Simulation Approach of Wireless OFDM and FBMC Signals

Comparison is given between OFDM and FBMC based wireless signal by ... Multi-service data user applications need large bandwidth supported by novel ... RoF link for wireless signal transmission make the system analysis efficient with an.
Télécharger le PDF
441KB taille 4 téléchargements 380 vues
commentaire
Report
ICTON 2017

We.C2.2

Simulation Approach of Wireless OFDM and FBMC Signals Transmission over Fiber Based on Equivalent Electrical Models Anne-Laure Billabert1, Ali Kabalan1, Salim Faci1, Rostom Zakaria2, Maina Moutaly1, and Catherine Algani1 1 ESYCOM, Le Cnam, 292 rue saint Martin, 75141 Paris Cédex 3, France 2 Cédric, Le Cnam, 292 rue saint Martin, 75141 Paris Cédex 3, France e-mail: [email protected] ABSTRACT In this paper, we present the impact of a Radio over fiber (RoF) link on the transmission of a signal dedicated to wireless link. Comparison is given between OFDM and FBMC based wireless signal by simulation with the electrical modelling approach of O/E and E/O devices and optical components. The considered link concerns the Intensity Modulation - Direct Detection (IM-DD) architecture where the wireless signal is transmitted at the intermediate frequency. The simulation system includes the noise sources and nonlinearity of each component making thus the easy study of the impact of each one separately. It is then possible to quantify the degradation of the transmitted signal in term of error vector magnitude (EVM) or bit error rate (BER). Thus, the system architecture can be optimized for various applications and furthermore depending on the number of users. Keywords: RoF link, Remote antenna unit, FBMC and OFDM coding, system simulation. 1. INTRODUCTION Multi-service data user applications need large bandwidth supported by novel technologies like FTTx (fiber to the x). In addition, the unlicensed millimeter wave band gives more potentiality of the broadband communication. The future 5G communication is confronted to network challenges for the transport of the multiplex signals. The networks must be low cost due to high density of the base stations, supporting high data rate, simple and flexible. Analog transmission over fiber (RoF, radio over fiber) is a lithe way to migrate the cost effective of the base stations to a share components in the central office. To realize this new network, high performance optical devices were developed and various experimental works on RoF performances were given. These experimental works were supported by simulation with commercial software or numerical computations. Some limitations of these solutions concern the connection between optical and electrical devices, and particularly for the integrated components. An alternative way to study the RoF links more efficiently is by developing an optical devices models and using a large potentiality offers by analog circuit/system design tools [1], [2]. This paper summarizes the electrical modeling of the optical devices to analyze and compares the transmission of wireless signal based on OFDM and FBMC over the fiber. These wireless signal are complained following IEEE 802.15.3c devoted to high data rate communication in home area network. An external modulation RoF link with Mach-Zehnder Modulator (MZM) is selected because of its better linearity compared to the other solutions. The signal transmission penalty is discussed by EVM evaluation for both signals. 2. MODELLING OF E/O, O/O AND O/E ELEMENTS OF ROF LINK The analog feature of the RoF link for wireless signal transmission make the system analysis efficient with an equivalent electrical models of these components. The integrated optical and electrical circuits can be optimized to enhance the system performances. The simulation of RoF link is then possible with an electrical simulator software where resourceful analysis tools are available. In this study, an IM-DD link with external modulation is modeled to analyze and design RoF link for optical transmission of wireless signal. 2.1 DFB Laser The electrical model of the laser is based on the rate equations governing the carrier, photon evolution inside the laser cavity and the optical phase [3]. The direct detection in the IM-DD link makes the use of only the two first rate equations sufficient. These equation are converted to electrical node equations where the output current of the model represents the optical power envelope. The Relative Intensity Noise (RIN) is taken into account by including Langevin noise forces into the rate equations. To have a realistic laser models, all physical parameters used in the rate equations are extracted from measurements. The electrical impedance mismatch is also integrated but with no effect in the case of external modulation. 2.2 Mach-Zehnder Modulator The MZM is chosen for this study thanks to its high linearity and wide bandwidth. The modulation of the biased voltage permits to modulate the phase shift between its two arms and thus the intensity of the recombined optical signal at its output. The high linearity is obtained when the modulator is biased in the middle of the linear region

