OPTICAL MIMO MULTIMODE FIBER LINKS

Channel Measurements and System Performance Analysis

Andreas Ahrens, Jens Pankow, Sebastian Aust and Steffen Lochmann

Hochschule Wismar, University of Technology, Business and Design, Philipp-M¨uller-Straße 14, 23966 Wismar, Germany

Keywords:

Multiple-Input Multiple-Output System, Singular-value Decomposition, Bit allocation, Wireless transmission,

Optical ﬁbre transmission, Multimode ﬁber.

Abstract:

Wireless communication is nowadays one of the areas attracting a lot of research activity due to the strongly

increasing demand in high-data rate transmission systems. The use of multiple antennas at both the transmitter

and receiver side has stimulated one of the most important technical breakthroughs in recent communications

allowing increasing the capacity and dropping the bit-error rate. However, multiple-input multiple-output

(MIMO) systems are not limited to wireless MIMO systems and can be observed in a huge variety of trans-

mission links and network parts and have attracted a lot of attention since the mid 90’s. In the ﬁeld of optical

MIMO transmission systems, multi-mode (MM) ﬁbre offers the possibility to transmit different signals by dif-

ferent mode groups. The perspective of the MIMO philosophy within the ﬁeld of optical transmission systems

is elaborated in this contribution based on channel measurements within a (2 × 2) MIMO system. For the

channel measurements the second optical window and a ﬁbre length of 1, 4 km was chosen. Computer simu-

lations on an overall data rate of 10,24 Gbps underline the potential of multi-mode ﬁbres in optical high-data

rate MIMO communication systems and show that in order to achieve the best bit-error rate, not necessarily

all MIMO layers have to be activated.

1 INTRODUCTION

The increasing desire for communication and infor-

mation interchange has attracted a lot of research

since Shannon’s pioneering work in 1948. A possible

solution was presented by Teletar and Foschini in the

mid 90’s, which revived the MIMO (multiple-input

multiple-output) transmission philosophy introduced

by van Etten in the mid 70’s (Telatar, 1999), (Fos-

chini, 1996), (van Etten, 1975), (van Etten, 1976).

Since the capacity of MIMO systems increases

linearly with the minimum number of antennas at

both the transmitter as well as the receiver side, wire-

less MIMO schemes have attracted substantial atten-

tion (McKay and Collings, 2005) and can be con-

sidered as an essential part of increasing both the

achievable capacity and integrity of future genera-

tions of wireless systems (K¨uhn, 2006), (Zheng and

Tse, 2003). However, the MIMO technique isn’t lim-

ited to wireless communication and a lot of scenar-

ios can be described and outperformed by the MIMO

technique.

In comparison to the wireless MIMO channel,

the optical ﬁbre is an important type of a ﬁxed-line

medium, which is used in several sections of telecom-

munication networks, where single- and multi-mode

ﬁbres are distinguished (Singer et al., 2008), (Winters

and Gitlin, 1990).

Optimizing the transmission on high-data rate

links is in particular of great practical interest for

delivering voice or video services in mobile IP (In-

ternet Protocol) based networks in the access do-

main. Unfortunately, the inherent modal dispersion

limits the maximum data speed within the multimode

ﬁber (MMF). In order to overcome this limitation,

the well-known single-input single-output systems,

also called SISO systems, should be transferred into

systems with multiple-inputs and multiple-outputs,

also called MIMO systems (Hsu and Tarighat, 2006),

(Singer et al., 2008). Taking ﬁnally into account that

delay-spread in wireless broadband MIMO transmis-

sion systems isn’t any longer a limiting parameter,

MMF links should be well suited for high-speed data

transmission (Raleigh and Ciofﬁ, 1998), (Raleigh and

Jones, 1999).

Different research groups, e.g. (Sch¨ollmann

and Rosenkranz, 2007), (Sch¨ollmann et al., 2008),

have adapted the MIMO technique on optical com-

128

Ahrens A., Pankow J., Aust S. and Lochmann S..

OPTICAL MIMO MULTIMODE FIBER LINKS - Channel Measurements and System Performance Analysis.

