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SNGCE, ECE DEPT CHAPTER 1 INTRODUCTION Due to the development of smart and portable devices Cyber-Physical Systems (CPSs) have attracted much attention. By utilizing the wireless networks to interact among cyber and physical components, CPSs can improve many smart systems such as smart grid, medical, and traffic control systems which construct smart society. Although the development of wireless techniques gives us fascinatingly convenient communication facilities, the network capacity of the backhaul network will gradually, if not drastically, diminish as it is not enough to meet the demands of the mobile users, number of whom has been increasing by leaps and bounds. In order to deal with such a mass users demands, the optical network is considered by many researchers to be suitable for the backhaul network because it is capable of providing stable communication and huge bandwidth. Therefore, the Fiber- Wireless (FiWi) network which integrates optical networks and wireless access networks attract more attention for supporting next generation CPSs in a future ubiquitous network. 1

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ABSTRACT Orthogonal frequency-division multiplexing (OFDM) has been widely used in fiber-wireless (FiWi) networks, but it suffers from reduced spectral efficiency, high peak to- average power ratio (PAPR), and severe sensitivity to carrier frequency offset (CFO). Here a flexible multi-block OFDM (MB-OFDM) transmission scheme is proposed to simultaneously solve these problems. First, one guard interval is shared by multiple blocks by exploiting the slow time-varying property of the fiber-wireless channel, so the spectral efficiency could be typically improved by about 10%. Second, the proposed scheme further divides every data block into multiple small sub-blocks, whereby each sub-block is generated by an inverse fast Fourier transform (IFFT) of smaller size accordingly.

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SNGCE, ECE DEPT

CHAPTER 1

INTRODUCTION

Due to the development of smart and portable devices Cyber-Physical Systems (CPSs) have attracted much attention. By utilizing the wireless networks to interact among cyber and physical components, CPSs can improve many smart systems such as smart grid, medical, and traffic control systems which construct smart society. Although the development of wireless techniques gives us fascinatingly convenient communication facilities, the network capacity of the backhaul network will gradually, if not drastically, diminish as it is not enough to meet the demands of the mobile users, number of whom has been increasing by leaps and bounds. In order to deal with such a mass users demands, the optical network is considered by many researchers to be suitable for the backhaul network because it is capable of providing stable communication and huge bandwidth. Therefore, the Fiber- Wireless (FiWi) network which integrates optical networks and wireless access networks attract more attention for supporting next generation CPSs in a future ubiquitous network.

Traditionally, optical communications and wireless technologies have gained rapid progresses respectively along their own paths. The optical fiber systems could provide extensive high-capacity but cannot reach everywhere, while the wireless systems could offer ubiquitous coverage but are usually bandwidth-constrained . Therefore, by integrating the huge capacity of optical fiber subsystems with the mobility and ubiquity of wireless subsystems, fiber-wireless (FiWi) networks are becoming the widely recognized powerful solution to future high-speed broadband access in the next decade.

For the FiWi networks , orthogonal frequency-division multiplexing (OFDM) has become the prominent transmission scheme due to its robustness against wireless multipath fading channels as well as the low complexity implementation. On the other hand, OFDM has recently gained considerable interest in future high-speed optical fiber networks with the data rate beyond 100 Gb/s . Also, OFDM is tolerent to optical impairments including chromatic dispersion (CD) and polarization mode dispersion (PMD) .

However, OFDM suffers from three main drawbacks for FiWi networks : 1) The reduced spectral efficiency due to the overhead of guard interval, usually in the form of cyclic prefix (CP), for every OFDM symbol; 2) The high peak-to-average power ratio (PAPR) of OFDM signals makes FiWi networks sensitive to the nonlinear distortion caused by either the laser diode or the Mach-Zehnder modulator (MZM);3) The severe sensitivity to the residual carrier frequency offset (CFO) would cause large inter-carrier-interference (ICI) degrading the system performance.

To improve the spectral efficiency, the length of the guard interval (e.g., in the form of CP) between adjacent OFDM symbols is reduced in the reduced-guard-interval OFDM (RGI-OFDM) scheme at the cost of increased receiver complexity . To reduce the PAPR, clipping or active constellation extension (ACE) can be used with signal distortion while selected mapping (SLM) and trellis shaping (TS) without signal distortion would add redundancy to the original OFDM signals . To mitigate the sensitivity to CFO, the windowing technique or the polynomial cancellation coding (PCC) based self-ICI cancellation scheme could be used at the cost of reducing the transmission efficiency. To solve all of those problems above, in this paper we propose the flexible multi-block OFDM (MB-OFDM) transmission scheme for future high speed FiWi networks to simultaneously improve the spectral efficiency, reduce the PAPR and mitigate the vulnerability to CFO.

