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标题与描述中的核心知识点是关于SMACK（一种智能确认方案）在无线网络广播消息中的应用。此方案由Aveek Dutta、Dola Saha、Dirk Grunwald和Douglas Sicker四位作者提出，他们分别来自美国科罗拉多大学电气、计算机和能源工程系以及计算机科学系。这篇文章发表在了2009年的SIGCOMM会议上，重点讨论了如何利用同时传输（Simultaneous Transmission）来减少可靠组播的成本，并加速常见的群组通信原语如任意播（Anycast）、广播、领导选举等。 ### 关键技术点详解： #### 同时传输（Simultaneous Transmission）的优势：传统的无线网络协议设计通常将干扰和同时传输视为问题，试图避免它们的发生。然而，作者们提出了一个新颖的观点，即通过合理设计，无线协议可以利用同时传输显著降低可靠组播的成本。这不仅提高了效率，还加快了群组通信的速度。 #### 可靠广播机制：研究者展示了如何在一个基础设施和对等网络中使用原型可重构硬件实现可靠的广播。通过简单频谱感知技术，能够区分多个传输，进而验证了同时传输的概念。 #### 组通信问题的解决：文章进一步描述了如何将这一机制扩展到解决更广泛的群组通信问题，包括但不限于任意播、广播和领导选举等。作者们还探讨了构建创新协议所面临的挑战，这些协议既能提高速度，又能保持可靠性。 #### 软件定义无线电（SDR）与正交频分复用（OFDM）的应用：文中提及的关键技术包括软件定义无线电和正交频分复用，这些都是实现高效无线通信的重要手段。软件定义无线电允许动态配置无线通信系统的功能，而正交频分复用则是一种高效的频谱利用技术，它能提高数据传输速率并减少多径衰落的影响。 #### 协议验证与性能考量：研究的通用术语涵盖了设计、性能和验证等方面，表明作者们在提出新方案时考虑了其实现的可行性、效率以及正确性验证。 ### 结论： SMACK方案为无线网络设计提供了一种全新的视角，通过有效利用同时传输，不仅减少了可靠组播的开销，还提升了群组通信的效率。这一成果对于推动无线网络技术的发展具有重要意义，尤其是对于那些依赖于组播和群组通信的应用场景。 ### 意义与应用前景：这项研究不仅对学术界有着重要的理论贡献，也为工业界提供了实际应用的可能性。例如，在物联网、车联网以及大规模无线传感器网络中，SMACK方案的应用有望显著提升网络性能，降低能耗，从而推动整个行业的进步。此外，随着5G和未来6G技术的发展，对高效率、低延迟的无线通信需求日益增长，SMACK方案有望成为实现这一目标的关键技术之一。

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SMACK - A SMart ACKnowledgment Scheme for Broadcast

Messages in Wireless Networks

Aveek Dutta

, Dola Saha

, Dirk Grunwald

1,2

, Douglas Sicker

Department of Electrical, Computer and Energy Engineering

Department of Computer Science

University of Colorado

Boulder, CO 80309-0430 USA

{Aveek.Dutta, Dola.Saha, Dirk.Grunwald, Douglas.Sicker}@colorado.edu

ABSTRACT

Network protocol designers, both at the physical and network level,

have long considered interference and simultaneous transmission

in wireless protocols as a problem to be avoided. This, coupled

with a tendency to emulate wired network protocols in the wireless

domain, has led to artiﬁcial limitations in wireless networks.

In this paper , we argue that wireless protocols can exploit si-

multaneous transmission to reduce the cost of reliable multicast

by orders of magnitude. With an appropriate application inter-

face, simultaneous transmission can also greatly speed up common

group communication primitives, such as anycast, broadcast, leader

election and others.

The proposed method precisely ﬁts into the domain of directly

reachable nodes where many group communication mechanisms

are commonly used in routing protocols and other physical-layer

mechanisms. We demonstrate how simultaneous transmission can

be used to implement a reliable broadcast for an infrastructure

and peer-to-peer network using a prototype reconﬁgurable hard-

ware. We also validate the notion of using simple spectrum sensing

techniques to distinguish multiple transmissions. We then describe

ho w the mechanism can be extended to solve group communication

problems and the challenges inherent to build innovative protocols

which are faster and reliable at the same time.

