Post on 20-Jan-2023
transcript
Paper on:
Performance analysis of OFDM basedsystem with optical components forimproving bandwidth efficiency and
reducing ICI and ISI
Submitted By:
Parasharam Neungare
parshuramneungare@gmail.com
8108698126
Rohit Bhoir rohitbhoir.105@gmail.com
9867330663
Rinika Doiphode rinika.doiphode@gmail.com 9773428586Yogesh Kesarkar ykesarkar@gmail.com 902907882
Under the Guidance of:
Prof. B. U. Rindhe (HoD EXTC)
DEPARTMENT OFELECTRONICS AND TELECOMMUNICATION ENGINEERING
Abstract: Orthogonal Frequency DivisionMultiplexing (OFDM) is a frequencydivision multiplexing scheme utilizedas a digital multi-carrier modulationtechnique. OFDM has many advantagesover other modulation techniques suchas a high resistance to inter-symbolinterference (ISI) and is robustagainst fading caused by multipathpropagation. Optical fiber cable as atransmission media is used fordistortion less transmission of dataat a very higher data speed. OFCcable has a lot of advantages overother media and OFDM over OFC cablewill provide data speeds at a veryhigh speed and with very less losses.The main aim of our project is todesign Network Components for OFDMbased optical networks. Variousparameters considering the advantagesand disadvantages of OFDM and OFCcables will be taken intoconsideration while designing thesystem such that it delivers highperformance with minimum distortionand low bit error rate (BER). We will be using RSoft’s OptSim &SIMULINK software for implementationof our project to study opticaltransmitter and receiver for OFDMbased optical networks.
Keywords: OOFDM, ISI, OSNR, MZIM, EDFA,SOA,DCF, CO-OFDM, SMF,MMF,BER,WDM
Aim:This paper will focus on “Performance analysis of OFDM based system with optical components for improving bandwidth efficiency and reducingICI and ISI”.
INTRODUCTIONWireless communications is an emerging field, which has seen enormous growth in the last several years. The huge uptake rate of mobile phone technology, Wireless Local Area Networks (WLAN) and the exponential growthof the Internet have resulted in an increased demand for new methods of obtaining high capacity wireless networks. Most WLAN systems currently use the IEEE802.11b standard, which provides a maximum data rate of 11 Mbps. Newer WLAN standards such as IEEE802.11a and HiperLAN2are based on Orthogonal FrequencyDivision Multiplexing (OFDM) technology and provide a much higher data rate of 54 Mbps. However systems of the near future will require WLANs with data rates greater than 100 Mbps,and so there is a need to furtherimprove the spectral efficiency and data capacity of OFDM systemsin WLAN applications.
BASIC PRINCIPLE OF OFDMA single carrier system modulatesinformation onto one carrier using frequency, phase or amplitude adjustment of the carrier. Frequency division multiplexing (FDM) extends the concept of single carrier modulation by using multiple subcarriers within the same single channel. The total data rate to be sent in the channel isdivided between various
subcarriers. The data do not haveto be divided evenly nor they have to originate from the same information source. Advantages include using separate modulationand demodulation customized to a particular type of data, or sending out banks of dissimilar data that can be best sent using multiple, and possibly different modulation schemes. Current National Television Systems Committee (NTSC) television and Frequency Modulation (FM) stereo multiplex are good examples of FDM. It offers an advantage over single-carrier modulation in terms of narrowband frequency interference, since this interference will only affect oneof the frequency subbands. The other subcarriers will not be affected by the interference. Since each subcarrier has a lowerinformation rate, the data symbolperiods in a digital system will be longer, adding some additionalimmunity to impulse noise and reflections. FDM systems usually require a guard band between modulated subcarriers to prevent the spectrum of one subcarrier from interfering with another. These guard bands lower the system’s effective information rate when compared to a single carrier system with similar modulation.
