Chapter the existing PON system with minimal modification,

Chapter
5

Demonstration of 25 Gbit/s PAM4 Hybrid Photonic-Wireless Transmission

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Introduction

The most encouraging way to achieve acceptable
delivery of wireless and baseband signals was Optical-Wireless access
technologies. The transmission distance and bandwidth increase while the power
budget decrease at the same time.

 A gigabit
passive optical network (GPON) is a solution to simplify the architecture to
apply to the ROF system. Through that we will able to afford higher bandwidth
at a longer connection distance 59.

While Radio-over-fiber
(RoF) systems and Passive Optical Network (PON) are promising candidates in
wireless and wired access networks, the primary concern is to transmit both
radio-frequency (RF) and baseband signals on a single wavelength over a single
fiber in a cost-effective way with acceptable performance. A most straightforward
solution is to build the RoF link based on the existing PON system with minimal
modification, which will be seamlessly integrated into
the existing optical distribution network. Recently, a high loss-budget
amplifier-less 100G-PON is demonstrated 60, on
which our proposed architecture is based. To achieve a low cost and complexity
link, 10G-class transmitter is promising, which requires advanced modulation
with high spectrum efficiency like four-level pulse amplitude modulation
(PAM4).

In this chapter, we
experimentally demonstrate the transmission of a 25 Gbit/s PAM4 signal with the
proposed architecture integrating a Ka-band hybrid photonic-wireless link and
an amplifier-less wired link. In our wireless link, the generated PAM4 signal
is delivered over 40-km SMF before up-converting to 25GHz Ka-band signal,
achieving a BER less than the HD-FEC threshold of 3.8×10-3 after
digital coherent down-conversion with the help of volterra equalization. In addition,
we also demonstrate an amplifier-less wired link, which is similar to the
typical Passive Optical Network (PON), to prove the feasibility of the
integration architecture of PON and RoF link, achieving receiver sensitivity at
-21.5 dBm and about 26dB loss budget with simple linear feed-forward
equalization.

A 4-level pulse amplitude modulation
(PAM4) scheme take placed Non-return to-zero (NRZ) signaling, because PAM4 used
half the bandwidth to transmit the same load compare with NRZ signal.

While PAM4 agitates signal integrity,
it increased complexity as well. This means that we face with new problems and
issues. More accurate measurements and design verification before generate are
some requirement at the beginning as principal.

5.1 High Data Rates beyond NRZ and
PAM4 Bandwidth Demands

With the aim of increasing the bit rate within the minimum bandwidth to
increase the performance of NG-PON, different modulation schemes are apparently
needed.

As illustrated in
figure 3, PAM4 signal uses four separate levels to encode two bits (00, 01, 10,
11) in each symbol, while  NRZ encodes
one bit, either 0 or 1.

 

Figure 3: Comparison between NRZ and PAM4.

 

5.1.1 PAM4 Complexity

We face potential nonlinearities by
using PAM4 and it brings an incremental leap in signal complexity.  “Eye compression” is differences in the eye
heights of the three eye openings, “timing skew” happen when the centers of the
three eyes are not regulated in the same line.

5.1.2 PAM4 ISI and Equalization

The frequency content of both NRZ and PAM4
signals consist of square wave harmonics and sub-harmonics from sequences of
identical symbols. The frequency components have precisely coordinated amplitudes and phases.

The amplitudes and phases of each
different frequency component has influenced by the channel.  By de-correlating its frequency and phase configuration,
the channel causes ISI. We can get rid of ISI by inverting the channel response
through Equalization. The problem is equalization in PAM4 systems is more
complicated than in NRZ systems, since PAM4 has 4 levels.

 

5.2 System Description

5.2.1 Methodology

Experiments and simulations are different methods to carry out research
in the field of optical systems and wireless. The idea of diving into two paths is rooted in the
problem of accessing to users and the need to use wireless system is inevitable
without modifying CO or transmission line.

If we cannot access an end-to-end fibre connection
and t is not economical to investigate a new fibre link, as might be the case in difficult-to-access terrains
or if an already existing fibre connection fails, a permanent wireless
connection could help.

There are several
optical techniques for generating and transporting signals over fiber and
wireless connection. Therefore, depending on
the transmission method used, the RAU could be simple or in a complicated structure.

Thus, heterodyne
photonic is configured because it is cost efficient and easy to
implement. While we can transfer data in typical PON, wireless transmission in
other path is aggregated. So it is reasonable to
introduce the low cost and less complexity RAU and CO. And also, the down
converting methods could let us flexibly rearrange the spectrum when the
antenna receives signals.

