based broadcasting of large number of television channels. They are also ... to digital TV in the United States, HDTV is definitely the future. ... between central-base station (CBS) and low power Radio .... frequencies are less than 10 GHz [28].
Radio over Fiber in Multimedia Access Networks Xavier Fernando, Senior Member, IEEE Department of Electrical and Computer Engineering, Ryerson University, Canada (Invited Paper)
Abstract— Dominant broadband access technologies today are Digital Subscriber Line (DSL) and hybrid fiber coaxial (HFC) networks. DSL leads, with global subscribers exceeding 100 million, while cable-modem subscribers worldwide total 55 million in 2005 [1]. Recently, fiber-to-the-home (FTTH) and fiber-tothe-curb (FTTC) scenarios are also on the rise. In all these access networks, optical fiber plays an imperative role. Fiber carries multimedia signals such as video, audio and Internet data with a combination of subcarrier multiplexed radio over fiber (SCM-ROF) and digital baseband formats. Despite the IPTV hype, the most bandwidth hungry video (especially HDTV) is predominantly transmitted in SCM-ROF format. In addition, wireless broadband multimedia access using the ROF technique is also on the rise. Newly released 71-76 GHz, 81-86 GHz, and 92-95 GHz bands for wireless multimedia delivery will trigger wide deployment of ROF technology. Therefore, ROF plays key role in both wired and wireless broadband multimedia access networks [2]. However, there are number of issues in the design and deployment of ROF links for multimedia access. Nonlinearity of the optical modulator (either direct or external); loss due to electrical-optical-electrical conversion and increasing intensity noise are key concerns. Some of our recent work involves with adaptive digital nonlinearity and noise cancelation [3], optical domain reduction in unmodulated carrier [4] and all optical de-multiplexing of closely spaced RF signals by Bragg grating filters [5] and multi system signals over fiber [6].
I. I NTRODUCTION Access networks, services and driving technologies currently undergo changes that have never been seen before. Differences between telecommunications and media delivery domains blur day by day. Integrated multimedia access is the key for revenue generation, customer satisfaction and ultimately for the survival of providers. There is a trade war in the home access scenario between the traditional voice telecommunication companies (Telcos) and coaxial cable television (CATV) providers. Both these sectors are now geared to provide bundled multimedia (video, voice and data) services over their networks, to enter the so called ’triple play’. More and more households have large-screen, high definition televisions (HDTV); large number of HDTV channels are also available. High quality HDTV needs good SNR and high bit rate. As a result both these players need more bandwidth than ever before. Wireless access technologies are also on the transform. Digital video/audio broadcasts over terrestrial or Satellite networks (DVB and DAB) are geared to merge with wireless access networks [7]. Wireless cellular network subscribers are no longer satisfied with just voice; they expect different types of data and streaming video. As a result, the demand for multimedia wireless services is also on the rise.
