Investigation of Optical Millimeter-Wave and Heterodyne Techniques in Radio-over-Fibre Systems 1
A. Bahrami1, Student member, IEEE, W. P. Ng1, Senior member, IEEE, Z. Ghassemlooy1, Senior member, IEEE, C. Qiao2, Senior member, IEEE
Optical Communications Research Group, NCRLab, Northumbria University, Newcastle upon Tyne, UK, 2 University at Buffalo The State University of New York, USA
[email protected]
Abstract- Radio over Fibre (RoF) has been proposed as an alternative medium to extend the coverage of radio frequency (RF) millimetre wave (MMW). The main challenges are the fibre attenuation and chromatic dispersion which degrade the signal power and induces inter-symbol interference (ISI), respectively. Two electrical modulation schemes are employed in this paper, namely binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK) at electrical bandwidth of 30 GHz. The proposed technique using optical continuous wave (CW) laser as the optical carrier to transmit BPSK and QPSK signals at 30 GHz. The performance of the proposed method than compared to optical MMW carrier at 60 GHz in order to carry 30 GHz BPSK and QPSK electrical signals. The results indicate the improvement in transmission distance of up to 20 km for both BPSK and QPSK modulation schemes at signal to noise ratio (SNR) of 11 dB and 16 dB respectively for bit error rate (BER) of 10-5 in the proposed method. These results compared to 15 km and 10 km in case of optical MMW using BPSK and MMW QPSK systems at SNR of 18 dB and 26 dB to achieve the same BER value.
I. I NTRODUCTION Nowadays, higher carrier frequencies are required in order to provide the platform for the ever increasing demand of transmitting higher bit rates. The concept of the millimetre wave (MMW) has been introduced to provide communication systems with a suitable carrier that can support data rates in the Gb/s region. One option for providing wireless access to high data rate broadband networks is to utilize MMW as the signal carrier. Frequency bands at 30 GHz and 60 GHz offer reasonable bandwidth between stationary sites [1]. However, there is a considerable amount of attenuation in wireless RF systems at the MMW range. For instance, typically at 20 GHz the attenuation caused by dew is about 1.5 dB/km while at 27 GHz the attenuation is above 3 dB [1]. Multipath fading [2] and dust [3] can also have destructive effects on the signal propagation through the air. Moreover, for radio frequency (RF) MMW, atmospheric effects such as molecular absorbers in water (H2O) and oxygen (O2) introduce up to 10 dB/km of loss at 40 GHz [4]. Radio over Fibre (RoF) is a technique where light is modulated by a RF signal and transmitted over an optical link in order to extend the
Indoor wireless RF MMW
LD
MZM
Central Station MMW- RoF
Base Station Fig. 1: Comparing MMW in RF and RoF. LD: Laser diode, MZM: Mach-Zehnder Modulator, Electrical link: Optical link:
wireless access range. The RoF system is designed to supply wireless systems with higher bandwidths, which will accordingly improve its performance. Fig. 1 illustrates the use of MMW in RF communication systems where signal can be used in indoor applications over very short distances. Fig. 1 also depicts MMW for RoF systems, such as the internet coverage, which is used to transmit the signal to longer distances between base stations (BS) and central stations (CS). The proposed alternative is transmitting the electrical MMW over a CW laser or an optical MMW in RoF systems. There are four techniques to generate the optical MMW carrier [5] including the optical heterodyne [6], external modulation [7], optical transceiver and up-and down-conversions [8]. One of the major challenges in using MMW in RoF systems is the fibre chromatic dispersion (CD) which can cause considerable signal degradation even at short ranges [9]. Although MMW signal has been used extensively in RoF communication systems, only short ranges (~ few meters) and medium ranges (~ 1 to 3 km) has been recorded in the
literature [10, 11]. In this paper, an optical CW LD has been incorporated in order to increase the transmission distance. There are several advantages of using lower frequencies: 1. More costly effective. 2. Using simple intensity modulation scheme. 3. Transmission of multiple channels over a single link at the same RF carrier such as multiple inputmultiple output (MIMO) channels. The remainder of the paper is organised as follows. The following section presents the system specifications. In section III, the results of the BPSK and QPSK simulations incorporating MMW and heterodyning are introduced. Finally Section IV concludes the findings of the paper.
