We demonstrate the improvement of the transmission performance based on tone-reservation technique in a multiple intermediate-frequency-over-fiber (IFoF) based mobile fronthaul. The tone-reservation technique can suppress nonlinear distortion by eliminating the high peak components of orthogonal frequency-division multiplexing (OFDM) signal. To prevent the regrowth of peak, we employ tone-reservation after multiplexing IF carriers. Furthermore, we use an out-of-band signal as the reserved tones to avoid any modification of a mobile signal. The impact of the number of IF carriers on peak-to-average power ratio (PAPR) characteristics is presented via numerical simulation. For the multi-IFoF based mobile fronthaul, we experimentally investigate the transmission performance of 36-IF carriers of the long term evolution-advanced (LTE-A) signals mapped with 64-quadrature amplitude modulation (QAM). It is clearly observed that the clipping-induced nonlinear distortion is dramatically suppressed by using tone-reservation. As a result, the transmission performance of 36-IF carriers of the LTE-A signal is improved by an error-vector-magnitude (EVM) of 4% (from 9.7% to 5.7%) after 20-km transmission.
© 2015 Optical Society of America
As an attractive next-generation mobile access network, a centralized radio access network (C-RAN) has been developed owing to its superior efficiency in both energy and cost aspects [1, 2]. In the C-RAN, the networks between a digital unit (DU) and a radio unit (RU), which are mobile fronthaul, primarily employ a digital optical communication method based on interfacing technologies such as a common public radio interface (CPRI) . In order to increase cell throughput, many operators are trying to use carrier aggregation, massive multiple-input multiple-output (MIMO), and coordinated multipoint transmission (CoMP). These capacity increase results in transport limitation in current mobile fronthaul link. For example, the required line data-rate for digital transmission is around 45 Gb/s to support long term evolution-advanced (LTE-A) service exploiting 3 carrier aggregations (CA), 3 sectors, and 4x4 MIMO system . Up to recent days, there have been substantial efforts to resolve this issue such as CPRI compression, radio over Ethernet, free-space optical transmission, and analog radio-over-fiber (RoF) [5–8]. Analog RoF based on multiple intermediate frequency over fiber (IFoF) could reduce transport capacity requirement in fronthaul link significantly . The each mobile baseband signal is mapped on to a single IF carrier, and multiplexed in the frequency domain as shown in Fig. 1. The multiple numbers of mobile baseband signals are transmitted through the optical fiber with analog waveform. The technical feasibilities of multi-IFoF system was examined by simulations and experiments [8–10].
LTE-A systems employ orthogonal frequency-division multiplexing (OFDM) as the downlink signal format. The OFDM signal format has a high peak-to-average power ratio (PAPR) characteristic owing to the superposition of many individual subcarriers . A high PAPR leads to a severe degradation of the system performance. To reduce PAPR, a number of approaches, such as selective mapping (SLM), partial transmit sequences (PTS), and discrete Fourier transform spread OFDM (DFT-spread OFDM), were proposed [11–13]. These techniques present a superior nonlinear performance not only in wireless communication but also in optical communication systems [14, 15]. In multi-IFoF systems, the reduction of PAPR should be also performed because the transmission performance can be limited by inherent nonlinearities in a directly-modulated laser (DML) where a clipping phenomenon occurs. This phenomenon introduces nonlinear distortion when a signal approaches the threshold current. These clipping distortions cause intermodulation products, which result in inter-carrier interference, a high out-of-band harmonic distortion power, and the degradation of the performance of bit-error-rate (BER). However, the several PAPR reduction techniques reported previously cannot provide a significant improvement of performance for multi-IFoF systems because multiplexing intermediate frequency (IF) carriers results in the regrowth of peak. In other words, although the PAPR reduction is performed to achieve a low PAPR characteristic, the PAPR of OFDM is increased again owing to the superposition of multiple IF carriers. This superposition makes the amplitude of the OFDM signals have a Gaussian distribution . As a result, the transmission performance in multi-IFoF systems is limited by the clipping distortion.
