Average Power Reduction Techniques for Multiple-Subcarrier Intensity-Modulated Optical Signals Roy You and Joseph M

Home , Intensity modulation

2164 IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 49, NO. 12, DECEMBER 2001 Average Power Reduction Techniques for Multiple-Subcarrier Intensity-Modulated Optical Signals Roy You and Joseph M. Kahn, Fellow, IEEE

Abstract—We describe two classes of simple, effective tech- carrier [6], [7]. The MSM electrical signal may be modulated niques for reducing the average optical power requirement in onto the optical carrier using intensity, frequency, or phase intensity-modulated optical systems using multiple BPSK or modulation. Most current practical MSM systems use intensity QPSK subcarriers. The first class of techniques involves block coding between the information bits to be transmitted and the modulation (IM) with direct detection (DD), owing to its simple symbol amplitudes modulated onto the subcarriers in order to implementation. MSM permits asynchronous multiplexing of increase the minimum value of the multiple-subcarrier electrical numerous, possibly heterogeneous information streams, and waveform. The second class of techniques involves replacing the permits a receiver to demodulate only the streams of interest. fixed dc bias by a bias signal that varies on a symbol-by-symbol For this reason, MSM with IM/DD is widely used in optical basis. These two classes of techniques can be applied separately or in tandem. The reduction in power requirement increases fiber distribution of video signals [6]. with the number of subcarriers and, with eight subcarriers, can The main drawback of MSM with IM/DD is poor optical av- be as high as about 3.6 dB and 3.2 dB with BPSK and QPSK, erage power efficiency. This arises because the MSM electrical respectively. The techniques described here are applicable as long signal is a sum of modulated sinusoids, and thus takes on both as all subcarriers originate from a single transmitter and are negative and positive values. Optical intensity (instantaneous symbol-synchronized. power) must be nonnegative. Hence, a dc bias must be added to Index Terms—Intensity modulation, optical communication, the MSM electrical signal in order to modulate it onto the inten- subcarrier multiplexing. sity of an optical carrier. As the number of subcarriers increases, the minimum value of the MSM electrical signal decreases (be- I. INTRODUCTION comes more negative), and the required dc bias increases. Since the average optical power is proportional to this dc bias, the op- N ELECTRICAL and RF communication systems, mul- tical average-power efficiency worsens as the number of sub- ticarrier modulation (MCM), in which multiple digital I carriers increases [6], [7]. It is worth noting that the peak-to-av- streams are modulated onto carriers at different frequencies, erage optical power ratio of a MSM IM signal is about two, in- permits transmission with minimal intersymbol interference dependent of the number of subcarriers. (ISI) on frequency-selective channels [1]. Electrical MCM sys- Optical fiber MSM IM/DD systems typically use tens to hun- tems frequently use tens to hundreds of carriers. On electrical dreds of subcarriers [6]. In an effort to reduce the optical average channels, the average-power requirement of a MCM system is power requirement, often the bias is reduced to the point that approximately the same as a single-carrier system, assuming some clipping occurs. Techniques intended to reduce the im- ISI is not present. Electrical MCM systems employing a large pact of clipping noise in optical fiber MSM systems have been number of carriers have the drawback of a high peak-to-average investigated [8], [9]. power ratio (PAR), which can lead to nonlinear distortion Optical wireless (OW) transmission with IM/DD is an attrac- and clipping in systems with peak-power limitations. Several tive option for high-speed indoor and outdoor links [7], [10], techniques for reducing the PAR in electrical MCM systems [11]. MSM with has been considered for use in IM/DD OW have been proposed [2]–[5]. systems, because the use of several narrow-band subcarriers On optical communication channels, multiple-subcarrier promises to minimize ISI on multipath channels [7], [12], and modulation (MSM) involves modulation of multiple digital because MSM can provide immunity to fluorescent-light noise and/or analog information sources onto different electrical near dc [13]. In most applications of OW, particularly those subcarriers, which are then modulated onto a single optical using portable transmitters, eye safety and power consumption limit the available average optical power. This dictates that in Paper approved for publication by O. K. Tonguz, the Editor for Optical Trans- MSM systems, the number of subcarriers should be kept small, mission Systems of the IEEE Communications Society. Manuscript received e.g., under ten. October 15, 1999; revised June 15, 2000. This work was supported by the Na- In this paper, we discuss two classes of techniques for tional Science Foundation under Grant ECS-9710065 and by the Sharp Corpo- ration. This work was presented in part at IEEE International Conference on reducing the optical average power requirement in IM/DD Communications, New Orleans, LA, June 18–22, 2000. MSM systems using BPSK or QPSK digital modulation. The The authors are with the Department of Electrical Engineering and Com- first class of techniques involves block coding between the puter Sciences, University of California, Berkeley, CA 94720 USA (e-mail: [email protected]). information bits to be transmitted and the symbol amplitudes Publisher Item Identifier S 0090-6778(01)10624-0. modulated onto subcarriers, in order to increase the minimum

