Tamas Cseh

Improved receiver techniques for Radio over Multimode fiber systems

Multimode fiber links can offer a useful approach for low cost and short Radio over Fiber (RoF) systems. However, in these connections the distortion effects are higher than in a singlemode fiber system. These effects can be compensated by a suitable optical receiver. In this paper an improved receiver structure is investigated for Radio over Multimode fiber systems. N-QAM radio signal transmission is simulated and measured on multimode fiber connections. Several improved compensating reception methods are compared with simple detection. From the different improved methods and installations an optimum solution is chosen for Radio over Multimode Fiber systems.

Pulsed mode red VCSEL for high speed VLC communication

VCSELs (Vertical-cavity surface-emitting lasers) offer a number of unique advantages: a threshold current that is low and stable with temperature so that electricity is converted to light with a very high efficiency; the ability to be modulated at very high frequencies; a circular output beam with low divergence, simplifying the optical system. Lifetime parameters are not degraded when overdriving its normal operating current in a low duty cycle application. The mentioned properties of VCSELs makes them excellent for usage in high speed free space optical links. However till the end of the last decade 850 nm VCSEL dominated the high speed area. Because of the laser safety regulations, the power limits restricted the 850 nm VCSEL usage in free space optical links. The fabrication problems of visible light producing VCSEL has been solved recently, and high speed 650 nm (red) VCSELs are available on market. In this paper we will show how a pulsed mode laser driver can exploit the unique properties of VCSELs in low cost, low power and high speed visible light communication (VLC) links.

Optical transmission of OFDM NQAM signals by direct laser modulation

The OFDM modulation method is widely used in several wireless applications because they are robust against multi-path distortions in the radio channel. However OFDM is sensitive to the channel nonllnearities. The OFDM signals are usually transmitted over an optical link to the wireless link. In this method the radio signals are transposed into the optical domain. In low cost systems direct laser modulation is applied. The nonlinearity of this type of modulation can cause distortion and this way limitation on the optical transmission. In this paper we present investigations on the limitations by simulations and experiments with the result of conditions for optimum operation.

Fundamental investigation of extending 4G-LTE signal over MMF/SMF-FSO under controlled turbulence conditions

In this paper, we propose a hybrid radio over multi-mode/single-mode fibre and free space optics (RoMMF/SMF-FSO) system to enhance the 4th generation-long term evolution (4G-LTE) signal performance when used in local area access networks. A single mode fibre pigtailed gradient index lens is used to minimise the modal effect of the optical link at different turbulence strengths. The proposed system is characterised in terms of the system transfer function, error vector magnitude (EVM), and optical launch power (OLP). Results demonstrate the robustness of the proposed scheme in terms of increased available passband bandwidth by 2 GHz and improving the transmission shape by reducing the system transfer function ripple by more than 5 dB. Successful transmission of the 4G-LTE signal is achieved over a MMF/SMF-FSO channel with a 1.5 dB power penalty under the higher turbulence level (i.e., Rytov variance of 0.1).

Efficient compensation methods for modal dispersion in radio over multimode fiber links

The multimode fiber (MMF) is a proper choice for short range optical fiber connections. Several applications need cost effective solutions. These connections can apply MMFs, however, the transmission parameters are degraded by the fiber modal dispersion. Therefore any efficient method to compensate modal dispersion is very important. In this paper mode filtering technique is introduced for that purpose. Two types of mode filters are proposed and tested by simulations and measurements. By the application of mode filters significant improvement is achieved in the transmission of QPSK and QPSK-OFDM signals. E.g. in case of a 500m long multimode fiber link transmitting signals with 500Mb/s bit rate the error vector magnitude (EVM) is improved from 40% to 5%.

Optical transmission of OFDM m-QAM RF signals: Comparison of direct and external modulations

For interconnections between the stations of mobile networks optical links are frequently used. The mobile networks require new modulation methods to achieve higher transmission capacity. For that purpose the m-QAM modulation method is a proper approach. On the other hand, the signal transmission in the mobile network is strongly affected by multipath wave propagation. The OFDM modulation can counterbalance that problem. Therefore an OFDM m-QAM RF signal is transposed into the optical domain by an optical modulator or by direct laser modulation. However, both m-QAM and OFDM are sensitive to the modulation nonlinearity. In this paper the nonlinear effect of direct and external optical modulations is investigated by simulations and experiments obtaining this way conditions for optimum operation.

Adaptation of Mode Filtering Technique in 4G-LTE Hybrid RoMMF-FSO for Last-Mile Access Network

This paper demonstrates a hybrid radio over multimode fibre and free space optics (RoMMF-FSO) system that can be used to extend the transmission range of the fourth generation long-term evolution (4G-LTE) signal in access networks. A single mode filtering technique (SMFT) is used to enhance 4G-LTE performance. The proposed scheme is evaluated in terms of the system transfer function, laser beam profile, and error vector magnitude (EVM). We show that using SMFT increases the RoMMF-FSO system bandwidth by 2 GHz and improves the received optical power by 13.6 dB. Moreover, the proposed system enhances the EVM by 4%. The measured results show that by using a 1 km MMF instead of a 1 km SMF will marginally increase the measured EVM from ∼6.6% to ∼7% with a 0.2 dB power penalty with respect to the LTE EVM limit of 12.5% as is specified for 16-quadrature amplitude modulation. The proposed system is validated practically under atmospheric turbulence conditions to mimic the outdoor environment. Measured EVM results are verified theoretically through transmitting LTE signals with turbulent using log-normal model. We also show that for a FSO link span of 500 m to meet the EVM target of 12.5% the SNR power penalties are ∼2 dB and ∼11 dB for Rytov variance of 1.2 × 10−4 and 0.1, respectively, compared with no turbulence.

Experimental optimization of the hybrid RoMMF-FSO system using mode filtering techniques

This paper proposes an optimum hybrid radio over multi-mode fiber and free space optics (RoMMF-FSO) system to enhance the last mile bottleneck issues of wireless communication systems. Single mode fiber (SMF) and gradient index (GRIN) lenses are used as mode filtering techniques to optimize the system performance by mitigating the modal effects produced by multimode optical fiber networks. We characterize the system based on the beam profile, transfer function, and error vector magnitude (EVM) measurements. The results reveal that both techniques can improve the EVM performance significantly by ~19% and ~30% using a SMF filter and SMF-GRIN lenses, respectively. Furthermore, the analysis of the obtained measurements show the SMF-GRIN lenses is more effective than SMF only with lower EVM values and negligible difference in the optical launch power.

Integrated microstrip diplexers for radio over fiber

The aim of the paper is to present a new integrated microstrip diplexer applicable in radio over fiber (RoF) links. That integrated microstrip diplexer has a much simpler construction with better characteristics compared to the widely used ones. The attenuation of the diplexer is significantly reduced by integrating the divider with the filters without using isolators between them. Two diplexer samples were designed, constructed and tested. One diplexer is separating (or combining) signals with frequencies of 1,3 GHz and 2,4 GHz. The frequencies of the other diplexer are 2,4 GHz and 2,6 GHz. Due to the integrated construction the size and the attenuation of the diplexers are much smaller compared to the well known, published approaches.

Experimental demonstration of transmitting LTE over FSO for in-building POF networks

In this paper, we propose a new scheme for the integration between the outdoor and indoor 4G long term evolution networks employing the radio-over free space optics technology for the rural environments applications to reduce the deployment cost. We experimentally characterise the system performance under the atmospheric fading effects including thick fog and a moderate turbulence regime considering 8% error-vector-magnitude as the figure of merit.