In a cognitive radio (CR) network, it is extremely important to implement periodic spectrum sensing in order to prevent secondary users (SUs) from interfering with primary users (PUs). Contention from SU transmitters may increase the back-off interval to the point that the time remaining to the beginning of next sensing interval is too short to transmit a frame. In this paper, we propose an energy-efficient medium access control (MAC) protocol that allows a single SU sender to transmit a sequence of packets to its intended receivers until the next spectrum sensing interval begins. The sender, which is allowed to monopolize the channel for a certain length of time, sends to a restricted number of intended receivers in the wake state while all other nodes switch to sleep mode to reduce overall power consumption. We provide an analytical model for throughput and power consumption performance of cognitive networks under our proposed MAC protocol. Through performance analysis and extensive simulation, we show that our proposed energy saving protocol efficiently reduces power consumption while retaining high throughput performance.

Multi-user multiple-input multiple-output (MIMO) technology makes it possible for wireless nodes to successfully receive multiple packets from simultaneous transmitters in wireless networks. As it can provide more transmission opportunities without causing collisions, the network throughput performance can be dramatically improved. In this paper, we propose an asynchronous medium access control (MAC) protocol, which enables senders to independently start their transmissions if the access point (AP) can receive more simultaneous packets up to its multi-packet reception capability. This asynchronous protocol makes the multi-user MIMO channel more efficiently used, especially in wireless networks where transmission durations are dynamically varying due to different packet sizes and transmission rates. Through our performance analysis and extensive simulations, we show that the proposed asynchronous MAC protocol achieves significantly higher uplink throughput performance in multi-user MIMO wireless networks.

In cognitive networks, secondary users (SUs) are regulated to prevent them from accessing a channel if primary users (PUs) are currently occupying the channel. However, there is no strict regulation about how to coordinate the channel access among multiple SUs belonging to heterogeneous networks with different channel access policies. In this paper, we consider the coexistence problem for two specific SUs of carrier sense multiple access (CSMA)-based IEEE 802.11 network and centralized scheduling-based IEEE 802.16 network. We propose an inter frame spacing (IFS) adaptation method that allows IEEE 802.11 SUs to fully utilize idle intervals between successive IEEE 802.16 frames while not interfering with the following IEEE 802.16 frame. Through various simulations, we show that our proposed method significantly improves the throughput of IEEE 802.11 networks without harmful interference to IEEE 802.16 systems.

In multihop wireless networks, packets of a flow originating from a source node are relayed by intermediate nodes (relay nodes) and travel towards their destination along a multihop wireless path. Since the traffic forwarding capability of each node varies according to its level of contention, ideally, a node should not transmit more packets to its relay node than the corresponding relay node can forward. Instead, each node should yield its channel access opportunity to its neighbor nodes so that all the nodes can evenly share the channel and have similar forwarding capabilities. In this manner, nodes can utilize the wireless channel effectively, and further increase the end-to-end throughput of a multihop path. We propose a fully distributed contention window adaptation (CWA) mechanism, which adjusts the channel access probability depending on the difference between the incoming and outgoing traffic at each node, in order to equate the traffic forwarding capabilities among all the nodes in the path. We implement the proposed adaptive contention algorithm on Madwifi Linux kernel driver for Wi-Fi interface with Atheros chipset and carry out an empirical study in our division building. The experiment results demonstrate how the proposed mechanism can improve end-to-end throughput performance in the multihop wireless networks.

This paper addresses the power control problem in wireless multiple access communication systems with a multi-packet reception (MPR) channel. Driven by advances in signal processing and multiuser detection (MUD) technologies such as UWB and MIMO, it has become possible for a wireless node to simultaneously receive several packets from other transmitters. In this case, the receiver node can determine the set of transmitters from which it wants to receive data and instruct them to adjust their transmit power levels in order to maximize the overall network capacity by exploiting the MPR capability. We first show that the efficiency of the MPR channel utilization can be maximized by equalizing the transmission durations of simultaneously transmitting nodes, and then propose a power control that increases/decreases the transmit power of the nodes to equalize the transmission durations. Under the proposed power control, a node with a long transmission duration is allowed to use a higher transmit power if the next higher transmission rate is achievable with the increased transmit power. Through extensive simulations, we show that the proposed power control significantly improves the overall network capacity in multiple access.

In multihop wireless networks, packets of a traffic flow originating from a source node are relayed by intermediate nodes (relay nodes) and travel toward their destination node along a multihop wireless path. If a source node transmits excessive packets at a high rate, its relay node may not have enough channel access opportunities to forward them, resulting in a significant throughput decrease on an end-to-end perspective of multihop path. In this paper, we propose a probabilistic packet dropping scheme that regulates the traffic load offered by source nodes and reduces their interference with neighboring relay nodes on the multihop paths. Through ns-2 simulations, we show that the proposed scheme can significantly improve the end-to-end throughput performance without exploiting topological information under chain and cross topologies.