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Polarization multiplexing of two MIMO RoF signals and one baseband signal over a single wavelength
Optics & Laser Technology 76 (2016) 70–78
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Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Polarization multiplexing of two MIMO RoF signals and one baseband signal over a single wavelength M.A. Elmagzoub a,n, Abu Bakar Mohammad a, Redhwan Q. Shaddad b, Samir A. Al-Gailani a a b
Lightwave Communication Research Group, InfoComm Research Alliance, Universiti Teknologi Malaysia, 81310 Johor, Malaysia Communication and Computer Engineering Department, Faculty of Engineering and Information Technology, Taiz University, Taiz, Yemen
art ic l e i nf o
a b s t r a c t
Article history: Received 15 May 2015 Received in revised form 22 July 2015 Accepted 28 July 2015 Available online 7 August 2015
Next-generation (NG) access networks require simultaneous provision of wired and wireless services and high data rates to meet the large demands of mobility and multiple services. In this paper, we propose a novel spectral efficient radio over fiber (RoF) scheme to simultaneously provide two spatially multiplexed multiple input multiple output (MIMO) wireless signals with a baseband (BB) wired signal in one wavelength using a centralized light source. The proposed scheme can be applicable to wavelength division multiplexed passive optical networks (WDM-PONs). The BB signal is modulated at a low extinction ratio (ER). The modulated light is re-used to modulate two MIMO signals that have the same carrier frequency that is combined optically using polarization-division-multiplexing (PDM). The data rate for each MIMO stream was 1.25 Gb/s, and the data rate was 2.5 Gb/s for the BB signal. Error free performance with a bit error rate (BER) of 10 9 was achieved for all three signals after 20 km and 60 km through single mode fiber (SMF) for 16-QAM and 4-QAM for the MIMO signals, respectively. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Polarization division multiplexing (PDM) Radio-over-fiber (RoF) Multiple-input multiple-output (MIMO)
1. Introduction Next-generation (NG) access networks require large bandwidths and high data rates for both wired and wireless access to support ever-increasing video-based interactive and multimedia services [1,2]. It is highly beneficial to simultaneously transmit baseband (BB) and wireless signals using the same infrastructure, such as passive optical network (PON). In particular, wavelength division multiplexed (WDM) PON offers high transport capacity, high bandwidth per subscriber, protocol transparency line rate independence, high security, and upgradability [3]. Radio-over-fiber (RoF) technology has been demonstrated to have great potential for the application of future broadband wireless access networks due to the seamless integration of the sufficient bandwidth of optical fiber communication and the high mobility of wireless radio communication [4–6]. For wireless broadband transmission, multiple-input–multiple-output (MIMO) is an indispensable technique for all new wireless standards and systems that require a high data rate [7]. MIMO systems are designed to improve the transmission range and reliability and to deliver higher data transmission rates than single-input single-output (SISO) systems [8]. Therefore, to build any RoF-PON system for a NG-PON, a MIMO technique has to be considered [7]. In spite of n
Corresponding author. Fax: þ60 75536155. E-mail address: [email protected] (M.A. Elmagzoub).
http://dx.doi.org/10.1016/j.optlastec.2015.07.023 0030-3992/& 2015 Elsevier Ltd. All rights reserved.
the importance of the MIMO technique, to the best of our knowledge, all of the proposed RoF systems are considering SISO with BB data or MIMO-RoF without BB data [9]. Transmission of wired and multiple wireless services is demonstrated by electrically combining the signals at different central frequencies, which is not valid for MIMO signals that have the same central frequency [10]. In [11], both the BB signal and the RoF signal were multiplexed and de-multiplexed in the optical domain; hence, the operation speed is not limited by the electronic bottleneck caused by digital signal processing (DSP). This is achieved by using an optical carrier suppression (OCS) technique to modulate the RoF signal; then, the optical carrier is re-used to modulate the BB signal. The output optical spectrum for this scheme is similar to the optical spectrum of an optical double sideband with carrier (ODSB þC) modulation technique, which requires twice the bandwidth compared to the optical single sideband with carrier (OSSB þC) modulation technique. In [12], the transmission of two wireless MIMO signals with the same carrier frequency over fiber in an RoF system using an optical single sideband frequency-translation (OSSB-FT) technique was proposed and demonstrated. The drawback of this method is dedicating a wavelength for each MIMO stream. In [7,13,14], polarization-division-multiplexing (PDM) is used to carry each MIMO stream at a different polarization of the same wavelength, taking advantage of the two polarizations of the wavelength. It is more attractive and cost-effective to deploy RoF-PON that supports a wired and MIMO wireless technique in the same
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Fig. 1. Generic architecture of the BB and MIMO-RoF WDM-PON.
infrastructure, such as in WDM-PON and as shown in Fig. 1. In this paper, BB wired and MIMO wireless signals are provided simultaneously in a single wavelength using PDM and a low extinction ratio (ER) to modulate the BB signal. In the proposed scheme, PDM is used due to its ability to optically multiplex two RF signals with the same center frequency without interfering with each other. Additionally, modulating the BB signal at a low ER is applicable in access networks where the transmission distance is limited. The novel proposed scheme improves the spectrum efficiency of a WDM-PON and makes providing a multi-gigabit level of wireless and wired signals practical. The rest of the paper is organized as follows: Section 2 elaborates the principles of the proposed method. In Section 3, a mathematical model illustrates how the novel BB and RF signals multiplexing scheme operates in the proposed system. The simulation considerations and the results are covered in Section 4. Finally, Section 5 concludes the paper.
