Performance Analysis of Indoor Infrared Wireless Systems Using OOK CDMA on Diffuse Channels

6a6 h(t; a) = u(dent of received signal [4]. At the receiver, we assume a receiver lter g(t) to be matched only to the transmitted pulse waveform. The correlation between the received signal and c(l) of the desired user is computed. The OOK decoder uses a simple slicer.
c(l)
Bits in Transmitter Pulse LD
X(t)
Other User’s Interference Multipath channel
Bits out Receive Filter
h(t)
Y(t) c(l)
systems use intensity modulation and direct detection(IM/DD). In IM/DD X (t) is an instantaneous optical power. The received photocurrent Y (t) is the integral of the received optical power over the detector surface, multiplied by the photodetector responsivity. Our channel model for IM/DD can be summarized by,
2 System Model
2.1 Di use channel
Multipath distortion is characterized by a linear, time-invariant channel impulse response. We adopt the ceiling-bounce functional model
i k;j d k;j i k;j d k;j
2.2 OOK CDMA
In OOK CDMA, each frame is divided into F chips where F is length T . This paper employs optical orthogonal codes (OOC) with maximum values of auto-correlation and cross-correlation, a, c, bounded by one as the signature codes sequence [3]. We assume each user to be assigned an optical code sequence c(l) of OOC of length F and weight K . Here l is the number of chip shifts. For OOC with length F , weight K , a = c = 1, the maximum number of code sequences is given by
Dept. of Information and Computer Science, Keio University 3-14-1 Hiyoshi, Kohoku, Yokohama, 223-8522 Japan Dept. of Electrical Engineering Science University of Tokyo 2641 Yamazaki, Noda, 278-8510 Japan
Y (t) = rX (t)
h(t) + n(t):
Photo Detecter
n(t)
(4)
Fig. 1: A simpli ed block diagram of the OOK CDMA to obtain the channel impulse response h(t) in evaluating the e ect of multipath distortion on ISI [4]. The the channel impulse response in the ceiling-bounce functional model is given by 11 D (h(t; a)) 13 (1) where u(t) is the unit-step function and D(h(t; a)) is the delay spread. The channel dc gain of di use channel is given by Z1 G0 = h(t; a)dt: (2) 01
3 Performance Analysis
We assume the received power of any interfering user to be equal to that of the desired user. We analyze the BER performance with considering the e ect of ISI and multiple access interference (MAI) in the chip synchronous case. Let us denote the amount of ISI when the interference or the desired pulse at the k th chip causes the interference to the j th chip by p ( ) or p ( ) , respectively. p ( ) and p ( ) are given by
Performance Analysis of Indoor Infrared Wireless Systems Using OOK CDMA on Di use Channels
yRyoko y z
Matsuo,
yMichihito Matsuo, z3Tomoaki Ohtsuki, 3Tomoyuki Udagawa and y3Iwao Sasase
c
pi(k;j ) = pd(k;j ) = hj 0k
k
(k j )
(5)
where h is an equivalent discrete-time impulse response of the system given by
3
TAO Shin-Kawasaki Research Center
11th Floor Shin-Kawasaki Mitsui Build.890-12 Kashimada,Saiwai-ku,Kawasaki,211-0958 Japan
Abstract{ We analyze the bit error rate (BER) performance of indoor infrared wireless systems using on-o keying (OOK) code division multiple access (CDMA) on di use channels by taking the e ects of intersymbol interference (ISI) over several chips into account. We evaluate how much the transmitted power should be increased on di use channels to compensate the performance degradation due to the ISI.
1 Introduction
The optical wireless LAN can typically use line-of-sight (LOS) or di use channels. The major advantage of using di use channels is the robustness against shadowing and the misalignment between a transmitter and a receiver. However, di use channels are subject to multipath dispersion that causes intersymbol interference (ISI) [1]. As a multiple access technique in infrared wireless systems, time-division multiple access (TDMA) is not desirable, because it is a synchronous technique where a common frame timing must be provided and users cannot communicate independently and simultaneously.
Wavelength-division multiple access (WDMA) requires each user to have an independent wavelength together with an optical tunable reception lter. Such a complex structure is expensive and generally not desirable for infrared wireless communication. Code division multiple access (CDMA) can realize random access using the same wavelength [2]. Infrared wireless system using OOK CDMA has been proposed,and performance is analyzed on LOS channels and di use channels when the e ect of ISI is limited [5]. However, the e ect of ISI a ecting beyond a chip is not considered. In this paper, we analyze the performance of indoor infrared wireless systems using OOK CDMA on di use channels by taking the e ects of ISI over several chips into account. We evaluate how much the transmitted power should be increased on di use channels to compensate the performance degradation due to the ISI.