## Design and simulation of 25 Gb/s optical OFDM transceiver ASICs |

Optics Express, Vol. 19, Issue 26, pp. B337-B342 (2011)

http://dx.doi.org/10.1364/OE.19.00B337

Acrobat PDF (856 KB)

### Abstract

We select the optimum design parameters for real-time optical OFDM transceivers running at 25 Gb/s and analyze power consumption and ASIC footprint for a variety of configurations based on synthesis for a 65nm standard-cell library. Experiments quantify the effects of modulation format and the number of IFFT/FFT points used in transceivers.

© 2011 OSA

## 1. Introduction

1. Y. Ma, Q. Yang, Y. Tang, S. Chen, and W. Shieh, “1-Tb/s single-channel coherent optical OFDM transmission with orthogonal-band multiplexing and subwavelength bandwidth access,” J. Lightwave Technol. **28**(4), 308–315 (2010). [CrossRef]

2. Y. Benlachtar, P. M. Watts, R. Bouziane, P. Milder, R. Koutsoyannis, J. C. Hoe, M. Püschel, M. Glick, and R. I. Killey, “Real-time digital signal processing for the generation of optical orthogonal frequency-division-multiplexed signals,” IEEE J. Sel. Top. Quantum Electron. **16**(5), 1235–1244 (2010). [CrossRef]

4. R. Schmogrow, M. Winter, B. Nebendahl, D. Hillerkuss, J. Meyer, M. Dreschmann, M. Huebner, J. Becker, C. Koos, W. Freude, and J. Leuthold, “101.5 Gbit/s real-time OFDM transmitter with 16QAM modulated subcarriers,” in *Optical Fiber Communication Conference*, OSA Technical Digest (CD) (Optical Society of America, 2011), paper OWE5.

5. R. Bouziane, P. Milder, R. Koutsoyannis, Y. Benlachtar, C. R. Berger, J. C. Hoe, M. Püschel, M. Glick, and R. I. Killey, “Design studies for an ASIC implementation of an optical OFDM transceiver,” in *2010 36th European Conference and Exhibition on Optical Communication (ECOC)*, (IEEE, 2010), paper Tu.5.A.4.

*M*-QAM modulation, for values of

*M*ranging from 2

^{2}to 2

^{8}. Next, we fix

*M*to 4, and vary the IFFT/FFT size in the range of 64 to 1024 points. Based on our ASIC synthesis results, we determine the power consumption and required area of all digital signal processing (DSP) transceivers and examine how the various parameters affect their area and power. Lastly, we perform post-synthesis simulation to verify correct functionality.

## 2. System structure

*M*-QAM modulation formats are investigated, for

*M*= 2

*,*

^{m}*m*= 2 to 8 inclusive. The symbols are then fed to a 128-point IFFT whose bit precision was varied from

*n*= 6 to 20 bits (for each of the real and imaginary parts of the complex data). The output of the IFFT is clipped and scaled to be interfaced with a

*k*-bit digital-to-analog converter (DAC). The resolution of the DAC,

*k*, was varied from 3 to 12. The system uses the discrete multitone (DMT) modulation format with 50 data subcarriers (causing the FFT to produce purely real data).

*k*-bit analog-to-digital converter (ADC) first converts the signal into the digital domain. We keep the ADC resolution

*k*the same as the DAC resolution. After serial-to-parallel conversion, the OFDM signal is scaled to

*p*bits and fed to a 128-point FFT. The FFT precision,

*p*, is also varied from 6 to 20 (independent of the IFFT precision

*n*). The output of the FFT (128 complex words) is equalized with 1-tap equalizers then fed to a symbol de-mapping block to translate from QAM symbols back to the original binary data. Later we will consider the effects of changing the number of IFFT/FFT points.

