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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 26 — Dec. 12, 2011
  • pp: B983–B988
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Energy efficient OFDM transceiver design based on traffic tracking and adaptive bandwidth adjustment

Jingjing Zhang, Junqiang Hu, Dayou Qian, and Ting Wang  »View Author Affiliations


Optics Express, Vol. 19, Issue 26, pp. B983-B988 (2011)
http://dx.doi.org/10.1364/OE.19.00B983


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Abstract

This paper proposes a novel scheme to reduce energy consumption of digital transceivers with OFDM as the modulation scheme. The energy consumption of transceivers is saved based on two key techniques: 1) adaptively tune the bandwidth and sampling rate of the OFDM signal; 2) selectively power off individual block of parallel modules in the transceiver. Performance analysis illustrate that the newly designed transceiver consumes a significantly less amount of energy as compared to the conventional transceiver.

© 2011 OSA

1. Introduction

This paper proposes a novel energy-efficient optical transceiver design which adapts its operating rate to the real-time incoming traffic and its energy consumption. As compared to existing solutions, the proposed approach does not need the upper layer functions, thus simplifying the network control. The main idea is to adaptively enable/disable individual block in parallel processing modules according to the dynamic network traffic load. The target system uses digital transceiver with OFDM (Orthogonal Frequency Domain Multiplexing) as the modulation scheme. As recent studies show that OFDM has become a good candidate for optical communications from access to transport networks [11

11. D. Qian, M. Huang, S. Zhang, P. Ji, Y. Shao, F. Yaman, E. Mateo, T. Wang, Y. Inada, T. Ogata, and Y. Aoki, “Transmission of 115*100G PDM-8QAM-OFDM channels with 4Bits/s/HZ spectral efficiency over 10181km,” in ECOC, (PDP2011).

,12

12. Y. Ma, Q. Yang, Y. Tang, S. Chen, and W. Shieh, “1-Tbit/s single-channel coherent optical OFDM transmission with orthogonal-band multiplexing and subwavelength bandwidth access,” J. Lightwave Technol. 4, 308–315 (2010).

], the proposed approach in this paper provides a promising solution to achieve energy efficiency in future telecom networks.

2. Principles of the solution

Our approach is based on two facts in the system. First, with OFDM, the bandwidth in use can be instantly adjusted, and the sampling rate can be changed accordingly. Second, in high-speed implementation, a system usually contains multiple identical modules to work in parallel. Thus, varying the sampling rate can facilitate putting some modules into the power-saving mode.

2.1. OFDM bandwidth vs. FFT size and the sampling rate

Fast Fourier Transform (FFT) [13

13. S. Weinstein and P. Ebert, “Data transmission by frequency-division multiplexing using the discrete fourier transform,” IEEE Trans. Commun. Technol. 19, 628–634 (1971). [CrossRef]

] has become a common and efficient way to generate and demodulate OFDM signals in the digital domain. At the transmitter side, inverse FFT (IFFT) is used to convert signals from the frequency domain to the time domain before transmission. Denote X(k) as the frequency-domain signal to be modulated by the kth subcarrier. Then, the corresponding time domain signal is x(n)=(1/N)k=0N1X(k)exp(2πikn/N), where n = 0, ... , N – 1. If the lower half bandwidth is used while the upper half frequency subcarriers are all set to zero, we can obtain the time domain signal, i.e., x(m)=(1/2M)k=0M1X(k)exp(2πikm/M), where x′(m) is generated at point x′(m) = x(2m) (m = 0, 1, ... , M – 1), and M = N/2. This is the IFFT equation with only half samples without the consideration of the amplitude difference. This equation shows that if the duration of an OFDM frame is constant, the time-domain signal can be generated using half size FFT with half sampling rate when the bandwidth is reduced to a half. The same principle can be applied to the receiver side which uses FFT to convert the signal back to the frequency domain for processing. Thus, it is feasible to reduce the sampling rate and the processing complexity when a lower bandwidth is enough to accommodate the incoming traffic.

2.2. Hardware implementation architecture

Usually, the digitizing module, i.e., the analog-to-digital converter (ADC) at the receiver side, works at the rate below 10G samples per second. To meet the requirement of higher throughput such as 100Gbit/s link, multiple sub-ADCs are arranged to work in parallel, with a track (or sample)-and-hold (T&H) module being placed in front of each sub-ADC. The phases of the tracking clocks of these T&H modules are interleaved with each other so as to provide evenly distributed sampling points at a higher sampling rate. Fig. 1 illustrates an example of the high-speed ADC structure containing 4 sub-ADC channels, each of which is connected to one T&H module. The clock generator distributes the clock signal with phase shifts of 0°, 90°, 180°, and 270° to the four T& H modules, respectively. By doing so, the input signal can be evenly sampled.