PROBLEM DEFINITIONIt is known that OFDM is spectrally very efficient and robust in dealing with the
frequency selective wireless fading channels, yet some combinations of spread spectrum techniques and OFDM are being considered to develop even bettersystems [17]. Though some works give details of their performance, it is important to compare them against OFDM in the same test conditions, which include among others non ideal receiver operations. Thus the first problem addressed is the performance evaluations and enhancements to multicarrier spread spectrum techniques for indoor conditions. This analysis has been extended to the conditions when the receiver cannot obtain perfect frequency synchronization and there exists channel estimation error. Though OFDM brings several bene¯ts, yet its performance depends on the channel estimation accuracy and residual phase error due to remaining uncorrected carrier frequency. Therefore pilot sub carriers are embedded among data sub carriers [11] so that good estimates of these errors can be obtained. Good estimation of the phase errors helps in better compensation of the errors which leads to improved performance. Since pilot sub carriers do not carry information bits they are overhead and causes loss in bandwidth e±ciency. Investigationof methods of reducing this loss in bandwidth e±ciency to improve the spectral e±ciency of the system is thus considered as a
problem area in this thesis. One of the strengths of OFDM is its closely packed sub carriers. The sub carrier bandwidth is a primary design parameter in OFDM systems. Carrier frequency and Doppler frequency spread cause ICI which severely degrade the performance of OFDM based systems. There are several algorithms to estimate and compensate the carrier. However, the Doppler frequency spread consists of multiple frequenc andcannot be compensated by carrier compensation algorithms. Algorithms to address ICI due to Doppler are very complex from implementation point of view [21,22]. The ratio of the maximum uncorrectable residual carrier and the maximum Doppler frequencyspread to the sub carrier spacingis an important factor on which the ICI depends. The maximum value of the ratio is usually kept within 2% [23]. Thus once the maximum distortion is known the maximum value of sub carrier bandwidth gets decided. The larger the maximum distortions, the larger is the value of the sub carrier bandwidth needed in order to keep the ratio within the limits mentioned above. The useful signal duration of the symbol is inversely proportional to the sub carrier spacing. Therefore the larger thesub carrier spacing the smaller is the useful signal duration.
Basic OFDM transmitter and receiver:-Transmitter:-
An OFDM carrier signal is the sumof a number of orthogonal sub-carriers, with baseband data on each sub-carrier being independently modulated commonly using some type of quadrature amplitude modulation (QAM) or phase-shift keying (PSK). Thiscomposite baseband signal is typically used to modulate a main RF carrier.
is a serial stream of binary digits. By inverse multiplexing, these are first demultiplexed into parallel streams, and eachone mapped to a (possibly complex) symbol stream using somemodulation constellation (QAM, PSK, etc.). Note that the constellations may be different, so some streams may carry a higher bit-rate than others.An inverse FFT is computed on each set of symbols, giving a set
of complex time-domain samples. These samples are then quadrature-mixed to passbandin the standard way. The real andimaginary components are first converted to the analogue domain using digital-to-analogue converters (DACs); the analogue signals are then used to modulate cosine and sine waves atthe carrierfrequency, , respectively. These signals are then summed to give the transmission signal, .
Receiver:-
The receiver picks up the signal , which is then quadrature-mixed down to basebandusing cosine and sine waves at the carrier frequency. This also creates signals centered on , so low-pass filters are used to reject these. The baseband signals are then sampled and
digitised using analog-to-digitalconverters (ADCs), and a forward FFT is used to convert back to the frequency domain.This returns parallel streams, each of which is converted to a binary stream using an appropriate symbol detector. These streams are then re-combined into a serial stream, , which is an estimate of the original binary stream at the transmitter.
DESIGN CONSIDERATIONS FOR OFDM SYSTEM:-
Bandwidth
The wider the bandwidth, themore probable that the system overcome the correlation bandwidth of thechannel. Short delay echoes are the main problems to overcome, and as these are always present there is no hard bound.
The narrower the bandwidth, the more likely it is that the whole signal will be affected
1.5MHz is used in the mobileenvironment and in portable radio reception.