value of the MSM electrical waveform, and thus decrease the III. POWER-EFFICIENT MULTIPLE-SUBCARRIER bias that must be added to it. These techniques are somewhat TRANSMISSION SCHEME analogous to those used for PAR reduction in MCM electrical systems [2]–[5]. The second class of techniques involves Fig. 1(a) and (b) depict the transmitter and receiver replacing the fixed dc bias by a bias signal that varies on a design used in the proposed MSM transmission scheme symbol-by-symbol basis. During each symbol, the minimum with QPSK. The transmitter and receiver are identical for bias required to achieve nonnegativity is utilized. The use of a BPSK, except that all sine branches are omitted. Refer- symbol-by-symbol bias for a single quadrature amplitude-mod- ring to Fig. 1(a), the transmitter uses a set of subcarrier ulated subcarrier has been considered in [14]. The two classes frequencies , . During each symbol in- of techniques proposed here can be applied separately or in terval, of duration , it transmits a vector of information tandem. They are effective and easy to implement, but do bits , , where require that all subcarriers originate from a single transmitter . A block coder maps to a corresponding vector and that all subcarriers be synchronized at the symbol level. of symbol amplitudes, . For QPSK, each such vector is of When the number of subcarriers is small, as in OW applica- the form tions, the computations required to implement the proposed techniques can be performed off-line and implemented via table (4) lookup. By contrast, in MCM electrical systems using a large number of carriers, PAR reduction techniques often require that where , , . computations be performed on a real-time basis; see, e.g., [4]. Similarly, for BPSK The remainder of this paper is organized as follows. The IM/DD channel model is described in Section II. Our techniques for power-efficient MSM transmission are presented in (5) Section III, and their performance is evaluated and compared to existing techniques in Section IV. Concluding remarks are where , . Defining a pulse shape given in Section V. , the MSM electrical signal for QPSK is

II. IM/DD OPTICAL CHANNEL MODEL

Indoor OW channels with IM/DD can be described by [7] (6) The MSM electrical signal for BPSK is identical to (6), except (1) that the sine terms are omitted. Since can be positive or negative, the transmitter adds a where the symbol represents convolution. The channel input baseband bias signal is the transmitted optical intensity, which must be nonnegative (7)

