High peak-to-average power ratio of the transmit signal is a major drawback of multicarrier transmission such as OFDM or DMT. This article describes some of the important PAPR reduction techniques for multicarrier transmission including amplitude clipping and filtering, coding, partial transmit sequence, selected mapping, interleaving, tone reservation, tone injection, and active constellation extension. Also, we make some remarks on the criteria for PAPR reduction technique selection and briefly address the problem of PAPR reduction in OFDMA and MIMO-OFDM.

An overview of peak-to-average power ratio reduction techniques for multicarrier transmission

The authors propose a very effective and flexible peak power reduction scheme for orthogonal frequency division multiplexing (OFDM) with almost vanishing redundancy. This new method works with arbitrary numbers of subcarriers and unconstrained signal sets. The core of the proposal is to combine partial transmit sequences (PTS) to minimise the peak-to-average power ratio distortion.

OFDM with reduced peak-to-average power ratio by optimum combination of partial transmit sequences

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

Network information theory promises high gains over simple point-to-point communication techniques, at the cost of higher complexity. However, lack of structured coding schemes limited the practical application of these concepts so far. One of the basic elements of a network code is the binning scheme. Wyner (1974, 1978) and other researchers proposed various forms of coset codes for efficient binning, yet these schemes were applicable only for lossless source (or noiseless channel) network coding. To extend the algebraic binning approach to lossy source (or noisy channel) network coding, previous work proposed the idea of nested codes, or more specifically, nested parity-check codes for the binary case and nested lattices in the continuous case. These ideas connect network information theory with the rich areas of linear codes and lattice codes, and have strong potential for practical applications. We review these developments and explore their tight relation to concepts such as combined shaping and precoding, coding for memories with defects, and digital watermarking. We also propose a few novel applications adhering to a unified approach.

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Nested linear/lattice codes for structured multiterminal binning

Recent theoretical results describing the sum-capacity when using multiple antennas to communicate with multiple users in a known rich scattering environment have not yet been followed with practical transmission schemes that achieve this capacity. We introduce a simple encoding algorithm that achieves near-capacity at sum-rates of tens of bits/channel use. The algorithm is a variation on channel inversion that regularizes the inverse and uses a "sphere encoder" to perturb the data to reduce the energy of the transmitted signal. The paper is comprised of two parts. In this second part, we show that, after the regularization of the channel inverse introduced in the first part, a certain perturbation of the data using a "sphere encoder" can be chosen to further reduce the energy of the transmitted signal. The performance difference with and without this perturbation is shown to be dramatic. With the perturbation, we achieve excellent performance at all signal-to-noise ratios. The results of both uncoded and turbo-coded simulations are presented.

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A vector-perturbation technique for near-capacity multiantenna multiuser communication-part II: perturbation

The limitations of present automatic and adaptive equalisers stem from the use of feedforward transversal filters. These drawbacks may be obviated by using a feedback transversal filter, the inverse filter, but this is only suitable for limited use since it can be an unstable circuit. It is shown that modulo arithmetic may be used in the inverse filter to eliminate completely the possibility of instability, and a very simple automatic or adaptive equalisation system is presented. Some interesting properties of the modulo inverse filter are included.

New automatic equaliser employing modulo arithmetic

A selective scrambling technique is proposed for reducing the peak to average power ratio (PAPR) in QPSK-OFDM systems, while incurring negligible redundancy. Formulation of an appropriate function for selecting between scrambled sequences is discussed, and simulation shows that scrambling reduces the PAPR of the OFDM signal to typically 2% of the maximum possible value.

Peak to average power reduction for OFDM schemes by selective scrambling

In an earlier transactions paper, Rosnes and Ytrehus presented an efficient algorithm for determining all stopping sets of low-density parity-check (LDPC) codes, up to a specified weight, and also gave results for a number of well-known codes including the family of IEEE 802.16e LDPC codes, commonly referred to as the WiMax codes. It is the purpose of this short paper to review the algorithm for determining the initial part of the stopping set weight spectrum (which includes the codeword weight spectrum), and to provide some improvements to the algorithm. As a consequence, complete stopping set weight spectra up to weight 32 (for selected IEEE 802.16e LDPC codes) can be provided, while in previous work only stopping set weights up to 28 are reported. In the published standard for the IEEE 802.16e codes there are two methods of construction presented, depending upon the code rate and the code length. We compare the stopping sets of the resulting codes and provide complete stopping set weight spectra (up to five terms) for all IEEE 802.16e LDPC codes using both construction methods.

Addendum to “An Efficient Algorithm to Find All Small-Size Stopping Sets of Low-Density Parity-Check Matrices”

The paper describes a high-capacity blind video watermarking system invariant to geometrical attacks such as shift, rotation, scaling and cropping. A spatial domain reference watermark is used to obtain invariance to geometric attacks by employing image registration techniques to determine and invert the attacks. A second, high-capacity watermark, which carries the data payload, is embedded in the wavelet domain according to a human visual system model. This is protected by a state-of-the-art error correction code (turbo code). For a false detection probability of 10/sup -8/, the proposed system is invariant to scaling up to 180%, rotation up to 70/spl deg/, and arbitrary aspect ratio changes up to 200% on both axes. Furthermore, the system is virtually invariant to any shifting, cropping, or combined shifting and cropping. The system is also robust to MPEG2 compression, even when combined with shifting and cropping.

DWT-based high-capacity blind video watermarking, invariant to geometrical attacks

The present disclosure includes systems and methods for direct packet communications and store and forward packet communications including packets which have attributes which determine the lifetime of the packet contents and these lifetimes are optionally a function of the recipient. Example methods are given featuring the transmission of packets with limited lifetime, the storing and retransmission of packets to one or more recipients and confirmation of deletion of packet contents. It is also shown that cryptography may be employed to ensure that timed presentation of packet contents to recipients takes place and is authenticated by the sender.

Martin Tomlinson

Papers

New automatic equaliser employing modulo arithmetic

Peak to average power reduction for OFDM schemes by selective scrambling

Addendum to “An Efficient Algorithm to Find All Small-Size Stopping Sets of Low-Density Parity-Check Matrices”

DWT-based high-capacity blind video watermarking, invariant to geometrical attacks

Method and Apparatus for Time Limited Messages in Packet Communications