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Monday, July 4, 2011

4G: The Challenges Ahead

The idea of a highly interconnected wireless knowledge society is certainly appealing. Although the user is always considered the ultimate beneficiary, terminal and infrastructure manufacturers, operators, and service providers see in this paradigm an unmatched opportunity for their businesses. In its widest sense the 4G concept appears to be the most ambitious ever, and the very first attempt to realize a true knowledge society. The challenges that researchers and developers of 4G will encounter are not few nor are the problems always easy to solve.

On the other hand, in terms of social impact and technology development, the reward is well worth the necessary efforts.
Multiple 4G definitions.
There is a clear lack of a universal, well-accepted, and unique definition of 4G. Regardless of huge efforts and several years of discussions, there is no worldwide consensus on the definition of 4G. Proposals and visions by ITU and WWRF should be respected and considered as authoritative suggestions for the benefit of all involved partners. Fortunately, developing parties are starting to identify some common characteristics and target capabilities required for 4G systems. Nevertheless, additional efforts should be made in order to ultimately converge towards a common understanding on what is 4G. This will be the first step to guarantee sound foundations and a global footprint for 4G. The importance of a global 4G standard is undeniable for the whole 4G value chain, from manufacturers, carriers, and service providers down to the users.

User-centric approach
Putting the user in the center of the scene appears to be the most logical approach to 4G development. In other words, technology is developed to fulfill users’ need and expectations. However, 4G is not necessarily targeting an immediate future and thus, understanding and predicting user needs at a larger time scale is difficult, if not impossible. Moreover, this is time consuming and requires considerable efforts, as there is not just one user profile. Failing to predict the users’ needs may lead in the worst case to 4G technology unable to fulfill user expectations. An opposite approach, creating first a generic technology to then trying to match the user needs to that technology is not a better option, in terms of risks. These are somewhat extreme cases, but 4G designers should understand the risk involved. One should not forget also the trendsetter role of industry. It is not only the user who dictates what industry does, industry also create trends that user will follow. It well could be that 4G will emerge from a middle way approach.

Seamless interconnectivity.
If 4G is a network of networks, then how do make an array of heterogeneous networks work and appear as a single network regardless of where it is accessed? Seamless vertical (internetwork) and horizontal (intranet-work) handovers are fundamental for achieving ubiquitous connectivity without interruption in the wireless ecosystem. The convergence and integration of existing, evolving, and newly developed technologies is critical to the success of 4G. Since the component parts of 4G (e.g., access schemes for different networks) are being developed independently by different parties, development of interconnection techniques supporting seamless communications should be started at an early stage.

Several services and applications in 4G will be delay sensitive, particular those exploiting real-time multimedia-based communications. Special care should be put in fulfilling requirements for transmission delays particularly when the contents could be reached from an array of possible networks with possible different access architectures. Connection delays should also be short enough, regardless of the network type or access architecture. Typical maximum accepted values for connection and transmission delays in 4G networks are 500 ms and 50 ms, respectively.

Scalability of services in a world of heterogeneous terminals.
4G terminals will have different capabilities (supported data rate, resolution, processing power, size). How do you efficiently scale up and down services to terminals with different capabilities? This appears to be a difficult task as terminal range is broad and there will be numerous manufacturers and service providers.

Exchange of complexity for simplicity.
4G networks will be complex, but in order to penetrate into the market with success, the user should experience 4G as a simple and intuitive system. Clearly the challenge is hiding networking complexity from users.

Technology fragmentation. By definition 4G encompasses several complementary technologies. As these technologies are developed, they tend to overlap in capabilities, resulting in competition. A severely fragmented 4G will also fragment the market, with negative effects on manufacturers, operators, and users. Technology fragmentation is inherent to 4G, but should lead to complementary systems rather than competing ones.

It is difficult to design a concrete wireless communications system without the knowledge of the frequency band allocated for operation. Knowledge of channel behavior is essential for the system designer. The availability of paired/unpaired bands also has a profound impact on the duplexing mode to be used. Moreover, since 4G assumes the use of MIMO systems, it is difficult if not impossible to design future multi-antenna-based systems without knowing the spectrum in which the system will be operating (and hence channel behavior). Scarcity of spectrum is another challenge to be taken into account. In addition to the development of communications techniques with higher spectral efficiency, new promising techniques like cognitive radio need to be studied and possibly applied in 4G.