(2) In general, is the sum of a constant and a baseband The average optical power is given by the mean value of pulse-amplitude modulation (PAM) signal corresponding to the as second term in (7). In this baseband PAM signal, the amplitude of symbol depends, in general, both on , the vector of (3) subcarrier amplitudes for symbol , and on , the vectors in past and future symbols. The bias is chosen so in contrast to electrical channels, where the average power is that , where is a nonnegative scale the mean value of . The channel output is the re- factor. The average optical power is ceived photocurrent. Here, is the photodetector responsivity. The channel is a fixed, linear system having impulse response (8) and frequency response . In this study, we will ne- glect multipath distortion, so that , where With the proper choice of , , and , is an impulse function. In OW systems with IM/DD, re- each of the subcarriers is orthogonal to the others and to the ceiver thermal noise and intense ambient shot noise can be mod- time-varying bias signal . eled as Gaussian and independent of the transmitted signal. We Fig. 1(b) shows the receiver used under the proposed MSM will model the noise as Gaussian, independent of , and techniques with QPSK; for BPSK, the receiver is identical, white, with two-sided power spectral density (PSD) . The re- but omits the sine branches. Like a standard QPSK MSM sults obtained here are applicable also to fiber-optic channels receiver, the receiver of Fig. 1(b) uses a bank of hard decision provided that dispersion is negligible and the noise is white, devices to obtain a vector of detected symbol amplitudes, Gaussian, and independent of the transmitted optical signal. . In the receiver of Fig. 1(b), 2166 IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 49, NO. 12, DECEMBER 2001

Fig. 1. Power-efficient multiple-subcarrier modulation scheme: (a) transmitter and (b) channel and receiver. QPSK modulation is assumed in this figure. For simplicity, we suppress the possible dependence of the baseband PAM amplitude on past and future symbols. however, a hard-decision block decoder maps the vector to a to guarantee that the subcarriers and time-varying bias signal vector of detected information bits, . are mutually orthogonal, we choose , the set of subcarrier In the remainder of this paper, unless otherwise noted, we frequencies to have an integral number of cycles per symbol in- consider a rectangular transmit pulse shape terval. We consider both “dense” and “coarse” packing of subcarrier frequencies, which are described by

(9) (dense packing) (10) and There are two reasons for this choice. In IM/DD optical systems, (coarse packing) (11) the rectangular pulse yields high average-power efficiency [7]. Also, choosing the rectangular pulse makes it very straightfor- respectively, where . With our choices of ward to design the block code and bias signal to minimize the and , , regardless of how we choose the signal transmit power requirement. In (7), the baseband PAM ampli- set . Hence, (8) simplifies to tude in symbol depends only on and not on , , so that we let ; , . In order (12) YOU AND KAHN: AVERAGE POWER REDUCTION TECHNIQUES FOR MULTIPLE-SUBCARRIER IM OPTICAL SIGNALS 2167

