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A hybrid CATV/OFDM long-reach PON architecture is proposed and experimentally demonstrated. By multiplexing the OFDM and CATV signals in different frequency bands, the overall network structure is simplified and the available bandwidth for the OFDM signals is increased. Furthermore, by carefully adjusting the driving voltage of the employed CATV transmitter, the overall transmission performance can be optimized by eliminating a large amount of unwanted distortions. Unlike the current fiber optical CATV transport system in which the CATV signals need to be amplified in every 30 ~40 km transmission span, the proposed architecture has successfully extended the transmission span to 60 km. This can practically remove the needs of electrical power provisioning and monitoring between the central office (CO) and each local exchange (LE). Good transmission performances of carrier-to-noise ratio (CNR), composite second-order (CSO), composite triple beat (CTB) and bit error rate (BER) were obtained for the transmitted CATV and OFDM signals. All of these experimental results prove the practice and efficiency of the proposed architecture in providing simplicity and extended reach services to customers.
©2012 Optical Society of America
1. Introduction
Fiber to the X (FTTx) architectures are widely developed and deployed to provide triple play multimedia services worldwide. With the characteristics of large network capacity and long transmission distance, various state of the art fiber optical network architectures have been proposed to provide broadband Internet services and CATV services over last mile connection [1,2]. A single FTTx central office (CO) can be employed to service various clients within roughly 20 km radius region where is traditionally serviced by multiple cable network COs. By reducing the number of the COs, the capital expense (CAPEX) as well as the operating expense (OPEX) can be greatly reduced [3,4]. In order to further extend this advantage, there has been growing interests in developing a longer-reach passive optical network (PON) to reduce the cost of high bandwidth connections over access and metro link [5–7]. By using such long-reach PON architectures, network traffics originating at the CO could be directly transmitted to a local exchange (LE) and then to subscribers’ premises without using any optical amplifier between the CO and the LE. This can resourcefully eliminate the needs of electrical power provisioning and monitoring as well as the backup battery maintenance between the CO and each LE [8,9]. Therefore, the long-reach PON technologies are rapidly evolved to provide simplicity and extended reach services for customers.
Basing on the long-reach PON architectures, various telecommunication applications, such as Internet access, Internet phone and video conference, can be easily transmitted from the CO to the consumers’ premises. However, when an internet service provider (ISP) tries to embed widely spread CATV signals into such long-reach PON architecture, the received signal quality is unacceptable. Currently the fiber optical CATV transport systems need to amplify the CATV signals in every 30~40 km transmission [10–13]. When the transmission span between two optical amplifiers extends 40 km, the received CATV signals will interfere with various distortions, such as the composite second order distortion and composite triple beat distortion, so some unwanted noises, such as “diagonal stripes” and “thin white horizontal stripes”, will present in clients’ TV screens [14,15]. To overcome this limitation, the CATV signals in the current long-reach PON architectures are normally sampled and converted into digital streams and are transmitted alone with other downstream digital signals using time division multiplexing (TDM) method in a CO. Subsequently they are recomposed back at the receiver end using another TDM scheme. In this case, complicate TDM technique, analog-to-digital (A/D) and digital-to-analog (D/A) converters are required in a long-reach PON architecture and most of the downstream network bandwidth is occupied by the digitized CATV signals. All of these will seriously limit the long-reach PON development. In order to reduce the capital cost associated with the A/D and D/A equipments as well as to simplify the operation complexity related to the TDM scheme, a novel 60km-reach PON architecture is proposed to simultaneously deliver both CATV signals and orthogonal frequency-division multiplexing (OFDM) signals by a CATV transmitter. By reasonably multiplexing downstream OFDM signals with the original CATV signals in different frequency bands and adjusting the driving voltage of the employed CATV transmitter, good transmission performances of carrier-to-noise ratio (CNR), composite second-order (CSO) and composite triple beat (CTB) were obtained for CATV signals transmission. Although the CATV signals with high power requirement in nature will cause high power harmonic signals at the frequency band of the transmitted OFDM signals, the inherent advantages of the OFDM signals can efficiently mitigate this effect resulting in acceptable bit error rate (BER) performance. All of the experimental results prove that the proposed hybrid CATV/OFDM long-reach PON architecture can be a suitable candidate to merge optical access and metro networks with less network element and network complexity.
2. Experimental setup
Figures 1(a) and 1(b) show the schematic diagrams of a normal hybrid CATV/OFDM long-reach PON architecture and the proposed hybrid CATV/OFDM long-reach PON architecture, respectively. Generally a CO is required to simultaneously provide 77 or more TV programs for clients. If all of these TV programs are digitized and transmitted by a normal long-reach PON architecture, high-speed A/D and D/A converters are required in the head-end and the optical network units (ONUs), respectively. Besides, the digitized TV programs will occupy a large amount of network capacity, so the available downstream bandwidth for the OFDM signals is limited. Comparing with the normal one, the proposed architecture can directly combine and transmit the CATV and OFDM signals over a long distance fiber. By taking off the required TDM technology and the high-speed A/D and D/A converters from the head-end and the ONUs, the overall network cost and the required CATV bandwidth are reduced. The reserved bandwidth can be employed to transmit more OFDM signals.
Fig. 1 The schematic diagrams of (a) a normal hybrid CATV/OFDM long-reach PON architecture and (b) the proposed hybrid CATV/OFDM long-reach PON architecture.
