Wireless Communications IEEE Volume 18 Issue 1 February 2011 issue Volume 18 Issue 1 February 2011

87 Pages·2011·7.43 MB·English

by  A Publication of the IEEE Communications Society

February 2011 Wireless Cover 1 2/7/11 12:20 PM Page 1
• IMS EMERGENCY SERVICES: A PRELIMINARY STUDY
IEEE February 2011, Vol. 18, No. 1 • MIXING NETWORK CODING AND COOPERATION FOR RELIABLE
WIRELESS COMMUNICATIONS
• SPECTRUM SENSING FOR COGNITIVE RADIO SYSTEMS
• COOPERATIVE COMMUNICATION IN MULTIHOP COGNITIVE RADIO NETWORKS
WIRELESS BASED ON MULTICARRIER MODULATION
COMMUNICATIONS • TOPOLOGICAL-BASED ARCHITECTURES FOR WIRELESS MESH NETWORK
IMS EMERGENCY
SERVICES
®
A Publication of the IEEE Communications Society
In cooperation with IEEE Computer and VehicularTechnology Societies
LYT-TOC-Feb 2/7/11 11:41 AM Page 1
FEBRUARY 2011/VOL . 18, NO. 1
IEEE
WIRELESS
COMMUNICATIONS
ACCEPTED FROM OPEN CALL 46
CHALLENGES, OPPORTUNITIES, AND
6
SOLUTIONS FOR CONVERGED SATELLITE AND
IMS EMERGENCY SERVICES:
TERRESTRIAL NETWORKS
A PRELIMINARY STUDY
TARIK TALEB, YASSINE HADJADJ-AOUL, AND TOUFIK AHMED
YI-BING LIN, MENG-HSUN TSAI, AND YUAN-KUANG TU
54
15
INTERFERENCE COORDINATION FOR
MIXING NETWORK
OFDM-BASED MULTIHOP
CODING AND COOPERATION FOR
LTE-ADVANCED NETWORKS
RELIABLE WIRELESS COMMUNICATIONS
KAN ZHENG, BIN FAN, JIANHUA LIU, YICHENG LIN,
FRANCESCO ROSSETTO AND MICHELE ZORZI AND WENBO WANG
22 64
SECURING UNDERWATER DISTRIBUTED AUTOMATED
WIRELESS COMMUNICATION NETWORKS INCIDENT DETECTION WITH VGRID
MARI CARMEN DOMINGO BEHROOZ KHORASHADI, FRED LIU, DIPAK GHOSAL,
30 MICHAEL ZHANG, AND CHEN-NEE CHUAH
SPECTRUM SENSING FOR 74
COGNITIVE RADIO SYSTEMS: TOPOLOGICAL-BASED ARCHITECTURES FOR
TECHNICAL ASPECTS AND WIRELESS MESH NETWORKS
STANDARDIZATION ACTIVITIES OF THE AMIR ESMAILPOUR, NIDAL NASSER, AND TARIK TALEB
IEEE P1900.6 WORKING GROUP
82
KLAUS MOESSNER, HIROSHI HARADA, CHEN SUN,
YOHANNES D. ALEMSEGED, HA NGUYEN TRAN, SYNCHRONIZATION OF
DOMINIQUE NOGUET, RYO SAWAI, AND NAOTAKA SATO MULTIHOP WIRELESS SENSOR NETWORKS AT
THE APPLICATION LAYER
38
ÁLVARO MARCO, ROBERTO CASAS, JOSÉ LUIS SEVILLANO
MULTICARRIER MODULATION AND RAMOS, VICTORIAN COARASA, ÁNGEL ASENSIO,
COOPERATIVE COMMUNICATION IN AND MOHAMMAD S. OBAIDAT
MULTIHOP COGNITIVE RADIO NETWORKS
TAO LUO, FEI LIN, TAO JIANG, MOHSEN GUIZANI,
AND WEN CHEN MESSAGE FROM THE EDITOR-IN-CHIEF — 2
SCANNING THE LITERATURE — 4
Cover image: Getty Images
IEEE Wireless Communications • February 2011 1
LYT-EDIT NOTE-February 2/7/11 10:39 AM Page 2
MESSAGE FROM THE ED I TOR- IN -CH I E F
CALL FOR MORE WIRELESS INNOVATIONS
irst of all, I am happy to report that my munications and networking, and decision mak-
term as the Editor-in-Chief has been ing and control. The second technology on the
F extended for one more year. I also want top 11 list is social networking, which has close
to report that we have been working on ties to wireless technologies as most users have
moving into Manuscript Central, which will already been doing their social networking with
be a great step toward a more efficient smartphones or mobile devices, leading to the
paper handling process. As a new year ever more popular mobile social networking. As
starts, we also look forward to more excit- we can see, wireless technologies have already
ing news on wireless communications, net- revolutionized the way we live and have trans-
working, and their applications. You may formed our society into a completely different
have already read the first issue of IEEE kind for everything imaginable. We, the engi-
Spectrum this year. Among the top 11 iden- neers and scientists, are at center stage to uti-
tified technologies (although it is not clear lize the technologies we have created to make a
why it is 11 rather than the more commonly significantly different society, good or bad, and
used 10, perhaps to avoid a copyright law- it is up to us to reshape it and our living envi-
suit from David Letterman?), smartphones ronments for better quality of life. We need you
