University of Cambridge

INDOOR COVERAGE CHALLENGES IN THE WiMAX TECHNOLOGY

INDOOR COVERAGE CHALLENGES IN THE WiMAX TECHNOLOGY

Dr.Hari Ramakrishna

Professor, Department of CSE,

Chaitanya Bharathi Institute of technology

Gandipet -500 075, Hyderabad,

dr.hariramakrishna@rediffmail.com

K.Ravi

Asst. Professor

Dept. of Informatics

Alluri Institute of Management Sciences

kolipakaravi@yahoo.co.in

K.Anil Kumar

Assoc. Professor

Dept. of Informatics

Alluri Institute of Management Sciences

kak_9920020@yahoo.com



ABSTRACT

Now a day’s mobile telephone usage is higher here than anywhere else in the world and the mobile devices are used inside the buildings, and the mobile networks operators are provide indoor coverage with their service. In order to provide the indoor coverage without having too much extra costs like additional cell sites, optimum network planning to be performed to aid the coverage area by using network models.

In this paper we provide the concepts of what is Indoor Coverage, the difficulties in Indoor Coverage. How the WiFi and WiMAX technologies are used for improved capacity to provide Indoor Coverage. This paper also covers the models and architecture to provide better indoor coverage.

Keywords: Indoor Coverage, Indoor Traffic, WiMAX, Wi-Fi, BTS, Antenna, Modem

1. INTRODUCTION

Mobile telephone usage is higher here than anywhere else in the world and the millions of subscribers demand a quality service wherever they go. Unfortunately for the operator, these subscribers generally go to the places hardest to cover with a macro cellular wireless network, due to unique topology, high-rise office buildings, subway stations, dense urban streets, shopping and entertainment areas with irregular busy hours, hundreds of railway lines, and small apartments in large apartment buildings.

Great coverage outside doesn’t always mean perfect coverage inside. Outdoor coverage (the macro network) has improved steadily over time with more cell sites and improved dropped call rates. But in-building coverage – in offices, commercial buildings and residences, can often be impacted by things beyond your carrier’s control. Our cell phone numbers and email addresses have increasingly become a way of staying connected no matter where we are. But building materials like concrete, cinderblock, steel, brick and tinted glass, and building locations in shadowed areas, can limit the penetration of cellular and PCS signals into your location. Even changes in weather and environmental conditions outside, such tree foliage changes, or terrain can affect the signal inside.

Many WiMAX operators are at a crucial stage in planning or deploying WiMAX networks. Their wait to obtain the necessary spectrum licenses, to get the initial delivery of Mobile WiMAX products, or to select a vendor is either over or about to end. Finally they are ready to start rolling out their networks and test them in real-life environments, with paying subscribers, on loaded networks.

As operators have progressed through network planning or deployment, the hottest issues in the Radio Access Network (RAN) have become outdoor reach and indoor coverage. As subscribers start using their network, providing the high capacity density that new devices and applications require will be another key target.

These are recurring issues for all new wireless data technologies, as the underlying physics they need to overcome to get good coverage and capacity are the same. WiMAX is no exception, despite the fact that techniques like Orthogonal Frequency Division Multiple Access (OFDMA) and Multiple Input Multiple Output (MIMO) will enhance coverage and capacity.

2. DIFFICULTIES IN INDOOR COVERAGE

The combination of high capacity requirements and extensive network access from indoor locations makes it a challenge to build a mobile broadband network. This is not a challenge unique to WiMAX operators: Long Term Evolution (LTE) or Ultra Mobile Broadband (UMB) operators will have to face a very similar situation in due course. WiMAX indoor coverage adds a significant burden to the existing network resources, as the penetration loss due to the first wall in a building usually ranges between 10 to 20 dB.

To extend the coverage range and improve indoor coverage, WiMAX changes its modulation scheme dynamically as a function of subscriber location. But there is a stiff price to pay: increased range and indoor reach results in lower throughput.

Devices located outdoors close to the Base Transceiver Station (BTS) will use the most efficient modulation scheme, Quadrature Amplitude Modulation (QAM), which has a higher spectral efficiency. However most of the subscribers are likely to be in indoor locations or towards the cell edge, where Quadrature Phase Shift Keying (QPSK) is used instead and has a lower spectral efficiency. The impact of modulation is substantial. When using QPSK 1/2 the data rate can be as little as 20% of the maximum throughput available with QAM 5/6 as shown in Figure 1.

