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Abstract
We report on a record spectral efficient terahertz communication system using a coherent radio-over-fiber (CRoF) approach. High spectral efficient back-to-back and wireless THz transmission around 325 GHz is experimentally demonstrated using a 64-QAM-OFDM modulation format and a 10 GHz wide wireless channel resulting in a data rate of 59 Gbit/s.
© 2017 Optical Society of America
1. Introduction
According to a recent CISCO forecast, global IP traffic will reach 2.3 ZB (zettabyte) by 2020 and wireless mobile devices will account for two-thirds of the total IP traffic. This is due to a steady, strong increase in mobile internet usage, and the fact that the overall number of devices connected to IP networks is expected to rise up to three times the global population by 2020 [1]. New applications such as Mobile Video, Internet of Things (IoT), machine-to-machine (M2M) communications, and vehicle-to-vehicle or vehicle-to-infrastructure (V2X) advancements are further pulling the technological developments towards high capacity and low-latency wireless systems.
In order to meet these new scale requirements, millimeter-wave (mm-wave) wireless technologies are foreseen for next generation (5G) mobile communications. New standards, such as Wireless-Gigabit (WiGig, IEEE 802.11ad), are announced for broadband wireless access making use e.g. of the 7 GHz license-free band at 60 GHz. But despite the available spectrum in the mm-wave region is being much wider than in legacy microwave wireless systems (e.g. 3G/4G), it still may not satisfy the upcoming applications which greedily demand extremely high wireless data rate. As an example, streaming an uncompressed 8k video (7680 x 4320, 120 fps, non-interlaced, RGB, 12-bits per component) from a “wireless beamer” to a screen, would require a data rate of 143.33 Gbit/s. Thus, a spectral efficiency of about 20 bit/s/Hz would be necessary to support such a real-time application within the regulated 7 GHz bandwidth at 60 GHz. Such high spectral efficiencies cannot easily be achieved with sufficient Signal-to-Noise-Ratio (SNR) at mm-wave bands. To the best of our knowledge, there is no wireless system demonstration that would support such high spectral efficiency and throughput today.
Consequently, the Terahertz (THz) frequency range has recently attracted a lot of interest for wireless communications [2] despite the somewhat lower output power and sensitivity of state-of-the-art emitters and receivers. The main reason is that the electromagnetic spectrum above 275 GHz (USA) or 300 GHz (Europe) is currently an almost unallocated region [3–5]. There is hope that regulatory bodies may open up even wider bandwidths for wireless services to support the development of ultra-high capacity short-to-medium range wireless systems that could eventually provide wireless data rates beyond 100 Gbit/s.
Consequently, different research groups are investigating techniques to increase wireless spectral efficiencies and data rates for THz communications. Most of the recent works have been focused on developing photonic and electronic technologies for the WR3 band (220 – 325 GHz) [2], [6–11].
In [6], Song et al. reported on a photonic 300 GHz wireless link using a set of two free-running externally modulated lasers and a high-frequency photodiode (PD) for optical-to-electrical (O/E) conversion. A Schottky barrier detector (SBD) had been used as wireless receiver. They achieved 24 Gbit/s wireless transmission at 300 GHz using a simple optical on-off keying (OOK) modulation format. Although the spectral efficiency wasn’t indicated, it was most likely to be around 0.5 bit/s/Hz. A similar system and modulation format had also been utilized in [7] achieving 40 Gbit/s wireless transmission at 300 GHz. Again, spectral efficiency had not been mentioned but was likely to be around 0.5 bit/s/Hz due to OOK modulation. In [8], Kanno et al. reported a THz communication system with a higher spectral efficiency of about 2 bit/s/Hz. By employing a quite complex optical comb generator for THz generation and an optical in-phase/quadrature (I/Q) modulator for single-sideband quadrature phase-shift keying (SSB-QPSK) modulation, they demonstrated 40 Gbit/s wireless transmission at 300 GHz. Also Kallfass et al. developed a THz link using SSB-QPSK modulation with a spectral efficiency of about 2 bit/s/Hz. The group demonstrated an all-electronic 340 GHz wireless system supporting data rates up to 64 Gbit/s in [9]. By multiplexing four SSB-QPSK wireless channels at carrier frequencies between 390.5 GHz and 428 GHz, a total data rate of 60 Gbit/s was successfully transmitted in [10]. However, the overall spectral efficiency was still limited, requiring a channel bandwidth of at least 30 GHz. Besides this rather inefficient use of spectrum, also the costly integration for multiplexing parallel wireless channels is a drawback. To our knowledge, the highest spectral efficiency achieved at THz band so far was demonstrated by Wang et al. in [11]. A 340 GHz wireless system supporting a higher-order 16-quadrature amplitude modulation (QAM) modulation format was demonstrated. The overall spectral efficiency achieved in this case was 2.86 bit/s/Hz. However, this high spectral efficiency has only been achieved within a very narrow frequency band resulting in a limited maximum data rate of only 3 Gbit/s.
