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A novel optical single-sideband frequency translation technique for transmission of wireless MIMO signals over fiber-wireless system
Optics & Laser Technology 47 (2013) 347–354
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A novel optical single-sideband frequency translation technique for transmission of wireless MIMO signals over fiber-wireless system Redhwan Q. Shaddad a,b,n, Abu Bakar Mohammad a, Abdulaziz M. Al-Hetar a,b, Samir A. Al-Gailani a,c a
Lightwave Communications Research Group (LCRG), Infocomm Research Alliance, Universiti Teknologi Malaysia, 81310 Johor, Malaysia Communication and Computer Engineering Department, Faculty of Engineering and Information Technology, Taiz University, Yemen c Industrial Technical Institute, Mua’lla, Aden, Yemen b
a r t i c l e i n f o
abstract
Article history: Received 3 August 2012 Received in revised form 12 September 2012 Accepted 19 September 2012 Available online 17 October 2012
The fiber-wireless (FiWi) access network is a powerful hybrid architecture of optical backhaul and wireless front-end to support high data rates and throughput with minimal time delay. By using radio over fiber (ROF) technique, the optical fiber is well adapted to propagate multiple wireless services having different carrier frequencies. However, multiple wireless signals which have the same carrier frequency cannot propagate over a single optical fiber on the same wavelength, such as multi-input multi-output (MIMO) signals. A novel optical single-sideband frequency translation technique is designed and simulated to solve this problem. 240 Mb/s 802.11n MIMO signals are proposed to transport over FiWi system using the proposed approach at 2.4 GHz and 5.0 GHz carrier frequencies. The crosstalk between MIMO signals with the same carrier frequency is excluded, since each MIMO signal is carried on a specific optical wavelength. Error vector magnitude (EVM) values of 29.83 dB (for 2.4 GHz) and 28.41 dB (for 5.0 GHz) have been achieved for bit error rate (BER) 10 5 in the proposed FiWi system. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Fiber wireless access network Radio over fiber Optical single-sideband frequency translation
1. Introduction Next generation access networks are projected to support high data rate, broadband multiple services, scalable bandwidth, and flexible communications for manifold end-users. There are many emerging optical and wireless access technologies proposed for these requirements [1–3]. The optical fiber access networks provide high-bandwidth digital services and long-distance communication, but less ubiquitous. The wireless access networks provide flexible and ubiquitous communication with a low deployment cost. However, its deployment scalability is limited by the spectrum and range limitations [4–6]. The FiWi access network is a powerful hybrid architecture of optical backhaul and wireless front-end. This hybrid FiWi access network supports high data rates and throughput with minimal time delay [7]. The FiWi networks are achieved by integrating optical access technologies (such as, passive optical networks (PONs)) and wireless access technologies (such as, cellular systems, worldwide interoperability for microwave access (WiMAX), and wireless fidelity (WiFi)) using ROF techniques [7–10].
n Corresponding author at: Lightwave Communications Research Group (LCRG), Infocomm Research Alliance, Universiti Teknologi Malaysia, 81310 Johor, Malaysia. Tel.: þ 60 172559558; fax: þ 60 75536155. E-mail address: [email protected] (R.Q. Shaddad).
0030-3992/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2012.09.028
For broadband access services, there is strong competition among several access technologies. Among the various emerging optical and wireless access technologies, the orthogonal frequency division multiplexing (OFDM) based technology is the most promising technology because it provides high transmission capacity, efficient bandwidth access, high flexibility on dynamic bandwidth allocation, and robust dispersion tolerance in both optical and wireless links [1,11]. Since the bandwidth demand of the wireless end user (WEU) increases dramatically in the wireless front-end, the optical backhaul is used to provide broadband interconnections between the central office (CO) and all the access points (APs). In this integrated network, complexity of the AP structure is reduced by moving the routing, bandwidth allocation, and processing functionalities to the CO. The FiWi system can offer essential benefits to future service providers [7]. Fig. 1 shows architecture of a FiWi access network. The optical backhaul is a tree network connecting the CO and wireless frontend. The optical backhaul comprises of an optical line terminal (OLT) at the CO, a standard single mode fiber (SSMF), a remote node (RN), and multiple APs. The wireless front-end consists of widespread APs to penetrate numerous WEUs. In this study, the wireless front-end is established using WiFi technology based on the IEEE 802.11n wireless local area network (WLAN). The 802.11n WLAN enhances the system capacity on a time-varying multipath fading channel by using a MIMO OFDM technique [12]. Two basic concepts are employed in 802.11n to increase the
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2. Transport of wireless MIMO signals over fiber
Fig. 1. FiWi access network architecture.
physical layer (PHY) data rates: MIMO and 40 MHz bandwidth channels. There are four spatial streams and four antennas instead of a single spatial stream and one antenna, which increase the data rate. These streams are multiplexed at the transmitter and demultiplexed at the receiver by using a spatial division multiplexing (SDM) technique. The 802.11n WLAN has many interesting characteristics such as high coverage, great throughput, high reliability and operates in both 2.4 GHz and 5.0 GHz radio frequency (RF) bands [13]. The transport of the wireless signal in the FiWi system is subject to several impairments, including dispersion effects in the fiber link and multipath fading in the wireless link [7]. The spectral efficient OFDM transmission provides an effective solution to eliminate intersymbol interference (ISI) caused by dispersive channels [11]. It is a known fact that multiple wireless signals having the same carrier frequency, such as MIMO signals specified in the 802.11n standard, cannot propagate over a single optical fiber on the same wavelength. A traditional solution using wavelength division multiplexing is expensive for this particular application, since the system then requires several optical sources and photodetectors [14]. In this paper, a new architecture of the FiWi system is proposed and designed based on an optical backhaul which transports wireless MIMO OFDM signals over fiber. A novel optical single-sideband frequency translation technique is designed and simulated to transport MIMO signals specified in the 802.11n standard over a single optical fiber to provide 240 Mb/s data rate at the WEU. The performance of the proposed FiWi system is analyzed in terms of BER, EVM, and signal-to-noise ratio (SNR). MATLAB, OptiSystem 11.0 and Advanced Design System (ADS) 2008 software tools are used to simulate the proposed FiWi system, where the 802.11n signals are modulated at the carrier frequencies of 2.4 GHz and 5.0 GHz. This paper is organized as follows: in Section 2, the literature review for transport of wireless MIMO signals over optical fiber is introduced, the principles of the proposed novel method are elaborated in Section 3, the FiWi system design and simulation considerations are covered in Section 4, Section 5 is dedicated to describe the result analysis and discussion, and finally, Section 6 concludes the paper and suggests future work.