are ranked on top. Unfortunately, the col- YUGUANG MICHAEL FANG to invent new technologies, including wireless
umn does not spell out the drivers of future technologies, and we need new ideas and new
smartphone technologies: the deep integration technologies of innovations from you. Our magazine offers you the platform
smartphones with smart environments and human interactions for you to spread the word. We are searching for articles on
for ubiquitous data collection, processing, data mining, com- new innovations in the wireless area and solicit new special
IEEE
Director of Magazines IEEE Wireless Communications (ISSN 1536-1284) is pub-
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LYT-EDIT NOTE-February 2/7/11 10:39 AM Page 3
MESSAGE FROM THE ED I TOR- IN -CH I E F
issues on hot topics that are of great interest to our reader- Bin Fan, Yicheng Lin, and Wenbo Wang, presents an
ship. Some of the topics we are particularly interested in are overview of the interference coordination strategies for
wireless technologies in cyber-physical systems, social net- OFDM-based multihop cellular networks and proposes sever-
works, smart grids, healthcare and medical systems, urban al static or semi-static interference coordination schemes
sensing and public safety, and cloud computing systems. With based on the framework of Long Term Evolution (LTE)-
the upcoming electronic paper handling process, I hope we Advanced networks with multihop relaying to improve cover-
can make the publication process much faster. For those who age and increase the data rate over cell edge areas.
are engaging in hot wireless innovations, I encourage you to “Distributed Automated Incident Detection with Vgrid,”
organize special issues to share your excitement with our gen- by Behrooz Khorashadi, Fred Liu, Dipak Ghosal, Chen-Nee
eral audiences. Chuah, and Michael Zhang, studies an ad hoc distributed
Due to the unforeseen delay in getting articles for the automated incident detection algorithm for highway traffic
planned special issue, we have decided to accommodate arti- using vehicles that are equipped with wireless communica-
cles from our open call to cut the long queue of accepted tions, processing, and storage capabilities. By requesting vehi-
papers. We have selected 10 articles in this issue; their brief cles with such capability to broadcast beacon information
summaries follow. containing their speed, location, and lane information, the
“IMS Emergency Services: A Preliminary Study,” by Yi- detection algorithm can make better decisions and yield bet-
Bing Lin, Meng-Hsun Tsai and Yuan-Kuang Tu, presents a ter performance.
study on how to support emergency call and walkie-talkie ser- “Topological-Based Architectures for Wireless Mesh Net-
vices over IP Multimedia Subsystem in wireless cellular sys- work,” by Amir Esmailpour, Nidal Nasser, and Tarik Taleb,
tems. provides an overview on architectural design for wireless mesh
“Mixing Network Coding and Cooperation for Reliable networks, summarizes the state-of-the-art research findings,
Wireless Communications,” by Francesco Rossetto and and calls for further research on this topic.
Michele Zorzi, gives an overview of how to take advantage of “Synchronization of Multihop Wireless Sensor Networks at
both cooperation and network coding to improve the perfor- the Application Layer,” by Álvaro Marco, Roberto Casas,
mance and error correction capabilities of radio networks and José Luis Sevillano, Victorián Coarasa, Ángel Asensio, and
highlights the main challenges for future research. Mohammad S. Obaidat, proposes a method for accurate syn-
“Securing Underwater Wireless Communication Net- chronization of large multihop networks, which operates at
works,” by Mari Carmen Domingo, reviews some important the application layer while minimizing message exchange.
security design issues specific to underwater wireless commu- I hope you enjoy reading these articles. I also wish you a
nication networks and discusses possible research challenges. productive 2011!
