The performance of code-division multiple access (CDMA) as a multiple-access technique for optical wireless communications is evaluated. The information data is modulated through dual-amplitude pulse interval modulation (DAPIM). Optical orthogonal code is used as the signature sequence. The upper bound and lower bound on the exact probability of errors per slot and per packet are derived in the presence of ambient background noise. We investigate the performance characteristics for various system parameters, including the SNR of the source information bits, receiver threshold, DAPIM order, and number of users.
For optical wireless networking, it is desirable to employ nondirected links whose performance depends on the reflection characteristics of the indoor surfaces. The non-Lambert reflection pattern, Phong model, is considered in the calculation of the multipath impulse response function as well as Lambert reflection pattern. The bit error rate (BER) of the optical wireless direct-sequence spread spectrum (DSSS) system using biorthogonal Walsh codes is investigated on a non-Lambert reflection channel. Simulation results demonstrate that biorthogonal DSSS systems can combat multipath dispersion with small power penalties very well. And by comparison, we show how much the error of the performance prediction for the biorthogonal DSSS system will occur when using Lamertian approximation on the non-Lambert channel.
In this paper, we present an infrared wireless indoor communication system that bases on Ethernet network. The bit rate of Ethernet is 10Mbps, but after Manchester coding, in the physical layer the actual bit rate is 20Mbps. In our designs, the transmitter uses laser diodes (LDs). The transmitter consists of differential input circuit, LD driver circuit. The receiver consists of a coated truncated spherical concentrator whose field of view (FOV) is 40 degree, a large area Si PIN photo-detector followed by transimpedance amplifier, second-stage amplifier, low-pass filter (LPF), high-pass filter (HPF), limiting amplifier and differential output circuit. The network is constructed as a base-terminals configuration and two transit wavelengths are used for base and terminals respectively to avoid collision. Experimental testing was conducted in a room with size 5m × 5m × 3m and the network could work well.
In the wireless infrared communications it is necessary to place a non-imaging optic concentrator in front of the planar PIN photo-detector in order to achieve more optic energy that can result in greater electrical SNR ratio. Incidence rays, which have different directions relative to the axis of concentrator, will produce different optic gain, and then it changes the channel impulse response compared with the one which is made in a simple planar PIN detector system. In addition, the field of view (FOV) will dramatically affect the system performance. Decreasing the FOV of concentrator will reduce the multipath-induced intersymbol interference (ISI), but at the same time it also reduces the total optic energy that is received. This paper demonstrates the effect of concentrator on impulse response of IR wireless indoor channel and compares the differences of the impulse responses of diffuse channel, basing on whether a hemisphere concentrator or a truncated spherical concentrator has been used. The paper also presents the relationships between FOV and system bandwidth and between FOV and average optical energy. A conclusion of optimal result is drawn.
The performance of high-speed non-directed diffuse indoor infrared wireless communication is mainly affected by intersymbol interference (ISI) due to multipath dispersion. The direct-sequence spread spectrum systems are applied to combat multipath dispersion. However, when the data rate is close to the channel 3dB bandwith, the spread spectrum technique may have a worse performance than OOK transmission as it is affected by more severe ISI. Exploiting muli-spot link, we can improve the performance of spread spectrum technique to be much better than OOK modulation because a higher channel bandwith are obtained. Both the analytical and simulation results are presented.
The free-space optical (FSO) communication technology has the advantages of broadband, high-security, immunity to radio, low cost and license-free, etc. It has become a good solution for the "last-mile" access networks. In this paper, We give an outline for designs of the transmitter and receiver circuits, as well as the optical system. The technical parameters calculations of a particular FSO system are also presented.
This paper investigates the equalized performance of digital pulse interval modulation (DPIM) in the presence of multipath propagation and additive white Gaussian noise. The results for slot error rate (SER) versus electronics SNR with and without equalizer are presented and made by Monte Carlo simulation with a simplified model of the generated received signal, which depends on the impulse response of the channel. Simulation results show that the performance of DPIM system can be improved dramatically by using an equalizer for a severe intersymbol interference (ISI) channel.
The impulse response of the wireless infrared communication channel determines performances of communication system, such as multipath penalty over the maximum baud rate and hidden-station situations. It is significant to correctly describe the impulse response of the working channel for design and performance evaluation of communication systems.
In this paper, details of a simulation package that obtains impulse responses for arbitrary wireless infrared indoor channels are presented. Capability of estimating the effects of shapes of room or objects, surface reflection and transmission characteristics on impulse responses is the most important feature of this package. The common simulation models just consider empty cubic rectangular room, with surfaces assumed to be Lambertian reflection pattern. With several examples given in this paper, the conclusions can be drawn that the common simulations models are not adequate to describe the real complicated communication environment and not suitable to the requirement of accurate performance prediction for the communication systems. The results give a demonstration that the use of the common simulation models to approximate the real wireless channels can introduce several decibels in the evaluation of the path loss.
This paper introduces a new modulation scheme, dual-amplitude pulse interval modulation (DAPIM) for optical wireless communications. We present the expression for bandwidth requirement, slot and packet error rates, and optical power requirements in a nondispersive channel. We find that DAPIM achieves improved bandwidth and/or power efficiency compared with OOK, PPM, and PIM. DAPIM with 4 bits and 5 bits per symbol is attractive in terms of optical power and bandwidth requirements.
As infrared transmission has the advantages of wide bandwidth, immunity to radio interference, high security and license-free, it is an attractive alternative to radio for wireless local area networks LAN. The design of an interface between the high-speed optical wireless transceiver (>20Mbps) and the optical wireless LAN is presented in this paper. We transplant the IEEE 802.11 medium access control (MAC) layer protocol to our system, define and implement a new physical (PHY) layer management module. This management module can also provide us an experimental platform to analyze the performance of the modulation schemes, such as L-PPM, PIM, DH-PIM, etc. Due to its characteristics of flexibility, simplicity and low cost, the management module is a good choice for the indoor optical wireless LANs.