Toward seamless networking in indoor environments in millimeter wave bands

Indoor Environments in Millimeter

Wave Bands

Indoor Environments in Millimeter

Wave Bands

PROEFSCHRIFT

ter verkrijging van de graad van doctor

aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus Prof. dr. ir. J.T. Fokkema,

voorzitter van het College voor Promoties,

in het openbaar te verdedigen

op dinsdag 2 september 2008 om 15:00 uur

door

Bao Linh DANG

elektrotechnisch ingenieur

geboren te Hanoi, Vietnam.

Prof. dr. ir. I.G.M.M. Niemegeers Samenstelling promotiecommissie:

Rector Magnificus voorzitter

Prof. dr. ir. I.G.M.M. Niemegeers Technische Universiteit Delft, promotor Prof. ir. A.M.J. Koonen Technische Universiteit Eindhoven Prof. dr. ir. S. Heemstra de Groot Technische Universiteit Delft Prof. dr. ir. P.G.M. Baltus Technische Universiteit Eindhoven Prof. dr. J.R. Long Technische Universiteit Delft Prof. dr. H. S. Jamadagni Indian Institute of Science

Prof. dr. ir. A.J. van der Veen Technische Universiteit Delft, reservelid

This work is supported by the IOP Generieke Communicatie program of SenterNovem, an agency of the Ministry of Economic Affairs of the Netherlands.

isbn 978-90-79746-01-9

Copyright © 2008 by Bao Linh Dang

All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without the prior permission of the author.

T

he future home and office networking environments are predicted to be dominated by a variety of multimedia services like wireless HDTV, wireless home entertain-ment, virtual wireless office etc. To support these applications, the required data rate offered to the user has to be in the order of hundreds of mega-bits-per-second, justify-ing the need for a short-range gigabit wireless system. Since as much as 5 GHz of spectrum in the 60 GHz band has been allocated worldwide, much attention has been recently paid to the band as a potential candidate for the radio layer of future indoor networks.

This thesis investigates the possibility of applying the 60 GHz band to the indoor networking environment at the system and link control levels. The work presented in this thesis aims at developing novel system concepts for seamless and cost-effective broadband local area networks operating in the 60 GHz band in particular and millimeter-wave bands in general. Different from many efforts today targeting point-to-point cable replacement solutions, this thesis attempts to apply the 60 GHz band to a broader context, i.e., the local area network environment in which multiple users and multiple applications share the network resource simultaneously.

The utilization of this millimeter wave band, however, leads to smaller radio cell cover-age due to propagation losses and line-of-sight requirements. Indoor networks operating in this band will comprise of a large number of pico-cells corresponding to that many of antenna stations. As a result, the cost of many antenna stations has become a major contributor for the total cost of the network’s infrastructure. To reduce the system’s cost, it is therefore crucial that the complexity of an antenna station is simplified. Radio over fiber (RoF) techniques can be employed to achieve this goal. Particularly, this thesis is based on a RoF technique called Optical Frequency Multiplication (OFM) that is able to generate pure millimeter wave carriers remotely. Instead of placing all the signal processing func-tions in antenna stafunc-tions, it is now possible to concentrate those complex funcfunc-tions in a single processing block, i.e., the central station (CS).

Since the propagation of 60 GHz signals is highly obstructed by objects, it is generally

difficult to obtain good signal coverage in the indoor environment. In this thesis, we pro-pose the novel Extended Cell concept that is able to overcome the problem of insufficient signal coverage. In this solution, a number of adjacent radio cells are grouped into an extended cell. Furthermore, all the antenna stations in an extended cell are set to oper-ate in the same radio channel. By controlling the cells to be included into an extended cell, overlap areas between extended cells can be created in transitional areas in the floor to ensure seamless handovers of ongoing connections. To optimize the performance of the system with regard to variable realtime traffic patterns, we propose an algorithm to dynamically form extended cells based on the actual traffic under each cell.

The performance of two Medium Access Control protocols, i.e., IEEE 802.11 represent-ing the distributed protocol family and IEEE 802.16 representrepresent-ing the centralized protocol one, when applied to the proposed RoF and extended cell based architecture have also been discussed in this thesis. A major effect when an optical distribution system is inserted in a traditional wireless network is the additional propagation delay introduced by the fiber links. This additional propagation delay can exceed the timing boundary of the MAC protocols and eventually stop them from working. Another problem also arises when the utilization of the 60 GHz band is combined with the extended cell concept. Specifically, mobile stations in a room will be completely hidden to other stations in other rooms. We show that the throughput of both protocols degrades when the length of optical distribu-tion network increases, but this degradadistribu-tion is not significant. With regards to the hidden terminal problem, the performance of distributed and carried sense based protocols is severely affected.

Finally, this thesis concerns the issues of maintaining quality of services (QoS) when the MAC protocols are applied to the proposed network architecture. We have shown that the proposed architecture does not affect the service differentiation mechanisms of the MAC protocols. Moreover, in this architecture, since the central station has full control of the network, QoS and mobility control algorithms can also be greatly simplified to improve the performance of the network.

Summary

i

1 Introduction

1

1.1 Future Broadband Indoor Networks . . . 2

1.2 Problem Statement . . . 6

1.3 Context of this Thesis . . . 8

1.4 Thesis Outline . . . 9

2 In-building Networking and the 60 GHz Band

11 2.1 The 60 GHz Band and The In-building Environment . . . 12

2.1.1 The characteristics of the 60 GHz band . . . 12

2.1.2 In-building networks operating in the 60 GHz band - The corner effect . . . 16

2.1.3 The 60 GHz channel . . . 18

2.2 Radio over Fiber - the Fusion of the Two Worlds . . . 20

2.2.1 The Optical Frequency Multiplication (OFM) technique . . 22

2.3 Orthogonal Frequency Division Multiplexing . . . 25

2.3.1 OFDM principles . . . 26

2.3.2 OFDM design . . . 28

2.4 MAC Protocols for Multimedia Wireless Networks . . . 28

2.4.1 Distributed MAC protocols . . . 30

2.4.2 Centralized MAC protocols . . . 32

2.5 Mobility Support in Communication Networks . . . 33

2.5.1 Handover Trigger Decision Schemes . . . 34 iii

2.5.2 Seamless handover and the required overlap area . . . 37

2.6 Related Work . . . 38

2.6.1 Characteristics of the 60 GHz band . . . 38

2.6.2 Architectures for networks operating in mm-wave bands . 39 2.6.3 MAC protocols for RoF networks operating in the 60 GHz band . . . 42