Fig. 1 An example of the high-speed ADC structure

Attributed to the parallelization of signal processing logic as well as the interleaving of low-speed sub-ADCs, it is possible to power-off or hibernate part of these modules when the required sampling rate is low.

3. Energy efficient transceiver design

Based on the feasibility of individually powering off or hibernating each instance of the parallel modules by using techniques such as power gating [14

14. D. Chiou, S. Chen, S. Chang, and C. Yeh, “Timing driven power gating,” in Proceedings of the 43rd annual Design Automation Conference, (ACM, 2006), pp. 121–124. [CrossRef]

], we propose an energy-efficient digital transceiver with OFDM as the modulation scheme. Fig. 2 describes the system block diagram of the proposed transceiver.

Fig. 2 The system block diagram of the proposed transceiver

Receiver: At the receiver side, during the initialization stage, the input signal is assumed to be with full-bandwidth, and both the ADC and the digital processing logic operate at the full speed. The processing decision module receives the information carried by control subcarriers, and further extracts the control message including the allocated bandwidth information. Then, the receiver is able to track the bandwidth allocated by the transmitter and adjust its functional modules including the sampling rate of ADC according to the specified w value. Consider ADC as an example. When the bandwidth is reduced to a half and there are 2 · k sub-ADCs work actively in interleaved mode, k sub-ADCs can be put in the power-saving mode, and the other k sub-ADCs remain staying in the active mode such that 1/2 of the original sampling rate can be guaranteed. The reduced sampling rate further allows half of the current active signal processing instances stay in the power saving mode.

One advantage of the proposed solution is that the receiver can be blindly started without knowledge of the bandwidth in use. It further implies that no feedback control is necessary from the receiver to the transmitter. The robustness of the system can therefore be enhanced, and the reaction time can be reduced.

Bandwidth allocation: The decision making module at the transmitter side makes its decision according to its internal queue length. Denote q as the queue length, thl as the lower bound of the queue length to reduce the bandwidth to a half, and thu is the upper bound of the queue length to double the bandwidth. A simple bandwidth allocation scheme can be as follows. If q < thl, and the bandwidth being used is not the lowest, the bandwidth is reduced to a half; if q > thu, and the bandwidth is not fully utilized, the allocated bandwidth is doubled. Figure 3 illustrates the process of adapting bandwidth. At the end of frame i, the transmitter finds that its queue length exceeds the upper bound thu. Then, it notifies the receiver to double the band-width in frame i + 1. It takes some time for the receiver to switch data rates. In this example, the rate is doubled at frame j, and the switching time equals to ji – 1 frames. The sampling rate should be doubled at frame j + 1 as well.

Fig. 3 An illustration of the bandwidth adaptation process

4. Performance analysis

Overhead analysis: In the proposed system, additional logic is required for sample distribution/reassembly and power control modules. The control logic takes less than 5% of overall resource, which is negligible as compared to the resource-consuming signal processing logic (e.g., FFT/IFFT). Besides the logic overhead, some bandwidth is required to convey allocated bandwidth information between transmitters and receivers. Consider OFDM signal with 200 subcarriers achieved from 256-point IFFT conversion. If the control information is carried by a single subcarrier in multiple OFDM frames, the bandwidth overhead used for control signals is only 0.5% of the total network bandwidth. Therefore, both the logic overhead and the bandwidth overhead can be negligible in the proposed system.

Power efficiency and delay performances: We investigate the power efficiency and delay performances of our proposed system by simulations. Consider a pair of 100 Gigabit Ethernet transceiver, and a self-similar traffic profile with packet size being uniformly distributed between 64 bytes and 1518 bytes. Receiver can support three data rates: 25Gbit/s, 50Gbit/s, and 100Gbit/s.

In Fig. 4(a), we do not consider the time which takes a transceiver to switch from one rate to another. thl and thu are set as 2500 bytes and 15000 bytes, respectively. Traffic transmission in 10 seconds is simulated. Denote T(25), T(50), and T(100) as the time duration that the transceiver operates at 25Gbit/s, 50Gbit/s, and 100Gbit/s, respectively. Denote P(25), P(50), and P(100) as the power consumption of the transceiver when it stays in operates at 25Gbit/s, 50Gbit/s, and 100Gbit/s, respectively. Assume P(25) : P(50) : P(100) = 1 : 2 : 4. With conventional transceivers, the energy consumption in these 10 seconds equals to 10s·P(100). With the proposed energy-efficient transceivers, the energy consumption equals to T(25) · P(25) + T(50) · P(50) + T(100) · P(100). The normalized power saving is then defined as
10sP(100)[T(25)P(25)+T(50)P(50)+T(100)P(100)]10sP(100)
(1)
It is shown that the transceiver almost operates at 25Gbit/s all the time when the traffic load is 17Gbit/s although the input self-similar traffic is rather bursty. With the increase of the traffic load, the time that the transceiver operates at 100Gbit/s keeps increasing. The time that the transceiver operates at 50Gbit/s first increases when the traffic load is below 65Gbit/s and then decrease when the load is higher than 65Gbit/s. Regarding packet delay, it increases with the increase of the traffic load, but always keeps below 25 μs even when the data rate is as high as 96bit/s. When the traffic load is 17Gbit/s, over 70% of the total energy can be saved. When the traffic load increases to 65Gbit/s, around 30% of the energy can be saved.