A. Wired Multimedia Access Traditional telephony over twisted pair last mile (limited to 4 KHz), has been dramatically changed with the advent of digital subscriber line (DSL) modems. Currently much higher bit rates are possible over twisted pair lines (Table I-A). Technology SDSL ADSL VDSL VDSL
Speed 2.3 Mbps 8/1.5 Mbps 14/14 Mbps 14/14 Mbps
Distance 4.5 km 4.5 km 1.2 km 1.2 km
Table I-A: DSL bit rates and distances DSL modems are very successful for Internet access. However, bandwidth hungry video delivery over DSL is the concern. From Table I-A, MPEG-2 quality video (4 Mbps) can be provided over VDSL (Very High Speed DSL) and ADSL (Asymmetric DSL) but not over SDSL (Symmetric DSL). Nevertheless, the distance is limited. Hence, the video (plus audio and data) signals are usually transmitted over fiber in subcarrier multiplexed radio over fiber (SCM-ROF) manner to the curb and then delivered over twisted pair last mile to the user. On the other hand, CATV providers usually enjoy more bandwidth than twisted-pair with their traditional hybrid-fibercoaxial (HFC) networks. They are well known for SCM-ROF based broadcasting of large number of television channels. They are also very successful in providing Internet over HFC using DOCSIS or similar protocols. Recently they have started offering home telephone using digital voice modems over HFC in an attempt to enter the so called triple play. There is some interest on Internet Protocol Television (IPTV) because of the promising interactive nature. IPTV delivers video signals using IP technology over a broadband connection. Currently, most operational IPTV networks are deployed on Telco network architectures using various DSL technologies to bridge the last mile. This is especially true in Europe with relatively short network loop lengths, which readily accommodate DSL. Note that despite the IPTV hype, the majority of IPTV services use MPEG-2 compression that has compromised quality. With 2006 declared as the transition year to digital TV in the United States, HDTV is definitely the future. Without the ability to provide HDTV with interactive services (video on demand), DolbyT M quality surround sound audio and some data (such as closed captioning) no media provider can
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survive. Although, the uncompressed bit rates for big screen HDTV is very high1 , typically in the Gbps range, good quality HDTV is provided by 19 or 38 Mb/s with complex video and audio compression technologies [8]. Generally, the video signal could be either radio frequency (RF) video or IP (Internet Protocol) video. The IP video is similar to Internet traffic with additional streaming requirement. However, the RF video signal is analog and subcarrier multiplexing is needed to transmit multiple television channels. Furthermore, the RF video is a proven technology that already dominates existing fiber-coaxial networks and has better quality. Nippon Telephone and Telegraph (NTT) expects RF video to continue to play a key role even in the fast emerging FTTH networks [1]. Therefore, SCM RF video (and SCM-ROF) will play imminent role in multimedia delivery for years to come, especially on the wake of HDTV. B. Wireless Multimedia Access Optical fiber based micro cellular wireless access (Fi-Wi) systems have been investigated by many researchers [9]. The Fi-Wi systems will be especially useful at the newly released 71-76 GHz, 81-86 GHz, and 92-95 GHz bands for wireless multimedia delivery, as the cell size will be inherently limited at these extremely high frequencies. Interest on Fi-Wi systems started in early nineties [10]. There were large number of publications since then. Fi-Wi systems enable rapid deployment of micro cellular networks and enable service to special areas such as subway stations, mines, airport concourses and supermarkets. For example, a Fi-Wi system named BriteCellT M with 500 remote antennae was installed for the Sydney 2000 Olympics in order to handle the expected massive wireless traffic. Just during the first few minutes of the opening ceremony, over 175,000 calls were made by the 110,000 spectators in the stadium. This traffic could not have been handled without the BriteCellT M FiWi system. The Chinese Government initiated Beyond Third Generation (B3G) project called the Future Technologies for a Universal Radio Environment (FUTURE) also uses a novel ROF cell as its basis for broadband communications [11]. Furthermore, there