Fourth Generation Wireless Networks And Interconnecting Standards
Third generation wireless networks are commencing their services in several countries soon. This generation includes several standards of IMT-2000 such as UMTS, CDMA2000, UWC-136, and EGPRS. The new generation wireless cellular networks although include new IP-based and mobility supported features still will suffer from a divergence between standards which limits the roaming of users between heterogeneous networks and thus limits the mobility of users. The issues of QoS such as perceived data rate, delay for message delivery, and the cost are yet to be addressed completely in the third generation wireless network. Interconnection of wireless cellular networks of different standards with the wired networks and other type of wireless networks such as satellite networks in an efficient and cost effective way calls for a new generation of wireless networks.
This special issue will address the state-of-the-art of proposals and research activities toward the fourth generation networks and how these networks can provide global seamless roaming between heterogeneous wireless and wired networks. Introduction of the inter working units (IWU) between networks of different standards (2G, 3G, and 4G) and between satellite and terrestrial wireless networks could be considered as the main issue in these networks. How the fourth generation networks can change the roaming capability of the previous generations at an affordable cost and perceived quality and how they boost the progress toward real personal communications are the issues to be discussed in this special issue. The special issue looks at different technologies, different protocols, and different network architectures that will be supported in the fourth generation wireless networks. Applications and services such as location services to be supported in the fourth generation wireless networks as well as challenges in this migration are of particular interests in this special issue.
Suggested topics for this special issue on “Fourth Generation Wireless Networks and Interconnecting Standards” include but not limited to:
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Feature of 4th Generation of Wireless Network
In the course of the accelerated contention of wireless technologies with wired technologies, broadband wireless service has become a reality, and wireless Internet is attainable. However, Quality of Service and cost remain as deficiencies of wireless systems. Despite the freedom of mobility, in the data arena, wireless technologies have enjoyed limited popularity to speak of. The true advantage of mobility in the context of broadband services is exemplified by the capability to deliver location specific services to the mobile user. Expanding the user’s range of sight through the fog, and around the corner, as well as increasing the user’s visibility to those around him, are among the objectives.
This feature topic issue will address the state-of-the-art of proposals and research activities toward the next generation broadband wireless networks and how these networks can provide global seamless roaming between heterogeneous wireless, satellite, and wired networks. Introduction of the inter working units (IWU) between networks of different standards (2G, 3G, and 4G) and between satellite and terrestrial wireless networks could be considered as the main issue in these networks. The next generation broadband wireless networks will provide new services to users such as Internet connectivity and navigation through satellite and terrestrial networks with perceived level of quality and at an affordable cost.
The feature topic issue looks at different technologies, different protocols, and different network architectures that will be supported in the next generation broadband wireless networks. The choice between IP-based and ATM-based technologies for wireless networks, optimum integration between satellite and terrestrial networks, and solutions for medium access protocols, routing, location management, handoff management, QoS management, error control, and interoperability are going to be explored for the next generation broadband wireless networks in this feature topic issue. The feature topic in particular aims to gather state-of-the-art of the activities in defining new services and applications for the next generation broadband wireless networks with emphasis on satellite-based and navigation services.
The articles for this Feature Topic issue will be solicited through an open call-for-papers and invitation to the experts in the field from industry and academia. Suggested topics for this FT issue include but not limited to:
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Requirements
(1) Broadband communications
Up until now, the traffic carried by mobile communication systems has mainly been voice communications. The second-generation (2G) system, the personal digital cellular (PDC) system, introduced imode services [2] that have brought about the currently popular form of Internet access, electronic commerce, and e-mail, which are mainly text-based data communications via a cellular phone. The IMT-2000 system offers high bit-rate transmission services from 64 to 384 kbit/s, and the proportion of data to voice traffic is expected to increase. Moreover, the rising popularity of broadband services such as ADSL (asymmetric digital subscriber line) and optical fiber access systems and office and home LANs is likely to lead to a demand for comparable services in the mobile communications environment.
(2) Low cost
To make broadband services available so that users can exchange various kinds of information, it is necessary to lower charges dramatically to keep the cost
at or below that of existing services. The IMT-2000system aimed at lowering the bit cost and establishing economical rates, but the 4G system requires a broadband channel and an even lower bit cost.
Wide service area
One feature of mobile communications is that it is available for use anytime and anywhere. These capabilities are also important for future mobile communications. When a new system is first introduced, it is generally difficult to provide such an extensive service area as the existing system, but customers will not buy the new terminals if they have restricted service areas. Moreover, to support terminals that have relatively large display screens, such as personal digital assistants (PDAs) and personal computers with wireless capability, especially ones used with advanced services, which will often be used indoors, we need to provide better coverage of indoor service areas.
