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  • 1. OPTICAL WIRELESS COMMUNICATIONS Multiple-Subcarrier Modulation in Optical Wireless Communications Tomoaki Ohtsuki, Tokyo University of Science ABSTRACT This article overviews multiple-subcarrier modulation techniques in optical wireless communications. The basic principles and characteristics of MSM techniques in optical wireless communications are presented. MSM optical wireless systems are explained where some block codes that convert information bits to be transmitted to the symbol amplitudes of subcarriers are used to increase the minimum value of the MS electrical waveform. MSM optical communications systems using subcarrier signal point sequences (SSPS) that can improve the power efficiency of MSM systems are also explained. The performance of MSM optical communications systems is presented in the intensity modulation with direct detection (IM/DD) channel without dispersion and in atmospheric optical communications where the effects of scintillation exist. INTRODUCTION Recently, multicarrier modulation (MCM) has attracted much attention in many fields, such as wired and wireless electrical and radio frequency (RF) communications, optical fiber communications, optical wireless communications, and so on. In MCM, multiple digital streams are modulated onto different carriers at different frequencies. MCM permits transmission with minimal intersymbol interference (ISI) on frequencyselective and multipath channels. Thus, MCM can realize high-speed communications. In electrical and RF communications, orthogonal frequency-division multiplexing (OFDM) has attracted much attention [1]. OFDM has been used or under consideration in many applications, such as digital TV, local area networks (LANs), asymmetric digital subscriber line (ADSL), nextgeneration mobile communications (i.e., fourth generation, 4G), and so on. In optical fiber communications, subcarrier multiplexing (SCM) is a popular technique [2]. In SCM, multiple signals are multiplexed in the RF domain and transmitted by a single wavelength. A major application of SCM in optical fiber systems is analog cable television (CATV) distribution. Since SCM can be implemented at low cost, it has been applied to transmit multichannel digital optical signals using direct detection (DD) for LANs. 74 0163-6804/03/$17.00 © 2003 IEEE In optical wireless communications, multiple-subcarrier modulation (MSM) has attracted much attention, because it can realize high-speed communications without equalization [3]. Note that MSM is referred to as SCM in the field of optical fiber networks. MSM can be more bandwidth-efficient than singlecarrier modulation, such as on-off keying (OOK) and M-ary pulse position modulation (PPM). The multiple subcarrier (MS) electrical signal may be modulated onto the optical carrier using intensity, frequency, or phase modulation. Most current practical MSM systems use intensity modulation with DD (IM/DD) due to its simple implementation. IM/DD MSM systems are attractive, because the use of several narrowband subcarriers promises to minimize ISI on multipath channels, and MSM can provide immunity to fluorescent-light noise near dc. The main drawback of IM/DD MSM systems is poor optical average power efficiency. This arises because the MS electrical signal is a sum of modulated sinusoids and thus takes on both negative and positive values. Optical intensity (instantaneous power) must be nonnegative. Hence, a dc bias must be added to the MS electrical signal to modulate it onto the intensity of an optical carrier. In MSM systems, the block coder maps the information bits to be transmitted to the symbol amplitudes modulated onto the subcarriers. Some block codes have been proposed for MSM optical wireless systems to reduce the dc bias and improve the power efficiency of the systems [4, 5]. In conventional MSM systems, one symbol is transmitted with one subcarrier, while in an MSM system using subcarrier signal point sequences (SSPS), one symbol is transmitted with multiple subcarriers, which means that each symbol is composed of signal points on some subcarriers [5]. This technique has been shown to be effective to reduce the dc bias and to improve the power efficiency of MSM systems. The following sections outline the basic system diagram of MSM optical wireless systems, the block codes used in MSM systems, a system diagram of MSM systems using SSPS, and the performance of MSM systems on channels without dispersion and in atmospheric optical communications. IEEE Communications Magazine • March 2003