According to a recent review on THz communications [2], higher spectral efficiencies beyond 2.86 bit/s/Hz were only achieved at sub-THz frequencies. In [12], we reported on a radio-over-fiber (RoF) mm-wave wireless system exhibiting a higher spectral efficiency beyond those of conventional OOK systems. By employing an electrical I/Q modulator and a 16-QAM-orthogonal frequency division multiplexing (OFDM) modulation, we achieved a spectral efficiency of 3.86 bit/s/Hz and overall 27 Gbit/s wireless transmission at 60 GHz. In [13], König et al. demonstrated a single-channel sub-THz wireless system operating at 237.5 GHz using a photonic transmitter and an electronic receiver. They also achieved a spectral efficiency around 4 bit/s/Hz and an impressive total data rate of 100 Gbit/s. However, their system approach required sophisticated optical filtering in the photonic transmitter and a non-state-of-the-art wireless monolithic microwave integrated circuit (MMIC) receiver. Further mm-wave wireless transmission experiments employing also 16-QAM modulation as well as polarization division multiplexing were reported in [14,15].
In [16], we presented a compact coherent radio-over-fiber (CRoF) wireless system architecture that allowed the remote optical-to-RF conversion of an optical baseband (BB) signal using a free-running local oscillator (LO) laser in the remote radio access unit (RAU) and a SBD in the wireless receiver. In [17], we demonstrated a 60 GHz CRoF wireless system architecture that allows transmitting complex I/Q-modulated signals and we successful achieved wireless transmission of spectrally efficient 512-QAM-OFDM signals in the 60 GHz band.
Here, we report on a THz communication system based on a CRoF architecture with a free-running local oscillator laser in the RAU for direct optic-to-THz conversion and a simple SBD wireless receiver. Using this THz system set-up, we demonstrate high-throughput data transmission over a 2 cm long WR3.4 waveguide for back-to-back transmission and over a 5 cm long wireless link using two directive horn antennas. In both cases, a high spectral efficient 64-QAM-OFDM data signal centered at ~325 GHz carrier frequency with a wireless bandwidth of 10 GHz resulting in a data rate of 59 Gbit/s were successfully transmitted.
2. CRoF system architecture for complex I/Q-modulated wireless signals
Conventional analog RoF system architectures with heterodyne RAU optical receivers suffer from the fact that phase locking of the lasers for heterodyne generation of the low-phase noise wireless carrier is a necessity [18]. In [16], we proposed a CRoF approach employing coherent heterodyne optical detection in the RAU for wireless signal generation. Envelope detection was utilized in the wireless receiver, a technique that had been demonstrated to be robust against laser phase noise and RF carrier drift for spectrally inefficient on-off keying modulation in [16]. Furthermore, this approach allows relocating the LO laser from the central station into the remote RAU because phase locking between the lasers is not necessary. In [17], the system architecture was improved in order to enable the use of spectrally efficient complex modulation formats, such as OFDM. Wireless transmission of record high spectral efficiency 512-QAM-OFDM signal at 60 GHz was demonstrated with the proposed CRoF system.
Figure 1 shows the proposed CRoF architecture. In order to enable complex and spectrally efficient I/Q-modulation formats, a complex BB signal m(t), conveying data, is up-converted in a real-valued passband signal to an intermediate frequency (IF). Then, the IF band signal is modulated on an optical carrier using an intensity Mach-Zehnder-Modulator (MZM). After the fiber transmission, the optical IF band signal is combined with an optical LO (Reference) laser which may be located in the RAU using an optical 3-dB coupler. At the output of the coupler, a high-frequency photodiode is used for optical heterodyne generation of the wireless signal. In the wireless receiver, only an envelope detector (ED) is required to recover the high-frequency wireless signal back to the IF band.
