Jeong-woo Cho

Joint network-wide opportunistic scheduling and power control in multi-cell networks

We present a unified analytical framework that maximizes generalized utilities of a wireless network by network-wide opportunistic scheduling and power control. That is, base stations in the network jointly decide mobile stations to be served at the same time as the transmission powers of base stations are coordinated to mitigate the mutually interfering effect. Although the maximization at the first glance appears to be a mixed, twofold and nonlinear optimization requiring excessive computational complexity, we show that the maximization can be transformed into a pure binary optimization with much lower complexity. To be exact, it is proven that binary power control of base stations is necessary and sufficient for maximizing the network-wide utilities under a physical layer regime where the channel capacity is linear in the signal-to-interference-noise ratio. To further reduce the complexity of the problem, a distributed heuristic algorithm is proposed that performs much better than existing opportunistic algorithms. Through extensive simulations, it becomes clear that network-wide opportunistic scheduling and power control is most suitable for fairness-oriented networks and under loaded networks. We believe that our work will serve as a cornerstone for network-wide scheduling approaches from theoretical and practical standpoints.

Joint Network-wide Opportunistic Scheduling and Power Control in Multi-cell Networks

We present a unified analytical framework that maximizes generalized utilities of a wireless network by network-wide opportunistic scheduling and power control. That is, base stations in the network jointly decide mobile stations to be served at the same time as the transmission powers of base stations are coordinated to mitigate the mutually interfering effect. Although the maximization at the first glance appears to be a mixed, twofold and nonlinear optimization requiring excessive computational complexity, we show that the maximization can be transformed into a pure binary optimization with much lower complexity. To be exact, it is proven that binary power control of base stations is necessary and sufficient for maximizing the network-wide utilities under a physical layer regime where the channel capacity is linear in the signal-to-interference-noise ratio. To further reduce the complexity of the problem, a distributed heuristic algorithm is proposed that performs much better than existing opportunistic algorithms. Through extensive simulations, it becomes clear that network-wide opportunistic scheduling and power control is most suitable for fairness-oriented networks and underloaded networks.

On the Asymptotic Validity of the Decoupling Assumption for Analyzing 802.11 MAC Protocol

Performance evaluation of the 802.11 MAC protocol is classically based on the decoupling assumption, which hypothesizes that the backoff processes at different nodes are independent. This decoupling assumption results from mean field convergence and is generally true in transient regime in the asymptotic sense (when the number of wireless nodes tends to infinity), but, contrary to widespread belief, may not necessarily hold in stationary regime. The issue is often related with the existence and uniqueness of a solution to a fixed point equation; however, it was also recently shown that this condition is not sufficient; in contrast, a sufficient condition is a global stability property of the associated ordinary differential equation. In this paper, we give a simple condition that establishes the asymptotic validity of the decoupling assumption for the homogeneous case (all nodes have the same parameters). We also discuss the heterogeneous and the differentiated service cases and formulate a new ordinary differential equation. We show that the uniqueness of a solution to the associated fixed point equation is not sufficient; we exhibit one case where the fixed point equation has a unique solution but the decoupling assumption is not valid in the asymptotic sense in stationary regime.

On the validity of the fixed point equation and decoupling assumption for analyzing the 802.11 mac protocol

Performance evaluation of the 802.11 MAC protocol is classically based on the decoupling assumption, which hypothesizes that the backoff processes at different nodes are independent. A necessary condition for the validity of this approach is the existence and uniqueness of a solution to a fixed point equation. However, it was also recently pointed out that this condition is not sufficient; in contrast, a necessary and sufficient condition is a global stability property of the associated ordinary differential equation. Such a property was established only for a specific case, namely for a homogeneous system (all nodes have the same parameters) and when the number of backoff stages is either 1 or infinite and with other restrictive conditions. In this paper, we give a simple condition that establishes the validity of the decoupling assumption for the homogeneous case. We also discuss the heterogeneous and the differentiated service cases and show that the uniqueness condition is not sufficient; we exhibit one case where the fixed point equation has a unique solution but the decoupling assumption is not valid.

Dynamic buffer management scheme based on rate estimation in packet-switched networks

While traffic volume of real-time applications is rapidly increasing, current routers do not guarantee minimum QoS values of fairness and drop packets in random fashion. If routers provide a minimum QoS, resulting less delays, more fairness, and smoother sending rates, TCP-friendly rate control (TFRC) can be adopted for real-time applications. We propose a dynamic buffer management scheme that meets the requirements described above, and can be applied to TCP flow and to data flow for transfer of real-time applications. The proposed scheme consists of a virtual threshold function, an accurate and stable per-flow rate estimation, a per-flow exponential drop probability, and a dropping strategy that guarantees fairness when there are many flows. Moreover, we introduce a practical definition of active flows to reduce the overhead coming from maintaining per-flow states. We discuss how proposed scheme motivates real-time applications to adopt TFRC.