  • 2. Symbol amplitudes a1c Information bits x1 g(t) sinω1t aNc each subcarrier is lower than that of a single carrier LED/ LD Σ with the same g(t) x(t) = A[s(t) + b(t)] MSM optical signal cosωNt aNs xK Σ MSM electrical signal g(t) Block coder xK–1 symbol rate of s(t) cosω1t a1s x2 In MSM the A g(t) Symbol-by- b(a1c,a1s,...,aNc,aNs) g(t) symbol bias computation sinωNt Σ Thus, MSM experiences little b(t) distortion and has Bias signal no need of using b0 Fixed bias total bit rate. equalization. (a) Detected symbol aplitudes ^1c a g(–t) Detected information bits t = iT cosω1t ^1s a g(–t) x(t) MSM optical signal rh(t) Optical channel, photodetector ^ x2 t = iT Σ y(t) sinω1t Photo current n(t) Noise g(–t) ^Nc a Block decoder t = iT cosωNt ^Ns a g(-t) ^ x1 t = iT ^ xK–1 ^ xK sinωNt (b) I Figure 1. A multiple-subcarrier modulation system with QPSK: a) transmitter; b) receiver. MSM OPTICAL COMMUNICATION SYSTEM MSM is a scheme where multiple signals (subcarriers) are multiplexed in the electrical domain, which modulates a single optical carrier (wavelength) [3]. High-speed single-carrier modulation schemes, such as OOK and M-ary PPM, are wideband signals and thus suffer from ISI due to multipath dispersion when the symbol rate exceeds about 10 Mbaud [6]. In MSM the symbol rate of each subcarrier is lower than that of a single carrier with the same total bit rate. Thus, MSM experiences little distortion and has no need to use equalization. MSM has another advantage in that it can exploit microwave devices that are more mature than optical devices; the stability of a microwave oscillator and frequency selectivity of a microwave filter are much better than their optical counterparts. In addition, the low phase noise of an RF oscillator makes coherent detection in the RF domain easier than optical coherent detection, and thus advanced modulation formats can be applied easily. Figure 1a and b depicts the transmitter and IEEE Communications Magazine • March 2003 receiver design used in the MSM transmission scheme with quaternary phase shift keying (QPSK). The transmitter uses a set of N subcarriers {ω n , n = 1, …, N}. During each symbol interval of duration T, it transmits a vector of K information bits. A block coder maps a vector of K information bits to a corresponding vector of symbol amplitudes, a(m). Since the MSM signal s(t) can be positive or negative, the transmitter adds a baseband bias signal b(t) so that the MSM signal is nonnegative: x(t) = A [s(t) + b(t)] 0, where A is a nonnegative scale factor. The average optical power is P = AE [s(t)] + AE [b(t)]. In general, b(t) is the sum of a constant and a baseband pulse-amplitude modulation (PAM) signal. Considering a rectangular transmit pulse shape and the subcarrier frequencies ω n = n(2π/T), n = 1, …, N, E [s(t)] = 0, regardless of how we choose the signal set {a(m)}. Hence, the average optical power just depends on the bias signal: P = AE [b(t)] [4]. If we choose the bias signal properly, we can decrease the average optical power. There are two biasing schemes: fixed bias and time-varying bias. When using fixed bias, we use fixed bias that has the same magnitude with the smallest value of an MSM signal. When using a time- 75
  • 3. No. Input bits SSPS Bias (V) 1 0 0 0 0 0 0 1.3156 2 0 0 1 3 6 1 1.3156 3 0 1 0 6 4 2 1.3156 4 0 1 1 1 2 3 1.3156 5 1 0 0 4 0 4 1.3156 6 1 0 1 7 6 5 1.3156 7 1 1 0 2 4 6 1.3156 8 1 1 1 5 2 7 1.3156 I Table 1. Subcarrier signal point sequences (SSPS) (N = 3, K = 3, 8PSK). varying bias, we use the smallest allowable symbol-by-symbol bias. Note that the average optical power with time-varying bias is generally smaller than that with fixed bias. BLOCK CODES FOR AN MSM OPTICAL COMMUNICATION SYSTEM In the MSM systems, the block coder maps the vector of K information bits to be transmitted to the vector of symbol amplitudes modulated onto the N subcarriers. Some block codes have been proposed for MSM optical communication systems to improve their power efficiency [4]. We will briefly explain some block codes. Normal Block Code — Under the normal block code, all N subcarriers are used for transmission