Fig. 4 Simulation results. (a). Performances vs. load (Stabilizing time=0, upper bound=25k bytes, lower bound=2.5k bytes), (b). Performances vs. stabilizing time (load=18.3Gbit/s, upper bound=25k bytes, lower bound=2.5k bytes)

In real implementations, the modules inside the receiver may take some time to transit between the sleep mode and the active mode. We refer this transition time to as “stabilizing time”. Power consumption of the modules during the stabilizing period is assumed to be the same with that in working mode. In Fig. 4(b), the traffic load is set as 66Gbit/s. Fig. 4(b) illustrates that with the increase of the stabilizing time, the time that the transceiver operates in 25Gbit/s and 50Gbit/s keeps decreasing, and the time that the transceiver operates at 100Gbit/s keeps increasing. When the stabilizing time is as large as 6μs, the transceiver operates at 100Gbit/s most of the time even if the load is only 66Gbit/s since the large stabilizing time prohibits the transceiver to switch down a lower rate once it increases to a higher rate. When the stabilizing time equals zero, the power saving is over 30%. With the increase of the stabilizing time, power saving decreases. When the stabilizing time is as large as 6μs, the power saving is reduced to less than 10%. Although the time that transceivers operate at high rate increases with the increase of the stabilizing time, delay performances generally increases with the increase of the stabilizing time as shown in the figure because the large amount of time that the transceiver spent in switching status cannot be used for data transmission.

5. Conclusion

We proposed an energy-efficient OFDM transceiver which dynamically adjust its power-on individual parallel modules according to the network traffic load for energy saving. The proposed receiver is able to blindly start and track the bandwidth decision made by transmitter without feedback control or higher-layer operation. Simulation results shows that signficant amount energy can be saved especialy when during the off-peak traffic hours with low network traffic load.

References and links

1.

A. Plepys, “The grey side of ICT,” Environmental Impact Assessment Review 22, 509–523 (2002). [CrossRef]

2.

J. Zhang and N. Ansari, “Towards energy-efficient 1G-EPON and 10G-EPON with sleep-aware MAC control and scheduling,” IEEE Commun. Mag. 49, S33–S38 (2011). [CrossRef]

3.

http://www.ams-ix.net/.

4.

F. Idzikowski, S. Orlowski, C. Raack, H. Woesner, and A. Wolisz, “Saving energy in IP-over-WDM networks by switching off line cards in low-demand scenarios,” in 14th Conference on Optical Network Design and Modeling (ONDM), (IEEE, 2010). [CrossRef]

5.

Y. Zhang, M. Tornatore, P. Chowdhury, and B. Mukherjee, “Time-aware energy conservation in IP-over-WDM networks,” in Photonics in Switching, (Optical Society of America, 2010).

6.

J. Chabarek, J. Sommers, P. Barford, C. Estan, D. Tsiang, and S. Wright, “Power awareness in network design and routing,” in The 27th Conference on Computer Communications (INFOCOM), (IEEE, 2008), pp. 457–465. [CrossRef]

7.

C. Li, Y. Ofek, and M. Yung, “Time-driven priority flow control for real-time heterogeneous internetworking,” in The fifteenth Annual Joint Conference of the IEEE Computer Societies, (IEEE, 1996), vol. 1, pp. 189–197.

8.

Y. Chen and A. Jaekel, “Energy efficient grooming of scheduled sub-wavelength traffic demands,” in Optical Fiber Communication Conference, (Optical Society of America, 2011).

9.

M. Feng, K. Hinton, R. Ayre, and R. Tucker, “Energy efficiency in optical IP networks with multi-layer switching,” in Optical Fiber Communication Conference, (Optical Society of America, 2011).

10.

C. Cavdar, “Energy-efficient connection provisioning in WDM optical networks,” in Optical Fiber Communication Conference, (Optical Society of America, 2011).

11.

D. Qian, M. Huang, S. Zhang, P. Ji, Y. Shao, F. Yaman, E. Mateo, T. Wang, Y. Inada, T. Ogata, and Y. Aoki, “Transmission of 115*100G PDM-8QAM-OFDM channels with 4Bits/s/HZ spectral efficiency over 10181km,” in ECOC, (PDP2011).

