Ashish Pandharipande

Principles of OFDM

After an introduction to the main principles of OFDM the author discusses the basic OFDM model, channel shortening, synchronization, and loading algorithms.

Optimum DMT-based transceivers for multiuser communications

The paper considers discrete multitone (DMT) modulation for multiuser communications when multiple users are supported by the same system, a zero-padding redundancy is employed at the transmitter output, and linear redundancy removal is used at the channel output. These users may have differing quality of service (QoS) requirements, as quantified by bit rate and symbol-error rate specifications, and are each assigned an equal number of subchannels. Our goal is to minimize the transmitted power, given the QoS specifications and subject to the knowledge of the channel and the second-order statistics of the colored interference at the receiver input. In particular, we find an optimum bit-loading scheme that distributes the bit rate transmitted across the various subchannels belonging to each user, and, subject to this bit allocation, we determine the precise subchannels assigned to each user, the optimum transceiver characterized by the input/output block transforms, and the redundancy removal operation. A major conclusion is that even though the optimum bit-rate allocation differs from the single-user case, the optimum transceiver does not. Further, it is determined entirely by the channel and interference conditions, and is unaffected by the QoS requirements.

Optimum multiflow biorthogonal DMT with unequal subchannel assignment

We consider the design of biorthogonal, as opposed to orthonormal, discrete multitone (DMT) systems supporting multiple services in a single antennae setting. The services may have differing quality of service (QoS) requirements, quantified by bit rate and symbol error rate specifications. Different users on the system can be potentially assigned different number of subchannels. Our goal is to minimize the transmitted power given the QoS specifications for the different users, subject to the knowledge of the channel and colored interference at the receiver input of the DMT system. We find an optimum bit loading scheme that distributes the bit rate transmitted across the various subchannels, where the precise subchannels are assigned to each user, and an optimum transceiver. Key conclusions are i) relaxing the orthonormality constraint yields no performance improvement; ii) the optimum transceiver is unaffected by changing service characteristics, and depends only on the channel and interference conditions; iii) the QoS requirements, the number of users, and the number of subchannels assigned to the different users only affect bitloading and subchannel assignment.

Optimal transceivers for DMT based multiuser communication

This paper considers discrete multitone (DMT) modulation for multiuser communications where different users are supported by the same system. These users may have differing quality of service (QoS) requirements, as quantified by their respective bit rate and symbol error rate specifications. Our goal is to minimize the transmitted power given the QoS specifications for the different users, subject to the knowledge of colored interference at the receiver input. In particular we find an optimum bit loading scheme that distributes the bit rate transmitted across the various subchannels belonging to the different users, and subject to this bit allocation, determine an optimum transceiver.

On biorthogonal nonuniform filter banks and tree structures

This paper concerns biorthogonal nonuniform filter banks. It is shown that a tree structured filter bank is biorthogonal if it is equivalent to a tree structured filter bank whose matching constituent levels on the analysis and synthesis sides are themselves biorthogonal pairs. We then show that a stronger statement can be made about dyadic filter banks in general: That a dyadic filter bank is biorthogonal if both the analysis and synthesis banks can be decomposed into dyadic trees. We further show that these decompositions are stability and FIR preserving. These results, derived for filter banks having filters with rational transfer functions, thus extend some of the earlier comparable results for orthonormal filter banks.

Optimum biorthogonal DMT systems for multi-service communication

This paper considers the design of biorthogonal DMT multicarrier transceiver systems supporting multiple services. The supported user services may have differing quality of service (QoS) requirements, quantified in this paper by bit rate and symbol error rate specifications. To reflect their service priorities, different users on the system can be potentially assigned different number of subchannels. Our goal is to minimize the transmitted power given the QoS specifications for the different users, subject to the knowledge of colored interference at the receiver input of the DMT system. In particular we find an optimum bit loading scheme that distributes the bit rate transmitted across the various subchannels belonging to the different users, and subject to this bit allocation, determine an optimum transceiver. This work differs from our prior work where the same number of subchannels were assigned to each user.

Subband coding of cyclostationary signals with static bit allocation

We consider the optimal orthonormal subband coding of zero mean wide sense cyclostationary signals (WSCS), with N-periodic second order statistics. An M-channel uniform filterbank, with N-periodic analysis and synthesis filters, is used as the subband coder. A static bit allocation scheme is used. An average variance condition is used to measure the output distortion. The conditions for maximizing the coding gain parallel those for the case when the signals are wide sense stationary (WSS) and the analysis and synthesis filters and the bit allocation time invariant.

Optimum multiuser OFDM systems with unequal subchannel assignment

This paper considers the design of multicarrier transceiver systems like OFDM/DMT supporting multiple users. The supported users may have differing quality of service (QoS) requirements, quantified by their respective bit rate and symbol error rate specifications. Different users on the system can be potentially assigned different number of subchannels, to reflect their service priorities. Our goal is to minimize the transmitted power given the QoS specifications for the different users, subject to the knowledge of colored interference at the receiver input of the OFDM system. In particular we find an optimum bit loading scheme that distributed the bit rate transmitted across the various subchannels belonging to the different users, and subject to this bit allocation, determine an optimum transceiver.

Timing estimation in multiple-antenna systems over Rayleigh flat-fading channels

In this paper, we investigate the problem of timing estimation in multiple-antenna systems with the aid of training signals. A slow, independent and identically distributed (i.i.d.) Rayleigh flat-fading channel model is considered. We derive two maximum likelihood (ML) timing estimators based on two different approaches, namely treating the channel deterministic and random, respectively, and present the corresponding Cramer-Rao bounds (CRBs). Then the optimal designs of training signals based on some figures of merit associated with the CRBs are discussed.

A new algorithm for optimum bit loading in subband coding

In this paper, we present an efficient bit loading algorithm for subband coding. The goal is to effect an optimal distribution of positive integer bit values among various subchannels to achieve a minimum distortion error variance. Existing algorithms in the literature grow with the total number of bits that must be distributed. The novelty of our algorithm lies in the fact that its complexity is independent of the total number of bits to be allocated.