Multimode Fiber-fed Indoor Wireless Networks Nathan J. Gomes, Anjali Das, Anthony Nkansah, Majlinda Mjeku, and David Wake (Invited paper) Broadband and Wireless Communications Group, Department of Electronics, University of Kent, Canterbury, Kent, CT2 7NT, United Kingdom (email:
[email protected])
Abstract — A summary of results obtained for radio over multimode fiber links and system demonstrations is presented. It is stated that when using low-cost technologies such as pre-installed MMF and VCSELs, although adequate link performance is achievable, with careful design when taking into account the wireless path, MAC protocol effects may need to be considered for WLAN operation. For the simultaneous transmission of multiple signals, practical considerations of crosstalk and interference may cause an increase in the necessary complexity of remote antenna units. Index Terms — multimode fiber, radio over fiber, VCSELs, wireless LAN.
I. INTRODUCTION Multimode fiber continues to be deployed in significant quantities inside buildings, and it is predicted that the fastest growing part of the optical communications market will be that targeting legacy multimode fiber for installed lengths up to 300m [1]. Inbuilding radio over fiber systems using distributed antennas to provide coverage and capacity need to make use of this installed legacy multimode fiber (MMF). The multimode fiber can be used to provide wireless LAN (WLAN) and mobile telephony (GSM, UMTS) services. There are many systems now installed that transport mobile telephony signals by radio over fiber, albeit using single-mode fiber for (generally) outdoor, longer distance applications. Some newer systems are now using the same antenna units to transmit/receive WLAN signals from closely located access points (that is, over short lengths of coaxial or twisted-pair cable). However, the distribution of both types of signal over MMF distribution networks has only been demonstrated by research groups up to now, e.g. [2 - 9]. The growth in WLAN usage has led to much interest in WLAN transmission over MMF for indoor distribution networks. An advantage of such a system is that the access points (APs) do not need to be located in different rooms of the building, rather in the central “machine room”, making maintenance and upgrading simpler. One can also imagine scenarios where the APs can be switched to different remote antenna units (RAUs) to cater for variations in demand. This
switching could be carried out based on longer term needs or trends using patch panels, or more dynamically using RF switching matrices. However, there are, of course, some technical and performance limitations to the radio over MMF links which need to be overcome. In this paper, we will review some of the work that has been carried out to demonstrate how performance limitations can be overcome, and to demonstrate the operation of MMF networked wireless systems. In section II, experiments that have shown high quality WLAN signal transmission over MMF, and some experiments which have demonstrated the WLAN operation when using a MMF distribution network, are described. In section III, we look more closely at how the WLAN MAC protocol might be affected by MMF feeds. Finally, in section IV, we discuss the transmission of WLAN and other signals (such as UMTS, GSM) over MMF. II. WLAN OVER MMF: TRANSMISSION EXPERIMENTS It was shown that MMF could be used for transmission over RF carriers beyond the modal dispersion limited 3dB bandwidth [10 - 12]. For a distributed antenna system (DAS) to be implemented for WLANs, the DAS must incur low incremental cost in comparison to the whole WLAN. Thus, low-cost components such as VCSELs are attractive, and it was, indeed, shown that VCSELs could provide lowdistortion performance, close to that of Fabry-Perot and distributed feedback (DFB) lasers in analog links [13]. A combination of the use of VCSELs and MMF was demonstrated in [14], where a spurious free dynamic range (SFDR) of 94 dBHz2/3 in a frequency range 1 – 8 GHz was measured. In [2], WLAN signal transmission over MMF at 2.4 GHz and 5 GHz was reported with good signal performance. However, [2 - 4, 13, 14] only characterized the MMF link performance. The inclusion of the wireless link as demonstrated in [5, 6] poses additional restrictions, as discussed in the following paragraphs. In [5] uncooled DFB lasers were used to demonstrate the remoting (fiber link) lengths achievable with MMF. Although, wireless transmission (and simultaneous
transmission of IEEE 802.11a/b/g signals) was carried out, the main emphasis was on the quality (EVM) of the signals after transport over the MMF links. While most of the experiments above have characterized the performance of the radio over MMF links, and full wireless-optical links, using error vector magnitude (EVM) and packet error rate (PER) measurements, comparing these to the quoted maximum tolerable values in the standards, in the networks field many performance measurements of WLANs examine the throughput (and signal strength) [15]. This type of characterization was carried out in [6] for a complete WLAN over MMF system (that is, with full operation of the WLAN). The system used in the characterization is shown in Fig.1. The RFin and RFout of the Central Unit were connected to the antenna output of a WLAN AP through a circulator to separate downlink and uplink paths. An attenuator was required before the downlink CU laser (which was an 850 nm VCSEL). In the remote antenna unit (RAU), RF amplification was needed for both the downlink and the uplink. For the latter, an 850 nm VCSEL was again employed as the optical source. The RAU used two separate antennas for downlink transmission and uplink reception with their separation such that the isolation between them was about 25dB. The antennas, in fact, had been designed for multiband operation. The MMF used was different in the uplink and downlink, but standard OM1/OM2 grade fibers (not high bandwidth Gigabit Ethernet grade) of lengths of 300m were employed.