12.

Y. Ma, Q. Yang, Y. Tang, S. Chen, and W. Shieh, “1-Tbit/s single-channel coherent optical OFDM transmission with orthogonal-band multiplexing and subwavelength bandwidth access,” J. Lightwave Technol. 4, 308–315 (2010).

13.

S. Weinstein and P. Ebert, “Data transmission by frequency-division multiplexing using the discrete fourier transform,” IEEE Trans. Commun. Technol. 19, 628–634 (1971). [CrossRef]

14.

D. Chiou, S. Chen, S. Chang, and C. Yeh, “Timing driven power gating,” in Proceedings of the 43rd annual Design Automation Conference, (ACM, 2006), pp. 121–124. [CrossRef]

OCIS Codes
(060.4080) Fiber optics and optical communications : Modulation
(060.4250) Fiber optics and optical communications : Networks
(060.4510) Fiber optics and optical communications : Optical communications

ToC Category:
Subsystems for Optical Networks

History
Original Manuscript: October 3, 2011
Revised Manuscript: November 29, 2011
Manuscript Accepted: November 30, 2011
Published: December 12, 2011

Virtual Issues
European Conference on Optical Communication 2011 (2011) Optics Express

Citation
Jingjing Zhang, Junqiang Hu, Dayou Qian, and Ting Wang, "Energy efficient OFDM transceiver design based on traffic tracking and adaptive bandwidth adjustment," Opt. Express 19, B983-B988 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-26-B983


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References

  1. A. Plepys, “The grey side of ICT,” Environmental Impact Assessment Review22, 509–523 (2002). [CrossRef]
  2. J. Zhang and N. Ansari, “Towards energy-efficient 1G-EPON and 10G-EPON with sleep-aware MAC control and scheduling,” IEEE Commun. Mag.49, S33–S38 (2011). [CrossRef]
  3. http://www.ams-ix.net/ .
  4. F. Idzikowski, S. Orlowski, C. Raack, H. Woesner, and A. Wolisz, “Saving energy in IP-over-WDM networks by switching off line cards in low-demand scenarios,” in 14th Conference on Optical Network Design and Modeling (ONDM), (IEEE, 2010). [CrossRef]
  5. Y. Zhang, M. Tornatore, P. Chowdhury, and B. Mukherjee, “Time-aware energy conservation in IP-over-WDM networks,” in Photonics in Switching, (Optical Society of America, 2010).
  6. J. Chabarek, J. Sommers, P. Barford, C. Estan, D. Tsiang, and S. Wright, “Power awareness in network design and routing,” in The 27th Conference on Computer Communications (INFOCOM), (IEEE, 2008), pp. 457–465. [CrossRef]
  7. C. Li, Y. Ofek, and M. Yung, “Time-driven priority flow control for real-time heterogeneous internetworking,” in The fifteenth Annual Joint Conference of the IEEE Computer Societies, (IEEE, 1996), vol. 1, pp. 189–197.
  8. Y. Chen and A. Jaekel, “Energy efficient grooming of scheduled sub-wavelength traffic demands,” in Optical Fiber Communication Conference, (Optical Society of America, 2011).
  9. M. Feng, K. Hinton, R. Ayre, and R. Tucker, “Energy efficiency in optical IP networks with multi-layer switching,” in Optical Fiber Communication Conference, (Optical Society of America, 2011).
  10. C. Cavdar, “Energy-efficient connection provisioning in WDM optical networks,” in Optical Fiber Communication Conference, (Optical Society of America, 2011).
  11. D. Qian, M. Huang, S. Zhang, P. Ji, Y. Shao, F. Yaman, E. Mateo, T. Wang, Y. Inada, T. Ogata, and Y. Aoki, “Transmission of 115*100G PDM-8QAM-OFDM channels with 4Bits/s/HZ spectral efficiency over 10181km,” in ECOC, (PDP2011).
  12. Y. Ma, Q. Yang, Y. Tang, S. Chen, and W. Shieh, “1-Tbit/s single-channel coherent optical OFDM transmission with orthogonal-band multiplexing and subwavelength bandwidth access,” J. Lightwave Technol.4, 308–315 (2010).
  13. S. Weinstein and P. Ebert, “Data transmission by frequency-division multiplexing using the discrete fourier transform,” IEEE Trans. Commun. Technol.19, 628–634 (1971). [CrossRef]
  14. D. Chiou, S. Chen, S. Chang, and C. Yeh, “Timing driven power gating,” in Proceedings of the 43rd annual Design Automation Conference, (ACM, 2006), pp. 121–124. [CrossRef]

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