Figure 1: Impact of modulation on data rate in a WiMAX network

To make things worse, indoor subscribers are more likely to have longer sessions and to use more bandwidth-intensive applications, simply because indoor locations are more hospitable to data users. Over time, the higher number of users and mobile devices, coupled with the more extensive use of high-throughput applications, will further increase the overall need for capacity in the network, as mobile devices use network resources more heavily, because of their smaller antenna and more limited power.

WiMAX networks provide capacity constrained and higher capacity density will be required due to the combination of high levels of indoor usage coupled with the increasing adoption of mobile devices, which typically cannot use the most efficient modulation in indoor locations, and with the increased per-user traffic levels.

A higher density of BTSs will be required to meet the requirements for both indoor coverage and capacity. The new cell sites added need to make efficient use of spectrum and network resources and effectively have to pack more traffic capacity into the same area. To accomplish this, new network architectures are needed and BTSs will have to move closer to the indoor locations where subscribers are.

3. HIERARCHICAL POSITIONING ARCHITECTURE

Assume that the hand-held devices carried by the mobile users are capable of receiving signals transmitted from heterogeneous wireless technologies, such as WLANs and infrared. Figure 2 shows the architecture of the proposed hierarchical indoor positioning service. As an example, the mobile device is equipped with appropriate network interfaces to receive the ID signals transmitted by WLAN access points and infrared signal transmitters. The accurate positions of the WLAN access points and the infrared signal transmitters are given in advance. A program, called the location client, is installed in each mobile host.

Figure 2: Architecture of the proposed hierarchical indoor positioning service

The ID signals are received, the location client of the mobile host delivers the signal strengths and IDs of all detected WLAN access points, or just the ID of the infrared signal transmitter to the location server. The location server will estimate the position of the mobile host, and then deliver the estimated position back to the location client. Then the location-based services corresponding to the estimated position will be retrieved to serve the mobile user. The estimated positions will be stored in the database, so that the location server is able to trace the historical moving path of each mobile user. As a mobile host receives the ID signal of an infrared signal transmitter, the distance error of the estimated position is only several centimeters to several meters from the infrared signal transmitter. However, as the ID signal of a WLAN access point is received, the distance error of the estimated position is about 10 to 100 meters from the WLAN access point.

Figure 3: Functional structure of the location client and the location server

The position information provided by the infrared signal transmitter is more accurate than that provided by WLAN access points, because the transmission distance of the infrared positioning devices is much smaller than the WLAN access points. In general, WLAN access points can be deployed to cover the whole indoor environment easily, but it is difficult for infrared signal transmitters. Therefore, the positions estimated by the location server are almost based on the signal information of WLAN access points. Once the infrared signal is detected, the position of the mobile host will be adjusted immediately to the position of the infrared signal transmitter, because the infrared technology is more accurate.

The proposed hierarchical positioning architecture is based on the client/server model.

The functional structure of the location server and the location client is shown in Figure 3. The location client implemented in the hand-held devices consists of signal collector module, location module, and location-based service module. The signal collector module is responsible for detecting the positioning signals for heterogeneous wireless technologies. The location module is responsible for delivering the collected signal information, such as Received Signal Strength Indicator (RSSI) and Infrared ID (IRID), to the location server, and receiving the estimated position from the location server.

After receiving the estimated position, the location-based service module immediately offers the corresponding services or actions for the mobile user. The location server consists of positioning engine and database. The positioning engine is responsible for estimating the position of the mobile user according to the signal information delivered from the location client. Basically, the position can be estimated according to the measured RSSI values of at least three access points.

Then the positioning engine performs the crosscheck and the adjustment for the estimated position, according to the signal information of other heterogeneous wireless technologies, so as to improve the accuracy. Another mission of the positioning engine is to transmit the estimated position back to the location module of the location client. The database is responsible for recording the estimated positions, so that the moving path of each mobile user can be traced and monitored.

