Frequency Domain Equalization and Adaptive OFDM vs. Single Carrier Modulation

Introduction

The basic recipe followed in this article is as follows:

• 1.

  For investigating the transmission of digital signals we have used wideband frequency-selective radio channels.

  • It has been observed that the frequency-selective fading caused by multipath time delay spread degrades the performance of digital communication channels by causing inter-symbol interference, thus results in an irreducible BER and imposes an upper limit on the data symbol rate.

• 2.

  We have compared the performance of single carrier and multi-carrier modulation schemes for a frequency-selective fading channel considering un-coded modulation scheme.

  • Our analysis shows that the un-coded OFDM loses all frequency diversity present in the channel which results in a dip in the channel. As a result of this, the information data on the subcarriers, affected by the dip, is erased. Further, this erased information cannot be recovered from the other carriers. Consequently, it results in a poor Bit Error Rate (BER) performance. However, we can recover frequency diversity and improve the BER performance by adding sufficiently strong coding which spreads the information over multiple subcarriers.

Alternatively, the performance of OFDM can also be improved significantly by using different modulation schemes for the individual sub-carriers. In this scenario, the modulation schemes have to be adapted to the prevailing channel transfer function. Moreover, each modulation scheme provides a trade off between spectral efficiency and the bit error rate. The spectral efficiency can be maximized by choosing the highest modulation scheme that will give an acceptable (BER). In a multipath radio channel, frequency selective fading can result in large variation in the received power of each carrier.

The article is organized as follows: In section II the fixed and adaptive OFDM transmitters are described. A brief description of a single carrier system with frequency domain equalization is given in section III. Section IV deals with the simulations results. Finally, the main conclusions are drawn in section V.

Adaptive Ofdm Transmission

Figure 1 shows the block diagram of the OFDM transmitter used. As can be seen, Binary data is, first, fed to a modulator which generates complex symbols on its output. The modulator either uses a fixed signal alphabet (QAM) or adapts the signal alphabets of the individual OFDM sub-carriers. Both, signal alphabets and power distribution can be optimized corresponding to the channel transfer function. Because of the slow variation of transfer function with time (as shown by the propagation measurements of radio channels with fixed antennas), it is safe to assume that the instantaneous channel transfer function can be estimated at the receiver and can be communicated back to the transmitter. The third block transforms the symbols into time-domain using inverse fast Fourier transform (FFT) at the transmitter. The next block inserts the guard interval. The output signal is transmitted over the radio channel. At the receiver, the cyclic extension is removed and the signal is transformed back into frequency domain with an FFT. Prior to demodulation, the signal is equalized in frequency domain with the inverse of the transfer function of the radio channel corresponding to a zero-forcing equalizer.

Figure 1.

Block diagram of a) an OFDM and b) a single carrier transmission system with frequency domain equalization

In this article, we have considered two different adaptive modulator/demodulator pairs A and B. In modulator A, the distribution of bits on the individual sub-carriers is adapted to the shape of the transfer function of the radio channel. Modulator B optimizes simultaneously both, the distribution of bits and the distribution of signal power with respect to frequency.