Communication Signal Processing Group, Department of Electrical & Electronics, ... wireless communication possesses two main attractive advantages over its.
Optical Wireless Communication Front-Ends M.F.L.Abdullah, Roger Green, Mark Leeson Communication Signal Processing Group, Department of Electrical & Electronics, School Engineering, University Warwick Coventry, CV4 7AL, United Kingdom Tel: +4424765 22333, +442476523133, +4476523908 e-mail: M.F.L.Abdullah, RogerGreen, M.S.LeesonOwatwick.ac.uk Abstract: Infrared wireless communication possesses two main attractive advantages over its radio frequency counterpart, namely the abundance of unregulated spectrum in 700nm 1500nm region and the ease with which the IR radiation can be confined. Integrating
microwave electronics and optics, it is possible to provide wideband communication services but it is well known that the signal level in an optical wireless receiver is weakest at the front end. Therefore, this paper identifies the technical obstacles and limitations in indoor infrared optical wireless front-ends, in addition to techniques for mitigating these effects, showing that infrared is a viable alternative to radio for certain applications. 1. INTRODUCTION
Today, wireless infrared transmission, or optical wireless, has entered homes, offices, industry and health care, with applications in the field of remote control, telemetry and local communication. One of the prime motivators for considering the use of an optical carrier in the wireless context is the demand for greater transmission bandwidths. This is due the fact that radio frequency spectrum is already exceedingly congested and frequency allocations of sufficient bandwidths are extremely hard to obtain [l]. As a medium for short-range wireless communication, infrared radiation has several advantages over radio. The primary advantage is an abundance of unregulated bandwidth, with a range of more than I30THz. In addition, being similar in wavelength, infrared light shares many of the features of visible light; in particular, infrared radiation does not pass through walls or other opaque barriers, so that an infrared signal is confined to the room in which it originates. More importantly, it allows neighboring rooms to use independent infrared links without interference. Furthermore, infrared links using intensity modulation and direct detection receivers do not suffer from multipath fading [2]. Infrared does have some drawbacks as well, offering a limited range because the noise from ambient light is high (Fig. 1). Also, the square-law nature of a direct-detection receiver doubles the effective path loss in dB when compared to a linear detector. Moreover, strict power limitations due to eye and skin safety considerations restrict the transmitter output power. Infrared is also susceptible to blocking either from objects or personnel resulting in loss of the communication link. These differences between radio and infrared are summarized in Table 1.
0-7803-8246-1/04/$20.00 02004 IEEE.
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Propem Multipath Fading Multipath Dispersion Source of Bandwidth Limitation Source of Dominant Noise Security Range Input X(t) Represents Path Loss
I
Infrared No Yes
Radio Yes Yes
High Photodiode capacitance, Multipath dispersion Ambient background light
Regulatory
High Low Power High
I
Interference from other users Low High Amplitude High
2. DESIGN CHALLENGE In designing a front end receiver amplifier, two design choices have to be made. One of these is concerned with the amplifier structure and the other is the circuit realization of the structure. One of the main noise mechanisms in most preamplifiers employing large area detectors is the f' noise due to the low pass filter formed by the detector capacitance associated with the input impedance of the amplifier [3]. Therefore, to achieve a wide bandwidth design, techniques to reduce the effective detector capacitance are required, and this will be discussed in Section 3.
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It is well known that the signal level in an optical wireless receiver is weakest at the front end. This is where the system signal-to noise ratio is determined and system performance level established. In order to achieve an acceptable level of received optical power set by the minimum signal-to-noise, it is then convenient to use large collection area detectors to maximize the amount of energy received and improve the link budget as shown in Fig 2. One way to achieve this is to use an optical concentrator on the top of the photodetector but unfortunately, with this approach the size of the photodetector used will be larger than that used for an optical fiber receiver.
Fig Z : Photodetector surface area [Z]and photodiode equivalent circuit diagram [4] Another major factor associated with photodetector application is the capacitance that limits response time, since a larger photodetector has a higher capacitance. The high capacitance of such devices will reduce the bandwidth of the receiver. Furthermore, there is the problem of increasing the load resistance, which will lead to an increase in thermal noise, which limits the signal-to-noise ratio (SNR), or decreasing the load resistance to achieve higher bandwidth.
3. FRONT END ENHANCEMENT TECHNIQUE DESIGN Preamplifier design for wireless infrared links and preamplifier design for fiber-optic links have a number of similarities. There are two fundamental differences, however, which deserve special attention. First, a wireless infrared receiver must contend with an enormous amount of background radiation from sun, incandescent and fluorescent lights, as shown in Fig 1. No such background light exists within the confines of an optical fiber. Second, unlike the highly concentrated beam of light that emanates from an optical fiber, the received optical signal in a non-directed wireless system will be spread over a wide region, so a large-area photodetector will be required to collect sufficient signal energy [ 5 ] . There are two major designs that are popular for the front end preamplifier: i) ii)
integrating amplifier trans impedance amplifier
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A much improved version of a front-end, incorporated within a transimpedance amplifier is shown in Fig 3(a), the topology being known as the bootstrapped transimpedance amplifier (BTA). The BTA is an attractive design as it reduces the effective detector capacitance, c d , seen by the signal [ 6 ] . The output of the emitter follower stage is feedback to the photodetector by a bootstrapping capacitor, C3. Fig 3(b) shows a modification of Fig 3(a), by placing a capacitor, Cq in series with the emitter resistor in the second gain stage, with a feedback resistor &. By varying the capacitor, the bandwidth of the circuit can be controlled. The output results of Fig 3(a) and Fig 3(b) are shown in Fig 4(a) and Fig 4(b). In each case the amplifier output is taken from the collector at Q3.
w.
