Essay: INTRODUCTION TO OFDM

OFDM is a technique in which multiple carriers can be modulated at the same time. Multi-carrier modulation is a method in which we send data by breaking it into number of components, and transmitting each of the components over individual carrier signals. The single carrier has narrow bandwidth, but the complex signal can have broad bandwidth. Due to the high data rate transmission and robustness against fading, orthogonal frequency division multiplexing (OFDM) is a favourable technique in the present broadband wireless communication system. Orthogonal frequency division multiplexing (OFDM) technology is to break a high-rate data sequence into various low rate data sequences that are transmitted simultaneously over a number of subcarrier.
As the symbol duration increases for the lower rate parallel carriers, the dispersion in time due to multipath delay spread also decreases. In OFDM systems, the subcarriers are made to overlap with each other, which is carefully designed so that the subcarriers are orthogonal to each other. In OFDM, the subcarrier frequencies are chosen such that to maintain orthogonality between the subcarriers, which results in eliminating crosstalk between the subcarriers and guard intervals are not required. The bandwidth efficiency of OFDM is another advantage.
Figure 1 Subcarrier Arrangement in OFDM
1.2 Applications of OFDM
1.2.1 Wireless LAN Network
A Wireless Local Area Network (WLAN) is used to link several equipments through some wireless distribution method, and mostly providing a connection via an access point to the wider Internet. This gives users the facility to roam around within a local coverage area and still be coupled to the network.
1.2.2 Digital Audio Broadcasting (DAB)
DAB can carry “radio text” from the station which gives real time information. Advanced programme guides can also be send. This feature also exists in FM in the form of the RDS (However, not all Frequency modulating receivers allow radio stations to be stored by name.) DAB receivers can display day time which is encoded into transmissions, and thus is automatically corrected when roaming between time zones and when changing to or from Daylight saving. This is not implemented on all the receivers, and some receivers display time only when they are in Standby mode. Some radios offer a stop facility on live broadcasts, duplicating the broadcast stream on local memory, although this function is limited.
1.2.3 High Definition Television (HDTV)
HDTV is a digital TV broadcasting pattern where the broadcast sends the wide screen pictures with more detail and quality than found in a standard analog TV, or any other digital television patterns. HDTV is a type of Digital Television (DTV) broadcast, and it is the best quality DTV format available. There are many types of HDTV displays such as direct-view, plasma, rear screen, and front screen projection. HDTV tuner is required to view the HDTV and the most detailed HDTV format is 1080i.
1.3 GMSK Modulation Technique
Gaussian Minimum Shift Keying (GMSK) is one of the most widely used digital modulation technique. In GMSK, the phase of the carrier signal is continuously varied by a contrary signal, which has been shaped by the Gaussian shaping filter. Since it is kind of MSK, it has modulation index of value 0.5. In GMSK, the side lobe levels of the spectrum which were introduced in MSK are further minimized by passing the modulating NRZ data waveform through a pre modulation Gaussian pulse-shaping filter. The Gaussian filter width is determined by the bandwidth-time product BT (e.g., BT = 0.3 for GSM and BT = 0.5 for CDPD). As the BT product decreases, the bandwidth of the spectrum becomes narrow but this increases the inter symbol interference.
1.3.1 Advantages of GMSK:
• Improved spectral efficiency
• Reduced mail lobe over MSK
• Self synchronizing capability
• Constant envelope over entire bandwidth
• Good BER performance
• High noise immunity
• Remain undistorted even when amplifies by a non- linear amplifier.
1.3.2 Disadvantages of GMSK:
Although GMSK reduces the side lobes of the spectrum, the use of Gaussian shaping filter may cause inter symbol, interference. Power spectral density of MSK does not fall fast hence will not reduce interference completely between adjacent channels. Hence GMSK is designed with number of BT factors. BT of 0.3 gives robustness against adjacent channels.