2. Principle of the proposed BB wired/MIMO wireless integrated RoF-PON Fig. 2 shows the schematic diagram of the proposed system with the corresponding spectra where the signal transmitter in the central office (CO) and a receiver in the optical network unit (ONU) (or remote antenna unit (RAU)) are arranged, which can be implemented using a WDM-PON architecture. The BB data are modulated by the MZM at a low ER. The modulated light from the BB data is used to modulate two 5 GHz 16-QAM MIMO wireless signals (or 4-QAM). Because the ER of the modulated light is very low, the two MIMO signals can be modulated using the same wavelength. In addition, the BB signal will not be affected by the MIMO signals because an OSSB þC modulation technique is used to modulate the RF wireless signals. Therefore, the RF signals will be carried in one of the sidebands, and the BB signal is carried in the carrier. Two dual drive MZMs (DD-MZM) are used to perform
OSSBþ C for the two MIMO signals. PDM is used to combine the two MIMO wireless signals. First, the laser diode light is adjusted using a polarization controller (PC) to be polarized at polarization x (Pol-x). Then, after the light is modulated by the BB data using the MZM, a power splitter is used to equally divide the light into two portions. Each portion of the modulated light is sent to a DDMZM through a PC. As illustrated in Fig. 2, the upper portion of the modulated light wave is intensity modulated using a DD-MZM and a 16-QAM MIMO1 signal, whereas the lower portion (Pol-y) is modulated using a 16-QAM MIMO2 signal and DD-MZM to perform the OSSBþ C modulation technique and to generate an optical spectra. The two MIMO signals are then polarization multiplexed at a polarization beam combiner (PBC). The multiplexed optical signals are transmitted to the ONU through a length of single mode fiber (SMF) where they are polarization de-multiplexed at the polarization beam splitter (PBS), as shown again in Fig. 2. The BB and MIMO1 signals are then converted into an electrical signal at the upper PD (Pol-x) and are then split into two paths. In the first path, the MIMO1 signal is filtered out using a low-pass filter (LPF), and only the wired signal is obtained and delivered to the BB receiver. In the second path, the electrical signal is sent to the MIMO1 antenna through a RF amplifier. The MIMO1 antenna can act as a bandpass filter to block the wired signal because its spectral response is for a 5 GHz signal and the BB signal has a very low frequency. The MIMO2 signal is received by the lower PD (Pol-y) and delivered through the RF amplifier to the MIMO2 antenna. In this paper, to verify the performance of our proposed RoF link, the two 5 GHz 16-QAM signals are directly demodulated without wireless transmission.
3. Mathematical model This section presents the mathematical model for the BB and RF signals multiplexing scheme in the proposed architecture. The upper portion of Fig. 2 (Pol-x) is considered in this mathematical
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Fig. 2. Schematic diagram of the proposed system with the corresponding spectra.
model to demonstrate the novel optical multiplexing principle for the BB signal modulated using the low ER and the RF signal. The lightwave from the LD, which can be expressed as Ein (t ) = Ac e jwc t , is first intensity modulated by the BB data using
the single-arm MZM. The output is expressed as
Eout1 (t ) = A (t ) e jwc t .
(1)
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Eout2 (t ) =
73
α A (t ) e jwc t × ⎡⎣ e jmh cos ω m t + je (jmh sin (ω m t ))⎤⎦. 2
(3)
From the Jacobi-Anger expansions [18] ∞ ∞ e jmh cos θ = ∑n =−∞ j n Jn (mh ) e jnθ and e jmh sin θ = ∑n =−∞ Jn (mh ) e jnθ , where Jn is the nth-order Bessel function of the modulation index. Based on the Jacobi-Anger expansions, Eq. (3) can be written as
Eout2 (t ) =
α A (t ) e jwc t 2
⎡1 −jω m t + J ( m ) + jJ ( m ) e jω m t ⎤ h h +1 0 ⎥ ⎢ j J−1 ( mh ) e ⎥ × ⎢ + jJ ( mh ) e−jω m t ⎥ ⎢ −1 ⎥ ⎢ + jJ ( m ) + jJ ( m ) e jω m t ⎦ ⎣ h h +1 0
where the values of Jn (mh ) are neglected for n ¼ 72, 73, …, 71 because their values are too small. α Eout2 (t ) = A (t ) e jwc t × ⎡⎣ J0 ( mh )(1 + j ) + 2jJ+1 ( mh ) e jω m t ⎤⎦. 2 π
π
Because 2e j 2 = 2j and 1 + j = the following:
Fig. 3. The effect of the BB signal ER on the two MIMO signals.
Eout2 (t ) =
⎡ α j A (t ) ⎢ 2 J0 ( mh ) e ⎣ 2
2 e j 4 , Eout2 (t ) can be rearranged as
(w t + π4 ) + 2J+1 ( mh ) e j ((w + ω c
c
m)t +
π 2
)⎤⎥. ⎦
(4)
Eq. (4) can be further simplified as the following:
⎛ j Eout2 (t ) = αA (t ) × ⎜ C1e ⎝ where C1 =
1
J (mh ) 2 0
(w t + π4 ) + C2 e j (w + ω c
c
m)t +
π⎞ 2⎟
⎠
(5)
and C2 = J+1 (mh ) according to Eq. (4). The two
components obtained from Eq. (5) represents the optical carrier (OC) and the upper optical sideband (UOSB) with initial phases π 4
and π , respectively. It should also be noted that in the OSSB þC 2 modulation scheme, the frequency spacing between the OC and the UOSB is precisely equal to the RF carrier [17]. The second modulator effect on the BB signal is very limited because it is only attenuated by the DD-MZM insertion loss and the small value of the constant C1. However, from Eq. (5), the UOSB is greatly affected by the BB signal because it is multiplied by A(t). A(t) represents the intensity modulation of the BB signal, which is defined as the following [19]:
Fig. 4. The performance of the MIMO signals for the BtB and 20 km cases.
Then, this signal is modulated again by the RF signal using the OSSB þC modulation technique via the DD-MZM. The OSSB þC is performed by applying a π/2 phase difference between the two RF arms of the DD-MZM biased at the quadrature point [15]. The RF signal has a sinusoidal clock Vm (t ) = VRF cos ωm t , where ωm = 2πfRF . This sinusoidal carrier can be modulated with any form of spectral efficient digital modulation, for example 16-QAM. The optical field of the output signal Eout2 (t ) from the DD-MZM can be mathematically expressed as the following [16,17]: Eout 2 (t ) =
α A (t ) e jwc t 2
⎞ ⎡ ⎛⎜ π ⎛ j V cos ω m t⎟ j π ⎜V − V cos ⎠ + e Vπ ⎝ dc RF × ⎢ e ⎝ Vπ RF ⎢⎣
⎞⎤ ωmt + π ⎟ 2 ⎠⎥
(
)
⎥⎦
(2)
where α is the insertion loss of the DD-MZM, Vπ is the required driving voltage to achieve a phase shift of π radians and Vdc is the DC bias voltage. For the OSSBþ C scheme, the DC voltage is biased at Vπ [17]. In this proposed design, Vπ = 4 and Vdc = 2. The 2
modulation index is defined as mh =
πVRF . Vπ
By substituting these
values, Eq. (2) can be rewritten as the following:
Eout2 (t ) =
⎡ π⎛ α A (t ) e jwc t × ⎢ e jmh cos ω m t + e j 2 ⎜ e 2 ⎝ ⎣
(
π
(
−jmh cos ω m t + π 2
π
)) ⎞⎟ ⎤⎥. ⎠⎦
Because cos (ωm t + 2 ) = − sin ωm t and e j 2 = j , Eout2 (t ) can be rearranged as the following:
A (t ) = cos (Δθ (t ))
(6)
where Δθ is the phase difference between the two branches of the single-arm MZM and is defined as the following:
Δθ (t ) =
π (0.5 − M (BB (t ) − 0.5)) 2
(7)
with
M=1−
⎛ 1 ⎞ π tan−1⎜ ⎟ ⎝ ER ⎠ 4
(8)
where ER is the extinction ratio (maximum-to-minimum-modulated light power) and BB(t) is the electrical input signal. The electrical input signal is normalized between 0 and 1. From Eqs. (5) and (8), it is clear that the RF signal in the UOSB is directly affected by the BB signal ER. For example, if the BB signal is modulated at a very high ER, the signal that enters the DD-MZM will be lost a portion of the time (when the BB signal is 0), which will highly degrade the RF signal in the reception. In contrast, if the BB signal is modulated at a low ER, there will be optical power in both cases (0 and 1) to modulate the RF signal. Additionally, if the BB signal ER is less than a certain threshold, the BB signal cannot be detected after the fiber degradation. An optimum ER is found in Section 4, which results in a good performance from the two signals (BB and RF). The optically modulated signal (Eq. (5)) propagates along an SMF with a propagation constant of β (w ) and an amplitude
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Fig. 5. Constellation diagrams of the MIMO signals for the (a) transmitted signal, (b) MIMO1 BtB, (c) MIMO1 20 km, (d) MIMO2 BtB, and (e) MIMO2 20 km.
