Experimental Demonstration of a Single-carrier Frequency Division Multiple Address based PON (SCFDMA-PON)

Architecture Juhao Li (1), Cheng Zhang (1), Fan Zhang (1), Yongqi He (1) and Zhangyuan Chen (1)

(1) State Key Lab. of Advanced Optical Communication Systems & Networks, Peking University, Beijing, 100871, China

((juhao_li@pku.edu.cn, fzhang@pku.edu.cn, chenzhy@pku.edu.cn))

Abstract We introduce a novel architecture for next generation passive optical network base on the Single-carrier Frequency Division Multiple Address (SC-FDMA) technique. Both downstream and upstream SC-FDMA transmissions are experimentally demonstrated.

Introduction The passive optical network (PON) is considered as a promising solution for future broadband access networks. Due to the growing bandwidth demand, next-generation PON technologies at the traffic higher than 10Gb/s have been widely discussed. Several access technologies have been proposed, such as Time Division Multiple Access (TDMA), Wavelength Division Multiplexing (WDM), Optical Code Division Multiplexed Access (O-CDMA), and Orthogonal Frequency Division Multiplexing Access (OFDMA)1. Among them, the OFDMA scheme is impressing for its highest 108Gb/s single wavelength downstream/upstream data rate in PON systems2.

In this paper, we propose a novel PON architecture employing the Single-carrier Frequency Division Multiple Address (SC-FDMA) technique. The SC-FDMA is a modified form of OFDMA which has a similar throughput performance and overall complexity as OFDMA. SC-FDMA is currently employed for the uplink multiple access scheme in the Long Term Evolution (LTE) of cellular systems by the Third Generation Partnership Project (3GPP)3. A principal advantage of SC-FDMA is the lower PAPR than that of OFDMA. In this paper, we show by experiments that SC-FDMA is feasible for both downstream and upstream PON

transmission.

Technique principle

As a modified form, the baseband digital signal processing (DSP) method of the SC-FDMA has much in common with that of the OFDMA. Fig. 1 shows the transmitter and receiver structure for SC-FDMA. We can see that the only difference between them is the presence of the discrete Fourier transform (DFT) in the SC-FDMA transmitter and the inverse DFT (IDFT) in the SC-FDMA receiver. For this reason, SC-FDMA is sometimes referred to as DFT-spread OFDMA. At the transmitter, the first step to modulate the SC-FDMA symbol is to perform an M-point DFT to produce the frequency domain representation of mapped quadrature amplitude modulation (QAM) or Phase Shift Keying (PSK) signals. It then maps each of the M DFT outputs to one of the N (N> M) orthogonal subcarriers that can be transmitted. After subcarrier mapping, an IDFT transforms the subcarriers to a complex time domain signal. Before the signal is transmitted, cyclic prefix (CP) is inserted in order to provide a guard time to prevent inter-block interference. At the receiver, after CP is removed from each block, the N-point DFT transforms the signals into frequency domain and channel equalization is performed. Unlike

S/PData

P/SAddCP LPFM-point

DFTN-pointIDFT

Subcarriermapping

Transmitted signalQAM or

PSK mapping

(a)

(b) Fig. 1: DSP block diagrams for (a) the SC-FDMA coder (b) the SC-FDMA decoder

ECOC 2010, 19-23 September, 2010, Torino, Italy

978-1-4244-8535-2/10/$26.00 ©2010 IEEE

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OFDMA scheme in which decision is performed in the frequency domain, the equalized SC-FDMA signal is transformed into time domain by the M-point IDFT for decision.

We can see that subcarrier mapping enables spectrum division for different terminals and is essential for SC-FDMA. The mapping can be localized or distributed3. Fig.2 shows a localized SC-FDMA scheme for a typical PON architecture. In OFDMA each optical network unit (ONU) uses a set of subcarriers to transmit or receive its data. In SC-FDMA, the ONUs employ single-carrier transmission, but each single carrier is frequency-domain shifted to occupy a specific part of the whole available bandwidth. For upstream transmission, each ONU should fill unwanted subcarriers with zeroes during the subcarrier mapping. Due to the DFT guaranteed orthogonality, the optical line terminal (OLT) can simultaneous received data from all ONUs without inter-subcarrier interference.

Splitter

Fig. 2: Frequency spectrum division for SCFDMA-PON

Moreover, if the traffic is organized with SC-FDMA frames, each of which consists of multiple SC-FDMA symbols, the resource allocation may be two-dimensional in both frequency and time domains. The resource structure can be expressed as shown in Fig. 3.

