Impact of Optical Modulators in LTE RoF System with Nonlinear Compensator for Enhanced Power Budget

Abstract

Nonlinear propagation compensation for LTE-RoF system is experimentally demonstrated with proposed DMFD and EMFD methods. We discovered that the RoF dithering frequency requirement have to be much smaller with an average improvement of ~5.35 dB.OCIS codes: (060.0060) Fiber Optics and Optical Communications; General; (060.5625) Radio Frequency Photonics Introduction and the Proposed SystemThe 3 rd generation partnership program (3GPP) long term evolution (LTE) technology experiences limited cell radius of 1 km with a cell edge throughput of < 20 Mb/s [1]. We have established a cell edge throughput improvement and adjacent cell transmission with the relay node (RN) and radio-over-fibre (RoF) interface connecting to eNodeB (eNB) of LTE [2,3]. The system design in [2,3] has revealed that LTE-RoF system attains improved quality of service (QoS) in the optimum optical launch power (OLP) region ranged between ~-3 and ~2 dBm. At OLP of < -3dBm known as the linear region, the impairment depends on the positive frequency chirp (PFC) and the chromatic dispersion (CD) that degrades QoS, which can be compensated with an erbium doped fibre amplifier (EDFA). The nonlinear region initiates from OLP of > 2 dBm, the nonlinear impairments are self phase modulation (SPM) and stimulated Brillouin scattering (SBS), which severely deteriorates QoS of LTE-RoF system. Due to the high power requirement after photodetection (PD) in RN for signal distribution to user equipments (UEs), it is vital that LTE-RoF operates in the nonlinear region (high OLP) to enhance the power budget in RN.The proposed LTE-RoF system is only SPM and SBS dependent [3], however since we are aiming on providing higher power budget, which corresponds to higher OLP, SBS becomes the dominant nonlinear factor. An index grating is formed when the propagating signal has a narrower linewidth than the SBS linewidth and subsequently energizes the back-reflecting Stokes wave [4]. An enhanced SBS threshold optical fibre was proposed as a solution in [5], but replacing the existing single mode fibre (SMF) infrastructure would not be cost effective [3]. Cross phase modulation based SBS compensation was proposed in [6], however this method cannot be adopted for the proposed single wavelength based LTE-RoF system [2,3]. Therefore, for the first time, we are proposing the usage of direct modulation (DM) and external modulation (EM) based frequency dithering (FD) methods for SBS compensation in RoF system. Since FD operates based on the frequency chirping, the investigation on the effectiveness of FD with two different optical modulators is crucial as DM induces PFC and contrariwise for EM. FD method was first introduced for the baseband system [7] where FD frequency f D has to be at least twice bigger than the signal frequency f m , {f D > 2f m }. Conversely, since RoF operates with a pass-band frequency f RF , we have discovered that

Demonstration of Mobile Fronthaul Test Bed Based on RoF Technology Supporting Two Frequency Assignments and 2 × 2 MIMO Antennas

“…The transmission characteristics of multi-IF carriers with an LTE baseband signal were also mathematically investigated in [11]. Most of the previous investigations were mainly focused solely on the performance of the optical transmission [15]- [18].…”

A Novel Scheme for Orthogonal Frequency Division Multiplexing-Radio over Fiber Based on Modulator and Dithering Technique: Impact of Self Phase Modulation and Group Velocity Dispersion

“…Recently, a novel scheme based on MZM dual drive to combine hybrid coupler and double parallel MZM an optical single side band that has been conducted [1,2]. Investigated Direct Modulation (DM) and External Modulation (EM), and direct modulator on QAM (Quadrature Amplitude Modulation) has exhibits to mitigate SBS (stimulated Brillouin scattering) and optimization nonlinear compensated for LTE (Long Term Evolution)-RoF with varied launch power for enhanced power budget [3]. Another line of research is aimed to demonstrate of the nonlinear compensation LTE-RoF based on DMFD (Direct Modulation Frequency Dithering) and EMFD (External Modulation Frequency Dithering) also to mitigate SBS including analysis of QPSK (Quadrature Phase Sift Keying), 16 QAM, and 64 QAM scheme [4].…”

Spectral Shape Impact of Nonlinear Compensator Signal in LTE RoF System

“…The principle of operation is based on simple amplifying and forwarding RNs with RoF acting as the interface between the eNB and RN [4][5][6][7][8]. In other words, instead of eNB, the RN delivers the LTE signal to the UE at the cell edge.…”

“…As a solution, we recently proposed a direct modulation based frequency dithering (DMFD) method for nonlinear compensation specifically designed for the LTE RoF system [5], which enabled the use of higher optical launch powers. Further optimizations of this method were carried out in the perspective of optical modulators [6] and dithering frequency f d with higher order modulations [7]. In [6], it is shown that DMFD method introduces an additional ~3 dB optical power penalty in comparison to external modulation while imposing a lower cost.…”

“…Further optimizations of this method were carried out in the perspective of optical modulators [6] and dithering frequency f d with higher order modulations [7]. In [6], it is shown that DMFD method introduces an additional ~3 dB optical power penalty in comparison to external modulation while imposing a lower cost. It is also shown in [7] that f d has to meet the condition of {f L < f d < f RF }, where f L and f RF are the dithering boundary limit and the LTE carrier frequency, respectively.…”