Wireless Communications

Information Theory and Reliable Communication

We propose a variable-rate and variable-power MQAM modulation scheme for high-speed data transmission over fading channels. We first review results for the Shannon capacity of fading channels with channel side information, where capacity is achieved using adaptive transmission techniques. We then derive the spectral efficiency of our proposed modulation. We show that there is a constant power gap between the spectral efficiency of our proposed technique and the channel capacity, and this gap is a simple function of the required bit-error rate (BER). In addition, using just five or six different signal constellations, we achieve within 1-2 dB of the maximum efficiency using unrestricted constellation sets. We compute the rate at which the transmitter needs to update its power and rate as a function of the channel Doppler frequency for these constellation sets. We also obtain the exact efficiency loss for smaller constellation sets, which may be required if the transmitter adaptation rate is constrained by hardware limitations. Our modulation scheme exhibits a 5-10-dB power gain relative to variable-power fixed-rate transmission, and up to 20 dB of gain relative to nonadaptive transmission. We also determine the effect of channel estimation error and delay on the BER performance of our adaptive scheme. We conclude with a discussion of coding techniques and the relationship between our proposed modulation and Shannon capacity.

Variable-rate variable-power MQAM for fading channels

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

In this paper, we present a finite-granularity, loading algorithm for a discrete multitone (DMT) modulation system. The proposed algorithm offers significant implementational advantages over the well-known water-pouring method and the earlier Hughes-Hartogs algorithm, while typically suffering only negligible performance degradation relative to the optimal solution. We also present simulation results of this loading algorithm applied to the newly proposed asymmetric digital subscriber lines (ADSL) service. >

A practical discrete multitone transceiver loading algorithm for data transmission over spectrally shaped channels

In a multicarrier data transmission system the length of a symbol is often limited by the maximum permissible delay of data from input to output. If the length of the impulse response of the channel is not negligible compared to the permitted symbol length, the impulse response can be shortened by passing the received signal through a time-domain equalizer before modulation. The best performance for a given computational complexity can then be achieved by appending to each block of samples of the transmit signal a cyclic prefix that has the same length as the shortened impulse response. Several algorithms for designing the time-domain equalizer, which use adaptation in the frequency domain and windowing in the time domain in order to minimize the mean squared error of the equalized response, are described. >

Equalizer training algorithms for multicarrier modulation systems

A variety of bi-directional data transmission systems that facilitate communications between a plurality of remote units (15) and a central unit (10) using a frame based discrete multi-carrier transmission scheme are disclosed. In each of the systems, frames transmitted from the plurality of remote units (15) are synchronized at the central unit (10). A variety of novel modem arrangements and methods for coordinating communications between a plurality of remote units (15) and a central unit (10) to facilitate multi-point-to-point transmission are disclosed. The invention has application in a wide variety of data transmission schemes including Asymmetric Digital Subscriber Line systems that includes the transmission of signals over twisted pair, fiber and/or hybrid telephone lines, cable systems that includes the transmission of signals over a coaxial cable, and digital cellular television systems that include the transmission of radio signals.

Method and apparatus for coordinating multi-point to point communications in a multi-tone data transmission system

A variety of methods of coordinating communications between a plurality of remote units and a central unit to facilitate communications using a frame based discrete multi-tone (DMT) transmission scheme am disclosed. In one aspect of the invention, synchronized quiet times are periodically provided in the upstream communication stream. The synchronized quiet times are used to handle a variety of control type functions such as synchronization of new remote units, transmission channel quality checking and handling data transfer requests. The information received is used to facilitate the dynamic allocation of bandwidth during use. In another aspect, a data request signal may be used to indicate either a desire to transmit at a particular data rate or a desire to transmit a particular amount of information. In the former case, the central unit allocates sufficient sub-channels to the remote unit to facilitate transmission at a requested data rate that is specified in the data request information. In the latter case, the central unit allocates one or more sub-channels for an amount of time sufficient to transmit an amount information that is specified in the data request information.

Methods for coordinating upstream discrete multi-tone data transmissions

A receiver or receiver system for high speed data communications having a radio frequency noise canceller (112) is disclosed. The radio frequency noise canceller (112) removes radio-frequency noise from received signals over a transmission medium by adaptively estimating the radio-frequency noise during data transmission. In one embodiment, the radio frequency noise canceller includes: an adaptive filter (208) for producing a noise cancellation signal by filtering a reference noise signal based on filter parameters, a subtractor (202) for subtracting the noise cancellation signal from the first signal to produce the second signal; and an update circuit (204) for enabling, at predetermined times, modification of the parameters of the adaptive filter based on the then existing second signal. A method for removing radio frequency noise is also disclosed.

John A. C. Bingham

Papers

A practical discrete multitone transceiver loading algorithm for data transmission over spectrally shaped channels

Equalizer training algorithms for multicarrier modulation systems

Method and apparatus for coordinating multi-point to point communications in a multi-tone data transmission system

Methods for coordinating upstream discrete multi-tone data transmissions

Radio frequency noise canceller