OFDM Orthogonal Frequency Division Multiplexing

OFDMOrthogonal Frequency Division MultiplexingAditra Vito AbdulkadirArief NurwantoFadilla Putri IrintkaKenneth Keulana JudaKrishadi AnanggaM Rah Adi SatrioWidoseno Nur Sukma AtriDefinitionOrthogonal frequency division multiplexing (OFDM) is a technique, method or scheme for digital multi-carrier modulation using many closely spaced subcarriers- a previously modulated signal modulated into another signal of higher frequency and bandwidth.Why OFDM?In contrast to conventional Frequency Division Multiplexing, the spectral overlapping among sub- carriers are allowed in OFDM since orthogonality will ensure the subcarrier separation at the receiver, providing better spectral efficiency and the use of steep band pass filter was eliminated. OFDM transmission system offers possibilities for alleviating many of the problems encountered with single carrier systems. It has the advantage of spreading out a frequency selective fade over many symbols. This effectively randomizes burst errors caused by fading or impulse interference so that instead of several adjacent symbols being Completely destroyed, many symbols are only slightly distorted. This allows successful reconstruction of majority of them even without forward error correction. Because of dividing an entire signal bandwidth into many narrow subbands, the frequency response over individual subbands is relatively flat due to subband are smaller than coherence bandwidth of the channel. Thus, equalization is potentially simpler than in a single carrier system and even equalization may be avoided altogether if Differential encoding is implemented.

3AdvantagesMakes efficient use of the spectrum by allowing overlap.By dividing the channel into narrowband flat fading subchannels, OFDM is more resistant to frequency selective fading than single carrier systems are.Using adequate channel coding and interleaving one can recover symbols lost due to the frequency selectivity of the channel.Channel equalization becomes simpler than by using adaptive equalization techniques with single carrier systems.It is possible to use maximum likelihood decoding with reasonable complexity.OFDM is computationally efficient by using FFT techniques to implement the modulation and demodulation functions.Is less sensitive to sample timing offsets than single carrier systems are.Provides good protection against cochannel interference and impulsive parasitic noise.5DisadvantagesThe OFDM signal has a noise like amplitude with a very large dynamic range; hence it requires RF power amplifiers with a high peak to average power ratio. It is more sensitive to carrier frequency offset and drift than single carrier systems are due to leakage of the DFT. It is sensitive to Doppler shift. It requires linear transmitter circuitry, which suffers from poor power efficiency. It suffers loss of efficiency caused by cyclic prefix. Principles of OFDMOverviewUsing all orthogonal subcarrier so that data rate can be increased with each sub-channel requires a longer symbol period.By implementing this, OFDM can overcome Inter-symbol Interference (ISI)OFDM is based on the concept of frequency-division multiplexing (FDD), the method of transmitting multiple data streams over a common broadband medium. That medium could be radio spectrum, coax cable, twisted pair, or fiber-optic cable. Each data stream is modulated onto multiple adjacent carriers within the bandwidth of the medium, and all are transmitted simultaneously. A good example of such a system is cable TV, which transmits many parallel channels of video and audio over a single fiber-optic cable and coax cable.

Q: Is that how OFDM works today?A: Sort of. The FDD technique is typically wasteful of bandwidth or spectrum because to keep the parallel modulated carriers from interfering with one another, you have to space them with some guard bands or extra space between them. Even then, very selective filters at the receiving end have to be able to separate the signals from one another. What researchers discovered is that with digital transmissions, the carriers could be more closely spaced to one another and still separate. That meant less spectrum and bandwidth waste.Orthogonal Frequency Division Multiplexing (OFDM) is a technique for transmitting large amounts of digital data over a radio wave The technology works by splitting the radio signal into multiple smaller sub-signals that are then transmitted simultaneously at different frequencies to the receiver. OFDM reduces the amount of crosstalk in signal transmissions.