4. WIMAX AND INDOOR COVERAGE

WiMAX in general promises ubiquitous connectivity, access for both fixed and mobile devices and supports bandwidth-hungry applications without sweat. However, similar to other wireless broadband technologies in the market, the underlying issue for WiMAX remains in providing high performance indoor coverage. Here lies the challenge, as most users would connect to WiMAX while indoors. In fact, according to Senza Fili Consulting, 75% of WiMAX Operators estimate that over 80% of their subscribers will connect to the WiMAX network while indoors.

More often than not, indoor users have longer sessions and use more bandwidth intensive applications, resulting in the need for Operators to ensure high capacity in addition to optimized indoor coverage. Therefore, for best-in-class user experience, improving indoor coverage is becoming a very crucial task.

4.1 INDOOR COVERAGE HALLENGE IN WIMAX

Demand for extensive indoor coverage and for high capacity density translates into a need for a high BTS density, with the number of BTSs increasing as more users sign up for services, using multiple devices and additional applications. Deploying additional multi-sector, macro BTSs is often not feasible or cost effective. For operators it becomes progressively more difficult and expensive to accommodate a higher number of macro cells.

The equipment cost, although significant, is often not the major obstacle to denser macro deployments. Site acquisition, site preparation, and installation can have an even larger impact on the overall capex. This may make it impossible for operators that operate in areas with a high density of users to close their business case or it may unnecessarily delay profitability.

As the number of BTSs and traffic increases, operators also have to carefully monitor the growth of backhaul costs. If traffic from each BTS has to be transported by a wire line solution, opex costs may escalate quickly if each BTS has its own link. Depending on the location, the connection to the wire line infrastructure (perhaps DSL or fiber) may be expensive and may not be easily accessible from some locations (e.g. from lampposts).

Conversely, wireless backhaul may cap operating backhaul costs significantly, as it then enables the operator to aggregate traffic efficiently and to skip the recurring backhaul fees associated with each deployed BTS, but may require additional spectrum and additional equipment investment. Furthermore, relay stations with integrated wireless backhaul support eliminate the need for the costly and lengthy process of preparing multiple sites for backhaul.

4.2 BEYOND THE MACRO BTS

In the early stages of most WiMAX deployments, a network architecture centered on multi-sector, macro BTSs prevails, as it effectively achieves good outdoor coverage with a limited number of BTSs. However, a network architecture centered around macro cells often does not scale well when requirements for indoor coverage and high capacity become more pressing. The cost of hardware, site acquisition and preparation, and installation quickly outgrows the revenue opportunity, making it difficult to achieve a positive ROI.

To address the new coverage and capacity requirements, operators need to deploy more compact, smaller BTSs that support cost-effective backhaul solutions. They also come with less onerous site acquisition requirements and costs, as they can be installed in more accessible locations (lampposts, building walls), closer to street level, and they use less power. Because of their reduced size and weight, it is easier for the operators to gain access to cell site locations and to meet existing regulatory requirements.

The upcoming IEEE 802.16j standard will bring further support for integrated WiMAX wireless backhaul. It demonstrates industry commitment to providing a standard-based solution for more compact BTSs that relies on on-band wireless backhaul.

Table 1: WiMAX BTS options

WiMAX operators have started to explore alternative network architectures, which can be used to augment the initial macro infrastructure, and which are also increasingly used by cellular operators to boost capacity and indoor coverage. These options rely on different types of BTSs (Table 1) that form an underlay network that is used to extend indoor coverage and boost the capacity available to indoor subscribers. They are:

  • Microcells or Pico cells in outdoor locations, in close proximity to the buildings they cover. Outdoor microcells and Pico cells are smaller and less expensive than macro, multi-sector BTSs, but they typically have a more limited range and, if they have a single sector, more limited capacity. They are typically used in urban areas with a high concentration of users and traffic. They are deployed in dense networks, where each cell covers a small area, ensuring good coverage and high capacity. They can be located on lampposts, rooftops, or on building walls. Microcells and Pico cells may use wire line or wireless backhaul. Mesh topologies may be adopted where wireless backhaul is fully integrated into the BTS.
  • Pico cells in public indoor locations or within enterprise buildings, to provide high density capacity and extensive in-building coverage. Indoor solutions are often the only way to provide deep in-building coverage. They are smaller than microcells and outdoor Pico cells, and their cost and ease of installation is even lower, as they can be mounted on walls or ceilings. Picocells can use wireless or wireline backhaul, depending on the reliability and cost of the connectivity options available. Inexpensive wireline broadband links, where available, often prove to be the most cost-effective solutions. In buildings where wireline connectivity is not available to the operator or are overly expensive, wireless backhaul can be used instead.
  • Extension in residential and small business coverage with femtocells. Self-installable, low-cost femtocells can be used to improve coverage in homes or small offices. They rely on the existing DSL or cable modem broadband connection for backhaul. The operator is usually not directly involved with the installation of femtocells. Subscribers typically purchase them in a store and install them at home or in the office.

