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Thursday, November 15, 2012

The Road Ahead with 4G- The different enablers

The contrast in capabilities between IMT-Advanced and its predecessor is remarkably exciting. IMT Advanced was designed to address the increasing demand for mobile traffic. To put the demand expectations into perspective, it is helpful to note that a 26-fold increase in mobile data traffic is expected by 2015, reaching a rate of 6.3 exabytes per month. With the world population estimated to grow to a range of 7.2 to 7.5 Billion people, an estimate is made there will be as much as 1 mobile unit or device per capita connecting wirelessly. Estimates are also predicting that 1.3GB per month generated per smartphone, with video taking up to two thirds of the traffic. In 2020, more than 50 Billion devices will be connected to the Internet and serving a population in the range of 7.5 to 7.9 Billion – almost six devices per capita .

Towards this vision, the earlier deployments of IMT-Advanced would have been made, with great advancements made in both the wireless and the wired Internet. The deployments will provide substantial understanding and experience of how OFDMA operates in practice, which is currently lacking. The use of heterogeneous access networks is also projected to be the norm, with different access technologies aimed at different connection requirements. In the meantime, policies and technologies currently investigated for combating spectrum will slowly emerge, and initial large scale realizations of the adaptive and opportunistic software-defined or cognitive radios will be made. Together with these physical layer advances, especially in cooperative MIMO communication, dynamic spectrum access and allocation will open the door for higher capacity communication.

It is these capacities that will make possible high bandwidth transmissions, both in the downlink and the uplink, in addition to supporting the transport of massive amounts of information. At the radio access interworking and backhaul level, advances will facilitate a more capable network-end management of network functionalities that is fitting to the multitude of devices to be communicating through the network.

At a larger scale, earlier forms of network intelligence will appear that will facilitate much desirable characteristics of network autonomy. Such characteristics include the currently deliberated aspects of self-optimization and self healing.

This autonomy will depend on processing massive amount of information that will be already traversing the network, generated either by passive sensing or through active Machine-to-Machine (M2M) communications. Many of the recently starting initiatives will ensures that such processing is made in a manner that preserves the integrity and privacy of the processed information, while achieving the desired network performance and user satisfaction objectives. At the same time, operators and vendors will begin employing mechanisms for reducing energy requirements, both per-unit and for networks as a whole. Such “greener operation”, however, will not be at the cost of network reliability.

In this the enablers of this vision, together with the challenges faced to realize its practicality are discussed;

Network Capacity
Several technologies have been sought in order to enhance the capacity of access networks at a cost efficient manner.Without doubt, the choice of multi-carrier access techniques will offer both great flexibility and reliability in such direction. However, it is in advanced antenna and network configurations that substantial capacity gains are achieved. Already, the notion of small cells – through the in-band femtocells or out-of-band WiFi networks – are already beginning to play an important role in today’s networks. The importance of small cells in the next few years can be highlighted by estimates of the amount of data they are expected to support 800 million terabytes per month by 2015.
Other advances will come at higher costs, including the use of relaying techniques and cooperative MIMO. It is generally understood that such “meshed” wireless communications can provide substantial gains. Relaying, for example, combats path loss and shadowing loss through the breaking down of the wireless link into smaller and reliable segments. Similarly with MIMO, which have shown great versatility in either mitigating interference or enhancing the reliability of the wireless link. And while for some of these advances the limits on possible gains are yet to be figured , the practicality of achieving these gains will be slowly evaluated over the next ten years as they are introduced to actual deployments. Certain issues, such as finding deployable mechanisms for resource allocations, remain unresolved. More critically, it will be important to demonstrate that capacity gains made exhibit reliability and cost efficiency.

LTE and LTE-Advanced are complemented by an IP-based network core, the EPC. There is also strong IP-based internetworking in WiMAX. Such support will be crucial in creating heterogeneous network composites – not only for user access, but for generalized device access. Work within the 3GPP and 3GPP2, in addition to the efforts in IEEE 802.21 or Media Independent Handover, are all aimed at supporting inter-technology handovers at the access level. There are also efforts including those of the IEEE P1900 working group that are aimed at, among other things, enhancing operational coexistence between the different radio technologies.

