OTh3A.4.pdf
OFC/NFOEC Technical Digest © 2013 OSA
Energy-Efficient OFDMA-PON Exploiting Modular OLT/ONU Digital Signal Processing Konstantinos Kanonakis and Ioannis Tomkos Athens Information Technology (AIT), 0.8 km Markopoulou Av, Athens, 19002, Greece. e-mail:
[email protected]
Abstract: We propose and evaluate a novel system design and MAC framework for OFDMAPONs leveraging modular DSP which offers significant power savings for the ONU and the OLT while providing controllable trade-off with packet delay. OCIS codes: (060.2330, 060.4250).
1. Introduction The recent introduction of the Orthogonal Frequency Division Multiple Access Passive Optical Network (OFDMAPON) concept [1,2] has been driven by the unique combination of both physical (PHY) layer and networking benefits enabled by OFDM technology, including improved transmission performance and flexible dynamic bandwidth allocation at sub-wavelength granularity. The latter is achieved by the use of a large number of subcarriers, groups of which form virtual transmission pipes per ONU. To the negative side, OFDMA-PON involves more advanced Digital Signal Processing (DSP) than TDMA-PONs, which heavily affects its power consumption. At the same time, the need for energy efficiency in optical access networks has lately become extremely pressing. Most relevant solutions for TDMA-PONs evolve around the use of sleep modes, whereby part of the ONU circuitry is set to a power-saving mode when it does not have traffic to sent [3]. However such schemes always imply a heavy trade-off between energy-efficiency and Quality of Service (QoS) performance. Moreover, they are not applicable to the OLT, since on the one hand there is always some traffic to/from the ONUs, and on the other the OLT has also to ensure communication of control messages vital to system operation. In this paper we propose for the first time a complete framework for improving the energy-efficiency of OFDMA-PONs. We first present a detailed model for estimating the power consumption of an OFDMA-PON OLT and ONU. Then a modular design is proposed for those devices, which allows switching off or adjusting OLT/ONU DSP modules when lower bitrates are required. Accordingly, we introduce a scheme at the MAC layer which dynamically adapts ONU power consumption based on temporal fluctuations of user traffic. This is combined with a dynamic subcarrier allocation algorithm operating on the basis of the ONU power consumption states. The benefits of the proposed concept are multi-fold; First of all, it can provide significant reduction in the overall network power consumption during normal operation and at a controllable ONU delay penalty. Moreover, in contrast to TDMA-PON, considerable savings are also possible for the OLT, by reducing the overall offered bandwidth during off-peak periods. Finally, in our proposed scheme there is always an active bidirectional transmission pipe per ONU, thus inherently eliminating the risk of losing control messages. 2. Proposed OFDMA-PON System Design and Power Consumption Modeling From the various PHY-layer OFDM techniques proposed, in this work we consider Intensity-Modulation DirectDetection (IM/DD), which has been shown to one of the most power-efficient and cost-effective solutions for OFDMA-PONs. Fig.1(a) shows the main building blocks in the IM/DD OFDMA-PON OLT and ONU receiving and transmitting paths. Compared to TDMA-PON, OFDMA-PON requires extra DSP blocks, namely the N-point Inverse Fast Fourier Transform (N-IFFT) and Digital-to-Analog Converter (DAC) at the Tx, and the ADC and NFFT at the Rx. For these blocks we adopt the scheme proposed in [4], whereby the ADC at the Rx is composed of parallel modules. By switching off some of them, the sampling rate is reduced, while the N-FFT blocks also have to follow by adjusting their number of points N. As a result, there are log + 1 possible Power Consumption States (PCS) for the Rx depending on the number of active modules, assuming efficient power-of-two N-FFT implementations. The Tx can also switch between the same number of states by adjusting its N-IFFT. In Fig.1(b) we provide a breakdown of power consumption for the OLT and ONU, assuming an IM/DD OFDMA-PON with a symmetrical aggregate bitrate of 10 Gbps, 32 ONUs and a maximum of 2.5 Gbps per ONU. The total number of upstream/downstream data subcarriers is = 256, each offering a bitrate of
= 39.0625 using BPSK modulation (in order to support > 40 km reach). The values are based on estimations performed within the GreenTouch consortium, with the participation of a large number of industrial and academic partners. As seen in Fig.1(b), the power consumption for all presented blocks is fixed, apart from the DSP modules which are adjustable.
