Implementation of a Multi-Gbit / s and GFDM-based Optical-Wireless 5 G Network

We report the implementation of an optical-wireless 5G network based on generalized frequency division multiplexing (GFDM) and multi-Gbit/s communication. Dual-drive MachZehnder modulator was employed, enabling simultaneously RF signals transport using two 5G candidate bands, namely: 26 GHz band for providing a femtocell with 2 Gbit/s throughput; 700 MHz band for enabling rural access applying a supercell. A vector signal generator provides the broadband 26 GHz signal. The Brazilian GFDM-based 5G transceiver generates the lower-frequency signal, with the advantage of low out-of-band emission. An experimental digital performance analysis illustrates the suitability of the proposed solution to address 5G requirements.


I. INTRODUCTION
The fifth-generation (5G) of wireless communications is predicted to provide novel services and address diverse challenging requirements, such as extremely high throughput for enhanced mobile broadband (emBB) applications, ultra-reliable low latency (URLL) communications, massive machine-type communication (mMTC) and long range for the remote area access [1], [2].Basically, multi-Gbit/s individual user experience, billions of devices and connections due to growth of Internet of things (IoT), as well as reliable and broadband mobile access in rural areas were envisioned.
Therefore, a flexible physical layer must support multi-service 5G networks, convergence of different technologies and innovative modulation schemes [3]- [5].
Multiple waveforms have been presented in the literature and the generalized frequency division multiplexing (GFDM) has been shown as a promising 5G solution in terms of out-of-band (OOB) emission and complexity [6][7][8].Essentially, it concerns a non-orthogonal waveform that uses subcarriers and subsymbols for transmitting M-ary quadrature amplitude modulation symbols (M-QAM).Another key advantage from GFDM is the flexibility, since this waveform might be modified to cover the fourth-generation (4G) technology [8].It means GFDM is backwards-compatible with the Implementation of a Multi-Gbit/s and GFDMbased Optical-Wireless 5G Network R. M. Borges The GFDM waveform is the core of the 5G transceiver.The innovative modulation scheme uses a non-orthogonal waveform that transmits N = MK QAM symbols per block, using M subsymbols per subcarrier and K subcarriers.Therefore, the GFDM symbol is defined as where dk,m is the QAM symbol transmitted at the k th subcarrier and m th subsymbol, g[n] is the transmit prototype filter and 〈(⋅)〉N is the modulo N operator [6][7][8].The advantage of low OOB emission was one of the main reasons for applying GFDM, since this feature is essential for allowing the coexistence among mobile networks operating with different technologies.Furthermore, the capability of covering LTE waveforms as corner cases makes GFDM even more attractive.For  For short, DPD identifies the EA transfer function and imposes an opposite response, aiming the output linear behavior.
In the receiver side, an automatic gain control block normalizes the input level, and after synchronization and channel estimation, the transmitter reverse process is carried out [17].The GFDM demodulator performs the QAM demapping and delivers the received bits to the channel decoder, which provides only the relevant information to the Ethernet interface.It is worth mentioning that the prototype can operate in burst data transfer mode (BDTM), in which the RF signal is available only when there is useful information to be transmitted, or in continuous data transfer mode (CDTM), when the transmitting antennas are fed all the time.The 2x2 MIMO (multiple input multiple output) feature is also available and the transceiver parameters can be set according to channel conditions or application requirements, including transmission power, number of subcarriers and subsymbols, modulation order, frequency, bandwidth, code rates and periodicity of synchronization.

IV. MULTI-GBIT/S AND GFDM-BASED OPTICAL-WIRELESS 5G NETWORK
The proposed multi-Gbit/s and GFDM-based optical-wireless 5G network concept is represented in Fig. 5, which relies on the use of a dual-drive Mach-Zehnder modulator (DD-MZM) for simultaneously modulating an optical carrier (1560 nm) with the sub-6 GHz and mm-wave signals from the 5G network.The Brazilian transceiver generates the first RF-driven signal (RF1), corresponding to a 0 dBm GFDM signal at 734 MHz with DPD on and a commercial vector signal generator (VSG) provides a 2 dBm 26 GHz signal, giving rise to the second RF-driven signal (RF2).
The main advantage of using DD-MZM is to mitigate interferences between RF1 and RF2 that come from the modulation process nonlinearity, since the distinct RF signals are independently modulated 585 at its upper and bottom arms, with distinct and optimized bias voltages.
The two RF signals are recovered by using a broadband photodetector (XPDV2150RA) and then separated by a diplexer, after being distributed by a 12.5 km optical link.Subsequently, electrical amplifiers are applied for both bands in order to increase the power to be radiated by the antennas, namely: 9 dBi gain Yagi-Uda and panel antennas for the transmission and reception in the lowerband, respectively; 25 and 13 dBi gain horn antennas for the transmission and reception in the higherband, respectively.In this way, a rural supercell has been implemented using the 734 MHz signal, whereas a high throughput indoor femtocell has been realized in the 26 GHz band.Fig. 6 displays photographs of the OWN implementation, including indoor and rural outdoor environments.A vector signal analyzer (VSA) and a second GFDM transceiver were used for evaluating the received RF signals.V. CONCLUSIONS We have successfully implemented and experimentally evaluated a multi-Gbit/s and GFDM-based optical-wireless 5G network, capable to provide a supercell at 734 MHz for rural applications and an instance, GFDM can cover orthogonal frequency division multiplexing (OFDM) by making M = 1, K > 1 and g[n] as a rectangular pulse.Fig. 2(a) presents GFDM and OFDM spectra in a case where central subcarriers are switched off.One can note the GFDM OOB emission was approximately 25 dB below that generated by OFDM.

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Higher power levels, e.g.0 dBm, lead to an SNR reduction (inset iii) and additional MER degradation, which comes from the EA noise figure and nonlinear distortions.However, the DPD use has reduced the nonlinearities and enabled reaching SNR = 44.5 dB and MER = 38.2dB for PRF = 0 dBm, as shown in Fig.4(c).The 256-QAM constellation presents some omitted symbols because the pseudo-random sequence starts every two symbols and it is insufficient to form all binary combinations.Nevertheless, this does not hamper the digital performance analysis, since the missing symbols are not accounted for the MER estimation.

Fig. 4 .
Fig. 4. Digital performance analysis at 100 Mbit/s: (a) MER as a function of RF power without using EA; (b) MER as a function of RF power by using a 30 dB EA at PD output; (c) Constellation and received electrical spectrum for PRFout = 0 dBm with DPD on.The insets i, ii, and iv are constellations, whereas the inset iii refers to an electrical spectrum.

Fig. 8 .
Fig. 8. Analysis of the 734 MHz GFDM signal obtained at a high school in a rural area with 7.51 km range: (a) Constellation; (b) Performance parameters.

TABLE I .
SUMMARY OF THE MEASURED PERFORMANCE PARAMETERS AT THE HIGH SCHOOL Distance = 7.51 km Rx Level = -55.76dBm (outdoor)