Single Cycle Subcarrier Modulation - IEEE Xplore

© 2009 OSA/OFC/NFOEC 2009 a1950_1.pdf OTuE1.pdf OTuE1.pdf

Single Cycle Subcarrier Modulation Andreas O. J. Wiberg Department of Electrical and Computer Engineering, Jacobs School of Engineering University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0407, USA e-mail: [email protected]

Bengt-Erik Olsson Ericsson Research, Ericsson AB, SE-431 84 Mölndal, Sweden

Peter A. Andrekson Department of Microtechnology and Nanoscience, Chalmers University of Technology, SE-412 96 Göteborg, Sweden

Abstract: We report experimental demonstration of a concept of subcarrier modulation with the symbol rate and carrier frequency being identical, and thereby synchronous. The concept is demonstrated with 2.5 Gsymbols/s signals generated with binary digital electronics. ©2009 Optical Society of America OCIS codes: (060.4510) Optical communications; (060.4080) Modulation.

1. Introduction Multilevel modulation has obtained considerable attention in optical communication [1]. However, multilevel optical phase and amplitude links have complex transmitters and receivers. The receiver complexity is due to the fact that the optical phase is lost upon photodetection and only the envelop of the signal is received and therefore coherent or interferometric detection is required. From this point of view it is attractive to consider an alternative modulation method where information is modulated on RF-subcarriers which are transmitted over the optical link [2]. Using such technique, much of the complexity in the transmitter and receiver is moved to the electrical domain. Usually the subcarrier frequency is many times larger than the symbol rate and has been shown for 10 [3], [4] and 20 Gb/s links [5]. The subcarriers do not have to be orthogonal unless orthogonal frequency division multiplexing (OFDM) is used which is orthogonal by definition. OFDM is often implemented as massive subcarrier modulation and has been suggested for long distance transmission since its potentially spectrally efficiency and allows relatively simple mitigation of optical transmission impairments [6]. In this paper we present a concept where we use a symbol rate that is equal to the subcarrier frequency, and we define this as single cycle subcarrier modulation. This concept is thus a novel hybrid of traditional subcarrier modulation and base-band data transmission and allows recovery of both the optical amplitude and phase. The scheme is transparent to any modulation format and more spectrally efficient compared to traditional multi-carrier systems. Since the information spectral content is close to baseband the spectral efficiency is approximately half of that of baseband data, at the same symbol rate. Furthermore, optical components used for baseband transmission could be used in this concept and the requirements on electrical components are relaxed with respect to the lower cut-off frequency, compared to electronics for baseband communication since no frequency components are present close to DC. Digital modulation of an RF/sub-carrier is usually done with an IQ-modulator, with which an arbitrary signal could be generated, e.g. phase shift keying (PSK) or QAM by using digital-to-analog converters (DACs) with sufficiently high sample rate and bandwidth. However, with very fast DACs available, the modulated RF-subcarrier could be created directly, and the need for an IQ-modulator is eliminated. Furthermore, we present and demonstrate the concept of single cycle subcarrier transmission and show that phase modulated signals (m-PSK) could be generated with binary digital electronics. Digital electronic data patterns have been shown for bitrates up to 165 Gb/s [7] which opens the possibility to digitally generate subcarrier and symbols at high symbol/subcarrier rate. The signals created with binary digital electronics are compared with benchmark signals created with an arbitrary waveform generator (AWG) Moreover, we show experimentally that also m-QAM signals successfully can be created and transmitted with this concept. 2. Principle of operation The principle of operation is to use one time period of sinusoidal subcarrier signal with frequency fc and time period Tc=fc-1, to create a set of m symbols by changing its phase and amplitude within the time period, i.e. each symbol created with one subcarrier oscillation and is therefore synchronous. The signal can be modulated with an IQ-modulator or be directly synthesized with digital circuits, where the modulation and subcarrier are simultaneously created. The detection and demodulation will both differ from baseband communication in the

