Bandwidth Improvement of Digitized RoF System Using Track-and-Hold Amplifier Yizhuo Yang #1 , Christina Lim #1 , Ampalavanapillai Nirmalathas #1*2 # Department of Electrical and Electronic Engineering, The University of Melbourne * National ICT Australia â Victoria Research Laboratory (NICTA-VRL) (
[email protected]) AbstractâThe implementation of high-frequency digitized RoF system is limited by the insufficient analog bandwidth of the current ADCs. In this paper, we proposed a T/H amplifier assisted scheme, which can relax the high analog bandwidth requirement on ADCs and offer a better SNR. Experimentally demonstration was realized by using a commercial-available T/H amplifier, ADC, clock generator, and FPGA. An OFDM signal at 20-GHz was successfully digitized using an ADC with 4.5-GHz bandwidth, and delivered via a 10-km 8-Gbps digital optical link. BER at 10-10, 10- 6 and 10-4 were observed when 4QAM, 8QAM and 16QAM modulation formats were transmitted respectively. A conventional single-tap frequency domain equalizer is used in the receiver for OFDM demodulation, in order to compensate for the uneven frequency response due to the lowpass filter characteristic of the bandpass sampling process. KeywordsâDigitized radio-over-fiber; Track-and-hold; Bandpass sampling; Analog bandwidth I. INTRODUCTION A growing user demand for access âanywhereâ to data services over mobile and wireless networks is necessitating a sustained improvement of the wireless access network towards the provision of wireless connectivity at high data rates to be able to deliver applications such as real-time data intensive streaming [1]. However, the current wireless services operating in the lower microwave frequencies (2-5 GHz) are overloading the already congested microwave region. This drives the move to new wireless technologies operating at higher frequencies with larger bandwidths, such as the millimeter-wave (mm- wave) range. Nevertheless, the large propagation loss of wireless signals at these frequencies leads to the needs for a large number of base stations (BS). Moreover, previous work show that the base stations emit approximately two-thirds of the total CO2 emissions of these wireless access networks [2]. Therefore base station simplicity and efficiency are the key features of future broadband wireless networks [3]. Radio- over-fiber (RoF) technique is highly recognized as an energy- efficient and economical-attractive solution to the last-mile access systems, which locates the wireless signals processing functionalities at the central office while keeping the antenna base stations relatively simple with only O/E conversion and RF amplification [4, 5]. However, the traditional RoF link based on analog transmission has stringent requirement on the linearity and bandwidth of the optoelectronic devices. High link linearity is essential to provide sufficient link gain to accommodate for wireless signals transmission over the air interface, and this requirement will be even more stringent if transmitting OFDM modulated signal with high PAPR. In addition, issues regarding optical impairments, performance optimization, cost and energy efficiency and integration with existing systems have to be also considered [6]. Recently we proposed an alternative technique based on the transmission of digitized RF signals which takes advantage of the better performance digital optical links and the benefits of direct RF distribution to realize simpler base stations [7, 8]. In the digitized RF over fiber (DRoF) technique, the analog-to- digital converter (ADC) and digital-to-analog converter (DAC) are located in the base stations and enable more of the receiver/transmitter functions to be performed in the digital domain. Bandpass sampling is employed to offer a lower sampling rate (proportional to the wireless message bandwidth) to digitize the RF signals, which can be provided by the current ADC technology [9], and also to relocate RF frequency signal to IF frequency with no physical needs of local oscillators (LO) and mixers. Therefore it not only takes advantage of high-performance digital optical link but also simplifies the architecture of the base stations. The maturity of CMOS based ADC, digital signal processors and electronic devices make it more and more feasible to implement such digitized RoF base station with huge potential improvements on the cost and energy efficiency. In the proposed digitized RoF technique, although the requirement for high sampling rate is relaxed for the ADC, the wireless carrier frequency is still far larger than the required sampling rate. The key limitation of this scheme for high frequency wireless signals transmission is the insufficient analog bandwidth of current ADCs to accommodate for the high frequency wireless signals. Despite the availability of high-speed ADSs with high sampling rate, only a handful offer analog bandwidth beyond GHz. Furthermore to maintain high linearity in the higher frequency range is technically very challenging. In this paper, we propose a track-and-hold (T/H) amplifier assisted digitized RoF system, which enables the use of ADCs with relatively low analog bandwidth (4.5 GHz) for high-frequency wireless signal transmission (up to 20 GHz) using digitized RF delivery over fiber. We have experimentally shown that the track-and-hold amplifier not only expands the 9 7 8 - l - 4 6 7 3 - 2 8 6 5 - 4 / 1 2 - $ 3 1 . 