In evaluating the average transmit power requirement, we will throughput is equivalent to that obtained when subcarrier often consider the minimum value of the MSM electrical signal frequencies are reserved. Hence, it may be said that described by over the symbol interval subcarrier frequencies are “effectively reserved” for maximizing the minimum value of the MSM electrical signal . (13) Under the minimum-power block code, , for QPSK and , for BPSK. To describe this block code, we first sort all possible For BPSK, the sine terms are omitted in the expression for . vectors of symbol amplitudes in order of decreasing .For We now describe various options for the design of the block QPSK, we have code and of the bias signal. (17) A. Block Code For BPSK, this ordering holds for . The In this subsection, we consider how to design the block code signal set uses the first vectors , utilized in the transmitter, which is shown in Fig. 1(a). This code which have maximum . This represents the optimal choice maps , the vector of information bits to be transmitted, of signal set for a given and , independent of the biasing to , the vector of symbol amplitudes modulated onto the technique. For a given , , and biasing technique, the optical subcarriers. Recall that , where . average power requirement under minimum-power block 1) Normal Block Code: Under this block code, all sub- coding always lower-bounds the average power requirement carriers are used for transmission of information bits. Hence, under the reserved-subcarrier block coding. The major draw- , for QPSK and , for back of minimum-power block coding is that, in general, each BPSK. Each information bit can be mapped independently to the receiver needs to demodulate all of the subcarriers, even to corresponding symbol amplitude, i.e., a component of the vector extract only a subset of the transmitted information bits. . At the receiver, each detected symbol amplitude, i.e., each component of the vector , can be mapped independently to an B. Baseband Bias Signal information bit. If the MSM signal conveys multiple, indepen- In this subsection, we consider how to design the bias signal dent bit streams to different users on different subcarrier fre- . The bias signal must be chosen so that when it is added quencies, each user’s receiver only needs to demodulate those to the MSM electrical signal , the sum is nonnegative, i.e., subcarriers conveying the desired bit streams. , where is a nonnegative scale 2) Reserved-Subcarrier Block Code: The reserved-subcar- factor. It is convenient to break into the sum of a constant rier block code is similar in spirit, though not implementation, and a baseband PAM signal, as indicated in (7). to the technique described in [4]. Under this block code, , 1) Fixed Bias: When using a fixed bias signal, we set subcarrier frequencies are reserved with the goal of maximizing , , and we use a fixed bias . With an arbitrary the minimum value of the MSM electrical signal , thereby pulse shape , the smallest allowable is given by minimizing the average optical power . Hence, in this case, , for QPSK and , (18) for BPSK. In reserved-subcarrier block coding, we define a set of reserved subcarriers and the average optical power is (14) (19) The block code is described as follows. We encode an informa- For the rectangular assumed here, the smallest allowable tion bit vector by freely choosing the symbol amplitudes fixed bias is given by on the unreserved subcarriers (20) (15) Note that when using fixed bias with reserved-subcarrier block We then choose the symbol amplitudes on the reserved subcar- coding, the average transmit power is minimized if we choose riers to maximize , the minimum value of the MSM elec- , the set of reserved subcarriers, to minimize . trical signal over the symbol interval 2) Time-Varying Bias: With a time-varying bias, is (16) chosen to be of the form (7), i.e., the sum of a constant and a baseband PAM signal. For an arbitrary pulse shape ,to minimize the average optical power , it may be necessary to For each choice of , , and biasing technique, there exists use both terms. In this case, using (8), the average transmitted an optimal set of reserved subcarriers, though this set is often optical power is not unique. Under reserved-subcarrier block coding, each user’s receiver only needs to demodulate those subcarriers conveying the desired bit streams. 3) Minimum-Power Block Code: Minimum-power block (21) coding is similar to the technique described in [2]. Under this With a rectangular , the baseband PAM component of block code, no fixed set of subcarriers are reserved, but the is piecewise constant and can describe the dc component 2168 IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 49, NO. 12, DECEMBER 2001 of even when we set . Hence, with rectangular and time-varying bias, we will choose . The smallest allowable symbol-by-symbol bias is given by

(22) Using (12), we find the average transmit power to be

(23) Note that when using time-varying bias with reserved-subcarrier block coding, the average transmit power is minimized if we choose , the set of reserved subcarriers, to minimize .

IV. PERFORMANCE EVALUATION In this section, we evaluate the bandwidth and power requirements of the proposed MSM block coding and biasing schemes, comparing them to ON–OFF keying (OOK). For each scheme, represents the information bit rate, represents the total electrical bandwidth required at the receiver, and represents the probability of information bit error. We first consider a reference system using OOK with rectangular pulses of duration [described by (9)] and symbol rate . Following [7], we have , , and , where is the Gaussian function [15]. For OOK, the bandwidth requirement is taken to equal the first null in the PSD of the transmitted signal. For the proposed MSM scheme, with either dense or coarse frequency packing, the information bit rates are given by

(BPSK) (24) and (QPSK) (25)

With either BPSK or QPSK, the electrical bandwidth require- Fig. 2. Normalized optical power requirement versus number of subcarriers ments are given by for various block-coding and biasing schemes. These results assume a rectangular transmit pulse g@tA and are valid for either 3 a nP%a or 3 a nR%a (dense packing) (26) . The normalized power requirement is the power required by a scheme divided by the power required by OOK, assuming a fixed bit rate . Solid lines and connect the actual power requirements at different x. Dashed lines represent linear least-squares fits to the power requirements for normal block coding, (coarse packing) (27) corresponding to the analytical expressions given. (a) BPSK. (b) QPSK.