Table 1 shows the comparison of GMSK with other techniques
Table 1 Comparison of GMSK with Other Conventional Techniques
 
1.4 Planning of the project
For this particular thesis work following steps has been followed:
In the proposed project, input image of dimensions 600×400 is transmitted through channel and received at the receiver. In the transmitter, the image undergoes convolution coding, interleaving, conversion, GMSK modulation, IFFT, pilot insertion and cyclic extension. In the channel, Additive White Gaussian Noise is added to the signal. The image is received at the receiver after performing removing of cyclic extension, pilot exertion, FFT, GMSK de-modulation, conversion, de-interleaving, and decoding.
Then the noise added signal undergoes removal of cyclic extension, pilot exertion, FFT, de-modulation, conversion, de-interleaving, decoding and the original image can be received at the receiver. The system performance can be evaluated by using BER Vs SNR plot.
This system performance using GMSK encoding technique is then compared with the performance using QPSK encoding technique.
Chapter 2
LITERATURE REVIEW
1. Yinsheng Liu et al., “Channel Estimation for OFDM”, IEEE 2014.
In this paper, the author reviewed the traditional channel estimation techniques such as CFR based channel estimation, PM based channel estimation, Iterative channel estimation etc.
The author concludes that the Iterative channel estimation can greatly improve the system performance.
2. Dungun Kim et al., “Filter and Forward Relay Design for MIMO OFDM Systems”, IEEE July 2014
In this paper, the author proposed the Filter and Forward relay design for MIMO OFDM systems. Various Parameters have been considered as a matter of design criteria. The first parameter is to minimize the mean square error (MSE) and secondly to maximize the rate. The proposed relay design is then compared to the conventional AF relay design.
The author concludes by providing the numerical results which shows the effectiveness of the proposed relay design over the conventional AF relay design.
3. Deergha Aggarwal et al., “PAPR Reduction Using Precoding and Companding Techniques for OFDM Systems”, 2015 ICACEA.
OFDM suffers from a limitation of very high Peak to Average Power Reduction Ratio (PAPR).This may lead to non linear distortions. This paper proposed some PAPR reduction techniques. Comparative analysis of precoding technique and Companding techniques has been done.
The author concludes that both the techniques cut back PAPR significantly. PAPR is reduced about 4dB as compared to the original OFDM. By comparing the results, it is concluded that Precoding technique is better than companding.
4. Md. Alamgir Hossain et al., “Low-Complexity Blind Phase Noise Compensation in OFDM Systems”, (ICEEICT) 2014, IEEE.
OFDM suffers from the effect of Phase Noise. Various pilot based techniques have been used earlier to remove the effect of phase noise but this decreases the system bandwidth efficiency. In this paper, author presented a blind phase noise compensation method which retains the system bandwidth with reduced computational complexity.
The author shows that the performance of the proposed technique is better as compare to the conventional techniques.
5. Navdeep Singh Randhawa et al., “A Survey of Equalization Techniques for an Effective Equalizer Design in MIMO-OFDM Systems”, 2015 [ICCPCT].
In this paper, the author has provided the survey of various equalization techniques which are used to remove ISI and improves error performance.
The paper describes the role of equalization and in future developing the techniques which would involve optimization and work on complexity.
6. H. A. Rahim et al., “Design and Simulation of OFDM System Using DPSK Technique for Wireless LAN”, ICCCE 2010.
In this paper, the author proposed the M-ary DPSK modulation technique to implement the OFDM transceiver. The Bit Error Rate performance is evaluated in AWGN channel and it is compared to the theoretical rate.
From the simulation results, the author concludes that the DPSK technique obtains the favourable bit error rate. However, it reduces the transmission capacity.
7. H.O.Qrabil et al., “Design and Implementation of OFDM Transceiver System Using M-PSK Encoding Technique”, 4th International Conference on Power Engineering, Energy and Electrical Drives Istanbul, Turkey, 13-17 May 2013.