5. INDOOR COVERAGE IMPROVEMENT STRATEGIES FOR WIMAX

Generally, the wireless broadband industry focuses on the backend system (Radio Access Network or Core Network) to optimize network, particularly in improving indoor coverage. As far as WiMAX is concerned, WiMAX Modems are often treated as a connectivity access device for end users, whose role is merely to transmit and receive. It is time the device receives more credit and is trusted with a more important role – improving indoor coverage.

There are 4 methods which can be employed by WiMAX Modems to play a part in improving indoor coverage:

1)      Improving uplink reception (through next generation antenna technologies)

2)      Use of appropriate antenna type

3)      Optimal modem placement

4)      Boosting indoor coverage with WiFi

Figure 4: WiMAX role in improving indoor coverage

5.1 IMPROVING UPLINK RECEPTION

There are many technologies introduced by base stations such as 4T4R MIMO A, radio unit on top of tower to reduce feeder loss and higher transmission power. Unfortunately, these technologies do very little to boost uplink reception which is often the bottleneck that limits indoor coverage. Uplink connection is usually weaker than downlink, as uplink connection is enabled by an indoor modem transmitter which has lower power (200mW) compared to that of a base station transmitter (10W). Hence, the coverage of uplink connection is always limited.

Figure 5 illustrates the downlink and uplink connection coverage using various antenna technologies. The loss of uplink connection (Point A) comes at a distance much earlier than loss of downlink connection (Points B, C & D) and at this point, indoor modems can no longer connect to the base station. Even though MIMO A and Beam forming can extend downlink reception (Points B, C & D), these technologies do not contribute in boosting uplink reception.

Figure 5: Uplink and downlink connection coverage of different antenna technologies

There are several technologies that can improve uplink performance. One of the popular method is Switched Tx Diversity which requires an extra antenna and includes an algorithm to determine transmission based on the path of the better antenna. This method allows the modem to transmit radio signals from the best antenna to improve overall transmission signal strength, with the slight tradeoff of an extra switch and minimal loss of power.

An improved method available is Dual Transmitter using Cyclic Delay Diversity (2Tx CDD) which requires two power amplifiers (PA) and two antennas. This method can further improve the overall transmission signal strength. Aside from 2Tx CDD, an alternate method with added performance is 2Tx Spatial Time Coding (STC). However WiMAX R1.5 base stations must be able to support STC for users to enjoy better uplink performance.

Figure 6 explains how uplink performance can be extended via Switched Tx Diversity (Point E), Dual Transmitter using Cyclic Delay Diversity (Point F) and 2Tx Spatial Time Coding (Point G). It is important to note that only one of these technologies can be used at any one time.

Figure 6: Uplink performance can be extended via next generation antenna technologies

5.2 USE OF APPROPRIATE ANTENNA TYPE

Antenna design is often regarded as a black art. There are many factors that can affect antenna performance. For example, factors such as material, length, type and antenna design contribute to the actual antenna gain.

The common type of antennas used is patch antennas and omni antennas. Patch antennas are made up of one or several conductive plates that are spaced above and parallel to a ground plane. This design enables patch antennas to have radiation patterns that are very directional. On the other hand, omni antennas are made from a piece of conductive material generally orthogonal to the ground plane. This design enables omni antennas to radiate signals perpendicular to the antenna uniformly.

Figure 7 below illustrates the 3D radiation pattern for patch and omni antennas. The color red specifies the most sensitive location or area with the highest gain relative to the antenna. From the diagram, it is obvious that patch antenna has strong directionality, hence, the modem has to be placed correctly to ensure that the modem surface that emits radiation patterns face the base station for optimum performance.