2.7 Chapter Summary . . . 44

I

Network Infrastructure Design

47

3 A Seamless Infrastructure for 60 GHz In-building Networks

49 3.1 Problem Description . . . 50

3.2 Broadband In-building Networks at 60 GHz - The Architecture . 51 3.2.1 The concept of Extended Cells (EC) . . . 53

3.2.2 Physical design considerations . . . 55

3.3 Simulation Study of the Proposed Architecture . . . 56

3.3.1 Simulation Setup . . . 56

3.3.2 Results and Discussion . . . 59

3.4 Chapter Summary . . . 63

4 Extended Cell Planning and Dynamic Extended Cell Formation

67 4.1 Problem Description . . . 68

4.2 Mobility Modeling and Traffic Analysis . . . 71

4.2.1 Crossing Rate Analysis . . . 71

4.2.2 Approximate Traffic Analysis . . . 75

4.2.3 Numerical Results . . . 77

4.3 Dynamic Extended Cell Formation . . . 79

4.3.1 Dynamic Extended Cell Formation . . . 79

4.3.2 Simulation study . . . 84

4.4 Chapter Summary . . . 89

II

Multiple MAC Protocols Support

91

5 IEEE 802.11 Networks Employing Radio over Fiber

93 5.1 Problem Description . . . 94

5.2 IEEE 802.11 Fundamentals . . . 95

5.2.1 Distributed Coordination Function . . . 96

5.2.2 QoS enhancement for IEEE 802.11 . . . 98

5.3 Feasibility of IEEE 802.11 Employing Radio over Fiber . . . 101

5.4 Theoretical Performance Analysis . . . 103

5.4.1 When the Extended Cell Concept is not Applied . . . 104

5.4.2 When the Extended Cell Concept is Applied . . . 105

5.5 Theoretical and Simulation Results . . . 106

5.5.1 Performance of an IEEE 802.11 network employing RoF . 106 5.5.2 Simulation Results . . . 109

5.6 Performance of IEEE 802.11e EDCA employing RoF . . . 112

5.7 Chapter Summary . . . 115

6 IEEE 802.16 Network employing Radio over Fiber

117 6.1 IEEE 802.16 Fundamentals . . . 118

6.2 Feasibility of IEEE 802.16 employing Radio over Fiber . . . 120

6.3 QoS for IEEE 802.16 Networks Employing RoF and the EC concept 125 6.3.1 The QoS mechanism defined by the standard . . . 126

6.3.2 The Uplink Package Scheduling (UPS) scheme . . . 128

6.4 Mobility . . . 131

6.4.1 Handover Pre-registration - Network topology acquisition 133 6.4.2 Actual Handover . . . 135

6.4.3 Simulation study of the proposed handover mechanism . 137 6.5 Chapter Summary . . . 140

7 Summary and Outlook

143 7.1 Contributions of the Thesis . . . 144

7.1.1 A seamless infrastructure for broadband indoor networks at 60 GHz . . . 144

7.1.2 Multiple standards support for RoF networks . . . 146

7.2 Future research directions . . . 148

7.2.1 A seamless infrastructure for broadband indoor networks at 60 GHz - the EC concept . . . 148

7.2.2 IEEE 802.11 networks employed RoF . . . 149

7.2.3 IEEE 802.16 networks employed RoF . . . 149

A Acronyms

151

Bibliography

153

Samenvatting (Summary in Dutch)

163

Acknowledgements

165

Chapter

1

Introduction

W

ith the rapid evolution of electronics, information and telecommunication technologies, first-mile networks are experiencing a shift from wired to wireless network infrastructures. Nowadays, not only do wireless technologies provide flexibility and mobility, but they also offer higher data rates by raising carrier frequencies and by introducing advanced signal processing techniques. In particular, we are witnessing the penetration of broadband wireless communica-tions into every household. Two aspects are driving this trend, i.e., the booming of wireless technologies that are able to substitute the wired counterparts and the proliferation of entertaining multimedia applications, such as wireless High Def-inition TV (HDTV), Internet Protocol TV (IPTV), wireless home entertainment, online gaming, virtual wireless office etc.

On the way to realize future multimedia wireless indoor networks, one of the key challenges has always been the limited bandwidth that constrains the maximum datarate that the networks can deliver. In fact the bandwidth require-ment has never been more critical since a number of new multimedia services requires datarates of up to several hundreds of Mbps [96][98][44]. To deliver this massive required data rate, either the spectrum efficiency of the network must be increased or more spectrum space is required. In the quest for more available bandwidth, much attention has been paid to the 60 GHz band where as much as 5 GHz of spectrum has been allocated worldwide. This unprecedented amount of available spectrum holds the potential for much higher data rate ever compared to other bandwidth-limited channels that are currently used.

In this thesis, the challenging issues of applying the 60 GHz band to the 1

Application Capacity per User (Mbps) Uncompressed Ultra HDTV (7680x4320) 47776

Current Uncompressed HDTV (1920x1080) 2986 Current Compressed HDTV (1920x1080 - MPEG2) 20

Wireless LAN Bridge 100-1000

Virtual Reality 450

Wireless IEEE 1394 300

High quality video conference 10-100

Wireless surveillance camera 4-10

Wireless Video Phone 1.5

Wireless remote control 0.01

Wireless embedded systems 0.01

Table 1.1:Wireless bandwidth-hungry applications [95]

indoor networking environment at the link and system levels will be the center of investigation. At the system level, this thesis investigates the cell planning problem in order to create a seamless communication environment at 60 GHz band and to optimize the utilization of the radio resource. At the link level, the performance of state-of-the-art Medium Access Control (MAC) protocols, i.e., IEEE 802.11 and IEEE 802.16, when applied to such environment is analyzed.

This chapter introduces the readers to the research topics covered by this thesis. First, the trends in broadband indoor networks development are presented in Section 1.1. Next, Section 1.2 discusses the problem descriptions that motivate the work discussed in this thesis.

1.1 F

UTURE

B

ROADBAND

I

NDOOR

N

ETWORKS

We are living in a world of information and collaboration. The increase in de-mand for information sharing has led to the development of new networking technologies. In the last few years, we have witnessed the booming of a wide range of networking technologies serving a wide range of application domains, from GSM networks targeting low-rate voice transmission over large distances to the Ultra Wide Band (UWB) technology designed to haul gigabit of information over a couple of meters. However, there are still many application domains that are calling for better networking technologies.

In the indoor environment, there exists a multitude of multimedia devices that need to be connected. The indoor networks today are typically designed to support the sharing of data, Internet access and peripherals. However, the future indoor networks are predicted to be dominated by advanced multimedia applications such as HDTV, IPTV, multiplayer gaming, online virtual worlds, realtime video conferencing etc. The traffic pattern of indoor networks is also predicted to be more symmetrical since resource sharing is one of the most important applications. In [36], the author has pointed out the six key application categories for indoor networks.

• Peripheral and Devices Sharing: Devices such as scanners and printers can be accessed from any location in the house.

• File Sharing: Calendars, address books, documents etc. can be accessed from any devices in the home.

• Broadband Internet Sharing: The ability to share broadband Internet connec-tion in the house is of special importance. This is a key driver for a total solution for connected homes.

• Communication and Collaboration: Instant Messaging, VoIP, Online Games etc. are predicted to be ubiquitous. These applications will be present in many home appliances beyond the computers.

• Gaming, Audio and Video Distribution: Movies, TV Programs, Audio Files etc. can be distributed to any devices in the home.

• Sensing, Remote Control and Monitoring: There has always been an increasing demand for these applications. Traditional applications such as automation of lighting, security, surveillance etc. and new applications like demand-side energy management, health care monitoring will be an important part of a home-network.

The first four categories are typical in the traditional Local Area Network environment. However, the fourth, the fifth and the sixth categories are rapidly emerging as the indispensable part of home networks. In Table 1.1, a number of popular applications and their corresponding estimated datarates are recapit-ulated. The table shows that the required datarates for some applications can be in the range of several hundred of Mbps. In a multi-user and multi-tasking

Wireless Carrier Maximum

Technology Frequency (GHz) Datarate (Mbps)

IEEE 802.11 2.4 2 ETSI HomeRF 2.4 1.6 IEEE 802.11b 2.4 11 IEEE 802.11a 5 54 ETSI HiperLAN/2 5 54 IEEE 802.11g 2.4 54 IEEE 802.15.1 (Bluetooth) 2.4 3 LMDS 26 IEEE 802.16 2-60 54 IEEE 802.15.3 (UWB) 60 2000 WirelessHD 60 4000

Table 1.2:The evolution of wireless standards

network, a large variety of applications co-exist and thus impose huge datarate requirements on the network’s infrastructure.

