Multiple Input Multiple Output Orthogonal Frequency Division Multiplexing Based Photonic

Access Point

Nur Shazwani Mohd Noor, Norliziani Zamuri, Razali Ngah, Teguh Prakoso Wireless Communication Center (WCC) Universiti Teknologi Malaysia (UTM)

Skudai, Johor, Malaysia shaz_wani219@yahoo.com.my, liziani_86@yahoo.com, razalin@fke.utm.my

Abstract—� Rapid increasing activity in Fiber To The Home (FTTH) deployment in Malaysia and general desired to eventually migrate to these systems inspire to the reduction in overall cost and components required to deploy FTTH. This paper proposed implementation of Multiple Input Multiple Output Orthogonal Frequency Division Multiplexing (MIMO- OFDM) in FTTH to provide higher capacity gain, high data rate , low complexity system and triple play service (voice, video and data). The complete MIMO OFDM based Photonic Access Point (PAP) system to accommodate Wireless Local Area Network (WLAN) 802.11n standard was implemented in a two transmits and receives antenna using simulation with Optisystem software. Perfect constellation on each 64QAM points at receiver part and wide eye opening shows that the system is reliable for FTTH application.

Keywords - MIMO-OFDM; Photonic Access Point; Optisystem

I. INTRODUCTION

Fiber To The Home (FTTH) has been introduced for years ago, however recently it receive high attention by user, service provider, governments and also researchers with driving factor of broadband usage and faster speed compare to fixed wired and wireless connection. Wireless communication is ubiquitous, however it not suitable and effective for transfer large data to many user. In FTTH an information data is transmit by sending light pulse that form a modulated electromagnetic carrier wave through fiber cable by Radio over Fiber concept (RoF) and the RF signal was radiated through the air [1]. Optical fiber enables the user experience higher speed more than 100Mbits/s and better throughput broadband services compared to Digital Subscriber Line (DSL) that use copper wire. However, fully connection by fiber optics might be difficult to be done due to time, cost, geographical problem and infrastructure. Therefore optical-wireless access is an alternative to provide broadband access. New edition in wireless standard known as WLAN 802.11n was introduce to improve network throughput to meet the consumer demand from previous standards - 802.11a and 802.11g. WLAN 802.11n use MIMO-OFDM can support data rate from 54 Mbit/s to 600 Mbit/s.

MIMO-OFDM is widely employ for next-generation that occupy high data rate wireless systems because OFDM offers

high spectrum usage efficiency and tolerance to multipath fading compare to conventional system that use only single carrier. MIMO antennas bring advantages by improve the capacity, transmission power and energy, spectral efficiency and system throughput.

Recently, many researchers have focused on integrating radio signals over fibers optic, referred to RoF [2] [3] [4]. Some issue and future research regarding RoF challenges was discussed in [1]. The challenges include the type of modulation techniques, compactness and cost of RBS, demand for high data rate, noise and also shortfall in fiber link itself. FTTH has trigged due to escalation in wireless demands and makes RoF as a convincing technology to support PhAP with (wavelength division multiplexing) WDM technique. Further RoF implementation in FTTH was presented in [5]. Exploration of point-to-multipoint access network architecture for FTTH applications by implementation of enhanced double weight optical code division multiple access codes (EDWOCDMA) was proposed in reference [6]. Beside triple play service, technologies now are aim for quintuple-play service and next generation like UWB, LTE and WiMax [7]. Many researches also attracted to investigate MIMO-OFDM based on WLAN in terms of evaluate the performance in throughput [8] and bandwidth efficient [9]. In [10] the optical wireless access using MIMO-OFDM was presented but using lower modulation level than 64QAM. It combines optical polarization division multiplexing (PDM) with spatial multiplexing techniques. Moreover, this research does not include the newest wireless standards.

RoF technology with external modulated signal was used in designation of CBS and PhAP. The specifications of the system are based on IEEE 802.11n at frequency of 2.4GHz and 144Mbps bit rate. PhAP was presented to replace Radio Access Point (RAP). Conventional FTTH only has a wireless connection inside the house from Optical Network Terminal (ONT) to user equipment while in this paper we have proposed the optical-wireless access from PhAP to FTB. Lastly electrical constellation diagram of 64 QAM, and BER analyzer was analyzed to evaluate the performance of the proposed system design.

