Sundeep Venkatraman

The Constrained Capacity of Single and Multiuser Channels with Unknown Fading

This work presents a general technique to compute tight upper and lower bounds on the constrained capacity of both the point to point and multiuser Rayleigh flat fading channel with no channel state information (CSI) at the transmitter or the receiver. The paper begins by proving that the optimal input distribution for the point-to-point fading channel must have uniform i.i.d phase and therefore, for constant envelope transmitters, the only design parameter is the density of the driving constellation. The paper then presents plots of the (essentially coincident) upper and lower bounds on the constrained capacity for BPSK,QPSK and 8-PSK inputs as a function of SNR for the point to point channel. These allow the designer to judge the benefits of different channel alphabet choices and to compare the efficiency of the final coding system with the capacity of this channel and modulation alphabet combination. The paper then goes on to extend these methods to multiuser fading channels.

Tight upper and lower bounds on the constrained capacity of non-coherent multi-antenna channels

This paper first presents an optimal receiver and coding scheme for a multi-user, multi-receiver, non-coherent Rayleigh flat fading channel which achieves the channels constrained capacity (i.e., the capacity for a fixed input distribution). The paper then goes on to analyze the optimal receiver and uses it to generate tight upper and lower bounds on the constrained capacity. Knowing this constrained capacity is essential to judge the efficiency of coding schemes for the channel; it performs the same function as BPSK capacity for the AWGN channel in judging LDPC codes. The paper then goes on to present a practicable correlation based receiver that uses a novel iterative channel estimation technique. The performance of this receiver remains close to the constrained capacity as long as the per-user SNR is low; there is no requirement on the aggregate SNR. With 50 users, 50 receivers and a brickwall fading channel with a coherence length of 200 symbols, for example, the correlation based receiver achieves a sum rate of 36 bits/sec/Hz, significantly greater than any current cellular standards.

Information rates of the noncoherent frequency selective fading channel

This paper examines ways of improving channel estimation using successive decoding for the time varying frequency selective fading channel in a point to point wireless communication system. The channel state information (CSI) is not known a priori at either the transmitter or the receiver. The paper first presents the conventional pilot-symbol aided channel estimation (PACE) scheme and then the sequential successive decoding scheme. The conventional PACE scheme involves a big loss in achievable rates, while the sequential successive decoding scheme has a large delay. Finally, these two schemes are compared with three interleaved pilot schemes, in which interleaved codewords are decoded successively. These interleaved pilot schemes have significantly reduced delays, but still maintain good achievable rates.

Information Rates for Multiantenna Systems With Unknown Fading

This paper presents analytical upper and lower bounds on the information rate of a multiuser Rayleigh fading channel with no channel state information (CSI) at the transmitters or the receivers. These bounds are shown to converge whenever an individual users data rate is small compared with the bandwidth, e.g., when users can employ CDMA. The amount of spreading required for a given degree of convergence depends on the number of receive antennas. The number of users can be sufficient for the aggregate spectral density to be large. The paper presents exact analytical expressions for the information rates of both the block fading and the continuous correlated fading channel models in this regime. Finally, the results are extended to more general channel models.

Large wireless systems with unknown fading

This paper presents results on the capacity of large wireless systems with unknown fading. The system model consists of many independent transmitters who have to communicate with a large number of receivers via wireless links. The receivers in the system are connected to a wired backbone. The paper presents an upper bound on the per user capacity of the large network which allows for full cooperation among the receivers. The paper also presents constructive lower bounds on per user capacity which meet the upper bounds. Sometimes, the lower bounds require no receiver cooperation and sometimes they require medium distance receiver cooperation. The results of this paper are fundamental and result from the randomness of the fading present in the network rather than any peculiarities in the system model.

Achievable rates for the noncoherent frequency selective fading channel

This paper investigates achievable rates for the time varying frequency selective fading channel in a single-input and single-output (SISO) system. The channel state information (CSI) is not known a priori at either the transmitter or the receiver. The paper begins by presenting a general simulation based technique to compute lower and upper bounds on the achievable rates. The paper also derives an analytical lower bound. Finally, the paper computes the loss associated with pilot based schemes by comparing their achievable rates with the bounds.

The Performance of BICM With a Single Decision Feedback for the AWGN Channel

This paper studies the performance of bit interleaved coded modulation (BICM) with a single decision feedback. The separation of demodulation and decoding in BICM inevitably reduces the channels capacity. Altering the conventional bit mapping in BICM and allowing a single decision feedback greatly reduces the capacity loss. The extra complexity introduced by the single decision feedback appears well justified for dense constellations.

The performance of BICM with a single decision feedback for the AWGN channel

This paper studies the performance of bit interleaved coded modulation (BICM) with a single decision feedback. The separation of demodulation and decoding in BICM inevitably reduces the channels capacity. Altering the conventional bit mapping in BICM and allowing a single decision feedback greatly reduces the capacity loss. The extra complexity introduced by the single decision feedback appears well justified for dense constellations.

Information rates for multiantenna systems with unknown fading

This paper presents analytical upper and lower bounds on the information rate of a multiuser Rayleigh fading channel with no channel state information (CSI) at the transmitters or the receivers. These bounds are shown to converge whenever an individual users data rate is small compared with the bandwidth, e.g., when users can employ CDMA. The amount of spreading required for a given degree of convergence depends on the number of receive antennas. The number of users can be sufficient for the aggregate spectral density to be large. The paper presents exact analytical expressions for the information rates of both the block fading and the continuous correlated fading channel models in this regime. Finally, the results are extended to more general channel models.