Moreover, as the users’ demand for better services increases rapidly, next to the capacity required for actual applications, there is much additional capacity needed for Quality of Service (QoS) provisioning, dynamic resource allocations, security protocols for data integrity etc [95]. Consequently, a future-proof system is expected to be able to offer datarate in the range of Gbps to support multi-user scenarios in which multiple multi-users watch, play and share concurrently in an indoor setting [44; 45].

To cope with these datarates and QoS demands, on one hand, we have wit-nessed the massive penetration of last-mile access technologies such as x-DSL and cable, in the business and household environments. Fiber-to-the-home (FTTH) deployments are nowadays also becoming very popular as a future-proof infras-tructure to provide high speed and triple play support, comprising both switched Ethernet-based and passive optical network (PON)-based architectures [39][26]. For example, ITU-T recommendation G.983 series (G-PON) provides efficient triple play service support with datarate up to 2.488 Gbps downstream and 1.244 Gbps upstream. Furthermore, a number of broadband wireless access systems, such as IEEE 802.16, are also emerging and are capable of delivering datarates up to 1 Gbps for fixed access applications. Many of the advanceed signal processing

techniques, e.g., MIMO, space-time coding etc., are being included in upcoming standards, like IEEE 802.16m and IEEE 802.11n, to improve the system’s capacity. On the other hand, a large amount of effort from the telecommunication industry has also been spent on making broadband wireless Local Area Net-work (LAN) systems a reality. A number of WLAN technologies such as IEEE 802.11a/b/g/n has been standardized and deployed successfully. Nowadays, houses, buildings, airports, shopping malls etc. can be easily covered by a num-ber of cheap and easy-to-deploy Access Points (AP). The WLAN market has been unquestionably conquered by the standard series IEEE 802.11 [56]. Operating in the 2.4 Ghz and 5 Ghz bands, the IEEE 802.11a/g versions of the standard can provide datarates up to 54 Mbps. To cope with the increasing demands in terms of datarates and QoS support from emerging multimedia applications, the IEEE 802.11n amendment is being standardized to improve the system performance by employing Multiple Input Multiple Output (MIMO) technique. The expected actual datarate offered by the standard can be as high as 200 Mbps [105]. How-ever, as pointed out earlier, this offered datarate is not high enough for future multimedia applications.

Wireless Personal Area Networking (PAN) is also an indispensable part of the ubiquitous communication landscape. WPAN and WLAN technologies are expected to complement each other to provide connectivity to end-users in the indoor environment. Among the state-of-the-art standards, the IEEE 802.15.3 family concentrates on multimedia applications and offers datarates up to 2 Gbps in the 57-64 GHz band. Table 1.2 presents a summary of some current and emerging wireless standards and their operating frequencies.

In addition to the bandwidth requirement, mobility is also another major factor in shaping the future home networks. According to a recent report from IDC [3], laptops will overtake desktop PCs as the dominant form of computers in 2011. Also reported by the Pew Internet and American Life Project [6], in the US, 80% of laptops had wireless capabilities and 88% of laptops users have a wireless network at home. The above figures partly reflect the need of an infrastructure that is able to offer seamless connectivity to the end-users.

From the discussions above, it is clear that the demands for data rates and QoS support will steadily increase. Moreover, since it is difficult to increase the spectrum efficiency to reach gigabit-per-second levels while maintaining the simplicity of the system to be cost-effective [17], some short-range and high speed wireless systems, e.g., IEEE 802.15.3, WirelessHD etc. have been designed

to operate in the licensed-free, millimeter-wave bands, i.e., 17 GHz and 60 GHz. It is also predicted that in the near future, a wide diversity of heterogeneous networks will be coexisting. As a result, there is a growing interest in developing infrastructures that are able to support multiple standards and provide seamless mobility.

1.2 P

ROBLEM

S

TATEMENT

Generally, there are two main approaches to achieve the required huge datarates from future multimedia applications; either to increase the available spectrum or the spectrum efficiency. While many techniques, e.g., diversity, coding schemes etc. are being extensively studied to improve the spectrum efficiency, researchers are also investigating the possibility of exploiting vacant frequency ranges, e.g. 17GHz or 60 GHz. The 60 GHz band is of special interest since there is as much as 5GHz of spectrum available in this band. Besides the vast available spectrum, this band also presents many other attractive properties, e.g., large oxygen absorp-tion enabling transmission at higher power levels, short propagaabsorp-tion distances enabling higher frequency reuse and millimeter wavelength enabling small size antennas [48].

However, the migration to such an aspiring radio band imposes many chal-lenges for the design of a reliable indoor network infrastructure. Due to the huge propagation loss at 60 GHz, this band has been proposed for short-range broad-band communication in the indoor environment. In this environment, a radio cell is typically confined to a room, where walls and floors can be automatically defined as reliable boundaries [48; 95]. Thus, at least one AP is required in a confined indoor area, such as a room, a hall, a corridor etc. In addition, the com-plexity of the signal processing functions, e.g., macro-diversity, Multiple-input Multiple output (MIMO) etc., required for each antenna station is also growing. Consequently, a large number of APs is required to provide a certain geograph-ical area with wireless access coverage [68], which augments enormously the infrastructure cost and the network management complexity. Therefore, a flexible

and cost-effective network architecture is desirable to make the radio access in this band economically viable.

Since the propagation of millimeter radio wave is strongly hindered by atten-uation, in an indoor environment, a radio cell typically spans only a room. As a result, an overlap area between two adjacent cells exists only around open areas

such as doors or windows. Moreover, these overlap areas are often narrow and directional. In a multi-channel communication system, where handovers (HO) are required when a mobile station (MS) roams from one cell to another, these overlap areas might be too small to allow the mobile station sufficient time to trig-ger and complete a handover. It is therefore crucial to design new mobility strategies

that enlarge the overlap areas in order to enable a seamless communication environment.

To address the problems of signal propagation at millimeter wave bands and to simplify the complexity of APs, Radio over Fiber (RoF) has for long been iden-tified as one of the most promising solutions [98]. By integrating the wireless and wired networks, the capacity and transparency of optical networks can be combined with the flexibility and mobility of wireless access networks to form a seamless and scalable communication solution. Moreover, this combination en-ables the concepts of in-house millimeter-wave networks where each room is a separate picocell. By using such architecture, a large part of the complexity of a traditional AP can be transferred to a central station (CS). Signals are gener-ated and processed at the CS and transferred to simplified antenna station (AS) transparently. Additionally, since all the processing functions of the network are now concentrated in the CS, system maintenance and upgrading can now be performed at a single point, i.e., the CS, without any modifications to the large number of antenna stations.

A major effect when an optical distribution system is inserted in a traditional wire-less network is the additional propagation delay introduced by the fiber links. This additional propagation delay can exceed the timing boundary of the network’s MAC protocol and eventually stop the network from working properly. For cen-trally scheduled MAC schemes, such as HiperLAN/2 and IEEE 802.16, the effect of the additional propagation delay is less severe since the timing between dif-ferent phases are allowed to be adjusted by the central station. In other words, the central station is capable of scheduling the traffic in the networks in order to minimize the effect of the extra delay. However, for distributed control protocols,

such as IEEE 802.11 and IEEE 802.15.3, the additional delay poses a challenge to the design of the fiber distribution system.