Stabilized Max-Min Flow Control Using PID and PII2 Controllers

This paper describes an analytical framework for the weighted max-min flow control of elastic flows in packet networks using PID and PII 2 controller when flows experience heterogeneous round-trip delays. Our algorithms are scalable in that routers do not need to store any per-flow information of each flow and they use simple first come first serve (FCFS) discipline, stable in that the stability is proven rigorously when there are flows with heterogeneous round-trip delays. We first suggest two closed-loop system models that approximate our flow control algorithms in continuous-time domain where the purpose of the first algorithm is to achieve the target queue length and that of the second is to achieve the target utilization. The slow convergence [1] of many rate-based flow control algorithms, which use queue lengths as input signals, can be resolved by the second algorithm. Based on these models, we find the conditions for controller gains that stabilize closed-loop systems when round-trip delays are equal and extend this result to the case of heterogeneous round-trip delays with the help of Zero exclusion theorem. We simulate our algorithms with optimal gain sets for various configurations including a multiple bottleneck network to verify the usefulness and extensibility of our algorithms.

A distributed max-min flow control algorithm for multi-rate multicast flows

We present a distributed algorithm to compute the bandwidth max-min fair rates in a multi-rate multicast network. The significance of the algorithm, compared to previous algorithms (Sakar, S. and Tassiulas L., 1999, 2000; Kar, K. et al., 2001), is that it is more scalable (it does not require each link to maintain the saturation status of all sessions and virtual sessions travelling through it) it is more stable (it converges asymptotically to the desired equilibrium, satisfying the minimum plus max-min fairness, even in the presence of heterogeneous round-trip delays) and it has explicit link buffer control (the buffer occupancy of every bottlenecked link in the network asymptotically converges to the pre-defined value). In addition, we propose an efficient feedback consolidation algorithm which is computationally simpler than its hard-synchronization based counterpart and eliminates unnecessary consolidation delay by preventing it from awaiting backward control packets (BCPs) that do not directly contribute to the session rate. Through simulations, we verify the performance of the proposed multi-rate multicast flow control scheme based on these two algorithms.

Capacity of interference-limited ad hoc networks with infrastructure support

In this letter, we consider the capacity of ad hoc networks with infrastructure support. Although Grossglauser-Tse mobile network model enables /spl Theta/(1) per-node throughput scaling, the mobility assumption may be too unrealistic to be accepted in some practical situations. One of the key observations we acquired is that the infrastructure support plays the same role played by the mobility in the Grossglauser-Tse model. We show that nodes can utilize the randomly located infrastructure support instead of mobility when nodes are nearly static. In this case, we show that the per-node throughput of /spl Theta/(1) is still achievable when the number of access points grows linearly with respect to the number of nodes.

Basic theorems on the backoff process in 802.11

Since its introduction, the performance of IEEE 802.11 has attracted a lot of research attention and the center of the attention has been the throughput. For throughput analysis, in the seminal paper by Kumar et al. [8], they axiomized several remarkable observations based on a fixed point equation (FPE). Above all, one of the key findings of [8] is that the full interference model, also called the single-cell model [8], in 802.11 networks leads to the backoff synchrony property which implies the backoff process can be completely separated and analyzed through the FPE technique. To date, however, only the uniqueness of the fixed point has been proven to hold under some mild assumptions [8, Theorem 5.1], and we still need an answer to the following fundamental question: Q1:“Exactly under which conditions the fixed point equation technique is valid?” (to be answered in Theorem 1) An intriguing notion, called short-term fairness, has been introduced in some recent works [2,4]. It can be easily seen that this notion pertains to a purely backoff-related argument also owing to the backoff synchrony property in the full interference model [8]. The two papers [2,4] considered the same situation where only two wireless nodes contend for the medium. The former [2] claimed and conjectured through simulations that the summation of the backoff values generated per a packet, denoted by Ω, is uniformly distributed because the initial backoff is uniformly distributed while the latter [4] conjectured based on their experiments that Ω is exponentially distributed in the sense that its coefficient of variation (CV) is one. This left room for misunderstandings about the backoff distribution: Q2:“One of the two works is incorrect?” (to be answered in Theorem 2) In addition, the two works [2,4] defined P[z|ζ] as the probability that other nodes transmits z packets while a tagged node is transmitting ζ packets and acquired the expression of P[z|ζ] valid only for the two node case. It is natural to ask the following pertinent questions: Q3:“Can we develop an analytical model for short-term fairness?” (to be answered in Theorem 3) Q4:“When does the short-term fairness undergo a dramatic

System Spectral Efficiency and Stability of 3G Networks: A Comparative Study

CDMA2000, WCDMA and WiMAX are three widely used 3G technologies. Since they share the same goal, which is to provide broader coverage and higher throughput in 3G networks, an impartial comparison of their performance is indispensable. However, they are based on different design principles and methodologies, which make the comparison challenging. This paper presents a comparative study of these technologies, with focus on system spectral efficiency and stability in 3G networks. Specifically, the paper presents a framework for the comparison based on the common set of configurations adopted by these technologies, which include channel models, system parameters and key algorithms. Through extensive simulations, the system spectral efficiency and stability of CDMA2000 1x EVDO Rev.A, WCDMA HSDPA/HSUPA and Mobile WiMAX are compared. It is found that while WiMAX can provide highest throughput, the two CDMA-based technologies achieve higher system spectral efficiency, especially on the downlink. Regarding system stability, it is observed that CDMA2000 1x EV-DO Rev.A can operate under higher interference levels than WCDMA HSDPA/HSUPA and Mobile WiMAX. In addition, the comparison on system spectral efficiency between CDMA2000, WCDMA and WiMAX is also conducted when relevant enhanced technologies, i.e., MIMO and interference cancelation, are adopted. We believe that our work will serve as a cornerstone for a fair comparison between the various technologies for prospective 3G networks.