of information bits. Hence, K = 2N, M = 22N for QPSK. Each information bit can be mapped independently to the corresponding symbol amplitude. At the receiver, each detected symbol amplitude (i.e., each component of the vector of the symbol amplitudes) can be mapped independently to an information bit. Reserved-Subcarrier Block Code [4] — Under the reserved-subcarrier block code, L subcarriers are reserved with the goal of maximizing the minimum value of the MSM electrical signal s(t), thereby minimizing the average optical power P. Hence, in this case, K = 2 = (N – L), M = 22(N – L) for QPSK. In reserved-subcarrier block coding, we encode an information bit vector by freely choosing the symbol amplitudes on the unreserved subcarriers. We then choose the symbol amplitudes on the reserved subcarriers to maximize the minimum value of the MSM electrical signal over the symbol interval. For each choice of N and L, there exists an optimal set of reserved subcarriers, although this set is often not unique. Minimum-Power Block Code [4] — Under the minimum-power block code, no fixed set of subcarriers is reserved, but L > 0 subcarriers are reserved for maximizing the minimum value of the MSM electrical signal s(t). Under the minimumpower block code, K = 2(N – L), M = 22(N – L) for QPSK. For a given N and L, the average optical 76 power under minimum-power block coding always lower bounds the average optical power requirement under reserved-subcarrier block coding. MSM OPTICAL COMMUNICATION SYSTEMS WITH SUBCARRIER SIGNAL POINT SEQUENCES In the conventional MSM optical systems, one symbol is transmitted with one subcarrier, while in the MSM system with subcarrier signal point sequences (SSPS) [5], one symbol is transmitted with multiple subcarriers, which means that each symbol is composed of signal points on some subcarriers. At the transmitter in MSM systems with SSPS, each SSPS consisting of N points is selected according to input data. The SSPS having a large minimum value and large Euclidean distances are used, so the required dc bias is minimized and the error rate performance is improved. Each subcarrier has a respective signal point selected according to the information data. The selected sequence is mapped to the corresponding signal point of each subcarrier. The receiver detects the transmitted symbols based on maximum likelihood sequence estimation (MLSE). According to the Euclidean distance between the received SSPS and the locally generated SSPS, the detector selects the signal point sequence that shows the minimum value of Euclidean distance as the transmitted symbol. The MSM systems with SSPS using 2K sequences can transmit K bits by one MSM-SSPS symbol. Table 1 shows a set of SSPS and the bias values for three subcarriers (N = 3) where each signal point corresponds to a signal point of an 8-PSK signal constellation. In this table, 8 SSPS having a minimum bias value of 1. 3156 have been selected from all possible sequences 83. PERFORMANCE ON IM/DD CHANNEL WITHOUT DISPERSION In this section we show the bandwidth and power requirements of MSM systems with some block codes, comparing them to the single-carrier system with OOK. For each scheme, Rb represents the information bit rate and B represents the total electrical bandwidth required at the receiver. The required bit error probability is set to 10–6. For comparison, the required power is normalized with that of OOK at the same bit rate R b . The system using OOK with rectangular pulses of duration T and a symbol rate of 1/T is considered as a reference system. The electrical bandwidth requirement is given by B = (N + 1)/T. The number of subcarriers is set to 1 ≤ N ≤ 8 in all the systems, because in optical wireless communications, eye safety and power consumption limit the available average optical power; thus, the number of subcarriers should be kept small. The number of reserved subcarriers L is set to L = 1 in the MSM system with the reserved subcarrier block coding (Reserved-MS). As for the bias signal, fixed bias and time-varying bias are used for all the systems. IEEE Communications Magazine • March 2003
