This paper addresses the joint design of transmit and receive beamforming or linear processing (commonly termed linear precoding at the transmitter and equalization at the receiver) for multicarrier multiple-input multiple-output (MIMO) channels under a variety of design criteria. Instead of considering each design criterion in a separate way, we generalize the existing results by developing a unified framework based on considering two families of objective functions that embrace most reasonable criteria to design a communication system: Schur-concave and Schur-convex functions. Once the optimal structure of the transmit-receive processing is known, the design problem simplifies and can be formulated within the powerful framework of convex optimization theory, in which a great number of interesting design criteria can be easily accommodated and efficiently solved, even though closed-form expressions may not exist. From this perspective, we analyze a variety of design criteria, and in particular, we derive optimal beamvectors in the sense of having minimum average bit error rate (BER). Additional constraints on the peak-to-average ratio (PAR) or on the signal dynamic range are easily included in the design. We propose two multilevel water-filling practical solutions that perform very close to the optimal in terms of average BER with a low implementation complexity. If cooperation among the processing operating at different carriers is allowed, the performance improves significantly. Interestingly, with carrier cooperation, it turns out that the exact optimal solution in terms of average BER can be obtained in closed form.

IEEE 802.11 based wireless networks have seen rapid growth and deployment in the recent years. Critical to the 802.11 MAC operation, is the handoff function which occurs when a mobile node moves its association from one access point to another. In this paper, we present an empirical study of this handoff process at the link layer, with a detailed breakup of the latency into various components. In particular, we show that a MAC layer function - probe is the primary contributor to the overall handoff latency. In our study, we observe that the latency is significant enough to affect the quality of service for many applications (or network connections). Further we find a large variation in the latency with from one handoff to another and also among APs and STAs used from different vendors. In this study, we account for this variation and also draw the guidelines for future handoff schemes.

Reducing the energy consumption by wireless communication devices is perhaps the most important issue in the widely-deployed and exponentially-growing IEEE 802.11 Wireless LANs (WLANs). TPC (Transmit Power Control) and PHY (physical layer) rate adaptation have been recognized as two most effective ways to achieve this goal. The emerging 802.11h standard, which is an extension to the current 802.11 MAC and the high-speed 802.11a PHY, will provide a structured means to support intelligent TPC. In this paper, we propose a novel scheme, called MiSer, that minimizes the communication energy consumption in 802.11a/h systems by combining TPC with PHY rate adaptation. The key idea is to compute offline an optimal rate-power combination table, and then at runtime, a wireless station determines the most energyefficient transmission strategy for each data frame by a simple table lookup. Another key contribution of this paper is to provide a rigorous analysis of the relation among different radio ranges and TPC’s effect on the interference in 802.11a/h systems, which justifies MiSer’s approach to ameliorating the TPC-caused interference by transmitting the CTS frames at a stronger power level. Our simulation results show that MiSer delivers about 20 % more data per unit of energy consumption than the PHY rate adaptation scheme without TPC, while outperforming single-rate TPC schemes significantly thanks to the excellent energy-saving capability of PHY rate adaptation.

In recent years, positioning systems for indoor areas using the existing wireless local area network infrastructure have been suggested. Such systems make use of location fingerprinting rather than time or direction of arrival techniques for determining the location of mobile stations. While experimental results related to such positioning systems have been presented, there is a lack of analytical models that can be used as a framework for designing and deploying the positioning systems. In this paper, we present an analytical model for analyzing such positioning systems. We develop the framework for analyzing a simple positioning system that employs the Euclidean distance between a sample signal vector and the location fingerprints of an area stored in a database. We analyze the effect of the number of access points that are visible and radio propagation parameters on the performance of the positioning system and provide some preliminary guidelines on its design.

This paper presents a new MAC protocol called RMAC that supports reliable multicast for wireless ad hoc networks. By utilizing the busy tone mechanism to realize multicast reliability, RMAC has the following three novelties: (1) it uses a variable-length control frame to stipulate an order for the receivers to respond, such that the problem of feedback collision is solved; (2) it extends the traditional usage of busy tone for preventing data frame collisions into the multicast scenario; and (3) it introduces a new usage of busy tone for acknowledging data frames. In addition, we also generalize RMAC into a comprehensive MAC protocol that provides both reliable and unreliable services for all the three modes of communications: unicast, multicast, and broadcast. Our evaluation shows that RMAC achieves high reliability with very limited overhead. We also compare RMAC with other reliable multicast MAC protocols, showing that RMAC not only provides higher reliability but also involves lower cost.

