rate (BER) performance of layered asymmetrically clipped optical fast orthogonal frequency division multiplexing (ACO-FOFDM) for intensity-modulated and direct-detected (IM/DD) optical transmission systems. Layered ACO-FOFDM can compensate the weakness of traditional ACO-FOFDM in low spectral efficiency, the utilization of discrete cosine transform in FOFDM system instead of fast Fourier transform in OFDM system can reduce the computational complexity without any influence on BER performance. The BER performances of layered ACO-FOFDM system with improved receiver based on diversity combining and DC-offset FOFDM (DCO-FOFDM) system with optimal DC-bias are compared at the same spectral efficiency. Simulation results show that under different optical bit energy to noise power ratios, layered ACO-FOFDM system with improved receiver has 2.86, 5.26 and 5.72 dB BER performance advantages at forward error correction limit over DCO-FOFDM system when the spectral efficiencies are 1, 2 and 3 bits/s/Hz, respectively. Layered ACO-FOFDM system with improved receiver based on diversity combining is suitable for application in the adaptive IM/DD systems with zero DC-bias.     

Analysis of LDPC with optimum combining

Low density parity check (LDPC) codes have shown exceptionally good performance for single antenna systems over a wide class of channels. LDPC when implemented with a single input multiple output system with maximum ratio combining is optimum from the standpoint of maximising signal-to-noise ratio at combiner output without the presence of interferer. Optimum combining outperforms maximal ratio combining (MRC) in the presence of interferer(s). In this article, the performance of the LDPC codes with multiple receiver antennas with optimum combining in the presence of single interferer is investigated. The simulation results showed that LDPC codes of irregular construction are able to give high coding and diversity gain with optimum combining. The proposed LDPC optimum combined (LDPC–OC) system in Rayleigh fading channel in the presence of a single interferer improves the signal to interferer plus noise ratio by 2.62 dB with four receiver antennas and by 1.98 dB when the number of receiver antennas is three.     

Channel estimation for OFDM systems with transmitter diversity inmobile wireless channels

Transmitter diversity is an effective technique to improve wireless communication performance. In this paper, we investigate transmitter diversity using space-time coding for orthogonal frequency division multiplexing (OFDM) systems in high-speed wireless data applications. We develop channel parameter estimation approaches, which are crucial for the decoding of the space-time codes, and we derive the MSE bounds of the estimators. The overall receiver performance using such a transmitter diversity scheme is demonstrated by extensive computer simulations. For an OFDM system with two transmitter antennas and two receiver antennas with transmission efficiency as high as 1.475 bits/s/Hz, the required signal-to-noise ratio is only about 7 dB for a 1% bit error rate and 9 dB for a 10% word error rate assuming channels with two-ray, typical urban, and hilly terrain delay profiles, and a 40-Hz Doppler frequency. In summary, with the proposed channel estimator, combining OPDM with transmitter diversity using space-time coding is a promising technique for highly efficient data transmission over mobile wireless channels     

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     

High-speed power-efficient indoor wireless infrared communication using code combining .II

For pt.I see ibid., vol.50, no.7, p.1098-1109 (2002). We examine an infrared link composed of a multibeam transmitter and a direction-diversity receiver, employing code combining. The latter represents an added dimension to the conventional diversity concepts, which are limited to combining the individual received symbols. Rate-compatible punctured convolutional codes are used to encode intensity-modulated on-off keying (OOK) optical power, to create an adaptive environment for efficient utilization of channel spectral bandwidth, to provide a means for accurate channel estimation, and to maintain a guaranteed bit error rate (BER) performance at all receiver positions. It is shown that a BER not exceeding 10/sup -9/ with 99% probability can be achieved at bit rates up to a few hundreds of megabits per second, at very low transmitted power levels.     

Analysis of infrared wireless links employing multibeamtransmitters and imaging diversity receivers

We analyze the improvements obtained in wireless infrared (IR) communication links when one replaces traditional single-element receivers by imaging receivers and diffuse transmitters by multibeam (quasi-diffuse) transmitters. This paper addresses both line-of-sight (LOS) and nonline-of-sight (non-LOS) IR links. We quantify link performance in terms of the transmitter power required to achieve a bit error rate (BER) not exceeding 10^-9 with 95% probability. Our results indicate that in LOS links, imaging receivers can reduce the required transmitter power by up to 13 dB compared to single-element receivers. In non-LOS links, imaging receivers and multibeam transmitters can reduce the required transmitter power by more than 20 dB. Furthermore we discuss the use of multibeam transmitters and imaging receivers to implement space-division multiple access (SDMA). In a representative example with two users transmitting at a power sufficient to achieve a BER not exceeding 10^-9 with 95% probability in the absence of cochannel interference, when SDMA is employed, the system can achieve a BER not exceeding 10^-9 with a probability of about 88%