Orthogonal Frequency Division Multiplexing

OFDM is a transmission technique that uses several frequencies are mutually perpendicular (orthogonal). This technology is actually already been proposed in about 1950, and the preparation of the basic theories of OFDM has been completed around 1960. In 1966, OFDM has been patented in the U.S. Then in the 1970s, appeared several papers have proposed to apply the DFT (Discrete Fourier Transform) in OFDM, and since 1985, appeared several papers which think about the application of OFDM technology is in wireless communication.Lately this OFDM technology again be the talk of communications experts, this can not be separated from the rapid development of LSI technology. Because prior to LSI technology develops, although theoretically highly promising, but OFDM is considered less applicable because it is too complicated.

The basic principle of OFDMOFDM is a transmission technique with much frequency (multicarrier), using the Discrete Fourier transformation (DFT). The way it works is as follows. Rows of information data to be transmitted is converted into parallel form, so that when the original bit rate is R, then the bit rate in each parallel path is R / M where M is the number of parallel lines (equal to the number of sub-carrier). After that, the modulation performed on each sub-carrier. This modulation may be BPSK, QPSK, QAM or another, but the three techniques are often used in OFDM. Then the signal has been modulated is applied to the Inverse Discrete Fourier Transform (IDFT), for the manufacture of OFDM symbols. Use of this IDFT allows the allocation of frequencies that are perpendicular (orthogonal), on this matter will be explained further. After the OFDM symbols are converted back into serial form, and then the signal sent.




Where Re (.) Is the real part of the equation, f (t) is the response of the filter transmission implus, T is the symbol period, vo is the frequency (carrier frequency) in radians, j is the phase (carrier phase), and bn is data that has been modulated information which becomes the input of the IDFT.




For simplicity, the discussion about the state of the signal when melewai communication line (channel) will be discussed in another section. While at the receiving station, performed the operation opposite to what is done at the sending station. Starting from the conversion from serial to parallel, then parallel signal conversion with Fast Fourier Transform (FFT), after the demodulation, parallel to serial conversion, and finally back into the form of information data....

What is the Orthogonal?The term orthogonal in Orthogonal Frequency Division Multiplexing (OFDM) implies a mathematical relationship between the frequencies being used. With mathematical equations may be expressed as follows, two sets of orthogonal signal when said,



The use of orthogonal frequency in OFDM allows overlap between the frequency without causing interference with each other. There is a set of orthogonal signals, one that quite often we use is the sine signal, as shown in




Excellence
a. Efficient in the use of frequenciesTo clarify the difference OFDM, both in basic operations and in terms of spectrum efficiency, with a single carrier system, and also with conventional multicarrier system, can be seen in picture 3 . From these images can be seen, that OFDM is a kind of multicarrier (FDM), but has a frequency usage efficiency is much better. In OFDM the frequency overlap between adjacent allowed, because each is mutually orthogonal, whereas the conventional multicarrier systems to prevent interference between adjacent frequencies need to tuck the frequency barrier (guard band), where it has the side effect of decreasing the transmission speed when compared to with a single carrier system with the same broad spectrum. So that one characteristic of OFDM is the high level of efficiency in the use of frequencies. In addition to the conventional multicarrier band pass filter is also needed as much as the frequency used, while in OFDM is to use FFT only




b. Facing strong frequency selective fadingAnother main character is robust in the face of OFDM frequency selective fading. By using OFDM technology, despite lines of communication that is used have characteristics frequencyselective fading (where the bandwidth of the channel is narrower than the bandwidth of the transmission so that the resulting weakening of the power received is not uniform at some particular frequency), but each sub-carrier of OFDM system is only experiencing flat fading (weakening the power received in a uniform). Attenuation caused by flat fading is more easily controlled, so that the performance of the system is easy to be upgraded.
OFDM technology can change the frequency selective fading into flat fading, because even though the overall system has a very high-speed transmission that has a wide bandwidth, due to transmission using the subcarriers (carrier frequency) with a number of very much, so the speed of transmission in each subcarrier is very low and bandwidth of each subcarrier is very narrow, narrower than the coherence bandwidth (width than the bandwidth that have relatively similar characteristics). The change of frequency selective fading into flat fading can be illustrated as






c. Not sensitive to signal delayAnother advantage is that, with the low speed of transmission in each subcarrier symbol means the period becomes longer sehinnga system sensitivity to delay spread (the spread of the signals that arrive late) to be relatively reduced.
WeaknessAs a Man-made systems, OFDM technology certainly did not escape from these shortcomings. Among them, a very prominent and has long been a topic of research is the frequency offset and nonlinear distortion (nonlinear distortion).a. Frequency OffsetThis system is very sensitive to carrier frequency offset caused by jitter on the carrier wave (carrier wave) and also to the Doppler effect caused by the movement either by the station sending and receiving stations.b. Nonlinear DistortionTechnology OFDM is a modulation system using multi-frequency and multi-amplitude, so that the system is easily contaminated by the nonlinear distortion that occurs in the transmission power amplifier.c. Signal SynchronizationAt the receiving station, determining the start point to begin operating the Fast Fourier Transform (FFT) when the OFDM signal arrives at the receiving station is a relatively difficult. Or in other words, a synchronization than the OFDM signal is difficult.