Wireless Communications

Broadband wireless access systems deployed in residential and business environments are likely to face hostile radio propagation environments, with multipath delay spread extending over tens or hundreds of bit intervals. Orthogonal frequency-division multiplex (OFDM) is a recognized multicarrier solution to combat the effects of such multipath conditions. This article surveys frequency domain equalization (FDE) applied to single-carrier (SC) modulation solutions. SC radio modems with frequency domain equalization have similar performance, efficiency, and low signal processing complexity advantages as OFDM, and in addition are less sensitive than OFDM to RF impairments such as power amplifier nonlinearities. We discuss similarities and differences of SC and OFDM systems and coexistence possibilities, and present examples of SC-FDE performance capabilities.

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Frequency domain equalization for single-carrier broadband wireless systems

The authors discuss the potential of OFDM signaling, with its limitations and inherent problems, as well as another potential technique that has so far been overlooked: single-carrier transmission with frequency-domain equalization. The carrier synchronisation issue is dealt with before the authors introduce coded-OFDM (COFDM), which makes use of channel coding and frequency-domain interleaving. >

Transmission techniques for digital terrestrial TV broadcasting

Relying on basic tools such as eigensignals of linear time-invariant systems, linear and circular block convolution, and fast Fourier transforms (FFTs), this article develops a systematic discrete-time framework and designs novel systems for single- and multiuser wireless multicarrier communications-a field rich in signal processing challenges that holds great potential in various applications including audio/video broadcasting, cable television, modem design, multimedia services, mobile local area networks, and future-generation wideband cellular systems. Wireless multicarrier (MC) communication systems utilize multiple complex exponentials as information-bearing carriers. MC transmissions thus retain their shape and orthogonality when propagating through linear time-dispersive media, precisely as eigensignals do when they pass through linear time-invariant (LTI) systems.

Wireless multicarrier communications

Single carrier frequency division multiple access (SC FDMA), a modified form of orthogonal FDMA (OFDMA), is a promising technique for high data rate uplink communications in future cellular systems. SC-FDMA has similar throughput performance and essentially the same overall complexity as OFDMA. A principal advantage of SC-FDMA is the peak-to-average power ratio (PARR), which is lower than that of OFDMA. SC FDMA is currently a strong candidate for the uplink multiple access scheme in the long term evolution of cellular systems under consideration by the third generation partnership project (3GPP). In this paper, we give an overview of SC-FDMA. We also analyze the effects of subcarrier mapping on throughput and PARR. Among the possible subcarrier mapping approaches, we find that localized FDMA (LFDMA) with channel-dependent scheduling (CDS) results in higher throughput than interleaved FDMA (JFDMA). However, the PARR performance of IFDMA is better than that of LFDMA. As in other communications systems there are complex tradeoffs between design parameters and performance in an SC-FDMA system

Single carrier FDMA for uplink wireless transmission

For mobile radio and terrestrial broadcast applications, we compare orthogonal frequency-division multiplexing (OFDM) and single-carrier transmission with frequency-domain equalization. With respect to our earlier results, we include channel coding and frequency-domain interleaving which are necessary for OFDM systems on multipath fading channels characterized by deep notches in the signal spectrum. Our results indicate that a single-carrier system with a frequency-domain equalizer achieves similar performance to coded-OFDM, while alleviating the carrier synchronization and nonlinear distortion problems inherent to this technique.

Frequency-domain equalization of mobile radio and terrestrial broadcast channels

An analysis made of the performance of predistortion, equalization, and intersymbol interference (ISI) cancellation techniques in compensating for the transmit amplifier nonlinearity in digital microwave radio systems. The study is carried out using the 64 QAM and 256 QAM signal formats and two values of the roll-off factor in the Nyquist pulse shaping. The simulated compensation techniques include three types of predistortion, two ISI cancellers, and several nonlinear equalizers with or without decision feedback. A basic result is that decision-feedback equalizers do not offer any significant advantage over nonrecursive equalizers. It is also shown that ISI cancelers with a memoryless equalizer as first-stage decision device do not perform any better than nonlinear equalizers of similar complexity. Another contribution an improved fifth-order analog signal predistortion technique is analyzed. The gain that can be achieved using a modified 256 QAM signal constellation that is more robust to nonlinear distortion is quantified. >

Analysis of predistortion, equalization, and ISI cancellation techniques in digital radio systems with nonlinear transmit amplifiers

The authors present an efficient data predistortion technique with memory for compensation of high-power amplifier (HPA) nonlinearities in digital microwave radio systems employing quadrature amplitude modulation (QAM) signal formats. A practical implementation method is described which trades off performance against complexity and which makes it possible to implement this kind of predistorter in 256-QAM, and higher-level QAM systems. Using the 16-, 64-, and 256-QAM signal constellations, it is shown that the proposed technique achieves a considerably higher performance than that of conventional memoryless data predistortion of the predistortion technique with memory based on finite-order inverses of nonlinear systems. Specifically, numerical results show that the proposed technique achieves a gain that is in excess of 2 dB over conventional memoryless data predistortion. >

A data predistortion technique with memory for QAM radio systems

We introduce the concept of orthogonal frequency-division multiple access (OFDMA) and investigate its application to send upstream information from subscriber premises to the cable head-end in cable TV (CATV) networks. The carrier and timing synchronizations needed to ensure orthogonality between different subscriber signals are achieved by sending common pilot signals from the head-end. Whereas N demodulators are required to accommodate N simultaneous users in conventional FDMA systems, OFDMA only requires a single demodulator followed by an N-point discrete Fourier transform (DFT). We analyze the performance of this technique and compare it to time-division multiple access (TDMA) which appears in several international standards. Our results indicate that OFDMA is an attractive multiple access technique which offers increased robustness to narrowband interference, impulse noise, and other signal impairments which characterize typical CATV networks.

Georges Karam

Papers

Transmission techniques for digital terrestrial TV broadcasting

Frequency-domain equalization of mobile radio and terrestrial broadcast channels

Analysis of predistortion, equalization, and ISI cancellation techniques in digital radio systems with nonlinear transmit amplifiers

A data predistortion technique with memory for QAM radio systems

Orthogonal frequency-division multiple access and its application to CATV networks