JTh2A.08.pdf
OFC/NFOEC Technical Digest © 2013 OSA
Demonstration of Fully Functional MIMO Wireless LAN
Transmission over GI-MMF for In-building Networks
Jim (Shihuan) Zou, Haoshuo Chen, Frans Huijskens, Zizheng Cao, Eduward Tangdiongga and Ton Koonen
COBRA Research Institute, Eindhoven University of Technology, 5600 MB, Eindhoven, The Netherlands
E-mail: s.zou@tue.nl
Abstract: We propose a low-cost optically-fed architecture capable of increasing the capacity and
overall coverage of IEEE 802.11n MIMO WLAN system for in-building networks. A fully
functional transmission over GI-MMF was demonstrated employing 2×3 MIMO configuration.
OCIS codes: (060.2330) Fiber optics communications; (060.5625) Radio frequency photonics
1. Introduction
The multiple-input and multiple-output (MIMO) technology adopted in the IEEE 802.11n standard is widely
exploited in wireless communication as it significantly enhances data throughput and link reach without additional
bandwidth or transmit power. Traditionally, a large number of wireless access points (APs) in the WLAN have to be
deployed inside buildings to guarantee sufficient coverage in every room. However, such an approach is powerhungry and costly especially in large buildings. Nevertheless, the radio-over-fiber (RoF) technique is a promising
substitute benefiting from large bandwidth, low loss and immunity to EM interference. Thanks to the optically-fed
infrastructure, all the APs connected remotely by fibers from a central station can be positioned wherever the
wireless coverage is required. However new challenges to adapt the existing protocol have come up as MIMO radio
signals are provided from the central station to remote antennas. Only a few approaches have been discussed
recently [1-4], but they did not accomplish a full functionality of the MIMO WLAN transmission.
In this paper, we propose a bidirectional radio over graded-index multimode fiber (GI-MMF) architecture with a
single sideband multiplexing in the radio frequency domain to handle the MIMO radio signals transmission. In
addition to the characterization of the physical layer, TCP and UDP traffic patterns are measured for both directions
simultaneously with respect to the distance between and the mobile device (MD) and antennas. A significant
improvement of throughput is observed, in comparison with the traditional single-input and single-output (SISO)
configuration. Moreover, unicast of a UDP-based video streaming is explored in order to evaluate the system
performance for a practical application.
2. Proposed system and experimental setup
The proposed RoF-enabled system architecture for delivering IEEE 802.11n MIMO radio signals is shown in Fig. 1.
The residential gateway (RG) is a central station hosting a number of APs to provide exclusive capacity to remote
antenna units (RAUs) inside each room throughout the building via optical fibers. The RAUs can be much simpler
and cheaper to cover the many picocells, because only electrical/optical conversions and antenna units are required
in such a case. On the other hand, the wide bandwidth of optical fiber is capable of distributing multi-service from
different operators on the same optical infrastructure, as well as it enables an easy system upgrade. The commercial
802.11n AP contains three MIMO signals at the same frequency carrier, therefore two MIMO signals are mixed with
a low frequency local oscillator (LO) then passing through a diplexer, of which two passbands filter out the upper
sideband and the lower sideband, respectively. The third MIMO signal is only attenuated to equalize the power level,
and then combined with the other two mixed signals to directly modulate the laser diode (LD) via an electrical
circulator. Each AP block at RG uses an individual wavelength to carry the MIMO signals, where λnd and λnu are
Fig. 1. Principle of the RoF-enabled system for the in-building wireless access
JTh2A.08.pdf
OFC/NFOEC Technical Digest © 2013 OSA
dedicated for downlinks and uplinks (n = 1, 2, 3, …) corresponding to RAUs, respectively. After the photo-detector
(PD), the functionality of RAU is almost as the same as the one of RG but in a reverse way to recover the original
MIMO signals. Comparing to the approach dedicated to the downlink in [2], it is doable, in fact, to reuse the same
electrical path at both RG and RAU for the uplink in the other way around, because basically all the passive
electrical components in the scheme can be operated bidirectionally. The only exception is the amplifier at the
antenna side, therefore a bidirectional amplifier is used with the automatic gain control (AGC).
(DE)MUX
(DE)MUX
Fig. 2. Experimental setup for a 2×3 MIMO channel measurement applying the proposed concept
The proof-of-concept experimental setup based on the proposed concept is shown in Fig. 2. Due to the limitation
of components, only two MIMO signals from the AP are carried out using the single sideband multiplexing, and two
separate bandpass filters (BPF) are used instead of the diplexer, whereas the third channel is terminated. However,
such a configuration is already able to prove and exhibit the proposed system, because the third channel is relatively
a straight forward transmission. The AP is forced in the 802.11n mode with a bandwidth of 40 MHz per channel.
The reason of choosing the 7th WLAN channel with center frequency of 2.442 GHz is to prevent the congestion
from the existing WLAN APs in our building during the experiment. Besides, RG and RAU are isolated in two
different laboratory rooms and are only linked by the fiber, in order to eliminate the leaking interference from the
AP. Note that IEEE 802.11n is a half-duplex communication system according to the standard, hence the electrical
circulator is substituted for a splitter/combiner. A DFB laser operating at 1310 nm with direct modulation and a
multimode InGaAs PD are used for the downlink, while a multimode VCSEL source operating at 850 nm and a
multimode GaAs PD are used for the uplink, transmitting MIMO signals over 100 m GI-MMF with 50 μm fiber
core diameter. So far in the experiment, two omni-directional antennas with 5dBi gain and two directional antennas
with 6 dBi gain are used for wireless transmitting and receiving, respectively, in order to prevent the positive
feedback loop in between. Finally the AP is connected by a server via LAN cable, while a laptop with three in-built
antennas was used to demonstrate that the entire infrastructure is completely transparent to the end user.