cannot be increased arbitrarily. This is due not only to the well-known dynamic range considerations, where for the uplink, in particular, transmissions from mobile users close to the antenna must be taken into account alongside those from users at the edge of the targeted wireless range, but also due to crosstalk problems. With crosstalk occurring both at the circulator at the AP and in the RAU, e.g. between transmit and receive antennas, a loop is easily set up. Preventing oscillations (singing) in this loop is the first consideration, and then simply preventing interference between the transmissions must be considered. The calculations presented in [6] permit the design of radio over fiber systems taking the gain and dynamic range of uplink and downlink (wireless and optical) paths into account, while minimizing any loop gain and interference problems, ensuring that lasers are not driven into nonlinear regions, and limiting transmitted noise. While [6] has demonstrated WLAN operation, further work is necessary: for example, to demonstrate the use of more than one RAU from an AP, and to demonstrate multiple mobile users. These factors will probably influence performance due to the MAC protocol, and such issues are discussed in the following section. III. WLAN OVER MMF: MAC PROTOCOL QUESTIONS WLAN MAC protocol issues are important as the fiber link adds considerable propagation delay to the transmission of fragments, acknowledgements and other signals (such as the RTS, CTS signals). Delay is an important parameter as the MAC protocol relies on sensing and transmitting after certain interframe delays. However, up to now, the full range of these issues have not been studied in detail for WLAN over fiber systems. 30
Basic fiber-fed distributed antenna system Maximum Fiber Length (km)
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Taking into account variations due to multipath effects, the measurements of signal strength in [6] verified link budget calculations which can be used in the design of such systems. The throughput measurements showed that the maximum throughput achievable was close to the expected levels for WLANs (approaching 22 Mbps for 54 Mbps data rate operation, and 6 Mbps for operation at 11 Mbps) [16]. The throughput decreased for greater wireless ranges as the system design had been carried out for wireless ranges of up to 5m. The work in [6] highlights a number of issues that must be considered for WLAN (and other radio system) transmission over fiber, which become more apparent when the complete optical-wireless link is considered. First, although the link gain must be optimized to counteract optical and wireless path losses and to maintain SNR, the RF amplifier gains used to do this
Theoretical results Simulation Results
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A principal concern that has been studied is the effect of the acknowledgement (ACK) time-out interval [9]. If the fiber length is too great such that the round-trip
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propagation delay is greater than the ACK time-out interval it is obvious that the WLAN cannot function. Fig. 2 shows the expected maximum fiber lengths that can be tolerated for a range of ACK time-out intervals. The calculations were verified with simulations in the network simulator, OPNET. An air propagation delay of 1 µs has been assumed; in most cases it will not be so large, but this has only the same effect as 100m of fiber in the path. It is interesting to note that in most cases the ACK time-out interval is a set parameter in the WLAN chipsets and its value can vary considerably. Control over this parameter could be useful for WLAN over fiber operation. While the extension of the ACK time-out interval can allow the WLAN to operate using longer fiber lengths, the throughput performance will degrade if delays increase [17].
of each user when there are two or three users is seen, as expected, the variability in the mobile devices makes general conclusions difficult. Clearly, such results need to be compared to simple WLAN cases (without the fiber links).