Another problem arises with the utilization of the 60 GHz band for the indoor environment. As mentioned above, at the millimeter wave bands, a radio cell is typically

confined in a room. Consequently, mobile stations in a radio cell will be completely hidden to other stations in other cells/rooms.This hidden terminal problem is especially se-vere in distributed control networks, e.g., IEEE 802.11, where all stations contend

for the channel. In these networks, whenever a station has a packet to send and it senses that the radio medium is idle, the station sends out the packet. However,

due to the hidden terminal problem, other stations in other cells can also send their packets at the same time, leading to transmission collisions.After a random backoff interval, all the stations have to resend their packet. Consequently, this problem reduces the network’s throughput and increases the packet delay.

1.3 C

ONTEXT OF THIS

T

HESIS

The work reported in this thesis has been carried out at Delft University of Technology, within the framework of the "Broadband wireless in-house networks employing radio over fiber" project [67]. The project is supported by the IOP GENCOM program of the Senter Novem agency (Dutch Ministry of Economics Affairs) and in collaboration with Eindhoven University of Technology. The main goal of the project is to investigate and develop new cost-effective system concepts for high-capacity wireless in-house networks including residential buildings, office buildings, airport buildings, etc. With these new concepts, a major reduction of installation, operation and maintenance costs is to be achieved, by simplifying significantly the antenna stations and by consolidating the signal processing, which is traditionally located in the antenna stations, as much as possible in a central processing unit [67].

This thesis mainly concerns with RoF-based indoor networks operating at 60 GHz band in particular and at millimeter-wave bands in general. Although there has been a number of RoF techniques proposed in the literature, this the-sis is based on a new millimeter-wave signal delivery technique called Optical Frequency Multiplying (OFM) developed by the COBRA Institute, Eindhoven University of Technology [69; 83]. The technique is capable of generating very high and pure microwave signals over multimode fiber and possesses many advantages, such as multi-standard support, dynamic radio link adaptation etc. This thesis is an attempt to address the research problems stated in Section 1.2. In general, this thesis aims at developing new system concepts which are required for handling the microwave signals efficiently and for creating an in-building seamless network environment which supports mobility of the users. In addition, network self-configuration and Quality-of-Service (QoS) aspects of the applications will also be investigated in this dissertation. The research presented in this thesis is based on the trends and visions that have been discussed earlier

and will be elaborated more in Chapter 2.

1.4 T

HESIS

O

UTLINE

Figure 1.1 illustrates the organization of this thesis. The thesis is organized into two main parts. From the system perspective, the first part concerns the problems of designing an indoor network architecture that supports seamless mobility at 60 GHz. In the second part, the performance of two state-of-the-art MAC protocols when applied to the architecture proposed in the first part is analyzed.

The thesis starts off with Chapter 1 describing the research problems that motivate the work presented in the following chapters. In Chapter 2, a broad range of background materials is recapitulated to provide a basis for the choices and assumptions made in this thesis. These background materials are system-atically organized to provide readers with an overview picture about indoor networking at 60 GHz. Related works are also presented in Chapter 2 to clarify the contributions of this thesis.

The first part contains two chapters, i.e., a seamless infrastructure for 60 GHz in-building networks (Chapter 3) and Extended Cells planning and Dynamic cell formation (Chapter 4). Specifically, a network architecture based on the novel concept of Extended Cells (EC) is proposed in Chapter 3 to solve the problem of insufficient coverage of 60 GHz radio in the indoor environment. By introducing the concept of extended cells, we discuss that the system has large enough overlap areas between cells and thus guarantees a seamless communication environment operating at 60 GHz.

However, Chapter 3 discusses only the static extended cell planning method that partitions a building floor into a number of fixed extended cells. The imme-diate consequence from this static method is that the extended cell plan is unable to adapt to various traffic patterns under the network. As a result, the traffic is light in some cell leading to the waste of the radio resource while it is too heavy in other cells leading to the degradation of QoS.

In Chapter 4, we move one step further to present a dynamic Extended Cells formation algorithm to optimize the utilization of the radio resource. By doing so, we discuss that the system is able to react to traffic variations.

In the second part of the thesis, the problems of applying multiple MAC pro-tocols to the architecture proposed in Chapter 3 and 4 are discussed. Representing the distributed MAC protocol family, the performance of the IEEE 802.11 MAC

Figure 1.1:The organization of the thesis

protocol is presented in Chapter 5.

Similarly, Chapter chapter:wimax provides the performance analysis of the centralized IEEE 802.16 MAC protocol. Moreover, a QoS architecture and a mo-bility mechanism for IEEE 802.16 networks are also presented in this chapter.

Finally, Chapter 7 articulates the main contributions and future research di-rections.

Chapter

2

In-building Networking and the 60

GHz Band

A

s introduced in Chapter 1, to accommodate higher data-rate and stringent Quality of Service (QoS) requirements of emerging applications, current wireless technologies are evolving towards ever higher carrier frequencies. This, however, leads to smaller radio cell coverage due to propagation losses and the line-of-sight requirement. More antennas will be required to cover a certain site, e.g., office buildings, hospitals, airports etc. In addition, the complexity of the signal processing functions, e.g., macro-diversity, Multiple-input Multiple output (MIMO) etc., required for each antenna station is also increasing. Consequently, the cost of many antenna stations is becoming a major part of the total costs of the infrastructure and the task of simplifying antenna stations is also becom-ing increasbecom-ingly important [68]. In these micro/pico-cellular networks, mobility management is also playing an indispensable role as a mobile user will have to perform frequent handovers (HO). This thesis relies on Radio over Fiber (RoF) techniques to simplify antenna stations, and thus to reduce the system cost. Fur-thermore, we concentrate on the 60 GHz band due to its potential of delivering high datarates.

Section 2.1 will first discuss about the characteristics of the 60 GHz band when utilized to provide connectivity for indoor environments. Next, the RoF technique proposed for the system design will be discussed in Section 2.2. Section 2.3 reca-pitulates the features of the Orthogonal Frequency Division Multiplex (OFDM)

modulation technique that can be used to overcome the multipath destructive ef-fects in the indoor environment. Next, a classification of state-of-the-art Medium Access Control (MAC) protocols is discussed in Section 2.4. Section 2.5 presents the popular handover techniques for providing seamless connectivity for mobile users. Finally, the work related to this thesis will be summarized in Section 2.6

2.1 T

HE

60 GH

Z

B

AND AND

T

HE

I

N

-

BUILDING

E

NVIRONMENT Due to the characteristics resembling visible lights, the propagation of signals in the 60 GHz band is especially constrained in the indoor environment. Signals can be easily blocked by people, walls, furniture etc. This poses serious challenges to the design of a seamless indoor network operating at the 60 GHz band. In this section, we provide the basic characteristics of this millimeter-wave band.

2.1.1 The characteristics of the 60 GHz band

2.1.1.1 Worldwide regulations and standardization

57GHz 58 59 60 61 62 63 64 65 66GHz US & Canada (57.05-64) Europe (57-66) Japan (59-66) Korea (57-64) Australia (59.4-62.9)

Figure 2.1:Available unlicensed frequency bands around 60 GHz in different regions

Currently, two standardization bodies are working to standardize the 60 GHz short-range high-datarate links. Specifically, the IEEE 802.15.3c Task Group [7][40] is considering a 60 GHz physical layer design for the WPAN IEEE 802.15.3-2003 standard. More recently, the ECMA International TC32-TG20 Task Group [2] has also started considering 60 GHz communication systems.

