A wireless broadcast network model with secrecy constraints is investigated, in which a source node broadcastsKconfidential message flows toKuser nodes, with each message intended to be decoded accurately by one user and to be kept secret from all other users (who are thus considered to be eavesdroppers with regard to all other messages but their own). The source maintains a queue for each message flow if it is not served immediately. The channel from the source to theKusers is modeled as a fading broadcast channel, and the channel state information is assumed to be known to the source and the corresponding receivers. Two eavesdropping models are considered. For a collaborative eavesdropping model, in which the eavesdroppers exchange their outputs, the secrecy capacity region is obtained, within which each rate vector is achieved by using a time-division scheme and a source power control policy over channel states. A throughput optimal queue-length-based rate scheduling algorithm is further derived that stabilizes all arrival rate vectors contained in the secrecy capacity region. Moreover, the network utility function is maximized via joint design of rate control, rate scheduling, power control, and secure coding. More precisely, a source controls the message arrival rate according to its message queue, the rate scheduling selects a transmission rate based the queue length vector, and the rate vector is achieved by power control and secure coding. These components work jointly to solve the network utility maximization problem. For a noncollaborative eavesdropping model, in which eavesdroppers do not exchange their outputs, an achievable secrecy rate region is derived based on a time-division scheme, and the queue-length-based rate scheduling algorithm and the corresponding power control policy are obtained that stabilize all arrival rate vectors in this region. The network utility maximizing rate control vector is also obtained.
A wireless broadcast network model with secrecy constraints is investigated, in which a source node broadcastsKconfidential message flows toKuser nodes, with each message intended to be decoded accurately by one user and to be kept secret from all other users (who are thus considered to be eavesdroppers with regard to all other messages but their own). The source maintains a queue for each message flow if it is not served immediately. The channel from the source to theKusers is modeled as a fading broadcast channel, and the channel state information is assumed to be known to the source and the corresponding receivers. Two eavesdropping models are considered. For a collaborative eavesdropping model, in which the eavesdroppers exchange their outputs, the secrecy capacity region is obtained, within which each rate vector is achieved by using a time-division scheme and a source power control policy over channel states. A throughput optimal queue-length-based rate scheduling algorithm is further derived that stabilizes all arrival rate vectors contained in the secrecy capacity region. Moreover, the network utility function is maximized via joint design of rate control, rate scheduling, power control, and secure coding. More precisely, a source controls the message arrival rate according to its message queue, the rate scheduling selects a transmission rate based the queue length vector, and the rate vector is achieved by power control and secure coding. These components work jointly to solve the network utility maximization problem. For a noncollaborative eavesdropping model, in which eavesdroppers do not exchange their outputs, an achievable secrecy rate region is derived based on a time-division scheme, and the queue-length-based rate scheduling algorithm and the corresponding power control policy are obtained that stabilize all arrival rate vectors in this region. The network utility maximizing rate control vector is also obtained.
A secure wireless broadcast network model is investigated, in which a source node broadcasts K confidential message flows to N user nodes, with each message intended to be decoded accurately by one user and to be kept secret from all of other users (who are thus considered to be eavesdroppers with regard to all other messages but their own). The source maintains a queue for each message flow if it is not served immediately. The channel from the source to the K users is modelled as a fading broadcast channel, and the channel state information is assumed to be known to all nodes. Two eavesdropping models are considered. For a collaborative eavesdropping model, in which the eavesdroppers exchange their outputs, the secrecy capacity region is obtained, within which each rate vector is achieved by using a time-division scheme and a source power control policy over channel states. A throughput optimal queue length based scheduling algorithm is further derived that stabilizes all arrival rate vectors contained in the secrecy capacity region. At each packet time slot, the queue length vector determines the power control policy over the channel states at the source, and hence determines the secrecy rate allocation among users. For a non-collaborative model, in which eavesdroppers do not exchange their outputs, the time-division scheme provides an achievable secrecy rate region, and the queue length based scheduling algorithm stabilizes all arrival rate vectors in this region.