A definite trend that is to grow over the coming years is the addition of satellite networks to the existing heterogeneity. Traditionally, and despite their great bandwidths, satellites have been avoided for user- and device level access due to both their cost and delay characteristics. However, there is currently great interest in near-space (1722 km) satellites called High Altitude Platforms (HAP).

The delay characteristics for HAPs will be functional for terrestrial application. HAPs will also be characterized by wide coverage, offering reasonable coverage overlays for IMT-Advanced networks. Already, the ITU-R has issued the minimum performance requirements for HAPs providing 3G service in certain regions.

Cognitive Radio and Dynamic Spectrum
Software-Defined Radios (SDR) were initially defined so as to facilitate changing the characteristics and capabilities of a radio interface simply through reprogramming. Its evolution, Cognitive Radio (CR), was one where the programmability of the SDR can be made over-the-air and on-the-fly. What is more, however, is that a CR had sufficient processing capability to autonomously understand and react to various elements of the radio’s context of operation. Among other things, these characteristics include identifying whether the current spectrum band of operation is the best spectrum available for the radio’s active communication, and whether there are bands that are available and, for example, would offer greater bandwidth or better transmission quality. For CR to perform, it requires more than simply identifying whether or not a particular spectrum band is busy – rather, it becomes important that the radio recognizes what entity is utilizing that spectrum, and know for how long will this utilization will take place.

Such distinction is greatly important, especially in light of recent international cooperation between the different Telecommunications Regulatory Authorities (TRA) of the different countries and the ITU-R. These cooperations are at spectrum harmonization, refarming and reallocation. In addition, many countries now recognize primary and secondary users for certain bands, allowing for cooperative arrangements and coexistences between the different spectrum users, both licensed and unlicensed. It thus becomes possible for a secondary user to utilize spectrum “holes” or “empty spots” in a primary user’s band or, depending on the band and mode of communication, for both primary and secondary users to operate in the same band. Such cognition, however, is not limited to licensed bands. Bluetooth, for example, is already instilled with adaptability so as to overcome from other devices in the ISM band such as WiFi network elements or microwaves.

Network Intelligence
Services utilizing network and location analytics are already emerging in the smartphone applications market. Meanwhile, the proliferation of various sensing and actuating platforms, for example, ANT+ and IQRF, that interface directly with mainstream smartphone and network access types will soon allow for more valuable services that are more prompt, reliable and relevant. In this interweaved connectivity between context and personal preferences (both through settings and through non-invasive profiling), in addition to the service infrastructure of social networking platforms, the users’ wireless and mobile experience will become much more enhanced. Another dimension of interest is that of utilizing network information to discern physical properties. Many examples of this have been displayed, both in research and industry. One of the commercial examples involves utilizing network traffic levels in recognizing actual street congestions.

For the considerations of access network operation, however, functionalities that employ network analytics include instilling reliable wireless communication, interference management and mitigation, power management, resource allocation, and reduced energy. Both LTE and WiMAX support various mechanisms for autonomous operation of network entities, and have made provisions for selfoptimization in various aspects of their respective standards. For example, the operation of femtocells cannot do without autonomy, especially given the ad hoc nature of their deployment. Another example involves the required processing capabilities for Coordinated Multipoint Transmission (CoMP).

Access Network Architecture

The introduction of 3GPP’s X2 interface marked a particular evolutionary step in the design of access network infrastructure. Traditionally, base stations were connected to network cores in centralized star configuration, with each base station directly and independently connected to the access core. Such configuration, exercised up until the earlier releases of UTRAN, results in substantial handover latencies, especially when it came to IP-based mobility. Similarly with WiMAX, which is neutral to the choice of network core, support has been made to realizing flat architectures. A direct advantage of flat architecture is greatly reduced handover latency times, which was mandated by the IMT-Advanced requirements letter.

This advantage, consequently, results in reduced disruptions for multimedia IP handover as the users traverse the network. Through internetworking base stations, user context can be transported from a serving base station to the target one without having to go back to the network core. As was observed, additional optimizations are also possible in instances where the user terminal moved between a base station and its children relay stations.

