It is shown that, particularly for terrestrial cellular telephony, the interference-suppression feature of CDMA (code division multiple access) can result in a many-fold increase in capacity over analog and even over competing digital techniques. A single-cell system, such as a hubbed satellite network, is addressed, and the basic expression for capacity is developed. The corresponding expressions for a multiple-cell system are derived. and the distribution on the number of users supportable per cell is determined. It is concluded that properly augmented and power-controlled multiple-cell CDMA promises a quantum increase in current cellular capacity. >

On the capacity of a cellular CDMA system

The objective of this survey is to provide the readers and practitioners in the industry with a broader understanding of the high peak-to-average power ratio (PAPR) problem in orthogonal frequency division multiplexing (OFDM) systems and generate a taxonomy of the available solutions to mitigate the problem. Beginning with a description of OFDM systems, the survey describes the most commonly encountered impediment of OFDM systems, the PAPR problem and consequent impact on power amplifiers leading to nonlinear distortion. The survey clearly defines the metrics based on which the performance of PAPR reduction schemes can be evaluated. A taxonomy of PAPR reduction schemes classifies them into signal distortion, multiple signaling and probabilistic, and coding techniques with further classification within each category. We also provide complexity analyses for a few PAPR reduction methods to demonstrate the differences in complexity requirements between different methods. Moreover, the paper provides insights into the transmitted power constraint by showing the possibility of satisfying the constraint without added complexity by the use of companding transforms with suitably chosen companding parameters. The rapid growth in multimedia-based applications has triggered an insatiable thirst for high data rates and hence increased demand on OFDM-based wireless systems that can support high data rates and high mobility. As the data rates and mobility supported by the OFDM system increase, the number of subcarriers also increases, which in turn leads to high PAPR. As future OFDM-based systems may push the number of subcarriers up to meet the higher data rates and mobility demands, there will be also a need to mitigate the high PAPR that arises, which will likely spur new research activities. The authors believe that this survey will serve as a valuable pedagogical resource for understanding the current research contributions in the area of PAPR reduction in OFDM systems, the different techniques that are available for designers and their trade-offs towards developing more efficient and practical solutions, especially for future research in PAPR reduction schemes for high data rate OFDM systems.

Peak-To-Average Power Ratio Reduction in OFDM Systems: A Survey And Taxonomy

A method and apparatus for allocating subcarriers in an orthogonal frequency division multiple access (OFDMA) system is described. In one embodiment, the method comprises allocating at least one diversity cluster of subcarriers to a first subscriber and allocating at least one coherence cluster to a second subscriber.

Ofdma with adaptive subcarrier-cluster configuration and selective loading

In recent years single carrier modulation (SCM) has again become an interesting and complementary alternative to multicarrier modulations such as orthogonal frequency division multiplexing (OFDM). This has been largely due to the use of nonlinear equalizer structures implemented in part in the frequency domain by means of fast Fourier transforms, bringing the complexity close to that of OFDM. Here a nonlinear equalizer is formed with a linear filter to remove part of intersymbol interference, followed by a canceler of remaining interference by using previous detected data. Moreover, the capacity of SCM is similar to that of OFDM in highly dispersive channels only if a nonlinear equalizer is adopted at the receiver. Indeed, the study of efficient nonlinear frequency domain equalization techniques has further pushed the adoption of SCM in various standards. This tutorial paper aims at providing an overview of nonlinear equalization methods as a key ingredient in receivers of SCM for wideband transmission. We review both hybrid (with filters implemented both in time and frequency domain) and all-frequency-domain iterative structures. Application of nonlinear frequency domain equalizers to a multiple input multiple output scenario is also investigated, with a comparison of two architectures for interference reduction. We also present methods for channel estimation and alternatives for pilot insertion. The impact on SCM transmission of impairments such as phase noise, frequency offset and saturation due to high power amplifiers is also assessed. The comparison among the considered frequency domain equalization techniques is based both on complexity and performance, in terms of bit error rate or throughput.

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Single Carrier Modulation With Nonlinear Frequency Domain Equalization: An Idea Whose Time Has Come—Again

A method and apparatus for subcarrier selection for systems is described. In one embodiment, a method for subcarrier for a system employing orthogonal frequency division multiple access (OFDMA) comprises partitioning subcarriers (101) into groups of at least one cluster of subcarriers (102), receiving an indication of a selection by the subscriber of one or more groups (103) in the groups, and allocating at least one cluster in the one or more groups of clusters selected by the subcarrier for use in communication with the subscriber (104, 105).