Categories and Subject Descriptors

C.2.2 [COMPUTER-COMMUNICATION NETWORKS]: Net-

work Protocols—Protocol Veriﬁcation;C.3[SPECIAL-

PURPOSE AND APPLICATION-BASED SYSTEMS]: Signal

Processing Systems

General Terms

Design, Performance, Veriﬁcation

Keywords

Software Deﬁned Radio, Orthogonal Frequency Division Multi-

plexing

Permission to make digital or hard copies of all or part of this work for

personal or classroom use is granted without fee provided that copies are

not made or distributed for proﬁt or commercial adv antage and that copies

bear this notice and the full citation on the ﬁrst page. To copy otherwise, to

republish, to post on servers or to redistribute to lists, requires prior speciﬁc

permission and/or a fee.

SIGCOMM’09, August 17–21, 2009, Barcelona, Spain.

1. INTRODUCTION

Noise and interference are fundamental aspects of communica-

tions, and are exceptionally important for wireless communications

because it’s more difﬁcult to contain propagation without waveg-

uides such as wires and ﬁbers. Avoiding interference or noise is a

fundamental design objective that limits the scope of simultaneous

multi-user communication. Conventional single carrier communi-

cation focuses on decoding the strongest signal while discarding

anything else as noise or interference.

Multi-user communication requires some form of orthogonal

channel for modulation that allows multiple parties to communi-

cate simultaneously. There are a number of ways to implement

orthogonal channels - code division multiple-access (CDMA) has

been adopted as a very reliable multiple access techniques by us-

ing specially designed codes with strong auto-correlation proper-

ties. With spatial frequency reuse, frequencies are allocated in a

way such that signals from far away communicating pairs will be

so strongly attenuated that they won’t interfere in local communi-

cation. Time division multiplexing, or taking turns using a channel,

is another method.

In this paper, we focus on using orthogonal frequency division

modulation (OFDM) to provide distinct orthogonal signals. OFDM

is a mechanism that splits the available spectrum into a number of

orthogonal non-interfering subchannels. Being orthogonal, each of

the subcarriers can be treated as an information carrying medium

without signiﬁcant interference with another subcarrier. Variants of

the OFDM wav eform are used in a number of current wireless (and

wired) physical layers, including the 802.11a/g. Under OFDM,

different nodes can also communicate on different subcarriers,as

used in WiMax, which employs “scalable OFDMA” where users

use different subcarriers or set of subcarriers to transmit data over

the same medium and at the same carrier frequency.

The ability to distinguish multiple simultaneous transmissions

requires either the signal structure to be fairly simple or the de-

coding/detection mechanism to be complex. In this paper we fo-

cus on a set of network primitives that calls for a very simple an-

swer typically in binary; in the form of yes or no. Empowered

by subcarrier transmission using OFDM we can either transmit a

1 or a 0 to convey these binary answers. Not only is this form

of signaling simple, the detection of such a multiuser communica-

tion can be accomplished using spectrum sensing and energy detec-

tion. Simultaneous transmissions can be an advantage in a number

of network applications that call for multiple nodes to participate

and also use simple information. Examples include route requests,

leader election, network management and other operations involv-

ing broadcast or multicast messages. Not only does simultaneous

transmission make the message exchange faster, it also allows such

exchanges to be reliable.

To demonstrate that the complexity in implementing this form

of multiuser communication is indeed tractable, we implemented

the protocol in a prototype hardware platform. Using FPGA based

Software Deﬁned Radios (SDR) we demonstrate the ability to de-

tect multiple tone transmissions using Fourier transform and energy

detection. The contributions of this paper are:

• We describe the practical constraints on using simultaneous

communication for a wireless mesh network.

• We describe how simultaneous reception can be used to

greatly improve protocol performance.

• We demonstrate the practicality of the system using a Soft-

ware Deﬁned Radio implementation of our protocol.

The rest of the paper is organized as follows. Section §2 provides

some background theory on OFDM signal structure and its relation

to Fourier transform. Section §3 explains the protocol functionality

and its efﬁciency. Section §4 describes the robustness of the pro-

tocol. In section §5 we present the challenges and issues involved

in implementing the protocol using SDRs. This is followed by the

actual hardware implementation and design aspects in section §6.

To evaluate the hardware and the protocol performance we present

a set of experiments in section §7. The results from the experi-

ments have been analyzed in §8. To demonstrate the usefulness of

this physical layer protocol to higher layer protocols we present a

few applications in §10. Prior work related to this paper has been

investigated in section §11. Finally we conclude the paper in §12.

2. OFDM AND FOURIER TRANSFORM

Orthogonal Frequency Division Multiplexing (OFDM) [6] is a

special type of Multicarrier Modulation (MCM), where the data

stream is divided into several bit streams and the modulated sub-

carriers are spaced closely, although overlapping in such a manner

that they do not interfere with each other . Using the Fourier Trans-

form and its inverse, the signal is efﬁciently converted from the

time domain to the frequency domain and vice versa. Even though

the technology is prevalent for approximately 20 years, and stan-

dards like 802.11a/g and 802.16 have embraced OFDM/OFDMA

modulation techniques, we have not found any intelligent use of

the technology other than simply using it as a medium of transmis-

sion at higher data rates.

