Xiangying Yang

Transmission capacity of wireless ad hoc networks with outage constraints

In this paper, upper and lower bounds on the transmission capacity of spread-spectrum (SS) wireless ad hoc networks are derived. We define transmission capacity as the product of the maximum density of successful transmissions multiplied by their data rate, given an outage constraint. Assuming that the nodes are randomly distributed in space according to a Poisson point process, we derive upper and lower bounds for frequency hopping (FH-CDMA) and direct sequence (DS-CDMA) SS networks, which incorporate traditional modulation types (no spreading) as a special case. These bounds cleanly summarize how ad hoc network capacity is affected by the outage probability, spreading factor, transmission power, target signal-to-noise ratio (SNR), and other system parameters. Using these bounds, it can be shown that FH-CDMA obtains a higher transmission capacity than DS-CDMA on the order of M/sup 1-2//spl alpha//, where M is the spreading factor and /spl alpha/>2 is the path loss exponent. A tangential contribution is an (apparently) novel technique for obtaining tight bounds on tail probabilities of additive functionals of homogeneous Poisson point processes.

Transmission Capacity of Wireless Ad Hoc Networks With Successive Interference Cancellation

The transmission capacity (TC) of a wireless ad hoc network is defined as the maximum spatial intensity of successful transmissions such that the outage probability does not exceed some specified threshold. This work studies the improvement in TC obtainable with successive interference cancellation (SIC), an important receiver technique that has been shown to achieve the capacity of several classes of multiuser channels, but has not been carefully evaluated in the context of ad hoc wireless networks. This paper develops closed-form upper bounds and easily computable lower bounds for the TC of ad hoc networks with SIC receivers, for both perfect and imperfect SIC. The analysis applies to any multiuser receiver that cancels the K strongest interfering signals by a factor z isin [0, 1]. In addition to providing the first closed-form capacity results for SIC in ad hoc networks, design-relevant insights are made possible. In particular, it is shown that SIC should be used with direct sequence spread spectrum. Also, any imperfections in the interference cancellation rapidly degrade its usefulness. More encouragingly, only a few - often just one - interfering nodes need to be canceled in order to get the vast majority of the available performance gain.

alpha-Optimal User Association and Cell Load Balancing in Wireless Networks

In this paper we develop a framework for user association in infrastructure-based wireless networks, specifically focused on flow-level cell load balancing under spatially inhomogeneous traffic distributions. Our work encompasses several different user association policies: rate-optimal, throughput- optimal, delay-optimal, and load-equalizing, which we collectively denote α-optimal user association. We prove that the optimal load vector ρ∗ that minimizes a generalized system performance function is the fixed point of a certain mapping. Based on this mapping we propose and analyze an iterative distributed user association policy that adapts to spatial traffic loads and converges to a globally optimal allocation.

Mobile relay and group mobility for 4G WiMAX networks

Relay is one of the key features being considered for IMT-Advanced systems. The relay architectures defined in IEEE 802.16m and 3GPP LTE-Advanced are optimized only for non-mobile relay, i.e., the Relay Station is attached to a designated Base Station and becomes a part of the fixed access network. A mobile relay architecture, where relay may switch attached BS according to operation demand, will promise more resilient relay deployment. In this paper, we first highlight three use cases where mobile relays can offer useful deployment options. Next, we propose an enhanced handover mechanism for relay-based group mobility by extending the IEEE 802.16m specification on relay. To this end, we describe the handover architecture for mobile relay highlighting the C-plane and U-plane enhancements required in order to support group mobility of mobile stations. Using illustration and comparisons, we show that mobile relay will offer significant benefits of signaling overhead reduction and provide users seamless mobility experience compared to fixed relays.

Transmission capacity of CDMA ad-hoc networks

Spread spectrum technologies are appropriate for ad-hoc networking because they permit interference averaging and tolerate colocated simultaneous transmissions. We develop analytical results on the transmission capacity of a CDMA ad-hoc network. Transmission capacity is defined as the maximum permissible density of simultaneous transmissions that allows a certain probability of successful reception. We obtain closed-form upper and lower bounds on the transmission capacity for both frequency hopped (FH-CDMA) and direct sequence (DS-CDMA) implementations of CDMA. Our analysis shows that FH-CDMA obtains a higher transmission capacity than DS-CDMA on the order of M/sup 1-2//spl alpha//, where M is the spreading factor and /spl alpha/>2 is the path loss exponent. The interpretation is that FH-CDMA is generally preferable to DS-CDMA for ad-hoc networks.