is plenty of dark (unused) or dim (partly used) fiber already in place in most major cities. This fiber can be adapted for Fi-Wi, which translates into reduced initial capital costs. In a Fi-Wi system, optical fibers transmit the RF signal between central-base station (CBS) and low power Radio Access Point (RAP). The RAP then transmits the broadband RF signal to customer units over the air. The RAP only implements optical to RF conversion for the downlink and RF to optical conversion for the uplink. Signal format dependent functions such as modulation, equalization and error correction are implemented at the CBS and at the portable to make the RAP simple and cost effective. This arrangement also makes the ROF link transparent to wireless system specifics and enables easy upgrades. 1 For example, bit rate for 1080i60 is 1080×1920×30×2×10 = 1.244160 Gbps
Several researchers have been investigating various issues of the Fi-Wi system [12]. There are specialized techniques such as subcarrier superimposing [13]; frequency modulation [14] and super frequency modulation [15]. However, intensity modulation and direct detection (IM/DD) is the most common approach. Hence, fundamental limits of IM/DD fiber optic links (loss, noise and nonlinearity [16]) also apply to Fi-Wi systems. II. M ULTIMEDIA ROF - U NIVERSAL M ODEL From the above discussion it is obvious that there are several occasions when a single fiber needs to transmit different subcarrier multiplexed RF signals (RF video channels/wireless radio) simultaneously. In this paper, we use a universal model to describe the SCM-ROF link with electrical last mile. The electrical last mile could be copper (in a fiber based DSL or HFC network) or air (in a fiber-wireless system) or nothing at all (in FTTH systems). This unified approach will make it easy to understand general issues. In this context, we refer ROF link as a fiber optic link that carries radio frequency subcarriers (this definition excludes digital baseband links such as SONET and Ethernet). The RF subcarrier may, in turn carry video, audio, wireless signal or data. The ROF link could be followed by copper wire, air or nothing at all. A. Nonlinear Distortion Despite having enough bandwidth to support several GHz of RF signal, nonlinear distortion of the optical modulator has been a key issue focused by many authors with IM/DD systems [17]. Let us assume the modulating SCM radio signal is s(t). This s(t) may N carry N number of multimedia sub channels s(t) = i=1 si (t). Considering direct intensity modulation at the laser diode, the instantaneous optical power output P (t) from the laser in response to input electrical signal s(t) is given as, P (t) = [1 + ms(t) + a1 m2 s2 (t) + a2 m3 s3 (t)]Po .
(1)
Here, Po is the mean optical power and m is the modulation depth; a1 and a2 are nonlinearity coefficients. Typically, the modulation depth m needs to be kept low to avoid clipping and saturation at the laser diode. The nonlinear distortion in HFC CATV systems is a well investigated issue (with PAL and NTSC analog video signals) [18]. However, in addition to traditional harmonic and intermodulation distortion, the nonlinearity will also create AM-AM and AM-PM type distortion when the RF signal has digital modulation like in ATSC (Advanced Television Systems Committee) standards. The AM-PM distortion could especially occur even in low RF power levels [19] and could severely increase the bit error rate and decrease the quality of the received video. The optical transmitter consists of either directly modulated laser diode or external (Mach Zehnder or electro absorption) modulator. Unfortunately all these have nonlinear characteristics. Traditional approach is to linearize the optical transmitter by opto-electronic means. This approach is device dependent
s(t)
ROF 1/Lop SNR
r(t)
+
OSNR
nop(t) + nel(t)
Gop u(t) Elec 1/Lel
Fig. 1. Radio over Fiber link with electrical last mile block diagram; ’Elec’ refers to the last mile that could be twisted pair or coaxial cable or air