These values of correspond to the first null in the PSD of the MSM signal above the highest subcarrier frequency. While (29) depends explicitly on the amplitude scaling factor , In the proposed MSM scheme, , a vector of infor- we would like to relate to the average optical power and to mation bits, is mapped to , a vector of symbol amplitudes. and . As shown by (12), is proportional to , and depends Assuming an additive white Gaussian noise (AWGN) channel on , , and the choice of block coding and biasing schemes and the hard-decision receiver shown in Fig. 1(b), is trans- via its dependence on . The probability of information mitted through a binary symmetric channel characterized by the bit error is given by crossover probability (28) (30) . This definition of holds for QPSK; for BPSK, the definition should omit the term . It is easily The relationship between and depends on the block code shown that [the choice of ] and on the mapping between and . In the proposed scheme, the block (BPSK or QPSK) (29) code is designed to minimize subject to fixed and not necessarily to minimize subject to fixed (i.e., YOU AND KAHN: AVERAGE POWER REDUCTION TECHNIQUES FOR MULTIPLE-SUBCARRIER IM OPTICAL SIGNALS 2169

Fig. 3. Normalized optical power requirement versus normalized bandwidth requirement for various block coding and biasing schemes. These results assume a rectangular transmit pulse g@tA. The normalized power requirement is the power required by a scheme divided by the power required by OOK, assuming a fixed bit rate . (a) BPSK, 3 a nP%a. (b) QPSK, 3 a nP%a. (c) BPSK, 3 a nR%a. (d) QPSK, 3 a nR%a. fixed ). Nonetheless, given any block code, it is always pos- Fig. 2 presents the normalized power requirement versus sible to choose the mapping between and so that the total number of subcarrier frequencies for various MSM .1 In evaluating the proposed MSM schemes, we assume schemes with BPSK and QPSK. In Fig. 2(a), we see that BPSK, conservatively that with normal block coding and fixed bias, has a normalized power requirement of , while normal block (BPSK or QPSK) (31) coding and time-varying bias lowers this to approximately In comparing various modulation techniques, we consider the .For , this amounts to a power normalized bandwidth requirement and the normalized savings of about 3.0 dB. For , reserved-subcarrier power requirement , which represents the average op- block coding with time-varying bias and minimum-power tical power required by a particular scheme to achieve an ar- block coding with time-varying bias are the best-performing bitrary specified BER compared to that required by OOK to techniques. The latter offers power savings up to about 0.6 dB achieve the same BER, assuming a fixed bit rate .2 more than normal block coding with time-varying bias. Referring to Fig. 2(b), we see that QPSK, with normal 1For simplicity, consider BPSK and arbitrary x. When v aH, the codewords (i.e., the f g) lie on the P vertices of an x-dimensional hypercube. When block coding and fixed bias, has a normalized power require- vbH, the worst-case block code has its codewords on the P vertices of ment of about . Normal block coding and a hypercube of dimension x v. This occurs when the symbol amplitudes time-varying bias lowers this to about , on v subcarriers are identical for all codewords. For this worst-case block code, a p, assuming Gray coding between the fx g and f g. For any other which represents a power savings of about 2.4 dB when . block code, it is possible to find a mapping such that p. Similar arguments For , reserved-subcarrier block coding with time-varying can be used in the case of QPSK to justify the assertion that p. bias and minimum-power block coding with time-varying bias 2 IH log @a A We actually consider , which has units of optical deci- are the best-performing techniques, and the latter offers power bels. 2170 IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 49, NO. 12, DECEMBER 2001

