Siddharth Ray

On Noncoherent MIMO Channels in the Wideband Regime: Capacity and Reliability

We consider a multiple-input multiple-output (MIMO) wideband Rayleigh block-fading channel where the channel state is unknown to both the transmitter and the receiver and there is only an average power constraint on the input. We compute the capacity and analyze its dependence on coherence length, number of antennas and receive signal-to-noise ratio (SNR) per degree of freedom. We establish conditions on the coherence length and number of antennas for the noncoherent channel to have a near-coherent performance in the wideband regime. We also propose a signaling scheme that is near-capacity achieving in this regime. We compute the error probability for this wideband noncoherent MIMO channel and study its dependence on SNR, number of transmit and receive antennas and coherence length. We show that error probability decays inversely with coherence length and exponentially with the product of the number of transmit and receive antennas. Moreover, channel outage dominates error probability in the wideband regime. We also show that the critical as well as cutoff rates are much smaller than channel capacity in this regime

On Jamming in the Wideband Regime

We consider the problem of jamming in non-coherent wideband fading channels. While the problem is well understood for coherent channels, the results for the coherent case do not generalize in the non-coherent regime. We show that energy-limited jammers do not affect capacity in the wideband regime. We also propose a training based transmission scheme that is able to achieve the wideband limit in the presence of a jammer

Fiber Aided Wireless Network Architecture

We introduce the concept of a fiber aided wireless network architecture (FAWNA), which allows high-speed mobile connectivity by leveraging the speed of optical networks. Specifically, we consider a single-input, multiple-output (SIMO) FAWNA, which consists of a SIMO wireless channel interfaced with an optical fiber channel through wireless-optical interfaces. We propose a design where the received wireless signal at each interface is sampled and quantized before being sent over the fiber. The capacity of our scheme approaches the capacity of the architecture, exponentially with fiber capacity. We also show that for a given fiber capacity, there is an optimal operating wireless bandwidth and number of interfaces. We show that the optimal way to divide the fiber capacity among the interfaces is to ensure that each interface gets enough rate so that its noise is dominated by front end noise rather than by quantizer distortion. We also show that rather than dynamically change rate allocation based on channel state, a less complex, fixed rate allocation scheme can be adopted with very small loss in performance.

On Separation for Multiple Access Channels

We examine the issue of separation for multiple access channels. We demonstrate that source-channel separation holds for noisy multiple access channels, when the channel operates over a common finite field. This robustness of separation is predicated on the fact that noise and inputs are independent, and we examine the loss from failure of separation when noise is input dependent.

On Optimal Signaling and Jamming Strategies in Wideband Fading Channels

We investigate the fundamental limits on communication performance in wideband fading channels, in the presence of a jammer. We show that energy-limited Gaussian jammers cannot affect the error exponents in the wideband regime. However, if the sender uses an impulsive training scheme, which is close to optimum in the wideband regime, the jammer may be able to interfere with sounding signals if the location of those signals is known. In the latter case, the jammer significantly reduces the wideband capacity limit

Wideband non-coherent MIMO capacity

We consider a multiple-input, multiple-output (MIMO) wideband Rayleigh block fading channel, where the channel state is unknown to both the transmitter and the receiver. With only an average power constraint, we compute the capacity of this channel and consider its interaction with the coherence length, number of transmit and receive antennas and receive signal-to-noise ratio (SNR) per degree of freedom. We establish how large the coherence length has to be in order for the non-coherent channel to have a near coherent performance in the wideband regime. More specifically, we show that if the coherence length of the channel is above a certain SNR (bandwidth) dependent threshold, the non-coherent and coherent capacities are the same in the large bandwidth regime. We also propose a signaling scheme that is near-optimal in the wideband regime

Random coding in noise-free multiple access networks over finite fields

A two transmitter single receiver multiple access noise-free network is considered where interference is additive and the transmit and receive alphabet size is the same. We consider two performance metrics - code rate and sum rate. Code rate is defined as the ratio of the symbols recovered, after multiple access interference, to the symbols sent by the transmitters. The sum rate is the number of symbols successfully received per unit time. A packet by packet coding scheme is presented where we determine how these rates change with redundancy. We propose a coding mechanism that maximizes the code and sum rates and show that it suffices to code at only one of the transmitters and that systematic codes are sufficient for this purpose. The development is independent of the alphabet size the symbols are defined over. We also show that we can achieve maximum rates by using a random code. This allows us to choose codes in a random fashion and the scheme achieves optimality with probability tending to 1 exponentially with the code length.

On MIMO capacity in the ultra-wideband regime

We consider multiple antenna communication over a wideband, non-coherent Rayleigh block fading channel. As the available power is spread over a large bandwidth, the signal-to-noise ratio (SNR) per degree of freedom is low for these channels. While the capacity of multiple antenna channels in the high SNR regime has recently become well understood, the low SNR regime has been scantily studied. We explore the interaction among transmit antennas, coherence time, SNR and receive antennas for time-varying channels at low SNR, with no channel state information at either the transmitter or receiver. We obtain a necessary criterion the coherence length of the non-coherent channel must satisfy, for a certain rate to be achievable. This derivation also tells us how large the coherence length necessarily needs to be, for the non-coherent channel to have a near coherent performance.

A SIMO Fiber Aided Wireless Network Architecture

The concept of a fiber aided wireless network architecture (FAWNA) is introduced in [Ray et al., Allerton Conference 2005], which allows high-speed mobile connectivity by leveraging the speed of optical networks. In this paper, we consider a single-input, multiple-output (SIMO) FAWNA, which consists of a SIMO wireless channel and an optical fiber channel, connected through wireless-optical interfaces. We propose a scheme where the received wireless signal at each interface is quantized and sent over the fiber. Though our architecture is similar to that of the classical CEO problem, our problem is different from it. We show that the capacity of our scheme approaches the capacity of the architecture, exponentially with fiber capacity. We also show that for a given fiber capacity, there is an optimal operating wireless bandwidth and an optimal number of wireless-optical interfaces. The wireless-optical interfaces of our scheme have low complexity and do not require knowledge of the transmitter code book. They are also extendable to FAWNAs with large number of transmitters and interfaces and, offer adaptability to variable rates, changing channel conditions and node positions

On Error Probability for Non-coherent MIMO Channels in the Wideband Regime

We consider a multiple-input, multiple-output (MIMO) wideband Rayleigh block fading channel where the channel state is unknown to both the transmitter and the receiver and there is only an average power constraint on the input. We compute the error probability and study its dependence on receive signal-to-noise ratio (SNR), number of transmit and receive antennas and coherence length. We show that error probability decays inversely with coherence length and exponentially with the product of the number of transmit and receive antennas. Moreover, channel outage dominates error probability in the wideband regime. We also show that the critical as well as cut-off rates are much smaller than channel capacity in this regime