Fu-Te Hsu

Random Access Game in Fading Channels With Capture: Equilibria and Braess-like Paradoxes

The Nash equilibrium point of the transmission probabilities in a slotted ALOHA system with selfish nodes is analyzed. The system consists of a finite number of heterogeneous nodes, each trying to minimize its average transmission probability (or power investment) selfishly while meeting its average throughput demand over the shared wireless channel to a common base station (BS). We use a game-theoretic approach to analyze the network under two reception models: one is called power capture, the other is called signal-to-interference-plus-noise ratio (SINR) capture. It is shown that, in some situations, Braess-like paradoxes may occur. That is, the performance of the system may become worse instead of better when channel state information (CSI) is available at the selfish nodes. In particular, for homogeneous nodes, we analytically presented that Braess-like paradoxes occur in the power capture model, and in the SINR capture model with the capture ratio larger than one and the noise to signal ratio sufficiently small.

When does the AP deployment incentivize a user to offload cellular data: An energy efficiency viewpoint

In this paper, we consider a cellular network supplemented with some WiFi access points (APs) for mobile data offloading. In the network, a mobile user can transmit its data directly to the base station or attempt to use WiFi networks to offload some data traffic from the cellular network. From an energy efficiency viewpoint, we derive the condition of incentivizing a user to use the WiFi networks in a general framework. Using our results, AP deployment problems of minimum deployment and maximum coverage are reconsidered, and we propose algorithms to solve the problems. In addition, we discover an interesting property of the problems that the optimal deployment in a target region, which is divided into grids with AP coverage, can be achieved when at most one grid is covered by more than one AP.

Channel-aware ALOHA with SINR capture: When is the knowledge of channel not helpful?

We consider an ALOHA network in which users have the knowledge of their own channel state information (CSI). In each time slot, each user transmits its packet to the base station (BS) according to a CSI-dependent transmission scheme. A packet is successfully received if its signal to interference plus noise ratio (SINR) is larger than the capture ratio. We analyze the throughput of this network under a threshold-based transmission scheme in which a user transmits only when its channel gain is higher than a threshold. It is shown that in some cases the conventional ALOHA transmission scheme without CSI may outperform the threshold-based transmission scheme in terms of the maximum possible throughput. We give the condition for this phenomenon and find that this phenomenon will not occur if the BS is capable of receiving three or more packets simultaneously. Besides, we present a transmission scheme in which a user only transmits when its channel gain lies in an interval. This scheme includes the threshold-based one as a special case.

Power allocation strategy against jamming attacks in Gaussian fading multichannel

In this paper, we consider a jamming game in wireless networks in which a malicious user tries to jam the transmissions of a user in Gaussian fading multichannel. Both the user and the jammer allocate power to multiple channels with their total power constraints in order to maximize their utilities. We model the utility of the user as a weighted sum of successfully received packets on all channels, and the goal of the jammer is trying to minimize the users utility. The power allocation strategies at Nash equilibrium of this zero-sum game are derived in closed form and some properties of robustness and worst-case guarantee are presented.

Analysis of a Reservation-Based Random Access Network: Throughput Region and Power Consumption

In this paper, we analytically study a random access network that uses the request-to-send and clear-to-send (RTS/CTS) handshake for reservation of transmission time. In the network, nodes initiate data transmission to a common base station (BS) by sending an RTS packet according to a transmission probability. The RTS packet of a node specifies the length of the nodes requested data transmission interval, and will be successfully received by the BS if its signal to interference plus noise ratio (SINR) is higher than the capture ratio. The BS will then reply with a CTS packet to grant this node the requested data transmission interval and inform the other nodes not to interrupt. The transmission probabilities of RTS packets of all nodes will determine the average throughput and power consumption of each node. The set of all possible throughputs that can be achieved by the network is called the throughput region. We characterize the throughput region and provide an upper bound on the total transmission power consumption over the throughput region at the optimal operating point. Specifically, the upper bound corresponds to one of three points in the throughput region depending on the fraction of time occupied by the RTS packets.

Exploiting channel state information in slotted ALOHA with SINR capture

In this paper, we consider a channel-aware ALOHA network in which users have the knowledge of their own channel state information (CSI) and exploit this decentralized CSI by utilizing CSI-based transmission schemes. The system throughput is investigated with multi-region transmission schemes under the signal-to-interference-plus-noise ratio (SINR) capture model. We discover that the two-region transmission scheme performs well enough for maximizing the throughput.

Cheat-Proof and Optimal Access Controls in a Pairwise Interaction Network

We derive both the optimal and the game-theoretic (cheat-proof) solutions in closed form, to a problem of distributed access with power control in a pairwise interaction network. In this network, each node has an energy constraint in a fixed time unit for using different power levels in each access attempt competing with a randomly selected node. The one with larger power level wins the access. The optimal control for nodes in the network to maximize the number of wining the access as well as the game-theoretic (cheat-proof) control in which no one will deviate to attain higher number of wining the access are fully characterized in this paper.

Throughput improvement in ALOHA networks with power capture and a cheat-proof access control

In this paper, we consider the problem of power level selections under an average power constraint in slotted ALOHA networks. In the network, each node chooses a power level from a pre-determined power level set based on a probability for each access attempt. The derivation of the probability for maximizing the throughput is difficult. In addition, nodes may not always obey the given algorithm in a distributed network because a misbehaving node might gain better performance at the expense of the others. To prevent the potential of misbehavior, we derive a cheat-proof algorithm based on game theory, which will make no nodes in the network want to deviate from the algorithm. Further, we show the throughput loss of the cheat-proof algorithm compared to the optimal one, and show the occurrence of unjust situation if the property of cheat-proof is not taken into account.

Equilibria of a shared medium access network with RTS/CTS handshake mechanism

We characterize the Nash equilibrium point of the request probability in a network using the Request-to-send / Clear-to-send (RTS/CTS) handshake mechanism for channel reservation, where each node tries to minimize its average power investment selfishly while meeting its average throughput demand over the shared wireless channel to a common base station. The feasible region of throughput demands is then maximized by the optimal data transmission periods and meanwhile the average power investment at the better Nash equilibrium is reduced. In addition, the performance of the optimum design is compared to that in a network without the RTS/CTS handshake mechanism.

Space-time codes for multiple access systems with low MMSE decoding complexity

We study a class of space-time codes constructed by linear dispersion encoding that allow low-complexity linear minimum mean square error (MMSE) decoding for a multiple access system. Considering that the information symbols are drawn from square QAM constellations, we optimize rate-one space time code designs with MMSE decoding to achieve minimum bit error rate (BER) for any channel realization. In addition, the performance is further improved with a simple MMSE successive interference cancellation (MMSE-SIC) method. Simulation results compare the proposed design with existing rate-one designs and show that simple rate-one circulant designs are also good candidates for deployment in multiple access systems.