Previously, many PAPR reduction techniques to support RoF systems were studied [17–19]. Among them, a multi-dimensional crest factor reduction was proposed to support multi-band RoF systems . They reduce a high-peak signal by using a clip-and-filter technique. By using the estimated peak of the total power of multiple bands, peak clipping is applied to each band individually with the reduction of out-of-band power by filtering . However, clipping causes in-band distortion, which reduces the overall transmission performance, and out-of-band radiation, which leads to the regrowth of peak owing to filtering .
In this study, we improve the transmission performance in the multi-IFoF system using a tone-reservation technique. The tone-reservation technique provides a significant improvement of the transmission performance by cancelling the peaks causing nonlinear distortions without any side information and without the distortion of the transmitted data symbols . To prevent the regrowth of peak, we investigate and design the technique of the PAPR reduction after the frequency up-conversion and the multiplexing procedure of IF carriers using the tone-reservation technique for supporting the multi-IFoF based mobile fronthaul. Furthermore, we use an out-of-band data signal instead of utilizing the unoccupied subcarrier of the OFDM signal as the reserved tones to implement mobile fronthaul without any modification of a mobile signal (i.e., the LTE-A signal). The impact of the number of IF carriers on the PAPR characteristic is examined by using numerical simulation. We also experimentally evaluate the advantages of the tone-reservation technique in the multi-IFoF system. The results of the error-vector-magnitude (EVM) measurements show that the tone-reservation technique dramatically improves the measured EVM by 4% (9.7% to 5.7%) over standard single-mode fiber (SSMF) of 20 km.
2. Principle of tone-reservation in multi-IFoF mobile fronthaul
2.1 Mobile fronthaul based on multi-IFoF technology
Figure 2 depicts the detailed configuration of the multi-IFoF system for mobile fronthaul. At DU, the modulated baseband LTE signals are digitally mapped onto each IF by digital IQ modulation . The multi-IF carrier signal is converted into the analog signal by a digital-to-analog converter (DAC). After that, a directly modulated laser diode converts the electrical signal to the optical signal and transmits the signal to RU through an optical fiber link. At RU, optical-to-electrical conversion is performed by using a photodetector (PD), and then the signal is up-converted onto the desired RF region for air transmission. The nonlinearities due to a high PAPR occur in a directly modulated laser diode, which is not operated linearly when the peak signal approaches the threshold current. Keeping under its well-behaved operating conditions is important for the practical implementations of the next-generation mobile fronthaul based on the multi-IFoF technology.
2.2 PAPR characteristics
Next, we investigated the impact of the number of IF carriers on the PAPR characteristic of the various OFDM systems. Figure 3 shows the complementary cumulative distribution function (CCDF) of PAPR for OFDM systems with an employed DFT-spread scheme as the PAPR reduction technique. Previously, it was reported that the poor PAPR characteristic of OFDM systems arose from the noise-like characteristics of the OFDM signal waveforms. Therefore, its amplitudes can be well-approximated by a Gaussian function. On the other hand, the instantaneous power of the OFDM signal after the PAPR reduction does not have a Gaussian distribution providing a lower PAPR characteristic . Because the multiplexing procedure of multiple IF carriers is similar to the superposition process of multiple subcarriers in a conventional OFDM system, the OFDM signal with the PAPR reduction technique has different CCDF characteristics depending on the number of IF carriers. Therefore, its PAPR characteristics regrow eventually, as shown in Fig. 3(b). In other words, the conventional PAPR reduction techniques such as DFT-spread scheme could not be applied in multi-IFoF system directly, and PAPR reduction technique performed after the multiplexing procedure of IF carriers is required. Otherwise, the high-peak components of the OFDM signal experienced strong clipping-induced nonlinear distortions when we operated the laser diode near the regime of threshold current.
