Optical OFDM. a Hype or is It for Real

8/3/2019 Optical OFDM. a Hype or is It for Real

1/4

Optical OFDM, a hype or is it for real?Sander L. Jansen

1, Itsuro Morita

2, Kamyar Forozesh

3, Sebastian Randel

4, Dirk van den Borne

1

and Hideaki Tanaka2

1: Nokia Siemens Networks, Munich, Germany, email: [email protected]

2: KDDI R&D Laboratories, Saitama, Japan3: Department of Astronomy and Space Physics, Uppsala University, Uppsala, Sweden

4: Siemens AG, Corporate Technology, Munich, Germany

Abstract During the last two years, numerous researchers have dedicated their work in optical OFDM and,

consequently, the number of papers has grown exponentially. In this paper we evaluate the suitability of OFDM

for long-haul 100GbE applications.

Introduction

Optical OFDM is currently a hot topic in the fiber-optic

research community and the number of optical OFDM

research papers published in international

conferences and journals has grown exponentially

over the last couple of years [1-3]. However, thequestion if this modulation format is really a viable

candidate for next generation fiber-optic transmission

systems is up till now only partially answered.

There exist several different definitions of OFDM in

the fiber-optic community. In this paper, we refer to

OFDM as a digital multicarrier technique. As the

subcarriers are generated in the digital domain, these

systems typically consist of many subcarriers

(typically more than 50) where channel estimation is

realized by periodically inserting training symbols [1-

3]. We therefore exclude in the paper coherent WDM

systems that sometimes are referred to as OFDM

systems [4]. Coherent WDM systems typically havefew subcarriers that are generated in the optical

domain. These systems typically do not use training

symbols, but rely on blind channel estimation instead.

Such systems have more in common with single-

carrier coherent systems and its evaluation is out of

scope of this manuscript.

In this paper we assess how the performance of

OFDM scales with coherently detected single-carrier

QPSK [9, 12, 13], another promising modulation

formats for long-haul transmission. Even though

many different OFDM systems have been proposed,

we restrict ourselves to coherent detected OFDM, as

this modulation format is most suited for long-haul

transmission [1-2].

100GbE OFDM system design

For the transport of 100GbE, overhead is required

onto the payload data. Although 100GbE has not

been standardized yet, it is foreseen that 7%

overhead needs to be allocated for FEC and about

4% for the Ethernet protocol (64B/66B coding). This

results in a raw data rate Rraw of 111 Gb/s.

Apart from the Ethernet and FEC related overhead,

an OFDM system has additional overheads caused

by cyclic prefix, training symbols and in some cases

pilot subcarriers. Especially at high data rates it is

essential to minimize these OFDM related overhead

as the system overhead significantly increases the

bandwidth requirements at the transmitter and

receiver.

Training symbols (TS)The overhead for training symbols is dependent on

the channel dynamics. In a fiber-optic transmission

system channel fluctuations are for instance caused

by polarization changes. The more stable a channel is

the less training symbols are required. In most

coherent transmission systems reported to date, the

training symbol overhead is about 2% to 4% [1-3].

Pilot subcarriers (PS)

Whether pilot subcarriers for phase noise (PN)

compensation are required or not is dependent on the

phase noise compensation technique. For fiber-optic

transmission system two phase noise compensation

schemes have been proposed, namely pilot

subcarrier [2] and RF-pilot based PN compensation

[5]. With pilot subcarrier PN compensation, typically

10% or more of the subcarriers must be allocated for

phase noise compensation [2]. In [5] we proposed

RF-pilot based PN compensation, in which a low-

power RF-pilot tone is used to revert the phase noise

impairments at the receiver. A major advantage of

this technology is that pilot subcarriers are not

required, saving about 10% of overhead. Another

advantage is that this compensation method is more

robust towards phase noise and it has been shown

that even PN compensation of conventional DFB

lasers can be realized with only a minor penalty [6].

Cyclic prefix (CP)

The required cyclic prefix (CP) is dependent on the

chromatic dispersion that is to be compensated for.