The authors are grateful to Universiti Teknologi Malaysia (UTM) for the financial assistance via Research University Grant with Vote Number of 02H31.

2012 IEEE Asia-Pacific Conference on Applied Electromagnetics (APACE 2012), December 11 - 13, 2012, Melaka, Malaysia

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II. SYSTEM DESCRIPTION

Optisystem software enables researchers to simulate and analyze the transmission layer of optical networks with advanced technologies and components based on real application. The current FTTH system consists of optical line transmission (OLT), CBS, optical fiber link, Access Point (AP) and end user. OLT as transmitter input connect to CBS through 20km Single Mode Fiber (SMF). In CBS, signal processing and network management was done by RoF system, instead of RAP to reduce the cost and complexity. CBS consists of photodetector PIN (PD), QAM sequence generator, Quadrature modulator, CW laser and optical modulator. Then the signals propagate to AP through shorter SMF fiber with length of 0.5km. From the AP, another fiber cable of 15m is required either overhead or underground installation connect to Fiber Termination Box (FTB) inside the house. Typical wireless digital communication used 64 QAM modulation techniques because it distributes the point more evenly, constellations are more distinct and higher BER compared with lower level of modulation. Quadrature modulator produces the RF modulated signal at frequency of 2.4GHz. In conventional FTTH, only one channel of 193.05THz was used and maximum bit rate of 54 Mbps.

Instead of implement fiber optic fully from OLT towards the user, this paper proposed a replacement of hybrid optical-wireless access from AP to the FTB by introducing PhAP with MIMO antenna and OFDM modulation. In addition, FTB located outside the house for reduce the space required. Bit rate of 144Mbps set for the simulation of two spatial streams with 20MHz bandwidth. Intensity modulation is employ to transmit radio signal through optical link by modulate the laser transmitter. Continuos-Wave (CW) Laser Array at wavelength 1554nm and 1553.6nm are lasing with external modulation using Lithium Niobate Mach-Zehnder Modulator (LiNb MZM). This component simulates a LiNb MZM based on measured parameters. LiNb MZM is used to avoid distortions in the signal. Each source with different wavelength and optical power of 10dBm were transmitted to the fiber through WDM multiplexer. LiNb MZM and WDM are considering as optical modulator.

TABLE I. DESIGN PARAMETER

Since ITU-R designated 900MHz, 2.4GHz and 5GHz

frequency band as unlicensed for ISM communities, therefore 2.4 GHz is chosen in IEEE 802.11n WLAN. Although 2.4GHz is full, frequency reuse technique could be use. 802.11 n can transmit in 2.4 GHz band with 14 available channel, however not all channel are legal and non overlap with other channel. Only three channel non overlaps which are 1,6,11 at 2.412GHz, 2.437GHz and 2.462 GHz respectively and bandwidth of 22MHz usually been referring to design channel reuse pattern.

Design parameter setting is shown in Table I, value of sequence length of 4096 was choose to match with the value of bit per symbol and number of samples is the multiplication of sequence length and sample per bit [11].�The optical signal received at PhAP is split using WDM demultiplexer before detected by PD. The PD is used to convert the optical signal back to RF signal so that it can be transmitted by antenna. Optical amplifier of 10dB is used to boost up the optical signal before the signals enter the optical fiber. For link reliability, this project employs an optical fiber link of 500 meter.�

Figure 1: MIMO-OFDM transmitter

Parameter Value

Bit rate 144Mbps Sequence length 4096 bits

Sample per bit 64

Number of samples 8192 PD PIN 1 PD PIN 2

193.05 THz (1554 nm) 193.1 THz (1553.6 nm)

OFDM Modulator FFT Point Clock rate Number of subcarrier

64 20MHz 32

Modulation technique 64QAM Frequency 2.4GHz QAM sequence generator 6 bit Fiber Link 0.5km

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Figure 2: MIMO OFDM receiver.