Bit Rate
On each carrier, the modulation system used is QPSK, with the carriers
are separated by a gap of around 1/Ts, where Ts is symbol period. The
maximum bit rate available is so 2bit/s/Hz of the bandwidth.
For Digital Audio Broadcasting (DAB), this brings the useful bit-rate down about 1 bit/s/Hz of thebandwidth. Therefore DAB system will provide just less than 1.5Mbit/s of useful data.
Carriers
About the number of carriers, themore they are, the greater is theresolution of the diversity offered by the system
OFDM Advantages:-
E ciently deals with multi-ffipath fading
E ciently deals with ffichannel delay spread
Enhanced channel capacity
E ective solution to ffintersymbol interference caused by a dispersive channel
High bandwidth efficiency
Scalable to high data rates.
Has excellent ICI performance.
Does not require channel equalization.
ISI a ects at most one ffsymbol, and equalization is simpli edfi
OFDM has excellent robustness in multi-path environments.
OFDM Disadvantages:-
High sensitivity inter-channel interference (ICI)
(a) OFDM is sensitive to frequency, clock and phase o setff
(b) The OFDM time-domain signal has a relatively large peak-to-average ratio
tends to reduce the power e ciency of the RF ampli erffi fi
non-linear ampli cation fidestroys the orthogonality of the OFDM signal and intro- duced out-of-band radiation
Very sensitive to frequency errors (Transmitter and Receiver o set)ff
Bandwidth and power loss due to the guard interval can be signi cantfi
The OFDM signal has a noise like amplitude with a very large dynamic range, therefore it requires RF
power ampli ers with a high fipeak to average power ratio.
More complex than single-carrier modulation.
Application:-
DAB(Digital Audio Broadcasting):
DAB-OFDM forms the basis forthe digital audio broadcasting (DAB) standard in the European market and is the next step in evolution beyond FM radio broadcasting providing interference free transmission.
HDTV(High De nition fiTelevision)
Wireless LAN
IEEE 802.16 and IEEE 802.20
Wireless ATM transmission system
Improvement of Bandwidth Efficiency using Equilizers:-
The high computational efficiency of FFT-baseddemodulation in the conventional OFDMsystem inherently relies on the data that are free of both interchannel interference and interblock interference. This results in loss intwo aspects. In the frequency domain,to avoid interchannel interference the total bandwidth must be larger than the Nyquist rate, thus considerably reducing the efficiency of bandwidth usage. In the time domain, in order for the collected
data to be free of interblock interference, a guard interval longerthan channel delay has to be used in each OFDM period. There have been several studies to increase the efficiency of bandwidth usage for theconventional OFDM system. These studies focus on the reduction of therequired guard interval while still retaining the characteristics of the conventional OFDM that uses ICI- and IBI-free data and FFT in demodulation. In this paper, we consider channel equalization of bandwidth-efficient OFDM over multipath fading channels.
Conventional OFDM system withbandwidth efficiency=67%
Proposed OFDM system withBandwidth efficiency= 100%
Comparison:-
ICI Cancellation:-
Inter-carrier interference (ICI) which results from thefrequency offset degrades the performance of the OFDM system.
Two methods were explored inthis project for mitigation of the ICI.
The ICI self-cancellation (SC) scheme
The extended Kalman filtering (EKF) method for estimation and cancellation of the frequency offset has been investigated
1) The ICI self-cancellation (SC) scheme :-
IMPROVED DFT-IDFT-BASED ICI-SELF CANCELLATIONSCHEME .For the proposed scheme the system model is shown in Fig.
In this scheme, a block of N point input data is equally divided into two parts each of N/2 data points, and then eachdata block is discrete Fourier transformed. After this, the N point data is fed to the IDFT block.
Magnitude of ICI components of standardOFDM, DFT-based, and proposed schemes
By analyzing the ICI coefficients and BER plots for different schemes, it can be concluded that the proposed scheme produces better ICI reduction as compared to theDFT-based ICI-self cancellation scheme for small values of normalized frequency offset (ε < 0.3) without anyreduction in the spectral efficiency ofthe system. For large values of frequency offset there is only slight improvement in the performance of the proposed scheme in terms of BER. Thus, the proposed scheme reduces the ICI effectively as compared to the DFT-based ICI selfcancellationscheme.