OTh3A.4.pdf
OFC/NFOEC Technical Digest © 2013 OSA
The number of modules assumed for the OLT/ONU Rx is = 8, (i.e. 4 possible PCS states for the OLT/ONU Rx/Tx). The maximum required sampling rates for the OLT and ONU are 20 GSps and 5 GSps respectively (2.5 GSps and 0.625 GSps per module) since - due to the IM scheme - only half of the transmitted subcarriers carry data. Therefore, for the N-FFT/IFFT blocks, N ranged between 64-512 (OLT) and 16-128 (ONU). For example, when the ONU Rx is in state ≥ 1, 2 ADC modules of 0.625 GSps are switched on and a 2 ∙ 16 -FFT is used, while the ONU Tx in state k implies a 2 ∙ 16 -IFFT. The maximum number of upstream/downstream data subcarriers "#$ "#$ and ,% respectively] that can be used by ONU & is 2 ∙ 8 when the Rx/Tx is in state . [ ,! As a result of the adjustable DSP, the power consumed by the ONU can range from 2.13 W up to 3.11 W depending on its exact PCS, as shown in Fig.1(b). In other words, power savings of more than 30% are possible at the user side by reducing the ONU upstream/downstream rates to the minimum. As will be explained below, the bursty nature of ONU traffic can be exploited to dynamically adjust their power consumption. Such a dynamic scheme is not commendable for the OLT due to smoothness of aggregated traffic, while possible incorrect adjustment decisions would degrade the QoS for a large number of users. However, time-of-day OLT power savings of up to 18% can be achieved (i.e. from 10.45W down to 8.56W) by appropriately adjusting the OLT Rx/Tx PCS, either during less busy periods (e.g. at night) or when fewer ONUs are registered in the OFDMA-PON. (a)
(b)
Receiving Path
Transmitting Path
APD/TIA (+ BM Rx)
(X)-GPON Digital
ADC
Power (mW)
OLT
ONU
Component
Power (mW)
N-IFFT
10 GHz APD/TIA
250
2.5 GHz DML + Driver
150
Burst-Mode Rx
1530
DAC
Down Converter
200
ADC Driver Amp.
200
DAC Driver Amp.
N-FFT
(Up Converter)
(X)-GPON Digital
DML (EML) + Driver
DSP Modules
ADC Driver Amp.
GPON Digital
460
XG-PON Digital
700
16-/128-FFT
49 /430
64-/512-FFT
273/781
8*0.625 GSps ADC
250
8*2.5 GSps ADC
1000
Minimum Total Power (W)
Transmitting Path
Component
Fixed
(Down Converter)
Receiving Path
10 GHz EML + Driver
1800
Up Converter
200
DAC Driver Amp.
200
GPON Digital
230
XG-PON Digital
2770
16-/128-IFFT
49 /430
64-/512-FFT
273/781
5 GSps DAC
110
20 GSps DAC
440
OLT
ONU
Maximum Total Power (W)
OLT
ONU
OLT
ONU
7.45
1.89
7.45
1.89
1.11
0.24
3
1.22
Fig. 1. (a) The main blocks contributing in power consumption for the OFDMA-PON OLT/ONU receiving and transmitting paths and (b) OFDMA-PON OLT/ONU power consumption breakdown using input from the GreenTouch consortium.
3. Proposed OFDMA-PON MAC for ONU Dynamic Power Consumption State Adaptation (DPCSA) For the OFDMA-PON MAC we assume a framework similar to that proposed in [5]. Once every scheduling cycle '( , the OLT assigns upstream/downstream subcarriers for each ONU by sending the respective downstream control messages. At the end of the cycle, ONUs report the number of bytes in their queues. The OLT can use the reports [) * denotes the bytes reported at the end of cycle *] together with the bytes received during cycle *, + ,! * , to calculate the average arrival rate at the ONU during cycle *, , ,! * as: , ,! * = max00, 1+ ,! * + ) 2 − ) 2 4⁄'( 6. The downstream arrival rate towards each ONU is simply calculated as , ,% * = + ,% * ⁄'( . Based on this information, the OLT can extract the ONU & upstream subcarrier requirements in cycle * as: 789 >!#7 >!#7 "#$ ,! * = min1max0? >!#7 * = ,! + ONU. Then, the OLT assigns the upstream subcarriers in a weighted fashion as follows: ,!
789 789 789 min @1 ,! * C∑ B B,! * 4 ∙ , ,! * D. The same process takes place for downstream subcarrier assignment. "#$ "#$ Note that since ,! and ,% are dependent on the ONU Tx/Rx PCS, the choice of the latter affects not only ONU power consumption but also QoS (due to the limitations it imposes in their maximum number of assigned subcarriers). For this reason, we propose the Dynamic PCS Adaptation (DPCSA) algorithm which is performed once every 'E ('E = ∙ '( , ≥ 1). DPCSA selects the PCS state for each ONU, independently for their Tx and Rx. The new Rx/Tx PCS is communicated to the ONUs via control messages and applies for scheduling cycles. At the end of scheduling cycle *, the new PCS for the ONU Tx, G ,! * , is selected as follows (the same for the downstream): E
"#$ P G ,! * = minHI