The three main contributions made in this paper are :1) Unlike the conventional OFDM transmission scheme whereby each inverse fast Fourier transform (IFFT) data block has its own guard interval within every OFDM symbol, the proposed MB-OFDM scheme adopts only one guard interval in the form of zero padding (ZP) for multiple blocks by exploiting the slow time-varying property of the fiber-wireless channel .In this way, the spectral efficiency could be typically improved by about 10%.

2) Every data block within the MB-OFDM transmission frame is further divided into multiple sub-blocks, whereby each sub-block is generated by an IFFT of smaller size accordingly. This mechanism makes MB-OFDM a flexible compromise between the multi-carrier and single-carrier transmissions, so the PAPR of the MB-OFDM signal could be reduced, and the sensitivity to CFO could also be alleviated.

3) The hybrid-domain channel equalization is proposed to demodulate the MB-OFDM signal, whereby the received signal in the form of multi-carrier is firstly regarded as a single-carrier signal which is recovered in the time domain, and then the FFT of reduced size is used to restore the transmitted multi-carrier signal in the frequency domain.

CHAPTER 2LITERATURE SURVEYWith the increase in the use of wireless devices bandwidth demand is increasing day by day. FiWi network which use both wireless as well as optical network with huge bandwidth can be considered as a solution to the increasing bandwidth usage. The advantage of using OFDM transmission technique for FiWi network is studied in the following section.

2.1 REVIEW ON BACKPAPERS

2.1.1 FIBER-WIRELESS NETWORKS AND SUBSYSTEM TECHNOLOGIESHybrid fiber-wireless networks incorporating WDM technology for fixed wireless access operating in the sub-millimeter- wave and millimeter-wave (mm-wave) frequency regions are being actively pursued to provide unmetered connectivity for ultrahigh bandwidth communications. The architecture of such radio networks requires a large number of antenna base-stations with high throughput to be deployed to maximize the geographical coverage with the main switching and routing functionalities located in a centralized location. The transportation of mm-wave wireless signals within the hybrid network is subject to several impairments including low opto-electronic conversion efficiency, fiber chromatic dispersion and also degradation due to nonlinearities along the link. One of the major technical challenges in implementing such networks lies in the mitigation of these various optical impairments that the wireless signals experience within the hybrid network. This paper presents an overview of different techniques to optically transport mm-wave wireless signals and to overcome impairments associated with the transport of the wireless signals. This also reviews the different designs of subsystems for integrating fiber-wireless technology onto existing optical infrastructure.

Millimeter-wave (mm-wave) fiber-wireless systems have the advantage of being able to exploit the large unused bandwidth in the wireless spectrum as well as the inherent large bandwidth of the optical fiber. This can provide high data rates with minimum delay. There are three possible methods to transport the mm wave wireless signal over the optical link RF-over-fiber, IF-over-fiber, and Baseband-over-fiber. Comparing the three transport schemes, ultimately there is a trade-off between the complexity in the RF electronic and the optoelectronic interfaces within the base-station. One of the key challenges in implementing mm-wave fiber-wireless access systems is to efficiently distribute the wireless signals while maintaining a functionally simple and compact BS design. Amongst the schemes, RF-over-fiber transport scheme has the potential to simplify the BS design for mm-wave fiber-wireless systems.It is well-established that the total capacity and throughput of a mm-wave fiber-wireless system can be greatly enhanced with efficient optical fiber architectures using wavelength-division multiplexing (WDM) optical networking technology. With the incorporation of WDM in mm-wave fiber-wireless approaches, a fast deployment route for these systems may be achieved. By leveraging the optical network infrastructure already existing in the access and metro network domains, unused fibers may be utilized as the means of communications between the CO and the antenna BSs. It is therefore equally important that mm-wave fiber-wireless access technology can coexist with other optical access technologies; being able to merge/integrate within the existing infrastructure and ensure transparency in the remote access nodes.Millimeter-wave fiber-wireless technology has been actively pursued and investigated in the past two decades. In this paper, we have provided an overview of the research progress in this area ranging from signal transportation, impairments to WDM based subsystem interfaces and optical-wireless integrated access infrastructure. In presenting this overview, we have focused the discussions on techniques and technologies that have been investigated and demonstrated for the implementation of high-performance mm-wave fiber-wireless systems.