“Spectrum Sensing for Cognitive Radio Systems: Technical
Aspects and Standardization Activities of IEEE 1900.6 Work- BIOGRAPHY
ing Group,” by Klaus Moessner, Hiroshi Harada, Chen Sun, YUGUANG MICHAEL FANG [F’08] (
LYT-SCANNING-February 2/7/11 10:40 AM Page 4
SCANN ING TH E L I T ERATU R E
EDITED BY YANCHAO ZHANG
Using Classification to Protect the input and output data traces used to con- ing reliability of the pulse oximeters
struct it may be private data of partici- (median 80.85 percent, range 0.46–97.69
Integrity of Spectrum Measure-
pants (e.g., their individual food intake, percent). Sensing failures usually
ments in White Space Networks lifestyle choices, and resulting weight). occurred in short bursts, although longer
O. Fatemieh, A. Farhadi, R. Chandra, and This paper proposes and experimentally periods were also present due to sensor
C. Gunter, in the 18th Annual Network studies a technique that attempts to keep disconnections. The authors show that
& Distributed System Security Sympo- such input and output data traces pri- the sensing reliability could be signifi-
sium (NDSS), San Diego, CA, February vate, while allowing accurate model con- cantly improved through oversampling
2010 struction. This is significantly different and by implementing a disconnection
from perturbation-based techniques in alarm system that incurs minimal inter-
The emerging paradigm for using the that no noise is added. The main contri- vention cost. A retrospective data analy-
wireless spectrum more efficiently is bution of the paper is to show a certain sis indicated that the system provided
based on enabling secondary users to data transformation at the client side sufficient temporal resolution to support
exploit white space frequencies that are that helps keeping the client data private the detection of clinical deterioration in
not occupied by primary users. A key while not introducing any additional three patients who suffered from signifi-
enabling technology for forming net- error to model construction. The authors cant clinical events including transfer to
works over white spaces is distributed particularly focus on linear regression intensive care units.
spectrum measurements to identify and models which are widely used in partici-
assess the quality of unused channels. patory sensing applications. They use the
CodeOn: Cooperative Popular Content
This spectrum availability data is often data set from a map-based participatory
aggregated at a central base station or sensing service to evaluate their scheme. Distribution for Vehicular Networks
database to govern the usage of spec- The service in question is a green naviga- Using Symbol Level Network Cod-
trum. This process is vulnerable to tion service that constructs regression
ing
integrity violations if the devices are models from participant data to predict
malicious and misreport spectrum sens- the fuel consumption of vehicles on road M. Li, Z. Yang, and W. Lou, IEEE Jour-
ing results. This paper presents CUSP, a segments. They evaluate the proposed nal on Selected Areas in Communica-
new technique based on machine learn- mechanism by providing empirical evi- tions (JSAC), special issue on Vehicular
ing that uses a trusted initial set of signal dence that: i) an individual data trace is Communication Networks, vol. 29, no. 1,
propagation data in a region as input to generally hard to reconstruct with any January 2011
build a classifier using support vector reasonable accuracy, and ii) the regres- Driven by both safety concerns and com-
machines. The classifier is subsequently sion model constructed using the trans- mercial interests, one of the key services
used to detect integrity violations. Using formed traces has a much smaller error offered by vehicular networks is popular
classification eliminates the need for than one based on additive data-pertur- content distribution (PCD). The funda-
arbitrary assumptions about signal propa- bation schemes. mental challenges to achieve high speed
gation models and parameters or thresh- content downloading come from the
olds in favor of direct training data. highly dynamic topology of vehicular ad
Reliable Clinical Monitoring Using Wire-
Extensive evaluations using TV transmit- hoc network (VANET) and the lossy
ter data from the FCC, terrain data from less Sensor Networks: Experiences nature of the vehicular wireless commu-
NASA, and house density data from the In A Step-Down Hospital Unit nications. This paper introduces