0 0 ®2012 IEEE 115 eackerman Typewritten Text eackerman Typewritten Text 115 eackerman Typewritten Text 978-1-4673-2865-4/12-$31.00 ©2012 IEEE operational bandwidth of an ADC but also improves the performance due to lower jitter noise. Experimental demonstration of this T/H assisted digitized RoF system was implemented using commercially available T/H amplifier, ADC board (with an analog bandwidth of 4.5 GHz), clock generator and FPGA with a centralized clock at 1 GHz. The performance of the system was quantified using an OFDM signal at 20 GHz via an 8 Gbps digital optical link with 4QAM, 8QAM and 16QAM modulation formats. After the transmission over a 10 km of single-mode-fiber (SMF), the signal was demodulated, processed and performance measured at the receiver. The uneven frequency response due to the lowpass filter characteristic of the bandpass sampling process is overcome using a conventional single-tap frequency domain equalizer in the OFDM demodulation at the receiver. II. PROPOSED SCHEME Track-and-hold Quantizer Clock generator Buffer Anti-alias filter Track-and-hold ADC Clock generator Anti-alias filter ADC T/H amplifier Low bandwidth ADC 1 2 3 1 2 3 f 1 f 2 t 3 (a) (b) (c) Fig.1. (a) ADC architecture; (b) proposed T/H and low- bandwidth ADC scheme; (c) signal formats at different points. An ADC consists of two major parts (Fig. 1a): track-and- hold (T/H) and quantizer. In the digitized RoF technique associating with bandpass sampling ADC, the frequency relocation occurs in the T/H, while digitization happens in the quantizer. Fig. 1c gives the signal formats at different points within the ADC. The RF signal is downconverted to IF frequency before going into the quantizer. Therefore, the high requirement on the ADC analog bandwidth that prevents the digitized RoF technique for high-frequency RF signal distribution can be removed if the frequency relocation process is done before the ADC. Here, we propose a new scheme that takes advantage of an additional track-and-hold amplifier in conjunction with the ADC, as shown in Fig. 1b. Besides bringing the RF frequency down to within the range of the ADC analog bandwidth, there is another benefit of using the T/H amplifier. Jitter noise of the ADC is identified as one of the major issues in digitized RoF technique [8, 10]. The T/H amplifier can offer very low random sample jitter which minimizes jitter induced signal-to-noise ratio (SNR) degradation at high frequencies. This jitter is significantly better than that typically obtained from present ADCs. As a result, the new scheme can provide an extension in the input analog bandwidth, an improvement on the linearity at high frequency and a better SNR for the digitized RoF link, comparing to using a standalone ADC. In this paper, we use HMC5640BLC4B Ultra Wideband Track-and-Hold Amplifier in our experimental demonstrations. To demonstrate the proposed scheme, we tested the performance of the digitized RoF link with and without the T/H amplifier (Fig. 2). A digital sampling oscilloscope (DSO) with an analog bandwidth of 15 GHz is employed as an ADC. The input RF signal is 4QAM modulated OFDM signal. The BER performances are shown in Fig. 3. It is evident that by using the T/H amplifier, the analog bandwidth is extended to 20 GHz from 15 GHz. A significant BER improvement is achieved in the proposed scheme as a result of a lower jitter noise. RF signal MZM DFBbias PDADC (DSO) T/H Amplifier Matlab Demodulation Digital signal Digital signal Fig.2. Simplified digitized RoF scheme -15 -10 -5 0 10 12 14 16 18 20 L o g (B E R ) RF frequency (GHz) with T/H without T/H Fig.3. Performance improvement of T/H amplifier. III. HIGH FREQUENCY DROF TRANSMISSION BASED ON COMMERCIAL-AVAILABLE ADC AND T/H AMPLIFIER A. Experimental setup Fig. 4 illustrates the experimental setup of the T/H amplifier assisted digitized RoF system for high-frequency RF delivery based on commercial-available devices. RF signal was generated by mixing the baseband OFDM signal (1 GHz bandwidth) with the LO (20 GHz). In the base station, the signal-ended RF signal was first converted to a differential signal using a balun (Picosecond 5315A) since the input of the T/H amplifier was required to be differential. After that, the signal was bandpass-sampled by the T/H amplifier evaluation 116 eackerman Typewritten Text 116 eackerman Typewritten Text board (HMC5640BLC4B) with a sampling clock at 1 GHz, where the RF signal was downconverted to IF frequency. The IF signal was digitized by a low-bandwidth ADC with the same sampling clock as used in the T/H amplifier. Here, we used ADC-EV10AS150 with an analog bandwidth of 4.5 GHz and bit resolution of 7-bit. The ADC produced parallel digital data flows which would be sent into a FPGA (Altera DE3). The FPGA has the role of performing necessary digital signal processing and generating serial digital data stream. At the same time, the FPGA also generates a control signal for the ADC to ensure its operation. A clock generator (AD9516) was employed to provide synchronized clock signals for the T/H amplifier evaluation board, the ADC and the FPGA. To this point, the 20 GHz OFDM signals has been converted to an 8 Gbps digital data including preambles, which was loaded into a signal generator to drive a Mach-Zehnder