Optical signal
90 90°
Electrical signal
LD
DD-MZMZ
BPSK/QPSK
SMF
signal
II. SYSTEM SPECIFICATIONS
Heterodyne down conversion
PD
Demodulator
BER
A. T ransmission using hete rody ne m ethod In Fig. 2, the Mach-Zehnder modulator (MZM) is the dualdrive (DD-MZM) biased at a quadrature biasing point. The biasing point, VDC is defined as a ratio of biasing voltage, Vbias, to switching voltage, Vπ, which governs the operating point of MZM. In this case VDC = Vbias / Vπ = 0.5 V where Vbias is the biasing voltage applied to the MZM and Vπ is the switching voltage required to make π phase shift in optical combiner in DD-MZM. This biasing point generates an optical single sideband (OSSB) which is less vulnerable to the fibre CD and attenuation [12]. The electrical signal can be transmitted in RoF system using a CW optical carrier instead of a MMW signal. The combination of a CW carrier and the OSSB modulation scheme results in lower CD and attenuation [13]. At the receiving end following photodetector, the signal is down-converted to a lower frequency by multiplying the received signal with a local oscillator and then passing it through a low-pass filter. In such circumstances, the signal can be transmitted over a longer distances. In this paper, the received signal at 30 GHz is detected and then down-converted to 12 GHz with a local oscillator operating at 18 GHz. Equation 1 [14], models the DD-MZM where E0(t) and Ei(t) are the output and input optical fields of MZM respectively, and V(t) is the input electrical signal. Depending on the biasing conditions of MZM, different modulation scheme can be achieved. Comparing OSSB and optical double side band (ODSB) modulation methods, lower CD values are achieved in case of the OSSB transmission due to the halved optical signal spectral bandwidth. Due to the fact that transmission at lower frequency is subjected to less quality degradation than in higher frequencies, using
Fig. 2: A block diagram of heterodyne method. LD: laser diode; DD-MZM: Dual-drive Mach-Zehnder modulator; PD: Photodetector. Optical signal Electrical signal
90° LD
SD-MZM
DD-MZM
~
BPSK/QPSK signal
Signal generator
BER
Demodulator
SMF
PD
Fig.3: A block diagram of 60 GHz MMW system. LD: laser diode; SD-MZM: single drive Mach-Zehnder modulator; DD-MZM: dualdrive Mach-Zehnder modulator; PD: Photodetector.
heterodyne method would be better for long haul transmission. The information of the relative arrival time of the various signal frequencies remains as part of the electrical output signal. Consequently, the fibre CD can be compensated in the electrical domain following the detection [12]. The optical carrier frequency and the modulation format (for either modulation technique) are key elements in the signal degradation. The length of the SMF in high bit rate long haul optical communication systems is limited by the bit rate and CD [16].
(V ( t ) V bias ) (V ( t ) V bias ) j j E i (t ) (V ( t ) V bias ) V V e E i cos E 0 (t ) e 2 V 2 (V ( t ) V bias ) E 0 ( t ) E i cos e V 2
(V ( t ) V bias ) j V 2
(1)
(2)
10
In equation 3 the fibre impulse response is presented as [15]
Theoratical BPSK Sim ulated for 5 km
10
-1
BPSK Sim ulated for 10 km BPSK Sim ulated for 12 km
(3)
III. RESULTS A ND DISCUSSION The bit error rate (BER) against the signal-to-noise ratio (SNR) for the BPSK modulation scheme using the optical MMW at 60 GHz over four different ranges of SMF is depicted in Fig. 4. The BER performance for QPSK modulation scheme using the optical MMW and the heterodyne method is illustrated in Fig. 5. Referring to Fig. 4, the maximum achieved distance for MMW BPSK is 15
BER
10
10
-3
-4
0
2
4
6
8
10
12
14
16
18
SNR - dB
Fig. 4. The BER value for optical MMW using BPSK modulation scheme. 10
0 Theoratical QPSK heterody ne for 5 km
10
10 BER
A method of generating MMW in forms of the optical double side band with suppressed carrier (ODSB-SC) has been used in [10]. A 60 GHz photonic link system has been used to transmit the signal to the BS with a bit rate of 6 Gb/s. In order to investigate the effect of fibre attenuation and CD on MMW, the same model is modified as follows. RoF system is used to deliver the MMW signal over a RF wireless channel. In order to generate the optical MMW signal in the form of DSB-SC, a Mach-Zehnder modulator (MZM) with a biased voltage Vbias of 3.5 V is used. In order to achieve a minimum bias point of MZM, VDC = 1, and the MZM switching voltage Vπ should be at 3.5 V. At a minimum bias point, the maximum power is transferred to the first sideband, which is ideal for generating MMW due to the suppression of the carrier and therefore, there is no need for filtering [13]. Biasing single-drive Mach-Zehnder modulator (SD-MZM) ensures that both fundamental and even modes of the optical carrier are suppressed; see Fig. 3. Equation 2, defines the output of SD-MZM. In ODSB-SC the separation between the two harmonics of ω0 ± ωrf where ω0 is the optical carrier and ωrf is the RF frequency, produces optical MMW at the frequency which is twice of the RF frequency, ωrf. The generated optical MMW at 60 GHz is externally modulated by the BPSK or QPSK electrical signal using DD-MZM. In this configuration, SDMZM is driven by a sinusoidal signal at 30 GHz to create a 60 GHz MMW in the form of DSB-SC. The modulated MMW at the optical frequency is transmitted over a single mode fibre (SMF) and received by a photodetector with a responsivity of 0.59 A/W [10]. The major difference between optical MMW and heterodyne method is the frequency band is used. In the proposed heterodyne method data rate has been reduced to 1.2 Gb/s and using optical CW laser as the carrier where as in optical MMW the data rate is 6 Gb/s and the optical MMW is 60 GHz. Sacrificing bit rate in heterodyne method provide longer transmission distance compared to RoF system with optical MMW as the carrier.
