A multiuser bit-loading problem is investigated in multicarrier communication systems. Assuming knowledge of all the channel gains, we propose a multiuser bit-loading algorithm that attempts to minimize the total power to transmit a target rate-sum of all users. It is shown that this algorithm is optimal when the interference among users is nonexistent or strong. The simulation results show that the proposed algorithm finds a near-optimal solution for very-high-speed digital subscriber line systems.

This paper investigates the multiuser bit and power allocation problem in discrete multi-tone digital subscriber line modems. A spectrum-management center with knowledge of direct and crosstalk-coupled channel gains allocates the bits and the available power to the subchannels for all users in a common binder. The center uses a multiuser discrete bit-loading algorithm that attempts to minimize the total transmit power given a target sum-rate. This algorithm extends the greedy algorithm for the single-user channel to the multi-user channel. Simulation results for the upstream transmission in very high-speed digital subscriber line show that the total power can be reduced using the multi-user discrete bit-loading algorithm instead of applying. the single-user greedy algorithm iteratively.

This paper investigates the rate and power control problem in a frequency-selective interference channel. The objective of the rate and power control is to maximize the achievable rate region, given an average power constraint for each user. We then propose a centralized algorithm with which a spectrum-management center determines the data rate and power of each user. The proposed algorithm is based on the multi-user discrete bit-loading algorithm that considers the bit and power allocation over frequency and users simultaneously. Simulation results for the upstream transmission in very high-speed digital subscriber line systems show that the proposed rate and power control algorithm enlarges the rate region achieved by the existing distributed algorithms.

The Shannon capacity equation defines the total data transmission capacity of a transmission channel as a function of receiver SNR and bandwidth. When the frequency-overlapped (FO) duplex mode of operation with an echo canceller is implemented on a digital subscriber loop (DSL) channel, then the near-end receiver SNR that determines the inbound capacity is not only a function of the far-end transmitted signal power, but also of the residual near-end transmit signal power that leaks through the echo canceller. This 'coupling' occurs on both ends of the copper loop for upstream and downstream reception. Consequently, maximization of the data transmission capacity is a "bidirectional" problem. This problem has not been analyzed in the literature. We define the notion of the total bidirectional data capacity of a copper loop and formulate the maximum bidirectional capacity that can be achieved when a power budget is allocated to each of the two transmitters. We then compute the channel capacity by an integer linear programming (ILP) technique for various channel and noise conditions and also compare the results with the data rates that are obtained by a modified "water-filling" algorithm designed for FO duplex operation and with the channel capacities of the conventional frequency-division (FD) simplex transmission.

Discrete multi-tone (DMT) based digital subscriber loop (DSL) modems must allocate power and information bits to the discrete tones for quadrature amplitude modulation. The bit allocation (also referred to as bit-loading) algorithms in the literature are designed for modems that operate in the simplex mode in which the upstream and downstream signals use nonoverlapping frequency bands. However, greater data transmission capacity can be achieved by using overlapping frequency bands and echo cancelled duplex operation. The conventional bit-loading algorithms are not well suited for the allocation of data bits for transmission in the duplex mode, and the problem of bit loading for duplex operation has not yet been considered in the literature. This article introduces an algorithm suitable for echo-canceled duplex DMT modems that use overlapping frequency bands for upstream and downstream transmissions for the purpose of transmitting equal data rates in both directions. Moreover, the algorithm performs the bit loading while jointly minimizing a weighted sum of the power used by the central office and the customer premise transmitters. Simulation results are included to show the higher data rates that are possible with the duplex operation as a function of varying levels of echo return loss enhancement.