The fact that the component sinusoids of an OFDM signal can be

easily aggregated to form time domain signals as in eq. 1 empowers

us to use any part of the spectrum by suitably selecting the spectral

coefﬁcients x(k).

X(n)=

N−1

i=0

x(k)sin(

2πkn

) − j

N−1

i=0

x(k)cos(

2πkn

)

(1)

Here, X(n) is the value of the signal at time n which is composed

of frequencies denoted by 2πkn/N, k is the index of frequency

ov er N spectral components that divides the available bandwidth

with equal spacing and x(k) gives the v alue of the spectrum at k

frequency.

This leads to the notion of non-contiguous OFDM (NC-OFDM)

which can degenerate to even a single frequency or tone. A Fourier

transform of such an NC-OFDM signal reveals the spectral energy

and can be detected using fairly simple methods.

The simplicity of OFDM and ease of implementation of such a

system has led us to innovate the ne wer protocols and signaling

methods described in the this paper.

3. SMART ACKNOWLEDGMENTS

In this paper, we focus on speeding group communication using

simultaneous transmission and r eception. There are many types

of group communications, the most common of which is broadcast

or multicast. Conventional infrastructure wireless networks (e.g.,a

standard WiFi network) usually only use broadcast packets to trans-

late wired broadcasts into wireless packets. The standard 802.11

physical layer doesn’t provide a method for determining if a broad-

cast was delivered; thus such broadcasts are typically transmitted

at the lowest modulation rate (in an effort to increase the reliabil-

ity of reception). Since broadcast messages are exchanged without

ackno wledgment control frames, there is a limited scope for the

source or the access point (AP) to reliably ensure the reception of

the message at the host nodes.

In “ad hoc” networks, broadcast messages are used for many pur-

poses. Typical applications include host discov ery, network main-

tenance, route discovery, etc. For example, wireless protocols such

as AODV [18] periodically broadcast a routing table to “neigh-

boring nodes” (meaning those that can hear the message). Nodes

also periodically transmit “hello” messages to determine if nodes

are still reachable. These messages are typically “unicast” mes-

sages, because there is no way to safely determine if they’ve been

received.

Reliable broadcast messages, “hello” link maintenance messages

and many other communications share a common pattern: a mes-

sage is sent and one or more nodes should “vote” on the trans-

mitted message. For reliable broadcasts, the vote is an acknowl-

edgment that “I have received and can decode the message”. If

a node has not receiv ed the message, the sender would retransmit

it. Link maintenance messages are almost identical, except that

if a formerly “adjacent” node does not receive the message, it is

removed from the node neighbors table (with no retransmission).

Many other protocols, such as voting protocols, can map to a simi-

lar query followed by a yes/no decision from other nodes.

Some of these protocols concerning a single network “link” have

an analogous extension to a “network” counterpart. For exam-

ple, there is considerable work on providing reliable network-wide

support for broadcast packets in wireless networks, as well as dis-

tributed leader election.

3.1 SMACK - Reliable Link Layer Broad-

casts

For any reliable broadcast mechanism to be reliable, there must

be a clearly deﬁned set of nodes in the network; Figure 1(a) shows

a single access point and multiple clients. Each client is assigned

a unique “membership number”. For our implementation we have

chosen the OFDM based physical layer for 802.11a/g as the under-

lying signaling method. Figure 1(b) shows a schematic illustration

of the properties of the OFDM waveform that are needed. A given

bandwidth, such as the 2.4Ghz band used by 802.11g, is subdivided

into a number of subcarriers around a center frequency; that center

frequency is the “channel” to which an 802.11 radio is set.

In 802.11g, 53 subcarriers remain for data modulation. Nor-

mally, a single transmitter modulates all subcarriers to send high

bandwidth data. In our protocol, since we only need to transmit

a “yes” or “no”, we assign subcarriers to individual nodes, as il-

lustrated in Figure 1(b); different clients are assigned subcarrier

bins labeled as f

,...,f

where n depends on the number

of users and the number of subcarriers available. The orthogonality

of individual subcarriers allows us to use each of them as sepa-

rate data carriers for different hosts. Using multicarrier modulation

techniques allows the AP to receive ACKs from a greater num-

ber of clients in the shortest possible time, dramatically reducing

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