2) The extended Kalman filtering (EKF) method:-the Extended Kalman Filter (EKF) method statistically estimate the frequency offset and correct the offset using the estimated value at the receiver.
For M=2 PSK modulation in 5200 transmitted bits 1135 errors occur with a BER of 0.047473 for offset of 0.3 and EBNO = 15 for Basic OFDM. In the similar conditioned using self cancellation scheme 905 errors occur witha BER of 0.17407. If EKF estimation is used then only 4 errors occur with BER of0.00076923.
ISI Cancellation:- The residual ISI can severely
degrade the performance of the OFDM systems.
A very effective algorithm, RISIC,was introduced to cancel the residual ISI.
The RISIC algorithm is capable of removing residual ISI almost completely on static or slowly fading ISI channels.
Two techniques:-1) Application of a 16-QAM
modulation technique in the OFDM system with a view to reducing the ISI.
2) Use of the RISIC algorithm to suppress ISI that is longer than the guard interval.
1) Application of a 16-QAM modulationtechnique in the OFDM system with
a view to reducing the ISI.
we have considered only one path (AWGN channel) between OFDM transmitter and receiver which corrupts the transmitted signal. After getting the output from FFT block, it is possible tomake a comparison between the original and received 16-QAM constellation. By observing the received constellation, a large amount of symbol errors arises which leads to high noise power, consequently, generates the ISI in the system. By increasing the value of SNR upto a certain level, we can reduce the Symbol Error Rate (SER) as well as the ISI.
Conclusion:- The conventional computationally efficient OFDM system has a very low efficiency of bandwidth usage and hasto use insufficient statistics in channel equalization, which causes degradation in symbol error performance. We consider the OFDM that possesses a high efficiency of bandwidth usage, allows interchannel interference and interblock
interference to exist in data samples, and uses sufficient statistics in channel equalization. Two methods were explored in this paper for mitigation of the ICI. One is the ICI self-cancellation (SC) scheme and anotheris the extended Kalman filtering (EKF) method for estimation and cancellation ofthe frequency offset has been investigated in this paper, and compared with basic OFDM and self cancellation. The choice of which method to employ depends on the specific application. For example, self cancellation does not require very complex hardware or softwarefor implementation. However, it is not bandwidth efficient as there is a redundancy of 2 for each carrier. On the other hand, the EKF method does not reduce bandwidth efficiency as the frequency offset can be estimated from the preamble of the data sequence in eachOFDM frame. However, it has the most complex implementation of the other methods. In addition, this method requires a training sequence to be sent before the data symbols for estimation ofthe frequency offset. In this paper, we have demonstrated the application of a 16- QAM modulation technique in the OFDM system with a view to reducing the ISI. The experiment has been successful through adding the guard intervals between the consecutive symbols. Although we could not bring the SER reduced to zero our designed system still works properly.
AcknowledgmentWe would like to express our deep
regards and gratitude to the College,H.O.D Prof . B. U. Rindhe & allfaculty members. We would also liketo thank the teaching & non -teaching staff and our comrades who
have helped us all the time in oneway or other.
References:
[1] Liu, Hui and Li, Guoqing(2005). OFDM-Based BroadbandWireless Networks, WileyPublications
[2] Optical Fibercommunication, Gerd Keiser, 2nd
edition, McGraw-Hillpublications “UnderstandingOptical Communications” byHarry J. R. Dutton.
[3] Yariv, OpticalElectronics. Saunders, HRW,fourth ed., 1991.
[4] OFDM and MC-CDMA, Aprimer, L. Hanzo and T.Keller,IEEE press.
[5] OFDM for OpticalCommunications, Jean Armstrong,Senior Member, IEEE
[6]W. Shieh and C. Athaudage,"Coherent optical orthogonalfrequency divisionmultiplexing," Electronics Letters,vol. 42, pp.