DOI: 10.5220/0003437201280132

In Proceedings of the International Conference on Data Communication Networking and Optical Communication System (OPTICS-2011), pages

128-132

ISBN: 978-989-8425-69-0

 2011 SCITEPRESS (Science and Technology Publications, Lda.)

munication channels. The experimental equaliza-

tion of crosstalk within frequency non-selective op-

tical MIMO systems has attracted a lot of research

(Sch¨ollmann and Rosenkranz, 2007), (Sch¨ollmann

et al., 2008). By contrast, frequency selective

MIMO links require substantial further research,

where spatio-temporal vector coding (STVC) intro-

duced by RALEIGH for wireless MIMO channels

seems to be an appropriate candidate for optical trans-

mission channels too (Raleigh and Ciofﬁ, 1998),

(Raleigh and Jones, 1999).

In this contribution the spatial multiplexing (SM)

is implemented at the transmitter side via different

sources launching light with different offsets into the

ﬁbre. At the receiver side, i.e. at the ﬁbre end, various

spatial ﬁlters are implemented (Pankow et al., 2011).

By launching light with different offsets into the ﬁ-

bre, different mode groups are activated, which prop-

agate along the ﬁbre with different speed and atten-

uation. Together, with the crosstalk between the dif-

ferent mode groups, the classical MIMO channel is

formed, where the most beneﬁcial choice of the num-

ber of activated MIMO layers and the number of bits

per symbol offer a certain degree of design freedom,

which substantially affects the performance (Ahrens

and Benavente-Peces, 2009).

Against this background, the novel contribution

of this paper is that based on channel measurements

within a (2 × 2) optical MIMO system, the perspec-

tive of the MIMO philosophy within the ﬁeld of op-

tical transmission systems is elaborated. Our results

show that even for relatively long (e. g. 1,4 km) trans-

mission lengths high data rates (e. g. 10,24 Gbps)

are feasible and that the choice of the number of bits

per symbol and the number of activated MIMO layers

substantially affects the performance of a MIMO sys-

tem, suggesting that not all MIMO layers have to be

activated in order to achieve the best BER.

The remaining part of this paper is organized as

follows: Section 2 introducesthe optical MIMO chan-

nel. The crosstalk impact within the optical MIMO

channel is studied in section 3, while the associated

performance results are presented and interpreted in

section 4. Finally, section 5 provides some conclud-

ing remarks.

2 OPTICAL MIMO CHANNEL

In order to comply with the demand on increas-

ing available data rates, systems with multiple in-

puts and multiple outputs, also called MIMO sys-

tems (multiple-input multiple-output), have become

indispensable and can be considered as an essential

10µm

Figure 1: Forming the optical MIMO channel (left: light

launch positions at the transmitter side with a given eccen-

tricity ξ, right: spatial conﬁguration at the receiver side as a

function of the mask radius r).

part of increasing both the achievable capacity and

integrity of future generations of communication sys-

tems (K¨uhn, 2006), (Zheng and Tse, 2003).

In this work the potential of the MIMO philoso-

phy in optical channels is elaborated, based on chan-

nel measurements. Basically, light launched at differ-

ent positions within the ﬁbre activates different mode

groups, which propagate along the ﬁbre with differ-

ent speed. Low order mode groups, activated by light

launched into the center of the ﬁbre, lead to a power

radiation pattern concentrated at the center of the ﬁber

end whereas higher order modes, activated by light

launched at an off-center position, e. g., with a given

eccentricity ξ, within the ﬁbre, lead to power radiation

pattern concentrated at the off-center of the ﬁber-end.

Therefore, by launching light into the ﬁbre with given

eccentricities, as highlighted in Fig. 1, different spa-

tially separated power distribution pattern can be ob-

tained at the receiver side to form the optical MIMO

channel.

According to Fig. 1, the optical input TX

was ad-

justed to launch light into the center of the core (center

launch condition), whereas for the optical input TX

a given eccentricity ξ was chosen (off-center launch

condition). The activated modes can be separated at

the ﬁbre end by the corresponding power distribu-

tion pattern. Fig. 2 illustrates the simulated power

distribution pattern by activating low- and high-order

modes. The simulations are in good agreement with

the measured power radiation pattern as depicted in

Fig. 3 for different parameters of the eccentricity ξ.

Now, spatial ring ﬁlters at the end of the transmission

line as depicted in Fig. 1 have been applied for chan-

nel separation. These spatial ﬁlters have been pro-

duced by depositing a metal layer at ﬁber end-faces

and subsequent ion milling (Pankow et al., 2011),

(Pankow et al., 2010).