and does not give universal solution. However, when the baseband information is digital, adaptive digital predistortion schemes could be deployed to overcome the nonlinearity [20], [21]. Fortunately, digital predistortion for nonlinear systems is well researched area because of the frequently encountered high power RF power amplifier nonlinearity in terrestrial and satellite wireless systems [22]. These techniques need to be suitably modified and applied to ROF links. 1) Nonlinear Distortion in Fi-Wi System: The nonlinear distortion plays a severe role in Fi-Wi systems because the nonlinear ROF link is concatenated with time varying multipath wireless channel [23]. Wireless systems generally require wide dynamic range for transmission to account for wireless channel impairments and varying number of active users. Nevertheless, ROF links have much less spurious free dynamic range due to the nonlinear distortion. There have been several attempts to compensate for the optical transmitter nonlinearity in Fi-Wi systems. The wireless channel and the ROF link parameters are unknown and it is preferable to use adaptive techniques to simultaneously overcome nonlinear distortion and multipath dispersion [3]. The Fiber-Wireless uplink is a Wiener type system because it consists of a linear dynamic system (wireless channel) followed by a static nonlinear system (optical link). The concatenated Fi-Wi uplink should be estimated using a suitable technique. One way is to use the autocorrelation properties of a PN sequence training set [24]. PN sequences are preferred because of their use in spread spectrum systems. Once the channel is estimated, the imperfections need to be equalized. The inverse of a Wiener system is a Hammerstein system. Therefore, the compensation could be done using something similar to the Hammerstein type decision feedback equalizer proposed in [25]. This technique is useful for the uplink where it is preferred to have the signal processing algorithms implemented in the receiver, which is at the central base station. On the other hand, predistortion nonlinearity compensation is the best for fiber wireless downlink, because now the signal processing is done at the transmitter which
is again at the central base station. This will not increase the portable unit complexity. This approach hence introduces asymmetry in complexity. When multiple users share the ROF link, they will have a common static nonlinear channel, in series with individual dynamic wireless channels. The received signal is the sum of the signals corresponding all users in a CDMA environment. Channel estimation and equalization in this scenario should also address multiuser interference cancelation. This is an interesting research issue [26]. B. Losses and Attenuation Referring to Fig. 1, let us assume PRF,in is the input RF signal power. Then it depends on the mean square value of the modulating stochastic radio signal s(t); PRF,in = s2 (t)/2. The input RF power undergoes several losses before reaching the end user. Note that optical loss in fiber appears squared in the electrical domain. The first loss happens in the directly modulated laser transmitter. This loss originates from the modulation gain of the laser Gm measured in mW(output optical power)/mA(injected current). Usually Gm is small and it depends on the external and internal efficiencies of the laser. Hence, the optical output power from the laser is [27], (2) Popt,laser = (Gm PRF,in /Zin ), where, Zin is the input impedance of the laser transmitter (typically 50 or 75 Ohms). Note that the square root in (2) and the factor 2 in (3) and (4) account for the fact that optical power is proportional to the electrical current. This signal is transmitted over the fiber that will introduce attenuation and dispersion. Almost all ROF links use single mode fiber. Hence, the fiber dispersion is not an issue with ROF links up to several tens of kilometers when the RF frequencies are less than 10 GHz [28]. Fiber attenuation is a function of wavelength. Modern fibers offer as low as 0.2 dB/km loss at 1550 nm. Connectors and splices will add few more dB loss. The optical losses together can be named as OL including fiber attenuation and connector losses2 . In a point-to-point fiber link, OL = 2(n1 lc + n2 lsp + αlf )
dB
(3)
where, lf is the fiber length; n1 lc is the connector loss with n1 connectors; n2 lsp is the splicing loss with n2 splices, and α is the fiber attenuation in dB/km. The OL could, however, be very large with passive optical networks (PON) despite their attractiveness [2] . The power is lost every time the power is split. In this case, OL = 2(n1 lc + n2 lsp + n3 lsplit + αlf )
dB
(4)
where, there are n3 splitters each with loss lsplit . Note that, each 3-dB split will introduce a 6-dB loss in the electrical domain. 2 usually
OL is available in dB, hence (3) and (4) are given in log scale.
