7 research outputs found
Writing on Fading Paper and Causal Transmitter CSI
A wideband fading channel is considered with causal channel state information
(CSI) at the transmitter and no receiver CSI. A simple orthogonal code with
energy detection rule at the receiver (similar to [6]) is shown to achieve the
capacity of this channel in the limit of large bandwidth. This code transmits
energy only when the channel gain is large enough. In this limit, this capacity
without any receiver CSI is the same as the capacity with full receiver CSI--a
phenomenon also true for dirty paper coding. For Rayleigh fading, this capacity
(per unit time) is proportional to the logarithm of the bandwidth. Our coding
scheme is motivated from the Gel'fand-Pinsker [2,3] coding and dirty paper
coding [4]. Nonetheless, for our case, only causal CSI is required at the
transmitter in contrast with dirty-paper coding and Gel'fand-Pinsker coding,
where non-causal CSI is required.
Then we consider a general discrete channel with i.i.d. states. Each input
has an associated cost and a zero cost input "0" exists. The channel state is
assumed be to be known at the transmitter in a causal manner. Capacity per unit
cost is found for this channel and a simple orthogonal code is shown to achieve
this capacity. Later, a novel orthogonal coding scheme is proposed for the case
of causal transmitter CSI and a condition for equivalence of capacity per unit
cost for causal and non-causal transmitter CSI is derived. Finally, some
connections are made to the case of non-causal transmitter CSI in [8]
Modulation and Estimation with a Helper
The problem of transmitting a parameter value over an additive white Gaussian
noise (AWGN) channel is considered, where, in addition to the transmitter and
the receiver, there is a helper that observes the noise non-causally and
provides a description of limited rate to the transmitter and/or
the receiver. We derive upper and lower bounds on the optimal achievable
-th moment of the estimation error and show that they coincide for
small values of and for low SNR values. The upper bound relies on a
recently proposed channel-coding scheme that effectively conveys
bits essentially error-free and the rest of the rate - over the same AWGN
channel without help, with the error-free bits allocated to the most
significant bits of the quantized parameter. We then concentrate on the setting
with a total transmit energy constraint, for which we derive achievability
results for both channel coding and parameter modulation for several scenarios:
when the helper assists only the transmitter or only the receiver and knows the
noise, and when the helper assists the transmitter and/or the receiver and
knows both the noise and the message. In particular, for the message-informed
helper that assists both the receiver and the transmitter, it is shown that the
error probability in the channel-coding task decays doubly exponentially.
Finally, we translate these results to those for continuous-time power-limited
AWGN channels with unconstrained bandwidth. As a byproduct, we show that the
capacity with a message-informed helper that is available only at the
transmitter can exceed the capacity of the same scenario when the helper knows
only the noise but not the message.Comment: This work has been submitted to the IEEE for possible publication.
Copyright may be transferred without notice, after which this version may no
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Physical-Layer Security: Wide-band Communications & Role of Known Interference
Data security is of such paramount importance that security measures have been implemented across all layers of a communication network. One layer at which security has not been fully developed and studied is the physical layer, the lowest layer of the protocol stack. Towards establishing fundamental limits of secure communications at the physical layer, we address in this dissertation two main problems. First, we study secure communication in the wide-band regime, and second we study the role of known interference in secure communication.
The concept of channel capacity per unit cost was introduced by Verdu´ in 1990 to study the limits of cost-efficient wide-band communication. It was shown that orthogonal signaling can achieve the channel capacity per unit cost of memoryless stationary channels with a zero-cost input letter. The first part of this dissertation introduces the concept of secrecy capacity per unit cost to study cost-efficient wide- band secrecy communication. For degraded memoryless stationary wiretap channels, it is shown that an orthogonal coding scheme with randomized pulse position and constant pulse shape achieves the secrecy capacity per unit cost with a zero-cost input letter. For general memoryless stationary wiretap channels, the performance of orthogonal codes is studied, and the benefit of further randomizing the pulse shape is demonstrated via a simple example. Furthermore, the problem of secure communication in a MIMO setting is considered, and a single-letter expression for the secrecy capacity per unit cost is obtained for the MIMO wiretap channel.
Recently there has been a lot of success in using the deterministic approach to provide approximate characterization of Gaussian network capacity. The second part of this dissertation takes a deterministic view and revisits the problem of wiretap channel with side information. A precise characterization of the secrecy capacity is obtained for a linear deterministic model, which naturally suggests a coding scheme which we show to achieve the secrecy capacity of the degraded Gaussian model (dubbed as “secret writing on dirty paper”) to within half a bit. The success of this approach allowed its application to the problem of “secret key agreement via dirty paper coding”, where also a suggested coding scheme achieves the secret-key capacity to within half a bit
Cooperative communication in wireless networks: algorithms, protocols and systems
Current wireless network solutions are based on a link abstraction where a
single co-channel transmitter transmits in any time duration. This model severely
limits the performance that can be obtained from the network. Being inherently an
extension of a wired network model, this model is also incapable of handling the
unique challenges that arise in a wireless medium. The prevailing theme of this
research is to explore wireless link abstractions that incorporate the broadcast and
space-time varying nature of the wireless channel. Recently, a new paradigm for
wireless networks which uses the idea of 'cooperative transmissions' (CT) has garnered
significant attention. Unlike current approaches where a single transmitter transmits
at a time in any channel, with CT, multiple transmitters transmit concurrently after
appropriately encoding their transmissions. While the physical layer mechanisms for
CT have been well studied, the higher layer applicability of CT has been relatively
unexplored. In this work, we show that when wireless links use CT, several network
performance metrics such as aggregate throughput, security and spatial reuse can
be improved significantly compared to the current state of the art. In this context,
our first contribution is Aegis, a framework for securing wireless networks against
eavesdropping which uses CT with intelligent scheduling and coding in Wireless Local
Area networks. The second contribution is Symbiotic Coding, an approach to encode
information such that successful reception is possible even upon collisions. The third
contribution is Proteus, a routing protocol that improves aggregate throughput in
multi-hop networks by leveraging CT to adapt the rate and range of links in a flow.
Finally, we also explore the practical aspects of realizing CT using real systems.PhDCommittee Chair: Sivakumar, Raghupathy; Committee Member: Ammar, Mostafa; Committee Member: Ingram, Mary Ann; Committee Member: Jayant, Nikil; Committee Member: Riley, Georg