attenuation γ , where ω is the angular frequency. β′(w ) is the time delay, which is calculated by the first derivative of β (w ). Therefore, the output lightwave at the end of the SMF of length of z can be approximated as the following [8,20]:
After the signal is directly detected by a PD, the photocurrent for the received signal can be written as the following equation according to the square-law PD [8,17]:
EOFibre (z, t ) = αe−γz A ( t − β′( wc + ω m ) z ) ×
* IOFibre (z, t ) = μ |EOFibre ( z, t ) |2 = μEOFibre (z, t ) × EOFibre (z, t )
⎧ ⎨ C1e j ⎩
(w t + β (w )z+ π4 ) + C2 e j ((w + ω c
c
c
m ) t + β (wc + ω m ) z +
π 2
⎫. ⎬ ⎭
)
(9)
where μ is the responsivity of the PD.
(10)
M.A. Elmagzoub et al. / Optics & Laser Technology 76 (2016) 70–78
Fig. 6. The BB signal performance for the BtB and 20 km cases.
75
Fig. 8. The effect of the BB signal ER on the two MIMO signals.
IOFibre (z, t ) = μα 2e−2γz A2 ( t − β′( wc + ω m ) z ) × ⎧ π π C e j (wc t + β (wc ) z + 4 ) + C2 e j ((wc + ω m ) t + β (wc + ω m ) z + 2 ) × ⎪ ⎪ 1 ⎨ ⎪ C e−j (wc t + β (wc ) z + 4π ) + C e−j ((wc + ω m ) t + β (wc + ω m ) z + 2π ) 1 2 ⎪ ⎩
( (
)
)
⎫ ⎪ ⎪ ⎬ ⎪ ⎪ ⎭
IOFibre (z, t ) = μα 2e−2γz A2 ( t − β′( wc + ω m ) z ) × ⎧ 2 π π ⎫ ⎪ C1 + C22 + C1C2 e j (wc t + β (wc ) z + 4 − wc t − ω m t − β (wc + ω m ) z − 2 )⎪ ⎬ ⎨ π π ⎪ ⎪ ⎭ ⎩ +C1C2 e−j (wc t + β (wc ) z + 4 − wc t − ω m t − β (wc + ω m ) z − 2 ) IOFibre (z, t ) = μα 2e−2γz A2 ( t − β′( wc + ω m ) z ) × ⎧ 2 π ⎫ ⎪ C1 + C22 + C1C2 e j (ω m t − β (wc ) z + β (wc + ω m ) z + 4 )⎪ ⎨ ⎬. π ⎪ ⎪ ⎩ +C1C2 e−j (ω m t − β (wc ) z + β (wc + ω m ) z + 4 ) ⎭ Because β (wc ) z , for a specified frequency, and z are constants, the total shift constant θsc can be assumed to be the following:
θsc = − β ( wc ) z + β ( wc + ω m ) z +
π . 4
Fig. 9. The performance of the MIMO signals for the BtB, 20 km and 60 km cases.
IOFibre (z, t ) = μα 2e−2γz A2 ( t − β′( wc + ω m ) z ) × ⎧ C2 + C2 + C C e j (wm t + θsc ) ⎫ 1 2 1 2 ⎨ ⎬ . ⎪ ⎪ ⎩ +C1C2 e−j (ω m t + θsc ) ⎭ ⎪
This shift constant is produced by the optical SMF and the optical modulator. Therefore,
⎪
Because cos θ =
e jθ + e−jθ , 2
Fig. 7. Eye diagrams of the BB signal for (a) BtB, and (b) 20 km.
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Fig. 10. Constellation diagrams of MIMO1 for (a) transmitted signal, (b) BtB, (c) 20 km, and (d) 60 km.
(
)
IOFibre (z, t ) = μα 2e−2γz A2 t − β′ ( wc + ω m ) z ×
{
Fig. 11. The BB signal performance for the BtB, 20 km and 60 km cases.
C12
+
C22
}
+ 2C1C2 cos ( ω m t + θsc ) .
(11)
According to Eq. (11), the photocurrent is composed of the DC components (BB) and the RF component (MIMO1) at ωm after transmission. The detected signal is then passed through the bandpass filter (BPF) with a center frequency of fRF , so the DC component is removed for the wireless MIMO reception or can be filtered out using the RF signal antenna. Then, each detected wireless MIMO signal with the carrier frequency fRF is directly amplified and propagated by using a MIMO antenna technique through the wireless channel. The wireless end-user will receive the MIMO signals and demodulate them using the suitable QAM demodulation and MIMO decoding techniques. For the BB signal reception, the MIMO1 signal is filtered out using a LPF, and only the wired BB signal is obtained. In the proposed scheme, PDM is used due to its ability to optically multiplex two RF signals with the same center frequency
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77
Fig. 12. Eye diagrams of the BB signal for (a) BtB, (b) 20 km, and (c) 60 km.