be applied separately or together. When the number of subcarriers is small, the computation required to implement the proposed techniques can be performed off-line and implemented via table lookup. The proposed techniques yield a reduction in average power requirement that increases with the number of subcarriers. With eight subcarriers, this reduction can be as high as about 3.6 and 3.2 dB with BPSK and QPSK, respectively. These two classes of techniques are applicable as long as all subcarriers originate from a single transmitter and are symbol-synchronized. Several problems remain for future study. These include: 1) block coding and time-varying biasing techniques for nonrect- Fig. 4. Choice of best reserved subcarrier under reserved-subcarrier block angular transmit pulse shapes; 2) efficient implementation of the coding with v aI, with fixed or time-varying bias. These results assume a rectangular transmit pulse g@tA, and are valid for either 3 a nP%a or proposed techniques when the number of subcarriers becomes 3 a nR%a. In many cases, the best reserved subcarrier is not unique. large, and table lookup becomes impractical; 3) obtaining ana- (a) BPSK. (b) QPSK. lytical bounds on the transmit power requirements with block coding and/or time-varying bias; 4) design of block codes that savings up to about 0.8 dB more than normal block coding with minimize the transmit power required to achieve a specified time-varying bias. information bit-error probability; 5) utilizing information con- In Fig. 3, we plot the normalized power requirement tained in the time-varying bias signal to enhance detection ef- versus the normalized bandwidth requirement for all of the ficiency [14]; and 6) possible impairment caused by a time- block-coding and biasing schemes, comparing these to OOK. varying bias signal on channels exhibiting memory or nonlin- Fig. 3(a) and (b) consider the dense frequency packing with earity. BPSK and QPSK, respectively, while Fig. 3(c) and (d) consider the coarse frequency packing with BPSK and QPSK, ACKNOWLEDGMENT respectively. Fig. 3 illustrates the bandwidth expansion ratio incurred by reserved-subcarrier or minimum-power block The authors would like to thank V. Anantharam for discus- codings, which is . With ,at , this sions on analytical bounds for the transmit power requirement. ratio is 2, while at , it is only 8/7. In applications where bandwidth is at a premium, when is small, it may be most REFERENCES attractive to use normal block coding with time-varying bias. [1] J. A. C. Bingham, “Multicarrier modulation for data transmission: An As increases, it becomes increasingly attractive to use the idea whose time has come,” IEEE Commun. Mag., vol. 28, pp. 5–14, other two block codings with time-varying bias. May 1990. [2] S. Fragiacomo, C. Matrakidis, and J. J. O’Reilly, “Multicarrier transmis- Fig. 4 indicates the best subcarrier to reserve with fixed bias sion peak-to-average power reduction using simple block code,” Elec- or time-varying bias, for various , considering reserved-sub- tron. Lett., vol. 34, no. 10, pp. 953–954, May 1998. carrier block coding with . In Fig. 4(a), we see that, for [3] R. van Nee and A. de Wild, “Reducing the peak-to-average power ratio BPSK with fixed bias, for most , there are several equivalent of OFDM,” in Proc. 1998 IEEE Conf. on Vehic. Technol., Ottawa, ON, Canada, May 18–21, 1998, pp. 2072–2076. best reserved subcarriers. By contrast, with time-varying bias, [4] J. Tellado and J. M. Cioffi, “Efficient algorithms for reducing PAR in for most , the best reserved subcarrier is unique, and it is best multicarrier systems,” in Proc. 1998 IEEE Intl. Symp. on Info. Thy.,New to reserve subcarrier 1. In Fig. 4(b), we see that, for QPSK with York, NY, Aug. 16–21, 1998, p. 191. fixed or time-varying bias, for most , the best reserved sub- [5] D. Wulich and L. Goldfeld, “Reduction of peak factor in orthogonal multicarrier modulation by amplitude limiting and coding,” IEEE on carrier is unique. With time-varying bias, for most , it is best Commun., vol. 47, pp. 18–21, Jan. 1999. to reserve subcarrier 1. [6] T. E. Darcie, “Subcarrier multiplexing for lightwave networks and video distribution systems,” IEEE J. Select. Areas Commun., vol. 8, pp. 1240–1248, Sept. 1990. V. C ONCLUDING REMARKS [7] J. R. Barry, Wireless Infrared Communications. Boston, MA: Kluwer, 1994. We have described two classes of simple, effective techniques [8] Q. Pan and R. J. Green, “Preclipping AM/QAM hybrid lightwave sys- for reducing the average optical power requirement in MSM tems with bandstop filtering,” IEEE Photon. Technol. Lett., vol. 8, pp. IM/DD systems using BPSK or QPSK modulation. The first 1079–1081, Aug. 1996. [9] Q. Shi, “Error performance of OFDM-QAM in subcarrier multiplexed class of techniques involves block coding between the informa- fiber-optic transmission,” IEEE Photon. Technol. Lett., vol. 9, pp. tion bits to be transmitted and the symbol amplitudes modulated 845–847, June 1997. onto the subcarriers, in order to increase the minimum value [10] F. R. Gfeller and U. H. Bapst, “Wireless in-house data communication of the MSM electrical waveform. This class of techniques in- via diffuse infrared radiation,” Proc. IEEE, vol. 67, pp. 1474–1486, Nov. 1979. creases the bandwidth required for transmission at a given bit [11] D. J. T. Heatley, D. R. Wisely, I. Neild, and P. Cochrane, “Optical wire- rate, but the bandwidth expansion ratio decreases as the number less: The story so far,” IEEE Commun. Mag., vol. 36, pp. 72–74, 79–82, of subcarriers increases. The second class of techniques replaces Dec. 1998. [12] J. B. Carruthers and J. M. Kahn, “Multiple-subcarrier modulation for the fixed dc bias by a time-varying bias signal and does not en- nondirected wireless infrared communication,” IEEE J. Select. Areas tail a bandwidth expansion. These two classes of techniques can Commun., vol. 14, pp. 538–546, Apr. 1996. YOU AND KAHN: AVERAGE POWER REDUCTION TECHNIQUES FOR MULTIPLE-SUBCARRIER IM OPTICAL SIGNALS 2171