Figure 7: Patch and omni antenna radiation pattern

However, omni antenna radiates signals uniformly in one plane and does not need to face the base station in a pre-conceived manner. Hence, it is ideal for indoor usage where the exact location of the nearest base station is difficult to determine.

5.3 OPTIMAL MODEM PLACEMENT

It is important to note that WiMAX signals are emitted through radio waves and careful indoor placement can significantly boost indoor coverage.

WiMAX Operators should educate users on where and how to place their indoor modem. Firstly, by simply placing the indoor modem near the window that faces the nearest base station as shown in Figure 8 can improve the antenna performance dramatically. This is because radio wave penetration loss for glass (6dB) is much lower than the penetration loss for concrete walls (13dB or more).

Figure 8: A WiMAX indoor modem facing base station

Secondly, placing the modem near the window as opposed to a distance away from the window yields better throughput as a result of improved indoor coverage. From observation and trial runs conducted.

5.4 BOOSTING INDOOR COVERAGE WITH WIFI

Some users might express that is it not always convenient to restrict computer usage to an area that is next to the window. Additionally, they might want the convenience and flexibility of sharing the wireless broadband connection through WiFi.

Therefore, using WiFi to complement WiMAX can provide advantages that improve indoor coverage. One of the ways of going about this is to use a WiMAX-WiFi combination modem also known as WiMAX Integrated Access Device (IAD) which enables WiMAX-In-WiFi-Out. WiFi and WiMAX transmitters are placed within the same modem so that the transmitters are able to connect to the respective WiFi-enabled devices and WiMAX base stations simultaneously.

For example, as illustrated in Figure 9 below, users can place the WiMAX indoor modem at an optimal location (generally next to a window) and enjoy WiMAX through the flexibility of multiple WiFi-enabled devices within the perimeters of the home or small office.

Figure 9: Place the WiMAX for better indoor coverage

However, having both WiMAX and WiFi in the same device comes with a price. In many countries, especially in Asia and US, WiMAX is offered on the 2.3GHz and 2.5GHz frequency band which almost coincides with the frequency band of WiFi which is 2.4GHz. When WiMAX and WiFi share approximate radio frequencies, interference can occur and jeopardize connectivity. To overcome the issue of interference, a carefully designed modem is required to allow both wireless technologies to co-exist in the same device.

The advantage of having the WiFi-WiMAX combination modem is that antennas can be optimally designed to isolate radio interferences in a highly controlled manner. In addition, since antennas are stationary within the modem, there is better control over the WiFi and WiMAX radio signals to ensure users gain the best WiFi and WiMAX connectivity in the same location.

6. LTE MODEL FOR INDOOR COVERAGE

The challenges presented by 3G/Universal Mobile Telecommunications System (UMTS) to the even more demanding requirements of LTE. The Third Generation Partnership Project (3GPP) developed the specification for LTE as an evolution of UMTS. It is set to provide a number of advantages including increased capacity and reduced latency, improved spectrum efficiency and cell edge performance as well as offering both GSM/HSPA and CDMA/EVDO service providers a migration path to a 4G platform that addresses interoperability issues.

However, the benefits of the LTE evolution will only be felt if service providers are able to effectively reach their customers. In ABI Research’s 2009 report, it forecast that by 2013 more than 67% of all handsets shipped will be 3G+ capable. The volume of mobile traffic originating from inside buildings is already in excess of 60% for voice calls and is set to grow above 90% for data sessions. Taking the technical issues and user patterns together, it can be seen that the majority of the mobile data revenue opportunity is to be found in buildings. It is therefore essential to have ubiquitous in-building wireless coverage and is an issue that has to be addressed for both the service provider and subscriber alike.

Poor indoor wireless coverage is now recognized globally as one of the biggest obstacles facing mobile subscribers today. This is particularly acute with data services and with LTE licenses being issued in frequencies as high as 2.6GHz with a promise of 100Mbps download and 50Mbps upload speeds per cell. There are many issues to be considered for effective in-building wireless coverage with some of the key questions to be asked by those looking to deploy a solution for LTE.