For wireless broadband transmission, the MIMO radio system has been developed [13] and implemented using multiple transmit/receive antennas. MIMO system is distinguished by improving transmission range/reliability, and delivering higher data transmission rates over the single-input single-output (SISO) system. One of the main problems of wireless link is a channel fading, especially in the multipath channels. The merge of the throughput improvement and path diversity proposed by MIMO technique [15] with the OFDM immunity to dispersive fading of channel is considered as a favorable combination for the broadband wireless access network [16,17]. By using ROF techniques, the optical fiber is well adapted to pass multiple wireless signals having different carrier frequencies. However, multiple wireless signals which have the same carrier frequency cannot propagate over an optical fiber, such as MIMO signals feeding multiple antennas in FiWi system. This problem starts once multiple MIMO signals are combined and modulated onto a single optical carrier. Individual MIMO signals could not be separated and recovered thereafter with regular electrical filtering. The solution for this problem was proposed using WDM and sub-carriers multiplexing (SCM) [14,18] techniques. These techniques are not cost-effective, since multiple optical sources and photodetectors are required. Transmission of three wireless MIMO signals all with 2.44 GHz carrier frequency over an optical fiber is proposed and demonstrated using an electrical single-sideband frequency-translation technique [19]. The proposed approach decreased the maximum crosstalk level between the different MIMO channels as compared to transport the same signals by SCM technique. In this paper, we propose and demonstrate transport of two wireless MIMO signals with the same carrier frequency over a fiber in the FiWi system using an optical single-sideband frequency translation technique. The novel approach does not need low-frequency local oscillators (LOs) at the OLT and the AP as compared to [19]. One optical source is enough to transport several MIMO signals over the optical fiber by using the optical single-sideband frequency translation technique. The FiWi system based on the new approach can also support the wavelength reuse technique, so one optical source is enough to generate the optical carrier which is reused at the AP, and many wavelengths which convey number of MIMO signals over the SSMF [20].
3. Principles of optical single-sideband frequency translation for transport of wireless MIMO signals over fiber The block diagram of the proposed approach is shown in Fig. 2(a). This technique uses the optical double sideband (ODSB) technique to generate three wavelengths for each laser diode (LD): the two single-sideband wavelengths (such as ld11, ld12 as shown in Fig. 2(a)), and same optical carrier frequency (such as l1). The two single-sideband wavelengths ld11, ld12 are used to modulate two MIMO signals separately. The wavelength l1 is reused to generate uplink wavelength at the AP, using the optical carrier suppression and separation (OCSS) modulation scheme [8,21]. A dual-arm modulator (DAM) is used to generate the three wavelengths from the LD. Fig. 2(b) shows the wavelength of the LD as the inset (i), and the three generated wavelengths from the DAM as the inset (ii). Two optical interleavers (ILS) are used after the DAM to separate the three generated wavelengths into two channel groups: one continuous wave (CW) wavelength l1, and other single-sidebands with the two downlink wavelengths ld11, ld12 (as shown in Fig. 2(b) inset (iii)) which are used to modulate the MIMO signals at the external intensity optical modulators
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(IMs). The channel spacing between each downlink wavelength and the CW wavelength is fo, so the channel spacing of the generated downlink wavelength pair is determined by twice the frequency of the sinusoidal clock 2 fo, as illustrated in Fig. 2(b) inset (iii). The type of the used ILs is 1:2 WDM interleaver, since each IL separates even channels from odd channels across a WDM comb onto two different output ports. The amplitude spectrum at either of the two output ports has a pass-band and a stop-band periodically with a free-spectral range (FSR) [22]. In this design, the WDM ILs are implemented by using 2:2 AWG routers in WDM demultiplexing mode. Because the number of channels per output port is small, the worst-case frequency offset has small value, which can be neglected [22]. The OLT implements an 802.11n MIMO data processor. The MIMO data processor generates two MIMO signals X1, X2 with the same carrier frequency f1 as shown in Fig. 2(b) inset (iv) and (v), respectively. The two MIMO signals X1, X2 are biased to be compatible with the nature of the optical signals and then optically modulated by the two IMs on the two assigned wavelengths ld11, ld12, respectively. In this method, each IM modulates the MIMO signal X1, X2 by using optical single sideband with carrier (OSSBþC) modulation scheme as illustrated in Fig. 2(b) as the insets (vi) and (vii), respectively. The OSSBþC modulation method enhances optical spectral efficiency for transporting modulated RF signals over optical fiber, and overcomes the fiber chromatic dispersion problem [7,8]. The modulated optical signals with the downlink wavelength pair (ld11, ld12) are amplified by an optical amplifier (such as erbium doped fiber amplifier (EDFA)) and then coupled with the optical CW signals with wavelength l1) as shown in Fig. 2 inset (viii). The coupled optical signal propagates along an SSMF. The AP receives the optical downstream, interleaves it into the CW optical signal with the wavelength (l1), and the two modulated optical signals with the wavelengths (ld11, ld12) as shown in Fig. 2(b) in the insets (ix–xi). There is a small additive noise which is produced by the optical segment. The AP then converts the two modulated optical signals directly to the suitable electrical signals by using an optical receiver for each signal. The electrical signals are then band-pass filtered according to the allocated RF frequency f1 by using BPFs to get the MIMO OFDM signals (X0 1, X0 2). The wireless MIMO signals then propagate to the WEU using 2 2 MIMO technique.
