Cellular over optical wireless networks G. Katz, S. Arnon, P. Goldgeier, Y. Hauptman and N. Atias Abstract: The development of optical wireless communication systems is accelerating as a costeffective alternative to fibre optic links and wireless technologies. The optical wireless technology is used mostly in wide bandwidth data transmission applications. This technology can also be useful for cellular applications. The evolution of cellular networks, for increased coverage and capacity and for the evolution towards 3G, leads towards the necessity of increasing the number of base stations. The idea of using wireless optical links to implement additional base stations is introduced. A detailed analysis and measured results of optical wireless links utilised to transmit cellular signals are presented.
1
Introduction
The development of optical wireless communication (OWC) systems is accelerating, as these systems provide a cost-effective alternative to fibre optic systems. OWC systems have many advantages over other wireless technologies. These advantages include much higher data rates and no required Federal Communications Commission (FCC) licensing or frequency allocation. This is very important because in some large urban areas it has become very difficult or impossible to obtain frequency allocation for microwave transmission. OWC is an attractive alternative to street excavations required to install fibre optic cables, which are logistically complex and expensive. However, the primary disadvantage of OWC is its vulnerability to atmospheric effects, such as attenuation and scintillation, which reduce link performance [1]. Using excess optical power margins to support fading loss can reduce this problem. A theoretical modelling of an analogue optical link scheme can be used to estimate the optimal power margin suitable for specific area conditions. 2
Cellular applications
The analogue optical link can be used for remote locating of a cellular antenna from the base station. This solution enables expansion of the network. The expansion can be implemented by adding data transceivers to existing base stations and placing only antennas instead of complete base stations at the required locations. At the central, or donor, base station, the cellular RF signal is modulated by the laser diode using AM modulation and converted into an optical signal. At the remote station, the received optical signal is demodulated by a photodetector and converted back to an electrical RF signal. To reconstruct the original cellular RF signals, the optical link must have low gain ripple, low noise figure and a high dynamic range. # The Institution of Engineering and Technology 2006 IEE Proceedings online no. 20050040 doi:10.1049/ip-opt:20050040 Paper first received 26th April and in revised form 21st November 2005 G. Katz and S. Arnon are with the Satellite and Wireless Communication Laboratory, Electrical and Computer Engineering Department, Ben-Gurion University of the Negev, P.O Box 653, IL-84105 Beer-Sheva, Israel P. Goldgeier, Y. Hauptman and N. Atias are with Celerica, 21 Yagea Kapayim St., Petach, Tikva 49130, Israel E-mail:
[email protected] IEE Proc.-Optoelectron., Vol. 153, No. 4, August 2006
The dynamic range requirement of the cellular system is unique for each cellular transmitting method (i.e. requirements for CDMA are more lenient than for GSM) and cell size (i.e. requirements are more lenient for small cells). 3 Transferring RF signal over free space optical link Although the classical usage of free space optical transmission is for digital data, it is sometimes advantageous to transmit analogue signals [2]. A technology base for optical analogue signal transmission already exists in the commercial fibre optic application, particularly for cable television industry [3]. Several modulation techniques such as AM, FM and PM can be applied for optical analogue transmission. The FM and PM techniques have some disadvantages, such as non-flat laser transfer functions causing undesired incidental IM and so on [4]. AM modulation can be simply implemented in two techniques: external and direct modulation. Because direct modulation is easier to implement [4], the direct modulation approach is chosen in this paper. One of the most critical parameters in analogue links is linearity, usually characterised by the spurious free dynamic range (SFDR). The SFDR can be defined as the input power range, where the output signal is above the noise floor and the intermodulation product is below the noise floor. 3.1
Theoretical analysis
The SFDR can be defined by 2 SFDR ¼ ðIP3o f 10 log BWÞ or 3 2 SFDR ¼ ðIP3in þ G f 10 log BWÞ 3
ð1Þ
where IP3o and IP3in are the third-order intercept points at the system output and input, respectively, f the noise floor at the system output and BW the system bandwidth. In a direct modulated link, the laser typically dominates nonlinearity. The gain (G) in an optical link is a function of the slope efficiency of the laser (m), end-to-end loss (loss), the photodiode responsivity (r), the APD multiplication factor (M ) and the ratio of the output to input impedance 195
(Ro/Ri , where Ro is the photodetector output resistance and Ri is the laser diode input resistance). The delivery power gain of the optical link in decibels can be presented as follows G ¼ 20 logðrMmÞ 2 loss(dB) þ 10 log
Ro ðdBÞ Ri
ð2Þ
Using a system with a trans impedance amplifier (TIA), Rf replaces Ro in the above equation and the resulting gain increases. Typically Rf ’ Ro . 60, thus providing a 18 dB gain increase. Neglecting degrading atmospheric effects of the free space, the noise floor is derived from three noise sources. The normally possible noises are relative intensity noise (RIN), shot noise (Ps) and thermal noise (Pth). Therefore the noise floor can be defined as [5] f ¼ PRIN þ Pth þ Ps ðW=HzÞ
ð3Þ
where PRIN ¼
P2pd 10RIN=10
r2 Rf ðW/HzÞ
ð4Þ
Pth ¼ KT ðW/HzÞ
ð5Þ
Ps ¼ 2eðrPpd þ IdrÞM 2þF Rf ðW/HzÞ
ð6Þ
K is the Boltzmann’s constant, T the temperature in Kelvin, e the electrical charge, Idr is the photodetector dark current, F the excess noise factor and Ppd the optical power incident on the photodiode.