Careful network design, however, is required in order to achieve these desirable characteristics. Design considerations would include aspects such as where is it best to connect the access network to the core or the identifying topology configurations that match the projected traffic load while achieving certain levels of reliability. Looking beyond IMT-Advanced networks, interest has already started in what is called “ultra-flat architectures”, wherein substantial processing is migrated from the network core to the network edges – the base stations.

Such migration, however, will largely depend on substantial advances taking place not in terms (of) processing capabilities, but also in inference frameworks. In such instances, the issues such as identifying the best location for a certain functionality, become more prominent.

Radio Resource Management

Radio resource management (RRM) functionalities oversee the allocation and maintenance of network resource to the various devices during network operation. RRM functionalities in IMT-Advanced comprise both traditional and emerging modules, including modules for admission control, scheduling, resource reservation (for various prioritization objectives), spectrum management, ARQ/HARQ, and routing. The various modules comprise different elements of an overall framework, and are expected to operate in a cohesive manner, serving specific overall operational objectives. Designing frameworks for IMT-Advanced networks, however, is not without challenges. By requirements, IMT-Advanced networks are expected to deal with certain characteristics among which are an immense magnitude of traffic from both users and devices, a range of traffic requirements for various services and applications, a range of mobility speeds, and different types of access technologies and modes. A definite problem of traditional framework designs is that they do not scale.

Complexity, hence, becomes a key issue to overcome when designing such frameworks, and one that is prominent at the different levels of network management. For example, there is  difficulty of scheduling multi-carrier access techniques, both OFDMA and SC-FDMA.

And while the separation of the time and frequency aspects of resource allocations does lead to significant operational optimization, scheduling becomes more cumbersome when introducing advances such as MIMO, either at the single cell or the multiple cell level. Another example of complexity can be found at a higher management level, and has to do with admission control of connections or flows. IMT-Advanced networks will employ different modes of operation, including point-to-multipoint, where a base station communicates directly to the device, relaying where the base stations communicate with the devices through one or more relay stations, or femtocells where the devices connect through the Internet.

Meanwhile, IMT-Advanced networks will support access heterogeneity, which adds the selection of access technology to the possible connection choices. In addition, the flexibility in spectrum allocations will also make possible varying the spectrum band through which the device is connected, that is, a spectrum handover. Considering that more than 50 Billion devices will be connected in the future, the importance of simplifying network selection mechanisms becomes more pressing.

This complexity issue has already been tackled in several ways. For example, the above noted notion of small cells “opens up” the capacities at the network end – a strong leverage when considering different connection possibilities. At the same time, the introduction of flat architectures have also simplified the considerations of the RRM as they have forced the decision making to be more localized, focusing only at users within the cell and the technologies overlaying the cell’s coverage. Within the research, much work has addressed the possibility of Common RRM, whereby the resources of overlaid access technologies can be jointly managed – a powerful advance that is viable for technologies administered by a single operator. Advances are expected in the AAA that would further facilitate inter-operator resource agreements and management. These advances, however, will take a longer time to realize.

Green Wireless Access

By some estimates, cellular networks consume 0.5% of world-wide energy consumption, with 1% consumed by the user handsets and 99% consumed by the network. Meanwhile, multiple-interface phones (Cellular with WiFI, Bluetooth, ANT+, etc.) have been observed to deplete their batteries much faster when all the radios are active all the time. Not surprisingly, then, that several initiatives and research projects have focused on reducing the energy requirements of wireless and mobile networks over the past few years. The projects, in general, vary in their approaches and their objectives. Some, for example, have focused on energy reduction through interference management – reducing the energy requirements of mobile handsets to reliably transmit its data. Network design plays an important role, whereby the location of the fixed base stations and the trajectory of the mobile stations are decided in a manner that also reduces handset energy expenditure. Meanwhile, energy can definitely be added to the considerations of network selection. Advances in dynamic spectrum allocation will also play a major role.

These enhancements, however, focus on handset energy expenditure. To alleviate some of the network expenditure, it is possible (to) utilize renewable energy sources such as solar and wind turbines. More advanced mechanisms, however, can also be employed. For example, it is possible to deploy high density access configurations whereby the all base stations would be turned in instances of high demand, and only a portion of the base stations would operate when the demand decreases. Naturally, a small coverage would be used when all base stations are turned on, and a wider coverage when only a portion is operating.
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