Peter J. W. Melsa

Impulse response shortening for discrete multitone transceivers

In discrete multitone (DMT) transceivers an intelligent guard time sequence, called a cyclic prefix (CP), is inserted between symbols to ensure that samples from one symbol do not interfere with the samples of another symbol. The length of the CP is determined by the length of the impulse response of the effective physical channel. Using a long CP reduces the throughput of the transceiver, To avoid using a long CP, a short time-domain finite impulse response (FIR) filter is used to shorten the effective channels impulse response. This paper explores various methods of determining the coefficients for this time-domain filter. An optimal shortening and a least-squares (LS) approach are developed for shortening the channels impulse response. To provide a computationally efficient algorithm a variation of the LS approach is explored. In full-duplex transceivers the length of the effective echo path impacts the computational requirements of the transceiver. A new paradigm of joint shortening is introduced and three methods are developed to jointly shorten the channel and the echo impulse responses in order to reduce the length of the CP and reduce computational requirements for the echo canceller.

Echo cancellation for asymmetrical digital subscriber lines

Discrete multitone modulation has been chosen to provide the high speed link for asymmetrical digital subscriber lines (ADSL). Modulating bidirectional digital traffic over the existing twisted pair loop plant necessitates the used of a high performance echo canceller. This paper investigates an echo canceller structure for ADSL. A combination frequency domain echo canceller and a time domain echo synthesizer are detailed along with timing and fixed point effects. Detailed simulation results show the canceller performance is more than adequate and does not limit the transceivers performance. Complexity issues are addressed and show that an effective canceller can be implemented with only modest computational requirements. Hence, there exists an effective and efficient echo canceller for ADSL.<<ETX>>

A neural network solutions for routing in three stage interconnection networks

A neural network solution to the problem of routing calls through a three-stage interconnection network is presented. The neural network is shown, via a theorem with proof, to select an open path through the interconnection network if one exists. The solution uses a Hopfield network with a binary threshold rather than a sigmoidal function. The weights of the neural network are fixed for all time, and thus are independent of the current state of the interconnection network. It is possible to implement various routing strategies through selection of inputs to the neural network, again independently of the weights. The convergence proof is based on a hypercube analysis technique that defines and locates all local minima of the neural network energy function. When one or more open paths exist, it is shown that all local minima correspond to such paths, and therefore convergence to a minimum is equivalent to selection of an open path. When no such path is available, the energy function is unimodal and the neural network converges to a null state indicating that the interconnection network is blocked.<<ETX>>

A neural network solution for call routing with preferential call placement

A neural network solution to the problem of routing calls through a three-stage interconnection network is presented. The solution uses a Hopfield network with a binary threshold, rather than a sigmoidal function. An important feature of this solution is that the weights of the neural network are fixed for all time and are independent of the current state of the interconnection network. It is possible to implement various preferential call placement strategies through selection of external inputs to the neural network, again independent of the weights. The performance of the call placement strategy is guaranteed. The operation of the neural network solution is discussed in terms of a digital logic implementation that could be used to realize the same functionality.<<ETX>>

Joint optimal impulse response shortening

In discrete multitone a cyclic prefix (CP) is inserted between symbols to ensure that samples from one symbol do not interfere with the samples of another symbol. The CP length is determined by the impulse response length of the effective physical channel. The cyclic prefix reduces the throughput of the transceiver. When the transceiver is used in a full-duplex environment the portion of the receive signal arising from echo must be reduced through the use of echo cancellation, the complexity of which is directly related to the length of the echo path impulse response. To alleviate both problems a short time-domain FIR filter can be used to shorten both the effective channel and echo impulse responses. This paper explores a joint optimal algorithm for determining the coefficients of this time-domain filter, resulting in reduced ISI and echo canceller complexity. For the simulations a configuration is chosen which is representative of that encountered by the remote terminal in the asymmetrical digital subscriber lines (ADSL) environment.

Joint impulse response shortening

In discrete multitone (DMT) transceivers an intelligent guard time sequence, called a cyclic prefix, is inserted between symbols to ensure that samples from one symbol do not interfere with the samples of another symbol. The length of the cyclic prefix is determined by the length of the impulse response of the effective physical channel. Using a long cyclic prefix reduces the throughput of the transceiver. To avoid using a long cyclic prefix, a short time-domain FIR filter is used to shorten the effective channels impulse response. In full duplex transceivers echo cancellation may be required. The complexity of the echo canceller is directly related to the length of the echo path. This paper explores a joint shortening approach for determining the coefficients for this time-domain filter to jointly shorten both the channel and echo path impulse responses.

Asynchronous communication and symbol synchronization in multipoint-to-point multicarrier systems

Discrete multitone (DMT) has been selected as the modulation technique for multipoint-to-point systems such as cable telephony and in home networks over conventional single-carrier techniques by several vendors, because it provides superior impulse noise immunity, frequency diversity, and spectral efficiency for many communication systems. This paper provides novel solutions for two key issues in such DMT and OFDM systems. First, a technique for in-band asynchronous communication, essential for adding new remote units, is presented. Second, a robust symbol synchronization technique, especially for aligning the transmissions of the remote units at the headend (the central site), is presented. Both techniques are highly efficient and cause negligible degradation in SNR.