8ModulationSerial-to-parallel conversion

1, 1, -1, -1, 1, 1, 1, -1, 1, -1, -1, -1, -1, 1, -1, -1, -1, 1,

FFT and IFFT

Forward FFT takes a random signal, multiplies it successively by complex exponentials over therange of frequencies, sums each product and plots the results as a coefficient of that frequency.The coefficients are called a spectrum and represent how much of that frequency is present inthe input signal. The results of the FFT in common understanding is a frequency domain signal.14

The functional block diagram of how the signal is modulated/demodulated is given below

Orthogonal sub carrierThe OFDM scheme differs from traditional FDM in the following interrelated ways:1.Multiple carriers (called subcarriers) carry the information stream,2.The subcarriers are orthogonal to each other, and3.A guard interval is added to each symbol to minimize the channel delay spread and inter symbol interference.

The following figure illustrates the main concepts of an OFDM signal and the inter-relationship between the frequency and time domains. In the frequency domain, multiple adjacent tones or subcarriers are each independently modulated with complex data. An Inverse FFT transform is performed on the frequency-domain subcarriers to produce the OFDM symbol in the time-domain. Then in the time domain, guard intervals are inserted between each of the symbols to prevent inter-symbol interference at the receiver caused by multi-path delay spread in the radio channel. Multiple symbols can be concatenated to create the final OFDM burst signal. At the receiver an FFT is performed on the OFDM symbols to recover the original data bits.19

In OFDM the frequency spacing between adjacent sub-carriers is f=1/(NT)f =1/(NT) is the minimum frequency separation that is necessary to ensure orthogonality between the subcarriers over the signalling interval of length NTIn OFDM the frequency spectrum of each sub-carrier overlaps the frequency spectrum of adjacent subcarriers

20

The use of orthogonal subcarriers allows more subcarriers per bandwidth resulting in an increase in spectral efficiency. In a perfect OFDM signal, Orthogonality prevents interference between overlapping carriers. In FDM systems, any overlap in the spectrums of adjacent signals will result in interference. In OFDM systems, the subcarriers will interfere with each other only if there is a loss of orthogonality. For example, frequency error will cause the subcarrier frequencies to shift so that the spectral nulls will no longer be aligned resulting in inter-subcarrier-interference21FadingFading effect occurs when the path from the transmitter to the receiver has either reflection or obstructions When fading occurs, the signal reach the transmitter by using several path with slightly different delay and gainThe time delay causes phase shift in the signal causing it to be degradedFading

Fading

24Fading

Intercarrier Interference Problem and SolutionsInter-carriers interference (ICI) is a special problem in the OFDM systemICI is different from the co-channel interference in MIMO systems.The co-channel interference is caused by reused channels in other cells, while ICI results from the other sub-channels in the same data block of the same user.Doppler EffectThe relative motion between receiver and transmitter, or mobile medium among them, would result in the Doppler effect, a frequency shift in narrow-band communications.the Doppler effect would influence the quality of a cell phone conversation in a moving carThere are 3 kind of dopler effect models:the classical modelthe uniform modelthe two-ray model

Classical modelIn classical model, the transmitter was assumed to be fixed with vertically polarized antenna. There was no Light of Sight (NLOS) pathIn classical model the spectrum of this kind of Doppler shift could be given:

Uniform model and Two ray modelThe uniform model is much simpler. Both velocity and angle are supposed to be uniformly distributedThe power spectrum could be written as:

The two-ray model assumed that there were only two paths between the transmitterthe resulting power spectrum is given as:

Synchronization ErrorIt can be assumed that most of the wireless receivers cannot make perfect frequency synchronization.In fact, practical oscillators for synchronization are usually unstable, which introduce frequency offset.the oscillator frequency offset varies from 20 ppm (Parts Per Million) to 100 ppm.Provided an OFDM system operates at 5 GHz, the maximum offset would be 100 KHz to 500 KHz (20-100 ppm.) Hence, the frequency offset could not be ignored.

the frequency offset can be normalized by the reciprocal of symbol durationif a system has a bandwidth of 10 MHz, and the number of subcarriers is 128, then the subcarrier frequency spacing would be 10M/128= 78 KHz.

