ADVANTAGES OF SC-FDE OVER OFDM

D. J. SANGHVI COLLEGE OF ENGINEERINGADVANTAGES OF SC-FDE OVER OFDMSUBJECT: MILLIMETER WAVE COMMUNICATION SYSTEMS

ElijahZac4/1/2014

INTRODUCTION

This report is written so as to emphasize the advantages of a single-single-carrier frequency domain equalizer(SC-FDE) with and without decision feedback equalization (DFE) compared to the orthogonal frequency division multiplexing (OFDM) technology. SC-FDE is a promising technique and it solves the power consumption issue of power amplifier (PA) in high rate (beyond gigabit/s) millimetre wave wireless systems, and lets the millimeter wave related products and devices become commercially viable. The issue of power consumption of PA in millimeter wave wireless systems is identified and the potential benefits in using the SC-FDE technique due to its inherently low peak-to-average power ratio (PAPR) to replace the OFDM technique are emphasized.

OFDM DRAWBACKS AND SOLUTION

SC-FDE gives the millimeter wave communication systems distinct advantages over its competitor, OFDM. It is known that OFDM suffers from a number of drawbacks, including a large PAPR, intolerance to amplifier nonlinearities, and high sensitivity to carrier frequency offsets. These problems become extremely severe when radio frequency goes to the millimeter wave range. The OFDM technology does have a significant advantage of offering the robustness against frequency selective fading. An alternative solution to ISI mitigation is the use of single-carrier (SC) modulation combined with frequency domain equalization (FDE). The complexity and performance of SC-FDE systems are comparable to that of OFDM while avoiding the above drawbacks associated with multi-carrier (MC) implementation. On the other hand, FDE does not represent an optimal solution to signal detection over ISI channels and SC systems cannot certainly offer the same flexibility as OFDM systems in the management of bandwidth and energy resources, both in single and multi-user communications. All these considerations have made a detailed analysis of SC-FDE techniques as well as a careful comparison between SC-FDE and OFDM much more critical.

TRANSMITTER AND RECEIVER OF OFDM

The transmitter and receiver block diagrams of OFDM wireless systems are shown in Figures below.

OFDM TRANSMITTER:

The transmitter comprises a quadrature amplitude modulation (QAM) modulator, an inverse fast Fourier transformation (IFFT), a cyclic prefix insertion module, a radio frequency (RF) modulator, and an antenna. Firstly, an input signal to be modulated and transmitted is sent to the QAMmodule. The IFFT module applies an inverse fast Fourier transformation (FFT) transformation on the signal received from the QAM module. The cyclic prefix insertion module inserts a cyclic prefix into the signal received from the IFFT module. The RF module converts the signal received from the cyclic prefix insertion module into a modulated RF signal, which is then radiated by the antenna.

OFDM RECEIVER

The receiver comprises an antenna, a RF receiver, a cyclic prefix removal module, a FFT module, a channel equalizer, a channel estimation module, and a QAM demodulator. The antenna converts the received electromagnetic signal into a RF electrical signal, which is then converted into a baseband signal by the RF receiver.After removing the inserted cyclic prefix by the cyclic prefix removal module, the received signal is then passed through the FFT module. The channel estimation module estimates the channel quality and other characteristics of the dynamic wireless channel between the transmitter and receiver, which are required by the channel equalizer to compensate for the frequency response of the wireless channel. The output of the channel equalizer is then passed to the QAM demodulator for recovering the transmitted data.

TRANSMITTER AND RECEIVER OF SC-FDE

In contrast to the MC nature of OFDM wireless systems shown in Figures above, the transmitter and receiver block diagrams of SC-FDE wireless systems are depicted in Figures below. The transmitter and receiver of SC-FDE operate based on single carrier.

SC-FDE TRANSMITTER

The transmitter of SC-FDE wireless systems shown above appears to be similar to that of OFDM systems illustrated, except that it does not contain an IFFT module.

SC-FDE RECEIVER

Likewise, the receiver of SC-FDE systems depicted in Figure above looks very similar to that of OFDM, except that it has an extra IFFT module between the channel equalizer and the QAM demodulator. It is clear that the complexity of SC-FDE wireless systems depicted in above Figure is almost the same as that of OFDM wireless systems. For a SC-FDE system, the transmitter and receiver can be merged in a single mobile device. Moreover, the whole transceiver can be integrated in a semiconductor chip, which also comprises additional modules to extend the functions for the customer.

ADVANTAGES OF SC-FDE OVER OFDMThe main advantages of SC-FDE systems are listed as follows:

The energy of individual symbols is transmitted over the whole available frequency spectrum. Therefore, narrow band notches within the channel transfer function have only a small impact on the performance. For OFDM systems, however, the narrow band notches would degrade the performance of transmitted symbols assigned over the relevant subcarriers. Of course, the diversity can be regained partly by utilizing a soft decision error control decoder with some performance penalty. Lower PAPR ratio for the radiated signal makes the PA from the transmitter (Tx) side more energy efficient and cheaper, especially for the millimeter wave wireless systems. SC-FDE is robust to the effect of phase noise, which makes the local oscillator (LO) simpler, especially for the millimeter wave wireless systems. A smaller number of analogue-to-digital converter (ADC) bits, significantly reduces the design and system cost for high rate applications. The carrier frequency error between the Tx side and the receiver (Rx) side can destroy the orthogonality between subcarriers and introduce the inter-subcarrier interference for OFDM systems. However, it has less effect on single-carrier systems with frequency domain equalization.Therefore, SC-FDE is more suitable to the user scenarios where a simple transmitter with less power consumption is desired, while at the same time the receiver can be complex and have relatively high power consumption, such as high definition television scenarios. In terms of power consumption and environment impact, SC-FDE with its low PAPR ratio is definitely an attractive technology for millimeter wave wireless systems.

PAPR COMPARISON BETWEEN SC-FDE AND OFDM

An OFDM system has to back off considerably more than SC-FDE. If multiple power amplifiers are used, the power back-off is not necessary, but the power consumption increases with higher PAPR. For Class A amplifiers, the power consumption is proportional to the peak power. The higher the PAPR is, the higher the power consumption will be. Also, the cost increases almost proportionally with the power consumption. An example below demonstrates that a SC-FDE has lower PAPR than an OFDM system.

When PAPR is considered at Pr (PAPR>)=0.01 and Psat is assumed to be 16 dBm, a PAPR comparison between SC-FDE and OFDMis shown in Figure above. It is clear that PAPR gain (between SC-FDE and OFDM) for quadrature phase shift keying (QPSK), non-square 8-ary QAM and 16-ary QAM is about 4.7 dB, 3.6 dB, and 3.0 dB, respectively.

PERFORMANCE COMPARISON BETWEEN SC-FDE AND OFDM

Performance comparisons between OFDM and SC-FDE wireless systems are shown below. When a nonlinear PA model is used and about 5.5 dB output back-off (OBO) is considered, the performance comparison between OFDM and SCFDE wireless systems is illustrated in the figure below, whereby Rc is the code rate. It is concluded from the figure below that better packet error rate (PER) performance can be achieved when the SC-FDE technology is used for millimeter wave wireless personal area networks (WPAN).

CONCLUSION

SC-FDE is a promising technique and it solves the power consumption issue of power amplifier (PA) in high rate (beyond gigabit/s) millimetre wave wireless systems, and lets the millimeter wave related products and devices become commercially viable. The issue of power consumption of PA in millimeter wave wireless systems was identified and the potential benefits in using the SC-FDE technique due to its inherently low peak-to-average power ratio (PAPR) to replace the OFDM technique have been emphasized.