2.3 Operating principle of tone-reservation technique in multi-IFoF system
Figure 4 shows the conceptual diagram of the proposed tone-reservation technique in the multi-IFoF system. Tone-reservation exploits an unused frequency range as the reserved tones to generate a peak-cancelling signal . This approach can reduce overall PAPR of the OFDM signals without any additional distortions of the effective data [25, 26]. Figure 4(a) shows the operating principle of the generation of the peak-cancelling signal based on tone-reservation. Iterative clipping and filtering under the tone-reservation constraints are employed to generate the peak-cancelling signal efficiently. During each iteration, first, the time-domain OFDM signal is clipped to a predefined threshold digitally. Next, the clipped signal is filtered to remove the clipping noise on the data signal bands. Therefore, the peak-cancelling signal is simply the clipping noise filtered on the data signal bands, which makes the tone-reservation have no in-band distortions, unlike for the clip-and-filter scheme . Finally, the generated peak-cancelling signal is combined with the original signal to remove the high peaks of the OFDM signal. Unlike a conventional tone-reservation technique, which exploits unused subcarriers of a baseband OFDM signal , we use out of the signal band for the reserved tones, as illustrated in Fig. 4(d). Therefore, it does not require an additional modification of a mobile signal (i.e., the LTE-A signal). Since the data bands and the reserved tones are located at mutually exclusive bands, the receiver can simply ignore non-data-bearing tones (reserved) using a filter after the frequency conversion onto the desired RF region. As a result, there is no observable degradation of performance from distortions to data-bands. Figures 4(e) and 4(f) show the pulse shapes of the OFDM signal with/without the tone-reservation technique. Contrary to other PAPR reduction techniques, such as the DFT-spread, SLM, and PTS techniques, which eliminate all high peaks (upper-side peaks and lower-side peaks), we primarily focus on the elimination of lower-side peaks to mitigate the clipping-induced nonlinear distortion, which is a strong limitation factor of performance in multi-IFoF systems [8, 9]. Therefore, the pulse shape of the OFDM signal with tone-reservation has a form similar to that of the clipped signal, as illustrated in Fig. 4(f).
3. Experimental setup and results
To confirm the feasibility of the tone-reservation technique in multi-IFoF systems, we performed a series of experiments on the system depicted in Fig. 5. We used downlink transmission instead of uplink transmission since single carrier frequency division multiplexing access (SC-FDMA) is typically used for uplink due to its low PAPR. To conduct the transmission of 36-IF carriers of the LTE-A signals simultaneously, we generated the 36 LTE-A baseband signals mapped with 64-quadrature amplitude modulation (QAM), in which each signal had a bandwidth of 20 MHz. The 36 IF-carriers are corresponding to 3 CA, 3 sectors, and 4x4 MIMO system. The 36 independent LTE-A signals were generated based on LTE E-UTRA Test Model 3.1 defined in the technical specification of the 3rd generation partnership project (3GPP) . The 36-IF carriers of the LTE-A signals were up-converted digitally and multiplexed with a frequency spacing of 40 MHz through off-line digital signal processing (DSP). To generate peak-cancelling signal, we used 10 iterative clipping and filtering since EVM performance was almost similar after 10 iterations. To remove clipping noise on data signal band, an ideal rectangular filter was employed. A Keysight Technologies arbitrary waveform generator (AWG) 8190A was used for the generation of the multi-IF LTE-A RF signal. The DAC in the AWG was operated at 14-bit resolution and a sampling rate of 8 GS/s. RF attenuator and amplifier were employed to control the optical modulation index (OMI) of the optical multi-IF LTE-A signal. A commercially available directly-modulated laser diode module was used for electrical-to-optical conversion. The transmitted optical signal of DML had a power of + 3 dBm, which is launched into a 20-km SSMF. The received power at PIN-photodiode was set as −10 dBm by using a variable optical attenuator (VOA). A Rohde & Schwarz FSW vector signal analyzer (VSA) was employed to measure the transmission performance in terms of EVM.
First, we evaluated the transmission performance of the tone-reservation based multi-IFoF system under back-to-back (BTB) condition. Figure 6 shows the EVM performances as functions of OMI per channel (OMI/ch) for 36-IF carriers of the LTE-A signals. Among 36-IF carriers, we examined the first (1st), center (18th) and last (36th) channels. As shown in Fig. 6, the transmission performances at a low OMI/ch were limited by shot noise mainly whereas degradation of performance occurred at a high OMI/ch because of clipping distortion. The dynamic range satisfying an EVM of 8%, which is a minimum requirement for 64-QAM in 3GPP, was improved by 4.5% (from 11% to 15.5%) at the last channel owing to the tone-reservation technique . Although the tone-reservation technique was employed, performance improvement at an optimal OMI/ch of each carrier was not observed. This result occurred because of the fact that the tone-reservation technique reduced the signal power of data signal owing to the insertion of the reserved tones in the out-of-band LTE-A signals. However, it is important for the practical implementation of mobile fronthaul that flatness of the system performance resulting from the fluctuation of OMI/ch is maintained. Therefore, the tone-reservation technique can improve the dynamic range of the multi-IFoF system without the loss of data rate and the degradation of the performance of transmission.