Fig. 1 Schematic description of the CRoF down-link system architecture enabling complex I/Q-modulation.
Assuming for simplicity quasi-ideal operation conditions by neglecting noise, ideal responsivities, and perfect optical SSB modulation, the optical fields associated with the reference LO laser Er(t) and signal laser Es(t) can be written as:
where Er0(t) and Es0(t) are the optical field amplitudes, ωr and ωs are the angular frequencies, and φr and φs, the optical phase fluctuations of the reference and signal laser, respectively.The MZM, with a half-wave voltage of Vπ, is driven at quadrature point, operating in a linear region. Applying small signal approximation, the optical field at the input of the photodiode Erec(t) is a linear combination of the electrical OFDM spectrum plus the two optical carriers:
Note that the 3-dB coupler, which is used for combining the two laser signals at the input of the photodiode, divides the optical field in Eq. (3) by a factor of √2. To overcome this power loss, a specific coherent photonic mixer based on a balanced photodiode could be used [17].Considering that the output of the photodiode is connected to a WR3.4 waveguide with a lower cut-off frequency of ~200 GHz, the terms outside the THz band can be neglected and the total RF photocurrent iPD(t) can thus be expressed as follows:
where ωTHz = |ωs - ωr| represents the angular THz-wave frequency and Δφ(t) = φs(t) - φr(t) - π/2 represents the phase noise of the THz-wave signal and the phase shift introduced by the 3-dB coupler.By assuming SSB modulation and by applying the Hilbert transformation, iPD(t) can be expressed as:
Here a(t) and θ(t) denote the envelope and phase of the BB data signal related with m(t) by a(t) = │m(t)│and θ(t) = arg(m(t)). Finally, modelling the ED as a square-law device and neglecting high-order-frequency components yield the output current of the envelope detector to be:As can be seen from Eq. (6), the detected signal includes a DC component (first term), a signal-to-signal beating interference (SSBI) component (second term) and the desired signal at the intermediate frequency (third term). Note that in case the signal centered at IF overlaps with the SSBI that extends from DC up to the signal bandwidth, the SNR will be decreased.From the model and, especially, from Eq. (6), we can extract the following important conclusions:
Firstly, to avoid the receiver SNR degradation due to the SSBI signal, the IF of the OFDM signal, m(t) should be at least 3/2 times the signal bandwidth.
Secondly, one of the key advantages of the envelope detection approach is the tolerance against phase noise and frequency drift of the THz-wave signal which allows to use free-running remote located LO lasers. This in turn also enables higher energy efficiency because the LO laser can now be placed in the RAU thus avoiding optical power loss due to fiber transmission as it is the case in conventional centralized RoF architectures [16, 17, 19, 20].
3. Terahertz CRoF system experimental set-up and results
Figure 2 shows the experimental set-up of the THz CRoF system. In the central station, the IF band OFDM signal was generated off-line by using Matlab and an arbitrary waveform generator (AWG) with a sampling rate of 60 GSa/s. The inverse fast Fourier transform (IFFT) length of the OFDM BB I/Q signal was 512, and the number of subcarriers was 84. Each subcarrier was modulated using a 64-QAM signal format. The total data rate of the 64-QAM-OFDM signal was 59.06 Gbit/s. The BB OFDM I/Q signal was up-converted to a 15 GHz IF before being loaded into the AWG. Due to the up-conversion process, the spectral efficiency of the optical BB OFDM signal of about 6/bit/s/Hz is reduced by half, i.e. the RF spectral efficiency is about 3 bit/s/Hz. Still, the RF spectral efficiency represents a record figure for THz communications, to our knowledge. The IF signal was amplified by a broadband power amplifier and fed into a 25 GHz bandwidth MZM, which was subsequently used to modulate the optical carrier. The optical carrier was generated by a tunable laser set at 193.6 THz. The power of the optical signal at the input of the MZM was + 14 dBm.
The modulated optical signal exiting the MZM was transported over a short piece of fiber to the remote RAU. There, the OFDM modulated optical signal was combined with the LO laser using a 3-dB optical coupler. The LO laser was another tunable laser at a frequency of 193.9 THz providing an optical power of + 6 dBm. Figure 3 shows the optical spectrum of the combined signal and reference (LO) laser with a difference frequency of ~325 GHz.