2.1.2 OFDM FOR OPTICAL COMMUNICATION

Orthogonal frequency division multiplexing (OFDM) is a modulation technique which is now used in most new and emerging broadband wired and wireless communication systems because it is an effective solution to inter symbol interference caused by a dispersive channel. Very recently a number of researchershave shown that OFDM is also a promising technology for optical communications. This paper gives a tutorial overview of OFDM highlighting the aspects that are likely to be important in optical applications. To achieve good performance in optical systems OFDM must be adapted in various ways. The constraints imposed by single mode optical fiber, multimode optical fiber and optical wireless are discussed and the new forms of optical OFDM which have been developed are outlined. The main drawbacks of OFDM are its high peak to average power ratio and its sensitivity to phase noise and frequency offset. The impairments that these cause are described and their implications for optical systems discussed.This paper presents a tutorial introduction to OFDM. A typical OFDM transmitter and receiver are described and the roles of the main signal processing blocks explained. The time and frequency domain signals at various points in the system are described. It is shown that if a cyclic prefix is added to each OFDM symbol, any linear distortion introduced by the channel can be equalized by a single tap equalizer. This process is explained by considering the effect of a simple two-path channel on the component of the transmitted signal due to one subcarrier frequency. Throughout the description particular emphasis is given to those aspects of an OFDM system that are often misunderstood.Although the theoretical basis for OFDM was laid several decades ago, and OFDM became the basis of many communications standards for wireless and wired applications in the 1990s, it is only very recently that the application of OFDMto optical communication has been considered. This is partly because of the apparent incompatibility between OFDM modulation and conventional optical systems. A number of forms of OFDM which overcome these incompatibilities in various ways have been developed for a variety of optical applications. These are classified into those appropriate for optical wireless and multimode fiber applications and those for single mode fiber. For the former, intensity modulation should be used, while for the latter the OFDM signal should be carried on the field of optical signal. OFDM has a number of drawbacks including its high peak-to average power ratio and sensitivity to frequency offset and phase noise. These are described and their likely implications for optical communications discussed.In conclusion, OFDM is a very promising technology for optical communications, but the very different constraints introduced open up many new interesting avenues for research.

2.1.3 OFDM VERSUS SINGLE CARRIER TRANSMISSION FOR 100 Gbps OPTICAL COMMUNICATION This paper analyze the orthogonal frequency division multiplexing (OFDM) technique in long-haul next generation optical communication links and compare it with the well-established single-carrier (SC) data transmission using high-level modulation formats and coherent detection. The analysis of the two alternative solutions is carried out in the 100 Gbps scenario, which is commonly considered to be the next upgrade of existing optical links, with special emphasis on quaternary phase-shift keying (QPSK) modulations. The comparison between OFDM and SC takes into account the main linear and nonlinear impairments of the optical channel, e.g., group velocity dispersion (GVD), polarization mode dispersion (PMD), self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave mixing (FWM), as well as the phase noise due to transmit and receive lasers, their relativefrequency offset, other synchronization aspects, the overall complexity, the power and spectral efficiency, and the technological constraints.OFDM is an effective way to decompose a time-dispersive channel into a set of independent and parallel sub channels .Hence, when OFDM is employed and the cyclic prefix is long enough to ensure perfect compensation of the dispersion a trivial SbS detector can be used for data detection. This is the reason for the recent widespread use of OFDM in several wireless communication systems, where, as mentioned, SC transmissions with SbS detection are not able to achieve channel capacity.The main advantage of OFDM, which made it a winning technology for a large number of communication standards like DVB, DSL, WiMax, is the possibility to match the transmitted signal spectrum to the particular channel characteristics, like non-flat amplitude responses, due to fading, or multipath, etc. The effective exploitation of the channel is also possible through several techniques which improve OFDM efficiency, like bit and power loading, pulse shaping, channel shortening. All these peculiar features of OFDM have a lower impact in optical communication, as the channel amplitude response is almost flat (neglecting minor impairments like polarization dependent losses and imperfections in the frequency responses of electrical and optical filters), and the high data rates prevent taking advantage from sophisticated signal processing.The main reasons that drive the interest in OFDM are the simple compensation of linear impairments and the implicit parallelization given by the IFFT/FFT operations, which lowers the processing speed at the receiver. As demonstrated, a proper design of a SC transmission system can provide the same benefits without incurring in the tight constraints of OFDM in terms of frequency and phase errors, in its penalty due to nonlinear effects, in its lack of spectral and energy efficiency, in its difficult implementation due to technological limits.A possible advantage of OFDM could be its scalability with higher bit rates, but, it is almost impossible nowadays to implement a convenient OFDM system even for 100 Gbps. As a conclusion, there are nowadays significant technological limits to a cheap and convenient implementation of OFDM, whereas SC modulations take advantage of a consolidated know-how and technology, relegating the efforts for realtime implementations only to the progress in the speed of optical and electronic devices.

