1. Introduction

With the explosive development of streaming and cloud applications, the data-intensive communication service raises stricter requirements for the access network [1–3]. Integrating the advantages of both the validated high capacity of fiber transmission and the wide-range coverage of wireless transmission, radio-over-fiber (RoF) systems have been drawing attention as a candidate access technology for future mobile communication [4–6]. Meanwhile, abundant available bandwidth in the millimeter-wave (mm-wave) band has a high potential to achieve transmission rates beyond gigabits per second (Gb/s) in RoF systems [7–11]. To further enhance the capacity and spectral efficiency of mm-wave RoF systems, research on applying high-order quadrature amplitude-modulation (QAM) formats in the mm-wave signal generation has been widely studied [12–15]. Nevertheless, the increase of modulation orders also results in the decline of the tolerance to the interference, such as noise and non-linear effects. This is mainly due to the decreased Euclidean distance between signal constellation points [16]. Delta-sigma modulation (DSM) has recently been regarded as an integral solution to convert complicated analog signals into simple digital pulse trains. Through the integration of over-sampling and noise shaping, DSM can optimize the in-band signal-to-noise ratio (SNR) of the transmitted signal and help generate the high-order QAM mm-wave signal more compatible with tolerance-limited transmission systems [17–22].

In addition, orthogonal-frequency-division-multiplexing (OFDM) technology has been widely employed in mm-wave RoF systems due to its high spectral efficiency, resistance to multipath effects, and resilience to interference [23–26]. Integrating OFDM modulation and high-order QAM signals ensures a higher data rate and transmission capacity. Therefore, the wireless mm-wave transmission of OFDM-modulated QAM signals can effectively enhance the system performance of RoF systems [27,28]. However, the fly in the ointment is the inherent high peak-to-average power ratio (PAPR) of OFDM-modulated signals [29]. For the transmitted OFDM-modulated signals in the RoF systems, the excessive PAPR cannot be ignored. On the other hand, Single-carrier (SC) modulation is adopted as a candidate scheme in RoF systems to achieve a lower PAPR [30,31]. To a certain degree, simultaneous wireless mm-wave transmission of both SC-modulated and OFDM-modulated signals can reduce the overall PAPR of the RoF system and simplify the system structure, especially in digital signal processing (DSP). In a word, the simultaneous wireless mm-wave transmission of SC-modulated and OFDM-modulated high-order QAM signals can not only enhance multi-modulation access flexibility but also further increase the spectrum efficiency and transmission capacity while optimizing the structure of the RoF system.

In this letter, we propose a novel scheme for the simultaneous wireless mm-wave transmission of both SC-modulated and OFDM-modulated high-order QAM signals enabled by bandpass delta-sigma modulation (BP-DSM) and heterodyne detection. At the transmitter side, both transmitted high-order QAM signals are converted into on-off keying (OOK) signals by two parallel BP-DSMs. Then, the two OOK signals are combined in an optical in-phase/quadrature (I/Q) modulator to generate the desired optical quadrature-phase-shift-keying (QPSK) mm-wave signals, containing complete information of both transmitted high-order QAM signals. Thus, the application of BP-DSM imparts a modulation-agnostic attribute to the simultaneous wireless transmission of both SC-modulated and OFDM-modulated high-order QAM signals. The heterodyne detection based on a single photodetector (PD) is utilized to detect the generated QPSK mm-wave signals. The design of a single photodetector and the subsequent DSP chain also simplifies the complexity of the system without degrading the system performance [8]. To verify the superiority of our proposed scheme, we experimentally demonstrate the simultaneous wireless mm-wave transmission of both SC-modulated 256/512 QAM and OFDM-modulated 512QAM signals at 30-GHz mm-wave band. After transmission over 10-km single-mode fiber (SMF) link and 1-m wireless link, the bit-error rates (BERs) of both SC-modulated and OFDM-modulated high-order QAM signals can reach the hard decision forward error correction (HD-FEC) threshold of 3.8 × 10⁻³, and both signals can be successfully demodulated and recovered. In addition, the performance impact between SC-modulated and OFDM-modulated high-order QAM signals has been proven to be virtually negligible in our proposed scheme.