BPSK Sim ulated for 15 km
-2
10
10
QPSK heterody ne for 10 km
-1
QPSK heterody ne for 20 km QPSK m m -wave for 5 km QPSK m m -wave for 10 km
-2
-3
-4
0
5
10
15
20
25
30
SNR - dB
Fig. 5. The BER value for optical MMW and heterodyne methods using QPSK modulation scheme. 10
10
10
0
T heorat ical BP SK het erodyne for 5 km BP SK het erodyne for 10 km
-1
BP SK het erodyne for 20 km
-2
BER
where α is the single mode fibre (SMF) attenuation (0.2 dB/km), L is the length of the fibre, D is the chromatic dispersion value (17ps/nm-km), λ0 is the operating wavelength (λ0 = 1.55 μm), c is the velocity of the light, B is the bit rate and f is the optical frequency. B. Transmi ssi on using M M W metho d
10
10
10
-3
-4
0
2
4
6
8
10
12
SNR - dB
Fig. 6. The BER value for BPSK modulation scheme using heterodyne down-conversion at 12 GHz. 0.9 BP SK m m -wave 10 km 0.8
BP SK heterody ne 10 km QPSK m m -wave 10 km
0.7
QPSK m heterody ne 10 km
0.6 EVM(%)
2 2 0 LB 2 f L j D c B H ( f ) 10 10 e
0
0.5 0.4 0.3 0.2 0.1 0
0
5
10
15
20
25
30
SNR - dB
Fig. 7. The EVM values for BPSK and QPSK modulation schemes for optical MMW at 60 GHz and heterodyne BPSK and QPSK at 12 GHz
km shows longer transmission distance compared to MMW QPSK at 10 km. In Fig. 6, we plot the BER for BPSK modulation scheme using the heterodyne down conversion at 12 GHz over three different SMF links. The maximum transmission distance for BPSK over the MMW carrier is 15 km while in case of QPSK it is 10 km. To achieve a BER of
10-5 the QPSK modulation scheme requires higher SNR value by 26 dB at a distance of 10 km compared to the SNR of 12 dB for the BPSK at the same distance. The results indicate that higher order modulation schemes reduce the transmission distance. In case of MMW at longer distances for both modulation schemes, the signal cannot be detected without using an amplifier. On the contrary, in case of the heterodyning method, the signal can be detected (for both modulation schemes) over 20 km with SNR values of 11dB and 16 dB, respectively in order to achieve a BER value of 10-5. One of the methods of measuring the quality of the communication system with regard to the receiver sensitivity is the error vector magnitude (EVM), which is a measurement of demodulator performance in the presence of impairments. Lower values of EVM indicate improved signal reception. For different modulations schemes and systems such as worldwide interoperability for microwave access (WiMAX) [17] BER of 10-5 is has been considered by the convention as the error free communication, thus adopted in this work. Figure 7 illustrates the EVM values for both modulation schemes using MMW and CW carriers with the heterodyne detection method at a distance of 10 km. The results indicate that at distance of 10 km the BPSK method requires 10 dB of SNR in order to achieve BER of 10-5. These results confirm that the effect of CD on the optical CW carrier is more severe at the receiver end compared to the optical MMW. However, for BPSK over the same fibre length and the BER using the optical MMW the required SNR value is increased to 12 dB. The SNR penalty increases for the QPSK modulation scheme. For the heterodyne method the required SNR value is 14 dB to achieve the desired BER, whereas in case of the optical MMW this value increases to 26 dB (i.e. 12 dB penalty compared to heterodyne method).