U.S. Census Bureau for areas in Illinois CodeOn, a novel push-based PCD
O. Chipara1, C. Lu, T. Bailey, and G.
and Pennsylvania show that CUSP is scheme where contents are actively
Roman, in the 8th ACM Conference on
effective against attackers of varying broadcasted to vehicles from road side
Embedded Networked Sensor Systems
sophistication, while accommodating access points and further distributed
(SenSys), Zurich, Switzeland, Novem-
regional terrain and shadowing diversity. among vehicles using a cooperative
ber 2010
VANET. In CodeOn, we employ a
This paper presents the design, deploy- recent technique, symbol level network
Privacy-Preserving Regression
ment, and empirical study of a wireless coding (SLNC), to combat the lossy
Modeling of Participatory Sensing Data clinical monitoring system that collects wireless transmissions. Through exploit-
H. Ahmadi, N. Pham, R. Ganti, T. pulse and oxygen saturation readings ing symbol level diversity, SLNC is
Abdelzaher, S. Nath, and J. Han, in from patients. The primary contribution robust to transmission errors and
the 8th ACM Conference on Embedded of this paper is an in-depth clinical trial encourages more aggressive concurrent
Networked Sensor Systems (SenSys), that assesses the feasibility of wireless transmissions. In order to fully enjoy the
Zurich, Switzeland, November 2010 sensor networks for patient monitoring benefits of SLNC, we propose a suite of
in general hospital units. The authors techniques to maximize the downloading
Many participatory sensing applications present a detailed analysis of the system rate, including a prioritized and localized
use data collected by participants to con- reliability from a long-term hospital relay selection mechanism where the
struct a public model of a system or phe- deployment over seven months involving selection criteria are based on the use-
nomenon. For example, a health 41 patients in a step-down cardiology fulness of vehicle-possessed contents,
application might compute a model relat- unit. The network achieved high reliabil- and a lightweight medium access proto-
ing exercise and diet to amount of weight ity (median 99.68 percent, range col that naturally exploits the abundant
loss. While the ultimately computed 95.21–100 percent). The overall reliabili- concurrent transmission opportunities.
model could be public, the individual ty of the system was dominated by sens- We also propose additional mechanisms
4 IEEE Wireless Communications • February 2011
LYT-SCANNING-February 2/7/11 10:40 AM Page 5
SCANN ING TH E L I T ERATU R E
to reduce the protocol overhead without tive throughput in the original transmis- transient contact with APs, the authors
sacrificing the performance. sion. As such, an extra and “free” coor- proactively push content to the APs the
dination channel can be built. vehicles are likely to visit in the near
Free Side Channel: Bits over Inter- Based on this idea, the authors pro- future. In this way, vehicles can enjoy
pose a DC-MAC to leverage this “free” the full wireless capacity instead of
ference channel for efficient medium access in being bottlenecked by Internet connec-
K. Wu, H. Tan, Y. Liu, J. Zhang, Q. Zhang, a multiple-user wireless network. They tivity, which is either slow or even
and L. Ni, in the 16th Annual International theoretically analyze the capacity of this unavailable. The authors develop a new
Conference on Mobile Computing and channel under different environments algorithm for predicting the APs that
Networking (MobiCom), Chicago, Illinois, with various modulation schemes. will soon be visited by the vehicles. They
September 2010 then develop a replication scheme that
Enabling High-Bandwidth Vehicular leverages the synergy among (i) Internet
Interference is a critical issue in wire- connectivity (which is persistent but has
less communications. In a typical multi- Content Distribution limited coverage and low bandwidth),
ple-user environment, different users U. Shevade, Y. Chen, L. Qiu, Y. Zhang, (ii) local wireless connectivity (which
may severely interfere with each other. V. Chandar, M. Han, H. Song, and Y. has high bandwidth but transient dura-
Coordination among users therefore is Seung, in the 6th International Conference tion), (iii) vehicular relay connectivity
an indispensable part of interference on emerging Networking EXperiments and (which has high bandwidth but high
management in wireless networks. It is Technologies (CoNEXT), Philadelphia, PA, delay), and (iv) mesh connectivity
known that coordination among multi- November 2010 among APs (which has high bandwidth
ple nodes is a costly operation, taking a but low coverage). The authors demon-
significant amount of valuable commu- This paper presents VCD, a novel sys- strate the effectiveness of the VCD sys-
nication resources. In this paper, the tem for enabling high-bandwidth con- tem using trace-driven simulation and
authors have an interesting observation tent distribution in vehicular networks. Emulab emulation based on real taxi
that by generating intended patterns, In VCD, a vehicle opportunistically traces. They further deploy VCD in two
some simultaneous transmissions (i.e., communicates with nearby access points vehicular networks, one using 802.11b
“interference”) can be successfully (APs) to download the content of inter- and the other using 802.11n, to demon-
decoded without degrading the effec- est. To fully take advantage of such strate its effectiveness.