Diversified services and ease of use
The target subscriber base for mobile communications comprises various types of users. In the future, we expect to enhance the system performance and functionality to introduce a variety of services that include not only ordinary telephone services, but also services that transfer information utilizing all five senses. These services must be made easier for anyone to use.
2.2 Design objectives
The design objectives for meeting the above requirements are shown in Fig. 1. Considering that video and data communications will be the main features,the 4G system must provide even higher transmission rates and larger capacity (i.e., both number of users and traffic volume) than IMT-2000. Also, considering that the video transmission quality in current broadcasting is achieved by a transmission rate of several megabits per second, that LAN transmission rates are from 10 to 100 Mbit/s, and that the rate of ADSL is several megabits per second, the design objective is a transfer rate of approximately 100 Mbit/s in an outdoor mobile environment and gigabitclass rates indoors. It will not be possible to accommodate future mobile communication traffic unless a transmission capacity of at least ten times that of the IMT-2000 is achieved. To ensure throughput for communications between terminals and achieve highlevel realtime communications, it is necessary to achieve a low transfer delay time of 50 ms. Also, assuming that future services will be based on Internet protocol (IP) networks, efficient transmission of IP packets over wireless connections is also a necessity. While increased capacity is also effective in lowering the bit cost, the cost per bit must be reduced to between 1/10 and 1/100 of the current levels by reducing the infrastructure equipment, operation, and construction costs. The design objectives described above focus on services that have higher performance than existing services, yet are easy to use. It is necessary to pioneer new markets by making use of the capabilities and performance of the 4G system, such as integration with indoor wireless LAN and wired systems, and by implementing a mechanism for introducing new services in a short time.
working. Accordingly, interconnection and handover between such various access systems are required in addition to handover and roaming within one mobile communication system.
System configuration
(1) IP-based connection configuration
The 4G system will be configured for connection to IP networks, considering efficient transmission of IP packets, co-existence with other access systems, ease of system introduction, expandability, and other such factors. IP networks can also connect with or accommodate wireless access systems other than 4G systems. The 4G wireless access point (hereinafter 4GAP) will be connected to an access router (AR) as shown in Fig. 3 and will have wireless control functions for wireless transmission, handover, etc., allowing communication with mobile nodes operating on IP. The 4G-APs will form their respective cells. When a mobile node moves between cells, handover will be accomplished by simply switching access points and wireless areas if the two 4G-APs are connected to the same AR. If the 4G-APs belong to different ARs, then the packet transmission route on the IP network must be changed rapidly. The cooperative operation of 4G-AP switching and IP routing is important for smooth handover. For handover between a 4G-AP and an AP of another system, the mobile node must have functions for accessing both systems. Handover will be performed by monitoring and comparing different systems to select the one that is more suitable.
(2) Cell classification and configuration according to communication environment
The 4G system has cells for outdoors, indoors, and inside moving vehicles, as shown in Fig. 3. Outdoor cells cover a wide area, unlike the hotspot areas of wireless LANs, and allow high-rate packet transfer for fast-moving terminals. Indoor areas are covered by indoor APs, because the radio waves to/from outdoor base stations suffer large attenuation. Indoor APs are designed not only to provide a high rate transfer and simple operation, but also to compete with expected future wireless LANs. Furthermore, cells within moving vehicles such as buses and trains (moving cells/networks) are served by a mobile router (MR) that has wireless functions and relays signals between a base station and each terminal in the vehicle, rather than the terminals individually communicating directly with the base station in the conventional method. This configuration is designed to achieve efficiency in terms of terminal transmission power, transmission rate, control signal volume, etc. A multi-hop connection, which is effective in expanding the cell size, is being investigated as a way to overcome dead spots caused by shadowing. Data transmission via relay stations is expected to allow communications even when the effects of limited terminal transmitting power and radio wave propagation attenuation are large,
(3) Multimedia communications
Conventional IP networks have provided mainly best-effort services, but with realtime applications expected to increase as multimedia communication
diversifies, the importance of services that take into account quality of service (QoS) is also expected to increase. The 4G system configuration allows for a mechanism that guarantees the transmission rate to some extent and that prioritizes packet transfer by the packet type in cooperation with the IP network for QoS-aware packet transmission on a mobile radio link, which is the bottleneck.
