Plugging the Deadzones – Het Nets, Radio Clouds & Cooperative networks
Wireless telecommunication market is witnessing a shift in business models and market structure as a result of the deployment of new broadband access technologies, spectrum management techniques, policy-based network management, and the drive of new entrants to compete against the incumbents. If you look at operators in the US today – Verizon has 2G, 3G and now LTE; AT&T is rapidly looking to deploy LTE and so is Sprint with its technology refresh program. There are reasons why all the operators need to look intrinsically as well to plug their coverage holes. Deploying new technologies is very exciting but the need to plug holes in the current technologies to address – traffic offload, inter-technology handovers and IP as a layer to guarantee the QOS needed to continue as a data or VOIP session is an often isolated. There have been technologies like IMS around for quite some time that operators have been reluctant to utilize the benefits due to high initial investment needed. But as the market matures and the need for connectivity all the time overtakes the cost, incubating technologies like IMS, Software defined radios, Inter-radio technology solutions will have to take place both on the access side as well as on the backhaul (transport) side. In the last few years we have seen an emerging trend in the use of Microwave as a backhaul tool mainly due to Clearwire and its innovative approach of using microwave rings to aggregate the cellsite traffic.
Coverage Maps for top four Operators in the US
Addressing Dead zones
By far the biggest factor for wireless churn has been coverage holes, other than of course the iPhone! The fact that the customer experience has been bad is the biggest factor that influences customer churn. So how can an operator address this? What can be done that would mitigate these dead zones – building new cell sites in these coverage deficient areas are one. But in cities like San Francisco, Los Angeles or New York zoning is difficult and getting space, power and leases signed might take up to two years. There are other ways and solutions to get around these bottlenecks – Strand mounted femtocells, public Wi-Fi and Relay technologies with LTE and 4G technology.
Radio Clouds – Wi-Fi/Femtocells, Relays & Mesh Networks
In a few years from now, there will be an overlap of coverage between different radio technologies, by the same operator within the same area and the handset or device has to be intelligent enough to make the decision to use the Radio infrastructure. The device or handset will have many radios built in and the software stack will have to make the decision based not only on the received signal strength but the capacity limitations while moving seamlessly between one another to be able to use the best QOS for the type of service used. For example a browsing service will not be as data hungry as a video call or a YouTube video.
Wi-Fi offloading by operators has become the norm now by AT&T and other operators to reduce the overloading of the Macro cells. Here is one of my older analyses for offloading solutions using Wi-Fi as well as Femtocells.
A long-term potential option is around a forthcoming 3GPP standard in Rel.10 called SIPTO, standing for Selective IP Traffic Offload. This particularly relates to femtocell and RAN offload, where particular traffic is routed via different connections directly to the Internet, or via the operators core. A related standard is called LIPA (Local IP Access), which relates to the breakout of connectivity to the local network in the home or office.
The Traffic Offload Function (TOF) is the key element in the network here, which will be needed to perform a variety of tasks including policy enforcement and assorted “border” functions such as dealing with Network Address Translation (NAT) headers and lawful interception.
It remains unclear exactly what the mechanism and algorithms for selection policy will be in SIPTO, or how these are constructed and maintained. Exactly what traffic goes back via the operator core network, and what goes straight to the local network or Internet? In some scenarios, it might be done with shallow inspection, such as involving connections to particular APNs, or via particular ports.
Relay Transmission Schemes in LTE
There are two different terminologies used for Relay’s – Type-I and Type-II being the first two and the others are non-transparency and transparency types. Specifically, a Type-I (or non-transparency) RS can help a remote UE unit, which is located far away from an eNB (or a BS), to access the eNB. So a Type-I RS needs to transmit the common reference signal and the control information for the eNB, and its main objective is to extend signal and service coverage. Type-I RSs mainly perform IP packet forwarding in the network layer (layer 3) and can make some contributions to the overall system capacity by enabling communication services and data transmissions for remote UE units. On the other hand, a Type-II (or transparency) RS can help a local UE unit, which is located within the coverage of an eNB (or a BS) and has a direct communication link with the eNB, to improve its service quality and link capacity. So a Type-II RS does not transmit the common reference signal or the control information, and its main objective is to increase the overall system capacity by achieving multipath diversity and transmission gains for local UE units.