Open Cal l f rom the IEEE Communications Society
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IEEE Wireless Communications • February 2011 5
www.comsoc.org/editor
LIN LAYOUT 2/7/11 12:04 PM Page 6
ACCEPTED FR O M OPEN CALL
IMS EMERGENCY SERVICES:
A PRELIMINARY STUDY
YI-BING LIN, NATIONAL CHIAO TUNG UNIVERSITY
MENG-HSUN TSAI, NATIONAL CHENG KUNG UNIVERSITY
YUAN-KUANG TU, CHUNGHWA TELECOM
ABSTRACT trols the connection for media channels in a
media gateway (MGW, 4, Fig. 1). The MGW
Emergency call and walkie-talkie are two ser- connects toward the legacy fixed and mobile net-
Fixed vices utilized in emergency situations. During works to provide user data transport. The home
network Typhoon Morakot in 2009, we experienced the subscriber server (HSS, 2, Fig. 1) is the master
deficiency of emergency call service that cannot database containing all user-related mobile sub-
continuously track callers in real time and scription and location information. In 2004
(3) MGC walkie-talkie communications where a speaker Chunghwa Telecom deployed the first commer-
may not be granted the permission to talk. These cial IMS in Taiwan to provision commercial
issues can be resolved by the emergency call and telecommunications services such as voice, video,
GW push-to-talk over cellular services in IP Multime- and Internet-based multimedia services. The ini-
dia Subsystem. This article conducts a prelimi- tial capacity was 125,000 subscribers. Current
nary study on how these two services can be IMS capacity can accommodate about 500,000
effectively exercised in IMS. subscribers in daily commercial operation.
During Typhoon Morakot in August 2009,
INTRODUCTION Taiwan experienced serious damage from flood-
ing and mudslides (Fig. 2, left), and the rescue
Deficiencies in IP Multimedia Subsystem (IMS) supports IP- missions solely relied on GSM and satellite com-
based multimedia services. IMS was originally munications that offer basic services (Fig. 2, right)
communications designed by the Third Generation Partnership such as emergency call without location tracking.
Project (3GPP) to deliver Internet services over From Typhoon Morakot, we learned that it is
during emergencies general packet radio service (GPRS) in 3G net- desirable to accommodate emergency services in
can be resolved by works such as Universal Mobile Telecommunica- IMS with a 3G network including emergency call
tions System (UMTS). IMS was later updated to and push-to-talk over cellular (PoC).
the emergency call support other access networks including wireless A GSM user in Taiwan can make an emer-
LAN, CDMA2000, and fixed line. For the pur- gency call by dialing 110, 112, or 119. However,
and the Push-to-talk pose of this article, Fig. 1 illustrates a simplified the existing GSM emergency call service only
IMS network architecture (the reader is referred identifies the location of the caller at the time of
over Cellular services to [1] for detailed descriptions of IMS). call setup and does not track the user’s location
The IMS (Fig. 1b) connects to both mobile during the call. In Typhoon Morakot many peo-
in IP Multimedia and fixed telecommunications networks (Fig. 1a) ple waiting for rescue could not be accurately
Subsystem network. for fixed mobile convergence (FMC). IMS is not located through their phone calls, which
intended to standardize applications or services. increased the difficulty of the rescue missions.