The received optical signal at the receiver illuminates the photo-detector, which produces a detected current iD (t) = P (t) where is the detector responsivity. Total detected current iD (t) is the sum of the mean (DC) current ID (t) and the ac component id (t). The fiber terminates at the radio access point (RAP) where the optical signal is converted to an electrical signal u(t). At the RAP, the RF output power from the detector is given by, Zin ) 2Zout
(6)
2 >= 2qID B is the shot noise variance, In (6), < Ishot ID is the detected current at the optical receiver, q is the charge of an electron and B is the RF signal bandwidth. The relative intensity noise power (RIN) can be given as 2 2 >= PRIN ID B, where PRIN is the RIN parameter3 . < IRIN 2 Next, < Ith >= 4KB To B/RL is the thermal noise power where KB is the Boltzman’s constant, To is the absolute temperature, and RL is the load resistance. Note that shot noise increases with the mean optical power and RIN increases 2 as a square of the optical power. It is also shown that IRIN increases with modulating RF signal power [30]. It is hard to reduce shot and RIN noise power, but thermal noise can be made small with better electronic design approaches. After detection, the electrical signal is amplified with an electrical amplifier at the optical receiver with gain Gop . Then it is transmitted over the last mile (copper wire or air) that has loss Lel . There will be more noise nel added to this signal in the last mile. Therefore, the equivalent noise at the customer premises n(t) can be given as,
n(t) =
nop (t)Gop + nel Lel
rn2 (t) =
(5)
The square law detection introduces the squaring effect. Zout is the output RF impedance of the optical receiver (typically 50 or 75 Ohm) and Lop is the optical loss in linear scale; Lop = 10OL/10 . Note that when the input to the laser and the output of the optical receiver are matched to the same RF impedance (Zout = Zin = 50Ω), the last term reduces to 1/2. This 50% loss is due to the simple resistive impedance matching between the high impedance photo detector and the 50 ohm system. Enhanced impedance matching techniques using reactive elements (inductors and capacitors) can be used to reduce this loss. However, reactive matching also reduces the receiver bandwidth, hence not very suitable for CATV systems [27]. The optical link noise, nop , is added at the optical receiver, where the signal u(t) is weak due to the fiber and conversion losses. The optical signal to noise ratio (OSNR) is defined at this point (Figure 1). Considering only the dominant noise processes, the optical link noise power n2op is the sum of shot, thermal and relative intensity noise (RIN) powers. 2 2 2 > + < IRIN > + < Ith > n2op =< Ishot
rs2 (t)
SNR =
Gop Lel
= PRF,out
Gop Lel
2
2 (8)
n2op (t) + n2el (t)
Zin ) 2 G2m PRF,in Lop ( 2Z out 2 Lel n2op (t) + n2el (t) G op
(9)
(10)
Number of issues can be observed from (10). First, it is always good to keep the electrical and optical noise powers in the same order to reduce the overall noise. Hence, large amplification in the optical receiver will boost the optical noise n2op and will not improve the overall SNR. The optical link noise nop increases with not only bandwidth, but also with the mean optical power. Figure 2 shows the simulation results of a fiber wireless system. An indoor wireless channel model specified in IEEE 802.20 Permanent Document (IEEE C802.20-03/70) with path loss Lel was used for simulation. Fiber loss of 0.5 dB/km and typical values for noise parameters were considered. Two DSCDMA RF signals with 5 MHz and 1.25 MHz bandwidth were considered. It can be seen that for short fiber length, the 5 MHz signal has better BER performance, while for long fibers, the 1.25 MHz signal performs better. This is mainly because the attenuation in the fiber is high and OSNR becomes limiting with long fibers. The OSNR is smaller over 5 MHz bandwidth (compared to 1.25 MHz) because nop is high. BER vs Fiber Length
−5
10
BW = 5MHz BW = 1.25MHz
Average Bit Error Rate
PRF,out = 2 G2m PRF,in Lop (
Note that the relative weights of optical and electrical noise depends on the values of Gop and Lel . At the customer premises, the received signal power, noise power and the SNR can be respectively shown as,
−6
10
−7
10
0
5
10
15
20
25
Fiber Length (km)
(7)
3 The P RIN is also widely referred as RIN and given in dB/Hz [29]. However, we use the notation PRIN to avoid confusion
Fig. 2. BER performance versus fiber length for single user; Gop = 100dB; 100 m indoor wireless channel with one floor
C. Improved Expression for RIN Calculated F-P transmission vs measured one (A210403C)
The widely used expression [29] gives the mean square value for the noise current IRIN due to RIN as 2 IRIN = PRIN 2 Po2 B
0 -3
(11)
−108.6
−108.8
Transm ission
The parameter PRIN is generally assumed constant for a given laser diode. Note that, according to (11), the intensity noise
(b)
-6 -9 -12 -15 -18 -21
−109
-24
2
E[s (t)] = 0.5 Intensity Noise Power (dBm)
−109.2
-27
1536.5305
−109.4
1536.5325
1536.5345
1536.5365
1536.5385
1536.5405
1536.5425
1536.5445
1536.5465
Wavelength (nm) −109.6
2 E[s (t)] = 0.3
Fig. 4.