2. Principle

Based on our proposed scheme, the principle of the simultaneous wireless mm-wave transmission of both SC-modulated and OFDM-modulated high-order QAM signals enabled by DSM is illustrated in Fig. 1. Both SC-modulated and OFDM-modulated high-order QAM signals are generated via software programming at the transmitter side, represented as QAM1 and QAM2, respectively. The modulation orders of the QAM1 and QAM2 signals can be the same or different. The schematic spectrum of the QAM1 signal is illustrated in Fig. 1(a). Subsequently, both QAM1 and QAM2 signals are concurrently fed into two parallel BP-DSMs. Each BP DSM comprises an interpolation filter, a digital upconverter, and a 1-bit discrete-time BP DSM for the united application of oversampling and noise shaping [32]. As illustrated in Fig. 1(b), the interpolation filter executes an oversampling operation to broaden the Nyquist zone and therefore reduce the in-band quantization noise. It also performs an anti-image filtering operation to suppress images caused by oversampling. The interpolation filter's oversampling ratio (OSR) is set to match the OSR of the subsequent discrete-time BP DSM. For the QAM1 signal, the digital upconverter facilitates up-conversion from the baseband to f_s, and the schematic spectrum of the upconverter output signal with a frequency of f_s is illustrated in Fig. 1(c). The up-conversion with the same or different frequency is applicable equally to the QAM2 signal. Then, the parallel discrete-time BP DSMs simultaneously implement the 1-bit quantization and convert both QAM1 and QAM2 signals into the corresponding digital sequences with levels of +1 and −1 exclusively, constituting two independent OOK signals. The schematic spectrum of the signals after BP-DSM is illustrated in Fig. 1(d). The noise shaping enabled by BP-DSMs implements a continual compression and displacement of in-band noise towards higher frequency, and thereby the power spectrum of the noise is shaped while maintaining constant signal power. The two generated OOK signals, represented as OOK1 and OOK2 signals, are subsequently fed into a digital-to-analog converter (DAC). With the aid of an optical I/Q modulator, two analog OOK signals modulate the optical continuous-wavelength (CW) lightwave, emanating from a laser operating at a frequency of f_c, to generate an optical QPSK signal encompassing information of both QAM1 and QAM2 signals.

 

Fig. 1. Principle of the simultaneous wireless mm-wave transmission of both SC-modulated and OFDM-modulated high-order QAM signals enabled by the BP-DSM and the heterodyne detection. (a) The schematic spectrum of the QAM1 signal; (b) The schematic spectrum of the interpolation filter output signal; (c) The schematic spectrum of the upconverter output signal with a frequency of f_s; (d) The schematic spectrum of the signals after 1-bit BP-DSM; (e) The schematic spectrum of the BPF input signals; (f) The schematic spectrum of the downconverter output signal; (g) The schematic spectrum of the BPF output signals.

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Subsequently, the heterodyne detection is utilized to convert the generated optical QPSK signal into an electrical QPSK-modulated mm-wave signal. With the aid of an optical coupler (OC), the optical QPSK signal is first coupled with an optical local oscillator (LO) signal, emanated from another laser operating at a frequency of f_LO, to generate an optical QPSK-modulated mm-wave signal with a carrier frequency of f_c-f_LO. The generated optical mm-wave signal is then fed into a PD to implement the optical-to-electrical conversion and generate an electrical QPSK-modulated mm-wave signal. We are able to freely adjust the frequency of the generated electrical mm-wave carrier by setting the frequency spacing between the optical CW lightwave and the optical LO signal. At the receiver side, the generated electrical QPSK-modulated mm-wave signal is then fed into an analog-to-digital converter (ADC). After down-conversion and QPSK demodulation, the demodulated OOK1 and OOK2 signals, as illustrated in Fig. 1(e), are fed into two parallel digital bandpass filters (BPFs). Each BPF comprises a digital downconverter and a decimation filter for the united application of down-conversion and filtering operation [32]. The digital downconverter implements a down-conversion operation of OOK1 signals from f_s to the baseband. The schematic spectrum of the received baseband signal is illustrated in Fig. 1(f). The down-conversion is applicable equally to the OOK2 signal upconverted at the transmitter side. As illustrated in Fig. 1(g), the decimation filter in each BPF, functioning as a low-pass filter (LPF), effectively suppresses the shaped out-of-band noise while preserving the original transmitted signals. Consequently, both SC-modulated and OFDM-modulated high-order QAM signals can be successfully recovered after two parallel BPFs.