  • 4. 7 3.5 Normal Reserved Min. power SSPS 3 Normalized power requirement Preq/Pook (optical dB) Normalized power requirement Preq/Pook (optical dB) 6 Normal Reserved Min. power SSPS 5 4 3 2 1 0 2.5 2 1.5 1 0.5 0 -1 -0.5 1 2 3 4 5 Total number of subcarriers (N) (a) 6 7 8 1 2 3 4 5 Total number of subcarriers (N) (b) 6 7 8 I Figure 2. Normalized power requirement versus the number of subcarriers for the systems with BPSK: a) the fixed bias and b)the timevarying bias. NORMALIZED POWER REQUIREMENT VS. THE NUMBER OF SUBCARRIERS We compare the performance in terms of the number of subcarriers, because it affects the required speed of the demodulation electronics and also influences the multipath immunity of the signal. Figure 2 shows the normalized power requirement vs. the total number of subcarriers for systems with binary PSK (BPSK) and fixed bias and time-varying bias. The normalized power requirement of the MSM-SSPS is smaller than those of MSM systems with normal block code (Normal-MS), reserved subcarrier block code (the Reserved-MS), and minimum power block code (Min. Power-MS) for all N and both biasing schemes. Comparing the performance with both biasing schemes, the performance of the system with time-varying bias is better than that of the system with fixed bias. As shown in this figure, MSM systems are less power-efficient than single- carrier systems with OOK. However, MSM systems are well suited to transmission of multiplexed bitstreams from a base station to some receivers. Through simultaneous transmission of several narrowband subcarriers, MSM systems can enable very high aggregate bit rates without requiring equalization to overcome ISI. Moreover, SM and MSM systems can achieve much greater immunity than OOK or M-ary PPM to near-dc noise from fluorescent lights. NORMALIZED POWER REQUIREMENT VS. NORMALIZED BANDWIDTH REQUIREMENT We also compare the performance in terms of the bandwidth requirement, because it is a measure of the electrical bandwidth required to pass the electrical signal resulting from DD. Figure 3 shows the normalized power requirement vs. the normalized bandwidth IEEE Communications Magazine • March 2003 requirement for systems with BPSK and fixed bias and time-varying bias. MSM-SSPS can reduce the normalized power requirement greatly at the same normalized bandwidth requirement as other systems. Comparing the performance with both biasing schemes, the performance of the system with time-varying bias is better than that of the system with fixed bias. As shown in this figure, MSM systems are less bandwidth-efficient than single-carrier systems with OOK or M-ary PPM. This is because the BPSK subcarrier used here requires twice the bandwidth of an OOK signal. ATMOSPHERIC OPTICAL COMMUNICATIONS USING MSM In this section we show the performance of atmospheric optical communications using MSM. Atmospheric optical communications are attractive due to their immunity to radio interference, urban noise, and electronic impulses. Atmospheric optical communications may be found, for instance, in LANs and wireless local loop (WLL) as information bridges between buildings containing cable or wireless subnetworks, or temporary quick connects for new outlying users. The atmospheric propagation path is characterized by molecular absorption, aerosol scattering, and turbulence [7]. In clear weather the molecular constituents of the atmosphere give rise to a variety of absorption bands. Turbulence effects in clear weather are due to the refractive index variation caused by microthermal fluctuations that cause fluctuation of the received optical beam. The primary influence of turbulence on the DD system is log-normal intensity scintillation [8]. To achieve good performance in atmospheric channels, atmospheric optical communications systems using MSM have been proposed [8, 9]. 77
  • 5. 7 3.5 N=8 5 4 N=8 3 2 N=8 N=2 N=1 1 N=2 0 Normal Reserved Min. power SSPS N=8 3 Normalized power requirement Preq/Pook (optical dB) N=8 6 Normalized power requirement Preq/Pook (optical dB) Normal Reserved Min. power SSPS N=8 2.5 N=8 2 N=2 N=1 1.5 N=8 1 N=2 0.5 0 -0.5 -1 0 0.5 1 1.5 2 2.5 3 Normalized bandwidth requirement (B/Rb) (Hz/b/s) 0 3.5 0.5 1 1.5 2 2.5 3 3.5 Normalized bandwidth requirement (B/Rb) (Hz/b/s) (a) (b) I Figure 3. Normalized power requirement versus normalized bandwidth requirement for the systems with BPSK: a)the fixed bias and b)the time-varying bias. gle-SM with BPSK has a better bit error rate (BER) than the system using BPPM. The performance advantage of the system using single-SM with BPSK over that using BPPM is about 3 dB in all cases. Note that both systems do not require dynamic estimation of the receiver threshold. In the presence of scintillation, both systems thus have clear advantages over the system using OOK, which does require dynamic estimation of the threshold. 