Semi Hierarchical Cellular Systems | NETWORK ARCHITECTURES

As OFDM (orthogonal frequency division multiplexing) technology has been considered for most next-generation mobile wireless networks, such as IEEE 802.16-WiMAX and 3G LTE (Long-Term Evolution), and these systems are susceptible to the inter-cell interference problem, there have been numerous approaches proposed to enhance the performance of cell-edge MSs. Recently, the 802.16m and 3G LTE Advanced standard groups proposed ways of supporting cell-edge MSs by deploying various network (multi-BS) MIMO antenna techniques as well as fractional frequency reuse schemes. In case fractional frequency reuse is adopted, the designated neighboring BSs of a cell to which a certain cell-edge MS is attached, should avoid concurrent transmission over the same set of channels. If macro-diversity or network MIMO is used, one or more neighboring BSs should serve a certain cell-edge MS at the same time, thus requiring concurrent transmissions over the same set of channels from multiple BSs to the same MS.

Such network-level radio resource coordination is easier to achieve, when there is an upper-level management entity like the ASN gateway. When BSs fulfill RRM in a flat architecture, the intercell RRM should be coordinated at least by one BS, rather than in a distributed way. Therefore, we present a simple solution of semi hierarchical cellular systems in Figure 1 where a super BS plays a role of a coordinator for intercell RRM. The super BS can be selected among a set of BSs in either a fixed or a dynamic way.
Figure 1: Semi hierarchical cellular systems



The 3G LTE standard (3GPP Release 8, 2009) has defined a simple network architecture of E-UTRAN (evolved universal terrestrial radio access network). The E-UTRAN consists of eNBs (evolved node Bs) which are interconnected with each other by the X2 interface. As presented in Figure 1, each eNB is connected with a S-GW (serving gateway). The S-GW terminates the S1 interface between an eNB and the MME (mobility management entity). The eNB hosts the functions of RRM and dynamic resource allocation as in other BSs, while the S-GW hosts the function of mobility anchoring by assigning an IP address to an end host. This architecture is similar to the Profile C of WiMAX ASN, since most RRMfunctions are fulfilled by the eNB in a flat manner while some mobility functions are fulfilled by the S-GW in a hierarchical manner.

Figure 1: The overall architecture of 3G-LTE E-UTRAN


Similarly, it is also a controversial issue how to implement the access network in the IEEE 802.11 wireless LAN systems. A subnet is composed of an AR and APs, where the hierarchical structure is also similar to cellular networks. Three types of APs are considered—Fat AP, Thin AP, and Fit AP—according to the role assigned to the AP (Sridhar, 2006). The Fat AP provides router-like functions, so there is no backhauling of traffic. This scenario is very close to the all-IP networking. In contrast, the Thin AP is close to the BS in the WiMAX Profile A. The primary role of Thin APs is to receive and transmit wireless traffic, but in this case, a group of APs are managed by a centralized access controller which acts as an ASN gateway in the WiMAX ASN. In the Fit AP architecture, MAC functions are split between the AP and the access controller, so this architecture is compromised between the Fat AP and the Thin AP models.

WiMAX ASN Profiles

The WiMAX standard has defined three different profiles, Profile A, B, and C, for an Access Service Network (ASN) which consists of multiple BSs and an ASN gateway (WiMAX Forum 2008). The relation between a BS and an ASN gateway is also similar to that between a BTS and a BSC in GSM systems. A hierarchical ASN is defined in Profile A and C, whereas a flat ASN is defined in Profile B. Profile A is a hierarchical structure that is similar to traditional cellular networks. 

As shown in Figure 1, the radio resource controller (RRC) and the radio resource agent (RRA) are implemented at the ASN gateway and the BS, respectively, so most radio resources are managed by the ASN gateway. In Profile B, the functionalities of a BS and an ASN gateway are co-located on the same platform/solution, which makes the architecture flat. That is, R6 defined for the link between an ASN gateway and a BS does not exist. In Profile C (Figure 2), the RRC is implemented at each BS, so all the RRM functions are performed at each BS as in a flat architecture, although it is still based on a hierarchical structure. Thus, mobility can be managed by the ASN gateway or other upper entities.