In this paper, the author presents the implementation of OFDM transceiver using M-PSK encoding techniques. Basically, QPSK is used to implement the OFDM. The author concludes by showing the simulation results that OFDM is implemented successfully using matlab simulation tools and further adding an option to use QAM instead of M-PSK
8. Rajesh Bansode et al., “Design, Simulation and Performance Evaluation of 4×4 MIMO Transceiver Systems Using 16 QAM”, ICWAC 2013.
In this paper, the author proposed a technique of implementing MIMO OFDM transceiver using 64 point FFT and encoded by 16QAM modulation technique.The author concludes that the proposed MIMO OFDM can deliver four times more data rate without additional power and bandwidth.
9. Behrouz Maham et al., “Impact of Transceiver I/Q Imbalance on Transmit Diversity of Beamforming OFDM Systems”, IEEE March 2012.
In this paper, the author presented the effect of I/Q imbalances on MIMO OFDM systems.
The author concludes by showing the results that these imbalances lead to high error rate at higher SNR.
10. Eonpyo Hong et al., “Peak-to-Average Power Ratio Reduction for
MISO OFDM Systems with Adaptive All-Pass Filters”, IEEE October 2011.
In this paper, the author proposed the PAPR technique based on Adaptive All Pass Filter for MISO OFDM systems. And the results are compared with the SLM technique.
The author concludes that the PAPR reduction performance of AAPF technique is same as that of SLM with less system complexity.
11. Kyung-Hwa Kim et al., “An ICI Suppression Scheme Based on the Correlative Coding for Alamouti SFBC-OFDM System with Phase Noise”, IEEE July 2011
In this paper, the author analyzed the effect of phase noise for space frequency block coded OFDM. Correlative coding is used to analyze the effect of Inter carrier Interference.
The author concludes that by using the Correlative coding, the ICI power is reduced to a great extent. The carrier to interference ratio (CIR) and bit error performance is also improved.
12. Gaurav Chandra et al., “Analyzing the Effect of Modulation Order and Sub-bands on PTS and SLM PAPR Reduction Technique in Various 802.11 Standards Using Different Modulator”, IJSER.May-2013.
In this paper, the author proposed the PAPR reduction technique to remove distortions. Partial Transmit Sequence (PTS) and Selective Mapping (SLM) techniques are used with various modulators such as QPSK, QAM and BPSK.
The author concludes that the PTS technique performs much better than SLM. Out of the three modulators, the QAM technique performs better.
13. Dharma Devi et al., “BER Performance of GMSK Using Matlab”, IJARCET, April 2013.
In this paper, the author introduced the technique to reduce ISI in GMSK. To improve the performance of GMSK, optimum filters and vertibi decoding is used.
The author concludes that the BER performance of GMSK is better as compared to MSK. The BER can further be reduced by increasing number of transmitting symbol.
14. S.S Ghorpade et al., “Behavior of OFDM Systems Using Matlab Simulation”, IJITR, April 2013.
In this paper, the author proposed the designing of OFDM transmitter and receiver and Matlab is used for simulation. The OFDM is implemented using QAM modulation technique.
By the simulation results, the author concludes that the OFDM transceiver is successfully simulated using Matlab. To reduce PAPR, wide range power amplifiers can be used at the front end of the transmitter.
15. Zain ul Abidin Jaffri et al., “Designing and Simulating the OFDM Transceiver”, IJESET, Feb 2014.
In this paper, the OFDM transceiver is designed using Matlab simulation. The OFDM is analyzed under AWGN environment. The BER performance is evaluated for AWGN channel.
The author concludes that OFDM is successfully implemented. Some improvements like equalization or using channel other than AWGN can be done.
16. S.S Ghorpade et al., “Performance Analysis of Multiband OFDM and Pulsed OFDM using Matlab Simulation”, IJSR, May 2014
To reduce the complexity and power consumption of multiband OFDM system, the author proposes a approach. Pulsed OFDM has been studied and it is found that it has lower complexity and power consumption as compare to multiband OFDM.
By presenting various simulation results, the author concludes that the complexity of MB-OFDM system can be reduced using pulsed OFDM. Even better spreading techniques can be used.