The required cyclic prefix time g of an OFDM signal

is dependent on the bandwidth of the OFDM band

and can be defined as:2

/ fcDBdg = , (1)

where D represents the chromatic dispersion of the

desired transmission distance [s/m], c is the speed of

light [m/s], f represents the center frequency of the

OFDM band [Hz] and Bd [Hz] is the effective

Mo.3.E.3

Vol. 1 - 49

ECOC 2008, 21-25 September 2008, Brussels, Belgium

978-1-4244-2229-6/08/$25.00 2008 IEEE


2/4

bandwidth of the modulated OFDM signal:

)(log

)1)(1)(1(

)(log 22

min

M

R

M

RB PSCPTSrawalno

d

+++== , (2)

where Rnominal and Rraw are the nominal and raw data

rate per polarization [b/s], respectively, M is theconstellation size and CP, TS and PS are the cyclic

prefix, training symbol and pilot subcarrier overheads,

respectively. From (2) it can be concluded that the

bandwidth Bd is dependent on the CP overhead,

which is defined as [5]:

gt

g

Cyclic

= , (3)

Where t is the total OFDM symbol time (including the

cyclic prefix). Combining (1), (2) and (3), results in:

t

gtg

PSTSrawR

M

c

fD

2

2

2

)1)(1(

)(log

++

= . (4)

From Eq. (4) it can be concluded that for a certain

data rate and constellation size, the reach of an

OFDM transmission system is dependent on the

guard time (cyclic prefix), and the OFDM symbol size.

For coherently detected OFDM systems the OFDM

symbol size is typically limited by the effectiveness of

the phase noise compensation scheme as well as the

linewidth of the transmitter and local oscillator laser.

For a conventional phase noise compensation

scheme (using pilot subcarriers), this results in a

maximum OFDM symbol length of about 10-15 ns [2]

whereas with RF-aided phase noise compensation,

an OFDM symbol length of longer than 100 ns can berealized [1]. The dispersion tolerance, expressed in

km of SSMF, as a function of the guard time is shown

in Fig. 1 for several OFDM symbol rates. This Figure

clearly demonstrates the advantage of using long

OFDM symbol lengths. For a system with a 10ns

OFDM symbol length the reach is far below 1000-km

SSMF whereas the OFDM system with 100ns symbol

length reaches to more than 5000km.

t

= 10ns

t = 50ns

t

= 100ns

Fig. 1: Dispersion tolerance for 100GbE PDM-OFDM in onesingle band as a function of the guard time for severalOFDM symbol lengths.

A method to further increase the dispersion tolerance

is by using multi-band OFDM. Basically, the OFDM

band is split up into several independently modulated

OFDM bands. In [5] we showed that by using multi-

band OFDM the CP overhead can be significantly

reduced. Fig. 2 shows the CP overhead required for

2,000-km SSMF as a function of the number of

OFDM bands for a system with 10ns and 100ns

symbol length. For a system with 100ns OFDM

symbol length, the CP overhead is already reduced

from 9.1% to 4.3% by using two OFDM bands. In [1],

four OFDM bands were used, reducing the total

OFDM related overheads for 100GbE OFDM to ~8%

t

= 100ns

t

= 10ns

Fig. 2: Cyclic prefix overhead required for 2,000 km SSMFas a function of the number of bands for a system with 10nsand 100ns OFDM symbol length.

Transmission performanceIn this section we will review the linear and nonlinear

performance of coherently detected OFDM and

compare this to the performance of coherently

detected single carrier systems.

OSNR sensitivity

Both at 40-Gb/s [7, 8] and 100-Gb/s [1, 7] similar BER

sensitivities have been reported for single carrier and

OFDM transmission systems. The OSNR sensitivity

for coherent detected systems is for a fixed nominal

data rate independent of the number of subcarriers

that is used. It is thus not surprising that the OSNR

sensitivity of single carrier and multi-carrier (OFDM)systems are comparable at the same data rate.

DAC/ADC sampling/bandwidth requirements

In all systems that employ coherent detection with

digital equalization ADCs are required at the receiver.

The bandwidth requirements of these receivers are

dependent only on the data rate and constellation

size, not on the number of subcarriers. Therefore,

single carrier and OFDM transmission systems have

the same bandwidth requirements for the ADCs.

In order to prevent high frequency distortion and

noise components from being mirrored into the

baseband signal oversampling is required [9]. A greatadvantage of OFDM with respect to a single carrier

system is that it is straightforward to reduce the

required oversampling to for instance a factor of 1.3.

Oversampling in an OFDM system is typically realized

by inserting unmodulated OFDM channels at high

frequencies [5] and thus unlike single carrier systems

no complex re-sampling is required at the receiver.

However, the main disadvantage of digital OFDM is

that DACs are required at the transmitter for the

generation of the signal. The DACs increase the cost

and complexity of the system, but at the same time

make it also easier to adaptively scale to higher level

modulation formats [1, 3].

Mo.3.E.3

Vol. 1 - 50



3/4

The use of multi-band OFDM as discussed in the

previous section does not only provide a reduction of

the CP overhead, in [5, 8] we have shown that the

required bandwidth for both DACs and ADCs can be

relaxed as well. This comes at the cost however of a

more complex transmitter and receiver layout.