4th Generation of Wireless Network infrastructures
A “hot” item, frequently debated in the wireless community these days, is whether there is such a thing as a “fourth generation” (4G) of wireless systems that is likely to appear after the successful deployment of the current third-generation (3G) systems, say five to ten years from now. This new generation of wireless systems is supposed
to complement and replace 3G systems, as well as second- generation (2G) systems that have already been in use for about a decade. A “classic” approach would design such a “system” in the same way as previous generations of wireless systems, that is, yet again focus on higher data rates (now beyond 2 Mb/s) and find new frequency bands for a worldwide standard (e.g., [1]). For a number of reasons, however, it is not obvious that the roadmap is this straightforward. One of the main concerns is that 4G wireless infrastructures will be deployed in an environment where many other types of wireless, and wired, communications systems are already present. Furthermore, some people argue that future wireless communications will become focused on services and user needs, thereby forcing the mixture of available wireless infrastructure elements to be used in a more transparent way [2, 3]. In that case, the previously so important air interface standard and frequency band issues will become secondary concerns.
By definition it is difficult to make precise statements on the nature of this kind of vision. An important factor contributing to this uncertainty is that we have very limited knowledge about the future environment in which a 4G wireless infrastructure should function. Which of today’s systems will still exist when a potential 4G infrastructure is deployed? Which systems and solutions will be considered successful then? What technical bottlenecks will be apparent 10 years from now? What market impact will 3G wireless systems have? How will this affect user behavior and user demand? How much money are prospective users willing to pay for services provided over this infrastructure?
As these questions indicate, defining relevant research topics with regard to future systems is not an easy task. Nevertheless, experience tells us that fundamental research related to 4G systems has to be carried out today in order to make it possible to deploy them a decade from now. We can thus formulate the key issue treated in this article:
How can reasonably relevant research questions related to future wireless infrastructures be identified? This article presents some of the results of the Fourth Generation Wireless project (4GW) of the Personal Computing and Communications program (PCC), the major Swedish cadmic research effort on future communications systems, launched in late 1997 [4]. In 4GW a scenario-based approach s been used to tackle the issue of identifying suitable research topics. In the article we present this method. We also I have an overview of some research results from the project. Finally, we conclude these results in terms of a vision of what4G wireless infrastructures might become.
Identifying Reasonable Assumptions Perhaps the most difficult issue in any scientific research endeavor is to identify reasonable assumptions. Most research therefore takes for granted assumptions that are common to the tradition in which it is conducted, that is, follows certain paradigms [5]. In general this is a very effective approach, but
when a study aims very far into the future, a more critical appreciation of the assumptions becomes necessary simply because they are likely to change over the time period the study spans. However, there is also a more fundamental reason. The assumptions taken for granted in the study are in part based on conditions external to the study. Implicitly, the researcher therefore also assumes these external determinants to remain stable over the course of his study. This is clearly not the case in the 4GW project.
An overview of the 4GW work process. Scenarios were created using literature
and other current knowledge sources. Key research issues, critical for the
success or failure of the scenarios, were formulated and researched.
How then does one handle future uncertainty in research projects aiming to provide results useful 10 or more years from now? The approach chosen in the 4GW project has been to work with scenarios. A scenario is a tool to explore a possible, plausible future by identifying key technical and social developments required for it to be realized. The point of a scenario is not to predict the future, but to create an awareness of which future developments are possible. It is thereby possible to both prepare for what the future will hold and identify the developments needed to influence the direction the future will take.
The research model used in the 4GW project is outlined in Fig. 1. As the figure demonstrates, the process began with the creation of techno-socio-economic scenarios based on literature studies and more informal sources of knowledge. The studied literature consisted of scenario methodology, as well as scenario work done by others (e.g., Ericsson [6] and Siemens [7]). The informal experience-based knowledge was gathered through Delphi interviews with academics and industry professionals. The resulting scenarios have been used as important input for the formulation of some basic assumptions that are an expression of the expectations and visions of the entire PCC program. From the basic assumptions, a number of working assumptions have been drawn. They in turn represent the more operational goals of the research program, and have been used to formulate the actual research problems of the 4GW project. By working in a multistage process, it has been possible to translate “fuzzy” societal developments into consequences for technologically defined research problems.
Three scenarios have been formulated: Pocket Computing, Big Brother, and Anything Goes. They address user behavior and lifestyle, telecommunications market evolution, development of supporting technologies, and evolution of values and society. The scenarios are outputs that portray the essence of what the world might become. The three scenarios are outlined below. More extensive narrative descriptions of the scenarios can found in a full report [8] published by 4GW in 1998.