17. Enis Kocan et al., “Performance Evaluation of OFDM Amplify and Forward Relay System with Subcarrier Permutation”, IEEE, May 2010.
In this paper, the author analyzed the error probability of the OFDM AF relay system with subcarrier permutation. Two permutation schemes are considered- Best to best SCP and best to worst SCP. The BER expression is derived.
The author concludes that the best to best SCP is better at low SNR values and best to worst is better at medium and high SNR values.
18. Suchita Varade et al., “Performance Analysis of MIMO OFDM System Using Space Time Turbo Codes and Adaptive Beamforming”, IJCA, May 2011.
In this paper, the author proposed the adaptive beamforming technique to analyze the MIMO OFDM system. Two beamforming algos are used- LMS and LLMS. The analysis is done over AWGN channel.
The author concludes that the use of LMS and LLMS algorithm enhances the BER performance. LLMS algorithm performs better as compare to LMS.
19. T.S.Gagandeep et al., “Performance Analysis of Extended SNR Estimation for OFDM System under AWGN and Rayleigh Channel”,
IJAEST, 2011.
In this paper, the author proposed various channel estimation algorithms to evaluate the performance of OFDM system under AWGN and Rayleigh channel. The comparison analysis of the techniques is shown.
The author concludes that the enhanced periodic sequence estimator improves the SNR estimation and also obtains better BER performance.
20. Ahasanun Nessa et al., “Performance Analysis of Two Hop Cooperative MIMO Transmission with Best Relay Selection in Rayleigh Fading Channel”, IJIT, January 2011.
In this paper, the author proposed the analysis of BER performance for a two hop MIMO transmission system. The BER comparison of the best relay and the single relay is done.
The author concludes that the single relay system reduces the system complexity and increases coverage.
21. Mohamad Aoude et al., “Performance Analysis of QPSK OFDM with Fading, Frequency Offset and Channel Estimation Error”, JTAIT, 2010.
In this paper, the author analyzed the effect of fading. Frequency offset and channel estimation error for OFDM implemented by QPSK modulation technique.
The author concludes that the system bandwidth efficiency increases but leads to complexity.
Channel estimation error and timing jitter must be minimized.
22. Srabani Mohapatra et al., “Performance Enhancement of OFDM System with ICI Reduction Technique”, WCE 2009
In this paper, the author introduced the ICI reduction technique. Various pulse shaping functions like sinc function, improved sinc power pulse techniques are considered.
By comparing the various parameters like ICI power, SIR and BER for various techniques, the author concludes that Improved Sinc Power Pulse technique is better than the other ICI reduction techniques.
23. D.K Sharma et al., “Studies on Performance of Pulse Shaped OFDM Signal”, IJCSCT, Jan. 2011.
In this paper, the author analyzed the various pulsed shaped OFDM signal like rectangular, Gaussian, Hamming etc. The effect of these waveforms on the performance of OFDM is analyzed. Power spectral density curve is evaluated for every pulse signal and then compared.
24. V.Jagan naveen et al., “Performance Analysis of MC-CDMA AND OFDM in Wireless Rayleigh Channel”, IJAST, 2010.
In this paper, the author calculates the effect of Doppler spread on Bit Error Rate of multi carrier CDMA and Orthogonal Frequency Division Multiplexing. The author concludes that for the systems having large sub carriers, Multi Carrier CDMA has no advantage over OFDM.
25. Neenu Joseph et al., “FPGA based Realization of OFDM Transceiver system for Software Defined Radio”, 2014 ICDCS.
In this paper, the author proposed a technique to implement OFDM transmitter and receiver using FPGA (Field Programmable Array) for Software Defined Ratio systems. FSK, BPSK and QPSK modulation techniques are used for implementation.
The author concludes that OFDM is implemented successfully using FPGA. Further, PAPR reduction techniques can be used.
26. Peng Cheng et al., “V-OFDM: On Performance Limits over Multi-Path Rayleigh Fading Channels”, IEEE 2011.
The paper presents the study of vector OFDM over multipath Rayleigh fading channels. The author shows the numerical results and concludes that OFDM performs well when vector length is equal to number of taps.