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electrical domain because IQ-demodulation and detection is needed which increases the complexity compared to if only amplitude modulation was used, but the optical components are the same as for baseband communication. Binary digital electronic single cycle subcarrier generation A subcarrier signal with frequency fc and time period Tc=fc-1 can be created with binary digital electronics with bandwidth Bd = 2fc and lowpass filter the signal, Fig. 1. Schematic of PSK symbol generation with binary digital circuits. for example with 2 bits per symbol by setting a series of 101010.... Moreover, by changing the order of the ones and zeros the created subcarrier signal will change its phase and is consequently binary phase modulated (BPSK), which is shown in Fig.1a. From this a set of symbols can be defined, with timeslot TS, which for example BPSK ”1” is 01 and ”0” is 10. Note that by creating the modulated signal in this manner the symbol rate and the subcarrier frequency is identical. Furthermore, this can be expanded to yield form-ary PSK symbols by increasing the bandwidth of the binary digital electronics circuits while the symbol rate is kept constant. The relationship between bandwidth of the electronics, symbol rate and number of phase states, m, is Ts=m.Td or Bd=m.Bs, if the symbol rate and the carrier frequency are equal, where Td is the bit time slot and Bs is the symbol rate. The different symbols for m =2, 4, 8 are shown in Fig.1. It is apparent that for many phase levels the bandwidth of the binary digital electronic circuits has to be very large, but the bandwidth of the analog equipment (electrical amplifiers, modulator or directly modulated laser, photo detector) in the link has only to be the subcarrier frequency plus the symbol bandwidth. For example if 20 Gb/s data is sent with 16-PSK at 5 GSymbol/s, the digital electronics are required to be clocked at 80 GHz, but bandwidth of the analog electronics and the transmission link has to be only 10 GHz. 3. Experimental and results The proposed technique was verified in two stages, first in an electrical back-to-back (BTB) setup, and then in a setup where the signal was modulated onto an optical carrier and received with Fig. 2. The setup used in the experiment and schematics of the electrical and an optical receiver, which is shown in Fig.2. The optical spectra. transmission link consisted of a Mach-Zehnder modulator (MZM), a few meters of standard single mode optical fiber, and a photo receiver. In order to emulate binary digital electronics a 40 Gb/s programmable pulse pattern generator (PPG) was used for signal generation. The PPG was programmed with a bit sequence corresponding to different symbols as shown in Fig.1, using a symbol-PRBS with 210 symbols. To generate a benchmark signal an arbitrary waveform generator (AWG) was used with 20 GSample/s and 10 bits of amplitude resolution. The AWG was programmed with a computer generated signal with 2 17 random symbols using Matlab. The generated signal sequences were continuously repeated. All signals were detected by a real-time oscilloscope with 3 GHz bandwidth and 20 GSample/s. In order to limit the bandwidth of system two low-pass filters with 7.5 GHz bandwidth were used after signal generation and before the oscilloscope. The detected trace was transferred to a computer where it was demodulated in software. The software demodulation algorithm was developed in Matlab and consists of carrier recovery using iterative cross-correlation, followed by synchronous resampling of the data, IQ-demodulation and presentation of constellation diagrams. Generation of PSK modulated data was demonstrated with a PPG with a subcarrier frequency of 2.5 GHz and a symbol rate of 2.5 Gsymbols/s. The corresponding bit rate for QPSK is 5 Gb/s, and 10 Gb/s for 16-PSK. The results of the electric back-to-back (BTB) and optical transmission are shown in Fig.3.a,c&e. The successful demodulation of the signals shows that this concept could be generated with binary digital electronics. It is clearly seen that decision points are concentrated around the ideal point, but with an irregular distribution. The results for the data sent and detected by the optical transmitter and receiver link are presented in Fig.3.b,d&f. One can see that that the constellation diagrams are practically not affected by the transmission, apart from a more oval distribution of the symbols dots in the constellation diagrams. This is due to the bandwidth limitation and phase response of the electrical and optical components, which results in intersymbol interference.


The PPG generated signals were compared with signals generated with an AWG, which had the same symbol rate and carrier frequency of 2.5 GHz. The setup was similar as above apart from the AWG. QPSK and 16-PSK were generated and transmitted over the link and the results after demodulation from the BTB and fiber transmission are shown in Fig.3g-j. The symmetric spreading of the symbol dots in the constellation diagrams indicates that thermal noise is the limiting factor. This is in contrast to the PPG generated signals which were irregularly distributed. This is explained by better jitter performance of the binary digital electronics of the PPG which results better signal-to-noise ratio. To further investigate the possibilities of single cycle subcarrier modulation, a 16-QAM signal was generated with the AWG with a subcarrier frequency of 2.5 GHz and a symbol rate of 2.5 Gsymbols/s. The corresponding bit rate for 16-QAM is 10 Gb/s and the results after demodulation from the BTB and fiber transmission are shown in Fig.3.k&l. 4. Discussion The spectral efficiency for single cycle modulation is Fig. 3. Constellation diagrams binary digital electronics electrically k/2 bits/s/Hz and the optical spectral efficiency is (PPG) generated signals (a-f) and for AWG generated signals k/4 bits/s/Hz, where k is the number of bits per symbol. The (g-l), back-to-back and after transmission for 2.5 GSymbols/s worse spectral efficiency in the optical domain is due to the two on a 2.5 GHz subcarrier. sidebands created with modulation, seen in Fig.2. However, this can be overcome by using, single sideband modulation with a dual arm MZM which has been shown for subcarrier transmission by [4], [8]. The timing jitter requirement of the binary digital electronics used for signal generation naturally increases with number of phase levels. For example, in order to create a 2.5 GSymbol/s QPSK signal, 10 Gb/s electronics are required and the timing jitter of the electronic signal has to be approximately four times smaller compared to the requirements at 2.5 Gb/s NRZ ASK. However, this timing jitter requirement will be present in all multilevel phase modulated subcarrier systems. The asymmetric distribution of the symbol dots in Fig.3, indicates that it is not purely noise that limits the performance of the system and that ordinary additive white Gaussian noise (AWGN) channel theory is not applicable. Depending of what is causing the spreading of the symbol dots, the theoretical relation with bit errors will differ. Further work will include derivation of theory and determination of the fundamental limits of the concept. Furthermore, testing the concept with directly modulated lasers, which are less linear than an external modulator and transmission over multi mode fibers would be of interest to investigate further. Pre- or post compensation using a digital equalizer could also easily be applied to the presented concept, to reduce transmission induced impairments. It should, however, be noted that in order to allow efficient mitigation of optical transmission impairments the optical transmitter must be single side-band since otherwise the quadratic detection will remove phase information about e.g. chromatic dispersion. It is therefore important to further investigate the transmission properties in longer optical links. 5. Conclusion The concept of a novel hybrid form of subcarrier and baseband modulation with the symbol rate equal to the subcarrier frequency, here called single cycle subcarrier modulation is presented. The concept has been experimentally demonstrated with signals generated with binary digital electronics subcarrier frequency of 2.5 GHz and 2.5 Gsymbols/s for m-PSK. The demonstration also show that m-PSK or m-QAM with a corresponding throughput of 10 Gb/s is achievable using 16-PSK or 16-QAM with