[13] R. Narasimhan, M. D. Audeh, and J. M. Kahn, “Effect of electronic- Joseph M. Kahn (M’90–SM’98–F’00) received the ballast fluorescent lighting on wireless infrared links,” Proc. Inst. Elect. A.B., M.A., and Ph.D. degrees in physics from the Eng., pt. J, vol. 143, pp. 347–354, Dec. 1996. University of California at Berkeley (UC Berkeley) [14] S. Hranilovic and D. A. Johns, “A multilevel modulation scheme for in 1981, 1983, and 1986, respectively. high-speed wireless infrared communications,” in Proc. IEEE Int. Symp. He is a Professor in the Department of Electrical Circuits and Systems, Orlando, FL, May 30–June 2, 1999. Engineering and Computer Sciences at UC Berkeley. [15] J. G. Proakis, Digital Communications, 3rd ed. New York: McGraw- In 2000, he co-founded StrataLight Communica- Hill, 1995. tions, Inc., where he is currently Chief Scientist. From 1987 to 1990, he was a Member of Technical Staff in the Lightwave Communications Research Department of At&T Bell Laboratories, where he performed research on multi-gigabit-per-second coherent optical fiber Roy You received the B.S. degree from the Univer- transmission systems, setting several world records for receiver sensitivity. sity of California at Berkeley in 1997. He is currently He joined the faculty of UC Berkeley in 1990. His current research interests working toward the Ph.D. degree in communications include wireless communication using antenna arrays, free-space optical at the same university. communication, optical fiber communication, and wireless communication for His research interests include digital communica- networks of sensors based on micro-electromechanical systems. tion, optical communication systems, and informa- Prof. Kahn received the National Science Foundation Presidential Young In- tion theory. vestigator Award in 1991. He is a member of the IEEE Communications Society, the IEEE Information Theory Society, and the IEEE Lasers and Electro-Optics Society. From 1993–2000, he served as a Technical Editor of IEEE Personal Communications Magazine.