100 BPSK BPPM 10–1 10–2 Bit error probability 10–3 σS2 = 0.1 10–4 σS2 = 0.03 CONCLUSIONS 10–5 10–6 σS2 = 0.01 10–7 10–8 10–9 10–10 0 5 10 15 20 25 30 Eb/N0 without scintillation (dB) I Figure 4. Bit error probability vs. Eb/N0 without scintillation for the atmospheric optical communication systems using single-SM with BPSK and using BPPM. Figure 4 shows the bit error probabilities of atmospheric optical communicationd systems using single-SM with BPSK and using BPPM for some values of the logarithm variance of the 2 scintillation σs . In this figure the performance of the systems is assumed to be dominated by scintillation in clear weather. The system using sin- 78 This article has outlined the basic principles and characteristics of multiple subcarrier modulation techniques in optical wireless communications. Although general MSM systems are not as power-efficient as single-carrier systems, an MSM system using subcarrier signal point sequences was presented to improve the power efficiency of MSM systems. The performance of subcarrier modulation schemes was also presented in atmospheric optical communications. It was shown that SM schemes are effective to improve the bit error probability in atmospheric optical communications. Therefore, MSM techniques are shown to be attractive in optical wireless communications. REFERENCES [1] J. A. C. Bingham, “Multicarrier Modulation for Data Transmission: An Idea Whose Time Has Come,” IEEE Commun. Mag., vol. 28, May 1990, pp. 5–14. [2] T. E. Darcie, “Subcarrier Multiplexing for Lightwave Networks and Video Distribution Systems,” IEEE JSAC, vol. 8, Sept. 1990, pp. 1240–48. [3] J. M. Kahn and J. R. Barry, “Wireless Infrared Communications,” Proc. IEEE, vol. 85, Feb. 1997, pp. 265–98. [4] R. You and J. M. Kahn, “Average Power Reduction Techniques for Multiple-Subcarrier Intensity-Modulated Optical Signals,” IEEE Trans. Commun., vol. 49, Dec. 2001, pp. 2164–71. IEEE Communications Magazine • March 2003
  • 6. [5] S. Teramoto and T. Ohtsuki, “Multiple-subcarrier Optical Communication Systems with Subcarrier Signal Point Sequence,” IEEE GLOBECOM 2002, Taipei, Taiwan, Nov. 2002. [6] J. B. Carruthers and J. M. Kahn, “Multiple-Subcarrier Modulation for Nondirected Wireless Infrared Communication,” IEEE JSAC, vol. 14, Apr. 1996, pp. 538–46. [7] R. S. Lawrence and J. W. Strohbehn, “A Survey of Clear Air Propagation Effects Relevant to Optical Communications,“ Proc. IEEE, vol. 58, Oct. 1970, pp. 1523–45. [8] W. Huang et al., “Atmospheric Optical Communication System Using Subcarrier PSK Modulation,” Trans. IEICE, vol. E76-B, no. 9, Sept. 1993, pp. 1169–76. [9] T. Ohtsuki, “Turbo-coded Atmospheric Optical Communication Systems,“ IEEE ICC 2002, New York, NY, Apr.–May 2002, pp. 2938–42. BIOGRAPHY TOMOAKI OHTSUKI [SM] (ohtsuki@ee.noda.tus.ac.jp) received B.E., M.E., and Ph.D. degrees in electrical engineering from IEEE Communications Magazine • March 2003 Keio University, Yokohama, Japan, in 1990, 1992, and 1994, respectively. From 1994 to 1995 he was a postdoctoral fellow and visiting researcher in electrical engineering at Keio University. From 1993 to 1995 he was a special researcher of fellowships of the Japan Society for the Promotion of Science for Japanese Junior Scientists. From 1995 to 1999 he was an assistant professor at Science University of Tokyo. From 1998 to 1999 he was with the Department of Electrical Engineering and Computer Sciences, University of California, Berkeley. Since 2000 he has been a lecturer (tenured) at Science University of Tokyo (now Tokyo University of Science). He is engaged in research on optical communication systems, wireless communication systems, and information theory. He is a recipient of the 1997 Inoue Research Award for Young Scientists, the 1997 Hiroshi Ando Memorial Young Engineering Award, Erricson Young Scientist Award 2000, 2002 Funai Information and Science Award for Young Scientist, and the 1st IEEE Asia-Pacific Young Researcher Award 2001. He is a member of the IEICE Japan and the Society of Information Theory and Its Applications (SITA). 79