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4. FiWi system design FiWi system design comprises of an OLT at the CO connected to a wireless AP through an SSMF in the downstream direction as shown in Fig. 2(a). The AP then transmits the wireless MIMO signals through the wireless channels to the MIMO end-user. In this paper, the FiWi system is designed to transport wireless 802.11n MIMO signals over fiber using a novel optical single-sideband frequency translation technique. In the CO, the WiFi 802.11n data processor generates two MIMO signals. The OFDM technique is implemented with a 64-Quadrature amplitude modulation (QAM). The 802.11n processor is designed to provide a 240 Mb/s data rate with a channel bandwidth of 40 MHz. The two MIMO signals (X1, X2) are modulated on RF carrier frequency f1 which is selected from the 2.4 GHz or 5.0 GHz RF frequency bands. The LD generates an optical signal with a wavelength l1 ¼ 1552.500 nm (193.100 GHz). The DAM, with a clock frequency fo ¼25 GHz, is used to produce the two downlink wavelengths {ld11 ¼ 1552.725 nm (193.075 GHz), and ld12 ¼1552.323 nm (193.125 GHz)} as shown in Fig. 3. The two downlink wavelengths ld11 and ld12 are used to modulate the two MIMO signals X1, X2 separately at IMs. Furthermore, a CW optical signal with optical carrier l1 ¼ 1552.500 nm (193.100 GHz) is created at the CO. This signal is reused at the AP to produce uplink wavelengths. Fig. 3 shows the input optical power spectra of the down/uplink channels to the SSMF as the inset (i). The wireless MIMO-OFDM signals (X1 and X2) with the RF carrier frequency of 5.0 GHz are upconverted over upper singlesideband of the optical carrier wavelengths ld11 and ld12 using OSSBþ C modulation scheme as shown in Fig. 3 in the insets (ii) and (iii), respectively. The frequency separation between each downlink wavelength and its optical single-sideband is 50 GHz as shown in Fig. 3. The three optical signals with wavelengths (ld11, ld12 and l1) are coupled and then propagated along a 20 km SSMF with attenuation of 0.2 dB/km and dispersion coefficient of 17 ps/nm/km. In this design, the launched optical power into the fiber has a suitable value (3.701 dBm) to avoid signal impairment by fiber nonlinearity [23]. The AP receives the optical downstream, interleaves it to the modulated optical signals, and then converts it directly to electrical signals by using PDs with power sensitivity of 30 dBm. The electrical signals are band-pass filtered according to the allocated RF frequency by using BPFs to get the MIMO
Fig. 2. Transport of wireless MIMO signals over optical fiber using the optical single-sideband frequency translation (a) block diagram of the proposed FiWi system and (b) power spectra of the signals according to the indicated insets in the structure.
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Fig. 2. Continued.
OFDM signals (X0 1, X0 2) which are the same as the transmitted signals (X1, X2) with small noise generated by the optical elements and the optical link. The wireless MIMO signals (X0 1, X0 2) propagate through MIMO channel to the WEU using 2 2 MIMO OFDM technique. The AP supports data rate up to 240 Mb/s through 100 m outdoor wireless link with high spectral efficiency of 6. The general specification of the optical backhaul and wireless frontend of the proposed FiWi system is summarized and shown in Table 1.
The SNR and the BER are estimated according to [24]. The MIMO system has been considered over the fiber-wireless channel. The purpose of the MIMO channel estimation, in the receiver, is to identify the optical channel and wireless channel between each pair of transmit and receive antennas. For MIMO channel estimation, a high throughput long training field (HT-LTF) is provided in the high throughput-mixed format (HT-MF) preamble of the IEEE 802.11n. This technique is called training symbol-based channel estimation [12,17], since a long training symbol is transmitted for
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Fig. 3. Optical power spectra of the allocated channels at the OLT.
Table 1 The general specification of the proposed FiWi system. Parameter
Value
Optical Backhaul Transmitted power of LD Receiver sensitivity Fiber length Attenuation Dispersion coefficient Optical modulation method
5 dBm 30 dBm 20 km 0.2 dB/km 17 ps/nm/km OSSB þC
Wireless Front-end Transmitted power of the AP Carrier frequencies Channel bandwidth Data rate of the AP Link range Channel type Radio technology Size of MIMO system Modulation Coding scheme Bandwidth efficiency
16 dBm 2.4 GHz or 5.0 GHz band 40 MHz 240 Mb/s 100 m Fading channel OFDM MIMO 2 2 MIMO 64-QAM Convolutional code of rate 2/3 6
each spatial stream indicated by the selected modulation and coding scheme (MCS) [13]. In the simulation design, the frequency selective behaviors caused by chromatic dispersion in the optical link and channel fading in the wireless link are characterized in the training symbols at the IEEE 802.11n data processor in the OLT. The
training symbol-based channel estimation is followed by the maximum likelihood (ML) MIMO detection process which determines the transmitted data symbols from the received signal [15,17]. In contrast, the training-based channel estimation has the relatively low computational complexity at the receiver and draws more interests in the OFDM system [25].
5. Results and discussion The group of the wavelengths (ld11, l1 and ld12) are received at the AP and then demultiplexed. The AP downconverts the optical signals at the interleaved downlink wavelengths (ld11 and ld12) to the corresponding MIMO OFDM signals and bandpass filter them according to the RF carrier frequency. The detected electrical signal is filtered by using 8th order Butterworth BPFs, with a 60 MHz bandwidth, and center frequencies 2.4 GHz or 5.0 GHz. The frequency responses of these BPFs are illustrated in Fig. 4(a) and (b) for the center frequencies 2.4 GHz and 5.0 GHz, respectively. These filters have a frequency response as determined by the following transfer function [26] B n Hðf Þ ¼ a:
2
B p 2k þ 1 Pn1 k ¼ 0 j f f c 2 :exp j 2 1 þ n
ð1Þ
where H(f) is the transfer function of the BPF, f is the frequency parameter, n is the order of the filter, a is the insertion loss (a has
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Fig. 4. (a,b) The frequency response of the BPFs, and (c–f) the spectra of the transmitted wireless signals X0 1 and X0 2; at 2.4 GHz and 5.0 GHz frequencies.