27. Kwanghoon Kim el al., “Rate-Compatible SFBC-OFDM under Rapidly Time-Varying Channels”, IEEE, 2011.
The author proposed a Polynomial Cancellation Code to suppress the Inter Carrier Interference. This technique is compared with the earlier proposed techniques. The author shows the simulation results and concludes that the proposed technique offers high channel capacity and low bit error rate.
28. Gaurav Bansal et al., “Adaptive Power Loading for OFDM-Based Cognitive Radio Systems with Statistical Interference Constraint”, IEEE, 2011.
In this paper, an optimal power allocation algorithm is designed for OFDM based cognitive radio systems. The numerical results show that the proposed algorithm can achieve high transmission rate as compare to traditional techniques.
29. Maryam Sabbaghian et al., “Near Shannon Limit and Low Peak to Average Power Ratio Turbo Block Coded OFDM”, IEEE 2011.
In this paper, a technique is proposed that generates low PAPR codeword. The author shows that the technique provides low PAPR and high error correcting capability.
30. G. Hacioglu et al., “A Transmit Diversity Method for DVB-H and IEEE 802.20”, IEEE 2011.
The author proposed transmit diversity technique for OFDM systems. The author shows that the proposed technique provides more gain than the traditional techniques.
Chapter 3
SYSTEM DESIGN
3.1 Evolution of OFDM
Orthogonal Frequency Division Multiplexing (OFDM) is an alternative wireless modulation technology to Code Division Multiple Access. OFDM has the potential to increase the capacity of CDMA systems. OFDM is a modulation scheme that transmits the digital data efficiently over the radio channel, even in multipath environments. The name ‘OFDM’ is derived from the fact that the digital data is transmitted using many carriers, each carrier having a different frequency (Frequency Division Multiplexing) and the carriers are orthogonal with each other, hence the name Orthogonal Frequency Division Multiplexing.
The OFDM development started in the 1950 with the introduction of Frequency Division Multiplexing (FDM) for data communication. In 1966 Chang obtained a patent for the structure of OFDM and published the concept of using OFDM for data communications. In 1971 Weinstein introduced the concept of using a Discrete Fourier Transform (DFT) for the generation and reception of OFDM signals, eliminating the requirement of analog subcarriers oscillators. OFDM can be easily implemented with the use of Fast Fourier Transform (FFT), which is an effective implementation of the DFT. Recently the advances in integrated circuit technology have made the implementation of OFDM cost effective. It 1980 the work began on the development of OFDM for commercial use, with the evolution of the Digital Audio Broadcasting (DAB) system. In 1995 U.K and Sweden replace FM audio broadcasting to DAB, which provides high quality digital audio and information
3.2 Comparison of FDM and OFDM
OFDM is a special class of frequency division multiplexing (FDM)
As OFDM is a special case of FDM but more spectral efficient than FDM. OFDM uses the principles of FDM to allow multiple messages to be sent over a single radio channel. An easy example of FDM is the use of various frequencies for each FM (Frequency Modulation) radio stations. All stations transmit at the same time but do not interfere with each other because they transmit over different carrier frequencies. To avoid interference guard band is also provide. Additionally they are bandwidth limited and are spaced sufficiently far part in frequency so that their transmitted signals do not overlap in the frequency domain. The required bandwidth is greatly reduced by removing the guard bands and allowing the signal to overlap with each other. In order to demodulate the signal, a Discrete Fourier transform is needed. Fast Fourier transform chips are commercially available, making this relatively easy operation.
OFDM has resilience to multipath, so effect of ISI can be reduced. As multipath environment results in interference and frequency selective fading, OFDM is able to overcome by utilizing its parallel, low bandwidth utilization. This makes OFDM able to handle the harsh condition of the mobile wireless environment.