2.1.4. PHASE NOISE SUPPRESSION OF OPTICAL OFDM SIGNALS IN 60-GHz RoF TRANSMISSION SYSTEM

The dispersion-induced phase noise (PN) in an OFDM RoF system at 60 GHz leads to not only subcarrier phase rotation (PRT) but also inter carrier interference (ICI) to severely degrade the transmission performance, when a commercial cost-effective DFB laser with the linewidth of several MHz is adopted. To mitigate both PRT and ICI, a post PN suppression algorithm is proposed, and it does not require any bandwidth-consuming pilot tone. For a 25.78-Gbps 16-QAM OFDM RoF signal using the laser with 1.8-MHz linewidth, employing the algorithm can extend the maximum transmission distance which corresponds to 3-dBm power penalty at the BER of 2x 10^-3 from 75 km to more than 115 km, i.e. 50% increment of transmission distance.Orthogonal frequency-division multiplexing (OFDM) has been considered as a prominent format for a variety of digital communication systems owing to its robustness against frequency selective fading and inter-symbol interference caused by both multipath in air-link and chromatic dispersion in optical fibers . Besides, highly spectral efficiency and flexibility of OFDM makes it attractive for narrowband applications (i.e. 7-GHz license-free band at 60 GHz) beyond the 10-Gb/s regime. However, an OFDM signal is very sensitive to PN which will destroy the orthogonality among subcarriers. Hence, the dispersion-induced PN from nonzero laser linewidth causes not only phase rotation (PRT) on each subcarrier but also intercarrier interference (ICI) among subcarrier to greatly reduce the fiber transmission distance. In particular, due to the increasing interest in optically amplified long-reach passive optical networks (PONs) , the required reach of a hybrid RoF/PON system which is expected to provide various services is up to 100 km.In this work, by simplifying the profile of dispersion, use the algorithm from the area of wireless communication to estimate and suppress both PRT and ICI. The proposed PN suppression (PNS) algorithm does not require any bandwidth-consuming pilot tones to allow full utilization of 7-GHz unlicensed band at 60 GHz. Experimental results show that the algorithm can suppress the performance degradation caused by dispersion-induced PN to extend the transmission distance and/or to reduce the requirement of laser linewidth. Employing the algorithm, the 25.78-Gbps 16-QAM OFDM signals at 60 GHz can reach the BER of 2 X10^-3 after 100-km transmission, and the extension of transmission distance is more than 40 km using the laser with 1.3 or 1.8-MHz linewidth.

2.1.5 FIBER NON-LINEARITY MITIGATION BY PAPR REDUCTION IN COHERENT OPTICAL OFDM SYSTEMS VIA ACTIVE CONSTELLATION EXTENSIONOrthogonal Frequency-Division Multiplexing (OFDM) has become a state-of-the-art technology in wireless communications over the last 10 years, and more recently has been proposed in optical systems. Coherent optical OFDM (CO-OFDM) has been demonstrated to combat fiber chromatic dispersion and polarization-mode dispersion using the robust equalization that is a major advantage of OFDM systems.One of the disadvantages of OFDM is a high peak-to-average power ratio (PAPR) due to a central limit theorem effect when modulating many symbols via an IFFT. In COOFDM systems, the PAPR problem increases the presence of nonlinear effects, which are proportional to the instantaneous signal power . This paper consider active constellation extension (ACE), an intelligent PAPR reduction algorithm we first introduced for wireless OFDM, to mitigate nonlinear effects. This paper demonstrates its effectiveness through simulation, and with such a scheme and find that the nonlinearity threshold is improved by 1.5 dB and 0.8 dB, respectively, at a reach of 400 km and 1040 km. This nonlinearity threshold enhancement is directly translated into OSNR margin improvement.Many techniques exist to reduce the PAPR of an OFDM symbol, with the most basic being to simply clip the signal. This results in distortion which movesconstellation points from their original locations, and for a majority of symbols, causes the symbol to move closer to other constellation points, resulting in an increased BER.Assume the upper-right constellation point is to be transmitted on some OFDM subcarrier. If extending that point anywhere into the semi-infinite grey region reduces the PAPR, then we have reduced the peak power while not increasing the BER. In fact, extending outwards reduces the BER for that symbol (along with an associated increase in the OFDM symbol average power). This is the main idea of ACE: reduce the PAPR by maintaining the minimum distance between constellation points and only allowing extension away from the maximum-likelihood decision boundaries. This results in a convex optimization problem, in which a suboptimal solution is obtained using an efficient smart gradient-project approach. A main advantage of the ACE algorithm is that it does not modify the symbol decision regions, and thus does not require extra overhead for OFDM reception, nor does it need any transmitted side-information like many PAPR techniques require .