without the signals interfering with each other as proposed and demonstrated in [7] and [14]. Moreover, unlike [21], no electrical frequency shifting will be required at the ONU. The primary disadvantage is that this system is valid for only two MIMO streams. Additionally, modulating the BB signal at a low ER is applicable for access networks where the transmission distance is limited. For the QAM signals, the bit error rate (BER) is obtained from its error vector magnitude (EVM). The EVMs are calculated considering the following equation [8]:
⎡ M Stx, k − Srx, k ⎢∑ EVM (dB) = 10. log10 ⎢ k = 1M 2 ∑ S ⎣ K = 1 tx, k
2⎤
⎥ ⎥ ⎦
(12)
where EVM is the value of the difference between a collection of received symbols and transmitted or ideal symbols, Stx, k is the corresponding transmitted symbol of the constellation associated with the kth symbol, Srx, k is the received symbol associated with Stx, k , and M is the number of symbols for the inphase-quadrature constellation. The BER is calculated according to [22]: BER =
⎡ ⎢ ε = erfc ⎢ ⎣
k=
1 3. log2Q 2 . . 2 2 Q2 − 1 ( k. EVMrms ) . log2M
(
)
Stx, max M
(
∑i = 1 Stx, i /M
)
⎤ ⎥ ⎥, ⎦
(1 − Q−1) *ε, log2Q
, (13)
where Q is the number of signal levels within each branch of the constellation diagram, log2M is the number of bits encoded into one QAM symbol, and K is the modulation format-dependent factor giving the relationship between the maximum field magnitude and the overall average M field magnitudes defined by the constellation diagram for the chosen modulation format. The Stx, i is the ideal transmitted field vector, and Stx, max is the field vector of the outermost constellation point.
The length of the SMF was 20 km with an attenuation of 0.2 dB/km and a dispersion coefficient of 17 ps/nm/km. The insertion loss of the DD-MZM was 4 dB. Because the quality of the three signals was dependent on the BB signal ER, the suitable value for the ER was found. From Fig. 3, it is clear that the three signals can obtain error-free transmission when the ER is in the range between 0.15 dB and 0.25 dB. Errorfree transmission here is defined as the transmission of a signal with a BER of no more than 10 9. This agrees with our theoretical analysis, which shows that increasing the ER will highly degrade the MIMO signal performance. Fig. 4 shows the measured BER in terms of the receiver optical power into the PD in both back to back (BtB) and 20 km SMF transmission cases for the MIMO signals. For the BtB MIMO signal case, we observed that the receiver sensitivity achieved 23 dBm and 22.3 dBm for the MIMO1 signal (Pol-x) and for the MIMO2 signal (Pol-y), respectively. This 0.7 dB power penalty between Pol-x and Pol-y could be attributed to the different performances of the optical and electrical components. After a transmission of 20 km, the power penalty was 2 dB at 10 9 BER. Additionally, the power penalty between Pol-x and Pol-y increased to 1 dB, which could be attributed to the polarization mode dispersion. Fig. 5 shows the constellation diagrams for the MIMO signals for the case of transmitted and received signals. For the received signal, BtB and 20 km SMF transmission cases are considered. For the BtB case, clear scatter-plots are achieved, although each constellation point appears to have some expansion due to the noise of the LD, PD and power loss and noise introduced by the passive optical components. After a transmission over 20 km SMF, the clusters in the constellations are still clearly separated. In the BtB BB signal case, as shown in Fig. 6, when the BER is 10 9, the measured received power is 12.2 dBm. After a transmission of 20 km the power penalty is 3 dB at 10 9 BER. The eye diagrams of the BB signal are also shown in Fig. 7. The relatively high power required at the PD and the power penalty required to achieve 10 9 BER performance are due to the low ER used to modulate the BB signal.
4. Simulation experiment and results
4.2. 4-QAM MIMO signals
4.1. 16-QAM MIMO signals
The system performance can be greatly improved when a 4-QAM modulation is used instead of 16-QAM, and a transmission distance of 60 km SMF is achievable with error free performance. The BB signal ER can be increased because the 4-QAM signal is more robust to changes in its modulation power compared with the 16-QAM signal. Fig. 8 shows the BER performance at a transmission distance of 60 km when the ER is varied. The three signals can obtain error-free transmission when the ER is in the range between 0.63 dB and 0.93 dB.
MATLAB and OptiSystem 13 software tools were used to simulate the proposed system, which shown in Fig. 2. Each channel of the two MIMO signals was 1.25 Gb/s 16-QAM at the same carrier frequency fc ¼5 GHz. The BB signal was a 2.5 Gb/s non-return-tozero (NRZ) signal generated using a pseudo-random bit sequence (PRBS) generator. The total input power of the three signals was 10 dBm and was launched from LD at a wavelength of 1552.52 nm.
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Fig. 9 shows the BER curves of the MIMO signals at BtB and following a 20 km and 60 km SMF transmission. For the BtB case, the receiver sensitivity at 10 9 achieved 29.3 dBm and 28.8 dBm for MIMO1 and MIMO2, respectively. A negligible power penalty (approximately 0.5 dB) was induced after the 20 km SMF transmission. After a transmission of 60 km, the power penalties were 3 dB and 2 dB at 10 9 BER for MIMO1 and MIMO2, respectively. The transmitted and received constellations of the MIMO1 signal in the three cases (BtB, 20 km and 60 km) are shown in Fig. 10. In the cases of the 20 km and 60 km transmission, the received signal also maintained clear scatter-plots even though the distances between the constellation points were becoming shorter due to the noise and fiber dispersion. Fig. 11 presents the BB signal transmission BER performance for the BtB and the 20 km and 60 km SMF transmission cases. In the BtB case, when the BER was 10 9, the measured received power was 20.4 dBm. A negligible power penalty was observed within the 20 km transmission, and the power penalty at a BER of 10 9 was 1.2 dB after transmission over 60 km SMF. Fig. 12 shows the eye diagrams for the BB signal, which indicates that a good performance was maintained, and the eye diagram is still wide open after the 60 km transmission.
5. Conclusions In this paper, three signals, namely, two 1.25 Gb/s MIMO wireless signals and a 2.5 Gb/s BB wired signal, have been optically multiplexed and transmitted successfully with error-free transmission using one wavelength. This was achieved by using PDM and low ER to modulate the BB signal. Transmission over 20 km and 60 km SMF were achieved when 16-QAM and 4-QAM were used for the MIMO signals, respectively. The physical layer performance was reported in terms of the BER and received power. In addition, constellation and eye diagrams were analyzed in this study. The theoretically derived equations and simulation results revealed that the performance of the MIMO signals was greatly dependent on the BB signal’s ER. An optimum ER range was found to obtain error free transmission for the three combined signals. The proposed technique provides a spectrally efficient and reliable RoF system for the simultaneous provision of MIMO and BB signals at high data rates. The simulation results verify that our scheme could be a promising candidate for future converged wired and wireless networks.
Acknowledgments This work was supported by a research grant from the Ministry of Science Technology and Innovation, Malaysia under Vote no. 73720. The authors would like to thank the Research Management Centre (RMC) at the Universiti Teknologi Malaysia, for approving the presentation of this work.