An erbium doped fiber amplifier (EDFA) was used to amplify the combined optical signal exiting the 3-dB coupler. After the EDFA, the amplified optical signal was detected by a 100 GHz bandwidth uni-traveling-carrier photodiode (UTC-PD) with a WR3.4 waveguide output and a responsivity of 0.15 A/W @ 325 GHz. This way, an OFDM-modulated THz signal at ~325 GHz was generated due to the square law detection of the UTC-PD. The RF spectrum of the generated THz OFDM signal at the output of the UTC-PD has been measured using an external mixer from OML (V03VNA2-T/R). To keep the signal inside the calibrated frequency range of the WR3.4 external mixer (220 to 325 GHz), the carrier frequency was slightly reduced to 322 GHz for this measurement. The measured spectrum shown in Fig. 4 shows the THz carrier and the lower 10 GHz wide sideband centered 15 GHz below the THz carrier. The upper sideband, centered at 337 GHz, is outside the operational frequency of the external OML WR3.4 mixer. As can be seen, as the system is based on a small signal modulation scheme, the spectral power density of the OFDM sideband is low. Furthermore, it is not flat, as one would expect for an OFDM signal (grey line), but it drops towards higher frequencies. This behavior can be traced back to the spectral response of the UTC-PD and the THz link which is shown in Fig. 5.
Fig. 4 Measured spectrum (lower sideband and carrier) of the transmitted OFDM THz signal. Blue dots represent measurements, grey line illustrates the expected spectra.
To measure the frequency response of THz link in a back-to-back configuration, the WR3.4 input of the SBD was directly connected to the WR3.4 output of UTC-PD without amplifier. Then the difference frequency of the reference laser was detuned to change the RF frequency and no data modulation was applied to the signal laser. The back-to-back link response was measured for two different photocurrent levels of 2 mA and 4 mA, as shown in Fig. 5.
As can be seen, the lower cut-off frequency of the WR3.4 waveguide is around ~200 GHz, as expected. Furthermore, it can be seen from Fig. 5 that the link response drops towards higher frequencies. This is mainly due to the response of the UTC-PD since the sensitivity of the SBD even increases towards higher frequencies. Thus, the shape of the non-ideal lower sideband observed in Fig. 4, is explained mainly by the frequency response of the UTC-PD performance.
As mentioned before, one key advantage of the presented CRoF approach is that it allows using un-locked lasers since the receiver is tolerant against the slow carrier frequency drift. To demonstrate that un-locked lasers were employed, the carrier frequency drift over one hour was measured and is illustrated in Fig. 6. As can be seen, the non-phase locked RoF system has to deal with a slow carrier frequency drift of up to 28.3 MHz/min with an absolute maximum carrier drift of approximately +/- 20 MHz.
In addition to back-to-back measurements, also data transmission experiments over a wireless THz link were carried out. Figure 7 shows a photograph of the two WR3.4 horn antennas used for wireless transmission. Here, the distance between the tips of the horn antennas is 5 cm and the distance between the feeding points of the two antennas is 14 cm. On the left hand side, one can see the UTC-PD connected to a 6 cm long WR3.4 antenna with a directivity of 26 dBi. The receiver in the right consists of the 3 cm long WR3.4 antenna with a directivity of 25 dBi, a WR3.4 low noise amplifier (LNA) with a gain of 28 dB and a noise figure of 12.5 dB as well as the SBD with an RF responsivity of 2000 V/W @ 325 GHz and an IF bandwidth of 40 GHz.
To recover the transmitted OFDM signal after wireless transmission, the THz signal was down-converted to IF using the 40 GHz IF bandwidth envelope detector. The use of the SBD envelope detector significantly simplifies the construction of the THz receiver since no electrical LO is needed as it is the case for heterodyne receivers. Finally, the down-converted 15 GHz IF OFDM signal was amplified by a LNA and captured by an 80 GSa/s real-time-oscilloscope.