III. MIMO OFDM BASED PHOTONICS ACCESS POINT (PhAP)

This MIMO transmitter antenna was placed at PhAP that locates between Central Base Station (CBS) and end user while MIMO receiver antenna installed on the FTB at premises as shown in Fig.1 and 2 respectively. PhAP also consists of PD and RF amplifier. The optical signal received at PhAP is split using WDM demultiplexer before detected by two PD. Conventional access point is referring to RF signal only, however this PhAP was design for the compatibility with optoelectronic components. RF signal then is amplified by RF Amplifier and propagate through wireless link by MIMO antenna [1]. MIMO require the transmission of several radio channels between the central unit and the remote units at exactly the same radio carrier frequency using separate RoF links on the same fiber but on separate optical wavelengths using WDM. The receive signal of two transmitter antenna are represent by eq.1 and 2 ����� � � ����� � ��� � ����� (1) � � � ����� � �� � ���� (2)

r is the combination of each signal, N is number of antenna and h{i,j} is channel weight to present the multipath. The proposed FTTH system is assume to be environment static, no reflection and only direct path from transmitter at PhAP to FTB at user house, therefore the channel coefficient is constant. The transmitted signal from transmit antenna to receive antenna experienced path loss in free space for outdoor propagation. Optical- wireless in FTTH for a small micro cell with distance from 0.5km to 3km and the antenna height is above medium roof-top level. According to Recommendation ITU-R P.1411, free space path loss model is given by:

L outdoor = 32.4 + 20 log d + 20 log f (3)

where d is distance in kilometers while f refer to frequency in MHz. By considering distance, d from transmit antenna to receive antenna is 10 meter and frequency, f is 2.4GHz, the loss of the outdoor propagation is 60dB. Spatial multiplexing of 2x2 antennas is used for the MIMO antenna to simultaneously transmit each data over different antennas to increase the maximum data rate. The receiving

antenna will receive a superposition of the transmitted signals and demodulation process was done. The MIMO antenna was modeled by two-port network S-parameter (S2P) as function of frequency since Optisytem is software that computes simulation based on time domain. Table II describes S2P data form antenna return loss (S11) and gain (S21) for transmit and receive antenna [12].

TABLE II: ANTENNA PARAMETER

OFDM modulator is a subsystem used to transmit

multicarrier where a data stream is carried with many lower-rate subcarrier tones without increase the bandwidth in the communication link. In OFDM modulator as exhibit in Fig.3, data from QAM sequence generator will first shifted into 64 multicarrier formats mapped into available frequency spectrum. Inverse Fourier Transform (IFFT) was used to find the symbols according to time waveform and then cyclic prefix add to each symbol. The data need to be interpolating either in step, linear or cubic form for conversion from digital to analog signal. RF spectrum analyzer was attaches at the output of quadrature modulator to view the 802.11n spectrum. The spectrum with 20MHz bandwidth is referring to suggested spectrum mask to ensure the transmission process is within the channel. Before transmit the signal, it must be shifted to serial data and filtered to reduce the noise while entering to next block.

Figure 3: OFDM modulator subsystem

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IV. RESULT AND DISCUSSION

Output signal quality for all digital demodulation at the receiver end can be analyzed by using electrical constellation diagram. Some of common impairment that can be identify in diagnose the link by constellation diagram such as I/Q gain, symbol clock error, group delay, phase noise and compression [13]. An Electrical constellation diagram was connected to M-ary Pulse Generator in order to view the representation of digital modulation in complex plane. Modulation part will produce 64 small bright points, however due to noise and interference, at the demodulation part, each point will form a small cluster Fig.4(a) shows the magnitude of I and Q for system without MIMO- OFDM. The symbols are affect by the noise and shift from its ideal location, this due to signal conversion from electrical to optical and vice versa that happens since this system use three type of fiber optic, which has different length of 20km and 0.5km SMF and 15m MMF.

Simulation from our proposed design was presented in Fig.4(b). It can clearly seen the 64 symbol are demodulated independently according to their respective point. By adding OFDM modulation in the design, it reduce the intersymbol interference (ISI) and inter-channel interference (ICI) that exist during the signal propogation. The error in symbol constellation and accuracy of OFDM transmitter are calculate by error vector magnitue (EVM) and the standard EVM for 802.11n devices varies from -22dB to -28 dB refering to their coding rate. By interconnecting a BER Analyzer at 3R Regenerator, the system produced an eye diagram with Q factor of 14.95 and minimum BER is 7.48e-051 as in Fig.5. Q factor indicate the signal quality, it will increase proportional to the standard deviation that related with noise, overshoot and ringing. The wide opening of the eye diagram shows a good performance. BER analyzer measures the performance of the MIMO-OFDM system based on the signal before and after propagation. The performance can be improved by increase the transmit power.