been setting to one in the simulation design), B is the 3 dB filter bandwidth, and fc is the filter center frequency. The filtered signal is amplified by using RF amplifier providing a suitable transmitted power level (16 dBm). At the AP, the transmitted MIMO OFDM signals X0 1 and X0 2 at the RF carrier frequency of 2.4 GHz have spectra shown in Fig. 4(c) and (d), respectively. In addition, the spectra of the transmitted MIMO OFDM signals X0 1 and X0 2 at 5.0 GHz carrier frequency are shown in Fig. 4(e) and (f), respectively. There is no crosstalk between the MIMO OFDM signals which have the same frequency, because each signal is carried on an independent wavelength with a large channel spacing. The bandwidths of the MIMO OFDM signals X0 1 and X0 2 are 40 MHz allocated on the same RF carrier frequency. These signals are transmitted on the wireless channel to the MIMO WEU. The BER was analyzed to evaluate the system performance. Fig. 5 shows BER versus SNR for the received signal at the receiver of the WEU. The additive white Gaussian noise (AWGN) channel and fading channel models were simulated in the wireless MIMO OFDM system, since 20 km SSMF with a dispersion coefficient of 17 ps/nm/km is considered in the optical backhaul. The SNR required for a BER of 10 5 as specified for RF transmission can be determined for the plots in Fig. 5(a) under AWGN channel for
2.4 GHz and 5.0 GHz carrier frequencies. The SNR values of 15 dB and 16.5 dB achieved BER of 10 5 at the carrier frequencies of 2.4 GHz and 5.0 GHz, respectively. In addition, BER versus SNR, under fading channel for 2.4 GHz and 5.0 GHz carrier frequencies, is shown in Fig. 5(b). The SNR values of 28 dB and 30 dB achieved BER of 10 5 at the carrier frequencies of 2.4 GHz and 5.0 GHz, respectively. There is a small difference between the required SNR to achieve BER of 10 5 at the carrier frequencies of 2.4 GHz and 5.0 GHz for the both wireless channel models. The optical signal-to-noise ratio (OSNR) is estimated at the optical receiver entrance point with 0.1 nm noise bandwidth. The BER performance versus the OSNR is shown in Fig. 6. When the AWGN wireless channel is taken into account, low OSNR values (17.26 dB and 19.24 dB) are required to achieve a minimum BER of 10 5 at the WEU for the frequencies 2.4 GHz and 5.0 GHz, respectively, as shown in Fig. 6(a). To realize a good BER performance considering the fading channel in wireless link, high OSNR values (32.61 dB and 34.11 dB) are required for the frequencies 2.4 GHz and 5.0 GHz, respectively, as shown in Fig. 6(b). The optical amplifier generates the most noise which is produced by the optical backhaul [27]. The created noise from the wireless channel and receiver front-end dominates the whole FiWi system
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Fig. 5. BER versus SNR at the receiver of the WEU considering (a) the wireless AWGN channel and (b) the wireless fading channel.
performance, especially when the wireless fading channel is considered [24]. Fig. 7(a) and (b) shows the simulated 240 Mb/s 64-QAM constellation diagrams for the transmitted and received signals from the data processor at the OLT to the wireless end-user at the RF carrier frequencies of 2.4 GHz and 5.0 GHz, respectively. Clear scatter-plots are achieved for the both RF frequencies, for a SNR of 30 dB and BER close to 10 5, which is well detected and undistorted, indicating little degradation due to the transmission over the FiWi link. The EVMs of 29.83 dB (for 2.4 GHz) and 28.41 dB (for 5.0 GHz) achieved the good performance of the proposed system. The EVMs are calculated considering the following equation [28]: " EVMðdBÞ ¼ 10:log10
2
SM k ¼ 1 9Stx,k Srx,k 9 2
SM k ¼ 1 9Stx,k 9
# ð2Þ
where EVM is the value of the difference between a collection of received symbols and transmitted or ideal symbols, Stx,k is the corresponding transmitted symbol of the constellation associated with the kth symbol, Srx,k is the received symbol associated with Stx,k, and M is the number of the symbols for the InphaseQuadrature constellation.
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Fig. 6. BER versus OSNR considering (a) the wireless AWGN channel and (b) the wireless fading channel.
6. Conclusions and future work The hybrid FiWi system is proposed and demonstrated based on the transmission of wireless 802.11n MIMO signals over fiber by using an efficient novel optical single-sideband frequency translation technique. The proposed FiWi system supports the wavelength reuse technique, so one optical source is enough to generate the optical carrier which is reused at the AP, and many wavelengths for carrying multiple wireless MIMO signals over the fiber. The crosstalk between different MIMO signals with the same RF carrier frequency is excluded, since each MIMO signal is carried on a specific optical wavelength. The physical layer performance has been reported in terms of the BER, the EVM, and the SNR. The proposed FiWi system achieved transmission of wireless 802.11n MIMO signals over 20 km SSMF and along 100 m outdoor MIMO wireless channel. This was investigated using RF carrier frequencies of 2.4 GHz and 5.0 GHz to provide 240 Mb/s data rate with a channel bandwidth of 40 MHz. In future, this system can be redesigned by using higher data rate up to 600 Mb/s based on IEEE 802.11n. In addition, multiple 2 2 MIMO signals with different frequencies on the same frequency band of 2.4 GHz or 5.0 GHz could be multiplexed and then transmitted over fiber using the proposed novel approach.
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Fig.7. The constellation diagram of the transmitted and received signals from the data processor at OLT to the WEU, considering (a) 2.4 GHz and (b) 5.0 GHz.
Acknowledgments We greatly appreciate Universiti Teknologi Malaysia and Photonics Research Laboratory for providing the facilities which enabled this work to be accomplished. We would also like to thank the Ministry of Science, Technology and Innovation (MOSTI), Malaysia for sponsoring this work under project vote number 73720. References [1] Chow CW, Yeh C, Wang C, Wu C, Chi S, Lin C. Studies of OFDM signal for broadband optical access networks. IEEE Journal on Selected Areas in Communications 2010;28(6):800–7. [2] Shaddad, R, Mohammad, AB, Idrus, S, Al-hetar, A, Al-geelani, N. Emerging optical broadband access networks from TDM PON to OFDM PON, In: Proceedings of PIERS 2012, Kuala Lumpur, Malaysia, 2012. p. 102–6.