In this paper, secure transmission of information over fading broadcast channels is studied in the presence of statistical queueing constraints. Effective capacity is employed as a performance metric to identify the secure throughput of the system, i.e., effective secure throughput. It is assumed that perfect channel side information (CSI) is available at both the transmitter and the receivers. Initially, the scenario in which the transmitter sends common messages to two receivers and confidential messages to one receiver is considered. For this case, the effective secure throughput region, which is the region of constant arrival rates of common and confidential messages that can be supported by the buffer-constrained transmitter and fading broadcast channel, is defined. It is proven that this effective throughput region is convex implying that time-sharing between any two viable transmission and power control strategies results in effective throughput values inside the region. Then, the optimal power control policies that achieve the boundary points of the effective secure throughput region are investigated and an algorithm for the numerical computation of the optimal power adaptation schemes is provided. Additionally, the throughput region achieved by time-division multiplexing of common and confidential messages is explored. Subsequently, the special case in which the transmitter sends only confidential messages to one receiver is addressed in more detail. For this case, effective secure throughput is formulated and two different power adaptation policies are studied. These power adaptation policies are compared with the opportunistic ones that are optimal in the absence of quality of service (QoS) constraints. It is shown that opportunistic schemes, in which data transmission with high rates and high power occurs only when the main channel is much better than the eavesdropper channel, are no longer optimal under buffer constraints, and the transmitter should send the data at a certain moderate rate and power even when the main channel strength is comparable to that of the eavesdropper channel to avoid buffer overflows.
In this paper, we consider a scenario where a source node wishes to broadcast two confidential messages to two receivers, while a wire-tapper also receives the transmitted signal. This model is motivated by wireless communications, where individual secure messages are broadcast over open media and can be received by any illegitimate receiver. The secrecy level is measured by the equivocation rate at the eavesdropper. We first study the general (non-degraded) broadcast channel with an eavesdropper. We present an inner bound on the secrecy capacity region for this model. This inner bound is based on a combination of random binning, and the Gelfand-Pinsker binning. We further study the situation in which the channels are degraded. For the degraded broadcast channel with an eavesdropper, we present the secrecy capacity region. Our achievable coding scheme is based on Covers superposition scheme and random binning. We refer to this scheme as the Secret Superposition Scheme. Our converse proof is based on a combination of the converse proof of the conventional degraded broadcast channel and Csiszar Lemma. We then assume that the channels are Additive White Gaussian Noise (AWGN) and show that the Secret Superposition Scheme with Gaussian codebook is optimal. The converse proof is based on Costas entropy power inequality. Finally, we use a broadcast strategy for the slowly fading wire-tap channel when only the eavesdroppers channel is fixed and known at the transmitter. We derive the optimum power allocation for the coding layers, which maximizes the total average rate.
Future wireless communication systems are rapidly transforming to satisfy everincreasing and varying mobile user demands. Cross-layer networking protocols have the potential to play a crucial role in this transformation by jointly addressing the requirements of user applications together with the time-varying nature of wireless networking. As wireless communications becoming an integral and crucial part of our daily lives with many of our personal data is being shared via wireless transmissions, the issue of keeping personal transactions confidential is at the forefront of any network design. Wireless communications is especially prone to attacks due to its broadcast nature. The conventional cryptographical methods can only guarantee secrecy with the assumption that it is computationally prohibitive for the eavesdroppers to decode the messages. On the other hand, information-theoretical secrecy as defined by Shannon in his seminal work has the potential to provide perfect secrecy regardless of the computational power of the eavesdropper. Recent studies has shown that information-theoretical secrecy is possible over noisy wireless channels. In this thesis, we aim to design simple yet provably optimal cross-layer algorithms taking into account information-theoretical secrecy as a Quality of Service (QoS) requirement. Our work has the potential to improve our understanding the interplay between the secrecy and networking protocols. In most of this thesis, we consider a