(a) (b) Figure 4: Constellation diagram at (a) without MIMO-OFDM and (b) Proposed MIMO-OFDM based PhAP.

Figure 5: Eye diagram result

V. CONCLUSION

The MIMO OFDM based on Photonic Access Point (PhAP) in WLAN 802.11n has been explained. The simulation of conventional FTTH and proposed design are accessed via simulation using Optisystem software. As a result, constellation diagram and BER analyzer show that the proposed Photonic Access Point (PhAP) with MIMO-OFDM results in a better performance compare to FTTH system without MIMO- OFDM. Details discussion on system design and the result prove that this system is convincible for FTTH application. In addition, this design also reliable and efficient to be implements in non line of sight environment that experience multipath. The system can be further improved by considering the effect of fading channel and noise.

ACKNOWLEDGMENT

The authors are grateful to Universiti Teknologi Malaysia (UTM) for the financial assistance via Research University Grant with Vote Number of 02H31.

REFERENCES [1] R.Abdolee, R.Ngah, V.Vakilian and T.A.Rahman, “ Application of

Radio-Over-Fiber (ROF) in mobile communication,” Asia Pacific Conference On Applied Electromagnetics Proceddings, Melaka, Malaysia,pp. 1-5, December 2007.

[2] M.Munsor, “Radio Access Point Design For Radio Over Fiber Technology,” Master of Engineering Thesis. Universiti Teknologi Malaysia, Skudai, April 2008.

[3] K.Pardeep and R.S.Kaler, “Radio Over Fiber Networks”,National Conference on Challenges & Opportunities in Information Technology (COIT-2007) RIMT-IET, Mandi Gobindgarh, March 2007.

[4] D.Opati, “Radio over Fiber Technology for Wireless Access,” GSDC Croatia Ericsson Nikola Tesla d.d. Krapinska 45, HR-10001 Zagreb, 2009.

[5] C.Rodrigues, A.Gamelas, F. Carvalho, A. Cartaxo, “Evolution of FTTH Networks Based on Radio-Over-Fibre,” ICTON 2011

[6] A.Z.G.Zahid, Feras N. Hasoon , H. Bakarman, S. Shaari, “Implementing EDW in Point to Multipoint Optical Access Network forFTTH Applications,” IEEE 9th Malaysia International Conference on Communications, Kuala Lumpur, Malaysia, pp 371- 375, December 2009.

[7] M.Morant, T. Quinlan, S.Walker, R.Llorento “ Real World FTTH Optical-to-Radio Interface Performance for Bi-directional Multi Format OFDM Wireless Signal Transmission” , Optical Fiber Communication Conference and Exposition (OFC/NFOEC), 2011 and the National Fiber Optic Engineers Conference .

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[8] M. B. Othman,L. Deng, X. Pang, J. Caminos eatl, “MIMO-OFDM WDM PON with DM-VCSEL for femtocells application”, OPTICS EXPRESS, Vol. 19, No. 26 ,2011

[9] Tim C.W. Schenk, Guido Dolmans and Isabella Modonesi (2004). “Throughput ofma MIMO OFDM based WLAN system”. Proc. Symposium IEEE Benelux Chapter on Communications and Vehicular Technology, 2004 (SCVT2004), Gent, Belgium. November 2004.

[10] 2x2 MIMO-OFDM Gigabit fiber wireless access system based on polarization division multiplexed WDM-PON.

[11] Optiwave, “Optisystem 6.0 Tutorials- Volume 1 and 2”, Optiwave, 2007.

[12] T. Prakoso, R.Ngah, Z. Ghassemlooy, T.A. Rahman, “ Antenna Representation in Two- Port Network Scattering Parameter, Microwave and Optical Technology Letters,53(6) 1404- 1409, 2011.

[13] LitePoint Corporation, Practical Manufacturing Testing of 802.11 OFDM Wireless Devices Handbook, United State of America, 2010.

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