Anything Goes! —
The diversity of telecommunications equipment has increased dramatically, as well as the possibilities of manufacturing cheap coexisting products. Manufacturing companies have become dominant in the telecom world. They advocate open de facto standards, and use software solutions to create flexible multistandard equipment. Because of dramatic price reductions, both residential and business environments have wireless LAN solutions. They are operated by a multitude of operators, and the end users have great freedom of choice in selecting where to purchase wireless services. Competition between operators, as well as equipment providers is fierce, and new wireless products and services appear all the time. Services and equipment are affordable for almost everyone in the industrialized world, which tends to narrow the social gaps in society. Equipment manufacturers, large and small, dominate the telecommunications scene.
Big Brother —
As more and more personal information is available in the information infrastructure, personal integrity, and privacy have become major concerns for the ordinary user. There is a widespread call for regulation and government intervention to ensure information integrity and secure networks. All citizens and companies wishing to deal with any aspect of computing and communication need some kind of regulatory approval. In the private sphere, most public information services use broadcasting. The complexity of products and services has increased, and thus also the cost. Service, transport, and equipment providers have been reduced to a few large actors (brands) that, in the public eye, can be trusted. Regulators dominate the telecommunications scene.
Pocket Computing —
Technological development continues at a high pace throughout the world, but due to financial and educational differences, society is divided between those who can follow the development and those who cannot. Parts of the population have access to a multitude of advanced services, whereas others use simple services adapted to their needs. Service providers offer a wide range of different services (which may include specialized hardware) tailored to various user groups. Mobile multimedia services mainly focus on high-end consumer and business needs. Global solutions are available, but much too expensive to be affordable for the average user. Cultural and educational differences between nations, and different strata in society, have led to political instability and unrest. A few operators and some very large manufacturers use standards to maintain their strategic position, and dominate the telecommunications scene.
Focal Areas for 4GW Research
Using the process model in Fig. 1, a number of research problems relevant to wireless infrastructures have been derived. A key recurrent problem is to provide high data
rates everywhere in a way that is affordable to the general public. The first part of this challenge, designing wireless system with high data rates, has attracted considerable interest in the research community. Our view, however, is that the real challenge is to combine this with affordability. As was shown in [9], the cost of providing wireless bandwidth everywhere, with the current “cellular” design paradigm, is essentially proportional to the data rate; that is, the cost per transmitted bit is almost constant, independent of the instantaneous data rate of the system. This is of course a devastating blow to high-bit-rate consumers, using, for example, high-quality sound and video applications.
The 4GW project has conducted a number of feasibility studies focusing on techniques and architectures that, if used to their full potential, could significantly change the cost and performance of wireless systems. The project has participants from various information and communication technology research fields. While the project work is conducted in a cooperative fashion, project members also belong to their own research tradition. Problems, methodology, tools, study objects, and so on vary between these traditions. Below follows an account of the subprojects of 4GW. Each has been formulated to study or challenge one of the working assumptions described in the previous section. Together with his advisors, one Ph. D. student has performed the research associated with each subproject.
Broadband OFDM Air Interface Design
The working assumptions state that user-deployed access points and self-planning capabilities will be key factors in making the 4GW infrastructure economically viable. Short-range broadband wireless systems play an important role in this context. In several countries, the 60 GHz unlicensed band has been proposed for this purpose, offering at least 5 GHz of available bandwidth. In a 60 GHz system, our research shows that coverage is not the main limitation in indoor office deployments, but rather that unstable handover situations are caused by the fact that interference occurs in short bursts. Using a ray tracing simulated channel, we have studied the dynamics of the 60 GHz time-varying channel in particular situations typical for office environments. The studies have also been extended to shopping mall environments. The results give an insight into the time variations of the signal-to-interference ratio. However, the simulations were based on a single-frequency network and omni directional antennas. Indirectly, we have showed that diversity at the terminal side is a prerequisite for functioning systems. Using directional antennas and dynamic resource allocation will decrease the interference issues, but the problems due to the short timescale variation of the interference will always remain more difficult to handle than in lower frequency bands.
The impact of human body shadowing on the 60 GHz channel has also been studied. This is a particularly important problem when considering imperfect installation of the infrastructure. The strong attenuation of the human body at 60 GHz considerably decreases the received power and changes the character of the multipath fading statistics, so the resulting error floor increases with the shadowing density. This can be described with a modified Saleh and Valenzuela indoor channel model [10, 11]. Exploiting site diversity can considerably improve system performance, since it effectively reduces the shadowing probability. Despite the difficult propagation situation at 60 GHz, it appears feasible to design wireless systems for high data rates that function in office areas or public hot spots of high-density population.