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A novel optical single-sideband frequency translation technique for transmission of wireless MIMO signals over fiber-wireless system Redhwan Q. Shaddad a,b,n, Abu Bakar Mohammad a, Abdulaziz M. Al-Hetar a,b, Samir A. Al-Gailani a,c a
Lightwave Communications Research Group (LCRG), Infocomm Research Alliance, Universiti Teknologi Malaysia, 81310 Johor, Malaysia Communication and Computer Engineering Department, Faculty of Engineering and Information Technology, Taiz University, Yemen c Industrial Technical Institute, Mua’lla, Aden, Yemen b
a r t i c l e i n f o
abstract
Article history: Received 3 August 2012 Received in revised form 12 September 2012 Accepted 19 September 2012 Available online 17 October 2012
The fiber-wireless (FiWi) access network is a powerful hybrid architecture of optical backhaul and wireless front-end to support high data rates and throughput with minimal time delay. By using radio over fiber (ROF) technique, the optical fiber is well adapted to propagate multiple wireless services having different carrier frequencies. However, multiple wireless signals which have the same carrier frequency cannot propagate over a single optical fiber on the same wavelength, such as multi-input multi-output (MIMO) signals. A novel optical single-sideband frequency translation technique is designed and simulated to solve this problem. 240 Mb/s 802.11n MIMO signals are proposed to transport over FiWi system using the proposed approach at 2.4 GHz and 5.0 GHz carrier frequencies. The crosstalk between MIMO signals with the same carrier frequency is excluded, since each MIMO signal is carried on a specific optical wavelength. Error vector magnitude (EVM) values of 29.83 dB (for 2.4 GHz) and 28.41 dB (for 5.0 GHz) have been achieved for bit error rate (BER) 10 5 in the proposed FiWi system. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Fiber wireless access network Radio over fiber Optical single-sideband frequency translation
1. Introduction Next generation access networks are projected to support high data rate, broadband multiple services, scalable bandwidth, and flexible communications for manifold end-users. There are many emerging optical and wireless access technologies proposed for these requirements [1–3]. The optical fiber access networks provide high-bandwidth digital services and long-distance communication, but less ubiquitous. The wireless access networks provide flexible and ubiquitous communication with a low deployment cost. However, its deployment scalability is limited by the spectrum and range limitations [4–6]. The FiWi access network is a powerful hybrid architecture of optical backhaul and wireless front-end. This hybrid FiWi access network supports high data rates and throughput with minimal time delay [7]. The FiWi networks are achieved by integrating optical access technologies (such as, passive optical networks (PONs)) and wireless access technologies (such as, cellular systems, worldwide interoperability for microwave access (WiMAX), and wireless fidelity (WiFi)) using ROF techniques [7–10].
n Corresponding author at: Lightwave Communications Research Group (LCRG), Infocomm Research Alliance, Universiti Teknologi Malaysia, 81310 Johor, Malaysia. Tel.: þ 60 172559558; fax: þ 60 75536155. E-mail address: [email protected] (R.Q. Shaddad).
0030-3992/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2012.09.028
For broadband access services, there is strong competition among several access technologies. Among the various emerging optical and wireless access technologies, the orthogonal frequency division multiplexing (OFDM) based technology is the most promising technology because it provides high transmission capacity, efficient bandwidth access, high flexibility on dynamic bandwidth allocation, and robust dispersion tolerance in both optical and wireless links [1,11]. Since the bandwidth demand of the wireless end user (WEU) increases dramatically in the wireless front-end, the optical backhaul is used to provide broadband interconnections between the central office (CO) and all the access points (APs). In this integrated network, complexity of the AP structure is reduced by moving the routing, bandwidth allocation, and processing functionalities to the CO. The FiWi system can offer essential benefits to future service providers [7]. Fig. 1 shows architecture of a FiWi access network. The optical backhaul is a tree network connecting the CO and wireless frontend. The optical backhaul comprises of an optical line terminal (OLT) at the CO, a standard single mode fiber (SSMF), a remote node (RN), and multiple APs. The wireless front-end consists of widespread APs to penetrate numerous WEUs. In this study, the wireless front-end is established using WiFi technology based on the IEEE 802.11n wireless local area network (WLAN). The 802.11n WLAN enhances the system capacity on a time-varying multipath fading channel by using a MIMO OFDM technique [12]. Two basic concepts are employed in 802.11n to increase the
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2. Transport of wireless MIMO signals over fiber
Fig. 1. FiWi access network architecture.
physical layer (PHY) data rates: MIMO and 40 MHz bandwidth channels. There are four spatial streams and four antennas instead of a single spatial stream and one antenna, which increase the data rate. These streams are multiplexed at the transmitter and demultiplexed at the receiver by using a spatial division multiplexing (SDM) technique. The 802.11n WLAN has many interesting characteristics such as high coverage, great throughput, high reliability and operates in both 2.4 GHz and 5.0 GHz radio frequency (RF) bands [13]. The transport of the wireless signal in the FiWi system is subject to several impairments, including dispersion effects in the fiber link and multipath fading in the wireless link [7]. The spectral efficient OFDM transmission provides an effective solution to eliminate intersymbol interference (ISI) caused by dispersive channels [11]. It is a known fact that multiple wireless signals having the same carrier frequency, such as MIMO signals specified in the 802.11n standard, cannot propagate over a single optical fiber on the same wavelength. A traditional solution using wavelength division multiplexing is expensive for this particular application, since the system then requires several optical sources and photodetectors [14]. In this paper, a new architecture of the FiWi system is proposed and designed based on an optical backhaul which transports wireless MIMO OFDM signals over fiber. A novel optical single-sideband frequency translation technique is designed and simulated to transport MIMO signals specified in the 802.11n standard over a single optical fiber to provide 240 Mb/s data rate at the WEU. The performance of the proposed FiWi system is analyzed in terms of BER, EVM, and signal-to-noise ratio (SNR). MATLAB, OptiSystem 11.0 and Advanced Design System (ADS) 2008 software tools are used to simulate the proposed FiWi system, where the 802.11n signals are modulated at the carrier frequencies of 2.4 GHz and 5.0 GHz. This paper is organized as follows: in Section 2, the literature review for transport of wireless MIMO signals over optical fiber is introduced, the principles of the proposed novel method are elaborated in Section 3, the FiWi system design and simulation considerations are covered in Section 4, Section 5 is dedicated to describe the result analysis and discussion, and finally, Section 6 concludes the paper and suggests future work.
