Electrical and optical clock and data recovery in optical ......Received: 3 September 2015 Revised: 20 July 2016 Accepted: 29 August 2016 DOI 10.1002/dac.3202 RESEARCH ARTICLE Electrical
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Received: 3 September 2015 Revised: 20 July 2016 Accepted: 29 August 2016
DOI 10.1002/dac.3202
R E S E A R C H A R T I C L E
Electrical and optical clock and data recovery in optical accessnetworks: a comparative study
Esraa Abd El-Khaleq1 Yasmine El-Sayed2 Tawfik Ismail2* Hassan Mostafa1
1Department of Electronics and Communication,
Faculty of Engineering, Cairo University, Giza,
Egypt2Department of Engineering Applications of Laser,
National Institute of Laser Enhanced Science,
Cairo University, Giza, Egypt
CorrespondenceTawfik Ismail, Department of Electronics and
has been calculated to perform the comparative study between
the non-CDR, FBG-based OCDR, DBRL-based OCDR, and
ADCDR cases. Once again, it should be highlighted that the
eye diagram is computed for each individual set of transmis-
sion rate and transmission distance and is not shown in the
following sections for space limitations and because the BER
is enough to describe the system performance.
4.2 Performance evaluation at constant rateand variable transmission distance
To evaluate the performance of the proposed CDRs, we study
the impact of changing the transmission distance while fix-
ing the data rate to 10 Gbps. Figure 12 plots the BER versus
the distance (10-70 km) at 10-Gbps transmission rate. In
this case, the transmission distance cannot be varied above
70 km because the BER reaches 1 (i.e., all data are received
in error). As expected, for a given BER, the 3 CDR designs
provide much better performance than the non-CDR case.
ABD EL-KHALEQ ET AL. 7
(A) (B)
(C) (D)
FIGURE 11 The resulting eye diagram of the received signal at 1-Gbps transmission rate and distance of 150 km for the case of (A) non-CDR, (B)
FBG-based OCDR, (C) DBRL-based OCDR, and (D) ADCDR. ADCDR indicates all-digital clock and data recovery; CDR, clock and data recovery; DBRL,
distributed Bragg reflector laser; FBG, fiber Bragg grating; OCDR, optical clock and data recovery
FIGURE 12 BER versus the distance at a constant transmission rate that equals 10 Gbps. ADCDR indicates all-digital clock and data recovery; BER, bit
error rate; CDR, clock and data recovery; DBRL, distributed Bragg reflector laser; FBG, fiber Bragg grating; OCDR, optical clock and data recovery
The DBRL-based OCDR exhibits better performance than the
FBG-based OCDR. For example, the maximum transmission
distance for the minimum BER in the case of DBRL-based
OCDR is improved by a factor of 2X compared with the
FBG-based OCDR case. This transmission distance improve-
ment at higher transmission rates is caused by the fact that
the DBRL-based OCDR uses 2 FBG modules around the
erbium-doped fiber.
8 ABD EL-KHALEQ ET AL.
It is interesting to figure out that the ADCDR provides
much higher transmission distance improvement at 10 Gbps
by factors of 5 and 2.5 compared with the FBG-based OCDR
and the DBRL-based OCDR, respectively. This shows that
using the ADCDR is very promising at higher transmission
rates compared with the 2 OCDR designs. Moreover, in the
non-CDR case, the link is not working, and accordingly, at
such a large transmission rate (10 Gbps), the use of the CDR
is not optional.
4.3 Performance evaluation at constant transmissiondistance and variable rate
Figure 13 represents the BER versus the transmission rate
when it is varied from 1 to 10 Gbps and maintains the trans-
mission distance at 50 km. At this fixed distance of 50 km,
the non-CDR, FBG-based OCDR, and DBRL-based OCDR
show approximately the same BER performance up to a
transmission rate of 6 Gbps. At transmission rates higher
than 6 Gbps, the FBG-based OCDR exhibits slightly bet-
ter performance compared with the non-CDR case whereas
the DBRL-based OCDR exhibits better performance than
the non-CDR and FBG-based OCDR cases. However, the 3
cases (i.e., non-CDR, FBG-based OCDR, and DBRL-based
OCDR) have very poor BER compared with the ADCDR case
when the transmission rate exceeds 6 Gbps. The ADCDR
shows an excellent performance up to 9 Gbps at a fixed
distance of 50 km. Additionally, Figure 14 shows the BER
versus the transmission rate (1-10 Gbps) at transmission dis-
tance that equals 100 km.