 

5.2.1.1 Ka band

 

The frequency range between 26.5 GHz and 40 GHz , which is the lower
edge of the mmW range, two bands around 28 GHz and 36 GHz have been assigned
for usage in mobile communication networks , that introduced as Ka-band in the
IEEE with an additional allocation in the adjacent K-band at 24 GHz 61.

These bands both are suitable for indoor and outdoor communications in
terms of channel characterizations 60,61.

The efficient use thereof has being a considerable problem even if these
bands allow for larger channel bandwidths and we have to pay attention to the
complexity of transmitting and receiving equipment.

5.2.1.2 Volterra filter

To compensate distortions Volterra filter have been applied simultaneously.
In sequence the three kernels Volterra filter, the k-th sample of the output
signal is expressed as 62

In our work, the nonlinear distortions are mainly recompensed by the
digital Volterra filter.

5.2.1.3 Equalization

 

We can use several signal processing methods at the receiver side to ease
the ISI difficulties which happened by delay spread .This technique is termed
as “equalization”.

Usually equalizers are small, inexpensive, and certainly tuneable, so
they can be employed digitally afterward A/D transformation.

Equalization techniques are classified in to two groups: linear and
nonlinear. The linear methods are almost the simplest to use and to comprehend
theoretically.

Most of the wireless applications use nonlinear equalizers, because
linear equalization techniques faced with noise boost which is not a big deal
in nonlinear category.

Equalizers can also be classified as
symbol-by-symbol (SBS) or sequence estimators (SE). SBS equalizers eliminate ISI
from every figure and sense every figure separately at next step.

5.3 Experimental setup and results

The following section presents the experimental set
up, then discuses about the results. In the current study we
investigate the data transmission over wireless and wired connection using
10G-class device, and use PPG to generate PAM4 signal and transmitted it with
40km SMF.

Non return to zero (NRZ) signaling has been utilized for PON, because of
the high price of 25G components. We considered spectrally compact modulation
formats such as four level pulse amplitude modulation (PAM4), which enable to
reuse the cheaper 10G components 63. Laser chirp and fiber chromatic dispersion
(CD) are limitation to the performance of DML transmitters in the C-band and
Ka-band. 

 

5.3.1
Direct Modulated Lasers

In high-capacity links, external optical
modulators and advanced modulation formats are more useful than Direct
Modulated Lasers (DML). Because DMLs are known to have serious dispersion
penalty due to modulation induced frequency chirp.

Three main factors cause signal distortions beside the DML chirp affect
at a DML-based transmission systems:

limited bandwidth of
device and fiber chromatic dispersion (CD);signal-to-signal
beating noise (SSBN); fiber nonlinearities
62.

5.3.2
Network Architecture

The proposed architecture of two-paths  fiber-wireless convergence systems based on a
10G class components has shown in Figure 5.4.1.The optical signal is generated by EML and is fed a
pseudo-random bit sequence (PRBS15) non-return-to-zero (NRZ) signal. This
signal transmitted over 40km (SMF) fibre and divided into wired and hybrid
photonic wireless path.

 

Figure 5.4.1
Proposed architecture for hybrid optical fiber-wireless link in the Ka-band and
wired link. CO: central office; RAU: radio access unit

 

In first step, we initialize the experimental set-up
in the Figure 5.4.1. we investigated data transmission over wireless and wired
connection using 10G class device to generate PAM4 signal and transmitted it
through 40km SMF, representing a typical optical link. The output power of the
10G-class EML was 4.3dBm at the operating wavelength of 1550 nm.

Tuneable Dispersion Compensator Module (TDC) is used
for compensating the chromatic dispersion and obtains a clearer eye diagram. It is a device with
low insertion loss, low Phase Ripple, low group delay Ripple, low PDL and low
PMD. It’s colorless and in support of data transmission rates of 10Gb/s, 40Gb/s
and beyond. The dispersion slope
can strongly limit the usable bandwidth, which is important in the case of
wavelength division multiplexing. The optical power after TDC is -1.3 dBm. The
signal was transmitted over 40km SMF and divided into wired and hybrid photonic
wireless paths.

Figure
5.4.2 Experimental setup

The wired path contains a VOA to control the output
power which has range of -12dB to -21dBm optical power. The signal has been
recovered by using a 10-G APD photo-detector.

(b)

(a)

 

Figure 5.4.3 Experimental setup (a) eye diagram
after EML, (b) eye diagram after EDFA

 

Figures 5.4.3 (a) and (b) show the eye diagrams before
and after transmission over fiber link. From these eye diagrams, we have
noticed after the transmission over a 40km SMF, the eyes look clearly open. We
could observe clear eye diagram at the end of wired path and after digital
signal processing.

5.3.3 Bit
Error Rate

In our experiments, in order to evaluate the link performance, we digitized and
recorded the received data with a real-time oscilloscope for offline digital signal
processing and signal quality evaluation.