The unlicensed frequency bands around 60 GHz have been allocated in dif-ferent regulatory regions (Figure 2.1). In Europe, as much as 9 GHz of spectrum has been allocated (57–66 GHz). In the USA and Canada, 6.95 GHz of bandwidth

(57.05–64 GHz) is assigned for unlicensed use. Also, 7 GHz of bandwidth (59–66 GHz) is available in Japan. However, under the Japanese regulations the signal bandwidth is limited to 2.5 GHz. Similarly, 7 GHz (57–64 GHz) of spectrum was recommended in Korea without any limitation on the types of applications to be supported. The smallest amount of spectrum of 3.5 GHz (59.4–62.9 GHz) was allocated in Australia. 5 10 15 20 25 30 10 20 30 40 50 G_TX (dBi) PTX (dBm) Portable Mobile

US peak power limit US average power limit Japan average power limit Autralia peak power limit

Figure 2.2:Allowed transmit power vs transmit antenna gain in different regions

The allowed transmit power is also regulated differently in different regu-latory domains (Figure 2.2). In USA and Canada, the peak transmit power is limited to 500 mW (27 dBm). Also, the average and the peak transmit power densities are limited to 9 and 18 µW/cm2, respectively, measured at 3m. This translates to equivalent EIRP of 40 and 43 dBm. However, regulations governing human exposure limits (defined for mobile and portable devices) will put a more stringent limitation on allowed transmit power (Figure 2.2). In Japan, the average transmit power is limited to 10 mW (10 dBm) and the transmit antenna gain is limited to 47 dBi. In Australia, the peak transmit power is limited to 10 mW (10 dBm), and the EIRP is limited to 52 dBm. Considering worldwide regulations, the transmit power is limited to 10 dBm and the EIRP is limited to 25 dBm and 37 dBm for portable and mobile devices, respectively.

2.1.1.2 Capacity limits of 60 GHz systems 0 5 10 15 20 106 107 108 109 1010 1011 Distance (m) Capacity Limits (bps) _{NLOS−1, Gt = 20dBi}

NLOS−1, Gt = 10dBi NLOS−2, Gt = 10dBi NLOS−1, B = 5GHz NLOS−1

NLOS−2

Figure 2.3:Shannon capacity limits for indoor networks at 60 GHz with omni-directional antennas

In this section, a study on the achievable capacity of different 60 GHz sys-tems with omni-directional antennas is presented. The achievable capacity of a network C can be calculated from the famous Shannon formula [89].

C = B log2(SN R + 1). (2.1)

In Equation 2.1, SNR is the signal to noise ratio at the receiver and B is the bandwidth of the system. The received power PRxcan be expressed as

PRx= PT + GT+ GR− P L(d) − IL, (2.2)

where PTis the transmit power, GTand GRrepresent the transmit and receive

antenna gain respectively, P L(d) is the path loss and finally ILis the

implementa-tion loss. The path loss is usually modeled over the log-distance in the following formula: [107]

P L0(dB) n Ω(dB)

NLOS-1 56.7 3.8 3.3

NLOS-2 71 2.7 2.7

Table 2.1:The log-distance model parameters of the two environments

where P L0 is the reference path loss at the distance of 1m, d denotes the

distance between the transmitter and receiver, n is the loss exponent and XΩis a

zero means Gaussian distributed random variable with a standard deviation Ω. The noise power at the receiver including the thermal noise and the noise figure (NF ) of the receiver can be calculated as follows

N = kT B + N F. (2.4)

In Equation 2.4, k denotes the Boltzmann constant and T represents the re-ceiver temperature. For this study, two non line-of-sight (NLOS) office environ-ments are used, i.e., NLOS-1 and NLOS-2. The log-distance model parameters of the two environments are shown in Table 2.1 [107]. For the default system con-figuration, the transmit power PT is set to be 10 dBm, both transmit and receive

antenna gains, GT and GR, are assumed to be 0 dBi and the system bandwidth

Bis 1.75 GHz.

Figure 2.3 shows the results for different system configurations. As can be seen in the figure, the capacity of the default configuration in both environments is below 1 Gbps when the distance d is further than 5 m. Interestingly, when the bandwidth B is increased to 5 GHz, the capacity is improved but still below 1Gbps when the distance increases beyond 5 m. However, when the transmit antenna gain GT is increased to 10 dBi, the capacity at 5m is increased up to

5 Gbps. When the transmit antenna gain GT is further increased, the capacity

decreases more slowly. In Figure 2.3, when GT is 20 dBi, the system achievable

capacity is 7 Gbps at 10 m. As shown in Figure 2.2, this system configuration, i.e., PT = 10 dBm and GT = 20 dBi, is allowed worldwide.

The detailed 60 GHz wireless physical layer and antenna designs are out of the scope of this dissertation. In the rest of this thesis, we assume that in a non-obstructed room with the size of 10 × 10 m, it is possible to offer good signal coverage at 60 GHz. As discussed above, this can be accomplished either by increasing the transmit power PT or the antenna gain GT.

2.1.2 In-building networks operating in the 60 GHz band - The

corner effect

One of the main drawbacks that prevents the 60 GHz band from being deployed for LANs is the considerable propagation attenuation of signals. This is espe-cially severe in the indoor environment, where signals can be easily obstructed by the movement of people, furniture, walls etc. A person standing in between a line-of-sight connection can easily cause 20 dB attenuation from the link budget [95]. As a result, in an indoor network operating in the 60 GHz band, walls can be considered as reliable cell boundaries. To obtain a good coverage, at least one Access Point (AP) is required per one closed indoor area, e.g., a room, a hall or a corridor. This configuration poses the problem of insufficient overlap areas be-tween cells since overlap areas exist only around open areas, e.g. windows, doors etc. Moreover, overlap areas are normally narrow and directional. Subsequently, a mobile user will lose connection when he/she goes out of a room and turns immediately. We term this problem as the “corner effect” [31].

Figure 2.4:The overlap area simulation

The in-building environment can be characterized by grids of rooms, corri-dors, hallways etc. Rooms are typically of the order of a few meters. Geograph-ically, a floor layout is well-planned and exposes some levels of regularity. For example, rooms or office spaces in an office building or in a living quarter are

0 50 100 150 −100 −95 −90 −85 −80 −75 −70 Receiver Position Signal Strength (dB) AT1 AT2 Rx Sensitivity A C D B HO Triggered HO Complete Hysteresis HO Latency

Figure 2.5:Signal Strength at 60 GHz [31]

normally arranged along corridors and have the same size and structure. As mentioned above, a 60 GHz radio cell typically spans a room and is separated from neighboring cells by walls. An overlap area between two adjacent radio cells exists only around doors or windows. Due to the high propagation loss caused by walls, the overlap area is normally narrow and directional. To verify this claim, a propagation simulation at the 60 GHz band has been carried out using the popular ray-tracing software named Radiowave Propagation Simulator 5.3 (RPS) [1]. This simulation package has been shown to be accurate in terms of statistical properties [97].

Figure 2.4 shows the simulation configuration. The floor plan of the Wireless and Mobile Communication (WMC) group, TU Delft is considered in this analy-sis. Two rows of 5×8 m office-rooms are arranged along a long corridor. A student lab (15 × 8 m) situates in the middle of the lower row. Walls are made of concrete and are 10 cm thick. Doors are assumed to be completely open. Two transmitters operating at 60 GHz are placed on the floor. The transmitter AT 1 is placed in the center of a room and another transmitter AT 2 is placed in the corridor under the ceiling (3 m high). A mobile user moves from the point A inside the room to the point B in the corridor. The point C is where the user crosses the door and the

user loses the line-of-sight connection with AT1 at the point D.

