OFDM Overview

OFDM is a special case of multi-carrier modulation where multiple low data rate carriers are combined by a transmitter to form a composite high data rate transmission and may be viewed as multiplexing technique. Fundamental concept of OFDM is the orthogonality of subcarriers in the frequency domain which is defined by

(1)

where f1(t) and f2(t) are the subcarrier frequencies and said to be orthogonal over the period T. While conventional FDM uses the frequency spacing of 2/T between neighboring subcarriers, OFDM uses the frequency spacing of 1/T, which is the minimum frequency spacing for orthogonality, between neighboring subcarriers by allowing the subcarrier spectra to overlap so that OFDM improves the spectral efficiency. Figure 1 shows the comparison of subcarrier spacing between the conventional FDM and OFDM.

Figure 1: Comparison of frequency spacing between conventional FDM and OFDM

Basic block diagram of OFDM transmitter and receiver is shown in Figure 2, where subcarrier modulation and demodulation can be efficiently implemented by IFFT (inverse fast Fourier transform) and FFT (fast Fourier transform) operation, respectively.

Figure 2: Basic block diagram of OFDM transmitter and receiver

One of main advantages of OFDM over single carrier (SC) system is the robustness against delay spread. Channel delay spread causes inter-symbol-interference (ISI) which cause irreducible error floor, hence limiting maximum data rate. However, symbol duration of each subcarrier in OFDM is  N times longer than that of single carrier system. Therefore, OFDM is more robust to delay spread. Although OFDM suffers from less ISI than SC, it still experiences some ISI. This ISI can be totally avoided by using cyclic prefix whose length is equal to or longer than maximum channel delay spread (for the channel which has K sample-spaced taps, the number of cyclic prefix guard samples should be Ng ≥ K - 1) as shown in Figure 3. Due to cyclic prefix, linear convolution of the channel impulse response and the signal becomes cyclic convolution.

Figure 3: Cyclic prefix for avoiding ISI

In addition, OFDM has several advantages over single carrier (SC) systems.

• Robustness against narrowband interference (impulse noise) owing to frequency diversity

• In a slow time-varying channel, implementation of adaptive bit loading or modulation is possible according to the signal-to-noise ratio (SNR) of a particular subcarrier.

• Supports single frequency network: since the main and relay broadcast transmitters can use the same carriers, it is possible to exploit receiver diversity in certain areas and therefore attractive for broadcast applications.

 On the other hand, there are some problems to be solved for a practical application.

• Since the time-domain OFDM signal is a sum of several sinusoids, it has a high peak-to-average power ratio (PAPR) which can cause nonlinear distortion of the signal at the transmit amplifier. (In-band and out-of-band distortion)
• High sensitivity to frequency offset errors which destroy orthogonality among subcarriers as shown in Figure 4.

Figure 4: Effect of frequency offset