Access architectures.
The suitability and limitations of conventional and nonconventional access architectures in different 4G radio networks need to be better understood. Network access architectures include infrastructure (e.g., single and multihop cellular) and infrastructureless (e.g., ad hoc) based concepts. Cooperative techniques are emerging also as a very promising network access approach.

Efficient resource allocation.
Management of available resources is crucial to guarantee a truly effective utilization of always-scarce time, frequency, and spatial resources. This is particularly true for 4G systems where multicarrier and multi-antenna systems are likely to be used. Finding efficient, fair, and simple-to-implement strategies to allocate resources in multiuser environments is far from trivial.

Interference Management.
Multiple access interference in heterogeneous networks with multiples operational air interfaces, heterogeneous terminals and heterogeneous services could be an issue. In addition to the interference issues at network level, also potential interference problems within the terminal should be considered. Indeed, multi-standard and multi-antenna terminals pose serious challenges for terminal designers, as it could be very difficult to manage and control RF interference within small form factor terminals.

Battery technology.
The energy density of current batteries is rather low, and if such batteries were used in 4G terminals they will not provide enough operative time, particularly if rich-content multimedia and high-speed communications are used. Fuel cells have one order of magnitude higher energy densities and so they have the potential to solve this problem.

Security issues.
Multimedia-based rich-content communication poses significant security risks for the user, for the information being transferred, and for the content rights. These are fundamental issues that need to be solved before associated services and applications are introduced. Also, since we are considering a highly heterogeneous network where information can be accessed from different places and through different air interfaces, strategies for management of user and information security should be considered.

Interlayer design.
Work needs to be done at all OSI layers, separately and jointly. Unlike in previous generations, scarcity and a high demand for resources in 4G leads us to seek efficient solutions to better use the frequency, time, and spatial resources. Cross-layer design and optimization are unique and important techniques that the 4G research and development community is relying on.

It is expected that the cost of 4G infrastructure should be significantly lower than that corresponding to 3G networks. A target cost of one order of magnitude lower is often mentioned. However, as the prospective band to be allocated to 4G appears to be at higher frequencies than in current 2G and 3G systems, cell coverage is expected to be smaller, leading to the need of more base stations to cover the same area. Also, higher data throughputs shrinks cells significantly. New access architectures, e.g., based on relaying stations, have the potential to solve theses problems. It is clear that lower infrastructure and terminal costs will help to increase 4G network penetration, but considerable research and development efforts need to be done to achieve these goals. Cost per (received) bit of information should also be much lower than in current systems (e.g., one hundred times or even cheaper than at present). Services and their use should be attractively priced. The nature of the information that 4G networks will transfer data requires new pricing strategies.

Synergetic efforts.
In order to become a reality, technical challenges have to be solved by worldwide research and development forces. More than ever, concepts and technical solutions should be developed jointly by industry and academia.

Open 4G Standards
In order to succeed 4G should be an open standard and thus, industry should strive to continue with the common practice of 2G, 3G and IEEE wireless standards, among others.

Convergence is the word that can be best associated with 4G. Convergence can be interpreted in several ways: convergence of wireless and wired networks; convergence of communications, consumer electronics, and computing; and convergence of services, as illustrated below;
From the 4G terminals perspective, one can see a clear trend where several functions and capabilities converge into a single terminal, a multimode (multi-standard), multifunction terminal. 4G will materialize the so-called three-screen convergence, bringing together TV, PC and mobile phone screens into a single portable device. These screens, perhaps the most notorious technology icons of our time, are watched and scrutinized day after day by billions of people worldwide, mostly at their home, working places and gradually on the move. 4G will not only fuse these separately developed technologies but also and more importantly, it will ultimately free them from any physical connection to the information sources. This is called the flying screen concept, as this screen is capable of displaying any content, anywhere the user is, at anytime, and on the move. The illustration below illustrates the three-screen convergence in 4G and the concept of flying screen.
Convergence will also take place at the network level, where several heterogeneous networks will appear as to be merged into a single network of networks, as shown in below. Indeed, a 4G network will encompass the entire network hierarchy, from a very wide coverage broadcasting network down to personal networks, and will consider wireless wide area networks, wireless metropolitan networks, wireless local area networks, and personal networks as well. To the user this 4G network will appear as a single, simple, and all-reaching network.

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