In upcoming very high-speed wireless LANs (WLANs), the physical (PHY) layer rate may reach 600 Mbps. To achieve high efficiency at the medium access control (MAC) layer, we identify fundamental properties that must be satisfied by any CSMA-/CA-based MAC layers and develop a novel scheme called aggregation with fragment retransmission (AFR) that exhibits these properties. In the AFR scheme, multiple packets are aggregated into and transmitted in a single large frame. If errors happen during the transmission, only the corrupted fragments of the large frame are retransmitted. An analytic model is developed to evaluate the throughput and delay performance of AFR over noisy channels and to compare AFR with similar schemes in the literature. Optimal frame and fragment sizes are calculated using this model. Transmission delays are minimized by using a zero-waiting mechanism where frames are transmitted immediately once the MAC wins a transmission opportunity. We prove that zero-waiting can achieve maximum throughput. As a complement to the theoretical analysis, we investigate the impact of AFR on the performance of realistic application traffic with diverse requirements by simulations. We have implemented the AFR scheme in the NS-2 simulator and present detailed results for TCP, VoIP, and HDTV traffic. The AFR scheme described was developed as part of the IEEE 802.11n working group work. The analysis presented here is general enough to be extended to proposed schemes in the upcoming 802.11n standard. Trends indicated in this paper should extend to any well-designed aggregation schemes.

In this paper, we characterize the achievable rate region for any 802.11-scheduled static multi-hop network. To do so, we first characterize the achievable edge-rate region, that is, the set of edge rates that are achievable on the given topology. This requires a careful consideration of the inter-dependence among edges, since neighboring edges collide with and affect the idle time perceived by the edge under study. We approach this problem in two steps. First, we consider two-edge topologies and study the fundamental ways by which they interact. Then, we consider arbitrary multi-hop topologies, compute the effect that each neighboring edge has on the edge under study in isolation, and combine to get the aggregate effect. We then use the characterization of the achievable edge-rate region to characterize the achievable rate region. We verify the accuracy of our analysis by comparing the achievable rate region derived from simulations with the one derived analytically. We make a couple of interesting and somewhat surprising observations while deriving the rate regions. First, the achievable rate region with 802.11 scheduling is not necessarily convex. Second, the performance of 802.11 is surprisingly good. For example, in all the topologies used for model verification, the max-min allocation under 802.11 is at least 64 % of the max-min allocation under a perfect scheduler.

We consider the medium access control (MAC) layer for very high-speed Wireless LANs, which is designed to support rich multimedia applications such as highdefinition television. In such networks, the physical (PHY) layer data rate is proposed to exceed 216Mbps. The legacy MAC layer, however, greatly restricts the performance improvement due to its overhead. It has been shown that MAC utilizes less than 20 % of the transportation ability provided by the PHY layer. To mitigate this inefficiency, we propose an Aggregation with Fragment Retransmission (AFR) scheme, which supports transmissions of very large frames and partial retransmissions in the case of errors. Aggregation allows for increased performance despite pertransmission overhead while partial retransmission alleviates the risk of losing the entire frame. Extensive simulations show that AFR fundamentally outperforms the legacy MAC protocol. It is particularly effective for applications with high data rates and large packet sizes such as HDTV and high-rate UDP traffic. For applications with very low data rates and small packet sizes such as Voice over IP, AFR performs slightly better. 1.

Abstract — Applications involving teams of mobile robots will require robots within the system to form connections to other members with certain quality of service (QoS) requirements. We present a distributed control architecture that allows robots participating in routing a QoS flow to maintain the required level of service while addressing secondary objectives. A distributed control system preserves global properties using “best-effort”, error-suppressing controllers. We outline a routing protocol that dynamically reconfigures a flow by recruiting neighboring robots if it believes a routing fault may occur. We evaluate the control architecture and dynamic configuration protocol in simulation, using the ns-2 network simulator and Player/Stage robot simulator platforms. I.

Abstract 1 — In this paper we study the problem of using the rate adaptation technique to achieve energy efficiency in an IEEE 802.11-based multi-hop network. Specifically, we formulate it as an optimization problem, i.e., minimizing the total transmission power over transmission data rates, subject to the traffic requirements of all the nodes in a multi-hop network. Interestingly, we can show that this problem is actually a well-known multiple-choice knapsack problem, which is proven to be an NP-hard problem. So, instead of finding an optimal solution, which is NP-hard, we seek a sub-optimal solution. Our key technique to attack this problem is distributed cooperative rate adaptation. Here, we promote node cooperation due to our observation that the inequality in non-cooperative channel contention among nodes caused by hidden terminal phenomenon in a multi-hop network tends to result in energy inefficiency. Under this design philosophy, we propose a distributed cooperative rate adaptation (CRA) scheme and prove that it converges. Simulation results show that our CRA scheme can reduce the power consumption up to 86 % as compared to the existing (non-cooperative) algorithm. Keywords-rate adaptation; energy efficiency; IEEE 802.11; cooperation; wireless multi-hop network 1 This work is performed when Kun Wang is a visiting student in Microsoft Research Asia. 1 I.