3. Demonstration and results
The bandwidth available in the link is determined by the effects of differential mode-group delay in the MMF
employed in the setup. The characterization of bandwidth for 100 m GI-MMF at 850 nm is shown in Fig. 3(a). The 3
dB bandwidth is about 3.3 GHz which is right fit for the proposed scheme. Comparing to the transmission at 850 nm,
the bandwidth is much larger than 3.3 GHz in the case of DFB laser working at 1310 nm [6]. The received optical
power of the downlink is 2.8 dBm, and two MIMO signals are recovered and amplified before antennas, as shown in
Fig. 3(b), where MIMO 1 and MIMO 2 represent signals re-mixed from lower sideband and upper sideband,
respectively. The received optical power of the uplink is only -5.8 dBm, due to the low-power VCSEL. The
recovered MIMO signals before the AP are shown in Fig. 3(c). Although the power level of signals is less than -55
dBm, it is sufficient for the AP thanks to its high receiving sensitivity. Moreover for each MIMO channel (MIMO 1
and MIMO 2), we use a vector signal generator (VSG) to generate IEEE 802.11g frames and then measure the
receiving error vector magnitude (EVM) as a function of sub-carriers, as shown in Fig. 4, where Fig. 4(a)-(b) and
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S21 (dBo)
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Fig. 3. (a) transfer function measured at 850 nm over 100 m GI-MMF; (b)-(c) RF spectra of MIMO signals at the receiving point
JTh2A.08.pdf
EVM (dB)
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OFC/NFOEC Technical Digest © 2013 OSA
(c)
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Fig. 4. (a)-(d) received EVM of 802.11g WLAN signals vs. sub-carriers for each MIMO channel (inset: IQ constellation)
(c)-(d) represent the measurement of the downlink and uplink, respectively.
After the optimization of the physical layer, the IP traffics are loaded at both server and laptop using Iperf tool,
and all the throughputs in the following measurements are net data rates without any overheads. The default TCP
packages are transmitted with the window size of 64 Kbytes. Fig. 5(a) shows the throughputs measured against three
different distances between the laptop and antenna units, where the solid line and dot line denote the downlink and
uplink traffic, respectively. The received signal strength is varied from -49 dBm down to -61 dBm. The decrease of
data rates and the instability of uplink in the further position are mainly due to the lack of AGC amplifiers, leading
to a lower modulation index of the VCSEL. Secondly, Fig. 5(b) shows the throughput and jitter employing VoIPlike UDP packages with the data bandwidth of 30 Mb/s. The total package loss is less than 1.2%, and the jitter is
well within the requirement of 30 ms [5]. To study the improvement of capacity introduced by the MIMO technique,
we also measure the throughput transmitting TCP and UDP packages in a SISO configuration by simply blocking
one MIMO channel in the setup, as shown in Fig. 5(c), about 10 Mb/s for TCP packages and 15 Mb/s for UDP
packages are obtained with the help of MIMO technique. Furthermore, a 720p high-definition (HD) video with a
data rate of 45 Mb/s is streamed from the RG to the laptop using UDP protocol, and the snapshots taken from the
transmitter and receiver at the same frame of the video are illustrated in Fig. 5(d). We can clearly observe that the
video signal transmitted over the proposed system has the same high performance without any visible distortion.
30
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Uplink
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Fig. 5. IP-based evaluation: (a) throughput of TCP packages as a function to the time measured with three wireless distances; (b) measured
throughput and jitter of UDP packages; (c) measured throughput of both TCP and UDP packages in a SISO configuration; (d) snapshot of the HD
streaming video from the source (top), and from the laptop after transmission (bottom)
4. Conclusion
We proposed and experimentally demonstrated a simple and low-cost scheme to distribute the IEEE 802.11n MIMO
signals over GI-MMF for the in-building network. We not only optimized the physical layer, but also evaluated
bidirectional performance transmitting IP-based packages, achieving a practical and fully functional application. As
also mentioned in [2], an efficient single-sideband frequency-translation technique is feasible and scalable for
transmitting even more than three MIMO channels, and it is easy to be integrated on a compact electrical circuit
board, which will reduce the total cost and complexity.
Funding from the Netherlands Innovative Research Program (IOP) project MEANS is gratefully acknowledged.
5. References
[1] C.P. Liu, et al., “Transmission of wireless MIMO-Type signals over a single optical fiber without WDM,” TMTT 58, pp. 3094-3102 (2010).
[2] Ton Koonen, et al., “Techniques for flexible radio-over-fibre networks,” Photonics in Switching 2009, 15-19 Sept., pp. 1-4.
[3] Andrey Kobyakov, et al., “MIMO radio signals over fiber in picocells for increased WLAN coverage,” in Proc. OFC/NFOEC 2008, JWA113.
[4] Lukasz Maksymiuk, et al., “Successful IEEE 802.11n 2-channel MIMO transmission over standard graded index multimode fiber in
passband,” in Proc. QBSC 2012, pp. 15-18.
[5] Michael Sauer, et al., “Radio Over Fiber for Picocellular Network Architectures,” JLT 25(11), pp. 3301-3320 (2007).
[6] M.G. Larrode, et al., “Overcoming Modal Bandwidth Limitation in Radio-over-Multimode Fiber Links,” PTL 18(22), pp. 2428-2430 (2006).