CHAPTER 3METHODOLOGY

In this section, the system architecture of the FiWi networks is rstly outlined, and then the proposed MB-OFDM frame structure for FiWi networks is addressed in detail. Finally, the signal model over the ber-wireless channels is presented.

3.1 FiWi SYSTEM ARCHITECTURE

Overall system architecture of the FiWi networks integrating the ber subsystems and the wireless subsystems is shown in Fig. 1. The wireless base stations and access points in wireless radio subsystems could provide ubiquitous and mobile access for end users, while the ber subsystem could offer very high throughput by exploiting the central ofces (CO) to interconnect all the wireless base stations and access points. In addition, most high-complexity processing originally implemented in the wireless base stations and access points can be shifted to the central ofces, thus more dense wireless antennas could be deployed with low cost, and obviously increased data rate and enhanced coverage are expected for future high-speed FiWi networks.

Fig 1 : System architecture of the FiWi network

3.2 PROPOSED MB-OFDM FRAME STRUCTURE

The frame structure comparison between the conventional OFDM and the proposed MB-OFDM schemes for FiWi networks is illustrated in Fig. 2.

Fig 2 : Time-domain frame structure comparison: (a) The conventional OFDM scheme; (b) The proposed MB-OFDM scheme.

As shown in Fig. 2 (a), the transmission frame for most existing OFDM-based FiWi networks is composed of one preamble used for joint timing/frequency synchronization and channel estimation, and the following S payload OFDM symbols using the CP of length M as the guard interval of the IFFT data block of length N. Since the ber-wireless channel is varying slowly, the channel information obtained during the preamble could be used to demodulate the following S OFDM symbols. For the mth OFDM symbol, the time-domain IFFT data block generated by applying the N-point IFFT to the frequency-domain data as

(1)

On the contrary, as shown in Fig. 2 (b), the proposed MB-OFDM transmission frame is composed of one preamble and the following two parts:1) One super-block data of length SN comprising S data blocks without guard interval (e.g., CP in most OFDM systems) among them. The mth data block is further divided into G sub- blocks , and each sub-block of length = N/G is generated by -point IFFT;2) One ZP z = 0M1, which is used as the guard interval of the super-block data d. Essentially, the super-block data d and the following ZP z form a virtual" ZP-OFDM symbol, whereby the data part is composed of S blocks corresponding to the S IFFT data blocks of the conventional OFDM symbols (here, we use the term virtual" to distinguish the proposed frame structure from the standard ZP-OFDM signal structure, whereby every IFFT data block has a ZP as the guard interval) .

Fig 3 : Transmitter architecture comparison: (a) The conventional OFDM scheme; (b) The proposed MB-OFDM scheme.

The preamble we have used here could achieve accurate timing and frequency synchronization due to the specially designed time-domain structure, and it can also be used for channel estimation. Normally, the length of the preamble is the same as that of a normal OFDM symbol.

The rst key difference between the proposed MB-OFDM scheme and the conventional one is the guard interval. Traditionally, each OFDM symbol has its own CP (or ZP in ZP- OFDM systems) as the guard interval, and there are totally S CPs as shown in Fig. 2 (a). Since the length of guard interval is usually large enough to combat the composite effects of wireless multipath as well as optical CD and PMD in FiWi networks the frequently inserted CPs obviously reduce the spectral efciency. However, the proposed scheme utilizes only one ZP as the guard interval for all of the S data blocks. The only one guard interval instead of S guard intervals could also be efciently used for reliable data demodulation, thus the proposed MB-OFDM scheme could achieve higher spectral efciency.The second obvious difference between the proposed MB- OFDM solution and the conventional one lies in the way to generate the time-domain payload data. As illustrated in Fig. 2 and further explained in Fig. 3, the gth sub-block (g =0 ,1,,G1) with in the mth data block dm is generated by applying the point IFFT ( = N/G) to the corresponding frequency-domain data as below : (2)