The experimental configuration of our proposed hybrid CATV/OFDM long-reach PON architecture is illustrated in Fig. 2 . To simulate multi-carrier CATV programs, 77 standard NTSC-channels (CH2-78; 55.25-547.25MHz; 6MHz/CH) were generated by a Matrix SX-16 signal generator. Furthermore, to evaluate the OFDM signals transmission, a 312.5Mbps QPSK OFDM signals (16 subcarriers, 693.36-849.61 MHz, 8-samples cyclic prefix) are generated by a Tektronix arbitrary waveform generator (AWG). Both the CATV and OFDM signals, as shown in the Fig. 2 insert (a), are combined by a 2 × 1 RF coupler, passed through an electrical band-pass filter (700MHz-1.2GHz, BPF) to filter out the OFDM sidelobes and then fed into a CATV transmitter for downstream transmission, in which the emitted lightwave is located at 1558.37 nm and the output power is 7 dBm, as shown in the Fig. 2 insert (b). The downstream optical signal is then amplified by an optical amplifier (OA) and communicated to a LE through a span of 60 km single-mode fiber (SMF).
Inside the LE, another OA was placed to boost up the downstream signal and an adjustable attenuator was used to simulate the power splitter apart from the 32 split ratio where a real splitter was used. Subsequently, the downstream signal in an ONU was split by a 1 × 2 optical splitter. 90% of the optical power was received and analyzed by HP-8591C CATV analyzer. The rest 10% was fed into a photodiode (PD) with a 3-dB bandwidth of 10GHz. The detected signal was firstly passed through a BPF (700MHz ~1.45GHz) to remove the spurious and was captured by a communication signal analyzer (Tektronix CSA744B). As shown in the Fig. 2 insert (c), the received OFDM signal was processed off-line with a MATLAB program to evaluate the BER performances and the corresponding constellation map.
3. Experimental results and discussions
Figure 3(a) , 3(b) and 3(c) show the CNR, CSO and CTB performances under various CATV channel numbers, respectively. The CNR is computed by measuring the power difference between the carrier and the noise levels, so the higher optical power is received, the better CNR performance can be obtained. It is clear from the Fig. 3(a) that the measured CNR values with OFDM signals off/on are all above 46/44 dB which is higher than the 43 dB threshold value and can effectively prevent “snow” phenomenon in consumers’ TV screens. Similarly, the CSO and CTB values are evaluated in relation to the power variation between the carrier and the second/triple order beat levels. The measured CSO and CTB values are higher than 59.8 dB and 60.6 dB which are much higher than the typical requirement, 53 dB, to prevent “diagonal stripes” and “thin white horizontal stripes” phenomena, respectively. In a fiber optical CATV transport system, the CSO and CTB distortions are in proportion to the chromatic dispersion. When the transmission distance is increased from 60 km to a longer distance (e.g. 100 km) the additional chromatic dispersion will degrade the final CATV performances. Fortunately, it is possible to reduce the unwanted distortions by carefully shifting the MZM driving voltage to a suitable value.
The detail CSO distortions to carrier ratio can be given by [16]:
where J1 and J0 are Bessel function expansions to derivate the amplitude of second and third order intermodulation, A is the designate tests channel amplitude, Vπ is the half wave voltage of a Mach-Zehnder modulator (MZM), Vdc is the applied dc voltage, NCSO is the product counts of the CSO distortions. When the Vdc are set to quadrature points where the Vdc equal to mVπ, (m = ± 0.5, ± 1.5, ± 2.5, ...), second and other high even-order distortions of the CATV transmitter can be eliminated, which can efficiently optimized the transmission performance. To match the experimental setup with this equation, the Vdc of the MZM inside the employed CATV transmitter is set to 2.1V (0.5Vπ). The measured CSO and CTB values prove that the transmitted 77 CATV channels can always satisfy the standard requirements whether the OFDM signals on or off.To evaluate the transmitted OFDM signals performance, its BER curves (estimated from the modulation error ratio, MER) and constellation diagram are presented in Fig. 4 . In the transport system, the data streams of the OFDM signals are modulated in QPSK format. They are insensitive to amplitude noise. Although parts of the CATV second/third order distortions are overlapping with the OFDM signals, the receiver circuit can still retrieve the transmitted OFDM signals by ignoring those noises. The received optical power levels for the downstream OFDM signal at the BER of 10−9 are −12.7 dBm and −13.6dBm at 60 km and 0 km transmission scenarios, respectively. A small power penalty of 0.9 dB proves that the OFDM signals and CATV signals can coexist in an optical carrier without seriously interfering with each other.
4. Conclusions
A hybrid CATV/OFDM long-reach PON architecture is proposed and experimentally demonstrated. Normally, the CATV signals in a fiber optical transport system need to be amplified in every 30 ~40 km transmission span. When they are transmitted over a long-reach PON architecture, they are generally sampled and converted into digital streams before transmitted along with other downstream digital signals. In this case, complicate TDM technique and A/D, D/A converters are required in the head-end and the ONUs. Besides, the digitized TV programs will occupy most of the network capacity resulting in limited bandwidth for the other services. In order to overcome these weaknesses, we propose a novel architecture to successfully extend the transmission span to 60 km. In this case, the CATV signals can be directly transmitted from the CO to the ONUs. No more TDM scheme and A/D, D/A converters are required in the system. Besides, by carefully setting up the driving voltage of the employed CATV transmitter and multiplexing the OFDM and CATV signals in suitable frequency bands, the transmission performance is optimized and the network structure is simplified. Good experimental results demonstrate that the proposed hybrid CATV/OFDM long-reach PON architecture is a practical and efficient architecture in simultaneously transmitting the CATV and OFDM signals over access and metro link.
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