## 3. System characterization

^{−3}. Following the procedure outlined above and considering the required EVM that corresponds to 10

^{−3}BER for different modulation formats, we search for optimum design parameters. The results are listed in Table 1 . We then repeat the simulations within an optical link and calculate the resulting BER. If we were to target an optical system with greater levels of impairment, we would require greater precision in the signal converters as well as higher arithmetic precision within the IFFT and FFT.

## 4. ASIC design and synthesis

*w*represent the number of samples produced or consumed each clock cycle by a given transmitter or receiver;

*w*is thus a measure of parallelism within the transceiver. Let

*f*be the system’s clock frequency. So,

*w*×

*f*must equal the required sampling rate. For example, to reach 8 GS/s, a transmitter could produce 128 samples per cycle at 62.5 MHz, or 64 samples per cycle at 125 MHz, and so on. Based on this idea, we have constructed a hardware generator that takes as input the parameters from Table 1 as well as values for

*w*and

*f*. The generator then produces synthesizable Verilog descriptions of the corresponding transmitters and receivers, using FFT and IFFT cores created by the Spiral hardware generation tool [6].

*w*= 128, 64, 32 …, and decreasing until

*f*reaches 800 MHz. Then, we synthesize each design using Synopsys Design Compiler, targeting a 65 nm standard cell library. Figure 3 shows the area and power of each synthesized design. Points closest to the lower-left corner are the most area- and power-efficient. The transmitter designs span a power range from 30 to 400 mW and an area range from 0.6 to 2 mm

^{2}. Receivers in general require more area and power due to the higher precision required for the FFT and the use of complex multipliers for equalization. However, we observe that at low modulation formats, the difference is very small or can be reversed.

*w*), while the rightmost instance corresponds to the lowest frequency and largest

*w*. So, as

*w*increases, the power requirement decreases (due to lower frequencies) while area increases (due to increased parallelism within the implementation). This allows the designer to balance between these two cost metrics by choosing the value of

*w*that gives the desired area/power tradeoff.

## 5. ASIC transceivers with different (I)FFT sizes

*w*and

*f*as in Section 4). We considered

*w*= 32, 64, and 128 samples per cycle, giving clock frequency

*f*= 1000, 500, and 250 MHz (respectively). We performed bit precision simulations to determine the required precision for each transform, observing that for a QPSK system with BER of 10

^{−3}or better, the IFFT/FFT precisions of 12/14 are still adequate for all considered transform sizes. We then synthesize each design targeting a commercial 65 nm standard-cell library and show the corresponding area and power consumption in Fig. 4 .

*n*is indicated with data labels. Each value of

*n*has three design points (the three

*w/f*pairs as described above), with the highest values of

*w*on the right. Both graphs closely follow an observable pattern except in the points where

*n ≤ w*(the three lowest-power points in each graph). These points have lower relative cost because special optimizations can be applied to FFTs when this condition holds.

*w*produces larger designs that require less power. Then, by comparing one line to the others, we are able to observe the large effect of changing the (I)FFT size. For example, we see that the

*w*= 32 transmitter design for

*n*= 1,024 is 4.7 times larger and 3.6 times higher power than the corresponding design for

*n*= 64. Comparing with Fig. 3 and 4, we see that the (I)FFT size can have a much larger effect on the power and area of the OFDM transceiver than the modulation format.

## 6. Post synthesis and optical link simulation

## 7. Conclusions

## References and links

1. | Y. Ma, Q. Yang, Y. Tang, S. Chen, and W. Shieh, “1-Tb/s single-channel coherent optical OFDM transmission with orthogonal-band multiplexing and subwavelength bandwidth access,” J. Lightwave Technol. |

2. | Y. Benlachtar, P. M. Watts, R. Bouziane, P. Milder, R. Koutsoyannis, J. C. Hoe, M. Püschel, M. Glick, and R. I. Killey, “Real-time digital signal processing for the generation of optical orthogonal frequency-division-multiplexed signals,” IEEE J. Sel. Top. Quantum Electron. |