The authors conduct Instead, it provides a standard approach for PoC is a walkie-talkie-like service defined in
voice/multimedia application access from user Open Mobile Alliance (OMA) specifications [3,
a preliminary study equipment (UE; 1, Fig. 1) in wireline and wire- 4]. In 2004 Chunghwa Telecom first launched
less networks. This goal is achieved by having this service in Asia using 2.5G technology. Our
on how these two horizontal control that isolates the access net- experience with 2.5G PoC included long PoC
works from the service and application networks call setup time (13 s) and handoff time (2.5–3 s
services can be (Fig. 1c). for reconnecting a PoC client when it moved
effectively exercised In the IMS, the transport of user data is sepa- from one base station to another). These prob-
rated from that for control signals, where IETF lems have been resolved by IMS-based PoC
in IMS. protocols such as Session Initiation Protocol established on 3G networks. For example, the
(SIP) [2] are used to ease the integration with 3G PoC call setup time is less than 6 s. Although
the Internet. For example, the call session con- it is not clear if PoC will be a successful residen-
trol function (CSCF, 5, Fig. 1) is a SIP server, tial service, it has proven useful for business cor-
which is responsible for call control. The media porations and government organizations such as
gateway control function (MGCF, 3, Fig. 1) con- the National Security Bureau in Taiwan.
6 1536-1284/11/$25.00 © 2011 IEEE IEEE Wireless Communications • February 2011
LIN LAYOUT 2/7/11 12:04 PM Page 7
In the past three years, we have studied emer-
gency call [5] and PoC [6]. During Typhoon
Morakot in 2009, emergency calls and walkie-
talkies were important means of communications (a) (1) UE
in rescue missions. Clearly, it is desirable to sup-
port emergency services in IMS for disastrous Fixed
Mobile network
events. Therefore, this article conducts a prelimi- network
nary investigation on IMS emergency call and PoC.
(b) (2) HSS
LOCATION TRACKING FOR EMERGENCY CALL (3) MGCF
An important feature of emergency call is that
the system can track the location of a calling UE (4) MGW
(1, Fig. 1) during the conversation. To support
IMS emergency call, three network nodes are
deployed. When the UE originates an emergen- (5) CSCF
cy call, the call is established by a special CSCF
called an emergency-CSCF (E-CSCF, 5, Fig. 1),
which dispatches the call to the nearest public (c)
safety answering point (PSAP) based on the
location information of the UE. The PSAP (7, (8) Fire department
Fig. 1) is an IMS application server that process-
(9) PoC server
es emergency calls according to the types of Police department
emergency events. For example, in a fire event (7) PSAP (6) GMLC
the PSAP connects the UE (the caller) to the
fire department (8, Fig. 1). The PSAP interacts Figure 1. A simplified IMS network architecture (dashed lines: control signal-
with the E-CSCF by using SIP, and the voice ing; solid lines: user data/control signaling): a) fixed and mobile telecom net-
conversation path is set up through the MGW works; b) IP Multimedia Subsystem; c) service and application networks.
(4, Fig. 1) to the UE by using Real-Time Trans-
port Protocol (RTP) [7]. The gateway mobile
location center (GMLC, 6, Fig. 1) supports a
location service (LCS) [8]. Through Signaling resolve these issues, the active location reporting
System Number 7 (SS7) Mobile Application Part scheme was proposed in [5]. This scheme reports
(MAP) [1], the GMLC interacts with the HSS the UE’s location upon change of its SA. This
and mobile network to obtain the accurate loca- section describes location polling and active
tion of UE. The GMLC provides the location location reporting, and comments on their per-
information to the PSAP and E-CSCF. formance.
The LCS merits further discussion. This ser-
vice utilizes one or more positioning methods EMERGENCY CALL SETUP
between the mobile network (Fig. 1a) and UE to Figure 3 illustrates IMS emergency call setup
determine the location of the UE [9]. The cell- message flow with the following steps [10]:
ID-based positioning method determines the Step A.1 The UE establishes IP connectivity to
UE’s position based on the coverage of service the IMS through the mobile network [1].
areas (SAs). An SA includes one or more cells Step A.2 The UE sends a SIP INVITE message
(base stations). The observed time difference of to the E-CSCF. This message includes the
arrival (OTDOA) and uplink time difference of supported positioning methods of the UE
arrival (U-TDOA) positioning methods utilize (cell-ID-based in our example).
trilateration to determine the UE’s position Step A.3 The E-CSCF uses the received infor-
based on the time differences between downlink mation to select a GMLC and sends the Emer-
and uplink signal arrivals, respectively. The gency Location Request message to the
Assisted Global Positioning System (A-GPS) GMLC.