The signal strength contributed from AT 1 and AT 2 is collected along the user’s path and is shown in Figure 2.5. As predicted, the user receives good signals from AT 1 from the point A to D. After the point D, the user loses the line-of-sight connection with AT1 and consequently, the signal strength drops sharply. Contrarily, signal strength from AT 2 rises when the user starts seeing AT 2 (at the point C). The distance between the point C and D determines how much time it takes for the user to cross the overlap area. To guarantee a seamless multi-channel communication environment, a handover is required to be triggered and completed within this amount of time. The distance between the point C and D can be very short when a user makes a sharp turn as he/she gets out of the door. In this case, the system will not have enough time to trigger and to complete a handover, thus resulting in packet losses (a break in call) or even a call drop.

Assuming the average speed of a mobile user is 2 m/s, a handover will have to be performed every 5 s as the mobile user passes through the grid of picocells. Due to the corner effect, a call might be dropped or it might experience a number of breaks as the user might take a number of turns along the path from one room to another. For realtime services, such as voice or interactive video conference, frequent interruptions are not acceptable.

2.1.3 The 60 GHz channel

2.1.3.1 Multipath and Frequency selective fading

Multiple paths in the propagation channel create multipath interference. The delay and attenuation associated with each multipath component corresponds to path length differences due to surface reflections, scattering by small objects, and propagation through different mediums. Since the arrival time and the delays (τm) of these different paths are distinctive, the frequency response of the channel

H(f )will exhibit amplitude fluctuation and thus distort the signal waveform. In a digital communication system, a channel is considered to be frequency-selective if the multipath delays are distinguishable within a symbol period. If the channel bandwidth is sufficiently narrow, the channel frequency response can be approximated as constant and the channel does not suffer from frequency selective fading. In other words, the wireless channel is considered to be flat if the following conditions are satisfied.

δτ≪ Ts (2.5)

Bc ≫ Bs. (2.6)

In the above equations, δτ is the rms delay spread of the channel, Ts is

the symbol period, Bsis the system bandwidth and Bc represents the channel

coherent bandwidth. A common rule of thumb is that a channel is flat if Ts≥ 10δτ

[89].

In the 60 GHz band, the scattering effect is drastically reduced since scattering occurs when objects are similar in dimension to the operating wavelength. Trans-mission through most objects is also reduced as a consequence of the limited ability to penetrate solid substances. However, reflection effects are amplified. Reflection occurs on objects larger than the dimensions of operating wavelength implying that objects traditionally acting as scattering objects now become re-flectors at 60 GHz [33].

In the 60 GHz band, the typical rms delay spread in an office environment is about 18 − 20ns and the excess delay spread for 30dB attenuation is 70 − 100ns. The coherent bandwidth is about 10 MHz [41].

2.1.3.2 Time selective fading

Due to the motion of different objects participating in a communication system, e.g., the transmitters, receivers, scatterers etc., different transmission paths expe-rience different Doppler shifts. This effect causes the signals to vary in time and also broadens the signal spectrum, i.e., frequency dispersion. Define the channel coherent time as

Tc=

1 fD

where fDis the maximum Doppler frequency. The channel will be termed as time selectiveor frequency dispersive if the coherent time Tc is comparable to the

symbol period Ts. On contrary, the channel will be time nonselective or slow fading

if the following condition is satisfied

The maximum Doppler frequency of a signal modulated on a 60 GHz carrier is around 200 Hz at a walking speed of 1 m/s. Is it predicted that for a communi-cation system operating in the 60 GHz band, a fairly large bandwidth (BS) will

be used. As a result, the system bandwidth will likely be larger than the coherent bandwidth of the channel (BS > BC). Since the wavelength of the carrier is in

the order of millimeters, all the movement under the network can be considered to be fast causing Doppler spread. Such a 60 GHz system will experience both

frequencyand time selective fading.

2.2 R

ADIO OVER

F

IBER

-

THE

F

USION OF THE

T

WO

W

ORLDS To address the problems of signal propagation in millimeter wave bands and to simplify the complexity of APs, Radio over Fiber (RoF) is one of the most promising solutions [98]. By integrating the wireless and wired networks, the capacity and transparency of the optical world can be combined with the flex-ibility and mobility of the wireless counterpart to form a seamless and scalable communication environment. In this hybrid environment, analog radio signals are transported over an optical fiber distribution network lying between a cen-tral processing station (CS) and remotely located antenna sites. By using such an architecture, a large part of the complexity of the antenna sites can now be transferred to the CS. As a result, a reduction in the system cost can be made because of two reasons. Firstly, the remote antenna sites are required to perform only simple functions and thus are small in size and low in cost. Secondly, since all the processing functions are now concentrated in the CS, the maintenance and upgrading tasks are also easier and therefore cheaper.

The RoF links lie within the physical layer of the wireless system to be sup-ported, and thus, it becomes a transparent extension of the wireless access do-main. This enables the possibility of allocating dynamically radio resources from the CS and thus optimizing the spectrum utilization. Additionally, mobility man-agement strategies can also be efficiently performed from the CS in combination with a proper radio resource management scheme. Figure 2.6 illustrates such an in-building RoF network. The CS typically performs all the signal processing, switching, routing, medium access control (MAC) and resource management functions. Electrical signals are processed by the CS, converted to the optical for-mat and then delivered to an antenna site via the optical distribution network. At the antenna site, optical signals are then simply converted back to the electrical

CS MS MS MS POF Twisted pair network Coax Cable network Fiber network Satellite dish

Figure 2.6:Broadband In-house network using Radio over Fiber technique. (CS: Central Station; POF: Polymer Optical Fiber; MS: Mobile Station)

format and radiated out to the destined mobile stations. The main advantages of the RoF technology can be summarized as follows [9][60][69][94].

• The system control functions, such as frequency allocation, modulation and demodulation scheme, are located within the CS, simplifying the design of the BS. The primary functions of the BSs are optical/RF conversion, RF amplification, and RF/optical conversion.

• This centralized network architecture allows a dynamic radio resource con-figuration and capacity allocation. Moreover, centralized upgrading is also possible.

• Due to simple BS structure, its reliability is higher and system maintenance becomes simple.

(modulation, radio frequency, bit rate and so on) and protocol. Thus, mul-tiple services on a single fiber can be supported at the same time.

• Large distances between the CS and the BS are possible

In order to design a reliable RoF-based infrastructure, a RoF technique must (a) be capable of remotely generating wireless radio signals and (b) allow a reli-able radio signal transmission over the optical distribution network. Since RoF technology was first demonstrated for cordless or mobile telephone service in 1990 [23], many RoF techniques have been proposed. However, in this thesis, our proposed infrastructure will be based on a RoF technique called Optical Frequency Multiplication (OFM) [43]. The OFM method [66] satisfies these two aforementioned requirements by generating the microwave signals with a sin-gle laser source and low frequency electronics and presenting high tolerance to dispersion impairments in transmission over single mode [72] and multimode [71] fiber links. In the following section, we review a number of functionalities enabled by the OFM technique, which makes it possible to design a reliable RoF-based infrastructure for broadband wireless networks.