For wireless broadband transmission, the MIMO radio system has been developed [13] and implemented using multiple transmit/receive antennas. MIMO system is distinguished by improving transmission range/reliability, and delivering higher data transmission rates over the single-input single-output (SISO) system. One of the main problems of wireless link is a channel fading, especially in the multipath channels. The merge of the throughput improvement and path diversity proposed by MIMO technique [15] with the OFDM immunity to dispersive fading of channel is considered as a favorable combination for the broadband wireless access network [16,17]. By using ROF techniques, the optical fiber is well adapted to pass multiple wireless signals having different carrier frequencies. However, multiple wireless signals which have the same carrier frequency cannot propagate over an optical fiber, such as MIMO signals feeding multiple antennas in FiWi system. This problem starts once multiple MIMO signals are combined and modulated onto a single optical carrier. Individual MIMO signals could not be separated and recovered thereafter with regular electrical filtering. The solution for this problem was proposed using WDM and sub-carriers multiplexing (SCM) [14,18] techniques. These techniques are not cost-effective, since multiple optical sources and photodetectors are required. Transmission of three wireless MIMO signals all with 2.44 GHz carrier frequency over an optical fiber is proposed and demonstrated using an electrical single-sideband frequency-translation technique [19]. The proposed approach decreased the maximum crosstalk level between the different MIMO channels as compared to transport the same signals by SCM technique. In this paper, we propose and demonstrate transport of two wireless MIMO signals with the same carrier frequency over a fiber in the FiWi system using an optical single-sideband frequency translation technique. The novel approach does not need low-frequency local oscillators (LOs) at the OLT and the AP as compared to [19]. One optical source is enough to transport several MIMO signals over the optical fiber by using the optical single-sideband frequency translation technique. The FiWi system based on the new approach can also support the wavelength reuse technique, so one optical source is enough to generate the optical carrier which is reused at the AP, and many wavelengths which convey number of MIMO signals over the SSMF [20].
3. Principles of optical single-sideband frequency translation for transport of wireless MIMO signals over fiber The block diagram of the proposed approach is shown in Fig. 2(a). This technique uses the optical double sideband (ODSB) technique to generate three wavelengths for each laser diode (LD): the two single-sideband wavelengths (such as ld11, ld12 as shown in Fig. 2(a)), and same optical carrier frequency (such as l1). The two single-sideband wavelengths ld11, ld12 are used to modulate two MIMO signals separately. The wavelength l1 is reused to generate uplink wavelength at the AP, using the optical carrier suppression and separation (OCSS) modulation scheme [8,21]. A dual-arm modulator (DAM) is used to generate the three wavelengths from the LD. Fig. 2(b) shows the wavelength of the LD as the inset (i), and the three generated wavelengths from the DAM as the inset (ii). Two optical interleavers (ILS) are used after the DAM to separate the three generated wavelengths into two channel groups: one continuous wave (CW) wavelength l1, and other single-sidebands with the two downlink wavelengths ld11, ld12 (as shown in Fig. 2(b) inset (iii)) which are used to modulate the MIMO signals at the external intensity optical modulators
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(IMs). The channel spacing between each downlink wavelength and the CW wavelength is fo, so the channel spacing of the generated downlink wavelength pair is determined by twice the frequency of the sinusoidal clock 2 fo, as illustrated in Fig. 2(b) inset (iii). The type of the used ILs is 1:2 WDM interleaver, since each IL separates even channels from odd channels across a WDM comb onto two different output ports. The amplitude spectrum at either of the two output ports has a pass-band and a stop-band periodically with a free-spectral range (FSR) [22]. In this design, the WDM ILs are implemented by using 2:2 AWG routers in WDM demultiplexing mode. Because the number of channels per output port is small, the worst-case frequency offset has small value, which can be neglected [22]. The OLT implements an 802.11n MIMO data processor. The MIMO data processor generates two MIMO signals X1, X2 with the same carrier frequency f1 as shown in Fig. 2(b) inset (iv) and (v), respectively. The two MIMO signals X1, X2 are biased to be compatible with the nature of the optical signals and then optically modulated by the two IMs on the two assigned wavelengths ld11, ld12, respectively. In this method, each IM modulates the MIMO signal X1, X2 by using optical single sideband with carrier (OSSBþC) modulation scheme as illustrated in Fig. 2(b) as the insets (vi) and (vii), respectively. The OSSBþC modulation method enhances optical spectral efficiency for transporting modulated RF signals over optical fiber, and overcomes the fiber chromatic dispersion problem [7,8]. The modulated optical signals with the downlink wavelength pair (ld11, ld12) are amplified by an optical amplifier (such as erbium doped fiber amplifier (EDFA)) and then coupled with the optical CW signals with wavelength l1) as shown in Fig. 2 inset (viii). The coupled optical signal propagates along an SSMF. The AP receives the optical downstream, interleaves it into the CW optical signal with the wavelength (l1), and the two modulated optical signals with the wavelengths (ld11, ld12) as shown in Fig. 2(b) in the insets (ix–xi). There is a small additive noise which is produced by the optical segment. The AP then converts the two modulated optical signals directly to the suitable electrical signals by using an optical receiver for each signal. The electrical signals are then band-pass filtered according to the allocated RF frequency f1 by using BPFs to get the MIMO OFDM signals (X0 1, X0 2). The wireless MIMO signals then propagate to the WEU using 2 2 MIMO technique.