method speeds up GPS positioning by down- Steps A.4 and A.5 The GMLC exchanges the SS7
loading GPS information through the mobile MAP_SEND_ROUTING_INFO_FOR_LCS and
network. In Chunghwa Telecom A-GPS is uti- acknowledgment message pair with the HSS
lized for location-based services. to identify the mobile network node responsi-
Without loss of generality, we consider the ble for connection to the UE. In UMTS this
cell-ID-based method. After emergency call node is a serving GPRS support node (SGSN).
setup, the PSAP may need to monitor the UE’s Step A.6 The GMLC sends the SS7 MAP_PRO-
location in real time. In the 3GPP 23.167 specifi- VIDE_SUBSCRIBER_LOCATION message.
cation [10], the UE’s location is tracked through Step A.7 The mobile network and UE exercise
polling, where the PSAP periodically queries the the cell-ID-based positioning procedure to
UE’s location. For description purposes, we obtain the location estimate information of
refer to the 3GPP 23.167 approach as the loca- the UE (i.e., the SA identity of the UE).
tion polling scheme. In this scheme, if the UE Step A.8 The mobile network returns the SA iden-
does not change its location between two tity to the GMLC through SS7 MAP_PRO-
queries, the second query is wasted (this is called VIDE_SUBSCRIBER_LOCATION_ack message.
redundant polling). On the other hand, if the UE Step A.9 The GMLC selects a suitable PSAP
has visited several locations between two loca- according to the SA of the UE and replies the
tion queries, the PSAP may lose track of the UE Emergency Location Response message (with
in this time period (this is called mistracking). To the selected PSAP address) to the E-CSCF.
IEEE Wireless Communications • February 2011 7
LIN LAYOUT 2/7/11 12:04 PM Page 8
Redundant polling
creates extra
network traffic
without providing
useful location infor-
mation. Furthermore,
mis-tracking may
result in wrong
positioning in case
of emergency
Figure 2. Telecommunications services in Typhoon Morakot: (left) deploying temporary cables in flooded
situations. These areas; (right) GSM/satellite communications through a vehicular base station.
issues are resolved
by Active Location Steps A.10–A.12 The E-CSCF forwards the SIP need to query the HSS to identify the mobile
INVITE to the PSAP to set up the call. The network node responsible for connection to the
Reporting. PSAP and the UE exchange the 200 OK and UE (i.e., steps B.2 and B.3 are eliminated). The
the SIP ACK messages through the E-CSCF. active location reporting scheme is illustrated in
After the PSAP has received the SIP ACK Fig. 5 with the following steps:
message, the emergency call is established. Step C.1 When the UE moves to a new SA, the
Step A.13 The GMLC sends the location infor- mobile network detects this movement at the
mation obtained at step A.8 to the PSAP after cell tracking mode and then triggers the posi-
the call has been established, where the PSAP tioning procedure.
address is resolved by the GMLC at step A.9. Step C.2 After the positioning procedure is exe-
cuted, the UE’s SA identity is obtained.
LOCATION POLLING Step C.3 The mobile network sends the SS7
UE may move during an emergency call, and the MAP_SUBSCRIBER_LOCATION_REPORT
PSAP needs to monitor the UE’s location in real message with the SA identity to the GMLC.
time. In location polling, the PSAP periodically Step C.4 From the UE-PSAP mapping table, the
queries the UE’s location. In each query, the fol- GMLC retrieves the PSAP address of the UE
lowing steps are executed (Fig. 4) [8]: stored at step A.9 and then sends the updated
Step B.1 The PSAP sends the Location Informa- location information to the PSAP.
tion Request message to the GMLC. When the emergency call is terminated, the
Steps B.2–B.6 These steps retrieve the UE’s SA following steps are executed:
identity, which are similar to steps A.4–A.8 in Step C.5 When the IMS call is released, the UE
Fig. 3. moves from the cell-connected mode to the
Step B.7 The GMLC returns the SA identity of idle mode, and the mobile network no longer
the UE to the PSAP. tracks the movement of the UE.
Steps B.8–B.10 When the emergency call is Step C.6 The E-CSCF sends the Emergency
terminated, the E-CSCF exchanges the Emer- Location Release message to the GMLC to
gency Location Release and Response mes- terminate location tracking.
sage pair with the GMLC to terminate Step C.7 The GMLC returns the Emergency
location tracking. Location Response message to the E-CSCF
and then deletes the (UE, PSAP) mapping
ACTIVE LOCATION REPORTING from the UE-PSAP table.