Note that the G N/G-point IFFTs in Fig. 3(b) can be actually implemented by a single N/G-point IFFT for G times to generate G sub-block .Fig. 4 compares the frequency-domain signal structure of the proposed MB-OFDM with the conventional OFDM schemes with the same signal bandwidth B. For the conventional OFDM system where each time-domain IFFT data block is generated by N-point IFFT, the subcarrier-spacing f as shown in Fig. 4(a) should be

(3)where T is the duration of the corresponding time-domain IFFT data block, Ts is the baseband sampling period, and B =1/Ts is the signal bandwidth. However, without changing the signal bandwidth (and the baseband sampling period as well), the proposed MB-OFDM scheme generates each sub block by the -point IFFT (= N/G), so the subcarrier- spacing as shown in Fig. 4(b) becomes (4)where = Ts is the duration of the corresponding sub-block in the time domain. It is clear that the subcarrier spacing would be enlarged when the smaller IFFT size is used.

Fig 4 : Comparison of frequency-domain signal structure: (a) The conven- tional OFDM with the N-point IFFT; (b) The proposed MB-OFDM with the N-point IFFT.

In conventional OFDM-based FiWi networks, the IFFT size is usually congured large to keep the overhead caused by the frequently inserted CPs within a certain degree, so the resulting subcarrier spacing is usually much smaller than the system coherent bandwidth . Therefore, it is possible to reduce the IFFT size to some extent without sacricing the system performance. However, could not be too small. In the extreme case of =1, the proposed MB-OFDM scheme becomes the single-carrier transmission, and the merits of multi-carrier transmission would vanish. Therefore, the proposed MB-OFDM scheme provides a exible compromise between the conventional OFDM multi-carrier and single- carrier transmissions by conguring the appropriate parameter according to the system requirements.The design of using smaller IFFT size does not sacrice the spectral efciency since multiple data blocks in MB-OFDM share only one guard interval. On the contrary, a conventional OFDM system usually adopts the relatively large IFFT size N because it requires every IFFT block to have its own guard interval. The direct reason of using the reduced IFFT size to generate the time-domain sub-blocks in MB-OFDM is to reduce the PAPR and alleviate the sensitivity to residual CFO of OFDM signals.

3.3 CHANNEL MODEL FOR FIBER-WIRELESS SYSTEMS

After the ber-wireless channel, the received data corresponding to the transmitted super- block data d should be (5)where dnl is the (nl)th entry of the super-block data d as shown in Fig. 3, is the time-domain channel impulse response (CIR) of the ber-wireless link, hl denotes the gain of the lth path including the wireless multi- path effect as well as the optical impairment effects like CD, PMD, and polarization dependent loss (PDL) , L denotes the channel length, wn presents the additive white Gussian noise (AWGN) caused by the optical amplied spontaneous emission (ASE) and electronic noise. Note that the ZP length M should be larger than the channel length L to avoid the interferences between adjacent transmission frames.

3.4 RECEIVER DESIGN

The receiver architecture of the proposed MB-OFDM transmission scheme is shown in Fig 5. After ber-wireless front end and analog-to-digital (A/D) conversion, the preamble is used for joint timing and frequency synchronization, and it can also be used for channel estimation . Then, we propose the hybrid-domain channel equalization to remove the ber-wireless channel impairments. Finally, constellation demapping and channel decoding are used to recover the transmitted signal. Motivated by the fact that the MB-OFDM scheme actually unies the modeling of the multi-carrier and single- carrier transmissions, we propose the hybrid-domain channel equalization of the MB-OFDM signal as illustrated by the dashed block of Fig. 5. Unlike the classical frequency-domain equalization (FDE) where one N-point FFT is applied to obtain the frequency-domain data for each OFDM symbol, the proposed hybrid-domain channel equalization is composed of the time-domain signal detection with a pair of SN-point FFT/IFFT and then the frequency-domain signal recovery with SG point FFTs.

Fig 5 : Receiver architecture of the proposed MB-OFDM scheme with reduced IFFT size and guard interval.