OFDM transmission is restricted to Digital modulation scheme. Each of the carriers in a FDM transmission can use either an analogue or digital modulation scheme. There is no coexistence between the transmission and so one station could transmit using FM and another using FSK technique. In a single OFDM transmission all the subcarriers are synchronized to each other, limiting the transmission to digital modulation schemes. Since multiple carriers form a single OFDM transmission, they are referred to as ‘subcarriers’.
3.3 OFDM Operation
Definition of Orthogonality: Two periodic signals are said to be orthogonal when the integral of their product, over a bit period, is equal to zero. This is true for certain sinusoids as shown in the equation 1 and 2 below:
Continuous time:
∫_0^tâ–’cos⁡〖(2 πnft)*cos⁡(2πmft)dt=0 〗 ; n ≠ m (1)
Discrete time:
−_o^(N-1)▒〖cos(2πkn/n)*cos(2πkm/N)〗 = 0 ; n ≠m (2)
The general structure of OFDM transceiver system is illustrated in Figure 2.
Figure 2 Block Diagram of OFDM Transceiver System
OFDM signals are basically digitally generated due to the difficulty in creating large banks of phase lock oscillators and receivers in the analog domain. Figure 2 shows the block diagram of a typical OFDM transceiver. The transmitter converts digital data to be transmitted, into a series of subcarrier amplitude and phase. It then converts the frequency domain signal into the time domain using an Inverse Discrete Fourier Transform (IDFT). The Inverse Fast Fourier Transform (IFFT) has the same function as that of IDFT, except that it is much more efficient. When the OFDM signal is to be transmitted, the time domain signal is then mixed up to the required frequency.
The receiver performs the opposite operation as that of the transmitter, combines the RF signal to original signal for further processing, then Fast Fourier Transform (FFT) converts the signal from time domain to the frequency domain. The amplitude and phase of the OFDM carriers is then picked out and converted back to digital data. The IFFT and the FFT are opposite function and are choose depending on the condition whether the signal is being received or generated. If the signal is not dependent on the difference, then the terms FFT and IFFT are used interchangeably.
3.4 Research Challenges
OFDM has various advantages such as resistance to multipath delay, high spectral efficiency in comparison to FDM or TDM and also provides better system throughput. But, it has some related problems as well. These problems can limit its effectiveness. So, we have to overcome these limitations.
Sensitivity to Carrier Frequency Offset (CFO) CFO causes number of destructions including phase noise added by nonlinear channels, distortion and rotation of each of the subcarriers and inter-carrier interference (ICI). ICI, disturbance in the orthogonality of the sub-carriers is caused due to the respective movement between transmitter and receiver which leads to Doppler frequency shifts. Another disastrous effect caused by CFO in OFDM systems is the degradation of signal amplitude. The sinc functions are shifted and are not sampled at the peak. A number of techniques have been developed to reduce the sensitivity to frequency offset.
Phase Noise Another related problem with OFDM systems is the effect of phase noise. Phase noise is present in all the practical oscillators and it proves itself in the form of random phase modulation of the carrier. The effect of both phase noise and frequency offsets is unpleasant in OFDM than single-carrier systems. The use of efficient frequency and phase estimation schemes can help in reducing these effects.
Peak to Average Power Ratio (PAPR) OFDM signal exhibits a very high PAPR, which is due to the addition of sine waves and variable envelope. Therefore, RF power amplifiers have to be operated in a very large linear region. Otherwise, the signal peaks part gets into the non-linear region causing signal distortion. This signal distortion causes modulation among the subcarriers and out-of-band interference. A simple way to avoid this is to use an RF amplifier of large variable range but this makes the transmitter costly. Thus, it is highly required to reduce the PAPR. Many techniques have been introduced to reduce the problems caused by PAPR. Theoretically, the difference of PAPR between any multicarrier and single-carrier is a function of number of subcarriers N i.e. D (dB) = 10logN. When N = 1000, the difference can be as large as 30dB, though this theoretical value hardly occurs. Disordered input data lowers the chances of reaching its peak limit, especially, when the constellation size is large.