Together, with the crosstalk between the different

mode groups, the classical MIMO channel is formed

(Fig. 4) (Pankow et al., 2011), (Pankow et al., 2010).

OPTICAL MIMO MULTIMODE FIBER LINKS - Channel Measurements and System Performance Analysis

129

Figure 2: Simulated mean power distribution pattern (left:

by the LP

mode, right: by activating all solutions of LP

modes); the dotted line represents the 50µm core size.

Figure 3: Measured mean power distribution pattern as

a function of the light launch position (left: eccentricity

ξ = 0µm, right: eccentricity ξ = 18µm); the dotted line rep-

resents the 50µm core size.

s 1

(t)

s 2

(t)

k 1

(t)

k 2

(t)

Figure 4: Electrical MIMO system model (example: n = 2).

3 CROSSTALK IMPACT

In this section it is studied, how the crosstalk impact

depends on the MIMO system conﬁguration. There-

fore at this point the eccentricity of the transmitter

side MIMO conﬁguration as well as the mask ra-

dius of the ring ﬁlter conﬁguration at the receiver side

are investigated in an exemplary system according to

Fig. 5. It is assumed, that each MMF input is fed

by a system with identical mean properties with re-

spect to transmit ﬁltering, pulse frequency f

= 1/T

the number of signalling levels and the mean transmit

power P

q 1

(t)

q 2

(t)

(f)

s 1

(t)

s 2

(t)

(f)

k 1

(t)

k 11

(t)

k 12

(t)

Figure 5: Electrical system model of transmitter and MMF

with crosstalk (example: n = 2).

The source signals u

(t) and u

(t) traverse the

transmit ﬁlters with the transfer function G

( f). Then

the wanted transmit signal u

(t) passes the MMF,

modelled by the transfer function G

( f) and causes

the signal u

k11

(t) with power P

k11

at the MMF out-

put, whereas the crosstalk signal u

k12

(t) (with power

k12

), which is fed into the MMF with a given ec-

centricity ξ, originates at the MMF output, after the

transmit signal u

(t) passed the ﬁlter with the trans-

fer function G

( f), which models the coupling from

optical input 2 to the output 1 (Fig. 5).

In general, the MIMO performance is affected by

both the mask radius r and the eccentricity ξ. As

highlighted in Fig. 6, the power of the wanted signal

k11

(t) at the MMF output, distributed in the inner

ring, increases monotonically with rising mask radius

r, whereas at the same time the power of the wanted

signal u

k22

(t), distributed in the outer ring, decreased

with increasing mask radius and increased eccentric-

ity. In addition a channel asymmetry can be observed

which is caused by the larger differential mode atten-

uation of the higher order mode groups. From this

point of view it can be concluded that a mask radius

in the range of 5µm to 15µm should be chosen in order

to have adequate power at both outputs.

From a practical point of view the power P

k12

(ξ,r)

of the crosstalk signal u

k12

(t) at the MMF output is an

interesting indicator for the strength of the crosstalk

disturbance, which depends on the mask radius r

and the eccentricity ξ. In order to assess the effect

of crosstalk on the wanted signal not only the pure

crosstalk signal power is of interest, but rather the be-

haviour of the powers of the wanted signal and the

crosstalk signal to each other. This behaviour may be

investigated by a signal-to-crosstalk-interferenceratio

(SCIR)

k11

(ξ,r) =

k11

(0,r)

k12

(ξ,r)

and ρ

k22

(ξ,r) =

k22

(ξ,r)

k21

(0,r)

(1)

Since MIMO makes use of the interference for chan-

nel improvement the SCIR should not be chosen as

high as possible like in orthogonal transmission. Re-

ferring to Fig. 7 this means for lower order modes

OPTICS 2011 - International Conference on Optical Communication Systems

130

0 5 10 15 20 25

0.2

0.4

0.6

0.8

r (inµm) →

normalized output power →

k11

(0µm, r)/P

max

k22

(10 µm,r)/P

max

k22

(14 µm,r)/P

max

k22

(18 µm,r)/P

max

Figure 6: Measured electrical signal power P

kνµ

(ξ,r) at the

MMF output as a function of the mask radius r for given

parameters of the eccentricity ξ.