In cellular applications, SC-FDMA is not recommended as the downstream technique,

because OFDMA performs better in the presence of severe multipath signal propagation. The immunity to multipath derives from the fact that an OFDMA system transmits information on multiple orthogonal frequency carriers, which is more robust against frequency selective fading than the single-carrier approach. But optical fibre channel differs from wireless channel. In this paper, we experimentally verify the feasibility of SCFDMA-PON with both downstream and upstream SC-FDMA transmission.

1fSC-FDMA symbol number n

Frequency

Resource Element

nf

Fig. 3: Basic time-frequency resource structure for SC-

FDMA frame

Experiment Setup

Figure 4 shows the experimental setup to validate the SCFDMA-PON architecture. The baseband SC-FDMA signal is generated with three times upsampling and up-converted to 2.5GHz by digital I-Q modulation in Matlab. The FFT size is 256 and from which 204 subcarriers are used for data transmission. The CP size is 16 and QPSK is used for constellation mapping. The generated waveform is uploaded into a Tektronix AWG7122B whose waveforms are continuously output at a sample rate of 10Gs/s (8 bits DAC), the total bit rate for two ONUs is 5Gb/s (2.5Gb/s for each). An intensity Mach-Zehnder modulator (MZM) is utilized to convert the SC-FDMA signal to double-side-band (DSB) optical signal. The optical distribution network is

(a) (b) Fig 4: Experimental setup for downstream and upstream SCFDMA-PON (a) transmitter and (b) receiver

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emulated with 22.2 km SSMF, a 10dB optical attenuator, a variable optical attenuator (VOA) and a 1:2 splitter. The optical DSB signal is converted to electrical RF signal by a photodiode and then is amplified before sampled by a real-time digital storage oscilloscope (Tektronix DPO72004B) at the sampling rate of 25 Gs/s. The sampled data are decoded in offline process.

For downstream traffic, data from OLT can be a whole single-carrier band or multiple single-carrier bands assigned to each ONUs. In the experiment, the latter case is preferred and two single-carrier bands for two ONUs are generated independently and combined in Matlab. For upstream traffic, the two output channels of AWG independently modulate two intensity MZMs to emulate two ONUs traffic. The downlink laser wavelength is 1550nm while the uplink ones of ONU-1 and ONU-2 are set to 1550 nm and 1557nm, respectively.

Experiment Results and discussion

(a) (b)

(c) (d)

Fig. 5: Signal spectrums. (a) downstream for ONU1&2; (b) simultaneous ONU1&2 upstream of SC-FDMA signals; (c)

single ONU1 upstream of SC-FDMA signals; (d) single ONU2 upstream of SC-FDMA signals.

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-5

-4

-3

-2

downstream ONU1 downstream ONU2

logB

ER

Received optical power (dBm)

Fig. 6: SC-FDMA downstream BER performance

Figure 5 shows the received downstream and upstream SC-FDMA signal spectrum. The 2.6~4 GHz frequency band is assigned to ONU-1, while the 1~ 2.4GHz frequency band is assigned to ONU-2. A small guard band about 200MHz is put between two ONUs’ signals to minimize the possible band interference due to nonlinear conponments. Fig. 6 and Fig. 7 show the bit error rate (BER) versus received optical power for downstream and upstream traffic, respectively. The different BER performance between the two ONUs’ upstream comes from different MZM modulation linearity. And power plenty of ONU2 upstream between single and combined transmission is due to the noise floor of ONU1 upstream.

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-5

-4

-3

-2

-1 ONU-2 upstream only ONU-2 upstream with ONU1 ONU-1 upstream only ONU-1 upstream with ONU2

logB

ER

Received optical power (dBm) Fig. 7: SC-FDMA upstream BER performance

Conclusions We have proposed a novel PON architecture employing the SC-FDMA technique and showed the first experimental demonstration of 5Gb/s optical SCFDMA-PON traffic.

Acknowledgment

This work was supported by National Basic Research Program of China (973 Program, No. 2010CB328201 and 2010CB328202), National Natural Science Foundation of China (NSFC, No. 60907030, No. 60877045 No. 60932004 and No.60736003).

References 1. D. Qian, et al, Proc. ECOC’07, paper 5.4.1

(2007). 2. D.Qian, et al. Proc. OFC’09, paper PDP5

(2009). 3. 3GPP, Technical Specification TR 25.814

V7.1.0 Sep, Release 7,(2006).

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