978-1-5386-0859-3/17/$31.00 ©2017 IEEE

1

ICTON 2017

We.C2.2

of its characteristic [4]. The equivalent electrical model of the MZM is based on the power static response implemented by equation based module available in ADS (SDD, Symbolically Defined Device). The linearity of the MZM is considered through the sine variation of the optical power with the biased voltage. 2.3 Optical Fiber The equivalent electrical model of the optical fiber takes into account the optical losses and the chromatic dispersion effect. Input and output signals of the model, which are an information of current, correspond to the envelope of the optical power. 2.4 PIN Photodetector At the output of the optical fiber, the modulated optical power is photodetected by a photodetector like a PIN diodes. When the incident optical power is low compared to the diode power saturation, the equivalent model includes a controlled source to generate the electrical current proportional to the optical power envelope and RC circuits for the bandwidth. The mismatch circuit, packaging parasitic elements and load are added to the model. 3. SYSTEM SIMULATION OF ROF LINK ADS offers the possibility to associate the analog and digital environments by co-simulation method. The Data Flow Controller (DF) is used to manage digital functions such as QAM, Fourier transform or filtering and link through specific components with the analog environment managed by an envelope simulator. Wireless signal generation, modulation and signal detection are then possible in the same software. 3.1 Wireless Signal The OFDM/FBMC modulation signal is generated according to the IEEE 802.15.3c standard. This signal is distributed across the RoF link at an intermediate frequency of 3 GHz due to the limited bandwidth of the RoF link components. 3.1.1 OFDM Signal OFDM is widely used in ultra-wideband (ULB) wireless communication systems [5]. The digital data is transmitted at a high rate by spreading them over a large number of subcarriers, each of which is modulated at low rate. The frequencies of the subcarriers are chosen such that the signals are mathematically orthogonal over an OFDM symbol period. Thus, even if the subcarriers overlap in the frequency domain, they can always be separated at the reception. A guard interval called a cyclic prefix is used between OFDM symbols to suppress the Inter Symbols Interference (ISI). The spectrum and time frame of transmitted OFDM signal are shown on Fig. 1.

(a) (b) Figure 1. OFDM signal in: (a) frequency and (b) time domains. 3.1.2 FBMC Signal During the last decade, Filter-Bank Multi-Carrier (FBMC) modulation has attracted a lot of research interest thanks to the good features that it can offer. Indeed, FBMC is considered to be a viable alternative to OFDM for emerging wireless networks [6]. The appealing feature of FBMC lies in the fact that subcarrier signals are filtered to be well localized in both time and frequency domain. This translates into a steep side-lobe decay, allowing a flexible spectrum usage and offering an increased resilience against time and frequency misalignment between users [7]. However, the filters applied on each subcarrier introduce inter-symbol interference (ISI) that makes symbol detection more difficult. To overcome this issue, QAM symbols are staggered by time-shifting the imaginary components by T/2 with respect to the real ones, and the filter is designed so that the interference is always orthogonal to the data symbol [7]. That is, when the transmitted data symbol is real-valued, the value of the accompanied interference is pure imaginary, and vice versa. At the receiver side, the symbol detection in a given subcarrier is performed at each period of T/2. Then, after channel equalization, the self-interference terms are removed by simply taking either the real or the imaginary parts of the equalized symbols. Finally, the real and imaginary values of the equalized symbols are recombined to form the received QAM symbols. Although the self-interference is always orthogonal to the desired data

2

ICTON 2017

We.C2.2

symbols, its presence severely deteriorates the system performance when considering, for instance, system nonlinearities or/and channel estimation errors [7]. Indeed, FBMC-based systems are very sensitive to residual phase rotation errors [8]. Figure 2 shows the spectrum and time frame of FBMC transmitted signal. As predicted, a higher spectral efficiency is achieved with FBMC modulation technique.

(a) (b) Figure 2. FBMC signal in: (a) frequency and (b) time domains. 3.2 System Architecture Fig. 3 shows the block diagram of the MZM based RoF link for EVM measurement of the transmitted OFDM wireless signal. Electrical amplification and attenuation are placed at the input of the RoF link to control the input power. LNA amplifier positioned at the output of the photodiode is used to increase the detected current. The link budget and non-linearity can be analyzed depending on MZM biased voltage and laser optical power as done in [9].