Chromatic dispersion and PMD tolerance

From (4) it can be concluded that the CD tolerance of

an OFDM transmission system is dependent on the

bandwidth of the signal (which scales with the

nominal data rate) and the allocated CP. In an OFDM

transmission system practically arbitrary amounts of

CD can be compensated for as long as the OFDM

symbol length is not restricted and sufficient CP is

allocated. Basically, the increase in CP causes a rise

of the CP overhead, which can be mitigated by

choosing a long enough OFDM symbol length. In

single carrier systems, the amount of dispersion

tolerance is dependent on the number of taps that is

used in the frequency domain compensation [7]. This

value scales with the dispersion that is to be

compensated for. Thus the receiver complexity scales

with dispersion for single carrier systems, whereas

the overhead increases in an OFDM system. This

holds for the PMD tolerance as well, although PMD

compensation is less stringent for a coherent receiver

as it generally has a much shorter impulse response.

In coherent detected transmission systems, PMD and

CD provide a combined penalty [7, 8]. PMD however,

causes depolarization as well, thus in order to realize

a large PMD tolerance a polarization diverse receiveris required. It has been shown for that both single

carrier [7] and multi-carrier (OFDM) [8] systems can

offer a practically unlimited tolerance towards PMD.

Narrowband filtering tolerance

OFDM is known to have a well defined optical

spectrum. Because of this narrow optical spectrum, it

is often stated that the obtainable spectral efficiency

of an OFDM signal is higher than that of its single

carrier counterpart [4]. But from information theory we

know that this is not per definition true [10]. In [11] we

have investigated the narrowband filtering tolerance

of 120-Gb/s polarization division multiplexed (PDM)OFDM. The simulation results are shown in Fig. 3. In

this simulation the optical signal-to-noise ratio

(OSNR) is fixed to 10.6-dB, resulting in a BER of

1x10-3

. Because of the confined spectrum of the

OFDM signal a negligible BER penalty is observed as

long as the optical filter is broader than the OFDM

band (30-GHz for 120-Gb/s PDM-OFDM). At

narrower bandwidths the subcarriers located on the

sides of the OFDM signal are attenuated and thereby

the SNRs of the subcarriers located at these

frequencies are severely degraded. As a result, a

steep increase in BER is observed when the filter

bandwidth is lower than 30 GHz. The 30 GHz filter

tolerance is comparable to that observed for single

carrier 100GbE [12]. It can thus be concluded that

even though the unfiltered spectrum of PDM-OFDM is

significantly smaller, it has a similar tolerance towards

narrowband optical filtering compared to single carrier

PDM-modulated signals with coherent detection (for

the same constellation size M).

Spectral width of 120Gb/s

PDM-OFDM with QPSK

Fig. 3: BER as a function of optical filtering bandwidth of120-Gb/s PDM-OFDM.

Nonlinear tolerance

Probably the main disadvantage of OFDM is that

compared to single carrier systems the peak to

average power ratio (PAPR) is significantly higher.

One would therefore expect the nonlinear tolerance of

OFDM to be lower resulting in a limited reach.

However, when comparing the reach on a

transmission link without DCF of 40 Gb/s PDM-OFDM

[8] with that of single carrier 40-Gb/s QPSK [13], a

similar reach is observed. The main reason for this is

that although single carrier systems have lower PAPR

in back-to-back configuration, the uncompensated

chromatic dispersion along the transmission link

causes the PAPR of single carrier systems to rise to

similar values of OFDM systems, resulting in a similar

nonlinear tolerance.

Recently, we have investigated the impact of

nonlinear impairments in a periodic compensated

dispersion map [14]. These simulations showed that

the nonlinear tolerance of OFDM is significantly

reduced in periodically compensated dispersion

maps. Fig. 4 shows the required OSNR after

1,200-km transmission for a BER of 10-3

as a function

of the launch power. Simulated is one polarization of

the 40-Gb/s PDM-OFDM experiment with the OFDM

configuration as reported in [8]. In order to evaluate

the influence of SPM and XPM both single channeland DWDM (50-GHz channel spacing) are simulated.

Three dispersion maps are compared, namely without

dispersion compensation, with fully periodic

compensation and with a dispersion map that is

optimal for 10-Gb/s OOK systems. The highest

nonlinear tolerance is observed for the dispersion

map without inline dispersion compensation. For the

single channel simulations, the maximum tolerable

launch power for a 1-dB OSNR penalty

is -3.1 dBm, -5.7 dBm and -5.8 dBm for the dispersion

map without DCF, the fully periodic and the 10G

optimized dispersion map (10G OPT), respectively.

Thus for both periodically compensated dispersion

Mo.3.E.3

Vol. 1 - 51


978-1-4244-2229-6/08/$25.00 2008 IEEE


4/4

Optical OFDM. a Hype or is It for Real

Documents

Transcript of Optical OFDM. a Hype or is It for Real