wireless cellular architecture, where all nodes participate in communication with a base station. When a node is transmitting a confidential messages, other legitimate nodes are considered as eavesdroppers, i.e., all eavesdroppers are internal. We characterize the region of achievable open and confidential data rate pairs for a single and then a multi-node scenario. We define the notion of confidential opportunistic scheduler, which schedules a node that has the largest instantaneous confidential information rate, with respect to the best eavesdropper node, which has the largest mean cross-channel rate. Having defined the operational limits of the system, we then develop dynamic joint scheduling and flow control algorithms when perfect and imperfect channel state information (CSI) is available. The developed algorithms are simple index policies, in which scheduling and flow control decisions are given in each time instant independently. In real networks, instantaneous CSI is usually unavailable due to computational and communication overheads associated with obtaining this information. Hence, we generalize our model for the case where only the distributions of direct- and crosschannel CSI are available at the transmitter. In order to provide end-to-end reliability, Hybrid Automatic Retransmission reQuest (HARQ) is employed. The challenge of using HARQ is that the dynamic control policies proposed in the preceding chapter are no longer optimal, since the decisions at each time instant are no longer independent. This is mainly due to the potential of re-transmitting a variant of the same message successively until it is decoded at the base station. We solve this critical issue by proposing a novel queuing model, in which the messages transmitted the same number of times previously are stored in the same queue with scheduler selecting a head-of-line message from these queues. We prove that with this novel queuing model, the dynamic control algorithms can still be optimal. We then shift our attention to providing confidentiality in multi-hop wireless networks, where there are multiple source-destination pairs communicating confidential messages, to be kept confidential from the intermediate nodes. For this case, we propose a novel end-to-end encoding scheme, where the confidential information is encoded into one very long message. The encoded message is then divided into multiple packets, to be combined at the ultimate destination for recovery, and being sent over different paths so that each intermediate node only has partial view of the whole message. Based on the proposed end-to-end encoding scheme, we develop two different dynamic policies when the encoded message is finite and asymptotically large, respectively. When the encoded message has finite length, our proposed policy chooses the encoding rates for each message, based on the instantaneous channel state information, queue states and secrecy requirements. Also, the nodes keep account of the information leaked to intermediate nodes as well the information reaching the destination in order to provide confidentiality and reliability. We demonstrate via simulations that our policy has a performance asymptotically approaching that of the optimal policy with increasing length of the encoded message. All preceding work assumes that the nodes are altruistic and/or well-behaved, i.e., they cooperatively participate into the communication of the confidential messages. In the final chapter of the thesis, we investigate the case with non-altruistic nodes, where non-altruistic nodes provide a jamming service to nodes with confidential communication needs and receiving in turn the right to access to the channel. We develop optimal resource allocation and power control algorithms maximizing the aggregate utility of both nodes with confidential communication needs as well as the nodes providing jamming service.
The use of wireless networks has grown significantly in contemporary \ntimes, and continues to develop further. The broadcast nature of \nwireless communications, however, makes them particularly vulnerable \nto eavesdropping. Unlike traditional solutions, which usually handle \nsecurity at the application layer, the primary concern of this \ndissertation is to analyze and develop solutions based on coding \ntechniques at the physical-layer. \n \nFirst, in chapter 2, we consider a scenario where a source node \nwishes to broadcast two confidential messages to two receivers, \nwhile a wire-tapper also receives the transmitted signal. This model \nis motivated by wireless communications, where individual secure \nmessages are broadcast over open media and can be received by any \nillegitimate receiver. The secrecy level is measured by the \nequivocation rate at the eavesdropper. We first study the general \n(non-degraded) broadcast channel with an eavesdropper, and present \nan inner bound on the secrecy capacity region for this model. This \ninner bound is based on a combination of random binning, and the \nGelfand-Pinsker binning. We further study the situation in which the \nchannels are degraded. For the degraded broadcast channel with an \neavesdropper, we present the secrecy capacity region. Our