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Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Polarization multiplexing of two MIMO RoF signals and one baseband signal over a single wavelength M.A. Elmagzoub a,n, Abu Bakar Mohammad a, Redhwan Q. Shaddad b, Samir A. Al-Gailani a a b
Lightwave Communication Research Group, InfoComm Research Alliance, Universiti Teknologi Malaysia, 81310 Johor, Malaysia Communication and Computer Engineering Department, Faculty of Engineering and Information Technology, Taiz University, Taiz, Yemen
art ic l e i nf o
a b s t r a c t
Article history: Received 15 May 2015 Received in revised form 22 July 2015 Accepted 28 July 2015 Available online 7 August 2015
Next-generation (NG) access networks require simultaneous provision of wired and wireless services and high data rates to meet the large demands of mobility and multiple services. In this paper, we propose a novel spectral efficient radio over fiber (RoF) scheme to simultaneously provide two spatially multiplexed multiple input multiple output (MIMO) wireless signals with a baseband (BB) wired signal in one wavelength using a centralized light source. The proposed scheme can be applicable to wavelength division multiplexed passive optical networks (WDM-PONs). The BB signal is modulated at a low extinction ratio (ER). The modulated light is re-used to modulate two MIMO signals that have the same carrier frequency that is combined optically using polarization-division-multiplexing (PDM). The data rate for each MIMO stream was 1.25 Gb/s, and the data rate was 2.5 Gb/s for the BB signal. Error free performance with a bit error rate (BER) of 10 9 was achieved for all three signals after 20 km and 60 km through single mode fiber (SMF) for 16-QAM and 4-QAM for the MIMO signals, respectively. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Polarization division multiplexing (PDM) Radio-over-fiber (RoF) Multiple-input multiple-output (MIMO)
1. Introduction Next-generation (NG) access networks require large bandwidths and high data rates for both wired and wireless access to support ever-increasing video-based interactive and multimedia services [1,2]. It is highly beneficial to simultaneously transmit baseband (BB) and wireless signals using the same infrastructure, such as passive optical network (PON). In particular, wavelength division multiplexed (WDM) PON offers high transport capacity, high bandwidth per subscriber, protocol transparency line rate independence, high security, and upgradability [3]. Radio-over-fiber (RoF) technology has been demonstrated to have great potential for the application of future broadband wireless access networks due to the seamless integration of the sufficient bandwidth of optical fiber communication and the high mobility of wireless radio communication [4–6]. For wireless broadband transmission, multiple-input–multiple-output (MIMO) is an indispensable technique for all new wireless standards and systems that require a high data rate [7]. MIMO systems are designed to improve the transmission range and reliability and to deliver higher data transmission rates than single-input single-output (SISO) systems [8]. Therefore, to build any RoF-PON system for a NG-PON, a MIMO technique has to be considered [7]. In spite of n
Corresponding author. Fax: þ60 75536155. E-mail address: [email protected] (M.A. Elmagzoub).
http://dx.doi.org/10.1016/j.optlastec.2015.07.023 0030-3992/& 2015 Elsevier Ltd. All rights reserved.
the importance of the MIMO technique, to the best of our knowledge, all of the proposed RoF systems are considering SISO with BB data or MIMO-RoF without BB data [9]. Transmission of wired and multiple wireless services is demonstrated by electrically combining the signals at different central frequencies, which is not valid for MIMO signals that have the same central frequency [10]. In [11], both the BB signal and the RoF signal were multiplexed and de-multiplexed in the optical domain; hence, the operation speed is not limited by the electronic bottleneck caused by digital signal processing (DSP). This is achieved by using an optical carrier suppression (OCS) technique to modulate the RoF signal; then, the optical carrier is re-used to modulate the BB signal. The output optical spectrum for this scheme is similar to the optical spectrum of an optical double sideband with carrier (ODSB þC) modulation technique, which requires twice the bandwidth compared to the optical single sideband with carrier (OSSB þC) modulation technique. In [12], the transmission of two wireless MIMO signals with the same carrier frequency over fiber in an RoF system using an optical single sideband frequency-translation (OSSB-FT) technique was proposed and demonstrated. The drawback of this method is dedicating a wavelength for each MIMO stream. In [7,13,14], polarization-division-multiplexing (PDM) is used to carry each MIMO stream at a different polarization of the same wavelength, taking advantage of the two polarizations of the wavelength. It is more attractive and cost-effective to deploy RoF-PON that supports a wired and MIMO wireless technique in the same
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71
Fig. 1. Generic architecture of the BB and MIMO-RoF WDM-PON.
infrastructure, such as in WDM-PON and as shown in Fig. 1. In this paper, BB wired and MIMO wireless signals are provided simultaneously in a single wavelength using PDM and a low extinction ratio (ER) to modulate the BB signal. In the proposed scheme, PDM is used due to its ability to optically multiplex two RF signals with the same center frequency without interfering with each other. Additionally, modulating the BB signal at a low ER is applicable in access networks where the transmission distance is limited. The novel proposed scheme improves the spectrum efficiency of a WDM-PON and makes providing a multi-gigabit level of wireless and wired signals practical. The rest of the paper is organized as follows: Section 2 elaborates the principles of the proposed method. In Section 3, a mathematical model illustrates how the novel BB and RF signals multiplexing scheme operates in the proposed system. The simulation considerations and the results are covered in Section 4. Finally, Section 5 concludes the paper.
2. Principle of the proposed BB wired/MIMO wireless integrated RoF-PON Fig. 2 shows the schematic diagram of the proposed system with the corresponding spectra where the signal transmitter in the central office (CO) and a receiver in the optical network unit (ONU) (or remote antenna unit (RAU)) are arranged, which can be implemented using a WDM-PON architecture. The BB data are modulated by the MZM at a low ER. The modulated light from the BB data is used to modulate two 5 GHz 16-QAM MIMO wireless signals (or 4-QAM). Because the ER of the modulated light is very low, the two MIMO signals can be modulated using the same wavelength. In addition, the BB signal will not be affected by the MIMO signals because an OSSB þC modulation technique is used to modulate the RF wireless signals. Therefore, the RF signals will be carried in one of the sidebands, and the BB signal is carried in the carrier. Two dual drive MZMs (DD-MZM) are used to perform
OSSBþ C for the two MIMO signals. PDM is used to combine the two MIMO wireless signals. First, the laser diode light is adjusted using a polarization controller (PC) to be polarized at polarization x (Pol-x). Then, after the light is modulated by the BB data using the MZM, a power splitter is used to equally divide the light into two portions. Each portion of the modulated light is sent to a DDMZM through a PC. As illustrated in Fig. 2, the upper portion of the modulated light wave is intensity modulated using a DD-MZM and a 16-QAM MIMO1 signal, whereas the lower portion (Pol-y) is modulated using a 16-QAM MIMO2 signal and DD-MZM to perform the OSSBþ C modulation technique and to generate an optical spectra. The two MIMO signals are then polarization multiplexed at a polarization beam combiner (PBC). The multiplexed optical signals are transmitted to the ONU through a length of single mode fiber (SMF) where they are polarization de-multiplexed at the polarization beam splitter (PBS), as shown again in Fig. 2. The BB and MIMO1 signals are then converted into an electrical signal at the upper PD (Pol-x) and are then split into two paths. In the first path, the MIMO1 signal is filtered out using a low-pass filter (LPF), and only the wired signal is obtained and delivered to the BB receiver. In the second path, the electrical signal is sent to the MIMO1 antenna through a RF amplifier. The MIMO1 antenna can act as a bandpass filter to block the wired signal because its spectral response is for a 5 GHz signal and the BB signal has a very low frequency. The MIMO2 signal is received by the lower PD (Pol-y) and delivered through the RF amplifier to the MIMO2 antenna. In this paper, to verify the performance of our proposed RoF link, the two 5 GHz 16-QAM signals are directly demodulated without wireless transmission.