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4. FiWi system design FiWi system design comprises of an OLT at the CO connected to a wireless AP through an SSMF in the downstream direction as shown in Fig. 2(a). The AP then transmits the wireless MIMO signals through the wireless channels to the MIMO end-user. In this paper, the FiWi system is designed to transport wireless 802.11n MIMO signals over fiber using a novel optical single-sideband frequency translation technique. In the CO, the WiFi 802.11n data processor generates two MIMO signals. The OFDM technique is implemented with a 64-Quadrature amplitude modulation (QAM). The 802.11n processor is designed to provide a 240 Mb/s data rate with a channel bandwidth of 40 MHz. The two MIMO signals (X1, X2) are modulated on RF carrier frequency f1 which is selected from the 2.4 GHz or 5.0 GHz RF frequency bands. The LD generates an optical signal with a wavelength l1 ¼ 1552.500 nm (193.100 GHz). The DAM, with a clock frequency fo ¼25 GHz, is used to produce the two downlink wavelengths {ld11 ¼ 1552.725 nm (193.075 GHz), and ld12 ¼1552.323 nm (193.125 GHz)} as shown in Fig. 3. The two downlink wavelengths ld11 and ld12 are used to modulate the two MIMO signals X1, X2 separately at IMs. Furthermore, a CW optical signal with optical carrier l1 ¼ 1552.500 nm (193.100 GHz) is created at the CO. This signal is reused at the AP to produce uplink wavelengths. Fig. 3 shows the input optical power spectra of the down/uplink channels to the SSMF as the inset (i). The wireless MIMO-OFDM signals (X1 and X2) with the RF carrier frequency of 5.0 GHz are upconverted over upper singlesideband of the optical carrier wavelengths ld11 and ld12 using OSSBþ C modulation scheme as shown in Fig. 3 in the insets (ii) and (iii), respectively. The frequency separation between each downlink wavelength and its optical single-sideband is 50 GHz as shown in Fig. 3. The three optical signals with wavelengths (ld11, ld12 and l1) are coupled and then propagated along a 20 km SSMF with attenuation of 0.2 dB/km and dispersion coefficient of 17 ps/nm/km. In this design, the launched optical power into the fiber has a suitable value (3.701 dBm) to avoid signal impairment by fiber nonlinearity [23]. The AP receives the optical downstream, interleaves it to the modulated optical signals, and then converts it directly to electrical signals by using PDs with power sensitivity of 30 dBm. The electrical signals are band-pass filtered according to the allocated RF frequency by using BPFs to get the MIMO
Fig. 2. Transport of wireless MIMO signals over optical fiber using the optical single-sideband frequency translation (a) block diagram of the proposed FiWi system and (b) power spectra of the signals according to the indicated insets in the structure.
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Fig. 2. Continued.
OFDM signals (X0 1, X0 2) which are the same as the transmitted signals (X1, X2) with small noise generated by the optical elements and the optical link. The wireless MIMO signals (X0 1, X0 2) propagate through MIMO channel to the WEU using 2 2 MIMO OFDM technique. The AP supports data rate up to 240 Mb/s through 100 m outdoor wireless link with high spectral efficiency of 6. The general specification of the optical backhaul and wireless frontend of the proposed FiWi system is summarized and shown in Table 1.
The SNR and the BER are estimated according to [24]. The MIMO system has been considered over the fiber-wireless channel. The purpose of the MIMO channel estimation, in the receiver, is to identify the optical channel and wireless channel between each pair of transmit and receive antennas. For MIMO channel estimation, a high throughput long training field (HT-LTF) is provided in the high throughput-mixed format (HT-MF) preamble of the IEEE 802.11n. This technique is called training symbol-based channel estimation [12,17], since a long training symbol is transmitted for
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Fig. 3. Optical power spectra of the allocated channels at the OLT.
Table 1 The general specification of the proposed FiWi system. Parameter
Value
Optical Backhaul Transmitted power of LD Receiver sensitivity Fiber length Attenuation Dispersion coefficient Optical modulation method
5 dBm 30 dBm 20 km 0.2 dB/km 17 ps/nm/km OSSB þC
Wireless Front-end Transmitted power of the AP Carrier frequencies Channel bandwidth Data rate of the AP Link range Channel type Radio technology Size of MIMO system Modulation Coding scheme Bandwidth efficiency
16 dBm 2.4 GHz or 5.0 GHz band 40 MHz 240 Mb/s 100 m Fading channel OFDM MIMO 2 2 MIMO 64-QAM Convolutional code of rate 2/3 6
each spatial stream indicated by the selected modulation and coding scheme (MCS) [13]. In the simulation design, the frequency selective behaviors caused by chromatic dispersion in the optical link and channel fading in the wireless link are characterized in the training symbols at the IEEE 802.11n data processor in the OLT. The
training symbol-based channel estimation is followed by the maximum likelihood (ML) MIMO detection process which determines the transmitted data symbols from the received signal [15,17]. In contrast, the training-based channel estimation has the relatively low computational complexity at the receiver and draws more interests in the OFDM system [25].
5. Results and discussion The group of the wavelengths (ld11, l1 and ld12) are received at the AP and then demultiplexed. The AP downconverts the optical signals at the interleaved downlink wavelengths (ld11 and ld12) to the corresponding MIMO OFDM signals and bandpass filter them according to the RF carrier frequency. The detected electrical signal is filtered by using 8th order Butterworth BPFs, with a 60 MHz bandwidth, and center frequencies 2.4 GHz or 5.0 GHz. The frequency responses of these BPFs are illustrated in Fig. 4(a) and (b) for the center frequencies 2.4 GHz and 5.0 GHz, respectively. These filters have a frequency response as determined by the following transfer function [26] B n Hðf Þ ¼ a:
2
B p 2k þ 1 Pn1 k ¼ 0 j f f c 2 :exp j 2 1 þ n
ð1Þ
where H(f) is the transfer function of the BPF, f is the frequency parameter, n is the order of the filter, a is the insertion loss (a has
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Fig. 4. (a,b) The frequency response of the BPFs, and (c–f) the spectra of the transmitted wireless signals X0 1 and X0 2; at 2.4 GHz and 5.0 GHz frequencies.