3. Experimental setup

Figure 2 illustrates the experimental setup of our proposed scheme for the simultaneous wireless mm-wave transmission of both SC-modulated and OFDM-modulated high-order QAM signals. The generation and recovery processes of SC-modulated and OFDM-modulated high-order QAM signals are both implemented via software programming. The dashed areas show the DSP at the transmitter and receiver sides, respectively. At the transmitter side, two independent pseudo-random binary sequences (PRBSs) are individually QAM mapped and SC/OFDM modulated, and thereby an SC-modulated 256/512 QAM signal and an OFDM-modulated 512QAM signal are generated. The generated high-order QAM signals are then up-sampled by a factor of two before passing through an RC filter with a roll-off factor of 1. Then, the generated signals are utilized as input signals of two parallel BP-DSMs. In each BP-DSM, the interpolation filter executes an oversampling operation with an OSR of 10. The upconverters implement the up-conversion of both high-order QAM signals from baseband to 25GHz to obtain the maximum quantization SNR [33]. The generated signals are then quantized by 4^th-order 1-bit BP-DSM for the noise shaping. As the principle states, the BP-DSMs implement a constant push from in-band noise to out-of-band noise, eventually converting both high-order signals to the corresponding 10-Gbaud OOK signals. The calculated spectrum of the OFDM-modulated signals after BP-DSM is illustrated in Fig. 2(a). Subsequently, two generated OOK signals are uploaded into an arbitrary waveform generator (AWG) operating at 100 GSa/s sampling rate with a bandwidth of 35GHz. The output signals of AWG are simultaneously amplified by two parallel electrical amplifiers (EAs) with a 23dB gain before driving an optical I/Q modulator. As the optical input signal of the optical I/Q modulator, the CW lightwave from an external cavity laser (ECL) has a central wavelength of 1550 nm and a narrow linewidth less than 100 kHz. The optical I/Q modulator generates the desired optical QPSK signal encompassing information of both high-order QAM signals. The measured optical spectrum of the generated QPSK signal in 0.02-nm resolution is illustrated in Fig. 2(b). Then, the optical QPSK signal is amplified by an erbium-doped fiber amplifier (EDFA) and coupled with an optical LO signal from ECL2 by an OC. As illustrated in Fig. 2(c), the frequency spacing between the optical CW lightwave and the optical LO signal is 30GHz, and thereby a 30-GHz optical QPSK-modulated mm-wave signal is generated.

 

Fig. 2. The experimental setup of dual high-order QAM vector mm-wave signal generation and detection. (a) The calculated spectrum of the OFDM-modulated signals after BP-DSM; (b) The measured optical spectrum of the optical I/Q modulator output signal; (c) The measured optical spectrum of the OC output signal; (d) The calculated spectrum of the received signal in OSC after transmission over 10-km SMF link and 1-m wireless link.

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After transmission over 10-km SMF link, the optical QPSK-modulated mm-wave signal is fed into a variable optical attenuator (VOA) to control the input power into PD. A single PD with a bandwidth of 50GHz enables the optical-to-electrical conversion and generates a 30-GHz electrical QPSK-modulated mm-wave signal. After amplified by an EA with a gain of 23dB, the signal is fed into a pair of horn antennas (HAs) for transmission over 1-m wireless link. Each HA has a gain of 25 dBi and an operating frequency range from 22 GHz to 50 GHz. After transmission over 1-m wireless link, the received signal is amplified by another low noise EA with a gain of 23dB and eventually captured by a digital storage oscilloscope (OSC) operating at 120 GSa/s sampling rate with an electrical bandwidth of 50GHz. The calculated spectrum of the received signal in OSC after transmission over 10-km SMF link and 1-m wireless link is shown in Fig. 2(d). With the aid of a common DSP chain, both SC-modulated 256/512 QAM signals and OFDM-modulated 512QAM signals can be demodulated and recovered from the captured electrical QPSK-modulated mm-wave signal. The offline DSP at the receiver side includes down-conversion, resampling, the Gram–Schmidt orthogonalization procedure (GSOP) and retiming, constant modulus algorithm (CMA) equalization, carrier recovery including frequency offset estimation (FOE) and carrier phase estimation (CPE), QPSK hard-decision, BPF operation, down-sampling, OFDM demodulation for the OFDM-modulated signal, QAM demapping, and BER calculation. After the QPSK hard decision, two recovered OOK signals are then fed into two parallel BPFs. Subsequently, two parallel BPFs implement the down-conversion of obtained OOK signals to baseband signals and further eliminate the out-of-band noise, as the principle states. Then, the SC-modulated 256/512QAM signals are QAM demapped, while the OFDM-modulated 512QAM signal requires an extra OFDM demodulation. Both original high-order QAM signals can be successfully demodulated and recovered. Ultimately, the BER is calculated to evaluate the transmission performance. For both SC-modulated and OFDM-modulated high-order QAM signals, 2048 symbols are transmitted and the average BER value of multiple calculations ensures the validity of our experiment results.