The slow time-varying property of channel in FiWi networks will be exploited to achieve the cyclicity reconstruction of the received virtual" ZP-OFDM symbol of large size by extending the classical overlap and add (OLA) algorithm used for a single standard ZP-OFDM symbol of small size. The OLA algorithm simply adds the tail" caused by the multipath effect in the ZP, to the front part of the received super-block data r of length SN, so the obtained dat actually becomes the cyclic convolution between the transmitted super-block data d and the CIR h as (6)

where w denotes the AWGN vector. Therefore, ZP in the proposed MB-OFDM acts similarly to the CP in conventional OFDM, whereby the cyclicity reconstruction could be achieved by directly removing the CP. Second, a pair of SN-point FFT/IFFT is used to recover the time-domain transmitted super-block data d after compensating for the ber-wireless channel impairments in the frequency domain. The SN-point FFT is applied to the time- domain signal in (6) to produce the frequency-domain signal (7)where = is the FFT output of d (note that distinguishes the direct stacking of since different IFFT sizes are used), = ,denotes the channel frequency response (CFR) corresponding to the CIR h, and = denotes the frequency-domain AWGN. Then, the time-domain transmitted super-block data d could be recovered as (8)where is obtained by the preamble-based channel estimation . From (8) we can nd that the inter-symbol interference caused by the multipath channel effect over the entire super-block d has been compensated for.Third, as illustrated in Fig. 6, the recovered time-domain super-block data is divided into SG length-N/ G sub-blocks according to the frame structure at the transmitter, and then SG-point FFTs are used to restore the transmitted signal in the frequency domain.

CHAPTER 4PERFORMANCE ANALYSIS

The performance analysis of the proposed MB-OFDM transmission scheme in terms of spectral efciency, PAPR reduction, and the computational complexity is done. The potential extensions of the proposed scheme are also discussed.

4.1 SPECTRAL EFFICIENCY

Due to the overhead caused by the preamble, especially the guard intervals, the spectral efciency of the OFDM based FiWi networks normalized by the ideal case without any overhead can be expressed in the percentage form as

where K is the number of guard intervals used in the transmission frame, so K = S for the conventional OFDM scheme while K =1 for the proposed MB-OFDM scheme, denotes the preamble length, and usually is adopted.Table I compares the spectral efciency between the conventional and proposed OFDM schemes for FiWi networks, whereby the typical values N = 256,M = 32are used for both schemes. It is clear that about 10% higher spectral efciency could be achieved by the proposed scheme due to the removal of (S 1) guard intervals.TABLE 1COMPARISON OF SPECTRAL EFFICIENCY

4.2 PAPR REDUCTION

It is highly desired to reduce the PAPR of OFDM signals in FiWi networks, since PAPR is the main source of nonlinear effect in those systems. The PAPR of the OFDM signal generated by a N-point IFFT is dened as

(10)Since the input frequency-domain data are random, the maximal PAPR could be as large as PAPRmax = 10log10N in the worst case that all the data are added in phase during the IFFT process to generate the corresponding time-domain signal . Such extreme case seldom appears in practice, so the widely used metric for evaluating PAPR is the complementary cumulative distribution function (CCDF) Fc(), which is dened as the probability that the PAPR is larger than a given threshold . It has been proved that Fc() = Pr(PAPR >)=1Pr(PAPR ) =, (11)which indicates that the CCDF is only dependent on the IFFT size N for a given threshold . Thus, the smaller IFFT size = N/G in the proposed MB-OFDM scheme would result in lower CCDF of the PAPR. It is worth noting that the improvement of PAPR by reducing the IFFT size has no penalty of signal distortion or bandwidth extension as the existing PAPR reduction techniques.The PAPR reduction due to smaller IFFT size could also be intuitively explained by the fact that the proposed MB- OFDM is essentially a compromise between the multi-carrier and signal-carrier transmission schemes, whereby multi-carrier signal has high PAPR while single-carrier signal enjoys low PARR.

4.3 COMPUTATIONAL COMPLEXITY

At the transmitter side, Fig. 3 indicates that the proposed scheme utilizes SG N/G point IFFTs instead of SN-point IFFTs to generate the time domain data. Considering the complexity of a N-point IFFT in terms of the number of multiplications is log2N, the proposed MB-OFDM scheme has the complexity of SGlog2()= log2(), instead of log2N for the conventional OFDM scheme. Clearly, a computation reduction of log2G could be achieved. At the receiver side, the hybrid-domain channel equalization requires a simple addition for OLA-based cyclicity reconstruction, a pair of SN-point FFT/IFFT for single-carrier signal demodulation, and SGpoint FFTs for multi-carrier signal recovery. By contrast, the conventional OFDM scheme only needs S N-point FFTs to produce the frequency-domain signals for the S length-N IFFT data blocks (note that the preamble-based channel estimate could be used to demodulate all S OFDM symbols). Therefore, the receiver of the proposed MB-OFDM needs SN log2 more times of multiplications than the conventional OFDM. However, since FFT/IFFT could be efciently realized by powerful modern digital processors, the increased complexity is affordable, especially for the proposed scheme having higher spectral efciency, reduced PAPR, and alleviated sensitivity to the residual CFO.

