Principle of ofdm pdf

Because the symbol duration increases for lower rate parallel subcarriers, the relative amount of dispersion in time caused by multipath delay spread is decreased. Intersymbol interference is eliminated almost completely by introducing a guard time in every OFDM symbol.

In the guard time , the symbol is cyclically extended to avoid intercarrier interference. In OFDM design, a number of parameters are up for consideration, such as the number of subcarriers, guard time, symbol duration, subcarrier spacing, modulation type per subcarrier. The choice of parameters is influenced by system requirements such as available bandwidth, required bit rate, tolerable delay spread, and Doppler values. Some requirement are conflicting.

For instance, to get a good delay spread tolerance, a large number of subcarriers with small subcarrier spacing is desirable, but the opposite is true for a good tolerance against Doppler spread and phase noise.

In this representation, the real and imaginary parts correspond to the in-phase and quadrature parts of the OFDM signal, which have to be multiplied by a cosine and sine of the desired carrier frequency to produce the final OFDM signal. Figure 1 shows the operation of the OFDM modular in block diagram. In this example, all subcarriersn have the phase and amplitude, but in practice the amplitudes and phases may be modulated differently for each subcarrier. Note that each subcarrier has exactly an integer number of cycles in the interval T , and the number of cycles between adjacent subcarries differs by exactly one.

This properly accounts for the orthogoality between subcarriers. By looking at the intermediate result, it can be seen that a complex carrier is integrated over T seconds.

For the demodulated subcarrier j, this integration gives the desired output j d multiplied by a constant factor T , which is the QAM value for that particular subcarriers. Hence, the spectrum of a single symbol is a convolution of group of Dirac pulses located at the subcarrier frequencies with the spectrum of a square pulse that is one for a Tsecond period and zero otherwise. This effect is shown in figure which shows the overlapping sinc spectra of individual subcarriers.

At the maximum of each subcarrier spectrum, all other subcarrier spectra are zero. Because an OFDM receiver calculates the spectrum values at those points that correspond to the maxima of individual subcarrier, it can demodulate each subcarrier free from any interference from the other subcarriers. Notice that the pulse shape is present in frequency domain and note in the time domain, for which the Nyquist criterion usually is applied. Therefore, instead of intersymbol interference ISI , it is intercarrier interference ICI that avoided by having the maximum of one subcarrier spectrum correspond to zero crossing of all the others.

Generation of subcarriers using the IFFT. In practice, this transform can be implemented very efficiently by the inverse Fast Fourier transform IFFT as shown in figure 4 and 5. One of the most important reasons to do OFDM is the efficient way it deals with multipath delay spread. By dividing the input data stream in s N subcarriers, the symbol duration is made s N times smaller, which also reduces the relative multipath delay spread, relative to symbol time, by the same factor.

To eliminate intersymbol interference almost completely, a guard time is introduced for each OFDM symbol. The guard time is chosen larger than the expected delay spread, such that multipath components from one symbol cannot interfere with the next symbol.

The guard time could consist of no signal at all. In that case, however, the problem of intercarrier ICI would arise. ICI is crosstalk between different subcarriers,which means they are no longer orthogonal.

This effect is illustrated in figure 6 in this example, a subcarrier 1 and a delayed subcarrier 2 are shown. When an OFDM receiver tries to demodulate the first subcarrier, it will encounter some interference from the second subcarrier, because within the FFT interval, there is no integer number of cycles difference between subcarrier 1and 2. At the same time, there will be crosstalk from the first to the second subcarrier for the same reason.

This ensures that delayed replicas of the OFDM symbol always have an integer number of cycles within the FFT interval, as long as the delay is smaller than the guard time. As result, multipath signals with delays smaller than the guard time cannot cause ICI. As an example of how multipath effects OFDM, figure 8 shows received signal for tow-ray channel, where the dotted curve is a delayed replica of the solid curve.

Three separate subcarriers are shown during three symbol intervals. In reality, an OFDM receiver only sees the sum of all these signals, but showing the separate components makes it more clear what the effect of multipath is. From the fig that there can be degree phase jumps at the symbol boundaries. For the dotted curve, these phase jumps occur at a certain delay after the first path. In this particular example, this multipath delay is smaller than the guard time, which means there are no phase transition during the FFT interval.

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