6.1 DIFFERENT SOLUTIONS

Traditionally, cellular networks have been designed using an ‘outside in’ approach, where the service provider uses the macro and micro networks to penetrate buildings. With most sessions originating in-building coupled with modern, eco-friendly and energy efficient building techniques and materials, penetration of buildings from the macro network is no longer viable. Therefore service providers, building owners and enterprises must deploy systems to provide effective wireless coverage from within the building. This is the only way to maintain good signal strength and meet the level of service and data rates demanded by today’s mobile broadband subscribers, while ensuring efficient use of network infrastructure for the service provider.

LTE comes with the option of antenna diversity provided by Multiple-In-Multiple-Out (MIMO) technology. MIMO was developed for outdoor deployments and there is a lot of debate as to the need for MIMO deployments in a building. There is also uncertainty as to whether it is cost effective. However, there may be specific projects where MIMO could be a benefit, so it is important that any coverage solution deployed should have this flexibility available. Careful planning of antenna locations and link budgets of the coverage solution will ensure the cell edge performance needed by the handset user delivers the appropriate bandwidth.

6.2 FREQUENCY AND DUPLEXING

The LTE standard allows for both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) variants with licences being considered across a wide range of frequencies including 700MHz, 800MHz, 900MHz, 1800MHz, 2100MHz and 2600MHz. The ABI 2009 report on in-building wireless has found that LTE deployments in China will almost certainly be TDD. This provides a challenge when selecting an appropriate in-building wireless coverage technology that has the flexibility to support all of these options and variants. In multi-operator deployments, it is possible that multiple frequencies and both duplexing schemes are required on the same system. In addition, such a multi-operator deployment may also need to support existing 2G and 3G services at the same time.

There are three main options available to improve in-building wireless coverage including distributed repeaters, distributed radio solutions and Distributed Antenna Systems (DAS). DAS is typically favored in moderate to large infrastructures for being able to offer improved and unified indoor wireless coverage for multiple services at lower capital expenditure and running costs.

6.3 DISTRIBUTED ANTENNA SYSTEM

Distributed antenna system (DAS) comprises a network of antennas, which are placed throughout a building to provide dedicated in-building coverage. Traditionally, there are two types of DAS available, passive and active. Hybrid solutions are also in use where active units are distributed in a building with each feeding a small passive antenna network.

Passive DAS consists of a network of coaxial cables, couplers and power splitters to distribute wireless signals throughout buildings. ABI Research identifies in its 2009 report that passive DAS systems are known for suffering higher losses at higher frequencies
and are therefore not easily suitable for LTE. It also recognizes that buildings above 20,000m2 will need an active DAS deployment.

Active DAS takes service feeds from a base station or repeater and distributes amplified wireless signals inside buildings over fibre optic and RF cable, which connect to multiple remote antenna units placed in various areas of the building. In the past there has been a question with active DAS solutions relating to their ability to support TDD and multiple frequencies simultaneously on a single hardware infrastructure. With the need for additional hardware overlays in order to add in services at a later date, there are hidden cost implications for upgrading many active DAS solutions.

More recently, another cost effective DAS option has been introduced which has taken a truly wideband, active approach. This alternative DAS simultaneously supports any number or combination of wireless services, protocols, duplex schemes or frequencies on one system without the need for service specific overlays. The system has the ability to support any service type, which also provides peace of mind by future-proofing new investments in in-building wireless infrastructure, allowing new services to be added without extra components or costly upgrades.

7. SUMMARY

Increasingly WiMAX operators have started to look for solutions that address their stringent coverage and capacity requirements with scalable, flexible and cost-effective network architecture. The strategy that operators choose to enhance coverage and capacity will have a substantial impact on their network plans from the very beginning. As the initial network plan may facilitate or hinder future infrastructure expansion, it is crucial to start addressing the issues of indoor coverage and high capacity density during the early stages of network planning.

There is no single solution. Operators need to take a hard look at their market, and understand what the market requirements and specific challenges of their physical environment are. Most likely, each operator will find that a different solution is the best to get the coverage and capacity it will need in a cost-effective way. There is no established path yet and a limited selection of products. But high capacity and enhanced coverage are issues whose importance is rapidly escalating, as demand for WiMAX services grows and mobile devices make their appearance in the market.

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