CHAPTER 5SIMULATION RESULTS

This section provides simulation results to evaluate the performance of the proposed MB-OFDM transmission scheme for future high-speed FiWi networks. In the simulation, we choose the LDPC code due to its low complexity and very low error oor resulted from the absence of 4-cycles in its graph. A 2.5 Gbps random data stream is coded by LDPC, and then 16 QAM modulation is adopted for constellation mapping. The constellation signal is used to generate the MB-OFDM transmission frame with the parameters N = 256,M = N/8 = 32 ,S = 10 ,G =8. The obtained OFDM signal is up-converted to 5.65 GHz , which is then used to modulate the continuous-wave external cavity laser (ELC) with the linewidth of 100 kHz at MZM. After that, the erbium-doped ber amplier (EDFA) is used to boost the optical OFDM signal, and the optical lter with 0.8 nm bandwidth is used to lter out the outband noise. After the transmission through a 20 km single-mode ber (SMF) with the attenuation of 0.2 dB/km, the optical CD of 40 ps/nm/km and the PMD with the mean differential group delay of 50 ps, a 10 GHz bandwidth photodiode (PD) is used to convert the optical OFDM signal into radio frequency signal. At the wireless receiver which is 5 m away from the wireless transmitter, a 20 dB gain low-noise amplier (LNA) is followed by a 40 GSa/s analog-to-digital convertor (ADC) to capture the wireless OFDM signal, and then the digital receiver algorithms as detailed in Chapter 3 are used for signal recovery. The sum-product algorithm (SPA) for LDPC decoding is adopted for iterative soft-decision decoding with the iteration number of 50. The Indoor A channel model dened by ITU is used to emulate the wireless multipath channel, and the received OSNR is dened with the 0.1 nm noise bandwidth.Fig. 6 shows the CCDF of the PAPR when different IFFT sizes are used. When N = 256, the IFFT size in conventional OFDM transmission scheme is 256. However, the proposed MB-OFDM scheme could adopt obviously reduced IFFT size = N/G to generate the corresponding sub-blocks of length as

Fig 6 : CCDF of the PAPR when different IFFT sizes are used.

shown in Figs. 2 and 3. The theoretical CCDF (11) is also included in Fig. 7 as the benchmark for comparison. It is clear that the reduced IFFT size could lead to the PAPR reduction, e.g., for the clipping ratio of 0.01, the PAPR is reduced by 1.2 dB and 1.9 dB when = 32and = 16 respectively. We could also observe that the simulated CCDF coincides with the theoretical result (11).

CONCLUSION

In this paper, the multi-block OFDM (MB-OFDM) transmission scheme is proposed for high-speed FiWi networks to simultaneously achieve improved spectral efciency, reduced PAPR and mitigated sensitivity to CFO. One key idea is that the slow time-varying property of the ber-wireless channel can be utilized to improve the spectral efciency, which is achieved by sharing one guard interval among multiple data blocks.. The above merits, which have been demonstrated by the simulation results, are achieved at the cost of increased yet affordable receiver complexity. The proposed scheme could also be easily extended to other ber/wireless/FiWi networks. The future work is to incorporate the proposed scheme with wireless multiple-input multiple-output (MIMO) and optical polarization division multiplexing (PDM) techniques to further improve the system capacity and reliability.

FUTURE WORKS

The proposed MB-OFDM scheme, or at least some key components of this scheme, can be directly used or easily extended to other OFDM-based wireless and optical communication systems. First, the MB-OFDM scheme is also valid when CP instead of ZP is used as the guard interval in ber/wireless/FiWi networks with the simple modication: replace ZP with CP and put the CP in front of the super-block data. As a result, the cyclicity reconstruction via OLA is not necessary any more, but other procedures at the transmitter and receiver remain unchanged. Thus, higher spectral efciency, reduced PAPR and mitigated vulnerability to CFO could still be achieved.Second, single-carrier transmission could also be used for FiWi networks, and the proposed MB-OFDM scheme becomes a single-carrier solution when G = N (i.e., = 1). Thus, the idea of using only one guard interval for multiple data blocks can be also used by single-carrier systems to achieve higher spectral efciency. Third, for some applications, especially for wireless communication systems, the channel may change very fast, then it is preferred to reduce the number of data blocks within a super-block to combat the fast channel variation. In the extreme case, S =1could be congured without improving the spectral efciency. However, the smaller IFFT size used in the proposed MB-OFDM scheme could still be used to reduce the PAPR and mitigate the sensitivity to the residual CFO.

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