Figure 1: WiMAX ASN Profile A

Figure 2: WiMAX ASN Profile C

Cross-Layer Reference Communication Model

Some activities within UCWW, such as end-to-end (E2E) hot access network change (HAC) based on user-driven ABS&S policies, require cross-layer protocol functionality. Other examples include E2E reconfigurability (Z. Boufidis et. al., 2004, Sept), service adaptability (Houssos, N., et. al. 2003), E2E QoS support (Politis, C. al., 2004), ABC&S (O'Droma, M., Ganchev, I, et. al., 2006), user/network/service/terminal profile management, 3P-AAA and related 3P-C&B, and WBC & ADA operation. While this seems to contradict the layering architecture model for designing, planning, implementing and analysing communication protocols, nonetheless, it is the reality and it is worthwhile to structurally allow for it with suitable modifications of the reference models. Such a suitably modified reference communication model is presented in Figure 1. 

It has similarities with the B-ISDN/ATM reference communication model in that it is a 3D model consisting of three planes: user plane, control plane, and management plane. The new central element, which intersects all three planes, is added to allow for structured cross-layer functionality. This cross-layer core cylinder is a modification ofthat proposed in (Ganchev, I., O'Droma, M., et. al., 2006) and may be visualized as consisting of several parallel mini cylinders each with its own dedicated functionality, e.g., corresponding to the activities already listed above with cross-layer protocol functionality. Formal reflecting of these activities and their cross-layer functionalities into this model will assist their formal design and analysis, and facilitate development of formal and open primitives and APIs.

Figure 1: The proposed cross-layer reference communication model


In existing cellular networks, an access network consists of many entities for supporting radio resource management and mobility management. For example, in 2G GSM/GPRS networks, the base station subsystem (BSS) consists of the base transceiver system (BTS) that handles the physical layer and the base station controller (BSC) that handles radio resource management and handoff. Also, the mobile service center (MSC) fulfills upper layer functionality and acts as the visitor location register (VLR) that is required to update the location of every MS for paging. Protocols defined in each layer in GSM systems are exhibited in Figure 1, where several protocols are defined for communication between any two entities.

Figure 1: An example of protocol stacks in GSM systems

4G networks, in contrast, will make such a complicated protocol stack much simpler, by enabling IP packets to traverse between a base station (BS) and a mobile station (MS). Each BS may need to perform all the functionalities required in BSS, BSC, and MSC. This makes the BS play a role of an access router (AR). This architecture is shown in Figure 2. It incurs high overhead, however, especially when an MS configures a mobile IP (MIP) address for handoff. As it is known that it takes several seconds to run the MIP handoff (Yokotaet al, 2002), MIP hinders an MS from carrying out smooth handoff. In addition, the 4G network is expected to have a small cell radius due to use of high frequency band, which possibly results in short cell residence time. For this matter, reducing the latency in performing the MIP handoff is still a challenging issue. For instance, a fast handoff scheme(Koodli, 2004) proposes to decrease the address resolution delay by pre-configuration.

Figure 2: The pure all-IP 4G Network

Another feature of such all-IP networks is their flat architecture. All the radio resource management and mobility management will be performed at each BS independently of the other BSs. Unlike traditional cellular networks of a hierarchical architecture, the flat all-IP network can be operated flexibly but at the cost of complexity in terms of intercell RRM (e.g., coordination among cells). There are increasing demands for intercell RRM for efficient network management; for example, fractional frequency planning for OFDMA wireless networks is needed to improve cell-edge performance. The upper entity such as the BSC in hierarchical cellular networks could be a good coordinator for such a scheme.

To alleviate the difficulty in radio resource and mobility management of all-IP cellular networks, a semi (i.e., subnet-based) all-IP cellular network can be considered as shown in Figure 3, an example of a simple network where an AR manages several BSs. The functionality of an AR is separated from that of a BS in order that each undertakes L3 and L2 protocols, respectively. This relation is similar to that between BSC and BTS in GSM networks. Then, an MS moving within the subnet (i.e., changing BSs) performs L2 handoff without changing MIP attachment. The MS only needs to trigger L3 handoff, when it moves into another AR area.