As shown in Fig. 7, we also investigated the transmission performances of 36-IF carriers of the LTE-A signals as functions of channel index at an OMI/ch of 10% for BTB and 20-km transmission with their constellations diagram at 36th IF carrier. In back-to-back, it was observed that the EVM performances deteriorate as an increasing of IF carrier frequency. The measured EVM difference between IF carrier is more significant without tone-reservation than with tone-reservation. These are mostly caused by clipping-induced nonlinear distortion, which generates more crosstalk components in high frequency band. The non-flat frequency response of transmitter generates additional EVM degradation . After 20-km transmission through SSMF, degradation of performance was observed for both systems. This is because interaction between DML-induced chirp and dispersion of fiber increases the intermixing noise at high frequency band [31, 32]. The phase noise due to direct modulation of laser is changed to amplitude noise by dispersion as the signal transmitted through fiber, which generates intermixing noise after direct-detection. As a result, the LTE-A signals at 24th to 36th channels cannot achieve an EVM of 8% if tone-reservation is not employed. On the other hand, the tone-reservation technique enables 36-IF carriers of the LTE-A signals to get an EVM of below 8% for entire IF channels. As shown in Figs. 7(b)–7(d), we obtained more clear constellations by using tone-reservation technique.
Next, we compared the RF spectra of the received LTE-A signals of 36th IF carrier with and without the tone-reservation techniques for BTB and 20-km transmission at an OMI/ch of 10%, as shown in Fig. 8, where the received RF powers are normalized to measure nonlinear distortion components. At both BTB and 20-km transmission, the tone-reservation technique suppresses 5 dB of in-band intermodulation products. On the other hand, the intermixing noise caused by dispersion is located in signal band both with and without tone-reservations while giving rise to performance degradation for both systems. Thus, the performance improvement is derived from the suppression of the nonlinear distortions induced by clipping phenomenon.
Finally, EVM performances of the LTE-A signal of 36th IF carrier as a function of the received optical power are evaluated for BTB and 20-km transmission at an OMI/ch of 10%, as illustrated in Fig. 9(a). Then their constellation diagrams are shown at a received power of -3 dBm in Fig. 9(b)–9(e). For BTB configuration, we obtain an EVM of below 8% both with tone- and without tone-reservation when the received optical power is higher than -14 dBm. Although the received optical power is increased up to -3 dBm, the LTE-A signal without tone-reservation cannot achieve EVM requirement. This is because its transmission performance is mainly dominated by the clipping-induced nonlinear distortions and the dispersion-induced intermixing noise in the optical link. As a result, it is observed that the constellation points of the LTE-A signals with tone-reservation are much clearer than those of the LTE-A signals without tone-reservation.
We demonstrated the improvement of transmission performance in the IFoF based mobile fronthaul by using the tone-reservation technique. We also showed that the reduction of PAPR in the baseband OFDM signal was not sufficient to suppress the nonlinear distortion of laser diode owing to the regrowth of PAPR after multiple-IF multiplexing. To reduce the clipping-induced nonlinear distortions in laser diode and to improve the transmission performance of the multi-IF based LTE-A signals, we used the tone-reservation technique after the frequency up-conversion and multiplexing IF carriers. We demonstrated experimentally that the EVM performance of 36-IF carriers of the LTE-A signal was improved by 4% after the transmission through 20-km SSMF by using tone-reservation. In addition, the EVM for entire 36-IF carriers with the LTE-A signal was maintained below 8%, which is a minimum requirement for the 64-QAM LTE-A signal in 3GPP. Thus, the tone-reservation technique could be used for enhancing the tolerance to the nonlinearities of laser diode in the multi-IFoF based mobile fronthaul.
This work was supported by the IT R&D programs of MSIP (Ministry of Science, ICT and Future Planning), Korea [15ZI1300, Development of compact radio & dense digital base station technologies based on RoF for mobile communication systems].
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