The fifth generation (5G) is expected to offer higher capacity, offer more access points,be lower cost than current wireless communication system. The multi-input and multi-output (MIMO) radio over fiber system (RoF) with orthogonal frequency division multiplexing using offset quadrature amplitude modulation (OFDM/OQAM) has attracted more and more attentions, for its high spectrum efficiency (SE), high capacity and cost effective. To transmit multi-channel signals, multi-dimensional multiplexing methods have been investigated, including wavelength division multiplexing(WDM), polarization division multiplexing(PDM) and spatial division multiplexing(SDM). In the SDM using mode division multiplexing(MDM), the higher mode crosstalk is an issue when demodulating the MIMO signals. Meanwhile, the channel time delay caused by differential mode group delay (DMGD) will induce crosstalk to the OFDM/OQAM signals which are without use of circle prefix(CP). As a result, in the MDM transmission system, both the mode crosstalk and DMGD should be concerned for a better transmission performance.

Prof. Songnian Fu, Prof. Ming Tang, Prof. Lei Deng and Ph.D student Qi Mo, Ph. D student Jiale He with the Wuhan National Lab for Optoelectronics, Huazhong University of Science and Technology investigated and fabricated a 2km elliptical core few-mode fiber. This fiber has a cladding diameter of 125μm, with horizontal and vertical radius of core 8μm and 6.2μm, respectively, guiding three modes (LP01, LP11a and LP11b). The attenuation of LP01 mode and LP11a mode is 0.23dB/km and 0.27dB/km, respectively. Moreover, the DMGD between LP01 and LP11a mode is 12.88ns. To reduce the crosstalk of OFDM/OQAM signal induced by the channel time domain, which is consist of DMGD and physical link difference, the time-offset crosstalk cancellation (TCC) algorithm is also proposed. This algorithm can expand the tolerance of channel delay to 60ns for OFDM/OQAM signals without use of CP. The system also realizes the radio over elliptical core few mode fiber transmissions with data rate of 4.46Gb/s under 600MHz wireless bandwidth. However, for the CP-OFDM under the same situation, 25% CP is need to mitigate the influence of channel delay. For OFDM/OQAM signals, this algorithm has the 18.7% SE improvement.

Figure 1. Experimental setup of a MIMO OFDM/OQAM radio over fibers.

Figure 2. Experimental results

This work not only demonstrate the feasibility of space division multiplexing using elliptical core few-mode fiber for RoF application but also validate the propose TCC algorithm can effectively mitigate the time offset induce crosstalk for OFDM/OQAM signals. Besides, this work also plays a reference role in the analyzing future 5G mobile communication

This work is supported by the National “863” Program (No. 2015AA016904) and National Natural Science Foundation of China (NSFC) (No.61675083, No.61307091, No.61275069 and No.61331010). This paper is published in Optical Letters (Vol. 41, No. 19, pp. 4546-4549, 2016).