Multi-carrier communications with group-based subcarrier allocation

PROBLEM TO BE SOLVED: To reduce a considerable deterioration in characteristics on a propagation path of high correlation in a spatial multiplex transmission method. SOLUTION: The signal transmission system for transmitting a signal from a transmitter 10 having a transmission antenna to a receiver 20 having a reception antenna, is provided with a propagation path estimating means 1 for estimating a propagation path estimate indicating a status of a propagation path between the transmission antenna and the reception antenna for each transmission mode designating the transmission antenna to be used for signal transmission, a communication capacity estimating means 2 for estimating communication capacity for each transmission mode from the propagation path estimate, a transmission mode determining means 3 for determining the transmission mode fulfilling desired communication capacity on the basis of the estimated communication capacity for each transmission mode, and a transmitting signal generating means 4 for generating a transmitting signal to be transmitted in accordance with the determined transmission mode. COPYRIGHT: (C)2004,JPO&NCIPI

Signal transmission system, signal transmission method and transmitter

A frequency diversity communication system is provided whereby the number of hopping channels is reduced and a more continuous communication achieved as a mobile station travels from cell to cell by the combination of chips of symbols from the mobile sources. The transmitter divides each symbol of an input symbol sequence into a number K of chips, which are modulated with mutually different carrier frequencies. The receiver demodulates the signal sent by the transmitter by local frequencies corresponding respectively to the K different carrier frequencies. According to this invention, the carrier phase is coherent for each frequency of the received wave. Moreover, because the carrier phase is coherent, the outputs for each frequency of the received wave can be combined with the use of a training signal. The receiver also performs an estimation of the maximum likelihood of the transmitted symbol to eliminate the need for decoding. It is also provided that the order of the chips of the symbols is rearranged within a burst.

Frequency diversity transmitter and receiver

An impluse response hmn(q) of each transmission path is estimated from N received signals rm (m=1, . . . , M) and a known signal (for a number of users equal to N, n=1, . . . , N). M×N matrix H (q) having hmn(q) as an element and a Q×Q matrix H having H(q) as an element are determined (where Q represents a number of multipaths of each transmitted wave and q=0, . . . , Q−1). A soft decision value b′n(k) is determined from decoded λ2 [bn(k)], and this is used to generate an interference component matrix B′(k) to generate an interference replica H·B′(k). The interference replica H·B′(k) is subtracted from a received matrix y(k) to determine y′(k). y(k) and H are used to determine an adaptive filter coefficient wn(k) to be applied to an n-th user in order to eliminate residual interference components in y′(k) according to the minimum mean square error criteria. y(k) is passed through wn(k) to provide a log-likelihood ratio as a received signal from the user n from which interferences are eliminated.

Turbo-reception method and turbo-receiver

A new space-time turbo equalization algorithm is derived for frequency-selective multiple-input-multiple-output (MIMO) channels with unknown interference. The algorithm is an extension of our proposed MIMO equalization algorithm , which performs joint channel estimation, multiple users' signal detection, and decoding, all in an iterative manner. This paper's proposed algorithm uses estimates of the correlation matrix of composite unknown interference-plus-noise components to suppress the unknown interference while effectively separating multiple users' signals to be detected (referred to as "known user" later). The correlation matrix of the composite unknown interference-plus-noise components can be estimated by time averaging the instantaneous empirical correlation matrix over the training period. Since the iterative channel estimation yields better channel estimates as more iterations are performed, thereby the estimate of the correlation matrix of the unknown interference-plus-noise components also becomes more accurate. This results in better signal detection performances, even in the presence of unknown interferers. A series of computer simulations show that this paper's proposed algorithm can properly separate known users' signals while suppressing unknown interference.

A MIMO turbo equalizer for frequency-selective channels with unknown interference

A receiver that receives a signal from a transmitter, including: a plurality of antennas; a channel estimation unit; a channel accumulation unit; a transmission weight generator; a feedback information transmission unit; and a reception weight generator. The plurality of antennas receive a second signal from the transmitter, the second signal being transmitted by using first feedback information. The channel estimation unit estimates a communication channel state on the basis of the second signal, and then outputs second channel estimation information. The reception weight generator generates reception weight information to be multiplied by the second signal, on the basis of first channel estimation information accumulated in the channel accumulation unit and a first transmission weight.

Shigeru Tomisato

Papers

Signal transmission system, signal transmission method and transmitter

Frequency diversity transmitter and receiver

Turbo-reception method and turbo-receiver

A MIMO turbo equalizer for frequency-selective channels with unknown interference

Multiple-output multiple-input (MIMO) communication system, MIMO receiver and MIMO receiving method