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Fig. 3 OPNET results for effect of fiber propagation delay on WLAN throughput for different fragmentation thresholds.
The MAC (DCF – distributed coordination function) protocol used in WLANs relies on acknowledgements, and on sensing the medium, which becomes problematic with long propagation delays compared to frame (fragment) transmission times. The effect of the fragmentation threshold on the throughput as the delay (fiber length) increases, as predicted by OPNET simulations, is shown in Fig. 3. Larger fragments mean that the medium is being utilized for a greater proportion of time, but this proportion decreases as the delay in waiting for acknowledgements rises. The abrupt threshold in Fig. 3 shows the maximum tolerable delays/fiber lengths as used in Fig. 2. These results are again based on a single user case, and further investigation for multiple users is required. Experimental results for the throughput for multiple users for a WLAN over (300m) MMF system similar to that in [6] are shown in Fig. 4. In fact, the throughput for single users is seen to be variable (probably depending on protocol parameters set in the chipsets employed); while a general reduction in the throughput
IV. MULTIPLE SYSTEM CONFIGURATIONS As stated in section I, up to now commercial radio over fiber systems have been used for the transport of mobile telephony signals (GSM, UMTS) using singlemode fiber (SMF). The use of MMF would be of interest to neutral-host providers for in-building applications, and overlaying the same fiber distribution network with transport of WLAN signals could prove highly cost effective. In [18], simultaneous transmission of four different wireless/mobile system signals (GSM, PHS, UMTS and WLAN) was demonstrated, with the system signals being paired so that each pair modulated a separate electro-absorption modulator; the two optical signals were then coupled together. Transport was over SMF. For operation over MMF, and using short wavelengths, the transmission of the WLAN signals at 2.4 GHz and 5 GHz in [2] was simultaneous, and the transmission of GSM-1800 and WCDMA signals over a VCSEL-MMF link was demonstrated in [3]. But, these
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Microwave Photonics, pp. 139–142, Budapest, Hungary, Sep. 2003. [5] P. Hartmann, X. Qian, R.V. Penty, I.H. White, “Broadband multimode fibre (MMF) based IEEE 802.11 a/b/g WLAN distribution system”, 2004 Int. Topical Mtg. on Microwave Photonics, pp. 173-176, Oganquit, Maine, 2004 [6] A. Das, A. Nkansah, N.J. Gomes, I.J. Garcia, J.C. Batchelor, D. Wake, “Design of low-cost multimode fiber fed indoor wireless networks”, to be published in IEEE Microwave Theory Tech [7] M. Sauer, A. Kobyakov, J. E. Hurley, J. George, “Experimental study of radio frequency transmission over standard and high-bandwidth multimode optical fibers”, 2005 Int. Topical Mtg. on Microwave Photonics, pp. 99102, Seoul, Korea, Oct. 2005 [8] A. Nkansah, A. Das, I.J. Garcia, C. Lethien, J-P. Vilcot, N.J. Gomes, J.C. Batchelor, D. Wake, “Simultaneous transmission of dual-band radio signals over a multimode fibre fed indoor wireless network”, submitted to IEEE Microw. Wireless Comp. Lett. [9] M. Garcia Larrode, A.M.J. Koonen, J.J. Vegas Olmos, G.J. Rijckenberg, L. Dang Bao, I. Niemegeers, “Transparent transport of wireless communication signals in radio-overfibre systems”, 10th European Conf. Networks and Optical Communications, NOC 2005, London, UK, July 2005 [10] L. Raddatz, D. Hardacre, I. H. White, R. V. Penty, D. G. Cunningham, M. R. T. Tan, S.