The upper curve represents an APD scheme, and the lower a PIN scheme (M ¼ 1). For the PIN scheme, Fig. 1 shows that for low optical losses within the range of 0– 10 dB, a constant SFDR is obtained because the RIN is the dominant noise. At the intermediate level within the range of 10 –20 dB, the shot noise is the main contributor with a 1 dB drop in SFDR for every 1 dB decrease in optical attenuation. For high optical attenuation, above 20 dB, the thermal noise of the receiver is the limiting factor, yielding a 2 dB drop in SFDR for each 1 dB drop in optical attenuation. For APD scheme, for optical losses above 15 dB, the SFDR decreases in a more moderate way, because the shot noise is the main contributor with a 1 dB drop in SFDR for every 1 dB decrease in optical attenuation. 3.3
Erbium-doped fibre amplifier improvement
Another candidate for increasing the link gain margin (GM) is the use of an optical amplifier. Specifically, the erbiumdoped fibre amplifier (EDFA) at 1.5 mm can be used to boost the optical output power from a laser and achieve an optical gain as high as 30 dB. Today’s commercial EDFAs can transmit several watts of output power, with high gain and low noise. The optical amplifier produces an amplified spontaneous emission (ASE) noise that contributes to the total noise floor. Furthermore, another noise source is generated through amplification, resulting in the mixing of the signal and the ASE –ASE beat noise. Hence, the total noise floor is [7] f ¼ PRIN þ Pth þ Ps þ Psp þ Psigs ðW/HzÞ
ð7Þ
where 3.2
Simulation results
The (third order) SFDR is calculated as a function of optical loss and the results are shown in Fig. 1. For these calculations, we used following parameters. DFB laser: Ri ¼ 50 V m ¼ 0.16 mW/mA, IP3in ¼ 33 dB m, RIN ¼ 2155 dB, Pout ¼ 10 dB m. PIN diode: Idr ¼ 1 nA, Rf ¼ 50 V, r ¼ 0.9 A/W, BW ¼ 100 kHz. APD multiplication was chosen to be optimal by Senior [6] 2þF ¼ Mop
4KT FeRo rPpd
ð6Þ
Fig. 1 SFDR against optical loss for APD and PIN photodiode 196
Pspsp ¼ ðrhyFn GÞ2 Dyopt Psigs ¼ 2ðrGÞ2 hyFn Psig
Rf ðW/HzÞ loss Rf ðW/HzÞ loss
ð8Þ ð9Þ
Fn is the optical amplifier noise figure, h the Planck’s constant, y the optical frequency, Psig the optical amplifier input signal and y opt the optical bandwidth of the spontaneous-emission noise. Fig. 2 shows a calculated performance comparison of the original scheme employing a DFB laser and a PIN photodetector with a similar link that also includes an EDFA booster amplifier with optical output of 27 dBm and Fn of 5 dB.
Fig. 2 SFDR against optical loss with and without EDFA IEE Proc.-Optoelectron., Vol. 153, No. 4, August 2006
As shown, the use of the EDFA allows for a link GM increase of 25 dB.
atmospheric condition configuration.
3.4
4
Clear weather loss
The clear weather loss (CWL) is the total optical loss in clear weather conditions. The CWL is the sum of geometrical spreading loss and optical coupling loss between the laser to optical head and optical head to photodetector. The geometrical spreading loss for a line-of-sight link is simply the ratio of the surface area of the receive aperture to the footprint on surface area of the transmitting beam at the receiver. The geometrical spreading loss is given by Kim et al. [1] Gloss ¼
SAR SAT þ p=4 ðuRÞ2
ð10Þ
where SAR is the surface area of receive aperture, SAT the surface area of transmitter aperture, R the optical head range and u the transmitter divergence. Typical geometrical spreading loss parameters at a distance of R ¼ 500 m are SAR ¼ 0.02 M2, u ¼ 1 mrad resulting in a geometrical loss of Gloss ¼ 10 dB (SAT is neglected). Assuming a typical end-to-end coupling loss of 4 dB, the overall CWL is 14 dB. 3.5 Performance requirements for cellular application
and
the
practical
system
Practical system performances
4.1
System configuration
A full duplex optical link was implemented and tested in transferring cellular signals in both wavelengths, 1550 and 810 nm. The system includes two optical heads at a distance of 500 m, a linear DFB laser at wavelength of 1550 nm, an InGaAs PIN photodiode with a diameter of active area of 300 mm, and in the second link, a GaAlAs single mode laser at wavelength of 810 nm, a Si APD photodiode with a diameter of active area of 500 mm with a multiplication factor of up to 100. The system includes also RF gain control units and stabilising mechanisms. Special considerations were taken in order to ensure low optical reflections to minimise the non-linearities. 4.2
Measured performances
To measure the link performance, the following parameters were tested † Basic RF parameters against optical loss: gain, flatness, noise figure, SFDR, QPSK constellation curves. The optical loss was set using optical attenuators 15 –40 dB. † Long term measurements of RF parameter stability at various weather conditions.