3. Mathematical model This section presents the mathematical model for the BB and RF signals multiplexing scheme in the proposed architecture. The upper portion of Fig. 2 (Pol-x) is considered in this mathematical
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Fig. 2. Schematic diagram of the proposed system with the corresponding spectra.
model to demonstrate the novel optical multiplexing principle for the BB signal modulated using the low ER and the RF signal. The lightwave from the LD, which can be expressed as Ein (t ) = Ac e jwc t , is first intensity modulated by the BB data using
the single-arm MZM. The output is expressed as
Eout1 (t ) = A (t ) e jwc t .
(1)
M.A. Elmagzoub et al. / Optics & Laser Technology 76 (2016) 70–78
Eout2 (t ) =
73
α A (t ) e jwc t × ⎡⎣ e jmh cos ω m t + je (jmh sin (ω m t ))⎤⎦. 2
(3)
From the Jacobi-Anger expansions [18] ∞ ∞ e jmh cos θ = ∑n =−∞ j n Jn (mh ) e jnθ and e jmh sin θ = ∑n =−∞ Jn (mh ) e jnθ , where Jn is the nth-order Bessel function of the modulation index. Based on the Jacobi-Anger expansions, Eq. (3) can be written as
Eout2 (t ) =
α A (t ) e jwc t 2
⎡1 −jω m t + J ( m ) + jJ ( m ) e jω m t ⎤ h h +1 0 ⎥ ⎢ j J−1 ( mh ) e ⎥ × ⎢ + jJ ( mh ) e−jω m t ⎥ ⎢ −1 ⎥ ⎢ + jJ ( m ) + jJ ( m ) e jω m t ⎦ ⎣ h h +1 0
where the values of Jn (mh ) are neglected for n ¼ 72, 73, …, 71 because their values are too small. α Eout2 (t ) = A (t ) e jwc t × ⎡⎣ J0 ( mh )(1 + j ) + 2jJ+1 ( mh ) e jω m t ⎤⎦. 2 π
π
Because 2e j 2 = 2j and 1 + j = the following:
Fig. 3. The effect of the BB signal ER on the two MIMO signals.
Eout2 (t ) =
⎡ α j A (t ) ⎢ 2 J0 ( mh ) e ⎣ 2
2 e j 4 , Eout2 (t ) can be rearranged as
(w t + π4 ) + 2J+1 ( mh ) e j ((w + ω c
c
m)t +
π 2
)⎤⎥. ⎦
(4)
Eq. (4) can be further simplified as the following:
⎛ j Eout2 (t ) = αA (t ) × ⎜ C1e ⎝ where C1 =
1
J (mh ) 2 0
(w t + π4 ) + C2 e j (w + ω c
c
m)t +
π⎞ 2⎟
⎠
(5)
and C2 = J+1 (mh ) according to Eq. (4). The two
components obtained from Eq. (5) represents the optical carrier (OC) and the upper optical sideband (UOSB) with initial phases π 4
and π , respectively. It should also be noted that in the OSSB þC 2 modulation scheme, the frequency spacing between the OC and the UOSB is precisely equal to the RF carrier [17]. The second modulator effect on the BB signal is very limited because it is only attenuated by the DD-MZM insertion loss and the small value of the constant C1. However, from Eq. (5), the UOSB is greatly affected by the BB signal because it is multiplied by A(t). A(t) represents the intensity modulation of the BB signal, which is defined as the following [19]:
Fig. 4. The performance of the MIMO signals for the BtB and 20 km cases.
Then, this signal is modulated again by the RF signal using the OSSB þC modulation technique via the DD-MZM. The OSSB þC is performed by applying a π/2 phase difference between the two RF arms of the DD-MZM biased at the quadrature point [15]. The RF signal has a sinusoidal clock Vm (t ) = VRF cos ωm t , where ωm = 2πfRF . This sinusoidal carrier can be modulated with any form of spectral efficient digital modulation, for example 16-QAM. The optical field of the output signal Eout2 (t ) from the DD-MZM can be mathematically expressed as the following [16,17]: Eout 2 (t ) =
α A (t ) e jwc t 2
⎞ ⎡ ⎛⎜ π ⎛ j V cos ω m t⎟ j π ⎜V − V cos ⎠ + e Vπ ⎝ dc RF × ⎢ e ⎝ Vπ RF ⎢⎣
⎞⎤ ωmt + π ⎟ 2 ⎠⎥
(
)
⎥⎦
(2)
where α is the insertion loss of the DD-MZM, Vπ is the required driving voltage to achieve a phase shift of π radians and Vdc is the DC bias voltage. For the OSSBþ C scheme, the DC voltage is biased at Vπ [17]. In this proposed design, Vπ = 4 and Vdc = 2. The 2
modulation index is defined as mh =
πVRF . Vπ
By substituting these
values, Eq. (2) can be rewritten as the following:
Eout2 (t ) =
⎡ π⎛ α A (t ) e jwc t × ⎢ e jmh cos ω m t + e j 2 ⎜ e 2 ⎝ ⎣
(
π
(
−jmh cos ω m t + π 2
π
)) ⎞⎟ ⎤⎥. ⎠⎦
Because cos (ωm t + 2 ) = − sin ωm t and e j 2 = j , Eout2 (t ) can be rearranged as the following:
A (t ) = cos (Δθ (t ))
(6)
where Δθ is the phase difference between the two branches of the single-arm MZM and is defined as the following:
Δθ (t ) =
π (0.5 − M (BB (t ) − 0.5)) 2
(7)
with
M=1−
⎛ 1 ⎞ π tan−1⎜ ⎟ ⎝ ER ⎠ 4
(8)
where ER is the extinction ratio (maximum-to-minimum-modulated light power) and BB(t) is the electrical input signal. The electrical input signal is normalized between 0 and 1. From Eqs. (5) and (8), it is clear that the RF signal in the UOSB is directly affected by the BB signal ER. For example, if the BB signal is modulated at a very high ER, the signal that enters the DD-MZM will be lost a portion of the time (when the BB signal is 0), which will highly degrade the RF signal in the reception. In contrast, if the BB signal is modulated at a low ER, there will be optical power in both cases (0 and 1) to modulate the RF signal. Additionally, if the BB signal ER is less than a certain threshold, the BB signal cannot be detected after the fiber degradation. An optimum ER is found in Section 4, which results in a good performance from the two signals (BB and RF). The optically modulated signal (Eq. (5)) propagates along an SMF with a propagation constant of β (w ) and an amplitude
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Fig. 5. Constellation diagrams of the MIMO signals for the (a) transmitted signal, (b) MIMO1 BtB, (c) MIMO1 20 km, (d) MIMO2 BtB, and (e) MIMO2 20 km.