−109.8
Fiber Bragg Grating Sub Picometer bandpass filter response
−110 E[s2(t)] = 0.1 −110.2
−110.4
−110.6 0.1
0.2
0.3
0.4
0.5 0.6 Modulation Index m
0.7
0.8
0.9
1
Fig. 3. Relative intensity noise in radio over fiber link with improved expression
power does not change unless PRIN , Po , B or changes. However, in our observations and many other occasions ([19] [31], [32], [33] and [34]) the RIN is found to be changing with the optical modulation index m that reflects the modulating signal power. In [30], an improved expression for the variance of RIN is derived by incorporating the influence of m in a directly modulated ROF link. This expression is, 2 = PRIN 2 Po2 B[1 + m2 s2 (t) ] IRIN (12) When s(t) consists of N number of frequencies such as in a subcarrier multiplexed system, (12) can be rewritten as, 2 IRIN = PRIN 2 Po2 B[1 +
N
m2i s2i (t) ]
(13)
i=1
The derived expression is general in the sense that it does not assume device dependent parameters and is independent of RF signal format and frequency. It better explains the typical behavior of RIN under large signal modulation conditions. It reduces to the conventional expression (11) for small m. Figure 3 shows the intensity noise power using the new expression. Note that intensity noise will not vary with m according to the traditional expression. D. Carrier to Sideband Ratio Reduction ROF multimedia access networks promise tremendous bandwidth. However, the power handling capability of the fiber is very limited. There is a large loss due to E/O and
O/E conversion alone [35]. Fiber loss appears squared in the electrical domain. The optical noise is added to the signal at the end of the fiber where the signal is weak. Optical amplification generally does not work very well in IM/DD ROF links because energy in the subcarriers is small (m is low to avoid nonlinear distortion). As a first step to solve these problems, fiber Bragg grating (FBG) based sub-picometer optical bandpass filter shown in Fig. 4 is reported in [4]. This filter has number of applications. It can reduce the unmodulated subcarrier, which means increasing m without the nonlinearity issue. High carrier to sideband ratio also means better receiver sensitivity and low quantum noise. E. All Optical Demultiplexing The same filter can also be used for all optical demultiplexing of closely spaced RF signals, as close as 50 MHz [5]. Experimental results show no significant distortion in the filtered RF signal due to the filter. Figure 5 shows how this filter was used to separate the upper sideband modulating signal from the double sideband spectrum. The modulating signal frequency was 2.4 GHz. Sidebands of the double sideband spectrum is not very well shown because of the limited resolution of the optical spectrum analyzer. III. C ONCLUSION In this paper, an overview of subcarrier multiplexed radio over fiber (SCM-ROF) systems with respect to multimedia delivery, especially high quality video delivery is given. It is highlighted that although the fiber has abundant bandwidth, there are number of issues to be addressed. For a comprehensive solution, optical and electrical domain understanding, modeling and signal processing is important. This multi disciplinary nature makes SCM-ROF multimedia systems research challenging and interesting.
2.4 GHz BPSK Modulated Double Sideband (DSB) and Filtered Upper Sideband (USB) Signals 5 DSB Signal USB Signal 0
Power [dB]
−5
2.4 GHz RF Signal Power: 5dBm Modulation: BPSK Symbol Rate: 1Msymbols/s Filter: sqr cos/0.35 (alpha)
−10
−15
−20
−25
−30 1560.35
1560.4
1560.45
1560.5
1560.55
1560.6
1560.65
1560.7
Wavelength [nm]
Fig. 5. Double Side Band and Single Side Band spectrums with all optical filtering (the USB signal is shifted for clarity)
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