To resolve redundant polling and mistracking Note that steps C.1 and C.2 in active location
issues in location polling, the active location reporting automatically detects the movement of
reporting scheme was proposed in [5], which UE, which is different from steps B.1–B.5 in
reports the UE’s location upon change of its SA. location polling.
This scheme introduces a new locationEstimate-
Type initiateActiveReport (to trigger active loca- PRELIMINARY PERFORMANCE EVALUATION
tion reporting) in the MAP_PROVIDE_ It is clear that redundant polling creates extra
SUBSCRIBER_LOCATION message (at step A.6). network traffic without providing useful location
Since the IP connectivity exists during the IMS information. Furthermore, mistracking may
emergency call, the UE is in the cell-connected result in wrong positioning in case of emergency
state and is tracked by the mobile network at the situations. These issues are resolved by active
cell level [1]. Therefore, the mobile network can location reporting. However, it is desirable to
detect when the UE moves from one base station evaluate the performance of location polling to
to another, and report the new SA identity to the justify the modifications to the existing location
GMLC. In this approach the GMLC maintains a tracking procedure in active location reporting.
UE-PSAP mapping table to store the (UE, Suppose that the SA residence time has a
PSAP) pair at step A.9. The GMLC does not Gamma distribution with mean 1/μ and variance
8 IEEE Wireless Communications • February 2011
LIN LAYOUT 2/7/11 12:04 PM Page 9
In the PoC service
several predefined
Mobile
network group members
participate in one
PSAP GMLC E-CSCF HSS UE
PoC session. Since
A.1. IMS connection
PoC utilizes
establishment
A.2. SIP INVITE half-duplex
A.3. Emergency Location Request communications,
only one PoC
Report initial location
member speaks
A.4. MAP_SEND_ROUTING_INFO_FOR_LCS
at a time,
A.5. MAP_SEND_ROUTING_INFO_FOR_LCS_ack
and others listen.
A.6. MAP_PROVIDE-SUBSCRIBER_LOCATION
A.7. Positioning
A.8. MAP_PROVIDE-SUBSCRIBER_LOCATION_ack
A.9. Emergency location response
Establish emergency call
A.10. SIP INVITE
A.11. 200 OK
A.11. 200 OK
A.12. SIP ACK
A.12. SIP ACK
A.13. Location information
Figure 3. IMS emergency call setup.
Vm (other distributions have shown similar es. Similarly, when λ is large (e.g., λ ≥ 5 μ),
results [5]). The inter-query interval is a fixed effect 1 is significant, and β is a decreasing func-
period 1/λ in location polling. Several output tion of Vm. The impact of Vm on β is more sub-
measures are studied. Let α be the mistracking tle when λ = μ. In this case β increases and then
probability. An SA crossing is mistracked if decreases as Vm increases, which implies that
there is no query between this and the next SA when Vm is small, effect 2 is more significant. On
crossings (i.e., the system does not know that the the other hand, when Vm is large, effect 1 domi-
user has visited this SA). Let β be the probability nates. An important observation is that when
that redundant queries exist between two SA 0.1/μ2 < Vm < 10/μ2, both α and β values are
crossings. The larger the α or β values, the worse non-negligibly large, and poor performance of
the performance of location polling. location polling cannot be ignored.
Figure 6a shows intuitive results that as the Besides mistracking and redundant polling
polling frequency λ increases, α decreases and β probabilities, we would also like to investigate
increases. We describe two effects of Vm: the following output measures:
• Effect 1: When the SA residence time intervals • Ti: The expected period in which the PSAP
become more irregular (i.e., Vm increases), cannot correctly track the UE’s location (i.e.,
more SA residence time intervals without any Ti is the period between an SA crossing and
query are observed. when the next query arrives). In this period
• Effect 2: When Vm increases, if a query arrives the system does not know the user’s correct
in an SA residence time interval, more than location.
one query will tend to arrive in this interval. • NR: The expected number of redundant
Effect 1 implies that as Vm increases, more queries between two SA crossings (i.e., NR is
SA crossings are mistracked, and we have a non- the number of queries issued within two con-
trivial observation that α increases as Vm increas- secutive SA crossings).
IEEE Wireless Communications • February 2011 9