If the receiver frequency offset is 1 KHz

normalized frequency =1/78=1.3%If the normalized frequency offset is larger than 1, only the decimapart needs to be considered.Multipath Fadingthe multipath fading does not cause ICI, but it will make the ICI problem worse.Because there are many time-delayed versions of received signals with different gains and different phase offsets, the ICI is more complicated to calculate.Solutions for ICIThere are 3 scheme were proposed to overcome ICI problem:CFO estimationwindowing techniqueICI self-cancellationIn order to compensate CFO, CFO must be estimated at first. Once a precise CFO estimate is obtained, a perfect equalizer then can be designed to eliminate ICI.Signal processing methods are applied to solve this problem (proposed MUSIC-based and ESPRIT-based algorithms)Windowingwindowing is Windowing is a popular method of reducing the spectral sidelobes of OFDM.windowing is capable to reduce the bandwidth of the channel matrix.windowing mitigate the ICI induced by time-varying frequency-selective channels.Hanning window, the Nyquist window, and the Kaiser window are used for windowing.To reduce out of band energy, it is common to apply a Raised Cosine filter to the front and back of an OFDM symbol. It is also common to overlap the windowed portions of adjacent symbols to eliminate amplitude dips.36The ICI self-cancellation has introduced by Yuping Zhao and Gustav Hangman in 2001 to combat supress ICI in OFDM.The ICI self-cancellation scheme is a method involving with encoded redundancy.The main idea is to modulate the input data symbol onto a group of subcarriers with predefined coifficient such that the generated ICI within the group cancel each other.ApplicationHigh spectral efficiency: provides more data servicesResiliency to RF interference: good performance in unregulated frequency bandsLower multi-path distortion: works in complex indoor environments as well as at speed in vehicles.

The ISM Band (Industrial Scientific and Medical) is a set of frequency ranges that are unregulated.Typical RF transmitters in the ISM band-Analog Cordless Phones (900MHz)-Microwave Ovens (2.45 GHz)-Bluetooth Devices (2.45 GHz)-Digital Cordless Phones (2.45 GHz or 5.8 GHz)- Wireless LAN (2.45 GHz or 5.8 GHz)

Single Carrier Single SymbolBluetooth, GSM, CDMA and other communications standards use a single carrier to transmit a single symbol at a time.Data troughput is achieved by using a very fast symbol rate.W-CDMA 3.14 Msymbols/secBluetooth 1 Msymbols/sec A primary disadvantages is that fast symbol rates are more susceptible to Multi-path distortion.

Digital Audio BroadcastingDigital Audio Broadcasting (DAB) is a digital radio technology for broadcasting radio stations, used in several countries, especially in Europe.

The DAB transmitted data consists of signals number sampled at a rate of 48 kHz with a 22-bit resolution.

This signal is then compressed at rates ranging from 32 to 384 kbps, depending upon the desired quality.

DAB uses differential QPSK modulation for the sub-carriers.

DVB-H (Digital Video Broadcast to Handheld)DVB-H is one of the established mobile TV formats.Can operate for transmission on 5, 6, 7 or 8 MHz bandwidth.

(DVB-H Receiver)

Modulator adalah suatu rangkaian yang berfungsi melakukan proses modulasi, yaitu proses menumpangkan data pada frekuensi gelombang pembawa (carrier signal) ke sinyal informasi/pesan agar bisa dikirim ke penerima melalui media tertentu (kabel atau udara), biasanya berupa gelombang sinus.46Advantages of DVB-H-Carriers - In DVB-H, carriers can use any additional spectrum that they might own for DVB-H broadcasting and be an infrastructure player.-Spectrum Availability - In U.S., DVB-H will be organized using clear and ready-for-use spectrum available today, without interfering with existing analog TV stations or other TV or wireless services.

Reference:Advance network Computing and communication 3ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING MODULATION AND INTER-CARRIER INTERFERENCE CANCELLATION, Thesis, by Yao Xiao B.S., Dalian University of Technology, 1998 M.S., Institute of Automation, C.A.S, 2001 May 2003WINDOWING TECHNIQUES FOR ICI MITIGATION IN MULTICARRIER SYSTEMS, Journal, Luca Rugini and Paolo BanelliIntroduction to Orthogonal Frequency Division Multiplex Technology (Keithley.com)Orthogonal Frequency Division Multiplexing and its Applications (Beena R.Ballal, Ankit Chadha, Neha Satam) IJSR.

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