Zooming in more on Figure 14 shows that the ADCDR
performance is actually slightly worse than that for both
the OCDR cases and non-CDR cases. This is because the
ADCDR is unable to recover the data at this high transmis-
sion rate when the data are highly attenuated. Therefore, the
ADCDR should be designed carefully when integrated with
high-speed optical access networks of large distances. How-
ever, the ADCDR speed can be enhanced by using several
ADCDR blocks working in parallel. This design allows the
data to be divided into several parallel paths at lower transmis-
sion rates (i.e., a 10-Gbps data can be divided into 10 parallel
CDRs, each with a transmission rate of 1 Gbps). This can
be performed by using the programmability advantage of the
FPGA in the case of the ADCDR. It should be noted that this
parallel solution is more expensive if applied to the OCDR.
FIGURE 13 BER versus transmission rate for transmission length that equals 50 km. ADCDR indicates all-digital clock and data recovery; BER, bit error
rate; CDR, clock and data recovery; DBRL, distributed Bragg reflector laser; FBG, fiber Bragg grating; OCDR, optical clock and data recovery
FIGURE 14 BER versus transmission rate transmission length that equals 100 km. ADCDR indicates all-digital clock and data recovery; BER, bit error rate;
CDR, clock and data recovery; DBRL, distributed Bragg reflector laser; FBG, fiber Bragg grating; OCDR, optical clock and data recovery
ABD EL-KHALEQ ET AL. 9
TABLE 2 The maximum allowable transmission distance at transmission rates of 10, 25, and 40 Gbps for afixed BER of 10 − 2.00
Tx Rate, Gbps ADCDR, km FBG-OCDR, km DBRL-OCDR, km Non-CDR, km
10 58 18 30 12
25 0 4 8 3
40 0 1 5 1
Abbreviations: ADCDR, all-digital clock and data recovery; BER, bit error rate; CDR, clock and data recovery; DBRL,
distributed Bragg reflector laser; FBG, fiber Bragg grating; OCDR, optical clock and data recovery; Tx, transmission.
Quantitatively, the comparative study between the 4 cases
(i.e., ADCDR, FBG-based OCDR, DBRL-based OCDR, and
non-CDR) showing the maximum allowable transmission
distance at specific transmission rate is summarized in Table
reftable:TI. These results emphasize the following design
insights:
1. For a transmission rate of 10 Gbps, the 3 CDR types
improve the transmission distance and the ADCDR
is superior compared with the 2 OCDR types. The
DBRL-based OCDR provides better performance com-
pared with the FBG-based OCDR.
2. When the transmission rate is increased to 25 and 40 Gbps,
the ADCDR fails to provide the required recovery, and
accordingly, several parallel ADCDR designs should be
used. These parallel ADCDR blocks can be performed
by the FPGA programmability feature and can be even
configured depending on the link transmission rate.
3. At high transmission rates of 25 and 40 Gbps, the
DBRL-based OCDR and the FBG-based OCDR are still
improving the transmission distance compared with the
non-CDR case.
4. The main recommendation of this work is to use the
ADCDR for links with transmission rates lower than
10 Gbps and the OCDR for higher speeds. In case the
ADCDR is to be used at higher transmission rates, par-
allel paths of the ADCDR should be used. These parallel
paths solution is more expensive in the OCDR case com-
pared with the programmable FPGA implementation of
the ADCDR.
5. To increase the transmission rate by using ADCDR, a
SerDes technique is used.24 We successfully implemented
10 ADCDR using Vertex-7 with data rate 1 Gbps per CDR.
5 CONCLUSIONS
In this paper, we have designed and implemented 2 types of
CDRs, electrical and optical. The electrical CDR is imple-
mented by using Vertex-7 FPGA while the OCDR is imple-
mented using DBRL and FBG. The 2 designs are tested by
embedding them with single mode optical fiber, and then
we studied the performance of the ADCDR and OCDR with
data rates up to 40 Gbps over transmission distances of up to
100 km. The simulation and practical results show that the
system performance is significantly improved by using the
proposed ADCDR compared with the OCDR in transmission
rates lower than 10 Gbps. However, by increasing the data
rate to more than 10 Gbps, the OCDR is given a better perfor-
mance than ADCDR. Otherwise, it is possible to use a parallel
set of ADCDRs to increase the overall system data rate. In
this case, 10-Gbps data transmission is implemented by using
10 parallel links. Each link uses a separate CDR with data
rate of 1 Gbps. This parallel ADCDR is achieved by using
the programmability advantage of the Vertex-7 FPGA. The
future work includes implementing the proposed ADCDR
with SerDes using Vertix-7 FPGA with 32 channels and
1.25-GHz speed. This work can be used for high-speed optical
access network with 40-Gbps transmission rate.
ACKNOWLEDGMENTS
This work was funded and supported by the National
Telecommunication Regulatory Authority of Egypt and Cairo
University.
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