Amplify and Forward — An RS receives the signal from the eNB (or UE) at the first phase. It amplifies this received signal and forwards it to the UE (or eNB) at the second phase. This Amplify and Forward (AF) scheme is very simple and has very short delay, but it also amplifies noise.
Selective Decode and Forward — An RS decodes (channel decoding) the received signal from the eNB (UE) at the first phase. If the decoded data is correct using cyclic redundancy check (CRC), the RS will perform channel coding and forward the new signal to the UE (eNB) at the second phase. This DCF scheme can effectively avoid error propagation through the RS, but the processing delay is quite long.
Demodulation and Forward — An RS demodulates the received signal from the eNB (UE) and makes a hard decision at the first phase (without decoding the received signal). It modulates and forwards the new signal to the UE (eNB) at the second phase. This Demodulation and Forward (DMF) scheme has the advantages of simple operation and low processing delay.
A wireless mesh network is a communications network made up of radio nodes organized in a mesh topology. Wireless mesh networks often consist of mesh clients, mesh routers and gateways. The mesh clients are often laptops, cell phones and other wireless devices while the mesh routers forward traffic to and from the gateways which may but need not connect to the Internet. The coverage area of the radio nodes working as a single network is sometimes called a mesh cloud. Access to this mesh cloud is dependent on the radio nodes working in harmony with each other to create a radio network. A mesh network is reliable and offers redundancy. When one node can no longer operate, the rest of the nodes can still communicate with each other, directly or through one or more intermediate nodes. Wireless mesh networks can be implemented with various wireless technology including 802.11, 802.15, 802.16, cellular technologies or combinations of more than one type and can self form and self heal.
A wireless mesh network can be seen as a special type of wireless ad-hoc network. A wireless mesh network often has a more planned configuration, and may be deployed to provide dynamic and cost effective connectivity over a certain geographic area. An ad-hoc network, on the other hand, is formed ad hoc when wireless devices come within communication range of each other.
Seamless Mobility for Inter-RAT technologies
Seamless mobility needs to address things like low delay, seamless hand offs, multi hopping, fading and last but not the least bandwidth. Simply put, inter-technology mobility is the ability to support movement of a device between differing radio access network types. There are many variations of this definition. In particular, 3GPP defines two: Inter-RAT (Radio Access Technology) mobility, which refers to mobility between LTE and earlier 3GPP technologies and Inter-Technology mobility which refers to mobility between LTE and non-3GPP technologies.
Inter-technology mobility can be supported in a variety of ways. The most basic form of inter-technology mobility can be provided by a multi-technology device without any inter-technology support from the operator’s network(s). In this case, the user or the device selects which technology to use and initiates access to that technology. If the selected technology becomes unavailable, the user or device must select another technology, initiate access to it and re-establish communications with the applications that were in use. This primitive form of inter-technology mobility can be marginally acceptable for some applications (e.g. email and web browsing) and works for nomadic users. For other session-based applications (e.g., web-based financial transactions and VPN access), it seriously degrades the user experience since it typically results in loss of intermediate application results and requires users to re-authenticate themselves with the applications.
For high bandwidth applications such as video on demand or video streaming and for applications with stringent QoS requirements, this basic inter-technology capability is completely unacceptable. For example when using this simple form of inter-technology mobility, a streaming video application would require the user to reinitiate the stream from the beginning whenever the boundary between two access technologies is crossed. Or for a video telephony application, video calls would be dropped whenever a boundary between access networks is crossed and would have to be reinitiated in the new access network.
A much more useful form of inter-technology mobility supports data session continuity across multiple technologies. With data session continuity, users are able to maintain their application sessions as they move between different access technologies. Unlike the primitive form of inter-technology mobility described above, no user actions are required to support the change in access technology. Applications are unaware that an access network change has occurred when data session continuity is supported and thus there is no impact on the user’s log-on status or other applications data.