modulator (MZM). A distributed feedback laser (DFB) was used to provide the CW optical source for the MZM. The output of the MZM was a digitally modulated optical signal which was then transmitted over 10 km SMF. In the central office, the optical signal was detected using a PIN receiver and captured by the DSO. The captured digital data was processed off-line which included the reconstruction of the baseband OFDM signal and data demodulation. Fig. 5 shows a picture of the electronic devices (T/H amplifier, ADC, clock generator, balun, and FPGA) in the proposed digitized RoF base station. ADC FPGA Clock T/H Amplifier Balun Fig.5. Picture of electronic devices in the base station B. Analysis and result In the digitized RoF system, the frequency relocation is realized by manipulating the frequency image of a bandpass sampled signal, where the frequency spectrum consists of evenly generated alias components in each Nyquist window. In this case, there will be alias of the original RF signal between N and (N+1) GHz; and we are only interested in the low- frequency image (0-1 GHz). However, the amplitude in each Nyquist window is very different and is shaped by a sinc function response. In other words, after the digitized RoF link the RF signal is not only relocated to the lower frequency, but also filtered by an equivalent lowpass filter governed by the sinc function. Fig. 6 illustrates this frequency shaping effect. Fig. 6a is the spectrum of the original RF signal, which is then copied to each Nyquest window (Fig. 6b) and then passes through a lowpass filter with a frequency response shown in Fig. 6c. Fig. 6d shows the final IF spectrum after lowpass filtering. 0 Fs0 Fs RF a b c d -6 0 G a in in ( d B ) Fig.6. (a)RF spectrum; (b) frequency images after sampling; (c) lowpass filter; (d) final spectrum after frequency relocation. As can be seen, due to the uneven frequency response of the lowpass filter, the OFDM signal is shaped and distorted. Therefore it is essential to compensate for this distortion for optimized link performance. In our experiments, a single-tap frequency domain equalizer that used in conventional OFDM systems [11] was employed to compensate for this signal distortion. Fig. 7a and b give the 4QAM, 8QAM and 16QAM MZM DFB Base station bias PD Clock Generator (AD 9516) Local Oscillator Central office Sampling Oscilloscope OFDM demodulation Matlab Balun (5315A) Differential Splitter T/H AMPLIFIER (HMC5640BLC4B) Bandpass Sampling ADC (EV10AS150A) Quantization FPGA (Altera DE3) DSP P/S conversion Signal capture Fig.4. Experimental demonstration. 117 eackerman Typewritten Text 117 eackerman Typewritten Text eackerman Typewritten Text eackerman Typewritten Text eackerman Typewritten Text eackerman Typewritten Text constellations of the demodulated OFDM signals using the proposed DRoF scheme with and without equalization. When no equalization is applied, the signal can hardly be demodulated specially for high-order modulation schemes; and the constellations become much clearer as a result of the single-tap frequency domain equalizer. a b Fig.7. Constellations of 4QAM, 8QAM, and 16 QAM with (a) and without (b) equalizer used at the receiver side. -15 -10 -5 0 10 12 14 16 18 20 L o g (B E R ) RF frequency (GHz) 4QAM 8QAM 16QAM Fig.8. BER performance of the proposed scheme Fig. 8 illustrates the BER performance of the proposed T/H amplifier assisted digitized RoF link based on commercial components, evaluation boards and with equalization. For 4QAM, 8QAM and 16QAM modulated OFDM signals, the BERs are around 10 -10 , 10 -6 and 10 -4 respectively. Small performance degradation is observed when we increase the RF carrier frequency. Therefore, by taking advantage of the track- and-hold amplifier, we have extended the bandwidth of the digitized RoF system to 20 GHz using an ADC with an analog bandwidth of 4.5 GHz. IV. CONCLUSION The application of digitized RoF technique in high- frequency radio-over-fiber system is limited by the insufficient analog bandwidth of the current ADCs. In this paper, we proposed to use a T/H amplifier in the digitized RoF link, which can not only extend the analog bandwidth of the system but also provide a better SNR due to lower jitter noise. We have experimentally demonstrated such T/H assisted digitized RoF system using commercial-available T/H amplifier, ADC, clock generator, and FPGA. With the assistance of T/H amplifier, an OFDM signal at 20 GHz was digitized using an ADC with 4.5 GHz bandwidth, and delivered via an 8 Gbps digital optical link. After transmission over 10-km SMF, the signal is demodulated with BER at 10 -10 , 10 -6 and 10 -4 respectively for 4QAM, 8QAM and 16QAM modulation formats. To compensate for the uneven frequency response due to the lowpass filter characteristic of the bandpass sampling process, a conventional single-tap frequency domain equalizer was used in the OFDM demodulation at the receiver. REFERENCES [1] V. G. Cerf, and M. P. Singh, âInternet predictions,â IEEE Internet Comput., vol. 14, no. 1, pp. 10â11, Jan.âFeb. 2010. [2] M. Deruyck, W. Vereecken, E. Tanghe, W. Joseph, M. Pickavet, L. Martens, and P. Demeester, âPower consumption in wireless access networks,â 2010 European Wireless Conference, 2010. [3] H. Chettat, L. M. Simohamed, Y. Bouslimani, and H. 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