been setting to one in the simulation design), B is the 3 dB filter bandwidth, and fc is the filter center frequency. The filtered signal is amplified by using RF amplifier providing a suitable transmitted power level (16 dBm). At the AP, the transmitted MIMO OFDM signals X0 1 and X0 2 at the RF carrier frequency of 2.4 GHz have spectra shown in Fig. 4(c) and (d), respectively. In addition, the spectra of the transmitted MIMO OFDM signals X0 1 and X0 2 at 5.0 GHz carrier frequency are shown in Fig. 4(e) and (f), respectively. There is no crosstalk between the MIMO OFDM signals which have the same frequency, because each signal is carried on an independent wavelength with a large channel spacing. The bandwidths of the MIMO OFDM signals X0 1 and X0 2 are 40 MHz allocated on the same RF carrier frequency. These signals are transmitted on the wireless channel to the MIMO WEU. The BER was analyzed to evaluate the system performance. Fig. 5 shows BER versus SNR for the received signal at the receiver of the WEU. The additive white Gaussian noise (AWGN) channel and fading channel models were simulated in the wireless MIMO OFDM system, since 20 km SSMF with a dispersion coefficient of 17 ps/nm/km is considered in the optical backhaul. The SNR required for a BER of 10 5 as specified for RF transmission can be determined for the plots in Fig. 5(a) under AWGN channel for
2.4 GHz and 5.0 GHz carrier frequencies. The SNR values of 15 dB and 16.5 dB achieved BER of 10 5 at the carrier frequencies of 2.4 GHz and 5.0 GHz, respectively. In addition, BER versus SNR, under fading channel for 2.4 GHz and 5.0 GHz carrier frequencies, is shown in Fig. 5(b). The SNR values of 28 dB and 30 dB achieved BER of 10 5 at the carrier frequencies of 2.4 GHz and 5.0 GHz, respectively. There is a small difference between the required SNR to achieve BER of 10 5 at the carrier frequencies of 2.4 GHz and 5.0 GHz for the both wireless channel models. The optical signal-to-noise ratio (OSNR) is estimated at the optical receiver entrance point with 0.1 nm noise bandwidth. The BER performance versus the OSNR is shown in Fig. 6. When the AWGN wireless channel is taken into account, low OSNR values (17.26 dB and 19.24 dB) are required to achieve a minimum BER of 10 5 at the WEU for the frequencies 2.4 GHz and 5.0 GHz, respectively, as shown in Fig. 6(a). To realize a good BER performance considering the fading channel in wireless link, high OSNR values (32.61 dB and 34.11 dB) are required for the frequencies 2.4 GHz and 5.0 GHz, respectively, as shown in Fig. 6(b). The optical amplifier generates the most noise which is produced by the optical backhaul [27]. The created noise from the wireless channel and receiver front-end dominates the whole FiWi system
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Fig. 5. BER versus SNR at the receiver of the WEU considering (a) the wireless AWGN channel and (b) the wireless fading channel.
performance, especially when the wireless fading channel is considered [24]. Fig. 7(a) and (b) shows the simulated 240 Mb/s 64-QAM constellation diagrams for the transmitted and received signals from the data processor at the OLT to the wireless end-user at the RF carrier frequencies of 2.4 GHz and 5.0 GHz, respectively. Clear scatter-plots are achieved for the both RF frequencies, for a SNR of 30 dB and BER close to 10 5, which is well detected and undistorted, indicating little degradation due to the transmission over the FiWi link. The EVMs of 29.83 dB (for 2.4 GHz) and 28.41 dB (for 5.0 GHz) achieved the good performance of the proposed system. The EVMs are calculated considering the following equation [28]: " EVMðdBÞ ¼ 10:log10
2
SM k ¼ 1 9Stx,k Srx,k 9 2
SM k ¼ 1 9Stx,k 9
# ð2Þ
where EVM is the value of the difference between a collection of received symbols and transmitted or ideal symbols, Stx,k is the corresponding transmitted symbol of the constellation associated with the kth symbol, Srx,k is the received symbol associated with Stx,k, and M is the number of the symbols for the InphaseQuadrature constellation.
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Fig. 6. BER versus OSNR considering (a) the wireless AWGN channel and (b) the wireless fading channel.
6. Conclusions and future work The hybrid FiWi system is proposed and demonstrated based on the transmission of wireless 802.11n MIMO signals over fiber by using an efficient novel optical single-sideband frequency translation technique. The proposed FiWi system supports the wavelength reuse technique, so one optical source is enough to generate the optical carrier which is reused at the AP, and many wavelengths for carrying multiple wireless MIMO signals over the fiber. The crosstalk between different MIMO signals with the same RF carrier frequency is excluded, since each MIMO signal is carried on a specific optical wavelength. The physical layer performance has been reported in terms of the BER, the EVM, and the SNR. The proposed FiWi system achieved transmission of wireless 802.11n MIMO signals over 20 km SSMF and along 100 m outdoor MIMO wireless channel. This was investigated using RF carrier frequencies of 2.4 GHz and 5.0 GHz to provide 240 Mb/s data rate with a channel bandwidth of 40 MHz. In future, this system can be redesigned by using higher data rate up to 600 Mb/s based on IEEE 802.11n. In addition, multiple 2 2 MIMO signals with different frequencies on the same frequency band of 2.4 GHz or 5.0 GHz could be multiplexed and then transmitted over fiber using the proposed novel approach.
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Fig.7. The constellation diagram of the transmitted and received signals from the data processor at OLT to the WEU, considering (a) 2.4 GHz and (b) 5.0 GHz.
Acknowledgments We greatly appreciate Universiti Teknologi Malaysia and Photonics Research Laboratory for providing the facilities which enabled this work to be accomplished. We would also like to thank the Ministry of Science, Technology and Innovation (MOSTI), Malaysia for sponsoring this work under project vote number 73720. References [1] Chow CW, Yeh C, Wang C, Wu C, Chi S, Lin C. Studies of OFDM signal for broadband optical access networks. IEEE Journal on Selected Areas in Communications 2010;28(6):800–7. [2] Shaddad, R, Mohammad, AB, Idrus, S, Al-hetar, A, Al-geelani, N. Emerging optical broadband access networks from TDM PON to OFDM PON, In: Proceedings of PIERS 2012, Kuala Lumpur, Malaysia, 2012. p. 102–6.
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