2.2.1 The Optical Frequency Multiplication (OFM) technique

φ

fsweep λ0 IM-data CS AS MZI radio signals fsc BPF radio signals fRF=n×fsw±fsc

Figure 2.7:OFM implementation schematic

The Optical Frequency Multiplication (OFM) principle is based on harmonics generation by FM-IM conversion through a periodic band pass filter [69]. At the CS, a continuous wave laser source ω0 is frequency modulated (FM) by a

sinusoid with sweep frequency fsw, intensity modulated by the radio signal

(data) at low frequency subcarrier fsc < fsw/2, passed through a periodic band

pass filter (e.g., a Mach-Zehnder interferometer (MZI)), launched into the optical fiber link and recovered at the AT by a photodetector (Figure 2.7). At the output of the photodetector, radio frequency components at every harmonic of fsw are

obtained (fharmonic = n · fsw) (Figure 2.8, no data), with relative amplitudes

depending on fsw, the frequency modulation index and the free spectral range

(FSR) of the MZI. When radio data is applied, the radio signals are obtained up-converted double-sided along with the generated harmonics at fRF = n·fsw±fsc

(n indicates the nth_{harmonic), at the AT.}

38.4GHz

< 100 Hz 38.4GHz

< 100 Hz

Figure 2.8:harmonics generation with fsw=6.4GHz [72]

Exploiting the features of the Optical Frequency Multiplication (OFM) tech-nique, a reliable RoF physical layer can be designed, comprising bidirectional RF transmission, increased cell capacity allocation, multi-standard support, remote LO delivery and an inband control channel for dynamic radio link adaptation and remote antenna controlling [70]. The proposed scheme can be easily integrated in WDM-PON architectures, allowing a flexible convergence of wireless services with broadband access optical networks.

2.2.1.1 Increased Cell Capacity Allocation and Multi-standard Support

As explained before, in a RoF link employing OFM, any radio signal at low fre-quency subcarrier fsc< fsw/2can be introduced by intensity modulation at the

CS, transparently transmitted to the AT, and recovered up-converted along with the desired harmonic. On the condition that the maximum RF bandwidth (fsw/2)

is not exceeded, different wireless signals can be transmitted simultaneously in a subcarrier multiplexing (SCM) scheme [43] (Figure 2.9). Hence, at the AT, the ob-tained up-converted signals can be selected at the same or at different harmonic bands. This opens the possibility of increasing the cell capacity of a wireless system at the AT, without the need of installing new costly TRX’s (provided that the

… fsw 2 · fsw 3 · fsw n · fsw fsw/2 … fsw 2 · fsw 3 · fsw n · fsw fsw/2 … n · fsw+fsci n · fsw … n · fsw+ fsw/2 … n · fsw+fsci n · fsw … n · fsw+ fsw/2 … fsc1fsc2fsc3 fsci fsw/2 … fsc1fsc2fsc3 fsci fsw/2

Figure 2.9:RF bandwidth capacity

2nd_{harmonic 6}th_harmonic CH1 (200MHz) recovered at 5.8GHz 6 GHz 6 GHz 18 GHz18 GHz CH2 (300MHz) recovered at 17.7GHz 2nd_{harmonic 6}th_harmonic CH1 (200MHz) recovered at 5.8GHz 6 GHz 6 GHz 18 GHz18 GHz CH2 (300MHz) recovered at 17.7GHz

Figure 2.10: multiple standard support (simultaneous transmission/upconversion of QAM signals to 5.8GHz and 17.7GHz (fsw=3GHz, fsc1=200MHz, fsc2=300MHz)

AT is equipped with the appropriate broadband filtering and RF amplification covering the whole or a part of the harmonic band). Also, a proper selection of the fsw and fsc’s at the CS enables the simultaneous recovery of the wireless

signals at different harmonic bands (Figure 2.10). In this way, multiple wireless

standardscan be simultaneously and transparently transmitted to the same AT, e.g. WiFi and WiMax, in a single OFM link, employing only one laser source and low frequency electronics at the CS.

2.2.1.2 Dynamic Radio Link Adaptation with OFM

Dynamic radio link adaptation to the physical medium is a key feature in wireless transmission to guarantee system performance. Thus, a RoF link has to support this adaptability without incurring additional signal degradation along the opti-cal path, being as independent as possible of the radio link adaptation procedures. Whereas link/MAC and baseband adaptation can be controlled from the CS, the

RF adaptation may occur either at the CS or at the AT. In the last case, adaptive remote AT configuration might be necessary. Hence, a trade-off between AT-simplicity and minimum level of AT-intelligence arises. OFM enables a flexible mechanism for the dynamic radio link adaptation support [43]:

• Dynamic carrier frequency allocation can be easily performed from the CS by tuning low frequency subcarriers.

• Transmit power can be remotely controlled from the CS and adjusted at the AT, to alleviate optical dynamic range requirements in the RoF link. For this purpose, an in-band control channel has to be transmitted simultaneously with the wireless data channel from the CS to the AT.

In a more general approach, an in-band control channel in the same optical link may enable other mechanisms for remote antenna configuration and controlling, during network optimization and dynamic resource allocation.

2.2.1.3 Flexible Wireless-Optical Convergence

OFM has the advantage of generating microwave carriers with the use of a sin-gle laser source. When bidirectional transmission is considered, two separate wavelengths (λDL and λUL) compose the RoF link. Thus, the extension of this

OFM-based RoF link towards a distribution antenna system design implies the multiplexing of wavelength pairs per AT. This scheme can be easily integrated in wavelength division multiplexing passive optical network (WDM-PON) architec-tures, which are nowadays very popular in fibre-to-the-home (FTTH) broadband access [70], provided that the wavelength grid is wide enough (at least as wide as the optical spectrum broadening produced by the optical frequency modulation) to avoid overlapping between WDM downlink channels.

2.3 O

RTHOGONAL

F

REQUENCY

D

IVISION

M

ULTIPLEXING

In the last decade, Orthogonal Frequency Division Multiplexing (OFDM) has be-come a popular choice for many wireless systems. It is a multi-carrier modulation scheme that offers nice equalization properties. One of the biggest advantages of an OFDM modem is the ability to convert dispersive broadband channels into parallel narrowband subchannels, thus significantly simplifying equalization at the receiver end. For severely frequency selective channels (as with the large

bandwidth channels for high data rate applications in 60 GHz) the total complex-ity of OFDM to eliminate ISI is less than half of that of single carrier time-domain equalization [41][52]. OFDM also offers high spectral efficiency when adapting the power and signal constellation over each frequency flat component of the frequency selective channel. Moreover, OFDM is capable of flexibly and opti-mally allocating power and rate among the narrowband subcarriers. This fea-ture is especially important for wireless broadband where channels suffer from frequency-selective fading.

2.3.1 OFDM principles

s(t) s_0,m s_Nc-1,m F₀(t-mT_s) F_Nc-1(t-mT_s) serial to parallel x(t) s_Nc-2,m F_Nc-2(t-mT_s)

...

Figure 2.11:Multi-carrier modulation

In single carrier systems, data is sent serially over the channel by modulating a single carrier at a baud rate of R symbols per second. As a result, the symbol period is 1/R. As discussed above, in a frequency selective fading (or time disper-sive) channel, the time dispersion can be larger than the symbol period and thus causes Inter Symbol Interference (ISI). In a single carrier communication system, a complex equalizer is then required to compensate for the channel distortion.

In a multi-carrier system, the total bandwidth BSis divided into a number of

subbands (Nc), each of width ∆f = BS/Nc. Now, the data stream is partitioned

into blocks of Ncdata symbols that are then transmitted in parallel by modulating

TS= Nc/R. The principle of multi-carrier modulation is presented in Figure 2.11.

The modulated signal is the sum of the modulated carriers:

x(t) = +∞ X m=0 Nc−1 X k=0 sk,mFk(t − mTs) ! (2.8) where sk,mis the input data symbol modulating the kthsubcarrier in the mth

signalling interval and Fk(t)is the waveform of the kthsubcarrier [79].

observation window CP CP current symbol time next symbol previous symbol

Figure 2.12:Cyclic Prefix guard interval

To avoid ISI, a zero-padded guard interval can be inserted between neigh-boring symbols to extend the observation window to include τmax. However,

zero-padding can destroy the orthogonality of the subcarriers [52]. To overcome this problem, Peled and Ruiz [86] introduced the cyclic prefix (CP) that essentially is a copy of the last part of the OFDM symbol and is prepended to the transmitted symbol (Figure 2.12). However, the cyclic prefix must be larger than the excess delay δτin order to eliminate ISI.