0 5 10 15 20 25

−10

r (inµm) →

10 · lg(ρ

) (in dB) →

(0µm,r)/P

(14 µm,r)

(14 µm,r)/P

(0,r)

(0µm,r)/P

(10 µm,r)

(10 µm,r)/P

(0,r)

Figure 7: Electrical signal-to-crosstalk-interference ratio

(SCIR) ρ

at the MMF outputs as a function of the mask

radius r and the eccentricity ξ.

(output 1) a movement towards larger mask radiuses

and vice versa for higher order mode groups (output

2).

Though a relationship between the spatial mode

location and the channel’s impulse response needs to

be established for an exact BER (bit-error rate) trade-

off it can already been concluded from Fig. 6 and

Fig. 7 that mask radiuses in the range of about 5µm

to 15µm should be used for further BER analyses.

4 PERFORMANCE ANALYSIS

For BER analysis, a frequency selective SDM (spa-

tial division multiplexing) MIMO link, composed of

inputs and n

outputs, is considered. The block-

oriented system for frequency selective channels is

modelled by

u = H· c+ w . (2)

In (2), the transmitted signal vector c is mapped

by the channel matrix H onto the received vector

u. Finally, the vector of the additive, white Gaus-

sian noise (AWGN) is deﬁned by w (Pankow et al.,

2011), (Ahrens and Benavente-Peces, 2009). The in-

terference between the different input’s data streams,

which is introduced by the off-diagonal elements of

the channel matrix H, requires appropriate signal

processing strategies. A popular technique is based

on the singular-value decomposition (SVD) (Haykin,

2002) of the system matrix H, which transfers the

whole system into independent, non-interferinglayers

having unequal gains (Pankow et al., 2011), (Ahrens

and Benavente-Peces, 2009).

In this contribution the efﬁciency of ﬁxed trans-

mission modes is studied regardless of the channel

quality. Assuming predeﬁned transmission modes, a

ﬁxed data rate can be guaranteed.

For numerical analysis it is assumed, that each

optical input within the MMF is fed by a system

with identical mean properties with respect to trans-

mit ﬁltering and pulse frequency f

= 1/T

. Within

this work, the pulse frequency f

is chosen to be

= 5, 12 GHz. The average transmit power is sup-

posed to be P

= 1V

and as an external disturbance a

white Gaussian noise with a power spectral density N

is assumed. In order to transmit at a ﬁxed data rate, an

Table 1: Investigated transmission modes.

throughput layer 1 layer 2

2 bit/s/Hz 4 0

2 bit/s/Hz 2 2

appropriate number of MIMO layers has to be used,

which depends on the speciﬁc transmission mode, as

detailed in Tab. 1 for the investigated (2 × 2) optical

MIMO system.

The obtained BER curves are depicted in Fig. 8 for

the different ASK (amplitude shift keying) constella-

tion sizes of Tab. 1. For the investigations, an eccen-

tricity of ξ = 10µm and a mask radius of r = 15µm

was assumed, which was found to be beneﬁcial for

minimizing the overall BER at a ﬁxed data rate. As-

suming a uniform distribution of the transmit power

over the number of activated MIMO layers, it turns

out that not all MIMO layers have to be activated in

order to achieve the best BERs.

5 CONCLUSIONS

In this paper the perspective of the MIMO philosophy

OPTICAL MIMO MULTIMODE FIBER LINKS - Channel Measurements and System Performance Analysis

131

20 25 30 35 40

−6

−4

−2

10 · log

) (indB) →

bit-error rate →

4 ASK, SISO

(4,0) ASK, MIMO

(2,2) ASK, MIMO

Figure 8: BER when using the transmission modes intro-

duced in Tab. 1 and transmitting 2 bit/s/Hz over frequency

selective optical MIMO channels.

within the ﬁeld of optical transmission systems is in-

vestigated. Our results, obtained by channel measure-

ments and computer simulations, show the potential

of MIMO techniques in the ﬁeld of optical transmis-

sion systems. In combination with appropriateMIMO

signal processing strategies, an improvement in the

overall BER was obtained.

ACKNOWLEDGEMENTS

The authors wish to thank their co-worker, Mr. Ralph

Bornitz, for supporting the measurement campaign.

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