3. | X. Q. Jin, R. P. Giddings, E. Hugues-Salas, and J. M. Tang, “Real-time demonstration of 128-QAM-encoded optical OFDM transmission with a 5.25bit/s/Hz spectral efficiency in simple IMDD systems utilizing directly modulated DFB lasers,” Opt. Express |

4. | R. Schmogrow, M. Winter, B. Nebendahl, D. Hillerkuss, J. Meyer, M. Dreschmann, M. Huebner, J. Becker, C. Koos, W. Freude, and J. Leuthold, “101.5 Gbit/s real-time OFDM transmitter with 16QAM modulated subcarriers,” in |

5. | R. Bouziane, P. Milder, R. Koutsoyannis, Y. Benlachtar, C. R. Berger, J. C. Hoe, M. Püschel, M. Glick, and R. I. Killey, “Design studies for an ASIC implementation of an optical OFDM transceiver,” in |

6. | P. Milder, F. Franchetti, J. C. Hoe, and M. Püschel, “Formal datapath representation and manipulation for implementing DSP transforms,” in |

**OCIS Codes**

(060.4080) Fiber optics and optical communications : Modulation

(060.4510) Fiber optics and optical communications : Optical communications

**ToC Category:**

Subsystems for Optical Networks

**History**

Original Manuscript: October 3, 2011

Revised Manuscript: October 30, 2011

Manuscript Accepted: November 2, 2011

Published: November 18, 2011

**Virtual Issues**

European Conference on Optical Communication 2011 (2011) *Optics Express*

**Citation**

Peter A. Milder, Rachid Bouziane, Robert Koutsoyannis, Christian R. Berger, Yannis Benlachtar, Robert I. Killey, Madeleine Glick, and James C. Hoe, "Design and simulation of 25 Gb/s optical OFDM transceiver ASICs," Opt. Express **19**, B337-B342 (2011)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-26-B337

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### References

- Y. Ma, Q. Yang, Y. Tang, S. Chen, and W. Shieh, “1-Tb/s single-channel coherent optical OFDM transmission with orthogonal-band multiplexing and subwavelength bandwidth access,” J. Lightwave Technol.28(4), 308–315 (2010). [CrossRef]
- Y. Benlachtar, P. M. Watts, R. Bouziane, P. Milder, R. Koutsoyannis, J. C. Hoe, M. Püschel, M. Glick, and R. I. Killey, “Real-time digital signal processing for the generation of optical orthogonal frequency-division-multiplexed signals,” IEEE J. Sel. Top. Quantum Electron.16(5), 1235–1244 (2010). [CrossRef]
- X. Q. Jin, R. P. Giddings, E. Hugues-Salas, and J. M. Tang, “Real-time demonstration of 128-QAM-encoded optical OFDM transmission with a 5.25bit/s/Hz spectral efficiency in simple IMDD systems utilizing directly modulated DFB lasers,” Opt. Express17(22), 20484–20493 (2009). [CrossRef] [PubMed]
- R. Schmogrow, M. Winter, B. Nebendahl, D. Hillerkuss, J. Meyer, M. Dreschmann, M. Huebner, J. Becker, C. Koos, W. Freude, and J. Leuthold, “101.5 Gbit/s real-time OFDM transmitter with 16QAM modulated subcarriers,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2011), paper OWE5.
- R. Bouziane, P. Milder, R. Koutsoyannis, Y. Benlachtar, C. R. Berger, J. C. Hoe, M. Püschel, M. Glick, and R. I. Killey, “Design studies for an ASIC implementation of an optical OFDM transceiver,” in 2010 36th European Conference and Exhibition on Optical Communication (ECOC), (IEEE, 2010), paper Tu.5.A.4.
- P. Milder, F. Franchetti, J. C. Hoe, and M. Püschel, “Formal datapath representation and manipulation for implementing DSP transforms,” in Proceedings of the 45th annual Design Automation Conference (ACM, 2008), pp. 385–390.

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