As can be seen in Figure 2.12, any multipath components with delay smaller than δτ will maintain their complex waveform within the observation window,

thus preserving the orthogonality of the waveforms. At the receiver side, the CP will be discarded to eliminate ISI. The choice of the CP length L is a tradeoff. On one hand, L is desired to be large to combat ISI. On the other hand, L should be constrained as the CP will be discarded at the receiver end causing a loss of transmission energy. As a rule of thumb, L should be chosen such that the energy loss is below 10% [41].

2.3.2 OFDM design

In a wireless channel, that is both time and frequency dispersive, the OFDM parameters should be carefully selected [41][103]. To avoid frequency selective fading (see Section 2.1.3.1 and thus eliminate ISI, Ncshould be chosen such that

the subchannel bandwidth is much smaller than the coherent bandwidth of the channel. In other words, the following condition must be satisfied:

△f = BS Nc

≪ Bc. (2.9)

The ISI effect is reduced when Nc is increased. However, since the symbol

duration (TS) is also increased and can be larger than the coherent time (Tc) of

the channel, the channel frequency response of the channel can fluctuate during the transmission of a symbol, making the detection at the receiver difficult. This gives rise to time selective fading as explained in Section 2.1.3.2. Hence, the number of subcarriers Nc is bounded by the coherent time of the channel. We

have:

µtTS= µt

Nc

R ≪ Tc (2.10)

where µt = N +L_N is the time domain efficiency factor and L is the length of

the CP. Combining Equation 2.9 and 2.10, the number of subcarrier should be selected in the following range.

BS

Bc

≪ Nc≪ TcR

µt (2.11)

2.4 MAC P

ROTOCOLS FOR

M

ULTIMEDIA

W

IRELESS

N

ETWORKS

Medium Access Control (MAC) protocols play an indispensable role in any com-munication system. They are designed to make efficient use of a channel, re-gardless of the type of the transmission medium, i.e., wireline or wireless. By defining rules for accessing the medium, MAC protocols coordinate mobile de-vices and thus enable efficient and fair sharing of the available bandwidth. With the booming of different networking applications ranging from best effort data transmission to realtime multimedia services, QoS requirements, such as delay, throughput, jitter etc., are becoming more and more significant in the design of a MAC protocol. Moreover, the nature of the wireless medium and of the mobile devices also bring about new challenging issues, especially with regards to the

characteristics of the wireless channel. Generally, the radio link presents problems such as noise, interference, and limited channel bandwidth (BW). Subsequently, a wireless MAC protocol has to address several induced problems including location-dependent carrier sensing, hidden and exposed terminal problems, time varying channel or power saving for mobile devices etc [49][93].

State-of-the-art MAC protocols can be broadly classified into two groups, i.e., distributed and centralized protocols as illustrated in Figure 2.13 [49]. Distributed protocols can be used in any network architecture, while centralized protocols can be used in centralized architectures only. Distributed MAC protocols are based on random access, while centralized MAC protocols offer a wider range of access techniques that can be random, guaranteed or hybrid. These access techniques differ primarily by their complexity, their efficiency and the overhead they add on the communication channel. Some are more convenient for delay sensitive services while others are suitable for delay tolerant transmission. In the following subsections, this classification will be discussed in more details. Furthermore, the protocols that are capable of supporting both realtime multimedia and delay tolerant data traffic will also be identified.

Wireless MAC protocols

Distributed MAC protocols Centralized MAC protocols

Random Access Random Access Hybrid Access Guaranteed Access

RRA Demand Assignment

2.4.1 Distributed MAC protocols

ALOHA was the first distributed MAC protocol proposed for wireless packet networks. The protocol operates as follows. A station that has a packet to send will send the packet immediately. If more than one stations transmit at the same time, their packets will be collided and the stations have to resend the packets after random intervals. As the number of stations in the network and the offer traffic increase, the collision rate also increases. As a result, the efficiency of this protocol is very low, i.e., its maximum throughput is merely 18% [64]. To reduce the collision rate, all the state-of-the-art distributed protocols rely on the carrier sensing to detect ongoing transmission. They are referred as Carrier Sense Multiple Access (CSMA) protocols. CSMA-based protocols themselves do not support QoS. Although many modifications can be employed to enhance the QoS capability of distributed MAC protocols, QoS requirements still cannot be entirely guaranteed since every station is free to contend for the medium.

The basis operation of any CSMA protocol is as follows. Initially, a station that has data to transmit has to sense the medium. However, different from ALOHA, the station still has to wait for an amount of time before transmitting when the medium is sensed to be idle. Otherwise, if the medium is busy, the station defers its transmission for a random duration and retries to send at a later time. Ev-ery packet that has been successfully transmitted requires an acknowledgement (ACK). If the ACK is not received by the sender in a due time, the packet is considered to be lost and the sender has to resend the packet.

The term ”carrier sensing” refers to techniques used to detect ongoing trans-mission. However, in a wireless network, since a station can only sense the medium within its vicinity, i.e., within the carrier sensing range, the station can-not detect all the activities taking place elsewhere in the network. Consequently, two serious problems arise in distributed control networks, namely the hidden and exposed terminal problems.

A hidden terminal is the one that is within the range of the receiver, but out of the range of the sender [65]. In Figure 2.14, MS A is transmitting to MS B. Meanwhile, MS C has also a packet to transmit to MS B. Since MS A is not in the range of MS C, MS C will sense an idle channel. Subsequently, it will start its transmission that then causes collision at MS B. It is therefore said that MS C is hidden to MS A and vice versa. The hidden terminal problem increases the collision rate and reduces the efficiency of the network.

MS A

MS B

MS D

Carrier Sensing Range

Hidden Terminal Ongoing Transmission Contending Transmission MS C Exposed Terminal MS E

Figure 2.14:The hidden and exposed terminals problems

but out of the range of the receiver. In Figure 2.14, MS A is transmitting to MS B. At the same time, MS C has a packet to send to MS E. However, it senses that the medium is busy due to the transmission from MS A to B and thus, it will defer its transmission to avoid possible collision. In this scenario, the station MS C can actually start the transmission to MS E without causing any collision since MS B is out of the range of MS C. It is therefore said that MS C is an exposed terminal to MS A. Similarly, this exposed terminal problem increases the number of unnecessary backoffs and thus reduces the efficiency of the network.

Nowadays, the two most popular CSMA-based protocols for distributed wire-less networks are Distributed Foundation Wirewire-less MAC (DFWMAC) and Elimi-nation Yield - Non-Preemptive Multiple Access (EY-NPMA). While DFWMAC is the basis access protocol for the IEEE 802.11 standard [101], EY-NPMA is used for HIPERLAN developed by ETSI [10]. The classification and performance analysis of the protocols have been extensively studied in [49][64][65]. To enhance the QoS capability of IEEE 802.11 networks, the amendment IEEE 802.11e was also approved in 2005 [57]. It has been shown that IEEE 802.11e can provide effective service differentiation between different types of traffic [84]. Although both IEEE 802.11 and ETSI HIPERLAN can be employed for indoor multimedia networks, this thesis will only concentrate on the IEEE 802.11 standard due to its wide popularity. Its operational details will be elaborated more in Chapter 5.