The IETF (Internet Engineering Task Force) Mobile IP (MIP) protocol, which was defined over five years ago, was intended to address the data session continuity issue. Unfortunately it functions completely at the IP level and has no way to address the time required for authentication and log-in when moving into a new access network. Without some form of mitigation this will cause severe disruption of many applications and thus significantly degrading the user experience.
Single Transmit Device – MIP-based
This is simple MIP-based mobility using a device that is only capable of communicating in one technology at a time. Two examples of this approach are the single transmitter versions of the non-optimized inter-technology handover procedure defined in the 3GPP standards for inter-technology mobility between WiMAX and LTE and between EVDO and LTE. Since the device can only communicate with one technology, it must break its connection with the source network before it can establish a connection with the target. Depending on the technology, the signaling associated with getting access to and authenticating on the target network can be quite time consuming and cause a significant gap in the user’s session.
Access Network Interconnect
Access Network Interconnect, requires the source and target access networks to be intimately connected in some way so that they can exchange control messages to help guide the movement of the device from one access technology to the other and to reduce the time that device is unavailable on either network. Historically this approach has been available for different generations of the same root technology such as cdma2000 and EVDO or UMTS and GSM, and this approach is being carried forward to provide mobility between LTE and GSM or UMTS. In all these cases the old and new technologies were controlled by the same standardization body, and the interworking can be just as easily viewed as a backwards compatibility requirement as an inter-technology mobility requirement.
With the introduction of LTE however, the limitation of access network interconnection to technologies covered by the same standards body is changing. LTE standards body, 3GPP, is working closely with the EVDO standards body, 3GPP2, to define inter-technology handover procedures that include mechanisms for interconnecting the LTE and EVDO RANs. Handover mechanisms that include exchange of information between the source and target RANs are generally referred to as optimized handover in the LTE standards. Optimized handover will support low-delay inter-technology handovers that can support demanding applications such as VoIP and video streaming. Currently LTE-EVDO optimized handover has made the most progress in the standards process.
Dual-Transmit Devices – MIP based
For environments where the level of inter-standards cooperation is less pronounced or where there is an urgent need to get inter-technology mobility deployed quickly, the Dual-Transmit Device (DTD) approach is attractive. In this approach the device does a true make-before-break handover to prevent data loss or the need for retransmission. The device uses its second transmitter to register and authenticate on the target network while maintaining its existing data session on the source network. Once the preliminary work is completed, and the device is ready to receive data on the new network, it uses a supported Internet protocol such as Mobile IP to move the data stream from the source to the destination network. The LTE standards accommodate the use of MIP in combination with DTDs to support efficient inter-technology mobility between LTE and Wi-Fi. The WiMAX Forum is also standardizing the use of DTDs with MIP with the primary goal of supplying mobility between EVDO and WiMAX.
Dual Transmit Device – SIP based
Due to a variety of practical, technical and business factors, MIP can be difficult to implement in some environments leading to the fourth approach of DTDs coupled with the Session Initiation Protocol. SIP can often be used in conjunction with dual-transmit devices instead of MIP. Additionally, SIP is the only choice if there is a need to move data sessions between devices as well as between technologies – e.g. a requirement to move a video session from a plasma screen supported by a set-top box connected to a DSL link to an LTE mobile device. An obvious drawback to this approach is that it is only applicable for those applications based on SIP. Also it will not work for any application that is sensitive to a change in a correspondent’s IP address (e.g. many applications based on TCP). Some additional standardization effort is needed to support inter-device and inter-technology mobility with SIP and IMS.
Cooperation is known as an effective strategy in nature to achieve individual or common goals by forming cooperative groups. As the crossover between nature and engineering has always been fruitful, let us see how pooled resources and cooperative concepts for wireless networks advocating mobile devices to cluster in a peer to peer fashion help us in forming a mesh network. Cooperation networks are advocated to overcome the most critical problems in mobile communication – energy consumption, security, and higher data rates.