attenuation γ , where ω is the angular frequency. β′(w ) is the time delay, which is calculated by the first derivative of β (w ). Therefore, the output lightwave at the end of the SMF of length of z can be approximated as the following [8,20]:
After the signal is directly detected by a PD, the photocurrent for the received signal can be written as the following equation according to the square-law PD [8,17]:
EOFibre (z, t ) = αe−γz A ( t − β′( wc + ω m ) z ) ×
* IOFibre (z, t ) = μ |EOFibre ( z, t ) |2 = μEOFibre (z, t ) × EOFibre (z, t )
⎧ ⎨ C1e j ⎩
(w t + β (w )z+ π4 ) + C2 e j ((w + ω c
c
c
m ) t + β (wc + ω m ) z +
π 2
⎫. ⎬ ⎭
)
(9)
where μ is the responsivity of the PD.
(10)
M.A. Elmagzoub et al. / Optics & Laser Technology 76 (2016) 70–78
Fig. 6. The BB signal performance for the BtB and 20 km cases.
75
Fig. 8. The effect of the BB signal ER on the two MIMO signals.
IOFibre (z, t ) = μα 2e−2γz A2 ( t − β′( wc + ω m ) z ) × ⎧ π π C e j (wc t + β (wc ) z + 4 ) + C2 e j ((wc + ω m ) t + β (wc + ω m ) z + 2 ) × ⎪ ⎪ 1 ⎨ ⎪ C e−j (wc t + β (wc ) z + 4π ) + C e−j ((wc + ω m ) t + β (wc + ω m ) z + 2π ) 1 2 ⎪ ⎩
( (
)
)
⎫ ⎪ ⎪ ⎬ ⎪ ⎪ ⎭
IOFibre (z, t ) = μα 2e−2γz A2 ( t − β′( wc + ω m ) z ) × ⎧ 2 π π ⎫ ⎪ C1 + C22 + C1C2 e j (wc t + β (wc ) z + 4 − wc t − ω m t − β (wc + ω m ) z − 2 )⎪ ⎬ ⎨ π π ⎪ ⎪ ⎭ ⎩ +C1C2 e−j (wc t + β (wc ) z + 4 − wc t − ω m t − β (wc + ω m ) z − 2 ) IOFibre (z, t ) = μα 2e−2γz A2 ( t − β′( wc + ω m ) z ) × ⎧ 2 π ⎫ ⎪ C1 + C22 + C1C2 e j (ω m t − β (wc ) z + β (wc + ω m ) z + 4 )⎪ ⎨ ⎬. π ⎪ ⎪ ⎩ +C1C2 e−j (ω m t − β (wc ) z + β (wc + ω m ) z + 4 ) ⎭ Because β (wc ) z , for a specified frequency, and z are constants, the total shift constant θsc can be assumed to be the following:
θsc = − β ( wc ) z + β ( wc + ω m ) z +
π . 4
Fig. 9. The performance of the MIMO signals for the BtB, 20 km and 60 km cases.
IOFibre (z, t ) = μα 2e−2γz A2 ( t − β′( wc + ω m ) z ) × ⎧ C2 + C2 + C C e j (wm t + θsc ) ⎫ 1 2 1 2 ⎨ ⎬ . ⎪ ⎪ ⎩ +C1C2 e−j (ω m t + θsc ) ⎭ ⎪
This shift constant is produced by the optical SMF and the optical modulator. Therefore,
⎪
Because cos θ =
e jθ + e−jθ , 2
Fig. 7. Eye diagrams of the BB signal for (a) BtB, and (b) 20 km.
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M.A. Elmagzoub et al. / Optics & Laser Technology 76 (2016) 70–78
Fig. 10. Constellation diagrams of MIMO1 for (a) transmitted signal, (b) BtB, (c) 20 km, and (d) 60 km.
(
)
IOFibre (z, t ) = μα 2e−2γz A2 t − β′ ( wc + ω m ) z ×
{
Fig. 11. The BB signal performance for the BtB, 20 km and 60 km cases.
C12
+
C22
}
+ 2C1C2 cos ( ω m t + θsc ) .
(11)
According to Eq. (11), the photocurrent is composed of the DC components (BB) and the RF component (MIMO1) at ωm after transmission. The detected signal is then passed through the bandpass filter (BPF) with a center frequency of fRF , so the DC component is removed for the wireless MIMO reception or can be filtered out using the RF signal antenna. Then, each detected wireless MIMO signal with the carrier frequency fRF is directly amplified and propagated by using a MIMO antenna technique through the wireless channel. The wireless end-user will receive the MIMO signals and demodulate them using the suitable QAM demodulation and MIMO decoding techniques. For the BB signal reception, the MIMO1 signal is filtered out using a LPF, and only the wired BB signal is obtained. In the proposed scheme, PDM is used due to its ability to optically multiplex two RF signals with the same center frequency
M.A. Elmagzoub et al. / Optics & Laser Technology 76 (2016) 70–78
77
Fig. 12. Eye diagrams of the BB signal for (a) BtB, (b) 20 km, and (c) 60 km.
