Simultaneous transmission of W-CDMA and WIMAX signals over 300 m 50 μm multimode fibres using low cost 850 nm devices.

Rubén Alemany, Valentín Polo and Javier Martí Nanophotonics Technology Center, Universidad Politecnica de Valencia, Camino de Vera S/N Ed. IDI-5 (8G),

Access A, Level 1, Valencia 46022, Spain. Email: rualser@ntc.upv.es

Abstract Simultaneous transmission of WCDMA and WIMAX signals over 300 m multimode fibre links for indoor coverage has been demonstrated employing low cost 850 nm devices.

Introduction There is an increasing demand of wireless communication systems for provision of broadband in-building coverage. Distributed Antenna Systems (DASs) are proposed to achieve such objective. DAS can distribute simultaneously different wireless systems to remote antenna units (AUs) [1] allowing several advantages already discussed in previous papers [2].

The channel frequencies in most widely deployed wireless communication systems are in the range of some GHz, and the typical distances required for in-building coverage make optical fibre the favourite media to transport and distribute the signals [3]. The existence of pre-installed multi-mode fibres (MMF) and the good performance of the associated devices, for instance 850nm Vertical-Cavity Surface-Emitting Lasers (VCSELs), at low-cost [4], make multimode technology very suitable to implement indoor DAS.

In this paper, some low-cost 850 optical sources and detectors have been evaluated and their responses along different fibre lengths and types have been measured. After that, single and dual band transmission for WCDMA and WIMAX signals over 300 m of 50 μm diameter MMF has been demonstrated.

Preliminary Characterization First of all, the E/O/E transfer function including different types and lengths of MMF for several combinations of transmitters and receivers for 850 nm were characterized as shown in figure 1. Some of them accomplished with the bandwidth requirements imposed by our application. Finally, the selected devices were from Truelight, models TTR-1F45-447 VCSEL and the TRR-8F55-643 PIN-TIA, due to the better Spurious Free Dynamic Range (SFDR) characteristic. Both devices were manually pigtailed using 50 μm MMF in the transmitter and 62,5 μm in the receiver. In figure 1, the measured E/O/E transfer functions without fibre and for 300 m fibre link using 50 and 62.5 μm MMF are shown.

0 1 2 3 4 5

x 109

-50

-40

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-10

0

Frequency [Hz]

S21

(dB)

300 m MMF TRANSFER FUNCTION

TRUELIGHT + 62.5um fibre

TRUELIGHT

TRUELIGHT + 50 um fibre

Figure 1. Set up and E/O/E transfer function

measurement.

Figure 2 shows the measured SFDR at 2GHz including 300 m 50 μm MMF. The measured SFDR is 84.6 dB Hz2/3 using the devices mentioned before.

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ower

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SFDR

Figure 2. SFDR measured for 300 m 50 μm MM

fibre link and Truelight devices.

Experimental Setup The simultaneous transmission of WCDMA and WIMAX signals has been demonstrated. The specifications for both signals are shown in table 1.

WCDMA WIMAX

2.14GHz 3.5GHz

QPSK 64QAM

5MHz BW 3.5MHz BW

RRC filter, α=0.22

Table 1. WCDMA and WIMAX signals specifications.

The setup used for the experiment is shown in figure 3, where the signals have been generated using two RF arbitrary waveform generators (Agilent E4438B). The combined signal modulates directly

P.6.20ECOC 2008, 21-25 September 2008, Brussels, Belgium

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the VCSEL. The driver used for the modulation, controls the averaged optical power and its operating current has been adjusted to 17mA, corresponding to an output optical power of -6.4dBm. The optical signal was transmitted along different fibre lengths. At the end of the link the PIN-TIA converts the signal to the electrical domain. A second commercial amplifier (G= 27 dB, 5 dB NF) is used to boost the signal power. The signal quality has been measured using the FSQ40 Vector Signal Analyzer (VSA) from Rohde & Schwarz.

Figure 3. Schematic of the experimental setup used for the simultaneous W-CDMA and WIMAX

transmission.

Results At the beginning, single transmission using WCDMA was demonstrated. Figure 4 shows the measured EVM as a function of the input channel power for 62.5 and 50 μm diameter core fibres being the link length 500 m. The distortions generated in the VCSEL source start to dominate around 0 dBm input power.

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25

Channel power [dBm]

EV

M (%

)

WCDMA EVM @ 500m50 �m MMF62.5 �m MMFEVM limit

Figure 4. WCDMA EVM vs RF input power for 62.5

and 50μm 500m MMF link.

Figure 5 shows the EVM for single WCDMA transmission over 50 μm link length for -12dBm input RF channel power. It can be observed how it remains almost constant with length.

After this, dual transmission was experimented for different 50 μm fibre lenghts. In the case of 300 m of fibre, the input electrical powers to the combiner were -15 and +4dBm, respectively, for the WCDMA and WIMAX channels. These unbalanced powers were forced to compensate the higher losses at 3.5 GHz. The worst measured Error Vector Magnitude (EVM) at the output of the amplifier was 3.1% and -31.44 dB with a channel power of -9 and 1.57 dBm

for WCDMA and WIMAX respectively. So, the measured signals quality satisfy the specifications defined by the corresponding standards at the input of the antenna in terms of spectral mask and EVM which are 17.5% for QPSK modulation in the case of WCDMA, and -31dB for 64QAM modulation in the case of WIMAX [5-6].

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0.5

1

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2.5

3

Fibre length [m]

EV

M (%

)

WCDMA Signal quality Vs fibre length

Figure 5. WCDMA EVM vs 50μm MMF link length.

Figure 6 shows the measured EVM as a function of the fibre link length for both channels. Again, the measured EVM complies the specifications defined by each standard.

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Received WIMAX signal quality

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Fibre length [m]

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Received W-CDMA Signal Quality

Figure 6. EVM vs link length for the WIMAX

Channel (left) and WCDMA (right)

Conclusions Several low cost 850nm sources and detectors have been implemented, and the best combination of transmitter and receiver was used to transmit simultaneously WCDMA and WIMAX signals over different MMF link lengths for wireless DAS for in-door coverage, satisfying the specifications of the standards at the antenna input.

References 1. Anthony Nkansah, et al IEEE Microwave, Vol.

16 (2006), 1531-1309. 2. Anjali Das, et al IEEE MTT, Vol. 54 (2006),

0018-9480. 3. Michael Sauer, et al MWP (2005), T3-2, pp 99-

102. 4. Tamás Marozsák, et al Journal of

Telecommunications and information technology (2003), pp 36-40.

5. 3GPP TS 25.104 V7.7.0 (2007-06); Base Station (BS) radio transmission and reception (FDD).

6. IEEE 802.16 2004; Part 16: Air interface for fixed Broadband Wireless Access System.

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