Figure 3. Measurement bench for Radio over Fiber link with MZM external modulator – AWG: arbitrary wave generator, Amp: 20GHz electric amplifier, Att: electric attenuator, OSC: 12GS/s digital oscilloscope 4. SIMULATION RESULTS The transmission penalties of the OFDM and FBMC signals are carried out in terms of EVM for transmission rates of 3 Gb/s (QPSK) and 6 Gb/s (16-QAM). The signals are constructed following the compliance of IEEE 802.15.3c standard. Only measurement with OFDM signal was carried out and presented in Fig. 4 for the MZM biased at quadrature and laser optical power of 10 mW. The photodiode is 25 GHz New Focus’s with a responsivity of 0.66 A/W. The detected DC current is 1.8 mA because of the 3dB insertion loss of the MZM. A short length SMF is used making thus a free-effect of optical attenuation and dispersion at a frequency of 3 GHz. The EVM is evaluated for different signal input power modulating the MZM. It is shown high EVM for low RF power due to noise level at the detection. The minimum value is reached for RF power between -5 and 5 dBm. Beyond 5 dBm, the limitation power of the experimental setup didn’t permit to attain the MZM saturation. System simulations are fulfilled by co-simulation method where the time domain wireless signals (Fig. 1b, 2b) are used as an envelope information for the analog simulation. Good agreement is obtained with measurement for the EVM curve with OFDM wireless signal for both QPSK and 16-QAM modulations. Simulation results of EVM with FBMC are close to EVM OFDM with QPSK in the RF power range (Fig. 4a). With 16-QAM, the EVM is a little better with OFDM and especially for low RF power since the equalization process becomes less accurate more complex modulation format. But, the main advantage of the FBMC is its optimal spectral allocation which would be very advantageous for multiband signals transmission. For wireless signal emission, the EVM limit required at the output of the transmitter is determined by the IEEE 802.15.3c standard according to the transmission rate. They are 20% and 9% for QPSK and 16-QAM, respectively. Considering no additional effect got from antenna, a dynamic range greater than 22 dB is obtained with the QPSK format and more than 14 dB with the 16-QAM for both signals. But, this range is higher for both

3

ICTON 2017

We.C2.2

modulation types when experimental setup limitation is cancelled. Figure 4c shows decrease in the EVM when AWG noise is suppressed from simulation for 16-QAM. The EVM curve intersects with a limit value of 9% at a power of -10 dBm either than -6 dBm for the previous simulations. The influence of the wireless channel between access points and users can be added to analyze the global system. This propagation channel is based on the model of Saleh-Valenzuela taking into account multipath and free space absorption effects was introduced in this type of system simulation as in [11].

(a) (b) (c) Figure 4. EVM measurement and simulation results with: (a) QPSK and (b) 16-QAM modulation formats, (c) effect of AWG noise on EVM with 16QAM. 5. CONCLUSION Electrical modeling of optical components on ADS has been allowed to fully analyze the RoF links. The analog characteristics of the optical link can be easily determined from analog circuit design tools. The EVM of the transmitted Wireless signal can be either approach from the signal to noise ratio or from co-simulation method once system non-linearities are present. EVM simulations of the transmitted wireless OFDM signal are very close to measured values when all electrical components and setup impairments are included. This means that a realistic system can fully be analyzed and optimized with this method. The RoF link effect on the wireless signals transmission is similar for both OFDM and FBMC modulation methods except that side lobe suppression ratio of FBMC is much higher than the conventional OFDM which should be benefit for multiband transmission. REFERENCES [1] A-L. Billabert et al.: Simulation of microwave optical links and demonstration of noise figure lower than electrical losses, Int. J. Microw. Wireless Technol., vol. 2, no. 06, pp. 497-503, 2010. [2] W-E. Kassa et al.: Electrical modeling of semiconductor laser diode for heterodyne RoF system simulation, IEEE J. of Quantum Electronics, vol. 49, no. 10, pp. 894-900, 2013. [3] R.S. Tucker and D.J. Pope: Microwave circuit models of semiconductor injection laser, IEEE Trans. Microw. Theory and Techniques, vol. 29, no. 3, Mar. 1982. [4] A. Hilt: Microwave harmonic generation in fiber-optical links, in Proc. Microwaves, Radar and Wireless Communications, MIKON-2000, Wroclaw, 2000. [5] P. Banelli et al.: Modulation formats and waveforms for 5G networks: Who will be the heir of OFDM?, IEEE Signal Processing Magazine, vol. 31, no. 6, pp. 80–93, 2014. [6] A. I. Pérez-Neira et al.: MIMO signal processing in offset-QAM based filter bank multicarrier systems, IEEE Transactions on Signal Proc., vol. 64, no. 21, pp. 5733-5762, Nov. 2016. [7] P. Siohan, C. Siclet, and N. Lacaille: Analysis and design of OFDM/OQAM systems based on filter bank theory, IEEE Transactions on Signal Processing, vol. 50, no. 5, pp. 1170-1183, May 2002. [8] R. Zakaria and D. Le Ruyet: SER analysis by Gaussian interference approximation for FBMC system in the presence of phase error, in Proc. 2015 IEEE International Conference on Communications (ICC), London, 2015, pp. 2662-2667. [9] A. Kabalan et al.: Direct and external modulation of IF over fiber for 60 GHz wireless applications, Int. J. of Microw. Wireless Technol., vol. 8, pp. 1-6, 2015.

4