Exploiting the MAC layer flexibility of WiMAX to systematically enhance TCP performance

TCP performance over wireless network has been extensively studied in the literature and many cross-layer optimization schemes are proposed. Nevertheless, new technologies such as WiMAX, based on IEEE 802.16, offer new dimensions for system optimization that limited research has explored. In this paper, we show that the flexible WiMAX MAC layer allows cross-layer optimization to be done at the fine granularity of each connection or each service class. We also highlight that in a WiMAX broadband wireless access network, aggregate system performance is more important than the performance of an individual application instance, particularly from a network operator point of view. To this end, we propose an asymmetric link adaptation strategy for TCP-based applications. Coupled with ARQ, the aggregate system performance is enhanced without significantly compromising the performance of individual TCP flow. In addition, we argue that schedulers should be designed specifically for different service classes. We propose a scheduler for TCP-based best-effort service class that achieves both good aggregate throughput and fairness. The proposed design concepts not only help to achieve a good end-to-end TCP performance along with enhanced spectral efficiency, but also open many possibilities for optimizing the performance of other applications with similar techniques in WiMAX

Adaptive Polling Service for Next-Generation IEEE 802.16 WiMAX Networks

WiMAX technology, based on IEEE 802.16 standard, is a promising broadband wireless technology for future 4G network. WiMAX provides radio access with centralized control, where bandwidth resources are managed and allocated by base station (BS) for both uplink (UL) and downlink (DL). Current WiMAX standard defines several bandwidth request (BWReq) mechanisms to serve a variety of applications, which however may not be efficient for on-off bursty traffic patterns. In this paper, we propose a novel adaptive polling service (aPS), with the advantage of significantly reducing the signaling overhead associated with the bandwidth request process without compromising the delay performance of real-time applications. Extensive simulations demonstrate that aPS outperforms existing scheduling services in terms of overall efficiency for a wide range of applications.

Inducing spatial clustering in MAC contention for spread spectrum ad hoc networks

This paper proposes a new principle for designing MAC protocols for spread spectrum based ad hoc networks -- inducing spatial clustering in contending transmitters/receivers. We first highlight the advantages of spread spectrum in handling quality of service (QoS) requirements, enhancing energy efficiency, and enabling spatial multiplexing of bursty traffic. Then, based on stochastic geometric models and simulation, we show how idealized contention resolution among randomly distributed nodes results in clustering of successful transmitters and receivers, in turn leading to efficient spatial reuse. This motivates the central idea of the paper which is to explicitly induce clustering among contending nodes to achieve even better spatial reuse. We propose two distributed mechanisms to realize such clustering and show substantial capacity gains over simple random access/ALOHA-like and even RTS/CTS based protocols. We examine under what regimes such gains can be achieved, and how clustering and contention resolution mechanisms should be optimized to do so. We propose the design of ad hoc networks supporting hop-by-hop relaying on different spatial scales. By allowing nodes to relay beyond the set of nearest neighbors using varying transmission ranges (scales), one can reduce the number of hops between a source and destination so as to meet end-to-end delay requirements. To that end we propose a multi-scale MAC clustering and power control mechanism to support transmissions with different ranges while achieving high spatial reuse. The considerations, analysis and simulations included in this paper suggest that the principle of inducing spatial clustering in contention has substantial promise towards achieving high spatial reuse, QoS, and energy efficiency in spread spectrum ad hoc networks.

Transmission capacity of CDMA ad hoc networks employing successive interference cancellation

Upper and lower bounds on the transmission capacity of direct-sequence CDMA wireless ad hoc networks are derived. The transmission capacity is a stochastic measure of the allowable number of transmissions per unit area, and is a generalization of previous measures of ad hoc network capacity. Successive interference cancellation (SIC) is attractive for DS-CDMA ad hoc networks since the dominant nearby interferers can be cancelled. Our closed-form results cleanly summarize the dependence of ad hoc network capacity on pathloss, spreading, outage probability, and interference cancellation accuracy. Other multiple access schemes, such as CSMA and DS-CDMA without SIC, are special cases. Perfect interference cancellation increases transmission capacity by nearly two orders of magnitude. Furthermore, cancelling just the strongest interferer generally gives the majority of the capacity gain, so the latency and complexity cost of SIC should be negligible.

Inducing multiscale clustering using multistage MAC contention in CDMA ad hoc networks

This paper proposes a new principle for designing MAC protocols for CDMA-based ad hoc networks-inducing spatial clustering in contending transmitters/receivers. We first highlight the advantages of CDMA in handling quality of service (QoS) requirements, enhancing energy efficiency, and enabling spatial multiplexing of bursty traffic. Then, based on stochastic geometric models and simulation, we show how idealized contention resolution among randomly distributed nodes results in clustering of successful transmitters and receivers, in turn leading to efficient spatial reuse. This motivates the central idea of the paper which is to explicitly induce clustering among contending nodes to achieve even better spatial reuse. We propose two distributed mechanisms to realize such clustering and show substantial capacity gains over simple random access/ALOHA-like and even RTS/CTS-based protocols. We examine under what regimes such gains can be achieved, and how clustering and contention resolution mechanisms should be optimized to do so. We propose the design of ad hoc networks supporting hop-by-hop relaying on different spatial scales. By allowing nodes to relay beyond the set of nearest neighbors using varying transmission distances (scales), one can reduce the number of hops between a source and destination so as to meet end-to-end delay requirements. To that end we propose a multi-scale MAC clustering and power control mechanism to support transmissions with different ranges while achieving high spatial reuse. The considerations, analysis and simulations included in this paper suggest that the principle of inducing spatial clustering in contention has substantial promise towards achieving high spatial reuse, QoS, and energy efficiency in CDMA ad hoc networks.