Figure 3: The subnet-based 4G network

A main difference is that the former is decentralized while the latter is centralized. Since the pure all-IP network incurs a L3 protocol in the end access link, it requires a long handoff latency and high signaling overhead. However, the architecture is simple and cost-efficient for implementation. On the other hand, the subnet-based all-IP network implements hierarchical architecture, so it is possible to fulfill efficient resource management in spite of its inflexibility. Both network architectures are being considered in WiMAX and 3G-LTE systems, which are described in the following.


  • At the WBC Service Layer - Besides the intelligent software architecture already mentioned, other issues for future investigation include:

    • Agent environment: JADE has been used to date to act as an agent environment in the heterogeneous WBC software architecture. However, JADE is a heavy agent platform with a big footprint for executing both the SD collecting, clustering, scheduling, indexing, broadcasting on the server side, and the SD discovery and association on the mobile terminal side. In addition, it does not fully support the BDI agent. Therefore, investigation into lightweight BDI-based Java agent platforms (WBC-BDI) is recommended. Formatting the communication language's messages with WBC-ASN is also recommended, as well as ensuring that the agent platform functions correctly in the following environments: J2SE (Sun Java 2 platform, standard edition 2003), J2ME (Sun J2ME Specification 2009), Android (Google Android Software Development Kit 2008), WinCE (Windows Embedded CE Overview 2008), etc.

    • SD formatting: In order to encode SDs in a more compact way, an efficient abstract syntax notation language based on ASN.1 (WBC-ASN) is suggested. Any design should take into account the requirement for minimizing decoder's power consumption.

    • Rule engine: Resolution of the need to improve the flexibility and scalability could be approached by designing an intelligent SD self-organization lightweight Java rule engine. In suggesting this, we also recommend that the rules configuration file here could be defined with WBC-ASN.

    • ADP: Designing with system scalability in mind, the route of developing the ADP protocol in Java, together with a Java-based Reed-Solomon algorithm being fully implemented is suggested as worthy of investigation.

    • Profile design: To increase security and privacy for WBC-SPs, and mobility and personalization for mobile users, investigation of the benefits from this perspective of a well-structured rule-based profile developed and formatted with WBC-ASN is suggested.

  • At the WBC Link Layer and Physical Layer - Potential broadcast platform solutions include WBC over DVB-H, over DRM, over DAB, etc. Investigations in the technical realization configurations for each have yet to be undertaken. There is little doubt about the potential consumer base into which WBC advertisement may be pushed. Today, for instance, there is an expectation of 300 million DVB-H capable handsets operational by 2009/10.


  • Extending 3P-AAA into the area of wireless Ad Hoc networks. This can yield significant 3P-AAA use cases influencing the 3P-C&B requirements. Typical Ad Hoc domain scenarios involving hot-zone wireless heterogeneous architectures are envisaged where mobile terminals use multi-hop techniques to get to a hot zone using intermediate mobile terminals (the latter should benefit from their role in such scenarios, i.e., be paid properly).

  • Research to date has identified and established the basic charging scenarios in CBM-based UCWW by employing inter-3P-AAA-SP signalling. However, when the inter-3P-AAA-SP signalling involves Internet usage, then charging interactions can experience high network latency. To eliminate this problem either further optimization is needed in the 3P-AAA-SP signalling (i.e., compressing the messages where possible) or a new ‘charging agent’ concept should be developed. This new concept would result in the following: (1) the charging occurs in the metering domain (TSP/ANP), (2) the charging agent is downloaded from the 3P-AAA-SP to provide the charging function in the TSP/ANP domain, (3) the charging agent imports the charging rule set from the 3P-AAA-SP, (4) the charging agent imports segments of the consumer account into the metering domain.

  • Elaboration of the C&B framework to support dynamic reconfiguration of applicable metering and pricing policies for specific service, specific user or combination of both, and to support various pricing models according to the service profile, user profile and location, and one-stop billing schemes.

  • Implementation of a C&B system prototype as a discrete service that can be provided by a trusted third-party authentication, authorization and accounting service providers (3P-AAA-SPs).

  • Running trial experiments with the designed prototype in a 4G testbed environment showing good interfacing with the 3P-AAA service, WBC&ADA services, and other (new) types of 4G services (e.g., consumer-oriented ICC service).
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