-Y. Wang, “High bandwidth data transmission in multimode fibre links using subcarrier multiplexing with VCSELs”, Electron. Lett., vol. 34, pp. 686 – 688, 1998 [11] D. Wake, S. Dupont, J-P. Vilcot, A.J.Seeds, “32-QAM radio transmission over multimode fibre beyond the fibre bandwidth”, 2001 Int. Topical Mtg. on Microwave Photonics, Volume supplement, PDP, Paper2 ,Long Beach, CA, Jan. 2002 [12] E. J. Tyler, M. Webster, R. V. Penty, I. H. White, “Penalty free subcarrier modulated multimode fiber links for datacomm applications beyond the bandwidth limit”, IEEE Photon. Technol. Lett., vol. 14, pp. 110-112, Jan. 2002. [13] R. V. Dalal, R. J. Ram, R. Helkey, H. Rousell, K. D. Choquette, “Low distortion analog signal transmission using vertical cavity lasers”, Electron. Lett., vol. 34, pp. 1590-1591, Aug. 1998. [14] C. Carlsson, H. Martinsson, A. Larsson, A. Alping, “High performance microwave link using a multimode VCSEL and high-bandwidth multimode fiber”, 2001 Int. Topical Mtg. on Microwave Photonics, pp.81-84, Long Beach, CA, Jan. 2002. [15] See, e.g., M. Boulmalf, H. El-Sayed, A. Soufyan, “Measured throughput and SNR of IEEE802.11g in a small enterprise environment”, IEEE 61st Semiannual Vehicular Technology Conference, Stockholm, Sweden, May 2005 [16] R. Seide, “Capacity, coverage, and deployment considerations for IEEE 802.11g”, Cisco Systems white paper, San Jose, CA, Oct. 2003. [17] K. K. Leung, B. McNair, L. J. Cimini, Jr., J. H. Winters, “Outdoor IEEE 802.11 cellular networks: MAC protocol design and performance”, IEEE Int. Conf. Comms., ICC 2002, vol. 1, pp. 595-599, Apr. 2002 [18] P. K. Tang, L. C. Ong, A. Alphones, B. Luo, M. Fujise, “PER and EVM measurements of a radio-over-fiber network for cellular and WLAN system applications”, J. Lightwave Technol., vol. 22, pp 2370 – 2376, 2004
experiments measured the radio over fiber path only, and did not demonstrate the operation of the wireless path. As stated in section II, the wireless path adds further restrictions to the design, such as in radio signal power. In [8], different pairs of DPRS (DECT packet radio service), GSM, UMTS and 802.11g signals were transmitted over radio over MMF links with the additional wireless paths. Transmission of all signals was demonstrated, although for some systems only short distances (a few meters) were achieved. However, the demonstration system was not optimized for wireless range. Other considerations were also highlighted. The interference from the downlink signal being coupled back into the uplink can be reduced by filtering in FDD systems, and by switched amplifiers/echo suppression in TDD systems; but this could result in a very complex AU for multiband systems. VII. SUMMARY Multimode fibre-fed indoor wireless networks have generated a great deal of interest in the last few years. While radio over fiber transmission experiments have demonstrated the possibility of implementation of such networks, there are still issues to be considered. The demonstration of systems with a wireless path, using inexpensive components has been carried out, but demonstrates that careful design is required. The MAC protocol is affected by the additional fiber delay, with the extent of this problem unclear in widely ranging scenarios. The simultaneous transmission of both FDD (e.g. GSM) and TDD (e.g. WLAN) signals might also lead to additional RAU complexity. ACKNOWLEDGEMENT The authors wish to acknowledge the advice and help of their co-workers in the EU project “ROSETTE”, under the umbrella of which many of the experimental measurements in this paper were carried out, particularly J-P Vilcot, C Lethien, J Batchelor, B Sanz and I Garcia. We acknowledge part funding for this work from the “ROSETTE” and EU FP6 “ISIS” projects. REFERENCES [1] see, e.g., Ovum-RHK press release: http://www.ovum.com/go/content/c,377,63629 [2] M.Y.W. Chia, B. Luo, M.L. Yee, E.J.Z Hao, “Radio over multimode fibre transmission for wireless LAN using VCSELs”, Electron. Lett., vol. 39, pp. 1143 – 1144, July 2003 [3] R. E. Schuh, A. Alping, E. Sundberg, “Penalty-free GSM1800 and WCDMA radio-over-fibre transmission using multimode fibre and 850nm VCSEL”, Electron. Lett., vol. 39, pp. 512-514, Mar. 2003. [4] H. Sasai, T. Niiho, K. Tanaka, K. Utsumi, “Radio over fiber transmission performance of OFDM signal for dual-band wireless LAN systems”, 2003 Int. Topical Mtg. on
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