The link requirements for transfering cellular signals are SFDR of 55 dB at 1 MHz bandwidth for CDMA [8] and SFDR of 65 dB at 200 KHz bandwidth for GSM [9]. In Fig. 3, calculation of the SFDR of the optical link is presented as a function of the optical loss for each cellular technique using the EDFA scheme, with the parameters mentioned above. In optical wireless systems, it is of interest to calculate the GM for each cellular technique (CDMA and GSM). The GM is defined by the difference between the maximum optical losses required to achieve a specific SFDR and clear weather optical losses. It can be shown from Fig. 3 that the GM for CDMA and GSM is 34 and 30 dB, respectively. We can now use the GM result to estimate the performance in a specific location by the local
1. Gain/flatness – remains constant: +0.5 dB. 2. Noise figure – remains at þ7 dB for optical loss , 25 dB with a pre-gain above 35 dB. þ11 dB at 25 dB optical loss and þ21 dB at 35 dB optical loss. 3. DQPSK constellation curves – the error vector magnitude remained unchanged for optical loss ,25 dB. 4. SFDR (Fig. 4). 5. RF stability – all RF parameters were stabilised to less than 1% at all weather conditions during the test.
Fig. 3 SFDR against optical loss for GSM and CDMA systems using EDFA
Fig. 4 SFDR against optical loss measurement results for APD (circle), PIN (pulse) and theoretical (continuous lines)
IEE Proc.-Optoelectron., Vol. 153, No. 4, August 2006
The RF testing results are as follows.
Changing the optical link design can vary the performancelimiting factor. For example, in the case of bandwidth of 1.5 GHz and photodetector diameter of 300 mm, the
197
receiver is a low front end (Ro ¼ 50 V). But in the case of a smaller bandwidth and photodetector diameter, the receiver used is a high front end (e.g. Ro ¼ 500 V). In this case the SFDR increases by 6.66 dB [2/3 10 log(500/50)]. Although this system may provide high-bandwidth communication channels over hundreds of metres, it is affected by fog, scattering and atmospheric turbulence. These conditions reduce the available received power. Therefore a system with excess optical power margin is required to allow for fading loss. 5
Conclusion
OWC is an attractive solution for a specific communication configuration, as it provides a cost-effective alternative to a fibre optic system. Furthermore, OWC provides high data rates and requires neither FCC licensing nor frequency allocation, as opposed to other wireless techniques. The excess optical power margin should be designed high enough to support fading loss. Here, inclusive performance analysis for analogue OWC link is presented leading to the decisive conclusion that a wireless configuration can be satisfying for both CDMA and GSM cellular techniques. The satisfying conclusion shows that the EDFA scheme delivers gain
198
margin of 34 dB for CDMA technique and gain margin of 30 dB for GSM technique. 6
Reference
1 Kim, I.I., Stiger, R., Koontz, J.A., Moursud, C., Barclay, M., Adhikari, P., Schuster, J., Korevaar, E., Ruigrok, R., and Decusatis, C.: ‘Wireless optical transmission of fast Ethernet, FDDI, ATM, and ESCON protocol data using the TerreLink laser communication system’, Opt. Eng, 1998, 37, (12), pp. 3143–3155 2 Einarsoon, G.: ‘Principles of lightwave communications’ (Wiley, England, 1996) 3 Nazarathy, M., Berger, J., Ley, A.J., Levy, I.M., and Kagan, Y.: ‘Progress in externally modulated AM CATV transmission systems’, J. Lightwave Technol, 1993, 11, (1), pp. 82– 105 4 Kaufmann, J.: ‘Analog link design’, Proc. SPIE: Free Space Commun. Technol., 1996, VIII 2699, pp. 38–49 5 Alexander, S.B.: ‘Optical communication receiver design’ (SPIE Optical Engineering Press, Bellingham, Washington, 1997) 6 Senior, J.M.: ‘Optical fiber communications: principles and practices’ (Prentice-Hall, New York, 1992, 2nd edn.) 7 Agrawal, G.P.: ‘Fiber optic communication systems’ (John Wiley & Sons, 1997, 2nd edn.) 8 Emura, K.: ‘Enabling technologies for SCM based optically fed wireless communication system’, Opt. Quantum Electron, 1998, 30, pp. 1089–1101 9 Huzinker, S., and Baechtold, W.: ‘Cellular remote antenna feeding: optical fiber or coaxial cable’, Electron. Lett., 1998, 34, (11), pp. 1038–1040
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