Cooperative communications and networking represent a new paradigm which involves both transmission and distributed processing, promising significant increase of capacity and diversity gain in wireless networks. The cooperation among nodes, as in the case of wireless sensor networks, allows a distributed space-time signal processing which enables environmental monitoring, localization techniques, distributed measurements, and others, with a reduced complexity or energy consumption per node.
Cooperative Networks support a layered approach globally and this means that the functionality of the system is grouped into distinct layers that form logically separate subsystems. Any layered model has at least three layers: application, connectivity, and access. The application layer can be further divided into service application and service support sub layers. The connectivity layer can be further divided into network control and transport (IP) sub layers. And the access layer may contain several independent access networks that can also function simultaneously. The layers should have well defined interfaces and be functionally independent of each other. Such an approach is required to ensure easy adaptation of heterogeneous access technologies, related technology changes, and flexible support for rapid service innovation. Because of the need for cross-layer optimization (e.g., for power management, QoS support, etc), the layered approach in Cooperative networks is considered as a design principle rather than as a universal design pattern, meaning that every system may potentially have its own layered architecture.
Independent Functional Blocks, Modularization and Reuse
Cooperative networks architecture should define each function as an independent functional block. This promotes component-based modular architecture, where different building blocks can be combined as needed to realize complex systems. Functionalities required to process and route user data, handle control signaling, and deal with network management can be separated into different functional blocks in the user, control, and management planes, respectively. Each layer of the architecture has its own separate set of functions for each plane; hence, nine different functional blocks can be identified. Realization of the functionalities as independent building blocks allows the introduction of new functions when needed without changing the whole architecture.
The Cooperative networks architecture should ensure connectivity between all the entities of a network in a consistent manner across all access technologies for any service. This requires consistent support for device mobility, QoS, authentication, authorization, and accounting (AAA) and so on. The connectivity layer provides cooperation across various realizations of networks, called cooperative connectivity, and shall be independent of the various transport technologies used to link the nodes of the network together. By separating access and transport the Cooperative networks architecture makes it transparent to the common and standardized transport infrastructure and hides the technology from the end user, while facilitating the most efficient usage of spectrum resources. The user should be able to seamlessly roam across different access technologies and administrative domains without any manual user intervention. Cooperative networks architecture should also support connection of subscribers to private IP networks through Network Address Translators (NATs) and Simple Traversal UDP through NATs (STUNs).
An End- To – End Architecture
The Cooperative networks architecture includes the endpoints (i.e., terminals) as part of the communication system, and support end-to-end (E2E) negotiation and fulfillment of QoS parameters, security settings, and so on. This E2E approach does not mandate that all functionality should be located in the endpoints. On the contrary, the functionality can also be provided in hop-by-hop, and/or edge-to-edge manner subject to proper and lawful termination of transport connections. The Cooperative networks architecture should ensure that the interoperability of (and communication between) heterogeneous endpoints is maintained.
Self Organizing Cooperative networks
The Cooperative networks architecture is expected to comprise a number of networks exhibiting different capabilities in terms of coverage, capacity, transmission rates, transmission delay, and transmission cost. Networks might be in a cooperative or competitive relationship with each another. Part of the access domain in the wireless world may not be centrally organized in the future and may even provide infrastructure less connectivity. Nodes may come and go and be loosely associated with each other, forming alliances whenever and wherever. Such nodes that could be part of the personal, the local, and even the global sphere would temporarily cooperate to provide connectivity in an ad hoc manner or share application resources. A network that is a member of the Cooperative networks has to build relations with other networks to provide expected global connectivity and support the demanded access versatility. Relations between cooperative networks are supposed to be established dynamically. The creation of these dynamic relationships is a first aspect of self organization. The dynamic organization of relations between networks and individual network elements is supposed to result in a Cooperative networks structure that adapts its topology to meet the demands of varying traffic patterns and transmission demands. To ease administration and operation, the network nodes should mostly be self-configuring, and resources should be distributed among them dynamically to cope with varying traffic volumes and traffic characteristics
Cooperative networks and Seamless Mobility are still under the standarization process by the academia and the standards bodies, much farther away from commercial launch.