Smart Antennas
In order to provide high date rates at a low cost, smart antenna systems have been proposed for short-range WLAN-type systems. Using the 60 GHz band requires an increased number of access points, but may allow inexpensive radio access equipment. Systems at 5 GHz offer greater range, and have the advantage that several users can share one access point, which offers flexibility for the operator at the cost of more complex access points. Our research results so far show that dual arrays at above 5 GHz, in indoor environments, fulfill the 4GW requirements of link capacity. Furthermore, we have found that it is feasible to deploy an antenna array on the user terminal, since one wavelength (~ 50 mm) is sufficient element separation to utilize the rich scattering characteristics of the channel (Fig. 2). The results have been derived from analyses and capacity computations on measured multiple input- multiple-output channel data [12]. The results indicate that operation at 5 GHz is an important alternative in 4G wireless systems. In addition to further work in this area (e.g., to map the network properties), an infrastructure study is needed in order to compare coverage and QoS vs. infrastructure cost for the proposed systems.
Wireless Infrastructure Architecture
The assumption in the program is that high data- rate wireless services can only be provided at a low cost if infrastructure deployment costs are reduced by some orders of magnitude. In current cellular systems, large sums are spent for antenna site acquisition, network planning, and installation of base station transceivers, while hardware components are continuously getting cheaper. If wireless networks could be deployed according to the wireless LAN paradigm (i.e., by customers themselves wherever wireless access is desired) and still offer sufficiently high data rates and guarantee adequate coverage, large cost savings would be possible. The high data rates i1ntended for 4G infrastructures will require the use of unlicensed spectrum with sufficient bandwidth to accommodate such high capacities. Acceptable bandwidth can be, found for example, around 17 and 60 GHz [13]. Propagation at these frequencies suffers high free-space loss, strong shadowing by humans, and high attenuation by common building materials. The number of wireless access points (APs) required to achieve sufficient coverage is therefore high.
REFA’s air interface, a 128-carrier orthogonal frequency division multiplexed (OFDM) air interface with 130 Mb/s link layer throughput and a 50 MHz channel bandwidth, was adopted for the purpose of making comparisons. Three characteristic environments — an office setting, a shopping mall, and a campus area — were used to evaluate system performance.
Our results show that user deployment is indeed a viable alternative to traditional infrastructure installation methods. In particular, dense networks, typically needed to satisfy the high-capacity demands in, say, office environments, are tolerant
of arbitrary placement of the APs, as long as they are reasonably uniformly distributed over the entire area. In densely populated large buildings such as shopping malls, train stations, or airports, user deployment also achieves acceptable performance, although AP placement will require some coarse preplanning. Our results indicate that 17 GHz systems should be recommended for such scenarios since 60 GHz systems achieve very limited cell radii, hence requiring an extremely high number of APs to achieve adequate coverage. Outdoor scenarios are normally not suited to the user deployment approach. Even for 17 GHz systems, rather sophisticated network planning is necessary to attain sufficient coverage.
ITU-R activities
In 2000, the year in which the prospect of introducing the IMT-2000 system came into view, the international telecommunication union (ITU) began research on future development of IMT-2000 and other systems. In the ITU radio communication sector (ITU-R), investigation of Q.229/8 on future development of IMT-2000 and systems beyond IMT-2000 was assigned to study group 8 (SG8) working party 8F (WP8F), which was established in November 1999, and work on this topic began in March 2000. At the world radio communication conference held in June 2000 (WRC-2000), ITU-R resolved to conduct research on future systems, including spectrum requirements, to investigate the research situation at WRC-2003, and to review spectrum requirements at subsequent WRCs. ITU-R WP8F formulated a recommendation regarding a future vision to give direction to future technological development. The recommendation was approved at the February 2003 meeting of SG8 and forwarded to a higher-level organization, the radio communication assembly (RA). In RA, it was approved as the framework recommendation in June 2003. In WRC-2003 held in July, approval was given for the agenda items of WRC-2007 to include the frequency assignment for systems beyond IMT- 2000. In that recommendation, “systems beyond IMT-2000” is considered to cover all future mobile communication systems, including the current IMT- 2000 and its enhanced versions. The various wireless access systems will need to cooperate via the network so that users can use the full range of capabilities of the systems beyond IMT-2000 without being aware of individual wireless access systems. Furthermore, there is now recognition of the need for a new wireless access system and a frequency band for it to operate in to cover the performance region that cannot be achieved by advanced IMT-2000 systems (transmission rates of approximately 100 Mbit/s during high-speed movement and approximately 1 Gbit/s when not moving, although these bit rates assume sharing by users and the specific values are research targets). Furthermore, the target time for implementation of the new wireless access system is 2010 [6]. In the future, the study of spectrum requirements and research on specific technological issues are expected to make progress.
Conclusion
We outlined research projects toward the 4G system. We described system requirements, topics for study, and a basic approach to the system configuration. We also presented trends related to standardization in this field.