We have measured the BER curve by changing the
received optical power with digital down-conversion performed in digital signal
processing (DSP) part. We have measured for both wired and wireless links the
output power along the 40-km fiber link; we got a good BER as illustrated in
Fig 5.4.4 which shows the
performance of the system.

At the receiver, we compared the recorded bit
sequence with the originally sent bit sequence and counted the number of bit
errors. This number divided by the number of compared bits is then an estimate
for the BER. However, if the signal quality is high, the BER is small; a
significant amount of time is needed to count enough errors to determine a
reliable BER value.

(a)

(b)

 

 

Figure 5.4.4
.Results: (a) BER versus input power into APD for wired path, (b) BER versus
input power into PIN for wireless path

For fiber length that
is 40 km, the BER performance remains acceptable. For wireless path, we
achieved BER under HD-FEC limit @ 3.8e-3 when the received optical power is 5
dBm with volterra equalization, while stay under SD-FEC limit @ 2e-2 until
received power decreased to 1.5 dBm. For wired path, we achieved receiver
sensitivity at -21.5 dBm at HD-FEC limit.

 

5.3.4 The hybrid photonic
wireless path

The hybrid photonic wireless path starts with an EDFA
to amplify the signal and an Optical Tuneable Filter (OTF) to narrow the
bandwidth. The optical input power to the Finisar 40GHz PIN is controlled
through two variable optical attenuators (VOA), allowing separate adjustment of
signal and LO power.

 

5.3.4.1Optical
spectrum at Hybrid photonic up conversion

To convert optical signal to RF signal, a distributed
feedback (DFB) laser was used
as tuneable local oscillator (LO) for hybrid photonic up conversion on a PIN.
DFB offers high efficiency and remarkable spectral purity. The modulated data
signal at 1538.087nm and the un-modulated LO at 1538.280nm are mixed by a
coupler then sent to the 40GHz

PIN. The frequency spacing between the unmodulated
optical local oscillator and the modulated data carrier

defines the generated
wireless carrier frequency after optical

Figure 4.4.5 Results: Optical
spectrum before PIN

heterodyning, which is
25 GHz, as shown in Fig. 5.4.5.

 

While recalling the output of EML is 4.3 dBm, we
achieved a loss budget of 25.8 dB with simple linear feed-forward equalization.

An ADC output signal
is transformed to a part of spectrum at smaller frequencies by a digital down
converter with an equalizer. This is followed by decimation. Sampling rate is
decreased to perform correcting operations on the signals and a specific
frequency pass band is selected on the down converter to carry out the
equalization. This way, computational burden is lowered enough for the down
converter to work in real time mode. It also boosts the efficiency of the Bit
Error Rate in digital formats.

The Fig. 4 shows the eye diagram after digital
down-conversion before and after equalization.

(a)

(c)

(d)

(b)

 

 

 

 

Figure 4. Eye diagram (a) wired path
without equalization, (b) wired path with equalization, (c) wireless path
without equalization, (d) wireless path with equalization

 

5.4 Conclusion

We have analyzed
and experimentally demonstrated a Ka-band 25-Gb/s PAM-4 over 40-km SMF based on
10G-class EML and a hybrid photonic wireless link employing heterodyne photonic
up-conversion has been demonstrated. In order to reach a simple architecture, TDC
is applied at CO to control the dispersion.

The
transmission characteristic through the single mode RoF system has been
evaluated. The experiments have also shown that signals can be transmitted
through RoF system for remote signal distribution as the same time with wired
path, without degradation of BER performance.

Another
contribution of our experiment is that the proposed structure would be a
practical and low-cost solution for typical wired and wireless transmission
that is EML-based four-channel PAM-4 combined with heterodyne photonic
up-conversion and Volterra filter.

 

 

 

 

 

 

 

 

 

 

 

 

Chapter 6

Conclusions

For integration of wireless and optical access, RoF is the most
effective technology. Combination of fiber optics and radio, and is a way to allocate
radio frequency as a broadband or baseband signal over fiber.

In comparison with wireless systems, RoF has some outstanding headlines
and characteristics which make it totally preferable;

RoF is less complicated and entails smaller
base stationsRoF-based networks can have centralized architecture

Too many researches have been done about this topic at physical layer;
but upper layer network architecture and system resource management problems
deserve more research and investigate than it has received.

Besides, in our study we have proposed access network architecture based
on RoF and 10G-Class devices which presents seamless integration of hybrid Ka-band
radio access units into existing optical distribution system, such as PON and WDM
point-to-point links for arrear rural and remote areas that provides an
efficient bandwidth management.

As a result, regarding to the system resource administration, RoF founded
wireless networks are further capable than systems which structured just on wireless
network.

 

 

 

 

 

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