without the signals interfering with each other as proposed and demonstrated in [7] and [14]. Moreover, unlike [21], no electrical frequency shifting will be required at the ONU. The primary disadvantage is that this system is valid for only two MIMO streams. Additionally, modulating the BB signal at a low ER is applicable for access networks where the transmission distance is limited. For the QAM signals, the bit error rate (BER) is obtained from its error vector magnitude (EVM). The EVMs are calculated considering the following equation [8]:
⎡ M Stx, k − Srx, k ⎢∑ EVM (dB) = 10. log10 ⎢ k = 1M 2 ∑ S ⎣ K = 1 tx, k
2⎤
⎥ ⎥ ⎦
(12)
where EVM is the value of the difference between a collection of received symbols and transmitted or ideal symbols, Stx, k is the corresponding transmitted symbol of the constellation associated with the kth symbol, Srx, k is the received symbol associated with Stx, k , and M is the number of symbols for the inphase-quadrature constellation. The BER is calculated according to [22]: BER =
⎡ ⎢ ε = erfc ⎢ ⎣
k=
1 3. log2Q 2 . . 2 2 Q2 − 1 ( k. EVMrms ) . log2M
(
)
Stx, max M
(
∑i = 1 Stx, i /M
)
⎤ ⎥ ⎥, ⎦
(1 − Q−1) *ε, log2Q
, (13)
where Q is the number of signal levels within each branch of the constellation diagram, log2M is the number of bits encoded into one QAM symbol, and K is the modulation format-dependent factor giving the relationship between the maximum field magnitude and the overall average M field magnitudes defined by the constellation diagram for the chosen modulation format. The Stx, i is the ideal transmitted field vector, and Stx, max is the field vector of the outermost constellation point.
The length of the SMF was 20 km with an attenuation of 0.2 dB/km and a dispersion coefficient of 17 ps/nm/km. The insertion loss of the DD-MZM was 4 dB. Because the quality of the three signals was dependent on the BB signal ER, the suitable value for the ER was found. From Fig. 3, it is clear that the three signals can obtain error-free transmission when the ER is in the range between 0.15 dB and 0.25 dB. Errorfree transmission here is defined as the transmission of a signal with a BER of no more than 10 9. This agrees with our theoretical analysis, which shows that increasing the ER will highly degrade the MIMO signal performance. Fig. 4 shows the measured BER in terms of the receiver optical power into the PD in both back to back (BtB) and 20 km SMF transmission cases for the MIMO signals. For the BtB MIMO signal case, we observed that the receiver sensitivity achieved 23 dBm and 22.3 dBm for the MIMO1 signal (Pol-x) and for the MIMO2 signal (Pol-y), respectively. This 0.7 dB power penalty between Pol-x and Pol-y could be attributed to the different performances of the optical and electrical components. After a transmission of 20 km, the power penalty was 2 dB at 10 9 BER. Additionally, the power penalty between Pol-x and Pol-y increased to 1 dB, which could be attributed to the polarization mode dispersion. Fig. 5 shows the constellation diagrams for the MIMO signals for the case of transmitted and received signals. For the received signal, BtB and 20 km SMF transmission cases are considered. For the BtB case, clear scatter-plots are achieved, although each constellation point appears to have some expansion due to the noise of the LD, PD and power loss and noise introduced by the passive optical components. After a transmission over 20 km SMF, the clusters in the constellations are still clearly separated. In the BtB BB signal case, as shown in Fig. 6, when the BER is 10 9, the measured received power is 12.2 dBm. After a transmission of 20 km the power penalty is 3 dB at 10 9 BER. The eye diagrams of the BB signal are also shown in Fig. 7. The relatively high power required at the PD and the power penalty required to achieve 10 9 BER performance are due to the low ER used to modulate the BB signal.
4. Simulation experiment and results
4.2. 4-QAM MIMO signals
4.1. 16-QAM MIMO signals
The system performance can be greatly improved when a 4-QAM modulation is used instead of 16-QAM, and a transmission distance of 60 km SMF is achievable with error free performance. The BB signal ER can be increased because the 4-QAM signal is more robust to changes in its modulation power compared with the 16-QAM signal. Fig. 8 shows the BER performance at a transmission distance of 60 km when the ER is varied. The three signals can obtain error-free transmission when the ER is in the range between 0.63 dB and 0.93 dB.
MATLAB and OptiSystem 13 software tools were used to simulate the proposed system, which shown in Fig. 2. Each channel of the two MIMO signals was 1.25 Gb/s 16-QAM at the same carrier frequency fc ¼5 GHz. The BB signal was a 2.5 Gb/s non-return-tozero (NRZ) signal generated using a pseudo-random bit sequence (PRBS) generator. The total input power of the three signals was 10 dBm and was launched from LD at a wavelength of 1552.52 nm.
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Fig. 9 shows the BER curves of the MIMO signals at BtB and following a 20 km and 60 km SMF transmission. For the BtB case, the receiver sensitivity at 10 9 achieved 29.3 dBm and 28.8 dBm for MIMO1 and MIMO2, respectively. A negligible power penalty (approximately 0.5 dB) was induced after the 20 km SMF transmission. After a transmission of 60 km, the power penalties were 3 dB and 2 dB at 10 9 BER for MIMO1 and MIMO2, respectively. The transmitted and received constellations of the MIMO1 signal in the three cases (BtB, 20 km and 60 km) are shown in Fig. 10. In the cases of the 20 km and 60 km transmission, the received signal also maintained clear scatter-plots even though the distances between the constellation points were becoming shorter due to the noise and fiber dispersion. Fig. 11 presents the BB signal transmission BER performance for the BtB and the 20 km and 60 km SMF transmission cases. In the BtB case, when the BER was 10 9, the measured received power was 20.4 dBm. A negligible power penalty was observed within the 20 km transmission, and the power penalty at a BER of 10 9 was 1.2 dB after transmission over 60 km SMF. Fig. 12 shows the eye diagrams for the BB signal, which indicates that a good performance was maintained, and the eye diagram is still wide open after the 60 km transmission.
5. Conclusions In this paper, three signals, namely, two 1.25 Gb/s MIMO wireless signals and a 2.5 Gb/s BB wired signal, have been optically multiplexed and transmitted successfully with error-free transmission using one wavelength. This was achieved by using PDM and low ER to modulate the BB signal. Transmission over 20 km and 60 km SMF were achieved when 16-QAM and 4-QAM were used for the MIMO signals, respectively. The physical layer performance was reported in terms of the BER and received power. In addition, constellation and eye diagrams were analyzed in this study. The theoretically derived equations and simulation results revealed that the performance of the MIMO signals was greatly dependent on the BB signal’s ER. An optimum ER range was found to obtain error free transmission for the three combined signals. The proposed technique provides a spectrally efficient and reliable RoF system for the simultaneous provision of MIMO and BB signals at high data rates. The simulation results verify that our scheme could be a promising candidate for future converged wired and wireless networks.
Acknowledgments This work was supported by a research grant from the Ministry of Science Technology and Innovation, Malaysia under Vote no. 73720. The authors would like to thank the Research Management Centre (RMC) at the Universiti Teknologi Malaysia, for approving the presentation of this work.
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