achievable \ncoding scheme is based on Cover's superposition scheme and random \nbinning. We refer to this scheme as the Secret Superposition Scheme. \nOur converse proof is based on a combination of the converse proof \nof the conventional degraded broadcast channel and Csiszar Lemma. We \nthen assume that the channels are Additive White Gaussian Noise and \nshow that the Secret Superposition Scheme with Gaussian codebook is \noptimal. The converse proof is based on Costa's entropy power \ninequality. Finally, we use a broadcast strategy for the slowly \nfading wire-tap channel when only the eavesdropper's channel is \nfixed and known at the transmitter. We derive the optimum power \nallocation for the coding layers, which maximizes the total average \nrate. \n \n \nSecond, in chapter 3 , we consider the \nMultiple-Input-Multiple-Output (MIMO) scenario of a broadcast \nchannel where a wiretapper also receives the transmitted signal via \nanother MIMO channel. First, we assume that the channels are \ndegraded and the wiretapper has the worst channel. We establish the \ncapacity region of this scenario. Our achievability scheme is the \nSecret Superposition Coding. For the outerbound, we use notion of \nthe enhanced channels to show that the secret superposition of \nGaussian codes is optimal. We show that we only need to enhance the \nchannels of the legitimate receivers, and the channel of the \neavesdropper remains unchanged. We then extend the result of the \ndegraded case to a non-degraded case. We show that the secret \nsuperposition of Gaussian codes, along with successive decoding, \ncannot work when the channels are not degraded. We develop a Secret \nDirty Paper Coding scheme and show that it is optimal for this \nchannel. We then present a corollary generalizing the capacity \nregion of the two receivers case to the case of multiple receivers. \nFinally, we investigate a scenario which frequently occurs in the \npractice of wireless networks. In this scenario, the transmitter and \nthe eavesdropper have multiple antennae, while both intended \nreceivers have a single antenna (representing resource limited \nmobile units). We characterize the secrecy capacity region in terms \nof generalized eigenvalues of the receivers' channels and the \neavesdropper's channel. We refer to this configuration as the MISOME \ncase. We then present a corollary generalizing the results of the \ntwo receivers case to multiple receivers. In the high SNR regime, we \nshow that the capacity region is a convex closure of rectangular \nregions. \n \n \nFinally, in chapter 4, we consider a K-user secure Gaussian \nMultiple-Access-Channel with an external eavesdropper. We establish \nan achievable rate region for the secure discrete memoryless MAC. \nThereafter, we prove the secrecy sum capacity of the degraded \nGaussian MIMO MAC using Gaussian codebooks. For the non-degraded \nGaussian MIMO MAC, we propose an algorithm inspired by the \ninterference alignment technique to achieve the largest possible \ntotal Secure-Degrees-of-Freedom . When all the terminals are \nequipped with a single antenna, Gaussian codebooks have shown to be \ninefficient in providing a positive S-DoF. Instead, we propose a \nnovel secure coding scheme to achieve a positive S-DoF in the single \nantenna MAC. This scheme converts the single-antenna system into a \nmultiple-dimension system with fractional dimensions. The \nachievability scheme is based on the alignment of signals into a \nsmall sub-space at the eavesdropper, and the simultaneous separation \nof the signals at the intended receiver. We use tools from the field \nof Diophantine Approximation in number theory to analyze the \nprobability of error in the coding scheme. We prove that the total \nS-DoF of fracK−1K can be achieved for almost all channel \ngains. For the other channel gains, we propose a multi-layer coding \nscheme to achieve a positive S-DoF. As a function of channel gains, \ntherefore, the achievable S-DoF is discontinued.
In this paper, we first consider a scenario where a source node wishes to broadcast two confidential messages for two respective receivers, while a wire-taper also receives the transmitted signal. We assume that the signals are transmitted over additive white Gaussian noise channels. We characterize the secrecy capacity region of this channel. Our achievable coding scheme is based on superposition coding and the random binning. We refer to this scheme as secret superposition coding. The converse proof combines the converse proof for the conventional Gaussian broadcast channel and the perfect secrecy constraint. This capacity region matches the capacity region of the broadcast channel without security constraint. It also matches the secrecy capacity of the wire-tap channel. Based on the rate characterization of the secure Gaussian broadcast channel, we then use a multilevel coding approach for the slowly fading wire-tap. We assume that the transmitter only knows the eavesdropper's channel. In this approach, source node sends secure layered coding and the receiver viewed as a continuum ordered users. We derive optimum power allocation for the layers which maximizes the total average rate.