Fig 3 : (a) BTA Circuit [6]
Fig 4: (a) BTA Bandwidth
kar
@) Modified BTA circuit
(b) Modified BTA Bandwidth
Assuming that the gain stages and the emitter follower can be approximated by a simplified amplifier model, R,, R, >> R, and we consider frequencies where CI and C, are short circuits, the transimpedance gain, A, for the circuit is approximated by equation (1):
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A
v,
= _=
‘ I,
+ Z,R,) - j d 4 Z f 2 R l l 2 Z f ( l + A , ) - m 2 C , Z f 2 R , C d + jmZ,Rl(Cd + C , ) + j d d Z ,
(1)
where A, is the voltage gain of the amplifiers stages and Zr is Rg // C2 in Fig 3(b) Equation (1) shows that the receiver bandwidth is determined not only by the &Cd time constant but by a complex function of R7, C2 and C4. The modified circuit shows that the bandwidth can be vaned by varying capacitor C4, thus modifying the second stage gain.
4. PREAMPLIFIER OPTIMIZATION TECHNIQUES
Since the receiver is the critical part of an optical wireless receiver front end, it often dictates the overall system performance. An optimally configured receiver has a wide dynamic range, high sensitivity and bandwidth enhancement. The modified BTA discussed in section 3, has the function of bandwidth enhancement ability only. Furthermore the circuit requires a highly sensitive variable capacitor to maintain the bandwidth adjustment. Fig 5 shows a block diagram of the improved version of an optimal configuration receiver.
ePHOTODIODE
Ri
A
VOLTAGE CONTROLLED FILTER (VCF) PREAMPLIFIER
b
AUTOMATIC GAIN CONTROL (AGC) Fig 5: Front-End - An Optical Wireless Receiver
The VCF acts as a variable resistor whose resistance value is controlled by the applied signal voltage. It controls the required bandwidth enhancement when in series with a capacitor. In this case the capacitor is valued at IOpF. The AGC adjusts the gain required depending on the applied signal. It consists of a single JFET, in series with a capacitor, which act as a “shunt” between the AGC circuit and the preamplifier. The preamplifier consists of a current mirror circuit in conjunction with an emitter follower. Fig 6 shows the Gain (dB) versus Frequency plot for the configuration of Fig 5 . The plot shows that the bandwidth adjustment for the above arrangement displays a ratio of some 1O:l over the frequency range lMHz to 20MHz. A gain-bandwidth product (G = Ao, = AjdBx f,dB), is plotted to verify the frequency range, as shown in Fig 7, since this is the basic performance parameter of the model amplifier.
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Fig 6: Gain (dB) versus Frequency Plot
GBP versus Bandwidth 5000
6 4000
-8
3000
(3
1000
7 1
2000
-
0 2.17 3.09 28.9 42.2 47.1 51.5 50.3 51.5 57.5 51.5
Frequency (MHz)
Fig 7: Gain-Bandwidth Product graph for circuit Fig 5.
5. CONCLUSION
This paper has presented a new horizon of the main issues associated with the physical layer of a wireless infrared communication system, in particular the front end. It has highlighted the significant problems of high ambient light levels and some major problems that could affect the effectiveness of better bandwidth. It has also presented two methods and results of amplifier configurations for bandwidth adjustment. Design is as much an art as it is a science, and ultimately the question of what design technique is best rests with the designer. 6. REFERENCE
[ I ] A.M.Street, P.N.Stavrinou, DCO’Brien and D.J.Edwards, “ Indoor optical wireless systems -a review” Optical and Quantum Electronics 29 (1997), pp : 349-378 [2] John R.Bany, “Wireless Infrared Communications” Kluwer Academic Publishers, 1994 [3] R.Ramirez Iniguez, S.M.Idrus and R.J.Green, “Receiver amplifiers for optical wireless communication systems” PREP 2001, 2001 [4] Gerame J, “Photodiode Amplifiers - Op Amp Solutions”McGraw Hill, 1995 [5] Stephen B.Alexander, “Optical Communication Receiver Design” IEE Telecommunications Series, Vol 37 and SPIE Tutorial Texts in Optical Engineering, Vol TT22, 1997 [6] P.P. Smyth, M.McCullagh, D.Wisely, D.Wood, S.Ritchie, P.Eardley, S . Cassidy, “Optical Wireless Local Area Networks - Enabling Technologies”, BT Tech. J., 11, 2, 1993,pp 56-64.
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