4. Experimental results

Figure 3 illustrates the measured BER versus the input optical power into PD for both SC-modulated 512QAM and OFDM-modulated 512QAM signals over different transmission scenarios, including (a) no fiber link and no wireless link (BTB), (b) only 1-m wireless link, as well as (c) 10-km SMF link and 1-m wireless link. With increased input power into PD, all BER curves have a downward trend and can reach the HD-FEC threshold of 3.8 × 10⁻³ for all three scenarios. In the BTB scenario, the obtained QPSK signal can implement error-free transmission when the input power into PD is larger than #x2212;12dBm, and both SC-modulated 512QAM and OFDM-modulated 512QAM signals have best BERs less than the HD-FEC threshold when the input power into PD is larger than #x2212;13dBm. There is almost no performance difference between scenario (b) and scenario (c), which indicates that 10-km SMF link causes a negligible power penalty. At the HD-FEC threshold of 3.8 × 10⁻³, for the obtained QPSK signal, the wireless link and 10-km SMF link introduce a total power penalty of 2.1dB compared to the BTB scenario. Meanwhile, the wireless link and 10-km SMF link introduce a total power penalty of 1.9dB for the SC-modulated 512QAM signal, while the wireless link and 10-km SMF link introduce a total power penalty of 1.5dB for the OFDM-modulated 512QAM signal. After transmission over 10-km SMF link and 1-m wireless link, the BER curves of both SC-modulated 512QAM and OFDM-modulated 512QAM signals become flat when the input power into PD is larger than #x2212;10dBm and the best BERs are 1.8 × 10⁻³ and 4.2 × 10⁻⁴, respectively. The BER of the obtained QPSK signal has already reached 0 when the input power into PD is larger than #x2212;10dBm. The reason that prevents further enhancement of both SC-modulated 512QAM and OFDM-modulated 512QAM signals is the residual in-band noise that cannot be completely eliminated, as shown in Fig. 1(g).

 

Fig. 3. The measured BER versus input optical power into PD for both SC-modulated 51QAM and OFDM-modulated 512 QAM signals over different transmission scenarios.

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In addition, to further investigate the interplay between SC-modulated and OFDM-modulated high-order QAM signals in our proposed scheme, we also conduct a comparative experiment to change the modulation order of the SC-modulated signal from 512 to 256. Figure 4 illustrates the BER difference after transmission over 10-km SMF link and 1-m wireless link when converting the SC-modulated signal from 512QAM to 256QAM while the OFDM-modulated signal remains employing 512QAM modulation. The overall tendency of BER performance curves is consistent with the curves in Fig. 3. It can be seen that the performance of the QPSK signal and the OFDM-modulated 512QAM signal almost remain invariable when converting the SC-modulated signal from 512QAM to 256QAM. When the input power into PD is larger than #x2212;11dBm and #x2212;10 dBm, the QPSK signal carrying information from both SC-modulated 256QAM and OFDM-modulated 512QAM signals, as well as the QPSK signal carrying information from both SC-modulated 512QAM and OFDM-modulated 512QAM signals can implement error-free transmission over 10-km SMF link and 1-m wireless link, respectively. Nevertheless, the SC-modulated 256QAM signal has a better BER than the SC-modulated 512QAM signal at all input powers. At the HD-FEC threshold of 3.8 × 10⁻³, the SC-modulated 256QAM signal has a receiver sensitivity gain of 0.6dB compared to the SC-modulated 512QAM signal. Figures 4(a) and 4(b) show the best-recovered constellation of both SC-modulated 256QAM and SC-modulated 512QAM signals with BERs of 1.8 × 10⁻³ and 4.2 × 10⁻⁴, respectively. This is due to the different tolerance of different-order QAM signals to the in-band noise. The results indicate that SC-modulated and OFDM-modulated high-order QAM signals are mutually independent in our proposed scheme, and the BER performance of the QPSK signal is transparent to the QAM modulation formats of both SC-modulated and OFDM-modulated signals.

 

Fig. 4. The measured BER versus input optical power into PD for different SC-modulated high-order QAM signals after transmission over 1-m wireless link and 10-km SMF link. (a) The best-recovered SC-modulated 256QAM constellation at #x2212;8dBm after transmission over 10-km SMF link and 1-m wireless link; (b) The best-recovered SC-modulated 512QAM constellation at #x2212;8dBm after transmission over 10-km SMF link and 1-m wireless link.

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5. Conclusions

In this paper, we validate the feasibility of the simultaneous wireless mm-wave transmission of both SC-modulated and OFDM-modulated high-order QAM signals enabled by BP-DSM. Based on our proposed scheme, we experimentally demonstrate the simultaneous wireless mm-wave transmission of both SC-modulated 256/512QAM and OFDM-modulated 512QAM signals at 30-GHz mm-wave band, achieving BERs below the HD-FEC threshold of 3.8 × 10⁻³ after transmission over 10-km SMF link and 1-m wireless link. In addition, we further investigate the performance interplay between SC-modulated and OFDM-modulated high-order QAM signals. The experiment result indicates that 1) the impact between dual vector signals with hybrid SC/OFDM modulation is virtually negligible; 2) the performance of the generated QPSK mm-wave signal is transparent to the QAM modulation formats of both SC-modulated and OFDM-modulated signals in our proposed scheme.