compared to the case of MMW. [1]
[2]
[3] [4] [5]
[6]
[7] [8] [9] [10]
[11]
[12]
IV. CONCLUSION In this paper two modulation schemes of BPSK and QPSK over the 60 GHz MMW system and the heterodyne method at 12 GHz have been investigated. It has been concluded that the transmission of the signal at MMW experiences excessive degradation in the signal power thus resulting in shorter transmission distance with a data rate of 6 Gb/s. On the other hand, using the heterodyne method enables the communication link to perform better in terms of the transmission distance but at the expense of lower data rates (1.2 Gb/s). We have illustrated that in heterodyne method for BPSK modulation scheme, the transmission distance of 20 km achieved with the SNR of 11 dB compared to 15 km transmission distance at SNR of 18 dB for BPSK using MMW. In the case of QPSK modulation scheme using heterodyne method, the transmission distance of 20 km achieved at SNR of 16 dB whereas in QPSK over optical MMW transmission distance of 10 km at SNR of 26 dB is recorded. We showed that for BPSK and QPSK modulation schemes employing heterodyne detection the EVM is almost 0% for a BER of 10-5 and at lower SNR values
[13]
[14] [15]
[16] [17]
REFERENCES
R. E. Henning and J. R. Stanton, "Effects of Dew on MillimeterWave Propagation," in Southeastcon '96. 'Bringing Together Education, Science and Technology'., Proceedings of the IEEE , 1996, pp. 684-687. A. A. Ali and M. A. Alhaider, "Effect of Multipath Fading on Millimetre Wave Propagation: A Field Study," Microwaves, Antennas and Propagation, IEE Proceedings H, vol. 140, pp. 343346, 1993. D. Wikner, "Millimeter-Wave Propagation Measurement Through a Dust Tunnel," U.S.Army Research Laboratory March 2008. R. A. Bohlander, R. W. McMillan, and J. J. Gallagher, "Atmospheric Effects on Near-Millimeter-Wave Propagation," Proceedings of the IEEE, vol. 73, pp. 49-60, 1985. N. Mohamed, S. M. Idrus, and A. B. Mohammad, "Review on System Architectures for the Millimeter-Wave Generation Techniques for RoF Communication Link," in RF and Microwave Conference, 2008. RFM 2008. IEEE International , 2008, pp. 326330. R. Hofstetter, H. Schmuck, and R. Heidemann, "Dispersion Effects in Optical Millimeter-Wave Systems Using Self-Heterodyne Method for Transport and Generation," Microwave Theory and Techniques, IEEE Transactions on, vol. 43, pp. 2263-2269, 1995. G. H. Smith, D. Novak, and Z. Ahmed, "Technique for Optical SSB Generation to Overcome Dispersion Penalties in Fibre-Radio Systems," Electronics Letters, vol. 33, pp. 74-75, 1997. K. J. Williams and R. D. Esman, "Optically Amplified Downconverting Link with Shot-Noise-Limited Performance," Photonics Technology Letters, IEEE, vol. 8, pp. 148-150, 1996. D. Wake, A. Nkansah, and N. J. Gomes, "Radio Over Fiber Link Design for Next Generation Wireless Systems," Journal of Lightwave Technology, vol. 28, pp. 2456-2464, 2010. M. Weiss, M. Huchard, A. Stohr, B. Charbonnier, S. Fedderwitz, and D. S. Jager, "60-GHz Photonic Millimeter-Wave Link for Short- to Medium-Range Wireless Transmission Up to 12.5 Gb/s," Lightwave Technology, Journal of, vol. 26, pp. 2424-2429, 2008. Y. Hsueh, Z. Jia, H. Chien, A. Chowdhury, J. Yu, and G. Chang, "Multiband 60-GHz Wireless over Fiber Access System with High Dispersion Tolerance Using Frequency Tripling Technique," Lightwave Technology, Journal of, vol. PP, pp. 1-1, 2011. M. Sieben, J. Conradi, and D. E. Dodds, "Optical single sideband transmission at 10 Gb/s using only electrical dispersion compensation," Lightwave Technology, Journal of, vol. 17, pp. 1742-1749, 1999. W. Zhang, J. A. R. Williams, and I. Bennion, "Optical single sideband generation with full RF-band operation," in Lasers and Electro-Optics, 2006 and 2006 Quantum Electronics and Laser Science Conference. CLEO/QELS 2006. Conference on , 2006, pp. 1-2. L. N. Binh, Optical Fibre Communications Systems: Theory and Practice with MATLAB and Simulink modes : Taylor & Francis Group, 2010. A. F. Elrefaie, R. E. Wagner, D. A. Atlas, and D. G. Daut, "Chromatic dispersion limitations in coherent lightwave transmission systems," Lightwave Technology, Journal of, vol. 6, pp. 704-709, 1988. A.Ghatak and K. Thyagarajan, Introduction to fibre Optics: Cambridge University press, 1998. O. Arafat and K. Dimyati, "Performance Parameter of Mobile WiMAX: A Study on the Physical Layer of Mobile WiMAX under Different Communication Channels; Modulation Technique," in Computer Engineering and Applications (ICCEA), 2010 Second International Conference on.