Next, the captured IF signal was demodulated using off-line signal processing in Matlab. For a systematically system performance evaluation, we first measured the SNR and error vector magnitude (EVM) for different data bandwidths for back-to-back, i.e. without the RF amplifier and the antennas. The measured SNR (and EVM) for a bandwidth of 1 GHz, 5 GHz, and 10 GHz were 31.9 dB (2.55%), 24.63 dB (5.87%), and 21.4 dB (8.51%), respectively. In all cases the IF was set at 1.5 times the data bandwidth, i.e. 1.5 GHz, 7.5 GHz, and 15 GHz. The modulation format was 64-QAM-OFDM, except for the 1 GHz case where 128-QAM-OFDM has been used. This shows that the SNR directly scales with the data bandwidth, as expected. Next, we investigated the impact of the RF amplifier and the wireless transmission as well as an additional 2 km fiber-optic transmission of the optical IF data signal on the system performance. Here, the modulation format, signal bandwidth and intermediate frequency were 64-QAM-OFDM, 10 GHz and 15 GHz IF, respectively. When adding only the RF amplifier (i.e. no antennas), the SNR (EVM) dropped from 21.4 dB (8.51%) to 19.37 dB (10.75%). With wireless transmission over 5 cm (wireless distance between the antenna feeding points was 14 cm), the SNR (EVM) figure remains almost unchanged at 19.42 dB (10.69%). In addition, no penalty was observed when adding an additional 2 km long fiber transmission of the IF band optical signal. Actually, we measured a slightly improved SNR (EVM) of 19.7 dB, (10.34%) in that case. The slightly better SNR after 2 km fiber transmission compared to the back-to-back case is considered to be caused at least partly by the negative chirp of the MZM. In addition, drift of the multifold operational driving parameters may have also affected this.
The frequency spectra of the received IF signals are shown in Fig. 8. For both cases, i.e. for back-to-back transmission (left) and for 2 km single mode fiber with 5 cm THz wireless transmission (right), clear 10 GHz wide OFDM signals were observed. In addition, from Fig. 8, one can observe the SSBI signal from DC up to the signal bandwidth of 10 GHz and a second harmonic signal centered at two times the IF, i.e. centered at 30 GHz. Note that the second harmonic term does not look like a symmetrical double sideband signal because of the 3-dB bandwidth limitation of the baseband amplifier in front of the oscilloscope. In order to fully avoid performance degradation due to SSBI, an IF of fIF = 15 GHz was selected.
Fig. 8 IF spectrum after down-conversion using the envelope detector for back-to-back (left) and after 2 km fiber and 5 cm wireless transmission (right).
The constellation diagrams of the demodulated OFDM signals are shown in Fig. 9. As can be seen, good signals were successfully recovered. The bit error rates (BER) of the recovered signals estimated from the SNR for back-to-back transmission and for 2 km fiber and 5 cm THz wireless transmission is about 3 × 10−3 and 1 × 10−2, respectively. If soft-decision forward error correction (SD-FEC) is applied, the maximum allowed pre-FEC for error-free transmission of a 64-QAM signal with 7% overhead is 2 × 10−2 [21]. Thus, we conclude that error-free transmission can be achieved with the presented system using advanced SD-FEC technology.
Fig. 9 Constellation of the demodulated 64-QAM-OFDM modulated signal at 59 Gbit/s for back-to-back (left) and after 2 km fiber and 5 cm wireless transmission (right).
Figure 10 shows the SNR versus subcarrier number of the demodulated 64-QAM-OFDM signal for the back-to-back experiment. As can be observed from the demodulated signal, a fairly uniform subcarrier SNR within ~3 dB fluctuation was obtained across the entire band of the signal.
4. Conclusion
A THz wireless communication system based on a CRoF approach is demonstrated in this article. The proposed architecture employs THz envelope detection in the wireless receiver using a SBD. It is theoretically shown that the approach benefits from the insensitivity against optical phase noise and RF carrier drift. Experimentally, wireless transmission around 0.325 THz of a 10 GHz wide 64-QAM-OFDM data signal at a 15 GHz IF is successfully demonstrated for back-to-back and for 5 cm wireless transmission. The SNR of the received and down-converted OFDM signal for back-to-back and for 5 cm THz wireless transmission with an additional 2 km long fiber in-front of the UTC-PD is 21.4 dB corresponding to a BER of 3 × 10−3 and 19.7 dB corresponding to a BER of 1 × 10−2, respectively. In both cases, error-free transmission can be achieved using advanced soft-decision FEC techniques with 7% overhead. The total transmitted data rate is about 59 Gbit/s. To the authors’ best knowledge, this is the first transmission of such ultra-wideband and high spectral efficient data signals in the THz domain.
Funding
Deutsche Forschungsgemeinschaft (DFG) (Tera50 project); European Commission (EC) (FIWIN5G and RAPID projects).
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